| The extraocular muscles of mammals represent a functionally heterogeneous group of skeletal muscles that are responsible for generating not only rapid saccadic eye movements, but slow vergence and pursuit movements as well. The functional diversity of this muscle group is reflected in its contractile protein composition. These studies address the expression and regulation of one of these muscle proteins, myosin, in rat extraocular muscle. Mature extraocular muscle exhibits a unique myosin isoform phenotype, including the retention of developmental isoforms, expression of a tissue-specific isoform, and longitudinal variations in the expression of some developmental and fast myosins. Because of this unusual myosin profile, it was of interest to determine how this phenotype is established developmentally. Data from these studies demonstrated that several of the myosin isoform transitions in developing eye muscle occurred concomitantly with key developmental landmarks associated with visual system maturation. When it is considered that the visual system provides a major excitatory drive to the oculomotor system, it is not surprising that the developing visual system may play a role in eye muscle maturation. To test this hypothesis, visual system development was disrupted and consequences for myosin expression in the extraocular muscles were assessed. Two visual system perturbations were exploited in these studies, monocular deprivation and dark rearing, and each visual insult has a distinct and differential effect of eye muscle properties. Monocular deprivation elicited a shift in the expression of fast myosin isoforms, with an increase in the expression of IIA. Monocular deprivation did not affect expression of the tissue-specific myosin. Dark rearing, on the other hand, yielded a dramatic reduction in levels of the extraocular muscle-specific MyHC RNA, suggesting that the expression of this isoform may be tightly linked to activity levels of the oculomotor system. Other myosin isoforms are also affected by dark rearing, with the most dramatic change being a significant decrease in slow myosin. Together, these data confirm the hypothesis that visual input to the oculomotor system during development does play a role in modulating myosin gene expression during eye muscle maturation. |
