Essential role of endocytosis for Interleukin-4 receptor mediated JAK/STAT signalling

Many important signalling cascades operate through specialized signalling endosomes, but a corresponding mechanism has as yet not been described for hematopoietic cytokine receptors. Based on live cell affinity measurements we recently proposed that ligand induced Interleukin-4 receptor (IL-4R) complex formation and thus JAK/STAT pathway activation requires a local, subcellular increase in receptor density. Here we show that this concentration step is provided by the internalization of IL-4R subunits through a constitutive, Rac1/Pak and actin mediated endocytosis route that causes IL-4R subunits to become enriched by about two orders of magnitude within a population of cortical endosomes. Consistently, ligand induced receptor dimers are preferentially detected within these endosomes. IL-4 signalling can be blocked by pharmacological inhibitors targeting the actin polymerization machinery driving receptor internalization, placing endocytosis unambigously upstream of receptor activation. Together these observations demonstrate a role for endocytosis that is mechanistically distinct from the scaffolding function of signalling endosomes in other pathways.