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The satellite cell is the major tissue-resident stem cell underlying muscle regeneration; however, multiple non-satellite myogenic progenitors as well as non-myogenic populations that support the muscle regenerative process have been identified. PW1 is expressed in satellite cells as well as in a subset of interstitial cells with myogenic potential termed PICs (PW1+ interstitial cells). Microarray profiling revealed that PICs express a broad range of genes common to mesenchymal stem cells, whereas satellite cells express genes consistent with a committed myogenic progenitor. Isolated PICs from both young and adult muscles can differentiate into smooth and skeletal muscle and fat whereas satellite cells are restricted to a skeletal muscle fate. We demonstrate that the adipogenic potential of PICs corresponds to a subpopulation that expresses platelet derived growth factor receptor alpha (PDGFRα) and overlaps with the recently described interstitial adipogenic progenitors. By contrast, PICs with myogenic potential do not express PDGFRα. Moreover, we observe a discrete and transient population of juvenile PICs based upon SCA1 expression that disappears by 3 weeks of postnatal development coincident with a switch in the cellular and genetic mechanisms underlying postnatal muscle growth.

Vascular associated endothelial cell (ECs) progenitors are still poorly studied and their role in the newly forming vasculature at embryonic or postnatal stage remains elusive. In the present work, we first defined a set of genes highly expressed during embryo development and strongly downregulated in the adult mouse. In this group, we then concentrated on the progenitor cell marker Peg3/PW1. By in vivo staining of the vasculature we found that only a subset of cells coexpressed endothelial markers and PW1. These cells were quite abundant in the embryo vasculature but declined in number at postnatal and adult stages. Using a reporter mouse for PW1 expression, we have been able to isolate PW1-positive (PW1posECs) and negative endothelial cells (PW1negECs). PW1-positive cells were highly proliferative in comparison to PW1negECs and were able to form colonies when seeded at clonal dilution. Furthermore, by RNAseq analysis, PW1posECs expressed endothelial cell markers together with mesenchymal and stem cell markers. When challenged by endothelial growth factors in vitro, PW1posECs were able to proliferate more than PW1negECs and to efficiently form new vessels in vivo. Taken together these data identify a subset of vessel associated endothelial cells with characteristics of progenitor cells. Considering their high proliferative potential these cells may be of particular importance to design therapies to improve the perfusion of ischemic tissues or to promote vascular repair.

Adult somatic stem cells, also called progenitors, are essential for tissue regeneration and homeostasis. Stem cells have been identified in almost all tissues including those with little regenerative capacity. It is a central interest in the field of regenerative medicine to understand the signals that govern stem cell function, particularly in tissues that typically do not regenerate. Progenitors are present in small numbers in most tissues and reside in a specific cellular context that is referred to as the stem cell niche. In this article, we survey the stem cell niche in several tissues and examine its role in stem cell function. While the anatomical appearance of the stem cell niche differs among tissues, common features exist that serve similar roles. We review here the stem cell niche in two tissues capable of robust regeneration (skeletal muscle and skin) and two tissues that display poor regenerative capacity (CNS and heart). We propose that an increased understanding of the stem cell niche, particularly in tissues that are ‘poor regenerators,’ will have a significant impact upon regenerative medicine. An understanding of the molecular signals that are transmitted between the progenitors and neighboring niche cells will help identify new therapeutic targets to augment regenerative capacity as well as increase our fundamental understanding of these important cellular players.

International audience; Specific muscles are spared in many degenerative myopathies. Most notably, the extraocular muscles (EOMs) do not show clinical signs of late stage myopathies including the accumulation of fibrosis and fat. It has been proposed that an altered stem cell niche underlies the resistance of EOMs in these pathologies, however, to date, no reports have provided a detailed characterization of the EOM stem cell niche. PW1/Peg3 is expressed in progenitor cells in all adult tissues including satellite cells and a subset of interstitial non-satellite cell progenitors in muscle. These PW1-positive interstitial cells (PICs) include a fibroadipogenic progenitor population (FAP) that give rise to fat and fibrosis in late stage myopathies. PICs/FAPs are mobilized following injury and FAPs exert a promyogenic role upon myoblasts in vitro but require the presence of a minimal population of satellite cells in vivo. We and others recently described that FAPs express promyogenic factors while satellite cells express antimyogenic factors suggesting that PICs/FAPs act as support niche cells in skeletal muscle through paracrine interactions. We analyzed the EOM stem cell niche in young adult and aged wild-type mice and found that the balance between PICs and satellite cells within the EOM stem cell niche is maintained throughout life. Moreover, in the adult mdx mouse model for Duchenne muscular dystrophy (DMD), the EOM stem cell niche is unperturbed compared to normal mice, in contrast to Tibialis Anterior (TA) muscle, which displays signs of ongoing degeneration/regeneration. Regenerating mdx TA shows increased levels of both PICs and satellite cells, comparable to normal unaffected EOMs. We propose that the increase in PICs that we observe in normal EOMs contributes to preserving the integrity of the myofibers and satellite cells. Our data suggest that molecular cues regulating muscle regeneration are intrinsic properties of EOMs.

The myocardium responds to injury by recruiting cardiac progenitor cells (CPCs) to the injured tissue to promote cardiac repair. Although different classes of CPCs have been identified, their contribution in physiological and pathological conditions remains unclear. PW1 gene has recently been proposed as a marker of resident adult stem and progenitor cell populations in several adult tissues. Our goal was to characterize and determine the role of PW1+ population in the heart. Here, we employ immunostaining and fluorescence-activated cell sorting (FACS) analysis in PW1-reporter mouse to perform qualitative and quantitative analyses of PW1+ population in the heart. We first found that PW1+ cells are mainly located in the epicardium and myocardial interstitium of normal hearts. The average percentage of PW1+ cells, as assessed by FACS, was 1.56±1.41%. A subset of PW1+ cells also co-express other CPC markers such as Sca-1 (52±22%) or PDGFR1α (43±14%). In contrast, a very small proportion of PW1+ cells co-express c-kit (6±5%). To investigate the contribution of PW1+ cells in pathological conditions, we then performed myocardial infarction (MI) by LAD ligation in PW1-reporter mice. We found that MI resulted in a 3-fold increase in the number of PW1+ cells in infarcted mice compared with sham-operated groups, at 1 week post-MI (1.16%±0.47% in sham versus 3.43%±0.82 in MI). This population preferentially localized in the injured myocardium and border area. PW1+ cells were isolated by FACS from the whole infarcted heart from PW1-reporter mice. In vitro differentiation assays reveal that purified PW1+ cells are multipotent and can spontaneously differentiate into smooth muscle cells, endothelial cells and cardiomyocyte-like cells. Taken together, our data identify a novel PW1+ cardiac progenitor population with the potential to undergo differentiation into multiple cardiac lineages, suggesting their involvement in cardiac repair in normal and pathological conditions. The discovery of a novel population of cardiac progenitor cells, augmented following MI and with cardiogenic potential, provides a novel target for therapeutic approaches aimed at improving cardiac regeneration.