We have identified from mouse the first mammalian ␤-carotene 15,15-dioxygenase (␤-CD), a crucial enzyme in development and metabolism that governs the de novo entry of vitamin A from plant-derived precursors. ␤-CD is related to the retinal pigment epithelium-expressed protein RPE65 and belongs to a diverse family that includes the plant 9-cis-epoxycarotenoid dioxygenase and bacterial lignostilbene dioxygenases. ␤-CD expression in Escherichia coli cells engineered to produce ␤-carotene led to the accumulation of all-trans-retinal at the expense of ␤-carotene, confirming that ␤-CD catalyzed the central cleavage of this vitamin A precursor. Purified recombinant ␤-CD protein cleaves ␤-carotene in vitro with a V max of 36 pmol of retinal/mg of enzyme/ min and a K m of 6 M. Non-provitamin A carotenoids were also cleaved, although with much lower activity. By Northern analysis, a 2.4-kilobase (kb) message was observed in liver, kidney, small intestine, and testis, tissues important in retinoid/carotenoid metabolism. This message encoded a 63-kDa cytosolic protein expressed in these tissues. A shorter transcript of 1.8 kb was found in testis and skin. Developmentally, the 2.4-kb mRNA was abundant at embryonic day 7, with lower expression at embryonic days 11, 13, and 15, suggesting a critical role for this enzyme in gastrulation. Identification of ␤-CD in an accessible model organism will create new opportunities to study vitamin A metabolism.In vertebrates, vitamin A in its various oxidative and isomeric forms is essential for embryonic development (1), pattern formation (2, 3), and vision (4). Retinoic acid, through its interaction with the nuclear retinoic acid receptor and retinoid X receptor, profoundly affects cell differentiation and development. Because animals are unable to synthesize vitamin A de novo from endogenous isoprenoid precursors, they must instead derive it from cleavage of ␤-carotene and certain other carotenoids with an unsubstituted ␤-ring (e.g. ␥-and ␣-carotenes, ␤-zeacarotene, and ␤-cryptoxanthin). It is generally accepted that central cleavage of ␤-carotene by a putative dioxygenase gives rise to two molecules of all-trans-retinal, whereas eccentric cleavage with subsequent processing leading to a single molecule of retinoic acid from an apocarotenal is quantitatively far less important (5). ␤-Carotene cleavage activity is reported highest in the intestinal mucosa, but is found at high activity levels in liver, kidney, lung, and fat tissues, among other sites. However, an inability to purify the protein catalyzing this reaction has hindered thorough investigation of this crucial first step in vitamin A metabolism.Because of a loose similarity between the mammalian protein RPE65 and the neoxanthin cleavage enzymes of plants, our laboratories have considered the hypothesis that the putative ␤-CD 1 would belong to an emerging family of carotenoidcleaving dioxygenases known mainly from examples in plants (6), but with members also in bacteria and Metazoa. The first described representative was a ba...

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After visual-pigment bleaching, single isolated rod photoreceptors of Ambystoma tigrinum recover their sensitivity to light when supplied with 11-cis-retinal from liposomes or with li-cis-retinal bound to interphotoreceptor retinoid-binding protein. Bleached rods do not recover sensitivity, or do so only very slowly, after exposure to 11-cis-retinol. The latter retinoid is "toxic" in that rods actually lose sensitivity in its presence. In contrast, bleached isolated cone cells recover sensitivity when either retinoid is supplied. It is suggested that the major pathway for rhodopsin regeneration during dark adaptation in the intact eye is transport of ll-cis-retinal from the pigment epithelium to the retina. The results also suggest that there may be separate pathways for visual-pigment regeneration in rods and cones during dark adaptation.Regeneration of visual pigment during dark adaptation or during maintained illumination requires retinoid isomerization from trans to cis form and conversion from alcohol to aldehyde before retinoid can be bound to opsin to reconstitute active pigment (1). The pigment epithelium (PE) has long been known to be involved in regeneration (2), implying a "visual cycle" that involves shuttling of retinoid from retina to PE and back again during cycles of light and dark. During light adaptation, there is indeed a progressive loss of retinoid from the retina, and an increase in the retinoid content of the PE. During darkness, the retinoid flow is reversed (3). Fulton and Rando (4) have presented strong evidence for localization of the retinoid-isomerizing system in PE rather than retina, but it remains uncertain which cells are involved in effecting the alcohol-to-aldehyde change that must take place during the visual cycle.We now report a difference in the use of retinol and retinal by rod and cone cells, and a "toxic" effect of retinol on rod cell function. We suggest that 11-cis-retinal is the major form of retinoid transported from PE to retina. Alternatively, ifthe alcohol form is transported it must be converted to the aldehyde in a cell type other than the rod photoreceptor (which may be cones, or possibly Muller cells).Pepperberg et al. (5) showed that the ordinarily permanent desensitization due to bleaching in isolated retina could be reversed by treatment with 11-cis-retinal. In their work, retinal was applied in ethanolic solution. Subsequently, it was shown (6) that liposomes could also be used to deliver retinoid. Our studies on isolated photoreceptors have employed both liposomes and interphotoreceptor retinoidbinding protein (IRBP). IRBP is a protein found at high concentration in the interphotoreceptor matrix, where its unique location and retinoid-binding properties (7,8) make it likely that it is involved in retinoid movement between retina and PE. In support of this, we demonstrate here the transfer of retinoid between IRBP and photoreceptor cells. MATERIALS AND METHODSIsolated photoreceptor cells from dark-adapted, larval tiger salamanders (Ambystoma tigrin...

Vascular endothelial growth factor (VEGF) is a major agent in choroidal and retinal neovascularization, events associated with age-related macular degeneration (AMD) and diabetic retinopathy. Retinal pigment epithelium (RPE), strategically located between retina and choroid, plays a critical role in retinal disorders. We have examined the effects of various growth factors on the expression and secretion of VEGF by human retinal pigment epithelial cell cultures (HRPE). RT-PCR analyses revealed the presence of three isoforms of mRNA corresponding to VEGF 121, 165, and 189 that were up regulated by TGF-beta1. TGF-beta1, beta2, and beta3 were the potent inducers of VEGF secretion by HRPE cells whereas bFGF, PDGF, TGF-alpha, and GM-CSF had no effects. TGF-beta receptor type II antibody significantly reversed induction of VEGF secretion by TGF-beta. In contrast activin, inhibin and BMP, members of TGF-beta super family, had no effects on VEGF expression in HRPE. VEGF mRNA levels and protein secretion induced by TGF-beta were significantly inhibited by SB203580 and U0126, inhibitors of MAP kinases, but not by staurosporine and PDTC, protein kinase C and NF-kappaB pathway inhibitors, respectively. TGF-beta also induced VEGF expression by fibroblasts derived from human choroid of eye. TGF-beta induction of VEGF secretion by RPE and choroid cells may play a significant role in choroidal neovascularization (CNV) in AMD. Since the secretion of VEGF by HRPE is regulated by MAP kinase pathways, MAP kinase inhibitors may have potential use as therapeutic agents for CNV in AMD.

Pathogenic effector T cells in experimental autoimmune uveitis (EAU) are T helper type 1–like, and interleukin (IL)-12 is required for their generation and function. Therefore, we expected that IL-12 administration would have disease-enhancing effects. Mice were immunized with a uveitogenic regimen of the retinal antigen interphotoreceptor retinoid-binding protein, treated with IL-12 (100 ng/d for 5 d), and EAU was assessed by histopathology. Unexpectedly, IL-12 treatment failed to enhance EAU in resistant strains and downregulated disease in susceptible strains. Only treatment during the first, but not during the second, week after immunization was consistently protective. High levels of interferon γ (IFN-γ) were present in the serum during IL-12 treatment, but subsequent antigen-specific IFN-γ production in protected mice was diminished, as were IL-5 production, lymph node cell proliferation, and serum antibody levels. Treated mice had fewer cells and evidence of enhanced apoptosis in the draining lymph nodes. Unlike wild-type mice, IFN-γ–deficient, inducible nitric oxide synthase (iNOS)-deficient, and Bcl-2lck transgenic mice were poorly protected by IL-12, whereas IL-10–deficient mice were protected. We conclude that administration of IL-12 aborts disease by curtailing development of uveitogenic effector T cells. The data are compatible with the interpretation that IL-12 induces systemic hyperinduction of IFN-γ, causing activation of iNOS and production of NO, which mediates protection at least in part by triggering Bcl-2 regulated apoptotic deletion of the antigen-specific T cells as they are being primed.

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Previous studies demonstrated the specific association of heme oxygenase (HO)‐1 protein to the neurofibrillary pathology of Alzheimer's disease (AD). In this study, we used reverse transcription‐polymerase chain reaction methods to show the increased expression of HO‐1 but not HO‐2 mRNA transcripts in cerebral cortex and cerebral vessels from subjects with AD compared with age‐matched non‐AD controls. Neither the HO‐1 nor the HO‐2 mRNA level was altered in the cerebellum, a brain region usually spared from the pathological alterations of AD. There was no clear evidence that the expression of HO‐1 in these tissues was related to postmortem interval, cause of death, or the age of the subjects studied. Using immunoblotting methods, we further showed that HO‐1 protein content was increased in neocortical and vascular samples from AD subjects compared with controls. Our findings suggest the specific induction of HO‐1 mRNA and protein in the cerebral cortex and cerebral vessels but not HO‐2 mRNA or protein in association with the pathological lesions of the disease.

Immunologically privileged retinal antigens can serve as targets of experimental autoimmune uveitis (EAU), a model for human uveitis. The tolerance status of susceptible strains, whose target antigen is not expressed in the thymus at detectable levels, is unclear. Here, we address this issue directly by analyzing the consequences of genetic deficiency versus sufficiency of a uveitogenic retinal antigen, interphotoreceptor retinoid-binding protein (IRBP). IRBP-knockout (KO) and wild-type (WT) mice on a highly EAU-susceptible background were challenged with IRBP. The KO mice had greatly elevated responses to IRBP, an altered recognition of IRBP epitopes, and their primed T cells induced exacerbated disease in WT recipients. Ultrasensitive immunohistochemical staining visualized sparse IRBP-positive cells, undetectable by conventional assays, in thymi of WT (but not of KO) mice. IRBP message was PCR amplified from these cells after microdissection. Thymus transplantation between KO and WT hosts demonstrated that this level of expression is functionally relevant and sets the threshold of immune (and autoimmune) reactivity. Namely, KO recipients of WT thymi generated reduced IRBP-specific responses, and WT recipients of KO thymi developed enhanced responses and a highly exacerbated disease. Repertoire culling and thymus-dependent CD25+ T cells were implicated in this effect. Thus, uveitis-susceptible individuals display a detectable and functionally significant tolerance to their target antigen, in which central mechanisms play a prominent role.