Opsin1-2, Gqα and arrestin levels at Limulus rhabdoms are controlled by diurnal light and a circadian clock

Abstract

in white-eyed Drosophila maintained under artificial light. Here we tested whether protein levels at rhabdoms change significantly in the highly pigmented lateral eyes of wild-caught Limulus polyphemus maintained in natural diurnal illumination and whether these changes are under circadian control. We found that rhabdomeral levels of opsins (Ops1-2), the G protein activated by rhodopsin (G q α) and arrestin change significantly from day to night and that nighttime levels of each protein at rhabdoms are significantly influenced by signals from the animalʼs central circadian clock. Clock input at night increases Ops1-2 and G q α and decreases arrestin levels at rhabdoms. Clock input is also required for a rapid decrease in rhabdomeral Ops1-2 beginning at sunrise. We found further that dark adaptation during the day and the night are not equivalent. During daytime dark adaptation, when clock input is silent, the increase of Ops1-2 at rhabdoms is small and G q α levels do not increase. However, increases in Ops1-2 and G q α at rhabdoms are enhanced during daytime dark adaptation by treatments that elevate cAMP in photoreceptors, suggesting that the clock influences dark-adaptive increases in Ops1-2 and G q α at Limulus rhabdoms by activating cAMP-dependent processes. The circadian regulation of Ops1-2 and G q α levels at rhabdoms probably has a dual role: to increase retinal sensitivity at night and to protect photoreceptors from light damage during the day.

Opsins inLimuluseyes: Characterization of three visible light-sensitive opsins unique to and co-expressed in median eye photoreceptors and a peropsin/RGR that is expressed in all eyes

“…A separate question is whether the visible light-sensitive opsins expressed in the LEs and VEs (LpOps1-2 and -5) are expressed in MEs. A puzzling finding mentioned in the Introduction is that although we routinely detect LpOps1-2 and -5 transcripts in ME cDNA with PCR using primers that amplify across an intron (Smith et al, 1993;Katti et al, 2010), we do not detect LpOps1-2 or -5-ir in ME rhabdoms using antibodies that clearly detect these proteins in LE and VE (Katti et al 2010;Battelle et al, 2013) (present study, Fig. 5B).…”

“…In LE retinular cells, for example, roughly 50% of rhabdomeric LpOps1-2 is shed during the day in response to light and signals from an internal circadian clock, and much appears to be degraded through a lysosomal pathway (Chamberlain and Barlow, 1979;Sacunas et al, 2002;Katti et al, 2010). However, by 4 h after sunset, the same amount of LpOps1-2 that was removed during the day is restored to rhabdoms (Battelle, 2013;Battelle et al, 2013). A similar daytime loss of r-opsins has been documented for VE photoreceptors (Katti et al, 2010).…”

“…, are co-expressed in LE retinular cells and giant ventral photoreceptors (Katti et al, 2010). However, interestingly, the relative levels of LpOps1-2 and LpOps5 proteins in rhabdoms change dramatically from day to night under the influence of light and signals from an internal circadian clock (Katti et al, 2010;Battelle et al, 2013;Battelle, 2013). Changes in the relative levels of these co-expressed opsins in rhabdoms will not alter the spectral tuning of the cell, but might influence the dynamics of the photoresponse.…”

Opsin expression in Limulus eyes: A UV opsin is expressed in each eye type and co-expressed with a visible light-sensitive opsin in ventral larval eyes

“…Our failure to detect LpUVOps1-ir on western blots of LE membranes (Fig. 5) is probably because eccentric cell membranes comprise only a small fraction of the total membrane in LEs, with most originating from the larger and more numerous retinular cells that express the visible-light-sensitive opsins LpOps1-2 and LpOps5 (Katti et al, 2010;Battelle et al, 2013). Finding LpUVOps1 in LEs was surprising because prior spectral studies did not detect UV sensitivity in these eyes (Nolte and Brown, 1970; but see Wasserman, 1969).…”

“…As an independent test of whether the UV opsin-containing cells actually responded to light, rhabdomeral arrestin was measured in the same rhabdoms. Rhabdomeral arrestin increases in L. polyphemus photoreceptors in response to light (Battelle et al, 2013). In these rhabdoms, arrestin-ir was significantly higher during the day compared with during the night (mean ± s.e.m.…”