Altered Cross-Bridge Kinetics and Increased Force Production in Presence of a Myosin Heavy Chain IIb Mutant

Abstract

Hereditary myosinopathies have emerged as a new group of diseases and are characterized by skeletal muscle weakness. These diseases are notably caused by mutations in genes encoding myosin heavy chain (MyHC) isoforms. A new mouse model with a L342Q MyHC IIb mutation has been developed to mimic such myosinopathies. Homozygous mice experience severe skeletal muscle paralysis and, because of ethical reasons, are killed thirteen days after birth. Heterozygous animals have a normal lifespan and no apparent weakness. The underlying mechanisms for such absence of deteriorated phenotype in heterozygous rodents remain unknown. In the current study, we aimed at investigating this particular point by evaluating whether and how the presence of 50% of L342Q MyHC IIb mutants affects the contractile function. We compared isolated membrane-permeabilized muscle fibers from heterozygote (L342Q+/-) and wild-type young adult mice. Results showed, during full activation, at pCa 4.5, an increase in force production in L342Q+/- when compared with wild-type animals (p<0.05) whilst stiffness was unaffected. Additionally, we observed a preserved rate of force redevelopment (ktr) together with an increase in maximum unloaded shortening velocity (V0) (p<0.05). All these findings suggest an enhanced strong myosin binding to actin as well as a faster myosin cross-bridge detachment, in presence of the MyHC IIb mutants. Results also highlighted, during rigor activation, an unchanged force-generating capacity and decreased stiffness in L342Q+/- when compared with wild-type rodents (p<0.05). This indicates that myosin cross-bridges carrying the mutant may not function properly but, at the same time, may perturb the wild-type myosin heads leading to an accelerated cross-bridge detachment and increased force generation.