Photoreceptors

Vision begins in the photoreceptors of the retina, where light energy is absorbed and converted to a neural response, a process known as phototransduction. In vertebrates, this process occurs in two classes of photoreceptors: rods and cones, which function in dim and bright light, respectively. Both rods and cones are polarized, having two biochemically and morphologically distinct compartments: the inner segment and the outer segment (figure 7.1).

Figure 7.1.  

Schematic representation of rod and cone photoreceptors. The outer segments contain the phototransduction machinery of the cells, and the inner segments perform the metabolic functions of the cells. The two compartments are joined by the connecting cilia.

The outer segment (OS) of photoreceptors is composed of a dense packing of disk-shaped membranes. In rods, these disks are pinched off from the plasma membrane. In cones, the disks form as invaginations of the plasma membrane, as in rods, but are not pinched off and maintain their continuity with the plasma membrane. These disks contain a very high concentration of the visual pigment rhodopsin, accounting for one third of the dry weight of the OS. In addition to rhodopsin, the OS are rich in proteins necessary for the visual transduction pathway as well as structural proteins responsible for maintaining the organization of the OS.

The inner segment contains the metabolic and synthetic machinery of the cell. The distal end of the inner segment, known as the ellipsoid, contains a dense packing of mitochondria. The mitochondria provide the substantial energy required by photoreceptors. For a dark-adapted rod, this requirement has been estimated at 90,000 ATP molecules per second per cell, ranking it as one of the most energetically demanding cells of the entire body. The proximal inner segment, known as the myoid, contains the synthetic machinery of the cell, including the endoplasmic reticulum and Golgi complex. Although responsible for providing all the photoreceptor components, the synthetic machinery is largely devoted to producing rhodopsin-containing vesicles to replace the disk membranes that are continuously phagocytosed at the distal end by the retinal pigmented epithelium.⁹¹

The inner and outer segments of rod and cone photoreceptors are connected by a narrow cilium through which must pass all components that travel between the segments (figure 7.2). The basic structure of the cilium is typical of nonmotile sensory cells, containing a “9 × 2 + 0” arrangement of microtubule doublets that extends from the basal body in the inner segment into the outer segment. The microtubule doublet composition is quickly lost within the proximal outer segment with the microtubules projecting as singlets into the distal outer segment, stopping short of the extreme distal tip.^13,31,69,82 This structure is collectively known as the axoneme and provides a cytoskeletal framework for structural support and for transporting molecules between the photoreceptor compartments using kinesin and dynein motors.^46,79,84,86 Microtubules are also abundant in the inner segment, functioning in the transport of macromolecular components from the endoplasmic reticulum and Golgi complex to the connecting cilium. The microtubules that form the axoneme contain acetylated tubulin and are thus more stable than the inner segment microtubules.^6,70 Microtubules also run the length of rod OS along the multiple incisures of the disks. These incisures are invaginations along the outer edge of the disks in rods and likely have a structural role. Interestingly, the number of incisures is quite variable between species, ranging from just one or a few invaginations in rodents and cows to several dozen in frogs and primates (summarized in Ref. 19).

Figure 7.2.  

The cytoskeleton of photoreceptors is composed principally of microtubules and microfilaments. The axoneme in the connecting cilium is composed of nine microtubule doublets, typical of nonmotile sensory cilia, and project to nearly the distal end of the outer segment. Microfilaments are found in the connecting cilia as well. In the inner segment, microtubules form the molecular train tracks between the Golgi complex and the connecting cilium.

In addition to microtubules, the photoreceptor cytoskeleton also utilizes microfilaments.^10,11,26,87 In the connecting cilium, these microfilaments have myosin VIIa associated with them, suggesting that they are not simply structural elements, but also function in moving cellular contents.^12,89 In addition to the connecting cilium, microfilaments are also found in calycal processes. These processes are fingerlike extensions of the inner segment that project around the basal portion of the OS. The calycal processes often align with the grooves formed by the multiple incisures in the OS disks.⁸⁷

There are two broad classes of cargoes that move through the connecting cilium: biosynthetic cargo destined for the outer segment (for example, phospholipid membranes and visual pigment) and proteins that show substantial intersegmental redistribution in response to light. How some of these cargoes utilize the ciliary cytoskeleton is beginning to be revealed. Recent studies have shown that kinesin II is a motor protein that is located in the cilium⁸⁴ and is at least involved in the transport of opsin-containing vesicles to the outer segment.^46,86 A similar role for myosin VIIa has also been demonstrated, although how the two motor proteins function together, whether in parallel or in series, is uncertain.^38,39,86,89 Additionally, an intraflagellar transport (IFT) particle has been identified in photoreceptor cilia, much like the IFT particles in the sensory cilia of Caenorhabditis and Chlamydomonas.⁶² In these organisms, the IFT particle has been shown to transport cargo along the axoneme, moved by kinesin in the plus direction and transported by dynein in the minus direction. The presence of a homologous molecular “raft” in photoreceptors argues strongly for kinesin transport of protein/vesicle cargoes on the raft from the inner segment to the outer segment along the axoneme microtubules. Further, the ciliary localization of dynein 1b/2⁶² suggests that the raft may also be used for retrograde transport to the inner segment. At this point, no specific cargoes have been associated with the IFT particle in photoreceptors.

Although the basic ciliary structure has been the subject of decades of study, new components of the cilium and new roles of ciliary elements continue to be identified. For example, RP1, RPGR, and RPGRIP are newly recognized proteins that localize to the cilia and appear to be involved in protein/vesicle transport through the cilium.^29,65 Further, an immunoscreen of retinal cDNA's using anti-axoneme-enriched antibodies identified several potentially new components of the connecting cilium.⁷¹ Clearly, there is much work to be done before the photoreceptor cilium and cytoskeleton are fully understood. Importantly, defects in ciliary proteins can be a significant cause of retinal degeneration. For example, RP1 is a member of the doublecortin family of proteins, and defects in this protein lead to severe retinitis pigmentosa.³⁷ This relationship emphasizes the role that protein trafficking through the cilium plays in the health of photoreceptor cells.