Researchers have discovered a cellular mechanism that helps nocturnal mammals see in the dark. Mice, cats, deer, lemurs, and other mammals that are active at night remodel the DNA within their eyes to turn photoreceptor cells into light-collecting lenses, according to a study published today (Apr. 16) in Cell.

Image: striatic and Animal Photos!

In nearly all eukaryotic nuclei, chromatin -- the structural building block of chromosomes -- is spatially separated into distinct compartments. Condensed, non-coding heterochromatin is usually localized to the periphery of the nucleus, while extended, active euchromatin typically resides in the nuclear interior. This "conventional" pattern is nearly universal, and probably helps cells regulate essential nuclear functions such as how and when genes are expressed.

But some nuclei are special. In 2006, a team led by Didier Devys, a molecular biologist at the Institute of Genetics and Molecular and Cellular Biology in Illkirch, France, showed that mouse rod photoreceptor cells have a different arrangement in which the chromatin is "inverted." In these cells, but not in other mouse cell types, heterochromatin is shunted to the interior, where it is enveloped by a thin ring of euchromatin. With this layout, all transcription takes place at the nuclear margins rather than at the core of nucleus as per usual.

Boris Joffe and Irina Solovei, nuclear biologists working in Thomas Cremer's lab at the Ludwig-Maximilians University in Munich, Germany, independently observed the same phenomenon, and they set out to determine why this was so. In the new study, they investigated mouse retinas dissected at various ages, and showed that rod cells have the conventional chromatin structure at birth, but gradually the chromatin shuffles around the nucleus to create the inverted pattern after around four weeks.

Looking beyond the mouse, Joffe and Solovei mapped the nuclear architecture of nearly 40 mammal species and found that all the nocturnal animals had the same inverted pattern as mice, also a nocturnal species, whereas all the diurnal animals had the conventional arrangement. The researchers also inspected a few bird and other vertebrate species, but found no inverted architecture outside of nocturnal mammals.

The Munich researchers then teamed up with Leo Peichl, a neurobiologist at the Max Planck Institute for Brain Research in Frankfurt, and mapped the nuclear arrangements onto the phylogenetic tree of mammals. They showed that the inverted pattern arose early in the evolution of mammals, probably as an adaptation to nocturnal vision to help ancient mammals escape predation from dinosaurs and other large reptiles, which were mostly active in the daytime. The conventional pattern was then "reacquired" many times over in all the diurnal mammals, including humans.

This indicates that the conventional architecture is under strong selective pressure in most well-lit environments, said Joffe. "We proved that there is some great advantage in the organization of the nuclei that we observe in most eukaryotic cells," he told The Scientist.

Finally, Joffe and Solovei partnered with Jochen Guck, a biophysicist at the University of Cambridge, UK, to measure differences in how the two nuclear arrangements physically interact with light. Using live retinal cells from mice (a typical nocturnal species) and pigs (a typical diurnal species), they showed that inverted rod nuclei scattered light much less than rod nuclei with the conventional pattern.

Computer simulations also indicated that when the inverted nuclei are stacked on end -- the organization found in the rod columns of nocturnal animals' retinas -- light is transmitted with very low scatter. Thus, nuclei with inverted chromatin act as converging lenses to focus light -- a necessary adaptation for seeing at dusk and dawn. "At very low light conditions [nocturnal mammals] need to be able to salvage every little photon that comes in," said Guck.

The finding that physical properties can remodel chromatin is "striking," said Devys. But he noted that the researchers discovered only a correlation between the layout of the nucleus and the circadian lifestyle of different mammals. "They don't have conclusions about what this chromatin organization would do at the transcriptional level," he said.


Related stories:

Visual system surprise
[7th July 2008]

Seeing the light
[6th September 2002]

Photoreceptor biology
[19th August 1996]