| Summary: | 博士 === 國防醫學院 === 生命科學研究所 === 100 === Ca2+ is an essential element that involved many physiological functions, including muscle contraction, synaptic transmission, cell signaling transduction and others in animals (Vangheluwe et al., 2009; Peacock, 2010). The regulation of Ca2+ uptake and emission are closely associated with the bone structure in vertebrates (Witten and Huysseune, 2009). The bone could support the body, provide sustained force for support and protect the organs (Datta et al., 2008). Therefore, to maintain Ca2+ homeostasis in vertebrates is important issue.
Terrestrial vertebrates mainly obtain Ca2+ from food, but aquatic vertebrates like fish, absorb Ca2+ from surrounding water. However, both terrestrial and aquatic vertebrates share the similar mechanism of Ca2+ uptake in specific ionocytes. Fish are continuously facing fluctuating environmental Ca2+ concentrations, and therefore it is critical for fish to control Ca2+ flow into or out of their bodies (Pan et al., 2005).
Many previous studies proposed the hormonal control of Ca2+ uptake in teleosts mainly based on physiological data (Evans et al. 2005; Hwang and Perry 2010). However, those previous studies did not clearly illustrate the regulatory mechanisms and pathways of the hormonal control of Ca2+ uptake probably due to the research limitations in the species studied (Hwang and Perry 2010). Those species lack well-developed genetic database and applicability of various molecular physiological approaches, and this appears to increase the difficulty to study the details of endocrine regulatory mechanisms.
Recently, zebrafish have become a model for research on ion regulation and the related endocrine controls because of the advantages in bioinformation and molecular physiology. Therefore, zebrafish provide a good model to further explore the regulatory mechanism and pathway of endocrine control in teleost Ca2+ uptake.
In this study, zebrafish was used as a model to explore the vitamin D and cortisol control of Ca2+ uptake in teleosts. The present study was delineated in the following 2 chapters.
Chapter I: Differential action of paralogous vitamin D receptors in calcium handling of zebrafish (Danio rerio)
Vitamin D is a well-known calciotropic endocrine in mammals. In carp and cod, vitamin D was reported to elevate the serum Ca2+ level (Swarup et al., 1991; Sundell et al., 1993), suggesting a similar calciotropic effect of vitamin D in teleosts. According to the current model, active transcellular Ca2+ transport is carried out through the apical epithelium channels (ECaC), and basolateral plasma membrane Ca2+-ATPase (PMCA) and the Na+/Ca2+ exchanger (NCX) in specific ionocytes in mammals and teleosts (Hoenderop et al., 2005; Hwang and Lee, 2007). It is unclear how vitamin D controls teleost Ca2+ uptake mechanism through regulations of the Ca2+-related transporters. Furthermore, vitamin D spreads its function mainly through vitamin D receptor (VDR). So far, there is no convincing data to reveal the physiological functions of 2 paralogous VDRs in teleosts. In this chapter, the regulatory mechanisms and pathways of vitamin D in Ca2+ uptake of zebrafish were dissected by treating zebrafish with exogenous vitamin D and knocking down the translation of VDRs.
Chapter II: Reverse effect of mammalian hypocalcemic cortisol in fish: cortisol stimulates Ca2+ uptake via glucocorticoid receptor-mediated vitamin D metabolism
Glucocorticoid (GC) causes hypocalcemic effects on mammals because it induces malabsorption and malemission of Ca2+ in the intestines and kidneys (Lukert and Raisz, 1990; Patschan et al., 2001; McLaughlin et al., 2002). In trout, cortisol (glucocorticoid) could stimulate branchial Ca2+ uptake and ecac transcript (Shahsavarani and Perry, 2006; Kelly and Wood, 2008), suggesting that calciotropic effect of cortisol in teleosts is different from the case in mammals; whereas, it is still unclear whether cortisol controls only ECaC or includes other Ca2+ transporters (NCX and PMCA). On the other hand, teleosts may lack aldosterone synthase (Baker, 2003; Nelson, 2003), and therefore cortisol is the main corticosteroid (CS). Some in vitro studies demonstrated the activation of teleost glucorcoticoid receptor (GR) or/and mineralocorticoid receptor (MR) by cortisol (Colombe et al., 2000; Bury et al., 2003), implying the physiological regulation by cortisol through GR or/and MR. Nevertheless, there is no study to demonstrate that cortisol regulates Ca2+ uptake mechanism through GR or/and MR. In chapter I, we dissected the mechanisms how vitamin D stimulates Ca2+ uptake in zebrafish. In mammals, glucocorticoid reveals different effects on vitamin D metabolism (Seeman et al., 1980; Bikle et al., 1993; Cosman et al., 1994); therefore it is evolutionally and physiologically important to explore the relationship between cortisol and vitamin D in zebrafish. In this chapter, the regulatory mechanisms and pathways of cortisol in Ca2+ uptake of zebrafish are investigated by exogenous cortisol treatment and translational knockdown of GR/MR.
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