GLT-1 (EAAT2; slc1a2) is the major glutamate transporter in the brain, and is predominantly expressed in astrocytes, but at lower levels also in excitatory terminals. We generated a conditional GLT-1 knock-out mouse to uncover cell-type-specific functional roles of GLT-1. Inactivation of the GLT-1 gene was achieved in either neurons or astrocytes by expression of synapsin-Cre or inducible human GFAPCreERT2. Elimination of GLT-1 from astrocytes resulted in loss of ϳ80% of GLT-1 protein and of glutamate uptake activity that could be solubilized and reconstituted in liposomes. This loss was accompanied by excess mortality, lower body weight, and seizures suggesting that astrocytic GLT-1 is of major importance. However, there was only a small (15%) reduction that did not reach significance of glutamate uptake into crude forebrain synaptosomes. In contrast, when GLT-1 was deleted in neurons, both the GLT-1 protein and glutamate uptake activity that could be solubilized and reconstituted in liposomes were virtually unaffected. These mice showed normal survival, weight gain, and no seizures. However, the synaptosomal glutamate uptake capacity (V max ) was reduced significantly (40%). In conclusion, astrocytic GLT-1 performs critical functions required for normal weight gain, resistance to epilepsy, and survival. However, the contribution of astrocytic GLT-1 to glutamate uptake into synaptosomes is less than expected, and the contribution of neuronal GLT-1 to synaptosomal glutamate uptake is greater than expected based on their relative protein expression. These results have important implications for the interpretation of the many previous studies assessing glutamate uptake capacity by measuring synaptosomal uptake.

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Oxidative stress has been implicated in the pathogenesis of Huntington's disease (HD), however, the origin of the oxidative stress is unknown. System xc- plays a role in the import of cystine to synthesize the antioxidant glutathione. We found in the STHdhQ7/Q7 and STHdhQ111/Q111 striatal cell lines, derived from neuronal precursor cells isolated from knock-in mice containing 7 or 111 CAG repeats in the huntingtin gene, that there is a decrease in system xc- function. System xc- is composed of two proteins, the substrate specific transporter, xCT, and an anchoring protein, CD98. The decrease in function in system xc- that we observed is caused by a decrease in xCT mRNA and protein expression in the STHdhQ111/Q111 cells. In addition we found a decrease in protein and mRNA expression in the transgenic R6/2 HD mouse model at 6 weeks of age. STHdhQ111/Q111 cells have lower basal levels of GSH and higher basal levels of ROS. Acute inhibition of system xc- causes greater increase in oxidative stress in the STHdhQ111/Q111 cells than in the STHdhQ7/Q7 cells. These results suggest that a defect in the regulation of xCT may be involved in the pathogenesis of HD by compromising xCT expression and increasing susceptibility to oxidative stress.

Excitotoxicity may contribute to the pathogenesis of Huntington’s disease. High affinity Na+ dependent glutamate transporters, residing in the plasma membrane, clear glutamate from the extracellular space and are the primary means of prevention against excitotoxicity. Many reports suggest that Huntington’s disease is associated with a decrease in the expression and function of glutamate transporters. We studied the expression and function of these transporters in a cellular model of Huntington’s disease, STHdhQ111/Q111 and STHdhQ7/Q7 cells. We found that only GLT-1b and EAAC1 were expressed in these cell lines and only EAAC1 significantly contributed to the glutamate uptake. Surprisingly, there was an increase in Na+-dependent glutamate uptake in STHdhQ111/Q111 cells accompanied by an increase in surface expression of EAAC1 We studied the influence of the Akt pathway on EAAC1 mediated uptake, since EAAC1 surface expression is influenced by Akt and previous studies have shown increased Akt expression in STHdhQ111/Q111 cells. Glutamate uptake was inhibited by Akt pathway inhibitors in both the STHdhQ7/Q7 and the STHdhQ111/Q111 cell lines, and, in fact, we have found no difference in Akt activation between the two cell lines under our conditions of culture. Therefore a difference in Akt activation does not seem to explain the increase in EAAC1 mediated uptake in the STHdhQ111/Q111 cells.

The central nervous system has traditionally been viewed as an immune privilege site with increased tolerance towards antigens that would induce rejection and limited immune responses against CNS antigens. 1 Sir Peter Medawar established this concept when performing his transplantation experiments demonstrating delayed rejection of skin graft in the brain. 2 He postulated that the immune privilege function of the brain arose primarily from (i) lack of drainage of CNS antigens to mount immune responses; (ii) isolation of the brain from peripheral infiltration through the blood brain barrier; and (iii) the relative absence of antigen presenting cells within the CNS. However, further studies demonstrated that rather than isolation and ignorance of the CNS by the immune system, it is rather tightly regulated interactions between immune cells and the CNS that control the apparent immune privilege status of the brain. 3,4 Likely, these controlled interactions are in place to limit neuronal damages while ensuring protection of the brain from infections and facilitate tissue repair.The anatomical sites where most of these interactions appear to take place are particularly located at the borders of the CNS. While the brain parenchyma is virtually devoid of immune cells, with exception of microglia and mast cells, its borders are populated with numerous innate and adaptive immune cells that actively communicate with the periphery to ensure surveillance and protection of the CNS.Yet we still know very little about the establishment, maintenance, and function of these brain borders. New technical advances and a deeper understanding of neuroimmune interactions are driving the field toward the definition of compartmentalized interactions between the brain and the immune system to allow for synergic control and maintenance of brain function in health and disease.Here, we will discuss the establishment and maintenance of the three primary brain borders, the meninges, the choroid plexus, and the blood brain barrier. We then dive into the described roles of the