Peripheral circadian clocks--a conserved phenotype?

Area of Science:

• Chronobiology
• Molecular Biology
• Physiology

Background:

• The mammalian circadian system, orchestrated by the suprachiasmatic nucleus (SCN) master pacemaker, synchronizes peripheral clocks for adaptation.
• A prevailing assumption is that peripheral circadian clocks maintain conserved phenotypes (period, robustness) under normal conditions.
• Understanding peripheral clock diversity is crucial for comprehending circadian regulation beyond the SCN.

Purpose of the Study:

• To investigate whether peripheral circadian clocks exhibit conserved phenotypes regarding gene expression and rhythm robustness.
• To assess the influence of the SCN and non-SCN time cues on peripheral clock characteristics.
• To determine tissue-specific differences in the functional ~24-hour clock cell populations.

Main Methods:

• Measured daily wheel-running activity (WRA) as a marker of SCN output in 84 male rats.
• Assessed mRNA expression of clock genes (rPer2, rBmal1) and a clock-controlled gene (rDbp) in olfactory bulbs (OBs), liver, tail skin, and white blood cells (WBCs).
• Analyzed temporal patterns, rhythm robustness, and potential differences in functional ~24-hour clock cells across tissues.

Main Results:

• Peripheral circadian clocks demonstrated significant differences in clock gene expression patterns, rhythm robustness, and potentially the number of functional ~24-hour clock cells.
• The liver exhibited the most robust clock output, suggesting a higher number of functional ~24-hour clock cells, while OBs showed the least.
• Circadian clock phenotypes are tissue-specific, influenced by both SCN signals and tissue-specific sensitivity to non-SCN time cues.

Conclusions:

• The circadian phenotype in peripheral tissues is not universally conserved and is highly tissue-specific.
• The SCN output significantly shapes clock phenotypes in the OBs and liver, whereas tail skin and WBCs are also influenced by other time cues.
• While core clock gene transcription forms the basic circadian phenotype, post-transcriptional and translational modifications further refine robustness and periodicity.