Early mitochondrial degeneration in the development of disuse-induced muscle atrophy

Skeletal muscle disuse and subsequent atrophy occurs with multiple pathological stimuli such as bedrest and spaceflight. It is known that during disuse glycolytic muscles appear largely protected while mitochondria‐dense oxidative muscles undergo rampant wasting, suggesting a possible mitochondrial component to this wasting. However, the role of mitochondrial degeneration in the development of disuse‐induced muscle atrophy is not well understood. Purpose To examine the development of mitochondrial network degeneration using the fluorescent reporter pMitoTimer during the development of disuse‐induced muscle atrophy. Methods At 10 weeks of age, C57BL6/J mice underwent hindlimb suspension for 24, 48, 72, or 168 hours. Control mice were singly housed under normal cage conditions. To examine mitochondrial network quality the fluorescent reporter pMitoTimer was electroporated into the Flexor Digitorum Brevis (FDB) muscle two weeks prior to hindlimb suspension. pMitoTimer has been previously validated to assess mitochondrial quality using ratio of red:green fluorescence intensity with red fluorescence indicating degeneration. Additionally, detection of pure red puncta suggests completely degenerated mitochondria tagged for mitophagic degradation. At the appropriate timepoint, each animal was humanely euthanized and hindlimb muscles collected. The FDB was immediately fixed in 4% paraformaldehyde and imaged on Nikon TiS epifluorescent microscope with LED based light source and standardized image acquisition parameters. pMitoTimer fluorescence was quantified for ratio of red:green fluorescence intensity and pure red puncta using a previously developed Matlab program. Data were statistically analyzed by one‐way ANOVA followed by Tukey's posthoc test when significant effects were found and α was set at 0.05. Results At 24 hours HS weights of soleus, plantaris and gastrocnemius muscles were not different from control. At 48 hours, however, weights of soleus, plantaris and gastrocnemius muscles were 11–14% lower than load‐bearing controls and at 168 hours weights of these muscles were ~16–39% lower than control with the greatest differences seen in the soleus. At 24 hours, pMitoTimer red:green ratio was ~37% greater compared to control and at 48 hours ~55% greater compared to control with no other statistical differences observed. Number of pMitoTimer pure red puncta was 5X greater in 48 hour HS mice compared to control and 24 hour with no other differences observed. Conclusions Degeneration of the mitochondrial network measured by the fluorescent reporter pMitoTimer occurs prior to the onset of muscle atrophy following hindlimb suspension‐induced muscle disuse. These data may provide evidence for a novel mechanism for the instigation of muscle atrophy with disuse. Support or Funding Information This study was supported by National Institutes of Health R15AR069913. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .