Assembly of rhabdomeric membrane from smooth endoplasmic reticulum can be activated by light in chromophore-deprived photoreceptors of Manduca sexta

Ultrastructure and localization of a visual Gq protein in hypertrophied epitoke ocelli of Perinereis brevicirris (Polychaeta, Annelida)

Functional ultrastructural changes in the rhabdomeric photoreceptors of the cerebral ocelli are described for normal and sexually mature (epitoke) Perinereis brevicirris (Polychaeta, Annelida). With sexual maturation, the cerebral ocelli hypertrophied, increasing in volume to 5.5 times that of ocelli in the normal state, and the thickness of the retinal layer increased up to 10 times. Perinereis ocelli have a pigmented retinal layer consisting of at least two cell types: photoreceptor cell (PR) and pigmented supporting cells (PS). In epitoke ocelli, PR bear well-developed rhabdomeric microvilli, multilamellar bodies, and numerous cytoplasmic membranous structures, including vesicles, smooth endoplasmic reticulum, and secondary lysosomes. Localization of a visual Gq protein in the ocelli was studied with anti-GqC antibody. The antibody strongly labeled not only microvilli and multilamellar bodies throughout the retinal layer, but also secondary lysosomes and vesicles in the cytoplasm of the PR in the epitoke ocelli, although labeling was observed only in the microvilli and multilamellar bodies in normal ocelli. Reverse transcription/polymerase chain reaction analysis revealed that the amount of G protein alpha subunit mRNA in the epitoke head increased by roughly twice that of the normal head. Since Gq protein is essential for phototransduction in Perinereis ocelli, these results suggest that the sites are involved in photoreceptive membrane turnover, which occurs much more extensively in epitoke ocelli. Thus, epitoke ocelli may represent a model system for studying rhabdomeric photoreceptive membrane turnover.

Carotenoid replacement in Drosophila: freeze-fracture electron microscopy

Because of the consequent lack of photopigment chromophore, carotenoid/ retinoid (vitamin A) deprivation during the larval period of Drosophila leads to decreased rhodopsin in adult photoreceptors. Decreased density of P-face particles in photoreceptor membrane (rhabdomeric microvilli) is a prominent ultrastructural feature of this rhodopsin deficiency. When adults are fed carotenoid, the rhabdomeric P-face particle density-which reflects the concentration of rhodopsin-increases halfway to the replete control level during the first 12 hours, and is fully restored by 2 days. Based on freeze-fracture replicas, there is a continuity of membrane between rhabdomeric microvilli and the parent retinula cell. That confluence is relevant to turnover of photoreceptive membrane. Microvillar and retinula cell P-face particle densities covary. The relevance of the demonstration of rapid recovery from chromophore depletion is discussed in relation to hypotheses that the chromophore and/or related retinoids regulate opsin gene transcription, and/or post-translational processing and deployment from the endoplasmic reticulum to the rhabdomere.

Expression of opsin mRNA in normal and vitamin A deficient retinas of the sphingid moth Manduca sexta

Two distinct opsin-encoding cDNAs, designated MANOP1 and MANOP2, were isolated as 3' fragments from the sphingid moth Manduca sexta. They were obtained by reverse transcription of retinal RNA and amplification with the polymerase chain reaction (PCR) using a degenerate primer designed to an amino-acid sequence conserved in arthropod opsins. The cDNA fragments labelled bands at approximately 1.8 kb on Northern blots of retinal RNA extracts. Levels of opsin message were compared in retinas from normal moths, whose diets were fortified with carotenoid precursors of the Manduca rhodopsin chromophore, 3-hydroxyretinal, and those reared on carotenoid/retinoid (vitamin A) deficient diets. The chromophore-depleted retinas contained more opsin mRNA;this was particularly true for MANOP2. Thus, the chromophore is not required for opsin gene transcription in Manduca.