A role for the myogenic determination gene Myf5 in adult regenerative myogenesis

A distinct profile of myogenic regulatory factor detection within Pax7+ cells at S phase supports a unique role of Myf5 during posthatch chicken myogenesis

Satellite cells are skeletal muscle stem cells that provide myogenic progeny for myofiber growth and repair. Temporal expression of muscle regulatory factors (MRFs) and the paired box transcription factor Pax7 defines characteristic phases of proliferation (Pax7(+)/MyoD(+)/myogenin(-)) and differentiation (Pax7(-)/MyoD(+)/myogenin(+)) during myogenesis of satellite cells. Here, using bromodeoxyuridine (BrdU) labeling and triple immunodetection, we analyzed expression patterns of Pax7 and the MRFs MyoD, Myf5, or myogenin within S phase myoblasts prepared from posthatch chicken muscle. Essentially, all BrdU incorporation was restricted to Pax7(+) cells, of which the majority also expressed MyoD. The presence of a minor BrdU(+)/Pax7(+)/myogenin(+) population in proliferation stage cultures suggests that myogenin up-regulation is alone insufficient for terminal differentiation. Myf5 was detected strictly within Pax7(+) cells and decreased during S phase while MyoD presence persisted in cycling cells. This study provides novel data in support of a unique role for Myf5 during posthatch myogenesis.

Progenitors of skeletal muscle satellite cells express the muscle determination gene, MyoD

Satellite cells are tissue-specific stem cells responsible for skeletal muscle growth and regeneration. Although satellite cells were identified almost 50 years ago, the identity of progenitor populations from which they derive remains controversial. We developed MyoD(iCre) knockin mice, and used Cre/lox lineage analysis to determine whether satellite cell progenitors express MyoD, a marker of myogenic commitment. Recombination status of satellite cells was determined by confocal microscopy of isolated muscle fibers and by electron microscopic observation of muscle tissue fixed immediately following isolation, using R26R-EYFP and R26R (beta-gal) reporter mice, respectively. We show that essentially all adult satellite cells associated with limb and body wall musculature, as well as the diaphragm and extraocular muscles, originate from MyoD+ progenitors. Neonatal satellite cells were Cre-recombined, but only a small minority exhibited ongoing Cre expression, indicating that most satellite cells had expressed MyoD prenatally. We also show that satellite cell development in MyoD-null mice is not due to functional compensation by MyoD non-expressing lineages. The results suggest that satellite cells are derived from committed myogenic progenitors, irrespective of the anatomical location, embryological origin, or physiological properties of associated musculature.

Skeletal muscle as a paradigm for regenerative biology and medicine

Tissue development and regeneration share common features, since modules of regulatory pathways and transcription factors that are crucial for prenatal development are redeployed for tissue reconstruction after trauma. Regenerative medicine has therefore gained important insights through the study of developmental and regenerative biology. Moreover, diverse experimental models have been used to investigate the regeneration process in different tissues and organs. Paradoxically, little is known regarding the relative contribution of stem cells with respect to the supporting tissue during tissue regeneration. Particular attention will be given to mouse models using distinct injury paradigms to investigate the regenerative biology of skeletal muscle. An understanding of the response of stem and parenchymal cells is crucial for the development of clinical strategies to combat the normal decline in tissue performance during aging or its reconstitution after trauma and during disease. This review addresses these issues, focusing on muscle regeneration and how different factors, including genes, cells and the environment, impinge on this process.

Niche regulation of muscle satellite cell self-renewal and differentiation

Muscle satellite cells have been shown to be a heterogeneous population of committed myogenic progenitors and noncommitted stem cells. This hierarchical composition of differentiating progenitors and self-renewable stem cells assures the extraordinary regenerative capacity of skeletal muscles. Recent studies have revealed a role for asymmetric division in satellite cell maintenance and offer novel insights into the regulation of satellite cell function by the niche. A thorough understanding of the molecular regulation and cell fate determination of satellite cells and other potential stem cells resident in muscle is essential for successful stem cell-based therapies to treat muscular diseases.

The ubiquitin-proteasome and the mitochondria-associated apoptotic pathways are sequentially downregulated during recovery after immobilization-induced muscle atrophy

Immobilization produces morphological, physiological, and biochemical alterations in skeletal muscle leading to muscle atrophy and long periods of recovery. Muscle atrophy during disuse results from an imbalance between protein synthesis and proteolysis but also between apoptosis and regeneration processes. This work aimed to characterize the mechanisms underlying muscle atrophy and recovery following immobilization by studying the regulation of the mitochondria-associated apoptotic and the ubiquitin-proteasome-dependent proteolytic pathways. Animals were subjected to hindlimb immobilization for 4-8 days (I4 to I8) and allowed to recover after cast removal for 10-40 days (R10 to R40). Soleus and gastrocnemius muscles atrophied from I4 to I8 to a greater extent than extensor digitorum longus and tibialis anterior muscles. Gastrocnemius muscle atrophy was first stabilized at R10 before being progressively reduced until R40. Polyubiquitinated proteins accumulated from I4, whereas the increased ubiquitination rates and chymotrypsin-like activity of the proteasome were detectable from I6 to I8. Apoptosome and caspase-3 or -9 activities increased at I6 and I8, respectively. The ubiquitin-proteasome-dependent pathway was normalized early when muscle stops to atrophy (R10). By contrast, the mitochondria-associated apoptotic pathway was first downregulated below basal levels when muscle started to recover at R15 and completely normalized at R20. Myf 5 protein levels decreased from I4 to I8 and were normalized at R10. Altogether, our results suggest a two-stage process in which the ubiquitin-proteasome pathway is rapidly up- and downregulated when muscle atrophies and recovers, respectively, whereas apoptotic processes may be involved in the late stages of atrophy and recovery.

Pax3 regulation of FGF signaling affects the progression of embryonic progenitor cells into the myogenic program

Pax3/7-dependent stem cells play an essential role in skeletal muscle development. We now show that Fgfr4 lies genetically downstream from Pax3 and is a direct target. In chromatin immunoprecipitation (ChIP)-on-chip experiments, Pax3 binds to a sequence 3' of the Fgfr4 gene that directs Pax3-dependent expression at sites of myogenesis in transgenic mouse embryos. The activity of this regulatory element is also partially dependent on E-boxes, targets of the myogenic regulatory factors, which are expressed as progenitor cells enter the myogenic program. Other FGF signaling components, notably Sprouty1, are also regulated by Pax3. In vivo manipulation of Sprouty expression reveals that FGF signaling affects the balance between Pax-positive progenitor cells and committed myoblasts. These results provide new insight into the Pax-initiated regulatory network that modulates stem cell maintenance versus tissue differentiation.

Skeletal muscle stem cells

In this review we shall discuss recent publications on the heterogeneity of muscle stem cells, signaling pathways that affect their behaviour and regulatory mechanisms that underlie their myogenic fate, with reference to insights provided by work on skeletal muscle formation in the embryo as well as the adult, with the mouse as a model of reference.

Specific requirements of MRFs for the expression of muscle specific microRNAs, miR-1, miR-206 and miR-133

The expression of three microRNAs, miR-1, miR-206 and miR-133 is restricted to skeletal myoblasts and cardiac tissue during embryo development and muscle cell differentiation, which suggests a regulation by muscle regulatory factors (MRFs). Here we show that inhibition of C2C12 muscle cell differentiation by FGFs, which interferes with the activity of MRFs, suppressed the expression of miR-1, miR-206 and miR-133. To further investigate the role of myogenic regulators (MRFs), Myf5, MyoD, Myogenin and MRF4 in the regulation of muscle specific microRNAs we performed gain and loss-of-function experiments in vivo, in chicken and mouse embryos. We found that directed expression of MRFs in the neural tube of chicken embryos induced ectopic expression of miR-1 and miR-206. Conversely, the lack of Myf5 but not of MyoD resulted in a loss of miR-1 and miR-206 expression. Taken together our results demonstrate differential requirements of distinct MRFs for the induction of microRNA gene expression during skeletal myogenesis.

Different autonomous myogenic cell populations revealed by ablation of Myf5-expressing cells during mouse embryogenesis

The development of myogenic cells is mainly determined by expression of two myogenic factors, Myf5 and Myod1 (MyoD), which genetically compensate for each other during embryogenesis. Here, we demonstrate by conditional cell ablation in mice that Myf5 determines a distinct myogenic cell population, which also contains some Myod1-positive cells. Ablation of this lineage uncovers the presence of a second autonomous myogenic lineage, which superseded Myf5-dependent myogenic cells and expressed Myod1. By contrast, ablation of myogenin-expressing cells erased virtually all differentiated muscle cells, indicating that some aspects of the myogenic program are shared by most skeletal muscle cells. We conclude that Myf5 and Myod1 define different cell lineages with distinct contributions to muscle precursor cells and differentiated myotubes. Individual myogenic cell lineages seem to substitute for each other within the developing embryo.

Satellite and stem cells in muscle growth and repair

The FASEB summer research conference on Skeletal Muscle Satellite and Stem Cells, organized by Thomas Rando, Giulio Cossu and Jeffrey Chamberlain, was held in Indian Wells, California, in July. An international array of researchers gathered to share numerous new insights into the cellular and molecular regulation of stem cells and satellite cells in skeletal muscle biology. The conference is unique in that it brings together investigators from diverse backgrounds, who work on the growth and repair of skeletal muscle in humans and model systems, in health and disease.

Skeletal muscle satellite cells and adult myogenesis

Research focusing on the canonical adult myogenic progenitor, the skeletal muscle satellite cell, is still an ever-growing field 46 years from their initial description. Recent publications revealed numerous new aspects of satellite cell biology, starting from their developmental life to their role as the principal self-renewing myogenic stem cell in adult skeletal muscle and finally their loss during aging. The myogenic potential of satellite cells is under the molecular control of specific paired-box and bHLH transcription factors whose tightly orchestrated balance accounts for an effective skeletal muscle regeneration. New reports also demonstrate satellite cells relationships with blood vessels and the high myogenic potential of stem cell subsets related to both lineages.