Abstract
Purpose:
There is considerable evidence that the initial injury in glaucoma occurs in the optic nerve head (ONH), where activated astrocytes produce mediators that damage axons and trigger inflammatory cell recruitment. We discovered that ONH astrocytes in human and mouse eyes constitutively express high levels of a critical regulator of inflammation called NLRP3, which is part of a multi-protein complex that contains the adapter protein ASC and activates caspase-1 to produce IL-1β and IL-18. We hypothesize that activation of the NLRP3 inflammasome in ONH astrocytes initiates inflammation and axonal damage in glaucoma.
Methods:
NLRP3 expression was assessed by immunofluorescence and PCR of human and mouse ONH. Intracameral microbead injection was used to elevate IOP in WT C57BL/6J mice and different B6 KO mice (NLRP3 KO, ASC KO, IL-1R KO, and IL-18 KO). Controls were uninjected and saline-only injected eyes. IOP was monitored by rebound tonometry. At designated time points, mice were euthanized and the ONH was processed for: (ii) RGC counts (ß-III tubulin), (iii) axon counts (PPD), (iv) immunostaining (GFAP, IL-1β, Iba1), and (v) RT-PCR.
Results:
Normal human and mouse ONH possessed astrocytes with high levels of NLRP3 but no evidence of inflammasome activation. Mice with elevated IOP displayed additional increases in NLRP3 and ASC, plus activated caspase-1. To demonstrate a functional role of the activated inflammasome in glaucoma, microbead-induced elevated IOP was induced in a series of KO mice that targeted inflammasome components (NLRP3 and ASC), or products (IL-1β and IL-18). All WT and KO mice displayed a similar increase in IOP for 21 days as compared with controls. The absence of NLRP3 or ASC resulted in a significant reduction in RGC and axon loss, as compared to WT controls. In addition, loss of IL-1R or IL-18 also produced a significant reduction in RGC and axon loss. The reduced RGC loss in NLRP3 KO mice correlated with a reduced infiltration of Iba1+ cells into the ONH as compared to WT controls.
Conclusions:
The components of the NLRP3 inflammasome are constitutively expressed in normal ONH astrocytes but remain inactive. Elevated IOP triggers activation of the inflammasome and production of IL-1β / IL-18, which are critical mediators of inflammation and necessary for development of glaucoma. Blocking inflammasome activation leads to neuroprotection in the presence of elevated IOP.