Effects Of Aqp4 Deficiency On Light-induced Retinal Damage

Aquaporins (AQPs) are integral membrane proteins that serve as channels in the transfer of water, and in some cases, small solutes across the membrane. 1992 Agre and associates serendipitously identified a member of the water channel family (CHIP28, subsequently renamed AQP1). So far, there are 13 members of the mammalian aquaporin family (AQP0-AQP12). In the central nervous system, aquaporin-4 (AQP4) is the most abundant isoform , in addition it is localized at spinal cord,eye,lung,kidney and gastrointestinal tract. In the retina, AQP4 is expressed strongly by Müller cells and astrocytes. Müller cells are the most principal glia cell in the vertabrate's retina. They cross the entire thickness of retina, and form a compact"neuron-glial"net with neurons. According the research on Müller cells, it was found that they exert important influence on the development, nutrition and metabolism of retinal neurons. However, Müller cells respond to a wide variety of pathological stimuli and undergo reactive gliosis. This leads to a blockade of the neuroprotective glia–neuronal interactions, which in turn causes a dramatic worsening of the survival conditions of the neurons. Further, reactive Müller cells actively contribute to neuronal cell death, by releasing toxic substances and cell death-mediating cytokines; finally, excessive glial cell proliferation causes retinal damage and detachment, which results in blindness.Light as an environmental factor has been shown to be toxic to rod photoreceptors if the retina is exposed to either high light intensities or to continuous light over a long period of time. In particular, short wavelength light is responsible for solar retinitis, playing a role in the pathogenesis of age-related macular degeneration, and (as a component of the light of ophthalmologic instruments) contributing to the development of macular edema after surgery. In response to various different pathological stimuli, Müller cells show a reactive gliosis. Recent experiments showed that excessive light may cause degenerative alterations both in the outer and inner retina. A disturbance of the Müller cell-mediated potassium and water homeostasis in the retina after exposure to blue light may contribute to the degenerative alterations in the inner retina. Meanwhile the roles of AQP4 in the ocular physical and pathological conditions have gained increased attention. Distinct roles for AQP4 in neural signal transduction and cellular swelling have been proposed. AQP4 deletion in mice was associated with mildly impaired light-induced retinal potentials. AQP4 knockout mice were protected after retinal ischemia. Motivated by this body of indirect evidence, we investigate the hypothesis that AQP4 in retinal Müller cells is involved in the pathophysiology of light-induced retinal damage by using AQP4 knockout mice.PURPOSE: To investigate the influence of AQP4 deletion on light-induced retinal damage and further the underlying mechanisms.METHODS: two-month-old female AQP4^+/+ and AQP4^-/- CD1 mice were divided into 4 groups: Control group, post-light 1 day group (PL1), post-light 3 days group (PL3) and post-light 7 days group (PL7). Each group contained 6 mice. All the groups except the control group were continually exposed to intense white light (16,000±2,000lux) since 8:00am to 20:00pm for 6 days. Morphologic, morphmetric, ERG and GFAP immunofluorescence studies were performed to evaluate the light-induced retinal degeneration in AQP4^+/+ and AQP4^-/- mice.RESULTS: 1) ERG and morphology of retinas in AQP4^+/+ and AQP4^-/- mice were indistinguishable before light exposure. After 6 days light exposure, the ERG of the retina in AQP4^-/- mice was irreversibly impaired. The ERG was mildly reduced at 3 days after light exposure in AQP4^+/+ mice, but returned to the baseline at 7 days after light exposure. The thickness of outer nuclear layers (ONL) and inner nuclear layers (INL) showed rapid degeneration in AQP4^-/- mice, while there was no morphological change in the retina of AQP4^+/+ mice. 2) Before light exposure, Müller cells were glial fibrillary acidic protein (GFAP) negative in both genotypic mice, only the astrocytes resided in the inner limiting membrane. At 3 days after light exposure, the soma and processes of Müller cell were GFAP-positive in AQP4^-/- mice. In addition, the processes showed hypertrophy and extended from the NFL to the INL. Elevated GFAP staining in soma was also noted in AQP4^+/+ mice, but the processes of Müller cell showed no significant change.CONCLUSION: AQP4 knockout resulted in hypersensitivity to the light-induced retinal degeneration, indicating AQP4 be a target for the treatment of retinal degeneration diseases and a new avenue to explore the mechanism of retinal light damage.The major contributions of the present study lie in:AQP4 knockout mice have been used to investigate the effects of AQP4 in light-induced retinal damage, which helps to obtain direct evidence that AQP4 knockout increased sensitivity to retinal light damage via modulating Müller cell gliosis.