Cloning and functional expression of the canine anaphylatoxin C5a receptor. Evidence for high interspecies variability.

A cDNA clone, DTJP03, encoding an orphan receptor, was isolated from a canine thyroid library, and found to exhibit 68.6% amino-acid identity with the recently described human C5a receptor. This relatively low similarity first suggested that DTJP03 encoded either a C5a receptor subtype, or the presumably related C3a receptor. Binding studies performed on membranes from COS-7 cells expressing the recombinant receptor demonstrated that DTJP03 encoded a high-affinity C5a receptor, with a Kd of 1.2 nM. C3a was unable to compete for C5a binding. Intracellular free calcium concentrations were measured by Quin-2 fluorescence assays in Chinese hamster ovary cells stably transfected with the canine C5a receptor. C5a addition elicited an increase in the intracellular calcium concentration. Extracellular EGTA partially prevented this response, suggesting that activation of the C5a receptor promotes both the release of calcium from intracellular stores, and the influx of extracellular calcium. Genes encoding C5a-receptor subtypes were subsequently searched for by PCR in genomic DNA from human, canine, rat and bovine sources. The result was the amplification of a single gene fragment from each species, with about 70% identity between any two of them. The canine C5a receptor has therefore to be considered as orthologous to the human C5a receptor described previously. The low similarity between C5a receptors from different mammalian species is quite unusual for a G-protein-coupled receptor.