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TISSUE-SPECIFIC STEM CELLS

Prospective Isolation of Skeletal Muscle Stem Cells with a Pax7 Reporter

^aLillehei Heart Institute and Department of Pediatrics, University of Minnesota, Minneapolis, Minnesota, USA;
Departments of ^bDevelopmental Biology and
^cMolecular Biology, University of Texas Southwestern Medical Center, Dallas, Texas, USA

Key Words. Pax7 • Experimental models • Muscle stem cells • Muscular dystrophy

Correspondence: Correspondence: Michael Kyba, Ph.D., Lillehei Heart Institute and Department of Pediatrics, 4-126 Nils Hasselmo Hall, 312 Church Street SE, Minneapolis, Minnesota 55455, USA. Telephone: 612-626-5869; Fax: 612-624-8118; e-mail: kyba{at}umn.edu

Received on December 4, 2007; accepted for publication on September 3, 2008.

First published online in STEM CELLS EXPRESS  September 18, 2008.

Muscle regeneration occurs through activation of quiescent satellite cells whose progeny proliferate, differentiate, and fuse to make new myofibers. We used a transgenic Pax7-ZsGreen reporter mouse to prospectively isolate stem cells of skeletal muscle by flow cytometry. We show that Pax7-expressing cells (satellite cells) in the limb, head, and diaphragm muscles are homogeneous in size and granularity and uniformly labeled by certain cell surface markers, including CD34 and CD29. The frequency of the satellite cells varies between muscle types and with age. Clonal analysis demonstrated that all colonies arising from single cells within the Pax7-sorted fraction have myogenic potential. In response to injury, Pax7⁺ cells reduce CD34, CD29, and CXCR4 expression, increase in size, and acquire Sca-1. When directly isolated and cultured in vitro, Pax7⁺ cells display the hallmarks of activation and proliferate, initially as suspension aggregates and later distributed between suspension and adherence. During in vitro expansion, Pax7 (ZsGreen) and CD34 expression decline, whereas expression of PSA-NCAM is acquired. The nonmyogenic, Pax7^neg cells expand as Sca1⁺ PDGR⁺ PSA-NCAM^neg cells. Satellite cells expanded exclusively in suspension can engraft and produce dystrophin⁺ fibers in mdx^–/– mice. These results establish a novel animal model for the study of muscle stem cell physiology and a culture system for expansion of engraftable muscle progenitors.

Disclosure of potential conflicts of interest is found at the end of this article.