Lamin-A is Mechanosensitive to Matrix Stiffness and Couples to the Retinoic Acid Pathway in Differentiation

Abstract

Matrix elasticity helps direct lineage specification of human bone marrow-derived MSCs in culture toward bone, fat, or other tissue types with mechanisms based in part on myosin-II generated stresses. We found with both gel systems and nano-collagen films that matrix rigidity leads to higher levels of nucleoskeletal protein lamin-A. Stem cell differentiation into fat on soft matrix was enhanced by low lamin-A levels, while differentiation into bone on stiff matrix was enhanced by high lamin-A levels. Our results show that lamin-A transcription was regulated by the vitamin A/retinoic acid (RA) pathway that plays a role in development and regeneration but lamin-A protein regulates nuclear translocation of the RA receptor gamma isoform (RARG), increasing fourfold from soft to stiff matrix and suppressed by lamin-A knockdown. We also overexpressed the membrane protein SUN2, which shuttles from the endoplasmic reticulum (ER) to the inner nuclear envelope, where it cross-links nesprins and the cytoskeleton to lamin-A (based on co-IP). SUN2 overexpression appears to saturate cross-linking sites, leading to nuclear rounding, decreased lamin-A levels, and higher RARG in the cytoplasm. Based on RA pathway effects on lamin-A expression in cells on stiff matrix, we expected antagonists to RA and specifically RARG to increase osteogenesis, and we confirm this. Moreover, the increased osteogenesis with antagonists was nullified by lamin-A knockdown, consistent with a specific role in stiff tissue differentiation.