Force-Generating Capacity of Human Myosin Isoforms Extracted from Single Muscle Fiber Segments

Abstract

Muscle, motor unit and muscle fibre type-specific differences in force-generating capacity have been investigated for many years, but there is still no consensus regarding specific differences between slow- and fast- twitch muscles, which may relate to a number of different confounding factors disguising the myosin function, i.e. the molecular motor protein. The aim of this study is to evaluate the force-generating capacity of specific myosin isoforms extracted from human muscle fiber segments in a modified single fiber in vitro motility assay, in which an internal load (α-actinin, actin-binding protein) was added in different concentrations to inhibit force generation of the myosin. After the negative linear relationship between the fraction of moving filaments and the α-actinin concentration was plotted, both the slope and x-axis intercept were used as force index to evaluate force production. The force-generating capacity of the β/slow myosin isoform (type I) was weaker (p<0.05) than the fast myosin isoform (type II), but the force-generating capacity of the different fast myosin isoforms types IIa, IIx or IIax were inseparable. In conclusion, (i) a significant difference in force-generating capacity was observed between human slow and fast myosin isoforms; (ii) the modified single fiber in vitro motility assay presents a unique possibility to measure the force-generating capacity of specific myosin isoforms, and also provides a platform for studies on myosin function in the growing disease entity called “myosinopathies” as well as in the different post-translational modifications.