Oxidative stress and the ubiquitin proteolytic system in age-related macular degeneration

Abstract

AMD is a leading cause of irreversible vision loss in people over 60 years of age. Although the pathogenesis of this disease is multifactorial, clinical studies have revealed that oxidative damage is a significant etiological factor. The ubiquitin proteolytic system (UPS) plays a major cytoprotective role in the retina. It accomplishes this largely by degrading oxidatively-damaged proteins to prevent their toxic accumulation. In this review, we discuss numerous features of the UPS in the retina and propose various ways that components of the UPS can be harnessed for therapeutic intervention in AMD. We discuss published work describing the distribution of various UPS enzymes in different retinal cell types and present new findings describing the localization of the class III ubiquitin conjugating enzymes. These enzymes are functional homologues of a pair of yeast enzymes that mediate the degradation of misfolded and oxidatively-damaged proteins. We also discuss recent work showing that only newly synthesized proteins which have incurred oxidative damage are targeted for degradation by the UPS whereas the turnover of oxidatively-damaged, long-lived proteins is largely unchanged. Additionally, we review recent work describing how polyubiquitylation influences the sorting of damaged proteins into one of two novel intracellular compartments. Finally, we discuss how the UPS modulates the stability and activity of Nrf2, the major anti-oxidant transcription factor in the retina.

Fig. 51.1

Cartoon of the enzyme cascade that attaches Ub to substrates. The types of enzymes that cooperate to transfer Ub to substrates are illustrated. Of note, E3 ligases can be single polypeptides or alternatively, multi-subunit complexes

Fig. 51.2

Differential expression of class III E2s in the mouse eye. Paraffin-embedded sections from 8-month old SvEv129 mice were immunolabeled with the indicated antibodies. The black arrowheads highlight specific labeling in each panel. Serial sections from the same eye were labeled individually with each antibody

Fig. 51.3

The stability and activity of Nrf2 are redox-sensitive. Nrf2 is constitutively degraded by CUL3^Keap1 and the 26S proteasome in the absence of stress but is stabilized in response to oxidative stress. Stabilized Nrf2 induces the expression of a battery of anti-oxidant genes

Fig. 51.4

Nrf2 stability is controlled by cellular redox status and by the UPS. a-Nrf2 western blot demonstrating that endogenous Nrf2 in RPE-1 cells is stabilized by oxidative stress (TBHQ) or by proteasome inhibition (MG132). ETOH and DMSO are the vehicles for TBHQ and MG132, respectively. The asterisk marks a non-specific band

Fig. 51.5

Nrf2 ubiquitylation is mediated by the multi-subunit E3 ligase CUL3^Keap1. Step 1: Ub is activated by the E1 enzyme in an ATP-dependent manner. Step 2: The activated Ub is transferred to an E2 enzyme. Step 3: The Ub-charged E2 is recruited to the ROC1 component of CUL3^Keap1 and directly transfers Ub to Nrf2. Nrf2 is recruited to CUL3^Keap1 by the Keap1 substrate adaptor. The small ball on CUL3 is Nedd8, a Ub-like protein that regulates the activity of CUL3-based E3 ligases