Retinal and optic nerve degeneration in liver X receptor β knockout mice

Abstract

The retina is an extension of the brain. Like the brain, neurodegeneration of the retina occurs with age and is the cause of several retinal diseases including optic neuritis, macular degeneration, and glaucoma. Liver X receptors (LXRs) are expressed in the brain where they play a key role in maintenance of cerebrospinal fluid and the health of dopaminergic neurons. Herein, we report that LXRs are expressed in the retina and optic nerve and that loss of LXRβ, but not LXRα, leads to loss of ganglion cells in the retina. In the retina of LXRβ^-/- mice, there is an increase in amyloid A4 and deposition of beta-amyloid (Aβ) aggregates but no change in the level of apoptosis or autophagy in the ganglion cells and no activation of microglia or astrocytes. However, in the optic nerve there is a loss of aquaporin 4 (AQP4) in astrocytes and an increase in activation of microglia. Since loss of AQP4 and microglial activation in the optic nerve precedes the loss of ganglion cells, and accumulation of Aβ in the retina, the cause of the neuronal loss appears to be optic nerve degeneration. In patients with optic neuritis there are frequently AQP4 autoantibodies which block the function of AQP4. LXRβ^-/- mouse is another model of optic neuritis in which AQP4 antibodies are not detectable, but AQP4 function is lost because of reduction in its expression.

Keywords: aquaporin 4; nuclear receptor; optic neuritis; retinal degeneration; β amyloid.

Fig. 1.

Loss of ganglion cells in LXRβ^−/− mice. The ganglion cells appear normal in both 6-mo-old WT and LXRβ^−/− mice, and the nerve layer (arrow) is visible in both (A–D). In 16-mo-old WT mouse eyes, the ganglion cells are still normal, and the nerve fiber layer is visible (E and F). In 16-mo-old LXRβ^−/− mouse eyes, the number of ganglion cells is markedly reduced (***P < 0.001) as is the nerve fiber layer (G–I). n = 4. (Scale bars: A, C, E, and G, 200 μm; B, D, F, and H, 50 μm.)

Fig. 2.

Expression of LXRα and LXRβ in the retina and optic nerve. In the retina, LXRα is expressed in the ganglion and amacrine cells. There is no expression in the bipolar cells (A and B). LXRβ is expressed in the ganglion cells but its expression in amacrine cells is more limited than LXRα. Some bipolar cells express LXRβ (C and D). Small cells in the optic nerve express both LXRα and LXRβ (E–H). n = 4. (Scale bars: A–H, 50 μm.)

Fig. 3.

Microglia and astrocytes in the retina of LXRβ^−/− mice. There were few microglial (Iba1-positive) cells in the retina of 6-mo-old WT or LXRβ^−/− mice (A and B), and there was no increase in the number of these cells in16-mo-old mice of either genotype (C and D). At either 6 mo (E and F) or 16 mo (G and H) of age, there were few astrocytes in the retina, and there was no detectable difference between WT and LXRβ^−/− mouse retinas. n = 4. (Scale bars: A–H, 20 μm.)

Fig. 4.

Accumulation of Aβ in the retina of LXRβ^−/− mice. There were only occasional Aβ deposits in the retina of 6-mo-old WT or LXRβ^−/− mice (A–D). However, in 16-mo-old mice there was a large amount of Aβ deposition in and around the ganglion cells in the retina of LXRβ^−/− mice but very little in the WT mice at this age. (***P < 0.001) (E–I). n = 4. (Scale bars: A, C, E, and G, 50 μm; B, D, F, and H, 20 μm.)

Fig. 5.

AQP4 in astrocytes in the optic nerve. In 6-mo-old WT mice, GFAP (green) and AQP4 (red) were strongly expressed at the feet and branches of astrocytes in the optic nerve (A–C). AQP4 expression was lower in the optic nerve of 6-mo-old LXRβ^−/− mice (D–F). In 16-mo-old mice, GFAP and AQP4 were still strongly expressed in the optic nerve of WT mice (G–I), but their expression was reduced in the optic nerve of LXRβ^−/− mice (*P < 0.05) (J–M). GFAP (green) was colocalized with AQP4 (red); the colocalized color is orange. The nuclei are counterstained with DAPI (C, F, I, and L). n = 4. (Scale bars: A–L, 50 μm.)

Fig. 6.

Increased activation of microglia in the optic nerve of LXRβ^−/− mice. There were Iba1-positive resting microglial cells in the optic nerve of 6-mo-old WT mice (A and B). The Ibal-1–positive microglial cells in the optic nerve of 6-mo-old LXRβ^−/− mice are ameboid in shape, indicating that they are activated (C and D). In 16-mo-old mice, the number of active Iba1-positive microglial cells was higher in LXRβ^−/− than in WT mice (***P < 0.001) (E–I). n = 4. (Scale bars: A, C, E, and G, 50 μm; B, D, F, and H, 20 μm.)

Fig. 7.

Loss of oligodendrocytes in the optic nerve of LXRβ^−/− mice. There were Oligo2-positive (red) cells and GS-positive (green) oligodendrocyte cells in the optic nerve of 6-mo-old WT mice (A–C). Although there was no difference in the number of Oligo2-positive cells in the optic nerve of 6-mo-old LXRβ^−/− mice, there were fewer GS-positive oligodendrocytes (D–F). In 16-mo-old mice, there were fewer Oligo2-positive and GS-positive oligodendrocytes (***P < 0.001) in the optic nerve of 16-mo-old LXRβ^−/− mice (G–M). n = 4. (Scale bars: A–L, 50 μm.)