We have previously identified a decrease with age in the activity of a lysosomal pathway involved in the selective degradation of soluble cytosolic proteins in most types of mammalian cells. This autophagic pathway, known as chaperone-mediated autophagy (CMA), is preferentially activated under stress conditions such as nutritional stress or exposure to different toxic derivatives. Under these conditions, an amino acid motif in the substrate proteins is recognized by a cytosolic chaperone complex, which targets the substrate to the lysosomal membrane. After docking on a receptor protein at the membrane, the substrate is unfolded and crosses the lysosomal membrane assisted by a luminal resident chaperone. Once translocated, the substrate protein is rapidly degraded by the lysosomal hydrolases. The ability of cells to upregulate CMA in response to these stressors decreases as they age, likely resulting in poor removal of proteins normally degraded by this autophagic pathway. We have identified a decrease with age in the levels of the CMA receptor as the primary defect responsible for CMA failure.
The main peculiarity of CMA, when compared to other forms of autophagy, is its selectivity. Only soluble proteins containing the CMA targeting motif are degraded through this pathway. This selectivity confers CMA the ability to remove particular proteins from inside cells without altering neighboring ones. In fact, we have found that activation of CMA is part of the defensive response orchestrated by most cells during oxidative injury. Oxidizing conditions promote higher rates of CMA by affecting both the substrates and the lysosomal compartment directly. Oxidized substrates are more efficiently internalized into lysosomes and, independent of this effect on the substrates, lysosomes from cells exposed to mild-oxidative stress also show enhanced ability for substrate translocation. This novel role of CMA in the removal of oxidized cytosolic proteins during mild-oxidative stress reinforces the contribution of the age-related failure in CMA to accumulation of damaged proteins in old tissues. In fact, in livers from old rodents, we have found a striking correlation between the increase in the levels of oxidized proteins in the cytosol as the animal ages and the decrease in the amount of oxidized proteins detected inside CMA active lysosomes.
Our group has undertaken two different approaches to restore CMA in old rodents. On one hand, we have generated a bitransgenic mouse line in which levels of the CMA receptor at the lysosomal membrane can be regulated at wish. By preventing the decrease in the receptor levels with age, we intend to maintain proper CMA activity in old rodents. As a second approach we are investigating the effect of caloric restriction on CMA activity. We have found a constitutive activation of this autophagic pathway in caloric restricted rodents till advanced ages. This continuous activation of CMA may contribute to the proper removal of oxidized and other damaged proteins in these animals. These two interventions to enhance CMA activity should allow us in the future to analyze possible beneficial effects of the restoration of this form of autophagy in old organisms.