Opsin co-expression in Limulus photoreceptors: differential regulation by light and a circadian clock

Abstract

SUMMARYA long-standing concept in vision science has held that a single photoreceptor expresses a single type of opsin, the protein component of visual pigment. However, the number of examples in the literature of photoreceptors from vertebrates and invertebrates that break this rule is increasing. Here, we describe a newly discovered Limulus opsin, Limulus opsin5, which is significantly different from previously characterized Limulus opsins, opsins1 and 2. We show that opsin5 is co-expressed with opsins1 and 2 in Limulus lateral and ventral eye photoreceptors and provide the first evidence that the expression of coexpressed opsins can be differentially regulated. We show that the relative levels of opsin5 and opsin1 and 2 in the rhabdom change with a diurnal rhythm and that their relative levels are also influenced by the animal's central circadian clock. An analysis of the sequence of opsin5 suggests it is sensitive to visible light (400-700nm) but that its spectral properties may be different from that of opsins1 and 2. Changes in the relative levels of these opsins may underlie some of the dramatic day-night changes in Limulus photoreceptor function and may produce a diurnal change in their spectral sensitivity. Supplementary material available online at

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

“…For completeness, these findings are plotted together with previous results (Katti et al, 2010) showing that RhOps1-2 levels roughly double from day to night while RhOps5 levels do not change significantly. The rays of the rhabdom clearly change shape from day to night.…”

“…It cross-reacts with Limulus Ops2 and thus the immunoreactivity (ir) observed with this antibody is referred to here as Ops1-2-ir. Anti-Limulus Ops5, a mouse monoclonal antibody (2-449), was generated against the C terminus of Limulus Ops5 and is specific for this opsin (Katti et al, 2010). Anti-Limulus Arr, a mouse monoclonal antibody (4D10), was generated against the N terminus of Arr (Battelle et al, 2000a).…”

“…The Journal of Experimental Biology 216 (10) These procedures were described in detail elsewhere (Katti et al, 2010). Briefly, ommatidia in immunostained cross-sections of LEs (Fig.2) were viewed with a confocal microscope (Leica SP2 or SP5; Leica Microsystems, Mannheim, Germany).…”

“…One slice was fixed immediately in the light. The other three were preincubated in organ culture medium (OCM) (Katti et al, 2010) for 10min in the light, and then incubated for 4h in the dark at room temperature under one of three conditions: (1) OCM; (2) OCM plus OA (40μm) and the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX) (1mmoll…”

“…In a previous study we showed that rhabdomeral levels of Limulus opsin 1-2 (RhOps1-2), the most abundant opsins in LE photoreceptors (retinular cells), changed dramatically from day to night and that the normal nighttime increase in RhOps1-2 required clock input (Katti et al, 2010). These results are confirmed and extended in the present study, in which we also examined diurnal and circadian changes in rhabdomeral levels of arrestin (RhArr) and G q α (RhG q α), the alpha subunit of the G protein activated by rhabdomeral opsins.…”