Upregulation of the Small Heat Shock Protein B–Crystallin Protects Retinal Function During Destructive Endophthalmitis

Abstract

Purpose: : Staphylococcus aureus (S. aureus) is a leading cause of endophthalmitis that is frequently associated with loss of retinal function. While research has focused on bacterial virulence factors contributing to ocular damage during infection, studies examining host protection of ocular tissue are limited. Therefore, we explored the hypothesis that upregulation of a survival gene in the retina, the small heat shock protein αB–crystallin, inhibits endophthalmitis–induced destruction and preserves retinal function.

Methods and Results: : Previously, we developed a model of endophthalmitis in which C57BL/6 mice received an intravitreal inoculation of 500 or 5000 CFU of S. aureus. In mice receiving 500 CFU, retinal function (as determined by ERG) decreased only transiently, recovering by 72h post infection. By contrast, in mice receiving 5000 CFU, retinal function was lost completely by 24h and never recovered. Here, we demonstrate that mice given an inoculation of 500 CFU display increased expression of αB–crystallin (∼21 kD) in the retina. Unexpectedly, mice injected with 5000 CFU display a cleaved ∼17 kD fragment of αB–crystallin in the retina within 24h. Appearance of the smaller fragment coincides with inflammation, loss of retinal function and increased expression of apoptotic markers in the retina, specifically activation of caspase–3 and cleavage of its downstream target PARP. Further studies demonstrate that αB–crystallin in the retina is cleaved by a metalloprotease released from S. aureus, since S.aureus supernatant cleaves full–length recombinant αB–crystallin in vitro and addition of a metalloprotease–specific inhibitor (EDTA) blocks cleavage.

Conclusions: : Increased expression of αB–crystallin during a S. aureus infection protects retinal tissue. However, S. aureus blocks this protection by releasing a protease that cleaves αB–crystallin, which renders it non–functional and unable to prevent retinal destruction. These data imply that inhibition of S.aureus–derived metalloproteases will prevent cleavage of αB–crystallin, thereby conferring retinal resistance to destructive endophthalmitis.

Keywords: apoptosis/cell death • endophthalmitis • inflammation