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Transport of Proteins to the Photoreceptor Outer Segment
Large amounts of protein are trafficked between the inner and outer segments of photoreceptor cells, along the axonemal linkage. Gene mutations that intefere with this trafficking typically cause retinal degenerations and often syndromic ciliopathies. The Williams laboratory is studying ciliary proteins that have transport or regulatory roles in delivering opsin and other outer segment proteins. Approaches include microscopy (such as live cell imaging and EM) and biochemistry, using cell culture and in vivo analysis.
Fig. 1. Diagram of a photoreceptor and an overlying RPE cell, illustrating the pathway for the turnover of opsin and other disk membrane proteins. Opsin is synthesized in the inner segment (IS), transported to the connecting cilium (CC), and incorporated into disk membranes of the outer segment (OS) as the membranes are formed. To compensate for the addition of new disks, older ones at the tip are ingested by the RPE, which then degrades them.
Roles of Motor Proteins in the Transport of Organelles in the RPE
Melanosomes, phagosomes, and endolysosomes are transported to key locations in the RPE cells. Interactions between phagosomes and endolysosomes are essential for efficient degradation, with defects leading to RPE pathogenesis and symptoms of macular degeneration. Dr. Williams’ laboratory is investigating the roles of motor proteins in the transport of these organelles. A major focus is on how the action of the different motors is coordinated. Approaches include microscopy (such as live cell imaging and EM) and biochemistry, using primary cultures of RPE cells as well as in vivo analysis.
Gene Therapy for Retinal Degeneration in Usher Syndrome
Dr. Williams’ laboratory is investigating gene therapy for preventing the vision loss associated with different types of Usher Syndrome, which is an inherited recessive disorder. Approaches involve augmentation of gene function, or direct editing of the mutant gene.
Fig. 2. Primary culture of RPE cells, showing the melanosome autofluorescence in red, due to excitation with 633-nm light, and phalloidin labeling of actin filaments in green. From Gibbs et al., 2009.
Studies on RPE cells derived from stem cells
In collaboration with the Broad Stem Cell Research Center at UCLA and clinical ophthalmologists, the Williams laboratory is studying RPE cells that have been derived from stem cells, including induced pluripotent stem cells (iPS cells) from patients with retinal degenerations. The objectives of these studies are to increase our understanding of the disease pathogenesis, and to develop cell lines that can be used for the transplantation of RPE cells into diseased retinas.