Research into the physiology of myosins - a personal odyssey.

Abstract

During my PhD, I worked on the neural regulation of mechanical properties fast and slow muscles. This led me to believe that myosins in fast and slow muscles are structurally distinct and that motor nerves regulate the expression of myosin genes. I devised a method for separating intact fast and slow myosins by gel electrophoresis and confirmed their neural regulation. The electrophoresis method was subsequently improved and used to analyse skeletal and cardiac myosin isoforms in various vertebrate species, including marsupials. This led to the discovery of neonatal myosin heavy chain (MyHC), α and β cardiac MyHCs and of the regulation of cardiac MyHCs by thyroid hormone. Antibodies were raised against 2A, 2X, 2B, masticatory and extraocular MyHCs and used to study the expression and regulation of MyHCs in jaw, laryngeal and Extraocular Muscle (EOM) fibres. Antibodies against masticatory myosin enabled the sequencing of masticatory MyHC and masticatory light chain 2 genes. Cross-bridge kinetics of fibres with different myosin isoforms were analysed. Different MyHC isoforms found in jaw-closing muscles across various species reflected evolutionary adaptations to diverse dietary intake, while MyHC expression changes in cardiac and laryngeal muscles with body mass reflected adaptations to changes in their specific metabolic rate. Transplantation experiments on masticatory and EOMs and cross-innervation experiments between laryngeal and somitic muscles revealed that their capacity to express masticatory or extraocular MyHC were myogenically determined but neural impulse patterns also influence MyHC expression. EOMs are the most complex, expressing 11 MyHC isoforms. Some EOM fibres express faster MyHCs in the endplate zone but slower MyHCs at the end segments, an arrangement helping to linearize the saccade. I suggested that during development, primary and secondary extraocular myotubes specify the synaptic inputs of the innervating neurons to generate impulse patterns which regulate the expression of their MyHCs.