Elementary response triggered by transducin in retinal rods

Abstract

G protein-coupled receptor (GPCR) signaling is crucial for many physiological processes. A signature of such pathways is high amplification, a concept originating from retinal rod phototransduction, whereby one photoactivated rhodopsin molecule (Rho*) was long reported to activate several hundred transducins (GT*s), each then activating a cGMP-phosphodiesterase catalytic subunit (GT*·PDE*). This high gain at the Rho*-to-GT* step has been challenged more recently, but estimates remain dispersed and rely on some nonintact rod measurements. With two independent approaches, one with an extremely inefficient mutant rhodopsin and the other with WT bleached rhodopsin, which has exceedingly weak constitutive activity in darkness, we obtained an estimate for the electrical effect from a single GT*·PDE* molecular complex in intact mouse rods. Comparing the single-GT*·PDE* effect to the WT single-photon response, both in Gcaps−/− background, gives an effective gain of only ∼12–14 GT*·PDE*s produced per Rho*. Our findings have finally dispelled the entrenched concept of very high gain at the receptor-to-G protein/effector step in GPCR systems.

Cell-autonomous light sensitivity via Opsin3 regulates fuel utilization in brown adipocytes

“…Our results reveal that the lack of Opn3 in brown adipose cells leads to impaired lipolysis and decreased mitochondrial activity even in darkness, presumably reflecting some basal constitutive Opn3 activity that exists in adipose tissue without light (as exemplified by rhodopsin; [51]). We employed the RNA-seq approach and identified the top 30 most significantly different genes between WT and Opn3-KO cells in the dark (ranked by p-value) (S1 Table).…”

“…This highly conserved ERY motif is located at the cytoplasmic end of the third transmembrane domain of class A GPCRs (rhodopsin family) [45,46] and couples receptor conformational states to G protein activation in several GPCRs [47][48][49], including rhodopsin [50][51][52]. As such, ERY motif mutants of rhodopsin [50,51], V2 vasopressin receptor [53], and the gonadotropin-releasing hormone receptor [54] have all led to defective signal transduction by affecting the Guanosine diphosphate (GDP)-release step in the respective G protein activation cascades. For example, the interaction between the REY mutant in rhodopsin and its downstream Gt protein decreases by 7,000 times with little or no change in the mutant protein's expression and retinal morphology [51].…”

“…As such, ERY motif mutants of rhodopsin [50,51], V2 vasopressin receptor [53], and the gonadotropin-releasing hormone receptor [54] have all led to defective signal transduction by affecting the Guanosine diphosphate (GDP)-release step in the respective G protein activation cascades. For example, the interaction between the REY mutant in rhodopsin and its downstream Gt protein decreases by 7,000 times with little or no change in the mutant protein's expression and retinal morphology [51]. The same may apply to the Opn3-GM mutant in question.…”

Diminished Cone Sensitivity in cpfl3 Mice Is Caused by Defective Transducin Signaling

“…The relationship between the amount of transducin and photoreceptor sensitivity is unclear, with evidence indicating a simple linear relationship 23 or a decidedly nonlinear relation such that the decrease in sensitivity is much less than the decrease in transducin concentration. 24 Such a nonlinear relation could exist if the rate of activation were less affected by the rate of encounter of holo-transducin with light-activated rhodopsin than by the rate of production of activated transducin-α once binding has occurred. There is no indication, however, that a two-fold to four-fold decrease in transducin concentration could produce a larger than two to four times decrease in sensitivity, much less the 30 to 250 times decrease we have observed for cpfl3 cones.…”

Dark noise and retinal degeneration from D190N-rhodopsin

“…D190N-rhodopsin | spontaneous isomerization | protein misfolding | night blindness | retinitis pigmentosa O ur dark-adapted visual system has an exceedingly low threshold for perceiving light (1), imparted largely by rhodopsin in several ways, namely, the high density of rhodopsin packed into each rod to ensure a high photon-capture ability (2), rhodopsin's substantial signal amplification to provide a sizeable single-photon response (3)(4)(5), and rhodopsin's low spontaneous excitation to minimize false signaling, or dark noise (6,7). As the most abundant protein in rods, rhodopsin also supports the structure of the outer segment, sustaining photoreceptor health and survival (8)(9)(10)(11).…”