Mesenchymal stem cells can give rise to several cell types, but variations depending on isolation method and tissue source have led to controversies about their usefulness in clinical medicine. Here we show that vascular endothelial cells can transform into multipotent stem-like cells by an ALK2 receptor-dependent mechanism. In lesions from patients with Fibrodysplasia Ossificans Progressiva, a disease where heterotopic ossification occurs as a result of activating ALK2 mutations, or from a mutant ALK2 transgenic mouse model, chondrocytes and osteoblasts express endothelial markers. Tie2-Cre lineage tracing also suggests an endothelial origin of these cells. Expressing mutant ALK2 in endothelial cells, or treatment with the ALK2 ligands TGF-β2 or BMP4, causes endothelial-mesenchymal transition and acquisition of a stem cell-like phenotype. In selective media, these cells differentiate into osteoblasts, chondrocytes, or adipocytes. The process is inhibited by ALK2-specific siRNA. Conversion of endothelial cells to stem-like cells may provide a novel approach to tissue engineering.

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Heterotopic ossification (HO) is a disabling condition associated with neurologic injury, inflammation, and overactive BMP signaling. The inductive factors involved in lesion formation are unknown. We found that the expression of the neuro-inflammatory factor Substance P (SP) is dramatically increased in early lesional tissue in patients who have either fibrodysplasia ossificans progressiva (FOP) or acquired HO, and in three independent mouse models of HO. In Nse-BMP4, a mouse model of HO, robust HO forms in response to tissue injury; however null mutations of the preprotachykinin gene encoding SP prevent HO. Importantly, ablation of SP+ sensory neurons, treatment with an antagonist of SP receptor NK1r, deletion of NK1r gene, or genetic down-regulation of NK1r-expressing mast cells also profoundly inhibits injury-induced HO. These observations establish a potent neuro-inflammatory induction and amplification circuit for BMP-dependent HO lesion formation, and identify novel molecular targets for prevention of HO.

Hypoxia and inflammation are implicated in the episodic induction of heterotopic endochondral ossification (HEO); however, the molecular mechanisms are unknown. HIF-1α integrates the cellular response to both hypoxia and inflammation and is a prime candidate for regulating HEO. We investigated the role of hypoxia and HIF-1α in fibrodysplasia ossificans progressiva (FOP), the most catastrophic form of HEO in humans. We found that HIF-1α increases the intensity and duration of canonical bone morphogenetic protein (BMP) signaling through Rabaptin 5 (RABEP1)-mediated retention of Activin A receptor, type I (ACVR1), a BMP receptor, in the endosomal compartment of hypoxic connective tissue progenitor cells from patients with FOP. We further show that early inflammatory FOP lesions in humans and in a mouse model are markedly hypoxic, and inhibition of HIF-1α by genetic or pharmacologic means restores canonical BMP signaling to normoxic levels in human FOP cells and profoundly reduces HEO in a constitutively active Acvr1Q207D/+ mouse model of FOP. Thus, an inflammation and cellular oxygen-sensing mechanism that modulates intracellular retention of a mutant BMP receptor determines, in part, its pathologic activity in FOP. Our study provides critical insight into a previously unrecognized role of HIF-1α in the hypoxic amplification of BMP signaling and in the episodic induction of HEO in FOP, and further identifies HIF-1α as a therapeutic target for FOP and perhaps non-genetic forms of HEO.

Phenotypic heterogeneity has been observed among mesenchymal stem/stromal cell (MSC) populations, but specific genes associated with this variability have not been defined. To study this question, we analyzed two distinct isogenic MSC populations isolated from umbilical cord blood (UCB1 and UCB2). The use of isogenic populations eliminated differences contributed by genetic background. We characterized these UCB MSCs for cell morphology, growth kinetics, immunophenotype, and potential for differentiation. UCB1 displayed faster growth kinetics, higher population doublings, and increased adipogenic lineage differentiation compared to UCB2. However, osteogenic differentiation was stronger for the UCB2 population. To identify MSC-specific genes and developmental genes associated with observed phenotypic differences, we performed expression analysis using Affymetrix microarrays and compared them to bone marrow (BM) MSCs. We compared UCB1, UCB2, and BM and identified distinct gene expression patterns. Selected clusters were analyzed demonstrating that genes of multiple developmental pathways, such as transforming growth factor-beta (TGF-beta) and wnt genes, and markers of early embryonic stages and mesodermal differentiation displayed significant differences among the MSC populations. In undifferentiated UCB1 cells, multiple genes were significantly up-regulated (p < 0.0001): peroxisome proliferation activated receptor gamma (PPARG), which correlated with adipogenic differentiation capacities, hepatocyte growth factor (HGF), and stromal-derived factor 1 (SDF1/CXCL12), which could both potentially contribute to the higher growth kinetics observed in UCB1 cells. Overall, the results confirmed the presence of two distinct isogenic UCB-derived cell populations, identified gene profiles useful to distinguish MSC types with different lineage differentiation potentials, and helped clarify the heterogeneity observed in these cells.