Peg3/PW1 Is a Marker of a Subset of Vessel Associated Endothelial Progenitors.

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. Stem Cells 2017;35:1328-1340.

Defining skeletal muscle resident progenitors and their cell fate potentials.

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.

Inhibition of the Activin Receptor Type-2B Pathway Restores Regenerative Capacity in Satellite Cell-Depleted Skeletal Muscle.

Degenerative myopathies typically display a decline in satellite cells coupled with a replacement of muscle fibers by fat and fibrosis. During this pathological remodeling, satellite cells are present at lower numbers and do not display a proper regenerative function. Whether a decline in satellite cells directly contributes to disease progression or is a secondary result is unknown. In order to dissect these processes, we used a genetic model to reduce the satellite cell population by ~70-80% which leads to a nearly complete loss of regenerative potential. We observe that while no overt tissue damage is observed following satellite cell depletion, muscle fibers atrophy accompanied by changes in the stem cell niche cellular composition. Treatment of these mice with an Activin receptor type-2B (AcvR2B) pathway blocker reverses muscle fiber atrophy as expected, but also restores regenerative potential of the remaining satellite cells. These findings demonstrate that in addition to controlling fiber size, the AcvR2B pathway acts to regulate the muscle stem cell niche providing a more favorable environment for muscle regeneration.

The extraocular muscle stem cell niche is resistant to ageing and disease.

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.