Selective overexpression of excitatory amino acid transporter 2 (EAAT2) in astrocytes enhances neuroprotection from moderate but not severe hypoxia-ischemia

Reinventing the Penumbra - the Emerging Clockwork of a Multi-modal Mechanistic Paradigm

The concept of the ischemic penumbra was originally defined as the area around a necrotic stroke core and seen as the tissue at imminent risk of further damage. Today, the penumbra is generally considered as time-sensitive hypoperfused brain tissue with decreased oxygen and glucose availability, salvageable tissue as treated by intervention, and the potential target for neuroprotection in focal stroke. The original concept entailed electrical failure and potassium release but one short of neuronal cell death and was based on experimental stroke models, later confirmed in clinical imaging studies. However, even though the basic mechanisms have translated well, conferring brain protection, and improving neurological outcome after stroke based on the pathophysiological mechanisms in the penumbra has yet to be achieved. Recent findings shape the modern understanding of the penumbra revealing a plethora of molecular and cellular pathophysiological mechanisms. We now propose a new model of the penumbra, one which we hope will lay the foundation for future translational success. We focus on the availability of glucose, the brain's central source of energy, and bioenergetic failure as core pathophysiological concepts. We discuss the relation of mitochondrial function in different cell types to bioenergetics and apoptotic cell death mechanisms, autophagy, and neuroinflammation, to glucose metabolism in what is a dynamic ischemic penumbra.

Neuronal MML-1/MXL-2 regulates systemic aging via glutamate transporter and cell nonautonomous autophagic and peroxidase activity

Accumulating evidence has demonstrated the presence of intertissue-communication regulating systemic aging, but the underlying molecular network has not been fully explored. We and others previously showed that two basic helix-loop-helix transcription factors, MML-1 and HLH-30, are required for lifespan extension in several longevity paradigms, including germlineless Caenorhabditis elegans. However, it is unknown what tissues these factors target to promote longevity. Here, using tissue-specific knockdown experiments, we found that MML-1 and its heterodimer partners MXL-2 and HLH-30 act primarily in neurons to extend longevity in germlineless animals. Interestingly, however, the downstream cascades of MML-1 in neurons were distinct from those of HLH-30. Neuronal RNA interference (RNAi)-based transcriptome analysis revealed that the glutamate transporter GLT-5 is a downstream target of MML-1 but not HLH-30. Furthermore, the MML-1-GTL-5 axis in neurons is critical to prevent an age-dependent collapse of proteostasis and increased oxidative stress through autophagy and peroxidase MLT-7, respectively, in long-lived animals. Collectively, our study revealed that systemic aging is regulated by a molecular network involving neuronal MML-1 function in both neural and peripheral tissues.

MicroRNA-mediated regulation of reactive astrocytes in central nervous system diseases

Astrocytes (AST) are abundant glial cells in the human brain, accounting for approximately 20-50% percent of mammalian central nervous system (CNS) cells. They display essential functions necessary to sustain the physiological processes of the CNS, including maintaining neuronal structure, forming the blood-brain barrier, coordinating neuronal metabolism, maintaining the extracellular environment, regulating cerebral blood flow, stabilizing intercellular communication, participating in neurotransmitter synthesis, and defending against oxidative stress et al. During the pathological development of brain tumors, stroke, spinal cord injury (SCI), neurodegenerative diseases, and other neurological disorders, astrocytes undergo a series of highly heterogeneous changes, which are called reactive astrocytes, and mediate the corresponding pathophysiological process. However, the pathophysiological mechanisms of reactive astrocytes and their therapeutic relevance remain unclear. The microRNAs (miRNAs) are essential for cell differentiation, proliferation, and survival, which play a crucial role in the pathophysiological development of CNS diseases. In this review, we summarize the regulatory mechanism of miRNAs on reactive astrocytes in CNS diseases, which might provide a theoretical basis for the diagnosis and treatment of CNS diseases.

The role of excitatory amino acid transporter 2 (EAAT2) in epilepsy and other neurological disorders

Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS). Excitatory amino acid transporters (EAATs) have important roles in the uptake of glutamate and termination of glutamatergic transmission. Up to now, five EAAT isoforms (EAAT1-5) have been identified in mammals. The main focus of this review is EAAT2. This protein has an important role in the pathoetiology of epilepsy. De novo dominant mutations, as well as inherited recessive mutation in this gene, have been associated with epilepsy. Moreover, dysregulation of this protein is implicated in a range of neurological diseases, namely amyotrophic lateral sclerosis, alzheimer's disease, parkinson's disease, schizophrenia, epilepsy, and autism. In this review, we summarize the role of EAAT2 in epilepsy and other neurological disorders, then provide an overview of the therapeutic modulation of this protein.

The Role of DNA Methylation in Stroke Recovery

Epigenetic alterations affect the onset of ischemic stroke, brain injury after stroke, and mechanisms of poststroke recovery. In particular, DNA methylation can be dynamically altered by maintaining normal brain function or inducing abnormal brain damage. DNA methylation is regulated by DNA methyltransferase (DNMT), which promotes methylation, DNA demethylase, which removes methyl groups, and methyl-cytosine–phosphate–guanine-binding domain (MBD) protein, which binds methylated DNA and inhibits gene expression. Investigating the effects of modulating DNMT, TET, and MBD protein expression on neuronal cell death and neurorepair in ischemic stroke and elucidating the underlying mechanisms can facilitate the formulation of therapeutic strategies for neuroprotection and promotion of neuronal recovery after stroke. In this review, we summarize the role of DNA methylation in neuroprotection and neuronal recovery after stroke according to the current knowledge regarding the effects of DNA methylation on excitotoxicity, oxidative stress, apoptosis, neuroinflammation, and recovery after ischemic stroke. This review of the literature regarding the role of DNA methylation in neuroprotection and functional recovery after stroke may contribute to the development and application of novel therapeutic strategies for stroke.

Activation of astroglial CB1R mediates cerebral ischemic tolerance induced by electroacupuncture

There are no effective treatments for stroke. The activation of endogenous protective mechanisms is a promising therapeutic approach, which evokes the intrinsic ability of the brain to protect itself. Accumulated evidence strongly suggests that electroacupuncture (EA) pretreatment induces rapid tolerance to cerebral ischemia. With regard to mechanisms underlying ischemic tolerance induced by EA, many molecules and signaling pathways are involved, such as the endocannabinoid system, although the exact mechanisms have not been fully elucidated. In the current study, we employed mutant mice, neuropharmacology, microdialysis, and virus transfection techniques in a middle cerebral artery occlusion (MCAO) model to explore the cell-specific and brain region-specific mechanisms of EA-induced neuroprotection. EA pretreatment resulted in increased ambient endocannabinoid (eCB) levels and subsequent activation of ischemic penumbral astroglial cannabinoid type 1 receptors (CB₁R) which led to moderate upregulation of extracellular glutamate that protected neurons from cerebral ischemic injury. These findings provide a novel cellular mechanism of EA and a potential therapeutic target for ischemic stroke.

Expression of miR-200c corresponds with increased reactive oxygen species and hypoxia markers after transient focal ischemia in mice

Embolic stroke results in a necrotic core of cells destined to die, but also a peri-ischemic, watershed penumbral region of potentially salvageable brain tissue. Approaches to effectively differentiate between the ischemic and peri-ischemic zones is critical for novel therapeutic discovery to improve outcomes in survivors of stroke. MicroRNAs are a class of small non-coding RNAs regulating gene translation that have region- and cell-specific expression and responses to ischemia. We have previously reported that global inhibition of cerebral microRNA-200c after experimental stroke in mice is protective, however delineating the post-stroke sub-regional and cell-type specific patterns of post-stroke miR-200c expression are necessary to minimize off-target effects and advance translational application. Here, we detail a novel protocol to visualize regional miR-200c expression after experimental stroke, complexed with visualization of regional ischemia and markers of oxidative stress in an experimental stroke model in mice. In the present study we demonstrate that the fluorescent hypoxia indicator pimonidazole hydrochloride, the reactive-oxygen-species marker 8-hydroxy-deoxyguanosine, neuronal marker MAP2 and NeuN, and the reactive astrocyte marker GFAP can be effectively complexed to determine regional differences in ischemic injury as early as 30 min post-reperfusion after experimental stroke, and can be effectively used to distinguish ischemic core from surrounding penumbral and unaffected regions for targeted therapy. This multi-dimensional post-stroke immunofluorescent imaging protocol enables a greater degree of sub-regional mechanistic investigation, with the ultimate goal of developing more effective post-stroke pharmaceutical therapy.

Glial Cells as Therapeutic Approaches in Brain Ischemia-Reperfusion Injury

Ischemic stroke is the second cause of mortality and the first cause of long-term disability constituting a serious socioeconomic burden worldwide. Approved treatments include thrombectomy and rtPA intravenous administration, which, despite their efficacy in some cases, are not suitable for a great proportion of patients. Glial cell-related therapies are progressively overcoming inefficient neuron-centered approaches in the preclinical phase. Exploiting the ability of microglia to naturally switch between detrimental and protective phenotypes represents a promising therapeutic treatment, in a similar way to what happens with astrocytes. However, the duality present in many of the roles of these cells upon ischemia poses a notorious difficulty in disentangling the precise pathways to target. Still, promoting M2/A2 microglia/astrocyte protective phenotypes and inhibiting M1/A1 neurotoxic profiles is globally rendering promising results in different in vivo models of stroke. On the other hand, described oligodendrogenesis after brain ischemia seems to be strictly beneficial, although these cells are the less studied players in the stroke paradigm and negative effects could be described for oligodendrocytes in the next years. Here, we review recent advances in understanding the precise role of mentioned glial cell types in the main pathological events of ischemic stroke, including inflammation, blood brain barrier integrity, excitotoxicity, reactive oxygen species management, metabolic support, and neurogenesis, among others, with a special attention to tested therapeutic approaches.

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

A plethora of neurological disorders shares a final common deadly pathway known as excitotoxicity. Among these disorders, ischemic injury is a prominent cause of death and disability worldwide. Brain ischemia stems from cardiac arrest or stroke, both responsible for insufficient blood supply to the brain parenchyma. Glucose and oxygen deficiency disrupts oxidative phosphorylation, which results in energy depletion and ionic imbalance, followed by cell membrane depolarization, calcium (Ca²⁺) overload, and extracellular accumulation of excitatory amino acid glutamate. If tight physiological regulation fails to clear the surplus of this neurotransmitter, subsequent prolonged activation of glutamate receptors forms a vicious circle between elevated concentrations of intracellular Ca²⁺ ions and aberrant glutamate release, aggravating the effect of this ischemic pathway. The activation of downstream Ca²⁺-dependent enzymes has a catastrophic impact on nervous tissue leading to cell death, accompanied by the formation of free radicals, edema, and inflammation. After decades of “neuron-centric” approaches, recent research has also finally shed some light on the role of glial cells in neurological diseases. It is becoming more and more evident that neurons and glia depend on each other. Neuronal cells, astrocytes, microglia, NG2 glia, and oligodendrocytes all have their roles in what is known as glutamate excitotoxicity. However, who is the main contributor to the ischemic pathway, and who is the unsuspecting victim? In this review article, we summarize the so-far-revealed roles of cells in the central nervous system, with particular attention to glial cells in ischemia-induced glutamate excitotoxicity, its origins, and consequences.

The role of astrocytic glutamate transporters GLT-1 and GLAST in neurological disorders: Potential targets for neurotherapeutics

Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS) which initiates rapid signal transmission in the synapse before its re-uptake into the surrounding glia, specifically astrocytes. The astrocytic glutamate transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) and their human homologs excitatory amino acid transporter 1 (EAAT1) and 2 (EAAT2), respectively, are the major transporters which take up synaptic glutamate to maintain optimal extracellular glutamic levels, thus preventing accumulation in the synaptic cleft and ensuing excitotoxicity. Growing evidence has shown that excitotoxicity is associated with various neurological disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), manganism, ischemia, schizophrenia, epilepsy, and autism. While the mechanisms of neurological disorders are not well understood, the dysregulation of GLAST/GLT-1 may play a significant role in excitotoxicity and associated neuropathogenesis. The expression and function of GLAST/GLT-1 may be dysregulated at the genetic, epigenetic, transcriptional or translational levels, leading to high levels of extracellular glutamate and excitotoxicity. Consequently, understanding the regulatory mechanisms of GLAST/GLT-1 has been an area of interest in developing therapeutics for the treatment of neurological disorders. Pharmacological agents including β-lactam antibiotics, estrogen/selective estrogen receptor modulators (SERMs), growth factors, histone deacetylase inhibitors (HDACi), and translational activators have shown significant efficacy in enhancing the expression and function of GLAST/GLT-1 and glutamate uptake both in vitro and in vivo. This comprehensive review will discuss the regulatory mechanisms of GLAST/GLT-1, their association with neurological disorders, and the pharmacological agents which mediate their expression and function. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Combined Gene Therapy to Reduce the Neuronal Damage in the Mouse Model of Focal Ischemic Injury

Research into stroke is driven by frustration over the limited available therapeutics. Targeting a single aspect of this multifactorial disease contributes to the therapeutic boundaries. To overcome this, we devised a novel multifactorial-cocktail treatment, using lentiviruses encoding excitatory amino acid transporter 2 (EAAT2(, glutamate dehydrogenase 2 (GDH2), and nuclear factor E2-related factor 2 (Nrf2) genes, that acts synergistically to address the effected excito-oxidative axis. Here, we used the vasoconstrictor endothelin-1 (ET-1) to induce focal ischemic injury in mice by direct injection into the striatum. Mice treated with the mixture of these three genes show significant improvement in body balance, motor coordination, and decreased motor asymmetry compared to each gene separately. These results demonstrate that overexpression of the combined EAAT2, GDH2, and NRF2 genes can provide neuroprotection after ischemic injury.

Potentially Common Therapeutic Targets for Multiple Sclerosis and Ischemic Stroke

Ischemic stroke (IS) and multiple sclerosis (MS) are two pathologies of the central nervous system (CNS). At the first look, this appears to be the only similarity between the two diseases, as they seem quite different. Indeed IS has an acute onset compared to MS which develops chronically; IS is consecutive to blood clot migrating to cerebral blood vessels or decrease in cerebral blood flow following atherosclerosis or decreases in cardiac output, whereas MS is an immune disease associated with neurodegeneration. However, both pathologies share similar pathologic pathways and treatments used in MS have been the object of studies in IS. In this mini-review we will discuss similarities between IS and MS on astrocytes and neuroinflammation hallmarks emphasizing the potential for treatments.

Emerging Roles of Sirtuins in Ischemic Stroke

Ischemic stroke is one of the leading causes of death worldwide. It is characterized by a sudden disruption of blood flow to the brain causing cell death and damage, which will lead to neurological impairments. In the current state, only one drug is approved to be used in clinical setting and new therapies that confer ischemic neuroprotection are desperately needed. Several targets and pathways have been indicated to be neuroprotective in ischemic stroke, among which the sirtuin family of nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases has emerged as important modulators of several processes in the normal physiology and pathological conditions such as stroke. Recent studies have identified some members of the sirtuin family are able to ameliorate the devastating consequences of ischemic stroke by conferring neuroprotection by means of reducing neuronal cell death, oxidative stress, and neuroinflammation whereas some sirtuins are found to be detrimental in the pathophysiology of ischemic stroke. This review summarizes implications of sirtuins in ischemic stroke and the experimental evidences that demonstrate the potential of sirtuin modulators as neuroprotective therapy for ischemic stroke.

Astrocyte-specific insulin-like growth factor-1 gene transfer in aging female rats improves stroke outcomes

Middle aged female rats sustain larger stroke infarction and disability than younger female rats. This older group also shows age-related reduction of insulin like growth factor (IGF)-1 in serum and in astrocytes, a cell type necessary for poststroke recovery. To determine the impact of astrocytic IGF-1 for ischemic stroke, these studies tested the hypothesis that gene transfer of IGF-1 to astrocytes will improve stroke outcomes in middle aged female rats. Middle aged (10-12 month old), acyclic female rats were injected with recombinant adeno-associated virus serotype 5 (AAV5) packaged with the coding sequence of the human (h)IGF-1 gene downstream of an astrocyte-specific promoter glial fibrillary acidic protein (GFAP) (AAV5-GFP-hIGF-1) into the striatum and cortex. The AAV5-control consisted of an identical shuttle vector construct without the hIGF-1 gene (AAV5-GFAP-control). Six to eight weeks later, animals underwent transient (90 min) middle cerebral artery occlusion via intraluminal suture. While infarct volume was not altered, AAV5-GFAP-hIGF-1 treatment significantly improved blood pressure and neurological score in the early acute phase of stroke (2 days) and sensory-motor performance at both the early and late (5 days) acute phase of stroke. AAV5-GFAP-hIGF-1 treatment also reduced circulating serum levels of GFAP, a biomarker for blood brain barrier permeability. Flow cytometry analysis of immune cells in the brain at 24 hr poststroke showed that AAV5-GFAP-hIGF-1 altered the type of immune cells trafficked to the ischemic hemisphere, promoting an anti-inflammatory profile. Collectively, these studies show that targeted enhancement of IGF-1 in astrocytes of middle-aged females improves stroke-induced behavioral impairment and neuroinflammation.

Impact of age and vector construct on striatal and nigral transgene expression

Therapeutic protein delivery using viral vectors has shown promise in preclinical models of Parkinson's disease (PD) but clinical trial success remains elusive. This may partially be due to a failure to include advanced age as a covariate despite aging being the primary risk factor for PD. We investigated transgene expression following intracerebral injections of recombinant adeno-associated virus pseudotypes 2/2 (rAAV2/2), 2/5 (rAAV2/5), 2/9 (rAAV2/9), and lentivirus (LV) expressing green fluorescent protein (GFP) in aged versus young adult rats. Both rAAV2/2 and rAAV2/5 yielded lower GFP expression following injection to either the aged substantia nigra or striatum. rAAV2/9-mediated GFP expression was deficient in the aged striatonigral system but displayed identical transgene expression between ages in the nigrostriatal system. Young and aged rats displayed equivalent GFP levels following LV injection to the striatonigral system but LV-delivered GFP was deficient in delivering GFP to the aged nigrostriatal system. Notably, age-related transgene expression deficiencies revealed by protein quantitation were poorly predicted by GFP-immunoreactive cell counts. Further, in situ hybridization for the viral CβA promoter revealed surprisingly limited tropism for astrocytes compared to neurons. Our results demonstrate that aging is a critical covariate to consider when designing gene therapy approaches for PD.

Targeting Glial Mitochondrial Function for Protection from Cerebral Ischemia: Relevance, Mechanisms, and the Role of MicroRNAs

Astrocytes and microglia play crucial roles in the response to cerebral ischemia and are effective targets for stroke therapy in animal models. MicroRNAs (miRs) are important posttranscriptional regulators of gene expression that function by inhibiting the translation of select target genes. In astrocytes, miR expression patterns regulate mitochondrial function in response to oxidative stress via targeting of Bcl2 and heat shock protein 70 family members. Mitochondria play an active role in microglial activation, and miRs regulate the microglial neuroinflammatory response. As endogenous miR expression patterns can be altered with exogenous mimics and inhibitors, miR-targeted therapies represent a viable intervention to optimize glial mitochondrial function and improve clinical outcome following cerebral ischemia. In the present article, we review the role that astrocytes and microglia play in neuronal function and fate following ischemic stress, discuss the relevance of mitochondria in the glial response to injury, and present current evidence implicating miRs as critical regulators in the glial mitochondrial response to cerebral ischemia.

Adeno-Associated Virus-Based Gene Therapy for CNS Diseases

Gene therapy is at the cusp of a revolution for treating a large spectrum of CNS disorders by providing a durable therapeutic protein via a single administration. Adeno-associated virus (AAV)-mediated gene transfer is of particular interest as a therapeutic tool because of its safety profile and efficiency in transducing a wide range of cell types. The purpose of this review is to describe the most notable advancements in preclinical and clinical research on AAV-based CNS gene therapy and to discuss prospects for future development based on a new generation of vectors and delivery.

Astrocytes, therapeutic targets for neuroprotection and neurorestoration in ischemic stroke

Astrocytes are the most abundant cell type within the central nervous system. They play essential roles in maintaining normal brain function, as they are a critical structural and functional part of the tripartite synapses and the neurovascular unit, and communicate with neurons, oligodendrocytes and endothelial cells. After an ischemic stroke, astrocytes perform multiple functions both detrimental and beneficial, for neuronal survival during the acute phase. Aspects of the astrocytic inflammatory response to stroke may aggravate the ischemic lesion, but astrocytes also provide benefit for neuroprotection, by limiting lesion extension via anti-excitotoxicity effects and releasing neurotrophins. Similarly, during the late recovery phase after stroke, the glial scar may obstruct axonal regeneration and subsequently reduce the functional outcome; however, astrocytes also contribute to angiogenesis, neurogenesis, synaptogenesis, and axonal remodeling, and thereby promote neurological recovery. Thus, the pivotal involvement of astrocytes in normal brain function and responses to an ischemic lesion designates them as excellent therapeutic targets to improve functional outcome following stroke. In this review, we will focus on functions of astrocytes and astrocyte-mediated events during stroke and recovery. We will provide an overview of approaches on how to reduce the detrimental effects and amplify the beneficial effects of astrocytes on neuroprotection and on neurorestoration post stroke, which may lead to novel and clinically relevant therapies for stroke.

Hypoxia preconditioning induced HIF-1α promotes glucose metabolism and protects mitochondria in liver I/R injury

BACKGROUND

Ischemia and reperfusion (I/R) injury is one of the main lesions after liver transplantation. This study aims to detect hypoxia-induced HIF-1α protects transplanted liver against I/R injury by promoting glucose metabolism to decrease mitochondrial injury and apoptosis on rat model.

METHODS

The rats were given a treatment of 90 min non-lethal hypoxic preconditioning to induce and increase the HIF-1α expression. The autologous orthotopic liver transplantation model was used to imitate liver I/R injury.

RESULTS

Hypoxic-induced HIF-1α was detected to increase in liver tissue after 90-minute hypoxic environment (HP vs. Ctrl, *P<0.001). After operation, the expression of HIF-1α in liver tissue was also stayed at a high level. At 24h after operation, several genes were promoted, such as the levels of HK-2 (HP vs. AT, 24h, *P=0.004), Lactate dehydrogenase (LDHA) (HP vs. AT, 24h, *P=0.003), pyruvate dehydrogenase kinase (PDK-1) (HP vs. AT, 24h, *P=0.007), even the NF-κB and Erk pathways. From the TUNEL assay, the apoptosis in hypoxic preconditioning liver tissue was decreased compared with non-HP operative group at 12h after operation. The expressions of cleaved-caspase 3 (HP vs. AT, *P=0.0119) and PARP (HP vs. AT, *P=0.0134) in HP group were also significantly lower than AT group.

CONCLUSION

The hypoxia-induced HIF-1α could promote glucose metabolism to protect hepatocellular mitochondria from damage. It could be a useful way to protect liver against I/R injuries and inflammatory injury, and particularly promote the recovery of graft function.

Glutamate transporter EAAT2: regulation, function, and potential as a therapeutic target for neurological and psychiatric disease

Glutamate is the predominant excitatory neurotransmitter in the central nervous system. Excitatory amino acid transporter 2 (EAAT2) is primarily responsible for clearance of extracellular glutamate to prevent neuronal excitotoxicity and hyperexcitability. EAAT2 plays a critical role in regulation of synaptic activity and plasticity. In addition, EAAT2 has been implicated in the pathogenesis of many central nervous system disorders. In this review, we summarize current understanding of EAAT2, including structure, pharmacology, physiology, and functions, as well as disease relevancy, such as in stroke, Parkinson's disease, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, major depressive disorder, and addiction. A large number of studies have demonstrated that up-regulation of EAAT2 protein provides significant beneficial effects in many disease models suggesting EAAT2 activation is a promising therapeutic approach. Several EAAT2 activators have been identified. Further understanding of EAAT2 regulatory mechanisms could improve development of drug-like compounds that spatiotemporally regulate EAAT2.

GLT1 overexpression in SOD1(G93A) mouse cervical spinal cord does not preserve diaphragm function or extend disease

Amyotrophic lateral sclerosis (ALS) is characterized by relatively rapid degeneration of both upper and lower motor neurons, with death normally occurring 2-5years following diagnosis primarily due to respiratory paralysis resulting from phrenic motor neuron (PhMN) loss and consequent diaphragm denervation. In ALS, cellular abnormalities are not limited to MNs. For example, decreased levels and aberrant functioning of the major central nervous system (CNS) glutamate transporter, GLT1, occur in spinal cord and motor cortex astrocytes of both humans with ALS and in SOD1(G93A) rodents, a widely studied ALS animal model. This results in dysregulation of extracellular glutamate homeostasis and consequent glutamate excitotoxicity, a primary mechanism responsible for MN loss in ALS animal models and in the human disease. Given these observations of GLT1 dysfunction in areas of MN loss, as well as the importance of testing therapeutic strategies for preserving PhMNs in ALS, we evaluated intraspinal delivery of an adeno-associated virus type 8 (AAV8)-Gfa2 vector to the cervical spinal cord ventral horn of SOD1(G93A) ALS mice for focally restoring intraspinal GLT1 expression. AAV8 was specifically injected into the ventral horn bilaterally throughout the cervical enlargement at 110days of age, a clinically-relevant time point coinciding with phenotypic/symptomatic disease onset. Intraspinal delivery of AAV8-Gfa2-GLT1 resulted in robust transduction primarily of GFAP(+) astrocytes that persisted until disease endstage, as well as a 2-3-fold increase in total intraspinal GLT1 protein expression in the ventral horn. Despite this robust level of astrocyte transduction and GLT1 elevation, GLT1 overexpression did not protect PhMNs, preserve histological PhMN innervation of the diaphragm NMJ, or prevent decline in diaphragmatic respiratory function as assessed by phrenic nerve-diaphragm compound muscle action potential (CMAP) recordings compared to control AAV8-Gfa2-eGFP injected mice. In addition, AAV-Gfa2-GLT1 did not delay forelimb disease onset, extend disease duration (i.e. time from either forelimb or hindlimb disease onsets to endstage) or prolong overall animal survival. These findings suggest that focal restoration of GLT1 expression in astrocytes of the cervical spinal cord using AAV delivery is not an effective therapy for ALS.

MCT4-mediated expression of EAAT1 is involved in the resistance to hypoxia injury in astrocyte-neuron co-cultures

Hypoxic stressors contribute to neuronal death in many brain diseases. Astrocyte processes surround most neurons and are therefore anatomically well-positioned to shield them from hypoxic injury. Excitatory amino acid transporters (EAATs), represent the sole mechanism of active reuptake of glutamate into the astrocytes and neurons and are essential to dampen neuronal excitation following glutamate release at synapses. Glutamate clearance impairment from any factors is bound to result in an increase in hypoxic neuronal injury. The brain energy metabolism under hypoxic conditions depends on monocarboxylate transporters (MCTs) that are expressed by neurons and glia. Previous co-immunoprecipitation experiments revealed that MCT4 directly modulate EAAT1 in astrocytes. The reduction in both surface proteins may act synergistically to induce neuronal hyperexcitability and excitotoxicity. Therefore we hypothesized that astrocytes would respond to hypoxic conditions by enhancing their expression of MCT4 and EAAT1, which, in turn, would enable them to better support neurons to survive lethal hypoxia injury. An oxygen deprivation (OD) protocol was used in primary cultures of neurons, astrocytes, and astrocytes-neurons derived from rat hippocampus, with or without MCT4-targeted short hairpin RNA (shRNA) transfection. Cell survival, expression of MCT4, EAAT1, glial fibrillary acidic protein and neuronal nuclear antigen were evaluated. OD resulted in significant cell death in neuronal cultures and up-regulation of MCT4, EAAT1 expression respectively in primary cell cultures, but no injury in neuron-astrocyte co-cultures and astrocyte cultures. However, neuronal cell death in co-cultures was increased exposure to shRNA-MCT4 prior to OD. These findings demonstrate that the MCT4-mediated expression of EAAT1 is involved in the resistance to hypoxia injury in astrocyte-neuron co-cultures.

Inducible glutamate oxaloacetate transaminase as a therapeutic target against ischemic stroke

SIGNIFICANCE

Glutamate serves multi-faceted (patho)physiological functions in the central nervous system as the most abundant excitatory neurotransmitter and under pathological conditions as a potent neurotoxin. Regarding the latter, elevated extracellular glutamate is known to play a central role in ischemic stroke brain injury.

RECENT ADVANCES

Glutamate oxaloacetate transaminase (GOT) has emerged as a new therapeutic target in protecting against ischemic stroke injury. Oxygen-sensitive induction of GOT expression and activity during ischemic stroke lowers glutamate levels at the stroke site while sustaining adenosine triphosphate levels in brain. The energy demands of the brain are among the highest of all organs underscoring the need to quickly mobilize alternative carbon skeletons for metabolism in the absence of glucose during ischemic stroke. Recent work builds on the important observation of Hans Krebs that GOT-mediated metabolism of glutamate generates tri-carboxylic acid (TCA) cycle intermediates in brain tissue. Taken together, outcomes suggest GOT may enable the transformative switch of otherwise excitotoxic glutamate into life-sustaining TCA cycle intermediates during ischemic stroke.

CRITICAL ISSUES

Neuroprotective strategies that focus solely on blocking mechanisms of glutamate-mediated excitotoxicity have historically failed in clinical trials. That GOT can enable glutamate to assume the role of a survival factor represents a paradigm shift necessary to develop the overall significance of glutamate in stroke biology.

FUTURE DIRECTIONS

Ongoing efforts are focused to develop the therapeutic significance of GOT in stroke-affected brain. Small molecules that target induction of GOT expression and activity in the ischemic penumbra are the focus of ongoing studies.

Gene therapy of the central nervous system: general considerations on viral vectors for gene transfer into the brain

The last decade has nourished strong doubts on the beneficial prospects of gene therapy for curing fatal diseases. However, this climate of reservation is currently being transcended by the publication of several successful clinical protocols, restoring confidence in the appropriateness of therapeutic gene transfer. A strong sign of this present enthusiasm for gene therapy by clinicians and industrials is the market approval of the therapeutic viral vector Glybera, the first commercial product in Europe of this class of drug. This new field of medicine is particularly attractive when considering therapies for a number of neurological disorders, most of which are desperately waiting for a satisfactory treatment. The central nervous system is indeed a very compliant organ where gene transfer can be stable and successful if provided through an appropriate strategy. The purpose of this review is to present the characteristics of the most efficient virus-derived vectors used by researchers and clinicians to genetically modify particular cell types or whole regions of the brain. In addition, we discuss major issues regarding side effects, such as genotoxicity and immune response associated to the use of these vectors.

Tumor necrosis factor-α promotes the expression of excitatory amino-acid transporter 2 in astrocytes: Optimal concentration and incubation time

The aim of the present study was to determine whether tumor necrosis factor (TNF)-α regulates the expression levels of excitatory amino-acid transporters (EAATs) in primary astrocytes and its roles in brain ischemia. Exogenous TNF-α was administered to pure cultured astrocytes and the expression levels of EAAT1, EAAT2 and glial fibrillary acidic protein (GFAP) were evaluated. The results showed that TNF-α at 10 ng/ml enhanced the expression of EAAT2 in a time-dependent manner, while the expression levels of EAAT1 and GFAP did not change. To determine whether the elevation in the levels of the EAAT2 protein induced by TNF-α had a beneficial effect on ischemic insult, TNF-α was applied to in vitro models of cerebral ischemia; the treatment was observed to increase neuronal viability. The present results suggest that a relatively short-term application of an optimal concentration of TNF-α may protect neurons against ischemic injury by elevating the expression of EAAT2 in astrocytes.

Overexpression of the astrocyte glutamate transporter GLT1 exacerbates phrenic motor neuron degeneration, diaphragm compromise, and forelimb motor dysfunction following cervical contusion spinal cord injury

A major portion of spinal cord injury (SCI) cases affect midcervical levels, the location of the phrenic motor neuron (PhMN) pool that innervates the diaphragm. While initial trauma is uncontrollable, a valuable opportunity exists in the hours to days following SCI for preventing PhMN loss and consequent respiratory dysfunction that occurs during secondary degeneration. One of the primary causes of secondary injury is excitotoxic cell death due to dysregulation of extracellular glutamate homeostasis. GLT1, mainly expressed by astrocytes, is responsible for the vast majority of functional uptake of extracellular glutamate in the CNS, particularly in spinal cord. We found that, in bacterial artificial chromosome-GLT1-enhanced green fluorescent protein reporter mice following unilateral midcervical (C4) contusion SCI, numbers of GLT1-expressing astrocytes in ventral horn and total intraspinal GLT1 protein expression were reduced soon after injury and the decrease persisted for ≥6 weeks. We used intraspinal delivery of adeno-associated virus type 8 (AAV8)-Gfa2 vector to rat cervical spinal cord ventral horn for targeting focal astrocyte GLT1 overexpression in areas of PhMN loss. Intraspinal delivery of AAV8-Gfa2-GLT1 resulted in transduction primarily of GFAP(+) astrocytes that persisted for ≥6 weeks postinjury, as well as increased intraspinal GLT1 protein expression. Surprisingly, we found that astrocyte-targeted GLT1 overexpression increased lesion size, PhMN loss, phrenic nerve axonal degeneration, and diaphragm neuromuscular junction denervation, and resulted in reduced functional diaphragm innervation as assessed by phrenic nerve-diaphragm compound muscle action potential recordings. These results demonstrate that GLT1 overexpression via intraspinal AAV-Gfa2-GLT1 delivery exacerbates neuronal damage and increases respiratory impairment following cervical SCI.

Advances in astrocyte-targeted approaches for stroke therapy: an emerging role for mitochondria and microRNAS

Astrocytes are critical regulators of neuronal function and an effective target for stroke therapy in animal models. Identifying individual targets with the potential for simultaneous activation of multiple downstream pathways that regulate astrocyte homeostasis may be a necessary element for successful clinical translation. Mitochondria and microRNAs each represent individual targets with multi-modal therapeutic potential. Mitochondria regulate metabolism and apoptosis, while microRNAs have the capacity to bind and inhibit numerous mRNAs. By combining strategies targeted at maintaining astrocyte function during and following cerebral ischemia, a synergistic therapeutic effect may be achieved.

Effect of early stress on hippocampal gray matter is influenced by a functional polymorphism in EAAT2 in bipolar disorder

Current views on the pathogenesis of psychiatric disorders focus on the interplay between genetic and environmental factors, with individual variation in vulnerability and resilience to hazards being part of the multifactorial development of illness. The aim of the study is to investigate the effect of glutamate transporter polymorphism SLC1A2-181A>C and exposure to Adverse Childhood Experiences (ACE) on hippocampal gray matter volume of patients with bipolar disorder (BD). Patients exposed to higher levels of ACE reported lower gray matter volume. The effect of SLC1A2-181A>C revealed itself only among patients exposed to lower levels of ACE, with T/T homozygotes showing the lowest, and G/G the highest, gray matter volume. The greatest difference between high and low exposures to ACE was observed in carriers of the G allele. Since the mutant G allele has been associated with a reduced transcriptional activity and expression of the transporter protein, we could hypothesize that after exposure to highest levels of ACE G/G homozygotes are more vulnerable to stress reporting the highest brain damage as a consequence of an excess of free glutamate.

Neuroprotection by astrocytes in brain ischemia: importance of microRNAs

Astrocytes have been shown to protect neurons and increase their survival in many pathological settings. Manipulating astrocyte functions is thus an important strategy to enhance neuronal survival and improve outcome following cerebral ischemia. Increasing evidence supports the involvement of microRNAs (miRNA), some of them being astrocyte-enriched, in the regulation of cerebral ischemia. This mini review will focus on several recently reported astrocyte-enriched miRNAs (miR-181 and miR-29 families and miR-146a), their validated targets, regional expression and effects on outcome after cerebral ischemia.

Involvement of pGluR1, EAAT2 and EAAT3 in offspring depression induced by prenatal stress

It is widely known that prenatal stress (PS) exposure causes depression-like behaviour to offspring, as well as maladaptive responses including neurobiological and physiological changes. However, the underlying mechanism of PS induced juvenile-onset depression remains largely unravelled. The inadequacies of monoamine deficiency hypothesis, the emerging evidence of altered glutamate neurotransmission in mood disorders, as well as our previous studies inspired us to assess the potential role of glutamatergic system in the pathogenesis of juvenile depression. In this research, we examined the expression of phosphorylated GluR1 subunit of ionotropic receptor alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), the Na+-dependent glutamate transporters excitatory amino acid transporter 2 (EAAT2) and EAAT3 in the hippocampus, striatum and frontal cortex of 1-month-old rat offspring after mid and late PS exposure. Prenatally stressed offspring rats showed significantly prolonged duration of immobility and shortened immobility latency in tail suspension test. We also detected that PS significantly altered the expression of glutamate receptor and glutamate transporters of these depressed rats. In brief, the changes of phosphorylated GluR1 subunit of AMPAR protein level in the hippocampus and frontal cortex, as well as markedly decreased EAAT2 mRNA expression in the hippocampus, striatum and frontal cortex and EAAT3 mRNA expression in the hippocampus of stressed rats were both observed. These results underpinned that glutamate receptors and glutamate transporters might be involved in the progress of depression-like behaviour in juvenile rat offspring induced by PS.

Chronic brain ischemia induces the expression of glial glutamate transporter EAAT2 in subcortical white matter

Glutamate plays a central role in brain physiology and pathology. The involvement of excitatory amino acid transporters (EAATs) in neurodegenerative disorders including acute stroke has been widely studied, but little is known about the role of glial glutamate transporters in white matter injury after chronic cerebral hypoperfusion. The present study evaluated the expression of glial (EAAT1 and EAAT2) and neuronal (EAAT3) glutamate transporters in subcortical white matter and cortex, before and 3-28 days after the ligation of bilateral common carotid arteries (LBCCA) in rat brain. K-B staining showed a gradual increase of demyelination in white matter after ischemia, while there was no cortical involvement. Between 3 and 7 days after LBCCA, a significant increase in EAAT2 protein levels was observed in the ischemic brain and the number of EAAT2-positive cells also significantly increased both in the cortical and white matter lesions. EAAT2 was detected in glial-fibrillary acidic protein (GFAP)-positive astrocytes in both the cortex and white matter, but not in neuronal and oligodendroglial cells. EAAT1 was slightly elevated after ischemia only in the white matter, but EAAT3 was at almost similar levels both in the cortex and white matter after ischemia. A significant increase in EAAT2 expression level was also noted in the deep white matter of chronic human ischemic brain tissue compared to the control group. Our findings suggest important roles for up-regulated EAAT2 in chronic brain ischemia especially in the regulation of high-affinity of extracellular glutamate and minimization of white matter damage.

Up-regulation of astroglial heme oxygenase-1 by a synthetic (S)-verbenone derivative LMT-335 ameliorates oxygen-glucose deprivation-evoked injury in cortical neurons

Excessive generation of free radicals is regarded as a major detrimental factor in cerebral ischemic insults. Neurons are particularly vulnerable to oxidative stress due to their limited anti-oxidant capacity. As an important source of antioxidants in the brain, astroglia are now thought to be attractive targets for pharmacological interventions to reduce neuronal oxidative stress in ischemic stroke. In the present study, we synthesized a novel antioxidant, the (1S)-(-)-verbenone derivative LMT-335, and investigated its anti-ischemic activities. In rat cortical neuronal/glial co-cultures, LMT-335 significantly reduced oxygen-glucose deprivation (OGD)/reoxygenation (R)-induced neuronal injury. Although it did not inhibit N-methyl-d-aspartate-induced excitotoxicity, LMT-335 significantly reduced OGD/R-evoked intracellular oxidative stress. However, the oxygen radical absorbance capacity assay and 1,1-diphenyl-2-picrylhydrazyl assay showed that the free radical scavenging activities of LMT-335 were lower than those of trolox. Instead, LMT-335 significantly increased the astroglial expression of heme oxygenase-1 (HO-1), a well-known anti-oxidant stress protein, as evidenced by immunocytochemistry and immunoblot analyses. Moreover, a selective HO-1 inhibitor, tin protoporphyrin IX (SnPP), significantly blocked the anti-ischemic effect of LMT-335. The present findings indicate that LMT-335 exerts neuroprotective effects against OGD/R by up-regulation of HO-1 in astroglial cells. Our data suggest that astroglial HO-1 represents a potential therapeutic target for the treatment of ischemic stroke.

Glutamate transporter EAAT2: a new target for the treatment of neurodegenerative diseases

Glutamate is the primary excitatory amino acid neurotransmitter in the CNS. The concentration of glutamate in the synaptic cleft is tightly controlled by interplay between glutamate release and glutamate clearance. Abnormal glutamate release and/or dysfunction of glutamate clearance can cause overstimulation of glutamate receptors and result in neuronal injury known as excitotoxicity. The glial glutamate transporter EAAT2 plays a major role in glutamate clearance. Dysfunction or reduced expression of EAAT2 has been documented in many neurodegenerative diseases. In addition, many studies in animal models of disease indicate that increased EAAT2 expression provides neuronal protection. Here, we summarize these studies and suggest that EAAT2 is a potential target for the prevention of excitotoxicity. EAAT2 can be upregulated by transcriptional or translational activation. We discuss current progress in the search for EAAT2 activators, which is a promising direction for the treatment of neurodegenerative diseases.

Effects of pregabalin on the activity of glutamate transporter type 3

BACKGROUND

Pregabalin, (S)-3-aminomethyl-5-methyl hexanoic acid, is a ligand for the α2δ subunit (a component of voltage-gated calcium channels) and has analgesic and anticonvulsant properties. Glutamate uptake by glutamate transporters may be a mechanism for these properties. We investigated the effects of pregabalin on the activity of the neuronal glutamate transporter type 3 (EAAT3).

METHODS

EAAT3 was expressed in Xenopus laevis oocytes. Two-electrode voltage clamping was used to record membrane currents before, during, and after applying l-glutamate (30 μM) in the presence or absence of pregabalin. Currents were also measured in oocytes pretreated with a protein kinase C (PKC) activator (phorbol-12-myristate-13-acetate, PMA), PKC inhibitors (chelerythrine or staurosporine), or a phosphatidylinositol-3-kinase (PI3K) inhibitor wortmannin.

RESULTS

The exposure of the oocytes injected with EAAT3 mRNA to serial concentrations of pregabalin (0.06-60 μM) significantly increased their responses to 30 μM l-glutamate. A kinetic study showed that pregabalin significantly increased V(max) without changing K(m). Treatment of oocytes with PMA, pregabalin, or pregabalin plus PMA significantly increased transporter currents vs controls, but treatment with PMA plus pregabalin did not increase the responses further vs PMA or pregabalin alone. In addition, pretreatment of oocytes with two PKC inhibitors (chelerythrine or staurosporine), or inhibitor wortmannin, significantly reduced basal and pregabalin-enhanced EAAT3 activity.

CONCLUSIONS

Pregabalin increased EAAT3 activity and PKC and PI3K were involved. This may explain the analgesic effect of pregabalin in neuropathic pain.

Astrocytes: targets for neuroprotection in stroke

In the past two decades, over 1000 clinical trials have failed to demonstrate a benefit in treating stroke, with the exception of thrombolytics. Although many targets have been pursued, including antioxidants, calcium channel blockers, glutamate receptor blockers, and neurotrophic factors, often the focus has been on neuronal mechanisms of injury. Broader attention to loss and dysfunction of non-neuronal cell types is now required to increase the chance of success. Of the several glial cell types, this review will focus on astrocytes. Astrocytes are the most abundant cell type in the higher mammalian nervous system, and they play key roles in normal CNS physiology and in central nervous system injury and pathology. In the setting of ischemia astrocytes perform multiple functions, some beneficial and some potentially detrimental, making them excellent candidates as therapeutic targets to improve outcome following stroke and in other central nervous system injuries. The older neurocentric view of the central nervous system has changed radically with the growing understanding of the many essential functions of astrocytes. These include K+ buffering, glutamate clearance, brain antioxidant defense, close metabolic coupling with neurons, and modulation of neuronal excitability. In this review, we will focus on those functions of astrocytes that can both protect and endanger neurons, and discuss how manipulating these functions provides a novel and important strategy to enhance neuronal survival and improve outcome following cerebral ischemia.

Functional response of hippocampal CA1 pyramidal cells to neonatal hypoxic-ischemic brain damage

Perinatal hypoxic-ischemic (H-I) is a major cause of brain injury in the newborn. The hippocampus is more sensitive to H-I injury than the other brain regions. It is believed that H-I brain damage causes a loss of neurons in the central nervous system. The patterns of neuronal death include apoptosis and necrosis. With regard to the responses of neurons, the neural functional changes should be earlier than the morphologic changes. The aim of the present study is to evaluate the electrophysiological characteristics and the synaptic transmission functions. Seven-day-old Sprague-Dawley rat pups were randomly divided into sham operation and H-I groups. The patch clamp, immunohistochemistry and Western blotting techniques were used to achieve this objective. The results of the study showed a decrease in neuronal excitability and a significant increase in the frequency of spontaneous excitatory postsynaptic currents and the duration of EPSCs in the CA1 pyramidal cells of H-I brain damage rats. The glutamate transporter subtype 1 (GLT-1) expression level of the hippocampal CA1 area in the H-I group was decreased compared with the control. There was no difference in the amplitude of excitatory postsynaptic currents and should be no difference in the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), N-methyl-D-aspartate receptor (NMDAR) and synaptophysin between the control and H-I brain injury group. These results revealed that changes of electrophysiological characteristics and synaptic functions occur instantly after H-I brain damage in the hippocampal pyramidal cells of neonatal rats. The failure to eliminate glutamate should be one of the important factors of excitotoxicity injury on hippocampal CA1 pyramidal cells, while neuronal excitation was not increased in the H-I brain injury model.

Chronic perinatal hypoxia reduces glutamate-aspartate transporter function in astrocytes through the Janus kinase/signal transducer and activator of transcription pathway

The cellular and molecular mechanisms that govern the response of the perinatal brain to injury remain largely unexplored. We investigated the role of white matter astrocytes in a rodent model of diffuse white matter injury produced by exposing neonatal mice to chronic hypoxia-a paradigm that mimics brain injury in premature infants. We demonstrate the absence of reactive gliosis in the immature white matter following chronic hypoxia, as determined by astrocyte proliferation index and glial fibrillary acidic protein levels. Instead, Nestin expression in astrocytes is transiently increased, and the glial-specific glutamate transporters glutamate-aspartate transporter (GLAST) and glutamate transporter 1 (GLT-1) are reduced. Finally, we demonstrate that Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling-which is important in both astrocyte development and response to injury-is reduced in the white matter following hypoxia, as well as in primary astrocytes exposed to hypoxia in vitro. Hypoxia and JAK/STAT inhibition reduce glutamate transporter expression in astrocytes, but unlike hypoxia JAK/STAT inhibition downregulates GLAST expression without affecting GLT-1, as demonstrated in vitro by treatment with JAK inhibitor I and in vivo by treatment with the JAK/STAT inhibitor AG490 [(E)-2-cyano-3-(3,4-dihydrophenyl)-N-(phenylmethyl)-2-propenamide]. Our findings (1) demonstrate specific changes in astrocyte function after perinatal hypoxia, which might contribute to the particular pathogenesis of perinatal white matter injury, (2) provide evidence that at least part of these changes result from a disturbance of the JAK/STAT pathway by hypoxia, and (3) identify JAK/STAT signaling as a potential therapeutic target to restore normal GLAST expression and uptake of glutamate after perinatal brain injury.

Excitatory amino acid transporter 2 and excitatory amino acid transporter 1 negatively regulate calcium-dependent proliferation of hippocampal neural progenitor cells and are persistently upregulated after injury

Using a transgenic mouse (Mus musculus) in which nestin-expressing progenitors are labeled with enhanced green fluorescent protein, we previously characterized the expression of excitatory amino acid transporter 2 (GltI) and excitatory amino acid transporter 1 (Glast) on early neural progenitors in vivo. To address their functional role in this cell population, we manipulated their expression in P7 neurospheres isolated from the dentate gyrus. We observed that knockdown of GltI or Glast was associated with decreased bromodeoxyuridine incorporation and neurosphere formation. Moreover, we determined that both glutamate transporters regulated progenitor proliferation in a calcium-dependent and metabotropic glutamate receptor-dependent manner. To address the relevance of this in vivo, we utilized models of acquired brain injury, which are known to induce hippocampal neurogenesis. We observed that GltI and Glast were specifically upregulated in progenitors following brain injury, and that this increased expression was maintained for many weeks. Additionally, we found that recurrently injured animals with increased expression of glutamate transporters within the progenitor population were resistant to subsequent injury-induced proliferation. These findings demonstrate that GltI and Glast negatively regulate calcium-dependent proliferation in vitro and that their upregulation after injury is associated with decreased proliferation after brain trauma.

Human nasal olfactory epithelium as a dynamic marker for CNS therapy development

Discovery of new central nervous system (CNS) acting therapeutics has been slowed down by the lack of useful applicable biomarkers of disease or drug action often due to inaccessibility of relevant human CNS tissue and cell types. In recent years, non-neuronal cells, such as astrocytes, have been reported to play a highly significant role in neurodegenerative diseases, CNS trauma, as well as psychiatric disease and have become a target for small molecule and biologic therapies. We report the development of a method for measuring pharmacodynamic changes induced by potential CNS therapeutics using nasal olfactory neural tissue biopsy. We validated this approach using a potential astrocyte-targeted therapeutic, thiamphenicol, in a pre-clinical rodent study as well as a phase 1 human trial. In both settings, analysis of the olfactory epithelial tissue revealed biological activity of thiamphenicol at the drug target, the excitatory amino acid transporter 2 (EAAT2). Therefore, this biomarker approach may provide a reliable evaluation of CNS glial-directed therapies and hopefully improve throughput for nervous system drug discovery.

Nuclear factor-κB contributes to neuron-dependent induction of glutamate transporter-1 expression in astrocytes

The glutamate transporter-1 [GLT-1 (excitatory amino acid transporter 2)] subtype of glutamate transporter ensures crisp excitatory signaling and limits excitotoxicity in the CNS. Astrocytic expression of GLT-1 is regulated during development, by neuronal activity, and in neurodegenerative diseases. Although neurons activate astrocytic expression of GLT-1, the mechanisms involved have not been identified. In the present study, astrocytes from transgenic mice that express enhanced green fluorescent protein (eGFP) under the control of a bacterial artificial chromosome (BAC) containing a very large region of DNA surrounding the GLT-1 gene (BAC GLT-1 eGFP mice) were used to assess the role of nuclear factor-κB (NF-κB) in neuron-dependent activation of the GLT-1 promoter. We provide evidence that neurons activate NF-κB signaling in astrocytes. Transduction of astrocytes from the BAC GLT-1 eGFP mice with dominant-negative inhibitors of NF-κB signaling completely blocked neuron-dependent activation of a NF-κB reporter construct and attenuated induction of eGFP. Exogenous expression of p65 and/or p50 NF-κB subunits induced expression of eGFP or GLT-1 and increased GLT-1-mediated transport activity. Using wild-type and mutant GLT-1 promoter reporter constructs, we found that NF-κB sites at -583 or -251 relative to the transcription start site were required for neuron-dependent reporter activation. Electrophoretic mobility shift and supershift assays reveal that p65 and p50 interact with these same sites ex vivo. Finally, chromatin immunoprecipitation showed that p65 and p50 interact with these sites in adult cortex, but not in kidney (a tissue that expresses no detectable GLT-1). Together, these studies strongly suggest that NF-κB contributes to neuron-dependent regulation of astrocytic GLT-1 transcription.

Pre-conditioning induces the precocious differentiation of neonatal astrocytes to enhance their neuroprotective properties

Hypoxic preconditioning reprogrammes the brain's response to subsequent H/I (hypoxia–ischaemia) injury by enhancing neuroprotective mechanisms. Given that astrocytes normally support neuronal survival and function, the purpose of the present study was to test the hypothesis that a hypoxic preconditioning stimulus would activate an adaptive astrocytic response. We analysed several functional parameters 24 h after exposing rat pups to 3 h of systemic hypoxia (8% O₂). Hypoxia increased neocortical astrocyte maturation as evidenced by the loss of GFAP (glial fibrillary acidic protein)-positive cells with radial morphologies and the acquisition of multipolar GFAP-positive cells. Interestingly, many of these astrocytes had nuclear S100B. Accompanying their differentiation, there was increased expression of GFAP, GS (glutamine synthetase), EAAT-1 (excitatory amino acid transporter-1; also known as GLAST), MCT-1 (monocarboxylate transporter-1) and ceruloplasmin. A subsequent H/I insult did not result in any further astrocyte activation. Some responses were cell autonomous, as levels of GS and MCT-1 increased subsequent to hypoxia in cultured forebrain astrocytes. In contrast, the expression of GFAP, GLAST and ceruloplasmin remained unaltered. Additional experiments utilized astrocytes exposed to exogenous dbcAMP (dibutyryl-cAMP), which mimicked several aspects of the preconditioning response, to determine whether activated astrocytes could protect neurons from subsequent excitotoxic injury. dbcAMP treatment increased GS and glutamate transporter expression and function, and as hypothesized, protected neurons from glutamate excitotoxicity. Taken altogether, these results indicate that a preconditioning stimulus causes the precocious differentiation of astrocytes and increases the acquisition of multiple astrocytic functions that will contribute to the neuroprotection conferred by a sublethal preconditioning stress.

Cellular changes underlying hyperoxia-induced delay of white matter development

Impaired neurological development in premature infants frequently arises from periventricular white matter injury (PWMI), a condition associated with myelination abnormalities. Recently, exposure to hyperoxia was reported to disrupt myelin formation in neonatal rats. To identify the causes of hyperoxia-induced PWMI, we characterized cellular changes in the white matter (WM) using neonatal wild-type 2-3-cyclic nucleotide 3-phosphodiesterase-enhanced green fluorescent protein (EGFP) and glial fibrillary acidic protein (GFAP)-EGFP transgenic mice exposed to 48 h of 80% oxygen from postnatal day 6 (P6) to P8. Myelin basic protein expression and CC1(+) oligodendroglia decreased after hyperoxia at P8, but returned to control levels during recovery between P12 and P15. At P8, hyperoxia caused apoptosis of NG2(+)O4(-) progenitor cells and reduced NG2(+) cell proliferation. This was followed by restoration of the NG2(+) cell population and increased oligodendrogenesis in the WM after recovery. Despite apparent cellular recovery, diffusion tensor imaging revealed WM deficiencies at P30 and P60. Hyperoxia did not affect survival or proliferation of astrocytes in vivo, but modified GFAP and glutamate-aspartate transporter expression. The rate of [(3)H]-d-aspartic acid uptake in WM tissue was also decreased at P8 and P12. Furthermore, cultured astrocytes exposed to hyperoxia showed a reduced capacity to protect oligodendrocyte progenitor cells against the toxic effects of exogenous glutamate. This effect was prevented by 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide treatment. Our analysis reveals a role for altered glutamate homeostasis in hyperoxia-induced WM damage. Understanding the cellular dynamics and underlying mechanisms involved in hyperoxia-induced PWMI will allow for future targeted therapeutic intervention.

The influence of epileptic neuropathology and prior peripheral immunity on CNS transduction by rAAV2 and rAAV5

Adeno-associated virus (AAV) provides a promising platform for clinical treatment of neurological disorders owing to its established efficacy and lack of apparent pathogenicity. To use viral vectors in treating neurological disease, however, transduction must occur under neuropathological conditions. Previous studies in rodents have shown that AAV5 more efficiently transduces cells in the hippocampus and piriform cortex than AAV2. Using the kainic acid (KA) model of temporal lobe epilepsy and AAV2 and 5 carrying a hybrid chicken β-actin promoter driving green fluorescent protein (GFP), we found that limbic seizure activity caused substantial neuropathology and resulted in a significant reduction in subsequent AAV5 transduction. Nonetheless, this reduced transduction still was greater than AAV2 transduction in control rats. Although KA seizures compromise blood-brain barrier function, potentially increasing exposure of target tissue to circulating neutralizing antibodies, we observed no interaction between KA seizure-induced damage and immunization status on AAV transduction. Finally, while we confirmed the near total neuronal-specific transgene expression for both serotypes in control rats, AAV5-GFP expression was increasingly localized to astrocytes in seizure-damaged areas. Thus, the pathological milieu of the injured brain can reduce transduction efficacy and alter viral tropism- both relevant concerns when considering viral vector gene therapy for neurological disorders.

Preconditioning and postconditioning for neuroprotection: the most recent evidence

Stroke is a leading cause of morbidity and mortality, with perioperative stroke being an important complication in the practice of anaesthesia. Unfortunately, pharmacological treatment options are very limited and often not applicable in the perioperative period. The notion of applying a subtoxic stimulus prior to an otherwise lethal event is termed preconditioning. The main focus of the article is on describing the different concepts of preconditioning, including remote ischaemic preconditioning and anaesthetic preconditioning, as well as postconditioning and summarizing the most recent discoveries in this exciting field.

The role of excitatory amino acid transporters in cerebral ischemia

Glutamate is an excitatory neurotransmitter that plays a major role in the pathogenesis of ischemia brain injury. The regulation of glutamate neurotransmission is carried out by excitatory amino acid transporters (EAATs) that act through reuptake of glutamate into cells. EAATs may also release glutamate into the extracellular space in a calcium-independent manner during ischemia and dysfunction of EAATs is specifically implicated in the pathology of cerebral ischemia. Recent studies show that up-regulation of EAAT2 provides neuroprotection during ischemic insult. This review summarizes current knowledge regarding the role of EAATs in cerebral ischemia.

Glia as transmitter sources and sensors in health and disease

Glial cells express a bewildering array of neurotransmitter receptors. To illustrate the complexity of expression, we have assayed non-glutamatergic neurotransmitter receptor mRNA in isolated rat optic nerve, a preparation devoid of neurons and neuronal synapses and from which relatively pure "glial" RNA can be isolated. Of the 44 receptor subunits examined which span the GABA-A, nicotinic, adreno- and glycine receptor families, over three quarters were robustly expressed in this mixed population of white matter glial cells, with several expressed at higher levels than found in control whole brain RNA. In addition to the complexity of glial receptor expression, numerous neurotransmitter release mechanisms have been identified. We have focused on glutamate release from astrocytes, which can occur via at least seven distinct pathways and which is implicated in excitotoxic injury and are neurons and glia. Recent findings suggest that non-glutamatergic receptors can also mediate acute glial injury are also discussed.