From inner segment to outer segment: Palmitoylation of photoreceptor Na+, K+-ATPase and the importance of PRCD in photoreceptor outer segment morphogenesis

Abstract

Photoreceptors are specialized neuroepithelial cells which are optimized for efficient capture of light and initiation of visual transduction. These cells have several compartments which are very important for proper visual function and segregation of cellular processes, including the outer segment (OS), inner segment (IS), nucleus, and synapse. The IS houses all of the cellular organelles and biosynthetic molecular machinery the cell requires and is the site of protein synthesis. The light-sensing OS is a highly modified, primary cilium, which contains many stacks of double membranous discs which house proteins required for formation and maintenance of OS structure, as well as phototransduction. These structural and phototransduction proteins are synthesized in the IS and subsequently trafficked to the OS through the narrow connecting cilium. Many of these proteins undergo post-translation lipid modifications for proper subcellular localization and association with the OS disc membranes. While the proteins involved in the phototransduction cascade are crucial for generating signals of vision, regulation of the photocurrent is needed for proper depolarization and hyperpolarization of the photoreceptor neuron, a process which is required for the cell to transduce electrical impulses to downstream neurons. Many ion channels, exchangers, and pumps, including the CNG channels and the NCKX in the OS, as well as the Na+, K+-ATPase and the HCN channel in the IS, are involved in this process of maintaining the photocurrent in both dark and light. One small OS disc-specific protein whose function has not yet been elucidated is progressive rod-cone degeneration, or PRCD. Previous studies in our lab have identified that PRCD is post-translationally lipid modified by S-palmitoylation on its sole cysteine, which is required for its stability and localization to the OS. Though PRCD has been shown to be important in OS disc morphogenesis and maintenance, its specific role in this process remains unclear. In this dissertation, I utilize a Prcd-KO animal our lab generated using CRISPR/Cas9 genome editing in order to investigate photoreceptor function, OS ultrastructure, and rhodopsin packaging into disc membranes in the absence of PRCD. Furthermore, I utilize acyl resin-assisted capture and mass spectrometry in order to identify novel proteins in the retina which undergo S-palmitoylation and validate palmitoylation profiles of several proteins using additional well-established techniques in the field. In Chapter 1 of this dissertation, I review the functional and structural needs of photoreceptor neurons, as well as the roles of both PRCD and the b2-subunit of the retinal Na+, K+-ATPase