The role of Interleukin-33 in retinal degenerative diseases
Müller cells are the principal glial cells in the retina, and they play an essential role in maintaining retinal homeostasis by supporting neuronal and vascular function. In disease settings, dysregulation of Müller cells leading to their prolonged activation is known to be critical for neurovascular pathology, especially in degenerative conditions such as diabetic retinopathy (DR). Interleukin-33 (IL-33) is expressed in the nuclei of Müller cells in the healthy retina. This cytokine has diverse roles in immunity and inflammation, although its role in Müller cell pathophysiology remains largely unknown. This PhD project has investigated the role of endogenous IL-33 to Müller cell function with emphasis on retinal neuronal degeneration and inflammation, using murine models of experimentally induced retinal detachment (RD) and chemical-induced diabetes. Firstly, immunohistochemistry showed that IL-33 was localised to the nuclei of Müller cells in normal retina. In comparison to wild type (WT) control mice, it was shown that genetic deletion of IL-33 (IL-33KO) did not affect the retinal morphology, function or inflammation. Primary Müller cells (PMC) from WT and IL-33KO mice were isolated and characterised. A novel, spontaneously immortalised murine Müller glial cell line QMMuC-1 was also generated. Following RD, in WT mice, the expression of the inflammatory cytokines IL-1β, IL-6, CCL2, TNF∝, TGFβ and IL-33 were significantly increased at day 1 and then reduced gradually and returned to basal level by day 28. In IL-33KO mice, RD induced an exacerbated inflammation with increased CCL2, TNF∝, TGFβ and GFAP expression and sustained immune cells infiltration was exhibited for at least 28 days. Electroretinography (ERG) revealed lower a- and b-wave amplitudes at day 28 post-RD in IL-33KO mice compared to WT RD mice. The numbers of cone photoreceptor cells and their synapses were significantly lower in the retina of IL-33KO mice subjected to experimental RD compared to WT controls. PMCs from IL-33KO mice expressed significantly less inflammatory cytokines including CCL2 and IL-6 compared to WT Müller cells. However, IL-33KO Müller cells failed to upregulate inflammatory cytokines under hypoxia and LPS conditions. Finally, deletion of IL-33 and activation of macrophages caused an upregulation of pro-inflammatory cytokines and exacerbated cell death in retinal explants. In the context of experimental diabetes, there was no difference in the degree of hyperglycaemia between WT and IL-33KO mice, but in the retina the ERG a- and b-wave amplitudes were significantly lower when IL-33 was absent. This was mirrored by significantly thinner retinal layers in IL-33KO mice when compared to WT diabetic controls. Immunohistochemistry revealed severe photoreceptor degeneration and Müller cell activation in the retina of diabetic IL-33KO mice compared to WT mice. Subsequently in IL-33KO mice, diabetes was associated with enhanced retinal inflammation, as indicated by increased IL-1β, IL-6, CCL2, TGFβ, IL-18 expression and infiltration of immune cells, an effect which was sustained after 6 months of diabetes. In parallel in vitro studies, PMCs isolated from WT and IL-33KO mice showed no alteration in inflammatory response following high glucose treatment. In summary, deletion of IL-33 results in more severe photoreceptor degeneration, persistent Müller gliosis and sustained inflammation following induction of RD and diabetes in mice. These results suggest that IL-33 plays a significant protective role as it negatively regulates Müller cell activation and protects photoreceptors from inflammation-mediated damage occurring under stress conditions such as RD and DR. Further study on intracellular and extracellular IL-33 in retinal degenerative disease settings may uncover opportunities to develop novel therapies that can prevent exacerbation of inflammation-linked degenerative pathology in important retinal diseases.
Interleukin-33, IL-33, Diabetic Retinopathy, Diabetes, Cytokines, neurodegeneration, Müller cells, Muller, cellline, QMMuC-1, josy augustine, gliosis, glial activation, microglia, macrophages, photoreceptors
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