Expression of the mammalian calcium signaling response to Trypanosoma cruzi in Xenopus laevis oocytes

Abstract

Infective stages of the protozoan parasite Trypanosoma cruzi contain a soluble factor that induces elevation in the intracellular free Ca 2+ concentration ([Ca 2+] i) of mammalian cells. The process is pertussis toxin (PTx)-sensitive, and involves phospholipase C (PLC) activation, inositol 1,4,5-trisphosphate (IP 3) formation and Ca 2+ release from intracellular stores (Tardieux I, et al. J Exp Med 1994;179:1017–1022; Rodriguez A, et al. J Cell Biol 1995;129:1263–1273). We now report that a molecule exposed on the surface of the target cells is required to trigger the signaling cascade, and that a response with identical characteristics can be induced in Xenopus laevis oocytes injected with mRNA from normal rat kidney (NRK) fibroblasts. Xenopus oocytes do not show an endogenous response to the trypomastigote Ca 2+ signaling factor, but a vigorous response in the form of a propagating Ca 2+ wave is expressed after injection of NRK cell mRNA. As previously demonstrated for mammalian cells, the response is inhibited when injected oocytes are pretreated with PTx, implicating G αi or G αo trimeric G-proteins, and with thapsigargin, which depletes intracellular Ca 2+ stores. Moreover, the [Ca 2+] i transients triggered by the T. cruzi soluble factor in mRNA-injected oocytes are blocked by the same inhibitors of the parasite oligopeptidase B that abolish the [Ca 2+] i response in NRK cells (Burleigh B, Andrews NW. J Biol Chem 1995;270:5172–5180; Burleigh BA et al. J Cell Biol 1997;136:609–620). The NRK mRNA fraction that induces expression of the [Ca 2+] i response to the T. cruzi signaling factor contains messages from 1.5 to 2.0 kb, a size range consistent with the family of seven-transmembrane G-protein-coupled receptors.