Graded and areal expression patterns of regulatory genes and cadherins in embryonic neocortex independent of thalamocortical input

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.

Histone methylation mediated by NSD1 is required for the establishment and maintenance of neuronal identities

Appropriate histone modifications emerge as essential cell fate regulators of neuronal identities across neocortical areas and layers. Here we showed that NSD1, the methyltransferase for di-methylated lysine 36 of histone H3 (H3K36me2), controls both area and layer identities of the neocortex. Nsd1-ablated neocortex showed an area shift of all four primary functional regions and aberrant wiring of cortico-thalamic-cortical projections. Nsd1 conditional knockout mice displayed defects in spatial memory, motor learning, and coordination, resembling patients with the Sotos syndrome carrying NSD1 mutations. On Nsd1 loss, superficial-layer pyramidal neurons (PNs) progressively mis-expressed markers for deep-layer PNs, and PNs remained immature both morphologically and electrophysiologically. Loss of Nsd1 in postmitotic PNs causes genome-wide loss of H3K36me2 and re-distribution of DNA methylation, which accounts for diminished expression of neocortical layer specifiers but ectopic expression of non-neural genes. Together, H3K36me2 mediated by NSD1 is required for the establishment and maintenance of region- and layer-specific neocortical identities.

Visuo-spatial imagery in dreams of congenitally and early blind: a systematic review

Background

The presence of visual imagery in dreams of congenitally blind people has long been a matter of substantial controversy. We set to systematically review body of published work on the presence and nature of oneiric visuo-spatial impressions in congenitally and early blind subjects across different areas of research, from experimental psychology, functional neuroimaging, sensory substitution, and sleep research.

Methods

Relevant studies were identified using the following databases: EMBASE, MEDLINE and PsychINFO.

Results

Studies using diverse imaging techniques and sensory substitution devices broadly suggest that the "blind" occipital cortex may be able to integrate non-visual sensory inputs, and thus possibly also generate visuo-spatial impressions. Visual impressions have also been reported by blind subjects who had near-death or out-of-body experiences.

Conclusion

Deciphering the mechanistic nature of these visual impression could open new possibility in utilization of neuroplasticity and its potential role for treatment of neurodisability.

Activation of Sonic Hedgehog Signaling Promotes Differentiation of Cortical Layer 4 Neurons via Regulation of Their Cell Positioning

Neuronal subtypes in the mammalian cerebral cortex are determined by both intrinsic and extrinsic mechanisms during development. However, the extrinsic cues that are involved in this process remain largely unknown. Here, we investigated the role of sonic hedgehog (Shh) in glutamatergic cortical subtype specification. We found that E14.5-born, but not E15.5-born, neurons with elevated Shh expression frequently differentiated into layer 4 subtypes as judged by the cell positioning and molecular identity. We further found that this effect was achieved indirectly through the regulation of cell positioning rather than the direct activation of layer 4 differentiation programs. Together, we provided evidence that Shh, an extrinsic factor, plays an important role in the specification of cortical superficial layer subtypes.

Multiple Functions of the Dmrt Genes in the Development of the Central Nervous System

The Dmrt genes encode the transcription factor containing the DM (doublesex and mab-3) domain, an intertwined zinc finger-like DNA binding module. While Dmrt genes are mainly involved in the sexual development of various species, recent studies have revealed that Dmrt genes, which belong to the DmrtA subfamily, are differentially expressed in the embryonic brain and spinal cord and are essential for the development of the central nervous system. Herein, we summarize recent studies that reveal the multiple functions of the Dmrt genes in various aspects of vertebrate neural development, including brain patterning, neurogenesis, and the specification of neurons.

COUP-TFI specifies the medial entorhinal cortex identity and induces differential cell adhesion to determine the integrity of its boundary with neocortex

Development of cortical regions with precise, sharp, and regular boundaries is essential for physiological function. However, little is known of the mechanisms ensuring these features. Here, we show that determination of the boundary between neocortex and medial entorhinal cortex (MEC), two abutting cortical regions generated from the same progenitor lineage, relies on COUP-TFI (chicken ovalbumin upstream promoter-transcription factor I), a patterning transcription factor with graded expression in cortical progenitors. In contrast with the classical paradigm, we found that increased COUP-TFI expression expands MEC, creating protrusions and disconnected ectopic tissue. We further developed a mathematical model that predicts that neuronal specification and differential cell affinity contribute to the emergence of an instability region and boundary sharpness. Correspondingly, we demonstrated that high expression of COUP-TFI induces MEC cell fate and protocadherin 19 expression. Thus, we conclude that a sharp boundary requires a subtle interplay between patterning transcription factors and differential cell affinity.

Variation of Human Neural Stem Cells Generating Organizer States In Vitro before Committing to Cortical Excitatory or Inhibitory Neuronal Fates

Better understanding of the progression of neural stem cells (NSCs) in the developing cerebral cortex is important for modeling neurogenesis and defining the pathogenesis of neuropsychiatric disorders. Here, we use RNA sequencing, cell imaging, and lineage tracing of mouse and human in vitro NSCs and monkey brain sections to model the generation of cortical neuronal fates. We show that conserved signaling mechanisms regulate the acute transition from proliferative NSCs to committed glutamatergic excitatory neurons. As human telencephalic NSCs develop from pluripotency in vitro, they transition through organizer states that spatially pattern the cortex before generating glutamatergic precursor fates. NSCs derived from multiple human pluripotent lines vary in these early patterning states, leading differentially to dorsal or ventral telencephalic fates. This work furthers systematic analyses of the earliest patterning events that generate the major neuronal trajectories of the human telencephalon.

Micali et al. report that human telencephalic NSCs in vitro transition through the organizer states that pattern the neocortex. Human pluripotent lines vary in organizer formation, generating divergent neuronal differentiation trajectories biased toward dorsal or ventral telencephalic fates and opening further analysis of the earliest cortical specification events.

Abnormal neocortex arealization and Sotos-like syndrome-associated behavior in Setd2 mutant mice

Proper formation of area identities of the cerebral cortex is crucial for cognitive functions and social behaviors of the brain. It remains largely unknown whether epigenetic mechanisms, including histone methylation, regulate cortical arealization. Here, we removed SETD2, the methyltransferase for histone 3 lysine-36 trimethylation (H3K36me3), in the developing dorsal forebrain in mice and showed that Setd2 is required for proper cortical arealization and the formation of cortico-thalamo-cortical circuits. Moreover, Setd2 conditional knockout mice exhibit defects in social interaction, motor learning, and spatial memory, reminiscent of patients with the Sotos-like syndrome bearing SETD2 mutations. SETD2 maintains the expression of clustered protocadherin (cPcdh) genes in an H3K36me3 methyltransferase-dependent manner. Aberrant cortical arealization was recapitulated in cPcdh heterozygous mice. Together, our study emphasizes epigenetic mechanisms underlying cortical arealization and pathogenesis of the Sotos-like syndrome.

Precision in the development of neocortical architecture: From progenitors to cortical networks

Of all brain regions, the 6-layered neocortex has undergone the most dramatic changes in size and complexity during mammalian brain evolution. These changes, occurring in the context of a conserved set of organizational features that emerge through stereotypical developmental processes, are considered responsible for the cognitive capacities and sensory specializations represented within the mammalian clade. The modern experimental era of developmental neurobiology, spanning 6 decades, has deciphered a number of mechanisms responsible for producing the diversity of cortical neuron types, their precise connectivity and the role of gene by environment interactions. Here, experiments providing insight into the development of cortical projection neuron differentiation and connectivity are reviewed. This current perspective integrates discussion of classic studies and new findings, based on recent technical advances, to highlight an improved understanding of the neuronal complexity and precise connectivity of cortical circuitry. These descriptive advances bring new opportunities for studies related to the developmental origins of cortical circuits that will, in turn, improve the prospects of identifying pathogenic targets of neurodevelopmental disorders.

Lhx2 Expression in Postmitotic Cortical Neurons Initiates Assembly of the Thalamocortical Somatosensory Circuit

Cortical neurons must be specified and make the correct connections during development. Here, we examine a mechanism initiating neuronal circuit formation in the barrel cortex, a circuit comprising thalamocortical axons (TCAs) and layer 4 (L4) neurons. When Lhx2 is selectively deleted in postmitotic cortical neurons using conditional knockout (cKO) mice, L4 neurons in the barrel cortex are initially specified but fail to form cellular barrels or develop polarized dendrites. In Lhx2 cKO mice, TCAs from the thalamic ventral posterior nucleus reach the barrel cortex but fail to further arborize to form barrels. Several activity-regulated genes and genes involved in regulating barrel formation are downregulated in the Lhx2 cKO somatosensory cortex. Among them, Btbd3, an activity-regulated gene controlling dendritic development, is a direct downstream target of Lhx2. We find that Lhx2 confers neuronal competency for activity-dependent dendritic development in L4 neurons by inducing the expression of Btbd3.

Enhanced expression of Pafah1b1 causes over-migration of cerebral cortical neurons into the marginal zone

Mutations of PAFAH1B1 cause classical lissencephaly in humans. In addition, duplications and triplications of PAFAH1B1 are found in individuals with intellectual disability and other neurological disorders suggesting that proper brain development is highly sensitive to the PAFAH1B1 dosage. To examine the effect of PAFAH1B1 over-dosage in neural development, especially in migration of neurons and layer formation during cerebral cortical development, we overexpressed Pafah1b1 in migrating neurons in the mouse embryonic cortex using in utero electroporation. Enhanced expression of Pafah1b1 in radially-migrating neurons resulted in their over-migration into the marginal zone. Neurons that invaded the marginal zone were oriented abnormally. Layer distribution of Pafaha1b1-overexpressing neurons shifted more superficially than control neurons. Some of the Pafaha1b1-overexpressing future layer 4 neurons changed their positions to layers 2/3. Furthermore, they also changed their layer marker expression from layer 4 to layers 2/3. These results suggest that overexpression of Pafah1b1 affects the migration of neurons and disrupts layer formation in the developing cerebral cortex, and further support the idea that appropriate dosage of Pafah1b1 is crucial for the proper development of the brain.

Charting the protomap of the human telencephalon

The cerebral cortex is divided stereotypically into a number of functionally distinct areas. According to the protomap hypothesis formulated by Rakic neural progenitors in the ventricular zone form a mosaic of proliferative units that provide a primordial species-specific cortical map. Positional information of newborn neurons is maintained during their migration to the overlying cortical plate. Much evidence has been found to support this hypothesis from studies of primary cortical areas in mouse models in particular. Differential expansion of cortical areas and the introduction of new functional modules during evolution might be the result of changes in the progenitor cells. The human cerebral cortex shows a wide divergence from the mouse containing a much higher proportion of association cortex and a more complicated regionalised repertoire of neuron sub-types. To what extent does the protomap hypothesis hold true for the primate brain? This review summarises a growing number of studies exploring arealised gene expression in the early developing human telencephalon. The evidence so far is that the human and mouse brain do share fundamental mechanisms of areal specification, however there are subtle differences which could lead us to a better understanding of cortical evolution and the origins of neurodevelopmental diseases.

Emx1 Is Required for Neocortical Area Patterning

Establishing appropriate area patterning in the neocortex is a critical developmental event, and transcription factors whose expression is graded across the developing neural axes have been implicated in this process. While previous reports suggested that the transcription factor Emx1 does not contribute to neocortical area patterning, those studies were performed at perinatal ages prior to the emergence of primary areas. We therefore examined two different Emx1 deletion mouse lines once primary areas possess mature features. Following the deletion of Emx1, the frontal and motor areas were expanded while the primary visual area was reduced, and overall the areas shifted posterio-medially. This patterning phenotype was consistent between the two Emx1 deletion strategies. The present study demonstrates that Emx1 is an area patterning transcription factor and is required for the specification of the primary visual area.

Identity of neocortical layer 4 neurons is specified through correct positioning into the cortex

Many cell-intrinsic mechanisms have been shown to regulate neuronal subtype specification in the mammalian neocortex. However, how much cell environment is crucial for subtype determination still remained unclear. Here, we show that knockdown of Protocadherin20 (Pcdh20), which is expressed in post-migratory neurons of layer 4 (L4) lineage, caused the cells to localize in L2/3. The ectopically positioned "future L4 neurons" lost their L4 characteristics but acquired L2/3 characteristics. Knockdown of a cytoskeletal protein in the future L4 neurons, which caused random disruption of positioning, also showed that those accidentally located in L4 acquired the L4 characteristics. Moreover, restoration of positioning of the Pcdh20-knockdown neurons into L4 rescued the specification failure. We further suggest that the thalamocortical axons provide a positional cue to specify L4 identity. These results suggest that the L4 identity is not completely determined at the time of birth but ensured by the surrounding environment after appropriate positioning.

Pbx Regulates Patterning of the Cerebral Cortex in Progenitors and Postmitotic Neurons

We demonstrate using conditional mutagenesis that Pbx1, with and without Pbx2(+/-) sensitization, regulates regional identity and laminar patterning of the developing mouse neocortex in cortical progenitors (Emx1-Cre) and in newly generated neurons (Nex1-Cre). Pbx1/2 mutants have three salient molecular phenotypes of cortical regional and laminar organization: hypoplasia of the frontal cortex, ventral expansion of the dorsomedial cortex, and ventral expansion of Reelin expression in the cortical plate of the frontal cortex, concomitant with an inversion of cortical layering in the rostral cortex. Molecular analyses, including PBX ChIP-seq, provide evidence that PBX promotes frontal cortex identity by repressing genes that promote dorsocaudal fate.

The effects of lifelong blindness on murine neuroanatomy and gene expression

Mammalian neocortical development is regulated by neural patterning mechanisms, with distinct sensory and motor areas arising through the process of arealization. This development occurs alongside developing central or peripheral sensory systems. Specifically, the parcellation of neocortex into specific areas of distinct cytoarchitecture, connectivity and function during development is reliant upon both cortically intrinsic mechanisms, such as gene expression, and extrinsic processes, such as input from the sensory receptors. This developmental program shifts from patterning to maintenance as the animal ages and is believed to be active throughout life, where the brain’s organization is stable yet plastic. In this study, we characterize the long-term effects of early removal of visual input via bilateral enucleation at birth. To understand the long-term effects of early blindness we conducted anatomical and molecular assays 18 months after enucleation, near the end of lifespan in the mouse. Bilateral enucleation early in life leads to long-term, stable size reductions of the thalamic lateral geniculate nucleus (LGN) and the primary visual cortex (V1) alongside a increase in individual whisker barrel size. Neocortical gene expression in the aging brain has not been previously identified; we document cortical expression of multiple regionalization genes. Expression patterns of Ephrin A5, COUP-TFI, and RZRβ and patterns of intraneocortical connectivity (INC) are altered in the neocortices of aging blind mice. Sensory inputs from different modalities during development likely play a major role in the development of cortical areal and thalamic nuclear boundaries. We suggest that early patterning by prenatal retinal activity combined with persistent gene expression within the thalamus and cortex is sufficient to establish and preserve a small but present LGN and V1 into late adulthood.

The role of T-cadherin in axonal pathway formation in neocortical circuits

Cortical efferent and afferent fibers are arranged in a stereotyped pattern in the intermediate zone (IZ). Here, we studied the mechanism of axonal pathway formation by identifying a molecule that is expressed in a subset of cortical axons in the rat. We found that T-cadherin (T-cad), a member of the cadherin family, is expressed in deep-layer cell axons projecting to subcortical structures, but not in upper layer callosal axons projecting to the contralateral cortex. Ectopic expression of T-cad in upper layer cells induced axons to project toward subcortical structures via the upper part of the IZ. Moreover, the axons of deep-layer cells in which T-cad expression was suppressed by RNAi projected towards the contralateral cortex via an aberrant route. These results suggest that T-cad is involved in axonal pathway formation in the developing cortex.

Broad integration of expression maps and co-expression networks compassing novel gene functions in the brain

Using a recently invented technique for gene expression mapping in the whole-anatomy context, termed transcriptome tomography, we have generated a dataset of 36,000 maps of overall gene expression in the adult-mouse brain. Here, using an informatics approach, we identified a broad co-expression network that follows an inverse power law and is rich in functional interaction and gene-ontology terms. Our framework for the integrated analysis of expression maps and graphs of co-expression networks revealed that groups of combinatorially expressed genes, which regulate cell differentiation during development, were present in the adult brain and each of these groups was associated with a discrete cell types. These groups included non-coding genes of unknown function. We found that these genes specifically linked developmentally conserved groups in the network. A previously unrecognized robust expression pattern covering the whole brain was related to the molecular anatomy of key biological processes occurring in particular areas.

Development and evolution of cortical fields

The neocortex is the brain structure that has been subjected to a major size expansion, in its relative size, during mammalian evolution. It arises from the cortical primordium through coordinated growth of neural progenitor cells along both the tangential and radial axes and their patterning providing spatial coordinates. Functional neocortical areas are ultimately consolidated by environmental influences such as peripheral sensory inputs. Throughout neocortical evolution, cortical areas have become more sophisticated and numerous. This increase in number is possibly involved in the complexification of neocortical function in primates. Whereas extensive divergence of functional cortical fields is observed during evolution, the fundamental mechanisms supporting the allocation of cortical areas and their wiring are conserved, suggesting the presence of core genetic mechanisms operating in different species. We will discuss some of the basic molecular mechanisms including morphogen-dependent ones involved in the precise orchestration of neurogenesis in different cortical areas, elucidated from studies in rodents. Attention will be paid to the role of Cajal-Retzius neurons, which were recently proposed to be migrating signaling units also involved in arealization, will be addressed. We will further review recent works on molecular mechanisms of cortical patterning resulting from comparative analyses between different species during evolution.

Parcellation of the thalamus into distinct nuclei reflects EphA expression and function

Intercellular signaling via the Eph receptor tyrosine kinases and their ligands, the ephrins, acts to shape many regions of the developing brain. One intriguing consequence of Eph signaling is the control of mixing between discrete cell populations in the developing hindbrain, contributing to the formation of segregated rhombomeres. Since the thalamus is also a parcellated structure comprised of discrete nuclei, might Eph signaling play a parallel role in cell segregation in this brain structure? Analyses of expression reveal that several Eph family members are expressed in the forming thalamus and that cells expressing particular receptors form cellular groupings as development proceeds. Specifically, expression of receptors EphA4 or EphA7 and ligand ephrin-A5 is localized to distinct thalamic domains. EphA4 and EphA7 are often coexpressed in regions of the forming thalamus, with each receptor marking discrete thalamic domains. In contrast, ephrin-A5 is expressed by a limited group of thalamic cells. Within the ventral thalamus, EphA4 is present broadly, occasionally overlapping with ephrin-A5 expression. EphA7 is more restricted in its expression and is largely nonoverlapping with ephrin-A5. In mutant mice lacking one or both receptors or ephrin-A5, the appearance of the venteroposterolateral (VPL) and venteroposteromedial (VPM) nuclear complex is altered compared to wild type mice. These in vivo results support a role for Eph family members in the definition of the thalamic nuclei. In parallel, in vitro analysis reveals a hierarchy of mixing among cells expressing ephrin-A5 with cells expressing EphA4 alone, EphA4 and EphA7 together, or EphA7 alone. Together, these data support a model in which EphA molecules promote the parcellation of discrete thalamic nuclei by limiting the extent of cell mixing.

Geniculocortical input drives genetic distinctions between primary and higher-order visual areas

Studies of area patterning of the neocortex have focused on primary areas, concluding that the primary visual area, V1, is specified by transcription factors (TFs) expressed by progenitors. Mechanisms that determine higher-order visual areas (V(HO)) and distinguish them from V1 are unknown. We demonstrated a requirement for thalamocortical axon (TCA) input by genetically deleting geniculocortical TCAs and showed that they drive differentiation of patterned gene expression that distinguishes V1 and V(HO). Our findings suggest a multistage process for area patterning: TFs expressed by progenitors specify an occipital visual cortical field that differentiates into V1 and V(HO); this latter phase requires geniculocortical TCA input to the nascent V1 that determines genetic distinctions between V1 and V(HO) for all layers and ultimately determines their area-specific functional properties.

Neocortical arealization: evolution, mechanisms, and open questions

The mammalian neocortex is a structure with no equals in the vertebrates and is the seat of the highest cerebral functions, such as thoughts and consciousness. It is radially organized into six layers and tangentially subdivided into functional areas deputed to the elaboration of sensory information, association between different stimuli, and selection and triggering of voluntary movements. The process subdividing the neocortical field into several functional areas is called "arealization". Each area has its own cytoarchitecture, connectivity, and peculiar functions. In the last century, several neuroscientists have investigated areal structure and the mechanisms that have led during evolution to the rising of the neocortex and its organization. The extreme conservation in the positioning and wiring of neocortical areas among different mammalian families suggests a conserved genetic program orchestrating neocortical patterning. However, the impressive plasticity of the neocortex, which is able to rewire and reorganize areal structures and connectivity after impairments of sensory pathways, argues for a more complex scenario. Indeed, even if genetics and molecular biology helped in identifying several genes involved in the arealization process, the logic underlying the neocortical bauplan is still beyond our comprehension. In this review, we will introduce the present knowledge and hypotheses on the ontogenesis and evolution of neocortical areas. Then, we will focus our attention on some open issues, which are still unresolved, and discuss some recent studies that might open new directions to be explored in the next few years.

Role for Lhx2 in corticogenesis through regulation of progenitor differentiation

The neocortex represents the brain region that has undergone a major increase in its relative size during the course of mammalian evolution. The larger cortex results from a corresponding increase in progenitor cell number. The progenitors giving rise to neocortex are located in the ventricular zone of the dorsal telencephalon and highly express Lhx2, a LIM-homeodomain transcription factor. The neocortex fails to form in the Lhx2 constitutive knockout, indicating a role for Lhx2 in corticogenesis, but mid-embryonic lethality of the Lhx2 knockout requires the use of conditional strategies for further studies. Therefore, to explore Lhx2 function in neocortical progenitors, we generated mice with Lhx2 conditionally deleted from cortical progenitors at the onset of neurogenesis. We find that Lhx2 is critical for maintaining the proliferative state of neocortical progenitors during corticogenesis. In the conditional knockouts, the neocortex is formed but is significantly smaller than wild type. We find that deletion of Lhx2 leads to significantly decreased numbers of cortical progenitors and premature neuronal differentiation. A likely mechanism is indicated by our findings that Lhx2 is required for the expression of Hes1 in cortical progenitors, a key effector in the Notch signaling pathway that maintains the proliferative progenitor state. We conclude that Lhx2 regulates the balance between proliferation and differentiation in cortical progenitors and through this mechanism Lhx2 controls cortical size.

Development and critical period plasticity of the barrel cortex

In primary sensory neocortical areas of mammals, the distribution of sensory receptors is mapped with topographic precision and amplification in proportion to the peripheral receptor density. The visual, somatosensory and auditory cortical maps are established during a critical period in development. Throughout this window in time, the developing cortical maps are vulnerable to deleterious effects of sense organ damage or sensory deprivation. The rodent barrel cortex offers an invaluable model system with which to investigate the mechanisms underlying the formation of topographic maps and their plasticity during development. Five rows of mystacial vibrissa (whisker) follicles on the snout and an array of sinus hairs are represented by layer IV neural modules ('barrels') and thalamocortical axon terminals in the primary somatosensory cortex. Perinatal damage to the whiskers or the sensory nerve innervating them irreversibly alters the structural organization of the barrels. Earlier studies emphasized the role of the sensory periphery in dictating whisker-specific brain maps and patterns. Recent advances in molecular genetics and analyses of genetically altered mice allow new insights into neural pattern formation in the neocortex and the mechanisms underlying critical period plasticity. Here, we review the development and patterning of the barrel cortex and the critical period plasticity.

Convergent repression of Foxp2 3'UTR by miR-9 and miR-132 in embryonic mouse neocortex: implications for radial migration of neurons

MicroRNAs (miRNAs) are rapidly emerging as a new layer of regulation of mammalian brain development. However, most of the miRNA target genes remain unidentified. Here, we explore gene expression profiling upon miRNA depletion and in vivo target validation as a strategy to identify novel miRNA targets in embryonic mouse neocortex. By this means, we find that Foxp2, a transcription factor associated with speech and language development and evolution, is a novel miRNA target. In particular, we find that miR-9 and miR-132 are able to repress ectopic expression of Foxp2 protein by targeting its 3' untranslated region (3'UTR) in vivo. Interestingly, ectopic expression of Foxp2 in cortical projection neurons (a scenario that mimics the absence of miRNA-mediated silencing of Foxp2 expression) delays neurite outgrowth in vitro and impairs their radial migration in embryonic mouse neocortex in vivo. Our results uncover a new layer of control of Foxp2 expression that may be required for proper neuronal maturation.

'Til Eph do us part': intercellular signaling via Eph receptors and ephrin ligands guides cerebral cortical development from birth through maturation

Eph receptors, the largest family of surface-bound receptor tyrosine kinases and their ligands, the ephrins, mediate a wide variety of cellular interactions in most organ systems throughout both development and maturity. In the forming cerebral cortex, Eph family members are broadly and dynamically expressed in particular sets of cortical cells at discrete times. Here, we review the known functions of Eph-mediated intercellular signaling in the generation of progenitors, the migration of maturing cells, the differentiation of neurons, the formation of functional connections, and the choice between life and death during corticogenesis. In synthesizing these results, we posit a signaling paradigm in which cortical cells maintain a life history of Eph-mediated intercellular interactions that guides subsequent cellular decision-making.

The emergence of somatotopic maps of the body in S1 in rats: the correspondence between functional and anatomical organization

Most of what we know about cortical map development and plasticity comes from studies in mice and rats, and for the somatosensory cortex, almost exclusively from the whisker-dominated posteromedial barrel fields. Whiskers are the main effector organs of mice and rats, and their representation in cortex and subcortical pathways is a highly derived feature of murine rodents. This specialized anatomical organization may therefore not be representative of somatosensory cortex in general, especially for species that utilize other body parts as their main effector organs, like the hands of primates. For these reasons, we examined the emergence of whole body maps in developing rats using electrophysiological recording techniques. In P5, P10, P15, P20 and adult rats, multiple recordings were made in the medial portion of S1 in each animal. Subsequently, these functional maps were related to anatomical parcellations of S1 based on a variety of histological stains. We found that at early postnatal ages (P5) medial S1 was composed almost exclusively of the representation of the vibrissae. At P10, other body part representations including the hindlimb and forelimb were present, although these were not topographically organized. By P15, a clear topographic organization began to emerge coincident with a reduction in receptive field size. By P20, body maps were adult-like. This study is the first to describe how topography of the body develops in S1 in any mammal. It indicates that anatomical parcellations and functional maps are initially incongruent but become tightly coupled by P15. Finally, because anatomical and functional specificity of developing barrel cortex appears much earlier in postnatal life than the rest of the body, the entire primary somatosensory cortex should be considered when studying general topographic map formation in development.

Bilateral enucleation alters gene expression and intraneocortical connections in the mouse

Background

Anatomically and functionally distinct sensory and motor neocortical areas form during mammalian development through a process called arealization. This process is believed to be reliant on both activity-dependent and activity-independent mechanisms. Although both mechanisms are thought to function concurrently during arealization, the nature of their interaction is not understood. To examine the potential interplay of extrinsic activity-dependent mechanisms, such as sensory input, and intrinsic activity-independent mechanisms, including gene expression in mouse neocortical development, we performed bilateral enucleations in newborn mice and conducted anatomical and molecular analyses 10 days later. In this study, by surgically removing the eyes of the newborn mouse, we examined whether early enucleation would impact normal gene expression and the development of basic anatomical features such as intraneocortical connections and cortical area boundaries in the first 10 days of life, before natural eye opening. We examined the acute effects of bilateral enucleation on the lateral geniculate nucleus of the thalamus and the neocortical somatosensory-visual area boundary through detailed analyses of intraneocortical connections and gene expression of six developmentally regulated genes at postnatal day 10.

Results

Our results demonstrate short-term plasticity on postnatal day 10 resulting from the removal of the eyes at birth, with changes in nuclear size and gene expression within the lateral geniculate nucleus as well as a shift in intraneocortical connections and ephrin A5 expression at the somatosensory-visual boundary. In this report, we highlight the correlation between positional shifts in ephrin A5 expression and improper refinement of intraneocortical connections observed at the somatosensory-visual boundary in enucleates on postnatal day 10.

Conclusions

Bilateral enucleation induces a positional shift of both ephrin A5 expression and intraneocortical projections at the somatosensory-visual border in only 10 days. These changes occur prior to natural eye opening, suggesting a possible role of spontaneous retinal activity in area border formation within the neocortex. Through these analyses, we gain a deeper understanding of how extrinsic activity-dependent mechanisms, particularly input from sensory organs, are integrated with intrinsic activity-independent mechanisms to regulate neocortical arealization and plasticity.

The role of Pax6 in forebrain development

Pax6 encodes a highly conserved transcriptional regulator with two DNA-binding motifs, a paired domain and a paired-like homeodomain. Humans carrying PAX6 loss-of-function mutations suffer from abnormal development of the eyes (congenital aniridia) and brain. Small eye mice carrying Pax6 loss-of-function mutations provide a good model for these human conditions. Their analysis has demonstrated the critical importance of this transcription factor in multiple cell types and at several key stages of forebrain development. In the forebrain, Pax6 is critical for the establishment of the pallial-subpallial boundary, which separates dorsal (future cerebral cortex) and ventral (future striatum) telencephalic regions. Levels of Pax6 expression are critically important for cortical progenitor proliferation and its presence in a rostro-lateral(high) to caudo-medial(low) gradient in the cortex is necessary to establish rostro-lateral identities. Furthermore, axon guidance is disrupted in Pax6⁻/⁻ mutants: the majority of thalamocortical axons fail to enter the ventral telencephalon and those that do are unable to innervate their cortical targets. The extent to which the effects of Pax6 later in development are secondary to its effects in early patterning and proliferation remains largely unknown. This is likely to be clarified by future studies on the molecular mechanisms of action of Pax6 and, in particular, the identification of its downstream target genes. Such studies should also help generate an increasingly coherent understanding of how this pleiotropic transcription factor becomes involved in so many facets of neural development.

Transcriptional mechanisms of EphA7 gene expression in the developing cerebral cortex

The patterning of cortical areas is controlled by a combination of intrinsic factors that are expressed in the cortex and external signals such as inputs from the thalamus. EphA7 is a guidance receptor that is involved in key aspects of cortical development and is expressed in gradients within developing cortical areas. Here, we identified a regulatory element of the EphA7 promoter, named pA7, that can recapitulate salient features of the pattern of expression of EphA7, including cortical gradients. Using a pA7-Green fluorescent Protein (GFP) mouse reporter line, we isolated cortical neuron populations displaying different levels of EphA7/GFP expression. Transcriptome analysis of these populations enabled to identify many differentially expressed genes, including 26 transcription factors with putative binding sites in the pA7 element. Among these, Pbx1 was found to bind directly to the EphA7 promoter in the developing cortex. All genes validated further were confirmed to be expressed differentially in the developing cortex, similarly to EphA7. Their expression was unchanged in mutant mice defective for thalamocortical projections, indicating a transcriptional control largely intrinsic to the cortex. Our study identifies a novel repertoire of cortical neuron genes that may act upstream of, or together with EphA7, to control the patterning of cortical areas.

How do barrels form in somatosensory cortex?

The somatosensory cortex of many rodents, lagomorphs, and marsupials contains distinct cytoarchitectonic features named "barrels" that reflect the pattern of large facial whiskers on the snout. Barrels are composed of clustered thalamocortical afferents relaying sensory information from one whisker surrounded by cell-dense walls or "barrels" in layer 4 of the cortex. In many ways, barrels are a simple and relatively accessible canonical cortical column, making them a common model system for the examination of cortical development and function. Despite their experimental accessibility and popularity, we still lack a basic understanding of how and why barrels form in the first place. In this review, we will examine what is known about mechanisms of barrel development, focusing specifically on the recent literature using the molecular-genetic power of mice as a model system for examining brain development.

Annual Research Review: Development of the cerebral cortex: implications for neurodevelopmental disorders

The cerebral cortex has a central role in cognitive and emotional processing. As such, understanding the mechanisms that govern its development and function will be central to understanding the bases of severe neuropsychiatric disorders, particularly those that first appear in childhood. In this review, I highlight recent progress in elucidating genetic, molecular and cellular mechanisms that control cortical development. I discuss basic aspects of cortical developmental anatomy, and mechanisms that regulate cortical size and area formation, with an emphasis on the roles of fibroblast growth factor (Fgf) signaling and specific transcription factors. I then examine how specific types of cortical excitatory projection neurons are generated, and how their axons grow along stereotyped pathways to their targets. Next, I address how cortical inhibitory (GABAergic) neurons are generated, and point out the role of these cells in controlling cortical plasticity and critical periods. The paper concludes with an examination of four possible developmental mechanisms that could contribute to some forms of neurodevelopmental disorders, such as autism.

Cadherin expression in the somatosensory cortex: evidence for a combinatorial molecular code at the single-cell level

Cadherin superfamily genes play a role in a wide variety of developmental processes and mature functions of the vertebrate brain. In the present study, we mapped in situ the expression pattern of five classic cadherins (Cdh4, Cdh6, Cdh7, Cdh8, Cdh11) and eight δ-protocadherins (Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, Pcdh11, Pcdh17 and Pcdh19) in the primary somatosensory cortex of the adult mouse. All of these cadherins show layer-specific expression profiles in primary somatosensory cortex. Some cadherins (for example, Cdh4, Cdh7, Pcdh8) mark subsets of cells within a given lamina, while other cadherins (Cdh11 and Pcdh10) are expressed more widely in multiple layers. Results from tyramide-based double-fluorescence in situ hybridization (FISH) provide evidence that most single neurons express more than one cadherin in a combinatorial fashion in all layers of cerebral cortex. This combinatorial code is rather comprehensive because pairwise expression of cadherins can assume any type of combination (complementarity, partial or complete overlap, subset-specific expression, cell-size specific expression, etc.). We propose that the combinatorial expression of multiple cadherin genes contributes to the molecular specification of the vast complexity of neurons in cerebral cortex.

A lifespan analysis of intraneocortical connections and gene expression in the mouse I

A hallmark of mammalian evolution is the structural and functional complexity of the cerebral cortex. Within the cerebral cortex, the neocortex, or isocortex, is a 6-layered complexly organized structure that is comprised of multiple interconnected sensory and motor areas. These areas and their precise patterns of connections arise during development, through a process termed arealization. Intrinsic, activity-independent and extrinsic, activity-dependent mechanisms are involved in the development of neocortical areas and their connections. The intrinsic molecular mechanisms involved in the establishment of this sophisticated network are not fully understood. In this report (I) and the companion report (II), we present the first lifespan analysis of ipsilateral intraneocortical connections (INCs) among multiple sensory and motor regions, from the embryonic period to adulthood in the mouse. Additionally, we characterize the neocortical expression patterns of several developmentally regulated genes that are of central importance to studies investigating the molecular control of arealization from embryonic day 13.5 to postnatal day (P) 3 (I) and P6 to 50 (II). In this analysis, we utilize novel methods to correlate the boundaries of gene expression with INCs and developing areal boundaries, in order to better understand the nature of gene-areal relationships during development.

Absence of layer-specific cadherin expression profiles in the neocortex of the reeler mutant mouse

Cadherins are a superfamily of Ca(2+)-dependent cell surface glycoproteins that play a morphogenetic role in a wide variety of developmental processes. They provide a code of potentially adhesive cues for layer formation in mammalian cerebral cortex. One of the animal models used for studying corticogenesis is the reeler mouse. Previous investigations showed that radial neuronal migration is impaired in this mutant, possibly resulting in an inversion of cortical layers. However, the extent of this "outside-in" cortical layering remains unclear. In the present study, we investigated the mRNA expression of cadherins (Cdh4, Cdh6, Cdh7, Cdh8, Pcdh8, Pcdh9, Pcdh11, Pcdh17, and Pcdh19) in the cerebral cortex of wild-type (wt) mice and reeler mutants. All cadherins show a layer-specific expression profile in wt mice, but, in reeler cortex, cadherin-expressing cells are distributed widely across the radial dimension. The altered layering in reeler mutants completely disrupts the radial expression of cadherins, which is more patchy, rather than laminar. Regionalized gradient-like expression of cadherins is preserved. Our findings are compatible with a model, in which the ubiquitous dispersion of cadherin-expressing cells results from a dysgenesis of radial glial cells and a misrouting of migrating neuroblasts.

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.

The developmental integration of cortical interneurons into a functional network

The central goal of this manuscript is to survey our present knowledge of how cortical interneuron subtypes are generated. To achieve this, we will first define what is meant by subtype diversity. To this end, we begin by considering the mature properties that differentiate between the different populations of cortical interneurons. This requires us to address the difficulties involved in determining which characteristics allow particular interneurons to be assigned to distinct subclasses. Having grappled with this thorny issue, we will then proceed to review the progressive events in development involved in the generation of interneuron diversity. Starting with their origin and specification within the subpallium, we will follow them up through the first postnatal weeks during their integration into a functional network. Finally, we will conclude by calling the readers attention to the devastating consequences that result from developmental failures in the formation of inhibitory circuits within the cortex.

Decision by division: making cortical maps

In the past three decades, mounting evidence has revealed that specification of the basic cortical neuronal classes starts at the time of their final mitotic divisions in the embryonic proliferative zones. This early cell determination continues during the migration of the newborn neurons across the widening cerebral wall, and it is in the cortical plate that they attain their final positions and establish species-specific cytoarchitectonic areas. Here, the development and evolutionary expansion of the neocortex is viewed in the context of the radial unit and protomap hypotheses. A broad spectrum of findings gave insight into the pathogenesis of cortical malformations and the biological bases for the evolution of the modern human neocortex. We examine the history and evidence behind the concept of early specification of neurons and provide the latest compendium of genes and signaling molecules involved in neuronal fate determination and specification.

Synaesthesia and cortical connectivity

Synaesthesia is a heritable condition of involuntary sensory cross-activation whereby the presentation of a particular stimulus elicits a secondary sensory-perceptual experience. It is thought to be caused by aberrant cross-activation of one cortical area by another, but models differ as to whether this reflects functional or structural differences in the brains of synaesthetes. Here we consider these models in light of recent experimental findings and argue for structural differences in the brains of synaesthetes, which might be more widespread than expected. We also discuss several plausible developmental mechanisms that could link a putative genetic variant to altered cortical connectivity and illustrate how synaesthesia could be an informative model to investigate how patterns of connectivity between cortical areas are established.

Layer-specific expression of multiple cadherins in the developing visual cortex (V1) of the ferret

Cadherins are superfamily of Ca2+-dependent transmembrane glycoproteins with more than 100 members. They play a role in a wide variety of developmental mechanisms, including cell proliferation, cell differentiation, cell-cell recognition, neurite outgrowth and synaptogenesis. We cloned 16 novel members of the classic cadherin and delta-protocadherin subgroups from ferret brain. Their expression patterns were investigated by in situ hybridization in the developing primary visual cortex (V1) of the ferret. Fifteen out of the 16 cadherins are expressed in a spatiotemporally restricted fashion throughout development. Each layer of V1 can be characterized by the combinatorial expression of a subset of cadherins at any given developmental stage. A few cadherins are expressed by subsets of neurons in specific layers or by neurons dispersed throughout all cortical layers. Generally, the expression of protocadherins is more widespread, whereas that of classic cadherins is more restricted to specific layers. At the V1/V2 boundary, changes in layer-specific cadherin expression are observed. In conclusion, our results suggest that cadherins provide a code of potentially adhesive cues for layer formation in ferret V1. The persistence of expression in the adult suggests a functional role also in the mature cortex.

GAP-43 is critical for normal targeting of thalamocortical and corticothalamic, but not trigeminothalamic axons in the whisker barrel system

Mice lacking the growth-associated protein GAP-43 (KO) show disrupted cortical topography and no barrels. Whisker-related patterns of cells are normal in the KO brainstem trigeminal complex (BSTC), while the pattern in KO ventrobasal thalamus (VB) is somewhat compromised. To better understand the basis for VB and cortical abnormalities, we used small placements of DiI to trace axonal projections between BSTC, VB, and barrel cortex in wildtype (WT) and GAP-43 KO mice. The trigeminothalamic (TT) pathway consists of axons from cells in the Nucleus Prinicipalis that project to the contralateral VB thalamus. DiI-labeled KO TT axons crossed the midline from BSTC and projected to contralateral VB normally, consistent with normal BSTC cytoarchitecture. By contrast, the KO thalamocortical axons (TCA) projection was highly abnormal. KO TCAs showed delays of 1-2 days in initial ingrowth to cortex. Postnatally, KO TCAs showed multiple pathfinding errors near intermediate targets, and were abnormally fasciculated within the internal capsule (IC). Interestingly, most individually labeled KO TCAs terminated in deep layers instead of in layer IV as in WT. This misprojection is consistent with birthdating analysis in KO mice, which revealed that neurons normally destined for layer IV remain in deep cortical layers. Early outgrowth of KO corticofugal (CF) axons was similar for both genotypes. However, at P7 KO CF fibers remained bundled as they entered the IC, and exhibited few terminal branches in VB. Thus, the establishment of axonal projections between thalamus and cortex are disrupted in GAP-43 KO mice.

The magnificent compromise: cortical field evolution in mammals

The neocortex of mammals is composed of cortical fields that have a unique organization associated with the animal's ecological niche and lifestyle. Each cortical field has a specific pattern of connections with other cortical fields and brain structures, and together they comprise a neocortical network that generates a variety of behaviors. These networks and the behaviors they generate are variable across mammals, and are particularly complex in some species such as humans. Here I discuss the mechanisms that contribute to neocortical organization in mammals, and how this organization has been altered to generate the variability that exists in different lineages.

Area patterning of the mammalian cortex

Here we describe mechanisms regulating area patterning of developing mammalian neocortex, referred to as arealization. Current findings indicate an interplay between intrinsic genetic mechanisms and extrinsic information relayed to cortex by thalamocortical input. Intrinsic mechanisms are based on morphogens and signaling molecules secreted by patterning centers, positioned at the perimeter of dorsal telencephalon, that generate across nascent cortex the graded expression of transcription factors in cortical progenitors. Two major patterning centers are the commissural plate, which expresses Fgf8 and Fgf17, and the cortical hem, which expresses Bmps and Wnts. Four transcription factors, COUP-TFI, Emx2, Pax6, and Sp8, with graded expression across the embryonic cortical axes, are shown to determine sizes and positions of cortical areas by specifying or repressing area identities within cortical progenitors. They also interact to modify their expression, as well as expression of Fgf8. We review these mechanisms of arealization and discuss models and concepts of cortical area patterning.

Genetic and epigenetic contributions to the cortical phenotype in mammals

One aspect of cortical organization, cortical field size, is variable both within and across species. The observed variability arises from a variety of sources, including genes intrinsic to the neocortex and a number of extrinsic and epigenetic factors. Genes intrinsic to the cortex are directly involved in the development and specification of cortical fields and are regulated from both signaling centers located outside of the neocortex, which secrete diffusible molecules, and the expression of transcription factors within the neocortex. In addition, extrinsic factors such as the type, location and density of sensory receptor arrays and how these receptor arrays are utilized, are also strongly related to cortical field size. Epigenetic factors including the relative activity patterns generated by the different types of physical stimuli in a given environment also contribute to differences in cortical organization, including cortical field size. Since both genetic and epigenetic factors contribute to cortical organization, some aspects of the cortical phenotype evolve, while other aspects of the cortical phenotype persist only if the environment in which an individual develops is relatively stable.

COUP-TFI regulates the balance of cortical patterning between frontal/motor and sensory areas

We used cortex-specific deletion of the transcription factor gene COUP-TFI (also known as Nr2f1) in mice to demonstrate previously unknown fundamental roles for it in patterning mammalian neocortex into areas. The highest COUP-TFI expression is observed in the cortical progenitors and progeny in parietal and occipital cortex that form sensory areas, and the lowest expression was observed in frontal cortex that includes motor areas. Cortical deletion of COUP-TFI resulted in massive expansion of frontal areas, including motor, to occupy most of neocortex, paralleled by marked compression of sensory areas to caudal occipital cortex. These area patterning changes are preceded and paralleled by corresponding changes in molecular markers of area identity and altered axonal projections to maintain patterned area-specific input and output connections. We conclude that COUP-TFI is required for balancing patterning of neocortex into frontal/motor and sensory areas by acting in its expression domain to repress frontal/motor area identities and to specify sensory area identities.