What does the intestinal microbiota have to do with eye diseases such as retinitis pigmentosa? It is difficult to believe that there could be a connection. Yet, a gene involved in some hereditary retinal diseases also appears to play a role in the integrity of the mucous membranes of the intestine. According to what one suggests study published on CellIn fact, the alteration of the CRB1 gene (Crumbs homolog 1), responsible for conditions such as retinitis pigmentosa, in addition to altering the permeability of the eye, also increases that of the intestinal barrier. In this way, the migration of microbiota bacteria to the eye is promoted, which is further damaged. The research provides the first clear evidence of the actual existence of a gut-eye axis, which until now had only been hypothesized.
Hereditary and genetic retinal dystrophies
Genetic retinal dystrophies (one of the most common is retinitis pigmentosa) are the most frequent cause of severe vision loss or blindness in working-age populations in industrialized countries. Suffice it to say that around 5 million people worldwide suffer from it. They are caused by genetic mutations, including those of the CRB1 gene, which codes for a protein present in the membranes of retinal cells in mammalian eyes and preserves their integrity.
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Intestinal bacteria, in the eye
Precisely by studying the mutation of the CRB1 gene and the consequent alteration of the permeability of the retinal barrier, a joint China-UK study made an interesting discovery: the presence of intestinal bacteria inside the eye. “Even a few bacteria that reach the eye are able to persist, colonize it and cause damage, as it is an organ that is difficult for the immune system to patrol”, he explains to Salute Maria Rescignohead of the Mucosal Immunology and Microbiota Laboratory of Humanitas and professor of Humanitas University.
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The study therefore wanted to verify whether the CRB1 gene also had a role in the intestine: they actually discovered that the change in its expression due to the mutation could also change intestinal permeability. As a consequence, intestinal bacteria reach the eye through the blood. To confirm their observation, the researchers treated animal models carrying the retinopathy gene with a broad-spectrum antibiotic and saw that this could successfully control retinal damage. Furthermore, a second confirmation came when, with the reintroduction of normal CRB1 expression in the lower gastrointestinal tract, they showed a significant reduction in retinal lesions. “These experiments demonstrate in an indirect and elegant way that these bacteria actually have an intestinal origin – comments Rescigno – Clearly, we need to demonstrate that preclinical studies can be translated into humans”.
There are many studies on experimental animal models that also have relevance in humans. “As regards the microbiota, it is not certain that the same microorganisms of the animal are also found in human ocular disease, because the microbiota of the mouse model is different from the human microbiota – specifies Rescigno – However, the microorganisms could have similar characteristics, producing responses comparable inflammatory conditions”.
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The gut-eye axis
At the end of the work, the authors suggest using antibiotics to prevent the worsening of retinopathy. According to Rescigno, however, antibiotics would only cure one aspect: “It is unthinkable to take antibiotics for life as prevention. Rather, we should have a correct diet, perhaps trying to encourage a more anti-inflammatory microbial population, which is therefore able to limit intestinal permeability. Furthermore, it is known that good nutrition also has a positive effect on vision. The important aspect of this study is that it has demonstrated that there is an intestine-eye axis” – concludes the expert. This axis has always been hypothesized and there are several cases in which ocular diseases have been associated with intestinal dysbiosis, but it is not clear that a cause-effect relationship exists. “This is a very interesting discovery because it makes us understand that, like other pathologies already studied, eye diseases can also be associated with intestinal dysfunctions, opening up new research perspectives.”
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