Scleraxis-Lineage Cells Contribute to Ectopic Bone Formation in Muscle and Tendon

Abstract

The pathologic development of heterotopic ossification (HO) is well described in patients with extensive trauma or with hyperactivating mutations of the bone morphogenetic protein (BMP) receptor ACVR1. However, identification of progenitor cells contributing to this process remains elusive. Here we show that connective tissue cells contribute to a substantial amount of HO anlagen caused by trauma using post-natal, tamoxifen-inducible, scleraxis-lineage restricted reporter mice (Scx-creERT2/tdTomatofl/fl). When the scleraxis-lineage is restricted specifically to adults prior to injury marked cells contribute to each stage of the developing HO anlagen and co-express markers of endochondral ossification (Osterix, SOX9). Furthermore, these adult pre-injury restricted cells co-expressed mesenchymal stem cell markers including PDGFRα, Sca1, and S100A4 in HO. When constitutively active ACVR1 (caACVR1) was expressed in scx-cre cells in the absence of injury (Scx-cre/caACVR1fl/fl), tendons and joints formed HO. Post-natal lineage-restricted, tamoxifen-inducible caACVR1 expression (Scx-creERT2/caACVR1fl/fl) was sufficient to form HO after directed cardiotoxin-induced muscle injury. These findings suggest that cells expressing scleraxis within muscle or tendon contribute to HO in the setting of both trauma or hyperactive bone morphogenetic protein receptor (e.g. caACVR1) activity.

Novel Model of Tendon Regeneration Reveals Distinct Cell Mechanisms Underlying Regenerative and Fibrotic Tendon Healing

“…The specific localization of ScxGFP re-expression and aberrant cartilage formation to the tendon stubs suggest that these events may be interrelated, and indeed we found Scx lin cells within the cartilage masses, consistent with two recent studies that also show a contribution of tenocytes to heterotopic ossification, either in the context of injury or via constitutive activation of the BMP receptor, ACVR13233. During embryonic development, an early pool of bipotent Scx + / Sox9 + progenitors give rise to either the cartilage or tendon components of the skeletal enthesis3435.…”

Activin-A enhances mTOR signaling to promote aberrant chondrogenesis in fibrodysplasia ossificans progressiva

“…In FOP, however, we found what we believe to be a new signaling pathway for Activin-A that initiates HO formation by mistransducing BMP signaling. MSCs, which are cells of origin of HO in FOP (67,68), respond abnormally to an Activin-A-evoked signal by taking on an osteo/chondrogenic lineage that is consistent with the response to bone fracture. Because Activin-A activates mTOR signaling in FOP, this response by MSCs could be related to the alert state.…”

Stem cells and heterotopic ossification: Lessons from animal models

“…However, lineage tracing[41,42,65,93] and transplantation[65] studies clearly demonstrated that satellite cells do not contribute to BMP-induced HO in vivo. Further, targeted expression of caACVR1[38,94] or ACVR1(R206H)[70] in satellite cells is not sufficient to induce HO. Collectively, these studies strongly argue against a direct role for satellite cells in acquired or genetic forms of HO, and illustrate the need for caution when interpreting results from cell culture systems, which may represent relatively permissive environments that are not necessarily reflective of in vivo settings.…”

“…Howell et al[66] observed that Scx-expressing cells give rise to cartilage following tendonectomy. Similarly, Scx-lineage-labeled cells accounted for ~40% of definitive cartilage and ~25% of definitive intratendinous bone following burn/tendonectomy[94]. That an apparent minority of Scx+ cells contributed to tendon HO suggests either the involvement of additional progenitor cell populations or lineage labeling of only a fraction of total Scx+ cells.…”

“…Dey et al[38] proposed that Scx-expressing HO progenitors are anatomically restricted to tendons and ligaments, whereas Agarwal et al reported Scx-Cre-lineage-labeled cells within muscle tissue, and injury-induced, intramuscular HO was observed when caACVR1 expression was induced by a ScxCreER T2 driver[94]. Both groups demonstrated spontaneous intratendinous HO when caACVR1 expression was driven by Scx-Cre[38,94]. Interestingly, however, Dey et al reported that heterotopic cartilage, but not bone, was lineage-labeled[38].…”