Signal transduction in macrophages by glycosylphosphatidylinositols of Plasmodium , Trypanosoma , and Leishmania : Activation of protein tyrosine kinases and protein kinase C by inositolglycan and diacylglycerol moieties

The perturbation of various glycosylphosphatidylinositol (GPI)-anchored surface proteins imparts profound regulatory signals to macrophages, lymphocytes and other cell types. The specific contribution of the GPI moieties to these events however is unclear. This study demonstrates that purified GPIs of Plasmodium falciparum, Trypanosoma brucei, and Leishmania mexicana origin are sufficient to initiate signal transduction when added alone to host cells as chemically defined agonists. GPIs (10 nM-1 microM) induce rapid activation of the protein tyrosine kinase (PTK) p59(hck) in macrophages. The minimal structural requirement for PTK activation is the evolutionarily conserved core glycan sequence Man alpha1-2Man alpha1-6Man alpha1-4GlcN1-6myo-inositol. GPI-associated diacylglycerols independently activate the calcium-independent epsilon isoform of protein kinase C. Both signals collaborate in regulating the downstream NF-kappa B/rel-dependent gene expression of interleukin 1alpha, tumor necrosis factor (TNF) alpha, and inducible NO synthase. The alkylacyl-glycerol-containing iM4 GIPL of L. mexicana, however, is unable to activate protein kinase C and inhibits TNF expression in response to other agonists, establishing signaling specificity among structurally distinct GPIs. GPI alone appears sufficient to mimic the activities of malaria parasite extracts in the signaling pathway leading to TNF expression. A mAb to GPI blocks TNF induction by parasite extracts indicating that GPI is a necessary agent in this response. As protozoal GPIs are closely related to their mammalian counterparts, the data indicate that GPIs do indeed constitute a novel outside-in signaling system, acting as both agonists and second messenger substrates, and imparting at least two separate signals through the structurally distinct glycan and fatty acid domains. These activities may underlie aspects of pathology and immune regulation in protozoal infections.