The Endoplasmic Reticulum of Dorsal Root Ganglion Neurons Contains Functional TRPV1 Channels*

Transient receptor potential vanilloid type 1 (TRPV1) is a plasma membrane Ca2+ channel involved in transduction of painful stimuli. Dorsal root ganglion (DRG) neurons express ectopic but functional TRPV1 channels in the endoplasmic reticulum (ER) (TRPV1ER). We have studied the properties of TRPV1ER in DRG neurons and HEK293T cells expressing TRPV1. Activation of TRPV1ER with capsaicin or other vanilloids produced an increase of cytosolic Ca2+ due to Ca2+ release from the ER. The decrease of [Ca2+]ER was directly revealed by an ER-targeted aequorin Ca2+ probe, expressed in DRG neurons using a herpes amplicon virus. The sensitivity of TRPV1ER to capsaicin was smaller than the sensitivity of the plasma membrane TRPV1 channels. The low affinity of TRPV1ER was not related to protein kinase A- or C-mediated phosphorylations, but it was due to inactivation by cytosolic Ca2+ because the sensitivity to capsaicin was increased by loading the cells with the Ca2+ chelator BAPTA. Decreasing [Ca2+]ER did not affect the sensitivity of TRPV1ER to capsaicin. Disruption of the TRPV1 calmodulin-binding domains at either the C terminus (Δ35AA) or the N terminus (K155A) increased 10-fold the affinity of TRPV1ER for capsaicin, suggesting that calmodulin is involved in the inactivation. The lack of TRPV1 sensitizers, such as phosphatylinositol 4,5-bisphosphate, in the ER could contribute to decrease the affinity for capsaicin. The low sensitivity of TRPV1ER to agonists may be critical for neuron health, because otherwise Ca2+ depletion of ER could lead to ER stress, unfolding protein response, and cell death.