Regulation of myosin expression during myotome formation

Abstract

The first skeletal muscle fibers to form in vertebrate embryos appear in the somitic myotome. PCR analysis and in situ hybridization with isoform-specific probes reveal differences in the temporal appearance and spatial distribution of fast and slow myosin heavy chainmRNA transcripts within myotomal fibers. Embryonic fast myosin heavy chain was the first isoform expressed, followed rapidly by slow myosin heavy chains 1 and 3, with slow myosin heavy chain 2 appearing several hours later. Neonatal fast myosin heavy chain is not expressed in myotomal fibers. Although transcripts of embryonic fast myosin heavy chain were always distributed throughout the length of myotomal fibers, the mRNA for each slow myosin heavy chain isoform was initially restricted to the centrally located myotomal fiber nuclei. As development proceeded, slow myosin heavy chain transcripts spread throughout the length of myotomal fibers in order of their appearance. Explants of segments from embryos containing neural tube, notochord and somites 7-10, when incubated overnight, become innervated by motor neurons from the neural tube and express all four myosin heavy chain genes. Removal of the neural tube and/or notochord from explants prior to incubation or addition of d-tubocurare to intact explants prevented expression of slow myosin chain 2 but expression of genes encoding the other myosin heavy chain isoforms was unaffected. Thus, expression of slow myosin heavy chain 2 is dependent on functional innervation, whereas expression of embryonic fast and slow myosin heavy chain 1 and 3are innervation independent. Implantation of sonic-hedgehog-soaked beads in vivo increased the accumulation of both fast and slow myosin heavy chain transcripts, as well as overall myotome size and individual fiber size, but had no effect on myotomal fiber phenotype. Transcripts encoding embryonic fast myosin heavy chain first appear ventrolaterally in the myotome, whereas slow myosin heavy chain transcripts first appear in fibers positioned midway between the ventrolateral and dorsomedial lips of the myotome. Therefore, models of epaxial myotome formation must account for the positioning of the oldest fibers in the more ventral-lateral region of the myotome and the youngest fibers in the dorsomedial region.

Calcineurin signaling in avian cardiovascular development

“…4C). The differential localization of VHMC1 expression in the somites at this stage is consistent with diversified muscle fiber types in the newly differentiated somitic muscle (Sacks et al, 2003). Therefore, the shift in VMHC1 localization from the somite core, where slow fibers predominate, to the rostral-caudal somite edges in CsA-treated embryos likely reflects altered skeletal muscle fiber-type distribution as a result of calcineurin inhibition in vivo.…”

“…However, differential localization of myosin heavy chain isoform gene expression within the somites was affected with CsA treatment. In untreated embryos, VHMC1 expression is predominant in the somitic core region enriched for slow muscle fibers (Sacks et al, 2003) and lower expression is evident at the rostral and caudal edges of the somites (Fig. 4F).…”

The pattern of MyoD and contractile protein localization in primary epaxial myotome reflects the dynamic progression of nascent myoblast differentiation

“…Arrow marked "v" shows the vector of growth of the myotome and dermomyotome through asymmetric cell divisions in the DML. ventro-lateral position in the primary epaxial myotome (Sacks et al, 2003). That finding is fully consistent with those of the present study and it identifies a fourth zone, M4, containing more mature myotomal fibers (nucleus e) that may undergo further growth due to myocyte hypertrophy.…”

“…The developmental timing of the appearance of desmin in wing level somites have been reported (Kaehn et al, 1988;Borman and Yorde, 1994) as has the later appearance of myosin relative to desmin (Furst et al, 1989;Shimada et al, 1996). Recently, the localization of slow and fast myosin isoforms in the myotome was determined indicating the spatial relationship of older myotome fibers (Sacks et al, 2003). The localized transcription of the muscle determination factor genes in the somite has been studied extensively (Sassoon et al, 1989;de la Brousse and Emerson, 1990;Lyons et al, 1990;Ott et al, 1991;Pownall and Emerson, 1992;Kiefer and Hauschka, 2001;Pownall et al, 2002;Summerbell et al, 2002) and the localization of their cognate proteins has been documented by immunolocalization (Cusella-De Angelis et al, 1992;Smith et al, 1994;Venters et al, 1999).…”

Novel murine clonal cell lines either express slow or mixed (fast and slow) muscle markers following differentiation in vitro

“…However, Dusterhoft and Pette (1993) used satellite cells isolated from the slow Soleus muscle that are more likely to express slow MHC isoforms (MHC1 and MHC2a) in culture, as we also showed here for clones isolated from Soleus muscle. The expression of the MHC1 isoform observed in all our cultured cell lines might reflect a natural process occurring in vivo during myogenesis: in chick (Sacks et al, 2003) as in mouse (Lyons et al, 1990), the slow MHC isoform is expressed together with developmental MHC isoforms in the embryonic muscles. The separation between fast and slow myofibers appears to occur later, in secondary fibers (Lyons et al, 1990).…”