Transfer of glutamine between astrocytes and neurons

The export of glutamine from astrocytes, and the uptake of glutamine by neu rons, are integral steps in the glutamate-glutamine cycle, a major pathway for the replenishment of neuronal glutamate, We review here the functional and molecular identification of the transporters that mediate this transfer . The emerging picture of glutamine transfer in adult brain is of a dominan t pathway mediated by system N transport (SN1) in astrocytes and system A t ransport (SAT/ATA) in neurons. The participating glutamine transporters are functionally and structurally related, sharing the following properties: ( a) unlike many neutral amino acid transporters which have proven to be obli gate exchangers, these glutamine transporters mediate net substrate transfe r energized by coupling to ionic gradients; (b) they are sensitive to small pH changes in the physiological range; (c) they are susceptible to adaptiv e and humoral regulation; (d) they are related structurally to the AAAP (am ino acid and auxin permeases) family of transporters. A key difference betw een SN1 and the SAT/ATA transporters is the ready reversibility of glutamin e fluxes via SN1 under physiological conditions, which allows SN1 both to s ustain a glutamine concentration gradient in astrocytes and to mediate the net outward flux of glutamine. It is likely that the ASCT2 transporter, an obligate exchanger of neutral amino acids, displaces the SN1 transporter as the main carrier of glutamine export in proliferating astrocytes.