3D printing of osteocytic Dll4 integrated with PCL for cell fate determination towards osteoblasts in vitro

Since 3D printed hard materials could match the shape of bone, cell survival and fate determination towards osteoblasts in such materials have become a popular research target. In this study, a scaffold of hard material for 3D fabrication was designed to regulate developmental signal (Notch) transduction guiding osteoblast differentiation. We established a polycaprolactone (PCL) and cell-integrated 3D printing system (PCI3D) to reciprocally print the beams of PCL and cell-laden hydrogel for a module. This PCI3D module holds good cell viability of over 87%, whereas cells show about sixfold proliferation in a 7-day culture. The osteocytic MLO-Y4 was engineered to overexpress Notch ligand Dll4, making up 25% after mixing with 75% stromal cells in the PCI3D module. Osteocytic Dll4, unlike other delta-like family members such as Dll1 or Dll3, promotes osteoblast differentiation and the mineralization of primary mouse and a cell line of bone marrow stromal cells when cultured in a PCI3D module for up to 28 days. Mechanistically, osteocytic Dll4 could not promote osteogenic differentiation of the primary bone marrow stromal cells (BMSCs) after conditional deletion of the Notch transcription factor RBPjκ by Cre recombinase. These data indicate that osteocytic Dll4 activates RBPjκ-dependent canonical Notch signaling in BMSCs for their oriented differentiation towards osteoblasts. Additionally, osteocytic Dll4 holds a great potential for angiogenesis in human umbilical vein endothelial cells within modules. Our study reveals that osteocytic Dll4 could be the osteogenic niche determining cell fate towards osteoblasts. This will open a new avenue to overcome the current limitation of poor cell viability and low bioactivity of traditional orthopedic implants.