Intrinsic Control of Axon Regeneration

Spotlight on plasticity-related genes: Current insights in health and disease

The development of the central nervous system is highly complex, involving numerous developmental processes that must take place with high spatial and temporal precision. This requires a series of complex and well-coordinated molecular processes that are tighly controlled and regulated by, for example, a variety of proteins and lipids. Deregulations in these processes, including genetic mutations, can lead to the most severe maldevelopments. The present review provides an overview of the protein family Plasticity-related genes (PRG1-5), including their role during neuronal differentiation, their molecular interactions, and their participation in various diseases. As these proteins can modulate the function of bioactive lipids, they are able to influence various cellular processes. Furthermore, they are dynamically regulated during development, thus playing an important role in the development and function of synapses. First studies, conducted not only in mouse experiments but also in humans, revealed that mutations or dysregulations of these proteins lead to changes in lipid metabolism, resulting in severe neurological deficits. In recent years, as more and more studies have shown their involvement in a broad range of diseases, the complexity and broad spectrum of known and as yet unknown interactions between PRGs, lipids, and proteins make them a promising and interesting group of potential novel therapeutic targets.

The LRRK2-G2019S mutation attenuates repair of brain injury partially by reducing the release of osteopontin-containing monocytic exosome-like vesicles

BACKGROUND

Brain injury has been suggested as a risk factor for neurodegenerative diseases. Accordingly, defects in the brain's intrinsic capacity to repair injury may result in the accumulation of damage and a progressive loss of brain function. The G2019S (GS) mutation in LRRK2 (leucine rich repeat kinase 2) is the most prevalent genetic alteration in Parkinson's disease (PD). Here, we sought to investigate how this LRRK2-GS mutation affects repair of the injured brain.

METHODS

Brain injury was induced by stereotaxic injection of ATP, a damage-associated molecular pattern (DAMP) component, into the striatum of wild-type (WT) and LRRK2-GS mice. Effects of the LRRK2-GS mutation on brain injury and the recovery from injury were examined by analyzing the molecular and cellular behavior of neurons, astrocytes, and monocytes.

RESULTS

Damaged neurons express osteopontin (OPN), a factor associated with brain repair. Following ATP-induced damage, monocytes entered injured brains, phagocytosing damaged neurons and producing exosome-like vesicles (EVs) containing OPN through activation of the inflammasome and subsequent pyroptosis. Following EV production, neurons and astrocytes processes elongated towards injured cores. In LRRK2-GS mice, OPN expression and monocytic pyroptosis were decreased compared with that in WT mice, resulting in diminished release of OPN-containing EVs and attenuated elongation of neuron and astrocyte processes. In addition, exosomes prepared from injured LRRK2-GS brains induced neurite outgrowth less efficiently than those from injured WT brains.

CONCLUSIONS

The LRRK2-GS mutation delays repair of injured brains through reduced expression of OPN and diminished release of OPN-containing EVs from monocytes. These findings suggest that the LRRK2-GS mutation may promote the development of PD by delaying the repair of brain injury.

Augmenting fibronectin levels in injured adult CNS promotes axon regeneration in vivo

In an attempt to repair injured central nervous system (CNS) nerves/tracts, immune cells are recruited into the injury site, but endogenous response in adult mammals is insufficient for promoting regeneration of severed axons. Here, we found that a portion of retinal ganglion cell (RGC) CNS projection neurons that survive after optic nerve crush (ONC) injury are enriched for and upregulate fibronectin (Fn)-interacting integrins Itga5 and ItgaV, and that Fn promotes long-term survival and long-distance axon regeneration of a portion of axotomized adult RGCs in culture. We then show that, Fn is developmentally downregulated in the axonal tracts of optic nerve and spinal cord, but injury-activated macrophages/microglia upregulate Fn while axon regeneration-promoting zymosan augments their recruitment (and thereby increases Fn levels) in the injured optic nerve. Finally, we found that Fn's RGD motif, established to interact with Itga5 and ItgaV, promotes long-term survival and long-distance axon regeneration of adult RGCs after ONC in vivo, with some axons reaching the optic chiasm when co-treated with Rpl7a gene therapy. Thus, experimentally augmenting Fn levels in the injured CNS is a promising approach for therapeutic neuroprotection and axon regeneration of at least a portion of neurons.

Eml1 promotes axonal growth by enhancing αTAT1-mediated microtubule acetylation

Microtubule stabilization is critical for axonal growth and regeneration, and many microtubule-associated proteins are involved in this process. In this study, we found that the knockdown of echinoderm microtubule-associated protein-like 1 (EML1) hindered axonal growth in cultured cortical and dorsal root ganglion neurons. We further revealed that EML1 facilitated the acetylation of microtubules and that the impairment of axonal growth due to EML1 inhibition could be restored by treatment with deacetylase inhibitors, suggesting that EML1 affected tubulin acetylation. Moreover, we verified an interaction between EML1 and the alpha-tubulin acetyltransferase 1, which is responsible for the acetylation of alpha-tubulin. We thus proposed that EML1 might regulate microtubule acetylation and stabilization via alpha-tubulin acetyltransferase 1 and then promote axon growth. Finally, we verified that the knockdown of EML1 in vivo also inhibited sciatic nerve regeneration. Our findings revealed a novel effect of EML1 on microtubule acetylation during axonal regeneration.

CX3CL1-CX3CR1 axis protects retinal ganglion cells by inhibiting microglia activation in a distal optic nerve trauma model

BACKGROUND

The chemokine CX3CL1 has been reported to play an important role in optic nerve protection, but the underlying mechanism is still unclear. CX3CR1, the only receptor of CX3CL1, is specifically expressed on retinal microglia, whose activation plays a role in the pathological process of optic nerve injury. This study aimed to evaluate whether CX3CL1 exerts optic neuroprotection by affecting the activation of microglia by combining with CX3CR1.

METHODS

A mouse model of distal optic nerve trauma (ONT) was used to evaluate the effects of the CX3CL1-CX3CR1 axis on the activation of microglia and survival or axonal regeneration of retinal ganglion cells (RGCs). The activation of microglia, loss of RGCs, and damage to visual function were detected weekly till 4 weeks after modeling. CX3CL1 was injected intravitreally immediately or delayed after injury and the status of microglia and RGCs were examined.

RESULTS

Increases in microglia activation and optic nerve damage were accompanied by a reduced production of the CX3CL1-CX3CR1 axis after the distal ONT modeling. Both immediate and delayed intravitreal injection of CX3CL1 inhibited microglia activation, promoted survival of RGCs, and improved axonal regenerative capacity. Injection with CX3CL1 was no longer effective after 48 h post ONT. The CX3CL1-CX3CR1 axis promotes survival and axonal regeneration, as indicated by GAP43 protein and gene expression, of RGCs by inhibiting the microglial activation after ONT.

CONCLUSIONS

The CX3CL1-CX3CR1 axis could promote survival and axonal regeneration of RGCs by inhibiting the microglial activation after optic nerve injury. The CX3CL1-CX3CR1 axis may become a potential target for the treatment of optic nerve injury. Forty-eight hours is the longest time window for effective treatment after injury. The study is expected to provide new ideas for the development of targeted drugs for the repair of optic nerve.

Injury distance limits the transcriptional response to spinal injury

The ability of neurons to sense and respond to damage is fundamental to homeostasis and nervous system repair. For some cell types, notably dorsal root ganglia (DRG) and retinal ganglion cells (RGCs), extensive profiling has revealed a large transcriptional response to axon injury that determines survival and regenerative outcomes. In contrast, the injury response of most supraspinal cell types, whose limited regeneration constrains recovery from spinal injury, is mostly unknown. Here we employed single-nuclei sequencing in mice to profile the transcriptional responses of diverse supraspinal cell types to spinal injury. Surprisingly, thoracic spinal injury triggered only modest changes in gene expression across all populations, including corticospinal tract (CST) neurons. Moreover, CST neurons also responded minimally to cervical injury but much more strongly to intracortical axotomy, including upregulation of numerous regeneration and apoptosis-related transcripts shared with injured DRG and RGC neurons. Thus, the muted response of CST neuron to spinal injury is linked to the injury's distal location, rather than intrinsic cellular characteristics. More broadly, these findings indicate that a central challenge for enhancing regeneration after a spinal injury is the limited sensing of distant injuries and the subsequent modest baseline neuronal response.

MircoRNA-25-3p in skin precursor cell-induced Schwann cell-derived extracellular vesicles promotes axon regeneration by targeting Tgif1

Nerve injury often leads to severe dysfunction because of the lack of axon regeneration in adult mammal. Intriguingly a series of extracellular vesicles (EVs) have the obvious ability to accelerate the nerve repair. However, the detailed molecular mechanisms to describe that EVs switch neuron from a transmitter to a regenerative state have not been elucidated. This study elucidated the microRNA (miRNA) expression profiles of two types of EVs that promote nerve regeneration. The functions of these miRNAs were screened in vitro. Among the 12 overlapping miRNAs, miR-25-3p was selected for further analysis as it markedly promoted axon regeneration both in vivo and in vitro. Furthermore, knockdown experiments confirmed that PTEN and Klf4, which are the major inhibitors of axon regeneration, were the direct targets of miR-25-3p in dorsal root ganglion (DRG) neurons. The utilization of luciferase reporter assays and functional tests provided evidence that miR-25-3p enhances axon regeneration by targeting Tgif1. Additionally, miR-25-3p upregulated the phosphorylation of Erk. Furthermore, Rapamycin modulated the expression of miR-25-3p in DRG neurons. Finally, the pro-axon regeneration effects of EVs were confirmed by overexpressing miR-25-3p and Tgif1 knockdown in the optic nerve crush model. Thus, the enrichment of miR-25-3p in EVs suggests that it regulates axon regeneration, proving a potential cell-free treatment strategy for nerve injury.

Functional Recovery Associated with Dendrite Regeneration in PVD Neuron of Caenorhabditis elegans

PVD neuron of Caenorhabditis elegans is a highly polarized cell with well-defined axonal, and dendritic compartments. PVD neuron operates in multiple sensory modalities including the control of both nociceptive touch sensation and body posture. Although both the axon and dendrites of this neuron show a regeneration response following laser-assisted injury, it is rather unclear how the behavior associated with this neuron is affected by the loss of these structures. It is also unclear whether neurite regrowth would lead to functional restoration in these neurons. Upon axotomy, using a femtosecond laser, we saw that harsh touch response was specifically affected leaving the body posture unperturbed. Subsequently, recovery in the touch response is highly correlated to the axon regrowth, which was dependent on DLK-1/MLK-1 MAP Kinase. Dendrotomy of both major and minor primary dendrites affected the wavelength and amplitude of sinusoidal movement without any apparent effect on harsh touch response. We further correlated the recovery in posture behavior to the type of dendrite regeneration events. We found that dendrite regeneration through the fusion and reconnection between the proximal and distal branches of the injured dendrite corresponded to improved recovery in posture. Our data revealed that the axons and dendrites of PVD neurons regulate the nociception and proprioception in worms, respectively. It also revealed that dendrite and axon regeneration lead to the restoration of these differential sensory modalities.

Spinal microcircuits go through multiphasic homeostatic compensations in a mouse model of motoneuron degeneration

In neurological conditions affecting the brain, early-stage neural circuit adaption is key for long-term preservation of normal behaviour. We tested if motoneurons and respective microcircuits also adapt in the initial stages of disease progression in a mouse model of progressive motoneuron degeneration. Using a combination of in vitro and in vivo electrophysiology and super-resolution microscopy, we found that, preceding muscle denervation and motoneuron death, recurrent inhibition mediated by Renshaw cells is reduced in half due to impaired quantal size associated with decreased glycine receptor density. Additionally, higher probability of release from proprioceptive Ia terminals leads to increased monosynaptic excitation to motoneurons. Surprisingly, the initial impairment in recurrent inhibition is not a widespread feature of inhibitory spinal circuits, such as group I inhibitory afferents, and is compensated at later stages of disease progression. We reveal that in disease conditions, spinal microcircuits undergo specific multiphasic homeostatic compensations to preserve force output.

Chronic chemogenetic inhibition of TRPV1 bladder afferent promotes micturition recovery post SCI

Spinal cord injury often results in chronic loss of micturition control, which is featured by bladder hyperreflexia and detrusor sphincter dyssynergia. Previous studies showed that treatment of capsaicin reduces non-voiding bladder contractions in multiple animal injury models and human patients. However, its underlying neural mechanisms remain largely unknown. Here, by injecting a RetroAAV into the bladder wall, we specifically targeted TRPV1+, a capsaicin receptor, bladder afferent neurons. Morphometric analysis revealed borderline increase of the soma size and significant spinal axon sprouting of TRPV1+ bladder afferent neurons post a complete T8 spinal cord crush. We further demonstrated that chronic chemogenetic inhibition of these DRG neurons improved micturition recovery after SCI by increasing voiding efficiency and alleviating bladder hyperreflexia, along with reduced morphological changes caused by injury. Our study provided novel insights into the structural and functional changes of TRPV1+ bladder afferent post SCI and further supports the clinical use of capsaicin as an effective treatment to improve bladder functions in patients with SCI.

Perspectives on the Novel Multifunctional Nerve Guidance Conduits: From Specific Regenerative Procedures to Motor Function Rebuilding

Peripheral nerve injury potentially destroys the quality of life by inducing functional movement disorders and sensory capacity loss, which results in severe disability and substantial psychological, social, and financial burdens. Autologous nerve grafting has been commonly used as treatment in the clinic; however, its rare donor availability limits its application. A series of artificial nerve guidance conduits (NGCs) with advanced architectures are also proposed to promote injured peripheral nerve regeneration, which is a complicated process from axon sprouting to targeted muscle reinnervation. Therefore, exploring the interactions between sophisticated NGC complexes and versatile cells during each process including axon sprouting, Schwann cell dedifferentiation, nerve myelination, and muscle reinnervation is necessary. This review highlights the contribution of functional NGCs and the influence of microscale biomaterial architecture on biological processes of nerve repair. Progressive NGCs with chemical molecule induction, heterogenous topographical morphology, electroactive, anisotropic assembly microstructure, and self-powered electroactive and magnetic-sensitive NGCs are also collected, and they are expected to be pioneering features in future multifunctional and effective NGCs.

Cell-free fat extract promotes axon regeneration and retinal ganglion cells survival in traumatic optic neuropathy

Injuries to axons within the central nervous system (CNS) pose a substantial clinical challenge due to their limited regenerative capacity. This study investigates the therapeutic potential of Cell-free fat extract (CEFFE) in CNS injury. CEFFE was injected intravitreally after the optic nerve was crushed. Two weeks post-injury, quantification of regenerated axons and survival rates of retinal ganglion cells (RGCs) were performed. Subsequently, comprehensive gene ontology (GO) an-notation elucidated the cellular origins and functional attributes of CEFFE components. Molecular mechanisms underlying CEFFE's therapeutic effects were explored through Western blotting (WB). Additionally, levels of inflammatory factors within CEFFE were determined using enzyme-linked immunosorbent assay (ELISA), and histological staining of microglia was conducted to assess its impact on neuroinflammation. CEFFE demonstrated a significant capacity to promote axon re-generation and enhance RGCs survival. GO annotation revealed the involvement of 146 proteins within CEFFE in axonogenesis and neurogenesis. WB analysis unveiled the multifaceted pathways through which CEFFE exerts its therapeutic effects. Elevated levels of inflammatory factors were detected through ELISA, and CEFFE exhibited a modulatory effect on microglial activation in the retinal tissue following optic nerve crush (ONC). The present study highlights the therapeutic promise of CEFFE in the management of CNS injuries, exemplified by its ability to foster axon regeneration and improve RGCs survival.

Interleukin-4 from curcumin-activated OECs emerges as a central modulator for increasing M2 polarization of microglia/macrophage in OEC anti-inflammatory activity for functional repair of spinal cord injury

Microglia/macrophages are major contributors to neuroinflammation in the central nervous system (CNS) injury and exhibit either pro- or anti-inflammatory phenotypes in response to specific microenvironmental signals. Our latest in vivo and in vitro studies demonstrated that curcumin-treated olfactory ensheathing cells (aOECs) can effectively enhance neural survival and axonal outgrowth, and transplantation of aOECs improves the neurological outcome after spinal cord injury (SCI). The therapeutic effect is largely attributed to aOEC anti-inflammatory activity through the modulation of microglial polarization from the M1 to M2 phenotype. However, very little is known about what viable molecules from aOECs are actively responsible for the switch of M1 to M2 microglial phenotypes and the underlying mechanisms of microglial polarization. Herein, we show that Interleukin-4 (IL-4) plays a leading role in triggering the M1 to M2 microglial phenotype, appreciably decreasing the levels of M1 markers IL‑1β, IL‑6, tumour necrosis factor-alpha (TNF-α) and inducible nitric oxide synthase (iNOS) and elevating the levels of M2 markers Arg-1, TGF-β, IL-10, and CD206. Strikingly, blockade of IL-4 signaling by siRNA and a neutralizing antibody in aOEC medium reverses the transition of M1 to M2, and the activated microglia stimulated with the aOEC medium lacking IL-4 significantly decreases neuronal survival and neurite outgrowth. In addition, transplantation of aOECs improved the neurological function deficits after SCI in rats. More importantly, the crosstalk between JAK1/STAT1/3/6-targeted downstream signals and NF-κB/SOCS1/3 signaling predominantly orchestrates IL-4-modulated microglial polarization event. These results provide new insights into the molecular mechanisms of aOECs driving the M1-to-M2 shift of microglia and shed light on new therapies for SCI through the modulation of microglial polarization.

Arsenic-induced IGF-1 signaling impairment and neurite shortening: The protective roles of IGF-1 through the PI3K/Akt axis

We recently reported that arsenic caused insulin resistance in differentiated human neuroblastoma SH-SY5Y cells. Herein, we further investigated the effects of sodium arsenite on IGF-1 signaling, which shares downstream signaling with insulin. A time-course experiment revealed that sodium arsenite began to decrease IGF-1-stimulated Akt phosphorylation on Day 3 after treatment, indicating that prolonged sodium arsenite treatment disrupted the neuronal IGF-1 response. Additionally, sodium arsenite decreased IGF-1-stimulated tyrosine phosphorylation of the IGF-1 receptor β (IGF-1Rβ) and its downstream target, insulin receptor substrate 1 (IRS1). These results suggested that sodium arsenite impaired the intrinsic tyrosine kinase activity of IGF-1Rβ, ultimately resulting in a reduction in tyrosine-phosphorylated IRS1. Sodium arsenite also reduced IGF-1 stimulated tyrosine phosphorylation of insulin receptor β (IRβ), indicating the potential inhibition of IGF-1R/IR crosstalk by sodium arsenite. Interestingly, sodium arsenite also induced neurite shortening at the same concentrations that caused IGF-1 signaling impairment. A 24-h IGF-1 treatment partially rescued neurite shortening caused by sodium arsenite. Moreover, the reduction in Akt phosphorylation by sodium arsenite was attenuated by IGF-1. Inhibition of PI3K/Akt by LY294002 diminished the protective effects of IGF-1 against sodium arsenite-induced neurite retraction. Together, our findings suggested that sodium arsenite-impaired IGF-1 signaling, leading to neurite shortening through IGF-1/PI3K/Akt.

Identification and Analysis of Axolotl Homologs for Proteins Implicated in Human Neurodegenerative Proteinopathies

Neurodegenerative proteinopathies such as Alzheimer's Disease are characterized by abnormal protein aggregation and neurodegeneration. Neuroresilience or regenerative strategies to prevent neurodegeneration, preserve function, or restore lost neurons may have the potential to combat human proteinopathies; however, the adult human brain possesses a limited capacity to replace lost neurons. In contrast, axolotls (Ambystoma mexicanum) show robust brain regeneration. To determine whether axolotls may help identify potential neuroresilience or regenerative strategies in humans, we first interrogated whether axolotls express putative proteins homologous to human proteins associated with neurodegenerative diseases. We compared the homology between human and axolotl proteins implicated in human proteinopathies and found that axolotls encode proteins highly similar to human microtubule-binding protein tau (tau), amyloid precursor protein (APP), and β-secretase 1 (BACE1), which are critically involved in human proteinopathies like Alzheimer's Disease. We then tested monoclonal Tau and BACE1 antibodies previously used in human and rodent neurodegenerative disease studies using immunohistochemistry and western blotting to validate the homology for these proteins. These studies suggest that axolotls may prove useful in studying the role of these proteins in disease within the context of neuroresilience and repair.

MicroRNA-9 promotes axon regeneration of mauthner-cell in zebrafish via her6/ calcium activity pathway

MicroRNA (miRNA), functioning as a post-transcriptional regulatory element, plays a significant role in numerous regulatory mechanisms and serves as a crucial intrinsic factor influencing axon regeneration. Prior investigations have elucidated the involvement of miRNA-9 in various processes, however, its specific contribution to axon regeneration in the central nervous system (CNS) remains uncertain. Hence, the zebrafish Mauthner axon regeneration model was employed to manipulate the expression of miRNA-9 in single cells, revealing that upregulation of miRNA-9 facilitated axon regeneration. Additionally, her6, a downstream target gene of miRNA-9, was identified as a novel gene associated with axon regeneration. Suppression of her6 resulted in enhanced Mauthner axon regeneration, as evidenced by the significantly improved regenerative capacity observed in her6 knockout zebrafish. In addition, modulation of her6 expression affects intracellular calcium levels in neurons and promoting her6 expression leads to a decrease in calcium levels in vivo using the new NEMOf calcium indicator. Moreover, the administration of the neural activity activator, pentylenetetrazol (PTZ) partially compensated for the inhibitory effect of her6 overexpression on the calcium level and promoted axon regeneration. Taken together, our study revealed a role for miRNA-9 in the process of axon regeneration in the CNS, which improved intracellular calcium activity and promoted axon regeneration by inhibiting the expression of downstream target gene her6. In our study, miRNA-9 emerged as a novel and intriguing target in the intricate regulation of axon regeneration and offered compelling evidence for the intricate relationship between calcium activity and the facilitation of axon regeneration.

Genes in Axonal Regeneration

This review explores the molecular and genetic underpinnings of axonal regeneration and functional recovery post-nerve injury, emphasizing its significance in reversing neurological deficits. It presents a systematic exploration of the roles of various genes in axonal regrowth across peripheral and central nerve injuries. Initially, it highlights genes and gene families critical for axonal growth and guidance, delving into their roles in regeneration. It then examines the regenerative microenvironment, focusing on the role of glial cells in neural repair through dedifferentiation, proliferation, and migration. The concept of "traumatic microenvironments" within the central nervous system (CNS) and peripheral nervous system (PNS) is discussed, noting their impact on regenerative capacities and their importance in therapeutic strategy development. Additionally, the review delves into axonal transport mechanisms essential for accurate growth and reinnervation, integrating insights from proteomics, genome-wide screenings, and gene editing advancements. Conclusively, it synthesizes these insights to offer a comprehensive understanding of axonal regeneration's molecular orchestration, aiming to inform effective nerve injury therapies and contribute to regenerative neuroscience.

O-GlcNAc signaling increases neuron regeneration through one-carbon metabolism in Caenorhabditis elegans

Cellular metabolism plays an essential role in the regrowth and regeneration of a neuron following physical injury. Yet, our knowledge of the specific metabolic pathways that are beneficial to neuron regeneration remains sparse. Previously, we have shown that modulation of O-linked β-N-acetylglucosamine (O-GlcNAc) signaling, a ubiquitous post-translational modification that acts as a cellular nutrient sensor, can significantly enhance in vivo neuron regeneration. Here, we define the specific metabolic pathway by which O-GlcNAc transferase (ogt-1) loss of function mediates increased regenerative outgrowth. Performing in vivo laser axotomy and measuring subsequent regeneration of individual neurons in C. elegans, we find that glycolysis, serine synthesis pathway (SSP), one-carbon metabolism (OCM), and the downstream transsulfuration metabolic pathway (TSP) are all essential in this process. The regenerative effects of ogt-1 mutation are abrogated by genetic and/or pharmacological disruption of OCM and the SSP linking OCM to glycolysis. Testing downstream branches of this pathway, we find that enhanced regeneration is dependent only on the vitamin B12 independent shunt pathway. These results are further supported by RNA sequencing that reveals dramatic transcriptional changes by the ogt-1 mutation, in the genes involved in glycolysis, OCM, TSP, and ATP metabolism. Strikingly, the beneficial effects of the ogt-1 mutation can be recapitulated by simple metabolic supplementation of the OCM metabolite methionine in wild-type animals. Taken together, these data unearth the metabolic pathways involved in the increased regenerative capacity of a damaged neuron in ogt-1 animals and highlight the therapeutic possibilities of OCM and its related pathways in the treatment of neuronal injury.

Histone methyltransferase Ezh2 coordinates mammalian axon regeneration via regulation of key regenerative pathways

Current treatments for neurodegenerative diseases and neural injuries face major challenges, primarily due to the diminished regenerative capacity of neurons in the mammalian CNS as they mature. Here, we investigated the role of Ezh2, a histone methyltransferase, in regulating mammalian axon regeneration. We found that Ezh2 declined in the mouse nervous system during maturation but was upregulated in adult dorsal root ganglion neurons following peripheral nerve injury to facilitate spontaneous axon regeneration. In addition, overexpression of Ezh2 in retinal ganglion cells in the CNS promoted optic nerve regeneration via both histone methylation-dependent and -independent mechanisms. Further investigation revealed that Ezh2 fostered axon regeneration by orchestrating the transcriptional silencing of genes governing synaptic function and those inhibiting axon regeneration, while concurrently activating various factors that support axon regeneration. Notably, we demonstrated that GABA transporter 2, encoded by Slc6a13, acted downstream of Ezh2 to control axon regeneration. Overall, our study underscores the potential of modulating chromatin accessibility as a promising strategy for promoting CNS axon regeneration.

Optic nerve regeneration: Potential treatment approaches

The optic nerve, predominantly constituted by the axons of retinal ganglion cells (RGCs), lacks the ability to regenerate and re-establish function after injury. RGCs are crucial for visual function, and thus, RGC death contributes to the development of numerous progressive neurodegenerative optic neuropathies including glaucoma, ischemic optic neuropathy, and optic neuritis. Regenerating optic nerve axons poses numerous challenges due to factors such as the intricate and inhibitory conditions that exist within their environment, intrinsic breaks to regeneration, and the geometric tortuosity that offers physical hindrance to axon growth. However, recent research advancements offer hope for clinically meaningful regeneration for those who suffer from optic nerve damage. In this review, we highlight the current treatment approaches for optic nerve axon regeneration.

Auditory robustness and resilience in the aging auditory system of the desert locust

After overexposure to loud music, we experience a decrease in our ability to hear (robustness), which usually recovers (resilience). Here, we exploited the amenable auditory system of the desert locust, Schistocerca gregaria, to measure how robustness and resilience depend on age. We found that gene expression changes are dominated by age as opposed to noise exposure. We measured sound-evoked nerve activity for young and aged locusts directly, after 24 hours and 48 hours after noise exposure. We found that both young and aged locusts recovered their auditory nerve function over 48 hours. We also measured the sound-evoked transduction current in individual auditory neurons, and although the transduction current magnitude recovered in the young locusts after noise exposure, it failed to recover in the aged locusts. A plastic mechanism compensates for the decreased transduction current in aged locusts. We suggest key genes upregulated in young noise-exposed locusts that mediate robustness to noise exposure and find potential candidates responsible for compensatory mechanisms in the auditory neurons of aged noise-exposed locusts.

Chemogenetic stimulation of intact corticospinal tract during rehabilitative training promotes circuit rewiring and functional recovery after stroke

BACKGROUND

Neuromodulatory techniques have been proven to enhance functional recovery after stroke in patients and animals, such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS). However, the success and feasibility of these approaches were often variable, largely due to a lack of target specificity.

OBJECTIVE

We explored the effects of specific chemogenetic stimulation of intact corticospinal tract during rehabilitative training on functional recovery after stroke in mice.

METHODS

We developed a viral-based intersectional targeting approach that allows specific chemogentic activation of contralateral hindlimb corticospinal neurons (CSNs) in a photothrombotic stroke model.

RESULTS

We demonstrated that specific chemogenetic activation of CSNs, when combined with daily rehabilitation training, leads to significant skilled motor functional recovery via promoting corticospinal tract (CST) axons midline crossing sprouting from intact to the denervated spinal hemicord, and rewiring new functional circuits by new synapse formation. Mechanistically, we revealed that combined chemogenetic stimulation of CSNs and daily rehabilitation training significantly enhanced the mTOR activity of CSNs.

CONCLUSIONS

Our findings highlight the great potential of specific neural activation protocols in combination with motor training for the recovery of skilled motor functions after stroke.

Unleashing Axonal Regeneration Capacities: Neuronal and Non-neuronal Changes After Injuries to Dorsal Root Ganglion Neuron Central and Peripheral Axonal Branches

Peripheral nerves obtain remarkable regenerative capacity while central nerves can hardly regenerate following nerve injury. Sensory neurons in the dorsal root ganglion (DRG) are widely used to decipher the dissimilarity between central and peripheral axonal regeneration as axons of DRG neurons bifurcate into the regeneration-incompetent central projections and the regeneration-competent peripheral projections. A conditioning peripheral branch injury facilitates central axonal regeneration and enables the growth and elongation of central axons. Peripheral axonal injury stimulates neuronal calcium influx, alters the start-point chromatin states, increases chromatin accessibility, upregulates the expressions of regeneration-promoting genes and the synthesis of proteins, and supports axonal regeneration. Following central axonal injury, the responses of DRG neurons are modest, resulting in poor intrinsic growth ability. Some non-neuronal cells in DRGs, for instance satellite glial cells, also exhibit diminished injury responses to central axon injury as compared with peripheral axon injury. Moreover, DRG central and peripheral axonal branches are respectively surrounded by inhibitory glial scars generated by central glial cells and a permissive microenvironment generated by Schwann cells and macrophages. The aim of this review is to look at changes of DRG neurons and non-neuronal cells after peripheral and central axon injuries and summarize the contributing roles of both neuronal intrinsic regenerative capacities and surrounding microenvironments in axonal regeneration.

Mimicked Spinal Cord Fibers Trigger Axonal Regeneration and Remyelination after Injury

Spinal cord injury (SCI) causes tissue structure damage and composition changes of the neural parenchyma, resulting in severe consequences for spinal cord function. Mimicking the components and microstructure of spinal cord tissues holds promise for restoring the regenerative microenvironment after SCI. Here, we have utilized electrospinning technology to develop aligned decellularized spinal cord fibers (A-DSCF) without requiring synthetic polymers or organic solvents. A-DSCF preserves multiple types of spinal cord extracellular matrix proteins and forms a parallel-oriented structure. Compared to aligned collagen fibers (A-CF), A-DSCF exhibits stronger mechanical properties, improved enzymatic stability, and superior functionality in the adhesion, proliferation, axonal extension, and myelination of differentiated neural progenitor cells (NPCs). Notably, axon extension or myelination has been primarily linked to Agrin (AGRN), Laminin (LN), or Collagen type IV (COL IV) proteins in A-DSCF. When transplanted into rats with complete SCI, A-DSCF loaded with NPCs improves the survival, maturation, axon regeneration, and motor function of the SCI rats. These findings highlight the potential of structurally and compositionally biomimetic scaffolds to promote axonal extension and remyelination after SCI.

Synergistic effect of chemogenetic activation of corticospinal motoneurons and physical exercise in promoting functional recovery after spinal cord injury

Single therapeutic interventions have not yet been successful in restoring function after spinal cord injury. Accordingly, combinatorial interventions targeting multiple factors may hold greater promise for achieving maximal functional recovery. In this study, we applied a combinatorial approach of chronic chemogenetic neuronal activation and physical exercise including treadmill running and forelimb training tasks to promote functional recovery. In a mouse model of cervical (C5) dorsal hemisection of the spinal cord, which transects almost all descending corticospinal tract axons, combining selective activation of corticospinal motoneurons (CMNs) by intersectional chemogenetics with physical exercise significantly promoted functional recovery evaluated by the grid walking test, grid hanging test, rotarod test, and single pellet-reaching tasks. Electromyography and histological analysis showed increased activation of forelimb muscles via chemogenetic stimuli, and a greater density of vGlut1+ innervation in spinal cord grey matter rostral to the injury, suggesting enhanced neuroplasticity and connectivity. Combined therapy also enhanced activation of mTOR signaling and reduced apoptosis in spinal motoneurons, Counts revealed increased numbers of detectable choline acetyltransferase-positive motoneurons in the ventral horn. Taken together, the findings from this study validate a novel combinatorial approach to enhance motor function after spinal cord injury.

Anatomical Diversity of the Adult Corticospinal Tract Revealed by Single-Cell Transcriptional Profiling

The corticospinal tract (CST) forms a central part of the voluntary motor apparatus in all mammals. Thus, injury, disease, and subsequent degeneration within this pathway result in chronic irreversible functional deficits. Current strategies to repair the damaged CST are suboptimal in part because of underexplored molecular heterogeneity within the adult tract. Here, we combine spinal retrograde CST tracing with single-cell RNA sequencing (scRNAseq) in adult male and female mice to index corticospinal neuron (CSN) subtypes that differentially innervate the forelimb and hindlimb. We exploit publicly available datasets to confer anatomic specialization among CSNs and show that CSNs segregate not only along the forelimb and hindlimb axis but also by supraspinal axon collateralization. These anatomically defined transcriptional data allow us to use machine learning tools to build classifiers that discriminate between CSNs and cortical layer 2/3 and nonspinally terminating layer 5 neurons in M1 and separately identify limb-specific CSNs. Using these tools, CSN subtypes can be differentially identified to study postnatal patterning of the CST in vivo, leveraged to screen for novel limb-specific axon growth survival and growth activators in vitro, and ultimately exploited to repair the damaged CST after injury and disease.SIGNIFICANCE STATEMENT Therapeutic interventions designed to repair the damaged CST after spinal cord injury have remained functionally suboptimal in part because of an incomplete understanding of the molecular heterogeneity among subclasses of CSNs. Here, we combine spinal retrograde labeling with scRNAseq and annotate a CSN index by the termination pattern of their primary axon in the cervical or lumbar spinal cord and supraspinal collateral terminal fields. Using machine learning we have confirmed the veracity of our CSN gene lists to train classifiers to identify CSNs among all classes of neurons in primary motor cortex to study the development, patterning, homeostasis, and response to injury and disease, and ultimately target streamlined repair strategies to this critical motor pathway.

Fluorescence imaging of peripheral nerve function and structure

Peripheral nerve injuries are common and can cause catastrophic consequences. Although peripheral nerves have notable regenerative capacity, full functional recovery is often challenging due to a number of factors, including age, the type of injury, and delayed healing, resulting in chronic disorders that cause lifelong miseries and significant financial burdens. Fluorescence imaging, among the various techniques, may be the key to overcome these restrictions and improve the prognosis because of its feasibility and dynamic real-time imaging. Intraoperative dynamic fluorescence imaging allows the visualization of the morphological structure of the nerve so that surgeons can reduce the incidence of medically induced injury. Axoplasmic transport-based neuroimaging allows the visualization of the internal transport function of the nerve, facilitating early, objective, and accurate assessment of the degree of regenerative repair, allowing early intervention in patients with poor recovery, thereby improving prognosis. This review briefly discusses peripheral nerve fluorescent dyes that have been reported or could potentially be employed, with a focus on their role in visualizing the nerve's function and anatomy.

Genetic Features of Young and Aged Animals After Peripheral Nerve Injury: Implications for Diminished Regeneration Capacity

The spontaneous regeneration capacity of peripheral nerves is fundamentally reduced with advancing age, leading to severe and long-term functional loss. The cellular and molecular basis underlying incomplete and delayed recovery of aging peripheral nerves is still murky. Here, we collected sciatic nerves of aged rats at 1d, 4d, and 7d after nerve injury, systematically analyzed the transcriptional changes of injured sciatic nerves, and examined the differences of injury responses between aged rats and young rats. RNA sequencing revealed that sciatic nerves of aged and young rats exhibit distinctive expression patterns after nerve injury. Acute and vigorous immune responses, including motivated B cell receptor signaling pathway, occurred in injured sciatic nerves of both aged and young rats. Different from young rats, aged rats have more CD8+ T cells and B cells in normal state and the elevation of M2 macrophages seemed to be more robust in sciatic nerves, especially at later time points after nerve injury. Young rats, on the other hand, showed strong and early up-regulation of cell cycle-related genes. These identified unique transcriptional signatures of aged and young rats help the understanding of aged-associated injury responses in the wound microenvironments and provide essential basis for the treatment of regeneration deficits in aged population.

Syntaphilin Inactivation Can Enhance Axonal Mitochondrial Transport to Improve Spinal Cord Injury

Mitochondria are important organelle of eukaryotic cells. They consists of a large number of different proteins that provide most of the ATP and supply power for the growth, function, and regeneration of neurons. Therefore, smitochondrial transport ensures that adequate ATP is supplied for metabolic activities. Spinal cord injury (SCI), a detrimental condition, has high morbidity and mortality rates. Currently, the available treatments only provide symptomatic relief for long-term disabilities. Studies have implicated mitochondrial transport as a critical factor in axonal regeneration. Hence, enhancing mitochondrial transports could be beneficial for ameliorating SCI. Syntaphilin (Snph) is a mitochondrial docking protein that acts as a "static anchor," and its inhibition enhances mitochondrial transports. Therefore, Snph as a key mediator of mitochondrial transports, may contribute to improving axonal regeneration following SCI. Herein, we examine Snph's biological effects and its relation to mitochondrial pathway. Then, we elaborate on mitochondrial transports after SCI, the possible role of Snph in SCI, and some possible therapeutic approaches by Snph.

Unveiling adcyap1 as a protective factor linking pain and nerve regeneration through single-cell RNA sequencing of rat dorsal root ganglion neurons

BACKGROUND

Severe peripheral nerve injury (PNI) often leads to significant movement disorders and intractable pain. Therefore, promoting nerve regeneration while avoiding neuropathic pain is crucial for the clinical treatment of PNI patients. However, established animal models for peripheral neuropathy fail to accurately recapitulate the clinical features of PNI. Additionally, researchers usually investigate neuropathic pain and axonal regeneration separately, leaving the intrinsic relationship between the development of neuropathic pain and nerve regeneration after PNI unclear. To explore the underlying connections between pain and regeneration after PNI and provide potential molecular targets, we performed single-cell RNA sequencing and functional verification in an established rat model, allowing simultaneous study of the neuropathic pain and axonal regeneration after PNI.

RESULTS

First, a novel rat model named spared nerve crush (SNC) was created. In this model, two branches of the sciatic nerve were crushed, but the epineurium remained unsevered. This model successfully recapitulated both neuropathic pain and axonal regeneration after PNI, allowing for the study of the intrinsic link between these two crucial biological processes. Dorsal root ganglions (DRGs) from SNC and naïve rats at various time points after SNC were collected for single-cell RNA sequencing (scRNA-seq). After matching all scRNA-seq data to the 7 known DRG types, we discovered that the PEP1 and PEP3 DRG neuron subtypes increased in crushed and uncrushed DRG separately after SNC. Using experimental design scRNA-seq processing (EDSSP), we identified Adcyap1 as a potential gene contributing to both pain and nerve regeneration. Indeed, repeated intrathecal administration of PACAP38 mitigated pain and facilitated axonal regeneration, while Adcyap1 siRNA or PACAP6-38, an antagonist of PAC1R (a receptor of PACAP38) led to both mechanical hyperalgesia and delayed DRG axon regeneration in SNC rats. Moreover, these effects can be reversed by repeated intrathecal administration of PACAP38 in the acute phase but not the late phase after PNI, resulting in alleviated pain and promoted axonal regeneration.

CONCLUSIONS

Our study reveals that Adcyap1 is an intrinsic protective factor linking neuropathic pain and axonal regeneration following PNI. This finding provides new potential targets and strategies for early therapeutic intervention of PNI.

Glia instruct axon regeneration via a ternary modulation of neuronal calcium channels in Drosophila

A neuron's regenerative capacity is governed by its intrinsic and extrinsic environment. Both peripheral and central neurons exhibit cell-type-dependent axon regeneration, but the underlying mechanism is unclear. Glia provide a milieu essential for regeneration. However, the routes of glia-neuron signaling remain underexplored. Here, we show that regeneration specificity is determined by the axotomy-induced Ca2+ transients only in the fly regenerative neurons, which is mediated by L-type calcium channels, constituting the core intrinsic machinery. Peripheral glia regulate axon regeneration via a three-layered and balanced modulation. Glia-derived tumor necrosis factor acts through its neuronal receptor to maintain calcium channel expression after injury. Glia sustain calcium channel opening by enhancing membrane hyperpolarization via the inwardly-rectifying potassium channel (Irk1). Glia also release adenosine which signals through neuronal adenosine receptor (AdoR) to activate HCN channels (Ih) and dampen Ca2+ transients. Together, we identify a multifaceted glia-neuron coupling which can be hijacked to promote neural repair.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Chronic stress hinders sensory axon regeneration via impairing mitochondrial cristae and OXPHOS

Spinal cord injury (SCI) often leads to physical limitations, persistent pain, and major lifestyle shifts, enhancing the likelihood of prolonged psychological stress and associated disorders such as anxiety and depression. The mechanisms linking stress with regeneration remain elusive, despite understanding the detrimental impact of chronic stress on SCI recovery. In this study, we investigated the effect of chronic stress on primary sensory axon regeneration using a preconditioning lesions mouse model. Our data revealed that chronic stress-induced mitochondrial cristae loss and a decrease in oxidative phosphorylation (OXPHOS) within primary sensory neurons, impeding central axon regrowth. Corticosterone, a stress hormone, emerged as a pivotal player in this process, affecting satellite glial cells by reducing Kir4.1 expression. This led to increased neuronal hyperactivity and reactive oxygen species levels, which, in turn, deformed mitochondrial cristae and impaired OXPHOS, crucial for axonal regeneration. Our study underscores the need to manage psychological stress in patients with SCI for effective sensory-motor rehabilitation.

Osteopontin drives retinal ganglion cell resiliency in glaucomatous optic neuropathy

Chronic neurodegeneration and acute injuries lead to neuron losses via diverse processes. We compared retinal ganglion cell (RGC) responses between chronic glaucomatous conditions and the acute injury model. Among major RGC subclasses, αRGCs and intrinsically photosensitive RGCs (ipRGCs) preferentially survive glaucomatous conditions, similar to findings in the retina subject to axotomy. Focusing on an αRGC intrinsic factor, Osteopontin (secreted phosphoprotein 1 [Spp1]), we found an ectopic neuronal expression of Osteopontin (Spp1) in other RGCs subject to glaucomatous conditions. This contrasted with the Spp1 downregulation subject to axotomy. αRGC-specific Spp1 elimination led to significant αRGC loss, diminishing their resiliency. Spp1 overexpression led to robust neuroprotection of susceptible RGC subclasses under glaucomatous conditions. In contrast, Spp1 overexpression did not significantly protect RGCs subject to axotomy. Additionally, SPP1 marked adult human RGC subsets with large somata and SPP1 expression in the aqueous humor correlated with glaucoma severity. Our study reveals Spp1's role in mediating neuronal resiliency in glaucoma.

Oligodendrocyte precursor cells stop sensory axons regenerating into the spinal cord

Primary somatosensory axons stop regenerating as they re-enter the spinal cord, resulting in incurable sensory loss. What arrests them has remained unclear. We previously showed that axons stop by forming synaptic contacts with unknown non-neuronal cells. Here, we identified these cells in adult mice as oligodendrocyte precursor cells (OPCs). We also found that only a few axons stop regenerating by forming dystrophic endings, exclusively at the CNS:peripheral nervous system (PNS) borderline where OPCs are absent. Most axons stop in contact with a dense network of OPC processes. Live imaging, immuno-electron microscopy (immuno-EM), and OPC-dorsal root ganglia (DRG) co-culture additionally suggest that axons are rapidly immobilized by forming synapses with OPCs. Genetic OPC ablation enables many axons to continue regenerating deep into the spinal cord. We propose that sensory axons stop regenerating by encountering OPCs that induce presynaptic differentiation. Our findings identify OPCs as a major regenerative barrier that prevents intraspinal restoration of sensory circuits following spinal root injury.

Recovery of walking after paralysis by regenerating characterized neurons to their natural target region

Axon regeneration can be induced across anatomically complete spinal cord injury (SCI), but robust functional restoration has been elusive. Whether restoring neurological functions requires directed regeneration of axons from specific neuronal subpopulations to their natural target regions remains unclear. To address this question, we applied projection-specific and comparative single-nucleus RNA sequencing to identify neuronal subpopulations that restore walking after incomplete SCI. We show that chemoattracting and guiding the transected axons of these neurons to their natural target region led to substantial recovery of walking after complete SCI in mice, whereas regeneration of axons simply across the lesion had no effect. Thus, reestablishing the natural projections of characterized neurons forms an essential part of axon regeneration strategies aimed at restoring lost neurological functions.

Customization of the translational complex regulates mRNA-specific translation to control CNS regeneration

In the adult mammalian central nervous system (CNS), axons fail to regenerate spontaneously after injury because of a combination of extrinsic and intrinsic factors. Despite recent advances targeting the intrinsic regenerative properties of adult neurons, the molecular mechanisms underlying axon regeneration are not fully understood. Here, we uncover a regulatory mechanism that controls the expression of key proteins involved in regeneration at the translational level. Our results show that mRNA-specific translation is critical for promoting axon regeneration. Indeed, we demonstrate that specific ribosome-interacting proteins, such as the protein Huntingtin (HTT), selectively control the translation of a specific subset of mRNAs. Moreover, modulating the expression of these translationally regulated mRNAs is crucial for promoting axon regeneration. Altogether, our findings highlight that selective translation through the customization of the translational complex is a key mechanism of axon regeneration with major implications in the development of therapeutic strategies for CNS repair.

A spatially specified systems pharmacology therapy for axonal recovery after injury

There are no known drugs or drug combinations that promote substantial central nervous system axonal regeneration after injury. We used systems pharmacology approaches to model pathways underlying axonal growth and identify a four-drug combination that regulates multiple subcellular processes in the cell body and axons using the optic nerve crush model in rats. We intravitreally injected agonists HU-210 (cannabinoid receptor-1) and IL-6 (interleukin 6 receptor) to stimulate retinal ganglion cells for axonal growth. We applied, in gel foam at the site of nerve injury, Taxol to stabilize growing microtubules, and activated protein C to clear the debris field since computational models predicted that this drug combination regulating two subcellular processes at the growth cone produces synergistic growth. Physiologically, drug treatment restored or preserved pattern electroretinograms and some of the animals had detectable visual evoked potentials in the brain and behavioral optokinetic responses. Morphology experiments show that the four-drug combination protects axons or promotes axonal regrowth to the optic chiasm and beyond. We conclude that spatially targeted drug treatment is therapeutically relevant and can restore limited functional recovery.

X chromosome encoded histone demethylase UTX regulates mammalian axon regeneration via microRNA-124

Neurons in the mammalian central nervous system (CNS) gradually lose their intrinsic regeneration capacity during maturation mainly because of altered transcription profile. Recent studies have made great progress by identifying genes that can be manipulated to enhance CNS regeneration. However, as a complex process involving many genes and signaling networks, it is of great importance to deciphering the underlying neuronal chromatin and transcriptomic landscape coordinating CNS regeneration. Here we identify UTX, an X-chromosome associated gene encoding a histone demethylase, as a novel regulator of mammalian neural regeneration. We demonstrate that UTX acts as a repressor of spontaneous axon regeneration in the peripheral nerve system (PNS). In the CNS, either knocking out or pharmacological inhibiting UTX in retinal ganglion cells (RGCs) leads to significantly enhanced neuronal survival and optic nerve regeneration. RNA-seq profiling revealed that deleting UTX switches the RGC transcriptomics into a developmental-like state. Moreover, microRNA-124, one of the most abundant microRNAs in mature neurons, is identified as a downstream target of UTX and blocking endogenous microRNA124-5p results in robust optic nerve regeneration. These findings revealed a novel histone modification-microRNA epigenetic signaling network orchestrating transcriptomic landscape supporting CNS neural regeneration.

The Promising Role of a Zebrafish Model Employed in Neural Regeneration Following a Spinal Cord Injury

Spinal cord injury (SCI) is a devastating event that results in a wide range of physical impairments and disabilities. Despite the advances in our understanding of the biological response to injured tissue, no effective treatments are available for SCIs at present. Some studies have addressed this issue by exploring the potential of cell transplantation therapy. However, because of the abnormal microenvironment in injured tissue, the survival rate of transplanted cells is often low, thus limiting the efficacy of such treatments. Many studies have attempted to overcome these obstacles using a variety of cell types and animal models. Recent studies have shown the utility of zebrafish as a model of neural regeneration following SCIs, including the proliferation and migration of various cell types and the involvement of various progenitor cells. In this review, we discuss some of the current challenges in SCI research, including the accurate identification of cell types involved in neural regeneration, the adverse microenvironment created by SCIs, attenuated immune responses that inhibit nerve regeneration, and glial scar formation that prevents axonal regeneration. More in-depth studies are needed to fully understand the neural regeneration mechanisms, proteins, and signaling pathways involved in the complex interactions between the SCI microenvironment and transplanted cells in non-mammals, particularly in the zebrafish model, which could, in turn, lead to new therapeutic approaches to treat SCIs in humans and other mammals.

Maf1 is an intrinsic suppressor against spontaneous neural repair and functional recovery after ischemic stroke

INTRODUCTION

Spontaneous recovery after CNS injury is often very limited and incomplete, leaving most stroke patients with permanent disability. Maf1 is known as a key growth suppressor in proliferating cells. However, its role in neuronal cells after stroke remains unclear.

OBJECTIVE

We aimed to investigate the mechanistic role of Maf1 in spontaneous neural repair and evaluated the therapeutic effect of targeting Maf1 on stroke recovery.

METHODS

We used mouse primary neurons to determine the signaling mechanism of Maf1, and the cleavage-under-targets-and-tagmentation-sequencing to map the whole-genome promoter binding sites of Maf1 in isolated mature cortical neurons. Photothrombotic stroke model was used to determine the therapeutic effect on neural repair and functional recovery by AAV-mediated Maf1 knockdown.

RESULTS

We found that Maf1 mediates mTOR signaling to regulate RNA polymerase III (Pol III)-dependent rRNA and tRNA transcription in mouse cortical neurons. mTOR regulates neuronal Maf1 phosphorylation and subcellular localization. Maf1 knockdown significantly increases Pol III transcription, neurite outgrowth and dendritic spine formation in neurons. Conversely, Maf1 overexpression suppresses such activities. In response to photothrombotic stroke in mice, Maf1 expression is increased and accumulates in the nucleus of neurons in the peripheral region of infarcted cortex, which is the key region for neural remodeling and repair during spontaneous recovery. Intriguingly, Maf1 knockdown in the peri-infarct cortex significantly enhances neural plasticity and functional recovery. Mechanistically, Maf1 not only interacts with the promoters and represses Pol III-transcribed genes, but also those of CREB-associated genes that are critical for promoting plasticity during neurodevelopment and neural repair.

CONCLUSION

These findings indicate Maf1 as an intrinsic neural repair suppressor against regenerative capability of mature CNS neurons, and suggest that Maf1 is a potential therapeutic target for enhancing functional recovery after ischemic stroke and other CNS injuries.

Circadian clock regulator Bmal1 gates axon regeneration via Tet3 epigenetics in mouse sensory neurons

Axon regeneration of dorsal root ganglia (DRG) neurons after peripheral axotomy involves reconfiguration of gene regulatory circuits to establish regenerative gene programs. However, the underlying mechanisms remain unclear. Here, through an unbiased survey, we show that the binding motif of Bmal1, a central transcription factor of the circadian clock, is enriched in differentially hydroxymethylated regions (DhMRs) of mouse DRG after peripheral lesion. By applying conditional deletion of Bmal1 in neurons, in vitro and in vivo neurite outgrowth assays, as well as transcriptomic profiling, we demonstrate that Bmal1 inhibits axon regeneration, in part through a functional link with the epigenetic factor Tet3. Mechanistically, we reveal that Bmal1 acts as a gatekeeper of neuroepigenetic responses to axonal injury by limiting Tet3 expression and restricting 5hmC modifications. Bmal1-regulated genes not only concern axon growth, but also stress responses and energy homeostasis. Furthermore, we uncover an epigenetic rhythm of diurnal oscillation of Tet3 and 5hmC levels in DRG neurons, corresponding to time-of-day effect on axon growth potential. Collectively, our studies demonstrate that targeting Bmal1 enhances axon regeneration.

Enhancing structural plasticity of PC12 neurons during differentiation and neurite regeneration with a catalytically inactive mutant version of the zRICH protein

BACKGROUND

Studies of the molecular mechanisms of nerve regeneration have led to the discovery of several proteins that are induced during successful nerve regeneration. RICH proteins were identified as proteins induced during the regeneration of the optic nerve of teleost fish. These proteins are 2',3'-cyclic nucleotide, 3'-phosphodiesterases that can bind to cellular membranes through a carboxy-terminal membrane localization domain. They interact with the tubulin cytoskeleton and are able to enhance neuronal structural plasticity by promoting the formation of neurite branches.

RESULTS

PC12 stable transfectant cells expressing a fusion protein combining a red fluorescent protein with a catalytically inactive mutant version of zebrafish RICH protein were generated. These cells were used as a model to analyze effects of the protein on neuritogenesis. Differentiation experiments showed a 2.9 fold increase in formation of secondary neurites and a 2.4 fold increase in branching points. A 2.2 fold increase in formation of secondary neurites was observed in neurite regeneration assays.

CONCLUSIONS

The use of a fluorescent fusion protein facilitated detection of expression levels. Two computer-assisted morphometric analysis methods indicated that the catalytically inactive RICH protein induced the formation of branching points and secondary neurites both during differentiation and neurite regeneration. A procedure based on analysis of random field images provided comparable results to classic neurite tracing methods.

HSPB2 facilitates neural regeneration through autophagy for sensorimotor recovery after traumatic brain injury

Autophagy is a promising target for promoting neural regeneration, which is essential for sensorimotor recovery following traumatic brain injury (TBI). Whether neuronal heat shock protein B2 (HSPB2), a small molecular heat shock protein, reduces injury and promotes recovery following TBI remains unclear. In this study, we demonstrated that HSPB2 was significantly increased in the neurons of a TBI mouse model, patients, and primary neuron cultures subjected to oxygen/glucose deprivation and reperfusion treatment. Upon creating a tamoxifen-induced neuron-specific HSPB2 overexpression transgenic mouse model, we found that elevated HSPB2 levels promoted long-term sensorimotor recovery and alleviated tissue loss after TBI. We also demonstrated that HSPB2 enhanced white matter structural and functional integrity, promoted central nervous system (CNS) plasticity, and accelerated long-term neural remodeling. Moreover, we found that autophagy occurred around injured brain tissues in patients, and the pro-regenerative effects of HSPB2 relied on its autophagy-promoting function. Mechanistically, HSPB2 may regulate autophagy possibly by forming the HSPB2/BCL2-associated athanogene 3/sequestosome-1 complex to facilitate the clearance of erroneously accumulated proteins in the axons. Treatment with the autophagy inhibitor chloroquine during the acute stage or delayed induction of HSPB2 remarkably impeded HSPB2's long-term reparative function, indicating the importance of acute-stage autophagy in long-term neuro-regeneration. Our findings highlight the beneficial role of HSPB2 in neuro-regeneration and functional recovery following acute CNS injury, thereby emphasizing the therapeutic potential of autophagy regulation for enhancing neuro-regeneration.

Fibroblast exosomal TFAP2C induced by chitosan oligosaccharides promotes peripheral axon regeneration via the miR-132-5p/CAMKK1 axis

Chitosan and its degradation product, oligosaccharides, have been shown to facilitate peripheral nerve regeneration. However, the underlying mechanisms are not well understood. In this study, we analyzed the protein expression profiles in sciatic nerves after injury using proteomics. A group of proteins related to exosome packaging and transport is up-regulated by chitosan oligosaccharides (COS), implying that exosomes are involved in COS-induced peripheral nerve regeneration. In fact, exosomes derived from fibroblasts (f-EXOs) treated with COS significantly promoted axon extension and regeneration. Exosomal protein identification and functional studies, revealed that TFAP2C is a key factor in neurite outgrowth induced by COS-f-EXOs. Furthermore, we showed that TFAP2C targets the pri-miRNA-132 gene and represses miR-132-5p expression in dorsal root ganglion neurons. Camkk1 is a downstream substrate of miR-132-5p that positively affects axon extension. In rats, miR-132-5p antagomir stimulates CAMKK1 expression and improves axon regeneration and functional recovery in sciatic nerves after injury. Our data reveal the mechanism for COS in axon regeneration, that is COS induce fibroblasts to produce TFAP2C-enriched EXOs, which are then transferred into axons to promote axon regeneration via miR-132-5p/CAMKK1. Moreover, these results show a new facet of fibroblasts in axon regeneration in peripheral nerves.

Soluble Nogo-Receptor-Fc decoy (AXER-204) in patients with chronic cervical spinal cord injury in the USA: a first-in-human and randomised clinical trial

BACKGROUND

Spinal cord injury (SCI) causes neural disconnection and persistent neurological deficits, so axon sprouting and plasticity might promote recovery. Soluble Nogo-Receptor-Fc decoy (AXER-204) blocks inhibitors of axon growth and promotes recovery of motor function after SCI in animals. This first-in-human and randomised trial sought to determine primarily the safety and pharmacokinetics of AXER-204 in individuals with chronic SCI, and secondarily its effect on recovery.

METHODS

We conducted a two-part study in adults (aged 18-65 years) with chronic (>1 year) cervical traumatic SCI at six rehabilitation centres in the USA. In part 1, AXER-204 was delivered open label as single intrathecal doses of 3 mg, 30 mg, 90 mg, or 200 mg, with primary outcomes of safety and pharmacokinetics. Part 2 was a randomised, parallel, double-blind comparison of six intrathecal doses of 200 mg AXER-204 over 104 days versus placebo. Participants were randomly allocated (1:1) by investigators using a central electronic system, stratified in blocks of four by American Spinal Injury Association Impairment Scale grade and receipt of AXER-204 in part 1. All investigators and patients were masked to treatment allocation until at least day 169. The part 2 primary objectives were safety and pharmacokinetics, with a key secondary objective to assess change in International Standards for Neurological Classification of SCI (ISNCSCI) Upper Extremity Motor Score (UEMS) at day 169 for all enrolled participants. This trial is registered with ClinicalTrials.gov, NCT03989440, and is completed.

FINDINGS

We treated 24 participants in part 1 (six per dose; 18 men, six women), and 27 participants in part 2 (13 placebo, 14 AXER-204; 23 men, four women), between June 20, 2019, and June 21, 2022. There were no deaths and no discontinuations from the study due to an adverse event in part 1 and 2. In part 2, treatment-related adverse events were of similar incidence in AXER-204 and placebo groups (ten [71%] vs nine [69%]). Headache was the most common treatment-related adverse event (five [21%] in part 1, 11 [41%] in part 2). In part 1, AXER-204 reached mean maximal CSF concentration 1 day after dosing with 200 mg of 412 000 ng/mL (SD 129 000), exceeding those concentrations that were efficacious in animal studies. In part 2, mean changes from baseline to day 169 in ISNCSCI UEMS were 1·5 (SD 3·3) for AXER-204 and 0·9 (2·3) for placebo (mean difference 0·54, 95% CI -1·48 to 2·55; p=0·59).

INTERPRETATION

This study delivers the first, to our knowledge, clinical trial of a rationally designed pharmacological treatment intended to promote neural repair in chronic SCI. AXER-204 appeared safe and reached target CSF concentrations; exploratory biomarker results were consistent with target engagement and synaptic stabilisation. Post-hoc subgroup analyses suggest that future trials could investigate efficacy in patients with moderately severe SCI without prior AXER-204 exposure.

FUNDING

Wings for Life Foundation, National Institute of Neurological Disorders and Stroke, National Center for Advancing Translational Sciences, National Institute on Drug Abuse, and ReNetX Bio.

Regulation of Schwann cell proliferation and migration via miR-195-5p-induced Crebl2 downregulation upon peripheral nerve damage

Background

Schwann cells acquire a repair phenotype upon peripheral nerve injury (PNI), generating an optimal microenvironment that drives nerve repair. Multiple microRNAs (miRNAs) show differential expression in the damaged peripheral nerve, with critical regulatory functions in Schwann cell features. This study examined the time-dependent expression of miR-195-5p following PNI and demonstrated a marked dysregulation of miR-195-5p in the damaged sciatic nerve.

Methods

CCK-8 and EdU assays were used to evaluate the effect of miR-195-5 on Schwann cell viability and proliferation. Schwann cell migration was tested using Transwell and wound healing assays. The miR-195-5p agomir injection experiment was used to evaluate the function of miR-195-5p in vivo. The potential regulators and effects of miR-195-5p were identified through bioinformatics evaluation. The relationship between miR-195-5p and its target was tested using double fluorescence reporter gene analysis.

Results

In Schwann cells, high levels of miR-195-5p decreased viability and proliferation, while suppressed levels had the opposite effects. However, elevated miR-195-5p promoted Schwann cell migration determined by the Transwell and wound healing assays. In vivo injection of miR-195-5p agomir into rat sciatic nerves promote axon elongation after peripheral nerve injury by affecting Schwann cell distribution and myelin preservation. Bioinformatic assessment further revealed potential regulators and effectors for miR-195-5p, which were utilized to build a miR-195-5p-centered competing endogenous RNA network. Furthermore, miR-195-5p directly targeted cAMP response element binding protein-like 2 (Crebl2) mRNA via its 3'-untranslated region (3'-UTR) and downregulated Crebl2. Mechanistically, miR-195-5p modulated Schwann cell functions by repressing Crebl2.

Conclusion

The above findings suggested a vital role for miR-195-5p/Crebl2 in the regulation of Schwann cell phenotype after sciatic nerve damage, which may contribute to peripheral nerve regeneration.

Endothelial progenitor cell-derived exosomes promote anti-inflammatory macrophages via SOCS3/JAK2/STAT3 axis and improve the outcome of spinal cord injury

BACKGROUND

Macrophage in the spinal cord injury (SCI) area imparts a chronic pro-inflammation effect that challenges the recovery of SCI. Previously, endothelial progenitor cell-produced exosomes (EPC-EXOs) have been noticed to facilitate revascularization and inflammation control after SCI. However, their effects on macrophage polarization remained unclear. This study aimed to investigate the EPC-EXOs' role in macrophage polarization and reveal its underlying mechanism.

METHODS

We extracted the macrophages and EPC from the bone marrow suspension of C57BL/L mice by centrifugation. After cell identification, the EPC-EXOs were collected by ultra-high-speed centrifugation and exosome extraction kits and identified by transmission electron microscopy and nanoparticle tracking analysis. Then, macrophages were cultured with EPC-EXOs in different concentrations. We labeled the exosome to confirm its internalization by macrophage and detected the macrophage polarization marker level both in vitro and in vivo. We further estimated EPC-EXOs' protective effects on SCI by mice spinal cord tissue H&E staining and motor behavior evaluation. Finally, we performed RT-qPCR to identify the upregulated miRNA in EPC-EXOs and manipulate its expression to estimate its role in macrophage polarization, SOCS3/JAK2/STAT3 pathway activation, and motor behavior improvement.

RESULTS

We found that EPC-EXOs decreased the macrophages' pro-inflammatory marker expression and increased their anti-inflammatory marker expression on the 7 and 14 days after SCI. The spinal cord H&E staining results showed that EPC-EXOs raised the tissue-sparing area rate significantly after 28 days of SCI and the motor behavior evaluation indicated an increased BMS score and motor-evoked potential by EPC-EXOs treatment after SCI. The RT-qPCR assay identified that miR-222-3P upregulated in EPC-EXOs and its miRNA-mimic also decreased the pro-inflammatory macrophages and increased the anti-inflammatory macrophages. Additionally, miR-222-3P mimic activated the SOCS3/JAK2/STAT3 pathway, and SOCS3/JAK2/STAT3 pathway inhibition blocked miR-2223P's effects on macrophage polarization and mouse motor behavior.

CONCLUSION

Comprehensively, we discovered that EPC-EXOs-derived miR-222-3p affected macrophage polarization via SOCS3/JAK2/STAT3 pathway and promoted mouse functional repair after SCI, which reveals EPC-EXOs' role in modulation of macrophage phenotype and will provide a novel interventional strategy to induce post-SCI recovery.

Spinal cord injury: molecular mechanisms and therapeutic interventions

Spinal cord injury (SCI) remains a severe condition with an extremely high disability rate. The challenges of SCI repair include its complex pathological mechanisms and the difficulties of neural regeneration in the central nervous system. In the past few decades, researchers have attempted to completely elucidate the pathological mechanism of SCI and identify effective strategies to promote axon regeneration and neural circuit remodeling, but the results have not been ideal. Recently, new pathological mechanisms of SCI, especially the interactions between immune and neural cell responses, have been revealed by single-cell sequencing and spatial transcriptome analysis. With the development of bioactive materials and stem cells, more attention has been focused on forming intermediate neural networks to promote neural regeneration and neural circuit reconstruction than on promoting axonal regeneration in the corticospinal tract. Furthermore, technologies to control physical parameters such as electricity, magnetism and ultrasound have been constantly innovated and applied in neural cell fate regulation. Among these advanced novel strategies and technologies, stem cell therapy, biomaterial transplantation, and electromagnetic stimulation have entered into the stage of clinical trials, and some of them have already been applied in clinical treatment. In this review, we outline the overall epidemiology and pathophysiology of SCI, expound on the latest research progress related to neural regeneration and circuit reconstruction in detail, and propose future directions for SCI repair and clinical applications.