IN XENOPUS OOCYTES THE HUMAN C3A AND C5A RECEPTORS ELICIT A PROMISCUOUS RESPONSE TO THE ANAPHYLATOXINS

The Xenopus laevis oocyte has been widely utilized for cloning and fun ctional expression of G-protein coupled receptors (GPCR), This system was used for the functional expression and characterization of the rec ently identified human C3a receptor, Complementary RNA from the human C3a receptor was transcribed in vitro and microinjected into Xenopus o ocytes for functional characterization. A positive response to a synth etic C3a peptide agonist and to C3a, but not to platelet activating fa ctor or fMetLeuPhe was detected, In addition, a response of approximat ely one third the amplitude obtained with C3a was obtained with rC5a, Conversely, oocytes co-injected with the C5a receptor and total RNA is olated from U937 cells responded to C5a as well as to C3a and the C3a synthetic peptide. A functional response with the anaphylatoxin C3a re ceptor in oocytes was dependent on co-injection of a pertussis toxin s ensitive complementary human factor which could be supplied by co-inje ction of total RNA isolated from U937 cells. Oocytes expressing the an aphylatoxin C3a and C5a receptors responded to both agonists, in each case the response to the cognate ligand was substantially more robust than the response elicited by the other anaphylatoxin.