德州A&M大学的研究人员研究一种早年失明的罕见形式，辨认出相关的细胞和致病机转，这项研究结果将有助于治疗这种目前无药可医的失明性疾病。这项研究结果发表于3月5-9日的Proceedings of the National Academy of Sciences网络抢先版。
A&M大学的生物学家Brian Perkins在哈佛大学的博士后研究时，就研究了在感光细胞中负责运输的蛋白质，这二种感光细胞分别是锥状和杆状细胞，可以使包含人类在内的脊椎动物看到这个世界。
Brian针对已经辩论长达30年的问题进行研究：感光细胞的死亡是否导致无脉络膜症？这种疾病是一种X染色体的性联遗传疾病，男性罹患这种疾病的比例较女性高出许多，其发生率约为每100,000人中有一人罹患此症，患者从青少年其就会发生严重的视力缺损及夜盲，到了中年会变得完全失明。
研究人员利用突变的斑马鱼进行研究，并且将研究焦点放在一种特殊的蛋白质REP1，这种蛋白质有助于调控感光细胞及视网膜色素上皮的细胞内运输。如果REP1发生突变，就会影响视网膜色素上皮的细胞，导致感光细胞死亡。因此，如果利用疗法来矫正RPE，将可以抢救无脉络膜症患者逐渐缺损的感光细胞，甚至扭转疾病的病程。
(资料来源 : Bio.com)
Published online before print March 5, 2007, 10.1073/pnas.0605818104
PNAS | March 13, 2007 | vol. 104 | no. 11 | 4600-4605
OPEN ACCESS ARTICLE
Noncell-autonomous photoreceptor degeneration in a zebrafish model of choroideremia
Bryan L. Krock*, Joseph Bilotta,, and Brian D. Perkins*,
*Department of Biology, Texas A & M University, College Station, TX 77843; and Department of Psychology and Biotechnology Center, Western Kentucky University, Bowling Green, KY 42101
Edited by John E. Dowling, Harvard University, Cambridge, MA, and approved January 10, 2007 (received for review July 11, 2006)
Abstract
Choroideremia is an X-linked hereditary retinal degeneration resulting from mutations in the Rab escort protein-1 (REP1). The Rep1 protein facilitates posttranslational modification of Rab proteins, which regulate intracellular trafficking in the retinal pigment epithelium (RPE) and photoreceptors and are likely involved in the removal of outer segment disk membranes by the RPE. A critical question for potential treatment of choroideremia is whether photoreceptor degeneration results from autonomous defects in opsin transport within the photoreceptor or as a nonautonomous and secondary consequence of RPE degeneration. To address this question, we have characterized the retinal pathology in zebrafish rep1 mutants, which carry a recessive nonsense mutation in the REP1 gene. Zebrafish rep1 mutants exhibit degeneration of the RPE and photoreceptors and complete loss of visual function as measured by electroretinograms. In the mutant RPE, photoreceptor outer segment material was not effectively eliminated, and large vacuoles were observed. However, opsin trafficking in photoreceptors occurred normally. Mosaic analysis revealed that photoreceptor degeneration was nonautonomous and required contact with the mutant RPE as mutant photoreceptors were rescued in wild-type hosts and wild-type photoreceptors degenerated in mutant hosts. We conclude that mutations in REP1 disrupt cellular processes in the RPE, which causes photoreceptor death as a secondary consequence. These results suggest that therapies that correct the RPE may successfully rescue photoreceptor loss in choroideremia.
Choroideremia (CHM) is an X-linked form of retinal degeneration caused by mutations in the gene for Rab escort protein 1 (Rep1) (1, 2), a protein found in all tissues and highly expressed in the outer retina and retinal pigment epithelium (RPE). CHM causes night blindness in children and progresses to complete loss of vision in adults. CHM is one of the few hereditary blindness disorders that can be clinically identified before significant loss of visual function (3), suggesting that diagnosis and intervention during childhood may prevent further loss of vision.
Rep proteins play an essential role in the posttranslational modification of Rab proteins, the small GTP-binding proteins that are essential for many aspects of intracellular transport. Rep proteins bind newly synthesized Rab proteins and facilitate the addition of geranyl-geranyl groups, a modification essential for Rab function in intracellular trafficking (reviewed in ref. 4). In humans, Rep1 and its homolog, Rep2, are ubiquitously expressed and exhibit overlapping substrate specificity (5). Mutations in Rep1 prevent the modification of Rab proteins, thereby disrupting Rab-mediated intracellular trafficking in photoreceptors and the RPE. Because patients with CHM only experience age-related blindness, Rep2 appears to effectively compensate for the loss of Rep1 in all tissues except the eye (6). Interestingly, zebrafish do not contain a Rep2 ortholog, and the loss of Rep1 results in lethality at larval stages (7).
The development and survival of photoreceptors requires effective intracellular trafficking in both photoreceptors and the RPE. In the photoreceptor, proteins destined for the outer segment (e.g., opsin) travel from the Golgi to the connecting cilium via vesicular transport that is regulated by Rab8 and Rab6 (8, 9). In Xenopus laevis expressing dominant-negative forms of Rab8, rapid photoreceptor degeneration and defects in outer segment morphogenesis were observed (10). It has been proposed that mutations in Rep1 may lead to defects in opsin trafficking that contribute to photoreceptor degeneration (11). In the RPE, intracellular trafficking controls the phagocytosis and degradation of disk membranes shed from the apical tips of photoreceptor outer segments. Failure of the RPE to clear outer segment debris leads to a toxic environment surrounding the photoreceptors and causes death. It is believed that Rab proteins function during phagocytosis by the RPE, although the mechanism is not clear. It is known that Rab27a is a target of Rep1 and that Rab27a interacts with myosin VIIA in the transport of melanosomes (12, 13). Furthermore, cultured RPE cells that lack myosin VIIA exhibit defects in the phagocytosis of outer segment membranes (14). A tempting hypothesis states that loss of Rep1 disrupts the function of a Rab27a–myosinVIIA complex and causes defects in phagocytosis by the RPE. Because all retinal cells express Rep1, it is unknown whether CHM reflects a cell-autonomous degeneration of photoreceptors, a noncell-autonomous effect caused by RPE dysfunction, or a combination of both. Development of appropriate therapies requires a clear understanding of the tissue-specific contributions to disease.
Here, we report that zebrafish carrying a recessive nonsense mutation in rep1 (7) exhibit retinal phenotypes consistent with CHM. Using histological, functional, and embryonic manipulations, we found that rep1 mutants experience photoreceptor degeneration, loss of visual function, and defects in RPE pigmentation and outer segment phagocytosis. By producing genetically mosaic animals, we show that the loss of Rep1 in the RPE is sufficient to induce degeneration of wild-type photoreceptors. These findings provide insight into the pathology of the disease and have implications for the design of future therapies.
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