Targeting Caspase-12 to Preserve Vision in Mice With Inherited Retinal Degeneration

Exploring caspase functions in mouse models

Caspases are enzymes with protease activity. Despite being known for more than three decades, caspase investigation still yields surprising and fascinating information. Initially associated with cell death and inflammation, their functions have gradually been revealed to extend beyond, targeting pathways such as cell proliferation, migration, and differentiation. These processes are also associated with disease mechanisms, positioning caspases as potential targets for numerous pathologies including inflammatory, neurological, metabolic, or oncological conditions. While in vitro studies play a crucial role in elucidating molecular pathways, they lack the context of the body's complexity. Therefore, laboratory animals are an indispensable part of successfully understanding and applying caspase networks. This paper aims to summarize and discuss recent knowledge, understanding, and challenges in caspase knock-out mice.

Gene Therapy for Rhodopsin Mutations

Mutations in RHO, the gene for rhodopsin, account for a large fraction of autosomal-dominant retinitis pigmentosa (adRP). Patients fall into two clinical classes, those with early onset, pan retinal photoreceptor degeneration, and those who experience slowly progressive disease. The latter class of patients are candidates for photoreceptor-directed gene therapy, while former may be candidates for delivery of light-responsive proteins to interneurons or retinal ganglion cells. Gene therapy for RHO adRP may be targeted to the mutant gene at the DNA or RNA level, while other therapies preserve the viability of photoreceptors without addressing the underlying mutation. Correcting the RHO gene and replacing the mutant RNA show promise in animal models, while sustaining viable photoreceptors has the potential to delay the loss of central vision and may preserve photoreceptors for gene-directed treatments.

Endoplasmic reticulum stress: New insights into the pathogenesis and treatment of retinal degenerative diseases

Physiological equilibrium in the retina depends on coordinated work between rod and cone photoreceptors and can be compromised by the expression of mutant proteins leading to inherited retinal degeneration (IRD). IRD is a diverse group of retinal dystrophies with multifaceted molecular mechanisms that are not fully understood. In this review, we focus on the contribution of chronically activated unfolded protein response (UPR) to inherited retinal pathogenesis, placing special emphasis on studies employing genetically modified animal models. As constitutively active UPR in degenerating retinas may activate pro-apoptotic programs associated with oxidative stress, pro-inflammatory signaling, dysfunctional autophagy, free cytosolic Ca²⁺ overload, and altered protein synthesis rate in the retina, we focus on the regulatory mechanisms of translational attenuation and approaches to overcoming translational attenuation in degenerating retinas. We also discuss current research on the role of the UPR mediator PERK and its downstream targets in degenerating retinas and highlight the therapeutic benefits of reprogramming PERK signaling in preclinical animal models of IRD. Finally, we describe pharmacological approaches targeting UPR in ocular diseases and consider their potential applications to IRD.

Ultrahigh Resolution Mouse Optical Coherence Tomography to Aid Intraocular Injection in Retinal Gene Therapy Research

HR-SD-OCT is utilized to monitor the progression of photoreceptor degeneration in live mouse models, assess the delivery of therapeutic agents into the subretinal space, and to evaluate toxicity and efficacy in vivo. HR-SD-OCT uses near infrared light (800-880 nm) and has optics specifically designed for the unique optics of the mouse eye with sub-2-micron axial resolution. Transgenic mouse models of outer retinal (photoreceptor) degeneration and controls were imaged to assess the disease progression. Pulled glass microneedles were used to deliver sub retinal injections of adeno-associated virus (AAV) or nanoparticles (NP) via a trans-scleral and trans-choroidal approach. Careful positioning of the needle into the subretinal space was required prior to a calibrated pressure injection, which delivers fluid into the sub retinal space. Real time subretinal surgery was conducted on our retinal imaging system (RIS). HR-SD-OCT demonstrated progressive uniform retinal degeneration due to expression of a toxic mutant human mutant rhodopsin (P347S) (RHO^P347S) transgene in mice. HR-SD-OCT allows rigorous quantification of all the retinal layers. Outer nuclear layer (ONL) thickness and photoreceptor outer segment length (OSL) measurements correlate with photoreceptor vitality, degeneration, or rescue. The RIS delivery system allows real-time visualization of subretinal injections in neonatal (~P10-14) or adult mice, and HR-SD-OCT immediately determines success of delivery and maps areal extent. HR-SD-OCT is a powerful tool that can evaluate the success of subretinal surgery in mice, in addition to measuring vitality of photoreceptors in vivo. HR-SD-OCT can also be used to identify uniform animal cohorts to evaluate the extent of retinal degeneration, toxicity, and therapeutic rescue in preclinical gene therapy research studies.

Translational attenuation and retinal degeneration in mice with an active integrated stress response

An integrated stress response (ISR), identified in several different animal models of inherited retinal degeneration (IRD), is activated following various cellular stresses. The ISR results in the phosphorylation of eIF2α (p-eIF2α) and a consequent halt in protein synthesis. Although generally protective, persistent elevations in p-eIF2α could lead to cell demise. Therefore, we aimed to determine whether ISR activation is associated with diminished translation rates in mice with IRD. Retinal protein extracts from rd16 mice at different time points were analyzed and the retinal levels of protein synthesis were assessed using the SUnSET method. We found that rd16 mice experience persistent ISR activation: p-eIF2α, ATF4, and CHOP were significantly upregulated at P15 and P20. In agreement with ISR activation, we found that rd16 mice experience translational attenuation at P15. Similar to rd16, other IRD models, T17M RHO, and rd10 also demonstrated a decline in protein synthesis, correlating with p-eIF2α elevation. We then assessed the role of PERK and eIF2α in translational attenuation in rd16 using a PERK inhibitor, GSK2606414. We found that while the treatment significantly reduced p-eIF2α, it did not cause a complete recovery in translation. This suggests that eIF2α is not the only or even the primary point of translational control in IRD, and a second node of translational regulation comprising AKT and mTOR should be evaluated. Surprisingly, we found that AKT-mTOR signaling was diminished in rd16 and rd10 retinas, suggesting a potential link between AKT-mTOR and translational inhibition. Therefore, for the first time, this study shows translation attenuation in IRD models, and highlights the potential roles of eIF2α kinases and AKT-mTOR signaling that could grant valuable insight into the potential treatments for IRD.

GARP2 accelerates retinal degeneration in rod cGMP-gated cation channel β-subunit knockout mice

The Cngb1 locus-encoded β-subunit of rod cGMP-gated cation channel and associated glutamic acid rich proteins (GARPs) are required for phototransduction, disk morphogenesis, and rod structural integrity. To probe individual protein structure/function of the GARPs, we have characterized several transgenic mouse lines selectively restoring GARPs on a Cngb1 knockout (X1^−/−) mouse background. Optical coherence tomography (OCT), light and transmission electron microscopy (TEM), and electroretinography (ERG) were used to analyze 6 genotypes including WT at three and ten weeks postnatal. Comparison of aligned histology/OCT images demonstrated that GARP2 accelerates the rate of degeneration. ERG results are consistent with the structural analyses showing the greatest attenuation of function when GARP2 is present. Even 100-fold or more overexpression of GARP1 could not accelerate degeneration as rapidly as GARP2, and when co-expressed GARP1 attenuated the structural and functional deficits elicited by GARP2. These results indicate that the GARPs are not fully interchangeable and thus, likely have separate and distinct functions in the photoreceptor. We also present a uniform murine OCT layer naming nomenclature system that is consistent with human retina layer designations to standardize murine OCT, which will facilitate data evaluation across different laboratories.

TNFa knockdown in the retina promotes cone survival in a mouse model of autosomal dominant retinitis pigmentosa

Expression of T17M rhodopsin (T17M) in rods activates the Unfolded Protein Response (UPR) and leads to the development of autosomal dominant retinitis pigmentosa (adRP). The rod death occurs in adRP retinas prior to cone photoreceptor death, so the mechanism by which cone photoreceptors die remains unclear. Therefore, the goal of the study was to verify whether UPR in rods induces TNFa-mediated signaling to the cones and to determine whether the TNFa deficit could prevent adRP cone cell death. Primary rod photoreceptors and cone-derived 661W cells transfected with siRNA against TNFa were treated with tunicamycin to mimic activation of UPR in T17M retinas expressing normal and reduced TNFa levels. The 661W cells were then exposed to recombinant TNFa to evaluate cell viability. In vivo, the role of TNFa was assessed in T17M TNFa^+/- mice by electroretinography, optical coherence tomography, histology, immunohistochemistry, and a cytokine enzyme-linked immunosorbent assay. Rods overexpressed and secreted TNFa in response to UPR activation. The recombinant TNFa treatment lowered the number of viable cones, inducing cell death through elevation of pro-inflammatory cytokines and caspase-3/7 activity. The TNFa deficiency significantly protected adRP retinas. The photopic ERG amplitudes and the number of surviving cones dramatically increased in T17M TNFa^+/- mice. This neuroprotection was associated with a reduced level of pro-inflammatory cytokines. Our results indicate that rod photoreceptors, following UPR activation during adRP progression, secrete TNFa and signal a self-destructive program to the cones, resulting in their cell death. TNFa therefore holds promise as a therapeutic target for treatment of adRP.

Limited ATF4 Expression in Degenerating Retinas with Ongoing ER Stress Promotes Photoreceptor Survival in a Mouse Model of Autosomal Dominant Retinitis Pigmentosa

T17M rhodopsin expression in rod photoreceptors leads to severe retinal degeneration and is associated with the activation of ER stress related Unfolded Protein Response (UPR) signaling. Here, we show a novel role of a UPR transcription factor, ATF4, in photoreceptor cellular pathology. We demonstrated a pro-death role for ATF4 overexpression during autosomal dominant retinitis pigmentosa (ADRP). Based on our results in ATF4 knockout mice and adeno-associated viral (AAV) delivery of ATF4 to the retina, we validated a novel therapeutic approach targeting ATF4 over the course of retinal degeneration. In T17M rhodopsin retinas, we observed ATF4 overexpression concomitantly with reduction of p62 and elevation of p53 levels. These molecular alterations, together with increased CHOP and caspase-3/7 activity, possibly contributed to the mechanism of photoreceptor cell loss. Conversely, ATF4 knockdown retarded retinal degeneration in 1-month-old T17M Rhodopsin mice and promoted photoreceptor survival, as measured by scotopic and photopic ERGs and photoreceptor nuclei row counts. Similarly, ATF4 knockdown also markedly delayed retinal degeneration in 3-month-old ADRP animals. This delay was accompanied by a dramatic decrease in UPR signaling, the launching of anti-oxidant defense, initiation of autophagy, and improvement of rhodopsin biosynthesis which together perhaps combat the cellular stress associated with T17M rhodopsin. Our data indicate that augmented ATF4 signals during retinal degeneration plays a cytotoxic role by triggering photoreceptor cell death. Future ADRP therapy regulating ATF4 expression can be developed to treat retinal degenerative disorders associated with activated UPR.