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Mitochondrial remodeling through fusion and fission is crucial for progenitor cell differentiation but its role in myogenesis is poorly understood. Here, we characterized the function of mitofusin 2 (Mfn2), a mitochondrial outer membrane protein critical for mitochondrial fusion, in muscle progenitor cells (myoblasts). Mfn2 expression is upregulated during myoblast differentiation in vitro and muscle regeneration in vivo. Targeted deletion of Mfn2 gene in myoblasts (Mfn2MKO) increases oxygen‐consumption rates (OCR) associated with the maximal respiration and spare respiratory capacity, and increased levels of reactive oxygen species (ROS). Skeletal muscles of Mfn2MKO mice exhibit robust mitochondrial swelling with normal mitochondrial DNA content. Additionally, mitochondria isolated from Mfn2MKO muscles have reduced OCR at basal state and for complex I respiration, associated with decreased levels of complex I proteins NDUFB8 (NADH ubiquinone oxidoreductase subunit B8) and NDUFS3 (NADH ubiquinone oxidoreductase subunit S3). However, Mfn2MKO has no obvious effects on myoblast differentiation, muscle development and function, and muscle regeneration. These results demonstrate a novel role of Mfn2 in regulating mitochondrial complex I protein abundance and respiratory functions in myogenic progenitors and myofibers. [ABSTRACT FROM AUTHOR]

Blunted muscle hypertrophy and impaired regeneration with aging have been partly attributed to satellite cell (SC) dysfunction. However, true muscle regeneration has not yet been studied in elderly individuals. To investigate this, muscle injury was induced by 200 electrically stimulated (ES) eccentric contractions of the vastus lateralis (VL) of one leg in seven young (20‐31 years) and 19 elderly men (60‐73 years). This was followed by 13 weeks of resistance training (RT) for both legs to investigate the capacity for hypertrophy. Muscle biopsies were collected Pre‐ and Post‐RT, and 9 days after ES, for immunohistochemistry and RT‐PCR. Hypertrophy was assessed by MRI, DEXA, and immunohistochemistry. Overall, surprisingly comparable responses were observed between the young and elderly. Nine days after ES, Pax7+ SC number had doubled (P <.05), alongside necrosis and substantial changes in expression of genes related to matrix, myogenesis, and innervation (P <.05). Post‐RT, VL cross‐sectional area had increased in both legs (~15%, P <.05) and SCs/type II fiber had increased ~2‐4 times more with ES+RT vs RT alone (P <.001). Together these novel findings demonstrate "youthful" regeneration and hypertrophy responses in human elderly muscle. Furthermore, boosting SC availability in healthy elderly men does not enhance the subsequent muscle hypertrophy response to RT. [ABSTRACT FROM AUTHOR]

Our aim in the current study was to determine the necessity of satellite cells for long-term muscle growth and maintenance. We utilized a transgenic Pax7-DTA mouse model, allowing for the conditional depletion of > 90% of satellite cells with tamoxifen treatment. Synergist ablation surgery, where removal of synergist muscles places functional overload on the plantaris, was used to stimulate robust hypertrophy. Following 8 wk of overload, satellite celldepleted muscle demonstrated an accumulation of extracellular matrix (ECM) and fibroblast expansion that resulted in reduced specific force of the plantaris. Although the early growth response was normal, an attenuation of hypertrophy measured by both muscle wet weight and fiber cross-sectional area occurred in satellite cell-depleted muscle. Isolated primary myogenic progenitor cells (MPCs) negatively regulated fibroblast ECM mRNA expression in vitro, suggesting a novel role for activated satellite cells/MPCs in muscle adaptation. These results provide evidence that satellite cells regulate the muscle environment during growth.--Fry, C. S., Lee, J. D., Jackson, J. R., Kirby, T. J., Stasko, S. A., Liu, H., Dupont-Versteegden, E. E., McCarthy, J. J., Peterson, C. A. Regulation of the muscle fiber microenvironment by activated satellite cells during hypertrophy. [ABSTRACT FROM AUTHOR]