Craniofacial Muscle Fiber Specializations: A Comparative Approach

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

Craniofacial muscles collectively serve an extremely diverse set of functions. This set of muscles includes extraocular muscles which are responsible for highly precise eye movements during visual tracking of a moving object, laryngeal muscles that serve vocalizations and airway protection, and mandibular muscles that drive mastication. These muscles are highly complex with respect to structure, biochemical properties and physiological properties and, as such, are markedly different from limb muscles that serve locomotion. The overall goal of this project is to acquire a more comprehensive understanding of the molecular and cellular properties of craniofacial muscles that allow these muscles to serve such a vast array of functions and to do so with an extraordinary level of precision. Myosin is a large protein, being composed of two heavy chain and four light chain subunits, and is a key regulator of contractile properties of muscles. Whole craniofacial muscles which express "superfast" and "extraocular/laryngeal" myosin isoforms are well recognized as having distinct contractile properties. However, the specific functional significance of superfast myosin and of extraocular/laryngeal myosin remains unknown, in contrast to a large body of literature describing the roles of limb muscle myosins in the regulation of physiologic properties. The project aims are designed to test whether mammalian skeletal muscle fibers that are identified as expressing either "superfast" myosin, found in carnivore jaw-closing muscles, or "extraocular/laryngeal" myosin, expressed in extraocular and intrinsic laryngeal muscles, have distinct contractile properties. The contractile properties to be measured in single muscle fibers include the maximal velocity of shortening, the rate of tension redevelopment following a rapid slackening and re-stretch, tension-generating ability, stiffness and myofibrillar ATPase activity. The myosin heavy chain and light chain isoform composition of each fiber will subsequently be determined using an ultrasensitive gel electrophoresis protocol. The results will be utilized to calculate the rate constants for crossbridge attachment and detachment in fibers that are identified on the basis of their myosin isoform composition. The primary objective is to test whether crossbridge kinetics differ significantly between superfast, extraocular/laryngeal and limb muscle myosins. The results are expected to provide a mechanistic understanding of differences in crossbridge properties in fibers that express different myosin isoforms through the determination of fundamental crossbridge rate constants. This project will be based, in part, on a comparative approach. Single muscle fibers from jaw-closing muscles of carninvores (dogs, cats) will be compared to those from an omnivore (pig) that is known to not express superfast myosin. Differences in contractile properties between these two sources of muscle fibers will illustrate the unique properties imparted by superfast myosin to accommodate the feeding behavior of carnivores. This project will result in a more comprehensive understanding of a highly specialized group of muscles that has thus far received relatively little attention. In particular, the results are expected to provide valuable cellular and molecular insights into the function of muscles that serve the aggressive feeding style characteristic of carnivores and underlie the extremely precise motor functions associated with eye movements, vocalization and airway protection. Understanding the cellular and molecular bases of contraction in these highly specialized muscles will greatly expand our understanding of the complexity of muscle fibers among vertebrates and broaden the known repertoire of their contractile properties.