Photoreceptor cell death and rescue in retinal detachment and degenerations

Abstract

Photoreceptor cell death is the ultimate cause of vision loss in various retinal disorders, including retinal detachment (RD). Photoreceptor cell death has been thought to occur mainly through apoptosis, which is the most characterized form of programmed cell death. The caspase family of cysteine proteases plays a central role for inducing apoptosis, and in experimental models of RD, dying photoreceptor cells exhibit caspase activation; however, there is a paradox that caspase inhibition alone does not provide a sufficient protection against photoreceptor cell loss, suggesting that other mechanisms of cell death are involved. Recent accumulating evidence demonstrates that non-apoptotic forms of cell death, such as autophagy and necrosis, are also regulated by specific molecular machinery, such as those mediated by autophagy-related proteins and receptor-interacting protein kinases, respectively. Here we summarize the current knowledge of cell death signaling and its roles in photoreceptor cell death after RD and other retinal degenerative diseases. A body of studies indicate that not only apoptotic but also autophagic and necrotic signaling are involved in photoreceptor cell death, and that combined targeting of these pathways may be an effective neuroprotective strategy for retinal diseases associated with photoreceptor cell loss.

Keywords: Apoptosis; Autophagy; Degenerations; Macula; Necrosis; Neuroprotection; Neuroregeneration; Retina.

Figure 1. Caspase signaling

A. The structure of caspase family proteins and components of DISC and apoptosome. DED, Death effector domain; DD, Death domain; KD, Kinase domain; ID, Intermediate domain; RHIM, RIP homotypic interaction motif; p20, p20 subunit; p10, p10 subunit; CARD, Caspase activation and recruit domain; NBD, Nucleotide binding domain; LRR, Leucine rich repeats; PYD, Pyrin domain; NACHT, NAIP (neuronal apoptosis inhibitory protein), CIITA (MHC class II transcription activator), HET-E (incompatibility locus protein from Podospora anserina) and TP1 (telomerase-associated protein) domain. B. The extrinsic pathway is initiated by the biding of death ligands to their cell-surface death receptors. The death domains of these receptors recruit adaptor molecules such as FADD and caspase-8. Multimerization of caspase-8 in the DISC leads to its activation and cleavage of the downstream effector caspases such as caspase-3. On the other hand, the intrinsic pathway is triggered by the release of pro-apoptotic proteins from the mitochondria. Released cytochrome c forms the apoptosome with Apaf-1 and caspase-9. Additionally, released Smac enhances caspase activation through the neutralization of IAP proteins. Of note, there is a cross-talk between the extrinsic and intrinsic pathways. Active caspase-8 cleaves the BH3-only protein Bid, which in turn initiates the release of mitochondrial pro-apoptotic proteins. C. The inflammasome is formed in response to diverse danger signals such as ROS, ATP, amyloid-β and dsRNA. Activation of caspase-1 in the inflammasome leads to the cleavage of pro-ILβ and pro-IL18 into their mature pro-inflammatory forms and promotes their secretion.

Figure 2. AIF signaling

A. The structure of AIF protein. MLS, mitochondrial localization sequence; TMS, transmembrane segment; FAD, flavin adenine dinucleotide-binding domain; NADH, nicotinamide adenine dinucleotide hydride-binding domain; NLS, nuclear localization sequence. B. In healthy state, AIF is located in the mitochondrial inner membrane and modulates the vital mitochondrial function. AIF regulates the complex I activity, and thus is critical for the mitochondrial oxidative phosphorylation and cell survival. C. In contrast, under stressed condition, AIF translocates from the mitochondria into the nucleus, where it functions as a caspase-independent inducer of cell death. The mitochondrial release of AIF proceeds by a two-step process: MOMP and proteolytic cleavage. Released AIF interacts with a number of proteins such as cyclophilin A and binds directly to DNA and RNA in the nucleus, thereby inducing chromatinolysis.

Figure 3. RIP signaling

A. The structure of RIP signaling components. TRADD N, TRADD N-terminal domain; DD, Death domain; RING, really interesting new gene domain; Zn finger, Zinc finger domain; TRAF N, TRAF N-terminal domain; TRAF C, TRAF C-terminal domain; BIR, Baculovirus IAP repeat domain; CARD, Caspase activation and recruit domain; DED, Death effector domain; p20, p20 subunit; p10, p10 subunit; KD, Kinase domain; ID, Intermediate domain; RHIM, RIP homotypic interaction motif. B. In response to TNF-α stimulation, RIP1 is recruited to TNFR and forms a membrane associated complex with TRADD, TRAF2 and cIAPs. cIAPs ubiquitinate RIP1, which in turn mediate NF-κB activation. Nuclear translocation of p65/p50 subunits promotes the production of pro-survival genes such as cIAPs and c-FLIPs as well as deubiquitinating enzymes such as CYLD and A20, which act as a negative feedback loop in NF-κB signaling. C and D. RIP1 switches its function to a regulator of cell death when it is deubiquitinated by CYLD or A20. Deubiquitination of RIP1 abolishes its ability to activate NF-κB, and leads to the formation of cytosolic pro-death complexes. These complexes contain TRADD, FADD, RIP1, caspase-8, c-FLIP and/or RIP3, and mediates either apoptosis or necrosis depending on cellular conditions. Multimerization of caspase-8 in the DISC mediates a conformational change to its active form, thereby inducing apoptosis (C). The catalytic activity of caspase-8-c-FLIP heterodimer complex cleaves and inactivates RIP1 and RIP3. In conditions where caspases/c-FLIPs are inhibited or cannot be activated efficiently, RIP1 forms the necrosome with RIP3, thereby promoting necrosis (D).

Figure 4. TEM findings and proposed mechanisms of photoreceptor cell death after RD

A-C. TEM microphotographs show the outer nuclear layer of the mouse retina 3 days after RD. The eyes were treated with vehicle (A), Z-VAD (B) or Z-VAD plus Nec-1 (C). A: apoptotic cell. N: Necrotic cell. Scale bar, 5 μm. After RD, photoreceptor cell death occurs mainly through apoptosis (A). Caspase inhibition by the pan-caspase inhibitor Z-VAD decreases apoptosis but increases RIP kinase-dependent necrosis (B). Simultaneous blockade of both caspase and RIP kinase pathways provides efficient protection against photoreceptor cell loss (C).

Figure 5. Autophagy signaling

Starvation activates autophagy through inhibition of mTOR, which negatively regulates ULK1 and Class III PI3-kinase. Activation of Class III PI3-kinase in the Beclin-1-interacting complex is a key step in the formation of isolation membrane. Elongation of isolation membrane requires two ubiquitin-like conjugation systems: the Atg-5-Atg12 conjugation system and the LC3 conjugation system. Atg5-Atg12 complex and PE-conjugated LC3 (LC3-II) are recruited to isolation membrane, and facilitate the formation of autophagosome. After fusion with lysosome, the autophagosomal content is degraded in the autolysosome. Autophagy is critical for cell survival by mediating the clearance and recycling of defective proteins and organelles. Nonetheless, autophagy also induces cell death via excessive self-degradation.

Figure 6. Regulation of MOMP

A. The structure of Bcl-2 family proteins. BH, Bcl-2 homology domain; TM, transmembrane domain. B. Bax/Bak are localized in the cytosol in normal conditions. However, on activation by BH3 only proteins, Bax and Bak undergo conformational change, leading to the oligomerization and translocation to the mitochondrial outer membrane. The formation of Bax/Bak channels allow for the release of cytochrome c and other IMS proteins. C. PTPC is a polyprotein complex composed of VDAC, ANT and cyclophilin D. PTPC opening induces an influx of fluid into the matrix, which results in mitochondrial swelling and the rupture of mitochondrial outer membrane, thereby facilitating the non-selective release of mitochondrial proteins.

Figure 7. Inflammation and photoreceptor cell death

Cell death and Inflammation are interconnected. RD induces the production of cytokines and chemokines such as TNF-α and MCP-1, which mediate the activation of macrophages and microglial cells. Activated inflammatory cells infiltrate into the outer nuclear layer of the retina and stimulate photoreceptor cell death. Concomitantly, dying photoreceptor cells release numerous molecules such as ATP and HMGB1. These molecules are known as DAMPs, and amplify inflammatory response through the binding to TLRs and other cell surface receptors on inflammatory cells.