Prenatal ß-catenin/Brn2/Tbr2 transcriptional cascade regulates adult social and stereotypic behaviors.

Social interaction is a fundamental behavior in all animal species, but the developmental timing of the social neural circuit formation and the cellular and molecular mechanisms governing its formation are poorly understood. We generated a mouse model with mutations in two Disheveled genes, Dvl1 and Dvl3, that displays adult social and repetitive behavioral abnormalities associated with transient embryonic brain enlargement during deep layer cortical neuron formation. These phenotypes were mediated by the embryonic expansion of basal neural progenitor cells (NPCs) via deregulation of a ß-catenin/Brn2/Tbr2 transcriptional cascade. Transient pharmacological activation of the canonical Wnt pathway during this period of early corticogenesis rescued the ß-catenin/Brn2/Tbr2 transcriptional cascade and the embryonic brain phenotypes. Remarkably, this embryonic treatment prevented adult behavioral deficits and partially rescued abnormal brain structure in Dvl mutant mice. Our findings define a mechanism that links fetal brain development and adult behavior, demonstrating a fetal origin for social and repetitive behavior deficits seen in disorders such as autism.

Neurodevelopmental and neuropsychiatric behaviour defects arise from 14-3-3ζ deficiency.

Complex neuropsychiatric disorders are believed to arise from multiple synergistic deficiencies within connected biological networks controlling neuronal migration, axonal pathfinding and synapse formation. Here, we show that deletion of 14-3-3ζ causes neurodevelopmental anomalies similar to those seen in neuropsychiatric disorders such as schizophrenia, autism spectrum disorder and bipolar disorder. 14-3-3ζ-deficient mice displayed striking behavioural and cognitive deficiencies including a reduced capacity to learn and remember, hyperactivity and disrupted sensorimotor gating. These deficits are accompanied by subtle developmental abnormalities of the hippocampus that are underpinned by aberrant neuronal migration. Significantly, 14-3-3ζ-deficient mice exhibited abnormal mossy fibre navigation and glutamatergic synapse formation. The molecular basis of these defects involves the schizophrenia risk factor, DISC1, which interacts isoform specifically with 14-3-3ζ. Our data provide the first evidence of a direct role for 14-3-3ζ deficiency in the aetiology of neurodevelopmental disorders and identifies 14-3-3ζ as a central risk factor in the schizophrenia protein interaction network.

Lissencephaly and LIS1: insights into the molecular mechanisms of neuronal migration and development.

Lissencephaly is a severe human neuronal migration defect characterized by a smooth cerebral surface, mental retardation and seizures. LIS1 was first gene cloned in an organism important for neuronal migration, as it was deleted or mutated in patients with lissencephaly in a heterozygous fashion. Studies in model organisms, particularly Aspergillus nidulans, as well as those in the mouse, have uncovered an evolutionarily conserved pathway that involves LIS1 and cytoplasmic dynein. This pathway codes for proteins in a complex with cytoplasmic dynein and positively regulates its conserved function in nuclear migration. This complex appears to be important for proliferation and neuronal survival as well as neuronal migration. One of the components of this complex, NDEL1, is a phosphoprotein that is a substrate for CDK5 (or CDK2 in fibroblasts) and Aurora-A, two mitotic kinases. CDK5-phosphorylated NDEL1 binds to 14-3-3epsilon, which protects it from phosphatase attack. Interestingly, 14-3-3epsilon is located 1 Mb from LIS1 and is heterozygously deleted with LIS1 in patients with a severe form of lissencephaly, Miller-Dieker syndrome. Mouse models confirm that 14-3-3epsilon plays an important role in neuronal migration, and mice that are double heterozygotes for mutations in Lis1 and 14-3-3epsilon, display more severe neuronal migration defects. The identification of LIS1 as the first lissencephaly gene, and the first gene required for neuronal migration has revealed the importance of the regulation of cytoplasmic dynein in the control of neuronal migration by modulating nuclear migration in a pathway conserved in virtually all eukaryotes.

Reduced expression of the Sp4 gene in mice causes deficits in sensorimotor gating and memory associated with hippocampal vacuolization.

HF-1B/SP4:, a member of the Sp1 family of transcription factors, is expressed restrictively in the developing nervous system and most abundantly in adult hippocampus in mice. Here, we report the generation of hypomorphic Sp4 allele mice, in which the Sp4 deficiency can be rescued by the expression of Cre recombinase. Vacuolization was detected in the hippocampal gray matter of the mutant Sp4-deficient mice. Expression analysis of Sp4 mutant hippocampi revealed an age-dependent decrease in neurotrophin-3 expression in the dentate granule cells. Hypomorphic Sp4 mutant mice displayed robust deficits in both sensorimotor gating and contextual memory. The restoration of Sp4 expression, via a Cre-dependent rescue strategy, completely rescued all the observed molecular, histological and behavioral abnormalities. Our studies thus reveal a novel Sp4 pathway that is essential for hippocampal integrity and modulates behavioral processes relevant to psychiatric disorders.

Expanded characterization of the social interaction abnormalities in mice lacking Dvl1.

Dvl1 is one of three murine Dishevelled genes widely expressed in embryonic development and in the adult central nervous system. Dishevelled proteins are a necessary component of the Wnt and planar cell polarity developmental signaling pathways. We reported previously that mice deficient in Dvl1 exhibited abnormal social interaction and sensorimotor gating. To assess the validity of our earlier findings, we replicated the previous behavioral tests and included several new assays. The behaviors assessed included: social interaction, sensorimotor reflexes, motor activity, nociception, prepulse inhibition of acoustic startle (PPI) and learning and memory. Assessments with an explicit social component included: social dominance test, whisker trimming, nest building, home-cage huddling and ultrasonic vocalization rate analysis in pups. In addition, separate cohorts of wildtype and Dvl1-null mice were assessed for social recognition of a conspecific. Replicating the original report, Dvl1-null mice were impaired in several tasks containing an explicit social component. However, no impairment was observed in the social memory task. A previously observed deficit in PPI did not replicate in two institutions. In conclusion, we provide evidence that the social interaction phenotype of Dvl1-deficient mice has a strong genetic influence, but the sensorimotor gating deficit was subject to environmental influences. The specificity of observed social interaction deficits also suggests that lack of Dvl1 is associated with deficits in the recognition of social hierarchy and dominance.

A study of the nature of embryonic lethality in LIS1-/- mice.

Homozygous deletion of the Lis1 gene (Lis1(-/-)) in mouse resulted in early embryonic lethality immediately after embryo implantation by an undefined mechanism. We seek to define the nature of this demise. LIS1 (pafah1b1) is a 46 kDa protein with seven tryptophan-aspartate (WD) repeats. It docks with many proteins and has been implicated in microtubular function, cell division, intercellular transport, and nuclear and cellular motility. Combined Western and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analyses showed that LIS1 expression from the blastocyst stage required new transcription from the embryonic genome. Consequently, the death of post-implantation embryos may not reflect the first time during development that LIS1 was required, rather, it may reflect the first time following depletion of gametic stores that its actions were essential. Following culture of blastocysts in vitro for 96 hr the inner cell mass (ICM) of null embryos were significantly smaller than ICM of wild-type siblings. Normal blastocyst outgrowths after 96-hr culture had high levels of LIS1 expression in the outer cells of developing ICM and extensive expression in trophoblast cells. Lis1(-/-) embryos had significantly smaller trophoblast nuclei than wild-type embryos. The results show that LIS1 expression is required for the continued normal development of the ICM and optimal trophoblast giant cell formation.

Laminar disorganisation of mitral cells in the olfactory bulb does not affect topographic targeting of primary olfactory axons.

Primary olfactory neurons expressing the same odorant receptor protein typically project to topographically fixed olfactory bulb sites. While cell adhesion molecules and odorant receptors have been implicated in guidance of primary olfactory axons, the postsynaptic mitral cells may also have a role in final target selection. We have examined the effect of disorganisation of the mitral cell soma layer in mutant mice heterozygous for the beta-subunit of platelet activating factor acetylhydrolase (Lis1(-/+)) on the targeting of primary olfactory axons. Lis1(-/+) mice display abnormal lamination of neurons in the olfactory bulb. Lis1(-/+) mice were crossed with the P2-IRES-tau:LacZ line of transgenic mice that selectively expresses beta-galactosidase in primary olfactory neurons expressing the P2 odorant receptor. LacZ histochemistry revealed blue-stained P2 axons that targeted topographically fixed glomeruli in these mice in a manner similar to that observed in the parent P2-IRES-tau:LacZ line. Thus, despite the aberrant organisation of postsynaptic mitral cells in Lis1(-/+) mice, primary olfactory axons continued to converge and form glomeruli at correct sites in the olfactory bulb. Next we examined whether challenging primary olfactory axons in adult Lis(-/+) mice with regeneration would affect their ability to converge and form glomeruli. Following partial chemical ablation of the olfactory neuroepithelium with dichlobenil, primary olfactory neurons die and are replaced by newly differentiating neurons that project axons to the olfactory bulb where they converge and form glomeruli. Despite the aberrant mitral cell layer in Lis(-/+) mice, primary olfactory axons continued to converge and form glomeruli during regeneration. Together these results demonstrate that the convergence of primary olfactory axons during development and regeneration is not affected by gross perturbations to the lamination of the mitral cell layer. Thus, these results support evidence from other studies indicating that mitral cells do not play a major role in the convergence and targeting of primary olfactory axons in the olfactory bulb.

Direct removal in the mouse of a floxed neo gene from a three-loxP conditional knockout allele by two novel approaches.

The presence in an intron of the ploxP-neo-loxP cassette often results in severe interference with gene expression. Consequently, many investigators selectively remove the ploxP-neo-loxP cassette by transient expression of Cre in ES cells. Although effective, the added manipulation of the ES cells may reduce the likelihood that a clone will be able to transmit via the germline. Therefore, we developed two novel approaches that remove the ploxP-neo-loxP by Cre-mediated recombination in mouse. First, the ploxP-neo-loxP-containing mice were crossed with EIIa-Cre transgenic mice. Second, a Cre-expression plasmid was injected into pronuclei of fertilized eggs bearing the ploxP-neo-loxP allele. Both approaches produced mosaic mice with partial and complete excision. These mosaic mice were then mated, and the neo-less conditional knockout allele was found in the offspring after screening only a few litters. These procedures provide options for removing neo directly in the mouse in addition to the commonly used approach that deletes neo in ES cells.

Nucleotide sequence and structure of the mouse carbonic anhydrase III gene.

At least 14 distinct isozymes of carbonic anhydrase have been identified in mammals. These enzymes catalyze the hydration of carbon dioxide and are essential for regulation of cellular pH and carbon dioxide transport. Carbonic anhydrase III is highly expressed in certain tissues, including muscle and fat where it constitutes up to 25% of the soluble protein. We cloned a cDNA encoding mouse carbonic anhydrase III. This cDNA contains 1653 bp, consisting of 79 bp in the 5' UTR, a 780 bp open reading frame, and 794 bp of the 3' UTR, including two potential polyadenylation signals. Fluorescent in situ hybridization confirmed the existence of a single copy of the gene on chromosome 3. We then isolated the genomic DNA for mouse carbonic anhydrase III and analyzed its structure. The gene consists of seven exons and six introns which span 10.5 kb. The 5' flanking region of the genomic DNA is notable for a pyrimidine rich region consisting of two dinucleotide repeats containing 23 and 20 TC pairs separated by the same 15 bp spacer.

Unraveling human cancer in the mouse: recent refinements to modeling and analysis.

The ability to manipulate the mouse genome has made the mouse the primary mammalian genetic model organism. It has been possible to model human cancer in the mouse by overexpressing oncogenes or inactivating tumor suppressor genes, and these experiments have provided much of our in vivo understanding of cancer. However, these transgenic approaches do not always completely and accurately model human carcinogenesis. Recent developments in transgenic and knockout approaches have improved the accuracy of modeling somatic cancer in the mouse and analyzing the genomic instability that occurs in murine tumors. It is possible to use retroviral gene delivery, chromosome engineering and inducible transgenes to selectively manipulate the genome in a more precise spatial and temporal pattern. In addition, the development of powerful cytogenetic tools such as spectral karyotyping, fluorescence in situ hybridization and comparative genome hybridization have improved our ability to detect chromosomal rearrangements. Finally, global patterns of gene expression can be determined by microarray analysis to decipher complex gene patterns which occur in cancers. Several of these advances in mouse modeling of human cancer are discussed in this review.

Early embryonic lethality in PARP-1 Atm double-mutant mice suggests a functional synergy in cell proliferation during development.

PARP-1 and ATM are both involved in the response to DNA strand breaks, resulting in induction of a signaling network responsible for DNA surveillance, cellular recovery, and cell survival. ATM interacts with double-strand break repair pathways and induces signals resulting in the control of the cell cycle-coupled checkpoints. PARP-1 acts as a DNA break sensor in the base excision repair pathway of DNA. Mice with mutations inactivating either protein show radiosensitivity and high radiation-induced chromosomal aberration frequencies. Embryos carrying double mutations of both PARP-1 and Atm genes were generated. These mutant embryos show apoptosis in the embryo but not in extraembryonic tissues and die at embryonic day 8.0, although extraembryonic tissues appear normal for up to 10.5 days of gestation. These results reveal a functional synergy between PARP-1 and ATM during a period of embryogenesis when cell cycle checkpoints are not active and the embryo is particularly sensitive to DNA damage. These results suggest that ATM and PARP-1 have synergistic phenotypes due to the effects of these proteins on signaling DNA damage and/or on distinct pathways of DNA repair.

TBX1 is responsible for cardiovascular defects in velo-cardio-facial/DiGeorge syndrome.

Velo-cardio-facial syndrome (VCFS)/DiGeorge syndrome (DGS) is a human disorder characterized by a number of phenotypic features including cardiovascular defects. Most VCFS/DGS patients are hemizygous for a 1.5-3.0 Mb region of 22q11. To investigate the etiology of this disorder, we used a cre-loxP strategy to generate mice that are hemizygous for a 1.5 Mb deletion corresponding to that on 22q11. These mice exhibit significant perinatal lethality and have conotruncal and parathyroid defects. The conotruncal defects can be partially rescued by a human BAC containing the TBX1 gene. Mice heterozygous for a null mutation in Tbx1 develop conotruncal defects. These results together with the expression patterns of Tbx1 suggest a major role for this gene in the molecular etiology of VCFS/DGS.

Heterozygosity for a mutation in Brca1 or Atm does not increase susceptibility to ENU-induced mammary tumors in Apc(Min)/+ mice.

The proteins encoded by BRCA1 and ATM may be important in DNA repair and maintenance of genomic integrity. Women heterozygous for a mutation in BRCA1 have an increased incidence of breast cancer. Some evidence also suggests that female carriers of ATM mutations may be susceptible to breast cancer. However, mice carrying one mutant allele of Brca1 or ATM are not highly susceptible to breast cancer. We proposed that heterozygosity for a mutant allele of Brca1 or ATM may confer a decreased ability to repair DNA damage. Such a defect might lead to a heightened sensitivity to tumor development in susceptible animal models. Therefore, mice predispose to mammary tumor development might show an increased susceptibility if they also carry an ATM or Brca1 mutation. C57BL/6J (B6) MIN/+ mice are predisposed to mammary and intestinal tumors and exposure to the point mutagen ethylnitrosourea (ENU) markedly increases mammary tumor multiplicity and incidence. To test our hypothesis, B6.MIN/+ male mice were crossed with 129S6/SvEvTac females heterozygous for a mutant allele of either Brca1 or ATM. Female progeny from each cross were treated with ENU and followed for tumor development. Only MIN/+ F1 females developed mammary tumors and heterozygosity for a mutant Brca1 or ATM allele had no effect on mammary or intestinal tumor incidence and multiplicity. These results suggest that heterozygosity for a mutation in Brca1 or ATM: does not affect MIN-induced tumorigenesis in mice under these conditions. Additionally, exposure to a somatic point mutagen does not increase tumor development in mice carrying Brca1 or ATM mutations.

Extra-chromosomal telomeric DNA in cells from Atm(-/-) mice and patients with ataxia-telangiectasia.

Ataxia-telangiectasia (AT) is an autosomally recessive human genetic disease with pleiotropic defects such as neurological degeneration, immunodeficiency, chromosomal instability, cancer susceptibility and premature aging. Cells derived from AT patients and ataxia-telangiectasia mutated (ATM)-deficient mice show slow growth in culture and premature senescence. ATM, which belongs to the PI3 kinase family along with DNA-PK, plays a major role in signaling the p53 response to DNA strand breaks. Telomere maintenance is perturbed in yeast strains lacking genes homologous to ATM and cells from patients with AT have short telomeres. We examined the length of individual telomeres in cells from ATM(-/-) mice by fluorescence in situ hybridization. Telomeres were extensively shortened in multiple tissues of ATM(-/-) mice. More than the expected number of telomere signals was observed in interphase nuclei of ATM(-/-) mouse fibroblasts. Signals corresponding to 5-25 kb of telomeric DNA that were not associated with chromosomes were also noticed in ATM(-/-) metaphase spreads. Extrachromosomal telomeric DNA was also detected in fibroblasts from AT patients and may represent fragmented telomeres or by-products of defective replication of telomeric DNA. These results suggest a role of ATM in telomere maintenance and replication, which may contribute to the poor growth of ATM(-/-) cells and increased tumor incidence in both AT patients and ATM(-/-) mice.

Modeling human congenital disorder of glycosylation type IIa in the mouse: conservation of asparagine-linked glycan-dependent functions in mammalian physiology and insights into disease pathogenesis.

The congenital disorders of glycosylation (CDGs) are recent additions to the repertoire of inherited human genetic diseases. Frequency of CDGs is unknown since most cases are believed to be misdiagnosed or unrecognized. With few patients identified and heterogeneity in disease signs noted, studies of animal models may provide increased understanding of pathogenic mechanisms. However, features of mammalian glycan biosynthesis and species-specific variations in glycan repertoires have cast doubt on whether animal models of human genetic defects in protein glycosylation will reproduce pathogenic events and disease signs. We have introduced a mutation into the mouse germline that recapitulates the glycan biosynthetic defect responsible for human CDG type IIa (CDG-IIa). Mice lacking the Mgat2 gene were deficient in GlcNAcT-II glycosyltransferase activity and complex N-glycans, resulting in severe gastrointestinal, hematologic, and osteogenic abnormalities. With use of a lectin-based diagnostic screen for CDG-IIa, we found that all Mgat2-null mice died in early postnatal development. However, crossing the Mgat2 mutation into a distinct genetic background resulted in a low frequency of survivors. Mice deficient in complex N-glycans exhibited most CDG-IIa disease signs; however, some signs were unique to the aged mouse or are prognostic in human CDG-IIa. Unexpectedly, analyses of N-glycan structures in Mgat2-null mice revealed a novel oligosaccharide branch on the "bisecting" N-acetylglucosamine. These genetic, biochemical, and physiologic studies indicate conserved functions for N-glycan branches produced in the Golgi apparatus among two mammalian species and suggest possible therapeutic approaches to GlcNAcT-II deficiency. Our findings indicate that human genetic disease due to aberrant protein glycosylation can be modeled in the mouse to gain insights into N-glycan-dependent physiology and the pathogenesis of CDG-IIa.

Targeted mutagenesis of Smad1 reveals an essential role in chorioallantoic fusion.

The Smad family of intracellular signaling intermediates transduce signals downstream from the transforming growth factor beta (TGF-beta) family of receptor serine threonine kinases. The original member of this family, Smad1, has been shown to mediate signals from receptors for the bone morphogenetic proteins (BMPs), a large group of ligands in the TGF-beta superfamily that mediate important developmental events. We have targeted the Smad1 gene in mice and created mutants null at this locus. Smad1 mutant mice die at approximately 9.5 days postcoitum due to defects in allantois formation. In Smad1 mutant mice, the allantois fails to fuse to the chorion, resulting in a lack of placenta and failure to establish a definitive embryonic circulation. Although vasculogenesis is initiated in the mutant allantois, the vessels formed are disorganized, and VCAM-1 protein, a marker for distal allantois development, is not expressed. Smad1 null fibroblasts are still able to respond to BMP2, however, suggesting that the defect observed in the developing extraembryonic tissue is caused by a very specific loss of transcriptional activity regulated by Smad1. Our data further demonstrate that although highly similar structurally, Smad proteins are not functionally homologous.

Mice with a targeted mutation in the thyroid hormone beta receptor gene exhibit impaired growth and resistance to thyroid hormone.

Patients with mutations in the thyroid hormone receptor beta (TRbeta) gene manifest resistance to thyroid hormone (RTH), resulting in a constellation of variable phenotypic abnormalities. To understand the molecular basis underlying the action of mutant TRbeta in vivo, we generated mice with a targeted mutation in the TRbeta gene (TRbetaPV; PV, mutant thyroid hormone receptor kindred PV) by using homologous recombination and the Cre/loxP system. Mice expressing a single PV allele showed the typical abnormalities of thyroid function found in heterozygous humans with RTH. Homozygous PV mice exhibit severe dysfunction of the pituitary-thyroid axis, impaired weight gains, and abnormal bone development. This phenotype is distinct from that seen in mice with a null mutation in the TRbeta gene. Importantly, we identified abnormal expression patterns of several genes in tissues of TRbetaPV mice, demonstrating the interference of the mutant TR with the gene regulatory functions of the wild-type TR in vivo. These results show that the actions of mutant and wild-type TRbeta in vivo are distinct. This model allows further study of the molecular action of mutant TR in vivo, which could lead to better treatment for RTH patients.

Regulation of cytoplasmic dynein behaviour and microtubule organization by mammalian Lis1.

Whereas total loss of Lis1 is lethal, disruption of one allele of the Lis1 gene results in brain abnormalities, indicating that developing neurons are particularly sensitive to a reduction in Lis1 dosage. Here we show that Lis1 is enriched in neurons relative to levels in other cell types, and that Lis1 interacts with the microtubule motor cytoplasmic dynein. Production of more Lis1 in non-neuronal cells increases retrograde movement of cytoplasmic dynein and leads to peripheral accumulation of microtubules. These changes may reflect neuron-like dynein behaviours induced by abundant Lis1. Lis1 deficiency produces the opposite phenotype. Our results indicate that abundance of Lis1 in neurons may stimulate specific dynein functions that function in neuronal migration and axon growth.