βPix Guanine Nucleotide Exchange Factor Regulates Regeneration of Injured Peripheral Axons

Axon regeneration is essential for successful recovery after peripheral nerve injury. Although growth cone reformation and axonal extension are crucial steps in axonal regeneration, the regulatory mechanisms underlying these dynamic processes are poorly understood. Here, we identify βPix (Arhgef7), the guanine nucleotide exchange factor for Rac1 GTPase, as a regulator of axonal regeneration. After sciatic nerve injury in mice, the expression levels of βPix increase significantly in nerve segments containing regenerating axons. In regrowing axons, βPix is localized in the peripheral domain of the growth cone. Using βPix neuronal isoform knockout (NIKO) mice in which the neuronal isoforms of βPix are specifically removed, we demonstrate that βPix promotes neurite outgrowth in cultured dorsal root ganglion neurons and in vivo axon regeneration after sciatic nerve crush injury. Activation of cJun and STAT3 in the cell bodies is not affected in βPix NIKO mice, supporting the local action of βPix in regenerating axons. Finally, inhibiting Src, a kinase previously identified as an activator of the βPix neuronal isoform, causes axon outgrowth defects in vitro, like those found in the βPix NIKO neurons. Altogether, these data indicate that βPix plays an important role in axonal regrowth during peripheral nerve regeneration.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

Genome-Wide Analysis of Cyclic Nucleotide-Gated Channel Genes Related to Pollen Development in Rice

In the angiosperm, pollen germinates and rapidly expands the pollen tube toward the ovule. This process is important for plant double fertilization and seed setting. It is well known that the tip-focused calcium gradient is essential for pollen germination and pollen tube growth. However, little is known about the Ca²⁺ channels that play a role in rice pollen germination and tube growth. Here, we divided the 16 cyclic nucleotide-gated channel (CNGC) genes from rice into five subgroups and found two subgroups (clades II and III) have pollen-preferential genes. Then, we performed a meta-expression analysis of all OsCNGC genes in anatomical samples and identified three pollen-preferred OsCNGCs (OsCNGC4, OsCNGC5, and OsCNGC8). The subcellular localization of these OsCNGC proteins is matched with their roles as ion channels on the plasma membrane. Unlike other OsCNGCs, these genes have a unique cis-acting element in the promoter. OsCNGC4 can act by forming a homomeric complex or a heteromeric complex with OsCNGC5 or OsCNGC8. In addition, it was suggested that they can form a multi-complex with Mildew Resistance Locus O (MLO) protein or other types of ion transporters, and that their expression can be modulated by Ruptured Pollen tube (RUPO) encoding receptor-like kinase. These results shed light on understanding the regulatory mechanisms of pollen germination and pollen tube growth through calcium channels in rice.

Electrical recordings from dendritic spines of adult mouse hippocampus and effect of the actin cytoskeleton

Dendritic spines (DS) are tiny protrusions implicated in excitatory postsynaptic responses in the CNS. To achieve their function, DS concentrate a high density of ion channels and dynamic actin networks in a tiny specialized compartment. However, to date there is no direct information on DS ionic conductances. Here, we used several experimental techniques to obtain direct electrical information from DS of the adult mouse hippocampus. First, we optimized a method to isolate DS from the dissected hippocampus. Second, we used the lipid bilayer membrane (BLM) reconstitution and patch clamping techniques and obtained heretofore unavailable electrical phenotypes on ion channels present in the DS membrane. Third, we also patch clamped DS directly in cultured adult mouse hippocampal neurons, to validate the electrical information observed with the isolated preparation. Electron microscopy and immunochemistry of PDS-95 and NMDA receptors and intrinsic actin networks confirmed the enrichment of the isolated DS preparation, showing open and closed DS, and multi-headed DS. The preparation was used to identify single channel activities and “whole-DS” electrical conductance. We identified NMDA and Ca²⁺-dependent intrinsic electrical activity in isolated DS and in situ DS of cultured adult mouse hippocampal neurons. In situ recordings in the presence of local NMDA, showed that individual DS intrinsic electrical activity often back-propagated to the dendrite from which it sprouted. The DS electrical oscillations were modulated by changes in actin cytoskeleton dynamics by addition of the F-actin disrupter agent, cytochalasin D, and exogenous actin-binding proteins. The data indicate that DS are elaborate excitable electrical devices, whose activity is a functional interplay between ion channels and the underlying actin networks. The data argue in favor of the active contribution of individual DS to the electrical activity of neurons at the level of both the membrane conductance and cytoskeletal signaling.

Perspectives on Mechanisms Supporting Neuronal Polarity From Small Animals to Humans

Axon-dendrite formation is a crucial milestone in the life history of neurons. During this process, historically referred as “the establishment of polarity,” newborn neurons undergo biochemical, morphological and functional transformations to generate the axonal and dendritic domains, which are the basis of neuronal wiring and connectivity. Since the implementation of primary cultures of rat hippocampal neurons by Gary Banker and Max Cowan in 1977, the community of neurobiologists has made significant achievements in decoding signals that trigger axo-dendritic specification. External and internal cues able to switch on/off signaling pathways controlling gene expression, protein stability, the assembly of the polarity complex (i.e., PAR3-PAR6-aPKC), cytoskeleton remodeling and vesicle trafficking contribute to shape the morphology of neurons. Currently, the culture of hippocampal neurons coexists with alternative model systems to study neuronal polarization in several species, from single-cell to whole-organisms. For instance, in vivo approaches using C. elegans and D. melanogaster, as well as in situ imaging in rodents, have refined our knowledge by incorporating new variables in the polarity equation, such as the influence of the tissue, glia-neuron interactions and three-dimensional development. Nowadays, we have the unique opportunity of studying neurons differentiated from human induced pluripotent stem cells (hiPSCs), and test hypotheses previously originated in small animals and propose new ones perhaps specific for humans. Thus, this article will attempt to review critical mechanisms controlling polarization compiled over decades, highlighting points to be considered in new experimental systems, such as hiPSC neurons and human brain organoids.

Pax6 loss alters the morphological and electrophysiological development of mouse prethalamic neurons

Pax6 is a well-known regulator of early neuroepithelial progenitor development. Its constitutive loss has a particularly strong effect on the developing prethalamus, causing it to become extremely hypoplastic. To overcome this difficulty in studying the long-term consequences of Pax6 loss for prethalamic development, we used conditional mutagenesis to delete Pax6 at the onset of neurogenesis and studied the developmental potential of the mutant prethalamic neurons in vitro. We found that Pax6 loss affected their rates of neurite elongation, the location and length of their axon initial segments, and their electrophysiological properties. Our results broaden our understanding of the long-term consequences of Pax6 deletion in the developing mouse forebrain, suggesting that it can have cell-autonomous effects on the structural and functional development of some neurons.

Summary: Pax6 impacts neurite extension, axon initial segment properties and the ability to fire normal action potentials in maturing neurons, revealing actions extending beyond those previously characterised in progenitors.

WDR72 regulates vesicle trafficking in ameloblasts

As the hardest tissue in the human body, tooth enamel formation is a highly regulated process involving several stages of differentiation and key regulatory genes. One such gene, tryptophan-aspartate repeat domain 72 (WDR72), has been found to cause a tooth enamel defect when deleted or mutated, resulting in a condition called amelogenesis imperfecta. Unlike the canonical genes regulating tooth development, WDR72 remains intracellularly and is not secreted to the enamel matrix space to regulate mineralization, and is found in other major organs of the body, namely the kidney, brain, liver, and heart. To date, a link between intracellular vesicle transport and enamel mineralization has been suggested, however identification of the mechanistic regulators has yet to be elucidated, in part due to the limitations associated with studying highly differentiated ameloblast cells. Here we show compelling evidence that WDR72 regulates endocytosis of proteins, both in vivo and in a novel in vitro ameloblast cell line. We elucidate WDR72's function to be independent of intracellular vesicle acidification while still leading to defective enamel matrix pH extracellularly. We identify a vesicle function associated with microtubule assembly and propose that WDR72 directs microtubule assembly necessary for membrane mobilization and subsequent vesicle transport. Understanding WDR72 function provides a mechanistic basis for determining physiologic and pathologic tissue mineralization.

JIP3 links lysosome transport to regulation of multiple components of the axonal cytoskeleton

Lysosome axonal transport is important for the clearance of cargoes sequestered by the endocytic and autophagic pathways. Building on observations that mutations in the JIP3 (MAPK8IP3) gene result in lysosome-filled axonal swellings, we analyzed the impact of JIP3 depletion on the cytoskeleton of human neurons. Dynamic focal lysosome accumulations were accompanied by disruption of the axonal periodic scaffold (spectrin, F-actin and myosin II) throughout each affected axon. Additionally, axonal microtubule organization was locally disrupted at each lysosome-filled swelling. This local axonal microtubule disorganization was accompanied by accumulations of both F-actin and myosin II. These results indicate that transport of axonal lysosomes is functionally interconnected with mechanisms that control the organization and maintenance of the axonal cytoskeleton. They have potential relevance to human neurological disease arising from JIP3 mutations as well as for neurodegenerative diseases associated with the focal accumulations of lysosomes within axonal swellings such as Alzheimer’s disease.

Rafiq et al. report that disruption of JIP3-dependent control of axonal lysosome transport in human neurons results in unexpected changes to the organization of multiple cytoskeletal proteins. This study provides new insights that improve our understanding of intellectual disabilities caused by mutations in JIP3, and are relevant for neurodegenerative diseases associated with accumulations of lysosomes such as the Alzheimer’s disease

Visible-light-responsive reduced graphene oxide/g-C3N4/TiO2 composite nanocoating for photoelectric stimulation of neuronal and osteoblastic differentiation

rGO/g-C3N4/TiO2 nanocoating was fabricated on Ti-based implant for photoelectric stimulation of bone and nerve repair. The ternary nanocoating exerted greater photoelectric effects on enhancing osteoblastic differentiation and neurite outgrowth.

Morin improves functional recovery after spinal cord injury in rats by enhancing axon regeneration via the Nrf2/HO-1 pathway

Spinal cord injury (SCI) is a devastating neurological occurrence that usually leads to a loss of motor and sensory function in patients. Axon regeneration has been reported to be crucial for recovery after trauma to the nervous system. Morin, a natural bioflavonoid obtained from the Moraceae family, has previously been reported to exert neuroprotective effects. In our study, we investigated the protective effects of morin on PC12 cells and primary neurons treated with oxygen-glucose deprivation (OGD) and its function in an SCI model. In vitro experiments showed that treating neuronal cells with morin enhanced axonal regeneration after OGD treatment by regulating microtubule stabilization and protecting mitochondrial function. Mechanistically, morin protected neuronal cells exposed to OGD by activating the nuclear factor (erythroid-derived 2)-like 2 (Nrf2)/heme oxygenase 1 (HO-1) pathway. An in vivo study illustrated that oral morin administration improved microtubule stability and promoted axon regeneration in SCI rats. Taken together, this study showed that treatment with morin improves functional recovery after SCI and that morin may serve as a potential agent for treating SCI.

Quantitative mapping of dense microtubule arrays in mammalian neurons

The neuronal microtubule cytoskeleton underlies the polarization and proper functioning of neurons, amongst others by providing tracks for motor proteins that drive intracellular transport. Different subsets of neuronal microtubules, varying in composition, stability, and motor preference, are known to exist, but the high density of microtubules has so far precluded mapping their relative abundance and three-dimensional organization. Here, we use different super-resolution techniques (STED, Expansion Microscopy) to explore the nanoscale organization of the neuronal microtubule network in rat hippocampal neurons. This revealed that in dendrites acetylated microtubules are enriched in the core of the dendritic shaft, while tyrosinated microtubules are enriched near the plasma membrane, thus forming a shell around the acetylated microtubules. Moreover, using a novel analysis pipeline we quantified the absolute number of acetylated and tyrosinated microtubules within dendrites and found that they account for 65-75% and ~20-30% of all microtubules, respectively, leaving only few microtubules that do not fall in either category. Because these different microtubule subtypes facilitate different motor proteins, these novel insights help to understand the spatial regulation of intracellular transport.

The Histone Methyltransferase G9a Controls Axon Growth by Targeting the RhoA Signaling Pathway

The generation of axonal and dendritic domains is critical for brain circuitry assembly and physiology. Negative players, such as the RhoA-Rho coiled-coil-associated protein kinase (ROCK) signaling pathway, restrain axon development and polarization. Surprisingly, the genetic control of neuronal polarity has remained largely unexplored. Here, we report that, in primary cultured neurons, expression of the histone methyltransferase G9a and nuclear translocation of its major splicing isoform (G9a/E10+) peak at the time of axon formation. RNAi suppression of G9a/E10+ or pharmacological blockade of G9a constrains neuronal migration, axon initiation, and the establishment of neuronal polarity in situ and in vitro. Inhibition of G9a function upregulates RhoA-ROCK activity by increasing the expression of Lfc, a guanine nucleotide exchange factor (GEF) for RhoA. Together, these results identify G9a as a player in neuronal polarization.

TTC3-Mediated Protein Quality Control, A Potential Mechanism for Cognitive Impairment

The tetrapeptide repeat domain 3 (TTC3) gene falls within Down's syndrome (DS) critical region. Cognitive impairment is a common phenotype of DS and Alzheimer’s disease (AD), and overexpression of TTC3 can accelerate cognitive decline, but the specific mechanism is unknown. The TTC3-mediated protein quality control (PQC) mechanism, similar to the PQC system, is divided into three parts: it acts as a cochaperone to assist proteins in folding correctly; it acts as an E3 ubiquitin ligase (E3s) involved in protein degradation processes through the ubiquitin–proteasome system (UPS); and it may also eventually cause autophagy by affecting mitochondrial function. Thus, this article reviews the research progress on the structure, function, and metabolism of TTC3, including the recent research progress on TTC3 in DS and AD; the role of TTC3 in cognitive impairment through PQC in combination with the abovementioned attributes of TTC3; and the potential targets of TTC3 in the treatment of such diseases.

SUMOylation of α-tubulin is a novel modification regulating microtubule dynamics

Microtubules (MTs) are regulated by a number of known posttranslational modifications (PTMs) on α/β-tubulin to fulfill diverse cellular functions. Here, we showed that SUMOylation is a novel PTM on α-tubulin in vivo and in vitro. The SUMOylation on α-tubulin mainly occurred at Lys 96 (K96), K166, and K304 of soluble α-tubulin and could be removed by small ubiquitin-related modifier (SUMO)-specific peptidase 1. In vitro experiments showed that tubulin SUMOylation could reduce interprotofilament interaction, promote MT catastrophe, and impede MT polymerization. In cells, mutation of the SUMOylation sites on α-tubulin reduced catastrophe frequency and increased the proportion of polymerized α-tubulin, while upregulation of SUMOylation with fusion of SUMO1 reduced α-tubulin assembly into MTs. Additionally, overexpression of SUMOylation-deficient α-tubulin attenuated the neurite extension in Neuro-2a cells. Thus, SUMOylation on α-tubulin represents a new player in the regulation of MT properties.

New insights on epigenetic mechanisms supporting axonal development: histone marks and miRNAs

Mechanisms supporting axon growth and the establishment of neuronal polarity have remained largely disconnected from their genetic and epigenetic fundamentals. Recently, post-transcriptional modifications of histones involved in chromatin folding and transcription, and microRNAs controlling translation have emerged as regulators of axonal specification, growth, and guidance. In this article, we review novel evidence supporting the concept that epigenetic mechanisms work at both transcriptional and post-transcriptional levels to shape axons. We also discuss the role of splicing on axonal growth, as one of the most (if not the most) powerful post-transcriptional mechanism to diversify genetic information. Overall, we think exploring the gap between epigenetics and axonal growth raises new questions and perspectives to the development of axons in physiological and pathological contexts.

Tubulin modifying enzymes as target for the treatment oftau-related diseases

In the brain of patients with Alzheimer's disease (AD), the number and length of microtubules (MTs) are significantly and selectively reduced. MTs are involved in a wide range of cellular functions, and defects of the microtubular system have emerged as a unifying hypothesis for the heterogeneous and variable clinical presentations of AD. MTs orchestrate their numerous functions through the spatiotemporal regulation of the binding of specialised microtubule-associated proteins (MAPs) and molecular motors. Covalent posttranslational modifications (PTMs) on the tubulin C-termini that protrude at the surface of MTs regulate the binding of these effectors. In neurons, MAP tau is highly abundant and its abnormal dissociation from MTs in the axon, cellular mislocalization and hyperphosphorylation, are primary events leading to neuronal death. Consequently, compounds targeting tau phosphorylation or aggregation are currently evaluated but their clinical significance has not been demonstrated yet. In this review, we discuss the emerging link between tubulin PTMs and tau dysfunction. In neurons, high levels of glutamylation and detyrosination profoundly impact the physicochemical properties at the surface of MTs. Moreover, in patients with early-onset progressive neurodegeneration, deleterious mutations in enzymes involved in modifying MTs at the surface have recently been identified, underscoring the importance of this enzymatic machinery in neurology. We postulate that pharmacologically targeting the tubulin-modifying enzymes holds promise as therapeutic approach for the treatment of neurodegenerative diseases.

Localization and Expression of Sirtuins 1, 2, 6 and Plasticity-Related Proteins in the Recovery Period after a Photothrombotic Stroke in Mice

Sirtuins, class III histone deacetylases, are involved in the regulation of tissue repair processes and brain functions after a stroke. The ability of some isoforms of sirtuins to circulate between the nucleus and cytoplasm may have various pathophysiological effects on the cells. In present work, we focused on the role of non-mitochondrial sirtuins SIRT1, SIRT2, and SIRT6 in the restoration of brain cells following ischemic stroke. Here, using a photothrombotic stroke (PTS) model in mice, we studied whether local stroke affects the level and intracellular localization of SIRT1, SIRT2, and SIRT6 in neurons and astrocytes of the intact cerebral cortex adjacent to the ischemic ipsilateral hemisphere and in the analogous region of the contralateral hemisphere at different time points during the recovery period after a stroke. We evaluated the co-localization of sirtuins with growth-associated protein-43 (GAP-43), the presynaptic marker synaptophysin (SYN) and acetylated α-tubulin (Ac-α-Tub), that are associated with brain plasticity and are known to be involved in brain repair after a stroke. The results show that during the recovery period, an increase in SIRT1 and SIRT2 levels occurred. The increase of SIRT1 level was associated with an increase in synaptic plasticity proteins, whereas the increase of SIRT2 level was associated with an acetylated of α-tubulin, that can reduce the mobility of neurites. SIRT6 co-localized with GAP-43, but not with SYN. Moreover, we showed that SIRT1, SIRT2, and SIRT6 are not involved in the PTS-induced apoptosis of penumbra cells. Taken together, our results suggest that sirtuins functions differ depending on cell type, intracellular localization, specificity of sirtuins isoforms to different substrates and nature of post-translational modifications of enzymes.

Interaction Between CRIPT and PSD-95 Is Required for Proper Dendritic Arborization in Hippocampal Neurons

CRIPT, the cysteine-rich PDZ-binding protein, binds to the third PDZ domain of PSD-95 (postsynaptic density protein 95) family proteins and directly binds microtubules, linking PSD-95 family proteins to the neuronal cytoskeleton. Here, we show that overexpression of a full-length CRIPT leads to a modest decrease, and knockdown of CRIPT leads to an increase in dendritic branching in cultured rat hippocampal neurons. Overexpression of truncated CRIPT lacking the PDZ domain-binding motif, which does not bind to PSD-95, significantly decreases dendritic arborization. Conversely, overexpression of a full-length CRIPT significantly increases the number of immature and mature dendritic spines, and this effect is not observed when CRIPT∆PDZ is overexpressed. Competitive inhibition of CRIPT binding to the third PDZ domain of PSD-95 with PDZ3-binding peptides resulted in differential effects on dendritic arborization based on the origin of respective peptide sequence. These results highlight multifunctional roles of CRIPT during development and underscore the significance of the interaction between CRIPT and the third PDZ domain of PSD-95.

The cytoskeleton as a modulator of tension driven axon elongation

Throughout development, neurons are capable of integrating external and internal signals leading to the morphological changes required for neuronal polarization and axon growth. The first phase of axon elongation occurs during neuronal polarization. At this stage, membrane remodeling and cytoskeleton dynamics are crucial for the growth cone to advance and guide axon elongation. When a target is recognized, the growth cone collapses to form the presynaptic terminal. Once a synapse is established, the growth of the organism results in an increased distance between the neuronal cell bodies and their targets. In this second phase of axon elongation, growth cone-independent molecular mechanisms and cytoskeleton changes must occur to enable axon growth to accompany the increase in body size. While the field has mainly focused on growth-cone mediated axon elongation during development, tension driven axon growth remains largely unexplored. In this review, we will discuss in a critical perspective the current knowledge on the mechanisms guiding axon growth following synaptogenesis, with a particular focus on the putative role played by the axonal cytoskeleton.

Advances in the neurophysiology of magnocellular neuroendocrine cells

Hypothalamic magnocellular neuroendocrine cells have unique electrical properties and a remarkable capacity for morphological and synaptic plasticity. Their large somatic size, their relatively uniform and dense clustering in the supraoptic and paraventricular nuclei, and their large axon terminals in the neurohypophysis make them an attractive target for direct electrophysiological interrogation. Here, we provide a brief review of significant recent findings in the neuroplasticity and neurophysiological properties of these neurones that were presented at the symposium "Electrophysiology of Magnocellular Neurons" during the 13th World Congress on Neurohypophysial Hormones in Ein Gedi, Israel in April 2019. Magnocellular vasopressin (VP) neurones respond directly to hypertonic stimulation with membrane depolarisation, which is triggered by cell shrinkage-induced opening of an N-terminal-truncated variant of transient receptor potential vanilloid type-1 (TRPV1) channels. New findings indicate that this mechanotransduction depends on actin and microtubule cytoskeletal networks, and that direct coupling of the TRPV1 channels to microtubules is responsible for mechanical gating of the channels. Vasopressin neurones also respond to osmostimulation by activation of epithelial Na⁺ channels (ENaC). It was shown recently that changes in ENaC activity modulate magnocellular neurone basal firing by generating tonic changes in membrane potential. Both oxytocin and VP neurones also undergo robust excitatory synapse plasticity during chronic osmotic stimulation. Recent findings indicate that new glutamate synapses induced during chronic salt loading express highly labile Ca²⁺ -permeable GluA1 receptors requiring continuous dendritic protein synthesis for synapse maintenance. Finally, recordings from the uniquely tractable neurohypophysial terminals recently revealed an unexpected property of activity-dependent neuropeptide release. A significant fraction of the voltage-dependent neurohypophysial neurosecretion was found to be independent of Ca²⁺ influx through voltage-gated Ca²⁺ channels. Together, these findings provide a snapshot of significant new advances in the electrophysiological signalling mechanisms and neuroplasticity of the hypothalamic-neurohypophysial system, a system that continues to make important contributions to the field of neurophysiology.

A Combinatorial MAP Code Dictates Polarized Microtubule Transport

Many eukaryotic cells distribute their intracellular components asymmetrically through regulated active transport driven by molecular motors along microtubule tracks. While intrinsic and extrinsic regulation of motor activity exists, what governs the overall distribution of activated motor-cargo complexes within cells remains unclear. Here, we utilize in vitro reconstitution of purified motor proteins and non-enzymatic microtubule-associated proteins (MAPs) to demonstrate that MAPs exhibit distinct influences on the motility of the three main classes of transport motors: kinesin-1, kinesin-3, and cytoplasmic dynein. Further, we dissect how combinations of MAPs affect motors and unveil MAP9 as a positive modulator of kinesin-3 motility. From these data, we propose a general "MAP code" that has the capacity to strongly bias directed movement along microtubules and helps elucidate the intricate intracellular sorting observed in highly polarized cells such as neurons.

Locally Activating TrkB Receptor Generates Actin Waves and Specifies Axonal Fate

Actin waves are filamentous actin (F-actin)-rich structures that initiate in the somato-neuritic area and move toward neurite ends. The upstream cues that initiate actin waves are poorly understood. Here, using an optogenetic approach (Opto-cytTrkB), we found that local activation of the TrkB receptor around the neurite end initiates actin waves and triggers neurite elongation. During actin wave generation, locally activated TrkB signaling in the distal neurite was functionally connected with preferentially localized Rac1 and its signaling pathways in the proximal region. Moreover, TrkB activity changed the location of ankyrinG--the master organizer of the axonal initial segment-and initiated the stimulated neurite to acquire axonal characteristics. Taken together, these findings suggest that local Opto-cytTrkB activation switches the fate from minor to major axonal neurite during neuronal polarization by generating actin waves.

Correct Laminar Positioning in the Neocortex Influences Proper Dendritic and Synaptic Development

The neocortex is a 6-layered laminated structure with a precise anatomical and functional organization ensuring proper function. Laminar positioning of cortical neurons, as determined by termination of neuronal migration, is a key determinant of their ability to assemble into functional circuits. However, the exact contribution of laminar placement to dendrite morphogenesis and synapse formation remains unclear. Here we manipulated the laminar position of cortical neurons by knocking down doublecortin (Dcx), a crucial effector of migration, and show that misplaced neurons fail to properly form dendrites, spines, and functional glutamatergic and GABAergic synapses. We further show that knocking down Dcx in properly positioned neurons induces similar but milder defects, suggesting that the laminar misplacement is the primary cause of altered neuronal development. Thus, the specific laminar environment of their fated layers is crucial for the maturation of cortical neurons, and influences their functional integration into developing cortical circuits.

Disruption of SynGAP–dopamine D1 receptor complexes alters actin and microtubule dynamics and impairs GABAergic interneuron migration

Disruption of γ-aminobutyric acid (GABA)–ergic interneuron migration is implicated in various neurodevelopmental disorders, including autism spectrum disorder and schizophrenia. The dopamine D1 receptor (D1R) promotes GABAergic interneuron migration, which is disrupted in various neurological disorders, some of which are also associated with mutations in the gene encoding synaptic Ras–guanosine triphosphatase–activating protein (SynGAP). Here, we explored the mechanisms underlying these associations and their possible connection. In prenatal mouse brain tissue, we found a previously unknown interaction between the D1R and SynGAP. This D1R-SynGAP interaction facilitated D1R localization to the plasma membrane and promoted D1R-mediated downstream signaling pathways, including phosphorylation of protein kinase A and p38 mitogen-activated protein kinase. These effects were blocked by a peptide (TAT-D1Rpep) that disrupted the D1R-SynGAP interaction. Furthermore, disrupting this complex in mice during embryonic development resulted in pronounced and selective deficits in the tangential migration of GABAergic interneurons, possibly due to altered actin and microtubule dynamics. Our results provide insights into the molecular mechanisms regulating interneuron development and suggest that disruption of the D1R-SynGAP interaction may underlie SYNGAP1 mutation–related neurodevelopmental disorders.

Asymmetric confinement for defining outgrowth directionality

Directional connectivity is required to develop accurate in vitro models of the nervous system. This research investigated the interaction of murine neuronal outgrowths with asymmetric microstructured geometries to provide insights into the mechanisms governing unidirectional outgrowth bias. The structures were designed using edge-guidance and critical turning angle principles to study different prohibitive to permissive edge-guidance ratios. The different structures enable outgrowth in the permissive direction, while reducing outgrowth in the prohibitive direction. Outgrowth bias was probabilistic in nature, requiring multiple structures for effective unidirectional bias in primary hippocampal cultures at 14 days in vitro. Arrowhead structures with acute posterior corners were optimal, enabling 100% unidirectional outgrowth bias by virtue of re-routing and delay effects.

Loureirin B Promotes Axon Regeneration by Inhibiting Endoplasmic Reticulum Stress: Induced Mitochondrial Dysfunction and Regulating the Akt/GSK-3β Pathway after Spinal Cord Injury

Axon retraction greatly limits functional recovery after spinal cord injury (SCI) and neuron polarization, which affects processes including axon formation and development, is a promising target for promoting axon regeneration. Increasing microtubule stability has been demonstrated to improve intrinsic axon regeneration processes and is critically related to endoplasmic reticulum (ER)-mitochondria interactions. We used real-time polymerase chain reaction, Western blotting, and immunofluorescence to screen a variety of natural compounds, and found that Loureirin B (LrB) effectively promoted neuron polarization and axon regeneration in vitro and in vivo. LrB significantly inhibited ER stress and thereby promoted mitochondrial functions by regulating mitochondrial fusion. Further, LrB reactivated the Akt/GSK-3β pathway, which plays critical roles in cell survival and microtubule stabilization. Taken together, our results suggest that the effects of LrB on neuron regeneration involve the inhibition of ER stress-induced mitochondrial dysfunction and activation of the Akt/GSK-3β pathway, which further promotes microtubule stabilization. LrB may therefore be a promising candidate for facilitating recovery following SCI.

Calotropis gigantea Promotes Neuritogenesis and Synaptogenesis through Activation of NGF-TrkA-Erk1/2 Signaling in Rat Hippocampal Neurons

Calotropis gigantea (L.) R. Br (Apocynaceae) (commonly known as milkweed or crown flower) is a large shrub native to temperate regions of Asia, including China, Bangladesh and India and has a long history of use in traditional medicines. In this study, we investigated the neuromodulatory effects of the ethanol extracts of C. gigantea leaves (CGE) during synaptogenesis in the late stage of neuronal development and during early stage neuritogenesis in cultured rat hippocampal neurons. Maximum neuritogenic activity was achieved at a CGE concentration of 7.5[Formula: see text][Formula: see text]g/ml. At this concentration, CGE facilitated the early development of cytoarchitecture, as evidenced by increases in morphometric parameters, such as, the numbers, lengths, and number of branches of initial neurites, axon and dendrites. During the synaptogenic stage (DIV 14), immunocytochemistry (ICC) showed that CGE upregulated synaptic vesicle 2 (SV2, a marker of axon terminals) and postsynaptic density-95 (PSD-95, a postsynaptic marker) and their colocalization. CGE upregulated nerve growth factor (NGF) and activated extracellular signal-regulated kinase 1/2 (Erk1/2), which is blocked by a TrkA-specific inhibitor suggesting the neuritogenic and synaptogenic potential of CGE was due to the activation of NGF-TrkA-Erk1/2 signaling. Moreover, UPLC of CGE did not detect stigmasterol, an active component of C. gigantea. However, the chloroform-methanol and ethyl acetate subfractions of CGE exhibited initial neuritogenic activity, suggesting that multiple active components were responsible for the neurotrophic-mimetic properties of CGE. Our data prove the neuromodulatory ability of CGE and provide a means of identifying new active phytochemicals with potential nootropic, preventative or therapeutic effects on the human brain.

Tubulin post-translational modifications in developing dog primary neurons obtained with methods according to the 3Rs principles

Microtubules play a crucial role during neuronal morphogenesis regulating many functions. In the study of these phenomena in vitro cellular models have been employed, mainly resorting to housed experimental animals. Among alternative models in neurobiological study, recently dog caught particular attention. In fact, the complexity of the canine brain, the life long span and the neurodegenerative pathologies render the dog a species more close to humans than rodents. Lately, growing interest in the limitation of the use of experimental animals, has stimulated the search for alternative experimental protocols. Starting from fetal dog brain, obtained by alternative way of sampling, we set neuronal primary cultures. Through immunofluorescence, we examined the presence and the cellular distribution of tubulin post-translational modifications as tyrosinated and acetylated α-tubulin, as markers of dynamic and stable microtubule respectively. In addition, we evaluated the pattern of two associated proteins which may slide on these two tubulin modifications, i.e. CLIP-170 and Kinesin-1. A clear positivity for tyrosinated and acetylated α-tubulin, was found. As far as the motor proteins are concerned, we detected a prevalence of CLIP-170 compared to kinesin-1 with a better overlapping between tyrosinated α-tubulin and CLIP-170. Our findings highlighted some original data about the role of the microtubular network during early phases of canine neuronal morphogenesis. In addition, the experimental protocol underlined the utility of this alternative model that allows to bypass both the scarcity of commercial canine neuronal cell lines and the need to resort to experimental dogs, respecting the 3Rs principles (reduction, refinement, and replacement).

Graphene oxide-enhanced cytoskeleton imaging and mitosis tracking

Here we show that graphene oxide greatly enhances the imaging ability of a peptide probe that selectively targets microtubules of the cytoskeleton, thus enabling the dynamic tracking of mitosis in live cells.

The Role of the Microtubule Cytoskeleton in Neurodevelopmental Disorders

Neurons depend on the highly dynamic microtubule (MT) cytoskeleton for many different processes during early embryonic development including cell division and migration, intracellular trafficking and signal transduction, as well as proper axon guidance and synapse formation. The coordination and support from MTs is crucial for newly formed neurons to migrate appropriately in order to establish neural connections. Once connections are made, MTs provide structural integrity and support to maintain neural connectivity throughout development. Abnormalities in neural migration and connectivity due to genetic mutations of MT-associated proteins can lead to detrimental developmental defects. Growing evidence suggests that these mutations are associated with many different neurodevelopmental disorders, including intellectual disabilities (ID) and autism spectrum disorders (ASD). In this review article, we highlight the crucial role of the MT cytoskeleton in the context of neurodevelopment and summarize genetic mutations of various MT related proteins that may underlie or contribute to neurodevelopmental disorders.

Crafting of Neuroprotective Octapeptide from Taxol-Binding Pocket of β-Tubulin

Microtubules play a crucial role in maintaining the shape and function of neurons. During progression of Alzheimer's disease (AD), severe destabilization of microtubules occurs, which leads to the permanent disruption of signal transduction processes and memory loss. Thus, microtubule stabilization is one of the key requirements for the treatment of AD. Taxol, a microtubule stabilizing anticancer drug, has been considered as a potential anti-AD drug but was never tested in AD patients, likely because of its' toxic nature and poor brain exposure. However, other microtubule-targeting agents such as epothilone D (BMS-241027) and TPI-287 (abeotaxane) and NAP peptide (davunetide) have entered in AD clinical programs. Therefore, the taxol binding pocket of tubulin could be a potential site for designing of mild and noncytotoxic microtubule stabilizing molecules. Here, we adopted an innovative strategy for the development of a peptide based microtubule stabilizer, considering the taxol binding pocket of β-tubulin, by using alanine scanning mutagenesis technique. This approach lead us to a potential octapeptide, which strongly binds to the taxol pocket of β-tubulin, serves as an excellent microtubule stabilizer, increases the expression of acetylated tubulin, and acts as an Aβ aggregation inhibitor and neuroprotective agent. Further, results revealed that this peptide is nontoxic against both PC12 derived neurons and primary cortical neurons. We believe that our strategy and discovery of peptide-based microtubule stabilizer will open the door for the development of potential anti-AD therapeutics in near future.

Identification and characterization of the BRI2 interactome in the brain

BRI family proteins are ubiquitous type II transmembrane proteins but BRI2 is highly expressed in some neuronal tissues. Possible BRI2 functions include neuronal maturation and differentiation. Protein complexes appear to be important in mediating its functions. Previously described BRI2 interactors include the Alzheimer's amyloid precursor protein and protein phosphatase 1, but clearly the identification of novel interactors provides an important tool to understand the role and function of BRI2. To this end three rat brain regions (cerebellum, hippocampus, and cerebral cortex) were processed by BRI2 immunoprecipitation; co-precipitating proteins were identified by Nano-HPLC-MS/MS. The pool of the brain regions resulted in 511 BRI2 interacting proteins (BRI2 brain interactome) of which 120 were brain specific and 49 involved in neuronal differentiation. Brain region-specific analyses were also carried out for cerebellum, hippocampus, and cerebral cortex. Several novel BRI2 interactors were identified among them DLG4/PSD-95, which is singularly important as it places BRI2 in the postsynaptic compartment. This interaction was validated as well as the interaction with GAP-43 and synaptophysin. In essence, the resulting BRI2 brain interactome, associates this protein with neurite outgrowth and neuronal differentiation, as well as synaptic signalling and plasticity. It follows that further studies should address BRI2 particularly given its relevance to neuropathological conditions.

Differentiation between Oppositely Oriented Microtubules Controls Polarized Neuronal Transport

Microtubules are essential for polarized transport in neurons, but how their organization guides motor proteins to axons or dendrites is unclear. Because different motors recognize distinct microtubule properties, we used optical nanoscopy to examine the relationship between microtubule orientations, stability, and modifications. Nanometric tracking of motors to super-resolve microtubules and determine their polarity revealed that in dendrites, stable and acetylated microtubules are mostly oriented minus-end out, while dynamic and tyrosinated microtubules are oriented oppositely. In addition, microtubules with similar orientations and modifications form bundles that bias transport. Importantly, because the plus-end-directed Kinesin-1 selectively interacts with acetylated microtubules, this organization guides this motor out of dendrites and into axons. In contrast, Kinesin-3 prefers tyrosinated microtubules and can enter both axons and dendrites. This separation of distinct microtubule subsets into oppositely oriented bundles constitutes a key architectural principle of the neuronal microtubule cytoskeleton that enables polarized sorting by different motor proteins.

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Motor-based nanoscopy enables direct observation of microtubule (MT) polarity

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In neurons, MTs organize into polarized bundles that locally bias transport

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In dendrites, bundles of opposite orientation differ in stability and composition

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Dendritic MTs bias Kinesin-1 transport toward the soma, ensuring axon selectivity

A Feed-Forward Mechanism Involving the NOX Complex and RyR-Mediated Ca2+ Release During Axonal Specification

Physiological levels of ROS support neurite outgrowth and axonal specification, but the mechanisms by which ROS are able to shape neurons remain unknown. Ca²⁺, a broad intracellular second messenger, promotes both Rac1 activation and neurite extension. Ca²⁺ release from the endoplasmic reticulum, mediated by both the IP3R1 and ryanodine receptor (RyR) channels, requires physiological ROS levels that are mainly sustained by the NADPH oxidase (NOX) complex. In this work, we explore the contribution of the link between NOX and RyR-mediated Ca²⁺ release toward axonal specification of rat hippocampal neurons. Using genetic approaches, we find that NOX activation promotes both axonal development and Rac1 activation through a RyR-mediated mechanism, which in turn activates NOX through Rac1, one of the NOX subunits. Collectively, these data suggest a feedforward mechanism that integrates both NOX activity and RyR-mediated Ca²⁺ release to support cellular mechanisms involved in axon development.

SIGNIFICANCE STATEMENT

High levels of ROS are frequently associated with oxidative stress and disease. In contrast, physiological levels of ROS, mainly sustained by the NADPH oxidase (NOX) complex, promote neuronal development and axonal growth. However, the mechanisms by which ROS shape neurons have not been described. Our work suggests that NOX-derived ROS promote axonal growth by regulating Rac1 activity, a molecular determinant of axonal growth, through a ryanodine receptor (RyR)-mediated Ca²⁺ release mechanism. In addition, Rac1, one of the NOX subunits, was activated after RyR-mediated Ca²⁺ release, suggesting a feedforward mechanism between NOX and RyR. Collectively, our data suggest a novel mechanism that is instrumental in sustaining physiological levels of ROS required for axonal growth of hippocampal neurons.

Advances in understanding the role of disease-associated proteins in spinal muscular atrophy

INTRODUCTION

Spinal muscular atrophy (SMA) is a neurodegenerative disorder characterized by alpha motor neuron loss in the spinal cord due to reduced survival motor neuron (SMN) protein level. While the genetic basis of SMA is well described, the specific molecular pathway underlying SMA is still not fully understood. Areas covered: This review discusses the recent advancements in understanding the molecular pathways in SMA using different omics approaches and genetic modifiers identified in both vertebrate and invertebrate systems. The findings that are summarized in this article were deduced from original articles and reviews with a particular focus on the latest advancements in the field. Expert commentary: The identification of genetic modifiers such as PLS3 and NCALD in humans or of SMA modulators such as Elavl4 (HuD), Copa, Uba1, Mapk10 (Jnk3), Nrxn2 and Tmem41b (Stasimon) in various SMA animal models improved our knowledge of impaired cellular pathways in SMA. Inspiration from modifier genes and their functions in motor neuron and neuromuscular junctions may open a new avenue for future SMA combinatorial therapies.

Mechanosensing in hypothalamic osmosensory neurons

Osmosensory neurons are specialized cells activated by increases in blood osmolality to trigger thirst, secretion of the antidiuretic hormone vasopressin, and elevated sympathetic tone during dehydration. In addition to multiple extrinsic factors modulating their activity, osmosensory neurons are intrinsically osmosensitive, as they are activated by increased osmolality in the absence of neighboring cells or synaptic contacts. This intrinsic osmosensitivity is a mechanical process associated with osmolality-induced changes in cell volume. This review summarises recent findings revealing molecular mechanisms underlying the mechanical activation of osmosensory neurons and highlighting important roles of microtubules, actin, and mechanosensitive ion channels in this process.

Modulating motility of intracellular vesicles in cortical neurons with nanomagnetic forces on-chip

Vesicle transport is a major underlying mechanism of cell communication. Inhibiting vesicle transport in brain cells results in blockage of neuronal signals, even in intact neuronal networks. Modulating intracellular vesicle transport can have a huge impact on the development of new neurotherapeutic concepts, but only if we can specifically interfere with intracellular transport patterns. Here, we propose to modulate motion of intracellular lipid vesicles in rat cortical neurons based on exogenously bioconjugated and cell internalized superparamagnetic iron oxide nanoparticles (SPIONs) within microengineered magnetic gradients on-chip. Upon application of 6-126 pN on intracellular vesicles in neuronal cells, we explored how the magnetic force stimulus impacts the motion pattern of vesicles at various intracellular locations without modulating the entire cell morphology. Altering vesicle dynamics was quantified using, mean square displacement, a caging diameter and the total traveled distance. We observed a de-acceleration of intercellular vesicle motility, while applying nanomagnetic forces to cultured neurons with SPIONs, which can be explained by a decrease in motility due to opposing magnetic force direction. Ultimately, using nanomagnetic forces inside neurons may permit us to stop the mis-sorting of intracellular organelles, proteins and cell signals, which have been associated with cellular dysfunction. Furthermore, nanomagnetic force applications will allow us to wirelessly guide axons and dendrites by exogenously using permanent magnetic field gradients.

Ran-dependent TPX2 activation promotes acentrosomal microtubule nucleation in neurons

The microtubule (MT) cytoskeleton is essential for the formation of morphologically appropriate neurons. The existence of the acentrosomal MT organizing center in neurons has been proposed but its identity remained elusive. Here we provide evidence showing that TPX2 is an important component of this acentrosomal MT organizing center. First, neurite elongation is compromised in TPX2-depleted neurons. In addition, TPX2 localizes to the centrosome and along the neurite shaft bound to MTs. Depleting TPX2 decreases MT formation frequency specifically at the tip and the base of the neurite, and these correlate precisely with the regions where active GTP-bound Ran proteins are enriched. Furthermore, overexpressing the downstream effector of Ran, importin, compromises MT formation and neuronal morphogenesis. Finally, applying a Ran-importin signaling interfering compound phenocopies the effect of TPX2 depletion on MT dynamics. Together, these data suggest a model in which Ran-dependent TPX2 activation promotes acentrosomal MT nucleation in neurons.

Phosphatidylinositol 3,4-bisphosphate regulates neurite initiation and dendrite morphogenesis via actin aggregation

Neurite initiation is critical for neuronal morphogenesis and early neural circuit development. Recent studies showed that local actin aggregation underneath the cell membrane determined the site of neurite initiation. An immediately arising question is what signaling mechanism initiated actin aggregation. Here we demonstrate that local clustering of phosphatidylinositol 3,4-bisphosphate (PI(3,4)P₂), a phospholipid with relatively few known signaling functions, is necessary and sufficient for aggregating actin and promoting neuritogenesis. In contrast, the related and more extensively studied phosphatidylinositol 4,5-bisphosphate or phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) molecules did not have such functions. Specifically, we showed that beads coated with PI(3,4)P₂ promoted actin aggregation and neurite initiation, while pharmacological interference with PI(3,4)P₂ synthesis inhibited both processes. PI(3,4)P₂ clustering occurred even when actin aggregation was pharmacologically blocked, demonstrating that PI(3,4)P₂ functioned as the upstream signaling molecule. Two enzymes critical for PI(3,4)P₂ generation, namely, SH2 domain-containing inositol 5-phosphatase and class II phosphoinositide 3-kinase α, were complementarily and non-redundantly required for actin aggregation and neuritogenesis, as well as for subsequent dendritogenesis. Finally, we demonstrate that neural Wiskott-Aldrich syndrome protein and the Arp2/3 complex functioned downstream of PI(3,4)P₂ to mediate neuritogenesis and dendritogenesis. Together, our results identify PI(3,4)P₂ as an important signaling molecule during early development and demonstrate its critical role in regulating actin aggregation and neuritogenesis.

Hybrid Thin Film Organosilica Sol-Gel Coatings To Support Neuronal Growth and Limit Astrocyte Growth

Thin films of silica prepared by a sol-gel process are becoming a feasible coating option for surface modification of implantable neural sensors without imposing adverse effects on the devices' electrical properties. In order to advance the application of such silica-based coatings in the context of neural interfacing, the characteristics of silica sol-gel are further tailored to gain active control of interactions between cells and the coating materials. By incorporating various readily available organotrialkoxysilanes carrying distinct organic functional groups during the sol-gel process, a library of hybrid organosilica coatings is developed and investigated. In vitro neural cultures using PC12 cells and primary cortical neurons both reveal that, among these different types of hybrid organosilica, the introduction of aminopropyl groups drastically transforms the silica into robust neural permissive substrate, supporting neuron adhesion and neurite outgrowth. Moreover, when this organosilica is cultured with astrocytes, a key type of glial cells responsible for glial scar response toward neural implants, such cell growth promoting effect is not observed. These findings highlight the potential of organo-group-bearing silica sol-gel to function as advanced coating materials to selectively modulate cell response and promote neural integration with implantable sensing devices.

Back to the tubule: microtubule dynamics in Parkinson's disease

Cytoskeletal homeostasis is essential for the development, survival and maintenance of an efficient nervous system. Microtubules are highly dynamic polymers important for neuronal growth, morphology, migration and polarity. In cooperation with several classes of binding proteins, microtubules regulate long-distance intracellular cargo trafficking along axons and dendrites. The importance of a delicate interplay between cytoskeletal components is reflected in several human neurodegenerative disorders linked to abnormal microtubule dynamics, including Parkinson’s disease (PD). Mounting evidence now suggests PD pathogenesis might be underlined by early cytoskeletal dysfunction. Advances in genetics have identified PD-associated mutations and variants in genes encoding various proteins affecting microtubule function including the microtubule-associated protein tau. In this review, we highlight the role of microtubules, their major posttranslational modifications and microtubule associated proteins in neuronal function. We then present key evidence on the contribution of microtubule dysfunction to PD. Finally, we discuss how regulation of microtubule dynamics with microtubule-targeting agents and deacetylase inhibitors represents a promising strategy for innovative therapeutic development.

Membrane lipid rafts and neurobiology: age-related changes in membrane lipids and loss of neuronal function

A better understanding of the cellular physiological role that plasma membrane lipids, fatty acids and sterols play in various cellular systems may yield more insight into how cellular and whole organ function is altered during the ageing process. Membrane lipid rafts (MLRs) within the plasma membrane of most cells serve as key organizers of intracellular signalling and tethering points of cytoskeletal components. MLRs are plasmalemmal microdomains enriched in sphingolipids, cholesterol and scaffolding proteins; they serve as a platform for signal transduction, cytoskeletal organization and vesicular trafficking. Within MLRs are the scaffolding and cholesterol binding proteins named caveolin (Cav). Cavs not only organize a multitude of receptors including neurotransmitter receptors (NMDA and AMPA receptors), signalling proteins that regulate the production of cAMP (G protein-coupled receptors, adenylyl cyclases, phosphodiesterases (PDEs)), and receptor tyrosine kinases involved in growth (Trk), but also interact with components that modulate actin and tubulin cytoskeletal dynamics (e.g. RhoGTPases and actin binding proteins). MLRs are essential for the regulation of the physiology of organs such as the brain, and age-related loss of cholesterol from the plasma membrane leads to loss of MLRs, decreased presynaptic vesicle fusion, and changes in neurotransmitter release, all of which contribute to different forms of neurodegeneration. Thus, MLRs provide an active membrane domain that tethers and reorganizes the cytoskeletal machinery necessary for membrane and cellular repair, and genetic interventions that restore MLRs to normal cellular levels may be exploited as potential therapeutic means to reverse the ageing and neurodegenerative processes.

CRMPs Function in Neurons and Glial Cells: Potential Therapeutic Targets for Neurodegenerative Diseases and CNS Injury

Neurodegeneration in the adult mammalian central nervous system (CNS) is fundamentally accelerated by its intrinsic neuronal mechanisms, including its poor regenerative capacity and potent extrinsic inhibitory factors. Thus, the treatment of neurodegenerative diseases faces many obstacles. The degenerative processes, consisting of axonal/dendritic structural disruption, abnormal axonal transport, release of extracellular factors, and inflammation, are often controlled by the cytoskeleton. From this perspective, regulators of the cytoskeleton could potentially be a therapeutic target for neurodegenerative diseases and CNS injury. Collapsin response mediator proteins (CRMPs) are known to regulate the assembly of cytoskeletal proteins in neurons, as well as control axonal growth and neural circuit formation. Recent studies have provided some novel insights into the roles of CRMPs in several inhibitory signaling pathways of neurodegeneration, in addition to its functions in neurological disorders and CNS repair. Here, we summarize the roles of CRMPs in axon regeneration and its emerging functions in non-neuronal cells, especially in inflammatory responses. We also discuss the direct and indirect targeting of CRMPs as a novel therapeutic strategy for neurological diseases.

Rab GTPase signaling in neurite outgrowth and axon specification

Neurons are highly polarized cells that contain specialized subcellular domains involved in information transmission in the nervous system. Specifically, the somatodendritic compartment receives neuronal inputs while the axons convey information through the synapse. The establishment of asymmetric domains requires a specific delivery of components, including organelles, proteins, and membrane. The Rab family of small GTPases plays an essential role in membrane trafficking. Signaling cascades triggered by extrinsic and intrinsic factors tightly regulate Rab functions in cells, with Rab protein activation depending on GDP/GTP binding to establish a binary mode of action. This review summarizes the contributions of several Rab family members involved in trans-Golgi, early/late endosomes, and recycling endosomes during neurite development and axonal outgrowth. The regulation of some Rabs by guanine exchanging factors and GTPase activating proteins will also be addressed. Finally, discussion will be provided on how specific effector-mediated Rab activation modifies several molecules essential to neuronal differentiation. © 2016 Wiley Periodicals, Inc.

Coordinating neuronal actin-microtubule dynamics

The growth and migration of neurons require continuous remodelling of the neuronal cytoskeleton, providing a versatile cellular framework for force generation and guided movement, in addition to structural support. Actin filaments and microtubules are central to the dynamic action of the cytoskeleton and rapid advances in imaging technologies are enabling ever more detailed visualisation of the dynamic intracellular networks that they form. However, these filaments do not act individually and an expanding body of evidence emphasises the importance of actin-microtubule crosstalk in orchestrating cytoskeletal dynamics. Here, we summarise our current understanding of the structure and dynamics of actin and microtubules in isolation, before reviewing both the mechanisms and the molecular players involved in mediating actin-microtubule crosstalk in neurons.