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Nature Communications Nov 2020In the developing nervous system, axons navigate through complex terrains that change depending on when and where outgrowth begins. For instance, in the developing...
In the developing nervous system, axons navigate through complex terrains that change depending on when and where outgrowth begins. For instance, in the developing cochlea, spiral ganglion neurons extend their peripheral processes through a growing and heterogeneous environment en route to their final targets, the hair cells. Although the basic principles of axon guidance are well established, it remains unclear how axons adjust strategies over time and space. Here, we show that neurons with different positions in the spiral ganglion employ different guidance mechanisms, with evidence for both glia-guided growth and fasciculation along a neuronal scaffold. Processes from neurons in the rear of the ganglion are more directed and grow faster than those from neurons at the border of the ganglion. Further, processes at the wavefront grow more efficiently when in contact with glial precursors growing ahead of them. These findings suggest a tiered mechanism for reliable axon guidance.
Topics: Animals; Axon Guidance; Basic Helix-Loop-Helix Transcription Factors; Cell Movement; Cochlea; Female; Mice, Transgenic; Nerve Tissue Proteins; Neurites; Neuroglia; Neurons; Organ Culture Techniques; Pregnancy; Spiral Ganglion; Time-Lapse Imaging
PubMed: 33203842
DOI: 10.1038/s41467-020-19521-2 -
International Journal of Molecular... Oct 2020Neurons are mechanosensitive cells. The role of mechanical force in the process of neurite initiation, elongation and sprouting; nerve fasciculation; and neuron... (Review)
Review
Neurons are mechanosensitive cells. The role of mechanical force in the process of neurite initiation, elongation and sprouting; nerve fasciculation; and neuron maturation continues to attract considerable interest among scientists. Force is an endogenous signal that stimulates all these processes in vivo. The axon is able to sense force, generate force and, ultimately, transduce the force in a signal for growth. This opens up fascinating scenarios. How are forces generated and sensed in vivo? Which molecular mechanisms are responsible for this mechanotransduction signal? Can we exploit exogenously applied forces to mimic and control this process? How can these extremely low forces be generated in vivo in a non-invasive manner? Can these methodologies for force generation be used in regenerative therapies? This review addresses these questions, providing a general overview of current knowledge on the applications of exogenous forces to manipulate axonal outgrowth, with a special focus on forces whose magnitude is similar to those generated in vivo. We also review the principal methodologies for applying these forces, providing new inspiration and insights into the potential of this approach for future regenerative therapies.
Topics: Animals; Humans; Mechanotransduction, Cellular; Neuronal Outgrowth; Neurons
PubMed: 33126477
DOI: 10.3390/ijms21218009 -
Archives of Toxicology Dec 2020Exposure to environmental chemicals during in utero and early postnatal development can cause a wide range of neurological defects. Since current guidelines for...
Exposure to environmental chemicals during in utero and early postnatal development can cause a wide range of neurological defects. Since current guidelines for identifying developmental neurotoxic chemicals depend on the use of large numbers of rodents in animal experiments, it has been proposed to design rapid and cost-efficient in vitro screening test batteries that are mainly based on mixed neuronal/glial cultures. However, cell culture tests do not assay correct wiring of neuronal circuits. The establishment of precise anatomical connectivity is a key event in the development of a functional brain. Here, we expose intact embryos of the locust (Locusta migratoria) in serum-free culture to test chemicals and visualize correct navigation of identified pioneer axons by fluorescence microscopy. We define separate toxicological endpoints for axonal elongation and navigation along a stereotyped pathway. To distinguish developmental neurotoxicity (DNT) from general toxicity, we quantify defects in axonal elongation and navigation in concentration-response curves and compare it to the biochemically determined viability of the embryo. The investigation of a panel of recognized DNT-positive and -negative test compounds supports a rather high predictability of this invertebrate embryo assay. Similar to the semaphorin-mediated guidance of neurites in mammalian cortex, correct axonal navigation of the locust pioneer axons relies on steering cues from members of this family of cell recognition molecules. Due to the evolutionary conserved mechanisms of neurite guidance, we suggest that our pioneer axon paradigm might provide mechanistically relevant information on the DNT potential of chemical agents on the processes of axon elongation, navigation, and fasciculation.
Topics: Animals; Axon Guidance; Axons; Dose-Response Relationship, Drug; Embryo Culture Techniques; Embryo, Nonmammalian; Grasshoppers; Microscopy, Fluorescence; Necrosis; Nervous System; Neurotoxicity Syndromes; Toxicity Tests
PubMed: 33079231
DOI: 10.1007/s00204-020-02929-6 -
Cerebral Cortex (New York, N.Y. : 1991) Jan 2021A better understanding of genetic influences on early white matter development could significantly advance our understanding of neurological and psychiatric conditions...
A better understanding of genetic influences on early white matter development could significantly advance our understanding of neurological and psychiatric conditions characterized by altered integrity of axonal pathways. We conducted a genome-wide association study (GWAS) of diffusion tensor imaging (DTI) phenotypes in 471 neonates. We used a hierarchical functional principal regression model (HFPRM) to perform joint analysis of 44 fiber bundles. HFPRM revealed a latent measure of white matter microstructure that explained approximately 50% of variation in our tractography-based measures and accounted for a large proportion of heritable variation in each individual bundle. An intronic SNP in PSMF1 on chromosome 20 exceeded the conventional GWAS threshold of 5 x 10-8 (p = 4.61 x 10-8). Additional loci nearing genome-wide significance were located near genes with known roles in axon growth and guidance, fasciculation, and myelination.
Topics: Axons; Chromosomes, Human, Pair 20; Diffusion Magnetic Resonance Imaging; Diffusion Tensor Imaging; Female; Genome-Wide Association Study; Humans; Image Processing, Computer-Assisted; Infant; Infant, Newborn; Male; Myelin Sheath; Nerve Fibers; Phenotype; Polymorphism, Single Nucleotide; Proteasome Endopeptidase Complex; Regression Analysis; White Matter
PubMed: 33009551
DOI: 10.1093/cercor/bhaa266 -
Scientific Reports Sep 2020Intra-retinal axon guidance involves a coordinated expression of transcription factors, axon guidance genes, and secretory molecules within the retina. Pax6, the master...
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 Pax6 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.
Topics: Animals; Axon Fasciculation; Axon Guidance; Cell Differentiation; Extracellular Matrix; Female; Gene Expression Profiling; Gene Expression Regulation, Developmental; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; Neurogenesis; PAX6 Transcription Factor; Pregnancy; RNA-Seq; Receptor, EphB1; Retina; Retinal Ganglion Cells; Semaphorins
PubMed: 32999322
DOI: 10.1038/s41598-020-72828-4 -
Development (Cambridge, England) Aug 2020Fragile X mental retardation protein (FMRP) is an RNA-binding protein abundant in the nervous system. Functional loss of FMRP leads to sensory dysfunction and severe...
Fragile X mental retardation protein (FMRP) is an RNA-binding protein abundant in the nervous system. Functional loss of FMRP leads to sensory dysfunction and severe intellectual disabilities. In the auditory system, FMRP deficiency alters neuronal function and synaptic connectivity and results in perturbed processing of sound information. Nevertheless, roles of FMRP in embryonic development of the auditory hindbrain have not been identified. Here, we developed high-specificity approaches to genetically track and manipulate throughout development of the Atoh1 neuronal cell type, which is highly conserved in vertebrates, in the cochlear nucleus of chicken embryos. We identified distinct FMRP-containing granules in the growing axons of Atoh1 neurons and post-migrating NM cells. FMRP downregulation induced by CRISPR/Cas9 and shRNA techniques resulted in perturbed axonal pathfinding, delay in midline crossing, excess branching of neurites, and axonal targeting errors during the period of circuit development. Together, these results provide the first identification of FMRP localization and actions in developing axons of auditory neurons, and demonstrate the importance of investigating early embryonic alterations toward understanding the pathogenesis of neurodevelopmental disorders.
Topics: Animals; Auditory Pathways; Axons; Base Sequence; CRISPR-Cas Systems; Chick Embryo; Chickens; Dendrites; Fragile X Mental Retardation Protein; Neural Stem Cells; Presynaptic Terminals; RNA, Small Interfering; Rhombencephalon; Synapses; Time Factors
PubMed: 32747436
DOI: 10.1242/dev.188797 -
International Journal of Molecular... Jul 2020The development of neural circuits is a complex process that relies on the proper navigation of axons through their environment to their appropriate targets. While... (Review)
Review
The development of neural circuits is a complex process that relies on the proper navigation of axons through their environment to their appropriate targets. While axon-environment and axon-target interactions have long been known as essential for circuit formation, communication between axons themselves has only more recently emerged as another crucial mechanism. Trans-axonal signaling governs many axonal behaviors, including fasciculation for proper guidance to targets, defasciculation for pathfinding at important choice points, repulsion along and within tracts for pre-target sorting and target selection, repulsion at the target for precise synaptic connectivity, and potentially selective degeneration for circuit refinement. This review outlines the recent advances in identifying the molecular mechanisms of trans-axonal signaling and discusses the role of axon-axon interactions during the different steps of neural circuit formation.
Topics: Animals; Axons; Fasciculation; Growth Cones; Neural Conduction; Signal Transduction
PubMed: 32708320
DOI: 10.3390/ijms21145170 -
Development (Cambridge, England) Jun 2020Thalamocortical axons (TCAs) cross several tissues on their journey to the cortex. Mechanisms must be in place along the route to ensure they connect with their targets...
Thalamocortical axons (TCAs) cross several tissues on their journey to the cortex. Mechanisms must be in place along the route to ensure they connect with their targets in an orderly fashion. The ventral telencephalon acts as an instructive tissue, but the importance of the diencephalon in TCA mapping is unknown. We report that disruption of diencephalic development by Pax6 deletion results in a thalamocortical projection containing mapping errors. We used conditional mutagenesis to test whether these errors are due to the disruption of pioneer projections from prethalamus to thalamus and found that, although this correlates with abnormal TCA fasciculation, it does not induce topographical errors. To test whether the thalamus contains navigational cues for TCAs, we used slice culture transplants and gene expression studies. We found the thalamic environment is instructive for TCA navigation and that the molecular cues netrin 1 and semaphorin 3a are likely to be involved. Our findings indicate that the correct topographic mapping of TCAs onto the cortex requires the order to be established from the earliest stages of their growth by molecular cues in the thalamus itself.
Topics: Animals; Axons; Diencephalon; Embryo, Mammalian; Gene Expression Regulation, Developmental; Homeodomain Proteins; Mice; Mice, Inbred C57BL; Mice, Inbred CBA; Mice, Knockout; Mutagenesis; Netrin-1; Organ Culture Techniques; PAX6 Transcription Factor; Semaphorin-3A; Thalamus
PubMed: 32541009
DOI: 10.1242/dev.184523 -
Cell Reports Jan 2020Neuronal migration, axon fasciculation, and axon guidance need to be closely coordinated for neural circuit assembly. Spinal motor neurons (MNs) face unique challenges...
Neuronal migration, axon fasciculation, and axon guidance need to be closely coordinated for neural circuit assembly. Spinal motor neurons (MNs) face unique challenges during development because their cell bodies reside within the central nervous system (CNS) and their axons project to various targets in the body periphery. The molecular mechanisms that contain MN somata within the spinal cord while allowing their axons to exit the CNS and navigate to their final destinations remain incompletely understood. We find that the MN cell surface protein TAG-1 anchors MN cell bodies in the spinal cord to prevent their emigration, mediates motor axon fasciculation during CNS exit, and guides motor axons past dorsal root ganglia. TAG-1 executes these varied functions in MN development independently of one another. Our results identify TAG-1 as a key multifunctional regulator of MN wiring that coordinates neuronal migration, axon fasciculation, and axon guidance.
Topics: Animals; Axon Guidance; Axons; COS Cells; Cell Line; Cell Movement; Chlorocebus aethiops; Contactin 2; Fasciculation; Ganglia, Spinal; Gene Expression Regulation, Developmental; Mice; Mice, Inbred C57BL; Mice, Knockout; Motor Neurons; Neurogenesis; Signal Transduction; Spinal Cord
PubMed: 31995756
DOI: 10.1016/j.celrep.2019.12.085 -
Development (Cambridge, England) Feb 2020Axon ensheathment is fundamental for fast impulse conduction and the normal physiological functioning of the nervous system. Defects in axonal insulation lead to...
Axon ensheathment is fundamental for fast impulse conduction and the normal physiological functioning of the nervous system. Defects in axonal insulation lead to debilitating conditions, but, despite its importance, the molecular players responsible are poorly defined. Here, we identify RalA GTPase as a key player in axon ensheathment in larval peripheral nerves. We demonstrate through genetic analysis that RalA action through the exocyst complex is required in wrapping glial cells to regulate their growth and development. We suggest that the RalA-exocyst pathway controls the targeting of secretory vesicles for membrane growth or for the secretion of a wrapping glia-derived factor that itself regulates growth. In summary, our findings provide a new molecular understanding of the process by which axons are ensheathed , a process that is crucial for normal neuronal function.
Topics: Animals; Animals, Genetically Modified; Axon Fasciculation; Axons; Drosophila Proteins; Drosophila melanogaster; Larva; Locomotion; Monomeric GTP-Binding Proteins; Mutation; Myelin Sheath; Neuroglia; Peripheral Nerves; RNA Interference; Vesicular Transport Proteins
PubMed: 31969325
DOI: 10.1242/dev.174540