GLT-1: The elusive presynaptic glutamate transporter

Abstract

Historically, glutamate uptake in the CNS was mainly attributed to glial cells for three reasons: 1) none of the glutamate transporters were found to be located in presynaptic terminals of excitatory synapses; 2) the putative glial transporters, GLT-1 and GLAST are expressed at high levels in astrocytes; 3) studies of the constitutive GLT-1 knockout as well as pharmacological studies demonstrated that >90% of glutamate uptake into forebrain synaptosomes is mediated by the operation of GLT-1. Here we summarize the history leading up to the recognition of GLT-1a as a presynaptic glutamate transporter. A major issue now is understanding the physiological and pathophysiologial significance of the expression of GLT-1 in presynaptic terminals. To elucidate the cell-type specific functions of GLT-1, a conditional knockout was generated with which to inactivate the GLT-1 gene in different cell types using Cre/lox technology. Astrocytic knockout led to an 80% reduction of GLT-1 expression, resulting in intractable seizures and early mortality as seen also in the constitutive knockout. Neuronal knockout was associated with no obvious phenotype. Surprisingly, synaptosomal uptake capacity (Vmax) was found to be significantly reduced, by 40%, in the neuronal knockout, indicating that the contribution of neuronal GLT-1 to synaptosomal uptake is disproportionate to its protein expression (5–10%). Conversely, the contribution of astrocytic GLT-1 to synaptosomal uptake was much lower than expected. In contrast, the loss of uptake into liposomes prepared from brain protein from astrocyte and neuronal knockouts was proportionate with the loss of GLT-1 protein, suggesting that a large portion of GLT-1 in astrocytic membranes in synaptosomal preparations is not functional, possibly because of a failure to reseal. These results suggest the need to reinterpret many previous studies using synaptosomal uptake to investigate glutamate transport itself as well as changes in glutamate homeostasis associated with normal functions, neurodegeneration, and response to drugs.

Oleocanthal ameliorates amyloid-β oligomers’ toxicity on astrocytes and neuronal cells: In vitro studies

“…While GLT1 is mainly expressed in astrocytes, evidence showed GLT1 is also expressed in neurons and contributes to the neuronal uptake of glutamate (Danbolt et al, 2016, Rimmele and Rosenberg, 2016). Studies tested the effect of Aβo on neuronal GLT1 are lacking.…”

Comparison of Glutamate Turnover in Nerve Terminals and Brain Tissue During [1,6-13C2]Glucose Metabolism in Anesthetized Rats

“…Direct measurements of the 13 C enrichment of the vesicular glutamate in the nerve terminals will be required to resolve this issue. Pre-synaptic reuptake of glutamate into neurons has been shown [88] and nerve terminals express GLT-1 glutamate transporter [89]. Studies of mice with knockout of neuronal GLT-1 suggest that reuptake could account for up to 40% of synaptic clearance [89].…”

“…Pre-synaptic reuptake of glutamate into neurons has been shown [88] and nerve terminals express GLT-1 glutamate transporter [89]. Studies of mice with knockout of neuronal GLT-1 suggest that reuptake could account for up to 40% of synaptic clearance [89]. Considering the small extracellular diffusion volume of the synaptic cleft active zone (~44 x 10 −3 μm 3 or <50 pL; [90]), the rapid turnover of this pool (e.g., cleft glutamate clearance in <20 msec at Calyx synapses [87]), and high glutamate-glutamine flux [29], the 13 C enrichment of cleft glutamate would be expected to follow closely the enrichment of released glutamate.…”

The effects of ceftriaxone on cue-primed reinstatement of cocaine-seeking in male and female rats: estrous cycle effects on behavior and protein expression in the nucleus accumbens

“…GLT-1a, xCT and mGlu5 were blotted only in the total protein fraction due to limited amount of the biotinylated fraction permitting only two blots to be conducted on this tissue. We utilized an antibody that recognizes the GLT-1a splice variant, as this is the predominant isoform expressed in the rodent forebrain (see Rimmele and Rosenberg 2016) and was previously shown to be upregulated by ceftriaxone and decreased by cocaine (Rothstein et al 2005; LaCrosse et al 2016, 2017; Rothstein et al 2005). …”