CD45 is a haematopoietic cell-surface marker that, until recently, wasn't thought to be expressed on any other cell types. So, the observation that muscle-derived CD45+ stem cells can become muscle cells in response to Wnt signalling not only shows that these cells have a biological role in regeneration, but also indicates that targeting Wnt signalling might be effective in treating neuromuscular degenerative disease.

Michael Rudnicki's group, reporting in Cell, observed a tenfold increase in the proportion of cells expressing CD45 and the stem-cell marker Sca-1 (stem cell antigen-1) in regenerating tissues compared to uninjured cells; these CD45 and Sca-1-expressing cells also proliferated extensively. Induction of Myf5, MyoD, desmin and Pax7 was used to show that muscle-derived CD45+–Sca-1+ cells undergo myogenic specification in response to muscle damage. CD45+–Sca-1+ cells from uninjured skeletal muscle didn't form myogenic cells, but co-culturing them with primary myoblasts increased the frequency of myogenic differentiation.

Signalling through the Wnt pathway directs cell-fate determination in skeletal muscle — Wnts initiate myogenesis by inducing the expression of Pax7. As lithium can activate Wnt signalling, Rudnicki's group treated co-cultures of CD45+–Sca-1+ cells with lithium and saw a 15-fold increase in myogenic specification; even lithium-treated CD45+–Sca-1+ cells cultured without myoblasts underwent myogenic differentiation.

Gene-expression studies showed that messenger RNAs for Wnt5a, Wnt5b, Wnt7a and Wnt7b were induced in regenerating muscle. Further analysis of co-cultures showed that Wnt5a and Wnt5b were expressed in proliferating myoblasts but not differentiated myotubes. Wnt7a, by contrast, was expressed in myotubes but not myoblasts. Wnt5a, Wnt5b and Wnt7a were all expressed in single muscle fibres. Wnt7b mRNA, however, wasn't detected in any sample. So, combined signalling by Wnt5a, Wnt5b and Wnt7a — which are presumably secreted by myofibres and myoblasts in regenerating muscle — seems to induce the myogenic commitment of adult muscle-derived stem cells.

If this is true, then CD45+–Sca-1+ cells would be expected to express the Wnt receptor, Frizzled, which was indeed the case. And, confirming that the Wnt pathway was activated in regenerating muscle, high levels of the Wnt target β-catenin were found in CD45+–Sca-1+ cells. Next, the authors showed that there were increased β-catenin levels — along with increased MyoD and Pax7 expression — in CD45+–Sca-1+ cells co-cultured with a stable cell line expressing recombinant Wnt. This confirmed that Wnt signalling activates myogenic determination.

To determine how relevant Wnt signalling is in vivo, Rudnicki and colleagues injected the Wnt antagonists soluble-Frizzled-related proteins (sFRPs) 2 and 3 into regenerating mouse muscle. sFRP2 and sFRP3 inhibited the increase in CD45+–Sca-1+ cells, and the induction of Myf5, that was seen in regenerating cells following injury.

So, the authors propose that CD45+–Sca-1+ cells represent a main source of progenitor cells — thought to be stable and resident in muscles (rather than marrow-derived) — that have an important role in muscle regeneration. The molecular mechanisms by which they respond to Wnt signals to induce myogenesis aren't clear, and investigation of the transcriptional targets of Wnts in stem cells will be necessary. But “...the ability of injected sFRP proteins to attenuate the myogenic differentiation of CD45+ stem cells unequivocally underscores the clinical potential for modulating Wnt signalling in muscle tissue.”