The challenge of aging muscle and protein homeostasis
The aim of our laboratory has long been focused on studying chaperone-mediated autophagy (CMA), a highly selective form of autophagy that identifies and delivers specific proteins directly to lysosomes for degradation. By clearing damaged or unnecessary proteins, and modulating levels of functional proteins in response to different types of stressors, CMA preserves cellular proteostasis and function. Yet, mounting evidence has shown that CMA activity decreases with age in most organs, including skeletal muscle, and this could potentially disrupt the delicate protein balance required for their correct function. Loss of proteostasis and autophagy dysfunction are well-accepted drivers of aging.
Interest in skeletal muscle biology has been central to aging research, given that the loss of muscle mass (or sarcopenia) is a major driver of frailty and poor health outcomes in older adults.
These considerations motivated us to pose a central question: Could the decline in CMA that occurs with aging contribute directly to the loss of muscle function and integrity?
Unraveling the consequences of CMA loss in skeletal muscle
To investigate the physiological role of CMA in skeletal muscle, we separately analyzed the impact of this pathway in muscle fibers (our primary work) and muscle stem cells (in a separate collaborative study that addresses the contribution of CMA to muscle regeneration). Using a CMA reporter mouse, we found that CMA is upregulated in skeletal muscle fibers in response to stimuli such as starvation, exercise or recovery from muscle injury. However, dietary challenges and aging show an inhibitory effect on skeletal muscle CMA. To understand the consequences of this loss of CMA, we generated a mouse model with CMA specifically blocked in skeletal muscle. When maintained unchallenged, signs of muscle malfunction did not appear until one year of age, when the CMA-deficient mice developed pronounced muscle weakness, cellular damage, and remarkable structural abnormalities in both mitochondrial and sarcoplasmic reticulum. Earlier onset of this phenotype was noted when younger animals were subjected to muscle injury.
We used comparative proteomics to identify proteins that are normally degraded by CMA in skeletal muscle, and that might drive the phenotype when CMA is blocked. We discovered that a subset of calcium-handling proteins, including SERCA in the endoplasmic reticulum, was no longer properly degraded, and that calcium homeostasis within muscle was disrupted as a result.
From mouse models to human aging: CMA as a therapeutic target
At this point, our project took two complementary directions aimed to explore translatability of our findings in mice. First, we genetically restored CMA activity systemically in middle-aged mice. Remarkably, these animals showed preserved muscle function as they aged. Second, we analyzed the status of CMA in sarcopenic patients and found reduced CMA compared to age-matched healthy subjects. To explore whether CMA dysfunction is an early event in the age-related loss of skeletal muscle function, in collaboration with Luigi Ferrucci’s group, we analyzed samples from healthy older human subjects from the Genetic and Epigenetic Signatures of Translational Aging Laboratory Testing (GESTALT) project. Interestingly, we found that despite their initial preservation of function, multiple key CMA-related proteins were already reduced in aged human muscle.
These findings together point toward an exciting possibility: boosting CMA could emerge as a therapeutic strategy to combat muscle weakness during human aging.
Our interest continues now in investigating ways to modulate CMA in humans, both pharmacologically and through life-style interventions, seeking therapies that could one day prevent or treat sarcopenia in aging populations.