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Author(s): Dhanushika Ratnayake [sup.1] [sup.2] , Phong D. Nguyen [sup.3] [sup.4] , Fernando J. Rossello [sup.1] [sup.5] , Verena C. Wimmer [sup.6] [sup.7] , Jean L. Tan [sup.1] [sup.2] , [...], Skeletal muscle regenerates through the activation of resident stem cells. Termed satellite cells, these normally quiescent cells are induced to proliferate by wound-derived signals.sup.1. Identifying the source and nature of these cues has been hampered by an inability to visualize the complex cell interactions that occur within the wound. Here we use muscle injury models in zebrafish to systematically capture the interactions between satellite cells and the innate immune system after injury, in real time, throughout the repair process. This analysis revealed that a specific subset of macrophages 'dwell' within the injury, establishing a transient but obligate niche for stem cell proliferation. Single-cell profiling identified proliferative signals that are secreted by dwelling macrophages, which include the cytokine nicotinamide phosphoribosyltransferase (Nampt, which is also known as visfatin or PBEF in humans). Nampt secretion from the macrophage niche is required for muscle regeneration, acting through the C-C motif chemokine receptor type 5 (Ccr5), which is expressed on muscle stem cells. This analysis shows that in addition to their ability to modulate the immune response, specific macrophage populations also provide a transient stem-cell-activating niche, directly supplying proliferation-inducing cues that govern the repair process that is mediated by muscle stem cells. This study demonstrates that macrophage-derived niche signals for muscle stem cells, such as NAMPT, can be applied as new therapeutic modalities for skeletal muscle injury and disease. Specific macrophage populations provide a transient niche that activates muscle stem cells after muscle injury and supply proliferation-inducing cues that govern the repair process mediated by these cells in both zebrafish and mouse injury models.

Early skeletal muscle (the primary myotome, which is composed of mononucleated post-mitotic muscle fibres, or myocytes) is formed from the generation of muscle cells at the four borders of the [...], How dynamic signalling and extensive tissue rearrangements interact to generate complex patterns and shapes during embryogenesis is poorly understood (1-3). Here we characterize the signalling events taking place during early morphogenesis of chick skeletal muscles. We show that muscle progenitors present in somites require the transient activation of NOTCH signalling to undergo terminal differentiation. The NOTCH ligand Delta1 is expressed in a mosaic pattern in neural crest cells that migrate past the somites. Gain and loss of Delta1 function in neural crest modifies NOTCH signalling in somites, which results in delayed or premature myogenesis. Our results indicate that the neural crest regulates early muscle formation by a unique mechanism that relies on the migration of Delta1-expressing neural crest cells to trigger the transient activation of NOTCH signalling in selected muscle progenitors. This dynamic signalling guarantees a balanced and progressive differentiation of the muscle progenitor pool.

The early vertebrate skeletal muscle is a well-organized tissue in which the primitive muscle fibres, the myocytes, are all parallel and aligned along the antero-posterior axis of the embryo. How [...]

In vertebrates, somite differentiation is mediated in part by Sonic Hedgehog (Shh), secreted by the notochord and the floor plate. However, Shh-null mice display close to normal expression of molecular markers for dermomytome, myotome, and sclerotome, indicating that Shh might not be required for their initial induction. In this paper, we have addressed the capacity of Shh to regulate in vivo the expression of the somite differentiation markers Pax-l, MyoD, and Pax-3 after separation of paraxial mesoderm from axial structures. We show that Pax-l, which is lost under these experimental conditions, is rescued by Shh. In contrast, Shh maintains, but cannot induce MyoD expression, while Pax-3 expression is independent of the presence of axial structures or Shh. Finally, we demonstrate that Shh is a potent mitogen for somitic cells, supporting the idea that it may serve to expand subpopulations of cells within the somite.

In the retinotectal system, positional information has long been postulated to take the form of molecular gradients within both the retina and the tectum. Recent reports have implicated Mek4, a member of the Eph (also named class V) family of tyrosine kinase receptors (RTKs), and two ligands, RAGS and ELF-1, in this process. Here, we report the cloning and distribution pattern of QEK5, another member of the Eph family of RTKs, isolated from a quail cDNA library. During retinal differentiation, QEK5 transcripts accumulate in a ventral to dorsal gradient within the retinal neuroepithelium, where its expression becomes restricted to the ganglion and bipolar cell layers. Within the tectum, QEK5 transcripts are detectable in a posterior to anterior gradient in the ventricular layer and newly formed-superficial layers. The pattern of QEK5 expression in the retina and tectum is distinct from that of Mek4, suggesting that complex patterns of Eph RTKs and their ligands may play a role in cell-cell interactions involved in retinotectal projections and differentiation of the central nervous system.

A new avian cell receptor for fibroblast growth factor called FREK was observed in replicating skeletal muscle myoblasts from chick embryos but not in differentiated muscle cells. BrdU labeling techniques showed that muscle progenitors originating from the somite start FREK expression at three days of development. In limb myoblasts, FREK expression precedes expression of MyoD, indicating that FREK is expressed prior to differentiation. These results show that FREK expression can be used as a molecular marker for myoblast cell populations.

A model based on chick and mouse embryos was used to study the regulatory mechanisms of myostatin, a TGF-[beta] family member that is an important regulator of adult muscle size. Results demonstrate that myostatin acts in vivo to regulate the balance between proliferation and differentiation of embryonic muscle progenitors by promoting their terminal differentiation through the activation of p21 and MyoD.