Research

Protein Aggregation and Human Neurodegenerative Diseases
Many neurodegenerative diseases, such as Alzheimer’s disease (AD), Huntington’s disease (HD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), and transmissible spongiform encephalopathies (TSEs or prion diseases), involve selective neuronal loss with degeneration of specific regions of the brain . These diseases are characterized by the accumulation of intracellular or extracellular protein aggregates. The aggregates may consist of fibrillar structures containing β-sheet-rich misfolded protein, termed amyloids, that are typically characterized by their ability to bind aromatic dyes. A large body of evidence from pathology, genetics, animal models, and biochemical, biophysical, and cell biological studies indicates that protein aggregation is not the result of neuronal death but in fact contributes to it. There is general agreement that the most neurotoxic protein aggregates cause cell death by impairment of various cellular functions and eventually lead to degeneration of brain regions.  (see figure)

Molecular Chaperones and Protein Folding
Proper folding of newly translated proteins can be challenging, given the very high protein concentration (>300mg/ml) in the eukaryotic cytosol. Protein quality control mechanisms insure cells against a build up of abnormally folded proteins. The first line of defense is provided by the Molecular Chaperones. The term ‘Molecular Chaperone’ encompasses several families of highly conserved proteins that mediate the folding and assembly of other proteins, but are not components of the final functional structures. In addition to their function in ensuring correct folding of polypeptides de novo, molecular chaperones also function in the prevention of misfolding and aggregation. In general they recognize structural features normally buried in fully folded proteins and exposed in unfolded polypeptides such as hydrophobic residues or unstructured backbones regions. Many chaperones are highly over-expressed under conditions of stress, which trigger protein misfolding, and are therefore also known as stress proteins or heat shock proteins (Hsps). Cells may accumulate misfolded proteins as a result of cellular stresses such as heat shock, oxidative stress, viral infection, anoxia, and even normal aging – most likely due to a reduced availability or functional capacity of molecular chaperones. They are also the most potent suppressors of protein aggregation-related neurodegeneration.

Protein Aggregation and Disaggregation
Protein aggregation is inevitable in living cells and may be brought about by partial unfolding during thermal or oxidative stress and by alterations in primary structure caused by mutation, RNA modification, or translational misincorporation. Their accumulation is tightly linked to neuronal degeneration or organ failure in many protein deposition diseases. However, healthy unstressed cells do not accumulate aggregates due to their clearance by the protein disaggregation machinery, which includes chaperones of the Hsp100 family together with Hsp70/Hsp40. The Hsp100 chaperones are ‘protein remodeling factors’ of the AAA+ (ATPases associated with diverse activities) superfamily and form hexameric ring-shaped structures. They are well studied in unicellular eukaryotes (in baker’s yeast, Saccharomyces cerevisiae), plants, and bacteria. Surprisingly, they are absent in metazoa! It is highly likely that Hsp100s have been replaced by alternate mechanisms of protein disaggregation, but efforts towards identifying these mechanisms have not been made. In light of the numerous protein aggregation diseases, elucidation of the interplay between these mechanisms is crucial. We are interested in identifying such mechanisms.

Small Heat Shock Proteins (sHsps)
Members of the sHsp family protect cells from a variety of environmental conditions such as heat and oxidative stress by antagonizing protein aggregation. In vivo, a number of functions have been proposed for sHsps – enhancing stress resistance, regulating actin and intermediate filament dynamics, inhibiting apoptosis etc. sHsps share a conserved α-crystallin domain of 80–100 amino acids at their C terminus whereas their N-terminal regions are highly variable in sequence and length. It has been proposed that sHsps aid in refolding of denatured proteins by holding them in a reactivation-competent state. The sHsps form dynamic oligomeric structures ranging from 9-50 subunits and resemble a hollow soccer ball.
The human genome codes for 10 genes for sHsps differing between 45 and 85% in sequence. Mutations in human sHsps lead to several aggregation diseases – for example, mutations in αA-crystallin leads to cataracts, mutations in αB-crystallin leads to desmin-related myopathy, and missense mutations in HSP27 segregates in families with Charcot-Marie-Tooth disease. Both Hsp27 and αB-crystallin have been found in proteinaceous inclusions of Alzheimer’s and Parkinson’s diseases. Over-expression of sHsps is highly protective against toxicity induced by α-synuclein or polyglutamine in model systems. Our goal is to understand the mechanism of this protection.

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Medical College of Georgia
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Cancer Research Center | Centers and Institutes | Medical College of Georgia
Please email comments, suggestions or questions to:
Kavitha Krishnarao, kkrishnarao@mcg.edu 

June 29, 2006