Proteomic survey reveals altered energetic patterns and metabolic failure prior to retinal degeneration

Abstract

Inherited mutations that lead to misfolding of the visual pigment rhodopsin (Rho) are a prominent cause of photoreceptor neuron (PN) degeneration and blindness. How Rho proteotoxic stress progressively impairs PN viability remains unknown. To identify the pathways that mediateRhotoxicity in PNs,weperformedacomprehensiveproteomic profiling of retinasfromDrosophila transgenics expressingRh1P37H, the equivalent of mammalian RhoP23H, themostcommonRhomutation linked to blindness inhumans.Profiling of young Rh1P37H retinas revealed acoordinated upregulation of energy-producingpathwaysandattenuation of energy-consumingpathwaysinvolving target of rapamycin(TOR) signaling,whichwasreversed in older retinas at the onset of PNdegeneration.Weprobedthe relevance of these metabolicchangesto PNsurvival by using a combination of pharmacological and genetic approaches. Chronic suppression of TOR signaling, using the inhibitor rapamycin, strongly mitigated PN degeneration, indicating that TOR signaling activation by chronic Rh1P37H proteotoxic stress is deleterious for PNs. Genetic inactivation of the endoplasmic reticulum stress-induced JNK/TRAF1 axis as well as the APAF-1/caspase-9 axis, activated by damaged mitochondria, dramatically suppressed Rh1P37H-induced PN degeneration, identifying the mitochondria as novel mediators of Rh1P37H toxicity. Wethus propose that chronicRh1P37H proteotoxic stress distorts the energetic profile of PNsleading to metabolic imbalance, mitochondrial failure, and PN degeneration and therapies normalizing metabolic function might be used to alleviate Rh1P37H toxicity in the retina. Our study offers a glimpse into the intricate higher order interactions that underlie PN dysfunction and provides a useful resource for identifying other molecular networks that mediate Rho toxicity in PNs. © 2014 the authors.