Targeting of ALK2, a Receptor for Bone Morphogenetic Proteins, Using the Cre/lox System to Enhance Osseous Regeneration by Adipose-Derived Stem Cells.

Access to readily available autogenous tissue that regenerates bone would greatly improve clinical care. We believe the osteogenic phenotype caused by mutations in ALK2 can be harnessed in adipose-derived stem cells (ASCs) to improve bone tissue engineering. We set out to demonstrate that ALK2 may serve as a novel target to (a) improve in vitro ASC osteogenic differentiation and (b) enhance in vivo bone regeneration and calvarial healing. Transgenic mice were designed using the Cre/lox system to express constitutively active ALK2 (caALK2) with ubiquitously inducible Cre expression after tamoxifen exposure. ASCs from caALK2+/- and caALK2-/-(control) mice were exposed to tamoxifen and assessed for pro-osteogenic gene expression, bone morphogenetic protein (BMP) signaling, and osteogenic differentiation. Next, ASCs collected from these transgenic mice were analyzed in vivo using a calvarial defect model and analyzed by micro-computed tomography (micro-CT) and histology. ASCs from caALK2+/-mice had increased BMP signaling as demonstrated by upregulation of pSmad 1/5. ASCs from caALK2+/-mice had enhanced bone signaling and osteogenic differentiation compared with caALK2-/-mice (n=4, p<.05). Transcription of pro-osteogenic genes at day 7 was significantly higher in ASCs from caALK2-overexpressing mice (Alp, Runx2, Ocn, Opn) (n=4, p<.05). Using micro-CT and histomorphometry, we found that bone formation was significantly higher in mice treated with caALK2-expressing ASCs in vivo. Using a novel transgenic mouse model, we show that expression of constitutively active ALK2 receptor results in significantly increased ASC osteogenic differentiation. Furthermore, we demonstrate that this increased ASC differentiation can be harnessed to improve calvarial healing.