The physiological effects of stress and aging on chaperone-mediated autophagy
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Autophagy is the degradation of intracellular components by lysosomes. Three known autophagic pathways exist in mammalian cells: macroautophagy, microautophagy and chaperone-mediated autophagy (CMA). The combined actions of these pathways are responsible for the catabolism of a diverse range of substrates. Macroautophagy and microautophagy take in substrates through sequestration of regions of the cytosol in membrane-bound compartments. This method of cargo selection allows for high-capacity degradation, but lacks the ability to selectively target single proteins for degradation. Conversely, CMA is a highly selective pathway for the degradation of certain soluble cytosolic proteins that contain a specific peptide motif in their amino acid sequence. This motif is recognized by the cytosolic chaperone heat shock cognate protein of 70 kDa (hsc70) that delivers substrate proteins to a specific subpopulation of lysosomes competent for CMA. These lysosomes are distinct from other lysosomal populations in that they contain within their lumen the chaperone lys-hsc70. This chaperone facilitates the translocation of substrate proteins across the lysosomal membrane. Binding of substrates to the lysosome-associated membrane protein type 2A (LAMP-2A), the CMA receptor at the lysosomal membrane, is rate limiting for this pathway.;The highly selective nature of the CMA pathway makes it suitable for the removal of damaged or altered cytosolic proteins. In the first part of this work, we show that CMA is activated as part of the cellular response to oxidative stress and contributes to the removal of oxidized proteins. The selectivity of CMA is not limited to damaged proteins but can be used for regulatory purposes within the cell. In the second part of this work, we highlight this role and present evidence that CMA contributes to the maintenance of endoplasmic reticulum (ER) homeostasis through the selective turnover of ER chaperones. This form of degradation is particularly important in response to ER stress, when the intracellular levels of various ER chaperones are upregulated. These functions of CMA in response to stress and in the maintenance of organelle homeostasis suggest that compromised CMA activity, as observed during aging, could have negative consequences on general cellular function. Therefore, the prevention of CMA decline or the restoration of its function is a potential anti-aging intervention. However, in order to correct the age-dependent decline in CMA activity, a better understanding of the molecular mechanisms behind its defective function in old cells is needed. Consequently, in the last part of this study, we have investigated the reasons behind the decreased activity of this pathway with age and have identified a reduction in the stability of the CMA receptor protein at the lysosomal membrane in old cells as one of the main causes.;In summary, our work has revealed two previously unknown cellular functions of CMA - in the cellular response to oxidative stress and in the maintenance of ER homeostasis - and provided new insights into the basis of the age-dependent malfunctioning of this pathway. Our findings could help to set the basis of future anti-aging interventions through modulation of CMA.
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