Caspases in retinal ganglion cell death and axon regeneration

Retinal ganglion cells (RGC) are terminally differentiated CNS neurons that possess limited endogenous regenerative capacity after injury and thus RGC death causes permanent visual loss. RGC die by caspase-dependent mechanisms, including apoptosis, during development, after ocular injury and in progressive degenerative diseases of the eye and optic nerve, such as glaucoma, anterior ischemic optic neuropathy, diabetic retinopathy and multiple sclerosis. Inhibition of caspases through genetic or pharmacological approaches can arrest the apoptotic cascade and protect a proportion of RGC. Novel findings have also highlighted a pyroptotic role of inflammatory caspases in RGC death. In this review, we discuss the molecular signalling mechanisms of apoptotic and inflammatory caspase responses in RGC specifically, their involvement in RGC degeneration and explore their potential as therapeutic targets.


INTRODUCTION
Retinal ganglion cells (RGCs) in the ganglion cell layer (GCL) of the inner retina form axons of the optic nerve (ON), which partially decussate at the optic chiasm, project in the optic tract and synapse in the lateral geniculate nucleus (LGN) as well as the superior colliculus, pretectal nucleus and hypothalamus. Optic radiations relay visual information from the LGN to the visual cortex. 1 The neural retina is an outgrowth of the central nervous system (CNS); consequently after injury, there is limited endogenous axon regeneration and lost RGCs are not replaced, leading to irreversible visual loss.
Caspases, a family of cysteine aspartate proteases, have roles in neuronal pruning during development, inducing RGC death (through apoptosis and pyroptosis) after trauma and disease and promoting RGC axon regeneration. Such processes are attenuated by endogenous and pharmacological inhibitors as well as gene knockdown using short interfering RNA (siRNA) to both understand signalling mechanisms and develop therapeutics to prevent RGC death and promote axon regeneration.
Here we review caspases in apoptotic and pyroptotic RGC death, the novel role of caspases in RGC axon regeneration and the neuroprotective success of caspase-targeting interventions.
Caspase-8 also acts as a non-enzymatic scaffold in the assembly of a pro-inflammatory 'FADDosome' (caspase-8-FADD-RIPK1) complex, inducing NF-κB-dependent inflammation. 25 Uniquely, caspase-2 can act as both an initiator and an executioner caspase, depending on the apoptotic stimuli and does not fit into either the classically described intrinsic or extrinsic apoptotic pathways ( Figure 2) 26,27 ; its structure resembles that of an initiator caspase due to its caspase recruitment domain but can act as an executioner caspase in response to multiple triggers, including DNA damage, heat shock, endoplasmic reticulum and oxidative stress. [28][29][30][31][32] DNA damage induces PIDDosome formation: a protein complex that consists of adaptor protein RIP-associated ICH-1 homologous protein with a death domain (RAIDD) 33 and p53-induced protein with a death domain (PIDD), 30,34,35 which recruit and activate pro-caspase-2. Caspase-2 can also be activated at the DISC. Caspase-2 can also mediate apoptosis directly from the mitochondrial compartment. 36 Intrinsic pathway  Figure 1. Apoptotic caspases in the canonical intrinsic and extrinsic pathways. Death receptor activation mediates the extrinsic pathway. Fas-R and TRAIL-R recruit FADD 9,10 and pro-caspase-8, 11 forming the DISC, 9,12 leading to proximity-induced caspase-8 activation 11,12 and downstream activation of executioner caspase-3, -6 and -7. 5 Caspase-8 can also activate the intrinsic pathway through truncating BH3interacting domain death agonist (Bid) into tBid, which then promotes Bak and Bax mitochondrial membrane insertion, increasing MOMP and releasing apoptogenic factors, 13 including Apaf-1, Cytochrome C and second mitochondria-derived activator of caspase/direct inhibitor of apoptosis-binding protein with low pI (Smac/DIABLO). 14,15 Cytochrome C, Apaf-1 and pro-caspase-9 form the septameric apoptosome complex, 16,17 which activates caspase-9 and successively downstream executioner caspases. Smac/DIABLO indirectly promotes apoptosis by opposing XIAP inhibition of caspase-3, -7 and -9. 22 Caspase-8 can also form complex I at the TNF receptor, which upregulates the NF-κB survival inflammatory pathway; however, if survival signals are compromised (for example, IAPs) then complex I dissociates from the receptor forming complex IIa, which initiates caspase-8-dependent apoptosis. 19 Caspase-8 inhibits complex IIb formation and necroptosis and caspase-8 inhibition (for example, through z-IETD-fmk) induces complex IIb formation, causing necroptosis. 20 The 'ripoptosome' complex forms after cellular IAPs (cIAPs) or XIAP inhibition, causing caspase-8-dependent apoptosis and necroptosis. 23,24 Inflammatory caspases Inflammatory caspases (-1 or -11 in mice and -1, -4 and -5 in humans) can be activated in the inflammasome protein signalling complex ( Figure 3). 4,37,38 Inflammasomes are large multimeric protein complexes that sense pathogen-and host-derived danger signals and typically comprise of a Nod-like receptor (NLR), adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC) and caspase-1. [37][38][39] The main functions of the inflammasome are to activate caspase-1 to cleave precursor cytokines IL-1β and IL-18 into their mature active forms and induce pyroptosis (a lytic form of cell death). Active caspase-1 also cleaves gasdermin-D into its cytotoxic N-terminal fragment, which forms a plasma membrane pore, releasing pro-inflammatory cytokines. [40][41][42] Inflammasome activation is a two-step process: initial inflammasome priming is required for transcriptional upregulation of machinery including Nod-like-receptor pyrin domain containing 3 (NLRP3) and pro-IL-1β, 37,38 followed by the trigger, such as a pathogen-associated molecular pattern (PAMP) or a damage-associated molecular pattern (DAMP), which induces inflammasome assembly and activation.

CASPASES AND RGC DEATH
Caspase-dependent RGC death occurs after eye and brain injuries, in retinal and optic nerve degenerative disorders 61,62 and during development. 63,64 Common mechanisms of degeneration between different conditions could lead to broadly translatable therapeutics. Caspase involvement in RGC death in animal models, primary cell culture and human postmortem specimens are highlighted in this section. Relative efficacy of neuroprotection is shown for direct caspase inhibitors in Table 1 and upstream indirect inhibitors in Table 2.
Endogenous caspase activity and inhibition in RGC Development. Caspase-dependent apoptosis is important in pruning neuronal, including RGC, numbers after normal developmental overproduction, 63,65 causing an~50% reduction in RGC numbers shortly after cell birth, which can be prevented by broadspectrum caspase inhibitor, Boc-D-fmk. 66,67 Caspase-3 is pivotal in neuronal developmental apoptosis, with active caspase-3 colocalising to terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL)-positive RGC in 2-6-day chick embryos, 67 and caspase-3 inhibition, using z-DEVD-fmk, reducing TUNEL-positive cells by~50% and increasing RGC numbers, axons and GCL thickness. 67 Moreover, BARHL2, a member of the Barh gene family, which suppresses caspase-3 activation, is essential for developmental preservation of normal complement of RGC subtypes. 68 Supporting this, caspase-3 knockout mice express a brainspecific phenotype with excessive neuronal numbers and cellular disorganisation, dying at 1-3 weeks of age. 3,69 Similarly, caspase-9 knockout results in a selective CNS phenotype, characterised by severe brain malformations and high perinatal lethality without gross abnormality of other body parts. 70,71 Caspase-2 (NEDD2) gene expression is elevated during neurogenesis and downregulated in the mature brain and retina. 72,73 However, caspase-2 knockout mice develop normally and lack overt phenotypic abnormalities, with minimal CNS or retinal defects. The role of caspase-2 in RGC neurogenesis is therefore unclear. In more mature mouse retinae, there are no alterations in caspase-3, -6, -7, -8 or -9 expression between 6 and 24 weeks. 74 However, there was a reduction in cIAP-1 suggesting a possible role for caspases at this stage. 74 Induced caspase activity and anti-caspase treatment in RGC Optic neuritis. Multiple sclerosis (MS) is an autoimmune, demyelinating CNS disease and a major cause of non-traumatic disability in young adults. Optic neuritis involves ON inflammation and  Figure 3. Inflammatory caspase-1 is activated within the inflammasome protein complex; 4,37,38 which typically consists of a Nod-like receptor (NLR; such as Nod-like-receptor pyrin domain containing 3 (NLRP3)), adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC) and caspase-1. 37-39 Initial inflammasome priming is required for transcriptional upregulation of inflammasome machinery, such as NLRP3, pro-IL-1β and pro-IL-18. 37,38 A second signal then induces inflammasome assembly and activation. The NLRP3 inflammasome is activated by lysosomal rupture, reactive oxygen species (ROS), oxidised mitochondrial DNA (mtDNA) and cathepsin B. 38,43 Potassium (K + ) efflux is a common NLRP3-activation mechanism, induced by P2X7-mediated pore opening, pore-forming toxins, pannexin-1 or MLKLmediated pore opening. 44 The NLRP3 inflammasome activates caspase-1, which cleaves precursor cytokines IL-1β and IL-18 into their active forms and gasdermin-D into its N-terminal fragment. The N-terminal fragment of gasdermin-D forms a plasma membrane pore facilitating pro-inflammatory cytokines release and inducing pyroptosis. [40][41][42] Gram-negative bacterial lipopolysaccharide (LPS) can activate caspase-11, 40 which also cleaves gasdermin-D cleavage and indirectly activates the NLRP3 inflammasome via pannexin-1. 47 demyelination and is a common presenting feature of MS 75 associated with visual loss. The extent of visual recovery after acute optic neuritis is influenced by demyelination, axonal loss and RGC death. 76 The experimental autoimmune encephalomyelitis (EAE) model is the most common MS animal model induced by myelin oligodendrocyte glycoprotein (MOG) peptide administration causing autoimmunity, inflammation and neurodegeneration. 77,78 In the EAE rat model cleaved caspase-3 immunolocalised to Fluoro-Gold-labelled RGC suggesting that RGC die by apoptosis, 77 though in the EAE mouse model only fulllength caspase-3 immunostaining is present in the GCL. 78 RGC NADH dehydrogenase (mitochondrial electron transport chain) overexpression suppresses RGC death, rescuing 88% of RGC and reducing cleaved caspase-3 immunostaining in Thy1-labelled RGC. 79 Treatment with erythropoietin (EPO) reduces RGC death and active caspase-3 levels, supporting a critical role for caspase-3. 80 Various regulators upstream of caspase-3 are also neuroprotective (Table 2).
In a refined mouse model of MS, the MOGTCR × Thy1CFP mouse, which develops optic neuritis only, either spontaneously or following induction with Bordetella pertussis toxin, 81 RGC express active caspase-2 and intravitreal injection of a modified siRNA against caspase-2 (siCASP2) protects~80% of RGC against apoptosis and axonal degeneration, 81 suggesting a critical role for caspase-2 in RGC apoptosis after optic neuritis.
Traumatic optic neuropathy. Traumatic optic neuropathy (TON) is a major cause of visual loss after brain and eye injury. TON can be either directwhen the ON is crushed or severedor more commonly indirect, when brain or ocular injury causes secondary RGC death or ON injury. Spontaneous recovery occurs in a minority of patients. 82 However, the most common outcome is permanent blindness, and at present, there is no treatment that improves outcome. 83,84 Direct TON can be caused by penetrating injury, such as craniofacial fractures, or direct compression from orbital haemorrhage. 85 ON transection (ONT) and ON crush (ONC) in animal models can be used to study degenerative mechanisms and evaluate neuroprotective and regenerative therapies. 86,87 RGC death after ON injury is progressive and the severity is dependent upon type of lesion and distance from the eye. 88,89 After direct TON, RGC begin to degenerate 5 days after axotomy, 90 and 90% die between 7 and 14 days 86 107 and -1, 108 have all been detected in RGC after crush or axotomy, highlighting the crucial role played by caspases in axotomy-induced RGC death.

Role of caspases in RGCs CN Thomas et al
In addition, combined caspase-8 and -9 inhibition provides additive survival benefits compared with single inhibition, 90,97,102 which may suggest either that both intrinsic and extrinsic apoptotic pathways are activated following direct optic nerve injury or that there are increased off-target effects. Inhibition of caspase-8 can also promote caspase-independent RGC death, such as necroptosis. 20 Recent studies have indicated a pivotal role of caspase-2 in apoptotic RGC injury. 91,95,96,116,117 After ON axotomy and crush, active caspase-2 is exclusively localised to RGC, and its inhibition using siRNA provides significant neuroprotection. 91,95,96 For example, intravitreal administration of either siCASP2 91 or the pharmacological inhibitor z-VDVAD-fmk 95 protect 98% and 60% of RGC, respectively, for up to 30 days and 495% of RGC are protected from death for 12 weeks if siCASP2 is injected every 8 days. 116 Pharmacological inhibition with z-VDVAD-fmk also inhibits caspase-3 and -7, 59 though activation of these caspases was not affected. The siCASP2 is being developed by Quark Pharmaceuticals Inc. and is currently in Phase III clinical trials for ischaemic optic neuropathy and glaucoma. 116 NLRP3-induced neuroinflammation promotes RGC death after partial ONC. 108 NLRP3 expression is upregulated in retinal microglia and NLRP3 inflammasome activation upregulates retinal cleaved caspase-1 and IL-1β, which is prevented in NLRP3 knockout mice, in which RGC are protected against axotomyinduced RGC death. 108 The P2X7 ionotropic ATP-gated receptors are implicated in RGC degeneration; P2X7-mediated potassium efflux induces NLRP3 inflammasome formation and caspase-1 activation. 44 P2X7 receptor-deficient mice displayed delayed RGC loss and reduced phagocytic microglia at early time points after RGC axotomy. 118 Intravitreal administration of a selective PX27 receptor antagonist A438079 delayed RGC death, suggesting P2X7 receptor antagonism as a potential therapeutic strategy. 118 Caspase-11 expression is also upregulated in RGC after ONC and ONT. 107 Primary ocular blast injury. Although direct ON injury results in rapid RGC degeneration, indirect blast-induced TON is delayed and progressive. After explosive blast, the sonic blast-wave causes primary blast injury (PBI), which can cause indirect TON. 119,120 Secondary blast injury causes direct and indirect TON, when explosively propelled fragments impact the eye, head and ON. Blast injury represents a significant threat to military personnel in modern warfare causing visual loss. 121,122 Multiple studies have demonstrated increased cleaved caspase-3 in the GCL and ON between 3 and 72 h after whole animal 123,124 and direct local ocular blast exposures. 125 Moreover, caspase-3 activation displays a cumulative effect after multiple exposures, 124 which is comparable to repeated exposure in combat, potentially leading to worse structural and functional visual outcomes. 126 Additionally, an alternative model using trinitrotoluene (TNT) explosives detected active caspase-3 exclusively in photoreceptors and not RGC. 127 Other apoptotic markers, such as Bax, Bcl-xL and Cytochrome C are also elevated in the retina up to 24 h after blast injury. 125 DBA/ 2J mice lack ocular regulatory mechanism of immune privilege in the anterior chamber, 128 and are thus used as a closed globe injury model to approximate features of open globe injury, without complications of infection. 129 In this model, full-length inflammatory caspase-1 is immunolocalised to the inner nuclear layer (INL) and GCL in control retinae, but immunostaining declines after blast injury, 129 suggesting caspase-1 cleavage. However, necroptotic markers RIPK1 and RIPK3 have increased retinal expression, with RIPK1 localised to outer nuclear layer (ONL), INL and Müller glia and RIPK3 in the ONL, INL and GCL 3 and 28 days post-ocular PBI. 130 These findings suggest potential activation of necroptotic or pyroptotic death pathways.
Although caspase activation immediately follows blast injury, RGC death does not occur until later time points, 130 with retinal nerve fibre layer (RNFL) thickness unchanged for 3 months postblast. 131,132 Axonal degeneration at 28 days after ON demyelination 130 suggests that, as in direct TON, ON degeneration may precede RGC death. 133 Research into blast-induced RGC degeneration is in its infancy. However, roles for apoptotic and potentially inflammatory caspases in RGC death are apparent.
Ischaemic RGC death. Retinal ischaemia is a common cause of visual impairment and sight loss 147 and can be experimentally induced by clamping or ligation of the ophthalmic artery, raising intraocular pressure (IOP) or bilateral common carotid artery occlusion. [148][149][150][151] The degree of RGC loss after ischaemic injury is dependent upon the length of ischaemic interval and is progressive. For example, after 45 min of ligation, ischaemia induces~50% of RGC to degenerate over a 2-week period, whereas 120 min induces death of 99% over 3 months. 151 Ischaemic RGC degeneration is caspase dependent, evidenced by neuroprotection with broad-spectrum caspase inhibitors (Q-VD-OPH and Boc-aspartyl-fmk). 62 In Thy1-positive RGC, full-length caspase-2 expression is increased 1, 152 6,153,154 24 152,154 and 72 h 152 after ischaemia and antisense oligonucleotide inhibitor of caspase-2 (antisense Nedd-2 oligonucleotide 5′-QGCTCG GCGCCGCCATTTCCAGL-3′) protected inner retinal thickness at 7 days. 152 Brain-derived neurotrophic factor (BDNF) is also RGC neuroprotective and reduced caspase-2 expression. 153 Full-length caspase-3 immunolocalised to the GCL 4 h after injury 155 and preinjury intravitreal siRNA caspase-3 injection was RGC neuroprotective, 156 though other studies have found full-length caspase-3 to be exclusively in the INL and ONL. 152 Valproic acid, a broad-spectrum histone deacetylase inhibitor, protects RGC after ischaemic reperfusion (I/R) injury caused by raised IOP, 113,114,157 reducing cleaved caspase-3 and -12 expression. 114,157 Pannexin-1 is a mammalian cell membrane channel-forming protein that acts as a diffusional pathway for ions and small molecules. Pannexin-1 facilitates neurotoxicity in the ischaemic brain and retinal pannexin-1 gene knockout suppresses inflammasome-mediated caspase-1 activation and IL-1β production 3 h after ischaemic injury and reduces RGC degeneration at 14 days. 158 Administration of YVAD-fmk (caspase-1, -4 and -5) protects inner retinal morphology in some, but not all, studies, 152,154,155 leaving the role of caspase-1 in question. P2X receptor stimulation induces ATP influx, potassium ion efflux and downstream NLRP3 inflammasome and caspase-1 activation. 37,38 During stimulated ischaemia (oxygen/glucose deprivation) of human organotypic retinal cultures, P2X receptor stimulation causes RGC death, suggesting possible involvement of NLRP3 inflammasome and caspase-1. 159 RGC axon degeneration after central retinal artery occlusion is mediated by the mitochondrial intrinsic apoptotic pathway 160 cytosolic Bax, a pro-apoptotic Bcl-2 family member, levels are decreased at 3 and 6 h post injury, whereas mitochondrial Bax levels are elevated at 3, 6 and 24 h, suggesting that Bax translocates to the mitochondria. 160 In addition, cytosolic Cytochrome C levels are elevated at 3 h post injury but not at 6 and 24 h, and cleaved caspase-9 levels are elevated at 3 h. 160 RGC are protected by intravitreal caspase-6 and -8 inhibitors (z-VEID-fmk and z-IETD-fmk) and siRNA against caspase-6 and -8 (siCASP6 and siCASP8) after I/R injury. 161 Two different siRNA were used for each caspase making off-target effects unlikely. Caspase-6 inhibition may act indirectly by increasing retinal glial CNTF production. 96 Two weeks after ischaemia, z-VEID-fmk (caspase-6, but also -3 and -7) and z-IETD-fmk (caspase-8 but also -3, -6, and -10) protect only a small proportion of RGC, whereas both siCASP8 and siCASP6 administration elevate RGC survival by~60%. 161 This suggests that small peptide inhibitors are less effective, as they act as a competitive inhibitor for the caspase substrates, whereas siRNA gene knockdown reduces caspase gene expression and could affect non-apoptotic caspase roles, such as caspase-8 in complex IIb, 'FADDosome', 'ripoptosome' and inflammasome formation. 20 Glaucoma. Glaucoma is a complex, multifactorial disease affecting 460 million people worldwide 162 and is associated with raised IOP causing RGC death. Genetic background 163 and age 164 are also associated with disease development. Glaucoma is currently treated by IOP control; however, there is an unmet clinical need for a neuroprotective treatment.

CASPASES AND RGC AXON REGENERATION
In addition to promoting RGC survival, caspases promote RGC axon regeneration after ON injury. Pharmacological inhibition of caspase-6 and -8, using z-VEID-fmk and z-IETD-fmk, provide RGC neuroprotection and promote limited RGC axon regeneration, 61 with few axons extending 41000 μm beyond the lesion site. Similarly, few RGC axons regenerated through the lesion site with inhibition of caspase-6 by a dominant negative (CASP6 DN) 96 ; however, combined suppression of caspase-2 and -6 using siCASP2 and CASP6 DN promoted significant regeneration, with an average of 195 ± 9 axons growing beyond 1000 μm. 96 Although caspase-6 is localised to RGC and some microglia, the neuroprotective and pro-regenerative effects of caspase-6 inhibition are mediated indirectly by CNTF upregulation in retinal glia and are blocked by suppression of gp130 and the JAK/STAT pathway. 96 These studies reveal a novel non-apoptotic role for caspases and warrants further investigation.

CONCLUSION
Postmitotic CNS neurons, including RGC, do not regenerate their axons after trauma or injury; hence RGC trauma or disease can lead to permanent visual loss. Understanding the signalling pathways in RGC injury is vital for the development of therapeutic interventions, such as pharmacological inhibitors, RNA interference technology or gene therapies. Caspases, a family of cysteine aspartate proteases, mediate RGC death in physiology, such as during development, as well as trauma and disease, and their inhibition can prevent RGC death. Caspase-3 is implicated during RGC developmental pruning, whereas most apoptotic and inflammatory caspases are implicated in trauma and disease, with siRNA knockdown of caspase-2 providing the greatest neuroprotection after axotomy. Non-apoptotic roles of caspases, such as inflammatory pyroptotic death or facilitating formation of necroptotic complexes are also critical in RGC death. Caspases also have a novel role in RGC axon regeneration; in particular, caspase-6 inhibition mediates regeneration indirectly through CNTF upregulation in retinal glia. Understanding the key pathways for caspase-dependant RGC death is fundamental to the development and effective translation of neuroprotective treatments from preclinical studies to clinical practice.