Induction of retinal degeneration in a crab by light and okadaic acid in vitro: comparison with the Drosophila light-dependent retinal degeneration mutant w rdgBKS222

RdgC/PP5-related phosphatases: novel components in signal transduction

A great variety of cellular functions are regulated by protein serine/threonine phosphatases (PP). This review summarises the current knowledge of the structural features, patterns of expression and involvement in signal transduction pathways of protein serine/threonine phosphatases related to PP5 and RdgC. Designated now as PP5/RdgC subfamily by P. T. W. Cohen in her 1997 study published in Trends in Biochemical Sciences, (Vol. 22, pp. 245-251), this heterogeneous group comprises phosphatases PP5/PPT, containing regulatory domains with tetratricopeptide repeats, RdgC/PPEF, which possess Ca2+-binding EF hand-type sites, and, recently discovered in plants, PP7. PP5 is ubiquitously expressed and appears to be a multifunctional phosphatase involved in a number of different signalling pathways. In contrast, expression of RdgC/PPEF phosphatases and PP7 is confined primarily to specialised sensory cells in animals and plants, respectively, which may be indicative of their more specialised roles in sensory signal transduction.

Calcium-activated opsin phosphatase activity in retinal rod outer segments

We describe the presence in bovine retinal rod outer segments of a phosphatase which dephosphorylates phosphoopsin with an efficiency similar to that of PP2A, and which is stimulated by submicromolar levels of Ca2+ (half-maximal activation, 0.4-0.5 microM). This enzyme is designated CA2+ -activated opsin phosphatase (CAOP). CAOP has a molecular mass of 70-75 kDa as determined by gel filtration on Superose 12 and exhibits reversible Ca2+ -dependent oligomerization. An unidentified protein of approximately 25 kDa is necessary for full activity of CAOP and for cooperative binding of Ca2+ (h > 2). CAOP does not require Mg2+ and is inhibited by okadaic acid (median inhibitory concentration > 25 microM), which suggests that it is related to the PP1/2A/2b class of protein phosphatases. Like PP2B, CAOP is inhibited by trifluoperazine (median inhibitory concentration 40 microM), but calmodulin has no effect on CAOP activity, and CAOP is inhibited by mastoparan at much higher concentrations than PP2b. This combination of properties suggests that CAOP is not identical to any characterized protein phosphatase. Since the cytoplasmic concentration of Ca2+ -sensitive opsin phosphatase activity suggests that light-dependent Ca2+ levels may control rhodopsin dephosphorylation.

Drosophila retinal degeneration C (rdgC) encodes a novel serine/threonine protein phosphatase

The Drosophila retinal degeneration C (rdgC) gene is required to prevent light-induced retinal degeneration. Molecular analysis shows that the rdgC transcription unit encodes a novel serine/threonine protein phosphatase. Amino acids 153-393 define a domain that has 30% identity with the catalytic domains of types 1, 2A, and 2B serine/threonine protein phosphatases. A putative regulatory domain is appended that contains multiple potential Ca(2+)-binding sites or "EF hand motifs." Thus, the analysis suggests that the rdgC protein is a novel type of serine/threonine protein phosphatase that is directly regulated by Ca2+. rdgC is expressed in the visual systems of the fly, as well as in the mushroom bodies of the central brain.

Calcium channel blockers inhibit retinal degeneration in the retinal-degeneration-B mutant of Drosophila

Light accelerates degeneration of photoreceptor cells of the retinal degeneration B (rdgB) mutant of Drosophila. During early stages of degeneration, light stimuli evoke spikes from photoreceptors of the mutant fly; no spikes can be recorded from photoreceptors of the wild-type fly. Production of spike potentials from mutant photoreceptors was blocked by diltiazem, verapamil hydrochloride, and cadmium. Little, if any, effect of the (-)-cis isomer or (+)-cis isomer of diltiazem on the light response was seen. Further, the (+)-cis isomer was approximately 50 times more effective than the (-)-cis isomer in blocking the Ca2+ spikes, indicating that diltiazem action on the rdgB eye is mediated by means of blocking voltage-sensitive Ca2+ channels, rather than by blocking the light-sensitive channels. Application of the Ca(2+)-channel blockers (+)-cis-diltiazem and verapamil hydrochloride to the eyes of rdgB flies over a 7-day period largely inhibited light-dependent degeneration of the photoreceptor cells. Pulse labeling with [32P]phosphate showed much greater incorporation into eye proteins of [32P]phosphate in rdgB flies than in wild-type flies. Retarding the light-induced photoreceptor degeneration in the mutant by Ca(2+)-channel blockers, thus, suggests that toxic increase in intracellular Ca2+ by means of voltage-gated Ca2+ channels, possibly secondary to excessive phosphorylation, leads to photoreceptor degeneration in the rdgB mutant.

The distribution of actin immunoreactivity in rhabdomeres of tipulid flies in relation to extracellular membrane shedding

Rhabdomeres of tipulid flies lose membrane during turnover from a 'shedding zone' composed of microvillar tips. These distal domains lack intramicrovillar cytoskeletons and appear to be empty sacs of membrane. Recent concerns about the role of ninaC mechano-enzymes in the architecture of dipteran rhabdomeral microvilli and the dynamic role that they may play in the creation of shedding zones demand an examination of the distribution of actin in tipulid rhabdomeres. We compared rhabdomeres from tipulid retinae incubated before fixation for immunocytochemistry in a buffer without additives and a stabilising buffer that contained a cocktail of cysteine protease inhibitors; both were challenged by an anti-actin antibody for immunogold labelling after embedding in LR White Resin. Shedding zones thus processed collapse to structureless detritus. Stabilised and unstabilized shedding zones were immunonegative to anti-actin. To ensure that the negative results were not consequent upon conformational changes generated by the processing protocol, we examined microvilli of degenerating rhabdomeres of the Drosophila light-dependent retinal degeneration mutant rdgBKS222 (which separate and collapse without creating a shedding zone) and found the detritus they generate to be immunopositive to anti-actin. Stabilised and unstabilized regions of basal regions of tipulid rhabdomeres were equally immunopositive. We infer that (a) actin is absent from shedding zones; (b) actin is not degraded by microvillar cysteine proteases. The implications of these conclusions are discussed in relation to some functional models of arthropod photoreceptor microvilli.