Microglia increase as photoreceptors decrease in the aging avian retina

New Perspectives on Avian Models for Studies of Basic Aging Processes

Avian models have the potential to elucidate basic cellular and molecular mechanisms underlying the slow aging rates and exceptional longevity typical of this group of vertebrates. To date, most studies of avian aging have focused on relatively few of the phenomena now thought to be intrinsic to the aging process, but primarily on responses to oxidative stress and telomere dynamics. But a variety of whole-animal and cell-based approaches to avian aging and stress resistance have been developed-especially the use of primary cell lines and isolated erythrocytes-which permit other processes to be investigated. In this review, we highlight newer studies using these approaches. We also discuss recent research on age-related changes in neural function in birds in the context of sensory changes relevant to homing and navigation, as well as the maintenance of song. More recently, with the advent of "-omic" methodologies, including whole-genome studies, new approaches have gained momentum for investigating the mechanistic basis of aging in birds. Overall, current research suggests that birds exhibit an enhanced resistance to the detrimental effects of oxidative damage and maintain higher than expected levels of cellular function as they age. There is also evidence that genetic signatures associated with cellular defenses, as well as metabolic and immune function, are enhanced in birds but data are still lacking relative to that available from more conventional model organisms. We are optimistic that continued development of avian models in geroscience, especially under controlled laboratory conditions, will provide novel insights into the exceptional longevity of this animal taxon.

Migration and ramification of microglia in quail embryo retina organotypic cultures

Organotypic cultures of retina explants preserve the complex cellular microenvironment of the retina and have been used as a tool to assess the biological functions of some cell types. However, studies to date have shown that microglial cells activate quickly in response to the retina explantation. In this study, microglial cells migrated and ramified in quail embryo retina organotypic cultures (QEROCs) according to chronological patterns bearing a resemblance to those in the retina in situ, despite some differences in cell density and ramification degree. Retinal explants from quail embryos at 9 days of incubation (E9) proved to be the best in vitro system for reproducing a physiological-like behavior of microglial cells when cultured in Eagle's basal medium supplemented with horse serum. During the first week in vitro, microglial cells migrated tangentially in the vitreal part of QEROCs, and some began to migrate radially from 3 days in vitro (div) onward, ramifying in the inner and outer plexiform layers, thus mimicking microglia development in the retina in situ, although reaching a lower degree of ramification after 7 div. From 8 div onward, microglial cells rounded throughout the explant thickness simultaneously with the nonphysiological appearance of dead photoreceptors and round microglia in the outernuclear layer. Therefore, E9 QEROCs can be used during the first week in vitro as a model system for experimental studies of molecules putatively involved in microglial migration and ramification.

Age-dependent accumulation of lipofuscin in perivascular and subretinal microglia in experimental mice

Fundus autofluorescence (AF) imaging by confocal scanning laser ophthalmoscopy has been widely used by ophthalmologists in the diagnosis/monitoring of various retinal disorders. It is believed that fundus AF is derived from lipofuscin in retinal pigment epithelial (RPE) cells; however, direct clinicopathological correlation has not been possible in humans. We examined fundus AF by confocal scanning laser ophthalmoscopy and confocal microscopy in normal C57BL/6 mice of different ages. Increasingly strong AF signals were observed with age in the neuroretina and subretinal/RPE layer by confocal scanning laser ophthalmoscopy. Unlike fundus AF detected in normal human subjects, mouse fundus AF appeared as discrete foci distributed throughout the retina. Most of the AF signals in the neuroretina were distributed around retinal vessels. Confocal microscopy of retinal and choroid/RPE flat mounts demonstrated that most of the AF signals were derived from Iba-1+ perivascular and subretinal microglia. An age-dependent accumulation of Iba-1+ microglia at the subretinal space was observed. Lipofuscin granules were detected in large numbers in subretinal microglia by electron microscopy. The number of AF+ microglia and the amount of AF granules/cell increased with age. AF granules/lipofuscin were also observed in RPE cells in mice older than 12 months, but the number of AF+ RPE cells was very low (1.48 mm(-2) and 5.02 mm(-2) for 12 and 24 months, respectively) compared to the number of AF+ microglial cells (20.63 mm(-2) and 76.36 mm(-2) for 6 and 24 months, respectively). The fluorescence emission fingerprints of AF granules in subretinal microglia were the same as those in RPE cells. Our observation suggests that perivascular and subretinal microglia are the main cells producing lipofuscin in normal aged mouse retina and are responsible for in vivo fundus AF. Microglia may play an important role in retinal aging and age-related retinal diseases.

Activation of immature microglia in response to stab wound in embryonic quail retina

Activation of mature (ramified) microglia in response to injury in the adult central nervous system (CNS) is well documented. However, the response of immature (ameboid) microglia to injury in the developing CNS has received little attention. In this study, a stab wound was made in embryonic quail retinas at incubation days 7 and 9, and the response of retinal microglial cells was analyzed at different times between days 1 and 37 postinjury. The appearance of microglial cells within the wound occurred at the same time as the arrival of the first migrating ameboid microglial cells at an equivalent area in control retinas. Therefore, no specific attraction of microglia toward the wound was observed. Microglial cells in the wound had phenotypic features similar to those of activated microglia in the adult CNS. Thus, their shape was more compact compared with microglial cells outside the wound, expression of the molecule recognized by the QH1 antibody was up-regulated, and their lysosomal compartment was markedly increased. Transitional forms between normal ameboid and activated-like microglial cells were seen at the wound edge, supporting the view that ameboid microglia become activated when they contact the wound during the normal course of their migration in the retina. The microglial reaction was maintained within the wound at 37 days postinjury. In addition to the stab wound, secondary damage areas were found in experimental retinas. Activated cells could still be observed in these areas at 37 days postinjury.

The eye of the laboratory mouse remains anatomically adapted for natural conditions

Evolutionary effects of domestication have been demonstrated for several body systems, including the eye, and for several vertebrate species, including the mouse. Given the importance of the laboratory mouse to vision science, we wished to determine whether the anatomical and histological features of the eyes of laboratory mice are distinct from those of their naturally adapted, wild counterparts. We measured dimensions and masses of whole eyes and lenses from a wild population plus three inbred strains (C57BL/6J, NZB/BINJ, and DBA/1J) of the house house, Mus musculus, as well as wild and outbred laboratory-domesticated stock of the deer mouse, Peromyscus maniculatus. Histological preparations from these eyes were used to determine outer nuclear layer thickness, linear density of the ganglion cell layer, and for indirect immunofluorescence evaluation of cone opsin expression. For all of these traits, no statistically significant differences were found between any laboratory strain and its wild counterpart. The evolutionary effects of domestication of mice therefore do not include changes to the eye in any variable measured, supporting the continued use of this animal as a model for a naturally adapted visual system.

Anatomical and functional evidence for progressive age-related decline in parasympathetic control of choroidal blood flow in pigeons

The choroid receives extensive parasympathetic innervation, which in birds arises largely from the ciliary ganglion (CG). Since age-related changes in parasympathetic regulation of choroidal blood flow (ChBF) could contribute to age-related retinal decline, we used anatomical and functional methods to determine if ChBF control by the CG shows age-related decline in pigeons. The efficacy of the choroidal vasodilatory response to activation of the CG preganglionic input from the medial subdivision of the nucleus of Edinger-Westphal (EWM) was assessed using laser Doppler flowmetry (LDF). The EWM receives bisynaptic retinal input, and electrical stimulation of EWM or light stimulation of the retina in young animals produces dramatic choroidal vasodilation. Transcleral LDF was therefore used to measure both basal ChBF and the increases in ChBF elicited by electrical stimulation of EWM or by retinal illumination in 0.5-18 year old pigeons. Fixed cryostat sections of the eye from 0.5 to 22 year old pigeons were immunolabeled for the 3A10 neurofilament-associated antigen to determine if intrachoroidal nerve fibers arising from CG exhibited age-related loss. We focused on superior choroid, since it is the primary target for CG nerve fibers. There was a marked age-related loss in the ChBF vasodilatory response elicited by either EWM stimulation or retinal illumination, as was also true for basal ChBF. A progressive decrease in choroidal nerve fibers of CG origin, to 17% of youthful abundance by 22 years of age, was also observed. The evoked ChBF increase, and basal ChBF, achieved 50% of their age-related decline between the ages of 3 and 4 years, while half the loss in CG innervation of choroid was later, occurring by 10 years. Age-related loss of choroidal nerve fibers occurs in parallel with but more slowly than the reduction in basal ChBF and the choroidal vasodilation that can be elicited via natural (light) or electrical activation of the central neural input to CG choroidal neurons. The prominent age-related decline in parasympathetic control of ChBF early in the pigeon life span could contribute to the age-related retinal decline observed in pigeons.

Long term dietary supplementation with zeaxanthin reduces photoreceptor death in light-damaged Japanese quail

The purpose of these studies was to evaluate the effects of light damage on Japanese quail whose retinal carotenoids had been experimentally manipulated through altered diets. The birds were raised 6 months on a commercial turkey diet (T), on a custom carotenoid-deficient diet (C-) containing 90% less carotenoid than the T diet, or on Z+ diet [the C- diet supplemented with natural zeaxanthin (35mgkg(-1) food)]. Equal numbers of males and females on each diet were exposed to nine intervals (1hr on, 2hr off) of 3200lux cool white light, then placed in the dark for 14hr before tissue collection. One retina was immediately frozen for HPLC analysis; the other eye was immediately fixed and processed for microscopy. There were no significant differences in the retinal carotenoid concentrations in hatch-mates that were and were not exposed to light. Supplementation resulted in three- to four-fold increases in retinal zeaxanthin and no change in retinal lutein or alpha-tocopherol, but the C- diet did not reduce the retinal carotenoid concentration in C- birds below that in T birds. The light-exposed retinas had significant numbers of apoptotic photoreceptors and photoreceptor ghosts. The number of ghosts was negatively correlated with the number of dying photoreceptors (P<0.05), and with retinal concentrations of zeaxanthin, alpha-tocopherol or gamma-tocopherol (P<0.04, 0.02, 0.04, respectively), but not with lutein. The number of dying photoreceptors was positively correlated with alpha-tocopherol and the sum alpha-tocopherol plus zeaxanthin (P<0.1; P0.04). Photoreceptor death was semi-quantitatively scored, assuming that ghosts were formed by removal of apoptotic photoreceptors with nuclear condensation. Stepwise regression produced a good model (r(2)=0.67;P <0.0001) for predicting death scores from retinal concentrations of zeaxanthin (Standard Coefficient=-0.76) and lutein (Standard Coefficients=+0.43). Absence of lutein in gender-specific analyses suggests lutein served as surrogate marker for gender. Combined analysis of the C- and T birds also demonstrated that dying photoreceptors were negatively correlated with retinal zeaxanthin. These data confirm our previous report that retinal carotenoids prevent photoreceptor cell death, and provide the first direct evidence that retinal zeaxanthin protects photoreceptors from light-induced death.