-
The Journal of Comparative Neurology May 2018Prior to forming and refining synaptic connections, axons of projection neurons navigate long distances to their targets. While much is known about guidance cues for...
Prior to forming and refining synaptic connections, axons of projection neurons navigate long distances to their targets. While much is known about guidance cues for axon navigation through intermediate choice points, whether and how axons are organized within tracts is less clear. Here we analyze the organization of retinal ganglion cell (RGC) axons in the developing mouse retinogeniculate pathway. RGC axons are organized by both eye-specificity and topography in the optic nerve and tract: ipsilateral RGC axons are segregated from contralateral axons and are offset laterally in the tract relative to contralateral axon topographic position. To identify potential cell-autonomous factors contributing to the segregation of ipsilateral and contralateral RGC axons in the visual pathway, we assessed their fasciculation behavior in a retinal explant assay. Ipsilateral RGC neurites self-fasciculate more than contralateral neurites in vitro and maintain this difference in the presence of extrinsic chiasm cues. To further probe the role of axon self-association in circuit formation in vivo, we examined RGC axon organization and fasciculation in an EphB1 mutant, in which a subset of ipsilateral RGC axons aberrantly crosses the midline but targets the ipsilateral zone in the dorsal lateral geniculate nucleus on the opposite side. Aberrantly crossing axons retain their association with ipsilateral axons in the contralateral tract, indicating that cohort-specific axon affinity is maintained independently of guidance signals present at the midline. Our results provide a comprehensive assessment of RGC axon organization in the retinogeniculate pathway and suggest that axon self-association contributes to pre-target axon organization.
Topics: Amino Acids; Animals; Animals, Newborn; Axons; Embryo, Mammalian; Eye; Fasciculation; Functional Laterality; In Vitro Techniques; Intermediate Filaments; Luminescent Proteins; Mice; Mice, Inbred C57BL; Mice, Transgenic; Optic Nerve; Receptor, EphB1; Retinal Ganglion Cells; Serotonin Plasma Membrane Transport Proteins; Visual Pathways
PubMed: 29322522
DOI: 10.1002/cne.24392 -
FEBS Open Bio Jan 2018Fasciculation and elongation zeta-1 (FEZ1) protein is involved in axon outgrowth and is highly expressed in the brain. It has multiple interaction partners, with...
Fasciculation and elongation zeta-1 (FEZ1) protein is involved in axon outgrowth and is highly expressed in the brain. It has multiple interaction partners, with functions varying from the regulation of neuronal development and intracellular transport mechanisms to transcription regulation. One of its interactors is retinoic acid receptor (RAR), which is activated by retinoic acid and controls many target genes and physiological process. Based on previous evidence suggesting a possible nuclear role for FEZ1, we wanted to deepen our understanding of this function by addressing the FEZ1-RAR interaction. We performed binding experiments and assessed the interface of interaction between both proteins. We found that FEZ1-RAR interacted with a similar magnitude as RAR to its responsive element DR5 and that the interaction occurred in the coiled-coil region of FEZ1 and in the ligand-binding domain of RAR. Furthermore, cellular experiments were performed in order to confirm the interaction and screen for induced target genes from an 86-gene panel. The analysis of gene expression showed that only in the presence of retinoic acid did FEZ1 induce gene expression. This finding is consistent with data from the literature showing the gene functionally involved in development and acute myeloid leukemia, as is FEZ1.
PubMed: 29321952
DOI: 10.1002/2211-5463.12338 -
PLoS Genetics Nov 2017Axon-guidance by Slit-Roundabout (Robo) signaling at the midline initially guides growth cones to synaptic targets and positions longitudinal axon tracts in discrete...
Axon-guidance by Slit-Roundabout (Robo) signaling at the midline initially guides growth cones to synaptic targets and positions longitudinal axon tracts in discrete bundles on either side of the midline. Following the formation of commissural tracts, Slit is found also in tracts of the commissures and longitudinal connectives, the purpose of which is not clear. The Slit protein is processed into a larger N-terminal peptide and a smaller C-terminal peptide. Here, I show that Slit and Slit-N in tracts interact with Robo to maintain the fasciculation, the inter-tract spacing between tracts and their position relative to the midline. Thus, in the absence of Slit in post-guidance tracts, tracts de-fasciculate, merge with one another and shift their position towards the midline. The Slit protein is proposed to function as a gradient. However, I show that Slit and Slit-N are not freely present in the extracellular milieu but associated with the extracellular matrix (ECM) and both interact with Robo1. Slit-C is tightly associated with the ECM requiring collagenase treatment to release it, and it does not interact with Robo1. These results define a role for Slit and Slit-N in tracts for the maintenance and fasciculation of tracts, thus the maintenance of the hardwiring of the CNS.
Topics: Animals; Axons; Drosophila; Drosophila Proteins; Extracellular Matrix; Fasciculation; Gene Expression Regulation, Developmental; Growth Cones; Nerve Tissue Proteins; Receptors, Immunologic; Signal Transduction; Spinal Cord; Roundabout Proteins
PubMed: 29155813
DOI: 10.1371/journal.pgen.1007094 -
Scientific Reports Oct 2017During nervous system development growing axons can interact with each other, for example by adhering together in order to produce bundles (fasciculation). How does such...
During nervous system development growing axons can interact with each other, for example by adhering together in order to produce bundles (fasciculation). How does such axon-axon interaction affect the resulting axonal trajectories, and what are the possible benefits of this process in terms of network function? In this paper we study these questions by adapting an existing computational model of the development of neurons in the Xenopus tadpole spinal cord to include interactions between axons. We demonstrate that even relatively weak attraction causes bundles to appear, while if axons weakly repulse each other their trajectories diverge such that they fill the available space. We show how fasciculation can help to ensure axons grow in the correct location for proper network formation when normal growth barriers contain gaps, and use a functional spiking model to show that fasciculation allows the network to generate reliable swimming behaviour even when overall synapse counts are artificially lowered. Although we study fasciculation in one particular organism, our approach to modelling axon growth is general and can be widely applied to study other nervous systems.
Topics: Animals; Axon Fasciculation; Larva; Models, Biological; Spinal Cord; Synapses; Xenopus laevis
PubMed: 29051550
DOI: 10.1038/s41598-017-13804-3 -
ELife Sep 2017Nervous system function relies on precise synaptic connections. A number of widely-conserved cell adhesion proteins are implicated in cell recognition between synaptic...
Nervous system function relies on precise synaptic connections. A number of widely-conserved cell adhesion proteins are implicated in cell recognition between synaptic partners, but how these proteins act as a group to specify a complex neural network is poorly understood. Taking advantage of known connectivity in , we identified and studied cell adhesion genes expressed in three interacting neurons in the mating circuits of the adult male. Two interacting pairs of cell surface proteins independently promote fasciculation between sensory neuron HOA and its postsynaptic target interneuron AVG: BAM-2/neurexin-related in HOA binds to CASY-1/calsyntenin in AVG; SAX-7/L1CAM in sensory neuron PHC binds to RIG-6/contactin in AVG. A third, basal pathway results in considerable HOA-AVG fasciculation and synapse formation in the absence of the other two. The features of this multiplexed mechanism help to explain how complex connectivity is encoded and robustly established during nervous system development.
Topics: Animals; Caenorhabditis elegans; Cell Adhesion; Connectome; Gene Expression Profiling; Male; Nerve Net; Neural Cell Adhesion Molecules; Neurons
PubMed: 28901288
DOI: 10.7554/eLife.29257 -
Seminars in Cell & Developmental Biology Sep 2017The δ-protocadherins comprise a small family of homophilic cell adhesion molecules within the larger cadherin superfamily. They are essential for neural development as... (Review)
Review
The δ-protocadherins comprise a small family of homophilic cell adhesion molecules within the larger cadherin superfamily. They are essential for neural development as mutations in these molecules give rise to human neurodevelopmental disorders, such as schizophrenia and epilepsy, and result in behavioral defects in animal models. Despite their importance to neural development, a detailed understanding of their mechanisms and the ways in which their loss leads to changes in neural function is lacking. However, recent results have begun to reveal roles for the δ-protocadherins in both regulation of neurogenesis and lineage-dependent circuit assembly, as well as in contact-dependent motility and selective axon fasciculation. These evolutionarily conserved mechanisms could have a profound impact on the robust assembly of the vertebrate nervous system. Future work should be focused on unraveling the molecular mechanisms of the δ-protocadherins and understanding how this family functions broadly to regulate neural development.
Topics: Animals; Cadherins; Humans; Models, Biological; Nerve Net; Nervous System Diseases; Phylogeny; Synapses
PubMed: 28751249
DOI: 10.1016/j.semcdb.2017.07.037 -
Development (Cambridge, England) Jul 2017Visual information is relayed from the eye to the brain via retinal ganglion cell (RGC) axons. Mice lacking NRP1 or NRP1-binding VEGF-A isoforms have defective RGC axon...
Visual information is relayed from the eye to the brain via retinal ganglion cell (RGC) axons. Mice lacking NRP1 or NRP1-binding VEGF-A isoforms have defective RGC axon organisation alongside brain vascular defects. It is not known whether axonal defects are caused exclusively by defective VEGF-A signalling in RGCs or are exacerbated by abnormal vascular morphology. Targeted NRP1 ablation in RGCs with a knock-in allele reduced axonal midline crossing at the optic chiasm and optic tract fasciculation. In contrast, -mediated endothelial NRP1 ablation induced axon exclusion zones in the optic tracts without impairing axon crossing. Similar defects were observed in and mice, which have vascular defects as a result of their expression of single VEGF-A isoforms. Ectopic midline vascularisation in endothelial and mutants caused additional axonal exclusion zones within the chiasm. As and assays demonstrated that vessels do not repel axons, abnormally large or ectopically positioned vessels are likely to present physical obstacles to axon growth. We conclude that proper axonal wiring during brain development depends on the precise molecular control of neurovascular co-patterning.
Topics: Animals; Axons; Blood Vessels; Body Patterning; Central Nervous System; Diencephalon; Endothelial Cells; Gene Knockdown Techniques; Homeodomain Proteins; Mice, Inbred C57BL; Mutation; Neovascularization, Physiologic; Neuropilin-1; Optic Chiasm; Retinal Ganglion Cells; Transcription Factor Brn-3B; Vascular Endothelial Growth Factor A; Visual Pathways
PubMed: 28676569
DOI: 10.1242/dev.151621 -
Science Signaling Jun 2017Slit proteins act as repulsive axon guidance cues by activating receptors of the Roundabout (Robo) family. During early neurogenesis in , Slit prevents the growth cones...
Slit proteins act as repulsive axon guidance cues by activating receptors of the Roundabout (Robo) family. During early neurogenesis in , Slit prevents the growth cones of longitudinal tract neurons from inappropriately crossing the midline, thus restricting these cells to trajectories parallel to the midline. Slit is expressed in midline glial cells, and Robo is present in longitudinal axon tracts and growth cones. We showed that the enzyme Mummy (Mmy) controlled Slit-Robo signaling through mechanisms that affected both the ligand and the receptor. Mmy was required for the glycosylation of Slit, which was essential for Slit secretion. Mmy was also required for maintaining the abundance and spatial distribution of Robo through an indirect mechanism that was independent of Slit secretion. Moreover, secretion of Slit was required to maintain the fasciculation and position of longitudinal axon tracts, thus maintaining the hardwiring of the nervous system. Thus, Mmy is required for Slit secretion and for maintaining Robo abundance and distribution in the developing nervous system in .
Topics: Animals; Animals, Genetically Modified; Axons; Crosses, Genetic; Drosophila Proteins; Drosophila melanogaster; Glycosylation; Growth Cones; Ligands; Nerve Tissue Proteins; Neuroglia; Neurons; Nucleotidyltransferases; Receptors, Immunologic; Signal Transduction; Roundabout Proteins
PubMed: 28634210
DOI: 10.1126/scisignal.aam5841 -
ELife Apr 2017While axon fasciculation plays a key role in the development of neural networks, very little is known about its dynamics and the underlying biophysical mechanisms. In a...
While axon fasciculation plays a key role in the development of neural networks, very little is known about its dynamics and the underlying biophysical mechanisms. In a model system composed of neurons grown ex vivo from explants of embryonic mouse olfactory epithelia, we observed that axons dynamically interact with each other through their shafts, leading to zippering and unzippering behavior that regulates their fasciculation. Taking advantage of this new preparation suitable for studying such interactions, we carried out a detailed biophysical analysis of zippering, occurring either spontaneously or induced by micromanipulations and pharmacological treatments. We show that zippering arises from the competition of axon-axon adhesion and mechanical tension in the axons, and provide the first quantification of the force of axon-axon adhesion. Furthermore, we introduce a biophysical model of the zippering dynamics, and we quantitatively relate the individual zipper properties to global characteristics of the developing axon network. Our study uncovers a new role of mechanical tension in neural development: the regulation of axon fasciculation.
Topics: Animals; Axon Fasciculation; Axons; Biophysical Phenomena; Cell Adhesion; Cells, Cultured; Mice; Models, Biological; Olfactory Mucosa; Stress, Mechanical
PubMed: 28422009
DOI: 10.7554/eLife.19907 -
The Journal of Neuroscience : the... Apr 2017Membrane excitability in the axonal growth cones of embryonic neurons influences axon growth. Voltage-gated K (Kv) channels are key factors in controlling membrane...
Membrane excitability in the axonal growth cones of embryonic neurons influences axon growth. Voltage-gated K (Kv) channels are key factors in controlling membrane excitability, but whether they regulate axon growth remains unclear. Here, we report that Kv3.4 is expressed in the axonal growth cones of embryonic spinal commissural neurons, motoneurons, dorsal root ganglion neurons, retinal ganglion cells, and callosal projection neurons during axon growth. Our (cultured dorsal spinal neurons of chick embryos) and (developing chick spinal commissural axons and rat callosal axons) findings demonstrate that knockdown of Kv3.4 by a specific shRNA impedes axon initiation, elongation, pathfinding, and fasciculation. In cultured dorsal spinal neurons, blockade of Kv3.4 by blood depressing substance II suppresses axon growth via an increase in the amplitude and frequency of Ca influx through T-type and L-type Ca channels. Electrophysiological results show that Kv3.4, the major Kv channel in the axonal growth cones of embryonic dorsal spinal neurons, is activated at more hyperpolarized potentials and inactivated more slowly than it is in postnatal and adult neurons. The opening of Kv3.4 channels effectively reduces growth cone membrane excitability, thereby limiting excessive Ca influx at subthreshold potentials or during Ca-dependent action potentials. Furthermore, excessive Ca influx induced by an optogenetic approach also inhibits axon growth. Our findings suggest that Kv3.4 reduces growth cone membrane excitability and maintains [Ca] at an optimal concentration for normal axon growth. Accumulating evidence supports the idea that impairments in axon growth contribute to many clinical disorders, such as autism spectrum disorders, corpus callosum agenesis, Joubert syndrome, Kallmann syndrome, and horizontal gaze palsy with progressive scoliosis. Membrane excitability in the growth cone, which is mainly controlled by voltage-gated Ca (Cav) and K (Kv) channels, modulates axon growth. The role of Cav channels during axon growth is well understood, but it is unclear whether Kv channels control axon outgrowth by regulating Ca influx. This report shows that Kv3.4, which is transiently expressed in the axonal growth cones of many types of embryonic neurons, acts to reduce excessive Ca influx through Cav channels and thus permits normal axon outgrowth.
Topics: Action Potentials; Animals; Axons; Calcium; Chick Embryo; Corpus Callosum; Electroporation; Ganglia, Spinal; Gene Knockdown Techniques; Growth Cones; Motor Neurons; Neurons; Potassium Channels, Voltage-Gated; Rats; Retinal Ganglion Cells
PubMed: 28320840
DOI: 10.1523/JNEUROSCI.1076-16.2017