Abstract
Inhibitory glycinergic neurotransmission is terminated by sodium and chloride-dependent plasma membrane glycine transporters (GlyTs). The mainly glial glycine transporter GlyT1 is primarily responsible for the completion of inhibitory neurotransmission and the neuronal glycine transporter GlyT2 mediates the reuptake of the neurotransmitter that is used to refill synaptic vesicles in the terminal, a fundamental role in the physiology and pathology of glycinergic neurotransmission. Indeed, inhibitory glycinergic neurotransmission is modulated by the exocytosis and endocytosis of GlyT2. We previously reported that constitutive and Protein Kinase C (PKC)-regulated endocytosis of GlyT2 is mediated by clathrin and that PKC accelerates GlyT2 endocytosis by increasing its ubiquitination. However, the role of ubiquitination in the constitutive endocytosis and turnover of this protein remains unexplored. Here, we show that ubiquitination of a C-terminus four lysine cluster of GlyT2 is required for constitutive endocytosis, sorting into the slow recycling pathway and turnover of the transporter. Ubiquitination negatively modulates the turnover of GlyT2, such that increased ubiquitination driven by PKC activation accelerates transporter degradation rate shortening its half-life while decreased ubiquitination increases transporter stability. Finally, ubiquitination of GlyT2 in neurons is highly responsive to the free pool of ubiquitin, suggesting that the deubiquitinating enzyme (DUB) ubiquitin C-terminal hydrolase-L1 (UCHL1), as the major regulator of neuronal ubiquitin homeostasis, indirectly modulates the turnover of GlyT2. Our results contribute to the elucidation of the mechanisms underlying the dynamic trafficking of this important neuronal protein which has pathological relevance since mutations in the GlyT2 gene (SLC6A5) are the second most common cause of human hyperekplexia.
Highlights
Inhibitory glycine neurotransmission is terminated by specific transporters, glycine transporters (GlyTs) (GlyT1 and GlyT2), which mediate the reuptake of glycine from the synaptic cleft
We recently demonstrated a direct relationship between the ubiquitination of lysine 791 in the C-terminus of the neuronal glycine transporter GlyT2 and its Protein Kinase C (PKC)-dependent endocytosis
We previously demonstrated that GlyT2 is recycled between the cell surface and cell interior via constitutive and PKC-regulated clathrin-dependent endocytosis, resulting in the localization of a large proportion of the transporter in a subset of Rab11-positive endosomes in central nervous system (CNS) nerve terminals under steady-state conditions [14,16]
Summary
Inhibitory glycine neurotransmission is terminated by specific transporters, GlyTs (GlyT1 and GlyT2), which mediate the reuptake of glycine from the synaptic cleft. GlyTs belong to the neurotransmitter:sodium symporter family (SLC6 gene family), which includes transporters for most of the neurotransmitters (serotonin, dopamine, norepinephrine and GABA) in the central nervous system (CNS) [1]. By mediating the synaptic recycling of glycine, the neuronal transporter GlyT2 preserves the quantal glycine content in synaptic vesicles and assists GlyT1 in regulating glycine levels at the synaptic cleft. Gene deletion studies suggest that modification of glycine transporter activity may be beneficial in several human disorders, including neuromotor deficiencies (startle disease, myoclonus), pain and epilepsy [2,3,4]. A microdeletion in SLC6A5 as cause of startle disease in Irish Wolfhounds has been reported [10]
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