Shannon

Molecular Mechanisms of Circadian Rhythms

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Circadian Clocks

In order to synchronize with the Earth’s 24-hour day-night cycle, many organisms possess biological clocks which regulate physiological processes.(1) In vertebrate animals, biological clocks exist in most organs and tissues, monitored by a master clock located in the brain.(2)

In cells throughout the body, gene expression and protein levels oscillate in a 24-hour pattern, in turn affecting functions including sleep, body temperature, and hormone production.(3) External factors such as light-dark cycles influence circadian rhythms by activating gene expression.(4) Because circadian cycles modulate many biological functions, circadian disruption from environmental stressors can lead to dysregulation in sleep, metabolism, hormones, and organ function.

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This investigation sought to explore the mechanisms of biological clocks at a molecular scale.

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Genes & Proteins

Circadian rhythms are maintained by the cyclical expression of several genes, which then activate other genes or proteins within organs such as the heart, brain and liver. The core clock genes CLOCK and BMAL1 are responsible for these physiological changes.(5) These genes also drive the expression of circadian genes including PER, CRY, and CHRONO.

As these protein complexes accumulate in the cytoplasm, they form complexes which can enter the nucleus.(6) Upon doing so, the complexes inhibit CLOCK and BMAL transcription, leading to downregulation of the protein complexes themselves, at which point CLOCK and BMAL are no longer inhibited, and the cycle begins again, having taken approximately 24 hours.(7)

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Double-time gene Controls Length of Circadian Rhythms Through Period

This study identified a gene double-time (dbt) critical to the maintenance of a 24-hour circadian cycle. Researchers compared circadian rhythms in flies with mutations in the gene double-time, as well as wild type flies. They observed Drosophila flies' timed locomotor activity to measure circadian cycle length. The flies were entrained to a 24-hour light-dark cycle, then moved to an all-dark environment. Mutations to this gene caused variation in the length of Drosophila circadian rhythms, ranging from 18.0 to 26.8 hours. To investigate the molecular mechanisms behind this relationship, the researchers measured protein and mRNA levels for other circadian genes. In particular, they looked at PER and TIM, both of which have a self-inhibitory feedback loop. Under normal conditions, PER proteins are relatively unstable and rely on binding with TIM to remain stable. However in organisms with largely reduced DBT function, the study found continuously increasing levels of PER, indicating that DBT acted to destabilize and cause the breakdown of these proteins. Since the self-inhibitory loop was responsible for maintaining circadian rhythms, double-time is necessary to fix the rate at which PER accumulates and therefore the length of the cycle.

Article: https://www.cell.com/fulltext/S0092-8674(00)81224-6#secd32622841e1013

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Synchronized Circadian Rhythms in the Absence of Bmal1

This experiment sought to determine whether circadian rhythms can be maintained in BMAL- rats. These organisms lack the mechanisms for traditional circadian cycling, yet may be entrained to a 24-hour light-dark pattern. The scientists tested whether this rhythm could be induced ex vivo using liver cells and skin fibroblasts from both BMAL- and BMAL+ rats. These tissue samples were given a glucocorticoid hormone to synchronize the circadian clocks of their contained cells, then measured gene expression every 3 hours for 3 days. The researchers found several thousand oscillating genes even in BMAL- rats, indicating that circadian clocks can remain synchronized in peripheral tissues without canonical circadian cycles. However, the genes discovered had little overlap with the oscillating genes in BMAL+ rats, suggesting that an independent mechanism is responsible for this synchronicity. The researchers hypothesized that ETS transcription factors may be at play, since they were expressed in peaks around subjective dawn and dusk times.

Article: https://science.sciencemag.org/content/367/6479/800