Abstract
Recent observation identifies subendothelial (mural) cells expressing MCAM, a specific system of clonogenic, self-renewing, osteoprogenitors (a.k.a, “mesenchymal stem cells”) in the microvascular compartment of post-natal human bone marrow (BM). In this study, we used MCAM/CD146, as a marker to localize, isolate and assay subendothelial clonogenic cells from the microvasculature of postnatal human skeletal muscle. We show here that these cells share with their BM counterpart, anatomic position (subendothelial/adventitial) and ex vivo clonogenicity (CFU-Fs). When assayed under the stringent conditions, these cells display a high spontaneous myogenic potential (independent of co-culture with myoblasts or of in vivo fusion with local myoblasts), which is otherwise only attained in cultures of satellite cells. These muscle-derived mural cells activated a myogenic program in culture. Cultured CD146+ cells expressed the myogenic factors (Pax7, Pax3 and Myf5), NCAM/CD56, desmin as well as proteins characteristic of more advanced myogenic differentiation, such as myosin heavy chain. In vivo, these cells spontaneously generate myotubes and myofibrils. These data identify the anatomy and phenotype of a novel class of committed myogenic progenitor in human post-natal skeletal muscle of subendothelial cells associated with the abluminal surface of microvascular compartment distinct from satellite cells.
Highlights
A stable tissue that does not undergo significant steady state turnover, skeletal muscle has a limited capacity for repair and regeneration [1, 2]
Using fluorescence-activated cell sorting (FACS) analysis of cellular suspension, as obtained by the digestion of muscle tissue, we analyzed the expression of CD34 as a marker of endothelial cells, NCAM, a marker of satellite cells, and Alkaline Phosphatase (ALP), a pericyte and muscle “mesenchymal stem cells” marker [26], along with CD146
To determine if skeletal muscle would include clonogenic adherent cells (Muscle-Colony Forming Units-Fibroblastic (CFU-Fs), M-CFU-Fs), similar to those found in the postnatal bone marrow (BM), we conducted Colony Forming Efficiency (CFE) assays with single cell suspensions generated by digesting muscle fragments with collagenase
Summary
A stable tissue that does not undergo significant steady state turnover, skeletal muscle has a limited capacity for repair and regeneration [1, 2]. Like its physiological postnatal growth, and its hypertrophy in response to exercise, muscle repair and regeneration are thought to reflect the biological activity of local progenitor cells [3]. Satellite cells, located beneath the basal lamina of myofibers, are the best characterized myogenic progenitors in postnatal muscle [4]. Resume mitotic activity and generate a pool of myoblasts able to fuse into newly formed myofibers [4, 5]. In spite of their unrivalled myogenic potential, satellite cells are unable to cross vascular wall, limiting their efficacy in in vivo transplantation [6,7,8]. Satellite cells are not easy to isolate and expand in culture: only recently they have been isolated from the mouse [9, 10], but not from humans
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