An Evolutionarily Conserved Regulatory Pathway of Muscle Mitochondrial Network Organization

Abstract

Mitochondrial networks provide coordinated energy distribution throughout muscle cells. However, pathways specifying mitochondrial network-type separately from contractile fiber-type remain unclear. Here, we show that natural energetic demands placed on Drosophila melanogaster muscles yield native cell-types among which contractile and mitochondrial network-types are regulated independently. Proteomic analyses of indirect flight, jump, and leg muscles together with muscles misexpressing known fiber-type specification factor salm identified transcription factors H15 and cut as potential mitochondrial network regulators. We demonstrate H15 operates downstream of salm regulating flight muscle contractile and mitochondrial network-type. Conversely, H15 regulates mitochondrial network configuration but not contractile type in jump and leg muscles. Further, we find that cut regulates salm expression in flight muscles and mitochondrial network configuration in leg muscles. These data indicate cell type-specific regulation of muscle mitochondrial network organization separately from contractile type, mitochondrial content, and mitochondrial size through an evolutionarily conserved pathway involving cut, salm, and H15.

Mitochondrial network configuration influences sarcomere and myosin filament structure in striated muscles

“…Moreover, intrasarcomere heterogeneity in myosin shape results in variable filamentto-filament lattice spacing along the length of the sarcomere where myosin filaments are closer together near the filament ends than in the middle in mice. Additionally, we demonstrate that both myosin filament linearity and intrasarcomere lattice spacing are regulated in Drosophila by muscle and mitochondrial network specification factors salm 29,36,39 and H15 4,36 . Finally, we show that acute swelling of mitochondria in mouse skeletal muscle results in increased curvature of adjacent myosin filaments.…”

“…The six muscle types with greater than 20% intrasarcomere CSA heterogeneity in Fig. 1 (mouse late postnatal and all three mature muscles, Drosophila leg, and human vastus lateralis) are all known to have large proportions of their mitochondrial networks which wrap perpendicularly around the sarcomere adjacent to the Z-disks, whereas the remaining five muscle types with less than 20% heterogeneity all have mitochondrial networks primarily oriented in parallel to the contractile apparatus 11,18,19,36,39,48,49 . Thus, we hypothesized that sarcomere structure is linked to mitochondrial network configuration.…”

“…Conversely, both the jump and flight muscles have parallel mitochondrial networks (Fig. 2g, h) whereas the leg has gridlike mitochondrial networks 36,39 (Fig. 2j).…”

“…Though there is relatively little information available regarding the variability in metabolism across tubular muscles 35 , recent work has shown Drosophila tubular muscles can vary in mitochondrial content from ~2-3% to greater than 20% of cell volume 4 with mitochondrial networks arranged in either parallel or grid-like configurations 36 . Additionally, transcription factors involved in the determination of fly muscle cell fate such as salm 29 , cut 37,38 , and H15 36 , are now also known to regulate mitochondrial network structure in both tubular and fibrillar muscles 36,39 . However, it remains unclear in both mammalian and insect muscles how altering mitochondrial network configuration may impact the structure/function relationships of adjacent subcellular structures.…”

“…Increasing mitochondrial volume in striated muscle largely comes at the expense of the contractile apparatus 40 . Thus, we aimed to determine how displacement of contractile volume is coordinated with the varying mitochondrial network configurations that occur across different muscle cell types 11,[17][18][19][20]36,39 . We hypothesized that mitochondria arranged in long, parallel rows between the sarcomeres effectively displace contractile volume uniformly along the length of the sarcomere, whereas mitochondria wrapped perpendicularly near the sarcomere ends only require local displacement of sarcomere structure in order to maximize efficient utilization of limited cellular volume.…”