Pathophysiological relevance of chaperone-mediated autophagy

Abstract

The lysosomal system degrades intracellular proteins and organelles by a process collectively termed autophagy. Chaperone-mediated autophagy (CMA) is a selective type of autophagy for the degradation of certain cytosolic proteins under stress conditions. CMA activity decreases with age but the cellular consequences of this loss of function were unknown. The goal of this study was to reproduce the CMA impairment in cultured cells and elucidate the contribution of defective CMA to the phenotype of aging. We have developed a method of RNA interference (RNAi) against the lysosome associated membrane protein type 2A (LAMP-2A) to selectively and efficiently block CMA in cultured mouse fibroblasts. CMA-defective cells display an initial disregulation of the other types of autophagy, such as macroautophagy, as well as other proteolytic systems. This initial decline in macroautophagy activity is followed by a state of upregulation of this form of autophagy to accommodate for cellular proteolysis requirements under basal conditions---pointing toward the existence of cross-talk among the different autophagic pathways. However, compensation for CMA by other proteolytic systems is not complete and these cells are more susceptible to stressors, suggesting that, although protein turnover was maintained, the selectivity of CMA is necessary as part of the cellular response to stress.;In light of the role of CMA in handling of altered intracellular proteins, in the second part of this work, we have investigated the possible contribution of altered autophagy to the pathogenesis of four protein conformational disorders: Alzheimer's disease, Huntington's disease, Parkinson's disease, and Juvenile Neuronal Ceroid Lipofuscinosis. Comparative analysis using animal and cellular models for these disorders has revealed that the primary autophagic defect is different in each disorder. We have also determined that cell-type differences in the ability to respond to the pathogenic protein, resulting from differences in the prevalence of the main autophagic pathways in each cell type, were behind the observed alterations.;In conclusion, our findings provide evidence that CMA is an essential component of the cellular response to stress and that cross-talk of CMA with other autophagic pathways could be an important determinant of disease progression in protein conformational disorders.