Visual impairment and blindness affects millions of people worldwide each year. And, as the general age of the population increases, rates of vision loss are not getting any better. Despite our knowledge of these visual conditions, there is still a lack of understanding on how these visual problems arise. The question then becomes: How can we better treat them?
With such an important question to be answered, there is a growing need for animal models in research. One particular application for zebrafish has evolved in the study of retinal pigment epithelium (RPE) as it relates to prevalent diseases, such as age-related macular degeneration (AMD). By understanding critical points of pathogenesis, researchers may be one step closer to unlocking new treatment modalities.
Zebrafish as a Model Organism
According to Dr. Jeffrey Gross, PhD, director of research at the Department of Ophthalmology, University of Pittsburgh School of Medicine, zebrafish are an ideal model for studying visual developments. “Zebrafish provide a robust system for modeling human diseases,” said Dr. Gross. “Since cell types are similar and the architecture is the same, they are valuable for understanding mechanisms of disease.”
Zebrafish are a type of freshwater fish belonging to the minnow family. Native to South Asia, zebrafish have become widely used specimens in research and drug development. Compared to mice, the eyes of zebrafish develop at a faster rate with features that more closely resemble those of human eyes.
“Compared to mice which have a rod-rich retina, zebrafish have a cone-rich retina similar to the human retina,” share Dr. Gross. “The zebrafish retinal structure is arranged in the same way as that in humans, so you’re modeling the same process.”
As a useful animal model, zebrafish are easy to care for without incurring high laboratory costs. Because they can produce up to 200 eggs for every mating, zebrafish can provide a large sample size needed for identifying structures of interest and other potential mutant variations. “We can look at 10 to 20 embryos at a time versus a much smaller sample in mice,” said Dr. Gross. “Along with other factors, this makes zebrafish useful for developmental biology, imaging, and genomics.”
Zebrafish utilize vision as a defense mechanism against predators. Their resourceful vision system and 360-view allows them to remain alert to predator shapes while also detecting potential food sources. Additionally, because of their regenerative abilities within their ocular systems, zebrafish have opened many doors for new treatment methods.
Regenerating Zebrafish RPE after Genetic Ablation
As a specialized, pigmented monolayer of cells, the RPE separates the retina from the choroid and acts as a critical component of vision. The dysfunction and degeneration of the RPE is believed to play an important role in the pathology of diseases, notably AMD. In AMD, RPE dysfunction and degeneration can result in the loss of cone photoreceptors and eventually, the loss of vision.
In one of his recent studies1 published this year, Dr. Gross and his team examined the mechanisms of RPE regeneration in zebrafish. They studied this mechanism by developing a transgenic model that implemented ablation of mature RPE. Using this model, they found for the first time, that stimulating endogenous RPE regeneration may provide a possibility for treating RPE degenerative diseases in humans.
“We sought to understand how the RPE regenerates in order to see how regeneration may be stimulated in a human eye,” explained Dr. Gross. “We want to see whether the strategies it employs could be used to keep RPE functioning in diseased eyes to retain vision longer. We could then identify genes, pathways and potential compounds which could help preserve and restore vision.”
Interestingly, compared to other models for RPE regeneration, their zebrafish model more closely resembles the degeneration and loss of visual function seen in late-stage AMD. Other models use non cell-specific injury methods such as debridement or laser photocoagulation. However, the zebrafish model produces RPE and photoreceptor degeneration which may provide a more applicable clinical picture for evaluating RPE regeneration in mammals.
Making Waves for Research in Ocular Disease
Several studies have used zebrafish models to explore the therapeutic potential of new drugs for pathological conditions from cardiovascular disease to infectious diseases. In recent years, studies in ocular diseases have shown an even stronger potential with zebrafish. Similar disease phenotypes have been replicated in zebrafish, mirroring several human vision conditions such as glaucoma, cataracts and diabetic retinopathy.
One study in Human Genetics and Human Mutation journal found a crystalline gene mutation using zebrafish animal models.2 The protein crystalline, is known to influence refractive power by regulating lens and cornea transparency. Mutations of the CRYBA2 and CRYGC gene were ultimately found to be linked to the formation of cataracts in humans.
Other studies, such as the one found in Human Molecular Genetics, examined the genetics of glaucoma. Experts later found a link between a FOXC1 gene mutation and increased intraocular pressure in zebrafish. Anterior segment defects and severe myopia were also exhibited due to mutations in the FOXx1 gene, predominantly expressed in the periocular mesenchymal cells.3
Lead investigator, Bo Chen, PhD, an associate professor of Ophthalmology and Director of the Ocular Stem Cell Program at the Icahn School of Medicine at Mount Sinai, found that zebrafish could repair retinal damage within a few days. Dr. Chen and his colleagues explored evidence for vision restoration as a result of Müller glial cell derived regeneration.4 Due in part to Müller glial cells, the freshwater fish can replenish damaged retinal nerve cells and essentially repair vision.
While Müller glial cells do not possess regenerative properties in the human retina, these cells may provide deeper insight for future research in retinal degeneration. Plus, with the additional understanding of RPE regeneration provided by Dr. Gross’s study, the framework behind photoreceptor degeneration may be further laid out.
The Future of Zebrafish
Retinal disease is a growing problem that can only be tackled with greater understanding of the underlying mechanisms of disease. With an increased need for adequate studies comes the need for effective animal models. Because of their similar anatomical eye structure to the human eye, zebrafish can provide a new lens into the pathology behind ocular conditions.
Zebrafish are easy to maintain, relatively quick to generate and display rapid retinal development. With several studies already replicating human vision disorders in zebrafish, the zebrafish model can only gain more traction moving forward.
References:
1 Hanovice NJ, Leach LL, Slater K, et al. Regeneration of the zebrafish retinal pigment epithelium after widespread genetic ablation. PLoS Genet. 2019; 15(1): e1007939.
2 Reis LM, Tyler RC, Muheisen S, et al. Whole exome sequencing in dominant cataract identifies a new causative factor, CRYBA2, and a variety of novel alleles in known genes. Hum Genet. 2013;132(7):761–770.
3 McMahon C, Semina EV, Link BA. Using zebrafish to study the complex genetics of glaucoma. Comp Biochem Physiol C Toxicol Pharmacol. 2004;138(3):343-350.4 Yao K, Qiu S, Wang YV, et al. Restoration of vision after de novo genesis of rod photoreceptors in mammalian retinas. Nature. 2018;560(7719):484-488.
