(352g) In Silico Model of Suppression and Desynchronization of Peripheral Clock Genes in Human Endotoxemia

Circadian rhythmicity is maintained in the human body through a hierarchical web of cell autonomous and self-sustained molecular clocks that drive transcriptional oscillations to a group of genes commonly known as clock genes. These biological clocks are synchronized with environmental light/dark cycles and provide circadian information to numerous physiological processes including sleep/wake activity, feeding rhythms, and body temperature in order to optimize organism’s performance (Dibner, Schibler et al. 2010). The immune system is also under circadian regulation since a wide range of immune parameters such as the level of red blood cells, peripheral blood mononuclear cells and cytokines undergo daily fluctuations. Data further suggest that the immune system can also provide inputs to clock genes network that can adjust its activity and ultimately modulate numerous physiological rhythms such as circadian production of cytokines and hormones. Importantly, acute as well as chronic stress conditions induce significant alterations in the rhythmic expression of peripheral clock genes disturbing their synchronized dynamics further uncoupling them from light mediated body rhythm (Haimovich, Calvano et al. 2010).This loss of body’s synchronization by environmental cues, may negatively impact health by impairing host defense against viral infection and other diseases. To further assess these effects, in this work we discuss a mathematical model of human endotoxemia where we evaluate the responses of peripheral clock genes in response to endotoxin stimulus mostly relative to their intercellular synchronization, as well as the rhythmic implications to critical immune compartments such as cortisol’s and pro-/anti-inflammatory cytokines circadian rhythm.

Elective administration of endotoxin (lipopolysaccharides, LPS) to otherwise healthy human volunteers has been used to study systemic inflammation and gain insight into behaviour of inflammatory mediators encountered in acute, as well as chronic, inflammatory diseases. Human endotoxemia precipitates signs and symptoms similar to what is observed clinically in sepsis patients and early trauma patients.  While we do not argue that the human endotoxin challenge model precisely replicates an acute infectious or septic condition, we believe that endotoxemia does serve as a useful model of TLR4 agonist-induced systemic inflammation by providing a reproducible experimental platform tying systemic inflammation to physiological signal generation (Haimovich, Reddell et al. 2010). Based on our prior work on modeling human endotoxemia (Scheff, Mavroudis et al. 2011), we aim to develop a multi-compartment mathematical model that incorporates the hypothalamic pituitary adrenal (HPA) gland’s mediated circadian secretion of cortisol, and the rhythmic secretion of pro- (P) and anti- (A) inflammatory cytokines that is mediated by the peripheral clock gene (PCG) network of immune cells. Administration of LPS leads to the activation of the NF-kB signaling pathway that ultimately leads to the upregulation of the pro-inflammatory cytokines (Scheff, Calvano et al. 2010). Increase pro-inflammatory cytokines further mediates the inflammatory signal to HPA axis that regulates the circadian expression of cortisol and ultimately the expression of anti-inflammatory cytokines that serve as the immunoregulatory component of the system in order to restore homeostasis. Recently, we showed that cortisol circadian rhythm apart from playing a key anti-inflammatory role in the inflammatory response, it synchronizes the expression of a population of peripheral clock genes in an amplitude and frequency dependent manner (Mavroudis, Scheff et al. 2012). In addition, recent experimental data indicate that pro-inflammatory cytokines such as TNF-α and IL-1 which rhythmic production is mediated by PCGs (Keller, Mazuch et al. 2009), downregulate the expression of E-box mediated transcription of PCGs (Cavadini, Petrzilka et al. 2007) forming in such a way a feedback regulatory mechanism between clock genes and pro-inflammatory cytokines.

For this paper, we aim to investigate the bidirectional link between P and PCGs. We hypothesize that pro-inflammatory cytokines inhibit the E-box mediated transcription of Per and Cry clock genes by blocking CLOCK/BMAL1 transcription factor binding to this region. The circadian secretion of P is mediated by the ensemble rhythm of a population of PCGs that are entrained by F. The endotoxin stimulus in our model is hypothesized to drive the upregulation of P that further induces F that apart from entraining PCGs, is assumed to regulate the A compartment of our model that ultimately leads to inhibition of the pro-inflammatory response by inhibiting P. Our model indicates that administration of LPS ultimately leads to a loss of intercellular phase coherency to the population of PCGs. This intercellular desynchronization is time of day dependent, pointing higher values at these times of day that PCGs reach their zenith levels.

Our work represents a step towards understanding the biological mechanisms of human endotoxemia. By evaluating the underlying dynamics resulting from the link between P and PCGs we start deciphering the mechanistic details leading to body’s loss of rhythmicity on stress conditions. The connections between circadian clock and immune system of the body is of high importance with strong current interest and our work contributes to this field with new interesting facts.

References:

Cavadini, G., S. Petrzilka, et al. (2007). "TNF-α suppresses the expression of clock genes by interfering with E-box-mediated transcription." Proceedings of the National Academy of Sciences 104(31): 12843-12848.

Dibner, C., U. Schibler, et al. (2010). "The mammalian circadian timing system: organization and coordination of central and peripheral clocks." Annu Rev Physiol 72: 517-49.

Haimovich, B., J. Calvano, et al. (2010). "In vivo endotoxin synchronizes and suppresses clock gene expression in human peripheral blood leukocytes *." Critical Care Medicine 38(3): 751-758 10.1097/CCM.0b013e3181cd131c.

Haimovich, B., M. T. Reddell, et al. (2010). "A novel model of common Toll-like receptor 4- and injury-induced transcriptional themes in human leukocytes." Crit Care 14(5): R177.

Keller, M., J. Mazuch, et al. (2009). "A circadian clock in macrophages controls inflammatory immune responses." Proceedings of the National Academy of Sciences 106(50): 21407-21412.

Mavroudis, P. D., J. D. Scheff, et al. (2012). "Entrainment of peripheral clock genes by cortisol." Physiol Genomics.

Scheff, J. D., S. E. Calvano, et al. (2010). "Modeling the influence of circadian rhythms on the acute inflammatory response." J Theor Biol 264(3): 1068-76.

Scheff, J. D., P. D. Mavroudis, et al. (2011). "Modeling autonomic regulation of cardiac function and heart rate variability in human endotoxemia." Physiol Genomics 43(16): 951-64.