Many physiological processes present near-24h oscillations, even in constant conditions (1). For example, the hormone cortisol is at high levels in the blood in the morning, and low levels in the evening; another well defined rhythm is that of body temperature, which in human is high in the day and at its lowest point in the late night. These daily rhythms are called circadian rhythms, and they are generated by circadian clocks. The research in the Laboratory of Molecular Chronobiology aims at deciphering how these clocks work, and more specifically, to find an answer to three questions: What are the mechanisms or gears of these biological clocks? How can these clocks control physiology? How are these clocks controlled by the environment?
The master circadian clock in mammals such as mice or humans is located in the brain, more specifically in a region of the hypothalamus called the suprachiasmatic nucleus. However, recent research has shown that clocks are also present in other brain regions and in most peripheral organs too. During evolution, circadian clocks have evolved by giving an advantage to living organisms, by adapting their physiology to the cyclically changing environment (e.g. light/dark, warm/cold daily cycles), and by organizing in time the processes occurring in various tissues and within cells. Thus, it is not a surprise that disturbances of circadian clocks have been associated with various diseases, including cancer, metabolic diseases and mood disorders.
1. The identification and study of clock components
This project addresses molecular mechanisms at play in circadian clocks in mice. Circadian clocks are made up of so-called "clock genes" (2), and animals or human subjects with mutated clock genes present rhythm disturbances, even arrhythmicity in some cases. The work in the laboratory in the past years has led to the identification of new clock genes. One example of this is the finding that proteins of the REV-ERB and ROR families can act as clock components (3, 4). Surprisingly, the analysis of the expression of these new clock genes in mouse tissues has highlighted striking tissue-specific differences, which underlines the specificities of circadian rhythm role and regulation across the body. Our work has also provided important information on how clock proteins work together in generating rhythms: the studies on the modification and fate of CLOCK, BMAL1 and CRY proteins has added new layers of complexity in our knowledge of the circadian clockwork (5). In line with the aims of finding new clock components and unravelling new modes of regulation in the clock, we recently initiated the study of the circadian role of USP2, a protein known to remove ubiquitin tags from proteins (in collaboration with Dr. Simon Wing, McGill University). Tagging of clock proteins with ubiquitin, and the subsequent degradation of these tagged proteins, has been shown to be critical for circadian timing (2). We now found that the reverse reaction (removal of the ubiquitin tag) is important for the clock too: USP2 binds to and deubiquitinates the clock protein PER1 and that mice devoid of USP2 present abnormal rhythms (6).
2. The circadian control of the immune response
What is the role of clocks in peripheral organs? In collaboration with Dr. Nathalie Labrecque (Maisonneuve-Rosemont Hospital, University of Montreal), we study circadian rhythms in lymphoid organs, i.e. organs involved in the development and function of immune system cells. More specifically we found that lymph nodes possess a circadian clock, and we are now studying how this immunological clock can control the presentation of antigens to T cells the initial event of the immune response to pathogens. We found that T cells from lymph nodes taken at different times of the day show different levels of proliferation when stimulated via their T cell receptor in vitro, and that this is associated with a rhythm of an important protein in this process, called ZAP-70 (7). Furthermore, we found that mice immunized during the day with an antigen carried by antigen-presenting cells respond more strongly to this antigen than if immunization is done during the night (7). In addition to studying how the clock regulates the immune response, we also study how the circadian clock responds to a state of inflammation, and showed that inflammation has strong tissue-, time- and gene-specific effects on clock genes in rats (8).
3. The regulation of human peripheral clocks
In order to study the regulation of human central and peripheral circadian clocks by environmental cues, we teamed up with Dr. Diane Boivin (Douglas Institute, McGill University). We assess the function of human peripheral oscillators by quantifying the expression of clock genes in human samples, in particular white blood cells. We found that clock genes can indeed be used to study the function of human circadian clocks under different environmental conditions (9). Building on these results, we applied this technology to subjects undergoing a protocol simulating night shift work, and they were able to show that clock gene expression in white blood cells becomes aligned to a simulated night shift after several days of bright light exposure, but that their response is slower than that of the central clock (10). We also looked at clock gene expression in post-mortem brain tissue, and found that rhythms of clock gene RNAs differ greatly between the brains of Alzheimer’s disease patients compared to those of control aged subjects, which might underlie the sleep and circadian rhythms of these patients (11).
1. Cermakian, N. & Boivin, D. B. (2003) Brain Res Brain Res Rev 42, 204-20.
2. Duguay, D. & Cermakian, N. (2009) Chronobiol Int 26, 1479-513.
3. Guillaumond, F., Dardente, H., Giguère, V. & Cermakian, N. (2005) J Biol Rhythms 20, 391-403.
4. Mongrain, V., Ruan, X., Dardente, H., Fortier, E. E. & Cermakian, N. (2008) Genes Cells 13, 1197-210.
5. Dardente, H., Fortier, E. E., Martineau, V. & Cermakian, N. (2007) Biochem J 402, 525-536.
6. Yang, Y. et al. (2012) Biology Open, publiée en ligne, doi: 10.1242/bio.20121990.
7. Fortier, E.E. et al. (2011) J Immunol 187:6291-300.
8. Westfall, S., Aguilar-Valles, A., Mongrain, V., Luheshi, G.N. & Cermakian, N. (2013) PLoS ONE 8, e59808.
9. James, F. O., Boivin, D. B., Charbonneau, S., Belanger, V. & Cermakian, N. (2007) Chronobiol Int 24, 1009-34.
10. James, F. O., Cermakian, N. & Boivin, D. B. (2007) Sleep 30, 1427-36.
11. Cermakian, N., Waddington Lamont, E., Boudreau, P. & Boivin, D.B. (2011) J Biol Rhythms 26, 160-170.