Genetic deletion of the neuronal glutamate transporter, EAAC1, results in decreased neuronal death after pilocarpine-induced status epilepticus

Dysfunction of EAAT3 Aggravates LPS-Induced Post-Operative Cognitive Dysfunction

Numerous results have revealed an association between inhibited function of excitatory amino acid transporter 3 (EAAT3) and several neurodegenerative diseases. This was also corroborated by our previous studies which showed that the EAAT3 function was intimately linked to learning and memory. With this premise, we examined the role of EAAT3 in post-operative cognitive dysfunction (POCD) and explored the potential benefit of riluzole in countering POCD in the present study. We first established a recombinant adeno-associated-viral (rAAV)-mediated shRNA to knockdown SLC1A1/EAAT3 expression in the hippocampus of adult male mice. The mice then received an intracerebroventricular microinjection of 2 μg lipopolysaccharide (LPS) to construct the POCD model. In addition, for old male mice, 4 mg/kg of riluzole was intraperitoneally injected for three consecutive days, with the last injection administered 2 h before the LPS microinjection. Cognitive function was assessed using the Morris water maze 24 h following the LPS microinjection. Animal behavioral tests, as well as pathological and biochemical assays, were performed to clarify the role of EAAT3 function in POCD and evaluate the effect of activating the EAAT3 function by riluzole. In the present study, we established a mouse model with hippocampal SLC1A1/EAAT3 knockdown and found that hippocampal SLC1A1/EAAT3 knockdown aggravated LPS-induced learning and memory deficits in adult male mice. Meanwhile, LPS significantly inhibited the expression of EAAT3 membrane protein and the phosphorylation level of GluA1 protein in the hippocampus of adult male mice. Moreover, riluzole pretreatment significantly increased the expression of hippocampal EAAT3 membrane protein and also ameliorated LPS-induced cognitive impairment in elderly male mice. Taken together, our results demonstrated that the dysfunction of EAAT3 is an important risk factor for POCD susceptibility and therefore, it may become a promising target for POCD treatment.

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

Regulation of NRG-1-ErbB4 signaling and neuroprotection by exogenous neuregulin-1 in a mouse model of epilepsy

Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy. Dysregulation of glutamate transporters has been a common finding across animal models of epilepsy and in patients with TLE. In this study, we investigate NRG-1/ErbB4 signaling in epileptogenesis and the neuroprotective effects of NRG-1 treatment in a mouse model of temporal lobe epilepsy. Using immunohistochemistry, we report the first evidence for NRG-1/ErbB4-dependent selective upregulation of glutamate transporter EAAC1 and bihemispheric neuroprotection by exogeneous NRG-1 in the intrahippocampal kainic acid (IHKA) model of TLE. Our findings provide evidence that dysregulation of glutamate transporter EAAC1 contributes to the development of epilepsy and can be therapeutically targeted to reduce neuronal death following IHKA-induced status epilepticus (SE).

Constitutive Endocytosis of the Neuronal Glutamate Transporter Excitatory Amino Acid Transporter-3 Requires ARFGAP1

The eukaryotic endocytic pathway regulates protein levels available at the plasma membrane by recycling them into specific endosomal compartments. ARFGAP1 is a component of the coat protein I (COPI) complex but it also plays a role in promoting adapter protein-2 (AP-2) mediated endocytosis. The excitatory amino acid transporter-3 (EAAT3) mediates the reuptake of glutamate from the synaptic cleft to achieve rapid termination of synaptic transmission at glutamatergic synapses. In this study, we identified two interacting proteins of EAAT3 by mass spectrometry (MS) ARFGAP1 and ARF6. We explored the role of ARFGAP1 and ARF6 in the endocytosis of EAAT3. Our data revealed that ARFGAP1 plays a role in the recycling of EAAT3, by utilizing its GTPase activating protein (GAP) activity and ARF6 acting as the substrate. ARFGAP1 promotes cargo sorting of EAAT3 via a single phenylalanine residue (F508) located at the C-terminus of the transporter. ARFGAP1-promoted AP-2 dependent endocytosis is abolished upon neutralizing F508. We utilized a heterologous expression system to identify an additional motif in the C-terminus of EAAT3 that regulates its endocytosis. Impairment in endocytosis did not affect somatodendritic targeting in cultured hippocampal neurons. Our findings support a model where endocytosis of EAAT3 is a multifactorial event regulated by ARFGAP1, occurring via the C-terminus of the transporter, and is the first study to examine the role of ARFGAP1 in the endocytosis of a transport protein.

Reduced Slc1a1 expression is associated with neuroinflammation and impaired sensorimotor gating and cognitive performance in mice: Implications for schizophrenia

We previously reported a 84-Kb hemi-deletion copy number variant at the SLC1A1 gene locus that reduces its expression and appeared causally linked to schizophrenia. In this report, we characterize the in vivo and in vitro consequences of reduced expression of Slc1a1 in mice. Heterozygous (HET) Slc1a1^+/- mice, which more closely model the hemi-deletion we found in human subjects, were examined in a series of behavioral, anatomical and biochemical assays. Knockout (KO) mice were also included in the behavioral studies for comparative purposes. Both HET and KO mice exhibited evidence of increased anxiety-like behavior, impaired working memory, decreased exploratory activity and impaired sensorimotor gating, but no changes in overall locomotor activity. The magnitude of changes was approximately equivalent in the HET and KO mice suggesting a dominant effect of the haploinsufficiency. Behavioral changes in the HET mice were accompanied by reduced thickness of the dorsomedial prefrontal cortex. Whole transcriptome RNA-Seq analysis detected expression changes of genes and pathways involved in cytokine signaling and synaptic functions in both brain and blood. Moreover, the brains of Slc1a1^+/- mice displayed elevated levels of oxidized glutathione, a trend for increased oxidative DNA damage, and significantly increased levels of cytokines. This latter finding was further supported by SLC1A1 knockdown and overexpression studies in differentiated human neuroblastoma cells, which led to decreased or increased cytokine expression, respectively. Taken together, our results suggest that partial loss of the Slc1a1 gene in mice causes haploinsufficiency associated with behavioral, histological and biochemical changes that reflect an altered redox state and may promote the expression of behavioral features and inflammatory states consistent with those observed in schizophrenia.

The importance of the excitatory amino acid transporter 3 (EAAT3)

The neuronal excitatory amino acid transporter 3 (EAAT3) is fairly ubiquitously expressed in the brain, though it does not necessarily maintain the same function everywhere. It is important in maintaining low local concentrations of glutamate, where its predominant post-synaptic localization can buffer nearby glutamate receptors and modulate excitatory neurotransmission and synaptic plasticity. It is also the main neuronal cysteine uptake system acting as the rate-limiting factor for the synthesis of glutathione, a potent antioxidant, in EAAT3 expressing neurons, while on GABAergic neurons, it is important in supplying glutamate as a precursor for GABA synthesis. Several diseases implicate EAAT3, and modulation of this transporter could prove a useful therapeutic approach. Regulation of EAAT3 could be targeted at several points for functional modulation, including the level of transcription, trafficking and direct pharmacological modulation, and indeed, compounds and experimental treatments have been identified that regulate EAAT3 function at different stages, which together with observations of EAAT3 regulation in patients is giving us insight into the endogenous function of this transporter, as well as the consequences of altered function. This review summarizes work done on elucidating the role and regulation of EAAT3.

C3-induced release of neurotrophic factors from Schwann cells - potential mechanism behind its regeneration promoting activity

Previous studies revealed a peripheral nerve regeneration (PNR)(1) promoting activity of Clostridium botulinum C3(2) exoenzyme or a 26(mer) C-terminal peptide fragment covering amino acids 156-181 (C3(156-181)),(3) when delivered as one-time injection at the lesion site. The current study was performed to 1) investigate if prolonged availability of C3 and C3(156-181) at the lesion site can further enhance PNR in vivo and to 2) elucidate effects of C3 and C3(156-181) on Schwann cells (SCs)(4)in vitro. For in vivo studies, 10 mm adult rat sciatic nerve gaps were reconstructed with the epineurial pouch technique or autologous nerve grafts. Epineurial pouches were filled with a hydrogel containing i) vehicle, ii) 40 μM C3 or iii) 40 μM C3(156-181). Sensory and motor functional recovery was monitored over 12 weeks and the outcome of PNR further analyzed by nerve morphometry. In vitro, we compared gene expression profiles (microarray analysis) and neurotrophic factor expression (western blot analysis) of untreated rat neonatal SCs with those treated with C3 or C3(156-181) for 72 h. Effects on neurotrophic factor expression levels were proven in adult human SCs. Unexpectedly, prolonged delivery of C3 and C3(156-181) at the lesion site did not increase the outcome of PNR. Regarding the potential mechanism underlying their previously detected PNR promoting action, however, 6 genes were found to be commonly altered in SCs upon treatment with C3 or C3(156-181). We demonstrate significant down-regulation of genes involved in glutamate uptake (Eaac1,(5)Grin2a(6)) and changes in neurotrophic factor expression (increase of FGF-2(7) and decrease of NGF(8)). Our microarray-based expression profiling revealed novel C3-regulated genes in SCs possibly involved in the axonotrophic (regeneration promoting) effects of C3 and C3(156-181). Detection of altered neurotrophic factor expression by C3 or C3(156-181) treated primary neonatal rat SCs and primary adult human SCs supports this hypothesis.

Tangzhining exhibits a protective effect against cognitive dysfunction in diabetic rats

Previous studies have suggested that diabetes significantly impairs the cognitive function. Tangzhining (TZN), as a kind of Traditional Chinese Medicine (TCM), has been widely used to treat diabetes in China. However, the effect of TZN on treatment of diabetes-induced learning and memory deficits has not been well documented. The present study was to investigate the effect of TZN on diabetes-induced learning and memory deficits and delineate the underlying molecular mechanism. Diabetic rats were randomly grouped and treated with various doses of TZN (0.47, 0.94 and 1.4 g/kg) by intraperitoneal injection. Using the Morris water maze, TZN treatment (0.94 g/kg and 1.4 g/kg) reduced markedly the escape latency and path length of diabetic rats. The morphological changes of pyramidal cells in hippocampus of diabetic rats were apparently reversed and improved by TZN treatment, in comparison with that in diabetic rats without TZN treatment. Moreover, the results of Western blot analysis showed that TZN treatment significantly increased the protein expression of glutamic acid decarboxylase (GAD) and excitatory amino acid carrier 1 (EAAC1) in hippocampus of diabetic rats. Furthermore, TZN treatment increased the protein expression of N-methyl-D-aspartic acid (NMDA) receptor subunits including NR1 and NR2B. Taken together, our data suggest that TZN sustains the balance between glutamate (Glu) and GABA by regulating GAD and EAAC1, and maintains the NMDA receptors activity for learning and memory function through regulating the subunits NR1 and NR2B.