BMP-dependent, injury-induced stem cell niche as a mechanism of heterotopic ossification

Background: Heterotopic ossification (HO), either acquired (aHO) or hereditary, such as fibrodysplasia ossificans progressiva (FOP), is a serious condition without effective treatment. Understanding of the core process of injury-induced HO is still severely limited. Methods: Double-pulse thymidine analog labeling was used to explore the distinctive domains evolved in injury-induced lesions in an animal model of HO (Nse-BMP4). Histological studies were performed to see whether a similar zonal pattern is also consistently found in biopsies from patients with aHO and FOP. In vivo clonal analysis with Rainbow mice, genetic loss-of-function studies with diphtheria toxin A (DTA)-mediated depletion and lineage tracing with Zsgreen reporter mice were used to obtain further evidence that Tie2-cre-, Gli1-creERT-, and Glast-creERT-labeled cells contribute to HO as niche-dwelling progenitor/stem cells. Immunohistochemistry was used to test whether vasculature, neurites, macrophages, and mast cells are closely associated with the proposed niche and thus are possible candidate niche supportive cells. Similar methods also were employed to further understand the signaling pathways that regulate the niche and the resultant HO. Results: We found that distinctive domains evolved in injury-induced lesions, including, from outside-in, a mesenchymal stem cell (MSC) niche, a transient domain and an inner differentiated core in an animal model of HO (Nse-BMP4). A similar zonal structure was found in patients with aHO and FOP. In vivo clonal analysis with Rainbow mice and genetic loss-of-function studies with DTA provided evidence that Tie2-cre-, Gli1-creERT-, and Glast-creERT-labeled cells contribute to HO as niche-dwelling progenitor/stem cells; consistently, vasculature, neurites, macrophages, and mast cells are closely associated with the proposed niche and thus are possible candidate niche supportive cells. Further mechanistic study found that BMP and hedgehog (Hh) signaling co-regulate the niche and the resultant HO. Conclusions: Available data provide evidence of a potential core mechanism in which multiple disease-specific cellular and extracellular molecular elements form a unique local microenvironment, i.e., an injury-induced stem cell niche, which regulates the proliferation and osteogenic differentiation of mesenchymal stem cells (MSCs). The implication for HO is that therapeutic approaches must consider several different disease specific factors as parts of a functional unit, instead of treating one factor at a time.