Neural activity promotes long-distance, target-specific regeneration of adult retinal axons

Toward understanding the role of genomic repeat elements in neurodegenerative diseases

Neurodegenerative diseases cause great medical and economic burdens for both patients and society; however, the complex molecular mechanisms thereof are not yet well understood. With the development of high-coverage sequencing technology, researchers have started to notice that genomic repeat regions, previously neglected in search of disease culprits, are active contributors to multiple neurodegenerative diseases. In this review, we describe the association between repeat element variants and multiple degenerative diseases through genome-wide association studies and targeted sequencing. We discuss the identification of disease-relevant repeat element variants, further powered by the advancement of long-read sequencing technologies and their related tools, and summarize recent findings in the molecular mechanisms of repeat element variants in brain degeneration, such as those causing transcriptional silencing or RNA-mediated gain of toxic function. Furthermore, we describe how in silico predictions using innovative computational models, such as deep learning language models, could enhance and accelerate our understanding of the functional impact of repeat element variants. Finally, we discuss future directions to advance current findings for a better understanding of neurodegenerative diseases and the clinical applications of genomic repeat elements.

A p75 neurotrophin receptor-sparing nerve growth factor protects retinal ganglion cells from neurodegeneration by targeting microglia

BACKGROUND AND PURPOSE

Retinal ganglion cells (RGCs) are the output stage of retinal information processing, via their axons forming the optic nerve (ON). ON damage leads to axonal degeneration and death of RGCs, and results in vision impairment. Nerve growth factor (NGF) signalling is crucial for RGC operations and visual functions. Here, we investigate a new neuroprotective mechanism of a novel therapeutic candidate, a p75-less, TrkA-biased NGF agonist (hNGFp) in rat RGC degeneration, in comparison with wild type human NGF (hNGFwt).

EXPERIMENTAL APPROACH

Both neonate and adult rats, whether subjected or not to ON lesion, were treated with intravitreal injections or eye drops containing either hNGFp or hNGFwt. Different doses of the drugs were administered at days 1, 4 or 7 after injury for a maximum of 10 days, when immunofluorescence, electrophysiology, cellular morphology, cytokine array and behaviour studies were carried out. Pharmacokinetic evaluation was performed on rabbits treated with hNGFp ocular drops.

RESULTS

hNGFp exerted a potent RGC neuroprotection by acting on microglia cells, and outperformed hNGFwt in rescuing RGC degeneration and reducing inflammatory molecules. Delayed use of hNGFp after ON lesion resulted in better outcomes compared with treatment with hNGFwt. Moreover, hNGFp-based ocular drops were less algogenic than hNGFwt. Pharmacokinetic measurements revealed that biologically relevant quantities of hNGFp were found in the rabbit retina.

CONCLUSIONS AND IMPLICATIONS

Our data point to microglia as a new cell target through which NGF-induced TrkA signalling exerts neuroprotection of the RGC, emphasizing hNGFp as a powerful treatment to tackle retinal degeneration.

Visual stimulation and brain-derived neurotrophic factor (BDNF) have protective effects in experimental autoimmune uveoretinitis

AIMS

To investigate the therapeutic potential of visual stimulation (VS) and BDNF in murine experimental autoimmune uveoretinitis (EAU).

MAIN METHODS

Mice were immunized by subcutaneous injection of interphotoreceptor retinoid-binding protein in Freund's complete adjuvant and intravenous injection of pertussis toxin, and were then exposed to high-contrast VS 12 h/day (days 1-14 post-immunization). EAU severity was assessed by examining clinical score, visual acuity, inflammatory markers, and immune cells in the retina. The transcriptome of activated retinal cells was determined by RNA-seq using RNA immunoprecipitated in complex with phosphorylated ribosomal protein S6. The retinal levels of protein products of relevant upregulated genes were quantified. The effect of BDNF on EAU was tested in unstimulated mice by its daily topical ocular administration (days 8-14 post-immunization).

KEY FINDINGS

VS attenuated EAU development and decreased the expression of pro-inflammatory cytokines/chemokines and numbers of immune cells in the retina (n = 10-20 eyes/group for each analysis). In activated retinal cells of control mice (n = 30 eyes/group), VS upregulated genes encoding immunomodulatory neuropeptides, of which BDNF and vasoactive intestinal peptide (VIP) also showed increased mRNA and protein levels in the retina of VS-treated EAU mice (n = 6-10 eyes/group for each analysis). In unstimulated EAU mice, BDNF treatment mimicked the protective effects of VS by modulating the inflammatory and stem cell properties of Müller cells (n = 5 eyes/group for each analysis).

SIGNIFICANCE

VS effectively suppresses EAU, at least through enhancing retinal levels of anti-inflammatory and neuroprotective factors, VIP and BDNF. Our findings also suggest BDNF as a promising therapeutic agent for uveitis treatment.

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.

Axon guidance during mouse central nervous system regeneration is required for specific brain innervation

Reconstructing functional neuronal circuits is one major challenge of central nervous system repair. Through activation of pro-growth signaling pathways, some neurons achieve long-distance axon regrowth. Yet, functional reconnection has hardly been obtained, as these regenerating axons fail to resume their initial trajectory and reinnervate their proper target. Axon guidance is considered to be active only during development. Here, using the mouse visual system, we show that axon guidance is still active in the adult brain in regenerative conditions. We highlight that regenerating retinal ganglion cell axons avoid one of their primary targets, the suprachiasmatic nucleus (SCN), due to Slit/Robo repulsive signaling. Together with promoting regeneration, silencing Slit/Robo in vivo enables regenerating axons to enter the SCN and form active synapses. The newly formed circuit is associated with neuronal activation and functional recovery. Our results provide evidence that axon guidance mechanisms are required to reconnect regenerating axons to specific brain nuclei.

Membrane-anchored intracellular insulin receptor or insulin-like growth factor-1 receptor elicits ligand-independent downstream signaling

Neurodegenerative diseases including glaucoma affect insulin signaling, and insulin treatment has been shown to reverse the neurodegenerative loss of dendritic complexity in retinal ganglion cells. Therefore, strategies for enhancing or maintaining insulin signaling are worth pursuing to establish new therapies for these diseases. In the present study, we generated constitutively active insulin receptor (F-iIR) and insulin-like growth factor-1 receptor (F-iIGF1R) using a system that forces membrane localization of the intracellular domains of these receptors by farnesylation. Immunohistochemistry and Western blot analysis revealed that F-iIR and F-iIGF1R caused the activation of ERK and AKT in the absence of ligands in vitro. Our results suggest that in vivo effects of F-iIR and F-iIGF1R on the progression of neurodegenerative diseases should be investigated in the future.

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.

Modulating amacrine cell-derived dopamine signaling promotes optic nerve regeneration and preserves visual function

As part of the central nervous system, the optic nerve, composed of axons from retinal ganglion cells (RGCs), generally fails to regenerate on its own when injured in adult mammals. An innovative approach to promoting optic nerve regeneration involves manipulating the interactions between amacrine cells (ACs) and RGCs. Here, we identified a unique AC subtype, dopaminergic ACs (DACs), that responded early after optic nerve crush by down-regulating neuronal activity and reducing retinal dopamine (DA) release. Activating DACs or augmenting DA release with levodopa demonstrated neuroprotective effects and modestly enhanced axon regeneration. Within this context, we pinpointed the DA receptor D1 (DRD1) as a critical mediator of DAC-derived DA and showed that RGC-specific Drd1 overexpression effectively overcame subtype-specific barriers to regeneration. This strategy markedly boosted RGC survival and axon regeneration after crush and preserved vision in a glaucoma model. This study unveils the crucial role of DAC-derived DA signaling in optic nerve regeneration, holding promise for therapeutic insights into neural repair.

Comparative transcriptomic profiling reveals a role for Olig1 in promoting axon regeneration

The regenerative potential of injured axons displays considerable heterogeneity. However, the molecular mechanisms underlying the heterogeneity have not been fully elucidated. Here, we establish a method that can separate spinal motor neurons (spMNs) with low and high regenerative capacities and identify a set of transcripts revealing differential expression between two groups of neurons. Interestingly, oligodendrocyte transcription factor 1 (Olig1), which regulates the differentiation of various neuronal progenitors, exhibits recurrent expression in spMNs with enhanced regenerative capabilities. Furthermore, overexpression of Olig1 (Olig1 OE) facilitates axonal regeneration in various models, and down-regulation or deletion of Olig1 exhibits an opposite effect. By analyzing the overlapped differentially expressed genes after expressing individual Olig factor and functional validation, we find that the role of Olig1 is at least partially through the neurite extension factor 1 (Nrsn1). We therefore identify Olig1 as an intrinsic factor that promotes regenerative capacity of injured axons.

TET3 Contributes to Exercise-Induced Functional Axon Regeneration and Visual Restoration

Axons have intrinsically poor regenerative capacity in the mature central nervous system (CNS), leading to permanent neurological impairments in individuals. There is growing evidence that exercise is a powerful physiological intervention that can obviously enhance cell rejuvenate capacity, but its molecular mechanisms that mediate the axonal regenerative benefits remain largely unclear. Using the eye as the CNS model, here it is first indicated that placing mice in an exercise stimulation environment induced DNA methylation patterns and transcriptomes of retinal ganglion cell, promoted axon regeneration after injury, and reversed vision loss in aged mice. These beneficial effects are dependent on the DNA demethylases TET3-mediated epigenetic effects, which increased the expression of genes associated with the regenerative growth programs, such as STAT3, Wnt5a, Klf6. Exercise training also shows with the improved mitochondrial and metabolic dysfunction in retinas and optic nerves via TET3. Collectively, these results suggested that the increased regenerative capacity induced by enhancing physical activity is mediated through epigenetic reprogramming in mouse model of optic nerve injury and in aged mouse. Understanding the molecular mechanism underlying exercise-dependent neuronal plasticity led to the identification of novel targets for ameliorating pathologies associated with etiologically diverse diseases.

Engineering an in vitro retinothalamic nerve model

Understanding the retinogeniculate pathway in vitro can offer insights into its development and potential for future therapeutic applications. This study presents a Polydimethylsiloxane-based two-chamber system with axon guidance channels, designed to replicate unidirectional retinogeniculate signal transmission in vitro. Using embryonic rat retinas, we developed a model where retinal spheroids innervate thalamic targets through up to 6 mm long microfluidic channels. Using a combination of electrical stimulation and functional calcium imaging we assessed how channel length and electrical stimulation frequency affects thalamic target response. In the presented model we integrated up to 20 identical functional retinothalamic neural networks aligned on a single transparent microelectrode array, enhancing the robustness and quality of recorded functional data. We found that network integrity depends on channel length, with 0.5-2 mm channels maintaining over 90% morphological and 50% functional integrity. A reduced network integrity was recorded in longer channels. The results indicate a notable reduction in forward spike propagation in channels longer than 4 mm. Additionally, spike conduction fidelity decreased with increasing channel length. Yet, stimulation-induced thalamic target activity remained unaffected by channel length. Finally, the study found that a sustained thalamic calcium response could be elicited with stimulation frequencies up to 31 Hz, with higher frequencies leading to transient responses. In conclusion, this study presents a high-throughput platform that demonstrates how channel length affects retina to brain network formation and signal transmission in vitro.

Environmental enrichment attenuates depressive-like behavior in maternal rats by inhibiting neuroinflammation and apoptosis and promoting neuroplasticity

Gestational stress can exacerbate postpartum depression (PPD), for which treatment options remain limited. Environmental enrichment (EE) may be a therapeutic intervention for neuropsychiatric disorders, including depression, but the specific mechanisms by which EE might impact PPD remain unknown. Here we examined the behavioral, molecular, and cellular impact of EE in a stable PPD model in rats developed through maternal separation (MS). Maternal rats subjected to MS developed depression-like behavior and cognitive dysfunction together with evidence of significant neuroinflammation including microglia activation, neuronal apoptosis, and impaired synaptic plasticity. Expanding the duration of EE to throughout pregnancy and lactation, we observed an EE-associated reversal of MS-induced depressive phenotypes, inhibition of neuroinflammation and neuronal apoptosis, and improvement in synaptic plasticity in maternal rats. Thus, EE effectively alleviates neuroinflammation, neuronal apoptosis, damage to synaptic plasticity, and consequent depression-like behavior in mother rats experiencing MS-induced PPD, paving the way for new preventive and therapeutic strategies for PPD.

Noninvasive Light Flicker Stimulation Promotes Optic Nerve Regeneration by Activating Microglia and Enhancing Neural Plasticity in Zebrafish

Purpose

Forty-hertz light flicker stimulation has been proven to reduce neurodegeneration, but its effect on optic nerve regeneration is unclear. This study explores the effect of 40-Hz light flicker in promoting optic nerve regeneration in zebrafish and investigates the underlying mechanisms.

Methods

Wild-type and mpeg1:EGFP zebrafish were used to establish a model of optic nerve crush. Biocytin tracing and hematoxylin and eosin staining were employed to observe whether 40-Hz light flicker promotes regeneration of retinal ganglion cell axons and dendrites. Optomotor and optokinetic responses were evaluated to assess recovery of visual function. Immunofluorescence staining of mpeg1:EGFP zebrafish was performed to observe changes in microglia. Differentially expressed genes that promote optic nerve regeneration following 40-Hz light flicker stimulation were identified and validated through RNA-sequencing analysis and quantitative real-time PCR (qRT-PCR).

Results

Zebrafish exhibited spontaneous optic nerve regeneration after optic nerve injury and restored visual function. We observed that 40-Hz light flicker significantly activated microglia following optic nerve injury and promoted regeneration of retinal ganglion cell axons and dendrites, as well as recovery of visual function. Transcriptomics and qRT-PCR analyses revealed that 40-Hz light flicker increased the expression of genes associated with neuronal plasticity, including bdnf, npas4a, fosab, fosb, egr4, and ier2a.

Conclusions

To our knowledge, this study is the first to demonstrate that 40-Hz light flicker stimulation promotes regeneration of retinal ganglion cell axons and dendrites and recovery of visual function in zebrafish, which is associated with microglial activation and enhancement of neural plasticity.

A Comparative Investigation of Axon-Blood Vessel Growth Interaction in the Regenerating Sciatic and Optic Nerves in Adult Mice

The vascular and the nervous systems share similarities in addition to their complex role in providing oxygen and nutrients to all cells. Both are highly branched networks that frequently grow close to one another during development. Vascular patterning and neural wiring share families of guidance cues and receptors. Most recently, this relationship has been investigated in terms of peripheral nervous system (PNS) regeneration, where nerves and blood vessels often run in parallel so endothelial cells guide the formation of the Büngner bands which support axonal regeneration. Here, we characterized the vascular response in regenerative models of the central and peripheral nervous system. After sciatic nerve crush, followed by axon regeneration, there was a significant increase in the blood vessel density 7 days after injury. In addition, the optic nerve crush model was used to evaluate intrinsic regenerative potential activated with a combined treatment that stimulated retinal ganglion cells (RGCs) regrowth. We observed that a 2-fold change in the total number of blood vessels occurred 7 days after optic nerve crush compared to the uncrushed nerve. The difference increased up to a 2.7-fold change 2 weeks after the crush. Interestingly, we did not observe differences in the total number of blood vessels 2 weeks after crush, compared to animals that had received combined treatment for regeneration and controls. Therefore, the vascular characterization showed that the increase in vascular density was not related to the efficiency of both peripheral and central axonal regeneration.

In vivo imaging of the neuronal response to spinal cord injury: a narrative review

Deciphering the neuronal response to injury in the spinal cord is essential for exploring treatment strategies for spinal cord injury (SCI). However, this subject has been neglected in part because appropriate tools are lacking. Emerging in vivo imaging and labeling methods offer great potential for observing dynamic neural processes in the central nervous system in conditions of health and disease. This review first discusses in vivo imaging of the mouse spinal cord with a focus on the latest imaging techniques, and then analyzes the dynamic biological response of spinal cord sensory and motor neurons to SCI. We then summarize and compare the techniques behind these studies and clarify the advantages of in vivo imaging compared with traditional neuroscience examinations. Finally, we identify the challenges and possible solutions for spinal cord neuron imaging.

Mechanosensitive channel of large conductance enhances the mechanical stretching-induced upregulation of glycolysis and oxidative metabolism in Schwann cells

BACKGROUND

Physical exercise directly stretching the peripheral nerve promotes nerve regeneration; however, its action mechanism remains elusive. Our present study aimed to investigate the effects of mechanosensitive channel of large conductance (MscL) activated by mechanical stretching on the cultured Schwann cells (SCs) and explore the possible mechanism.

METHODS

Primary SCs from neonatal mice at 3-5 days of age were derived and transfected with the lentivirus vector expressing a mutant version of MscL, MscL-G22S. We first detected the cell viability and calcium ion (Ca2+) influx in the MscL-G22S-expressing SCs with low-intensity mechanical stretching and the controls. Proteomic and energy metabolomics analyses were performed to investigate the comprehensive effects of MscL-G22S activation on SCs. Measurement of glycolysis- and oxidative phosphorylation-related molecules and ATP production were respectively performed to further validate the effects of MscL-G22S activation on SCs. Finally, the roles of phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathway in the mechanism of energy metabolism modulation of SCs by MscL-G22S activation was investigated.

RESULTS

Mechanical stretching-induced MscL-G22S activation significantly increased the cell viability and Ca2+ influx into the SCs. Both the proteomic and targeted energy metabolomics analysis indicated the upregulation of energy metabolism as the main action mechanism of MscL-G22S-activation on SCs. MscL-G22S-activated SCs showed significant upregulation of glycolysis and oxidative phosphorylation when SCs with stretching alone had only mild upregulation of energy metabolism than those without stimuli. MscL-G22S activation caused significant phosphorylation of the PI3K/AKT/mTOR signaling pathway and upregulation of HIF-1α/c-Myc. Inhibition of PI3K abolished the MscL-G22S activation-induced upregulation of HIF-1α/c-Myc signaling in SCs and reduced the levels of glycolysis- and oxidative phosphorylation-related substrates and mitochondrial activity.

CONCLUSION

Mechanical stretching activates MscL-G22S to significantly promote the energy metabolism of SCs and the production of energic substrates, which may be applied to enhance nerve regeneration via the glia-axonal metabolic coupling.

Synergistic Protection of Retinal Ganglion Cells (RGCs) by SARM1 Inactivation with CNTF in a Rodent Model of Nonarteritic Anterior Ischemic Optic Neuropathy

We evaluated whether inhibiting sterile alpha and (Toll/interleukin receptor (TIR)) motif-containing 1 (SARM1) activity protects retinal ganglion cells (RGCs) following ischemic axonopathy (rodent nonarteritic anterior ischemic optic neuropathy: rNAION) by itself and combined with ciliary neurotrophic factor (CNTF). Genetically modified SARM1(-) rats were rNAION-induced in one eye and compared against equivalently induced wild-type animals of the same background. Optic nerve (ON) diameters were quantified using optical coherence tomography (SD-OCT). RGCs were quantified 30 d post-induction using retinal stereology for Brn3a(+) nuclei. ON sections were analyzed by TEM and immunohistochemistry. SARM1(-)(-) and WT animals were then bilaterally sequentially rNAION-induced. One eye received intravitreal vehicle injection following induction; the contralateral side received CNTF and was analyzed 30 d post-induction. Inhibiting SARM1 activity suppressed axonal collapse following ischemic axonopathy. SARM1(-) animals significantly reduced RGC loss, compared with WT animals (49.4 ± 6.8% RGC loss in SARM1(-) vs. 63.6 ± 3.2% sem RGC loss in WT; Mann-Whitney one-tailed U-test, (p = 0.049)). IVT-CNTF treatment vs. IVT-vehicle in SARM1(-) animals further reduced RGC loss by 24% at 30 d post-induction, but CNTF did not, by itself, improve long-term RGC survival in WT animals compared with vehicle (Mann-Whitney one-tailed t-test; p = 0.033). While inhibiting SARM1 activity is itself neuroprotective, combining SARM1 inhibition and CNTF treatment generated a long-term, synergistic neuroprotective effect in ischemic neuropathy. Combinatorial treatments for NAION utilizing independent neuroprotective mechanisms may thus provide a greater effect than individual treatment modalities.

Glial cells as a promising therapeutic target of glaucoma: beyond the IOP

Glial cells, a type of non-neuronal cell found in the central nervous system (CNS), play a critical role in maintaining homeostasis and regulating CNS functions. Recent advancements in technology have paved the way for new therapeutic strategies in the fight against glaucoma. While intraocular pressure (IOP) is the most well-known modifiable risk factor, a significant number of glaucoma patients have normal IOP levels. Because glaucoma is a complex, multifactorial disease influenced by various factors that contribute to its onset and progression, it is imperative that we consider factors beyond IOP to effectively prevent or slow down the disease's advancement. In the realm of CNS neurodegenerative diseases, glial cells have emerged as key players due to their pivotal roles in initiating and hastening disease progression. The inhibition of dysregulated glial function holds the potential to protect neurons and restore brain function. Consequently, glial cells represent an enticing therapeutic candidate for glaucoma, even though the majority of glaucoma research has historically concentrated solely on retinal ganglion cells (RGCs). In addition to the neuroprotection of RGCs, the proper regulation of glial cell function can also facilitate structural and functional recovery in the retina. In this review, we offer an overview of recent advancements in understanding the non-cell-autonomous mechanisms underlying the pathogenesis of glaucoma. Furthermore, state-of-the-art technologies have opened up possibilities for regenerating the optic nerve, which was previously believed to be incapable of regeneration. We will also delve into the potential roles of glial cells in the regeneration of the optic nerve and the restoration of visual function.

Selective deletion of zinc transporter 3 in amacrine cells promotes retinal ganglion cell survival and optic nerve regeneration after injury

Vision depends on accurate signal conduction from the retina to the brain through the optic nerve, an important part of the central nervous system that consists of bundles of axons originating from retinal ganglion cells. The mammalian optic nerve, an important part of the central nervous system, cannot regenerate once it is injured, leading to permanent vision loss. To date, there is no clinical treatment that can regenerate the optic nerve and restore vision. Our previous study found that the mobile zinc (Zn²⁺) level increased rapidly after optic nerve injury in the retina, specifically in the vesicles of the inner plexiform layer. Furthermore, chelating Zn²⁺ significantly promoted axonal regeneration with a long-term effect. In this study, we conditionally knocked out zinc transporter 3 (ZnT3) in amacrine cells or retinal ganglion cells to construct two transgenic mouse lines (VGAT^CreZnT3^fl/fl and VGLUT2^CreZnT3^fl/fl, respectively). We obtained direct evidence that the rapidly increased mobile Zn²⁺ in response to injury was from amacrine cells. We also found that selective deletion of ZnT3 in amacrine cells promoted retinal ganglion cell survival and axonal regeneration after optic nerve crush injury, improved retinal ganglion cell function, and promoted vision recovery. Sequencing analysis of reginal ganglion cells revealed that inhibiting the release of presynaptic Zn²⁺ affected the transcription of key genes related to the survival of retinal ganglion cells in postsynaptic neurons, regulated the synaptic connection between amacrine cells and retinal ganglion cells, and affected the fate of retinal ganglion cells. These results suggest that amacrine cells release Zn²⁺ to trigger transcriptomic changes related to neuronal growth and survival in reginal ganglion cells, thereby influencing the synaptic plasticity of retinal networks. These results make the theory of zinc-dependent retinal ganglion cell death more accurate and complete and provide new insights into the complex interactions between retinal cell networks.

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.

Suppressing DNMT3a Alleviates the Intrinsic Epigenetic Barrier for Optic Nerve Regeneration and Restores Vision in Adult Mice

The limited regenerative potential of the optic nerve in adult mammals presents a major challenge for restoring vision after optic nerve trauma or disease. The mechanisms of this regenerative failure are not fully understood1,2. Here, through small-molecule and genetic screening for epigenetic modulators3, we identify DNA methyltransferase 3a (DNMT3a) as a potent inhibitor of axon regeneration in mouse and human retinal explants. Selective suppression of DNMT3a in retinal ganglion cells (RGCs) by gene targeting or delivery of shRNA leads to robust, full-length regeneration of RGC axons through the optic nerve and restoration of vision in adult mice after nerve crush injury. Genome-wide bisulfite and transcriptome profiling in combination with single nucleus RNA-sequencing of RGCs revealed selective DNA demethylation and reactivation of genetic programs supporting neuronal survival and axonal growth/regeneration by DNMT3a deficiency. This was accompanied by the suppression of gene networks associated with apoptosis and inflammation. Our results identify DNMT3a as the central orchestrator of an RGC-intrinsic mechanism that limits optic nerve regeneration. Suppressing DNMT3a expression in RGCs unlocks the epigenetic switch for optic nerve regeneration and presents a promising therapeutic avenue for effectively reversing vision loss resulted from optic nerve trauma or diseases.

Schwann cell-derived extracellular vesicles as a potential therapy for retinal ganglion cell degeneration

Optic neuropathy is the leading cause of irreversible blindness and is characterized by progressive degeneration of retinal ganglion cells (RGCs). Several studies have demonstrated that transplantation of Schwann cells (SCs) is a promising candidate therapy for optic neuropathy and that intravitreally transplanted cells exert their effect via paracrine actions. Extracellular vesicle (EV)-based therapies are increasingly recognized as a potential strategy for cell replacement therapy. In this study, we aimed to investigate the neuroprotective and regenerative effects of SC-EVs following optic nerve injury. We found that SC-EVs were internalized by RGCs in vitro and in vivo without any transfection reagents. Intriguingly, SC-EVs significantly enhanced the survival and axonal growth of primary RGCs in a coculture system. In a rat optic nerve crush model, SC-EVs mitigated RGC degeneration, prevented RGC loss, and preserved the thickness of the ganglion cell complex, as demonstrated by the statistically significant improvement in RGC counts and thickness measurements. Mechanistically, SC-EVs activated the cAMP-response element binding protein (CREB) signaling pathway and regulated reactive gliosis in ONC rats, which is crucial for RGC protection and axonal regeneration. These findings provide novel insights into the neuroprotective and regenerative properties of SC-EVs, suggesting their potential as a cell-free therapeutic strategy and natural biomaterials for neurodegenerative diseases of the central nervous system.

Virtual Reality Visual Perceptual Plastic Training Promotes Retinal Structure and Macular Function Recovery in Glaucoma Patients

Twenty-seven glaucoma patients (54 eyes in total) with well-controlled intraocular pressure were trained with binocular virtual reality visual software for 3 months to investigate whether virtual reality visual perceptual plastic training promotes macular retinal structure and macular function recovery in glaucoma patients. The thickness of peripapillary retinal nerve fiber layer (pRNFL), macular ganglion cell layer-inner plexiform layer (mGCIPL), and mean macular sensitivity (mMS) were evaluated 3 months after training. The mean value of pRNFL thickness in glaucoma patients did not change significantly (Z = 0.642, p = 0.521), nor did the mean value (t = 1.916, p = 0.061) and minimum value (Z = 1.428, p = 0.153) of mGCIPL after 3 months. However, the significant increases were found in superior temporal mGCIPL thickness (t = 2.430, p = 0.019) as well as superior mGCIPL thickness (t = 2.262, p = 0.028). Additionally, the mMS was increased (Z = 2.259, p < 0.05), with the inferior square to be a more pronounced mMS increase (Z = 2.070, p = 0.038). In conclusion, virtual reality visual perceptual plastic training can increase the thickness of retinal ganglion cells complexes in the macular area of glaucoma patients and improve the macular function of the corresponding area. Clinical Trial registration number: ChiCTR1900027909.

Diversity in homeostatic calcium set points predicts retinal ganglion cell survival following optic nerve injury in vivo

Retinal ganglion cell (RGC) degeneration drives vision loss in blinding conditions. RGC death is often triggered by axon degeneration in the optic nerve. Here, we study the contributions of dynamic and homeostatic Ca2+ levels to RGC death from axon injury. We find that axonal Ca2+ elevations from optic nerve injury do not propagate over distance or reach RGC somas, and acute and chronic Ca2+ dynamics do not affect RGC survival. Instead, we discover that baseline Ca2+ levels vary widely between RGCs and predict their survival after axon injury, and that lowering these levels reduces RGC survival. Further, we find that well-surviving RGC types have higher baseline Ca2+ levels than poorly surviving types. Finally, we observe considerable variation in the baseline Ca2+ levels of different RGCs of the same type, which are predictive of within-type differences in survival.

Electrical, Electromagnetic, Ultrasound Wave Therapies, and Electronic Implants for Neuronal Rejuvenation, Neuroprotection, Axonal Regeneration, and IOP Reduction

The peripheral nervous system (PNS) of mammals and nervous systems of lower organisms possess significant regenerative potential. In contrast, although neural plasticity can provide some compensation, the central nervous system (CNS) neurons and nerves of adult mammals generally fail to regenerate after an injury or damage. However, use of diverse electrical, electromagnetic and sonographic energy waves are illuminating novel ways to stimulate neuronal differentiation, proliferation, neurite growth, and axonal elongation/regeneration leading to various levels of functional recovery in animals and humans afflicted with disorders of the CNS, PNS, retina, and optic nerve. Tools such as acupuncture, electroacupuncture, electroshock therapy, electrical stimulation, transcranial magnetic stimulation, red light therapy, and low-intensity pulsed ultrasound therapy are demonstrating efficacy in treating many different maladies. These include wound healing, partial recovery from motor dysfunctions, recovery from ischemic/reperfusion insults and CNS and ocular remyelination, retinal ganglion cell (RGC) rejuvenation, and RGC axonal regeneration. Neural rejuvenation and axonal growth/regeneration processes involve activation or intensifying of the intrinsic bioelectric waves (action potentials) that exist in every neuronal circuit of the body. In addition, reparative factors released at the nerve terminals and via neuronal dendrites (transmitter substances), extracellular vesicles containing microRNAs and neurotrophins, and intercellular communication occurring via nanotubes aid in reestablishing lost or damaged connections between the traumatized tissues and the PNS and CNS. Many other beneficial effects of the aforementioned treatment paradigms are mediated via gene expression alterations such as downregulation of inflammatory and death-signal genes and upregulation of neuroprotective and cytoprotective genes. These varied techniques and technologies will be described and discussed covering cell-based and animal model-based studies. Data from clinical applications and linkage to human ocular diseases will also be discussed where relevant translational research has been reported.

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

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.

Cellular and subcellular optogenetic approaches towards neuroprotection and vision restoration

Optogenetics is defined as the combination of genetic and optical methods to induce or inhibit well-defined events in isolated cells, tissues, or animals. While optogenetics within ophthalmology has been primarily applied towards treating inherited retinal disease, there are a myriad of other applications that hold great promise for a variety of eye diseases including cellular regeneration, modulation of mitochondria and metabolism, regulation of intraocular pressure, and pain control. Supported by primary data from the authors' work with in vitro and in vivo applications, we introduce a novel approach to metabolic regulation, Opsins to Restore Cellular ATP (ORCA). We review the fundamental constructs for ophthalmic optogenetics, present current therapeutic approaches and clinical trials, and discuss the future of subcellular and signaling pathway applications for neuroprotection and vision restoration.

Moving CNS axon growth and regeneration research into human model systems

Axon regeneration is limited in the adult mammalian central nervous system (CNS) due to both intrinsic and extrinsic factors. Rodent studies have shown that developmental age can drive differences in intrinsic axon growth ability, such that embryonic rodent CNS neurons extend long axons while postnatal and adult CNS neurons do not. In recent decades, scientists have identified several intrinsic developmental regulators in rodents that modulate growth. However, whether this developmentally programmed decline in CNS axon growth is conserved in humans is not yet known. Until recently, there have been limited human neuronal model systems, and even fewer age-specific human models. Human in vitro models range from pluripotent stem cell-derived neurons to directly reprogrammed (transdifferentiated) neurons derived from human somatic cells. In this review, we discuss the advantages and disadvantages of each system, and how studying axon growth in human neurons can provide species-specific knowledge in the field of CNS axon regeneration with the goal of bridging basic science studies to clinical trials. Additionally, with the increased availability and quality of 'omics datasets of human cortical tissue across development and lifespan, scientists can mine these datasets for developmentally regulated pathways and genes. As there has been little research performed in human neurons to study modulators of axon growth, here we provide a summary of approaches to begin to shift the field of CNS axon growth and regeneration into human model systems to uncover novel drivers of axon growth.

Postsynaptic neuronal activity promotes regeneration of retinal axons

The wiring of visual circuits requires that retinal neurons functionally connect to specific brain targets, a process that involves activity-dependent signaling between retinal axons and their postsynaptic targets. Vision loss in various ophthalmological and neurological diseases is caused by damage to the connections from the eye to the brain. How postsynaptic brain targets influence retinal ganglion cell (RGC) axon regeneration and functional reconnection with the brain targets remains poorly understood. Here, we established a paradigm in which the enhancement of neural activity in the distal optic pathway, where the postsynaptic visual target neurons reside, promotes RGC axon regeneration and target reinnervation and leads to the rescue of optomotor function. Furthermore, selective activation of retinorecipient neuron subsets is sufficient to promote RGC axon regeneration. Our findings reveal a key role for postsynaptic neuronal activity in the repair of neural circuits and highlight the potential to restore damaged sensory inputs via proper brain stimulation.

Adeno-associated virus mediated gene therapy for neuroprotection of retinal ganglion cells in glaucoma

Glaucoma is a group of diseases typically characterized by the degeneration of the optic nerve and is one of the world's leading causes of blindness. Although there is no cure for glaucoma, reducing intraocular pressure is an approved treatment to delay optic nerve degeneration and retinal ganglion cell (RGC) death in most patients. Recent clinical trials have evaluated the safety and efficacy of gene therapy vectors for the treatment of inherited retinal degenerations (IRDs), and the results are promising, generating enthusiasm for the treatment of other retinal diseases. While there have been no reports on successful clinical trials for gene therapy-based neuroprotective treatment of glaucoma, and only a few studies assessing the efficacy of gene therapy vectors for the treatment of Leber hereditary optic neuropathy (LHON), the potential for neuroprotective treatment of glaucoma and other diseases affecting RGCs is still widely recognized. Here, we review recent progress and cover current limitations pertaining to targeting RGCs with adeno-associated virus-based gene therapy for the treatment of glaucoma.

Gliotransmission and adenosine signaling promote axon regeneration

How glia control axon regeneration remains incompletely understood. Here, we investigate glial regulation of regenerative ability differences of closely related Drosophila larval sensory neuron subtypes. Axotomy elicits Ca2+ signals in ensheathing glia, which activates regenerative neurons through the gliotransmitter adenosine and mounts axon regenerative programs. However, non-regenerative neurons do not respond to glial stimulation or adenosine. Such neuronal subtype-specific responses result from specific expressions of adenosine receptors in regenerative neurons. Disrupting gliotransmission impedes axon regeneration of regenerative neurons, and ectopic adenosine receptor expression in non-regenerative neurons suffices to activate regenerative programs and induce axon regeneration. Furthermore, stimulating gliotransmission or activating the mammalian ortholog of Drosophila adenosine receptors in retinal ganglion cells (RGCs) promotes axon regrowth after optic nerve crush in adult mice. Altogether, our findings demonstrate that gliotransmission orchestrates neuronal subtype-specific axon regeneration in Drosophila and suggest that targeting gliotransmission or adenosine signaling is a strategy for mammalian central nervous system repair.

The RSK2-RPS6 axis promotes axonal regeneration in the peripheral and central nervous systems

Unlike immature neurons and the ones from the peripheral nervous system (PNS), mature neurons from the central nervous system (CNS) cannot regenerate after injury. In the past 15 years, tremendous progress has been made to identify molecules and pathways necessary for neuroprotection and/or axon regeneration after CNS injury. In most regenerative models, phosphorylated ribosomal protein S6 (p-RPS6) is up-regulated in neurons, which is often associated with an activation of the mTOR (mammalian target of rapamycin) pathway. However, the exact contribution of posttranslational modifications of this ribosomal protein in CNS regeneration remains elusive. In this study, we demonstrate that RPS6 phosphorylation is essential for PNS and CNS regeneration in mice. We show that this phosphorylation is induced during the preconditioning effect in dorsal root ganglion (DRG) neurons and that it is controlled by the p90S6 kinase RSK2. Our results reveal that RSK2 controls the preconditioning effect and that the RSK2-RPS6 axis is key for this process, as well as for PNS regeneration. Finally, we demonstrate that RSK2 promotes CNS regeneration in the dorsal column, spinal cord synaptic plasticity, and target innervation leading to functional recovery. Our data establish the critical role of RPS6 phosphorylation controlled by RSK2 in CNS regeneration and give new insights into the mechanisms related to axon growth and circuit formation after traumatic lesion.

Effects of constitutively active K-Ras on axon regeneration after optic nerve injury

Visual disturbance after optic nerve injury is a serious problem. Attempts have been made to enhance the intrinsic ability of retinal ganglion cells (RGCs) to regenerate their axons, and the importance of PI3K/Akt and RAF/MEK/ERK signal activation has been suggested. Since these signals are shared with oncogenic signaling cascades, in this study, we focused on a constitutively active form of K-Ras, K-Ras^V12, to determine if overexpression of this molecule could stimulate axon regeneration. We confirmed that K-Ras^V12 phosphorylated Akt and ERK in vitro. Intravitreal delivery of AAV2-K-Ras^V12 increased the number of surviving RGCs and promoted 1.0 mm of axon regeneration one week after optic nerve injury without inducing abnormal proliferative effects in the RGCs. In addition, AAV2-K-Ras^V12 induced robust RGC axon regeneration, reaching as far as approximately 2.5 mm from the injury site, in eight weeks. Our findings suggest that AAV2-K-Ras^V12 could provide a good model for speedy and efficient analysis of the mechanism underlying axon regeneration in vivo.

Human Retinal Ganglion Cells Respond to Evolutionarily Conserved Chemotropic Cues for Intra Retinal Guidance and Regeneration

Retinal ganglion cells (RGCs) connect the retina with the higher centers in the brain for visual perception. Their degeneration leads to irreversible vision loss in glaucoma patients. Since human RGCs (hRGCs) are born during fetal development and connections with the central targets are established before birth, the mechanism underlying their axon growth and guidance remains poorly understood. Here, using RGCs directly generated from human embryonic stem cells, we demonstrate that hRGCs express a battery of guidance receptors. These receptors allow hRGCs to read the spatially arrayed chemotropic cues in the developing rat retina for the centripetal orientation of axons toward the optic disc, suggesting that the mechanism of intra-retinal guidance is conserved in hRGCs. The centripetal orientation of hRGCs axons is not only in response to chemo-repulsion but also involves chemo-attraction, mediated by Netrin-1/DCC interactions. The spatially arrayed chemotropic cues differentially influence hRGCs physiological responses, suggesting that neural activity of hRGCs may facilitate axon growth during inter-retinal guidance. Additionally, we demonstrate that Netrin-1/DCC interactions, besides promoting axon growth, facilitate hRGCs axon regeneration by recruiting the mTOR signaling pathway. The diverse influence of Netrin-1/DCC interactions ranging from axon growth to regeneration may involve recruitment of multiple intracellular signaling pathways as revealed by transcriptome analysis of hRGCs. From the perspective of ex-vivo stem cell approach to glaucomatous degeneration, our findings posit that ex-vivo generated human RGCs are capable of reading the intra-retinal cues for guidance toward the optic disc, the first step toward connecting with the central target to restore vision.

CyclinD2-mediated regulation of neurogenic output from the retinal ciliary margin is perturbed in albinism

In albinism, aberrations in the ipsi-/contralateral retinal ganglion cell (RGC) ratio compromise the functional integrity of the binocular circuit. Here, we focus on the mouse ciliary margin zone (CMZ), a neurogenic niche at the embryonic peripheral retina, to investigate developmental processes regulating RGC neurogenesis and identity acquisition. We found that the mouse ventral CMZ generates predominantly ipsilaterally projecting RGCs, but this output is altered in the albino visual system because of CyclinD2 downregulation and disturbed timing of the cell cycle. Consequently, albino as well as CyclinD2-deficient pigmented mice exhibit diminished ipsilateral retinogeniculate projection and poor depth perception. In albino mice, pharmacological stimulation of calcium channels, known to upregulate CyclinD2 in other cell types, augmented CyclinD2-dependent neurogenesis of ipsilateral RGCs and improved stereopsis. Together, these results implicate CMZ neurogenesis and its regulators as critical for the formation and function of the mammalian binocular circuit.

Use of gene therapy for optic nerve protection: Current concepts

Gene therapy has become an essential treatment for optic nerve injury (ONI) in recent years, and great strides have been made using animal models. ONI, which is characterized by the loss of retinal ganglion cells (RGCs) and axons, can induce abnormalities in the pupil light reflex, visual field defects, and even vision loss. The eye is a natural organ to target with gene therapy because of its high accessibility and certain immune privilege. As such, numerous gene therapy trials are underway for treating eye diseases such as glaucoma. The aim of this review was to cover research progress made in gene therapy for ONI. Specifically, we focus on the potential of gene therapy to prevent the progression of neurodegenerative diseases and protect both RGCs and axons. We cover the basic information of gene therapy, including the classification of gene therapy, especially focusing on genome editing therapy, and then we introduce common editing tools and vector tools such as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) -Cas9 and adeno-associated virus (AAV). We also summarize the progress made on understanding the roles of brain derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), phosphatase-tensin homolog (PTEN), suppressor of cytokine signal transduction 3 (SOCS3), histone acetyltransferases (HATs), and other important molecules in optic nerve protection. However, gene therapy still has many challenges, such as misalignment and mutations, immunogenicity of AAV, time it takes and economic cost involved, which means that these issues need to be addressed before clinical trials can be considered.

Neuroprotection and Neuroregeneration Strategies Using the rNAION Model: Theory, Histology, Problems, Results and Analytical Approaches

Nonarteritic anterior ischemic optic neuropathy (NAION) is the most common cause of sudden optic nerve (ON)-related vision loss in humans. Study of this disease has been limited by the lack of available tissue and difficulties in evaluating both treatments and the window of effectiveness after symptom onset. The rodent nonarteritic anterior ischemic optic neuropathy model (rNAION) closely resembles clinical NAION in its pathophysiological changes and physiological responses. The rNAION model enables analysis of the specific responses to sudden ischemic axonopathy and effectiveness of potential treatments. However, there are anatomic and genetic differences between human and rodent ON, and the inducing factors for the disease and the model are different. These variables can result in marked differences in lesion development between the two species, as well as in the possible responses to various treatments. These caveats are discussed in the current article, as well as some of the species-associated differences that may be related to ischemic lesion severity and responses.

Research hotspots and trends for axon regeneration (2000-2021): a bibliometric study and systematic review

BACKGROUND

Axons play an essential role in the connection of the nervous system with the rest of the body. Axon lesions often lead to permanent impairment of motor and cognitive functions and the interaction with the outside world. Studies focusing on axon regeneration have become a research field with considerable interest. The purpose of this study is to obtain an overall perspective of the research field of axonal regeneration and to assist the researchers and the funding agencies to better know the areas of greatest research opportunities.

METHODS

We conducted a bibliometric analysis and Latent Dirichlet Allocation (LDA) analysis of the global literature on axon regeneration based on the Web of Science (WoS) over the recent 22 years, to address the research hotspots, publication trends, and understudied areas.

RESULTS

A total of 21,018 articles were included, which in the recent two decades has increased by 125%. Among the top 12 hotspots, the annual productions rapidly increased in some topics, including axonal regeneration signaling pathway, axon guidance cues, neural circuits and functional recovery, nerve conduits, and cells transplant. Comparatively, the number of studies on axon regeneration inhibitors decreased. As for the topics focusing on nerve graft and transplantation, the annual number of papers tended to be relatively stable. Nevertheless, the underlying mechanisms of axon regrowth have not been completely uncovered. A lack of notable research on the epigenetic programs and noncoding RNAs regulation was observed. The significance of cell-type-specific data has been highlighted but with limited research working on that. Functional recovery from neuropathies also needs further studies.

CONCLUSION

The last two decades witnessed tremendous progress in the field of axon regeneration. There are still a lot of challenges to be tackled in translating these technologies into clinical practice.

Rapid presynaptic maturation in naturally regenerating axons of the adult mouse olfactory nerve

Successful neuronal regeneration requires the reestablishment of synaptic connectivity. This process requires the reconstitution of presynaptic neurotransmitter release, which we investigate here in a model of entirely natural regeneration. After toxin-induced injury, olfactory sensory neurons in the adult mouse olfactory epithelium can regenerate fully, sending axons via the olfactory nerve to reestablish synaptic contact with postsynaptic partners in the olfactory bulb. Using electrophysiological recordings in acute slices, we find that, after initial recontact, functional connectivity in this system is rapidly established. Reconnecting presynaptic terminals have almost mature functional properties, including high release probability and strong capacity for presynaptic inhibition. Release probability then matures quickly, rendering reestablished terminals functionally indistinguishable from controls just 1 week after initial contact. These data show that successful synaptic regeneration in the adult mammalian brain is almost a "plug-and-play" process, with presynaptic terminals undergoing a rapid phase of functional maturation as they reintegrate into target networks.

Dynamic functional and structural remodeling during retinal regeneration in zebrafish

Introduction

Zebrafish regenerate their retinas following damage, resulting in restoration of visual function. Here we evaluate recovery of retinal function through qualitative and quantitative analysis of the electroretinogram (ERG) over time following retinal damage, in correlation to histological features of regenerated retinal tissue.

Methods

Retinas of adult zebrafish were lesioned by intravitreal injection of 10 μM (extensive lesion; destroys all neurons) or 2 μM (selective lesion; spares photoreceptors) ouabain. Unlesioned contralateral retinas served as controls. Function of retinal circuitry was analyzed at selected timepoints using ERG recordings from live zebrafish, and whole eyes were processed for histological analyses immediately thereafter.

Results

Qualitative and quantitative assessment of waveforms during retinal regeneration revealed dynamic changes that were heterogeneous on an individual level within each sampling time, but still followed common waveform recovery patterns on a per-fish and population-level basis. Early in the regeneration period (13-30 days post injury; DPI), for both lesion types, b-waves were essentially not detected, and unmasked increased apparent amplitudes, implicit times, and half-widths of a-waves (vs. controls). In control recordings, d-waves were not obviously detected, but apparent d-waves (OFF-bipolar responses) from regenerating retinas of several fish became prominent by 30DPI and dominated the post-photoreceptor response (PPR). Beyond 45DPI, b-waves became detectable, and the ratio of apparent d- to b-wave contributions progressively shifted with most, but not all, fish displaying a b-wave dominated PPR. At the latest timepoints (extensive, 90DPI; selective, 80DPI), recordings with measurable b-waves approached a normal waveform (implicit times and half-widths), but amplitudes were not restored to control levels. Histological analyses of the retinas from which ERGs were recorded showed that as regeneration progressed, PKCa + ON-bipolar terminals and parvalbumin + amacrine cell processes became more stereotypically positioned within the deep sublaminae of the INL over recovery time after each lesion type, consistent with the shift in PPR seen in the ERG recordings.

Discussion

Taken together, these data suggest that photoreceptor-OFF-bipolar component/connectivity may functionally recover and mature earlier during regeneration compared to the photoreceptor-ON-bipolar component, though the timeframe in which such recovery happens is heterogeneous on a per-fish basis. Collectively our studies suggest gradual restoration of ON-bipolar functional circuitry during retinal regeneration.

Restoring After Central Nervous System Injuries: Neural Mechanisms and Translational Applications of Motor Recovery

Central nervous system (CNS) injuries, including stroke, traumatic brain injury, and spinal cord injury, are leading causes of long-term disability. It is estimated that more than half of the survivors of severe unilateral injury are unable to use the denervated limb. Previous studies have focused on neuroprotective interventions in the affected hemisphere to limit brain lesions and neurorepair measures to promote recovery. However, the ability to increase plasticity in the injured brain is restricted and difficult to improve. Therefore, over several decades, researchers have been prompted to enhance the compensation by the unaffected hemisphere. Animal experiments have revealed that regrowth of ipsilateral descending fibers from the unaffected hemisphere to denervated motor neurons plays a significant role in the restoration of motor function. In addition, several clinical treatments have been designed to restore ipsilateral motor control, including brain stimulation, nerve transfer surgery, and brain–computer interface systems. Here, we comprehensively review the neural mechanisms as well as translational applications of ipsilateral motor control upon rehabilitation after CNS injuries.

A small molecule M1 promotes optic nerve regeneration to restore target-specific neural activity and visual function

Axon regeneration is an energy-demanding process that requires active mitochondrial transport. In contrast to the central nervous system (CNS), axonal mitochondrial transport in regenerating axons of the peripheral nervous system (PNS) increases within hours and sustains for weeks after injury. Yet, little is known about targeting mitochondria in nervous system repair. Here, we report the induction of sustained axon regeneration, neural activities in the superior colliculus (SC), and visual function recovery after optic nerve crush (ONC) by M1, a small molecule that promotes mitochondrial fusion and transport. We demonstrated that M1 enhanced mitochondrial dynamics in cultured neurons and accelerated in vivo axon regeneration in the PNS. Ex vivo time-lapse imaging and kymograph analysis showed that M1 greatly increased mitochondrial length, axonal mitochondrial motility, and transport velocity in peripheral axons of the sciatic nerves. Following ONC, M1 increased the number of axons regenerating through the optic chiasm into multiple subcortical areas and promoted the recovery of local field potentials in the SC after optogenetic stimulation of retinal ganglion cells, resulting in complete recovery of the pupillary light reflex, and restoration of the response to looming visual stimuli was detected. M1 increased the gene expression of mitochondrial fusion proteins and major axonal transport machinery in both the PNS and CNS neurons without inducing inflammatory responses. The knockdown of two key mitochondrial genes, Opa1 or Mfn2 , abolished the growth-promoting effects of M1 after ONC, suggesting that maintaining a highly dynamic mitochondrial population in axons is required for successful CNS axon regeneration.

Ex Vivo Integration of Human Stem Retinal Ganglion Cells into the Mouse Retina

Cell replacement therapies may be key in achieving functional recovery in neurodegenerative optic neuropathies diseases such as glaucoma. One strategy that holds promise in this regard is the use of human embryonic stem cell and induced pluripotent stem-derived retinal ganglion cells (hRGCs). Previous hRGC transplantation studies have shown modest success. This is in part due to the low survival and integration of the transplanted cells in the host retina. The field is further challenged by mixed assays and outcome measurements that probe and determine transplantation success. Thefore, we have devised a transplantation assay involving hRGCs and mouse retina explants that bypasses physical barriers imposed by retinal membranes. We show that hRGC neurites and somas are capable of invading mouse explants with a subset of hRGC neurites being guided by mouse RGC axons. Neonatal mouse retina explants, and to a lesser extent, adult explants, promote hRGC integrity and neurite outgrowth. Using this assay, we tested whether suppmenting cultures with brain derived neurotrophic factor (BDNF) and the adenylate cyclase activator, forskolin, enhances hRGC neurite integration, neurite outgrowth, and integrity. We show that supplementing cultures with a combination BDNF and forskolin strongly favors hRGC integrity, increasing neurite outgrowth and complexity as well as the invasion of mouse explants. The transplantation assay presented here is a practical tool for investigating strategies for testing and optimizing the integration of donor cells into host tissues.

Guidance landscapes unveiled by quantitative proteomics to control reinnervation in adult visual system

In the injured adult central nervous system (CNS), activation of pro-growth molecular pathways in neurons leads to long-distance regeneration. However, most regenerative fibers display guidance defects, which prevent reinnervation and functional recovery. Therefore, the molecular characterization of the proper target regions of regenerative axons is essential to uncover the modalities of adult reinnervation. In this study, we use mass spectrometry (MS)-based quantitative proteomics to address the proteomes of major nuclei of the adult visual system. These analyses reveal that guidance-associated molecules are expressed in adult visual targets. Moreover, we show that bilateral optic nerve injury modulates the expression of specific proteins. In contrast, the expression of guidance molecules remains steady. Finally, we show that regenerative axons are able to respond to guidance cues ex vivo, suggesting that these molecules possibly interfere with brain target reinnervation in adult. Using a long-distance regeneration model, we further demonstrate that the silencing of specific guidance signaling leads to rerouting of regenerative axons in vivo. Altogether, our results suggest ways to modulate axon guidance of regenerative neurons to achieve circuit repair in adult.