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 (SOD1G93A), a model of human amyotrophic lateral sclerosis showing constitutively excessive Glu exocytosis. Exposure of spinal cord synaptosomes from WT mice to a 35 mM KCl pulse increased the expression of GAT-1 at glutamatergic synaptosomal membranes and enhanced the GAT-1 heterotransporter-induced [3H]d-aspartate ([3H]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 [3H]d-Asp release in WT mice. Impaired endocytosis in Amph-I KO mice increased GAT-1 membrane expression and [3H]GABA uptake in spinal cord synaptosomes. Also the GAT-1 heterotransporter-evoked release of [3H]d-Asp was augmented in Amph-I KO mice. The constitutively excessive Glu exocytosis in SOD1G93A mice resulted in augmented GAT-1 expression at glutamatergic synaptosomal membranes and GAT-1 or GlyT-1/2 heterotransporter-mediated [3H]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 SOD1G93A mice, is due to the heterotransporter over-expression at the nerve terminal membrane, promoted by the excessive Glu exocytosis.