Abstract
The classical notion of a centralized clock that governs circadian rhythmicity has been challenged with the discovery of peripheral oscillators that enable organisms to cope with daily changes in their environment. The present study aimed to identify the molecular clock components in Atlantic cod (Gadus morhua) and to investigate their daily gene expression in fast skeletal muscle. Atlantic cod clock genes were closely related to their orthologs in teleosts and tetrapods. Synteny was conserved to varying degrees in the majority of the 18 clock genes examined. In particular, aryl hydrocarbon receptor nuclear translocator-like 2 (arntl2), RAR-related orphan receptor A (rora) and timeless (tim) displayed high degrees of conservation. Expression profiling during the early ontogenesis revealed that some transcripts were maternally transferred, namely arntl2, cryptochrome 1b and 2 (cry1b and cry2), and period 2a and 2b (per2a and per2b). Most clock genes were ubiquitously expressed in various tissues, suggesting the possible existence of multiple peripheral clock systems in Atlantic cod. In particular, they were all detected in fast skeletal muscle, with the exception of neuronal PAS (Per-Arnt-Single-minded) domain-containing protein (npas1) and rora. Rhythmicity analysis revealed 8 clock genes with daily rhythmic expression, namely arntl2, circadian locomotor output cycles kaput (clock), npas2, cry2, cry3 per2a, nuclear receptor subfamily 1, group D, member 1 (nr1d1), and nr1d2a. Transcript levels of the myogenic genes myogenic factor 5 (myf5) and muscleblind-like 1 (mbnl1) strongly correlated with clock gene expression. This is the first study to unravel the molecular components of peripheral clocks in Atlantic cod. Taken together, our data suggest that the putative clock system in fast skeletal muscle of Atlantic cod has regulatory implications on muscle physiology, particularly in the expression of genes related to myogenesis.
Highlights
Circadian clocks are ubiquitous time-keeping systems found in a wide range of organisms, enabling them to adjust to light (L) - dark (D) cycles over a 24 h period [1]
Partial sequences of 18 clock genes from both loops of the clock machinery were cloned in Atlantic cod: arntl1, arntl2, clock (JN643707), cry1a (JN643708), cry1b (KC204818), cry-dash (KC204819), cry2 (JN643709), cry3 (JX035865), npas1 (KC204820), npas2 (JX035866), nr1d1, nr1d2a, nr1d2b, per1 (JN643710), per2a (JN643711), per2b (JX035863), timeless (JN634817) and rora (KC204821)
The two Atlantic cod per2 genes clustered in two separate groups, along with other teleost orthologs, such G. aculeatus, O. latipes, O. niloticus, T. nigroviridis, T. rubripes and Paralicthys olivaceus
Summary
Circadian clocks are ubiquitous time-keeping systems found in a wide range of organisms, enabling them to adjust to light (L) - dark (D) cycles over a 24 h period [1]. The core machinery of the circadian clock consists of intracellular transcriptional-translational feedback loops regulated by clock genes and corresponding proteins [1]. The positive arm of the core clock is formed by two members: circadian locomotor output cycles kaput (Clock) and Aryl hydrocarbon receptor nuclear translocator-like (Arntl or Bmal). The positive arm of the core clock is formed by two members: circadian locomotor output cycles kaput (Clock) and Aryl hydrocarbon receptor nuclear translocator-like (Arntl or Bmal1) They are basic-helix-loop-helix (bHLH) transcription factors that heterodimerize via a PAS domain in the cytosol [6]. Clock:Arntl activates the orphan nuclear receptors Rev-erb a/b and RAR-related orphan receptor alpha (Rora) These genes are part of the core clock, as they link the feedback loops through their proteins that repress or activate arntl expression. This autoregulatory mechanism results in a cyclic, self-sustained expression of clock genes with an approximately 24-h period [1]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
