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Ching-Hwa Sung, Ya-Chu Hsu, Jen-Zen Chuang; Light regulates the outer segment protein transport and disc renewal of mammalian photoreceptors. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5955.
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The vertebrate photoreceptor outer segment (OS) is a modified cilium containing ~1,000 membranous discs to accommodate rhodopsin for light detection. Mammalian rod OS undergo constant and rapid renewal (every ~10 days). Nascent discs are formed at the base of the OS via the incorporation of proteins synthesized at and transported from the inner se through the connecting cilium, while distal discs are shed and phagocytosed by neighboring RPE cells. To date, the mechanism and environmental cues that regulate the disc renewal and OS protein transport of remains elusive
We used both constitutive and Cre-lox inducible expression system in transfected rods to trace the path and fate of two essential OS proteins, rhodopsin and peripherin-2/rds, in rats reared under different conditions. Both confocal and electron microscopies were employed to investigate the distribution of the reporter proteins.
Our results show that newly synthesized rhodopsin appears on Rab11-positive recycling endosome prior to reaching the OS. Rhodopsin primarily enters the OS in the dark. Photoexcitation of post-Golgi rhodopsins retains them in the inner segment. Daily-synthesized rhodopsins are packed into distinct “segments”; each OS has ~10 segments. The OS entry of disc-rim protein peripherin-2/rds follows a rhythm complementary from that of rhodopsin’s. Thus, cyclic light serves as a mechanism to alternate rhodopsin- and peripherin-2/rds- rich disc and “divide” the OS into ~10 segments.
Our data showed that the post-Golgi rhodopsin transits through intermediate compartments before entering the OS. Furthermore, the trafficking of rhodopsin from the inner segment to the OS is inhibited by light. Light differentially regulates the OS transport of rhodopsin and peripherin-2/rds in a complementary fashion. As a result, discs characterized by distinguishable protein composition are alternately stacked. We propose a model explaining how this specialized cytostructure of the OS participates the balance the quanta of discs added and removed daily, and, hence, maintaining the OS homeostasis.
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