Growth Cone Formation Research Articles

CCL5 is essential for axonogenesis and neuronal restoration after brain injury

BackgroundTraumatic brain injury (TBI) causes axon tearing and synapse degradation, resulting in multiple neurological dysfunctions and exacerbation of early neurodegeneration; the repair of axonal and synaptic structures is critical for restoring neuronal function. C-C Motif Chemokine Ligand 5 (CCL5) shows many neuroprotective activities.MethodA close-head weight-drop system was used to induce mild brain trauma in C57BL/6 (wild-type, WT) and CCL5 knockout (CCL5-KO) mice. The mNSS score, rotarod, beam walking, and sticker removal tests were used to assay neurological function after mTBI in different groups of mice. The restoration of motor and sensory functions was impaired in CCL5-KO mice after one month of injury, with swelling of axons and synapses from Golgi staining and reduced synaptic proteins-synaptophysin and PSD95. Administration of recombinant CCL5 (Pre-treatment: 300 pg/g once before injury; or post-treatment: 30 pg/g every 2 days, since 3 days after injury for 1 month) through intranasal delivery into mouse brain improved the motor and sensory neurological dysfunctions in CCL5-KO TBI mice.ResultsProteomic analysis using LC-MS/MS identified that the “Nervous system development and function”-related proteins, including axonogenesis, synaptogenesis, and myelination signaling pathways, were reduced in injured cortex of CCL5-KO mice; both pre-treatment and post-treatment with CCL5 augmented those pathways. Immunostaining and western blot analysis confirmed axonogenesis and synaptogenesis related Semaphorin, Ephrin, p70S6/mTOR signaling, and myelination-related Neuregulin/ErbB and FGF/FAK signaling pathways were up-regulated in the cortical tissue by CCL5 after brain injury. We also noticed cortex redevelopment after long-term administration of CCL5 after brain injury with increased Reelin positive Cajal-Rerzius Cells and CXCR4 expression. CCL5 enhanced the growth of cone filopodia in a primary neuron culture system; blocking CCL5’s receptor CCR5 by Maraviroc reduced the intensity of filopodia in growth cone and also CCL5 mediated mTOR and Rho signalling activation. Inhibiting mTOR and Rho signaling abolished CCL5 induced growth cone formation.ConclusionsCCL5 plays a critical role in starting the intrinsic neuronal regeneration system following TBI, which includes growth cone formation, axonogenesis and synaptogensis, remyelination, and the subsequent proper wiring of cortical circuits. Our study underscores the potential of CCL5 as a robust therapeutic stratagem in treating axonal injury and degeneration during the chronic phase after mild brain injury.Graphical

Gnao1 is a molecular switch that regulates the Rho signaling pathway in differentiating neurons

GNAO1 encodes G protein subunit alpha O1 (Gαo). Pathogenic variations in GNAO1 cause developmental delay, intractable seizures, and progressive involuntary movements from early infancy. Because the functional role of GNAO1 in the developing brain remains unclear, therapeutic strategies are still unestablished for patients presenting with GNAO1-associated encephalopathy. We herein report that siRNA-mediated depletion of Gnao1 perturbs the expression of transcripts associated with Rho GTPase signaling in Neuro2a cells. Consistently, siRNA treatment hampered neurite outgrowth and extension. Growth cone formation was markedly disrupted in monolayer neurons differentiated from iPSCs from a patient with a pathogenic variant of Gαo (p.G203R). This variant disabled neuro-spherical assembly, acquisition of the organized structure, and polarized signals of phospho-MLC2 in cortical organoids from the patient’s iPSCs. We confirmed that the Rho kinase inhibitor Y27632 restored these morphological phenotypes. Thus, Gαo determines the self-organizing process of the developing brain by regulating the Rho-associated pathway. These data suggest that Rho GTPase pathway might be an alternative target of therapy for patients with GNAO1-associated encephalopathy.

Role of Histamine H3 Receptor Antagonist Pitolisant in Early Neural Differentiation of Mouse Embryonic Stem Cells.

The histamine H3 receptor, prominently expressed in neurons with a minor presence in glial cells, acts as both an autoreceptor and an alloreceptor, controlling the release of histamine and other neurotransmitters. The receptor impacts various essential physiological processes. Our team's initial investigations had demonstrated that the histamine H3 receptor antagonists could facilitate nerve regeneration by promoting the histamine H1 receptors on primary neural stem cells (NSCs) in the traumatic brain injury mouse, which suggested the potential of histamine H3 receptor as a promising target for treating neurological disorders and promoting nerve regeneration. Pitolisant (PITO) is the only histamine H3 receptor antagonist approved by the Food and Drug Administration (FDA) for treating narcolepsy. However, there is no report on Pitolisant in neural development or regeneration, and it is urgent to be further studied in strong biological activity models in vitro. The embryonic stem (ES) cells were differentiated into neural cells in vitro, which replicated the neurodevelopmental processes that occur in vivo. It also provided an alternative model for studying neurodevelopmental processes and testing drugs for neurological conditions. Therefore, we aimed to elucidate the regulatory role of Pitolisant in the early differentiation of ES cells into neural cells. Our results demonstrated that Pitolisant could promote the differentiation of ES cells toward NSCs and stimulated the formation of growth cones. Furthermore, Pitolisant was capable of inducing the polarization of NSCs through the cAMP-LKB1-SAD/MARK2 pathway, but had no significant effect on later neuronal maturation. Pitolisant altered mitochondrial morphology and upregulated the levels of mitochondrion-related proteins TOM20, Drp1, and p-Drp1, and reversed the inhibitory effect of Mdivi-1 on mitochondrial fission during the early neural differentiation of ES cells. In addition, Pitolisant induced the increase in cytosolic Ca2+. Our study provided an experimental foundation for the potential application of histamine H3 receptor-targeted modulators in the field of neuroregeneration.

Trabid patient mutations impede the axonal trafficking of adenomatous polyposis coli to disrupt neurite growth

ZRANB1 (human Trabid) missense mutations have been identified in children diagnosed with a range of congenital disorders including reduced brain size, but how Trabid regulates neurodevelopment is not understood. We have characterized these patient mutations in cells and mice to identify a key role for Trabid in the regulation of neurite growth. One of the patient mutations flanked the catalytic cysteine of Trabid and its deubiquitylating (DUB) activity was abrogated. The second variant retained DUB activity, but failed to bind STRIPAK, a large multiprotein assembly implicated in cytoskeleton organization and neural development. Zranb1 knock-in mice harboring either of these patient mutations exhibited reduced neuronal and glial cell densities in the brain and a motor deficit consistent with fewer dopaminergic neurons and projections. Mechanistically, both DUB-impaired and STRIPAK-binding-deficient Trabid variants impeded the trafficking of adenomatous polyposis coli (APC) to microtubule plus-ends. Consequently, the formation of neuronal growth cones and the trajectory of neurite outgrowth from mutant midbrain progenitors were severely compromised. We propose that STRIPAK recruits Trabid to deubiquitylate APC, and that in cells with mutant Trabid, APC becomes hyperubiquitylated and mislocalized causing impaired organization of the cytoskeleton that underlie the neuronal and developmental phenotypes.

Trabid patient mutations impede the axonal trafficking of adenomatous polyposis coli to disrupt neurite growth.

ZRANB1 (human Trabid) missense mutations have been identified in children diagnosed with a range of congenital disorders including reduced brain size, but how Trabid regulates neurodevelopment is not understood. We have characterized these patient mutations in cells and mice to identify a key role for Trabid in the regulation of neurite growth. One of the patient mutations flanked the catalytic cysteine of Trabid and its deubiquitylating (DUB) activity was abrogated. The second variant retained DUB activity, but failed to bind STRIPAK, a large multiprotein assembly implicated in cytoskeleton organization and neural development. Zranb1 knock-in mice harboring either of these patient mutations exhibited reduced neuronal and glial cell densities in the brain and a motor deficit consistent with fewer dopaminergic neurons and projections. Mechanistically, both DUB-impaired and STRIPAK-binding-deficient Trabid variants impeded the trafficking of adenomatous polyposis coli (APC) to microtubule plus-ends. Consequently, the formation of neuronal growth cones and the trajectory of neurite outgrowth from mutant midbrain progenitors were severely compromised. We propose that STRIPAK recruits Trabid to deubiquitylate APC, and that in cells with mutant Trabid, APC becomes hyperubiquitylated and mislocalized causing impaired organization of the cytoskeleton that underlie the neuronal and developmental phenotypes.

Knockdown of Porf-2 restores visual function after optic nerve crush injury

Retinal ganglion cells (RGCs), the sole output neurons in the eyes, are vulnerable to diverse insults in many pathological conditions, which can lead to permanent vision dysfunction. However, the molecular and cellular mechanisms that contribute to protecting RGCs and their axons from injuries are not completely known. Here, we identify that Porf-2, a member of the Rho GTPase activating protein gene group, is upregulated in RGCs after optic nerve crush. Knockdown of Porf-2 protects RGCs from apoptosis and promotes long-distance optic nerve regeneration after crush injury in both young and aged mice in vivo. In vitro, we find that inhibition of Porf-2 induces axon growth and growth cone formation in retinal explants. Inhibition of Porf-2 provides long-term and post-injury protection to RGCs and eventually promotes the recovery of visual function after crush injury in mice. These findings reveal a neuroprotective impact of the inhibition of Porf-2 on RGC survival and axon regeneration after optic nerve injury, providing a potential therapeutic strategy for vision restoration in patients with traumatic optic neuropathy.

Rac1 and Rac3 GTPases and TPC2 are required for axonal outgrowth and migration of cortical interneurons.

Rho GTPases, among them Rac1 and Rac3, are major transducers of extracellular signals and are involved in multiple cellular processes. In cortical interneurons, the neurons that control the balance between excitation and inhibition of cortical circuits, Rac1 and Rac3 are essential for their development. Ablation of both leads to a severe reduction in the numbers of mature interneurons found in the murine cortex, which is partially due to abnormal cell cycle progression of interneuron precursors and defective formation of growth cones in young neurons. Here, we present new evidence that upon Rac1 and Rac3 ablation, centrosome, Golgi complex and lysosome positioning is significantly perturbed, thus affecting both interneuron migration and axon growth. Moreover, for the first time, we provide evidence of altered expression and localization of the two-pore channel 2 (TPC2) voltage-gated ion channel that mediates Ca2+ release. Pharmacological inhibition of TPC2 negatively affected axonal growth and migration of interneurons. Our data, taken together, suggest that TPC2 contributes to the severe phenotype in axon growth initiation, extension and interneuron migration in the absence of Rac1 and Rac3.

Peripheral nerve regeneration through nerve conduits evokes differential expression of growth-associated protein-43 in the spinal cord.

Growth-associated protein 43 plays a key role in neurite outgrowth through cytoskeleton remodeling. We have previously demonstrated that structural damage of peripheral nerves induces growth-associated protein 43 upregulation to promote growth cone formation. Conversely, the limited regenerative capacity of the central nervous system due to an inhibitory environment prevents major changes in neurite outgrowth and should be presumably associated with low levels of growth-associated protein 43 expression. However, central alterations due to peripheral nerve damage have never been assessed using the growth-associated protein 43 marker. In this study, we used the tubulization technique to repair 1 cm-long nerve gaps in the rat nerve injury/repair model and detected growth-associated protein 43 expression in the peripheral and central nervous systems. First, histological analysis of the regeneration process confirmed an active regeneration process of the nerve gaps through the conduit from 10 days onwards. The growth-associated protein 43 expression profile varied across regions and follow-up times, from a localized expression to an abundant and consistent expression throughout the regeneration tissue, confirming the presence of an active nerve regeneration process. Second, spinal cord changes were also histologically assessed, and no apparent changes in the structural and cellular organization were observed using routine staining methods. Surprisingly, remarkable differences and local changes appeared in growth-associated protein 43 expression at the spinal cord level, in particular at 20 days post-repair and beyond. Growth-associated protein 43 protein was first localized in the gracile fasciculus and was homogeneously distributed in the left posterior cord. These findings differed from the growth-associated protein 43 pattern observed in the healthy control, which did not express growth-associated protein 43 at these levels. Our results revealed a differential expression in growth-associated protein 43 protein not only in the regenerating nerve tissue but also in the spinal cord after peripheral nerve transection. These findings open the possibility of using this marker to monitor changes in the central nervous system after peripheral nerve injury.

Ketamine impairs growth cone and synaptogenesis in human GABAergic projection neurons via GSK-3β and HDAC6 signaling

The growth cone guides the axon or dendrite of striatal GABAergic projection neurons that protrude into the midbrain and cortex and form complex neuronal circuits and synaptic networks in a developing brain, aberrant projections and synaptic connections in the striatum related to multiple brain disorders. Previously, we showed that ketamine, an anesthetic, reduced dendritic growth, dendritic branches, and spine density in human striatal GABAergic neurons. However, whether ketamine affects the growth cone, the synaptic connection of growing striatal GABAergic neurons has not been tested. Using human GABAergic projection neurons derived from human inducible pluripotent stem cells (hiPSCs) and embryonic stem cells (ES) in vitro, we tested ketamine effects on the growth cones and synapses in developing GABAergic neurons by assessing the morphometry and the glycogen synthase kinase-3 (GSK-3) and histone deacetylase 6 (HDAC6) pathway. Ketamine exposure impairs growth cone formation, synaptogenesis, dendritic development, and maturation via ketamine-mediated activation of GSK-3 pathways and inhibiting HDAC6, an essential stabilizing protein for dendritic morphogenesis and synapse maturation. Our findings identified a novel ketamine neurotoxic pathway that depends on GSK-3β and HDAC6 signaling, suggesting that microtubule acetylation is a potential target for reducing ketamine’s toxic effect on GABAergic projection neuronal development.

Adhesion Energy Modulation Acts as an NGF Receptor Activator for Neuronal Differentiation in NGF‐Free Medium

Neuronal repair is promoted after a nerve injury by chemical and physical stimuli from their environment. Among them, local adhesion energy gradients generated on surfaces trigger cone formation and neurite outgrowth without any nerve growth factor (NGF) addition. In this study, the molecular mechanisms leading to neuronal differentiation after stimulation via energy gradients are investigated. For this purpose, PC12 cells are cultured on n-[3-(trimethoxysilyl)propyl] ethylendiamine (EDA) and n-hexyl trimethoxysilane (HTMS), two functionalized surfaces possessing local energy gradients but chemically different. These surfaces similarly trigger neurite and growth cone formation after 3 days in culture. Gene expression of PI3/Akt quantified through a microarray and the action of a specific inhibitor of the corresponding pathway reveal that PI3/Akt signaling pathway is induced and activated on these surfaces in the same way as NGF does after binding on its TrkA receptor. The biological downstream effectors of this signaling pathway are also upregulated, thereby promoting cell survival, growth cone formation, and neurite outgrowth. EDA and HTMS surfaces act as an ongoing stimulus of the TrkA receptor as the addition of 100 nM K252a, its specific inhibitor, leads to the inhibition of neurite growth. Taken together, these results suggest that functionalized surface with energy surface gradients could be useful to promote nerve repair after an injury.

The role of mitochondria in the recovery of neurons after injury.

The role of mitochondria in the recovery of neurons after injury.

Cdk5 and GSK3\u03b2 inhibit fast endophilin-mediated endocytosis

Endocytosis mediates the cellular uptake of micronutrients and cell surface proteins. Fast Endophilin-mediated endocytosis, FEME, is not constitutively active but triggered upon receptor activation. High levels of growth factors induce spontaneous FEME, which can be suppressed upon serum starvation. This suggested a role for protein kinases in this growth factor receptor-mediated regulation. Using chemical and genetic inhibition, we find that Cdk5 and GSK3β are negative regulators of FEME. They antagonize the binding of Endophilin to Dynamin-1 and to CRMP4, a Plexin A1 adaptor. This control is required for proper axon elongation, branching and growth cone formation in hippocampal neurons. The kinases also block the recruitment of Dynein onto FEME carriers by Bin1. As GSK3β binds to Endophilin, it imposes a local regulation of FEME. Thus, Cdk5 and GSK3β are key regulators of FEME, licensing cells for rapid uptake by the pathway only when their activity is low.

The βγ subunit of heterotrimeric G proteins interacts with actin filaments during neuronal differentiation

The βγ subunit of heterotrimeric G proteins, a key molecule in the G protein-coupled receptors (GPCRs) signaling pathway, has been shown to be an important factor in the modulation of the microtubule cytoskeleton. Gβγ has been shown to bind to tubulin, stimulate microtubule assembly, and promote neurite outgrowth of PC12 cells. In this study, we demonstrate that in addition to microtubules, Gβγ also interacts with actin filaments, and this interaction increases during NGF-induced neuronal differentiation of PC12 cells. We further demonstrate that the Gβγ-actin interaction occurs independently of microtubules as nocodazole, a well-known microtubule depolymerizing agent did not inhibit Gβγ-actin complex formation in PC12 cells. A confocal microscopic analysis of NGF-treated PC12 cells revealed that Gβγ co-localizes with both actin and microtubule cytoskeleton along neurites, with specific co-localization of Gβγ with actin at the distal end of these neuronal processes. Furthermore, we show that Gβγ interacts with the actin cytoskeleton in primary hippocampal and cerebellar rat neurons. Our results indicate that Gβγ serves as an important modulator of the neuronal cytoskeleton by interacting with both microtubules and actin filaments, and is likely to participate in various aspects of neuronal differentiation including axon and growth cone formation.

Adult Mouse Retina Explants: From ex vivo to in vivo Model of Central Nervous System Injuries.

In mammals, adult neurons fail to regenerate following any insult to adult central nervous system (CNS), which leads to a permanent and irreversible loss of motor and cognitive functions. For a long time, much effort has been deployed to uncover mechanisms of axon regeneration in the CNS. Even if some cases of functional recovery have been reported, there is still a discrepancy regarding the functionality of a neuronal circuit upon lesion. Today, there is a need not only to identify new molecules implicated in adult CNS axon regeneration, but also to decipher the fine molecular mechanisms associated with regeneration failure. Here, we propose to use cultures of adult retina explants to study all molecular and cellular mechanisms that occur during CNS regeneration. We show that adult retinal explant cultures have the advantages to (i) recapitulate all the features observed in vivo, including axon regeneration induced by intrinsic factors, and (ii) be an ex vivo set-up with high accessibility and many downstream applications. Thanks to several examples, we demonstrate that adult explants can be used to address many questions, such as axon guidance, growth cone formation and cytoskeleton dynamics. Using laser guided ablation of a single axon, axonal injury can be performed at a single axon level, which allows to record early and late molecular events that occur after the lesion. Our model is the ideal tool to study all molecular and cellular events that occur during CNS regeneration at a single-axon level, which is currently not doable in vivo. It is extremely valuable to address unanswered questions of neuroprotection and neuroregeneration in the context of CNS lesion and neurodegenerative diseases.

Non-Muscle Myosin II in Axonal Cell Biology: From the Growth Cone to the Axon Initial Segment.

By binding to actin filaments, non-muscle myosin II (NMII) generates actomyosin networks that hold unique contractile properties. Their dynamic nature is essential for neuronal biology including the establishment of polarity, growth cone formation and motility, axon growth during development (and axon regeneration in the adult), radial and longitudinal axonal tension, and synapse formation and function. In this review, we discuss the current knowledge on the spatial distribution and function of the actomyosin cytoskeleton in different axonal compartments. We highlight some of the apparent contradictions and open questions in the field, including the role of NMII in the regulation of axon growth and regeneration, the possibility that NMII structural arrangement along the axon shaft may control both radial and longitudinal contractility, and the mechanism and functional purpose underlying NMII enrichment in the axon initial segment. With the advances in live cell imaging and super resolution microscopy, it is expected that in the near future the spatial distribution of NMII in the axon, and the mechanisms by which it participates in axonal biology will be further untangled.

Progress on axon regeneration in model organisms

Different from neurons in the peripheral nervous system, mature neurons in the mammalian central nervous system often fail to regenerate after injury. Recent studies have found that calcium transduction, injury signaling, mitochondrial transportation, cytoskeletal remodeling and protein synthesis play essential roles in axon regeneration. Firstly, axon injury increases the intracellular concentration of calcium, and initiates the injury signaling pathways including cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) and dual leucine kinase (DLK), which are found to promote axon regeneration in multiple animal injury models. The second step for axonal regrowth is to rebuild growth cones. Overexpressing proteins that promote dynamics of microtubules and actin filaments is beneficial for the reassembly of cytoskeletons and initiation of new growth cones. Thirdly, mitochondria, the power factory for cells, also play important roles in growth cone formation and axonal extension. The last but not the least important step is the regulation of gene transcription and protein translation to sustain the regrowth of axons. This review summarizes important findings revealing the functions and mechanisms of these biological progresses.

S179. ADMINISTRATION OF ROTENONE DURING NEONATAL PERIOD INDUCES SCHIZOPHRENIA-LIKE BEHAVIOR

BackgroundThe neurodevelopment refers to the process that generates and shapes the nervous system of animals. It begins during the earliest stages of embryonic development and it is the last system to be completed after birth, which occurs during the end of adolescence. The majority of mechanisms that constitutes neurodevelopment, such as cell proliferation, migration and neural differentiation, axon growth cone and dendritic filopodia formation as well as synapse are ATP dependents, therefore mitochondria dependents. Hence, disturbs during nervous system development can lead to behavioral deficits and neuropsychiatric disorders, such as schizophrenia. Results of our research group showed that a mitochondria dysfunction during neurodevelopment, by Rotenone (a complex I inhibitor) administration, induces behavioral deficits, such as hyperlocomotion and decreased social interaction, when the rats were young adults (60 days old). Thus, the objective of this study was to evaluate the behavioral deficits in an early age (prodromal phase) and if the behavioral deficits presented at young adulthood could be attenuated or reverted by antipsychotic (Haldol – Hal) or psychostimulant (Metilphenidate – MPD). Notwithstanding, draw a parallel between this animal model and a neuropsychiatric disorder.MethodsTo reach this goal, we treated intraperitoneally wistar puppies (P) with Rotenone 0.1mg/kg (Rot), Saline (Sal) or Vehicle (Veh) (DMSO) during P5-P11. To verify the prodromal phase, we performed behavioral tests (open field, social interaction and contextual fear conditioning) at P35. Therefore, in order to analyze the behavior during the young adulthood (P60), each animal previously treated with Sal, Veh or Rot was then treated intraperitoneally, 30 min before the behavioral tests (open field, social interaction and contextual fear conditioning), with either Saline (Sal-Sal, Veh-Sal, Rot-Sal), Hal (Sal-Hal, Veh-Hal, Rot-Hal) or MPD (Sal-MPD, Veh-MPD, Rot-MPD).ResultsResults showed mean±SEM (n=8 for each group) and analyzed by One-Way Anova and Duncan post-hoc compared to control group (Sal-Sal 100%). At P35, we verified: i) no significant changes in locomotor activity between Rot and Sal; ii) decrease in social interaction after treatment with Rot (68.40%±3.077; p<0.05 in relation to Sal); iii) a reduction in freezing time in contextual fear conditioning task to Rot group (45.20±8.03; p<0.05 when compared to Sal). At P60, we noticed i) an increase in locomotor activity after treatment with Rot-Sal (134.88%±3.95; p<0.05 when compared to Sal-Sal) that was decreased by treatment with Rot-Hal (94.76%±15.74; p<0.05 when compared to Rot-Sal), but not with Rot-MPD (compared to Rot-Sal); ii) a decrease in social interaction at P60 after treatment with Rot-Sal (68.35%±5.87; p<0.05 when compared to Sal-Sal) that was reverted by treatment with Rot-Hal (100.80%±6.66; p<0.05 when compared to Rot-Sal) and also with Rot-MPD (100.6%±6,89; p<0.05 when compared to Rot-Sal); iii) in contextual fear conditioning the animals treated with Rot presented a decrease in freezing response (31.50%±7.36; p<0.05 when compared to Sal-Sal) that was reverted by treatment with Rot-Hal (81.21%±16.65; p<0.05 when compared to Rot-Sal).DiscussionWe concluded that neonatal treatment with Rot promote behavior deficits at young age (P35), such as deficit in social interaction and emotional memory; in young adulthood (P60) we also verified deficit in social interaction and emotional memory, in addition to hyperlocomotion, that are mostly reverted with treatment with Hal, but not with MPD. Altogether our data suggest a relationship between mitochondria inhibition during neurodevelopment leading to schizophrenia-like behavior.

Growth-Promoting Treatment Screening for Corticospinal Neurons in Mouse and Man

Neurons of the central nervous system (CNS) that project long axons into the spinal cord have a poor axon regenerative capacity compared to neurons of the peripheral nervous system. The corticospinal tract (CST) is particularly notorious for its poor regeneration. Because of this, traumatic spinal cord injury (SCI) is a devastating condition that remains as yet uncured. Based on our recent observations that direct neuronal interleukin-4 (IL-4) signaling leads to repair of axonal swellings and beneficial effects in neuroinflammation, we hypothesized that IL-4 acts directly on the CST. Here, we developed a tissue culture model for CST regeneration and found that IL-4 promoted new growth cone formation after axon transection. Most importantly, IL-4 directly increased the regenerative capacity of both murine and human CST axons, which corroborates its regenerative effects in CNS damage. Overall, these findings serve as proof-of-concept that our CST regeneration model is suitable for fast screening of new treatments for SCI.

Apolipoprotein E promotes white matter remodeling via the Dab1‐dependent pathway after traumatic brain injury

IntroductionAxonal injury results in long‐term neurological deficits in traumatic brain injury (TBI) patients. Apolipoprotein E (ApoE) has been reported to activate intracellular adaptor protein Disabled‐1 (Dab1) phosphorylation via its interaction with ApoE receptors. The Dab1 pathway acts as a regulator of axonal outgrowth and growth cone formation in the brain.AimsWe hypothesized that ApoE may alleviate axonal injury and regulate axonal regeneration via the Dab1 pathway after TBI.ResultsIn this study, we established a model of controlled cortical impact (CCI) to mimic TBI in vivo. Using diffusion tensor imaging to detect white matter integrity, we demonstrated that APOE‐deficient mice exhibited lower fractional anisotropy (FA) values than APOE+/+ mice at 28 days after injury. The expression levels of axonal regeneration and synapse plasticity biomarkers, including growth‐associated protein 43 (GAP43), postsynaptic density protein 95 (PSD‐95), and synaptophysin, were also lower in APOE‐deficient mice. In contrast, APOE deficiency exerted no effects on the levels of myelin basic protein (MBP) expression, oligodendrocyte number, or oligodendrocyte precursor cell number. Neurological severity score (NSS) and behavioral measurements in the rotarod, Morris water maze, and Y maze tests revealed that APOE deficiency caused worse neurological deficits in CCI mice. Furthermore, Dab1 activation downregulation by the ApoE receptor inhibitor receptor‐associated protein (RAP) or Dab1 shRNA lentivirus attenuated the beneficial effects of ApoE on FA values, GAP43, PSD‐95, and synaptophysin expression, and neurological function tests. Additionally, the effects of ApoE on axonal regeneration were further validated in vitro. In a mechanical scratch injury model of primary cultured neurons, recombinant ApoE protein treatment enhanced axonal outgrowth and growth cone formation in injured neurons; however, these effects were attenuated by Dab1 shRNA, consistent with the in vivo results.ConclusionCollectively, these data suggest that ApoE promotes axonal regeneration partially through the Dab1 pathway, thereby contributing to functional recovery following TBI.

Collapsin Response Mediator Protein 4 (CRMP4) Facilitates Wallerian Degeneration and Axon Regeneration following Sciatic Nerve Injury.

In contrast to neurons in the CNS, damaged neurons from the peripheral nervous system (PNS) regenerate, but this process can be slow and imperfect. Successful regeneration is orchestrated by cytoskeletal reorganization at the tip of the proximal axon segment and cytoskeletal disassembly of the distal segment. Collapsin response mediator protein 4 (CRMP4) is a cytosolic phospho-protein that regulates the actin and microtubule cytoskeleton. During development, CRMP4 promotes growth cone formation and dendrite development. Paradoxically, in the adult CNS, CRMP4 impedes axon regeneration. Here, we investigated the involvement of CRMP4 in peripheral nerve injury in male and female Crmp4−/− mice following sciatic nerve injury. We find that sensory axon regeneration and Wallerian degeneration are impaired in Crmp4−/− mice following sciatic nerve injury. In vitro analysis of dissociated dorsal root ganglion (DRG) neurons from Crmp4−/− mice revealed that CRMP4 functions in the proximal axon segment to promote the regrowth of severed DRG neurons and in the distal axon segment where it facilitates Wallerian degeneration through calpain-dependent formation of harmful CRMP4 fragments. These findings reveal an interesting dual role for CRMP4 in proximal and distal axon segments of injured sensory neurons that coordinately facilitate PNS axon regeneration.