Signal mingle: Micropatterns of BMP-2 and fibronectin on soft biopolymeric films regulate myoblast shape and SMAD signaling

International audienceIn vivo, bone morphogenetic protein 2 (BMP-2) exists both in solution and bound to the extracellular matrix (ECM). While these two modes of presentation are known to influence cell behavior distinctly, their role in the niche microenvironment and their functional relevance in the genesis of a biological response has sparsely been investigated at a cellular level. Here we used the natural affinity of BMP-2 for fibronectin (FN) to engineer cell-sized micropatterns of BMP-2. This technique allowed the simultaneous control of the spatial presentation of fibronectin-bound BMP-2 and cell spreading. These micropatterns induced a specific actin and adhesion organization around the nucleus, and triggered the phosphorylation and nuclear translocation of SMAD1/5/8 in C2C12 myoblasts and mesenchymal stem cells, an early indicator of their osteoblastic trans-differentiation. We found that cell spreading itself potentiated a BMP-2-dependent phosphorylation of SMAD1/5/8. Finally, we demonstrated that FN/BMP-2-mediated early SMAD signaling depended on LIM kinase 2 and ROCK, rather than myosin II activation. Altogether, our results show that FN/BMP-2 micropatterns are a useful tool to study the mechanisms underlying BMP-2-mediated mechanotransduction. More broadly, our approach could be adapted to other combinations of ECM proteins and growth factors, opening an exciting avenue to recreate tissue-specific niches in vitro. Due to their physiological relevance, bone morphogenetic proteins (BMPs) are widely studied for orthopedic clinical applications to enhance the healing of large bone defects 1,2 , as well as for developing new strategies in bone tissue engineering 3-5. BMPs are indeed highly potent growth factors (GFs) that play a crucial role in mor-phogenesis and tissue homeostasis during embryonic development and until adulthood 6,7. In particular, BMP-2 promotes the differentiation of mesenchymal stem cells (MSCs) and osteoblasts toward osteocytes 8,9 , and induces the trans-differentiation of myoblasts into osteoblasts 10. In addition, BMP-2 in solution plays a role in early adhesive events, including adhesion and migration through cytoskeletal reorganization 11,12. Recently, several studies have demonstrated that BMP-2 strongly interacts with extra-cellular matrix (ECM) proteins 13 , especially fibronectin (FN) due to its highly promiscuous GF-binding site (the 12th to 14th type III repeats) 14,15. Moreover, immobilized BMP-2 whether by physical adsorption (i.e. matrix-bound BMP-2 13,16) or by covalent grafting 17 was shown to regulate cell behavior quite distinctly from BMP-2 in solution. This effect is currently poorly known and is likely due to the close proximity and crosstalk of integrin-binding domains of FN and BMP-2 13,17-19. It has indeed been shown that the secretion of FN by cells is necessary for BMP-2-mediated signaling 13. A consequence of this association of BMP-2 with ECM proteins in vivo is that the spatially patterned presentation of BMPs b