Circadian rhythmicity and photic plasticity of myosin gene transcription in fast skeletal muscle of Atlantic cod (Gadus morhua) Carlo C. Lazado, Kazue Nagasawa 1 , Igor Babiak, Hiruni P.S. Kumaratunga 2 , Jorge M.O. Fernandes ⁎ Faculty of Biosciences and Aquaculture, University of Nordland, 8049 Bodø, Norway abstract article info Article history: Received 1 April 2014 Received in revised form 24 April 2014 Accepted 24 April 2014 Available online xxxx Keywords: Circadian rhythm Environmental plasticity Epigenetics Myosin Skeletal muscle Photoperiod The circadian rhythm is a fundamental adaptive mechanism to the daily environmental changes experienced by many organisms, including ﬁsh. Myosins constitute a large family of contractile proteins that are essential func- tional components of skeletal muscle. They are known to display thermal plasticity but the inﬂuence of light on myosin expression remains to be investigated in ﬁsh. In the present study, we have examined the circadian rhythmicity and photoperiodic plasticity of myosin gene transcription in Atlantic cod (Gadus morhua) fast skel- etal muscle. In silico mining of the Atlantic cod genome resulted in the identiﬁcation of 76 myosins representing different classes, many of which were hitherto uncharacterized. Among the 23 fast skeletal muscle myosin genes, myh_tc, myh_n1, myh_n4, myo18a_2, and myo18b_2 displayed circadian rhythmic expression and contained several circadian-related transcription factor binding sites (Creb, Mef2 and E-box motifs) within their putative promoter regions. Also, the circadian expression of these 5 myosins strongly correlated with the transcription pat- tern of clock genes in fast skeletal muscle. Under ex vivo conditions, myosin transcript levels lost their circadian rhythmicity. Nonetheless, different photoperiod regimes inﬂuenced the mRNA levels of myh_n4, myo18a_2 and myo18b_2 in fast skeletal muscle explants. Photoperiod manipulation in Atlantic cod juveniles revealed that continuous light signiﬁcantly elevated mRNA levels of several myosins in fast skeletal muscle when com- pared to natural photoperiod. The daily rhythmicity observed in some fast skeletal muscle myosin genes suggests that they may be under circadian clock regulation. In addition, the inﬂuence of photoperiod on their expression implies that myosins may be involved in the photic plasticity of muscle growth observed in Atlantic cod. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Most organisms adjust their behavior and physiology to the daily (circadian) cycle of day and night. This circadian rhythm is controlled by a complex molecular clock machinery that is highly conserved in the animal kingdom (Vatine et al., 2011; Katherine Tamai et al., 2003). The core system of the molecular clock is composed of interlocked auto-regulatory transcriptional–translational feedback loops that are regulated by clock genes and their proteins (Cahill, 2002; Dardente and Cermakian, 2007). In ﬁsh, the central clock is believed to be located in the pineal gland or retina (Falcón, 1999). Besides these organs, several tissues also express clock genes in a circadian rhythmic manner, thus in- dicating that there may be multiple peripheral oscillators (Whitmore et al., 1998, 2000; Tamai et al., 2005). It is believed that the components of the clock system do not only regulate the core members of the transcriptional–translational loop but they also are regulators of other genes (McCarthy et al., 2007). The genes that are under the clock coordination are termed clock- controlled genes and they are responsible for integrating the clock mechanism and physiological pathways, eventually orchestrating bio- logical processes in a circadian fashion (McCarthy et al., 2007; Amaral and Johnston, 2012). There is a paucity regarding the extent to what clock mechanisms regulate the transcriptional network in fast skeletal muscle in ﬁsh. Nevertheless, biological clocks are thought to play a key role in mammalian muscle physiology. For instance, myoD, a mem- ber of myogenic regulatory factors family, is believed to be under clock control. In mouse fast muscle, MyoD is expressed in a circadian rhythmic manner and the absence of a functional clock mechanism disrupts the rhythmicity of gene expression, as well as both Peroxisome proliferator activated receptor γ coactivator 1α (Pgc-1α) and Pgc-1β, leading to struc- tural and functional alterations at the cellular level in this tissue (Andrews et al., 2010). Further, the core enhancer (CE) in the promoter region of MyoD is necessary for its rhythmic expression, and the core clock proteins, Circadian locomotor output cycles kaput (CLOCK) and Brain and muscle Arnt-like protein-1 (BMAL1) bind to a conserved non-canonical E-box within the CE (Zhang et al., 2011). Moreover, in a Marine Genomics xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +47 75517736; fax: +47 75517457. E-mail address: jorge.fernandes@uin.no (J.M.O. Fernandes). 1 Present Address: Laboratory of Aquacultural Biology, Graduate School of Agricultural Science, Tohoku University, 1-1 Tsutsumidori-amamiyamachi, Sendai, 981-8555 Japan. 2 Present Address: Department of Zoology, Faculty of Applied Sciences, University of Sri Jayewardenepura, Nugegoda, Sri Lanka. MARGEN-00213; No of Pages 9 http://dx.doi.org/10.1016/j.margen.2014.04.011 1874-7787/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Marine Genomics journal homepage: www.elsevier.com/locate/margen Please cite this article as: Lazado, C.C., et al., Circadian rhythmicity and photic plasticity of myosin gene transcription in fast skeletal muscle of Atlantic cod (Gadus morhua), Mar. Genomics (2014), http://dx.doi.org/10.1016/j.margen.2014.04.011