Histochemistry and isomyosins of tail musculature in Xenopus

Xenopus muscle development: from primary to secondary myogenesis

Xenopus myogenesis is characterized by specific features, different from those of mammalian and avian systems both at the cellular level and in gene expression patterns. During early embryogenesis, after the initial molecular signals inducing mesoderm, the myogenic determination factors XMyoD and XMyf-5 are activated in presomitic mesoderm in response to mesoderm-inducing factors. After these first inductions of the myogenic program, forming muscles in Xenopus can have different destinies, some of these resulting in cell death before adulthood. In particular, it is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary "adult" multinucleated myofibers. This feature offers the unique opportunity to totally separate the molecular analysis of these two distinct types of myogenesis. The aim of this review is to summarize our knowledge on the cellular and molecular events as well as the epigenetic regulations involved in the construction of Xenopus muscles during development.

Xenopus bagpipe-related gene, koza, may play a role in regulation of cell proliferation

The homeobox gene koza is a new member of the vertebrate bagpipe-related gene family. Embryonic expression of koza is observed at highest levels in the muscle layer of the somites and, during later development, is restricted to the lateral somitic cells, which correspond to slow twitch muscle tissue. Expression of koza is also observed in the myocardial layer of the heart and in the cement gland. In each of these tissues, koza transcription commences only after the expression of terminal differentiation markers. By injection of synthetic mRNA, we show that overexpression of koza leads to an apparent decrease in the number of cells in the somites. No reduction in cell number is observed when koza is present in neural tissues, suggesting that koza exhibits some tissue specificity in regulation of cell proliferation. Embryonic manipulations show that restriction of koza expression to the slow twitch muscle layer is independent of axial structures but is, at least partly, regulated by signals arising in ectodermal tissue. Finally, in Drosophila, bagpipe expression is regulated by the hedgehog signaling pathway. By using ectopic expression, we show that koza transcription is positively regulated by banded hedgehog. This result indicates that regulation of bagpipe expression by hedgehog signaling is evolutionarily conserved.

Novel structural elements identified during tail resorption in Xenopus laevis metamorphosis: lessons from tailed frogs

When the tail of the Xenopus laevis tadpole resorbs at the end of metamorphosis, various cell types, including muscle, fibroblasts, skin, and spinal cord, are lost at about the same time. However, feeding frogs with tails can be produced by inhibiting thyroid hormone production at the climax of metamorphosis with the goitrogen methimazole. These tails lose their fast muscle preferentially, showing that the different cell types of the tail have different fates and confirming that more than one cell death program is involved in tail resorption. Both normal and methimazole tails contain "cords," novel structures that consist of two dorsal and two ventral parallel rows of slow muscle bundles joined by collagen fibers that run the length of the tail. The cords persist until the very end of tail resorption, being the last structure to dissolve. When thyroid hormone induces expression of proteolytic enzymes in the notochord sheath, the notochord, a structural rod that runs the length of the tail, begins to buckle, demonstrating that the tail is under tension. When sections of the tail that contain cords are surgically separated from the notochord, they contract in vitro, suggesting that the cords contribute to the tension that augments tail resorption.

Expression of myogenic regulatory factors during muscle development of Xenopus: myogenin mRNA accumulation is limited strictly to secondary myogenesis

To clarify the acquisition of the adult muscle pattern in Xenopus laevis, in situ hybridization and reverse transcriptase-polymerase chain reaction were used to correlate the time course of gene expression for myogenic regulatory factors (Myf-5, MyoD, and myogenin) with the expression of contractile protein (myosin heavy chain; MHC) genes during hindlimb formation compared with their expression in dorsal body muscles. After the precocious expression of Myf-5 and MyoD mRNA in limb bud (stage 50), myogenin mRNA strongly accumulated later at paddle stages (stages 52/53) concomitantly with the accumulation of both the larval and the adult MHC mRNAs. In dorsal body muscles, as early as stage 52, myogenin transcripts accumulated in a few small, secondary myofibers expressing the adult MHC mRNA that were located along the dorsomedial edge, but they were never detected in the large, primary myofibers of the body expressing the larval MHC mRNA. During metamorphosis, the areas expressing both the adult MHC and the myogenin transcripts gradually expanded from the dorsomedial edge to the ventral side of the dorsal body muscles, accounting for the progression of the secondary "adult" myogenesis described previously (Nishikawa and Hayashi [1994] Dev. Biol. 165:86-94). This work shows that, in Xenopus, the accumulation of myogenin mRNA is restricted to secondary myogenesis, including the formation of new muscles in developing limbs as well as in dorsal muscles during body remodeling. This shows that myogenin is not required for primary myogenesis, and it suggests a crucial role for myogenin in the terminal differentiation program, including myoblast fusion and the activation of adult-type muscle genes.

Intraspecific myosin light chain polymorphism in the white muscle of herring (Clupea harengus harengus, L.)

The myosin contained in white and red muscles of herring (Clupea harengus harengus) was purified, and its subunit composition analyzed by electrophoretic techniques. The only myosin isoform present in red muscles was made up of one type of heavy chain and two types of light chain. The native myosin from white muscles migrated as one wide band. Analysis of the extracts by SDS/glycerol/PAGE from white muscles revealed one main type of heavy chain. Light chains were identified by SDS-PAGE analysis of electrophoretically purified myosin, and two-dimensional electrophoresis of the extracts demonstrated differences in the light chain composition of white and red muscles. Using this methodology, light chain polymorphism was detected in white muscles among members of the same species.

Electrophoretic study of myosin isoforms in white muscles of some teleost fishes

1. White skeletal muscle myosin of four marine teleost fish species (cod, blue whiting, Norway haddock, and spotted wolf-fish) was analyzed by native, SDS-PAGE, and 2-dimensional electrophoresis. 2. Four types of native myosin were present in cod, blue whiting and Norway haddock. The second fastest migrating form was predominant. 3. Myosin from spotted wolf-fish also resolved into four forms. The fastest migrating form was hardly noticeable. The other three were present in apparently similar amounts. 4. In the myosin from each species there were three types of light chains. The pattern of light chains was species specific. 5. Apparently, there was only one type of heavy chain in myosin from cod, Norway haddock and spotted wolf-fish. One preparation of cod showed an extra band of higher electrophoretic mobility than the main band. In blue whiting we found two bands present in approximately equal amounts.