Heart cells have their own 24 hour clock

The biological clock, governed by a master clock in the hypothalamus in the brain and peripheral clocks located in nearly all of our tissues and organs, synchronises our bodies with the 24-hour day.

Scientist recently discovered the heart cells are governed by their own circadian rhythm and how the mechanism works.

According to a study published in 2021, led by researchers at the MRC Laboratory of Molecular Biology in Cambridge, the heart cells follow a clock dictated by a complex system of protein concentration and ion transfer. The importance of circadian rhythms within heart cells is neatly summarised by co-author, Dr John O’Neill. “It allows the heart to anticipate demand rather than simply react to it.” This means that the heart is already primed to beat faster before the demand arrives.

The body’s main circadian clock is located within the suprachiasmatic nuclei (SCN), within the hypothalamus. The SCN helps to coordinate the body’s daily biological clock and keep it aligned with external stimulus night and day. However, the SCN is not our only biological clock. Most cells have their own internal clocks these include cardiomyocytes (heart cells). This means the heart can have its own internal daily rhythm that is not dictated entirely by instructions from the brain.

What Dr O’Neill and his team found was that the heart’s internal circadian clock helps regulate its electrical activity through daily changes in intracellular protein and ion concentrations.

Approximately 6–20% of cellular proteins are under circadian control, meaning their abundance varies according to an approximately 24-hour cycle. An important regulator of this is mTORC, which is a protein complex that helps govern protein synthesis. So, when mTORC activity increases, concentration of soluble proteins inside the cell tends to increase and when its activity decreases the protein concentration decreases.

The total amount of protein inside the cell remained constant so alongside mTORC activity, protein redistribution inside the cell also has an effect. Relocation of proteins into different compartments of the cell can also change the protein concentration in the cytosol. This alongside proteins being produced and degraded effects the osmotic gradient between inside and outside the cell.

This is important because the concentration of soluble proteins inside the cytosol changes the physical environment inside the cell. If concentration of soluble proteins increases in the cytosol the water potential would decrease causing water to move into the cell from the surrounding by osmosis. This would cause the cell to swell and effect normal cellular function.

But researchers found that even with the daily changing of protein concentration in the cytosol the volume of the cell remained remarkably stable. In heart cells the changing in cytosol protein concentration is mediated by the cell changing its internal ion concentration. There are two major transport proteins involved, KCC transporters (move potassium and chloride ions out of the cell together) and NKCC transporters (move sodium, potassium and two chloride ions into the cell together). These two transporters are controlled by proteins called WNK kinases and OXSR1/SPAK. So how does it work? If cellular protein concentration increases, KCC activity moves potassium and chloride ions out of the cell which compensates for the extra soluble proteins inside the cell.

However, as the chloride ion concentration inside the decreases the KCC activity becomes limited because chloride ions needed for the potassium ion transport are outside the cell. Due to the low Cl- concentration inside the cell and high protein concentration WNK is activated. WNK phosphorylates (adds a phosphate group) to OXSR1 and SPAK which causes them to regulate the KCC and NKCC transporters. The KCC activity drops and the KNCC activity increases. At first this sounds counterproductive as KNCC moves ions into the cell, but it is providing an increase in chloride ion concentration so the KCC can continue pumping potassium ions out of the cell. This leads to an overall net loss of potassium from the cell meaning the number of osmotically active particles inside the cell decreases.

In short, potassium loss helps adjust the osmotic effect of increased soluble protein, while sodium and chloride ion transport helps maintain the ion gradients necessary for this process to continue. If the concentration of soluble proteins in the cytosol then decreases, the balance then shifts towards ion import rather than export.

Cardiomyocytes (heart cells), use difference in ion concentration across their membranes to generate electrical signals. Normally the inside of the cell is more negative compared to the outside of the cell. This is due to the concentration of potassium ions being higher inside the cells and the concentration of sodium ions being higher outside the cell. For an electrical signal to be generated the cell must become depolarised meaning the inside of the cell becomes less negative. The quicker it depolarises the more electrical impulses it can generate per minute. Research has showed as intracellular ion concentration increases it increases the speed at which the cell depolarises meaning more electrical signals are generated and a higher heart rate.

To investigate this, researchers cultured mouse cardiomyocytes on multielectrode arrays. This allowed them to measure the frequency of electrical impulses generated by the cells. They discovered that cardiomyocytes fired electrical impulses more frequently when intracellular ion concentration was higher. When they inhibited mTORC activity using a drug called torin1, these daily differences in electrical firing disappeared, but returned after the drug was removed.

To investigate whether this mechanism also influenced heart rate, they studied isolated mice hearts and found that their beating frequency varied depending on the time of day. These differences disappeared after mTORC inhibition. Daily variations in heart rate persisted in living mice even when the autonomic nervous system was blocked demonstrating that the heart must have an internal mechanism that contributes to its circadian rhythm.

However, the heart having its own circadian clock comes at a cost. Factors such as shift work, disrupted sleep and sudden changes in light exposure can cause circadian misalignment. This is when the body’s main biological clock in the SCN becomes uncoordinated with the clocks in the cardiomyocytes.

Research by Hayter et al. (2021) found that some electrical activity in the heart would adjust quickly and some would remain aligned with their original circadian rhythm when participants took on a night shift routine. This evidence suggests that changes in sleeping patterns can temporarily disrupt coordination of electrical activity within the heart. The researchers also found that at different times of day the susceptibility to potentially dangerous heart rhythms varied in mice. Although this does not directly prove that disruptions to the circadian rhythm in the heart causes arrhythmias in human shift workers, it raises important questions about the potential long-term consequences of irregular working hours.

These finding could have important implications for future medicine. Understanding how circadian rhythms change the electrical activity in the heart could help doctors interpret ECGs more accurately. Further research into whether the time-of-day treatments are administered effects their effectiveness and if the hearts internal clock could become a target for new therapies, may lead to more effective treatment for patients. However, before new clinical practices are brought into effect, further research is needed.

Ultimately, the research by Stangherlin et al. (2021) and Hayter et al. (2021) demonstrates that the heart is not only responding to instructions from the brain but also follows its own internal biological rhythm. Understanding how this rhythm works and what happens when it becomes disrupted may prove important in improving our understanding of cardiovascular health and disease.