Wnt 3a promotes proliferation and suppresses osteogenic differentiation of adult human mesenchymal stem cells

Multipotential adult mesenchymal stem cells (MSCs) are able to differentiate along several known lineages, and lineage commitment is tightly regulated through specific cellular mediators and interactions. Recent observations of a low/high bone-mass phenotype in patients expressing a loss-/gain-of-function mutation in LRP5, a coreceptor of the Wilt family of signaling molecules, Suggest the importance of Writ signaling in bone formation, possibly involving MSCs. To analyze the role of Wilt signaling in mesenchymal osteogenesis, we have profiled the expression of WNTs and their receptors, FRIZZLEDs(FZDs), and several secreted Writ inhibitors, such as SFRPs, and examined the effect of Wnt 3a, as a representative canonical Wnt member, during MSC osteogenesis in vitro. WNT11, FZD6, SFRP2, and SFRP3 are upregulated during MSC osteogenesis, while WNT9A and FZD7 are downregulated. MSCs also respond to exogenous Wnt 3a, based oil increased beta-catenin nuclearization and activation of a Wnt-responsive promoter, and the magnitude of this response depends on the MSC differentiation state. Writ 3a exposure inhibits MSC osteogenic differentiation, with decreased matrix mineralization and reduced alkaline phosphatase mRNA and activity. Wnt 3a treatment of fully osteogenically differentiated MSCs also Suppresses osteoblastic marker gene expression. The Wilt 3a effect is accompanied by increased cell number, resulting from both increased proliferation and decreased apoptosis, particularly during expansion of undifferentiated MSCs. The osteo-suppressive effects of Writ 3a are fully reversible, i.e., treatment prior to osteogenic induction does not compromise subsequent MSC osteogenesis. The results also showed that sFRP3 treatment attenuates some of the observed Wnt 3a effects on MSCs, and that inhibition of canonical Wilt signaling using a dominant negative TCF1 enhances MSC osteogenesis. Interestingly, expression of Wnt 5a, a non-canonical Wilt member, appeared to promote osteogenesis. Taken together, these findings suggest that canonical Writ signaling functions in maintaining an undifferentiated, proliferating progenitor MSC population, whereas non-canonical Wilts facilitate osteogenic differentiation. Release from canonical Wnt regulation is a prerequisite for MSC differentiation. Thus, loss-/gain-of-function Mutations of LRP5 would perturb Writ signaling and depress/promote bone formation by affecting the progenitor cell pool. Elucidating Wnt regulation of MSC differentiation is important for their potential application in tissue regeneration. Published 2004 Wiley-Liss, Inc.(dagger)