Mechanisms of Photoreceptor Death in Retinitis Pigmentosa

Abstract

Retinitis pigmentosa (RP) is the most common cause of inherited blindness and is characterised by the progressive loss of retinal photoreceptors. However, RP is a highly heterogeneous disease and, while much progress has been made in developing gene replacement and gene editing treatments for RP, it is also necessary to develop treatments that are applicable to all causative mutations. Further understanding of the mechanisms leading to photoreceptor death is essential for the development of these treatments. Recent work has therefore focused on the role of apoptotic and non-apoptotic cell death pathways in RP and the various mechanisms that trigger these pathways in degenerating photoreceptors. In particular, several recent studies have begun to elucidate the role of microglia and innate immune response in the progression of RP. Here, we discuss some of the recent progress in understanding mechanisms of rod and cone photoreceptor death in RP and summarise recent clinical trials targeting these pathways.

Keywords: apoptosis; autophagy; clinical trials; inherited retinal disease; microglia; photoreceptor; regulated necrosis; retinitis pigmentosa.

Figure 1

Cell death mechanisms in Retinitis pigmentosa (RP). (A) Schematic of a rod photoreceptor. (B) Both intrinsic pathways (through detection of cell damage) and extrinsic pathways (through interaction with immune cells and death receptors) can lead to apoptosis. Both pathways lead to the release of CytC from mitochondria and oligomerisation of CytC, APAF1 and pro-caspase-9 to form the apoptosome. Activated caspase-9 cleaves executioner caspases 3 and 7. (C) Photoreceptor death can occur by various forms of regulated necrosis. Necroptosis requires activation of RIP1/RIP3, leading to the activation and oligomerisation of MLKL, which forms pores in the cytoplasmic membrane. Parthanatos can occur following activation of PARP, which can be induced by high cGMP levels, leading to nuclear translocation of AIF. Some RP-causing mutations may lead to elevated Fe²⁺ and/or inhibition of GPX4, causing increased lipid oxidation and death by ferroptosis, although the mechanism is not known. (D) Macroautophagy can be initiated by endoplasmic reticulum (ER) stress via ATG12 activation, direct activation of ATG18/WIP12 and activation of beclin1. Phagophores engulf large cellular components and fuse with lysosomes to form autophagosomes and lysosomal enzymes then degrade the contents of autophagosomes. Proteins also enter lysosomes by hsc-70-assisted chaperone-mediated autophagy via the LAMP1 receptor and by microautophagy through invagination of the lysosomal membrane. (CytC = cytochrome C; APAF1 = apoptotic protease-activating factor 1; RIP = receptor-interacting serine/threonine-protein kinase; MLKL = mixed lineage kinase domain-like pseudokinase; PARP = poly-ADP ribose polymerase; AIF = apoptosis-inducing factor; GPX4 = glutathione peroxidase 4; ATG = autophagy-related protein; LAMP = lysosome-associated membrane glycoprotein).

Figure 1

Cell death mechanisms in Retinitis pigmentosa (RP). (A) Schematic of a rod photoreceptor. (B) Both intrinsic pathways (through detection of cell damage) and extrinsic pathways (through interaction with immune cells and death receptors) can lead to apoptosis. Both pathways lead to the release of CytC from mitochondria and oligomerisation of CytC, APAF1 and pro-caspase-9 to form the apoptosome. Activated caspase-9 cleaves executioner caspases 3 and 7. (C) Photoreceptor death can occur by various forms of regulated necrosis. Necroptosis requires activation of RIP1/RIP3, leading to the activation and oligomerisation of MLKL, which forms pores in the cytoplasmic membrane. Parthanatos can occur following activation of PARP, which can be induced by high cGMP levels, leading to nuclear translocation of AIF. Some RP-causing mutations may lead to elevated Fe²⁺ and/or inhibition of GPX4, causing increased lipid oxidation and death by ferroptosis, although the mechanism is not known. (D) Macroautophagy can be initiated by endoplasmic reticulum (ER) stress via ATG12 activation, direct activation of ATG18/WIP12 and activation of beclin1. Phagophores engulf large cellular components and fuse with lysosomes to form autophagosomes and lysosomal enzymes then degrade the contents of autophagosomes. Proteins also enter lysosomes by hsc-70-assisted chaperone-mediated autophagy via the LAMP1 receptor and by microautophagy through invagination of the lysosomal membrane. (CytC = cytochrome C; APAF1 = apoptotic protease-activating factor 1; RIP = receptor-interacting serine/threonine-protein kinase; MLKL = mixed lineage kinase domain-like pseudokinase; PARP = poly-ADP ribose polymerase; AIF = apoptosis-inducing factor; GPX4 = glutathione peroxidase 4; ATG = autophagy-related protein; LAMP = lysosome-associated membrane glycoprotein).

Figure 2

Pathways leading to rod photoreceptor death in RP. (A) Oxidative stress caused by increased extracellular O₂ or extracellular ROS produced by activated microglia can induce apoptosis via the p38/NFkB pathway. Increased intracellular ROS may also trigger necroptosis or parthanatos. The mTOR pathway may be inhibited in RP through increased PTEN activity, this down-regulates cell survival pathways and increases autophagy. DAMPS released by apoptotic or necroptotic cells cause further activation of microglia. (B) Raised cGMP levels lead to increased Ca²⁺ influx, resulting in calpain activation and death by apoptosis. ER stress resulting from raised Ca²⁺ or accumulation of mis-folded rhodopsin activates PERK, Ire1 and ATF6. While this can lead to protective responses such as translation inhibition or ERAD, sustained ER stress can also trigger apoptosis. ER stress can also lead to increased autophagy and this, in turn, can lead to the degradation of proteasome subunits. Reducing autophagy allows more mis-folded rhodopsin to be degraded by the proteasome, thus reducing ER stress. Calpain activation can inhibit autophagy through lysosomal membrane permeablisation (LMP) and the cleavage of autophagy proteins such as ATG5. (ROS = reactive oxygen species; NFkB = necrosis factor k B; mTOR = mammalian target of rapamycin; ERAD = endoplasmic-reticulum-associated protein degradation; DAMP = damage-associated molecular patterns; PTEN = phosphatase and tensin homolog; PERK = protein kinase RNA-like endoplasmic reticulum kinase).

Figure 3

Microglia have an active role in photoreceptor death in RP. DAMPS released by apoptotic or necrotic photoreceptors trigger the release of inflammatory factors and cytokines from Müller glia (MG). Activated MG increase the expression and secretion of CX3CL3, leading to the activation of resting microglia (M_R). Activated microglia (M_A) secrete complement components including C3, which is cleaved to C3b on damaged or dying photoreceptors, marking these cells for phagocytosis. Stressed/damaged photoreceptors also expose phosphatidylserine (PS) on the outer surface of the cell membrane and this is also recognised by phagocytic receptors on activated microglia. The further release of inflammatory factors from M_A and MG induces apoptotic or necroptotic cell death in nearby photoreceptors. These processes affect rods (yellow) more than cones (orange) in early disease stages.