When biologist Samer Hattar describes the development of eyes in newborn mice, he speaks of a microscopic death match. He is not being metaphorical. In the eyes' early stages of development, thousands of photosensitive cells fight for optimal position on the retina, and the combat is mortal— nearly 60 percent die in what amounts to a cellular struggle for survival. But this carnage is necessary because it results in the surviving cells forming an orderly mosaic across the retina's surface. This mosaic is essential to proper reception of the light signals that allow the mouse's brain to manage its circadian clock. Because human eyes contain the same neurons, known as intrinsically photosensitive retinal ganglion cells (ipRGCs), Hattar believes a similar death battle occurs in the eyes of human fetuses at about the 30-week mark in gestation.
"This is remarkable because it shows that cell death is not a passive process but an active one involved in setting up circuits in the nervous system," says Hattar.
A research team led by Hattar, who is an associate professor of biology in the Krieger School, initially set out to do an experiment that would help them understand jet lag. Jet lag occurs when the body's internal circadian clock goes out of sync with the external solar day. In the eye, ipRGCs, along with the more familiar rods and cones, detect the irradiance patterns of light and dark that regulate the body's clock, a process called circadian photoentrainment. Hattar knew that ipRGCs are produced in excess at the very earliest stages of eye development, before so many of them die as the retina develops. So if he blocked the normal cell death process, he could increase the number of ipRGCs that survive, resulting in the retina having greater light sensitivity. Because light information from the environment is necessary for shifting the internal biological clock, Hattar explains, "this should allow, we thought, the animal to be more sensitive to lower light levels and hence have better ability to adjust to jet lag."
Hattar's team studied mice that had been genetically modified to remove the Bax protein, a factor essential for cell death. This blocked the "death battle" and created the desired surplus of ipRGCs. But it also resulted in the cells forming clumps on the retina instead of the normal, orderly mosaic. This clumping, to the researchers' surprise, seemed to have no effect on regulating the mice's circadian clocks. But in another cohort of mice, the team knocked out both the Bax proteins and a photoreceptive protein called melanopsin, which also plays a role in photoentrainment. Those mice slept during both day and night, indicating their eyes had been made incapable of setting their circadian clocks.
From these results, the scientists concluded that the abnormal clumping of ipRGCs had impaired the rods and cones' ability to regulate the circadian clock, an effect that had been masked in the experiment until the melanopsin was knocked out. So the orderly mosaic of ipRGCs produced by the "death battle" is essential to the eye's ability to keep the body's clock in sync with the day/ night cycle, because without the mosaic, the rods and cones' input to ipRGCs does not function properly.
Hattar believes the team's study not only revealed more about the eye, but it will help scientists better understand how neural circuits are formed. A greater understanding of this circuit formation, he says, could be applied to any neurological function, perhaps leading to breakthroughs in treating neurological diseases including autism, Alzheimer's, Parkinson's, and psychiatric disorders where neural circuits influence behavior.