TISSUE HYPERPLASIA AND ENHANCED T-CELL SIGNALING VIA ZAP-70 IN C-CBL-DEFICIENT MICE

The c-Cbl protein is tyrosine phosphorylated and forms complexes with a nide range of signalling partners in response to various growth fact ors. How c-Cbl interacts with proteins, such as Grb2, phosphatidylinos itol 3-kinase, and phosphorylated receptors, is well understood, but i ts role in these complexes is unclear. Recently, the Caenorhabditis el egans Cbl homolog, Sli-1, was shown to act as a negative regulator of epidermal growth factor receptor signalling. This finding forced a rea ssessment of the role of Cbl proteins and highlighted the desirability of testing genetically whether c-Cbl acts as a negative regulator of mammalian signalling, Here we investigate the role of c-Cbl in develop ment and homeostasis in mice by targeted disruption of the c-Cbl locus . c-Cbl-deficient mice were viable, fertile, and outwardly normal in a ppearance. Bone development and remodelling also appeared normal in c- Cbl mutants, despite a previously reported requirement for c-Cbl in os teoclast function. However, consistent with a high level of expression of c-Cbl in the hemopoietic compartment, c-Cbl deficient mice display ed marked changes in their hemopoietic profiles, including altered T-c ell receptor expression, lymphoid hyperplasia, and primary splenic ext ramedullary hemopoiesis. The mammary fat pads of mutant female mice al so showed increased ductal density and branching compared to those of their mild-type littermates, indicating an unanticipated role for c-Cb l in regulating mammary growth. Collectively, the hyperplastic histolo gical changes seen in c-Cbl mutant mice are indicative of a normal rol e for c-Cbl in negatively regulating signalling events that control ce ll growth, Consistent with this view, we observed greatly increased in tracellular protein tyrosine phosphorylation in thymocytes following C D3 epsilon cross-linking. In particular, phosphorylation of ZAP-70 kin ase in thymocytes was uncoupled from a requirement for CD4-mediated Lc k activation. This study provides the first biochemical characterizati on of any organism that is deficient in a member of this unique protei n family. Our findings demonstrate critical roles for c-Cbl in hemopoi esis and in controlling cellular proliferation and signalling by the S yk/ZAP-70 family of protein kinases.