Figure 6.

The connecting cilium of photoreceptors and its transport machinery. (A) Schematic of a vertebrate rod photoreceptor consisting of the light-sensitive outer segment (OS) enclosed by the retinal pigment epithelium (RPE) and linked via the connecting cilium (CC) to the metabolically active inner segment (IS); (B,C) Schematics of the two ciliary transport systems in photoreceptors; (B) Cargo transport to the CC is mediated along microtubules (MT) by cytoplasmic dynein 1 motor proteins (minus-end-directed). Delivery from the IS to the OS is mediated by IFT B molecules (binding the cargo) and kinesin II motor proteins (plus-end-directed). Delivery back to the IS is mediated by IFT A molecules and cytoplasmic dynein 2 motor proteins (minus-end-directed); (C) In addition, a myosin 7a-driven transport along actin filaments is used for trafficking proteins to the OS; (D–G) Confocal laser scanning micrographs of a vertical cryostat section through an adult mouse retina double labeled for Centrin 3 (D, green) and Pericentrin (E, red) as markers for the cilia and the basal body complex (BBC), respectively; (F,G) As seen in the merge of the stainings with additional labeling of the cell nuclei with DAPI, centrin 3 and pericentrin partially colocalize at the ciliary region of the photoreceptors and at the centrosomes of other retinal cells; (H–J) Higher power views showing the partial colocalization of centrin 3 and pericentrin at the BBC of photoreceptor-connecting cilia. Scale bars: 20 µm (F), 1 µm (J).