Characterization of an epilepsy-associated variant of the human Cl-/HCO3(-) exchanger AE3.
Anion exchanger 3 (AE3), expressed in the brain, heart, and retina, extrudes intracellular HCO(3)(-) 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 approximately 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. AE3(fl)-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 AE3(fl)-A867D. Treatment with 8-bromo-cAMP (8-Br-cAMP) increased Cl(-)/HCO(3)(-) exchange activity of WT and AE3(fl)-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(-)/HCO(3)(-) exchange activity. No difference in Cl(-)/HCO(3)(-) 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 AE3(fl)-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.
Vilas
Gonzalo L
GL
Dept. of Physiology, Univ. of Alberta, Edmonton, Alberta T6G 2H7, Canada.
Johnson
Danielle E
DE
Freund
Paul
P
Casey
Joseph R
JR
D000818:Animals; D017920:Antiporters / Q000235:genetics* / Q000378:metabolism; D002460:Cell Line; D004827:Epilepsy / Q000235:genetics* / Q000378:metabolism*; D005786:Gene Expression Regulation; D006624:Hippocampus / Q000166:cytology; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D009154:Mutation; D009474:Neurons / Q000378:metabolism
D017920:Antiporters; C489237:SLC4A3 protein, human; C062002:Slc4a3 protein, mouse