Although it is mainly a glial protein

Although it is mainly a glial protein, 5–10% of total GLT-1 represents a neuronal form located at presynaptic terminals, but its physiological role remains elusive (for discussion, see (Rimmele and Rosenberg, 2016)). Nevertheless, this amount might be significant since GLT-1 has been estimated to be one on the most abundantly expressed proteins in Ro 3306 (1% of total protein) (Danbolt, 2001). Thus, neuronal GLT-1 might contribute significantly to replenishing glutamate stores in presynaptic terminals. All glutamate transporters display an uncoupled permeability to Cl− that might counterbalance the depolarizing effect of sodium. However, the ratio of Cl− conductance to glutamate transport varies between EAAT isoforms, being the greatest in EAAT4 and EAAT5 and the lowest in EAAT1, EAAT2 and EAAT3 (Cater et al., 2016). Therefore, for EAAT2/GLT-1 these currents might be insufficient to clamp the voltage in conditions of high glutamate release. Indeed, in glial cells, where GLT-1 and GLAST are the major contributors to glutamate uptake, the glial potassium channel Kir4.1 supports the function of glutamate transporters by re-equilibrating the membrane potential (Djukic et al., 2007). Consequently, we propose that K7.2/7.3 potassium channels might perform a similar function in neurons. Thus, independent of the biochemical data that support a strong interaction, our electrophysiological measurements revealed the opening of M-channels after addition of glutamate and GLT-1 activation. Moreover, co-expression of GLT-1 and Kv7.2/7.3 increased uptake capacity of the transporter, which was cut by the channel blocker XE-991. These observations, together with the previous localization of Kv7.2/7.3 in presynaptic terminals, support the idea of a functional crosstalk between GLT-1 and the potassium channel, similar to that existing for the glial forms of GLT-1 and Kir4.1.
Funding This work was supported by grants from the Spanish MINECO (SAF2014- 55686-R) and the “Fundación Ramón Areces”, the latter also providing an institutional grant to CBMSO.
Acknowledgements
Introduction Alzheimer’s disease (AD), which is the most common progressive neurodegenerative disease and the most common cause of dementia worldwide, is characterized by alteration of cognitive functions, coexistence of senile plaques and neurofibrillary tangles in the brain, and neuronal loss (Selkoe, 2001). The amyloid hypothesis posits that Aβ-related toxicity is considered the primary cause of synaptic dysfunction, and subsequent neurodegeneration underlies the progression characteristic of AD (Gerakis et al., 2016, Hardy and Selkoe, 2002). However, the molecular and cellular mechanisms of Aβ-induced synaptic dysfunction and pathological changes in the AD brain are unknown. Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS), affecting neuronal and glial function by acting on glutamate receptors (Al et al., 2016). It is critically involved in learning and memory (Zhang et al., 2016), and glutamatergic synapses are densely concentrated in the hippocampus. However, excess glutamate in the synaptic cleft may lead to neuronal cell death because of excitotoxicity (Karki et al., 2014, Ullensvang et al., 1997). In the brain, the critical function that transports glutamate is carried out by a family of 5 high-affinity Na+-dependent glutamate transporters referred to as GLAST (also known as EAAT1), GLT-1 (also known as EAAT2), EAAC1 (also known as EAAT3), EAAT4, and EAAT5. Among these transporters, GLT-1 is particularly important in the cortex and hippocampus, where it is responsible for clearing 80–90% of released extracellular glutamate (Danbolt, 2001, Haugeto et al., 1996, Lehre and Danbolt, 1998, Tanaka et al., 1997). Excitotoxicity is also normally prevented by GLAST, which efficiently removes excess neurotransmitters from the synaptic cleft (Anderson and Swanson, 2000, Danbolt, 2001). A previous study reported the unexpected observation that GLT-1 in terminals accounted for approximately half of the D-[3H]-aspartic acid uptake observed in hippocampal slices despite GLT-1 in axon terminals representing 10% of total GLT-1 protein (Furness et al., 2008). Thus, changes in synaptosomal glutamate uptake may reflect alterations in uptake mediated by GLT-1 in neurons (Petr et al., 2015). Several studies indicate that glutamate transporters are impaired in AD (Jacob et al., 2007, Lauderback et al., 2001). Down-regulation of glutamate uptake or abnormal expression of glutamate transporters GLAST and GLT-1 has been shown in transgenic mice developing AD-like pathology or in postmortem human brain tissue (Masliah et al., 2000, Scott et al., 2002, Thai, 2002). This evidence suggests that disruption of glutamate homeostasis contributes to development of neuropathological hallmarks and cognitive decline in AD. Neuronal vulnerability to glutamate excitotoxicity triggered by Aβ-induced sustained activation of glutamate receptors may be a potential mechanism of neurodegeneration in AD (Hynd et al., 2004). Although considerable evidence has suggested that glutamate neurotoxicity plays a role in AD pathophysiology, it is important to determine the underlying mechanism.