Regulation of the evolutionarily conserved muscle myofibrillar matrix by cell type dependent and independent mechanisms

Abstract

Skeletal muscles play a central role in human movement through forces transmitted by contraction of the sarcomere. We recently showed that mammalian sarcomeres are connected through frequent branches forming a singular, mesh-like myofibrillar matrix. However, the extent to which myofibrillar connectivity is evolutionarily conserved as well as mechanisms which regulate the specific architecture of sarcomere branching remain unclear. Here, we demonstrate the presence of a myofibrillar matrix in the tubular, but not indirect flight (IF) muscles within Drosophila melanogaster. Moreover, we find that loss of transcription factor H15 increases sarcomere branching frequency in the tubular jump muscles, and we show that sarcomere branching can be turned on in IF muscles by salm-mediated conversion to tubular muscles. Finally, we demonstrate that neurochondrin misexpression results in myofibrillar connectivity in IF muscles without conversion to tubular muscles. These data indicate an evolutionarily conserved myofibrillar matrix regulated by both cell-type dependent and independent mechanisms.

Identification of evolutionarily conserved regulators of muscle mitochondrial network organization

“…These results suggest that H15 regulates mitochondrial network configuration but not contractile type in the jump muscles. However, it should be noted that while H15 KD does not alter the tubular contractile type of the jump muscles, muscle-specific loss of H15 has recently been shown to increase sarcomere branching frequency 32 and myosin filament curvature 71 above the level of the wild-type jump muscles and closer to the levels of the wild type leg muscles. Those results are consistent with the interpretation here that H15 KD in the jump muscle results in a leg muscle-like phenotype based on the tubular contractile type and grid-like mitochondrial networks.…”

“…Mature striated muscles form relatively stable mitochondrial networks 27 comprised of many physically and electrically connected mitochondria 5 , 28 , 29 , and muscle mitochondrial networks display differences in mitochondrial content, size, and configuration depending on the energetic and contractile force requirements of a given muscle cell type 5 , 28 , 30 – 32 . In mammalian systems, muscle type is commonly classified by both contractile (i.e., fast- or slow-twitch) and metabolic (i.e., glycolytic or oxidative) types 33 , 34 with contractile type generally defined by myosin isoform composition or myofibrillar ATPase activity 35 – 37 and metabolic type often inferred based on mitochondrial content or enzyme activity 35 , 38 – 40 .…”

“…Though the indirect flight muscles are the largest and most well-studied muscles in Drosophila , the fibrillar (i.e., comprised of individual myofibrils) and asynchronous (i.e., one calcium cycle results in tens of muscle contractions) nature of these muscles are unlike any known mammalian muscle. Conversely, the majority of Drosophila muscles are tubular 70 (i.e., form cross-striated myofibrillar networks 32 ) and synchronous (i.e., one calcium cycle per contraction) more similar to mammalian skeletal muscles. Despite being classified as the same contractile type, tubular muscles can have different contractile characteristics (e.g., mechanical force/power 61 – 64 and sarcomere 71 or myofibrillar network 32 structure) consistent with the variable functions of each specific Drosophila muscle.…”

“…Conversely, the majority of Drosophila muscles are tubular 70 (i.e., form cross-striated myofibrillar networks 32 ) and synchronous (i.e., one calcium cycle per contraction) more similar to mammalian skeletal muscles. Despite being classified as the same contractile type, tubular muscles can have different contractile characteristics (e.g., mechanical force/power 61 – 64 and sarcomere 71 or myofibrillar network 32 structure) consistent with the variable functions of each specific Drosophila muscle. We hypothesized that, like mammals, the wide variety of contractile demands faced across Drosophila tubular muscles would also necessitate cells of varying oxidative or glycolytic natures, and in turn, muscles with different mitochondrial network configurations.…”

Muscle-fiber specific genetic manipulation of Drosophila sallimus severely impacts neuromuscular development, morphology, and physiology

“…From insects to humans the structure and function of the sarcomere is well conserved [18]. Given that most components of the sarcomere, including titin, are highly conserved among animals, insect models have flourished as an essential aspect of muscle research [19].…”