Ancient Roots

• Researchers at Johns Hopkins University School of Medicine appear to have uncovered an “ancient” light-sensing mechanism in modern mice (1) – and it is likely found in human retinas too. The team was researching the biochemical pathways of “non-image forming” photoreceptors (intrinsically photosensitive retinal ganglion cells or ipRGCs), when they discovered that one subtype (M4) didn’t use the previously discovered phospholipase C pathway but something novel: HCN-channel-mediated phototransduction. And, perhaps more surprisingly, subtype (M2) appears to use both mechanisms. “Some evolutionary biologists have proposed that […], through evolution, these two mechanisms separated into different cell types. Our research seems to provide evidence that photoreceptors containing both light-sensing mechanisms may still exist in modern mammals,” said King-Wai Yau, a professor of neuroscience, who led the study.