Role of Lipocalin-2 in iron homeostasis and inflammasome activation in pathogenesis of age-related macular degeneration (AMD)

Abstract

Purpose : Elevated iron levels in the retina are associated with inflammasome activation in wet AMD, but its role in early AMD remains elusive. Lipocalin-2 (LCN-2), an adipokine which regulates iron release from cells through the endo-lysosomal pathway, has been linked to AMD pathogenesis. This study was undertaken to determine if lysosomal dysfunction potentiates alterations in LCN-2-dependent iron homeostasis in RPE cells, leading to inflammasome activation and AMD development.

Methods : Cryba1 cKO mice lack the gene encoding βA3/A1-crystallin, specifically in RPE and have an early AMD-like phenotype with lysosomal dysfunction. RPE cells from cKO and floxed mice (5 and 10 months old), were used to perform RNA seq and western analysis to evaluate the expression of iron-related and oxidative stress genes and proteins in the NLRP3-dependent inflammasome pathway, respectively. Cryosections from human dry AMD and age-matched control retinas were immunostained with NLRP3 and IL-1β. RPE cells from floxed and cKO mice (3 weeks old) were cultured to ascertain the endo-lysosomal regulation of LCN-2-dependent iron homeostasis.

Results : We found differential expression of iron metabolism and oxidative stress-related genes like fth1, ftl1, fxn, sod2, and cat, concomitant with elevated protein expression of NLRP3, active caspase-1 and IL-1β in RPE from aged cKO mice, relative to control mice. Moreover, sections of eyes from human AMD subjects showed increased expression of NLRP3 and IL-1β in the RPE/ choroid compared to age-matched controls. In vitro studies revealed that an altered endo-lysosomal pathway in cKO RPE cells leads to abnormal iron sequestration, thereby triggering inflammasome activation.

Conclusions : It can be concluded from our results that lysosome dysfunction in the RPE of cKO mice triggers an LCN-2-dependent abnormality in iron sequestration/extracellular transport, leading to inflammasome activation and AMD pathogenesis. Therefore, targeting the endo-lysosomal pathway and facilitating LCN-2-dependent iron release from iron laden cells like RPE, could be of therapeutic importance in delaying the progression of AMD.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.