Astrocytes’ effect on internal clock discovered

Astrocytes, the star shaped cells that surround neurons, have been found to influence the body’s internal clock more than previously thought.

The regulation of the circadian rhythm, which lasts approximately 24 hours, was believed to be controlled by the suprachiasmatic nucleus (SCN) – a tiny region situated in the hypothalamus. Now researchers at Washington University in the US have proven that astrocytes (also known as astroglia) help influence the SCN.

Professor Erik Herzog, a neuroscientist at the university, said: “Asked to define the body's master clock, biologists would say it is two small spheres – the SCN – in the brain that consist of 20,000 neurons. They likely wouldn't even mention the 6,000 astroglia mixed in with the neurons.”

Scientists had previously discovered that almost all cells in the human body keep time, with stem cells one of a few notable exceptions. Herzog’s lab created a method to independently control astrocytes in the SCN to enable further understanding of how they work.

Herzog coupled a bioluminescent protein to a biological clock gene – a feedback loop in which the accumulating protein halts its own production upon reaching a certain level. Astrocytes were then isolated, placed in a glass dish and observed brightening and dimming rhythmically, showing that they were keeping time. The next challenge was observing the astrocytes in SCN samples and living animals.

A collaborator involved in the study created a gene that switched on a firefly luciferase gene whenever a clock gene was being expressed in a cell of interest. Matt Tso, one of Herzog’s graduate students, created a method to introduce this into viruses, so they could be used as vectors. Following on from this, researchers were able to view SCN slices expressing this clock gene in the same rhythmic pattern, proving astrocytes carry out the same function they did when isolated in vitro.

But the researchers were not finished – they used CRISPR-Cas9 technology to delete the clock gene in astrocytes in mice SCN and then observed what effect this would have. Mice run habitually, approximately every 23.7 hours, with deviations of no longer than 10 minutes from this schedule. Tso said: “When we deleted the gene in the astrocytes, we had good reason to predict the rhythm would remain unchanged. When people deleted this clock gene in neurons, the animals completely lost rhythm, which suggests that the neurons are necessary to sustain a daily rhythm.”

When the astrocyte clock was deleted, the SCN clock ran slower – with mice climbing into their wheels one hour later than usual daily. The researchers then began another experiment, a mouse with a mutation that made its internal clock run fast. They engineered the astrocytes to correct this but did not make the same change in neurons – so the neurons were still running fast even though the astrocytes were at a normal pace.

Mice with restored astrocyte clocks climbed into their wheels two hours later than mice with fast paced neurons and astrocytes. This surprised the researchers as the SCN was expected to follow the neurons that were running fast rather than the astrocytes.

Herzog and his team are planning further experiments and observing the connection between circadian rhythm and brain cancer, manic depression and other diseases. The research was published in Current Biology.