thus be a faithful monitor of its overall (locomotor)
Circadian clocks respond to light and numerous other factors (16, 17), including, for example,
social interactions (18, 19), drug administration
(20), temperature (21), or feeding protocols (22),
forcing the question of how these are being integrated to compute a central clock time. The finding that—in flies—proprioceptor activation, which
accompanies all forms of locomotor behavior,
can reset the clock offers a potential solution to
this problem: All environmental stimuli that lead
to changes in the animal’s behavior would also
affect the clock through the concomitant changes
in proprioceptor activation. A proprioceptive clock
entrainment would automatically weight environmental stimuli according to their respective ability
to generate locomotion, which is a good indicator
of the stimuli’s evolutionary importance. An animal’s activity can indeed affect its circadian clock
(23, 24), and some nonphotic influences, such
as certain drugs, only affect the clock if the animal is free to move (25). Proprioceptive feedback
to the clock offers a mechanism for such activity-dependent clock entrainment.
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Acknowledgments: This work was supported by grants from
the Biotechnology and Biological Sciences Research
Council (BB/H001204/1 to R.S. and BB/G004455/1 to
J. T.A.), the Human Frontier Science Program (to J. T.A.),
and the European Union FP6 Integrated Project “EUCLOCK”
(to R.S.). M.P. T. received funding from the Engineering
and Physical Sciences Research Council (EP/F500351/1).
Additional data, including raw data, are presented in
the supplementary materials. The authors thank CoMPLEX
students W. Ashworth and M. Ransley for their help,
W. Potter for technical support, and P. Dayan from the
Gatsby Computational Neuroscience Unit at University
College London for valuable discussions.
Materials and Methods
Figs. S1 to S10
9 September 2013; accepted 11 December 2013
Fig. 3. Phase shifts of PERIOD protein oscillations in central clock
neurons caused by rhythmic mechanical stimulation. (A) Schematic
representation of the environmental conditions before flies were transferred to
the bioluminescence counter. For the delay and advance experiment, flies
were transferred at ZT20 and ZT8, respectively, relative to the previous vibration onset. Colors and shadings referring to light and stimulus conditions are the same as in Fig. 1. (B) Normalized bioluminescence activity of FR
8.0-luc flies—which express a PER-LUC fusion construct in a subset of the
clock neurons (7, 8, 15)—after exposure to VS cycles (red) and control flies not
exposed to VS (gray). VS cycles that were delayed (Dt[vib] = +6 hours; experi-
mental flies, n = 15; controls, n = 11) relative to the initial LD entrainment
lead to an advance of the molecular oscillations (left), whereas VS cycles that
were advanced (Dt[vib] = –6 hours; experimental flies, n = 10; controls, n = 8)
relative to the initial LD entrainment lead to a delay (right) compared with
PER-LUC expression of control flies not exposed to VS stimuli. Bioluminescence
data was de-trended before fitting of a sinusoidal model (solid lines) (fig. S8)
(7). Both data and corresponding fit values have been normalized to the
maximum of the fit function to highlight the phase difference. Two additional
delay experiments (Dt[vib] = +6 hours; experimental flies, n = 17; controls, n =
25) and one advance experiment (Dt[vib] = –6 hours; experimental flies, n = 9;
controls, n = 10) were performed, and molecular phase shifts in the same
direction as shown in (B) were observed. Error bars represent SEM.