Early planarian brain regeneration is independent of blastema polarity mediated by the Wnt/β-catenin pathway

Analysis of anteroposterior (AP) axis specification in regenerating planarian flatworms has shown that Wnt/β-catenin signaling is required for posterior specification and that the FGF-like receptor molecule nou-darake (ndk) may be involved in restricting brain regeneration to anterior regions. The relationship between re-establishment of AP identity and correct morphogenesis of the brain is, however, still poorly understood. Here we report the characterization of two axin paralogs in the planarian Schmidtea mediterranea. Although Axins are well known negative regulators of Wnt/β-catenin signaling, no role in AP specification has previously been reported for axin genes in planarians. We show that silencing of Smed-axin genes by RNA interference (RNAi) results in two-tailed planarians, a phenotype previously reported after silencing of Smed-APC-1, another β-catenin inhibitor. More strikingly, we show for the first time that while early brain formation at anterior wounds remains unaffected, subsequent development of the brain is blocked in the two-tailed planarians generated after silencing of Smed-axin genes and Smed-APC-1. These findings suggest that the mechanisms underlying early brain formation can be uncoupled from the specification of AP identity by the Wnt/β-catenin pathway. Finally, the posterior expansion of the brain observed following Smed-ndk RNAi is enhanced by silencing Smed-APC-1, revealing an indirect relationship between the FGFR/Ndk and Wnt/β-catenin signaling systems in establishing the posterior limits of brain differentiation.     

Expression of secreted Wnt pathway components reveals unexpected complexity of the planarian amputation response

Regeneration is widespread throughout the animal kingdom, but our molecular understanding of this process in adult animals remains poorly understood. Wnt/β-catenin signaling plays crucial roles throughout animal life from early development to adulthood. In intact and regenerating planarians, the regulation of Wnt/β-catenin signaling functions to maintain and specify anterior/posterior (A/P) identity. Here, we explore the expression kinetics and RNAi phenotypes for secreted members of the Wnt signaling pathway in the planarian Schmidtea mediterranea. Smed-wnt and sFRP expression during regeneration is surprisingly dynamic and reveals fundamental aspects of planarian biology that have been previously unappreciated. We show that after amputation, a wounding response precedes rapid re-organization of the A/P axis. Furthermore, cells throughout the body plan can mount this response and reassess their new A/P location in the complete absence of stem cells. While initial stages of the amputation response are stem cell independent, tissue remodeling and the integration of a new A/P address with anatomy are stem cell dependent. We also show that WNT5 functions in a reciprocal manner with SLIT to pattern the planarian mediolateral axis, while WNT11-2 patterns the posterior midline. Moreover, we perform an extensive phylogenetic analysis on the Smed-wnt genes using a method that combines and integrates both sequence and structural alignments, enabling us to place all nine genes into Wnt subfamilies for the first time.     

Wnt/β-catenin signaling in hepatic organogenesis

Wnt/β-catenin signaling has come to the forefront of liver biology in recent years. This pathway regulates key pathophysiological events inherent to the liver including development, regeneration, and cancer, by dictating several biological processes such as proliferation, apoptosis, differentiation, adhesion, zonation and metabolism in various cells of the liver. This review will examine the studies that have uncovered the relevant roles of Wnt/β-catenin signaling during the process of liver development. We will discuss the potential roles of Wnt/β-catenin signaling during the phases of development, including competence, hepatic induction, expansion, and morphogenesis. In addition, we will discuss the role of negative and positive regulation of this pathway and how the temporal expression of Wnt/β-catenin can direct key processes during hepatic development. We will also identify some of the major deficits in the current understanding of the role of Wnt/β-catenin signaling in liver development in order to provide a perspective for future studies. Thus, this review will provide a contextual overview of the role of Wnt/β-catenin signaling during hepatic organogenesis.     

Investigating Tonic Wnt Signaling Throughout the Adult CNS and in the Hippocampal Neurogenic Niche of BatGal and Ins-TopGal Mice

Wnt/β-catenin signaling has a well-established role in the development of the central nervous system (CNS), and recent evidence is extending this role to include the regulation of adult hippocampal function, including neurogenesis within the dentate gyrus. While the neuroanatomical expression pattern of many canonical Wnt signaling components have been investigated, the sites of signal integration and functional downstream β-catenin activation remain comparatively less characterized in the adult CNS. Using two independent transgenic β-catenin-activated LacZ reporter mouse lines (BatGal and ins-TopGal), we demonstrate that Wnt/β-catenin signaling is active in discrete regions of the adult mouse CNS. Intriguingly, BatGal mice exhibit a broad pattern of reporter expression in the CNS, while expression in ins-TopGal mice is more restricted. Further investigation of these two lines reveals temporal differences in β-catenin-activated reporter expression during neurogenesis within the adult hippocampus. Ins-TopGal mice display peaks of Wnt/β-catenin-activated reporter expression during early and later stages of neurogenesis suggesting Wnt/β-catenin signaling plays an important role during both progenitor cell amplification as well as neuronal maturation, integration, and/or maintenance; however, results from BatGal mice are not as convincing. Thus our data using ins-TopGal mice are consistent with the idea that Wnt signaling plays diverse roles during adult hippocampal neurogenesis and support the idea that multiple transgenic reporter lines must be rigorously compared during scientific investigations.     

A LIM-homeobox gene is required for differentiation of Wnt-expressing cells at the posterior end of the planarian body

Planarians have high regenerative ability, which is dependent on pluripotent adult somatic stem cells called neoblasts. Recently, canonical Wnt/β-catenin signaling was shown to be required for posterior specification, and Hedgehog signaling was shown to control anterior-posterior polarity via activation of the Djwnt1/P-1 gene at the posterior end of planarians. Thus, various signaling molecules play an important role in planarian stem cell regulation. However, the molecular mechanisms directly involved in stem cell differentiation have remained unclear. Here, we demonstrate that one of the planarian LIM-homeobox genes, Djislet, is required for the differentiation of Djwnt1/P-1-expressing cells from stem cells at the posterior end. RNA interference (RNAi)-treated planarians of Djislet [Djislet(RNAi)] show a tail-less phenotype. Thus, we speculated that Djislet might be involved in activation of the Wnt signaling pathway in the posterior blastema. When we carefully examined the expression pattern of Djwnt1/P-1 by quantitative real-time PCR during posterior regeneration, we found two phases of Djwnt1/P-1 expression: the first phase was detected in the differentiated cells in the old tissue in the early stage of regeneration and then a second phase was observed in the cells derived from stem cells in the posterior blastema. Interestingly, Djislet is expressed in stem cell-derived DjPiwiA- and Djwnt1/P-1-expressing cells, and Djislet(RNAi) only perturbed the second phase. Thus, we propose that Djislet might act to trigger the differentiation of cells expressing Djwnt1/P-1 from stem cells.     

Wnt/β-catenin signaling regulates neuronal differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) have been demonstrated to be able to differentiate into neuron-like cells, but the precise mechanisms controlling this process are unclear. Using neuron-specific enolase (NSE) and nestin as neuronal markers, we examined the role of Wnt/β-catenin signaling in MSC neuronal differentiation in present study. The results indicated that the expression of β-catenin increased markedly during the neuronal differentiation of MSCs. Blocking Wnt signaling by treating MSCs with β-catenin siRNA could decrease the differentiation of MSCs into neuron-like cells and up-regulation of Wnt signaling by treating MSCs with Wnt-3a could promote neuronal differentiation of MSCs. Above results suggest that Wnt/β-catenin signaling may play a pivotal role in neuronal differentiation of MSCs. Our data broaden the knowledge of molecular mechanisms involved in the neuronal differentiation of MSCs and provide a potential target for directing differentiation of MSCs for clinical application.     

Wnt signaling is required for antero-posterior patterning of the planarian brain

Wnt signaling functions in axis formation and morphogenesis in various animals and organs. Here we report that Wnt signaling is required for proper brain patterning during planarian brain regeneration. We showed here that one of the Wnt homologues in the planarian Dugesia japonica, DjwntA, was expressed in the posterior region of the brain. When DjwntA-knockdown planarians were produced by RNAi, they could regenerate their heads at the anterior ends of the fragments, but formed ectopic eyes with irregular posterior lateral branches and brain expansion. This suggests that the Wnt signal may be involved in antero-posterior (A-P) patterning of the planarian brain, as in vertebrates. We also investigated the relationship between the DjwntA and nou-darake/FGFR signal systems, as knockdown planarians of these genes showed similar phenotypes. Double-knockdown planarians of these genes did not show any synergistic effects, suggesting that the two signal systems function independently in the process of brain regeneration, which accords with the fact that nou-darake was expressed earlier than DjwntA during brain regeneration. These observations suggest that the nou-darake/FGFR signal may be involved in brain rudiment formation during the early stage of head regeneration, and subsequently the DjwntA signal may function in A-P patterning of the brain rudiment.     

WNT5A signaling contributes to Aβ-induced neuroinflammation and neurotoxicity

Neurodegenration is a pathological hallmark of Alzheimer's disease (AD), but the underlying molecular mechanism remains elusive. Here, we present evidence that reveals a crucial role of Wnt5a signaling in this process. We showed that Wnt5a and its receptor Frizzled-5 (Fz5) were up-regulated in the AD mouse brain, and that beta-amyloid peptide (Aβ), a major constituent of amyloid plaques, stimulated Wnt5a and Fz5 expression in primary cortical cultures; these observations indicate that Wnt5a signaling could be aberrantly activated during AD pathogenesis. In support of such a possibility, we observed that inhibition of Wnt5a signaling attenuated while activation of Wnt5a signaling enhanced Aβ-evoked neurotoxicity, suggesting a role of Wnt5a signaling in AD-related neurodegeneration. Furthermore, we also demonstrated that Aβ-induced neurotoxicity depends on inflammatory processes, and that activation of Wnt5a signaling elicited the expression of proinflammatory cytokines IL-1β and TNF-α whereas inhibition of Wnt5a signaling attenuated the Aβ-induced expression of the cytokines in cortical cultures. Our findings collectively suggest that aberrantly up-regulated Wnt5a signaling is a crucial pathological step that contributes to AD-related neurodegeneration by regulating neuroinflammation.     

Distinct phases of Wnt/β-catenin signaling direct cardiomyocyte formation in zebrafish

Normal heart formation requires reiterative phases of canonical Wnt/β-catenin (Wnt) signaling. Understanding the mechanisms by which Wnt signaling directs cardiomyocyte (CM) formation in vivo is critical to being able to precisely direct differentiated CMs from stem cells in vitro. Here, we investigate the roles of Wnt signaling in zebrafish CM formation using heat-shock inducible transgenes that increase and decrease Wnt signaling. We find that there are three phases during which CM formation is sensitive to modulation of Wnt signaling through the first 24 h of development. In addition to the previously recognized roles for Wnt signaling during mesoderm specification and in the pre-cardiac mesoderm, we find a previously unrecognized role during CM differentiation where Wnt signaling is necessary and sufficient to promote the differentiation of additional atrial cells. We also extend the previous studies of the roles of Wnt signaling during mesoderm specification and in pre-cardiac mesoderm. Importantly, in pre-cardiac mesoderm we define a new mechanism where Wnt signaling is sufficient to prevent CM differentiation, in contrast to a proposed role in inhibiting cardiac progenitor (CP) specification. The inability of the CPs to differentiate appears to lead to cell death through a p53/Caspase-3 independent mechanism. Together with a report for an even later role for Wnt signaling in restricting proliferation of differentiated ventricular CMs, our results indicate that during the first 3days of development in zebrafish there are four distinct phases during which CMs are sensitive to Wnt signaling.     

Syndecan-4 inhibits Wnt/β-catenin signaling through regulation of low-density-lipoprotein receptor-related protein (LRP6) and R-spondin 3

Regulation of Wnt signaling is crucial for embryonic development and adult homeostasis. Here we study the role of Syndecan-4 (SDC4), a cell-surface heparan sulphate proteoglycan, and Fibronectin (FN), in Wnt/β-catenin signaling. Gain- and loss-of-function experiments in mammalian cell lines and Xenopus embryos demonstrate that SDC4 and FN inhibit Wnt/β-catenin signaling. Epistatic and biochemical experiments show that this inhibition occurs at the cell membrane level through regulation of LRP6. R-spondin 3, a ligand that promotes canonical and non-canonical Wnt signaling, is more prone to potentiate Wnt/β-catenin signaling when SDC4 levels are reduced, suggesting a model whereby SDC4 tunes the ability of R-spondin to modulate the different Wnt signaling pathways. Since SDC4 has been previously related to non-canonical Wnt signaling, our results also suggest that this proteoglycan can be a key component in the regulation of Wnt signaling.     

Epithelial Wnt/β-catenin signaling regulates palatal shelf fusion through regulation of Tgfβ3 expression

The canonical Wnt/β-catenin signaling plays essential role in development and diseases. Previous studies have implicated the canonical Wnt/β-catenin signaling in the regulation of normal palate development, but functional Wnt/β-catenin signaling and its tissue-specific activities remain to be accurately elucidated. In this study, we show that functional Wnt/β-catenin signaling operates primarily in the palate epithelium, particularly in the medial edge epithelium (MEE) of the developing mouse palatal shelves, consistent with the expression patterns of β-catenin and several Wnt ligands and receptors. Epithelial specific inactivation of β-catenin by the K14-Cre transgenic allele abolishes the canonical Wnt signaling activity in the palatal epithelium and leads to an abnormal persistence of the medial edge seam (MES), ultimately causing a cleft palate formation, a phenotype resembling that in Tgfβ3 mutant mice. Consistent with this phenotype is the down-regulation of Tgfβ3 and suppression of apoptosis in the MEE of the β-catenin mutant palatal shelves. Application of exogenous Tgfβ3 to the mutant palatal shelves in organ culture rescues the midline seam phenotype. On the other hand, expression of stabilized β-catenin in the palatal epithelium also disrupts normal palatogenesis by activating ectopic Tgfβ3 expression in the palatal epithelium and causing an aberrant fusion between the palate shelf and mandible in addition to severely deformed palatal shelves. Collectively, our results demonstrate an essential role for Wnt/β-catenin signaling in the epithelial component at the step of palate fusion during palate development by controlling the expression of Tgfβ3 in the MEE.     

Requirement of Wnt/β-catenin signaling in pronephric kidney development

The pronephric kidney controls water and electrolyte balance during early fish and amphibian embryogenesis. Many Wnt signaling components have been implicated in kidney development. Specifically, in Xenopus pronephric development as well as the murine metanephroi, the secreted glycoprotein Wnt-4 has been shown to be essential for renal tubule formation. Despite the importance of Wnt signals in kidney organogenesis, little is known of the definitive downstream signaling pathway(s) that mediate their effects. Here we report that inhibition of Wnt/β-catenin signaling within the pronephric field of Xenopus results in significant losses to kidney epithelial tubulogenesis with little or no effect on adjoining axis or somite development. We find that the requirement for Wnt/β-catenin signaling extends throughout the pronephric primordium and is essential for the development of proximal and distal tubules of the pronephros as well as for the development of the duct and glomus. Although less pronounced than effects upon later pronephric tubule differentiation, inhibition of the Wnt/β-catenin pathway decreased expression of early pronephric mesenchymal markers indicating it is also needed in early pronephric patterning. We find that upstream inhibition of Wnt/β-catenin signals in zebrafish likewise reduces pronephric epithelial tubulogenesis. We also find that exogenous activation of Wnt/β-catenin signaling within the Xenopus pronephric field results in significant tubulogenic losses. Together, we propose Wnt/β-catenin signaling is required for pronephric tubule, duct and glomus formation in Xenopus laevis, and this requirement is conserved in zebrafish pronephric tubule formation.     

Exosomes from human adipose-derived mesenchymal stem cells promote migration through Wnt signaling pathway in a breast cancer cell model

Recent studies have demonstrated that the Wnt/β-catenin signaling plays an important role in stem cell aging. However, the mechanisms of cell senescence induced by Wnt/β-catenin signaling are still poorly understood. Our preliminary study has indicated that activated Wnt/β-catenin signaling can induce MSC aging. In this study, we reported that the Wnt/β-catenin signaling was a potent activator of reactive oxygen species (ROS) generation in MSCs. After scavenging ROS with N-acetylcysteine, Wnt/β-catenin signaling-induced MSC aging was significantly attenuated and the DNA damage and the expression of p16^INK4A, p53, and p21 were reduced in MSCs. These results indicated that the Wnt/β-catenin signaling could induce MSC aging through promoting the intracellular production of ROS, and ROS may be the main mediators of MSC aging induced by excessive activation of Wnt/β-catenin signaling.     

Retinoic acid and Wnt/β-catenin have complementary roles in anterior/posterior patterning embryos of the basal chordate amphioxus

A role for Wnt/β-catenin signaling in axial patterning has been demonstrated in animals as basal as cnidarians, while roles in axial patterning for retinoic acid (RA) probably evolved in the deuterostomes and may be chordate-specific. In vertebrates, these two pathways interact both directly and indirectly. To investigate the evolutionary origins of interactions between these two pathways, we manipulated Wnt/β-catenin and RA signaling in the basal chordate amphioxus during the gastrula stage, which is the RA-sensitive period for anterior/posterior (A/P) patterning. The results show that Wnt/β-catenin and RA signaling have distinctly different roles in patterning the A/P axis of the amphioxus gastrula. Wnt/β-catenin specifies the identity of the ends of the embryo (high Wnt = posterior; low Wnt = anterior) but not intervening positions. Thus, upregulation of Wnt/β-catenin signaling induces ectopic expression of posterior markers at the anterior tip of the embryo. In contrast, RA specifies position along the A/P axis, but not the identity of the ends of the embryo—increased RA signaling strongly affects the domains of Hox expression along the A/P axis but has little or no effect on the expression of either anterior or posterior markers. Although the two pathways may both influence such things as specification of neuronal identity, interactions between them in A/P patterning appear to be minimal.     

Effects of 17β-estradiol and bisphenol A on the formation of reproductive organs in planarians

Planarians have a remarkable capacity for regeneration after ablation, and they reproduce asexually by fission. However, some planarians can also reproduce and maintain their sexual organs. During the regenerative process, their existing sexual organs degenerate and new ones develop. However, little is known about hormonal regulation during the development of reproductive organs in planarians. In this study, we investigated the effects of 17β-estradiol (a steroid) and bisphenol A (an endocrine disrupter) on the formation of sexual organs in the hermaphroditic planarian Dugesia ryukyuensis. Under control conditions, all worm tissues regenerated into sexual planarians with sexual organs within 4 weeks after ablation. However, in the presence of bisphenol A or 17β-estradiol, although they apparently regenerated into sexual planarians, the yolk glands, which are one of the female sexual organs, failed to regenerate even 7 weeks after ablation. These data suggest that planarians have a steroid hormone system, which plays a key role in the formation and maturation of sexual organs.