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

Abstract

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 Wnt family of signaling molecules, suggest the importance of Wnt signaling in bone formation, possibly involving MSCs. To analyze the role of Wnt signaling in mesenchymal osteogenesis, we have profiled the expression of WNTs and their receptors, FRIZZLEDs (FZDs), and several secreted Wnt 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 on increased beta-catenin nuclearization and activation of a Wnt-responsive promoter, and the magnitude of this response depends on the MSC differentiation state. Wnt 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 Wnt 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 Wnt 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 Wnt signaling using a dominant negative TCF1 enhances MSC osteogenesis. Interestingly, expression of Wnt 5a, a non-canonical Wnt member, appeared to promote osteogenesis. Taken together, these findings suggest that canonical Wnt signaling functions in maintaining an undifferentiated, proliferating progenitor MSC population, whereas non-canonical Wnts facilitate osteogenic differentiation. Release from canonical Wnt regulation is a prerequisite for MSC differentiation. Thus, loss-/gain-of-function mutations of LRP5 would perturb Wnt 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.

Predominant role of PDGF receptor transactivation in Wnt3a-induced osteoblastic cell proliferation

“…In vitro studies have shown that Wnt3a promotes the proliferation and suppresses the differentiation of adult human mesenchymal stem cells. (27,28) Whether this effect is also observed in more differentiated osteoblastic cells is unknown, and the molecular mechanism involved in this response has not been investigated. As shown in Fig.…”

“…Studies using various in vitro systems have suggested that the Wnt/b-catenin pathway promotes osteoblastogenesis and chondrogenesis. (12,(24)(25)(26) In contrast, other studies have reported that Wnts either enhance the proliferation and/or suppress the differentiation of mesenchymal progenitor cells, (27)(28)(29) preosteoblasts, (30) or cementoblasts. (31) Other studies in Wnt signaling revealed a more complex transduction network than previously thought.…”

Atherosclerotic Calcification: Wnt Is the Hint

“…Interestingly, the chromosomal location of Wnt11 to 11q13.5 is near previously linked markers of high bone mass 138, 139. In addition, Wnt11 expression is upregulated during mesenchymal osteogenesis 140. Wnt11 activation has been associated with bone canonical Wnt/β‐catenin signaling and noncanonical signaling 141.…”

“…Canonical Wnt ligands Wnt3a and Wnt9a promote MSC proliferation and maintenance of the progenitor cell pool. Meanwhile, noncanonical signaling, mediated by Wnt5a and Wnt11, is upregulated during MSC osteogenic and chondrogenic differentiation144 and is involved in regulating and promoting MSC differentiation 140. In this model, MSCs are maintained in the progenitor state by canonical signaling and will undergo differentiation on an exogenous signal involving noncanonical Wnt signaling that will release the cells from suppression of differentiation.…”

“…In this model, MSCs are maintained in the progenitor state by canonical signaling and will undergo differentiation on an exogenous signal involving noncanonical Wnt signaling that will release the cells from suppression of differentiation. These cells are then able to undergo osteogenic, chondrogenic, or adipogenic lineage commitment 140. This model suggests that Wnt/β‐catenin signaling can promote osteogenesis by expansion of the progenitor cell compartment 140…”