Co-existing feedback loops generate tissue-specific circadian rhythms.

Area of Science:

• Molecular Biology
• Systems Biology
• Chronobiology

Background:

• Circadian rhythms are endogenous biological processes influencing daily physiological functions.
• Mammalian circadian clocks rely on complex gene regulatory feedback loops for autonomous rhythm generation.
• The precise design principles and tissue-specific variations of these core clock networks remain incompletely understood.

Purpose of the Study:

• To investigate the fundamental design principles of mammalian circadian clocks across different tissues.
• To identify the essential feedback loops responsible for generating circadian rhythms.
• To explore potential tissue-specific differences in clockwork mechanisms.

Main Methods:

• Utilized global optimization techniques to fit mathematical models to experimental circadian gene expression data.
• Analyzed multiple tissue-specific datasets to compare clock model parameters.
• Extracted and identified key feedback loop motifs across various model versions.

Main Results:

• Multiple parameter sets accurately reproduced experimental circadian gene expression data for all investigated tissues.
• Identified auto-inhibitions of Period (Per) and Cryptochrome (Cry) genes, Bmal1-Rev-erb-α loops, and repressilator motifs as key rhythm generators.
• Discovered significant differences in essential feedback loop compositions between mammalian tissues, such as the suprachiasmatic nucleus and liver.

Conclusions:

• Mammalian tissue clocks employ distinct sets of synergistic feedback loops, indicating tissue-specific design principles.
• The suprachiasmatic nucleus clock models are characterized by Per and Cry auto-inhibitions.
• Liver clock models feature synergistic loops integrated within a repressilator motif, potentially explaining functional organ differences.