Rhabdomere turnover and rhodopsin cycle: maintenance of retinula cells inDrosophila melanogaster

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Abstract
Visual receptor maintenance inDrosophila involves turnover of membrane. Coated pits derived from rhabdomere and plasmalemma, coated vesicles and multivesicular bodies (MVBs) of about 0.5 μm diameter characterize the early autophagic steps. Smaller electron dense bodies (0.15 μm) 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 ultrastuctural 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.