Acid-sensing ion channels: Regulation and physiologic function

Acid-sensing ion channels (ASICs) are proton (H+)-gated ion channels that produce transient cation currents in response to extracellular acid. ASICs are expressed widely in the brain, and contribute to learning, memory, and fear-related behaviors. Inappropriate activation of ASICs during prolonged acidosis also induces neuronal damage during brain ischemia. However, little is known about how ASICs contribute to neuronal function or how ASICs are regulated by endogenous modulators.; For my thesis, I began by investigating the role of sulfhydryl compounds on ASIC activity. I determined that glutathione, of which concentration increases in ischemic brains, potentiated ASIC1a-mediated H+-gated currents by increasing apparent proton sensitivity and slowing channel desensitization. This potentiation of ASIC1a is due to the relief of tonic inhibition by transition metal ions, and the reduction of redox-sensitive residues. These results suggest that endogenous sulfhydryl compounds such as glutathione potentiate ASICs in neurons, and may exacerbate acidotoxic neuronal death during ischemia.; ASIC1 knockout mice display defects in learning and memory, suggesting a role of ASICs in synaptic function. I investigated how ASICs contribute to synaptic transmission by comparing synaptic responses between wild-type and ASIC knockout hippocampal neurons in microisland culture. I determined that neurons from ASIC1 knockout mice have an increased probability of neurotransmitter release and altered short-term plasticity. Further, transfection of ASIC1a into ASIC1 knockout neurons restored release probability. These results suggest that ASIC1a regulates basal synaptic transmission and short-term plasticity by modulating neurotransmitter release at glutamatergic synapses, and that ASIC1a contributes to normal learning and memory through its presynaptic function.; I also investigated the physiologic role of ASICs in the cerebellum. Both ASIC1 and ASIC2 are expressed in the cerebellum including Purkinje cells, which have intrinsic firing property required for normal cerebellar function. Neither ASIC1 knockout nor ASIC2 knockout mice showed any difference in tonic firing of Purkinje cell in acute cerebellar slices compared to wild-type mice. However, the tonic firing rate of Purkinje cells was increased significantly in ASIC1/ASIC2 double knockout mice. These results suggest that both ASIC1 and ASIC2 can contribute to normal cerebellar function by regulating Purkinje cell firing independently.; Together, these studies describe, for the first time, how ASICs are regulated by glutathione, one of the ischemia-related signals, and how ASICs regulate the glutamatergic synaptic transmission of hippocampal neurons and the tonic firing of cerebellar Purkinje cells. These results discussed herein provide significant insights on the pathologic mechanism of ischemic neuronal death, and the physiologic roles of ASICs in synaptic function and neuronal excitability.