TRPV1Δ1-283: A Candidate Osmoreceptor Channel

Abstract

Mammalian osmosensory neurons exposed to hypertonicity display a shrinking-induced increase in non-selective cation conductance. That causes membrane depolarization and excitation triggering homeostatic responses such as thirst and antidiuretic hormone release. The molecular identity of the osmoreceptor channel is not known, but appears to involve a capsaicin-insensitive N-terminal variant of the transient receptor potential vanilloid type 1 (TRPV1) channel (Ciura and Bourque, 2006; Sharif-Naeini et al., 2006). Here, we report that heterologous expression of TRPV1 variant Δ1-283, which lacks exons 1-5, confers osmosensory characteristics similar to those found in native osmosensory neurons. Whole cell voltage clamp recordings were used to examine the effects of hyperosmolality (+mannitol) on human embryonic kidney (HEK293) cells expressing Δ1-283 TRPV1, GFP or wild type TRPV1. Dynamic imaging confirmed that all HEK cells exposed to hypertonicity underwent visible shrinking. However in contrast to GFP (n = 10) and TRPV1 (n = 32) transfected cells, which were unresponsive, ∼41% (84/206) of cells transfected with Δ1-283 TRPV1 showed a progressive and sustained increase in non-selective cation current and membrane conductance when exposed to a hyperosmotic stimuli lasting 2-5 minutes. The effects of osmolality were dose-dependent (+5 to +80 mosmol/kg), mimicked by suction induced cell shrinking (n=6), and were abolished by Ruthenium Red. Moreover, addition of capsaicin caused robust responses in cells expressing TRPV1, but had no effect on those expressing Δ1-283 TRPV1. Imaging experiments on cells loaded with Fura-2 revealed that ∼21% of the cells transfected with Δ1-283 TRPV1 display reversible increases in intracellular [Ca2+], whereas ∼17% shown an increase without reversion when exposed to a +40 mosmol/kg hyperosmotic stimulus. These results suggest Δ1-283 TRPV1 channels could mediate mechanically-induced increases in cation current associated with hypertonicity-induced shrinking in osmosensory neurons.