Extracellular Matrix and αβ Integrin Signaling Control the Maintenance of Bone Formation Capacity by Human Adipose-derived Stromal Cells

Stromal vascular fraction (SVF) cells of human adipose tissue have the capacity to generate osteogenic grafts with intrinsic vasculogenic properties. However, adipose-derived stromal/stem cells (ASC), even after minimal monolayer expansion, display poor osteogenic capacity in vivo. We investigated whether ASC bone-forming capacity may be maintained by culture within a self-produced extracellular matrix (ECM) that recapitulates the native environment. SVF cells expanded without passaging up to 28 days (Unpass-ASC) deposited a fibronectin-rich extracellular matrix and displayed greater clonogenicity and differentiation potential in vitro compared to ASC expanded only for 6 days (P0-ASC) or for 28 days with regular passaging (Pass-ASC). When implanted subcutaneously, Unpass-ASC produced bone tissue similarly to SVF cells, in contrast to P0- and Pass-ASC, which mainly formed fibrous tissue. Interestingly, clonogenic progenitors from native SVF and Unpass-ASC expressed low levels of the fibronectin receptor α integrin (CD49e), which was instead upregulated in P0- and Pass-ASC. Mechanistically, induced activation of αβ integrin in Unpass-ASC led to a significant loss of bone formation in vivo. This study shows that ECM and regulation of αβ-integrin signaling preserve ASC progenitor properties, including bone tissue-forming capacity, during in vitro expansion.