Patterning and specification of the cerebral cortex

Diverse axonal morphologies of individual callosal projection neurons reveal new insights into brain connectivity

In the mature brain, functionally distinct areas connect to specific targets, mediating network activity required for function. New insights are still occurring regarding how specific connectivity occurs in the developing brain. Decades of work have revealed important insights into the molecular and genetic mechanisms regulating cell type specification in the brain. This work classified long-range projection neurons of the cerebral cortex into three major classes based on their primary target (e.g. subcortical, intracortical, and interhemispheric projections). However, painstaking single-cell mapping reveals that long-range projection neurons of the corpus callosum connect to multiple and overlapping ipsilateral and contralateral targets with often highly branched axons. In addition, their scRNA transcriptomes are highly variable, making it difficult to identify meaningful subclasses. This work has prompted us to reexamine how cortical projection neurons that comprise the corpus callosum are currently classified and how this stunning array of variability might be achieved during development.

Subclass-specific expression patterns of MET receptor tyrosine kinase during development in medial prefrontal and visual cortices

Met encodes a receptor tyrosine kinase (MET) that is expressed during development and regulates cortical synapse maturation. Conditional deletion of Met in the nervous system during embryonic development leads to deficits in adult contextual fear learning, a medial prefrontal cortex (mPFC)-dependent cognitive task. MET also regulates the timing of critical period plasticity for ocular dominance in primary visual cortex (V1). However, the underlying circuitry responsible remains unknown. Therefore, this study determines the broad expression patterns of MET throughout postnatal development in mPFC and V1 projection neurons (PNs), providing insight into similarities and differences in the neuronal subtypes and temporal patterns of MET expression between cortical areas. Using a transgenic mouse line that expresses green fluorescent protein (GFP) in Met⁺ neurons, immunofluorescence and confocal microscopy were performed to visualize MET-GFP⁺ cell bodies and PN subclass-specific protein markers. Analyses reveal that the MET expression is highly enriched in infragranular layers of mPFC, but in supragranular layers of V1. Interestingly, temporal regulation of the percentage of MET⁺ neurons across development not only differs between cortical regions but also is distinct between lamina within a cortical region. Further, MET is expressed predominantly in the subcerebral PN subclass in mPFC, but the intratelencephalic PN subclass in V1. The data suggest that MET signaling influences the development of distinct circuits in mPFC and V1 that underlie subcerebral and intracortical functional deficits following Met deletion, respectively.

BAF (mSWI/SNF) complex regulates mediolateral cortical patterning in the developing forebrain

Early forebrain patterning entails the correct regional designation of the neuroepithelium, and appropriate specification, generation, and distribution of neural cells during brain development. Specific signaling and transcription factors are known to tightly regulate patterning of the dorsal telencephalon to afford proper structural/functional cortical arealization and morphogenesis. Nevertheless, whether and how changes of the chromatin structure link to the transcriptional program(s) that control cortical patterning remains elusive. Here, we report that the BAF chromatin remodeling complex regulates the spatiotemporal patterning of the mouse dorsal telencephalon. To determine whether and how the BAF complex regulates cortical patterning, we conditionally deleted the BAF complex scaffolding subunits BAF155 and BAF170 in the mouse dorsal telencephalic neuroepithelium. Morphological and cellular changes in the BAF mutant forebrain were examined using immunohistochemistry and in situ hybridization. RNA sequencing, Co-immunoprecipitation, and mass spectrometry were used to investigate the molecular basis of BAF complex involvement in forebrain patterning. We found that conditional ablation of BAF complex in the dorsal telencephalon neuroepithelium caused expansion of the cortical hem and medial cortex beyond their developmental boundaries. Consequently, the hippocampal primordium is not specified, the mediolateral cortical patterning is compromised, and the cortical identity is disturbed in the absence of BAF complex. The BAF complex was found to interact with the cortical hem suppressor LHX2. The BAF complex suppresses cortical hem fate to permit proper forebrain patterning. We provide evidence that BAF complex modulates mediolateral cortical patterning possibly by interacting with the transcription factor LHX2 to drive the LHX2-dependent transcriptional program essential for dorsal telencephalon patterning. Our data suggest a putative mechanistic synergy between BAF chromatin remodeling complex and LHX2 in regulating forebrain patterning and ontogeny.

The neuroecology of the water-to-land transition and the evolution of the vertebrate brain

The water-to-land transition in vertebrate evolution offers an unusual opportunity to consider computational affordances of a new ecology for the brain. All sensory modalities are changed, particularly a greatly enlarged visual sensorium owing to air versus water as a medium, and expanded by mobile eyes and neck. The multiplication of limbs, as evolved to exploit aspects of life on land, is a comparable computational challenge. As the total mass of living organisms on land is a hundredfold larger than the mass underwater, computational improvements promise great rewards. In water, the midbrain tectum coordinates approach/avoid decisions, contextualized by water flow and by the animal's body state and learning. On land, the relative motions of sensory surfaces and effectors must be resolved, adding on computational architectures from the dorsal pallium, such as the parietal cortex. For the large-brained and long-living denizens of land, making the right decision when the wrong one means death may be the basis of planning, which allows animals to learn from hypothetical experience before enactment. Integration of value-weighted, memorized panoramas in basal ganglia/frontal cortex circuitry, with allocentric cognitive maps of the hippocampus and its associated cortices becomes a cognitive habit-to-plan transition as substantial as the change in ecology. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Wiring of higher-order cortical areas: Spatiotemporal development of cortical hierarchy

A hierarchical development of cortical areas was suggested over a century ago, but the diversity and complexity of cortical hierarchy properties have so far prevented a formal demonstration. The aim of this review is to clarify the similarities and differences in the developmental processes underlying cortical development of primary and higher-order areas. We start by recapitulating the historical and recent advances underlying the biological principle of cortical hierarchy in adults. We then revisit the arguments for a hierarchical maturation of cortical areas, and further integrate the principles of cortical areas specification during embryonic and postnatal development. We highlight how the dramatic expansion in cortical size might have contributed to the increased number of association areas sustaining cognitive complexification in evolution. Finally, we summarize the recent observations of an alteration of cortical hierarchy in neuropsychiatric disorders and discuss their potential developmental origins.

Physiological significance of multipolar cells generated from neural stem cells and progenitors for the establishment of neocortical cytoarchitecture

Neurogenesis encompasses an entire set of events that leads to the generation of newborn neurons from neural stem cells and more committed progenitor cells, including cell division, the production of migratory precursors and their progeny, differentiation and integration into circuits. In particular, the precise control of neuronal migration and morphological changes is essential for the development of the neocortex. Postmitotic cells within the intermediate zone have been found to transiently assume a characteristic "multipolar" morphology, after which a multipolar-to-bipolar transition occurs before the cells enter the cortical plate; however, the importance of this multipolar phase in the establishment of mature cortical cytoarchitecture and the precise genetic control of this phase remains largely unknown. Thus, this review article focuses on the multipolar phase in the developing neocortex. It begins by summarizing the molecular mechanism that underlies multipolar migration for the regulation of each step in multipolar phase in intermediate zone. The physiological significance of this multipolar phase in the establishment of mature cortical lamination and neurodevelopmental disorders associated with migration defects is then described.

The legacy of Drosophila imaginal discs

The study of Drosophila imaginal discs has contributed to a number of discoveries in developmental and cellular biology. In addition to the elucidation of the role of tissue compartments and organ-specific master regulator genes during development, imaginal discs have also become well established as models for studying cellular interactions and complex genetic pathways. Here, we review key discoveries resulting from investigations of these epithelial precursor organs, ranging from cell fate determination and transdetermination to tissue patterning. Furthermore, the design of increasingly sophisticated genetic tools over the last decades has added value to the use of imaginal discs as model systems. As a result of tissue-specific genetic screens, several components of developmentally regulated signaling pathways were identified and epistasis revealed the levels at which they function. Discs have been widely used to assess cellular interactions in their natural tissue context, contributing to a better understanding of growth regulation, tissue regeneration, and cancer. With the continuous implementation of novel tools, imaginal discs retain significant potential as model systems to address emerging questions in biology and medicine.

Cross-modal plasticity in sensory deprived animal models: From the thalamocortical development point of view

Over recent decades, our understanding of the plasticity of the central nervous system has expanded enormously. Accordingly, it is now widely accepted that the brain can adapt to changes by reorganizing its circuitry, both in response to external stimuli and experience, as well as through intrinsic mechanisms. A clear example of this is the activation of a deprived sensory area and the expansion of spared sensory cortical regions in individuals who suffered peripheral sensory loss. Despite the efforts to understand these neuroplastic changes, the mechanisms underlying such adaptive remodeling remains poorly understood. Progress in understanding these events may be hindered by the highly varied data obtained from the distinct experimental paradigms analyzed, which include different animal models and neuronal systems, as well as studies into the onset of sensory loss. Here, we will establish the current state-of-the-art describing the principal observations made according to the time of sensory deprivation with respect to the development of the thalamocortical connectivity. We will review the experimental data obtained from animal models where sensory deprivation has been induced either before or after thalamocortical axons reach and invade their target cortical areas. The anatomical and functional effects of sensory loss on the primary sensory areas of the cortex will be presented. Indeed, we consider that the comparative approach of this review is a necessary step in order to help deciphering the processes that underlie sensory neuroplasticity, for which studies in animal models have been indispensable. Understanding these mechanisms will then help to develop restorative strategies and prostheses that will overcome the functional loss.

Deletion of Prdm8 impairs development of upper-layer neocortical neurons

Upper-layer (UL) neocortical neurons are the most prominent distinguishing features of the mammalian neocortex compared with those of the avian dorsal cortex and are vastly expanded in primates. However, little is known about the identities of the genes that control the specification of UL neurons. Here, we found that Prdm8, a member of the PR (PRDI-BF1 and RIZ homology) domain protein family, was specifically expressed in the postnatal UL neocortex, particular those in late-born RORß-positive layer IV neurons. We generated homozygous Prdm8 knockout (Prdm8 KO) mice and found that the deletion of Prdm8 causes growth retardation and a reduced brain weight, although the brain weight-to-body weight ratio is unchanged at postnatal day 8 (P8). Immunohistochemistry showed that the relative UL thickness, but not the thickness of the deep layer (DL), was significantly reduced in Prdm8 KO mice compared with wild-type (WT) mice. In addition, we found that a number of late-born Brn2-positive UL neurons were significantly decreased in Prdm8 KO mice. To identify genes regulated by Prdm8 during neocortical development, we compared expression profiling analysis in Prdm8 KO and WT mice, and identified some candidate genes. These results suggest that the proper expression of Prdm8 is required for the normal development and construction of UL neurons in the mammalian neocortex.

Quantitative and functional interrogation of parent-of-origin allelic expression biases in the brain

The maternal and paternal genomes play different roles in mammalian brains as a result of genomic imprinting, an epigenetic regulation leading to differential expression of the parental alleles of some genes. Here we investigate genomic imprinting in the cerebellum using a newly developed Bayesian statistical model that provides unprecedented transcript-level resolution. We uncover 160 imprinted transcripts, including 41 novel and independently validated imprinted genes. Strikingly, many genes exhibit parentally biased--rather than monoallelic--expression, with different magnitudes according to age, organ, and brain region. Developmental changes in parental bias and overall gene expression are strongly correlated, suggesting combined roles in regulating gene dosage. Finally, brain-specific deletion of the paternal, but not maternal, allele of the paternally-biased Bcl-x, (Bcl2l1) results in loss of specific neuron types, supporting the functional significance of parental biases. These findings reveal the remarkable complexity of genomic imprinting, with important implications for understanding the normal and diseased brain.

Role of EphA/ephrin--a signaling in the development of topographic maps in mouse corticothalamic projections

Corticothalamic (CT) feedback outnumbers thalamocortical projections and regulates sensory information processing at the level of the thalamus. It is well established that EphA7, a member of EphA receptor family, is involved in the topographic mapping of CT projections. The aim of the present study was to dissect the precise impact of EphA7 on each step of CT growth. We used in utero electroporation-mediated EphA7 overexpression in developing somatosensory CT axons to dissect EphA7/ephrin-A-dependent mechanisms involved in regulating both initial targeting and postnatal growth of the CT projections. Our data revealed that topographic maps of cortical afferents in the ventrobasal complex and medial part of the posterior complex in the thalamus become discernible shortly after birth and are fully established by the second postnatal week. This process starts with the direct ingrowth of the CT axons to the designated areas within target thalamic nuclei and by progressive increase of axonal processes in the terminal zones. Large-scale overproduction and elimination of exuberant widespread axonal branches outside the target zone was not observed. Each developmental event was coordinated by spatially and temporally different responsiveness of CT axons to the ephrin-A gradient in thalamic nuclei, as well as by the matching levels of EphA7 in CT axons and ephrin-As in thalamic nuclei. These results support the concept that the topographic connections between the maps in the cerebral cortex and corresponding thalamic nuclei are genetically prespecified to a large extent, and established by precise spatiotemporal molecular mechanisms that involve the Eph family of genes.

Only scratching the cell surface: extracellular signals in cerebrum development

Numerous roles have been identified for extracellular signals such as Fibroblast Growth Factors (FGFs), Transforming Growth Factor-βs (TGFβs), Wingless-Int proteins (WNTs), and Sonic Hedgehog (SHH) in assigning fates to cells during development of the cerebrum. However, several fundamental questions remain largely unexplored. First, how does the same extracellular signal instruct precursor cells in different locations or at different stages to adopt distinct fates? And second, how does a precursor cell integrate multiple signals to adopt a specific fate? Answers to these questions require knowing the mechanisms that underlie each cell type's competence to respond to certain extracellular signals. This brief review provides illustrative examples of potential mechanisms that begin to bridge the gap between cell surface and cell fate during cerebrum development.

Coupling diffusion imaging with histological and gene expression analysis to examine the dynamics of cortical areas across the fetal period of human brain development

As a prominent component of the human fetal brain, the structure of the cerebral wall is characterized by its laminar organization which includes the radial glial scaffold during fetal development. Diffusion tensor imaging (DTI) is useful to quantitatively delineate the microstructure of the developing brain and to clearly identify transient fetal layers in the cerebral wall. In our study, the spatio-temporal microstructural changes in the developing human fetal cerebral wall were quantitatively characterized with high-resolution DTI data of postmortem fetal brains from 13 to 21 gestational weeks. Eleven regions of interest for each layer in the entire cerebral wall were included. Distinctive time courses of microstructural changes were revealed for 11 regions of the neocortical plate. A histological analysis was also integrated to elucidate the relationship between DTI fractional anisotropy (FA) and histology. High FA values correlated with organized radial architecture in histological image. Expression levels of 17565 genes were quantified for each of 11 regions of human fetal neocortex from 13 to 21 gestational weeks to identify transcripts showing significant correlation with FA change. These correlations suggest that the heterogeneous and regionally specific microstructural changes of the human neocortex are related to different gene expression patterns.

Pallio-pallial tangential migrations and growth signaling: new scenario for cortical evolution?

Observations accruing in recent years imply that the areal patterning and size dimensioning of the mammalian neocortex are influenced by diverse sets of tangentially migrating glutamatergic neurons that invade the cortical plate and, in so doing, modify the properties of the neopallial proliferative compartments. This developmental scenario sheds new light upon the old issue of how the mammalian neocortex evolved its more complex structure from nonmammalian antecedent forms. In reviewing these novelties, I first point out the topological position of the neopallial island as a central component of the pallium in all gnathostomes, surrounded by a ring of prospective allocortical pallial regions and a more distant set of peripheral neighboring forebrain areas. Early patterning arises from the periphery via passive planar signaling. This process probably establishes the pallium field and its basic island plus allocortical ring organization, as well as a rough prepatterning of some regional subareas. Afterwards, patterning and modulated growth are also actively influenced by the convergence of separate streams of tangentially migrating subpial cells (partly peripheral and partly allocortical in origin) which collectively form the Cajal-Retzius neuronal population in layer I. Effects of these cells include the inside-out stratification of the cortical plate and they may also contribute to the evolutionary emergence of the 6-layered neocortical structure. The most recent addition to our knowledge of pallio-pallial migrations is the existence of a subsequent deep tangential migration of ventropallial cells into the neopallial primordium, whose signaling influence upon local progenitors magnifies the cortex population by 20%. These glutamatergic cells dispersedly invade the entire cortex but largely die postnatally. The crucial implications of these data for comparative thinking on mammalian neocortex evolution and interpretation of potential homologs in sauropsids are explored. Finally, a new conjecture regarding a possible role of the hitherto disregarded lateral pallium is advanced.

Neuronal DNA content variation (DCV) with regional and individual differences in the human brain

It is widely assumed that the human brain contains genetically identical cells through which postgenomic mechanisms contribute to its enormous diversity and complexity. The relatively recent identification of neural cells throughout the neuraxis showing somatically generated mosaic aneuploidy indicates that the vertebrate brain can be genomically heterogeneous (Rehen et al. [2001] Proc. Natl. Acad. Sci. U. S. A. 98:13361-13366; Rehen et al. [2005] J. Neurosci. 25:2176-2180; Yurov et al. [2007] PLoS ONE:e558; Westra et al. [2008] J. Comp. Neurol. 507:1944-1951). The extent of human neural aneuploidy is currently unknown because of technically limited sample sizes, but is reported to be small (Iourov et al. [2006] Int. Rev. Cytol. 249:143-191). During efforts to interrogate larger cell populations by using DNA content analyses, a surprising result was obtained: human frontal cortex brain cells were found to display "DNA content variation (DCV)" characterized by an increased range of DNA content both in cell populations and within single cells. On average, DNA content increased by approximately 250 megabases, often representing a substantial fraction of cells within a given sample. DCV within individual human brains showed regional variation, with increased prevalence in the frontal cortex and less variation in the cerebellum. Further, DCV varied between individual brains. These results identify DCV as a new feature of the human brain, encompassing and further extending genomic alterations produced by aneuploidy, which may contribute to neural diversity in normal and pathophysiological states, altered functions of normal and disease-linked genes, and differences among individuals.

A novel role for Dbx1-derived Cajal-Retzius cells in early regionalization of the cerebral cortical neuroepithelium

Patterning of the cerebral cortex during embryogenesis depends not only on passive diffusion of morphogens but also on signal delivery by Cajal-Retzius neurons that migrate over long distances.

Patterning of the cortical neuroepithelium occurs at early stages of embryonic development in response to secreted molecules from signaling centers. These signals have been shown to establish the graded expression of transcription factors in progenitors within the ventricular zone and to control the size and positioning of cortical areas. Cajal-Retzius (CR) cells are among the earliest generated cortical neurons and migrate from the borders of the developing pallium to cover the cortical primordium by E11.5. We show that molecularly distinct CR subtypes distribute in specific combinations in pallial territories at the time of cortical regionalization. By means of genetic ablation experiments in mice, we report that loss of septum Dbx1-derived CR cells in the rostromedial pallium between E10.5 and E11.5 results in the redistribution of CR subtypes. This leads to changes in the expression of transcription factors within the neuroepithelium and in the proliferation properties of medial and dorsal cortical progenitors. Early regionalization defects correlate with shifts in the positioning of cortical areas at postnatal stages in the absence of alterations of gene expression at signaling centers. We show that septum-derived CR neurons express a highly specific repertoire of signaling factors. Our results strongly suggest that these cells, migrating over long distances and positioned in the postmitotic compartment, signal to ventricular zone progenitors and, thus, function as modulators of early cortical patterning.

Patterning of the cerebral cortex occurs early during embryonic development in response to secreted molecules or morphogens produced at signaling centers. These morphogens establish the graded expression of transcription factors (TFs) in progenitor cells and control the size and positioning of cortical areas in the postnatal animal. CR cells are among the earliest born cortical neurons and play a crucial role in cortical lamination. They are generated at signaling centers and migrate over long distances to cover its entire surface. We show that three different CR subtypes distribute in specific proportions in cortical territories. Genetic ablation of one subpopulation leads to a highly dynamic redistribution of the two others. This results in defects in expression of transcription factors and in progenitor cell proliferation, which correlate with the resulting changes in the size and positioning of cortical areas. Given our additional evidence that CR subtypes express specific repertoires of signaling factors, the ablation phenotypes point to a novel early role for CR cells as mediators of cortical patterning and suggest that CR cells are able to signal to progenitor cells. Our data thus add to the conventional model that morphogens act by passive diffusion and point to a strategy of morphogen delivery over long distance by migrating cells.

Regionally specified human neural progenitor cells derived from the mesencephalon and forebrain undergo increased neurogenesis following overexpression of ASCL1

Human neural progenitor cells (hNPC) derived from the developing brain can be expanded in culture and subsequently differentiated into neurons and glia. They provide an interesting source of tissue for both modeling brain development and developing future cellular replacement therapies. It is becoming clear that hNPC are regionally and temporally specified depending on which brain region they were isolated from and its developmental stage. We show here that hNPC derived from the developing cortex (hNPC(CTX)) and ventral midbrain (hNPC(VM)) have similar morphological characteristics and express the progenitor cell marker nestin. However, hNPC(CTX) cultures were highly proliferative and produced large numbers of neurons, whereas hNPC(VM) divided slowly and produced fewer neurons but more astrocytes. Microarray analysis revealed a similar expression pattern for some stemness markers between the two growing cultures, overlaid with a regionally specific profile that identified some important differentially expressed neurogenic transcription factors. By overexpressing one of these, the transcription factor ASCL1, we were able to regain neurogenesis from hNPC(VM) cultures, which produced larger neurons with more neurites than hNPC(CTX) but no fully mature dopamine neurons. Thus, hNPC are regionally specified and can be induced to undergo neurogenesis following genetic manipulation. Although this restores neuronal production with a region-specific phenotype, it does not restore full neurochemical maturation, which may require additional factors.

Lability of neuronal lineage decisions is revealed by acute exposures to ethanol

Developing neurons pass through periods of sensitivity to environmental factors, e.g., alterations induced by ethanol are defined when the exposure occurs. We tested the hypothesis that timely episodic prenatal exposure to ethanol can change the lineage of cortical neurons. This study exploited mice in which many layer V neurons expressed a Thy1-YFPh transgene and endogenously fluoresced yellow. Fetuses were exposed to ethanol or saline on gestational day (G) 14 (when layer V neurons were generated) or on G 15 or 17 (when these layer V neurons were migrating). Fetuses dosed on G 14 exhibited an increased frequency of YFP+ neurons across cortex. This contrasted with a decreased frequency following ethanol exposure on G 17. Ethanol did not affect overall density of layer V neurons or their generation. Thus, the magnitude and valence of ethanol-induced changes in YFP+ neurons are time-dependent. Cell lineage is defined at the time of origin and the window of lability for this definition continues into the early post-mitotic (migratory) period.

Dopamine, cocaine and the development of cerebral cortical cytoarchitecture: a review of current concepts

Exposure of the developing fetus to cocaine produces lasting adverse effects on brain structure and function. Animal models show that cocaine exerts its effects by interfering with monoamine neurotransmitter function and that dopamine is cocaine's principal monoamine target in the fetal brain. This review will examine the role of dopamine receptor signaling in the regulation of normal development of the cerebral cortex, the seat of higher cognitive functions, and discuss whether dopamine receptor signaling mechanisms are the principal mediators of cocaine's deleterious effects on the ontogeny of cerebral cortical cytoarchitecture.

The Limbic System in Mammalian Brain Evolution

Previous accounts of mammalian brain allometry have relied largely on data from primates, insectivores and bats. Here we examine scaling of brain structures in carnivores, ungulates, xenarthrans and sirenians, taxa chosen to maximize potential olfactory and limbic system variability. The data were compared to known scaling of the same structures in bats, insectivores and primates. Fundamental patterns in brain scaling were similar across all taxa. Marine mammals with reduced olfactory bulbs also had reduced limbic systems overall, particularly in those structures receiving direct olfactory input. In all species, a limbic factor with olfactory and non-olfactory components was observed. Primates, insectivores, ungulate and marine mammals collectively demonstrate an inverse relationship between isocortex and limbic volumes, but terrestrial carnivores have high relative volumes of both, and bats low relative volumes of both. We discuss developmental processes that may provide the mechanistic bases for understanding these findings.

Genetic analysis of anterior posterior expression gradients in the developing mammalian forebrain

Intrinsic regulatory factors play critical roles in early cortical patterning, including the development of the anteroposterior (A-P) axis. To identify genes that are differentially expressed along the A-P axis of the developing cerebral cortex, we analyzed gene expression in presumptive frontal, parietal, and occipital cerebral walls of E12.5 mouse using complementary DNA microarrays. We identified 106 genes, including expressed sequence tags (ESTs), expressed in an A-P gradient in the embryonic brain and screened 88 by in situ hybridization for confirmation. Central nervous system (CNS) expression patterns of many of these genes were previously unknown. Others, such as Sfrp1, CoupTF1, and FABP7, were expressed in a manner consistent with previous studies, providing independent confirmation. Two related transcription factors, previously not implicated in CNS development, Fhl1 and Fhl2, were observed to be enriched in posterior and anterior telencephalon, respectively. We studied patterning gradients in Fhl1 knockout mice but observed no changes in gene expression related to A-P regionalization in the Fhl1 knockout mice. These data provide an important set of new candidates for studies of cortical patterning and maturation.

Methylmercury elicits rapid inhibition of cell proliferation in the developing brain and decreases cell cycle regulator, cyclin E

The developing brain is highly sensitive to methylmercury (MeHg). Still, the initial changes in cell proliferation that may contribute to long-term MeHg effects are largely undefined. Our previous studies with growth factors indicate that acute alterations of the G1/S-phase transition can permanently affect cell numbers and organ size. Therefore, we determined whether an environmental toxicant could also impact brain development with rapid (6-7h) effects on DNA synthesis and cell cycle machinery in neuronal precursors. In vivo studies in newborn rat hippocampus and cerebellum, two regions of postnatal neurogenesis, were followed by in vitro analysis of two precursor models, cortical and cerebellar cells, focusing on the proteins that regulate the G1/S transition. In postnatal day 7 (P7) pups, a single subcutaneous injection of MeHg (3microg/g) acutely (7h) decreased DNA synthesis in the hippocampus by 40% and produced long-term (2 weeks) reductions in total cell number, estimated by DNA quantification. Surprisingly, cerebellar granule cells were resistant to MeHg effects in vivo at comparable tissue concentrations, suggesting region-specific differences in precursor populations. In vitro, MeHg altered proliferation and cell viability, with DNA synthesis selectively inhibited at an early timepoint (6h) corresponding to our in vivo observations. Considering that G1/S regulators are targets of exogenous signals, we used a well-defined cortical cell model to examine MeHg effects on relevant cyclin-dependent kinases (CDK) and CDK inhibitors. At 6h, MeHg decreased by 75% levels of cyclin E, a cell cycle regulator with roles in proliferation and apoptosis, without altering p57, p27, or CDK2 nor levels of activated caspase 3. In aggregate, our observations identify the G1/S transition as an early target of MeHg toxicity and raise the possibility that cyclin E degradation contributes to both decreased proliferation and eventual cell death.

Molecular approaches to brain asymmetry and handedness

In the human brain, distinct functions tend to be localized in the left or right hemispheres, with language ability usually localized predominantly in the left and spatial recognition in the right. Furthermore, humans are perhaps the only mammals who have preferential handedness, with more than 90% of the population more skillful at using the right hand, which is controlled by the left hemisphere. How is a distinct function consistently localized in one side of the human brain? Because of the convergence of molecular and neurological analysis, we are beginning to consider the puzzle of brain asymmetry and handedness at a molecular level.

Expression of D2 dopamine receptor in the mouse brain

The neurotransmitter, dopamine, binds to dopamine receptor (DR), and is involved in several functions of the brain, such as initiation and execution of movement, emotion, prolactin secretion, etc. Of all the five DRs, D2 dopamine receptor has maximal affinity for dopamine. D2 has a short isoform, D2S, and a long isoform D2L. D2L is longer than D2S by 29 amino acid residues. We studied the expression of the gene and protein of D2 receptor in the cerebral and cerebellar cortices of the brain of new born, developing, adult, and old male mice to find out: (i) at what stage of development, expression of the gene peaks and (ii) if it undergoes any changes as the animal ages, which may account for the neurodegenerative changes and symptoms of Parkinson's and other diseases seen in old age. RT-PCR and Western blot studies show that peak expression of D2 gene occurs in the cerebral and cerebellar cortices around 15-day after birth. We speculate that the majority of dopaminergic synapses are established and possibly become functional in the brain around 15-day after birth. The expression of D2 receptor is upregulated in the cerebral cortex in old mice. However, it is down-regulated in the cerebellar cortex.

Progenitors resume generating neurons after temporary inhibition of neurogenesis by Notch activation in the mammalian cerebral cortex

The mammalian cerebral cortex comprises six layers of neurons. Cortical progenitors in the ventricular zone generate neurons specific to each layer through successive cell divisions. Neurons of layer VI are generated at an early stage, whereas later-born neurons occupy progressively upper layers. The underlying molecular mechanisms of neurogenesis, however, are relatively unknown. In this study, we devised a system where the Notch pathway was activated spatiotemporally in the cortex by in vivo electroporation and Cre-mediated DNA recombination. Electroporation at E13.5 transferred DNA to early progenitors that gave rise to neurons of both low and upper layers. Forced expression of a constitutively active form of Notch (caNotch) at E13.5 inhibited progenitors from generating neurons and kept progenitors as proliferating radial glial cells. After subsequent transfection at E15.5 of a Cre expression vector to remove caNotch, double-transfected cells, in which caNotch was excised, migrated into the cortical plate and differentiated into neurons specific to upper layers. Bromodeoxyuridine-labeling experiments showed that the neurons were born after Cre transfection. These results indicate that cortical progenitors that had been temporarily subjected to Notch activation at an early stage generated neurons at later stages, but that the generation of low-layer neurons was skipped. Moreover, the double-transfected cells gave rise to upper-layer neurons, even after their transplantation into the E13.5 brain, indicating that the developmental state of progenitors is not halted by caNotch activity.

A morphogenetic model for the development of cortical convolutions

The convolutions of the mammalian cortex are one of its most intriguing characteristics. Their pattern is very distinctive for different species, and there seems to be a remarkable relationship between convolutions and the architectonic and functional regionalization of the cerebral cortex. Yet the mechanisms behind the development of convolutions and their association with the cortical regionalization are poorly understood. Here we propose a morphogenetic model for the development of cortical convolutions based on the structure of the cortex as a closed surface with glial and axonal fibres pulling radially, the fundamental mechanical properties of cortex and fibres (elasticity and plasticity), and the growth of the cortical surface. The computer simulations of this model suggest that convolutions are a natural consequence of cortical growth. The model reproduces several aspects of convolutional development, such as the relationship between cortical surface and brain volume among mammals, the period of compensation in the degree of convolution observed in gyrencephalic brains and the dependence of the degree of convolution on cortical thickness. We have also studied the effect of early cortical regionalization on the development of convolutions by introducing geometric, mechanic and growth asymmetries in the model. The morphogenetic model is thus able to reproduce the gradients in the degree of convolution, the development of primary, secondary and tertiary convolution, and the overproduction of sulci observed in animals with altered afferent cortical connections.

Distribution of the limbic system-associated membrane protein (LAMP) in pigeon forebrain and midbrain

The limbic system-associated membrane protein (LAMP) is an adhesion molecule involved in specifying regional identity during development, and it is enriched in the neuropil of limbic brain regions in mammals but also found in some somatic structures. Although originally identified in rat, LAMP is present in diverse species, including avians. In this study, we used immunolabeling with a monoclonal antibody against rat LAMP to examine the distribution of LAMP in pigeon forebrain and midbrain. LAMP immunolabeling was prominent in many telencephalic regions previously noted as limbic in birds. These regions include the hippocampal complex, the medial nidopallium, and the ventromedial arcopallium. Subpallial targets of these pallial regions were also enriched in LAMP, such as the medial-most medial striatum. Whereas some telencephalic areas that have not been regarded as limbic were also LAMP-rich (e.g., the hyperpallium intercalatum and densocellulare of the Wulst, the mesopallium, and the intrapeduncular nucleus), most nonlimbic telencephalic areas were LAMP-poor (e.g., field L, the lateral nidopallium, and somatic basal ganglia). Similarly, in the diencephalon and midbrain, prominent LAMP labeling was observed in such limbic areas as the dorsomedial thalamus, the hypothalamus, the ventral tegmental area, and the central midbrain gray, as well as in a few nonlimbic areas such as nucleus rotundus, the shell of the nucleus pretectalis, the superficial tectum, and the parvocellular isthmic nucleus. Thus, as in mammals, LAMP in birds appears to be enriched in most known forebrain and midbrain limbic structures but is present as well in some somatic structures.

Intermediate targets in formation of topographic projections: inputs from the thalamocortical system

Topography of axonal projections has been generally thought to arise from positional information located within the projecting and targeted structures, independent of events along the path or within the axonal bundle. Recent evidence suggests that in the projection from the dorsal thalamus to the neocortex, initial rostrocaudal targeting of axons is regulated at the level of an intermediate target, the subcortical telencephalon. In this system, thalamic axons are spatially positioned within the subcortical telencephalon, partly via interactions between EphAs and ephrin-As, and this positioning apparently determines the rostrocaudal level of the neocortex that the axons will initially target.

Overexpression of genes in the CA1 hippocampus region of adult rat following episodes of global ischemia

Ischemic stress is associated with marked changes in gene expression in the hippocampus--albeit little information exists on the activation of nonabundant genes. We have examined the expression of several known genes and identified novel ones in the adult rat hippocampus after a mild, transient, hypovolemic and hypotensive, global ischemic stress. An initial differential screening using a prototype array to assess gene expression after stress followed by a suppression subtractive hybridization protocol and cDNA microarray revealed 124 nonoverlapped transcripts predominantly expressed in the CA1 rat hippocampus region in response to ischemic stress. About 78% of these genes were not detected with nonsubtracted probes. Reverse transcription polymerase chain reaction (RT-PCR) and in situ hybridization on these 124 transcripts confirmed the differential expression of at least 83. Most robustly expressed were gene sequences NFI-B, ATP1B1, RHOGAP, PLA2G4A, BAX, CASP3, P53, MAO-A, FRA1, HSP70.2, and NR4A1 (NUR77), as well as sequence tags of unknown function. New stress-related genes of similar functional motifs were identified, reemphasizing the importance of functional grouping in the analysis of multiple gene expression profiles. These data indicate that ischemia elicits expression of an array of functional gene clusters that may be used as an index for stress severity and a template for target therapy design.

Novel neuronal phenotypes from neural progenitor cells

We report the first isolation of progenitor cells from the hypothalamus, a derivative of the embryonic basal plate that does not exhibit neurogenesis postnatally. Neurons derived from hypothalamic progenitor cells were compared with those derived from progenitor cultures of hippocampus, an embryonic alar plate derivative that continues to support neurogenesis in vivo into adulthood. Aside from their different embryonic origins and their different neurogenic potential in vivo, these brain regions were chosen because they are populated with cells of three different categories: Category I cells are generated in both hippocampus and hypothalamus, Category II cells are generated in the hypothalamus but are absent from the hippocampus, and Category III is a cell type generated in the olfactory placode that migrates into the hypothalamus during development. Stem-like cells isolated from other brain regions, with the ability to generate neurons and glia, produce neurons of several phenotypes including gabaergic, dopaminergic, and cholinergic lineages. In the present study, we extended our observations into neuroendocrine phenotypes. The cultured neural precursors from 7-week-old rat hypothalamus readily generated neuropeptide-expressing neurons. Hippocampal and hypothalamic progenitor cultures converged to indistinguishable populations and produced neurons of all three categories, confirming that even short-term culture confers or selects for immature progenitors with enough plasticity to elaborate neuronal phenotypes usually inhibited in vivo by the local microenvironment. The range of phenotypes generated from neuronal precursors in vitro now includes the peptides found in the neuroendocrine system: corticotropin-releasing hormone, growth hormone-releasing hormone, gonadotropin-releasing hormone, oxytocin, somatostatin, thyrotropin-releasing hormone, and vasopressin.

Neuronal birthdate-specific gene transfer with adenoviral vectors

The multilayered structure of the cerebral cortex has been studied in detail. Early-born neurons migrate into the inner layer and late-born neurons migrate into more superficial layers, thus establishing an inside-out gradient. The progenitor cells appear to acquire layer-specific properties at the time of neuronal birth; however, the molecular mechanisms of cell-fate acquisition are still unclear, because it has been difficult to identify a cohort of birthdate-related progenitor cells. Using replication-defective adenoviral vectors, we successfully performed "pulse gene transfer" into progenitor cells in a neuronal birthdate-specific manner. When adenoviral vectors were injected into the midbrain ventricle of mouse embryos between embryonic day 10.5 (E10.5) and E14.5, the adenoviral vectors introduced a foreign gene into a specific cohort of birthdate-related progenitor cells. The virally infected cohorts developed normally into cortical neurons and formed the canonical cortical layers in an inside-out manner. This technique allows us to distinguish a cohort of birthdate-related progenitor cells from other progenitor cells with different birthdates and to introduce a foreign gene into specific subsets of cortical layers by performing adenoviral injection at specific times. This adenovirus-meditated gene transfer technique will enable us to examine the properties of each subset of progenitor cells that share the same neuronal birthdate.

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.

Molecular regionalization of the neocortex is disrupted in Fgf8 hypomorphic mutants

The neocortex is divided into multiple areas with specific architecture, molecular identity and pattern of connectivity with the dorsal thalamus. Gradients of transcription factor expression in the cortical primordium regulate molecular regionalization and potentially the patterning of thalamic projections. We show that reduction of Fgf8 levels in hypomorphic mouse mutants shifts early gradients of gene expression rostrally, thereby modifying the molecular identity of rostral cortical progenitors. This shift correlates with a reduction in the size of a molecularly defined rostral neocortical domain and a corresponding rostral expansion of more caudal regions. Despite these molecular changes, the topography of projections between the dorsal thalamus and rostral neocortex in mutant neonates appears the same as the topography of wild-type littermates. Overall, our study demonstrates the role of endogenous Fgf8 in regulating early gradients of transcription factors in cortical progenitor cells and in molecular regionalization of the cortical plate.

Nature versus nurture revisited: an old idea with a new twist

The nature versus nurture debate has recently resurfaced with the emergence of the field of developmental molecular neurobiology. The questions associated with "nature" have crystallized into testable hypotheses regarding patterns of gene expression during development, and those associated with "nurture" have given over to activity-dependent cellular mechanisms that give rise to variable phenotypes in developing nervous systems. This review focuses on some of the features associated with complex brains and discusses the evolutionary and activity-dependent mechanisms that generate these features. These include increases in the size of the cortical sheet, changes in cortical domain and cortical field specification, and the activity-dependent intracellular mechanisms that regulate the structure and function of neurons during development. We discuss which features are likely to be genetically mediated, which features are likely to be regulated by activity, and how these two mechanisms act in concert to produce the wide variety of phenotypes observed for the mammalian neocortex. For example, the size of the cortical sheet is likely to be under genetic control, and regulation of cell-cycle kinetics through upregulation of genes such as beta-catenin can account for increases in the size of the cortical sheet. Similarly, intrinsic signaling genes or gene products such as Wnt, Shh, Fgf2, Fgf8 and BMP may set up a combinatorial coordinate system that guides thalamic afferents. Changes in peripheral morphology that regulate patterned activity are also likely to be under genetic control. Finally, the intracellular machinery that allows for activity-dependent plasticity in the developing CNS may be genetically regulated, although the specific phenotype they generate are not. On the other hand, aspects of neocortical organization such as sensory domain assignment, the size and shape of cortical fields, some aspects of connectivity, and details of functional organization are likely to be activity-dependent. Furthermore, the role of genes versus activity, and their interactions, may be different for primary fields versus non-primary fields.

The early topography of thalamocortical projections is shifted in Ebf1 and Dlx1/2 mutant mice

The prevailing model to explain the formation of topographic projections in the nervous system stipulates that this process is governed by information located within the projecting and targeted structures. In mammals, different thalamic nuclei establish highly ordered projections with specific neocortical domains and the mechanisms controlling the initial topography of these projections remain to be characterized. To address this issue, we examined Ebf1(-/-) embryos in which a subset of thalamic axons does not reach the neocortex. We show that the projections that do form between thalamic nuclei and neocortical domains have a shifted topography, in the absence of regionalization defects in the thalamus or neocortex. This shift is first detected inside the basal ganglia, a structure on the path of thalamic axons, and which develops abnormally in Ebf1(-/-) embryos. A similar shift in the topography of thalamocortical axons inside the basal ganglia and neocortex was observed in Dlx1/2(-/-) embryos, which also have an abnormal basal ganglia development. Furthermore, Dlx1 and Dlx2 are not expressed in the dorsal thalamus or in cortical projections neurons. Thus, our study shows that: (1) different thalamic nuclei do not establish projections independently of each other; (2) a shift in thalamocortical topography can occur in the absence of major regionalization defects in the dorsal thalamus and neocortex; and (3) the basal ganglia may contain decision points for thalamic axons' pathfinding and topographic organization. These observations suggest that the topography of thalamocortical projections is not strictly determined by cues located within the neocortex and may be regulated by the relative positioning of thalamic axons inside the basal ganglia.

Timing and plasticity of specification of CaM-Kinase II alpha expression by neocortical neurons

In this work, the differential expression of a chemical marker, the alpha-isoform of the calcium/calmodulin-dependent protein kinase II (CaM-Kinase II alpha) and the development of the spinal cord projection were used to determine in vivo the embryonic stages at which different aspects of the phenotype of neocortical cells are specified. We first performed a quantitative, immunocytochemical study on the levels of CaM-Kinase II alpha expression in the frontal, parietal and occipital cortical areas of control adult rats. We found that the levels of expression of CaM-Kinase II alpha were larger in the frontal and parietal areas than in the occipital areas. In addition, all layer V neurons identified as projecting to the spinal cord were CaM-Kinase II alpha immunopositive. We then grafted embryonic day (E) 12 or 14 cells from the presumptive frontal or occipital cortex of donor fetuses into the frontal or occipital cortex of newborn hosts. Cortical cells grafted at E12 differentiate neurons with molecular (CaM-Kinase II alpha) and connectivity (spinal cord projection) phenotypes appropriate to the cortical area where they complete their development whereas cells taken at E14 differentiate neurons with molecular and connectivity phenotypes appropriate to their cortical locus of origin. These findings suggest that E12 progenitors destined to generate layer V neurons are multipotent. The final phenotype of their progeny depends on regionalizing signals expressed in the environment. Later in corticogenesis, committed progenitors become unable to respond to regionalizing signals and generate neurons whose phenotype is appropriate to the initial cortical position of the precursor.

Differential display identifies neuroendocrine-specific protein-A (NSP-A) and interferon-inducible protein 10 (IP-10) as ethanol-responsive genes in the fetal rat brain

Fetal alcohol exposure is the most common nonhereditary cause of mental retardation in the western world. Rats prenatally treated with ethanol liquid diet exhibit extensive defects in the brain that accurately model those observed in humans. To analyze the ethanol effects on gene expression during brain development, we performed mRNA differential display and two-dimensional electrophoresis on gestational day (G) 13 and G 16 brain from rats treated with ethanol liquid diet. Using mRNA differential display followed by a variety of quantitative analyses, three genes were confirmed to be ethanol-responsive. Among them was Neuroendocrine-Specific Protein-A (NSP-A), which is known to be affected by thyroid hormone in the cortex at this developmental time. However, two additional genes known to be thyroid hormone-responsive were unaffected by ethanol, indicating that interference with thyroid hormone action may not be a predominant pathway by which alcohol induces damage in the fetal brain. The observation that interferon-inducible protein-10 (IP-10) is up-regulated in ethanol-treated fetal brain may indicate the presence of a disease process recruiting CD8+ T-cells capable of interfering with myelination. The result of two-dimensional (2D) electrophoresis and Western analyses demonstrated that few changes in the abundance of individual proteins or the phosphorylation of proteins at threonine and tyrosine were induced by prenatal ethanol exposure. A critical analysis of the approaches used in the present study may be important for future studies in this field.

Functional synaptic projections onto subplate neurons in neonatal rat somatosensory cortex

Subplate neurons (SPn) play an important role in the formation of thalamocortical connections during early development and show glutamatergic and GABAergic spontaneous synaptic activity. We characterized these synaptic inputs by performing whole-cell recordings from SPn in somatosensory cortical slices of postnatal day 0-3 rats. At -70 mV, electrical stimulation of the thalamocortical afferents elicited in 68% of the SPn a monosynaptic CNQX-sensitive postsynaptic current (PSC). These fast PSCs were mediated by AMPA receptors, because they were prolonged by cyclothiazide and blocked by GYKI 52466. On membrane depolarization, thalamocortical stimulation elicited in 50% of the cells an additional slow monosynaptic component mediated by NMDA receptors. Stimulation of the cortical plate evoked in 72% of SPn a monosynaptic AMPA receptor-mediated PSC with an additional NMDA component at depolarized membrane potentials and in 40% of the investigated cells polysynaptic responses, depending on GABA(A) and NMDA receptors. Stimulation of the subplate elicited in 67% of SPn a monosynaptic dual-component PSC mediated by AMPA and NMDA receptors activated at -70 mV and in 12% of SPn a monosynaptic single-component PSC mediated by AMPA receptors with an additional NMDA component activated at depolarized membrane potentials. A monosynaptic GABAergic response could be observed in 68% of SPn after stimulation of the subplate. In gramicidin-perforated patch recordings, bath application of GABA caused membrane depolarization to -40 mV and elicited action potentials. These results demonstrate that SPn receive distinct functional synaptic inputs arising from the thalamus, cortical plate, and subplate, indicating that SPn are capable of integrating and processing information from cortical and subcortical regions.

Transgenic and knock-out mouse pups: the growing need for behavioral analysis

Few laboratories working with transgenic and knockout mice analyze the neurobehavioral consequences of genetic manipulation in early ontogeny. However, the study of behavioral endpoints during the early postnatal period in genetically modified mice is important not only to assess possible developmental abnormalities, but also to better understand and disentangle the effects of genetic manipulations in adulthood. We propose that the assessment of neurobehavioral development represents an appropriate strategy to identify possible compensatory and/or unexpected effects. Nowadays, a large number of experimental protocols that take into account the practical constraints imposed by the peculiar physiological and behavioral responses of an immature subject are available to assess the neurobehavioral profile of developing mice. While this knowledge should be applied to the field of transgenic and knock-out mice in general, it should be recommended, in particular, for the study of mouse models of neurodevelopmental disorders.

Characterization of two novel nuclear BTB/POZ domain zinc finger isoforms. Association with differentiation of hippocampal neurons, cerebellar granule cells, and macroglia

BTB/POZ (broad complex tramtrack bric-a-brac/poxvirus and zinc finger) zinc finger factors are a class of nuclear DNA-binding proteins involved in development, chromatin remodeling, and cancer. However, BTB/POZ domain zinc finger factors linked to development of the mammalian cerebral cortex, cerebellum, and macroglia have not been described previously. We report here the isolation and characterization of two novel nuclear BTB/POZ domain zinc finger isoforms, designated HOF(L) and HOF(S), that are specifically expressed in early hippocampal neurons, cerebellar granule cells, and gliogenic progenitors as well as in differentiated glia. During embryonic development of the murine cerebral cortex, HOF expression is restricted to the hippocampal subdivision. Expression coincides with early differentiation of presumptive CA1 and CA3 pyramidal neurons and dentate gyrus granule cells, with a sharp decline in expression at the CA1/subicular border. By using bromodeoxyuridine labeling and immunohistochemistry, we show that HOF expression coincides with immature non-dividing cells and is down-regulated in differentiated cells, suggesting a role for HOF in hippocampal neurogenesis. Consistent with the postulated role of the POZ domain as a site for protein-protein interactions, both HOF isoforms are able to dimerize. The HOF zinc fingers bind specifically to the binding site for the related promyelocytic leukemia zinc finger protein as well as to a newly identified DNA sequence.

Opposite actions of brain-derived neurotrophic factor and neurotrophin-3 on firing features and ion channel composition of murine spiral ganglion neurons

It is now well established that sensory neurons and receptors display characteristic morphological and electrophysiological properties tailored to their functions. This is especially evident in the auditory system, where cells are arranged tonotopically and are highly specialized for precise coding of frequency- and timing-dependent auditory information. Less well understood, however, are the mechanisms that give rise to these biophysical properties. We have provided insight into this issue by using whole-cell current-clamp recordings and immunocytochemistry to show that BDNF and NT-3, neurotrophins found normally in the cochlea, have profound effects on the firing properties and ion channel distribution of spiral ganglion neurons in the murine cochlea. Exposure of neurons to BDNF caused all neurons, regardless of their original cochlear position, to display characteristics of the basal neurons. Conversely, NT-3 caused cells to show the properties of apical neurons. These results are consistent with oppositely oriented gradients of these two neurotrophins and/or their high-affinity receptors along the tonotopic map, and they suggest that a combination of neurotrophins are necessary to establish the characteristic firing features of postnatal spiral ganglion neurons.

Regional differences in neurotrophin availability regulate selective expression of VGF in the developing limbic cortex

Gene and protein expression patterns in the cerebral cortex are complex and often change spatially and temporally through development. The signals that regulate these patterns are primarily unknown. In the present study, we focus on the regulation of VGF expression, which is limited to limbic cortical areas early in development but later expands into sensory and motor areas. We isolated neurons from embryonic day 17 rat cortex and demonstrate that the profile of VGF expression in perirhinal (expressing) and occipital (nonexpressing) populations in vitro is similar to that in the perinatal cortex in vivo. The addition of neutralizing neurotrophin antibodies indicates that endogenous brain-derived neurotrophic factor (BDNF) is necessary for the normal complement of VGF-expressing neurons in the perirhinal cortex, although endogenous neurotrophin-3 (NT-3) regulates the expression of VGF in a subpopulation of cells. ELISA analysis demonstrates that there is significantly more BDNF present in the perirhinal cortex compared with the occipital cortex in the perinatal period. However, the total amount of NT-3 is similar between the two regions and, moreover, there is considerably more NT-3 than BDNF in both areas, a finding seemingly in conflict with regional VGF expression. Quantification of the extracellular levels of neurotrophins in perirhinal and occipital cultures using ELISA in situ analysis indicates that perirhinal neurons release significantly more BDNF than the occipital population. Furthermore, the amount of NT-3 released by the perirhinal neurons is significantly less than the amount of BDNF. Local injection of BDNF in vivo into a normally negative VGF region results in robust ectopic expression of VGF. These data suggest that the local availability of specific neurotrophins for receptor occupation, rather than the total amount of neurotrophin, is a critical parameter in determining the selective expression of VGF in the developing limbic cortex.

Cloning, tissue expression pattern and genomic organization of latexin, a human homologue of rat carboxypeptidase A inhibitor

Latexin, a carboxypeptidase A inhibitor, is expressed in a cell type-specific manner in both central and peripheral nervous systems in the rat. It is used as a molecular marker for the regional specification of the neocortex. In this study, a cDNA was isolated from a human fetal brain cDNA library. The cDNA (LXN) contains an open reading frame encoding 222 amino acids. The comparison between the deduced amino acid sequences of LXN and latexins of rat and mouse revealed high sequence identity (84.2 and 84.7%, respectively). Northern blot analysis showed that LXN was expressed as a transcript of 1.3 kb in 15 out of 16 tissues examined, except in peripheral blood leukocyte. The expression levels were high in heart, prostate, ovary, kidney, pancreas, and colon, moderate or low in other tissues including brain. It is noteworthy that the tissue distribution of human LXN differs greatly to that of its homologue in the model animal, rat latexin. In addition, the LXN gene contains at least 6 exons and spans 5.9 kb according to the genomic sequence of the clone RP11-79M21 and the gap sequence cloned in this paper. LXN was assigned to 3q25-q26.2 according to the position of the marker SHGC-35682 found adjacent to LXN gene.

Ephrins stimulate neurite outgrowth during early cortical neurogenesis

The Eph receptor ligands, the ephrins, are membrane-bound molecules that play important roles in establishing intercellular communication after neurogenesis by regulating cell migration, axon pathfinding, and topographic mapping. In diverse systems, such as embryonic day 17.5 (E17.5) hippocampal and cortical neurons, repulsive/inhibitory mechanisms underlie these cellular effects. However, although ligand/receptor expression occurs far earlier, during brain neurogenesis, little is known about potential ephrin functions in initial process outgrowth. We have examined ligand/receptor expression in E13.5 cortex in vivo and in culture, using alkaline phosphatase (AP)-conjugated reagents and RNase protection assay. B ephrins are highly expressed, including B1, B2, and B3, whereas A ephrins exhibit low expression levels. In contrast, the Eph receptors demonstrate an opposite pattern, exhibiting high levels of Eph A3, A4, and A5 mRNA transcripts and low levels of the B-class receptors. To examine effects on neurite outgrowth, soluble ephrins were incubated with antihuman IgG antibody, producing oligomeric agonist complexes, and dried onto culture dishes. Unexpectedly, both ephrin A and B complexes increased process outgrowth: Seventy to eighty percent of neuronal precursors exhibited long neurites on ephrins, whereas only 5-10% of cells had neurites on IgG control substrates, indicating that ephrins stimulated neuritogenesis by early cortical neurons. These observations suggest that ephrin ligand/receptor systems play ontogenetic roles not previously considered, activating mechanisms other than cellular repulsion. Ephrin systems may induce initial process elaboration by early cortical neurons that is restricted at later stages by well-characterized repulsive signaling mechanisms.

Neocortex patterning by the secreted signaling molecule FGF8

A classic model proposes that the mammalian neocortex is divided into areas early in neurogenesis, but the molecular mechanisms that generate the area map have been elusive. Here we provide evidence that FGF8 regulates development of the map from a source in the anterior telencephalon. Using electroporation-mediated gene transfer in mouse embryos, we show that augmenting the endogenous anterior FGF8 signal shifts area boundaries posteriorly, reducing the signal shifts them anteriorly, and introducing a posterior source of FGF8 elicits partial area duplications, revealed by ectopic somatosensory barrel fields. These findings support a role for FGF signaling in specifying positional identity in the neocortex.

Gene expression differences in bipolar disorder revealed by cDNA array analysis of post-mortem frontal cortex

Previous studies have implicated a number of biochemical pathways in the etiology of bipolar disorder (BD). However, the precise abnormalities underlying this disorder remain to be established. To investigate novel factors that may be important in the pathophysiology of BD, we utilized cDNA expression arrays to examine differences in expression of up to 1200 genes known to be involved in potentially relevant biochemical processes. This investigation was undertaken in post-mortem samples of frontal cortex tissue from patients with BD and matched controls, obtained (n = 10/group) from the Stanley Foundation Neuropathology Consortium. Results include significant (greater than 35% change in signal intensity) differences between BD and controls in a number of genes (n = 24). Selected targets were analyzed by RT-PCR, which confirmed a decrease in transforming growth factor-beta1 (TGF-beta 1), and an increase in both caspase-8 precursor (casp-8) and transducer of erbB2 (Tob) expression in BD. We further observed a significant decrease of TGF-beta 1 mRNA levels in BD by RT-PCR in individual post-mortem samples. Given the neuroprotective role attributed to this inhibitory cytokine, our results suggest that the down-regulation of TGF-beta 1 may lead to various neurotoxic insults potentially involved in the etiology of certain mood disorders.