Mesenchymal stem cell mechanosensing in engineered fibrillar microenvironments

ECM mechanics influence basic cellular functions (eg. spreading, proliferation and differentiation), thereby affecting biological processes in development, tissue homeostasis, and pathogenesis. Synthetic hydrogel matrices, given the precise control they afford, have been crucial to studying cellular mechanosensing. However, a major caveat to our understanding is the difference between widely studied hydrogel systems and the topographically and mechanically more complex ECM cells routinely reside within in vivo. In contrast to the flat expanse of cell adhesive ligand and linear elastic, continuum behavior of typical gel systems, within the body, cell-scale mechanics and ligand availability are entwined, both defined by the presence and organization of fibrillar proteins composing the ECM. Given these striking differences, an understanding of how cells sense the mechanics of their surroundings in fibrillar contexts remains an open challenge. To contrast how cells sense ECM mechanics within gels and fibrillar contexts, we designed a material processable into flat gels or suspended networks of electrospun fibers. The moduli of these materials are tunable and cell adhesivity is defined through the addition of RGD. Interestingly, we find that the well-described relationships between stiffness and cell spreading or proliferation identified in studies employing gels may not translate to fibrillar networks. Furthermore, active cellular forces resulted in visible deformations and reorganization of fibrous networks in a stiffness-dependent manner; such remodeling events do not occur in gels. Taken together, we describe a novel material system and initial studies towards a fundamental understanding of how cells probe their surroundings in more physiologic, fibrillar contexts.