The adhesion molecule TAG-1 is required for proper migration of the superficial migratory stream in the medulla but not of cortical interneurons

Using the Allen gene expression atlas of the adult mouse brain to gain further insight into the physiological significance of TAG-1/Contactin-2

The anatomic gene expression atlas (AGEA) of the adult mouse brain of the Allen Institute for Brain Science is a transcriptome-based atlas of the adult C57Bl/6 J mouse brain, based on the extensive in situ hybridization dataset of the Institute. This spatial mapping of the gene expression levels of mice under baseline conditions could assist in the formation of new, reasonable transcriptome-derived hypotheses on brain structure and underlying biochemistry, which could also have functional implications. The aim of this work is to use the data of the AGEA (in combination with Tabula Muris, a compendium of single cell transcriptome data collected from mice, enabling direct and controlled comparison of gene expression among cell types) to provide further insights into the physiology of TAG-1/Contactin-2 and its interactions, by presenting the expression of the corresponding gene across the adult mouse brain under baseline conditions and to investigate any spatial genomic correlations between TAG-1/Contactin-2 and its interacting proteins and markers of mature and immature oligodendrocytes, based on the pre-existing experimental or bibliographical evidence. The across-brain correlation analysis on the gene expression intensities showed a positive spatial correlation of TAG-1/Contactin-2 with the gene expression of Plp1, Myrf, Mbp, Mog, Cldn11, Bace1, Kcna1, Kcna2, App and Nfasc and a negative spatial correlation with the gene expression of Cspg4, Pdgfra, L1cam, Ncam1, Ncam2 and Ptprz1. Spatially correlated genes are mainly expressed by mature oligodendrocytes (like Cntn2), while spatially anticorrelated genes are mainly expressed by oligodendrocyte precursor cells. According to the data presented in this work, we propose that even though Contactin-2 expression during development correlates with high plasticity events, such as neuritogenesis, in adulthood it correlates with pathways characterized by low plasticity.

Cell-autonomous impact of polysialic acid-producing enzyme ST8SIA2 on developmental migration and distribution of cortical interneurons

In humans, variations in the polysialic acid-producing enzyme ST8SIA2 and disturbances in the cortical inhibitory system are associated with neurodevelopmental psychiatric disorders such as schizophrenia and autism. In mice, the ST8SIA2-dependent formation of polysialic acid during embryonic development is crucial for the establishment of interneuron populations of the medial prefrontal cortex. However, the spatial pattern and the neurodevelopmental mechanisms of interneuron changes caused by loss of ST8SIA2 function have not been fully characterized. Here, we use immunohistochemical analysis to demonstrate that densities of parvalbumin-positive interneurons are not only reduced in the medial prefrontal cortex, but also in the adjacent motor and somatosensory cortices of St8sia2-deficient male mice. These reductions, however, were confined to the rostral parts of the analyzed region. Mice with conditional knockout of St8sia2 under the interneuron-specific Lhx6 promoter, but not mice with a deletion under the Emx1 promoter that targets cortical excitatory neurons and glia, largely recapitulated the area-specific changes of parvalbumin-positive interneurons in the anterior cortex of St8sia2^-/- mice. Live imaging of interneuron migration in slice cultures of the developing cortex revealed a comparable reduction of directional persistence accompanied by increased branching of leading processes in slice cultures obtained from St8sia2^-/- embryos or from embryos with interneuron-specific ablation of St8sia2. Together, the data demonstrate a cell-autonomous impact of ST8SIA2 on cortical interneuron migration and the distribution of parvalbumin-positive interneurons in the anterior cortex. This provides a neurodevelopmental mechanism for how dysregulation of ST8SIA2 may lead to disturbed inhibitory balance as observed in schizophrenia and autism.

Ablation of CNTN2+ Pyramidal Neurons During Development Results in Defects in Neocortical Size and Axonal Tract Formation

Corticothalamic axons express Contactin-2 (CNTN2/TAG-1), a neuronal recognition molecule of the immunoglobulin superfamily involved in neurogenesis, neurite outgrowth, and fasciculation. TAG-1, which is expressed transiently by cortical pyramidal neurons during embryonic development, has been shown to be fundamental for axonal recognition, cellular migration, and neuronal proliferation in the developing cortex. Although Tag-1^−/− mice do not exhibit any obvious defects in the corticofugal system, the role of TAG-1+ neurons during the development of the cortex remains elusive. We have generated a mouse model expressing EGFP under the Tag-1 promoter and encompassing the coding sequence of Diptheria Toxin subunit A (DTA) under quiescence with no effect on the expression of endogenous Tag-1. We show that while the line recapitulates the expression pattern of the molecule, it highlights an extended expression in the forebrain, including multiple axonal tracts and neuronal populations, both spatially and temporally. Crossing these mice to the Emx1-Cre strain, we ablated the vast majority of TAG-1+ cortical neurons. Among the observed defects were a significantly smaller cortex, a reduction of corticothalamic axons as well as callosal and commissural defects. Such defects are common in neurodevelopmental disorders, thus this mouse could serve as a useful model to study physiological and pathophysiological cortical development.

Multiplexed Metabolic Labeling of Glycoconjugates in Polarized Primary Cerebral Cortical Neurons

The spatial distribution of cell-surface glycoconjugates in the brain changes continuously, reflecting neurophysiology especially in the developing phase, but their functions and fates mostly remain unexplored. Their spatiotemporal distribution is particularly important in polarized neuronal cells, such as cerebral cortical neurons composed of a soma and neurites. In this work, we dually labeled sialic acid (Sia5Ac) and N-acetylgalactosamine/glucosamine (GalNAc/GlcNAc) by a neurocompatible strategy of metabolic glycan labeling, metabolism-by-tissues (MbT), and obtained the multiplexed information on their spatiotemporal distribution on polarized cortical neurons. The analyses showed the preferentially distinct distribution of each saccharide set at the late developmental stage after randomized, heterogeneous distribution at the early stage, suggesting that Sia5Ac and GalNAc/GlcNAc are translocated anisotropically during neuronal development.

PAR3-PAR6-atypical PKC polarity complex proteins in neuronal polarization

Polarity is a fundamental feature of cells. Protein complexes, including the PAR3-PAR6-aPKC complex, have conserved roles in establishing polarity across a number of eukaryotic cell types. In neurons, polarity is evident as distinct axonal versus dendritic domains. The PAR3, PAR6, and aPKC proteins also play important roles in neuronal polarization. During this process, either aPKC kinase activity, the assembly of the PAR3-PAR6-aPKC complex or the localization of these proteins is regulated downstream of a number of signaling pathways. In turn, the PAR3, PAR6, and aPKC proteins control various effector molecules to establish neuronal polarity. Herein, we discuss the many signaling mechanisms and effector functions that have been linked to PAR3, PAR6, and aPKC during the establishment of neuronal polarity.

Manduca Contactin Regulates Amyloid Precursor Protein-Dependent Neuronal Migration

Amyloid precursor protein (APP) was originally identified as the source of β-amyloid peptides that accumulate in Alzheimer's disease (AD), but it also has been implicated in the control of multiple aspects of neuronal motility. APP belongs to an evolutionarily conserved family of transmembrane proteins that can interact with a variety of adapter and signaling molecules. Recently, we showed that both APP and its insect ortholog [APPL (APP-Like)] directly bind the heterotrimeric G-protein Goα, supporting the model that APP can function as an unconventional Goα-coupled receptor. We also adapted a well characterized assay of neuronal migration in the hawkmoth, Manduca sexta, to show that APPL-Goα signaling restricts ectopic growth within the developing nervous system, analogous to the role postulated for APP family proteins in controlling migration within the mammalian cortex. Using this assay, we have now identified Manduca Contactin (MsContactin) as an endogenous ligand for APPL, consistent with previous work showing that Contactins interact with APP family proteins in other systems. Using antisense-based knockdown protocols and fusion proteins targeting both proteins, we have shown that MsContactin is selectively expressed by glial cells that ensheath the migratory neurons (expressing APPL), and that MsContactin-APPL interactions normally prevent inappropriate migration and outgrowth. These results provide new evidence that Contactins can function as authentic ligands for APP family proteins that regulate APP-dependent responses in the developing nervous system. They also support the model that misregulated Contactin-APP interactions might provoke aberrant activation of Goα and its effectors, thereby contributing to the neurodegenerative sequelae that typify AD.

SIGNIFICANCE STATEMENT

Members of the amyloid precursor protein (APP) family participate in many aspects of neuronal development, but the ligands that normally activate APP signaling have remained controversial. This research provides new evidence that members of the Contactin family function as authentic ligands for APP and its orthologs, and that this evolutionarily conserved class of membrane-attached proteins regulates key aspects of APP-dependent migration and outgrowth in the embryonic nervous system. By defining the normal role of Contactin-APP signaling during development, these studies also provide the framework for investigating how the misregulation of Contactin-APP interactions might contribute to neuronal dysfunction in the context of both normal aging and neurodegenerative conditions, including Alzheimer's disease.

Contactin-2/TAG-1, active on the front line for three decades

Contactin-2/transiently expressed axonal surface glycoprotein-1 (TAG-1) is a cell adhesion molecule belonging to the immunoglobulin superfamily (IgSF). It has six immunoglobulin-like extracellular domains and four fibronectin III-like ones, with anchoring to the cell membrane through glycosylphosphatidyl inositol. Contactin-2/TAG-1 is expressed in specific neurons transiently on the axonal surface during the fetal period. In postnatal stages, Contactin-2/TAG-1 is expressed in cerebellar granule cells, hippocampal pyramidal cells, and the juxtaparanodal regions of myelinated nerve fibers. In the embryonic nervous system, Contactin-2/TAG-1 plays important roles in axonal elongation, axonal guidance, and cellular migration. In the postnatal nervous system, it also plays an essential role in the formation of myelinated nerve fibers. Moreover, Contactin-2/TAG-1 has been linked to autoimmune diseases of the human nervous system. Taken together, Contactin-2/TAG-1 plays a central role in a variety of functions from development to disease.

Neurexins 1-3 Each Have a Distinct Pattern of Expression in the Early Developing Human Cerebral Cortex

Neurexins (NRXNs) are presynaptic terminal proteins and candidate neurodevelopmental disorder susceptibility genes; mutations presumably upset synaptic stabilization and function. However, analysis of human cortical tissue samples by RNAseq and quantitative real-time PCR at 8-12 postconceptional weeks, prior to extensive synapse formation, showed expression of all three NRXNs as well as several potential binding partners. However, the levels of expression were not identical; NRXN1 increased with age and NRXN2 levels were consistently higher than for NRXN3. Immunohistochemistry for each NRXN also revealed different expression patterns at this stage of development. NRXN1 and NRXN3 immunoreactivity was generally strongest in the cortical plate and increased in the ventricular zone with age, but was weak in the synaptogenic presubplate (pSP) and marginal zone. On the other hand, NRXN2 colocalized with synaptophysin in neurites of the pSP, but especially with GAP43 and CASK in growing axons of the intermediate zone. Alternative splicing modifies the role of NRXNs and we found evidence by RNAseq for exon skipping at splice site 4 and concomitant expression of KHDBRS proteins which control this splicing. NRXN2 may play a part in early cortical synaptogenesis, but NRXNs could have diverse roles in development including axon guidance, and intercellular communication between proliferating cells and/or migrating neurons.

The role of Gpi-anchored axonal glycoproteins in neural development and neurological disorders

This review article focuses on the Contactin (CNTN) subset of the Immunoglobulin supergene family (IgC2/FNIII molecules), whose components share structural properties (the association of Immunoglobulin type C2 with Fibronectin type III domains), as well as a general role in cell contact formation and axonal growth control. IgC2/FNIII molecules include 6 highly related components (CNTN 1-6), associated with the cell membrane via a Glycosyl Phosphatidyl Inositol (GPI)-containing lipid tail. Contactin 1 and Contactin 2 share ~50 (49.38)% identity at the aminoacid level. They are components of the cell surface, from which they may be released in soluble forms. They bind heterophilically to multiple partners in cis and in trans, including members of the related L1CAM family and of the Neurexin family Contactin-associated proteins (CNTNAPs or Casprs). Such interactions are important for organising the neuronal membrane, as well as for modulating the growth and pathfinding of axon tracts. In addition, they also mediate the functional maturation of axons by promoting their interactions with myelinating cells at the nodal, paranodal and juxtaparanodal regions. Such interactions also mediate differential ionic channels (both Na⁺ and K⁺) distribution, which is of critical relevance in the generation of the peak-shaped action potential. Indeed, thanks to their interactions with Ankyrin G, Na⁺ channels map within the nodal regions, where they drive axonal depolarization. However, no ionic channels are found in the flanking Contactin1-containing paranodal regions, where CNTN1 interactions with Caspr1 and with the Ig superfamily component Neurofascin 155 in cis and in trans, respectively, build a molecular barrier between the node and the juxtaparanode. In this region K⁺ channels are clustered, depending upon molecular interactions with Contactin 2 and with Caspr2. In addition to these functions, the Contactins appear to have also a role in degenerative and inflammatory disorders: indeed Contactin 2 is involved in neurodegenerative disorders with a special reference to the Alzheimer disease, given its ability to work as a ligand of the Alzheimer Precursor Protein (APP), which results in increased Alzheimer Intracellular Domain (AICD) release in a γ-secretase-dependent manner. On the other hand Contactin 1 drives Notch signalling activation via the Hes pathway, which could be consistent with its ability to modulate neuroinflammation events, and with the possibility that Contactin 1-dependent interactions may participate to the pathogenesis of the Multiple Sclerosis and of other inflammatory disorders.

Systematic substrate identification indicates a central role for the metalloprotease ADAM10 in axon targeting and synapse function

Metzincin metalloproteases have major roles in intercellular communication by modulating the function of membrane proteins. One of the proteases is the a-disintegrin-and-metalloprotease 10 (ADAM10) which acts as alpha-secretase of the Alzheimer's disease amyloid precursor protein. ADAM10 is also required for neuronal network functions in murine brain, but neuronal ADAM10 substrates are only partly known. With a proteomic analysis of Adam10-deficient neurons we identified 91, mostly novel ADAM10 substrate candidates, making ADAM10 a major protease for membrane proteins in the nervous system. Several novel substrates, including the neuronal cell adhesion protein NrCAM, are involved in brain development. Indeed, we detected mistargeted axons in the olfactory bulb of conditional ADAM10-/- mice, which correlate with reduced cleavage of NrCAM, NCAM and other ADAM10 substrates. In summary, the novel ADAM10 substrates provide a molecular basis for neuronal network dysfunctions in conditional ADAM10-/- mice and demonstrate a fundamental function of ADAM10 in the brain.

Tag1 deficiency results in olfactory dysfunction through impaired migration of mitral cells

The olfactory system provides mammals with the abilities to investigate, communicate and interact with their environment. These functions are achieved through a finely organized circuit starting from the nasal cavity, passing through the olfactory bulb and ending in various cortical areas. We show that the absence of transient axonal glycoprotein-1 (Tag1)/contactin-2 (Cntn2) in mice results in a significant and selective defect in the number of the main projection neurons in the olfactory bulb, namely the mitral cells. A subpopulation of these projection neurons is reduced in Tag1-deficient mice as a result of impaired migration. We demonstrate that the detected alterations in the number of mitral cells are well correlated with diminished odor discrimination ability and social long-term memory formation. Reduced neuronal activation in the olfactory bulb and the corresponding olfactory cortex suggest that Tag1 is crucial for the olfactory circuit formation in mice. Our results underpin the significance of a numerical defect in the mitral cell layer in the processing and integration of odorant information and subsequently in animal behavior.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.

Crosstalk between intracellular and extracellular signals regulating interneuron production, migration and integration into the cortex

During embryogenesis, cortical interneurons are generated by ventral progenitors located in the ganglionic eminences of the telencephalon. They travel along multiple tangential paths to populate the cortical wall. As they reach this structure they undergo intracortical dispersion to settle in their final destination. At the cellular level, migrating interneurons are highly polarized cells that extend and retract processes using dynamic remodeling of microtubule and actin cytoskeleton. Different levels of molecular regulation contribute to interneuron migration. These include: (1) Extrinsic guidance cues distributed along migratory streams that are sensed and integrated by migrating interneurons; (2) Intrinsic genetic programs driven by specific transcription factors that grant specification and set the timing of migration for different subtypes of interneurons; (3) Adhesion molecules and cytoskeletal elements/regulators that transduce molecular signalings into coherent movement. These levels of molecular regulation must be properly integrated by interneurons to allow their migration in the cortex. The aim of this review is to summarize our current knowledge of the interplay between microenvironmental signals and cell autonomous programs that drive cortical interneuron porduction, tangential migration, and intergration in the developing cerebral cortex.

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Pioneering axons regulate neuronal polarization in the developing cerebral cortex

The polarization of neurons, which mainly includes the differentiation of axons and dendrites, is regulated by cell-autonomous and non-cell-autonomous factors. In the developing central nervous system, neuronal development occurs in a heterogeneous environment that also comprises extracellular matrices, radial glial cells, and neurons. Although many cell-autonomous factors that affect neuronal polarization have been identified, the microenvironmental cues involved in neuronal polarization remain largely unknown. Here, we show that neuronal polarization occurs in a microenvironment in the lower intermediate zone, where the cell adhesion molecule transient axonal glycoprotein-1 (TAG-1) is expressed in cortical efferent axons. The immature neurites of multipolar cells closely contact TAG-1-positive axons and generate axons. Inhibition of TAG-1-mediated cell-to-cell interaction or its downstream kinase Lyn impairs neuronal polarization. These results show that the TAG-1-mediated cell-to-cell interaction between the unpolarized multipolar cells and the pioneering axons regulates the polarization of multipolar cells partly through Lyn kinase and Rac1.

Neuronal migration and its disorders affecting the CA3 region

In this review, we focus on CA3 neuronal migration disorders in the rodent. We begin by introducing the main steps of hippocampal development, and we summarize characteristic hippocampal malformations in human. We then describe various mouse mutants showing structural hippocampal defects. Notably, genes identified in human cortical neuronal migration disorders consistently give rise to a CA3 phenotype when mutated in the mouse. We successively describe their molecular, physiological and behavioral phenotypes that together contribute to a better understanding of CA3-dependent functions. We finally discuss potential factors underlying the CA3 vulnerability revealed by these mouse mutants and that may also contribute to other human neurological and psychiatric disorders.

BACE1 activity regulates cell surface contactin-2 levels

Background

Although BACE1 is a major therapeutic target for Alzheimer’s disease (AD), potential side effects of BACE1 inhibition are not well characterized. BACE1 cleaves over 60 putative substrates, however the majority of these cleavages have not been characterized. Here we investigated BACE1-mediated cleavage of human contactin-2, a GPI-anchored cell adhesion molecule.

Results

Our initial protein sequence analysis showed that contactin-2 harbors a strong putative BACE1 cleavage site close to its GPI membrane linker domain. When we overexpressed BACE1 in CHO cells stably transfected with human contactin-2, we found increased release of soluble contactin-2 in the conditioned media. Conversely, pharmacological inhibition of BACE1 in CHO cells expressing human contactin-2 and mouse primary neurons decreased soluble contactin-2 secretion. The BACE1 cleavage site mutation 1008MM/AA dramatically impaired soluble contactin-2 release. We then asked whether contactin-2 release induced by BACE1 expression would concomitantly decrease cell surface levels of contactin-2. Using immunofluorescence and surface-biotinylation assays, we showed that BACE1 activity tightly regulates contactin-2 surface levels in CHO cells as well as in mouse primary neurons. Finally, contactin-2 levels were decreased in Alzheimer’s disease brain samples correlating inversely with elevated BACE1 levels in the same samples.

Conclusion

Our results clearly demonstrate that mouse and human contactin-2 are physiological substrates for BACE1. BACE1-mediated contactin-2 cleavage tightly regulates the surface expression of contactin-2 in neuronal cells. Given the role of contactin-2 in cell adhesion, neurite outgrowth and axon guidance, our data suggest that BACE1 may play an important role in these physiological processes by regulating contactin-2 surface levels.

New insights into the roles of the contactin cell adhesion molecules in neural development

In vertebrates, the contactin (CNTN) family of neural cell recognition molecules includes six related cell adhesion molecules that play non-overlapping roles in the formation and maintenance of the nervous system. CNTN1 and CNTN2 are the prototypical members of the family and have been involved, through cis- and trans-interactions with distinct cell adhesion molecules, in neural cell migration, axon guidance, and the organization of myelin subdomains. In contrast, the roles of CNTN3-6 are less well characterized although the generation of null mice and the recent identification of a common extracellular binding partner have considerably advanced our grasp of their physiological roles in particular as they relate to the wiring of sensory tissues. In this review, we aim to present a summary of our current understanding of CNTN functions and give an overview of the challenges that lie ahead in understanding the roles these proteins play in nervous system development and maintenance.

Production and organization of neocortical interneurons

Inhibitory GABA (γ-aminobutyric acid)-ergic interneurons are a vital component of the neocortex responsible for shaping its output through a variety of inhibitions. Consisting of many flavors, interneuron subtypes are predominantly defined by their morphological, physiological, and neurochemical properties that help to determine their functional role within the neocortex. During development, these cells are born in the subpallium where they then tangentially migrate over long distances before being radially positioned to their final location in the cortical laminae. As development progresses into adolescence, these cells mature and form chemical and electrical connections with both glutamatergic excitatory neurons and other interneurons ultimately establishing the cortical network. The production, migration, and organization of these cells are determined by vast array of extrinsic and intrinsic factors that work in concert in order to assemble a proper functioning cortical inhibitory network. Failure of these cells to undergo these processes results in abnormal positioning and cortical function. In humans, this can bring about several neurological disorders including schizophrenia, epilepsy, and autism spectrum disorders. In this article, we will review previous literature that has revealed the framework for interneuron neurogenesis and migratory behavior as well as discuss recent findings that aim to elucidate the spatial and functional organization of interneurons within the neocortex.

A forward genetic screen in mice identifies mutants with abnormal cortical patterning

Formation of a 6-layered cortical plate and axon tract patterning are key features of cerebral cortex development. Abnormalities of these processes may be the underlying cause for a range of functional disabilities seen in human neurodevelopmental disorders. To identify mouse mutants with defects in cortical lamination or corticofugal axon guidance, N-ethyl-N-nitrosourea (ENU) mutagenesis was performed using mice expressing LacZ reporter genes in layers II/III and V of the cortex (Rgs4-lacZ) or in corticofugal axons (TAG1-tau-lacZ). Four lines with abnormal cortical lamination have been identified. One of these was a splice site mutation in reelin (Reln) that results in a premature stop codon and the truncation of the C-terminal region (CTR) domain of reelin. Interestingly, this novel allele of Reln did not display cerebellar malformation or ataxia, and this is the first report of a Reln mutant without a cerebellar defect. Four lines with abnormal cortical axon development were also identified, one of which was found by whole-genome resequencing to carry a mutation in Lrp2. These findings demonstrated that the application of ENU mutagenesis to mice carrying transgenic reporters marking cortical anatomy is a sensitive and specific method to identify mutations that disrupt patterning of the developing brain.

Distinct cis regulatory elements govern the expression of TAG1 in embryonic sensory ganglia and spinal cord

Cell fate commitment of spinal progenitor neurons is initiated by long-range, midline-derived, morphogens that regulate an array of transcription factors that, in turn, act sequentially or in parallel to control neuronal differentiation. Included among these are transcription factors that regulate the expression of receptors for guidance cues, thereby determining axonal trajectories. The Ig/FNIII superfamily molecules TAG1/Axonin1/CNTN2 (TAG1) and Neurofascin (Nfasc) are co-expressed in numerous neuronal cell types in the CNS and PNS – for example motor, DRG and interneurons - both promote neurite outgrowth and both are required for the architecture and function of nodes of Ranvier. The genes encoding TAG1 and Nfasc are adjacent in the genome, an arrangement which is evolutionarily conserved. To study the transcriptional network that governs TAG1 and Nfasc expression in spinal motor and commissural neurons, we set out to identify cis elements that regulate their expression. Two evolutionarily conserved DNA modules, one located between the Nfasc and TAG1 genes and the second directly 5′ to the first exon and encompassing the first intron of TAG1, were identified that direct complementary expression to the CNS and PNS, respectively, of the embryonic hindbrain and spinal cord. Sequential deletions and point mutations of the CNS enhancer element revealed a 130bp element containing three conserved E-boxes required for motor neuron expression. In combination, these two elements appear to recapitulate a major part of the pattern of TAG1 expression in the embryonic nervous system.

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.

Cell-autonomous and cell-to-cell signalling events in normal and altered neuronal migration

The cerebral cortex is a complex six-layered structure that contains an important diversity of neurons, and has rich local and extrinsic connectivity. Among the mechanisms governing the cerebral cortex construction, neuronal migration is perhaps the most crucial as it ensures the timely formation of specific and selective neuronal circuits. Here, we review the main extrinsic and extrinsic factors involved in regulating neuronal migration in the cortex and describe some environmental factors interfering with their actions.

Absence of CNTNAP2 leads to epilepsy, neuronal migration abnormalities, and core autism-related deficits

Although many genes predisposing to autism spectrum disorders (ASD) have been identified, the biological mechanism(s) remain unclear. Mouse models based on human disease-causing mutations provide the potential for understanding gene function and novel treatment development. Here, we characterize a mouse knockout of the Cntnap2 gene, which is strongly associated with ASD and allied neurodevelopmental disorders. Cntnap2(-/-) mice show deficits in the three core ASD behavioral domains, as well as hyperactivity and epileptic seizures, as have been reported in humans with CNTNAP2 mutations. Neuropathological and physiological analyses of these mice before the onset of seizures reveal neuronal migration abnormalities, reduced number of interneurons, and abnormal neuronal network activity. In addition, treatment with the FDA-approved drug risperidone ameliorates the targeted repetitive behaviors in the mutant mice. These data demonstrate a functional role for CNTNAP2 in brain development and provide a new tool for mechanistic and therapeutic research in ASD.

The effects of Tag-1 on the maturation of mouse cerebellar granule neurons

The cell adhesion molecule Tag-1 is highly expressed in immature cerebellar granule neurons (CGNs) during axonogenesis and is down-regulated prior to onset of radial migration. However, its precise role(s) during development of mammalian CGNs has been unclear. Here we studied the effects of anti-Tag-1 function blocking antibodies on the development of mouse CGNs in primary cell culture and in situ. Interfering antibodies inhibited axon formation by mouse CGNs in both cell cultures and in cerebellar slices. Effects on axon extension in cell cultures were observed under conditions of homotypic cell-cell contact, consistent with inhibition of cell adhesion activity. Further, when used as a substratum Tag-1 protein strongly stimulated neurite outgrowth by CGNs. Antagonism of Tag-1 also enhanced CGN migration in modified Boyden chamber assays. Radial migration was inhibited by Tag-1 antibodies in cerebellar slices, possibly reflecting a block in early CGN maturation in situ. These findings are consistent with a regulatory role for Tag-1 in axon emergence as well as migratory behavior by developing mouse CGNs.

PAF-AH Catalytic Subunits Modulate the Wnt Pathway in Developing GABAergic Neurons

Platelet-activating factor acetylhydrolase 1B (PAF-AH) inactivates the potent phospholipid platelet-activating factor (PAF) and is composed of two catalytic subunits (α1 and α2) and a dimeric regulatory subunit, LIS1. The function of the catalytic subunits in brain development remains unknown. Here we examined their effects on proliferation in the ganglionic eminences and tangential migration. In α1 and α2 catalytic subunits knockout mice we noticed an increase in the size of the ganglionic eminences resulting from increased proliferation of GABAergic neurons. Our results indicate that the catalytic subunits act as negative regulators of the Wnt signaling pathway. Overexpression of each of the PAF-AH catalytic subunits reduced the amount of nuclear beta-catenin and provoked a shift of this protein from the nucleus to the cytoplasm. In the double mutant mice, Wnt signaling increased in the ganglionic eminences and in the dorsal part of the cerebral cortex. In situ hybridization revealed increased and expanded expression of a downstream target of the Wnt pathway (Cyclin D1), and of upstream Wnt components (Tcf4, Tcf3 and Wnt7B). Furthermore, the interneurons in the cerebral cortex were more numerous and in a more advanced position. Transplantation assays revealed a non-cell autonomous component to this phenotype, which may be explained in part by increased and expanded expression of Sdf1 and Netrin-1. Our findings strongly suggest that PAF-AH catalytic subunits modulate the Wnt pathway in restricted areas of the developing cerebral cortex. We hypothesize that modulation of the Wnt pathway is the evolutionary conserved activity of the PAF-AH catalytic subunits.

Two separate subtypes of early non-subplate projection neurons in the developing cerebral cortex of rodents

The preplate of the cerebral cortex contains projection neurons that connect the cortical primordium with the subpallium. These are collectively named pioneer neurons. After preplate partition, most of these pioneer neurons become subplate neurons. Certain preplate neurons, however, never associate with the subplate but rather with the marginal zone. In the present overview, we propose a novel classification of non-subplate pioneer neurons in rodents into two subtypes. In rats, the neurons of the first subtype are calbindin⁺ (CB), calretinin⁺ (CR) and L1⁺ and are situated in the upper part of the preplate before its partition. Neurons of the second subtype are TAG-1⁺ and are located slightly deeper to the previous population in the preplate. After the preplate partition, the CB⁺, CR⁺ and L1⁺ neurons remain in the marginal zone whereas TAG-1⁺ neurons become transiently localized in the upper cortical plate. In mice, by contrast, calcium binding proteins did not label pioneer neurons. We define in mice two subtypes of non-subplate pioneer neurons, either L1⁺ or TAG-1⁺/cntn2⁺. We propose these to be the homologues of the two subtypes of non-subplate pioneer neurons of rats. The anatomical distribution of these neuron populations is similar in rats and mice. The two populations of non-subplate pioneer neurons differ in their axonal projections. Axons of L1⁺ pioneer neurons project to the ganglionic eminences and the anterior preoptic area, but avoid entering the posterior limb of the internal capsule towards the thalamus. Axons of TAG-1⁺ pioneer neurons project to the lateral parts of the ganglionic eminences at the early stages of cortical histogenesis examined.

Preferential localization of neural cell recognition molecule NB-2 in developing glutamatergic neurons in the rat auditory brainstem

NB-2 is a neuronal cell recognition molecule that is preferentially expressed in auditory pathways. Mice deficient in the NB-2 gene exhibit aberrant responses to acoustic stimuli. Here we examined the expression and localization of NB-2 in the auditory brainstem during development in the rat. NB-2 was strongly expressed in the ventral cochlear nucleus (VCN), ventral acoustic stria, lateral and medial superior olivary complex (SOC), superior paraolivary nucleus, medial nucleus of the trapezoid body (MNTB), ventrolateral lemniscus, and central nucleus of the inferior colliculus (CIC). In the VCN and CIC, NB-2 was expressed in the regions that are known to respond to high frequencies. In situ hybridization combined with immunohistochemistry suggested that NB-2 is expressed only in neurons. NB-2 was colocalized with glutamatergic elements in the neuropil and the calyces of Held but not with glycinergic or GABAergic elements. NB-2 expression in the SOC was first detected at embryonic day (E)19, reached a maximum level at postnatal day (P)7, and declined thereafter. Immunolabeling with VGLUT1 and VGLUT2, markers for mature and premature glutamatergic synapses, respectively, in combination with NB-2 immunolabeling revealed that NB-2 is expressed at glutamatergic synapses. Collectively, our findings suggest that NB-2 plays a key role in maturation of glutamatergic synapses in the brainstem during the final stages of auditory development.

Contactins: emerging key roles in the development and function of the nervous system

Contactins are a subgroup of molecules belonging to the immunoglobulin superfamily that are expressed exclusively in the nervous system. The subgroup consists of six members: contactin, TAG-1, BIG-1, BIG-2, NB-2 and NB-3. Since their identification in the late 1980s, contactin and TAG-1 have been studied extensively. Axonal expression and the neurite extension activity of contactin and TAG-1 attracted researchers to study the function of these molecules in axon guidance during development. After the exciting discovery of the molecular function of contactin and TAG-1 in myelination earlier this decade, these two molecules have come to be known as the principal molecules in the function and maintenance of myelinated neurons. In contrast, the function of the other four members of this subgroup remained unknown until recently. Here, we will give an overview of contactin function, including recent progress on BIG-2, NB-2 and NB-3.

The cell adhesion molecule Tag1, transmembrane protein Stbm/Vangl2, and Lamininalpha1 exhibit genetic interactions during migration of facial branchiomotor neurons in zebrafish

Interactions between a neuron and its environment play a major role in neuronal migration. We show here that the cell adhesion molecule Transient Axonal Glycoprotein (Tag1) is necessary for the migration of the facial branchiomotor neurons (FBMNs) in the zebrafish hindbrain. In tag1 morphant embryos, FBMN migration is specifically blocked, with no effect on organization or patterning of other hindbrain neurons. Furthermore, using suboptimal morpholino doses and genetic mutants, we found that tag1, lamininalpha1 (lama1) and stbm, which encodes a transmembrane protein Vangl2, exhibit pairwise genetic interactions for FBMN migration. Using time-lapse analyses, we found that FBMNs are affected similarly in all three single morphant embryos, with an inability to extend protrusions in a specific direction, and resulting in the failure of caudal migration. These data suggest that tag1, lama1 and vangl2 participate in a common mechanism that integrates signaling between the FBMN and its environment to regulate migration.

Impairment of learning and memory in TAG-1 deficient mice associated with shorter CNS internodes and disrupted juxtaparanodes

The cell adhesion molecule TAG-1 is expressed by neurons and glial cells and plays a role in axon outgrowth, migration and fasciculation during development. TAG-1 is also required for the clustering of Kv1.1/1.2 potassium channels and Caspr2 at the juxtaparanodes of myelinated fibers. Behavioral examination of TAG-1 deficient mice (Tag-1(-/-)) showed cognitive impairments in the Morris water maze and novel object recognition tests, reduced spontaneous motor activity, abnormal gait coordination and increased response latency to noxious stimulation. Investigation at the molecular level revealed impaired juxtaparanodal clustering of Caspr2 and Kv1.1/1.2 in the hippocampus, entorhinal cortex, cerebellum and olfactory bulb, with diffusion into the internode. Caspr2 and Kv1.1 levels were reduced in the cerebellum and olfactory bulb. Moreover, Tag-1(-/-) mice had shorter internodes in the cerebral and cerebellar white matter. The detected molecular alterations may account for the behavioural deficits and hyperexcitability in these animals.

Comparison of slow and fast neocortical neuron migration using a new in vitro model

BACKGROUND

Mutations, toxic insults and radiation exposure are known to slow or arrest the migration of cortical neurons, in most cases by unknown mechanisms. The movement of migrating neurons is saltatory, reflecting the intermittent movement of the nucleus (nucleokinesis) within the confines of the plasma membrane. Each nucleokinetic movement is analogous to a step. Thus, average migration speed could be reduced by lowering step frequency and/or step distance.

RESULTS

To assess the kinetic features of cortical neuron migration we developed a cell culture system that supports fiber-guided migration. In this system, the majority of fiber-apposed cells were neurons, expressed age-appropriate cortical-layer specific markers and migrated during a 30 min imaging period. Comparison of the slowest and fastest quartiles of cells revealed a 5-fold difference in average speed. The major determinant of average speed in slower cells (6-26 microm/hr) was step frequency, while step distance was the critical determinant of average speed in faster cells (>26 microm/hr). Surprisingly, step distance was largely determined by the average duration of the step, rather than the speed of nucleokinesis during the step, which differed by only 1.3-fold between the slowest and fastest quartiles.

CONCLUSION

Saltatory event frequency and duration, not nucleokinetic speed, are the major determinants of average migration speed in healthy neurons. Alteration of either saltatory event frequency or duration should be considered along with nucleokinetic abnormalities as possible contributors to pathological conditions.

BIG-2 mediates olfactory axon convergence to target glomeruli

Olfactory sensory neurons expressing a given odorant receptor converge axons onto a few topographically fixed glomeruli in the olfactory bulb, leading to establishment of the odor map. Here, we report that BIG-2/contactin-4, an axonal glycoprotein belonging to the immunoglobulin superfamily, is expressed in a subpopulation of mouse olfactory sensory neurons. A mosaic pattern of glomerular arrangement is observed with strongly BIG-2-positive, weakly positive, and negative axon terminals in the olfactory bulb, which is overlapping but not identical with those of Kirrel2 and ephrin-A5. There is a close correlation between the BIG-2 expression level and the odorant receptor choice in individual sensory neurons. In BIG-2-deficient mice, olfactory sensory neurons expressing a given odorant receptor frequently innervate multiple glomeruli at ectopic locations. These results suggest that BIG-2 is one of the axon guidance molecules crucial for the formation and maintenance of functional odor map in the olfactory bulb.

Axonin-1/TAG-1 is required for pathfinding of granule cell axons in the developing cerebellum

BACKGROUND

Neural development consists of a series of steps, including neurogenesis, patterning, cell migration, axon guidance, and finally, synaptogenesis. Because all these steps proceed in a constantly changing environment, functional gene analyses during development have to take time into account. This is quite challenging, however, as loss-of-function approaches based on classic genetic tools do not allow for the precise temporal control that is required for developmental studies. Gene silencing by RNA interference (RNAi) in combination with the chicken embryo or with cultured embryos opens new possibilities for functional gene analysis in vivo. Axonin-1/TAG-1 is a cell adhesion molecule of the immunoglobulin superfamily with a well defined temporal and spatial expression pattern in the developing vertebrate nervous system. Axonin-1/TAG-1 was shown to promote neurite outgrowth in vitro and to be required for commissural and sensory axon pathfinding in vivo.

RESULTS

To knock down axonin-1 in a temporally and spatially controlled manner during development of the nervous system, we have combined RNAi with the accessibility of the chicken embryo even at late stages of development. Using ex ovo RNAi, we analyzed the function of axonin-1/TAG-1 in cerebellar development. Axonin-1 is expressed in postmitotic granule cells while they extend their processes, the parallel fibers. In the absence of axonin-1 these processes still extend but no longer in a parallel manner to each other or to the pial surface of the cerebellum.

CONCLUSION

Axonin-1/TAG-1 is required for the navigation, but not for the elongation, of granule cell processes in the developing cerebellum in vivo.

Transient axonal glycoprotein-1 (TAG-1) and laminin-alpha1 regulate dynamic growth cone behaviors and initial axon direction in vivo

BACKGROUND

How axon guidance signals regulate growth cone behavior and guidance decisions in the complex in vivo environment of the central nervous system is not well understood. We have taken advantage of the unique features of the zebrafish embryo to visualize dynamic growth cone behaviors and analyze guidance mechanisms of axons emerging from a central brain nucleus in vivo.

RESULTS

We investigated axons of the nucleus of the medial longitudinal fascicle (nucMLF), which are the first axons to extend in the zebrafish midbrain. Using in vivo time-lapse imaging, we show that both positive axon-axon interactions and guidance by surrounding tissue control initial nucMLF axon guidance. We further show that two guidance molecules, transient axonal glycoprotein-1 (TAG-1) and laminin-alpha1, are essential for the initial directional extension of nucMLF axons and their subsequent convergence into a tight fascicle. Fixed tissue analysis shows that TAG-1 knockdown causes errors in nucMLF axon pathfinding similar to those seen in a laminin-alpha1 mutant. However, in vivo time-lapse imaging reveals that while some defects in dynamic growth cone behavior are similar, there are also defects unique to the loss of each gene. Loss of either TAG-1 or laminin-alpha1 causes nucMLF axons to extend into surrounding tissue in incorrect directions and reduces axonal growth rate, resulting in stunted nucMLF axons that fail to extend beyond the hindbrain. However, defects in axon-axon interactions were found only after TAG-1 knockdown, while defects in initial nucMLF axon polarity and excessive branching of nucMLF axons occurred only in laminin-alpha1 mutants.

CONCLUSION

These results demonstrate how two guidance cues, TAG-1 and laminin-alpha1, influence the behavior of growth cones during axon pathfinding in vivo. Our data suggest that TAG-1 functions to allow growth cones to sense environmental cues and mediates positive axon-axon interactions. Laminin-alpha1 does not regulate axon-axon interactions, but does influence neuronal polarity and directional guidance.

Structure-function analysis of protein complexes involved in the molecular architecture of juxtaparanodal regions of myelinated fibers

Demyelinating disorders, including multiple sclerosis (MS), are common causes of neurological disability. One critical step towards the management and therapy of demyelinating diseases is to understand the basic functions of myelinating glia and their relationship with axons. Axons and myelinating glia, oligodendrocytes in the central (CNS) and Schwann cells in the peripheral (PNS) nervous systems, reciprocally influence each other's development and trophism. These interactions are critical for the formation of distinct axonal domains in myelinated fibers that ensure the rapid propagation of action potentials. Macromolecular complexes mediating axo-glial interactions in these domains have been identified, consisting of members of the immunoglobulin superfamily (IgSF) of adhesion molecules and the neurexin/NCP superfamily as well as other proteins. We have investigated the molecular details of axo-glial interactions in the juxtaparanodal region of myelinated fibers by utilizing domain-specific GFP constructs and immunoprecipitation assays on transfected cells. We have shown that the immunoglobulin domains of the IgSF member TAG-1/Cnt-2 are necessary and sufficient for the direct, cis interaction of this protein with Caspr2 and potassium channels.

Pax6-/- mice have a cell nonautonomous defect in nonradial interneuron migration

The mammalian neocortex comprises two major neuronal subtypes; interneurons derived from the ganglionic eminence (GE) and projection neurons from the cortical ventricular zone (VZ). These separate origins necessitate distinct pathways of migration. Using mouse genetics and embryonic forebrain slice culture assays, we sought to identify substrates and/or guidance molecules for nonradial cell migration (NRCM). Mice carrying a mutation in Pax6 (Sey(-/-)), a paired domain transcription factor, are reported to have increased numbers of cortical inhibitory interneurons, suggesting that Pax6 could induce inhibitors of interneuron development or alternatively play a repressive role in guiding NRCM and/or specifying interneurons. Unexpectedly, we found a cell nonautonomous reduction in the distance Sey-/- neurons migrated, reflecting a disorganized migration, with frequent changes in direction. In contrast, no difference in the number of nonradially migrating GE cells was observed in Sey-/- mice. Our data indicate that the increased numbers of interneurons observed in Sey-/- do not result from an increased rate or number of nonradially migrating cells; instead, loss of Pax6 results in the ectopic specification of interneurons in the cortical VZ. Further, our data indicate that the known axonal disorganization in Sey-/- mice contributes to the observed reduced distance of NRCM.

Control of tangential/non-radial migration of neurons in the developing cerebral cortex

Projection neurons in the developing cerebral cortex of rodents are basically born near the ventricle and migrate radially to beneath the marginal zone, whereas their cortical interneurons are generated in the ventral telencephalon and migrate tangentially to the cortex. The origins and migratory profiles of each interneuron subtype have been studied extensively in the last decade, and an enormous effort has been made to clarify the cellular and molecular mechanisms that regulate interneuron migration. More recently, the interaction between projection neurons and migrating interneurons, including how they are incorporated into their proper layers, has begun to be analyzed. In this review, I outline the most recent findings in regard to these issues and discuss the mechanisms underlying the development of cortical cytoarchitecture.

Cell and molecular mechanisms involved in the migration of cortical interneurons

Since the discovery that the vast majority of the GABA-containing interneurons of the cerebral cortex arise in the subpallium, considerable effort has been put into the description of the precise origin of these neurons in subdivisions of the ganglionic eminence and in the migratory routes they follow on their way to the developing cortex. More recently, studies have focused on the molecular and cellular mechanisms that guide their migration. Investigations of the molecular mechanisms involved have demonstrated important roles for numerous transcription factors, motogenic factors and guidance molecules. Here, we review results of very recent analyses of the underlying cellular mechanisms and specifically of the movement of the nucleus, cytoplasmic components and neuritic processes during interneuron migration.