Semaphorin-6A controls guidance of corticospinal tract axons at multiple choice points

<p>Abstract</p> <p>Background</p> <p>The trajectory of corticospinal tract (CST) axons from cortex to spinal cord involves a succession of choice points, each of which is controlled by multiple guidance molecules. To assess the involvement of transmembrane semaphorins and their plexin receptors in the guidance of CST axons, we have examined this tract in mutants of <it>Semaphorin-6A </it>(<it>Sema6A</it>), <it>Plexin-A2 </it>(<it>PlxnA2</it>) and <it>Plexin-A4 </it>(<it>PlxnA4</it>).</p> <p>Results</p> <p>We describe defects in CST guidance in <it>Sema6A </it>mutants at choice points at the mid-hindbrain boundary (MHB) and in navigation through the pons that dramatically affect how many axons arrive to the hindbrain and spinal cord and result in hypoplasia of the CST. We also observe defects in guidance within the hindbrain where a proportion of axons aberrantly adopt a ventrolateral position and fail to decussate. This function in the hindbrain seems to be mediated by the known Sema6A receptor PlxnA4, which is expressed by CST axons. Guidance at the MHB, however, appears independent of this and of the other known receptor, PlxnA2, and may depend instead on Sema6A expression on CST axons themselves at embryonic stages.</p> <p>Conclusion</p> <p>These data identify Sema6A as a major contributor to the guidance of CST axons at multiple choice points. They highlight the active control of guidance at the MHB and also implicate the inferior olive as an important structure in the guidance of CST axons within the hindbrain. They also suggest that Sema6A, which is strongly expressed by oligodendrocytes, may affect CST regeneration in adults.</p

Pathology of a mouse mutation in peripheral myelin protein P0 is characteristic of a severe and early onset form of human Charcot-Marie-Tooth type 1B disorder

Mutations in the gene of the peripheral myelin protein zero (P0) give rise to the peripheral neuropathies Charcot-Marie-Tooth type 1B disease (CMT1B), Déjérine-Sottas syndrome, and congenital hypomyelinating neuropathy. To investigate the pathomechanisms of a specific point mutation in the P0 gene, we generated two independent transgenic mouse lines expressing the pathogenic CMT1B missense mutation Ile106Leu (P0sub) under the control of the P0 promoter on a wild-type background. Both P0sub-transgenic mouse lines showed shivering and ultrastructural abnormalities including retarded myelination, onion bulb formation, and dysmyelination seen as aberrantly folded myelin sheaths and tomacula in all nerve fibers. Functionally, the mutation leads to dispersed compound muscle action potentials and severely reduced conduction velocities. Our observations support the view that the Ile106Leu mutation acts by a dominant-negative gain of function and that the P0sub-transgenic mouse represents an animal model for a severe, tomaculous form of CMT1B

Mutation of Semaphorin-6A Disrupts Limbic and Cortical Connectivity and Models Neurodevelopmental Psychopathology

Psychiatric disorders such as schizophrenia and autism are characterised by cellular disorganisation and dysconnectivity across the brain and can be caused by mutations in genes that control neurodevelopmental processes. To examine how neurodevelopmental defects can affect brain function and behaviour, we have comprehensively investigated the consequences of mutation of one such gene, Semaphorin-6A, on cellular organisation, axonal projection patterns, behaviour and physiology in mice. These analyses reveal a spectrum of widespread but subtle anatomical defects in Sema6A mutants, notably in limbic and cortical cellular organisation, lamination and connectivity. These mutants display concomitant alterations in the electroencephalogram and hyper-exploratory behaviour, which are characteristic of models of psychosis and reversible by the antipsychotic clozapine. They also show altered social interaction and deficits in object recognition and working memory. Mice with mutations in Sema6A or the interacting genes may thus represent a highly informative model for how neurodevelopmental defects can lead to anatomical dysconnectivity, resulting, either directly or through reactive mechanisms, in dysfunction at the level of neuronal networks with associated behavioural phenotypes of relevance to psychiatric disorders. The biological data presented here also make these genes plausible candidates to explain human linkage findings for schizophrenia and autism

Mutations in the extracellular domain of the neural cell adhesion molecule L1 impair protein trafficking in vitro and in vivo

Rünker AE. Mutations in the extracellular domain of the neural cell adhesion molecule L1 impair protein trafficking in vitro and in vivo. Bielefeld (Germany): Bielefeld University; 2002.Das neurale Zelladhäsionsmolekül L1 ist ein Mitglied der Immunglobulin Superfamilie und erfüllt wichtige Funktionen im sich entwickelnden und adulten Nervensystem. So ist L1 an verschiedenen Prozessen, wie der Migration und dem Überleben von Neuronen, dem Auswachsen, der Bündelung und der Wegfindung von Axonen und der synaptischen Plastizität, beteiligt. Mutationen in allen Regionen des L1 Gens können beim Menschen zu gravierenden neurologischen Syndromen des sogenannten L1 Spektrums führen. Gemeinsame Merkmale dieser Syndrome sind eine erhöhte Sterblichkeit, geistige Behinderung und verschiedene Missbildungen des Gehirns. Patienten mit Punktmutationen in der extrazellulären Domäne von L1 entwickeln häufig einen schwereren Phänotyp als solche mit Mutationen im zytoplasmatischen Bereich. Um Einblicke in die Ursachen für das häufige Auftreten von schwerwiegenden Punktmutationen in der extrazellulären Domäne zu gewinnen, wurden in dieser Studie die funktionellen Konsequenzen von Mutationen sowohl der extrazellulären als auch der intrazellulären Domäne von L1 untersucht. Die Expression von mutierten L1 Konstrukten wurde in CHO- (chinese hamster ovarian) Zellen analysiert. Es konnte gezeigt werden, dass die pathogene L1 Punktmutation C264Y (L1C264Y), die in der extrazellulären Domäne liegt und zu einem schweren Phänotyp beim Menschen führt, nicht auf der Zelloberfläche exprimiert wird. Ähnliche Resultate ergaben sich für ein L1 Konstrukt mit einer Deletion der putativen homophilen Bindungsstelle von L1 (L1[Delta]hbs). Im Gegensatz dazu zeigte eine intrazellulär deletierte L1-Variante normale L1-Mengen auf der Zelloberfläche. Das Molekulargewicht von L1C264Y- und L1[Delta]hbs-Proteinen war reduziert, da die N-gekoppelten Oligosaccharide der Proteine keine Golgi-typische Modifikationen aufwiesen. Diese Beobachtungen lassen vermuten, dass die mutierten Proteine im endoplasmatischen Reticulum (ER) zurückgehalten und nicht weiter zum Golgi-Apparat transportiert werden. Um die Konsequenzen einer humanpathologischen Mutation auf die Expression und Funktion von L1 in vivo zu untersuchen, wurde eine transgene Mauslinie erzeugt, in der die extrazellulär gelegene Punktmutation C264Y unter Kontrolle des L1-Promotors vor einem L1-defizienten Hintergrund exprimiert wird. In diesen Mausmutanten war das L1C264Y-Protein in neuronalen Zellkörpern lokalisiert und wies einen abnormalen Glykosylierungsgrad auf. Diese Ergebnisse stimmen mit den Befunden der Zellkultur-Untersuchungen überein. Eine phänotypische Analyse der L1C264Y-transgenen Mäuse ergab keine Unterschiede zu L1-defizienten Mäusen, d.h. beide Mutanten zeigten eine verringerte Überlebenswahrscheinlichkeit, eine Reduktion des corticospinalen Traktes, Fehler in der Wegfindung von corticospinalen Axonen, sowie abnormale unmyelinisierte Fasern in peripheren Nerven. Diese Befunde legen nahe, dass die transgenen Mäuse funktionale Null-Mutanten repräsentieren. Die Ergebnisse der in vitro und in vivo Untersuchungen deuten darauf hin, dass eine Akkumulation des mutierten L1 Proteins im ER, gefolgt von einer Degradation des Proteins, den zugrundeliegenden molekularen Pathomechanismus der L1C264Y Mutation darstellt. Demnach könnte eine gestörte Zelloberflächenexpression ursächlich für das häufige Auftreten von schweren pathogenen Punktmutationen im humanen L1 Gen sein.The neural cell adhesion molecule L1, a member of the immunoglobulin superfamily, performs important functions in the developing and adult nervous system. L1 is implicated in neuronal migration and survival, elongation, fasciculation and pathfinding of axons, and synaptic plasticity. Mutations in all parts of the L1 gene might cause serious neurological syndromes in humans, characterized by increased mortality, mental retardation and various malformations of the nervous system. Patients with missense mutations in the extracellular domain of L1 often develop severe phenotypes, while mutations in the cytoplasmic domain usually cause moderate phenotypes. In an attempt to understand the reasons for the frequent occurrence of severe extracellular missense mutations, this study addressed the functional consequences of extracellular and cytoplasmic L1 mutations. To this aim, we used mutated L1 constructs to study their expression in CHO cells. The L1 missense mutation C264Y (L1C264Y), located in the extracellular domain and causing a severe phenotype in humans, was not expressed at the cell surface. Similar results were obtained for a L1 construct with a deletion of the putative homophilic binding side (L1[Delta]hbs). In contrast, an intracellularly truncated form of L1 showed normal levels of cell surface expression. L1C264Y and L1[Delta]hbs protein had a reduced molecular weight due to the lack of Golgi-type modified N-glycans. These observations suggest that both mutated L1 variants are retained within the endoplasmic reticulum (ER). To study the expression and the functional consequences of a human pathogenic missense mutation in vivo, a transgenic mouse line was generated expressing the extracellular missense mutation C264Y under the control of the L1 promoter in a L1-deficient background. In these mutant mice, the L1C264Y protein was located intracellularly to neuronal cell bodies and displayed an abnormal glycosylation state, in line with the results obtained in cell culture experiments. Analysis of the L1C264Y transgenic mice revealed no phenotypical differences to L1-deficient mice, i.e. both mutants showed reduced survival, a reduced size of the corticospinal tract and pathfinding errors of corticospinal axons, and abnormalities of unmyelinated fibers in peripheral nerves. Together, these data indicate that the transgenic mice represent functional null mutants. We suggest an ER retention followed by degradation of the mutated L1 protein as the most likely underlying molecular pathomechanism of the L1C264Y missense mutation, ultimately resulting in the loss of L1 function. The combined in vitro and in vivo observations corroborate the view that impaired cell surface expression of mutated variants of L1 is a potential explanation for the high number of severe pathogenic mutations identified within the human L1 gene

Myelin-specific T helper 17 cells promote adult hippocampal neurogenesis through indirect mechanisms [version 2; referees: 1 approved, 2 approved with reservations]

CD4+ T cells provide a neuro-immunological link in the regulation of adult hippocampal neurogenesis, but the exact mechanisms underlying enhanced neural precursor cell proliferation and the relative contribution of different T helper (Th) cell subsets have remained unclear. Here, we explored the pro-proliferative potential of interleukin 17-producing T helper (Th17) cells, a developmentally and functionally distinct Th cell subset that is a key mediator of autoimmune neurodegeneration. We found that base-line proliferation of hippocampal precursor cells in a T cell-deficient mouse model of impaired hippocampal neurogenesis can be restored upon adoptive transfer with homogeneous Th17 populations enriched for myelin-reactive T cell receptors (TCR). In these experiments, enhanced proliferation was independent of direct interactions of infiltrating Th17 cells with precursor cells or neighboring cells in the hippocampal neurogenic niche. Complementary studies in immunocompetent mice identified several receptors for Th17 cell-derived cytokines with mRNA expression in hippocampal precursor cells and dentate gyrus tissue, suggesting that Th17 cell activity in peripheral lymphoid tissues might promote hippocampal neurogenesis through secreted cytokines

The C264Y missense mutation in the extracellular domain of L1 impairs protein trafficking in vitro and in vivo

The neural cell adhesion molecule L1, a member of the immunoglobulin superfamily, performs important functions in the developing and adult nervous system and is implicated in neuronal migration and survival, elongation, fasciculation and pathfinding of axons, and synaptic plasticity. This view is in line with the fact that mutations in the L1 gene result in severe neurological syndromes in humans. Patients with missense mutations in the extracellular domain of L1 often develop severe phenotypes. Here, we characterized in vitro and in vivo the missense mutation C264Y, which is located in the extracellular domain of L1 and causes a severe phenotype in humans. Transfection studies in vitro demonstrate that L1 carrying this missense mutation is not expressed at the cell surface but instead is located intracellularly, most likely within the endoplasmic reticulum. Lack of cell surface expression of L1 with a C264Y mutation was confirmed in a transgenic mouse line expressing the C264Y mutation under the control of the L1 promoter in an L1-deficient background. Analysis of these transgenic mice indicates that they represent functional null mutants, phenotypically indistinguishable from L1-deficient mice. These observations corroborate the view that impaired cell surface expression of mutated variants of L1 is a potential explanation for the high number of severe pathogenic mutations identified within the human L1 gene

A protocol for isolation and enriched monolayer cultivation of neural precursor cells from mouse dentate gyrus

In vitro assays are valuable tools to study the characteristics of adult neural precursor cells under controlled conditions with a defined set of parameters. We here present a detailed protocol based on our previous original publication (Babu et al., Enriched monolayer precursor cell cultures from micro-dissected adult mouse dentate gyrus yield functional granule cell-like neurons, PLoS One 2007, 2:e388) to isolate neural precursor cells from the hippocampus of adult mice and maintain and propagate them as adherent monolayer cultures. The strategy is based on the use of Percoll density gradient centrifugation to enrich precursor cells from the micro-dissected dentate gyrus. Based on the expression of Nestin and Sox2, a culture-purity of more than 98% can be achieved. The cultures are expanded under serum-free conditions in Neurobasal A medium with addition of the mitogens EGF and FGF2 as well as the supplements Glutamax-1 and B27. Under differentiation conditions, the precursor cells reliably generate approximately 30% neurons with appropriate morphological, molecular and electrophysiological characteristics that might reflect granule cell properties as their in vivo counterpart. We also highlight potential modifications to the protocol