Neuroscientists at Johns Hopkins Medicine believe they have found a solution to a 30-year biological mystery. They used genetically engineered mice to determine how one mutation in the gene for the light-sensing protein rhodopsin leads to congenital stationary night blindness. This condition is present from birth.The rhodopsin gene mutation G90D causes poor vision in low-light settings, according to a study published on May 14 in the Proceedings of the National Academy of Sciences. This mutation leads to abnormal background electrical activity that desensitizes the eye’s rods, which are responsible for nighttime vision, resulting in night blindness. The study’s authors believe that identifying this unusual electrical activity could lead to potential targets for therapeutic interventions. Understanding these electrical events could also help scientists learn more about how the eye’s rods and cones function.According to King-Wai Yau, Ph.D., a professor in the department of neuroscience at the Johns Hopkins University School of Medicine, the research was conducted by Yau and postdoctoral fellow Zuying Chai. Yau explains that the G90D mutation in rhodopsin is known to create background electrical noise that desensitizes rods, but the exact nature of this “noise” and its molecular source have remained unresolved for nearly 30 years. He states, “We were able to contribute to solving the mechanism of this disease using a mouse model with a very low expression level of G90D rhodopsin.” The researchers compared the low expression level of G90D in genetically engineered mice.The authors found that the level of G90D in human patients with night blindness may be the main cause of the disease due to its unusual electrical activity with low amplitude but extremely high frequency. In addition to this unusual electrical noise, rhodopsin also produces spontaneous thermal isomerization, where the thermal energy inside the rhodopsin molecule triggers random activation.
The researchers discovered that G90D rhodopsin has a significantly higher spontaneous-isomerization rate than normal rhodopsin, but it does not have a strong enough effect on rod adaptation to cause night blindness in humans. Rods are normally very sensitive to light, but in individuals with night blindness, they struggle to accurately detect light changes and do not work well in the dark. As a result, those with night blindness need brighter light to see in low-light conditions. Despite knowing about the G90D mutation for many years, researchers have not been able to fully understand it.The cause of night blindness was challenging to determine because previous mouse models with the same mutation had high levels of background noise, leading to effects similar to background light that the mouse’s rods quickly adapted to. This made it hard for researchers to accurately measure the effects of the mutation’s signaling. To address this issue, the scientists at Johns Hopkins Medicine genetically modified mice to have a low expression of G90D, equivalent to .1% of the normal rhodopsin found in the natural mouse population. This adjustment allowed the researchers to differentiate between various types of activity in mice.The scientists were able to observe that the retinal rods with the G90D mutation reacted as if there was very little or no background light present. They used a high-resolution method to measure the electrical activity in individual rods in the mouse retina. This was done using an ultra-tiny glass pipette, filled with saline solution capable of conducting electricity, to access the rods. According to Yau, the technique used, known as suction-pipette recording, allowed them to record the activity at such a high resolution that they could actually see individual events such as the isomerization of a rhodopsin molecule.
ates, we can observe it, as it results in a change in electrical current.”
G90D is one of four mutations of rhodopsin linked to night blindness. Lead researcher Chai suggests that the next steps will involve determining how the other rhodopsin mutations, T94I, A292E, and A295V, contribute to this condition.
“The mechanism that causes G90D night blindness could be similar in the three other rhodopsin mutations that cause this condition,” Chai explains.
Other researchers involved in the study include Yaqing Ye, Daniel Silverman, and Randall Reed from Johns Hopkins, as well as Kasey Rose, Alana Madura, and Jeannie Chen from the University of Southern CalifornThe study was funded by the National Institutes of Health grant EY006837, the António Champalimaud Vision Award from Portugal, the Multiphoton Imaging Core at Johns Hopkins, the Daniel Nathans Scientific Innovator Award from the Johns Hopkins University School of Medicine, and the Beckman-Argyros Vision Award from the Arnold and Mabel Beckman Foundation.
