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Glutamate transporter-1 (GLT-1) is the main glutamate transporter in the central nervous system, and its concentration severely decreases in neurodegenerative diseases. The number of transporters in the plasma membrane reflects the balance between their insertion and removal, and it has been reported that the regulated endocytosis of GLT-1 depends on its ubiquitination triggered by protein kinase C (PKC) activation. Here, we identified serine 520 of GLT-1 as the primary target for PKC-dependent phosphorylation, although elimination of this serine did not impair either GLT-1 ubiquitination or endocytosis in response to phorbol esters. In fact, we present evidence indicating that the ubiquitin ligase Nedd4-2 mediates the PKC-dependent ubiquitination and down-regulation of GLT-1. Overexpression of Nedd4-2 increased the ubiquitination of the transporter and promoted its degradation. Moreover, phorbol myristate acetate enhanced Nedd4-2 phosphorylation and the formation of GLT-1·Nedd4-2 complexes, whereas siRNA knockdown of Nedd4-2 prevented ubiquitination, endocytosis, and the concomitant decrease in GLT-1 activity triggered by PKC activation. These results indicate that GLT-1 endocytosis is independent of its phosphorylation and that Nedd4-2 mediates PKC-dependent down-regulation of the transporter.

The glycine transporter GLYT1 regulates both glycinergic and glutamatergic neurotransmission by controlling the reuptake of glycine at synapses. Trafficking of GLYT1 to and from the cell surface is critical for its function. Activation of PKC down-regulates the activity of GLYT1 through a mechanism that has so far remained uncharacterized. Here we show that GLYT1b undergoes fast constitutive endocytosis that is accelerated by phorbol esters. Both constitutive and regulated endocytosis occur through a dynamin 2- and clathrin-dependent pathway, accumulating in the transporter in transferrin-containing endosomes. A chimera with the extracellular and transmembrane domains of the nerve growth factor receptor and the COOH-terminal tail of GLYT1 was efficiently internalized through this clathrin pathway, suggesting the presence of molecular determinants for GLYT1b endocytosis in its COOH-terminal tail. Extensive site-directed mutagenesis in this region of the chimera highlighted the involvement of lysine residues in its internalization. In the context of the full-length transporter, lysine 619 played a prominent role in both the constitutive and phorbol 12-myristate 13-acetate-induced endocytosis of GLYT1b, suggesting the involvement of ubiquitin modification of GLYT1b during the internalization process. Indeed, we show that GLYT1b undergoes ubiquitination and that this process is stimulated by phorbol 12-myristate 13-acetate. In addition, this endocytosis is impaired in an ubiquitination-deficient cell line, further evidence that constitutive and regulated endocytosis of GLYT1b is ubiquitin-dependent. It remains to be determined whether GLYT1b recycling might be affected in pathologies involving alterations to the ubiquitin system, thereby interfering with its influence on inhibitory and excitatory neurotransmission.