Role of Cl-HCO3 exchanger AE3 in intracellular-pH homeostasis in cultured murine hippocampal neurons, and in crosstalk to adjacent astrocytes

Abstract

The anion exchanger AE3, expressed in hippocampal (HC) neurons but not astrocytes, contributes to intracellular pH (pH_i) 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 pH_i in 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 pH_i homeostasis in HC neurons, co-cultured with astrocytes. We find that the presence of AE3 increases the acidification rate constant during pH_i recovery 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 pH_i later during MAc in both neurons and astrocytes. Paradoxically, AE3 enhances intracellular re-alkalization after MAc removal in neurons and astrocytes, and pH_i recovery from an ammonium-prepulse-induced acid load in neurons. The effects of AE3 knockout on astrocytic pH_i homeostasis in MAc-related assays require the presence of neurons, and are consistent with the hypothesis that the AE3 KO 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.

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