Mammalian target of rapamycin (mTOR) activation increases axonal growth capacity of injured peripheral nerves

Disruption of Core Stress Granule Protein Aggregates Promotes CNS Axon Regeneration

Depletion or inhibition of core stress granule proteins, G3BP1 in mammals and TIAR-2 in C. elegans , increases axon regeneration in injured neurons that show spontaneous regeneration. Inhibition of G3BP1 by expression of its acidic or 'B-domain' accelerates axon regeneration after nerve injury bringing a potential therapeutic intervention to promote neural repair in the peripheral nervous system. Here, we asked if G3BP1 inhibition is a viable strategy to promote regeneration in the injured mammalian central nervous system where axons do not regenerate spontaneously. G3BP1 B-domain expression was found to promote axon regeneration in both the mammalian spinal cord and optic nerve. Moreover, a cell permeable peptide to a subregion of G3BP1's B-domain (rodent G3BP1 amino acids 190-208) accelerated axon regeneration after peripheral nerve injury and promoted the regrowth of reticulospinal axons into the distal transected spinal cord through a bridging peripheral nerve graft. The rodent and human G3BP1 peptides promoted axon growth from rodent and human neurons cultured on permissive substrates, and this function required alternating Glu/Asp-Pro repeats that impart a unique predicted tertiary structure. These studies point to G3BP1 granules as a critical impediment to CNS axon regeneration and indicate that G3BP1 granule disassembly represents a novel therapeutic strategy for promoting neural repair after CNS injury.

The promotive effect of activation of the Akt/mTOR/p70S6K signaling pathway in oligodendrocytes on nerve myelin regeneration in rats with spinal cord injury

PURPOSE

Akt/mTOR/p70S6K signaling pathway promotes motor function recovery after spinal cord injury (SCI) in both neurons and astrocytes. But the role and mechanism of this pathway in oligodendrocytes during nerve repair following SCI has not been researched. This study aimed to investigate the effect and mechanism of this signaling pathway in oligodendrocytes on nerve myelin regeneration and motor function recovery in rats with SCI.

METHODS

After inhibiting or activating this signaling pathway, Western blotting and double immunofluorescence labeling were used to determine the levels of the signaling molecules in this pathway and myelin formation-related proteins in the plane of the thoracic segment of the injured spinal cord. The level of motor function recovery was evaluated and the oligodendrocytes involved in nerve myelin regeneration were studied. Primary oligodendrocytes were isolated and cultured in vitro, then MBP, PLP, and MOG were measured with reverse transcription-quantitative polymerase chain reaction (RT-qPCR).

RESULTS

Akt/mTOR/p70S6K signaling pathway was activated after SCI compared with the sham-operated rats, prominently elevated levels of the pathway components were observed in the SC79-treated group. The activation of the signaling pathway significantly increased the expression levels of myelin formation-related proteins, including MBP, PLP, and MOG, and improved the Basso, Beattie, and Bresnahan (BBB) scores in the injured spinal cord. Conversely, rapamycin suppressed the expression of these signaling molecules and reduced the levels of myelin formation-related proteins.

CONCLUSION

Akt/mTOR/p70S6K signaling pathway activation can contribute to nerve myelin regeneration and has the potential to improve the regenerative environment and motor function, as well as the potential to promote repair of SCI.

Mesencephalic astrocyte-derived neurotrophic factor promotes axonal regeneration and the motor function recovery after sciatic nerve injury

Peripheral nerve injuries have common clinical problems that are often accompanied by sensory and motor dysfunction and failure of axonal regeneration. Although various therapeutic approaches have been attempted, full functional recovery and axonal regeneration are rarely achieved in patients. In this study, we investigated the effects of recombinant adeno-associated virus (AAV) of mesencephalic astrocyte-derived neurotrophic factor (AAV-MANF) or placental growth factor (AAV-PlGF) transduced into mesenchymal stem cells (hMSC-MANF and hMSC-PlGF), which were then transplanted using human decellularized nerves (HDN) into sciatic nerve injury model. Our results showed that both AAV-MANF and AAV-PlGF were expressed in MSCs transplanted into the injury site. Behavioral measurements performed 2, 4, 6, 8, and 12 weeks after injury indicated that MANF facilitated the rapid and improved recovery of sensory and motor functions than PlGF. In addition, immunohistochemical analysis was used to quantitatively analyze the myelination of neurofilaments, Schwann cells, and regrowth axons. Both hMSC-MANF and hMSC-PlGF increased axon numbers and immunoreactive areas of axons and Schwann cells compared with the hMSC-GFP group. However, hMSC-MANF significantly improved the thickness of axons and Schwann cells compared with hMSC-PlGF. G-ratio analysis also showed a marked increase in axon myelination in axons thicker than 2.0 μm treated with MANF than that treated with PlGF. Our study suggests that transplantation of hMSC transduced with AAV-MANF has a potential to provide a novel and efficient strategy for promoting functional recovery and axonal regeneration in peripheral nerve injury.

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.

Intrinsic control of DRG sensory neuron diversification by Pten

Phosphatase and tensin homolog (PTEN) modulates intracellular survival and differentiation signaling pathways downstream of neurotrophin receptors in the developing peripheral nervous system (PNS). Although well-studied in the context of brain development, our understanding of the in vivo role of PTEN in the PNS is limited to models of neuropathic pain and nerve injury. Here, we assessed how alterations in PTEN signaling affects the development of peripheral somatosensory circuits. We found that sensory neurons within the dorsal root ganglia (DRG) in Pten heterozygous ( Pten Het ) mice exhibit defects in neuronal subtype diversification. Abnormal DRG differentiation in Pten Het mice arises early in development, with subsets of neurons expressing both progenitor and neuronal markers. DRGs in Pten Het mice show dysregulation of both mTOR and GSK-3β signaling pathways downstream of PTEN. Finally, we show that mice with an autism-associated mutation in Pten ( Pten Y68H/+ ) show abnormal DRG development. Thus, we have discovered a crucial role for PTEN signaling in the intrinsic diversification of primary sensory neuron populations in the DRG during development.

Development of a simultaneous and noninvasive measuring method using high-frame rate videography and motion analysis software for the assessment of facial palsy recovery in a rat model

BACKGROUND

For the development of new therapeutic and reconstructive methods for facial nerve palsy, it is critical to validate them in animal models. This study developed a novel evaluation method using a high-speed camera and motion analysis software for rat facial paralysis models. The validity of the new method was verified using normal rats and rats with facial paralysis.

METHODS

The whisker movement was recorded using a high-frame video camera. The video files were processed using motion analysis software, and the angular velocities were measured. The score was calculated as the percentage of movement on the side that had palsy compared with the movement on the normal side. Normal rats were used to examine which of the four indices of angular velocity is appropriate for this evaluation method. Using this method, two types of facial nerve palsy models were compared. Furthermore, the three agents that were predicted to promote axon regeneration from previous studies were evaluated.

RESULTS

The two averages of the protraction and retraction movement velocities of the whiskers were considered as the most appropriate indicators for this new method. Compared with the saline group, all agent groups showed significant differences in the improvement of facial palsy recovery.

CONCLUSIONS

This method is an evaluation method for the effects of therapeutic intervention for facial nerve paralysis in real time without sacrificing animals.

Advances in the mTOR signaling pathway and its inhibitor rapamycin in epilepsy

INTRODUCTION

Epilepsy is one of the most common and serious brain syndromes and has adverse consequences on a patient's neurobiological, cognitive, psychological, and social wellbeing, thereby threatening their quality of life. Some patients with epilepsy experience poor treatment effects due to the unclear pathophysiological mechanisms of the syndrome. Dysregulation of the mammalian target of the rapamycin (mTOR) pathway is thought to play an important role in the onset and progression of some epilepsies.

METHODS

This review summarizes the role of the mTOR signaling pathway in the pathogenesis of epilepsy and the prospects for the use of mTOR inhibitors.

RESULTS

The mTOR pathway functions as a vital mediator in epilepsy development through diverse mechanisms, indicating that the it has great potential as an effective target for epilepsy therapy. The excessive activation of mTOR signaling pathway leads to structural changes in neurons, inhibits autophagy, exacerbates neuron damage, affects mossy fiber sprouting, enhances neuronal excitability, increases neuroinflammation, and is closely associated with tau upregulation in epilepsy. A growing number of studies have demonstrated that mTOR inhibitors exhibit significant antiepileptic effects in both clinical applications and animal models. Specifically, rapamycin, a specific inhibitor of TOR, reduces the intensity and frequency of seizures. Clinical studies in patients with tuberous sclerosis complex have shown that rapamycin has the function of reducing seizures and improving this disease. Everolimus, a chemically modified derivative of rapamycin, has been approved as an added treatment to other antiepileptic medicines. Further explorations are needed to evaluate the therapeutic efficacy and application value of mTOR inhibitors in epilepsy.

CONCLUSIONS

Targeting the mTOR signaling pathway provides a promising prospect for the treatment of epilepsy.

Gliotransmission and adenosine signaling promote axon regeneration

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

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

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

Epigenetic and epitranscriptomic regulation of axon regeneration

Effective axonal regeneration in the adult mammalian nervous system requires coordination of elevated intrinsic growth capacity and decreased responses to the inhibitory environment. Intrinsic regenerative capacity largely depends on the gene regulatory network and protein translation machinery. A failure to activate these pathways upon injury is underlying a lack of robust axon regeneration in the mature mammalian central nervous system. Epigenetics and epitranscriptomics are key regulatory mechanisms that shape gene expression and protein translation. Here, we provide an overview of different types of modifications on DNA, histones, and RNA, underpinning the regenerative competence of axons in the mature mammalian peripheral and central nervous systems. We highlight other non-neuronal cells and their epigenetic changes in determining the microenvironment for tissue repair and axon regeneration. We also address advancements of single-cell technology in charting transcriptomic and epigenetic landscapes that may further facilitate the mechanistic understanding of differential regenerative capacity in neuronal subtypes. Finally, as epigenetic and epitranscriptomic processes are commonly affected by brain injuries and psychiatric disorders, understanding their alterations upon brain injury would provide unprecedented mechanistic insights into etiology of injury-associated-psychiatric disorders and facilitate the development of therapeutic interventions to restore brain function.

Knockout of TSC2 in Nav1.8+ neurons predisposes to the onset of normal weight obesity

OBJECTIVE

Obesity and nutrient oversupply increase mammalian target of rapamycin (mTOR) signaling in multiple cell types and organs, contributing to the onset of insulin resistance and complications of metabolic disease. However, it remains unclear when and where mTOR activation mediates these effects, limiting options for therapeutic intervention. The objective of this study was to isolate the role of constitutive mTOR activation in Nav1.8-expressing peripheral neurons in the onset of diet-induced obesity, bone loss, and metabolic disease.

METHODS

In humans, loss of function mutations in tuberous sclerosis complex 2 (TSC2) lead to maximal constitutive activation of mTOR. To mirror this in mice, we bred Nav1.8-Cre with TSC2fl/fl animals to conditionally delete TSC2 in Nav1.8-expressing neurons. Male and female mice were studied from 4- to 34-weeks of age and a subset of animals were fed a high-fat diet (HFD) for 24-weeks. Assays of metabolism, body composition, bone morphology, and behavior were performed.

RESULTS

By lineage tracing, Nav1.8-Cre targeted peripheral sensory neurons, a subpopulation of postganglionic sympathetics, and several regions of the brain. Conditional knockout of TSC2 in Nav1.8-expressing neurons (Nav1.8-TSC2KO) selectively upregulated neuronal mTORC1 signaling. Male, but not female, Nav1.8-TSC2KO mice had a 4-10% decrease in body size at baseline. When challenged with HFD, both male and female Nav1.8-TSC2KO mice resisted diet-induced gains in body mass. However, this did not protect against HFD-induced metabolic dysfunction and bone loss. In addition, despite not gaining weight, Nav1.8-TSC2KO mice fed HFD still developed high body fat, a unique phenotype previously referred to as 'normal weight obesity'. Nav1.8-TSC2KO mice also had signs of chronic itch, mild increases in anxiety-like behavior, and sex-specific alterations in HFD-induced fat distribution that led to enhanced visceral obesity in males and preferential deposition of subcutaneous fat in females.

CONCLUSIONS

Knockout of TSC2 in Nav1.8+ neurons increases itch- and anxiety-like behaviors and substantially modifies fat storage and metabolic responses to HFD. Though this prevents HFD-induced weight gain, it masks depot-specific fat expansion and persistent detrimental effects on metabolic health and peripheral organs such as bone, mimicking the 'normal weight obesity' phenotype that is of growing concern. This supports a mechanism by which increased neuronal mTOR signaling can predispose to altered adipose tissue distribution, adipose tissue expansion, impaired peripheral metabolism, and detrimental changes to skeletal health with HFD - despite resistance to weight gain.

Characterization of circular RNAs in dorsal root ganglia after central and peripheral axon injuries

In central nervous system, axons fail to regenerate after injury while in peripheral nervous system, axons retain certain regenerative ability. Dorsal root ganglion (DRG) neuron has an ascending central axon branch and a descending peripheral axon branch stemming from one single axon and serves as a suitable model for the comparison of growth competence following central and peripheral axon injuries. Molecular alterations underpin different injury responses of DRG branches have been investigated from many aspects, such as coding gene expression, chromatin accessibility, and histone acetylation. However, changes of circular RNAs are poorly characterized. In the present study, we comprehensively investigate circular RNA expressions in DRGs after rat central and peripheral axon injuries using sequencing analysis and identify a total of 33 differentially expressed circular RNAs after central branch injury as well as 55 differentially expressed circular RNAs after peripheral branch injury. Functional enrichment of host genes of differentially expressed circular RNAs demonstrate the participation of Hippo signaling pathway and Notch signaling pathway after both central and peripheral axon injuries. Circular RNA changes after central axon injury are also linked with apoptosis and cellular junction while changes after peripheral axon injury are associated with metabolism and PTEN-related pathways. Altogether, the present study offers a systematic evaluation of alterations of circular RNAs in rat DRGs following injuries to the central and peripheral axon branches and contributes to the deciphering of essential biological activities and mechanisms behind successful nerve regeneration.

mTOR-neuropeptide Y signaling sensitizes nociceptors to drive neuropathic pain

Neuropathic pain is a refractory condition that involves de novo protein synthesis in the nociceptive pathway. The mTOR is a master regulator of protein translation; however, mechanisms underlying its role in neuropathic pain remain elusive. Using the spared nerve injury-induced neuropathic pain model, we found that mTOR was preferentially activated in large-diameter dorsal root ganglion (DRG) neurons and spinal microglia. However, selective ablation of mTOR in DRG neurons, rather than microglia, alleviated acute neuropathic pain in mice. We show that injury-induced mTOR activation promoted the transcriptional induction of neuropeptide Y (Npy), likely via signal transducer and activator of transcription 3 phosphorylation. NPY further acted primarily on Y2 receptors (Y2R) to enhance neuronal excitability. Peripheral replenishment of NPY reversed pain alleviation upon mTOR removal, whereas Y2R antagonists prevented pain restoration. Our findings reveal an unexpected link between mTOR and NPY/Y2R in promoting nociceptor sensitization and neuropathic pain.

Unleashing Intrinsic Growth Pathways in Regenerating Peripheral Neurons

Common mechanisms of peripheral axon regeneration are recruited following diverse forms of damage to peripheral nerve axons. Whether the injury is traumatic or disease related neuropathy, reconnection of axons to their targets is required to restore function. Supporting peripheral axon regrowth, while not yet available in clinics, might be accomplished from several directions focusing on one or more of the complex stages of regrowth. Direct axon support, with follow on participation of supporting Schwann cells is one approach, emphasized in this review. However alternative approaches might include direct support of Schwann cells that instruct axons to regrow, manipulation of the inflammatory milieu to prevent ongoing bystander axon damage, or use of inflammatory cytokines as growth factors. Axons may be supported by a growing list of growth factors, extending well beyond the classical neurotrophin family. The understanding of growth factor roles continues to expand but their impact experimentally and in humans has faced serious limitations. The downstream signaling pathways that impact neuron growth have been exploited less frequently in regeneration models and rarely in human work, despite their promise and potency. Here we review the major regenerative signaling cascades that are known to influence adult peripheral axon regeneration. Within these pathways there are major checkpoints or roadblocks that normally check unwanted growth, but are an impediment to robust growth after injury. Several molecular roadblocks, overlapping with tumour suppressor systems in oncology, operate at the level of the perikarya. They have impacts on overall neuron plasticity and growth. A second approach targets proteins that largely operate at growth cones. Addressing both sites might offer synergistic benefits to regrowing neurons. This review emphasizes intrinsic aspects of adult peripheral axon regeneration, emphasizing several molecular barriers to regrowth that have been studied in our laboratory.

Protective effects and mechanism of puerarin targeting PI3K/Akt signal pathway on neurological diseases

Neurological diseases impose a tremendous and increasing burden on global health, and there is currently no curative agent. Puerarin, a natural isoflavone extracted from the dried root of Pueraria montana var. Lobata (Willd.) Sanjappa and Predeep, is an active ingredient with anti-inflammatory, antioxidant, anti-apoptotic, and autophagy-regulating effects. It has great potential in the treatment of neurological and other diseases. Phosphatidylinositol 3-kinases/protein kinase B (PI3K/Akt) signal pathway is a crucial signal transduction mechanism that regulates biological processes such as cell regeneration, apoptosis, and cognitive memory in the central nervous system, and is closely related to the pathogenesis of nervous system diseases. Accumulating evidence suggests that the excellent neuroprotective effect of puerarin may be related to the regulation of the PI3K/Akt signal pathway. Here, we summarized the main biological functions and neuroprotective effects of puerarin via activating PI3K/Akt signal pathway in neurological diseases. This paper illustrates that puerarin, as a neuroprotective agent, can protect nerve cells and delay the progression of neurological diseases through the PI3K/Akt signal pathway.

Repurposing SGLT2 Inhibitors for Neurological Disorders: A Focus on the Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder with a substantially increasing incidence rate. It is characterized by repetitive behavior, learning difficulties, deficits in social communication, and interactions. Numerous medications, dietary supplements, and behavioral treatments have been recommended for the management of this condition, however, there is no cure yet. Recent studies have examined the therapeutic potential of the sodium-glucose cotransporter 2 (SGLT2) inhibitors in neurodevelopmental diseases, based on their proved anti-inflammatory effects, such as downregulating the expression of several proteins, including the transforming growth factor beta (TGF-β), interleukin-6 (IL-6), C-reactive protein (CRP), nuclear factor κB (NF-κB), tumor necrosis factor alpha (TNF-α), and the monocyte chemoattractant protein (MCP-1). Furthermore, numerous previous studies revealed the potential of the SGLT2 inhibitors to provide antioxidant effects, due to their ability to reduce the generation of free radicals and upregulating the antioxidant systems, such as glutathione (GSH) and superoxide dismutase (SOD), while crossing the blood brain barrier (BBB). These properties have led to significant improvements in the neurologic outcomes of multiple experimental disease models, including cerebral oxidative stress in diabetes mellitus and ischemic stroke, Alzheimer's disease (AD), Parkinson's disease (PD), and epilepsy. Such diseases have mutual biomarkers with ASD, which potentially could be a link to fill the gap of the literature studying the potential of repurposing the SGLT2 inhibitors' use in ameliorating the symptoms of ASD. This review will look at the impact of the SGLT2 inhibitors on neurodevelopmental disorders on the various models, including humans, rats, and mice, with a focus on the SGLT2 inhibitor canagliflozin. Furthermore, this review will discuss how SGLT2 inhibitors regulate the ASD biomarkers, based on the clinical evidence supporting their functions as antioxidant and anti-inflammatory agents capable of crossing the blood-brain barrier (BBB).

The role of autophagic kinases in regulation of axonal function

Autophagy is an essential process for maintaining cellular homeostasis. Highlighting the importance of proper functioning of autophagy in neurons, disruption of autophagy is a common finding in neurodegenerative diseases. In recent years, evidence has emerged for the role of autophagy in regulating critical axonal functions. In this review, we discuss kinase regulation of autophagy in neurons, and provide an overview of how autophagic kinases regulate axonal processes, including axonal transport and axonal degeneration and regeneration. We also examine mechanisms for disruption of this process leading to neurodegeneration, focusing on the role of TBK1 in pathogenesis of Amyotrophic Lateral Sclerosis.

It takes two to tango: Widening our understanding of the onset of schizophrenia from a neuro-angiogenic perspective

Schizophrenia is a chronic debilitating mental disorder characterized by perturbations in thinking, perception, and behavior, along with brain connectivity deficiencies, neurotransmitter dysfunctions, and loss of gray brain matter. To date, schizophrenia has no cure and pharmacological treatments are only partially efficacious, with about 30% of patients describing little to no improvement after treatment. As in most neurological disorders, the main descriptions of schizophrenia physiopathology have been focused on neural network deficiencies. However, to sustain proper neural activity in the brain, another, no less important network is operating: the vast, complex and fascinating vascular network. Increasing research has characterized schizophrenia as a systemic disease where vascular involvement is important. Several neuro-angiogenic pathway disturbances have been related to schizophrenia. Alterations, ranging from genetic polymorphisms, mRNA, and protein alterations to microRNA and abnormal metabolite processing, have been evaluated in plasma, post-mortem brain, animal models, and patient-derived induced pluripotent stem cell (hiPSC) models. During embryonic brain development, the coordinated formation of blood vessels parallels neuro/gliogenesis and results in the structuration of the neurovascular niche, which brings together physical and molecular signals from both systems conforming to the Blood-Brain barrier. In this review, we offer an upfront perspective on distinctive angiogenic and neurogenic signaling pathways that might be involved in the biological causality of schizophrenia. We analyze the role of pivotal angiogenic-related pathways such as Vascular Endothelial Growth Factor and HIF signaling related to hypoxia and oxidative stress events; classic developmental pathways such as the NOTCH pathway, metabolic pathways such as the mTOR/AKT cascade; emerging neuroinflammation, and neurodegenerative processes such as UPR, and also discuss non-canonic angiogenic/axonal guidance factor signaling. Considering that all of the mentioned above pathways converge at the Blood-Brain barrier, reported neurovascular alterations could have deleterious repercussions on overall brain functioning in schizophrenia.

Study on Antidepressant Effect and Mechanism of Crocin Mediated by the mTOR Signaling Pathway

Crocin is a monomer of Chinese traditional herbs extracted from saffron, relieving depression-like behavior. However, its underlying mechanism of action remains unclear. Herein, we explored whether crocin's antidepressant effect depended on the mammalian target of the rapamycin (mTOR) signaling pathway. The model of PC12 cells injury was established by corticosterone, the changes in cell survival rate were tested by the CCK-8 method, and the changes in cellular morphology were observed under a fluorescence microscope. The depression model was established by chronic unpredictable mild stress (CUMS), and its antidepressant effect was estimated by open field test (OFT), forced swimming test (FST), and tail suspension test (TST). Western blot was used to monitor the protein expression. The results showed that crocin could effectively improve cell survival rate and cellular synaptic growth, alleviate the depressive behavior of CUMS mice, and promote the expression of BDNF, P-mTOR, P-ERK, and PSD95. However, when rapamycin was pretreated, the antidepressant effects of crocin were inhibited. In summary, crocin plays a significant antidepressant effect. After pretreatment with rapamycin, the anti-depression effect of crocin was significantly inhibited. It is suggested that the mechanism of the anti-depression effect of crocin may be related to the mTOR signaling pathway.

Panax notoginseng Saponins Stimulates Neurogenesis and Neurological Restoration After Microsphere-Induced Cerebral Embolism in Rats Partially Via mTOR Signaling

P. Notoginseng Saponins (PNS), the main active component of herbal medicine Panax notoginseng, has been widely used to treat cerebrovascular diseases. It has been acknowledged that PNS exerted protection on nerve injuries induced by ischemic stroke, however, the long-term impacts of PNS on the restoration of neurological defects and neuroregeneration after stroke have not been thoroughly studied and the underlying molecular mechanism of stimulating neurogenesis is difficult to precisely clarify, much more in-depth researches are badly needed. In the present study, cerebral ischemia injury was induced by microsphere embolism (ME) in rats. After 14 days, PNS administration relieved cerebral ischemia injury as evidenced by alleviating neurological deficits and reducing hippocampal pathological damage. What’s more, PNS stimulated hippocampal neurogenesis by promoting cell proliferation, migration and differentiation activity and modulated synaptic plasticity. Increased number of BrdU/Nestin, BrdU/DCX and NeuroD1-positive cells and upregulated synapse-related GAP43, SYP, and PSD95 expression were observed in the hippocampus. We hypothesized that upregulation of brain-derived neurotrophic factor (BDNF) expression and activation of Akt/mTOR/p70S6K signaling after ME could partially underlie the neuroprotective effects of PNS against cerebral ischemia injury. Our findings offer some new viewpoints into the beneficial roles of PNS against ischemic stroke.

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

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

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

The metabolomic profiling identifies N, N-dimethylglycine as a facilitator of dorsal root ganglia neuron axon regeneration after injury

Identifying novel molecules involved in axon regeneration of neurons in the peripheral nervous system (PNS) will be of benefit in obtaining a therapeutic strategy for repairing axon damage both in the PNS and the central nervous system (CNS). Metabolism and axon regeneration are tightly connected. However, the overall metabolic processes and the landscape of the metabolites in axon regeneration of PNS neurons are uncovered. Here, we used an ultra high performance liquid tandem chromatography quadrupole time of flight mass spectrometry (UHPLC-QTOFMS)-based untargeted metabolomics to analyze dorsal root ganglia (DRG) metabolic characteristics at different time points post sciatic nerve injury and acquired hundreds of differentially changed metabolites. In addition, the results reveal that several metabolic pathways were significantly altered, such as 'Histidine metabolism', 'Glycine serine and threonine metabolism', 'Arginine and proline metabolism', 'taurine and hypotaurine metabolism' and so on. Given metabolite could alter a cell's or an organism's phenotype, further investigation demonstrated that N, N-dimethylglycine (DMG) has a promoting effect on the regenerative ability post injury. Overall, our data may serve as a resource useful for further understanding how metabolites contribute to axon regeneration in DRG during sciatic nerve regeneration and suggest DMG may be a candidate drug to repair nerve injury.

Neuronal mTOR Outposts: Implications for Translation, Signaling, and Plasticity

The kinase mTOR is a signaling hub for pathways that regulate cellular growth. In neurons, the subcellular localization of mTOR takes on increased significance. Here, we review findings on the localization of mTOR in axons and offer a perspective on how these may impact our understanding of nervous system development, function, and disease. We propose a model where mTOR accumulates in local foci we term mTOR outposts, which can be found in processes distant from a neuron’s cell body. In this model, pathways that funnel through mTOR are gated by local outposts to spatially select and amplify local signaling. The presence or absence of mTOR outposts in a segment of axon or dendrite may determine whether regional mTOR-dependent signals, such as nutrient and growth factor signaling, register toward neuron-wide responses. In this perspective, we present the emerging evidence for mTOR outposts in neurons, their putative roles as spatial gatekeepers of signaling inputs, and the implications of the mTOR outpost model for neuronal protein synthesis, signal transduction, and synaptic plasticity.

Axon Regeneration: A Subcellular Extension in Multiple Dimensions

Axons are a unique cellular structure that allows for the communication between neurons. Axon damage compromises neuronal communications and often leads to functional deficits. Thus, developing strategies that promote effective axon regeneration for functional restoration is highly desirable. One fruitful approach is to dissect the regenerative mechanisms used by some types of neurons in both mammalian and nonmammalian systems that exhibit spontaneous regenerative capacity. Additionally, numerous efforts have been devoted to deciphering the barriers that prevent successful axon regeneration in the most regeneration-refractory system-the adult mammalian central nervous system. As a result, several regeneration-promoting strategies have been developed, but significant limitations remain. This review is aimed to summarize historic progression and current understanding of this exciting yet incomplete endeavor.

BET protein inhibition in macrophages enhances dorsal root ganglion neurite outgrowth in female mice

Peripheral nerve regeneration is limited after injury, especially in humans, due to the large distance the axons have to grow in the limbs. This process is highly dependent on the expression of neuroinflammatory factors produced by macrophages and glial cells. Given the importance of the epigenetic BET proteins on inflammation, we aimed to ascertain if BET inhibition may have an effect on axonal outgrowth. For this purpose, we treated female mice with JQ1 or vehicle after sciatic nerve crush injury and analyzed target reinnervation. We also used dorsal root ganglion (DRG) culture explants to analyze the effects of direct BET inhibition or treatment with conditioned medium from BET-inhibited macrophages. We observed that although JQ1 produced an enhancement of IL-4, IL-13, and GAP43 expression, it did not have an effect on sensory or motor reinnervation after crush injury in vivo. In contrast, JQ1 reduced neurite growth when interacting directly with DRG neurons ex vivo, whereas conditioned medium from JQ1-treated macrophages promoted neurite outgrowth. Therefore, BET-inhibited macrophages secrete pro-regenerative factors that induce neurite outgrowth, and that may counteract the direct inhibition of BET proteins in neurons in vivo. Finally, we observed an activation of the STAT6 pathway in DRG explants treated with conditioned medium from JQ1-treated macrophages. In conclusion, this study demonstrates that BET protein inhibition in macrophages provides a mechanism to enhance axonal outgrowth. However, specific targeting of BET proteins to macrophages will be needed to efficiently enhance functional recovery after nerve injury.

Paligenosis: Cellular Remodeling During Tissue Repair

Complex multicellular organisms have evolved specific mechanisms to replenish cells in homeostasis and during repair. Here, we discuss how emerging technologies (e.g., single-cell RNA sequencing) challenge the concept that tissue renewal is fueled by unidirectional differentiation from a resident stem cell. We now understand that cell plasticity, i.e., cells adaptively changing differentiation state or identity, is a central tissue renewal mechanism. For example, mature cells can access an evolutionarily conserved program (paligenosis) to reenter the cell cycle and regenerate damaged tissue. Most tissues lack dedicated stem cells and rely on plasticity to regenerate lost cells. Plasticity benefits multicellular organisms, yet it also carries risks. For one, when long-lived cells undergo paligenotic, cyclical proliferation and redif-ferentiation, they can accumulate and propagate acquired mutations that activate oncogenes and increase the potential for developing cancer. Lastly, we propose a new framework for classifying patterns of cell proliferation in homeostasis and regeneration, with stem cells representing just one of the diverse methods that adult tissues employ.

Coupling axonal mRNA transport and local translation to organelle maintenance and function

Neuronal homeostasis requires the transport of various organelles to distal compartments and defects in this process lead to neurological disorders. Although several mechanisms for the delivery of organelles to axons and dendrites have been elucidated, exactly how this process is orchestrated is not well-understood. In this review, we discuss the recent literature supporting a novel paradigm - the co-shuttling of mRNAs with different membrane-bound organelles. This model postulates that the tethering of ribonucleoprotein complexes to endolysosomes and mitochondria allows for the spatiotemporal coupling of organelle transport and the delivery of transcripts to axons. Subcellular translation of these "hitchhiking" transcripts may thus provide a proximal source of proteins required for the maintenance and function of organelles in axons.

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).

Autophagy and apoptosis cascade: which is more prominent in neuronal death?

Autophagy and apoptosis are two crucial self-destructive processes that maintain cellular homeostasis, which are characterized by their morphology and regulated through signal transduction mechanisms. These pathways determine the fate of cellular organelle and protein involved in human health and disease such as neurodegeneration, cancer, and cardiovascular disease. Cell death pathways share common molecular mechanisms, such as mitochondrial dysfunction, oxidative stress, calcium ion concentration, reactive oxygen species, and endoplasmic reticulum stress. Some key signaling molecules such as p53 and VEGF mediated angiogenic pathway exhibit cellular and molecular responses resulting in the triggering of apoptotic and autophagic pathways. Herein, based on previous studies, we describe the intricate relation between cell death pathways through their common genes and the role of various stress-causing agents. Further, extensive research on autophagy and apoptotic machinery excavates the implementation of selective biomarkers, for instance, mTOR, Bcl-2, BH3 family members, caspases, AMPK, PI3K/Akt/GSK3β, and p38/JNK/MAPK, in the pathogenesis and progression of neurodegenerative diseases. This molecular phenomenon will lead to the discovery of possible therapeutic biomolecules as a pharmacological intervention that are involved in the modulation of apoptosis and autophagy pathways. Moreover, we describe the potential role of micro-RNAs, long non-coding RNAs, and biomolecules as therapeutic agents that regulate cell death machinery to treat neurodegenerative diseases. Mounting evidence demonstrated that under stress conditions, such as calcium efflux, endoplasmic reticulum stress, the ubiquitin-proteasome system, and oxidative stress intermediate molecules, namely p53 and VEGF, activate and cause cell death. Further, activation of p53 and VEGF cause alteration in gene expression and dysregulated signaling pathways through the involvement of signaling molecules, namely mTOR, Bcl-2, BH3, AMPK, MAPK, JNK, and PI3K/Akt, and caspases. Alteration in gene expression and signaling cascades cause neurotoxicity and misfolded protein aggregates, which are characteristics features of neurodegenerative diseases. Excessive neurotoxicity and misfolded protein aggregates lead to neuronal cell death by activating death pathways like autophagy and apoptosis. However, autophagy has a dual role in the apoptosis pathways, i.e., activation and inhibition of the apoptosis signaling. Further, micro-RNAs and LncRNAs act as pharmacological regulators of autophagy and apoptosis cascade, whereas, natural compounds and chemical compounds act as pharmacological inhibitors that rescue neuronal cell death through inhibition of apoptosis and autophagic cell death.

Peptide ligands targeting FGF receptors promote recovery from dorsal root crush injury via AKT/mTOR signaling

Background: Fibroblast growth factor receptors (FGFRs) are key targets for nerve regeneration and repair. The therapeutic effect of exogenous recombinant FGFs in vivo is limited due to their high molecular weight. Small peptides with low molecular weight, easy diffusion, low immunogenicity, and nontoxic metabolite formation are potential candidates. The present study aimed to develop a novel low-molecular-weight peptide agonist of FGFR to promote nerve injury repair.

Methods: Phage display technology was employed to screen peptide ligands targeting FGFR2. The peptide ligand affinity for FGFRs was detected by isothermal titration calorimetry. Structural biology-based computer virtual analysis was used to characterize the interaction between the peptide ligand and FGFR2. The peptide ligand effect on axon growth, regeneration, and behavioral recovery of sensory neurons was determined in the primary culture of sensory neurons and dorsal root ganglia (DRG) explants in vitro and a rat spinal dorsal root injury (DRI) model in vivo. The peptide ligand binding to other membrane receptors was characterized by surface plasmon resonance (SPR) and liquid chromatography-mass spectrometry (LC-MS)/MS. Intracellular signaling pathways primarily affected by the peptide ligand were characterized by phosphoproteomics, and related pathways were verified using specific inhibitors.

Results: We identified a novel FGFR-targeting small peptide, CH02, with seven amino acid residues. CH02 activated FGFR signaling through high-affinity binding with the extracellular segment of FGFRs and also had an affinity for several receptor tyrosine kinase (RTK) family members, including VEGFR2. In sensory neurons cultured in vitro, CH02 maintained the survival of neurons and promoted axon growth. Simultaneously, CH02 robustly enhanced nerve regeneration and sensory-motor behavioral recovery after DRI in rats. CH02-induced activation of FGFR signaling promoted nerve regeneration primarily via AKT and ERK signaling downstream of FGFRs. Activation of mTOR downstream of AKT signaling augmented axon growth potential in response to CH02.

Conclusion: Our study revealed the significant therapeutic effect of CH02 on strengthening nerve regeneration and suggested a strategy for treating peripheral and central nervous system injuries.

Azoxystrobin Impairs Neuronal Migration and Induces ROS Dependent Apoptosis in Cortical Neurons

Fungicides often cause genotoxic stress and neurodevelopmental disorders such as autism (ASD). Fungicide-azoxystrobin (AZOX) showed acute and chronic toxicity to various organisms, and remained a concern for ill effects in developing neurons. We evaluated the neurotoxicity of AZOX in developing mouse brains, and observed prenatal exposure to AZOX reduced neuronal viability, neurite outgrowth, and cortical migration process in developing brains. The 50% inhibitory concentration (IC50) of AZOX for acute (24 h) and chronic (7 days) exposures were 30 and 10 μM, respectively. Loss in viability was due to the accumulation of reactive oxygen species (ROS), and inhibited neurite outgrowth was due to the deactivation of mTORC1 kinase activity. Pretreatment with ROS scavenger- N-acetylcysteine (NAC) reserved the viability loss and forced activation of mTORC1 kinase revived the neurite outgrowth in AZOX treated neurons. Intra-amniotic injection of AZOX coupled with in utero electroporation of GFP-labelled plasmid in E15.5 mouse was performed and 20 mg/kg AZOX inhibited radial neuronal migration. Moreover, the accumulation of mitochondria was significantly reduced in AZOX treated primary neurons, indicative of mitochondrial deactivation and induction of apoptosis, which was quantified by Bcl2/Bax ratio and caspase 3 cleavage assay. This study elucidated the neurotoxicity of AZOX and explained the possible cure from it.

Newly regenerated axons via scaffolds promote sub-lesional reorganization and motor recovery with epidural electrical stimulation

Here, we report the effect of newly regenerated axons via scaffolds on reorganization of spinal circuitry and restoration of motor functions with epidural electrical stimulation (EES). Motor recovery was evaluated for 7 weeks after spinal transection and following implantation with scaffolds seeded with neurotrophin producing Schwann cell and with rapamycin microspheres. Combined treatment with scaffolds and EES-enabled stepping led to functional improvement compared to groups with scaffold or EES, although, the number of axons across scaffolds was not different between groups. Re-transection through the scaffold at week 6 reduced EES-enabled stepping, still demonstrating better performance compared to the other groups. Greater synaptic reorganization in the presence of regenerated axons was found in group with combined therapy. These findings suggest that newly regenerated axons through cell-containing scaffolds with EES-enabled motor training reorganize the sub-lesional circuitry improving motor recovery, demonstrating that neuroregenerative and neuromodulatory therapies cumulatively enhancing motor function after complete SCI.

Systematic investigation and comparison of US FDA-approved immunosuppressive drugs FK506, cyclosporine and rapamycin for neuromuscular regeneration following chronic nerve compression injury

Aim: To compare therapeutic benefits of different immunophilin ligands for treating nerve injuries. Materials & methods: Cyclosporine, FK506 and rapamycin, were evaluated first in vitro on a serum-free culture of embryonic dorsal root ganglia followed by a new in vivo model of chronic nerve compression. Results: Outcomes of the in vitro study have shown a potent effect of cyclosporine and FK506, on dorsal root ganglia axonal outgrowth, comparable to the effect of nerve growth factor. Rapamycin exhibited only a moderate effect. The in vivo study revealed the beneficial effects of cyclosporine, FK506 and rapamycin for neuromuscular regeneration. Cyclosporine showed the better maintenance of the tissues and function. Conclusion: Cyclosporine, FK506 and rapamycin drugs showed potential for treating peripheral nerve chronic compression injuries.

MicroRNAs 21 and 199a-3p Regulate Axon Growth Potential through Modulation of Pten and mTor mRNAs

Increased mTOR activity has been shown to enhance regeneration of injured axons by increasing neuronal protein synthesis, while PTEN signaling can block mTOR activity to attenuate protein synthesis. MicroRNAs (miRs) have been implicated in regulation of PTEN and mTOR expression, and previous work in spinal cord showed an increase in miR-199a-3p after spinal cord injury (SCI) and increase in miR-21 in SCI animals that had undergone exercise. Pten mRNA is a target for miR-21 and miR-199a-3p is predicted to target mTor mRNA. Here, we show that miR-21 and miR-199a-3p are expressed in adult dorsal root ganglion (DRG) neurons, and we used culture preparations to test functions of the rat miRs in adult DRG and embryonic cortical neurons. miR-21 increases and miR-199a-3p decreases in DRG neurons after in vivo axotomy. In both the adult DRG and embryonic cortical neurons, miR-21 promotes and miR-199a-3p attenuates neurite growth. miR-21 directly bound to Pten mRNA and miR-21 overexpression decreased Pten mRNA levels. Conversely, miR-199a-3p directly bound to mTor mRNA and miR-199a-3p overexpression decreased mTor mRNA levels. Overexpressing miR-21 increased both overall and intra-axonal protein synthesis in cultured DRGs, while miR-199a-3p overexpression decreased this protein synthesis. The axon growth phenotypes seen with miR-21 and miR-199a-3p overexpression were reversed by co-transfecting PTEN and mTOR cDNA expression constructs with the predicted 3′ untranslated region (UTR) miR target sequences deleted. Taken together, these studies indicate that injury-induced alterations in miR-21 and miR-199a-3p expression can alter axon growth capacity by changing overall and intra-axonal protein synthesis through regulation of the PTEN/mTOR pathway.

Toll-Like Receptor 9-Mediated Neuronal Innate Immune Reaction Is Associated with Initiating a Pro-Regenerative State in Neurons of the Dorsal Root Ganglia Non-Associated with Sciatic Nerve Lesion

One of the changes brought about by Wallerian degeneration distal to nerve injury is disintegration of axonal mitochondria and consequent leakage of mitochondrial DNA (mtDNA)—the natural ligand for the toll-like receptor 9 (TLR9). RT-PCR and immunohistochemical or Western blot analyses were used to detect TLR9 mRNA and protein respectively in the lumbar (L4-L5) and cervical (C7-C8) dorsal root ganglia (DRG) ipsilateral and contralateral to a sterile unilateral sciatic nerve compression or transection. The unilateral sciatic nerve lesions led to bilateral increases in levels of both TLR9 mRNA and protein not only in the lumbar but also in the remote cervical DRG compared with naive or sham-operated controls. This upregulation of TLR9 was linked to activation of the Nuclear Factor kappa B (NFκB) and nuclear translocation of the Signal Transducer and Activator of Transcription 3 (STAT3), implying innate neuronal immune reaction and a pro-regenerative state in uninjured primary sensory neurons of the cervical DRG. The relationship of TLR9 to the induction of a pro-regenerative state in the cervical DRG neurons was confirmed by the shorter lengths of regenerated axons distal to ulnar nerve crush following a previous sciatic nerve lesion and intrathecal chloroquine injection compared with control rats. The results suggest that a systemic innate immune reaction not only triggers the regenerative state of axotomized DRG neurons but also induces a pro-regenerative state further along the neural axis after unilateral nerve injury.

Defining Spatial Relationships Between Spinal Cord Axons and Blood Vessels in Hydrogel Scaffolds

Positively charged oligo(poly(ethylene glycol) fumarate) (OPF+) hydrogel scaffolds, implanted into a complete transection spinal cord injury (SCI), facilitate a permissive regenerative environment and provide a platform for controlled observation of repair mechanisms. Axonal regeneration after SCI is critically dependent upon nutrients and oxygen from a newly formed blood supply. Our objective was to investigate fundamental characteristics of revascularization in association with the ingrowth of axons into hydrogel scaffolds, thereby defining spatial relationships between axons and the neovasculature. A novel combination of stereologic estimates and precision image analysis techniques quantitate neurovascular regeneration in rats. Multichannel hydrogel scaffolds containing Matrigel-only (MG), Schwann cells (SCs), or SCs with rapamycin-eluting poly(lactic co-glycolic acid) microspheres (RAPA) were implanted for 6 weeks following complete spinal cord transection. Image analysis of 72 scaffold channels identified a total of 2494 myelinated and 4173 unmyelinated axons at 10 μm circumferential intervals centered around 708 individual blood vessel profiles. Blood vessel number, density, volume, diameter, intervessel distances, total vessel surface and cross-sectional areas, and radial diffusion distances were compared. Axon number and density, blood vessel surface area, and vessel cross-sectional areas in the SC group exceeded that in the MG and RAPA groups. Individual axons were concentrated within a concentric radius of 200-250 μm from blood vessel walls, in Gaussian distributions, which identified a peak axonal number (Mean Peak Amplitude) corresponding to defined distances (Mean Peak Distance) from each vessel, the highest concentrations of axons were relatively excluded from a 25-30 μm zone immediately adjacent to the vessel, and from vessel distances >150 μm. Higher axonal densities correlated with smaller vessel cross-sectional areas. A statistical spatial algorithm was used to generate cumulative distribution F- and G-functions of axonal distribution in the reference channel space. Axons located around blood vessels were definitively organized as clusters and were not randomly distributed. A scoring system stratifies 5 direct measurements and 12 derivative parameters influencing regeneration outcomes. By providing methods to quantify the axonal-vessel relationships, these results may refine spinal cord tissue engineering strategies to optimize the regeneration of complete neurovascular bundles in their relevant spatial relationships after SCI. Impact statement Vascular disruption and impaired neovascularization contribute critically to the poor regenerative capacity of the spinal cord after injury. In this study, hydrogel scaffolds provide a detailed model system to investigate the regeneration of spinal cord axons as they directly associate with individual blood vessels, using novel methods to define their spatial relationships and the physiologic implications of that organization. These results refine future tissue engineering strategies for spinal cord repair to optimize the re-development of complete neurovascular bundles in their relevant spatial architectures.

Traumatic Brain Injury: Mechanistic Insight on Pathophysiology and Potential Therapeutic Targets

Traumatic brain injury (TBI) causes brain damage, which involves primary and secondary injury mechanisms. Primary injury causes local brain damage, while secondary damage begins with inflammatory activity followed by disruption of the blood-brain barrier (BBB), peripheral blood cells infiltration, brain edema, and the discharge of numerous immune mediators including chemotactic factors and interleukins. TBI alters molecular signaling, cell structures, and functions. Besides tissue damage such as axonal damage, contusions, and hemorrhage, TBI in general interrupts brain physiology including cognition, decision-making, memory, attention, and speech capability. Regardless of the deep understanding of the pathophysiology of TBI, the underlying mechanisms still need to be assessed with a desired therapeutic agent to control the consequences of TBI. The current review gives a brief outline of the pathophysiological mechanism of TBI and various biochemical pathways involved in brain injury, pharmacological treatment approaches, and novel targets for therapy.

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.

Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling

Historically, the field of regenerative medicine has aimed to heal damaged tissue through the use of biomaterials scaffolds or delivery of foreign progenitor cells. Despite 30 years of research, however, translation and commercialization of these techniques has been limited. To enable mammalian regeneration, a more practical approach may instead be to develop therapies that evoke endogenous processes reminiscent of those seen in innate regenerators. Recently, investigations into tadpole tail regrowth, zebrafish limb restoration, and the super-healing Murphy Roths Large (MRL) mouse strain, have identified ancient oxygen-sensing pathways as a possible target to achieve this goal. Specifically, upregulation of the transcription factor, hypoxia-inducible factor one alpha (HIF-1α) has been shown to modulate cell metabolism and plasticity, as well as inflammation and tissue remodeling, possibly priming injuries for regeneration. Since HIF-1α signaling is conserved across species, environmental or pharmacological manipulation of oxygen-dependent pathways may elicit a regenerative response in non-healing mammals. In this review, we will explore the emerging role of HIF-1α in mammalian healing and regeneration, as well as attempts to modulate protein stability through hyperbaric oxygen treatment, intermittent hypoxia therapy, and pharmacological targeting. We believe that these therapies could breathe new life into the field of regenerative medicine.

Experimental Model Systems for Understanding Human Axonal Injury Responses

Neurons are structurally unique and have dendrites and axons that are vulnerable to injury. Some neurons in the peripheral nervous system (PNS) can regenerate their axons after injuries. However, most neurons in the central nervous system (CNS) fail to do so, resulting in irreversible neurological disorders. To understand the mechanisms of axon regeneration, various experimental models have been utilized in vivo and in vitro. Here, we collate the key experimental models that revealed the important mechanisms regulating axon regeneration and degeneration in different systems. We also discuss the advantages of experimenting with the rodent model, considering the application of these findings in understanding human diseases and for developing therapeutic methods.

A new ketogenic formulation improves functional outcome and reduces tissue loss following traumatic brain injury in adult mice

Rationale: Traumatic brain injury (TBI) leads to neurological impairment, with no satisfactory treatments available. Classical ketogenic diets (KD), which reduce reliance on carbohydrates and provide ketones as fuel, have neuroprotective potential, but their high fat content reduces compliance, and experimental evidence suggests they protect juvenile brain against TBI, but not adult brain, which would strongly limit their applicability in TBI. Methods: We designed a new-KD with a fat to carbohydrate plus protein ratio of 2:1, containing medium chain triglycerides (MCT), docosahexaenoic acid (DHA), low glycaemic index carbohydrates, fibres and the ketogenic amino acid leucine, and evaluated its neuroprotective potential in adult TBI. Adult male C57BL6 mice were injured by controlled cortical impact (CCI) and assessed for 70 days, during which they received a control diet or the new-KD. Results: The new-KD, that markedly increased plasma Beta-hydroxybutyrate (β-HB), significantly attenuated sensorimotor deficits and corrected spatial memory deficit. The lesion size, perilesional inflammation and oxidation were markedly reduced. Oligodendrocyte loss appeared to be significantly reduced. TBI activated the mTOR pathway and the new-KD enhanced this increase and increased histone acetylation and methylation. Conclusion: The behavioural improvement and tissue protection provide proof of principle that this new formulation has therapeutic potential in adult TBI.

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.

Candidate Strategies for Development of a Rapid-Acting Antidepressant Class That Does Not Result in Neuropsychiatric Adverse Effects: Prevention of Ketamine-Induced Neuropsychiatric Adverse Reactions

Non-competitive N-methyl-D-aspartate/glutamate receptor (NMDAR) antagonism has been considered to play important roles in the pathophysiology of schizophrenia. In spite of severe neuropsychiatric adverse effects, esketamine (racemic enantiomer of ketamine) has been approved for the treatment of conventional monoaminergic antidepressant-resistant depression. Furthermore, ketamine improves anhedonia, suicidal ideation and bipolar depression, for which conventional monoaminergic antidepressants are not fully effective. Therefore, ketamine has been accepted, with rigorous restrictions, in psychiatry as a new class of antidepressant. Notably, the dosage of ketamine for antidepressive action is comparable to the dose that can generate schizophrenia-like psychotic symptoms. Furthermore, the psychotropic effects of ketamine precede the antidepressant effects. The maintenance of the antidepressive efficacy of ketamine often requires repeated administration; however, repeated ketamine intake leads to abuse and is consistently associated with long-lasting memory-associated deficits. According to the dissociative anaesthetic feature of ketamine, it exerts broad acute influences on cognition/perception. To evaluate the therapeutic validation of ketamine across clinical contexts, including its advantages and disadvantages, psychiatry should systematically assess the safety and efficacy of either short- and long-term ketamine treatments, in terms of both acute and chronic outcomes. Here, we describe the clinical evidence of NMDAR antagonists, and then the temporal mechanisms of schizophrenia-like and antidepressant-like effects of the NMDAR antagonist, ketamine. The underlying pharmacological rodent studies will also be discussed.

Melatonin promotes Schwann cell proliferation and migration via the shh signalling pathway after peripheral nerve injury

Peripheral nerve injury (PNI) is a common and incurable disease in the clinic, but the effects of available treatments are still not satisfactory. Therefore, it is necessary to explore new treatment methods. To explore the effect and mechanism of melatonin in peripheral nerve regeneration, we administered melatonin to mice with PNI by intraperitoneal injection. We applied microarray analysis to detect differentially expressed genes of mice with sciatic nerve injury after melatonin application. Then, we conducted gene ontology and protein-protein interactions to screen out the key genes related to peripheral nerve regeneration. Cell biology and molecular biology experiments were performed in Schwann cells in vitro to verify the key genes identified by microarray analysis. Our results showed that a total of 598 differentially expressed genes were detected after melatonin subcutaneously injecting into mice with sciatic nerve injury. Bioinformatics analysis showed that Shh may be the key gene for the promotion of peripheral nerve regeneration by melatonin. In vitro, the proliferation and migration abilities of schwann cells in the melatonin group were significantly higher than those of Schwann cells in the control group; while after treating with both melatonin and luzindole (a Shh signalling pathway inhibitor), the proliferation and migration abilities of Schwann cells decreased compared with the melatonin group. Our study suggests that melatonin might improve the proliferation and migration of Schwann cells via the Shh signalling pathway after PNI, thus promoting peripheral nerve regeneration. Our study provides a new approach and target for the clinical treatment of PNI.

Schwann Cell Role in Selectivity of Nerve Regeneration

Peripheral nerve injuries result in the loss of the motor, sensory and autonomic functions of the denervated segments of the body. Neurons can regenerate after peripheral axotomy, but inaccuracy in reinnervation causes a permanent loss of function that impairs complete recovery. Thus, understanding how regenerating axons respond to their environment and direct their growth is essential to improve the functional outcome of patients with nerve lesions. Schwann cells (SCs) play a crucial role in the regeneration process, but little is known about their contribution to specific reinnervation. Here, we review the mechanisms by which SCs can differentially influence the regeneration of motor and sensory axons. Mature SCs express modality-specific phenotypes that have been associated with the promotion of selective regeneration. These include molecular markers, such as L2/HNK-1 carbohydrate, which is differentially expressed in motor and sensory SCs, or the neurotrophic profile after denervation, which differs remarkably between SC modalities. Other important factors include several molecules implicated in axon-SC interaction. This cell–cell communication through adhesion (e.g., polysialic acid) and inhibitory molecules (e.g., MAG) contributes to guiding growing axons to their targets. As many of these factors can be modulated, further research will allow the design of new strategies to improve functional recovery after peripheral nerve injuries.

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.

Comparative gene expression profiling between optic nerve and spinal cord injury in Xenopus laevis reveals a core set of genes inherent in successful regeneration of vertebrate central nervous system axons

BACKGROUND

The South African claw-toed frog, Xenopus laevis, is uniquely suited for studying differences between regenerative and non-regenerative responses to CNS injury within the same organism, because some CNS neurons (e.g., retinal ganglion cells after optic nerve crush (ONC)) regenerate axons throughout life, whereas others (e.g., hindbrain neurons after spinal cord injury (SCI)) lose this capacity as tadpoles metamorphose into frogs. Tissues from these CNS regions (frog ONC eye, tadpole SCI hindbrain, frog SCI hindbrain) were used in a three-way RNA-seq study of axotomized CNS axons to identify potential core gene expression programs for successful CNS axon regeneration.

RESULTS

Despite tissue-specific changes in expression dominating the injury responses of each tissue, injury-induced changes in gene expression were nonetheless shared between the two axon-regenerative CNS regions that were not shared with the non-regenerative region. These included similar temporal patterns of gene expression and over 300 injury-responsive genes. Many of these genes and their associated cellular functions had previously been associated with injury responses of multiple tissues, both neural and non-neural, from different species, thereby demonstrating deep phylogenetically conserved commonalities between successful CNS axon regeneration and tissue regeneration in general. Further analyses implicated the KEGG adipocytokine signaling pathway, which links leptin with metabolic and gene regulatory pathways, and a novel gene regulatory network with genes regulating chromatin accessibility at its core, as important hubs in the larger network of injury response genes involved in successful CNS axon regeneration.

CONCLUSIONS

This study identifies deep, phylogenetically conserved commonalities between CNS axon regeneration and other examples of successful tissue regeneration and provides new targets for studying the molecular underpinnings of successful CNS axon regeneration, as well as a guide for distinguishing pro-regenerative injury-induced changes in gene expression from detrimental ones in mammals.

Ultrasound Stimulation Increases Neurite Regeneration in Injured Dorsal Root Ganglion Neurons through Mammalian Target of Rapamycin Activation

Ultrasound stimulation (US) is reported to be a safe and useful technology for improving injured nerve regeneration. However, the intracellular mechanisms underlying its stimulatory effects are only partially understood. Mammalian target of rapamycin (mTOR) signaling is involved in neuronal survival and axonal outgrowth. In this study, we investigated the effect of US on regeneration of injured dorsal root ganglion (DRG) neurons and activation of the mTOR pathway. We showed that US significantly increased neurite regeneration and enhanced mTOR activation. Moreover, the expression of growth-associated protein-43 (GAP-43), a crucial factor for axonal outgrowth and regeneration in neurons, was significantly increased by US. These data suggest that US-induced neurite regeneration is mediated by upregulation of mTOR activity, which promotes the regeneration of injured DRG neurons.

Lab review: Molecular dissection of the signal transduction pathways associated with PTEN deletion-induced optic nerve regeneration

BACKGROUND

Permanent loss of vital functions after central nervous system (CNS) injury occurs in part because axons in the adult mammalian CNS do not regenerate after injury. PTEN was identified as a prominent intrinsic inhibitor of CNS axon regeneration about 10 years ago. The PTEN negatively regulated PI3K-AKT-mTOR pathway, which has been intensively explored in diverse models of axon injury and diseases and its mechanism for axon regeneration is becoming clearer.

OBJECTIVE

It is timely to summarize current knowledge about the PTEN/AKT/mTOR pathway and discuss future directions of translational regenerative research for neural injury and neurodegenerative diseases.

METHODS

Using mouse optic nerve crush as an in vivo retinal ganglion cell axon injury model, we have conducted an extensive molecular dissection of the PI3K-AKT-mTORC1/mTORC2 pathway to illuminate the cross-regulating mechanisms in axon regeneration.

RESULTS

AKT is the nodal point that coordinates both positive (PI3K-PDK1-pAKT-T308) and negative (PI3K-mTORC2-pAKT-S473) signals to regulate adult CNS axon regeneration through two parallel pathways, activating mTORC1 and inhibiting GSK3β. However, mTORC1/S6K1-mediated feedback inhibition after PTEN deletion prevents potent AKT activation.

CONCLUSIONS

A key permissive signal from an unidentified AKT-independent pathway is required for stimulating the neuron-intrinsic growth machinery. Future studies into this complex neuron-intrinsic balancing mechanism involving necessary and permissive signals for axon regeneration is likely to lead to safe and effective regenerative strategies for CNS repair.

Brain NMDA Receptors in Schizophrenia and Depression

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP), dizocilpine (MK-801) and ketamine have long been considered a model of schizophrenia, both in animals and humans. However, ketamine has been recently approved for treatment-resistant depression, although with severe restrictions. Interestingly, the dosage in both conditions is similar, and positive symptoms of schizophrenia appear before antidepressant effects emerge. Here, we describe the temporal mechanisms implicated in schizophrenia-like and antidepressant-like effects of NMDA blockade in rats, and postulate that such effects may indicate that NMDA receptor antagonists induce similar mechanistic effects, and only the basal pre-drug state of the organism delimitates the overall outcome. Hence, blockade of NMDA receptors in depressive-like status can lead to amelioration or remission of symptoms, whereas healthy individuals develop psychotic symptoms and schizophrenia patients show an exacerbation of these symptoms after the administration of NMDA receptor antagonists.