Development transitions of thin filament proteins in rat extraocular muscles

Extraocular muscles are a unique subset of striated muscles. During postnatal development, the extraocular muscles undergo a number of myosin isoform transitions that occur between postnatal day P10 (P10) and P15. These include: (1) loss of embryonic myosin from the global layer resulting in the expression restricted to the orbital layer; (2) the onset of expression of extraocular myosin and the putative tonic myosin (myh 7b/14); and (3) the redistribution of nonmuscle myosin IIB from a subsarcolemmal position to a sarcomeric distribution in the slow fibers of the global layer. For this study, we examined the postnatal appearance and distribution of α-actinin, tropomyosin, and nebulin isoforms during postnatal development of the rat extraocular muscles. Although sarcomeric α-actinin is detectable from birth, α-actinin 3 appears around P15. Both tropomyosin-1 and -2 are present from birth in the same distribution as in the adult animal. The expression of nebulin was monitored by gel electrophoresis and western blots. At P5-10, nebulin exhibits a lower molecular mass than observed P15 and later during postnatal development. The changes in α-actinin 3 and nebulin expression between P10 and P15 coincide with transitions in myosin isoforms as detailed above. These data point to P10–P15 as the critical period for the maturation of the extraocular muscles, coinciding with eyelid opening.