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

Summary

Deficiency of the photopigment chromophore, resulting from carotenoid/retinoid (vitamin A) deprivation, that severely impairs the visual function of Manduca sexta also leads to the hypertrophy of smooth endoplasmic reticulum in the photoreceptors. The excess endomembrane accumulates in the stacked cisternae of myeloid bodies. Although 11-cis retinal promotes substantial recovery of function in the retinas of deprived moths maintained in darkness, the myeloid bodies remain. When such recovering photoreceptors were exposed to light of moderate intensities, the amount of endomembrane diminished to normal levels over a period of several hours, while rhabdomeres grew larger. Since there was no endocytolysis, the myeloid bodies must have provided the membrane for rhabdomere enlargement. Bright light similarly mobilized the myeloid bodies in deprived receptors. Thus the persistence of myeloid bodies in moderately illuminated chromophoredeficient receptors is a consequence of their insensitivity. However, the initial hypertrophy of endomembrane does not appear to result from the lack of adequate stimulation: normal, chromophore-replete photoreceptors maintained in darkness from before the period of retinal development had large rhabdomeres and no myeloid bodies. The development of myeloid bodies during the differentiation of vitamin A-deprived photoreceptors appears to entail an influence of the chromophore at another level of receptor cell function.