德州A&M大學的研究人員研究一種早年失明的罕見形式,，辨認出相關的細胞和致病機轉,，這項研究結果將有助于治療這種目前無藥可醫(yī)的失明性疾病。這項研究結果發(fā)表于3月5-9日的Proceedings of the National Academy of Sciences網(wǎng)絡搶先版,。
A&M大學的生物學家Brian Perkins在哈佛大學的博士后研究時,，就研究了在感光細胞中負責運輸?shù)牡鞍踪|，這二種感光細胞分別是錐狀和桿狀細胞,，可以使包含人類在內(nèi)的脊椎動物看到這個世界,。
Brian針對已經(jīng)辯論長達30年的問題進行研究：感光細胞的死亡是否導致無脈絡膜癥？這種疾病是一種X染色體的性聯(lián)遺傳疾病,，男性罹患這種疾病的比例較女性高出許多,，其發(fā)生率約為每100,000人中有一人罹患此癥，患者從青少年其就會發(fā)生嚴重的視力缺損及夜盲,，到了中年會變得完全失明,。
研究人員利用突變的斑馬魚進行研究，并且將研究焦點放在一種特殊的蛋白質REP1,，這種蛋白質有助于調控感光細胞及視網(wǎng)膜色素上皮的細胞內(nèi)運輸,。如果REP1發(fā)生突變，就會影響視網(wǎng)膜色素上皮的細胞,，導致感光細胞死亡,。因此,，如果利用療法來矯正RPE，將可以搶救無脈絡膜癥患者逐漸缺損的感光細胞,，甚至扭轉疾病的病程,。
(資料來源 : 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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