Distinct Modulations of Human Capsaicin Receptor by Protons and Magnesium through Different Domains

Shu Wang,Kinning Poon,Robert E. Oswald,Huai-hu Chuang

doi:10.1074/jbc.m109.058727

Abstract

The capsaicin receptor (TRPV1) is a nonselective cation channel that integrates multiple painful stimuli, including capsaicin, protons, and heat. Protons facilitate the capsaicin- and heat-induced currents by decreasing thermal threshold or increasing agonist potency for TRPV1 activation (Tominaga, M., Caterina, M. J., Malmberg, A. B., Rosen, T. A., Gilbert, H., Skinner, K., Raumann, B. E., Basbaum, A. I., and Julius, D. (1998) Neuron 21, 531-543). In the presence of saturating capsaicin, rat TRPV1 (rTRPV1) reaches full activation, with no further stimulation by protons. Human TRPV1 (hTRPV1), a species ortholog with high homology to rTRPV1, is potentiated by extracellular protons and magnesium, even at saturating capsaicin. We investigated the structural basis for protons and magnesium modulation of fully capsaicin-bound human receptors. By analysis of chimeric channels between hTRPV1 and rTRPV1, we found that transmembrane domain 1-4 (TM1-4) of TRPV1 determines whether protons can further open the fully capsaicin-bound receptors. Mutational analysis identified a titratable glutamate residue (Glu-536) in the linker between TM3 and TM4 critical for further stimulation of fully liganded hTRPV1. In contrast, hTRPV1 TM5-6 is required for magnesium augmentation of capsaicin efficacy. Our results demonstrate that capsaicin efficacy of hTRPV1 correlates with the extracellular ion milieu and unravel the relevant structural basis of modulation by protons and magnesium.

Highlights

Many mammalian TRPV1 agonists exhibit weak receptor activation, even if they may bind the receptor comparably to capsaicin [13, 14]
The first four transmembrane segments (TM1– 4) of TRPV1 dictate whether the activity of a fully capsaicin-bound receptor can be further enhanced by protons, and a glutamate residue (Glu-536) in the linker between TM3 and TM4 of Human TRPV1 (hTRPV1) is critical in the modulation by protons
Further extracellular acidification only led to inhibition of 100 ␮M capsaicin-evoked rat TRPV1 (rTRPV1) currents (Fig. 1, B and C, and Table 1)

Summary

Introduction

Many mammalian TRPV1 agonists exhibit weak receptor activation, even if they may bind the receptor comparably to capsaicin [13, 14]. Pain sensation arising from TRPV1 activation by chemical agonists in humans could conceivably be different from that in rodents, the model species frequently used in biomedical research. Because of its therapeutic implication, a better understanding of how hTRPV1 opens in response to the extracellular ionic milieu is important. Both protons and magnesium enhance the efficacy of capsaicin as an agonist for hTRPV1. The first four transmembrane segments (TM1– 4) of TRPV1 dictate whether the activity of a fully capsaicin-bound receptor can be further enhanced by protons, and a glutamate residue (Glu-536) in the linker between TM3 and TM4 of hTRPV1 is critical in the modulation by protons. A concerted action of the capsaicin-binding domain and the permeation pathway converges at the final stage of hTRPV1 opening

Methods

Results

Conclusion