Adam9-deficient retinal pigment epithelium pseudopods maintain photoreceptor outer segment renewal despite subretinal space expansion
Tylor R. Lewis, Carson M. Castillo, Sebastien Phan, Camilla R. Shores, Kylie K. Hayase, Keun-Young Kim, Mark H. Ellisman, Oleg Alekseev, Marie E. Burns, Vadim Y. Arshavsky
Tylor R. Lewis, Carson M. Castillo, Sebastien Phan, Camilla R. Shores, Kylie K. Hayase, Keun-Young Kim, Mark H. Ellisman, Oleg Alekseev, Marie E. Burns, Vadim Y. Arshavsky
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Research Article Cell biology Neuroscience Ophthalmology

Adam9-deficient retinal pigment epithelium pseudopods maintain photoreceptor outer segment renewal despite subretinal space expansion

Abstract

Vision begins in the outer segment compartment of photoreceptor cells, which is constantly renewed through the addition of membrane material at its base and ingestion of mature membranes at its tip by the retinal pigment epithelium (RPE). The close apposition of outer segments to the RPE is believed to be critical for maintaining this renewal process. Yet, in several retinal diseases, expansion of the subretinal space separating photoreceptors from the RPE does not immediately impact photoreceptor functionality. Here, we analyzed outer segment function and renewal in the Adam9-knockout mouse characterized by a major expansion of the subretinal space. Surprisingly, photoreceptor-RPE separation affected neither the sensitivity of photoreceptor light responses nor the normal rate of outer segment renewal in this mouse prior to the onset of photoreceptor degeneration. The latter is achieved through the formation of elongated RPE pseudopods extending across the enlarged subretinal space to ingest outer segment tips. This work suggests that pseudopod formation may underlie the persistence of photoreceptor function in human diseases accompanied by photoreceptor-RPE separation, such as vitelliform macular dystrophy or age-related macular degeneration associated with subretinal drusenoid deposits.

Authors

Tylor R. Lewis, Carson M. Castillo, Sebastien Phan, Camilla R. Shores, Kylie K. Hayase, Keun-Young Kim, Mark H. Ellisman, Oleg Alekseev, Marie E. Burns, Vadim Y. Arshavsky

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Figure 6

Loss of ADAM9 leads to the formation of elongated RPE pseudopods supporting outer segment renewal.

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Loss of ADAM9 leads to the formation of elongated RPE pseudopods support...
(A) Representative TEM images of the outer segment–RPE interface in P30 WT and Adam9–/– mice. For each condition, 3 retinas from separate mice were analyzed. Black arrowheads indicate phagosomes. OS, outer segment. Scale bar: 1 μm. (B) Quantification of phagosome number (normalized to RPE length) and phagosome size at P30 and P180. For phagosome number, the RPE length analyzed was as follows: WT, P30 1: 497 μm; WT, P30 2: 520 μm; WT, P30 3: 420 μm; Adam9–/–, P30 1: 445 μm; Adam9–/–, P30 2: 433 μm; Adam9–/–, P30 3: 543 μm; WT, P180 1: 566 μm; WT, P180 2: 646 μm; WT, P180 3: 652 μm; Adam9–/–, P180 1: 642 μm; Adam9–/–, P180 2: 604 μm; Adam9–/–, P180 3: 629 μm. Unpaired t test showed a statistically significant difference in phagosome number between WT and Adam9–/– mice at P180 (*P = 0.0430) but not P30 (P = 0.0543). For phagosome size, 25 phagosomes from each sample were analyzed. Unpaired t test showed no statistically significant difference between phagosome size of WT and Adam9–/– mice at either P30 (P = 0.6883) or P180 (P = 0.6173). (C) Representative TEM images of pseudopods in P30 Adam9–/– mice at various stages of their formation. Scale bar: 1 μm. (D) Quantification of pseudopod number normalized to the RPE length analyzed. For each time point, at least 3 retinas from separate mice were analyzed. The RPE length analyzed was as follows: Adam9–/–, P30 1: 437 μm; Adam9–/–, P30 2: 394 μm; Adam9–/–, P30 3: 464 μm; Adam9–/–, P180 1: 390 μm; Adam9–/–, P180 2: 409 μm; Adam9–/–, P180 3: 394 μm, Adam9–/–, P180 4: 447 μm. Unpaired t test showed a statistically significant reduction in pseudopod number at P180 (**P = 0.0016). Data are presented as mean ± SEM.