The TRPM1 channel in ON-bipolar cells is gated by both the α and the βγ subunits of the G-protein Go

Abstract

Transmission from photoreceptors to ON bipolar cells in mammalian retina is mediated by a sign-inverting cascade. Upon binding glutamate, the metabotropic glutamate receptor mGluR6 activates the heterotrimeric G-protein Gαoβ3γ13, and this leads to closure of the TRPM1 channel (melastatin). TRPM1 is thought to be constitutively open, but the mechanism that leads to its closure is unclear. We investigated this question in mouse rod bipolar cells by dialyzing reagents that modify the activity of either Gαo or Gβγ and then observing their effects on the basal holding current. After opening the TRPM1 channels with light, a constitutively active mutant of Gαo closed the channel, but wild-type Gαo did not. After closing the channels by dark adaptation, phosducin or inactive Gαo (both sequester Gβγ) opened the channel while the active mutant of Gαo did not. Co-immunoprecipitation showed that TRPM1 interacts with Gβ3 and with the active and inactive forms of Gαo. Furthermore, bioluminescent energy transfer assays indicated that while Gαo interacts with both the N- and the C- termini of TRPM1, Gβγ interacts only with the N-terminus. Our physiological and biochemical results suggest that both Gαo and Gβγ bind TRPM1 channels and cooperate to close them.

The Transduction Cascade in Retinal ON-Bipolar Cells: Signal Processing and Disease

“…In attempt to reconcile these findings a more recent study employed a panel of manipulations, which sequester and activate both Gβγ and Gα o under both dark- and light-adaptated conditions. This study reported evidence that both Gα and Gβγ are involved in regulation of native ON-BC transduction channels (Xu et al 2016). Furthermore, evidence in transfected cells suggest that both Gα and Gβγ are capable of interacting directly with the TRPM1 protein (Xu et al 2016).…”

“…This study reported evidence that both Gα and Gβγ are involved in regulation of native ON-BC transduction channels (Xu et al 2016). Furthermore, evidence in transfected cells suggest that both Gα and Gβγ are capable of interacting directly with the TRPM1 protein (Xu et al 2016). The proposed model (Figure 2) indicates that the interaction of TRPM1 with both free Gβγ and Gα o -GTP is required to close fully transduction channels, and that re-arrangement between the subunits upon their inactivation removes the inhibitory constraint allowing channels to open (Xu et al 2016).…”

“…Furthermore, evidence in transfected cells suggest that both Gα and Gβγ are capable of interacting directly with the TRPM1 protein (Xu et al 2016). The proposed model (Figure 2) indicates that the interaction of TRPM1 with both free Gβγ and Gα o -GTP is required to close fully transduction channels, and that re-arrangement between the subunits upon their inactivation removes the inhibitory constraint allowing channels to open (Xu et al 2016). It should be noted, however, that these studies did not use physiologically-relevant combinations of Gβγ, and did not assess the influence of additional components of the ion channel complex.…”

“…Upon GTP hydrolysis, G proteins rearrange forming inactive heterotrimer allowing the TRPM1 channel to open. Modified from (Xu et al 2016). …”

TRPM3_miR-204: a complex locus for eye development and disease

“…Conversely, they are inhibited by (1) certain clinically approved drugs including nonsteroidal anti-inflammatory drugs (NSAIDs, e.g., mefenamic acid), antibiotics (e.g., voriconazole, which also inhibits TRPM1 channels), sex-steroids (e.g., progesterone), and synthetic drug-like antagonists [40,41,51,61,[67][68][69][70][71][72]; (2) naturally occurring plant-derived secondary metabolites (e.g., citrus flavanones) [73][74][75]; and (3) Gprotein coupled receptor βγ subunits [76][77][78][79]. By contrast, TRPM1 channels were found to be inhibited by interaction with both Gα or Gβγ subunits [80].…”