Transcriptome Analysis of Retinal and Choroidal Pathologies in Aged BALB/c Mice Following Systemic Neonatal Murine Cytomegalovirus Infection

Our previous studies have shown that systemic neonatal murine cytomegalovirus (MCMV) infection of BALB/c mice spread to the eye with subsequent establishment of latency in choroid/RPE. In this study, RNA sequencing (RNA-Seq) analysis was used to determine the molecular genetic changes and pathways affected by ocular MCMV latency. MCMV (50 pfu per mouse) or medium as control were injected intra-peritoneally (i.p.) into BALB/c mice at <3 days after birth. At 18 months post injection, the mice were euthanized, and the eyes were collected and prepared for RNA-Seq. Compared to three uninfected control eyes, we identified 321 differentially expressed genes (DEGs) in six infected eyes. Using the QIAGEN Ingenuity Pathway Analysis (QIAGEN IPA), we identified 17 affected canonical pathways, 10 of which function in neuroretinal signaling, with the majority of DEGs being downregulated, while 7 pathways function in upregulated immune/inflammatory responses. Retinal and epithelial cell death pathways involving both apoptosis and necroptosis were also activated. MCMV ocular latency is associated with upregulation of immune and inflammatory responses and downregulation of multiple neuroretinal signaling pathways. Cell death signaling pathways are also activated and contribute to the degeneration of photoreceptors, RPE, and choroidal capillaries.

Retinal and Choroidal Pathologies in Aged BALB/c Mice Following Systemic Neonatal Murine Cytomegalovirus Infection

Although pathologies associated with acute virus infections have been extensively studied, the effects of long-term latent virus infections are less well understood. Human cytomegalovirus, which infects 50% to 80% of humans, is usually acquired during early life and persists in a latent state for the lifetime. The purpose of this study was to determine whether systemic murine cytomegalovirus (MCMV) infection acquired early in life disseminates to and becomes latent in the eye and if ocular MCMV can trigger in situ inflammation and occurrence of ocular pathology. This study found that neonatal infection of BALB/c mice with MCMV resulted in dissemination of virus to the eye, where it localized principally to choroidal endothelia and pericytes and less frequently to the retinal pigment epithelium (RPE) cells. MCMV underwent ocular latency, which was associated with expression of multiple virus genes and from which MCMV could be reactivated by immunosuppression. Latent ocular infection was associated with significant up-regulation of several inflammatory/angiogenic factors. Retinal and choroidal pathologies developed in a progressive manner, with deposits appearing at both basal and apical aspects of the RPE, RPE/choroidal atrophy, photoreceptor degeneration, and neovascularization. The pathologies induced by long-term ocular MCMV latency share features of previously described human ocular diseases, such as age-related macular degeneration.

Ocular cytomegalovirus latency exacerbates the development of choroidal neovascularization

Age-related macular degeneration (AMD) is a complex, multifactorial, progressive disease which represents a leading cause of irreversible visual impairment and blindness in older individuals. Human cytomegalovirus (HCMV), which infects 50-80% of humans, is usually acquired during early life and persists in a latent state for the life of the individual. In view of its previously described pro-angiogenic properties, we hypothesized that cytomegalovirus might be a novel risk factor for progression to an advanced form, neovascular AMD, which is characterized by choroidal neovascularization (CNV). The purpose of this study was to investigate if latent ocular murine cytomegalovirus (MCMV) infection exacerbated the development of CNV in vascular endothelial growth factor (VEGF)-overexpressing VEGF-A^hyper mice. Here we show that neonatal infection with MCMV resulted in dissemination of virus to various organs throughout the body including the eye, where it localized principally to the choroid in both VEGF-overexpressingVEGF-A^hyper and wild-type(WT) 129 mice. By 6 months post-infection, no replicating virus was detected in eyes and extraocular tissues, although virus DNA was still present in all eyes and extraocular tissues of both VEGF-A^hyper and WT mice. Expression of MCMV immediate early (IE) 1 mRNA was detected only in latently infected eyes of VEGF-A^hyper mice, but not in eyes of WT mice. Significantly increased CNV was observed in eyes of MCMV-infected VEGF-A^hyper mice compared to eyes of uninfected VEGF-A^hyper mice, while no CNV lesions were observed in eyes of either infected or uninfected WT mice. Protein levels of several inflammatory/angiogenic factors, particularly VEGF and IL-6, were significantly higher in eyes of MCMV-infected VEGF-A^hyper mice, compared to uninfected controls. Initial studies of ocular tissue from human cadavers revealed that HCMV DNA was present in four choroid/retinal pigment epithelium samples from 24 cadavers. Taken together, our data suggest that ocular HCMV latency could be a significant risk factor for the development of AMD. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Bone marrow transplantation transfers age-related susceptibility to neovascular remodeling in murine laser-induced choroidal neovascularization

PURPOSE

Neovascular remodeling (NVR), the progression of small capillaries into large-caliber arterioles with perivascular fibrosis, represents a major therapeutic challenge in neovascular age-related macular degeneration (AMD). Neovascular remodeling occurs after laser-induced choroidal neovascularization (CNV) in aged but not young mice. Additionally, bone marrow-derived cells, including macrophages, endothelial precursor cells, and mesenchymal precursor cells, contribute to CNV severity. In this study, we investigated the impact of aged bone marrow transplantation (BMT) on the degree of fibrosis, size, and vascular morphology of CNV lesions in a mouse model of laser-induced CNV.

METHODS

Young (2 months) and old (16 months) mice were transplanted with green fluorescent protein (GFP)-labeled bone marrow isolated from either young or old donors. Laser CNV was induced 1 month following transplant, and eyes were analyzed via choroidal flat mounts and immunohistochemistry 1 month postlaser. The identity of cells infiltrating CNV lesions was determined using specific markers for the labeled transplanted cells (GFP+), macrophages (F4/80+), perivascular mesenchymal-derived cells (smooth muscle actin, SMA+), and endothelial cells (CD31+).

RESULTS

Bone marrow transplantation from aged mice transferred susceptibility to NVR into young recipients. Inversely, transplantation of young marrow into old mice prevented NVR, preserving small size and minimal fibrosis. Mice with NVR demonstrated a greater relative contribution of marrow-derived SMA+ perivascular mesenchymal cells as compared to other cells.

CONCLUSIONS

Our findings indicate that the status of bone marrow is an important determining factor of neovascular severity. Furthermore, we find that perivascular mesenchymal cells, rather than endothelial cells, derived from aged bone marrow may contribute to increased CNV severity in this murine model of experimental neovascularization.

Repetitive Nonlethal Oxidant Injury to Retinal Pigment Epithelium Decreased Extracellular Matrix Turnover In Vitro and Induced Sub-RPE Deposits In Vivo

PURPOSE

To determine the impact of repetitive nonlethal oxidant injury with hydroquinone (HQ) on regulation of cell membrane blebbing and molecules, which are essential in extracellular matrix turnover (ECM) maintenance, especially matrix metalloproteinase (MMP)-2, tissue inhibitor of MMP (TIMP)-2, and type IV collagen in cultured RPE. In addition, to determine whether chronic oral HQ causes induction of sub-RPE deposit formation in a mouse model.

METHODS

An ARPE-19 cell line stably expressing membrane-targeted green fluorescent protein (GFP) was challenged by exposure to HQ (100 microM). Repetitive acute (6 hours every 3 days for 4 weeks) or transient (6 hours followed by a recovery phase, every 5 days for 6 weeks) exposure to HQ were evaluated. An MTS assay, cell counts, and bromodeoxyuridine (BrdU) incorporation were used to detect cell viability and proliferation. Supernatants and cell homogenates were collected to assess MMP-2 and TIMP-2 activity by zymography and reverse zymography, proteins by Western blot, and type IV collagen accumulation by ELISA and immunostaining. Expression of MMP-2 and type IV collagen was examined by real-time RT-PCR on total RNA. Sixteen-month-old C57BL/6 female mice were fed a regular fat diet, with or without HQ (0.8%) in the drinking water, for 4 months. The eyes were removed for transmission electron microscopy of the retina and choroid after treatment. Semiquantitative grading of deposit severity was performed.

RESULTS

In vitro, high doses of HQ (400-250 microM) killed a significant fraction of RPE cells ( approximately 60% of control). Low doses (50-100 microM) were nonlethal but induced significant blebbing. Both nonlethal repetitive acute and transient exposure to HQ were associated with diminished MMP-2 activity and increased collagen type IV accumulation. In vivo, mice exposed to oral HQ demonstrated moderately thick basal laminar deposits and a variable degree of deposits within Bruch's membrane (BrM). These homogeneous sub-RPE deposits accumulated in the eyes, consistent with early laminar deposits.

CONCLUSIONS

In cultured RPE, nonlethal injury with HQ upregulated nonlethal blebbing and decreased ECM turnover. Similarly, in vivo exposure to oral HQ induced nonlethal bleb injury and sub-RPE deposits. These data support the hypothesis that HQ may regulate blebbing and molecules that influence ECM turnover. This study suggests that HQ may be another type of oxidant that causes injury to the RPE and may explain the association between environmental oxidants and early AMD.

Cigarette Smoke–Related Oxidants and the Development of Sub-RPE Deposits in an Experimental Animal Model of Dry AMD

PURPOSE

Oxidative injury to the retinal pigment epithelium (RPE) has been proposed to be an important injury stimulus relevant to the accumulation of subretinal deposits in age-related macular degeneration (AMD). Cigarette smoking is a major risk factor for AMD, and cigarette smoke-related tar contains high concentrations of a potent oxidant, hydroquinone (HQ). This study was an investigation of the effects of cigarette smoke (CS) and HQ in the development of sub-RPE deposits in an experimental mouse model.

METHODS

Sixteen-month-old C57BL/6 female mice were fed a high-fat diet (HFD) for 4.5 months. Mice were divided into two major experimental groups, one to examine the effects of cigarette smoke and one to study the effects of a defined cigarette smoke component such as HQ. In the first group, mice eyes were exposed to blue-green light (positive controls) or to whole cigarette smoke. A third group with no intervention served as the negative control. In the second experimental group, animals received a purified diet with HQ (0.8%) with low or high fat content for 4.5 months. Mice in both groups were euthanatized at 4.5 months and eyes processed for transmission electron microscopy.

RESULTS

As previously demonstrated by our laboratory and others, most mice fed an HFD without other oxidant exposure demonstrated normal morphology or, in a few cases, small nodular basal laminar deposits. Eyes of mice exposed to whole cigarette smoke or to HQ in the food demonstrated a variable degree of basal laminar deposits and diffusely thickened Bruch's membrane. The choriocapillaris endothelium was variably hypertrophic.

CONCLUSIONS

Exposure to cigarette smoke or the smoke-related redox molecule, HQ, results in the formation of sub-RPE deposits, thickening of Bruch's membrane, and accumulation of deposits within Bruch's membrane. Smoke-related oxidants may be another oxidative injury stimulus to the choriocapillaris and RPE, and may explain the association between cigarette smoking and early AMD.

Blood-derived macrophages infiltrate the retina and activate Muller glial cells under experimental choroidal neovascularization

Inflammation is a major mechanism in the pathogenesis of age-related macular degeneration, the most important cause of blindness in the elderly. Previous studies have focused on the role of macrophages in regulating the growth of pathological new vessels over the retina, called choroidal neovascularization (CNV). However, no research has been done to evaluate the role of inflammation as a mechanism of vision loss and retinal degeneration in the retina underlying CNV. In other neuropathological conditions, hematogenous macrophages and/or resident microglia contribute to neurodegeneration. We have combined laser-induced CNV in mice and bone marrow transplantation with GFP-labeled bone marrow to determine the relative role of recruited blood-derived macrophages versus resident microglia in the retina associated with CNV. Using these chimeric mice, we have found that many GFP-labeled cells infiltrated the retina underlying CNV but not the retina unaffected by CNV. Immunostaining for the cell adhesion molecules VCAM 1, ICAM 1, and PECAM was strongly upregulated in retinal blood vessels under CNV. All GFP-labeled cells were immunoreactive for the macrophage marker F4/80. Most (70%) of the F4/80 immunoreactive cells were GFP-labeled under CNV. The density of resident microglia did not increase. Most GFP-labeled cells were found in close proximity to activated Muller cells. Depleting circulating macrophages with clodronic acid diminished the density of F4/80 immunoreactive cells as well as the density of pERK immunoreactive Muller cells in the retina under CNV. Thus, recruitment of blood-derived macrophages more than resident microglia seems to be associated with CNV.

Photoreceptor synapses degenerate early in experimental choroidal neovascularization

Severe visual loss in patients with age-related macular degeneration is associated with the development of choroidal neovascularization (CNV). The pathogenic mechanisms for CNV formation have been extensively investigated, but remarkably little research has addressed the mechanisms for dysfunction of the retina in CNV. Using laser-induced CNV in mice, we evaluated the mechanisms of retinal dysfunction. At 3 days, 1 week, 2 weeks, and 4 weeks after laser application, retinas under experimental CNV were characterized physiologically (ERG recordings, synaptic uptake of the exocytotic marker FM1-43, and light-induced translocation of transducin), histologically, and immunohistochemically. ERG amplitudes were reduced by 20% at 1 week after CNV. Depolarization-induced FM1-43 uptake in photoreceptor synapses was selectively reduced by 45% at 1 week after CNV. Although photoreceptor outer segments were shortened by 36%, light adaptation as measured by transducin translocation was mostly preserved. Early in CNV (3 days to 1 week), Muller cells demonstrated induction of c-fos and pERK expression. Also, the density of macrophage-like, F4/80 immunoreactive cells increased approximately 3-fold. Minimal photoreceptor death occurred during the first week, and was variable thereafter. At later times in CNV formation (> or =2 weeks), expression of photoreceptor synaptic markers was reduced in the outer plexiform layer, indicating loss of photoreceptor synaptic terminals. ERG amplitudes, synaptic uptake of FM1-43, and the induction of c-fos and pERK in Muller cells were altered within 1 week of experimental CNV, suggesting that during CNV formation, deficits in retinal function, in particular photoreceptor synaptic function, precede degeneration of photoreceptor terminals and photoreceptor cell death.

Gender and estrogen supplementation increases severity of experimental choroidal neovascularization

Observational clinical studies suggest that post-menopausal women may be at risk for more severe age-related macular degeneration, and that estrogen loss due to menopause may contribute. We sought to determine the effect of gender and estrogen status on the severity of choroidal neovascularization (CNV) in a mouse model for experimental choroidal neovascularization. Laser-induced CNV was performed in mice with or without estrogen supplementation. At various times, eyes were removed for analysis of severity of CNV lesions or for extraction of choroidal mRNA to evaluate iNOS, TNF-alpha, MMP-9, and ER-alpha expression, which are molecules relevant to angiogenic processes. Also, splenic macrophages were analysed for iNOS to determine the effect of estrogen treatment in vitro. Finally, laser-induced CNV was performed in iNOS -/- mice. Our result showed that aged female mice had significantly larger CNV than age-matched males. Ovariectomy in adult mice did not increase severity, but paradoxically estrogen supplementation after ovariectomy did increase CNV severity. More severe CNV were associated with a significant decrease in choroidal iNOS mRNA. Splenic macrophages from estrogen supplemented mice showed a significant increased in TNF-alpha mRNA expression (eight fold difference compared to the control) but only a mild change in iNOS mRNA levels (2-3 fold difference). In vitro data further showed that nitric oxide production in splenic macrophages at different estrogen levels was not different from controls. Finally, CNV severity was significantly more severe in iNOS -/- mice, compared to iNOS +/+ mice after laser treatment. In conclusion, aged female mice developed more severe CNV than do males. Estrogen replacement seems to increase severity, possibly by suppressing the upregulation of choroidal iNOS and activating macrophages. The putative beneficial or detrimental role of estrogen biology in age-related macular degeneration must be more carefully evaluated and may vary with the stage of age-related macular degeneration (atrophic or neovascular) as well as with the specific target cell type (monocytes vs. endothelial cell or vascular smooth muscle cell).

Quantitative enumeration of vascular smooth muscle cells and endothelial cells derived from bone marrow precursors in experimental choroidal neovascularization

Choroidal neovascularization (CNV) is characterized by the subretinal invasion of a pathologic new vessel complex from the choriocapillaris. Although CNV is traditionally considered to consist of endothelial cells, the cellular population of CNV is likely more complex in nature, comprising several different cell types. In addition, recent studies suggest that the CNV cell population has a dual origin (circulating versus resident populations). In this study we sought to determine the contribution and origin of different cell types in experimental CNV. Laser-induced CNV was performed on chimeric mice generated by reconstituting C57BL/6 mice with bone marrow from green fluorescent protein (GFP)-transgenic mice. In these mice, bone marrow-derived cells are GFP-labeled. Immunofluorescence staining was used to examine both flatmount preparations of the choroid and cross sections of the posterior pole for macrophages, endothelial cells, vascular smooth muscle cells, retinal pigment epithelial (RPE) cells, lymphocytes, or neutrophils at day 3, 7, 14 and 28 post-laser (n=5 per group). Cell types present in CNV included macrophages (20% of the cells in CNV), endothelial cells (25%), vascular smooth muscle cells (11%), RPE cells (12%) and non-labeled cells (32%). The macrophage population was mostly derived from circulating monocytes at all timepoints studied (70% were GFP labeled), while endothelial and vascular smooth muscle cells were partly bone marrow derived (50-60% were GFP labeled), and RPE cells appeared to be entirely derived from preexisting tissue resident cells. These results demonstrate that bone marrow-derived progenitor cells contribute significantly to the vascular and inflammatory components of CNV. Knowledge of the cellular composition and origin might help understand the pathogenic mechanisms controlling CNV severity as well as indicate potential targets for therapeutic intervention.

The association of prior cytomegalovirus infection with neovascular age-related macular degeneration

PURPOSE

To determine if prior exposure to pathogens associated with vascular disease, cytomegalovirus, Chlamydia pneumoniae, and Helicobacter pylori correlates with neovascular age-related macular degeneration (AMD).

DESIGN

An experimental study.

METHODS

SETTING

Institutional. Bascom Palmer Eye Institute, October 2001 to December 2002.

PATIENT POPULATION

150 patients (47 neovascular amd, 36 dry amd, and 67 non-amd controls) were included in the study. exclusion criteria included hiv infection, malignancy, recent acute illness requiring hospitalization within 6 months, or immunosuppressive illness.

PROCEDURE

Serum samples were obtained for analysis of cytomegalovirus, chlamydia pneumoniae, and helicobacter pylori igg antibody titers by elisa.

MAIN OUTCOME MEASURE

Comparison of the distribution of igg titers between patients with wet amd, dry amd, and controls.

RESULTS

The average cytomegalovirus IgG titer was higher in patients with wet AMD versus controls (p = 0.02, Student t-test, two-tailed) and patients with dry AMD (p = 0.06). Twenty-six (55%) of 47 subjects with wet AMD had high cytomegalovirus IgG titers compared with 14 (39%) of 36 patients with dry AMD (odds ratio [OR] = 2.23, 95% confidence interval [CI] = 0.77 to 6.44) and 23 (34%) of 67 control patients (OR = 2.49, 95% CI = 0.98 to 6.33). There was no major difference in the distribution of titers for Chlamydia pneumoniae IgG and Helicobacter pylori IgG in wet and dry AMD patients. Five of 47 patients with wet AMD (11%) had high antibody titers to all three pathogens, compared with only 1 of 36 patients with dry AMD (3%) (OR = 4.17, 95% CI = 0.46 to 37.36).

CONCLUSIONS

There was a significant association of high cytomegalovirus IgG titer with neovascular AMD compared with dry AMD and control patients. Chronic infection with cytomegalovirus may be a novel risk factor for the progression from dry to neovascular AMD.

Monocyte activation in patients with age-related macular degeneration: a biomarker of risk for choroidal neovascularization?

OBJECTIVE

To evaluate the activation state of macrophage function in patients with age-related macular degeneration (AMD) by quantifying the production of the proinflammatory and angiogenic factor tumor necrosis factor alpha (TNF-alpha) and by correlating its expression with dry and wet AMD.

METHODS

Circulating monocytes were obtained from the blood of patients with AMD or age-matched control subjects by gradient centrifugation. The monocytes were then analyzed for either TNF-alpha release from cultured macrophages in response to retinal pigment epithelium-derived blebs and cytokines or TNF-alpha messenger RNA content by reverse transcriptase-polymerase chain reaction.

RESULTS

In human monocytes obtained from controls and AMD patients, TNF-alpha was expressed by freshly isolated monocytes and produced by macrophages in culture after stimulation with retinal pigment epithelium-derived blebs. However, wide variability in TNF-alpha expression was observed among different patients. Patients with monocytes that expressed the greatest amount of TNF-alpha demonstrated higher prevalence of choroidal neovascularization.

CONCLUSIONS

Both controls and AMD patients vary in the activation state (defined as TNF-alpha expression) of circulating monocytes. Partially active monocytes, defined as high TNF-alpha expression, may be a biomarker to identify patients at risk for formation of choroidal neovascularization.

CLINICAL RELEVANCE

Early diagnostic testing may prove useful to detect those patients who will progress to the more severe complications of the disease.

Nicotine increases size and severity of experimental choroidal neovascularization

PURPOSE

Cigarette smoking is the strongest environmental risk factor for all forms of age-related macular degeneration (AMD). In the present study, the influence of nicotine on the severity of choroidal neovascularization (CNV) in a mouse model of neovascular AMD and its effects on vascular smooth muscle cells derived from mouse choroid were investigated.

METHODS

A mouse model for CNV was used to study the effects of nicotine in young and middle-aged mice. Nicotine was administered orally in the drinking water to achieve serum levels consistent with those of chronic smokers. Hexamethonium, a nonspecific nicotinic receptor antagonist, was injected subconjunctivally to counteract the effects of nicotine. A mouse choroidal vascular smooth muscle cell line was exposed to nicotine, vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), or a combination of one of the factors and nicotine. Cell growth was determined by cell counts, and the activity of matrix metalloproteinase (MMP)-2 and -9 was quantified by gel zymography.

RESULTS

Nicotine administration resulted in increased size and vascularity of CNV, and older mice developed a greater relative increase than younger mice. This effect was blocked by subconjunctival hexamethonium. Choroidal vascular smooth muscle cells demonstrated a statistically significant increase in growth after exposure to a combination of PDGF and nicotine. Nicotine also reversed VEGF-induced suppression of MMP-2 activity.

CONCLUSIONS

Nicotine increases size and severity of experimental CNV in the present mouse model, possibly by potentiating PDGF-mediated upregulation of proliferation of choroidal smooth muscle cells or by other mechanisms. These results suggest that non-neuronal nicotinic receptor activation probably mediates some of the harmful effects of cigarette smoking in wet AMD.

Basal laminar deposit formation in APO B100 transgenic mice: complex interactions between dietary fat, blue light, and vitamin E

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.

Bone Marrow-Derived Progenitor Cells Contribute to Experimental Choroidal Neovascularization

PURPOSE

The pathogenesis of choroidal neovascularization (CNV) is postulated to be driven by angiogenesis, a process in which the cellular components of the new vessel complex are derived from cells resident within an adjacent preexisting capillary. Recently, an alternative paradigm, termed postnatal vasculogenesis, has been shown to contribute to some forms of neovascularization. In vasculogenesis, the cellular components of the new vessel complex are derived from circulating vascular progenitors from bone marrow. In the current study, transplantation of green fluorescent protein (GFP)-labeled bone marrow and laser-induced CNV were combined to examine the contribution of vasculogenesis to the formation of CNV.

METHODS

Ten adult C57BL/6 female mice were used as recipients for bone marrow transplantation. Bone marrow was obtained from three C57BL/6 female mice transgenic for the beta-actin promoter GFP. One month after bone marrow transplantation, CNV was induced in recipient mice by making four separate burns in the choroid of each eye with a red diode laser. Four weeks after CNV was induced, eyes of recipient mice were processed for immunohistochemistry to detect GFP and markers for vascular smooth muscle cells (alpha-smooth muscle actin, desmin, and NG2 chondroitin sulfate proteoglycan), endothelial cells (CD31, BS-1 lectin), or macrophages (F4/80).

RESULTS

GFP-labeled cells represented 17% of the total cell population in the lesion. Many of the GFP-labeled cells were immunoreactive for alpha-smooth muscle actin (39%), desmin, NG2, CD31 (41%), BS-1 lectin, or F4/80. GFP-labeled cells were morphologically indistinguishable from cells normally present in CNV lesions.

CONCLUSIONS

This study is the first to demonstrate that bone marrow-derived progenitor cells are a source of endothelial and smooth musclelike cells in CNV.

Macrophage depletion diminishes lesion size and severity in experimental choroidal neovascularization

PURPOSE

Macrophage recruitment to the choroid has been proposed to contribute to the pathogenesis of choroidal neovascularization (CNV) in AMD. The study was conducted to determine whether treatment with clodronate liposomes (CL(2)MDP-lip), which cause depletion of blood monocytes and lymph node macrophages, diminishes the severity of neovascularization in a mouse model of laser-induced CNV.

METHODS

Laser-induced CNV was performed in female 16-month-old C57BL/6 mice. Macrophages were depleted by use of CL(2)MDP-lip intraperitoneally and subcutaneously 72 and 24 hours before and every 2 to 3 days after laser injury. Control mice received injections of either PBS alone or PBS liposomes. Blood monocyte and choroidal macrophage depletion were documented by flow cytometry and choroidal flatmount preparation analysis, respectively. Two weeks after laser injury, mice were injected intravenously with fluoresceinated dextran. The right eyes were removed and prepared for flatmount analysis of CNV surface area (in relative disc areas or DA), vascularity (relative fluorescence), and cellularity (propidium iodide stain). The mice were then perfused with 10% formaldehyde, and the left eyes were removed for histopathology. The means of the various parameters for four CNV lesions per eye were calculated. Fluorescein angiography was also performed.

RESULTS

Flow cytometry of circulating monocytes and immunohistochemical analysis of choroidal macrophage density confirmed the effective depletion of blood monocytes and choroidal macrophages respectively in CL(2)MDP-lip-treated mice. Compared with the control, flatmount analysis of macrophage depleted mice demonstrated a significant reduction in size of the CNV area (2.8 +/- 0.5 DA vs. 1.4 +/- 0.1 DA; P < 0.043). The treated group also revealed less vascularity (1.6 +/- 0.1 units vs. 1.1 +/- 0.0 units; P < 0.0092) and cellularity of CNV lesions (3.3 +/- 0.6 DA vs. 1.7 +/- 0.1 DA, P < 0.04). Histopathology revealed that, in the macrophage-depleted group, CNV was smaller in diameter (1270 +/- 73 pixels vs. 770 +/- 82 pixels, P < 0.0006) and thickness (120 +/- 7 pixels vs. 96 +/- 7 pixels, P < 0.019).

CONCLUSIONS

Macrophage depletion using CL(2)MDP-lip reduces size, cellularity, and vascularity of CNV. This observation supports the hypothesis that macrophages contribute to the severity of CNV lesions.

Female gender, estrogen loss, and Sub-RPE deposit formation in aged mice

PURPOSE

Estrogen status influences the incidence and severity of many diseases in women. Because women with early menopause appear at risk for worse ARMD, estrogen deficiency may also contribute to the onset or severity of ARMD in women. It has been observed that aged male C57BL/6 mice fed a high-fat diet and briefly exposed to blue-green light exhibit development of significant sub-RPE deposits and mild Bruch's membrane (BrM) thickening. This model was used in an attempt to delineate the role of gender and estrogen status in this model.

METHODS

C57BL/6 male and female mice of 9 or 16 months were fed a high-fat diet for 4.5 months. Several groups of 9-month-old female mice underwent estrogen depletion by ovariectomy, with or without supplementation with exogenous 17beta-estradiol. After 4 weeks of a high-fat diet, the eyes were exposed to seven 5-second doses of nonphototoxic levels of blue-green light over 2 weeks. Three and a half months after cessation of blue light treatment, transmission electron microscopy was performed to assess severity of deposits, BrM changes, and choriocapillaris endothelial morphology. In some mice, gelatin zymography and Western blot analyses were performed on protein extracts of freshly isolated RPE to determine the effect of estrogen on matrix metalloproteinase (MMP)-2 activity in the RPE.

RESULTS

Both male and female 16-month-old mice showed qualitatively similar basal laminar deposit morphology, but the severity of thickness, continuity, and content was significantly greater in female mice. Aged female mice also demonstrated a trend toward more severe endothelial changes and increased BrM thickening compared with age-matched male mice. Ovariectomized middle-aged mice showed more severe deposits than sham-surgery control animals. However, ovariectomized mice that received high-dose estrogen supplementation also showed significant deposits, although they had thinner BrMs than did the estrogen-deficient mice. Loss of RPE MMP-2 activity correlated with deposit severity, with estrogen-deficient mice expressing less MMP-2 than ovary-intact control mice.

CONCLUSIONS

Female gender in aged mice and estrogen deficiency in middle-aged mice appears to increase the severity of sub-RPE deposit formation. Estrogen deficiency may increase susceptibility to formation of sub-RPE deposits by dysregulating turnover of BrM, contributing to collagenous thickening and endothelial changes. Estrogen supplementation at the dosages used in this study does not appear to protect against formation of sub-RPE deposits.

The role of aging, high fat diet and blue light exposure in an experimental mouse model for basal laminar deposit formation

We sought to investigate the role of aging as a susceptibility factor for the capacity of dietary fat intake to increase the development of subretinal deposits. Mice of various ages (2, 9 and 16 months) were fed a normal diet or a diet high in saturated and unsaturated fats for a total four and a half months. Some eyes were also exposed to non-phototoxic levels of blue-green light. The outer retina and choroid were examined by light and transmission electron microscopy, and the characteristics, frequency and severity of subRPE deposits was determined. Aged mice fed normal diets developed only very mild subretinal deposits. However, many eyes of mice aged 9 months or older at the time of initiation of diet developed frequent basal laminar deposits of moderate severity, and only 16 month old mice developed more severe deposits after exposure to blue-green light. Some eyes in this older group also developed endothelial invasion into Bruch's membrane. None of the eyes developed classic drusen or linear deposits. These observations demonstrate that age increases the capacity of dietary fat, especially in the presence of environmental light, to induce subRPE deposits.

Age as an independent risk factor for severity of experimental choroidal neovascularization

PURPOSE

For many vascular diseases, aging appears to be an independent risk factor for severity of vascular complications, and blood vessels of aged individuals often demonstrate exaggerated repair responses to injury. This study was undertaken to determine the influence of aging on the severity of neovascularization in a mouse model of laser-induced choroidal neovascularization (CNV).

METHODS

CNV was induced in young (2-month-old) and aged (16-month-old) C57BL/6 mice by making four separate choroidal burns in each eye with a diode red laser (650 nm). At 1, 2, and 4 weeks, the left eyes were removed for histopathology, and the right eyes were removed for flatmount analysis of CNV surface area, vascularity, and cellularity.

RESULTS

Aged mice demonstrated a much larger area of CNV than did young mice (3.81 +/- 1.28 vs. 1.36 +/- 0.99 disc areas, P < 0.001) at 2 weeks, when the lesions showed maximum growth. Aged mice also demonstrated higher ratios for vascularity and cellularity of the CNV (1.34 +/- 0.06 vs. 1.03 +/- 0.11, P < 0.0001 and 4.06 +/- 1.19 vs. 1.91 +/- 0.81, P < 0.002 at 2 weeks, respectively). Histopathology revealed that CNV in older eyes was larger, thicker, and more cellular than in young eyes.

CONCLUSIONS

In mice, age is associated with more severe CNV, defined as larger surface area, greater vascularity, and greater cellularity. Age-related systemic susceptibility factors, independent of local changes in the retina, may contribute to the greater severity of CNV in older than in younger individuals.