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

Date:
August 7, 2020
Source:
UT Southwestern Medical Center
Summary:
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 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.



FULL STORY ==========================================================================
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." This research was supported by funds from
the HHMI, the National Institutes of Health (R01 NS106657, R01 GM114424, T32-HLO9701, F32-AG064886), and The Welch Foundation (AU-1971-20180324).


========================================================================== Story Source: Materials provided by UT_Southwestern_Medical_Center. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yongli Shan, John H. Abel, Yan Li, Mariko Izumo, Kimberly H. Cox,
Byeongha Jeong, Seung-Hee Yoo, David P. Olson, Francis J. Doyle,
Joseph S. Takahashi. Dual-Color Single-Cell Imaging of the
Suprachiasmatic Nucleus Reveals a Circadian Role in Network
Synchrony. Neuron, 2020; DOI: 10.1016/j.neuron.2020.07.012 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200807111938.htm

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