Eyes need light in order for human to see. What happens at the cellular level once light enters the retina? Conversely, when things aren’t working in the retina, what’s going at the cellular level that is causing the malfunction? Two studies at the National Eye Institute at the National Institutes of Health looked at these things and what they found can lead to better disease models and pave the way for treatments for diseases like Stargardt Disease.
Recharging the Retina
Scientists at the National Eye Institute (NEI) learned that a part of a protein known as RPE65 turns into a spiral shape when it encounters intracellular membranes. This process converts Vitamin A into a usable form by the photoreceptor cells.
When light hits the photoreceptor pigments, known as opsins, this triggers a series of chemical reactions that lead to visual signals being sent to the brain. The retinal pigment epithelium (RPE), which is support tissue located adjacent to the photoreceptors, recharges the opsins to restore their sensitivity to light. RPE65 converts the used-up Vitamin A derivative all-trans retinol into a photosensitive 11-cis retinal. Changes in the RPE65 gene are associated with early-onset blinding disorder.
It is the interaction of the RPE65 with the RPE cell’s endoplasmic reticulum, which is a network of sac-like structures and tubes located in cell’s cytoplasm, that is essential to making the 11-cis retinal. What scientists didn’t know was the mechanism that binds RPE65 to the cell’s endoplasmic reticulum.
Scientists looked to biochemistry to learn more and what they found was in a liquid solution. Namely, a specific region of RPE65 doesn’t have structure. When this region comes into contact with membranes, it quickly forms a spiral shape. This change allows RPE65 to bind to the RPE cells’ endoplasmic reticulum where the 11-cis retinal is made from the all-trans retinol.
There’s more. When a specific amino acid in a previously uncharacterized region of RPE65 was modified by a specific lipid, it increased the speed of the formation of the spiral locking into place and helping its insertion into the cell membrane. This research clarifies the function of RPE65 and scientist hope that this information will lead to more accurate disease models.
Cause of Stargardt Disease
Speaking of models, scientists at NEI used stem cell models to learn more about Stargardt Disease. Stargardt’s leads to a loss of central and night vision and the vision loss is linked to the toxic build-up of lipid-rich deposits in the RPE. Previous research showed that this disease is caused by different mutations in the ABCA4 gene. Scientists in this study used stem-cells made from skin cells and found direct evidence that Stargardt-related ABCA4 gene mutations affect the RPE.
What makes this model so valuable to researchers is that Stargardt Disease is rare, it affects 1 out of 10,000 people in the U.S., so having more cells to study leads to the development of treatments for this disease. Prior to this study, scientists used mouse models to learn about Stargardt disease. The effectiveness of the mouse model was limited due to the wide genetic variability of the disease in humans. The human model helped scientists to learn if the ABCA4 gene mutations directly affected the RPE independent of the photoreceptors.
In order to create this model, scientists took skin cells from Stargardt patients, turned them into stem cells and then had the stem cells become RPE cells. When they looked at these RPE cells, researchers detected the ABCA4 protein on the RPE cell membrane. They then studied the function of the ABCA4 protein by using the gene editing technology CRISPR/Cas9 to create RPE that doesn’t have the ABCA4 protein.
What they found was that the lack of ABCA4 didn’t affect the development of the patient-derived RPE. However, when the RPE that lacked the ABCA4 was exposed to normal photoreceptor outer segments, the RPE cells amassed intracellular lipid deposits. Additional tests of the ABCA4 knockouts showed signs of defective RPE lipid metabolism and reduced ability to digest photoreceptors outer segments, which leads to lipid deposits in the RPE cells. This suggests that these lipid deposits may contribute to RPE atrophy and that leads to photoreceptor degeneration.
This discovery suggests that therapies for Stargardt’s need to both correct for the loss of the ABCA4 function in the photoreceptors and target the RPE cells.
Once again, research shows that there’s more to vision and vision disease than meets the eye.