Neocortical morphometry in Huntington's disease: Indication of the coexistence of abnormal neurodevelopmental and neurodegenerative processes

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We found shallower central, intraparietal and left intermediate frontal sulci in HD.

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Shallow calcarine fissure is further evidence of primary cortical degeneration in HD.

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Healthy subjects show strong asymmetry in length of posterior Sylvian fissure (pSF).

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Absence of pSF asymmetry in HD indicates genetic interplay with neurodevelopment.

Huntington's disease (HD) is an inherited, autosomal dominant disorder that is characteristically thought of as a degenerative disorder. Despite cellular and molecular grounds suggesting HD could also impact normal development, there has been scarce systems-level data obtained from in vivo human studies supporting this hypothesis. Sulcus-specific morphometry analysis may help disentangle the contribution of coexisting neurodegenerative and neurodevelopmental processes, but such an approach has never been used in HD. Here, we investigated cortical sulcal depth, related to degenerative process, as well as cortical sulcal length, related to developmental process, in early-stage HD and age-matched healthy controls. This morphometric analysis revealed significant differences in the HD participants compared with the healthy controls bilaterally in the central and intra-parietal sulcus, but also in the left intermediate frontal sulcus and calcarine fissure. As the primary visual cortex is not connected to the striatum, the latter result adds to the increasing in vivo evidence for primary cortical degeneration in HD. Those sulcal measures that differed between HD and healthy populations were mainly atrophy-related, showing shallower sulci in HD. Conversely, the sulcal morphometry also revealed a crucial difference in the imprint of the Sylvian fissure that could not be related to loss of grey matter volume: an absence of asymmetry in the length of this fissure in HD. Strong asymmetry in that cortical region is typically observed in healthy development. As the formation of the Sylvian fissure appears early in utero, and marked asymmetry is specifically found in this area of the neocortex in newborns, this novel finding likely indicates the foetal timing of a disease-specific, genetic interplay with neurodevelopment.

Disturbances in the positioning, proliferation and apoptosis of neural progenitors contribute to subcortical band heterotopia formation

Cortical malformations are commonly associated with intractable epilepsy and other developmental disorders. Our studies utilize the tish rat, a spontaneously occurring genetic model of subcortical band heterotopia (SBH) associated with epilepsy, to evaluate the developmental events underlying SBH formation in the neocortex. Our results demonstrate that Pax6(+) and Tbr2(+) progenitors are mislocalized in tish(+/-) and tish(-/-)- neocortex throughout neurogenesis. In addition, mislocalized tish(-/-) progenitors possess a longer cell cycle than wild type or normally-positioned tish(-/-) progenitors, owing to a lengthened G(2)+M+G(1) time. This mislocalization is not associated with adherens junction breakdown or loss of radial glial polarity in the ventricular zone (VZ), as assessed by immunohistochemistry against phalloidin (to identify F-actin), aPKC-λ and Par3. However, vimentin immunohistochemistry indicates that the radial glial scaffold is disrupted in the region of the tish(-/-) heterotopia. Moreover, lineage tracing experiments using in utero electroporation in tish(-/-) neocortex demonstrate that mislocalized progenitors do not retain contact with the ventricular surface and that ventricular/subventricular zone (SVZ) progenitors produce neurons that migrate into both the heterotopia and cortical plate (CP). Taken together, these findings define a series of developmental errors contributing to SBH formation that differs fundamentally from a primary error in neuronal migration.

ASPP2 binds Par-3 and controls the polarity and proliferation of neural progenitors during CNS development

Cell polarity plays a key role in the development of the central nervous system (CNS). Interestingly, disruption of cell polarity is seen in many cancers. ASPP2 is a haplo-insufficient tumor suppressor and an activator of the p53 family. In this study, we show that ASPP2 controls the polarity and proliferation of neural progenitors in vivo, leading to the formation of neuroblastic rosettes that resemble primitive neuroepithelial tumors. Consistent with its role in cell polarity, ASPP2 influences interkinetic nuclear migration and lamination during CNS development. Mechanistically, ASPP2 maintains the integrity of tight/adherens junctions. ASPP2 binds Par-3 and controls its apical/junctional localization without affecting its expression or Par-3/aPKC lambda binding. The junctional localization of ASPP2 and Par-3 is interdependent, suggesting that they are prime targets for each other. These results identify ASPP2 as a regulator of Par-3, which plays a key role in controlling cell proliferation, polarity, and tissue organization during CNS development.

Pax6-/- mice have a cell nonautonomous defect in nonradial interneuron migration

The mammalian neocortex comprises two major neuronal subtypes; interneurons derived from the ganglionic eminence (GE) and projection neurons from the cortical ventricular zone (VZ). These separate origins necessitate distinct pathways of migration. Using mouse genetics and embryonic forebrain slice culture assays, we sought to identify substrates and/or guidance molecules for nonradial cell migration (NRCM). Mice carrying a mutation in Pax6 (Sey(-/-)), a paired domain transcription factor, are reported to have increased numbers of cortical inhibitory interneurons, suggesting that Pax6 could induce inhibitors of interneuron development or alternatively play a repressive role in guiding NRCM and/or specifying interneurons. Unexpectedly, we found a cell nonautonomous reduction in the distance Sey-/- neurons migrated, reflecting a disorganized migration, with frequent changes in direction. In contrast, no difference in the number of nonradially migrating GE cells was observed in Sey-/- mice. Our data indicate that the increased numbers of interneurons observed in Sey-/- do not result from an increased rate or number of nonradially migrating cells; instead, loss of Pax6 results in the ectopic specification of interneurons in the cortical VZ. Further, our data indicate that the known axonal disorganization in Sey-/- mice contributes to the observed reduced distance of NRCM.

Hyperconnectivity of local neocortical microcircuitry induced by prenatal exposure to valproic acid

Exposure to valproic acid (VPA) during embryogenesis can cause several teratogenic effects, including developmental delays and in particular autism in humans if exposure occurs during the third week of gestation. We examined the postnatal effects of embryonic exposure to VPA on microcircuit properties of juvenile rat neocortex using in vitro electrophysiology. We found that a single prenatal injection of VPA on embryonic day 11.5 causes a significant enhancement of the local recurrent connectivity formed by neocortical pyramidal neurons. The study of the biophysical properties of these connections revealed weaker excitatory synaptic responses. A marked decrease of the intrinsic excitability of pyramidal neurons was also observed. Furthermore, we demonstrate a diminished number of putative synaptic contacts in connection between layer 5 pyramidal neurons. Local hyperconnectivity may render cortical modules more sensitive to stimulation and once activated, more autonomous, isolated, and more difficult to command. This could underlie some of the core symptoms observed in humans prenatally exposed to valproic acid.

A normal radial glial scaffold is necessary for migration of interneurons during neocortical development

The relationship between radial glia and neurons migrating tangentially from the ganglionic eminence (GE) has been suggested but not firmly established. To study this relationship we used a ferret model of cortical dysplasia where radial glia are highly disorganized. To produce this, an antimitotic, methylazoxy methanol (MAM) is injected on the 24th day of gestation (E24 MAM). Neurons migrating away from the GE in MAM-treated animals tend to remain in the intermediate zone (IZ) and do not reach the cortical plate (CP) as they do in normal ferret slices. We recently observed that the disrupted radial glia after MAM treatment could be restored toward their normal morphology by exogenous application of neuregulin1 (NRG1). We demonstrate here that when E24 MAM slices are treated with NRG1, the distribution of cells arising from the GE was similar to normal slices. In a second paradigm, we disrupted radial glia by adding ciliary neurotrophic factor (CNTF) to the culture media of normal ferret slices; CNTF induces acute differentiation of radial glia into astrocytes. After CNTF exposure, few tangentially migrating cells reach the CP compared to untreated slices. These results show that interneurons fail to reach the CP by disrupted normal radial glia and restoring the normal radial glial scaffold is sufficient to allow migrating cells to invade the CP. Our results suggest an important role for radial glia by controlling directly or indirectly the migration of interneurons to the CP, their main target.

Altered distribution of KCC2 in cortical dysplasia in patients with intractable epilepsy

PURPOSE

To examine the distribution of KCC2, a neuron-specific K(+)-Cl(-) cotransporter, in human cortical dysplasia (CD).

METHODS

The immunohistochemical expression of KCC2 was investigated in 18 CD specimens obtained during epilepsy surgery. The histopathologic diagnoses were focal CD (FCD) type I (eight cases), FCD type II (six cases), and hemimegalencephaly (HME; four cases). Tissue sections were immunostained for KCC2 and compared with control sections.

RESULTS

In the mature nondysplastic cortex, all the layers showed diffuse neuropil staining for KCC2. The somata were stained much less, although subcortical ectopic neurons displayed dense staining in the cytosol (intrasomatic staining). In FCD type I, the cortex showed neuropil staining for KCC2 with less-stained somata. Aberrant giant pyramidal neurons were also less stained at the soma, whereas immature neurons showed intrasomatic staining. Increased numbers of ectopic neurons with intrasomatic staining were noted in the subcortical white matter. In FCD type II, dysmorphic neurons displayed dense intrasomatic staining with reduced staining of the neighboring neuropils. Balloon cells did not stain for KCC2. Dysmorphic neurons in HME also showed intrasomatic staining.

CONCLUSIONS

Neurons in CD tissues expressed KCC2. However, the subcellular distribution of KCC2 was altered, which might have affected the ionic homeostasis of Cl(-) and K(+) involved in epileptic activity within CD tissues.

Prenatal exposure to bisphenol A affects adult murine neocortical structure

Prenatal exposure to low-doses of bisphenol A (BPA) has been shown to affect murine neocortical development by accelerating neuronal differentiation/migration through disrupting thyroid hormone function. We therefore studied whether prenatal exposure to low-doses of BPA affected organization of adult neocortical structures. Pregnant mice were injected with 20 microg/kg of BPA daily from embryonic day 0.5 (E0.5) and bromodeoxyuridine (BrdU) was injected at E12.5, E14.5 and at E16.5, and the fetal brains were analyzed after birth. The BrdU-positive cells labeled at E14.5 were significantly increased in the Vth and VIth cortical layers of BPA-treated mice at postnatal 3 weeks (P3W), whereas they were confined to the IVth layer of control mice, though such differences disappeared at P12W. The thalamocortical projections demonstrated by DiI-labeling were abnormal at P3W and P12W in BPA-treated mice. These results indicate that BPA might affect not only neocortical development but also thalamocortical connections.

Neocortical and cerebellar developmental abnormalities in conditions of selective elimination of peroxisomes from brain or from liver

Defects in the formation of the cerebral cortex and the cerebellum are a prominent feature of the peroxisome biogenesis disorder Zellweger syndrome and in mouse models for this disease. The aim of the present study was to investigate the impact of liver and brain peroxisomes on neurodevelopment by analyzing mice with tissue-selective elimination of peroxisomes. To this end, Pex5-loxP mice were bred with albumin/alpha-fetoprotein (Alfp)-Cre and nestin (Nes)-Cre mice. Local elimination of peroxisomes from the brain in Nes-Pex5 knockout mice caused a delay of cortical neuronal migration and of the formation of cerebellar folia and fissures. Migration of granule cells from the external granular layer was retarded, as was the polarization and branching of Purkinje cells, resulting in a less complex branching pattern and a smaller dendritic tree at P21. The Alfp-Pex5 knockout mice were affected differently, displaying a partial arrest of neuronal migration in the cerebral neopallium in the postnatal period despite of the incomplete elimination of peroxisomes from liver during embryonic development. Major abnormalities were seen in the formation of the cerebellum of these liver knockout mice, including hypotrophy, impaired foliation, a delay of granule cell migration, increased cell death, and stunted Purkinje cell arborization. In conclusion, these data demonstrate that absence of peroxisomal function both from liver and brain impairs cortical neuronal migration and maturation of the cerebellum, but different pathogenic mechanisms might be involved.

Layer-specific markers as probes for neuron type identity in human neocortex and malformations of cortical development

Malformations of cortical development (MCDs) are heterogeneous disorders caused by abnormalities of cell proliferation, apoptosis, cell migration, cortical organization, and axon pathfinding. In severe MCDs, the cerebral cortex can appear completely disorganized, or may be replaced by aberrant laminar patterns, as in "4-layered" types of lissencephaly and polymicrogyria. Little is known about the abnormal layers in MCDs and whether they bear any relation to normal cortical layers or how MCDs affect specific neuron types. Normally, each layer contains a defined mixture of different types of pyramidal and nonpyramidal neurons. The neuron types are distinguished by molecular expression as well as morphologic, neurochemical, and electrophysiologic criteria. Patterns of layer-specific mRNA and protein expression reflect the segregation of different neuron types into different layers (e.g. corticospinal projection neurons in layer V). Numerous layer-specific markers have been described in rodent cortex, and increasing numbers are being documented in human and monkey cortex. Applied to MCDs, layer-specific markers have the potential to reveal new insights on pathogenesis, treatment possibilities, and genotype-phenotype correlations. However, much work remains before layer-specific markers become practical tools in diagnostic neuropathology. Additional markers, more extensive documentation of normal expression, and better antibodies compatible with paraffin-embedded tissues will be necessary.

Disruption of neuronal migration by RNAi of Dyx1c1 results in neocortical and hippocampal malformations

The brains of individuals with developmental dyslexia have neocortical neuronal migration abnormalities including molecular layer heterotopias, laminar dysplasias, and periventricular nodular heterotopias (PNH). RNA interference (RNAi) of Dyx1c1, a candidate dyslexia susceptibility gene, disrupts neuronal migration in developing embryonic neocortex. Using in utero electroporation, we cotransfected cells in the rat neocortical ventricular zone (VZ) at E14/15 with short hairpin RNA vectors targeting Dyx1c1 along with either plasmids encoding enhanced green fluorescent protein or plasmids encoding monomeric red fluorescent protein only. RNAi of Dyx1c1 resulted in pockets of unmigrated neurons resembling PNH. The pattern of migration of transfected neurons was bimodal, with approximately 20% of the neurons migrating a short distance from the VZ and another 40% that migrated past their expected lamina. Approximately 25% of the transfected brains had hippocampal pyramidal cell migration anomalies. Molecular layer ectopias, which were not related to injection site artifacts, were also seen in 25% of the animals. These results support the hypothesis that targeted disruption of the candidate dyslexia susceptibility gene, Dyx1c1, results in neuronal migration disorders similar to those seen in the brains of dyslexics.

GABAergic synaptic inhibition is reduced before seizure onset in a genetic model of cortical malformation

Malformations of the neocortex are a common cause of human epilepsy; however, the critical issue of how disturbances in cortical organization render neurons epileptogenic remains controversial. The present study addressed this issue by studying inhibitory structure and function before seizure onset in the telencephalic internal structural heterotopia (tish) rat, which is a genetic model of heightened seizure susceptibility associated with a prominent neocortical malformation. Both normally positioned (normotopic) and misplaced (heterotopic) pyramidal neurons in the tish neocortex exhibited lower resting membrane potentials and a tendency toward higher input resistance compared with pyramidal neurons from control brains. GABAergic synaptic transmission was attenuated in the tish cortex, characterized by significant reductions in the frequency of spontaneous IPSCs (sIPSCs) and miniature IPSCs recorded from pyramidal neurons. In addition, the amplitudes of sIPSCs were reduced in the tish neocortex, an effect that was more profound in the normotopic cells. Immunohistochemical assessment of presynaptic GABAergic terminals showed a reduction in terminals surrounding pyramidal cell somata in normotopic and heterotopic tish neocortex. The attenuation of inhibitory innervation was more prominent for normotopic neurons and was associated with a reduction in a subset of GABAergic interneurons expressing the calcium-binding protein parvalbumin. Together, these findings indicate that key facets of inhibitory GABAergic neurotransmission are disturbed before seizure onset in a brain predisposed to developing seizures. Such alterations represent a rational substrate for reduced seizure thresholds associated with certain cortical malformations.

Neuronal production and precursor proliferation defects in the neocortex of mice with loss of function in the canonical Wnt signaling pathway

To better understand the function of the Wnt pathway in the developing telencephalon, we analyzed neocortical development in low density lipoprotein receptor-related protein (LRP) 6 mutants. LRP6 mutant mice are hypomorphic for the canonical Wnt signaling pathway and have hypoplasia of the developing neocortex. While early telencephalic morphogenesis is largely intact in these mice, probably due to compensation by LRP5, the mutant mice develop a dramatically thinner cortical plate. There is a prominent reduction of neurogenesis leading to a thin cortical plate. Reduced proliferation late in gestation probably also contributes to the hypoplasia. Although there are marked decreases in the numbers of layer 6 and layers 2-4 neurons all laminar identities are generated and there is no evidence of compensatory increases in layer 5 neurons. In addition, LRP6 mutants have partial penetrance of a complex of cortical dysmorphologies resembling those found in patients with developmental forms of epilepsy and mental retardation. These include ventricular and marginal zone heterotopias and cobblestone lissencephaly. This analysis demonstrates that canonical Wnt signaling is required for a diverse array of developmental processes in the neocortex in addition to the previously known roles in regulating precursor proliferation and patterning.

Reeler homozygous mice exhibit enhanced susceptibility to epileptiform activity

PURPOSE

Seizures are observed frequently in humans with diffuse neuronal migration disorders. The reeler mutant mouse also exhibits a diffuse disruption of migration, yet no pro-epileptic phenotype has been reported for this model. Whether this disparity reflects a phenotypic difference that can be used to delineate the mechanisms associated with increasing seizure susceptibility or reflects a paucity of knowledge is unclear. Consequently, this study examined whether seizure susceptibility is altered in reeler mutant mice.

METHODS

In vivo (minimal electroshock delivered transcorneally) and in vitro techniques (field-potential recordings in neocortical and hippocampal brain slice preparations exposed to bicuculline methiodide) were used to determine whether the susceptibility to epileptiform activity is enhanced in reeler homozygous mice relative to controls. Adult (3-7 months) male reeler homozygotes (rl/rl) and controls (+/?) were identified based on their behavioral phenotype and were used in all experiments.

RESULTS

Minimal electroshock revealed that rl/rl mice, compared with controls, exhibited a lower threshold for electroshock-induced seizures (4.5 +/- 0.52 vs. 6.7 +/- 0.35 mA), and a higher incidence of behavioral seizures (median seizure score, class 4 vs. class 0) when animals were subjected to a 5-mA electroshock stimulus. Additionally, neocortical and hippocampal slices from rl/rl mice were more likely to generate spontaneous epileptiform activity after bicuculline application, compared with controls, and the duration of the epileptiform events elicited in 10-30 muM bicuculline was longer in slices from rl/rl mice.

CONCLUSIONS

These data demonstrate that rl/rl mice have enhanced seizure susceptibility that is in part intrinsic to the malformed neocortex and hippocampus. Thus in contrast to prior belief, most animal models of diffuse neuronal migration disorders do exhibit a pro-epileptic phenotype.

Neocortical neuronal arrangement in Miller Dieker syndrome

Miller Dieker syndrome (MDS, type I lissencephaly) is a neuronal migration disorder, which is caused by deletions along the short arm of chromosome 17 (17p13.3). Recent studies would suggest that the cortical lamination in MDS is inverted, based on morphological criteria. The present neuropathological study examines the cerebral cortex from a 33-week old fetus with MDS using both neuronal and laminar-specific markers. These expression studies demonstrate a relatively preserved cortex and cortical lamination, overlying a layer of immature neurons in MDS brain. The findings are consistent with both a migratory and proliferative defect, giving rise to lissencephaly. Moreover, characterization of such rare human malformations of cortical development by immunohistochemical techniques will provide a greater understanding of the underlying mechanisms.

Imaging the neocortex in epilepsy with double inversion recovery imaging

The neocortices of 10 patients with partial seizures and acquired lesions, 14 patients with malformations of cortical development (MCD) and 33 patients with partial seizures and normal conventional MRI were quantitatively evaluated using whole brain double inversion recovery imaging (DIR) and Statistical Parametric Mapping (SPM). Compared to a group of 30 control subjects, DIR and objective voxel-by-voxel statistical comparison identified regions of significantly abnormal DIR signal intensity (DSI) in 9 out of 10 patients with acquired nonprogressive cerebral lesions and partial seizures. In all 9 patients, the areas of abnormal DSI concurred with abnormalities identified on visual inspection of conventional MRI. In all 14 patients with MCD, SPM detected regions of significantly abnormal DSI; all of which corresponded to abnormalities identified on visual inspection of conventional MRI. In addition, in both groups, there were areas that were normal on conventional imaging, which demonstrated abnormal DSI. Voxel-by-voxel statistical analysis identified significantly abnormal DSI in 15 of the 33 patients with cryptogenic focal epilepsy. In 10 of these, the areas of abnormal DSI concurred with epileptic EEG abnormality and clinical seizure semiology. Group analysis of MRI-negative patients with electroclinical seizure onset localising to the left temporal and left and right frontal regions revealed significantly abnormal DSI within the white matter of each respective lobe. DIR analysed using SPM was sensitive in patients with MCDs and acquired cerebral damage. Significant abnormalities in DSI in individual and grouped MRI-negative patients suggest that occult epileptogenic cerebral lesions are associated with subtle structural abnormalities. DIR is, therefore, a useful quantitative MRI technique for characterising epileptic foci and may contribute to presurgical evaluation.

Early callosal absence disrupts the establishment of normal neocortical structure in Swiss mice

In the present study, we tested the hypothesis that the ontogenetic development of the corpus callosum is relevant for the establishment of a normal neocortical structure. To that effect, neocortical morphology (thickness and neuronal density) was analyzed in adult Swiss mice rendered acallosal by midline transection at the first postnatal day (Acallosal group) and in non-manipulated mice. The neocortical thicknesses and neuronal densities of layers II+III through VI were measured in area 6 and at the 17/18a border, both of which present abundant callosal inputs, and in the relatively acallosal area 17. For the thickness measure, significant differences between Non-manipulated and Acallosal groups were only found in the areas that receive massive callosal connections. In area 6, Acallosal mice presented a reduced thickness of layer V, while at the 17/18a border, these mice presented a reduced thickness of layers II+III when compared to non-manipulated ones. No statistical difference between acallosal and non-manipulated mice was found regarding the neuronal density measure. The reduced cortical thickness associated with a comparatively normal neuronal density in neocortical regions which normally have abundant callosal connections suggest a reduction in the number of cortical neurons in acallosal mice. Altogether, the present data indicate that the input provided by callosal axons is necessary for the normal development of the neocortex.

Increased discharge threshold after an interictal spike in human focal epilepsy

It is commonly assumed that interictal spikes (ISs) in focal epilepsies set off a period of inhibition that transiently reduces tissue excitability. Post-spike inhibition was described in experimental models but was never demonstrated in the human epileptic cortex. In the present study post-spike excitability was retrospectively evaluated on intracerebral stereo-electroencephalographic recordings performed in the epileptogenic cortex of five patients suffering from drug-resistant focal epilepsy secondary to Taylor-type neocortical dysplasias. Patients typically presented with highly periodic interictal spiking activity at 2.33 +/- 0.87 Hz (mean +/- SD) in the dysplastic region. During the stereo-electroencephalographic procedure, low-frequency stimulation at 1 Hz was systematically performed for diagnostic purposes to identify the epileptogenic zone. The probability of evoking an IS during the interspike period in response to 1-Hz stimuli delivered close to the ictal-onset zone was examined. Stimuli that occurred early after a spontaneous IS (within 70% of the inter-IS period) had a very low probability of generating a further IS. On the contrary, stimuli delivered during the late inter-IS period had the highest probability of evoking a further IS. The generation of stimulus-evoked ISs is occluded for several hundred milliseconds after the occurrence of a preceding spike discharge. As previously shown in animal models, these findings suggest that, during focal, periodic interictal spiking, human neocortical excitability is phasically controlled by post-spike inhibition.

Heterotopia formation in rat but not mouse neocortex after RNA interference knockdown of DCX

Subcortical band heterotopia (SBH) or double cortex is associated with significant impairments in neocortical function including mental retardation and epilepsy. Mutant alleles of DCX in humans typically cause SBH in females and lissencephaly in males, whereas Dcx null mutations in mice neither disrupt neocortical neuronal migration nor cause SBH formation. In utero RNA interference (RNAi) of Dcx in rats, in contrast, creates an animal model of SBH. Possible explanations for the discrepancies in results following loss of Dcx function include species differences and/or differences between RNAi knockdown and genetic deletion. We have carried out a series of in utero RNAi experiments to investigate possible species differences between rat and mouse to determine the molecular specificity of RNAi against Dcx and to identify the cellular constituents of SBH in the rat model. In utero RNAi in the rat consistently leads to both the formation of SBH and laminar displacement of transfected cells in normotopic cortex, whereas the same treatment in mouse fails to induce SBH but does create laminar displacement. Induction of SBH and impaired radial migration following RNAi against Dcx is rescued by overexpression of Dcx. Thus, both disruptions induced by RNAi are specific to interference of Dcx. SBHs contain transfected pyramidal cells as well as nontransfected cell types, including neocortical interneurons and glia. Together these results indicate that there is a species difference between rat and mouse with respect to RNAi-induced SBH formation and that SBH formation involves the recruitment of several unaltered cell types.

Abnormal neocortical development in mice lacking cGMP-dependent protein kinase I

Cyclic GMP-dependent protein kinase type I (cGKI) is a key signaling intermediate important for synaptic potentiation in the hippocampus and cerebellum, but its expression and function in cortical development have not been elucidated. The expression of cGKI in the developing mouse neocortex was evaluated by immunofluorescence labeling, and effect of cGKI deletion on cortical development was studied in adult cGKI knockout mice. cGKI was expressed at highest levels at embryonic stages in young neurons and radial glial fibers, corresponding to the major period of radial migration and laminar development of pyramidal neurons (embryonic day E13.5-E14.5), declining upon maturation (E17.5-postnatal day P28). The cerebral cortex of homozygous null mutant mice lacking cGKI exhibited heterotopic collections of neurons in the upper cortical layers and abnormal invaginations of layer I, in accord with a neuronal migration or positioning defect. Some cGKI mutant mice displayed defects in midline development resulting in partial fusion of cerebral hemispheres with adjacent neuronal heterotopias. Apical dendrites of cortical pyramidal neurons were misoriented in the cerebral cortex of cGKI null mutants, as shown in reporter mice expressing yellow fluorescent protein in layer V pyramidal neurons and by Golgi impregnation. These results demonstrate a role for cGKI signaling in cortical development related to neuronal migration/positioning that is important for dendritic orientation and connectivity.

Constructing circuits: neurogenesis and migration in the developing neocortex

Our knowledge of the proliferation, migration, and differentiation of neurons has changed dramatically over the last 10 years. Whereas traditionally it was thought that glial and neuronal cells were separate cell lines with different lineages, we now know that this is not true. Radial glia are a type of neural stem cell that generate excitatory pyramidal neurons directly through asymmetric cell division in the ventricular zone (VZ) of the telencephalon and indirectly through the symmetric division of daughter intermediate precursor cells that divide in the subventricular zone (SVZ). Moreover, pyramidal neurons, once thought to migrate only along radial guide fibers to the developing layers of the cortex, have been shown to proceed through four distinct stages of migration during which they change shape, direction, and speed. Gamma-aminobutyric acid (GABAergic) inhibitory interneurons, on the other hand, are generated not in the cortex, but in the medial ganglionic eminence and migrate tangentially to their final cortical destinations. Evidence suggests that GABA activation may play a role in coordinating the generation and migration of both pyramidal and interneuron populations. At the end of neurogenesis, radial glial cells translocate to the cortex and transform into astrocytes. Although they do not actively divide in the adult brain, astrocytes may retain the potential to generate new neurons. These new findings have increased our understanding of the mechanisms underlying certain developmental disorders and, in doing so, reveal potentially useful modes of therapeutic intervention.

Alterations in the phenotype of neocortical pyramidal cells in the Dyrk1A+/- mouse

The gene encoding the dual-specificity tyrosine-regulated kinase DYRK1A maps to the chromosomal segment HSA21q22.2, which lies within the Down syndrome critical region. The reduction in brain size and behavioral defects observed in mice lacking one copy of the murine homologue Dyrk1A (Dyrk1A+/-) support the idea that this kinase may be involved in monosomy 21 associated mental retardation. However, the structural basis of these behavioral defects remains unclear. In the present work, we have analyzed the microstructure of cortical circuitry in the Dyrk1A+/- mouse and control littermates by intracellular injection of Lucifer Yellow in fixed cortical tissue. We found that labeled pyramidal cells were considerably smaller, less branched and less spinous in the cortex of Dyrk1A+/- mice than in control littermates. These results suggest that Dyrk1A influences the size and complexity of pyramidal cells, and thus their capability to integrate information.

Genomic characterisation of a Fgf-regulated gradient-based neocortical protomap

Recent findings support a model for neocortical area formation in which neocortical progenitor cells become patterned by extracellular signals to generate a protomap of progenitor cell areas that in turn generate area-specific neurons. The protomap is thought to be underpinned by spatial differences in progenitor cell identity that are reflected at the transcriptional level. We systematically investigated the nature and composition of the protomap by genomic analyses of spatial and temporal neocortical progenitor cell gene expression. We did not find gene expression evidence for progenitor cell organisation into domains or compartments, instead finding rostrocaudal gradients of gene expression across the entire neocortex. Given the role of Fgf signalling in rostrocaudal neocortical patterning, we carried out an in vivo global analysis of cortical gene expression in Fgfr1 mutant mice, identifying consistent alterations in the expression of candidate protomap elements. One such gene, Mest, was predicted by those studies to be a direct target of Fgf8 signalling and to be involved in setting up, rather than implementing, the progenitor cell protomap. In support of this, we confirmed Mest as a direct transcriptional target of Fgf8-regulated signalling in vitro. Functional studies demonstrated that this gene has a role in establishing patterned gene expression in the developing neocortex, potentially by acting as a negative regulator of the Fgf8-controlled patterning system.

Neuropathological spectrum of cortical dysplasia in children with severe focal epilepsies

Cortical dysplasias comprise a variable spectrum of clinical, neuroradiological and histopathological findings. We report about a cohort of 25 pediatric patients (mean age 8.1+/-4.8 years) with severe drug-resistant early onset focal epilepsies (mean duration 2.1+/-0.4 years), mental/psychomotor retardation, and multilobar epileptogenesis. Compared to age-matched biopsy controls, microscopical inspection of neurosurgically resected specimens revealed dysplastic neurons with/without balloon cells in only 7 patients. According to Palmini's classification system, these lesions were categorized as focal cortical dysplasia (FCD) type II. All other patients presented with rather subtle but statistically significant neuroanatomical abnormalities. We identified increased numbers of ectopic neurons in white matter and cortical gliosis. However, most intriguing was our finding of a microcolumnar arrangement of cortical neurons in layer III. These microcolumns can be statistically defined as vertical lining of more than eight neurons (two times standard deviation of cell countings obtained from controls). In addition, neuronal perikarya were significantly smaller in epilepsy patients. Although histological abnormalities occurring during postnatal maturation of the brain challenge any neuropathological classification in this group of young patients, we propose that these findings are classified according to FCD type I. Our observations support a concept compatible with regional loss of high-order brain organization.

The NMDA receptor NR2B subunit contributes to epileptogenesis in human cortical dysplasia

Cortical dysplasia (CD) is often associated with pharmacoresistant epilepsy. Previous studies showed increased expression of the NMDA receptor subunit NR2B in dysplastic and epileptic human neocortex. We tested the hypothesis that differential increase of NR2B constitutes an epileptogenic mechanism in humans. Dysplastic neocortex and lateral temporal lobe regions resected for treatment of pharmacoresistant seizures were processed for electrophysiological, histological, and immunocytochemical studies. Assignment to the "dysplastic" (n = 8) and "non-dysplastic" (n = 8) groups was based on histology. Neurons in "dysplastic" samples differentially stained for NR2B. Western blot (n = 6) showed an immunoreactive band for NR2B in three out of four "dysplastic" samples. Epileptiform field potentials (EFP) were elicited in vitro by omission of magnesium from the bath. EFP in "dysplastic" slices were characterized by multiple afterdischarges, occurring at a significantly higher repetition rate than EFP in non-dysplastic slices. The NR2B-specific NMDA receptor inhibitor ifenprodil (10muM) suppressed EFP in dysplastic slices. In non-dysplastic slices, burst repetition rate did not change with ifenprodil application. In both dysplastic and non-dysplastic slices, EFP were suppressed by a non-specific NMDAR antagonist (APV) or AMPA receptor antagonist (CNQX). These results provide additional evidence that the differential expression of NR2B in dysplastic human neocortex may play a role in the expression of in-situ epileptogenesis in human CD. NR2B may constitute a target for new diagnostic and pharmacotherapeutic approaches.

Quantitative MR imaging of the neocortex

This article provides an overview of novel MR image analysis methods applied to the quantitative assessment of the neocortex in various forms of epilepsy. Postacquisition processing methods, such as voxel-based morphometry and texture analysis, involve the use of computer software to manipulate, enhance, and classify image information in a digital format. These techniques have the potential to demonstrate subtle abnormalities that are not identified by eye because of anatomic variability. Information provided by quantitative MR imaging of the neocortex may be important for the identification of accurate predictors of surgical outcome and may refine the selection of surgical candidates, particularly those with "nonlesional" neocortical epilepsy.

Whole-brain T2 mapping demonstrates occult abnormalities in focal epilepsy

OBJECTIVES

To examine the cerebral structure of 14 patients with partial seizures and acquired lesions, 20 patients with malformations of cortical development (MCDs), and 45 patients with partial seizures and normal conventional MRI using whole-brain T2 mapping and statistical parametric mapping (SPM).

METHODS

T2 maps were calculated, and individual patients were compared with a group of 30 control subjects using SPM.

RESULTS

T2 mapping and objective voxel-by-voxel statistical comparison identified regions of increased T2 signal in all 14 patients with acquired nonprogressive cerebral lesions and partial seizures. In all of these, the areas of increased T2 signal concurred with abnormalities identified on visual inspection of conventional MRI. In 18 of 20 patients with MCDs, SPM detected regions of increased T2 signal, all of which corresponded to abnormalities identified on visual inspection of conventional MRI. In addition, in both groups, there were areas that were normal on conventional imaging, which demonstrated abnormal T2 signal. Voxel-by-voxel statistical analysis identified increased T2 signal in 23 of the 45 patients with cryptogenic focal epilepsy. In 20 of these, the areas of increased T2 signal concurred with epileptiform EEG abnormality and clinical seizure semiology. Group analysis of MRI-negative patients with electroclinical seizure onset localizing to the left and right temporal and left and right frontal regions revealed increased T2 signal within the white matter of each respective lobe.

CONCLUSIONS

T2 mapping analyzed using statistical parametric mapping was sensitive in patients with malformations of cortical development and acquired cerebral damage. Increased T2 signal in individual and grouped MRI-negative patients suggests that minor structural abnormalities exist in occult epileptogenic cerebral lesions.

[Neocortex formation in mice developing after prenatal serotonin depletion]

The objective of this study was a detailed investigation of structural changes taking place in murine neocortex during its formation and stratification after prenatal serotonin depletion. The study was carried out in murine embryos of F1 (C58BL/CBA) hybrid strain. For depletion of endogenous serotonin in mice, pchlorophenylalanin, an inhibitor of tryptophan hydroxylase - a key enzyme of serotonin synthesis, was used. Brain of the offspring was studied on postnatal days 1, 5 and 10 (n = 10-15 for each time point). Intact animals of respective developmental stages were used as a control. The study demonstrated that prenatal inhibition of serotonin synthesis resulted in malformation of all neocortical layers, disorders of neuronal growth, development and differentiation, changes in their shape and dimensions. During postnatal development, the loss of a significant numbers of cells was observed in the brain structures studied of serotonin-depleted animals.

Primary central white matter hypoplasia of the neocortex

Leukodystrophies, or diseases of the white matter, represent acute or ongoing damage to the oligodendrocytes of the central nervous system. Early childhood white matter disease is most commonly observed after hypoxic ischemic insults, with acute magnetic resonance imaging changes followed by atrophy or periventricular leukomalacia. Dysmyelination occurring in the setting of inborn errors of metabolism is characterized by progressive changes with high signal intensity in white matter on magnetic resonance imaging. This report presents a patient whose magnetic resonance imaging scans demonstrated hypoplasia of myelin in the telencephalon, without clinical or magnetic resonance imaging evidence of inflammatory dysmyelination. Clinical features included intractable seizures, severe hypotonia, and dysmorphic facial features coupled with a static failure to gain developmental milestones. Together, the clinical and magnetic resonance imaging findings are evidence of a primary failure of myelination in the neocortex.

Microanatomy of the dysplastic neocortex from epileptic patients

Focal cortical dysplasia (FCD) is a pathology that is characterized by the abnormal development of the neocortex. Indeed, a wide range of abnormalities in the cortical mantle have been associated with this pathology, including cytoarchitectonic alterations and the presence of dysmorphic neurons, balloon cells and ectopic neurons in the white matter. FCD is commonly associated with epilepsy, and hence we have studied the ultrastructure of cortical tissue resected from three subjects with intractable epilepsy secondary to cortical dysplasia to identify possible alterations in synaptic circuitry, using correlative light and electron microscopic methods. While the balloon cells found in this tissue do not appear to receive synaptic contacts, the ectopic neurons in the white matter were abnormally large and were surrounded by hypertrophic basket formations immunoreactive for the calcium-binding protein parvalbumin. Furthermore, these basket formations formed symmetrical (inhibitory) synapses with both the somata and the proximal portion of the dendrites of these giant ectopic neurons. A quantitative analysis revealed that in the dysplastic tissue, the density of excitatory and inhibitory synapses was different from that of the normal adjacent cortex. Both increases and decreases in synaptic density were observed, as well as changes in the proportion of excitatory and inhibitory synapses. However, we could not establish a common pattern of changes, either in the same patients or between different patients. These results suggest that cortical dysplasia leads to multiple changes in excitatory and inhibitory synaptic circuits. We discuss the possible relationship between these alterations and epilepsy, bearing in mind the possible limitations that preclude the extrapolation of the results to the whole population of epileptic patients with dysplastic neocortex.

Regulation of neocortical interneuron development and the implications for neurodevelopmental disorders

Neurodevelopmental disorders typically have complex endophenotypes, which can include abnormalities in neuronal excitability, processing of complex information, as well as behaviors such as anxiety and social interactions. Converging experimental and clinical evidence suggests that altered interneuron development may underlie part of the pathophysiological process of such disorders. Consistent with this, mice with abnormal hepatocyte growth factor signaling exhibit disturbances in the development of specific interneuron subclasses that are paralleled by seizure activity and a complex behavioral phenotype. Mutations in molecules that regulate different aspects of interneuron development could provide the heterogeneity in genetic susceptibility that, when combined with environmental disturbances, results in a phenotypic spectrum that serves as the hallmark pathophysiology for autism, mental retardation, schizophrenia and other neurodevelopmental disorders.

A moderate and transient deficiency of maternal thyroid function at the beginning of fetal neocorticogenesis alters neuronal migration

Epidemiological studies and case reports show that even a relatively minor degree of maternal hypothyroxinemia during the first half of gestation is potentially dangerous for optimal fetal neurodevelopment. Our experimental approach was designed to result in a mild and transient period of maternal hypothyroxinemia at the beginning of corticogenesis. Normal rat dams received the goitrogen 2-mercapto-1-methyl-imidazole for only 3 d, from embryonic d 12 (E12) to E15. Maternal thyroid hormones decreased transiently to 70% of normal serum values, without clinical signs of hypothyroidism. Dams were injected daily with 5-bromo-2'-deoxyuridine (BrdU) during 3 d, from E14-E16 or E17-E19. Their pups were tested for audiogenic seizure susceptibility 39 d after birth (P39) and killed at P40. Cells that had incorporated BrdU were identified by immunocytochemistry, and quantified: numerous heterotopic cells were found, whether labeled at E14-E16 or E17-E19, that were identified as neurons. The cytoarchitecture and the radial distribution of BrdU-labeled neurons was significantly affected in the somatosensory cortex and hippocampus of 83% of the pups. The radial distribution of gamma-aminobutyric acidergic neurons was, however, normal. The infusion of dams with T4 between E13 and E15 avoided these alterations, which were not prevented when the T4 infusion was delayed to E15-E18. In total, 52% of the pups born to the goitrogen-treated dams responded to an acoustic stimulus with wild runs, followed in some by seizures. When extrapolated to man, these results stress the need for prevention of hypothyroxinemia before midpregnancy, however moderate, and whichever the underlying cause.

Subtle Microscopic Abnormalities in Hippocampal Sclerosis Do Not Predict Clinical Features of Temporal Lobe Epilepsy

PURPOSE

Subtle microdysplastic features are found in some patients with hippocampal sclerosis (HS) and refractory temporal lobe epilepsy. The significance of these findings is unknown. We investigated their frequency, relation to the pattern of HS, and clinical associations.

METHODS

One-hundred forty patients with histologically confirmed HS (mean age at operation, 35 years; 85 women) were analyzed. The presence of HS and subtle structural abnormalities (SSAs) in the mesial temporal lobe and in the lateral neocortical tissue was assessed in detail. Antecedents, seizure characteristics, two verbal memory tests, and outcome in HS patients with and without SSAs were determined.

RESULTS

SSAs were found in 60 (43%) of the 140 HS patients, being mesial only in 32 of the 60 cases, and lateral only in nine cases; the remaining 19 cases had both mesial and lateral abnormalities. The frequency of SSA was not related to the pattern of HS or other tested variables. Prolonged febrile convulsions were present in 26 (44%) patients with SSAs, and in 26 (34%) patients (not significant) without SSAs. The outcome after surgery did not differ between patients with SSAs (incidence rate ratio for seizure recurrence, 0.9; 95% confidence interval, 0.5-1.6) compared with patients without SSAs (reference ratio, 1).

CONCLUSIONS

Forty-three percent of HS patients have SSAs in their lobectomy specimens. The presence of SSAs does not predict clinical characteristics, such as presence of prolonged febrile convulsions, postsurgical outcome, or neuropsychological performance, nor does it correlate with the histologic pattern of HS.

Defective neocortical development in Fyn-tyrosine-kinase-deficient mice

Fyn tyrosine kinase is involved in the tyrosine-phosphorylation of Disabled-1 in the Reelin signaling pathway, and absence of Fyn is expected to result in a reeler-like phenotype. Thus, this study investigated neocortical development in Fyn-deficient mice. Bromodeoxyuridine labeling revealed the under-migration of later-generated neurons despite the normal placement of earlier-generated neurons. Calbindin- and alpha-calcium/calmodulin-dependent protein kinase II-immunohistochemistry showed that layer II-III neurons were aberrantly stratified, but the neurons in the deeper layers showed little evidence of abnormality. Fyn was intensely expressed in the leading process of migratory cortical neurons generated in the later stage. These findings strongly suggest that Fyn is required for the migration of later-generated neurons, but that it is dispensable for the Reelin-dependent inside-out layer formation.

[GABA-, serotonin-immunoreactive structures and Ca2+-binding protein in the neocortex of reeler mutant mice]

To investigate the neurotransmitter organization of abnormally formed neocortex in the reeler mutant mice, an immunohistochemical study of GABA-, serotoninergic structures and Ca2+-binding protein was performed. GABA-ergic structures were identified according to the localization of glutamate decarboxylase (GAD), the key enzyme involved in GABA synthesis. The cells that were morphologically and immunohistochemically identical to Cajal-Retzius neurons, were found to have an unusual localization in the neocortex of reeler mutant mice, i.e. under the layer I, but not in its upper third, as seen in normal cerebral cortex. GAD-immunoreactive label was shown to accumulate in the neuropil of the middle and deep layers, while layer I contained only single granules. In contrast to what was observed in normal mice, serotonin-immunopositive fibers did not form superficial and deep plexuses in the neocortex, however they reached their targets. Thus, in abnormally formed neocortex, lacking the appropriate cytoarchitectonic organization, the structure of both intrinsic and projectional neurotransmitter systems was disturbed.

Expression of the transcription factor, tailless, is required for formation of superficial cortical layers

The gene tailless (tlx) encodes a forebrain-restricted transcription factor that is robustly expressed in progenitor cells of the ventricular and subventricular zones during neurogenesis. To investigate the role of tlx in neocortical development we generated a targeted deletion of tlx by homologous recombination. Here we compared the lamination, connectivity and patterning of cortical regions in adult tlx-/- mice and their wild-type littermates. We found first that neocortical thickness is reduced by 20% in mutant animals; most of this reduction is due to a diminution of supragranular layers, while layer I and layers IV through VI are relatively intact cytoarchitecturally. Consistent with this, the cross-sectional area of the corpus callosum is reduced by over 40%. Second, thalamocortical and intrinsic excitatory circuits in tlx-/- mice exhibit an essentially normal distribution from layer IV to the white matter, but are reduced superficial to layer IV. Finally, within parietal cortex of mutant mice a vibrissa-like pattern of cortical barrels is present in the expected rostro-caudal location. These observations indicate that loss of tlx function most severely affects generation and differentiation of neurons destined for superficial cortical layers. Thus, tlx may be important in sustaining the progenitor cell population throughout late prenatal development. Establishment of functional cortical areas, and development of basic patterns of thalamocortical and intra-cortical circuits occurs independently of tlx function.

CXCR4 regulates interneuron migration in the developing neocortex

The chemotactic factors directing interneuron migration during cerebrocortical development are essentially unknown. Here we identify the CXC chemokine receptor 4 (CXCR4) in interneuron precursors migrating from the basal forebrain to the neocortex and demonstrate that stromal cell-derived factor-1 (SDF-1) is a potent chemoattractant for isolated striatal precursors. In addition, we show that CXCR4 is present in early generated Cajal-Retzius cells of the cortical marginal zone. In mice with a null mutation in CXCR4 or SDF-1, interneurons were severely underrepresented in the superficial layers and ectopically placed in the deep layers of the neocortex. In contrast, the submeningeal positioning of Cajal-Retzius cells was unaffected. Thus, our findings suggest that SDF-1, which is highly expressed in the embryonic leptomeninx, selectively regulates migration and layer-specific integration of CXCR4-expressing interneurons during neocortical development.

Aberrant lamination in the cerebral cortex of mouse embryos lacking DNA topoisomerase IIbeta

We have examined corticogenesis in mouse embryos lacking DNA topoisomerase IIbeta (IIbeta) in the brain or in all tissues. The absence of IIbeta, a type II DNA topoisomerase normally expressed in postmitotic cells in the developing cortex, severely affects cerebral stratification: no subplate is discernible, and neurons born at later stages of corticogenesis fail to migrate to the superficial layers. This abnormal pattern of neuron positioning in the cerebral cortex is reminiscent of that observed in mouse mutants defective in the reelin-signaling pathway. Significantly, the level of reelin in the neocortex is much reduced when IIbeta is absent. These results implicate a role of IIbeta in brain development. The enzyme may be required in implementing particular genetic programs in postmitotic cells, such as reelin expression in Cajal-Retzius cells, perhaps through its action on nucleoprotein structure of particular chromosomal regions.

Abnormal distributions of callosal commissural and corticothalamic neurons in the cerebral neocortex of Shaking Rat Kawasaki

Shaking Rat Kawasaki (SRK) is an autosomal recessive mutant rat recognized by unstable gait and tremor and by early death around the time of weaning. We previously reported that corticospinal tract neurons are malpositioned in the motor cortex of the SRK rat [Ikeda and Terashima (1997) J. Comp. Neurol. 383, 370-380]. In the present study, we examined the distribution pattern of callosal commissural (CC) and corticothalamic (CT) neurons of SRK and normal rats with the injection of horseradish peroxidase (HRP) into the contralateral hemisphere or wheat germ agglutinin-conjugated HRP into the ventral lateral thalamic nucleus. The intracortical distribution pattern of retrogradely labeled CC and CT neurons in the motor cortex of SRK rat was abnormal: CC neurons were more deeply situated and CT neurons were more superficially situated in the SRK cortex than the corresponding components in the normal cortex. Most of labeled CC and CT neurons had abnormal dendritic configurations. Statistical analysis revealed that the difference of the mean intracortical position of CC and CT neurons of the SRK was significantly different from the normal counterparts (Student's t-test, P<0.01). Taken together with previous findings, our data demonstrate that the abnormal cytoarchitecture of SRK cortex resembles the reeler cortex.

Impaired cell cycle control of neuronal precursor cells in the neocortical primordium of presenilin-1-deficient mice

Recent studies have implicated presenilin-1 (PS-1) in the processing of the amyloid precursor protein and Notch-1. We show that PS-1 has biological effects on differentiation and cell cycle control of neuronal precursor cells in vivo using PS-1-deficient mice. The expression of Class III beta-tubulin was upregulated throughout the neocortical primordia of PS-1-deficient E14 embryos, especially on the ventricular surface. The increased speed of migration of the immature neurons from the ventricular zone outward in the PS-1-deficient neocortical primordia was indicated by an in vivo bromodeoxyuridine (BrdU)-labeling assay and a DiI-labeling assay in slice culture. Furthermore, we investigated the cell cycle of neuronal precursor cells in the neocortical ventricular zone using an in vivo cumulative BrdU-labeling assay. The length of the cell cycle in the neocortical precursor cells of wild-type mice was 11.4 hr whereas that of the PS-1-deficient mice was 15.4 hr. Among all phases of the cell cycle, S-phase exhibited the most prominent change in length, increasing from 2.4 hr in the wild-type mice to 7.4 hr in the mutant mice. The distribution of beta-catenin was specifically affected in the ventricular zone of the PS-1-deficient mice. These findings suggest that PS-1 is involved in the differentiation and the cell cycle control of neuronal precursor cells in the ventricular proliferating zone of the neocortical primordium.

Neocortical system abnormalities in autism: an fMRI study of spatial working memory

OBJECTIVE

To test the hypothesis that deficits in spatial working memory in autism are due to abnormalities in prefrontal circuitry.

METHODS

Functional MRI (fMRI) at 3 T was performed in 11 rigorously diagnosed non-mentally retarded autistic and six healthy volunteers while they performed an oculomotor spatial working memory task and a visually guided saccade task.

RESULTS

Autistic subjects demonstrated significantly less task-related activation in dorsolateral prefrontal cortex (Brodmann area [BA] 9/46) and posterior cingulate cortex (BA 23) in comparison with healthy subjects during a spatial working memory task. In contrast, activation of autistic individuals was not reduced in other regions comprising the neural circuitry for spatial working memory including the cortical eye fields, anterior cingulate cortex, insula, basal ganglia, thalamus, and lateral cerebellum. Autistic subjects also did not demonstrate reduced activation in any brain regions while performing visually guided saccades.

CONCLUSION

Impairments in executive cognitive processes in autism may be subserved by abnormalities in neocortical circuitry as evidenced by decreased activation in prefrontal and posterior cingulate circuitry during a spatial working memory task.

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.

Layer I ectopias and increased excitability in murine neocortex

Cortical dysplasias are associated with both epilepsy and cognitive impairments in humans. Similarly, several animal models of cortical dysplasia show that dysplasia causes increased seizure susceptibility and behavioral deficits in vivo and increased levels of excitability in vitro. As most current animal models involve either global disruptions in cortical architecture or the induction of lesions, it is not yet clear whether small spontaneous neocortical malformations are also associated with increased excitability or seizure susceptibility. Small groups of displaced neurons in layer I of the neocortex, ectopias, have been identified in patients with cognitive impairments, and similar malformations occur sporadically in some inbred lines of mice where they are associated with behavioral and sensory-processing deficits. In a previous study, we characterized the physiology of cells within neocortical ectopias, in one of the inbred lines (NXSM-D/Ei) and showed that the presence of multiple ectopias is associated with the generation of spontaneous epileptiform activity in slices. In this study, we use extracellular recordings from brain slices to show that even single-layer I ectopias are associated with higher excitability. Specifically, slices that contain single ectopias display epileptiform activity at significantly lower concentrations of the GABA(A) receptor antagonist bicuculline than do slices without ectopias (either from opposite hemispheres or animals without ectopias). Moreover, because removal of ectopias from slices does not restore normal excitability, enhanced excitability is not generated within the ectopia. Finally, we show that in vivo, mice with ectopias are more sensitive to the convulsant pentylenetetrazole than are mice without ectopias. Together these results suggest that alterations in cortical hemispheres containing focal layer I malformations increase cortical excitability and that even moderately small spontaneous cortical dysplasias are associated with increased excitability in vitro and in vivo.

Types and three-dimensional distribution of neuronal ectopias in the brain of mice prenatally subjected to X-irradiation

The types and three-dimensional distribution of neocortical ectopias following prenatal exposure to X-irradiation were studied by a histological examination and computer reconstruction techniques. Pregnant ICR mice were subjected to X-irradiation at a dose of 1.5 Gy on embryonic day 13. The brains from 30-day-old mice were serially sectioned on the frontal plane at 15 microns, stained with HE and observed with a microscope. The image data for the sections were input to a computer, and then reconstructed to three-dimensional brain structures using the Magellan 3.6 program. Sectional images were then drawn on a computer display at 240 microns intervals, and the positions of the different types of neocortical ectopias were marked using color coding. Three types of neocortical ectopias were recognized in the irradiated brains. Neocortical Lay I ectopias were identified as small patches in the caudal occipital cortex, and were located more laterally in the neocortex in caudal sections than in the rostral sections. Periventricular ectopias were located more rostrally than Lay I ectopias, and were found from the most caudal extent of the presumed motor cortex to the most caudal extent of the lateral ventricle. Hippocampal ectopias appeared as continuous linear bands, and were frequently associated with the anterior parts of the periventricular ectopias.

A hypomorphic allele of dab1 reveals regional differences in reelin-Dab1 signaling during brain development

The disabled 1 (Dab1) p80 protein is essential for reelin signaling during brain development. p80 has an N-terminal domain for association with reelin receptors, followed by reelin-dependent tyrosine phosphorylation sites and about 310 C-terminal residues of unknown function. We have generated mutant mice that express only a natural splice form of Dab1, p45, that lacks the C-terminal region of p80. The normal development of these mice implies that the receptor-binding region and tyrosine phosphorylation sites of p80 are sufficient for reelin signaling. However, a single copy of the truncated gene does not support normal development of the neocortex and hippocampus. The CA1 region of the hippocampus is split into two well-organized layers, while the marginal zone of the neocortex is invaded by late-born cortical plate neurons. The haploinsufficiency of the p45 allele of Dab1 implies that the C terminus of p80 affects the strength of reelin-Dab1 signaling, yet there is no apparent change in reelin-dependent tyrosine phosphorylation of p45 relative to p80. Therefore, we suggest that the C-terminal region of Dab1 p80 is involved in signaling to downstream effector molecules. Furthermore, the presence of late-born cortical plate neurons in the marginal zone reveals a requirement for reelin-Dab1 signaling in late-born cortical plate neurons, and helps distinguish models for the cortical inversion in the reeler mutant mouse.

disabled-1 functions cell autonomously during radial migration and cortical layering of pyramidal neurons

Genetic mosaics offer an excellent opportunity to analyze complex gene functions. Chimeras consisting of mutant and wild-type cells provide not only the avenue for lineage-specific gene rescue but can also distinguish cell-autonomous from non-cell-autonomous gene functions. Using an independent genetic marker for wild-type cells, we constructed Dab1(+/+) <--> Dab1(-/-) chimeras with the aim of discovering whether or not the function of Dab1 during neuronal migration and cortical layering is cell autonomous. Dab1(+/+) cells were capable of radial migration and columnar formation in a Dab1(-/-)environment. Most Dab1(+/+) cells segregated to the superficial part of the mutant cortex, forming a multilayered supercortex. Neuronal birth-dating studies indicate that supercortex neurons were correctly layered, although adjacent mutant cortex neurons were in reversed order. Immunocytochemistry using Emx1, a marker for pyramidal neurons, indicates that the vast majority of Dab1(+/+) neurons in the supercortex were Emx1 immunoreactive. Confirmation of the pyramidal phenotype was demonstrated by the absence of GABA immunoreactivity among Dab1(+/+) cells in the supercortex. Myelin staining using 2'3'-cyclic nucleotide 3'-phosphodiesterase showed the supercortex was supported by a secondary white matter from which thick fiber tracts appear connected to the underlying mutant white matter. The presence of Dab1(+/+) cells failed to rescue inversion of cortical layers and the abnormal infiltration of the marginal zone by Dab1(-/-) cells. Conversely, mutant cells did not impose a mutant phenotype on adjacent wild-type neurons. These results suggest that Dab1 functions cell autonomously with respect to radial migration and cortical layering of pyramidal neurons.

Time course of the effects of prenatal gamma irradiation on the dorsal lateral geniculate nucleus of Swiss mice

A previous study reported that adult mice irradiated at the 16th embryonic day present a severe neuronal number reduction in the dorsal lateral geniculate thalamic nucleus. In the present study, we investigated the time course of the effects of prenatal irradiation on this thalamic nucleus. One day after irradiation, a great number of pyknotic figures were seen mainly in the cerebral proliferative zones. In the geniculate nucleus, only scattered pyknotic figures were identified. On the first week after birth, the geniculate nucleus presented frequent pyknotic figures. From five days after birth onwards, a severe shrinkage of the occipital cortex and a great reduction in the geniculate nucleus neuronal number were found. On the second week after birth this neuronal number reduction reached as high as 75%. At each postnatal analyzed age, severe volumetric geniculate nucleus shrinkage was combined to non-significant neuronal density variations. The presence of few pyknotic figures in the geniculate nucleus one day after irradiation and its delayed neuronal loss indicate an indirect effect of irradiation. We suggest that the effect upon the geniculate nucleus is secondary to the damage of the occipital cortex. A possible interpretation for thalamic neuronal loss is that geniculate neurons fail to establish cortical arbors after major target loss. In this case, the loss of trophic support should also be considered.

Aberrant apoptosis in the neurological mutant Flathead is associated with defective cytokinesis of neural progenitor cells

Flathead is a rat neurological mutant which is phenotypically characterized by a flattened cranium, resting tremor, ataxia, progressive paralysis of the hind limbs, and death at 3-4 weeks after birth. Previous studies showed that rats homozygous for the mutation have a dramatically reduced brain size caused by a burst of apoptosis that begins after embryonic day 16 (E16) and which peaks at about E18. Late-developing structures such as the dentate gyrus, internal granule layer of the cerebellum, and superficial layers of the neocortex are severely depleted of cells. In the present study we have found that neurons and glia are both affected by the mutation. Immunohistochemical analysis with TAG-1, a marker for migratory neurons, revealed reduced staining in Fh neocortex and cerebellum, indicating that the mutation affects neuronal migration or a developmental event prior to it. Analysis of acutely dissociated neocortical cultures showed an accumulation of nestin-positive progenitor cells. Moreover, a substantial proportion of these progenitor cells were multinucleated with the nuclei organized as rosettes. Such multinucleated cells were also found in intact sections of the neocortex and the cerebellum where their presence was restricted to proliferative zones. Within the neocortex, the abundance of multinucleated progenitors is highest at E18 and decreases thereafter, thus correlating with the profile of cell death. This, along with the dramatically higher frequency of apoptosis among multinucleated cells, suggests that the aberrant cell death in Fh is due to defective cytokinesis that occurs in progenitor cells during late stages of brain development.