INOSITOL PHOSPHOLIPID AND INVERTEBRATE PHOTORECEPTORS

Evidence for the involvement of inositol trisphosphate but not cyclic nucleotides in visual transduction in Hermissenda eye

Although several second messengers are known to be involved in invertebrate photoresponses, the mechanism underlying invertebrate phototransduction remains unclear. In the present study, brief injection of inositol trisphosphate into Hermissenda photoreceptors induced a transient Na+ current followed by burst activity, which accurately reproduced the native photoresponse. Injection of Ca2+ did not induce a significant change in the membrane potential but potentiated the native photoresponse. Injection of a Ca2+ chelator decreased the response amplitude and increased the response latency. Injection of cGMP induced a Ca2+-dependent, transient depolarization with a short latency. cAMP injection evoked Na+-dependent action potentials without a rise in membrane potential. Taken together, these results suggest that phototransduction in Hermissenda is mediated by Na+ channels that are directly activated by inositol trisphosphate without mobilization of cytosolic Ca2+.

Blue and ultraviolet light-absorbing opsin from the retinal pigment epithelium

The retinal pigment epithelium (RPE) contains an abundant opsin that is distinct from rhodopsin and cone visual pigments and is able to bind the retinaldehyde chromophore. The putative retinal G protein-coupled receptor (RGR) was isolated in digitonin solution from bovine RPE microsomes and copurified consistently with a minor 34-kDa protein. The absorption spectrum of RGR revealed endogenous pH-sensitive absorbance in the blue and near-ultraviolet regions of light. Membrane-bound RGR was incubated with exogenously added all-trans-retinal and formed two long-lived pH-dependent photopigments with absorption maxima of 469 +/- 2.4 and 370 +/- 7.3 nm. The effects of hydrogen ion concentration suggest that the blue and near-UV photopigments are tautomeric forms of RGR, in which an all-trans-retinal Schiff base is protonated or unprotonated, respectively. The RPE pigment was also demonstrable by its reactivity to hydroxylamine in the dark. The retinaldehyde-RGR conjugate at neutral pH favors the near-UV pigment and is a novel light-absorbing opsin in the vertebrate eye.

Molecular, biochemical, and electrophysiological characterization of Drosophila norpA mutants

Inositol phosphate signaling has been implicated in a wide variety of eukaryotic cellular processes. In Drosophila, the phototransduction cascade is mediated by a phosphoinositide-specific phospholipase C (PLC) encoded by the norpA gene. We have characterized eight norpA mutants by electroretinogram (ERG), Western, molecular, and in vitro PLC activity analyses. ERG responses of the mutants show allele-dependent reductions in amplitudes and retardation in kinetics. The mutants also exhibit allele-dependent reductions in in vitro PLC activity levels and greatly reduced or undetectable NorpA protein levels. Three carry a missense mutation and five carry a nonsense mutation within the norpA coding sequence. In missense mutants, the amino acid substitution occurs at residues highly conserved among PLCs. These substitutions reduce the levels of both the NorpA protein and the PLC activity, with the reduction in PLC activity being greater than can be accounted for simply by the reduction in protein. The effects of the mutations on the amount and activity of the protein are much greater than their effects on the ERG, suggesting an amplification of the transduction signal at the effector (NorpA) protein level. Transgenic flies were generated by germline transformation of a null norpA mutant using a P-element construct containing the wild-type norpA cDNA driven by the ninaE promoter. Transformed flies show rescue of the electrophysiological phenotype in R1-R6 photoreceptors, but not in R7 or R8. The degeneration phenotype of R1-R6 photoreceptors is also rescued.

Invertebrate phosphatidylinositol-specific phospholipases C and their role in cell signaling

Phosphatidylinositol-specific phospholipase C (PLC) is a family of enzymes that occupy a pivotal role in one of the largest classes of cellular signaling pathways known. Mammalian PLC enzymes have been divided into four major classes and a variety of subclasses based on their structural characteristics and immunological differences. There have been five invertebrate PLC-encoding genes cloned thus far and these fall within three of the four major classes used in categorizing mammalian PLC. Four of these invertebrate genes have been cloned from Drosophila melanogaster and one is from Artemia, a brine shrimp. Structural characteristics of the invertebrate enzymes include the presence of highly conserved Box X and Box Y domains found in major types of mammalian PLC as well as novel features. Two of the invertebrate PLC genes encode multiple splice-variant subtypes which is a newly emerging level of diversity observed in mammalian enzymes. Studies of the invertebrate PLCs have contributed to the identification of the physiological functions of individual isozymes. These identified roles include cellular processes such as phototransduction, olfaction, cell growth and differentiation.

Multiple subtypes of phospholipase C are encoded by the norpA gene of Drosophila melanogaster

The norpA gene of Drosophila melanogaster encodes a phosphatidylinositol-specific phospholipase C that is essential for phototransduction. Besides being found abundantly in retina, norpA gene products are expressed in a variety of tissues that do not contain phototransduction machinery, implying that norpA is involved in signaling pathways in addition to phototransduction. We have identified a second subtype of norpA protein that is generated by alternative splicing of norpA RNA. The alternative splicing occurs at a single exon that is excluded from mature norpA transcripts when a substitute exon of equal size is retained. The net difference between the two subtypes of norpA protein is 14 amino acid substitutions occurring between amino acid positions 130 and 155 of the enzyme. Results from Northern analyses suggest that norpA subtype I transcripts are most abundantly expressed in adult retina, while subtype II transcripts are most abundant in adult body. Moreover, norpA subtype I RNA can be detected by the reverse transcription-polymerase chain reaction in extracts of adult head tissue but not adult body nor at earlier stages of Drosophila development. Conversely, norpA subtype II RNA can be detected by reverse transcription-polymerase chain reaction throughout development as well as in heads and bodies of adults. Furthermore, norpA subtype I RNA is easily detected in retina using tissue in situ hybridization analysis, while subtype II RNA is not detectable in retina but is found in brain. Since only norpA subtype I RNA is found in retina, we conclude that subtype I protein is utilized in phototransduction. Since norpA subtype II RNA is not found in retina but is expressed in a variety of tissues not known to contain phototransduction machinery, subtype II protein is likely to be utilized in signaling pathways other than phototransduction. The amino acid differences between the two subtypes of norpA protein may reflect the need for each subtype to interact with signaling components of different signal-generating pathways.

Phospholipase C rescues visual defect in norpA mutant of Drosophila melanogaster

Mutations in the norpA gene of Drosophila melanogaster severely affect the light-evoked photoreceptor potential with strong mutations rendering the fly blind. The norpA gene has been proposed to encode phosphatidylinositol-specific phospholipase C (PLC), which enzymes play a pivotal role in one of the largest classes of signaling pathways known. A chimeric norpA minigene was constructed by placing the norpA cDNA behind an R1-6 photoreceptor cell-specific rhodopsin promoter. This minigene was transferred into norpAP24 mutant by P-element-mediated germline transformation to determine whether it could rescue the phototransduction defect concomitant with restoring PLC activity. Western blots of head homogenates stained with norpA antiserum show that norpA protein is restored in heads of transformed mutants. Moreover, transformants exhibit a large amount of measurable PLC activity in heads, whereas heads of norpAP24 mutant exhibit very little to none. Immunohistochemical staining of tissue sections using norpA antiserum confirm that expression of norpA protein in transformants localizes in the retina, more specifically in rhabdomeres of R1-6 photoreceptor cells, but not R7 or R8 photoreceptor cells. Furthermore, electrophysiological analyses reveal that transformants exhibit a restoration of light-evoked photoreceptor responses in R1-6 photoreceptor cells, but not in R7 or R8 photoreceptor cells. This is the strongest evidence thus far supporting the hypothesis that the norpA gene encodes phospholipase C that is utilized in phototransduction.

Activation and regeneration of rhodopsin in the insect visual cycle

Light absorption by rhodopsin generates metarhodopsin, which activates heterotrimeric guanine nucleotide-binding proteins (G proteins) in photoreceptor cells of vertebrates and invertebrates. In contrast to vertebrate metarhodopsins, most invertebrate metarhodopsins are thermally stable and regenerate rhodopsin by absorption of a second photon. In experiments with Rh1 Drosophila rhodopsin, the thermal stability of metarhodopsin was found not to be an intrinsic property of the visual pigment but a consequence of its interaction with arrestin (49 kilodaltons). The stabilization of metarhodopsin resulted in a large decrease in the efficiency of G protein activation. Light absorption by thermally stable metarhodopsin initially regenerated an inactive rhodopsin-like intermediate, which was subsequently converted in the dark to active rhodopsin. The accumulation of inactive rhodopsin at higher light levels may represent a mechanism for gain regulation in the insect visual cycle.

Membrane association of phospholipase C encoded by the norpA gene of Drosophila melanogaster

Severe mutations within the norpA gene of Drosophila abolish the photoreceptor potential and render the fly blind by deleting phospholipase C, an essential component of the phototransduction pathway. To study the membrane association of phospholipase C, we have utilized biochemical assays of phospholipase C activity, which predominant measurable phospholipase C activity in head homogenates has been shown to be encoded by norpA, as well as antisera generated against the major gene product of norpA to examine its subcellular distribution before and during phototransduction. We find that both phospholipase C activity and the norpA protein are predominantly associated with membrane fractions in heads of both light- and dark-adapted flies. Moreover, phospholipase C activity as well as norpA protein can be easily extracted from membrane preparations of light- or dark-adapted flies using high salt, indicating that the norpA protein is peripherally localized on the membrane. These data suggest that the norpA encoded phospholipase C of Drosophila is a permanent peripheral membrane protein. If this is indeed the case, then it would mean that the reversible redistribution of phospholipase C from the cytosol to the membrane, as observed in epidermal growth factor receptor stimulation of mammalian phospholipase C gamma, is not a universal mechanism utilized by all types of phosphatidylinositol-specific phospholipase C.

The rpa (receptor potential absent) visual mutant of the blowfly (Calliphora erythrocephala) is deficient in phospholipase C in the eye

The rpa (receptor potential absent) mutation of the blowfly, Calliphora erythrocephala, reduces the light-evoked responses of photoreceptor cells and renders the fly blind. This phenotype is similar to the phenotype caused by norpA mutations in Drosophila which have been shown to occur within a gene encoding phospholipase C. In Western blots, norpA antiserum stains a protein in homogenates of wild-type Calliphora eye and head that is similar in molecular weight to the norpA protein. Very little staining of this protein is observed in similar homogenates of rpa mutant. Moreover, norpA antiserum strongly stains retina in immunohistochemical assays of wild-type adult head, but not in rpa mutant. Furthermore, eyes of rpa mutant have a reduced amount of phospholipase C activity compared to eye of wild-type Calliphora. These data suggest that the rpa mutation occurs in a phospholipase C gene of the blowfly that is homologous to the norpA gene of Drosophila.

Cyclic 3',5'-nucleotide phosphodiesterase: cytochemical localization in photoreceptor cells of the fly Calliphora erythrocephala

The distribution of cyclic 3',5'-nucleotide phosphodiesterase activity was determined in photoreceptor cells of the fly Calliphora erythrocephala. With cAMP as substrate, staining was most intense within the phototransducing region of these cells, the rhabdomeral microvilli and also in the extracellular space surrounding the microvilli and in the mitochondria. With cGMP as substrate, the intensity within the rhabdomeres was less marked, while their extracellular surroundings were stained heavily. Thus, compared to cGMP, cAMP is the better substrate for the phosphodiesterase in the rhabdomeres of the fly. For comparison, the same cytochemical method was used to localize the well-known phosphodiesterase activity in retinal tissue of the mouse. Under the same conditions as used for fly photoreceptors, a very intense reaction product was obtained in rod outer segments. With regard to the conflicting reports concerning the light-stimulated changes of cyclic nucleotides in invertebrate photoreceptor cells, the results presented here further argue for an important role of a cyclic nucleotide in the process of phototransduction of invertebrates.

Concentrations of phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate within the distal segment of squid photoreceptors

Although inositol trisphosphate (InsP3) is a key substance in phototransduction of invertebrate photoreceptors, its intracellular concentration remains unknown. The purpose of this study was to assay its concentration and the concentration of its precursor, phosphatidylinositol bisphosphate (PtdInsP2), within squid photoreceptors. Rhabdomeric membranes were purified and their PtdInsP2 content measured with a phosphate assay after the extracted phospholipids were deacylated and separated by ion-exchange chromatography. At least 75% of the total PtdInsP2 found in the retinal homogenate was associated with the plasma membranes of the rhabdomeric microvilli, where PtdInsP2 was 3.1 +/- 0.7% of the total phospholipids, a level comparable to values published for rat brain. In terms of rhodopsin, microvillar membranes contained 3.7 +/- 0.9 mol PtdInsP2/mol rho. The InsP3 content of living retinas was measured with a radioreceptor assay. The basal content of dark-adapted retinas was 0.15 +/- 0.05 InsP3/rho, equivalent to 30 +/- 9 nmol/g tissue that is about twice that of rat brains. Flash illumination (approximately 1 ms in duration) that photoactivated 1% of rhodopsin increased the level about fivefold to 0.68 +/- 0.22 InsP3/rho. Corresponding decrease in PtdInsP2 was undetectable as it was within measurement errors. For PtdInsP2, the measured content corresponds to 5.6 +/- 1.4 mM within the volume of rhabdomere. Maximal light-induced concentration of InsP3 is calculated to be 1.2 +/- 0.4 mM within the cytoplasm of the distal segment. Each photoactivated rhodopsin leads to the formation of < or = 500 InsP3 molecules when measured 15 s after the flash.(ABSTRACT TRUNCATED AT 250 WORDS)