Introduction

Alpha synuclein (α-syn) protein was originally identified as a precursor protein for the non-β-amyloid component (NAC) of Alzheimer’s disease plaques [1]. The α-syn, encoded by the SNCA gene, is a small (14 kD) presynaptic nerve terminal protein, abundant in the brain, and its function is not known. Parkinson’s disease (PD) and related disorders known as synucleinopathies share a common pathological lesion composed of protein inclusions in the cytoplasm of selected populations of neurons and glia (i.e., oligodendrocytes), known as Lewy bodies (LBs) and Lewy neurites, and glial cytoplasmic inclusions (GCIs), respectively [2–6]. Aggregates of the insoluble α-syn protein are the major component of LBs [7], Lewy neurites, and GCIs. In addition to PD, these groups of disorders include dementia with Lewy bodies (DLB), Alzheimer disease (AD) with Lewy bodies (LBV/AD), neurodegeneration with brain iron accumulation (NBIA) type I, pure autonomic failure (PAF), and multiple system atrophy (MSA). SNCA was the first gene implicated in familial Parkinson’s disease (fPD) [8]. Over the last decade, genome wide association studies (GWASs) and candidate gene-based approaches [9] have implicated SNCA as a highly significant genetic risk factor for synucleinopathies including sporadic PD [10–23], DLB [24], MSA [25, 26], and LBV/AD [27, 28]. However, while coding missense mutations and multiplication of SNCA locus led to the fPD [8, 29–31], the precise variants within SNCA gene that contribute to the sporadic forms of PD, DLB, MSA, and other synucleinopathies and their molecular mechanisms of action remain elusive.

The molecular mechanisms through which the SNCA gene elicits synucleinopathies and the underlying genetic factors have been studied most extensively in relation to the etiology of sporadic PD. To date, accumulating evidence has been reported in both in vitro systems and in vivo models, suggesting that the α-syn expression levels are critical for the development of the disease. In this review, we will describe recent advances in understanding the role of SNCA gene and its expression levels in sporadic PD (spPD) and other synucleinopathies. We will also discuss several regulatory mechanisms of SNCA gene expression and possible means for manipulating SNCA overexpression for therapeutic approaches.

Alpha-synuclein overexpression

The role of α-syn expression levels in disease pathogenesis has been studied in a variety of biological systems. Here, we review major approaches and current advancements.

Cellular model systems

Cell-based models, including established immortalized cell lines such as HEK293 cells [32] and SH-SY5Y neuroblastoma cells [33, 34], and primary neuronal cultures [35–37], have been widely used to study the effect of α-syn expression levels on cellular phenotypes. Although overexpression of α-syn in those mammalian cell culture systems did not show evidence for aggregation, it was shown that expression levels of α-syn are crucial for the neurodegenerative process, and it was suggested that there is a threshold level above which the detrimental function of α-syn emerge [38]. Xu et al. showed that α-syn overexpression in dopaminergic neurons leads to apoptosis, while an opposite neuroprotective effect was observed in non-dopaminergic cortical neurons. They suggested that the vulnerability of the dopaminergic cell population was associated to dopamine (DA) production [39]. It was also shown that α-syn overexpression induces oxidative stress in SH-SY5Y cells through unclear mechanisms [40]. The effect of α-syn overexpression on oligodendrocytes was evaluated in rodent oligodendroglial cell line expressing wild-type (WT) human α-syn and revealed that α-syn delays oligodendrocyte progenitor cell (OPC) maturation by severely down-regulating myelin-gene regulatory factor and myelin basic protein [41].

Modeling human neurodegenerative diseases by using induced pluripotent stem cells (iPSCs) leverages the use of cell culture systems in neurodegenerative research. The iPSC approach uses patient-derived cells and differentiates these cells into disease-relevant neuronal populations. In particular, the differentiation into dopaminergic and cholinergic neurons could address specific gaps in the mechanisms of neurodegeneration of PD and DLB, respectively, while, iPSC-derived oligodendrocytes represent a powerful tool for modeling MSA. The vulnerability of the dopaminergic neurons to overexpression of SNCA has been confirmed by differentiating iPSCs with SNCA triplication into dopaminergic neurons [42, 43]. The iPSC-derived dopaminergic neurons carrying the SNCA triplication showed an approximately twofold increase in α-syn levels compared to the control cells. α-syn accumulation coincided with elevated oxidative stress markers and conferred increased vulnerability to oxidative stress-induced cell death [44]. These studies introduced the iPSC-derived dopaminergic neurons as a model to investigate the effects of SNCA overexpression on PD-related phenotypes. Another study that evaluated the consequences of SNCA triplication using iPSCs, reported that iPSC-derived neural precursor cells (NPCs) displayed overall normal cellular and mitochondrial morphology but showed substantial changes in growth, viability, cellular energy metabolism, and stress resistance especially when challenged by starvation or toxicant challenge. Importantly, knockdown of SNCA overexpression resulted in reversal of the observed phenotypic changes [45]. Deriving iPSCs from both genetic and idiopathic patients could advance our understanding whether a given mutation or a particular genetic background confers selective susceptibility in a specific neuronal-type population. It has been reported recently that SNCA triplication interferes with the differentiation of iPSCs into dopaminergic and GABAergic neurons. This study showed that increased level of α-syn affects messenger RNA (mRNA) expression of genes implicated in neuronal differentiation leading to delayed maturation [46]. In unpublished work, we have developed a system to evaluate the effect of up-regulation of SNCA mRNA on cholinergic neuronal differentiation and maturation (Fig. 1). SNCA-mRNA and protein expressions in both the SNCA triplication and the control iPSC lines increased along the maturation process (Fig. 1a), and although the SNCA triplication line exhibited twofold overexpression of SNCA compared to the control cell line (Fig. 1a–c), there was no effect on the maturation process and the neurite outgrowth (Fig. 1d). A deep characterization of this cell line is underway, and the implication to DLB warrants further investigations. A new study pioneered the generation of oligodendrocytes from MSA and familial PD iPSC lines. Expression of SNCA transcript and protein was detected in those oligodendrocyte lineage cells, offering cellular models for studying the functional implication of α-syn during oligodendrocyte development and in MSA [47]. Collectively, iPSC models provide access to cell types of interest that were previously unobtainable in sufficient quantity or quality and presents exciting promises for the elucidation of synucleionopathies. However, cellular model systems lack the microenvironment (interaction with other cell type and microvasculature), occurs in the intact brain tissue, and lack the whole organism in vivo factors necessary to trigger the progression of the pathology and to evaluate phenotypic outcomes in the organism level. Furthermore, the maturation period of the differentiated neurons should be considered when establishing an iPSC model system to study neurodegenerative diseases such as synucleinopathies. For example, Crompton et al. reported that the iPSC-derived cholinergic neurons were maturated for almost 50 days when they observed neuronal specific markers that demonstrated that the cells exhibited characteristics of cholinergic neurons present in the human brains [48]. It is also important to note that iPSC-derived neurons have to undergo a prolonged maturation in culture [49], or other “aging” protocols [50, 51], in order to be comparable to the mature adult brain.

Fig. 1
figure 1

Generation of cholinergic neurons from SNCA triplication and control iPSC lines. Induced pluripotent stem cells (iPSCs) derived from a Parkinson’s disease patient with the triplication of the human SNCA genomic locus (SNCA-Tri) and from a healthy individual (control) were differentiated to basal forebrain cholinergic neurons (BFCNs). Following forebrain patterning, neural precursor cells (NPCs) were passaged at day 16+. BFCNs were next maturated for 5–7 weeks. a SNCA-mRNA expression levels at different stages of the neuronal differentiation. Levels of SNCA-mRNA were measured by real-time RT-PCR and calculated relatively to the geometric mean of GAPDH-mRNA and PPIA-mRNA reference control using the 2−ΔCT method. The blue bars represent SNCA-mRNA fold expression levels measured in the control cell lines, and the red bars represent SNCA-mRNA fold expression levels measured in the SNCA-Tri lines. Immunolabeling for α-syn protein (Alexa-488) was performed in matured cholinergic neurons (BFCNs, day 35) from control (b) and SNCA-Tri (c). Immunocytochemistry was analyzed with by confocal microscopy end fragment. d Phase-contrast micrograph of representative mature cholinergic neurons (day 50) differentiated from SNCA-Tri iPSC. The arrows indicate the neuritis outgrowth

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Animal model systems

Developing animal model systems represent an in vivo approach to understand disease mechanisms. Both overexpression and down-regulation of α-syn have been modeled in several animal systems [52]. The limitation in the use of these animal models is mainly related to the difficulties in recapitulating the pathological progression and mimicking disease phenotypes. Nevertheless, animal models represent an important tool in understanding the pathogenesis mechanisms and processes leading to the disorder in a whole organism level and support the identification of novel therapeutic targets and potential therapies [53].

Caenorhabditis elegans does not contain a homolog to the human α-syn protein, providing an opportunity to explore the role of human α-syn in relation to dopaminergic neuronal vulnerability. Transgenic C. elegans have been developed, overexpressing WT and mutated α-syn in specific subsets of neurons [54]. Overexpression of WT human α-syn led to moderate dopaminergic neuronal loss.

Transgenic Drosophila melanogaster overexpressing WT or mutant human α-syn recapitulated PD-related pathological features including dopaminergic neuron loss, filamentous intraneuronal α-syn inclusions, and locomotor dysfunction [55]. These phenotypes were reverted by L-DOPA or DA agonists, suggesting the usefulness of these models for genetic screens.

Transgenic rodent models have been the most widely used animal model to study the role of SNCA gene in PD and related diseases. Several rat model systems overexpressing the human α-syn have been established and showed phenotypes closely related to those in human PD [56, 57]. Many mouse lines overexpressing human-SNCA (hSNCA) under various promoters have been created. For example, transgenic mice expressing WT hSNCA140 under the control of platelet-derived growth factor-β promoter were characterized by a reduction in tyrosine hydroxylase and DA in the striatum and progressive accumulation of α-syn and ubiquitin-immunoreactive inclusions. This model suggests that α-syn accumulation may play a causal role in PD and related conditions [58]. Mice overexpressing SNCA under the Thy1 promoter are characterized by a region-specific increase of most hSNCA transcripts (SNCA140, SNCA126, SNCA112, and SNCA98) when compared to WT mice, consistent with the region-specific elevation in hSNCA transcript level in PD patients [59]. Using the same model system, Subramaniam et al. discovered early and regionally selective alterations in mitochondrial function and oxidative stress [60]. Altered mitochondrial functions in the striatum and substantia nigra are likely due to endogenous factors that render neurons in these regions less able to cope with α-syn-induced mitochondrial dysfunction. This data is in agreement with previous findings in the postmortem substantia nigra of patients with idiopathic PD [61] but reveals, for the first time, that α-syn alters mitochondrial function to a greater extent in the regions containing nigrostriatal dopaminergic neurons than in other brain regions, specifically the cerebral cortex. Interestingly, transgenic mice overexpressing SNCA under BAC promoter show deficits in DA neurotransmission and motor impairments in the absence of α-syn aggregates, suggesting that PD-related phenotypes may be associated with much earlier deficits in DA neurotransmission [62], and possibly, the damage due to α-syn overexpression occurs earlier than the apparent accumulation of aggregates. The role of α-syn in MSA has been evaluated as well by the creation of transgenic animal models. Mice models overexpressing human WT α-syn in oligodendrocytes under the control of the 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNP) promoter [63] or the proteolipid protein (PLP) promoter [64] recapitulated features of MSA such as GCI-like α-syn aggregates in oligodendrocytes, oligodendroglial and neuronal loss, and slowly progressive motor impairment.

In primates, transgenic monkeys overexpressing WT or mutant α-syn showed chronic and progressive features of human PD such as motor impairments, α-syn inclusions, and dystrophic neurites [65].

Human studies

Copy number variations [31, 66–69] in the SNCA gene have been identified in only a few families with an early-onset, autosomal dominant form of PD. Genomic triplication of the region containing SNCA results in four fully functional copies of SNCA and twofold overexpression of SNCA mRNA and protein. This leads to high penetrance of an early-onset PD phenotype with cognitive impairment and autonomic dysfunction [31, 70, 71]. Duplications of the wild-type SNCA gene result in a 1.5-fold elevation of SNCA expression and, compared with the triplication, a slightly later onset of heritable PD that is characterized by a lower penetrance and a “milder” phenotype with slower progression [66–69], demonstrating the dose-dependent effect of SNCA on disease etiology.

The role of SNCA levels was also demonstrated in the idiopathic form of sporadic PD (spPD). Elevated levels of SNCA-mRNA have been reported in midbrain tissues [72] and in dopaminergic neurons of the substantia nigra [73] from spPD postmortem brains compared to controls, suggestive of a general role for SNCA overexpression in PD. Moreover, the role of SNCA overexpression was also demonstrated in other synucleinopathies. Analysis of SNCA mRNA expression in human temporal cortex in PD and DLB demonstrated a correlation between the number of α-syn-immunoreactive LBs and the abundance of SNCA-mRNA [74]. It was also shown that oligodendrocytes isolated from MSA brains expressed elevated levels of SNCA-mRNA compared to control [75]. Recent gene expression analysis demonstrated that SNCA-mRNA levels were significantly higher in temporal cortex from LBV/AD brains compared with AD controls [27, 76]. While the studies discussed above are strongly agreeable regarding the pathogenic effect of overexpression of SNCA, other studies presented contradicting findings. For example, Neystat et al. showed a 15 to 20 % reduction of SNCA-mRNA expression in substantia nigra from PD cases relative to normal, but the same group reported no alteration in expression of SNCA-mRNA in homogenates of frontal cortex [77]. Another group reported an approximately 50 % reduction in SNCA-mRNA expression at the cellular level in substantia nigra neurons and frontal cortex neurons in PD [78]. Noteworthy, these results reflect the SNCA-mRNA of the “surviving” neurons in the disease-affected brains; therefore, we need to interpret the outcomes from gene expression analyses of neuropathological degenerative brain tissues with caution. Differences in SNCA level between PD and normal controls were evaluated in human tissues/cells other than brains as well. A mRNA expression study of peripheral lymphocytes showed no significant alteration of SNCA-mRNA expression in between spPD and controls [79]. The quantification of α-syn protein levels in blood also provided conflicting data. While some studies showed increased levels of α-syn in the plasma of PD patients compared to the controls [80], others reported decreased [81] or no significant differences [82] in plasma. These conflicting results might be attributable to the different methodologies and experimental designs of the studies and also imply a possible tissue-/cell type-specific effect. Furthermore, elevation in SNCA-mRNA levels might contribute to disease pathogenesis in many but, perhaps, not all cases of spPD.

In normal primate brains, expression of α-syn increases with age in parallel with decreased tyrosine hydroxylase expression [83]. Accordingly, some investigators have hypothesized that PD and related diseases represent accelerated variants of the normal aging process, where inappropriate degradation or overexpression of α-syn plays a central role [84, 85]. This observation provided further support to the pathogenic effect of SNCA overexpression.

Intervention approaches for SNCA down-regulation and the impact

Specific targeting of α-syn expression levels represents an attractive neuroprotective strategy, and manipulations of SNCA levels have had beneficial effects. RNA interference (RNAi) approach has been tested in vitro in neuron-like cell cultures and in vivo in rodent and other animal models [86–89]. Different effects of RNAi strategies have been reported in different cell lines. Down-regulation of SNCA in MN9D cells decreases cell viability [89]. Two adeno-associated virus (AAV) gene-silencing vectors have also been tested for efficiency and specificity of silencing and toxicity in 293T, PC12, and SH-SY5Y cells. One vector was embedded in a microRNA backbone while the second was not [90]. Both vectors silenced hSNCA to the same extent and with high specificity. However, the mir30-embedded vector was significantly less toxic [90]. Knockdown of SNCA in a triplication NPCs by short hairpin RNA (shRNA) resulted in reversal of the observed phenotypic abnormalities in growth, viability, cellular energy metabolism, and stress resistance [45]. Knocking down SNCA levels was also evaluated in animal models. siRNAs against SNCA injected directly into the monkey substantia nigra achieved over 50 % down-regulation of expression in nigrostriatal dopaminergic neurons [91]. A new siRNA approach, “expression-control RNAi” (ExCont-RNAi) treatment of PD flies, demonstrated a significant improvement in their motor function [92]. This study suggested that α-syn overexpression is associated with the degree of motor dysfunction and that it is possible to determine a threshold between benign and malignant levels of α-syn. shRNA strategies have been used in rat models with both lentiviral [87] and AAV deliveries [88]. Lentiviral delivery was able to silence specifically ectopic α-syn expression, and no toxicity was reported as a consequence of these treatments. The use of AAV vectors caused a significant loss of nigrostriatal dopaminergic neurons. However, there was also neurotoxicity associated with robust reduction of SNCA levels mediated by siRNA in those rat models [88, 93].

In conclusion, the role of SNCA overexpression in PD pathogenesis on the one hand, and the need to maintain normal physiological levels of α-syn protein on the other, emphasizes the so-far unmet need to determine the threshold between physiological and pathological α-syn levels and underscores the importance of understanding the mechanisms and factors that participate in the expression regulation of the SNCA gene.

Regulatory mechanisms of SNCA gene expression

Transcriptional and post-transcriptional mechanisms regulate SNCA gene expression and could have an important role in the development of synucleinopathies. Here, we describe the current knowledge of molecular mechanisms that control SNCA expression levels.

Transcription

Several groups have studied the transcription regulation of SNCA and have identified few putative transcription factors (TFs) that mediate SNCA expression. We identified the transcriptional activator, poly(ADP-ribose) transferase/polymerase-1 (PARP-1), that binds to Rep1 and regulates SNCA expression via this interaction [94]. Transcription factors of the GATA family [95] and ZSCAN21 [96] bind to elements within intron 1 and the promoter [97] region of SNCA and have been proposed to play a role as inducers of transcription. Clough et al. suggested that a signaling pathway involving ERK/PI3-mediated ZSCAN induced SNCA transcriptional activation [96, 98, 99]. Sterling et al. identified cis-regulating, evolutionarily conserved genomic elements in the SNCA locus that modulated the expression of a reporter gene and found five factors (PITX3, OTX2, NR3C1, AR, and TBP) that interacted with the SNCA promoter [100]. Additional regulatory areas may be located outside the promoter regions, such as intronic or intergenic regions [100]. The regulatory role of intronic regions has been confirmed by combining in silico and wet bench approaches that led to the identification of an intronic cytosine-thymine (CT)-rich region in SNCA gene that influences SNCA transcript levels [76].

microRNAs

Modulation of SNCA-mRNA levels by endogenous microRNAs (miRNAs) was proposed a post-transcriptional mechanism of SNCA regulation. Two miRNAs—miR-7 and miR-153—that are abundantly expressed in the brain have been implicated in the regulation of SNCA transcript levels. In rodent primary neurons, both miR-7 and miR-153 down-regulated SNCA levels and showed an additive effect [101]. In PD human brains, miR-34b and miR-34c are down-regulated [102, 103]. The effect of these miRNAs has been investigated in SH-SY5Y cells. Both miR-34b and miR-34c decrease SNCA-mRNA levels and α-syn protein by targeting the 3′ untranslated region (3′UTR) of SNCA mRNA [104].

Splicing

Alternative splicing is another post-transcriptional mechanism that regulates expression of SNCA transcripts. At least six different SNCA transcript variants have been described for SNCA gene, SNCA140, SNCA126, SNCA115, SNCA112, SNCA98, and SNCA67 [59, 105], of which SNCA112, SNCA126, and SNCA98 arise from alternative splicing. No one has yet discovered the biological and pathological significance of the different SNCA isoforms. However, specific isoforms have been associated with intracellular aggregation [106] and are differently expressed in human synucleinopathies [107]. A deletion of either exon 3 or 5 predicts functional consequences; while exon 3 deletion (SNCA126) leads to the interruption of the N-terminal protein-membrane interaction domain which may lead to less aggregation, exon 5 deletion (SNCA112) may result in enhanced aggregation due to a significant shortening of the unstructured C-terminus [108, 109]. In frontal cortex of DLB, SNCA112 is increased markedly compared to the controls [110], while SNCA126 levels are decreased in the prefrontal cortex of DLB patients [111]. In contrast, SNCA126 expression showed increased in the frontal cortex of PD brains and no significant differences in MSA [107]. SNCA98 is a brain-specific splice variant that lacks both exons 3 and 5 and exhibits different expression levels in various areas of fetal and adult brains. Overexpression of SNCA98 has been reported in DLB, PD [112], and MSA [107] frontal cortices compared with controls.

Post-translation

Post-translational degradation also regulates α-syn protein levels; however, the specific degradation pathway is still controversial. It was suggested that α-syn clearance is related to α-syn levels, its assembly state, and to the homeostatic environment in which degradation occurs. The ubiquitin-proteasome system (UPS) and the autophagy lysosome pathway (ALP) influence α-syn turnover as well [113]. Interestingly, aging affects these pathways, leading to α-syn accumulation [114]. Increased α-syn levels promote the generation of aberrant species that impair UPS and ALP determining a bidirectional positive feedback loop, leading to neuronal death [115].

In this view, tight regulation of the expression levels of SNCA, mainly through the various transcriptional and post-transcriptional pathways, presents a promising avenue to fine-tune α-syn levels to the degree required for successful therapeutic intervention. Thus, continued investigations of the molecular mechanisms and trans-acting factors that regulate SNCA expression in a cell type-specific manner—neuronal (dopaminergic and cortical) and oligodendrocyte—and identification of their corresponding binding sequence elements will have a high impact on the development of therapeutic targets.

Genetic and epigenetic regulation of SNCA expression

Differential regulation of SNCA expression can be attributed, in part, to genetic variability across the SNCA genomic region and intergenic sequences. We highlight several examples below:

  • A polymorphic dinucleotide complex repeat site located ∼10 kb upstream of the SNCA transcription start site [116, 117], named Rep1, has been described [118]. The length of Rep1 appears to be associated with increased risk of PD [119]. The overwhelming majority of the reported association studies, including a large meta-analysis, have shown that the extended alleles of SNCA-Rep1 confer increased risk to develop late-onset, “idiopathic” PD, while the shorter allele is protective [20, 120–123]. We investigated the effect of the Rep1 polymorphism on SNCA expression and discovered that Rep1 regulates SNCA transcription in human brain tissues. These results have been confirmed using luciferase reporter assay and in a humanized mouse model. The PD risk Rep1 allele led to increased SNCA-mRNA levels, providing further support to the pathogenic effect of SNCA overexpression [118, 124, 125]. In support of this, we identified a factor, PARP-1, that binds specifically to Rep1 and modulates SNCA transcription [94], suggestion that the association of Rep1 alleles with sporadic PD may be mediated, in part, by the effect of PARP-1 on SNCA expression.

  • Recently, four distinct haplotypes within a highly polymorphic low-complexity CT-rich region in intron 4 of SNCA gene were identified. A specific haplotype conferred risk to develop LBV/AD. It was further demonstrated that the CT-rich site acts as an enhancer element, where the risk haplotype was significantly associated with elevated levels of SNCA mRNA [76].

  • The SNCA126 splicing variant may have a protective role. A poly-T variant in intron 2 of SNCA gene comprises three alleles (5T, 7T, and 12T), and the length of the poly-T stretch is directly associated with SNCA126 expression levels in the normal brain, influencing the splicing efficiency of SNCA exon 3. Whereas the shortest 5T allele was associated with lower expression of SNCA126-mRNA, the longest 12T led to the highest SNCA126-mRNA levels [126]. The same study also reported that the 12T allele-carrying genotypes accumulated with increasing age in the normal population, while the frequency of 5T allele-carrying genotypes decreased in successive age groups until reaching zero in the oldest group. Collectively, these observations imply that the longest poly-T allele has a protective effect in aging, presumably via its association with higher SNCA126-mRNA levels.

  • Single-nucleotide polymorphisms (SNPs) tagging the SNCA 3′ showed significant effects on the relative levels of SNCA112-mRNA (exon 5 in-frame skipping) from total SNCA transcript levels in human brain tissues. The reported disease risk alleles were correlated with increased expression ratio of SNCA112-mRNA from total SNCA-mRNA [127]. Interestingly, it has been suggested that exon 5 deletion (SNCA112) results in enhanced α-syn aggregation due to a significant shortening of the unstructured C-terminus [108, 109]. This study provided evidence for functional consequences of PD-associated SNCA gene variants at the 3′ region, suggesting that genetic regulation of SNCA splicing plays an important role in the development of the disease. A follow-up analysis using bioinformatic tools found potential splicing enhancer/silencer cis-elements within the sequences surrounding the SNPs and identified SR protein-binding motifs (in particular for SRp40) that might be created or disrupted by these SNPs. Further empirical studies to determine the definite functional variant/s within SNCA 3′ and to establish their association with PD pathology are necessary [128].

  • The minor allele of SNP rs17016074 at the SNCA 3′UTR increased luciferase expression in SH-SY5Y neuroblastoma cells. SNPs in the 3′UTR including rs17016074 were associated with increased PD risk. These findings demonstrated that the 3′UTR of human SNCA, as a whole, and rs17016074, in particular, are loci of potential importance for disease development possibly via post-transcriptional effect on SNCA expression levels [128].

  • SNP rs10024743 in SNCA 3′UTR lies within a target site for miR-34b and was found to lower the miR-34b-mediated repression of the α-syn protein. This study suggested that down-regulation of miR-34b and miR-34c in the brain, as well as a SNP in the 3′UTR of SNCA gene, can increase α-syn expression, possibly contributing to PD pathogenesis [104].

GWAS, candidate gene-based studies, and meta-analyses [9, 14, 20, 24–26, 121, 129–134] showed that polymorphisms of SNCA are associated with spPD, DLB, MSA, and other synucleinopathies. The examples above demonstrated that non-coding genetic variants in SNCA genomic region contribute to the development of synucleinopathies possibly via cis-regulation of the gene expression and suggested that changes in SNCA expression profile is the molecular mechanism that mediates the reported genetic associations.

Epigenetic regulation of SNCA expression, in particular DNA methylation, was suggested to play a key role [135]. However, there are conflicting reports regarding the link between DNA methylation and PD pathogenesis. It was shown that methylation of intron 1 decreased SNCA transcription [136, 137]. Interestingly, in PD and DLB human brains, reduced DNA methylation has been reported, leading to increased α-syn expression [136, 137]. Conversely, no differences have been reported by comparing DNA methylation levels of CpG islands in SNCA intron 1 between normal and PD leukocytes [138].

Conclusion remarks

We reported experimental evidence, from different disciplines, of the role of α-syn overexpression in the pathogenesis of synucleinopathies in general and PD in detail. Herein, we propose a model, whereas even subtle changes in the α-syn expression are able to trigger the onset of synucleinopathies. The changes in the α-syn expression can be attributed to different causes as outlined in Fig. 2 and are potentially cell type-specific. Built on evidence described in this review, the upper panel of Fig. 2 summarizes possible genetic mechanisms—cis-genetic factors and trans-acting modulators—leading to alter regulation of SNCA expression. This review focused on genetic regulation of gene expression and epigenomic to some extent; however, other factors including, environmental, aging, and gender may also contribute to changes in α-syn levels. The schematic model presents several plausible mechanisms of regulation of gene expression and cell type-specific threshold of vulnerability to SNCA overexpression that may underlie the commonality and differences among synucleinopathies (Fig. 2). Interestingly, the genetic associations of SNCA gene with PD and MSA are distinct from the reported association of SNCA with DLB. Consequently, the relation of the SNCA gene with the different diseases in the broader spectrum of synucleinopathies might represent a case of allelic heterogeneity and pleiotropy. This implies that common and distinct regulatory mechanism of SNCA gene expression might be involved in the etiology of synucleinopathies.

Fig. 2
figure 2

A schematic model of the contribution of SNCA expression to synucleinopathies. Changes in the SNCA expression can be attributed to different factors. The upper panel shows the cis-genetic variants across the SNCA genomic locus and their interactions with trans-acting factors, including transcription and splicing machineries and microRNAs. The middle panel presents plausible factors affecting the endpoint cellular level of α-syn protein. These factors include, but not limited to, genetic, epigenetic, environment, gender, and age and may also interact with one another and cross-influence each other. The thresholds among physiological and pathological levels of SNCA, designated in gray vertical lines, need to be determined. The thresholds of SNCA expression levels leading to the different synucleinopathy diseases could be cell type and disease specific. Dopaminergic neurons, cortical neurons (mainly cholinergic), and oligodendrocytes, indicated in the lower panel, may exhibit different vulnerability to α-syn overexpression. In addition, interactions with other causal genes may determine the particular disease path. Collectively, the model suggests that common and distinct regulatory mechanisms and vulnerability thresholds of SNCA gene expression underlie the etiology of synucleinopathies

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It is widely agreeable that reducing α-syn levels represents an attractive strategy to counteract the detrimental effect of the overexpression. However, there are major gaps in the current knowledge, including the precise causal genetic variants and their mechanisms of action underlying the different synucleinopathies, the functional role of the different SNCA transcript variants in the pathogenesis of synucleionopathies, and the expression threshold above which α-syn acquires a toxic effect that lead to the initiation of the disease processes. Emerging technologies and novel model systems, including iPSC-derived neurons and genome editing by CRISPR/Cas9, will support advancements to fill in those gaps in knowledge that in return will result in the development of precision medicine, including biomarkers for preclinical diagnosis and effective treatment approaches for synucleinopathies in general and to each disease in particular.