Interplay of the Norrin and Wnt7a/Wnt7b signaling systems in blood-brain barrier and blood-retina barrier development and maintenance.

ß-Catenin signaling controls the development and maintenance of the blood-brain barrier (BBB) and the blood-retina barrier (BRB), but the division of labor and degree of redundancy between the two principal ligand-receptor systems-the Norrin and Wnt7a/Wnt7b systems-are incompletely defined. Here, we present a loss-of-function genetic analysis of postnatal BBB and BRB maintenance in mice that shows striking threshold and partial redundancy effects. In particular, the combined loss of Wnt7a and Norrin or Wnt7a and Frizzled4 (Fz4) leads to anatomically localized BBB defects that are far more severe than observed with loss of Wnt7a, Norrin, or Fz4 alone. In the cerebellum, selective loss of Wnt7a in glia combined with ubiquitous loss of Norrin recapitulates the phenotype observed with ubiquitous loss of both Wnt7a and Norrin, implying that glia are the source of Wnt7a in the cerebellum. Tspan12, a coactivator of Norrin signaling in the retina, is also active in BBB maintenance but is less potent than Norrin, consistent with a model in which Tspan12 enhances the amplitude of the Norrin signal in vascular endothelial cells. Finally, in the context of a partially impaired Norrin system, the retina reveals a small contribution to BRB development from the Wnt7a/Wnt7b system. Taken together, these experiments define the extent of CNS region-specific cooperation for several components of the Norrin and Wnt7a/Wnt7b systems, and they reveal substantial regional heterogeneity in the extent to which partially redundant ligands, receptors, and coactivators maintain the BBB and BRB.

Loss of Wnt16 Leads to Skeletal Deformities and Downregulation of Bone Developmental Pathway in Zebrafish.

Wingless-type MMTV integration site family, member 16 (wnt16), is a wnt ligand that participates in the regulation of vertebrate skeletal development. Studies have shown that wnt16 can regulate bone metabolism, but its molecular mechanism remains largely undefined. We obtained the wnt16-/- zebrafish model using the CRISPR-Cas9-mediated gene knockout screen with 11 bp deletion in wnt16, which led to the premature termination of amino acid translation and significantly reduced wnt16 expression, thus obtaining the wnt16-/- zebrafish model. The expression of wnt16 in bone-related parts was detected via in situ hybridization. The head, spine, and tail exhibited significant deformities, and the bone mineral density and trabecular bone decreased in wnt16-/- using light microscopy and micro-CT analysis. RNA sequencing was performed to explore the differentially expressed genes (DEGs). Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis found that the down-regulated DEGs are mainly concentrated in mTOR, FoxO, and VEGF pathways. Protein-protein interaction (PPI) network analysis was performed with the detected DEGs. Eight down-regulated DEGs including akt1, bnip4, ptena, vegfaa, twsg1b, prkab1a, prkab1b, and pla2g4f.2 were validated by qRT-PCR and the results were consistent with the RNA-seq data. Overall, our work provides key insights into the influence of wnt16 gene on skeletal development.

Wnt16 attenuates osteoarthritis progression through a PCP/JNK-mTORC1-PTHrP cascade.

OBJECTIVES: Wnt16 is implicated in bone fracture and bone mass accrual both in animals and humans. However, its functional roles and molecular mechanism in chondrocyte differentiation and osteoarthritis (OA) pathophysiology remain largely undefined. In this study, we analysed its mechanistic association and functional relationship in OA progression in chondrocyte lineage. METHODS: The role of Wnt16 during skeletal development was examined by Col2a1-Wnt16 transgenic mice and Wnt16fl/fl;Col2a1-Cre (Wnt16-cKO) mice. OA progression was assessed by micro-CT analysis and Osteoarthritis Research Society International score after anterior cruciate ligament transection (ACLT) surgery with Wnt16 manipulation by adenovirus intra-articular injection. The molecular mechanism was investigated in vitro using 3D chondrocyte pellet culture and biochemical analyses. Histological analysis was performed in mouse joints and human cartilage specimens. RESULTS: Wnt16 overexpression in chondrocytes in mice significantly inhibited chondrocyte hypertrophy during skeletal development. Wnt16 deficiency exaggerated OA progression, whereas intra-articular injection of Ad-Wnt16 markedly attenuated ACLT-induced OA. Cellular and molecular analyses showed that, instead of ß-catenin and calcium pathways, Wnt16 activated the planar cell polarity (PCP) and JNK pathway by interacting mainly with AP2b1, and to a lesser extend Ror2 and CD146, and subsequently induced PTHrP expression through phosphor-Raptor mTORC1 pathway. CONCLUSIONS: Our findings indicate that Wnt16 activates PCP/JNK and crosstalks with mTORC1-PTHrP pathway to inhibit chondrocyte hypertrophy. Our preclinical study suggests that Wnt16 may be a potential therapeutic target for OA treatment.

A canonical to non-canonical Wnt signalling switch in haematopoietic stem-cell ageing.

Many organs with a high cell turnover (for example, skin, intestine and blood) are composed of short-lived cells that require continuous replenishment by somatic stem cells. Ageing results in the inability of these tissues to maintain homeostasis and it is believed that somatic stem-cell ageing is one underlying cause of tissue attrition with age or age-related diseases. Ageing of haematopoietic stem cells (HSCs) is associated with impaired haematopoiesis in the elderly. Despite a large amount of data describing the decline of HSC function on ageing, the molecular mechanisms of this process remain largely unknown, which precludes rational approaches to attenuate stem-cell ageing. Here we report an unexpected shift from canonical to non-canonical Wnt signalling in mice due to elevated expression of Wnt5a in aged HSCs, which causes stem-cell ageing. Wnt5a treatment of young HSCs induces ageing-associated stem-cell apolarity, reduction of regenerative capacity and an ageing-like myeloid-lymphoid differentiation skewing via activation of the small Rho GTPase Cdc42. Conversely, Wnt5a haploinsufficiency attenuates HSC ageing, whereas stem-cell-intrinsic reduction of Wnt5a expression results in functionally rejuvenated aged HSCs. Our data demonstrate a critical role for stem-cell-intrinsic non-canonical Wnt5a signalling in HSC ageing.

WNT16 antagonises excessive canonical WNT activation and protects cartilage in osteoarthritis.

OBJECTIVE: Both excessive and insufficient activation of WNT signalling results in cartilage breakdown and osteoarthritis. WNT16 is upregulated in the articular cartilage following injury and in osteoarthritis. Here, we investigate the function of WNT16 in osteoarthritis and the downstream molecular mechanisms. METHODS: Osteoarthritis was induced by destabilisation of the medial meniscus in wild-type and WNT16-deficient mice. Molecular mechanisms and downstream effects were studied in vitro and in vivo in primary cartilage progenitor cells and primary chondrocytes. The pathway downstream of WNT16 was studied in primary chondrocytes and using the axis duplication assay in Xenopus. RESULTS: WNT16-deficient mice developed more severe osteoarthritis with reduced expression of lubricin and increased chondrocyte apoptosis. WNT16 supported the phenotype of cartilage superficial-zone progenitor cells and lubricin expression. Increased osteoarthritis in WNT16-deficient mice was associated with excessive activation of canonical WNT signalling. In vitro, high doses of WNT16 weakly activated canonical WNT signalling, but, in co-stimulation experiments, WNT16 reduced the capacity of WNT3a to activate the canonical WNT pathway. In vivo, WNT16 rescued the WNT8-induced primary axis duplication in Xenopus embryos. CONCLUSIONS: In osteoarthritis, WNT16 maintains a balanced canonical WNT signalling and prevents detrimental excessive activation, thereby supporting the homeostasis of progenitor cells.

Regulation of angiogenesis by a non-canonical Wnt-Flt1 pathway in myeloid cells.

Myeloid cells are a feature of most tissues. Here we show that during development, retinal myeloid cells (RMCs) produce Wnt ligands to regulate blood vessel branching. In the mouse retina, where angiogenesis occurs postnatally, somatic deletion in RMCs of the Wnt ligand transporter Wntless results in increased angiogenesis in the deeper layers. We also show that mutation of Wnt5a and Wnt11 results in increased angiogenesis and that these ligands elicit RMC responses via a non-canonical Wnt pathway. Using cultured myeloid-like cells and RMC somatic deletion of Flt1, we show that an effector of Wnt-dependent suppression of angiogenesis by RMCs is Flt1, a naturally occurring inhibitor of vascular endothelial growth factor (VEGF). These findings indicate that resident myeloid cells can use a non-canonical, Wnt-Flt1 pathway to suppress angiogenic branching.

A somitic Wnt16/Notch pathway specifies haematopoietic stem cells.

Haematopoietic stem cells (HSCs) are a self-renewing population of cells that continuously replenish all blood and immune cells during the lifetime of an individual. HSCs are used clinically to treat a wide array of diseases, including acute leukaemias and congenital blood disorders, but obtaining suitable numbers of cells and finding immune-compatible donors remain serious problems. These difficulties have led to an interest in the conversion of embryonic stem cells or induced pluripotent stem cells into HSCs, which is not possible using current methodologies. To accomplish this goal, it is critical to understand the native mechanisms involved in the specification of HSCs during embryonic development. Here we demonstrate in zebrafish that Wnt16 controls a novel genetic regulatory network required for HSC specification. Non-canonical signalling by Wnt16 is required for somitic expression of the Notch ligands deltaC (dlc) and deltaD (dld), and these ligands are, in turn, required for the establishment of definitive haematopoiesis. Notch signalling downstream of Dlc and Dld is earlier than, and distinct from, known cell-autonomous requirements for Notch, strongly suggesting that novel Notch-dependent relay signal(s) induce the first HSCs in parallel to other established pathways. Our results demonstrate that somite-specific gene expression is required for the production of haemogenic endothelium.

Non-canonical Wnt5a/Ror2 signaling regulates kidney morphogenesis by controlling intermediate mesoderm extension.

Congenital anomalies of the kidney and urinary tract (CAKUT) affect about 1 in 500 births and are a major cause of morbidity in infants. Duplex collecting systems rank among the most common abnormalities of CAKUT, but the molecular basis for this defect is poorly understood. In mice, conditional deletion of Wnt5a in mesoderm results in bilateral duplex kidney and ureter formation. The ureteric buds (UBs) in mutants emerge as doublets from the intermediate mesoderm (IM)-derived nephric duct (ND) without anterior expansion of the glial cell line-derived neurotrophic factor (Gdnf) expression domain in the surrounding mesenchyme. Wnt5a is normally expressed in a graded manner at the posterior end of the IM, but its expression is down-regulated prior to UB outgrowth at E10.5. Furthermore, ablation of Wnt5a in the mesoderm with an inducible Cre at E7.5 results in duplex UBs, whereas ablation at E8.5 yields normal UB outgrowth, demonstrating that Wnt5a functions in IM development well before the formation of the metanephros. In mutants, the posterior ND is duplicated and surrounding Pax2-positive mesenchymal cells persist in the nephric cord, suggesting that disruption of normal ND patterning prompts the formation of duplex ureters and kidneys. Ror2 homozygous mutants, which infrequently yield duplex collecting systems, show a dramatic increase in incidence with the additional deletion of one copy of Wnt5a, implicating this receptor in non-canonical Wnt5a signaling during IM development. This work provides the first evidence of a role of Wnt5a/Ror2 signaling in IM extension and offers new insights into the etiology of CAKUT and possible involvement of Wnt5a/Ror2 mutations.

Abnormal primary and permanent dentitions with ectodermal symptoms predict WNT10A deficiency.

BACKGROUND: The WNT10A protein is critical for the development of ectodermal appendages. Variants in the WNT10A gene may be associated with a spectrum of ectodermal abnormalities including extensive tooth agenesis. METHODS: In seven patients with severe tooth agenesis we identified anomalies in primary dentition and additional ectodermal symptoms, and assessed WNT10A mutations by genetic analysis. RESULTS: Investigation of primary dentition revealed peg-shaped crowns of primary mandibular incisors and three individuals had agenesis of at least two primary teeth. The permanent dentition was severely affected in all individuals with a mean of 21 missing teeth. Primary teeth were most often present in positions were succedaneous teeth were missing. Furthermore, most existing molars had taurodontism. Light, brittle or coarse hair was reported in all seven individuals, hyperhidrosis of palms and soles in six individuals and nail anomalies in two individuals. The anomalies in primary dentition preceded most of the additional ectodermal symptoms. Genetic analysis revealed that all seven individuals were homozygous or compound heterozygous for WNT10A mutations resulting in C107X, E222X and F228I. CONCLUSIONS: We conclude that tooth agenesis and/or peg-shaped crowns of primary mandibular incisors, severe oligodontia of permanent dentition as well as ectodermal symptoms of varying severity may be predictors of bi-allelic WNT10A mutations of importance for diagnosis, counselling and follow-up.

Wnt11 patterns a myocardial electrical gradient through regulation of the L-type Ca(2+) channel.

Electrical gradients are critical for many biological processes, including the normal function of excitable tissues, left-right patterning, organogenesis and wound healing. The fundamental mechanisms that regulate the establishment and maintenance of such electrical polarities are poorly understood. Here we identify a gradient of electrical coupling across the developing ventricular myocardium using high-speed optical mapping of transmembrane potentials and calcium concentrations in the zebrafish heart. We excluded a role for differences in cellular excitability, connexin localization, tissue geometry and mechanical inputs, but in contrast we were able to demonstrate that non-canonical Wnt11 signals are required for the genesis of this myocardial electrical gradient. Although the traditional planar cell polarity pathway is not involved, we obtained evidence that Wnt11 acts to set up this gradient of electrical coupling through effects on transmembrane Ca(2+) conductance mediated by the L-type calcium channel. These data reveal a previously unrecognized role for Wnt/Ca(2+) signalling in establishing an electrical gradient in the plane of the developing cardiac epithelium through modulation of ion-channel function. The regulation of cellular coupling through such mechanisms may be a general property of non-canonical Wnt signals.

Ectodermal Wnt signaling regulates abdominal myogenesis during ventral body wall development.

Defects of the ventral body wall are prevalent birth anomalies marked by deficiencies in body wall closure, hypoplasia of the abdominal musculature and multiple malformations across a gamut of organs. However, the mechanisms underlying ventral body wall defects remain elusive. Here, we investigated the role of Wnt signaling in ventral body wall development by inactivating Wls or ß-catenin in murine abdominal ectoderm. The loss of Wls in the ventral epithelium, which blocks the secretion of Wnt proteins, resulted in dysgenesis of ventral musculature and genito-urinary tract during embryonic development. Molecular analyses revealed that the dermis and myogenic differentiation in the underlying mesenchymal progenitor cells was perturbed by the loss of ectodermal Wls. The activity of the Wnt-Pitx2 axis was impaired in the ventral mesenchyme of the mutant body wall, which partially accounted for the defects in ventral musculature formation. In contrast, epithelial depletion of ß-catenin or Wnt5a did not resemble the body wall defects in the ectodermal Wls mutant. These findings indicate that ectodermal Wnt signaling instructs the underlying mesodermal specification and abdominal musculature formation during ventral body wall development, adding evidence to the theory that ectoderm-mesenchyme signaling is a potential unifying mechanism for the origin of ventral body wall defects.

Functional intestinal stem cells after Paneth cell ablation induced by the loss of transcription factor Math1 (Atoh1).

Intestinal epithelium has the capacity to self-renew and generate differentiated cells through the existence of two types of epithelial stem cells: active crypt base columnar cells (CBCs) and quiescent +4 cells. The behaviors of these cells are regulated both by intrinsic programs and by extrinsic signals sent by neighboring cells, which define the niche. It is clear that the ß-catenin pathway acts as an essential intrinsic signal for the maintenance and proliferation of CBC, and it was recently proposed that Paneth cells provide a crucial niche by secreting Wingless/Int (Wnt) ligands. Here, we examined the effect of disrupting the intestinal stem cell niche by inducible deletion of the transcription factor Math1 (Atoh1), an essential driver of secretory cell differentiation. We found that complete loss of Paneth cells attributable to Math1 deficiency did not perturb the crypt architecture and allowed the maintenance and proliferation of CBCs. Indeed, Math1-deficient crypt cells tolerated in vivo Paneth cell loss and maintained active ß-catenin signaling but could not grow ex vivo without exogenous Wnt, implying that, in vivo, underlying mucosal cells act as potential niche. Upon irradiation, Math1-deficient crypt cells regenerated and CBCs continued cycling. Finally, CBC stem cells deficient in adenomatous polyposis coli (Apc) and Math1 were able to promote intestinal tumorigenesis. We conclude that in vivo, Math1-deficient crypts counteract the absence of Paneth cell-derived Wnts and prevent CBC stem cell exhaustion.

Wnt5a-Ror-Dishevelled signaling constitutes a core developmental pathway that controls tissue morphogenesis.

Wnts make up a large family of extracellular signaling molecules that play crucial roles in development and disease. A subset of noncanonical Wnts signal independently of the transcription factor ß-catenin by a mechanism that regulates key morphogenetic movements during embryogenesis. The best characterized noncanonical Wnt, Wnt5a, has been suggested to signal via a variety of different receptors, including the Ror family of receptor tyrosine kinases, the Ryk receptor tyrosine kinase, and the Frizzled seven-transmembrane receptors. Whether one or several of these receptors mediates the effects of Wnt5a in vivo is not known. Through loss-of-function experiments in mice, we provide conclusive evidence that Ror receptors mediate Wnt5a-dependent processes in vivo and identify Dishevelled phosphorylation as a physiological target of Wnt5a-Ror signaling. The absence of Ror signaling leads to defects that mirror phenotypes observed in Wnt5a null mutant mice, including decreased branching of sympathetic neuron axons and major defects in aspects of embryonic development that are dependent upon morphogenetic movements, such as severe truncation of the caudal axis, the limbs, and facial structures. These findings suggest that Wnt5a-Ror-Dishevelled signaling constitutes a core noncanonical Wnt pathway that is conserved through evolution and is crucial during embryonic development.

Wnt5a regulates distinct signalling pathways by binding to Frizzled2.

Wnt5a regulates multiple intracellular signalling cascades, but how Wnt5a determines the specificity of these pathways is not well understood. This study examined whether the internalization of Wnt receptors affects the ability of Wnt5a to regulate its signalling pathways. Wnt5a activated Rac in the beta-catenin-independent pathway, and Frizzled2 (Fz2) and Ror1 or Ror2 were required for this action. Fz2 was internalized through a clathrin-mediated route in response to Wnt5a, and inhibition of clathrin-dependent internalization suppressed the ability of Wnt5a to activate Rac. As another action of Wnt5a, it inhibited Wnt3a-dependent lipoprotein receptor-related protein 6 (LRP6) phosphorylation and beta-catenin accumulation. Wnt3a-dependent phosphorylation of LRP6 was enhanced in Wnt5a knockout embryonic fibroblasts. Fz2 was also required for the Wnt3a-dependent accumulation of beta-catenin, and Wnt5a competed with Wnt3a for binding to Fz2 in vitro and in intact cells, thereby inhibiting the beta-catenin pathway. This inhibitory action of Wnt5a was not affected by the impairment of clathrin-dependent internalization. These results suggest that Wnt5a regulates distinct pathways through receptor internalization-dependent and -independent mechanisms.

ß-Catenin is essential for Müllerian duct regression during male sexual differentiation.

During male sexual differentiation, the transforming growth factor-ß (TGF-ß) signaling molecule anti-Müllerian hormone (AMH; also known as Müllerian inhibiting substance, MIS) is secreted by the fetal testes and induces regression of the Müllerian ducts, the primordia of the female reproductive tract organs. Currently, the molecular identity of downstream events regulated by the AMH signaling pathway remains unclear. We found that male-specific Wnt4 expression in mouse Müllerian duct mesenchyme depends upon AMH signaling, implicating the WNT pathway as a downstream mediator of Müllerian duct regression. Inactivation of ß-catenin, a mediator of the canonical WNT pathway, did not affect AMH signaling activation in the Müllerian duct mesenchyme, but did block Müllerian duct regression. These data suggest that ß-catenin mediates AMH signaling for Müllerian duct regression during male sexual differentiation.

Epigenetic mechanism causes Wnt9b deficiency and nonsyndromic cleft lip and palate in the A/WySn mouse strain.

BACKGROUND: The heritable multifactorial etiology of human nonsyndromic cleft lip with or without cleft palate (CL ± P) is not understood. CL ± P occurs in 15% of neonates in the homozygous A/WySn mouse strain, with a multifactorial genetic etiology, the clf1 and clf2 variant genes. Clf1 acts as a mutant allele of Wnt9b but its coding sequence is normal. An IAP (intracisternal A particle) retrotransposon inserted near the Wnt9b gene is associated with clf1. METHODS: Transcription of noncoding sequence between the IAP and the Wnt9b gene was examined in A/WySn embryos. The levels of Wnt9b transcript and of an "IAP antisense" transcript initiated in the IAP and extending into the noncoding interval were assayed in A/WySn and C57BL/6J whole embryos or heads across embryonic days 8 to 12. Methylation of the 5' LTR of the IAP was examined in E12 A/WySn embryo heads. RESULTS: Mean Wnt9b transcript levels were lower in A/WySn than in C57BL/6J at all ages examined and lower in CL ± P embryos than in their normal littermates. The "IAP antisense" transcript was found in all A/WySn embryos and was highest in CL ± P embryos. The IAP at Wnt9b was generally unmethylated in CL ± P embryos and approximately 50% methylated in normal littermates. CONCLUSION: The clf1 mutation in A/WySn is a "metastable epiallele", in which stochastic deficiency in some individuals of DNA methylation of a retrotransposon uniquely inserted near the Wnt9b gene allows transcriptional activity of the retrotransposon and interference with transcription from Wnt9b. Methylation of metastable epialleles should be investigated in human nonsyndromic CL ± P.

Dkk1 and MSX2-Wnt7b signaling reciprocally regulate the endothelial-mesenchymal transition in aortic endothelial cells.

OBJECTIVE: Endothelial cells (ECs) can undergo an endothelial-mesenchymal transition with tissue fibrosis. Wnt- and Msx2-regulated signals participate in arteriosclerotic fibrosis and calcification. We studied the impact of Wnt7, Msx2, and Dkk1, a Wnt7 antagonist, on endothelial-mesenchymal transition in primary aortic ECs. APPROACH AND RESULTS: Transduction of aortic ECs with vectors expressing Dkk1 suppressed EC differentiation and induced a mineralizing myofibroblast phenotype. Dkk1 suppressed claudin 5, PECAM, cadherin 5 (Cdh5), Tie1, and Tie2. Dkk1 converted the cuboidal cell monolayer into a spindle-shaped multilayer and inhibited EC cord formation. Myofibroblast and osteogenic markers, SM22, type I collagen, Osx, Runx2, and alkaline phosphatase, were upregulated by Dkk1 via activin-like kinase/Smad pathways. Dkk1 increased fibrotic mineralization of aortic ECs cultured under osteogenic conditions--the opposite of mesenchymal cell responses. Msx2 and Wnt7b maintained morphology and upregulated markers of differentiated ECs. Deleting EC Wnt7b with the Cdh5-Cre transgene in Wnt7b(fl/fl);LDLR(-/-) mice upregulated aortic osteogenic genes (Osx, Sox9, Runx2, and Msx2) and nuclear phospho-Smad1/5, and increased collagen and calcium accumulation. CONCLUSIONS: Dkk1 enhances endothelial-mesenchymal transition in aortic ECs, whereas Wnt7b and Msx2 signals preserve EC phenotype. EC responses to Dkk1, Wnt7b, and Msx2 are the opposite of mesenchymal responses, coupling EC phenotypic stability with osteofibrogenic predilection during arteriosclerosis.

Wnt5a is necessary for normal kidney development in zebrafish and mice.

BACKGROUND: Wnt5a is important for the development of various organs and postnatal cellular function. Little is known, however, about the role of Wnt5a in kidney development, although WNT5A mutations were identified in patients with Robinow syndrome, a genetic disease which includes developmental defects in kidneys. Our goal in this study was to determine the role of Wnt5a in kidney development. METHODS: Whole-mount in situ hybridization was used to establish the expression pattern of Wnt5a during kidney development. Zebrafish with wnt5a knockdown and Wnt5a global knockout mice were used to identify kidney phenotypes. RESULTS: In zebrafish, wnt5a knockdown resulted in glomerular cyst formation and dilated renal tubules. In mice, Wnt5a global knockout resulted in pleiotropic, but severe, kidney phenotypes, including agenesis, fused kidney, hydronephrosis and duplex kidney/ureter. CONCLUSIONS: Our data demonstrated the important role of Wnt5a in kidney development. Disrupted Wnt5a resulted in kidney cysts in zebrafish and pleiotropic abnormal kidney development in mice.

[Cilia and renal cysts]. / Cils et kystes rénaux.

Advances in genomics, bioinformatics and the creation of model organisms have identified many genes associated with polycystic kidney diseases. Historically, these genes were not necessarily associated with ciliopathies, but it appeared that many connections can be made between the cystic kidney disease and function of the primary cilium. Indeed, the proteins encoded by these genes are localized to the cilium itself, to the basal body or are known to regulate the expression and localization of ciliary proteins. The goal of this article is to describe the multiple cellular processes that may lead to the development of renal cysts if they are deregulated. These include changes in proliferation rate, cell polarity or signaling pathways involved in embryonic kidney development. To highlight the role of the primary cilium in cystogenesis, I will discuss several studies investigating the function of ciliary genes and cilia in the kidneys of different model organisms.

Wnt/ß-catenin signaling maintains the mesenchymal precursor pool for murine sinus horn formation.

RATIONALE: Canonical (ß-catenin [Ctnnb1]-dependent) wingless-related MMTV integration site (Wnt) signaling plays an important role in the development of second heart field-derived structures of the heart by regulating precursor cell proliferation. The signaling pathways that regulate the most posterior elongation of the heart, that is, the addition of the systemic venous return from a Tbx18(+) precursor population, have remained elusive. OBJECTIVE: To define the role of Ctnnb1-dependent Wnt signaling in the development of the cardiac venous pole. METHODS AND RESULTS: We show by in situ hybridization analysis that Wnt pathway components are expressed and canonical Wnt signaling is active in the developing sinus horns. We analyzed sinus horn (Tbx18(cre))-specific Ctnnb1 loss- and gain-of-function mutant embryos. In Ctnnb1-deficient embryos, the dorsal part of the sinus horns is not myocardialized but consists of cells with at least partial fibroblast identity; the sinoatrial node is unaffected. Stabilization of Ctnnb1 in this domain results in the formation of undifferentiated cell aggregates. Analysis of cellular changes revealed a role of canonical Wnt signaling in proliferation of the Tbx18(+) mesenchymal progenitor cell population. CONCLUSIONS: Wnt/ß-catenin signaling maintains the Tbx18(+)Nkx2-5(-) mesenchymal precursor pool for murine sinus horn formation.