ERM proteins and cell morphology in the retina

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The long-term objective of the lab is to study the mechanisms regulating development and maintenance of the photoreceptor outer segment.*** Each cell type of the body must acquire and maintain a specific morphology adapted to its function. Our lab focuses on photoreceptors, the light-sensing cells of the eye, and their specialized sensory endings, the outer segments. Outer segments are expanded and modified cilia containing stacks of flattened membranous disks laden with opsin photopigments. To build and renew an outer segment requires repeated evagination of the plasma membrane, each time giving rise to a new disk. However, the mechanisms driving and controlling disk expansion are unclear. At the same time, microvilli-like protrusions, called calyceal processes, extend perpendicular to the disks and surround the base of the outer segment. The purpose of calyceal processes is unknown, but their disruption is associated with photoreceptor degeneration. *** In this grant, we look at how the actin cytoskeleton and plasma membrane co-operate to create the complex morphology of the photoreceptor outer segment. Importantly, calyceal processes are supported by a core bundle of actin filaments, but outer segment disks evaginate without any cytoskeleton. Actin is however present adjacent to newly forming disks and is required for disk initiation. Therefore, the actin cytoskeleton is necessary for both calyceal processes and outer segments, but in distinct ways. As OS disks do not have an actin core, we propose that they form through blebbing, a cellular process whereby actomyosin contraction leads to increased intracellular pressure and release of the membrane from the cytoskeleton. In contrast, the microvilli-like CPs form by extension of their actin core. ERM (Ezrin, Radixin, Moesin) proteins act as linkers between the cytoskeleton and membrane and have well-described roles in both blebbing and microvilli formation. We hypothesize that the ERM protein Ezrin anchors calyceal processes, but at the base of the outer segment is periodically inhibited to allow for disk evagination.*** We use zebrafish as our model system to study photoreceptors because of their many experimental advantages, well-developed retina and excellent vision. Here, we will take advantage of both in vitro and in vivo assays to examine how the ERM protein Ezrin and the actin cytoskeleton promote growth of the outer segment, by asking three main questions:1) Does blebbing drive evagination of photoreceptor outer segment disks? 2) Do calyceal processes limit growth of OS disks and stabilize the OS? 3) What are the specific functions of actin and Ezrin in calyceal process stabilization and outer segment disk evagination?***Impact: This research will improve our understanding of how membrane-cytoskeletal interactions control the morphologesis of complex cells.