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Circadian rhythms in glucocorticoid signalling and pulmonary inflammation
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
The circadian clock drives ~24hr rhythms in a variety of processes, from gene expression through to behaviour, facilitating anticipation of daily changes in the external environment and temporal separation of internal processes. This pacemaker is a critical regulator of immune function and many inflammatory diseases show time-of-day variation in symptom severity. Disruption of the pacemaker by manipulation of the daily cycle of light and dark exposure (experimental ‘jet lag’) is known to exacerbate inflammatory responses to innate immune challenge, and recent evidence has highlighted immuno-modulatory roles for components of the molecular oscillator in peripheral tissues. The adrenal-derived glucocorticoid hormones are potent anti-inflammatory molecules and are capable of modulating circadian oscillations in peripheral tissues. This, along with their rhythmic secretion profile, makes them key candidates as mediators of circadian regulation of inflammatory signalling.Utilising adrenalectomy, timed glucocorticoid administration, hormone clamp and genetic targeting of the glucocorticoid receptor in mice, I present evidence for an interaction between glucocorticoid signalling and the circadian pacemaker in regulating the pulmonary inflammatory response to lipopolysaccharide (LPS) challenge. The neutrophilic response to aerosolised LPS exhibits a clear time-of-day effect in vivo, which is lost after disruption of endogenous glucocorticoid production via adrenalectomy. However, replacement of a rhythmic circulating glucocorticoid concentration with a flat daily average using a subcutaneous hormone clamp does not disrupt the inflammatory rhythm. Finally, a novel mouse strain was produced with disrupted expression of the glucocorticoid receptor (GR) in bronchial epithelial cells (Ccsp-GR-/-). These cells are critical regulators of circadian rhythmicity in the lung and drive rhythmic neutrophil influx in response to LPS stimulation through production of the chemokine CXCL5. Loss of GR in the bronchial epithelium was associated with a loss of rhythmic neutrophil influx after challenge, but anti-inflammatory sensitivity to the synthetic glucocorticoid dexamethasone remained.Collectively, these data show that appropriate temporal modulation of pulmonary inflammation requires functional glucocorticoid signalling, although the ligand itself does not need to oscillate. The retention of anti-inflammatory dexamethasone sensitivity suggests a role for cross-talk between the bronchial epithelium and additional cell populations, consistent with recent evidence for immuno-suppressive macrophage-epithelium communication in the lung. These are the first studies to dissect the mechanistic links between clocks, glucocorticoids and immunological responses in a target tissue.