Neuropeptide Interaction with the Extracellular Domain of the Acid Sensing Ion Channel 1A

Abstract

Acidic fluctuations occur within the brain and contribute to multiple physiological and patho-physiological processes. Neurons possess specific molecules to sense and respond to these pH changes. In particular, many neurons express the acid sensing ion channels (ASICs). ASICs are ligand-gated ion channels activated by extracellular protons. ASICs are permeable to cations and depolarize neurons to mediate acid-dependent neuronal signaling. Some ASIC channels are also permeable to calcium and these channels are thought to play a particularly important role in neuronal processes. Specifically, ASIC1a subunits form channels permeable to calcium and contribute to multiple behaviors, seizure termination, pain, as well as neuronal death initiated by prolonged acidosis following inflammation and stroke. ASIC1a undergoes a process called steady-state desensitization in which slow, incremental acidification causes the channels to desensitize rather than undergo robust activation. Induction of steady-state desensitization prevents ASIC1a-mediated neuronal death. Yet, specific neuropeptides present within the brain limit steady-state desensitization and allow ASIC1a to activate following slow, incremental acidification. Two types of neuropeptides, the RFamides and dynorphin-related peptides, interact directly with the extracellular domain of ASICs to modulate channel gating. In this work, we explore the mechanism and protein domains responsible for modulation of ASC1a steady-state desensitization by endogenous neuropeptides. We determined that (1) RFamide and dynorphins alter ASIC gating in distinct, but overlapping ways; (2) dynorphins can compete with the inhibitory spider toxin venom peptide PcTx1 for interaction with the channels; and (3) the addition of RFamide and dynorphins synergizes ASIC1a modulation. We also present evidence that specific protein domains are involved in endogenous neuropeptide modulation of ASIC1a. Together, these studies further define endogenous neuropeptides as important modulators of ASIC1a activity and present data indicating that distinct protein domains control neuropeptide effects.