Researchers at Johns Hopkins Medicine have discovered that a particular type of light-sensing cell in the retina uses two different pathways to transmit electrical “vision” signals to the brain. This discovery is a significant breakthrough in understanding how the cells work and may have ancient origins on the evolutionary scale.
The research, published in PNAS, shows that intrinsically photosensitive retinal ganglion cells (ipRGCs) use both microvillous and ciliary signaling pathways simultaneously. This is in contrast to other photoreceptors in the retina, which use either one or the other.
The team discovered this by exposing ipRGCs to brief pulses of bright light. The microvillous signaling pathway produced faster electrical responses and preceded a slower response by the ciliary pathway. The researchers found that all six subtypes of ipRGCs use both pathways at different percentages simultaneously.
It has been known that animal photoreceptors detect light by using a signaling pathway named for the cell’s origin. Photoreceptors of “microvillous” origin, similar to those in the fruit fly eye, use the enzyme phospholipase C to signal light detection — whereas, photoreceptors of ciliary origin, such as those in our rods and cones, use a cyclic-nucleotide pathway.
The researchers also found that ipRGCs use cAMP as the signaling messenger, similar to jellyfish, an animal much older on the evolutionary scale. This suggests that ipRGCs may have an ancient origin.
The findings shed light on a decades-long mystery about how such cells work and may eventually help scientists learn why people without sight can still sense light. The study was funded by a grant from the National Institutes of Health and a Beckman-Argyros Award in Vision Research.
This research was co-led by King-Wai Yau, Ph.D., professor in the Department of Neuroscience at the Johns Hopkins University School of Medicine, and postdoctoral fellow Guang Li. Other Johns Hopkins researchers who contributed to this study are Lujing Chen and Zheng Jiang.
Journal Reference
- Li, G., Chen, L., Jiang, Z., & Yau, K. (2023). Coexistence within one cell of microvillous and ciliary phototransductions across M1- through M6-IpRGCs. Proceedings of the National Academy of Sciences, 120(52), e2315282120. DOI: 10.1073/pnas.2315282120