CRYPTOCHROMES confer robustness, not rhythmicity, to circadian timekeeping

Area of Science:

• Chronobiology
• Molecular Biology
• Genetics

Background:

• Circadian rhythms are fundamental to mammalian physiology, driven by molecular clocks.
• Transcriptional feedback loops involving CLOCK, BMAL1, PERIOD (PER), and CRYPTOCHROME (CRY) proteins are the established model.
• CRY proteins are considered crucial for cellular timekeeping, as evidenced by behavioral arrhythmicity in CRY-deficient mice.

Purpose of the Study:

• To investigate the role of CRYPTOCHROME proteins in cellular and behavioral circadian rhythms.
• To challenge the established model by examining circadian function in the absence of CRY proteins.
• To identify alternative mechanisms sustaining circadian rhythms.

Main Methods:

• Analysis of locomotor activity rhythms in CRY-deficient (CKO) mice under specific environmental conditions.
• Measurement of PER2 protein rhythms in cells lacking CRY.
• Assessment of temperature compensation and period determination by CK1δ/ε activity.

Main Results:

• Locomotor rhythms and cellular PER2 rhythms were observed in CKO mice, contradicting previous assumptions.
• CRY-less oscillations exhibited variability and shorter periods compared to wild-type.
• Key circadian features like temperature compensation and period determination by CK1δ/ε remained intact.
• PER2 protein rhythms persisted for several cycles due to circadian variation in protein stability, independent of rhythmic Per2 transcription.

Conclusions:

• Circadian rhythms can be sustained in the absence of CRY proteins, suggesting they are not strictly essential for core timekeeping.
• The core circadian timekeeping mechanism appears to be post-translational, involving protein stability.
• While transcriptional feedback provides robustness, post-translational regulation is fundamental for circadian oscillations.