(534b) Peripheral Clock Gene Entrainment by Cortisol

  Peripheral Clock Gene Entrainment by Cortisol

The objective of this study is to assess the entrainment of peripheral blood
leukocyte (PBL) clock genes by cortisol and evaluate its entraining
characteristics. We demonstrate that the circadian characteristics of the
peripheral entrainer (cortisol) have significant implications on the dynamics
of clock genes. In the presence of a normal circadian pattern in cortisol, the PBLs
are entrained to a coherent diurnal pattern. As the amplitude of cortisol's
circadian rhythm is decreased slightly, the PBL oscillators remain entrained
but undergo a phase shift relative to cortisol's phase. And as the amplitude is
further decreased, eventually the PBLs fall out of sync and produce a flat
ensemble average. Understanding the relationship between the pattern of
cortisol secretion and its downstream effects on peripheral clock genes lays a
foundation for studying the implications of dysregulated cortisol secretion
(most notably under stress) on the function of PBLs.

To study the effect of cortisol in peripheral clock genes, we propose a
unified model involving the circadian production of cortisol, cortisol signal
transduction, and clock gene oscillations. We explore the ?two rates? model of
circadian cortisol production (Chakraborty, Krzyzanski et al. 1999) to generate the diurnal pattern in plasma cortisol concentration by switching between a high secretion rate during the morning when cortisol concentration is increasing and a low secretion rate the rest of the day when cortisol concentration decreases to a low resting level. Cortisol in plasma moves into the cytoplasm of PBLs, where it binds to glucocorticoid receptor (GR), forming an activated drug-receptor complex which subsequently translocates into the nucleus and acts as a transcription factor, regulating a wide range of genes (Yao, DuBois et al. 2006).

We further hypothesize that the peripheral circadian gene network involves a
limited set of critical genes, including Per1-3 and Cry1-2, which, in
collaboration with the CLOCK/BMAL1 heterocomplex, form an orchestrated feedback
loop that leads to a circadian rhythm. Since Per1 has a glucocorticoid
responsive element (GRE) in its promoter region and is known to be responsive
to glucocorticoids (Yamamoto, Nakahata et al. 2005), we hypothesize that the primary link between cortisol's circadian rhythms and PBL circadian rhythms is the transcriptional activation of Per1 by the cortisol-GR activated complex. This circadian model includes an equation representing Per/Cry gene transcripts, which was modified by including an additive term in the Per/Cry equation representing stimulus of Per1 transcription by activated GR.

The purpose of our modeling effort is to evaluate the implications of
alterations in the circadian patterns of cortisol release in the entrainment of
clock genes. Our results demonstrate that under homeostatic release of
cortisol, robust entrainment of clock genes is observed, as well as full
synchronization of the ensemble of PBLs. As the entraining characteristics of
cortisol are dysregulated, through a reduction in the amplitude of the
circadian pattern (a hallmark of stress), individual cells' clock genes fall
out of phase until eventually they become desynchronized. The loss of
synchronization is hypothesized to play a significant role in the attenuated
control effects clock genes exert on the homeostatic response.

Preliminary
model predictions are depicted in Figure 1. Our
model demonstrates the ability of cortisol's circadian variability to
synchronize clock genes (R_syn
= 1) in an amplitude-dependent
manner. It predicts that a population of cells falls out of sync in the absence
of a systemic cortisol cue, whereas a circadian cortisol rhythm induces
synchronization of PBL clock genes. Furthermore, as the entrainer (cortisol)
loses its circadian rhythmicity, our model predicts a regime of phase shift in
peripheral clock genes relative to cortisol before synchronization is lost at
very low circadian amplitudes. Similar phase shifted responses have been
observed in murine adipose tissues, where restricted feeding provokes a
disruption of glucocorticoid rhythmicity along with phase shifts in the
expression of peripheral clock genes (Zvonic, Ptitsyn et al. 2006).

Cortisol rhythmicity plays an
important role in entraining peripheral
clock genes and affecting the dynamics of the peripheral clock network. Our
work has significant clinical implications as cortisol is a key component of the
stress response and regulates the transcription of anti-inflammatory genes. In
human endotoxemia experiments, cortisol is acutely elevated and peripheral
clock genes in PBLs are suppressed (Haimovich, Calvano et al. 2010), which may have important effects on the inflammatory
response, given the interplay between clock genes and cytokine production in
peripheral immune cells (Coogan and Wyse 2008). Furthermore, in chronic stress when circadian rhythms in cortisol can
be lost (Lowry and Calvano 2008), peripheral circadian rhythms may be similarly desynchronized or
shifted as predicted by our model. The work presented here will serve as a
foundation for studying these interconnections between circadian rhythms and
the inflammatory response.

Figure 1: The degree of synchronization R_syn
(ratio of the variance of the mean field to the mean variance of each
oscillator, R_syn=1 is synchronized, 0 otherwise) and the standard
deviation of the distribution of cells' phases  both change as a function of
cortisol's amplitude.

  References

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Coogan, A. N.
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Haimovich, B.,
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Lowry, S. F.
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Yamamoto, T.,
Y. Nakahata, et al. (2005). "Acute physical stress elevates mouse period1
mRNA expression in mouse peripheral tissues via a glucocorticoid-responsive
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Yao, Z., D. C.
DuBois, et al. (2006). "Modeling circadian rhythms of glucocorticoid
receptor and glutamine synthetase expression in rat skeletal muscle." Pharm
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Zvonic, S., A.
A. Ptitsyn, et al. (2006). "Characterization of Peripheral Circadian
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