Time of Day Regulates Renal Mineralocorticoid Receptor Transcriptional Control of Electrolyte Balance

Abstract

Abstract The mineralocorticoid receptor (MR) has an established role in blood pressure control and cardiovascular homeostasis via many actions in the heart and kidney. We recently identified a role for the MR in controlling the circadian clock in cardiac cells and demonstrated that time-of-day impacts MR activation in the heart. While time dependent behaviours such as upright posture and fluid intake control aldosterone release via the renin-angiotensin-aldosterone system (RAAS), we hypothesise that the circadian clock controls aldosterone signalling by modifying MR transcriptional outcomes. Two established MR target genes and core circadian clock genes are period 1 (Per1) and period 2 (Per2). We have previously shown that a bolus dose of aldosterone (i.p.) induced cardiac expression of Per1 and Per2 in wildtype mice treated at 8AM (start of rest period) but not when administered at 8PM (start of active period). Whether MR activation in the kidney is similarly dependent on time of day and aligns with MR actions in the heart remains to be assessed. We also sought to determine if the MR directly regulates the molecular clock in a ligand-dependent manner. In contrast to the heart, renal Per1 and Per2 expression was not upregulated at four hours following aldosterone administration at either 8AM or 8PM. Interestingly, aldosterone administered at 8AM, but not at 8PM, significantly down-regulated expression of the circadian clock gene, nuclear receptor reverse strand c-ERBA (ReverbA); ReverbA was not regulated in the heart following aldosterone treatment. Investigation of renal-MR target genes involved in sodium ion transfer, showed a down-regulation of epithelial sodium channel 1 alpha (ENaC-α), by aldosterone administered only at 8AM. Conversely, aldosterone administered at 8AM induced gene expression of FKBP Prolyl Isomerase 5 (Fkbp5), an immunophilin that is important in nuclear trafficking of the MR. These data provide new insights for tissue sensitivity to MR activation in the heart and the kidney, which may play differing roles in contributing to regulating circadian patterns of several cardiovascular and renal parameters. Moreover, our data suggest that in addition to the accepted entrainment role of the glucocorticoid receptor (GR), the MR may also play a role in the entrainment of the circadian clock in peripheral tissues. Ongoing mechanistic studies will determine whether the MR can directly control the periodicity of the renal cellular clock, as it does in the heart. Key outcomes will be assessed in population databases to determine the clinical relevance of these findings. We propose that understanding of time-of-day dependent vulnerability to MR signalling in the heart versus the kidney may offer the rationale for the development of novel temporal or tissue specific-MR modulators in the management of cardiovascular disease.