Embryonic and fetal limb myogenic cells are derived from developmentally distinct progenitors and have different requirements for beta-catenin

Muscle stem cells in developmental and regenerative myogenesis

PURPOSE OF REVIEW

Skeletal muscle development serves as a paradigm for cell lineage specification and cell differentiation. Adult skeletal muscle has high regenerative capacity, with satellite cells the primary source of this capability. The present review describes recent findings on developmental and adult myogenesis with emphasis on emerging distinctions between various muscle groups and stages of myogenesis.

RECENT FINDINGS

Muscle progenitors of the body are derived from multipotent cells of the dermomyotome and express the transcription factors Pax3 and Pax7. These cells self-renew or induce expression of myogenic regulatory factors (MRFs) and differentiate. The roles of Pax3, Pax7 and specific myogenic regulatory factor progenitor populations in trunk and limb myogenesis have been identified through cell ablation in the mouse. Various head muscles and associated satellite cells have differing developmental origins, and rely on distinct combinations of transcriptional regulators, than trunk and limb muscles. Several genetic and sorting protocols demonstrate that satellite cells are heterogeneous with some possessing stem cell properties; the relative roles of lineage and niche in these properties are being explored. Although cellular mechanisms of developmental, postnatal and adult regenerative myogenesis are thought to be similar, recent studies reveal distinct genetic requirements for embryonic, fetal, postnatal and adult regenerative myogenesis.

SUMMARY

Genetic determinants of formation or repair of various muscles during different stages of myogenesis are unexpectedly diverse. Future studies should illuminate these differences, as well as mechanisms that underlie stem cell properties of satellite cells.

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.

Nfix regulates fetal-specific transcription in developing skeletal muscle

Skeletal myogenesis, like hematopoiesis, occurs in successive developmental stages that involve different cell populations and expression of different genes. We show here that the transcription factor nuclear factor one X (Nfix), whose expression is activated by Pax7 in fetal muscle, in turn activates the transcription of fetal specific genes such as MCK and beta-enolase while repressing embryonic genes such as slow myosin. In the case of the MCK promoter, Nfix forms a complex with PKC theta that binds, phosphorylates, and activates MEF2A. Premature expression of Nfix activates fetal and suppresses embryonic genes in embryonic muscle, whereas muscle-specific ablation of Nfix prevents fetal and maintains embryonic gene expression in the fetus. Therefore, Nfix acts as a transcriptional switch from embryonic to fetal myogenesis.

Tbx4 and tbx5 acting in connective tissue are required for limb muscle and tendon patterning

Proper functioning of the musculo-skeletal system requires the precise integration of bones, muscles and tendons. Complex morphogenetic events ensure that these elements are linked together in the appropriate 3D configuration. It has been difficult, however, to tease apart the mechanisms that regulate tissue morphogenesis. We find that deletion of Tbx5 in forelimb (or Tbx4 in hindlimbs) specifically affects muscle and tendon patterning without disrupting skeletal development thus suggesting that distinct cues regulate these processes. We identify muscle connective tissue as the site of action of these transcription factors and show that N-Cadherin and β-Catenin are key downstream effectors acting in muscle connective tissue regulating soft-tissue morphogenesis. In humans, TBX5 mutations lead to Holt-Oram syndrome, which is characterised by forelimb musculo-skeletal defects. Our results suggest that a focus on connective tissue is required to understand the aetiology of diseases affecting soft tissue formation.

Wnt/Lef1 signaling acts via Pitx2 to regulate somite myogenesis

Wnt signaling has been implicated in somite, limb, and branchial arch myogenesis but the mechanisms and roles are not clear. We now show that Wnt signaling via Lef1 acts to regulate the number of premyogenic cells in somites but does not regulate myogenic initiation in the limb bud or maintenance in the first or second branchial arch. We have also analysed the function and regulation of a putative downstream transcriptional target of canonical Wnt signaling, Pitx2. We show that loss-of-function of Pitx2 decreases the number of myogenic cells in the somite, whereas overexpression increases myocyte number particularly in the epaxial region of the myotome. Increased numbers of mitotic cells were observed following overexpression of Pitx2 or an activated form of Lef1, suggesting an effect on cell proliferation. In addition, we show that Pitx2 expression is regulated by canonical Wnt signaling in the epaxial somite and second branchial arch, but not in the limb or the first branchial arch. These results suggest that Wnt/Lef1 signaling regulates epaxial myogenesis via Pitx2 but that this link is uncoupled in other regions of the body, emphasizing the unique molecular networks that control the development of various muscles in vertebrates.

Muscle stem cells

Despite being mainly composed of highly differentiated contractile fibers, the adult skeletal muscle possesses the remarkable ability to regenerate, following injury. The cells that are responsible for this capacity are the satellite cells, a small population of adult stem cells positioned under the basal lamina of muscle fibers and that can give rise to both differentiated myogenic cells while maintaining a stem cell pool by a self-renewal mechanism. We will discuss here recent publications on the developmental origin of muscle stem cells, on the signaling pathways that affect their proliferation and differentiation, with reference to works on skeletal muscle formation in the embryo as well as the adult, using the mouse and chick as reference models.

Genetic manipulations reveal dynamic cell and gene functions: Cre-ating a new view of myogenesis

Development of multicellular organisms is temporally and spatially complex. The Cre/loxP and Flp/FRT systems for genetic manipulation in mammals now enable researchers to explicitly examine in vivo the temporal and spatial role of cells and genes during development via cell lineage and ablation studies and conditional gene inactivation and activation. Recently we have used these methods to genetically dissect the role of Pax3(+) and Pax7(+) progenitor populations and the function of beta-catenin, an important regulator of myogenesis, in vertebrate limb myogenesis. Our lineage and ablation studies of Pax3(+) and Pax7(+) progenitors revealed surprising insights into myogenesis not apparent from Pax3 and Pax7 expression and functional studies. In addition, conditional inactivation and activation of beta-catenin in different progenitor populations and their progeny demonstrated that beta-catenin plays several cell-autonomous roles in myogenesis. Our studies highlight the hierarchical (i.e., genes versus cells), temporal and spatial complexity of development and demonstrate that manipulations of both cells and genes will be required to obtain a full understanding of the development of multicellular organisms.

Skeletal muscle stem cells in developmental versus regenerative myogenesis

Tissue and organ regeneration proceed in a coordinated manner to restore proper function after trauma. Vertebrate skeletal muscle has a remarkable ability to regenerate after repeated and complete destruction of the tissue, yet limited information is available on how muscle stem and progenitor cells, and other nonmuscle cells, reestablish homeostasis after the regenerative process. The genetic pathways that regulate the establishment of skeletal muscle in the embryo have been studied extensively, and many of the genes that govern muscle stem cell maintenance and commitment are redeployed during adult homeostasis and regeneration. Therefore, correlates can be made between embryonic muscle development and postnatal regeneration. However, there are some important distinctions between prenatal development and regeneration - in the context of the cells, niche, anatomy and the regulatory genes employed. The similarities and distinctions between these two scenarios are the focus of this review.

Embryonic vs. adult myogenesis: challenging the 'regeneration recapitulates development' paradigm

A popular theory in the stem cell field is that 'regeneration recapitulates development', or that adult stem cells function similarly to embryonic ones. In a recent Nature article, Lepper et al. questioned this approach, highlighting the differences in requirements for Pax7 during myogenesis for embryonic, juvenile and adult muscle.

Distinct and dynamic myogenic populations in the vertebrate embryo

Myogenic cells in the body of vertebrates derive from the dorsal somite, the dermomyotome, where multipotent cells are present. Regulation of cell fate choice is discussed, as is that of progenitor cell self-renewal once cells have entered the myogenic programme. Ongoing research on the formation of the first skeletal muscle, the myotome, is presented with emphasis on mechanisms controlling the early segregation of slow and fast muscle lineages that characterizes this process in the zebrafish embryo. Further insights into myogenic populations that contribute to trunk and limb development at different stages are summarized and the distinct regulatory networks that underlie the formation of head muscles are discussed.

The origin of embryonic and fetal myoblasts: a role of Pax3 and Pax7

Skeletal muscle is a heterogeneous tissue composed of individual muscle fibers, diversified in size, shape, and contractile protein content, to fulfill the different functional needs of the vertebrate body. This heterogeneity derives from and depends at least in part on distinct classes of myogenic progenitors; i.e., embryonic and fetal myoblasts and satellite cells whose origin and lineage relationship have been elusive so far. In this issue of Genes & Development, Hutcheson and colleagues (pp. 997-1013) provide a first answer to this question.