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Identification of Progenitor Cells That Contribute to Heterotopic Skeletogenesis

¹ Departments of Orthopaedic Surgery (V.Y.L., E.M.S., D.L.G., and F.S.K.), Genetics (E.M.S.), Medicine (F.S.K.), and Radiology (A.D.A.M.), and the Center for Research in FOP and Related Disorders (V.Y.L., E.M.S., D.L.G., and F.S.K.), the University of Pennsylvania School of Medicine, Hospital of the University of Pennsylvania, Silverstein Pavilion, 2nd Floor, 3400 Spruce Street, Philadelphia, PA 19104. E-mail address for F.S. Kaplan: Frederick.Kaplan{at}uphs.upenn.edu
² Department of Pathology, University of California School of Medicine, Box 0102, 505 Parnassus Avenue, San Francisco, CA 94143
³ The Center for Regenerative Biology, Department of Molecular and Cell Biology, Advanced Technology Laboratory, University of Connecticut, 1392 Storrs Road, Storrs, CT 06269. E-mail address for D.J. Goldhamer: david.goldhamer{at}uconn.edu

Investigation performed at the University of Pennsylvania, Philadelphia, Pennsylvania, and the University of Connecticut, Storrs, Connecticut

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the International Fibrodysplasia Ossificans Progressiva Association, the Center for Research in FOP and Related Disorders, the Ian Cali Endowment, the Weldon Family Endowment, the Rita Allen Foundation, the Isaac and Rose Nassau Professorship of Orthopaedic Molecular Medicine, the Orthopaedic Research and Education Foundation Zachary Friedenberg Clinician-Scientist Award, and the National Institutes of Health (R01-AR41916 and R01-AG20911). Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Methods: We used Cre/loxP lineage tracing methods in the mouse to identify cell lineages that contribute to all stages of heterotopic ossification. Specific cell populations were permanently labeled by crossing lineage-specific Cre mice with the Cre-dependent reporter mice R26R and R26R-EYFP. Two mouse models were used to induce heterotopic ossification: (1) intramuscular injection of BMP2/Matrigel and (2) cardiotoxin-induced skeletal muscle injury in transgenic mice that misexpress BMP4 at the neuromuscular junction. The contribution of labeled cells to fibroproliferative lesions, cartilage, and bone was evaluated histologically by light and fluorescence microscopy. The cell types evaluated as possible progenitors included skeletal muscle stem cells (MyoD-Cre), endothelium and endothelial precursors (Tie2-Cre), and vascular smooth muscle (Smooth Muscle Myosin Heavy Chain-Cre [SMMHC-Cre]).

Results: Vascular smooth muscle cells did not contribute to any stage of heterotopic ossification in either mouse model. Despite the osteogenic response of cultured skeletal myoblasts to BMPs, skeletal muscle precursors in vivo contributed minimally to heterotopic ossification (<5%), and this contribution was not increased by cardiotoxin injection, which induces muscle regeneration and mobilizes muscle stem cells. In contrast, cells that expressed the vascular endothelial marker Tie2/Tek at some time in their developmental history contributed robustly to the fibroproliferative, chondrogenic, and osteogenic stages of the evolving heterotopic endochondral anlagen. Importantly, endothelial markers were expressed by cells at all stages of heterotopic ossification. Finally, muscle injury and associated inflammation were sufficient to trigger fibrodysplasia ossificans progressiva-like heterotopic ossification in a setting of chronically stimulated BMP activity.

Conclusions: Tie2-expressing progenitor cells, which are endothelial precursors, respond to an inflammatory trigger, differentiate through an endochondral pathway, contribute to every stage of the heterotopic endochondral anlagen, and form heterotopic bone in response to overactive BMP signaling in animal models of fibrodysplasia ossificans progressiva. Thus, the ectopic skeleton is not only supplied by a rich vasculature, but appears to be constructed in part by cells of vascular origin. Further, these data strongly suggest that dysregulation of the BMP signaling pathway and an inflammatory microenvironment are both required for the formation of fibrodysplasia ossificans progressiva-like lesions.

Clinical Relevance: These cell lineage tracing studies provide new insight into the cellular pathophysiology of heterotopic ossification. Therapeutic regulation of specific cell lineages involved in BMP-induced heterotopic ossification holds promise for the treatment of fibrodysplasia ossificans progressiva and possibly other more common disorders of heterotopic ossification.

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