Molecular Architecture of Signal Complexes Regulating Immune Cell Function

Contents

1 Scaffolding of Proximal TCR Signaling 328

1.1 Transmembrane Adaptor Proteins 329

1.2 Slp-76 and Vav: Important Integrators of Complex Cellular

Responses 336

1.3 Integration of TCR Signaling by PI3K Signaling 339

2 Modulation of Proximal Signaling Events by cAMP 340

2.1 cAMP Modulates Immune Functions 340

2.2 A cAMP-PKA-Csk Inhibitory Pathway in Lipid Rafts Regulates

T-Cell Immune Function 341

2.3 cAMP Levels Are Increased in Lipid Rafts upon TCR Stimulation 343

2.4 PDE4 Is Recruited to Lipid Rafts upon TCR and CD28

Co-Stimulation 344

2.5 Control of cAMP Levels Is Implicated in Normal and Diseased

T-Cell Function 344

3 Protein Assembly Linking TCR to NF-kB Activation 345

4 Other Receptor Systems Controlling Immune Responses 348

4.1 Scaffolding of SLAM Receptor Signaling 349

4.2 Scaffolding of Toll-Like Receptor Signaling 350

5 Targeting of Anchored Signal Molecules as a Basis for Therapy 351

References 353

Abstract Signals transmitted via multichain immunoreceptors control the development, differentiation and activation of hematopoetic cells. The cytoplasmic parts of these receptors contain immunoreceptor tyrosine-based activation motifs (ITAMs) that upon phosphorylation by members of the Src tyrosine kinase family orchestrate a complex set of signaling events involving tyrosine phosphorylation, generation of second messengers like DAG, IP3 and Ca2+, activation of effector

K. Tasken

The Biotechnology Centre of Oslo, Gaustadalleen 21, PO Box 1125 Blindern,

0317 Oslo, Norway

[email protected]

E. Klussmann, J. Scott (eds.) Protein-Protein Interactions as New Drug Targets. Handbook of Experimental Pharmacology 186, © Springer-Verlag Berlin Heidelberg 2008

molecules like Ras and MAPKs and the translocation and activation of transcription factors like NFAT, API and NF-kB. Spatial and temporal organization of these signaling events is essential both to connect the receptors to downstream cascades as well as to control the functional outcome of the immune activation. Throughout this process control and fine-tuning of the different signals are necessary both for effective immune function and in order to avoid inappropriate or exaggerated immune activation and autoimmunity. This control includes modulating mechanisms that set the threshold for activation and reset the activation status after an immune response has been launched. One immunomodulating pathway is the cAMP-protein kinase A-Csk pathway scaffolded by a supramolecular complex residing in lipid rafts with the A kinase-anchoring protein (AKAP) ezrin, the Csk-binding protein PAG and a linker between the two, EBP50. Failure of correct scaffolding and loss of spatiotemporal control can potentially have severe consequences, leading to immune failure or autoimmunity. The clinical relevance of supramolecular complexes specifically organized by scaffolding proteins in regulating immune activity and the specter of genetic diseases linked to different signaling components suggest that protein-protein contact surfaces can be potential targets for drug intervention. It is also of interest to note that different pathogens have evolved strategies to specifically modulate signal integration, thereby rewiring the signal in a way beneficial for their survival. In addition to demonstrating the importance of different signal processes, these adaptations are elegant illustrations of the potential for drug targeting of protein assembly. This chapter reviews some of the important scaffolding events downstream of immunoreceptors with focus on signaling transduction through the T-cell receptor (TCR).

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