Insect retinal pigments: Spectral characteristics and physiological functions

Upon illumination, visual arrestin translocates from photoreceptor cell bodies to rhodopsin and membrane-rich photosensory compartments, vertebrate outer segments or invertebrate rhabdomeres, where it quenches activated rhodopsin. Both the mechanism and function of arrestin translocation are unresolved and controversial. In dark-adapted photoreceptors of the fruitfly Drosophila, confocal immunocytochemistry shows arrestin (Arr2) associated with distributed photoreceptor endomembranes. Immunocytochemistry and live imaging of GFP-tagged Arr2 demonstrate rapid reversible translocation to stimulated rhabdomeres in stoichiometric proportion to rhodopsin photoisomerization. Translocation is very rapid in normal photoreceptors (time constant <10 s) and can also be resolved in the time course of electroretinogram recordings. Genetic elimination of key phototransduction proteins, including phospholipase C (PLC), Gq, and the light-sensitive Ca²⁺-permeable TRP channels, slows translocation by 10- to 100-fold. Our results indicate that Arr2 translocation in Drosophila photoreceptors is driven by diffusion, but profoundly accelerated by phototransduction and Ca²⁺ influx.

Photoreceptors in metazoans can be grouped into two classes, with their photoreceptive membrane derived either from cilia or microvilli. Both classes use some form of the visual pigment protein opsin, which together with 11-cis retinaldehyde absorbs light and activates a G-protein cascade, resulting in the opening or closing of ion channels. Considerable attention has recently been given to the molecular evolution of the opsins and other photoreceptor proteins; much is also known about transduction in the various photoreceptor types. Here we combine this knowledge in an attempt to understand why certain photoreceptors might have conferred particular selective advantages during evolution. We suggest that microvillar photoreceptors became predominant in most invertebrate species because of their single-photon sensitivity, high temporal resolution, and large dynamic range, and that rods and a duplex retina provided primitive chordates and vertebrates with similar sensitivity and dynamic range, but with a smaller expenditure of ATP.

Seeing begins in the photoreceptors, where light is absorbed and signaled to the nervous system. Throughout the animal kingdom, photoreceptors are diverse in design and purpose. Nonetheless, phototransduction—the mechanism by which absorbed photons are converted into an electrical response—is highly conserved and based almost exclusively on a single class of photoproteins, the opsins. In this Review, we survey the G protein-coupled signaling cascades downstream from opsins in photoreceptors across vertebrate and invertebrate species, noting their similarities as well as differences.

Phototransduction in flies is the fastest known G protein-coupled signaling cascade, but how this performance is achieved remains unclear. Here, we investigate the mechanism and role of rhodopsin inactivation. We determined the lifetime of activated rhodopsin (metarhodopsin = M^∗) in whole-cell recordings from Drosophila photoreceptors by measuring the time window within which inactivating M^∗ by photoreisomerization to rhodopsin could suppress responses to prior illumination. M^∗ was inactivated rapidly (τ ∼20 ms) under control conditions, but ∼10-fold more slowly in Ca²⁺-free solutions. This pronounced Ca²⁺ dependence of M^∗ inactivation was unaffected by mutations affecting phosphorylation of rhodopsin or arrestin but was abolished in mutants of calmodulin (CaM) or the CaM-binding myosin III, NINAC. This suggests a mechanism whereby Ca²⁺ influx acting via CaM and NINAC accelerates the binding of arrestin to M^∗. Our results indicate that this strategy promotes quantum efficiency, temporal resolution, and fidelity of visual signaling.

Invertebrate microvillar photoreceptors depolarize in response to light by opening nonselective cation channels. Often the same cells can respond sensitively and rapidly to single photons, and yet light adapt to encode fluctuations of intensity under full sunlight. The underlying molecular mechanisms have been most extensively studied in the fruitfly Drosophila. Incident light is absorbed in the rhabdomere, a 1- to 2-μm diameter, 100-μm-long waveguide composed of ∼40 000 tightly packed microvilli, which contain the visual pigment rhodopsin and all the major components of the cascade. The extreme compartmentalization provided by this microvillar design is key to understanding the combination of sensitivity, rapid kinetics, and large dynamic range. Excitation is mediated by activation of G_q protein and phospholipase C (PLC) resulting in activation of two classes of Ca²⁺-permeable channel. These are encoded by the transient receptor potential (trp) and trp-like (trpl) genes, the founding members of the large and diverse TRP (light-activated nonspecific cation channel encoded by trp gene) superfamily of nonvoltage-gated cation channels. Several components of the cascade, including TRP, PLC, and protein kinase C (PKC) are assembled into a multimolecular signaling complex by the scaffolding protein INAD. Recent evidence suggests that the primary activator of the channels is likely to be a lipid messenger derived from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) by PLC. Candidates include diacylglycerol (DAG), polyunsaturated acids (PUFAs), or the reduction in PIP₂. Ca²⁺ influx via TRP channels is essential for rapid kinetics, amplification, and light adaptation, and mediates both positive and negative feedback via multiple downstream targets, including calmodulin (CaM), both TRP and TRPL (light-activated nonspecific cation channel encoded by trpl gene) channels, PKC, and PLC. Ca²⁺ influx is also required for proper functioning of the visual pigment cycle, which involves CaMKII-dependent phosphorylation of arrestin and Ca²⁺ CaM-dependent dephosphorylation of rhodopsin.

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of “dark adaptation” has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.