Direct regulation of the Xenopus engrailed-2 promoter by the Wnt signaling pathway, and a molecular screen for Wnt-responsive genes, confirm a role for Wnt signaling during neural patterning in Xenopus

Engrailed 1 deficiency induces changes in ciliogenesis during human neuronal differentiation

A key pathological feature of Parkinson's Disease (PD) is the progressive degeneration of dopaminergic neurons (DAns) in the substantia nigra pars compacta. Considering the major role of EN1 in the development and maintenance of these DAns and the implications from En1 mouse models, it is highly interesting to study the molecular and protective effect of EN1 also in a human cellular model. Therefore, we generated EN1 knock-out (ko) human induced pluripotent stem cell (hiPSCs) lines and analyzed these during neuronal differentiation. Although the EN1 ko didn't interfere with neuronal differentiation and generation of tyrosine hydroxylase positive (TH+) neurons per se, the neurons exhibited shorter neurites. Furthermore, mitochondrial respiration, as well as mitochondrial complex I abundance was significantly reduced in fully differentiated neurons. To understand the implications of an EN1 ko during differentiation, we performed a transcriptome analysis of human neuronal precursor cells (hNPCs) which unveiled alterations in cilia-associated pathways. Further analysis of ciliary morphology revealed an elongation of primary cilia in EN1-deficient hNPCs. Besides, also Wnt signaling pathways were severely affected. Upon stimulating hNPCs with Wnt which drastically increased EN1 expression in WT lines, the phenotypes concerning mitochondrial function and cilia were exacerbated in EN1 ko hNPCs. They failed to enhance the expression of the complex I subunits NDUFS1 and 3, and now displayed a reduced mitochondrial respiration. Furthermore, Wnt stimulation decreased ciliogenesis in EN1 ko hNPCs but increased ciliary length even further. This further highlights the relevance of primary cilia next to mitochondria for the functionality and correct maintenance of human DAns and provides new possibilities to establish neuroprotective therapies for PD.

NOTUM promotes thermogenic capacity and protects against diet-induced obesity in male mice

We recently showed that NOTUM, a liver-secreted Wnt inhibitor, can acutely promote browning of white adipose. We now report studies of chronic overexpression of NOTUM in liver indicating that it protects against diet-induced obesity and improves glucose homeostasis in mice. Adeno-associated virus (AAV) vectors were used to overexpress GFP or mouse Notum in the livers of male C57BL/6J mice and the mice were fed an obesifying diet. After 14 weeks of high fat, high sucrose diet feeding, the AAV-Notum mice exhibited decreased obesity and improved glucose tolerance compared to the AAV-GFP mice. Gene expression and immunoblotting analysis of the inguinal fat and brown fat revealed increased expression of beige/brown adipocyte markers in the AAV-Notum group, suggesting enhanced thermogenic capacity by NOTUM. A β3 adrenergic receptor agonist-stimulated lipolysis test suggested increased lipolysis capacity by NOTUM. The levels of collagen and C–C motif chemokine ligand 2 (CCL2) in the epididymal white adipose tissue of the AAV-Notum mice were significantly reduced, suggesting decreased fibrosis and inflammation, respectively. RNA sequencing analysis of inguinal white adipose of 4-week chow diet-fed mice revealed a highly significant enrichment of extracellular matrix (ECM) functional cluster among the down-regulated genes in the AAV-Notum group, suggesting a potential mechanism contributing to improved glucose homeostasis. Our in vitro studies demonstrated that recombinant human NOTUM protein blocked the inhibitory effects of WNT3A on brown adipocyte differentiation. Furthermore, NOTUM attenuated WNT3A’s effects on upregulation of TGF-β signaling and its downstream targets. Overall, our data suggest that NOTUM modulates adipose tissue function by promoting thermogenic capacity and inhibiting fibrosis through inhibition of Wnt signaling.

Wnt signaling regulates neural plate patterning in distinct temporal phases with dynamic transcriptional outputs

The process that partitions the nascent vertebrate central nervous system into forebrain, midbrain, hindbrain, and spinal cord after neural induction is of fundamental interest in developmental biology, and is known to be dependent on Wnt/β-catenin signaling at multiple steps. Neural induction specifies neural ectoderm with forebrain character that is subsequently posteriorized by graded Wnt signaling: embryological and mutant analyses have shown that progressively higher levels of Wnt signaling induce progressively more posterior fates. However, the mechanistic link between Wnt signaling and the molecular subdivision of the neural ectoderm into distinct domains in the anteroposterior (AP) axis is still not clear. To better understand how Wnt mediates neural AP patterning, we performed a temporal dissection of neural patterning in response to manipulations of Wnt signaling in zebrafish. We show that Wnt-mediated neural patterning in zebrafish can be divided into three phases: (I) a primary AP patterning phase, which occurs during gastrulation, (II) a mes/r1 (mesencephalon-rhombomere 1) specification and refinement phase, which occurs immediately after gastrulation, and (III) a midbrain-hindbrain boundary (MHB) morphogenesis phase, which occurs during segmentation stages. A major outcome of these Wnt signaling phases is the specification of the major compartment divisions of the developing brain: first the MHB, then the diencephalic-mesencephalic boundary (DMB). The specification of these lineage divisions depends upon the dynamic changes of gene transcription in response to Wnt signaling, which we show primarily involves transcriptional repression or indirect activation. We show that otx2b is directly repressed by Wnt signaling during primary AP patterning, but becomes resistant to Wnt-mediated repression during late gastrulation. Also during late gastrulation, Wnt signaling becomes both necessary and sufficient for expression of wnt8b, en2a, and her5 in mes/r1. We suggest that the change in otx2b response to Wnt regulation enables a transition to the mes/r1 phase of Wnt-mediated patterning, as it ensures that Wnts expressed in the midbrain and MHB do not suppress midbrain identity, and consequently reinforce formation of the DMB. These findings integrate important temporal elements into our spatial understanding of Wnt-mediated neural patterning and may serve as an important basis for a better understanding of neural patterning defects that have implications in human health.

Developmentally regulated Tcf7l2 splice variants mediate transcriptional repressor functions during eye formation

Tcf7l2 mediates Wnt/β-Catenin signalling during development and is implicated in cancer and type-2 diabetes. The mechanisms by which Tcf7l2 and Wnt/β-Catenin signalling elicit such a diversity of biological outcomes are poorly understood. Here, we study the function of zebrafish tcf7l2alternative splice variants and show that only variants that include exon five or an analogous human tcf7l2 variant can effectively provide compensatory repressor function to restore eye formation in embryos lacking tcf7l1a/tcf7l1b function. Knockdown of exon five specific tcf7l2 variants in tcf7l1a mutants also compromises eye formation, and these variants can effectively repress Wnt pathway activity in reporter assays using Wnt target gene promoters. We show that the repressive activities of exon5-coded variants are likely explained by their interaction with Tle co-repressors. Furthermore, phosphorylated residues in Tcf7l2 coded exon5 facilitate repressor activity. Our studies suggest that developmentally regulated splicing of tcf7l2 can influence the transcriptional output of the Wnt pathway.

Wnt/β-catenin signaling during early vertebrate neural development

The vertebrate central nervous system (CNS) is comprised of vast number of distinct cell types arranged in a highly organized manner. This high degree of complexity is achieved by cellular communication, including direct cell-cell contact, cell-matrix interactions, and cell-growth factor signaling. Among the several developmental signals controlling the development of the CNS, Wnt proteins have emerged as particularly critical and, hence, have captivated the attention of many researchers. With Wnts' evolutionarily conserved function as primordial symmetry breaking signals, these proteins and their downstream effects are responsible for simultaneously establishing cellular diversity and tissue organization. With their expansive repertoire of secreted agonists and antagonists, cell surface receptors, signaling cascades and downstream biological effects, Wnts are ideally suited to control the complex processes underlying vertebrate neural development. In this review, we will describe the mechanisms by which Wnts exert their potent effects on cells and tissues and highlight the many roles of Wnt signaling during neural development, starting from the initial induction of the neural plate, the subsequent patterning along the embryonic axes, to the intricately organized structure of the CNS. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1239-1259, 2017.

N-acetylcysteine ameliorates repetitive/stereotypic behavior due to its antioxidant properties without activation of the canonical Wnt pathway in a valproic acid-induced rat model of autism

N-acetylcysteine (NAC) is widely used as an antioxidant, and previous studies have suggested that it may have potential as an alternative therapeutic strategy for the treatment of patients with autism. However, the exact effects of NAC administration on the development of autism, as well as the molecular mechanisms underlying its actions, have yet to be fully elucidated. The present study aimed to investigate the effects of NAC on the oxidative status of rats in a valproic acid (VPA)‑induced model of autism, and to examine the involvement of the canonical Wnt signaling pathway in the actions of NAC. Rats exposed to VPA were monitored for behavioral changes, and oxidative stress indicators and key molecules of the canonical Wnt pathway were investigated using colorimetric and western blot analysis, respectively. The present results demonstrated that NAC ameliorated repetitive and stereotypic activity in autism model rats. Furthermore, NAC was revealed to relieve oxidative stress, as demonstrated by the increased glutathione and reduced malondialdehyde levels compared with VPA‑treated rats. However, NAC did not appear to affect the activity of the canonical Wnt signaling pathway. The present findings suggested that the beneficial effects of NAC in autism may be associated with its antioxidative properties, and may not be mediated by the canonical Wnt pathway. However, it may be hypothesized that the canonical Wnt pathway can be indirectly regulated by NAC through the activation of other signaling pathways or upstream factors. Taken together, the present study has contributed to the elucidation of the molecular mechanisms that underlie the actions of NAC in autism, suggesting its potential for the development of novel therapeutic strategies for the treatment of patients with autism.

Knowledge-Guided Bioinformatics Model for Identifying Autism Spectrum Disorder Diagnostic MicroRNA Biomarkers

Autism spectrum disorder (ASD) is a severe neurodevelopmental disease with a high incidence and effective biomarkers are urgently needed for its diagnosis. A few previous studies have reported the detection of miRNA biomarkers for autism diagnosis, especially those based on bioinformatics approaches. In this study, we developed a knowledge-guided bioinformatics model for identifying autism miRNA biomarkers. We downloaded gene expression microarray data from the GEO Database and extracted genes with expression levels that differed in ASD and the controls. We then constructed an autism-specific miRNA-mRNA network and inferred candidate autism biomarker miRNAs based on their regulatory modes and functions. We defined a novel parameter called the autism gene percentage as autism-specific knowledge to further facilitate the identification of autism-specific biomarker miRNAs. Finally, 11 miRNAs were screened as putative autism biomarkers, where eight miRNAs (72.7%) were significantly dysregulated in ASD samples according to previous reports. Functional enrichment results indicated that the targets of the identified miRNAs were enriched in autism-associated pathways, such as Wnt signaling (in KEGG and IPA), cell cycle (in KEGG), and glioblastoma multiforme signaling (in IPA), thereby supporting the predictive power of our model.

EN2 in Prostate Cancer

Despite extensive efforts to identify a clinically useful diagnostic biomarker in prostate cancer, no new test has been approved by regulatory authorities. As a result, this unmet need has shifted to biomarkers that additionally indicate presence or absence of "significant" disease. EN2 is a homeodomain-containing transcription factor secreted by prostate cancer into the urine and can be detected by enzyme-linked immunoassay. EN2 may be an ideal biomarker because normal prostate tissue and benign prostatic hypertrophic cells do not secrete EN2. This review discusses the enormous potential of EN2 to address this unmet need and provide the urologist with a simple, inexpensive, and reliable prostate cancer biomarker.

CD44 functions in Wnt signaling by regulating LRP6 localization and activation

Wnt reception at the membrane is complex and not fully understood. CD44 is a major Wnt target gene in the intestine and is essential for Wnt-induced tumor progression in colorectal cancer. Here we show that CD44 acts as a positive regulator of the Wnt receptor complex. Downregulation of CD44 expression decreases, whereas CD44 overexpression increases Wnt activity in a concentration-dependent manner. Epistasis experiments place CD44 function at the level of the Wnt receptor LRP6. Mechanistically, CD44 physically associates with LRP6 upon Wnt treatment and modulates LRP6 membrane localization. Moreover, CD44 regulates Wnt signaling in the developing brain of Xenopus laevis embryos as shown by a decreased expression of Wnt targets tcf-4 and en-2 in CD44 morphants.

Engrailed homeoproteins in visual system development

Engrailed is a homeoprotein transcription factor. This family of transcription factors is characterized by their DNA-binding homeodomain and some members, including Engrailed, can transfer between cells and regulate protein translation in addition to gene transcription. Engrailed is intimately involved in the development of the vertebrate visual system. Early expression of Engrailed in dorsal mesencephalon contributes to the development and organization of a visual structure, the optic tectum/superior colliculus. This structure is an important target for retinal ganglion cell axons that carry visual information from the retina. Engrailed regulates the expression of Ephrin axon guidance cues in the tectum/superior colliculus. More recently it has been reported that Engrailed itself acts as an axon guidance cue in synergy with the Ephrin system and is proposed to enhance retinal topographic precision.

Mouse IDGenes: a reference database for genetic interactions in the developing mouse brain

The study of developmental processes in the mouse and other vertebrates includes the understanding of patterning along the anterior–posterior, dorsal–ventral and medial– lateral axis. Specifically, neural development is also of great clinical relevance because several human neuropsychiatric disorders such as schizophrenia, autism disorders or drug addiction and also brain malformations are thought to have neurodevelopmental origins, i.e. pathogenesis initiates during childhood and adolescence. Impacts during early neurodevelopment might also predispose to late-onset neurodegenerative disorders, such as Parkinson’s disease. The neural tube develops from its precursor tissue, the neural plate, in a patterning process that is determined by compartmentalization into morphogenetic units, the action of local signaling centers and a well-defined and locally restricted expression of genes and their interactions. While public databases provide gene expression data with spatio-temporal resolution, they usually neglect the genetic interactions that govern neural development. Here, we introduce Mouse IDGenes, a reference database for genetic interactions in the developing mouse brain. The database is highly curated and offers detailed information about gene expressions and the genetic interactions at the developing mid-/hindbrain boundary. To showcase the predictive power of interaction data, we infer new Wnt/β-catenin target genes by machine learning and validate one of them experimentally. The database is updated regularly. Moreover, it can easily be extended by the research community. Mouse IDGenes will contribute as an important resource to the research on mouse brain development, not exclusively by offering data retrieval, but also by allowing data input.

Database URL:http://mouseidgenes.helmholtz-muenchen.de.

Identification of mechanosensitive genes during skeletal development: alteration of genes associated with cytoskeletal rearrangement and cell signalling pathways

Background

Mechanical stimulation is necessary for regulating correct formation of the skeleton. Here we test the hypothesis that mechanical stimulation of the embryonic skeletal system impacts expression levels of genes implicated in developmentally important signalling pathways in a genome wide approach. We use a mutant mouse model with altered mechanical stimulation due to the absence of limb skeletal muscle (Splotch-delayed) where muscle-less embryos show specific defects in skeletal elements including delayed ossification, changes in the size and shape of cartilage rudiments and joint fusion. We used Microarray and RNA sequencing analysis tools to identify differentially expressed genes between muscle-less and control embryonic (TS23) humerus tissue.

Results

We found that 680 independent genes were down-regulated and 452 genes up-regulated in humeri from muscle-less Spd embryos compared to littermate controls (at least 2-fold; corrected p-value ≤0.05). We analysed the resulting differentially expressed gene sets using Gene Ontology annotations to identify significant enrichment of genes associated with particular biological processes, showing that removal of mechanical stimuli from muscle contractions affected genes associated with development and differentiation, cytoskeletal architecture and cell signalling. Among cell signalling pathways, the most strongly disturbed was Wnt signalling, with 34 genes including 19 pathway target genes affected. Spatial gene expression analysis showed that both a Wnt ligand encoding gene (Wnt4) and a pathway antagonist (Sfrp2) are up-regulated specifically in the developing joint line, while the expression of a Wnt target gene, Cd44, is no longer detectable in muscle-less embryos. The identification of 84 genes associated with the cytoskeleton that are down-regulated in the absence of muscle indicates a number of candidate genes that are both mechanoresponsive and potentially involved in mechanotransduction, converting a mechanical stimulus into a transcriptional response.

Conclusions

This work identifies key developmental regulatory genes impacted by altered mechanical stimulation, sheds light on the molecular mechanisms that interpret mechanical stimulation during skeletal development and provides valuable resources for further investigation of the mechanistic basis of mechanoregulation. In particular it highlights the Wnt signalling pathway as a potential point of integration of mechanical and molecular signalling and cytoskeletal components as mediators of the response.

Midbrain dopaminergic neurons: a review of the molecular circuitry that regulates their development

Dopaminergic (DA) neurons of the ventral midbrain (VM) play vital roles in the regulation of voluntary movement, emotion and reward. They are divided into the A8, A9 and A10 subgroups. The development of the A9 group of DA neurons is an area of intense investigation to aid the generation of these neurons from stem cell sources for cell transplantation approaches to Parkinson's disease (PD). This review discusses the molecular processes that are involved in the identity, specification, maturation, target innervation and survival of VM DA neurons during development. The complex molecular interactions of a number of genetic pathways are outlined, as well as recent advances in the mechanisms that regulate subset identity within the VM DA neuronal pool. A thorough understanding of the cellular and molecular mechanisms involved in the development of VM DA neurons will greatly facilitate the use of cell replacement therapy for the treatment of PD.

Synthesis of novel inhibitors blocking Wnt signaling downstream of β-catenin

Large scale screening of libraries consisting of natural and small molecules led to the identification of many small molecule inhibitors repressing Wnt/β-Catenin signaling. However, targeted synthesis of novel Wnt pathway inhibitors has been rarely described. We developed a modular and expedient way to create the aromatic ring system with an aliphatic ring in between. Our synthesis opens up the possibility, in principle, to substitute all positions at the ring system with any desired substituent. Here, we tested five different haloquinone analogs carrying methoxy- and hydroxy-groups at different positions. Bona fide Wnt activity assays in cell culture and in Xenopus embryos revealed that two of these compounds act as potent inhibitors of aberrant activated Wnt/β-Catenin signaling.

A review of the evidence for the canonical Wnt pathway in autism spectrum disorders

Microdeletion and microduplication copy number variations are found in patients with autism spectrum disorder and in a number of cases they include genes that are involved in the canonical Wnt signaling pathway (for example, FZD9, BCL9 or CDH8). Association studies investigating WNT2, DISC1, MET, DOCK4 or AHI1 also provide evidence that the canonical Wnt pathway might be affected in autism. Prenatal medication with sodium-valproate or antidepressant drugs increases autism risk. In animal studies, it has been found that these medications promote Wnt signaling, including among others an increase in Wnt2 gene expression. Notably, the available genetic information indicates that not only canonical Wnt pathway activation, but also inhibition seems to increase autism risk. The canonical Wnt pathway plays a role in dendrite growth and suboptimal activity negatively affects the dendritic arbor. In principle, this provides a logical explanation as to why both hypo- and hyperactivity may generate a similar set of behavioral and cognitive symptoms. However, without a validated biomarker to stratify for deviant canonical Wnt pathway activity, it is probably too dangerous to treat patients with compounds that modify pathway activity.

Xaml1/Runx1 is required for the specification of Rohon-Beard sensory neurons in Xenopus

Lower vertebrates develop a unique set of primary sensory neurons located in the dorsal spinal cord. These cells, known as Rohon-Beard (RB) sensory neurons, innervate the skin and mediate the response to touch during larval stages. Here we report the expression and function of the transcription factor Xaml1/Runx1 during RB sensory neurons formation. In Xenopus embryos Runx1 is specifically expressed in RB progenitors at the end of gastrulation. Runx1 expression is positively regulated by Fgf and canonical Wnt signaling and negatively regulated by Notch signaling, the same set of factors that control the development of other neural plate border cell types, i.e. the neural crest and cranial placodes. Embryos lacking Runx1 function fail to differentiate RB sensory neurons and lose the mechanosensory response to touch. At early stages Runx1 knockdown results in a RB progenitor-specific loss of expression of Pak3, a p21-activated kinase that promotes cell cycle withdrawal, and of N-tub, a neuronal-specific tubulin. Interestingly, the pro-neural gene Ngnr1, an upstream regulator of Pak3 and N-tub, is either unaffected or expanded in these embryos, suggesting the existence of two distinct regulatory pathways controlling sensory neuron formation in Xenopus. Consistent with this possibility Ngnr1 is not sufficient to activate Runx1 expression in the ectoderm. We propose that Runx1 function is critically required for the generation of RB sensory neurons, an activity reminiscent of that of Runx1 in the development of the mammalian dorsal root ganglion nociceptive sensory neurons.

An X chromosome-wide association study in autism families identifies TBL1X as a novel autism spectrum disorder candidate gene in males

BACKGROUND

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder with a strong genetic component. The skewed prevalence toward males and evidence suggestive of linkage to the X chromosome in some studies suggest the presence of X-linked susceptibility genes in people with ASD.

METHODS

We analyzed genome-wide association study (GWAS) data on the X chromosome in three independent autism GWAS data sets: two family data sets and one case-control data set. We performed meta- and joint analyses on the combined family and case-control data sets. In addition to the meta- and joint analyses, we performed replication analysis by using the two family data sets as a discovery data set and the case-control data set as a validation data set.

RESULTS

One SNP, rs17321050, in the transducin β-like 1X-linked (TBL1X) gene [OMIM:300196] showed chromosome-wide significance in the meta-analysis (P value = 4.86 × 10-6) and joint analysis (P value = 4.53 × 10-6) in males. The SNP was also close to the replication threshold of 0.0025 in the discovery data set (P = 5.89 × 10-3) and passed the replication threshold in the validation data set (P = 2.56 × 10-4). Two other SNPs in the same gene in linkage disequilibrium with rs17321050 also showed significance close to the chromosome-wide threshold in the meta-analysis.

CONCLUSIONS

TBL1X is in the Wnt signaling pathway, which has previously been implicated as having a role in autism. Deletions in the Xp22.2 to Xp22.3 region containing TBL1X and surrounding genes are associated with several genetic syndromes that include intellectual disability and autistic features. Our results, based on meta-analysis, joint analysis and replication analysis, suggest that TBL1X may play a role in ASD risk.

Transmembrane protein 198 promotes LRP6 phosphorylation and Wnt signaling activation

Wnt/β-catenin signaling is fundamental in embryogenesis and tissue homeostasis in metazoans. Upon Wnt stimulation, cognate coreceptors LRP5 and LRP6 ([LRP5/6] low-density lipoprotein receptor-related proteins 5 and 6) are activated via phosphorylation at key residues. Although several kinases have been implicated, the LRP5/6 activation mechanism remains unclear. Here, we report that transmembrane protein 198 (TMEM198), a previously uncharacterized seven-transmembrane protein, is able to specifically activate LRP6 in transducing Wnt signaling. TMEM198 associates with LRP6 and recruits casein kinase family proteins, via the cytoplasmic domain, to phosphorylate key residues important for LRP6 activation. In mammalian cells, TMEM198 is required for Wnt signaling and casein kinase 1-induced LRP6 phosphorylation. During Xenopus embryogenesis, maternal and zygotic tmem198 mRNAs are widely distributed in the ectoderm and mesoderm. TMEM198 is required for Wnt-mediated neural crest formation, antero-posterior patterning, and particularly engrailed-2 expression in Xenopus embryos. Thus, our results identified TMEM198 as a membrane scaffold protein that promotes LRP6 phosphorylation and Wnt signaling activation.

Stem cell potential in Parkinson's disease and molecular factors for the generation of dopamine neurons

Parkinson's disease (PD) involves the loss of dopamine (DA) neurons, making it the most expected neurodegenerative disease to be treated by cell replacement therapy. Stem cells are a promising source for cell replacement therapy due to their ability to self-renew and their pluripotency/multipotency that allows them to generate various types of cells. However, it is challenging to derive midbrain DA neurons from stem cells. Thus, in this review, I will discuss the molecular factors that are known to play critical roles in the generation and survival of DA neurons. The developmental process of DA neurons and functions of extrinsic soluble factors and homeodomain proteins, forkhead box proteins, proneural genes, Nurr1 and genes involved in epigenetic control are discussed. In addition, different types of stem cells that have potential for future cell replacement therapy are reviewed.

Targets and effects of yessotoxin, okadaic acid and palytoxin: a differential review

In this review, we focus on processes, organs and systems targeted by the marine toxins yessotoxin (YTX), okadaic acid (OA) and palytoxin (PTX). The effects of YTX and their basis are analyzed from data collected in the mollusc Mytilus galloprovincialis, the annelid Enchytraeus crypticus, Swiss CD1 mice and invertebrate and vertebrate cell cultures. OA and PTX, two toxins with a better established mode of action, are analyzed with regard to their effects on development. The amphibian Xenopus laevis is used as a model, and the Frog Embryo Teratogenesis Assay-Xenopus (FETAX) as the experimental protocol.

Involvement of Neptune in induction of the hatching gland and neural crest in the Xenopus embryo

Neptune, a Krüppel-like transcription factor, is expressed in various regions of the developing Xenopus embryo and it has multiple functions in the process of development in various organs. In situ hybridization analysis showed that Neptune is expressed in the boundary region between neural and non-neural tissues at the neurula stage, but little is known about the function of Neptune in this region. Here, we examined the expression and function of Neptune in the neural plate border (NPB) in the Xenopus embryo. Depletion of Neptune protein in developing embryos by using antisense MO caused loss of the hatching gland and otic vesicle as well as malformation of neural crest-derived cranial cartilages and melanocytes. Neptune MO also suppressed the expression of hatching gland and neural crest markers such as he, snail2, sox9 and msx1 at the neurula stage. Subsequent experiments showed that Neptune is necessary and sufficient for the differentiation of hatching gland cells and that it is located downstream of pax3 in the signal regulating the differentiation of these cells. Thus, Neptune is a new member of hatching gland specifier and plays a physiological role in determination and specification of multiple lineages derived from the NPB region.

Requirement of prorenin receptor and vacuolar H+-ATPase-mediated acidification for Wnt signaling

Wnt/beta-catenin signaling is important in stem cell biology, embryonic development, and disease, including cancer. However, the mechanism of Wnt signal transmission, notably how the receptors are activated, remains incompletely understood. We found that the prorenin receptor (PRR) is a component of the Wnt receptor complex. PRR functions in a renin-independent manner as an adaptor between Wnt receptors and the vacuolar H+-adenosine triphosphatase (V-ATPase) complex. Moreover, PRR and V-ATPase were required to mediate Wnt signaling during antero-posterior patterning of Xenopus early central nervous system development. The results reveal an unsuspected role for the prorenin receptor, V-ATPase activity, and acidification during Wnt/beta-catenin signaling.

Wnt signaling and the polarity of the primary body axis

How animals establish and pattern the primary body axis is one of the most fundamental problems in biology. Data from diverse deuterostomes (frog, fish, mouse, and amphioxus) and from planarians (protostomes) suggest that Wnt signaling through beta-catenin controls posterior identity during body plan formation in most bilaterally symmetric animals. Wnt signaling also influences primary axis polarity of pre-bilaterian animals, indicating that an axial patterning role for Wnt signaling predates the evolution of bilaterally symmetric animals. The use of posterior Wnt signaling and anterior Wnt inhibition might be a unifying principle of body plan development in most animals.

The posteriorizing gene Gbx2 is a direct target of Wnt signalling and the earliest factor in neural crest induction

Wnt signalling is required for neural crest (NC) induction; however, the direct targets of the Wnt pathway during NC induction remain unknown. We show here that the homeobox gene Gbx2 is essential in this process and is directly activated by Wnt/beta-catenin signalling. By ChIP and transgenesis analysis we show that the Gbx2 regulatory elements that drive expression in the NC respond directly to Wnt/beta-catenin signalling. Gbx2 has previously been implicated in posteriorization of the neural plate. Here we unveil a new role for this gene in neural fold patterning. Loss-of-function experiments using antisense morpholinos against Gbx2 inhibit NC and expand the preplacodal domain, whereas Gbx2 overexpression leads to transformation of the preplacodal domain into NC cells. We show that the NC specifier activity of Gbx2 is dependent on the interaction with Zic1 and the inhibition of preplacodal genes such as Six1. In addition, we demonstrate that Gbx2 is upstream of the neural fold specifiers Pax3 and Msx1. Our results place Gbx2 as the earliest factor in the NC genetic cascade being directly regulated by the inductive molecules, and support the notion that posteriorization of the neural folds is an essential step in NC specification. We propose a new genetic cascade that operates in the distinction between anterior placodal and NC territories.

Expression of the genes siamois, engrailed-2, bmp4 and myf5 during Xenopus development in presence of the marine toxins okadaic acid and palytoxin

The present investigation examines the effects of the marine toxins, okadaic acid (OA) and palytoxin (PTX), on some genes involved in the neural and muscular specification and patterning of Xenopus laevis. The RT-PCR analyses performed at different stages of embryonic and larval development (stages 11-47) demonstrated that both toxins induce an over-expression of the genes siamois and engrailed-2 and a different behaviour in bmp4 and myf5. Indeed, OA provoked a significant increase in bmp4 in the earliest stage (11) examined, a down-regulation from stages 12 to 17, and a renewed increase from the beginning of hatching onwards (stages 35-47). In contrast, myf5 was up-regulated in all stages up to 35. PTX induced an over-expression of both bmp4 and myf5 during the embryonic and early larval development stages. The results show that PTX induces an increase in expression levels in all tested genes, while the response to OA seems to be more stage-dependent, with the embryonic development stage more sensitive to the toxin than the larval stages.

Developmental expression and regulation of the chemokine CXCL14 in Xenopus

Chemokines are a family of proteins originally identified for their activity promoting the recruitment of leukocytes to inflammatory sites. Recent evidence indicates that chemokines and their receptors may also regulate key developmental processes. In this paper we report the expression and regulation of the chemokine CXCL14 during Xenopus laevis embryogenesis. CXCL14 is first detected in several ectoderm derivatives, the dorsal aspect of the retina, the cement gland and the hatching gland. Later in development, additional domains of expression include the head mesenchyme and the medial ventral aspect of the otic vesicle. CXCL14 expression in the ectoderm is regulated by both Bmp and canonical Wnt signaling. In the hatching gland CXCL14 is co-expressed with the transcription factor Pax3. Using gain of function and knockdown approaches in whole embryos and animal explants we show that Pax3 is both necessary and sufficient for CXCL14 expression in this domain of the ectoderm.

Estradiol activates beta-catenin dependent transcription in neurons

Estradiol may fulfill a plethora of functions in neurons, in which much of its activity is associated with its capacity to directly bind and dimerize estrogen receptors. This hormone-protein complex can either bind directly to estrogen response elements (ERE's) in gene promoters, or it may act as a cofactor at non-ERE sites interacting with other DNA-binding elements such as AP-1 or c-Jun. Many of the neuroprotective effects described for estrogen have been associated with this mode of action. However, recent evidence suggests that in addition to these “genomic effects”, estrogen may also act as a more general “trophic factor” triggering cytoplasmic signals and extending the potential activity of this hormone. We demonstrated that estrogen receptor alpha associates with β-catenin and glycogen synthase kinase 3 in the brain and in neurons, which has since been confirmed by others. Here, we show that the action of estradiol activates β-catenin transcription in neuroblastoma cells and in primary cortical neurons. This activation is time and concentration-dependent, and it may be abolished by the estrogen receptor antagonist ICI 182780. The transcriptional activation of β-catenin is dependent on lymphoid enhancer binding factor-1 (LEF-1) and a truncated-mutant of LEF-1 almost completely blocks estradiol TCF-mediated transcription. Transcription of a TCF-reporter in a transgenic mouse model is enhanced by estradiol in a similar fashion to that produced by Wnt3a. In addition, activation of a luciferase reporter driven by the engrailed promoter with three LEF-1 repeats was mediated by estradiol. We established a cell line that constitutively expresses a dominant-negative LEF-1 and it was used in a gene expression microarray analysis. In this way, genes that respond to estradiol or Wnt3a, sensitive to LEF-1, could be identified and validated. Together, these data demonstrate the existence of a new signaling pathway controlled by estradiol in neurons. This pathway shares some elements of the insulin-like growth factor-1/Insulin and Wnt signaling pathways, however, our data strongly suggest that it is different from that of both these ligands. These findings may reveal a set of new physiological roles for estrogens, at least in the Central Nervous System (CNS).

Zebrafish gbx1 refines the midbrain-hindbrain boundary border and mediates the Wnt8 posteriorization signal

Background

Studies in mouse, Xenopus and chicken have shown that Otx2 and Gbx2 expression domains are fundamental for positioning the midbrain-hindbrain boundary (MHB) organizer. Of the two zebrafish gbx genes, gbx1 is a likely candidate to participate in this event because its early expression is similar to that reported for Gbx2 in other species. Zebrafish gbx2, on the other hand, acts relatively late at the MHB. To investigate the function of zebrafish gbx1 within the early neural plate, we used a combination of gain- and loss-of-function experiments.

Results

We found that ectopic gbx1 expression in the anterior neural plate reduces forebrain and midbrain, represses otx2 expression and repositions the MHB to a more anterior position at the new gbx1/otx2 border. In the case of gbx1 loss-of-function, the initially robust otx2 domain shifts slightly posterior at a given stage (70% epiboly), as does MHB marker expression. We further found that ectopic juxtaposition of otx2 and gbx1 leads to ectopic activation of MHB markers fgf8, pax2.1 and eng2. This indicates that, in zebrafish, an interaction between otx2 and gbx1 determines the site of MHB development. Our work also highlights a novel requirement for gbx1 in hindbrain development. Using cell-tracing experiments, gbx1 was found to cell-autonomously transform anterior neural tissue into posterior. Previous studies have shown that gbx1 is a target of Wnt8 graded activity in the early neural plate. Consistent with this, we show that gbx1 can partially restore hindbrain patterning in cases of Wnt8 loss-of-function. We propose that in addition to its role at the MHB, gbx1 acts at the transcriptional level to mediate Wnt8 posteriorizing signals that pattern the developing hindbrain.

Conclusion

Our results provide evidence that zebrafish gbx1 is involved in positioning the MHB in the early neural plate by refining the otx2 expression domain. In addition to its role in MHB formation, we have shown that gbx1 is a novel mediator of Wnt8 signaling during hindbrain patterning.

The 48-kDa alternative translation isoform of PP2A:B56epsilon is required for Wnt signaling during midbrain-hindbrain boundary formation

Alternative translation is an underappreciated post-transcriptional regulation mechanism. Although only a small number of genes are found to be alternatively translated, most genes undergoing alternative translation play important roles in tumorigenesis and development. Protein phosphatase 2A (PP2A) is involved in many cellular events during tumorigenesis and development. The specificity, localization, and activity of PP2A are regulated by B regulatory subunits. B56epsilon, a member of the B56 regulatory subunit family, is involved in multiple signaling pathways and regulates a number of developmental processes. Here we report that B56epsilon is alternatively translated, leading to the production of a full-length form and a shorter isoform that lacks the N-terminal 76 amino acid residues of the full-length form. Alternative translation of B56epsilon occurs through a cap-dependent mechanism. We provide evidence that the shorter isoform is required for Wnt signaling and regulates the midbrain/hindbrain boundary formation during Xenopus embryonic development. This demonstrates that the shorter isoform of B56epsilon has important biological functions. Furthermore, we show that the N-terminal sequence of B56epsilon, which is not present in the shorter isoform, contains a nuclear localization signal, whereas the C terminus of B56epsilon contains a nuclear export signal. The shorter isoform, which lacks the N-terminal nuclear localization signal, is restricted to the cytoplasm. In contrast, the full-length form can be localized to the nucleus in a cell type-specific manner. The finding that B56epsilon is alternatively translated adds a new level of regulation to PP2A holoenzymes.

Canonical Wnt signaling controls proliferation of retinal stem/progenitor cells in postembryonic Xenopus eyes

Vertebrate retinal stem cells, which reside quiescently within the ciliary margin, may offer a possibility for treatment of degenerative retinopathies. The highly proliferative retinal precursor cells in Xenopus eyes are confined to the most peripheral region, called the ciliary marginal zone (CMZ). Although the canonical Wnt pathway has been implicated in the developing retina of different species, little is known about its involvement in postembryonic retinas. Using a green fluorescent protein-based Wnt-responsive reporter, we show that in transgenic Xenopus tadpoles, the canonical Wnt signaling is activated in the postembryonic CMZ. To further investigate the functional implications of this, we generated transgenic, hormone-inducible canonical Wnt pathway activating and repressing systems, which are directed to specifically intersect at the nuclear endpoint of transcriptional Wnt target gene activation. We found that postembryonic induction of the canonical Wnt pathway in transgenic retinas resulted in increased proliferation in the CMZ compartment. This is most likely due to delayed cell cycle exit, as inferred from a pulse-chase experiment on 5-bromo-2'-deoxyuridine-labeled retinal precursors. Conversely, repression of the canonical Wnt pathway inhibited proliferation of CMZ cells. Neither activation nor repression of the Wnt pathway affected the differentiated cells in the central retina. We conclude that even at postembryonic stages, the canonical Wnt signaling pathway continues to have a major function in promoting proliferation and maintaining retinal stem cells. These findings may contribute to the eventual design of vertebrate, stem cell-based retinal therapies. Disclosure of potential conflicts of interest is found at the end of this article.

Studying Wnt signaling in Xenopus

Xenopus is an established and powerful model system for the study of Wnt signaling in vertebrates. Above all, the relatively large size of the embryos enables microinjection experiments, which have led to key discoveries not only about the functional role of Wnt signaling in vertebrate embryos, but also about the molecular mechanisms of Wnt signaling in vertebrate cells. A major advantage of the Xenopus model is the ability to obtain large numbers of embryos, which develop relatively rapidly and which can be studied in natural separation from sentient adult parental animals. In order to obtain Xenopus embryos, ovulation in females is induced with a simple hormone injection, the eggs collected and fertilized with sperm from males. The Xenopus model system has been further strengthened by recent advances such as morpholino technology and efficient transgenic methods, as well as the development of Xenopus tropicalis as a diploid genetic model system with a shorter generation time and a genome similar to higher vertebrates.

Laser capture-microarray analysis of Lim1 mutant kidney development

The Lim1 gene has essential functions during several stages of kidney development. In particular, a tissue-specific knockout in the early metanephric mesenchyme results in the formation of the earliest nephron precursor, the renal vesicle, but failure of this structure to progress to the next stage, the comma-shaped body. To better understand the molecular nature of this developmental arrest, we used a laser capture microdissection-microarray strategy to examine the perturbed gene expression pattern of the mutant renal vesicles. Among the genes found differently expressed were Chrdl2, an inhibitor of BMP signaling, the proapoptotic factor Bmf, as well as myob5, an atypical myosin that modulates chemokine signaling, and pdgfrl, which is important in epithelial folding. Of particular interest, the microarray data indicated that the Dkk1 gene, which encodes an inhibitor of Wnt signaling, was downregulated ninefold in mutants. This was confirmed by in situ hybridizations. It is interesting to note that Lim1 and Dkk1 mutant mice have striking similarities in phenoytpe. These results suggest that the Dkk1 gene might be a key downstream effector of Lim1 function.

Mutations of the Wnt antagonist AXIN2 (Conductin) result in TCF-dependent transcription in medulloblastomas

Medulloblastomas (MBs) represent the most common malignant brain tumors in children. Most MBs develop sporadically in the cerebellum, but their incidence is highly elevated in patients with familial adenomatous polyposis coli. These patients carry germline mutations in the APC tumor suppressor gene. APC is part of a multiprotein complex involved in the Wnt signaling pathway that controls the stability of beta-catenin, the central effector in this cascade. Previous genetic studies in MBs have identified mutations in genes coding for beta-catenin and its partners, APC and AXIN1, which cause activation of Wnt signaling. The pathway is negatively controlled by the tumor suppressor AXIN2 (Conductin), a scaffold protein of this signaling complex. To investigate whether alterations in AXIN2 may also be involved in the pathogenesis of sporadic MBs, we performed a mutational screening of the AXIN2 gene in 116 MB biopsy samples and 11 MB cell lines using single-strand conformation polymorphism and sequencing analysis. One MB displayed a somatic, tumor-specific 2 bp insertion in exon 5, leading to carboxy-terminal truncation of the AXIN2 protein. This tumor biopsy showed nuclear accumulation of beta-catenin protein, indicating an activation of Wnt signaling. In 2 further MB biopsies, mutations were identified in exon 5 (Glu408Lys) and exon 8 (Ser738Phe) of the AXIN2 gene, which are due to predicted germline mutations and rare polymorphisms. mRNA expression analysis in 22 MBs revealed reduced expression of AXIN2 mRNA compared to 8 fetal cerebellar tissues. Promoter hypermethylation could be ruled out as a major cause for transcriptional silencing by bisulfite sequencing. To study the functional role of AXIN2 in MBs, wild-type AXIN2 was overexpressed in MB cell lines in which the Wnt signaling pathway was activated by Wnt-3a. In this assay, AXIN2 inhibited Wnt signaling demonstrated in luciferase reporter assays. In contrast, overexpression of mutated AXIN2 with a deleted C-terminal DIX-domain resulted in an activation of the Wnt signaling pathway. These findings indicate that mutations of AXIN2 can lead to an oncogenic activation of the Wnt pathway in MBs.

c-Myc transforms human mammary epithelial cells through repression of the Wnt inhibitors DKK1 and SFRP1

c-myc is frequently amplified in breast cancer; however, the mechanism of myc-induced mammary epithelial cell transformation has not been defined. We show that c-Myc induces a profound morphological transformation in human mammary epithelial cells and anchorage-independent growth. c-Myc suppresses the Wnt inhibitors DKK1 and SFRP1, and derepression of DKK1 or SFRP1 reduces Myc-dependent transforming activity. Myc-dependent repression of DKK1 and SFRP1 is accompanied by Wnt target gene activation and endogenous T-cell factor activity. Myc-induced mouse mammary tumors have repressed SFRP1 and increased expression of Wnt target genes. DKK1 and SFRP1 inhibit the transformed phenotype of breast cancer cell lines, and DKK1 inhibits tumor formation. We propose a positive feedback loop for activation of the c-myc and Wnt pathways in breast cancer.

The activity of Pax3 and Zic1 regulates three distinct cell fates at the neural plate border

In Xenopus, the neural plate border gives rise to at least three cell populations: the neural crest, the preplacodal ectoderm, and the hatching gland. To understand the molecular mechanisms that regulate the formation of these lineages, we have analyzed the role of two transcription factors, Pax3 and Zic1, which are among the earliest genes activated in response to neural plate border-inducing signals. At the end of gastrulation, Pax3 and Zic1 are coexpressed in the neural crest forming region. In addition, Pax3 is expressed in progenitors of the hatching gland, and Zic1 is detected in the preplacodal ectoderm. Using gain of function and knockdown approaches in whole embryos and animal explants, we demonstrate that Pax3 and Zic1 are necessary and sufficient to promote hatching gland and preplacodal fates, respectively, whereas their combined activity is essential to specify the neural crest. Moreover, we show that by manipulating the levels of Pax3 and Zic1 it is possible to shift fates among these cells. These findings provide novel information on the mechanisms regulating cell fate decisions at the neural plate border.

The ups and downs of Wnt signaling in prevalent neurological disorders

In order to function properly, the brain must be wired correctly during critical periods in early development. Mistakes in this process are hypothesized to occur in disorders like autism and schizophrenia. Later in life, signaling pathways are essential in maintaining proper communication between neuronal and non-neuronal cells, and disrupting this balance may result in disorders like Alzheimer's disease. The Wnt/beta-catenin pathway has a well-established role in cancer. Here, we review recent evidence showing the involvement of Wnt/beta-catenin signaling in neurodevelopment as well as in neurodegenerative diseases. We suggest that the onset/development of such pathological conditions may involve the additive effect of genetic variation within Wnt signaling components and of molecules that modulate the activity of this signaling cascade.

Frizzled7 mediates canonical Wnt signaling in neural crest induction

The neural crest is a multipotent cell population that migrates from the dorsal edge of the neural tube to various parts of the embryo where it differentiates into a remarkable variety of different cell types. Initial induction of neural crest is mediated by a combination of BMP, Wnt, FGF, Retinoic acid and Notch/Delta signaling. The two-signal model for neural crest induction suggests that BMP signaling induces the competence to become neural crest. The second signal involves Wnt acting through the canonical pathway and leads to expression of neural crest markers such as slug. Wnt signals from the neural plate, non-neural ectoderm and paraxial mesoderm have all been suggested to play a role in neural crest induction. We show that Xenopus frizzled7 (Xfz7) is expressed in the dorsal ectoderm including early neural crest progenitors and is a key mediator of the Wnt inductive signal. We demonstrate that Xfz7 expression is induced in response to a BMP antagonist, noggin, and that Xfz7 can induce neural crest specific genes in noggin-treated ectodermal explants (animal caps). Morpholino-mediated or dominant negative inhibition of Xfz7 inhibits Wnt induced Xslug expression in the animal cap assay and in the whole embryo leading to a loss of neural crest derived pigment cells. Full-length Xfz7 rescues the morpholino-induced phenotype, as does activated beta-catenin, suggesting that Xfz7 is signaling through the canonical pathway. We therefore demonstrate that Xfz7 is regulated by BMP antagonism and is required for neural crest induction by Wnt in the developing vertebrate embryo.

Maternal XTcf1 and XTcf4 have distinct roles in regulating Wnt target genes

Wnt signaling pathways have essential roles in developing embryos and adult tissue, and alterations in their function are implicated in many disease processes including cancers. The major nuclear transducers of Wnt signals are the Tcf/LEF family of transcription factors, which have binding sites for both the transcriptional co-repressor groucho, and the co-activator beta-catenin. The early Xenopus embryo expresses three maternally inherited Tcf/LEF mRNAs, and their relative roles in regulating the expression of Wnt target genes are not understood. We have addressed this by using antisense oligonucleotides to deplete maternal XTcf1 and XTcf4 mRNAs in oocytes. We find that XTcf1 represses expression of Wnt target genes ventrally and laterally, and activates their expression dorsally. Double depletions of XTcf1 and XTcf3 suggest that they act cooperatively to repress Wnt target genes ventrally. In contrast, XTcf4 has no repressive role but is required to activate expression of Xnr3 and chordin in organizer cells at the gastrula stage. This work provides evidence for distinct roles for XTcfs in regulating Wnt target gene expression.

Tcf- and Vent-binding sites regulate neural-specific geminin expression in the gastrula embryo

Vertebrate neural development has been extensively investigated. However, it is unknown for any vertebrate gene how the onset of neural-specific expression in early gastrula embryos is transcriptionally regulated. geminin expression is among the earliest markers of dorsal, prospective neurectoderm at early gastrulation in Xenopus laevis. Here, we identified two 5' sequence domains that are necessary and sufficient to drive neural-specific expression during gastrulation in transgenic Xenopus embryos. Each domain contained putative binding sites for the transcription factor Tcf, which can mediate Wnt signaling and for Vent homeodomain proteins, transcriptional repressors that mediate BMP signaling. Results from embryos transgenic for constructs with mutated Tcf or Vent sites demonstrated that signaling through the Tcf sites was required for dorsal-specific expression at early gastrulation, while signaling through the Vent sites restricted geminin expression to the prospective neurectoderm at mid-gastrulation. Consistent with these results, geminin 5' regulatory sequences and endogenous Xgem responded positively to Wnt signaling and negatively to BMP signaling. The two 5' sequence domains were also conserved among geminin orthologs. Together, these results demonstrate that signaling through Tcf and Vent binding sites regulates transcription of geminin in prospective neurectoderm during gastrulation.

Xenopus connexins: how frogs bridge the gap

Animal species use specialized cell-to-cell channels, called gap junctions, to allow for a direct exchange of ions and small metabolites between their cells' cytoplasm. In invertebrates, gap junctions are formed by innexins, while vertebrates use connexin (Cx) proteins as gap-junction-building blocks. Recently, innexin homologs have been found in vertebrates and named pannexins. From progress in the different genome projects, it has become evident that every class of vertebrates uses their own unique set of Cxs to build their gap junctions. Here, we review all known Xenopus Cxs with respect to their expression, regulation, and function. We compare Xenopus Cxs with those of zebrafish and mouse, and provide evidence for the existence of several additional, non-identified, amphibian Cxs. Finally, we identify two new Xenopus pannexins by screening EST libraries.