Retinal and nonocular abnormalities in Cyp27a1(-/-)Cyp46a1(-/-) mice with dysfunctional metabolism of cholesterol.

Cholesterol elimination from nonhepatic cells involves metabolism to side-chain oxysterols, which serve as transport forms of cholesterol and bioactive molecules modulating a variety of cellular processes. Cholesterol metabolism is tissue specific, and its significance has not yet been established for the retina, where cytochromes P450 (CYP27A1 and CYP46A1) are the major cholesterol-metabolizing enzymes. We generated Cyp27a1(-/-)Cyp46a1(-/-) mice, which were lean and had normal serum cholesterol and glucose levels. These animals, however, had changes in the retinal vasculature, retina, and several nonocular organs (lungs, liver, and spleen). Changes in the retinal vasculature included structural abnormalities (retinal-choroidal anastomoses, arteriovenous shunts, increased permeability, dilation, nonperfusion, and capillary degeneration) and cholesterol deposition and oxidation in the vascular wall, which also exhibited increased adhesion of leukocytes and activation of the complement pathway. Changes in the retina included increased content of cholesterol and its metabolite, cholestanol, which were focally deposited at the apical and basal sides of the retinal pigment epithelium. Retinal macrophages of Cyp27a1(-/-)Cyp46a1(-/-) mice were activated, and oxidative stress was noted in their photoreceptor inner segments. Our findings demonstrate the importance of retinal cholesterol metabolism for maintenance of the normal retina, and suggest new targets for diseases affecting the retinal vasculature.

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.

Posttranslational modification by an isolevuglandin diminishes activity of the mitochondrial cytochrome P450 27A1.

Posttranslational modification by isolevuglandins (isoLGs), arachidonate oxidation products, is an important yet understudied process associated with altered protein properties. This type of modification is detected in cytochrome P450 27A1 (CYP27A1), a multifunction enzyme expressed in almost every cell and involved in the metabolism of cholesterol and other sterols. Previously, the CYP27A1 Lys(358)-isoLG adduct was found in human retina afflicted with age-related macular degeneration. Yet, the effect of Lys(358) modification on enzyme activity was not investigated. Herein, we characterized catalytic properties of Lys(358) as well as Lys(476) CYP27A1 mutants before and after isoLG treatment and quantified the extent of modification by multiple reaction monitoring. The K358R mutant was less susceptible to isoLG-induced loss of catalytic activity than the wild type (WT), whereas the K476R mutant was nearly as vulnerable as the WT. Both mutants showed less isoLG modification than WT. Thus, modification of Lys(358), a residue involved in redox partner interactions, is the major contributor to isoLG-associated loss of CYP27A1 activity. Our data show the specificity of isoLG modification, provide direct evidence that isoLG adduction impairs enzyme activity, and support our hypothesis that isoLG modification in the retina is detrimental to CYP27A1 enzyme activity, potentially disrupting cholesterol homeostasis.

High-throughput fluorescence polarization assay to identify inhibitors of Cbl(TKB)-protein tyrosine kinase interactions.

The casitas B-lineage lymphoma (Cbl) proteins play an important role in regulating signal transduction pathways by functioning as E3 ubiquitin ligases. The Cbl proteins contain a conserved tyrosine kinase binding (TKB) domain that binds more than a dozen proteins, including protein tyrosine kinases (PTKs), in a phosphorylation-dependent manner. The cell surface expression levels of the PTKs are regulated by Cbl-mediated ubiquitination, internalization, and degradation. Dysfunction in this signaling cascade has resulted in prolonged activation of the PTKs and, therefore, has been implicated in inflammatory diseases and various cancers. Due to this negative regulatory function, Cbl has been largely ignored as a therapeutic target. However, recent studies, such as the identification of (i) gain of function c-Cbl mutations in subsets of myeloid cancer and (ii) c-Cbl as a prostate basal cell marker that correlates with poor clinical outcome, suggest otherwise. Here we report the development of a competitive high-throughput fluorescence polarization assay in a 384-well format to identify inhibitors of Cbl(TKB). The high-throughput screen readiness of the assay was demonstrated by screening the Prestwick Chemical Library.

Mass spectrometry detection of isolevuglandin adduction to specific protein residues.

The aging process seems to be associated with oxidative stress and hence increased production of lipid peroxidation products, including isolevuglandins (isoLGs). The latter are highly reactive γ-ketoaldehydes which can form covalent adducts with primary amino groups of enzymes and proteins and alter the properties of these biomolecules. Yet little is currently known about amino acid-containing compounds affected by isoLG modification in different age-related pathological processes. To facilitate the detection of these biomolecules, we developed a strategy in which the purified enzyme (or protein) of interest is first treated with authentic isoLG in vitro to evaluate whether it contains reactive lysine residues prone to modification with isoLGs. The data obtained serve as a basis for making the "GO/NO GO" decision as to whether to pursue a further search of this isoLG modification in a biological sample. In this chapter, we describe the conditions for the in vitro isoLG modification assay and how to use mass spectrometry to identify the isoLG-modified peptides and amino acid residues. Our studies were carried out on cytochrome P450 27A1, an important metabolic enzyme, and utilized iso[4]levuglandin E2 as a prototypical isoLG. The isoLG-treated cytochrome P450 was subjected to proteolysis followed by liquid chromatography-tandem mass spectrometry for peptide separation and analysis by Mascot, a proteomics search engine, for the presence of modified peptides. The developed protocol could be applied to characterization of other enzymes/proteins and other types of unconventional posttranslational protein modification.

Abnormal vascularization in mouse retina with dysregulated retinal cholesterol homeostasis.

Several lines of evidence suggest a link between age-related macular degeneration and retinal cholesterol maintenance. Cytochrome P450 27A1 (CYP27A1) is a ubiquitously expressed mitochondrial sterol 27-hydroxylase that plays an important role in the metabolism of cholesterol and cholesterol-related compounds. We conducted a comprehensive ophthalmic evaluation of mice lacking CYP27A1. We found that the loss of CYP27A1 led to dysregulation of retinal cholesterol homeostasis, including unexpected upregulation of retinal cholesterol biosynthesis. Cyp27a1-/- mice developed retinal lesions characterized by cholesterol deposition beneath the retinal pigment epithelium. Further, Cyp27a1-null mice showed pathological neovascularization, which likely arose from both the retina and the choroid, that led to the formation of retinal-choroidal anastomosis. Blood flow alterations and blood vessel leakage were noted in the areas of pathology. The Cyp27a1-/- retina was hypoxic and had activated Müller cells. We suggest a mechanism whereby abolished sterol 27-hydroxylase activity leads to vascular changes and identify Cyp27a1-/- mice as a model for one of the variants of type 3 retinal neovascularization occurring in some patients with age-related macular degeneration.

Spatial distribution of the pathways of cholesterol homeostasis in human retina.

BACKGROUND: The retina is a light-sensitive tissue lining the inner surface of the eye and one of the few human organs whose cholesterol maintenance is still poorly understood. Challenges in studies of the retina include its complex multicellular and multilayered structure; unique cell types and functions; and specific physico-chemical environment. METHODOLOGY/PRINCIPAL FINDINGS: We isolated specimens of the neural retina (NR) and underlying retinal pigment epithelium (RPE)/choroid from six deceased human donors and evaluated them for expression of genes and proteins representing the major pathways of cholesterol input, output and regulation. Eighty-four genes were studied by PCR array, 16 genes were assessed by quantitative real time PCR, and 13 proteins were characterized by immunohistochemistry. Cholesterol distribution among different retinal layers was analyzed as well by histochemical staining with filipin. Our major findings pertain to two adjacent retinal layers: the photoreceptor outer segments of NR and the RPE. We demonstrate that in the photoreceptor outer segments, cholesterol biosynthesis, catabolism and regulation via LXR and SREBP are weak or absent and cholesterol content is the lowest of all retinal layers. Cholesterol maintenance in the RPE is different, yet the gene expression also does not appear to be regulated by the SREBPs and varies significantly among different individuals. CONCLUSIONS/SIGNIFICANCE: This comprehensive investigation provides important insights into the relationship and spatial distribution of different pathways of cholesterol input, output and regulation in the NR-RPE region. The data obtained are important for deciphering the putative link between cholesterol and age-related macular degeneration, a major cause of irreversible vision loss in the elderly.