Abstract 2107: GSK-3α and GSK-3β Play Contrasting Roles in Mediating Cardiomyocyte Differentiation in Bone Marrow Derived Mesenchymal Stem Cells
Signaling mechanisms facilitating cardiomyocyte (CM) differentiation from bone marrow (BM)-derived mesenchymal stem cells (MSCs) are not well understood. We have shown previously that glycogen synthase kinase-3β (GSK-3β) facilitates expression of CM marker genes in mouse BM-MSCs. GSK-3 has two isoforms, GSK-3α and GSK-3β, which have 97% identical amino acids in the catalytic domain but differ substantially in the N- and C- terminals. Increasing lines of evidence suggest that GSK-3α and GSK-3β play distinct roles in cell growth/differentiation. We investigated the role of GSK-3 isoforms in mediating differentiation of MSCs. Although overexpression of GSK-3β in MSCs potently induced mRNA and protein expression of markers of CM differentiation, including Nkx2.5, cardiac troponin C and atrial natriuretic factor, that of GSK-3α exhibited a negligible effect upon cardiomyocyte differentiation. Instead, GSK-3α induced expression of nestin, a marker of neuronal differentiation, and Sox9, a marker of chondrocyte differentiation, though GSK-3β did not. Downregulation of GSK-3α by shRNA-GSK-3α induced expression of CM markers, whereas downregulation of GSK-3β by shRNA-GSK-3β or shRNA-scramble (control) did not induce CM markers. 5-Aza-induced expression of CM markers was abolished in the presence of shRNA-GSK-3β but was enhanced by shRNA-GSK-3α. GSK-3β-induced CM differentiation in MSCs was accompanied by downregulation of β-catenin, and it was abolished when downregulation of β-catenin was reversed by forced expression of β-catenin. On the other hand, CM differentiation due to GSK-3α knockdown was not accompanied by significant changes in β-catenin levels, but did exhibit an additive effect upon GSK-3β-induced CM differentiation. These results suggest that GSK-3β promotes, while GSK-3α inhibits CM differentiation in BM-derived MSCs. On the other hand, GSK-3α promotes, while GSK-3β inhibits MSC differentiation into non-myocyte lineages, including neuronal and chondrocyte lineages. Furthermore, GSK-3β stimulates CM differentiation through downregulation of β-catenin, whereas GSK-3α inhibits CM differentiation via a β-catenin-independent mechanism in BM-derived MSCs.