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

Neurobiol Dis. 2015 Feb;74:314-24. doi: 10.1016/j.nbd.2014.12.004. Epub 2014 Dec 10.

Abstract

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.

Keywords: Amyotrophic lateral sclerosis; GABA heterotransporter; Glutamate excitotoxicity; Glutamate release; Glycine heterotransporter; SOD1(G93A) mice; Transporter trafficking.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amyotrophic Lateral Sclerosis / metabolism*
  • Animals
  • Disease Models, Animal
  • Exocytosis / physiology*
  • Female
  • GABA Plasma Membrane Transport Proteins / metabolism*
  • Glutamic Acid / metabolism*
  • Glycine Plasma Membrane Transport Proteins / metabolism*
  • Humans
  • Male
  • Mice
  • Mice, Knockout
  • Mice, Transgenic
  • Nerve Tissue Proteins / deficiency
  • Nerve Tissue Proteins / genetics
  • Spinal Cord / metabolism
  • Superoxide Dismutase / genetics
  • Superoxide Dismutase / metabolism
  • Superoxide Dismutase-1
  • Synapses / metabolism
  • Synaptosomes / metabolism
  • gamma-Aminobutyric Acid / metabolism

Substances

  • GABA Plasma Membrane Transport Proteins
  • Glycine Plasma Membrane Transport Proteins
  • Nerve Tissue Proteins
  • SOD1 protein, human
  • Slc6a1 protein, mouse
  • Slc6a5 protein, mouse
  • Slc6a9 protein, mouse
  • amphiphysin
  • Glutamic Acid
  • gamma-Aminobutyric Acid
  • Sod1 protein, mouse
  • Superoxide Dismutase
  • Superoxide Dismutase-1