Isoform transition of contractile proteins related to muscle remodeling with an axial gradient during metamorphosis in Xenopus laevis.

For clarification of the mechanism of hormonally and spatially regulated larval-to-adult conversion of skeletal muscle, changes in the expression of muscle contractile proteins were examined during metamorphosis of Xenopus laevis. Analysis by electrophoresis revealed that isoforms of myosin heavy chain (MHC) switched from larval to adult type and adult-specific beta-tropomyosin (TM) appeared during metamorphosis in addition to preexisting alpha-TM. Distinct regional differences in isoform transition were apparent. Isoform changes started at stage 54 in the hindlimb and at stage 57 in the body. However, no change in the tail occurred. Immunohistochemical examination was performed to analyze isoform transition in dorsal body muscle. Before metamorphosis (stage 53), only a small number of muscle fibers at the dorsomedial part of dorsal muscle expressed "adult-type" muscle proteins. During metamorphosis the adult-type area gradually expanded from dorsal to ventral slides with an anteroposterior gradient with increase in adult-type (adult-type MHC and beta-TM-positive) muscle fibers. Thus, there is a gradient in isoform transition. In addition, adult-type fibers showed smaller diameter than larval-type fibers and DNA synthesis occurred in dorsal muscle with an anteroposterior gradient before the isoform transition, suggesting that new myogenesis of adult-type fibers proceeds with a gradient during metamorphosis. Also, degenerating fibers were observed only in the larval-type area. These results suggest that isoform transition is achieved by new proliferation of adult-type myoblasts and death of preexisting larval-type fibers, not by change in gene expression within the same cell.