Mechanical and Topographical Guidance of Stem Cell Differentiation in Vitro Via YAP-Dependent Mechanotransduction: A Biomimetic Approach Toward Annulus Fibrosus Regeneration

The current engineered bionic disc tissue is considered as a potential way for treating intervertebral disc degeneration disease, which yet still remains challenging due to the complex radial gradient of natural annulus fibrosus (AF) tissue in cell phenotype, biochemical composition, topographical features, and mechanical properties. Previously, we have found that the differentiation of annulus fibrosus-derived stem cells (AFSCs) could be regulated by the elasticity of scaffold. In this study, we attempted to examine the combined effect of both mechanical and topographical features on the gene expression of AFSCs. Cells were cultured on four types of poly(ether carbonate urethane) urea (PECUU) scaffolds with controlled elasticity and fiber size (soft, small size; stiff, small size; soft, large size and stiff, large size). Results showed that when the fiber size of scaffold was kept constant, the expression of collagen-I (Col-I) in AFSCs increased with scaffold elasticity, while the expression of collagen-II (Col-II) and aggrecan genes showed an opposite trend. Moreover, when scaffold elasticity was controlled, the gene expression of Col-I in AFSCs increased with fiber size. In contrast, the expression of Col-II and aggrecan decreased. Such substrate elasticity and topography dependent changes of AFSC were similar to the gradient characteristics of native AF tissue. In addition, increasing elasticity and fiber size of scaffolds promoted YAP activation and its nuclear translocation. The results illustrate unambiguously that the mechanical property is a potent regulator of AFSC differentiation. Moreover, we revealed that fiber size of scaffold significantly affects spreading area, focal adhesion and differentiation of AFSCs. Therefore, both mechanical property and topography features of scaffolds regulate AFSC differentiation, possibly through a YAP-dependent mechanotransduction mechanism.