Pinpointing the cells that keep the body's master circadian clock ticking
A new mouse model helps researchers study the roles of cell types in keeping time inside the body
August 7, 2020
Science Daily/UT Southwestern Medical Center
UT Southwestern scientists have developed a genetically engineered mouse and imaging system that lets them visualize fluctuations in the circadian clocks of cell types in mice. The method, described online in the journal Neuron, gives new insight into which brain cells are important in maintaining the body's master circadian clock. But they say the approach will also be broadly useful for answering questions about the daily rhythms of cells throughout the body.
"This is a really important technical resource for advancing the study of circadian rhythms," says study leader Joseph Takahashi, Ph.D., chair of the department of neuroscience at UT Southwestern Medical Center, a member of UT Southwestern's Peter O'Donnell Jr. Brain Institute, and an investigator with the Howard Hughes Medical Institute (HHMI). "You can use these mice for many different applications."
Nearly every cell in humans -- and mice -- has an internal circadian clock that fluctuates on a roughly 24-hour cycle. These cells help dictate not only hunger and sleep cycles, but biological functions such as immunity and metabolism. Defects in the circadian clock have been linked to diseases including cancer, diabetes, and Alzheimer's, as well as sleep disorders. Scientists have long known that a small part of the brain -- called the suprachiasmatic nucleus (SCN) -- integrates information from the eyes about environmental light and dark cycles with the body's master clock. In turn, the SCN helps keep the rest of the cells in the body in sync with each other.
"What makes the SCN a very special kind of clock is that it's both robust and flexible," says Takahashi. "It's a very strong pacemaker that doesn't lose track of time, but at the same time can shift to adapt to seasons, changing day lengths, or travel between time zones."
To study the circadian clock in both the SCN and the rest of the body, Takahashi's research group previously developed a mouse that had a bioluminescent version of PER2 -- one of the key circadian proteins whose levels fluctuate over the course of a day. By watching the bioluminescence levels wax and wane, the researchers could see how PER2 cycled throughout the animals' bodies during the day. But the protein is present in nearly every part of the body, sometimes making it difficult to distinguish the difference in circadian cycles between different cell types mixed together in the same tissue.
"If you observe a brain slice, for instance, almost every single cell has a PER2 signal, so you can't really distinguish where any particular PER2 signal is coming from," says Takahashi.
In the new work, the scientists overcame this problem by turning to a new bioluminescence system that changed color -- from red to green -- only in cells that expressed a particular gene known as Cre. Then, the researchers could engineer mice so that Cre, which is not naturally found in mouse cells, was only present in one cell type at a time.
To test the utility of the approach, Takahashi and his colleagues studied two types of cells that make up the brain's SCN -- arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) cells. In the past, scientists have hypothesized that VIP neurons hold the key to keeping the rest of the SCN synchronized.
When the research team looked at VIP neurons -- expressing Cre in just those cells, so that PER2 luminesced green in VIP cells, while red elsewhere -- they found that removing circadian genes from the neurons had little overall effect on the circadian rhythms of the VIP neurons, or the rest of the SCN. "Even when VIP neurons no longer had a functioning clock, the rest of the SCN behaved essentially the same," explains Yongli Shan, Ph.D., a UTSW research scientist and lead author of the study. Nearby cells were able to signal to the VIP neurons to keep them in sync with the rest of the SCN, he says.
When they repeated the same experiment on AVP neurons, however -- removing key clock genes -- not only did AVP neurons themselves show disrupted rhythms, but the entire SCN stopped synchronously cycling on its usual 24-hour rhythm.
"What this showed us was that the clock in AVP neurons is really essential for the synchrony of the whole SCN network," says Shan. "That's a surprising result and somewhat counterintuitive, so we hope it leads to more work on AVP neurons going forward."
Takahashi says other researchers who study circadian rhythms have already requested the mouse line from his lab to study the daily cycles of other cells. The mice might allow scientists to hone in on the differences in circadian rhythms between cell types within a single organ, or how tumor cells cycle differently than healthy cells, he says.
"In all sorts of complex or diseased tissues, this can let you see which cells have rhythms and how they might be similar or different from the rhythms of other cell types."
https://www.sciencedaily.com/releases/2020/08/200807111938.htm
Why intense light can protect cardiovascular health
The light boosts a critical gene that strengthens blood vessels
August 8, 2019
Science Daily/University of Colorado Anschutz Medical Campus
Researchers at the University of Colorado Anschutz Medical Campus have found that intense light amplifies a specific gene that bolsters blood vessels and offers protection against heart attacks.
"We already knew that intense light can protect against heart attacks, but now we have found the mechanism behind it," said the study's senior author Tobias Eckle, MD, PhD, professor of anesthesiology at the University of Colorado School of Medicine.
The study was published recently in the journal Cell Reports.
The scientists discovered that housing mice under intense light conditions for one week `robustly enhances cardio protection', which resulted in a dramatic reduction of cardiac tissue damage after a heart attack. They also found that humans could potentially benefit from a similar light exposure strategy.
In an effort to find out why, they developed a strategy to protect the heart using intense light to target and manipulate the function of the PER2 gene which is expressed in a circadian pattern in the part of the brain that controls circadian rhythms.
By amplifying this gene through light, they found that it protected cardiovascular tissues against low oxygen conditions like myocardial ischemia, caused by reduced oxygen flow to the heart.
They also discovered that the light increased cardiac adenosine, a chemical that plays a role in blood flow regulation.
Mice that were blind, however, enjoyed no cardio protection indicating a need for visual light perception.
Next, they investigated whether intense light had similar effects on healthy human volunteers. The subjects were exposed to 30 minutes of intense light measured in lumens. In this case, volunteers were exposed to 10,000 LUX, or lumens, on five consecutive days. Researchers also did serial blood draws.
The light therapy increased PER2 levels as it did in mice. Plasma triglycerides, a surrogate for insulin sensitivity and carbohydrate metabolism, significantly decreased. Overall, the therapy improved metabolism.
Eckle has long known that light plays a critical role in cardiovascular health and regulating biological processes. He pointed out that past studies have shown an increase in myocardial infarctions during darker winter months in all U.S. states, including sunnier places like Arizona, Hawaii and California. The duration of the light isn't as important as the intensity, he said.
"The most dramatic event in the history of earth was the arrival of sunlight," Eckle said. "Sunlight caused the great oxygen event. With sunlight, trillions of algae could now make oxygen, transforming the entire planet."
Eckle said the study shows, on a molecular level, that intensive light therapy offers a promising strategy in treating or preventing low oxygen conditions like myocardial ischemia.
He said if the therapy is given before high risk cardiac and non-cardiac surgery it could offer protection against injury to the heart muscle which can be fatal.
"Giving patients light therapy for a week before surgery could increase cardio protection," he said. "Drugs could also be developed that offer similar protections based on these findings. However, future studies in humans will be necessary to understand the impact of intense light therapy and its potential for cardio protection."
https://www.sciencedaily.com/releases/2019/08/190808115052.htm