Biomedical subjects

Christopher A Walsh

Publications and source records attributed to Christopher A Walsh.

67 records · Page 4Linked to original sources

PubMed2003

Mapping form and function in the human brain: the emerging field of functional neuroimaging in cortical malformations.

Malformations of cortical development (MCDs) are increasingly being recognized as a common cause of epilepsy in cases previously felt to be cryptogenic. MCDs occur when the normal process of cerebral cortical development is disrupted, and include disorders of neuronal proliferation, migration, and organization. Many have a genetic basis and the genes responsible for some MCDs have been identified. MCDs represent a unique and valuable substrate in functional brain mapping studies, since as developmental lesions they provide complementary information to studies performed on patients with acquired brain lesions. In recent years an increasing number of functional neuroimaging methods, including positron emission tomography, single photon emission computed tomography, magnetic resonance spectroscopy, and functional magnetic resonance imaging, have been applied to patients with MCDs. In this review we highlight some of the prominent findings in this emerging field by presenting the functional neuroimaging characteristics of selected MCDs.

Brain Diseases↗

PubMed2003

The DCX-domain tandems of doublecortin and doublecortin-like kinase.

The doublecortin-like domains (DCX), which typically occur in tandem, are novel microtubule-binding modules. DCX tandems are found in doublecortin, a 360-residue protein expressed in migrating neurons; the doublecortin-like kinase (DCLK); the product of the RP1 gene that is responsible for a form of inherited blindness; and several other proteins. Mutations in the gene encoding doublecortin cause lissencephaly in males and the 'double-cortex syndrome' in females. We here report a solution structure of the N-terminal DCX domain of human doublecortin and a 1.5 A resolution crystal structure of the equivalent domain from human DCLK. Both show a stable, ubiquitin-like tertiary fold with distinct structural similarities to GTPase-binding domains. We also show that the C-terminal DCX domains of both proteins are only partially folded. In functional assays, the N-terminal DCX domain of doublecortin binds only to assembled microtubules, whereas the C-terminal domain binds to both microtubules and unpolymerized tubulin.

Amino Acid Sequence↗

PubMed2003

Increased neuronal production, enlarged forebrains and cytoarchitectural distortions in beta-catenin overexpressing transgenic mice.

beta-Catenin can function in the decision of neural precursors to proliferate or differentiate during mammalian neuronal development and may regulate cerebral cortical size by controlling the generation of neural precursor cells. Mice expressing high levels of a stabilized beta-catenin transgene in neural precursors develop enlarged brains with expanded precursor populations, increased cerebral cortical surface area, and folds resembling sulci and gyri of higher mammals present at birth. Here we report the effects in adult mice expressing lower levels of the same stabilized beta-catenin transgene in neural precursors. Adult transgenic animals develop enlarged forebrains with thin cerebral cortices with increased surface area, expanded subventricular zones with subcortical aggregations of neurons and enlarged, distorted hippocampi. The brains from transgenic mice also show apparent arrest of neuronal migration and dramatic disorganization of the layering of the cerebral cortex. These findings suggest that beta-catenin can cause expansion of the precursor pool resulting in increased neuronal production and greater brain size and suggest a crucial role for beta-catenin in neuronal migration and cortical lamination.

Animals↗

PubMed2002

Filamin A and Filamin B are co-expressed within neurons during periods of neuronal migration and can physically interact.

Mutations in the X-linked gene Filamin A (FLNA) lead to the human neurological disorder, periventricular heterotopia (PH). Although PH is characterized by a failure in neuronal migration into the cerebral cortex with consequent formation of nodules in the ventricular and subventricular zones, many neurons appear to migrate normally, even in males, suggesting compensatory mechanisms. Here we characterize expression patterns for FlnA and a highly homologous protein Filamin B (FlnB) within the nervous system, in order to better understand their potential roles in cortical development. FlnA mRNA was widely expressed in all cortical layers while FlnB mRNA was most highly expressed in the ventricular and subventricular zones during development. In adulthood, widespread but reduced expression of FlnA and FlnB persisted throughout the cerebral cortex. FlnA and FlnB proteins were highly expressed in both the leading processes and somata of migratory neurons during corticogenesis. Postnatally, FlnA immunoreactivity was largely localized to the cell body with FlnB in the soma and neuropil during neuronal differentiation. In adulthood, diminished expression of both proteins localized to the cell soma and nucleus. Moreover, the putative FLNB homodimerization domain strongly interacted with itself or the corresponding homologous region of FLNA by yeast two-hybrid interaction, the two proteins co-localized within neuronal precursors by immunocytochemistry and the existence of FLNA-FLNB heterodimers could be detected by co-immunoprecipitation. These results suggest that FLNA and FLNB may form both homodimers and heterodimers and that their interaction could potentially compensate for the loss of FLNA function during cortical development within PH individuals.

Animals↗

PubMed2002

Mutations in the O-mannosyltransferase gene POMT1 give rise to the severe neuronal migration disorder Walker-Warburg syndrome.

Walker-Warburg syndrome (WWS) is an autosomal recessive developmental disorder characterized by congenital muscular dystrophy and complex brain and eye abnormalities. A similar combination of symptoms is presented by two other human diseases, muscle-eye-brain disease (MEB) and Fukuyama congenital muscular dystrophy (FCMD). Although the genes underlying FCMD (Fukutin) and MEB (POMGnT1) have been cloned, loci for WWS have remained elusive. The protein products of POMGnT1 and Fukutin have both been implicated in protein glycosylation. To unravel the genetic basis of WWS, we first performed a genomewide linkage analysis in 10 consanguineous families with WWS. The results indicated the existence of at least three WWS loci. Subsequently, we adopted a candidate-gene approach in combination with homozygosity mapping in 15 consanguineous families with WWS. Candidate genes were selected on the basis of the role of the FCMD and MEB genes. Since POMGnT1 encodes an O-mannoside N-acetylglucosaminyltransferase, we analyzed the possible implication of O-mannosyl glycan synthesis in WWS. Analysis of the locus for O-mannosyltransferase 1 (POMT1) revealed homozygosity in 5 of 15 families. Sequencing of the POMT1 gene revealed mutations in 6 of the 30 unrelated patients with WWS. Of the five mutations identified, two are nonsense mutations, two are frameshift mutations, and one is a missense mutation. Immunohistochemical analysis of muscle from patients with POMT1 mutations corroborated the O-mannosylation defect, as judged by the absence of glycosylation of alpha-dystroglycan. The implication of O-mannosylation in MEB and WWS suggests new lines of study in understanding the molecular basis of neuronal migration.

Abnormalities, Multiple↗

PubMed2002

ASPM is a major determinant of cerebral cortical size.

One of the most notable trends in mammalian evolution is the massive increase in size of the cerebral cortex, especially in primates. Humans with autosomal recessive primary microcephaly (MCPH) show a small but otherwise grossly normal cerebral cortex associated with mild to moderate mental retardation. Genes linked to this condition offer potential insights into the development and evolution of the cerebral cortex. Here we show that the most common cause of MCPH is homozygous mutation of ASPM, the human ortholog of the Drosophila melanogaster abnormal spindle gene (asp), which is essential for normal mitotic spindle function in embryonic neuroblasts. The mouse gene Aspm is expressed specifically in the primary sites of prenatal cerebral cortical neurogenesis. Notably, the predicted ASPM proteins encode systematically larger numbers of repeated 'IQ' domains between flies, mice and humans, with the predominant difference between Aspm and ASPM being a single large insertion coding for IQ domains. Our results and evolutionary considerations suggest that brain size is controlled in part through modulation of mitotic spindle activity in neuronal progenitor cells.

Anthropometry↗

PubMed2002

Doublecortin is required in mice for lamination of the hippocampus but not the neocortex.

Doublecortin (DCX) is a microtubule-associated protein that is required for normal neocortical and hippocampal development in humans. Mutations in the X-linked human DCX gene cause gross neocortical disorganization (lissencephaly or "smooth brain") in hemizygous males, whereas heterozygous females show a mosaic phenotype with a normal cortex as well as a second band of misplaced (heterotopic) neurons beneath the cortex ("double cortex syndrome"). We created a mouse carrying a targeted mutation in the Dcx gene. Hemizygous male Dcx mice show severe postnatal lethality; the few that survive to adulthood are variably fertile. Dcx mutant mice show neocortical lamination that is largely indistinguishable from wild type and show normal patterns of neocortical neurogenesis and neuronal migration. In contrast, the hippocampus of both heterozygous females and hemizygous males shows disrupted lamination that is most severe in the CA3 region. Behavioral tests show defects in context and cued conditioned fear tests, suggesting that deficits in hippocampal learning accompany the abnormal cytoarchitecture.

Animals↗

PubMed2002

Bilateral periventricular nodular heterotopia due to filamin 1 gene mutation: widespread glomeruloid microvascular anomaly and dysplastic cytoarchitecture in the cerebral cortex.

Bilateral periventricular nodular heterotopia (BPNH) is a neuronal migration disorder that is characterized by subependymal nodules of gray matter. Recently, a causative gene for BPNH, filamin 1, has been identified, and possible roles of the translated protein in cell migration and blood vessel development have been proposed. We report here the histopathological features of an autopsy case of BPNH with widespread glomeruloid microvascular anomaly and dysplastic cytoarchitecture in the cerebral cortex, in whom we found a novel exon 11 (Val528Met) filamin 1 mutation. Within the periventricular nodules, well-differentiated pyramidal neurons were randomly oriented. A small proportion of neurons were immunolabeled with antibodies raised against calbindin D-28k, parvalbumin, or calretinin. We used a carbocyanine dye (DiI) tracing technique to investigate the extent of fiber projections within and outside the nodules. The labeled fibers formed bundles that extended into the surrounding white matter. Connections between adjacent nodules were evident. Connections between the nodules and the cerebral cortex were also seen, with a small number of labeled fibers reaching the cortex. In the cerebral cortex, small closely packed vessels ran in a parallel fashion throughout all of the layers. Immunohistochemically, the inner rim of individual vessel lumina was labeled by an antibody against factor VIII, and the vessel walls were labeled by antibodies against actin and laminin. Astrocyte processes, labeled with an antibody to glial fibrillary acidic protein, invaded these vascular channels. Ultrastructurally, a network of basal lamina-like materials lined with endothelial cells was evident. The cytoarchitecture of the cerebral cortex was disturbed, in that the columnar neuronal arrangement was distorted around the malformed vessels. This case appears to represent an example of BPNH manifesting widespread developmental anomalies within the blood vessels and the cortical cytoarchitecture in the cerebrum.

Brain Diseases↗

PubMed2002

Regulation of cerebral cortical size by control of cell cycle exit in neural precursors.

Transgenic mice expressing a stabilized beta-catenin in neural precursors develop enlarged brains with increased cerebral cortical surface area and folds resembling sulci and gyri of higher mammals. Brains from transgenic animals have enlarged lateral ventricles lined with neuroepithelial precursor cells, reflecting an expansion of the precursor population. Compared with wild-type precursors, a greater proportion of transgenic precursors reenter the cell cycle after mitosis. These results show that beta-catenin can function in the decision of precursors to proliferate or differentiate during mammalian neuronal development and suggest that beta-catenin can regulate cerebral cortical size by controlling the generation of neural precursor cells.

Animals↗

PubMed2002

Evidence of common progenitors and patterns of dispersion in rat striatum and cerebral cortex.

To correlate clonal patterns in the rat striatum with adult neuronal phenotypes, we labeled striatal progenitors between embryonic day 14 (E14) and E19 with a retroviral library encoding alkaline phosphatase. In the adult striatum, the majority of E14-labeled neurons (87%) were members of discrete horizontal or radial cell clusters. Radial clusters accounted for only 23% of cell clusters but >34% of labeled cells. Striatal clones also demonstrated an unexpected widespread pattern of clonal dispersion. The majority of striatal clones were widely dispersed within the striatum, and 80% of clones were part of even larger clones that included cortical interneurons. Finally, we observed that PCR-positive cortical interneurons were members of clones containing both interneurons and pyramids (44%), exclusively interneuron clones (24%), or combined striatal-cortical clones (16%), consistent with the view that cortical interneurons have multiple origins in differentially behaving progenitor cells. Our data are also consistent with the notion that similar mechanisms underpin striatal and cortical development.

Alkaline Phosphatase↗

PubMed2002

An autosomal recessive form of bilateral frontoparietal polymicrogyria maps to chromosome 16q12.2-21.

Polymicrogyria is a cerebral cortical malformation that is grossly characterized by excessive cortical folding and microscopically characterized by abnormal cortical layering. Although polymicrogyria appears to have one or more genetic causes, no polymicrogyria loci have been identified. Here we describe the clinical and radiographic features of a new genetic form of polymicrogyria and localize the responsible gene. We studied two consanguineous Palestinian pedigrees with an autosomal recessive form of bilateral frontoparietal polymicrogyria (BFPP), using linkage analysis. Five affected children had moderate-to-severe mental retardation, developmental delay, and esotropia, and four of the five affected children developed seizures. Brain magnetic-resonance imaging revealed polymicrogyria that was most prominent in the frontal and parietal lobes but involved other cortical areas as well. A genomewide linkage screen revealed a single locus that was identical by descent in affected children in both families and showed a single disease-associated haplotype, suggesting a common founder mutation. The locus for BFPP maps to chromosome 16q12.2-21, with a minimal interval of 17 cM. For D16S514, the maximal pooled two-point LOD score was 3.98, and the maximal multipoint LOD score was 4.57. This study provides the first genetic evidence that BFPP is an autosomal recessive disorder and serves as a starting point for the identification of the responsible gene.

Adolescent↗

PubMed2002

Mapping of the mouse hyh gene to a YAC/BAC contig on proximal Chromosome 7.

Mice that are homozygous for the autosomal recessive hydrocephaly with hop gait (hyh) mutation on Chromosome (Chr) 7 have congenital hydrocephalus characterized by an interhemispheric cyst arising from the third ventricle and agenesis of the corpus callosum. Analysis of more than 500 backcross and intercross progeny maps the hyh locus to proximal Chr 7, approximately 13 cM centromeric to its originally reported map position. Analysis of recombinants at several MIT microsatellite markers localized the hyh locus between D7Mit75 and D7Mit56. Development of several new SSLP markers allowed us to refine the hyh candidate interval to a region defined by the cone-rod homeobox ( Crx) gene proximally and D7Mit56 distally. A contig of yeast artificial chromosome (YAC) clones and bacterial artificial chromosome (BAC) clones spanning this entire region has been developed, and a number of potential candidate genes for hyh within this interval have been identified. Gene content is conserved between this region of mouse Chr 7 and human Chr 19q13.3. Physical mapping of the regions around D7Mit75 and D7Mit56 has also determined the order of a number of MIT markers that remain unresolved on the Mouse Genome Database (MGD) map. Our physical map and transcript map may be useful for positional cloning of genes in this unusually gene-rich region of the genome.

Animals↗

PubMed2002

Smooth, rough and upside-down neocortical development.

Lissencephaly, which means 'smooth cortex', is caused by defective neuronal migration during development of the cerebral cortex and has devastating clinical consequences. 'Classical' lissencephaly seems to reflect mutations in regulators of the microtubule cytoskeleton, whereas 'cobblestone' lissencephaly is caused by mutations in genes needed for the integrity of the basal lamina of the central nervous system. Reelin, which is mutated in a third type of lissencephaly, may represent a unifying link because it encodes an extracellular protein that regulates neuronal migration and may also regulate the microtubule cytoskeleton.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗