Vesicular and Plasma Membrane Transporters for Neurotransmitters
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Downloaded from http://cshperspectives.cshlp.org/ on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press Vesicular and Plasma Membrane Transporters for Neurotransmitters Randy D. Blakely1 and Robert H. Edwards2 1Department of Pharmacology and Psychiatry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548 2Departments of Neurology and Physiology, UCSF School of Medicine, San Francisco, California 94143 Correspondence: [email protected] The regulated exocytosis that mediates chemical signaling at synapses requires mechanisms to coordinate the immediate response to stimulation with the recycling needed to sustain release. Two general classes of transporter contribute to release, one located on synaptic ves- icles that loads them with transmitter, and a second at the plasma membrane that both termi- nates signaling and serves to recycle transmitter for subsequent rounds of release. Originally identified as the target of psychoactive drugs, these transport systems have important roles in transmitter release, but we are only beginning to understand their contribution to synaptic transmission, plasticity, behavior, and disease. Recent work has started to provide a structural basis for their activity, to characterize their trafficking and potential for regulation. The results indicate that far from the passive target of psychoactive drugs, neurotransmitter transporters undergo regulation that contributes to synaptic plasticity. he speed and potency of synaptic transmis- brane, more active reuptake should help to re- Tsion depend on the immediate availability plenish the pool of releasable transmitter, but of synaptic vesicles filled with high concentra- may also reduce the extent and duration of sig- tions of neurotransmitter. In this article, we fo- naling to the postsynaptic cell. Conversely, loss cus on the mechanisms responsible for packag- of reuptake increases the activation of receptors ing transmitter into synaptic vesicles and for but results in the depletion of stores (Jones et al. reuptake from the extracellular space that both 1998). At the vesicle, steeper concentration gra- terminates synaptic transmission and recycles dients release more transmitter per vesicle but transmitter for future rounds of release. Collec- reduce the cytosolic transmitter available for re- tively, we refer to this entire process as the neu- filling, whereas more shallow gradients facilitate rotransmitter cycle. refilling but reduce the transmitter available for The recycling of neurotransmitter illustrates release. The way in which the nerve terminal a general, conceptual problem for the mecha- balances these competing factors thus has pro- nism of vesicular release. At the plasma mem- found consequences for synaptic transmission. Editors: Morgan Sheng, Bernardo Sabatini, and Thomas Su¨dhof Additional Perspectives on The Synapse available at www.cshperspectives.org Copyright # 2012 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a005595 Cite this article as Cold Spring Harb Perspect Biol 2012;4:a005595 1 Downloaded from http://cshperspectives.cshlp.org/ on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press R.D. Blakely and R.H. Edwards NEUROTRANSMITTER SYNTHESIS AND phorylation regulates tyrosine hydroxylase ac- TRANSPORT INTO SECRETORY VESICLES tivity through modulation of this feedback inhibition, and the enzyme undergoes stringent The ability of vesicles to concentrate neuro- regulation at transcriptional as well as post- transmitter was suspected by Bernard Katz and translational levels (Kaneda et al. 1991). Much coworkers when identifying quantal events as- less is known about the closely related enzyme sociated with the release of acetylcholine (ACh) tryptophan hydroxylase, which is involved in at the neuromuscular junction. The subsequent serotonin (5-hydroxyptryptamine, 5-HT) pro- identification of synaptic vesicles by electron duction. The biosynthetic enzyme for ACh, microscopy then provided a structural basis for choline acetyltransferase (ChAT), has similarly this phenomenon (Katz 1971). Julius Axelrod, received little attention, but the gene resides at Arvid Carlsson, and others extended the con- the same chromosomal locus as the vesicular cept of vesicular storage, finding that radio- ACh transporter (VAChT), and indeed shares labeled catecholamines became “stabilized” some of the same promoters (Erickson et al. after uptake (Carlsson 1963; Axelrod 1971). 1994; Cervini et al. 1995), indicating highly The amount of neurotransmitter released conserved, coordinate regulation of the two pro- per vesicle can also influence the postsynaptic teins. ChATalso appears to undergo regulation response. The extent of vesicle filling is partic- byphosphorylation(DobranskyandRylett2005), ularly important for volume transmission by butundermostcircumstances,theplasmamem- neuromodulators such as monoamines, Ach, brane choline transporter (CHT) is thought to and neuropeptides, in which release often oc- be rate-limiting (Ferguson et al. 2004; Sarter and curs at a distance from receptors. However, the Parikh 2005). amount of transmitter released per vesicle influ- GABA production relies on two, distinct bio- ences signaling even at classical synapses, where synthetic enzymes, glutamic acid decarboxylase receptors lie immediately under the release site. (GAD) of 65 kD and 67 kD. An autoantigen in Even high affinity NMDA receptors for gluta- diabetes, the 65 kD isoform associates directly mate are not saturated by the release of a single with vesicles through palmitoylation (Christgau synaptic vesicle at many synapses (Mainen et al. et al. 1992), whereas the 67 kD isoform is cyto- 1999; McAllister and Stevens 2000), indicating solic. However, the 67 kD isoform appears the potential for changes in vesicle filling to in- much more important for GABAergic neuro- fluence synaptic transmission, particularly for transmission than the 65 kD-animals lacking lower affinity ionotropic receptors or G protein- the 67 kD isoform die shortly after birth be- coupled receptors outside the synapse. There cause of cleft palate, but also show up to 80% re- are three major determinants of vesicle filling: duction in GABA levels (Asada et al. 1997; Con- the cytosolic concentration of transmitter, the þ die et al. 1997). In contrast, loss of the 65 kD H electrochemical driving force across the ves- isoform has minimal effect on inhibitory neu- icle membrane, and intrinsic properties of the rotransmission. Surprisingly, the amplitudes of vesicular transporter such as its ionic coupling. evoked and spontaneous inhibitory postsynap- tic currents (IPSCs) show no difference from controls, but the knockouts show increased syn- Neurotransmitter Biosynthesis aptic depression to sustained synaptic activa- The availability of cytosolic neurotransmitter tion (Tian et al. 1999), raising the interesting depends on specific biosynthetic enzymes. The possibility that vesicle filling might influence enzyme tyrosine hydroxylase has a rate-limiting release probability. role in catecholamine biosynthesis, and nor- Considering its general role in intermediary mally functions at a small fraction of its po- metabolism and protein synthesis, glutamate tential capacity because of allosteric feedback can be synthesized from multiple sources. How- inhibition by L-Dopa and its downstream prod- ever, the inability to regenerate most of these uct dopamine (DA) (Zigmond et al. 1989). Phos- sources (such as a-ketoglutarate from the tri- 2 Cite this article as Cold Spring Harb Perspect Biol 2012;4:a005595 Downloaded from http://cshperspectives.cshlp.org/ on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press Vesicular and Plasma Membrane Transporters carboxylic acid cycle) would result in the deple- astrocytes through the excitatory amino acid tion of glutamate stores by prolonged stimula- transporters (EAATs), glutamate is converted tion. For this reason, the glutamate released as into glutamine by glutamine synthetase, then a transmitter is generally considered to derive transferred back to neurons where it is con- from glutamine, which can be regenerated from verted to glutamate and ammonia through the glutamate through the so-called glutamine-glu- action of glutaminase (Fig. 1) (Albrecht et al. tamate cycle (Fig. 1). In contrast to most other 2007). Consistent with this cycle, inhibition of classical transmitters which undergo reuptake glutamine synthetase results in the shift of glu- directly into the nerve terminal, glutamate ap- tamate immunoreactivity from neurons to as- pears to recycle indirectly: after uptake into trocytes in the retina (Pow and Robinson 1994). ATP H+ H+ VMAT + NT NT + ++ VGLUT Cl– gln gln PMT + synthase Na+ Na EAAT1,2 NT glu EAAT3 Figure 1. Role of plasma membrane and vesicular neurotransmitter transporters in synaptic transmission. After the exocytotic release from synaptic vesicles, neurotransmitter is transported back into the terminal by Naþ and Cl2-dependent plasma membrane transporters (PMT), thereby regenerating the vesicular pools required to sus- tain release. In the case of glutamate, excitatory amino acid transporters (EAATs) are generally found on cells other than those directly involved in glutamate release; most of the uptake occurs into astrocytes, mediated by EAAT1 and 2, which do not couple stoichiometrically to the flux of Cl2. Nonetheless, other isoforms such as EAAT3can be expressed by neurons, although generally not at presynaptic sites or not by glutamate neu-