Role of molecular chaperones in neurodegenerative disorders

Many major neurodegenerative diseases, including Amyotrophic Lateral Sclerosis, Alzheimer's disease, Parkinson's disease, Huntington Disease and other polyglutamine expansion disorders, are associated with degeneration and death of specific neuronal populations due to accumulation of certain abnormal polypeptides. These misfolded species aggregate and form inclusion bodies and their neurotoxicity is associated with the aggregation. To handle a build-up of abnormal proteins cells employ a complicated machinery of molecular chaperones and various proteolytic systems. Chaperones facilitate refolding or degradation of misfolded polypeptides, prevent protein aggregation and play a role in formation of aggresome, a centrosome-associated body to which small cytoplasmic aggregates are transported. The ubiquitin-proteasome proteolytic system is critical for reducing the levels of soluble abnormal proteins, while autophagy plays the major role in clearing of cells from protein aggregates. Accumulation of the aggregation prone proteins activates signal transduction pathways that control cell death, including JNK pathway that controls viability of a cell in various models of Parkinson's and Huntington's diseases. The major chaperone Hsp72 can interfere with this signalling pathway, thus promoting survival. A very important consequence of a build-up and aggregation of misfolded proteins is impairment of the ubiquitin-proteasome degradation system and suppression of the heat shock response. Such an inhibition of the major cell defense systems may play a critical role in neurodegeneration. Here, it is suggested that these changes may reflect a senescence-like programme initiated by the aggregated abnormal polypeptides. Pathways that control the fate of misfolded proteins, for example molecular chaperones or proteolytic systems, may become interesting novel targets for therapy of neurodegenerative disorders.