Endoplasmic reticulum stress and the unfolded protein responses in retinal degeneration

Abstract

The endoplasmic reticulum (ER) is the primary intracellular organelle responsible for protein and lipid biosynthesis, protein folding and trafficking, calcium homeostasis, and several other vital processes in cell physiology. Disturbance in ER function results in ER stress and subsequent activation of the unfolded protein response (UPR). The UPR up-regulates ER chaperones, reduces protein translation, and promotes clearance of cytotoxic misfolded proteins to restore ER homeostasis. If this vital process fails, the cell will be signaled to enter apoptosis, resulting in cell death. Sustained ER stress also can trigger an inflammatory response and exacerbate oxidative stress, both of which contribute synergistically to tissue damage. Studies performed over the past decade have implicated ER stress in a broad range of human diseases, including neurodegenerative diseases, cancer, diabetes, and vascular disorders. Several of these diseases also entail retinal dysfunction and degeneration caused by injury to retinal neurons and/or to the blood vessels that supply retinal cells with nutrients, trophic and homeostatic factors, oxygen, and other essential molecules, as well as serving as a conduit for removal of waste products and potentially toxic substances from the retina. Collectively, such injuries represent the leading cause of blindness world-wide in all age groups. Herein, we summarize recent progress on the study of ER stress and UPR signaling in retinal biology and discuss the molecular mechanisms and the potential clinical applications of targeting ER stress as a new therapeutic approach to prevent and treat neuronal degeneration in the retina.

Keywords: apoptosis; cell death; endoplasmic reticulum stress; inflammation; retinal degeneration; unfolded protein response.

Figure 1. Activation of the UPR during ER stress

Upon accumulation of unfolded or misfolded proteins in the ER lumen, GRP78 dissociates from ER stress transducers including IRE1, PERK and ATF6. Loss of GRP78 binding allows oligomerization and autophosphorylation of IRE1. In addition, unfolded proteins can directly bind to IRE1 resulting in its activation. Activated IRE1 splices XBP1 mRNA through its RNase activity, generating spliced XBP1 encoding a transcription factor that upregulates ER chaperones and UPR genes involved in the ER-associated protein degradation (ERAD). PERK is activated in a similar manner to IRE. Activated PERK phosphorylates eIF2α, leading to a global attenuation of protein synthesis and a concomitant increase in ATF4 translation. ATF4 binds to the UPR response element (UPRE) and induces its target genes such as CHOP. Enhanced eIF2α phosphorylation further increases CHOP protein level by facilitating its translation. CHOP, in turn, suppresses eIF2 phosphorylation by upregulating GADD34 resulting in the recovery of protein synthesis. After the dissociation of GRP78, ATF6 translocates to Golgi apparatus, where it is activated by proteolysis. Activated ATF6 transcriptionally induces ERAD and other UPR target genes.

Figure 2. Role of ER stress and dysregulated UPR signaling in hydroquinone-induced RPE apopotosis

Hydroquinone induces ER stress resulting in activation of the PERK-UPR branch and upregulation of CHOP expression. Inhibition of CHOP by means of chemical chaperones, Ad-PERKDN, and Ad-GADD34ΔN (expressing constitutively active GADD34) that target ER hydroquinone suppresses the adaptive UPR signaling mediated by XBP1 in RPE cells. Loss of XBP1 exacerbates hydroquinone-induced RPE apoptosis through regulation of Bcl-2, caspase12, and CHOP. In contrast, overexpressing XBP1 protects RPE cells and ameliorates hydroquinone-ER stress. Mitochondrial dysfunction-associated cytochrome C release and ER stress-mediated pro-apoptotic events converge on caspase-3 activation ultimately leading to apoptosis and cell death.

Figure 3. Potential mechanisms of XBP1 deficiency-related RPE cell injury

XBP1 is a master regulator of the adaptive UPR in response to ER stress. Activation of XBP1 induces expression of ER-targeted genes including molecular chaperones such as GRP78, p58IPK, ERp29, and proteins involved in ER-associated degradation. Impaired expression and/or oactivation of XBP1 results in reduced ER folding capacity and increased ER stress. In addition, lack of XBP1 can lead to overactivation of IRE1 resulting in enhanced JNK and NF-kB activation and inflammation. As a transcription factor, XBP1 regulates a large number of non-ER-targeted genes. It also interacts with other signaling molecules and modulates vital cellular processes of oxidative stress, autophagy and apoptosis.