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.

Retinae of the crab Leptograpsus which had been maintained on a 12-h light/12-h dark cycle were cultured in vitro and exposed to 1 microM okadaic acid (OKA) at 0.75 h before light onset. Control retinae were subjected to the same routine and sampled at the same times without OKA treatment. At the concentration used, OKA totally inhibits types 1 and 2A protein phosphatases, minimally inhibits type 2B, and does not affect type 2C. 1 microM OKA provoked a diminution of rhabdom diameter measured at the level of the photoreceptor nuclei in the dark, some ommatidial cartridges being stripped of rhabdomeral microvilli altogether. After 1-h illumination (225-320 lux), further reduction of rhabdom diameter was modest in control retinae but precipitate in those treated with OKA. After 2 h, control rhabdom diameters showed a further, not significant, decline, but OKA had induced a resynthesis of massive structures with the light-microscopic appearance of rhabdoms. Electron microscopy revealed that they were heterogeneous and of the following kinds: (1) a minority of rhabdoms with normally disposed but distorted microvilli; (2) rhabdoms in the throes of events that parody normal assembly; and (3) rhabdomal volumes occupied by saccular organelles or by pleats or ruffles of irregular architecture. The cytoplasm of all such receptors was packed with free and bound ribosomes and endomembranes. The sequence of events parallels that seen during light-induced degeneration of photoreceptors of the Drosophila mutant w rdgBKS222. Preliminary experiments show that a protein kinase activator SC-9 mimics many of these effects in the dark in the presence of 1 microM OKA. As a working hypothesis, it is proposed that light activates protein kinases via diacylglycerols generated by the phototransduction cascade, and that in both crab retinas challenged with OKA and retinas of rdg BKS222 activation of a nuclear regulatory protein by hyperphosphorylation provokes a runaway transcription whose selectivity and extent remain to be determined.