The Rheb-mTOR pathway is upregulated in reactive astrocytes of the injured spinal cord

Impacts of PI3K/protein kinase B pathway activation in reactive astrocytes: from detrimental effects to protective functions

Astrocytes are the most abundant type of glial cell in the central nervous system. Upon injury and inflammation, astrocytes become reactive and undergo morphological and functional changes. Depending on their phenotypic classification as A1 or A2, reactive astrocytes contribute to both neurotoxic and neuroprotective responses, respectively. However, this binary classification does not fully capture the diversity of astrocyte responses observed across different diseases and injuries. Transcriptomic analysis has revealed that reactive astrocytes have a complex landscape of gene expression profiles, which emphasizes the heterogeneous nature of their reactivity. Astrocytes actively participate in regulating central nervous system inflammation by interacting with microglia and other cell types, releasing cytokines, and influencing the immune response. The phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway is a central player in astrocyte reactivity and impacts various aspects of astrocyte behavior, as evidenced by in silico , in vitro , and in vivo results. In astrocytes, inflammatory cues trigger a cascade of molecular events, where nuclear factor-κB serves as a central mediator of the pro-inflammatory responses. Here, we review the heterogeneity of reactive astrocytes and the molecular mechanisms underlying their activation. We highlight the involvement of various signaling pathways that regulate astrocyte reactivity, including the PI3K/AKT/mammalian target of rapamycin (mTOR), α v β 3 integrin/PI3K/AKT/connexin 43, and Notch/PI3K/AKT pathways. While targeting the inactivation of the PI3K/AKT cellular signaling pathway to control reactive astrocytes and prevent central nervous system damage, evidence suggests that activating this pathway could also yield beneficial outcomes. This dual function of the PI3K/AKT pathway underscores its complexity in astrocyte reactivity and brain function modulation. The review emphasizes the importance of employing astrocyte-exclusive models to understand their functions accurately and these models are essential for clarifying astrocyte behavior. The findings should then be validated using in vivo models to ensure real-life relevance. The review also highlights the significance of PI3K/AKT pathway modulation in preventing central nervous system damage, although further studies are required to fully comprehend its role due to varying factors such as different cell types, astrocyte responses to inflammation, and disease contexts. Specific strategies are clearly necessary to address these variables effectively.

Recent advances on signaling pathways and their inhibitors in spinal cord injury

Spinal cord injury (SCI) is a serious and disabling central nervous system injury. Its complex pathological mechanism can lead to sensory and motor dysfunction. It has been reported that signaling pathway plays a key role in the pathological process and neuronal recovery mechanism of SCI. Such as PI3K/Akt, MAPK, NF-κB, and Wnt/β-catenin signaling pathways. According to reports, various stimuli and cytokines activate these signaling pathways related to SCI pathology, thereby participating in the regulation of pathological processes such as inflammation response, cell apoptosis, oxidative stress, and glial scar formation after injury. Activation or inhibition of relevant pathways can delay inflammatory response, reduce neuronal apoptosis, prevent glial scar formation, improve the microenvironment after SCI, and promote neural function recovery. Based on the role of signaling pathways in SCI, they may be potential targets for the treatment of SCI. Therefore, understanding the signaling pathway and its inhibitors may be beneficial to the development of SCI therapeutic targets and new drugs. This paper mainly summarizes the pathophysiological process of SCI, the signaling pathways involved in SCI pathogenesis, and the potential role of specific inhibitors/activators in its treatment. In addition, this review also discusses the deficiencies and defects of signaling pathways in SCI research. It is hoped that this study can provide reference for future research on signaling pathways in the pathogenesis of SCI and provide theoretical basis for SCI biotherapy.

AKT2 modulates astrocytic nicotine responses in vivo

A better understanding of nicotine neurobiology is needed to reduce or prevent chronic addiction, ameliorate the detrimental effects of nicotine withdrawal, and increase successful cessation of use. Nicotine binds and activates two astrocyte-expressed nicotinic acetylcholine receptors (nAChRs), α4β2 and α7. We recently found that Protein kinase B-β (Pkb-β or Akt2) expression is restricted to astrocytes in mice and humans. To determine if AKT2 plays a role in astrocytic nicotinic responses, we generated astrocyte-specific Akt2 conditional knockout (cKO) and full Akt2 KO mice for in vivo and in vitro experiments. For in vivo studies, we examined mice exposed to chronic nicotine for two weeks in drinking water (200 μg/mL) and following acute nicotine challenge (0.09, 0.2 mg/kg) after 24 hrs. Our in vitro studies used cultured mouse astrocytes to measure nicotine-dependent astrocytic responses. We validated our approaches using lipopolysaccharide (LPS) exposure inducing astrogliosis. Sholl analysis was used to measure glial fibrillary acidic protein responses in astrocytes. Our data show that wild-type (WT) mice exhibit increased astrocyte morphological complexity during acute nicotine exposure, with decreasing complexity during chronic nicotine use, whereas Akt2 cKO mice showed increased astrocyte morphology complexity. In culture, we found that 100μM nicotine was sufficient for morphological changes and blocking α7 or α4β2 nAChRs prevented observed morphologic changes. Finally, we performed conditioned place preference (CPP) in Akt2 cKO mice and found that astrocytic AKT2 deficiency reduced nicotine preference compared to controls. These findings show the importance of nAChRs and Akt2 signaling in the astrocytic response to nicotine.

Non-canonical retinoid signaling in neural development, regeneration and synaptic function

Canonical retinoid signaling via nuclear receptors and gene regulation is critical for the initiation of developmental processes such as cellular differentiation, patterning and neurite outgrowth, but also mediates nerve regeneration and synaptic functions in adult nervous systems. In addition to canonical transcriptional regulation, retinoids also exert rapid effects, and there are now multiple lines of evidence supporting non-canonical retinoid actions outside of the nucleus, including in dendrites and axons. Together, canonical and non-canonical retinoid signaling provide the precise temporal and spatial control necessary to achieve the fine cellular coordination required for proper nervous system function. Here, we examine and discuss the evidence supporting non-canonical actions of retinoids in neural development and regeneration as well as synaptic function, including a review of the proposed molecular mechanisms involved.

FDA-Approved Kinase Inhibitors in Preclinical and Clinical Trials for Neurological Disorders

Cancers and neurological disorders are two major types of diseases. We previously developed a new concept termed "Aberrant Cell Cycle Diseases" (ACCD), revealing that these two diseases share a common mechanism of aberrant cell cycle re-entry. The aberrant cell cycle re-entry is manifested as kinase/oncogene activation and tumor suppressor inactivation, which are hallmarks of both tumor growth in cancers and neuronal death in neurological disorders. Therefore, some cancer therapies (e.g., kinase inhibition, tumor suppressor elevation) can be leveraged for neurological treatments. The United States Food and Drug Administration (US FDA) has so far approved 74 kinase inhibitors, with numerous other kinase inhibitors in clinical trials, mostly for the treatment of cancers. In contrast, there are dire unmet needs of FDA-approved drugs for neurological treatments, such as Alzheimer's disease (AD), intracerebral hemorrhage (ICH), ischemic stroke (IS), traumatic brain injury (TBI), and others. In this review, we list these 74 FDA-approved kinase-targeted drugs and identify those that have been reported in preclinical and/or clinical trials for neurological disorders, with a purpose of discussing the feasibility and applicability of leveraging these cancer drugs (FDA-approved kinase inhibitors) for neurological treatments.

Endothelin-1, over-expressed in SOD1G93A mice, aggravates injury of NSC34-hSOD1G93A cells through complicated molecular mechanism revealed by quantitative proteomics analysis

Endothelin-1 (ET-1), a secreted signaling peptide, is suggested to be involved in multiple actions in various tissues including the brain, but its role in amyotrophic lateral sclerosis (ALS) remains unknown. In this study, we detected the expression changes as well as the cellular localization of ET-1, endothelin A (ET-A) and endothelin B (ET-B) receptors in spinal cord of transgenic SOD1-G93A (TgSOD1-G93A) mice, which showed that the two ET receptors (ET-Rs) expressed mainly on neurons and decreased as the disease progressed especially ET-B, while ET-1 expression was up-regulated and primarily localized on astrocytes. We then explored the possible mechanisms underlying the effect of ET-1 on cultured NSC34-hSOD1G93A cell model. ET-1 showed toxic effect on motor neurons (MNs), which can be rescued by the selective ET-A receptor antagonist BQ-123 or ET-B receptor antagonist BQ-788, suggesting that clinically used ET-Rs pan-antagonist could be a potential strategy for ALS. Using proteomic analysis, we revealed that 110 proteins were differentially expressed in NSC34-hSOD1G93A cells after ET-1 treatment, of which 54 were up-regulated and 56 were down-regulated. Bioinformatic analysis showed that the differentially expressed proteins (DEPs) were primarily enriched in hippo signaling pathway-multiple species, ABC transporters, ErbB signaling pathway and so on. These results provide further insights on the potential roles of ET-1 in ALS and present a new promising therapeutic target to protect MNs of ALS.

Activation of Neuroinflammation via mTOR Pathway is Disparately Regulated by Differential Target Multiplexed and Traditional Low-Rate Spinal Cord Stimulation in a Neuropathic Pain Model

Introduction

Spinal cord stimulation (SCS) has been used for decades to treat neuropathic pain conditions with limited understanding of its mechanisms of action. The mTOR pathway is a well-known co-factor in chronic pain and has not been previously linked to SCS therapy. Proteomic and phosphorylation analyses allow capturing a broad view of tissue response to an injury model and subsequent therapies such as SCS. Here, we evaluated the effect of differential target multiplexed SCS programming (DTMP) and traditional low-rate spinal cord stimulation (LR-SCS) on the mTOR pathway using proteomic and phosphoproteomic analyses.

Methods

The spared nerve injury (SNI) model of neuropathic pain in animals was established followed by continuous treatment with either DTMP or LR-SCS for 48 hours. Control groups included sham-stimulated (No-SCS) and uninjured animals (No-SNI). Proteins were extracted from spinal cord tissue removed post-stimulation and subjected to liquid chromatography/tandem mass spectrometry to assess changes in protein expression and states of phosphorylation. Bioinformatics tools and literature were used to identify mTOR-related proteins in the various groups.

Results

Over 7000 proteins were identified and filtered to find 1451 and 705 proteins significantly affected by DTMP and LR-SCS (p < 0.05), respectively, relative to No-SCS. Literature and bioinformatic tools yielded 192 mTOR-related proteins that were cross-referenced to the list of DTMP and LR-SCS affected proteins. Of these proteins, 49 were found in the proteomic dataset. Eight of these proteins showed a significant response to the pain model, 25 were significantly modulated by DTMP, and 8 by LR-SCS. Phosphoproteomic analyses yielded 119 mTOR-related phosphoproteins affected by the injury model with a 66% reversal following DTMP versus a 58% reversal by LR-SCS.

Conclusion

Proteomic and phosphoproteomic analyses support the hypothesis that DTMP, and to a lesser extent LR-SCS, reverse injury induced changes of the mTOR pathway while treating neuropathic pain.

Human Primary Astrocytes Differently Respond to Pro- and Anti-Inflammatory Stimuli

For a long time, astrocytes were considered a passive brain cell population. However, recently, many studies have shown that their role in the central nervous system (CNS) is more active. Previously, it was stated that there are two main functional phenotypes of astrocytes. However, nowadays, it is clear that there is rather a broad spectrum of these phenotypes. The major goal of this study was to evaluate the production of some inflammatory chemokines and neurotrophic factors by primary human astrocytes after pro- or anti-inflammatory stimulation. We observed that only astrocytes induced by inflammatory mediators TNFα/IL-1a/C1q produced CXCL10, CCL1, and CXCL13 chemokines. Unstimulated astrocytes and those cultured with anti-inflammatory cytokines (IL-4, IL-10, or TGF-β1) did not produce these chemokines. Interestingly, astrocytes cultured in proinflammatory conditions significantly decreased the release of neurotrophic factor PDGF-A, as compared to unstimulated astrocytes. However, in response to anti-inflammatory cytokine TGF-β1, astrocytes significantly increased PDGF-A production compared to the medium alone. The production of another studied neurotrophic factor BDNF was not influenced by pro- or anti-inflammatory stimulation. The secretory response was accompanied by changes in HLA-DR, CD83, and GFAP expression. Our study confirms that astrocytes differentially respond to pro- and anti-inflammatory stimuli, especially to inflammatory cytokines TNF-α, IL-1a, and C1q, suggesting their role in leukocyte recruitment.

The PI3K/AKT signalling pathway in inflammation, cell death and glial scar formation after traumatic spinal cord injury: Mechanisms and therapeutic opportunities

Objects

Traumatic spinal cord injury (TSCI) causes neurological dysfunction below the injured segment of the spinal cord, which significantly impacts the quality of life in affected patients. The phosphoinositide 3kinase/serine‐threonine kinase (PI3K/AKT) signaling pathway offers a potential therapeutic target for the inhibition of secondary TSCI. This review summarizes updates concerning the role of the PI3K/AKT pathway in TSCI.

Materials and Methods

By searching articles related to the TSCI field and the PI3K/AKT signaling pathway, we summarized the mechanisms of secondary TSCI and the PI3K/AKT signaling pathway; we also discuss current and potential future treatment methods for TSCI based on the PI3K/AKT signaling pathway.

Results

Early apoptosis and autophagy after TSCI protect the body against injury; a prolonged inflammatory response leads to the accumulation of pro‐inflammatory factors and excessive apoptosis, as well as excessive autophagy in the surrounding normal nerve cells, thus aggravating TSCI in the subacute stage of secondary injury. Initial glial scar formation in the subacute phase is a protective mechanism for TSCI, which limits the spread of damage and inflammation. However, mature scar tissue in the chronic phase hinders axon regeneration and prevents the recovery of nerve function. Activation of PI3K/AKT signaling pathway can inhibit the inflammatory response and apoptosis in the subacute phase after secondary TSCI; inhibiting this pathway in the chronic phase can reduce the formation of glial scar.

Conclusion

The PI3K/AKT signaling pathway has an important role in the recovery of spinal cord function after secondary injury. Inducing the activation of PI3K/AKT signaling pathway in the subacute phase of secondary injury and inhibiting this pathway in the chronic phase may be one of the potential strategies for the treatment of TSCI.

Apelin alleviated neuroinflammation and promoted endogenous neural stem cell proliferation and differentiation after spinal cord injury in rats

Background

Spinal cord injury (SCI) causes devastating neurological damage, including secondary injuries dominated by neuroinflammation. The role of Apelin, an endogenous ligand that binds the G protein-coupled receptor angiotensin-like receptor 1, in SCI remains unclear. Thus, our aim was to investigate the effects of Apelin in inflammatory responses and activation of endogenous neural stem cells (NSCs) after SCI.

Methods

Apelin expression was detected in normal and injured rats, and roles of Apelin in primary NSCs were examined. In addition, we used induced pluripotent stem cells (iPSCs) as a carrier to prolong the effective duration of Apelin and evaluate its effects in a rat model of SCI.

Results

Co-immunofluorescence staining suggested that Apelin was expressed in both astrocytes, neurons and microglia. Following SCI, Apelin expression decreased from 1 to 14 d and re-upregulated at 28 d. In vitro, Apelin promoted NSCs proliferation and differentiation into neurons. In vivo, lentiviral-transfected iPSCs were used as a carrier to prolong the effective duration of Apelin. Transplantation of transfected iPSCs in situ immediately after SCI reduced polarization of M1 microglia and A1 astrocytes, facilitated recovery of motor function, and promoted the proliferation and differentiation of endogenous NSCs in rats.

Conclusion

Apelin alleviated neuroinflammation and promoted the proliferation and differentiation of endogenous NSCs after SCI, suggesting that it might be a promising target for treatment of SCI.

The role of Wnt/mTOR signaling in spinal cord injury

Spinal cord injury (SCI) is the most common disabling spinal injury, a complex pathologic process that can eventually lead to severe neurological dysfunction. The Wnt/mTOR signaling pathway is a pervasive signaling cascade that regulates a wide range of physiological processes during embryonic development, from stem cell pluripotency to cell fate. Numerous studies have reported that Wnt/mTOR signaling pathway plays an important role in neural development, synaptogenesis, neuron growth, differentiation and survival after the central nervous system (CNS) is damaged. Wnt/mTOR also plays an important role in regulating various pathophysiological processes after spinal cord injury (SCI). After SCI, Wnt/mTOR signal regulates the physiological and pathological processes of neural stem cell proliferation and differentiation, neuronal axon regeneration, neuroinflammation and pain through multiple pathways. Due to the characteristics of the Wnt signal in SCI make it a potential therapeutic target of SCI. In this paper, the characteristics of Wnt/mTOR signal, the role of Wnt/mTOR pathway on SCI and related mechanisms are reviewed, and some unsolved problems are discussed. It is hoped to provide reference value for the research field of the role of Wnt/mTOR pathway in SCI, and provide a theoretical basis for biological therapy of SCI.

Reactive Astrocytes in Central Nervous System Injury: Subgroup and Potential Therapy

Traumatic central nervous system (CNS) injury, which includes both traumatic brain injury (TBI) and spinal cord injury (SCI), is associated with irreversible loss of neurological function and high medical care costs. Currently, no effective treatment exists to improve the prognosis of patients. Astrocytes comprise the largest population of glial cells in the CNS and, with the advancements in the field of neurology, are increasingly recognized as having key functions in both the brain and the spinal cord. When stimulated by disease or injury, astrocytes become activated and undergo a series of changes, including alterations in gene expression, hypertrophy, the loss of inherent functions, and the acquisition of new ones. Studies have shown that astrocytes are highly heterogeneous with respect to their gene expression profiles, and this heterogeneity accounts for their observed context-dependent phenotypic diversity. In the inured CNS, activated astrocytes play a dual role both as regulators of neuroinflammation and in scar formation. Identifying the subpopulations of reactive astrocytes that exert beneficial or harmful effects will aid in deciphering the pathological mechanisms underlying CNS injuries and ultimately provide a theoretical basis for the development of effective strategies for the treatment of associated conditions. Following CNS injury, as the disease progresses, astrocyte phenotypes undergo continuous changes. Although current research methods do not allow a comprehensive and accurate classification of astrocyte subpopulations in complex pathological contexts, they can nonetheless aid in understanding the roles of astrocytes in disease. In this review, after a brief introduction to the pathology of CNS injury, we summarize current knowledge regarding astrocyte activation following CNS injury, including: (a) the regulatory factors involved in this process; (b) the functions of different astrocyte subgroups based on the existing classification of astrocytes; and (c) attempts at astrocyte-targeted therapy.

Modulation of the IGF1R-MTOR pathway attenuates motor neuron toxicity of human ALS SOD1G93A astrocytes

ALS (amyotrophic lateral sclerosis), the most common motor neuron disease, causes muscle denervation and rapidly fatal paralysis. While motor neurons are the most affected cells in ALS, studies on the pathophysiology of the disease have highlighted the importance of non-cell autonomous mechanisms, which implicate astrocytes and other glial cells. In ALS, subsets of reactive astrocytes lose their physiological functions and become toxic for motor neurons, thereby contributing to disease pathogenesis. Evidence of astrocyte contribution to disease pathogenesis are well established in cellular and animal models of familial ALS linked to mutant SOD1, where astrocytes promote motor neuron cell death. The mechanism underlying astrocytes reactivity in conditions of CNS injury have been shown to involve the MTOR pathway. However, the role of this conserved metabolic signaling pathway, and the potential therapeutic effects of its modulation, have not been investigated in ALS astrocytes. Here, we show elevated activation of the MTOR pathway in human-derived astrocytes harboring mutant SOD1, which results in inhibition of macroautophagy/autophagy, increased cell proliferation, and enhanced astrocyte reactivity. We demonstrate that MTOR pathway activation in mutant SOD1 astrocytes is due to post-transcriptional upregulation of the IGF1R (insulin like growth factor 1 receptor), an upstream positive modulator of the MTOR pathway. Importantly, inhibition of the IGF1R-MTOR pathway decreases cell proliferation and reactivity of mutant SOD1 astrocytes, and attenuates their toxicity to motor neurons. These results suggest that modulation of astrocytic IGF1R-MTOR pathway could be a viable therapeutic strategy in SOD1 ALS and potentially other neurological diseases.Abbreviations: ACM: astrocyte conditioned medium; AKT: AKT serine/threonine kinase; ALS: amyotrophic lateral sclerosis; BrdU: thymidine analog 5-bromo-2'-deoxyuridine; CNS: central nervous system; EIF4EBP1/4EBP1: eukaryotic translation initiation factor 4E binding protein 1; GFAP: glial fibrillary acidic protein; IGF1R: insulin like growth factor 1 receptor; INSR: insulin receptor; iPSA: iPSC-derived astrocytes; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta;MTOR: mechanistic target of rapamycin kinase; NES: nestin; PPK1: 3-phosphoinositide dependent protein kinase 1; PI: propidium iodide; PPP: picropodophyllotoxin; PTEN: phosphatase and tensin homolog; S100B/S100β: S100 calcium binding protein B; SLC1A3/ EAAT1: solute carrier family 1 member 3; SMI-32: antibody to nonphosphorylated NEFH; SOD1: superoxide dismutase 1; TUBB3: tubulin beta 3 class III; ULK1: unc-51 like autophagy activating kinase 1.

PAF Physiology in Target Organ Systems—A Deep Dive to Understand the PAF Mystery in Pathogenesis of Disease

The purpose of this literature review is to gain an overview of the role of platelet-activating factor (PAF) within each of the body systems and how it contributes to normal and pathophysiological states. The review showed that there are multiple functions of PAF that are common to several body systems; however, there is little evidence to explain why PAF has this affect across multiple systems. Interestingly, there seems to be conflicting research as to whether PAF is an overall protective or pathogenic pathway. Within this research, it was found that there are different pathways depending on the specific body system, as well as between body systems. However, one universal function reported in the literature is of PAF as a pro-inflammatory molecule. Overall, this review identified five major functions of PAF: vasoconstriction, increased inflammation, vascular remodeling, increased edema, and endothelial permeability.

mTOR pathway: A potential therapeutic target for spinal cord injury

Spinal cord injury (SCI) is the most common disabling spinal injury, and the complex pathological process can eventually lead to severe neurological dysfunction. Many studies have reported that the mammalian target of rapamycin (mTOR) signaling pathway plays an important role in synaptogenesis, neuron growth, differentiation, and survival after central nervous system injury. It is also involved in various traumatic and central nervous system diseases, including traumatic brain injury, neonatal hypoxic-ischemic brain injury, Alzheimer's disease, Parkinson's disease, and cerebral apoplexy. mTOR has also been reported to play an important regulatory role in various pathophysiological processes following SCI. Activation of mTOR signals after SCI can regulate physiological and pathological processes, such as proliferation and differentiation of neural stem cells, regeneration of nerve axons, neuroinflammation, and glial scar formation, through various pathways. Inhibition of mTOR activity has been confirmed to promote repair in SCI. At present, many studies have reported that Chinese herbal medicine can inhibit the SCI-activated mTOR pathway to improve the microenvironment and promote nerve repair after SCI. Due to the role of the mTOR pathway in SCI, it may be a potential therapeutic target for SCI. This review is focused on the pathophysiological process of SCI, characteristics of the mTOR pathway, role of the mTOR pathway in SCI, role of inhibition of mTOR on SCI, and role and significance of inhibition of mTOR by related Chinese herbal medicine inhibitors in SCI. In addition, the review discusses the deficiencies and solutions to mTOR and SCI research shortcomings. This study hopes to provide reference for mTOR and SCI research and a theoretical basis for SCI biotherapy.

Astrocyte Role in Temporal Lobe Epilepsy and Development of Mossy Fiber Sprouting

Epilepsy affects approximately 50 million people worldwide, with 60% of adult epilepsies presenting an onset of focal origin. The most common focal epilepsy is temporal lobe epilepsy (TLE). The role of astrocytes in the presentation and development of TLE has been increasingly studied and discussed within the literature. The most common histopathological diagnosis of TLE is hippocampal sclerosis. Hippocampal sclerosis is characterized by neuronal cell loss within the Cornu ammonis and reactive astrogliosis. In some cases, mossy fiber sprouting may be observed. Mossy fiber sprouting has been controversial in its contribution to epileptogenesis in TLE patients, and the mechanisms surrounding the phenomenon have yet to be elucidated. Several studies have reported that mossy fiber sprouting has an almost certain co-existence with reactive astrogliosis within the hippocampus under epileptic conditions. Astrocytes are known to play an important role in the survival and axonal outgrowth of central and peripheral nervous system neurons, pointing to a potential role of astrocytes in TLE and associated cellular alterations. Herein, we review the recent developments surrounding the role of astrocytes in the pathogenic process of TLE and mossy fiber sprouting, with a focus on proposed signaling pathways and cellular mechanisms, histological observations, and clinical correlations in human patients.

Involvement of mTOR Pathways in Recovery from Spinal Cord Injury by Modulation of Autophagy and Immune Response

Traumatic spinal cord injury (SCI) is untreatable and remains the leading cause of disability. Neuroprotection and recovery after SCI can be partially achieved by rapamycin (RAPA) treatment, an inhibitor of mTORC1, complex 1 of the mammalian target of rapamycin (mTOR) pathway. However, mechanisms regulated by the mTOR pathway are not only controlled by mTORC1, but also by a second mTOR complex (mTORC2). Second-generation inhibitor, pp242, inhibits both mTORC1 and mtORC2, which led us to explore its therapeutic potential after SCI and compare it to RAPA treatment. In a rat balloon-compression model of SCI, the effect of daily RAPA (5 mg/kg; IP) and pp242 (5 mg/kg; IP) treatment on inflammatory responses and autophagy was observed. We demonstrated inhibition of the mTOR pathway after SCI through analysis of p-S6, p-Akt, and p-4E-BP1 levels. Several proinflammatory cytokines were elevated in pp242-treated rats, while RAPA treatment led to a decrease in proinflammatory cytokines. Both RAPA and pp242 treatments caused an upregulation of LC3B and led to improved functional and structural recovery in acute SCI compared to the controls, however, a greater axonal sprouting was seen following RAPA treatment. These results suggest that dual mTOR inhibition by pp242 after SCI induces distinct mechanisms and leads to recovery somewhat inferior to that following RAPA treatment.

New insights into the effects of vinpocetine against neurobehavioral comorbidities in a rat model of temporal lobe epilepsy via the downregulation of the hippocampal PI3K/mTOR signalling pathway

OBJECTIVES

As one of the most frequent worldwide neurological disorders, epilepsy is an alteration of the central nervous system (CNS) characterized by abnormal increases in neuronal electrical activity. The mammalian target of rapamycin (mTOR) signalling pathway has been investigated as an interesting objective in epilepsy research. Vinpocetine (VNP), a synthesized derivative of the apovincamine alkaloid, has been used in different cerebrovascular disorders. This study aimed to examine the modulatory effects of VNP on neurobehavioral comorbidities via the mTOR signalling pathway in a lithium-pilocarpine (Li-Pil) rat model of seizures.

METHODS

In male Wistar rats, seizures were induced with a single administration of pilocarpine (60 mg/kg; i.p.) 20 hours after the delivery of a single dose of lithium (3 mEq/kg; i.p.). VNP (10 mg/kg; i.p.) was administered daily for 14 consecutive days before Li-Pil administration.

KEY FINDINGS

VNP had a protective effect against Li-Pil-induced seizures. VNP improved both the locomotor and cognitive abilities, moreover, VNP exerted a neuroprotective action, as verified histologically and by its inhibitory effects on hippocampal glutamate excitotoxicity, mTOR pathway, and inflammatory and apoptotic parameters.

CONCLUSIONS

VNP is a valuable candidate for epilepsy therapy via its modulation of the mechanisms underlying epileptogenesis with emphasis on its modulatory effect on mTOR signalling pathway.

Metformin ameliorates the status epilepticus- induced hippocampal pathology through possible mTOR modulation

The initial precipitating injury such as SE progresses to chronic epilepsy through multiple epileptogenic processes. Early epileptogenic events are generally characterized by neuroinflammation, neurodegeneration and abnormal neurogenesis in the hippocampus. Metformin has exhibited anti-inflammatory and neuroprotective properties in numerous studies. The current study attempts to investigate the effect of metformin on seizure-induced inflammation and neuronal degeneration, and the involvement of the mTOR pathway. Status epilepticus (SE) was induced in male Wistar rats with systemic administration of Lithium (127 mg/kg) and Pilocarpine (30 mg/kg). In test rats, Metformin 100 mg/kg or 200 mg/kg was administered orally for 7 days, followed by SE induction. Results indicate that metformin did not alter the SE profile significantly which was evident by the behavioural scoring and electroencephalogram (EEG) recordings. However, metformin 200 mg/kg attenuated the SE-induced glial activation (p < 0.01), up regulated mRNA levels of proinflammatory cytokines (p < 0.001) and chemokines (p < 0.001) and enhanced BBB permeability (p < 0.05). In addition, metformin ameliorated the insult-induced region-specific neuronal damage (p < 0.01) and restored the hippocampal neuronal density. Metformin significantly inhibited phosphorylated S6 ribosomal protein (phospho-S6rp) (p < 0.05), thus demonstrating that the beneficial effects might be partly mediated by the mTOR pathway. The study thus reiterates that mTOR signalling is one of the mechanisms involved in inflammation and neurodegeneration in early epileptogenesis following SE.

Metformin: A Prospective Alternative for the Treatment of Chronic Pain

Metformin (biguanide) is a drug widely used for the treatment of type 2 diabetes. This drug has been used for 60 years as a highly effective antihyperglycemic agent. The search for the mechanism of action of metformin has produced an enormous amount of research to explain its effects on gluconeogenesis, protein metabolism, fatty acid oxidation, oxidative stress, glucose uptake, autophagy and pain, among others. It was only up the end of the 1990s and beginning of this century that some of its mechanisms were revealed. Metformin induces its beneficial effects in diabetes through the activation of a master switch kinase named AMP-activated protein kinase (AMPK). Two upstream kinases account for the physiological activation of AMPK: liver kinase B1 and calcium/calmodulin-dependent protein kinase kinase 2. Once activated, AMPK inhibits the mechanistic target of rapamycin complex 1 (mTORC1), which in turn avoids the phosphorylation of p70 ribosomal protein S6 kinase 1 and phosphatidylinositol 3-kinase/protein kinase B signaling pathways and reduces cap-dependent translation initiation. Since metformin is a disease-modifying drug in type 2 diabetes, which reduces the mTORC1 signaling to induce its effects on neuronal plasticity, it was proposed that these mechanisms could also explain the antinociceptive effect of this drug in several models of chronic pain. These studies have highlighted the efficacy of this drug in chronic pain, such as that from neuropathy, insulin resistance, diabetic neuropathy, and fibromyalgia-type pain. Mounting evidence indicates that chronic pain may induce anxiety, depression and cognitive impairment in rodents and humans. Interestingly, metformin is able to reverse some of these consequences of pathological pain in rodents. The purpose of this review was to analyze the current evidence about the effects of metformin in chronic pain and three of its comorbidities (anxiety, depression and cognitive impairment).

Reactive Astrogliosis: Implications in Spinal Cord Injury Progression and Therapy

Astrocytes are the most populous glial cells in the central nervous system (CNS). They are essential to CNS physiology and play important roles in the maintenance of homeostasis, development of synaptic plasticity, and neuroprotection. Nevertheless, under the influence of certain factors, astrocytes may also exert detrimental effects through a process of reactive astrogliosis. Previous studies have shown that astrocytes have more than one type of polarization. Two types have been extensively researched. One is a damaging change that occurs under inflammation and has been termed A1 astrocyte, while the other is a restorative change that occurs under ischemic induction and was termed A2 astrocyte. Researchers are now increasingly paying attention to the role of astrocytes in spinal cord injury (SCI), degenerative diseases, chronic pain, neurological tumors, and other CNS disorders. In this review, we discuss (a) the characteristics of polarized astrocytes, (b) the relationship between astrocyte polarization and SCI, and (c) new implications of reactive astrogliosis for future SCI therapies.

Protective Mechanism and Treatment of Neurogenesis in Cerebral Ischemia

Stroke is the fifth leading cause of death worldwide and is a main cause of disability in adults. Neither currently marketed drugs nor commonly used treatments can promote nerve repair and neurogenesis after stroke, and the repair of neurons damaged by ischemia has become a research focus. This article reviews several possible mechanisms of stroke and neurogenesis and introduces novel neurogenic agents (fibroblast growth factors, brain-derived neurotrophic factor, purine nucleosides, resveratrol, S-nitrosoglutathione, osteopontin, etc.) as well as other treatments that have shown neuroprotective or neurogenesis-promoting effects.

The Influence of Neuron-Extrinsic Factors and Aging on Injury Progression and Axonal Repair in the Central Nervous System

In the aging western population, the average age of incidence for spinal cord injury (SCI) has increased, as has the length of survival of SCI patients. This places great importance on understanding SCI in middle-aged and aging patients. Axon regeneration after injury is an area of study that has received substantial attention and made important experimental progress, however, our understanding of how aging affects this process, and any therapeutic effort to modulate repair, is incomplete. The growth and regeneration of axons is mediated by both neuron intrinsic and extrinsic factors. In this review we explore some of the key extrinsic influences on axon regeneration in the literature, focusing on inflammation and astrogliosis, other cellular responses, components of the extracellular matrix, and myelin proteins. We will describe how each element supports the contention that axonal growth after injury in the central nervous system shows an age-dependent decline, and how this may affect outcomes after a SCI.

Role of the PGE2 receptor in ischemia-reperfusion injury of the rat retina

Purpose

To investigate the function and expression of the PGE2 receptors EP1-4 in rat retinal ischemia-reperfusion (I/R) injury and to determine the regulatory role of resveratrol (RES) in this process.

Methods

In vitro, we stimulated primary astrocytes extracted from the optic disc of rats with epidermal growth factor (EGF) and RES, and detected the location of EP1-4 expression with immunofluorescence. The expression of antiglial fibrillary acidic protein (GFAP), EGF receptor (EGFR), inducible NOS (iNOS), and EP1-4 in astrocytes was detected with western blotting. In vivo, we established an I/R injury model and RES treatment model with Sprague-Dawley rats. Changes in the thickness of the inner retina were observed with hematoxylin and eosin (H&E) staining. EP1-4 localization in the retina was observed with immunohistochemistry. The expression of COX-2, iNOS, and EP1-4 in the control and model groups was detected with western blotting.

Results

In this study, immunofluorescence and immunohistochemistry showed that EP1-4 are expressed in astrocytes and the rat retina. EGF stimulation increased the expression of EGFR, iNOS, EP1, EP2, and EP4 in astrocytes. The expression of EP1-4 was statistically significantly increased on the third day after model induction, and EP1-4 expression decreased to normal levels on day 7. EGF and RES mediated the decrease in the expression of EP2. RES treatment significantly reduced retinal damage and RGC loss, as demonstrated by the relatively intact tissue structure on day 7 observed with H&E staining. Moreover, inflammation was associated with this I/R injury model, as demonstrated by the early induction of proinflammatory mediators, and this inflammation was significantly attenuated after RES treatment.

Conclusions

These results indicate that the COX-2/PGE2/EPs pathway is involved in retinal damage and astrocyte inflammation. In addition, the results suggest that the neuroprotective effects of RES may be associated with decreased production of inflammatory mediators. These results suggest that the PGE2 receptor may be a key factor in the treatment of neurodegenerative diseases, and that RES may be used as a possible therapeutic strategy for glaucoma.

Roles of mTOR Signaling in Tissue Regeneration

The mammalian target of rapamycin (mTOR), is a serine/threonine protein kinase and belongs to the phosphatidylinositol 3-kinase (PI3K)-related kinase (PIKK) family. mTOR interacts with other subunits to form two distinct complexes, mTORC1 and mTORC2. mTORC1 coordinates cell growth and metabolism in response to environmental input, including growth factors, amino acid, energy and stress. mTORC2 mainly controls cell survival and migration through phosphorylating glucocorticoid-regulated kinase (SGK), protein kinase B (Akt), and protein kinase C (PKC) kinase families. The dysregulation of mTOR is involved in human diseases including cancer, cardiovascular diseases, neurodegenerative diseases, and epilepsy. Tissue damage caused by trauma, diseases or aging disrupt the tissue functions. Tissue regeneration after injuries is of significance for recovering the tissue homeostasis and functions. Mammals have very limited regenerative capacity in multiple tissues and organs, such as the heart and central nervous system (CNS). Thereby, understanding the mechanisms underlying tissue regeneration is crucial for tissue repair and regenerative medicine. mTOR is activated in multiple tissue injuries. In this review, we summarize the roles of mTOR signaling in tissue regeneration such as neurons, muscles, the liver and the intestine.

Combinatorial tissue engineering partially restores function after spinal cord injury

Hydrogel scaffolds provide a beneficial microenvironment in transected rat spinal cord. A combinatorial biomaterials-based strategy provided a microenvironment that facilitated regeneration while reducing foreign body reaction to the three-dimensional spinal cord construct. We used poly lactic-co-glycolic acid microspheres to provide sustained release of rapamycin from Schwann cell (SC)-loaded, positively charged oligo-polyethylene glycol fumarate scaffolds. The biological activity and dose-release characteristics of rapamycin from microspheres alone and from microspheres embedded in the scaffold were determined in vitro. Three dose formulations of rapamycin were compared with controls in 53 rats. We observed a dose-dependent reduction in the fibrotic reaction to the scaffold and improved functional recovery over 6 weeks. Recovery was replicated in a second cohort of 28 animals that included retransection injury. Immunohistochemical and stereological analysis demonstrated that blood vessel number, surface area, vessel diameter, basement membrane collagen, and microvessel phenotype within the regenerated tissue was dependent on the presence of SCs and rapamycin. TRITC-dextran injection demonstrated enhanced perfusion into scaffold channels. Rapamycin also increased the number of descending regenerated axons, as assessed by Fast Blue retrograde axonal tracing. These results demonstrate that normalization of the neovasculature was associated with enhanced axonal regeneration and improved function after spinal cord transection.

In search of antiepileptogenic treatments for post-traumatic epilepsy

Post-traumatic epilepsy (PTE) is diagnosed in 20% of individuals with acquired epilepsy, and can impact significantly the quality of life due to the seizures and other functional or cognitive and behavioral outcomes of the traumatic brain injury (TBI) and PTE. There is no available antiepileptogenic or disease modifying treatment for PTE. Animal models of TBI and PTE have been developed, offering useful insights on the value of inflammatory, neurodegenerative pathways, hemorrhages and iron accumulation, calcium channels and other target pathways that could be used for treatment development. Most of the existing preclinical studies test efficacy towards pathologies of functional recovery after TBI, while a few studies are emerging testing the effects towards induced or spontaneous seizures. Here we review the existing preclinical trials testing new candidate treatments for TBI sequelae and PTE, and discuss future directions for efforts aiming at developing antiepileptogenic and disease-modifying treatments.

Minocycline Before Aortic Occlusion Reduces Hindlimb Motor Impairment, Attenuates Spinal Cord Damage and Spinal Astrocytosis, and Preserve Neuronal Cytoarchitecture in the Rat

OBJECTIVES

Spinal cord ischemia secondary to trauma or a vascular occlusive event is a threatening phenomenon. The neuroprotective properties of minocycline have been shown in several models of central nervous system diseases and after spinal cord ischemia; however, the benefit of using the drug requires additional confirmation in different animal models. Astrocytes are essential as regulators of neuronal functions and for providing nutrients. The authors hypothesized that astrocytes in the spinal cord may be an important target for minocycline action after ischemia and thus in the prevention of secondary spreading damage.

DESIGN

A prospective, randomized animal study.

SETTING

University research laboratory, single institution.

PARTICIPANTS

Adult male Sprague Dawley rats, weighing between 400 and 450 g.

INTERVENTIONS

A model of spinal cord ischemia in the rat was used for this study to determine whether a single, high-dose (10 mg/kg) of minocycline protects against damage to the neuronal cytoskeleton, both in the white and gray matter, and whether it reduces glial fibrillary acidic protein levels, which is an index for prevention of astrocyte activation during ischemia. Thirty minutes before thoracic aorta occlusion, minocycline was administered for 18 minutes using a 2 F Fogarty catheter.

MEASUREMENTS AND MAIN RESULTS

Minocycline given prophylactically significantly mitigated severe hindlimb motor impairment and reduced glial fibrillary acidic protein plus astrocytosis in both the white and gray matter of the spinal cord, caudal to the occlusion. Neuronal histologic cytoarchitecture, which was severely and significantly compromised in control animals, was preserved in the minocycline-treated animals.

CONCLUSIONS

This study's data imply that minocycline may attenuate reactive astrocytosis in response to injury with better neurologic outcome in a model of spinal cord ischemia in rats. The data suggest that future use of minocycline, clinically, might be advantageous in surgeries with a potential risk for paraplegia due to spinal cord ischemia.

Effects of rapamycin and curcumin on inflammation and oxidative stress in vitro and in vivo - in search of potential anti-epileptogenic strategies for temporal lobe epilepsy

Background

Previous studies in various rodent epilepsy models have suggested that mammalian target of rapamycin (mTOR) inhibition with rapamycin has anti-epileptogenic potential. Since treatment with rapamycin produces unwanted side effects, there is growing interest to study alternatives to rapamycin as anti-epileptogenic drugs. Therefore, we investigated curcumin, the main component of the natural spice turmeric. Curcumin is known to have anti-inflammatory and anti-oxidant effects and has been reported to inhibit the mTOR pathway. These properties make it a potential anti-epileptogenic compound and an alternative for rapamycin.

Methods

To study the anti-epileptogenic potential of curcumin compared to rapamycin, we first studied the effects of both compounds on mTOR activation, inflammation, and oxidative stress in vitro, using cell cultures of human fetal astrocytes and the neuronal cell line SH-SY5Y. Next, we investigated the effects of rapamycin and intracerebrally applied curcumin on status epilepticus (SE)—induced inflammation and oxidative stress in hippocampal tissue, during early stages of epileptogenesis in the post-electrical SE rat model for temporal lobe epilepsy (TLE).

Results

Rapamycin, but not curcumin, suppressed mTOR activation in cultured astrocytes. Instead, curcumin suppressed the mitogen-activated protein kinase (MAPK) pathway. Quantitative real-time PCR analysis revealed that curcumin, but not rapamycin, reduced the levels of inflammatory markers IL-6 and COX-2 in cultured astrocytes that were challenged with IL-1β. In SH-SY5Y cells, curcumin reduced reactive oxygen species (ROS) levels, suggesting anti-oxidant effects. In the post-SE rat model, however, treatment with rapamycin or curcumin did not suppress the expression of inflammatory and oxidative stress markers 1 week after SE.

Conclusions

These results indicate anti-inflammatory and anti-oxidant properties of curcumin, but not rapamycin, in vitro. Intracerebrally applied curcumin modified the MAPK pathway in vivo at 1 week after SE but failed to produce anti-inflammatory or anti-oxidant effects. Future studies should be directed to increasing the bioavailability of curcumin (or related compounds) in the brain to assess its anti-epileptogenic potential in vivo.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1247-9) contains supplementary material, which is available to authorized users.

Rapamycin mediates mTOR signaling in reactive astrocytes and reduces retinal ganglion cell loss

Damage and loss of retinal ganglion cells (RGCs) can cause visual impairment. The underlying molecular mechanisms that mediate RGC death in ischemic retinal diseases are still unclear. In this study, we sought to understand the neuroprotective effect of rapamycin, the selective inhibitor of mTORC1, on RGC survival and the cellular mechanics that mediate this effect. Recent studies have reported that the epidermal growth factor (EGF) receptor shows an increase in expression in astrocytes after injury, and this receptor can promote their transformation into reactive astrocytes. Our results, along with previous works from others, show the colocalization of phosphor-EGF receptors with the astrocyte marker glial fibrillary acidic proteins in reactive astrocytes in the injured retina. In our in vitro studies, using primary astrocyte cultures of the optic nerve head of rats, showed that rapamycin significantly blocked EGF-induced mTOR signaling mainly through the PI3K/Akt pathway in primary astrocytes, but not through the MAPK/Erk pathway. Additionally, rapamycin dramatically inhibited the activation of mTOR signaling in our ratinal ischemia-reperfusion (I/R) injury model in vivo. Astrocyte activation was assessed by immunostaining retinal flat mounts or cross sections with antibody against GFAP, and we also used western blots to detect the expression of GFAP. Taken together, these results revealed that rapamycin decreases the activation of astrocytes after retinal ischemia-reperfusion injury. Furthermore, rapamycin can improve retinal RGC survival in rats during I/R, as detected by FluoroGold labeling. Our data reveals the neuroprotective effects of rapamycin in an experimental retina injury model, possibly through decreasing glial-dependent intracellular signaling mechanisms for suppressing apoptosis of RGCs. Our study also presents an approach to targeting reactive astrocytes for the treatment of optic neurodegenerations.

A Single Dose of Atorvastatin Applied Acutely after Spinal Cord Injury Suppresses Inflammation, Apoptosis, and Promotes Axon Outgrowth, Which Might Be Essential for Favorable Functional Outcome

The aim of our study was to limit the inflammatory response after a spinal cord injury (SCI) using Atorvastatin (ATR), a potent inhibitor of cholesterol biosynthesis. Adult Wistar rats were divided into five experimental groups: one control group, two Th9 compression (40 g/15 min) groups, and two Th9 compression + ATR (5 mg/kg, i.p.) groups. The animals survived one day and six weeks. ATR applied in a single dose immediately post-SCI strongly reduced IL-1β release at 4 and 24 h and considerably reduced the activation of resident cells at one day post-injury. Acute ATR treatment effectively prevented the excessive infiltration of destructive M1 macrophages cranially, at the lesion site, and caudally (by 66%, 62%, and 52%, respectively) one day post-injury, whereas the infiltration of beneficial M2 macrophages was less affected (by 27%, 41%, and 16%). In addition, at the same time point, ATR visibly decreased caspase-3 cleavage in neurons, astrocytes, and oligodendrocytes. Six weeks post-SCI, ATR increased the expression of neurofilaments in the dorsolateral columns and Gap43-positive fibers in the lateral columns around the epicenter, and from day 30 to 42, significantly improved the motor activity of the hindlimbs. We suggest that early modulation of the inflammatory response via effects on the M1/M2 macrophages and the inhibition of caspase-3 expression could be crucial for the functional outcome.

Beneficial Effects of Resveratrol-Mediated Inhibition of the mTOR Pathway in Spinal Cord Injury

Spinal cord injury (SCI) causes a high rate of morbidity and disability. The clinical features of SCI are divided into acute, subacute, and chronic phases according to its pathophysiological events. The mammalian target of rapamycin (mTOR) signaling pathway plays an important role in cell death and inflammation in the acute phase and neuroregeneration in the subacute/chronic phases at different times. Resveratrol has the potential of regulating cell growth, proliferation, metabolism, and angiogenesis through the mTOR signaling pathway. Herein, we explicate the role of resveratrol in the repair of SCI through the inhibition of the mTOR signaling pathway. The inhibition of the mTOR pathway by resveratrol has the potential of serving as a neuronal restorative mechanism following SCI.

PTEN negatively regulates the cell lineage progression from NG2+ glial progenitor to oligodendrocyte via mTOR-independent signaling

Oligodendrocytes (OLs), the myelin-forming CNS glia, are highly vulnerable to cellular stresses, and a severe myelin loss underlies numerous CNS disorders. Expedited OL regeneration may prevent further axonal damage and facilitate functional CNS repair. Although adult OL progenitors (OPCs) are the primary players for OL regeneration, targetable OPC-specific intracellular signaling mechanisms for facilitated OL regeneration remain elusive. Here, we report that OPC-targeted PTEN inactivation in the mouse, in contrast to OL-specific manipulations, markedly promotes OL differentiation and regeneration in the mature CNS. Unexpectedly, an additional deletion of mTOR did not reverse the enhanced OL development from PTEN-deficient OPCs. Instead, ablation of GSK3β, another downstream signaling molecule that is negatively regulated by PTEN-Akt, enhanced OL development. Our results suggest that PTEN persistently suppresses OL development in an mTOR-independent manner, and at least in part, via controlling GSK3β activity. OPC-targeted PTEN-GSK3β inactivation may benefit facilitated OL regeneration and myelin repair.

A role for ErbB signaling in the induction of reactive astrogliosis

Reactive astrogliosis is a hallmark of many neurological disorders, yet its functions and molecular mechanisms remain elusive. Particularly, the upstream signaling that regulates pathological responses of astrocytes is largely undetermined. We used a mouse traumatic brain injury model to induce astrogliosis and revealed activation of ErbB receptors in reactive astrocytes. Moreover, cell-autonomous inhibition of ErbB receptor activity in reactive astrocytes by a genetic approach suppressed hypertrophic remodeling possibly through the regulation of actin dynamics. However, inhibiting ErbB signaling in reactive astrocytes did not affect astrocyte proliferation after brain injury, although it aggravated local inflammation. In contrast, active ErbB signaling in mature astrocytes of various brain regions in mice was sufficient to initiate reactive responses, reproducing characterized molecular and cellular features of astrogliosis observed in injured or diseased brains. Further, prevalent astrogliosis in the brain induced by astrocytic ErbB activation caused anorexia in animals. Therefore, our findings defined an unrecognized role of ErbB signaling in inducing reactive astrogliosis. Mechanistically, inhibiting ErbB signaling in reactive astrocytes prominently reduced Src and focal adhesion kinase (FAK) activity that is important for actin remodeling, although ErbB signaling activated multiple downstream signaling proteins. The discrepancies between the results from loss- and gain-of-function studies indicated that ErbB signaling regulated hypertrophy and proliferation of reactive astrocytes by different downstream signaling pathways. Our work demonstrated an essential mechanism in the pathological regulation of astrocytes and provided novel insights into potential therapeutic targets for astrogliosis-implicated diseases.

Disorders of Astrocytes: Alexander Disease as a Model

Astrocytes undergo important phenotypic changes in many neurological disorders, including strokes, trauma, inflammatory diseases, infectious diseases, and neurodegenerative diseases. We have been studying the astrocytes of Alexander disease (AxD), which is caused by heterozygous mutations in the GFAP gene, which is the gene that encodes the major astrocyte intermediate filament protein. AxD is a primary astrocyte disease because GFAP expression is specific to astrocytes in the central nervous system (CNS). The accumulation of extremely large amounts of GFAP causes many molecular changes in astrocytes, including proteasome inhibition, stress kinase activation, mechanistic target of rapamycin (mTOR) activation, loss of glutamate and potassium buffering capacity, loss of astrocyte coupling, and changes in cell morphology. Many of these changes appear to be common to astrocyte reactions in other neurological disorders. Using AxD to illuminate common mechanisms, we discuss the molecular pathology of AxD astrocytes and compare that to astrocyte pathology in other disorders.

Rheb in neuronal degeneration, regeneration, and connectivity

The small GTPase Rheb was originally detected as an immediate early response protein whose expression was induced by NMDA-dependent synaptic activity in the brain. Rheb's activity is highly regulated by its GTPase activating protein (GAP), the tuberous sclerosis complex protein, which stimulates the conversion from the active, GTP-loaded into the inactive, GDP-loaded conformation. Rheb has been established as an evolutionarily conserved molecular switch protein regulating cellular growth, cell volume, cell cycle, autophagy, and amino acid uptake. The subcellular localization of Rheb and its interacting proteins critically regulate its activity and function. In stem cells, constitutive activation of Rheb enhances differentiation at the expense of self-renewal partially explaining the adverse effects of deregulated Rheb in the mammalian brain. In the context of various cellular stress conditions such as oxidative stress, ER-stress, death factor signaling, and cellular aging, Rheb activation surprisingly enhances rather than prevents cellular degeneration. This review addresses cell type- and cell state-specific function(s) of Rheb and mainly focuses on neurons and their surrounding glial cells. Mechanisms will be discussed in the context of therapy that interferes with Rheb's activity using the antibiotic rapamycin or low molecular weight compounds.

The MNK-eIF4E Signaling Axis Contributes to Injury-Induced Nociceptive Plasticity and the Development of Chronic Pain

Injury-induced sensitization of nociceptors contributes to pain states and the development of chronic pain. Inhibiting activity-dependent mRNA translation through mechanistic target of rapamycin and mitogen-activated protein kinase (MAPK) pathways blocks the development of nociceptor sensitization. These pathways convergently signal to the eukaryotic translation initiation factor (eIF) 4F complex to regulate the sensitization of nociceptors, but the details of this process are ill defined. Here we investigated the hypothesis that phosphorylation of the 5' cap-binding protein eIF4E by its specific kinase MAPK interacting kinases (MNKs) 1/2 is a key factor in nociceptor sensitization and the development of chronic pain. Phosphorylation of ser209 on eIF4E regulates the translation of a subset of mRNAs. We show that pronociceptive and inflammatory factors, such as nerve growth factor (NGF), interleukin-6 (IL-6), and carrageenan, produce decreased mechanical and thermal hypersensitivity, decreased affective pain behaviors, and strongly reduced hyperalgesic priming in mice lacking eIF4E phosphorylation (eIF4E^S209A ). Tests were done in both sexes, and no sex differences were found. Moreover, in patch-clamp electrophysiology and Ca²⁺ imaging experiments on dorsal root ganglion neurons, NGF- and IL-6-induced increases in excitability were attenuated in neurons from eIF4E^S209A mice. These effects were recapitulated in Mnk1/2^-/- mice and with the MNK1/2 inhibitor cercosporamide. We also find that cold hypersensitivity induced by peripheral nerve injury is reduced in eIF4E^S209A and Mnk1/2^-/- mice and following cercosporamide treatment. Our findings demonstrate that the MNK1/2-eIF4E signaling axis is an important contributing factor to mechanisms of nociceptor plasticity and the development of chronic pain.SIGNIFICANCE STATEMENT Chronic pain is a debilitating disease affecting approximately one in three Americans. Chronic pain is thought to be driven by changes in the excitability of peripheral nociceptive neurons, but the precise mechanisms controlling these changes are not elucidated. Emerging evidence demonstrates that mRNA translation regulation pathways are key factors in changes in nociceptor excitability. Our work demonstrates that a single phosphorylation site on the 5' cap-binding protein eIF4E is a critical mechanism for changes in nociceptor excitability that drive the development of chronic pain. We reveal a new mechanistic target for the development of a chronic pain state and propose that targeting the upstream kinase, MAPK interacting kinase 1/2, could be used as a therapeutic approach for chronic pain.

"mTOR Signaling Pathway": A Potential Target of Curcumin in the Treatment of Spinal Cord Injury

The purpose of this review is to discuss the possibility of the treatment of spinal cord injury (SCI) with curcumin via regulating the mTOR signaling pathway, which may provide another strong support for curcumin to be a promising medicine applied to the treatment of SCI. Curcumin is termed as a multifunctional targeting therapy drug that regulates the mTOR signaling pathway in the treatment of numerous diseases. Previous research has already revealed that mTOR signaling pathway plays a vital role in prognosis, which involves the axon regeneration and autophagy. This review discusses a potential mechanism that curcumin suppresses the activation of this pathway and ameliorates the microenvironment of axons regeneration which would provide a new way that induces autophagy appropriately.

The Unwanted Cell Migration in the Brain: Glioma Metastasis

Cell migration is identified as a highly orchestrated process. It is a fundamental and essential phenomenon underlying tissue morphogenesis, wound healing, and immune response. Under dysregulation, it contributes to cancer metastasis. Brain is considered to be the most complex organ in human body containing many types of neural cells with astrocytes playing crucial roles in monitoring both physiological and pathological functions. Astrocytoma originates from astrocytes and its most malignant type is glioblastoma multiforme (WHO Grade IV astrocytoma), which is capable to infiltrate widely into the neighboring brain tissues making a complete resection of tumors impossible. Very recently, we have reviewed the mechanisms for astrocytes in migration. Given the fact that astrocytoma shares many histological features with astrocytes, we therefore attempt to review the mechanisms for glioma cells in migration and compare them to normal astrocytes, hoping to obtain a better insight into the dysregulation of migratory mechanisms contributing to their metastasis in the brain.

BAMBI inhibits inflammation through the activation of autophagy in experimental spinal cord injury

Autophagy plays an important role in the progression of spinal cord injury (SCI). In this study, we aimed to examine the effects and potential mechanisms of action of BMP and activin membrane-bound inhibitor (BAMBI) in the progression of SCI. A rat model of SCI was established and the rats were injected with pLentiH1-BAMBI shRNA and pAd-BAMBI in the gray and white matter of the spinal cord at T8. After 14 days, motor function evaluation was measured according to the Basso Beattie Bresnahan (BBB) method and the number of motor neuron cell accounts in the anterior horns was measured by Nissl staining. The protein expression of levels light chain 3B (LC3B), Beclin 1, Bim and p62 were measured by western blot analysis. The concentrations of interleukin (IL)-1β, IL-6 and IL-10 were measured by ELISA. The results revealed that BAMBI expression was significantly decreased in the rats with SCI. The BBB scores and the number of motor neuron cell accounts in the anterior horns were significantly decreased in the pLentiH1-BAMBI shRNA injection group, and were increased in the pAd-BAMBI injection group, suggesting a neuroprotective effect of BAMBI in SCI. Moreover, the increased expression levels of LC3B and Beclin 1, and the decreased expression of Bim and p62 indicated that autophagy was significantly induced in the pAd-BAMBI injection group. Moreover, the overexpression of BAMBI also decreased the expression of transforming growth factor-β (TGF-β) and mammalian target of rapamycin (mTOR), and decreased the concentrations of IL-1β, IL-6 and IL-10. These results indicate that BAMBI plays a neuroprotective role by decreasing inflammation and activating autophagy in rats with SCI.

MiR-17 targets PTEN and facilitates glial scar formation after spinal cord injuries via the PI3K/Akt/mTOR pathway

OBJECTIVES

We attempted to discover the regulatory role of miR-17 and PTEN in glial scar formation accompanied with spinal cord injuries.

METHODS

We established a spinal cord injury (SCI) model in mice which were transfected with different groups of adenoviruses: miR-17 mimics, miR-17 inhibitors and PTEN cDNAs. The improvement of hind limb functions was assessed using the 21-point Basso-Beattie-Bresnahan (BBB) locomotion scale. Immunohistochemistry was used to detect the expression levels of glial fibrillary acidic protein (GFAP), Vimentin and neurofilaments. The expression of miR-17 was quantified using Real time-PCR (RT-PCR). Western blot was conducted to detect the expressions of PTEN, PI3K, Akt, mTOR and S6. Finally, dual luciferase reporter gene assay was conducted to confirm the target relationship between miR-17 and PTEN.

RESULTS

The model group exhibited significantly increased expression levels of GFAP, Vimentin, miR-17, PTEN, PI3K, Akt and mTOR. The above trend was enhanced by the transfection of miR-17 mimics (P<0.05). By contrast, the transfection of miR-17 inhibitors significantly down-regulated the expression of GFAP, Vimentin, PTEN, PI3K, Akt, mTOR and p-S6 whereas the expression of GFAP, Vimentin, PI3K, Akt, mTOR and p-S6 in the cells transfected with PTEN cDNAs significantly decreased (P<0.05). Also, the transfection of miR-17 inhibitors and PTEN cDNAs alleviated the astrogliosis in SCI lesions, contributed to the regeneration of nerve filament and improved the functional recovery of the hind limb of mice. Finally, the targeting relationship between miR-17 and PTEN was verified by the dual luciferase reporter gene assay.

CONCLUSION

MiR-17 is able to target PTEN and stimulate the PI3K/Akt/mTOR pathway. The formation of glial scar resulted from spinal cord injuries can be reduced either by inhibiting miR-17 or by overexpressing PTEN.

Current Understanding of Platelet-Activating Factor Signaling in Central Nervous System Diseases

Platelet-activating factor (PAF) is a bioactive lipid mediator which serves as a reciprocal messenger between the immune and nervous systems. PAF, a pluripotent inflammatory mediator, is extensively expressed in many cells and tissues and has either beneficial or detrimental effects on the progress of inflammation-related neuropathology. Its wide distribution and various biological functions initiate a cascade of physiological or pathophysiological responses during development or diseases. Current evidence indicates that excess PAF accumulation in CNS diseases exacerbates the inflammatory response and pathological consequences, while application of PAF inhibitors or PAFR antagonists by blocking this signaling pathway significantly reduces inflammation, protects cells, and improves the recovery of neural functions. In this review, we integrate the current findings of PAF signaling in CNS diseases and elucidate topics less appreciated but important on the role of PAF signaling in neurological diseases. We propose that the precise use of PAF inhibitors or PAFR antagonists that target the specific neural cells during the appropriate temporal window may constitute a potential therapy for CNS diseases.

Modeling Alexander disease with patient iPSCs reveals cellular and molecular pathology of astrocytes

Alexander disease is a fatal neurological illness characterized by white-matter degeneration and formation of Rosenthal fibers, which contain glial fibrillary acidic protein as astrocytic inclusion. Alexander disease is mainly caused by a gene mutation encoding glial fibrillary acidic protein, although the underlying pathomechanism remains unclear. We established induced pluripotent stem cells from Alexander disease patients, and differentiated induced pluripotent stem cells into astrocytes. Alexander disease patient astrocytes exhibited Rosenthal fiber-like structures, a key Alexander disease pathology, and increased inflammatory cytokine release compared to healthy control. These results suggested that Alexander disease astrocytes contribute to leukodystrophy and a variety of symptoms as an inflammatory source in the Alexander disease patient brain. Astrocytes, differentiated from induced pluripotent stem cells of Alexander disease, could be a cellular model for future translational medicine.

Neuronal RARβ Signaling Modulates PTEN Activity Directly in Neurons and via Exosome Transfer in Astrocytes to Prevent Glial Scar Formation and Induce Spinal Cord Regeneration

Failure of axonal regeneration in the central nervous system (CNS) is mainly attributed to a lack of intrinsic neuronal growth programs and an inhibitory environment from a glial scar. Phosphatase and tensin homolog (PTEN) is a major negative regulator of neuronal regeneration and, as such, inhibiting its activity has been considered a therapeutic target for spinal cord (SC) injuries (SCIs). Using a novel model of rat cervical avulsion, we show that treatment with a retinoic acid receptor β (RARβ) agonist results in locomotor and sensory recovery. Axonal regeneration from the severed roots into the SC could be seen by biotinylated dextran amine labeling. Light micrographs of the dorsal root entry zone show the peripheral nervous system (PNS)-CNS transition of regrown axons. RARβ agonist treatment also resulted in the absence of scar formation. Mechanism studies revealed that, in RARβ-agonist-treated neurons, PTEN activity is decreased by cytoplasmic phosphorylation and increased secretion in exosomes. These are taken up by astrocytes, resulting in hampered proliferation and causing them to arrange in a normal-appearing scaffold around the regenerating axons. Attribution of the glial modulation to neuronal PTEN in exosomes was demonstrated by the use of an exosome inhibitor in vivo and PTEN siRNA in vitro assays. The dual effect of RARβ signaling, both neuronal and neuronal-glial, results in axonal regeneration into the SC after dorsal root neurotmesis. Targeting this pathway may open new avenues for the treatment of SCIs.

SIGNIFICANCE STATEMENT

Spinal cord injuries (SCIs) often result in permanent damage in the adult due to the very limited capacity of axonal regeneration. Intrinsic neuronal programs and the formation of a glial scar are the main obstacles. Here, we identify a single target, neuronal retinoic acid receptor β (RARβ), which modulates these two aspects of the postinjury physiological response. Activation of RARβ in the neuron inactivates phosphatase and tensin homolog and induces its transfer into the astrocytes in small vesicles, where it prevents scar formation. This may open new therapeutic avenues for SCIs.

The role of the PI3K/Akt/mTOR pathway in glial scar formation following spinal cord injury

Several studies suggest that glial scars pose as physical and chemical barriers that limit neurite regeneration after spinal cord injury (SCI). Evidences suggest that the activation of the PI3K/Akt/mTOR signaling pathway is involved in glial scar formation. Therefore, inhibition of the PI3K/Akt/mTOR pathway may beneficially attenuate glial scar formation after SCI. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulates the PI3K/Akt/mTOR pathway. Therefore, we hypothesized that the overexpression of PTEN in the spinal cord will have beneficial effects after SCI. In the present study, we intrathecally injected a recombinant adenovirus carrying the pten gene (Ad-PTEN) to cause overexpression of PTEN in rats with contusion injured spinal cords. The results suggest overexpression of PTEN in spinal cord attenuated glial scar formation and led to improved locomotor function after SCI. Overexpression of PTEN following SCI attenuated gliosis, affected chondroitin sulfate proteoglycan expression, and improved axon regeneration into the lesion site. Furthermore, we suggest that the activation of the PI3K/Akt/mTOR pathway in astrocytes at 3 days after SCI may be involved in glial scar formation. Because delayed treatment with Ad-PTEN enhanced motor function recovery more significantly than immediate treatment with Ad-PTEN after SCI, the results suggest that the best strategy to attenuate glial scar formation could be to introduce 3 days after SCI. This study's findings thus have positive implications for patients who are unable to receive immediate medical attention after SCI.

Deletion of mTOR in Reactive Astrocytes Suppresses Chronic Seizures in a Mouse Model of Temporal Lobe Epilepsy

Germline and somatic mutations in key genes of the mammalian target of rapamycin (mTOR) pathway have been identified in seizure-associated disorders. mTOR mutations lead to aberrant activation of mTOR signaling, and, although affected neurons are critical for epileptogenesis, the role of mTOR activation in glial cells remains poorly understood. We previously reported a consistent activation of the mTOR pathway in astrocytes in the epileptic foci of temporal lobe epilepsy. In this study, it was demonstrated that mTOR deletion from reactive astrocytes prevents increases in seizure frequency over the disease course. By using a tamoxifen-inducible mTOR conditional knockout system and kainic acid, a model was developed that allowed astrocyte-specific mTOR gene deletion in mice with chronic epilepsy. Animals in which mTOR was deleted from 44 % of the astrocyte population exhibited a lower seizure frequency compared with controls. Down-regulation of mTOR significantly ameliorated astrogliosis in the sclerotic hippocampus but did not rescue mossy fiber sprouting. In cultured astrocytes, the mTOR pathway modulated the stability of the astroglial glutamate transporter 1 (Glt1) and influenced the ability of astrocytes to remove extracellular glutamate. Taken together, these data indicate that astrocytes with activated mTOR signaling may provide conditions that are favorable for spontaneous recurrent seizures.