In vivo visualization of dendritic cells, macrophages, and microglial cells responding to laser-induced damage in the fundus of the eye

Neurofibromatosis-1 heterozygosity increases microglia in a spatially and temporally restricted pattern relevant to mouse optic glioma formation and growth

Whereas carcinogenesis requires the acquisition of driver mutations in progenitor cells, tumor growth and progression are heavily influenced by the local microenvironment. Previous studies from our laboratory have used Neurofibromatosis-1 (NF1) genetically engineered mice to characterize the role of stromal cells and signals to optic glioma formation and growth. Previously, we have shown that Nf1+/- microglia in the tumor microenvironment are critical cellular determinants of optic glioma proliferation. To define the role of microglia in tumor formation and maintenance further, we used CD11b-TK mice, in which resident brain microglia (CD11b+, CD68+, Iba1+, CD45low cells) can be ablated at specific times after ganciclovir administration. Ganciclovir-mediated microglia reduction reduced Nf1 optic glioma proliferation during both tumor maintenance and tumor development. We identified the developmental window during which microglia are increased in the Nf1+/- optic nerve and demonstrated that this accumulation reflected delayed microglia dispersion. The increase in microglia in the Nf1+/- optic nerve was associated with reduced expression of the chemokine receptor, CX3CR1, such that reduced Cx3cr1 expression in Cx3cr1-GFP heterozygous knockout mice led to a similar increase in optic nerve microglia. These results establish a critical role for microglia in the development and maintenance of Nf1 optic glioma.

Microglia in the healthy and degenerating retina: insights from novel mouse models

In contrast to the tremendous amount of research data from the central nervous system, relatively little is known about microglial homeostasis in the retina. This may be explained by a strong research bias towards important brain pathologies including Alzheimer's disease, Parkinson's disease, and Multiple Sclerosis. In addition, there are specific technical limitations which hampered the analysis of retinal microglia, including their relatively small number in ocular tissue. The lack of experimental tools also prevented direct visualization and molecular analysis of this specialized neuronal macrophage population. Over the last few years, this situation has changed considerably as more and more retinal disorders have come into focus. Many rare monogenic forms as well as more prevalent complex disorders, in particular the age-related macular degeneration involves innate immune mechanisms. As a consequence, new genetic and experimental mouse models have been developed that mimic various forms of human retinal degeneration. In conjunction with these disease models, novel macrophage/microglia-specific reporter mice were established that allow the monitoring of retinal microglia in situ and in vivo. This review summarizes recent findings from these mouse models and thereby provides an overview of microglial homeostasis in the healthy and degenerating retina. Based on this knowledge, microglia-targeted therapies are envisioned which could delay or attenuate degenerative retinal disease.

Regulation of dynamic behavior of retinal microglia by CX3CR1 signaling

PURPOSE

Microglia in the central nervous system display a marked structural dynamism in their processes in the resting state. This dynamic behavior, which may play a constitutive surveying role in the uninjured neural parenchyma, is also highly responsive to tissue injury. The role of CX3CR1, a chemokine receptor expressed in microglia, in regulating microglia morphology and dynamic behavior in the resting state and after laser-induced focal injury was examined.

METHODS

Time-lapse confocal imaging of retinal explants was used to evaluate the dynamic behavior of retinal microglia labeled with green fluorescent protein (GFP). Transgenic mice in which CX3CR1 signaling was ablated (CX3CR1(GFP/GFP)/CX3CR1(-/-)) and preserved (CX3CR1(+/GFP)/CX3CR1(+/-)) were used.

RESULTS

Retinal microglial density, distribution, cellular morphology, and overall retinal tissue anatomy were not altered in young CX3CR1(-/-) animals. In the absence of CX3CR1, retinal microglia continued to exhibit dynamic motility in their processes. However, rates of process movement were significantly decreased, both under resting conditions and in response to tissue injury. In addition, microglia migration occurring in response to focal laser injury was also significantly slowed in microglia lacking CX3CR1.

CONCLUSIONS

CX3CR1 signaling in retinal microglia, though not absolutely required for the presence of microglial dynamism, plays a role in potentiating the rate of retinal microglial process dynamism and cellular migration. CX3CL1 signaling from retinal neurons and endothelial cells likely modulates dynamic microglia behavior so as to influence the level of microglial surveillance under basal conditions and the rate of dynamic behavior in response to tissue injury.

Cooperative phagocytes: resident microglia and bone marrow immigrants remove dead photoreceptors in retinal lesions

Phagocytosis is essential for the removal of photoreceptor debris following retinal injury. We used two mouse models, mice injected with green fluorescent protein-labeled bone marrow cells or green fluorescent protein-labeled microglia, to study the origin and activation patterns of phagocytic cells after acute blue light-induced retinal lesions. We show that following injury, blood-borne macrophages enter the eye via the optic nerve and ciliary body and soon migrate into the injured retinal area. Resident microglia are also activated rapidly throughout the entire retina and adopt macrophage characteristics only in the injured region. Both blood-borne- and microglia-derived macrophages were involved in the phagocytosis of dead photoreceptors. No obvious breakdown of the blood-retinal barrier was observed. Ccl4, Ccl12, Tgfb1, Csf1, and Tnf were differentially expressed in both the isolated retina and the eyecup of wild-type mice. Debris-laden macrophages appeared to leave the retina into the general circulation, suggesting their potential to become antigen-presenting cells. These experiments provide evidence that both local and immigrant macrophages remove apoptotic photoreceptors and cell debris in the injured retina.

Progress and perspectives on the role of RPE cell inflammatory responses in the development of age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. The etiology of AMD remains poorly understood and no treatment is currently available for the atrophic form of AMD. Atrophic AMD has been proposed to involve abnormalities of the retinal pigment epithelium (RPE), which lies beneath the photoreceptor cells and normally provides critical metabolic support to these light-sensing cells. Cumulative oxidative stress and local inflammation are thought to represent pathological processes involved in the etiology of atrophic AMD. Studies of tissue culture and animal models reveal that oxidative stress-induced injury to the RPE results in a chronic inflammatory response, drusen formation, and RPE atrophy. RPE degeneration in turn causes a progressive degeneration of photoreceptors, leading to the irreversible loss of vision. This review describes some of the potential major molecular and cellular events contributing to RPE death and inflammatory responses. In addition, potential target areas for therapeutic intervention will be discussed and new experimental therapeutic strategies for atrophic AMD will be presented.

Ex vivo dynamic imaging of retinal microglia using time-lapse confocal microscopy

PURPOSE

Retinal microglia have been implicated in the pathogenesis of various retinal diseases, but their basic function and cellular phenotype remain incompletely understood. Here, the authors used a novel ex vivo retinal imaging preparation to examine the behavioral phenotype of living retinal microglia in intact tissue and in response to injury.

METHODS

Fluorescence-labeled microglia in retinal explants from CX3CR1(+/GFP) transgenic mice were observed using time-lapse confocal imaging. High spatial and temporal resolution imaging parameters were used to follow dynamic microglial behavior in real time.

RESULTS

Under normal conditions, resting retinal microglia are not static in structure but instead exhibit extensive structural dynamism in their cellular processes. Process movements are highly random in direction but are balanced to maintain overall cellular symmetry and arbor size. At rest, however, these exuberant process movements do not result in overt cellular migration. After focal laser injury, microglial processes increase significantly in their motility and direct themselves toward the injury site. Microglia rapidly transition their morphologies from symmetric to polarized toward the laser lesion. Microglia also transition from a fixed to a migratory phenotype, translocating through tissue while retaining their ramified morphology.

CONCLUSIONS

Retinal microglia normally occupying uninjured tissue display a continuous, dynamic behavior that suggests functions of tissue surveillance and intercellular communication. Microglial behavior is highly regulated by, and immediately responsive to, focal tissue injury and may constitute a therapeutic cellular response to focal laser photocoagulation. Ex vivo live imaging in the retina is an experimental approach well suited to the study of dynamic aspects of microglial physiology.