OU-MRU Vol.12 June 2015

Okayama University Medical Research Updates （OU-MRU）

2015.06 Vol.12

Source: Okayama University, Center for Public Affairs and Communications
For immediate release: 26 June 2015
Okayama University research: Molecular features of the circadian clock system in fruit flies(Okayama, 26 June) Studies of mutant fruit flies reveal how both photoreceptors and thevisual system influence circadian clock neurons in response to changing light-dark cycles.
The results are reported in the Journal of Neuroscience give new insights into sleep
disorders including jet-lag.
Our bodies can respond to shifts in day and night patterns by reprogramming our so-called‘circadian clock’ – a molecular system which responds to light-dark cycles (LD cycles). Now,
Taishi Yoshii and Kenji Tomioka at the Graduate School of Natural Science and Technology,
Okayama, in collaboration with scientists in Germany, have revealed how protein
photoreceptors called ‘cryptochromes’ (CRY), together with the visual system, influencecircadian clock neurons in Drosophila, or fruit flies[1].
Circadian clock neurons can be divided into two groups; morning neurons (M) and evening
neurons (E). CRY proteins are expressed in most clock neurons and send signals in response
to light, but the specific role of CRY in different types of clock neurons is unclear.
Yoshii and colleagues generated mutant fly-lines, some without CRY and others expressing
CRY in different neuron subsets. They also wanted to determine the influence of the eyesand visual system signals on the neurons. Their aim was to test the flies’ ability tosynchronise to changes in LD cycles, a process known as LD entrainment.
They exposed the flies to an 8-hour delay in the 16-hour/8-hour LD-cycle. Control flies
responded to the shift within a day, but those flies without CRY, and without eyes, wereincapable of any LD entrainment. Mutant flies with E neurons were able to re-entrain, but
their response was slow. The team discovered this entrainment ability was due to amolecular cycling process involving a protein called par domain protein 1 (PDP1), which is
triggered by the visual input pathways, independent of CRY.
When the team then expressed CRY in the E neurons the LD entrainment process sped up
considerably. If CRY was expressed only in M neurons, no LD entrainment occurred. Theresults indicate that CRY expression in E neurons is important to LD entrainment and thatmolecular cycling of PDP1 supports this process.
BackgroundCryptochromes
Circadian clock neurons respond to signals from a class of proteins called ‘cryptochromes’(CRY), which are found in animals and plants. The CRY protein is photoreceptive; when
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OU-MRU Vol.12 June 2015

exposed to blue light, CRY undergoes conformational changes, allowing it to bind to acircadian clock protein called TIMELESS (TIM). This action triggers the degradation of TIM,
thereby resetting the body’s circadian clock.
Previous research has shown that Drosophila (fruit flies) are highly light-sensitive – TIM
degradation occurs even after exposure to a short, weak light pulse during darkness.
However, how CRY and the visual system help the body to respond to changes in light-darkcycles (so-called LD entrainment) has not been fully explored.
Methodology
To determine whether separate clock neurons have different functions in CRY-dependent
light responses, the researchers generated various Drosophila fruit fly lines, with different
CRY expression in individual subsets of clock neurons. They also used control flies with noalterations, null-CRY lines to examine the effect of knocking out CRY altogether, and flies
with null-CRY and no eyes, to examine the influence of the visual system in the LDentrainment process. The team also analyzed par domain protein 1 (PDP1), a protein
important for the molecular mechanism of the circadian clock.
They exposed all the different fly groups to an 8-hour shift in LD cycle. Control flies shifted
their circadian clock within a day. The mutant flies exhibited different patterns of LD
entrainment, depending on CRY expression and visual inputs.
They found PDP1 worked independently of CRY to help in LD entrainment, but only inevening (E) neurons. Null-CRY E neurons were still capable of LD entrainment, but it wasvery slow. Reinstating CRY into two particular E neurons (fifth s-LNv and LNd), sped up theprocess considerably.
Future work
CRY expression in E neurons is particularly important for the rapid adjustment of thecircadian clock in the event of LD cycle changes. Further work is needed to understand howmorning (M) neurons factor in the LD entrainment of E neurons, and whether M cells (andanother subset of neurons called PDF neurons) can contribute to LD entrainment in theabsence of CRY.

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OU-MRU Vol.12 June 2015

Figure caption
The circadian neurons responsible for response to changing light-dark cycles. The brain andthe distribution of the circadian clock neurons of the fruit fly Drosophila melanogaster (Left).
The enlarged picture of E neurons (fifth s-LNv and LNd) (Right).
ReferenceT. Yoshii, C. Hermann-Luibl, C. Kistenpfennig, B. Schmid, K. Tomoika, & C. Helfrich-Förster.
Cryptochrome-dependent and –independent circadian entrainment circuits in Drosophila.
The Journal of Neuroscience 35 (15) April 15th 2015, 35(15):6131-6141･6131.
DOI:10.1523/JNEUROSCI.0070-15.2015
http://www.ncbi.nlm.nih.gov/pubmed/25878285
Correspondence toAssociate Professor Taishi Yoshii, Ph.D.
Laboratory of Chronobiology, Graduate School of Natural
Science and Technology, Okayama University,
Tsushima-Naka 3-1-1, Kita-ku, Okayama 700-8530, Japan.
E-mail: yoshii@okayama-u.ac.jp.

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OU-MRU Vol.12 June 2015

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