Glaucoma-associated polymorphism M98K-OPTN sensitizes retinal cells to protein homeostasis stress through p62-mediated caspase activation

Abstract

M98K polymorphism of OPTN is significantly associated with glaucoma in certain populations. This raises the possibility that M98K-OPTN alone is not sufficient to cause glaucoma, and it may require cooperation with other genetic or environmental factors to induce glaucoma. Loss of vision in glaucoma occurs due to the degeneration of retinal ganglion cells. Here, we have tested the hypothesis that M98K-OPTN may enhance the sensitivity of retinal cells to protein homeostasis stress. For this purpose, we have used M98K-OPTN expressing and wild-type (WT)-OPTN expressing clones of retinal 661W cells. Upon induction of protein homeostasis stress by a proteasome inhibitor MG132 (1–2 μM), M98K-OPTN expressing cells showed reduced survival, and enhanced caspase-8, caspase-9, and caspase-3 activation in comparison with WT-OPTN expressing cells. Compared to WT-OPTN expressing cells, M98K-OPTN expressing cells showed enhanced formation of p62/SQSTM1-positive aggregates and enhanced p62 protein level under conditions of protein homeostasis stress. Knockdown of p62 resulted in reduced caspase-9, caspase-8, and caspase-3 activation in M98K-OPTN expressing cells treated with proteasome inhibitor. Our results suggest that M98K-OPTN modulates protein homeostasis stress-induced signalling that mediates p62-dependent caspase activation, which leads to enhanced sensitivity of M98K-OPTN expressing retinal cells to protein homeostasis stress.