A Ciliary Branched Actin Network Drives Photoreceptor Disc Morphogenesis

The light-detecting organelle of the photoreceptor cell is a modified primary cilium, called the outer segment. The outer segment houses hundreds of light-sensitive membrane, “discs,” that are continuously renewed by the constant formation of new discs at the outer segment base and the phagocytosis of old ones from outer segment tips by the retinal pigment epithelium. In this chapter, we describe how an actin cytoskeleton network, residing precisely at the site of disc formation, provides the driving force that pushes out the ciliary plasma membrane to form each disc evagination that subsequently can mature into a bona fide disc. We highlight the functions of actin-binding proteins, particularly PCARE and Arp2/3, that are known to participate in disc formation. Finally, we describe a working model of disc formation built upon the many studies focusing on the role of actin during disc morphogenesis.