Calcitonin Receptor Signaling Inhibits Muscle Stem Cells from Escaping the Quiescent State and the Niche.

Calcitonin receptor (Calcr) is expressed in adult muscle stem cells (muscle satellite cells [MuSCs]). To elucidate the role of Calcr, we conditionally depleted Calcr from adult MuSCs and found that impaired regeneration after muscle injury correlated with the decreased number of MuSCs in Calcr-conditional knockout (cKO) mice. Calcr signaling maintained MuSC dormancy via the cAMP-PKA pathway but had no impact on myogenic differentiation of MuSCs in an undifferentiated state. The abnormal quiescent state in Calcr-cKO mice resulted in a reduction of the MuSC pool by apoptosis. Furthermore, MuSCs were found outside their niche in Calcr-cKO mice, demonstrating cell relocation. This emergence from the sublaminar niche was prevented by the Calcr-cAMP-PKA and Calcr-cAMP-Epac pathways downstream of Calcr. Altogether, the findings demonstrated that Calcr exerts its effect specifically by keeping MuSCs in a quiescent state and in their location, maintaining the MuSC pool.

Doublecortin marks a new population of transiently amplifying muscle progenitor cells and is required for myofiber maturation during skeletal muscle regeneration.

Muscle satellite cells are indispensable for muscle regeneration, but the functional diversity of their daughter cells is unknown. Here, we show that many Pax7(+)MyoD(-) cells locate both beneath and outside the basal lamina during myofiber maturation. A large majority of these Pax7(+)MyoD(-) cells are not self-renewed satellite cells, but have different potentials for both proliferation and differentiation from Pax7(+)MyoD(+) myoblasts (classical daughter cells), and are specifically marked by expression of the doublecortin (Dcx) gene. Transplantation and lineage-tracing experiments demonstrated that Dcx-expressing cells originate from quiescent satellite cells and that the microenvironment induces Dcx in myoblasts. Expression of Dcx seems to be necessary for myofiber maturation because Dcx-deficient mice exhibited impaired myofiber maturation resulting from a decrease in the number of myonuclei. Furthermore, in vitro and in vivo studies suggest that one function of Dcx in myogenic cells is acceleration of cell motility. These results indicate that Dcx is a new marker for the Pax7(+)MyoD(-) subpopulation, which contributes to myofiber maturation during muscle regeneration.

Isolation, characterization, and molecular regulation of muscle stem cells.

Skeletal muscle has great regenerative capacity which is dependent on muscle stem cells, also known as satellite cells. A loss of satellite cells and/or their function impairs skeletal muscle regeneration and leads to a loss of skeletal muscle power; therefore, the molecular mechanisms for maintaining satellite cells in a quiescent and undifferentiated state are of great interest in skeletal muscle biology. Many studies have demonstrated proteins expressed by satellite cells, including Pax7, M-cadherin, Cxcr4, syndecan3/4, and c-met. To further characterize satellite cells, we established a method to directly isolate satellite cells using a monoclonal antibody, SM/C-2.6. Using SM/C-2.6 and microarrays, we measured the genes expressed in quiescent satellite cells and demonstrated that Hesr3 may complement Hesr1 in generating quiescent satellite cells. Although Hesr1- or Hesr3-single knockout mice show a normal skeletal muscle phenotype, including satellite cells, Hesr1/Hesr3-double knockout mice show a gradual decrease in the number of satellite cells and increase in regenerative defects dependent on satellite cell numbers. We also observed that a mouse's genetic background affects the regenerative capacity of its skeletal muscle and have established a line of DBA/2-background mdx mice that has a much more severe phenotype than the frequently used C57BL/10-mdx mice. The phenotype of DBA/2-mdx mice also seems to depend on the function of satellite cells. In this review, we summarize the methodology of direct isolation, characterization, and molecular regulation of satellite cells based on our results. The relationship between the regenerative capacity of satellite cells and progression of muscular disorders is also summarized. In the last part, we discuss application of the accumulating scientific information on satellite cells to treatment of patients with muscular disorders.

Imatinib attenuates severe mouse dystrophy and inhibits proliferation and fibrosis-marker expression in muscle mesenchymal progenitors.

Imatinib mesylate inhibits signaling of tyrosine kinase receptors, including PDGFRα, and has been used for human cancer therapy. Recent studies have indicated that imatinib is also effective in treatment of some chronic diseases with fibrosis. Fibrosis is the feature of Duchenne muscular dystrophy. It has been reported that imatinib attenuates fibrosis in mdx mice. Recently we revealed that PDGFRα is specifically expressed in muscle mesenchymal progenitors, which are the origin of muscle fibrosis. Here, we show that imatinib ameliorates the muscular pathology of DBA/2-mdx, a more severe mouse muscular dystrophy. In addition, imatinib inhibits both the proliferation and fibrosis marker expression induced by PDGF-AA in muscle mesenchymal progenitors in vitro. Importantly, the effective dose of imatinib on muscle mesenchymal progenitors did not inhibit myoblast proliferation. These results suggest that imatinib targets mesenchymal progenitors, and that a therapeutic strategy targeting mesenchymal progenitors could be a potential treatment for muscular dystrophies.