Release of iron by human retinal pigment epithelial cells

Alpha-synuclein modulates retinal iron homeostasis by facilitating the uptake of transferrin-bound iron: Implications for visual manifestations of Parkinson's disease

Aggregation of α-synuclein (α-syn) in neurons of the substantia nigra is diagnostic of Parkinson's disease (PD), a neuro-motor disorder with prominent visual symptoms. Here, we demonstrate that α-syn, the principal protein involved in the pathogenesis of PD, is expressed widely in the neuroretina, and facilitates the uptake of transferrin-bound iron (Tf-Fe) by retinal pigment epithelial (RPE) cells that form the outer blood-retinal barrier. Absence of α-syn in knock-out mice (α-syn(-/-)) resulted in down-regulation of ferritin in the neuroretina, indicating depletion of cellular iron stores. A similar phenotype of iron deficiency was observed in the spleen, femur, and brain tissue of α-syn(-)(/-) mice, organs that utilize mainly Tf-Fe for their metabolic needs. The liver and kidney, organs that take up significant amounts of non-Tf-bound iron (NTBI), showed minimal change. Evaluation of the underlying mechanism in the human RPE47 cell line suggested a prominent role of α-syn in the uptake of Tf-Fe by modulating the endocytosis and recycling of transferrin (Tf)/transferrin-receptor (TfR) complex. Down-regulation of α-syn in RPE cells by RNAi resulted in the accumulation of Tf/TfR complex in common recycling endosomes (CREs), indicating disruption of recycling to the plasma membrane. Over-expression of exogenous α-syn in RPE cells, on the other hand, up-regulated ferritin and TfR expression. Interestingly, exposure to exogenous iron increased membrane association and co-localization of α-syn with TfR, supporting its role in iron uptake by the Tf/TfR complex. Together with our observations indicating basolateral expression of α-syn and TfR on RPE cells in vivo, this study reveals a novel function of α-syn in the uptake of Tf-Fe by the neuroretina. It is likely that retinal iron dyshomeostasis due to impaired or altered function of α-syn contributes to the visual symptoms associated with PD.

Cd-Cysteine Nanorods as a Fluorescence Sensor for Determination of Fe (III) in Real Samples

A new Cd-Cysteine complex nanorods (Cd-Cys NRs) were synthesized in one step at room temperature, and its morphology, structure and spectral properties were characterized by transmission electron microscopy (TEM), elemental analysis (EA), X-Ray diffraction (XRD), solid state and normal UV-Vis, Fourier transform infrared (FTIR) and spectrofluorometry. The developed Cd-Cys NRs were used as a fluorescence sensor for detection of Fe (III) in different aqueous matrices. The selectivity and sensitivity of the fabricated nano-sensor based on its fluorescence quenching in the presence of Fe (III) were probed according to the Stern-Volmer equation. The detection limit of the method was in micro-molar per liter range. Cd-Cys NRs response tested in different complex samples such as Rosemary plant leaves, exhibited a well-defined response. Anticoagulation measurements were performed to evaluate their blood biocompatibility.

Ferritin polarization and iron transport across monolayer epithelial barriers in mammals

Epithelial barriers are found in many tissues such as the intestine, kidney and brain where they separate the external environment from the body or a specific compartment from its periphery. Due to the tight junctions that connect epithelial barrier-cells (EBCs), the transport of compounds takes place nearly exclusively across the apical or basolateral membrane, the cell-body and the opposite membrane of the polarized EBC, and is regulated on numerous levels including barrier-specific adapted trafficking-machineries. Iron is an essential element but toxic at excess. Therefore, all iron-requiring organisms tightly regulate iron concentrations on systemic and cellular levels. In contrast to most cell types that control just their own iron homeostasis, EBCs also regulate homeostasis of the compartment they enclose or the body as a whole. Iron is transported across EBCs by specialized transporters such as the transferrin receptor and ferroportin. Recently, the iron storage protein ferritin was also attributed a role in the regulation of systemic iron homeostasis and we gathered evidence from the literature and original data that ferritin is polarized in EBC, suggesting also a role for ferritin in iron trafficking across EBCs.

Pretreatment with pyridoxamine mitigates isolevuglandin-associated retinal effects in mice exposed to bright light

The benefits of antioxidant therapy for treating age-related macular degeneration, a devastating retinal disease, are limited. Perhaps species other than reactive oxygen intermediates should be considered as therapeutic targets. These could be lipid peroxidation products, including isolevuglandins (isoLGs), prototypical and extraordinarily reactive γ-ketoaldehydes that avidly bind to proteins, phospholipids, and DNA and modulate the properties of these biomolecules. We found isoLG adducts in aged human retina but not in the retina of mice kept under dim lighting. Hence, to test whether scavenging of isoLGs could complement or supplant antioxidant therapy, we exposed mice to bright light and found that this insult leads to retinal isoLG-adduct formation. We then pretreated mice with pyridoxamine, a B6 vitamer and efficient scavenger of γ-ketoaldehydes, and found that the levels of retinal isoLG adducts are decreased, and morphological changes in photoreceptor mitochondria are not as pronounced as in untreated animals. Our study demonstrates that preventing the damage to biomolecules by lipid peroxidation products, a novel concept in vision research, is a viable strategy to combat oxidative stress in the retina.

Retinal iron homeostasis in health and disease

Iron is essential for life, but excess iron can be toxic. As a potent free radical creator, iron generates hydroxyl radicals leading to significant oxidative stress. Since iron is not excreted from the body, it accumulates with age in tissues, including the retina, predisposing to age-related oxidative insult. Both hereditary and acquired retinal diseases are associated with increased iron levels. For example, retinal degenerations have been found in hereditary iron overload disorders, like aceruloplasminemia, Friedreich's ataxia, and pantothenate kinase-associated neurodegeneration. Similarly, mice with targeted mutation of the iron exporter ceruloplasmin and its homolog hephaestin showed age-related retinal iron accumulation and retinal degeneration with features resembling human age-related macular degeneration (AMD). Post mortem AMD eyes have increased levels of iron in retina compared to age-matched healthy donors. Iron accumulation in AMD is likely to result, in part, from inflammation, hypoxia, and oxidative stress, all of which can cause iron dysregulation. Fortunately, it has been demonstrated by in vitro and in vivo studies that iron in the retinal pigment epithelium (RPE) and retina is chelatable. Iron chelation protects photoreceptors and retinal pigment epithelial cells (RPE) in a variety of mouse models. This has therapeutic potential for diminishing iron-induced oxidative damage to prevent or treat AMD.

How does the macula protect itself from oxidative stress?

Oxidative stress has been hypothesized to contribute to the development of age-related macular degeneration (AMD), the most common cause of blindness in the United States. At present, there is no treatment for early disease. Reactive oxygen species (ROS) play a physiological role in the retinal pigment epithelium (RPE), a key cell type in this disease, but with excessive ROS, oxidative damage or excessive innate immune system activation can result. The RPE has developed a robust antioxidant system driven by the transcription factor Nrf2. Impaired Nrf2 signaling can lead to oxidative damage or activate the innate immune response, both of which can lead to RPE apoptosis, a defining change in AMD. Several mouse models simulating environmental stressors or targeting specific antioxidant enzymes such as superoxide dismutase or Nrf2, have simulated some of the features of AMD. While ROS are short-lived, oxidatively damaged molecules termed oxidation specific epitopes (OSEs), can be long-lived and a source of chronic stress that activates the innate immune system through pattern recognition receptors (PRRs). The macula accumulates a number of OSEs including carboxyethylpyrrole, malondialdehyde, 4-hydroxynonenal, and advanced glycation endproducts, as well as their respective neutralizing PRRs. Excessive accumulation of OSEs results in pathologic immune activation. For example, mice immunized with the carboxyethylpyrrole develop cardinal features of AMD. Regulating ROS in the RPE by modulating antioxidant systems or neutralizing OSEs through an appropriate innate immune response are potential modalities to treat or prevent early AMD.

Ferroxidase hephaestin's cell-autonomous role in the retinal pigment epithelium

Hephaestin (Heph) is a ferroxidase protein that converts ferrous to ferric iron to facilitate cellular iron export by ferroportin. Many tissues express either Heph or its homologue, ceruloplasmin (Cp), but the retina expresses both. In mice, a combined systemic mutation of Heph and systemic knockout of Cp (Cp(-/-), Heph(sla/sla)) causes retinal iron accumulation and retinal degeneration, with features of human age-related macular degeneration; however, the role of Heph and Cp in the individual retinal cells is unclear. Herein, we used conditional knockout mice to study Heph's role in retinal pigment epithelial (RPE) and photoreceptor cells. Loss of both Heph and Cp from RPE cells alone results in RPE cell iron accumulation and degeneration. We found, however, that RPE iron accumulation in these conditional knockout mice is not as great as in systemic knockout mice. Photoreceptor-specific Heph knockout indicates that the additional iron in the RPE cells does not result from loss of ferroxidases in the photoreceptors, and Cp and Heph play minor roles in photoreceptors. Instead, loss of ferroxidases in other retinal cells causes retinal iron accumulation and transfer of iron to the RPE cells. Cp and Heph are necessary for iron export from the retina but are not essential for iron import into the retina. Thus, our studies, revise how we think about iron import and export from the retina.

Iron overload in diabetic retinopathy: a cause or a consequence of impaired mechanisms?

Iron is an essential ion for life, playing a central role in many metabolic processes. The most important property of free iron is its capacity to be reversibly oxidized and reduced, but at same time this make it highly pro-oxidant molecule. In this regard, iron is able to generate powerful reactive oxygen species (ROS). For this reason, careful control on iron availability is central to the maintenance of normal cell function in the retina. In the diabetic eye there is an impairment of iron homeostasis, thus leading to iron overload. The mechanisms involved in this process include: (1) Destruction of heme molecules induced by hyperglycemia (2) Intraretinal and vitreal hemorrhages (3) Overexpression of the renin-angiotensin system. The main consequences of iron overload are the following: (1) Retinal neurodegeneration due to the increase of oxidative stress (2) Increase of AGE-RAGE binding (3) Defective phagocytosis of retinal pigment epithelium, which generates the accumulation of autoantigens and the synthesis of proinflammatory cytokines. Further studies addressed to explore not only the role of iron in the pathogenesis of diabetic retinopathy, but also to design novel therapeutic strategies based on the regulation of iron homeostasis are needed.

Iron, the retina and the lens: a focused review

This review is focused on iron metabolism in the retina and in the lens and its relation to their respective age-related pathologies, macular degeneration (AMD) and cataract (ARC). Several aspects of iron homeostasis are considered first in the retina and second in the lens, paying particular attention to the transport of iron through the blood-retinal barrier and through the lens epithelial cell barrier, to the immunochemistry of iron-related proteins and their expression in both the retina and the lens, and to the nature of the photochemical damage caused by UV light on both tissues. A comparative overview of some iron related parameters (total iron, transferrin (Tf), transferrin saturation and total iron binding capacity), in plasma and ocular tissues and fluids of three animal species is also presented. Based on results selected from the literature reviewed, and our own results, a scheme for the overall circulation of iron within and out of the eye is proposed, in which, (i) iron is pumped from the retina to the vitreous body by a ferroportin/ferroxidase-mediated process at the endfeet of Müller cells, (ii) vitreal Tf binds this iron and the complex diffuses towards the lens, (iii) the iron/Tf complex is incorporated into the lens extracellular space probably at the lens equator and moves to the epithelial-fiber interface, (iv) upon interaction with Tf receptors of the apical pole of lens epithelial cells, the iron/Tf complex is endocytosed and iron is exported as Fe(3+) by a ferroportin/ferroxidase-mediated process taking place at the basal pole of the epithelial cells, and (v) Fe(3+) is bound to aqueous humor Tf and drained with the aqueous humor into systemic blood circulation for recycling. The proposed scheme represents an example of close cooperation between the retina and the lens to maintain a constant flow of iron within the eye that provides an adequate supply of iron to ocular tissues and secures the systemic recycling of this element. It does not discount the existence of additional ways for iron to leave the eye through the blood-retinal barrier. In this review both AMD and ARC are recognized as multifactorial diseases with an important photoxidative component, and exhibiting a remarkable similitude of altered local iron metabolism. The epidemiological relationship between ARC and ferropenic anemia is explained on the basis that hepcidin, the hormone responsible for the anemia of chronic inflammation, could paradoxically cause intracellular iron overload in the lens by interfering with the proposed ferroportin/ferroxidase-mediated export of iron at the basal side of the anterior lens epithelium. Other authors have suggested that a similar situation is created in the retina in the case of AMD.

Iron metabolism in the eye: a review

This review article covers all aspects of iron metabolism, which include studies of iron levels within the eye and the processes used to maintain normal levels of iron in ocular tissues. In addition, the involvement of iron in ocular pathology is explored. In each section there is a short introduction to a specific metabolic process responsible for iron homeostasis, which for the most part has been studied in non-ocular tissues. This is followed by a summary of our current knowledge of the process in ocular tissues.

Iron homeostasis and toxicity in retinal degeneration

Iron is essential for many metabolic processes but can also cause damage. As a potent generator of hydroxyl radical, the most reactive of the free radicals, iron can cause considerable oxidative stress. Since iron is absorbed through diet but not excreted except through menstruation, total body iron levels buildup with age. Macular iron levels increase with age, in both men and women. This iron has the potential to contribute to retinal degeneration. Here we present an overview of the evidence suggesting that iron may contribute to retinal degenerations. Intraocular iron foreign bodies cause retinal degeneration. Retinal iron buildup resulting from hereditary iron homeostasis disorders aceruloplasminemia, Friedreich's ataxia, and panthothenate kinase-associated neurodegeneration cause retinal degeneration. Mice with targeted mutation of the iron exporter ceruloplasmin have age-dependent retinal iron overload and a resulting retinal degeneration with features of age-related macular degeneration (AMD). Post mortem retinas from patients with AMD have more iron and the iron carrier transferrin than age-matched controls. Over the past 10 years much has been learned about the intricate network of proteins involved in iron handling. Many of these, including transferrin, transferrin receptor, divalent metal transporter-1, ferritin, ferroportin, ceruloplasmin, hephaestin, iron-regulatory protein, and histocompatibility leukocyte antigen class I-like protein involved in iron homeostasis (HFE) have been found in the retina. Some of these proteins have been found in the cornea and lens as well. Levels of the iron carrier transferrin are high in the aqueous and vitreous humors. The functions of these proteins in other tissues, combined with studies on cultured ocular tissues, genetically engineered mice, and eye exams on patients with hereditary iron diseases provide clues regarding their ocular functions. Iron may play a role in a broad range of ocular diseases, including glaucoma, cataract, AMD, and conditions causing intraocular hemorrhage. While iron deficiency must be prevented, the therapeutic potential of limiting iron-induced ocular oxidative damage is high. Systemic, local, or topical iron chelation with an expanding repertoire of drugs has clinical potential.

N-tert-butyl hydroxylamine, a mitochondrial antioxidant, protects human retinal pigment epithelial cells from iron overload: relevance to macular degeneration

Age-related macular degeneration (AMD) is the leading cause of severe visual impairment in the elderly in developed countries. AMD patients have elevated levels of iron within the retinal pigment epithelia (RPE), which may lead to oxidative damage to mitochondria, disruption of retinal metabolism, and vision impairment or loss. As a possible model for iron-induced AMD, we investigated the effects of excess iron in cultured human fetal RPE cells on oxidant levels and mitochondrial cytochrome c oxidase (complex IV) function and tested for protection by N-tert-butyl hydroxylamine (NtBHA), a known mitochondrial antioxidant. RPE exposure to ferric ammonium citrate resulted in a time- and dose-dependent increase in intracellular iron, which increased oxidant production and decreased glutathione (GSH) levels and mitochondrial complex IV activity. NtBHA addition to iron-overloaded RPE cells led to a reduction of intracellular iron content, oxidative stress, and partial restoration of complex IV activity and GSH content. NtBHA might be useful in AMD due to its potential to reduce oxidative stress, mitochondrial damage, and age-related iron accumulation, which may damage normal RPE function and lead to loss of vision.

Disruption of ceruloplasmin and hephaestin in mice causes retinal iron overload and retinal degeneration with features of age-related macular degeneration

Mechanisms of brain and retinal iron homeostasis have become subjects of increased interest after the discovery of elevated iron levels in brains of patients with Alzheimer's disease and retinas of patients with age-related macular degeneration. To determine whether the ferroxidase ceruloplasmin (Cp) and its homolog hephaestin (Heph) are important for retinal iron homeostasis, we studied retinas from mice deficient in Cp and/or Heph. In normal mice, Cp and Heph localize to Müller glia and retinal pigment epithelium, a blood-brain barrier. Mice deficient in both Cp and Heph, but not each individually, had a striking, age-dependent increase in retinal pigment epithelium and retinal iron. The iron storage protein ferritin was also increased in Cp-/-Heph-/Y retinas. After retinal iron levels had increased, Cp-/-Heph-/Y mice had age-dependent retinal pigment epithelium hypertrophy, hyperplasia and death, photoreceptor degeneration, and subretinal neovascularization, providing a model of some features of the human retinal diseases aceruloplasminemia and age-related macular degeneration. This pathology indicates that Cp and Heph are critical for CNS iron homeostasis and that loss of Cp and Heph in the mouse leads to age-dependent retinal neurodegeneration, providing a model that can be used to test the therapeutic efficacy of iron chelators and antiangiogenic agents.

Membranes of retinal pigment epithelial cells in vitro are damaged in the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions

The ferrous ions released from haemoglobin and storage-transferrin ions cause oxidative stress in the eyes. We observed the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions in the retinal pigment epithelial (RPE) cells in vitro, and investigated how the ferrous ions influenced RPE in vitro and the photoreceptor outer segment discs. We obtained isolated photoreceptor outer segment discs using sucrose gradient of specific gravity after homogenizing porcine retinas. After bovine RPE cells were cultured with isolated photoreceptor outer segment discs containing FeCl2 for 5 and 24 h, we incubated the specimens with rhodamine phalloidin, antimouse alpha-tubulin antibody and antimouse Ig G (FITC and rhodamine labelled). We observed the specimens by a laser scanning microscopy, and made the ultrathin sections with or without 2% uranyl acetate and 2% lead acetate for examination by transmission electron microscopy. Actin filaments and microtubules of specialized cells such as RPE cells were actively involved in phagocytosis of the photoreceptor outer segment discs. Microtubules were damaged during the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions. The peroxidation increased the granular and aggregated autofluorescence of the photoreceptor outer segment discs. The membranes of the disc and the phagosomes, and lysosomes in RPE cells were damaged by ferrous ions and had fine particles with high electron density staining without uranium acetate and lead citrate. The cytoskeletons such as actin filaments and microtubules, and the membranes of the phagosomes and the lysosomes in RPE cells in vitro were damaged during the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions.

Morphogenesis of the retinal pigment epithelium: toward understanding retinal degenerative diseases

The phenotype of an epithelial cell is defined by a unique combination of morphology, gene and protein expression, and protein localization. Results indicate that the terminal differentiation of the RPE cell can be described in part by changes in the polarity of its surface proteins alpha v beta 5 integrin, Na,K-ATPase, N-CAM, and EMMPRIN. Changes in protein/gene expression and protein localization in late stages of RPE development identify alpha v beta 5 integrin as a key player in RPE phagocytosis, and N-CAM and EMMPRIN as potentially important molecules in other RPE functions necessary for photoreceptor survival. By studying the trafficking of the later two proteins it is shown that entry into an apical or basolateral pathway in RPE cells cannot be predicted by the distribution of a given protein in other epithelial cells, and that this distribution may change through the course of RPE development. The mechanisms used by RPE and other epithelia to establish and maintain their specific polarity properties are fundamental to the formation and maintenance of their specific epithelial phenotype. The ability to therapeutically direct molecules incorporated into RPE by gene therapy into apical or basal surfaces requires an understanding of protein localization and expression. Furthermore, evidence is provided that assays capitalizing on changes in gene/protein expression and protein localization during the late stages of RPE development can prove a productive way of identifying proteins used by RPE for photoreceptor support. This approach can continue to be exploited to identify other proteins essential for the mission of the RPE cell, that may thus be likely candidates for participation in retinal degenerative disease.

Expression of the protective proteins hemopexin and haptoglobin by cells of the neural retina

The blood-retinal barrier, consisting of retinal pigment epithelial cells and retinal endothelial cells, prevents hemopexin and haptoglobin, anti-oxidant protective plasma proteins normally synthesized by the liver, from entering the neural retina. If present, these proteins must, therefore, be made locally. The cell types within the retina in which hemopexin and haptoglobin mRNAs are made have been investigated. RNA was extracted from both the neural retina and pigment epithelium obtained by dissection of human donor eyes as well as from cultured pigment epithelial and photoreceptor cells. The mRNAs for both haptoglobin and hemopexin were detected, using reverse-transcriptase polymerase chain reaction, in the neural retina and cultured photoreceptors but not in pigment epithelial cells. The cellular location of these mRNAs was determined using in situ hybridization of sections of human retina which revealed that haptoglobin mRNA was located principally in the photoreceptor cells, cells of the inner nuclear layer and some cells of the ganglion cell layer. Hemopexin mRNA, previously shown to be made in the human neural retina (Hunt et al., 1996. Journal of Cellular Physiology 168: 71-80), is expressed by most of the cells of neural retina including the photoreceptors and, notably, the ganglion cells.

Heme-mediated reactive oxygen species toxicity to retinal pigment epithelial cells is reduced by hemopexin

Catalysis of the formation of reactive oxygen species (RO2S) by low molecular weight complexes of iron has been implicated in several pathological conditions in the retina since photoreceptors and retinal pigment epithelial cells are likely to be especially sensitive to RO2S. Since protective proteins cannot cross the blood-retinal barrier, it is likely that the retina performs its own protective functions by synthesizing proteins that bind iron and nonprotein iron complexes, the major catalysts of RO2S generation. Investigations were carried out to determine whether pigment epithelial cells are themselves sensitive to iron-generated RO2S and whether apo-transferrin and apo-hemopexin, known to be made locally in the retina, can perform a protective function. In 51Cr release assays, the toxicity of exogenous RO2S including hydrogen peroxide or superoxide (generated by xanthine oxidase/hypoxanthine) to human retinal pigment epithelial cells was inhibited by the iron chelators, desferrioxamine and apo-transferrin. Free but not protein-bound ferric iron and heme exacerbated the toxic effect. The toxic effect of heme was abolished by the heme-scavenging, extracellular antioxidant, apo-hemopexin, and also by exogenous bovine serum albumin. In addition, heme toxicity was inhibited by a 3 h preincubation of cells with either heme, apo-hemopexin, or heme-hemopexin 24 h prior to the toxicity assay. It is concluded, first, that toxic effects of iron and heme can be prevented by apo-transferrin or apo-hemopexin and, second, that exposure of RPE cells to free heme or hemopexin sets in motion a series of biochemical events resulting in protection against oxidative stress. It is probable that these include heme oxygenase induction.

Hemopexin in the human retina: protection of the retina against heme-mediated toxicity

The existence of the blood-retinal barrier means that proteins that protect the retina from damage by reactive oxygen species must either be made locally or specifically transported across the barrier cells; however, such transepithelial transport does not seem to occur. Among the circulatory proteins that protect against iron-catalyzed production of free radicals are apo-transferrin, which binds ferric iron and has previously been shown to be made by cells of the neural retina (Davis and Hunt, 1993, J. Cell Physiol., 156:280-285), and the extracellular antioxidant, apo-hemopexin, which binds free heme (iron-protoporphyrin IX). Since hemorrhage and heme release can be important contributing factors in retinal disease, evidence of a hemopexin-based retinal protection system was sought. The human retina has been shown to contain apo-hemopexin which is probably synthesized locally since its mRNA can be detected in retinal tissue dissected from human donor eyes. It is likely that the retina contains a mechanism for the degradation of hemopexin-bound heme since the blood-retinal barrier also precludes the exit of heme-hemopexin from the retina. Retinal pigment epithelial cells have been found to bind and internalize heme-hemopexin in a temperature-dependent, saturable, and specific manner, analogous to the receptor-mediated endocytic system of hepatoma cells. Moreover, the binding of heme-hemopexin to the cells stimulates the expression of heme oxygenase-1, metallothionein-1, and ferritin.

Transferrin is made and bound by photoreceptor cells

Retinal pigment epithelial cells, which form one aspect of the blood-retinal barrier, control the access of blood-borne components such as diferric transferrin to the neural retina. It has recently been shown that RPE cells remove iron from diferric transferrin in a low pH compartment and subsequently release it in a low molecular weight form that can be chelated by apo-transferrin (Hunt and Davis: J. Cell Physiol. 152:102-110, 1992). It is now shown that photoreceptor cells can bind diferric transferrin to receptors on their inner segments. Moreover, polymerase chain reaction and in situ hybridization show that cells of the neural retina, particularly photoreceptors, make apo-transferrin.