Basal Laminar Deposit Formation in APO B100 Transgenic Mice: Complex Interactions between Dietary Fat, Blue Light, and Vitamin E

Abstract

purpose. Dietary fat intake has been proposed as a mechanism of sub-RPE deposit formation. It has been demonstrated recently that sub-RPE deposits develop in 16- to 18-month-old C57BL/6 mice fed a high-fat diet and exposed to blue-green light. Hyperlipidemia also develops in these mice after they consume a high-fat diet. Because hyperlipidemia also develops in young C57BL/6 mice that overexpress APO B100, the major apolipoprotein in LDL cholesterol, this research was conducted to determine whether high-fat diet and plasma hyperlipidemia correlate with formation of basal laminar deposits (BLD) in young transgenic mice.

methods. APO B100 and wild-type C57BL/6 2-month-old mice were fed a high-fat diet for 4.5 months. After the first month, the right eyes were exposed to seven 5-second doses of nonphototoxic levels of blue-green light (20 mJ of argon 488 nm) over 2 weeks. Three months later, transmission electron microscopy (TEM) of the retina was performed to evaluate whether sub-RPE deposits correlate with plasma cholesterol and triglyceride levels. Several eyes were stained with filipin to detect cholesterol and osmium-thiocarbohydrazide-osmium (OTO) to detect neutral lipids in Bruch’s membrane (BrM). A third group of APO B100 2-month-old mice were pretreated with vitamin E subcutaneously twice a week throughout the experiment and underwent the same light-exposure protocol.

results. Mice fed a high-fat diet had a more elevated plasma triglyceride and cholesterol level than those that consumed a regular diet. Young APO B100 mice fed a high-fat diet had blood lipid levels higher than those in young wild-type mice that consumed high-fat diets, and these two groups had higher lipid levels than animals with regular diets, as shown previously in wild-type C57BL/6 (old and young). Eyes of APO B100 mice treated with blue-green light showed a high frequency of “moderate BLD”, whereas the nonexposed eyes did not. In contrast, no BLD formed in either eye of the wild-type young mice fed a high-fat diet. In individual affected mice, only a weak correlation was observed between deposit severity and plasma lipid concentration. None of the eyes in mice with sustained hyperlipidemia with or without BLD demonstrated obvious widespread neutral lipid or cholesterol deposition in BLD or BrM. However, vitamin E–treated mice showed minimal formation of BLD.

conclusions. Although a high-fat diet is a necessary precondition for this model of BLD, the findings demonstrated a convincing direct correlation between plasma lipidemia and deposit severity. The results suggest that age, as shown in previous studies, and high-fat predispose to formation of BLD by altering hepatic and/or RPE lipid metabolism in ways more complicated than plasma hyperlipidemia alone.

Age-related macular degeneration (AMD) is the most important cause of vision loss among the elderly. ¹ The initial stage of the disease is characterized by the accumulation of lipid rich deposits under the RPE and within Bruch’s membrane (BrM). ² The three important deposits that have been recognized include basal laminar deposits (BLD), basal linear deposits, and drusen. ³ Although all three deposits are clearly associated with aging, the interrelationship between the pathogenic conditions that cause the various types of deposit remains unknown. 

Several groups have demonstrated that in aging and in AMD, BrM contains increasing amounts of various lipids, including unesterified and esterified cholesterol, ^{4 5 6} neutral fats, ^{6 7 8} phospholipids, ^{6 7 8} unsaturated fatty acids, ⁹ and peroxidized lipids. ⁹ Epidemiologic data indicate that dietary fat, especially polyunsaturated fats, is associated with AMD. ¹⁰ Recently, work from our and other laboratories has identified an association between high-fat diet, ocular blue-light exposure, and the development of sub-RPE deposits, especially BLD. ^{11 12 13} In our previous research, old, but not young, C57BL/6 mice were susceptible to deposit formation. The susceptibility correlated with the development of plasma hyperlipidemia after consumption of a high-fat diet, which only occurred in old C57BL/6 mice. ¹⁴ We concluded that an age-related abnormality in lipid metabolism might be related to the susceptibility of the RPE to blue-light–induced deposit formation. 

The mechanism of abnormal lipid metabolism and sub-RPE deposit formation is unknown, but at least three pathogenic possibilities have been proposed. Marshall has postulated that plasma lipids directly infiltrate BrM. ¹⁵ Alternatively, Curcio et al. have proposed that lipid infiltration may be the result of abnormal RPE lipid metabolism (Curcio C, et al. IOVS 2002;43:ARVO E-Abstract 862). Finally, dietary lipids significantly accumulate within cell membranes of the retina, the RPE, and probably the choriocapillaris endothelium. Increased content of polyunsaturated fatty acids (PUFAs) can render cells more susceptible to lipid peroxidation. ¹⁶  

We sought to provide additional evidence of a relationship of dietary fat and abnormal lipid metabolism with sub-RPE deposit formation. In humans, apolipoprotein B100 (APO B100) is the lipoprotein associated with LDL cholesterol particles. ¹⁷ Conceptually, the APO B100 molecule is inserted in the phospholipid membrane of the LDL particle and contains the LDL receptor-binding domain, which is involved in the uptake and internalization of plasma LDL by tissues. ¹⁸ Mice typically express only a splice variant (APO B48), which lacks the binding site for the LDL receptor and limits internalization of LDL particles. ¹⁹ Transgenic mice expressing the human APO B100 molecule have been developed and demonstrate a marked increase in LDL cholesterol on challenge with a high-fat diet and increased transport of LDL cholesterol into peripheral tissues. ²⁰  

We tested the hypothesis that the age-related susceptibility of C57BL/6 mice to high-fat-diet–induced BLD was directly related to abnormal lipid biology, but not necessarily to the aging eye. Therefore, we determined the susceptibility of young APO B100 transgenic mice to high-fat-diet–induced BLD, evaluated BLD for the presence of cholesterol and triglyceride deposition, and determined whether pretreatment with the antioxidant α-tocopherol (vitamin E) reduces the severity and frequency of BLD formation. 

Material and Methods

Mice

C57BL/6 mice and APO B transgenic mice, 2 months old, were purchased from the National Institute of Aging (Bethesda, MD) and Taconic Farms (Germantown, NY), respectively. As shown previously, plasma hyperlipidemia develops in C57BL/6 wild-type mice fed a high-fat diet. ¹⁴ APO B100 transgenic mice overexpress human APO B100, driven by a liver-specific promoter, ²¹ and demonstrate hyperlipidemia with elevation in plasma cholesterol and triglycerides, including LDL, VLDL, and plasma nonesterified fatty acids, when fed diets high in saturated and polyunsaturated fats (but not when fed a normal diet). The provisions of the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research were observed and all experiments were approved by the Division of Veterinary Resources. 

Diet

We have developed a model of sub-RPE deposit formation based on feeding mice a high-fat diet followed by exposure to blue-green light. ¹⁴ Experimental mice were switched from the regular diet (Diet 5001; PMI Nutrition International Test Diet, Richmond, IN) to the high-fat chow (Diet 5015; PMI Nutrition International Test Diet) at the beginning of the experiments to allow tissue distribution of dietary lipid. The high-fat diet more than doubled the relative percentage of calories from fat but maintained the same number of calories per gram of food (by reducing the percentage of calories from carbohydrates). Dietary fat was increased from 11% of total calories to 25%, equally divided between increased saturated and unsaturated fat. The diets contained the following constituents: 64% carbohydrates, 30% protein, 6.5% total fat, 3.6% unsaturated fat, 1.2% linoleic acid, and 0.05 IU/g vitamin E in the regular chow and 55% carbohydrates, 18% protein, 26% total fat, 8.2% unsaturated fat, 2.5% linoleic acid, and 0.04 IU/g vitamin E in the high-fat chow. 

Experimental Model for Sub-RPE Deposits

Briefly, in the right eye, a repetitive exposure to nonphototoxic levels of argon laser 488 nm blue-green light was added to induce transient RPE oxidant production 4 weeks after starting the high-fat chow. Seven 5-second exposures to 20 mJ of argon laser were given 2 to 3 days apart over a 2-week period. The delivery system used a probe producing a 200-μm spot. Retinal illumination required a specially designed biconcave contact lens to neutralize the optical power of the natural lens and to enlarge the retinal spot size. The high-fat chow diet was continued for three additional months, after the 2-week exposure to blue-green light. Then, mice were killed and the eyes immediately removed for transmission electron microscopy (TEM) with special lipid staining. 

Experimental Protocol

To determine the effect of plasma hyperlipidemia in the formation of BLD, the deposit model was performed in wild-type C57BL/6 (n = 10) and transgenic APO B100 mice (n = 10). The effect of aging shown in previous work was eliminated in this experiment by choosing mice of a young age (2 months). Another group of 2-month-old APO B100 mice (n = 10) were given vitamin E (90 IU twice a week, subcutaneously) to investigate the protective effect of this vitamin on lipid peroxidation after a diet high in PUFAs and exposure to blue-green light. Control animals for this last experiment were given soybean (vehicle) using the same route of administration and dose. 

Serum Levels of Cholesterol and Triglycerides

Total plasma cholesterol and triglycerides were measured by collecting 0.8 mL of whole blood in heparin by cardiac puncture at the time of euthanasia and submitting the samples for analysis by a chemistry analyzer (Beckman Instruments, Fullerton, CA). Sub-RPE deposit severity was correlated with plasma cholesterol and triglyceride levels. For the studies to correlate plasma lipid levels with BLD severity, we also included BLD severity scores from 3- to 6-month-old APO B100 mice and old wild-type C57BL/6 mice (n = 6 or 7 per age group). 

Histology and TEM

Mice were killed by anesthetic overdose and perfused with saline followed by a mixture of 3% glutaraldehyde and 2% paraformaldehyde. The eyes were immediately enucleated and the corneas removed and fixed overnight in 3% glutaraldehyde and 2% paraformaldehyde in PBS (0.1 M, pH 7.3). The lens was removed, and the posterior segment (retina, choroid, and sclera) was quadrisected to contain the perioptic nerve portion at the apex and ciliary body at the base. The superotemporal quadrant of retina, choroid, and sclera was sectioned for electron microscopy. The tissue was fixed in 1% osmium tetroxide for 1 hour, rinsed in PBS, dehydrated in EtOH, and embedded in Spur’s resin. Thick and ultrathin sections (0.7–1.0 μm) were cut on a microtome (MT-2 Porter Blum; Sorvall, Newtown, CT). Thick sections were stained with toluidine blue and examined by light microscopy. Ultrathin sections were stained with 4% uranyl acetate and lead citrate and then examined with a transmission electron microscope (model CX-100; JEOL, Tokyo, Japan). 

Semiquantitative Grading System

For each specimen, a single cross section was examined, and low-power transmission electron micrographs were made of the entire section from the perioptic to the ciliary body portion (usually approximately 10 micrographs). For semiquantitative scoring, two to four representative high-power micrographs were made of each low-power section by an individual unaware of the experimental conditions. The high-power micrographs were graded by two independent examiners for the presence and severity of BLD. A severity score of 0 to 15 points was determined for each section by summation of the median scores of all the micrographs from a section containing one of the five different categories of abnormality (from 0 to 3 points for each): continuity of BLD, maximum thickness of BLD, nature of deposit content (homogeneous, banded structures, membranous debris, and granular material), presence of BrM abnormalities, and assessment of other choriocapillaris endothelial damage or invasion. BrM thickness was also directly measured in three different standardized locations in each image and averaged to provide a mean score for that micrograph. The mean of 10 high-power micrographs was used to assign an average BrM thickness for an individual specimen. Groups were compared by determining the median values, and the Mann-Whitney test was used for statistical analysis of the differences. In addition, the frequency of BLD was determined using two different criteria. “Any BLD” was defined as the presence of any discrete focal nodule of homogenous material of intermediate electron density between the RPE cell membrane and BrM in at least one micrograph within a section. “Moderate BLD” was defined as the presence in at least three micrographs of the following: continuous BLD extending under two or more cells, deposit thickness equaling 20% or more of RPE cell cross-sectional thickness, and/or the presence of any banded structures within the BLD. 

Lipid Staining

Eyes and mesentery (used as control tissue) were removed and placed in fixative overnight. The eye was dissected, and the retina was quadrisected as described previously. One of two techniques was used for lipid staining before eyes were embedded for sectioning and stained for TEM. Filipin staining was used for unesterified cholesterol. Filipin (5 mg) was dissolved in 1 mL of dimethylformamide and then diluted to 100 mL in PBS. Tissue for analysis was incubated in this solution for 30 minutes, washed, fixed in 2.5% glutaraldehyde, and postfixed in 1% OsO₄. OTO was used to visualize neutral lipid deposition, mostly triglycerides. Tissue to be analyzed was incubated for 30 minutes in 2% OsO₄, washed in PBS buffer, and incubated in 1.5% aqueous thiocarbohydrazide for 5 minutes. Then, the tissue was washed and reincubated in OsO₄ for 30 minutes. 

Results

First, we confirmed that APO B100 mice were susceptible to increase lipidemia after they were fed a high-fat diet. As shown in Table 1 , young APO B100 mice demonstrated a moderate increase in plasma cholesterol compared with young wild-type mice fed a normal diet and a slight increase in serum triglycerides. On being fed a high-fat diet, both wild-type and transgenic mice demonstrated increases in both parameters. Levels achieved in young APO B100 mice were actually greater than those observed in wild-type older mice in previous studies. 

Next, we evaluated the development of sub-RPE deposits in the different groups. As expected, neither young wild-type nor young transgenic mice fed normal diets demonstrated any evidence of abnormal morphology or deposit accumulation (data not shown). In addition, young wild-type mice fed a high-fat diet, with or without exposure to blue-green light, failed to demonstrate deposit formation (Fig. 1A) . In contrast, APO B100 transgenic mice, fed a high-fat diet showed development of deposits, but only after exposure to blue-green light (Fig. 1B) . All fat-fed, light-exposed APO B100 mice demonstrated at least focal deposit formation (Fig. 2A) , and 60% of mice had moderately severe deposits. The deposits generally were characterized by the formation of mild to moderately thick homogenous material between the RPE plasma membrane and basal membrane. Banded structures, resembling fibrous long-spacing collagen, were frequent. In addition, BrM was typically thickened with an occasional accumulation of small circular profiles. The choriocapillaris endothelium demonstrated apparent hypertrophy and occasional protrusion of processes into BrM (Fig. 2B) . Reduplication of the basal membrane was occasionally observed. No evidence of explicit choroidal neovascular formation was observed. 

To evaluate whether the deposit severity correlated directly with plasma lipidemia, we plotted the deposit severity score (DSS) of wild-type C57BL/6 or APO B100 mice and correlated the results with cholesterol and triglycerides. Although there was a statistically significant Pearson correlation coefficient (DSS versus cholesterol r = 0.41, P < 0.01 and DSS versus triglycerides r = 0.17, P < 0.05), the results demonstrated a high degree of scatter indicating a relatively weak direct correlation between plasma lipid levels and BLD severity among the individual mice fed a high-fat diet (Fig. 3) . 

LDL cholesterol particles contain many different kinds of lipids, especially triglycerides and cholesterol. Because both of these lipids have been associated with AMD, we evaluated their direct deposition within BrM or within the deposits. Figure 4A demonstrates filipin staining of BLD, used to detect unesterified cholesterol. As seen in Figure 4B , the photoreceptor outer segments demonstrated significant punctuate uptake of filipin, consistent with high levels of unesterified cholesterol in the outer segments. In contrast, minimal filipin deposits were observed within BLD and BrM (Fig. 4C) . OTO staining of BLD for triglyceride was also performed (Fig. 5A) and no staining was demonstrated within BLD or BrM. In contrast, the mesenteric adipocytes used as the control demonstrated significant uptake, indicating that the technique was functional (Fig. 5B) . We conclude that we were unable to identify prominent deposition of either unesterified cholesterol or triglyceride within BrM or the deposits. 

Finally, we performed a preliminary experiment to determine whether deposit formation might be related to lipid peroxidation rather than actual lipid deposition. Subcutaneous injections of α-tocopherol twice weekly for 4.5 months prevented the accumulation of deposits, compared with untreated mice (Fig. 6) . 

A semiquantitative analyses of deposit severity was performed (Table 2) . Compared with the wild-type controls with median severity score of 0, APO B100 mice fed a high-fat diet and exposed to blue-green light had a significantly increased median severity score (5.5). In contrast, APO B100 mice pretreated with α-tocopherol demonstrated a reduced frequency and severity of deposits (score, 0.5). BrM thickness was measured in the latter study. There was a statistically significant difference between the control and the vitamin E–treated group (1.16 ± 0.21 μm vs. 0.69 ± 0.09 μm, P < 0.05 by t-test). These data suggests that the antioxidant properties of α-tocopherol appear to prevent blue-green-light–mediated formation of deposits. 

Discussion

The mechanism of deposit formation in age-related macular degeneration remains highly controversial and probably involves multiple pathogenic mechanisms. Two prominent paradigms are the “response-to-injury paradigm” and the “barrier paradigm.” The response to injury paradigm proposes that the RPE is the target of various specific injury responses, such as oxidant injury and immune injury. ^{16 22 23 24} The subsequent response to injury is the formation of deposits. 

The barrier hypothesis proposes that deposit formation is secondary to a defect in transport through BrM caused by the accumulation of lipids. ^{25 26} Research has established the age-related accumulation of various lipids, including phospholipid, triglycerides, and unesterified cholesterol, within BrM to be part of normal aging—perhaps more severe in the presence of AMD. ^{6 8 9} Nevertheless, the source of these lipids remains controversial. Originally, it was thought that BrM lipid accumulation was the result of deposition from circulating LDL cholesterol or other plasma sources. Recently, however, Curcio et al. have postulated that BrM lipids may in fact be derived from the RPE (Curcio C, et al. IOVS 2002;43:ARVO E-Abstract 862). Conceivably, these RPE-derived lipids are the product of specific secretion, exocytosis, or blebbing. 

In this study, our results demonstrated that transgenic mice with overexpression of APO B100 and increased LDL cholesterol in the blood, are more susceptible to blue-light–induced formation of deposits. Deposits did not form in the absence of blue-light exposure, and they were prevented by pretreatment with the antioxidant α-tocopherol. These observations suggest that high-fat diet and blue-light exposure induces oxidant injury, which is related to the formation of deposits. In addition, the data suggest a poor correlation between plasma lipid concentration and deposit formation, and provide no evidence of significant unesterified cholesterol or triglyceride accumulation. Taking these findings together, we believe that they support the hypothesis that defects in hepatic and possibly RPE lipid metabolism in response to dietary lipid challenge play a role in the formation of deposits, but the pathogenesis is much more complex than mere deposition into BrM. 

Our failure to detect unesterified cholesterol by filipin and triglyceride deposition by OTO staining does not rule out the presence of both of these lipids within BrM. TEM imaging of deposits is a relatively insensitive way to characterize lipid deposition, and thus more sophisticated techniques using other fluorescent staining methods or direct extraction and HPLC may demonstrate the quantitative increase of deposition of lipids in this model. Also, the processing of tissue may result in the loss of some lipids from the specimens. Nonetheless, our data also failed to show a pronounced accumulation within BLD. 

Other genetically manipulated mouse models of hyperlipidemia, especially with APO E abnormalities, have given contradictory results regarding deposit formation. Dithmar et al. ²⁷ showed the APO E null mice did not show formation of BLD or degeneration of the RPE, but electron-lucent particles were present in BrM, consistent with a mild form of BLD. Kliffen et al. ¹² showed that typical BLD developed in mice expressing mutant APO E3 after consuming a fat diet, without light exposure. APO E is important in hepatic processing of VLDL remnants, and its absence results in increased VLDL in the blood. ^{17 18} Also, APO E isoforms may contribute to transport and solubilization of extracellular lipids in tissues and therefore may play a role in BrM lipid transport. ^{12 28}  

Our study differs significantly from these others. APO B100 is the normal ligand for endothelial uptake of LDL cholesterol particles, and it is absent in normal mice. ¹⁹ LDL cholesterol particles are the physiologic means by which triglycerides, fatty acids, and lipophilic antioxidants are transported into tissues. Their content is strongly influenced by dietary intake and hepatic processing of chylomicrons. Overexpression APO B100 in the mouse mimics the physiology of moderate hypercholesterolemia in humans and facilitates endothelial uptake of LDL particles. ^{20 21} The susceptibility of young APO B100 transgenic and C57Bl/6 mice to BLD rules out age dependence as an absolute necessity in deposit formation, but confirms the complicated relationship between plasma lipid, antioxidants, and formation of sub-RPE deposits. 

The determination of a role for α-tocopherol in preventing sub-RPE deposits should be considered preliminary. In particular, we used subcutaneous administration of vitamin E to guarantee adequate sustained doses. After oral intake, vitamin E is transported to the liver by chylomicrons and incorporated into LDL particles by specific vitamin E transport proteins. ²⁹ In our experiments, vitamin E assimilation would first necessitate drainage of the lymph and gaining access to lymph nodes and macrophage-rich tissues before entry into the circulation with subsequent uptake by the liver. Conceivably, our administration method may have affected anti-inflammatory and antioxidant mechanisms more than after oral ingestion. Future studies will more specifically address the protective mechanism of vitamin E in this model. Of note, the Age-Related Eye Disease Study (AREDS), in which a cocktail of antioxidants including vitamin E was used to treat patients with dry macular degeneration, failed to demonstrate that antioxidants reverse or prevent the progression of drusen formation. ³⁰ Nevertheless, in this animal model, the treatment was started before the onset of BLD, a stage in which deposits are not detectable. The preventive mechanism of vitamin E against preclinical formation of sub-RPE deposits was not directly tested in this clinical study. 

Although we favor the hypothesis that PUFAs are incorporated into the endothelial cell and/or RPE cell membrane where they become substrates for light-induced lipid peroxidation, we recognize that other mechanisms may also play a role in this process. Studies are under way in our laboratory, testing in vitro and in vivo the mechanisms involved in the response-to-injury paradigm in which lipid peroxidation and immune mechanisms have an important role in the pathogenesis of drusen formation. Because LDL lipoproteins active in the delivery of fatty acids, phospholipids, antioxidants, and cholesterol to peripheral tissue, changes in LDL content may be more important than increases in LDL particle concentrations. 

 Supported by National Eye Institute Grant EY/AI 13318.

 Submitted for publication August 20, 2003; revised September 3, 2003; accepted September 4, 2003.

 Disclosure: D.G. Espinosa-Heidmann, None; J. Sall, None; E.P. Hernandez, None; S.W. Cousins, None

 Presented at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, May 2000.

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Table 1.

 

Association between Diet and Plasma Lipidemia

Mouse Strain	Normal Diet			High Fat Diet
		Cholesterol	Triglycerides	Cholesterol	Triglycerides
Young C57BL/6	49	69	102	78
Young APO B100	79	79	125	101

Figure 1.

 

Transmission electron micrograph of the outer retina and choroid from 2-month-old fat-fed C57BL/6 and APO B100 mice exposed to blue-green light. (A) Specimen from a young wild-type C57BL/6 female mouse showed no abnormalities. (B) TEM confirmed a range of histologic abnormalities of the RPE, BrM, and choriocapillaris (CC) in the APO B100 transgenic mice exposed to a high-fat diet and blue-green light. Moderately thick sub-RPE deposits are present (under dotted line) containing many banded structures (white asterisk) consistent with BLD. BrM revealed marked collagenous thickening, and the CC showed apparent endothelial thickening (black asterisk) with loss of fenestration (arrows). Magnification, ×25,000.

Figure 2.

 

Transmission electron micrograph of the outer retina and choroid of a 2-month-old fat-fed APO B100 mouse, with and without exposure to blue-green light. (A) Specimen from an APO B100 young female mouse consuming a high-fat diet but not exposed to blue-green light revealed focal homogeneous deposits (arrow) between the RPE cell membrane and its basement membrane. In eyes not exposed to blue-green light, the deposit accumulation was typically thin, patchy, or nodular and of a homogenous material, without banded or membranous content. BrM and choriocapillaris (CC) appeared normal. (B) Another specimen from an APO B100 young female mouse that consumed a high-fat diet and was exposed to blue-green light showed moderately thick and continuous sub-RPE deposits. Often, these deposits contained oval-shaped banded structures resembling fibrous long-spacing collagen (white asterisk). In general, BrM changes were observed as diffuse thickening involving both the inner and outer collagenous layers. Abnormalities were observed in the choriocapillaris, such as apparent hypertrophy of the endothelial cell body (arrowheads), reduplication of the basement membrane (black asterisks), and protrusion of a cell process (arrow) into the basement membrane and outer collagenous zone, without definite classic choroidal neovascular membrane. Magnification, ×25,000.

Figure 3.

 

Correlation of the deposit severity score with plasma lipidemia. The deposit severity scores of wild-type C57BL/6 and APO B100 young mice were plotted and the results compared with serum cholesterol (A) and triglyceride (B) levels. Although there was a statistically significant Pearson correlation coefficient, the results demonstrated a high degree of scatter, indicating a relatively weak direct correlation between plasma lipid levels and BLD severity among the individual mice fed a high-fat diet.

Figure 4.

 

Filipin staining shown in low- (A) and high- (C) magnification transmission electron micrographs of the outer retina and choroid of a 2-month-old fat-fed APO B100 mouse exposed to blue-green light. (A) A low-magnification micrograph showed no filipin stain that was used to detect unesterified cholesterol in the BLD and BrM. (B) Photoreceptor outer segments (POS) demonstrated significant punctuate uptake of filipin, consistent with high levels of unesterified cholesterol in the outer segments and in the intestines, which were used as a positive control (arrowheads). (C) Higher magnification of another APO B100 specimen that clearly showed no filipin staining of sub-RPE deposits (asterisk). CC, choriocapillaris. Magnification: (A) 7,200; (B) POS, ×25,000; Adipocyte, ×14,500; (C) ×25,000.

Figure 5.

 

OTO staining of a transmission electron micrograph of the outer retina and choroid of the eye of a 2-month-old fat-fed APO B100 mouse exposed to blue-green light. (A) High-magnification micrograph showed no evident OTO staining of sub-RPE deposits ( ). (B) Mesenteric adipocytes demonstrated significant punctuate uptake of OTO stain (arrowhead), consistent with high levels of triglycerides in the intestines used as the positive control CC, choriocapillaris. Magnification: (A) ×25,000; (B) ×18,000.

Figure 6.

 

TEM of the outer retina and choroid from 2 month-old fat-fed APO B100 mice exposed to blue-green light and treated with vehicle (soybean) or a vitamin E supplement. (A) Control APO B100 mouse showed the same changes as those described the in RPE (sub-RPE deposits under dotted line), BrM (thickening), and choriocapillaris (CC; endothelial hypertrophy with loss of fenestration) of transgenic mice fed a high-fat diet and exposed to blue-green light. (B) Specimen from a young APO B100 fat-fed female mouse exposed to blue-green light and treated with subcutaneous vitamin E showed no abnormalities in the RPE, BrM, and CC. Magnification: (A, B) ×25,000.

Table 2.

 

Effect of HFD, blue light, and vitamin E on deposit severity in C57BL/6 and APO B100 mice

Mouse Strain	Vitamin E	Eye Exposure to Blue Light	Frequency of Any BLD	Frequency of Moderate BLD	Median Score “12 point”
C57BL/6	No	No	0	0	0
	No	Yes	0	0	0
APO B100	No	No	20	0	0.5
	No	Yes	100	60	5.5*
APO B100	Yes	Yes	30	0	0.5, †