Vitellogenin is an acute phase protein with bacterial-binding and inhibiting activities
Introduction
Vitellogenin (Vg) constitutes the precursor of major yolk proteins in all egg-laying organisms. In non-mammalian vertebrates, Vg is usually synthesized in the liver of female animals, released into the bloodstream and transported via circulation to the ovary, where it is internalized by the maturing oocytes and proteolytically cleaved to form the yolk proteins, providing the energy reserves for later developing embryos and larvae (Arukwe and Goksøyr 2003; Finn and Kristoffersen 2007; Matozzo et al. 2008). However, the roles of Vg appear to extend beyond this nutritional function. For example, in the advanced eusocial honeybee, Vg has been shown to be involved in the social organization, temporal division of labor and foraging specialization, regulation of hormonal dynamics and change in gustatory responsiveness (Amdam et al. 2003; Amdam et al. 2006; Guidugli et al. 2005; Nelson et al. 2007). Besides, the honeybee Vg has been demonstrated to be able to reduce oxidative stress by scavenging free radicals, thereby prolonging the lifespan in the facultatively sterile worker castes and reproductive queen castes (Seehuus et al. 2006). Similar antioxidant activity has also been observed for the nematode (Caenorhabditis elegans) Vg (Nakamura et al. 1999).
Another novel function of Vg is linked with the host immune defense. Vg has been demonstrated to possess both hemagglutinating and antibacterial activities in the protochordate Branchiostoma belcheri as well as in the bony fish rosy barb Puntius conchonius (Shi et al. 2006; Zhang et al. 2005). More recently, piscine Vg has been revealed to be a multivalent pattern recognition molecule capable of identifying non-self components including lipopolysaccharide (LPS), peptidoglycan (PGN), lipoteichoic acid (LTA) and glucan, and to act as an opsonin that can enhance macrophage phagocytosis (Li et al. 2008; Liu et al. 2009). However, the potential immunological role of Vg in vivo remains open. Interestingly, injection of Escherichia coli into the male rosy barb has been shown to be able to lead to an increase in serum Vg (Shi et al. 2006). This suggests that Vg may be related to infection-resistant immunity, but little is known regarding the dynamics of Vg synthesis as well as the significance of serum Vg in the male fish. We therefore sought to address these issues in this study.
Section snippets
Fish
Zebrafish, Danio rerio aged approximately 5 months were purchased from a local fish dealer. The male fish were selected and acclimated for at least 3 days before experiments. They were fed and cared following the established protocols of Westerfield (1995).
Injection and sampling
Pilot experiments revealed that intraperitoneal (i.p.) injection of LPS and LTA (Sigma-Aldrich, USA) at a dose of 5 μg/g was able to induce a significant increase in Vg mRNA within 1 h in male D. rerio (data not shown). Therefore, in the
Up-regulation of Vg1 expression by LPS and LTA
qRT-PCR assays for determination of Vg1 expression and its response to injection with LPS and LTA in D. rerio are presented. The expression levels of the gene were given in fold increase compared to the same gene expression level in control fish. The dissociation curve of amplification products showed only a single peak, indicating that the amplification was specific (data not shown).
Injection of LPS soon resulted in a significant up-regulation of Vg1 expression (Fig. 1A). The expression level
Discussion
Acute-phase response comprises a complex series of reactions initiated in response to infection, physical trauma, or malignancy, which forms part of the host's first line of systemic defense and begins to act well in advance of any antibody-mediated immune responses. It involves a large number of acute phase proteins functioning in a variety of defense-related activities such as preventing ongoing tissue damage, isolating and destroying the infective agents and activating the repair processes
Acknowledgements
This work was supported by Grants (30730072; 2006CB101805) of Natural Science Foundation of China (NSFC) and Ministry of Science and Technology (MOST) of China.
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These authors contributed equally to this work.