Characterization of L-alpha-aminoadipic acid transport in cultured rat astrocytes

Mature astrocytes as source for astrocyte repopulation after deletion in the medial prefrontal cortex: Implications for depression

The adult brain retains a high repopulation capacity of astrocytes after deletion, and both mature astrocytes in the neocortex and neural stem cells in neurogenic regions possess the potential to generate astrocytes. However, the origin and the repopulation dynamics of the repopulating astrocytes after deletion remain largely unclear. The number of astrocytes is reduced in the medial prefrontal cortex (mPFC) of patients with depression, and selective elimination of mPFC astrocytes is sufficient to induce depression-like behaviors in rodents. However, whether astrocyte repopulation capacity is impaired in depression is unknown. In this study, we used different transgenic mouse lines to genetically label different cell types and demonstrated that in the mPFC of normal adult mice of both sexes, mature astrocytes were a major source of the repopulating astrocytes after acute deletion induced by an astrocyte-specific toxin, L-alpha-aminoadipic acid (L-AAA), and astrocyte regeneration was accomplished within two weeks accompanied by reversal of depression-like behaviors. Furthermore, re-ablation of mPFC astrocytes post repopulation led to reappearance of depression-like behaviors. In adult male mice subjected to 14-day chronic restraint stress, a well-validated mouse model of depression, the number of mPFC astrocytes was reduced; however, the ability of mPFC astrocytes to repopulate after L-AAA-induced deletion was largely unaltered. Our study highlights a potentially beneficial role for repopulating astrocytes in depression and provides novel therapeutic insights into enhancing local mature astrocyte generation in depression.

High preoperative blood oxaloacetate and 2-aminoadipic acid levels are associated with postoperative delayed neurocognitive recovery

Introduction

This study aimed to identify preoperative blood biomarkers related to development of delayed neurocognitive recovery (dNCR) following surgery.

Methods

A total of 67 patients (≥65 years old) who underwent head and neck tumor resection under general anesthesia were assessed using the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA). Preoperative serum metabolomics were determined using widely targeted metabolomics technology.

Results

Of the 67 patients, 25 developed dNCR and were matched to 25 randomly selected patients from the remaining 42 without dNCR. Differential metabolites were selected using the criteria of variable importance in projection > 1.0 in orthogonal partial least squares discrimination analysis, false discovery rate <0.05, and fold-change >1.2 or <0.83 to minimize false positives. Preoperative serum levels of oxaloacetate (OR: 1.054, 95% CI: 1.027-1.095, P = 0.001) and 2-aminoadipic acid (2-AAA) (OR: 1.181, 95% CI: 1.087-1.334, P = 0.001) were associated with postoperative dNCR after adjusting for anesthesia duration, education, and age. Areas under the curve for oxaloacetate and 2-AAA were 0.86 (sensitivity: 0.84, specificity: 0.88) and 0.86 (sensitivity: 0.84, specificity: 0.84), respectively. High levels of preoperative oxaloacetate and 2-AAA also were associated with postoperative decreased MoCA (β: 0.022, 95% CI: 0.005-0.04, P = 0.013 for oxaloacetate; β: 0.077, 95%CI: 0.016-0.137, P = 0.014 for 2-AAA) and MMSE (β: 0.024, 95% CI: 0.009-0.039, P = 0.002 for oxaloacetate; β: 0.083, 95% CI: 0.032-0.135, P = 0.002 for 2-AAA) scores after adjusting for age, education level, and operation time.

Conclusion

High preoperative blood levels of oxaloacetate and 2-AAA were associated with increased risk of postoperative dNCR.

Clinical trial registration

https://classic.clinicaltrials.gov/ct2/show/NCT05105451, identifier NCT05105451.

Understanding the Role of the Glial Scar through the Depletion of Glial Cells after Spinal Cord Injury

Spinal cord injury (SCI) is a condition that affects between 8.8 and 246 people in a million and, unlike many other neurological disorders, it affects mostly young people, causing deficits in sensory, motor, and autonomic functions. Promoting the regrowth of axons is one of the most important goals for the neurological recovery of patients after SCI, but it is also one of the most challenging goals. A key event after SCI is the formation of a glial scar around the lesion core, mainly comprised of astrocytes, NG2⁺-glia, and microglia. Traditionally, the glial scar has been regarded as detrimental to recovery because it may act as a physical barrier to axon regrowth and release various inhibitory factors. However, more and more evidence now suggests that the glial scar is beneficial for the surrounding spared tissue after SCI. Here, we review experimental studies that used genetic and pharmacological approaches to ablate specific populations of glial cells in rodent models of SCI in order to understand their functional role. The studies showed that ablation of either astrocytes, NG2⁺-glia, or microglia might result in disorganization of the glial scar, increased inflammation, extended tissue degeneration, and impaired recovery after SCI. Hence, glial cells and glial scars appear as important beneficial players after SCI.

Combination Therapy of Mesenchymal Stem Cell Transplantation and Astrocyte Ablation Improve Remyelination in a Cuprizone-Induced Demyelination Mouse Model

Astrocytes display an active, dual, and controversial role in multiple sclerosis (MS), a chronic inflammatory demyelination disorder. However, mesenchymal stem cells (MSCs) can affect myelination in demyelinating disorders. This study aimed to investigate the effect of single and combination therapies of astrocyte ablation and MSC transplantation on remyelination in the cuprizone (CPZ) model of MS. C57BL/6 mice were fed 0.2% CPZ diet for 12 weeks. Astrocytes were ablated twice by L-a-aminoadipate (L-AAA) at the beginning of weeks 13 and 14 whereas MSCs were injected in the corpus callosum at the beginning of week 13. Motor coordination and balance were assessed through rotarod test whereas myelin content was evaluated by Luxol-fast blue (LFB) staining and transmission electron microscopy (TEM). Glial cells were assessed by immunofluorescence staining while mRNA expression was evaluated by quantitative real-time PCR. Combination treatment of ablation of astrocytes and MSC transplantation (CPZ + MSC + L-AAA) significantly decreased motor coordination deficits better than single treatments (CPZ + MSCs or CPZ + L-AAA), in comparison to CPZ mice. In addition, L-AAA and MSCs treatment significantly enhanced remyelination compared to CPZ group. Moreover, combination therapy caused a significant decrease in the number of GFAP⁺ and Iba-1⁺ cells, whereas oligodendrocytes were significantly increased in comparison to CPZ mice. Finally, MSC administration resulted in a significant upregulation of BDNF and NGF mRNA expression levels. Our data indicate that transient ablation of astrocytes along with MSCs treatment improve remyelination through enhancing oligodendrocytes and attenuating gliosis in a chronic demyelinating mouse model of MS.

Glutathione peroxidase 4-dependent glutathione high-consumption drives acquired platinum chemoresistance in lung cancer-derived brain metastasis

BACKGROUND

Platinum-based chemotherapy is effective in inducing shrinkage of primary lung cancer lesions; however, it shows finite therapeutic efficacy in patients suffering from brain metastasis (BM). The intrinsic changes of BM cells, which contribute to the poor results remain unknown.

METHODS

Platinum drug-sensitivity was assessed by utilizing a preclinical BM model of PC9 lung adenocarcinoma cells in vitro and in vivo. High consumption of glutathione (GSH) and two associated upregulated proteins (GPX4 and GSTM1) in BM were identified by integrated metabolomics and proteomics in cell lines and verified by clinical serum sample. Gain-of-function and rescue experiments were implemented to reveal the impact and mechanism of GPX4 and GSTM1 on the chemosensitivity in BM. The interaction between GPX4 and GSTM1 was examined by immunoblotting and immunoprecipitation. The mechanism of upregulation of GPX4 was further uncovered by luciferase reporter assay, immunoprecipitation, and electrophoretic mobility shift assay.

RESULTS

The derivative brain metastatic subpopulations (PC9-BrMs) of parental cells PC9 developed obvious resistance to platinum. Radically altered profiles of BM metabolism and protein expression compared with primary lung cancer cells were described and GPX4 and GSTM1 were identified as being responsible for the high consumption of GSH, leading to decreased chemosensitivity by negatively regulating ferroptosis. Besides, GSTM1 was found regulated by GPX4, which was transcriptionally activated by the Wnt/NR2F2 signaling axis in BM.

CONCLUSIONS

Collectively, our findings demonstrated that Wnt/NR2F2/GPX4 promoted acquired chemoresistance by suppressing ferroptosis with high consumption of GSH. GPX4 inhibitor was found to augment the anticancer effect of platinum drugs in lung cancer BM, providing novel strategies for lung cancer patients with BM.

Astrocyte ablation induced by La-aminoadipate (L-AAA) potentiates remyelination in a cuprizone demyelinating mouse model

Chronic demyelination in the central nervous system (CNS) is accompanied by an increase in the number of reactive astrocytes and astrogliosis. There are controversial issues regarding astrocytes and their roles in demyelinating diseases in particular for multiple sclerosis (MS). We aimed to evaluate possible roles for pharmacologic astrocyte ablation strategy using La-aminoadipate (L-AAA) on remyelination in a cuprizone model of demyelination. Male C57BL/6 mice were fed with 0.2% cuprizone for 12 weeks followed by 2-week administration of L-AAA through a cannula inserted 1 mm above the corpus callosum. Rotarod test showed a significant decrease in the range of motor coordination deficits after ablation of astrocytes in mice receiving cuprizone. Results of Luxol fast blue (LFB) and transmission electron microscopy (TEM) for evaluation of myelin content within the corpus callosum revealed a noticeable rise in the percentage of myelinated areas and in the number of myelinated fibers after L-AAA administration in the animals. Astrocyte ablation reduced protein expressions for GFAP (an astrocyte marker) and Iba-1 (a microglial marker), but increased expression of Olig2 (an oligodendrocyte marker) assessed by immunofluorescence. Finally, expression of genes related to recruitment of microglia (astrocyte chemokines CXCL10 and CXCL12) and suppression of oligodendrocyte progenitor cell (OPC) differentiation (astrocyte peptides ET-1 and EDNRB) showed a considerable decrease after administration of L-AAA (for all p < 0.05). These results are indicative of improved remyelination after ablation of astrocytes possibly through hampering microgliosis and astrogliosis and a further rise in the number of matured Olig2⁺ cells.

Astrocytes Maintain Glutamate Homeostasis in the CNS by Controlling the Balance between Glutamate Uptake and Release

Glutamate is one of the most prevalent neurotransmitters released by excitatory neurons in the central nervous system (CNS); however, residual glutamate in the extracellular space is, potentially, neurotoxic. It is now well-established that one of the fundamental functions of astrocytes is to uptake most of the synaptically-released glutamate, which optimizes neuronal functions and prevents glutamate excitotoxicity. In the CNS, glutamate clearance is mediated by glutamate uptake transporters expressed, principally, by astrocytes. Interestingly, recent studies demonstrate that extracellular glutamate stimulates Ca²⁺ release from the astrocytes’ intracellular stores, which triggers glutamate release from astrocytes to the adjacent neurons, mostly by an exocytotic mechanism. This released glutamate is believed to coordinate neuronal firing and mediate their excitatory or inhibitory activity. Therefore, astrocytes contribute to glutamate homeostasis in the CNS, by maintaining the balance between their opposing functions of glutamate uptake and release. This dual function of astrocytes represents a potential therapeutic target for CNS diseases associated with glutamate excitotoxicity. In this regard, we summarize the molecular mechanisms of glutamate uptake and release, their regulation, and the significance of both processes in the CNS. Also, we review the main features of glutamate metabolism and glutamate excitotoxicity and its implication in CNS diseases.

Inhibitors of the NMDA-Nitric Oxide Signaling Pathway Protect Against Neuronal Atrophy and Synapse Loss Provoked by l-alpha Aminoadipic Acid-treated Astrocytes

The impact of treating astrocytes with the astrocytic toxin l-alpha amino adipic acid (L-AAA) on neuronal outgrowth, complexity and synapse formation was assessed, using a model of astrocyte-neuronal interaction. Treatment of rat primary cortical neurons with conditioned media (CM) derived from astrocytes treated with L-AAA reduced neuronal complexity and synapse formation. L-AAA provoked a reduction in the expression of glial fibrillary acid protein (GFAP) and a reduction in ATP-linked mitochondrial respiration in astrocytic cells. As the NMDA-R/PSD-95/NOS signaling pathway is implicated in regulating the structural plasticity of neurons, treatment of neuronal cultures with the neuronal nitric oxide synthase (nNOS) inhibitor 1-[2-(trifluoromethyl)phenyl] imidazole (TRIM) [100 nM] was assessed and observed to protect against L-AAA-treated astrocytic CM-induced reduction in neuronal complexity and synapse loss. Treatment with the NMDA-R antagonist ketamine protected against the CM-induced loss of synapse formation whereas the novel PSD-95/nNOS inhibitors 2-((1H-benzo[d] [1,2,3]triazol-5-ylamino) methyl)-4,6-dichlorophenol (IC87201) and 4-(3,5-dichloro-2-hydroxy-benzylamino)-2-hydroxybenzoic acid (ZL006) protected against synapse loss with partial protection against reduced neurite outgrowth. Furthermore, L-AAA delivery to the pre-limbic cortex (PLC) of mice was found to increase dendritic spine density and treatment with ZL006 reduced this effect. In summary, L-AAA-induced astrocyte impairment leads to a loss of neuronal complexity and synapse loss in vitro and increased dendritic spine density in vivo that may be reversed by inhibitors of the NMDA-R/PSD-95/NOS pathway. The results have implications for understanding astrocytic-neuronal interaction and the search for drug candidates that may provide therapeutic approaches for brain disorders associated with astrocytic histopathology.

Cysteine, Glutathione, and Thiol Redox Balance in Astrocytes

This review discusses the current understanding of cysteine and glutathione redox balance in astrocytes. Particular emphasis is placed on the impact of oxidative stress and astrocyte activation on pathways that provide cysteine as a precursor for glutathione. The effect of the disruption of thiol-containing amino acid metabolism on the antioxidant capacity of astrocytes is also discussed.

Improved retinal function in RCS rats after suppressing the over-activation of mGluR5

Müller cells maintain retinal synaptic homeostasis by taking up glutamate from the synaptic cleft and transporting glutamine back to the neurons. To study the interaction between Müller cells and photoreceptors, we injected either DL-α-aminoadipate or L-methionine sulfoximine–both inhibitors of glutamine synthetase–subretinally in rats. Following injection, the a-wave of the electroretinogram (ERG) was attenuated, and metabotropic glutamate receptor 5 (mGluR5) was activated. Selective antagonism of mGluR5 by 2-methyl-6-(phenylethynyl)-pyridine increased the ERG a-wave amplitude and also increased rhodopsin expression. Conversely, activation of mGluR5 by the agonist, (R,S)-2-chloro-5-hydroxyphenylglycine, decreased both the a-wave amplitude and rhodopsin expression, but upregulated expression of G_q alpha subunit and phospholipase C βIII. Overexpression of mGluR5 reduced the inward-rectifying potassium ion channel (K_ir) current and decreased the expression of K_ir4.1 and aquaporin-4 (AQP4). Further experiments indicated that mGluR5 formed a macromolecular complex with these two membrane channels. Lastly, increased expression of mGluR5 was found in Royal College of Surgeons rats–a model of retinitis pigmentosa (RP). Inhibition of mGluR5 in this model restored the amplitude of ERG features, and reduced the expression of glial fibrillary acidic protein. These results suggest that mGluR5 may be worth considering as a potential therapeutic target in RP.

Inhibition of astrocyte metabolism is not the primary mechanism for anaesthetic hypnosis

Astrocytes have been promoted as a possible mechanistic target for anaesthetic hypnosis. The aim of this study was to explore this using the neocortical brain slice preparation. The methods were in two parts. Firstly, multiple general anaesthetic compounds demonstrating varying in vivo hypnotic potency were analysed for their effect on "zero-magnesium" seizure-like event (SLE) activity in mouse neocortical slices. Subsequently, the effect of astrocyte metabolic inhibition was investigated in neocortical slices, and compared with that of the anaesthetic drugs. The rationale was that, if suppression of astrocytes was both necessary and sufficient to cause hypnosis in vivo, then inhibition of astrocytic metabolism in slices should mimic the anaesthetic effect. In vivo anaesthetic potency correlated strongly with the magnitude of reduction in SLE frequency in neocortical slices (R(2) 37.7 %, p = 0.002). Conversely, SLE frequency and length were significantly enhanced during exposure to both fluoroacetate (23 and 20 % increase, respectively, p < 0.01) and aminoadipate (12 and 38 % increase, respectively, p < 0.01 and p < 0.05). The capacity of an anaesthetic agent to reduce SLE frequency in the neocortical slice is a good indicator of its in vivo hypnotic potency. The results do not support the hypothesis that astrocytic metabolic inhibition is a mechanism of anaesthetic hypnosis.

EAAT and Xc⁻ Exchanger Inhibition Depletes Glutathione in the Transformed Human Lens Epithelial Cell Line SRA 01/04

PURPOSE

Maintaining the high glutathione (GSH; tripeptide of glutamate, cysteine and glycine) levels in the lens cortex promotes lens health. The role of glutamate/aspartate (Glu/Asp) transporters and the cystine (Cys)/Glu exchanger (Xc(-) exchanger) in maintaining GSH in transformed human lens epithelial cells (SRA 01/04) was investigated.

METHODS

Detection and differentiation of excitatory amino acid transporters (EAAT1-5) and the Xc(-) exchanger was performed by the uptake of radiolabeled l-Glu, d-Asp and l-Cys in the presence and absence of Na(+), substrate-specific inhibition studies and Western-blot analysis. Reductions in GSH levels post-inhibition of Xc(-) exchanger and EAAT activities by substrate inhibitors demonstrated the roles of EAAT and Xc(-) exchanger in maintaining GSH.

RESULTS

Glu and d-Asp uptake in HLEC was Na(+)-dependent. Strong inhibition by substrate-specific Glu/Asp uptake inhibitors and weak inhibition by kainic acid (KA) was consistent with Na(+)-dependent EAAT1/3/4/5 activity and weak EAAT2 activity, respectively. Na(+)-independency and Glu inhibition of Cys uptake were consistent with Xc(-) exchanger activity, but inhibition of Na(+)-dependent Cys uptake by N-acetylcysteine suggests Cys uptake by EAAT3. EAAT1-5 and xCT (Xc(-) exchanger light chain) immunoreactive peptides were detected by Western-blot analysis of HLEC lysates. EAAT and Xc(-) exchanger inhibition by substrate antagonists depleted GSH concentrations by 15-28% (p's ≤ 0.02), while GSH synthesis inhibition by buthionine sulfoximine depleted GSH by 33% (p = 0.008).

CONCLUSION

Inhibition of Glu and Cys uptake by EAAT and Xc(-) exchanger antagonists depletes GSH in human lens epithelial cells. These in vitro results support pivotal roles for EAAT and Xc(-) exchanger activities in maintaining GSH and protection against oxidative stress in cortical lens epithelium.

Lysine metabolism in mammalian brain: an update on the importance of recent discoveries

The lysine catabolism pathway differs in adult mammalian brain from that in extracerebral tissues. The saccharopine pathway is the predominant lysine degradative pathway in extracerebral tissues, whereas the pipecolate pathway predominates in adult brain. The two pathways converge at the level of ∆(1)-piperideine-6-carboxylate (P6C), which is in equilibrium with its open-chain aldehyde form, namely, α-aminoadipate δ-semialdehyde (AAS). A unique feature of the pipecolate pathway is the formation of the cyclic ketimine intermediate ∆(1)-piperideine-2-carboxylate (P2C) and its reduced metabolite L-pipecolate. A cerebral ketimine reductase (KR) has recently been identified that catalyzes the reduction of P2C to L-pipecolate. The discovery that this KR, which is capable of reducing not only P2C but also other cyclic imines, is identical to a previously well-described thyroid hormone-binding protein [μ-crystallin (CRYM)], may hold the key to understanding the biological relevance of the pipecolate pathway and its importance in the brain. The finding that the KR activity of CRYM is strongly inhibited by the thyroid hormone 3,5,3'-triiodothyronine (T3) has far-reaching biomedical and clinical implications. The inter-relationship between tryptophan and lysine catabolic pathways is discussed in the context of shared degradative enzymes and also potential regulation by thyroid hormones. This review traces the discoveries of enzymes involved in lysine metabolism in mammalian brain. However, there still remain unanswered questions as regards the importance of the pipecolate pathway in normal or diseased brain, including the nature of the first step in the pathway and the relationship of the pipecolate pathway to the tryptophan degradation pathway.

Acid fibroblast growth factor and peripheral nerve grafts regulate Th2 cytokine expression, macrophage activation, polyamine synthesis, and neurotrophin expression in transected rat spinal cords

Spinal cord injury elicits an inflammatory response that recruits macrophages to the injured spinal cord. Quantitative real-time PCR results have shown that a repair strategy combining peripheral nerve grafts with acidic fibroblast growth factor (aFGF) induced higher interleukin-4 (IL-4), IL-10, and IL-13 levels in the graft areas of rat spinal cords compared with transected spinal cords at 10 and 14 d. This led to higher arginase I-positive alternatively activated macrophage (M2 macrophage) responses. The gene expression of several enzymes involved in polyamine biosynthesis pathways was also upregulated in the graft areas of repaired spinal cords. The treatment induced a twofold upregulation of polyamine levels at 14 d, as confirmed by HPLC. Polyamines are important for the repair process, as demonstrated by the observation that treatment with inhibitors of arginase I and ornithine decarboxylase attenuates the functional recoveries of repaired rats. After 14 d, the treatment also induced the expression of neurotrophin nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), as well as M2 macrophages within grafted nerves expressing BDNF. IL-4 was upregulated in the injury sites of transected rats that received aFGF alone compared with those that received nerve grafts alone at 10 d. Conversely, nerve graft treatment induced NGF and BDNF expression at 14 d. Macrophages expressing polyamines and BDNF may benefit axonal regeneration at 14 d. These results indicate that aFGF and nerve grafts regulate different macrophage responses, and M2 macrophages may play an important role in axonal regeneration after spinal cord injury in rats.

Inhibitory regulation of glutamate aspartate transporter (GLAST) expression in astrocytes by cadmium-induced calcium influx

After injury to the CNS, the accumulation of extracellular glutamate induces neuronal excitotoxicity, leading to secondary tissue damage. Astrocytes can reduce excess extracellular glutamate primarily through the astrocytic glutamate transporter-1 and the Na(+)-dependent glutamate/aspartate transporter (GLAST). In this study, we used an in vitro model of cadmium-induced cellular stress and found that glutamate uptake activity of astrocytes was suppressed because of cadmium-induced inhibition of GLAST expression. The blockage of cadmium-triggered Ca(2+) influx by Ca(2+) chelators elevated GLAST transcription and glutamate uptake activity in astrocytes, suggesting that the suppression of GLAST expression in cadmium-treated astrocytes was Ca(2+)-dependent. This was supported by the findings showing the reduction of GLAST mRNA in astrocytes after treatment with Ca(2+)-ionophore A23187. Cadmium reduced human GLAST promoter activity; however, it increased the binding of Ca(2+)-sensitive activator protein-1 (AP-1) and cAMP response element binding protein (CREB) to their specific elements derived from the human GLAST promoter. These results demonstrate that AP-1 and CREB may be coupled with Ca(2+)-dependent pathway triggered by cadmium to mediate the inhibition of GLAST transcription. Our results suggest that Ca(2+) influx into astrocytes after CNS injury could cause the down-regulation of GLAST expression, thus reducing the astrocytic glutamate uptake function, which in turn may exacerbate secondary damage after CNS injury.

Pharmacological disruption of the outer limiting membrane leads to increased retinal integration of transplanted photoreceptor precursors

Retinal degeneration is the leading cause of untreatable blindness in the developed world. Cell transplantation strategies provide a novel therapeutic approach to repair the retina and restore sight. Previously, we have shown that photoreceptor precursor cells can integrate and form functional photoreceptors after transplantation into the subretinal space of the adult mouse. In a clinical setting, however, it is likely that far greater numbers of integrated photoreceptors would be required to restore visual function. We therefore sought to assess whether the outer limiting membrane (OLM), a natural barrier between the subretinal space and the outer nuclear layer (ONL), could be reversibly disrupted and if disruption of this barrier could lead to enhanced numbers of transplanted photoreceptors integrating into the ONL. Transient chemical disruption of the OLM was induced in adult mice using the glial toxin, dl-alpha-aminoadipic acid (AAA). Dissociated early post-natal neural retinal cells were transplanted via subretinal injection at various time-points after AAA administration. At 3 weeks post-injection, the number of integrated, differentiated photoreceptor cells was assessed and compared with those found in the PBS-treated contralateral eye. We demonstrate for the first time that the OLM can be reversibly disrupted in adult mice, using a specific dose of AAA administered by intravitreal injection. In this model, OLM disruption is maximal at 72 h, and recovers by 2 weeks. When combined with cell transplantation, disruption of the OLM leads to a significant increase in the number of photoreceptors integrated within the ONL compared with PBS-treated controls. This effect was only seen in animals in which AAA had been administered 72 h prior to transplantation, i.e. when precursor cells were delivered into the subretinal space at a time coincident with maximal OLM disruption. These findings suggest that the OLM presents a physical barrier to photoreceptor integration following transplantation into the subretinal space in the adult mouse. Reversible disruption of the OLM may provide a strategy for increasing cell integration in future therapeutic applications.

Gliotoxins disrupt alanine metabolism and glutathione production in C6 glioma cells: a 13C NMR spectroscopic study

Gliotoxins are a group of amino acids that are toxic to astrocytes, and are substrates of high-affinity sodium-dependent glutamate transporters. In the present study, C6 glioma cells were preincubated for 20 h in the presence of 400 microM L-alpha-aminoadipate, L-serine-O-sulphate, D-aspartate or L-cysteate, as well as in the presence of the poorly transported L-glutamate uptake inhibitor, L-anti-endo-methanopyrrolidine dicarboxylate. In experiments following [3-13C]alanine metabolism, all toxins caused a decreased incorporation of label into glutamate. Production of labelled lactate changed only when cells were incubated in the presence of L-alpha-aminoadipate or L-serine-O-sulphate. Incubation with L-anti-endo-methanopyrrolidine dicarboxylate caused no change in the amount of label incorporated into either glutamate or lactate. When glutathione production was followed using 1 mM [2-13C]glycine, differential effects of the gliotoxins were revealed. Most notably, both L-serine-O-sulphate and L-alpha-aminoadipate caused significant increases in labelling of glutathione. Once again, L-anti-endo-methanopyrrolidine dicarboxylate was without effect. Overall, we have shown that the gliotoxins cause disruption to alanine metabolism and glutathione production in C6 glioma cells, but that there are notable differences in their mechanisms of action. In the absence of any disruption to metabolism by L-anti-endo-methanopyrrolidine dicarboxylate, it is concluded that their mode of action involves more than inhibition of glutamate transport.

Glial glutamate transporters limit spillover activation of presynaptic NMDA receptors and influence synaptic inhibition of Purkinje neurons

Glutamate transporters limit cross talk between excitatory synapses by removing synaptically released glutamate. However, the role of glutamate transporters in limiting the action of synaptically released glutamate at inhibitory synapses remains unknown. Single and paired whole-cell patch-clamp recordings were obtained from Purkinje neurons and Bergmann glia in mouse cerebellar slices to determine the function of neuronal and glial glutamate transporters at GABAergic synapses onto Purkinje neurons. NMDA and inhibition of glutamate transporters with DL-threo-beta-benzyloxyaspartic acid (TBOA) increased the frequency of miniature IPSCs (mIPSCs) by enhancing presynaptic NMDA receptor (NMDAR) activation, whereas inhibition of cystine-glutamate antiporters had no effect on mIPSCs. Selective inhibition of glutamate transporters in an individual Bergmann glial cell enhanced mIPSC frequency recorded in an adjacent Purkinje neuron significantly more than did postsynaptic transporter inhibition. TBOA did not change the frequency of spontaneous IPSCs (sIPSCs) but decreased their amplitude, as well as that of evoked IPSCs (eIPSCs), and enhanced the paired-pulse ratio. The action of NMDAR activation on eIPSCs but not that on mIPSCs was prevented by 100 microm ryanodine. Repetitive stimulations of climbing fibers resulted in a NMDAR-dependent reduction of sIPSC amplitude, and this effect was enhanced by TBOA even when postsynaptic glutamate transporters were blocked. These data indicate that glial glutamate transporters limit glutamate spillover from excitatory to inhibitory synapses and strongly influence synaptic inhibition of Purkinje neurons by controlling NMDAR activation on GABAergic terminals.

Heterogeneity between hippocampal and septal astroglia as a contributing factor to differential in vivo AMPA excitotoxicity

Astroglial participation in the regional differences of vulnerability to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced neurodegeneration was investigated in the rat hippocampus and medial septum using L-alpha-aminoadipate (alpha-AA) as a specific astroglial toxin. alpha-AA was microinjected in the hippocampus and the medial septum and a time-course study was carried out between 2 hr and 3 days. When compared to controls, microinjection of alpha-AA in the hippocampus induced within 3 days a reversible loss of glial fibrillary acidic protein (GFAP) immunostaining and a microglial reaction without any neuronal loss, whereas in the medial septum it caused no effects on astroglial, microglial, or neuronal populations. Differences in hippocampus and medial septum vulnerability were also evidenced when alpha-AA was co-injected with AMPA and neurodegeneration was assessed in terms of neuronal loss, glial reactions, calcification, and atrophy of the area. In the hippocampus, alpha-AA increased AMPA excitotoxicity with marked disorganization of all hippocampal subfields, increased neuronal loss, a more important astroglial reaction, a larger area of microgliosis, and a greater abundance of calcium deposits. By contrast, in the medial septum alpha-AA did not modify any parameter of the AMPA-induced lesion. In conclusion, the presence of different astroglial populations in hippocampus and medial septum results in a different participation to AMPA excitotoxicity that may determine, at least in part, the specific regional vulnerability to neurodegeneration.

Determination of alpha-aminoadipic acid in brain, peripheral tissues, and body fluids using GC/MS with negative chemical ionization

alpha-Aminoadipic acid (alphaAA) is a structural homolog of the excitatory amino acid glutamate and a natural product of lysine metabolism in mammalian cells. Under experimental conditions, alphaAA can influence various elements of glutamatergic neurotransmission. Moreover, as a selective inhibitor of kynurenine aminotransferase II, alphaAA is capable of decreasing the levels of the neuroinhibitory metabolite kynurenic acid in the brain. We now describe the identification of this potential endogenous neuromodulator in tissues and body fluids by gas chromatography/mass spectrometry (GC/MS) analysis of its pentafluorobenzyl (PFB) derivative. alphaAA was recovered from the GC column with a retention time of approximately 7 min. Subsequent MS analysis using electron capture with negative ionization revealed two separate ions for alphaAA (m/z 520, approximately 45% and m/z 322, approximately 55%). Both of these ions were positively identified with two different GC methodologies. In the rat, alphaAA levels ranged from 5 to 30 microM in various brain areas and from 8 to 40 microM in peripheral organs, whereas serum and urine contained only 1-2 microM alphaAA. Levels in the human brain were 18.7+/-2.4 microM (cortex) and 18.0+/-1.7 microM (striatum) alphaAA (n=9 each), and the mouse forebrain contained 8.3+/-1.9 microM alphaAA (n=6). Neuronal depletion, caused in rats by an intrastriatal injection of NMDA (300 nmol/2.5 microl), did not alter the striatal content of alphaAA, indicating that brain alphaAA resides at least in part in glial cells. alphaAA may therefore function as a glia-derived modulator of excitatory neurotransmission.

An NMR study of alterations in [1-13C]glucose metabolism in C6 glioma cells by gliotoxic amino acids

A series of glutamate analogues, known as gliotoxins, are toxic to astrocytes in culture, and are inhibitors or substrates of high affinity sodium-dependent glutamate transporters. The mechanisms by which these gliotoxins cause toxicity are not fully understood. The effects of a series of gliotoxic amino acids (L-alpha-aminoadipate, L-serine-O-sulphate, D-aspartate and L-cysteate) on metabolism of [1-13C]glucose were examined in C6 glioma cells using 13C nuclear magnetic resonance (NMR) spectroscopy. The cells were preincubated in the presence of sub toxic concentrations of each gliotoxin (400 micromol/l) for 20 h. This was followed by incubation (4 h) with [1-13C]glucose (5.5 mmol/l) in the presence and absence of each gliotoxin. The incorporation of 13C label into the observed metabolites was analysed. Following preincubation with L-alpha-aminoadipate, D-aspartate, and L-serine-O-sulphate there was a significant decrease in the incorporation of 13C label into glutamate, alanine and lactate from [1-13C]glucose. In the presence of L-cysteate production of labelled glutamate was decreased, while there was no significant effect on the concentrations of labelled lactate and alanine. There was no change in the quantity of LDH released into the medium after incubation of the cells with any of the gliotoxins. Overall these results indicate that the presence of gliotoxins profoundly alters the flux of glucose to lactate, alanine, aspartate and glutamate.

Cerebral cystine uptake: a tale of two transporters

Transport of cystine across the cell membrane is essential for synthesis of the major cellular antioxidant glutathione. Cystine uptake in the brain occurs by both the Na(+)-independent x(c)(-) cystine-glutamate exchanger and the X(AG)(-) family of high-affinity, Na(+)-dependent glutamate transporters. New evidence concerning the role of cystine transport in the defence against oxidative stress is described.

Dibutyryl-cAMP (dbcAMP) up-regulates astrocytic chloride-dependent L-[3H]glutamate transport and expression of both system xc(-) subunits

Recent studies have shown that N(6),2'-O-dibutyryladenosine 3':5' cyclic monophosphate (dbcAMP) increases the expression of specific subtypes of Na(+)-dependent glutamate transporters in cultured astrocytes. Our group also found that treatment of astrocytes with dbcAMP for several days increases the Na(+)-independent accumulation of L-[3H]glutamate. In this study, the properties of this Na(+)-independent accumulation were characterized, and the mechanism by which dbcAMP up-regulates this process was investigated. This accumulation was markedly reduced in the absence of Cl(-) and was also inhibited by several anion-exchange inhibitors, including 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 4,4'-dinitrostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, suggesting that this activity is mediated by a Cl(-)-dependent transporter. In addition, this activity was inhibited by micromolar concentrations of several inhibitors of another Cl(-)-dependent (Na(+)-independent) transport activity frequently referred to as system xc(-) (L-cystine, L-alpha-aminoadipate, L-homocysteate, quisqualate, beta-N-oxalyl-l-alpha,beta-diaminopropionate, ibotenate). This activity was competitively inhibited by several phenylglycine derivatives previously characterized as inhibitors of metabotropic glutamate receptor activation. The concentration-dependence for Na(+)-independent, Cl(-)-dependent L-[3H]glutamate uptake activity was compared for dbcAMP-treated and untreated astrocytes. Treatment with dbcAMP increased the V(max) of this Cl(-)-dependent transport activity by sixfold but had no effect on the K(m) value. System xc(-) requires two subunits, xCT and 4F2hc/CD98, to reconstitute functional activity. We found that dbcAMP caused a twofold increase in the levels of xCT mRNA and a sevenfold increase in the levels of 4F2hc/CD98 protein. This study indicates that dbcAMP up-regulates Cl(-)-dependent L-[3H]glutamate transport activity in astrocytes and suggests that this effect is related to increased expression of both subunits of system xc(-). Because this activity is thought to be important for the synthesis of glutathione and protection from oxidant injury, understanding the regulation of system xc(-) may provide alternate approaches to limit this form of injury.

Amino acids and their transporters in the retina

This review provides an overview of the distributions, properties and roles of amino acid transport systems in normal and pathological retinal tissues and discusses the roles of specific identified transporters in the mammalian retina. The retina is used in this context as a vehicle for describing neuronal and glial properties, which are in some, but not all cases comparable to those found elsewhere an the brain. Where significant departures are noted, these are discussed in the context of functional specialisations of the retina and its relationship to adjacent supporting tissues such as the retinal pigment epithelium. Specific examples are given where immunocytochemical labelling for amino acid transporters may yield inaccurate results, possibly because of activity-dependent conformation changes of epitopes in these proteins which render the epitopes more or less accessible to antibodies.

Visualising the activity of the cystine-glutamate antiporter in glial cells using antibodies to aminoadipic acid, a selectively transported substrate

The cystine-glutamate antiporter is a transport system that facilitates the uptake of cystine, concomitant with the release of glutamate. The cystine accumulated by this transporter is generally considered for use in the formation of the cysteine-containing antioxidant glutathione, which is abundant in many glial cells. This study used the simple strategy of generating an antibody to aminoadipic acid, a selective substrate for the cystine-glutamate antiporter. Stereospecific accumulation of aminoadipic acid into specific cell types in rat brain slice preparations was detected immunocytochemically. Strong accumulation was detected in astroglial cells in all brain regions studied including those in white matter tracts. Strong accumulation into radial glial cells, including the retinal Müller cells and the Bergmann glial cells was also observed. Glial accumulation was observed not only in cells within the blood brain barrier, but also outside such; anterior pituitary folliculostellate cell and intermediate lobe pituitary glial cells exhibited strong accumulation of aminoadipic acid. Interestingly, some glial cells such as the posterior pituitary glial cells (pituicytes) exhibited very little if any accumulation of aminoadipic acid. Within the brain labelling was not uniform. Particularly strong labelling was noted in some regions, such as the glial cells surrounding the CA1 pyramidal cells. By contrast, neurons never exhibited uptake of aminoadipic acid. Because cystine uptake is associated with glutamate release, it is suggested that this antiporter might contribute to release of glutamate from glial cells under some pathophysiological conditions.