Ectopic Expression of Delta FBJ Murine Osteosarcoma Viral Oncogene Homolog B Mediates Transdifferentiation of Adipose-like Spheroids into Osteo-like Microtissues

Differentiation and transdifferentiation strategies have a large role in the manipulation of cells in replacing dysfunctional cells and tissues. We developed adipose-like microtissues using gravity-enforced self-assembly of monodispersed human primary preadipocytes to determine their transdifferentiation capacity to form bone-like tissues. Using lentivirus-derived particles to induce ectopic bone morphogenetic protein (BMP)-2 and delta FBJ murine osteosarcoma viral oncogene homolog B (ΔFosB) gene expression, we demonstrated a time-dependent induction of osteoblast-specific genes and properties such as calcium deposits, bone-like extracellular matrix (ECM), and matrix mineralization. ΔFosB was able to trigger partial Pref-1-mediated de-differentiation of adipocytes, which also retained their adipocytic cell phenotype. Osteoblast-specific structures could be co-localized in the ECM of lipid-containing cells analyzed using immunofluorescence and transmission electron microscopy when BMP-2 and ΔFosB were co-expressed, suggesting that differentiated adipocytes are able to transdifferentiate into osteoblasts via a transient hybrid adipocyte-preadipocyte-osteoblast cell phenotype. Microtissues transgenic for BMP-2 and ΔFosB expression were able to reproduce bone matrix, which occurs to a lesser extent in conventional two-dimensional (2D) cultures but is known to play a decisive role in the development and function of bone in vivo . This demonstrates that ECM-inclusive studies are essential for future characterization assays. Therefore, 3D cultures provide a superior ex vivo system for the improved characterization of phenotypical and functional alterations resulting from interventions directed toward differentiation processes. Precise control of transdifferentiation of adipocytes into osteoblasts in a 3D culture mimicking in vivo tissue conditions as closely as possible will foster important advances in regenerative medicine and tissue engineering.