Evidence for an Age-Dependent Decline in Axon Regeneration in the Adult Mammalian Central Nervous System

Enhancement of motor functional recovery in thoracic spinal cord injury: voluntary wheel running versus forced treadmill exercise

JOURNAL/nrgr/04.03/01300535-202503000-00028/figure1/v/2024-06-17T092413Z/r/image-tiff Spinal cord injury necessitates effective rehabilitation strategies, with exercise therapies showing promise in promoting recovery. This study investigated the impact of rehabilitation exercise on functional recovery and morphological changes following thoracic contusive spinal cord injury. After a 7-day recovery period after spinal cord injury, mice were assigned to either a trained group (10 weeks of voluntary running wheel or forced treadmill exercise) or an untrained group. Bi-weekly assessments revealed that the exercise-trained group, particularly the voluntary wheel exercise subgroup, displayed significantly improved locomotor recovery, more plasticity of dopaminergic and serotonin modulation compared with the untrained group. Additionally, exercise interventions led to gait pattern restoration and enhanced transcranial magnetic motor-evoked potentials. Despite consistent injury areas across groups, exercise training promoted terminal innervation of descending axons. In summary, voluntary wheel exercise shows promise for enhancing outcomes after thoracic contusive spinal cord injury, emphasizing the role of exercise modality in promoting recovery and morphological changes in spinal cord injuries. Our findings will influence future strategies for rehabilitation exercises, restoring functional movement after spinal cord injury.

An effective pharmacological hydrogel induces optic nerve repair and improves visual function

Irreversible eye lesions, such as glaucoma and traumatic optic neuropathy, can cause blindness; however, no effective treatments exist. The optic nerve, in particular, lacks the capacity to spontaneously regenerate, requiring the development of an effective approach for optic nerve repair, which has proven challenging. Here, we demonstrate that a combination of the small molecules 3BDO and trichostatin A (TSA)-which regulate mTOR and HDAC, respectively-packaged in thermosensitive hydrogel for 4-week-sustained release after intravitreal injection, effectively induced optic nerve regeneration in a mouse model of optic nerve crush injury. Moreover, this combination of 3BDO and TSA also protected axon projections and improved visual responses in an old mouse model (11 months old) of glaucoma. Taken together, our data provide a new, local small molecule-based treatment for the effective induction of optic nerve repair, which may represent a foundation for the development of pharmacological methods to treat irreversible eye diseases.

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.

Deep scRNA sequencing reveals a broadly applicable Regeneration Classifier and implicates antioxidant response in corticospinal axon regeneration

Despite substantial progress in understanding the biology of axon regeneration in the CNS, our ability to promote regeneration of the clinically important corticospinal tract (CST) after spinal cord injury remains limited. To understand regenerative heterogeneity, we conducted patch-based single-cell RNA sequencing on rare regenerating CST neurons at high depth following PTEN and SOCS3 deletion. Supervised classification with Garnett gave rise to a Regeneration Classifier, which can be broadly applied to predict the regenerative potential of diverse neuronal types across developmental stages or after injury. Network analyses highlighted the importance of antioxidant response and mitochondrial biogenesis. Conditional gene deletion validated a role for NFE2L2 (or NRF2), a master regulator of antioxidant response, in CST regeneration. Our data demonstrate a universal transcriptomic signature underlying the regenerative potential of vastly different neuronal populations and illustrate that deep sequencing of only hundreds of phenotypically identified neurons has the power to advance regenerative biology.

PTEN deletion in spinal pathways via retrograde transduction with AAV-RG enhances forelimb motor recovery after cervical spinal cord injury; Sex differences and late-onset pathophysiologies

Spinal cord injuries (SCI) cause permanent functional impairments due to interruption of motor and sensory pathways. Regeneration of axons does not occur due to lack of intrinsic growth capacity of adult neurons and extrinsic inhibitory factors, especially at the injury site. However, some regeneration can be achieved via deletion of the phosphatase and tensin homolog (PTEN) in cells of origin of spinal pathways. Here, we deployed an AAV variant that is retrogradely transported (AAV-rg) to deliver gene modifying cargos to the cells of origin of multiple pathways interrupted by SCI, testing whether this promoted recovery of motor function. PTENf/f;RosatdTomato mice and control RosatdTomato mice received injections of different doses (number of genome copies, GCs) of AAV-rg/Cre into the cervical spinal cord at the time of a C5 dorsal hemisection injury. Forelimb grip strength was tested over time using a grip strength meter. PTENf/f;RosatdTomato mice with AAV-rg/Cre (PTEN-deleted) exhibited substantial improvements in forelimb gripping ability in comparison to controls. Of note, there were major sex differences in the extent of recovery, with male mice exhibiting greater recovery than females. However, at around 5-7 weeks post-injury/injection, many mice with SCI and AAV-rg-mediated PTEN deletion began to exhibit pathophysiologies involving excessive scratching of the ears and back of the neck and rigid forward extension of the hindlimbs. These pathophysiologies increased in incidence and severity over time. Our results reveal that although intra-spinal injections of AAV-rg/Cre in PTENf/f;RosatdTomato mice can enhance forelimb motor recovery after SCI, late-developing functional abnormalities occur with the experimental conditions used here. Mechanisms underlying late-developing pathophysiologies remain to be defined.

Identification of age-specific biomarkers of spinal cord injury: A bioinformatics analysis of young and aged mice models

BACKGROUND

Spinal cord injury (SCI) is a debilitating event that often results in long-term physical damage, disability, and a significant impact on a patient's quality of life. Past research has noted an age-dependent decline in regeneration post-SCI. This study seeks to identify potential biomarkers and enriched pathways in young and aged SCI mouse models.

METHODS

We retrieved the microarray data of spinal cord samples from SCI mice and control mice from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were identified using the R software and the Linear Models for Microarray Data (limma) package. The Gene Set Enrichment Analysis (GSEA) determined enrichment differences among gene sets. The Weighted Gene Co-expression Network Analysis (WGCNA) pinpointed clinically significant modules and hub genes in SCI. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis was employed to uncover significantly related pathways of crucial genes in SCI.

RESULTS

We found 2560 DEGs in the young SCI group, comprised of 1733 upregulated RNAs and 827 downregulated RNAs. In the aged SCI group, 3048 DEGs were revealed including 1856 upregulated and 1192 downregulated genes. The GSEA revealed 12 enriched signaling pathways in the young SCI group, such as IL6/JAK/STAT3 signaling, interferon alpha response, and interferon gamma response, and 21 signaling pathways in the aged SCI group such as IL6/JAK/STAT3 signaling, E2F targets, and angiogenesis-related pathways. The WGCNA identified the turquoise module as significantly associated with the clinical traits of both young and aged SCI, and revealed 3181 hub genes. Ultimately, 1858 significant genes in SCI were found, with associated signaling pathways including epithelial-mesenchymal transition (EMT), interferon gamma response, and KARS signaling.

CONCLUSION

Our study explored the RNA expression patterns and enriched signaling pathways in young and aged SCI mice. These findings may provide potential biomarkers for targeted SCI therapy.

Challenges in Translating Regenerative Therapies for Spinal Cord Injury

Regenerating the injured spinal cord is a substantial challenge with many obstacles that need to be overcome to achieve robust functional benefits. This abundance of hurdles can partly explain the limited success when applying regenerative intervention treatments in animal models and/or people. In this article, we elaborate on a few of these obstacles, starting with the applicability of animal models and how they compare to the clinical setting. We then discuss the requirement for combinatorial interventions and the associated problems in experimental design, including the addition of rehabilitative training. The article expands on differences in lesion sizes and locations between humans and common animal models, and how this difference can determine the success or failure of an intervention. An additional and frequently overlooked problem in the translation of interventions that applies beyond the field of neuroregeneration is the reporting bias and the lack of transparency in reporting findings. New data mandates are tackling this problem and will eventually result in a more balanced view of the field. Finally, we will discuss strategies to negotiate the challenging course of successful translation to facilitate successful translation of regeneration promoting interventions.

Golgi-dependent reactivation and regeneration of Drosophila quiescent neural stem cells

The ability of stem cells to switch between quiescent and proliferative states is crucial for maintaining tissue homeostasis and regeneration. In Drosophila, quiescent neural stem cells (qNSCs) extend a primary protrusion, a hallmark of qNSCs. Here, we have found that qNSC protrusions can be regenerated upon injury. This regeneration process relies on the Golgi apparatus that acts as the major acentrosomal microtubule-organizing center in qNSCs. A Golgi-resident GTPase Arf1 and its guanine nucleotide exchange factor Sec71 promote NSC reactivation and regeneration via the regulation of microtubule growth. Arf1 physically associates with its new effector mini spindles (Msps)/XMAP215, a microtubule polymerase. Finally, Arf1 functions upstream of Msps to target the cell adhesion molecule E-cadherin to NSC-neuropil contact sites during NSC reactivation. Our findings have established Drosophila qNSCs as a regeneration model and identified Arf1/Sec71-Msps pathway in the regulation of microtubule growth and NSC reactivation.

PTEN knockout using retrogradely transported AAVs transiently restores locomotor abilities in both acute and chronic spinal cord injury

Restoring function in chronic stages of spinal cord injury (SCI) has often been met with failure or reduced efficacy when regenerative strategies are delayed past the acute or sub-acute stages of injury. Restoring function in the chronically injured spinal cord remains a critical challenge. We found that a single injection of retrogradely transported adeno-associated viruses (AAVrg) to knockout the phosphatase and tensin homolog protein (PTEN) in chronic SCI can effectively target both damaged and spared axons and transiently restore locomotor functions in near-complete injury models. AAVrg's were injected to deliver cre recombinase and/or a red fluorescent protein (RFP) under the human Synapsin 1 promoter (hSyn1) into the spinal cords of C57BL/6 PTENFloxΔ/Δ mice to knockout PTEN (PTEN-KO) in a severe thoracic SCI crush model at both acute and chronic time points. PTEN-KO improved locomotor abilities in both acute and chronic SCI conditions over a 9-week period. Regardless of whether treatment was initiated at the time of injury (acute), or three months after SCI (chronic), mice with limited hindlimb joint movement gained hindlimb weight support after treatment. Interestingly, functional improvements were not sustained beyond 9 weeks coincident with a loss of RFP reporter-gene expression and a near-complete loss of treatment-associated functional recovery by 6 months post-treatment. Treatment effects were also specific to severely injured mice; animals with weight support at the time of treatment lost function over a 6-month period. Retrograde tracing with Fluorogold revealed viable neurons throughout the motor cortex despite a loss of RFP expression at 9 weeks post-PTEN-KO. However, few Fluorogold labeled neurons were detected within the motor cortex at 6 months post-treatment. BDA labeling from the motor cortex revealed a dense corticospinal tract (CST) bundle in all groups except chronically treated PTEN-KO mice, indicating a potential long-term toxic effect of PTEN-KO to neurons in the motor cortex which was corroborated by a loss of β-tubulin III labeling above the lesion within spinal cords after PTEN-KO. PTEN-KO mice had significantly more β-tubulin III labeled axons within the lesion when treatment was delivered acutely, but not chronically post-SCI. In conclusion, we have found that using AAVrg's to knockout PTEN is an effective manipulation capable of restoring motor functions in chronic SCI and can enhance axon growth of currently unidentified axon populations when delivered acutely after injury. However, the long-term consequences of PTEN-KO on neuronal health and viability should be further explored.

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

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

Morphological screens using aged primary adult neuronal, microglial, and astrocytic cultures to find novel neurotherapeutics

The average age of a patient with neurotraumatic injuries or neurodegenerative diseases has been increasing worldwide. The preclinical live animal models used for neurotrauma and neurodegenerative diseases are typically young adults, failing to represent the age of humans in the clinic. This dichotomy in age between human populations and animal models is likely to impede the understanding of the pathological mechanisms of most neurological disorders and the translation of their respective promising therapies. This lack of cohesion between animal models and patients in the clinic begins prior to in vivo testing, it starts during the in vitro drug screening phase. Conventional screening methods typically involve the use of stem cell derived neural cells, with some researchers using embryonic derived neural cells instead. These cells lack the fundamental characteristics present in aged neural cells, such as age-induced changes in process length and branching in microglia and how astrocytes respond to various insults. Various technologies and techniques have been developed recently that can help researchers use age-appropriate neural cells for their drug discovery endeavors. The use of age-appropriate neural cells during screening phases is hypothesized to significantly increase the translation rate of the hits to the geriatric patients suffering from neurotraumatic and neurodegenerative diseases.

Discovery of therapeutic targets for spinal cord injury based on molecular mechanisms of axon regeneration after conditioning lesion

BACKGROUND

Preinjury of peripheral nerves triggers dorsal root ganglia (DRG) axon regeneration, a biological change that is more pronounced in young mice than in old mice, but the complex mechanism has not been clearly explained. Here, we aim to gain insight into the mechanisms of axon regeneration after conditioning lesion in different age groups of mice, thereby providing effective therapeutic targets for central nervous system (CNS) injury.

METHODS

The microarray GSE58982 and GSE96051 were downloaded and analyzed to identify differentially expressed genes (DEGs). The protein-protein interaction (PPI) network, the miRNA-TF-target gene network, and the drug-hub gene network of conditioning lesion were constructed. The L4 and L5 DRGs, which were previously axotomized by the sciatic nerve conditioning lesions, were harvested for qRT-PCR. Furthermore, histological and behavioral tests were performed to assess the therapeutic effects of the candidate drug telmisartan in spinal cord injury (SCI).

RESULTS

A total of 693 and 885 DEGs were screened in the old and young mice, respectively. Functional enrichment indicates that shared DEGs are involved in the inflammatory response, innate immune response, and ion transport. QRT-PCR results showed that in DRGs with preinjury of peripheral nerve, Timp1, P2ry6, Nckap1l, Csf1, Ccl9, Anxa1, and C3 were upregulated, while Agtr1a was downregulated. Based on the bioinformatics analysis of DRG after conditioning lesion, Agtr1a was selected as a potential therapeutic target for the SCI treatment. In vivo experiments showed that telmisartan promoted axonal regeneration after SCI by downregulating AGTR1 expression.

CONCLUSION

This study provides a comprehensive map of transcriptional changes that discriminate between young and old DRGs in response to injury. The hub genes and their related drugs that may affect the axonal regeneration program after conditioning lesion were identified. These findings revealed the speculative pathogenic mechanism involved in conditioning-dependent regenerative growth and may have translational significance for the development of CNS injury treatment in the future.

PTEN deletion in spinal pathways via retrograde transduction with AAV-rg enhances forelimb motor recovery after cervical spinal cord injury; sex differences and late-onset pathophysiologies

Spinal cord injuries (SCI) cause permanent functional impairments due to interruption of motor and sensory pathways. Regeneration of axons does not occur due to lack of intrinsic growth capacity of adult neurons and extrinsic inhibitory factors, especially at the injury site. However, some regeneration can be achieved via deletion of the phosphatase and tensin homolog (PTEN) in cells of origin of spinal pathways. Here, we deployed an AAV variant that is retrogradely transported (AAV-rg) to deliver gene modifying cargos to the cells of origin of multiple pathways interrupted by SCI, testing whether this promoted recovery of motor function. PTEN f/f ;Rosa tdTomato mice and control Rosa tdTomato mice received injections of different doses (number of genome copies, GCs) of AAV-rg/Cre into the cervical spinal cord at the time of a C5 dorsal hemisection injury. Forelimb grip strength was tested over time using a grip strength meter. PTEN f/f ;Rosa tdTomato mice with AAV-rg/Cre (PTEN-deleted) exhibited substantial improvements in forelimb gripping ability in comparison to controls. Of note, there were major sex differences in the extent of recovery, with male mice exhibiting greater recovery than females. However, at around 5-7 weeks post-injury/injection, many mice with SCI and AAV-rg-mediated PTEN deletion began to exhibit pathophysiologies involving excessive scratching of the ears and back of the neck and rigid forward extension of the hindlimbs. These pathophysiologies increased in incidence and severity over time. Our results reveal that although intra-spinal injections of AAV-rg/Cre in PTEN f/f ;Rosa tdTomato mice can enhance forelimb motor recovery after SCI, late-developing functional abnormalities occur with the experimental conditions used here. Mechanisms underlying late-developing pathophysiologies remain to be defined.

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.

PTEN knockout using retrogradely transported AAVs restores locomotor abilities in both acute and chronic spinal cord injury

Restoring function in chronic stages of spinal cord injury (SCI) has often been met with failure or reduced efficacy when regenerative strategies are delayed past the acute or sub-acute stages of injury. Restoring function in the chronically injured spinal cord remains a critical challenge. We found that a single injection of retrogradely transported adeno-associated viruses (AAVrg) to knockout the phosphatase and tensin homolog protein (PTEN) in chronic SCI can effectively target both damaged and spared axons and restore locomotor functions in near-complete injury models. AAVrg's were injected to deliver cre recombinase and/or a red fluorescent protein (RFP) under the human Synapsin 1 promoter (hSyn1) into the spinal cords of C57BL/6 PTEN FloxΔ / Δ mice to knockout PTEN (PTEN-KO) in a severe thoracic SCI crush model at both acute and chronic time points. PTEN-KO improved locomotor abilities in both acute and chronic SCI conditions over a 9-week period. Regardless of whether treatment was initiated at the time of injury (acute), or three months after SCI (chronic), mice with limited hindlimb joint movement gained hindlimb weight support after treatment. Interestingly, functional improvements were not sustained beyond 9 weeks coincident with a loss of RFP reporter-gene expression and a near-complete loss of treatment-associated functional recovery by 6 months post-treatment. Treatment effects were also specific to severely injured mice; animals with weight support at the time of treatment lost function over a 6-month period. Retrograde tracing with Fluorogold revealed viable neurons throughout the motor cortex despite a loss of RFP expression at 9 weeks post-PTEN-KO. However, few Fluorogold labeled neurons were detected within the motor cortex at 6 months post-treatment. BDA labeling from the motor cortex revealed a dense corticospinal tract (CST) bundle in all groups except chronically treated PTEN-KO mice indicating a potential long-term toxic effect of PTEN-KO to neurons in the motor cortex. PTEN-KO mice had significantly more β - tubulin III labeled axons within the lesion when treatment was delivered acutely, but not chronically post-SCI. In conclusion, we have found that using AAVrg's to knockout PTEN is an effective manipulation capable of restoring motor functions in chronic SCI and can enhance axon growth of currently unidentified axon populations when delivered acutely after injury. However, the long-term consequences of PTEN-KO may exert neurotoxic effects.

TIR-1/SARM1 inhibits axon regeneration and promotes axon degeneration

Growth and destruction are central components of the neuronal injury response. Injured axons that are capable of repair, including axons in the mammalian peripheral nervous system and in many invertebrate animals, often regenerate and degenerate on either side of the injury. Here we show that TIR-1/dSarm/SARM1, a key regulator of axon degeneration, also inhibits regeneration of injured motor axons. The increased regeneration in tir-1 mutants is not a secondary consequence of its effects on degeneration, nor is it determined by the NADase activity of TIR-1. Rather, we found that TIR-1 functions cell-autonomously to regulate each of the seemingly opposite processes through distinct interactions with two MAP kinase pathways. On one side of the injury, TIR-1 inhibits axon regeneration by activating the NSY-1/ASK1 MAPK signaling cascade, while on the other side of the injury, TIR-1 simultaneously promotes axon degeneration by interacting with the DLK-1 mitogen-activated protein kinase (MAPK) signaling cascade. In parallel, we found that the ability to cell-intrinsically inhibit axon regeneration is conserved in human SARM1. Our finding that TIR-1/SARM1 regulates axon regeneration provides critical insight into how axons coordinate a multidimensional response to injury, consequently informing approaches to manipulate the response toward repair.

Probing regenerative heterogeneity of corticospinal neurons with scRNA-Seq

The corticospinal tract (CST) is clinically important for the recovery of motor functions after spinal cord injury. Despite substantial progress in understanding the biology of axon regeneration in the central nervous system (CNS), our ability to promote CST regeneration remains limited. Even with molecular interventions, only a small proportion of CST axons regenerate1. Here we investigate this heterogeneity in the regenerative ability of corticospinal neurons following PTEN and SOCS3 deletion with patch-based single cell RNA sequencing (scRNA-Seq)2,3, which enables deep sequencing of rare regenerating neurons. Bioinformatic analyses highlighted the importance of antioxidant response and mitochondrial biogenesis along with protein translation. Conditional gene deletion validated a role for NFE2L2 (or NRF2), a master regulator of antioxidant response, in CST regeneration. Applying Garnett4, a supervised classification method, to our dataset gave rise to a Regenerating Classifier (RC), which, when applied to published scRNA-Seq data, generates cell type- and developmental stage-appropriate classifications. While embryonic brain, adult dorsal root ganglion and serotonergic neurons are classified as Regenerators, most neurons from adult brain and spinal cord are classified as Non-regenerators. Adult CNS neurons partially revert to a regenerative state soon after injury, which is accelerated by molecular interventions. Our data indicate the existence of universal transcriptomic signatures underlying the regenerative abilities of vastly different neuronal populations, and further illustrate that deep sequencing of only hundreds of phenotypically identified CST neurons has the power to reveal new insights into their regenerative biology.

Regulation of axonal regeneration after mammalian spinal cord injury

One hundred years ago, Ramón y Cajal, considered by many as the founder of modern neuroscience, stated that neurons of the adult central nervous system (CNS) are incapable of regenerating. Yet, recent years have seen a tremendous expansion of knowledge in the molecular control of axon regeneration after CNS injury. We now understand that regeneration in the adult CNS is limited by (1) a failure to form cellular or molecular substrates for axon attachment and elongation through the lesion site; (2) environmental factors, including inhibitors of axon growth associated with myelin and the extracellular matrix; (3) astrocyte responses, which can both limit and support axon growth; and (4) intraneuronal mechanisms controlling the establishment of an active cellular growth programme. We discuss these topics together with newly emerging hypotheses, including the surprising finding from transcriptomic analyses of the corticospinal system in mice that neurons revert to an embryonic state after spinal cord injury, which can be sustained to promote regeneration with neural stem cell transplantation. These gains in knowledge are steadily advancing efforts to develop effective treatment strategies for spinal cord injury in humans.

Targeting PTEN but not SOCS3 resists an age-dependent decline in promoting axon sprouting

Axonal repair is critical for functional recovery after injury of the CNS. We previously reported that neuronal PTEN deletion exhibits an age-dependent decline in promoting axon regeneration from the corticospinal tract (CST). How sprouting of uninjured axons, a naturally occurring form of axonal repair, is impacted by age is unknown. We assessed CST sprouting after unilateral pyramidotomy in PTEN and/or SOCS3-deleted mice at different ages. While PTEN deletion enhances sprouting independently of age, SOCS3 deletion loses its sprouting-promoting effect with age. The synergistic effect of PTEN/SOCS3 co-deletion on CST sprouting is rapidly lost with increased age. Overall, promoting sprouting appears more robust across age than regeneration, yet distinct molecular pathways are differentially impacted by age. Importantly, six-week delayed PTEN deletion promotes CST sprouting across age groups, supporting a clinically relevant time frame for this neural repair strategy independently of age.

Improving translatability of spinal cord injury research by including age as a demographic variable

Pre-clinical and clinical spinal cord injury (SCI) studies differ in study design, particularly in the demographic characteristics of the chosen population. In clinical study design, criteria such as such as motor scores, neurological level, and severity of injury are often key determinants for participant inclusion. Further, demographic variables in clinical trials often include individuals from a wide age range and typically include both sexes, albeit historically most cases of SCI occur in males. In contrast, pre-clinical SCI models predominately utilize young adult rodents and typically use only females. While it is often not feasible to power SCI clinical trials to test multi-variable designs such as contrasting different ages, recent pre-clinical findings in SCI animal models have emphasized the importance of considering age as a biological variable prior to human experiments. Emerging pre-clinical data have identified case examples of treatments that diverge in efficacy across different demographic variables and have elucidated several age-dependent effects in SCI. The extent to which these differing or diverging treatment responses manifest clinically can not only complicate statistical findings and trial interpretations but also may be predictive of worse outcomes in select clinical populations. This review highlights recent literature including age as a biological variable in pre-clinical studies and articulates the results with respect to implications for clinical trials. Based on emerging unpredictable treatment outcomes in older rodents, we argue for the importance of including age as a biological variable in pre-clinical animal models prior to clinical testing. We believe that careful analyses of how age interacts with SCI treatments and pathophysiology will help guide clinical trial design and may improve both the safety and outcomes of such important efforts.

In vivo peptide-based delivery of a gene-modifying enzyme into cells of the central nervous system

We report on the successful delivery of the Cre recombinase enzyme in the neural cells of mice in vivo by simple coinjection with peptides derived from HIV-TAT. Cre delivery activates the expression of a reporter gene in both neurons and astrocytes of the cortex without tissue damage and with a transduction efficiency that parallels or exceeds that of a commonly used adeno-associated virus. Our data indicate that the delivery peptides mediate efficient endosomal leakage and cytosolic escape in cells that have endocytosed Cre. The peptides, therefore, act in trans and do not require conjugation to the payload, greatly simplifying sample preparation. Moreover, the delivery peptides are exclusively composed of natural amino acids and are consequently readily degradable and processed by cells. We envision that this approach will be beneficial to applications that require the transient introduction of proteins into cells in vivo.

Inducible motor neuron differentiation of human induced pluripotent stem cells in vivo

OBJECTIVES

Transplantation of neural progenitor cells (NPCs) derived from human-induced pluripotent stem cells (hiPSCs) is one of the promising treatment strategies for motor neuron diseases (MNDs). However, the inefficiency in committed differentiation of NPCs in vivo limits its application. Here, we tried to establish a potential therapeutic strategy for MNDs by in vivo directional differentiation of hiPSCs engineered with motor neuron (MN) specific transcription factors and Tet-On system.

MATERIALS AND METHODS

We engineered hiPSCs with three MN-specific transcription factors and Tet-On system. The engineered cells were directly transplanted into immunodeficient mice through subcutaneous, intra-spinal cord and intracerebroventricular injections. Following doxycycline (Dox) induction, teratoma formation, and motor MN differentiation were evaluated.

RESULTS

We generated genetically engineered hiPSCs, in which the expression of Ngn2, Isl1, and Lhx3 was controlled by a drug-inducible transgenic system. These cells showed normal pluripotency and proliferative capacity, and were able to directionally differentiate into mature motor neurons (MNs) and NPCs with high efficiency in spinal cords and cerebral lateral ventricles under the induction of Dox. The grafts showed long-term survival in the recipient mice without formation of teratoma.

CONCLUSIONS

The induced mature MNs and NPCs were expected to replace the damaged endogenous MNs directly, and play a role of de novo stem cell stock for long-term neuron damage repair, respectively. Therefore, in vivo directional differentiation of the hiPSCs engineered with MN-specific transcription factors and Tet-On system via Dox induction could be a potential therapeutic strategy for MNDs with high efficacy and safety.

Corrigendum: Purinergic signaling systems across comparative models of spinal cord injury

[This corrects the article DOI: 10.4103/1673-5374.338993].

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.

Self-delivering RNAi compounds as therapeutic agents in the central nervous system to enhance axonal regeneration after injury

The therapeutic use of RNAi has grown but often faces several hurdles related to delivery systems, compound stability, immune activation, and on-target/off-tissue effects. Self-delivering RNAi (sdRNA) molecules do not require delivery agents or excipients. Here we demonstrate the ability of sdRNA to reduce the expression of PTEN (phosphatase and tensin homolog) to stimulate regenerative axon regrowth in the injured adult CNS. PTEN-targeting sdRNA compounds were tested for efficacy in vivo by intravitreal injection after adult rat optic nerve injury. We describe critical steps in lead compound generation through the optimization of nucleotide modifications, enhancements for stability in biological matrices, and screening for off-target immunostimulatory activity. The data show that PTEN expression in vivo can be reduced using sdRNA and this enhances regeneration in adult CNS neurons after injury, raising the possibility that this method could be utilized for other clinically relevant nervous system indications.

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self-delivering siRNA (sdRNA) can decrease neuronal gene expression in vivo

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sdRNA can be successfully delivered in vivo without using vectors or excipients

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Phosphatase and tensin homolog (PTEN)-targeting sdRNA can enhance CNS neuronal regeneration after injury

Neuronal Redevelopment and the Regeneration of Neuromodulatory Axons in the Adult Mammalian Central Nervous System

One reason that many central nervous system injuries, including those arising from traumatic brain injury, spinal cord injury, and stroke, have limited recovery of function is that neurons within the adult mammalian CNS lack the ability to regenerate their axons following trauma. This stands in contrast to neurons of the adult mammalian peripheral nervous system (PNS). New evidence, provided by single-cell expression profiling, suggests that, following injury, both mammalian central and peripheral neurons can revert to an embryonic-like growth state which is permissive for axon regeneration. This “redevelopment” strategy could both facilitate a damage response necessary to isolate and repair the acute damage from injury and provide the intracellular machinery necessary for axon regrowth. Interestingly, serotonin neurons of the rostral group of raphe nuclei, which project their axons into the forebrain, display a robust ability to regenerate their axons unaided, counter to the widely held view that CNS axons cannot regenerate without experimental intervention after injury. Furthermore, initial evidence suggests that norepinephrine neurons within the locus coeruleus possess similar regenerative abilities. Several morphological characteristics of serotonin axon regeneration in adult mammals, observable using longitudinal in vivo imaging, are distinct from the known characteristics of unaided peripheral nerve regeneration, or of the regeneration seen in the spinal cord and optic nerve that occurs with experimental intervention. These results suggest that there is an alternative CNS program for axon regeneration that likely differs from that displayed by the PNS.

Immunoglobulin G is Increased in the Injured Spinal Cord in a Sex and Age Dependent Manner

There are limited studies examining age and sex as biological variables in the pathophysiology of spinal cord injury (SCI). The use of older animals and sex-balanced groups in SCI models is increasingly prioritized to better match clinical demographics. Including older animals in SCI studies is technically challenging, and outcomes are unpredictable with respect to biological and treatment responses. Incidental discoveries that are unrelated to the question under investigation often emerge while including age and sex as biological variables. When probing tissue homogenates on Western blots of 4- and 14-month-old (MO) mice, we identified a sex- and age-dependent increase in immunoglobulin G (IgG) within the spinal cords of older, 14-MO mice acutely after SCI, with females having more IgG compared with males. We further probed to determine whether differences in hemorrhage exist between sexes or ages by evaluating hemoglobin within spinal homogenates. Differences in hemoglobin between sexes and ages were not consistently observed. Because IgG was elevated in an age- and sex-dependent manner without of evidence of differences in hemorrhage, our findings point to potential pre-existing differences in IgG within mouse plasma in an age- and sex-dependent manner. This report has identified age- and sex-dependent differences in infiltrating IgG into the injured spinal cord environment that may affect injury and recovery processes. Our findings highlight that systemic contributions to SCI can be sex- and age-dependent and illustrate the value of reporting incidental discoveries.

Association of age with the timing of acute spine surgery-effects on neurological outcome after traumatic spinal cord injury

PURPOSE

To investigate the association of age with delay in spine surgery and the effects on neurological outcome after traumatic spinal cord injury (SCI).

METHODS

Ambispective cohort study (2011-2017) in n = 213 patients consecutively enrolled in a Level I trauma center with SCI care in a metropolitan region in Germany. Age-related differences in the injury to surgery interval and conditions associated with its delay (> 12 h after SCI) were explored using age categories or continuous variables and natural cubic splines. Effects of delayed surgery or age with outcome were analyzed using multiple logistic regression.

RESULTS

The median age of the study population was 58.8 years (42.0-74.6 IQR). Older age (≥ 75y) was associated with a prolonged injury to surgery interval of 22.8 h (7.2-121.3) compared to 6.6 h (4.4-47.9) in younger patients (≤ 44y). Main reasons for delayed surgery in older individuals were secondary referrals and multimorbidity. Shorter time span to surgery (≤ 12 h) was associated with higher rates of ASIA impairment scale (AIS) conversion (OR 4.22, 95%CI 1.85-9.65), as mirrored by adjusted spline curves (< 20 h 20-25%, 20-60 h 10-20%, > 60 h < 10% probability of AIS conversion). In incomplete SCI, the probability of AIS conversion was lower in older patients [e.g., OR 0.09 (0.02-0.44) for'45-59y' vs.' ≤ 44y'], as confirmed by spline curves (< 40y 20-80%, ≥ 40y 5-20% probability).

CONCLUSION

Older patient age complexifies surgical SCI care and research. Tackling secondary referral to Level I trauma centers and delayed spine surgery imposes as tangible opportunity to improve the outcome of older SCI patients.

The age factor in optic nerve regeneration: Intrinsic and extrinsic barriers hinder successful recovery in the short-living killifish

As the mammalian central nervous system matures, its regenerative ability decreases, leading to incomplete or non-recovery from the neurodegenerative diseases and central nervous system insults that we are increasingly facing in our aging world population. Current neuroregenerative research is largely directed toward identifying the molecular and cellular players that underlie central nervous system repair, yet it repeatedly ignores the aging context in which many of these diseases appear. Using an optic nerve crush model in a novel biogerontology model, that is, the short-living African turquoise killifish, the impact of aging on injury-induced optic nerve repair was investigated. This work reveals an age-related decline in axonal regeneration in female killifish, with different phases of the repair process being affected depending on the age. Interestingly, as in mammals, both a reduced intrinsic growth potential and a non-supportive cellular environment seem to lie at the basis of this impairment. Overall, we introduce the killifish visual system and its age-dependent regenerative ability as a model to identify new targets for neurorepair in non-regenerating individuals, thereby also considering the effects of aging on neurorepair.

Promoting axon regeneration in the central nervous system by increasing PI3-kinase signaling

Much research has focused on the PI3-kinase and PTEN signaling pathway with the aim to stimulate repair of the injured central nervous system. Axons in the central nervous system fail to regenerate, meaning that injuries or diseases that cause loss of axonal connectivity have life-changing consequences. In 2008, genetic deletion of PTEN was identified as a means of stimulating robust regeneration in the optic nerve. PTEN is a phosphatase that opposes the actions of PI3-kinase, a family of enzymes that function to generate the membrane phospholipid PIP₃ from PIP₂ (phosphatidylinositol (3,4,5)-trisphosphate from phosphatidylinositol (4,5)-bisphosphate). Deletion of PTEN therefore allows elevated signaling downstream of PI3-kinase, and was initially demonstrated to promote axon regeneration by signaling through mTOR. More recently, additional mechanisms have been identified that contribute to the neuron-intrinsic control of regenerative ability. This review describes neuronal signaling pathways downstream of PI3-kinase and PIP₃, and considers them in relation to both developmental and regenerative axon growth. We briefly discuss the key neuron-intrinsic mechanisms that govern regenerative ability, and describe how these are affected by signaling through PI3-kinase. We highlight the recent finding of a developmental decline in the generation of PIP₃ as a key reason for regenerative failure, and summarize the studies that target an increase in signaling downstream of PI3-kinase to facilitate regeneration in the adult central nervous system. Finally, we discuss obstacles that remain to be overcome in order to generate a robust strategy for repairing the injured central nervous system through manipulation of PI3-kinase signaling.

Aging and miR-155 in mice influence survival and neuropathic pain after spinal cord injury

Spinal cord injury (SCI) elicits chronic pain in 65% of individuals. In addition, SCI afflicts an increasing number of aged individuals, and those with SCI are predisposed to shorter lifespan. Our group previously identified that deletion of the microRNA miR-155 reduced neuroinflammation and locomotor deficits after SCI. Here, we hypothesized that aged mice would be more susceptible to pain symptoms and death soon after SCI, and that miR-155 deletion would reduce pain symptoms in adult and aged mice and improve survival. Adult (2 month-old) and aged (20 month-old) female wildtype (WT) and miR-155 knockout (KO) mice received T9 contusion SCI. Aged WT mice displayed reduced survival and increased autotomy - a symptom of spontaneous pain. In contrast, aged miR-155 KO mice after SCI were less susceptible to death or spontaneous pain. Evoked pain symptoms were tested using heat (Hargreaves test) and mechanical (von Frey) stimuli. At baseline, aged mice showed heightened heat sensitivity. After SCI, adult and aged WT and miR-155 KO mice all exhibited heat and mechanical hypersensitivity at all timepoints. miR-155 deletion in adult (but not aged) mice reduced mechanical hypersensitivity at 7 and 14 d post-SCI. Therefore, aging predisposes mice to SCI-elicited spontaneous pain and expedited mortality. miR-155 deletion in adult mice reduces evoked pain symptoms, and miR-155 deletion in aged mice reduces spontaneous pain and expedited mortality post-SCI. This study highlights the importance of studying geriatric models of SCI, and that inflammatory mediators such as miR-155 are promising targets after SCI for improving pain relief and longevity.

L-leucine promotes axonal outgrowth and regeneration via mTOR activation

Discovering safe and effective drugs that promote neuron regeneration is an essential strategy for the recovery of central nervous system injuries. In this study, we found that L-leucine, an essential amino acid obtained from both supplements and food sources, could dramatically boost axonal outgrowth and regeneration. First, the effects of L-leucine on neurons were evaluated by cell apoptosis, survival, and death assays, and the results showed no changes in these processes after treatment. By live cell imaging, L-leucine was found to remarkably increase axonal length and growth velocity after axotomy. We also verified that L-leucine enhanced p-mTOR/p-S6K activation in neurons by testing with an mTOR inhibitor, rapamycin. Thereafter, we investigated the effects of L-leucine on the spinal cord injury in vivo. A mouse model of spinal cord hemi-section was established, and L-leucine was administered by tail intravenous injection. Basso mouse scale values revealed that L-leucine could improve functional recovery after injury. It was also notable that L-leucine treatment promoted axon growth across chondroitin sulfate proteoglycan (CSPG) areas. Furthermore, we used CSPGs as inhibitory environmental cues and clarified that L-leucine significantly enhanced axonal outgrowth and regeneration by promoting p-mTOR and p-S6K activation. Therefore, our study is the first to report that L-leucine promotes axonal regeneration in vitro and in vivo and could be candidate drug for axonal re-growth and nervous functional recovery.

Diversity of Reactive Astrogliosis in CNS Pathology: Heterogeneity or Plasticity?

Astrocytes are essential for the development and homeostatic maintenance of the central nervous system (CNS). They are also critical players in the CNS injury response during which they undergo a process referred to as "reactive astrogliosis." Diversity in astrocyte morphology and gene expression, as revealed by transcriptional analysis, is well-recognized and has been reported in several CNS pathologies, including ischemic stroke, CNS demyelination, and traumatic injury. This diversity appears unique to the specific pathology, with significant variance across temporal, topographical, age, and sex-specific variables. Despite this, there is limited functional data corroborating this diversity. Furthermore, as reactive astrocytes display significant environmental-dependent plasticity and fate-mapping data on astrocyte subsets in the adult CNS is limited, it remains unclear whether this diversity represents heterogeneity or plasticity. As astrocytes are important for neuronal survival and CNS function post-injury, establishing to what extent this diversity reflects distinct established heterogeneous astrocyte subpopulations vs. environmentally dependent plasticity within established astrocyte subsets will be critical for guiding therapeutic development. To that end, we review the current state of knowledge on astrocyte diversity in the context of three representative CNS pathologies: ischemic stroke, demyelination, and traumatic injury, with the goal of identifying key limitations in our current knowledge and suggesting future areas of research needed to address them. We suggest that the majority of identified astrocyte diversity in CNS pathologies to date represents plasticity in response to dynamically changing post-injury environments as opposed to heterogeneity, an important consideration for the understanding of disease pathogenesis and the development of therapeutic interventions.

Age-Dependent Decline in Neuron Growth Potential and Mitochondria Functions in Cortical Neurons

The age of incidence of spinal cord injury (SCI) and the average age of people living with SCI is continuously increasing. However, SCI is extensively modeled in young adult animals, hampering translation of research to clinical applications. While there has been significant progress in manipulating axon growth after injury, the impact of aging is still unknown. Mitochondria are essential to successful neurite and axon growth, while aging is associated with a decline in mitochondrial functions. Using isolation and culture of adult cortical neurons, we analyzed mitochondrial changes in 2-, 6-, 12- and 18-month-old mice. We observed reduced neurite growth in older neurons. Older neurons also showed dysfunctional respiration, reduced membrane potential, and altered mitochondrial membrane transport proteins; however, mitochondrial DNA (mtDNA) abundance and cellular ATP were increased. Taken together, these data suggest that dysfunctional mitochondria in older neurons may be associated with the age-dependent reduction in neurite growth. Both normal aging and traumatic injury are associated with mitochondrial dysfunction, posing a challenge for an aging SCI population as the two elements can combine to worsen injury outcomes. The results of this study highlight this as an area of great interest in CNS trauma.

Swimming Exercise Promotes Post-injury Axon Regeneration and Functional Restoration through AMPK

Restoration of lost function following a nervous system injury is limited in adulthood as the regenerative capacity of nervous system declines with age. Pharmacological approaches have not been very successful in alleviating the consequences of nervous system injury. On the contrary, physical activity and rehabilitation interventions are often beneficial to improve the health conditions in the patients with neuronal injuries. Using touch neuron circuit of Caenorhabditis elegans, we investigated the role of physical exercise in the improvement of functional restoration after axotomy. We found that a swimming session of 90 min following the axotomy of posterior lateral microtubule (PLM) neuron can improve functional recovery in larval and adult stage animals. In older age, multiple exercise sessions were required to enhance the functional recovery. Genetic analysis of axon regeneration mutants showed that exercise-mediated enhancement of functional recovery depends on the ability of axon to regenerate. Exercise promotes early initiation of regrowth, self-fusion of proximal and distal ends, as well as postregrowth enhancement of function. We further found that the swimming exercise promotes axon regeneration through the activity of cellular energy sensor AAK-2/AMPK in both muscle and neuron. Our study established a paradigm where systemic effects of exercise on functional regeneration could be addressed at the single neuron level.

Regulation of UNC-40/DCC and UNC-6/Netrin by DAF-16 promotes functional rewiring of the injured axon

The adult nervous system has a limited capacity to regenerate after accidental damage. Post-injury functional restoration requires proper targeting of the injured axon to its postsynaptic cell. Although the initial response to axonal injury has been studied in great detail, it is rather unclear what controls the re-establishment of a functional connection. Using the posterior lateral microtubule neuron in Caenorhabditis elegans, we found that after axotomy, the regrowth from the proximal stump towards the ventral side and accumulation of presynaptic machinery along the ventral nerve cord correlated to the functional recovery. We found that the loss of insulin receptor DAF-2 promoted 'ventral targeting' in a DAF-16-dependent manner. We further showed that coordinated activities of DAF-16 in neuron and muscle promoted 'ventral targeting'. In response to axotomy, expression of the Netrin receptor UNC-40 was upregulated in the injured neuron in a DAF-16-dependent manner. In contrast, the DAF-2-DAF-16 axis contributed to the age-related decline in Netrin expression in muscle. Therefore, our study revealed an important role for insulin signaling in regulating the axon guidance molecules during the functional rewiring process.

Spinal cord regeneration: A brief overview of the present scenario and a sneak peek into the future

The central nervous system (CNS) portrays appreciable complexity in developing from a neural tube to controlling major functions of the body and orchestrated co-ordination in maintaining its homeostasis. Any insult or pathology to such an organized tissue leads to a plethora of events ranging from local hypoxia, ischemia, oxidative stress to reactive gliosis and scarring. Despite unravelling the pathophysiology of spinal cord injury (SCI) and linked cellular and molecular mechanism, the over exhaustive inflammatory response at the site of injury, limited intrinsic regeneration capability of CNS, and the dual role of glial scar halts the expected accomplishment. The review discusses major current treatment approaches for traumatic SCI, addressing their limitation and scope for further development in the field under three main categories- neuroprotection, neuro-regeneration, and neuroplasticity. We further propose that a multi-disciplinary combinatorial treatment approach exploring any two or all three heads simultaneously might alleviate the inhibitory milieu and ameliorate functional recovery.

EFA6 in Axon Regeneration, as a Microtubule Regulator and as a Guanine Nucleotide Exchange Factor

Axon regeneration after injury is a conserved biological process that involves a large number of molecular pathways, including rapid calcium influx at injury sites, retrograde injury signaling, epigenetic transition, transcriptional reprogramming, polarized transport, and cytoskeleton reorganization. Despite the numerous efforts devoted to understanding the underlying cellular and molecular mechanisms of axon regeneration, the search continues for effective target molecules for improving axon regeneration. Although there have been significant historical efforts towards characterizing pro-regenerative factors involved in axon regeneration, the pursuit of intrinsic inhibitors is relatively recent. EFA6 (exchange factor for ARF6) has been demonstrated to inhibit axon regeneration in different organisms. EFA6 inhibition could be a promising therapeutic strategy to promote axon regeneration and functional recovery after axon injury. This review summarizes the inhibitory role on axon regeneration through regulating microtubule dynamics and through affecting ARF6 (ADP-ribosylation factor 6) GTPase-mediated integrin transport.

Knocking Out Non-muscle Myosin II in Retinal Ganglion Cells Promotes Long-Distance Optic Nerve Regeneration

In addition to altered gene expression, pathological cytoskeletal dynamics in the axon are another key intrinsic barrier for axon regeneration in the central nervous system (CNS). Here, we show that knocking out myosin IIA and IIB (myosin IIA/B) in retinal ganglion cells alone, either before or after optic nerve crush, induces significant optic nerve regeneration. Combined Lin28a overexpression and myosin IIA/B knockout lead to an additive promoting effect and long-distance axon regeneration. Immunostaining, RNA sequencing, and western blot analyses reveal that myosin II deletion does not affect known axon regeneration signaling pathways or the expression of regeneration-associated genes. Instead, it abolishes the retraction bulb formation and significantly enhances the axon extension efficiency. The study provides clear evidence that directly targeting neuronal cytoskeleton is sufficient to induce significant CNS axon regeneration and that combining altered gene expression in the soma and modified cytoskeletal dynamics in the axon is a promising approach for long-distance CNS axon regeneration.

Respiratory axon regeneration in the chronically injured spinal cord

Promoting the combination of robust regeneration of damaged axons and synaptic reconnection of these growing axon populations with appropriate neuronal targets represents a major therapeutic goal following spinal cord injury (SCI). A key impediment to achieving this important aim includes an intrinsic inability of neurons to extend axons in adult CNS, particularly in the context of the chronically-injured spinal cord. We tested whether an inhibitory peptide directed against phosphatase and tensin homolog (PTEN: a central inhibitor of neuron-intrinsic axon growth potential) could restore inspiratory diaphragm function by reconnecting critical respiratory neural circuitry in a rat model of chronic cervical level 2 (C2) hemisection SCI. We found that systemic delivery of PTEN antagonist peptide 4 (PAP4) starting at 8 weeks after C2 hemisection promoted substantial, long-distance regeneration of injured bulbospinal rostral Ventral Respiratory Group (rVRG) axons into and through the lesion and back toward phrenic motor neurons (PhMNs) located in intact caudal C3-C5 spinal cord. Despite this robust rVRG axon regeneration, PAP4 stimulated only minimal recovery of diaphragm function. Furthermore, re-lesion through the hemisection site completely removed PAP4-induced functional improvement, demonstrating that axon regeneration through the lesion was responsible for this partial functional recovery. Interestingly, there was minimal formation of putative excitatory monosynaptic connections between regrowing rVRG axons and PhMN targets, suggesting that (1) limited rVRG-PhMN synaptic reconnectivity was responsible at least in part for the lack of a significant functional effect, (2) chronically-injured spinal cord presents an obstacle to achieving synaptogenesis between regenerating axons and post-synaptic targets, and (3) addressing this challenge is a potentially-powerful strategy to enhance therapeutic efficacy in the chronic SCI setting. In conclusion, our study demonstrates a non-invasive and transient pharmacological approach in chronic SCI to repair the critically-important neural circuitry controlling diaphragmatic respiratory function, but also sheds light on obstacles to circuit plasticity presented by the chronically-injured spinal cord.

The Role of Lipids, Lipid Metabolism and Ectopic Lipid Accumulation in Axon Growth, Regeneration and Repair after CNS Injury and Disease

Axons in the adult mammalian nervous system can extend over formidable distances, up to one meter or more in humans. During development, axonal and dendritic growth requires continuous addition of new membrane. Of the three major kinds of membrane lipids, phospholipids are the most abundant in all cell membranes, including neurons. Not only immature axons, but also severed axons in the adult require large amounts of lipids for axon regeneration to occur. Lipids also serve as energy storage, signaling molecules and they contribute to tissue physiology, as demonstrated by a variety of metabolic disorders in which harmful amounts of lipids accumulate in various tissues through the body. Detrimental changes in lipid metabolism and excess accumulation of lipids contribute to a lack of axon regeneration, poor neurological outcome and complications after a variety of central nervous system (CNS) trauma including brain and spinal cord injury. Recent evidence indicates that rewiring lipid metabolism can be manipulated for therapeutic gain, as it favors conditions for axon regeneration and CNS repair. Here, we review the role of lipids, lipid metabolism and ectopic lipid accumulation in axon growth, regeneration and CNS repair. In addition, we outline molecular and pharmacological strategies to fine-tune lipid composition and energy metabolism in neurons and non-neuronal cells that can be exploited to improve neurological recovery after CNS trauma and disease.

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.

Effect of Cannabinoid 2 Receptor Modulation on the Peripheral Immune Response in Central Nervous System Injury-Induced Immunodeficiency Syndrome

Introduction: Acute central nervous system (CNS) injury, such as stroke, spinal cord injury, or traumatic brain injury can result in dysregulated immune response, and the condition is known as CNS injury-induced immunodeficiency syndrome (CIDS). The endocannabinoid system is an important homeostatic regulator in the CNS and immune system. Activation of cannabinoid 2 receptors (CB2R) on immune cells has been reported to dampen inflammation, suggesting a potential role of CB2R in the peripheral immune response following CNS injury. In this study, we have investigated the effect of CB2R modulation on the peripheral immune response during CIDS. Materials and Methods: Experimental CNS injury was induced in C57BL/6 mice through intracerebral injection of the vasopressor peptide, endothelin-1. A selective CB2R agonist (HU308) was used as an early treatment before the onset of CIDS and AM630, a selective CB2R antagonist, was administered as a later-phase therapy to combat the systemic immunodeficiency following the CNS injury. The peripheral immune response to endotoxin was studied 24 h after the CNS injury using intravital microscopy to examine leukocyte activation within the intestinal microcirculation in mice. Brain infarct size, and plasma levels of cytokines and soluble adhesion molecules were measured as additional parameters for the assessment of treatment outcomes. Results: Our results showed that early CB2R activation with HU308 reduced brain injury size and restored leukocyte response to endotoxin in the peripheral microcirculation. Late CB2R inhibition with AM630 also improved the peripheral leukocyte response to endotoxin and did not exacerbate the extent of brain injury. Discussion: CB2R activation has the potential to mitigate CNS injury as an early treatment by limiting neuroinflammation and preventing the development of CIDS. At the later stage with already-established CIDS, treatment may require dampening CB2R activation to improve the patient's outcome.

Combining task-based rehabilitative training with PTEN inhibition promotes axon regeneration and upper extremity skilled motor function recovery after cervical spinal cord injury in adult mice

BACKGROUND

Conditional deletion of Pten in corticospinal neurons promotes axon sprouting and regeneration after spinal cord injury (SCI). However, regeneration studies targeted on PTEN inhibition seldom show motor function recovery. The promotion of functional recovery can be improved by rehabilitative training under a use-dependent plasticity mechanism.

PURPOSE

To investigate the combined effects of PTEN inhibition and rehabilitative training on axon regeneration and subsequent motor functional improvement after cervical spinal cord injury.

METHODS

Lentiviral particles (Lenti-PTEN-RNAi or Lenti-Scrambled-EGFP) were injected into the right sensorimotor mouse cortex in four experimental groups (PTEN RNAi + Training, PTEN RNAi, Control + Training, Control). Two weeks after injection, all mouse groups received a left C5 crush injury. We performed task-based rehabilitative training for 4 weeks on the PTEN RNAi + Training and Control + Training groups. Biotinylated dextran amine (BDA) was used for anterograde tracing of the dorsal corticospinal tract (dCST). We analysed axonal regeneration through immunohistochemical methods. A battery of behavioral tests was employed to assess functional recovery at Day3 and every other week after injury.

RESULTS

Combining rehabilitative training with PTEN inhibition induced more axon regeneration and synapse reformation in the spinal cord caudal to the lesion site. Rostral to the lesion, the transected dCST axons sprouted into gray matter upon contact. Furthermore, forelimb function was found to be improved after combination therapy during behavioral testing.

CONCLUSION

Combining task-based rehabilitative training with PTEN inhibition further promotes axon regeneration, synaptic plasticity and reorganization of the neural network, with significant improvement in forelimb skilled motor function after cervical spinal cord injury. Our study provides new therapeutic insights for spinal cord injury management in the future.

Axonal Organelles as Molecular Platforms for Axon Growth and Regeneration after Injury

Investigating the molecular mechanisms governing developmental axon growth has been a useful approach for identifying new strategies for boosting axon regeneration after injury, with the goal of treating debilitating conditions such as spinal cord injury and vision loss. The picture emerging is that various axonal organelles are important centers for organizing the molecular mechanisms and machinery required for growth cone development and axon extension, and these have recently been targeted to stimulate robust regeneration in the injured adult central nervous system (CNS). This review summarizes recent literature highlighting a central role for organelles such as recycling endosomes, the endoplasmic reticulum, mitochondria, lysosomes, autophagosomes and the proteasome in developmental axon growth, and describes how these organelles can be targeted to promote axon regeneration after injury to the adult CNS. This review also examines the connections between these organelles in developing and regenerating axons, and finally discusses the molecular mechanisms within the axon that are required for successful axon growth.

The genetic basis of inter-individual variation in recovery from traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of death among young people, and is increasingly prevalent in the aging population. Survivors of TBI face a spectrum of outcomes from short-term non-incapacitating injuries to long-lasting serious and deteriorating sequelae. TBI is a highly complex condition to treat; many variables can account for the observed heterogeneity in patient outcome. The limited success of neuroprotection strategies in the clinic has led to a new emphasis on neurorestorative approaches. In TBI, it is well recognized clinically that patients with similar lesions, age, and health status often display differences in recovery of function after injury. Despite this heterogeneity of outcomes in TBI, restorative treatment has remained generic. There is now a new emphasis on developing a personalized medicine approach in TBI, and this will require an improved understanding of how genetics impacts on long-term outcomes. Studies in animal model systems indicate clearly that the genetic background plays a role in determining the extent of recovery following an insult. A candidate gene approach in human studies has led to the identification of factors that can influence recovery. Here we review studies of the genetic basis for individual differences in functional recovery in the CNS in animals and man. The application of in vitro modeling with human cells and organoid cultures, along with whole-organism studies, will help to identify genes and networks that account for individual variation in recovery from brain injury, and will point the way towards the development of new therapeutic approaches.

Comparative gene expression profiling reveals the mechanisms of axon regeneration

Axons are vulnerable to injury, potentially leading to degeneration or neuronal death. While neurons in the central nervous system fail to regenerate, neurons in the peripheral nervous system are known to regenerate. Since it has been shown that injury-response signal transduction is mediated by gene expression changes, expression profiling is a useful tool to understand the molecular mechanisms of regeneration. Axon regeneration is regulated by injury-responsive genes induced in both neurons and their surrounding non-neuronal cells. Thus, an experimental setup for the comparative analysis between regenerative and nonregenerative conditions is essential to identify ideal targets for the promotion of regeneration-associated genes and to understand the mechanisms of axon regeneration. Here, we review the original research that shows the key factors regulating axon regeneration, in particular by using comparative gene expression profiling in diverse systems.

Neural circuit repair after central nervous system injury

Central nervous system injury often causes lifelong impairment of neural function, because the regenerative ability of axons is limited, making a sharp contrast to the successful regeneration that is seen in the peripheral nervous system. Nevertheless, partial functional recovery is observed, because axonal branches of damaged or undamaged neurons sprout and form novel relaying circuits. Using a lot of animal models such as the spinal cord injury model or the optic nerve injury model, previous studies have identified many factors that promote or inhibit axonal regeneration or sprouting. Molecules in the myelin such as myelin-associated glycoprotein, Nogo-A or oligodendrocyte-myelin glycoprotein, or molecules found in the glial scar such as chondroitin sulfate proteoglycans, activate Ras homolog A (RhoA) signaling, which leads to the collapse of the growth cone and inhibit axonal regeneration. By contrast, axonal regeneration programs can be activated by many molecules such as regeneration-associated transcription factors, cyclic AMP, neurotrophic factors, growth factors, mechanistic target of rapamycin or immune-related molecules. Axonal sprouting and axonal regeneration largely share these mechanisms. For functional recovery, appropriate pruning or suppressing of aberrant sprouting are also important. In contrast to adults, neonates show much higher sprouting ability. Specific cell types, various mouse strains and different species show higher regenerative ability. Studies focusing on these models also identified a lot of molecules that affect the regenerative ability. A deeper understanding of the mechanisms of neural circuit repair will lead to the development of better therapeutic approaches for central nervous system injury.