Insights into the desensitization mechanisms of acid-sensing ion channels.

Abstract

Acid-sensing ion channels (ASICs) are proton-gated cation channels contribute to synaptic transmission, learning and memory. ASICs are also involved in a wide range of diseases associated with prolonged tissue acidosis, such as ischemic stroke, chronic pain and neurological disorders. Besides activation, extracellular protonation of ASICs can elicit at least three types of desensitization: (i) reversible desensitization after activation, (ii) Steady-state desensitization (SSD), where the channels desensitize without apparent opening, and (iii) tachyphylaxis, a decrease in the response amplitude in a series of activations. The different mechanisms behind the various types of desensitization are not yet fully understood. Here, we use voltage-clamp fluorometry to track the conformational changes of the extracellular domain of the ASIC1a in parallel with recordings of the functional properties of the channel. We introduce selected mutations in ASIC1a to decrease desensitization after activation. These channels show a full or partial reduction in SSD but are more prone to exhibiting strong tachyphylaxis. While the mutations heavily impact all three types of desensitization at the level of the pore, the extracellular domain still undergoes conformational changes and remains highly sensitive to changes in pH. Additionally, the channels remain affected by known extracellular peptide modulators of ASIC1a. These observations highlight a possible decoupling between the extracellular domain and the pore during desensitization. Understanding the mechanism behind the conformational and functional changes not only during activation, but also desensitization might be particularly relevant for disorders with moderate tissue acidification and thus for pharmacological targeting.