Regulation of Induced Pluripotent Stem (iPS) Cell Induction by Wnt/β-Catenin Signaling

Wnt signaling has been implicated in promoting somatic cell reprogramming. However, its molecular mechanisms remain unknown. Here we report that Wnt/β-catenin enhances iPSCs induction at the early stage of reprogramming. The augmented reprogramming induced by β-catenin is not due to increased total cell population or activation of c-Myc. In addition, β-catenin interacts with reprogramming factors Klf4, Oct4, and Sox2, further enhancing expression of pluripotency circuitry genes. These studies reveal novel mechanisms underlying the regulation of reprogramming somatic cells to pluripotency by Wnt/β-catenin signaling.     

Smurf1-mediated Axin Ubiquitination Requires Smurf1 C2 Domain and Is Cell Cycle-dependent

Previously, Smad ubiquitination regulatory factor 1 (Smurf1)-mediated Lys29 (K29)-linked poly-ubiquitination of Axin has been identified as a novel regulatory process in Wnt/β-catenin signaling. In this work, we discovered that the C2 domain of Smurf1 is critical for targeting Axin for ubiquitination. We found that the C2 domain-mediated plasma membrane localization of Smurf1 is required for Axin ubiquitination, and interfering with that disturbs the co-localization of Smurf1 and Axin around the plasma membrane. Moreover, the C2 domain of Smurf1, rather than its WW domains, is involved in Smurf1''s interaction with Axin; and the putative PPXY motifs (PY motif) of Axin are not essential for such an interaction, indicating that Smurf1 binds to Axin in a non-canonical way independent of WW-PY interaction. Further, we found that Smurf1-Axin interaction and Axin ubiquitination are attenuated in the G2/M phase of cell cycle, contributing to an increased cell response to Wnt stimulation at that stage. Collectively, we uncovered a dual role of Smurf1 C2 domain, recruiting Smurf1 to membrane for accessing Axin and mediating its interaction with Axin, and that Smurf1-mediated Axin ubiquitination is subjected to the regulation of cell cycle.     

Wnt signaling and skeletal development

Wnt proteins are a family of secreted proteins that regulate many aspects of cellular functions. The discovery that mutations in low-density lipoprotein receptor-related protein 5, a putative Wnt coreceptor, could positively and negatively affect bone mass in humans generated an enormous amount of interest in the possible role of the Wnt signaling pathway in skeletal biology. Over the last decade, considerable progress has been made in determining the role of the canonical Wnt signaling pathway in various aspects of skeletal development. Furthermore, recent evidence indicates the important role of non-canonical Wnt signaling in skeletal development. In this review we discuss the current understanding of the role of Wnt signaling in chondrogenesis, osteoblastogenesis, and osteoclastogenesis.     

Essential roles of mesenchyme-derived beta-catenin in mouse Müllerian duct morphogenesis

Members of the Wnt family of genes such as Wnt4, Wnt5a, and Wnt7a have been implicated in the formation and morphogenesis of the Müllerian duct into various parts of the female reproductive tract. These WNT ligands elicit their action via either the canonical WNT/beta-catenin or the non-canonical WNT/calcium pathway and could possibly function redundantly in Müllerian duct differentiation. By using the Müllerian duct-specific anti-Müllerian hormone receptor 2 cre (Amhr2-cre) mouse line, we established a conditional knockout model that removed beta-catenin specifically in the mesenchyme of the Müllerian duct. At birth, loss of beta-catenin in the Müllerian duct mesenchyme disrupted the normal coiling of the oviduct in the knockout embryo, resembling the phenotype of the Wnt7a knockout. The overall development of the female reproductive tract was stunted at birth with a decrease in proliferation in the mesenchyme and epithelium. We also discovered that Wnt5a and Wnt7a expression remained normal, excluding the possibility that the phenotypes resulted from a loss of these WNT ligands. We examined the expression of Frizzled (Fzd), the receptors for WNT, and found that Fzd1 is one receptor present in the Müllerian duct mesenchyme and could be the putative receptor for beta-catenin activation in the Müllerian duct. In summary, our findings suggest that mesenchymal beta-catenin is a downstream effector of Wnt7a that mediates the patterning of the oviduct and proper differentiation of the uterus.     

Adiponectin stimulates proliferation of adult hippocampal neural stem/progenitor cells through activation of p38 mitogen-activated protein kinase (p38MAPK)/glycogen synthase kinase 3β (GSK-3β)/β-catenin signaling cascade

Adiponectin is the most abundant adipokine secreted from adipocytes. Accumulating evidence suggests that the physiological roles of adiponectin go beyond its metabolic effects. In the present study, we demonstrate that adiponectin receptors 1 and 2 (AdipoR1 and AdipoR2) are expressed in adult hippocampal neural stem/progenitor cells (hNSCs). Adiponectin treatment increases proliferation of cultured adult hNSCs in a dose- and time-dependent manner, whereas apoptosis and differentiation of adult hNSCs into neuronal or glial lineage were not affected. Adiponectin activates AMP-activated protein kinase and p38 mitogen-activated protein kinase (p38MAPK) signaling pathways in adult hNSCs. Pretreatment with the p38MAPK inhibitor SB203580, but not the AMP-activated protein kinase inhibitor Compound C, attenuates adiponectin-induced cell proliferation. Moreover, adiponectin induces phosphorylation of Ser-389, a key inhibitory site of glycogen synthase kinase 3β (GSK-3β), and this effect can be blocked by inhibition of p38MAPK with SB203580. Levels of total and nuclear β-catenin, the primary substrate of GSK-3β, were increased by adiponectin treatment. These results indicate that adiponectin stimulates proliferation of adult hNSCs, via acting on GSK-3β to promote nuclear accumulation of β-catenin. Thus, our studies uncover a novel role for adiponectin signaling in regulating proliferation of adult neural stem cells.     

Cross-regulation of signaling pathways: An example of nuclear hormone receptors and the canonical Wnt pathway

Predicting the potential physiological outcome(s) of any given molecular pathway is complex because of cross-talk with other pathways. This is particularly evident in the case of the nuclear hormone receptor and canonical Wnt pathways, which regulate cell growth and proliferation, differentiation, apoptosis, and metastatic potential in numerous tissues. These pathways are known to intersect at many levels: in the intracellular space, at the membrane, in the cytoplasm, and within the nucleus. The outcomes of these interactions are important in the control of stem cell differentiation and maintenance, feedback loops, and regulating oncogenic potential. The aim of this review is to demonstrate the importance of considering pathway cross-talk when predicting functional outcomes of signaling, using nuclear hormone receptor/canonical Wnt pathway cross-talk as an example.