Contributions of Vimentin to Skeletal Muscle Hypertrophy

Abstract

Our laboratory has performed various experiments examining the proteomic alterations that coincide with mechanical overload (MOV)-induced skeletal muscle hypertrophy. With this same intent we first sought to determine how 10 weeks of resistance training in 15 college-aged females affected protein concentrations in different tissue fractions. Training, which promoted significant lower body muscle- and fiber-level hypertrophy, notably increased sarcolemmal/membrane protein content (+10.1%, p<0.05). Sarcolemmal/membrane protein isolates were queried using mass-spectrometry-based proteomics, ~10% (38/387) of proteins were up-regulated (>1.5-fold, p<0.05), and one of these targets (the intermediate filament vimentin, VIM) warranted further investigation. VIM expression was first examined in the plantaris muscles of 4-month-old C57BL/6 mice following 10- and 20-days of MOV via synergist ablation. Relative to Sham (control) mice, VIM mRNA and protein content significantly increased in MOV mice. Immunohistochemistry corroborated these findings and showed that VIM was localized to the extracellular matrix (ECM). The 10- and 20-day MOV experiment was replicated in Pax7-DTA mice, and results indicated satellite cell depletion significantly blunted the presence of VIM in the ECM. A series of follow-up cell culture experiments supported that myoblasts, rather than myofibers, likely produce VIM in response to anabolic stimuli. Finally, a third 10- and 20-day MOV experiment was performed in C57BL/6 mice intramuscularly injected with either AAV9-scrambled (control) or AAV9-VIM shRNA. While VIM shRNA injections significantly blunted the presence of VIM (~50%), plantaris masses were similar between injection groups in response to MOV. However, a leftward (smaller) myofiber size shift in response to MOV was observed in VIM shRNA mice, and this coincided with appreciably more myofibers presenting a regeneration phenotype (MyHCemb-positive fibers with centrally located nuclei). Using a highly integrative approach, we propose that skeletal muscle VIM is a mechanosensitive target predominantly localized to the ECM and produced by satellite cells. Moreover, a disruption in VIM expression during MOV leads to dysfunctional skeletal muscle hypertrophy.