Restoration of chaperone -mediated autophagy in aging mice
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Chaperone-mediated autophagy (CMA) is one of the three major types of autophagy in mammalian cells, in which select cytosolic proteins are degraded in lysosomes after binding to a lysosomal membrane receptor--LAMP-2A. CMA is mostly activated under stress conditions. In aging, CMA activity decreases due to a decrease in the levels of LAMP-2A in the lysosomal membrane. The goal of this work was to maintain levels of LAMP-2A constant throughout a mouse's life span and to analyze the cellular consequences of this intervention. We have generated a bitransgenic mouse model, which overexpresses LAMP-2A specifically in the liver under the control of a tetracycline-regulated promoter. Expression of this exogenous copy of LAMP-2A was sufficient to compensate for the decrease in levels of endogenous LAMP-2A. Lysosomes from 22 month old transgenic mice displayed CMA activity at values close to those of 6 month old. Maintenance of proper CMA until advance ages resulted in a marked reduction in the levels of oxidized cytosolic proteins, aggregate proteins and of lipofuscin, supporting an improved ability to handle protein damage in these animals. In addition, old mice with maintained CMA activity showed lower levels of cellular damage in response to stress--as measured by blood levels of liver enzymes and TUNEL apoptotic index--and improved liver function (clearance rates of toxic circulating compounds) when compared with wild type littermates. Interestingly, preservation of CMA activity also improved macroautophagy and the ubiquitin proteasome system, suggesting that prevention of the decline in one proteolytic pathway could benefit the others by avoiding their overloading. Taken together, our results provide the first evidence in vivo of the physiological relevance of CMA as part of the cellular response to stress and the contribution of the decline in its activity with age to the accumulation of altered proteins characteristic of the aging phenotype.