The light sensing organelle of photoreceptor cells is a modified primary cilium, called the outer segment. The outer segment is massive compared to a prototypic primary cilia because it houses hundreds of flattened, light sensitive membrane “discs” that enable this organelle to detect as little of light as a single photon. To cope with photooxidative damage, photoreceptor cells must continuously renew their disc membranes. This is accomplished by the constant formation of new discs at the outer segment base and the engulfment of old discs from outer segment tips by the retinal pigment epithelium. The daily turnover of disc membranes is so enormous that the retinal pigment epithelial cells phagocytize more material than any other cells in the body. For this reason, even a slight defect in disc renewal can lead to inherited retinal dystrophy resulting in incurable blindness in humans.
Discs are formed by membrane evagination
Photoreceptor disc formation occurs at the base of the outer segment in a series of well-defined steps. First, the ciliary plasma membrane protrudes outward forming a broad membrane evagination which is connected to the cilium. Second, this membrane evagination completely flattens and expands until it is the full diameter of the outer segment. Finally, in a process resembling endocytosis, the outer segment plasma membrane wraps around the circumference of the evagination, enclosing it inside of the outer segment as a disc membrane.
How does the actin cytoskeleton drive disc formation?
The first step of disc formation is crucial and relies on the action of the actin cytoskeleton. The biochemical force generated by the polymerization of actin pushes on the ciliary plasma membrane to create each nascent disc evagination. The machinery driving this actin polymerization is called Arp2/3, a seven-subunit protein complex that induces actin polymerization in a branched manner. After F-actin pushes the membrane outwards, it must depolymerize to allow the evagination to flatten. This cycle occurs up to 100 times a day in each photoreceptor cell and is analogous to other Arp2/3 driven protrusions of the plasma membrane like the lamellipodia. Interestingly, the actin network driving disc formation has specialized proteins which are only found in photoreceptor cells and the precise molecular mechanisms of actin dynamics at the site of disc formation are unknown. This actin-dependent step is crucial for building the outer segment and defects in the process lead to dysmorphic outer segments, death of photoreceptor cells and complete blindness. Therefore, a major focus of the Spencer lab is to understand how this actin network functions at the molecular level.
How does PRCD function and what is the significance of photoreceptor-derived extracellular vesicles?
Loss of the photoreceptor disc specific protein, PRCD, causes a defect in disc formation leading to the release of a fraction of the disc membrane material as extracellular vesicles which accumulate like debris in the retina. Neuroimmune cells called microglia attempt to clear the vesicular debris by phagocytosis but photoreceptor cells progressively die surrounded by the accumulating extracellular vesicles. The Spencer Lab aims to elucidate the molecular mechanisms underpinning PRCD’s function during disc formation and the microglial clearance of the vesicular debris. Mutations of the Prcd gene are among the most common cause of blindness in dogs and are also identified in blind human patients suffering from incurable retinal degeneration. In addition to PRCD, numerous other animal models of retinal degeneration are marked by the accumulation of extracellular vesicles surrounding photoreceptor cells. Understanding the basic mechanisms of vesicle release by photoreceptors and associated neuroimmune response will advance the development of future therapies for these blinding retinal diseases.