Response: Skeletal Muscle Precursor Grafts in Dystrophic Mice

In his Correspondence, Terence Partridge raises two issues regarding our recent study investigating the stem cell properties of FACS-purified skeletal muscle precursors (SMPs) (Cerletti et al., 2008). First, he challenges our conclusion that SMPs represent a distinct myogenic cell population, and second, he questions our physiological assessment of the SMP-engrafted muscles of dystrophic mdx mice. However, as elaborated below, his point regarding the novelty of our cell isolation strategy is inaccurate, and more importantly, his reanalysis of our functional data uses inappropriate statistical methods that lead him to an erroneous conclusion. First, Dr. Partridge argues that SMPs are not a unique myogenic cell population because (1) SMPs share phenotypic markers with muscle satellite cells, and (2) a previous publication (Montarras et al., 2005), which he coauthored, isolated a similar population of cells using gene-targeted Pax3-GFP reporter mice and another cellsurface marker (CD34 expression). We agree that SMPs have properties similar to Pax3-GFP+ muscle satellite cells and properly credit this work in our paper (Cerletti et al., 2008, p. 42). However, Dr. Partridge seems to disregard direct evidence presented in our study that SMPs are a distinct subset of muscle satellite cells. We showed that most SMPs do express the canonical satellite cell marker Pax7, but that SMP markers (β1-integrin and CXCR4) are expressed by only ~80% of Pax7+ cells. Thus, we conclude that SMPs are a subpopulation of satellite cells, which unlike Pax3-GFP+ cells (Montarras et al., 2005) do not require specialized transgenic mouse strains for their isolation. More importantly, however, our work rigorously demonstrates distinctive functional and physiological properties of SMPs (Cerletti et al., 2008; Sherwood et al., 2004), an essential step in the effective characterization of any stem or progenitor cell population (Wagers and Weissman, 2004). In particular, we have shown that SMPs are the only subset of myofiber-associated cells that exhibits clonal myogenesis in vitro and a robust myogenic contribution in vivo. Our more recent experiments using intramuscular transplantation of single GFP+ SMPs further demonstrate that ~50% of muscles transplanted with a single SMP exhibit detectable myofiber engraftment (n = 10 muscles, M.C. and A.W., unpublished data). Given that previous studies indicate that 99% of myogenic donor cells perish almost immediately upon transplant (Beauchamp et al., 1999), these data clearly indicate that selection for SMP markers yields a unique, highly purified population of cells that is well suited to in vivo cell therapy. Response skeletal Muscle Precursor Grafts in Dystrophic Mice