Evidence from normal and degenerating photoreceptors that two outer segment integral membrane proteins have separate transport pathways

Abstract

Detachment of the neural retina from the retinal pigment epithelium induces photoreceptor degeneration. We studied the effects of this degeneration on the localization of two photoreceptor outer segment-specific integral membrane proteins, opsin and peripherin/rds, in rod photoreceptors. Results from laser scanning confocal microscopic and electron microscopic immunolocalization demonstrate that these two proteins, normally targeted to the newly-forming discs of the outer segments, accumulate in different sub-cellular compartments during photoreceptor degeneration: opsin immunolabeling increases throughout the photoreceptor cell's plasma membrane, while peripherin/rds immunolabeling occurs within cytoplasmic vesicles. The simplest hypothesis to explain our results is that these proteins are transported in different post-Golgi transport vesicles and separately inserted into the plasma membrane. More complex mechanisms involve having the two co-transported and then opsin finds its way into the plasma membrane but peripherin/rds does not, remaining behind in vesicles. Alternatively, both insert into the plasma membrane but peripherin/rds is recycled into cytoplasmic vesicles. We believe the data most strongly supports the first possibility. Although the transport pathways for these proteins have not been fully characterized, the presence of peripherin/rds-positive vesicles adjacent to the striated rootlet suggests a transport role for this cytoskeletal element. The accumulation of these proteins in photoreceptors with degenerated outer segments may also indicate that their rate of synthesis has exceeded the combined rates of their incorporation into newly forming outer segment disc membranes and their degradation. The accumulation may also provide a mechanism for rapid recovery of the outer segment following retinal reattachment and return of the photoreceptor cell to an environment favorable to outer segment regeneration.

Photoreceptor renewal: A role for peripherin/rds

“…Hence it is reasonable to infer the existence of two pools of transport vesicles, although only those vesicles bearing rhodopsin have been isolated and studied so far. The observation that peripherin/rds and rhodopsin are mislocalized in induced retinal detachment but remain segregated is consistent with this notion (Fariss et al, 1993). Some investigators have observed "cytoplasmic bridges" between the inner and outer segment (distinct from the connecting cilium), leading to the hypothesis that vesicles may bud from the apex of the inner segment to deliver components to the outer segment (Besharse and Horst, 1990).…”

“…Signal of Rhodopsin-The mislocalization of rhodopsin is involved in the pathology of various retinal degenerative diseases (Dryja et al, 1990;Fariss et al, 1993). Mutations and deletions within the distal eight amino acids of rhodopsin, later identified as the localization signal sequence, contribute to a disease cluster characterized by early onset and aggressive retinal degeneration (Sandberg et al, 1995).…”

“…Earlier evidence supporting two transport pathways, one for opsin and one for peripherin/ rds, comes from a series of confocal and electron microscopy studies (Fariss et al, 1993;1997). Upon retinal detachment-induced degeneration, opsin was found to accumulate in the plasma membrane of the rod cell, while peripherin/rds was most prominent in cytoplasmic vesicles.…”

Animal models of retinal detachment and reattachment: identifying cellular events that may affect visual recovery

“…This results in a bright signal from the ONL in retinas detached for more than a day, with the signal increasing in extent and intensity as detachment time increases. 21 Reattachment can stop this process; treatment with BDNF or hyperoxia can diminish it. 3,8 When the retina is reattached after 1 h of detachment there is almost no redistribution (Figure 5c).…”

The Ciliary Rootlet Maintains Long-Term Stability of Sensory Cilia

“…An additional role in the proper positioning or anchoring of cellular organelles is implicated by the association of photoreceptor rootlets with membranebound saccules, endoplasmic reticulum (ER), Golgi membranes, and mitochondria (19,24). Finally, a role for rootlets in intracellular protein transport has also been suggested (4). Provision of structural support, organization of cellular organelles and participation in intracellular trafficking seem plausible hypotheses, but none was tested directly.…”