| dc.description.abstract | Eosinophils are multifunctional leukocytes implicated in the pathogenesis of inflammatory processes including hypereosinophilic syndrome, eosinophilic esophagitis, and allergic asthma. Due to its role in the initiation and propagation of inflammatory responses, and as modulators between innate and adaptive immunity, eosinophils are emerging as critical cells in both health and disease. Eosinophil activation and inflammatory signaling are dependent on interleukin-5 (IL-5) and signaling by its receptor, IL-5R, comprised of a ligand specific alpha chain (IL-5Rα) and a shared signaling component, βc. Previously, our laboratory demonstrated the critical role of three membrane proximal lysine residues in the cytoplasmic domain of βc for Janus kinase (JAK) binding and optimal receptor ubiquitination, and such binding is responsible for receptor signaling and endocytosis following IL-5 stimulation. However, little is known about whether or not IL-5Rα itself is ubiquitinated or what the molecular determinants are. Since βc and IL-5Rα are both members of the Type I cytokine receptor family, with similar structural features, we hypothesized that IL-5Rα is also ubiquitinated and that lysine residues in homologous regions were important for this process.
To test this hypothesis, we performed site directed mutagenesis on four membrane proximal lysine residues within the cytoplasmic domain of IL-5Rα, generating six mutant receptors expressed in HEK293 cell lines with wild-type βc. Our data revealed that IL-5Rα is ubiquitinated under steady state conditions, highlighting a novel role of ubiquitin in stabilizing cell surface expression of receptors prior to cytokine stimulation. In addition, the binding of JAKs to IL-5Rα depended on the presence of all four lysine residues, and such binding resulted in optimal ubiquitination under basal conditions. Our most striking finding was the IL-5-induced deubiquitination of IL-5Rα that triggers receptor internalization, with this deubiquitination event being dependent on the presence of the third and fourth lysine residues (Lys^370 and Lys^379).
In sum, these findings reveal a unique regulatory role for ubiquitin in IL-5Rα biology, particularly the significance of receptor deubiquitination following cytokine stimulation. Due to the specific role of IL-5Rα within eosinophil biology, results from this study may provide more insight into IL-5Rα regulation and perhaps help identify novel molecular targets for controlling inappropriate signaling associated with eosinophilic inflammation. | en |
