TAG-1 Multifunctionality Coordinates Neuronal Migration, Axon Guidance, and Fasciculation.
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ABSTRACT:
SUBMITTER: Suter TACS
PROVIDER: S-EPMC7049094 | biostudies-literature | 2020 Jan
REPOSITORIES: biostudies-literature
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Project description:Neuronal migration and axon guidance constitute fundamental processes in brain development that are generally studied independently. Although both share common mechanisms of cell biology and biochemistry, little is known about their coordinated integration in the formation of neural circuits. Here we show that the development of the thalamocortical projection, one of the most prominent tracts in the mammalian brain, depends on the early tangential migration of a population of neurons derived from the ventral telencephalon. This tangential migration contributes to the establishment of a permissive corridor that is essential for thalamocortical axon pathfinding. Our results also demonstrate that in this process two different products of the Neuregulin-1 gene, CRD-NRG1 and Ig-NRG1, mediate the guidance of thalamocortical axons. These results show that neuronal tangential migration constitutes a novel mechanism to control the timely arrangement of guidance cues required for axonal tract formation in the mammalian brain.
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Project description:Intra-retinal axon guidance involves a coordinated expression of transcription factors, axon guidance genes, and secretory molecules within the retina. Pax6, the master regulator gene, has a spatio-temporal expression typically restricted till neurogenesis and fate-specification. However, our observation of persistent expression of Pax6 in mature RGCs led us to hypothesize that Pax6 could play a major role in axon guidance after fate specification. Here, we found significant alteration in intra-retinal axon guidance and fasciculation upon knocking out of Pax6 in E15.5 retina. Through unbiased transcriptome profiling between Pax6fl/fl and Pax6-/- retinas, we revealed the mechanistic insight of its role in axon guidance. Our results showed a significant increase in the expression of extracellular matrix molecules and decreased expression of retinal fate specification and neuron projection guidance molecules. Additionally, we found that EphB1 and Sema5B are directly regulated by Pax6 owing to the guidance defects and improper fasciculation of axons. We conclude that Pax6 expression post fate specification of RGCs is necessary for regulating the expression of axon guidance genes and most importantly for maintaining a conducive ECM through which the nascent axons get guided and fasciculate to reach the optic disc.
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Project description:Axon guidance involves the spatiotemporal interplay between guidance cues and membrane-bound cell-surface receptors, present on the growth cone of the axon. Netrin-1 is a prototypical guidance cue that binds to deleted in colorectal cancer (DCC), and it has been proposed that the guidance cue Draxin modulates this interaction. Here, we present structural snapshots of Draxin/DCC and Draxin/Netrin-1 complexes, revealing a triangular relationship that affects Netrin-mediated haptotaxis and fasciculation. Draxin interacts with DCC through the N-terminal four immunoglobulin domains, and Netrin-1 through the EGF-3 domain, in the same region where DCC binds. Netrin-1 and DCC bind to adjacent sites on Draxin, which appears to capture Netrin-1 and tether it to the DCC receptor. We propose the conformational flexibility of the single-pass membrane receptor DCC is used to promote fasciculation and regulate axon guidance through concerted Netrin-1/Draxin binding. VIDEO ABSTRACT.
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Project description:The accuracy of axonal pathfinding and the formation of functional neural circuitry are crucial for an organism to process, store, and retrieve information from internal networks as well as from the environment. Aberrations in axonal migration is believed to lead to loop formation and self-fasciculation, which can lead to highly dysfunctional neural circuitry and therefore self-avoidance of axons is proposed to be the regulatory mechanism for control of synaptogenesis. Here, we report the application of a newly developed non-contact optical method using a weakly-focused, near infrared laser beam for highly efficient axonal guidance, and demonstrate the formation of axonal loops in cortical neurons, which demonstrate that cortical neurons can self-fasciculate in contrast to self-avoidance. The ability of light for axonal nano-loop formation opens up new avenues for the construction of complex neural circuitry, and non-invasive guidance of neurons at long working distances for restoration of impaired neural connections and functions.
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Project description:RNA-binding proteins (RBPs) control multiple aspects of post-transcriptional gene regulation and function during various biological processes in the nervous system. To further reveal the functional significance of RBPs during neural development, we carried out an in vivo RNAi screen in the dorsal spinal cord interneurons, including the commissural neurons. We found that the NOVA family of RBPs play a key role in neuronal migration, axon outgrowth, and axon guidance. Interestingly, Nova mutants display similar defects as the knockout of the Dcc transmembrane receptor. We show here that Nova deficiency disrupts the alternative splicing of Dcc, and that restoring Dcc splicing in Nova knockouts is able to rescue the defects. Together, our results demonstrate that the production of DCC splice variants controlled by NOVA has a crucial function during many stages of commissural neuron development.
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Project description:BACKGROUND: Although originally identified as embryonic axon guidance cues, semaphorins are now known to regulate multiple, distinct, processes crucial for neuronal network formation including axon growth and branching, dendritic morphology, and neuronal migration. Semaphorin7A (Sema7A), the only glycosylphosphatidylinositol-anchored semaphorin, promotes axon growth in vitro and is required for the proper growth of the mouse lateral olfactory tract in vivo. Sema7A has been postulated to signal through two unrelated receptors, an RGD-dependent alpha1beta1-integrin and a member of the plexin family, plexinC1. beta1-integrins underlie Sema7A-mediated axon growth and Sema7A function in the immune system. Sema7A-plexinC1 interactions have also been implicated in immune system function, but the neuronal role of this ligand-receptor pair remains to be explored. To gain further insight into the function(s) of Sema7A and plexinC1 during neural development, we present here a detailed analysis of Sema7A and plexinC1 expression in the developing rat nervous system. RESULTS: In situ hybridization revealed select expression of Sema7A and plexinC1 in multiple neuronal systems including: the olfactory system, the hypothalamo-hypophysial system, the hippocampus, the meso-diencephalic dopamine system, and the spinal cord. Within these systems, Sema7A and plexinC1 are often expressed in specific neuronal subsets. In general, Sema7A transcript levels increase significantly towards adulthood, whereas plexinC1 expression decreases as development proceeds.PlexinC1, but not Sema7A, is strongly expressed by distinct populations of migrating neurons. In addition to neuronal expression, Sema7A and plexinC1 transcripts were detected in oligodendrocytes and ependymal cells, respectively. CONCLUSION: Sema7A and plexinC1 expression patterns are consistent with these proteins serving both cooperative and separate functions during neural development. The prominent expression of plexinC1 in several distinct populations of migrating neurons suggests a novel role for this plexin family member in neuronal migration.
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Project description:Heterozygous loss-of function mutations in CHD7 (chromodomain helicase DNA-binding protein 7) lead to CHARGE syndrome, a complex developmental disorder affecting craniofacial structures, peripheral nerves and several organ systems like eyes, ears, nose and heart. Recently, it was demonstrated that CHD7 is essential for the formation of multipotent migratory neural crest cells, which migrate from the neural tube to many regions of the embryo, where they differentiate into various tissues including craniofacial and heart structures. So far only few CHD7 target genes involved in neural crest cell development have been identified and the role of CHD7 in neural crest cell guidance and the regulation of mesenchymal-epithelial transition is unknown. Therefore, we undertook a genome-wide microarray expression analysis on wild-type and CHD7 deficient (Chd7Whi/+ and Chd7Whi/Whi) mouse embryos at day 9.5, the time point of neural crest cell migration. We identified 98 genes showing greater than two fold differences in expression (log2 fold-change) and a P-value to false discovery rate (FDR) < 0.05 between wild-type and Chd7Whi/Whi embryos. Interestingly, many misregulated genes are involved in neural crest cell and axon guidance like semaphorins and ephrin receptors. By performing knockdown experiments for Chd7 and one of its target genes, namely semaphorin3a in Xenopus laevis embryos, we could show abnormalities in the migration of neural crest cells in vivo. Additionally, we detected non-synonymous SEMA3A variations in 3 out of 45 CHD7 negative CHARGE patients suggesting a role for SEMA3A in the pathogenesis of CHARGE syndrome. To identify genes that are affected by the absence of functional Chd7 at the time point of neural crest cell migration, the expression profiles of E9.5 wild-type, Chd7Whi/+ and Chd7Whi/Whi female mouse embryos were compared by whole-genome microarray analysis. Mouse embryos of the same sex were used to avoid sex-dependent gene expression effects. We performed microarray analysis by using the Agilent-026655 Whole Mouse Genome Microarray 4x44K v2 (Agilent) on four biological replicates from each group.
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