Transient Receptor Potential Melastatin 1 (TRPM1) Is an Ion-conducting Plasma Membrane Channel Inhibited by Zinc Ions

Abstract

TRPM1 is the founding member of the melastatin subgroup of transient receptor potential (TRP) proteins, but it has not yet been firmly established that TRPM1 proteins form ion channels. Consequently, the biophysical and pharmacological properties of these proteins are largely unknown. Here we show that heterologous expression of TRPM1 proteins induces ionic conductances that can be activated by extracellular steroid application. However the current amplitudes observed were too small to enable a reliable biophysical characterization. We overcame this limitation by modifying TRPM1 channels in several independent ways that increased the similarity to the closely related TRPM3 channels. The resulting constructs produced considerably larger currents after overexpression. We also demonstrate that unmodified TRPM1 and TRPM3 proteins form functional heteromultimeric channels. With these approaches, we measured the divalent permeability profile and found that channels containing the pore of TRPM1 are inhibited by extracellular zinc ions at physiological concentrations, in contrast to channels containing only the pore of TRPM3. Applying these findings to pancreatic ␤ cells, we found that TRPM1 proteins do not play a major role in steroid-activated currents of these cells. The inhibition of TRPM1 by zinc ions is primarily due to a short stretch of seven amino acids present only in the pore region of TRPM1 but not of TRPM3. Combined, our data demonstrate that TRPM1 proteins are bona fide ion-conducting plasma membrane channels. Their distinct biophysical properties allow a reliable identification of endogenous TRPM1-mediated currents.

Flavanones That Selectively Inhibit TRPM3 Attenuate Thermal Nociception In Vivo

“…TRPM1 can also be activated by PregS. Unlike TRPM3, TRPM1 is strongly blocked by extracellularly applied ZnCl 2 (Lambert et al, 2011). When tested at a concentration of 10 mM, liquiritigenin had almost no inhibitory effect on TRPM1-mediated [Ca 21 ] i signals (Fig.…”

“…HEK293 cells were transiently transfected with a bicistronic expression plasmid, encoding Myc-tagged TRPM1 and enhanced green fluorescent protein, as previously described elsewhere (Lambert et al, 2011;Straub et al, 2013). For heat activation of TRPM3, HEK293 cells were transiently transfected with a bicistronic plasmid encoding TRPM3a2 and a green fluorescent protein protein.…”

Zinc Inactivates Melastatin Transient Receptor Potential 2 Channels via the Outer Pore

“…whether they are primarily involved in interactions with Zn 2ϩ , conformational changes, or both. During the preparation of this manuscript, a study was published showing that Zn 2ϩ induces reversible and concentration-dependent inhibition of the TRPM1 channel but no inhibition of the TRPM3 channel (53). Removal of seven TRPM1-specific residues in the large turret abolished Zn 2ϩ inhibition of the TRPM1 channel, whereas insertion of this short stretch into the TRPM3 channel transferred the sensitivity to inhibition by Zn 2ϩ .…”

Oligomeric State of Purified Transient Receptor Potential Melastatin-1 (TRPM1), a Protein Essential for Dim Light Vision

“…However, the currents have been quite small, and it is not clear that these expressed channels have the same properties as the ON bipolar cell transduction channel, nor is it clear that they are formed without participation of endogenous proteins. Interestingly, TRPM1 has been reported to form relatively robust channels when it is co-expressed with TRPM3 or when selected regions have been substituted with the corresponding regions of TRPM3 (54). Most recently, it was reported (23) that in HEK293 cells transiently transfected with TRPM1 the calcium influx in response to addition of extracellular calcium was not increased over that of untransfected cells.…”

“…Consistent with these observations, many attempts in our laboratory to demonstrate a calcium flux activity mediated by TRPM1 heterologously expressed in Sf9 cells, HEK cells, and CHO cells have failed. Nevertheless, there have been reports of cation currents induced by TRPM1 expression in mammalian cells (22,54,74). These currents are likely derived from the small fraction of TRPM1 that is localized at the plasma membrane.…”