Mechanisms of Integrin Signal Transduction

Integrins are a protein family of cell surface receptors, expressed in all cell types in the human body, except the red blood cells. Besides their importance in mediating physical connections with the surrounding environment, the integrin family members are also vital signalling mediators. They have no intrinsic kinase activity; instead the signals are transduced through conformational changes. In this thesis, work is presented which is focused on molecular mechanisms of integrin signal transduction. The signal transduction was first studied from a structural point of view, determining the transmembrane domain borders of a few selected integrin family members and ruling out a signalling model involving a “piston-like” movement. Then, downstream signalling events involved in the beta1 integrin-induced activation of Akt via the PI3kinase family were characterized. Our results identify a novel pathway for PI3K/Akt activation by beta1 integrins, which is independent of focal adhesion kinase (FAK), Src and EGF receptor. Furthermore, both beta1 integrins and EGF receptors induced phosphorylation of Akt at the regulatory sites Thr308 and Ser473, but only EGF receptor stimulation induced tyrosine phosphorylation of Akt. Finally, signals from beta1 integrins underlying the morphologic changes during cell spreading were studied. A rapid integrin-induced cell spreading dependent on actin polymerisation was observed by using total internal reflection fluorescence (TIRF) microscopy. This integrin-induced actin polymerisation was shown to be dependent on PI3K p110alpha catalytic subunit and to involve the conserved Lys756 in the beta1-integrin membrane proximal part