Drosophila model of Freeman-Sheldon Syndrome points to possible molecular mechanism undlerying effects of the syndrome on muscles

Biophys J. 2021 Jan 29:S0006-3495(21)00075-8. doi: 10.1016/j.bpj.2020.12.033.

Prolonged Myosin Binding Increases Muscle Stiffness in Drosophila Models of Freeman-Sheldon Syndrome.

Bell KM, Huang A, Kronert WA, Bernstein SI, Swank DM

Abstract: Freeman-Sheldon Syndrome (FSS) is characterized by congenital contractures resulting from dominant point mutations in the embryonic isoform of muscle myosin. To investigate its disease mechanism, we used Drosophila models expressing FSS myosin mutations Y583S or T178I in their flight and jump muscles. We isolated these muscles from heterozygous mutant Drosophila and performed skinned fiber mechanics. The most striking mechanical alteration was an increase in active muscle stiffness. Y583S/+ and T178I/+ fibers' elastic moduli increased 70% and 77%, respectively. Increased stiffness contributed to decreased power generation, 49% and 66%, as a result of increased work absorbed during the lengthening portion of the contractile cycle. Slower muscle kinetics also contributed to the mutant phenotype, as shown by 17% and 32% decreases in optimal frequency for power generation, and 27% and 41% slower muscle apparent rate constant 2πb. Combined with previous measurements of slower in vitro actin motility, our results suggest a rate reduction of at least one strongly-bound cross-bridge cycle transition that increases the time myosin spends strongly bound to actin, ton. Increased ton was further supported by decreased ATP affinity and a 16% slowing of jump muscle relaxation rate in T178I heterozygotes. Impaired muscle function caused diminished flight and jump ability of Y583S/+ and T178I/+ Drosophila. Based on our results, assuming that our model system mimics human skeletal muscle, we propose that one mechanism driving FSS is elevated muscle stiffness arising from prolonged ton in developing muscle fibers.

DOI: 10.1016/j.bpj.2020.12.033
PMID: 33524372

Fly model provides "first animal models for myosin-based Freeman-Sheldon syndrome (FSS)"

Rao DS, Kronert WA, Guo Y, Hsu KH, Sarsoza F, Bernstein SI. Reductions in ATPase activity, actin sliding velocity, and myofibril stability yield muscle dysfunction in Drosophila models of myosin-based Freeman-Sheldon syndrome. Mol Biol Cell. 2019 Jan 1;30(1):30-41. PubMed PMID: 30379605; PubMed Central PMCID: PMC6337914.

Abstract: "Using Drosophila melanogaster, we created the first animal models for myosin-based Freeman-Sheldon syndrome (FSS), a dominant form of distal arthrogryposis defined by congenital facial and distal skeletal muscle contractures. Electron microscopy of homozygous mutant indirect flight muscles showed normal (Y583S) or altered (T178I, R672C) myofibril assembly followed by progressive disruption of the myofilament lattice. In contrast, all alleles permitted normal myofibril assembly in the heterozygous state but caused myofibrillar disruption during aging. The severity of myofibril defects in heterozygotes correlated with the level of flight impairment. Thus our Drosophila models mimic the human condition in that FSS mutations are dominant and display varied degrees of phenotypic severity. Molecular modeling indicates that the mutations disrupt communication between the nucleotide-binding site of myosin and its lever arm that drives force production. Each mutant myosin showed reduced in vitro actin sliding velocity, with the two more severe alleles significantly decreasing the catalytic efficiency of actin-activated ATP hydrolysis. The observed reductions in actin motility and catalytic efficiency may serve as the mechanistic basis of the progressive myofibrillar disarray observed in the Drosophila models as well as the prolonged contractile activity responsible for skeletal muscle contractures in FSS patients."

Learn about Freeman-Sheldon syndrome online at the US NIH Genetic and Rare Diseases Information Center (GARD).