How do visual receptors survive the abuses they suffer? Visual receptors are under continuous frontal assault from an armada of deadly foes including light, which can initiate deleterious reactions, and a high level of oxidative catabolism, or biological rusting. The motley crew of these beleaguered cells includes highly reactive polyunsaturated fatty acids and vitamin A metabolites. The question of visual receptor maintenance has been pivotal in vision research for over three decades. Visual membranes are constantly being renewed, and I characterized the membrane turnover in Drosophila with electron microscopy. Multivesicular bodies (MVB's) were clearly seen to be merging with primary lysosomes to form secondary lysosomes for membrane breakdown. Rhodopsin mutants as well as transduction mutants such as norpA (no receptor potential) were used to determine the involvement of light initiated visual excitation in photoreceptor maintenance. Circadian rhythm mutants, the per (period alleles), were used to determine aspects of periodicity. Endocytosis (shibire) [dynamin] and lysosomal enzyme (acid phosphatase negative) mutants were used to dissect the early steps in membrane breakdown.

Papers in the receptor maintenance and membrane turnover series:

Stark, W.S., Sapp, R. and Schilly, D. Rhabdomere turnover and rhodopsin cycle: Maintenance of retinula cells in Drosophila melanogaster. Journal of Neurocytology, 1988, 17, 499-509. PubMed

Stark, W.S., Sapp, R., and Carlson, S.D. Photoreceptor maintenance and degeneration in the norpA (no receptor potential-A) mutant of Drosophila melanogaster. Journal of Neurogenetics, 1989, 5, 49-59. PubMed

Stark, W.S. and Sapp, R. Retinal degeneration and photoreceptor maintenance in Drosophila: rdgB and its interaction with outer mutants. In: Inherited and Environmentally Induced Retinal Degenerations: (ed: M.M.LaVail, R. E. Anderson and J. G. Hollyfield) New York: Alan R. Liss, Inc.1989. pp. 467-489 (Invited Chapter). PubMed

Zinkl, G., Maier, L., Studer, K., Sapp, R., Chen, D.-M., Stark, W. S. Microphotometric, ultrastructural and electrophysiological analyses of light dependent processes on visual receptors in white-eyed wild-type and norpA (no receptor potential) mutant Drosophila. Visual Neuroscience, 1990, 5, 429-439. PubMed

Sapp, R, J., Christianson, J. S., Stark, W. S. Turnover of membrane and opsin in visual receptors of normal and mutant Drosophila. Journal of Neurocytology, 1991, 20, 597-608. PubMed

Chen, D.-M., Christianson, J.S., Sapp, R.J., Stark, W.S. Visual receptor cycle in normal and period mutant Drosophila: A circadian rhytum and its entrainment. Visual Neuroscience, 1992, 9, 125-135. PubMed

Chen, D.-M., Stark, W. S. The effects of temperature on visual receptors in temperature-sensitive paralytic paralytic shibire (shits) mutant Drosophila. Journal of Insect Physiology 1993, 39, 385-392.

Coated pit suggesting autophagy from retinula cell plasmalemma. Also note rhabdomere tip. (see Stark et al., 1988).

Coated pit suggesting autophagy from rhabdomere. (see Stark et al., 1988)

An electron micrograph from a photoreceptor in the Drosophila compound eye showing a multivesicular body (MVB) surrounded by primary lysosomes, components of the autophagy process. Interestingly, section was stained for acid phosphatase in an acid phosphatase null mutant. (from Stark et al., 1988)

An electron micrograph of MVB's (compare with previous figure) where acid phosphatase staining reveals the expected punctate densification. (see Stark et al., 1988)

An electron micrograph showinc coated pits that fail to turn into vesicles in shibire photoreceptors (from Sapp et al., 1988)

An electron micrograph showing the synaptic area in shibire with a string of uptake pits that did not endocytose properly (from Sapp et al., 1991)

An electron micrograph of a photoreceptor in the Drosophila compound eye with part of the photoreceptive rhabdomere at the top and prominent rough endoplasmic reticulum

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