Introduction

Accumulation of protein aggregates in the cellular milieu is a major burden for neurons, and it greatly disturbs the nervous system homeostasis. These misfolded and aggregated proteins are hampering the activities and transmission of the neuronal cell. The accumulation of aggregates induces toxicity, which causes memory loss, cognitive decline, and impairment in the maturation of neuronal cells that result in the progression of several neurodegenerative disorders (NDDs), including Alzheimer’s disease (AD), Parkinson’s disease (PD), Amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD), and Multiple sclerosis (MS) [1, 2]. Excessive accumulation of abnormally aggregated/non-functional proteins in the cytoplasmic region of the cell leads to organelle damage, which is responsible for neuronal death in the central nervous system (CNS) and ultimately leads to cognitive defects and synaptic dysfunction. Apoptosis and autophagy are two degradation mechanisms that are currently known for eliminating the degraded components and quality control of cellular components, which is necessary for maintaining cellular homeostasis. Autophagy is defined as the lysosomal-dependent degradation process of cytoplasmic constituents, whereas, apoptosis is considered as programmed cell death (PCD) of cells. Autophagy is of three types, namely macroautophagy, microautophagy, and chaperone-mediated autophagy that occurs through the formation of autophagosomes followed by association with lysosomes leads to the formation of the autophagolysosomal complex. On the contrary, apoptosis is described as morphological and physiological changes required to maintain cellular homeostasis by inducing nuclear membrane destruction, DNA fragmentation, and generation of apoptotic bodies [3, 4]. Recent studies demonstrated that the perturbation of autophagic machinery causes accumulation of misfolded proteins, and excessive induction of apoptotic mechanism leads to neuronal death that is involved in the pathogenesis of NDDs [5, 6]. Excessive loss of neuronal cells leads to cognitive defects, impaired neurogenesis and neural differentiation, synaptic dysfunction, and memory impairment, which are characteristic features of NDDs [7, 8]. However, the molecular crosstalk between autophagic degradation and apoptotic cell death is a complicated phenomenon and has provided conflicting results but at the same time necessary for determining the fate of the cell. However, under physiological conditions, such as excessive oxidative stress, reactive oxygen species (ROS) production, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress, neuronal cells exhibit defective or incomplete autophagic degradation of misfolded protein aggregates and, therefore, apoptotic machinery that causes neuronal cell death. Extensive investigations identified the potential implementation of epigenetic regulator p53 and pro-angiogenic marker vascular endothelial growth factor (VEGF) in the modulation and regulation of both apoptosis and autophagy machinery.

Moreover, autophagy is known to have a dual effect on apoptosis, which involves inhibition and induction of the apoptosis pathway. Under stress conditions, apart from misfolded protein degradation, autophagic machinery, either itself or through apoptotic induction, causes cell death depending upon the exposure of a stress condition [9, 10]. Both autophagy and apoptosis pathways regulate brain homeostasis through the involvement of downstream targets such as the mammalian target of rapamycin (mTOR), Bcl and BH3 family of proteins, caspases, 5' AMP-activated protein kinase (AMPK), class III phosphatidylinositol 3-kinase (PI3K), and glycogen synthase kinase 3β (GSK3β). Recent studies explored the potential of biomolecules, long non-coding RNAs (LncRNAs), and micro-RNAs (miRNAs) as therapeutic modulators of these pathways involved in the pathogenesis and progression of NDDs.

Herein, we provided a comprehensive story derived from various literature sources to dissect the molecular mechanism between apoptosis and autophagy in NDDs. In the beginning, we have discussed about cell death pathways followed by the shared mechanism between three types of cell death pathways and the dual role of autophagy on apoptosis. The later part of the review discusses the molecular markers of cell death in NDDs with apoptosis and autophagy signaling. Finally, we discuss the potential application of miRNAs, LncRNAs, and biomolecules on different cell death pathways.

Overview of cell death pathways

Autophagic pathway: act as pro-death and pro-survival signaling cascade

Autophagy is a molecular phenomenon used to eliminate damaged organelle and protein aggregates, which is characterized by the formation of autophagosomes and interaction with the lysosome. Cytoplasmic component degradation in the lysosome is divided into three subtypes as follows: macroautophagy, microautophagy, and chaperone-mediated autophagy. The mechanism underlying autophagy includes phagophore membrane formation from the Golgi apparatus, mitochondria, plasma membrane, and ER, where misfolded proteins and degraded cytoplasmic material are wrapped, elongated and forms autophagosome. This autophagosome, through microtubule dynamics, transports to the lysosome, where the formation of autolysosome occurs through the fusion of autophagosome and lysosome [11, 12]. Further, autophagy is a multi-regulatory process initiated by two major clusters of proteins UN51-like Ser/Thr kinases (ULK) complex and PI3K complex. The ULK complex consists of ULK1/2 family, FAK family kinase interacting protein of 200 kDa, autophagy-related protein 13 (Atg13) whereas, PI3K complex consists of vacuolar protein sorting 34 (Vps34), p15 (Vps15), beclin1 (Atg6), and Barkor (Atg14) [13, 14]. Two ubiquitin complexes control the elongation and interaction of autophagosomes. Firstly, a complex Atg5/Atg7/Atg12 is formed due to covalent interaction between Atg5/Atg7 and Atg12. Secondly, this Atg5/Atg7/Atg12 complex interacts with Atg16 to form another complex, Atg5/Atg7/Atg12/Atg16, that is required for autophagosomes elongation. Another complex associated with the molecular marker of autophagosome is formed through the proteolytic cleavage of microtubule-associated protein 1 light chain 3 (LC3) with Atg4B to generate LC3-II [15,16,17,18]. However, autophagosomes require a motor and kinesin protein along with the recruitment of protein complexes known as the soluble NSF attachment protein receptor (SNAREs) for relocation along the microtubule, fusion with the lysosome, and protein degradation [19] (Fig. 1).

Fig. 1
figure 1

Molecular connection between apoptosis and necroptosis as cell death pathways opening a new area of research in the field of neurodegenerative disorders. However, whether autophagy is a pro-death or pro-survival pathway is still a matter of concern. From the past two decades, extensive research in this field has found the connection between three pathways involving different molecular mechanisms and biological processes. Mitochondrial dysfunction, genotoxic stress, oxidative stress, Ca2+ concentration, and ER stress were among the major external factors that activated the signaling cascade leading to cell death. Here, Bax and Bak were two pro-apoptotic proteins that activate the apoptosis pathway, while mTOR, TSC1, and TSC2 were important in regulating the autophagic pathway. Activation of mTOR causes inhibition of ULK1 complex that further inhibits the autophagic pathway. Similarly, phosphorylation of TSC1 and TSC2 causes inhibition of Rheb, which leads to mTOR activation and subsequently inhibition of autophagy pathway. Besides external stress factors, Atg12 leads to activation of autophagosomes, cytochrome C, and caspase, followed by necrosome leads to the autophagic, apoptotic, and necroptosis pathway, respectively

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Apoptosis pathway: intrinsic and extrinsic cell death machinery

Apoptosis, an important molecular phenomenon, which is also known as PCD, is involved in the maintenance of tissue homeostasis. Apoptosis is best described as nuclear morphological changes characterized by chromatin regulation, degradation of cytoskeletal proteins, nuclear membrane breakdown, DNA fragmentation, and generation of apoptotic bodies adjacent to the cell surface [20, 21]. The physical execution of apoptosis can be initiated by either the extrinsic or intrinsic apoptotic pathway. Moreover, death receptors and internal stimuli such as DNA damage, activation of pro-apoptotic factors of B-cell lymphoma 2 (Bcl-2) family, and upregulation of p53 play a major function in regulating the apoptotic pathway [22, 23]. Extrinsic apoptotic pathway induces the attachment of tumor necrosis factor (TNF) family receptor on the cell surface, which increases the recruitment of fas-associated death domain protein (FADD) and TNF-related apoptosis-inducing ligand (TRAIL) following the binding of initiator caspases (caspase 8 and caspase 9), which initiate its autoproteolytic processing. Initiation of autoproteolytic processing leads to activation of effector caspases (caspase 3 and caspase 7), resulting in cleavage of Bcl-2 homology region 3 (BH3) protein, which induces pro-apoptotic factors’ activation and alters inner mitochondria membrane permeability [24,25,26]. On the contrary, the intrinsic apoptotic pathway, also called the mitochondrial apoptotic pathway, is a death receptor-independent mechanism and requires Bcl-2 member proteins, consisting of the BH1-3 domain, to decide whether to undergo mitochondrial membrane permeabilization or not. Further, intrinsic apoptotic pathway causes sequestration of pro-apoptotic factors from mitochondria to cytosol, including cytochrome C, a second mitochondria-derived activator of caspases/direct IAP-binding protein with low PI (Smac/DIABLO), HtrA2/Omi, and apoptosis-inducing factors (AIFs), which results in the generation of apoptosome complex.

Bcl-2 associated X protein (Bax) and Bcl-2 homologous antagonist/killer (Bak), which have BH1-3 domain required for the execution of the mitochondrial apoptotic pathways in a caspase-dependent or caspase-independent manner [27,28,29]. Apoptosis is a highly regulated phenomenon controlled by the inhibitor of apoptosis proteins (IAP) and X-linked inhibitor of apoptosis protein (XIAP), which can interfere in the caspase activation process leading to caspase-dependent or caspase-independent apoptosis. Further, ROS, nitrogen–oxygen species (NOS), and DNA damage are considered to be inducers of apoptosis, resulting in the activation of signaling cascade that results in cell death in various disease models of NDDs. These agents lead to activation of janus kinases/signal transducer and activator of transcription protein (JAK/STAT) signaling pathway, through increased activity of cytokines, such as a nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway and PI3K-like kinases, respectively, which promotes cell apoptosis [30,31,32] (Fig. 1).

Necroptosis cell death machinery

Necroptosis is the well-characterized molecular phenomenon of unprogrammed cell death activated by cellular damage or pathogenic infiltration regulating necrosis mediated by receptor-interacting protein kinase 1 (RIPK1) and receptor-interacting protein kinase 3 (RIPK3). Activation of RIPK1 and RIPK3 eventually leads to plasma membrane permeabilization, activation of cytokines and chemokines, sequestering cell content, and exposure of damage-associated molecular patterns (DAMP’s) [33]. RIPK1 initiated a signaling cascade, which phosphorylates and activates RIPK3 that further phosphorylates and activates mixed lineage kinase domain-like (MLKL), forming a complex known as necrosome. Necrosome cause cell rupture because of the pore-forming ability of MLKL aggregates, modulation of ion channels, and the inflammasome formation in some cellular contexts [34,35,36,37]. Inhibition of RIPK1, RIPK3, and MLKL and activation of necrosome in concert with necrosis is the pharmacological feature of necroptosis. The number of literatures suggesting the role of caspase 8 inhibition in transferring the mitochondrial apoptotic pathway to necroptosis cell death pathway due to increased expression of RIPK3 and MLKL and initiation due to immune-based ligands [38]. Necroptosis resembles the apoptotic cell death pathways due to the implication of caspase 8 and death receptors such as TNF alpha, FADD, Tumor necrosis factor receptor type 1-associated DEATH domain protein (TRADD), and TNF receptor-associated factor 2 (TRAF2) and hence is called as alternative cell death signaling pathway [39]. In addition, FADD/RIP3 and FLIP/RIP3 knock out the model interplay between apoptosis and necroptosis due to the absence of FLIP and caspase 8-FLIP heterodimers. In another study function of protein kinase B (Akt) as a molecular switch between apoptosis and necroptosis through phosphorylation, production of TNFα, and blocking of pro-apoptotic factor response was demonstrated [40, 41] (Fig. 1).

Crosslinking autophagy and apoptosis signaling pathway

Calcium efflux and endoplasmic reticulum stress response

Misfolded protein aggregates cause activation of ER stress signaling, which involves the synthesis and degradation of proteins via autophagic pathway and endoplasmic-reticulum-associated protein degradation (ERAD) pathway. Further, eliminating the damaged organelle through apoptotic machinery decides the fate of the cell that depends on the intensity and time-duration of the implied stress condition [42]. During this process, molecular chaperone GRP78/BiP interacts with mechanistic UPR signaling molecules, namely activating transcription factor 6 (ATF6), protein kinase RNA (PKR), ER kinase (PERK), and inositol-requiring protein 1α (IRE1α). The complex between GRP78/BiP and UPR signaling molecules activate respective transducers and assist in the folding of accumulated proteins. However, PERK attenuates mRNA translation and thus inhibits the entry of newly synthesized protein in contact with the ER under stress conditions along with eIF2α activation [43]. Moreover, eIF2α phosphorylation causes protein synthesis inhibition mediated through a dedicated protein translational mechanism.

Under a high-stress environment, ATF4 causes both autophagy and apoptosis induction through regulation of Atg genes, and XIAP interacts with C/EBP homologous proteins (CHOP/GADD153) mediated through increased caspase activation [44]. Moreover, CHOP activates apoptotic pathways through increased expression of pro-apoptotic factors (such as BIM and death receptor 5), decreased expression of anti-apoptotic factors (Bcl members), and increased mitochondrial activity. Further, increased mitochondrial function leads to elevate cytochrome-c release from mitochondrial pores along with EROα and IP3R. Activation of EROα and IP3R causes an increase in mitochondrial calcium influx, which induces the apoptosis pathway [45]. However, under the ER stress environment, JNK mediated Bcl-2 phosphorylation leads to Beclin-1/Bcl-2 dissociation and autophagy activation, while a prolonged stress environment causes activation of the apoptotic pathway [46]. Further, ER stress increases calcium influx, which leads to AMPK activation and inhibits mTOR activity and thus induces the autophagy pathway. Similarly, ER stress also causes mitochondrial dysfunction through increased generation of mitochondrial pores leading to mitochondrial death via apoptotic machinery [47, 48]. Altogether, it may be concluded that ER stress regulates both autophagy and apoptosis machinery through modulating downstream targets and increased calcium ion concentration leading to mitochondrial dysfunction.

The implication of ubiquitin–proteasome system

UPS machinery is the major protein degradation pathway involved in neuronal regeneration and plasticity, whereas apoptosis and autophagy are the major regulatory signaling cascade involved in neuronal cell death that leads to neurodegeneration. Mounting evidence suggests the extensive crosstalk between autophagy, apoptosis, and UPS, which are involved in regulating brain homeostasis [49]. A recent study by Tsai et al., demonstrated that administration of Maackiain (MK) in the SH-SY5Y cell line prevents PD pathology through apoptosis inhibition and autophagic degradation due to increased PINK1/parkin expression and enhanced UPS machinery [50]. Similarly, Mudawal et al. demonstrated that dose-dependent administration of lindane in aged rats at 2.5 mg/kg concentration for 21 days causes alteration in apoptosis and autophagic markers expression. The study concluded that administration of lindane causes significant upregulation of Bax, Bad, caspase 3, caspase 9, ATG5, ATG12, LC-III levels, and causes a decrease in Bcl-2 expression. Thus, the analysis concluded that administration of lindane alters the expression of proteins associated with UPS machinery, autophagic cascade, and apoptotic pathway [51]. In post-traumatic brain injury, UPS machinery, axonal degeneration, apoptosis, and autophagic degradation play an important role, where enhanced expression of UCH-L1 modulates the autophagic pathway and UPS pathway. Congregation of UCH-L1 with TAT promotes neuronal transduction where it causes inhibition of K48-linkage polyubiquitination in the hippocampus but no effects on K65-linkage polyubiquitination. Further, the combination of UCH-L1 and TAT decreases autophagic degradation and neuronal apoptosis through decreased expression of Beclin-1 and LC3-II proteins [52].

Further, Guo et al. demonstrated the involvement of p-p38α as a central mediator of autophagy and apoptosis in response to UPS impairment. Reduced phosphorylation of p-p38α in response to BIRB796 causes a decrease in autophagic flux and neuronal apoptosis [53]. Likewise, the interaction between E3-ubiquitin ligase FBXO32/atrogin-1 and FOXO3A regulates autophagic and apoptotic cascade. Thus, administration of Endophilin-A in cultured neurons downregulates FBXO32 expression, which causes a decrease in neuronal apoptosis and increases autophagosome formation [54]. Similarly, administration of Trehalose in HD patients demonstrated a decrease in ROS levels, ubiquitinated protein expression, caspase 3 expression. Further, administration of Trehalose counteracts the decrease in LC-3 levels induced by Epoxomicin [55].

Moreover, Dietary restriction is known to regulate autophagic and apoptotic cell death through the involvement of UPS machinery. Shruthi et al. demonstrated that dietary restriction increases autophagic degradation in a spontaneous obese rat model and decreases Bax and p53 activity, thus preventing neurodegeneration [56]. Further, Xu et al., in SH-SY5Y cell culture, demonstrated that SIAH silencing through siRNA suppressed apoptosis, promoted cell proliferation, and decreases LC3-II expression [57]. Furthermore, XIAP, a ubiquitin E3 ligase, regulates mitochondrial depolarization, where XIAP in the absence of BH3 protein activates Bax-induced mitochondrial outer membrane potential (MOMP). XIAP targets the dysfunctional mitochondria for the autophagy-lysosomal pathway and delays cytochrome-C release, hence lowering the mitochondrial apoptotic potential [58]. Altogether, it may be concluded that UPS machinery regulates both apoptosis and autophagy signaling cascade through respective downstream targets in case of neurodegeneration.

Dual role of autophagy on the apoptotic signaling cascade

In the above sections, direct and indirect factors have been described through which the relationship between autophagy and apoptosis has been established, for instance, autophagic degradation of active caspases, the interaction between Beclin and proteins of family Bcl, expression activity of autophagic protein Atg, calpain-mediated cleavage of Atg, functional activity of cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein, and p53 mediated regulation. [59,60,61,62]. Autophagy helps in degrading misfolded and unfolded protein structures, but only up to a certain threshold beyond which it may cause cell death either directly or via regulation of apoptosis through common regulators. Several autophagic proteins were regulating apoptotic cascade through direct involvement with apoptotic machinery without activation of the entire autophagic process. Numerous studies demonstrated that genetic manipulation in the autophagic pathway regulates the activation of the Fas-dependent death-inducing signaling complex, which activates pro-apoptotic genes and initiates apoptotic pathways [63]. Moreover, ER stress induced by tunicamycin and thapsigargin regulates caspase 8 ubiquitination, which forms a complex containing caspase 8, Atg5, FADD, and translocation autophagosomal membrane. Further, this complex in the absence of caspase 9, Bax, and Bak leads to the activation of caspase 8 dependent apoptotic cell death. Moreover, knockdown of Atg5 and Atg7 resulted in the deficiency of caspase 8 dependent apoptosis [64, 65]. Different studies performed on the regulatory steps of autophagy concluded that inhibition of late steps of autophagy induced caspase 8 activation, which leads to induction of apoptosis rather than knockdown of Atg5 and Atg7 at early stages. Thus, activation of apoptosis due to early inhibition of autophagy contradicted findings of the experiments performed by Amir et al., 2013, which stated that inhibition of Atg7 leads to caspase-dependent apoptotic cell death [66, 67]. However, the molecular mechanism and factor that trigger autophagosomes to initiate caspase activation and the apoptotic pathway are still poorly understood. Moreover, autophagy is also capable of apoptosis induction by inhibiting the conserved family of cytosolic protein known as IAPs by activating caspases [68]. During stress conditions, Atg5 and Atg12 have been evolved as an important regulator of an apoptotic pathway independent of their specific functions in autophagy machinery, which is cleaved by calpains leading to translocation of its N-terminal fragment in mitochondria where it mediated the release of cytochrome c through pro-survival factors such as BCL and BCLXL. Further, mitophagy is the molecular phenomenon through which autophagy reduces the tendency of the cell to undergo an apoptotic pathway. Mitochondria, as an initiator of apoptosis, release pro-apoptotic factors, namely cytochrome c and SMAC, which cause the failure of mitochondrial bioenergetics due to the rupture of the mitochondrial membrane. Thus, removal of damaged mitochondria by the autophagic phenomenon can increase the threshold for apoptosis induction [69,70,71,72]. Altogether, autophagy is not only capable of attenuating apoptosis through damaged mitochondria but also the expression of caspases. Hou et al., demonstrated that autophagy inhibition mediated by Beclin-1 and Vps34 knockdown causes an increase in catalytic processing of caspase 8 prodomain, the release of cytochrome c, and generation of Annexin V-positive cells’ subpopulation in TRAIL-induced Bax-/-Hct cells and cisplatin-treated caspase 8 deficient mice cells [73]. Autophagy is considered a molecular phenomenon through which cells can evade apoptosis, but the molecular mechanism of such a process is poorly understood. However, different studies demonstrated the synergic effect of autophagy inhibitors and other drugs in estimating the relationship between autophagy and apoptosis. Fitzwalter et al. observed that autophagy regulating FOXO3a due to basal autophagy leads to a potential feedback loop, which on autophagy inhibition increases the expression of pro-apoptotic factors such as Bcl-2 Binding Component 3 (BBC3/PUMA), which sensitize apoptotic pathway [74]. Another study demonstrated that infracted high mobility group box 1 (HMGB1) upregulated autophagy by increasing the expression of proteins, including LC3, Beclin-1, and Atg7, along with the decrease in Bax, Bcl-2, Caspase 3, and mTOR expression activity [75]. Altogether it may be concluded that autophagy and apoptosis are two interconnected molecular phenomena in response to cellular stress. However, the mechanism is still not yet understood. The cytoprotective function of autophagy involves negative regulation of apoptosis and vice-versa. p53 is another important regulator of autophagy and apoptosis, which inhibits mTOR activity followed by downstream targets, regulates cell cycle progression and apoptosis pathway. This study observed that knockdown of p53 or autophagy inducers mediates the proteasomal degradation of p53 through the HDM3/E3 ubiquitin ligase system [76, 77].

Molecular phenomenon between apoptosis and autophagy

Involvement of p53 pathway

Tumor suppressor, TP53 gene encodes p53 protein from three transcription factor (TF) subunits such as p53, p63, and p73, which have a central role in transcriptional regulation involved in the pathogenesis of NDDs. P53, a gatekeeper of the cell, is activated by different post-translational modifications, namely acetylation, methylation, and ubiquitination. Further, it is known that p53 responds to a number of cell toxicity conditions, such as genotoxicity, oxidative stress, and metabolic stress [78,79,80,81]. p53 is a well-known regulator of autophagy and apoptotic cell death pathways during the DNA damage response and cell cycle arrest [82, 83]. Moreover, p53 also promotes the activation of both extrinsic and intrinsic apoptotic pathways. In the extrinsic pathway, nuclear p53 accelerates the expression of the APO-1/Fas receptor and the TRAIL receptor, whereas cytoplasmic p53 increases the caspase 3 and caspase 8 activities. In the intrinsic pathway, nuclear p53 is known to upregulate pro-apoptotic factors such as PIDD, BH3 only protein, p53 upregulated modulator of apoptosis (PUMA), Phorbol-12-myristate-13-acetate-induced protein 1 (NOXA), Bax, and BID leads to caspase 9 and caspase 8 activation. Likewise, cytoplasmic p53 translocates towards mitochondria, promoting the activity of Bax and Bak proteins after forming a complex with Bcl-2/Bcl-XL and activation of crucial apoptosome protein APAF1 [84,85,86,87]. Kim et al. demonstrated that depletion of intracellular zinc in N,N,N′,N′-tetrakis(2-pyridylmethyl) ethylenediamine (TPEN) induced mouse cortical neuronal cells regulate the apoptosis pathway by p53-induced protein synthesis, where poly(ADP-ribose) polymerase (PARP)-1 acts as an upstream effector of p53 induced neuronal apoptosis [88, 89].

Different studies have also demonstrated the effect of p53 on the autophagic cell death pathway through inhibition of the mTOR complex 1 by transcriptional activation of sestrin proteins and AMPK. Further, p53 induces the expression of damaged-regulated-autophagy-modulator (DRAM) through an unknown molecular mechanism that helps in regulating the expression of crucial autophagic genes such as LKB1 and ULK1/2 along with autophagosome maturation genes such as Atg4, Atg7, and Atg10 [90,91,92]. Moreover, p53 promotes the TFEB/TF binding to IGHM enhancer 3 (TFEB/TFE3) nuclear translocation during the DNA damage response through an increase in TF forkhead box O3a (FOXO3) expression and activity, which regulates upstream effectors of the autophagy pathway [93, 94]. However, further studies need to be done to understand the mechanism of p53 in autophagy. p53 mediated increase in autophagic cell death may be implemented in several neuronal cell death, but the precise mechanism should be defined before any concluding remarks. Lee et al. demonstrated the interrelation between apoptosis and autophagy in mouse embryo fibroblasts, where the deficiency of Atg7 leads to induce p53 dependent apoptosis. Moreover, Robin et al. demonstrated that the absence of p53 in Drosophila results in autophagic flux impairment, caspase activation, and mortality under oxidative stress [95] (Fig. 2A).

Fig. 2
figure 2

A Molecular mechanism of P53 involvement in the autophagic and apoptosis pathway. In the presence of oxidative stress and DNA damage response activation of MAPK. In the presence of oxidative stress and DNA damage response activation of MAPK, NF-KB, ATR, and ATM genes were carried out, which ultimately leads to the activation of p53. p53 activates m-TOR, HSF1, Sestrin ½, Bax, and NOXA, leading to the activation of different signaling cascades that regulate autophagic and apoptotic cell death pathways. Beclin-1 leads to the activation of PIP3 and Caspase 3, which activates autophagy and apoptosis, respectively. B VEGF is another essential protein that connects autophagy and apoptosis through different signaling molecules and cascades. VEGF leads to activation of VEGFR and VEGFR2, followed by activation of downstream targets such as PI3K/Akt and MAPKT of autophagic pathway and eNOS, BAD, and Bcl-2 of the apoptotic pathway. VEGF also acts on AMPK, HIF-1, Caspase 3, and Integrins, which further regulates downstream targets of signaling cascade such as TSC1, TSC2, PI3P, ATGG, mTOR, and Beclin that leads to the regulation of an autophagic pathway. Similarly, VEGF's interaction with PI3K/Akt, Ras, and EGFR activates pro-apoptotic factors that in turn activate signaling molecules like HSP90, cytochrome c, ASK1, MDM2, and Raf, leading to the initiation of apoptotic death pathway

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Angiogenic pathway: role of VEGF

VEGF is involved in biological processes, such as cell proliferation, cell migration, and tube formation, which can induce diseases such as NDDs, cancer, arthritis, and diabetes [96]. Recent studies demonstrated the antiproliferative, apoptotic, and autophagic effects of anti-angiogenic drugs targeting VEGF, which induces cellular and molecular responses during stress conditions. For instance, Liu et al. showed that apatinib, a highly selective inhibitor of vascular endothelial growth factor receptor 2 (VEGFR2) tyrosine kinase that is involved in the alteration of cell cycle arrest, apoptosis, and autophagy. Further, inhibition of autophagy increases apoptotic effect through direct binding between VEGFR2 and signal transducer and activator of transcription 3 (STAT3). Inhibition of VEGFR2 mediated by siRNA resulted in the downregulation of STAT3 and Bcl-2 reinforced autophagy and apoptosis induced by apatinib [97]. Further, Endostatin activates autophagy through decreased Bcl-2 expression and increased Beclin-1 expression in Eahy926 human endothelial cells [98]. Yang et al. 2014 demonstrated the inducing effect of convallatoxin on autophagy and apoptosis through increased cleavage of caspase 3 and PARP along with LC3 conversion. Moreover, convallatoxin inhibits the mTOR/p70S6K signaling pathway, resulting in autophagic induction and exerting anti-angiogenic activity in-vitro and in-vivo [99].

VEGF-B is the neuroprotectant lacking general angiogenic activity that rescues neurons from apoptosis in rat and mouse cell lines. VEGF-B inhibits the expression activity of BH3 proteins along with p53, a member of the caspase family mediated through activation of VEGFR1, thus hampering retinal neovascularization [100]. Similarly, Falk et al. 2009 demonstrated the neuroprotective implication of VEGF-B in the culture model of PD where expression of VEGF-B was upregulated while the activity of VEGF-A remains unaltered [101].

Moreover, the lentiviral-mediated expression of VEGF165 was found to be neuroprotective in both SHSY-5Y and rat primary striatal cultures, which attenuated DARPP-32+ mediated neuronal loss and rescued Exp-Htt aggregation [102]. Religa et al. 2013 studied the effect of VEGF on β-amyloid (Aβ) induced endothelial cells in-vitro. VEGF significantly prevents neuronal apoptosis and restored memory deficit in the transgenic AD mice model [103]. Further, the administration of batroxobin would exhibit neuroprotective effects in the spinal cord injury model mediated through neurotrophic factors and increased expression of VEGF, which reduces apoptosis [104]. Administration of VEGFR2 inhibitor PTK787/ZK222584 on primary cerebellar granule neurons prevented 1-methyl-4-phenylpyridinium ion (MPP+) induced neurotoxicity followed by neuronal apoptosis. Inhibition of VEGFR2 activates PI3K/Akt and ERK pathways, which play the opposite role in MPP+-induced neuronal apoptosis [105]. Studies in the past demonstrated the plausible function and mechanism of VEGF-B in neurodegeneration, altering mitochondrial dysfunction and neuronal cell apoptosis while lacking traditional angiogenic activity, especially in the PD model. VEGF also acts as a therapeutic target in NDDs and can be an interesting topic for crosstalk between oxidative stress and mitochondrial biogenesis [106] (Fig. 2B).

Molecular markers of neuronal cell death

Mammalian target of rapamycin

mTOR is the key signaling mechanism of cell growth and is considered as the master regulator of autophagy, protein synthesis [107], and mRNA translation [108], transcriptional regulation, and phosphorylation of other protein substrates. Inhibition of mTOR with rapamycin acts as an initiator for autophagy induction as mTOR activity inhibits autophagosome formation, which is crucial for the induction of autophagy signaling cascade. Alteration in autophagy cascade, possibly due to mTOR implication, has been observed in different neurological defects [109,110,111]. Further, the mTOR signaling cascade has been linked with the establishment of neuronal plasticity, shape, spine morphology, and axonal development. In an in-vitro study, it was demonstrated that activation of the mTOR signaling pathway induces the growth and branching of dendritic cells along with the reduction of dendritic complexity through mTOR or S6K1 knockdown. Further, in rat hippocampal neurons, it was observed that activation of both mTOR1 and mTOR2 signaling is required for neuronal development and organization along with the change in expression activity of Calcium/calmodulin (Ca2+/CaM) dependent protein kinase II [112,113,114]. Similarly, the mTOR pathway regulates axon outgrowth, as shown in mouse dorsal root ganglia neurons (DRGNs). Further, deletion of TSC2 and association of the mTOR with tuberin and GTP-binding protein Ras homolog enriched in the brain (RHEB) was found to promote axon outgrowth both in the in-vivo and in-vitro mouse model [115, 116]. Likewise, the mTOR signaling cascade modulates excitatory and inhibitory neurotransmission regulating synaptic plasticity as observed in the phosphatase and tensin homolog protein model of the knockout mouse. The mTOR pathway increases synaptic vesicles, synapse response, and the number of synapses both in glutamatergic and GABAergic neurons [117]. Likewise, the mTOR antagonist rapamycin treatment results in hippocampal neurons demonstrated long-term reduced potentiation promoted by high-frequency stimulations, together with inhibition of synaptic potentiation promoted by brain-derived neurotrophic factors (BDNF) [118]. Moreover, rapamycin prevented 3,5-dihydroxyphenylalanine induced metabotropic glutamate receptor (mGluR) mediated long-term potentiation through Akt and mTOR phosphorylation in CA1 hippocampal neurons [119]. Abundant evidence suggests the possible role of mTOR inhibition in the anti-aging effect through cellular senescence relevant to NDDs such as AD, PD, ALS, and HD [120]. In the 3XTg AD and S6K1 knockout mouse model, inhibition of the mTOR downstream signaling pathway resulted in decreased cognitive defects by reducing Aβ and Tau pathology [121]. In-vitro and in-vivo models have demonstrated rapamycin-mediated neuroprotection from synaptic toxicity, tau-induced neuronal cell death, and astrogliosis [122]. Altogether, rapamycin antagonist temsirolimus prevents tau-induced toxicity and the formation of neurofibrillary tangles via enhanced autophagy [123]. Several studies have demonstrated the effect of the increased number of autophagosomes in α-synuclein-induced dopaminergic cell death, suggesting a pivotal role of autophagy pathway induction in the PD model while inhibition of mTOR with rapamycin causes an increase in autophagy, which inhibits the accumulation of ubiquitinated α-synuclein [124, 125] (Fig. 3A).

Fig. 3
figure 3

A mTOR is an anti-autophagic molecule that acts on the ULK1 complex and P70S6K leads to activation of downstream signaling molecules to alter autophagy and apoptosis pathway. mTOR inhibits ULK complex followed by deactivation of autophagy, which inhibits mHTT and alpha-synuclein clearance and increases memory impairment, and cognitive decline. mTOR also increases SOD aggregate and ALS and decreases expression of P70S6K, which increases Aβ aggregation followed by Aβ toxicity, which causes memory impairment and ultimately leads to AD. B Neurotoxins cause oxidative stress, which activates AMPK, decreases phosphorylation of AMPK, activates p53, and increases mitochondrial dysfunction. ER stress increases the calcium influx, which activates calpain, caspase 4, and caspase 12 results in increased neuronal apoptosis. Mitochondrial dysfunction activates cytochrome-c followed by caspase 9 and caspase 3 activation, which increases Tau phosphorylation, causes synaptic loss and cognitive decline, ultimately leading to neuronal apoptosis. Activation of AMPK decreases neuronal autophagy, followed by alpha-synuclein degradation, and leads to neuroprotection. Similarly, deactivation of phosphorylated AMPK decreases P-CREB, which causes the release of inflammatory cytokines followed by activation of inflammation signaling cascade, and leads to neuronal apoptosis

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Involvement of Bcl-2 and BH3 family members

With the limitations of the apoptotic pathway in post-mitotic neuronal differentiation and maturation, Bcl-2 member was highly expressed in different forms with proliferating NPCs in the developing brain. However, the differentiated form in post-mitotic neurons, as demonstrated by restricted expression of Bak in post-mitotic neuronal differentiation, depends on Bax to promote neural apoptosis where the genetic knockout of Bax provides neuronal protection in multiple disorders [126,127,128,129,130,131,132,133]. Interestingly, N-Bak, an alternative splicing form of Bak characterized by additional exon and generation of BH3 only proteins due to translation, is expressed in neurons that further interact with anti-apoptotic protein Bcl-XL rather than Bax and induce apoptosis through Bax dependent pathway. Further, apart from neurotoxic function, N-Bak has neuroprotective abilities, as demonstrated in different studies [134,135,136]. For instance, Ginsenoside Re and Alcohol Dehydrogenase 1B suppresses Aβ induced neurotoxicity in SHSY-5Y cell culture and AD mouse model, respectively, through increased Bcl-2/Bax ratio, caspase inactivation, and reduced cytochrome-c release [137, 138]. He et al. demonstrated the potential implication of HECT, UBA, and WWE domain-containing 1 (Huwe1), an E3 ubiquitin ligase, in neuronal apoptosis. It was observed that induction of JNK inhibitor (SP600125) or a p38 mitogen-activated protein kinase (MAPK) inhibitor (SB203508) in pretreated Huwe1 increases caspase 3 cleavage, Bax and Bak expression, and p53 activity involved in the progression of neuronal apoptosis [139].

Moreover, myeloid leukemia cell differentiation protein (Mcl-1), an anti-apoptotic member of the Bcl-2 family, is highly expressed throughout the developing cortex regulating apoptotic pathways in differentiating and post-mitotic neuronal cells. A study concluded that deletion of Mcl-1 results in the induction of apoptosis, where GCN precursor does not depend on Mcl-1 for apoptosis [140, 141]. As compared to Mcl-1, the expression pattern of Bcl-XL is different, which is expressed at a low level in the developing brain and at a high level in post-mitotic differentiating neuronal cells where the genetic knockout of Bcl-XL is not able to induce apoptosis in the developing brain but induces cell death in post-mitotic differentiating cells [142]. Lauren et al., demonstrated the anti-apoptotic function of Mcl-1 and Bcl-XL in mouse embryonic CNS during different stages of neurogenesis promoting cell survival. The authors concluded that the sequential deletion of MCL-1 and BCL-x promotes cell survival during neurogenesis at embryonic day 10 in proliferating NPC and at day 11 within the post-mitotic cell population. The same study observed that in the double knockout mouse model, caspase-dependent apoptosis was initiated in non-proliferating and proliferating cell populations [143]. Bcl-2, another member of the Bcl family, is also widely expressed in developing and mature brain, but unlike Mcl-1, loss of Bcl-2 does not induce apoptosis but the result in progressive degeneration of the peripheral and facial neurons due to excessive accumulation of ROS involved in the regulation of oxidative stress pathways [144, 145]. Moreover, anti-apoptotic Bcl-w, whose expression is restricted during embryonic development but highly increased in post-mitotic differentiating neurons, regulates cell death signaling cascade. However, the deletion of Bcl-w neither induces neuronal apoptosis nor sensitizes hippocampal neurons; rather, Bcl-w plays a neuroprotective function in axons of sensory neurons during axonal degeneration [146,147,148,149,150].

BH3 is a pro-apoptotic protein highly expressed in the embryonic brain. At the same time, the expression reduces in the postnatal brain. However, BH3-interacting domain-containing protein 3 (Hrk/DP5), a neuronal-specific BH3 protein, is significantly expressed in the postnatal brain rather than the embryonic brain [151,152,153,154]. Different experimental studies demonstrated that consistent deletion or inhibition of BH3 proteins hampers neuronal apoptosis. Administration of arsenite causes deletion of PUMA, which causes an upregulated activity of BH3 only protein and leads to neuroprotection [155,156,157,158,159,160,161,162]. Post-translational modifications such as cleavage of Bid and dephosphorylation of Bad along with modifications in Bim, PUMA, NOXA, Bmf, and Hrk/DP5 activated BH3 only proteins transcriptionally induced by apoptotic stimuli. Interestingly, several apoptotic stimuli regulate TFs that activate BH3 only proteins such as Bim, PUMA, Hrk/DP5, and Bmf were transcriptionally activated by nerve growth factor (NGF) deprivation. Further, activation of activator protein 1 and TF c-Jun by phosphorylation result in Bim, PUMA, and Hrk/DP5 induction in response to neurotoxic elements [157, 160, 163,164,165,166,167,168,169,170]. Moreover, after the DNA damage response, the P53 signaling pathway stimulates PUMA and NOXA in response to seizures 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP+) induced neurotoxicity, NGF withdrawal, and Aβ aggregation in the mature brain and neuronal cells [155, 171,172,173,174]. Activation of FoxO1 and FoxO3a downstream targets such as AMPK, tribbles pseudokinase 3, macrophage stimulating 1, and cyclin-dependent kinase 5 (Cdk5/p35) mediate Bim induction in response to external stimuli such as NGF withdrawal, oxidative stress, and Aβ aggregation through nuclear translocation of FoxO TF either by Akt or 14–3-3 mediated inhibition or sequestering of FoxO TFs [175,176,177,178,179,180,181].

Moreover, ER stress induces PUMA and Bim activation through transactivating their promoters through the interaction between CHOP, Cdk4, and FoxO3a TFs in neuronal cells, which upregulates the B-Myb required for Bim activation and neuronal death [182,183,184,185,186]. In healthy neurons, survival pathways, including PI3K/Akt and MEK/ERK, represses the expression of BH3 only proteins through inhibition of FoxO3 or inhibition of Akt and ERK itself, which is involved in the induction of Bim activity via both transcriptional and post-transcriptional mechanism [178, 187, 188]. Further, MEK/ERK survival pathway promoted the proteasomal degradation of Bim via interaction with ubiquitin ligase tripartite motif-containing 2 through phosphorylation on ser65 by ERK1/2 followed by polyubiquitination and proteasomal degradation, which was found to be neuroprotective under stress conditions [189, 190]. ERK5 induces phosphorylation of Bad through CAMP-response element-binding protein (CREB) on ser112, ser136, ser155, and ser170 regulates Bad expression and pro-apoptotic functions in the mature and adult brain. Similarly, phosphorylation of ser112 by MEK/ERK/RSK pathway and on ser136 by Akt dissociates its interaction with Bcl-XL and increases its interaction with 14–3-3 regulatory protein to promote neuronal survival [188, 191,192,193].

AMPK and caspases

Being an essential regulator of neurodevelopment and neuroprotective activities, the mechanism of caspases in neuronal cell death is still not well defined [194]. Although, decreased expression of Caspase 3, an effector caspase, was observed in neuronal cell death caused by neuronal injury in the ischemic brain model. Further, neurodevelopment activity was observed in adults as compared to the neonate rodent model. However, mature neurons reflect both apoptotic and non-apoptotic pathways, but the maturation of neurons is also associated with decreased activity of the caspase family gene. Moreover, the activation of caspase 3 through the copper-induced ROS generation causes increased activity of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) expression leading to neuronal cell death in the P19 cell culture model [195]. Thus, caspase inhibition has the potential to minimize cell death caused by ER stress, oxidative stress, and calcium withdrawal in NDDs both in-vivo and in-vitro conditions through a decrease in expressions of upstream and downstream targets, such as PERK, heat shock 70 kDa protein 5, CHOP, PARP, HIV-1 TAR RNA binding protein (TRBP), PKC, TNFα and protein activator [196, 197]. A study found that downregulation of apoptotic protease activating factor 1 decreases the activation of effector caspases, possibly through apoptosome, leading to impaired neuronal development and reduced synaptic plasticity [198]. Likewise, in PD murine model, the NLR family pyrin domain containing 3 (NLRP3) antagonist kaempferol promoted neuroprotection through decreased expression of caspase 1 along with disruption in NLRP3-PYD and CARD domain-containing (PYCARD)-caspase 1 complex assembly [199]. Further, inhibition of caspase 1 via caspase 6 resulted in downregulating the proteolytic cleavage at D586 of mutant Htt, axon degeneration, and pathological lesions [200, 201] (Fig. 3B).

In different experimental studies, it was demonstrated that inhibition of caspase 1 and caspase 3 signaling pathway in microglia promotes neuroprotection through reduced neuroinflammation in microglia, reduced impaired cognition and regulation of neuronal cell apoptosis, possibly through a decrease in beta-secretase 1 expression and macrophage stimulating 1/JNK signaling cascade [202,203,204,205,206,207]. In the case–control study, two caspases 8 variants, that is p.K148R, and p.I298V are involved in neuronal cell loss, which on interaction with caspase 3 involved in synaptic plasticity, microglia inflammation, and memory impairment [208]. Extracellular adenosine increases the expression level of caspase 9, followed by caspase 3 through activation of two independent pathways. A1 adenosine receptor-mediated adenylate cyclase inhibition and adenosine uptake into cells/conversion to AMP/activation of AMPK are two independent pathways, which leads to astrocytoma cell death through the apoptotic pathway [209]. Moreover, Song et al., demonstrated the crosstalk between autophagy and apoptosis through AMPK and activated caspase. In this study, inhibition of the mTOR and the proteasome with rapamycin and Bortezomib respectively activates AMPK, which phosphorylate downstream target Beclin-1 resulted in autophagic cell death followed by its cleavage through activated caspase resulted in apoptotic cell death through mitochondrial dysfunction [210, 211].

Further, neurotoxins such as 6-hydroxydopamine, oxygen–glucose deprivation, and MPP+ increase oxidative stress, followed by an increase in autophagy and apoptosis. Inhibition of AMPK phosphorylation and the activation of mTOR phosphorylation with antioxidants, such as propofol and alpha-lipoic acid, downregulates autophagic and apoptotic cell death, which causes an increase in synaptic plasticity, cognitive ability, and neuroprotection [212,213,214,215]. Similarly, Meares et al., 2013 observed that in-vitro AMPK expression inhibits gene expression of C–C Motif chemokine ligand 2, TNFα, C-X-C motif chemokine 10 and inducible nitric oxide synthase (iNOS), mediated by IFN-γ through signal transducer and activator of transcription 1 [216]. Further, intraperitoneal treatment of lipopolysaccharide treated with 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) in the disturbed neuronal mouse model demonstrated a reduction in TNFα-mRNA expression level along with increased mRNA expression level of peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC-1α). The same study observed that after 24 h of lipopolysaccharide injection treatment with AICAR decreases glial fibrillary acidic protein (GFAP) activity. However, different studies demonstrated the detrimental effect of AMPK activation as treatment with AICAR increase apoptosis in SHSY-5Y and Neuro 2a cell culture models mediated through an increase in caspase 3 activity [217]. Likewise, it was found that Aβ induced neurotoxicity in human neural stem cells decreases cell viability by decreasing AMPK activation and expression of neuroprotective genes such as Bcl-2 and CREB. The same study also concluded that Aβ neurotoxicity causes an increase in caspase 3, caspase 9, and cytochrome c expression [218, 219]. Altogether, it may conclude that AMPK activation promotes apoptosis mediated through increased expression of pro-apoptotic genes such as caspases and cytochrome c.

PI3K/Akt/GSK3β pathway

GSK3 is ubiquitously expressed in the nervous system and involved in regulating neuronal plasticity, and neurological disorders with GSK3β remain the dominant form compared to GSK3α. Inhibition of GSK3β through Akt-dependent phosphorylation, PI3K activation, and PKC activation implicated in glutamate-induced N-methyl-D-aspartate receptor (NMDAR) dependent neuronal plasticity and facilitates the surface transport of potassium voltage-gated channel subfamily Q member 2 subunits that are involved in the regulation of neuronal excitability [220,221,222,223]. It has been considered that the PI3K/Akt/GSK3β pathway is involved in Aβ induced neurotoxicity, which causes memory impairment and learning deficits. However, the mechanism behind this rationale is poorly defined. Further, Akt-dependent inhibition of GSK3β found to reverse learning and memory deficits [224, 225]. It has been observed that GSK3β activity causes hyperphosphorylation of tau protein and accumulation of amyloid precursor protein, which leads to detachment of tau from microtubule and decreases amyloidogenic processing, respectively, resulting in neurite degeneration. Further, GSK3β has the potential to bind with NMDAR receptors, and modulating their function leads to the accumulation of Ca2+ ions causes degeneration of neurons, ultimately leading to neuronal cell death [226, 227]. Moreover, the active form of GSK3 was enhanced in patients suffering from PD, which is localized with the halo form of α-Synuclein, leading to memory impairment and neural degeneration [228]. Further, inhibition of GSK3 activity decreases the aggregation and phosphorylation of α-Synuclein and increases autophagic flux, while activation of GSK3β leads to impaired autophagy [229]. GSK3β has been found to regulate apoptosis through phosphorylating the downstream targets such as p53, Bax, p21, and initiate caspase cascade, which is regulated by many signaling events involved in the modification of mitochondrial activity [230, 231]. A study demonstrated that overexpression of inactive GSK3 mutant prevents apoptosis, which was later confirmed by studies using specific GSK3 inhibitors. Altogether, reduction in GSK3β serine 9 phosphorylation causes increased cytochrome c release and caspase 3 activity and direct involvement in cell death induced by PI3K/mTOR inhibitor and histone deacetylase inhibitor such as Trichostatin A in different cell lines [232,233,234]. Mcl-2, another Bcl-2 family member, stabilizes mitochondrial outer membrane permeabilization through Bim and Bid, followed by phosphorylation activity at serine 159 recognized for ubiquitination and degradation. [235, 236]. In neuronal cells, GSK3β dependent phosphorylation of Bcl-2 family member Bax at serine 163 induces its mitochondrial translocation exerting pro-apoptotic function [237, 238]. Mitochondria being the major producer and center of oxidative stress, undergo mitochondrial permeability transition resulting in apoptotic cell death due to GSK3 activation, which causes hyperphosphorylation of different downstream targets, namely oxidative damage associated cellular defense protein nuclear factor erythroid 2-related factor 2 (Nrf2) [239,240,241,242] (Fig. 4A).

Fig. 4
figure 4

A PI3K/Akt is a molecular marker that activates apoptosis and autophagy, which regulates neurodegenerative disorders. PI3K/Akt activates GSK3β, which acts on downstream signaling molecules involved in neurodegenerative diseases. TSC1 and TSC2 activate mTOR, decreasing neuronal autophagy, followed by an increase in neuronal toxicity, while activated GSK3β decreases NRF2 expression and activates neuroinflammation signaling cascade. Activation of P-CRMP2 and NMDAR mediated through GSK3β increases caspase 3 activations, and the calcium influx respectively lead to an increase in neuronal apoptosis, ultimately increases memory impairment and neuronal cell death. B Rotenone and MPTP activate P38 MAPK, which leads to activation of downstream signaling molecules such as JNK, ROS, and iNOS, followed by activation of the signaling mechanism of neurodegenerative disorders. Activation of JNK activates BIM, increases the release of inflammatory cytokines, and decreases expression of GSK3β, which further activates Cytochrome-C, inflammation signaling cascade, and neuronal toxicity, respectively, ultimately leads to neurodegeneration. P38 MAPK increases ROS causes oxidative stress leads to activation of caspase 1 and caspase 2, which increases neuronal apoptosis followed by memory impairment and cognitive decline involved in the pathogenesis of neurodegenerative disorders. Similarly, activation of iNOS releases NO causes mitochondrial dysfunction, which increases neuronal toxicity leads to neuronal cell death followed by neurodegeneration

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Moreover, the implication of GSK3 has been extensively studied in manipulating autophagy from the last few years. GSK3β inhibits autophagy by activating mTOR complex 1 through phosphorylation of mTOR associated scaffold protein raptor on serine 859. Inhibition of GSK3β activity inhibits mTOR complex 1 and raptor interaction and reduced phosphorylation of ULK1, followed by increased autophagic flux [243, 244]. Similarly, inhibition of GSK3β leads to an increase in AMP/ATP cause AMPK activation followed by autophagic induction through sequential phosphorylation of tuberin by AMPK and GSK3β, which causes mTOR inhibition [245,246,247]. Apart from its inhibition GSK3β in the absence of growth factors, activates acetyltransferase KAT/TIP60, followed by activation of the ULK1 complex to induce autophagy [248]. Inhibiting GSK3β expression through enhancing mTOR activity through overexpression of Aurora A kinase induces resistance to autophagic cell death while activation of GSK3β signal transduction pathway mediated by cadmium promotes autophagic cell death in ROS elevated conditions [249,250,251]. Further, pharmacological and genetic knockdown of GSK3β expression and Akt activation significantly alleviate autophagic cell death in a neuronal cell, while GSK3β mediated phosphorylation of MCL1 has been observed to induce axonal autophagy and axonal degeneration [252,253,254]. Inhibiting the activity of calpain, Akt, and GSK3β reduces the autophagosome number and increases microtubule stability in paeoniflorin-treated okadaic acid-induced tau hyperphosphorylated SH-SY5Y cell model [255]. Also, the Wnt3a ligand promotes AMPK activation, followed by GSK3β inhibition modulating the autophagic phenomenon in hippocampal neurons [256]. These data suggested that GSK3 has potential relevance in autophagic and apoptotic cell death and maybe a potential therapeutic target in NDDs.

p38 and JNK MAPK pathway

MAPK, due to its tremendous application in different cellular functions such as apoptosis, cell survival and proliferation, cell differentiation, inflammatory activities, and external ROS, has been considered as a potential therapeutic target against NDDs. p38 MAPK inhibitors have been considered as potential therapeutic agents against chronic inflammatory diseases, including AD, PD, ALS, and HD. MAPK causes phosphorylation of its downstream targets, including P38, c-Jun, and JNK signaling, which is linked with neuronal apoptosis, where c-Jun activation is required for NGF withdrawal-induced apoptosis. In contrast, inhibition of c-Jun activity protects neuronal cell death.

Moreover, MAP3K-ASK1 has been associated with JNK’s activation and promotes neuronal apoptosis in PC12 cells. However, different studies concluded that standalone JNK signaling was associated with reducing apoptotic cell death [257,258,259,260,261,262]. A series of experiments demonstrated the functional effect of MAPK inhibitors on HMGB1-induced neuronal apoptosis [263]. A study demonstrated that activator protein 1 and c-Jun act as both anti and pro-apoptotic factors depending on the level of stress and suggesting the implication of defective mitophagy in MAPK/c-Jun-induced apoptosis [264]. Further, activation of the JNK and P38 MAPK pathway leads to activation of NF-κB-induced phosphorylation activity, which leads to proteasome degradation. On the contrary, inhibition of p38 MAPK leads to impaired proinflammatory NF-κB transcriptional activity without altering its DNA binding activity. It downregulates the expression of inducible NO synthase through acetylation activity of p65 rather than phosphorylation activity [265]. An in-vitro study performed by Papademetrio et al. demonstrated the autophagy inhibition and apoptosis induction in both caspase-dependent and caspase-independent patterns in MIA PaCa-2 and PANC-1 cells. Although, administration of caffeic acid phenethyl ester reverses autophagic degradation and apoptotic cell death by inhibiting MAPK and NF-κB pathways. [266]. Recently, several studies concluded the protective effect of inhibitors, namely doxycycline, steppogenin, neferine, alantolactone, and indirubin, against lipopolysaccharide-induced primary microglial cells through inhibition of MAPK phosphorylation and NF-κB nuclear translocation. Altogether inhibition of MAPK and NF-κB pathways through the action of inhibitors lowers the expression of microglial activation markers, including IBA1, reduced ROS, NOS, and activation of proinflammatory cytokines [267,268,269,270,271]. The MAPK-activated protein kinase 2 complexes are known to regulate the phenomenon of inflammation through the production and activation of inflammatory mediators. It has been observed that MAPK-activated protein kinase 2 knockout mice are resistant to endotoxic stress and involved in the regulation of TNFα, Interleukin 6, Interleukin 8, and other regulatory cytokines involved in the process of neuroinflammation [272,273,274] (Fig. 4B).

Pharmacological intervention targeting apoptotic and autophagic machinery

Implementation of microRNAs in the regulation of cell-death pathway

The microRNAs are a family of 23–25 nucleotide sequences involved in transcriptional regulation that can be used as potential biomarkers in various diseases, including NDDs. miRNAs modulate several biological processes, such as cell cycle progression, apoptosis, autophagy, and inflammation [275, 276]. Various studies demonstrated the role of miRNA in neuronal cell death, regulating apoptosis and autophagy. However, the functional mechanism of miRNAs in these processes must be elucidated. Table 1 lists the miRNA that regulates autophagy and apoptosis cascade in the pathogenesis and progression of NDDs. For instance, H. Jia et al. demonstrated the effect of the miR-499-5p hypoxic-ischemic encephalopathy rat model, where it was found that the administration of miRNA significantly reduced the expression of C-reactive protein followed by a reduction in neuronal apoptosis. Further, the study indicated that miR-499-5p increases spatial learning ability, spatial memory, and locomotor functions [277]. Similarly, miR-217/138-5p, miR-15a, and miR-129-5p regulate the expression of sirtuin 1, TNFα, IL-1β, BDNF, and SOX6 through oxidative stress, inflammatory pathway, and Akt/GSK3β signaling cascade, which resulted in decreased neuronal apoptosis in MPP+-induced SH-SY5Y cells, oxygen–glucose deprivation neurons of rats, and AD rat model, respectively [278,279,280]. Likewise, miR-93 regulates the expression activity of the TLR4/NF-κB signaling pathway through inhibition of TNFα, IL-6, IL-1β, and VEGF, along with the decrease in pro-apoptotic molecules expression [281]. Further, H. Ge et al., demonstrated the neuroprotective effect of miR-410 in 6-hydroxydopamine-induced SH-SY5Y and PC12 cellular PD model through inhibition PTEN/Akt/mTOR signaling cascade. At the same time, Wang et al. studied that miR-124 exerts neuroprotective effects in the MPTP-induced PD model through the hedgehog signaling pathway targeting endothelin 2. Both studies demonstrated that induction of miRNA causes a reduction in apoptosis, caspase 3 expressions, and ROS activity [282, 283]. Similarly, Chen et al. demonstrated that miR-98 reduces Aβ aggregation and improves oxidative stress and mitochondrial dysfunction through a notch signaling pathway targeting Hes‑related with YRPW motif protein 2 and decreases hippocampal neuronal apoptosis in the AD mice model [284]. Moreover, in the SH-SY5Y cell line, miR-764 protected the neuronal cell from hydrogen peroxide-induced neuronal apoptosis through regulating ninjurin-2 expression and motor neuron functions [285]. Likewise, miR-429 and miR-34a regulate neuronal damage by inhibiting apoptotic expression in mouse cortical neurons and MPP-induced SH-SY5Y cells, respectively [286, 287]. Moreover, miRNA was also found to regulate the ER stress-induced apoptotic pathway. miR-211 inhibits ER stress and upregulates H3K27 methylation of the CHOP promoter leads to cell survival [288]. miR-378 and miR-155 regulate caspase -3 activity resulted in decreased apoptotic expression, whereas, miR-106b attenuates apoptotic pathway targeting caspase 7 expressions [289,290,291].

Table 1 Involvement of microRNAs (miRNAs) in apoptosis, autophagy, and apoptosis + autophagy signaling cascade in the pathogenesis and progression of neurological disease
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Further, miRNA also modulates the autophagic pathway by regulating different proteins and complexes involved in the signaling cascade. It was reported that miR-20a, miR-106b, miR-372, miR-26b, and miR-93 involved in the regulation of autophagy-mediated through ULK1 and ULK2 complex situated at the beginning of autophagic cascade [292,293,294]. Similarly, miR-338-5p, miR-30a, miR-376b, miR-216a, miR-630, miR-374a, and miR-17-5p suppress the autophagic pathway through negative regulation of class III PI3K complex [295,296,297,298,299,300] (Fig. 5).

Fig. 5
figure 5

microRNAs have been implemented to regulate autophagy and apoptosis signaling in the pathogenesis of neurological defects. Both overexpression and downregulation of different microRNAs known to regulate the expression of apoptotic and autophagic proteins by activating or inhibiting different signaling pathways that ultimately lead to the pathogenesis of NDDs

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Moreover, miR-101, miR-376b, miR-17, and miR-495 modulate ATG4D, ATG4, ATG7, and ATG3 expression, which resulted in autophagy inhibition [298, 301,302,303]. Several studies indicated the potential of miRNA as therapeutic agents in neuronal autophagy. A study conducted by Wang et al. demonstrated that overexpression of miR-9a-5p reverses neurological deficits in MACO rat and SH-SY5Y cell lines through decreased autophagy and ATG5 expression [304]. miR-96 and miR-204 alleviate cognitive impairment by suppressing autophagic signaling cascade and exerts neuroprotective effects through decreased expression of LC3, Beclin-1, and mTOR [305, 306]. Likewise, in the MPTP induced SH-SY5Y and PC-12 PD model, miR-124, miR-185, and miR-181b rescue memory deficits and cognitive decline through AMPK/mTOR and PTEN/Akt/mTOR pathway. In addition, Gong et al., 2016 demonstrated that miR-124 suppression significantly increased cell apoptosis and LC3-II/LC3-I ratio, whereas, overexpression of miR-124 decreases the percentage of apoptotic cells and LC3-II/LC3-I ratio. Similarly, overexpression of miR-185 and miR-181b significantly downregulates the LC3-II/LC3-I ratio and apoptosis [307,308,309]. Moreover, miR-212-5p prevents dopaminergic cell death in the MPTP induced PD mouse model (C57BL/6 mice) through SIRT2 inhibition resulting in increased p53 acetylation and reduced autophagy [310]. Similarly, miR-124 in MPTP induced SH-SY5Y PD cell culture model regulates p62/p38, Bim, and Bax expression level resulted in increased autophagy and decreased neuroinflammation [311, 312]. Additionally, Zhao et al., demonstrated that miR-326 inhibits NOS expression and promotes autophagy degradation through the JNK signaling cascade. miR-326 interacts with X‐box binding protein 1, resulting in increased expression of LC3-II, c-jun, and p-α-synuclein [313]. Similarly, miR-27a and miR-23b in post-traumatic brain injury attenuates neuronal deficits and improves cognitive impairment and neurological functions through altered neuronal autophagy by FOXO3a and ATG12 regulation, respectively [314, 315].

Long non-coding RNAs as a pharmacological target

LncRNAs are a set of RNAs having more than 200 nucleotides that regulate gene expression, transcriptional activity, epigenetic modifications, and translational control. Different studies indicate the involvement of altered LncRNAs expression in the progression and pathogenesis of neurological defects such as AD, PD, ischemic stroke, HD, traumatic brain injury, spinal cord injury, and ALS through the regulation of cell death pathways, namely apoptosis and autophagy. Table 2 discusses the different potential LncRNAs, which regulate the expression of both apoptosis and autophagy. For instance, LncRNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1), referred to as non-coding nuclear-enriched abundant transcript 2 (NEAT2), was found to be expressed in the in vitro model of ischemic stroke. Guo et al., 2017 demonstrated that down-regulation of MALAT1 suppresses neuronal apoptosis through downregulating Beclin-1 dependent autophagy degradation. In the same study, the authors concluded that the downregulation of autophagy is through regulation of the MALAT1-miR30a-Beclin-1 axis [359]. Similarly, Wu and Yi 2018 concluded that downregulation of MALAT1 reverses neurological defects by inhibiting excessive autophagy and apoptosis through regulating PI3K/Akt signaling pathway [360]. Further, LncRNA colorectal neoplasia differentially expressed (CRNDE) also regulates apoptosis and autophagy in different neurological defects. For instance, Chun-Hua et al. 2020 demonstrated that in hypoxic-ischemic (HI) brain damage (HIBD), silencing of CRNDE promotes autophagy and inhibits neuronal apoptosis both in-vivo and in-vitro conditions [361]. Likewise, downregulation of CRNDE in traumatic brain injury inhibits autophagy and apoptosis through regulation of GFAP, BrdU, NGF, and Nestin [362]. Wei-Lan et al. 2019 concluded that LncRNA small nucleolar RNA host gene 12 (SNHG12) inhibits miR-199a, which upregulated the activity of SIRT1 through activation of AMPK. Activation of AMPK leads to increased autophagy and decreases neuronal apoptosis [363]. Another member of the SNHG family known as SNHG14 was considered to be associated with the progression and pathogenesis of cerebral ischemia–reperfusion injury. Deng et al. 2020 in HT22 mouse hippocampal neuronal cells demonstrated that SNHG14 promotes neuronal injury through excessive mitophagy and neuronal apoptosis by regulating the miR-182-5p/BINP3 axis [338]. Likewise, Cao et al., 2020 concluded that LncRNA SNHG3 promotes autophagic degradation and neuronal cell apoptosis through increased activity of miR-485 and increased expression of ATG7 [364]. Thus, despite having several evidence, which concluded the potential role of LncRNAs in the regulation of apoptosis and autophagy, simultaneously in the pathogenesis and progression of neurological defects, there will be a need for in vivo studies (Fig. 6A).

Table 2 Involvement of long non-coding RNAs in apoptosis and autophagic cascades simultaneously
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Fig. 6
figure 6

A long non-coding RNAs have been implemented to regulate autophagic and apoptosis signaling cascade through modulation of different signaling cascades. For example, BACE1-AS, HAGLROS, MIAT, 17A, and MEG3 through miR-214-3p/ATG5 axis, PI3K/Akt/mTOR pathway, CUL4A-DDB1-REDD1 axis, GABAB signaling, and Beclin-1 signaling, respectively, increase autophagy and apoptosis simultaneously. Similarly, LncRNAs, such as HOTAIR, RMRP, and TCTN2 through miR-874-5p/ATG10 axis, PI3K/Akt/mTOR and miR-216b-Beclin-1 axis, respectively, lead to an increase in autophagy and decrease in the apoptosis pathway. R2Q1 B natural biomolecules act as a potential therapeutic agent in modulating autophagy and apoptosis pathway in the pathogenesis and progression of neurological disorders. For instance, Flavones and flavanols modulate mTOR, Akt, NF-κB signaling, and caspase 3, whereas phenolic acids and alkaloids modulate the expression of Atg3 and Beclin-1. Similarly, Flavanols, Flavanones, Isoflavones, Alkaloids, and Flavones regulate the activity of Bcl-2, whereas Lignines, Flavones, Flavanols, and Flavanones regulates the expression of caspase 9 and Atg3

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Small-molecule inhibitors in autophagy and apoptosis pathways in NDDs

Recent studies implicated the potential of cell death pathways, including the autophagic pathway and apoptosis pathway, in the progression and pathogenesis of various diseases such as cancer, cardiovascular, and NDDs. These emerging discoveries led to expanding the pharmacological interventions targeting PCD pathways and provided the opportunities for development and prosecutions of known drugs or novel compounds as a therapeutic approach. Autophagy and apoptosis were commonly involved in NDD progression mediated through different signaling cascades and molecules. Oxidative stress, calcium imbalance, mitochondrial dysfunction, AMPK signaling, inflammatory response, and ER stress are commonly involved pathways in the autophagic degradation of accumulated toxic proteins and neuronal apoptosis due to aggregated misfolded proteins. Further, recent studies have shown that upregulation of autophagy through autophagy inducers causes a decrease in the accumulation of misfolded proteins and delays the progression of NDDs. Likewise, inhibition of pro-apoptotic proteins and activating anti-apoptotic proteins through synthetic or natural molecules delay the progression of NDDs. Thus, induction of autophagic degradation and inhibition of apoptosis signaling cascade can be used as a therapeutic strategy for NDDs. Table 3 discusses the drugs that undergo clinical trials for induction of autophagy in the pathogenesis of NDDs.

Table 3 Autophagy inducer drugs undergo clinical trials in neurodegenerative disease involved different targets obtained from (https://clinicaltrials.gov/)
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Another study indicated that Apelin-13 reverses amyloid-induced memory deficits by inhibiting apoptosis and autophagy, whereas administration of malathion in N2a neuroblastoma cells increases neuronal apoptosis and decreases autophagic flux through inducing lysosomal membrane permeabilization [375, 376]. Apart from NDDs, modulation of autophagy and apoptosis pathways could be protective in other neurological diseases, such as spinal cord injury, sleep deprivation, traumatic brain injury, ischemic stroke, and epilepsy. For instance, Modafinil protects hippocampal neurons by inhibiting autophagy and apoptosis pathway in the mice model, whereas metformin protects neuronal cells against spinal cord injury through inhibition of autophagy and apoptosis cascade by regulating mTOR/p70S6K signaling pathway [377, 378]. Similarly, Ganoderma lucidum extracts reverse MPTP-induced neurodegeneration by inhibiting excessive autophagy and apoptosis by regulating oxidative stress and mitochondrial function [379]. Further, recent studies concluded the potential of flavanols, flavonols, flavones, flavones, and flavanones as therapeutic agents in the treatment of NDDs through reversing the effects of dysregulated autophagic degradation and apoptosis. For instance, Singh et al., demonstrated that administration of fisetin, a natural flavonol compound in D-galactosidase aged rats decreased the activity of pro-oxidants and increased the activity of antioxidants. Further, fisetin causes a decrease in neuronal cell apoptosis and upregulates the expression of autophagic genes, such as Atg-3 and Beclin-1 [380]. Likewise, Yang et al., demonstrated that administration of fisetin improves synaptic dysfunction through the decrease in neuronal apoptosis and neuroinflammation by inducing autophagy and activation of AMPK [381]. Further, administration of rapamycin leads to increased autophagy and protects the neuronal cell from oxidative stress and apoptotic cell death [382]. Further, catechin can protect hippocampal neuronal cell apoptosis by inhibiting the JNK/MLCK pathway and microglial activation [383, 384]. Further, theaflavin decreases neuronal apoptosis by inhibiting the inflammatory response and ROS-induced oxidative stress [385, 386]. Naringenin, a dietary flavanone, reduces apoptotic cell death, inhibits oxidative stress, and improves mitochondrial function through Nrf2/ARE signaling pathway [387, 388], whereas, naringin inhibits neuronal apoptosis through inhibiting oxido-nitrosative stress and neuroinflammatory response [389]. Meng et al. 2021 in a mouse model of AD, demonstrated that naringin could improve cognitive function through decreased neuronal cell death by MAPK/p38 pathway [390]. Further, Guo et al. 2020 demonstrated that administration of genistein promotes neuroprotection against Aβ-induced neuronal cell death through PI3K/Akt/Nrf2 signaling pathway, whereas Jiang et al., 2017 concluded that genistein attenuates isoflurane-induced neurotoxicity and improves spatial learning and memory abilities through cAMP/CREB and BDNF/PI3K/Akt pathway [391, 392]. Similarly, equol, a dietary daidzein attenuates neuronal cell death and promotes neuroprotection through inhibiting microglial activation and cell cycle reentry [393, 394]. Moreover, apigenin also promotes neuroprotection through inhibition of neuroinflammatory response and oxidative stress-induced neuronal apoptosis [395, 396]. Kim et al., 2021 concluded that administration of apigenin repressed scopolamine-induced neuronal damage and reduced cognitive impairment. The authors also concluded that neuronal protection by apigenin is the result of enhanced BDNF activity, which decreases neuronal apoptosis and amyloidogenesis [397]. Similarly, luteolin promotes neuroprotection through reduced neuronal cell apoptosis by regulating SIRT3/AMPK/mTOR and p62/Keap1/Nrf2 signaling pathway [398, 399]. In addition, administration of luteolin and apigenin causes activation of autophagic degradation through HMOX1 and mTOR/AMPK/ULK1 complex, respectively, which promotes neuroprotection [400]. Peruru and Dodoala in 2021 concluded that diosmin, a citrus flavonoid, promotes neuroprotection by suppressing NOX4 and its subunits [401]. Moreover, apart from the above-mentioned polyphenol compounds, studies demonstrated the protective effects of lignins and phenolic acid against neuronal apoptosis and autophagic cell death in NDDs and other neurological diseases. For example, caffeic acid phenethyl ester, a phenolic compound, prevented neuronal cell apoptosis against Aβ1-42 through the modulation of GSK3β in the mice model of AD, whereas, gallic acid protects from 6-OHDA induced neurotoxicity and cell apoptosis through inhibition of oxidative stress [402, 403]. Similarly, geraniin protected neuronal cells from apoptosis in PC12 cell culture against Aβ25-35 toxicity through the modulation of the NF-κB pathway, whereas arctigenin protected PC12 cell culture against ethanol‑induced nerve damage [404, 405]. Furthermore, recent studies demonstrated the protective effects of natural alkaloids in preventing neuronal cell viability [406,407,408,409]. For instance, tricyclic pyridine, an alkaloid from Fusarium lateritium SSF2, prevents neuronal cell apoptosis against glutamate-induced oxidative stress in the HT22 hippocampal neuronal cell line by inhibiting caspase 9 and caspase 3 [410]. Similarly, dendrobium alkaloids enhanced neural function through reduced neuronal cell death by modulating the expression of inflammatory cytokines [411]. Thus, from the evidence mentioned above, it might be concluded that targeting apoptosis or autophagy pathways could be beneficial for reverses neurological defects. Table 4 lists the natural and synthetic biomolecules in the regulation of autophagy and apoptosis machinery (Fig. 6B).

Table 4 List of natural and synthetic biomolecules that are involved in the regulation of autophagy and apoptosis pathway
Full size table

Conclusion and future perspectives

This review displayed the intricacies between two major cell death pathways, viz. apoptosis and autophagy in NDDs, which provide a great avenue for therapeutics. These two pathways have several common mechanisms, such as initiator and effector molecules, genes and proteins, and signaling pathways that form a connection. With the development of research technologies and specific inhibitors, our understanding of cell death pathways is ready to be executed. Herein, we tried to elaborate the knowledge about molecular phenomena between the two death pathways involved in NDDs, for instance, interactions between targets and pathological mechanisms of molecular targets involved in cell death pathways and autophagy. However, many critical issues must be resolved while targeting cell death pathways concerning autophagy as a therapeutic approach in NDDs. Investigating molecular targets, regulatory mechanisms, and signaling cascade is a matter of extensive research to maximize the potential of cell death pathways. Moreover, regulation of PCD and NDDs through miRNAs is a new direction for research in this field, where miRNA may target more than one component of the cell death pathways or sometimes may target more than one death pathway. In this review, we also discussed the molecular mechanism of autophagy and apoptosis in NDD’s while focusing on the molecular markers, signaling cascades, and shared mechanisms such as ER stress and Ca2+ concentration. Both autophagy and apoptosis can regulate each other mediated by inhibition of activation of apoptosis-associated caspases. However, to maximize the potential of cell death pathways as a therapeutic approach, further in-vitro and in-vivo studies are required.