Targeting SHIP1 and PI3Ky for a synergistic inhibition of mast cell activation

Abstract

Type I hypersensitivity reactions are immune system responses of the body triggered by the IgE/antigen-mediated activation of tissue localized mast cells but also by circulating basophils or eosinophils. Within minutes from their stimulation, mast cells release a plethora of preformed molecules whose activity induces vasodilatation and broncho-constriction and increases the vascular permeability, enhancing the recruitment of leukocytes. Such a hyperactivation of the immune system in response to a foreign molecule is commonly defined as allergic reaction. The stimulation of the high affinity IgE receptors, FceRI, expressed on the surface of mast cells is induced by the clustering of several IgE/FceRI complexes and promotes the formation of an intracellular signalosome that generates a cascate of signalling events. Class I phosphatydilinositol-3- kinases (PI3Ks) are activated downstream FceRI clustering and are responsible for the generation of PtdIns(3,4,5)P3 at the plasma membrane. On the other hand, the 5’-phosphatase SHIP1 is recruited via its SH2 domain at the plasma membrane where binds tyrosine-phosphorylated domains of several receptors and hydrolyses the PtdIns(3,4,5)P3 at the 5’ position of the inositol ring, generating PtdIns(3,4)P2. Being recognised by PH-domain bearing proteins, both PtdIns(3,4,5)P3 and PtdIns(3,4)P2 participates to the recruitment of a wide variety of effectors. PI3Kγ, the sole member of class IB PI3Ks, has been showed to have a pivotal role in mast cells recruitment to the tissues and degranulation as well as in systemic anaphylactic reactions; therefore PI3Kγ has been proposed as a pharmacological target for the treatment of inflammatory diseases. Interestingly, the current treatment of the allergic responses is based on the pharmacological amelioration of the symptoms and does not target the aetiology of the disease. In the present work we demonstrate the possibility of inhibiting PI3Kγ signalling in mast cells in order to modulate hypersensitivity responses without affecting the phisiological functionality of class I PI3Ks in the other tissues. In the first project described in the present manuscript, we show that during mast cells activation, the GPCR-mediated activation of PI3Kγinduces a signalling cascade that is not inhibited by the phosphatase activity of SHIP1 and therefore contribute to the reinforcement of the antigen-induced mast cells activation. Moreover we demonstrate that PI3Kγ and SHIP1 are two valid targets for a combined pharmacological inhibition of mast cells activation. In the second project described in the manuscript, we demonstrate the possibility to selectively modulate PI3Kγ activation in mast cells by blocking the plasma membrane localization of the monomeric GTPase Ras using farnesyltransferase inhibitor. We demonstrate that Ras is required for the activation of PI3Kγ in cells that express p84 as adaptor subunit (p110γ/p84 heterodimer), such as mast cells but not in cells where the PI3Kγ active complex is p110γ/p101. In the final part of the manuscript we described the generation of three genetically modified mouse strains we developed: p101 knock-out, p84 knock-out and p84 knock-in. The two p84-mutant mice were designed as novel tools for the analysis of the physiological p110γ/p84 signalling in vivo and ex vivo, while the generation of the p101 mutant has been already characterised by other groups but will be further used in our research.