Disk membrane synthesis in rod and cone photoreceptors

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The vertebrate retina contains photoreceptors of two principle types: rods and cones. Within these cells, photodetection occurs in an elaborate structure called the outer segment (OS), a modified sensory cilium. In rods, the OS consists of thousands of membranous discs surrounded by a plasma membrane and separated from the cell body by a connecting cilium. The disks contain high concentrations of rhodopsin. Rhodopsin-containing rod disc membranes are created at the base of the rod OS. Each disc is isolated from the others; new discs are displaced gradually towards the retinal pigment epithelium, where they are shed. In contrast, cone disc membranes are continuous with each other. The mechanisms underlying this remarkable process are still under investigation, with two principle competing models, and some basic premises under debate. The chief obstacles are lack of tools, and lack of insight into molecular players. We have developed new techniques for studying disc membrane biosynthesis, and will use these techniques in the following Aims: