title: Role of Cl--HCO3-exchanger AE3 in intracellular pH homeostasis in cultured murine hippocampal neurons, and in crosstalk to adjacent astrocytes.
abstract: A polymorphism of human AE3 is associated with idiopathic generalized epilepsy. Knockout of AE3 in mice lowers the threshold for triggering epileptic seizures. The explanations for these effects are elusive. Comparisons of cells from wild-type vs. AE3-/-mice show that AE3 (present in hippocampal neurons, not astrocytes; mediates HCO3-efflux) enhances intracellular pH (pHi) recovery (decrease) from alkali loads in neurons and, surprisingly, adjacent astrocytes. During metabolic acidosis (MAc), AE3 speeds initial acidification, but limits the extent of pHidecrease in neurons and astrocytes. AE3 speeds re-alkalization after removal of MAc in neurons and astrocytes, and speeds neuronal pHirecovery from an ammonium prepulse-induced acid load. We propose that neuronal AE3 indirectly increases acid extrusion in (a) neurons via Cl-loading, and (b) astrocytes by somehow enhancing NBCe1 (major acid extruder). The latter would enhance depolarization-induced alkalinization of astrocytes, and extracellular acidification, and thereby reduce susceptibility to epileptic seizures.
  
  
  
  The anion exchanger AE3, expressed in hippocampal (HC) neurons but not astrocytes, contributes to intracellular pH (pHi) regulation by facilitating the exchange of extracellular Cl-for intracellular HCO3-. The human AE3 polymorphism A867D is associated with idiopathic generalized epilepsy. Moreover, AE3 knockout (AE3-/-) mice are more susceptible to epileptic seizure. The mechanism of these effects has been unclear because the starting pHiin AE3-/-and wild-type neurons is indistinguishable. The purpose of the present study was to use AE3-/-mice to investigate the role of AE3 in pHihomeostasis in HC neurons, co-cultured with astrocytes. We find that the presence of AE3 increases the acidification rate constant during pHirecovery from intracellular alkaline loads imposed by reducing [CO2]. The presence of AE3 also speeds intracellular acidification during the early phase of metabolic acidosis (MAc), not just in neurons but, surprisingly, in adjacent astrocytes. Additionally, AE3 contributes to braking the decrease in pHilater during MAc in both neurons and astrocytes. Paradoxically, AE3 enhances intracellular re-alkalization after MAc removal in neurons and astrocytes, and pHirecovery from an ammonium prepulse-induced acid load in neurons. The effects of AE3 knockout on astrocytic pHihomeostasis in MAc-related assays require the presence of neurons, and are consistent with the hypothesis that the AE3 knockout reduces functional expression of astrocytic NBCe1. These findings suggest a new type of neuron-astrocyte communication, based on the expression of AE3 in neurons, which could explain how AE3 reduces seizure susceptibility.
authors.affiliation: Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, OH, 44106, USA.
authors.forename: Ahlam I
authors.initials: AI
authors.lastname: Salameh
authors.affiliation: Institute of Human Genetics, University Hospital Jena, Jena, Germany.
authors.forename: Christian A
authors.initials: CA
authors.lastname: Hübner
authors.affiliation: Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, OH, 44106, USA.
authors.forename: Walter F
authors.initials: WF
authors.lastname: Boron
mesh_terms: D000818:Animals; D017920:Antiporters / Q000235:genetics / Q000502:physiology*; D001253:Astrocytes / Q000502:physiology*; D002478:Cells, Cultured; D018920:Coculture Techniques; D006624:Hippocampus / Q000166:cytology; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009474:Neurons / Q000502:physiology*
chemical_list: D017920:Antiporters; C062002:Slc4a3 protein, mouse
keywords: Slc4a3; acid-base balance; acid-extrusion rate from cells; acid-loading rate of cells; anion exchanger