Noncanonical Wnt signaling and neural polarity

The role of Wnt signaling in the development of Alzheimer's disease: a potential therapeutic target?

Accumulating evidence supports a key role for Wnt signaling in the development of the central nervous system (CNS) during embryonic development and in the regulation of the structure and function of the adult brain. Alzheimer's disease (AD) is the most common form of senile dementia, which is characterized by β -amyloid (A β ) deposition in specific brain regions. However, the molecular mechanism underlying AD pathology remains elusive. Dysfunctional Wnt signaling is associated with several diseases such as epilepsy, cancer, metabolic disease, and AD. Increasing evidence suggests that downregulation of Wnt signaling, induced by A β , is associated with disease progression of AD. More importantly, persistent activation of Wnt signaling through Wnt ligands, or inhibition of negative regulators of Wnt signaling, such as Dickkopf-1 (DKK-1) and glycogen synthase kinase-3 β (GSK-3 β ) that are hyperactive in the disease state, is able to protect against A β toxicity and ameliorate cognitive performance in AD. Together, these data suggest that Wnt signaling might be a potential therapeutic target of AD. Here, we review recent studies related to the progression of AD where Wnt signaling might be relevant and participate in the development of the disease. Then, we focus on the potential relevance of manipulating the Wnt signaling pathway for the treatment of AD.

TAF4 controls differentiation of human neural progenitor cells through hTAF4-TAFH activity

Expression of general transcription factor and co-activator TAF4 varies during development and in the processes of cell differentiation with suggested connection to neurodegenerative diseases. Here, we show that expression of TAF4 alternative splice variants is different in various regions of the human brain, substantiating the role of alternative splicing of TAF4 in the regulation of neural development and brain function. Most of the described splicing events affect the TAFH homology domain of TAF4 (hTAF4-TAFH). Besides, differentiated towards neural lineages, normal human neural progenitors (NHNPs) lose canonical full-length TAF4 isoform. To study the effects of hTAF4-TAFH splicing on neuronal differentiation, we used RNAi approach to target hTAF4-TAFH-encoding domain in NHNPs. Results show that inactivation of hTAF4-TAFH domain accelerates differentiation of human neural progenitor cells. Conversely, enhanced expression of TAF4 suppresses differentiation and keeps neural progenitor cells in a stem cell-like state. Finally, we provide data on the involvement of TP53 and noncanonical WNT signaling pathways in mediating effects of TAF4 on neuronal differentiation. Overall, our data suggest that specific isoforms of TAF4 may selectively and efficiently control neurogenesis.

ACF7 is a hair-bundle antecedent, positioned to integrate cuticular plate actin and somatic tubulin

The precise morphology of the mechanosensitive hair bundle requires seamless integration of actin and microtubule networks. Here, we identify Acf7a (actin crosslinking family protein 7a) as a protein positioned to bridge these distinct cytoskeletal networks in hair cells. By imaging Acf7a-Citrine fusion protein in zebrafish and immunolabeling of vestibular and cochlear mouse hair cells, we show that Acf7a and ACF7 circumscribe, underlie, and are interwoven into the cuticular plate (CP), and they also encircle the basal body of the kinocilium. In cochlear hair cells, ACF7 localization is graded, with the highest concentration near each fonticulus--an area free of F-actin in the region of the CP that contains the basal body. During hair-cell development and regeneration, Acf7a precedes formation of the hair bundle and CP. Finally, electron tomography demonstrates that the ends of microtubules insert into the CP and are decorated with filamentous linkers connecting microtubules to the CP. These observations are consistent with ACF7 being a linker protein, which may shape the cytoskeleton of the hair cell early during hair-bundle genesis.

Wnt-5a increases NO and modulates NMDA receptor in rat hippocampal neurons

Wnt signaling has a crucial role in synaptic function at the central nervous system. Here we evaluate whether Wnts affect nitric oxide (NO) generation in hippocampal neurons. We found that non-canonical Wnt-5a triggers NO production; however, Wnt-3a a canonical ligand did not exert the same effect. Co-administration of Wnt-5a with the soluble Frizzled related protein-2 (sFRP-2) a Wnt antagonist blocked the NO production. Wnt-5a activates the non-canonical Wnt/Ca(2+) signaling through a mechanism that depends on Ca(2+) release from Ryanodine-sensitive internal stores. The increase in NO levels evoked by Wnt-5a promotes the insertion of the GluN2B subunit of the NMDA receptor (NMDAR) into the neuronal cell surface. To the best of our knowledge, this is the first time that Wnt-5a signaling is related to NO production, which in turn increases NMDARs trafficking to the cell surface.

Clusterin regulates β-amyloid toxicity via Dickkopf-1-driven induction of the wnt-PCP-JNK pathway

Although the mechanism of Aβ action in the pathogenesis of Alzheimer's disease (AD) has remained elusive, it is known to increase the expression of the antagonist of canonical wnt signalling, Dickkopf-1 (Dkk1), whereas the silencing of Dkk1 blocks Aβ neurotoxicity. We asked if clusterin, known to be regulated by wnt, is part of an Aβ/Dkk1 neurotoxic pathway. Knockdown of clusterin in primary neurons reduced Aβ toxicity and DKK1 upregulation and, conversely, Aβ increased intracellular clusterin and decreased clusterin protein secretion, resulting in the p53-dependent induction of DKK1. To further elucidate how the clusterin-dependent induction of Dkk1 by Aβ mediates neurotoxicity, we measured the effects of Aβ and Dkk1 protein on whole-genome expression in primary neurons, finding a common pathway suggestive of activation of wnt-planar cell polarity (PCP)-c-Jun N-terminal kinase (JNK) signalling leading to the induction of genes including EGR1 (early growth response-1), NAB2 (Ngfi-A-binding protein-2) and KLF10 (Krüppel-like factor-10) that, when individually silenced, protected against Aβ neurotoxicity and/or tau phosphorylation. Neuronal overexpression of Dkk1 in transgenic mice mimicked this Aβ-induced pathway and resulted in age-dependent increases in tau phosphorylation in hippocampus and cognitive impairment. Furthermore, we show that this Dkk1/wnt-PCP-JNK pathway is active in an Aβ-based mouse model of AD and in AD brain, but not in a tau-based mouse model or in frontotemporal dementia brain. Thus, we have identified a pathway whereby Aβ induces a clusterin/p53/Dkk1/wnt-PCP-JNK pathway, which drives the upregulation of several genes that mediate the development of AD-like neuropathologies, thereby providing new mechanistic insights into the action of Aβ in neurodegenerative diseases.

Wnts in adult brain: from synaptic plasticity to cognitive deficiencies

During development of the central nervous system the Wnt signaling pathway has been implicated in a wide spectrum of physiological processes, including neuronal connectivity and synapse formation. Wnt proteins and components of the Wnt pathway are expressed in the brain since early development to the adult life, however, little is known about its role in mature synapses. Here, we review evidences indicating that Wnt proteins participate in the remodeling of pre- and post-synaptic regions, thus modulating synaptic function. We include the most recent data in the literature showing that Wnts are constantly released in the brain to maintain the basal neural activity. Also, we review the evidences that involve components of the Wnt pathway in the development of neurological and mental disorders, including a special emphasis on in vivo studies that relate behavioral abnormalities to deficiencies in Wnt signaling. Finally, we include the evidences that support a neuroprotective role of Wnt proteins in Alzheimer’s disease. We postulate that deregulation in Wnt signaling might have a fundamental role in the origin of neurological diseases, by altering the synaptic function at stages where the phenotype is not yet established but when the cognitive decline starts.

WNT3 inhibits cerebellar granule neuron progenitor proliferation and medulloblastoma formation via MAPK activation

During normal cerebellar development, the remarkable expansion of granule cell progenitors (GCPs) generates a population of granule neurons that outnumbers the total neuronal population of the cerebral cortex, and provides a model for identifying signaling pathways that may be defective in medulloblastoma. While many studies focus on identifying pathways that promote growth of GCPs, a critical unanswered question concerns the identification of signaling pathways that block mitogenic stimulation and induce early steps in differentiation. Here we identify WNT3 as a novel suppressor of GCP proliferation during cerebellar development and an inhibitor of medulloblastoma growth in mice. WNT3, produced in early postnatal cerebellum, inhibits GCP proliferation by down-regulating pro-proliferative target genes of the mitogen Sonic Hedgehog (SHH) and the bHLH transcription factor Atoh1. WNT3 suppresses GCP growth through a non-canonical Wnt signaling pathway, activating prototypic mitogen-activated protein kinases (MAPKs), the Ras-dependent extracellular-signal-regulated kinases 1/2 (ERK1/2) and ERK5, instead of the classical β-catenin pathway. Inhibition of MAPK activity using a MAPK kinase (MEK) inhibitor reversed the inhibitory effect of WNT3 on GCP proliferation. Importantly, WNT3 inhibits proliferation of medulloblastoma tumor growth in mouse models by a similar mechanism. Thus, the present study suggests a novel role for WNT3 as a regulator of neurogenesis and repressor of neural tumors.

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.

Novel mutations in Lrp6 orthologs in mouse and human neural tube defects affect a highly dosage-sensitive Wnt non-canonical planar cell polarity pathway

Wnt signaling has been classified as canonical Wnt/β-catenin-dependent or non-canonical planar cell polarity (PCP) pathway. Misregulation of either pathway is linked mainly to cancer or neural tube defects (NTDs), respectively. Both pathways seem to antagonize each other, and recent studies have implicated a number of molecular switches that activate one pathway while simultaneously inhibiting the other thereby partially mediating this antagonism. The lipoprotein receptor-related protein Lrp6 is crucial for the activation of the Wnt/β-catenin pathway, but its function in Wnt/PCP signaling remains largely unknown. In this study, we investigate the role of Lrp6 as a molecular switch between both Wnt pathways in a novel ENU mouse mutant of Lrp6 (Skax26(m1Jus)) and in human NTDs. We demonstrate that Skax26(m1Jus) represents a hypermorphic allele of Lrp6 with increased Wnt canonical and abolished PCP-induced JNK activities. We also show that Lrp6(Skax26-Jus) genetically interacts with a PCP mutant (Vangl2(Lp)) where double heterozygotes showed an increased frequency of NTDs and defects in cochlear hair cells' polarity. Importantly, our study also demonstrates the association of rare and novel missense mutations in LRP6 that is an inhibitor rather than an activator of the PCP pathway with human NTDs. We show that three LRP6 mutations in NTDs led to a reduced Wnt canonical activity and enhanced PCP signaling. Our data confirm an inhibitory role of Lrp6 in PCP signaling in neurulation and indicate the importance of a tightly regulated and highly dosage-sensitive antagonism between both Wnt pathways in this process.

The ciliary cytoskeleton

Cilia and flagella are surface-exposed, finger-like organelles whose core consists of a microtubule (MT)-based axoneme that grows from a modified centriole, the basal body. Cilia are found on the surface of many eukaryotic cells and play important roles in cell motility and in coordinating a variety of signaling pathways during growth, development, and tissue homeostasis. Defective cilia have been linked to a number of developmental disorders and diseases, collectively called ciliopathies. Cilia are dynamic organelles that assemble and disassemble in tight coordination with the cell cycle. In most cells, cilia are assembled during growth arrest in a multistep process involving interaction of vesicles with appendages present on the distal end of mature centrioles, and addition of tubulin and other building blocks to the distal tip of the basal body and growing axoneme; these building blocks are sorted through a region at the cilium base known as the ciliary necklace, and then transported via intraflagellar transport (IFT) along the axoneme toward the tip for assembly. After assembly, the cilium frequently continues to turn over and incorporate tubulin at its distal end in an IFT-dependent manner. Prior to cell division, the cilia are usually resorbed to liberate centrosomes for mitotic spindle pole formation. Here, we present an overview of the main cytoskeletal structures associated with cilia and centrioles with emphasis on the MT-associated appendages, fibers, and filaments at the cilium base and tip. The composition and possible functions of these structures are discussed in relation to cilia assembly, disassembly, and length regulation.

Spatial-temporal expressions of Crumbs and Nagie oko and their interdependence in zebrafish central nervous system during early development

A vast number of apicobasal polarity proteins play essential roles in the polarization and morphogenesis of the neuroepithelia. Crumbs (Crb) type I transmembrane cell-cell adhesion proteins are among these proteins. Five crb genes have been identified in zebrafish. However, their expressional and functional differences during early neural development remain to be fully elucidated. Here, we study the spatial-temporal expression patterns and functions of Crb1, Crb2a, and Crb2b in the central nervous system (CNS) during the neurulation period. We show that: 1, the optic vesicle and undifferentiated retinal neuroepithelium only express Crb2a; 2, Crb1 and Crb2a expressions overlap extensively in the undifferentiated neural tube epithelium; 3, Crb2b expression is the weakest of the three and is restricted to the ventral-most regions of the anterior CNS; and 4, Nok and Crb proteins require each other for their apical localization in neuroepithelium. The commencements of Crb1, Crb2a, and Crb2b expressions follow a spatial-temporal spread from anterior to posterior and from ventral to dorsal and lag behind that of adherens junction components, such as ZO-1 and actin bundles. Genetic and morpholino suppression analyses suggest that in regions where these Crb expressions overlap, they are functionally redundant in maintaining apicobasal polarity of the undifferentiated neuroepithelium.

RNA deep sequencing analysis reveals that nicotine restores impaired gene expression by viral proteins in the brains of HIV-1 transgenic rats

Persons infected with HIV-1 often develop neurologic disorders despite receiving highly active anti-retroviral therapy. Although the underlying mechanism is largely undetermined, our previous RNA-seq-based study showed that the expression of many genes was altered in the central nervous system (CNS) of HIV-1 transgenic (HIV-1Tg) rats. Because nicotine, a natural agonist of nicotinic acetylcholine receptors, exhibits a neuroprotective effect, we presently tested the hypothesis that nicotine restores the expression of altered genes in the CNS of HIV-1Tg rats. Adult male HIV-1Tg and F344 control strain rats were injected with either nicotine (0.25 mg/kg) or saline subcutaneously twice a day for 17 days. Gene expression in the prefrontal cortex (PFC), dorsal hippocampus (HIP), and dorsal striatum (STR) was evaluated using the RNA deep sequencing technique. We found that about 20% of the altered genes in the HIV-1Tg rat were affected by nicotine in each brain region, with the expression of most restored. Analysis of the restored genes showed distinct pathways corrected by nicotine in different brain regions of HIV-1Tg rats. Specifically, the two most significantly restored pathways were Wnt/β-catenin signaling and ephrin B signaling in the PFC, cAMP-responsive element-binding protein (CREB) signaling and glutathione metabolism pathway in the HIP, and tricarboxylic acid (TCA) cycle and calcium signaling in the STR. Together, our findings indicate that cholinergic modulators such as nicotine have beneficial effects on HIV-1-induced neurologic deficits.

Neural tube defects: recent advances, unsolved questions, and controversies

Neural tube defects are severe congenital malformations affecting around one in every 1000 pregnancies. An innovation in clinical management has come from the finding that closure of open spina bifida lesions in utero can diminish neurological dysfunction in children. Primary prevention with folic acid has been enhanced through introduction of mandatory food fortification in some countries, although not yet in the UK. Genetic predisposition accounts for most of the risk of neural tube defects, and genes that regulate folate one-carbon metabolism and planar cell polarity have been strongly implicated. The sequence of human neural tube closure events remains controversial, but studies of mouse models of neural tube defects show that anencephaly, open spina bifida, and craniorachischisis result from failure of primary neurulation, whereas skin-covered spinal dysraphism results from defective secondary neurulation. Other malformations, such as encephalocele, are likely to be postneurulation disorders.

Wnt signaling: role in LTP, neural networks and memory

Wnt components are key regulators of a variety of developmental processes, including embryonic patterning, cell specification, and cell polarity. The Wnt signaling pathway participates in the development of the central nervous system and growing evidence indicates that Wnts also regulates the function of the adult nervous system. In fact, most of the key components including Wnts and Frizzled receptors are expressed in the adult brain. Wnt ligands have been implicated in the regulation of synaptic assembly as well as in neurotransmission and synaptic plasticity. Deregulation of Wnt signaling has been associated with several pathologies, and more recently has been related to neurodegenerative diseases and to mental and mood disorders. In this review, we focus our attention on the Wnt signaling cascade in postnatal life and we review in detail the presence of Wnt signaling components in pre- and postsynaptic regions. Due to the important role of Wnt proteins in wiring neural circuits, we discuss recent findings about the role of Wnt pathways both in basal spontaneous activities as well as in activity-dependent processes that underlie synaptic plasticity. Finally, we review the role of Wnt in vivo and we finish with the most recent data in literature that involves the effect of components of the Wnt signaling pathway in neurological and mental disorders, including a special emphasis on in vivo studies that relate behavioral abnormalities to deficiencies in Wnt signaling, as well as the data that support a neuroprotective role of Wnt proteins in relation to the pathogenesis of Alzheimer's disease.

The genetics of hair cell development and regeneration

Sensory hair cells are exquisitely sensitive vertebrate mechanoreceptors that mediate the senses of hearing and balance. Understanding the factors that regulate the development of these cells is important, not only to increase our understanding of ear development and its functional physiology but also to shed light on how these cells may be replaced therapeutically. In this review, we describe the signals and molecular mechanisms that initiate hair cell development in vertebrates, with particular emphasis on the transcription factor Atoh1, which is both necessary and sufficient for hair cell development. We then discuss recent findings on how microRNAs may modulate the formation and maturation of hair cells. Last, we review recent work on how hair cells are regenerated in many vertebrate groups and the factors that conspire to prevent this regeneration in mammals.

An autocrine Wnt5a-Ror signaling loop mediates sympathetic target innervation

During nervous system development, axon branching at nerve terminals is an essential step in the formation of functional connections between neurons and target cells. It is known that target tissues exert control of terminal arborization through secretion of trophic factors. However, whether the in-growing axons themselves produce diffusible cues to instruct target innervation remains unclear. Here, we use conditional mutant mice to show that Wnt5a derived from sympathetic neurons is required for their target innervation in vivo. Conditional deletion of Wnt5a resulted in specific deficits in the extension and arborization of sympathetic fibers in their final target fields, while no defects were observed in the overall tissue patterning, proliferation, migration or differentiation of neuronal progenitors. Using compartmentalized neuronal cultures, we further demonstrate that the Ror receptor tyrosine kinases are required locally in sympathetic axons to mediate Wnt5a-dependent branching. Thus, our study suggests an autocrine Wnt5a-Ror signaling pathway that directs sympathetic axon branching during target innervation.

Atypical protein kinase Cι is required for Wnt3a-dependent neurite outgrowth and binds to phosphorylated dishevelled 2

Previously we reported that Wnt3a-dependent neurite outgrowth in Ewing sarcoma family tumor cell lines was mediated by Frizzled3, Dishevelled (Dvl), and c-Jun N-terminal kinase (Endo, Y., Beauchamp, E., Woods, D., Taylor, W. G., Toretsky, J. A., Uren, A., and Rubin, J. S. (2008) Mol. Cell. Biol. 28, 2368-2379). Subsequently, we observed that Dvl2/3 phosphorylation correlated with neurite outgrowth and that casein kinase 1δ, one of the enzymes that mediate Wnt3a-dependent Dvl phosphorylation, was required for neurite extension (Greer, Y. E., and Rubin, J. S. (2011) J. Cell Biol. 192, 993-1004). However, the functional relevance of Dvl phosphorylation in neurite outgrowth was not established. Dvl1 has been shown by others to be important for axon specification in hippocampal neurons via an interaction with atypical PKCζ, but the role of Dvl phosphorylation was not evaluated. Here we report that Ewing sarcoma family tumor cells express PKCι but not PKCζ. Wnt3a stimulated PKCι activation and caused a punctate distribution of pPKCι in the neurites and cytoplasm, with a particularly intense signal at the centrosome. Knockdown of PKCι expression with siRNA reagents blocked neurite formation in response to Wnt3a. Aurothiomalate, a specific inhibitor of PKCι/Par6 binding, also suppressed neurite extension. Wnt3a enhanced the co-immunoprecipitation of endogenous PKCι and Dvl2. Although FLAG-tagged wild-type Dvl2 immunoprecipitated with PKCι, a phosphorylation-deficient Dvl2 derivative did not. This derivative also was unable to rescue neurite outgrowth when endogenous Dvl2/3 was suppressed by siRNA (González-Sancho, J. M., Greer, Y. E., Abrahams, C. L., Takigawa, Y., Baljinnyam, B., Lee, K. H., Lee, K. S., Rubin, J. S., and Brown, A. M. (2013) J. Biol. Chem. 288, 9428-9437). Taken together, these results suggest that site-specific Dvl2 phosphorylation is required for Dvl2 association with PKCι. This interaction is likely to be one of the mechanisms essential for Wnt3a-dependent neurite outgrowth.

Differential Expression of Wnt Pathway Genes in Sporadic Hepatocellular Carcinomas Infected With Hepatitis B Virus Identified With OligoGE Arrays

BACKGROUND

Epidemiological evidence has clearly indicated that chronic infection with the hepatitis B virus (HBV) is the major risk factor for developing hepatocellular carcinoma (HCC). Nonetheless, the mechanisms by which HBV contributes to the pathogenesis of HCC have not been fully elucidated.

OBJECTIVES

Our aim was to characterize differential gene expression profiles related to the Wnt signaling pathway between primary tumor and adjacent normal tissues in HCC patients with concomitant HBVinfection .

MATERIALS AND METHODS

An oligoGEArray® (an oligonucleotide-based gene expression array platform) containing 126 Wnt signaling pathway-related genes was used to compare gene expressions between primary HCC and adjacent non-tumorous liver tissues from 10 patients with HCC. Selected differential genes were identified with real-time RT-PCR and immunohistochemistry (IHC). In particular, the protein of the differential gene DVL3 (disheveled, dsh homolog 3 [Drosophila]) was chosen to investigate whether it is up regulated in primary tumor correlated with the clinic pathological characteristics of HCC patients. For this purpose we examined 56 HCC tissue samples via IHC for the presence of DVL3 protein.

RESULTS

Sixteen genes were identified with significant differential expression between HCC and adjacent non-tumorous liver tissue. These genes have been previously associated with the Frizzled signaling pathway, cell cycle, transcription, or protein degradation. All (100%) of the tumor samples results from 56 HCC patients tested were positive for DVL3 via IHC. Based on the intensity of DVL3 immunoreactivity, 25 (44.6%) and 31 (55.4%) of the patients were classified aslow and high-DVL3, respectively, which correlated with tumor stage (P = 0.029).

CONCLUSIONS

This study clarified a number of Wnt pathway-related genes which are dysregulated in HBV-associated HCC. These genes may be contributedto the frequent activation of the Wnt signaling pathway. Our results promote the role of the Wnt signaling pathway in HBV-associated HCC.

ROCK inhibitor (Y-27632) disrupts somitogenesis in chick embryos

AIM

In chick embryos, administration of cadmium (Cd) induces ventral body wall defects (VBWD) similar to human omphalocele. It has been shown that failure of proper VBW formation may be due to disruption of somite development during early embryogenesis. In the VBWD chick model, Cd causes abnormal cell death in the somitic region resulting in improperly developed somites and tortuosity of the neural tube. However, the exact molecular mechanisms leading to VBWD still remain unclear. Wnt signaling is crucial during embryogenesis and plays a key role in normal somite formation. The Rho-associated coiled-coil containing protein kinase (ROCK) is involved in the non-canonical Wnt pathway which controls actin cytoskeleton assembly and cell contractility, and contributes to several developmental processes including somitogenesis. ROCK gene expression levels have recently been reported to be significantly decreased in the Cd-induced VBWD chick model. We designed this study to investigate the hypothesis that administration of ROCK inhibitor (Y-27632) in the absence of Cd disrupts somitogenesis and could contribute to the development of VBWD during early embryogenesis.

METHODS

After 60 h of incubation chick embryos were transferred from eggs to culture dishes containing 20 μM of Y-27632 for experimental group (Y-27, n = 22) or chick saline for controls (n = 14). Following 24 h in the incubator they were assessed for stage development and gross abnormalities in morphology using the dissecting microscope. Western blot was performed to confirm Y-27632 inhibition of ROCK downstream signaling using an antibody against phosphorylated cofilin-2.

RESULTS

20 (90.9 %) embryos from Y-27 group and all controls were alive at examination. Morphological abnormalities were detected in 14 (70 %) Y-27 embryos. Somites appeared improperly developed, flattened in the cranio-caudal direction, and elongated in transverse direction in relation to controls. Chick embryos in Y-27 also presented with tortuosity of the neural tube in the lumbosacral region. Western blot analysis showed inhibition of cofilin-2 phosphorylation in affected embryos in comparison to controls.

CONCLUSION

Our study provides evidence that ROCK inhibitor results in the disruption of normal somitogenesis in chick embryos which may contribute to the failure of fusion of the anterior abdominal wall causing VBWD.

Copy number variation analysis implicates the cell polarity gene glypican 5 as a human spina bifida candidate gene

Neural tube defects (NTDs) are common birth defects of complex etiology. Family and population-based studies have confirmed a genetic component to NTDs. However, despite more than three decades of research, the genes involved in human NTDs remain largely unknown. We tested the hypothesis that rare copy number variants (CNVs), especially de novo germline CNVs, are a significant risk factor for NTDs. We used array-based comparative genomic hybridization (aCGH) to identify rare CNVs in 128 Caucasian and 61 Hispanic patients with non-syndromic lumbar-sacral myelomeningocele. We also performed aCGH analysis on the parents of affected individuals with rare CNVs where parental DNA was available (42 sets). Among the eight de novo CNVs that we identified, three generated copy number changes of entire genes. One large heterozygous deletion removed 27 genes, including PAX3, a known spina bifida-associated gene. A second CNV altered genes (PGPD8, ZC3H6) for which little is known regarding function or expression. A third heterozygous deletion removed GPC5 and part of GPC6, genes encoding glypicans. Glypicans are proteoglycans that modulate the activity of morphogens such as Sonic Hedgehog (SHH) and bone morphogenetic proteins (BMPs), both of which have been implicated in NTDs. Additionally, glypicans function in the planar cell polarity (PCP) pathway, and several PCP genes have been associated with NTDs. Here, we show that GPC5 orthologs are expressed in the neural tube, and that inhibiting their expression in frog and fish embryos results in NTDs. These results implicate GPC5 as a gene required for normal neural tube development.

Wnt signaling: role in Alzheimer disease and schizophrenia

Wnt signaling function starts during the development of the nervous system and is crucial for synaptic plasticity in the adult brain. Clearly Wnt effects in synaptic and plastic processes are relevant, however the implication of this pathway in the prevention of neurodegenerative diseases that produce synaptic impairment, is even more interesting. Several years ago our laboratory found a relationship between the loss of Wnt signaling and the neurotoxicity of the amyloid-β-peptide (Aβ), one of the main players in Alzheimer's disease (AD). Moreover, the activation of the Wnt signaling cascade prevents Aβ-dependent cytotoxic effects. In fact, disrupted Wnt signaling may be a direct link between Aβ-toxicity and tau hyperphosphorylation, ultimately leading to impaired synaptic plasticity and/or neuronal degeneration, indicating that a single pathway can account for both neuro-pathological lesions and altered synaptic function. These observations, suggest that a sustained loss of Wnt signaling function may be a key relevant factor in the pathology of AD. On the other hand, Schizophrenia remains one of the most debilitating and intractable illness in psychiatry. Since Wnt signaling is important in organizing the developing brain, it is reasonable to propose that defects in Wnt signaling could contribute to Schizophrenia, particularly since the neuro-developmental hypothesis of the disease implies subtle dys-regulation of brain development, including some core components of the Wnt signaling pathways such as GSK-3β or Disrupted in Schizophrenia-1 (DISC-1). This review focuses on the relationship between Wnt signaling and its potential relevance for the treatment of neurodegenerative and neuropsychiatric diseases including AD and Schizophrenia.

Tiam1 regulates the Wnt/Dvl/Rac1 signaling pathway and the differentiation of midbrain dopaminergic neurons

Understanding the mechanisms that drive the differentiation of dopaminergic (DA) neurons is crucial for successful development of novel therapies for Parkinson's disease, in which DA neurons progressively degenerate. However, the mechanisms underlying the differentiation-promoting effects of Wnt5a on DA precursors are poorly understood. Here, we present the molecular and functional characterization of a signaling pathway downstream of Wnt5a, the Wnt/Dvl/Rac1 pathway. First, we characterize the interaction between Rac1 and Dvl and identify the N-terminal part of Dvl3 as necessary for Rac1 binding. Next, we show that Tiam1, a Rac1 guanosine exchange factor (GEF), is expressed in the ventral midbrain, interacts with Dvl, facilitates Dvl-Rac1 interaction, and is required for Dvl- or Wnt5a-induced activation of Rac1. Moreover, we show that Wnt5a promotes whereas casein kinase 1 (CK1), a negative regulator of the Wnt/Dvl/Rac1 pathway, abolishes the interactions between Dvl and Tiam1. Finally, using ventral midbrain neurosphere cultures, we demonstrate that the generation of DA neurons in culture is impaired after Tiam1 knockdown, indicating that Tiam1 is required for midbrain DA differentiation. In summary, our data identify Tiam1 as a novel regulator of DA neuron development and as a Dvl-associated and Rac1-specific GEF acting in the Wnt/Dvl/Rac1 pathway.

Wound healing in development

Wound healing is the inherent ability of an organism to protect itself against injuries. Cumulative evidence indicates that the healing process patterns in part embryonic morphogenesis and may result in either organ regeneration or scarring, phenomena that are developmental stage- or age-dependent. Skin is the largest organ. Its morphogenesis and repair mechanisms have been studied extensively due not only to its anatomical location, which allows easy access and observation, but also to its captivating structure and vital function. Thus, this review will focus on using skin as a model organ to illustrate new insights into the mechanisms of wound healing that are developmentally regulated in mammals, with special emphasis on the role of the Wnt signaling pathway and its crosstalk with TGF-β signaling. Relevant information from studies of other organs is discussed where it applies, and the clinical impact from such knowledge and emerging concepts on regenerative medicine are discussed in perspective.

Neural tube defects--disorders of neurulation and related embryonic processes

Neural tube defects (NTDs) are severe congenital malformations affecting 1 in every 1000 pregnancies. 'Open' NTDs result from failure of primary neurulation as seen in anencephaly, myelomeningocele (open spina bifida), and craniorachischisis. Degeneration of the persistently open neural tube in utero leads to loss of neurological function below the lesion level. 'Closed' NTDs are skin-covered disorders of spinal cord structure, ranging from asymptomatic spina bifida occulta to severe spinal cord tethering, and usually traceable to disruption of secondary neurulation. 'Herniation' NTDs are those in which meninges, with or without brain or spinal cord tissue, become exteriorized through a pathological opening in the skull or vertebral column (e.g., encephalocele and meningocele). NTDs have multifactorial etiology, with genes and environmental factors interacting to determine individual risk of malformation. While over 200 mutant genes cause open NTDs in mice, much less is known about the genetic causation of human NTDs. Recent evidence has implicated genes of the planar cell polarity signaling pathway in a proportion of cases. The embryonic development of NTDs is complex, with diverse cellular and molecular mechanisms operating at different levels of the body axis. Molecular regulatory events include the bone morphogenetic protein and Sonic hedgehog pathways which have been implicated in control of neural plate bending. Primary prevention of NTDs has been implemented clinically following the demonstration that folic acid (FA), when taken as a periconceptional supplement, can prevent many cases. Not all NTDs respond to FA, however, and adjunct therapies are required for prevention of this FA-resistant category.

MicroRNA-206 regulates cell movements during zebrafish gastrulation by targeting prickle1a and regulating c-Jun N-terminal kinase 2 phosphorylation

During vertebrate gastrulation, both concurrent inductive events and cell movements are required for axis formation. Convergence and extension (CE) movements contribute to narrowing and lengthening the forming embryonic axis. MicroRNAs (miRNAs) play a critical role in regulating fundamental cellular functions and developmental processes, but their functions in CE movements are not well known. Zebrafish mir206 is maternally expressed and present throughout blastulation and gastrulation periods. Either gain or loss of function of mir206 leads to severe defects of convergent extension movements both cell autonomously and non-cell autonomously. Mosaic lineage tracing studies reveal that the formation of membrane protrusions and actin filaments is disturbed in mir206-overexpressing embryos or mir206 morphants. Mechanistically, mir206 targets prickle1a (pk1a) mRNA and as a result regulates c-Jun N-terminal protein kinase 2 (JNK2) phosphorylation. pk1a overexpression or knockdown can rescue convergent extension defects induced by mir206 overexpression or knockdown, respectively. Therefore, mir206 is an essential, novel regulator for normal convergent and extension movements by regulating mitogen-activated protein kinase (MAPK) JNK signaling.

Initiation of synapse formation by Wnt-induced MuSK endocytosis

In zebrafish, the MuSK receptor initiates neuromuscular synapse formation by restricting presynaptic growth cones and postsynaptic acetylcholine receptors (AChRs) to the center of skeletal muscle cells. Increasing evidence suggests a role for Wnts in this process, yet how muscle cells respond to Wnt signals is unclear. Here, we show that in vivo, wnt11r and wnt4a initiate MuSK translocation from muscle membranes to recycling endosomes and that this transition is crucial for AChR accumulation at future synaptic sites. Moreover, we demonstrate that components of the planar cell polarity pathway colocalize to recycling endosomes and that this localization is MuSK dependent. Knockdown of several core components disrupts MuSK translocation to endosomes, AChR localization and axonal guidance. We propose that Wnt-induced trafficking of the MuSK receptor to endosomes initiates a signaling cascade to align pre- with postsynaptic elements. Collectively, these findings suggest a general mechanism by which Wnt signals shape synaptic connectivity through localized receptor endocytosis.

Development of the bile ducts: essentials for the clinical hepatologist

Several cholangiopathies result from a perturbation of developmental processes. Most of these cholangiopathies are characterised by the persistence of biliary structures with foetal configuration. Developmental processes are also relevant in acquired liver diseases, as liver repair mechanisms exploit a range of autocrine and paracrine signals transiently expressed in embryonic life. We briefly review the ontogenesis of the intra- and extrahepatic biliary tree, highlighting the morphogens, growth factors, and transcription factors that regulate biliary development, and the relationships between developing bile ducts and other branching biliary structures. Then, we discuss the ontogenetic mechanisms involved in liver repair, and how these mechanisms are recapitulated in ductular reaction, a common reparative response to many forms of biliary and hepatocellular damage. Finally, we discuss the pathogenic aspects of the most important primary cholangiopathies related to altered biliary development, i.e. polycystic and fibropolycystic liver diseases, Alagille syndrome.

Effects of fulvestrant on biological activity and Wnt expression in rat GH3 cells

The present study investigated the influence of anti-estrogen treatment (fulvestrant) on pituitary adenoma cell line GH3 biological activity, the estrogen receptor α pathway, the WnT pathway, and mechanisms of decreased Wnt inhibitory factor-1 expression in GH3 cells. Results showed that fulvestrant suppressed GH3 cell proliferation and reduced hormone secretion in a dose-dependent manner. Estrogen receptor α and Wnt4 expression decreased, but Wnt inhibitory factor-1 expression increased in a dose-dependent manner following fulvestrant treatment, and β-catenin expression remained unchanged. Inhibitors of DNA methylation and histone modification upregulated Wnt inhibitory factor-1 expression. Results suggested that fulvestrant suppressed biological activity of GH3 cells via the estrogen receptor α and Wnt pathways. These results suggested that decreased Wnt inhibitory factor-1 expression in GH3 cells played a role in epigenetic mechanisms. Anti-estrogen therapies could provide novel treatments for growth hormone adenomas.

New factors controlling the balance between osteoblastogenesis and adipogenesis

The majority of conditions associated with bone loss, including aging, are accompanied by increased marrow adiposity possibly due to shifting of the balance between osteoblast and adipocyte differentiation in bone marrow stromal (skeletal) stem cells (MSC). In order to study the relationship between osteoblastogenesis and adipogenesis in bone marrow, we have characterized cellular models of multipotent MSC as well as pre-osteoblastic and pre-adipocytic cell populations. Using these models, we identified two secreted factors in the bone marrow microenviroment: secreted frizzled-related protein 1 (sFRP-1) and delta-like1 (preadipocyte factor 1) (Dlk1/Pref-1). Both exert regulatory effects on osteoblastogenesis and adipogenesis. Our studies suggest a model for lineage fate determination of MSC that is regulated through secreted factors in the bone marrow microenvironment that mediate a cross-talk between lineage committed cell populations in addition to controlling differentiation choices of multipotent MSC.

The many roles of PTK7: a versatile regulator of cell-cell communication

PTK7 (protein tyrosine kinase 7) is an evolutionarily conserved transmembrane receptor with functions in various processes ranging from embryonic morphogenesis to epidermal wound repair. Here, we review recent findings indicating that PTK7 is a versatile co-receptor that functions as a molecular switch in Wnt, Semaphorin/Plexin and VEGF signaling pathways. We focus in particular on the role of PTK7 in Wnt signaling, as recent data indicate that PTK7 acts as a Wnt co-receptor, which activates the planar cell polarity pathway, but inhibits canonical Wnt signaling.

The planar cell polarity pathway and parietal endoderm cell migration

Parietal endoderm (PE) migration is the first long-range migratory event in the mammalian embryo contributing to the parietal yolk sac. PE migration can be studied in vitro using the F9 teratocarcinoma stem cell model system. We have found that PE migration is directed and modulated via the Planar Cell Polarity (PCP) pathway through Rho/ROCK signaling. Wnt inhibition using sFRP results in a loss of orientation, visualized by Golgi apparatus localization, along with disorganized microtubules and a lack of robust focal adhesions. Small GTPases are downstream of PCP signaling and Rho/ROCK inhibition results in a loss of orientation, whereas inhibition of Rac does not affect PCP. Activation of canonical Wnt signaling combined with Wnt inhibition does not prevent loss of oriented migration. These data support a role for non-canonical Wnt/PCP signaling directing oriented migration of PE.

Differential expression of Wnts after spinal cord contusion injury in adult rats

BACKGROUND

Spinal cord injury is a major cause of disability that has no clinically accepted treatment. Functional decline following spinal cord injury is caused by mechanical damage, secondary cell death, reactive gliosis and a poor regenerative capacity of damaged axons. Wnt proteins are a family of secreted glycoproteins that play key roles in different developmental processes although little is known of the expression patterns and functions of Wnts in the adult central nervous system in normal or diseased states.

FINDINGS

Using qRT-PCR analysis, we demonstrate that mRNA encoding most Wnt ligands and soluble inhibitors are constitutively expressed in the healthy adult spinal cord. Strikingly, contusion spinal cord injury induced a time-dependent increase in Wnt mRNA expression from 6 hours until 28 days post-injury, and a narrow peak in the expression of soluble Wnt inhibitors between 1 and 3 days post-injury. These results are consistent with the increase in the migration shift, from day 1 to 7, of the intracellular Wnt signalling component, Dishevelled-3. Moreover, after an initial decrease by 1 day, we also found an increase in phosphorylation of the Wnt co-receptor, low-density lipoprotein receptor-related protein 6, and an increase in active β-catenin protein, both of which suffer a dramatic change, from a homogeneous expression pattern in the grey matter to a disorganized injury-induced pattern.

CONCLUSIONS

Our results suggest a role for Wnts in spinal cord homeostasis and injury. We demonstrate that after injury Wnt signalling is activated via the Wnt/β-catenin and possibly other pathways. These findings provide an important foundation to further address the function of individual Wnt proteins in vivo and the pathophysiology of spinal cord injury.

Wnt signaling in prostate development and carcinogenesis

BACKGROUND

The Wnt signaling pathway is crucial for cell fate decisions, stem cell renewal, regulation of cell proliferation and differentiation. Deregulated Wnt signaling is also implicated in a number of hereditary and degenerative diseases and cancer.

METHODS AND RESULTS

This review highlights the role of the Wnt pathway in the regulation of stem/progenitor cell renewal and prostate gland development and how this signaling is altered in prostate cancer. Recent evidence suggests that Wnt signaling regulates androgen activity in prostate cancer cells, enhances androgen receptor expression and promotes the growth of prostate cancer even after androgen ablation therapy. There is also strong evidence that Wnt signaling is enhanced in androgen-ablation resistant tumors and bone metastases.

CONCLUSIONS

Further study of the modulators of this pathway will be of therapeutic relevance as inhibition of Wnt signaling may have the potential to reduce the self-renewal and aggressive behaviour of prostate cancer while Wnt signaling activation might enhance stem cell activity when tissue regeneration is required.

Activation of non-canonical Wnt/JNK pathway by Wnt3a is associated with differentiation fate determination of human bone marrow stromal (mesenchymal) stem cells

The canonical Wnt signaling pathway can determine human bone marrow stromal (mesenchymal) stem cell (hMSC) differentiation fate into osteoblast or adipocyte lineages. However, its downstream targets in MSC are not well characterized. Thus, using DNA microarrays, we compared global gene expression patterns induced by Wnt3a treatment in two hMSC lines: hMSC-LRP5(T253) and hMSC-LRP5(T244) cells carrying known mutations of Wnt co-receptor LRP5 (T253I or T244M) that either enhances or represses canonical Wnt signaling, respectively. Wnt3a treatment of hMSC activated not only canonical Wnt signaling, but also the non-canonical Wnt/JNK pathway through upregulation of several non-canonical Wnt components e.g. naked cuticle 1 homolog (NKD1) and WNT11. Activation of the non-canonical Wnt/JNK pathway by anisomycin enhanced osteoblast differentiation whereas its inhibition by SP600125 enhanced adipocyte differentiation of hMSC. In conclusion, canonical and non-canonical Wnt signaling cooperate in determining MSC differentiation fate.

Cdx mediates neural tube closure through transcriptional regulation of the planar cell polarity gene Ptk7

The vertebrate Cdx genes (Cdx1, Cdx2 and Cdx4) encode homeodomain transcription factors with well-established roles in anteroposterior patterning. To circumvent the peri-implantation lethality inherent to Cdx2 loss of function, we previously used the Cre-loxP system to ablate Cdx2 at post-implantation stages and confirmed a crucial role for Cdx2 function in events related to axial extension. As considerable data suggest that the Cdx family members functionally overlap, we extended this analysis to assess the consequence of concomitant loss of both Cdx1 and Cdx2. Here, we report that Cdx1-Cdx2 double mutants exhibit a severely truncated anteroposterior axis. In addition, these double mutants exhibit fused somites, a widened mediolateral axis and craniorachischisis, a severe form of neural tube defect in which early neurulation fails and the neural tube remains open. These defects are typically associated with deficits in planar cell polarity (PCP) signaling in vertebrates. Consistent with this, we found that expression of Ptk7, which encodes a gene involved in PCP, is markedly reduced in Cdx1-Cdx2 double mutants, and is a candidate Cdx target. Genetic interaction between Cdx mutants and a mutant allele of Scrib, a gene involved in PCP signaling, is suggestive of a role for Cdx signaling in the PCP pathway. These findings illustrate a novel and pivotal role for Cdx function upstream of Ptk7 and neural tube closure in vertebrates.

Epithelial-mesenchymal interactions in biliary diseases

In most cholangiopathies, liver diseases of different etiologies in which the biliary epithelium is the primary target in the pathogenic sequence, the central mechanism involves inflammation. Inflammation, characterized by pleomorphic peribiliary infiltrate containing fibroblasts, macrophages, lymphocytes, as well as endothelial cells and pericytes, is associated to the emergence of "reactive cholangiocytes." These biliary cells do not possess bile secretory functions, are in contiguity with terminal cholangioles, and are of a less-differentiated phenotype. They have acquired several mesenchymal properties, including motility and ability to secrete a vast number of proinflammatory chemo/cytokines and growth factors along with de novo expression of a rich receptor machinery. These functional properties enable reactive cholangiocytes to establish intimate contacts and to mutually exchange a variety of paracrine signals with the different mesenchymal cell types populating the portal infiltrate. The extensive crosstalk between the epithelial and mesenchymal compartments is the driver of liver repair mechanisms in cholangiopathies, ultimately evolving toward portal fibrosis. Herein, the authors first review the properties of the different cell types involved in their interaction, and then analyze the underlying molecular mechanisms as they relate to liver repair in cholangiopathies.

Dishevelled is essential for neural connectivity and planar cell polarity in planarians

The Wingless/Integrated (Wnt) signaling pathway controls multiple events during development and homeostasis. It comprises multiple branches, mainly classified according to their dependence on β-catenin activation. The Wnt/β-catenin branch is essential for the establishment of the embryonic anteroposterior (AP) body axis throughout the phylogenetic tree. It is also required for AP axis establishment during planarian regeneration. Wnt/β-catenin-independent signaling encompasses several different pathways, of which the most extensively studied is the planar cell polarity (PCP) pathway, which is responsible for planar polarization of cell structures within an epithelial sheet. Dishevelled (Dvl) is the hub of Wnt signaling because it regulates and channels the Wnt signal into every branch. Here, we analyze the role of Schmidtea mediterranea Dvl homologs (Smed-dvl-1 and Smed-dvl-2) using gene silencing. We demonstrate that in addition to a role in AP axis specification, planarian Dvls are involved in at least two different β-catenin-independent processes. First, they are essential for neural connectivity through Smed-wnt5 signaling. Second, Smed-dvl-2, together with the S. mediterranea homologs of Van-Gogh (Vang) and Diversin (Div), is required for apical positioning of the basal bodies of epithelial cells. These data represent evidence not only of the function of the PCP network in lophotrocozoans but of the involvement of the PCP core elements Vang and Div in apical positioning of the cilia.

Cartilage biology, pathology, and repair

Osteoarthritis is one of the most common forms of musculoskeletal disease and the most prominent type of arthritis encountered in all countries. Although great efforts have been made to investigate cartilage biology and osteoarthritis pathology, the treatment has lagged behind that of other arthritides, as there is a lack of effective disease-modifying therapies. Numerous approaches for dealing with cartilage degradation have been tried, but enjoyed very little success to develop approved OA treatments with not only symptomatic improvement but also structure-modifying effect. In this review we discuss the most recent findings regarding the regulation of cartilage biology and pathology and highlight their potential therapeutic values.

Wingless-type family member 5A (Wnt-5a) stimulates synaptic differentiation and function of glutamatergic synapses

Growing evidence indicates that Wingless-type (Wnt) signaling plays an important role in the maturation of the central nervous system. We report here that Wingless-type family member 5A (Wnt-5a) is expressed early in development and stimulates dendrite spine morphogenesis, inducing de novo formation of spines and increasing the size of the preexisting ones in hippocampal neurons. Wnt-5a increased intracellular calcium concentration in dendritic processes and the amplitude of NMDA spontaneous miniature currents. Acute application of Wnt-5a increased the amplitude of field excitatory postsynaptic potentials (fEPSP) in hippocampal slices, an effect that was prevented by calcium-channel blockers. The physiological relevance of our findings is supported by studies showing that Wnt scavengers decreased spine density, miniature excitatory postsynaptic currents, and fEPSP amplitude. We conclude that Wnt-5a stimulates different aspects of synaptic differentiation and plasticity in the mammalian central nervous system.

Development of innervation to maxillary whiskers in mice

The maxillary vibrissal pad is a unique, richly innervated sensory apparatus. It is highly evolved in the rodent that it constitutes a major source of sensory information to the somatosensory cortex. In this report, indocarbocyanine tracing and immunofluorescence were used to study the embryonic and early neonatal development of innervation to maxillary vibrissal follicles in mice. The first sign of vibrissal follicle innervation occurred at embryonic day 12 (E12), when the lateral nasal and maxillary processes were penetrated by nerve branches with small terminal plexuses assuming the positions of vibrissal follicle primordia. Between E13 and E15, the nerve plexuses at the presumptive follicles grew in size and became more numerous with no signs of specific receptor subtype formation. By E17, the nerve plexuses had grown further in size and the region-specific receptor subtype specification developed. At birth (P0), the superficial vibrissal nerves began to innervate the apical part of the inner conical body, whereas the deep vibrissal nerve gave off the recurrent cavernous branches. At P3, all of the different sets of receptor subtypes had regional distributions, densities and morphologies comparable to those described in adult mice. A 3-day old mouse had all complements of sensory receptors necessary for somatosensory transduction as revealed not only by neuroanatomic tracing but also with immunofluorescence for several markers of neurosensory differentiation. Our data reveal a hitherto unknown time table for the development of peripheral sensory receptors in the vibrissal follicles. This time table parallels that of their central targets in the somatosensory barrel cortex, which develops at P4.

The Wnt/planar cell polarity protein-tyrosine kinase-7 (PTK7) is a highly efficient proteolytic target of membrane type-1 matrix metalloproteinase: implications in cancer and embryogenesis

PTK7 is an essential component of the Wnt/planar cell polarity (PCP) pathway. We provide evidence that the Wnt/PCP pathway converges with pericellular proteolysis in both normal development and cancer. Here, we demonstrate that membrane type-1 matrix metalloproteinase (MT1-MMP), a key proinvasive proteinase, functions as a principal sheddase of PTK7. MT1-MMP directly cleaves the exposed PKP(621)↓LI sequence of the seventh Ig-like domain of the full-length membrane PTK7 and generates, as a result, an N-terminal, soluble PTK7 fragment (sPTK7). The enforced expression of membrane PTK7 in cancer cells leads to the actin cytoskeleton reorganization and the inhibition of cell invasion. MT1-MMP silencing and the analysis of the uncleavable L622D PTK7 mutant confirm the significance of MT1-MMP proteolysis of PTK7 in cell functions. Our data also demonstrate that a fine balance between the metalloproteinase activity and PTK7 levels is required for normal development of zebrafish (Danio rerio). Aberration of this balance by the proteinase inhibition or PTK7 silencing results in the PCP-dependent convergent extension defects in the zebrafish. Overall, our data suggest that the MT1-MMP-PTK7 axis plays an important role in both cancer cell invasion and normal embryogenesis in vertebrates. Further insight into these novel mechanisms may promote understanding of directional cell motility and lead to the identification of therapeutics to treat PCP-related developmental disorders and malignancy.

The planar polarity protein Scribble1 is essential for neuronal plasticity and brain function

Scribble (Scrib) is a key regulator of apicobasal polarity, presynaptic architecture, and short-term synaptic plasticity in Drosophila. In mammals, its homolog Scrib1 has been implicated in cancer, neural tube closure, and planar cell polarity (PCP), but its specific role in the developing and adult nervous system is unclear. Here, we used the circletail mutant, a mouse model for PCP defects, to show that Scrib1 is located in spines where it influences actin cytoskeleton and spine morphing. In the hippocampus of these mutants, we observed an increased synapse pruning associated with an increased number of enlarged spines and postsynaptic density, and a decreased number of perforated synapses. This phenotype was associated with a mislocalization of the signaling pathway downstream of Scrib1, leading to an overall activation of Rac1 and defects in actin dynamic reorganization. Finally, Scrib1-deficient mice exhibit enhanced learning and memory abilities and impaired social behavior, two features relevant to autistic spectrum disorders. Our data identify Scrib1 as a crucial regulator of brain development and spine morphology, and suggest that Scrib1(crc/+) mice might be a model for studying synaptic dysfunction and human psychiatric disorders.

A Wnt-Frz/Ror-Dsh pathway regulates neurite outgrowth in Caenorhabditis elegans

One of the challenges to understand the organization of the nervous system has been to determine how axon guidance molecules govern axon outgrowth. Through an unbiased genetic screen, we identified a conserved Wnt pathway which is crucial for anterior-posterior (A/P) outgrowth of neurites from RME head motor neurons in Caenorhabditis elegans. The pathway is composed of the Wnt ligand CWN-2, the Frizzled receptors CFZ-2 and MIG-1, the co-receptor CAM-1/Ror, and the downstream component Dishevelled/DSH-1. Among these, CWN-2 acts as a local attractive cue for neurite outgrowth, and its activity can be partially substituted with other Wnts, suggesting that spatial distribution plays a role in the functional specificity of Wnts. As a co-receptor, CAM-1 functions cell-autonomously in neurons and, together with CFZ-2 and MIG-1, transmits the Wnt signal to downstream effectors. Yeast two-hybrid screening identified DSH-1 as a binding partner for CAM-1, indicating that CAM-1 could facilitate CWN-2/Wnt signaling by its physical association with DSH-1. Our study reveals an important role of a Wnt-Frz/Ror-Dsh pathway in regulating neurite A/P outgrowth.

Wnt-5a modulates recycling of functional GABAA receptors on hippocampal neurons

GABA(A) receptors (GABA(A)-Rs) play a significant role in mediating fast synaptic inhibition and it is the main inhibitory receptor in the CNS. The role of Wnt signaling in coordinating synapse structure and function in the mature CNS is poorly understood. In previous studies we found that Wnt ligands can modulate excitatory synapses through remodeling both presynaptic and postsynaptic regions. In this current study we provide evidence for the effect of Wnt-5a on postsynaptic GABA(A)-Rs. We observed that Wnt-5a induces surface expression and maintenance of this receptor in the neuronal membrane. The evoked IPSC recordings in rat hippocampal slice indicate that Wnt-5a can regulates postsynaptically the hippocampal inhibitory synapses. We found also that Wnt-5a: (a) induces the insertion and clustering of GABA(A)-Rs in the membrane; (b) increases the amplitude of GABA-currents due exclusively to postsynaptic mechanisms; (c) does not affect the endocytic process, but increases the receptor recycling. Finally, all these effects on the GABA(A)-Rs are mediated by the activation of calcium/calmodulin-dependent kinase II (CaMKII). Therefore, we postulate that Wnt-5a, by activation of CaMKII, induces the recycling of functional GABA(A)-Rs on the mature hippocampal neurons.

Prickle1b mediates interpretation of migratory cues during zebrafish facial branchiomotor neuron migration

The facial branchiomotor neurons undergo a characteristic tangential migration in the vertebrate hindbrain. Several signaling mechanisms have been implicated in this process, including the non-canonical Wnt/planar cell polarity (PCP) pathway. However, the role of this signaling pathway in controlling the dynamics of these neurons is unclear. Here, we describe the cellular dynamics of the facial neurons as they migrate, focusing on the speed and direction of migration, extension of protrusions, cell shape, and orientation. Furthermore, we show that the PET/LIM domain protein Prickle1b (Pk1b) is required for several aspects of these migratory behaviors, including cell orientation. However, we find that centrosome localization is not significantly affected by disruption of Pk1b function, suggesting that polarization of the neurons is not completely lost. Together, our data suggest that Pk1b function may be required to integrate the multiple migratory cues received by the neurons into polarization instructions for proper posterior movement.

A systems-based approach to investigate dose- and time-dependent methylmercury-induced gene expression response in C57BL/6 mouse embryos undergoing neurulation

BACKGROUND

Aberrations during neurulation due to genetic and/or environmental factors underlie a variety of adverse developmental outcomes, including neural tube defects (NTDs). Methylmercury (MeHg) is a developmental neurotoxicant and teratogen that perturbs a wide range of biological processes/pathways in animal models, including those involved in early gestation (e.g., cell cycle, cell differentiation). Yet, the relationship between these MeHg-linked effects and changes in gestational development remains unresolved. Specifically, current information lacks mechanistic comparisons across dose or time for MeHg exposure during neurulation. These detailed investigations are crucial for identifying sensitive indicators of toxicity and for risk assessment applications.

METHODS

Using a systems-based toxicogenomic approach, we examined dose- and time-dependent effects of MeHg on gene expression in C57BL/6 mouse embryos during cranial neural tube closure, assessing for significantly altered genes and associated Gene Ontology (GO) biological processes. Using the GO-based application GO-Quant, we quantitatively assessed dose- and time-dependent effects on gene expression within enriched GO biological processes impacted by MeHg.

RESULTS

We observed MeHg to significantly alter expression of 883 genes, including several genes (e.g., Vangl2, Celsr1, Ptk7, Twist, Tcf7) previously characterized to be crucial for neural tube development. Significantly altered genes were associated with development cell adhesion, cell cycle, and cell differentiation-related GO biological processes.

CONCLUSIONS

Our results suggest that MeHg-induced impacts within these biological processes during gestational development may underlie MeHg-induced teratogenic and neurodevelopmental toxicity outcomes.