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A L Joyner

Publications and source records attributed to A L Joyner.

At least 55 records · Page 3Linked to original sources
PubMed1996

Impaired motor learning performance in cerebellar En-2 mutant mice.

Mice homozygous for a null mutation in their En-2 gene exhibit cerebellar neuroanatomical alterations including absence and misplacements of specific fissures and size reduction. The present study investigated cerebellar function by comparing the behavior of age-matched homozygous and heterozygous En-2 mutant and wild-type mice. Motor function of the mutants was found normal in several situations. Habituation to novelty in the open field was not significantly different in mutants. However, in a motor learning paradigm, the rotating rod, the performance of homozygous mutant mice improved significantly less than that of the heterozygous mice which were also significantly impaired compared to wild-type mice. Unlike other cerebellar mutants in which severe motor or sensory defects are obvious, the En-2 mouse model offers a unique tool to study the role of cerebellum in complex behavioral phenomena, including motor learning, without confounding effects.

Animals↗
PubMed1996

Inactivation of Fac in mice produces inducible chromosomal instability and reduced fertility reminiscent of Fanconi anaemia.

Fanconi anaemia (FA) is an autosomal recessive disease characterized by bone marrow failure, variable congenital malformations and predisposition to malignancies. Cells derived from FA patients show elevated levels of chromosomal breakage and an increased sensitivity to bifunctional alkylating agents such as mitomycin C (MMC) and diepoxybutane (DEB). Five complementation groups have been identified by somatic cell methods, and we have cloned the gene defective in group C (FAC)(7). To understand the in vivo role of this gene, we have disrupted murine Fac and generated mice homozygous for the targeted allele. The -/- mice did not exhibit developmental abnormalities nor haematologic defects up to 9 months of age. However, their spleen cells had dramatically increased numbers of chromosomal aberrations in response to MMC and DEB. Homozygous male and female mice also had compromised gametogenesis, leading to markedly impaired fertility, a characteristic of FA patients. Thus, inactivation of Fac replicates some of the features of the human disease.

Animals↗
PubMed1996

Glucose intolerance but normal satiety in mice with a null mutation in the glucagon-like peptide 1 receptor gene.

Glucagon-like peptide 1 (GLP1) is postulated to regulate blood glucose and satiety, but the biological importance of GLP1 as an incretin and neuropeptide remains controversal. The regulation of nutrient-induced insulin secretion is dependent on the secretion of incretins, gut-derived peptides that potentiate insulin secretion from the pancreatic islets. To ascertain the relative physiological importance of GLP1 as a regulator of feeding behavior and insulin secretion, we have generated mice with a targeted disruption of the GLP1 receptor gene (GLP1R). These GLP1R-/- mice are viable, develop normally but exhibit increased levels of blood glucose following oral glucose challenge in association with diminished levels of circulating insulin. It is surprising that they also exhibit abnormal levels of blood glucose following intraperitoneal glucose challenge. Intracerebroventricular administration of GLP1 inhibited feeding in wild-type mice but not in GLP1R-/- mice; however, no evidence for abnormal body weight or feeding behavior was observed in GLP1R-/- mice. These observations demonstrate that GLP1 plays a central role in the regulation of glycemia; however, disruption of GLP1/GLP1R signaling in the central nervous system is not associated with perturbation of feeding behavior or obesity in vivo.

Animals↗
PubMed1996

Two Pax-binding sites are required for early embryonic brain expression of an Engrailed-2 transgene.

The temporally and spatially restricted expression of the mouse Engrailed (En) genes is essential for development of the midbrain and cerebellum. The regulation of En-2 expression was studied using in vitro protein-DNA binding assays and in vivo expression analysis in transgenic mice to gain insight into the genetic events that lead to regionalization of the developing brain. A minimum En-2 1.0 kb enhancer fragment was defined and found to contain multiple positive and negative regulatory elements that function in concert to establish the early embryonic mid-hindbrain expression. Furthermore, the mid-hindbrain regulatory sequences were shown to be structurally and functionally conserved in humans. The mouse paired-box-containing genes Pax-2, Pax-5 and Pax-8 show overlapping expression with the En genes in the developing brain. Significantly, two DNA-binding sites for Pax-2, Pax-5 and Pax-8 proteins were identified in the 1.0 kb En-2 regulatory sequences, and mutation of the binding sites disrupted initiation and maintenance of expression in transgenic mice. These results present strong molecular evidence that the Pax genes are direct upstream regulators of En-2 in the genetic cascade controlling mid-hindbrain development. These mouse studies, taken together with others in Drosophila and zebrafish on the role of Pax genes in controlling expression of En family members, indicate that a Pax-En genetic pathway has been conserved during evolution.

Animals↗
PubMed1995

Rescue of the En-1 mutant phenotype by replacement of En-1 with En-2.

The related mouse Engrailed genes En-1 and En-2 are expressed from the one- and approximately five-somite stages, respectively, in a similar presumptive mid-hindbrain domain. However, mutations in En-1 and En-2 produce different phenotypes. En-1 mutant mice die at birth with a large mid-hindbrain deletion, whereas En-2 mutants are viable, with cerebellar defects. To determine whether these contrasting phenotypes reflect differences in temporal expression or biochemical activity of the En proteins, En-1 coding sequences were replaced with En-2 sequences by gene targeting. This rescued all En-1 mutant defects, demonstrating that the difference between En-1 and En-2 stems from their divergent expression patterns.

Animals↗
PubMed1995

Inactivation of the mouse Huntington's disease gene homolog Hdh.

Huntington's disease (HD) is a dominant neurodegenerative disorder caused by expansion of a CAG repeat in the gene encoding huntingtin, a protein of unknown function. To distinguish between "loss of function" and "gain of function" models of HD, the murine HD homolog Hdh was inactivated by gene targeting. Mice heterozygous for Hdh inactivation were phenotypically normal, whereas homozygosity resulted in embryonic death. Homozygotes displayed abnormal gastrulation at embryonic day 7.5 and were resorbing by day 8.5. Thus, huntingtin is critical early in embryonic development, before the emergence of the nervous system. That Hdh inactivation does not mimic adult HD neuropathology suggests that the human disease involves a gain of function.

Animals↗
PubMed1995

Ultrasound backscatter microscope analysis of early mouse embryonic brain development.

The history of developmental and genetic analysis in the mouse has made it the model of choice for studying mammalian embryogenesis. Presently lacking is a simple technique for efficiently analyzing early mouse mutant phenotypes in utero. We demonstrate application of a real-time imaging method called ultrasound backscatter microscopy for visualizing mouse early embryonic neural tubes and hearts. This method was used to study live embryos in utero between 9.5 and 11.5 days of embryogenesis, with a spatial resolution close to 50 microns. Ultrasound backscatter microscope images of cultured embryos made it possible to visualize the heart chambers. This noninvasive imaging method was also used for analyzing a neural tube defect. The midhindbrain deletion associated with a null mutation of the Wnt-1 protooncogene was easily recognizable on ultrasound backscatter microscope images of 10.5- and 11.5-day embryos. Computer-generated volumetric renderings of the neural tube cavities were made from three-dimensional image data. This allowed a much clearer definition of the Wnt-1 mutant phenotype. These imaging techniques should be of considerable use in studying mouse development in utero.

Animals↗
PubMed1995

Genomic imprinting of Mash2, a mouse gene required for trophoblast development.

The mouse gene Mash2 encodes a transcription factor required for development of trophoblast progenitors. Mash2-homozygous mutant embryos die at 10 days postcoitum from placental failure. Here we show that Mash2 is genomically imprinted. First, Mash2+/- embryos inheriting a wild-type allele from their father die at the same stage as -/- embryos, with a similar placental phenotype. Second, the Mash2 paternal allele is initially expressed by groups of trophoblast cells at 6.5 and 7.5 days post-coitum, but appears almost completely repressed by 8.5 days post-coitum. Finally, we have genetically and physically mapped Mash2 to the distal region of chromosome 7, within a cluster of imprinted genes, including insulin-2, insulin-like growth factor-2 and H19.

Alleles↗
PubMed1995

A role for En-2 and other murine homologues of Drosophila segment polarity genes in regulating positional information in the developing cerebellum.

To gain insight into the molecular genetic basis of cerebellar patterning, the expression patterns of many vertebrate homologues of Drosophila segment polarity genes were examined during normal and abnormal cerebellar development, including members of the En, Wnt, Pax, Gli and Dvl gene families. Five of these genes were found to show transient, spatially restricted patterns of expression. Strikingly, expression of En-2, En-1, Wnt-7B and Pax-2 defined eleven similar sagittal domains at 17.5 dpc, reminiscent of the transient sagittal domains of expression of Purkinje cell markers which have been implicated in cerebellar afferent patterning. Postnatally, transient anterior/posterior differences in expression were observed for En-2, En-1, Gli and Wnt-7B dividing the cerebellum into anterior and posterior regions. The expression patterns of these genes were altered in cerebella of En-2 homozygous mutant mice, which show a cerebellar foliation patterning defect. Strikingly, four of the Wnt-7B expression domains that are adjacent to the En-2 domains are lost in En-2 mutant embryonic cerebella. These studies provide the first evidence of a potential network of regulatory genes that establish spatial cues in the developing cerebellum by dividing it into a grid of positional information required for patterning foliation and afferents. Taken together with previous gene expression studies, our data suggests that eleven sagittal domains and at least two anterior/posterior compartments are the basic elements of spatial information in the cerebellum.

Animals↗
PubMed1994

Essential role of Mash-2 in extraembryonic development.

The outer layer of the blastocyst, or trophectoderm, is the first cell lineage to differentiate in the mouse embryo, but little is known about the genetic control of its development. Lineage-specific transcription factors may be important in lineage specification, and the product of the Mash-2 gene fulfils the criteria for such a factor. Mash-2 is a mammalian member of the achaete-scute family which encodes basic-helix-loop-helix transcription factors and is strongly expressed in the extraembryonic trophoblast lineage. Mash-2 transcripts are found in the female germ line and in the embryo throughout preimplantation development, but are highly expressed later only in the ectoplacental cone, the chorion and their derivatives in the placenta. Mash-2 transcripts are not found in primary and secondary giant cells, yolk sac or allantois at any post-implantation stage, and are present only transiently and at low levels in the embryo during gastrulation. To analyse the role of Mash-2 in development, we have used gene targeting to generate mice having no Mash-2 function. We report here that Mash-2-/- embryos die from placental failure at 10 days postcoitum. In mutant placentas, spongiotrophoblast cells and their precursors are absent and chorionic ectoderm is reduced. We have rescued this placental mutant phenotype by constructing chimaeras with tetraploid wild-type embryos which contribute almost exclusively to extraembryonic tissues. Mash-2-/- embryos developed normally and adult Mash-2-/- mice were viable, demonstrating that Mash-2 has no major role in the embryo itself. Mash-2 is the first transcription factor shown to play a critical part in the development of the mammalian trophoblast lineage.

Animals↗
PubMed1994

Expression of three mouse homologs of the Drosophila segment polarity gene cubitus interruptus, Gli, Gli-2, and Gli-3, in ectoderm- and mesoderm-derived tissues suggests multiple roles during postimplantation development.

Three mouse genes, Gli, Gli-2, and Gli-3, which share a similar zinc finger domain with the products of the Drosophila segment polarity gene cubitus interruptus and the Caenorhabditis elegans sex-determining gene tra-1 were cloned and characterized. The expression patterns during postimplantation development of the three genes were analyzed by Northern blot, whole-mount, and section in situ hybridizations. Expression was first detected during gastrulation in both the ectoderm and mesoderm. Later in development, their expression became more restricted in various ectoderm- and mesoderm-derived tissues and was not detectable after completion of organogenesis. Interestingly, in the developing neural tube, Gli showed a narrow ventral domain of expression, whereas Gli-2 and Gli-3 showed a broad and dorsally restricted domain. Expression of these three Gli genes in various ectoderm- and mesoderm-derived tissues suggests that they play multiple roles during postimplantation development. Consistent with this hypothesis, a naturally occurring Gli-3 mutation, the mouse extra-toes mutant; shows defects in both mesoderm- and ectoderm-derived tissues.

Amino Acid Sequence↗
PubMed1994

Gene targeting and development of the nervous system.

Gene targeting provides a means of directly assaying the function of specific genes during mouse nervous system development. Generation of targeted mutant mice has provided the first evidence of developmental roles for genes whose function was suggested based on their expression, but for which appropriate assay systems were lacking. In other cases, where gene function was known, targeted mutations have revealed in which cell population, and at what developmental stage, particular genes are first indispensable. The existing targeted mutants suggest that an early mechanism of pattern formation in mammals involves regional control of proliferation or survival of neural precursors, and that later general functions, such as the control of differentiation of precursors, may be performed by different genes in distinct neural lineages. As many genes display complex temporal and spatial patterns of expression, analysis of the full range of functions of such genes will require the generation of a series of alleles, including stage- and tissue-specific mutations.

Animals↗
PubMed1994

Abnormal embryonic cerebellar development and patterning of postnatal foliation in two mouse Engrailed-2 mutants.

The cerebellum is an ideal system to study pattern formation in the central nervous system because of its simple cytoarchitecture and regular organization of folds and neural circuitry. Engrailed-2 (En-2) is expressed in a spatially restricted broad band around the mesencephalic-metencephalic junction, a region from which the cerebellum is derived. Mice homozygous for a targeted deletion of the En-2 homeobox, En-2hd, previously have been shown to have an altered adult cerebellar foliation pattern. To address whether the En-2hd allele was hypomorphic, we generated a putative null mutation that makes an N-terminal deletion (ntd). Mice homozygous for this new mutation, En-2ntd, display an identical cerebellar patterning defect, suggesting that both alleles represent null alleles. We also examined the developmental profile of En-2 homozygous mutant cerebellar foliation. This revealed a complex phenotype of general developmental delay and abnormal formation of specific fissures with the most severe morphological disruptions being limited to the posterior region of the cerebellum. The expression of two transgenes, which express lacZ in lobe-specific patterns in the cerebellum, also was found to be altered in En-2 homozygotes, suggesting possible lobe transformations. Finally, during embryogenesis there was a clear delay in fusion of the cerebellar rudiments at the midline by 15.5 d.p.c. This and the expression pattern of En-2 suggests that although cerebellar foliation is largely a postnatal process, the patterning of the cerebellum may begin during embryogenesis and that En-2 plays a critical role in this early process.

Animals↗
PubMed1994

Multiple developmental defects in Engrailed-1 mutant mice: an early mid-hindbrain deletion and patterning defects in forelimbs and sternum.

During mouse development, the homeobox-containing gene En-1 is specifically expressed across the mid-hindbrain junction, the ventral ectoderm of the limb buds, and in regions of the hindbrain, spinal cord, somites and somite-derived tissues. To address the function of En-1 during embryogenesis, we have generated mice homozygous for a targeted deletion of the En-1 homeobox. En-1 mutant mice died shortly after birth and exhibited multiple developmental defects. In the brains of newborn mutants, most of the colliculi and cerebellum were missing and the third and fourth cranial nerves were absent. A deletion of midhindbrain tissue was observed as early as 9.5 days of embryonic development and the phenotype resembles that previously reported for Wnt-1 mutant mice. In addition, patterning of the forelimb paws and sternum was disrupted, and the 13th ribs were truncated. The results of these studies suggest a cell autonomous role for En-1 in generation and/or survival of mid-hindbrain precursor cells and also a non-cell autonomous role in signalling normal development of the limbs and possibly sternum.

Animals↗
PubMed1994

Expression of mouse LSP1/S37 isoforms. S37 is expressed in embryonic mesenchymal cells.

Mouse LSP1 is a 330 amino acid intracellular F-actin binding protein expressed in lymphocytes and macrophages but not in non-hematopoietic tissues. A 328 amino acid LSP1-related protein, designated S37, is expressed in murine bone marrow stromal cells, in fibroblasts, and in a myocyte cell line. The two proteins differ only at their N termini, the first 23 amino acid residues of LSP1 being replaced by 21 different residues in S37. The presence of different amino termini suggests that the LSP1 and S37 proteins are encoded by transcripts arising through alternative exon splicing. Here we report the genomic organization of the Lsp1 gene and show that the distinct N termini of LSP1 and S37 are encoded by two alternatively used exons, each containing a translational start codon. We also demonstrate that alternative 3' acceptor sites are used in the splicing of exon 5. This results in LSP1 and S37 transcripts that either do or do not contain 18 bp encoding the 6 amino acids HLIRHQ of the acidic domain. Therefore, the Lsp1 gene encodes four protein isoforms: full-length LSP1 and S37 proteins, designated LSP1-I and S37-I and the same proteins without the HLIRHQ sequence, designated LSP1-II and S37-II. By in situ hybridization analysis we show that the S37 isoforms are expressed in mesenchymal tissue, but not in adjacent epithelial tissue, of several developing organs during mouse embryogenesis. This, together with our finding that S37 is an F-actin binding protein, suggests that S37 is a cytoskeletal protein of mesenchymal cells, which may play a role in mesenchyme-induced epithelial differentiation during organogenesis.

Actins↗
PubMed1994

MASH-1: a marker and a mutation for mammalian neural crest development.

The ability to generate mice containing null mutations in any cloned gene promises new insights into the molecular control of mammalian neural crest development. This approach has recently been applied to MASH-1, a transcription factor in the bHLH family that is a mammalian homologue of the Drosophila proneural genes achaete-scute. In wild-type embryos, this gene is expressed early in the development of the autonomic nervous system, in apparent precursors of sympathetic, parasympathetic, and enteric neurons (as well as in restricted regions of the central nervous system). A null mutation in the MASH-1 gene eliminates sympathetic and parasympathetic neurons and enteric neurons of the foregut (esophagus); however, enteric neurons of the stomach and hindgut are only partially affected. Analysis with other markers indicates that the mutation acts after neural crest cells have localized in the anlagen of the autonomic nervous system to prevent neuronal differentiation. The differentiation of autonomic glia appears unaffected. Thus, MASH-1 provides one of the most specific mutations affecting neural development in mammals, as well as a valuable marker to study the early segregation of neural crest cell lineages.

Adrenal Glands↗
PubMed1993

Mammalian achaete-scute homolog 1 is required for the early development of olfactory and autonomic neurons.

The mouse Mash-1 gene, like its Drosophila homologs of the achaete-scute complex (AS-C), encodes a transcription factor expressed in neural precursors. We created a null allele of this gene by homologous recombination in embryonic stem cells. Mice homozygous for the mutation die at birth with apparent breathing and feeding defects. The brain and spinal cord of the mutants appear normal, but their olfactory epithelium and sympathetic, parasympathetic, and enteric ganglia are severely affected. In the olfactory epithelium, neuronal progenitors die at an early stage, whereas the nonneuronal supporting cells are present. In sympathetic ganglia, the mutation arrests the development of neuronal precursors, preventing the generation of sympathetic neurons, but does not affect glial precursor cells. These observations suggest that Mash-1, like its Drosophila homologs of the AS-C, controls a basic operation in development of neuronal progenitors in distinct neural lineages.

Adrenal Medulla↗