Rhythms in the beat: Circadian Clock Regulation of Cardiac Electrophysiology

Each year, millions of people in the UK experience heart rhythm problems (arrhythmias). Given that cardiac arrhythmias are a leading cause of cardiovascular-related deaths, understanding the genetic, biological, and lifestyle factors which contribute to the occurrence of cardiac arrhythmias is clearly timely and important. It is well-established that adverse cardiac events and arrhythmias are more likely to occur at certain times of the day. Recent work has demonstrated that the heart exhibits an inherent daily rhythm in its function, which is not simply a consequence of our behaviour (sleep and low heart rate at night, high activity and heart rate in the day). Rather this rhythm is in part driven by the body's internal clock (circadian) system, which consists of a network of clocks located in the brain and peripheral organs (including the heart). However, how the circadian system is coupled to heart function is not well understood. We believe that a local clock within the heart, and specifically within the parts of the heart which propagate electrical signals (the cardiac conduction system), exerts a strong effect on cardiac responses across the day and in response to perturbation. Our recent work demonstrates that in mice and humans, influence of the clock is not uniform across the conduction system (unexpected for a highly coupled system), and that cardiac conduction is disturbed by paradigms that affect the body's clocks. Moreover, we provide evidence for the first time that the heart is inherently (i.e. a property of the heart itself, not in response to changes in behaviour or systemic factor) more vulnerable to arrhythmias at specific times of day. These findings have an important implication for cardiovascular health, especially given that circadian disruption (linked to shift work, light at night, aging, etc.) is common within our modern society. On a more fundamental level, our findings and that of others demonstrates that the circadian clocks in our brain and heart are important for cardiac function. Understanding the role of different clocks and how they interact within the heart, is critical to deepening our understanding of cardiovascular physiology.

To address these important aspects of circadian biology and cardiac function, we will use unique mouse models that allow clocks within the brain, the heart, or throughout the body to be selectively turned off. By studying cardiac conduction in these models, we can unravel how different clocks in the body help regulate heart rare (HR) and conduction, and define differences in heart function at different times of the day. We will also determine how clocks in the body help (or hinder) the heart's ability to deal with disruptive events, such as sudden shifts in the daily light-dark (LD) cycle and altered feeding schedules (similar to experiences of shift work). We will also test directly how the heart's electrical responses and gene expression differ at different times of day, and when the clock has been disabled. Since our pilot work indicates that the clock does indeed have a strong impact on heart function, we will determine whether the heart clock itself is disturbed by rapid changes in the LD environment, and whether clock function exists within the parasympathetic nervous system (a primary route through which the brain directs heart activity). Finally, we will determine whether disruptive behaviour patterns (mimicking rotating shift work) can increase the likelihood of electrical malfunction in the heart.

Together, these studies will greatly advance our understanding of cardiac physiology and inform the general public, health care sector and regulatory bodies with regard to how our behavioural, social and work routines can impact our heart. The work may also open potential new therapeutic options which target the body clock pharmacologically or non-pharmacologically (e.g. with light).

It has long been known that the circadian clock has a strong influence over cardiac physiology. What remains unclear is how different clocks within the body work together to ensure proper heart function. The impact of the clock on the cardiac conduction system, specifically, has also not been defined. This is important because potentially lethal cardiac arrhythmias show a strong time-of-day incidence. Moreover, given that modern lifestyles often undermine our internal clock system, understanding the impact of circadian dysfunction to cardiac health is pressing and important.

Our new data reveal that in both humans and mice, rhythmic influences over key sites along the cardiac conduction pathway (SA and AV nodes) are not equal, and that acute alterations in behavioural routine can desynchronise aspects of the cardiac conduction system. Moreover, we show for the first time that the heart shows a pronounced time-of-day dependent vulnerability to induced ventricular tachycardia. The current proposal aims to define how clocks in the brain and heart influence cardiac electrophysiology and how circadian disruption (achieved through both genetic and environmental approaches impact heart function. To this end, we will:

1) Use tissue specific genetic targeting in mice (removing clock function in the brain, the heart, or throughout the body) in combination with longitudinal ECG recording under well-defined circadian LD and feeding paradigms
2) Imaging of cardiac clock function in freely moving mice (via a novel luciferase reporter) will define whether the clock itself is disturbed by autonomic challenge or shift in the LD cycle
2) Test directly how cardiac conduction is defined by time of day or presence of the heart clock using established stimulation/recording protocols and optogentic manipulations in Langendorff heart preparations and non-bias transcriptional profiling (RNAseq)
3) Determine whether chronic circadian disruption increases vulnerability to arrhythmia

The research questions posed within this proposal are of major interest to ACADEMIC GROUPINGS in Biological, Biomedical, and Clinical Sciences. The academic community will benefit from elucidation of mechanisms involved in circadian biology, cardiovascular physiology, genetics, neuroscience, and electrophysiology. As such, research findings will impact greatly on the HEALTH CARE COMMUNITY. We will disseminate findings by publishing primary papers and reviews in high impact journals, and presenting work at national and international meetings. Because our research could have immediate implications for the impact of social and/or work routine of cardiovascular health, we will regularly engage with clinical communities. This is greatly facilitated by our close proximity to the Central Manchester Teaching Hospitals, and day-to-day engagement with senior cardiovascular and clinical colleagues (eg Keavney, Clarke, Ray). DB and AT are embedded in the cardiovascular research domain in UoM, which integrates basic research academics with clinical cardiologists.

Our findings will be of interest to the GENERAL PUBLIC due to general interest in the circadian biology, the impact of our behaviour (when we eat, sleep, and work), as well as the prevalence of cardiovascular disease, cardiac conduction disorders and heart attack within our society. Modern society is placing our body clocks under tremendous strain, with circadian disruption, sleep deprivation and lack of natural light all now commonplace. Our work is readily accessible to the general public due to relatively wide-spread understanding of the heart and our body clocks. We will capitalise on this to deliver our research to a wide audience. At its most basic, the work will engage sections of the population who wish to learn about their health and human physiology. We will deliver our research both from a broad perspective, and in relation to this project, to as wide demographic as possible through public engagement activities, school visits and mass media. Our work has regularly featured in broadcast, web-based and printed media.

The proposed research is of interest to PHARMACEUTICAL COMPANIES due to direct implications for human cardiac function and arrhythmia. We have worked to embed an understanding of the clock into clinical development programmes (GSK and Pfizer), as well as identifying novel targets and approaches to tackle circadian dysfunction. DB has been engaged in a successful collaboration with Pfizer for a number of years, working together with them on the development of clock directed therapeutics. DB has recently entered into a formal research collaboration with the IPA team at Qiagen to maximise statistical, bioinformatics, and 'multi-omics' approaches used in our research. Benefits of this research to the UK ECONOMY are guaranteed beyond the research activity and generation of intellectual property. Nevertheless, in the longer term, economic and public health benefits may arise. Cardiovascular disease and cardiac related morbidity are a massive burden on the national health care service.

Embedded within this proposal is the advancement of tools and approaches to analyse large quantities of ECG data in human and animal studies. Where this often relies on hours of manual scoring, these developments allow days to weeks of recordings to be analysed quickly and accurately - critical to understanding how human physiology and disease. We are currently working with UoM BIOELECTRONICS (A Casson, UoM) to develop tools for well-tolerated long-term ECG recording and analysis, a development which would facilitate access into the general population and primary care.

This proposal also offers a significant opportunity for high-level in vivo training of the associated PDRA, and PhD students involved. This is a significant benefit as a lack of in vivo research training has been highlighted as a weakness in UK bioscience. BSc/MSc students will also be exposed to this work.