Role of Septin7 in mitochondrial dynamics and oxidative metabolism in C2C12 skeletal muscle cells

Mitochondria are dynamic organelles that undergo fusion and fission. Key proteins are needed to create mitochondrial networks, as well as facilitate biogenesis, fragmentation or movement within the cell. Septins are considered as the fourth component of the cytoskeleton, providing attachment sites for proteins. Besides that, they have important roles in different cellular processes, including mitochondrial fission and fusion (remodelling). Septins form oligomeric complexes comprising various septin subgroups, which can create higher-order structures. Septin7 is the sole member of its subgroup. We aimed to examine how mitochondrial dynamics and oxidative phosphorylation (OXPHOS) are affected in Septin7 downregulated C2C12 (S7-KD) myoblasts and terminally differentiated myotubes compared to scrambled short hairpin RNA-transfected control cells. We detected altered expression of genes related to mitochondrial biogenesis (PGC1α), dynamics (DRP1, OPA1 and MFN2) and autophagy (PINK1 and BNIP3); furthermore, a significant decrease in differentiation-dependent mRNA expression of OXPHOS markers (ATP synthase, COX1 and SDH). Septin7 downregulation also affected the expression of post-translational modifications of MFN2 and DRP1. Functional measurements of OXPHOS revealed decreased O₂ consumption (flux) and higher O₂ concentration in Septin7 KD cultures following selective inhibition of electron transport complexes. We observed significant alterations in basal respiration and OXPHOS pathways in Septin7 KD cultures. Our results suggest that Septin7, as a cytoskeletal protein, could be a significant regulator of mitochondrial dynamics and oxidative metabolism. Therefore, these molecules, as mitochondrial dynamics modulators, can serve as potential therapeutic targets in diseases related to changes in mitochondrial function. KEY POINTS: Knockdown of Septin7 results in altered gene and protein expression of markers controlling mitochondrial dynamics. Diminished level of Septin7 causes decreased gene expression of members of oxidative phosphorylation. Knockdown of Septin7 has an impact on microRNAs involved in the regulation of mitochondrial markers. Septin7 has an impact on mitochondrial respiration.