Phosphatidylinositol 4-Phosphate 5-Kinase α Activation Critically Contributes to CD28-Dependent Signaling Responses

Abstract

CD28 is one of the most relevant costimulatory receptors that deliver both TCR-dependent and TCR-independent signals regulating a wide range of signaling pathways crucial for cytokine and chemokine gene expressions, T cell survival, and proliferation. Most of the CD28-dependent signaling functions are initiated by the recruitment and activation of class IA PI3Ks, which catalyze the conversion of phosphatidylinositol 4,5-biphosphate (PIP2) into phosphatidylinositol 3,4,5-triphosphate, thus generating the docking sites for key signaling proteins. Hence, PIP2 is a crucial substrate in driving the PI3K downstream signaling pathways, and PIP2 turnover may be an essential regulatory step to ensure the activation of PI3K following CD28 engagement. Despite some data evidence that CD28 augments TCR-induced turnover of PIP2, its direct role in regulating PIP2 metabolism has never been assessed. In this study, we show that CD28 regulates PIP2 turnover by recruiting and activating phosphatidylinositol 4-phosphate 5-kinases α (PIP5Kα) in human primary CD4+ T lymphocytes. This event leads to the neosynthesis of PIP2 and to its consumption by CD28-activated PI3K. We also evidenced that PIP5Kα activation is required for both CD28 unique signals regulating IL-8 gene expression as well as for CD28/TCR-induced Ca2+ mobilization, NF-AT nuclear translocation, and IL-2 gene transcription. Our findings elucidate a novel mechanism that involves PIP5Kα as a key modulator of CD28 costimulatory signals.

Phosphatidylinositol 4-Phosphate 5-Kinase β Controls Recruitment of Lipid Rafts into the Immunological Synapse

“…The results presented in this work, together with our previous findings on PIP5Ka relevance in CD28-mediated costimulatory signals (21,22), suggest that distinct PIP2 pools generated from distinct kinases have functionally unique roles in orchestrating CD28-mediated actin remodeling in human T lymphocytes. By examining both PIP5Ka and PIP5Kb localizations and deciphering their important role in the organization of the signaling compartment at the IS, we identified PIP5Kb as a key molecule that participates in tethering membrane rafts to the actin cytoskeleton during T lymphocyte activation, whereas PIP5Ka ensures the replenishment of the PIP2 pool necessary for the activation of phospholipase Cg1-and PI3K-regulated downstream signaling pathways (19,21,22).…”

“…Thus, similarly to what has been observed for PIP5Ka (21,22), CD28 mediates the membrane recruitment of PIP5Kb in a TCRindependent manner.…”

“…By examining both PIP5Ka and PIP5Kb localizations and deciphering their important role in the organization of the signaling compartment at the IS, we identified PIP5Kb as a key molecule that participates in tethering membrane rafts to the actin cytoskeleton during T lymphocyte activation, whereas PIP5Ka ensures the replenishment of the PIP2 pool necessary for the activation of phospholipase Cg1-and PI3K-regulated downstream signaling pathways (19,21,22). Interestingly, Lacalle et al (54) have recently demonstrated that PIP5Ka/PIP5Kb dimerization is essential for both membrane localization and PIP2 synthesis.…”

“…Primary CD4 + T cells express all three PIP5K isoforms (a, b, and g), with a specific subcellular localization (19). Human PIP5Ka accumulates in a sustained manner in the T:APC contact zone (20), where it regulates CD28-mediated actin polymerization events necessary for CD28 costimulatory signaling (21,22). PIP5Kg90 is predominantly found at the distal pole and in the uropod.…”

“…CD28 represents a key node in the regulation of PIP2 turnover by recruiting and activating PIP5Ka (21,22). To verify whether CD28 was also involved in PIP5Kb recruitment, Jurkat cells (Fig.…”