Lysosomal destabilization in retinal pigment epithelial cells activates the NLRP3 inflammasome and induces IL-1beta secretion

Age-related macular degeneration (AMD) is a leading cause of visual impairment and blindness, affecting over 30 million people worldwide. It is characterized by the appearance of insoluble deposits known as drusen in the outer retina, between the retinal pigment epithelium (RPE) and Bruch's membrane. Drusen are heterogeneously composed of many compounds, including cholesterol, amyloid-beta, and complement proteins. AMD also involves the accumulation of pigments collectively termed lipofuscin in RPE lysosomes. The underlying causes of AMD are unknown, but studies have implicated inflammatory processes in its pathogenesis.;The NLRP3 inflammasome is a multiprotein complex, consisting of NLRP3, ASC, and caspase-1, that plays an important role in inflammation. Inflammasome assembly activates caspase-1 via proteolysis of its precursor, procaspase-1. Caspase-1 mediates the maturation of inflammatory cytokines such as IL-1beta, as well as a form of cell death called pyroptosis. A myriad of chemically diverse agonists are known to activate the NLRP3 inflammasome, including insoluble particles and crystals, which destabilize phagolysosomes following internalization. As lysosomes are disrupted by the phagocytosis of certain drusen components, such as amyloid-beta, or the detergent-like effects of the lipofuscin constituent A2E, I sought to evaluate the hypothesis that the destabilization of RPE lysosomes activates the NLRP3 inflammasome.;Immunohistochemistry of human outer retinal sections revealed that NLRP3 was expressed in the RPE of AMD-affected eyes, but was not detectable in age-matched control retinas. The NLRP3 inflammasome components NLRP3, ASC, and procaspase-1 were expressed in the human RPE cell line ARPE-19 and in primary human RPE cells. Expression of the IL-1beta precursor, pro-IL-1beta, in RPE cells was induced by treatment with NF-kappaB agonists such as IL-1&agr;. Disruption of RPE lysosomes using the lysosomotropic agent L-leucyl-L-leucine methyl ester induced inflammasome activation, as evidenced by caspase-1 activation, processing and release of IL-1beta, and pyroptotic cell death. These results suggest a mechanism of AMD pathogenesis in which molecular changes associated with AMD lead to lysosomal damage, activating the NLRP3 inflammasome in RPE cells and mediating vision loss by inducing IL-1beta production and cell death. Therefore, the NLRP3 inflammasome may be a key mediator of AMD pathology and a potential target for therapeutic intervention.