Glutamine uptake by neurons: interaction of protons with system a transporters

Neutral amino acid transporter ASCT1 is preferentially expressed in L-Ser-synthetic/storing glial cells in the mouse brain with transient expression in developing capillaries

Nonessential amino acid L-Ser plays an essential role in neuronal survival and differentiation, through preferential expression of the L-Ser biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH), in particular in glial cells but not in neurons. To seek the molecular candidates responsible for glia-borne L-Ser transport, we performed histochemical analyses on amino acid transporter ASCT1, which prefers small neutral amino acids, such as Ala, Ser, Cys, and Thr, and mediates their obligatory exchange. At early developmental stages, neuroepithelial cells constituting the ventricular zone expressed ASCT1 mRNA and protein ubiquitously. Thereafter, ASCT1 expression was gradually downregulated in neuronal populations during the late embryonic and neonatal periods, whereas its high expression was transmitted to radial glial cells and then to astrocytes. High levels of ASCT1 were also detected in the olfactory ensheathing glia. The preferential glial expression of ASCT1 was consistent with that of 3PGDH, and their extensive colocalization was demonstrated at the cellular level. Moreover, high cellular contents of L-Ser were revealed in these glial cells by using a specific antibody to L-Ser. These results strongly suggest that a large amount of L-Ser is synthesized and stored in these glial cells and is released through ASCT1 in exchange for other extracellular substrates. In addition, we observed prominent expression of ASCT1 in capillary endothelial cells of embryonic and neonatal brains. Therefore, ASCT1 appears to be regulated to meet metabolic demands by differentiating and mature neurons through the transport of glia- and blood-borne small neutral amino acids.

Block of glutamate-glutamine cycle between astrocytes and neurons inhibits epileptiform activity in hippocampus

Recurrent epileptiform activity occurs spontaneously in cultured CNS neurons and in brain slices in which GABA inhibition has been blocked. We demonstrate here that pharmacological treatments resulting in either the block of glutamine production by astrocytes or the inhibition of glutamine uptake by neurons suppress or markedly decrease the frequency of spontaneous epileptiform discharges both in primary hippocampal cultures and in disinhibited hippocampal slices. These data point to an important role for the neuron-astrocyte metabolic interaction in sustaining episodes of intense rhythmic activity in the CNS, and thereby reveal a new potential target for antiepileptic treatments.

Glutamine uptake and metabolism to N-acetylaspartylglutamate (NAAG) by crayfish axons and glia

We have proposed that N-acetylaspartylglutamate (NAAG) or its hydrolytic product glutamate, is a chemical signaling agent between axons and periaxonal glia at non-synaptic sites in crayfish nerves, and that glutamine is a probable precursor for replenishing the releasable pool of NAAG. We report here, that crayfish central nerve fibers synthesize NAAG from exogenous glutamine. Cellular accumulation of radiolabel during in vitro incubation of desheathed cephalothoracic nerve bundles with [3H]glutamine was 74% Na(+)-independent. The Na(+)-independent transport was temperature-sensitive, linear with time for at least 4 h, saturable between 2.5 and 10 mM L-glutamine, and blocked by neutral amino acids and analogs that inhibit mammalian glutamine transport. Radiolabeled glutamine was taken up and metabolized by both axons and glia to glutamate and NAAG, and a significant fraction of these products effluxed from the cells. Both the metabolism and release of radiolabeled glutamine was influenced by extracellular Na(+). The uptake and conversion of glutamine to glutamate and NAAG by axons provides a possible mechanism for recycling and formation of the axon-to-glia signaling agent(s).

Cell-specific expression of the glutamine transporter SN1 suggests differences in dependence on the glutamine cycle

Glutamine is involved in a variety of metabolic processes, including recycling of the neurotransmitters glutamate and gamma-aminobutyric acid (GABA). The system N transporter SN1 mediates efflux as well as influx of glutamine in glial cells [Chaudhry et al. (1999), Cell, 99, 769-780]. We here report qualitative and quantitative data on SN1 protein expression in rat. The total tissue concentrations of SN1 in brain and in kidney are half and one-quarter, respectively, of that in liver, but the average concentration of SN1 could be higher in astrocytes than in hepatocytes. Light and electron microscopic immunocytochemistry shows that glutamatergic, GABAergic and, surprisingly, purely glycinergic boutons are ensheathed by astrocytic SN1 laden processes, indicating a role of glutamine in the production of all three rapid transmitters. A dedication of SN1 to neurotransmitter recycling is further supported by the lack of SN1 immunoreactivity in oligodendrocytes (cells rich in glutamine but without perisynaptic processes). All neuronal structures appear unlabelled implying that a different protein mediates glutamine uptake into nerve endings. In several regions, SN1 immunoreactivity is higher in association with GABAergic than glutamatergic synapses, in agreement with observations that exogenous glutamine increases output of transmitter glutamate but not GABA. Nerve terminals with low transmitter reuptake or high prevailing firing frequency are associated with high SN1 immunoreactivity in adjacent glia. Bergmann glia and certain other astroglia contain very low levels of SN1 immunoreactivity compared to most astroglia, including retinal Müller cells, indicating the possible existence of SN isoforms and alternative mechanisms for transmitter recycling.

The glutamine commute: take the N line and transfer to the A

The transfer of glutamine between cells contributes to signaling as well as to metabolism. The recent identification and characterization of the system N and A family of transporters has begun to suggest mechanisms for the directional transfer of glutamine, and should provide ways to test its physiological significance in diverse processes from nitrogen to neurotransmitter release.

Diuretics as Antiepileptic Drugs: Should We Go with the Flow?

Recent epidemiological and experimental studies have suggested that certain diuretics may have significant anticonvulsant actions. Potential anticonvulsant mechanisms are discussed in light of the effects of these diuretics on electrolyte balance and synaptic signaling.

Diuretics as Antiepileptic Drugs: Should We Go with the Flow?

Recent epidemiological and experimental studies have suggested that certain diuretics may have significant anticonvulsant actions. Potential anticonvulsant mechanisms are discussed in light of the effects of these diuretics on electrolyte balance and synaptic signaling.