Mechanisms of cerebral cortical patterning in mice and humans

An adhesion signaling axis involving Dystroglycan, β1-Integrin, and Cas adaptor proteins regulates the establishment of the cortical glial scaffold

The mature mammalian cortex is composed of 6 architecturally and functionally distinct layers. Two key steps in the assembly of this layered structure are the initial establishment of the glial scaffold and the subsequent migration of postmitotic neurons to their final position. These processes involve the precise and timely regulation of adhesion and detachment of neural cells from their substrates. Although much is known about the roles of adhesive substrates during neuronal migration and the formation of the glial scaffold, less is understood about how these signals are interpreted and integrated within these neural cells. Here, we provide in vivo evidence that Cas proteins, a family of cytoplasmic adaptors, serve a functional and redundant role during cortical lamination. Cas triple conditional knock-out (Cas TcKO) mice display severe cortical phenotypes that feature cobblestone malformations. Molecular epistasis and genetic experiments suggest that Cas proteins act downstream of transmembrane Dystroglycan and β1-Integrin in a radial glial cell-autonomous manner. Overall, these data establish a new and essential role for Cas adaptor proteins during the formation of cortical circuits and reveal a signaling axis controlling cortical scaffold formation.

Cerebral Organoids as an Experimental Platform for Human Neurogenomics

The cerebral cortex forms early in development according to a series of heritable neurodevelopmental instructions. Despite deep evolutionary conservation of the cerebral cortex and its foundational six-layered architecture, significant variations in cortical size and folding can be found across mammals, including a disproportionate expansion of the prefrontal cortex in humans. Yet our mechanistic understanding of neurodevelopmental processes is derived overwhelmingly from rodent models, which fail to capture many human-enriched features of cortical development. With the advent of pluripotent stem cells and technologies for differentiating three-dimensional cultures of neural tissue in vitro, cerebral organoids have emerged as an experimental platform that recapitulates several hallmarks of human brain development. In this review, we discuss the merits and limitations of cerebral organoids as experimental models of the developing human brain. We highlight innovations in technology development that seek to increase its fidelity to brain development in vivo and discuss recent efforts to use cerebral organoids to study regeneration and brain evolution as well as to develop neurological and neuropsychiatric disease models.

mRNA-Decapping Associated DcpS Enzyme Controls Critical Steps of Neuronal Development

Homozygous mutations in the gene encoding the scavenger mRNA-decapping enzyme, DcpS, have been shown to underlie developmental delay and intellectual disability. Intellectual disability is associated with both abnormal neocortical development and mRNA metabolism. However, the role of DcpS and its scavenger decapping activity in neuronal development is unknown. Here, we show that human neurons derived from patients with a DcpS mutation have compromised differentiation and neurite outgrowth. Moreover, in the developing mouse neocortex, DcpS is required for the radial migration, polarity, neurite outgrowth, and identity of developing glutamatergic neurons. Collectively, these findings demonstrate that the scavenger mRNA decapping activity contributes to multiple pivotal roles in neural development and further corroborate that mRNA metabolism and neocortical pathologies are associated with intellectual disability.

Making sense of mRNA landscapes: Translation control in neurodevelopment

Like all other parts of the central nervous system, the mammalian neocortex undergoes temporally ordered set of developmental events, including proliferation, differentiation, migration, cellular identity, synaptogenesis, connectivity formation, and plasticity changes. These neurodevelopmental mechanisms have been characterized by studies focused on transcriptional control. Recent findings, however, have shown that the spatiotemporal regulation of post-transcriptional steps like alternative splicing, mRNA traffic/localization, mRNA stability/decay, and finally repression/derepression of protein synthesis (mRNA translation) have become just as central to the neurodevelopment as transcriptional control. A number of dynamic players act post-transcriptionally in the neocortex to regulate these steps, as RNA binding proteins (RBPs), ribosomal proteins (RPs), long non-coding RNAs, and/or microRNA. Remarkably, mutations in these post-transcriptional regulators have been associated with neurodevelopmental, neurodegenerative, inherited, or often co-morbid disorders, such as microcephaly, autism, epilepsy, intellectual disability, white matter diseases, Rett-syndrome like phenotype, spinocerebellar ataxia, and amyotrophic lateral sclerosis. Here, we focus on the current state, advanced methodologies and pitfalls of this exciting and upcoming field of RNA metabolism with vast potential in understanding fundamental neurodevelopmental processes and pathologies. This article is categorized under: Translation > Translation Regulation RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

The impact of (ab)normal maternal environment on cortical development

The cortex in the mammalian brain is the most complex brain region that integrates sensory information and coordinates motor and cognitive processes. To perform such functions, the cortex contains multiple subtypes of neurons that are generated during embryogenesis. Newly born neurons migrate to their proper location in the cortex, grow axons and dendrites, and form neuronal circuits. These developmental processes in the fetal brain are regulated to a large extent by a great variety of factors derived from the mother - starting from simple nutrients as building blocks and ending with hormones. Thus, when the normal maternal environment is disturbed due to maternal infection, stress, malnutrition, or toxic substances, it might have a profound impact on cortical development and the offspring can develop a variety of neurodevelopmental disorders. Here we first describe the major developmental processes which generate neuronal diversity in the cortex. We then review our knowledge of how most common maternal insults affect cortical development, perturb neuronal circuits, and lead to neurodevelopmental disorders. We further present a concept of selective vulnerability of cortical neuronal subtypes to maternal-derived insults, where the vulnerability of cortical neurons and their progenitors to an insult depends on the time (developmental period), place (location in the developing brain), and type (unique features of a cell type and an insult). Finally, we provide evidence for the existence of selective vulnerability during cortical development and identify the most vulnerable neuronal types, stages of differentiation, and developmental time for major maternal-derived insults.

[Mechanism of axonal degeneration: from molecular signaling to the development of therapeutic applications]

Neurons communicate with other cells via long processes, i.e., axons and dendrites, functionally and morphologically specialized tree-like structures. Formation and maintenance of such processes play a crucial role in neuronal functions. Axons are particularly important for construction of neuronal network, and, together with synapses at the end of them, play a central role in transmission of information. Axonal degeneration, a phenomenon that once formed axons lose structural integrity, is most typically observed as "Wallerian degeneration", in which injured axonal segment (distal to the site of injury) degenerates. Different forms of axonal degeneration are also observed in a variety of contexts, including pathogenesis and progression of different neurodegenerative disorders, as well as neuronal network formation during development. Thus, understanding of regulatory mechanism of axonal degeneration is important in many aspects, such as for clarification of neuronal morphogenesis mechanism, and for development of neuroprotective therapy against neurological disorders. Here, I discuss recent progress in the research field of axonal degeneration mechanism.

The essential role of primary cilia in cerebral cortical development and disorders

Primary cilium, first described in the 19th century in different cell types and organisms by Alexander Ecker, Albert Kolliker, Aleksandr Kowalevsky, Paul Langerhans, and Karl Zimmermann (Ecker, 1844; Kolliker, 1854; Kowalevsky, 1867; Langerhans, 1876; Zimmermann, 1898), play an essential modulatory role in diverse aspects of nervous system development and function. The primary cilium, sometimes referred to as the cell's 'antennae', can receive wide ranging inputs from cellular milieu, including morphogens, growth factors, neuromodulators, and neurotransmitters. Its unique structural and functional organization bequeaths it the capacity to hyper-concentrate signaling machinery in a restricted cellular domain approximately one-thousandth the volume of cell soma. Thus enabling it to act as a signaling hub that integrates diverse developmental and homestatic information from cellular milieu to regulate the development and function of neural cells. Dysfunction of primary cilia contributes to the pathophysiology of several brain malformations, intellectual disabilities, epilepsy, and psychiatric disorders. This review focuses on the most essential contributions of primary cilia to cerebral cortical development and function, in the context of neurodevelopmental disorders and malformations. It highlights the recent progress made in identifying the mechanisms underlying primary cilia's role in cortical progenitors, neurons and glia, in health and disease. A future challenge will be to translate these insights and advances into effective clinical treatments for ciliopathies.

Dynamic and Cell-Specific DACH1 Expression in Human Neocortical and Striatal Development

DACH1 is the human homolog of the Drosophila dachshund gene, which is involved in the development of the eye, nervous system, and limbs in the fly. Here, we systematically investigate DACH1 expression patterns during human neurodevelopment, from 5 to 21 postconceptional weeks. By immunodetection analysis, we found that DACH1 is highly expressed in the proliferating neuroprogenitors of the developing cortical ventricular and subventricular regions, while it is absent in the more differentiated cortical plate. Single-cell global transcriptional analysis revealed that DACH1 is specifically enriched in neuroepithelial and ventricular radial glia cells of the developing human neocortex. Moreover, we describe a previously unreported DACH1 expression in the human striatum, in particular in the striatal medium spiny neurons. This finding qualifies DACH1 as a new striatal projection neuron marker, together with PPP1R1B, BCL11B, and EBF1. We finally compared DACH1 expression profile in human and mouse forebrain, where we observed spatio-temporal similarities in its expression pattern thus providing a precise developmental description of DACH1 in the 2 mammalian species.

Brain-specific deletion of histone variant H2A.z results in cortical neurogenesis defects and neurodevelopmental disorder

Defects in neurogenesis alter brain circuit formations and may lead to neurodevelopmental disorders such as autism and schizophrenia. Histone H2A.z, a variant of histone H2A, plays critical roles in chromatin structure and epigenetic regulation, but its function and mechanism in brain development remain largely unknown. Here, we find that the deletion of H2A.z results in enhanced proliferation of neural progenitors but reduced neuronal differentiation. In addition, neurons in H2A.z knockout mice exhibit abnormal dendrites during brain development. Furthermore, H2A.z^cKO mice exhibit serial behavioral deficits, such as decreased exploratory activity and impaired learning and memory. Mechanistically, H2A.z regulates embryonic neurogenesis by targeting Nkx2–4 through interaction with Setd2, thereby promoting H3K36me3 modification to activate the transcription of Nkx2–4. Furthermore, enforced expression of Nkx2–4 can rescue the defective neurogenesis in the H2A.z-knockdown embryonic brain. Together, our findings implicate the epigenetic regulation by H2A.z in embryonic neurogenesis and provide a framework for understanding how disruption in the H2A.z gene may contribute to neurological disorders.

Isolation and culture of porcine primary fetal progenitors and neurons from the developing dorsal telencephalon

The development of long-term surviving fetal cell cultures from primary cell tissue from the developing brain is important for facilitating studies investigating neural development and for modelling neural disorders and brain congenital defects. The field faces current challenges in co-culturing both progenitors and neurons long-term. Here, we culture for the first time, porcine fetal cells from the dorsal telencephalon at embryonic day (E) 50 and E60 in conditions that promoted both the survival of progenitor cells and young neurons. We applied a novel protocol designed to collect, isolate and promote survival of both progenitors and young neurons. Herein, we used a combination of low amount of fetal bovine serum, together with pro-survival factors, including basic fibroblast growth factor and retinoic acid, together with arabinofuranosylcytosine and could maintain progenitors and facilitate in vitro differentiation into calbindin 1+ neurons and reelin+ interneurons for a period of 7 days. Further improvements to the protocol that might extend the survival of the fetal primary neural cells would be beneficial. The development of new porcine fetal culture methods is of value for the field, given the pig's neuroanatomical and developmental similarities to the human brain.

Maintenance of Positional Identity of Neural Progenitors in the Embryonic and Postnatal Telencephalon

Throughout embryonic development and into postnatal life, regionally distinct populations of neural progenitor cells (NPCs) collectively generate the many different types of neurons that underlie the complex structure and function of the adult mammalian brain. At very early stages of telencephalic development, NPCs become organized into regional domains that each produce different subsets of neurons. This positional identity of NPCs relates to the regional expression of specific, fate-determining homeodomain transcription factors. As development progresses, the brain undergoes vast changes in both size and shape, yet important aspects of NPC positional identity persist even into the postnatal brain. How can NPC positional identity, which is established so early in brain development, endure the many dynamic, large-scale and complex changes that occur over a relatively long period of time? In this Perspective article, we review data and concepts derived from studies in Drosophila regarding the function of homeobox (Hox) genes, Polycomb group (PcG) and trithorax group (trxG) chromatin regulators. We then discuss how this knowledge may contribute to our understanding of the maintenance of positional identity of NPCs in the mammalian telencephalon. Similar to the axial body plan of Drosophila larvae, there is a segmental nature to NPC positional identity, with loss of specific homeodomain transcription factors causing homeotic-like shifts in brain development. Finally, we speculate about the role of mammalian PcG and trxG factors in the long-term maintenance of NPC positional identity and certain neurodevelopmental disorders.

Focal Cortical Dysplasia: Gene Mutations, Cell Signaling, and Therapeutic Implications

Focal cortical dysplasias (FCDs) are malformations of cortical development (MCDs) that are highly associated with medication-resistant epilepsy and are the most common cause of neocortical epilepsy in children. FCDs are a heterogeneous group of developmental disorders caused by germline or somatic mutations that occur in genes regulating the PI3K/Akt/mTOR pathway-a key pathway in neuronal growth and migration. Accordingly, FCDs are characterized by abnormal cortical lamination, cell morphology (e.g., cytomegaly), and cellular polarity. In some FCD subtypes, balloon cells express proteins typically seen in neuroglial progenitor cells. Because recurrent intractable seizures are a common feature of FCDs, epileptogenic electrophysiological properties are also observed in addition to local inflammation. Here, we will summarize the current literature regarding FCDs, addressing the current classification system, histopathology, molecular genetics, electrophysiology, and transcriptome and cell signaling changes.

Radial Structure Scaffolds Convolution Patterns of Developing Cerebral Cortex

Commonly-preserved radial convolution is a prominent characteristic of the mammalian cerebral cortex. Endeavors from multiple disciplines have been devoted for decades to explore the causes for this enigmatic structure. However, the underlying mechanisms that lead to consistent cortical convolution patterns still remain poorly understood. In this work, inspired by prior studies, we propose and evaluate a plausible theory that radial convolution during the early development of the brain is sculptured by radial structures consisting of radial glial cells (RGCs) and maturing axons. Specifically, the regionally heterogeneous development and distribution of RGCs controlled by Trnp1 regulate the convex and concave convolution patterns (gyri and sulci) in the radial direction, while the interplay of RGCs' effects on convolution and axons regulates the convex (gyral) convolution patterns. This theory is assessed by observations and measurements in literature from multiple disciplines such as neurobiology, genetics, biomechanics, etc., at multiple scales to date. Particularly, this theory is further validated by multimodal imaging data analysis and computational simulations in this study. We offer a versatile and descriptive study model that can provide reasonable explanations of observations, experiments, and simulations of the characteristic mammalian cortical folding.

Structural Imaging Changes and Behavioral Correlates in Patients with Crohn's Disease in Remission

Background: Crohn’s disease (CD) is a subtype of inflammatory bowel disease caused by immune-mediated inflammation in the gastrointestinal tract. The extent of morphologic brain alterations and their associated cognitive and affective impairments remain poorly characterized.

Aims: We used magnetic resonance imaging to identify structural brain differences between patients with Crohn’s disease in remission compared to age-matched healthy controls and evaluated for structural-behavioral correlates.

Methods: Nineteen patients and 20 healthy, age-matched controls were recruited in the study. Group differences in brain morphometric measures and correlations between brain measures and performance on a cognitive task, the verbal fluency (VF) task, were examined. Correlations between brain measures and cognitive measures as well as self-reported measures of depression, personality, and affective scales were examined.

Results: Patients showed significant cortical thickening in the left superior frontal region compared to controls. Significant group differences were observed in sub-cortical volume measures in both hemispheres. Investigation of brain-behavior correlations revealed significant group differences in the correlation between cortical surface area and VF performance, although behavioral performance was equivalent between the two groups. The left middle temporal surface area was a significant predictor of VF performance with controls showing a significant positive correlation between these measures, and patients showing the opposite effect.

Conclusion: Our results indicate key differences in structural brain measures in patients with CD compared to controls. Additionally, correlation between brain measures and behavioral responses suggest there may be a neural basis to the alterations in patients’ cognitive and affective responses.

Forty years of structural imaging in psychosis: promises and truth

OBJECTIVE

Since the first study published in the Lancet in 1976, structural neuroimaging has been used in psychosis with the promise of imminent clinical utility. The actual impact of structural neuroimaging in psychosis is still unclear.

METHOD

We present here a critical review of studies involving structural magnetic resonance imaging techniques in patients with psychosis published between 1976 and 2015 in selected journals of relevance for the field. For each study, we extracted summary descriptive variables. Additionally, we qualitatively described the main structural findings of each article in summary notes and we employed a biomarker rating system based on quality of evidence (scored 1-4) and effect size (scored 1-4).

RESULTS

Eighty studies meeting the inclusion criteria were retrieved. The number of studies increased over time, reflecting an increased structural imaging research in psychosis. However, quality of evidence was generally impaired by small samples and unclear biomarker definitions. In particular, there was little attempt of replication of previous findings. The effect sizes ranged from small to modest. No diagnostic or prognostic biomarker for clinical use was identified.

CONCLUSIONS

Structural neuroimaging in psychosis research has not yet delivered on the clinical applications that were envisioned.

Role of mechanical factors in cortical folding development

Deciphering mysteries of the structure-function relationship in cortical folding has emerged as the cynosure of recent research on brain. Understanding the mechanism of convolution patterns can provide useful insight into the normal and pathological brain function. However, despite decades of speculation and endeavors the underlying mechanism of the brain folding process remains poorly understood. This paper focuses on the three-dimensional morphological patterns of a developing brain under different tissue specification assumptions via theoretical analyses, computational modeling, and experiment verifications. The living human brain is modeled with a soft structure having outer cortex and inner core to investigate the brain development. Analytical interpretations of differential growth of the brain model provide preliminary insight into the critical growth ratio for instability and crease formation of the developing brain followed by computational modeling as a way to offer clues for brain's postbuckling morphology. Especially, tissue geometry, growth ratio, and material properties of the cortex are explored as the most determinant parameters to control the morphogenesis of a growing brain model. As indicated in results, compressive residual stresses caused by the sufficient growth trigger instability and the brain forms highly convoluted patterns wherein its gyrification degree is specified with the cortex thickness. Morphological patterns of the developing brain predicted from the computational modeling are consistent with our neuroimaging observations, thereby clarifying, in part, the reason of some classical malformation in a developing brain.

Neurodevelopment in new-onset juvenile myoclonic epilepsy over the first 2 years

OBJECTIVE

Adults with juvenile myoclonic epilepsy (JME) have subtle brain structural abnormalities in the frontothalamocortical network, poorer cognitive function, and worse long-term social outcomes, even when their seizures are controlled and/or remitted. The natural history of JME and development of abnormalities in brain structure and cognition from epilepsy onset has not been studied.

METHODS

The maturational trajectories of cognitive and brain development were prospectively compared between 19 children with new-onset JME in the first 2 years after diagnosis and 57 healthy controls.

RESULTS

Cognitive abilities of children with JME were similar to or worse than healthy controls at baseline but failed to reach the competence level of healthy controls at follow-up across most of the tested cognitive abilities. Abnormal patterns of brain development, as assessed by magnetic resonance imaging studies, were evident in children with JME and included attenuation of age-related decline in cortical volume, thickness, and surface area compared to typically developing children. The altered brain developmental trajectory in the JME group was evident in higher-association frontoparietotemporal brain regions (p < 0.05, corrected for multiple comparisons).

INTERPRETATION

Children with JME have abnormal structural brain development and impaired cognitive development early in the course of their epilepsy.

Nitric oxide signaling in the development and evolution of language and cognitive circuits

The neocortex underlies not only remarkable motor and sensory capabilities, but also some of our most distinctly human cognitive functions. The emergence of these higher functions during evolution was accompanied by structural changes in the neocortex, including the acquisition of areal specializations such as Broca's speech and language area. The study of these evolutionary mechanisms, which likely involve species-dependent gene expression and function, represents a substantial challenge. These species differences, however, may represent valuable opportunities to understand the molecular underpinnings of neocortical evolution. Here, we discuss nitric oxide signaling as a candidate mechanism in the assembly of neocortical circuits underlying language and higher cognitive functions. This hypothesis was based on the highly specific mid-fetal pattern of nitric oxide synthase 1 (NOS1, previously nNOS) expression in the pyramidal (projection) neurons of two human neocortical areas respectively involved in speech and language, and higher cognition; the frontal operculum (FOp) and the anterior cingulate cortex (ACC). This expression is transiently present during mid-gestation, suggesting that NOS1 may be involved in the development of these areas and the assembly of their neural circuits. As no other gene product is known to exhibit such exquisite spatiotemporal expression, NOS1 represents a remarkable candidate for these functions.

Neural progenitors, patterning and ecology in neocortical origins

The anatomical organization of the mammalian neocortex stands out among vertebrates for its laminar and columnar arrangement, featuring vertically oriented, excitatory pyramidal neurons. The evolutionary origin of this structure is discussed here in relation to the brain organization of other amniotes, i.e., the sauropsids (reptiles and birds). Specifically, we address the developmental modifications that had to take place to generate the neocortex, and to what extent these modifications were shared by other amniote lineages or can be considered unique to mammals. In this article, we propose a hypothesis that combines the control of proliferation in neural progenitor pools with the specification of regional morphogenetic gradients, yielding different anatomical results by virtue of the differential modulation of these processes in each lineage. Thus, there is a highly conserved genetic and developmental battery that becomes modulated in different directions according to specific selective pressures. In the case of early mammals, ecological conditions like nocturnal habits and reproductive strategies are considered to have played a key role in the selection of the particular brain patterning mechanisms that led to the origin of the neocortex.

Evolutionary development of neural systems in vertebrates and beyond

The emerging field of "neuro-evo-devo" is beginning to reveal how the molecular and neural substrates that underlie brain function are based on variations in evolutionarily ancient and conserved neurochemical and neural circuit themes. Comparative work across bilaterians is reviewed to highlight how early neural patterning specifies modularity of the embryonic brain, which lays a foundation on which manipulation of neurogenesis creates adjustments in brain size. Small variation within these developmental mechanisms contributes to the evolution of brain diversity. Comparing the specification and spatial distribution of neural phenotypes across bilaterians has also suggested some major brain evolution trends, although much more work on profiling neural connections with neurochemical specificity across a wide diversity of organisms is needed. These comparative approaches investigating the evolution of brain form and function hold great promise for facilitating a mechanistic understanding of how variation in brain morphology, neural phenotypes, and neural networks influences brain function and behavioral diversity across organisms.

Transcriptional dysregulation of neocortical circuit assembly in ASD

Autism spectrum disorders (ASDs) impair social cognition and communication, key higher-order functions centered in the human neocortex. The assembly of neocortical circuitry is a precisely regulated developmental process susceptible to genetic alterations that can ultimately affect cognitive abilities. Because ASD is an early onset neurodevelopmental disorder that disrupts functions executed by the neocortex, miswiring of neocortical circuits has been hypothesized to be an underlying mechanism of ASD. This possibility is supported by emerging genetic findings and data from imaging studies. Recent research on neocortical development has identified transcription factors as key determinants of neocortical circuit assembly, mediating diverse processes including neuronal specification, migration, and wiring. Many of these TFs (TBR1, SOX5, FEZF2, and SATB2) have been implicated in ASD. Here, I will discuss the functional roles of these transcriptional programs in neocortical circuit development and their neurobiological implications for the emerging etiology of ASD.

A sharp cadherin-6 gene expression boundary in the developing mouse cortical plate demarcates the future functional areal border

The mammalian cerebral cortex can be tangentially subdivided into tens of functional areas with distinct cyto-architectures and neural circuitries; however, it remains elusive how these areal borders are genetically elaborated during development. Here we establish original bacterial artificial chromosome transgenic mouse lines that specifically recapitulate cadherin-6 (Cdh6) mRNA expression profiles in the layer IV of the somatosensory cortex and by detailing their cortical development, we show that a sharp Cdh6 gene expression boundary is formed at a mediolateral coordinate along the cortical layer IV as early as the postnatal day 5 (P5). By further applying mouse genetics that allows rigid cell fate tracing with CreERT2 expression, it is demonstrated that the Cdh6 gene expression boundary set at around P4 eventually demarcates the areal border between the somatosensory barrel and limb field at P20. In the P6 cortical cell pellet culture system, neurons with Cdh6 expression preferentially form aggregates in a manner dependent on Ca(2+) and electroporation-based Cdh6 overexpression limited to the postnatal stages perturbs area-specific cell organization in the barrel field. These results suggest that Cdh6 expression in the nascent cortical plate may serve solidification of the protomap for cortical functional areas.

Transcriptional co-regulation of neuronal migration and laminar identity in the neocortex

The cerebral neocortex is segregated into six horizontal layers, each containing unique populations of molecularly and functionally distinct excitatory projection (pyramidal) neurons and inhibitory interneurons. Development of the neocortex requires the orchestrated execution of a series of crucial processes, including the migration of young neurons into appropriate positions within the nascent neocortex, and the acquisition of layer-specific neuronal identities and axonal projections. Here, we discuss emerging evidence supporting the notion that the migration and final laminar positioning of cortical neurons are also co-regulated by cell type- and layer-specific transcription factors that play concomitant roles in determining the molecular identity and axonal connectivity of these neurons. These transcriptional programs thus provide direct links between the mechanisms controlling the laminar position and identity of cortical neurons.

Genetic influences on cortical regionalization in the human brain

Animal data demonstrate that the development of distinct cortical areas is influenced by genes that exhibit highly regionalized expression patterns. In this paper, we show genetic patterning of cortical surface area derived from MRI data from 406 adult human twins. We mapped genetic correlations of areal expansion between selected seed regions and all other cortical locations, with the selection of seed points based on results from animal studies. "Marching seeds" and a data-driven, hypothesis-free, fuzzy-clustering approach provided convergent validation. The results reveal strong anterior-to-posterior graded, bilaterally symmetric patterns of regionalization, largely consistent with patterns previously reported in nonhuman mammalian models. Broad similarities in genetic patterning between rodents and humans might suggest a conservation of cortical patterning mechanisms, whereas dissimilarities might reflect the functionalities most essential to each species.

Increased β-catenin activity in the anterior neural plate induces ectopic mid-hindbrain characteristics

BACKGROUND

The early telencephalon shares molecular features with the early mid-hindbrain region. In particular, these two developing brain areas each have a signaling center that secretes FGFs and an adjacent one that secretes WNTs. WNTs and FGFs each play essential roles in regulating cell fates in both the telencephalon and mid-hindbrain. Despite this similarity, telencephalic and mid-hindbrain precursors express distinct genes and ultimately generate different cell types, tissue morphologies, and neural functions.

RESULTS

Here we show that genetically increasing the level of β-catenin, a mediator of canonical WNT signaling, in the anterior neural plate causes a loss of telencephalic characteristics and a gain of mid-hindbrain characteristics.

CONCLUSION

These results, together with previous ones demonstrating that increased WNT signaling in the anterior neural plate increases FGF expression, suggest that the levels of WNT and FGF signaling regulate telencephalic versus mid-hindbrain fates.

β-Catenin-dependent FGF signaling sustains cell survival in the anterior embryonic head by countering Smad4

Growing evidence suggests that FGFs secreted from embryonic signaling centers are key mediators of cell survival. However, the mechanisms regulating FGF-dependent cell survival remain obscure. At the rostral end of the embryo, for example, ablation of FGF signaling leads to the rapid death of the precursor cells that form the anterior head, including the telencephalon. Here, we outline a core genetic circuit that regulates survival in the embryonic mouse head: WNT signaling through β-catenin directly maintains FGF expression and requires FGF function in vivo to oppose proapoptotic TGF-β signaling through SMAD4. Moreover, these antagonistic pathways converge on the transcriptional regulation of apoptosis, and genes such as Cdkn1a, suggesting a mechanism for how signaling centers in the embryonic head regulate cell survival.

Candidate gene sequencing of LHX2, HESX1, and SOX2 in a large schizencephaly cohort

Schizencephaly is a malformation of cortical development characterized by gray matter-lined clefts in the cerebral cortex and a range of neurological presentations. In some cases, there are features of septo-optic dysplasia concurrently with schizencephaly. The etiologies of both schizencephaly and septo-optic dysplasia are thought to be heterogeneous, but there is evidence that at least some cases have genetic origin. We hypothesized that these disorders may be caused by mutations in three candidate genes: LHX2, a gene with an important cortical patterning role, and HESX1 and SOX2, genes that have been associated with septo-optic dysplasia. We sequenced a large cohort of patients with schizencephaly, some with features of septo-optic dysplasia, for mutations in these genes. No pathogenic mutations were observed, suggesting that other genes or non-genetic factors influencing genes critical to brain development must be responsible for schizencephaly. © 2010 Wiley-Liss, Inc.

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

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

Investigating gradients of gene expression involved in early human cortical development

The division of the neocortex into functional areas (the cortical map) differs little between individuals, although brain lesions in development can lead to substantial re-organization of regional identity. We are studying how the cortical map is established in the human brain as a first step towards understanding this plasticity. Previous work on rodent development has identified certain transcription factors (e.g. Pax6, Emx2) expressed in gradients across the neocortex that appear to control regional expression of cell adhesion molecules and organization of area-specific thalamocortical afferent projections. Although mechanisms may be shared, the human neocortex is composed of different and more complex local area identities. Using Affymetrix gene chips of human foetal brain tissue from 8 to 12.5 post-conceptional weeks [PCW, equivalent to Carnegie stage (CS) 23, to Foetal stage (F) 4], human material obtained from the MRC-Wellcome Trust Human Developmental Biology Resource (http://www.hdbr.org), we have identified a number of genes that exhibit gradients along the anterior-posterior axis of the neocortex. Gene probe sets that were found to be upregulated posteriorally compared to anteriorally, included EMX2, COUPTFI and FGF receptor 3, and those upregulated anteriorally included cell adhesion molecules such as cadherins and protocadherins, as well as potential motor cortex markers and frontal markers (e.g. CNTNAP2, PCDH17, ROBO1, and CTIP2). Confirmation of graded expression for a subset of these genes was carried out using real-time PCR. Furthermore, we have established a dissociation cell culture model utilizing tissue dissected from anteriorally or posteriorally derived developing human neocortex that exhibits similar gradients of expression of these genes for at least 72 h in culture.

Interpreting developmental changes in neuroimaging signals

The imaging of developmental changes in brain function is challenging, but great strides have been made in addressing many of the conceptual issues that this work raises. I highlight a set of issues that remain to be addressed in this literature. First, I argue that the appeal to developmental neurobiology is often misplaced, as it focuses on neurodevelopmental processes that are mostly completed by the age at which neuroimaging studies can be performed. Second, I argue that the concept of "normative" development needs to be reexamined, as it reflects fundamental value judgments about brain development that seem inappropriate for scientific investigation. Third, I examine the ways in which developmental changes are often interpreted, arguing that common interpretations, including the concepts of "efficiency" and "focalization" may be less useful than commonly supposed. To put developmental neuroimaging on stronger footing, we need to develop stronger connections between computational and neurobiological accounts of developmental changes.

Ontogenetic expression of sonic hedgehog in the chicken subpallium

Sonic hedgehog (SHH) is a secreted signaling factor that is implicated in the molecular patterning of the central nervous system (CNS), somites, and limbs in vertebrates. SHH has a crucial role in the generation of ventral cell types along the entire rostrocaudal axis of the neural tube. It is secreted early in development by the axial mesoderm (prechordal plate and notochord) and the overlying ventral neural tube. Recent studies clarified the impact of SHH signaling mechanisms on dorsoventral patterning of the spinal cord, but the corresponding phenomena in the rostral forebrain are slightly different and more complex. This notably involves separate Shh expression in the preoptic part of the forebrain alar plate, as well as in the hypothalamic floor and basal plates. The present work includes a detailed spatiotemporal description of the singular alar Shh expression pattern in the rostral preoptic forebrain of chick embryos, comparing it with FoxG1, Dlx5, Nkx2.1, and Nkx2.2 mRNA expression at diverse stages of development. As a result of this mapping, we report a subdivision of the preoptic region in dorsal and ventral zones; only the dorsal part shows Shh expression. The positive area impinges as well upon a median septocommissural preoptic domain. Our study strongly suggests tangential migration of Shh-positive cells from the preoptic region into other subpallial domains, particularly into the pallidal mantle and the intermediate septum.

A computational model of cerebral cortex folding

The geometric complexity and variability of the human cerebral cortex have long intrigued the scientific community. As a result, quantitative description of cortical folding patterns and the understanding of underlying folding mechanisms have emerged as important research goals. This paper presents a computational 3D geometric model of cerebral cortex folding initialized by MRI data of a human fetal brain and deformed under the governance of a partial differential equation modeling cortical growth. By applying different simulation parameters, our model is able to generate folding convolutions and shape dynamics of the cerebral cortex. The simulations of this 3D geometric model provide computational experimental support to the following hypotheses: (1) Mechanical constraints of the skull regulate the cortical folding process. (2) The cortical folding pattern is dependent on the global cell growth rate of the whole cortex. (3) The cortical folding pattern is dependent on relative rates of cell growth in different cortical areas. (4) The cortical folding pattern is dependent on the initial geometry of the cortex.

Changing numbers of neuronal and non-neuronal cells underlie postnatal brain growth in the rat

The rat brain increases >6x in mass from birth to adulthood, presumably through the addition of glial cells and increasing neuronal size, without the addition of neurons. To test this hypothesis, here we investigate quantitatively the postnatal changes in the total number of neuronal and non-neuronal cells in the developing rat brain, and examine how these changes correlate with brain growth. Total numbers of cells were determined with the isotropic fractionator in the brains of 53 Wistar rats, from birth to young adulthood. We find that at birth, >90% of the cells in the rat brain are neurons. Following a dormant period of approximately 3 days after birth, the net number of neurons in the cerebral cortex, hippocampus, and remaining tissue (excluding cerebellum and olfactory bulb) doubles during the first week, then is reduced by 70% during the second postnatal week, concurrently with net gliogenesis. A second round of net addition of 6 million neurons is observed in the cerebral cortex over the following 2 weeks. During the first postnatal week, brain growth relates mainly to increased numbers of neurons of larger average size. In the second and third weeks, it correlates with increased numbers of non-neuronal cells that are smaller in size than the preexisting neurons. Postnatal rat brain development is thus characterized by dramatic changes in the cellular composition of the brain, whose growth is governed by different combinations of cell addition and loss, and changes in average cell size during the first months after birth.

ZHX2 Interacts with Ephrin-B and regulates neural progenitor maintenance in the developing cerebral cortex

Neural progenitor cells in the ventricular zone of the developing mammalian cerebral cortex give rise to specialized cortical cell types via consecutive rounds of proliferation and differentiation, but the mechanisms by which progenitor cell self-renewal and differentiation are regulated during cortical development are not well understood. Here, we show that zinc-finger and homeodomain protein 2 (ZHX2) is specifically expressed in neural progenitor cells during cortical neurogenesis. ZHX2 binds to the cytoplasmic domain of ephrin-B1, which is expressed in cortical neural progenitors and plays a role in neural progenitor cell maintenance. ZHX2 acts as a transcriptional repressor in cell, and its repressor activity is enhanced by coexpression of an ephrin-B1 intracellular domain. Blocking ZHX2 function in cultured neural progenitor cells or in the embryonic cortex causes neuronal differentiation, whereas overexpression of ZHX2 and an ephrin-B1 intracellular domain disrupts the normal differentiation of cortical neural progenitor cells. This study identifies ZHX2 as a novel regulator of neural progenitor cell maintenance and suggests a potential nuclear mechanism of the ephrin-B function in the cortex.

Pathogenesis of holoprosencephaly

Holoprosencephaly (HPE), the most common human forebrain malformation, occurs in 1 in 250 fetuses and 1 in 16,000 live births. HPE is etiologically heterogeneous, and its pathology is variable. Several mouse models of HPE have been generated, and some of the molecular causes of different forms of HPE and the mechanisms underlying its variable pathology have been revealed by these models. Herein, we summarize the current knowledge on the genetic alterations that cause HPE and discuss some important questions about this disease that remain to be answered.

Progressive loss of PAX6, TBR2, NEUROD and TBR1 mRNA gradients correlates with translocation of EMX2 to the cortical plate during human cortical development

The transcription factors Emx2 and Pax6 are expressed in the proliferating zones of the developing rodent neocortex, and gradients of expression interact in specifying caudal and rostral identities. Pax6 is also involved in corticoneurogenesis, being expressed by radial glial progenitors that give rise to cells that also sequentially express Tbr2, NeuroD and Tbr1, genes temporally downstream of Pax6. In this study, using in situ hybridization, we analysed the expression of EMX2, PAX6, TBR2, NEUROD and TBR1 mRNA in the developing human cortex between 8 and 12 postconceptional weeks (PCW). EMX2 mRNA was expressed in the ventricular (VZ) and subventricular zones (SVZ), but also in the cortical plate, unlike in the rodent. However, gradients of expression were similar to that of the rodent at all ages studied. PAX6 mRNA expression was limited to the VZ and SVZ. At 8 PCW, PAX6 was highly expressed rostrally but less so caudally, as has been seen in the rodent, however this gradient disappeared early in corticogenesis, by 9 PCW. There was less restricted compartment-specific expression of TBR2, NEUROD and TBR1 mRNA than in the rodent, where the gradients of expression were similar to that of PAX6 prior to 9 PCW. The gradient disappeared for TBR2 by 10 PCW, and for NEUROD and TBR1 by 12 PCW. These data support recent reports that EMX2 but not PAX6 is more directly involved in arealization, highlighting that analysis of human development allows better spatio-temporal resolution than studies in rodents.

Self-organized formation of polarized cortical tissues from ESCs and its active manipulation by extrinsic signals

Here, we demonstrate self-organized formation of apico-basally polarized cortical tissues from ESCs using an efficient three-dimensional aggregation culture (SFEBq culture). The generated cortical neurons are functional, transplantable, and capable of forming proper long-range connections in vivo and in vitro. The regional identity of the generated pallial tissues can be selectively controlled (into olfactory bulb, rostral and caudal cortices, hem, and choroid plexus) by secreted patterning factors such as Fgf, Wnt, and BMP. In addition, the in vivo-mimicking birth order of distinct cortical neurons permits the selective generation of particular layer-specific neurons by timed induction of cell-cycle exit. Importantly, cortical tissues generated from mouse and human ESCs form a self-organized structure that includes four distinct zones (ventricular, early and late cortical-plate, and Cajal-Retzius cell zones) along the apico-basal direction. Thus, spatial and temporal aspects of early corticogenesis are recapitulated and can be manipulated in this ESC culture.

Differences in genetic and environmental influences on the human cerebral cortex associated with development during childhood and adolescence

In this report, we present the first regional quantitative analysis of age-related differences in the heritability of cortical thickness using anatomic MRI with a large pediatric sample of twins, twin siblings, and singletons (n = 600, mean age 11.1 years, range 5-19). Regions of primary sensory and motor cortex, which develop earlier, both phylogenetically and ontologically, show relatively greater genetic effects earlier in childhood. Later developing regions within the dorsal prefrontal cortex and temporal lobes conversely show increasingly prominent genetic effects with maturation. The observation that regions associated with complex cognitive processes such as language, tool use, and executive function are more heritable in adolescents than children is consistent with previous studies showing that IQ becomes increasingly heritable with maturity(Plomin et al. 1997: Psychol Sci 8:442-447). These results suggest that both the specific cortical region and the age of the population should be taken into account when using cortical thickness as an intermediate phenotype to link genes, environment, and behavior.

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

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

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

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

LUZP deficiency affects neural tube closure during brain development

LUZP is a leucine zipper-containing protein predominantly expressed in the brain. The functional significance of LUZP remains unknown. To explore the role of LUZP in brain development, a knockout mouse strain with a lacZ knock-in (Luzp-KO/lacZ-KI) has been established. LacZ reporter expression driven by the endogenous Luzp promoter was detected in the neuroepithelium and the cardiac tissue. Luzp(-/-) mice exhibited perinatal death, presumably due to the accompanied complex cardiovascular defects. Luzp(-/-) embryos displayed a cranial neural tube closure defect (NTD), with exposed brain tissues. Ectopic expression of Sonic-hedgehog, which is a protein known to be involved in neural tube closure, and elevated apoptosis were observed in the dorsal lateral neuroepithelium of the NTD Luzp(-/-) hindbrain. These findings assign a novel function of LUZP in the embryonic development of brain.

Gene expression atlas of the mouse central nervous system: impact and interactions of age, energy intake and gender

The transcriptional profiles of five regions of the central nervous system (CNS) of mice varying in age, gender and dietary intake were measured by microarray. The resulting data provide insights into the mechanisms of age-, diet- and gender-related CNS plasticity and vulnerability in mammals.

Background

The structural and functional complexity of the mammalian central nervous system (CNS) is organized and modified by complicated molecular signaling processes that are poorly understood.

Results

We measured transcripts of 16,896 genes in 5 CNS regions from cohorts of young, middle-aged and old male and female mice that had been maintained on either a control diet or a low energy diet known to retard aging. Each CNS region (cerebral cortex, hippocampus, striatum, cerebellum and spinal cord) possessed its own unique transcriptome fingerprint that was independent of age, gender and energy intake. Less than 10% of genes were significantly affected by age, diet or gender, with most of these changes occurring between middle and old age. The transcriptome of the spinal cord was the most responsive to age, diet and gender, while the striatal transcriptome was the least responsive. Gender and energy restriction had particularly robust influences on the hippocampal transcriptome of middle-aged mice. Prominent functional groups of age- and energy-sensitive genes were those encoding proteins involved in DNA damage responses (Werner and telomere-associated proteins), mitochondrial and proteasome functions, cell fate determination (Wnt and Notch signaling) and synaptic vesicle trafficking.

Conclusion

Mouse CNS transcriptomes responded to age, energy intake and gender in a regionally distinctive manner. The systematic transcriptome dataset also provides a window into mechanisms of age-, diet- and sex-related CNS plasticity and vulnerability.

Haploinsufficiency of Six3 fails to activate Sonic hedgehog expression in the ventral forebrain and causes holoprosencephaly

Holoprosencephaly (HPE), the most common forebrain malformation, is characterized by an incomplete separation of the cerebral hemispheres. Mutations in the homeobox gene SIX3 account for 1.3% of all cases of human HPE. Using zebrafish-based assays, we have now determined that HPE-associated Six3 mutant proteins function as hypomorphs. Haploinsufficiency of Six3 caused by deletion of one allele of Six3 or by replacement of wild-type Six3 with HPE-associated Six3 mutant alleles was sufficient to recapitulate in mouse models most of the phenotypic features of human HPE. We demonstrate that Shh is a direct target of Six3 in the rostral diencephalon ventral midline (RDVM). Reduced amounts of functional Six3 protein fail to activate Shh expression in the mutant RDVM and ultimately lead to HPE. These results identify Six3 as a direct regulator of Shh expression and reveal a crossregulatory loop between Shh and Six3 in the ventral forebrain.

Regulation of neural progenitor cell state by ephrin-B

Maintaining a balance between self-renewal and differentiation in neural progenitor cells during development is important to ensure that correct numbers of neural cells are generated. We report that the ephrin-B–PDZ-RGS3 signaling pathway functions to regulate this balance in the developing mammalian cerebral cortex. During cortical neurogenesis, expression of ephrin-B1 and PDZ-RGS3 is specifically seen in progenitor cells and is turned off at the onset of neuronal differentiation. Persistent expression of ephrin-B1 and PDZ-RGS3 prevents differentiation of neural progenitor cells. Blocking RGS-mediated ephrin-B1 signaling in progenitor cells through RNA interference or expression of dominant-negative mutants results in differentiation. Genetic knockout of ephrin-B1 causes early cell cycle exit and leads to a concomitant loss of neural progenitor cells. Our results indicate that ephrin-B function is critical for the maintenance of the neural progenitor cell state and that this role of ephrin-B is mediated by PDZ-RGS3, likely via interacting with the noncanonical G protein signaling pathway, which is essential in neural progenitor asymmetrical cell division.

The ups and downs of holoprosencephaly: dorsal versus ventral patterning forces

Holoprosencephaly (HPE), characterized by incomplete separation of forebrain and facial components into left and right sides, is a common developmental defect in humans. It is caused by both genetic and environmental factors and its severity covers a wide spectrum of phenotypes. The genetic interactions underlying inherited forms of HPE are complex and poorly understood. Animal models, in particular mouse mutants, are providing a growing understanding of how the forebrain develops and how the cerebral hemispheres become split into left and right sides. These insights, along with the characterization to date of some of the genes involved in human HPE, suggest that two distinct mechanisms underlie the major classes of HPE, 'classic' and midline interhemispheric (MIH). Disruption either directly or indirectly of the ventralizing effect of sonic hedgehog signaling appears central to all or most forms of classic HPE, while disruption of the dorsalizing effect of bone morphogenetic protein signaling may be key to cases of MIH HPE.

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

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

Targeted disruption of Mib2 causes exencephaly with a variable penetrance

Mib1 and Mib2 ubiquitin ligases are very similar in their domain construction. They partake in the Notch signaling pathway by ubiquitinating the Notch receptors Delta and Jagged prior to endocytosis. We have created a targeted mutation of Mib2 and show that its phenotype is a variable penetrance, failure to close the cranial neural tube. The penetrance depends on the genetic background but it appears that Mib2 is not completely essential in mouse development.