Hippocampal subfield abnormalities and memory functioning in children with fetal alcohol Spectrum disorders.

BACKGROUND: Prenatal alcohol exposure (PAE) affects early brain development and has been associated with hippocampal damage. Animal models of PAE have suggested that some subfields of the hippocampus may be more susceptible to damage than others. Recent advances in structural MRI processing now allow us to examine the morphology of hippocampal subfields in humans with PAE. METHOD: Structural MRI scans were collected from 40 children with PAE and 39 typically developing children (ages 8-16). The images were processed using the Human Connectome Project Minimal Preprocessing Pipeline (v4.0.1) and the Hippocampal Subfields package (v21) from FreeSurfer. Using a large dataset of typically developing children enrolled in the Human Connectome Project in Development (HCP-D) for normative standards, we computed age-specific volumetric z-scores for our two samples. Using these norm-adjusted hippocampal subfield volumes, comparisons were performed between children with PAE and typically developing children, controlling for total intracranial volume. Lastly, we investigated whether subfield volumes correlated with episodic memory (i.e., Picture Sequence Memory test of the NIH toolbox). RESULTS: Five subfields had significantly smaller adjusted volumes in children with PAE than in typically developing controls: CA1, CA4, subiculum, presubiculum, and the hippocampal tail. Subfield volumes were not significantly correlated with episodic memory. CONCLUSIONS: The results suggest that several regions of the hippocampus may be particularly affected by PAE. The finding of smaller CA1 volumes parallels previous reports in rodent models. The novel findings of decreased volume in the subicular cortex, CA4 and the hippocampal tail suggest avenues for future research.

Developmental, cellular, and behavioral phenotypes in a mouse model of congenital hypoplasia of the dentate gyrus.

In the hippocampus, a widely accepted model posits that the dentate gyrus improves learning and memory by enhancing discrimination between inputs. To test this model, we studied conditional knockout mice in which the vast majority of dentate granule cells (DGCs) fail to develop - including nearly all DGCs in the dorsal hippocampus - secondary to eliminating Wntless (Wls) in a subset of cortical progenitors with Gfap-Cre. Other cells in the Wlsfl/-;Gfap-Cre hippocampus were minimally affected, as determined by single nucleus RNA sequencing. CA3 pyramidal cells, the targets of DGC-derived mossy fibers, exhibited normal morphologies with a small reduction in the numbers of synaptic spines. Wlsfl/-;Gfap-Cre mice have a modest performance decrement in several complex spatial tasks, including active place avoidance. They were also modestly impaired in one simpler spatial task, finding a visible platform in the Morris water maze. These experiments support a role for DGCs in enhancing spatial learning and memory.

Hippocampal abnormalities and seizures: a 16-year single center review of sudden unexpected death in childhood, sudden unexpected death in epilepsy and SIDS.

Sudden Unexpected Death in Childhood (SUDC) is the unexplained death of children aged between 1 and 18 years old. Hippocampal abnormalities have previously been described in Sudden Unexpected Death in Epilepsy (SUDEP) and it is possible that SUDC shares similar pathogenic mechanisms with SUDEP. Our aim was to determine the prevalence of hippocampal abnormalities, history of seizures and demographic features in our caseload of SUDC, SUDEP and SIDS cases. A review of post-mortem reports from 2003 to 2018 was carried out to identify cases of SUDC, SUDEP and SIDS. Histological evidence of hippocampal abnormalities, patient demographics (age, gender), sleeping position, and past medical history (history of seizures and illness 72 hours prior to death) were recorded. Statistical analysis was performed to compare the three groups. 48 SUDC, 18 SUDEP and 358 SIDS cases were identified. Hippocampal abnormalities associated with temporal lobe epilepsy were found in 44.4% of SUDC cases. 5/15 SUDC cases with a history of seizures demonstrated hippocampal abnormalities. SUDC cases were also more likely to be found prone compared to SIDS cases. In comparison with SIDS, both SUDC and SUDEP cases were more likely to demonstrate hippocampal abnormalities (SUDC: (OR = 9.4, 95% CI: 3.1-29.1, p < 0.001; SUDEP: OR = 35.4, 95% CI: 8.3-151.5, p < 0.001). We found a potential link between hippocampal abnormalities and epileptic seizures in SUDC. A concerted effort should be directed towards consistent sampling and standardized description of the hippocampus and clinical correlation with a history of seizures/epilepsy in postmortem reports.

Maternal exposure to swainsonine impaired the early postnatal development of mouse dentate gyrus of offspring.

Neurogenesis in the dentate gyrus (DG) plays a key role in the normal of structure and function of the hippocampus-learning and memory. After eating the locoweeds, animals develop a chronic neurological disease called "locoism". Swainsonine (SW) is the main toxin in locoweeds. Studies have shown that SW induces neuronal apoptosis in vitro and impairs learning and memory in adult mouse. The present study explored effects of SW exposure to dams on the postnatal neurogenesis of DG of offspring. Pregnant ICR mice were orally gavaged with SW at a dose of 0, 5.6 or 8.4 mg/kg/day from gestation day 10 to postnatal day (PND) 21, respectively. We found that SW impaired the proliferation capacity of neural progenitor cells in the DG so that the number of newborn cells was reduced at PND 8. Using the postnatal in vivo electroporation, we showed that the dendritic branching and total length of granule cells were significantly decreased due to SW exposure. In addition, on PND 21, the density of NeuN-positive and Reelin-positive interneurons increased in the hilus, implying the disorder of neuronal migration. These results suggest that maternal exposure to SW, the neurogenesis of DG on offspring was disrupted, finally leading to the functional disorder of DG.

Dentate gyrus abnormalities in sudden unexplained death in infants: morphological marker of underlying brain vulnerability.

Sudden unexplained death in infants, including the sudden infant death syndrome, is likely due to heterogeneous causes that involve different intrinsic vulnerabilities and/or environmental factors. Neuropathologic research focuses upon the role of brain regions, particularly the brainstem, that regulate or modulate autonomic and respiratory control during sleep or transitions to waking. The hippocampus is a key component of the forebrain-limbic network that modulates autonomic/respiratory control via brainstem connections, but its role in sudden infant death has received little attention. We tested the hypothesis that a well-established marker of hippocampal pathology in temporal lobe epilepsy-focal granule cell bilamination in the dentate, a variant of granule cell dispersion-is associated with sudden unexplained death in infants. In a blinded study of hippocampal morphology in 153 infants with sudden and unexpected death autopsied in the San Diego County medical examiner's office, deaths were classified as unexplained or explained based upon autopsy and scene investigation. Focal granule cell bilamination was present in 41.2% (47/114) of the unexplained group compared to 7.7% (3/39) of the explained (control) group (p < 0.001). It was associated with a cluster of other dentate developmental abnormalities that reflect defective neuronal proliferation, migration, and/or survival. Dentate lesions in a large subset of infants with sudden unexplained death may represent a developmental vulnerability that leads to autonomic/respiratory instability or autonomic seizures, and sleep-related death when the infants are challenged with homeostatic stressors. Importantly, these lesions can be recognized in microscopic sections prepared in current forensic practice. Future research is needed to determine the relationship between hippocampal and previously reported brainstem pathology in sudden infant death.

Mice lacking α-tubulin acetyltransferase 1 are viable but display α-tubulin acetylation deficiency and dentate gyrus distortion.

α-Tubulin acetylation at Lys-40, located on the luminal side of microtubules, has been widely studied and used as a marker for stable microtubules in the cilia and other subcellular structures, but the functional consequences remain perplexing. Recent studies have shown that Mec-17 and its paralog are responsible for α-tubulin acetylation in Caenorhabditis elegans. There is one such protein known as Atat1 (α-tubulin acetyltransferase 1) per higher organism. Zebrafish Atat1 appears to govern embryo development, raising the intriguing possibility that Atat1 is also critical for development in mammals. In addition to Atat1, three other mammalian acetyltransferases, ARD1-NAT1, ELP3, and GCN5, have been shown to acetylate α-tubulin in vitro, so an important question is how these four enzymes contribute to the acetylation in vivo. We demonstrate here that Atat1 is a major α-tubulin acetyltransferase in mice. It is widely expressed in mouse embryos and tissues. Although Atat1-null animals display no overt phenotypes, α-tubulin acetylation is lost in sperm flagella and the dentate gyrus is slightly deformed. Furthermore, human ATAT1 colocalizes on bundled microtubules with doublecortin. These results thus suggest that mouse Atat1 may regulate advanced functions such as learning and memory, thereby shedding novel light on the physiological roles of α-tubulin acetylation in mammals.

Developmental characteristics of dendritic spines in the dentate gyrus of Fmr1 knockout mice.

Fragile X Syndrome (FXS) is the most common form of inherited mental retardation. The neuroanatomical phenotype of adult FXS patients, as well as adult Fmr1 knockout (KO) mice, includes elevated dendritic spine density and a spine morphology profile in neocortex that resembles younger individuals. Developmental studies in mouse neocortex have revealed a dynamic phenotype that varies with age, especially during the period of synaptic pruning. Here we investigated the hippocampal dentate gyrus to determine if the FXS spine phenotype is similarly tied to periods of maturation and pruning in this brain region. We used high-voltage electron microscopy to characterize Golgi-stained spines along granule cell dendrites in Fmr1 KO and wildtype (WT) mouse dentate gyrus at postnatal days 15, 21, 30, and 60. In contrast to neocortex, dendritic spine density was higher in Fmr1 KO mice across development. Interestingly, neither genotype showed specific phases of synaptogenesis or pruning, potentially explaining the phenotypic differences from neocortex. Similarly, although the KO mice showed a more immature morphological phenotype overall than WT (higher proportion of thin headed spines, lower proportion of mushroom and stubby spines), both genotypes showed gradual development, rather than impairments during specific phases of maturation. Finally, spine length showed a complex developmental pattern that differs from other brain regions examined, suggesting dynamic regulation by FMRP and other brain region-specific proteins. These findings shed new light on FMRP's role in development and highlight the need for new techniques to further understand the mechanisms by which FMRP affects synaptic maturation.

Effects of tamoxifen on morphological and ultrastructural aspects of developing hippocampus of rat.

BACKGROUND: Tamoxifen treatment induced cell death in the hippocampus formation of the prenatal and postnatal rat. The present study delineates the effect of tamoxifen on developing hippocampus in prenatal, postnatal and full term neonate rats received certain doses of the partial antagonist tamoxifen. METHODS: After perfusion and fixation, the brains were removed and processed for light and electron microscopy. The morphology, ultrastructure and the density of the neurons in different ages (E22, P1, P7 and P21) and in different areas of developing hippocampus including cornu ammonis (CA1 and CA3), dentate gyrus and subiculum were studied. RESULTS: These findings showed that in tamoxifen-treated groups, the cell number of pyramidal neurons of CA1 and subiculum significantly decreased comparing to control groups in E22, P1 and P7 but not in third weeks. The mitochondria of the above mentioned groups also showed a dilated feature with less cristae than control group and most of them were greatly enlarged and swollen into spherical shapes rather than the normal ovoid or rod shape. CONCLUSION: The present study shows that prenatal exposure to tamoxifen alters neurogenesis in developing rat hippocampus. These results demonstrated the non-neuroprotective roles of tamoxifen.

Ethanol neurotoxicity and dentate gyrus development.

Maternal alcohol ingestion during pregnancy adversely affects the developing fetus, often leading to fetal alcohol syndrome (FAS). One of the most severe consequences of FAS is brain damage that is manifested as cognitive, learning, and behavioral deficits. The hippocampus plays a crucial role in such abilities; it is also known as one of the brain regions most vulnerable to ethanol-induced neurotoxicity. Our recent studies using morphometric techniques have further shown that ethanol neurotoxicity appears to affect the development of the dentate gyrus in a region-specific manner; it was found that early postnatal ethanol exposure causes a transitory deficit in the hilus volume of the dentate gyrus. It is strongly speculated that such structural modifications, even transitory ones, appear to result in developmental abnormalities in the brain circuitry and lead to the learning disabilities observed in FAS children. Based on reports on possible factors deciding ethanol neurotoxicity to the brain, we review developmental neurotoxicity to the dentate gyrus of the hippocampal formation.

Aberrant dentate gyrus cytoarchitecture and fiber lamination in Lis1 mutant mice.

Mutant mice with a heterozygous deletion of LIS1, show varying degrees of hippocampal abnormality and enhanced excitability. To examine how LIS1 affects cytoarchitecture and fiber lamination in dentate gyrus (DG), we performed a series of immunohistochemistry studies. By using different neuronal- and glial-specific antibodies, we found that the majority of hippocampal cell populations were affected by heterozygous mutation of LIS1; some reelin-positive Cajal-Retzius cells were left undisturbed. Granule cell dispersion was significant in hippocampal sections from Lis1-deficient mice. However, the fiber termination of commissural/associational fibers and mossy fibers appeared relatively compact despite obvious granule cell dispersion and CA1-CA3 pyramidal cell disorganization. vGlut1-immunoreactive axon terminals were found aberrantly traversing the dispersed granule cell layer. Consistent with previous observations, we also found that immature granule cells in Lis1 mutants, here stained with antibodies to doublecortin (DCX) and Mash-1, are aberrantly located and bear an abnormal cellular morphology. Our findings suggest that LIS1 mutants exhibit abnormal cell positioning and aberrant hippocampal neurogenesis, but maintain relatively normal fiber termination patterns. The functional consequences of hippocampal granule cell dispersion could offer critical insight to the epileptic and cognitive disorder associated with LIS1 haploinsufficiency.

No long-term effect two years after intrauterine exposure to dexamethasone on dentate gyrus volume, neuronal proliferation and differentiation in common marmoset monkeys.

Glucocorticoids are prenatally administered to promote the maturation of the lungs. They, however, can affect neuronal proliferation and differentiation. In newborn marmoset monkeys, intrauterine hyperexposure to dexamethasone (DEX) resulted in a significantly decreased proliferation rate in the hippocampal dentate gyrus without affecting neuronal differentiation. In this study, marmoset monkeys received 5 mg/kg body weight DEX either during early (days 42-48) or late (days 90-96) pregnancy. The volume of the dentate granule cell layer as well as the proliferation and neuronal differentiation in the dentate gyrus of their 2-year-old offspring were investigated. The density of proliferating cells (Ki-67), apoptotic cells (in situ tailing) and cells differentiating to neurons (double cortin, TUC-4 and calretinin) were determined immunohistochemically. Analysis of the dentate granule cell layer volume showed no significant differences between early or late DEX-exposed marmosets and untreated control animals. Similarly, proliferation and neuronal differentiation in DEX-treated animals was not significantly different in comparison with controls. In summary, the decreased proliferation rate observed in newborn marmosets after intrauterine exposure to DEX was no longer detectable in their 2-year-old siblings suggesting no long-lasting effect of prenatal hyperexposure to DEX on neuronal proliferation and differentiation in the dentate gyrus of marmoset monkeys.

Microarray analysis of the fetal hippocampus in the Emx2 mutant.

Deficiency in the transcription factor Emx2 causes a specific alteration of hippocampal development, which has been well analyzed morphologically. We are currently using microarrays and in situ hybridization to characterize gene expression in the Emx2 mutant hippocampus. In this report on our preliminary results for the fetal stage, we identify a group of genes for most of which a putative relation to Emx2 pathways has not been previously recognized. Some candidates are development genes or are involved in functional maturation, and show expression in the hippocampal plate and/or developing dentate gyrus. A second class of candidates label neuronal, glial or vascular structures in the outer marginal zone, and likely represent markers for cell populations specifically absent in the mutant. Our results point at pathways and processes altered in the mutant, particularly the Notch and chemokine pathways, the processes of cell migration, axonal guidance and angiogenesis, and the relation of pia and Cajal-Retzius cells with hippocampal morphogenesis.

Postnatal development of entorhinodentate projection of the Reeler mutant mouse.

We anterogradely labeled entorhinodentate axons by the injection of biotin dextran amine into the entorhinal cortex of adult wildtype and reeler mice to clarify whether the course and terminal endings of the reeler entorhinal projection are normal or not. We found that in the reeler mouse, biotin dextran amine-labeled entorhinodentate fibers arising from the entorhinal cortex curved around the hippocampal fissure instead of crossing it, whereas in the wildtype mouse, they crossed the fissure as a perforant pathway. Next, we examined carbocyanine dye (DiI) labeling of the immature entorhinodentate projection and the developmental changes of the hippocampal fissure during early postnatal days based on the laminin and glial fibrillary acidic protein (GFAP) immunohistochemistry. Injection of DiI into the entorhinal area of the wildtype and reeler mice at postnatal day 1 resulted in anterograde labeling of pioneer axons passing through the hippocampal fissure. However, follower axons could not penetrate through the hippocampal fissure in reeler mice, whereas in the normal controls, many DiI-labeled axons continued to pass through the fissure. GFAP immunohistochemistry demonstrated that GFAP-immunopositive astrocytes were abundant along the hippocampal fissure both in the wildtype and reeler mice at birth. In the wildtype mouse, GFAP-positive neurons nearby the fissure were decreasing in number during the early postnatal days, whereas in the reeler mouse, many GFAP-positive astrocytes were continuing to accumulate there. This barrier made of astrocytes in the reeler mouse may obstruct the ingrowth of the follower axons arising from the entorhinal cortex through the hippocampal fissure, resulting in the abnormal course of the entorhinodentate axons in this mutant.

Anatomic and electrophysiologic evidence for a proconvulsive circuit in the dentate gyrus of reeler mutant mice, an animal model of diffuse cortical malformation.

Although cortical malformations (CMs) are often associated with epilepsy, the underlying mechanisms are unknown. The reeler mouse is a model of CM with enhanced susceptibility to epileptiform activity, including the in vitro dentate gyrus, a region normally resistant to seizures. In this study, field potential recordings in hippocampal slices and the Timm stain were used to examine mossy fiber distribution in the dentate gyrus. In artificial cerebrospinal fluid containing bicuculline, 100% of reeler slices and 0% of control slices had spontaneous and antidromic evoked prolonged negative field potential shifts that were blocked by glutamate receptor antagonists. Sections from reeler mice, but not controls, exhibited a dark band of Timm's stain at the molecular layer/granule cell layer border. These data reveal that mossy fiber distribution is altered in reeler mice and coincides with the presence of an abnormal proconvulsive glutamatergic circuit.

Dentate development in organotypic hippocampal slice cultures from p35 knockout mice.

Abnormal brain development, induced by genetic influences or resulting from a perinatal trauma, has been recognized as a cause of seizure disorders. To understand how and when these structural abnormalities form, and how they are involved in epileptogenesis, it is important to generate and investigate animal models. We have studied one such model, a mouse in which deletion of the p35 gene (p35-/-) gives rise to both structural disorganization and seizure-like function. We now report that aberrant dentate development can be recognized in the organotypic hippocampal slice culture preparation generated from p35-/- mouse pups. In these p35-/- cultures, an abnormally high proportion of dentate granule cells migrates into the hilus and molecular layer, and develops aberrant dendritic and axonal morphology. In addition, astrocyte formation in the dentate gyrus is disturbed, as is the distribution of GABAergic interneurons. Although the p35-/- brain shows widespread abnormalities, the disorganization of the hippocampal dentate region is particularly intriguing since a similar pathology is often found in hippocampi of temporal lobe epilepsy patients. The abnormal granule cell features occur early in development, and are independent of seizure activity. Further, these aberrant patterns and histopathological features of p35-/- culture preparations closely resemble those observed in p35 knockout mice in vivo. This culture preparation thus provides an experimentally accessible window for studying abnormal developmental factors that can result in seizure propensity.

Citron kinase is required for postnatal neurogenesis in the hippocampus.

The dentate gyrus is a site of continual neurogenesis in the postnatal mammalian brain. Here we investigated postnatal neurogenesis in the citron kinase (citron-K) null-mutant rat (flathead). The flathead rat has substantial deficits in embryonic neurogenesis that are due to failed cytokinesis and cell death. We report here the loss of citron-K function has an even severer effect on postnatal neurogenesis in the dentate gyrus. Analysis of phosphorylated histone H3 expression in postnatal neurogenic regions of the flathead mutant revealed a complete lack of mitotic cells in the dentate gyrus and a large reduction in the number of dividing cells in the flathead subventricular zone. Examination of 5-bromodeoxyuridine incorporation in the flathead rat revealed that the flathead rat had a 99% reduction in the number of newly generated cells in the dentate gyrus at postnatal day 10. In addition, doublecortin-positive cells were essentially absent from the postnatal flathead dentate gyrus which also lacked the vimentin- and nestin-positive radial glia scaffold that defines the neurogenic niche in the postnatal subgranular zone. Together these results indicate that postnatal neurogenesis in the dentate gyrus is eliminated by loss of citron-K function, and suggests that a citron-K-dependent progenitor lineage forms the postnatal neuronal progenitor population in the dentate gyrus.

Granule cell dispersion and aberrant neurogenesis in the adult hippocampus of an LIS1 mutant mouse.

Human type 1 lissencephaly is a severe brain malformation associated with cognitive dysfunction and intractable epilepsy. Mutant mice with a heterozygous deletion of LIS1 show varying degrees of hippocampal abnormality and enhanced excitability. Whether a reduction of LIS1 function affects adult hippocampal neurogenesis, and if so, whether aberrant neurogenesis contributes to the generation of a disorganized hippocampus remain unknown. Previous reports indicate the presence of multiple pyramidal cell layers and granule cell dispersion in LIS1 mutant mice. Here we observed disruption of the subgranular zone and glial fibrillary acidic protein-immunoreactive radial astrocytes in the dentate gyrus of adult LIS1 mice. Using pulse-chase bromodeoxyuridine (BrdU) labeling combined with neuronal and glial antibody staining we provide evidence for ectopic adult neurogenesis in LIS1 mice. A gradually decreased survival rate for these newborn granule cells was also demonstrated in LIS1 mice 7 days after BrdU injection. This reduced survival rate was associated with impaired neuronal differentiation 28 days after BrdU administration. Thus, LIS1 haploinsufficiency can lead to abnormal cell proliferation, migration and differentiation in the adult dentate gyrus.

Effect of prenatal exposure to an anti-inflammatory drug on neuron number in cornu ammonis and dentate gyrus of the rat hippocampus: a stereological study.

Prenatal exposed to an anti-inflammatory drug is a major problem for the developing central nervous system. It is not well known the effect of prenatal exposed to a non-steroidal anti-inflammatory drug on the hippocampus. Total neuron number in one side of the cornu ammonis (CA) and gyrus dentatus (GD) of the hippocampal formation in control and drug-treated (diclofenac sodium, DS) groups of male rats was estimated using the optical fractionator technique. Each main group has also two subgroups that are 4 weeks old (4W-old) and 20 weeks old (20W-old). In CA, no significant difference between 4W-old DS-treated and their control was found, but a significant difference was observed between 20W-old DS-treated and their controls. A decreasing of neuron number was 12% for 20W-old DS-treated group. In GD, a decreasing of the granule cell number in 4W-old of DS-treated group was seen but an increasing of granule cell number was found in the 20W-old drug-treated rats in comparison to its control group, 7% and 9%, respectively. Although an increasing of neuron number in CA at the control group was seen with age, from 4th week to 20th week (10%), age-dependent substantial granule cell decline (17%) was observed in GD. No age effect on the total cell numbers of CA and GD of the drug-treated groups was seen in comparison to 4W-old week and 20W-old. A pronounced neuron loss observed in the drug-treated group may be attributed to the neurotoxicity of diclofenac sodium (DS) on the developing hippocampal formation. Age-dependent neuron increase in the CA of 20W-old and neuron decline in GD of 20W-old control groups may be a result of a dual effect of saline injection during the fetal life, since these animals were exposed to a stress of 15-day-period of saline injection, prenatal stress. The reason of no age effect on CA and GD cell number in the drug-treated groups may be attributed to the depletion of the progenitor cells due to neurotoxicity of DS in the fetal life of these animals.

Bilateral agenesis of the hippocampal dentate gyrus in a neurologically normal adult.

BACKGROUND: Ontogenic development of granule cells in the hippocampal dentate gyrus is influenced by genes including WNT3, EMX2, NEUROD, and LEF1. Dentate granule cells continue to be generated from stem cell precursors postnatally and during adult life, and are implicated in normal and abnormal neurological function. Developmental privation of dentate granule cells is rare and essentially always occurs in the context of other neurodevelopmental abnormalities. We have found no previous reports of severe, selective agenesis of dentate granule cells in humans. METHODS: A gross and microscopic examination of the brain included appropriate histochemical and immunohistochemical preparations and examination of the hippocampal formation at multiple levels bilaterally. RESULTS: This neurologically normal 82-year-old man was found to have bilateral agenesis of the hippocampal dentate gyrus, no identifiable dentate granule cells, and moderate disorganization of the pyramidal cell layer of Ammon's horn. We found no neurodevelopmental abnormalities outside the hippocampus. CONCLUSIONS: The hippocampal architectural alterations in this patient are similar to those associated with a murine Lef1 mutation, but our human case does not have the other congenital deficits reported in the Lef1-null mouse. Bilateral agenesis of the hippocampal dentate gyrus, and apparent failure of regeneration of dentate granule cells from stem cells in adult life, may occur without overt clinical neurological deficits.

Rescue of the reeler phenotype in the dentate gyrus by wild-type coculture is mediated by lipoprotein receptors for Reelin and Disabled 1.

Reelin is a positional signal for the lamination of the dentate gyrus. In the reeler mutant lacking Reelin, granule cells are scattered all over the dentate gyrus. We have recently shown that the reeler phenotype of the dentate gyrus can be rescued in vitro by coculturing reeler hippocampal slices with slices from wild-type hippocampus. Here we studied whether Reelin from other brain regions can similarly induce this rescue effect and whether it is mediated via the Reelin receptors apolipoprotein E receptor 2 (ApoER2) and very-low-density lipoprotein receptor (VLDLR). We found that coculturing reeler hippocampal slices with slices from wild-type olfactory bulb, cerebellum, and neocortex rescued the reeler phenotype as seen before with hippocampal slices, provided that the Reelin-synthesizing cells of these regions were placed near the marginal zone of the reeler hippocampal slice. However, coculturing wild-type hippocampal slices with hippocampal slices from mutants deficient in ApoER2 and VLDLR did not rescue the reeler-like phenotype in these cultures. Similarly, no rescue of the reeler-like phenotype was observed in slices from mutants lacking Disabled 1 (Dab1), an adapter protein downstream of Reelin receptors. Conversely, reeler hippocampal slices were rescued by coculturing them with slices from Dab1(-/-) mutants or ApoER2(-/-)/VLDLR(-/-) mice. These findings show that Reelin from other brain regions can substitute for the loss of hippocampal Reelin and that rescue of the reeler phenotype observed in our coculture studies is mediated via lipoprotein receptors for Reelin and Dab1.