Cortical upper layer neurons derive from the subventricular zone as indicated bySvet1gene expression

COUP-TFI promotes radial migration and proper morphology of callosal projection neurons by repressing Rnd2 expression

During corticogenesis, late-born callosal projection neurons (CPNs) acquire their laminar position through glia-guided radial migration and then undergo final differentiation. However, the mechanisms controlling radial migration and final morphology of CPNs are poorly defined. Here, we show that in COUP-TFI mutant mice CPNs are correctly specified, but are delayed in reaching the cortical plate and have morphological defects during migration. Interestingly, we observed that the rate of neuronal migration to the cortical plate normally follows a low-rostral to high-caudal gradient, similar to that described for COUP-TFI. This gradient is strongly impaired in COUP-TFI(-/-) brains. Moreover, the expression of the Rho-GTPase Rnd2, a modulator of radial migration, is complementary to both these gradients and strongly increases in the absence of COUP-TFI function. We show that COUP-TFI directly represses Rnd2 expression at the post-mitotic level along the rostrocaudal axis of the neocortex. Restoring correct Rnd2 levels in COUP-TFI(-/-) brains cell-autonomously rescues neuron radial migration and morphological transitions. We also observed impairments in axonal elongation and dendritic arborization of COUP-TFI-deficient CPNs, which were rescued by lowering Rnd2 expression levels. Thus, our data demonstrate that COUP-TFI modulates late-born neuron migration and favours proper differentiation of CPNs by finely regulating Rnd2 expression levels.

FLRT2 and FLRT3 act as repulsive guidance cues for Unc5-positive neurons

Netrin-1 induces repulsive axon guidance by binding to the mammalian Unc5 receptor family (Unc5A-Unc5D). Mouse genetic analysis of selected members of the Unc5 family, however, revealed essential functions independent of Netrin-1, suggesting the presence of other ligands. Unc5B was recently shown to bind fibronectin and leucine-rich transmembrane protein-3 (FLRT3), although the relevance of this interaction for nervous system development remained unclear. Here, we show that the related Unc5D receptor binds specifically to another FLRT protein, FLRT2. During development, FLRT2/3 ectodomains (ECDs) are shed from neurons and act as repulsive guidance molecules for axons and somata of Unc5-positive neurons. In the developing mammalian neocortex, Unc5D is expressed by neurons in the subventricular zone (SVZ), which display delayed migration to the FLRT2-expressing cortical plate (CP). Deletion of either FLRT2 or Unc5D causes a subset of SVZ-derived neurons to prematurely migrate towards the CP, whereas overexpression of Unc5D has opposite effects. Hence, the shed FLRT2 and FLRT3 ECDs represent a novel family of chemorepellents for Unc5-positive neurons and FLRT2/Unc5D signalling modulates cortical neuron migration.

Hierarchical temporal processing deficit model of reality distortion and psychoses

We posit in this article that hierarchical temporal processing deficit is the underlying basis of reality distortion and psychoses. Schizophrenia is a prototypical reality distortion disorder in which the patient manifests with auditory hallucinations, delusions, disorganized speech and thinking, cognitive impairment, avolition and social and occupational dysfunction. Reality distortion can be present in many other disorders including bipolar disorder, major depression and even dementia. Conceptually, schizophrenia is a heterogeneous entity likely to be because of numerous causes similar to dementia. Although no single symptom or set of symptoms is pathognomonic, a cardinal feature in all patients with schizophrenia is chronic distortion of reality. The model that we have proposed accounts for the varied manifestations of reality distortion including hallucinations and delusions. In this paper we consider the implications of this model for the underlying biology of psychoses and also for the neurobiology of schizophrenia and suggest potential targets to consider for the etiology and pathophysiology of reality distortion, especially in the context of schizophrenia.

Genetic and developmental homology in amniote brains. Toward conciliating radical views of brain evolution

The six-layered neocortex is both a unique and a universal character of mammals. Historically, a major concern has been to determine its phylogenetic origins by establishing which structures, if any, correspond to it in the brains of other vertebrates. Two opposing hypotheses have been debated in the last years: (i) the neocortex arises entirely from the dorsal hemisphere of ancestral reptiles, and (ii) a large portion of it originates in the lateral hemisphere, from a structure termed the dorsal ventricular ridge (DVR), which expands significantly in reptiles and especially in birds. While developmental and genetic evidence strongly favors a dorsal origin of the neocortex, there are important similarities in the sensory connectivity to the neocortex and to the DVR, and more recently, in the phenotype of late-produced elements in both structures. It is proposed that, despite originating in different embryonic domains, the proliferative expansion of both the mammalian neocortex and the sauropsidian DVR is partly based on the amplification of similar developmental programs, possibly dependent on Pax6 activity or of related cascades that promote progenitor proliferation. While Pax6 activity is already present in the amphibian pallium, I propose that at some point(s) in amniote evolution it has been upregulated yielding brain expansion in both sauropsids and mammals. However, in the latter there has been an additional dorsalizing influence contributing to the development of the neocortex and restricting the expansion of the lateral hemisphere. Finally, a significant contribution to neocortical origins by anterior signaling centers secreting FGFs is suggested, by virtue of their association to olfactory development and their cortical patterning functions. This perspective fits a dynamical view of brain homology, where instead of searching for a one-to-one correspondence between components, emphasis is placed on changes in the modulation of conserved signaling centers and their corresponding morphogen gradients across species.

Development, specification, and diversity of callosal projection neurons

Callosal projection neurons (CPN) are a diverse population of neocortical projection neurons that connect the two hemispheres of the cerebral cortex via the corpus callosum. They play key roles in high-level associative connectivity, and have been implicated in cognitive syndromes of high-level associative dysfunction, such as autism spectrum disorders. CPN evolved relatively recently compared to other cortical neuron populations, and have undergone disproportionately large expansion from mouse to human. While much is known about the anatomical trajectory of developing CPN axons, and progress has been made in identifying cellular and molecular controls over midline crossing, only recently have molecular-genetic controls been identified that specify CPN populations, and help define CPN subpopulations. In this review, we discuss the development, diversity and evolution of CPN.

Induction of Toll-like receptor 3-mediated immunity during gestation inhibits cortical neurogenesis and causes behavioral disturbances

Maternal infection during pregnancy with a wide range of RNA and DNA viruses is associated with increased risk for schizophrenia and autism in their offspring. A common feature in these exposures is that virus replication induces innate immunity through interaction with Toll-like receptors (TLRs). We employed a mouse model wherein pregnant mice were exposed to polyinosinic-polycytidylic acid [poly(I  ⋅  C)], a synthetic, double-stranded RNA molecular mimic of replicating virus. Poly(I ⋅ C) inhibited embryonic neuronal stem cell replication and population of the superficial layers of the neocortex by neurons. Poly(I ⋅ C) also led to impaired neonatal locomotor development and abnormal sensorimotor gating responses in adult offspring. Using Toll-like receptor 3 (TLR3)-deficient mice, we established that these effects were dependent on TLR3. Inhibition of stem cell proliferation was also abrogated by pretreatment with the nonsteroidal anti-inflammatory drug (NSAID) carprofen, a cyclooxygenase (COX) inhibitor. Our findings provide insights into mechanisms by which maternal infection can induce subtle neuropathology and behavioral dysfunction, and they may suggest strategies for reducing the risk of neuropsychiatric disorders subsequent to prenatal exposures to pathogens and other triggers of innate immunity.

Maternal infection during gestation increases the risk of neuropsychiatric disorders in their offspring. Furthermore, work in animal models indicates that pre- or neonatal infections with a wide range of viruses results in similar neurodevelopmental outcomes. These observations are consistent with a mechanism whereby damage is mediated through common pathways. Exposure of pregnant mice to polyinosinic-polycytidylic acid [poly(I ⋅ C)], a synthetic, double-stranded RNA (dsRNA) molecular mimic of replicating virus, inhibited embryonic neuronal stem cell replication and led to behavioral abnormalities in their offspring. These effects were mediated through TLR3 and abrogated by pretreatment with the nonsteroidal anti-inflammatory drug (NSAID) carprofen. Our findings provide insights into mechanisms by which maternal infection can induce subtle neuropathology and may suggest strategies for reducing the risk of neuropsychiatric diseases following exposures to infectious agents and other triggers of innate immunity during gestation.

Beta-catenin signaling negatively regulates intermediate progenitor population numbers in the developing cortex

Intermediate progenitor cells constitute a second proliferative cell type in the developing mammalian cerebral cortex. Little is known about the factors that govern the production of intermediate progenitors. Although persistent expression of stabilized β-catenin was found to delay the maturation of radial glial progenitors into intermediate progenitors, the relationship between β-catenin signaling and intermediate progenitors remains poorly understood. Using a transgenic reporter mouse for Axin2, a direct target of Wnt/β-catenin signaling, we observed that β-catenin signaling is decreased in intermediate progenitor cells relative to radial glial progenitors. Conditional deletion of β-catenin from mouse cortical neural progenitors increased intermediate progenitor numbers, while conditional expression of stabilized β-catenin reduced the intermediate progenitor population. Together, these findings provide evidence that β-catenin signaling in radial progenitors negatively regulates intermediate progenitor cell number during cortical development.

Molecular analysis of neocortical layer structure in the ferret

Molecular markers that distinguish specific layers of rodent neocortex are increasingly employed to study cortical development and the physiology of cortical circuits. The extent to which these markers represent general features of neocortical cell type identity across mammals, however, is unknown. To assess the conservation of layer markers more broadly, we isolated orthologs for 15 layer-enriched genes in the ferret, a carnivore with a large, gyrencephalic brain, and analyzed their patterns of neocortical gene expression. Our major findings are: 1) Many but not all layer markers tested show similar patterns of layer-specific gene expression between mouse and ferret cortex, supporting the view that layer-specific cell type identity is conserved at a molecular level across mammalian superorders; 2) Our panel of deep layer markers (ER81/ETV1, SULF2, PCP4, FEZF2/ZNF312, CACNA1H, KCNN2/SK2, SYT6, FOXP2, CTGF) provides molecular evidence that the specific stratifications of layers 5 and 6 into 5a, 5b, 6a, and 6b are also conserved between rodents and carnivores; 3) Variations in layer-specific gene expression are more pronounced across areas of ferret cortex than between homologous areas of mouse and ferret cortex; 4) This variation of area gene expression was clearest with the superficial layer markers studied (SERPINE2, MDGA1, CUX1, UNC5D, RORB/NR1F2, EAG2/KCNH5). Most dramatically, the layer 4 markers RORB and EAG2 disclosed a molecular sublamination to ferret visual cortex and demonstrated a molecular dissociation among the so-called agranular areas of the neocortex. Our findings establish molecular markers as a powerful complement to cytoarchitecture for neocortical layer and cell-type comparisons across mammals.

The generation of superficial cortical layers is regulated by levels of the transcription factor Pax6

The ventricular zone (VZ) of the embryonic dorsal telencephalon is a major site for generating cortical projection neurons. The transcription factor Pax6 is highly expressed in apical progenitors (APs) residing in the VZ from the earliest stages of corticogenesis. Previous studies mainly focused on Pax6(-/-) mice have implicated Pax6 in regulating cortical progenitor proliferation, neurogenesis, and formation of superficial cortical layers. We analyzed the developing cortex of PAX77 transgenic mice that overexpress Pax6 in its normal domains of expression. We show that Pax6 overexpression increases cell cycle length of APs and drives the system toward neurogenesis. These effects are specific to late stages of corticogenesis, when superficial layer neurons are normally generated, in cortical regions that express Pax6 at the highest levels. The number of superficial layer neurons is reduced in postnatal PAX77 mice, whereas radial migration and lamina specification of cortical neurons are not affected by Pax6 overexpression. Conditional deletion of Pax6 in cortical progenitors at midstages of corticogenesis, by using a tamoxifen-inducible Emx1-CreER line, affected both numbers and specification of late-born neurons in superficial layers of the mutant cortex. Our analyses suggest that correct levels of Pax6 are essential for normal production of superficial layers of the cortex.

Wnt signaling and its downstream target N-myc regulate basal progenitors in the developing neocortex

Basal progenitors (also called non-surface dividing or intermediate progenitors) have been proposed to regulate the number of neurons during neocortical development through expanding cells committed to a neuronal fate, although the signals that govern this population have remained largely unknown. Here, we show that N-myc mediates the functions of Wnt signaling in promoting neuronal fate commitment and proliferation of neural precursor cells in vitro. Wnt signaling and N-myc also contribute to the production of basal progenitors in vivo. Expression of a stabilized form of beta-catenin, a component of the Wnt signaling pathway, or of N-myc increased the numbers of neocortical basal progenitors, whereas conditional deletion of the N-myc gene reduced these and, as a likely consequence, the number of neocortical neurons. These results reveal that Wnt signaling via N-myc is crucial for the control of neuron number in the developing neocortex.

Af9/Mllt3 interferes with Tbr1 expression through epigenetic modification of histone H3K79 during development of the cerebral cortex

Mutations of leukemia-associated AF9/MLLT3 are implicated in neurodevelopmental diseases, such as epilepsy and ataxia, but little is known about how AF9 influences brain development and function. Analyses of mouse mutants revealed that during cortical development, AF9 is involved in the maintenance of TBR2-positive progenitors (intermediate precursor cells, IPCs) in the subventricular zone and prevents premature cell cycle exit of IPCs. Furthermore, in postmitotic neurons of the developing cortical plate, AF9 is implicated in the formation of the six-layered cerebral cortex by suppressing a TBR1-positive cell fate mainly in upper layer neurons. We show that the molecular mechanism of TBR1 suppression is based on the interaction of AF9 with DOT1L, a protein that mediates transcriptional control through methylation of histone H3 lysine 79 (H3K79). AF9 associates with the transcriptional start site of Tbr1, mediates H3K79 dimethylation of the Tbr1 gene, and interferes with the presence of RNA polymerase II at the Tbr1 transcriptional start site. AF9 expression favors cytoplasmic localization of TBR1 and its association with mitochondria. Increased expression of TBR1 in Af9 mutants is associated with increased levels of TBR1-regulated expression of NMDAR subunit Nr1. Thus, this study identified AF9 as a developmental active epigenetic modifier during the generation of cortical projection neurons.

The glial nature of embryonic and adult neural stem cells

Glial cells were long considered end products of neural differentiation, specialized supportive cells with an origin very different from that of neurons. New studies have shown that some glial cells--radial glia (RG) in development and specific subpopulations of astrocytes in adult mammals--function as primary progenitors or neural stem cells (NSCs). This is a fundamental departure from classical views separating neuronal and glial lineages early in development. Direct visualization of the behavior of NSCs and lineage-tracing studies reveal how neuronal lineages emerge. In development and in the adult brain, many neurons and glial cells are not the direct progeny of NSCs, but instead originate from transit amplifying, or intermediate, progenitor cells (IPCs). Within NSCs and IPCs, genetic programs unfold for generating the extraordinary diversity of cell types in the central nervous system. The timing in development and location of NSCs, a property tightly linked to their neuroepithelial origin, appear to be the key determinants of the types of neurons generated. Identification of NSCs and IPCs is critical to understand brain development and adult neurogenesis and to develop new strategies for brain repair.

AP2gamma regulates basal progenitor fate in a region- and layer-specific manner in the developing cortex

An important feature of the cerebral cortex is its layered organization, which is modulated in an area-specific manner. We found that the transcription factor AP2gamma regulates laminar fate in a region-specific manner. Deletion of AP2gamma (also known as Tcfap2c) during development resulted in a specific reduction of upper layer neurons in the occipital cortex, leading to impaired function and enhanced plasticity of the adult visual cortex. AP2gamma functions in apical progenitors, and its absence resulted in mis-specification of basal progenitors in the occipital cortex at the time at which upper layer neurons were generated. AP2gamma directly regulated the basal progenitor fate determinants Math3 (also known as Neurod4) and Tbr2, and its overexpression promoted the generation of layer II/III neurons in a time- and region-specific manner. Thus, AP2gamma acts as a regulator of basal progenitor fate, linking regional and laminar specification in the mouse developing cerebral cortex.

Neocortical neurogenesis: morphogenetic gradients and beyond

Each of the five cellular layers of the cerebral neocortex is composed of a specific number of a single predominant 'class' of projection neuron. The projection neuron class is defined by its unique morphology and axonal projections to other areas of the brain. Precursor cell populations lining the embryonic lateral ventricles produce the projection neurons. The mechanisms regulating precursor cell proliferation also regulate total numbers of neurons produced at specific developmental periods and destined to a specific neocortical layer. Because the newborn neurons migrate relatively long distances to reach their final layer destinations, it is often assumed that the mechanisms governing acquisition of neuronal-class-specific characteristics, many of which become evident after neuron production, are independent of the mechanisms governing neuron production. We review evidence that suggests that the two mechanisms might be linked via operations of Notch1 and p27(Kip1), molecules known to regulate precursor cell proliferation and neuron production.

Minicolumnar width: Comparison between supragranular and infragranular layers

The minicolumn derives from the radial migration of neurons along glial scaffoldings during gestation. Investigators have presumed the minicolumn to be a single-cell wide structure based on their rectilinear migratory origin. The present study measures the width of minicolumnar cores in both supra- and infra-granular layers. Postmortem tissue was obtained from 9 brain areas in 7 normative individuals. Examined tissues were celloidin embedded and Nissl stained. Digital images were denoised and then analyzed with a step-wise algorithm involving region growing and recursive line tracing. Significant differences were noted between the minicolumnar core widths of supra- and infra-granular layers. A review of the literature on corticogenesis provides some ideas as to how these laminar differences in minicolumnar core width are engendered.

The transcriptional repressor RP58 is crucial for cell-division patterning and neuronal survival in the developing cortex

The neocortex and the hippocampus comprise several specific layers containing distinct neurons that originate from progenitors at specific development times, under the control of an adequate cell-division patterning mechanism. Although many molecules are known to regulate this cell-division patterning process, its details are not well understood. Here, we show that, in the developing cerebral cortex, the RP58 transcription repressor protein was expressed both in postmitotic glutamatergic projection neurons and in their progenitor cells, but not in GABAergic interneurons. Targeted deletion of the RP58 gene led to dysplasia of the neocortex and of the hippocampus, reduction of the number of mature cortical neurons, and defects of laminar organization, which reflect abnormal neuronal migration within the cortical plate. We demonstrate an impairment of the cell-division patterning during the late embryonic stage and an enhancement of apoptosis of the postmitotic neurons in the RP58-deficient cortex. These results suggest that RP58 controls cell division of progenitor cells and regulates the survival of postmitotic cortical neurons.

The level of the transcription factor Pax6 is essential for controlling the balance between neural stem cell self-renewal and neurogenesis

Neural stem cell self-renewal, neurogenesis, and cell fate determination are processes that control the generation of specific classes of neurons at the correct place and time. The transcription factor Pax6 is essential for neural stem cell proliferation, multipotency, and neurogenesis in many regions of the central nervous system, including the cerebral cortex. We used Pax6 as an entry point to define the cellular networks controlling neural stem cell self-renewal and neurogenesis in stem cells of the developing mouse cerebral cortex. We identified the genomic binding locations of Pax6 in neocortical stem cells during normal development and ascertained the functional significance of genes that we found to be regulated by Pax6, finding that Pax6 positively and directly regulates cohorts of genes that promote neural stem cell self-renewal, basal progenitor cell genesis, and neurogenesis. Notably, we defined a core network regulating neocortical stem cell decision-making in which Pax6 interacts with three other regulators of neurogenesis, Neurog2, Ascl1, and Hes1. Analyses of the biological function of Pax6 in neural stem cells through phenotypic analyses of Pax6 gain- and loss-of-function mutant cortices demonstrated that the Pax6-regulated networks operating in neural stem cells are highly dosage sensitive. Increasing Pax6 levels drives the system towards neurogenesis and basal progenitor cell genesis by increasing expression of a cohort of basal progenitor cell determinants, including the key transcription factor Eomes/Tbr2, and thus towards neurogenesis at the expense of self-renewal. Removing Pax6 reduces cortical stem cell self-renewal by decreasing expression of key cell cycle regulators, resulting in excess early neurogenesis. We find that the relative levels of Pax6, Hes1, and Neurog2 are key determinants of a dynamic network that controls whether neural stem cells self-renew, generate cortical neurons, or generate basal progenitor cells, a mechanism that has marked parallels with the transcriptional control of embryonic stem cell self-renewal.

Distinct roles of neuropilin 1 signaling for radial and tangential extension of callosal axons

Cortical excitatory neurons migrate from their origin in the ventricular zone (VZ) toward the pial surface. During migration, these neurons exhibit a stellate shape in the intermediate zone (IZ), transform into bipolar cells, and then initiate radial migration, extending a trailing process, which may lead to an axon. Here we examined the role of neuropilin 1 (NRP1) in these developmental events. Both NRP1 mRNA and protein were highly expressed in the IZ, where stellate-shaped cells were located. DiI labeling experiments showed that neuronal migration occurred normally in Nrp1 mutant mice up to embryonic day (E) 14.5, the latest day to which the mutant survives, with only subtle axonal defasciculation. However, interference with Nrp1 signaling at a later stage caused pathfinding errors: when a dominant negative form of Nrp1 was electroporated into the cortical VZ cells at E12.5 or E15.5 and examined perinatally, guidance errors were found in tangential axonal extension toward the midline. In contrast, no significant effect was noted on the migration of cortical excitatory neurons. These findings indicate that NRP1 plays an important role in the guidance of callosal axons originating from cortical excitatory neurons but does not support a role in their migration. Moreover, insofar as radial axonal extension within the cortical plate was unaffected, the present findings imply that molecular mechanisms for the axonal extension of excitatory neurons within the cortical plate are distinct from those in the white matter.

Molecular mechanisms of projection neuron production and maturation in the developing cerebral cortex

The cerebral cortex is a brain structure unique to mammals and highly adapted to process complex information. Through multiple developmental steps, the cerebral cortex is assembled as a huge diversity of neurons comprising a complex laminar structure, and with both local and long-distance connectivity within the nervous system. Key processes must take place during its construction, including: (i) regulation of the correct number of neurons produced by progenitor cells, (ii) temporal and spatial generation of neuronal diversity, and (iii) control of neuron migration and laminar positioning as well as terminal differentiation within the mature cortex. Here, we seek to highlight recent cellular and molecular findings underlying these sequential steps of neurogenesis, cell fate specification and migration during cortical development, with particular emphasis on cortical projection neurons.

A stem cell niche for intermediate progenitor cells of the embryonic cortex

The excitatory neurons of the mammalian cerebral cortex arise from asymmetric divisions of radial glial cells in the ventricular zone and symmetric division of intermediate progenitor cells (IPCs) in the subventricular zone (SVZ) of the embryonic cortex. Little is known about the microenvironment in which IPCs divide or whether a stem cell niche exists in the SVZ of the embryonic cortex. Recent evidence suggests that vasculature may provide a niche for adult stem cells but its role in development is less clear. We have investigated the vasculature in the embryonic cortex during neurogenesis and find that IPCs are spatially and temporally associated with blood vessels during cortical development. Intermediate progenitors mimic the pattern of capillaries suggesting patterns of angiogenesis and neurogenesis are coordinated during development. More importantly, we find that IPCs divide near blood vessel branch points suggesting that cerebral vasculature establishes a stem cell niche for intermediate progenitors in the SVZ. These data provide novel evidence for the presence of a neurogenic niche for intermediate progenitors in the embryonic SVZ and suggest blood vessels are important for proper patterning of neurogenesis.

In vivo MRI of altered brain anatomy and fiber connectivity in adult pax6 deficient mice

The impact of developmental ablation of Pax6 function on morphology and functional connectivity of the adult cerebrum was studied in cortex-specific Pax6 knockout mice (Pax6cKO) using structural magnetic resonance imaging (MRI), manganese-enhanced MRI, and diffusion tensor MRI in conjunction with fiber tractography. Mutants presented with decreased volumes of total brain and olfactory bulb, reduced cortical thickness, and altered layering of the piriform cortex. Tracking of major neuronal fiber bundles revealed a disorganization of callosal fibers with an almost complete lack of interhemispheric connectivity. In Pax6cKO mice intrahemispheric callosal fibers as well as intracortical fibers were predominantly directed along a rostrocaudal orientation instead of a left-right and dorsoventral orientation found in controls. Fiber disorganization also involved the septohippocampal connection targeting mostly the lateral septal nucleus. The hippocampus was rostrally extended and its volume was increased relative to that of the forebrain and midbrain. Manganese-induced MRI signal enhancement in the CA3 region suggested a normal function of hippocampal pyramidal cells. Noteworthy, several morphologic disturbances in gray and white matter of Pax6cKO mice were similar to observations in human aniridia patients. The present findings indicate an important role of Pax6 in the development of both the cortex and cerebral fiber connectivity.

Intermediate neuronal progenitors (basal progenitors) produce pyramidal-projection neurons for all layers of cerebral cortex

The developing cerebral cortex contains apical and basal types of neurogenic progenitor cells. Here, we investigated the cellular properties and neurogenic output of basal progenitors, also called intermediate neuronal progenitors (INPs). We found that basal mitoses expressing transcription factor Tbr2 (an INP marker) were present throughout corticogenesis, from embryonic day 10.5 through birth. Postnatally, Tbr2(+) progenitors were present in the dentate gyrus, subventricular zone (SVZ), and posterior periventricle (pPV). Two morphological subtypes of INPs were distinguished in the embryonic cortex, "short radial" in the ventricular zone (VZ) and multipolar in the SVZ, probably corresponding to molecularly defined INP subtypes. Unexpectedly, many short radial INPs appeared to contact the apical (ventricular) surface and some divided there. Time-lapse video microscopy suggested that apical INP divisions produced daughter INPs. Analysis of neurogenic divisions (Tis21-green fluorescent protein [GFP](+)) indicated that INPs may produce the majority of projection neurons for preplate, deep, and superficial layers. Conversely, proliferative INP divisions (Tis21-GFP(-)) increased from early to middle corticogenesis, concomitant with SVZ growth. Our findings support the hypothesis that regulated amplification of INPs may be an important factor controlling the balance of neurogenesis among different cortical layers.

Differences of migratory behavior between direct progeny of apical progenitors and basal progenitors in the developing cerebral cortex

Cerebral cortical neurons are known to be produced from both apical progenitors in the ventricular zone (VZ) and basal (intermediate) progenitors in the subventricular zone (SVZ). On the other hand, we have shown that many SVZ cells assume multipolar morphology and show a characteristic movement termed "multipolar migration." The relationship between multipolar cells and basal progenitors in the SVZ has yet to be investigated. Herein, we followed postmitotic cells generated in the VZ and found that they stayed for more than 10 h in the VZ after becoming postmitotic and then accumulated in the lower part of the SVZ (multipolar cell accumulation zone: MAZ) as multipolar cells (slowly exiting population: SEP), whereas basal progenitors rapidly migrated into the SVZ or intermediate zone (IZ) (rapidly exiting population: REP) with somal translocation morphology. Although REP reached the SVZ/IZ earlier than the SEP, REP stayed within in the SVZ/IZ, whereas SEP moved steadily and entered the CP prior to the REP. We also observed SEP to eventually differentiate into pyramidal neurons in layers II/III. This study provides in vivo evidence of differences in migration modes between postmitotic cells generated from apical progenitors and basal progenitors.

Cortex shatters the glass ceiling

Recreating developmental structures in vitro has been a primary challenge for stem cell biologists. Recent studies in Cell Stem Cell (Eiraku et al., 2008) and Nature (Gaspard et al., 2008) demonstrate that embryonic stem cells can recapitulate early cortical development, enabling them to generate specific cortical subtypes.

Tbr2 directs conversion of radial glia into basal precursors and guides neuronal amplification by indirect neurogenesis in the developing neocortex

T-brain gene-2 (Tbr2) is specifically expressed in the intermediate (basal) progenitor cells (IPCs) of the developing cerebral cortex; however, its function in this biological context has so far been overlooked due to the early lethality of Tbr2 mutant embryos. Conditional ablation of Tbr2 in the developing forebrain resulted in the loss of IPCs and their differentiated progeny in mutant cortex. Intriguingly, early loss of IPCs led to a decrease in cortical surface expansion and thickness with a neuronal reduction observed in all cortical layers. These findings suggest that IPC progeny contribute to the correct morphogenesis of each cortical layer. Our observations were confirmed by tracing Tbr2+ IPC cell fate using Tbr2::GFP transgenic mice. Finally, we demonstrated that misexpression of Tbr2 is sufficient to induce IPC identity in ventricular radial glial cells (RGCs). Together, these findings identify Tbr2 as a critical factor for the specification of IPCs during corticogenesis.

Expression of Leukaemia associated transcription factor Af9/Mllt3 in the cerebral cortex of the mouse

Mutations of leukaemia associated AF9/MLLT3 are implicated in neurodevelopmental diseases such as epilepsia and ataxia. This study shows for the first time, that murine Af9 is transcribed in various CNS structures including the subventricular zone (SVZ) of the cerebral cortex, hippocampus, cerebellar cortex, septum and various thalamic structures, the choroid plexus, and the midbrain/hindbrain boundary. Expression of Af9 in the SVZ overlaps with Svet1, Cux2, and partially with Tbr2, confining its activity to the neurogenic compartment of the SVZ. In contrast to Svet1 and Cux2 expression, Af9 transcription is not limited to upper layer neurons but is found in the entire cortical plate. As part of an extensive network of interacting proteins involved in epigenetic DNA modification, we could show overlapping expression of Af9 with Af4/Aff1 and Fmr2/Aff2, two genes that are also related to neurodevelopmental diseases, as well as with the highly homologous Enl.

The T-box transcription factor Eomes/Tbr2 regulates neurogenesis in the cortical subventricular zone

The embryonic subventricular zone (SVZ) is a critical site for generating cortical projection neurons; however, molecular mechanisms regulating neurogenesis specifically in the SVZ are largely unknown. The transcription factor Eomes/Tbr2 is transiently expressed in cortical SVZ progenitor cells. Here we demonstrate that conditional inactivation of Tbr2 during early brain development causes microcephaly and severe behavioral deficits. In Tbr2 mutants the number of SVZ progenitor cells is reduced and the differentiation of upper cortical layer neurons is disturbed. Neurogenesis in the adult dentate gyrus but not the subependymal zone is abolished. These studies establish Tbr2 as a key regulator of neurogenesis in the SVZ.

DCX and PSA-NCAM expression identifies a population of neurons preferentially distributed in associative areas of different pallial derivatives and vertebrate species

In adult rodents, doublecortin (DCX) and polysialylated neural cell adhesion molecule (PSA-NCAM) expression is mostly restricted to newly generated neurons. These molecules have also been described in prenatally generated cells of the piriform cortex and, to a lesser extent, neocortex (NC) of the rat. In addition, PSA-NCAM+ cells have been identified in several telencephalic regions of the lizard. Here, through immunohistochemistry and 3-dimensional reconstruction, we have investigated distribution, morphology, and phenotype of DCX/PSA-NCAM-expressing cells in the pallium of different mammals and in lizard. In all species, a population of nonnewly-generated pallial DCX+/PSA-NCAM+ cells shows common morphological and phenotypic characteristics, including expression of Tbr-1, a transcription factor expressed in pallial projection neurons, and preferential distribution in associative areas. In the guinea pig and rabbit, DCX+/PSA-NCAM+ elements are also abundant in the NC, particularly in areas implicated in nonspatial learning and memory networks. In reptiles, DCX+/PSA-NCAM+ cells are located in the lateral and medial cortex and dorsal ventricular ridge but not in the dorsal cortex. These data support the fact that coexpression of DCX+/PSA-NCAM+/Tbr-1+ in the adult brain identifies evolutionary conserved cell populations shared by different pallial derivatives including the mammalian NC.

The p21-activated kinase is required for neuronal migration in the cerebral cortex

The normal formation and function of the mammalian cerebral cortex depend on the positioning of its neurones, which occurs in a highly organized, layer-specific manner. The correct morphology and movement of neurones rely on synchronized regulation of their actin filaments and microtubules. The p21-activated kinase (Pak1), a key cytoskeletal regulator, controls neuronal polarization, elaboration of axons and dendrites, and the formation of dendritic spines. However, its in vivo role in the developing nervous system is unclear. We have utilized in utero electroporation into mouse embryo cortices to reveal that both loss and gain of Pak1 function affect radial migration of projection neurones. Overexpression of hyperactivated Pak1 predominantly caused neurones to arrest in the intermediate zone (IZ) with apparently misoriented and disorganized leading projections. Loss of Pak1 disrupted the morphology of migrating neurones, which accumulated in the IZ and deep cortical layers. Unexpectedly, a significant number of neurones with reduced Pak1 expression aberrantly entered into the normally cell-sparse marginal zone, suggesting their inability to cease migrating that may be due to their impaired dissociation from radial glia. Our findings reveal the in vivo importance of temporal and spatial regulation of the Pak1 kinase during key stages of cortical development.

The role of ATP signaling in the migration of intermediate neuronal progenitors to the neocortical subventricular zone

Most neurons of the cerebral cortex are generated in the germinal zones near the embryonic cerebral ventricle and migrate radially to the overlying cortical plate. Initially, all dividing cells are attached to the surface of the embryonic ventricle (ventricular zone) until a subset of dividing cells (basal or intermediate neuronal progenitors, INPs), recognized by their immunoreactivity to Tbr2, detach from the ventricular surface and migrate a short distance to establish a secondary proliferative compartment (the subventricular zone). The mechanism that regulates migration of the Tbr2(+) INPs from the ventricular to the subventricular zones is unknown. Here, we show that INPs, unlike the postmitotic neurons that tend to lose the ATP response, continue to express the purinergic P2Y1 receptor. Furthermore, blocking ATP signaling by the P2Y1 blockers, MRS2176, suramin, and apyrase, reduces Ca(2+) transients and retards INP migration to the subventricular zone. In addition, genetic knockdown of the P2Y1 receptor by in vivo application of short hairpin RNA selectively impairs the migration of INPs to the subventricular zone. Together, these results suggest that intercellular ATP signaling is essential for the migration of INPs and the proper formation of the subventricular zone. Interference of ATP signaling or abnormal Ca(2+) fluctuations in INPs may play a significant role in variety of genetic or acquired cortical malformations.

Foxg1 haploinsufficiency reduces the population of cortical intermediate progenitor cells: effect of increased p21 expression

Foxg1 is a transcription factor that is critical for forebrain development. Foxg1(+/Cre) mice were used to test the hypotheses 1) that the subventricular zone (SZ) generates supragranular neurons, 2) that Foxg1-regulated activities define the output from the SZ, and 3) that Foxg1 is involved in the suppression of p21-initiated cell-cycle exit. Foxg1(+/Cre) mice have thinner neocortices than wild-type controls, specifically in the supragranular layers, as detected by Brn2 immunostaining. Cell proliferation in the ventricular zone (VZ) and SZ was examined to investigate the reduction in upper layer neurons. The number of cycling VZ cells was similar in Foxg1(+/+) and Foxg1(+/Cre) brains. Interestingly, cell proliferation in the SZ and intermediate progenitor cell (IPC) production (noted by Tbr2-immunostaining) was reduced in Foxg1(+/Cre) brains. These decreases coincided with increased expression of the cell-cycle inhibitor p21 in the VZ and SZ. Furthermore, colocalization of p21 with markers of cell proliferation and IPCs indicated that p21 was temporally expressed to influence the proliferative fate of IPCs. Thus, the present data are consistent with the above hypotheses, particularly, that during corticogenesis, Foxg1-regulated activities enable the expansion of the IPC population likely through suppression of p21-dependent cell-cycle exit.

Pax6 directly modulate Sox2 expression in the neural progenitor cells

Pax6 is a key regulator in the neuronal fate determination as well as the proliferation of neural stem cells, but the mechanisms are still unknown. Our study shows that Pax6 regulate the proliferation of neural progenitor cells of cortical subventricular zone, through direct modulation of the Sox2 expression during the late developmental stage in mice. We found a dramatic decrease in the number of Sox2+ neural progenitor cells in the subventricular zone of E18.5 Pax6(-/-) mice. We confirmed that Pax6 could bind to the Sox2 promoter by chromatin immunoprecipitation assay and activate Sox2 expression by a luciferase reporter gene assay. Moreover, neural progenitors isolated from the Pax6(-/-) embryos showed a decreased neurosphere formation as well as proliferation.

The determination of projection neuron identity in the developing cerebral cortex

Here we review the mechanisms that determine projection neuron identity during cortical development. Pyramidal neurons in the mammalian cerebral cortex can be classified into two major classes: corticocortical projection neurons, which are concentrated in the upper layers of the cortex, and subcortical projection neurons, which are found in the deep layers. Early progenitor cells in the ventricular zone produce deep layer neurons that express transcription factors including Sox5, Fezf2, and Ctip2, which play important roles in the specification of subcortically projecting axons. Upper layer neurons are produced from progenitors in the subventricular zone, and the expression of Satb2 in these differentiating neurons is required for the formation of axonal projections that connect the two cerebral hemispheres. The Fezf2/Ctip2 and Satb2 pathways appear to be mutually repressive, thus ensuring that individual neurons adopt either a subcortical or callosal projection neuron identity at early times during development. The molecular mechanisms by which Satb2 regulates gene expression involves long-term epigenetic changes in chromatin configuration, which may enable cell fate decisions to be maintained during development.

Satb2 regulates callosal projection neuron identity in the developing cerebral cortex

Satb2 is a DNA-binding protein that regulates chromatin organization and gene expression. In the developing brain, Satb2 is expressed in cortical neurons that extend axons across the corpus callosum. To assess the role of Satb2 in neurons, we analyzed mice in which the Satb2 locus was disrupted by insertion of a LacZ gene. In mutant mice, beta-galactosidase-labeled axons are absent from the corpus callosum and instead descend along the corticospinal tract. Satb2 mutant neurons acquire expression of Ctip2, a transcription factor that is necessary and sufficient for the extension of subcortical projections by cortical neurons. Conversely, ectopic expression of Satb2 in neural stem cells markedly decreases Ctip2 expression. Finally, we find that Satb2 binds directly to regulatory regions of Ctip2 and induces changes in chromatin structure. These data suggest that Satb2 functions as a repressor of Ctip2 and regulatory determinant of corticocortical connections in the developing cerebral cortex.

Satb2 is a postmitotic determinant for upper-layer neuron specification in the neocortex

Pyramidal neurons of the neocortex can be subdivided into two major groups: deep- (DL) and upper-layer (UL) neurons. Here we report that the expression of the AT-rich DNA-binding protein Satb2 defines two subclasses of UL neurons: UL1 (Satb2 positive) and UL2 (Satb2 negative). In the absence of Satb2, UL1 neurons lose their identity and activate DL- and UL2-specific genetic programs. UL1 neurons in Satb2 mutants fail to migrate to superficial layers and do not contribute to the corpus callosum but to the corticospinal tract, which is normally populated by DL axons. Ctip2, a gene required for the formation of the corticospinal tract, is ectopically expressed in all UL1 neurons in the absence of Satb2. Satb2 protein interacts with the Ctip2 genomic region and controls chromatin remodeling at this locus. Satb2 therefore is required for the initiation of the UL1-specific genetic program and for the inactivation of DL- and UL2-specific genes.

Distinct behaviors of neural stem and progenitor cells underlie cortical neurogenesis

Neocortical precursor cells undergo symmetric and asymmetric divisions while producing large numbers of diverse cortical cell types. In Drosophila, cleavage plane orientation dictates the inheritance of fate-determinants and the symmetry of newborn daughter cells during neuroblast cell divisions. One model for predicting daughter cell fate in the mammalian neocortex is also based on cleavage plane orientation. Precursor cell divisions with a cleavage plane orientation that is perpendicular with respect to the ventricular surface (vertical) are predicted to be symmetric, while divisions with a cleavage plane orientation that is parallel to the surface (horizontal) are predicted to be asymmetric neurogenic divisions. However, analysis of cleavage plane orientation at the ventricle suggests that the number of predicted neurogenic divisions might be insufficient to produce large amounts of cortical neurons. To understand factors that correlate with the symmetry of cell divisions, we examined rat neocortical precursor cells in situ through real-time imaging, marker analysis, and electrophysiological recordings. We find that cleavage plane orientation is more closely associated with precursor cell type than with daughter cell fate, as commonly thought. Radial glia cells in the VZ primarily divide with a vertical orientation throughout cortical development and undergo symmetric or asymmetric self-renewing divisions depending on the stage of development. In contrast, most intermediate progenitor cells divide in the subventricular zone with a horizontal orientation and produce symmetric daughter cells. We propose a model for predicting daughter cell fate that considers precursor cell type, stage of development, and the planar segregation of fate determinants.

Transient expression of the conserved zinc finger gene INSM1 in progenitors and nascent neurons throughout embryonic and adult neurogenesis

INSM1 is a zinc-finger protein expressed in the developing nervous system and pancreas as well as in medulloblastomas and neuroendocrine tumors. With in situ hybridization combined with immunohistochemistry, we detected INSM1 mRNA in all embryonic to adult neuroproliferative areas examined: embryonic neocortex, ganglionic eminence, midbrain, retina, hindbrain, and spinal cord; autonomic, dorsal root, trigeminal and spiral ganglia; olfactory and vomeronasal organ epithelia; postnatal cerebellum; and juvenile to adult subgranular zone of dentate gyrus, subventricular zone, and rostral migratory stream leading to olfactory bulb. In most of these neurogenic areas, subsets of neuronal progenitors and nascent, but not mature, neurons express INSM1. For example, in developing cerebellum, INSM1 is present in proliferating progenitors of the outer external granule layer (EGL) and in postmitotic cells of the inner EGL, but not in mature granule cell neurons. Also, lining the neural tube from spinal cord to neocortex in mouse as well as human embryos, cells undergoing mitosis apically do not express INSM1. By contrast, nonsurface progenitors located in the basal ventricular and/or subventricular zones express INSM1. Whereas apical progenitors are proliferative and generate one or two additional progenitors, basal progenitors are thought to divide terminally and symmetrically to produce two neurons. The nematode ortholog of INSM1, EGL-46, is expressed during terminal symmetric neurogenic divisions and regulates the termination of proliferation. We propose that, in mice and humans, INSM1 is likewise expressed transiently during terminal neurogenic divisions, from late progenitors to nascent neurons, and particularly during symmetric neuronogenic divisions.

Er81 is a downstream target of Pax6 in cortical progenitors

Background

Although the transcription factor Pax6 plays an essential role in neurogenesis, layer formation and arealization in the developing mammalian cortex, the mechanisms by which it accomplishes these regulatory functions are largely unknown. Pax6 and the ETS family transcription factor Er81, which is presumed to play a role in the specification of a sublineage of layer 5 projection neurons, are expressed with a prominent rostrolateral-high to caudomedial-low gradient in cortical progenitors. In the absence of functional Pax6, progenitors do not express Er81 and the rostrolateral cortex lacks Er81-positive layer 5 neurons. In this study, we investigated the transcriptional regulation of Er81 and provide evidence that Er81 is a direct target of Pax6.

Results

We identified and analyzed the regulatory function of an evolutionarily conserved upstream DNA sequence in the putative mouse Er81 promoter. Three potential Pax6 binding sites were identified in this region. We found that the presence of one of these sites is necessary and sufficient for full activation of the Er81 promoter in Pax6-transfected HeLa cells, while other still unknown factors appear to contribute to Er81 promoter activity in cortical progenitors and neuronal cells. The results suggest that endogenous Pax6, which is expressed at the highest level in progenitors of the rostrolateral cortex, exerts region-specific control of Er81 activity, thus specifying a subpopulation of layer 5 projection neurons.

Conclusion

We conclude that the genetic interplay between the transcription factors, Pax6 and Er81, is responsible, in part, for the regional specification of a distinct sublineage of layer 5 projection neurons.

Development of the human cerebral cortex: Boulder Committee revisited

In 1970 the Boulder Committee described the basic principles of the development of the CNS, derived from observations on the human embryonic cerebrum. Since then, numerous studies have significantly advanced our knowledge of the timing, sequence and complexity of developmental events, and revealed important inter-species differences. We review current data on the development of the human cerebral cortex and update the classical model of how the structure that makes us human is formed.

Prospective isolation of functionally distinct radial glial subtypes--lineage and transcriptome analysis

Since the discovery of radial glia as the source of neurons, their heterogeneity in regard to neurogenesis has been described by clonal and time-lapse analysis in vitro. However, the molecular determinants specifying neurogenic radial glia differently from radial glia that mostly self-renew remain ill-defined. Here, we isolated two radial glial subsets that co-exist at mid-neurogenesis in the developing cerebral cortex and their immediate progeny. While one subset generates neurons directly, the other is largely non-neurogenic but also gives rise to Tbr2-positive basal precursors, thereby contributing indirectly to neurogenesis. Isolation of these distinct radial glia subtypes allowed determining interesting differences in their transcriptome. These transcriptomes were also strikingly different from the transcriptome of radial glia isolated at the end of neurogenesis. This analysis therefore identifies, for the first time, the lineage origin of basal progenitors and the molecular differences of this lineage in comparison to directly neurogenic and gliogenic radial glia.

Specific expression of long noncoding RNAs in the mouse brain

A major proportion of the mammalian transcriptome comprises long RNAs that have little or no protein-coding capacity (ncRNAs). Only a handful of such transcripts have been examined in detail, and it is unknown whether this class of transcript is generally functional or merely artifact. Using in situ hybridization data from the Allen Brain Atlas, we identified 849 ncRNAs (of 1,328 examined) that are expressed in the adult mouse brain and found that the majority were associated with specific neuroanatomical regions, cell types, or subcellular compartments. Examination of their genomic context revealed that the ncRNAs were expressed from diverse places including intergenic, intronic, and imprinted loci and that many overlap with, or are transcribed antisense to, protein-coding genes of neurological importance. Comparisons between the expression profiles of ncRNAs and their associated protein-coding genes revealed complex relationships that, in combination with the specific expression profiles exhibited at both regional and subcellular levels, are inconsistent with the notion that they are transcriptional noise or artifacts of chromatin remodeling. Our results show that the majority of ncRNAs are expressed in the brain and provide strong evidence that the majority of processed transcripts with no protein-coding capacity function intrinsically as RNAs.

COUP-TFI coordinates cortical patterning, neurogenesis, and laminar fate and modulates MAPK/ERK, AKT, and beta-catenin signaling

A major unsolved question in cortical development is how proliferation, neurogenesis, regional growth, regional identity, and laminar fate specification are coordinated. Here we provide evidence, using loss-of-function and gain-of-function manipulations, that the COUP-TFI orphan nuclear receptor promotes ventral cortical fate, promotes cell cycle exit and neural differentiation, regulates the balance of early- and late-born neurons, and regulates the balanced production of different types of layer V cortical projection neurons. We suggest that COUP-TFI controls these processes by repressing Mapk/Erk, Akt, and beta-catenin signaling.