Telomeres and time: Effects of circadian rhythm disruption on telomere dynamics

Biological rhythms are found throughout the tree of life, and new tools used to study them have demonstrated the pervasiveness of circadian regulation across processes in the body. The circadian system had evolved to interact with reliable alternation of 24 h day-night rhythmicity, but human lifestyles have become increasingly disconnected from geophysical time. The availability of light at night, an increasing demand for shift-work and travel across time zones all disrupt the circadian system in modern life. Such disruption has major impacts on health including reduced longevity and accelerated ageing. Captive animals such as nocturnal laboratory rodents can also experience severe circadian disruption, often being kept in inappropriate light regimes, and fed and disturbed during the hours of daylight. A clear understanding of how the adverse effects of circadian disruption come about is therefore needed.

The circadian clocks within the body ensure that different physiological processes take place at the most appropriate time of day. Temporal partitioning through circadian regulation ensures that processes such as DNA replication and cell division take place at a time when the risk of DNA damage from the Reactive Oxygen Species (ROS) produced during metabolism is minimised. One largely unstudied route whereby circadian disruption could affect long term health and the incidence of age related disease is through effects on telomere loss. Telomeres are protective caps at the end of linear chromosomes and enable cells to identify chromosome ends. They consist of tandem repeats of a non-coding sequence of bases. Some DNA is lost from the chromosome ends during cell division, and the amount lost can be increased by oxidative damage to the telomere. Telomeres in most body cells therefore shorten with each round of cell division. They eventually reach a critical length at which the cells stop dividing and often die. The rate of telomere loss differs among individuals and is accelerated by environmental influences. If the circadian orchestration of cell division, generation of ROS and antioxidant protection is disrupted, increased telomere shortening could give rise to premature aging.

In this project we will investigate whether circadian disruption accelerates telomere loss by disrupting the light cues that animals use to set their circadian rhythms. We will measure the consequences of this for the temporal patterning of key physiological and molecular process, patterns of cell division and telomere loss. We will use continuous light, which supresses the circadian rhythm, and using a simulated shift-work protocol. We will do this with three age classes of animal, since young animals, where telomere loss is generally higher, may be at more risk. We will use the zebra finch, a well-studied vertebrate that has daytime activity and telomere dynamics similar to humans, and whose telomere length is predictive of longevity. We will address the following questions:

1) Does circadian disruption increase telomere loss? We will compare telomere loss under control conditions to that of birds exposed to circadian disruption. We predict that telomeres will shorten more quickly under circadian disruption.

2) Are the effects on telomere loss associated with reduced temporal coordination in the body? We will continuously monitor activity patterns and the temporal pattern of key physiological parameters and the pattern of cell division. We will relate this to telomere loss and DNA damage.

3) Do the effects differ among age classes? We will apply the treatments to young, mid-age, and old birds.

4) Do the effects vary with the nature of the circadian disruption? We expect the effects of the clock suppression under constant light to be more severe than those of the shift-work protocol.

The circadian system coordinates processes across the body. Its disruption has serious health consequences for organisms, including humans, such as premature ageing and increased disease risk. One route whereby this could occur is via effects on telomere dynamics, changes to which are known to influence health and the occurrence of age-related diseases. Surprisingly, this has been little studied. This project will investigate telomere loss in response to circadian disruption. It will test the prediction that circadian disruption will accelerate telomere attrition by disrupting the temporal coordination of activity rhythms, clock components, Reactive Oxygen Species generation, antioxidant levels and cell division. Two types of circadian disruption, clock-suppressive constant bright light and repeated phase shifts mimicking shift work or repeated jet lag, will be applied to the zebra finch. This well-studied vertebrate has diurnal rhythmicity and telomere dynamics similar to humans. We have previously shown that its telomere length is predictive of longevity, and that telomere attrition is influenced by environmental factors. Applying the treatments to 3 age classes, we will compare telomere attrition at 3 time points in red blood cells by measuring telomere length using a well-established qPCR protocol. For the 3 time points, we will also establish 24 h profiles of plasma melatonin, expression of the clock gene Bmal1, and levels of DNA damage and antioxidants. We will measure the time profile of cell proliferation in each group using flow cytometry. This experimental set-up will enable us to examine 1) whether circadian disruption increases telomere loss, 2) whether this is associated with the degree of uncoupling of the cell cycle and circadian clock 3) whether the effects differ with age class and 4) whether they differ with the type of disruption.

Modern human lifestyles have become increasingly disconnected from the 24 h day-night environment in which circadian systems have evolved. Use of light at night, shift-work and travel across time zones all disrupt the circadian system. Inappropriate light and feeding regimes can also create welfare problems for managed animals. This project is concerned with the effect of disrupted circadian rhythms on long term health, and specifically examines the effect on telomere dynamics. Increased telomere loss is a mechanism that could underlie many of the effects reported for circadian disruption, but has been very little investigated in this context. Importantly, we will use a diurnal species, which is more similar to humans than the nocturnal rodents usually used in animal experiments on the health consequences of circadian disruption. We will use two levels of disruption - one that effectively removes cues and suppresses the circadian system, and another that mimics the continually shifting light cues to which human-shift workers are subject, and which have been associated with premature ageing and increased incidence of age-related diseases.. We will compare the effects of these two types of disruption, and also examine whether any of the effects on telomere dynamics are affected by the age of subjects.
The following groups would benefit from this research as described below.
i) Academic researchers. The work will open up new research areas by examining the effects of uncoupling physiological process, the temporal pattern of cell division and telomere loss. There is recognition that more diurnal models are needed in such circadian studies, and the project will provide this. The project will test the extent to which the disruption of the circadian patterns in the cell cycle are associated with increased telomere loss, opening up a new avenue for further research. Both PIs are involved in interdisciplinary networks that enhance their ability to communicate the work to target audiences.
ii) Health care community - those dealing with the health of shift workers may be stimulated to investigate mitigating processes.
iii) Policy makers - shift work is essential in modern societies for some occupations and there has been substantial recent concern over its effect. Circadian researchers have suggested improved schedules for shift-work, which are however only hesitatingly implemented. The results of this project could contribute to evidence-based policy development, for example on whether particular age classes are more vulnerable to telomere attrition. This could contribute to changing organisational culture and practices.
iv) Commercial sector - shift work constitutes a substantial proportion of the work mode in the UK, and hence, health and safety of the workers, as well as high-quality performance, are of great interest.
v) People who care for animals in different capacities - nocturnal and crepuscular species are kept as pets and also in zoos. Most commonly however, nocturnal rodents and crepuscular rabbits are kept in research establishments. That the essentially diurnal circadian rhythms in the husbandry systems might affect their health and welfare has received little attention. The results of this project will be of interest in the development of appropriate animal management regimes, new codes of practice and guidelines for the keeping of diurnal and nocturnal species.
vi) General public - the use of artificial light at night, involvement in shift work and travel across time zones has brought circadian disruption to private homes. Thus, the effect of the pattern of light exposure on health is of great interest to the general public, who will benefit from obtaining an improved understanding of its effects on quality of life, health and well-being.
These groups will benefit from the collection of robust scientific data, whether or not telomere loss is accelerated by circadian disruption.