Cytoskeletal dynamics and transport in growth cone motility and axon guidance

Cytoskeleton dysfunction of motor neuron in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by deletions or mutations of survival of motor neuron 1 (SMN1) gene. To date, the mechanism of selective cell death of motor neurons as a hallmark of SMA is still unclear. The severity of SMA is dependent on the amount of survival motor neuron (SMN) protein, which is an essential and ubiquitously expressed protein involved in various cellular processes including regulation of cytoskeletal dynamics. In this review, we discuss the effect of SMN ablation on cytoskeleton organization including actin dynamics, growth cone formation, axonal stability, neurite outgrowth, microtubule stability, synaptic vesicle dynamics and neurofilament protein release in SMA. We also summarized a list of critical proteins such as profilin-2 (PFN2), plastin-3 (PLS3), stathmin-1 (STMN1), microtubule-associated protein 1B (MAP1B) and neurofilament which play an important role in modulating cytoskeleton in SMA. Our aim is to highlight how cytoskeletal defects contribute to motor neuron degeneration in SMA disease progression and concentrating on cytoskeleton dynamics may be a promising approach to develop new therapy or biomarker.

Dynamic modelling and predictive position/force control of a plant-inspired growing robot

This paper presents the development and control of a dynamic model for a plant-inspired growing robot, termed the 'vine-robot', using the Euler-Lagrangian method. The unique growth mechanism of the vine-robot enables it to navigate complex environments by extending its body. We derive the dynamic equations of motion and employ model predictive control to regulate the task space position, orientation, and interaction forces. Simulation experiments are conducted to evaluate the performance of the proposed model and control strategy. The results demonstrate that the model effectively achieves sub-millimeter precision in the position control in both static and time varying refrence trajectroies, and sub micronewton in force control.

Biomimetic design and integrated biofabrication of an in-vitro three-dimensional multi-scale multilayer cortical model

The lack of accurate and reliable in vitro brain models hinders the development of brain science and research on brain diseases. Owing to the complex structure of the brain tissue and its highly nonlinear characteristics, the construction of brain-like in vitro tissue models remains one of the most challenging research fields in the construction of living tissues. This study proposes a multi-scale design of a brain-like model with a biomimetic cortical structure, which includes the macroscopic structural features of six layers of different cellular components, as well as micrometer-scale continuous fiber structures running through all layers vertically. To achieve integrated biomanufacturing of such a complex multi-scale brain-like model, a multi-material composite printing/culturing integrated bioprinting platform was developed in-house by integrating cell-laden hydrogel ink direct writing printing and electrohydrodynamic fiber 3D printing technologies. Through integrated bioprinting, multi-scale models with different cellular components and fiber structural parameters were prepared to study the effects of macroscopic and microscopic structural features on the directionality of neural cells, as well as the interaction between glial cells and neurons within the tissue model in a three-dimensional manner. The results revealed that the manufactured in vitro biomimetic cortical model achieved morphological connections between the layers of neurons, reflecting the structure and cellular morphology of the natural cortex. Micrometer-scale (10 μm) cross-layer fibers effectively guided and controlled the extension length and direction of the neurites of surrounding neural cells but had no significant effect on the migration of neurons. In contrast, glial cells significantly promoted the migration of surrounding PC12 cells towards the glial layer but did not contribute to the extension of neurites. This study provides a basis for the design and manufacture of accurate brain-like models for the functionalization of neuronal tissues.

"Mitotic" kinesin-5 is a dynamic brake for axonal growth

During neuronal development, neurons undergo significant microtubule reorganization to shape axons and dendrites, establishing the framework for efficient wiring of the nervous system. Previous studies from our laboratory demonstrated the key role of kinesin-1 in driving microtubule-microtubule sliding, which provides the mechanical forces necessary for early axon outgrowth and regeneration in Drosophila melanogaster. In this study, we reveal the critical role of kinesin-5, a mitotic motor, in modulating the development of postmitotic neurons. Kinesin-5, a conserved homotetrameric motor, typically functions in mitosis by sliding antiparallel microtubules apart in the spindle. Here, we demonstrate that the Drosophila kinesin-5 homolog, Klp61F, is expressed in larval brain neurons, with high levels in ventral nerve cord (VNC) neurons. Knockdown of Klp61F using a pan-neuronal driver leads to severe locomotion defects and complete lethality in adult flies, mainly due to the absence of kinesin-5 in VNC motor neurons during early larval development. Klp61F depletion results in significant axon growth defects, both in cultured and in vivo neurons. By imaging individual microtubules, we observe a significant increase in microtubule motility, and excessive penetration of microtubules into the axon growth cone in Klp61F-depleted neurons. Adult lethality and axon growth defects are fully rescued by a chimeric human-Drosophila kinesin-5 motor, which accumulates at the axon tips, suggesting a conserved role of kinesin-5 in neuronal development. Altogether, our findings show that at the growth cone, kinesin-5 acts as a brake on kinesin-1-driven microtubule sliding, preventing premature microtubule entry into the growth cone. This regulatory role of kinesin-5 is essential for precise axon pathfinding during nervous system development.

The Axonal Actin Filament Cytoskeleton: Structure, Function, and Relevance to Injury and Degeneration

Early investigations of the neuronal actin filament cytoskeleton gave rise to the notion that, although growth cones exhibit high levels of actin filaments, the axon shaft exhibits low levels of actin filaments. With the development of new tools and imaging techniques, the axonal actin filament cytoskeleton has undergone a renaissance and is now an active field of research. This article reviews the current state of knowledge about the actin cytoskeleton of the axon shaft. The best understood forms of actin filament organization along axons are axonal actin patches and a submembranous system of rings that endow the axon with protrusive competency and structural integrity, respectively. Additional forms of actin filament organization along the axon have also been described and their roles are being elucidated. Extracellular signals regulate the axonal actin filament cytoskeleton and our understanding of the signaling mechanisms involved is being elaborated. Finally, recent years have seen advances in our perspective on how the axonal actin cytoskeleton is impacted by, and contributes to, axon injury and degeneration. The work to date has opened new venues and future research will undoubtedly continue to provide a richer understanding of the axonal actin filament cytoskeleton.

Polyglutamine binding protein 1 regulates neurite outgrowth through recruiting N-WASP

Neurite outgrowth is a critical step in neural development, leading to the generation of neurite branches that allow individual neurons to make contacts with multiple neurons within the target region. Polyglutamine-binding protein 1 (PQBP1) is a highly conserved protein with a key role in neural development. Our recent mass spectrometric analysis showed that PQBP1 associates with neural Wiskott-Aldrich syndrome protein (N-WASP), an important actin polymerization-promoting factor involved in neurite outgrowth. Here, we report that the WW domain of PQBP1 directly interacts with the proline-rich domain of N-WASP. The disruption of this interaction leads to impaired neurite outgrowth and growth cone size. Furthermore, we demonstrate that PQBP1/N-WASP interaction is critical for the recruitment of N-WASP to the growth cone, but does not affect N-WASP protein levels or N-WASP-induced actin polymerization. Our results indicated that PQBP1 regulates neurite outgrowth by recruiting N-WASP to the growth cone, thus representing an alternative molecular mechanism via which PQBP1-mediates neurite outgrowth.

Mutation of the ALS-/FTD-Associated RNA-Binding Protein FUS Affects Axonal Development

Aberrant condensation and localization of the RNA-binding protein (RBP) fused in sarcoma (FUS) occur in variants of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Changes in RBP function are commonly associated with changes in axonal cytoskeletal organization and branching in neurodevelopmental disorders. Here, we asked whether branching defects also occur in vivo in a model of FUS-associated disease. We use two reported Xenopus models of ALS/FTD (of either sex), the ALS-associated mutant FUS(P525L) and a mimic of hypomethylated FUS, FUS(16R). Both mutants strongly reduced axonal complexity in vivo. We also observed an axon looping defect for FUS(P525L) in the target area, which presumably arises due to errors in stop cue signaling. To assess whether the loss of axon complexity also had a cue-independent component, we assessed axonal cytoskeletal integrity in vitro. Using a novel combination of fluorescence and atomic force microscopy, we found that mutant FUS reduced actin density in the growth cone, altering its mechanical properties. Therefore, FUS mutants may induce defects during early axonal development.

Molecular cartography within axons

Recent advances in imaging methods begin to further illuminate the inner workings of neurons. Views of the axonal landscape through the lens of in situ cryo-electron tomography (cryo-ET) provide a high-resolution atlas of the macromolecular organization in near-native conditions, leading to our growing understanding of the vital roles of compositional and structural organization in maintaining neuronal homeostasis. In this review, we discuss the latest observations concerning the fundamental components found within neuronal compartments, with special emphasis on the axon, branch points, and growth cone. We describe the similarity and difference in organization of organelles and molecules in varying compartments. Finally, we highlight outstanding questions on the dynamics and localization of various components along the axon that may be answered using orthogonal approaches.

CLUH maintains functional mitochondria and translation in motoneuronal axons and prevents peripheral neuropathy

Transporting and translating mRNAs in axons is crucial for neuronal viability. Local synthesis of nuclear-encoded mitochondrial proteins protects long-lived axonal mitochondria from damage; however, the regulatory factors involved are largely unknown. We show that CLUH, which binds mRNAs encoding mitochondrial proteins, prevents peripheral neuropathy and motor deficits in the mouse. CLUH is enriched in the growth cone of developing spinal motoneurons and is required for their growth. The lack of CLUH affects the abundance of target mRNAs and the corresponding mitochondrial proteins more prominently in axons, leading to ATP deficits in the growth cone. CLUH interacts with ribosomal subunits, translation initiation, and ribosome recycling components and preserves axonal translation. Overexpression of the ribosome recycling factor ABCE1 rescues the mRNA and translation defects, as well as the growth cone size, in CLUH-deficient motoneurons. Thus, we demonstrate a role for CLUH in mitochondrial quality control and translational regulation in axons, which is essential for their development and long-term integrity and function.

LncRNA TubAR complexes with TUBB4A and TUBA1A to promote microtubule assembly and maintain myelination

A long-standing hypothesis proposes that certain RNA(s) must exhibit structural roles in microtubule assembly. Here, we identify a long noncoding RNA (TubAR) that is highly expressed in cerebellum and forms RNA-protein complex with TUBB4A and TUBA1A, two tubulins clinically linked to cerebellar and myelination defects. TubAR knockdown in mouse cerebellum causes loss of oligodendrocytes and Purkinje cells, demyelination, and decreased locomotor activity. Biochemically, we establish the roles of TubAR in promoting TUBB4A-TUBA1A heterodimer formation and microtubule assembly. Intriguingly, different from the hypomyelination-causing mutations, the non-hypomyelination-causing mutation TUBB4A-R2G confers gain-of-function for an RNA-independent interaction with TUBA1A. Experimental use of R2G/A mutations restores TUBB4A-TUBA1A heterodimer formation, and rescues the neuronal cell death phenotype caused by TubAR knockdown. Together, we uncover TubAR as the long-elusive structural RNA for microtubule assembly and demonstrate how TubAR mediates microtubule assembly specifically from αβ-tubulin heterodimers, which is crucial for maintenance of cerebellar myelination and activity.

A modified motor-clutch model reveals that neuronal growth cones respond faster to soft substrates

Neuronal growth cones sense a variety of cues including chemical and mechanical ones to establish functional connections during nervous system development. Substrate-cytoskeletal coupling is an established model for adhesion-mediated growth cone advance; however, the detailed molecular and biophysical mechanisms underlying the mechanosensing and mechanotransduction process remain unclear. Here, we adapted a motor-clutch model to better understand the changes in clutch and cytoskeletal dynamics, traction forces, and substrate deformation when a growth cone interacts with adhesive substrates of different stiffnesses. Model parameters were optimized using experimental data from Aplysia growth cones probed with force-calibrated glass microneedles. We included a reinforcement mechanism at both motor and clutch level. Furthermore, we added a threshold for retrograde F-actin flow that indicates when the growth cone is strongly coupled to the substrate. Our modeling results are in strong agreement with experimental data with respect to the substrate deformation and the latency time after which substrate-cytoskeletal coupling is strong enough for the growth cone to advance. Our simulations show that it takes the shortest time to achieve strong coupling when substrate stiffness was low at 4 pN/nm. Taken together, these results suggest that Aplysia growth cones respond faster and more efficiently to soft than stiff substrates.

Cell adhesion and actin dynamics factors promote axonal extension and synapse formation in transplanted Drosophila photoreceptor cells

Vision is formed by the transmission of light stimuli to the brain through axons extending from photoreceptor cells. Damage to these axons leads to loss of vision. Despite research on neural circuit regeneration through transplantation, achieving precise axon projection remains challenging. To achieve optic nerve regeneration by transplantation, we employed the Drosophila visual system. We previously established a transplantation method for Drosophila utilizing photoreceptor precursor cells extracted from the eye disc. However, little axonal elongation of transplanted cells into the brain, the lamina, was observed. We verified axonal elongation to the lamina by modifying the selection process for transplanted cells. Moreover, we focused on N-cadherin (Ncad), a cell adhesion factor, and Twinstar (Tsr), which has been shown to promote actin reorganization and induce axon elongation in damaged nerves. Overexpression of Ncad and tsr promoted axon elongation to the lamina, along with presynaptic structure formation in the elongating axons. Furthermore, overexpression of Neurexin-1 (Nrx-1), encoding a protein identified as a synaptic organizer, was found to not only promote presynapse formation but also enhance axon elongation. By introducing Ncad, tsr, and Nrx-1, we not only successfully achieved axonal projection of transplanted cells to the brain beyond the retina, but also confirmed the projection of transplanted cells into a deeper ganglion, the medulla. The present study offers valuable insights to realize regeneration through transplantation in a more complex nervous system.

Adhesion-clutch between DCC and netrin-1 mediates netrin-1-induced axonal haptotaxis

The growth cone, a motile structure located at the tip of growing axons, senses extracellular guidance cues and translates them into directional forces that drive axon outgrowth and guidance. Axon guidance directed by chemical cues on the extracellular adhesive substrate is termed haptotaxis. Recent studies reported that netrin-1 on the substrate functions as a haptotactic axon guidance cue. However, the mechanism mediating netrin-1-induced axonal haptotaxis remains unclear. Here, we demonstrate that substrate-bound netrin-1 induces axonal haptotaxis by facilitating physical interactions between the netrin-1 receptor, DCC, and the adhesive substrates. DCC serves as an adhesion receptor for netrin-1. The clutch-linker molecule shootin1a interacted with DCC, linking it to actin filament retrograde flow at the growth cone. Speckle imaging analyses showed that DCC underwent either grip (stop) or retrograde slip on the adhesive substrate. The grip state was more prevalent on netrin-1-coated substrate compared to the control substrate polylysine, thereby transmitting larger traction force on the netrin-1-coated substrate. Furthermore, disruption of the linkage between actin filament retrograde flow and DCC by shootin1 knockout impaired netrin-1-induced axonal haptotaxis. These results suggest that the directional force for netrin-1-induced haptotaxis is exerted on the substrates through the adhesion-clutch between DCC and netrin-1 which occurs asymmetrically within the growth cone.

Nerve Regeneration and Gait Function Recovery with Implantation of Glucose/Mannose Conduits Using a Rat Model: Efficacy of Glucose/Mannose as a New Neurological Guidance Material

Therapy with clinical nerve guidance conduits often causes functional incompleteness in patients. With the aim of better therapeutic efficacy, nerve regeneration and gait function were investigated in this study using a novel nerve guidance conduit consisting of glucose/mannose. The glucose/mannose nerve guidance conduits were prepared by filling the conduits with the glucose/mannose aqueous solutions for different kinematic viscosity, which were applied to sciatic nerve defects (6 mm gap) in a rat model. The nerve regeneration effect and the gait function recovery with the fabricated nerve guidance conduits were examined. From the results of the XRD measurement, the glucose/mannose conduits were identified as crystal structures of cellulose type II. Young's modulus and the maximum tensile strength of the crystalline glucose/mannose conduits demonstrated good strength and softness for the human nerve. Above 4 weeks postoperative, macroscopic observation revealed that the nerve was regenerated in the defective area. In various staining results of the nerve tissue removed at 4 weeks postoperative, myelinated nerves contributing to gait function could not be observed in the proximal and distal sites to the central nerve. At 8-12 weeks postoperative, myelinated nerves were found at the proximal and distal sites in hematoxylin/eosin staining. Glia cells were confirmed by phosphotungstic acid-hematoxylin staining. Continuous nerve fibers were observed clearly in the sections of the regenerated nerves towards the longitudinal direction at 12 weeks postoperative. The angle between the metatarsophalangeal joint and the ground plane was approximately 93° in intact rats. At 4 weeks postoperative, walking was not possible, but at 8 weeks postoperative, the rats were able to walk, with an angle of 53°. At 12 weeks postoperative, the angle increased further, reaching 65°, confirming that the rats were able to walk more quickly than at 8 weeks postoperative. These results demonstrated that gait function in rats treated with glucose/mannose nerve guidance conduits was rapidly recovered after 8 weeks postoperative. The glucose/mannose nerve guidance conduit could be applied as a new promising candidate material for peripheral nerve regeneration.

Rasal1 regulates calcium dependent neuronal maturation by modifying microtubule dynamics

BACKGROUND

Rasal1 is a Ras GTPase-activating protein which contains C2 domains necessary for dynamic membrane association following intracellular calcium elevation. Membrane-bound Rasal1 inactivates Ras signaling through its RasGAP activity, and through such mechanisms has been implicated in regulating various cellular functions in the context of tumors. Although highly expressed in the brain, the contribution of Rasal1 to neuronal development and function has yet to be explored.

RESULTS

We examined the contributions of Rasal1 to neuronal development in primary culture of hippocampal neurons through modulation of Rasal1 expression using molecular tools. Fixed and live cell imaging demonstrate diffuse expression of Rasal1 throughout the cell soma, dendrites and axon which localizes to the neuronal plasma membrane in response to intracellular calcium fluctuation. Pull-down and co-immunoprecipitation demonstrate direct interaction of Rasal1 with PKC, tubulin, and CaMKII. Consequently, Rasal1 is found to stabilize microtubules, through post-translational modification of tubulin, and accordingly inhibit dendritic outgrowth and branching. Through imaging, molecular, and electrophysiological techniques Rasal1 is shown to promote NMDA-mediated synaptic activity and CaMKII phosphorylation.

CONCLUSIONS

Rasal1 functions in two separate roles in neuronal development; calcium regulated neurite outgrowth and the promotion of NMDA receptor-mediated postsynaptic events which may be mediated both by interaction with direct binding partners or calcium-dependent regulation of down-stream pathways. Importantly, the outlined molecular mechanisms of Rasal1 may contribute notably to normal neuronal development and synapse formation.

Genome-wide study reveals novel roles for formin-2 in axon regeneration as a microtubule dynamics regulator and therapeutic target for nerve repair

Peripheral nerves regenerate successfully; however, clinical outcome after injury is poor. We demonstrated that low-dose ionizing radiation (LDIR) promoted axon regeneration and function recovery after peripheral nerve injury (PNI). Genome-wide CpG methylation profiling identified LDIR-induced hypermethylation of the Fmn2 promoter, exhibiting injury-induced Fmn2 downregulation in dorsal root ganglia (DRGs). Constitutive knockout or neuronal Fmn2 knockdown accelerated nerve repair and function recovery. Mechanistically, increased microtubule dynamics at growth cones was observed in time-lapse imaging of Fmn2-deficient DRG neurons. Increased HDAC5 phosphorylation and rapid tubulin deacetylation were found in regenerating axons of neuronal Fmn2-knockdown mice after injury. Growth-promoting effect of neuronal Fmn2 knockdown was eliminated by pharmaceutical blockade of HDAC5 or neuronal Hdac5 knockdown, suggesting that Fmn2deletion promotes axon regeneration via microtubule post-translational modification. In silico screening of FDA-approved drugs identified metaxalone, administered either immediately or 24-h post-injury, accelerating function recovery. This work uncovers a novel axon regeneration function of Fmn2 and a small-molecule strategy for PNI.

Neuron navigators: A novel frontier with physiological and pathological implications

Neuron navigators are microtubule plus-end tracking proteins containing basic and serine rich regions which are encoded by neuron navigator genes (NAVs). Neuron navigator proteins are essential for neurite outgrowth, neuronal migration, and overall neurodevelopment along with some other functions as well. The navigator proteins are substantially expressed in the developing brain and have been reported to be differentially expressed in various tissues at different ages. Over the years, the research has found neuron navigators to be implicated in a spectrum of pathological conditions such as developmental anomalies, neurodegenerative disorders, neuropathic pain, anxiety, cancers, and certain inflammatory conditions. The existing knowledge about neuron navigators remains sparse owing to their differential functions, undiscovered modulators, and unknown molecular mechanisms. Investigating the possible role of neuron navigators in various physiological processes and pathological conditions pose as a novel field that requires extensive research and might provide novel mechanistic insights and understanding of these aspects.

Tunable hydrogel viscoelasticity modulates human neural maturation

Human-induced pluripotent stem cells (hiPSCs) have emerged as a promising in vitro model system for studying neurodevelopment. However, current models remain limited in their ability to incorporate tunable biomechanical signaling cues imparted by the extracellular matrix (ECM). The native brain ECM is viscoelastic and stress-relaxing, exhibiting a time-dependent response to an applied force. To recapitulate the remodelability of the neural ECM, we developed a family of protein-engineered hydrogels that exhibit tunable stress relaxation rates. hiPSC-derived neural progenitor cells (NPCs) encapsulated within these gels underwent relaxation rate-dependent maturation. Specifically, NPCs within hydrogels with faster stress relaxation rates extended longer, more complex neuritic projections, exhibited decreased metabolic activity, and expressed higher levels of genes associated with neural maturation. By inhibiting actin polymerization, we observed decreased neuritic projections and a concomitant decrease in neural maturation gene expression. Together, these results suggest that microenvironmental viscoelasticity is sufficient to bias human NPC maturation.

Customizing 3D Structures of Vertically Aligned Carbon Nanotubes to Direct Neural Stem Cell Differentiation

Neural tissue-related illnesses have a high incidence and prevalence in society. Despite intensive research efforts to enhance the regeneration of neural cells into functional tissue, effective treatments are still unavailable. Here, a novel therapeutic approach based on vertically aligned carbon nanotube forests (VA-CNT forests) and periodic VA-CNT micropillars produced by thermal chemical vapor deposition is explored. In addition, honeycomb-like and flower-like morphologies are created. Initial viability testing reveals that NE-4C neural stem cells seeded on all morphologies survive and proliferate. In addition, free-standing VA-CNT forests and capillary-driven VA-CNT forests are created, with the latter demonstrating enhanced capacity to stimulate neuritogenesis and network formation under minimal differentiation medium conditions. This is attributed to the interaction between surface roughness and 3D-like morphology that mimics the native extracellular matrix, thus enhancing cellular attachment and communication. These findings provide a new avenue for the construction of electroresponsive scaffolds based on CNTs for neural tissue engineering.

Actin cytoskeletal dynamics do not impose an energy drain on growth cone bioenergetics

The regulation of the intracellular level of ATP is a fundamental aspect of bioenergetics. Actin cytoskeletal dynamics have been reported to be an energetic drain in developing neurons and platelets. We addressed the role of actin dynamics in primary embryonic chicken neurons using luciferase assays, and by measurement of the ATP/ADP ratio using the ratiometric reporter PercevalHR and the ATP level using the ratiometric reporter mRuby-iATPSnFR. None of the methods revealed an effect of suppressing actin dynamics on the decline in the neuronal ATP level or the ATP/ADP ratio following shutdown of ATP production. Similarly, we find that treatments that elevate or suppress actin dynamics do not alter the ATP/ADP ratio in growth cones, the subcellular domain with the highest actin dynamics in developing neurons. Collectively, the data indicate that actin cytoskeletal dynamics are not a significant energy drain in developing neurons and that the ATP/ADP ratio is maintained when energy utilization varies. Discrepancies between prior work and the current data are discussed with emphasis on methodology and interpretation of the data.

Roles of Fibroblast Growth Factors in the Axon Guidance

Fibroblast growth factors (FGFs) have been widely studied by virtue of their ability to regulate many essential cellular activities, including proliferation, survival, migration, differentiation and metabolism. Recently, these molecules have emerged as the key components in forming the intricate connections within the nervous system. FGF and FGF receptor (FGFR) signaling pathways play important roles in axon guidance as axons navigate toward their synaptic targets. This review offers a current account of axonal navigation functions performed by FGFs, which operate as chemoattractants and/or chemorepellents in different circumstances. Meanwhile, detailed mechanisms behind the axon guidance process are elaborated, which are related to intracellular signaling integration and cytoskeleton dynamics.

Highly Specialized Mechanisms for Mitochondrial Transport in Neurons: From Intracellular Mobility to Intercellular Transfer of Mitochondria

The highly specialized structure and function of neurons depend on a sophisticated organization of the cytoskeleton, which supports a similarly sophisticated system to traffic organelles and cargo vesicles. Mitochondria sustain crucial functions by providing energy and buffering calcium where it is needed. Accordingly, the distribution of mitochondria is not even in neurons and is regulated by a dynamic balance between active transport and stable docking events. This system is finely tuned to respond to changes in environmental conditions and neuronal activity. In this review, we summarize the mechanisms by which mitochondria are selectively transported in different compartments, taking into account the structure of the cytoskeleton, the molecular motors and the metabolism of neurons. Remarkably, the motor proteins driving the mitochondrial transport in axons have been shown to also mediate their transfer between cells. This so-named intercellular transport of mitochondria is opening new exciting perspectives in the treatment of multiple diseases.

Cytoelectric Coupling: Electric fields sculpt neural activity and "tune" the brain's infrastructure

We propose and present converging evidence for the Cytoelectric Coupling Hypothesis: Electric fields generated by neurons are causal down to the level of the cytoskeleton. This could be achieved via electrodiffusion and mechanotransduction and exchanges between electrical, potential and chemical energy. Ephaptic coupling organizes neural activity, forming neural ensembles at the macroscale level. This information propagates to the neuron level, affecting spiking, and down to molecular level to stabilize the cytoskeleton, "tuning" it to process information more efficiently.

Moving through the crowd. Where are we at understanding physiological axon growth?

Axon growth enables the rapid wiring of the central nervous system. Understanding this process is a prerequisite to retriggering it under pathological conditions, such as a spinal cord injury, to elicit axon regeneration. The last decades saw progress in understanding the mechanisms underlying axon growth. Most of these studies employed cultured neurons grown on flat surfaces. Only recently studies on axon growth were performed in 3D. In these studies, physiological environments exposed more complex and dynamic aspects of axon development. Here, we describe current views on axon growth and highlight gaps in our knowledge. We discuss how axons interact with the extracellular matrix during development and the role of the growth cone and its cytoskeleton within. Finally, we propose that the time is ripe to study axon growth in a more physiological setting. This will help us uncover the physiologically relevant mechanisms underlying axon growth, and how they can be reactivated to induce axon 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.​.

Dephosphorylation of neural wiring protein shootin1 by PP1 phosphatase regulates netrin-1-induced axon guidance

Axon pathfinding is an essential step in neuronal network formation. Shootin1a is a clutch-linker molecule that is mechanically involved in axon outgrowth and guidance. It was previously shown that concentration gradients of axon guidance molecule netrin-1 in the extracellular environment elicit asymmetrically localized Pak1 kinase-mediated phosphorylation of shootin1a within axonal growth cones, which is higher on the netrin-1 source side. This asymmetric phosphorylation promotes shootin1a-mediated local actin-adhesion coupling within growth cones, thereby generating directional forces for turning the growth cone toward the netrin-1 source. However, how the spatial differences in netrin-1 concentration are transduced into the asymmetrically localized signaling within growth cones remains unclear. Moreover, the protein phosphatases that dephosphorylate shootin1a remain unidentified. Here, we report that protein phosphatase-1 (PP1) dephosphorylates shootin1a in growth cones. We found that PP1 overexpression abolished the netrin-1-induced asymmetric localization of phosphorylated-shootin1a as well as axon turning. In addition, we show PP1 inhibition reversed the asymmetrically localized shootin1a phosphorylation within growth cones under netrin-1 gradient, thereby changing the netrin-1-induced growth cone turning from attraction to repulsion. These data indicate that PP1-mediated shootin1a dephosphorylation plays a key role in organizing asymmetrically-localized phosphorylated shootin1a within growth cones, which regulates netrin-1-induced axon guidance.

Establishment of a schizophrenia classifier based on peripheral blood signatures and investigation of pathogenic miRNA-mRNA regulation

To date, the diagnosis of schizophrenia (SCZ) mainly relies on patients' or guardians' self-reports and clinical observation, and the pathogenesis of SCZ remains elusive. In this study, we sought to develop a reliable classifier for diagnosing SCZ patients and provide clues to the etiology and pathogenesis of SCZ. Based on the high throughput sequencing analysis of peripheral blood miRNA expression profile and weighted gene co-expression network analysis (WGCNA) in our previous study, we selected eleven hub miRNAs for validation by qRT-PCR in 51 SCZ patients and 51 controls. miR-939-5p, miR-4732-3p let-7d-3p, and miR-142-3p were confirmed to be significantly up-regulated, and miR-30e-3p and miR-23a-3p were down-regulated in SCZ patients. miR-30e-3p with the most considerable fold change and statistically significance was selected for targeting validation. We first performed bioinformatics prediction followed by qRT-PCR and verified the up-regulation of potential target mRNAs (ABI1, NMT1, HMGB1) expression. Next, we found that the expression level of ABI1 was significantly up-regulated in SH-SY5Y cells transfected with miR-30e-3p mimics. Lastly, we conducted a luciferase assay in 293T cells confirming that miR-30e-3p could directly bind with the 3'untranslated region (3'-UTR) of ABI1, revealing that miR-30e-3p might play a role in the polymerization of neuronal actin and the reconstruction of the cytoskeleton via the downstream regulation of ABI1. In addition, we constructed a classifier by a series of bioinformatics algorithms and evaluated its diagnostic performance. It appears that the classifier consists of miRNAs and mRNAs possess a better discrimination performance than individual miRNA or mRNA in SCZ.

Hypoxia-Induced Neurite Outgrowth Involves Regulation Through TRPM7

Transient receptor potential melastatin 7 (TRPM7) is a ubiquitously expressed divalent cation channel that plays a key role in cell functions such as ion homeostasis, cell proliferation, survival, and cytoskeletal dynamics and mediates cells death in hypoxic and ischemic conditions. Previously, TRPM7 was found to play a role in the neurite outgrowth and maturation of primary hippocampal neurons. Either knockdown of TRPM7 with target-specific shRNA or blocking channel conductance by a specific blocker waixenicin A enhanced axonal outgrowth in the primary neuronal culture. In this study, we investigated whether and how TPRM7 is involved in hypoxia-altered neurite outgrowth patterns in E16 hippocampal neuron cultures. We demonstrate that short-term hypoxia activated the MEK/ERK and PI3K/Akt pathways, reduced TRPM7 activity, and enhanced axonal outgrowth of neuronal cultures. On the other hand, long-term hypoxia caused a progressive retraction of axons and dendrites that could be attenuated by the TRPM7-specific inhibitor waixenicin A. Further, we demonstrate that in the presence of astrocytes, axonal retraction in long-term hypoxic conditions was enhanced, and TRPM7 block by waixenicin A prevented this retraction. Our data demonstrate the effect of hypoxia on TRPM7 activity and axonal outgrowth/retraction in cultures with or without astrocytes present.

Neuronal filopodia: From stochastic dynamics to robustness of brain morphogenesis

Brain development relies on dynamic morphogenesis and interactions of neurons. Filopodia are thin and highly dynamic membrane protrusions that are critically required for neuronal development and neuronal interactions with the environment. Filopodial interactions are typically characterized by non-deterministic dynamics, yet their involvement in developmental processes leads to stereotypic and robust outcomes. Here, we discuss recent advances in our understanding of how filopodial dynamics contribute to neuronal differentiation, migration, axonal and dendritic growth and synapse formation. Many of these advances are brought about by improved methods of live observation in intact developing brains. Recent findings integrate known and novel roles ranging from exploratory sensors and decision-making agents to pools for selection and mechanical functions. Different types of filopodial dynamics thereby reveal non-deterministic subcellular decision-making processes as part of genetically encoded brain development.

A synthetic pregnenolone analog promotes microtubule dynamics and neural development

BACKGROUND

Pregnenolone (P5) is a neurosteroid that promotes microtubule polymerization. It also reduces stress and negative symptoms of schizophrenia, promotes memory, as well as recovery from spinal cord injury. P5 is the first substance in the steroid-synthetic pathway; it can be further metabolized into other steroids. Therefore, it is difficult to differentiate the roles of P5 versus its metabolites in the brain. To alleviate this problem, we synthesized and screened a series of non-metabolizable P5 derivatives for their ability to polymerize microtubules similar to P5.

RESULTS

We identified compound #43 (3-beta-pregnenolone acetate), which increased microtubule polymerization. We showed that compound #43 modified microtubule dynamics in live cells, increased neurite outgrowth and changed growth cone morphology in mouse cerebellar granule neuronal culture. Furthermore, compound #43 promoted the formation of stable microtubule tracks in zebrafish developing cerebellar axons.

CONCLUSIONS

We have developed compound #43, a nonmetabolized P5 analog, that recapitulates P5 functions in vivo and can be a new therapeutic candidate for the treatment of neurodevelopmental diseases.

Accelerating Neurite Growth and Directing Neuronal Connections Constrained by 3D Porous Microtubes

Investigation of neural growth and connection is crucial in the field of neural tissue engineering. Here, using a femtosecond laser direct writing (fs-DLW) technique, we propose a directionally aligned porous microtube array as a culture system for accelerating the growth of neurons and directing the connection of neurites. These microtubes exhibited an unprecedented guidance effect toward the outgrowth of primary embryonic rat hippocampal neurons, with a wrap resembling the myelin sheaths of neurons. The speed of neurite growth inside these microtubes was significantly faster than that outside these microtubes. We also achieved selective/directing connection of neural networks inside the magnetic microtubes via precise microtube delivery to a gap between two neural clusters. This work not only proposes a powerful microtube platform for accelerated growth of neurons but also offers a new idea for constructing biological neural circuits by arranging the size, location, and pattern of microtubes.

Bioinspired micro- and nano-structured neural interfaces

The development of a functional nervous system requires neurons to interact with and promptly respond to a wealth of biochemical, mechanical and topographical cues found in the neural extracellular matrix (ECM). Among these, ECM topographical cues have been found to strongly influence neuronal function and behavior. Here, we discuss how the blueprint of the architectural organization of the brain ECM has been tremendously useful as a source of inspiration to design biomimetic substrates to enhance neural interfaces and dictate neuronal behavior at the cell-material interface. In particular, we focus on different strategies to recapitulate cell-ECM and cell-cell interactions. In order to mimic cell-ECM interactions, we introduce roughness as a first approach to provide informative topographical biomimetic cues to neurons. We then examine 3D scaffolds and hydrogels, as softer 3D platforms for neural interfaces. Moreover, we will discuss how anisotropic features such as grooves and fibers, recapitulating both ECM fibrils and axonal tracts, may provide recognizable paths and tracks that neuron can follow as they develop and establish functional connections. Finally, we show how isotropic topographical cues, recapitulating shapes, and geometries of filopodia- and mushroom-like dendritic spines, have been instrumental to better reproduce neuron-neuron interactions for applications in bioelectronics and neural repair strategies. The high complexity of the brain architecture makes the quest for the fabrication of create more biologically relevant biomimetic architectures in continuous and fast development. Here, we discuss how recent advancements in two-photon polymerization and remotely reconfigurable dynamic interfaces are paving the way towards to a new class of smart biointerfaces forin vitroapplications spanning from neural tissue engineering as well as neural repair strategies.

Computational simulations reveal that Abl activity controls cohesiveness of actin networks in growth cones

Extensive studies of growing axons have revealed many individual components and protein interactions that guide neuronal morphogenesis. Despite this, however, we lack any clear picture of the emergent mechanism by which this nanometer-scale biochemistry generates the multimicron-scale morphology and cell biology of axon growth and guidance in vivo. To address this, we studied the downstream effects of the Abl signaling pathway using a computer simulation software (MEDYAN) that accounts for mechanochemical dynamics of active polymers. Previous studies implicate two Abl effectors, Arp2/3 and Enabled, in Abl-dependent axon guidance decisions. We now find that Abl alters actin architecture primarily by activating Arp2/3, while Enabled plays a more limited role. Our simulations show that simulations mimicking modest levels of Abl activity bear striking similarity to actin profiles obtained experimentally from live imaging of actin in wild-type axons in vivo. Using a graph theoretical filament-filament contact analysis, moreover, we find that networks mimicking hyperactivity of Abl (enhanced Arp2/3) are fragmented into smaller domains of actin that interact weakly with each other, consistent with the pattern of actin fragmentation observed upon Abl overexpression in vivo. Two perturbative simulations further confirm that high-Arp2/3 actin networks are mechanically disconnected and fail to mount a cohesive response to perturbation. Taken together, these data provide a molecular-level picture of how the large-scale organization of the axonal cytoskeleton arises from the biophysics of actin networks.

Neuronal paxillin and drebrin mediate BDNF-induced force transduction and growth cone turning in a soft-tissue-like environment

Soft tissue environments govern neuronal morphogenesis. However, the precise molecular mechanisms underlying chemotropism-directed axonal growth cone movement in extremely soft environments remain unclear. Here, we show that drebrin, a growth cone T-zone protein, modulates growth cone turning in response to brain-derived neurotrophic factor (BDNF) coated on a soft substrate. Structurally, axonal growth cones of rodent hippocampal neurons grown on 0.1 kPa hydrogels possess an expanded T zone in which drebrin is highly integrated with both F-actin and microtubules. Biochemically, we identify paxillin as interacting with drebrin in cells grown on 0.1 kPa hydrogels but not on glass coverslips. When grown on 0.1 kPa substrates, growth cones asymmetrically exposed to BDNF-bound stripes exhibit enhanced paxillin-drebrin interaction on the side facing the stripes, an activity that is PKA and AAK1 dependent but independent of Src kinase. Functionally, we show that BDNF-induced growth cone turning and force generation on soft substrates require drebrin phosphorylation and paxillin-drebrin association.

Diaphragmatic paralysis in a neonate with circumferential skin creases Kunze type

Background

A range of clinical features have been confirmed with heterozygous mutations in Beta Tubulin (TUBB), including skin creases, facial deformities, abnormal cerebral structures, and intellectual disability, and were defined as Circumferential Skin Creases Kunze type (CSC‐KT).

Methods

Clinical information was obtained retrospectively on a neonate hospitalized in the Neonatal Intensive Care Unit, Wuhan Children’s Hospital. Genomic DNA was extracted from circulating leukocytes of the proband according to standard procedures.

Results

The neonate presented dyspnea resulting from diaphragmatic paralysis, accompanied by other typical features of CSC‐KT. Additionally, exome sequencing confirmed a new variant (NM_178,014. 4: c. 1114 A > G) in TUBB. We also summarized features described in previous cases, thus representing phenotype extension of CSC‐KT.

Conclusion

Our report is the youngest confirmed case, which could extend the current phenotype of CSC‐KT as well as the clinical diagnostic approach.

Human photoreceptors switch from autonomous axon extension to cell-mediated process pulling during synaptic marker redistribution

Photoreceptors (PRs) are the primary visual sensory cells, and their loss leads to blindness that is currently incurable. Although cell replacement therapy holds promise, success is hindered by our limited understanding of PR axon growth during development and regeneration. Here, we generate retinal organoids from human pluripotent stem cells to study the mechanisms of PR process extension. We find that early-born PRs exhibit autonomous axon extension from dynamic terminals. However, as PRs age from 40 to 80 days of differentiation, they lose dynamic terminals on 2D substrata and in 3D retinal organoids. Interestingly, PRs without motile terminals are still capable of extending axons but only by process stretching via attachment to motile non-PR cells. Immobile PR terminals of late-born PRs have fewer and less organized actin filaments but more synaptic proteins compared with early-born PR terminals. These findings may help inform the development of PR transplantation therapies.

Microtubule Dynamics Following Central and Peripheral Nervous System Axotomy

Disturbance in the neuronal network leads to instability in the microtubule (MT) railroad of axons, causing hindrance in the intra-axonal transport and making it difficult to re-establish the broken network. Peripheral nervous system (PNS) neurons can stabilize their MTs, leading to the formation of regeneration-promoting structures called "growth cones". However, central nervous system (CNS) neurons lack this intrinsic reparative capability and, instead, form growth-incompetent structures called "retraction bulbs", which have a disarrayed MT network. It is evident from various studies that although axonal regeneration depends on both cell-extrinsic and cell-intrinsic factors, any therapy that aims at axonal regeneration ultimately converges onto MTs. Understanding the neuronal MT dynamics will help develop effective therapeutic strategies in diseases where the MT network gets disrupted, such as spinal cord injury, traumatic brain injury, multiple sclerosis, and amyotrophic lateral sclerosis. It is also essential to know the factors that aid or inhibit MT stabilization. In this review, we have discussed the MT dynamics postaxotomy in the CNS and PNS, and factors that can directly influence MT stability in various diseases.

Selective axonal transport through branch junctions is directed by growth cone signaling and mediated by KIF1/kinesin-3 motors

Development and function of nerve cells rely on the orchestration of microtubule-based transport from the cell body into distal axonal terminals. Neurons often have highly elaborate branches innervating multiple targets, but how protein or membrane cargos navigate through branch junctions to specific branch targets is unknown. Here, we demonstrate that anterograde transport of membrane vesicles through axonal branch junctions is highly selective, which is influenced by branch length and more strongly by growth cone motility. Using an optogenetic tool, we demonstrate that signaling from the growth cone can rapidly direct transport through branch junctions. We further demonstrate that such transport selectivity is differentially regulated for different vesicles and mediated by the KIF1/kinesin-3 family motors. We propose that this transport regulation through branch junctions could broadly impact neuronal development, function, and regeneration.

Based on proteomics to explore the mechanism of mecobalamin promoting the repair of injured peripheral nerves

Mecobalamin is commonly used in the adjuvant intervention of various peripheral nerve injuries. Actin cytoskeleton plays a role in regeneration of myelin and axon. Therefore, the purpose of this study was to explore the possibility of mecobalamin regulating actin cytoskeleton in repairing nerve injury. In this study, a crush injury on the right sciatic nerve of two group of rats (12 in each group) was established. The control group was only given normal saline (i.g.), and the intervention group was given Mecobalamin 1mg/kg (i.g.). The rats were sacrificed on 28th day and the injured nerves were collected for proteomics. The result shows that regulation of actin cytoskeleton pathway changed significantly. The expression of protein Vav1 was verified by western blot and immunofluorescence. In the intervention group, the nerve fiber structure was complete, the axons were dense and symmetrical, the myelin sheath was compact and uniform in thickness, The positive rate of myelin basic protein (MBP) and βⅢ-Tubulin was higher than that in the control group. The findings of the study show that mecobalamin regulates the actin cytoskeleton in the repair of nerve damage and up-regulates vav1 in the regulation of actin cytoskeleton pathway.

In situ cryo-electron tomography reveals local cellular machineries for axon branch development

Neurons are highly polarized cells forming an intricate network of dendrites and axons. They are shaped by the dynamic reorganization of cytoskeleton components and cellular organelles. Axon branching allows the formation of new paths and increases circuit complexity. However, our understanding of branch formation is sparse due to the lack of direct in-depth observations. Using in situ cellular cryo-electron tomography on primary mouse neurons, we directly visualized the remodeling of organelles and cytoskeleton structures at axon branches. Strikingly, branched areas functioned as hotspots concentrating organelles to support dynamic activities. Unaligned actin filaments assembled at the base of premature branches accompanied by filopodia-like protrusions. Microtubules and ER comigrated into preformed branches to support outgrowth together with accumulating compact, ∼500-nm mitochondria and locally clustered ribosomes. We obtained a roadmap of events supporting the hypothesis of local protein synthesis selectively taking place at axon branches, allowing them to serve as unique control hubs for axon development and downstream neural network formation.

Structural Plasticity of the Hippocampus in Neurodegenerative Diseases

Neuroplasticity is the capacity of neural networks in the brain to alter through development and rearrangement. It can be classified as structural and functional plasticity. The hippocampus is more susceptible to neuroplasticity as compared to other brain regions. Structural modifications in the hippocampus underpin several neurodegenerative diseases that exhibit cognitive and emotional dysregulation. This article reviews the findings of several preclinical and clinical studies about the role of structural plasticity in the hippocampus in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. In this study, literature was surveyed using Google Scholar, PubMed, Web of Science, and Scopus, to review the mechanisms that underlie the alterations in the structural plasticity of the hippocampus in neurodegenerative diseases. This review summarizes the role of structural plasticity in the hippocampus for the etiopathogenesis of neurodegenerative diseases and identifies the current focus and gaps in knowledge about hippocampal dysfunctions. Ultimately, this information will be useful to propel future mechanistic and therapeutic research in neurodegenerative diseases.

Mathematical models of neuronal growth

The establishment of a functioning neuronal network is a crucial step in neural development. During this process, neurons extend neurites-axons and dendrites-to meet other neurons and interconnect. Therefore, these neurites need to migrate, grow, branch and find the correct path to their target by processing sensory cues from their environment. These processes rely on many coupled biophysical effects including elasticity, viscosity, growth, active forces, chemical signaling, adhesion and cellular transport. Mathematical models offer a direct way to test hypotheses and understand the underlying mechanisms responsible for neuron development. Here, we critically review the main models of neurite growth and morphogenesis from a mathematical viewpoint. We present different models for growth, guidance and morphogenesis, with a particular emphasis on mechanics and mechanisms, and on simple mathematical models that can be partially treated analytically.

The Role of Biomaterials in Peripheral Nerve and Spinal Cord Injury: A Review

Peripheral nerve and spinal cord injuries are potentially devastating traumatic conditions with major consequences for patients’ lives. Severe cases of these conditions are currently incurable. In both the peripheral nerves and the spinal cord, disruption and degeneration of axons is the main cause of neurological deficits. Biomaterials offer experimental solutions to improve these conditions. They can be engineered as scaffolds that mimic the nerve tissue extracellular matrix and, upon implantation, encourage axonal regeneration. Furthermore, biomaterial scaffolds can be designed to deliver therapeutic agents to the lesion site. This article presents the principles and recent advances in the use of biomaterials for axonal regeneration and nervous system repair.

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

A Review on the Role of Endogenous Neurotrophins and Schwann Cells in Axonal Regeneration

Injury to the peripheral nerve is traditionally referred to acquired nerve injury as they are the result of physical trauma due to laceration, stretch, crush and compression of nerves. However, peripheral nerve injury may not be completely limited to acquired physical trauma. Peripheral nerve injury equally implies clinical conditions like Guillain-Barré syndrome (GBS), Carpal tunnel syndrome, rheumatoid arthritis and diabetes. Physical trauma is commonly mono-neuropathic as it engages a single nerve and produces focal damage, while in the context of pathological conditions the damage is divergent involving a group of the nerve causing polyneuropathy. Damage to the peripheral nerve can cause a diverse range of manifestations from sensory impairment to loss of function with unpredictable recovery patterns. Presently no treatment option provides complete or functional recovery in nerve injury, as nerve cells are highly differentiated and inert to regeneration. However, the regenerative phenotypes in Schwann cells get expressed when a signalling cascade is triggered by neurotrophins. Neurotrophins are one of the promising biomolecules that are released naturally post-injury with the potential to exhibit better functional recovery. Pharmacological intervention modulating the expression of these neurotrophins such as brain-derived neurotrophic factor (BDNF) and pituitary adenylyl cyclase-activating peptide (PACAP) can prove to be a significant treatment option as endogenous compounds which may have remarkable innate advantage showing maximum 'biological relevance'.

Dynamic remodeling of ribosomes and endoplasmic reticulum in axon terminals of motoneurons

In neurons, the endoplasmic reticulum (ER) forms a highly dynamic network that enters axons and presynaptic terminals and plays a central role in Ca2+ homeostasis and synapse maintenance; however, the underlying mechanisms involved in regulation of its dynamic remodeling as well as its function in axon development and presynaptic differentiation remain elusive. Here, we used high-resolution microscopy and live-cell imaging to investigate rapid movements of the ER and ribosomes in axons of cultured motoneurons after stimulation with brain-derived neurotrophic factor. Our results indicate that the ER extends into axonal growth cone filopodia, where its integrity and dynamic remodeling are regulated mainly by actin and the actin-based motor protein myosin VI (encoded by Myo6). Additionally, we found that in axonal growth cones, ribosomes assemble into 80S subunits within seconds and associate with the ER in response to extracellular stimuli, which describes a novel function of axonal ER in dynamic regulation of local translation. This article has an associated First Person interview with Chunchu Deng, joint first author of the paper.

Molecular and cellular mechanisms of spastin in neural development and disease (Review)

Spastin is a microtubule (MT)‑severing enzyme identified from mutations of hereditary spastic paraplegia in 1999 and extensive studies indicate its vital role in various cellular activities. In the past two decades, efforts have been made to understand the underlying molecular mechanisms of how spastin is linked to neural development and disease. Recent studies on spastin have unraveled the mechanistic processes of its MT‑severing activity and revealed that spastin acts as an MT amplifier to mediate its remodeling, thus providing valuable insight into the molecular roles of spastin under physiological conditions. In addition, recent research has revealed multiple novel molecular mechanisms of spastin in cellular biological pathways, including endoplasmic reticulum shaping, calcium trafficking, fatty acid trafficking, as well as endosomal fission and trafficking. These processes are closely involved in axonal and dendritic development and maintenance. The current review presents recent biological advances regarding the molecular mechanisms of spastin at the cellular level and provides insight into how it affects neural development and disease.

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.

Strategies to neutralize RhoA/ROCK pathway after spinal cord injury

Regeneration is bungled following CNS injuries, including spinal cord injury (SCI). Inherent decay of permissive conditions restricts the regrowth of the mature CNS after an injury. Hypertrophic scarring, insignificant intrinsic axon-growth activity, and axon-growth inhibitory molecules such as myelin inhibitors and scar inhibitors constitute a significant hindrance to spinal cord repair. Besides these molecules, a combined absence of various mechanisms responsible for axonal regeneration is the main reason behind the dereliction of the adult CNS to regenerate. The neutralization of specific inhibitors/proteins by stymieing antibodies or encouraging enzymatic degradation results in improved axon regeneration. Previous efforts to induce regeneration after SCI have stimulated axonal development in or near lesion sites, but not beyond them. Several pathways are responsible for the axonal growth obstruction after a CNS injury, including SCI. Herein, we summarize the axonal, glial, and intrinsic factor which impedes the regeneration. We have also discussed the methods to stabilize microtubules and through this to maintain the proper cytoskeletal dynamics of growth cone as disorganized microtubules lead to the failure of axonal regeneration. Moreover, we primarily focus on diverse inhibitors of axonal growth and molecular approaches to counteract them and their downstream intracellular signaling through the RhoA/ROCK pathway.

The Axonal Glycolytic Pathway Contributes to Sensory Axon Extension and Growth Cone Dynamics

Understanding the bioenergetics of axon extension and maintenance has wide ranging implications for neurodevelopment and disease states. Glycolysis is a pathway consisting of 10 enzymes and separated into preparatory and payoff phases, the latter producing ATP. Using embryonic chicken sensory neurons, we report that glycolytic enzymes are found through the axon and the growth cone. Pharmacological inhibition of glycolysis in the presence of NGF impairs axon extension and growth cone dynamics within minutes without affecting axon maintenance. Experiments using microfluidic chambers show that the effect of inhibiting glycolysis on axon extension is local along distal axons and can be reversed by promoting mitochondrial respiration. Knockdown of GAPDH simplifies growth cone morphology and is rescued by shRNA-resistant GAPDH expression. Rescue of GAPDH using KillerRed fused to GAPDH followed by localized chromophore-assisted light inactivation of KillerRed-GAPDH in distal axons halts growth cone dynamics. Considering filament polymerization requires ATP, inhibition of glycolysis results in a paradoxical increase in axonal actin filament levels. The effect on actin filaments is because of enzymes before GAPDH, the first enzyme in the payoff phase. In the absence of NGF, inhibition of glycolysis along distal axons results in axon degeneration independent of cell death. These data indicate that the glycolytic pathway is operative in distal axons and contributes to the rate of axon extension and growth cone dynamics in the presence of NGF and that, in the absence of NGF, the axonal glycolytic pathway is required for axon maintenance.SIGNIFICANCE STATEMENT Elucidation of the sources of ATP required for axon extension and maintenance has implications for understanding the mechanism of neuronal development and diseases of the nervous system. While recent work has emphasized the importance of mitochondrial oxidative phosphorylation, the role of the glycolytic pathway in axon morphogenesis and maintenance remains minimally understood. The data reveal that the glycolytic pathway is required for normal sensory axon extension in the presence of NGF, while in the absence of NGF the glycolytic pathway is required for axon maintenance. The results have implications for the understanding of the bioenergetics of axon morphogenesis and plasticity and indicate that NGF has protective effects on sensory axon maintenance in hypoglycemic states.