The cell adhesion molecule neurofascin stabilizes axo-axonic GABAergic terminals at the axon initial segment

On the origin and nature of nongenetic information in eumetazoans

Nongenetic information implies all the forms of biological information not related to genes and DNA in general. Despite the deep scientific relevance of the concept, we currently lack reliable knowledge about its carriers and origins; hence, we still do not understand its true nature. Given that genes are the targets of nongenetic information, it appears that a parsimonious approach to find the ultimate source of that information is to trace back the sequential steps of the causal chain upstream of the target genes up to the ultimate link as the source of the nongenetic information. From this perspective, I examine seven nongenetically determined phenomena: placement of locus-specific epigenetic marks on DNA and histones, changes in snRNA expression patterns, neural induction of gene expression, site-specific alternative gene splicing, predator-induced morphological changes, and cultural inheritance. Based on the available evidence, I propose a general model of the common neural origin of all these forms of nongenetic information in eumetazoans.

Involvement of GABAA receptors containing α6 subtypes in antisecretory factor activity on rat cerebellar granule cells studied by two-photon uncaging

The antisecretory factor (AF) is an endogenous protein that counteracts intestinal hypersecretion and various inflammation conditions in vivo. It has been detected in many mammalian tissues and plasma, but its mechanisms of action are largely unknown. To study the pharmacological action of the AF on different GABA_A receptor populations in cerebellar granule cells, we took advantage of the two‐photon uncaging method as this technique allows to stimulate the cell locally in well‐identified plasma membrane parts. We compared the electrophysiological response evoked by releasing a caged GABA compound on the soma, the axon initial segment and neurites before and after administering AF‐16, a 16 amino acids long peptide obtained from the amino‐terminal end of the AF protein. After the treatment with AF‐16, we observed peak current increases of varying magnitude depending on the neuronal region. Thus, studying the effects of furosemide and AF‐16 on the electrophysiological behaviour of cerebellar granules, we suggest that GABA_A receptors, containing the α₆ subunit, may be specifically involved in the increase of the peak current by AF, and different receptor subtype distribution may be responsible for differences in this increase on the cell.

Mechanisms Underlying Target Selectivity for Cell Types and Subcellular Domains in Developing Neocortical Circuits

The cerebral cortex contains numerous neuronal cell types, distinguished by their molecular identity as well as their electrophysiological and morphological properties. Cortical function is reliant on stereotyped patterns of synaptic connectivity and synaptic function among these neuron types, but how these patterns are established during development remains poorly understood. Selective targeting not only of different cell types but also of distinct postsynaptic neuronal domains occurs in many brain circuits and is directed by multiple mechanisms. These mechanisms include the regulation of axonal and dendritic guidance and fine-scale morphogenesis of pre- and postsynaptic processes, lineage relationships, activity dependent mechanisms and intercellular molecular determinants such as transmembrane and secreted molecules, many of which have also been implicated in neurodevelopmental disorders. However, many studies of synaptic targeting have focused on circuits in which neuronal processes target different lamina, such that cell-type-biased connectivity may be confounded with mechanisms of laminar specificity. In the cerebral cortex, each cortical layer contains cell bodies and processes from intermingled neuronal cell types, an arrangement that presents a challenge for the development of target-selective synapse formation. Here, we address progress and future directions in the study of cell-type-biased synaptic targeting in the cerebral cortex. We highlight challenges to identifying developmental mechanisms generating stereotyped patterns of intracortical connectivity, recent developments in uncovering the determinants of synaptic target selection during cortical synapse formation, and current gaps in the understanding of cortical synapse specificity.

Myelination defects in the medial prefrontal cortex of Fkbp5 knockout mice

The hypothalamic-pituitary-adrenal (HPA) axis plays a principal role in stress response regulation and has been implicated in the etiology of stress-related disorders. The HPA axis regulates the normal synthesis and release of glucocorticoids; dysregulation of the HPA axis causes abnormal responses to stress. FK506-binding protein 5 (FKBP5), a co-chaperone of heat shock protein 90 in the glucocorticoid receptor (GR) molecular complex, is a key GR sensitivity regulator. FKBP5 single nucleotide polymorphisms are associated with dysregulated HPA axis and increased risk of stress-related disorders, including posttraumatic stress disorder (PTSD) and depression. In this study, we profiled the microRNAs (miRNAs) in the medial prefrontal cortex of Fkbp5 knockout (Fkbp5^-/- ) mice and identified the target genes of differentially expressed miRNAs using sequence-based miRNA target prediction. Gene ontology analysis revealed that the differentially expressed miRNAs were involved in nervous system development, regulation of cell migration, and intracellular signal transduction. The validation of the expression of predicted target genes using quantitative polymerase chain reaction revealed that the expression of axon development-related genes, specifically actin-binding LIM protein 1 (Ablim1), lemur tyrosine kinase 2 (Lmtk2), kinesin family member 5c (Kif5c), neurofascin (Nfasc), and ephrin type-A receptor 4 (Epha4), was significantly decreased, while that of brain-derived neurotrophic factor (Bdnf) was significantly increased in the brain of Fkbp5^-/- mice. These results suggest that axonal development-related genes can serve as potential targets in future studies focused on understanding the pathophysiology of PTSD.

Neuronal post-developmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture in C. elegans

Whereas remarkable advances have uncovered mechanisms that drive nervous system assembly, the processes responsible for the lifelong maintenance of nervous system architecture remain poorly understood. Subsequent to its establishment during embryogenesis, neuronal architecture is maintained throughout life in the face of the animal's growth, maturation processes, the addition of new neurons, body movements, and aging. The C. elegans protein SAX-7, homologous to the vertebrate L1 protein family of neural adhesion molecules, is required for maintaining the organization of neuronal ganglia and fascicles after their successful initial embryonic development. To dissect the function of sax-7 in neuronal maintenance, we generated a null allele and sax-7S-isoform-specific alleles. We find that the null sax-7(qv30) is, in some contexts, more severe than previously described mutant alleles, and that the loss of sax-7S largely phenocopies the null, consistent with sax-7S being the key isoform in neuronal maintenance. Using a sfGFP::SAX-7S knock-in, we observe sax-7S to be predominantly expressed across the nervous system, from embryogenesis to adulthood. Yet, its role in maintaining neuronal organization is ensured by post-developmentally acting SAX-7S, as larval transgenic sax-7S(+) expression alone is sufficient to profoundly rescue the null mutants' neuronal maintenance defects. Moreover, the majority of the protein SAX-7 appears to be cleaved, and we show that these cleaved SAX-7S fragments together, not individually, can fully support neuronal maintenance. These findings contribute to our understanding of the role of the conserved protein SAX-7/L1CAM in long-term neuronal maintenance, and may help decipher processes that go awry in some neurodegenerative conditions.

Interneuron Types and Their Circuits in the Basolateral Amygdala

The basolateral amygdala (BLA) is a cortical structure based on its cell types, connectivity features, and developmental characteristics. This part of the amygdala is considered to be the main entry site of processed and multisensory information delivered via cortical and thalamic afferents. Although GABAergic inhibitory cells in the BLA comprise only 20% of the entire neuronal population, they provide essential control over proper network operation. Previous studies have uncovered that GABAergic cells in the basolateral amygdala are as diverse as those present in other cortical regions, including the hippocampus and neocortex. To understand the role of inhibitory cells in various amygdala functions, we need to reveal the connectivity and input-output features of the different types of GABAergic cells. Here, I review the recent achievements in uncovering the diversity of GABAergic cells in the basolateral amygdala with a specific focus on the microcircuit organization of these inhibitory cells.

Autism Spectrum Disorder Risk Factor Met Regulates the Organization of Inhibitory Synapses

A common hypothesis explains autism spectrum disorder (ASD) as a neurodevelopmental disorder linked to excitatory/inhibitory (E/I) imbalance in neuronal network connectivity. Mutation of genes including Met and downstream signaling components, e.g., PTEN, Tsc2 and, Rheb are involved in the control of synapse formation and stabilization and were all considered as risk genes for ASD. While the impact of Met on glutamatergic synapses was widely appreciated, its contribution to the stability of inhibitory, GABAergic synapses is poorly understood. The stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin. Here, we show in vivo and in vitro that Met is necessary and sufficient for the stabilization of GABAergic synapses via induction of gephyrin clustering. Likewise, we provide evidence for Met-dependent gephyrin clustering via activation of mTOR. Our results support the notion that deficient GABAergic signaling represents a pathomechanism for ASD.

Molecular mechanisms of axo-axonic innervation

One of the most intriguing features of inhibitory synapses is the precision by which they innervate their target, not only at the cellular level but also at the subcellular level (i.e. axo-dendritic, axo-somatic, or axo-axonic innervation). In particular, in the cerebellum, cortex, and spinal cord, distinct and highly specialized GABAergic interneurons, such as basket cells, chandelier cells, and GABApre interneurons, form precise axo-axonic synapses, allowing them to directly regulate neuronal output and circuit function. In this article, we summarize our latest knowledge of the cellular and molecular mechanisms that regulate the establishment and maintenance of axo-axonic synapses in these regions of the CNS. We also detail the key roles of the L1CAM family of cell adhesion molecules in such GABAergic subcellular target recognition.

Intra-amygdala metaplasticity modulation of fear extinction learning

The amygdala is a key brain region involved in emotional memory formation. It is also responsible for memory modulation in other brain areas. Under extreme conditions, amygdala modulation may lead to the generation of abnormal plasticity and trauma-related psychopathologies. However, the amygdala itself is a dynamic brain region, which is amenable to long-term plasticity and is affected by emotional experiences. These alterations may modify the way the amygdala modulates activity and plasticity in other related brain regions, which in turn may alter the animal's response to subsequent challenges in what could be termed as "Behavioral metaplasticity."Because of the reciprocal interactions between the amygdala and other emotion processing regions, such as the medial prefrontal cortex (mPFC) or the hippocampus, experience-induced intra-amygdala metaplasticity could lead to alterations in mPFC-dependent or hippocampus-dependent behaviors. While initiated by alterations within the basolateral amygdala (BLA), such alterations in other brain regions may come to be independent of BLA modulation, thus establishing what may be termed "Trans-regional metaplasticity." In this article, we review evidence supporting the notions of intra-BLA metaplasticity and how this may develop into "Trans-regional metaplasticity." Future research is needed to understand how such dynamic metaplastic alterations contribute to developing psychopathologies, and how this knowledge may be translated into promoting novel interventions in psychopathologies associated with fear, stress, and trauma.

Shedding Light on Chandelier Cell Development, Connectivity, and Contribution to Neural Disorders

Chandelier cells (ChCs) are a unique type of GABAergic interneuron that selectively innervate the axon initial segment (AIS) of excitatory pyramidal neurons; the subcellular domain where action potentials are initiated. The proper genesis and maturation of ChCs is critical for regulating neural ensemble firing in the neocortex throughout development and adulthood. Recently, genetic and molecular studies have shed new light on the complex innerworkings of ChCs in health and disease. This review presents an overview of recent studies on the developmental origins, migratory properties, and morphology of ChCs. In addition, attention is given to newly identified molecules regulating ChC morphogenesis and connectivity as well as recent work linking ChC dysfunction to neural disorders, including schizophrenia, epilepsy, and autism spectrum disorder (ASD).

Cytoskeletal organization of axons in vertebrates and invertebrates

The maintenance of axons for the lifetime of an organism requires an axonal cytoskeleton that is robust but also flexible to adapt to mechanical challenges and to support plastic changes of axon morphology. Furthermore, cytoskeletal organization has to adapt to axons of dramatically different dimensions, and to their compartment-specific requirements in the axon initial segment, in the axon shaft, at synapses or in growth cones. To understand how the cytoskeleton caters to these different demands, this review summarizes five decades of electron microscopic studies. It focuses on the organization of microtubules and neurofilaments in axon shafts in both vertebrate and invertebrate neurons, as well as the axon initial segments of vertebrate motor- and interneurons. Findings from these ultrastructural studies are being interpreted here on the basis of our contemporary molecular understanding. They strongly suggest that axon architecture in animals as diverse as arthropods and vertebrates is dependent on loosely cross-linked bundles of microtubules running all along axons, with only minor roles played by neurofilaments.

Trafficking and Activity of Glutamate and GABA Receptors: Regulation by Cell Adhesion Molecules

The efficient targeting of ionotropic receptors to postsynaptic sites is essential for the function of chemical excitatory and inhibitory synapses, constituting the majority of synapses in the brain. A growing body of evidence indicates that cell adhesion molecules (CAMs), which accumulate at synapses at the earliest stages of synaptogenesis, are critical for this process. A diverse variety of CAMs assemble into complexes with glutamate and GABA receptors and regulate the targeting of these receptors to the cell surface and synapses. Presynaptically localized CAMs provide an additional level of regulation, sending a trans-synaptic signal that can regulate synaptic strength at the level of receptor trafficking. Apart from controlling the numbers of receptors present at postsynaptic sites, CAMs can also influence synaptic strength by modulating the conductivity of single receptor channels. CAMs thus act to maintain basal synaptic transmission and are essential for many forms of activity dependent synaptic plasticity. These activities of CAMs may underlie the association between CAM gene mutations and synaptic pathology and represent fundamental mechanisms by which synaptic strength is dynamically tuned at both excitatory and inhibitory synapses.

Interference With Complex IV as a Model of Age-Related Decline in Synaptic Connectivity

Age-related impairment of mitochondrial function may negatively impact energy-demanding processes such as synaptic transmission thereby triggering cognitive decline and processes of neurodegeneration. Here, we present a novel model for age-related mitochondrial impairment based on partial inhibition of cytochrome c oxidase subunit 4 (Cox4) of complex IV of the respiratory chain. miRNA-mediated knockdown of Cox4 correlated with a marked reduction in excitatory and inhibitory synaptic marker densities in vitro and in vivo as well as an impairment of neuronal network activity in primary neuronal cultures. Transcriptome analysis identified the deregulation of gene clusters, which link induced mitochondrial perturbation to impaired synaptic function and plasticity as well as processes of aging. In conclusion, the model of Cox4 deficiency reflects aspects of age-related dementia and might, therefore, serve as a novel test system for drug development.

Missing a beat: assessment of circadian rhythm abnormalities in bipolar disorder in the genomic era

Circadian rhythm abnormalities have been recognized as a central feature of bipolar disorder (BD) but a coherent biological explanation for them remains lacking. Using genetic mutation of 'clock genes', robust animal models of mania and depression have been developed that elucidate key aspects of circadian rhythms and the circadian clock-mood connection. However, translation of this knowledge into humans remains incomplete. In recent years, very large genome-wide association studies (GWAS) have been conducted and the genetic underpinnings of BD are beginning to emerge. However, these genetic studies in BD do not match well with the evidence from animal studies that implicate the circadian clock in mood regulation. Even larger GWAS have been conducted for circadian phenotypes including chronotype, rhythm amplitude, sleep duration, and insomnia. These studies have identified a diverse set of associated genes, including a minority with previously well-characterized functions in the circadian clock. Taken together, the data from recent GWAS of BD and circadian phenotypes indicate that the genetic organization of the circadian clock, both in health and in BD is complex. The findings from GWAS elucidate potentially novel circadian mechanism that may be partly distinct from those identified in animal models. Pleiotropy, epistasis and nongenetic factors may play important roles in regulating circadian rhythms, some of which may underlie circadian rhythm disturbances in BD.

Axo-axonic Innervation of Neocortical Pyramidal Neurons by GABAergic Chandelier Cells Requires AnkyrinG-Associated L1CAM

Among the diverse interneuron subtypes in the neocortex, chandelier cells (ChCs) are the only population that selectively innervate pyramidal neurons (PyNs) at their axon initial segment (AIS), the site of action potential initiation, allowing them to exert powerful control over PyN output. Yet, mechanisms underlying their subcellular innervation of PyN AISs are unknown. To identify molecular determinants of ChC/PyN AIS innervation, we performed an in vivo RNAi screen of PyN-expressed axonal cell adhesion molecules (CAMs) and select Ephs/ephrins. Strikingly, we found the L1 family member L1CAM to be the only molecule required for ChC/PyN AIS innervation. Further, we show that L1CAM is required during both the establishment and maintenance of innervation, and that selective innervation of PyN AISs by ChCs requires AIS anchoring of L1CAM by the cytoskeletal ankyrin-G/βIV-spectrin complex. Thus, our findings identify PyN-expressed L1CAM as a critical CAM required for innervation of neocortical PyN AISs by ChCs. VIDEO ABSTRACT.

Loss of Neurofascin-186 Disrupts Alignment of AnkyrinG Relative to Its Binding Partners in the Axon Initial Segment

The axon initial segment (AIS) is a specialized region within the proximal portion of the axon that initiates action potentials thanks in large part to an enrichment of sodium channels. The scaffolding protein ankyrinG (AnkG) is essential for the recruitment of sodium channels as well as several other intracellular and extracellular proteins to the AIS. In the present study, we explore the role of the cell adhesion molecule (CAM) neurofascin-186 (NF-186) in arranging the individual molecular components of the AIS in cultured rat hippocampal neurons. Using a CRISPR depletion strategy to ablate NF expression, we found that the loss of NF selectively perturbed AnkG accumulation and its relative proximal distribution within the AIS. We found that the overexpression of sodium channels could restore AnkG accumulation, but not its altered distribution within the AIS without NF present. We go on to show that although the loss of NF altered AnkG distribution, sodium channel function within the AIS remained normal. Taken together, these results demonstrate that the regulation of AnkG and sodium channel accumulation within the AIS can occur independently of one another, potentially mediated by other binding partners such as NF.

A dominant dendrite phenotype caused by the disease-associated G253D mutation in doublecortin (DCX) is not due to its endocytosis defect

Doublecortin (DCX) is a protein needed for cortical development, and DCX mutations cause cortical malformations in humans. The microtubule-binding activity of DCX is well-described and is important for its function, such as supporting neuronal migration and dendrite growth during development. Previous work showed that microtubule binding is not sufficient for DCX-mediated promotion of dendrite growth and that domains in DCX's C terminus are also required. The more C-terminal regions of DCX bind several other proteins, including the adhesion receptor neurofascin and clathrin adaptors. We recently identified a role for DCX in endocytosis of neurofascin. The disease-associated DCX-G253D mutant protein is known to be deficient in binding neurofascin, and we now asked if disruption of neurofascin endocytosis underlies the DCX-G253D-associated pathology. We first demonstrated that DCX functions in endocytosis as a complex with both the clathrin adaptor AP-2 and neurofascin: disrupting either clathrin adaptor binding (DCX-ALPA) or neurofascin binding (DCX-G253D) decreased neurofascin endocytosis in primary neurons. We then investigated a known function for DCX, namely, increasing dendrite growth in cultured neurons. Surprisingly, we found that the DCX-ALPA and DCX-G253D mutants yield distinct dendrite phenotypes. Unlike DCX-ALPA, DCX-G253D caused a dominant-negative dendrite growth phenotype. The endocytosis defect of DCX-G253D thus was separable from its detrimental effects on dendrite growth. We recently identified Dcx-R59H as a dominant allele and can now classify Dcx-G253D as a second Dcx allele that acts dominantly to cause pathology, but does so via a different mechanism.

Developmental seizures and mortality result from reducing GABAA receptor α2-subunit interaction with collybistin

Fast inhibitory synaptic transmission is mediated by γ-aminobutyric acid type A receptors (GABAARs) that are enriched at functionally diverse synapses via mechanisms that remain unclear. Using isothermal titration calorimetry and complementary methods we demonstrate an exclusive low micromolar binding of collybistin to the α2-subunit of GABAARs. To explore the biological relevance of collybistin-α2-subunit selectivity, we generate mice with a mutation in the α2-subunit-collybistin binding region (Gabra2-1). The mutation results in loss of a distinct subset of inhibitory synapses and decreased amplitude of inhibitory synaptic currents. Gabra2-1 mice have a striking phenotype characterized by increased susceptibility to seizures and early mortality. Surviving Gabra2-1 mice show anxiety and elevations in electroencephalogram δ power, which are ameliorated by treatment with the α2/α3-selective positive modulator, AZD7325. Taken together, our results demonstrate an α2-subunit selective binding of collybistin, which plays a key role in patterned brain activity, particularly during development.

The Neuroprotective Effect of Ethanol Intoxication in Traumatic Brain Injury Is Associated with the Suppression of ErbB Signaling in Parvalbumin-Positive Interneurons

Ethanol intoxication (EI) is a frequent comorbidity of traumatic brain injury (TBI), but the impact of EI on TBI pathogenic cascades and prognosis is unclear. Although clinical evidence suggests that EI may have neuroprotective effects, experimental support is, to date, inconclusive. We aimed at elucidating the impact of EI on TBI-associated neurological deficits, signaling pathways, and pathogenic cascades in order to identify new modifiers of TBI pathophysiology. We have shown that ethanol administration (5 g/kg) before trauma enhances behavioral recovery in a weight-drop TBI model. Neuronal survival in the injured somatosensory cortex was also enhanced by EI. We have used phospho-receptor tyrosine kinase (RTK) arrays to screen the impact of ethanol on TBI-induced activation of RTK in somatosensory cortex, identifying ErbB2/ErbB3 among the RTKs activated by TBI and suppressed by ethanol. Phosphorylation of ErbB2/3/4 RTKs were upregulated in vGlut2⁺ excitatory synapses in the injured cortex, including excitatory synapses located on parvalbumin (PV)-positive interneurons. Administration of selective ErbB inhibitors was able to recapitulate, to a significant extent, the neuroprotective effects of ethanol both in sensorimotor performance and structural integrity. Further, suppression of PV interneurons in somatosensory cortex before TBI, by engineered receptors with orthogonal pharmacology, could mimic the beneficial effects of ErbB inhibitors. Thus, we have shown that EI interferes with TBI-induced pathogenic cascades at multiple levels, with one prominent pathway, involving ErbB-dependent modulation of PV interneurons.

Memory Synapses Are Defined by Distinct Molecular Complexes: A Proposal

Synapses are diverse in form and function. While there are strong evidential and theoretical reasons for believing that memories are stored at synapses, the concept of a specialized “memory synapse” is rarely discussed. Here, we review the evidence that memories are stored at the synapse and consider the opposing possibilities. We argue that if memories are stored in an active fashion at synapses, then these memory synapses must have distinct molecular complexes that distinguish them from other synapses. In particular, examples from Aplysia sensory-motor neuron synapses and synapses on defined engram neurons in rodent models are discussed. Specific hypotheses for molecular complexes that define memory synapses are presented, including persistently active kinases, transmitter receptor complexes and trans-synaptic adhesion proteins.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

Spatio-temporal and dynamic regulation of neurofascin alternative splicing in mouse cerebellar neurons

Alternative splicing is crucial for molecular diversification, which greatly contributes to the complexity and specificity of neural functions in the central nervous system (CNS). Neurofascin (NF) is a polymorphic cell surface protein that has a number of splicing isoforms. As the alternative splicing of the neurofascin gene (Nfasc) is developmentally regulated, NF isoforms have distinct functions in immature and mature brains. However, the molecular mechanisms underlying the alternative splicing of Nfasc in neurons are not yet understood. Here, we demonstrate that, alongside developmental regulation, Nfasc alternative splicing is spatially controlled in the mouse brain. We then identified distinct Nfasc splicing patterns at the cell-type level in the cerebellum, with Nfasc186 being expressed in Purkinje cells and absent from granule cells (GCs). Furthermore, we show that high K⁺-induced depolarization triggers a shift in splicing from Nfasc140 to Nfasc186 in cerebellar GCs. Finally, we identified a neural RNA-binding protein, Rbfox, as a key player in neural NF isoform selection, specifically controlling splicing at exons 26−29. Together, our results show that Nfasc alternative splicing is spatio-temporally and dynamically regulated in cerebellar neurons. Our findings provide profound insight into the mechanisms underlying the functional diversity of neuronal cell-adhesive proteins in the mammalian CNS.

Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience

ALBRECHT, A., MÜLLER, I., ARDI, Z., ÇALIŞKAN, G., GRUBER, D., IVENS, S., SEGAL, M., BEHR, J., HEINEMANN, U., STORK, O., and RICHTER-LEVIN, G. Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience. NEUROSCI BIOBEHAV REV XXX-XXX, 2016.- Childhood adversity is among the most potent risk factors for developing mood and anxiety disorders later in life. Therefore, understanding how stress during childhood shapes and rewires the brain may optimize preventive and therapeutic strategies for these disorders. To this end, animal models of stress exposure in rodents during their post-weaning and pre-pubertal life phase have been developed. Such 'juvenile stress' has a long-lasting impact on mood and anxiety-like behavior and on stress coping in adulthood, accompanied by alterations of the GABAergic system within core regions for the stress processing such as the amygdala, prefrontal cortex and hippocampus. While many regionally diverse molecular and electrophysiological changes are observed, not all of them correlate with juvenile stress-induced behavioral disturbances. It rather seems that certain juvenile stress-induced alterations reflect the system's attempts to maintain homeostasis and thus promote stress resilience. Analysis tools such as individual behavioral profiling may allow the association of behavioral and neurobiological alterations more clearly and the dissection of alterations related to the pathology from those related to resilience.

GABAergic Synapses at the Axon Initial Segment of Basolateral Amygdala Projection Neurons Modulate Fear Extinction

Inhibitory synaptic transmission in the amygdala has a pivotal role in fear learning and its extinction. However, the local circuits formed by GABAergic inhibitory interneurons within the amygdala and their detailed function in shaping these behaviors are not well understood. Here we used lentiviral-mediated knockdown of the cell adhesion molecule neurofascin in the basolateral amygdala (BLA) to specifically remove inhibitory synapses at the axon initial segment (AIS) of BLA projection neurons. Quantitative analysis of GABAergic synapse markers and measurement of miniature inhibitory postsynaptic currents in BLA projection neurons after neurofascin knockdown ex vivo confirmed the loss of GABAergic input. We then studied the impact of this manipulation on anxiety-like behavior and auditory cued fear conditioning and its extinction as BLA related behavioral paradigms, as well as on long-term potentiation (LTP) in the ventral subiculum-BLA pathway in vivo. BLA knockdown of neurofascin impaired ventral subiculum-BLA-LTP. While this manipulation did not affect anxiety-like behavior and fear memory acquisition and consolidation, it specifically impaired extinction. Our findings indicate that modification of inhibitory synapses at the AIS of BLA projection neurons is sufficient to selectively impair extinction behavior. A better understanding of the role of distinct GABAergic synapses may provide novel and more specific targets for therapeutic interventions in extinction-based therapies.

Dual Function of NRP1 in Axon Guidance and Subcellular Target Recognition in Cerebellum

Subcellular target recognition in the CNS is the culmination of a multiple-step program including axon guidance, target recognition, and synaptogenesis. In cerebellum, basket cells (BCs) innervate the soma and axon initial segment (AIS) of Purkinje cells (PCs) to form the pinceau synapse, but the underlying mechanisms remain incompletely understood. Here, we demonstrate that neuropilin-1 (NRP1), a Semaphorin receptor expressed in BCs, controls both axonal guidance and subcellular target recognition. We show that loss of Semaphorin 3A function or specific deletion of NRP1 in BCs alters the stereotyped organization of BC axon and impairs pinceau synapse formation. Further, we identified NRP1 as a trans-synaptic binding partner of the cell adhesion molecule neurofascin-186 (NF186) expressed in the PC AIS during pinceau synapse formation. These findings identify a dual function of NRP1 in both axon guidance and subcellular target recognition in the construction of GABAergic circuitry.

Receptor tyrosine kinase EphA7 is required for interneuron connectivity at specific subcellular compartments of granule cells

Neuronal transmission is regulated by the local circuitry which is composed of principal neurons targeted at different subcellular compartments by a variety of interneurons. However, mechanisms that contribute to the subcellular localisation and maintenance of GABAergic interneuron terminals are poorly understood. Stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin and its interaction with the guanine nucleotide exchange factor collybistin. Lentiviral knockdown experiments in adult rats indicated that the receptor tyrosine kinase EphA7 is required for the stabilisation of basket cell terminals on proximal dendritic and somatic compartments of granular cells of the dentate gyrus. EphA7 deficiency and concomitant destabilisation of GABAergic synapses correlated with impaired long-term potentiation and reduced hippocampal learning. Reduced GABAergic innervation may be explained by an impact of EphA7 on gephyrin clustering. Overexpression or ephrin stimulation of EphA7 induced gephyrin clustering dependent on the mechanistic target of rapamycin (mTOR) which is an interaction partner of gephyrin. Gephyrin interactions with mTOR become released after mTOR activation while enhanced interaction with the guanine nucleotide exchange factor collybistin was observed in parallel. In conclusion, EphA7 regulates gephyrin clustering and the maintenance of inhibitory synaptic connectivity via mTOR signalling.

The Effect of Chronic Methamphetamine Exposure on the Hippocampal and Olfactory Bulb Neuroproteomes of Rats

Nowadays, drug abuse and addiction are serious public health problems in the USA. Methamphetamine (METH) is one of the most abused drugs and is known to cause brain damage after repeated exposure. In this paper, we conducted a neuroproteomic study to evaluate METH-induced brain protein dynamics, following a two-week chronic regimen of an escalating dose of METH exposure. Proteins were extracted from rat brain hippocampal and olfactory bulb tissues and subjected to liquid chromatography-mass spectrometry (LC-MS/MS) analysis. Both shotgun and targeted proteomic analysis were performed. Protein quantification was initially based on comparing the spectral counts between METH exposed animals and their control counterparts. Quantitative differences were further confirmed through multiple reaction monitoring (MRM) LC-MS/MS experiments. According to the quantitative results, the expression of 18 proteins (11 in the hippocampus and 7 in the olfactory bulb) underwent a significant alteration as a result of exposing rats to METH. 13 of these proteins were up-regulated after METH exposure while 5 were down-regulated. The altered proteins belonging to different structural and functional families were involved in processes such as cell death, inflammation, oxidation, and apoptosis.

Enrichment of GABAA Receptor α-Subunits on the Axonal Initial Segment Shows Regional Differences

Although it is generally recognized that certain α-subunits of γ-aminobutyric acid type A receptors (GABAARs) form enriched clusters on the axonal initial segment (AIS), the degree to which these clusters vary in different brain areas is not well known. In the current study, we quantified the density, size, and enrichment ratio of fluorescently labeled α1-, α2-, or α3-subunits aggregates co-localized with the AIS-marker ankyrin G and compared them to aggregates in non-AIS locations among different brain areas including hippocampal subfields, basal lateral amygdala (BLA), prefrontal cortex (PFC), and sensory cortex (CTX). We found regional differences in the enrichment of GABAAR α-subunits on the AIS. Significant enrichment was identified in the CA3 of hippocampus for α1-subunits, in the CA1, CA3, and BLA for α2-subunits, and in the BLA for α3-subunits. Using α-subunit knock-out (KO) mice, we found that BLA enrichment of α2- and α3-subunits were physiologically independent of each other, as the enrichment of one subunit was unaffected by the genomic deletion of the other. To further investigate the unique pattern of α-subunit enrichment in the BLA, we examined the association of α2- and α3-subunits with the presynaptic vesicular GABA transporter (vGAT) and the anchoring protein gephyrin (Geph). As expected, both α2- and α3-subunits on the AIS within the BLA received prominent GABAergic innervation from vGAT-positive terminals. Further, we found that the association of α2- and α3-subunits with Geph was weaker in AIS versus non-AIS locations, suggesting that Geph might be playing a lesser role in the enrichment of α2- and α3-subunits on the AIS. Overall, these observations suggest that GABAARs on the AIS differ in subunit composition across brain regions. As with somatodendritic GABAARs, the distinctive expression pattern of AIS-located GABAAR α-subunits in the BLA, and other brain areas, likely contribute to unique forms of GABAergic inhibitory transmission and pharmacological profiles seen in different brain areas.

Alterations in Brain Inflammation, Synaptic Proteins, and Adult Hippocampal Neurogenesis during Epileptogenesis in Mice Lacking Synapsin2

Synapsins are pre-synaptic vesicle-associated proteins linked to the pathogenesis of epilepsy through genetic association studies in humans. Deletion of synapsins causes an excitatory/inhibitory imbalance, exemplified by the epileptic phenotype of synapsin knockout mice. These mice develop handling-induced tonic-clonic seizures starting at the age of about 3 months. Hence, they provide an opportunity to study epileptogenic alterations in a temporally controlled manner. Here, we evaluated brain inflammation, synaptic protein expression, and adult hippocampal neurogenesis in the epileptogenic (1 and 2 months of age) and tonic-clonic (3.5-4 months) phase of synapsin 2 knockout mice using immunohistochemical and biochemical assays. In the epileptogenic phase, region-specific microglial activation was evident, accompanied by an increase in the chemokine receptor CX3CR1, interleukin-6, and tumor necrosis factor-α, and a decrease in chemokine keratinocyte chemoattractant/ growth-related oncogene. Both post-synaptic density-95 and gephyrin, scaffolding proteins at excitatory and inhibitory synapses, respectively, showed a significant up-regulation primarily in the cortex. Furthermore, we observed an increase in the inhibitory adhesion molecules neuroligin-2 and neurofascin and potassium chloride co-transporter KCC2. Decreased expression of γ-aminobutyric acid receptor-δ subunit and cholecystokinin was also evident. Surprisingly, hippocampal neurogenesis was reduced in the epileptogenic phase. Taken together, we report molecular alterations in brain inflammation and excitatory/inhibitory balance that could serve as potential targets for therapeutics and diagnostic biomarkers. In addition, the regional differences in brain inflammation and synaptic protein expression indicate an epileptogenic zone from where the generalized seizures in synapsin 2 knockout mice may be initiated or spread.

Giant ankyrin-G stabilizes somatodendritic GABAergic synapses through opposing endocytosis of GABAA receptors

GABAA-receptor-based interneuron circuitry is essential for higher order function of the human nervous system and is implicated in schizophrenia, depression, anxiety disorders, and autism. Here we demonstrate that giant ankyrin-G (480-kDa ankyrin-G) promotes stability of somatodendritic GABAergic synapses in vitro and in vivo. Moreover, giant ankyrin-G forms developmentally regulated and cell-type-specific micron-scale domains within extrasynaptic somatodendritic plasma membranes of pyramidal neurons. We further find that giant ankyrin-G promotes GABAergic synapse stability through opposing endocytosis of GABAA receptors, and requires a newly described interaction with GABARAP, a GABAA receptor-associated protein. We thus present a new mechanism for stabilization of GABAergic interneuron synapses and micron-scale organization of extrasynaptic membrane that provides a rationale for studies linking ankyrin-G genetic variation with psychiatric disease and abnormal neurodevelopment.

Dentate Gyrus Local Circuit is Implicated in Learning Under Stress--a Role for Neurofascin

The inhibitory synapses at the axon initial segment (AIS) of dentate gyrus granular cells are almost exclusively innervated by the axo-axonic chandelier interneurons. However, the role of chandelier neurons in local circuitry is poorly understood and controversially discussed. The cell adhesion molecule neurofascin is specifically expressed at the AIS. It is crucially required for the stabilization of axo-axonic synapses. Knockdown of neurofascin is therefore a convenient tool to interfere with chandelier input at the AIS of granular neurons of the dentate gyrus. In the current study, feedback and feedforward inhibition of granule cells was measured in the dentate gyrus after knockdown of neurofascin and concomitant reduction of axo-axonic input. Results show increased feedback inhibition as a result of neurofascin knockdown, while feedforward inhibition remained unaffected. This suggests that chandelier neurons are predominantly involved in feedback inhibition. Neurofascin knockdown rats also exhibited impaired learning under stress in the two-way shuttle avoidance task. Remarkably, this learning impairment was not accompanied by differences in electrophysiological measurements of dentate gyrus LTP. This indicates that the local circuit may be involved in (certain types) of learning.

A combined transgenic proteomic analysis and regulated trafficking of neuroligin-2

Synapses, the basic units of communication in the brain, require complex molecular machinery for neurotransmitter release and reception. Whereas numerous components of excitatory postsynaptic sites have been identified, relatively few proteins are known that function at inhibitory postsynaptic sites. One such component is neuroligin-2 (NL2), an inhibitory synapse-specific cell surface protein that functions in cell adhesion and synaptic organization via binding to neurexins. In this study, we used a transgenic tandem affinity purification and mass spectrometry strategy to isolate and characterize NL2-associated complexes. Complexes purified from brains of transgenic His6-FLAG-YFP-NL2 mice showed enrichment in the Gene Ontology terms cell-cell signaling and synaptic transmission relative to complexes purified from wild type mice as a negative control. In addition to expected components including GABA receptor subunits and gephyrin, several novel proteins were isolated in association with NL2. Based on the presence of multiple components involved in trafficking and endocytosis, we showed that NL2 undergoes dynamin-dependent endocytosis in response to soluble ligand and colocalizes with VPS35 retromer in endosomes. Inhibitory synapses in brain also present a particular challenge for imaging. Whereas excitatory synapses on spines can be imaged with a fluorescent cell fill, inhibitory synapses require a molecular tag. We find the His6-FLAG-YFP-NL2 to be a suitable tag, with the unamplified YFP signal localizing appropriately to inhibitory synapses in multiple brain regions including cortex, hippocampus, thalamus, and basal ganglia. Altogether, we characterize NL2-associated complexes, demonstrate regulated trafficking of NL2, and provide tools for further proteomic and imaging studies of inhibitory synapses.

Role of the axonal initial segment in psychiatric disorders: function, dysfunction, and intervention

The progress of developing effective interventions against psychiatric disorders has been limited due to a lack of understanding of the underlying cellular and functional mechanisms. Recent research findings focused on exploring novel causes of psychiatric disorders have highlighted the importance of the axonal initial segment (AIS), a highly specialized neuronal structure critical for spike initiation of the action potential. In particular, the role of voltage-gated sodium channels, and their interactions with other protein partners in a tightly regulated macromolecular complex has been emphasized as a key component in the regulation of neuronal excitability. Deficits and excesses of excitability have been linked to the pathogenesis of brain disorders. Identification of the factors and regulatory pathways involved in proper AIS function, or its disruption, can lead to the development of novel interventions that target these mechanistic interactions, increasing treatment efficacy while reducing deleterious off-target effects for psychiatric disorders.

Plasticity of GABAA receptor diffusion dynamics at the axon initial segment

The axon initial segment (AIS), a site of action potential initiation, undergoes activity-dependent homeostatic repositioning to fine-tune neuronal activity. However, little is known about the behavior of GABA_A receptors (GABA_ARs) at synapses made onto the axon and especially the AIS. Here, we study the clustering and lateral diffusion of GABA_ARs in the AIS under baseline conditions, and find that GABA_AR lateral mobility is lower in the AIS than dendrites. We find differences in axonal clustering and lateral mobility between GABA_ARs containing the α1 or α2 subunits, which are known to localize differentially to the AIS. Interestingly, we find that chronic activity driving AIS repositioning does not alter GABAergic synapse location along the axon, but decreases GABA_AR cluster size at the AIS. Moreover, in response to chronic depolarization, GABA_AR diffusion is strikingly increased in the AIS, and not in dendrites, and this is coupled with a decrease in synaptic residency time of GABA_ARs at the AIS. We also demonstrate that activation of L-type voltage-gated calcium channels is important for regulating GABA_AR lateral mobility at the AIS during chronic depolarization. Modulation of GABA_AR diffusion dynamics at the AIS in response to prolonged activity may be a novel mechanism for regulating GABAergic control of information processing.

Activity-dependent adaptations in inhibitory axons

Synaptic connections in our brains change continuously and throughout our lifetime. Despite ongoing synaptic changes, a healthy balance between excitation and inhibition is maintained by various forms of homeostatic and activity-dependent adaptations, ensuring stable functioning of neuronal networks. In this review we summarize experimental evidence for activity-dependent changes occurring in inhibitory axons, in cultures as well as in vivo. Axons form many presynaptic terminals, which are dynamic structures sharing presynaptic material along the axonal shaft. We discuss how internal (e.g., vesicle sharing) and external factors (e.g., binding of cell adhesion molecules or secreted factors) may affect the formation and plasticity of inhibitory synapses.

Brain inflammation induces post-synaptic changes during early synapse formation in adult-born hippocampal neurons

An inflammatory reaction in the brain is primarily characterized by activation of parenchymal microglial cells. Microglia regulate several aspects of adult neurogenesis, i.e. the continuous production of new neurons in the adult brain. Hippocampal neurogenesis is thought to be important for memory formation, but its role in brain diseases is not clear. We have previously shown that brain inflammation modulates the functional integration of newly formed hippocampal neurons. Here, we explored whether there is a defined time period during synaptic development when new neurons are susceptible to brain inflammation. Newly formed hippocampal neurons, born in an intact environment in the adult mouse brain, were exposed to lipopolysaccharide (LPS)-induced inflammation during either early or late phases of excitatory and inhibitory synaptogenesis. We used intra-hippocampal injections of GFP-retroviral vector (RV-GFP) to label the new neurons and ipsilateral LPS injection at either 1 or 4weeks post-RV-GFP injection. A single intra-hippocampal LPS injection induced an inflammatory response for at least 3weeks, including an acute transient pro-inflammatory cytokine release as well as a sub-acute and sustained change in microglial morphology. The general cytoarchitecture of the hippocampal dentate gyrus, including granule cell layer (GCL) volume, and astrocytic glial fibrillary acidic protein expression was not different compared to vehicle controls, and no Fluoro-Jade-positive cell death was observed. New neurons encountering this inflammatory environment exhibited no changes in their gross morphology. However, when inflammation occurred during early stages of synapse formation, we found a region-specific increase in the number of thin dendritic spines and post-synaptic density-95 (PSD-95) cluster formation on spines, suggesting an enhanced excitatory synaptic connectivity in the newborn neurons. No changes were observed in the expression of N-cadherin, an adhesion molecule primarily associated with excitatory synapses. At the inhibitory synapses, alterations due to inflammation were also evident during early but not later stages of synaptic development. Gephyrin, an inhibitory scaffolding protein, was down-regulated in the somatic region, while the adhesion molecules neuroligin-2 (NL-2) and neurofascin were increased in the somatic region and/or on the dendrites. The GABAA receptor-α2 subunit (GABAAR-α2) was increased, while pre/peri-synaptic GABA clustering remained unaltered. The disproportional changes in post-synaptic adhesion molecules and GABAA receptor compared to scaffolding protein expression at the inhibitory synapses during brain inflammation are likely to cause an imbalance in GABAergic transmission. These changes were specific for the newborn neurons and were not observed when estimating the overall expression of gephyrin, NL-2, and GABAAR-α2 in the hippocampal GCL. The expression of interleukin-1-type 1 receptor (IL-1R1) on preferentially the somatic region of new neurons, often in close apposition to NL-2 clusters, may indicate a direct interaction between brain inflammation and synaptic proteins on newborn neurons. In summary, this study provides evidence that adult-born hippocampal neurons alter their inhibitory and excitatory synaptic integration when encountering an LPS-induced brain inflammation during the initial stages of synapse formation. Changes at this critical developmental period are likely to interfere with the physiological functions of new neurons within the hippocampus.

The making of a node: a co-production of neurons and glia

Nodes of Ranvier are specialized axonal domains formed in response to a glial signal. Recent research advances have revealed that both CNS and PNS nodes form by several overlapping molecular mechanisms. However, the precise nature of these mechanisms and the hierarchy existing between them is considerably different in CNS versus PNS nodes. Namely, the Schwann cells of the PNS, which directly contact the nodal axolemma, secrete proteins that cluster axonodal components at the edges of the growing myelin segment. In contrast, the formation of CNS nodes, which are not contacted by the myelinating glia, is surprisingly similar to the assembly of the axon initial segment and depends largely on axonal diffusion barriers.

Organization and maintenance of molecular domains in myelinated axons

Over a century ago, Ramon y Cajal first proposed the idea of a directionality involved in nerve conduction and neuronal communication. Decades later, it was discovered that myelin, produced by glial cells, insulated axons with periodic breaks where nodes of Ranvier (nodes) form to allow for saltatory conduction. In the peripheral nervous system (PNS), Schwann cells are the glia that can either individually myelinate the axon from one neuron or ensheath axons of many neurons. In the central nervous system (CNS), oligodendrocytes are the glia that myelinate axons from different neurons. Review of more recent studies revealed that this myelination created polarized domains adjacent to the nodes. However, the molecular mechanisms responsible for the organization of axonal domains are only now beginning to be elucidated. The molecular domains in myelinated axons include the axon initial segment (AIS), where various ion channels are clustered and action potentials are initiated; the node, where sodium channels are clustered and action potentials are propagated; the paranode, where myelin loops contact with the axolemma; the juxtaparanode (JXP), where delayed-rectifier potassium channels are clustered; and the internode, where myelin is compactly wrapped. Each domain contains a unique subset of proteins critical for the domain's function. However, the roles of these proteins in axonal domain organization are not fully understood. In this review, we highlight recent advances on the molecular nature and functions of some of the components of each axonal domain and their roles in axonal domain organization and maintenance for proper neuronal communication.

A missense mutation in the ITGA8 gene, a cell adhesion molecule gene, is associated with schizophrenia in Japanese female patients

BACKGROUND

Cell adhesion molecules (CAMs) play pivotal role in the development of the central nervous system (CNS) and have also been reported to play role in the pathophysiology of schizophrenia. Missense mutations in the CAMs genes might alter the binding of their ligands, increasing the vulnerability to develop schizophrenia.

METHODS

We selected 15 missense mutations in the CAMs genes of the CNS reported in the Kyoto Encyclopedia of Genes and Genomes (KEGG) and examined the association between these mutations and schizophrenia in 278 patients and 284 control subjects (first batch). We also genotyped the positive single nucleotide polymorphisms (SNPs) in 567 patients and 710 control subjects (second batch) and in 635 patients and 639 control subjects (replication samples).

RESULTS

Genotypic and allelic distributions of rs2298033 in the ITGA8 gene between the schizophrenia and control groups were significantly different in the first batch (p=0.005 and 0.007, respectively). Gender-based analysis revealed that the allelic and genotypic distributions of rs2298033 in the ITGA8 were significantly different between the schizophrenia and control groups among females in both batches (p=0.010, 0.011 and 0.0086, 0.010, respectively) but not among males. Combine analysis of rs2298033 with the replication samples revealed a more significant differences (p=0.0032; 0.0035 in the overall subjects and p=0.0024; 0.0025 in the female subjects, respectively). The significant differences for rs2802808 of the NFASC gene were only observed in the female subgroup of the first batch.

CONCLUSION

These results suggest that the ITGA8 gene might have gender-specific roles in the development of schizophrenia. Further replication and functional studies are required to confirm these findings.

A comprehensive small interfering RNA screen identifies signaling pathways required for gephyrin clustering

The postsynaptic scaffold protein gephyrin is clustered at inhibitory synapses and serves for the stabilization of GABA(A) receptors. Here, a comprehensive kinome-wide siRNA screen in a human HeLa cell-based model for gephyrin clustering was used to identify candidate protein kinases implicated in the stabilization of gephyrin clusters. As a result, 12 hits were identified including FGFR1 (FGF receptor 1), TrkB, and TrkC as well as components of the MAPK and mammalian target of rapamycin (mTOR) pathways. For confirmation, the impact of these hits on gephyrin clustering was analyzed in rat primary hippocampal neurons. We found that brain-derived neurotrophic factor (BDNF) acts on gephyrin clustering through MAPK signaling, and this process may be controlled by the MAPK signaling antagonist sprouty2. BDNF signaling through phosphatidylinositol 3-kinase (PI3K)-Akt also activates mTOR and represses GSK3β, which was previously shown to reduce gephyrin clustering. Gephyrin is associated with inactive mTOR and becomes released upon BDNF-dependent mTOR activation. In primary neurons, a reduction in the number of gephyrin clusters due to manipulation of the BDNF-mTOR signaling is associated with reduced GABA(A) receptor clustering, suggesting functional impairment of GABA signaling. Accordingly, application of the mTOR antagonist rapamycin leads to disinhibition of neuronal networks as measured on microelectrode arrays. In conclusion, we provide evidence that BDNF regulates gephyrin clustering via MAPK as well as PI3K-Akt-mTOR signaling.

Regulation of adhesion by flexible ectodomains of IgCAMs

To perform their diverse biological functions the adhesion activities of the cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) might be regulated by local clustering, proteolytical shedding of their ectodomains or rapid recycling to and from the plasma membrane. Another form of regulation of adhesion might be obtained through flexible ectodomains of IgCAMs which adopt distinct conformations and which in turn modulate their adhesion activity. Here, we discuss variations in the conformation of the extracellular domains of CEACAM1 and CAR that might influence their binding and signaling activities. Furthermore, we concentrate on alternative splicing of single domains and short segments in the extracellular regions of L1 subfamily members that might affect the organization of the N-terminal located Ig-like domains. In particular, we discuss variations of the linker sequence between Ig-like domains 2 and 3 (D2 and D3) that is required for the horseshoe conformation.

Doublecortin (DCX) mediates endocytosis of neurofascin independently of microtubule binding

Doublecortin on X chromosome (DCX) is one of two major genetic loci underlying human lissencephaly, a neurodevelopmental disorder with defects in neuronal migration and axon outgrowth. DCX is a microtubule-binding protein, and much work has focused on its microtubule-associated functions. DCX has other reported binding partners, including the cell adhesion molecule neurofascin, but the functional significance of the DCX-neurofascin interaction is not understood. Neurofascin localizes strongly to the axon initial segment in mature neurons, where it plays a role in assembling and maintaining other axon initial segment components. During development, neurofascin likely plays additional roles in axon guidance and in GABAergic synaptogenesis. We show here that DCX can modulate the surface distribution of neurofascin in developing cultured rat neurons and thereby the relative extent of accumulation between the axon initial segment and soma and dendrites. Mechanistically, DCX acts via increasing endocytosis of neurofascin from soma and dendrites. Surprisingly, DCX increases neurofascin endocytosis apparently independently of its microtubule-binding activity. We additionally show that the patient allele DCXG253D still binds microtubules but is deficient in promoting neurofascin endocytosis. We propose that DCX acts as an endocytic adaptor for neurofascin to fine-tune its surface distribution during neuronal development.

Pinceau organization in the cerebellum requires distinct functions of neurofascin in Purkinje and basket neurons during postnatal development

Basket axon collaterals synapse onto the Purkinje soma/axon initial segment (AIS) area to form specialized structures, the pinceau, which are critical for normal cerebellar function. Mechanistic details of how the pinceau become organized during cerebellar development are poorly understood. Loss of cytoskeletal adaptor protein Ankyrin G (AnkG) results in mislocalization of the cell adhesion molecule Neurofascin (Nfasc) at the Purkinje AIS and abnormal organization of the pinceau. Loss of Nfasc in adult Purkinje neurons leads to slow disorganization of the Purkinje AIS and pinceau morphology. Here, we used mouse conditional knock-out techniques to show that selective loss of Nfasc, specifically in Purkinje neurons during early development, prevented maturation of the AIS and resulted in loss of Purkinje neuron spontaneous activity and pinceau disorganization. Loss of Nfasc in both Purkinje and basket neurons caused abnormal basket axon collateral branching and targeting to Purkinje soma/AIS, leading to extensive pinceau disorganization, Purkinje neuron degeneration, and severe ataxia. Our studies reveal that the Purkinje Nfasc is required for AIS maturation and for maintaining stable contacts between basket axon terminals and the Purkinje AIS during pinceau organization, while the basket neuron Nfasc in combination with Purkinje Nfasc is required for proper basket axon collateral outgrowth and targeting to Purkinje soma/AIS. Thus, cerebellar pinceau organization requires coordinated mechanisms involving specific Nfasc functions in both Purkinje and basket neurons.

Neurofascin: a switch between neuronal plasticity and stability

Neurofascin (NF) is a cell surface protein belonging to the immunoglobulin superfamily (IgSF). Different polypeptides of 186, 180, 166 and 155 kDa are generated by alternative splicing. Expression of these isoforms is temporally and spatially regulated and can be roughly grouped into embryonic, adult and glial expression. NF interacts with many different interaction partners both extra- and intracellularly. Interactions of NF166 and NF180 selectively regulate mechanisms of plasticity like neurite outgrowth and the formation postsynaptic components. By contrast, NF155 and NF186 confer stabilization of neural structures by interaction with voltage-gated sodium channels and ankyrinG at axon initial segments (AIS) or nodes of Ranvier as well as neuron-glia interactions at the paranodes. Alternatively spliced isoforms of neurofascin may therefore balance dynamic and stabilizing mechanisms of the CNS.