Role of Cajal-Retzius and subplate neurons in cerebral cortical development

A single-cell transcriptomic atlas of developing inhibitory neurons reveals expanding and contracting modes of diversification

The cerebral cortex relies on vastly different types of inhibitory neurons to compute. How this diversity emerges during development remains an open question. The rarity of individual inhibitory neuron types often leads to their underrepresentation in single-cell RNA sequencing (scRNAseq) datasets, limiting insights into their developmental trajectories. To address this problem, we developed a computational pipeline to enrich and integrate rare cell types across multiple datasets. Applying this approach to somatostatin-expressing (SST+) inhibitory neurons-the most diverse inhibitory cell class in the cortex-we constructed the Dev-SST-Atlas, a comprehensive resource containing mouse transcriptomic data of over 51,000 SST+ neurons. We identify three principal groups-Martinotti cells (MCs), non-Martinotti cells (nMCs), and long-range projecting neurons (LRPs)-each following distinct diversification trajectories. MCs commit early, with distinct embryonic and neonatal clusters that map directly to adult counterparts. In contrast, nMCs diversify gradually, with each developmental cluster giving rise to multiple adult cell types. LRPs follow a unique 'contracting' mode. Initially, two clusters are present until postnatal day 5 (P5), but by P7, one type is eliminated through programmed cell death, leaving a single surviving population. This transient LRP type is also found in the fetal human cortex, revealing an evolutionarily conserved feature of cortical development. Together, these findings highlight three distinct modes of SST+ neuron diversification-invariant, expanding, and contracting-offering a new framework to understand how the large repertoire of inhibitory neurons emerges during development.

Contribution of the serotonergic system to developmental brain abnormalities in autism spectrum disorder

This review highlights a key role of the serotonergic system in brain development and in distortions of normal brain development in early stages of fetal life resulting in cascades of abnormalities, including defects of neurogenesis, neuronal migration, neuronal growth, differentiation, and arborization, as well as defective neuronal circuit formation in the cortex, subcortical structures, brainstem, and cerebellum of autistic subjects. In autism, defects in regulation of neuronal growth are the most frequent and ubiquitous developmental changes associated with impaired neuron differentiation, smaller size, distorted shape, loss of spatial orientation, and distortion of cortex organization. Common developmental defects of the brain in autism include multiregional focal dysplastic changes contributing to local neuronal circuit distortion, epileptogenic activity, and epilepsy. There is a discrepancy between more than 500 reports demonstrating the contribution of the serotonergic system to autism's behavioral anomalies, highlighted by lack of studies of autistic subjects' brainstem raphe nuclei, the center of brain serotonergic innervation, and of the contribution of the serotonergic system to the diagnostic features of autism spectrum disorder (ASD). Discovery of severe fetal brainstem auditory system neuronal deficits and other anomalies leading to a spectrum of hearing deficits contributing to a cascade of behavioral alterations, including deficits of social and verbal communication in individuals with autism, is another argument to intensify postmortem studies of the type and topography of, and the severity of developmental defects in raphe nuclei and their contribution to abnormal brain development and to the broad spectrum of functional deficits and comorbid conditions in ASD.

Adult Upper Cortical Layer Specific Transcription Factor CUX2 Is Expressed in Transient Subplate and Marginal Zone Neurons of the Developing Human Brain

Cut-Like Homeobox 2 (Cux2) is a transcription factor involved in dendrite and spine development, and synapse formation of projection neurons placed in mouse upper neocortical layers. Therefore, Cux2 is often used as an upper layer marker in the mouse brain. However, expression of its orthologue CUX2 remains unexplored in the human fetal neocortex. Here, we show that CUX2 protein is expressed in transient compartments of developing neocortical anlage during the main fetal phases of neocortical laminar development in human brain. During the early fetal phase when neurons of the upper cortical layers are still radially migrating to reach their final place in the cortical anlage, CUX2 was expressed in the marginal zone (MZ), deep cortical plate, and pre-subplate. During midgestation, CUX2 was still expressed in the migrating upper cortical neurons as well as in the subplate (SP) and MZ neurons. At the term age, CUX2 was expressed in the gyral white matter along with its expected expression in the upper layer neurons. In sum, CUX2 was expressed in migratory neurons of prospective superficial layers and in the diverse subpopulation of transient postmigratory SP and MZ neurons. Therefore, our findings indicate that CUX2 is a novel marker of distinct transient, but critical histogenetic events during corticogenesis. Given the Cux2 functions reported in animal models, our data further suggest that the expression of CUX2 in postmigratory SP and MZ neurons is associated with their unique dendritic and synaptogenesis characteristics.

DOT1L promotes progenitor proliferation and primes neuronal layer identity in the developing cerebral cortex

Cortical development is controlled by transcriptional programs, which are orchestrated by transcription factors. Yet, stable inheritance of spatio-temporal activity of factors influencing cell fate and localization in different layers is only partly understood. Here we find that deletion of Dot1l in the murine telencephalon leads to cortical layering defects, indicating DOT1L activity and chromatin methylation at H3K79 impact on the cell cycle, and influence transcriptional programs conferring upper layer identity in early progenitors. Specifically, DOT1L prevents premature differentiation by increasing expression of genes that regulate asymmetric cell division (Vangl2, Cenpj). Loss of DOT1L results in reduced numbers of progenitors expressing genes including SoxB1 gene family members. Loss of DOT1L also leads to altered cortical distribution of deep layer neurons that express either TBR1, CTIP2 or SOX5, and less activation of transcriptional programs that are characteristic for upper layer neurons (Satb2, Pou3f3, Cux2, SoxC family members). Data from three different mouse models suggest that DOT1L balances transcriptional programs necessary for proper neuronal composition and distribution in the six cortical layers. Furthermore, because loss of DOT1L in the pre-neurogenic phase of development impairs specifically generation of SATB2-expressing upper layer neurons, our data suggest that DOT1L primes upper layer identity in cortical progenitors.

Experience-Dependent Regulation of Cajal-Retzius Cell Networks in the Developing and Adult Mouse Hippocampus

In contrast to their near-disappearance in the adult neocortex, Cajal-Retzius cells have been suggested to persist longer in the hippocampus. A distinctive feature of the mature hippocampus, not maintained by other cortical areas, is its ability to sustain adult neurogenesis. Here, we have investigated whether environmental manipulations affecting hippocampal postnatal neurogenesis have a parallel impact on Cajal-Retzius cells. We used multiple mouse reporter lines to unequivocally identify Cajal-Retzius cells and quantify their densities during postnatal development. We found that exposure to an enriched environment increased the persistence of Cajal-Retzius cells in the hippocampus, but not in adjacent cortical regions. We did not observe a similar effect for parvalbumin-expressing interneurons, which suggested the occurrence of a cell type-specific process. In addition, we did not detect obvious changes either in Cajal-Retzius cell electrophysiological or morphological features, when compared with what previously reported in animals not exposed to enriched conditions. However, optogenetically triggered synaptic output of Cajal-Retzius cells onto local interneurons was enhanced, consistent with our observation of higher Cajal-Retzius cell densities. In conclusion, our data reveal a novel form of hippocampal, cell type-specific, experience-dependent network plasticity. We propose that this phenomenon may be involved in the regulation of enrichment-dependent enhanced hippocampal postnatal neurogenesis.

New insights into a spectrum of developmental malformations related to mTOR dysregulations: challenges and perspectives

In recent years the role of the mammalian target of rapamycin (mTOR) pathway has emerged as crucial for normal cortical development. Therefore, it is not surprising that aberrant activation of mTOR is associated with developmental malformations and epileptogenesis. A broad spectrum of malformations of cortical development, such as focal cortical dysplasia (FCD) and tuberous sclerosis complex (TSC), have been linked to either germline or somatic mutations in mTOR pathway-related genes, commonly summarised under the umbrella term 'mTORopathies'. However, there are still a number of unanswered questions regarding the involvement of mTOR in the pathophysiology of these abnormalities. Therefore, a monogenetic disease, such as TSC, can be more easily applied as a model to study the mechanisms of epileptogenesis and identify potential new targets of therapy. Developmental neuropathology and genetics demonstrate that FCD IIb and hemimegalencephaly are the same diseases. Constitutive activation of mTOR signalling represents a shared pathogenic mechanism in a group of developmental malformations that have histopathological and clinical features in common, such as epilepsy, autism and other comorbidities. We seek to understand the effect of mTOR dysregulation in a developing cortex with the propensity to generate seizures as well as the aftermath of the surrounding environment, including the white matter.

Evidence for Brainstem Contributions to Autism Spectrum Disorders

Autism spectrum disorder (ASD) is a neurodevelopmental condition that affects one in 59 children in the United States. Although there is a mounting body of knowledge of cortical and cerebellar contributions to ASD, our knowledge about the early developing brainstem in ASD is only beginning to accumulate. Understanding how brainstem neurotransmission is implicated in ASD is important because many of this condition’s sensory and motor symptoms are consistent with brainstem pathology. Therefore, the purpose of this review was to integrate epidemiological, behavioral, histological, neuroimaging, and animal evidence of brainstem contributions to ASD. Because ASD is a neurodevelopmental condition, we examined the available data through a lens of hierarchical brain development. The review of the literature suggests that developmental alterations of the brainstem could have potential cascading effects on cortical and cerebellar formation, ultimately leading to ASD symptoms. This view is supported by human epidemiology findings and data from animal models of ASD, showing that perturbed development of the brainstem substructures, particularly during the peak formation of the brainstem’s monoaminergic centers, may relate to ASD or ASD-like behaviors. Furthermore, we review evidence from human histology, psychophysiology, and neuroimaging suggesting that brainstem development and maturation may be atypical in ASD and may be related to key ASD symptoms, such as atypical sensorimotor features and social responsiveness. From this review there emerges the need of future research to validate early detection of the brainstem-based somatosensory and psychophysiological behaviors that emerge in infancy, and to examine the brainstem across the life span, while accounting for age. In all, there is preliminary evidence for brainstem involvement in ASD, but a better understanding of the brainstem’s role would likely pave the way for earlier diagnosis and treatment of ASD.

Cajal-Retzius cells and GABAergic interneurons of the developing hippocampus: Close electrophysiological encounters of the third kind

In contrast to the large number of studies investigating the electrophysiological properties and synaptic connectivity of hippocampal pyramidal neurons, granule cells, and GABAergic interneurons, much less is known about Cajal-Retzius cells. In this review article, we discuss the possible reasons underlying this difference, and review experimental work performed on this cell type in the hippocampus, comparing it with results obtained in the neocortex. Our main emphasis is on data obtained with in vitro electrophysiology. In particular, we address the bidirectional connectivity between Cajal-Retzius cells and GABAergic interneurons, examine their synaptic properties and propose specific functions of Cajal-Retzius cell/GABAergic interneuron microcircuits. Lastly, we discuss the potential involvement of these microcircuits in critical physiological hippocampal functions such as postnatal neurogenesis or pathological scenarios such as temporal lobe epilepsy.

Cornu Ammonis Regions-Antecedents of Cortical Layers?

Studying neocortex and hippocampus in parallel, we are struck by the similarities. All three to four layered allocortices and the six layered mammalian neocortex arise in the pallium. All receive and integrate multiple cortical and subcortical inputs, provide multiple outputs and include an array of neuronal classes. During development, each cell positions itself to sample appropriate local and distant inputs and to innervate appropriate targets. Simpler cortices had already solved the need to transform multiple coincident inputs into serviceable outputs before neocortex appeared in mammals. Why then do phylogenetically more recent cortices need multiple pyramidal cell layers? A simple answer is that more neurones can compute more complex functions. The dentate gyrus and hippocampal CA regions-which might be seen as hippocampal antecedents of neocortical layers-lie side by side, albeit around a tight bend. Were the millions of cells of rat neocortex arranged in like fashion, the surface area of the CA pyramidal cell layers would be some 40 times larger. Even if evolution had managed to fold this immense sheet into the space available, the distances between neurones that needed to be synaptically connected would be huge and to maintain the speed of information transfer, massive, myelinated fiber tracts would be needed. How much more practical to stack the "cells that fire and wire together" into narrow columns, while retaining the mechanisms underlying the extraordinary precision with which circuits form. This demonstrably efficient arrangement presents us with challenges, however, not the least being to categorize the baffling array of neuronal subtypes in each of five "pyramidal layers." If we imagine the puzzle posed by this bewildering jumble of apical dendrites, basal dendrites and axons, from many different pyramidal and interneuronal classes, that is encountered by a late-arriving interneurone insinuating itself into a functional circuit, we can perhaps begin to understand why definitive classification, covering every aspect of each neurone's structure and function, is such a challenge. Here, we summarize and compare the development of these two cortices, the properties of their neurones, the circuits they form and the ordered, unidirectional flow of information from one hippocampal region, or one neocortical layer, to another.

Reelin signaling in development, maintenance, and plasticity of neural networks

The developing brain is formed through an orchestrated pattern of neuronal migration, leading to the formation of heterogeneous functional regions in the adult. Several proteins and pathways have been identified as mediators of developmental neuronal migration and cell positioning. However, these pathways do not cease to be functionally relevant after the embryonic and early postnatal period; instead, they switch from guiding cells, to guiding synapses. The outcome of synaptic guidance determines the strength and plasticity of neuronal networks by creating a scalable functional architecture that is sculpted by cues from the internal and external environment. Reelin is a multifunctional signal that coordinates cortical and subcortical morphogenesis during development and regulates structural plasticity in adulthood and aging. Gain or loss of function in reelin or its receptors has the potential to influence synaptic strength and patterns of connectivity, with consequences for memory and cognition. The current review highlights similarities in the signaling cascades that modulate neuronal positioning during development, and synaptic plasticity in the adult, with a focus on reelin, a glycoprotein that is increasingly recognized for its dual role in the formation and maintenance of neural circuits.

Growth defects in the dorsal pallium after genetically targeted ablation of principal preplate neurons and neuroblasts: a morphometric analysis

The present study delineates the large-scale, organic responses of growth in the dorsal pallium to targeted genetic ablations of the principal PP (preplate) neurons of the neocortex. Ganciclovir treatment during prenatal development [from E11 (embryonic age 11) to E13] of mice selectively killed cells with shared S-phase vulnerability and targeted expression of a GPT [golli promoter transgene; GPT linked to HSV-TK (herpes simplex virus-thymidine kinase), τ-eGFP and lacZ reporters] localized in PP neurons and their intermediate progenitor neuroblasts. The volume, area and thickness of the pallium were measured in an E12-P4 (postnatal age 4) longitudinal study with comparisons between ablated (HSV-TK(+/0)) and control (HSV-TK(0/0)) littermates. The extent of ablations was also systematically varied, and the effect on physical growth was assessed in an E18 cross-sectional study. The morphological evidence obtained in the present study supports the conclusion that genetically targeted ablations delay the settlement of the principal PP neurons of the dorsal pallium. This leads to progressive and substantial reductions of growth, despite compensatory responses that rapidly replace the ablated cells. These growth defects originate from inductive cellular interactions in the proliferative matrix of the ventricular zone of the pallium, but are amplified by subsequent morphogenic and trophic cellular interactions. The defects persist during the course of prenatal and postnatal development to demonstrate a constrained dose-response relationship with the extent of specific killing of GPT neurons. The defects propagate simultaneously in both the horizontal and vertical cytoarchitectural dimensions of the developing pallium, an outcome that produces a localized shortfall of volume in the telencephalic vesicles.

Reelin expression during embryonic development of the pig brain

Background

Reelin is an extracellular glycoprotein of crucial importance in the developmental organisation of neurons in the mammalian cerebral cortex and other laminated brain regions. The pig possesses a gyrencephalic brain that bears resemblance to the human brain. In order to establish an animal model for neuronal migration disorders in the pig, we have studied the expression pattern and structure of Reelin during pig brain development.

Results

We determined the sequence of pig Reelin mRNA and protein and identified a high degree of homology to human Reelin. A peak in Reelin mRNA and protein expression is present during the period of major neurogenesis and neuronal migration. This resembles observations for human brain development. Immunohistochemical analysis showed the highest expression of Reelin in the Cajal-Reztius cells of the marginal zone, in resemblance with observations for the developing brain in humans and other mammalian species.

Conclusions

We conclude that the pig might serve as an alternative animal model to study Reelin functions and that manipulation of the pig Reelin could allow the establishment of an animal model for human neuronal migration disorders.

Fate of cajal-retzius neurons in the postnatal mouse neocortex

Cajal–Retzius (CR) neurons play a critical role in cortical neuronal migration, but their exact fate after the completion of neocortical lamination remains a mystery. Histological evidence has been unable to unequivocally determine whether these cells die or undergo a phenotypic transformation to become resident interneurons of Layer 1 in the adult neocortex. To determine their ultimate fate, we performed chronic in vivo two-photon imaging of identified CR neurons during postnatal development in mice that express the green fluorescent protein (GFP) under the control of the early B-cell factor 2 (Ebf2) promoter. We find that, after birth, virtually all CR neurons in mouse neocortex express Ebf2. Although postnatal CR neurons undergo dramatic morphological transformations, they do not migrate to deeper layers. Instead, their gradual disappearance from the cortex is due to apoptotic death during the second postnatal week. A small fraction of CR neurons present at birth survive into adulthood. We conclude that, in addition to orchestrating cortical layering, a subset of CR neurons must play other roles beyond the third postnatal week.

Fetal pain and intrauterine analgesia/anesthesia: neuroanatomy, ontogenesis and physiology of pain perception

A magzat intrauterin élete során és perinatalisan számos, potenciálisan fájdalmas beavatkozáson eshet át. Az elmúlt években a magzati fájdalom létével és csillapításának szükségességével foglalkozó tudományos állásfoglalások kereteit túllépve széles körű társadalmi, politikai, vallási, erkölcsi, filozófiai vita bontakozott ki – leginkább a művi abortusz során a magzat által megélt szenvedést helyezve a középpontba. Munkánkban összefoglaljuk a magzati fájdalommal kapcsolatos orvosi szakirodalomban fellelhető ismereteket. Definiáljuk a fájdalmat, majd a szubjektív fájdalomérzet kialakulásában szerepet játszó neuroanatómiai struktúrák rövid ismertetése után utóbbiak ontogenezis során bekövetkező fejlődését ismertetjük. Ezt követően a noxa kiváltotta stresszválasz objektív (neuroendokrin-vegetatív, cardiovascularis, reflexes és viselkedési) komponensének intrauterin megfigyelhető bizonyítékait vesszük sorra.

Targeted ablation and reorganization of the principal preplate neurons and their neuroblasts identified by golli promoter transgene expression in the neocortex of mice

The present study delineates the cellular responses of dorsal pallium to targeted genetic ablation of the principal preplate neurons of the neocortex. Ganciclovir treatment during prenatal development (E11–E13; where E is embryonic day) of mice selectively killed cells with shared S-phase vulnerability and targeted expression of a GPT [golli promoter transgene, linked to HSV-TK (herpes simplex virus-thymidine kinase), τ-eGFP (τ-enhanced green fluorescent protein) and lacZ (lacZ galactosidase) reporters] localized in preplate neurons. Morphogenetic fates of attacked neurons and neuroblasts, and their successors, were assessed by multiple labelling in time-series comparisons between ablated (HSV-TK^+/0) and control (HSV-TK^0/0) littermates. During ablation generation, neocortical growth was suppressed, and compensatory reorganization of non-GPT ventricular zone progenitors of dorsal pallium produced replacements for killed GPT neuroblasts. Replacement and surviving GPT neuroblasts then produced replacements for killed GPT neurons. Near-normal restoration of their complement delayed the settlement of GPT neurons into the reconstituted preplate, which curtailed the outgrowth of pioneer corticofugal axons. Based on this evidence, we conclude that specific cell killing in ablated mice can eliminate a major fraction of GPT neurons, with insignificant bystander killing. Also, replacement GPT neurons in ablated mice originate exclusively by proliferation from intermediate progenitor GPT neuroblasts, whose complement is maintained by non-GPT progenitors for inductive regulation of the total complement of GPT neurons. Finally, GPT neurons in both normal and ablated mice meet all morphogenetic criteria, including the ‘outside-in’ vertical gradient of settlement, presently used to identify principal preplate neurons. In ablated mice, delayed organization of these neurons desynchronizes and isolates developing neocortex from the rest of the brain, and permanently impairs its connectivity.

Glycine receptors mediate excitation of subplate neurons in neonatal rat cerebral cortex

The development of the cerebral cortex depends on genetic factors and early electrical activity patterns that form immature neuronal networks. Subplate neurons (SPn) are involved in the construction of thalamocortical innervation, generation of oscillatory network activity, and in the proper formation of the cortical columnar architecture. Because glycine receptors play an important role during early corticogenesis, we analyzed the functional consequences of glycine receptor activation in visually identified SPn in neocortical slices from postnatal day 0 (P0) to P4 rats using whole cell and perforated patch-clamp recordings. In all SPn the glycinergic agonists glycine, beta-alanine, and taurine induced dose-dependent inward currents with the affinity for glycine being higher than that for beta-alanine and taurine. Glycine-induced responses were blocked by the glycinergic antagonist strychnine, but were unaffected by either the GABAergic antagonist gabazine, the N-methyl-d-aspartate-receptor antagonist d-2-amino-5-phosphonopentanoic acid, or picrotoxin and cyanotriphenylborate, antagonists of alpha-homomeric and alpha1-subunit-containing glycine receptors, respectively. Under perforated-patch conditions, glycine induced membrane depolarizations that were sufficient to trigger action potentials (APs) in most cells. Furthermore, glycine and taurine decreased the injection currents as well as the synaptic stimulation strength required to elicit APs, indicating that glycine receptors have a consistent excitatory effect on SPn. Inhibition of taurine transport and application of hypoosmolar solutions induced strychnine-sensitive inward currents, suggesting that taurine can act as a possible endogenous agonist on SPn. In summary, these results demonstrate that SPn express glycine receptors that mediate robust excitatory membrane responses during early postnatal development.

Genotype/phenotype correlations in two patients with 12q subtelomere deletions

Subtelomeric imbalances have been implicated in developmental delay and mental retardation (MR) and described for most chromosomes. This study reports the first detailed description of two individuals with de novo 12q subtelomere deletions and high-resolution mapping of their deletion size with oligonucleotide array CGH for genotype/phenotype comparisons. Patient 1 is an 8-year-old male with borderline mild MR, food-seeking behavior, obesity, no significant dysmorphic facial features, abnormal hair whorl pattern, brachydactyly and mild clinodactyly. Patient 2 is a 12-year-old male with mild MR, food-seeking behavior, obesity, short stature, mild dysmorphic facial features, multicystic kidney and unilateral cryptorchidism. Both patients share a deleted region of approximately 1.6 Mb, including 14 known genes, which perhaps contributed to their similar phenotypes. However, Patient 2 has more severe MR and organ system involvement, possibly due to the larger deletion size ( approximately 4.5 Mb) including an additional eight genes, although it is difficult to make phenotype/genotype correlations based on only two patients. Due to the relatively mild presentation of both of our patients, we propose that a proportion of individuals with subtelomeric imbalances may go undetected and therefore, recommend subtelomeric studies be carried out for cases of unexplained mild MR or isolated learning disability (LD) with behavioral problems in the absence of major dysmorphic features or birth defects. In addition, 12q subtelomeric deletions should be considered in the differential diagnosis of patients presenting with food-seeking behavior and resultant obesity, as well as those referred to rule out Prader-Willi syndrome.

Neurodevelopmental changes of fetal pain

Pain in the developing fetus is controversial because of the difficulty in measuring and interpreting pain during gestation. It has received increased attention lately because of recently introduced legislation that would require consideration of fetal pain during intentional termination of pregnancy. During development, sensory fibers are abundant by 20 weeks; a functional spinal reflex is present by 19 weeks; connections to the thalamus are present by 20 weeks; and connections to subplate neurons are present by 17 weeks with intensive differentiation by 25 weeks. These cells are important developmentally, but decline as a result of natural apoptosis. Mature thalamocortical projections are not present until 29 to 30 weeks, which has led many to believe the fetus does not experience emotional "pain" until then. Pain requires both nociception and emotional reaction or interpretation. Nociception causes physiologic stress, which in turn causes increases in catecholamines, cortisol, and other stress hormones. Physiological stress is different from the emotional pain felt by the more mature fetus or infant, and this stress is mitigated by pain medication such as opiates. The plasticity of the developing brain makes it vulnerable to the stressors that cause long-term developmental changes, ultimately leading to adverse neurological outcomes. Whereas evidence for conscious pain perception is indirect, evidence for the subconscious incorporation of pain into neurological development and plasticity is incontrovertible. Scientific data, not religious or political conviction, should guide the desperately needed research in this field. In the meantime, it seems prudent to avoid pain during gestation.

Proteomic analysis of hypoxia/ischemia-induced alteration of cortical development and dopamine neurotransmission in neonatal rat

Perinatal hypoxia/ischemia (HI) is a common cause of neurological deficits in children. Our goal was to elucidate the underlying mechanisms that contribute to the neurological sequelae of HI-induced brain injury. HI was induced by permanent ligation of the left carotid artery followed by 90 min of hypoxia (7.8% O2) in female P7 rats. A two-dimensional differential proteome analysis was used to assess changes in protein expression in cortex 2 h after HI. In total, 17 proteins reflecting a 2-fold or higher perturbation of expression after HI as compared to sham-treated pups were identified by mass spectrometry. Of the altered proteins, 14-3-3epsilon and TUC-2, both playing an important role in the development of the central nervous system, decrease after HI, consistent with an early disturbance of cortical development. Also affected, DARPP-32 and alpha-synuclein, two proteins important for dopamine neurotransmission, increased more than 2-fold 2 h after HI injury. The differential expression of these proteins was validated by individual Western blot assays. The expression of several metabolic enzymes and translational factors was also perturbed early after HI brain injury. These findings provide initial insights into the mechanisms underlying neurodegenerative events after HI and may allow for the rational design of therapeutic strategies that enhance neuronal adaptation and compensation after HI.

Signaling pathways regulating cell motility: a role in ethanol teratogenicity?

This article summarizes the proceedings of a symposium presented at the 2005 annual meeting of the Research Society on Alcoholism in Santa Barbara, California. The organizer and chair was Tara A. Lindsley. The presentations were (1) Ethanol and Neuron Migration in the CNS, by Michael W. Miller; (2) Ethanol and L1-mediated Neurite Outgrowth, by Yoav Littner and Cynthia F. Bearer; and (3) Ethanol and Axon Guidance, by Tara A. Lindsley.

Molecular and cellular mechanisms of altered GAD1/GAD67 expression in schizophrenia and related disorders

The 67 and 65 kDa isoforms of glutamic acid decarboxylase, the key enzymes for GABA biosynthesis, are expressed at altered levels in postmortem brain of subjects diagnosed with schizophrenia and related disorders, including autism and bipolar illness. The predominant finding is a decrease in GAD67 mRNA levels, affecting multiple brain regions, including prefrontal and temporal cortex. Postmortem studies, in conjunction with animal models, identified several mechanisms that contribute to the dysregulation of GAD67 in cerebral cortex. These include disordered connectivity formation during development, abnormal expression of Reelin and neural cell adhesion molecule (NCAM) glycoproteins, defects in neurotrophin signaling and alterations in dopaminergic and glutamatergic neurotransmission. These mechanisms are likely to operate in conjunction with genetic risk factors for psychosis, including sequence polymorphisms residing in the promoter of GAD1 (2q31), the gene encoding GAD67. We propose an integrative model, with multiple molecular and cellular mechanisms contributing to transcriptional dysregulation of GAD67 and cortical dysfunction in psychosis.

A transgenic marker mouse line labels Cajal–Retzius cells from the cortical hem and thalamocortical axons

Cajal-Retzius (CR) cells are among the earliest born cortical neurons and are required for normal cortical development in rodents and humans; however, their embryonic origin has been controversial. Recent genetic lineage studies and direct visualization of migration of CR cells have demonstrated multiple germinative sources for CR cells on the edges of the developing pallium. We generated transgenic mice using 5' untranslated regions of the Frizzled10 gene in order to mark the cortical hem (the most caudomedial edge of the telencephalic neuroepithelium) and found that these mice faithfully reproduce the previously described expression pattern of Frizzled10 mRNA in the cortical hem, dorsal thalamus and dorsal neural tube. In the cortical hem, expression of LacZ mRNA was confined to the ventricular zone and perdurance of LacZ protein served as lineage marker for CR cells derived from the hem during embryonic life. When these marker mice were crossed with FoxG1 (BF1) mutants, they confirmed the previous finding that in these mice the cortical hem is expanded leading to increased production of CR cells from the medial wall of the cortex.

Neurodevelopment, neuroplasticity, and new genes for schizophrenia

Schizophrenia is a complex, debilitating neuropsychiatric disorder. Epidemiological, clinical, neuropsychological, and neurophysiological studies have provided substantial evidence that abnormalities in brain development and ongoing neuroplasticity play important roles in the pathogenesis of the disorder. Complementing these clinical studies, a range of cytoarchitectural, morphometric, ultrastructural, immunochemical, and gene expression methods have been applied in investigations of postmortem brain tissues to characterize the cellular and molecular profile of putative developmental and plastic abnormalities in schizophrenia. While findings have been diverse and many are in need of replication, investigations focusing on higher cortical and limbic brain regions are increasingly demonstrating abnormalities in the structural and molecular integrity of the synaptic complex as well as glutamate-related receptors and signal transduction pathways that play critical roles in brain development, synaptogenesis, and synaptic plasticity. Most exciting have been recent associations of schizophrenia with specific genes, such as neuregulin-1, dysbindin-1, and AKT-1, which are vital to synaptic development, neurotransmission, and plasticity.

Epileptic-like convulsions associated with LIS-1 in the cytoskeletal control of neurotransmitter signaling in Caenorhabditis elegans

Cortical malformations are a collection of disorders affecting brain development. Mutations in the LIS1 gene lead to a disorganized and smooth cerebral cortex caused by failure in neuronal migration. Among the clinical consequences of lissencephaly are mental retardation and intractable epilepsy. It remains unclear whether the seizures result from aberrant neuronal placement, disruption of intrinsic properties of neurons, or both. The nematode Caenorhabditis elegans offers an opportunity to study such convulsions in a simple animal with a defined nervous system. Here we show that convulsions mimicking epilepsy can be induced by a mutation in a C. elegans lis-1 allele (pnm-1), in combination with a chemical antagonist of gamma-aminobutyric acid (GABA) neurotransmitter signaling. Identical convulsions were obtained using C. elegans mutants defective in GABA transmission, whereas none of these mutants or the antagonist alone caused convulsions, indicating a threshold was exceeded in response to this combination. Crosses between pnm-1 and fluorescent marker strains designed to exclusively illuminate either the processes of GABAergic neurons or synaptic vesicles surprisingly showed no deviations in neuronal architecture. Instead, presynaptic defects in GABAergic vesicle distribution were clearly evident and could be phenocopied by RNAi directed against cytoplasmic dynein, a known LIS1 interactor. Furthermore, mutations in UNC-104, a neuronal-specific kinesin, and SNB-1, a synaptic vesicle-associated protein termed synaptobrevin, exhibit similar convulsion phenotypes following chemical induction. Taken together, these studies establish C. elegans as a system to investigate subtle cytoskeletal mechanisms regulating intrinsic neuronal activity and suggest that it may be possible to dissociate the epileptic consequences of lissencephaly from the more phenotypically overt cortical defects associated with neuronal migration.

Steroid sulfatase (STS) expression in the human temporal lobe: enzyme activity, mRNA expression and immunohistochemistry study

Dehydroepiandrosterone (DHEA) and its sulfate (DHEAS) are suggested to be important neurosteroids. We investigated steroid sulfatase (STS) in human temporal lobe biopsies in the context of possible cerebral DHEA(S) de novo biosynthesis. Formation of DHEA(S) in mature human brain tissue has not yet been studied. 17 alpha-Hydroxylase/C17-20-lyase and hydroxysteroid sulfotransferase catalyze the formation of DHEA from pregnenolone and the subsequent sulfoconjugation, respectively. Neither their mRNA nor activity were detected, indicating that DHEA(S) are not produced within the human temporal lobe. Conversely, strong activity and mRNA expression of DHEAS desulfating STS was found, twice as high in cerebral neocortex than in subcortical white matter. Cerebral STS resembled the characteristics of the known placental enzyme. Immunohistochemistry revealed STS in adult cortical neurons as well as in fetal and adult Cajal-Retzius cells. Organic anion transporting proteins OATP-A, -B, -D, and -E showed high mRNA expression levels with distinct patterns in cerebral neocortex and subcortical white matter. Although it is not clear whether they are expressed at the blood-brain barrier and facilitate an influx rather than an efflux, they might well be involved in the transport of steroid sulfates from the blood. Therefore, we hypothesize that DHEAS and/or other sulfated 3beta-hydroxysteroids might enter the human temporal lobe from the circulation where they would be readily converted via neuronal STS activity.

Subplate neurons: bridging the gap to function in the cortex

The importance of subplate neurons in controlling multiple steps of cortex development was recognized more than a decade ago. However, knowledge of the precise function of subplate neurons in the establishment of cortical circuitry has been missing. A recent study has now started to dissect the functional and molecular mechanisms underlying the generation of transient neuronal circuitry in the visual cortex.