e-book Protein misfolding in neurodegenerative diseases. Mechanisms and therapeutic strategies

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Molecular chaperons in protein folding and quality control. Physiological and environmental stress such as aging, diseases, heat, heavy metal ions and UV exposure induce damages to cellular proteins and accelerate imbalances in the cellular proteome [13]. Originally, chaperons were identified as proteins that were overexpressed in response to stress [14]. Later, functions of these proteins were also found to be essential to maintain the balance between de-novo folding and regulation of degradation pathways under normal physiological conditions.

Nevertheless, despite of its constitutive levels, the expression and synthesis of these proteins greatly increases during the course of stress and therefore, these group of proteins are also known as heat shock proteins Hsps [7]. Classified according to the molecular weight and homologies, cells consist of several classes of chaperones such as Hsp40, Hsp60, Hsp70, Hsp90, Hsp and the small Hsps sHsps [15].

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In the cellular system, three classes of chaperones - Hsp70, Hsp60 and Hsp90 - largely participate in de-novo folding and refolding of proteins in an ATP dependent manner [16]. Chaperones start functioning from the point where stretches of nascent polypeptide chain get released from the polyribosomes [17]. Molecular chaperones recognize hydrophobic residues or unstructured peptide bonds exposed by non-native proteins.

Such hydrophobic residues or unstructured peptide bonds are buried inside during native confirmation.

Several chaperones such as trigger factors, ribosome binding Hsp70s, nascent chain associated complex NAC , ribosome associated complex RAC etc. However, co-translational folding is possible only for small and single domain proteins and majority of the cellular proteins are multi-domain and large. When such proteins are released in the cytoplasm, exposed non-native structures in the nascent proteins trigger self-aggregation [6]. Hsp70 along with several other molecular chaperones plays a central role in identification and stabilization of such newly synthesized unfolded proteins.

Hsp70 is a ubiquitously expressed, stress inducible multifunctional chaperone which mediates a variety of biological processes, including folding, targeting, degradation and interactions of proteins [16]. Successive rounds of Hsp70 mediated folding results in the formation of fully folded substrates. However, generation of terminally misfolded or slowly folding species is also possible during this phenomenon. In view of above, selective degradation of misfolded protein is needed to avoid accumulation of potentially toxic aggregates.

The folding pathway continues to correct the slowly folding or misfolded species for which Hspsubstrate complex recruits Hsp90 and other chaperones. Hsp90 is a crucial regulator of protein homeostasis which also functions via an ATP-dependent hydrolytic cycle [16].

Natural Animal Models of Neurodegenerative Protein Misfolding Diseases

Hsp90 acts downstream of Hsp70 and assists in achieving conformational maturation of various signal transduction proteins including kinases and many of the steroid hormone receptors [16]. Hop allows substrate transfer through its tetratricopeptide repeat TPR domain and acts as a direct link between Hsp70 and Hsp90 [21]. This implies the fact that chaperones also facilitate timely removal of misfolded proteins.

Quality control of misfolded proteins generated by Hsp70 and Hsp90 folding machinery is found to be further regulated by a different TPR domain-containing co-chaperones such as C terminus domain of Hsc70 interacting protein CHIP [22]. CHIP associates with Hsp70 or Hsp90 through its TPR domain and ubiquitinates misfolded substrates and target them for endoplasmic-reticulum-associated protein degradation pathway [22].

Interestingly, many of the Hsp90 substrates also include oncogenic proteins like kinases and transcription factors, documenting its role in tumor development [23].

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Therefore, inhibition of Hsp90 has emerged as a novel therapeutic approach in cancer treatment. As discussed earlier, protein folding in cytosol is generally achieved by Hsp70 along with downstream chaperones and chaperonins. Chaperonins are large multimember ring complexes that provide posttranslational folding of proteins in an isolated compartment, unimpaired by aggregation [24]. It has been proposed that Hsp70 and Hsp60 chaperone machinery act sequentially, where the previous acts upstream with nascent polypeptide and the latter functions downstream and assists in folding of those substrates which have failed to reach to the final states by Hsp70 alone [16].

Eukaryotic Hsp60 represents group I chaperonins which includes GroEL found in bacteria, mitochondria and chloroplast with its seven membered ring structure. GroEL functionally cooperates with GroES Hsp10 in eukaryotes which forms a lid on the folding cage and together they represent the most extensively studied protein folding machinery [25]. Interistingly, apical domain protrusion of TRiC replaces the role of lid during cage formation [27]. Interestingly, TRiC has also been found to prevent protein aggregation in some protein folding disease models [28].

One of the primary difference between GroEL and TRiC is that the previous acts strictly post-translationally while the later functions co-translationally to fold large proteins of 30 to kDa size. Role of sHsps and Hsp is also crucial in maintaining protein quality and cellular proteome homeostasis. Hsp family includes chaperones such as bacterial ClpB, yeast Hsp etc. Similar function is performed by Hsp in higher eukaryotes which refold and disaggregate the protein aggregates [32]. Molecular chaperones and neurodegenerative disorders. Neurodegenerative disorders are a class of late-onset progressive disorders characterized by presence of misfolded protein aggregates in affected neurons and selective degeneration of the brain.

The protein aggregates in each case are formed as a result of protein misfolding which might occur due to genetic or environmental stresses. Formation of protein aggregates -a common link between various neurodegenerative disorders.

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Conformational changes and protein misfolding result in amyloid-like aggregate formation which is a hallmark of most neurodegenerative diseases. For instance, HD is a dominantly inherited neurodegenerative disorder caused by expansion of CAG trinucleotide repeat in the coding region of the huntingtin htt gene [36]. The exact cellular function of Htt is still debatable, however, it has been proposed to participate in vesicular transport and cell signaling [37].

Mutant Htt with expanded poly Q repeats tends to misfold and aggregates in the form of inclusion bodies in the cells [38]. Inclusion body formation results in pathological changes in the basal ganglia and cortical neurons of the brain [38]. Types of mutations in cases of PD may range from missense mutations to gene duplication or triplication.

Therefore, in view of the fact that protein misfolding serves as the root cause in most of neurodegenerative disorders, next section of the article discusses about major events which lead to disease pathogenesis and also provides a brief overview of the role of chaperones in alleviating aggregate-mediated cytotoxicity. The presence of fibrillar protein aggregates marks the signature feature shared by most of the neurodegenerative disorders. Such aggregates were reported for the first time about a century ago by Alios Alzheimer in brain parenchymal tissues of mentally retarded patients [42].

In view of above, it has been proposed that misfolded protein monomers trigger the process of aggregate formation and disease pathogenesis [44]. Several studies suggest that undesirable conformational changes of corresponding proteins underlie the cause of disease pathogenicity [34]. One of the following reasons may result in transformation of a natively folded or even nascent polypeptide chain into a pathogenic misfolded conformation: a impairment of chaperone based protein folding machinery due to aging or environmental stresses eg.

CAG expansion in exon 1 of htt c stress mediated atypical post-translational modifications of the target protein eg. This shows that the information needed for protein misfolding except CAG expansion is not encoded by the genome in sporadic cases of neurodegenerative disorders. Therefore, the protein aggregates found in neurodegenerative disorders share common structural and biochemical features as per the altered confirmation of the constituent proteins.

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Shared mechanism of protein aggregate formation in neurodegenerative disorders. As a nascent polypeptide chain emerges out of the translating ribosome, numerous hydrophobic amino acid stretches get exposed to the cellular milieu, which must be stabilized to ensure non-aggregated states of the protein. At this point, the polypeptide chain can theoretically adopt countless number of conformations out of which only one is sufficiently stable under physiological conditions and could take up the biologically active state.

These species expose hydrophobic segments to the solvent, an event that sparks off the aggregation process [35]. The equilibrium between the native and non-native species shifts towards the non-native flank when the protein harbors mutations like CAG repeats or under conditions of cellular stress or aging.

In the log phase, other misfolded protein molecules or even pre-formed seeds are recruited to the initial nuclei leading to rapid polymerisation of the filament and consequent formation of mature fibrillar structures [46,47]. These fibrillar deposits may then be further organized into larger aggregates or amyloid plaques. While aggregates are mostly sequestered and deposited intracellularly in the form of inclusion bodies, amyloid plaques are generally found outside the cell [47].

Natural Animal Models of Neurodegenerative Protein Misfolding Diseases | Bentham Science

The intracellular inclusion bodies can be localized both in the cytoplasm as well as in the nucleus. The inclusion bodies are dense enough to sediment upon low speed centrifugation and could also be observed by light or electron microscopy [49]. However, intra-nuclear protein inclusions are key features of most inherited neurodegenerative disorders and have been found to be more toxic than their cytoplasmic counterparts [49].

It has been proposed that neuronal inclusions sequester several key transcription factors through hydrophobic interactions and thus deplete the level of cellular transcriptional machinery [12]. Subsequently, a progressive increase in cellular toxicity may lead to activation of cell death pathways. Compromised protein folding apparatus serves as one of the major cause for development of neurodegenerative disorders. Several reports have indicated an early impairment of stress response in most of these disorders [50].

For instance, brain samples of sporadic AD and PD patients have been found to harbour an abnormally S-nitrosylated form of the endoplasmic reticulum ER chaperone protein disulphide isomerase PDI [51]. As a consequence, the anomalous chaperone is incapable of mounting an efficient ER stress response which is a basic necessity in all protein misfolding disorders.

Similarly, a mutation in the gene encoding the co-chaperone for Grp78 a crucial ER chaperone has been shown to result in protein accumulation and neurodegeneration [52]. As discussed earlier, molecular chaperones represent key players of the protein quality control machinery and in this capacity they play essential role in eliminating toxic proteins by inhibiting or promoting the process of aggregate formation.

Interestingly, molecular chaperones have been shown to be associated with Htt aggregates as well as with Lewy bodies in brain tissues of affected individuals [54,55]. Moreover, transgenic mouse models of SCA1, 3 and 7 have also been reported to exhibit such association [56]. It has been suggested that chaperones may either block the initial oligomerization of the misfolded proteins by preventing fibril formation or they may promote amyloid formation in an attempt to constrain the toxic species into benign aggregates [57,58].

As discussed above, parts of the nascent polypeptides emerge out of the ribosome and associate themselves with Hsp70 chaperone to achieve favorable folding and to prevent undesirable interactions. Despite the available safeguards, if protein misfolding occurs due to one of the reasons discussed above, induction of additional Hsps takes place.