Neocortex patterning by the secreted signaling molecule FGF8

Does experience provide a permissive or instructive influence on the development of direction selectivity in visual cortex?

In principle, the development of sensory receptive fields in cortex could arise from experience-independent mechanisms that have been acquired through evolution, or through an online analysis of the sensory experience of the individual animal. Here we review recent experiments that suggest that the development of direction selectivity in carnivore visual cortex requires experience, but also suggest that the experience of an individual animal cannot greatly influence the parameters of the direction tuning that emerges, including direction angle preference and speed tuning. The direction angle preference that a neuron will acquire can be predicted from small initial biases that are present in the naïve cortex prior to the onset of visual experience. Further, experience with stimuli that move at slow or fast speeds does not alter the speed tuning properties of direction-selective neurons, suggesting that speed tuning preferences are built in. Finally, unpatterned optogenetic activation of the cortex over a period of a few hours is sufficient to produce the rapid emergence of direction selectivity in the naïve ferret cortex, suggesting that information about the direction angle preference that cells will acquire must already be present in the cortical circuit prior to experience. These results are consistent with the idea that experience has a permissive influence on the development of direction selectivity.

Visualization of Rostral Migratory Stream in the Developing Rat Brain by In Vivo Electroporation

Interneurons in the olfactory bulb (OB) are generated from neuronal precursor cells migrating from anterior subventricular zone (SVZa) not only in the developing embryo but also throughout the postnatal life of mammals. In the present study, we established an in vivo electroporation assay to label SVZa cells of rat both at embryonic and postnatal ages, and traced SVZa progenitors and followed their migration pathway and differentiation. We found that labeled cells displayed high motility. Interestingly, the postnatal cells migrated faster than the embryonic cells after applying this assay at different ages of brain development. Furthermore, based on brain slice culture and time-lapse imaging, we analyzed the detail migratory properties of these labeled precursor neurons. Finally, tissue transplantation experiments revealed that cells already migrated in subependymal zone of OB were transplanted back into rostral migratory stream (RMS), and these cells could still migrate out tangentially along RMS to OB. Taken together, these findings provide an in vivo labeling assay to follow and trace migrating cells in the RMS, their maturation and integration into OB neuron network, and unrecognized phenomena that postnatal SVZa progenitor cells with higher motility than embryonic cells, and their migration was affected by extrinsic environments.

Somatotopic Mapping of the Developing Sensorimotor Cortex in the Preterm Human Brain

In the mature mammalian brain, the primary somatosensory and motor cortices are known to be spatially organized such that neural activity relating to specific body parts can be somatopically mapped onto an anatomical "homunculus". This organization creates an internal body representation which is fundamental for precise motor control, spatial awareness and social interaction. Although it is unknown when this organization develops in humans, animal studies suggest that it may emerge even before the time of normal birth. We therefore characterized the somatotopic organization of the primary sensorimotor cortices using functional MRI and a set of custom-made robotic tools in 35 healthy preterm infants aged from 31 + 6 to 36 + 3 weeks postmenstrual age. Functional responses induced by somatosensory stimulation of the wrists, ankles, and mouth had a distinct spatial organization as seen in the characteristic mature homunculus map. In comparison to the ankle, activation related to wrist stimulation was significantly larger and more commonly involved additional areas including the supplementary motor area and ipsilateral sensorimotor cortex. These results are in keeping with early intrinsic determination of a somatotopic map within the primary sensorimotor cortices. This may explain why acquired brain injury in this region during the preterm period cannot be compensated for by cortical reorganization and therefore can lead to long-lasting motor and sensory impairment.

Development of tactile sensory circuits in the CNS

Molecular identification of neuronal types and genetic and imaging approaches to characterize their properties reveal morphological, physiological and dynamic aspects of sensory circuit development. Here we focus on the mouse tactile sensory circuitry, with particular emphasis on the main trigeminal pathway that connects the whiskers, the major tactile organ in rodents, to the neocortex. At each level of this pathway, neurogenesis, axonal elongation, pathfinding, target recognition and circuit reorganization including dendritic refinement of cortical layer 4 neurons occur contemporaneously and a multitude of molecular signals are used in differing combinations. We highlight recent advances in development of tactile circuitry and note gaps in our understanding.

In utero electroporation induces cell death and alters embryonic microglia morphology and expression signatures in the developing hypothalamus

Background

Since its inception in 2001, in utero electroporation (IUE) has been widely used by the neuroscience community. IUE is a technique developed to introduce plasmid DNA into embryonic mouse brains without permanently removing the embryos from the uterus. Given that IUE labels cells that line the ventricles, including radial fibers and migrating neuroblasts, this technique is an excellent tool for studying factors that govern neural cell fate determination and migration in the developing mouse brain. Whether IUE has an effect on microglia, the immune cells of the central nervous system (CNS), has yet to be investigated.

Methods

We used IUE and the pCIG2, pCIC-Ascl1, or pRFP-C-RS expression vectors to label radial glia lining the ventricles of the embryonic cortex and/or hypothalamus. Specifically, we conducted IUE at E14.5 and harvested the brains at E15.5 or E17.5. Immunohistochemistry, along with cytokine and chemokine analyses, were performed on embryonic brains with or without IUE exposure.

Results

IUE using the pCIG2, pCIC-Ascl1, or pRFP-C-RS vectors alone altered microglia morphology, where the majority of microglia near the ventricles were amoeboid and displayed altered expression signatures, including the upregulation of Cd45 and downregulation of P2ry12. Moreover, IUE led to increases in P2ry12⁻ cells that were Iba1⁺/IgG⁺ double-positive in the brain parenchyma and resembled macrophages infiltrating the brain proper from the periphery. Furthermore, IUE resulted in a significant increase in cell death in the developing hypothalamus, with concomitant increases in cytokines and chemokines known to be released during pro-inflammatory states (IL-1β, IL-6, MIP-2, RANTES, MCP-1). Interestingly, the cortex was protected from elevated cell death following IUE, implying that microglia that reside in the hypothalamus might be particularly sensitive during embryonic development.

Conclusions

Our results suggest that IUE might have unintended consequences of activating microglia in the embryonic brain, which could have long-term effects, particularly within the hypothalamus.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1213-6) contains supplementary material, which is available to authorized users.

A computational approach to identify cellular heterogeneity and tissue-specific gene regulatory networks

Background

The heterogeneity of cells across tissue types represents a major challenge for studying biological mechanisms as well as for therapeutic targeting of distinct tissues. Computational prediction of tissue-specific gene regulatory networks may provide important insights into the mechanisms underlying the cellular heterogeneity of cells in distinct organs and tissues.

Results

Using three pathway analysis techniques, gene set enrichment analysis (GSEA), parametric analysis of gene set enrichment (PGSEA), alongside our novel model (HeteroPath), which assesses heterogeneously upregulated and downregulated genes within the context of pathways, we generated distinct tissue-specific gene regulatory networks. We analyzed gene expression data derived from freshly isolated heart, brain, and lung endothelial cells and populations of neurons in the hippocampus, cingulate cortex, and amygdala. In both datasets, we found that HeteroPath segregated the distinct cellular populations by identifying regulatory pathways that were not identified by GSEA or PGSEA. Using simulated datasets, HeteroPath demonstrated robustness that was comparable to what was seen using existing gene set enrichment methods. Furthermore, we generated tissue-specific gene regulatory networks involved in vascular heterogeneity and neuronal heterogeneity by performing motif enrichment of the heterogeneous genes identified by HeteroPath and linking the enriched motifs to regulatory transcription factors in the ENCODE database.

Conclusions

HeteroPath assesses contextual bidirectional gene expression within pathways and thus allows for transcriptomic assessment of cellular heterogeneity. Unraveling tissue-specific heterogeneity of gene expression can lead to a better understanding of the molecular underpinnings of tissue-specific phenotypes.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2190-6) contains supplementary material, which is available to authorized users.

Differential gene regulatory plasticity between upper and lower layer cortical excitatory neurons

Neocortical projection neurons consist of intracortical connected upper layer (UL, layer II-IV) neurons and subcortical connected lower layer (LL, layer V-VI) neurons. Afferent activity from the thalamus regulates layer-specific gene expression during postnatal development, which is critical for the formation of proper neocortical cytoarchitecture. Here, we show that activity-dependent gene regulation is confined to UL cortical neurons, but not LL neurons, and that this distinction is likely due to epigenetic modifications of chromatin. We found that the immediate early genes (IEGs), EGR1 and c-FOS, are downregulated in all cortical laminar layers in the absence of afferent activity in vivo. Transcriptional assays demonstrated that EGR1 and c-FOS are able to bind to the promoters of UL- and LL-specific genes to induce transcription. Furthermore, we discovered that LL neurons express higher levels of heterochromatin markers, such as H3K9m3 and H4K20m3, compared to UL neurons. Our results suggest that differential epigenetic modifications of chromatin is an intrinsic mechanism that underlies the different sensitivities of cortical neurons to activity-dependent gene regulation.

The pleiotropic transcriptional regulator COUP-TFI plays multiple roles in neural development and disease

Transcription factors are expressed in a dynamic fashion both in time and space during brain development, and exert their roles by activating a cascade of multiple target genes. This implies that understanding the precise function of a transcription factor becomes a challenging task. In this review, we will focus on COUP-TFI (or NR2F1), a nuclear receptor belonging to the superfamily of the steroid/thyroid hormone receptors, and considered to be one of the major transcriptional regulators orchestrating cortical arealization, cell-type specification and maturation. Recent data have unraveled the multi-faceted functions of COUP-TFI in the development of several mouse brain structures, including the neocortex, hippocampus and ganglionic eminences. Despite NR2F1 mutations and deletions in humans have been linked to a complex neurodevelopmental disease mainly associated to optic atrophy and intellectual disability, its role during the formation of the retina and optic nerve remains unclear. In light of its major influence in cortical development, we predict that its haploinsufficiency might be the cause of other cognitive diseases, not identified so far. Mouse models offer a unique opportunity of dissecting COUP-TFI function in different regions during brain assembly; hence, the importance of comparing and discussing common points linking mouse models to human patients' symptoms.

Rostrocaudal Areal Patterning of Human PSC-Derived Cortical Neurons by FGF8 Signaling

The cerebral cortex is subdivided into distinct areas that have particular functions. The rostrocaudal (R-C) gradient of fibroblast growth factor 8 (FGF8) signaling defines this areal identity during neural development. In this study, we recapitulated cortical R-C patterning in human pluripotent stem cell (PSC) cultures. Modulation of FGF8 signaling appropriately regulated the R-C markers, and the patterns of global gene expression resembled those of the corresponding areas of human fetal brains. Furthermore, we demonstrated the utility of this culture system in modeling the area-specific forebrain phenotypes [presumptive upper motor neuron (UMN) phenotypes] of amyotrophic lateral sclerosis (ALS). We anticipate that our culture system will contribute to studies of human neurodevelopment and neurological disease modeling.

Sulcal pits and patterns in developing human brains

Spatial distribution and specific geometric and topological patterning of early sulcal folds have been hypothesized to be under stronger genetic control and are more associated with optimal organization of cortical functional areas and their white matter connections, compared to later developing sulci. Several previous studies of sulcal pit (putative first sulcal fold) distribution and sulcal pattern analyses using graph structures have provided evidence of the importance of sulcal pits and patterns as remarkable anatomical features closely related to human brain function, suggesting additional insights concerning the anatomical and functional development of the human brain. Recently, early sulcal folding patterns have been observed in healthy fetuses and fetuses with brain abnormalities such as polymicrogyria and agenesis of corpus callosum. Graph-based quantitative sulcal pattern analysis has shown high sensitivity in detecting emerging subtle abnormalities in cerebral cortical growth in early fetal stages that are difficult to detect via qualitative visual assessment or using traditional cortical measures such as gyrification index and curvature. It has proven effective for characterizing genetically influenced early cortical folding development. Future studies will be aimed at better understanding a comprehensive map of spatio-temporal dynamics of fetal cortical folding in a large longitudinal cohort in order to examine individual clinical fetal MRIs and predict postnatal neurodevelopmental outcomes from early fetal life.

Mechanical morphogenesis and the development of neocortical organisation

The development of complex neocortical organisations is thought to result from the interaction of genetic and activity-dependent processes. We propose that a third type of process - mechanical morphogenesis - may also play an important role. We review theoretical and experimental results in physics showing how even homogeneous growth can produce a variety of forms, in particular neocortical folding. The mechanical instabilities that produce these forms induce heterogeneous patterns of stress at the scale of the organ. We review the evidence showing how these stresses can influence cell proliferation, migration and apoptosis, cell differentiation and shape, migration and axonal guidance, and could thus be able to influence regional neocortical identity and connectivity.

Lhx2, an evolutionarily conserved, multifunctional regulator of forebrain development

A hundred years after Lhx2 ortholog apterous was identified as a critical regulator of wing development in Drosophila, LIM-HD gene family members have proved to be versatile and powerful components of the molecular machinery that executes the blueprint of embryogenesis across vertebrate and invertebrate species. Here, we focus on the spatio-temporally varied functions of LIM-homeodomain transcription factor LHX2 in the developing mouse forebrain. Right from its earliest known role in telencephalic and eye field patterning, to the control of the neuron-glia cell fate switch, and the regulation of axon pathfinding and dendritic arborization in late embryonic stages, LHX2 has been identified as a fundamental, temporally dynamic, always necessary, and often sufficient factor in a range of critical developmental phenomena. While Lhx2 mutant phenotypes have been characterized in detail in multiple brain structures, only recently have we advanced in our understanding of the molecular mechanisms by which this factor acts. Common themes emerge from how this multifunctional molecule controls a range of developmental steps in distinct forebrain structures. Examining these shared features, and noting unique aspects of LHX2 function is likely to inform our understanding of how a single factor can bring about a diversity of effects and play central and critical roles across systems and stages. The parallels in LHX2 and APTEROUS functions, and the protein complexes they participate in, offer insights into evolutionary strategies that conserve tool kits and deploy them to play new, yet familiar roles in species separated by hundreds of millions of years.

GDNF/GFRα1 Complex Abrogates Self-Renewing Activity of Cortical Neural Precursors Inducing Their Differentiation

The balance between factors leading to proliferation and differentiation of cortical neural precursors (CNPs) determines the correct cortical development. In this work, we show that GDNF and its receptor GFRα1 are expressed in the neocortex during the period of cortical neurogenesis. We show that the GDNF/GFRα1 complex inhibits the self-renewal capacity of mouse CNP cells induced by fibroblast growth factor 2 (FGF2), promoting neuronal differentiation. While GDNF leads to decreased proliferation of cultured cortical precursor cells, ablation of GFRα1 in glutamatergic cortical precursors enhances its proliferation. We show that GDNF treatment of CNPs promoted morphological differentiation even in the presence of the self-renewal-promoting factor, FGF2. Analysis of GFRα1-deficient mice shows an increase in the number of cycling cells during cortical development and a reduction in dendrite development of cortical GFRα1-expressing neurons. Together, these results indicate that GDNF/GFRα1 signaling plays an essential role in regulating the proliferative condition and the differentiation of cortical progenitors.

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GFRα1 receptor is expressed in the neocortex during the period of neurogenesis

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GDNF/GFRα1 complex inhibits self-renewing of cortical neuronal precursors

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GDNF and GFRα1 promote neurogenic differentiation of cortical neural progenitors

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Requirement of GFRα1 for proper dendrite development of cortical neurons

Untangling Cortical Complexity During Development

The cerebral cortex is composed of billions of morphologically and functionally distinct neurons. These neurons are produced and organized in a regimental fashion during development. The ability of neurons to encode and elicit complex cognitive and motor functions depends on their precise molecular processes, identity, and connectivity established during development. Elucidating the cellular and molecular mechanisms that regulate development of the neocortex has been a challenge for many years. The cerebral cortical neuronal subtypes are classified based on morphology, function, intrinsic synaptic properties, location, connectivity, and marker gene expression. Development of the neocortex requires an orchestration of a series of processes including the appropriate determination, migration and positioning of the neurons, acquisition of layer-specific transcriptional hallmarks, and formation of precise axonal projections and networks. Historically, fate mapping, genome-wide analysis, and transcriptome profiling have provided many opportunities for the characterization of neuronal subtypes. During the course of this review, we will address the regimental organization of the cerebral cortex, dissect the cellular subtypes that contribute to cortical complexity, and outline their molecular hallmarks to understand cellular diversity in the cerebral cortex with a focus on the excitatory neurons.

Gyrification of the cerebral cortex requires FGF signaling in the mammalian brain

Although it has been believed that the evolution of cortical folds was a milestone, allowing for an increase in the number of neurons in the cerebral cortex, the mechanisms underlying the formation of cortical folds are largely unknown. Here we show regional differences in the expression of fibroblast growth factor receptors (FGFRs) in the developing cerebral cortex of ferrets even before cortical folds are formed. By taking the advantage of our in utero electroporation technique for ferrets, we found that cortical folding was impaired in the ferret cerebral cortex when FGF signaling was inhibited. We also found that FGF signaling was crucial for producing Pax6-positive neural progenitors in the outer subventricular zone (OSVZ) of the developing cerebral cortex. Furthermore, we found that upper layers of the cerebral cortex were preferentially reduced by inhibiting FGF signaling. Our results shed light on the mechanisms of cortical folding in gyrencephalic mammalian brains.

Organizing activity of Fgf8 on the anterior telencephalon

The anterior part of the embryonic telencephalon gives rise to several brain regions that are important for animal behavior, including the frontal cortex (FC) and the olfactory bulb. The FC plays an important role in decision-making behaviors, such as social and cognitive behavior, and the olfactory bulb is involved in olfaction. Here, we show the organizing activity of fibroblast growth factor 8 (Fgf8) in the regionalization of the anterior telencephalon, specifically the FC and the olfactory bulb. Misexpression of Fgf8 in the most anterior part of the mouse telencephalon at embryonic day 11.5 (E11.5) by ex utero electroporation resulted in a lateral shift of dorsal FC subdivision markers and a lateral expansion of the dorsomedial part of the FC, the future anterior cingulate and prelimbic cortex. Fgf8-transfected brains had lacked ventral FC, including the future orbital cortex, which was replaced by the expanded olfactory bulb. The olfactory region occupied a larger area of the FC when transfection efficiency of Fgf8 was higher. These results suggest that Fgf8 regulates the proportions of the FC and olfactory bulb in the anterior telencephalon and has a medializing effect on the formation of FC subdivisions.

Inter-progenitor pool wiring: An evolutionarily conserved strategy that expands neural circuit diversity

Diversification of neuronal types is key to establishing functional variations in neural circuits. The first critical step to generate neuronal diversity is to organize the compartmental domains of developing brains into spatially distinct neural progenitor pools. Neural progenitors in each pool then generate a unique set of diverse neurons through specific spatiotemporal specification processes. In this review article, we focus on an additional mechanism, 'inter-progenitor pool wiring', that further expands the diversity of neural circuits. After diverse types of neurons are generated in one progenitor pool, a fraction of these neurons start migrating toward a remote brain region containing neurons that originate from another progenitor pool. Finally, neurons of different origins are intermingled and eventually form complex but precise neural circuits. The developing cerebral cortex of mammalian brains is one of the best examples of inter-progenitor pool wiring. However, Drosophila visual system development has revealed similar mechanisms in invertebrate brains, suggesting that inter-progenitor pool wiring is an evolutionarily conserved strategy that expands neural circuit diversity. Here, we will discuss how inter-progenitor pool wiring is accomplished in mammalian and fly brain systems.

Developing a de novo targeted knock-in method based on in utero electroporation into the mammalian brain

Genome-editing technology has revolutionized the field of biology. Here, we report a novel de novo gene-targeting method mediated by in utero electroporation into the developing mammalian brain. Electroporation of donor DNA with the CRISPR/Cas9 system vectors successfully leads to knock-in of the donor sequence, such as EGFP, to the target site via the homology-directed repair mechanism. We developed a targeting vector system optimized to prevent anomalous leaky expression of the donor gene from the plasmid, which otherwise often occurs depending on the donor sequence. The knock-in efficiency of the electroporated progenitors reached up to 40% in the early stage and 20% in the late stage of the developing mouse brain. Furthermore, we inserted different fluorescent markers into the target gene in each homologous chromosome, successfully distinguishing homozygous knock-in cells by color. We also applied this de novo gene targeting to the ferret model for the study of complex mammalian brains. Our results demonstrate that this technique is widely applicable for monitoring gene expression, visualizing protein localization, lineage analysis and gene knockout, all at the single-cell level, in developmental tissues.

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.

Genome Stability by DNA Polymerase β in Neural Progenitors Contributes to Neuronal Differentiation in Cortical Development

DNA repair is crucial for genome stability in the developing cortex, as somatic de novo mutations cause neurological disorders. However, how DNA repair contributes to neuronal development is largely unknown. To address this issue, we studied the spatiotemporal roles of DNA polymerase β (Polβ), a key enzyme in DNA base excision repair pathway, in the developing cortex using distinct forebrain-specific conditional knock-out mice, Emx1-Cre/Polβ ^fl/fl and Nex-Cre/Polβ ^fl/fl mice. Polβ expression was absent in both neural progenitors and postmitotic neurons in Emx1-Cre/Polβ ^fl/fl mice, whereas only postmitotic neurons lacked Polβ expression in Nex-Cre/Polβ ^fl/fl mice. We found that DNA double-strand breaks (DSBs) were frequently detected during replication in cortical progenitors of Emx1-Cre/Polβ ^fl/fl mice. Increased DSBs remained in postmitotic cells, which resulted in p53-mediated neuronal apoptosis. This neuronal apoptosis caused thinning of the cortical plate, although laminar structure was normal. In addition, accumulated DSBs also affected growth of corticofugal axons but not commissural axons. These phenotypes were not observed in Nex-Cre/Polβ ^fl/fl mice. Moreover, cultured Polβ-deficient neural progenitors exhibited higher sensitivity to the base-damaging agent methylmethanesulfonate, resulting in enhanced DSB formation. Similar damage was found by vitamin C treatment, which induces TET1-mediated DNA demethylation via 5-hydroxymethylcytosine. Together, genome stability mediated by Polβ-dependent base excision repair is crucial for the competence of neural progenitors, thereby contributing to neuronal differentiation in cortical development.SIGNIFICANCE STATEMENT DNA repair is crucial for development of the nervous system. However, how DNA polymerase β (Polβ)-dependent DNA base excision repair pathway contributes to the process is still unknown. We found that loss of Polβ in cortical progenitors rather than postmitotic neurons led to catastrophic DNA double-strand breaks (DSBs) during replication and p53-mediated neuronal apoptosis, which resulted in thinning of the cortical plate. The DSBs also affected corticofugal axon growth in surviving neurons. Moreover, induction of base damage and DNA demethylation intermediates in the genome increased DSBs in cultured Polβ-deficient neural progenitors. Thus, genome stability by Polβ-dependent base excision repair in neural progenitors is required for the viability and differentiation of daughter neurons in the developing nervous system.

Behavioral Consequences of a Bifacial Map in the Mouse Somatosensory Cortex

The whisker system is an important sensory organ with extensive neural representations in the brain of the mouse. Patterned neural modules (barrelettes) in the ipsilateral principal sensory nucleus of the trigeminal nerve (PrV) correspond to the whiskers. Axons of the PrV barrelette neurons cross the midline and confer the whisker-related patterning to the contralateral ventroposteromedial nucleus of the thalamus, and subsequently to the cortex. In this way, specific neural modules called barreloids and barrels in the contralateral thalamus and cortex represent each whisker. Partial midline crossing of the PrV axons, in a conditional Robo3 mutant (Robo3R3-5cKO) mouse line, leads to the formation of bilateral whisker maps in the ventroposteromedial, as well as the barrel cortex. We used voltage-sensitive dye optical imaging and somatosensory and motor behavioral tests to characterize the consequences of bifacial maps in the thalamocortical system. Voltage-sensitive dye optical imaging verified functional, bilateral whisker representation in the barrel cortex and activation of distinct cortical loci following ipsilateral and contralateral stimulation of the specific whiskers. The mutant animals were comparable with the control animals in sensorimotor tests. However, they showed noticeable deficits in all of the whisker-dependent or -related tests, including Y-maze exploration, horizontal surface approach, bridge crossing, gap crossing, texture discrimination, floating in water, and whisking laterality. Our results indicate that bifacial maps along the thalamocortical system do not offer a functional advantage. Instead, they lead to impairments, possibly due to the smaller size of the whisker-related modules and interference between the ipsilateral and contralateral whisker representations in the same thalamus and cortex.SIGNIFICANCE STATEMENT The whisker sensory system plays a quintessentially important role in exploratory behavior of mice and other nocturnal rodents. Here, we studied a novel mutant mouse line, in which the projections from the brainstem to the thalamus are disrupted. This led to formation of bilateral whisker maps in both the thalamus and the cortex. The two whisker maps crowd in a space normally devoted to the contralateral map alone and in a nonoverlapping fashion. Stimulation of the whiskers on either side activates the corresponding region of the map. Mice with bilateral whisker maps perform well in general sensorimotor tasks but show poor performance in specific tests that require whisker-dependent tactile discrimination. These observations indicate that contralateral, instead of bilateral, representation of the sensory space plays a critical role in acuity and fine discrimination during somesthesis.

Nucleocytoplasmic Shuttling of Histone Deacetylase 9 Controls Activity-Dependent Thalamocortical Axon Branching

During development, thalamocortical (TC) axons form branches in an activity-dependent fashion. Here we investigated how neuronal activity is converted to molecular signals, focusing on an epigenetic mechanism involving histone deacetylases (HDACs). Immunohistochemistry demonstrated that HDAC9 was translocated from the nucleus to the cytoplasm of thalamic cells during the first postnatal week in rats. In organotypic co-cultures of the thalamus and cortex, fluorescent protein-tagged HDAC9 also exhibited nuclueocytoplasmic translocation in thalamic cells during culturing, which was reversed by tetrodotoxin treatment. Transfection with a mutant HDAC9 that interferes with the translocation markedly decreased TC axon branching in the culture. Similarly, TC axon branching was significantly decreased by the mutant HDAC9 gene transfer in vivo. However, axonal branching was restored by disrupting the interaction between HDAC9 and myocyte-specific enhancer factor 2 (MEF2). Taken together, the present results demonstrate that the nucleocytoplasmic translocation of HDAC9 plays a critical role in activity-dependent TC axon branching by affecting transcriptional regulation and downstream signaling pathways.

Fate and freedom in developing neocortical circuits

The activity of neuronal circuits of the neocortex underlies our ability to perceive the world and interact with our environment. During development, these circuits emerge from dynamic interactions between cell-intrinsic, genetically determined programs and input/activity-dependent signals, which together shape these circuits into adulthood. Building on a large body of experimental work, several recent technological developments now allow us to interrogate these nature–nurture interactions with single gene/single input/single-cell resolution. Focusing on excitatory glutamatergic neurons, this review discusses the genetic and input-dependent mechanisms controlling how individual cortical neurons differentiate into specialized cells to assemble into stereotypical local circuits within global, large-scale networks.

Proper functioning of the neocortex – the center of higher-order brain functions – depends on the correct assembly of neocortical neural circuits during development. Here the author discusses how cell-intrinsic developmental programs and activity-dependent signals together shape the formation of neocortical circuits.

Sox11 Balances Dendritic Morphogenesis with Neuronal Migration in the Developing Cerebral Cortex

The coordinated mechanisms balancing promotion and suppression of dendritic morphogenesis are crucial for the development of the cerebral cortex. Although previous studies have revealed important transcription factors that promote dendritic morphogenesis during development, those that suppress dendritic morphogenesis are still largely unknown. Here we found that the expression levels of the transcription factor Sox11 decreased dramatically during dendritic morphogenesis. Our loss- and gain-of-function studies using postnatal electroporation and in utero electroporation indicate that Sox11 is necessary and sufficient for inhibiting dendritic morphogenesis of excitatory neurons in the mouse cerebral cortex during development. Interestingly, we found that precocious suppression of Sox11 expression caused precocious branching of neurites and a neuronal migration defect. We also found that the end of radial migration induced the reduction of Sox11 expression. These findings indicate that suppression of dendritic morphogenesis by Sox11 during radial migration is crucial for the formation of the cerebral cortex.

SIGNIFICANCE STATEMENT

Because dendritic morphology has profound impacts on neuronal information processing, the mechanisms underlying dendritic morphogenesis during development are of great interest. Our loss- and gain-of-function studies indicate that Sox11 is necessary and sufficient for inhibiting dendritic morphogenesis of excitatory neurons in the mouse cerebral cortex during development. Interestingly, we found that precocious suppression of Sox11 expression caused a neuronal migration defect. These findings indicate that suppression of dendritic morphogenesis by Sox11 during radial migration is crucial for the formation of the cerebral cortex.

Foxp2 Regulates Identities and Projection Patterns of Thalamic Nuclei During Development

The molecular mechanisms underlying the formation of the thalamus during development have been investigated intensively. Although transcription factors distinguishing the thalamic primordium from adjacent brain structures have been uncovered, those involved in patterning inside the thalamus are largely unclear. Here, we show that Foxp2, a member of the forkhead transcription factor family, regulates thalamic patterning during development. We found a graded expression pattern of Foxp2 in the thalamic primordium of the mouse embryo. The expression levels of Foxp2 were high in the posterior region and low in the anterior region of the thalamic primordium. In Foxp2 (R552H) knockin mice, which have a missense loss-of-function mutation in the forkhead domain of Foxp2, thalamic nuclei of the posterior region of the thalamus were shrunken, while those of the intermediate region were expanded. Consistently, Foxp2 (R552H) knockin mice showed changes in thalamocortical projection patterns. Our results uncovered important roles of Foxp2 in thalamic patterning and thalamocortical projections during development.

Mouse Fgf8-Cre-LacZ lineage analysis defines the territory of the postnatal mammalian isthmus

The isthmus is recognized as the most rostral segment of the hindbrain in non-mammalian vertebrates. In mammalian embryos, transient Fgf8 expression defines the developing isthmic region, lying between the midbrain and the first rhombomere, but there has been uncertainty about the existence of a distinct isthmic segment in postnatal mammals. We attempted to find if the region of early embryonic Fgf8 expression (which is considered to involve the entire extent of the prospective isthmus initially) might help to identify the boundaries of the isthmus in postnatal animals. By creating an Fgf8-Cre-LacZ lineage in mice, we were able to show that Fgf8-Cre reporter expression in postnatal mice is present in the same nuclei that characterize the isthmic region in birds. The 'signature' isthmic structures in birds include the trochlear nucleus, the dorsal raphe nucleus, the microcellular tegmental nuclei, the pedunculotegmental nucleus, the vermis of the cerebellum, rostral parts of the parabrachial complex and locus coeruleus, and the caudal parts of the substantia nigra and VTA. We found that all of these structures were labeled with the Fgf8-Cre reporter in the mouse brain, and we conclude that the isthmus is a distinct segment of the mammalian brain lying caudal to the midbrain and rostral to rhombomere 1 of the hindbrain.

Autism spectrum disorder-associated genes and the development of dentate granule cells

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by severe clinical symptoms such as the deficiency of the social communication, repetitive and stereotyped behaviors, and restricted interests. Although complex genetic and environmental factors are thought to contribute to the development of ASD, the precise etiologies are largely unknown. Neuroanatomical observations have been made of developmental abnormalities in different brain regions, including dentate gyrus of hippocampus, which is widely accepted as the center for learning and memory. However, little is known about what roles ASD-associated genes play in the development of hippocampal dentate granule cells. In this article, we summarized functions and pathophysiological significance of 6 representative ASD-associated genes, SEMA5A, PTEN, NLGN, EN-2, FMR1, and MECP2, by focusing on the development of dentate gyrus. We then introduced a recently developed gene transfer method directed to neonatal dentate granule cells. This new method will be useful for elucidating physiological as well as pathophysiological significance of ASD-associated genes in the development of hippocampal formation.

Pathophysiological analyses of periventricular nodular heterotopia using gyrencephalic mammals

Although periventricular nodular heterotopia (PNH) is often found in the cerebral cortex of people with thanatophoric dysplasia (TD), the pathophysiology of PNH in TD is largely unknown. This is mainly because of difficulties in obtaining brain samples of TD patients and a lack of appropriate animal models for analyzing the pathophysiology of PNH in TD. Here we investigate the pathophysiological mechanisms of PNH in the cerebral cortex of TD by utilizing a ferret TD model which we recently developed. To make TD ferrets, we electroporated fibroblast growth factor 8 (FGF8) into the cerebral cortex of ferrets. Our immunohistochemical analyses showed that PNH nodules in the cerebral cortex of TD ferrets were mostly composed of cortical neurons, including upper layer neurons and GABAergic neurons. We also found disorganizations of radial glial fibers and of the ventricular lining in the TD ferret cortex, indicating that PNH may result from defects in radial migration of cortical neurons along radial glial fibers during development. Our findings provide novel mechanistic insights into the pathogenesis of PNH in TD.

In Vitro, Ex Vivo and In Vivo Techniques to Study Neuronal Migration in the Developing Cerebral Cortex

Neuronal migration is a fundamental biological process that underlies proper brain development and neuronal circuit formation. In the developing cerebral cortex, distinct neuronal populations, producing excitatory, inhibitory and modulatory neurotransmitters, are generated in different germinative areas and migrate along various routes to reach their final positions within the cortex. Different technical approaches and experimental models have been adopted to study the mechanisms regulating neuronal migration in the cortex. In this review, we will discuss the most common in vitro, ex vivo and in vivo techniques to visualize and study cortical neuronal migration.

Quantitative assessment of fibroblast growth factor receptor 1 expression in neurons and glia

Background

Fibroblast growth factors (FGFs) and their receptors (FGFRs) have numerous functions in the developing and adult central nervous system (CNS). For example, the FGFR1 receptor is important for proliferation and fate specification of radial glial cells in the cortex and hippocampus, oligodendrocyte proliferation and regeneration, midline glia morphology and soma translocation, Bergmann glia morphology, and cerebellar morphogenesis. In addition, FGFR1 signaling in astrocytes is required for postnatal maturation of interneurons expressing parvalbumin (PV). FGFR1 is implicated in synapse formation in the hippocampus, and alterations in the expression of Fgfr1 and its ligand, Fgf2 accompany major depression. Understanding which cell types express Fgfr1 during development may elucidate its roles in normal development of the brain as well as illuminate possible causes of certain neuropsychiatric disorders.

Methods

Here, we used a BAC transgenic reporter line to trace Fgfr1 expression in the developing postnatal murine CNS. The specific transgenic line employed was created by the GENSAT project, tgFGFR1-EGFPGP338Gsat, and includes a gene encoding enhanced green fluorescent protein (EGFP) under the regulation of the Fgfr1 promoter, to trace Fgfr1 expression in the developing CNS. Unbiased stereological counts were performed for several cell types in the cortex and hippocampus.

Results

This model reveals that Fgfr1 is primarily expressed in glial cells, in both astrocytes and oligodendrocytes, along with some neurons. Dual labeling experiments indicate that the proportion of GFP+ (Fgfr1+) cells that are also GFAP+ increases from postnatal day 7 (P7) to 1 month, illuminating dynamic changes in Fgfr1 expression during postnatal development of the cortex. In postnatal neurogenic areas, GFP expression was also observed in SOX2, doublecortin (DCX), and brain lipid-binding protein (BLBP) expressing cells. Fgfr1 is also highly expressed in DCX positive cells of the dentate gyrus (DG), but not in the rostral migratory stream. Fgfr1 driven GFP was also observed in tanycytes and GFAP+ cells of the hypothalamus, as well as in Bergmann glia and astrocytes of the cerebellum.

Conclusions

The tgFGFR1-EGFPGP338Gsat mouse model expresses GFP that is congruent with known functions of FGFR1, including hippocampal development, glial cell development, and stem cell proliferation. Understanding which cell types express Fgfr1 may elucidate its role in neuropsychiatric disorders and brain development.

Building a lineage from single cells: genetic techniques for cell lineage tracking

Resolving lineage relationships between cells in an organism is a fundamental interest of developmental biology. Furthermore, investigating lineage can drive understanding of pathological states, including cancer, as well as understanding of developmental pathways that are amenable to manipulation by directed differentiation. Although lineage tracking through the injection of retroviral libraries has long been the state of the art, a recent explosion of methodological advances in exogenous labelling and single-cell sequencing have enabled lineage tracking at larger scales, in more detail, and in a wider range of species than was previously considered possible. In this Review, we discuss these techniques for cell lineage tracking, with attention both to those that trace lineage forwards from experimental labelling, and those that trace backwards across the life history of an organism.

A mutant with bilateral whisker to barrel inputs unveils somatosensory mapping rules in the cerebral cortex

In mammals, tactile information is mapped topographically onto the contralateral side of the brain in the primary somatosensory cortex (S1). In this study, we describe Robo3 mouse mutants in which a sizeable fraction of the trigemino-thalamic inputs project ipsilaterally rather than contralaterally. The resulting mixture of crossed and uncrossed sensory inputs creates bilateral whisker maps in the thalamus and cortex. Surprisingly, these maps are segregated resulting in duplication of whisker representations and doubling of the number of barrels without changes in the size of S1. Sensory deprivation shows competitive interactions between the ipsi/contralateral whisker maps. This study reveals that the somatosensory system can form a somatotopic map to integrate bilateral sensory inputs, but organizes the maps in a different way from that in the visual or auditory systems. Therefore, while molecular pre-patterning constrains their orientation and position, preservation of the continuity of inputs defines the layout of the somatosensory maps.

Temporal variations in early developmental decisions: an engine of forebrain evolution

Tight control of developmental timing is pivotal to many major processes in developmental biology, such as patterning, fate specification, cell cycle dynamics, cell migration and connectivity. Temporal change in these ontogenetic sequences is known as heterochrony, a major force in the evolution of body plans and organogenesis. In the last 5 years, studies in fish and rodents indicate that heterochrony in signaling during early development generates diversity in forebrain size and complexity. Here, we summarize these findings and propose that, additionally to spatio-temporal tuning of neurogenesis, temporal and quantitative modulation of signaling events drive pivotal changes in shape, size and complexity of the forebrain across evolution, participating to the generation of diversity in animal behavior and emergence of cognition.

Molecular investigations of development and diseases of the brain of higher mammals using the ferret

The brains of higher mammals such as primates and carnivores contain well-developed unique brain structures. Uncovering the physiological functions, developmental mechanisms and evolution of these brain structures would greatly facilitate our understanding of the human brain and its diseases. Although the anatomical and electrophysiological features of these brain structures have been intensively investigated, our knowledge about their molecular bases is still limited. To overcome this limitation, genetic techniques for the brains of carnivores and primates have been established, and molecules whose expression patterns correspond to these brain structures were identified recently. To investigate the functional roles of these molecules, rapid and efficient genetic manipulation methods for higher mammals have been explored. In this review, recent advances in molecular investigations of the brains of higher mammals are discussed, mainly focusing on ferrets (Mustela putorius furo).

An S116R Phosphorylation Site Mutation in Human Fibroblast Growth Factor-1 Differentially Affects Mitogenic and Glucose-Lowering Activities

Fibroblast growth factor-1 (FGF-1), a potent human mitogen and insulin sensitizer, signals through both tyrosine kinase receptor-mediated autocrine/paracrine pathways as well as a nuclear intracrine pathway. Phosphorylation of FGF-1 at serine 116 (S116) has been proposed to regulate intracrine signaling. Position S116 is located within a ∼17 amino acid C-terminal loop that contains a rich set of functional determinants including heparin∖heparan sulfate affinity, thiol reactivity, nuclear localization, pharmacokinetics, functional half-life, nuclear ligand affinity, stability, and structural dynamics. Mutational targeting of specific functionality in this region without perturbing other functional determinants is a design challenge. S116R is a non-phosphorylatable variant present in bovine FGF-1 and other members of the human FGF family. We show that the S116R mutation in human FGF-1 is accommodated with no perturbation of biophysical or structural properties, and is therefore an attractive mutation with which to elucidate the functional role of phosphorylation. Characterization of S116R shows reduction in NIH 3T3 fibroblast mitogenic stimulation, increase in fibroblast growth factor receptor-1c activation, and prolonged duration of glucose lowering in ob/ob hyperglycemic mice. A novel FGF-1/fibroblast growth factor receptor-1c dimerization interaction combined with non-phosphorylatable intracrine signaling is hypothesized to be responsible for these observed functional effects.

Akirin2 is essential for the formation of the cerebral cortex

Background

The proper spatial and temporal regulation of dorsal telencephalic progenitor behavior is a prerequisite for the formation of the highly-organized, six-layered cerebral cortex. Premature differentiation of cells, disruption of cell cycle timing, excessive apoptosis, and/or incorrect neuronal migration signals can have devastating effects, resulting in a number of neurodevelopmental disorders involving microcephaly and/or lissencephaly. Though genes encoding many key players in cortical development have been identified, our understanding remains incomplete. We show that the gene encoding Akirin2, a small nuclear protein, is expressed in the embryonic telencephalon. Converging evidence indicates that Akirin2 acts as a bridge between transcription factors (including Twist and NF-κB proteins) and the BAF (SWI/SNF) chromatin remodeling machinery to regulate patterns of gene expression. Constitutive knockout of Akirin2 is early embryonic lethal in mice, while restricted loss in B cells led to disrupted proliferation and cell survival.

Methods

We generated cortex-restricted Akirin2 knockouts by crossing mice harboring a floxed Akirin2 allele with the Emx1-Cre transgenic line and assessed the resulting embryos using in situ hybridization, EdU labeling, and immunohistochemistry.

Results

The vast majority of Akirin2 mutants do not survive past birth, and exhibit extreme microcephaly, with little dorsal telencephalic tissue and no recognizable cortex. This is primarily due to massive cell death of early cortical progenitors, which begins at embryonic day (E)10, shortly after Emx1-Cre is active. Immunostaining and cell cycle analysis using EdU labeling indicate that Akirin2-null progenitors fail to proliferate normally, produce fewer neurons, and undergo extensive apoptosis. All of the neurons that are generated in Akirin2 mutants also undergo apoptosis by E12. In situ hybridization for Wnt3a and Wnt-responsive genes suggest defective formation and/or function of the cortical hem in Akirin2 null mice. Furthermore, the apical ventricular surface becomes disrupted, and Sox2-positive progenitors are found to “spill” into the lateral ventricle.

Conclusions

Our data demonstrate a previously-unsuspected role for Akirin2 in early cortical development and, given its known nuclear roles, suggest that it may act to regulate gene expression patterns critical for early progenitor cell behavior and cortical neuron production.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-016-0076-8) contains supplementary material, which is available to authorized users.

Cerebral Cortex Electroporation to Study Projection Neuron Migration

Brain electroporation is a rapid and powerful approach to study neuronal development. In particular, this technique has become a method of choice for studying the process of radial migration of projection neurons in the embryonic cerebral cortex. This method has considerably helped to describe in detail the different steps of radial migration and to characterize the molecular mechanisms controlling this process. Delineating the complexities of neuronal migration is critical to our understanding not only of normal cerebral cortex formation but also of neurodevelopmental disorders resulting from neuronal migration defects. Here, we describe in detail the protocols to perform in utero or ex vivo electroporation of progenitor cells in the ventricular zone of the cerebral cortex with the aim of studying the process of radial migration of projection neurons during embryonic development. © 2016 by John Wiley & Sons, Inc.

How Wnt Signaling Builds the Brain: Bridging Development and Disease

Wnt/β-catenin signaling plays a crucial role throughout all stages of brain development and remains important in the adult brain. Accordingly, many neurological disorders have been linked to Wnt signaling. Defects in Wnt signaling during neural development can give rise to birth defects or lead to neurological dysfunction later in life. Developmental signaling events can also be hijacked in the adult and result in disease. Moreover, knowledge about the physiological role of Wnt signaling in the brain might lead to new therapeutic strategies for neurological diseases. Especially, the important role for Wnt signaling in neural differentiation of pluripotent stem cells has received much attention as this might provide a cure for neurodegenerative disorders. In this review, we summarize the versatile role of Wnt/β-catenin signaling during neural development and discuss some recent studies linking Wnt signaling to neurological disorders.

Distinct Genetic Influences on Cortical and Subcortical Brain Structures

This study examined the heritability of brain grey matter structures in a subsample of older adult twins (93 MZ and 68 DZ twin pairs; mean age 70 years) from the Older Australian Twins Study. The heritability estimates of subcortical regions ranged from 0.41 (amygdala) to 0.73 (hippocampus), and of cortical regions, from 0.55 (parietal lobe) to 0.78 (frontal lobe). Corresponding structures in the two hemispheres were influenced by the same genetic factors and high genetic correlations were observed between the two hemispheric regions. There were three genetically correlated clusters, comprising (i) the cortical lobes (frontal, temporal, parietal and occipital lobes); (ii) the basal ganglia (caudate, putamen and pallidum) with weak genetic correlations with cortical lobes, and (iii) the amygdala, hippocampus, thalamus and nucleus accumbens grouped together, which genetically correlated with both basal ganglia and cortical lobes, albeit relatively weakly. Our study demonstrates a complex but patterned and clustered genetic architecture of the human brain, with divergent genetic determinants of cortical and subcortical structures, in particular the basal ganglia.

Brain evolution by brain pathway duplication

Understanding the mechanisms of evolution of brain pathways for complex behaviours is still in its infancy. Making further advances requires a deeper understanding of brain homologies, novelties and analogies. It also requires an understanding of how adaptive genetic modifications lead to restructuring of the brain. Recent advances in genomic and molecular biology techniques applied to brain research have provided exciting insights into how complex behaviours are shaped by selection of novel brain pathways and functions of the nervous system. Here, we review and further develop some insights to a new hypothesis on one mechanism that may contribute to nervous system evolution, in particular by brain pathway duplication. Like gene duplication, we propose that whole brain pathways can duplicate and the duplicated pathway diverge to take on new functions. We suggest that one mechanism of brain pathway duplication could be through gene duplication, although other mechanisms are possible. We focus on brain pathways for vocal learning and spoken language in song-learning birds and humans as example systems. This view presents a new framework for future research in our understanding of brain evolution and novel behavioural traits.

Conservation of Distinct Genetically-Mediated Human Cortical Pattern

The many subcomponents of the human cortex are known to follow an anatomical pattern and functional relationship that appears to be highly conserved between individuals. This suggests that this pattern and the relationship among cortical regions are important for cortical function and likely shaped by genetic factors, although the degree to which genetic factors contribute to this pattern is unknown. We assessed the genetic relationships among 12 cortical surface areas using brain images and genotype information on 2,364 unrelated individuals, brain images on 466 twin pairs, and transcriptome data on 6 postmortem brains in order to determine whether a consistent and biologically meaningful pattern could be identified from these very different data sets. We find that the patterns revealed by each data set are highly consistent (p<10⁻³), and are biologically meaningful on several fronts. For example, close genetic relationships are seen in cortical regions within the same lobes and, the frontal lobe, a region showing great evolutionary expansion and functional complexity, has the most distant genetic relationship with other lobes. The frontal lobe also exhibits the most distinct expression pattern relative to the other regions, implicating a number of genes with known functions mediating immune and related processes. Our analyses reflect one of the first attempts to provide an assessment of the biological consistency of a genetic phenomenon involving the brain that leverages very different types of data, and therefore is not just statistical replication which purposefully use very similar data sets.

Although functional and anatomical connections among cortical regions have been intensively explored, genetically-mediated relationships between cortical regions have not been pursued to the same degree. Identifying genetic factors that mediate these relationships among different brain subcomponents can provide insight into how the human brain is organized and functions. We have assessed the genetic relationships among cortical regions using an integrated approach that considers twin data, genotype information among a large set of unrelated individuals, and gene expression measurements from postmortem neural tissues. We looked for evidence that subsets of cortical brain regions are under common or unique genetic control. We found that the patterns of genetic relationships are highly consistent across three independent data sets and multiple lines of evidence, suggesting that the patterning of cortical surface area is strongly mediated by genetic factors and, furthermore, likely reflects underlying anatomical and possibly functional relationships among cortical brain regions.

An essential role of SVZ progenitors in cortical folding in gyrencephalic mammals

Because folding of the cerebral cortex in the mammalian brain is believed to be crucial for higher brain functions, the mechanisms underlying its formation during development and evolution are of great interest. Although it has been proposed that increased neural progenitors in the subventricular zone (SVZ) are responsible for making cortical folds, their roles in cortical folding are still largely unclear, mainly because genetic methods for gyrencephalic mammals had been poorly available. Here, by taking an advantage of our newly developed in utero electroporation technique for the gyrencephalic brain of ferrets, we investigated the role of SVZ progenitors in cortical folding. We found regional differences in the abundance of SVZ progenitors in the developing ferret brain even before cortical folds began to be formed. When Tbr2 transcription factor was inhibited, intermediate progenitor cells were markedly reduced in the ferret cerebral cortex. Interestingly, outer radial glial cells were also reduced by inhibiting Tbr2. We uncovered that reduced numbers of SVZ progenitors resulted in impaired cortical folding. When Tbr2 was inhibited, upper cortical layers were preferentially reduced in gyri compared to those in sulci. Our findings indicate the biological importance of SVZ progenitors in cortical folding in the gyrencephalic brain.

Development of Cortical Maps: Perspectives From the Barrel Cortex

One approach to examining how higher sensory, motor, and cognitive faculties emerge in the neocortex is to elucidate the underlying wiring principles of the brain during development. The mammalian neocortex is a layered structure generated from a sheet of proliferating ventricular cells that progressively divide to form specific functional areas, such as the primary somatosensory (S1) and motor (M1) cortices. The basic wiring pattern in each of these functional areas is based on a similar framework, but is distinct in detail. Functional specialization in each area derives from a combination of molecular cues within the cortex and neuronal activity-dependent cues provided by innervating axons from the thalamus. One salient feature of neocortical development is the establishment of topographic maps in which neighboring neurons receive input relayed from neighboring sensory afferents. Barrels, which are prominent sensory units in the somatosensory cortex of rodents, have been examined in detail, and data suggest that the initial, gross formation of the barrel map relies on molecular cues, but the refinement of this topography depends on neuronal activity. Several excellent reviews have been published on the patterning and plasticity of the barrel cortex and the precise targeting of ventrobasal thalamic axons. In this review, the authors will focus on the formation and functional maturation of synapses between thalamocortical axons and cortical neurons, an event that coincides with the formation of the barrel map. They will briefly review cortical patterning and the initial targeting of thalamic axons, with an emphasis on recent findings. The rest of the review will be devoted to summarizing their understanding of the cellular and molecular mechanisms underlying thalamocortical synapse maturation and its role in barrel map formation.

Differentiation of human pluripotent stem cells into Medial Ganglionic Eminence vs. Caudal Ganglionic Eminence cells

Human pluripotent stem cells (PSCs) represent an opportunity to study human development in vitro, to model diseases in a dish, to screen drugs as well as to provide an unlimited and ethically unimpeded source of therapeutic cells. Cortical GABAergic interneurons, which are generated from Medial Ganglionic Eminence (MGE) cells and Caudal Ganglionic Eminence (CGE) cells during embryonic development, regulate cortical neural networks by providing inhibitory inputs. Their malfunction, resulting in failure to intricately regulate neural circuit balance, has been implicated in brain diseases, such as schizophrenia, autism and epilepsy. In this study, using combinatorial and temporal modulation of developmentally relevant dorsoventral and rostrocaudal signaling pathways, we efficiently generated MGE cells vs. CGE cells from human PSCs, which predominantly generate Parvalbumin-expressing or Somatostatin-expressing interneurons vs. Calretinin-expressing interneurons, respectively. Efficient generation of specific differentiated progenies of hPSCs as shown in this study will be a pivotal step to realize the full potential of hPSCs for regenerative medicine, developmental studies, disease modeling, bioassay, and drug screening.