Zebrafish skeletal muscle growth and repair: role of the muscle progenitor cell

Current evidence indicates that post‐embryonic muscle growth and regeneration is mediated almost entirely by stem cells derived from muscle progenitor cells (MPCs), known as satellite cells. Exhaustion and impairment of satellite cell activity is involved in the severe atrophy associated with degenerative muscle diseases (eg. Duchenne’s Muscular Dystrophy) and is the main cause of age‐associated muscle wasting. Understanding the molecular and cellular basis of satellite cell function in muscle generation and regeneration (myogenesis) is critical to the broader goal of developing treatments that may ameliorate such conditions. Considerable knowledge exists regarding the embryonic stages of amniote myogenesis due to the relative ease of examining this process. Much less is known about how postembryonic amniote myogenesis proceeds, how adult myogenesis relates to embryonic myogenesis on a cellular or genetic level. Of the studies focusing on post‐embryonic amniote myogenesis, most are post‐mortem and in vitro analyses, precluding the understanding of cellular behaviours and genetic mechanisms in an undisturbed in vivo setting. Zebrafish are optically clear throughout much of their post‐embryonic development, facilitating their use in live imaging of cellular processes. Zebrafish also possess a compartment of MPCs, which appear similar to satellite cell and persist throughout the post‐embryonic development of the fish, permitting their use in examining the contribution of these cells to muscle tissue growth and regeneration. Our work focuses on expanding the existing knowledge of post‐embryonic muscle growth and repair. Morphometric analysis applied to muscle sections taken from fish throughout their development demonstrates that zebrafish undergo both hyperplasia (addition of new muscle fibres to the myotome) and hypertrophy (growth of existing muscle fibres within the myotome) from early larvae to adulthood. Using transgenic expression to drive fluorophores that differ in their timeframe to become visible, we identify the zones of new growth throughout the myotome for various stages of fish development in a novel approach unbiased for fibre size. Additional fluorescent transgenic lines driven by MPC specific promoters allow us to investigate the myotomal location of MPCs with respect to these zones of growth. Combining these MPC marking transgenic lines with further morphometric analysis allows us to examine some of the cellular and genetic programs involved in the transition from an undifferentiated mononuclear MPCs into multinucleate muscle fibres within larval and adult skeletal muscle. Finally, we apply needle stick and laser ablation injury models to the aforementioned transgenic fish on both wildtype and mutant backgrounds to visualise the capacity of these MPCs to contribute to post‐embryonic skeletal muscle repair, and to interrogate the genetic program used by these myogenic cells. Our results indicate that zebrafish MPCs are functional equivalents to amniote satellite cells, contributing to both muscle growth and repair while utilising similar genetic pathways, and that these processes can be imaged in real time and in vivo. Therefore, zebrafish present a relevant and useful model for further study of skeletal muscle growth and repair in both injury and disease contexts

Zebrafish skeletal muscle growth and repair: role of the muscle progenitor cell

Current evidence indicates that post‐embryonic muscle growth and regeneration is mediated almost entirely by stem cells derived from muscle progenitor cells (MPCs), known as satellite cells. Exhaustion and impairment of satellite cell activity is involved in the severe atrophy associated with degenerative muscle diseases (eg. Duchenne’s Muscular Dystrophy) and is the main cause of age‐associated muscle wasting. Understanding the molecular and cellular basis of satellite cell function in muscle generation and regeneration (myogenesis) is critical to the broader goal of developing treatments that may ameliorate such conditions. Considerable knowledge exists regarding the embryonic stages of amniote myogenesis due to the relative ease of examining this process. Much less is known about how postembryonic amniote myogenesis proceeds, how adult myogenesis relates to embryonic myogenesis on a cellular or genetic level. Of the studies focusing on post‐embryonic amniote myogenesis, most are post‐mortem and in vitro analyses, precluding the understanding of cellular behaviours and genetic mechanisms in an undisturbed in vivo setting. Zebrafish are optically clear throughout much of their post‐embryonic development, facilitating their use in live imaging of cellular processes. Zebrafish also possess a compartment of MPCs, which appear similar to satellite cell and persist throughout the post‐embryonic development of the fish, permitting their use in examining the contribution of these cells to muscle tissue growth and regeneration. Our work focuses on expanding the existing knowledge of post‐embryonic muscle growth and repair. Morphometric analysis applied to muscle sections taken from fish throughout their development demonstrates that zebrafish undergo both hyperplasia (addition of new muscle fibres to the myotome) and hypertrophy (growth of existing muscle fibres within the myotome) from early larvae to adulthood. Using transgenic expression to drive fluorophores that differ in their timeframe to become visible, we identify the zones of new growth throughout the myotome for various stages of fish development in a novel approach unbiased for fibre size. Additional fluorescent transgenic lines driven by MPC specific promoters allow us to investigate the myotomal location of MPCs with respect to these zones of growth. Combining these MPC marking transgenic lines with further morphometric analysis allows us to examine some of the cellular and genetic programs involved in the transition from an undifferentiated mononuclear MPCs into multinucleate muscle fibres within larval and adult skeletal muscle. Finally, we apply needle stick and laser ablation injury models to the aforementioned transgenic fish on both wildtype and mutant backgrounds to visualise the capacity of these MPCs to contribute to post‐embryonic skeletal muscle repair, and to interrogate the genetic program used by these myogenic cells. Our results indicate that zebrafish MPCs are functional equivalents to amniote satellite cells, contributing to both muscle growth and repair while utilising similar genetic pathways, and that these processes can be imaged in real time and in vivo. Therefore, zebrafish present a relevant and useful model for further study of skeletal muscle growth and repair in both injury and disease contexts

Haematopoietic stem cell induction by somite-derived endothelial cells controlled by meox1

Haematopoietic stem cells (HSCs) are self-renewing stem cells capable of replenishing all blood lineages. In all vertebrate embryos that have been studied, definitive HSCs are generated initially within the dorsal aorta (DA) of the embryonic vasculature by a series of poorly understood inductive events1-3. Previous studies have identified that signalling relayed from adjacent somites coordinates HSC induction, but the nature of this signal has remained elusive. Here we reveal that somite specification of HSCs occurs via the deployment of a specific endothelial precursor population, which arises within a sub-compartment of the zebrafish somite that we have defined as the endotome. Endothelial cells of the endotome are specified within the nascent somite by the activity of the homeobox gene meox1. Specified endotomal cells consequently migrate and colonize the DA, where they induce HSC formation through the deployment of chemokine signalling activated in these cells during endotome formation. Loss of meox1 activity expands the endotome at the expense of a second somitic cell type, the muscle precursors of the dermomyotomal equivalent in zebrafish, the external cell layer. The resulting increase in endotome-derived cells that migrate to colonize the DA generates a dramatic increase in chemokine-dependent HSC induction. This study reveals the molecular basis for a novel somite lineage restriction mechanism and defines a new paradigm in induction of definitive HSCs.5 page(s