Updates and challenges of axon regeneration in the mammalian central nervous system

Roles of Kdm6a and Kdm6b in Regulation of Mammalian Neural Regeneration

Epigenetic regulation of neuronal transcriptomic landscape is emerging to be a key coordinator of mammalian neural regeneration. The roles of two histone 3 lysine 27 (H3K27) demethylases, Kdm6a/b, in controlling neuroprotection and axon regeneration are investigated here. Deleting either Kdm6a or Kdm6b leads to enhanced sensory axon regeneration in the peripheral nervous system (PNS), whereas in the central nervous system (CNS), only deleting Kdm6a in retinal ganglion cells (RGCs) significantly enhances optic nerve regeneration. Moreover, both Kdm6a and Kdm6b function to regulate RGC survival but with different mechanisms. Mechanistically, Kdm6a regulates RGC regeneration via distinct pathway from that of Pten, and co-deleting Kdm6a and Pten results in long distance optic nerve regeneration passing the optic chiasm. In addition, RNA-seq profiling reveals that Kdm6a deletion switches the RGC transcriptomics into a developmental-like state and suppresses several known repressors of neural regeneration. Klf4 is identified as a direct downstream target of Kdm6a-H3K27me3 signaling in both sensory neurons and RGCs to regulate axon regeneration. These findings not only reveal different roles of Kdm6a and Kdm6b in regulation of neural regeneration and their underlying mechanisms, but also identify Kdm6a-mediated histone demethylation signaling as a novel epigenetic target for supporting CNS neural regeneration.

Tppp3 is a novel molecule for retinal ganglion cell identification and optic nerve regeneration

Mammalian central nervous system (CNS) axons cannot spontaneously regenerate after injury, creating an unmet need to identify molecular regulators to promote axon regeneration and reduce the lasting impact of CNS injuries. While tubulin polymerization promoting protein family member 3 (Tppp3) is known to promote axon outgrowth in amphibians, its role in mammalian axon regeneration remains unknown. Here we investigated Tppp3 in retinal ganglion cells (RGCs) neuroprotection and axonal regeneration using an optic nerve crush (ONC) model in the rodent. Single-cell RNA sequencing identified the expression of Tppp3 in RGCs of mice, macaques, and humans. Tppp3 overexpression enhanced neurite outgrowth in mouse primary RGCs in vitro, promoted axon regeneration, and improved RGC survival after ONC. Bulk RNA sequencing indicated that Tppp3 overexpression upregulates axon regeneration genes such as Bmp4 and neuroinflammatory pathways. Our findings advance regenerative medicine by developing a new therapeutic strategy for RGC neuroprotection and axon regeneration.

Roles of Kdm6a and Kdm6b in regulation of mammalian neural regeneration

Epigenetic regulation of neuronal transcriptomic landscape is emerging to be a key coordinator of mammalian neural regeneration. Here we investigated roles of two histone 3 lysine 27 (H3K27) demethylases Kdm6a/b in controlling neuroprotection and axon regeneration. Deleting either Kdm6a or Kdm6b led to enhanced sensory axon regeneration in PNS, whereas in the CNS only deleting Kdm6a in retinal ganglion cells (RGCs) significantly enhanced optic nerve regeneration. Moreover, both Kdm6a and Kdm6b functioned to regulate RGC survival but with different mechanisms. Mechanistically, Kdm6a regulates RGC regeneration via distinct pathway from that of Pten and co-deleting Kdm6a and Pten resulted in long distance optic nerve regeneration passing the optic chiasm. In addition, RNA-seq profiling revealed that Kdm6a deletion switched the RGC transcriptomics into a developmental-like state and suppressed several known repressors of neural regeneration. Klf4 was identified as a direct downstream target of Kdm6a-H3K27me3 signaling in both sensory neurons and RGCs to regulate axon regeneration. These findings not only revealed different roles of Kdm6a and Kdm6b in regulation of neural regeneration and their underlying mechanisms, but also identified Kdm6a-mediated histone demethylation signaling as a novel epigenetic target for supporting CNS neural regeneration.

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.

Glycogen synthase kinase 3 signaling in neural regeneration in vivo

Glycogen synthase kinase 3 (GSK3) signaling plays important and broad roles in regulating neural development in vitro and in vivo. Here, we reviewed recent findings of GSK3-regulated axon regeneration in vivo in both the peripheral and central nervous systems and discussed a few controversial findings in the field. Overall, current evidence indicates that GSK3β signaling serves as an important downstream mediator of the PI3K-AKT pathway to regulate axon regeneration in parallel with the mTORC1 pathway. Specifically, the mTORC1 pathway supports axon regeneration mainly through its role in regulating cap-dependent protein translation, whereas GSK3β signaling might be involved in regulating N6-methyladenosine mRNA methylation-mediated, cap-independent protein translation. In addition, GSK3 signaling also plays a key role in reshaping the neuronal transcriptomic landscape during neural regeneration. Finally, we proposed some research directions to further elucidate the molecular mechanisms underlying the regulatory function of GSK3 signaling and discover novel GSK3 signaling-related therapeutic targets. Together, we hope to provide an updated and insightful overview of how GSK3 signaling regulates neural regeneration in vivo.

Appropriate patient population for future visual system axon regeneration therapies

A number of blinding diseases caused by damage to the optic nerve result in progressive vision loss or loss of visual acuity. Secondary glaucoma results from traumatic injuries, pseudoexfoliation or pigmentary dispersion syndrome. Progressive peripheral vision loss is common to all secondary glaucoma irrespective of the initial event. Axon regeneration is a potential therapeutic avenue to restore lost vision in these patients. In contrast to the usual approach of having the worst possible patient population for initial therapies, axon regeneration may require consideration of appropriate patient population even for initial treatment trials. The current state of axon regeneration therapies, their potential future and suitable patient population when ready is discussed in this perspective. The selection of patients are important for adoption of axon regeneration specifically in the areas of central nervous system regenerative medicine. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Neurological Diseases > Biomedical Engineering Metabolic Diseases > Molecular and Cellular Physiology.

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.

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.

Exploring the brain epitranscriptome: perspectives from the NSAS summit

Increasing evidence reinforces the essential function of RNA modifications in development and diseases, especially in the nervous system. RNA modifications impact various processes in the brain, including neurodevelopment, neurogenesis, neuroplasticity, learning and memory, neural regeneration, neurodegeneration, and brain tumorigenesis, leading to the emergence of a new field termed neuroepitranscriptomics. Deficiency in machineries modulating RNA modifications has been implicated in a range of brain disorders from microcephaly, intellectual disability, seizures, and psychiatric disorders to brain cancers such as glioblastoma. The inaugural NSAS Challenge Workshop on Brain Epitranscriptomics hosted in Crans-Montana, Switzerland in 2023 assembled a group of experts from the field, to discuss the current state of the field and provide novel translational perspectives. A summary of the discussions at the workshop is presented here to simulate broader engagement from the general neuroscience field.

Janus kinase inhibitors are potential therapeutics for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a poorly treated multifactorial neurodegenerative disease associated with multiple cell types and subcellular organelles. As with other multifactorial diseases, it is likely that drugs will need to target multiple disease processes and cell types to be effective. We review here the role of Janus kinase (JAK)/Signal transducer and activator of transcription (STAT) signalling in ALS, confirm the association of this signalling with fundamental ALS disease processes using the BenevolentAI Knowledge Graph, and demonstrate that inhibitors of this pathway could reduce the ALS pathophysiology in neurons, glia, muscle fibres, and blood cells. Specifically, we suggest that inhibition of the JAK enzymes by approved inhibitors known as Jakinibs could reduce STAT3 activation and modify the progress of this disease. Analysis of the Jakinibs highlights baricitinib as a suitable candidate due to its ability to penetrate the central nervous system and exert beneficial effects on the immune system. Therefore, we recommend that this drug be tested in appropriately designed clinical trials for ALS.

Roles of constitutively secreted extracellular chaperones in neuronal cell repair and regeneration

Protein quality control involves many processes that jointly act to regulate the expression, localization, turnover, and degradation of proteins, and has been highlighted in recent studies as critical to the differentiation of stem cells during regeneration. The roles of constitutively secreted extracellular chaperones in neuronal injury and disease are poorly understood. Extracellular chaperones are multifunctional proteins expressed by many cell types, including those of the nervous system, known to facilitate protein quality control processes. These molecules exert pleiotropic effects and have been implicated as playing important protective roles in a variety of stress conditions, including tissue damage, infections, and local tissue inflammation. This article aims to provide a critical review of what is currently known about the functions of extracellular chaperones in neuronal repair and regeneration and highlight future directions for this important research area. We review what is known of four constitutively secreted extracellular chaperones directly implicated in processes of neuronal damage and repair, including transthyretin, clusterin, α2-macroglobulin, and neuroserpin, and propose that investigation into the effects of these and other extracellular chaperones on neuronal repair and regeneration has the potential to yield valuable new therapies.

Downregulation of SF3B2 protects CNS neurons in models of multiple sclerosis

OBJECTIVE

Neurodegeneration induced by inflammatory stress in multiple sclerosis (MS) leads to long-term neurological disabilities that are not amenable to current immunomodulatory therapies.

METHODS AND RESULTS

Here, we report that neuronal downregulation of Splicing factor 3b subunit 2 (SF3B2), a component of U2 small nuclear ribonucleoprotein (snRNP), preserves retinal ganglion cell (RGC) survival and axonal integrity in experimental autoimmune encephalomyelitis (EAE)-induced mice. By employing an in vitro system recapitulating the inflammatory environment of MS lesion, we show that when SF3B2 levels are downregulated, cell viability and axon integrity are preserved in cortical neurons against inflammatory toxicity. Notably, knockdown of SF3B2 suppresses the expression of injury-response and necroptosis genes and prevents activation of Sterile Alpha and TIR Motif Containing 1 (Sarm1), a key enzyme that mediates programmed axon degeneration.

INTERPRETATION

Together, these findings suggest that the downregulation of SF3B2 is a novel potential therapeutic target to prevent secondary neurodegeneration in MS.

Enhanced neuronal differentiation by dynamic piezoelectric stimulation

Electroactive smart materials play an important role for tissue regenerative applications. Poly(vinylidene fluoride) (PVDF) is a specific subtype of piezoelectric electroactive material that generates electrical potential upon mechanical stimulation. This work focuses on the application of piezoelectric PVDF films for neural differentiation. Human neural precursor cells (hNPCs) are cultured on piezoelectric poled and non-poled β-PVDF films with or without a pre-coating step of poly-d-lysine and laminin (PDL/L). Subsequently, hNPCs differentiation into the neuronal lineage is assessed (MAP2⁺ and DCX⁺ ) under static or dynamic (piezoelectric stimulation) culture conditions. The results demonstrate that poled and coated β-PVDF films induce neuronal differentiation under static culture conditions which is further enhanced with mechanical stimulation. In silico calculations of the electrostatic potential of different domains of laminin, highlight the high polarity of those domains, which shows a clear preference to interact with the varying surface electric field of the piezoelectric material under mechanical stimulation. These interactions might explain the higher neuronal differentiation induced by poled β-PVDF films pre-coated with PDL/L under dynamic conditions. Our results suggest that electromechanical stimuli, such as the ones induced by piezoelectric β-PVDF films, are suitable to promote neuronal differentiation and hold great promise for the development of neuroregenerative therapies.

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

Retinal ganglion cell expression of cytokine enhances occupancy of NG2 cell-derived astrocytes at the nerve injury site: Implication for axon regeneration

Following injury in the central nervous system, a population of astrocytes occupy the lesion site, form glial bridges and facilitate axon regeneration. These astrocytes originate primarily from resident astrocytes or NG2+ oligodendrocyte progenitor cells. However, the extent to which these cell types give rise to the lesion-filling astrocytes, and whether the astrocytes derived from different cell types contribute similarly to optic nerve regeneration remain unclear. Here we examine the distribution of astrocytes and NG2+ cells in an optic nerve crush model. We show that optic nerve astrocytes partially fill the injury site over time after a crush injury. Viral mediated expression of a growth-promoting factor, ciliary neurotrophic factor (CNTF), in retinal ganglion cells (RGCs) promotes axon regeneration without altering the lesion size or the degree of lesion-filling GFAP+ cells. Strikingly, using inducible NG2CreER driver mice, we found that CNTF overexpression in RGCs increases the occupancy of NG2+ cell-derived astrocytes in the optic nerve lesion. An EdU pulse-chase experiment shows that the increase in NG2 cell-derived astrocytes is not due to an increase in cell proliferation. Lastly, we performed RNA-sequencing on the injured optic nerve and reveal that CNTF overexpression in RGCs results in significant changes in the expression of distinct genes, including those that encode chemokines, growth factor receptors, and immune cell modulators. Even though CNTF-induced axon regeneration has long been recognized, this is the first evidence of this procedure affecting glial cell fate at the optic nerve crush site. We discuss possible implication of these results for axon regeneration.

Novel adult cortical neuron processing and screening method illustrates sex- and age-dependent effects of pharmaceutical compounds

Neurodegenerative diseases and neurotraumatic injuries are typically age-associated disorders that can reduce neuron survival, neurite outgrowth, and synaptic plasticity leading to loss of cognitive capacity, executive function, and motor control. In pursuit of reducing the loss of said neurological functions, novel compounds are sought that promote neuron viability, neuritogenesis, and/or synaptic plasticity. Current high content in vitro screenings typically use cells that are iPSC-derived, embryonic, or originate from post-natal tissues; however, most patients suffering from neurodegenerative diseases and neurotrauma are of middle-age and older. The chasm in maturity between the neurons used in drug screens and those in a target population is a barrier for translational success of in vitro results. It has been historically challenging to culture adult neurons let alone conduct screenings; therefore, age-appropriate drug screenings have previously not been plausible. We have modified Miltenyi's protocol to increase neuronal yield, neuron purity, and neural viability at a reduced cost to expand our capacity to screen compounds directly in primary adult neurons. To our knowledge, we developed the first morphology-based screening system using adult cortical neurons and the first to incorporate age and sex as biological variables in a screen using adult cortical neurons. By using primary adult cortical neurons from mice that were 4 to 48 weeks old for screening pharmaceutical agents, we have demonstrated age- and sex-dependent effects on neuritogenesis and neuron survival in vitro. Utilizing age- and sex-appropriate in vitro models to find novel compounds increasing neuron survival and neurite outgrowth, made possible by our modified adult neuron processing method, will greatly increase the relevance of in vitro screening for finding neuroprotective compounds.

The Role of Spastin in Axon Biology

Neurons are highly polarized cells with elaborate shapes that allow them to perform their function. In neurons, microtubule organization—length, density, and dynamics—are essential for the establishment of polarity, growth, and transport. A mounting body of evidence shows that modulation of the microtubule cytoskeleton by microtubule-associated proteins fine tunes key aspects of neuronal cell biology. In this respect, microtubule severing enzymes—spastin, katanin and fidgetin—a group of microtubule-associated proteins that bind to and generate internal breaks in the microtubule lattice, are emerging as key modulators of the microtubule cytoskeleton in different model systems. In this review, we provide an integrative view on the latest research demonstrating the key role of spastin in neurons, specifically in the context of axonal cell biology. We focus on the function of spastin in the regulation of microtubule organization, and axonal transport, that underlie its importance in the intricate control of axon growth, branching and regeneration.

Reprogramming neurons for regeneration: The fountain of youth

Neurons in the central nervous system (CNS) are terminally differentiated cells that gradually lose their ability to support regeneration during maturation due to changes in transcriptomic and chromatin landscape. Similar transcriptomic changes also occur during development when stem cells differentiate into different types of somatic cells. Importantly, differentiated cells can be reprogrammed back to induced pluripotent stems cells (iPSCs) via global epigenetic remodeling by combined overexpression of pluripotent reprogramming factors, including Oct4, Sox2, Klf4, c-Myc, Nanog, and/or Lin28. Moreover, recent findings showed that many proneural transcription factors were able to convert non-neural somatic cells into neurons bypassing the pluripotent stage via direct reprogramming. Interestingly, many of these factors have recently been identified as key regulators of CNS neural regeneration. Recent studies indicated that these factors could rejuvenate mature CNS neurons back to a younger state through cellular state reprogramming, thus favoring regeneration. Here we will review some recent findings regarding the roles of genetic cellular state reprogramming in regulation of neural regeneration and explore the potential underlying molecular mechanisms. Moreover, by using newly emerging techniques, such as multiomics sequencing with big data analysis and Crispr-based gene editing, we will discuss future research directions focusing on better revealing cellular state reprogramming-induced remodeling of chromatin landscape and potential translational application.

Gas5 inhibition promotes the axon regeneration in the adult mammalian nervous system

Neurons in the peripheral nervous system (PNS) have robust regenerative capacity after axon injury, but the regenerative capacity is generally absent in the neurons of the central nervous system (CNS) in mammals. Increasing evidence highlighted the pivotal roles of long-noncoding RNAs (lncRNAs) in development and disease, but the role of LncRNA in triggering the regenerative capacity in CNS and PNS is not well studied. Here, we reported that lncRNA Gas5 is a suppressor for axon regeneration. Bioinformatics analysis shows that Gas5 is age-dependent up-regulated during DRG neurons development and down-regulated after sciatic nerve injury. In vitro, inhibiting the expression of Gas5 promotes the neurite growth of DRG neurons both in mice and rats. Consistently, Gas5 overexpression inhibits axon growth of mice DRG neurons. In vivo, Gas5 knockout(Gas5-/-) mice display enhanced nerve regeneration ability after sciatic nerve injury. RNA pull-down analysis indicates that Gas5 can interacts with soluble Vimentin, which is essential for peripheral nerve development and regeneration. Vimentin knockdown reverses the Gas5 silence-regulated axon pro-regeneration demonstrating that the function of Gas5 depending on Vimentin. Besides, inhibition of Gas5 expression can also enhance optic nerve regeneration indicating a potential pro-regenerative ability of Gas5 silence in CNS. Our study for the first time provides direct evidence in vivo that lncRNA plays a role in regulating central axon regrowth and Gas5 might be a novel therapeutic target for axon regeneration in both PNS and CNS.

RSK1 promotes mammalian axon regeneration by inducing the synthesis of regeneration-related proteins

In contrast to the adult mammalian central nervous system (CNS), the neurons in the peripheral nervous system (PNS) can regenerate their axons. However, the underlying mechanism dictating the regeneration program after PNS injuries remains poorly understood. Combining chemical inhibitor screening with gain- and loss-of-function analyses, we identified p90 ribosomal S6 kinase 1 (RSK1) as a crucial regulator of axon regeneration in dorsal root ganglion (DRG) neurons after sciatic nerve injury (SNI). Mechanistically, RSK1 was found to preferentially regulate the synthesis of regeneration-related proteins using ribosomal profiling. Interestingly, RSK1 expression was up-regulated in injured DRG neurons, but not retinal ganglion cells (RGCs). Additionally, RSK1 overexpression enhanced phosphatase and tensin homolog (PTEN) deletion-induced axon regeneration in RGCs in the adult CNS. Our findings reveal a critical mechanism in inducing protein synthesis that promotes axon regeneration and further suggest RSK1 as a possible therapeutic target for neuronal injury repair.

This study shows that p90 ribosomal S6 kinase 1 (RSK1) responds differentially to nerve injury in the peripheral and central nervous systems, and identifies it as a crucial regulator of axonal regeneration; mechanistically, RSK1 preferentially induces the synthesis of regeneration-related proteins via the RSK1-eEF2K-eEF2 axis.

A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System

Nerve axonal injury and associated cellular mechanisms leading to peripheral nerve damage are important topics of research necessary for reducing disability and enhancing quality of life. Model systems that mimic the biological changes that occur during human nerve injury are crucial for the identification of cellular responses, screening of novel therapeutic molecules, and design of neural regeneration strategies. In addition to in vivo and mathematical models, in vitro axonal injury models provide a simple, robust, and reductionist platform to partially understand nerve injury pathogenesis and regeneration. In recent years, there have been several advances related to in vitro techniques that focus on the utilization of custom-fabricated cell culture chambers, microfluidic chamber systems, and injury techniques such as laser ablation and axonal stretching. These developments seem to reflect a gradual and natural progression towards understanding molecular and signaling events at an individual axon and neuronal-soma level. In this review, we attempt to categorize and discuss various in vitro models of injury relevant to the peripheral nervous system and highlight their strengths, weaknesses, and opportunities. Such models will help to recreate the post-injury microenvironment and aid in the development of therapeutic strategies that can accelerate nerve repair.

Dorsal myelotomy in E15-E16 fetal rat: A promising paradigm in regeneration research, with serendipitous transcriptomic effects on development, of the primary afferent system

BACKGROUND

Using a multidisciplinary approach in neuroscience has tackled the regeneration enigma in the central nervous system for decades. Reported regeneration potential in mammals is lacking confirmation without a suitable paradigm. In vivo fetal experimentation offers an almost insurmountable drawback, though its feasibility has been shown long ago. New method: Following two former technical papers on fetal surgery, here we present dorsal myelotomy in fetal rat and antegrade HRP-tracing as a suitable paradigm to reveal the intrinsic regenerative program in DRG neurons. Moreover, disclosing the spatio-temporal development of the primary afferent system appeared as an unexpected feature of the design.

CONCLUSION

this paper underlines the feasibility that fetal experimentation may offer a new venue for research into the rat transcriptome.

Thermosensitive collagen/fibrinogen gels loaded with decorin suppress lesion site cavitation and promote functional recovery after spinal cord injury

The treatment of spinal cord injury (SCI) is a complex challenge in regenerative medicine, complicated by the low intrinsic capacity of CNS neurons to regenerate their axons and the heterogeneity in size, shape and extent of human injuries. For example, some contusion injuries do not compromise the dura mater and in such cases implantation of preformed scaffolds or drug delivery systems may cause further damage. Injectable in situ thermosensitive scaffolds are therefore a less invasive alternative. In this study, we report the development of a novel, flowable, thermosensitive, injectable drug delivery system comprising bovine collagen (BC) and fibrinogen (FB) that forms a solid BC/FB gel (Gel) immediately upon exposure to physiological conditions and can be used to deliver reparative drugs, such as the naturally occurring anti-inflammatory, anti-scarring agent Decorin, into adult rat spinal cord lesion sites. In dorsal column lesions of adult rats treated with the Gel + Decorin, cavitation was completely suppressed and instead lesion sites became filled with injury-responsive cells and extracellular matrix materials, including collagen and laminin. Decorin increased the intrinsic potential of dorsal root ganglion neurons (DRGN) by increasing their expression of regeneration associated genes (RAGs), enhanced local axon regeneration/sprouting, as evidenced both histologically and by improved electrophysiological, locomotor and sensory function recovery. These results suggest that this drug formulated, injectable hydrogel has the potential to be further studied and translated into the clinic.

The spatial arrangement of cells in a 3D-printed biomimetic spinal cord promotes directional differentiation and repairs the motor function after spinal cord injury

Spinal cord injury is a permanent destructive disease that causes devastating neurologic deficits and disability. Long-term complications are associated with low prognosis, mortality, and decreased quality of life. The functional recovery depends on the regeneration of neurons and the growth of medullated axons. Single treatment strategies, including cell transplantation, cannot adapt to a changeable microenvironment. Patients with spinal cord injuries need more effective, long-term, and stable treatment options. Therefore, we investigated the benefit of a combined-tissue engineering strategy by loading homologous bone mesenchymal stem cells (BMSCs) and Schwann cells in three-dimensional (3D) scaffolds. We placed BMSCs and Rat Schwann cells (RSCs) in specific spatial arrangements using cell gravity and the diffusion effect to promote the formation of intercellular connections and cell-directed differentiation. This novel bioengineering system allowed us to control multiple factors, including cell types, cell relative position, and axon growth direction in the scaffold. Our system facilitated motor function recovery by enhancing tissue mimicry and allowing the reconstruction of medullated axons. This new 3D-integrated printing platform is multi-function and can simulate biomimetic tissue using different types of materials and multi-cells scaffolds. We believe that this study can help promote the clinical development and application of 3D printing in the field of regenerative medicine.

In vivo glial trans-differentiation for neuronal replacement and functional recovery in central nervous system

The adult mammalian central nervous system (CNS) is deficient in intrinsic machineries to replace neurons lost in injuries or progressive degeneration. Various types of these neurons constitute neural circuitries wired to support vital sensory, motor, and cognitive functions. Based on the pioneer studies in cell lineage conversion, one promising strategy is to convert in vivo glial cells into neural progenitors or directly into neurons that can be eventually rewired for functional recovery. We first briefly summarize the well-studied regeneration-capable CNS in the zebrafish, focusing on their postinjury spontaneous reprogramming of the retinal Müller glia (MG). We then compare the signaling transductions, and transcriptional and epigenetic regulations in the zebrafish MGs with their mammalian counterparts, which perpetuate certain barriers against proliferation and neurogenesis and thus fail in MG-to-progenitor conversion. Next, we discuss emerging evidence from mouse studies, in which the in vivo glia-to-neuron conversion could be achieved with sequential or one-step genetic manipulations, such as the conversions from retinal MGs to interneurons, photoreceptors, or retinal ganglion cells (RGCs), as well as the conversions from midbrain astrocytes to dopaminergic or GABAergic neurons. Some of these in vivo studies showed considerable coverage of subtypes in the newly induced neurons and partial reestablishment in neural circuits and functions. Importantly, we would like to point out some crucial technical concerns that need to be addressed to convincingly show successful glia-to-neuron conversion. Finally, we present challenges and future directions in the field for better neural function recovery.

Mitochondrial Behavior in Axon Degeneration and Regeneration

Mitochondria are organelles responsible for bioenergetic metabolism, calcium homeostasis, and signal transmission essential for neurons due to their high energy consumption. Accumulating evidence has demonstrated that mitochondria play a key role in axon degeneration and regeneration under physiological and pathological conditions. Mitochondrial dysfunction occurs at an early stage of axon degeneration and involves oxidative stress, energy deficiency, imbalance of mitochondrial dynamics, defects in mitochondrial transport, and mitophagy dysregulation. The restoration of these defective mitochondria by enhancing mitochondrial transport, clearance of reactive oxidative species (ROS), and improving bioenergetic can greatly contribute to axon regeneration. In this paper, we focus on the biological behavior of axonal mitochondria in aging, injury (e.g., traumatic brain and spinal cord injury), and neurodegenerative diseases (Alzheimer's disease, AD; Parkinson's disease, PD; Amyotrophic lateral sclerosis, ALS) and consider the role of mitochondria in axon regeneration. We also compare the behavior of mitochondria in different diseases and outline novel therapeutic strategies for addressing abnormal mitochondrial biological behavior to promote axonal regeneration in neurological diseases and injuries.

Signaling pathways modulated by monocular enucleation in the superior colliculus of juvenile rats

Monocular eye enucleation (ME) is a classical paradigm to induce neural plasticity in retinal ganglion cells (RGCs) axons from the intact eye, especially when performed within the critical period of visual system development. However, the precise mechanisms underlying the axonal sprouting and synaptogenesis seen in this model remain poorly understood. In the present work, we investigated the temporal alterations in phosphorylation of three kinases related to axonal growth and synaptogenesis-GSK3β (an important repressor of axonal outgrowth), AKT, and ERK-in superior colliculus of rats submitted to ME during early postnatal development. Western blotting analysis showed an increase in pGSK3β, the inactive form of this enzyme, 24 and 48 hr after ME. Accordingly, an increase in pERK levels was detected 24 hr after ME, indicating that phosphorylation of these enzymes might be related to axonal reorganization induced by ME. Interestingly, AKT phosphorylation was increased just 1 week after ME, suggesting it may be involved in the stabilization of newly formed synapses, rising from the axonal reorganization of remaining eye. A better understanding of how signaling pathways are modulated in a model of intense axonal sprouting can highlight possible therapeutic targets in RGCs injuries in adult individuals, where axonal regrowth is nearly absent.

Roles of Non-coding RNAs in Central Nervous System Axon Regeneration

Axons in the central nervous system often fail to regenerate after injury due to the limited intrinsic regeneration ability of the central nervous system (CNS) and complex extracellular inhibitory factors. Therefore, it is of vital importance to have a better understanding of potential methods to promote the regeneration capability of injured nerves. Evidence has shown that non-coding RNAs play an essential role in nerve regeneration, especially long non-coding RNA (lncRNA), microRNA (miRNA), and circular RNA (circRNA). In this review, we profile their separate roles in axon regeneration after CNS injuries, such as spinal cord injury (SCI) and optic nerve injury. In addition, we also reveal the interactive networks among non-coding RNAs.