Teets, E. M. | Search bioRxiv

bioRxiv2024

Mylpf dosage is proportionate to fast-twitch myofibril size in the zebrafish embryo

Muscle becomes stronger by expanding sarcomeres from the cells edge to its center. Here we investigate how Mylpf (Myosin Light Chain Phosphorylatable Fast) abundance impacts sarcomere growth. The two zebrafish Mylpf genes (mylpfa and mylpfb) are exclusively expressed in fast-twitch muscle, with mylpfa expressed more abundantly than mylpfb. Mutations in both genes cause full mRNA and protein loss, and loss of sarcomere formation. The degree of sarcomere reduction in fast-twitch muscle is predicted by Mylpf protein dosage using a high-slope Hill equation. The sarcomere defect may be driven by thick filament localization, which is impaired in mylpfa-/-and obliterated in mylpfa-/-;mylpfb-/-. Slow-twitch muscle is spared, becoming larger and more active in the mutants. The mylpfa-/- defects can be rescued by Mylpfa-GFP, Mylpfb-GFP, and by human MYLPF-GFP with equivalent efficiency, but not by a MYLPF protein variant that causes Distal Arthrogryposis. These effects indicate that Mylpf dosage strengthens muscle by mediating myofilament localization, and thereby sarcomere and myofibril growth.

developmental biology↗

bioRxiv2020

Mutations in MYLPF cause a novel segmental amyoplasia that manifests as distal arthrogryposis

We identified ten persons in six consanguineous families with Distal Arthrogryposis (DA) who had congenital contractures, scoliosis, and short stature. Exome sequencing revealed that each affected person was homozygous for one of two different rare variants (c.470G>T, p.(Cys157Phe) or c.469T>C, p.(Cys157Arg)) affecting the same residue of myosin light chain, phosphorylatable, fast skeletal muscle (MYLPF). In a seventh family, a c.487G>A, p.(Gly163Ser) variant in MYLPF arose de novo in a father, who transmitted it to his son. In an eighth family comprised of seven individuals with dominantly-inherited DA, a c.98C>T, p.(Ala33Val) variant segregated in all four persons tested. Variants in MYLPF underlie both dominant and recessively inherited DA. Mylpf protein models suggest that the residues associated with dominant DA interact with myosin whereas the residues altered in families with recessive DA only indirectly impair this interaction. Pathological and histological exam of a foot amputated from an affected child revealed complete absence of skeletal muscle (i.e., segmental amyoplasia). To investigate the mechanism for this finding, we generated an animal model for partial MYLPF impairment by knocking out zebrafish mylpfa. The mylpfa mutant had reduced trunk contractile force and complete pectoral fin paralysis, demonstrating that mylpf impairment most severely affects limb movement. mylpfa mutant muscle weakness was most pronounced in an appendicular muscle and was explained by reduced myosin activity and fiber degeneration. Collectively, our findings demonstrate that partial loss of MYLPF function can lead to congenital contractures, likely as a result of degeneration of skeletal muscle in the distal limb.

genetics↗