| Liver X receptors (LXRs) were members of nuclear receptors family and expressed widely in mammalians. Two main subtypes of LXRs areLXRa (NR1H3) and LXRβ (NR1H2). LXRa was mainly expressed in liver, intestine, pancreas and other organs which were involved inmetabolism, and LXRβ was widely expressed all over the body and abundantlyexpressed in central nervous system and immune system. LXRs controlled cholesterol balanceand lipoprotein remodeling at cellular and whole-body levels, regulated the innate immune response, involved in the lamination of central nervous system and the pathogenesis of neural degenerative disease, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). Neural retina is the embryonic derivative of the forebrain, and LXRβ wasfound abundantly expressed in the retina,while the function ofLXRs in retina still has not been elucidated yet.In the previousstudies,it was demonstrated that loss of LXRβ caused a retraction of vertical processes of radial glial cells (RGCs) which was important for the longitudinal migration of neonatal neurons from the ventricular zone to the superficial cortical layers in a classic "inside-out"manner, and finally led tothe malformation of cerebral cortex. Muller cells are the specialized RGCs in the mammalian retina and have been proved to play vital role in the retinal development. Itshowed that damage ofthe Miiller cells during the early postnatal age mice usually resulted inthe ectopic distribution of photoreceptor cells. In vitro experiment showedthat the retinal progenitor cellsspheres co-cultured with the Miiller cells usually formed a normal retina-likelaminal structure, and the photoreceptors distributedin a regular out nuclear layer, on the other hand, the retinal progenitor cells spheres with no Miiller cells in the co-culture system resulted in "rosettes" structure which was mainly found in retinal diseases, it indicated that Muller cells participated in the development and lamination of retina and they might act as the scaffold and guidethe migration of retina neurons in the manner whatthe RGCs works in the developing brain.During the adult stage, Muller cells are important in maintaining the homeostasis of inner retina,whichare involved in clearance and metabolism of excitatory and inhibitory neurotransmitters, such as the glutamate and the y-aminobutyric acid (GABA). Muller cells are also involved in maintaining the ion and water homeostasis of the inner retinal tissue including the pH, regulating the retinal blood flow and the retinal glucose metabolism. Glutaminesynthetase (GS) is the key enzyme in the "glutamate-glutamine cycle" which is mainly expressed by Muller cells in the retina. The activity and expression of the GS influence the rate of glutamate uptake by Muller cells, the loss of GS in the Muller cellsusually leads to the accumulation of extracellular glutamate and cause an excitatory toxic to the neurons in the retina. Reduced expression of GS impacts electroretinogram (ERG), especially the b-wave amplitude. During retinal degeneration, Muller cells were over reactivated, which led to a malfunction of Muller cells. In some situations, such as ischemia and injuries, Muller cells can dedifferentiate and act as the potential retinal stem cells and differentiate into retinal neurons and glia, and this characteristic of Muller cells coincides with that of RGCs in the brain and the cerebellum. Whether LXRβ can regulate the function of Muller cells in the adult mice retina and affect the visual function still need to be elucidated.LXRs can regulate the inflammatory response in the central nervous system by suppressed activation of microglial cells and astrocytes. Dai et al. found that the dopaminergic neurons in the substantia nigra of LXRβ-/- mice were much more severely affected by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) than those of their wild-type littermates. On the other hand, the activated microglia and astrocytes inthe substantia nigra of LXRβ-/- mice were more than thatof wild-type littermates. In AD transgenic mice models, activation of LXRs by agonists could inhibit the activation of astrocytes and slows down the progress of AD.Thus, LXRs are usefulin protect the neurons in neural degenerative diseases by suppressing the inflammation. Gliosis is a common pathological change in several typesof retinal diseases, such as the proliferative vitreoretinopathy (PVR)and the proliferative diabetic retinopathy (PDR). Whether the activation of LXRs can alleviate the inflammatory response and gliosis in the degenerativeretina has not been clarified. In the mice model of alcohol exposure which the development of brain were affected, the loss of ganglion cells, bipolar cells and enhancement of glial cells activation were also significant in the retina. Therefore, we use this animal model to study the function of LXRs in the injuredretina.Based on those evidences, we promote the hypothesis as follows:LXR(3 mightaffect the function of Muller cells in the mice retina both at the development and at adult stage which lead to the impacts of the structure and visual function. Activation of LXRs by their agonistscan suppress the inflammatory response and protect the neurons from alcohol toxicity in the neonatal mice retina.Our main research results are showed as follows:1. Effects of LXRβon the lamination of early postnatal mice retina.Methods:Hematoxylin and eosin (HE) stain, BrdU marking, and immunohistochemistry. Results:At postnatal 7 days, the thickness of inner nuclear layer (INL) is pretty much thicker than that of wild-type (WT) littermates (p<0.01). The number of BrdU-labeledcell is equally between the LXRβ-/- mice and WT littermates. The numbers of BLBP-positive cells in LXRβ-/-mice retina increase dramaticallycompared with the WT mice, and less BLBP-positive cells are found in outer nuclear layer (ONL) of LXRβ-/- mice.2. Effects of LXRP on the structure and visual function of adult mice.Methods:scotopic-electroretinogram (ERG), immunohistochemistry and western-blot. Results:At 3M,7M and 10M, there are significantdecreasein amplitudes of the scotopic-ERG a-wave and b-wave in LXRβ-/- mice compared with WT littermates (p<0.01), while no obvious changes are found in the a-and b-wave latencies (p>0.05). At 3M and 7M, GS expression in the Muller cell end-feet of LXRβ-/- mice is significantly reduced by evaluating the optic density (p<0.05), and this result is confirmed by western-blot analysis too (p<0.01). The number of PKC-a positive bipolar cells is lower in LXRβ-/- mice retina than in WT littermates retina (p<0.05), and so is the western-blot analysis (p<0.05).There is no significant difference of Rhodopsin expression between LXRβ-/- mice and WT littermates.3. Protective effects of LXRs’agonists in the alcohol-induced retinal injured retina.Methods:Hematoxylin and eosin (HE) stain and immunohistochemistry. Results:At P6, the cell number in the ganglion cell layer (GCL) of alcohol exposed group is lower than the control-group (p<0.01) and the T0901317 (TO)-treated group (p<0.01), loss of cells in the GCL is found at P10 too. And at P6, NeuN-positive neurons in GCL of alcohol exposed group is lower than the control-group (p<0.01) and the TO-treated group (p<0.01), loss of NeuN-positive neurons in the GCL is found at P10 too.The expression of GFAP in the alcohol exposed group is higher than that of the control-group (p<0.01) and the T0-treated group (p<0.05) at P6 and P10. The GS expression and number ofPKC-α positive bipolar cells are decreased in alcohol exposed group compared with that of the control group and T0-treated group at P10.Based on the results, we concluded:1. Loss of LXRβ affects the maturation of Miiller cells in the retina of mice at P7 which leads to an abnormal migration of retinal rod cells and thickened INL.2. Loss LXRβ results in decreased GS expression and loss of PKC-α positive bipolarcell in the retina ofadult mice which finally caused an impact on ERG wave forms.3. Alcohol exposure of neonatal mice resulted inthe enhanced glia activity and reduced NeuN positive cells in the GCL, while the pretreatmentof LXRs agonist T0901317 can protect the ganglion cells in the retina from alcohol toxicity. |
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