Ciliary signaling cascades in photoreceptors

Abstract

For being a polarized neuron and having a sensory cilium, photoreceptors attract remarkable attention. This is due their highly polarized structure and active visual signal transduction cascades and for the enrichment of complex networks of proteins in the cilium. Structural and functional maintenance of the photoreceptor sensory cilium, also called outer segment, ensures that light signal is received and relayed appropriately to the brain. Any perturbations in the protein content of the outer segment result in photoreceptor dysfunction, degeneration and eventually, blindness. This review focuses on the importance of photoreceptor sensory cilium to carry out signal transduction cascade for vision.

Figure 1. Schematic representation of phototransduction components in photoreceptors

The outer segment (OS) is a modified cilium, with the basal body (BB) located at the apical region of the inner segment. The transition zone extends from the BB and gives rise to the axoneme, which continues into the outer segment. The disc membranes in the outer segment are enriched in rhodopsin and other proteins required for carrying out the phototransduction cascade. Part of the cascade also takes place in the overlaying RPE (retinal pigmented epithelium). In addition, RPE is also required for periodic disc shedding by phagocytosis. Only selected proteins in the outer segment are depicted in the inset. N: nucleus