Keywords

1 Introduction

The concept of proteostasis maintenance involves the biological pathways which are critical for post-mitotic cells, such as neurons. Disruption of the mechanisms of proteostasis is increasingly referred in many studies, to be involved in neurodegenerative diseases. Dysregulation of some of the key mechanisms which have been evolved to maintain cellular homeostasis, such as protein folding, as well as the presence of molecular chaperones and degradation mechanisms, are implicated as pathological hallmark of these diseases.

Protein misfolding and formation of toxic aggregates within affected neurons are thought to be involved in the pathogenesis many age-related diseases, such as Alzheimer’s disease. Whilst neurodegenerative diseases differ in the proteins which appear to misfold, one common feature is the disruption of the protein quality control system.

Chaperone machinery is involved in the assembly of the unique three-dimensional structure of proteins. Chaperones play a ubiquitous role in the cytoplasmic meshwork: they are highly abundant, they form a complex with all the elements of the cytoskeleton and they also attach to a plethora of other proteins [1]. Molecular chaperones serve to prevent protein misfolding, participate in the loose folding procedure, assist in folding and refolding of damaged proteins and target severely damaged proteins to degradation and sequester them to larger aggregates. Therefore, chaperones are key components to this protein quality control system [2].

The native folding of proteins can be disrupted by stress, molecular crowding and mutations [3]. Driven by ATP, chaperones gradually convert toxic misfolded protein substrates, into non-toxic, natively refolded. Stress and protein damages can lead to cellular dysfunction, causing degenerative diseases and aging [4]. These conditions activate cellular stress-response pathways, including the UPR, which counteract stress and help restore homeostasis to protein folding in the ER [5].

Chaperone complex disruption results in instability and incorrect subcellular localization of the signaling protein [6]. Besides the major role of chaperones, which is to target all misfolded proteins with hydrophobic surfaces, there are also specialized chaperones that stabilize several signal transduction networks important to cancer cells [7].

2 Why Proteins Misfold?

Misfolding may originate from improper interactions between regions of the folding polypeptide chain. Another factor is the crowded nature of the intracellular environment that may prevent folding from proceeding at a biologically relevant time scale [8]. Incompletely folded proteins, often tend to inappropriate interactions with other molecules [9] and aggregate in the cell due to high local concentration of nascent chains [10]. There may be several kinds of aggregates, including disordered or ‘amorphous’ aggregates, but amyloid fibrils are the most characteristic [11]. Deposition of abnormal protein aggregation characterizes most neurodegnerative disorders, not just AD and Parkinson’s, but also motor neuron diseases, as well as diseases of peripheral tissue like familial amyloid polyneuropathy, Charcot-Marie-Tooth [12–15].

3 How Chaperones Prevent Aggregation

The cellular strategy to counteract the aggregation of non-native proteins is the chaperone machinery. The heat shock proteins (HSPs) are the main chaperone classes which are referred to prevent the accumulation of misfolded conformers (e.g. HSP60 and HSP70) [16]. The reversibility of protein aggregation has been first observed to be mediated by heat-shock protein Hspl04 [17]. Chaperone binding blocks intermolecular aggregation of non-native polypeptides and may also prevent, or reverse intramolecular misfolding [18, 19]. Those proteins that do not succeed to fold correctly are selected for degradation by the protein control system. This is achieved through the proteasome and the chaperone mediated autophagy [20]. Increasing evidence show that perturbation of these functions plays a key role in the pathogenesis of severe human disorders [21, 22].

Moreover, the ability of maintaining cellular proteostasis declines with aging, which results in the accumulation of misfolded proteins, deposition of aggregates, cellular toxicity and eventually cell death [23–25]. The age-related accumulation of oxidized proteins has been proposed to be due to either, or both, increased protein oxidative damage and decreased oxidized protein degradation and repair [26]. This deterioration of proteostasis is a characteristic risk factor of many human pathologies and represents a hallmark which is considered to contribute to the aging process [27].

The role of chaperone function and the simultaneous protein oxidation, misfolding and aggregation in aged organisms, suggest that preservation of protein homeostasis and long-range protein organization have a major role in neurodegeneration and aging [1, 28, 29].

4 ER Stress and the Unfolded Protein Response

Although differential mechanisms have been described to be involved in different neurodegenerative conditions, endoplasmic reticulum (ER) stress is increasingly implicated as a key factor relevant to pathogenesis of AD and other neurodegenerative diseases [30, 31]. The accumulation of misfolded or deficiently modified proteins within the ER, disturbs ER homeostasis, giving rise to ER stress, which results in activation of the unfolded protein response (UPR) [32, 33]. The UPR involves [31, 34]:

  • up-regulation of protein chaperones to promote protein folding,

  • translational attenuation to reduce the load of proteins within the ER to prevent further accumulation of misfolded proteins,

  • up-regulation of ER-associated protein degradation (ERAD) and

  • autophagy to promote degradation of misfolded proteins

Therefore, ER stress triggering of UPR plays a pivotal role in maintaining cell proteostasis [35, 36]. In circumstances of chronic or prolonged ER stress, however, the UPR seems to trigger apoptotic signalling cascades [37, 38]. The ER response includes changes in specific proteins, which might cause translational attenuation, induction of ER chaperones and degradation of misfolded proteins [39]. In case of prolonged or aggravated ER stress, apoptotic signalling is stimulated, which leads to cell death [40]. The dual functions of ER stress appears to switch from pro-survival to pro-apoptosis [41–43].

5 Chaperones and Diseases

Molecular chaperones have a ubiquitous role in the protein quality control system. Misfolded proteins are usually refolded with the assistance of molecular chaperons and/or degraded by the ubiquitin-proteasome system. In many studied the protective effects of overexpressing chaperones in models of misfolding disease is promoted. The role of different chaperones, their function and their association in different neurological diseases is summarized in the table in Appendix section.

5.1 Alzheimer’s Disease

Amyloid fibrils, Aβ aggregation, are a well-characterized aggregation state associated with Alzheimer’s disease [44]. Several studies have investigated the role of heat shock proteins in AD [45]. Some studies suggest that the induction of small heat-shock proteins, Hsp70, protects neurons from protein aggregation and toxicity [46, 47]. In addition to HSP70 the complex of Hsp90 can also inhibit Aβ formation and slow the rate of aggregation [48]. Other studies showed that ubiquitin (a heat-shock protein, which labels damaged proteins and directs them for proteolytic degradation) is affected by Alzheimer’s disease, in neurons and in surrounding astrocytes [49]. The role that HSPs may act therapeutically as neuroprotective agents, by modulating innate immune activation is reviewed in [50].

5.2 Parkinson’s Disease

Parkinson’ s disease is the second most common movement disorder, characterized by motor impairments—bradykinesia, rigidity, and resting tremor, caused by a progressive degeneration of dopaminergic neurons in the substantia nigra [51]. An increasing number of evidence shows that endoplasmic reticulum stress and the unfolded protein response are also key elements of Parkinson’ s disease etiology [52]. These results suggest a proposed therapeutic strategy to ensure appropriate protein folding and to avoid ER stress. This requires an efficient chemical or molecular chaperone network to promote the appropriate folding of proteins [53]. A number of chaperone-based therapies are under development, which aim to prevent the formation of potentially toxic—synuclein oligomers and aggregates [54].

5.3 Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis also known as ALS is a neurodegenerative disease that affects the upper and lower motor neurons. Molecular analyses have shown that the disease is primarily caused due to a mutation either to SOD1 (Cu/Zn superoxide dismutase) or to FUS (fused in sarcoma) [55]. Mutant SOD1 is related to the Hsp70/Hsp90 network and its degradation seems to be regulated by CHIP, another co-chaperone [56]. Ubiquitinated SOD1 forms aggregates, that are associated with Hsc70 which might be able to protect Sod1 from degradation while Hsp70 favors it [57]. Therapeutic strategies that inhibit Hsp90 and increase Hsp70 activity are investigated in some cellular and animal ALS models [58].

5.4 Huntington’s Disease

Amyloid-like inclusions have been associated also with Huntington’s disease (HD), which is caused by expanded polyglutamine repeats in the Huntingtin protein [59]. HD is an autosomal, dominant and inherited neurodegenerative disease that is focused on the region of basal ganglia causing mental, emotional and motor problems to the patients. In Huntington’s, the responsible gene (IT15) for the production of protein huntingtin is mutated causing expanded repetition of the trinucleotide CAG, which in turn makes the produced polyglutamine strand toxic for the brain. As a result, neurons which contain the mutant protein begin to atrophy [60, 61]. The normal function of huntingtin is to maintain brain cells in a good condition and to help in intracellular procedures. The mutation at the beginning of the huntingtin gene causes the destabilization of the protein leading to problems; it interferes with the typical function of [62]. Mutant huntingtin (m-htt) affects many cytoplasmic proteins which are related to apoptosis, transcription, mitochondrial function and other vital for the cell procedures. M-htt joins together with other proteins forming protein aggregates. Protein aggregation is the phenomenon when misfolded proteins clump together inside or outside the cell. Those forms are toxic for the cell and reduce the Ubiquitin Proteasome System function (UPS) [63, 64].

5.5 Charcot-Marie-Tooth Disease

Autosomal Dominant Demyelinating Neuropathies CMT1 (Charcot-Marie-Tooth Disease type1) is associated with an autosomal dominant duplication on a chromosome that includes the peripheral myelin protein 22 gene (PMP22) [65]. When overexpressed in cultured cells, PMP22 has been observed to form protein aggregates [66]. In other studies were found that misfolded protein SIMPLE forms abnormal cytosolic aggregates. These findings suggest that demyelinating CMT may be a protein-misfolding disease of Schwann cells [67].

5.6 Creutzfeldt-Jakob Disease and Other Prion Encephalopathies

Prion proteins from mammalian species, are prone to amyloid-like prion diseases, like Creutzfeld-Jakob disease [68]. In many studies it is referred that chaperones try to block the contact surfaces of prion molecules [1]. Many chaperones, such as Hsp60, Hsp70, or its co-chaperone, Hsp40 were found to fight against prion aggregation [69, 70]. The “chaperone overload” hypothesis emphasises the need for efficient ways to enhance chaperone-capacity in ageing subjects and calls for the identification and future “repair” of silent mutations [1].

5.7 Conclusions

Researches suggest that protein aggregation is part of the cellular response to an imbalanced protein homeostasis. Intense research interest to unravel the pathophysiological significance of these protein aggresome has unveiled the important role of chaperones in proteostasis, by promoting the correct folding of proteins into their native conformations. Therefore, novel therapeutic strategies should aim at the role of chaperones to prevent aberrant protein misfolding and promoting maintenance of cellular homeostasis.