A systematic, genome-wide, phenotype-driven mutagenesis programme for gene function studies in the mouse

As the human genome project approaches completion, the challenge for mammalian geneticists is to develop approaches for the systematic determination of mammalian gene function. Mouse mutagenesis will be a key element of studies of gene function. Phenotype-driven approaches using the chemical mutagen ethylnitrosourea (ENU) represent a potentially efficient route for the generation of large numbers of mutant mice that can be screened for novel phenotypes. The advantage of this approach is that, in assessing gene function, no a priori assumptions are made about the genes involved in any pathway. Phenotype-driven mutagenesis is thus an effective method for the identification of novel genes and pathways. We have undertaken a genome-wide, phenotype-driven screen for dominant mutations in the mouse. We generated and screened over 26,000 mice, and recovered some 500 new mouse mutants. Our work, along with the programme reported in the accompanying paper, has led to a substantial increase in the mouse mutant resource and represents a first step towards systematic studies of gene function in mammalian genetics.

Mutation of Celsr1 disrupts planar polarity of inner ear hair cells and causes severe neural tube defects in the mouse

We identified two novel mouse mutants with abnormal head-shaking behavior and neural tube defects during the course of independent ENU mutagenesis experiments. The heterozygous and homozygous mutants exhibit defects in the orientation of sensory hair cells in the organ of Corti, indicating a defect in planar cell polarity. The homozygous mutants exhibit severe neural tube defects as a result of failure to initiate neural tube closure. We show that these mutants, spin cycle and crash, carry independent missense mutations within the coding region of Celsr1, encoding a large protocadherin molecule [1]. Celsr1 is one of three mammalian homologs of Drosophila flamingo/starry night, which is essential for the planar cell polarity pathway in Drosophila together with frizzled, dishevelled, prickle, strabismus/van gogh, and rhoA. The identification of mouse mutants of Celsr1 provides the first evidence for the function of the Celsr family in planar cell polarity in mammals and further supports the involvement of a planar cell polarity pathway in vertebrate neurulation.

Disruption of scribble (Scrb1) causes severe neural tube defects in the circletail mouse

Circletail is one of only two mouse mutants that exhibit the most severe form of neural tube defect (NTD), termed craniorachischisis. In this disorder, almost the entire brain and spinal cord is affected, owing to a failure to initiate neural tube closure. Craniorachischisis is a significant cause of lethality in humans, yet the molecular mechanisms involved remain poorly understood. Here, we report the identification of the gene mutated in circletail (Crc), using a positional cloning approach. This gene, Scrb1, encodes a member of the LAP protein family related to Drosophila scribble, with 16 leucine rich repeats and four PDZ domains. The Crc mutant contains a single base insertion that creates a frame shift and leads to premature termination of the Scrb1 protein. We report the expression pattern of Scrb1 during embryonic and fetal development, and show that Scrb1 expression closely mirrors the phenotypic defects observed in Crc/Crc mutants. In addition, circletail genetically interacts with the loop-tail mutant, and we reveal overlapping expression of Scrb1 with Vangl2, the gene mutated in loop-tail. The identification of the Crc gene further defines the nature of the genetic pathway required for the initiation of neural tube closure and provides an important new candidate that may be implicated in the aetiology of human NTDs.

Stabilization of Xist RNA mediates initiation of X chromosome inactivation

The onset of X inactivation is preceded by a marked increase in the level of Xist RNA. Here we demonstrate that increased stability of Xist RNA is the primary determinant of developmental up-regulation. Unstable transcript is produced by both alleles in XX ES cells and in XX embryos prior to the onset of random X inactivation. Following differentiation, transcription of unstable RNA from the active X chromosome allele continues for a period following stabilization and accumulation of transcript on the inactive X allele. We discuss the implications of these findings in terms of models for the initiation of random and imprinted X inactivation.

An N-ethyl-N-nitrosourea screen for genes involved in variegation in the mouse

We have developed a sensitized screen to identify genes involved in gene silencing, using random N-ethyl-N-nitrosourea mutagenesis on mice carrying a variegating GFP transgene. The dominant screen has produced six mutant lines, including both suppressors and enhancers of variegation. All are semidominant and five of the six are homozygous embryonic lethal. In one case, the homozygous lethality depends on sex: homozygous females die at midgestation and display abnormal DNA methylation of the X chromosome, whereas homozygous males are viable. Linkage analysis reveals that the mutations map to unique chromosomal locations. We have studied the effect of five of the mutations on expression of an endogenous allele known to be sensitive to epigenetic state, agouti viable yellow. In all cases, there is an effect on penetrance, and in most cases, parent of origin and sex-specific effects are detected. This screen has identified genes that are involved in epigenetic reprogramming of the genome, and the behavior of the mutant lines suggests a common mechanism between X inactivation and transgene and retrotransposon silencing. Our findings raise the possibility that the presence or absence of the X chromosome in mammals affects the establishment of the epigenetic state at autosomal loci by acting as a sink for proteins involved in gene silencing. The study demonstrates the power of sensitized screens in the mouse not only for the discovery of novel genes involved in a particular process but also for the elucidation of the biology of that process.

Loss of Atrx affects trophoblast development and the pattern of X-inactivation in extraembryonic tissues

ATRX is an X-encoded member of the SNF2 family of ATPase/helicase proteins thought to regulate gene expression by modifying chromatin at target loci. Mutations in ATRX provided the first example of a human genetic disease associated with defects in such proteins. To better understand the role of ATRX in development and the associated abnormalities in the ATR-X (alpha thalassemia mental retardation, X-linked) syndrome, we conditionally inactivated the homolog in mice, Atrx, at the 8- to 16-cell stage of development. The protein, Atrx, was ubiquitously expressed, and male embryos null for Atrx implanted and gastrulated normally but did not survive beyond 9.5 days postcoitus due to a defect in formation of the extraembryonic trophoblast, one of the first terminally differentiated lineages in the developing embryo. Carrier female mice that inherit a maternal null allele should be affected, since the paternal X chromosome is normally inactivated in extraembryonic tissues. Surprisingly, however, some carrier females established a normal placenta and appeared to escape the usual pattern of imprinted X-inactivation in these tissues. Together these findings demonstrate an unexpected, specific, and essential role for Atrx in the development of the murine trophoblast and present an example of escape from imprinted X chromosome inactivation.

ATRX belongs to a class of proteins that may modify how DNA is packaged into chromatin, altering the accessibility of other proteins in the nucleus to DNA. In this way, ATRX is thought to influence gene expression. Mutations in the ATRX gene, which is located on the female sex chromosome (X), provided the first example of a human disease (ATR-X syndrome) associated with defects in such proteins. Affected males (X^MUTY) have multiple developmental abnormalities in a wide variety of systems. Currently, it is not understood how proteins like ATRX influence cell biology. To address this question, the authors deleted the version of the gene in mice, Atrx. Although affected male mice (X^MUTY) started to develop normally, they died early in development because they failed to form a normal placenta. In the placenta, female mice normally inactivate the X chromosome that they inherit from their fathers (Xp), so if females inherit from their mother an X chromosome (Xm) that bears the abnormal copy of Atrx (Xm^MUTXp), one would predict that, like affected males, they would fail to form a normal placenta. The authors unexpectedly found this not to be so. They showed, instead, that in such females the normal, paternally derived Atrx gene is active. This study has therefore demonstrated an important facet of X-chromosome imprinting.

Neural plate morphogenesis during mouse neurulation is regulated by antagonism of Bmp signalling

Dorsolateral bending of the neural plate, an undifferentiated pseudostratified epithelium, is essential for neural tube closure in the mouse spinal region. If dorsolateral bending fails, spina bifida results. In the present study, we investigated the molecular signals that regulate the formation of dorsolateral hinge points (DLHPs). We show that Bmp2expression correlates with upper spinal neurulation (in which DLHPs are absent); that Bmp2-null embryos exhibit premature, exaggerated DLHPs;and that the local release of Bmp2 inhibits neural fold bending. Therefore,Bmp signalling is necessary and sufficient to inhibit DLHPs. By contrast, the Bmp antagonist noggin is expressed dorsally in neural folds containing DLHPs,noggin-null embryos show markedly reduced dorsolateral bending and local release of noggin stimulates bending. Hence, Bmp antagonism is both necessary and sufficient to induce dorsolateral bending. The local release of Shh suppresses dorsal noggin expression, explaining the absence of DLHPs at high spinal levels, where notochordal expression of Shh is strong. DLHPs`break through' at low spinal levels, where Shh expression is weaker. Zic2 mutant embryos fail to express Bmp antagonists dorsally and lack DLHPs, developing severe spina bifida. Our findings reveal a molecular mechanism based on antagonism of Bmp signalling that underlies the regulation of DLHP formation during mouse spinal neural tube closure.

Zic2 is required for neural crest formation and hindbrain patterning during mouse development

The Zic genes are the vertebrate homologues of the Drosophila pair rule gene odd-paired. It has been proposed that Zic genes play several roles during neural development including mediolateral segmentation of the neural plate, neural crest induction, and inhibition of neurogenesis. Initially during mouse neural development Zic2 is expressed throughout the neural plate while later on expression in the neurectoderm becomes restricted to the lateral region of the neural plate. A hypomorphic allele of Zic2 has demonstrated that in the mouse Zic2 is required for the timing of neurulation. We have isolated a new allele of Zic2 that behaves as a loss of function allele. Analysis of this mutant reveals two further functions for Zic2 during early neural development. Mutation of Zic2 results in a delay of neural crest production and a decrease in the number of neural crest cells that are produced. These defects are independent of mediolateral segmentation of the neurectoderm and of dorsal neurectoderm proliferation, both of which occur normally in the mutant embryos. Additionally Zic2 is required during hindbrain patterning for the normal development of rhombomeres 3 and 5. This work provides the first genetic evidence that the Zic genes are involved in neural crest production and the first demonstration that Zic2 functions during hindbrain patterning.

BMP-7 influences pattern and growth of the developing hindbrain of mouse embryos

The expression pattern of bone morphogenetic protein-7 (BMP-7) in the hindbrain region of the headfold and early somite stage developing mouse embryo suggests a role for BMP-7 in the patterning of this part of the cranial CNS. In chick embryos it is thought that BMP-7 is one of the secreted molecules which mediates the dorsalizing influence of surface ectoderm on the neural tube, and mouse surface ectoderm has been shown to have a similar dorsalizing effect. While we confirm that BMP-7 is expressed in the surface ectoderm of mouse embryos at the appropriate time to dorsalize the neural tube, we also show that at early stages of hindbrain development BMP-7 transcripts are present in paraxial and ventral tissues, within and surrounding the hindbrain neurectoderm, and only later does expression become restricted to a dorsal domain. To determine more directly the effect that BMP-7 may have on the developing hindbrain we have grafted COS cells expressing BMP-7 into the ventrolateral mesoderm abutting the neurectoderm in order to prolong BMP-7 expression in the vicinity of ventral hindbrain. Three distinct actvities of BMP-7 are apparent. Firstly, as expected from previous work in chick, BMP-7 can promote dorsal characteristics in the neural tube. Secondly, we show that it can also attenuate the expression of sonic hedgehog (Shh) in the floorplate without affecting Shh expression in the notochord. Finally, we find that ectopic BMP-7 appears to promote growth of the neurectoderm. These findings are discussed with respect to possible timing mechanisms necessary for the coordination of hindbrain dorsoventral patterning.

Induction of the mammalian node requires Arkadia function in the extraembryonic lineages

The early mammalian embryo is patterned by signals emanating from extraembryonic and embryonic signalling centres, most notably the anterior visceral endoderm (AVE) and the node, respectively. The AVE is responsible for anterior development, whereas further axis specification depends on the node, the equivalent of Spemann's organizer. Formation of the node, at the anterior primitive streak, depends on expression of the transcription factor HNF3beta (ref. 4). However, both the source and the nature of the signals responsible for inducing the node have been unknown. Here we describe a recessive lethal mutation, arkadia, generated using gene-trap mutagenesis. Mutant embryos establish an AVE but fail to maintain anterior embryonic structures and lack a node. The mutation has disrupted the Arkadia gene, which encodes a putative intracellular protein containing a RING domain. Arkadia is essential for HNF3beta expression in the anterior primitive streak. Analysis with chimaeras, however, shows that Arkadia functions within extraembryonic tissues, revealing that these are required to induce the node. Furthermore, our experiments show that Arkadia interacts genetically with the transforming growth factor (TGF)beta-like factor Nodal, implying that Nodal mediates the function of Arkadia in node induction.

APOE*E2 allele delays age of onset in PSEN1 E280A Alzheimer's disease

Alzheimer's disease (AD) age of onset (ADAOO) varies greatly between individuals, with unique causal mutations suggesting the role of modifying genetic and environmental interactions. We analyzed ~50 000 common and rare functional genomic variants from 71 individuals of the 'Paisa' pedigree, the world's largest pedigree segregating a severe form of early-onset AD, who were affected carriers of the fully penetrant E280A mutation in the presenilin-1 (PSEN1) gene. Affected carriers with ages at the extremes of the ADAOO distribution (30s-70s age range), and linear mixed-effects models were used to build single-locus regression models outlining the ADAOO. We identified the rs7412 (APOE*E2 allele) as a whole exome-wide ADAOO modifier that delays ADAOO by ~12 years (β=11.74, 95% confidence interval (CI): 8.07-15.41, P=6.31 × 10(-8), PFDR=2.48 × 10(-3)). Subsequently, to evaluate comprehensively the APOE (apolipoprotein E) haplotype variants (E1/E2/E3/E4), the markers rs7412 and rs429358 were genotyped in 93 AD affected carriers of the E280A mutation. We found that the APOE*E2 allele, and not APOE*E4, modifies ADAOO in carriers of the E280A mutation (β=8.24, 95% CI: 4.45-12.01, P=3.84 × 10(-5)). Exploratory linear mixed-effects multilocus analysis suggested that other functional variants harbored in genes involved in cell proliferation, protein degradation, apoptotic and immune dysregulation processes (i.e., GPR20, TRIM22, FCRL5, AOAH, PINLYP, IFI16, RC3H1 and DFNA5) might interact with the APOE*E2 allele. Interestingly, suggestive evidence as an ADAOO modifier was found for one of these variants (GPR20) in a set of patients with sporadic AD from the Paisa genetic isolate. This is the first study demonstrating that the APOE*E2 allele modifies the natural history of AD typified by the age of onset in E280A mutation carriers. To the best of our knowledge, this is the largest analyzed sample of patients with a unique mutation sharing uniform environment. Formal replication of our results in other populations and in other forms of AD will be crucial for prediction, follow-up and presumably developing new therapeutic strategies for patients either at risk or affected by AD.

Sexually dimorphic expression of protease nexin-1 and vanin-1 in the developing mouse gonad prior to overt differentiation suggests a role in mammalian sexual development

The mammalian sex-determining pathway is controlled by the presence or absence of SRY expression in the embryonic gonad. Expression of SRY in males is believed to initiate a pathway of gene expression resulting in testis development. In the absence of SRY, ovary development ensues. Several genes have now been placed in this pathway but our understanding of it is far from complete and several functional classes of protein appear to be absent. Sex-determining genes frequently exhibit sexually dimorphic patterns of expression in the developing gonad both before and after overt differentiation of the testis or ovary. In order to identify additional sex-determining or gonadal differentiation genes we have examined gene expression in the developing gonads of the mouse using cDNA microarrays constructed from a normalized urogenital ridge library. We screened for genes exhibiting sexually dimorphic patterns of expression in the gonad at 12.5 and 13.5 days post-coitum, after overt gonad differentiation, by comparing complex cDNA probes derived from male and female gonadal tissue at these stages on micro-arrays. Using in situ hybridization analysis we show here that two genes identified by this screen, protease nexin-1 (Pn-1) and vanin-1 (Vnn1), exhibit male-specific expression prior to overt gonadal differentiation and are detected in the somatic portion of the developing gonad, suggesting a possible direct link to the testis-determining pathway for both genes.

Zic2-associated holoprosencephaly is caused by a transient defect in the organizer region during gastrulation

The putative transcription factor ZIC2 is associated with a defect of forebrain development, known as Holoprosencephaly (HPE), in humans and mouse, yet the mechanism by which aberrant ZIC2 function causes classical HPE is unexplained. The zinc finger domain of all mammalian Zic genes is highly homologous with that of the Gli genes, which are transcriptional mediators of Shh signalling. Mutations in Shh and many other Hh pathway members cause HPE and it has been proposed that Zic2 acts within the Shh pathway to cause HPE. We have investigated the embryological cause of Zic2-associated HPE and the relationship between Zic2 and the Shh pathway using mouse genetics. We show that Zic2 does not interact with Shh to produce HPE. Moreover, molecular defects that are able to account for the HPE phenotype are present in Zic2 mutants before the onset of Shh signalling. Mutation of Zic2 causes HPE via a transient defect in the function of the organizer region at mid-gastrulation which causes an arrest in the development of the prechordal plate (PCP), a structure required for forebrain midline morphogenesis. The analysis provides genetic evidence that Zic2 functions during organizer formation and that the PCP develops via a multi-step process.

The ZIC gene family encodes multi-functional proteins essential for patterning and morphogenesis

The zinc finger of the cerebellum gene (ZIC) discovered in Drosophila melanogaster (odd-paired) has five homologs in Xenopus, chicken, mice, and humans, and seven in zebrafish. This pattern of gene copy expansion is accompanied by a divergence in gene and protein structure, suggesting that Zic family members share some, but not all, functions. ZIC genes are implicated in neuroectodermal development and neural crest cell induction. All share conserved regions encoding zinc finger domains, however their heterogeneity and specification remain unexplained. In this review, the evolution, structure, and expression patterns of the ZIC homologs are described; specific functions attributable to individual family members are supported. A review of data from functional studies in Xenopus and murine models suggest that ZIC genes encode multifunctional proteins operating in a context-specific manner to drive critical events during embryogenesis. The identification of ZIC mutations in congenital syndromes highlights the relevance of these genes in human development.

In vitro analysis of partial loss-of-function ZIC2 mutations in holoprosencephaly: alanine tract expansion modulates DNA binding and transactivation

Heterozygous loss-of-function mutations in ZIC2 result in the severe brain malformation known as holoprosencephaly (HPE), indicating that forebrain development is exquisitely sensitive to the activity of this poorly understood transcription factor. To identify the regions of ZIC2 that are essential for activity, we have assessed the ability of a variety of ZIC2 mutant proteins to function in in vitro assays. Two sources of information were used to design relevant mutations. First, phenotype producing mutations in human and in mouse ZIC2 were mimicked and secondly, a comparative sequence analysis of the C-terminal was carried out. Analysis of these mutations suggests that either a decrease or an increase in ZIC2 mediated transcriptional activity can produce a forebrain phenotype. In addition, the analysis reveals that the C-terminal of ZIC2 contains both activation and repression domains. This region of ZIC2 contains an alanine-tract, and expansion of this domain is associated with HPE. In vitro analysis of proteins with alterations in alanine-tract length illustrates that the C-terminal alanine-tract of ZIC2 influences the strength of DNA binding and alters transcriptional activity in a promoter-specific manner. This finding provides a possible mechanism by which alanine-tract expansion mutations could alter the function of other transcription factors.

Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair

Flightless I (Flii), a highly conserved member of the gelsolin family of actin-remodelling proteins associates with actin structures and is involved in cellular motility and adhesion. Our previous studies have shown that Flii is an important negative regulator of wound repair. Here, we show that Flii affects hemidesmosome formation and integrin-mediated keratinocyte adhesion and migration. Impaired hemidesmosome formation and sparse arrangements of keratin cytoskeleton tonofilaments and actin cytoskeleton anchoring fibrils were observed in Flii(Tg/+) and Flii(Tg/Tg) mice with their skin being significantly more fragile than Flii(+/-) and WT mice. Flii(+/-) primary keratinocytes showed increased adhesion on laminin and collagen I than WT and Flii(Tg/Tg) primary keratinocytes. Decreased expression of CD151 and laminin-binding integrins alpha3, beta1, alpha6 and beta4 were observed in Flii overexpressing wounds, which could contribute to the impaired wound re-epithelialization observed in these mice. Flii interacts with proteins directly linked to the cytoplasmic domain of integrin receptors suggesting that it may be a mechanical link between ligand-bound integrin receptors and the actin cytoskeleton driving adhesion-signaling pathways. Therefore Flii may regulate wound repair through its effect on hemidesmosome formation and integrin-mediated cellular adhesion and migration.

Cloning, mapping, and expression analysis of a gene encoding a novel mammalian EGF-related protein (SCUBE1)

The epidermal growth factor (EGF) superfamily comprises a diverse group of proteins that function as secreted signaling molecules, growth factors, and components of the extracellular matrix, many with a role in vertebrate development. We have isolated a novel mammalian gene encoding an EGF-related protein with a CUB (C1s-like) domain that defines a new mammalian gene family. The Scube1 (signal peptide-CUB domain-EGF-related 1) gene was isolated from a developing mouse urogenital ridge cDNA library and is expressed prominently in the developing gonad, nervous system, somites, surface ectoderm, and limb buds. We have mapped Scube1 to mouse chromosome 15 and show that it is orthologous to a human gene in the syntenic region of chromosome 22q13. We discuss the possible functions of this novel gene and its role in heritable disease in light of these data.

Expression of inhibin subunits and follistatin during postimplantation mouse development: decidual expression of activin and expression of follistatin in primitive streak, somites and hindbrain

Members of the activin family are believed to act as mesoderm-inducing factors during early amphibian development. Little is known, however, about mesoderm formation in the mammalian embryo, and as one approach to investigating this we have studied activin and follistatin expression during early mouse development. Activins are homo- or heterodimers of the beta A or beta B subunits of inhibin, itself a heterodimer consisting of one of the beta subunits together with an alpha subunit. Follistatin is a single-chain polypeptide which inhibits activin function. Expression of the inhibin alpha chain could not be detected in embryonic or extraembryonic tissues at any of the stages studied (5.5 to 8.5 days) and expression of the beta A and beta B subunits could only be observed in the deciduum in cells surrounding the embryo. Expression of follistatin could also be detected in the deciduum, but in a pattern complementary to that of the beta subunits. Embryonic expression of follistatin first occurred in the primitive streak, and at later stages transcripts were detectable in the somites and in rhombomeres 2, 4 and 6 of the hindbrain. These results are consistent with a role for activin in mesoderm formation in the mouse embryo, and suggest functions for follistatin in addition to its role as an inhibitor of activin.

Overlapping and distinct expression domains of Zic2 and Zic3 during mouse gastrulation

The Zic genes are the vertebrate homologues of the Drosophila Odd-paired gene. Mutations in two of these genes are associated with human congenital genetic disorders. Mutation of human and mouse Zic2 is associated with holoprosencephaly which is caused by a defect of ventral forebrain development and mutation of human and mouse Zic3 is associated with a X-linked heterotaxy syndrome that results from a failure of left-right axis formation. The embryological role of the Zic genes in these disorders is not well understood. Here we show that both of these genes are expressed prior to and throughout gastrulation. The genes show some broad similarities in their expression domains. Both genes however are also uniquely expressed in some tissues and these unique domains correlate with regions that potentially play a role in the aetiology of the respective genetic disorders. During primitive streak stages Zic2 is expressed transiently and uniquely in the node and the head process mesendoderm. The head process is known to be required for the establishment or maintenance of the ventral forebrain, which is the region disrupted in holoprosencephaly. Zic3 is not expressed in the node during primitive streak stages but is expressed in and around the node beginning from the head fold stages of development. This expression of Zic3 correlates well with the first steps in the establishment of the left-right axis. We also examined the expression of the closely related gene, Zic1, and did not detect any transcripts in gastrulation stage embryos.

Isolation of novel tissue-specific genes from cDNA libraries representing the individual tissue constituents of the gastrulating mouse embryo

A total of 5 conventional, directionally cloned plasmid cDNA libraries have been constructed from the entire embryonic region of the mid-gastrulation mouse embryo and from its four principal tissue constituents (ectoderm, mesoderm, endoderm and primitive streak). These libraries have been validated with respect to the number of independent clones, insert-size and appropriate representation of diagnostic marker genes. Subtractive hybridisation has been used to remove clones common to the Endoderm and Mesoderm cDNA libraries resulting in an Endoderm minus Mesoderm subtracted library. Probe prepared from this subtracted library has been hybridised to a grid containing approximately 18,500 Embryonic Region library clones. Three novel clones have been recovered as well as expected genes already known to be highly expressed in the primitive endoderm lineage at this stage of development. In situ hybridisation to early postimplantation embryos has revealed the expression patterns of these novel genes. One is highly expressed exclusively in visceral endoderm, one is expressed in ectodermal and endodermal tissues, and the third proves to be an early marker of prospective and differentiated surface ectoderm as well as being expressed in endoderm and its derivatives.

Initiating head development in mouse embryos: integrating signalling and transcriptional activity

The generation of an embryonic body plan is the outcome of inductive interactions between the progenitor tissues that underpin their specification, regionalization and morphogenesis. The intercellular signalling activity driving these processes is deployed in a time- and site-specific manner, and the signal strength must be precisely controlled. Receptor and ligand functions are modulated by secreted antagonists to impose a dynamic pattern of globally controlled and locally graded signals onto the tissues of early post-implantation mouse embryo. In response to the WNT, Nodal and Bone Morphogenetic Protein (BMP) signalling cascades, the embryo acquires its body plan, which manifests as differences in the developmental fate of cells located at different positions in the anterior–posterior body axis. The initial formation of the anterior (head) structures in the mouse embryo is critically dependent on the morphogenetic activity emanating from two signalling centres that are juxtaposed with the progenitor tissues of the head. A common property of these centres is that they are the source of antagonistic factors and the hub of transcriptional activities that negatively modulate the function of WNT, Nodal and BMP signalling cascades. These events generate the scaffold of the embryonic head by the early-somite stage of development. Beyond this, additional tissue interactions continue to support the growth, regionalization, differentiation and morphogenesis required for the elaboration of the structure recognizable as the embryonic head.

Stringent requirement of a proper level of canonical WNT signalling activity for head formation in mouse embryo

In mouse embryos, loss of Dickkopf-1 (DKK1) activity is associated with an ectopic activation of WNT signalling responses in the precursors of the craniofacial structures and leads to a complete truncation of the head at early organogenesis. Here, we show that ENU-induced mutations of genes coding for two WNT canonical pathway factors, the co-receptor LRP6 and the transcriptional co-activator β-catenin, also elicit an ectopic signalling response and result in loss of the rostral tissues of the forebrain. Compound mutant embryos harbouring combinations of mutant alleles of Lrp6, Ctnnb1 and Dkk1 recapitulate the partial to complete head truncation phenotype of individual homozygous mutants. The demonstration of a synergistic interaction of Dkk1, Lrp6 and Ctnnb1 provides compelling evidence supporting the concepts that (1) stringent regulation of the level of canonical WNT signalling is necessary for head formation, (2) activity of the canonical pathway is sufficient to account for the phenotypic effects of mutations in three different components of the signal cascade and (3) rostral parts of the brain and the head are differentially more sensitive to canonical WNT signalling and their development is contingent on negative modulation of WNT signalling activity.

Zinc fingers of the cerebellum (Zic): transcription factors and co-factors

The Zic genes encode zinc finger containing proteins that can bind proteins and DNA. The understanding of Zic molecular networks has been hampered by functional redundancy amongst family members, and because their loss-of-function phenotypes are indicative of a role in many signalling pathways. Recently molecular evidence has emerged confirming the pleiotropic nature of these proteins: they act both as classical transcription factors and as co-factors to directly and indirectly influence gene expression. It has long been known that germ-line mutation of the Zic genes in human and mouse causes a range of congenital disorders. Recently connections between Zic proteins and stem cell function have also emerged suggesting a role in adult onset diseases. The immediate challenge is to determine when and where these proteins act as transcription factors/co-factors during development and disease and how the switch between these roles is controlled.

Wnt signalling in mouse gastrulation and anterior development: new players in the pathway and signal output

Embryonic development and adult homeostasis are dependent upon the coordinated action of signal transduction pathways such as the Wnt signalling pathway which is used iteratively during these processes. In the early post-implantation mouse embryo, Wnt/β-catenin signalling activity plays a critical role in the formation of the primitive streak, progression of gastrulation and tissue patterning in the anterior-posterior axis. The net output of the signalling pathway is influenced by the delivery and post-translational modification of the ligands, the counteracting activities of the activating components and the negative modulators, and the molecular interaction of β-catenin, TCF and other factors regulating the transcription of downstream target genes.

Role of the transcription factor sox4 in insulin secretion and impaired glucose tolerance

OBJECTIVES

To identify, map, clone, and functionally validate a novel mouse model for impaired glucose tolerance and insulin secretion.

RESEARCH DESIGN AND METHODS

Haploinsufficiency of the insulin receptor and associated mild insulin resistance has been used to sensitize an N-ethyl-N-nitrosourea (ENU) screen to identify novel mutations resulting in impaired glucose tolerance and diabetes. The new impaired glucose tolerance 4 (IGT4) model was selected using an intraperitoneal glucose tolerance test and inheritance of the phenotype confirmed by generation of backcross progeny. Segregation of the phenotype was correlated with genotype information to map the location of the gene and candidates sequenced for mutations. The function of the SRY-related high mobility group (HMG)-box 4 (Sox4) gene in insulin secretion was tested using another ENU allele and by small interfering RNA silencing in insulinoma cells.

RESULTS

We describe two allelic autosomal dominant mutations in the highly conserved HMG box of the transcription factor Sox4. Previously associated with pancreas development, Sox4 mutations in the adult mouse result in an insulin secretory defect, which exhibits impaired glucose tolerance in association with insulin receptor(+/-)-induced insulin resistance. Elimination of the Sox4 transcript in INS1 and Min6 cells resulted in the abolition of glucose-stimulated insulin release similar to that observed for silencing of the key metabolic enzyme glucokinase. Intracellular calcium measurements in treated cells indicate that this defect lies downstream of the ATP-sensitive K(+) channel (K(ATP) channel) and calcium influx.

CONCLUSIONS

IGT4 represents a novel digenic model of insulin resistance coupled with an insulin secretory defect. The Sox4 gene has a role in insulin secretion in the adult beta-cell downstream of the K(ATP) channel.