Polarity of varicosity initiation in central neuron mechanosensation.
ABSTRACT: Little is known about mechanical regulation of morphological and functional polarity of central neurons. In this study, we report that mechanical stress specifically induces varicosities in the axons but not the dendrites of central neurons by activating TRPV4, a Ca2+/Na+-permeable mechanosensitive channel. This process is unexpectedly rapid and reversible, consistent with the formation of axonal varicosities in vivo induced by mechanical impact in a mouse model of mild traumatic brain injury. In contrast, prolonged stimulation of glutamate receptors induces varicosities in dendrites but not in axons. We further show that axonal varicosities are induced by persistent Ca2+ increase, disassembled microtubules (MTs), and subsequently reversible disruption of axonal transport, and are regulated by stable tubulin-only polypeptide, an MT-associated protein. Finally, axonal varicosity initiation can trigger action potentials to antidromically propagate to the soma in retrograde signaling. Therefore, our study demonstrates a new feature of neuronal polarity: axons and dendrites preferentially respond to physical and chemical stresses, respectively.
Project description:Neurons project axons to local and distal sites and can display heterogeneous morphologies with limited physical dimensions that may influence the structure of large organelles such as mitochondria. Using cryo-electron tomography (cryo-ET), we characterized native environments within axons and presynaptic varicosities to examine whether spatial restrictions within these compartments influence the morphology of mitochondria. Segmented tomographic reconstructions revealed distinctive morphological characteristics of mitochondria residing at the narrowed boundary between presynaptic varicosities and axons with limited physical dimensions (approximately 80 nm), compared to mitochondria in nonspatially restricted environments. Furthermore, segmentation of the tomograms revealed discrete organizations between the inner and outer membranes, suggesting possible independent remodeling of each membrane in mitochondria at spatially restricted axonal/varicosity boundaries. Thus, cryo-ET of mitochondria within axonal subcompartments reveals that spatial restrictions do not obstruct mitochondria from residing within them, but limited available space can influence their gross morphology and the organization of the inner and outer membranes. These findings offer new perspectives on the influence of physical and spatial characteristics of cellular environments on mitochondrial morphology and highlight the potential for remarkable structural plasticity of mitochondria to adapt to spatial restrictions within axons.
Project description:Establishing precise synaptic connectivity during development is crucial for neural circuit function. However, very few molecules have been identified that are involved in determining where and how many synapses form. The Plexin cell-surface molecules are a conserved family of axon guidance receptors that mediate axon fasciculation and repulsion during neural development, and later in development PlexinA receptors are involved in eliminating axonal branches and synapse numbers. Here we investigate the roles of PlexinA and PlexinB receptors in axonal branch and varicosity formation in Drosophila. We knocked down PlexinA or PlexinB expression using RNAi in identified mechanosensory neurons and analyzed axonal branching patterns and varicosity formations. Reducing PlexinA expression increased the axonal arbor complexity by increasing the number of branches and varicosities along the axon. In contrast, knocking down PlexinB expression decreased morphological complexity by decreasing the number of branches and the overall size of the axonal arbor, but did not reduce the number of varicosities. Our results demonstrate opposing roles for PlexinA and PlexinB in local wiring within a target region, where PlexinA functions to suppress excessive axonal branches and synapses and PlexinB facilitates axonal growth.
Project description:Plasticity in the central nervous system in response to injury is a complex process involving axonal remodeling regulated by specific molecular pathways. Here, we dissected the role of growth-associated protein 43 (GAP-43; also known as neuromodulin and B-50) in axonal structural plasticity by using, as a model, climbing fibers. Single axonal branches were dissected by laser axotomy, avoiding collateral damage to the adjacent dendrite and the formation of a persistent glial scar. Despite the very small denervated area, the injured axons consistently reshape the connectivity with surrounding neurons. At the same time, adult climbing fibers react by sprouting new branches through the intact surroundings. Newly formed branches presented varicosities, suggesting that new axons were more than just exploratory sprouts. Correlative light and electron microscopy reveals that the sprouted branch contains large numbers of vesicles, with varicosities in the close vicinity of Purkinje dendrites. By using an RNA interference approach, we found that downregulating GAP-43 causes a significant increase in the turnover of presynaptic boutons. In addition, silencing hampers the generation of reactive sprouts. Our findings show the requirement of GAP-43 in sustaining synaptic stability and promoting the initiation of axonal regrowth.
Project description:Microtubule polarity in axons and dendrites defines the direction of intracellular transport in neurons. Axons contain arrays of uniformly polarized microtubules with plus-ends facing the tips of the processes (plus-end-out), while dendrites contain microtubules with a minus-end-out orientation. It has been shown that cytoplasmic dynein, targeted to cortical actin, removes minus-end-out microtubules from axons. Here we have identified Spindly, a protein known for recruitment of dynein to kinetochores in mitosis, as a key factor required for dynein-dependent microtubule sorting in axons of Drosophila neurons. Depletion of Spindly affects polarity of axonal microtubules in vivo and in primary neuronal cultures. In addition to these defects, depletion of Spindly in neurons causes major collapse of axonal patterning in the third-instar larval brain as well as severe coordination impairment in adult flies. These defects can be fully rescued by full-length Spindly, but not by variants with mutations in its dynein-binding site. Biochemical analysis demonstrated that Spindly binds F-actin, suggesting that Spindly serves as a link between dynein and cortical actin in axons. Therefore, Spindly plays a critical role during neurodevelopment by mediating dynein-driven sorting of axonal microtubules.
Project description:Neuronal function requires axon-dendrite membrane polarity, which depends on sorting of membrane traffic during entry into axons. Due to a microtubule network of mixed polarity, dendrites receive vesicles from the cell body without apparent capacity for directional sorting. We found that, during entry into dendrites, axonally destined cargos move with a retrograde bias toward the cell body, while dendritically destined cargos are biased in the anterograde direction. A microtubule-associated septin (SEPT9), which localizes specifically in dendrites, impedes axonal cargo of kinesin-1/KIF5 and boosts kinesin-3/KIF1 motor cargo further into dendrites. In neurons and in vitro single-molecule motility assays, SEPT9 suppresses kinesin-1/KIF5 and enhances kinesin-3/KIF1 in a manner that depends on a lysine-rich loop of the kinesin motor domain. This differential regulation impacts partitioning of neuronal membrane proteins into axons-dendrites. Thus, polarized membrane traffic requires sorting during entry into dendrites by a septin-mediated mechanism that bestows directional bias on microtubules of mixed orientation.
Project description:Neurons are highly polarized cells that extend a single axon and several dendrites. Studies with cultured neurons indicate that the proximal portion of the axon, denoted as the axon initial segment (AIS), maintains neuronal polarity in vitro. The membrane-adaptor protein ankyrinG (ankG) is an essential component of the AIS. To determine the relevance of ankG for neuronal polarity in vivo, we studied mice with a cerebellum-specific ankG deficiency. Strikingly, ankG-depleted axons develop protrusions closely resembling dendritic spines. Such axonal spines are enriched with postsynaptic proteins, including ProSAP1/Shank2 and ionotropic and metabotropic glutamate receptors. In addition, immunofluorescence indicated that axonal spines are contacted by presynaptic glutamatergic boutons. For further analysis, double mutants were obtained by crossbreeding ankG(-/-) mice with L7/Purkinje cell-specific promoter 2 (PCP2) mice expressing enhanced green fluorescent protein (EGFP) in Purkinje cells (PCs). This approach allowed precise confocal microscopic mapping of EGFP-positive spiny axons and their subsequent identification at the electron microscopic level. Ultrastructurally, axonal spines contained a typical postsynaptic density and established asymmetric excitatory synapses with presynaptic boutons containing synaptic vesicles. In the shaft of spiny axons, typical ultrastructural features of the AIS, including the membrane-associated dense undercoating and cytoplasmic bundles of microtubules, were absent. Finally, using time-lapse imaging of organotypic cerebellar slice cultures, we demonstrate that nonspiny PC axons of EGFP-positive/ankG(-/-) mice acquire a spiny phenotype within a time range of only 3 days. Collectively, these findings demonstrate that axons of ankG-deficient mice acquire hallmark features of dendrites. AnkG thus is important for maintaining appropriate axo-dendritic polarity in vivo.
Project description:Axons and dendrites differ in both microtubule organization and in the organelles and proteins they contain. Here we show that the microtubule motor dynein has a crucial role in polarized transport and in controlling the orientation of axonal microtubules in Drosophila melanogaster dendritic arborization (da) neurons. Changes in organelle distribution within the dendritic arbors of dynein mutant neurons correlate with a proximal shift in dendritic branch position. Dynein is also necessary for the dendrite-specific localization of Golgi outposts and the ion channel Pickpocket. Axonal microtubules are normally oriented uniformly plus-end-distal; however, without dynein, axons contain both plus- and minus-end distal microtubules. These data suggest that dynein is required for the distinguishing properties of the axon and dendrites: without dynein, dendritic organelles and proteins enter the axon and the axonal microtubules are no longer uniform in polarity.
Project description:This study aimed at providing the first detailed morphological description, at the single-cell level, of the rat dorsal raphe nucleus neurons, including the distribution of the VGLUT3 protein within their axons. Electrophysiological guidance procedures were used to label dorsal raphe nucleus neurons with biotinylated dextran amine. The somatodendritic and axonal arborization domains of labeled neurons were reconstructed entirely from serial sagittal sections using a computerized image analysis system. Under anaesthesia, dorsal raphe nucleus neurons display highly regular (1.72 ± 0.50 Hz) spontaneous firing patterns. They have a medium size cell body (9.8 ± 1.7 µm) with 2-4 primary dendrites mainly oriented anteroposteriorly. The ascending axons of dorsal raphe nucleus are all highly collateralized and widely distributed (total axonal length up to 18.7 cm), so that they can contact, in various combinations, forebrain structures as diverse as the striatum, the prefrontal cortex and the amygdala. Their morphological features and VGLUT3 content vary significantly according to their target sites. For example, high-resolution confocal analysis of the distribution of VGLUT3 within individually labeled-axons reveals that serotonin axon varicosities displaying VGLUT3 are larger (0.74 ± 0.03 µm) than those devoid of this protein (0.55 ± 0.03 µm). Furthermore, the percentage of axon varicosities that contain VGLUT3 is higher in the striatum (93%) than in the motor cortex (75%), suggesting that a complex trafficking mechanism of the VGLUT3 protein is at play within highly collateralized axons of the dorsal raphe nucleus neurons. Our results provide the first direct evidence that the dorsal raphe nucleus ascending projections are composed of widely distributed neuronal systems, whose capacity to co-release serotonin and glutamate varies from one forebrain locus to the other.
Project description:The formation of synaptic connections with target cells and maintenance of axons are highly regulated and crucial for neuronal function. The atypical cadherin and G-protein-coupled receptor Flamingo and its orthologs in amphibians and mammals have been shown to regulate cell polarity, dendritic and axonal growth, and neural tube closure. However, the role of Flamingo in synapse formation and function and in axonal health remains poorly understood. Here we show that fmi mutations cause a significant increase in the number of ectopic synapses on muscles and result in the formation of novel en passant synapses along axons, and unique presynaptic varicosities, including active zones, within axons. The fmi mutations also cause defective synaptic responses in a small subset of muscles, an age-dependent loss of muscle innervation and a drastic degeneration of axons in 3rd instar larvae without an apparent loss of neurons. Neuronal expression of Flamingo rescues all of these synaptic and axonal defects and larval lethality. Based on these observations, we propose that Flamingo is required in neurons for synaptic target selection, synaptogenesis, the survival of axons and synapses, and adult viability. These findings shed new light on a possible role for Flamingo in progressive neurodegenerative diseases.
Project description:Neuronal polarity relies on the selective localization of cargo to axons or dendrites. The molecular motor kinesin-1 moves cargo into axons but is also active in dendrites. This raises the question of how kinesin-1 activity is regulated to maintain the compartment-specific localization of cargo. Our in vivo structure-function analysis of endogenous Drosophila melanogaster kinesin-1 reveals a novel role for autoinhibition in enabling the dendrite-specific localization of Golgi outposts. Mutations that disrupt kinesin-1 autoinhibition result in the axonal mislocalization of Golgi outposts. Autoinhibition also regulates kinesin-1 localization. Uninhibited kinesin-1 accumulates in axons and is depleted from dendrites, correlating with the change in outpost distribution and dendrite growth defects. Genetic interaction tests show that a balance of kinesin-1 inhibition and dynein activity is necessary to localize Golgi outposts to dendrites and keep them from entering axons. Our data indicate that kinesin-1 activity is precisely regulated by autoinhibition to achieve the selective localization of dendritic cargo.