The retinal acid phosphatase of a crab,Leptograpsus: Characterisation, and relation to the cyclical turnover of photoreceptor membrane

Light-stimulated rhabdom turnover in Limulus ventral photoreceptors maintained in vitro

The role of light in turnover of photosensitive membranes was studied in isolated photoreceptors maintained in vitro. Ventral photoreceptors of the horseshoe crab, Limulus polyphemus, were used since they have been the subjects of many in vitro physiological studies. This study shows that the two classes of ventral photoreceptors, the large and small photoreceptors (Herman: companion paper), differ in their morphological response to light. The rhabdom of small photoreceptors is remarkable for its regularity, independent of lighting condition. The photosensitive microvilli of the rhabdom of small photoreceptors are narrow and almost always tightly packed in a hexagonal arrangement. In contrast, the morphology of the rhabdom of the large ventral photoreceptors is different in the dark and in the light, and the rhabdom undergoes turnover during lighting transitions. When fully dark-adapted, the photosensitive microvilli of large photoreceptors are narrow and well organized, sometimes in a crystalline array. However, in the light-adapted state, the microvilli are much thicker and very irregular. The transitions between the dark and light-adapted states, examined at midday, are rapid. After 5 minutes light exposure, the microvilli are dilated at their bases and shed membranes are present in the cytoplasm. By 30 minutes after light onset, the appearance of the rhabdom of large photoreceptors is indistinguishable from fully light-adapted cells. The transition to the dark-adapted state is equally rapid. Even at 5 or 12 minutes after light offset, most microvilli are narrow and quite regular, and by 30 minutes, the rhabdom usually appears to be fully dark-adapted. These experiments show that both the synthetic and degradative phases of rhabdom renewal take place in isolated photoreceptors. No efferent neural activity is required to initiate turnover; rather, changes in illumination alone are sufficient to generate rhabdom turnover in large ventral photoreceptors in vitro.

Rhabdomere turnover and rhodopsin cycle: maintenance of retinula cells in Drosophila melanogaster

Visual receptor maintenance in Drosophila involves turnover of membrane. Coated pits derived from rhabdomere and plasmalemma, coated vesicles and multivesicular bodies (MVBs) of about 0.5 micron diameter characterize the early autophagic steps. Smaller electron dense bodies (0.15 micron) merge with MVBs. These are likely to be primary lysosomes as suggested by histochemistry for acid phosphatase in normal flies and an acid phosphatase deficient mutant. Aggregates of extracellular membranes confirm an earlier report that exocytotic shedding may also be employed in the fly with its open rhabdomeres. Microspectrophotometry was used to determine aspects of cycling of visual pigment to begin to correlate with what is known about membrane cycling at the ultrastructural level. Visual pigment decreases to about half 3 h after dawn then builds back gradually to maximum before dawn. Our fixations of tissue have been at the post-dawn period when autophagy may be high as inferred from visual pigment levels. In attempts to optimize our visualization of the constructive phase of the turnover process, we developed the paradigm of carotenoid 'replacement therapy'. Carotenoid replaced flies show an increase in visual pigment possibly associated with a streaming of membrane into the rhabdomere. Aged flies, studied to determine how effective maintenance is, have a unique accumulation of extracellular debris, and a small fraction of the receptors eventually degenerate.

Acyltransferase and acid hydrolase activities of the abalone photoreceptor cell

In order to study the synthesis and degradation processes of the photoreceptor membranes in the abalone, Nordotis discus, the localization of acyltransferase and acid hydrolase activities, respectively, were determined at the electron-microscopic level. Acyltransferase activity was localized on the cytoplasmic sides of thick (greater than 10 nm) membranes of the following organelles: a few cisternae at the trans (or concave) side of Golgi apparatus, Golgi and probably related vesicles, short tubules, curved pentalaminar disks and limiting membranes of the phagosomal multivesicular bodies; all organelles were scattered in the peri- to supranuclear cytoplasm. The phospholipids, which are major components of the photoreceptor membrane, are considered to be synthesized by these membranes. Acid phosphatase activity was localized in the lumina of Golgi cisternae and vesicles, lysosomes, and smaller multivesicular and related bodies, but not in multilamellar bodies. The matrices of the larger multivesicular bodies and of the pigment granule complexes showed arylsulfatase activity. Vesiculated and autophagocytosed photoreceptor microvilli seemed to be degraded by acid hydrolases, forming multivesicular and related bodies. Supporting cells also showed acyltransferase and acid hydrolase activities.