Photoreceptor function

PDA (prolonged depolarizing afterpotential)-defective mutants: the story of nina's and ina's--pinta and santa maria, too

Our objective is to present a comprehensive view of the PDA (prolonged depolarizing afterpotential)-defective Drosophila mutants, nina's and ina's, from the discussion of the PDA and the PDA-based mutant screening strategy to summaries of the knowledge gained through the studies of mutants generated using the strategy. The PDA is a component of the light-evoked photoreceptor potential that is generated when a substantial fraction of rhodopsin is photoconverted to its active form, metarhodopsin. The PDA-based mutant screening strategy was adopted to enhance the efficiency and efficacy of ERG (electroretinogram)-based screening for identifying phototransduction-defective mutants. Using this strategy, two classes of PDA-defective mutants were identified and isolated, nina and ina, each comprising multiple complementation groups. The nina mutants are characterized by allele-dependent reduction in the major rhodopsin, Rh1, whereas the ina mutants display defects in some aspects of functions related to the transduction channel, TRP (transient receptor potential). The signaling proteins that have been identified and elucidated through the studies of nina mutants include the Drosophila opsin protein (NINAE), the chaperone protein for nascent opsin (NINAA), and the multifunctional protein, NINAC, required in multiple steps of the Drosophila phototransduction cascade. Also identified by the nina mutants are some of the key enzymes involved in the biogenesis of the rhodopsin chromophore. As for the ina mutants, they led to the discovery of the scaffold protein, INAD, responsible for the nucleation of the supramolecular signaling complex. Also identified by the ina mutants is one of the key members of the signaling complex, INAC (ePKC), and two other proteins that are likely to be important, though their roles in the signaling cascade have not yet been fully elucidated. In most of these cases, the protein identified is the first member of its class to be so recognized.

Drosophila locus with gene-dosage effects on rhodopsin

Mutations that decrease the amplitude of the prolonged depolarizing afterpotential (PDA) in Drosophila melanogaster have been shown to have reduced rhodopsin content in the rhabdomeres of photoreceptor cells. In the present study, a genetic analysis of a class of third chromosome PDA-defective mutants localized the ninaE locus to the salivary band region 92A-B. In flies with only one copy of this region instead of the normal two copies, and in ninaE heterozygotes, the rhodopsin content of the major class of photoreceptors is reduced. Three doses of this region increase the rhodopsin content of these photoreceptors. These characteristics of the ninaE locus are expected of the structural gene encoding the major species of opsin in the Drosophila compound eye.

Degeneration of photoreceptors in rhodopsin mutants of Drosophila

Five different, well-characterized mutants of the R1-6 rhodopsin gene (ninaE), which corresponds to the rod opsin gene of vertebrates, have been examined morphologically as a function of age (up to 9 weeks) to determine whether or not the photoreceptors degenerate and to assess the pattern of degeneration. Structural deterioration of R1-6 photoreceptors with age has been found in all five mutants. The structural pattern of degeneration is similar in the five mutants, but the time course of degeneration is allele dependent and varies greatly among the five, with the strongest alleles causing the fastest degeneration. The degeneration appears to be independent of either the illumination cycle to which the animals are exposed or the presence of screening pigments in the eye. Although the degeneration first appears in R1-6 photoreceptors, eventually R7/8 photoreceptors, which correspond to cones of vertebrates, are also affected. In many of these mutants, striking proliferations of membrane processes have been observed in the subrhabdomeric region of R1-6 photoreceptors. It is hypothesized that (1) this accumulation of membranes may be caused by the failure of newly synthesized membranes that are inserted into the base of microvilli to be assembled into R1-6 rhabdomeres and (2) this failure may be caused by the extremely low concentration of normal R1-6 rhodopsin in the ninaE mutants.

Gene encoding cytoskeletal proteins in Drosophila rhabdomeres

The ninaC gene is one of eight nina (neither inactivation nor afterpotential) genes identified from mutations that drastically reduce the amount of rhodopsin in the compound eye of Drosophila melanogaster. The gene has been cytogenetically localized to the 27E-28B region of the second chromosome. NaDodSO4/PAGE analysis of eye proteins of flies carrying one, two, or three copies of the ninaC region shows that two eye-specific proteins of molecular weight 170,000 and 130,000 display a strong dependence on the dosage of the ninaC gene, although the dependence is evident only when the dosage is decreased and not when it is increased. All mutations in the ninaC gene studied to date have pronounced effects on these two polypeptides. These results suggest that the ninaC locus encodes these two polypeptides. Ultrastructural studies show that the polypeptides encoded by ninaC are very likely to be important components of the cytoskeletal structure of rhabdomeral microvilli.

Electrophysiological study of Drosophila rhodopsin mutants

Electrophysiological investigations were carried out on several independently isolated mutants of the ninaE gene, which encodes opsin in R1-6 photoreceptors, and a mutant of the ninaD gene, which is probably important in the formation of the rhodopsin chromophore. In these mutants, the rhodopsin content in R1-6 photoreceptors is reduced by 10(2)-10(6)-fold. Light-induced bumps recorded from even the most severely affected mutants are physiologically normal. Moreover, a detailed noise analysis shows that photoreceptor responses of both a ninaE mutant and a ninaD mutant follow the adapting bump model. Since any extensive rhodopsin-rhodopsin interactions are not likely in these mutants, the above results suggest that such interactions are not needed for the generation and adaptation of light-induced bumps. Mutant bumps are strikingly larger in amplitude than wild-type bumps. This difference is observed both in ninaD and ninaE mutants, which suggests that it is due to severe depletion of rhodopsin content, rather than to any specific alterations in the opsin protein. Lowering or buffering the intracellular calcium concentration by EGTA injection mimics the effects of the mutations on the bump amplitude, but, unlike the mutations, it also affects the latency and kinetics of light responses.

Freeze-fracture study of the Drosophila photoreceptor membrane: mutations affecting membrane particle density

The photoreceptor membrane of Drosophila melanogaster (wild type, vitamin A-deprived wild type, and the mutants ninaAP228, ninaBP315, and oraJK84) was studied by freeze-fracture electron microscopy. The three mutations caused a decrease in the number of particles on the protoplasmic face of the rhabdomeric membrane. The ninaAP228 mutation affected only the peripheral photoreceptors (R1-6), while the ninaBP315 mutation affected both the peripheral (R1-6) and the central photoreceptors (R7). The oraJK84 mutation, which essentially eliminates R1-6 rhabdomeres, was found to drastically deplete the membrane particles in the vestigial R1-6 rhabdomeres but not in the normal rhabdomeres of R7 photoreceptors, suggesting that the failure of the oraJK84 mutant to form normal R1-6 rhabdomeres may be due to a defect in a major R1-6 photoreceptor-specific protein in the mutant. In all cases in which both the rhabdomeric particle density and rhodopsin content were studied, the mutations or vitamin A deprivation was found to reduce both these quantities, supporting the idea that at least the majority of the rhabdomeric membrane particles are closely associated with rhodopsin. Vitamin A deprivation and the mutations also reduced the number of particles in the plasma membrane as in the rhabdomeric membrane, suggesting that both classes of membrane contain rhodopsin.

Genetics of the nervous system in Drosophila

The genetic analysis ofDrosophilaneurobiology has expanded into a considerable field. A host of genetic variants is now available for analyzing the development, structure, physiology, and chemistry of the fly's excitable cells and tissues and how they control the animal's behavior.