Skeletal muscle stem cell mitochondria are transferred to muscle fibers in response to a hypertrophic stimulus.

The fusion of skeletal muscle stem cell (MuSC) to myofibers during hypertrophy has exclusively focused on the transfer of the MuSC nucleus, leaving the fate of other MuSC organelles, such as mitochondria, largely unexplored. The objective of this study was to determine if MuSCs transfer their mitochondria upon myofiber fusion in response to a hypertrophic stimulus. To achieve this goal, we specifically labeled MuSC mitochondria with Dendra2 fluorescence by crossing the MuSC-specific CreER (Pax7CreER/CreER) mouse with the Rosa26-Dendra2 mouse to generate the Pax7-Dendra2 mouse. To induce the fusion of MuSC to myofibers, Pax7-Dendra2 mice underwent synergist ablation surgery to induce mechanical overload (MOV) of plantaris muscle for 3, 7 and 14 days. To track MuSC proliferation, a mini-osmotic pump was implanted at the time of MOV to continuously deliver EdU. At the designated time, plantaris muscles were excised and processed for immunohistochemistry to quantify Dendra2 + myofibers. There was a progressive increase in Dendra2-positive fibers across the MOV time course. Three distinct patterns or domains of Dendra2 fluorescence within myofibers were identified and designated as newly fused (NF), crescent (CS) or diffuse (DF). From these Dendra2 + domain types, we inferred MuSC fusion dynamics which indicated MuSC fusion occurred prior to mechanical overload day 3 (MOV-3) and preferentially with Type 2A fibers. Quantification of EdU + myonuclei found the majority of early (MOV < 3 days) MuSC fusion was division-independent, while proliferating MuSCs contributed primarily to later fusion events. The results of this study provide the first evidence that MuSC mitochondria are transferred to myofibers upon fusion during hypertrophy while, unexpectedly, revealing a greater complexity in MuSC fusion than previously recognized.