Light Activated Cells
© Chemical & Engineering News
Light can switch this azobenzene-modified version of the circadian clock regulator longdaysin between its active and inactive forms, allowing researchers to controllably shift cells’ circadian clocks by up to 4 h.
Most living things contain a circadian clock — a signaling system that keeps the cell's function tied to the periodicity of a 24-h cycle. Although scientists have identified many elements that regulate the clock's function, much about circadian rhythms and how they influence health and disease remains poorly understood.

Now, researchers have created a light-activated molecule that can reversibly shift the length of the clock's cycle in living cells and tissues. The tool provides a targeted, non-invasive way to study how the clock affects cellular physiology, says Ben Feringa, a chemist at the University of Groningen who co-led the work.

The team made use of an azobenzene moiety, which is commonly used as a photoswitch and which changes conformation upon exposure to light. Light converts azobenzene from the trans isomer, which is thermally stable, to the cis isomer, which is not. The cis isomer gradually reverts to trans on its own or can be converted photochemically. "It's just a change in geometry," Feringa says. In past work, Feringa's lab built light switches into other molecules including antibiotics and antitumor compounds.

In the new work, the researchers added an azobenzene light switch to a molecule called longdaysin that chronobiologist Tsuyoshi Hirota at Nagoya University had created previously. Longdaysin slows down the clock by interfering with the action of a key regulator of two genetic feedback loops that control the circadian clock.

After testing several variations of the azobenzene-modified longdaysin, the team landed on one in which violet light (400 nm) activates longdaysin, while green light (530 nm) switches longdaysin off.

The researchers tested the molecule in cultured human cells, in mouse spleen tissue, and in mouse brain sections containing the brain region central to regulating circadian rhythms. "By using 400 and 530 nm light, we can either extend clock time by up to 4 hours, or reduce it back," Feringa says.

"It's a very cool application of photopharmacology," says Dirk Trauner, a chemist at New York University who designs chemical switches but was not involved in the work.

One struggle for researchers studying the circadian clock in cultured cells is that even basic experimental procedures — say, taking cells out of the incubator to change the growth medium keeping them alive — "can reset the molecular rhythm," says Akhilesh Reddy, a chronobiologist at the University of Pennsylvania who was not involved in the work. The compound described in the study "is a good way to get around that" because it provides a noninvasive way to control the clock. Being able to use the compound in a living organism would make it even better, he adds.