Carotenoids regulation in polymorphic noble scallops Chlamys nobilis under different light cycle
Introduction
Noble scallop Chlamys nobilis is an important scallop species for aquaculture, mainly due to its fast growth, rich nutrition (rich in amino acids, LC-PUFA and carotenoids) and high market value (Zheng et al., 2010, Zheng et al., 2012; Tan et al., 2019a, Tan et al., 2019b, Tan et al., 2020a). C. nobilis showed polymorphism not only in shell color but also in tissue color (orange and white) (Zheng et al., 2012). The different color of C. nobilis is caused by a pigment called carotenoids. The total carotenoid contain (TCC) of orange strain (Nan'ao Golden Scallop) (88 to 274 μg/g) was significantly higher than that of brown strain (common brown scallops) (22 to 76 μg/g) (Tan et al., 2019a). Light is one of the most important environmental factors affecting the carotenoid content of C. nobilis (Maneiro et al., 2017). However, the regulatory mechanism of carotenoids under 24L/0D and 0L/24D conditions is still unclear.
Carotenoids are a large group (at least 1100 compounds have been described) of hydrophobic molecules synthesized in photosynthetic plants, some fungi and microorganisms (Yabuzaki, 2017). Most carotenoids contain a C40 backbone and 10 to 11 conjugated bonds (Stahl and Sies, 2003). Carotenoids have many biological functions in animals, including providing oxidative stress protection (Handelman, 1996; Stahl and Sies, 2003) and ultraviolet radiation protection (Caramujo et al., 2012). Carotenoids are very powerful antioxidants. For instance, one molecule of β-carotene can scavenge more than 100,000 singlet oxygen without being destroyed (Handelman, 1996). When exposed to singlet oxygen, carotenoids are excited into a triplet state and rapidly return to normal state by releasing excess energy as heat (Handelman, 1996). In addition to the above functions, carotenoids are involved in innate immunity (Tan et al., 2020b) and environmental tolerance of bivalves (Tan et al., 2020c), including high temperature (e.g. Cheng et al., 2019, Cheng et al., 2020), low temperature (e.g. Tan et al., 2019a, Tan et al., 2020d), low salinity (e.g. Wang et al., 2019) and etc.
Carotenoids cannot be biosynthesized by animals, so these compounds must be obtained from their diet (Fraser and Bramley, 2004; Tan et al., 2020a). In addition to C. nobilis, two scallop (Yesso scallop Patinopecten yessoensis (Li et al., 2010) and Bay scallop Argopecten irradians (Du et al., 2017)) showed polymorphism in shell color and tissue color, which was also caused by carotenoids. Some genes, including Scavenger Receptor class B (SRB) genes, β-carotene oxygenase (BCO) genes, and Retinoic acid receptors (RAR and RXR) genes have been identified to have higher expression levels in the orange muscle scallop strains than in the white muscle scallop strains (Liu et al., 2015; Li et al., 2019; Andre et al., 2019). These genes are believed to be involved in the absorption, deposition and metabolism of carotenoids in scallops.
Scavenger Receptor class B (SRB) is a cell surface high density lipoprotein (HDL) receptor that mediates HDL uptake (Acton et al., 1996). Intestinal absorption of carotenoids includes dispersion of carotenoids in fat emulsion and dissolution of carotenoids in mixed bile salt micelles, and then transfer to the absorption site (Tyssandier et al., 2001). There is growing evidence that SRB is involved in the absorption of carotenoids and vitamin A (Liu et al., 2015). SRB-1 was first identified in Drosophila, in which the ninaD (encode for SRB-1) mutant lacked visual chromophore and significantly reduced carotenoid levels (Zuker, 1996), suggesting that SRB-1 plays an important role in carotenoid absorption in Drosophila (Kiefer et al., 2002). To date, the role of SRB in the absorption of various carotenoids has been confirmed, including lutein (Reboul et al., 2005), carotene (van Bennekum et al., 2005), zeaxanthin and xanthophylls (During et al., 2008). In the noble scallop Chlamys nobilis, t SRB genes was significantly associated with the absorption of carotenoid (Liu et al., 2015).
The first animal β-carotene oxygenase (BCO) was identified in Drosophila melanogaster, where Nina B (encode for BCO) mutant had significantly higher carotenoid accumulation (von Lintig et al., 2001). Subsequently, BCO-1 (β-carotene-15, 15′‑oxygenase) and BCO-2 (β-carotene-9′, 10′‑oxygenase) in several vertebrates were subsequently characterized (e.g. Leung et al., 2009; Vage and Boman, 2010; Amengual et al., 2013). BCO-1 is a cytosolic enzyme evolutionary conserved from invertebrates and vertebrates and it cleaves β-carotene symmetrically at 15–15′ position to produce two retinal molecules (Amengual et al., 2013). In contrast to BCO-1, BCO-2 has a wide range of substrate specificity, including oxidative cleavage of β-carotene, α-carotene, β-cryptoxanthin, xanthophylls lutein and zeaxanthin, but not apocarotenoids (Amengual et al., 2013). BCO-2 cleaves carotenoids at the 9′–10′ carbon‑carbon double bonds into C13- and C27-apocarotenoids, or further into C14-dialdehyde (Amengual et al., 2011; Mein et al., 2011). In recent years, Li et al. (2019) cloned and characterized BCO-1 from Patinopecten yessoensis. Interestingly, scallop BCO-1 has a broad substrate spectrum similar to BCO-2. In addition, the inhibition of BCO-1 in Patinopecten yessoensis with white adductor muscle resulted orange pigmentation and significant carotenoid deposition in adductor muscle (Li et al., 2019).
Retinoic acid receptor (RAR) and retinoid X receptor (RXR) are nuclear receptor superfamily, mostly ligand-dependent transcription factors, which regulate the expression of specific gene subsets of biological functions, (Rochette-Egly and Germain, 2009; Volgeler et al., 2017). RAR and RXR have been shown to regulate the expression of SRB-1, in which LXRα/RXR and LXRβ/RXR induce SRB-I transcription in human and mouse hepatoma cell lines, and in 3T3-L1 preadipocytes independently of SREBP-1 (Malerød et al., 2002). Recently, Andre et al. (2019) demonstrated that in the evolutionary process, the mollusc RARs lost the ability to mediate signal transduction (unresponsive to retinoic acids), but retained the ability to form heterodimer with RXRs. For RXR of mollusks, they are responsive to retinoic acids and can mediate the transcription of target genes, most of which play a modulatory role in the form of RAR/RXR heterodimer (Andre et al., 2019).
The absorption, transport and metabolism of carotenoids in mollusks is a complex process, which is still poorly understood. In order to reveal the genetic mechanism of carotenoids absorption and metabolism under 24L/0D and 0L/24D conditions, the expression of RAR, RXR, SRB-1, SRB-4, BCO-1 and BCO-2 genes in tissues of polymorphic C. nobilis (Nan'ao Golden Scallop and common brown scallop) were analyzed under different light cycles. To our knowledge, this work is the first to study the effects of light cycle on the accumulation of carotenoids in bivalves and its molecular regulatory mechanism. The only significant difference between Nan'ao Golden Scallop and common brown scallop is that the carotenoid content of Nan'ao Golden Scallop is much higher than that of common brown scallops. This study will advance our understanding on the molecular regulation mechanism of carotenoids in bivalves under different light cycles, which is of great significance for the management strategy to be implemented in bivalve hatcheries and has a significant impact on the overall health of bivalve broodstocks and larvae.
Section snippets
Experimental design
A number of 108 individual of adult (17 month old) Nan'ao Golden Scallop and common brown scallops were collected from Nan'ao Marine Biology Experimental Station of Shantou University, China. The shell length and shell weight were 69.43 ± 5.17 mm and 69.58 ± 13.96 g, respectively. All scallops were acclimated to laboratory condition for 72 h and then each strain was divided into 9 groups (12 scallops per group). Three groups of each strain(triplicate) were randomly selected and maintain under
Changes of TCC in adductor muscle and hemolymph of Nan'ao Golden Scallop and common brown scallop maintained under different light cycle
The TCC of adductor muscle and hemolymph of Nan'ao Golden Scallop and common brown scallop maintained under different light cycle is illustrated in Fig. 1. Under 24L/0D, the TCC of adductor muscle of Nan'ao Golden Scallop was significantly higher (F(2, 87) = 3.552, P < 0.05) than that of Nan'ao Golden Scallop maintained under 12L/12D and 0L/24D. In the hemolymph, the TCC of Nan'ao Golden Scallop maintained 0L/24D treatment was significantly higer than that of 12L/12D and 24L/0D treatments.
In
Discussion
In marine bivalves, carotenoids are mainly deposited in gonads, and their concentrations are affected by gonad development and maturation (Zheng et al., 2012). However in recent years, accumulation of carotenoids has been observed in adductor muscle of marine scallops, including Noble scallop C. nobilis (Zheng et al., 2010), Yesso scallop Patinopecten yessoensis (Li et al., 2010) and Bay scallop Argopecten irradians (Du et al., 2017). Interestingly, high levels of carotenoids in scallop tissues
Conclusion
In conclusion, 24L/0D and 0L/24D treatments significantly increased the accumulation of TCC in the adductor muscle of Nan'ao Golden Scallop and common brown scallop, respectively. The TCC accumulation in adductor muscles of polymorphic C. nobilis was induced by the up-regulation of RAR, SRB-1, SRB-4, BCO-1 and BCO-2 genes. In hemolymph, 0L/24D treatment decreased the TCC accumulation in hemolymph of Nan'ao Golden Scallop, while the accumulation of TCC in hemolymph of common brown scallops was
Author statement
Light cycle is an important factor affecting the total carotenoid content (TCC) in tissues of noble scallops Chlamys nobilis, but its regulatory mechanism is still unclear. In this study, adult polymorphic C. nobilis (Nan'ao Golden Scallop and common brown scallop) were exposed to three different light cycles (24L/0D, 12L/12D and 0L/24D) for 10 days. The expression of carotenoid related genes (Scavenger Receptor class B (SRB) genes, β-carotene oxygenase (BCO) genes, and Retinoic acid receptors (
Declaration of Competing Interest
None.
Acknowledgements
Present study was financially supported by the National Natural Science Foundation of China (31872563), China Modern Agro-industry Technology Research System (CARS-49), Guangdong Provincial Department of Education (2017KCXTD014), China.
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