Slow myosins in muscle development

Muscle development and differentiation in the urodele Ambystoma mexicanum

Muscle differentiation has been widely described in zebrafish and Xenopus, but nothing is known about this process in amphibian urodeles. Both anatomical features and locomotor activity in urodeles are known to show intermediate features between fish and anurans. Therefore, a better understanding of myogenesis in urodeles could be useful to clarify the evolutionary changes that led to the formation of skeletal muscle in the trunk of land vertebrates. We report here a detailed morphological and molecular investigation on several embryonic stages of Ambystoma mexicanum and show that the first differentiating muscle fibers are the slow ones, originating from a myoblast population initially localized close to the notochord that forms a superficial layer on the somitic surface afterwards. Subsequently, fast fibers differentiation ensues. We also identified and cloned A. mexicanum Myf5 as a muscle-specific transcriptional factor likely involved in urodele muscle differentiation.

Prdm1 (Blimp-1) and the expression of fast and slow myosin heavy chain isoforms during avian myogenesis in vitro

Background

Multiple types of fast and slow skeletal muscle fibers form during early embryogenesis in vertebrates. In zebrafish, formation of the earliest slow myofibers in fin muscles requires expression of the zinc-finger transcriptional repressor Prdm1 (also known as Blimp1). To further understand how the role of Prdm1 in early myogenesis may vary through evolution and during development, we have now analyzed Prdm1 expression in the diverse types of myotubes that form in culture from somitic, embryonic, and fetal chicken myoblasts.

Principal Findings

In cultures of somitic, embryonic limb, and fetal limb chicken cells, we found that Prdm1 was expressed in all of the differentiated muscle cells that formed, including those that expressed only fast myosin heavy chain isoforms, as well as those that co-expressed both fast and slow myosin heavy chain isoforms. Prdm1 was also expressed in Pax7-positive myoblasts, as well as in non-myogenic cells in the cultures. Furthermore, though all differentiated cells in control somite cultures co-expressed fast and slow myosin heavy chains, antisense knockdown of Prdm1 expression inhibited the formation of these co-expressing cells in somite cultures.

Conclusions

In chicken myogenic cell cultures, Prdm1 was expressed in most Pax7-positive myoblasts and in all differentiated muscle cells, irrespective of the developmental stage of cell donor or the pattern of fast and slow myosin heavy chains expressed in the differentiated cells that were formed. Thus, Prdm1 was expressed in myogenic cells prior to terminal differentiation; and, after differentiation, Prdm1 expression was not limited to cells that expressed slow myosin heavy chain isoforms. In addition, Prdm1 appeared to be required for differentiation of the somitic myocytes, which are the earliest myocytes to form in the avian embryo.

Regulating fibrinolysis to engineer skeletal muscle from the C2C12 cell line

Muscles engineered from transformed cells would be a powerful model for the study of muscle physiology by allowing long-term in vitro studies of muscle adaptation. However, previously described methods either take >5 weeks to produce a tissue or use collagen as a scaffold, which decreases the specific force of the muscle, making it hard to measure the function of the constructs. The aim of this study was to rapidly engineer muscle using the C2C12 cell line in fibrin, which has a stiffness similar to muscle tissue, allowing accurate functional testing. Both the protease inhibitor aprotinin and the natural cross-linker genipin increased the length of time that muscle could be cultured, with genipin increasing the time in culture to 10 weeks. The function of the tissues was significantly affected by the batch of serum (64-78%) or thrombin (41%), the differentiation medium (78%), and the seeding protocol (38%), but was unaffected by initial cell number. Strikingly, different C2C12 clones produced up to a 3.6-fold variation in force production. Under optimal conditions, the tissues form in 10.4+/-0.3 days and remain fully functional for 5 weeks over which time they continue to mature. The optimized model described here provides rapid, reliable, and functional tissues that will be useful in the study of skeletal muscle physiology.

Hedgehog signaling is required for commitment but not initial induction of slow muscle precursors

In zebrafish, skeletal muscle precursors can adopt at least three distinct fates: fast, non-pioneer slow, or pioneer slow muscle fibers. Slow muscle fibers develop from adaxial cells and depend on Hedgehog signaling. We analyzed when precursors become committed to their fates and the step(s) along their differentiation pathway affected by Hedgehog. Unexpectedly, we find that embryos deficient in Hedgehog signaling still contain postmitotic adaxial cells that differentiate into fast muscle fibers instead of slow. We show that by the onset of gastrulation, slow and fast muscle precursors are already spatially segregated but uncommitted to their fates until much later, in the segmental plate when slow precursors become independent of Hedgehog. In contrast, pioneer and non-pioneer slow muscle precursors share a common lineage from the onset of gastrulation. Our results demonstrate that slow muscle precursors form independently of Hedgehog signaling and further provide direct evidence for a multipotent muscle precursor population whose commitment to the slow fate depends on Hedgehog at a late stage of development when postmitotic adaxial cells differentiate into slow muscle fibers.

Regulation of myosin expression during myotome formation

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 chain mRNA 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 3 are 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.