Characterization of an epilepsy-associated variant of the human Cl-/HCO3- exchanger AE3

Vilas GL, Johnson DE, Freund P, Casey JR. Characterization of an epilepsy-associated variant of the human Cl-/HCO3- exchanger AE3. Am J Physiol Cell Physiol 297: C526-C536, 2009. First published July 15, 2009; doi:10.1152/ajpcell.00572.2008.-Anion exchanger 3 (AE3), expressed in the brain, heart, and retina, extrudes intracellular HCO3- in exchange for extracellular Cl-. The SLC4A3 gene encodes two variants of AE3, brain or full-length AE3 (AE3(fl)) and cardiac AE3 (cAE3). Epilepsy is a heterogeneous group of disorders characterized by recurrent unprovoked seizures that affect about 50 million people worldwide. The AE3-A867D allele in humans has been associated with the development of IGE (IGE), which accounts for similar to 30% of all epilepsies. To examine the molecular basis for the association of the A867D allele with IGE, we characterized wild-type (WT) and AE3(fl)-A867D in transfected human embryonic kidney (HEK)-293 cells. AE3fl-A867D had significantly reduced transport activity relative to WT (54 +/- 4%, P < 0.01). Differences in expression levels or the degree of protein trafficking to the plasma membrane did not account for the defect of AE3fl-A867D. Treatment with 8-bromo-cAMP (8-Br-cAMP) increased Cl-/HCO3- exchange activity of WT and AE3fl-A867D to a similar degree, which was abolished by preincubation with the protein kinase A (PKA)-specific inhibitor H89. This indicates that PKA regulates WT and AE3(fl)-A867D Cl-/HCO3- exchange activity. No difference in Cl-/HCO3- exchange activity was found between cultures of mixed populations of neonatal hippocampal cells from WT and slc4a3(-/-) mice. We conclude that the A867D allele is a functional (catalytic) mutant of AE3 and that the decreased activity of AE3fl-A867D may cause changes in cell volume and abnormal intracellular pH. In the brain, these alterations may promote neuron hyperexcitability and the generation of seizures.