Cellular and molecular alterations in neurons and glial cells in inherited retinal degeneration

Molecular mechanisms underlying inherited photoreceptor degeneration as targets for therapeutic intervention

Retinitis pigmentosa (RP) is a form of retinal degeneration characterized by primary degeneration of rod photoreceptors followed by a secondary cone loss that leads to vision impairment and finally blindness. This is a rare disease with mutations in several genes and high genetic heterogeneity. A challenging effort has been the characterization of the molecular mechanisms underlying photoreceptor cell death during the progression of the disease. Some of the cell death pathways have been identified and comprise stress events found in several neurodegenerative diseases such as oxidative stress, inflammation, calcium imbalance and endoplasmic reticulum stress. Other cell death mechanisms appear more relevant to photoreceptor cells, such as high levels of cGMP and metabolic changes. Here we review some of the cell death pathways characterized in the RP mutant retina and discuss preclinical studies of therapeutic approaches targeting the molecular outcomes that lead to photoreceptor cell demise.

Prph2 knock-in mice recapitulate human central areolar choroidal dystrophy retinal degeneration and exhibit aberrant synaptic remodeling and microglial activation

Central areolar choroidal dystrophy is an inherited disorder characterized by progressive choriocapillaris atrophy and retinal degeneration and is usually associated with mutations in the PRPH2 gene. We aimed to generate and characterize a mouse model with the p.Arg195Leu mutation previously described in patients. Heterozygous (Prph2WT/KI) and homozygous (Prph2KI/KI) mice were generated using the CRISPR/Cas9 system to introduce the p.Arg195Leu mutation. Retinal function was assessed by electroretinography and optomotor tests at 1, 3, 6, 9, 12, and 20 months of age. The structural integrity of the retinas was evaluated at the same ages using optical coherence tomography. Immunofluorescence and transmission electron microscopy images of the retina were also analyzed. Genetic sequencing confirmed that both Prph2WT/KI and Prph2KI/KI mice presented the p.Arg195Leu mutation. A progressive loss of retinal function was found in both mutant groups, with significantly reduced visual acuity from 3 months of age in Prph2KI/KI mice and from 6 months of age in Prph2WT/KI mice. Decreased amplitudes in the electroretinography responses were observed from 1 month of age in Prph2KI/KI mice and from 6 months of age in Prph2WT/KI mice. Morphological analysis of the retinas correlated with functional findings, showing a progressive decrease in retinal thickness of mutant mice, with earlier and more severe changes in the homozygous mutant mice. We corroborated the alteration of the outer segment structure, and we found changes in the synaptic connectivity in the outer plexiform layer as well as gliosis and signs of microglial activation. The new Prph2WT/KI and Prph2KI/KI murine models show a pattern of retinal degeneration similar to that described in human patients with central areolar choroidal dystrophy and appear to be good models to study the mechanisms involved in the onset and progression of the disease, as well as to test the efficacy of new therapeutic strategies.

Exacerbated response to oxidative stress in the Retinitis Pigmentosa CerklKD/KO mouse model triggers retinal degeneration pathways upon acute light stress

The retina is particularly vulnerable to genetic and environmental alterations that generate oxidative stress and cause cellular damage in photoreceptors and other retinal neurons, eventually leading to cell death. CERKL (CERamide Kinase-Like) mutations cause Retinitis Pigmentosa and Cone-Rod Dystrophy in humans, two disorders characterized by photoreceptor degeneration and progressive vision loss. CERKL is a resilience gene against oxidative stress, and its overexpression protects cells from oxidative stress-induced apoptosis. Besides, CERKL contributes to stress granule-formation and regulates mitochondrial dynamics in the retina. Using the CerklKD/KO albino mouse model, which recapitulates the human disease, we aimed to study the impact of Cerkl knockdown on stress response and activation of photoreceptor death mechanisms upon light/oxidative stress. After acute light injury, we assessed immediate or late retinal stress response, by combining both omic and non-omic approaches. Our results show that Cerkl knockdown increases ROS levels and causes a basal exacerbated stress state in the retina, through alterations in glutathione metabolism and stress granule production, overall compromising an adequate response to additional oxidative damage. As a consequence, several cell death mechanisms are triggered in CerklKD/KO retinas after acute light stress. Our studies indicate that Cerkl gene is a pivotal player in regulating light-challenged retinal homeostasis and shed light on how mutations in CERKL lead to blindness by dysregulation of the basal oxidative stress response in the retina.

The New Era of Therapeutic Strategies for the Treatment of Retinitis Pigmentosa: A Narrative Review of Pathomolecular Mechanisms for the Development of Cell-Based Therapies

Retinitis pigmentosa, defined more properly as cone-rod dystrophy, is a paradigm of inherited diffuse retinal dystrophies, one of the rare diseases with the highest prevalence in the worldwide population and one of the main causes of low vision in the pediatric and elderly age groups. Advancements in and the understanding of molecular biology and gene-editing technologies have raised interest in laying the foundation for new therapeutic strategies for rare diseases. As a consequence, new possibilities for clinicians and patients are arising due to the feasibility of treating such a devastating disorder, reducing its complications. The scope of this review focuses on the pathomolecular mechanisms underlying RP better to understand the prospects of its treatment using innovative approaches.