Identification of a Calcium Permeable Human Acid-sensing Ion Channel 1 Transcript Variant

Abstract

The acid-sensing ion channels (ASICs) are proton-gated cation channels activated when extracellular pH declines. In rodents, the Accn2 gene encodes transcript variants ASIC1a and ASIC1b, which differ in the first third of the protein and display distinct channel properties. In humans, ACCN2 transcript variant 2 (hVariant 2) is homologous to mouse ASIC1a. In this article, we study two other human ACCN2 transcript variants. Human ACCN2 transcript variant 1 (hVariant 1) is not present in rodents and contains an additional 46 amino acids directly preceding the proposed channel gate. We report that hVariant 1 does not produce proton-gated currents under normal conditions when expressed in heterologous systems. We also describe a third human ACCN2 transcript variant (hVariant 3) that is similar to rodent ASIC1b. hVariant 3 is more abundantly expressed in dorsal root ganglion compared with brain and shows basic channel properties analogous to rodent ASIC1b. Yet, proton-gated currents from hVariant 3 are significantly more permeable to calcium than either hVariant 2 or rodent ASIC1b, which shows negligible calcium permeability. hVariant 3 also displays a small acid-dependent sustained current. Such a sustained current is particularly intriguing as ASIC1b is thought to play a role in sensory transduction in rodents. In human DRG neurons, hVariant 3 could induce sustained calcium influx in response to acidic pH and make a major contribution to acid-dependent sensations, such as pain.The acid-sensing ion channels (ASICs) 4 are a family of proton-gated cation channels expressed in neurons throughout the central and peripheral nervous system (1). There are four ASIC genes (Accn1-4) that produce at least six individual ASIC subunits in rodents (ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4). ASIC subunits have a characteristic topology with two transmembrane domains separated by a large cysteine-rich extracellular region (2, 3). Three individual ASIC subunits associate to form functional cation channels that are activated by decreases in extracellular pH (4, 5). ASIC currents are typically transient, inactivate even in the continued presence of acidic pH, and (except for ASIC1a) are not substantially permeable to calcium (6). The specific properties of an ASIC current, such as the pH necessary for activation and the kinetics of inactivation, are defined by the subunit composition of the channel (7). In the central nervous system, the ASIC1a subunit plays an important role (8). Genetic disruption or pharmacological inhibition of ASIC1a affects learning and memory, fear-related behaviors, pain, depression, and seizure duration in rodents (9 -15). ASIC1a also contributes to neuronal damage after cerebral ischemia in mice and mediates neuronal death following prolonged acidosis (16 -19). ASIC1a is thought to play a prominent role in neuronal death because it is uniquely permeable to calcium compared with other ASICs (16,20,21).In rodents, the Accn2 gene encodes both ASIC1a and ASIC1b. ASIC1b is a transcript variant expressed...

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