Mechanisms of Fractional Calcium Currents through TRPV1 Channels in Primary Sensory Neurons

Abstract

TRPV1 and TRPV2 channels are important molecular sensors on the plasma membrane in mammalian physiology (temperature) and diseases (pain) in sensory neurons. These nonselective cation channels permit Ca2+, Na+ and K+ influxes simultaneously through its pores to regulate intracellular Ca2+ homeostasis and Ca2+-dependent neural transmitter release. The Ca2+ influx can be determined by the fractional Ca2+ current (Pf) through a cation channel (Burnashev et al., 1995; Schneggenburger et al., 1993; Yu et al., 2004; Zhou and Neher, 1993). Here, we report (1) Opposite Ca2+ permeability was found as determined by the classic “Goldman-Hodgkin-Katz equation” of PCa/Na (TRPV1 = 7.6 > TRPV2 = 2.8) under non-physiological intra- and extracellular solutions(Caterina et al., 1999; Caterina et al., 1997), or by the “fractional Ca2+-current” of TRPV1 vs. TRPV2 (Pf = 5.5% vs. 22%) under physiological solutions; (2) The selective filter “GMGX” is similar in TRPV1 and TRPV2, except “X” (X = D for TRPV1 and E for V2). In TRPV1, switching native D to E of TRPV2, Pf(V1, D646E) was greatly increased toward Pf(V2) (from 5.5% to 13%), and vice versa (from 22% to 5.0%); (3) Mutations of two sites outside of the “GMGX”, reduced Pf by half; (4) In native neurons replacing TRPV1-WT (Pf = 5.5%) with TRPV1-D646E (Pf = 13%), the release mode of Ca2+-dependent single vesicle events was dramatically altered from partial (kiss-and-run) to full release as determined by TIRF-imaging, implicating a physilogical relevance of Pf(TRP) studies. Taken together, TRPV1-D646 (or TRPV2-E604) is the dominant site determining fractional Ca2+-influx through thermal sensitive TRP channels—a novel mechanism of TRPV channels in presynaptic neural transmitter release for temperature and pain sensation.