Mechanistic Insights into the Role of TRIM28 in the Regulation of Skeletal Muscle Size

Abstract

Protein homeostasis plays a critical role in the regulation of skeletal muscle size, and the maintenance of skeletal muscle size contributes significantly to disease prevention and quality of life. Over the past few decades, it has become widely accepted that skeletal muscle size is controlled by the net balance between the rates of protein synthesis and protein degradation. Despite this assertion, the mechanisms that modulate this balance and lead to changes in muscle size remain incompletely defined. Nevertheless, advancements have been made. For instance, a recent study from our lab revealed that the myofiber‐specific loss of a transcriptional intermediary factor named TRIM28 led to a significant reduction in basal myofiber size and attenuated the increase in myofiber size that occurs in response to mechanical overload. Moreover, rigorous follow‐up studies indicate that the TRIM28 knockout‐induced deficits in myofiber size are not driven by a decrease in the rate of protein synthesis, but that this phenotype is instead associated with elevated levels of canonical markers of protein degradation. Together, these observations led to our central hypothesis that TRIM28 confers its effects on myofiber size via the regulation of protein degradation. Thus, in an effort to gain further insight into this possibility, we performed a deep RNA‐sequencing analysis, the results of which led to the identification of Mettl21c and Mettl21e as two genes whose expression were significantly downregulated by the loss of TRIM28. We were intrigued by this discovery because recent reports have implicated Mettl21c and Mettl21e in the regulation of protein degradation, and it has been shown that the loss of these proteins in muscle promotes aberrant protein degradation and reduces muscle size. Combined with our initial observations, it became apparent that Mettl21c and Mettl21e might be important parts of the pathway via which TRIM28 regulates protein degradation to control myofiber size. To test this theory, we generated expression plasmids encoding HA‐Mettl21c and HA‐Mettl21e, and then used electroporation to demonstrate that the expression of Mettl21c and Mettl21e in myofibers is sufficient to induce a robust hypertrophic response in control animals, and that the hypertrophic effect of Mettl21c, but not Mettl21e, is conserved in muscles lacking TRIM28. Collectively, these findings provide evidence that TRIM28 regulates myofiber size via the regulation of protein degradation, and that Mettl21c, but not Mettl21e, might play an important role in the pathway via which TRIM28 confers this effect.