Genetic Approaches to Visual Transduction in Drosophila melanogaster

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

The regulations of Drosophila phototransduction

This is the first of two reviews that include some of the studies that we, members of the Pak lab and collaborators, carried out from 1998 to 2010 on the functional and physical interactions among several Drosophila phototransduction components. The report includes our studies on the regulations and/or the functions of arrestin II (Arr2), norpA (PLC), inactivation no afterpotential D (INAD), transient receptor potential (TRP), TRP-like (TRPL), inactivation no afterpotential E (INAE), and Porin.

In search of proteins that are important for synaptic functions in Drosophila visual system

This is the second of two reviews that include some of the studies we, members of the Pak laboratory and collaborators, did from 2000 to 2010 on the mutants that affect synaptic transmission in the Drosophila visual system. Of the five mutants we discuss, two turned out to also play roles in the larval neuromuscular junction. This review complements the one on phototransduction to give a fairly complete account of what we focused on during the 10-year period, although we also did some studies on photoreceptor degeneration in the early part of the decade. Besides showing the power of using a genetic approach to the study of synaptic transmission, the review contains some unexpected results that illustrate the serendipitous nature of research.

Rhythm defects caused by newly engineered null mutations in Drosophila's cryptochrome gene

Much of the knowledge about cryptochrome function in Drosophila stems from analyzing the cryb mutant. Several features of this variant's light responsiveness imply either that CRYb retains circadian-photoreceptive capacities or that additional CRY-independent light-input routes subserve these processes. Potentially to resolve these issues, we generated cry knock-out mutants (cry0's) by gene replacement. They behaved in an anomalously rhythmic manner in constant light (LL). However, cry0 flies frequently exhibited two separate circadian components in LL, not observed in most previous cryb analyses. Temperature-dependent circadian phenotypes exhibited by cry(0) flies suggest that CRY is involved in core pacemaking. Further locomotor experiments combined cry0 with an externally blinding mutation (norpAP24), which caused the most severe decrements of circadian photoreception observed so far. cryb cultures were shown previously to exhibit either aperiodic or rhythmic eclosion in separate studies. We found cry0 to eclose in a solidly periodic manner in light:dark cycles or constant darkness. Furthermore, both cry0 and cryb eclosed rhythmically in LL. These findings indicate that the novel cry0 type causes more profound defects than does the cryb mutation, implying that CRYb retains residual activity. Because some norpAP24 cry0 individuals can resynchronize to novel photic regimes, an as-yet undetermined light-input route exists in Drosophila.

How sensitive is a nose?

Odor sensitivity may not be due to odor-receptor (OR) binding but rather may be due to emergent properties of transduction pathways and the anatomical convergence of olfactory neurons. A recent study suggests that odor-OR interactions are brief and infrequently activate heterotrimeric GTP-binding proteins (G proteins); in contrast, visual receptors have long-lasting activation states and activate many G proteins. These differences may reflect strategies that evolved to accommodate very different signals, and the mechanisms described might be applicable for receptors across phyla. However, whereas visual receptors (rhodopsin) appeared before protostome-deuterostome separation, ORs may be independently derived in different phyla. Alternatively, phylum-distinct ORs may share common ancestry but be influenced by diversifying selection. Phylum-distinct ORs may imply phylum-specific OR mechanisms, whereas common ancestry may imply common mechanisms. Nonetheless, most animals detect a similar repertoire of olfactory signals, and OR mechanisms may be convergent on those signals independent of receptor relatedness. Thus, recent insights into the molecular characteristics of odor perception in frogs may well be relevant to such processes as how mosquitoes detect host odors for a malaria-transmitting blood meal.

The role of Drosophila ninaG oxidoreductase in visual pigment chromophore biogenesis

We previously reported (Sarfare, S., Ahmad, S. T., Joyce, M. V., Boggess, B., and O'Tousa, J. E. (2005) J. Biol. Chem. 280, 11895-11901) that the Drosophila ninaG gene encodes an oxidoreductase involved in the biosynthesis of the (3S)-3-hydroxyretinal serving as chromophore for Rh1 rhodopsin and that ninaG mutant flies expressing Rh4 as the major opsin accumulate large amounts of a different retinoid. Here, we show that this unknown retinoid is 11-cis-3-hydroxyretinol. Reversed phase high performance liquid chromatography coupled with a photodiode array UV-visible absorbance detector and mass spectrometer revealed a major product eluting at a retention time, t(r), of 3.5 min with a lambda(max) of approximately 324 nm and with a base peak in the mass spectrum at m/z 285. These observations are identical with those of the 3-hydroxyretinol standard. The base peak in the electrospray ionization mass spectrum arises from the loss of a water molecule from the protonated molecule at m/z 303 because of fragmentation in the ion source. These results suggest that 11-cis-3-hydroxyretinol is an intermediate required for chromophore biogenesis in Drosophila. We further show that ninaG mutants fed on retinal as the sole source of vitamin A are able to synthesize 3-hydroxyretinoids. Thus, the NinaG oxidoreductase is not responsible for the initial hydroxylation of the retinal ring but rather acts in a subsequent step in chromophore production. These data are used to review chromophore biosynthesis and propose that NinaG acts in the conversion of (3R)-3-hydroxyretinol to the 3S enantiomer.

Ceramidase expression facilitates membrane turnover and endocytosis of rhodopsin in photoreceptors

Transgenic expression of ceramidase suppresses retinal degeneration in Drosophila arrestin and phospholipase C mutants. Here, we show that expression of ceramidase facilitates the dissolution of incompletely formed and inappropriately located elements of rhabdomeric membranes in ninaE(I17) mutants lacking the G protein receptor Rh1 in R1-R6 photoreceptor cells. Ceramidase expression facilitates the endocytic turnover of Rh1. Although ceramidase expression aids the removal of internalized rhodopsin, it does not affect the turnover of Rh1 in photoreceptors maintained in dark, where Rh1 is not activated and thus has a slower turnover and a long half-life. Therefore, the phenotypic consequence of ceramidase expression in photoreceptors is caused by facilitation of endocytosis. This study provides mechanistic insight into the sphingolipid biosynthetic pathway-mediated modulation of endocytosis and suppression of retinal degeneration.