PIP5-Kinase Dependent PIP2 Generation Impairs T Cell Activation by Rigidifying the Actin Cortex
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Antigen-specific activation of T cells is initiated by the interaction of T cell receptors (TCRs) with the peptide-loaded MHC complex on antigen presenting cells (APCs). A specialized junction, the immunological synapse (IS), is formed at the T cell-APC interface, where cell surface receptors, signaling molecules as well as cytoskeletal elements rearrange into a highly organized structure (Monks et al., 1998; Grakoui et al., 1999; Cemerski et al., 2006; Fooksman et al., 2009). Dynamics actin and remodeling plays a pivotal role in IS formation and T cell activation (Wülfing et al. 1998, 2003; Dustin et al., 2000; Billadeau et al., 2007; Burkhardt et al., 2008; Gomez et al., 2008). It is believed that the activities of many actin regulatory proteins are spatially and temporally regulated by phosphatidylinositol 4,5-bisphosphate (PIP2) (Sechi et al, 2000; Yin et al., 2003; Logan et al., 2006). How actin dynamics is regulated by PIP2 in T cell activation is unclear. Generation of PIP2 at the plasma membrane can be accomplished by the three isoforms of the type I phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks): α, β, and (Ishihara et al., 1996; Loijens and Anderson, 1996; Oude Weernink et al., 2004). It is proposed that different PIP5K isoforms synthesize distinct pools of PIP2 in the plasma membrane and contribute to the versatility of PIP2 function (Kanaho et al., 2007; van den Bout et al., 2009). The roles of these PIP5Ks in T cell activation have not been addressed. The experiments in this thesis are designed to investigate how regulation of PIP2 levels by changing cellular PIP5K levels affects actin cytoskeletal dynamics at the IS and T cell activation. The major findings of this thesis are: PIP5K isoforms are differentially targeted during T cell activation by APCs. PIP5Kα and γ87 are mainly targeted to the IS, whereas β and γ90 isoforms accumulate at the uropod or distal pole region of the T cells. Overexpression of PIP5Ks caused defective actin polymerization at the periphery of the IS and rigid T cell morphology, impaired TCR proximal signaling and decreased IL-2 secretion. Knockdown of PIP5Kγ instead induced sustained peripheral actin accumulation at the IS, enhanced T cell proximal signaling and IL-2 secretion. Furthermore, we found that the activity of the actin-binding ERM proteins was impaired in PIP5K overexpressing T cells. Taken together, we propose that PIP5K dependent PIP2 production inhibits ERM inactivation triggered by TCR stimulation, which leads to increased T cell rigidity, defective actin dynamics and T cell effector function.