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

Erin N Hoagland,Thomas W Sherwood,Kirsten G Lee,Christopher J Walker,Candice C Askwith

doi:10.1074/jbc.m110.171330

Erin N Hoagland, Thomas W Sherwood + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m110.171330

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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.

Highlights

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]
ACCN2 transcript variant 1 differs from hVariant 2 in apparent alternate 3Ј splicing of coding exon 9, which results in a coding sequence containing an additional 138 in-frame nucleotides (Fig. 1, A and B) [32]. hVariant 1 was originally isolated from human brain and is represented within the expressed sequence tag data base (AW015707) [32]. hVariant 1 is predicted to produce a protein with an additional 46 amino acids located just prior to the proposed desensitization gate of the second transmembrane domain, a junction known to be critical for ASIC gating (Fig. 1C) [42,43,44]
Three human ACCN2 transcript variants are readily identifiable within the ENTREZ data base. hVariant 2 has been well studied and is homologous to ASIC1b than hVariant 2 (ASIC1a) in other species [5, 23, 31, 32]

Summary

Introduction

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]. The Accn2 gene encodes transcript variants ASIC1a and ASIC1b, which differ in the first third of the protein and display distinct channel properties. 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.

Results

Conclusion