Molecular basis of cytokine receptor activation

Cytokines are secreted soluble peptides that precisely regulate multiple cellular functions. Amongst these the GM-CSF/IL-3/IL-5 family of cytokines controls whether hematopoietic cells will survive or apoptose, proliferate, differentiate, migrate, or perform effector functions such as phagocytosis or reactive oxygen species release. Their potent and pleiotropic activities are mediated through binding to high affinity membrane receptors at surprisingly low numbers per cell. Receptor binding triggers a cascade of intracellular signaling events, including reversible phosphorylation of receptor subunits and associated signaling molecules, leading to multiple biological responses, with the prevention of apoptosis or "cell survival" being a key cellular function that underpins all others. Many chronic inflammatory diseases and a number of haematological malignancies are driven by deregulated GM-CSF, IL-3, or IL-5 cytokine receptor signaling, highlighting their importance in disease. A major step in understanding how these cytokine receptors function is to elucidate their three dimensional structure and to relate this to the many signaling pathways emanating from their receptors. We have recently solved the structure of the human GM-CSF receptor complexed to GM-CSF which revealed distinct forms of receptor assembly: a hexamer that comprises two molecules each of GM-CSF, GM-CSF receptor alpha chain and GM-CSF receptor beta chain; and an unexpected dodecamer in which two hexameric complexes associate through a novel site 4. This latter form is necessary to bring JAK2 molecules sufficiently close together to enable full receptor activation. In this review we focus on the most recent insights in cytokine receptor signaling, and in receptor assembly. The stage is now set to link distinct forms of cytokine receptor assembled structures to specific forms of cytokine receptor signaling and function. Armed with this knowledge it may be possible to map distinct cytokine receptor signaling pathways from the cell surface to the cell nucleus which may themselves become new therapeutic targets.