Thrombospondin-1 Expression in RPE and Choroidal Neovascular Membranes

Introduction Age-related macular degeneration (AMD) s the leading cause of blindness in the West or individuals more than 50 years of age[1-3].Severe visual loss in the late stages of AMD most commonly results from choroidal neovas- cularization (CNV), a process characterized by the growth of new vessels from the choriocapil- laris through Bruch′s membrane. These new vessels are prone to leakage and bleeding and may be associated with detachment of the retinal pigment epithelium (RPE). …     

缺氧诱导体外培养人视网膜色素上皮细胞中Toll样受体4的表达

目的:了解缺氧对人视网膜色素上皮(retinal pigmentepithelium,RPE)细胞Toll样受体4(Toll-like receptor4,TLR4)表达的影响。方法:采用200μmol/L CoCl2处理RPE细胞建立化学缺氧模型,以未处理细胞作对照,在缺氧处理后2,4,8,12和24h用免疫荧光法观察TLR4在细胞中的定位,用RT-PCR和Western blot检测细胞中TLR4表达水平。结果:共聚焦显微镜观察到正常对照组RPE细胞胞质内有微弱的TLR4荧光表达,缺氧后胞质内荧光表达明显增强,缺氧12h荧光强度达最强。RT-PCR和Western blot结果分别提示随缺氧时间延长,RPE细胞中TLR4mRNA和蛋白表达增强水平升高,缺氧至12h表达高峰最强(P<0.05),24h表达开始下降,各组差异有统计学意义。结论:缺氧可诱导RPE细胞中TLR4表达增强。     

Thrombospondin plays a vital role in the immune privilege of the eye

PURPOSE: The role of thrombospondin (TSP)-1 in TGF-beta activation and T-cell suppression was studied in the retinal pigment epithelial (RPE) cells, a monolayer of pigmented cells that line the subretinal space, an immune-privileged site in the eye. METHODS: Posterior eyecups were prepared by excising the anterior segment, lens, and retina from enucleated eyes of C57BL/6, thrombospondin-1 knockout (TSP-1KO), and TGF-beta2 receptor II double-negative (TGF-beta2 RII DN) mice, leaving behind a healthy monolayer of RPE resting on choroid and sclera. Serum-free medium was added to these RPE eyecups, and, after various time intervals, supernatants (SNs) were removed and tested. RESULTS: SNs of an ex vivo culture of RPE cells from C57BL/6 mice were shown to inhibit both antigen and anti-CD3 activation of T cells, partially due to constitutive production of TGF-beta and to the ability of RPE to activate the latent form of TGF-beta. Activation of TGF-beta was entirely dependent on TSP-1, also produced by RPE. SNs of RPE from TSP-1KO mice failed to inhibit T-cell activation. Ovalbumin (OVA)-specific delayed hypersensitivity (DH) was not impaired when OVA was injected either into the subretinal space or into the anterior chamber of TSP-1KO mice before OVA immunization. Moreover, experimental autoimmune uveoretinitis was significantly more intense in eyes of TSP-1KO mice and failed to undergo spontaneous resolution unlike wild-type mice. CONCLUSIONS: Production of both TSP-1 and active TGF-beta by RPE is essential to the creation and maintenance of immune privilege in the subretinal space and that the immune privilege limits the severity and duration of retinal inflammation due to autoimmunity.     

Thrombospondin Plays a Vital Role in the Immune Privilege of the Eye

Purpose: The role of thrombospondin (TSP)-1 in TGF-β activation and T-cell suppression was studied in the retinal pigment epithelial (RPE) cells, a monolayer of pigmented cells that line the subretinal space, an immune-privileged site in the eye. Methods: Posterior eyecups were prepared by excising the anterior segment, lens, and retina from enucleated eyes of C57BL/6, thrombospondin-1 knockout (TSP-1KO), and TGF-β2 receptor II double-negative (TGF-β2 RII DN) mice, leaving behind a healthy monolayer of RPE resting on choroid and sclera. Serum-free medium was added to these RPE eyecups, and, after various time intervals, supernatants (SNs) were removed and tested. Results: SNs of an ex vivo culture of RPE cells from C57BL/6 mice were shown to inhibit both antigen and anti-CD3 activation of T cells, partially due to constitutive production of TGF-β and to the ability of RPE to activate the latent form of TGF-β. Activation of TGF-β was entirely dependent on TSP-1, also produced by RPE. SNs of RPE from TSP-1KO mice failed to inhibit T-cell activation. Ovalbumin (OVA)-specific delayed hypersensitivity (DH) was not impaired when OVA was injected either into the subretinal space or into the anterior chamber of TSP-1KO mice before OVA immunization. Moreover, experimental autoimmune uveoretinitis was significantly more intense in eyes of TSP-1KO mice and failed to undergo spontaneous resolution unlike wild-type mice. Conclusions: Production of both TSP-1 and active TGF-β by RPE is essential to the creation and maintenance of immune privilege in the subretinal space and that the immune privilege limits the severity and duration of retinal inflammation due to autoimmunity.     

人视网膜色素上皮细胞对色素颗粒的吞噬作用

为检测培养的人视网膜色素上细胞对色素颗粒的吞噬活性,将从原代培养的ＲＰＥ细胞中提取的色素颗粒加入到培养的第８代ＲＰＥ细胞培养中,２４小时后,可见色素颗粒出现于细胞浆中,４天后,ＲＰＥ细胞胞浆中布满大量的色素颗粒。表明培养的ＲＰＥ细胞具有吞色素颗粒的活性。同时,该方法也为细胞移植提供了大量的细胞来源。     

反义RNA抑制人视网膜色素上皮细胞VEGF分泌的研究

目的　探索应用血管内皮生长因子 (VEGF12 1)的反义RNA抑制人视网膜色素上皮细胞 (RPE)VEGF表达的可行性。方法　构建编码反义hVEGF12 1真核表达质粒 ,转入人RPE细胞。将稳定表达外源基因的细胞分别置于大气氧环境 ( 2 1%O2 )或缺氧环境 ( 1%O2 )中培养 ,以空白细胞为对照 ,18h后收集上清液 ,ELISA测定其中VEGF的浓度 ,将此上清液加入人脐静脉血管内皮细胞 (HUVEC) ,观察其对细胞生长的影响。结果　反义VEGF转染能使RPE细胞VEGF的分泌量降低 62 7%。缺氧条件下 ,RPE细胞大量分泌VEGF ,18h时细胞培养上清液能刺激HUVEC细胞增殖 18 4% (P <0 0 1)。同样状态下 ,转染反义VEGF的RPE细胞培养上清液则无明显促内皮细胞生长作用 (P >0 0 1)。结论　本实验构建的反义VEGF12 1质粒能通过下调RPE细胞VEGF的蛋白分泌水平 ,有效抑制缺氧情况下该细胞上清液对血管内皮细胞的生长刺激作用     

Inhibition of retinal angiogenesis by peptides derived from thrombospondin-1

PURPOSE: Thrombospondin (TSP)1 is a tumor suppressor with activity that is associated with its ability to inhibit neovascularization. Previous studies have mapped this antiangiogenic activity to the type 1 repeats and the amino-terminal portion of the molecule within the procollagen-like domain. The present study was performed to investigate the ability of TSP-1 and peptides derived from the type 1 repeats to inhibit retinal angiogenesis. METHODS: TSP-1 and peptides with tryptophan-rich, heparin-binding sequences and transforming growth factor (TGF)-beta1 activation sequences were evaluated in two models of retinal angiogenesis: a retinal explant assay and a rat model of retinopathy of prematurity (ROP). RESULTS: Platelet-derived TSP-1 inhibited angiogenesis in both experimental models. Peptides from the native TSP-1 sequence, which contained both the tryptophan-rich repeat and the TGF-beta1 activation sequence, were the most potent inhibitors of endothelial cell outgrowth in the retinal explant assay. In contrast, a peptide containing only the tryptophan-rich, heparin-binding sequence was most active in inhibiting neovascular disease in the rat ROP model. CONCLUSIONS: These results indicate that the type 1 repeats of TSP-1 contain two subdomains that may independently influence the process of neovascularization, and that peptides derived from these type 1 repeats may be promising pharmacologic agents for treatment of retinal angiogenesis.     

体外培养人胚胎来源视网膜干细胞的诱导分化

目的 探讨培养的人胚胎来源视网膜干细胞向视网膜终末细胞分化的可能性。方法 来自16～20周人胚胎的视网膜干细胞进行无血清体外培养,并分别进行有血清条件下体外诱导和用含视网膜色素上皮的眼杯模拟体内条件诱导的观察,采用免疫荧光法检测干细胞和视网膜终末细胞表面抗原的表达,采用实时荧光定量PCR法检测诱导前后细胞nestin基因在mRNA水平的表达差异。结果 从人胚胎视网膜神经感觉层分离出的视网膜干细胞,在体外诱导的条件下,可表达视网膜终末细胞标记PKCα、GFAP、Thy1,少数细胞表达nestin和MAP2;在模拟体内环境诱导后,则不仅表达上述细胞标记,而且rhodopsin和syntaxin表达阳性。实时荧光定量PcR法检测显示：诱导后细胞nestin基因表达量较诱导前细胞明显降低。结论 RPE可以促进体外培养的视网膜干细胞向视杆细胞和无长突细胞分化。(中华眼科杂志,2004,40：448-452)     

Adrenomedullin in cultured human retinal pigment epithelial cells

PURPOSE: To determine whether adrenomedullin (ADM), a vasorelaxant peptide is produced and secreted by human retinal pigment epithelial (RPE) cells, whether ADM expression is regulated by inflammatory cytokines and a growth factor, and whether ADM has proliferative effects on these cells. METHODS: Production and secretion of ADM by cultured human RPE cells were examined by Northern blot analysis and radioimmunoassay. Regulation of the ADM expression by basic fibroblast growth factor, interferon (IFN)-gamma, tumor necrosis factor-alpha, interleukin (IL)1beta, or all-trans-retinoic acid was studied. In addition, proliferative effects of ADM on human RPE cells were examined by modified 3-(4,5-dimetylthiazol-2-yl)2,5-diphenyltetrazolium bromide (MTT) assay. RESULTS: ADM mRNA was expressed constitutively in all three human RPE cell lines (F-0202, D407, and ARPE-19) examined. Immunoreactive ADM was detected in the cultured media by radioimmunoassay. Sephadex G-50 column chromatography of the cultured medium showed a single peak eluting in the position of ADM-(1-52). Treatment with IFN-gamma or IL-beta increased ADM mRNA levels and immunoreactive-ADM levels in the medium in dose- and time-dependent manners in ARPE-19 cells. Exogenously added ADM increased the number of F-0202 cells and ARPE-19 cells, and the treatment with ADM antibody or ADM-(22-52) (an ADM antagonist) decreased it. CONCLUSIONS: Human RPE cells produced and secreted ADM. IFN-gamma and IL-1beta induced ADM expression in ARPE-19 cells. Furthermore, ADM stimulated proliferation of RPE cells. These results raise the possibility that ADM is related to the pathophysiology of some inflammatory and proliferative ocular diseases.     

Modulation of Nrf2-dependent antioxidant functions in the RPE by Zip2, a zinc transporter protein

PURPOSE: To characterize the involvement of Zip2, a zinc transporter protein, in the antioxidant functions of cultured human retinal pigment epithelial (RPE) cells. METHODS: The expression of zinc transporter proteins was determined by RT-PCR. Intracellular zinc concentration was assessed by staining with a zinc-sensitive dye followed by flow cytometry. Stable overexpression of the transporter protein Zip2 was achieved by transducing ARPE-19 cells with a retroviral vector containing the open reading frame of the human Zip2 gene. Activity of nuclear factor erythroid 2-related factor 2 (Nrf2) was measured using a dual luciferase assay after transient transfection of reporter plasmids containing the antioxidant response element (ARE). Glutamate-cysteine ligase (GCL) expression was measured by quantitative real-time RT-PCR. RESULTS: Cultured RPE cells could transport zinc with Zip2 as an influx transporter expressed in ARPE-19 cells and human RPE cells isolated from postmortem donor eyes. The mRNA level of Zip2 was influenced by intracellular and extracellular zinc concentrations. Overexpression of Zip2 resulted in increased Nrf2 activity, higher GCL expression, and increased glutathione synthesis. CONCLUSIONS: RPE cells can actively uptake zinc through the transporter Zip2, and the increased intracellular zinc upregulates the Nrf2-dependent antioxidant function.     

替德肝素抑制冷冻后视网膜色素上皮细胞分泌肝细胞生长因子的研究

目的探讨替德肝素(tedelparin)在抑制冷冻后的人视网膜色素上皮(humanretinapigmentepithelium,hRPE)细胞分泌肝细胞生长因子(humanhepatocytegrowthfactor,HGF)和生长中的作用。方法体外培养的RPE细胞在-80℃下进行冷冻,冷冻时间分为0、15和60s,随后继续体外培养(体外实验组)和注入正常兔眼玻璃体(体内实验组)并在第6天时取出一些兔眼的玻璃体液加入到正常RPE细胞培养液中孵育48h;每实验组再分为两个亚组替德肝素治疗(终浓度25μl/ml)组和未治疗组,在3d、6d时收集细胞培养上清和玻璃体样本,ELISA法测定HGF含量,MTT法测定48h后RPE细胞的增殖状态。结果在体外实验组,比较未冷冻组,冷冻后的RPE细胞随着冷冻时间延长HGF分泌水平增加(F=2736,P<001),替德肝素干预组HGF分泌水平下降(F=17950,P<001)。在体内实验组(兔玻璃体)随着冷冻时间延长HGF浓度较对照组明显增高(F=624,P<001),当玻璃体液(冷冻15s和60s,第6天)加入到正常RPE细胞培养液48h后刺激细胞增殖(P<001),在替德肝素干预组,细胞增殖明显减弱(F=4490,P<005)。结论冷冻可刺激RPE细胞在体外和体内玻璃体环境中HGF的高分泌,且随着冷冻时间增加更为显著。替德肝素可降低冷冻后RPE细胞分泌HGF水平和抑制促生长环境中的RPE细胞生长,具有预防PVR的作用。     

缺氧对培养的人视网膜色素上皮细胞中血管生成素-2表达的影响

背景脉络膜新生血管（CNV）是导致一些眼底疾病视力丧失的主要原因,而血管生成素2（Ang-2）与血管生成的关系较为密切。目前在缺氧条件下人视网膜色素上皮（RPE）细胞上Ang-2的表达情况与CNV发病的关系研究较少。目的观察缺氧对体外培养的人RPE细胞中Ang．2表达的影响,探讨Ang-2在CNV形成中的可能作用。方法人RPE细胞经培养和传代,取第4～7代细胞以5×10^7个／L密度接种于培养皿,常氧对照组不加CoCl2,缺氧组换以含终浓度为200μmol／LCoCl2的培养基5ml建立RPE细胞化学缺氧模型,分别于加药导致缺氧后0．5、1、2、4、6、12和24h终止缺氧。采用半定量逆转录聚合酶链反应（RT—PCR）法检测缺氧不同时间对体外培养的人RPE细胞Ang-2mRNA表达的影响;采用酶联免疫吸附反应（ELISA）法检测缺氧不同时间对体外培养的人RPE细胞上清液中Ang-2蛋白质量浓度的影响。结果人RPE细胞复苏后存活率达90％,第4代传代细胞中色素很少,细胞形态多为梭形。不同培养条件组人RPE细胞中Ang-2mRNA／β-actin mRNA值的总体比较差异有统计学意义（F=1086．30,P=0．00）;缺氧条件下培养0．5h后人RPE细胞中Ang-2mRNA／β-actin mRNA值开始明显增加,至缺氧培养后4～6h达高峰,与常氧对照组比较,差异均有统计学意义（P〈0．05）;缺氧培养24h后Ang-2mRNA／β-actin mRNA值接近常氧对照组。ELISA分析结果表明,不同培养条件下人RPE细胞培养基上清液中Ang-2蛋白的总体比较差异有统计学意义（F=1034．00,P=0．00）,缺氧条件下培养0．5h后人RPE细胞中Ang-2蛋白的质量浓度开始明显升高,6h达到高峰,与常氧对照组相比,差异均有统计学意义（P〈0．05）;缺氧培养24h后Ang-2蛋白的质量浓度接近常氧对照组。结论缺氧可显著上调体外培养的人RPE细胞中Ang-2的表达,Ang-2于缺氧早期呈高表达,提示Ang-2参与了CNV的形成,可能在CNV形成过程的早期阶段发挥作用。     

A novel serum-free method for culturing human prenatal retinal pigment epithelial cells

PURPOSE: Established techniques for culturing primary human retinal pigment epithelial (RPE) cells have facilitated the laboratory investigation of this multipurpose retinal cell layer. However, most culture methods involve the use of animal serum to establish and maintain RPE monolayers, which can complicate efforts to define and study factors involved in the maturation and function of these cells. Therefore, this study was conducted to develop a simple, serum-free system to propagate and sustain human RPE in vitro. METHODS: RPE was dissected from human prenatal donor eyes and cultured in serum-free defined medium containing the commercially formulated supplement B27 or N2. Cultures were grown initially as adherent tissue sections or suspended spherical aggregates and later expanded and maintained as monolayers. PCR, Western blot analysis, and immunocytochemistry were used to monitor gene and protein expression in established cultures, followed by examination of secretory products in RPE conditioned medium by ELISA and mass spectrometric analysis. RESULTS: In medium supplemented with B27, but not N2, RPE could be expanded up to 40,000-fold over six passages and maintained in culture for more than 1 year. In long-term cultures, typical cellular morphology and pigmentation were observed, along with expression of characteristic RPE markers. RPE monolayers also retained proper apical-basal orientation and secreted multiple factors implicated in the maintenance of photoreceptor health and the pathogenesis of age-related macular degeneration. CONCLUSIONS: Monolayer cultures of human prenatal RPE can be grown and maintained long term in the total absence of serum and still retain the phenotype, gene and protein expression profile, and secretory capacity exhibited by mature RPE cells.     

短发夹RNA对人视网膜色素上皮细胞血管内皮生长因子表达的抑制

目的探讨针对人血管内皮生长因子（VEGF）的短发夹RNA（shRNA）在体外对视网膜色素上皮（RUE）细胞VEGF表达的影响。方法用酶辅助显微分离法分离人RPE细胞,并用细胞角蛋白和S-100进行免疫组化鉴定。设计针对人VEGF的短发夹RNA（shRNAs,P1,P2）,P3为阴性对照即不含特异性shRNA,P1、P2退火后双链DNA连接到质粒pSilencer 4．1-CMV的BamHⅠ和Hjnd Ⅲ双酶切位点。转化细菌后,提取的质粒用EcoRⅠ和SamⅠ进行酶切鉴定。实验分5组,第1组：RPE细胞中在含有100μmol／LCoCl2,10％胎牛血清（FBS）的DMEM培养基中培养30h;第2组：在常规含有10％FBS的DMEM培养基中培养30h;第3、4、5组：分别P1、P2、P3转染细胞24h后在含有100μmoL／LCoCl2的10％FBS的DMEM培养基中培养30h;用免疫印迹法检测各组细胞VEGF表达水平。结果培养的细胞用细胞角蛋白和S-100免疫组化染色阳性。提取的质粒经酶切后片断相对分子质量分别为3300和1600,说明质粒成功地提取和纯化。VEGF表达水平1组明显高于2、3、4组（均P〈0．001）。乏氧（2组与1组比）可以明显增加RPE细胞中VEGF的表达。3、4组（P1和P2）对VEGF表达抑制分别为65．9％和52．4％。5组与1组比差异无统计学意义（P=0．147）。各组间β肌动蛋白表达量差异无统计学意义。结论针对人VEGF的特异性shRNA可以明显降低人RPE细胞VEGF的表达,为RNA干扰治疗新生血管性眼病,尤其是脉络膜新生血管奠定了基础。     

Modification of Isolation and Culture of Human Retinal Pigment Epithelial Cells

Purpose: To modify the isolation of human retinal pigment epithelial (RPE) cells and to increase the purification and production of cultured RPE cells.Methods: The human eyecups were fixed on a rubber holder. After digestion by trypsin, RPE cells were collected, then cultured and identified by morphology, im-munohistochemistry and electron microscopy.Results: The cultured RPE cells grew actively in the early stage with transparent nucleus and abundant melanin particles in cytoplasm. These cells were positive in DOPA oxidase reaction and in anti-pancytokeratin antibody staining. Cellular microvilli and tight junctions could be seen through transmission electron microscopy. Conclusion: We developed a rubber holder to fix the eyecup. Using this holder, more and purer cultured RPE cells can be obtained. These cultured RPE cells are similar to those in vivo in morphology and immunohistochemical staining. Eye Science 1999; 15: 187 - 190.     

无动物源性成分培养体系快速诱导人胚胎干细胞向视网膜色素上皮细胞的分化

背景 随着视网膜色素上皮(RPE)细胞视网膜下腔移植治疗年龄相关性黄斑变性(AMD)研究的开展,需要优化无动物源性成分(xeno-free)培养体系快速定向诱导人胚胎干细胞(hESCs)向RPE细胞分化以满足日渐增长的科研及临床需要. 目的 建立xeno-free培养体系,优化快速诱导hESCs向RPE细胞分化的方法. 方法 将hESCs克隆团接种至Vitronectin XFTM,在培养液中加入50 ng/ml noggin、10 ng/ml DKK-1以及10 ng/ml胰岛素样生长因子-1(IGF-1)培养2d,第2～4天将培养液中noggin质量浓度减至10 ng/ml,并加入5 ng/ml碱性成纤维细胞生长因子(bFGF),第4～6天移除培养液中的noggin和bFGF,第8～14天培养液中加入1 μmol/L CHIR99021.倒置显微镜下观察ESCs在分化为RPE细胞过程中的形态变化,免疫荧光染色检测细胞内特异性抗原的表达以对分化细胞进行鉴定,实时荧光定量PCR (qRT-PCR)法检测细胞分化过程中RPE细胞特异性蛋白mRNA的相对表达变化量.结果 分化培养第14天,部分细胞呈多角形并呈现铺路石样排列,且细胞内可见色素颗粒;培养第35天,诱导分化的细胞内表达RPE细胞特异性抗原Mitf及RPE65;培养至第60天,细胞内富含黑色素颗粒且呈规则六边形.与诱导分化前比较,诱导分化第7天和第14天hES-RPE细胞中Mitf mRNA的表达量分别增加了(3.43±2.77)倍和(8.91±2.83)倍,而RPE65 mRNA的表达量分别增加了(14.60±3.94)倍和(87.16±9.32)倍,分化第7天和第14天细胞中的Mitf mRNA和RPE65mRNA的相对表达量均明显高于分化前,差异均有统计学意义(均P＜0.05). 结论 hESCs可在含尼克酰胺、DKK-1、noggin、IGF-1和CHIR99021的xeno-free优化培养体系中快速分化为RPE细胞.     

VEGF-A regulates the expression of VEGF-C in human retinal pigment epithelial cells

AIM: To determine the expression and regulation of vascular endothelial growth factor C (VEGF-C), and its receptor VEGFR-3, in human retinal pigment epithelial (RPE) cells and to consider their angiogenic role in choroidal neovascularisation (CNV). METHOD: The expression of VEGF-C and VEGFR-3 in cultured human RPE was confirmed by immunostaining, PCR, western blotting, and ELISA. Cultured RPE cells were exposed to VEGF-A and glucose and VEGF-C and VEGFR-3 changes in gene expression determined by RT-PCR. Secreted VEGF-C protein in conditioned media from RPE was examined by western blotting and ELISA analysis. The ability of VEGF-C to elicit tube formation in choroidal endothelial cells was assayed by an in vitro Matrigel model. RESULT: VEGF-A and glucose upregulated VEGF-C mRNA expression and increased the secretion of VEGF-C protein into the culture medium. VEGF-A, but not glucose alone, stimulated VEGFR-3 mRNA expression. VEGF-C acted synergistically with VEGF-A to promote in vitro tube formation by choroidal endothelial cells. CONCLUSION: VEGF-A has a critical role in the orchestration of VEGF-C expression in RPE cells and the synergistic action of VEGF-C with VEGF-A may play an important part in the aetiology of CNV.     

Uptake, processing and release of retinoids by cultured human retinal pigment epithelium

Upon absorption of a photon, the 11-cis retinaldehyde chromophore of rhodopsin is isomerized and reduced to all-trans retinol (vitamin A) in the photoreceptor outer segments, whereupon it leaves the photoreceptors, and moves to the retinal pigment epithelium (RPE). To clarify the function of the RPE in the regeneration of 11-cis retinaldehyde, we delivered all-trans retinol to monolayer cultures of human RPE. During delivery the retinol was associated with its putative natural carrier, interphotoreceptor retinoid binding protein (IRBP). IRBP has been proposed as a carrier protein involved in the exchange of retinoids between the photoreceptors and the retinal pigment epithelium. The retinoid composition of RPE cells and culture medium was analyzed by HPLC following several incubation periods. The RPE monolayer was found to process all-trans retinol into two distinct end-products: all-trans retinyl palmitate, which remained within the RPE monolayer: and 11-cis retinaldehyde which was released into the culture medium. These results demonstrate retinoid isomerase, retinol oxidoreductase and retinyl ester synthetase activity in human RPE cells cultured under the appropriate conditions. They show that IRBP can serve as a carrier of retinol through an aqueous medium to the RPE, and they illustrate that the visual cycle can be studied in vitro.