Mechanical regulation of direct cell reprogramming

Direct conversion of differentiated cells into a completely different lineage has wide applications in regenerative medicine, disease modeling and drug screening. Although the roles of transcriptional factors and chemical compounds in direct reprogramming have been widely studied, the effects of extracellular matrix (ECM) and substrate stiffness on the reprogramming process are not well understood. Here we used induced neuronal reprogramming as a model system to investigate how the mechanical property of the ECM modulates this process. Transcriptional factors Brn2, Ascl1 and Myt1L were transduced into fibroblasts to generate induced neurons (iNs), but the efficiency was low, especially for adult cells. Culturing cells on soft substrates enhanced reprogramming efficiency. Surprisingly, we found a biphasic dependence with an intermediate stiffness promoting the most efficient direct conversion of fibroblasts into iNs, suggesting that the mechanical regulation of reprogramming process may have totally different mechanisms from stem cell differentiation. Interestingly, this stiffness effect was independent of the chemical components of ECM proteins. Further investigations suggest that several signaling pathways, cytoskeleton and nuclear matrix are involved in the mechanotransduction to cell reprogramming. Our findings provide insights into the roles of substrate stiffness in regulating direct reprogramming and the underlying mechanism, which will open a new avenue for the rational design of smart biomaterials for cell reprogramming.