Cortical neurons require Otx1 for the refinement of exuberant axonal projections to subcortical targets

GWAS meta-analysis reveals key risk loci in essential tremor pathogenesis

Essential tremor (ET) is a prevalent neurological disorder with a largely unknown underlying biology. In this genome-wide association study meta-analysis, comprising 16,480 ET cases and 1,936,173 controls from seven datasets, we identify 12 sequence variants at 11 loci. Evaluating mRNA expression, splicing, plasma protein levels, and coding effects, we highlight seven putative causal genes at these loci, including CA3 and CPLX1. CA3 encodes Carbonic Anhydrase III and carbonic anhydrase inhibitors have been shown to decrease tremors. CPLX1, encoding Complexin-1, regulates neurotransmitter release. Through gene-set enrichment analysis, we identify a significant association with specific cell types, including dopaminergic and GABAergic neurons, as well as biological processes like Rho GTPase signaling. Genetic correlation analyses reveals a positive association between ET and Parkinson's disease, depression, and anxiety-related phenotypes. This research uncovers risk loci, enhancing our knowledge of the complex genetics of this common but poorly understood disorder, and highlights CA3 and CPLX1 as potential therapeutic targets.

Revealing the Impact of Mitochondrial Fitness During Early Neural Development Using Human Brain Organoids

Mitochondrial homeostasis -including function, morphology, and inter-organelle communication- provides guidance to the intrinsic developmental programs of corticogenesis, while also being responsive to environmental and intercellular signals. Two- and three-dimensional platforms have become useful tools to interrogate the capacity of cells to generate neuronal and glia progeny in a background of metabolic dysregulation, but the mechanistic underpinnings underlying the role of mitochondria during human neurogenesis remain unexplored. Here we provide a concise overview of cortical development and the use of pluripotent stem cell models that have contributed to our understanding of mitochondrial and metabolic regulation of early human brain development. We finally discuss the effects of mitochondrial fitness dysregulation seen under stress conditions such as metabolic dysregulation, absence of developmental apoptosis, and hypoxia; and the avenues of research that can be explored with the use of brain organoids.

The Association of OTX1 rs17850223 Polymorphisms in Han Chinese Patients with Idiopathic Epilepsy

This study is aimed at investigating the association between orthodenticle homeobox 1 (OTX1) gene polymorphisms and idiopathic epilepsy in a cohort of Han Chinese patients. We carried out a case-control study on 147 patients with idiopathic epilepsy and 150 healthy controls. Genomic DNA was isolated from 1 ml of ethylene diamine tetraacetic acid (EDTA)-treated blood. The OTX1 coding sequence was divided into three parts and amplified using PCR, and the products were genotyped using the Sanger sequencing method. All OTX1 coding sequences were conserved except for rs17850223 located on the fifth exon. The frequency of the CC, CG, and GG genotypes showed no statistical differences between the idiopathic epileptic patients and the controls. The rs17850223 G allele distribution was also similar between the idiopathic epileptic patients and the controls. Interestingly, the frequency of the GG genotype was significantly higher in the patients with generalized seizures compared with that of the controls (12.2% vs. 2%, p = 0.012), and a greater distribution of the rs17850223 G allele was also seen in the patients with generalized seizures compared with controls (18.3% vs. 10%, p = 0.049). rs17850223 might play a critical role in Chinese idiopathic epileptic patients with generalized seizure activity.

CRMP2 mediates Sema3F-dependent axon pruning and dendritic spine remodeling

Regulation of axon guidance and pruning of inappropriate synapses by class 3 semaphorins are key to the development of neural circuits. Collapsin response mediator protein 2 (CRMP2) has been shown to regulate axon guidance by mediating semaphorin 3A (Sema3A) signaling; however, nothing is known about its role in synapse pruning. Here, using newly generated crmp2^-/- mice we demonstrate that CRMP2 has a moderate effect on Sema3A-dependent axon guidance in vivo, and its deficiency leads to a mild defect in axon guidance in peripheral nerves and the corpus callosum. Surprisingly, crmp2^-/- mice display prominent defects in stereotyped axon pruning in hippocampus and visual cortex and altered dendritic spine remodeling, which is consistent with impaired Sema3F signaling and with models of autism spectrum disorder (ASD). We demonstrate that CRMP2 mediates Sema3F signaling in primary neurons and that crmp2^-/- mice display ASD-related social behavior changes in the early postnatal period as well as in adults. Together, we demonstrate that CRMP2 mediates Sema3F-dependent synapse pruning and its dysfunction shares histological and behavioral features of ASD.

Rodent Models of Developmental Ischemic Stroke for Translational Research: Strengths and Weaknesses

Cerebral ischemia can occur at any stage in life, but clinical consequences greatly differ depending on the developmental stage of the affected brain structures. Timing of the lesion occurrence seems to be critical, as it strongly interferes with neuronal circuit development and determines the way spontaneous plasticity takes place. Translational stroke research requires the use of animal models as they represent a reliable tool to understand the pathogenic mechanisms underlying the generation, progression, and pathological consequences of a stroke. Moreover, in vivo experiments are instrumental to investigate new therapeutic strategies and the best temporal window of intervention. Differently from adults, very few models of the human developmental stroke have been characterized, and most of them have been established in rodents. The models currently used provide a better understanding of the molecular factors involved in the effects of ischemia; however, they still hold many limitations due to matching developmental stages across different species and the complexity of the human disorder that hardly can be described by segregated variables. In this review, we summarize the key factors contributing to neonatal brain vulnerability to ischemic strokes and we provide an overview of the advantages and limitations of the currently available models to recapitulate different aspects of the human developmental stroke.

Myrf ER-Bound Transcription Factors Drive C. elegans Synaptic Plasticity via Cleavage-Dependent Nuclear Translocation

Synaptic refinement is a critical step in nervous system maturation, requiring a carefully timed reorganization and refinement of neuronal connections. We have identified myrf-1 and myrf-2, two C. elegans homologs of Myrf family transcription factors, as key regulators of synaptic rewiring. MYRF-1 and its paralog MYRF-2 are functionally redundant specifically in synaptic rewiring. They co-exist in the same protein complex and act cooperatively to regulate synaptic rewiring. We find that the MYRF proteins localize to the ER membrane and that they are cleaved into active N-terminal fragments, which then translocate into the nucleus to drive synaptic rewiring. Overexpression of active forms of MYRF is sufficient to accelerate synaptic rewiring. MYRF-1 and MYRF-2 are the first genes identified to be indispensable for promoting synaptic rewiring in C. elegans. These findings reveal a molecular mechanism underlying synaptic rewiring and developmental circuit plasticity.

Focal Stroke in the Developing Rat Motor Cortex Induces Age- and Experience-Dependent Maladaptive Plasticity of Corticospinal System

Motor system development is characterized by an activity-dependent competition between ipsilateral and contralateral corticospinal tracts (CST). Clinical evidence suggests that age is crucial for developmental stroke outcome, with early lesions inducing a “maladaptive” strengthening of ipsilateral projections from the healthy hemisphere and worse motor impairment. Here, we investigated in developing rats the relation between lesion timing, motor outcome and CST remodeling pattern. We induced a focal ischemia into forelimb motor cortex (fM1) at two distinct pre-weaning ages: P14 and P21. We compared long-term motor outcome with changes in axonal sprouting of contralesional CST at red nucleus and spinal cord level using anterograde tracing. We found that P14 stroke caused a more severe long-term motor impairment than at P21, and induced a strong and aberrant contralesional CST sprouting onto denervated spinal cord and red nucleus. The mistargeted sprouting of CST, and the worse motor outcome of the P14 stroke rats were reversed by an early skilled motor training, underscoring the potential of early activity-dependent plasticity in modulating lesion outcome. Thus, changes in the mechanisms controlling CST plasticity occurring during the third postnatal week are associated with age-dependent regulation of the motor outcome after stroke.

Disruptions in asymmetric centrosome inheritance and WDR62-Aurora kinase B interactions in primary microcephaly

Recessive mutations in WD repeat domain 62 (WDR62) cause microcephaly and a wide spectrum of severe brain malformations. Disruption of the mouse ortholog results in microcephaly underlain by reduced proliferation of neocortical progenitors during late neurogenesis, abnormalities in asymmetric centrosome inheritance leading to neuronal migration delays, and altered neuronal differentiation. Spindle pole localization of WDR62 and mitotic progression are defective in patient-derived fibroblasts, which, similar to mouse neocortical progenitors, transiently arrest at prometaphase. Expression of WDR62 is closely correlated with components of the chromosome passenger complex (CPC), a key regulator of mitosis. Wild type WDR62, but not disease-associated mutant forms, interacts with the CPC core enzyme Aurora kinase B and staining of CPC components at centromeres is altered in patient-derived fibroblasts. Our findings demonstrate critical and diverse functions of WDR62 in neocortical development and provide insight into the mechanisms by which its disruption leads to a plethora of structural abnormalities.

The pathology of incipient polymicrogyria

OBJECTIVE

To characterise the early tissue changes of post encephaloclastic polymicrogyria in the human fetus.

METHODS

We identified and reviewed the clinical histories and autopsy pathology of post ischemic fetal cerebral cortical injury at less than 30weeks gestational age (GA). The histology of local cortical abnormalities was examined with neuronal, glial, microglial and vascular immunohistochemical markers.

RESULTS

We identified eight cases ranging from 18 to 29weeks GA: 5 cases show full thickness cortical infarcts and 3 show periSylvian post-ischemic necrosis of the cerebral cortex. The maximal age is less than 10weeks after injury. There are abnormalities in gross fissuration as early as one month after injury. Disruption of the pia limitans was associated with a microglial and glial response and full thickness cortical injury. Macrophages were often seen accumulating deep to abnormal cortex. Hyperplasia of the subpial granular cell layer was universal in perilesional cortex. Cajal Retzius neuron hyperplasia, aggregation, and both superficial and deep displacement were noted. Where there was loss and dispersal of early cortical pyramidal neurons there was usually no pseudolaminar necrosis. Radial glia by 18weeks GA showed altered growth patterns and lateral branching. Altered migration of primitive elements was often prominent. Particularly prior to 20weeks GA subadjacent subplate neurons showed striking hypertrophy.

CONCLUSIONS

The array of histological changes encompasses all tissue elements of the affected brains, early in the evolution polymicrogyria. Although subpial alterations were ubiquitous, not all changes are referable to alterations in the pia limitans. The role of the necroinflammatory response in the genesis of abnormal cytoarchitecture deserves further study.

Modeling ALS with motor neurons derived from human induced pluripotent stem cells

Directing the differentiation of induced pluripotent stem cells into motor neurons has allowed investigators to develop new models of amyotrophic lateral sclerosis (ALS). However, techniques vary between laboratories and the cells do not appear to mature into fully functional adult motor neurons. Here we discuss common developmental principles of both lower and upper motor neuron development that have led to specific derivation techniques. We then suggest how these motor neurons may be matured further either through direct expression or administration of specific factors or coculture approaches with other tissues. Ultimately, through a greater understanding of motor neuron biology, it will be possible to establish more reliable models of ALS. These in turn will have a greater chance of validating new drugs that may be effective for the disease.

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.

Understanding the molecular basis of autism in a dish using hiPSCs-derived neurons from ASD patients

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder characterized by deficits in social cognition, language development, and repetitive/restricted behaviors. Due to the complexity and heterogeneity of ASD and lack of a proper human cellular model system, the pathophysiological mechanism of ASD during the developmental process is largely unknown. However, recent progress in induced pluripotent stem cell (iPSC) technology as well as in vitro neural differentiation techniques have allowed us to functionally characterize neurons and analyze cortical development during neural differentiation. These technical advances will increase our understanding of the pathogenic mechanisms of heterogeneous ASD and help identify molecular biomarkers for patient stratification as well as personalized medicine. In this review, we summarize our current knowledge of iPSC generation, differentiation of specific neuronal subtypes from iPSCs, and phenotypic characterizations of human ASD patient-derived iPSC models. Finally, we discuss the current limitations of iPSC technology and future directions of ASD pathophysiology studies using iPSCs.

Orthodenticle Is Required for the Expression of Principal Recognition Molecules That Control Axon Targeting in the Drosophila Retina

Parallel processing of neuronal inputs relies on assembling neural circuits into distinct synaptic-columns and layers. This is orchestrated by matching recognition molecules between afferent growth cones and target areas. Controlling the expression of these molecules during development is crucial but not well understood. The developing Drosophila visual system is a powerful genetic model for addressing this question. In this model system, the achromatic R1-6 photoreceptors project their axons in the lamina while the R7 and R8 photoreceptors, which are involved in colour detection, project their axons to two distinct synaptic-layers in the medulla. Here we show that the conserved homeodomain transcription factor Orthodenticle (Otd), which in the eye is a main regulator of rhodopsin expression, is also required for R1-6 photoreceptor synaptic-column specific innervation of the lamina. Our data indicate that otd function in these photoreceptors is largely mediated by the recognition molecules flamingo (fmi) and golden goal (gogo). In addition, we find that otd regulates synaptic-layer targeting of R8. We demonstrate that during this process, otd and the R8-specific transcription factor senseless/Gfi1 (sens) function as independent transcriptional inputs that are required for the expression of fmi, gogo and the adhesion molecule capricious (caps), which govern R8 synaptic-layer targeting. Our work therefore demonstrates that otd is a main component of the gene regulatory network that regulates synaptic-column and layer targeting in the fly visual system.

Establishing neural circuits during brain development relies on the ability for neurons to find their appropriate targets, a task that depends on recognition molecules whose expression must be tightly controlled. Very little is known of the factors that control the expression of these molecules. Here we have used the genetically amenable fly retina to address this issue. We identify Orthodenticle, a protein involved in mammalian brain development, as a main component of a new genetic cascade controlling the expression of recognition molecules that govern the wiring of the fly retina. Our work raises the issue that Orthodenticle might play a similar role in mammals. It also sheds new light on the genetic basis for wiring a model neural circuit during development.

Deterministic progenitor behavior and unitary production of neurons in the neocortex

Radial glial progenitors (RGPs) are responsible for producing nearly all neocortical neurons. To gain insight into the patterns of RGP division and neuron production, we quantitatively analyzed excitatory neuron genesis in the mouse neocortex using Mosaic Analysis with Double Markers, which provides single-cell resolution of progenitor division patterns and potential in vivo. We found that RGPs progress through a coherent program in which their proliferative potential diminishes in a predictable manner. Upon entry into the neurogenic phase, individual RGPs produce ∼8–9 neurons distributed in both deep and superficial layers, indicating a unitary output in neuronal production. Removal of OTX1, a transcription factor transiently expressed in RGPs, results in both deep- and superficial-layer neuron loss and a reduction in neuronal unit size. Moreover, ∼1/6 of neurogenic RGPs proceed to produce glia. These results suggest that progenitor behavior and histogenesis in the mammalian neocortex conform to a remarkably orderly and deterministic program.

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Radial glial progenitors (RGPs) progress through a coherent proliferation program

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Individual RGPs produce a unitary output of neurons during the neurogenic phase

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OTX1 deletion in RGPs leads to a reduction in neuronal unit size

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A defined fraction of about one in six RGPs proceeds to gliogenesis after neurogenesis

Splicing variants of NOL4 differentially regulate the transcription activity of Mlr1 and Mlr2 in cultured cells

Mlr1 (Mblk-1-related protein-1) and Mlr2 are mouse homologs of transcription factor Mblk-1 (Mushroom body large-type Kenyon cell-specific protein-1), which we originally identified from the honeybee brain. In the present study, aiming at identifying coregulator(s) of Mlr1 and Mlr2 from the mouse brain, we used yeast two-hybrid screening of mouse brain cDNA library to search for interaction partners of Mlr 1 and Mlr2, respectively. We identified nucleolar protein 4 (NOL4) splicing variants as major interaction partners for both Mlr1 and Mlr2. Among the three murine NOL4 splicing variants, we further characterized NOL4-S, which lacks an N-terminal part of NOL4-L, and NOL4-SΔ, which lacks nuclear localization signal (NLS)-containing domain of NOL4-S. A GST pull-down assay revealed that Mlr1 interacts with both NOL4-S and NOL4-SΔ, whereas Mlr2 interacts with NOL4-S, but not with NOL4-SΔ. These results indicate that the NLS-containing domain of NO4-S Is necessary for in vitro binding with Mlr2, but not for that with Mlr1. Furthermore, a luciferase assay using Schneider's Line 2 cells revealed that transactivation activity of Mlr1 was significantly suppressed by both NOL4-S and NOL4-SΔ, with almost complete suppression by NOL4-SΔ. In contrast, transactivation activity of Mlr2 was significantly suppressed by NOL4-S but rather activated by NOL4-SΔ. Our findings suggest that transactivation activities of Mlr1 and Mlr2 are differentially regulated by splicing variants of NOL4, which are expressed in a tissue-selective manner.

Emergence of neuronal diversity from patterning of telencephalic progenitors

During central nervous system (CNS) development, hundreds of distinct neuronal subtypes are generated from a single layer of multipotent neuroepithelial progenitor cells. Within the rostral CNS, initial regionalization of the telencephalon marks the territories where the cerebral cortex and the basal ganglia originate. Subsequent refinement of the primary structures determines the formation of domains of differential gene expression, where distinct fate-restricted progenitors are located. To understand how diversification of neural progenitors and neurons is achieved in the telencephalon, it is important to address early and late patterning events in this context. In particular, important questions include: How does the telencephalon become specified and regionalized along the major spatial axes? Within each region, are the differences in neuronal subtypes established at the progenitor level or at the postmitotic stage? If distinct progenitors exist that are committed to subtype-specific neuronal lineages, how does the diversification emerge? What is the contribution of positional and temporal cues and how is this information integrated into the intrinsic programs of cell identity? WIREs For further resources related to this article, please visit the WIREs website.

Systematic expression analysis of Hox genes at adulthood reveals novel patterns in the central nervous system

Hox proteins are key regulators of animal development, providing positional identity and patterning information to cells along the rostrocaudal axis of the embryo. Although their embryonic expression and function are well characterized, their presence and biological importance in adulthood remains poorly investigated. We provide here the first detailed quantitative and neuroanatomical characterization of the expression of the 39 Hox genes in the adult mouse brain. Using RT-qPCR we determined the expression of 24 Hox genes mainly in the brainstem of the adult brain, with low expression of a few genes in the cerebellum and the forebrain. Using in situ hybridization (ISH) we have demonstrated that expression of Hox genes is maintained in territories derived from the early segmental Hox expression domains in the hindbrain. Indeed, we show that expression of genes belonging to paralogy groups PG2-8 is maintained in the hindbrain derivatives at adulthood. The spatial colinearity, which characterizes the early embryonic expression of Hox genes, is still observed in sequential antero-posterior boundaries of expression. Moreover, the main mossy and climbing fibres precerebellar nuclei express PG2-8 Hox genes according to their migration origins. Second, ISH confirms the presence of Hox gene transcripts in territories where they are not detected during development, suggesting neo-expression in these territories in adulthood. Within the forebrain, we have mapped Hoxb1, Hoxb3, Hoxb4, Hoxd3 and Hoxa5 expression in restricted areas of the sensory cerebral cortices as well as in specific thalamic relay nuclei. Our data thus suggest a requirement of Hox genes beyond their role of patterning genes, providing a new dimension to their functional relevance in the central nervous system.

Loss of spastin function results in disease-specific axonal defects in human pluripotent stem cell-based models of hereditary spastic paraplegia

Human neuronal models of hereditary spastic paraplegias (HSP) that recapitulate disease-specific axonal pathology hold the key to understanding why certain axons degenerate in patients and to developing therapies. SPG4, the most common form of HSP, is caused by autosomal dominant mutations in the SPAST gene, which encodes the microtubule-severing ATPase spastin. Here, we have generated a human neuronal model of SPG4 by establishing induced pluripotent stem cells (iPSCs) from an SPG4 patient and differentiating these cells into telencephalic glutamatergic neurons. The SPG4 neurons displayed a significant increase in axonal swellings, which stained strongly for mitochondria and tau, indicating the accumulation of axonal transport cargoes. In addition, mitochondrial transport was decreased in SPG4 neurons, revealing that these patient iPSC-derived neurons recapitulate disease-specific axonal phenotypes. Interestingly, spastin protein levels were significantly decreased in SPG4 neurons, supporting a haploinsufficiency mechanism. Furthermore, cortical neurons derived from spastin-knockdown human embryonic stem cells (hESCs) exhibited similar axonal swellings, confirming that the axonal defects can be caused by loss of spastin function. These spastin-knockdown hESCs serve as an additional model for studying HSP. Finally, levels of stabilized acetylated-tubulin were significantly increased in SPG4 neurons. Vinblastine, a microtubule-destabilizing drug, rescued this axonal swelling phenotype in neurons derived from both SPG4 iPSCs and spastin-knockdown hESCs. Thus, this study demonstrates the successful establishment of human pluripotent stem cell-based neuronal models of SPG4, which will be valuable for dissecting the pathogenic cellular mechanisms and screening compounds to rescue the axonal degeneration in HSP.

Generation and characterization of an Nse-CreERT2 transgenic line suitable for inducible gene manipulation in cerebellar granule cells

We created an Nse-CreER^T2 mouse line expressing the tamoxifen-inducible CreER^T2 recombinase under the control of the neuron-specific enolase (Nse) promoter. By using Cre reporter lines we could show that this Nse-CreER^T2 line has recombination activity in the granule cells of all cerebellar lobules as well as in postmitotic granule cell precursors in the external granular layer of the developing cerebellum. A few hippocampal dentate gyrus granule cells showed Cre-mediated recombination as well. Cre activity could be induced in both the developing and adult mouse brain. The established mouse line constitutes a valuable tool to study the function of genes expressed by cerebellar granule cells in the developing and adult brain. In combination with reporter lines it is a useful model to analyze the development and maintenance of the cerebellar architecture including granule cell distribution, migration, and the extension of granule cell fibers in vivo.

Ccm3, a gene associated with cerebral cavernous malformations, is required for neuronal migration

Loss of function of cerebral cavernous malformation 3 (CCM3) results in an autosomal dominant cerebrovascular disorder. Here, we uncover a developmental role for CCM3 in regulating neuronal migration in the neocortex. Using cell type-specific gene inactivation in mice, we show that CCM3 has both cell autonomous and cell non-autonomous functions in neural progenitors and is specifically required in radial glia and newly born pyramidal neurons migrating through the subventricular zone, but not in those migrating through the cortical plate. Loss of CCM3 function leads to RhoA activation, alterations in the actin and microtubule cytoskeleton affecting neuronal morphology, and abnormalities in laminar positioning of primarily late-born neurons, indicating CCM3 involvement in radial glia-dependent locomotion and possible interaction with the Cdk5/RhoA pathway. Thus, we identify a novel cytoplasmic regulator of neuronal migration and demonstrate that its inactivation in radial glia progenitors and nascent neurons produces severe malformations of cortical development.

Regulation of branching dynamics by axon-intrinsic asymmetries in Tyrosine Kinase Receptor signaling

Axonal branching allows a neuron to connect to several targets, increasing neuronal circuit complexity. While axonal branching is well described, the mechanisms that control it remain largely unknown. We find that in the Drosophila CNS branches develop through a process of excessive growth followed by pruning. In vivo high-resolution live imaging of developing brains as well as loss and gain of function experiments show that activation of Epidermal Growth Factor Receptor (EGFR) is necessary for branch dynamics and the final branching pattern. Live imaging also reveals that intrinsic asymmetry in EGFR localization regulates the balance between dynamic and static filopodia. Elimination of signaling asymmetry by either loss or gain of EGFR function results in reduced dynamics leading to excessive branch formation. In summary, we propose that the dynamic process of axon branch development is mediated by differential local distribution of signaling receptors.

DOI:http://dx.doi.org/10.7554/eLife.01699.001

In the human brain, 100 billion neurons form 100 trillion connections. Each neuron consists of a cell body with numerous small branch-like projections known as dendrites (from the Greek word for ‘tree’), plus a long cable-like structure called the axon. Neurons receive electrical inputs from neighboring cells via their dendrites, and then relay these signals onto other cells in their network via their axons.

The development of the brain relies on new neurons integrating successfully into existing networks. Axon branching helps with this by enabling a single neuron to establish connections with several cells, but it is unclear how individual neurons decide when and where to form branches. Now, Zschätzsch et al. have revealed the mechanism behind this process in the fruit fly, Drosophila.

Mutant flies that lack a protein called EGFR produce abnormal numbers of axon branches, suggesting that this molecule regulates branch formation. Indeed in fruit flies, just as in mammals, the developing brain initially produces excessive numbers of branches, which are subsequently pruned to leave only those that have formed appropriate connections. In Drosophila, an uneven distribution of EGFR between branches belonging to the same axon acts as a signal to regulate this pruning process.

To examine this mechanism in more detail, high-resolution four-dimensional imaging was used to study brains that had been removed from Drosophila pupae and kept alive in special culture chambers. Axon branching and loss could now be followed in real time, and were found to occur more slowly in brains that lacked EGFR. The receptor controlled the branching of axons by influencing the distribution of another protein called actin, which is a key component of the internal skeleton that gives cells their structure.

In addition to providing new insights into a fundamental aspect of brain development, the work of Zschätzsch et al. also highlights the importance of stochastic events in shaping the network of connections within the developing brain. These findings may well be relevant to ongoing efforts to map the human brain ‘connectome’.

DOI:http://dx.doi.org/10.7554/eLife.01699.002

Established monolayer differentiation of mouse embryonic stem cells generates heterogeneous neocortical-like neurons stalled at a stage equivalent to midcorticogenesis

Two existing and widely applied protocols of embryonic stem (ES) cell differentiation have been developed to enable in vitro generation of neurons resembling neocortical projection neurons in monolayer culture and from embryoid bodies. The monolayer approach offers advantages for detailed in vitro characterizations and potential mechanistic and therapeutic screening. We investigated whether mouse ES cells undergoing largely undirected neocortical differentiation in monolayer culture recapitulate progressive developmental programs of in vivo progenitor and postmitotic differentiation and whether they develop into specific neocortical subtypes. We find that ES-derived mitotic cells that have been dorsalized by the sonic hedgehog antagonist cyclopamine, and that express, as a total population, cardinal markers of telencephalic progenitors, are, in fact, molecularly heterogeneous. We next show that these progenitors subsequently generate small numbers of heterogeneous neocortical-like neurons that are "stalled" at an immature stage of differentiation, based on multiple developmental criteria. Although some aspects of neocortical development are recapitulated by existing protocols of ES cell differentiation, these data indicate that mouse ES-derived neocortical progenitors both are more heterogeneous than their in vivo counterparts and seemingly include many incorrectly specified progenitors. Furthermore, these ES-derived progenitors spontaneously differentiate into sparse, and incompletely and largely imprecisely differentiated, neocortical-like neurons that fail to adopt specific neuronal identities in vitro. These results provide both foundation and motivation for refining and enhancing directed differentiation of clinically important neocortical projection neuron subtypes.

Molecular logic of neocortical projection neuron specification, development and diversity

The sophisticated circuitry of the neocortex is assembled from a diverse repertoire of neuronal subtypes generated during development under precise molecular regulation. In recent years, several key controls over the specification and differentiation of neocortical projection neurons have been identified. This work provides substantial insight into the 'molecular logic' underlying cortical development and increasingly supports a model in which individual progenitor-stage and postmitotic regulators are embedded within highly interconnected networks that gate sequential developmental decisions. Here, we provide an integrative account of the molecular controls that direct the progressive development and delineation of subtype and area identity of neocortical projection neurons.

In and out from the cortex: development of major forebrain connections

In this review we discuss recent advances in the understanding of the development of forebrain projections attending to their origin, fate determination, and axon guidance. Major forebrain connections include callosal, corticospinal, corticothalamic and thalamocortical projections. Although distinct transcriptional programs specify these subpopulations of projecting neurons, the mechanisms involved in their axonal development are similar. Guidance by short- and long-range molecular cues, interaction with intermediate target populations and activity-dependent mechanisms contribute to their development. Moreover, some of these connections interact with each other showing that the development of these axonal tracts is a well-orchestrated event. Finally, we will recapitulate recent discoveries that challenge the field of neural wiring that show that these forebrain connections can be changed once formed. The field of reprogramming has arrived to postmitotic cortical neurons and has showed us that forebrain connectivity is not immutable and might be changed by manipulations in the transcriptional program of matured cells.

Organization of the gymnotiform fish pallium in relation to learning and memory: IV. Expression of conserved transcription factors and implications for the evolution of dorsal telencephalon

We have cloned the apteronotid homologs of FoxP2, Otx1, and FoxO3. There was, in the case of all three genes, good similarity between the apteronotid and human amino acid sequences: FoxP2, 78%; Otx1, 54%; FoxO3, 71%. The functional domains of these genes were conserved to a far greater extent, on average: FoxP2, 89%; Otx1, 76%; FoxO3, 82%. This led us to hypothesize that the cellular functions of these genes might also be conserved. We used in situ hybridization to examine the distribution of the mRNA transcripts of these genes in the apteronotid telencephalon. We confined our analysis to the pallial regions previously associated with learning about social signals, whose circuitry has been closely examined in the other articles of this series. We found that AptFoxP2 and AptOtx1 transcripts were expressed predominantly in the dorsocentral division of the pallium (DC); the dorsolateral division of the pallium (DL) contained only weakly labeled neurons. In both cases, the distribution of labeled neurons was very heterogeneous, and unlabeled neurons could be found adjacent to strongly labeled ones. In contrast, we found that most neurons in DL strongly expressed AptFoxO3 mRNA, although there was only weak expression in a small number of cells within DC. We briefly discuss the relevance of our results regarding the functional roles of AptFoxP2/AptOtx1-expressing neurons in DC for communication vs. foraging behavior. We extensively discuss the implications of our results for possible homologies between DL and DC and medial and dorsal pallium of tetrapods, respectively.

Lmo4 establishes rostral motor cortex projection neuron subtype diversity

The mammalian neocortex is parcellated into anatomically and functionally distinct areas. The establishment of area-specific neuronal diversity and circuit connectivity enables distinct neocortical regions to control diverse and specialized functional outputs, yet underlying molecular controls remain largely unknown. Here, we identify a central role for the transcriptional regulator Lim-only 4 (Lmo4) in establishing the diversity of neuronal subtypes within rostral mouse motor cortex, where projection neurons have particularly diverse and multi-projection connectivity compared with caudal motor cortex. In rostral motor cortex, we report that both subcerebral projection neurons (SCPN), which send projections away from the cerebrum, and callosal projection neurons (CPN), which send projections to contralateral cortex, express Lmo4, whereas more caudal SCPN and CPN do not. Lmo4-expressing SCPN and CPN populations are comprised of multiple hodologically distinct subtypes. SCPN in rostral layer Va project largely to brainstem, whereas SCPN in layer Vb project largely to spinal cord, and a subset of both rostral SCPN and CPN sends second ipsilateral caudal (backward) projections in addition to primary projections. Without Lmo4 function, the molecular identity of neurons in rostral motor cortex is disrupted and more homogenous, rostral layer Va SCPN aberrantly project to the spinal cord, and many dual-projection SCPN and CPN fail to send a second backward projection. These molecular and hodological disruptions result in greater overall homogeneity of motor cortex output. Together, these results identify Lmo4 as a central developmental control over the diversity of motor cortex projection neuron subpopulations, establishing their area-specific identity and specialized connectivity.

Conserved roles of ems/Emx and otd/Otx genes in olfactory and visual system development in Drosophila and mouse

The regional specialization of brain function has been well documented in the mouse and fruitfly. The expression of regulatory factors in specific regions of the brain during development suggests that they function to establish or maintain this specialization. Here, we focus on two such factors—the Drosophila cephalic gap genes empty spiracles (ems) and orthodenticle (otd), and their vertebrate homologues Emx1/2 and Otx1/2—and review novel insight into their multiple crucial roles in the formation of complex sensory systems. While the early requirement of these genes in specification of the neuroectoderm has been discussed previously, here we consider more recent studies that elucidate the later functions of these genes in sensory system formation in vertebrates and invertebrates. These new studies show that the ems and Emx genes in both flies and mice are essential for the development of the peripheral and central neurons of their respective olfactory systems. Moreover, they demonstrate that the otd and Otx genes in both flies and mice are essential for the development of the peripheral and central neurons of their respective visual systems. Based on these recent experimental findings, we discuss the possibility that the olfactory and visual systems of flies and mice share a common evolutionary origin, in that the conserved visual and olfactory circuit elements derive from conserved domains of otd/Otx and ems/Emx action in the urbilaterian ancestor.

Longitudinal RNA sequencing of the deep transcriptome during neurogenesis of cortical glutamatergic neurons from murine ESCs

Using paired-end RNA sequencing, we have quantified the deep transcriptional changes that occur during differentiation of murine embryonic stem cells into a highly enriched population of glutamatergic cortical neurons. These data provide a detailed and nuanced account of longitudinal changes in the transcriptome during neurogenesis and neuronal maturation, starting from mouse embryonic stem cells and progressing through neuroepithelial stem cell induction, radial glial cell formation, neurogenesis, neuronal maturation and cortical patterning. Understanding the transcriptional mechanisms underlying the differentiation of stem cells into mature, glutamatergic neurons of cortical identity has myriad applications, including the elucidation of mechanisms of cortical patterning; identification of neurogenic processes; modeling of disease states; detailing of the host cell response to neurotoxic stimuli; and determination of potential therapeutic targets. In future work we anticipate correlating changes in longitudinal gene expression to other cell parameters, including neuronal function as well as characterizations of the proteome and metabolome. In this data article, we describe the methods used to produce the data and present the raw sequence read data in FASTQ files, sequencing run statistics and a summary flatfile of raw counts for 22,164 genes across 31 samples, representing 3-5 biological replicates at each timepoint. We propose that this data will be a valuable contribution to diverse research efforts in bioinformatics, stem cell research and developmental neuroscience studies.

In vivo reprogramming of circuit connectivity in postmitotic neocortical neurons

The molecular mechanisms that control how progenitors generate distinct subtypes of neurons, and how undifferentiated neurons acquire their specific identity during corticogenesis, are increasingly understood. However, whether postmitotic neurons can change their identity at late stages of differentiation remains unknown. To study this question, we developed an electrochemical in vivo gene delivery method to rapidly manipulate gene expression specifically in postmitotic neurons. Using this approach, we found that the molecular identity, morphology, physiology and functional input-output connectivity of layer 4 mouse spiny neurons could be specifically reprogrammed during the first postnatal week by ectopic expression of the layer 5B output neuron-specific transcription factor Fezf2. These findings reveal a high degree of plasticity in the identity of postmitotic neocortical neurons and provide a proof of principle for postnatal re-engineering of specific neural microcircuits in vivo.

Anatomic and molecular development of corticostriatal projection neurons in mice

Corticostriatal projection neurons (CStrPN) project from the neocortex to ipsilateral and contralateral striata to control and coordinate motor programs and movement. They are clinically important as the predominant cortical population that degenerates in Huntington's disease and corticobasal ganglionic degeneration, and their injury contributes to multiple forms of cerebral palsy. Together with their well-studied functions in motor control, these clinical connections make them a functionally, behaviorally, and clinically important population of neocortical neurons. Little is known about their development. "Intratelencephalic" CStrPN (CStrPNi), projecting to the contralateral striatum, with their axons fully within the telencephalon (intratelencephalic), are a major population of CStrPN. CStrPNi are of particular interest developmentally because they share hodological and axon guidance characteristics of both callosal projection neurons (CPN) and corticofugal projection neurons (CFuPN); CStrPNi send axons contralaterally before descending into the contralateral striatum. The relationship of CStrPNi development to that of broader CPN and CFuPN populations remains unclear; evidence suggests that CStrPNi might be evolutionary "hybrids" between CFuPN and deep layer CPN-in a sense "chimeric" with both callosal and corticofugal features. Here, we investigated the development of CStrPNi in mice-their birth, maturation, projections, and expression of molecular developmental controls over projection neuron subtype identity.

Spatially coordinated kinase signaling regulates local axon degeneration

In addition to being a hallmark of neurodegenerative disease, axon degeneration is used during development of the nervous system to prune unwanted connections. In development, axon degeneration is tightly regulated both temporally and spatially. Here, we provide evidence that degeneration cues are transduced through various kinase pathways functioning in spatially distinct compartments to regulate axon degeneration. Intriguingly, glycogen synthase kinase-3 (GSK3) acts centrally, likely modulating gene expression in the cell body to regulate distally restricted axon degeneration. Through a combination of genetic and pharmacological manipulations, including the generation of an analog-sensitive kinase allele mutant mouse for GSK3β, we show that the β isoform of GSK3, not the α isoform, is essential for developmental axon pruning in vitro and in vivo. Additionally, we identify the dleu2/mir15a/16-1 cluster, previously characterized as a regulator of B-cell proliferation, and the transcription factor tbx6, as likely downstream effectors of GSK3β in axon degeneration.

Hierarchical clustering of gene expression patterns in the Eomes + lineage of excitatory neurons during early neocortical development

BACKGROUND

Cortical neurons display dynamic patterns of gene expression during the coincident processes of differentiation and migration through the developing cerebrum. To identify genes selectively expressed by the Eomes + (Tbr2) lineage of excitatory cortical neurons, GFP-expressing cells from Tg(Eomes::eGFP) Gsat embryos were isolated to > 99% purity and profiled.

RESULTS

We report the identification, validation and spatial grouping of genes selectively expressed within the Eomes + cortical excitatory neuron lineage during early cortical development. In these neurons 475 genes were expressed ≥ 3-fold, and 534 genes ≤ 3-fold, compared to the reference population of neuronal precursors. Of the up-regulated genes, 328 were represented at the Genepaint in situ hybridization database and 317 (97%) were validated as having spatial expression patterns consistent with the lineage of differentiating excitatory neurons. A novel approach for quantifying in situ hybridization patterns (QISP) across the cerebral wall was developed that allowed the hierarchical clustering of genes into putative co-regulated groups. Forty four candidate genes were identified that show spatial expression with Intermediate Precursor Cells, 49 candidate genes show spatial expression with Multipolar Neurons, while the remaining 224 genes achieved peak expression in the developing cortical plate.

CONCLUSIONS

This analysis of differentiating excitatory neurons revealed the expression patterns of 37 transcription factors, many chemotropic signaling molecules (including the Semaphorin, Netrin and Slit signaling pathways), and unexpected evidence for non-canonical neurotransmitter signaling and changes in mechanisms of glucose metabolism. Over half of the 317 identified genes are associated with neuronal disease making these findings a valuable resource for studies of neurological development and disease.

Ischemia-induced neural stem/progenitor cells express pyramidal cell markers

Adult brain-derived neural stem cells have acquired a lot of interest as an endurable neuronal cell source that can be used for central nervous system repair in a wide range of neurological disorders such as ischemic stroke. Recently, we identified injury-induced neural stem/progenitor cells in the poststroke murine cerebral cortex. In this study, we show that, after differentiation in vitro, injury-induced neural stem/progenitor cells express pyramidal cell markers Emx1 and CaMKIIα, as well as mature neuron markers MAP2 and Tuj1. 5-bromo-2-deoxyuridinine-positive neurons in the peristroke cortex also express such pyramidal markers. The presence of newly regenerated pyramidal neurons in the poststroke brain might provide a noninvasive therapeutic strategy for stroke treatment with functional recovery.

Modeling psychiatric disorders through reprogramming

Psychiatric disorders, including autism spectrum disorders and schizophrenia, are extremely heritable complex genetic neurodevelopmental disorders. It is now possible to directly reprogram fibroblasts from psychiatric patients into human induced pluripotent stem cells (hiPSCs) and subsequently differentiate these disorder-specific hiPSCs into neurons. This means that researchers can generate nearly limitless quantities of live human neurons with genetic backgrounds that are known to result in psychiatric disorders, without knowing which genes are interacting to produce the disease state in each patient. With these new human-cell-based models, scientists can investigate the precise cell types that are affected in these disorders and elucidate the cellular and molecular defects that contribute to disease initiation and progression. Here, we present a short review of experiments using hiPSCs and other sophisticated in vitro approaches to study the pathways underlying psychiatric disorders.

Molecular control of axon branching

Axon branching is a complex morphological process, the regulation of which we are just beginning to understand. Many factors known to be important for axon growth and guidance have emerged as key regulators of axon branching. The extrinsic factors implicated in axon branching include traditional axon guidance cues such as the slits, semaphorins, and ephrins; neurotrophins such as BDNF; the secreted glycoprotein Wnt; the extracellular matrix protein anosmin-1; and certain transmembrane cell adhesion molecules--as well as sensory experience and neuronal activity. Although less is known about the intracellular control of axon branching, in recent years significant advances have been made in this area. Kinases and their regulators, Rho GTPases and their regulators, transcription factors, ubiquitin ligases, and several microtubule and actin-binding proteins are now implicated in the control of axon branching. It is likely that many more branching regulators remain to be discovered, as do the links between extrinsic cues and intracellular signaling proteins in the control of axon branching.

Concise review: the promise of human induced pluripotent stem cell-based studies of schizophrenia

Schizophrenia (SCZD) is a heritable developmental disorder. Although the molecular mechanism of disease remains unclear, insights into the disorder have been made through a vast array of experimental techniques. Together, magnetic resonance brain imaging, pharmacological, and post-mortem pathological studies have observed decreased brain volume, aberrant neurotransmitter signaling, reduced dendritic arborization, and impaired myelination in SCZD. Genome-wide association studies (GWAS) have identified common single nucleotide polymorphisms as well as rare copy number variants that contribute to SCZD, while mouse models of candidate SCZD genes show behavioral abnormalities and anatomical perturbations consistent with human disease. The advent of human induced pluripotent stem cells (hiPSCs) makes it possible to study SCZD using live human neurons with a genetic predisposition toward SCZD, even without knowledge of the genes interacting to produce the disease state. SCZD hiPSC neurons show cellular defects comparable to those identified in post-mortem human and mouse studies, and gene expression changes are consistent with predictions made by GWAS. SCZD hiPSC neurons represent a new tool to look beyond phenotype and begin to dissect the molecular mechanisms of SCZD.

Fezf2 regulates telencephalic precursor differentiation from mouse embryonic stem cells

The mechanisms by which transcription factors control stepwise lineage restriction during the specification of cortical neurons remain largely unknown. Here, we investigated the role of forebrain embryonic zinc finger like (Fezf2) in this process by generating Fezf2 knockdown and tetracycline-inducible Fezf2 overexpression mouse embryonic stem cell (mESC) lines. The overexpression of Fezf2 at early time points significantly increased the generation of rostral forebrain progenitors (Foxg1(+), Six3(+)) and inhibited the expression of transcription factors which are expressed by the midbrain and caudal diencephalon (En1(+), Irx(+)). This effect was partially achieved by the regulation of Wnt signaling during this critical early time window. The role of Fezf2 in regulating the rostrocaudal patterning was further confirmed by the significant decrease in the expression of Foxg1 and Six3 and the increase in the expression of En1 when Fezf2 was knocked down. In addition, Fezf2 overexpression at later time points had little effect on the expression of Foxg1 and Six3. Instead, Fezf2 promotes the generation of dorsal telencephalic progenitors and deep-layer cortical neurons at later stages. Collectively, our data suggest that Fezf2 controls the specification of telencephalic progenitors from mESCs through differentially regulating the expression of rostrocaudal and dorsoventral patterning genes.

Altered gene expression in the emerging cerebellar primordium of Neurog1-/- mice

Expression of the basic helix-loop-helix (bHLH) transcription factor Neurogenin1 (Neurog1) coincides with the emergence of the cerebellum and Neurog1-expressing progenitors are fated to become Purkinje cells and later interneurons. However, the gene regulatory functions of Neurog1 in cerebellar development have not been characterized. We performed a genome-wide analysis of gene expression in the cerebellar primordium of E11.5 Neurog1 null (Neurog1-/-) mice to identify the Neurog1 transcriptome in the emerging cerebellum. This screen identified 117 genes differentially enriched in Neurog1-/- versus control sample sets with a high presence of gene sets enriched for functions in nervous system development. Hierarchical clustering revealed complete stratification of differentially expressed genes based on Neurog1 gene deletion status. In silico analysis of promoter regions identifies high probability Neurog1 regulatory (E-box) binding sites in 94 of the 117 differentially expressed genes and Pax6 binding motifs in 25 of these 94 promoters. Our data provide a framework for investigating Neurog1 transcriptional programs in early cerebellar development and suggest functional Neurog1-Pax6 cross-talk in the activation of downstream targets.

Karyopherins in nuclear transport of homeodomain proteins during development

Homeodomain proteins are crucial transcription factors for cell differentiation, cell proliferation and organ development. Interestingly, their homeodomain signature structure is important for both their DNA-binding and their nucleocytoplasmic trafficking. The accurate nucleocytoplasmic distribution of these proteins is essential for their functions. We summarize information on (a) the roles of karyopherins for import and export of homeoproteins, (b) the regulation of their nuclear transport during development, and (c) the corresponding complexity of homeoprotein nucleocytoplasmic transport signals. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Laminar and areal expression of unc5d and its role in cortical cell survival

The UNC-5 family of netrin receptors is known to regulate axon guidance, cell migration, and cell survival. We have previously demonstrated that unc5d, one of the UNC-5 family member genes, is specifically expressed in layer 4 of the developing rat neocortex (Zhong Y, Takemoto M, Fukuda T, Hattori Y, Murakami F, Nakajima D, Nakayama M, Yamamoto N. 2004. Identification of the genes that are expressed in the upper layers of the neocortex. Cereb Cortex. 14:1144-1152). However, the role of UNC5D in cortical development is still unknown. In this study, we revealed that unc5d was highly expressed in the primary sensory areas of the mouse neocortex at around postnatal day 7. Netrin-4 was also found to be predominantly expressed in layer 4 of the sensory cortex and sensory thalamic nuclei. Cell surface binding assay showed that netrin-4 protein bound to UNC5D-expressing cells. An in vitro study further demonstrated that cell death of unc5d-expressing layer 4 cells was reduced by exogenous application of netrin-4 protein, whereas UNC5D is not sufficient to mediate the effect of netrin-4 in deep layer cells. Taken together, these results suggest that UNC5D is primarily expressed by layer 4 cells in the primary sensory areas of the developing neocortex and may mediate the effect of netrin-4 on cortical cell survival in a lamina-specific manner.

A JIP3-regulated GSK3β/DCX signaling pathway restricts axon branching

Axon branching plays a critical role in establishing the accurate patterning of neuronal circuits in the brain. However, the mechanisms that control axon branching remain poorly understood. Here we report that knockdown of the brain-enriched signaling protein JNK-interacting protein 3 (JIP3) triggers exuberant axon branching and self-contact in primary granule neurons of the rat cerebellar cortex. JIP3 knockdown in cerebellar slices and in postnatal rat pups in vivo leads to the formation of ectopic branches in granule neuron parallel fiber axons in the cerebellar cortex. We also find that JIP3 restriction of axon branching is mediated by the protein kinase glycogen synthase kinase 3β (GSK3β). JIP3 knockdown induces the downregulation of GSK3β in neurons, and GSK3β knockdown phenocopies the effect of JIP3 knockdown on axon branching and self-contact. Finally, we establish doublecortin (DCX) as a novel substrate of GSK3β in the control of axon branching and self-contact. GSK3β phosphorylates DCX at the distinct site of Ser327 and thereby contributes to DCX function in the restriction of axon branching. Together, our data define a JIP3-regulated GSK3β/DCX signaling pathway that restricts axon branching in the mammalian brain. These findings may have important implications for our understanding of neuronal circuitry during development, as well as the pathogenesis of neurodevelopmental disorders of cognition.

The application of cortical layer markers in the evaluation of cortical dysplasias in epilepsy

The diagnostic criteria for focal cortical dysplasia type I (FCD I) remain to be well and consistently defined. Cortical layer-specific markers (CLM) provide a potential tool for the objective assessment of any dyslamination. We studied expression patterns of recognised CLM using immunohistochemistry for N200, ER81, Otx1, Map1b (subsets of V/VI projection neurones), Pax6, Tbr1, Tbr2 (differentially expressed in cortical neurones from intermediate progenitor cells), Cux 1 (outer cortical layers) and MASH1 (ventricular zone progenitors). Dysplasia subtypes included FCD I and II, dysplasias adjacent to hippocampal sclerosis (HS) or dysembryoplastic neuroepithelial tumours (DNTs); all were compared to neonatal and adult controls. Laminar expression patterns in normal cortex were observed with Tbr1, Map1b, N200 and Otx1. FCDI cases in younger patients were characterised by abnormal expression in layer II for Tbr1 and Otx1. FCDII showed distinct labelling of balloon cells (Pax6, ER81 and Otx1) and dysmorphic neurones (Tbr 1, N200 and Map1b) supporting origins from radial glia and intermediate progenitor cells, respectively. In temporal lobe sclerosis cases with dysplasia adjacent to HS, Tbr1 and Map1b highlighted abnormal orientation of neurones in layer II. Dyslamination was not confirmed in the perilesional cortex of DNT with CLM. Finally, immature cell types (Otx1, Pax6 and Tbr2) were noted in varied pathologies. One possibility is activation of progenitor cell populations which could contribute to the pathophysiology of these lesions.

Fluorescent in situ hybridization technique for cell type identification and characterization in the central nervous system

Central nervous system consists of a myriad of cell types. In particular, many subtypes of neuronal cells, which are interconnected with each other, form the basis of functional circuits. With the advent of genomic era, there have been systematic efforts to map gene expression profiles by in situ hybridization (ISH) and enhancer-trapping strategy. To make full use of such information, it is important to correlate "cell types" to gene expression. Toward this end, we have developed highly sensitive method of fluorescent dual-probe ISH, which is essential to distinguish two cell types expressing distinct marker genes. Importantly, we were able to combine ISH with retrograde tracing and antibody staining including BrdU staining that enables birthdating. These techniques should prove useful in identifying and characterizing the cell types of the neural tissues. In this article, we describe the methodology of these techniques, taking examples from our analyses of the mammalian cerebral cortex.