Biomedical subjects
Publications and source records attributed to T Curran.
Integrins link the extracellular matrix to the intracellular environment. They have been implicated in the Reelin pathway in cortical development. But new genetic studies have revealed that, while beta(1) integrin plays a role in formation of the basement membrane, it is not essential for neuronal migration.
Disabled-1 (Dab1) is an intracellular adapter protein that mediates the effect of Reelin on neuronal migration and cell positioning during mammalian brain development. To identify components of the Reelin-Dab1 signaling pathway, we searched for proteins that interact with Dab1 using a yeast two-hybrid strategy. We found that the Dab1 phosphotyrosine binding (PTB) domain interacts with a novel protocadherin, orthologous to human protocadherin 18. Mouse Pcdh18 (mPcdh18), which consists of four exons similar to other protocadherin family members, maps to chromosome 3. The deduced amino acid sequence of mPcdh18 contains six extracellular cadherin motifs, a single transmembrane region, and a large intracellular domain. The site of Dab1 interaction was localized to the C-terminal 243 residues of mPcdh18. Expression analyses revealed that mPcdh18 is present in a variety of tissues in the embryo, but in adult mice it is primarily expressed in lung and kidney. In embryonic brain, mPcdh18 expression is temporally and spatially regulated. Our results indicate that mPcdh18 participates in signaling pathways involving PTB-containing proteins and suggest that it may play a role during brain development.
The formation of synaptic connections requires the coordination of specific guidance molecules and spontaneous neuronal activity. The visual system has provided a useful model for understanding the role of these cues in shaping the precise connections from the neural retina to the brain. Here, we demonstrate that two essential genes in the Reelin signaling pathway function during the patterning of synaptic connectivity in the retina. Physiological studies of mice deficient in either reelin or disabled-1 reveal an attenuation of rod-driven retinal responses. This defect is associated with a decrease in rod bipolar cell density and an abnormal distribution of processes in the inner plexiform layer. These results imply that, in addition to its essential role during neuronal migration, the Reelin pathway contributes to the formation of neuronal circuits in the central nervous system.
As early-onset hypothyroidism produces audiogenic seizure susceptibility (AGS) in rodents, the role of TR alpha 1 and TR beta thyroid hormone receptors in AGS was investigated. AGS occurs in mice lacking specifically TR beta (Thrb(tm1/tm1)) and is marked by early onset and persistence, thereby differing from mouse strains where AGS is age-restricted. Thrb(tm1/tm1) mice display AGS whether on a mixed 129/Sv x C57BL/6J or congenic C57BL/6J background. 27% of wild-type mice on the mixed and 0% on the congenic background exhibited AGS. The inability of Thrb(tm1/tm1) mice to downregulate the response to sustained acoustic stimulation may reside in the brain or in the auditory system itself as Thrb(tm1/tm1) mice also display auditory deficits. The AGS phenotype identifies a novel neurological role for TR beta.
The study of mice with spontaneous and targeted mutations has uncovered a signaling pathway that controls neuronal positioning during mammalian brain development. Mice with disruptions in reelin, dab1, or both vldlr and apoER2 are ataxic, and they exhibit severe lamination defects within several brain structures. Reelin is a secreted extracellular protein that binds to the very low density lipoprotein receptor and the apolipoprotein E receptor 2 on the surface of neurons. Disabled-1 (Dab1), an intracellular adapter protein containing a PTB (phosphotyrosine binding) domain, is tyrosyl-phosphorylated during embryogenesis, but it accumulates in a hypophosphorylated form in mice lacking Reelin or both very low density lipoprotein receptor and apolipoprotein E receptor 2. Dab1 is rapidly phosphorylated when neurons isolated from embryonic brains are stimulated with Reelin, and several tyrosines have been implicated in this response. Mice with phenylalanine substitutions of all five tyrosines (Tyr(185), Tyr(198), Tyr(200), Tyr(220), and Tyr(232)) exhibit a reeler phenotype, implying that tyrosine phosphorylation is critical for Dab1 function. Here we report that, although Src can phosphorylate all five tyrosines in vitro, Tyr(198) and Tyr(220) represent the major sites of Reelin-induced Dab1 phosphorylation in embryonic neurons.
People often falsely recognize nonstudied lures that are semantically similar to previously studied words. Behavioral research suggests that such false recognition is based on high semantic overlap between studied items and lures that yield a feeling of familiarity, whereas true recognition is more often associated with the recollection of details. Despite this behavioral evidence for differences between true and false recognition, research measuring brain activity (PET, fMRI, ERP) has not clearly differentiated corresponding differences in brain activity. A median split was used to separate subjects into Good and Poor performers based on their discrimination of studied targets from similar lures. Only Good performers showed late (1000--1500 msec), right frontal event-related brain potentials (ERPs) that were more positive for targets and lures compared with new items. The right frontal differences are interpreted as reflecting postretrieval evaluation processes that were more likely to be engaged by Good than Poor performers. Both Good and Poor performers showed a parietal ERP old/new effect (400--800 msec), but only Poor performers showed a parietal old/lure difference. These results are consistent with the view that the parietal and frontal ERP old/new effects reflect dissociable processes related to recollection.
Brain malignancies represent the most common solid tumors in children, and they are responsible for significant mortality and morbidity. The molecular basis of the most common malignant pediatric brain tumor, medulloblastoma, is poorly understood. Mutations in several genes including the human homologue of the Drosophila segment polarity gene, patched (PTCH), the adenomatous polyposis coli gene (APC), beta-catenin, and p53 have been reported in subsets of hereditary and sporadic medulloblastoma. Inactivation of one Ptc allele in mice results in a 14% incidence of medulloblastoma. Here, we report a dramatic increase in the incidence (>95%) and accelerated development (prior to 12 weeks of age) of medulloblastoma in mice heterozygous for Ptc that lack p53. The acceleration of tumorigenesis in Ptc+/- mice is specific for loss of p53, because no change in tumor incidence was observed in Ptc+/- mice carrying a mutation in APC (Min+/-) or in Ptc+/- mice deficient in p19ARF. Thus, there is a specific interaction between p53 loss and heterozygosity of Ptc that results in medulloblastoma. This may be a consequence of increased genomic instability associated with loss of p53 function that may enhance the rate of acquisition of secondary mutations. Ptc+/- p53-/- mice provide a useful model for investigation of the molecular bases of medulloblastoma and for evaluation of the efficacy of therapeutic intervention strategies in a spontaneously arising endogenous brain tumor.
The effects of aging on visuospatial attention were investigated with event-related brain potentials (ERPs). A central arrow pointed towards (75% valid cues) or away from (25% invalid cues) the location of upcoming visual targets to which subjects made two choice discriminations. Young and older adults responded faster following valid than invalid cues. The absolute magnitude of the cueing effect was larger for older than young subjects, but cueing effects were similar between groups when estimated proportionally to overall response time. Under the present conditions, the electrophysiological manifestations of visuospatial attention were similar for young and older adults. Early ERP components following the target stimulus (P1, N1, Nd1) were slower for older than young subjects, but amplitude was similarly affected by cueing in each group. The temporal correspondence between component latencies and the observed cueing effects are consistent with theories positing that attention amplifies the sensory gain of early perceptual processes. The observation that aging slowed latency of the ipsilateral but not the contralateral P1, is consistent with age differences in interhemispheric transfer times. A broadly distributed 200-400 ms validity effect on ERP amplitude was similar between groups in timing, spatial distribution, and magnitude. The 200-400 ms attention effect appeared to be a modulation of the P3 in younger subjects, as earlier observed. However, the present study dissociated the 200-400 ms attention effects from the P3 component because the P3 did not peak until 526 ms in older subjects.
Although most adults are considered to be experts in the identification of faces, fewer people specialize in the recognition of other objects, such as birds and dogs. In this research, the neurophysiological processes associated with expert bird and dog recognition were investigated using event-related potentials. An enhanced early negative component (N170, 164 ms) was found when bird and dog experts categorized objects in their domain of expertise relative to when they categorized objects outside their domain of expertise. This finding indicates that objects from well-learned categories are neurologically differentiated from objects from lesser-known categories at a relatively early stage of visual processing.
The neurological mutant mouse reeler has played a critical role in the evolution of our understanding of normal brain development. From the earliest neuroanatomic studies of reeler, it was anticipated that the characterization of the gene responsible would elucidate important molecular and cellular principles governing cell positioning and the formation of synaptic circuits in the developing brain. Indeed, the identification of reelin has challenged many of our previous notions and has led to a new vision of the events involved in the migration of neurons. Several neuronal populations throughout the brain secrete Reelin, which binds to transmembrane receptors located on adjacent cells triggering a tyrosine kinase cascade. This allows neurons to complete migration and adopt their ultimate positions in laminar structures in the central nervous system. Recent studies have also suggested a role for the Reelin pathway in axonal branching, synaptogenesis, and pathology underlying neurodegeneration.
Some studies have shown that, although repetition increases the familiarity of a stimulus, it does not improve memory for its details. Because memory for associative information is thought to require memory for the details of study presentation, the effects of repetition on associative recognition were examined in the present study. The pattern of results was similar to that found for the recognition of item details: Repetition increased the familiarity of the individual items within each pair to a greater extent than it improved memory for their specific pairings.
In summary, contemporary pathophysiological models of OCD and related disorders implicate CSTC circuitry. In this chapter, we have reviewed relevant concepts related to implicit learning and more specifically, the use of an implicit sequence learning paradigm as a probe of striato-thalamic function. An initial PET investigation of patients with OCD confirmed a priori hypotheses of failure to recruit right striatum, despite the absence of a performance deficit (22). A modified version of the SRT was studied in conjunction with fMRI and yielded reliable right-lateralized striatal activation in a cohort of 10 male subjects, with clear spatial dissociation of caudate and putamen activation foci (119). Subsequent studies in our laboratory suggest that this paradigm also yields a reliable temporal window of thalamic deactivation, and hence a means for assessing thalamic gating in human subjects (120). Finally, as presented in this chapter, preliminary data from the fMRI-SRT in patients with OCD and TS as well as normal control subjects appear to replicate and extend the findings from our original PET-SRT study in OCD. Future investigations in our laboratory will seek to elaborate upon these preliminary results. In particular, we intend to study psychiatric comparison groups to establish the generalizability and/or specificity of these findings across disorders. Within OCD, we hope to explore the relationship between abnormal brain-activation patterns and symptom dimensions (34). Further, by studying subjects with remitted OCD who have been successfully treated, we hope to determine whether the observed brain-activation abnormalities represent state or trait markers. Finally, we have already begun to test a hypothesis of parallel processing deficiency in OCD by using a dual-task version of the SRT that makes simultaneous demands on implicit and explicit information processing systems (128). It is our hope that this program of research will yield new insights about OCD and related disorders, including TS. Most importantly, as other teams of investigators pursue complementary lines of inquiry, it is our wish that collective efforts in this field will lead to improved diagnosis and treatment, if not cure or prevention, for those who are afflicted with these illnesses.
BACKGROUND: Many patients with schizophrenia demonstrate memory deficits. We studied patterns of brain activity during episodic recognition of new and previously seen three-dimensional objects. METHODS: We used (15)O positron emission tomography to study regional cerebral blood flow in eight normal subjects and nine patients with schizophrenia during a visual object recognition task. RESULTS: In comparison with control subjects, patients with schizophrenia showed less regional cerebral blood flow increases in the pulvinar region of the right thalamus and the right prefrontal cortex during the recognition of new objects and significantly greater left prefrontal cortex regional cerebral blood flow increases during the recognition of previously seen objects. Patients with schizophrenia exhibited alarm rates to new objects similar to those of control subjects, but significantly lower recognition rates for previously seen objects. CONCLUSIONS: Schizophrenia is associated with attenuated right thalamic and right prefrontal activation during the recognition of novel visual stimuli and with increased left prefrontal cortical activation during impaired episodic recognition of previously seen visual stimuli. This study provides further evidence for abnormal thalamic and prefrontal cortex function in schizophrenia.
The organization of several laminated structures in the brain is controlled by a signaling pathway activated by Reelin, a large glycoprotein secreted by pioneer neurons in the developing brain. Reelin binds to transmembrane receptors, including VLDLR and ApoER2, and stimulates tyrosine phosphorylation of Disabled-1 (Dab1), which associates with an NPxY motif present in the cytoplasmic domain of the receptors. Disruption of reelin, dab1, or both the vldr and apoer2 genes results in similar cell positioning defects in laminated brain regions including the cerebellum, hippocampus, and cerebral cortex. Although retinal ganglion cells express reelin during development, there is no obvious disruption of cell positioning in the retina of reeler mice. Here, we examine the expression pattern of Dab1 as a first step toward understanding the function of the Reelin signaling pathway in neural retina. Immunohistochemical analysis of the adult retina revealed that Dab1 is expressed in a specific type of amacrine cell. These cells display a narrow dendritic field and they project to two distinct sublaminae within the inner plexiform layer. Dab1 co-localizes with the high-affinity glycine transporter, indicating that these amacrine cells are glycinergic. Cells that express Dab1 are surrounded by dopaminergic fibers originating from wide-field amacrine cells. These features are characteristic of type AII amacrine cells described in other mammalian species. Analysis of the retina at several stages of development revealed that Dab1 is expressed shortly after birth during the time at which AII amacrine cells extend neurites and form synaptic connections in the inner retina. This raises the possibility that the Reelin/Dab1 signaling pathway contributes to formation of intraretinal circuitry in the neural retina.
The expression of reelin mRNA was studied during embryonic brain development in the chick, by using in situ hybridization. Reelin was highly expressed in the olfactory bulb and in subpial neurons in the marginal zone of the cerebral cortex. In the diencephalon, the ventral division of lateral geniculate nuclei and perirotundal nuclei were strongly positive. High levels of expression were associated with some layers of the tectum and with the external granule cell layer of the cerebellum. A more moderate signal was detected in the septal nuclei, hyperstriatal fields, retina, habenular nuclei and hypothalamus, in some reticular nuclei of the mid- and hindbrain, and in the spinal cord. Little or no expression was observed in the cortical plate, Purkinje cells, or the inferior olivary complex. Comparison with reelin expression during mammalian and reptilian brain development reveals several evolutionarily conserved features that presumably define a homology. In addition, significant differences are noted, particularly in telencephalic fields. Most importantly, the developing chick cortex does not exhibit high levels of reelin expression in subpial Cajal-Retzius cells characteristic of the mammalian brain. These observations are compatible with an action of reelin on adhesion and/or of nucleokinesis at the level of target cells. They further suggest that, whereas the telencephalon of birds and archosaurs evolved primarily from dorsal ventricular ridge derivatives in which reelin is probably secondary, the increase in number of reelin-positive cells, and amplification of reelin expression played a key part in the evolution of the cortex in the synapsid lineage leading to mammals.
Cyclin-dependent kinase 5 (Cdk5) is much more than its name implies; it plays a role in neuronal migration, neurite outgrowth and degeneration. Recent evidence suggests that Cdk5 regulates neuronal adhesion and cytoskeletal dynamics.
Defects in a developmental signaling pathway involving mammalian homologues of the Drosophila segment polarity gene, patched (ptc) and its ligand, sonic hedgehog (shh), contribute to tumor formation in several tissues. Recently, a subset of medulloblastoma, the most common malignant brain tumor in children, was found to contain somatic mutations in the human ptc gene. In addition, basal cell nevus syndrome (BCNS), or Gorlin syndrome, which is characterized by developmental anomalies and a predisposition to skin and nervous system malignancies, is associated with germ-line mutation of ptc. Targeted disruption of both alleles of ptc in mice results in embryonic lethality. However, ptc+/- mice survive and develop spontaneous cerebellar brain tumors, suggesting that ptc may function as a tumor suppressor gene. Therefore, we investigated ptc+/-mice as a model for human medulloblastoma. We report that 14% of ptc+/- mice develop central nervous system tumors in the posterior fossa by 10 months of age, with peak tumor incidence occurring between 16 and 24 weeks of age. The tumors exhibited several characteristics of human medulloblastoma, including expression of intermediate filament proteins specific for neurons and glia. Full-length ptc mRNA was present in all tumors analyzed, indicating that there was no loss of heterozygosity at the ptc locus. Nucleotide sequence of ptc mRNA from four tumors failed to identify any mutations. However, a comparison of the normal ptc sequence from C57BL/6 and 129Sv mice did reveal several polymorphisms. High levels of glil mRNA and protein were detected in the tumors, suggesting that the shh/ptc pathway was activated despite the persistence of ptc expression. These data indicate that haploinsufficiency of ptc is sufficient to promote oncogenesis in the central nervous system.
NMDA receptors in mice were mutated by gene targeting to substitute asparagine (N) in position 598 of the NR1 subunit to glutamine (Q) or arginine (R). Animals expressing exclusively the mutated NR1 alleles, NR1(Q/Q) and NR1(-/R) mice, developed a perinatally lethal phenotype mainly characterized by respiratory failure. The dysfunctions were partially rescued in heterozygous mice by the presence of pure wild-type receptors. Thus, NR1(+/Q) mice exhibited reduced life expectancy, with females being impaired in nurturing; NR1(+/R) mice displayed signs of underdevelopment such as growth retardation and impaired righting reflex, and died before weaning. We analyzed the key properties of NMDA receptors, high Ca(2+) permeability, and voltage-dependent Mg(2+) block, in the mutant mice. Comparison of the complex physiological and phenotypical changes observed in the different mutants indicates that properties controlled by NR1 subunit residue N598 are important for autonomic brain functions at birth and during postnatal development. We conclude that disturbed NMDA receptor signaling mediates a variety of neurological phenotypes.