Activated CREB is sufficient to overcome inhibitors in myelin and promote spinal axon regeneration in vivo

Neuron-specific RNA-sequencing reveals different responses in peripheral neurons after nerve injury

Peripheral neurons are heterogeneous and functionally diverse, but all share the capability to switch to a pro-regenerative state after nerve injury. Despite the assumption that the injury response is similar among neuronal subtypes, functional recovery may differ. Understanding the distinct intrinsic regenerative properties between neurons may help to improve the quality of regeneration, prioritizing the growth of axon subpopulations to their targets. Here, we present a comparative analysis of regeneration across four key peripheral neuron populations: motoneurons, proprioceptors, cutaneous mechanoreceptors, and nociceptors. Using Cre/Ai9 mice that allow fluorescent labeling of neuronal subtypes, we found that nociceptors showed the greater regeneration after a sciatic crush, followed by motoneurons, mechanoreceptors, and, finally, proprioceptors. By breeding these Cre mice with Ribotag mice, we isolated specific translatomes and defined the regenerative response of these neuronal subtypes after axotomy. Only 20% of the regulated genes were common, revealing a diverse response to injury among neurons, which was also supported by the differential influence of neurotrophins among neuron subtypes. Among differentially regulated genes, we proposed MED12 as a specific regulator of the regeneration of proprioceptors. Altogether, we demonstrate that the intrinsic regenerative capacity differs between peripheral neuron subtypes, opening the door to selectively modulate these responses.

Calcium-Associated Proteins in Neuroregeneration

The dysregulation of intracellular calcium levels is a critical factor in neurodegeneration, leading to the aberrant activation of calcium-dependent processes and, ultimately, cell death. Ca2+ signals vary in magnitude, duration, and the type of neuron affected. A moderate Ca2+ concentration can initiate certain cellular repair pathways and promote neuroregeneration. While the peripheral nervous system exhibits an intrinsic regenerative capability, the central nervous system has limited self-repair potential. There is evidence that significant variations exist in evoked calcium responses and axonal regeneration among neurons, and individual differences in regenerative capacity are apparent even within the same type of neurons. Furthermore, some studies have shown that neuronal activity could serve as a potent regulator of this process. The spatio-temporal patterns of calcium dynamics are intricately controlled by a variety of proteins, including channels, ion pumps, enzymes, and various calcium-binding proteins, each of which can exert either positive or negative effects on neural repair, depending on the cellular context. In this concise review, we focus on several calcium-associated proteins such as CaM kinase II, GAP-43, oncomodulin, caldendrin, calneuron, and NCS-1 in order to elaborate on their roles in the intrinsic mechanisms governing neuronal regeneration following traumatic damage processes.

Brief Electrical Stimulation Promotes Recovery after Surgical Repair of Injured Peripheral Nerves

Injured peripheral nerves regenerate their axons in contrast to those in the central nervous system. Yet, functional recovery after surgical repair is often disappointing. The basis for poor recovery is progressive deterioration with time and distance of the growth capacity of the neurons that lose their contact with targets (chronic axotomy) and the growth support of the chronically denervated Schwann cells (SC) in the distal nerve stumps. Nonetheless, chronically denervated atrophic muscle retains the capacity for reinnervation. Declining electrical activity of motoneurons accompanies the progressive fall in axotomized neuronal and denervated SC expression of regeneration-associated-genes and declining regenerative success. Reduced motoneuronal activity is due to the withdrawal of synaptic contacts from the soma. Exogenous neurotrophic factors that promote nerve regeneration can replace the endogenous factors whose expression declines with time. But the profuse axonal outgrowth they provoke and the difficulties in their delivery hinder their efficacy. Brief (1 h) low-frequency (20 Hz) electrical stimulation (ES) proximal to the injury site promotes the expression of endogenous growth factors and, in turn, dramatically accelerates axon outgrowth and target reinnervation. The latter ES effect has been demonstrated in both rats and humans. A conditioning ES of intact nerve days prior to nerve injury increases axonal outgrowth and regeneration rate. Thereby, this form of ES is amenable for nerve transfer surgeries and end-to-side neurorrhaphies. However, additional surgery for applying the required electrodes may be a hurdle. ES is applicable in all surgeries with excellent outcomes.

Regeneration of neuromuscular synapses after acute and chronic denervation by inhibiting the gerozyme 15-prostaglandin dehydrogenase

To date, there are no approved treatments for the diminished strength and paralysis that result from the loss of peripheral nerve function due to trauma, heritable neuromuscular diseases, or aging. Here, we showed that denervation resulting from transection of the sciatic nerve triggered a marked increase in the prostaglandin-degrading enzyme 15-hydroxyprostaglandin dehydrogenase (15-PGDH) in skeletal muscle in mice, providing evidence that injury drives early expression of this aging-associated enzyme or gerozyme. Treating mice with a small-molecule inhibitor of 15-PGDH promoted regeneration of motor axons and formation of neuromuscular synapses leading to an acceleration in recovery of force after an acute nerve crush injury. In aged mice with chronic denervation of muscles, treatment with the 15-PGDH inhibitor increased motor neuron viability and restored neuromuscular junctions and function. These presynaptic changes synergized with previously reported muscle tissue remodeling to result in a marked increase in the strength of aged muscles. We further found that 15-PGDH aggregates defined the target fibers that are histopathologic hallmarks of human neurogenic myopathies, suggesting that the gerozyme may be involved in their etiology. Our data suggest that inhibition of 15-PGDH may constitute a therapeutic strategy to physiologically boost prostaglandin E2, restore neuromuscular connectivity, and promote recovery of strength after acute or chronic denervation due to injury, disease, or aging.

Rolipram-loaded PgP nanoparticle reduces secondary injury and enhances motor function recovery in a rat moderate contusion SCI model

Spinal cord injury (SCI) results in immediate axonal damage and cell death, as well as a prolonged secondary injury consist of a cascade of pathophysiological processes. One important aspect of secondary injury is activation of phosphodiesterase 4 (PDE4) that leads to reduce cAMP levels in the injured spinal cord. We have developed an amphiphilic copolymer, poly (lactide-co-glycolide)-graft-polyethylenimine (PgP) that can deliver Rolipram, the PDE4 inhibitor. The objective of this work was to investigate the effect of rolipram loaded PgP (Rm-PgP) on secondary injury and motor functional recovery in a rat moderate contusion SCI model. We observed that Rm-PgP can increase cAMP level at the lesion site, and reduce secondary injury such as the inflammatory response by macrophages/microglia, astrogliosis by activated astrocytes and apoptosis as well as improve neuronal survival at 4 weeks post-injury (WPI). We also observed that Rm-PgP can improve motor functional recovery after SCI over 4 WPI.

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

INTRODUCTION

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

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

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.

Regenerative Potential of Injured Spinal Cord in the Light of Epigenetic Regulation and Modulation

A spinal cord injury is a form of physical harm imposed on the spinal cord that causes disability and, in many cases, leads to permanent mammalian paralysis, which causes a disastrous global issue. Because of its non-regenerative aspect, restoring the spinal cord's role remains one of the most daunting tasks. By comparison, the remarkable regenerative ability of some regeneration-competent species, such as some Urodeles (Axolotl), Xenopus, and some teleost fishes, enables maximum functional recovery, even after complete spinal cord transection. During the last two decades of intensive research, significant progress has been made in understanding both regenerative cells' origins and the molecular signaling mechanisms underlying the regeneration and reconstruction of damaged spinal cords in regenerating organisms and mammals, respectively. Epigenetic control has gradually moved into the center stage of this research field, which has been helped by comprehensive work demonstrating that DNA methylation, histone modifications, and microRNAs are important for the regeneration of the spinal cord. In this review, we concentrate primarily on providing a comparison of the epigenetic mechanisms in spinal cord injuries between non-regenerating and regenerating species. In addition, we further discuss the epigenetic mediators that underlie the development of a regeneration-permissive environment following injury in regeneration-competent animals and how such mediators may be implicated in optimizing treatment outcomes for spinal cord injurie in higher-order mammals. Finally, we briefly discuss the role of extracellular vesicles (EVs) in the context of spinal cord injury and their potential as targets for therapeutic intervention.

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

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

Ligand-Induced Activation of GPR110 (ADGRF1) to Improve Visual Function Impaired by Optic Nerve Injury

It is extremely difficult to achieve functional recovery after axonal injury in the adult central nervous system. The activation of G-protein coupled receptor 110 (GPR110, ADGRF1) has been shown to stimulate neurite extension in developing neurons and after axonal injury in adult mice. Here, we demonstrate that GPR110 activation partially restores visual function impaired by optic nerve injury in adult mice. Intravitreal injection of GPR110 ligands, synaptamide and its stable analogue dimethylsynaptamide (A8) after optic nerve crush significantly reduced axonal degeneration and improved axonal integrity and visual function in wild-type but not gpr110 knockout mice. The retina obtained from the injured mice treated with GPR110 ligands also showed a significant reduction in the crush-induced loss of retinal ganglion cells. Our data suggest that targeting GPR110 may be a viable strategy for functional recovery after optic nerve injury.

CBP/p300 activation promotes axon growth, sprouting, and synaptic plasticity in chronic experimental spinal cord injury with severe disability

The interruption of spinal circuitry following spinal cord injury (SCI) disrupts neural activity and is followed by a failure to mount an effective regenerative response resulting in permanent neurological disability. Functional recovery requires the enhancement of axonal and synaptic plasticity of spared as well as injured fibres, which need to sprout and/or regenerate to form new connections. Here, we have investigated whether the epigenetic stimulation of the regenerative gene expression program can overcome the current inability to promote neurological recovery in chronic SCI with severe disability. We delivered the CBP/p300 activator CSP-TTK21 or vehicle CSP weekly between week 12 and 22 following a transection model of SCI in mice housed in an enriched environment. Data analysis showed that CSP-TTK21 enhanced classical regenerative signalling in dorsal root ganglia sensory but not cortical motor neurons, stimulated motor and sensory axon growth, sprouting, and synaptic plasticity, but failed to promote neurological sensorimotor recovery. This work provides direct evidence that clinically suitable pharmacological CBP/p300 activation can promote the expression of regeneration-associated genes and axonal growth in a chronic SCI with severe neurological disability.

Gabapentin inhibits the analgesic effects and nerve regeneration process induced by hepatocyte growth factor (HGF) in a peripheral nerve injury model: Implication for the use of VM202 and gabapentinoids for peripheral neuropathy

Hepatocyte growth factor (HGF) is a multifunctional protein that plays a critical role in the angiogenic, neurotrophic, antifibrotic, and antiapoptotic activities of various cell types. It has been previously reported that intramuscular injection of pCK-HGF-X7 (or VM202), a plasmid DNA designed to express both native isoforms of human HGF (Pyun et al., 2010), significantly reduced the level of neuropathic pain in clinical studies as well as in a variety of animal models. In clinical studies, it has been observed that pCK-HGF-X7 appeared to give much higher pain-relieving effects in subjects not taking pregabalin or gabapentin, α2δ1 calcium channel blockers frequently prescribed for reducing pain in patients with diabetic peripheral neuropathy. In this study, we tested the effects of gabapentin on HGF-mediated pain reduction and nerve regeneration in vivo. Consistent with the data from clinical studies, gabapentin administration inhibited the pain reduction and axon regeneration effects mediated by HGF expression from pCK-HGF-X7. In the context of nerve regenerative effects, treatment with gabapentin or EGTA, a Ca²⁺ chelator, inhibited HGF-mediated axon outgrowth of injured sciatic nerves in vivo. Taken together, i.m. injection of HGF-encoding plasmid DNA ameliorated pain symptoms and enhanced the regeneration of injured nerves, and these therapeutic effects of HGF were significantly hindered by gabapentin treatment, suggesting the possible involvement of Ca²⁺ in the pro-regenerative activities of native HGF derived from treatment with pCK-HGF-X7.

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.

The Role of ATF3 in Neuronal Differentiation and Development of Neuronal Networks in Opossum Postnatal Cortical Cultures

Activating transcription factor 3 (ATF3), a member of the ATF/cAMP response element-binding (CREB) family, is upregulated by various intracellular and extracellular signals such as injury and signals related to cell proliferation. ATF3 also belongs to the regeneration-associated genes (RAG) group of transcription factors. RAG and ATF/CREB transcription factors that play an important role in embryonic neuronal development and PNS regeneration may also be involved in postnatal neuronal differentiation and development, as well as in the regeneration of the injured CNS. Here we investigated the effect of ATF3 in differentiation, neural outgrowth, network formation, and regeneration after injury using postnatal dissociated cortical neurons derived from neonatal opossums (Monodelphis domestica). Our results show that RAG and ATF genes are differentially expressed in early differentiated neurons versus undifferentiated neurospheres and that many members of those families, ATF3 in particular, are upregulated in cortical cultures obtained from younger animals that have the ability to fully functionally regenerate spinal cord after injury. In addition, we observed different intracellular localization of ATF3 that shifts from nuclear (in neuronal progenitors) to cytoplasmic (in more mature neurons) during neuronal differentiation. The ATF3 inhibition, pharmacological or by specific antibody, reduced the neurite outgrowth and differentiation and caused increased cell death in early differentiating cortical neuronal cultures, suggesting the importance of ATF3 in the CNS development of neonatal opossums. Finally, we investigated the regeneration capacity of primary cortical cultures after mechanical injury using the scratch assay. Remarkably, neonatal opossum-derived cultures retain their capacity to regenerate for up to 1 month in vitro. Inhibition of ATF3 correlates with reduced neurite outgrowth and regeneration after injury. These results indicate that ATF3, and possibly other members of RAG and ATF/CREB family of transcription factors, have an important role both during cortical postnatal development and in response after injury.

Co-targeting B-RAF and PTEN Enables Sensory Axons to Regenerate Across and Beyond the Spinal Cord Injury

Primary sensory axons in adult mammals fail to regenerate after spinal cord injury (SCI), in part due to insufficient intrinsic growth potential. Robustly boosting their growth potential continues to be a challenge. Previously, we showed that constitutive activation of B-RAF (rapidly accelerated fibrosarcoma kinase) markedly promotes axon regeneration after dorsal root and optic nerve injuries. The regrowth is further augmented by supplemental deletion of PTEN (phosphatase and tensin homolog). Here, we examined whether concurrent B-RAF activation and PTEN deletion promotes dorsal column axon regeneration after SCI. Remarkably, genetically targeting B-RAF and PTEN selectively in DRG neurons of adult mice enables many DC axons to enter, cross, and grow beyond the lesion site after SCI; some axons reach ∼2 mm rostral to the lesion by 3 weeks post-injury. Co-targeting B-RAF and PTEN promotes more robust DC regeneration than a pre-conditioning lesion, which additively enhances the regeneration triggered by B-RAF/PTEN. We also found that post-injury targeting of B-RAF and PTEN enhances DC axon regeneration. These results demonstrate that co-targeting B-RAF and PTEN effectively enhances the intrinsic growth potential of DC axons after SCI and therefore may help to develop a novel strategy to promote robust long-distance regeneration of primary sensory axons.

Cellular Immune Signal Exchange From Ischemic Stroke to Intestinal Lesions Through Brain-Gut Axis

As a vital pivot for the human circulatory system, the brain-gut axis is now being considered as an important channel for many of the small immune molecules’ transductions, including interleukins, interferons, neurotransmitters, peptides, and the chemokines penetrating the mesentery and blood brain barrier (BBB) during the development of an ischemic stroke (IS). Hypoxia-ischemia contributes to pituitary and neurofunctional disorders by interfering with the molecular signal release and communication then providing feedback to the gut. Suffering from such a disease on a long-term basis may cause the peripheral system’s homeostasis to become imbalanced, and it can also lead to multiple intestinal complications such as gut microbiota dysbiosis (GMD), inflammatory bowel disease (IBD), necrotizing enterocolitis (NEC), and even the tumorigenesis of colorectal carcinoma (CRC). Correspondingly, these complications will deteriorate the cerebral infarctions and, in patients suffering with IS, it can even ruin the brain’s immune system. This review summarized recent studies on abnormal immunological signal exchange mediated polarization subtype changes, in both macrophages and microglial cells as well as T-lymphocytes. How gut complications modulate the immune signal transduction from the brain are also elucidated and analyzed. The conclusions drawn in this review could provide guidance and novel strategies to benefit remedies for both IS and relative gut lesions from immune-prophylaxis and immunotherapy aspects.

GPR68 Improves Nerve Damage and Myelination in an Immature Rat Model Induced by Sevoflurane Anesthesia by Activating cAMP/CREB to Mediate BDNF

Ovarian cancer G-protein-coupled receptor 1 (OGR1, also known as GPR68) is a member of proton-sensing G-protein-coupled receptors, involved in cardiovascular physiology, tumor biology, and asthma, and exerts a neuroprotective effect against brain ischemia. The effects of GPR68 on anesthesia-induced nerve damage and myelination were investigated in this study. First, 2-day old postnatal rats were exposed to 4.9% sevoflurane for 2 h. Data from hematoxylin and eosin staining and Nissl staining showed that sevoflurane induced pathological changes in the hippocampus with a reduced number of neurons. GPR68 was downregulated in the hippocampus of sevoflurane-induced rats. Second, sevoflurane-induced rats were injected with adeno-associated virus (AAV)-mediated overexpression of GPR68, and overexpression of GPR68 ameliorated sevoflurane-induced pathological changes, enhanced the number of neurons, and improved the learning and memory function. Moreover, overexpression of GPR68 increased the number of BrdU-positive and Olig2-positive cells and enhanced protein expression of Olig2 in sevoflurane-induced rats. Third, the number of myelin basic protein (MBP) positive cells and protein expression of MBP in sevoflurane-induced rats were also enhanced by injection with AAV-GPR68. Overexpression of GPR68 attenuated sevoflurane-induced neuronal apoptosis and oxidative stress in rats. Lastly, overexpression of GPR68 upregulated protein expression of the brain-derived neurotrophic factor (BDNF) by increasing cAMP and phosphorylated cAMP response element-binding protein (CREB). In conclusion, GPR68 alleviated sevoflurane-induced nerve damage and myelination through BDNF-mediated activation of the cAMP/CREB pathway.

Viral expression of constitutively active AKT3 induces CST axonal sprouting and regeneration, but also promotes seizures

Increasing the intrinsic growth potential of neurons after injury has repeatedly been shown to promote some level of axonal regeneration in rodent models. One of the most studied pathways involves the activation of the PI3K/AKT/mTOR pathways, primarily by reducing the levels of PTEN, a negative regulator of PI3K. Likewise, activation of signal transducer and activator of transcription 3 (STAT3) has previously been shown to boost axonal regeneration and sprouting within the injured nervous system. Here, we examined the regeneration of the corticospinal tract (CST) after cortical expression of constitutively active (ca) Akt3 and STAT3, both separately and in combination. Overexpression of caAkt3 induced regeneration of CST axons past the injury site independent of caSTAT3 overexpression. STAT3 demonstrated improved axon sprouting compared to controls and contributed to a synergistic improvement in effects when combined with Akt3 but failed to promote axonal regeneration as an individual therapy. Despite showing impressive axonal regeneration, animals expressing Akt3 failed to show any functional improvement and deteriorated with time. During this period, we observed progressive Akt3 dose-dependent increase in behavioral seizures. Histology revealed increased phosphorylation of ribosomal S6 protein within the unilateral cortex, increased neuronal size, microglia activation and hemispheric enlargement (hemimegalencephaly).

Phenotypic Screening Following Transcriptomic Deconvolution to Identify Transcription Factors Mediating Axon Growth Induced by a Kinase Inhibitor

After injury to the central nervous system (CNS), both neuron-intrinsic limitations on regenerative responses and inhibitory factors in the injured CNS environment restrict regenerative axon growth. Instances of successful axon regrowth offer opportunities to identify features that differentiate these situations from that of the normal adult CNS. One such opportunity is provided by the kinase inhibitor RO48, which dramatically enhances neurite outgrowth of neurons in vitro and substantially increased contralateral sprouting of corticospinal tract neurons when infused intraventricularly following unilateral pyramidotomy. The authors present here a transcriptomic deconvolution of RO48-associated axon growth, with the goal of identifying transcriptional regulators associated with axon growth in the CNS. Through the use of RNA sequencing (RNA-seq) and transcription factor binding site enrichment analysis, the authors identified a list of transcription factors putatively driving differential gene expression during RO48-induced neurite outgrowth of rat hippocampal neurons in vitro. The 82 transcription factor motifs identified in this way included some with known association to axon growth regulation, such as Jun, Klf4, Myc, Atf4, Stat3, and Nfatc2, and many with no known association to axon growth. A phenotypic loss-of-function screen was carried out to evaluate these transcription factors for their roles in neurite outgrowth; this screen identified several potential outgrowth regulators. Subsequent validation suggests that the Forkhead box (Fox) family transcription factor Foxp2 restricts neurite outgrowth, while FoxO subfamily members Foxo1 and Foxo3a promote neurite outgrowth. The authors' combined transcriptomic-phenotypic screening strategy therefore allowed identification of novel transcriptional regulators of neurite outgrowth downstream of a multitarget kinase inhibitor.

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.

The Jun-dependent axon regeneration gene program: Jun promotes regeneration over plasticity

The regeneration-associated gene (RAG) expression program is activated in injured peripheral neurons after axotomy and enables long-distance axon re-growth. Over 1000 genes are regulated, and many transcription factors are upregulated or activated as part of this response. However, a detailed picture of how RAG expression is regulated is lacking. In particular, the transcriptional targets and specific functions of the various transcription factors are unclear. Jun was the first-regeneration-associated transcription factor identified and the first shown to be functionally important. Here we fully define the role of Jun in the RAG expression program in regenerating facial motor neurons. At 1, 4 and 14 days after axotomy, Jun upregulates 11, 23 and 44% of the RAG program, respectively. Jun functions relevant to regeneration include cytoskeleton production, metabolic functions and cell activation, and the downregulation of neurotransmission machinery. In silico analysis of promoter regions of Jun targets identifies stronger over-representation of AP1-like sites than CRE-like sites, although CRE sites were also over-represented in regions flanking AP1 sites. Strikingly, in motor neurons lacking Jun, an alternative SRF-dependent gene expression program is initiated after axotomy. The promoters of these newly expressed genes exhibit over-representation of CRE sites in regions near to SRF target sites. This alternative gene expression program includes plasticity-associated transcription factors and leads to an aberrant early increase in synapse density on motor neurons. Jun thus has the important function in the early phase after axotomy of pushing the injured neuron away from a plasticity response and towards a regenerative phenotype.

PACAP-derived mutant peptide MPAPO protects trigeminal ganglion cell and the retina from hypoxic injury through anti-oxidative stress, anti-apoptosis, and promoting axon regeneration

The purpose of this study was to determine whether the MPAPO, derived peptide of pituitary adenylate cyclase-activating polypeptide (PACAP), would protect trigeminal ganglion cells (TGCs) and the mice retinas from a hypoxic insult. The nerve endings of the ophthalmic nerve of the trigeminal nerve are widely distributed in eye tissues. In TGCs after hypoxia exposure, we discovered that reactive oxygen species level, the contents of cytosolic cytochrome c and cleaved-caspase-3 were significantly increased, in the meanwhile, m-Calpain was activated and cytoskeleton proteins (αII-spectrin and Synapsin) were degraded, neurites of TGCs disappeared, but these effects were reversed in TGCs treated with MPAPO. The structure of the mice retinas after hypoxic exposure was disordered. Increased lipid peroxidation (LPO), decreased glutathione (GSH) levels, and decreased superoxide dismutase (SOD) activity, positive cells of terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), the disintegration of nerve fibers was examined in the retinas following a hypoxic insult. Disordered retina was attenuated with MPAPO eye drops, as well as hypoxia-induced apoptosis in the developing retina, increase in LPO, and decrease in GSH levels and SOD activity of the retina. Moreover, the disintegrated retinal nerve fibers were reassembled after MPAPO treatment. These results suggest that hypoxia induces oxidative stress, apoptosis, and neurites disruption, while MPAPO is remarkably protective against these adverse effects of hypoxia in TGCs and the developing retinas by specifically activating PAC1 receptor.

Nogo-A Modulates the Synaptic Excitation of Hippocampal Neurons in a Ca2+-Dependent Manner

A tight regulation of the balance between inhibitory and excitatory synaptic transmission is a prerequisite for synaptic plasticity in neuronal networks. In this context, the neurite growth inhibitor membrane protein Nogo-A modulates synaptic plasticity, strength, and neurotransmitter receptor dynamics. However, the molecular mechanisms underlying these actions are unknown. We show that Nogo-A loss-of-function in primary mouse hippocampal cultures by application of a function-blocking antibody leads to higher excitation following a decrease in GABA_ARs at inhibitory and an increase in the GluA1, but not GluA2 AMPAR subunit at excitatory synapses. This unbalanced regulation of AMPAR subunits results in the incorporation of Ca²⁺-permeable GluA2-lacking AMPARs and increased intracellular Ca²⁺ levels due to a higher Ca²⁺ influx without affecting its release from the internal stores. Increased neuronal activation upon Nogo-A loss-of-function prompts the phosphorylation of the transcription factor CREB and the expression of c-Fos. These results contribute to the understanding of the molecular mechanisms underlying the regulation of the excitation/inhibition balance and thereby of plasticity in the brain.

Genome-wide chromatin accessibility analyses provide a map for enhancing optic nerve regeneration

Retinal Ganglion Cells (RGCs) lose their ability to grow axons during development. Adult RGCs thus fail to regenerate their axons after injury, leading to vision loss. To uncover mechanisms that promote regeneration of RGC axons, we identified transcription factors (TF) and open chromatin regions that are enriched in rat embryonic RGCs (high axon growth capacity) compared to postnatal RGCs (low axon growth capacity). We found that developmental stage-specific gene expression changes correlated with changes in promoter chromatin accessibility. Binding motifs for TFs such as CREB, CTCF, JUN and YY1 were enriched in the regions of the chromatin that were more accessible in embryonic RGCs. Proteomic analysis of purified rat RGC nuclei confirmed the expression of TFs with potential role in axon growth such as CREB, CTCF, YY1, and JUND. The CREB/ATF binding motif was widespread at the open chromatin region of known pro-regenerative TFs, supporting a role of CREB in regulating axon regeneration. Consistently, overexpression of CREB fused to the VP64 transactivation domain in mouse RGCs promoted axon regeneration after optic nerve injury. Our study provides a map of the chromatin accessibility during RGC development and highlights that TF associated with developmental axon growth can stimulate axon regeneration in mature RGC.

Epitranscriptomic Analysis of m6A Methylome After Peripheral Nerve Injury

N6-methyladenosine (m6A) is one of the most plentiful internal RNA modifications, especially in eukaryotic messenger RNA (mRNA), which plays pivotal roles in the regulation of mRNA life cycle and nerve development. However, the mRNA m6A methylation pattern in peripheral nervous injury (PNI) has not been investigated. In this study, sciatic nerve samples were collected from 7 days after sciatic nerve injury (SNI) and control rats. Quantitative real-time PCR demonstrated that m6A-related methyltransferase/demethylase genes were remarkably upregulated in SNI group compared with control group. Methylated RNA immunoprecipitation sequencing (MeRIP-seq) was performed to reveal the m6A methylation landscape. The results showed that 4,014 m6A peaks were significantly altered, including 2,144 upregulated and 1,870 downregulated m6A peaks, which were corresponded to 1,858 genes. Moreover, 919 differentially expressed genes were identified by the conjoint analysis of MeRIP-seq and RNA-seq. GO and KEGG pathway analyses were performed to determine the biological functions and signaling pathways of the m6A-modified genes. Notably, these genes were mainly related to the immune system process, cell activation, and nervous system development in GO analysis. KEGG pathway analysis revealed that these genes were involved in the cell cycle, B cell receptor signaling pathway, axon guidance pathway, and calcium signaling pathway. Furthermore, the m6A methylation and protein expression levels of autophagy-related gene (Atg7) were increased, together with the activation of autophagy. These findings shed some light on the epigenetic regulation of gene expression, which may provide a new opinion to promote functional recovery after PNI.

PTEN Blocking Stimulates Corticospinal and Raphespinal Axonal Regeneration and Promotes Functional Recovery After Spinal Cord Injury

The long-term disabilities associated with spinal cord injury (SCI) are primarily due to the absence of robust neuronal regeneration and functional plasticity. The inability of the axon to regenerate after SCI is contributed by several intrinsic factors that trigger a cascade of molecular growth program and modulates axonal sprouting. Phosphatase and tensin homolog (PTEN) is one of the intrinsic factors contributing to growth failure after SCI, however, the underlying mechanism is not well known. Here, we developed a novel therapeutic approach for treating SCI by suppressing the action of PTEN in a mouse model of hemisection SCI. We have used a novel peptide, PTEN antagonistic peptide (PAP) to block the critical domains of PTEN to demonstrate its ability to potentially promote axon growth. PAP treatment not only enhanced regeneration of corticospinal axons into the caudal spinal cord but also promoted the regrowth of descending serotonergic axons in SCI mice. Furthermore, expression levels of p-mTOR, p-S6, p-Akt, p-Erk, p-GSK, p-PI3K downstream of PTEN signaling pathway were increased significantly in the spinal cord of SCI mice systemically treated with PAP than control TAT peptide-treated mice. Our novel strategy of administering deliverable compounds postinjury may facilitate translational feasibility for central nervous system injury.

The Integrin Signaling Network Promotes Axon Regeneration via the Src-Ephexin-RhoA GTPase Signaling Axis

Axon regeneration is an evolutionarily conserved process essential for restoring the function of damaged neurons. In Caenorhabditis elegans hermaphrodites, initiation of axon regeneration is regulated by the RhoA GTPase-ROCK (Rho-associated coiled-coil kinase)-regulatory nonmuscle myosin light-chain phosphorylation signaling pathway. However, the upstream mechanism that activates the RhoA pathway remains unknown. Here, we show that axon injury activates TLN-1/talin via the cAMP-Epac (exchange protein directly activated by cAMP)-Rap GTPase cascade and that TLN-1 induces multiple downstream events, one of which is integrin inside-out activation, leading to the activation of the RhoA-ROCK signaling pathway. We found that the nonreceptor tyrosine kinase Src, a key mediator of integrin signaling, activates the Rho guanine nucleotide exchange factor EPHX-1/ephexin by phosphorylating the Tyr-568 residue in the autoinhibitory domain. Our results suggest that the C. elegans integrin signaling network regulates axon regeneration via the Src-RhoGEF-RhoA axis.SIGNIFICANCE STATEMENT The ability of axons to regenerate after injury is governed by cell-intrinsic regeneration pathways. We have previously demonstrated that the Caenorhabditis elegans RhoA GTPase-ROCK (Rho-associated coiled-coil kinase) pathway promotes axon regeneration by inducing MLC-4 phosphorylation. In this study, we found that axon injury activates TLN-1/talin through the cAMP-Epac (exchange protein directly activated by cAMP)-Rap GTPase cascade, leading to integrin inside-out activation, which promotes axonal regeneration by activating the RhoA signaling pathway. In this pathway, SRC-1/Src acts downstream of integrin activation and subsequently activates EPHX-1/ephexin RhoGEF by phosphorylating the Tyr-568 residue in the autoinhibitory domain. Our results suggest that the C. elegans integrin signaling network regulates axon regeneration via the Src-RhoGEF-RhoA axis.

Planet of the AAVs: The Spinal Cord Injury Episode

The spinal cord injury (SCI) is a medical and life-disrupting condition with devastating consequences for the physical, social, and professional welfare of patients, and there is no adequate treatment for it. At the same time, gene therapy has been studied as a promising approach for the treatment of neurological and neurodegenerative disorders by delivering remedial genes to the central nervous system (CNS), of which the spinal cord is a part. For gene therapy, multiple vectors have been introduced, including integrating lentiviral vectors and non-integrating adeno-associated virus (AAV) vectors. AAV vectors are a promising system for transgene delivery into the CNS due to their safety profile as well as long-term gene expression. Gene therapy mediated by AAV vectors shows potential for treating SCI by delivering certain genetic information to specific cell types. This review has focused on a potential treatment of SCI by gene therapy using AAV vectors.

Learning and Stroke Recovery: Parallelism of Biological Substrates

Stroke is a debilitating disease. Current effective therapies for stroke recovery are limited to neurorehabilitation. Most stroke recovery occurs in a limited and early time window. Many of the mechanisms of spontaneous recovery after stroke parallel mechanisms of normal learning and memory. While various efforts are in place to identify potential drug targets, an emerging approach is to understand biological correlates between learning and stroke recovery. This review assesses parallels between biological changes at the molecular, structural, and functional levels during learning and recovery after stroke, with a focus on drug and cellular targets for therapeutics.

BRCA1-BARD1 Regulates Axon Regeneration in Concert with the Gqα-DAG Signaling Network

The breast cancer susceptibility protein BRCA1 and its partner BRCA1-associated RING domain protein 1 (BARD1) form an E3-ubiquitin (Ub) ligase complex that acts as a tumor suppressor in mitotic cells. However, the roles of BRCA1–BARD1 in postmitotic cells, such as neurons, remain poorly defined. Here, we report that BRC-1 and BRD-1, the Caenorhabditis elegans orthologs of BRCA1 and BARD1, are required for adult-specific axon regeneration, which is positively regulated by the EGL-30 Gqα–diacylglycerol (DAG) signaling pathway. This pathway is downregulated by DAG kinase (DGK), which converts DAG to phosphatidic acid (PA). We demonstrate that inactivation of DGK-3 suppresses the brc-1 brd-1 defect in axon regeneration, suggesting that BRC-1–BRD-1 inhibits DGK-3 function. Indeed, we show that BRC-1–BRD-1 poly-ubiquitylates DGK-3 in a manner dependent on its E3 ligase activity, causing DGK-3 degradation. Furthermore, we find that axon injury causes the translocation of BRC-1 from the nucleus to the cytoplasm, where DGK-3 is localized. These results suggest that the BRC-1–BRD-1 complex regulates axon regeneration in concert with the Gqα–DAG signaling network. Thus, this study describes a new role for breast cancer proteins in fully differentiated neurons and the molecular mechanism underlying the regulation of axon regeneration in response to nerve injury.

SIGNIFICANCE STATEMENT BRCA1–BRCA1-associated RING domain protein 1 (BARD1) is an E3-ubiquitin (Ub) ligase complex acting as a tumor suppressor in mitotic cells. The roles of BRCA1–BARD1 in postmitotic cells, such as neurons, remain poorly defined. We show here that Caenorhabditis elegans BRC-1/BRCA1 and BRD-1/BARD1 are required for adult-specific axon regeneration, a process that requires high diacylglycerol (DAG) levels in injured neurons. The DAG kinase (DGK)-3 inhibits axon regeneration by reducing DAG levels. We find that BRC-1–BRD-1 poly-ubiquitylates and degrades DGK-3, thereby keeping DAG levels elevated and promoting axon regeneration. Furthermore, we demonstrate that axon injury causes the translocation of BRC-1 from the nucleus to the cytoplasm, where DGK-3 is localized. Thus, this study describes a new role for BRCA1–BARD1 in fully-differentiated neurons.

Ligand-Induced GPR110 Activation Facilitates Axon Growth after Injury

Recovery from axonal injury is extremely difficult, especially for adult neurons. Here, we demonstrate that the activation of G-protein coupled receptor 110 (GPR110, ADGRF1) is a mechanism to stimulate axon growth after injury. N-docosahexaenoylethanolamine (synaptamide), an endogenous ligand of GPR110 that promotes neurite outgrowth and synaptogenesis in developing neurons, and a synthetic GPR110 ligand stimulated neurite growth in axotomized cortical neurons and in retinal explant cultures. Intravitreal injection of GPR110 ligands following optic nerve crush injury promoted axon extension in adult wild-type, but not in gpr110 knockout, mice. In vitro axotomy or in vivo optic nerve injury rapidly induced the neuronal expression of gpr110. Activating the developmental mechanism of neurite outgrowth by specifically targeting GPR110 that is upregulated upon injury may provide a novel strategy for stimulating axon growth after nerve injury in adults.

Hepatocyte growth factor is necessary for efficient outgrowth of injured peripheral axons in in vitro culture system and in vivo nerve crush mouse model

Hepatocyte growth factor (HGF) is a neurotrophic factor and its role in peripheral nerves has been relatively unknown. In this study, biological functions of HGF and its receptor c-met have been investigated in the context of regeneration of damaged peripheral nerves. Axotomy of the peripheral branch of sensory neurons from embryonic dorsal root ganglia (DRG) resulted in the increased protein levels of HGF and phosphorylated c-met. When the neuronal cultures were treated with a pharmacological inhibitor of c-met, PHA665752, the length of axotomy-induced outgrowth of neurite was significantly reduced. On the other hand, the addition of recombinant HGF proteins to the neuronal culture facilitated axon outgrowth. In the nerve crush mouse model, the protein level of HGF was increased around the injury site by almost 5.5-fold at 24 h post injury compared to control mice and was maintained at elevated levels for another 6 days. The amount of phosphorylated c-met receptor in sciatic nerve was also observed to be higher than control mice. When PHA665752 was locally applied to the injury site of sciatic nerve, axon outgrowth and injury mediated induction of cJun protein were effectively inhibited, indicating the functional involvement of HGF/c-met pathway in the nerve regeneration process. When extra HGF was exogenously provided by intramuscular injection of plasmid DNA expressing HGF, axon outgrowth from damaged sciatic nerve and cJun expression level were enhanced. Taken together, these results suggested that HGF/c-met pathway plays important roles in axon outgrowth by directly interacting with sensory neurons and thus HGF might be a useful tool for developing therapeutics for peripheral neuropathy.

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In in vitro primary eDRGs, axotomy-induced HGF/c-met pathway enhanced the neurite outgrowth process.

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Nerve injury induced the expression of HGF, consequently leading to the activation of c-met in peripheral axons.

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HGF/c-met pathway played an important role in the regeneration process of injured peripheral nerves.

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Additional supply of HGF, in the form of plasmid DNA, enhanced the regeneration of damaged peripheral nerves.

Role of extracellular vesicles in neurodegenerative diseases

Extracellular vesicles (EVs) are heterogeneous cell-derived membranous structures that arise from the endosome system or directly detach from the plasma membrane. In recent years, many advances have been made in the understanding of the clinical definition and pathogenesis of neurodegenerative diseases, but translation into effective treatments is hampered by several factors. Current research indicates that EVs are involved in the pathology of diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). Besides, EVs are also involved in the process of myelin formation, and can also cross the blood-brain barrier to reach the sites of CNS injury. It is suggested that EVs have great potential as a novel therapy for the treatment of neurodegenerative diseases. Here, we reviewed the advances in understanding the role of EVs in neurodegenerative diseases and addressed the critical function of EVs in the CNS. We have also outlined the physiological mechanisms of EVs in myelin regeneration and highlighted the therapeutic potential of EVs in neurodegenerative diseases.

Mesenchymal stem cell-derived exosomes: therapeutic opportunities and challenges for spinal cord injury

Spinal cord injury (SCI) often leads to serious motor and sensory dysfunction of the limbs below the injured segment. SCI not only results in physical and psychological harm to patients but can also cause a huge economic burden on their families and society. As there is no effective treatment method, the prevention, treatment, and rehabilitation of patients with SCI have become urgent problems to be solved. In recent years, mesenchymal stem cells (MSCs) have attracted more attention in the treatment of SCI. Although MSC therapy can reduce injured volume and promote axonal regeneration, its application is limited by tumorigenicity, a low survival rate, and immune rejection. Accumulating literature shows that exosomes have great potential in the treatment of SCI. In this review, we summarize the existing MSC-derived exosome studies on SCI and discuss the advantages and challenges of treating SCI based on exosomes derived from MSCs.

Ascending dorsal column sensory neurons respond to spinal cord injury and downregulate genes related to lipid metabolism

Regeneration failure after spinal cord injury (SCI) results in part from the lack of a pro-regenerative response in injured neurons, but the response to SCI has not been examined specifically in injured sensory neurons. Using RNA sequencing of dorsal root ganglion, we determined that thoracic SCI elicits a transcriptional response distinct from sciatic nerve injury (SNI). Both SNI and SCI induced upregulation of ATF3 and Jun, yet this response failed to promote growth in sensory neurons after SCI. RNA sequencing of purified sensory neurons one and three days after injury revealed that unlike SNI, the SCI response is not sustained. Both SCI and SNI elicited the expression of ATF3 target genes, with very little overlap between conditions. Pathway analysis of differentially expressed ATF3 target genes revealed that fatty acid biosynthesis and terpenoid backbone synthesis were downregulated after SCI but not SNI. Pharmacologic inhibition of fatty acid synthase, the enzyme generating palmitic acid, decreased axon growth and regeneration in vitro. These results support the notion that decreased expression of lipid metabolism-related genes after SCI, including fatty acid synthase, may restrict axon regenerative capacity after SCI.

The Mechanisms of Peripheral Nerve Preconditioning Injury on Promoting Axonal Regeneration

Two major factors contribute to the failure of axonal regrowth in the central nervous system (CNS), namely, the neuronal intrinsic regenerative capacity and the extrinsic local inhibitory microenvironments. However, a preconditioning peripheral nerve lesion could substantially enhance the regeneration of central axons following a subsequent spinal cord injury. In the present review, we summarize the molecular mechanisms of the preconditioning injury effect on promoting axonal regeneration. The injury signal transduction resulting from preconditioning peripheral nerve injury regulates the RAG expression to enhance axonal regeneration. Importantly, preconditioning peripheral nerve injury triggers interactions between neurons and nonneuronal cells to amplify and maintain their effects. Additionally, the preconditioning injury impacts mitochondria, protein, and lipid synthesis. All these coordinated changes endow axonal regeneration.

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.

Communications Between Peripheral and the Brain-Resident Immune System in Neuronal Regeneration After Stroke

Cerebral ischemia may cause irreversible neural network damage and result in functional deficits. Targeting neuronal repair after stroke potentiates the formation of new connections, which can be translated into a better functional outcome. Innate and adaptive immune responses in the brain and the periphery triggered by ischemic damage participate in regulating neural repair after a stroke. Immune cells in the blood circulation and gut lymphatic tissues that have been shaped by immune components including gut microbiota and metabolites can infiltrate the ischemic brain and, once there, influence neuronal regeneration either directly or by modulating the properties of brain-resident immune cells. Immune-related signalings and metabolites from the gut microbiota can also directly alter the phenotypes of resident immune cells to promote neuronal regeneration. In this review, we discuss several potential mechanisms through which peripheral and brain-resident immune components can cooperate to promote first the resolution of neuroinflammation and subsequently to improved neural regeneration and a better functional recovery. We propose that new insights into discovery of regulators targeting pro-regenerative process in this complex neuro-immune network may lead to novel strategies for neuronal regeneration.

AMPK controls the axonal regenerative ability of dorsal root ganglia sensory neurons after spinal cord injury

Regeneration after injury occurs in axons that lie in the peripheral nervous system but fails in the central nervous system, thereby limiting functional recovery. Differences in axonal signalling in response to injury that might underpin this differential regenerative ability are poorly characterized. Combining axoplasmic proteomics from peripheral sciatic or central projecting dorsal root ganglion (DRG) axons with cell body RNA-seq, we uncover injury-dependent signalling pathways that are uniquely represented in peripheral versus central projecting sciatic DRG axons. We identify AMPK as a crucial regulator of axonal regenerative signalling that is specifically downregulated in injured peripheral, but not central, axons. We find that AMPK in DRG interacts with the 26S proteasome and its CaMKIIα-dependent regulatory subunit PSMC5 to promote AMPKα proteasomal degradation following sciatic axotomy. Conditional deletion of AMPKα1 promotes multiple regenerative signalling pathways after central axonal injury and stimulates robust axonal growth across the spinal cord injury site, suggesting inhibition of AMPK as a therapeutic strategy to enhance regeneration following spinal cord injury.

Functional Electrical Stimulation and the Modulation of the Axon Regeneration Program

Neural injury in mammals often leads to persistent functional deficits as spontaneous repair in the peripheral nervous system (PNS) is often incomplete, while endogenous repair mechanisms in the central nervous system (CNS) are negligible. Peripheral axotomy elicits growth-associated gene programs in sensory and motor neurons that can support reinnervation of peripheral targets given sufficient levels of debris clearance and proximity to nerve targets. In contrast, while damaged CNS circuitry can undergo a limited amount of sprouting and reorganization, this innate plasticity does not re-establish the original connectivity. The utility of novel CNS circuitry will depend on effective connectivity and appropriate training to strengthen these circuits. One method of enhancing novel circuit connectivity is through the use of electrical stimulation, which supports axon growth in both central and peripheral neurons. This review will focus on the effects of CNS and PNS electrical stimulation in activating axon growth-associated gene programs and supporting the recovery of motor and sensory circuits. Electrical stimulation-mediated neuroplasticity represents a therapeutically viable approach to support neural repair and recovery. Development of appropriate clinical strategies employing electrical stimulation will depend upon determining the underlying mechanisms of activity-dependent axon regeneration and the heterogeneity of neuronal subtype responses to stimulation.

Dominant-Negative Attenuation of cAMP-Selective Phosphodiesterase PDE4D Action Affects Learning and Behavior

PDE4 cyclic nucleotide phosphodiesterases reduce 3′, 5′ cAMP levels in the CNS and thereby regulate PKA activity and the phosphorylation of CREB, fundamental to depression, cognition, and learning and memory. The PDE4 isoform PDE4D5 interacts with the signaling proteins β-arrestin2 and RACK1, regulators of β₂-adrenergic and other signal transduction pathways. Mutations in PDE4D in humans predispose to acrodysostosis, associated with cognitive and behavioral deficits. To target PDE4D5, we developed mice that express a PDE4D5-D556A dominant-negative transgene in the brain. Male transgenic mice demonstrated significant deficits in hippocampus-dependent spatial learning, as assayed in the Morris water maze. In contrast, associative learning, as assayed in a fear conditioning assay, appeared to be unaffected. Male transgenic mice showed augmented activity in prolonged (2 h) open field testing, while female transgenic mice showed reduced activity in the same assay. Transgenic mice showed no demonstrable abnormalities in prepulse inhibition. There was also no detectable difference in anxiety-like behavior, as measured in the elevated plus-maze. These data support the use of a dominant-negative approach to the study of PDE4D5 function in the CNS and specifically in learning and memory.

Pharmacologic and cellular therapies in the treatment of traumatic spinal cord injuries: A systematic review

OBJECTIVE

The objective of this review is to synthesize and consolidate the existing literature on the treatment of SCI, focusing on drugs in development and cellular therapeutics, including stem-cell treatments.

METHODS

Studies were identified through a systemic search of PubMed, Ovid MEDLINE, Embase and the Cochrane database from their respective inceptions through January 1, 2020. We used the keywords "spinal cord injuries", "therapeutics", "stem cells", and "pharmacology."

STUDY SELECTION

Studies that assessed treatment strategies for SCI were included.

DATA EXTRACTION AND SYNTHESIS

Data on SCIs were processed according to the Preferred Reporting Items for Systematic Reviews and meta-Analyses (PRISMA) guidelines.

FINDINGS

In total, 62 articles were found in the literature search and 13 clinical trials were identified and included in this study. This review article discusses the management and treatment of SCI with an emphasis on the pharmacology, molecular approaches, and the use of stem cells. Presently, none of the treatments examined has shown to be clearly effective.

CONCLUSIONS

Present management strategies of SCI are focused on improving spinal cord perfusion and decreasing secondary injuries such as hypoxia, inflammation, edema, excitotoxicity and disturbances of ion homeostasis. This review hopes to demonstrate the significant advances made in the field of SCI and the new methodologies and practices being employed by researchers to improve our knowledge of the pathology. Our hope is that by consolidating the past and current research, improvements can be made in the management, treatment, and outcomes for these patients and other who suffer from spinal pathologies.

Cortico-amygdala interaction determines the insular cortical neurons involved in taste memory retrieval

The insular cortex (IC) is the primary gustatory cortex, and it is a critical structure for encoding and retrieving the conditioned taste aversion (CTA) memory. In the CTA, consumption of an appetitive tastant is associated with aversive experience such as visceral malaise, which results in avoidance of consuming a learned tastant. Previously, we showed that levels of the cyclic-AMP-response-element-binding protein (CREB) determine the insular cortical neurons that proceed to encode a conditioned taste memory. In the amygdala and hippocampus, it is shown that CREB and neuronal activity regulate memory allocation and the neuronal mechanism that determines the specific neurons in a neural network that will store a given memory. However, cellular mechanism of memory allocation in the insular cortex is not fully understood. In the current study, we manipulated the neuronal activity in a subset of insular cortical and/or basolateral amygdala (BLA) neurons in mice, at the time of learning; for this purpose, we used an hM3Dq designer receptor exclusively activated by a designer drug system (DREADD). Subsequently, we examined whether the neuronal population whose activity is increased during learning, is reactivated by memory retrieval, using the expression of immediate early gene c-fos. When an hM3Dq receptor was activated only in a subset of IC neurons, c-fos expression following memory retrieval was not significantly observed in hM3Dq-positive neurons. Interestingly, the probability of c-fos expression in hM3Dq-positive IC neurons after retrieval was significantly increased when the IC and BLA were co-activated during conditioning. Our findings suggest that functional interactions between the IC and BLA regulates CTA memory allocation in the insular cortex, which shed light on understanding the mechanism of memory allocation regulated by interaction between relevant brain areas.

Phosphodiesterase-4 enzyme as a therapeutic target in neurological disorders

Phosphodiesterases (PDE) are a diverse family of enzymes (11 isoforms so far identified) responsible for the degradation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) which are involved in several cellular and biochemical functions. Phosphodiesterase 4 (PDE4) is the major isoform within this group and is highly expressed in the mammalian brain. An inverse association between PDE4 and cAMP levels is the key mechanism in various pathophysiological conditions like airway inflammatory diseases-chronic obstruction pulmonary disease (COPD), asthma, psoriasis, rheumatoid arthritis, and neurological disorders etc. In 2011, roflumilast, a PDE4 inhibitor (PDE4I) was approved for the treatment of COPD. Subsequently, other PDE4 inhibitors (PDE4Is) like apremilast and crisaborole were approved by the Food and Drug Administration (FDA) for psoriasis, atopic dermatitis etc. Due to the adverse effects like unbearable nausea and vomiting, dose intolerance and diarrhoea, PDE4 inhibitors have very less clinical compliance. Efforts are being made to develop allosteric modulation with high specificity to PDE4 isoforms having better efficacy and lesser adverse effects. Interestingly, repositioning PDE4Is towards neurological disorders including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), multiple sclerosis (MS) and sleep disorders, is gaining attention. This review is an attempt to summarize the data on the effects of PDE4 overexpression in neurological disorders and the use of PDE4Is and newer allosteric modulators as therapeutic options. We have also compiled a list of on-going clinical trials on PDE4 inhibitors in neurological disorders.

MicroRNA-19a-PTEN Axis Is Involved in the Developmental Decline of Axon Regenerative Capacity in Retinal Ganglion Cells

Irreversible blindness from glaucoma and optic neuropathies is attributed to retinal ganglion cells (RGCs) losing the ability to regenerate axons. While several transcription factors and proteins have demonstrated enhancement of axon regeneration after optic nerve injury, mechanisms contributing to the age-related decline in axon regenerative capacity remain elusive. In this study, we show that microRNAs are differentially expressed during RGC development and identify microRNA-19a (miR-19a) as a heterochronic marker; developmental decline of miR-19a relieves suppression of phosphatase and tensin homolog (PTEN), a key regulator of axon regeneration, and serves as a temporal indicator of decreasing axon regenerative capacity. Intravitreal injection of miR-19a promotes axon regeneration after optic nerve crush in adult mice, and it increases axon extension in RGCs isolated from aged human donors. This study uncovers a previously unrecognized involvement of the miR-19a-PTEN axis in RGC axon regeneration, and it demonstrates therapeutic potential of microRNA-mediated restoration of axon regenerative capacity in optic neuropathies.

Targeted Krüppel-Like Factor 4 Gene Knock-Out in Retinal Ganglion Cells Improves Visual Function in Multiple Sclerosis Mouse Model

Axonal demyelination injury and neuronal degeneration are the primary causes of visual disability in multiple sclerosis (MS)-linked optic neuritis patients. Immunomodulatory therapies targeting inflammation have failed to avert the disease progression and no therapies exist to prevent the neuronal deficits seen in MS to date. Neuroprotective strategies targeting oligodendrocytes and astroglia have shown limited success due to a lack of axonal regeneration from injured neurons. In this study, we used the chronic experimental autoimmune encephalomyelitis (EAE) mouse model of MS to investigate the axonal regenerative approach to improve the neuronal function. Our approach focused on targeted knock-out (KO) of the developmentally regulated axon growth inhibitory Krüppel-like factor 4 (Klf4) gene in retinal ganglion cells (RGCs) of Klf4^fl/flmice by intravitreal delivery of AAV2-Cre-ires-EGFP recombinant virus (1) at the time of EAE sensitization and (2) after the onset of optic neuritis-mediated visual defects in the mice. Klf4 gene KO performed simultaneous with EAE sensitization prevented the visual loss as assessed by pattern electroretinograms (PERGs) in the mice and protected the RGCs from EAE-mediated death. More importantly, however, Klf4 gene KO after the onset of optic neuritis also resulted in RGC neuroprotection with additional restoration of their function, thereby improving the visual function outcomes in the EAE model. This study establishes the efficacy of Klf4 targeted knock-down in EAE even after the onset of disease symptoms, and thus should be further explored as a potential treatment strategy for MS/optic neuritis patients.