Characterization of a C3a Receptor in Rainbow Trout and Xenopus: The First Identification of C3a Receptors in Nonmammalian Species

Department of Pathobiology, University of Pennsylvania School of Veterinary Medicine, Philadelphia, PA 19104, USA.
The Journal of Immunology (Impact Factor: 4.92). 09/2005; 175(4):2427-37. DOI: 10.4049/jimmunol.175.4.2427
Source: PubMed


Virtually nothing is known about the structure, function, and evolutionary origins of the C3aR in nonmammalian species. Because C3aR and C5aR are thought to have arisen from the same common ancestor, the recent characterization of a C5aR in teleost fish implied the presence of a C3aR in this animal group. In this study we report the cloning of a trout cDNA encoding a 364-aa molecule (TC3aR) that shows a high degree of sequence homology and a strong phylogenetic relationship with mammalian C3aRs. Northern blotting demonstrated that TC3aR was expressed primarily in blood leukocytes. Flow cytometric analysis and immunofluorescence microscopy showed that Abs raised against TC3aR stained to a high degree all blood B lymphocytes and, to a lesser extent, all granulocytes. More importantly, these Abs inhibited trout C3a-mediated intracellular calcium mobilization in trout leukocytes. A fascinating structural feature of TC3aR is the lack of a significant portion of the second extracellular loop (ECL2). In all C3aR molecules characterized to date, the ECL2 is exceptionally large when compared with the same region of C5aR. However, the exact function of the extra portion of ECL2 is unknown. The lack of this segment in TC3aR suggests that the extra piece of ECL2 was not necessary for the interaction of the ancestral C3aR with its ligand. Our findings represent the first C3aR characterized in nonmammalian species and support the hypothesis that if C3aR and C5aR diverged from a common ancestor, this event occurred before the emergence of teleost fish.

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    • "The positive charges of mammalian C3a C-termini have been suggested to interact with negatively charged amino acid residues, localized at the basis of the second extracellular loop of the specific C3aR (Chao et al., 1999; Wetsel et al., 2000). The remarkable differences found between human and teleost sequences in the second extracellular loop of the C3aR (Boshra et al., 2005) could account for a different ligand–receptor interaction mechanism. The inability of teleost C3a to promote chemotaxis could be associated with the failure to bind to the specific C3aR as in the mammalian model. "
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    • "The fold change and p-value for each time point is summarized in Table 2. rtIL-6 stimulation did not affect the expression of IRF-4 (Holland et al., 2010), ␤-defensin-1 (Casadei et al., 2009), TGF-␤1(Hardie et al., 1998), IL- 8 (Laing et al., 2002b), IL-11 (Wang et al., 2005) and IL-17C1(Wang et al., 2010b). A small but significant up-regulation by rtIL-6 was seen with CISH (Wang et al., 2010a,b,c), ␥IP (Laing et al., 2002a), C3aR (Boshra et al., 2005), complement C4 (Wang et al., 2003), IL-6 itself, IL-10 (Inoue et al., 2005) and IL-20 (Wang et al., 2010c). However , the expression of TNF-␣1 (Laing et al., 2001) and to a lesser extent TNF-␣2 (Zou et al., 2002) was down-regulated at multiple time points by rtIL-6. "
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