UNC-45 has a crucial role in maintaining muscle sarcomeres during aging in Caenorhabditis elegans
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As people live longer, age-related diseases, like sarcopenia, will become a greater public health concern. We use the model organism C. elegans to better understand the molecular mechanisms behind muscle maintenance. Muscle function is dependent on having properly organized and functioning thick filaments, which are primarily composed of myosin. The myosin head requires the chaperone UNC-45 to initially fold it after translation and is likely used to re-fold back to functionality after thermal or chemical stress induced unfolding. We observe an early onset of sarcopeania when UNC-45 is perturbed during adulthood. We observe that during adult aging, there is a sequential decline of HSP-90, UNC-45, and then myosin. Myosin and UNC-45 protein decline are independent of steady state mRNA levels. Loss of UNC-45 is correlated with an increase in phosphorylation of the protein. By mass spectrometry, S111 was identified as being phosphorylated and this modification may affect binding to HSP-90. A longevity mutant with delayed onset of sarcopenia also shows a delay in the loss of HSP-90, UNC-45, and myosin. We also see a decrease in UNC-45 protein, but not transcript, in an hsp-90 loss of function mutant, suggesting a role for HSP-90 in stabilizing UNC-45. This leads us to propose the model that during aging, a loss of HSP-90 leads to UNC-45 being post translationally modified, such as phosphorylation, and degraded, which then leads to a loss of myosin, and thus muscle mass and function. A better understanding of how myosin and its chaperone proteins are regulated and affected by aging will lead to better preventative care and treatment of sarcopenia and, possibly, the age-related decline of heart muscle function.
Graphical Abstract
In young adults, under normal conditions the UCS domain of UNC-45 (shown in green) is bound to the myosin head (in red) and the TPR domain (in yellow) is bound to HSP-90 (in purple). Under stress conditions, HSP-90 detaches from the TPR domain, causing a conformational change in UNC-45 that allows the Central domain (in blue) to bind to the myosin neck (in red) resulting in inhibition of the myosin power stroke while the UCS domain protects/re-folds the myosin head. HSP-90 can then rebind the TPR domain, causing the Central domain to release the myosin neck, allowing movement of the myosin motor. However, aged adults experience a loss of HSP-90 and UNC-45 (which has increased post translational modification with aging). The loss of the Myosin chaperones leads to increased aggregation and degradation of Myosin with age.This loss of Myosin at the thick filament results in decline in muscle mass and function, also known as sarcopenia. Note that only the myosin head and neck are shown for simplicity of illustration.
