Title: A circadian clock in the sinus node mediates day-night rhythms in Hcn4 and heart rate.

Authors: Alicia D'Souza, Yanwen Wang, Cali Anderson, Annalisa Bucchi, Mirko Baruscotti, Servé Olieslagers, Pietro Mesirca, Anne Berit Johnsen, Svetlana Mastitskaya, Haibo Ni, Yu Zhang, Nicholas Black, Charlotte Cox, Sven Wegner, Beatriz Bano-Otalora, Cheryl Petit, Eleanor Gill, Sunil Jit R J Logantha, Halina Dobrzynski, Nick Ashton, George Hart, Rai Zhang, Henggui Zhang, Elizabeth J Cartwright, Ulrik Wisloff, Matteo E Mangoni, Paula A da Costa Martins, Hugh D Piggins, Dario DiFrancesco, Mark R Boyett

Journal, date & volume: Heart Rhythm, 2021May, 18, 801-810

PubMed link: http://www.ncbi.nlm.nih.gov/pubmed/33278629

Abstract
Heart rate follows a diurnal variation, and slow heart rhythms occur primarily at night.The lower heart rate during sleep is assumed to be neural in origin, but here we tested whether a day-night difference in intrinsic pacemaking is involved.In vivo and in vitro electrocardiographic recordings, vagotomy, transgenics, quantitative polymerase chain reaction, Western blotting, immunohistochemistry, patch clamp, reporter bioluminescence recordings, and chromatin immunoprecipitation were used.The day-night difference in the average heart rate of mice was independent of fluctuations in average locomotor activity and persisted under pharmacological, surgical, and transgenic interruption of autonomic input to the heart. Spontaneous beating rate of isolated (ie, denervated) sinus node (SN) preparations exhibited a day-night rhythm concomitant with rhythmic messenger RNA expression of ion channels including hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4). In vitro studies demonstrated 24-hour rhythms in the human HCN4 promoter and the corresponding funny current. The day-night heart rate difference in mice was abolished by HCN block, both in vivo and in the isolated SN. Rhythmic expression of canonical circadian clock transcription factors, for example, Brain and muscle ARNT-Like 1 (BMAL1) and Cryptochrome (CRY) was identified in the SN and disruption of the local clock (by cardiomyocyte-specific knockout of Bmal1) abolished the day-night difference in Hcn4 and intrinsic heart rate. Chromatin immunoprecipitation revealed specific BMAL1 binding sites on Hcn4, linking the local clock with intrinsic rate control.The circadian variation in heart rate involves SN local clock-dependent Hcn4 rhythmicity. Data reveal a novel regulator of heart rate and mechanistic insight into bradycardia during sleep.