Exocytosis regulates trafficking of GABA and glycine heterotransporters in spinal cord glutamatergic synapses: a mechanism for the excessive heterotransporter-induced release of glutamate in experimental amyotrophic lateral sclerosis

The impact of synaptic vesicle endo-exocytosis on the trafficking of nerve terminal heterotransporters was studied by monitoring membrane expression and function of the GABA transporter-1 (GAT-1) and of type-1/2 glycine (Gly) transporters (GlyT-1/2) at spinal cord glutamatergic synaptic boutons. Experiments were performed by inducing exocytosis in wild-type (WT) mice, in amphiphysin-I knockout (Amph-I KO) mice, which show impaired endocytosis, or in mice expressing high copy number of mutant human SOD1 with a Gly93Ala substitution (SOD1(G93A)), a model of human amyotrophic lateral sclerosis showing constitutively excessive Glu exocytosis. Exposure of spinal cord synaptosomes from WT mice to a 35mM KCl pulse increased the expression of GAT-1 at glutamatergic synaptosomal membranes and enhanced the GAT-1 heterotransporter-induced [(3)H]d-aspartate ([(3)H]d-Asp) release. Similar results were obtained in the case of GlyT-1/2 heterotransporters. Preventing depolarization-induced exocytosis normalized the excessive GAT-1 and GlyT-1/2 heterotransporter-induced [(3)H]d-Asp release in WT mice. Impaired endocytosis in Amph-I KO mice increased GAT-1 membrane expression and [(3)H]GABA uptake in spinal cord synaptosomes. Also the GAT-1 heterotransporter-evoked release of [(3)H]d-Asp was augmented in Amph-I KO mice. The constitutively excessive Glu exocytosis in SOD1(G93A) mice resulted in augmented GAT-1 expression at glutamatergic synaptosomal membranes and GAT-1 or GlyT-1/2 heterotransporter-mediated [(3)H]d-Asp release. Thus, endo-exocytosis regulates the trafficking of GAT-1 and GlyT-1/2 heterotransporters sited at spinal cord glutamatergic nerve terminals. As a consequence, it can be hypothesized that the excessive GAT-1 and GlyT-1/2 heterotransporter-mediated Glu release, in the spinal cord of SOD1(G93A) mice, is due to the heterotransporter over-expression at the nerve terminal membrane, promoted by the excessive Glu exocytosis.

Internalization and degradation of the glutamate transporter GLT-1 in response to phorbol ester

Activation of protein kinase C (PKC) decreases the activity and cell surface expression of the predominant forebrain glutamate transporter, GLT-1. In the present study, C6 glioma were used as a model system to define the mechanisms that contribute to this decrease in cell surface expression and to determine the fate of internalized transporter. As was previously observed, phorbol 12-myristate 13-acetate (PMA) caused a decrease in biotinylated GLT-1. This effect was blocked by sucrose or by co-expression with a dominant-negative variant of dynamin 1, and it was attenuated by co-expression with a dominant-negative variant of the clathrin heavy chain. Depletion of cholesterol with methyl-beta-cyclodextrin, co-expression with a dominant-negative caveolin-1 mutant (Cav1/S80E), co-expression with dominant-negative variants of Eps15 (epidermal-growth-factor receptor pathway substrate clone 15), or co-expression with dominant-negative Arf6 (T27N) had no effect on the PMA-induced loss of biotinylated GLT-1. Long-term treatment with PMA caused a time-dependent loss of biotinylated GLT-1 and decreased the levels of GLT-1 protein. Inhibitors of lysosomal degradation (chloroquine or ammonium chloride) or co-expression with a dominant-negative variant of a small GTPase implicated in trafficking to lysosomes (Rab7) prevented the PMA-induced decrease in protein and caused an intracellular accumulation of GLT-1. These results suggest that the PKC-induced redistribution of GLT-1 is dependent upon clathrin-mediated endocytosis. These studies identify a novel mechanism by which the levels of GLT-1 could be rapidly down-regulated via lysosomal degradation. The possibility that this mechanism may contribute to the loss of GLT-1 observed after acute insults to the CNS is discussed.

Differential regulation of GLAST immunoreactivity and activity by protein kinase C: evidence for modification of amino and carboxyl termini

Many neurotransmitter transporters, including the GLT-1 and EAAC1 subtypes of the glutamate transporter, are regulated by protein kinase C (PKC) and these effects are associated with changes in cell surface expression. In the present study, the effects of PKC activation on the glutamate aspartate transporter (GLAST) subtype of glutamate transporter were examined in primary astrocyte cultures. Acute (30 min) exposure to the phorbol 12-myristate 13-acetate (PMA) increased (approximately 20%) transport activity but had the opposite effect on both total and cell surface immunoreactivity. Chronic treatment (6 or 24 h) with PMA had no effect on transport activity but caused an even larger decrease in total and cell surface immunoreactivity. This loss of immunoreactivity was observed using antibodies directed against three different cytoplasmic epitopes, and was blocked by the PKC antagonist, bisindolylmaleimide II. We provide biochemical and pharmacological evidence that the activity observed after treatment with PMA is mediated by GLAST. Two different flag-tagged variants of the human homolog of GLAST were introduced into astrocytes using lentiviral vectors. Although treatment with PMA caused a loss of transporter immunoreactivity, flag immunoreactivity did not change in amount or size. Together, these studies suggest that activation of PKC acutely up-regulates GLAST activity, but also results in modification of several different intracellular epitopes so that they are no longer recognized by anti-GLAST antibodies. We found that exposure of primary cultures of neurons/astrocytes to transient hypoxia/glucose deprivation also caused a loss of GLAST immunoreactivity that was attenuated by the PKC antagonist, bisindolylmaleimide II, suggesting that some acute insults previously thought to cause a loss of GLAST protein may mimic the phenomenon observed in the present study.

Rapid trafficking of the neuronal glutamate transporter, EAAC1: evidence for distinct trafficking pathways differentially regulated by protein kinase C and platelet-derived growth factor

The neuronal glutamate transporter, EAAC1, appears to both limit spillover between excitatory synapses and provide precursor for the synthesis of the inhibitory neurotransmitter, gamma-aminobutyric acid. There is evidence for a large intracellular pool of EAAC1 from which transporter is redistributed to the cell surface following activation of protein kinase C (PKC) or platelet-derived growth factor (PDGF) receptor by seemingly independent pathways. A variety of biotinylation strategies were employed to measure trafficking of EAAC1 to and from the plasma membrane and to examine the effects of phorbol ester and PDGF on these events. Biotinylation of cell surface protein under trafficking-permissive conditions (37 degrees C) resulted in a 2-fold increase in the amount of biotinylated EAAC1 within 15 min in C6 glioma and in primary neuronal cultures, suggesting that EAAC1 has a half-life of approximately 5-7 min for residence at the plasma membrane. Both phorbol ester and PDGF increased the amount of transporter labeled under these conditions. Using a reversible biotinylation strategy, a similarly rapid internalization of EAAC1 was observed in C6 glioma. Phorbol ester, but not PDGF, blocked this measure of internalization. Incubation at 18 degrees C, which blocks some forms of intracellular membrane trafficking, inhibited PKC- and PDGF-dependent redistribution of EAAC1 but had no effect on basal trafficking of EAAC1. These studies suggest that both PKC and PDGF accelerate delivery of EAAC1 to the cell surface and that PKC has an additional effect on endocytosis. The data also suggest that basal and regulated pools of EAAC1 exist in distinct compartments.

Signaling pathways take aim at neurotransmitter transporters

Neurotransmitter transporters are the target of various pharmacological agents used to treat psychological or cognitive conditions, such as depression and attention-deficit disorder. In addition, some of the effects of stimulant-type drugs of abuse result from inhibition of neurotransmitter transporters. Robinson describes the intersection between neurotransmitter transporters and signaling pathways. Neurotransmitter transporters can be regulated by altering the rate of internalization and insertion into the plasma membrane to control cell surface expression or by altering the activity of the transporters within the membrane. As the mechanisms governing regulation of these transporters become elucidated, new potential therapeutic targets may be revealed, given the many processes affected by the activity of neurotransmitter transporters.

Rottlerin, an inhibitor of protein kinase Cdelta (PKCdelta), inhibits astrocytic glutamate transport activity and reduces GLAST immunoreactivity by a mechanism that appears to be PKCdelta-independent

Protein kinase C (PKC) regulates the activity and/or cell surface expression of several different neurotransmitter transporters, including subtypes of glutamate transporters. In the present study, the effects of pharmacological inhibitors of PKC were studied in primary astrocyte cultures that express the glutamate aspartate transporter (GLAST) subtype of glutamate transporter. We found that general inhibitors of PKC, bisindolylmaleimide I (Bis I), bisindolylmaleimide II (Bis II), staurosporine and an inhibitor of classical PKCs, Gö6976, had no effect on Na+-dependent glutamate transport activity. However, rottlerin, a putative specific inhibitor of PKCdelta, decreased transport activity with an IC50 value (less than 10 micro m) that is comparable to that reported for inhibition of PKCdelta. The effect of rottlerin was very rapid (maximal effect within 5 min) and was due to a decrease in the capacity (Vmax) for transport. Rottlerin also caused a drastic loss of GLAST immunoreactivity within 5 min, suggesting that rottlerin accelerates GLAST degradation/proteolysis. Rottlerin had no effect on cell surface or total expression of the transferrin receptor, providing evidence that the effect on GLAST cannot be attributed to a non-specific internalization/degradation of plasma membrane proteins. Down-regulation of PKCdelta with chronic phorbol ester treatment did not block rottlerin-mediated inhibition of transport activity. These results suggest a novel mechanism for regulation of the GLAST subtype of glutamate transporter and indicate that there is a rottlerin target that is capable of controlling the levels of GLAST by controlling the rate of degradation or limited proteolysis. It appears that the target for rottlerin may not be PKCdelta.

Protein kinase C activation decreases cell surface expression of the GLT-1 subtype of glutamate transporter. Requirement of a carboxyl-terminal domain and partial dependence on serine 486

Na(+)-dependent glutamate transporters are required for the clearance of extracellular glutamate and influence both physiological and pathological effects of this excitatory amino acid. In the present study, the effects of a protein kinase C (PKC) activator on the cell surface expression and activity of the GLT-1 subtype of glutamate transporter were examined in two model systems, primary co-cultures of neurons and astrocytes that endogenously express GLT-1 and C6 glioma cells transfected with GLT-1. In both systems, activation of PKC with phorbol ester caused a decrease in GLT-1 cell surface expression. This effect is opposite to the one observed for the EAAC1 subtype of glutamate transporter (Davis, K. E., Straff, D. J., Weinstein, E. A., Bannerman, P. G., Correale, D. M., Rothstein, J. D., and Robinson, M. B. (1998) J. Neurosci. 18, 2475-2485). Several recombinant chimeric proteins between GLT-1 and EAAC1 transporter subtypes were generated to identify domains required for the subtype-specific redistribution of GLT-1. We identified a carboxyl-terminal domain consisting of 43 amino acids (amino acids 475-517) that is required for PKC-induced GLT-1 redistribution. Mutation of a non-conserved serine residue at position 486 partially attenuated but did not completely abolish the PKC-dependent redistribution of GLT-1. Although we observed a phorbol ester-dependent incorporation of (32)P into immunoprecipitable GLT-1, mutation of serine 486 did not reduce this signal. We also found that chimeras containing the first 446 amino acids of GLT-1 were not functional unless amino acids 475-517 of GLT-1 were also present. These non-functional transporters were not as efficiently expressed on the cell surface and migrated to a smaller molecular weight, suggesting that a subtype-specific interaction is required for the formation of functional transporters. These studies demonstrate a novel effect of PKC on GLT-1 activity and define a unique carboxyl-terminal domain as an important determinant in cellular localization and regulation of GLT-1.

Regulated trafficking of neurotransmitter transporters: common notes but different melodies

The activity of biogenic amine and amino acid neurotransmitters is limited by presynaptic and astrocytic Na(+)-dependent transport systems. Their functional importance is underscored by the observation that these transporters are the targets of broad classes of psychotherapeutic agents, including antidepressants and stimulants. Early studies suggested that the activity of these transporters can be fine tuned by a number of different signaling pathways. In the past five years, several groups have provided compelling evidence that changing the cell surface availability of these transporters contributes to this fine tuning. This regulated trafficking can result in rapid (within minutes) increases or decreases in the plasma membrane expression of these transporters and is independent of transcriptional or translational control mechanisms. Many of the same signaling molecules, including protein kinase C (PKC), tyrosine kinase, phosphatidylinositol 3-kinase (P13-K), and protein phosphatase, regulate the transporters for different neurotransmitters. In addition to these classical receptor activated pathways, transporter substrates also regulate activity and cell surface expression of these transporters. In fact, some of the transporters form complexes with signaling molecules. Given the functional and genetic similarities of these transporters, it is not surprising that the same signaling molecules regulate their trafficking, but except for the molecules, the actual effects on individual transporters are remarkably different. It is as if the same musical notes have been rearranged into several different melodies.