Glutamate transporter expression and function in a striatal neuronal model of Huntington’s disease

Abstract

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 importance of the excitatory amino acid transporter 3 (EAAT3)

“…It should therefore be noted that these studies are not directly comparable. In a murine cellular model of Huntington’s disease, plasma membrane EAAT3 was found to be upregulated (Petr et al, 2013), with a concomitant decrease in X c - expression later described by the same group (Frederick et al, 2014). This is consistent with an EAAT3 up-regulation in the neuronal Huntington’s cell line being a compensatory mechanism for maintaining Cys uptake during the observed X c - decrease, and a functional role of both transport systems.…”

Dysregulation of system xc− expression induced by mutant huntingtin in a striatal neuronal cell line and in R6/2 mice

“…A representative saturation analysis of both Na + -dependent and Na + -independent transport of glutamate can be see in Figure 1A. We observed an increase in Na + -dependent uptake in the STHdh Q111/Q111 , which is the subject of another publication (Petr et al, 2013), and a decrease in Na + -independent uptake (Fig. 1A).…”

“…Disruption of glutamate homeostasis has been implicated in HD (Ferrante et al, 2002; Miller et al, 2008; Petr et al, 2013) and other triplet repeat diseases (Custer et al, 2006). Glutamate transport, thought to be mediated primarily by a family of 5 genes ( eaat1-5 ) is important for clearing extracellular glutamate, with important consequences for excitatory signaling and neuronal survival (Rosenberg and Aizenman, 1989; Rosenberg et al, 1992).…”

“…Glutamate transport, thought to be mediated primarily by a family of 5 genes ( eaat1-5 ) is important for clearing extracellular glutamate, with important consequences for excitatory signaling and neuronal survival (Rosenberg and Aizenman, 1989; Rosenberg et al, 1992). Recent work suggests that a defect in trafficking to the plasma membrane (Li et al, 2010) and expression (Petr et al, 2013) of the neuronal cysteine and glutamate transporter EAAC1 (EAAT3) is impaired in HD, compromising the ability of neurons to synthesize glutathione. Decreased GSH levels may be an important source of oxidative stress in HD (Browne, 2008; Chen, 2011; Ribeiro et al, 2012).…”

“…Initially we confined ourselves to high affinity sodium dependent glutamate transport and transporters (Petr et al, 2013). In the course of that study, we observed a sodium independent component of glutamate transport in these cells.…”