The Vascular Circadian Clock in Chronic Kidney Disease

Abstract

Chronic kidney disease is associated with extremely high cardiovascular mortality. The circadian rhythms (CR) have an impact on vascular function. The disruption of CR causes serious health problems and contributes to the development of cardiovascular diseases. Uremia may affect the master pacemaker of CR in the hypothalamus. A molecular circadian clock is also expressed in peripheral tissues, including the vasculature, where it regulates the different aspects of both vascular physiology and pathophysiology. Here, we address the impact of CKD on the intrinsic circadian clock in the vasculature. The expression of the core circadian clock genes in the aorta is disrupted in CKD. We propose a novel concept of the disruption of the circadian clock system in the vasculature of importance for the pathology of the uremic vasculopathy.

Keywords: ICAM-1; VCAM-1; adhesion molecules; cardiovascular disease; ccl2; inflammation; thrombomodulin; uremia; vascular calcification.

Figure 1

The core components of the molecular circadian clock. Transcription–translation feedback loops of the circadian clock are shown. The transcription factors BMAL1 and CLOCK are core components of the molecular circadian clock positive limb. BMAL1 and CLOCK heterodimerize and bind to E-box elements in the promoters of period 1–3 (Per1–3) and cryptochrome 1–2 (Cry1–2) genes, which constitute the negative limb in the feedback loop. PER and CRY proteins dimerize and translocate back into the nucleus, hindering CLOCK and BMAL1 transcriptional activity and thereby repress their own expression. The main loop is modulated by an accessory feedback loop driven by the BMAL1/CLOCK induction of Rev-Erbα/β and RORα mediating opposing actions and repressing and activating BMAL1 gene expression, respectively. The circadian clock components regulate the expression of clock-controlled tissue-specific output genes, and about 4% of the aorta transcriptome shows circadian rhythmicity.

Figure 2

Differential effect of a feeding cue on the circadian clock in the aorta and liver. Many peripheral tissues are known to be entrained by the time of feeding, including the liver, a classical organ primarily responding to feeding cues rather than light input. The input of light and feeding were dissociated by restricting feeding to the habitual inactive period of the nocturnal rats (light period). The effect of restricted feeding on the circadian clock in the rat aorta and in the rat liver are shown. Expression of the core circadian clock genes Bmal1 and Per3 in the rat aorta (left) and liver (right) subjected to ad libitum feeding (top row, black, n = 38) and after 4 weeks of feeding restricted to ZT2-ZT12 (bottom row, blue, n = 39) was examined. As expected, the phase of the circadian clock in the liver was markedly shifted by restricted feeding. In contrast, the phase of the circadian clock in the aorta was not affected by the feeding time. Circadian rhythmicity was assessed by fitting data to a Cosinor regression model (solid lines). Gray areas indicate the dark period, and white areas indicate the light period. Zeitgeber time (ZT; “time-giver”) is the time since light onset. The red bar indicates the feeding time. Gene expression is normalized to the housekeeping gene Hprt1.

Figure 3

Disturbed expression of circadian clock genes in the uremic aorta after 8 weeks of CKD. Expression profiles of the core circadian clock genes in the aorta after 8 weeks of uremia (red lines) (n = 44) are compared to the normal controls (black lines) (n = 39). Dots represent each animal. Data are fitted by Cosinor regression, and the resulting p-values are shown within the figures. Gray areas indicate the dark period, and white areas indicate the light period. Zeitgeber time (ZT; “time-giver”) is the time since light onset. A significant difference in the Mesor (rhythm-adjusted mean) is indicated on top of the single figures. * Indicates a significant difference (p < 0.05) between the groups at specific time points. Data were previously presented in reference [48] and reprinted with permission from Kidney Int.

Figure 4

Graphic overview of the disturbances of the vascular circadian clock in chronic kidney disease (CKD). The circadian clock operates in the cells that comprise the vasculature, such as endothelial cells, VSMCs and fibroblasts. In CKD, the vascular circadian clock is disturbed and associated with a disturbance in the diurnal rhythm of chemokines and adhesion molecules such as VCAM-1 and ICAM-1, as well as of clotting factors, blood pressure, hormones and white blood cells, which may contribute to uremic vasculopathy by means of calcification, atherosclerosis, blood clotting, increased migration of immune cells into the vasculature and increased oxidative injury.