Macular degeneration in a community of rhesus monkeys. Ultrastructural observations

Spontaneous fundus lesions in elderly monkeys: An ideal model for age-related macular degeneration and high myopia clinical research

AIMS

Age-related macular degeneration (AMD) and high myopia are frequent causes of progressive visual impairment, so it is critical to identify animal models with resembling human retinal physiology, AMD and high myopia pathological features for therapeutic studies.

MAIN METHODS

We screened elderly cynomolgus monkeys for fundus lesions by slit-lamp biomicroscope combined with fundus pre-set lens and further examined positive cases by color fundus photography (CFP), optical coherence tomography (OCT), fundus fluorescein angiography (FFA), streak retinoscopy, and A-scan ultrasonography.

KEY FINDINGS

Among the 156 animals examined, 10 males and 5 females (30 eyes) exhibited fundus abnormalities (9.6% prevalence). Multi-modal imaging revealed drusen in 20 eyes of 11 animals (prevalence rate of 7.1%), tessellated fundus in 22 eyes of 11 animals, and myopia choroidal neovascularization (CNV) in 4 eyes of 3 animals.

SIGNIFICANCE

Aged cynomolgus monkeys exhibit spontaneous fundus lesions resembling human AMD and high myopia, which could be an ideal model for clinical research.

Innate Immunity in Age-Related Macular Degeneration

Multiple lines of investigation have demonstrated that inflammation plays significant roles in etiology of age-related macular degeneration (AMD). Although interventional trials in AMD therapy targeting inflammatory pathways have been conducted, they have not yet been successful and a detailed understanding as to why some have failed is still elusive. One limitation is the relative dearth of information on how immune cells interact with retinal cells to generate AMD phenotypes at each disease stage. Here, we summarize current research evidence and hypotheses regarding potential pathogenic roles of innate immune cells in the eye, which include resident retinal microglia, macrophages derived from infiltrating systemic monocytes, and macrophages resident in the choroid. We relate recent findings regarding the physiology, function, and cellular interactions involving innate immune cells in the retina and choroid to AMD-related processes, including: (1) drusen formation and regression, (2) the onset and spread of degeneration in late atrophic AMD, and (3) the initiation, growth, and exudation of neovascular vessels in late "wet" AMD. Understanding how innate immune cells contribute to specific AMD phenotypes can assist in generating a comprehensive view on the inflammatory etiology of AMD and aid in identifying anti-inflammatory therapeutic strategies and selecting appropriate clinical outcomes for the planned interventions.

Animal models of age related macular degeneration

Age related macular degeneration (AMD) is the leading cause of vision loss of those over the age of 65 in the industrialized world. The prevalence and need to develop effective treatments for AMD has lead to the development of multiple animal models. AMD is a complex and heterogeneous disease that involves the interaction of both genetic and environmental factors with the unique anatomy of the human macula. Models in mice, rats, rabbits, pigs and non-human primates have recreated many of the histological features of AMD and provided much insight into the underlying pathological mechanisms of this disease. In spite of the large number of models developed, no one model yet recapitulates all of the features of human AMD. However, these models have helped reveal the roles of chronic oxidative damage, inflammation and immune dysregulation, and lipid metabolism in the development of AMD. Models for induced choroidal neovascularization have served as the backbone for testing new therapies. This article will review the diversity of animal models that exist for AMD as well as their strengths and limitations.

Variation of cone photoreceptor packing density with retinal eccentricity and age

PURPOSE

To study the variation of cone photoreceptor packing density across the retina in healthy subjects of different ages.

METHODS

High-resolution adaptive optics scanning laser ophthalmoscope (AOSLO) systems were used to systematically image the retinas of two groups of subjects of different ages. Ten younger subjects (age range, 22-35 years) and 10 older subjects (age range, 50-65 years) were tested. Strips of cone photoreceptors, approximately 12° × 1.8° long were imaged for each of the four primary retinal meridians: superior, inferior, nasal, and temporal. Cone photoreceptors within the strips were counted, and cone photoreceptor packing density was calculated. Statistical analysis (three-way ANOVA) was used to calculate the interaction for cone photoreceptor packing density between age, meridian, and eccentricity.

RESULTS

As expected, cone photoreceptor packing density was higher close to the fovea and decreased with increasing retinal eccentricity from 0.18 to 3.5 mm (∼0.6-12°). Older subjects had approximately 75% of the cone density at 0.18 mm (∼0.6°), and this difference decreased rapidly with eccentricity, with the two groups having similar cone photoreceptor packing densities beyond 0.5 mm retinal eccentricity on average.

CONCLUSIONS

Cone packing density in the living human retina decreases as a function of age within the foveal center with the largest difference being found at our most central measurement site. At all ages, the retina showed meridional difference in cone densities, with cone photoreceptor packing density decreasing faster with increasing eccentricity in the vertical dimensions than in the horizontal dimensions.

Drusenoid maculopathy in rhesus monkeys (Macaca mulatta): effects of age and gender

PURPOSE

To compare drusenoid maculopathy in monkeys with human age-related macular degeneration, and evaluate the influence of age, gender and caloric restriction.

METHODS

Examination by indirect ophthalmoscopy, slit-lamp biomicroscopy and fundus photography, including in some cases fluorescein angiography, was performed on 61 male and 60 female rhesus macaques of ages 10-39 years. Fifty-four of the monkeys were maintained on a calorically restricted diet (approximately 30% lower than control levels) and 67 on an approximately ad libitum diet for 2-19 years, with all other environmental factors held constant. Maculopathies were graded on a 5-point scale and the effects of age, sex, and diet on prevalence and severity were examined. The retinas of six monkeys with macular drusen, 19-28 years old, were examined histologically.

RESULTS

Rhesus monkeys showed a high prevalence (61%) of drusenoid maculopathy. The prevalence and severity of the maculopathy increased with age (p = 0.012). Fully half of all monkeys aged 10-12 years had some detectable degree of drusen. This high prevalence in young adulthood indicates that drusen develop much earlier in rhesus monkeys than in humans, who develop early maculopathy most rapidly at 50-60 years of age, even when correcting for the 3-fold difference in lifespan. No neovascularization or geographic atrophy was found. Females had a higher prevalence and severity than males (p = 0.019). Calorically restricted monkeys had a slightly lower prevalence and severity at 10-12 years than controls, but the difference was not statistically significant. This is an on-going project, and differences between the caloric restricted and ad-lib groups may emerge as the animals age. Some monkeys developed severe maculopathy in their 20s, with others unaffected in their 30s. The histology of drusen resembled those in human retina.

CONCLUSION

Drusenoid maculopathy is common in rhesus monkeys, even in young adult life. Half of the rhesus monkeys examined have drusen at a much younger age than in humans. Severity of maculopathy was greater in female monkeys, a gender difference not consistently found in humans. No differences were detected due to caloric restriction, but a definitive test of this intervention will require a larger sample, longer period of observation, and/or an earlier institution of caloric restriction. Genetic factors are implied because with similar environments, some monkeys are affected at an early age, while older ones are not.

IMPG1 gene variation in rhesus macular drusen

Drusen is a hallmark of human age-related maculopathy. Rhesus macaques (Macaca mulatta) represent a natural model of age-related maculopathy with drusen. We have already mapped the macular drusen susceptibility locus in rhesus macaques to the homolog of human chromosome 6q14-15 and shown that a particular IMPG1 gene SNP haplotype was apparently associated with drusen formation in the rhesus macaques maintained by the Caribbean Primate Research Center (CPRC), Puerto Rico, USA. The aim of the present study was to verify this finding in the macaques kept at the German Primate Research Center (DPZ), Germany. The study group comprised 64 animals (34 affected, 30 unaffected). These monkeys were genotyped for all known variations in the IMPG1 gene and haplotype analysis was performed. A total absence of the previously identified risk haplotype of the IMPG1 gene, and a much lower drusen prevalence in comparison to the CPRC group, was observed in the DPZ samples. This prompted a re-analysis of the original disease association in the CPRC, which revealed that the implied risk haplotype was in fact a sequencing artifact. Taken together, the data highlight that additional factors, other than IMPG1 variation, must play a role in drusen pathogenesis in rhesus macaques.

Maculas, monkeys, models, AMD and aging

Age related macular degeneration (AMD) signs may be found reliably in monkeys (Macaca mulatta) bred selectively in Florida after 14 generations of inbreeding in a closed colony at the University of Puerto Rico. Progression, ultrastructure and functional losses are parallel to those found in humans.

An overview on the toxic morphological changes in the retinal pigment epithelium after systemic compound administration

Many medications that are administered systemically for nonocular conditions may evoke ocular toxicological complications. Therefore, the eye is routinely investigated histopathologically in preclinical in vivo toxicity studies. The retinal pigment epithelium is a likely target for systemically administered compounds, since the underlying choroid is highly vascularized. The specialized pigment epithelium has numerous functions that all maintain the integrity and function of photoreceptors. Consequently, toxic effects on the pigment epithelium will eventually affect the neural retina. The potential of pigment epithelial cells to respond to toxic injury is limited, but a standardized terminology to describe its morphological changes does not exist in the scientific literature. Detailed morphologic analysis, however, might allow early detection of retinotoxicity and may provide evidence on the underlying pathomechanism. We here review toxic effects on the pigment epithelium focusing in particular on the morphology of toxic cell injury. Morphological changes comprise hypertrophy, intracytoplasmic accumulation of cellular components, loss of cell polarity, degeneration, metaplasia, and formation of subretinal membranes. Some of these changes are reversible whereas others are permanent, leading to impaired function of the pigment epithelium and eventually to photoreceptor loss and retinal atrophy.

Mapping of a macular drusen susceptibility locus in rhesus macaques to the homologue of human chromosome 6q14-15

Rhesus macaques (Macaca mulatta) are a natural model for retinal drusen formation. The present study aimed at clarifying whether chromosomal regions homologous to candidate genes for drusen formation and progression in humans are also associated with a drusen phenotype in rhesus macaques. Some 42 genetic markers from seven chromosomal regions implicated in macular degeneration syndromes in humans were tested for whether they identified homologous, polymorphic sequences in rhesus DNA. This was found to be the case for seven markers, all of which were subsequently screened for the presence of potentially disease-predisposing alleles in 52 randomly chosen adult animals from the Cayo Santiago population of rhesus macaques (Caribbean Primate Research Center, PR, USA). The high drusen prevalence expected in the Cayo Santiago colony was confirmed in our sample in that 38 animals were found to have drusen (73%). Logistic regression analysis revealed that some alleles of the rhesus homologue of anonymous human marker D6S1036 were consistently over-represented among affected animals. Of two candidate genes located in the respective region, allelic variation in one (IMPG1) showed strong association with drusen formation. We conclude that one or more genes located at the rhesus homologue of human 6q14-15 are likely to play a role in retinal drusen formation, a finding that represents a first step towards the identification of genetic factors implicated in macular drusen formation in rhesus macaques. This is an important tool for the separation of genetic and environmental factors which must occur before satisfactory management methods can be developed.

Age-related macular degeneration: etiology, pathogenesis, and therapeutic strategies

Age-related macular degeneration is the principal cause of registered legal blindness among those aged over 65 in the United States, western Europe, Australia, and Japan. Despite intensive research, the precise etiology of molecular events that underlie age-related macular degeneration is poorly understood. However, investigations on parallel fronts are addressing this prevalent public health problem. Sophisticated biochemical and biophysical techniques have refined our understanding of the pathobiology of drusen, geographic atrophy, and retinal pigment epithelial detachments. Epidemiological identification of risk factors has facilitated an intelligent search for underlying mechanisms and fueled clinical investigation of behavior modification. Gene searches have not only brought us to the cusp of identifying the culpable gene loci in age-related macular degeneration, but also localized genes responsible for other macular dystrophies. Recent and ongoing investigations, often cued by tumor biology, have revealed an important role for various growth factors, particularly in the neovascular form of the condition. Transgenic and knockout studies have provided important mechanistic insights into the development of choroidal neovascularization, the principal cause of vision loss in age-related macular degeneration. This in turn has culminated in preclinical and clinical trials of directed molecular interventions.

Influence of arterial hypertension and diet-induced atherosclerosis on macular drusen

PURPOSE

To evaluate whether development of macular drusen is influenced by experimentally induced chronic arterial hypertension and atherosclerosis.

METHODS

The prospective experimental study included 93 eyes of 51 elderly rhesus monkeys. The total study group was divided into groups with experimental arterial hypertension ( n=22), diet-induced atherosclerosis ( n=10), or both arterial hypertension and atherosclerosis ( n=29) and a control group without arterial hypertension or atherosclerosis ( n=32). Using color wide-angle fundus photographs taken at the beginning and the end of the study, age-related macular degeneration was graded by counting the number and estimating the mean size of drusen in the macular region.

RESULTS

In the monkeys with arterial hypertension, the count and area of the macular drusen and the change in number and size of macular drusen during the follow-up period were significantly ( P<0.05) higher than in the monkeys of the control group. Correspondingly, the count and size of macular drusen were significantly ( P<0.05) correlated with the duration of arterial hypertension. In contrast, monkeys with only atherosclerosis did not vary significantly ( P>0.10) from monkeys of the control group in number and size of drusen. The macular drusen parameters were statistically ( P>0.20) independent of the duration of atherosclerosis. Similarly, the atherosclerotic monkeys and the monkeys of the control group did not vary significantly ( P>0.30) in the change of the macular drusen parameters during the follow-up period.

CONCLUSIONS

Development of macular drusen as part of age-related macular degeneration in rhesus monkeys may be associated with experimentally induced arterial hypertension. It does not seem to be influenced by diet-induced atherosclerosis.

Neural bases of visual deficits during aging

Visual abilities decline during normal (non-pathological) aging. Many of these visual declines cannot be attributed to optical changes and must therefore be due to changes in the retina or central visual pathways. These include declines in visual acuity and spatial contrast sensitivity (especially under low luminance levels), suprathreshold contrast vision and contrast gain, temporal-frequency contrast sensitivity and resolution, spatial-temporal interactions, hyperacuity, binocular processing, and sensitivity to motion. Certain aspects of these vision deficits and comparisons with neurophysiological and lesion-behavior studies in monkeys suggest hypotheses about the nature and location (e.g. magnocellular vs parvocellular pathways, specific visual structures, and so on) of the neural deficits. Despite the well-documented psychophysical deficits, available anatomical studies in humans and monkeys suggest that aging has only relatively minor effects on the retino-geniculo-striate pathway. Retinal photoreceptor losses are relatively restricted to rods, and there is compensation among the remaining rods for those that are lost. Although some retinal ganglion cells appear to be lost, the loss is small relative to individual-to-individual variability. In addition, there appear to be no massive cell losses in the LGN or striate cortex. Physiological results in the monkey LGN suggest that the functional properties of LGN neurons, and therefore their retinal inputs, are not significantly affected by aging. Retinal pattern-evoked ERG studies in humans likewise suggest that the physiological properties of the retina are little affected by aging. Comparisons between pattern-evoked ERG and cortical evoked potentials in the same individuals suggest that some neural change occurs between the retina and striate cortex, but the location and nature of this change is not known. Thus, we are far from being able to answer the question, What are the neural bases of visual deficits during aging? There are several possible reasons for this: (1) The neurobiological methods that have been brought to bear on the question have been fairly limited. (2) Investigations of neural changes may not have been guided sufficiently by what is known about the psychophysical changes that occur with aging. (3) Existing studies may not have examined the correct locations in the visual system. (4) There is large individual-to-individual variability in the effects of aging and, with the small samples of individuals that typically are available in neural studies of aging, the variability could obscure detection of aging-related changes. Suggestions are offered for ways in which future research can solve these problems.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary open angle glaucomas in the rhesus monkey

Rhesus monkeys from the closed Cayo Santiago colony of the University of Puerto Rico demonstrate elevated (> or = 22 mm Hg) intraocular pressure in a pattern which significantly favours certain maternal lineage groupings. The colony had remained genetically pure since 1938. Of nine matriarchal lineages (matrilines) examined, two had an incidence of ocular hypertension of more than 40% and six of more than 10%. Information on 18 matrilines is currently located in the colony data base which identifies each individual and its vital statistics. In 1990, six animals were moved to the laboratory in Florida. Among those from a low incidence matriline, we found abnormal optic nerve cups, pallor, reduced function of (mainly peripheral) fields, progression and loss of optic nerve axons in the presence of ocular hypertension. In another individual where the cup/disc ratio for the right eye was 0.7 and left eye 0.4 and outflow facility was normal, we excluded all other causes of optic nerve atrophy, and low tension glaucoma was diagnosed. This female was from a matriline with a low incidence of ocular hypertension. Relatively rapid aging (3-4 years/human year) monkeys with ocular hypertension and familial clustering produce a near ideal glaucoma research model.

A primate model for age related macular drusen

A closed colony of semi-free-ranging rhesus monkeys maintained in isolation since 1938 by the Caribbean Primate Research Center (CPRC) is being studied as a model for age related macular drusen. Of examined colony animals 57.7% of the monkeys and 47.3% of their eyes have drusen. The prevalence and severity of drusen are linearly related to increasing age and are significantly higher in specific maternal lineages (matrilines). An electrophysiological estimate indicates loss of function associated with drusen. Prevalence of drusen in CPRC females is almost twice that of males, while the prevalence among CPRC animals in general appears to be several times that of monkeys from continental US facilities. Evidence suggests that the frequency of endstage lesions is also similar to that in human populations. The CPRC matriline monkeys appear to provide the best model yet reported for human age related macular drusen.

Morphological assessment of visual dysfunction

The eye is an isolated unit but with a potentially high degree of sensitivity to toxic substances. The multiplicity of types of reaction to injury reflects the unique anatomical, physiological and biochemical features of the eye. The following are examples of such: The albino rat is not a good model for retinal toxicity because of problems of phototoxic retinopathy, the absence of pigment within the pigment epithelial layer and the high incidence of spontaneous retinal pathologies; The ocular toxicity of a compound cannot be anticipated from its chemical structure; Pharmacological side effects are similar between species, and are predictive for man; Mechanisms of ocular toxicity are poorly understood.