T-cell receptors (TCRs) upon binding to peptide-MHC ligands transduce signals in T lymphocytes.Tyrosine phosphorylations in the cytoplasmic domains of the CD3 (γδε) and ζ subunits of the TCR complex by Src family kinases initiate the signaling cascades via docking and activation of ZAP-70 kinase and other signaling components. We examined the role of the low-density detergent-insoluble membranes (DIMs) in TCR signaling. Using mouse thymocytes as a model, we characterized the structural organization of DIMs in detail. We then demonstrated that TCR engagement triggered an immediate increase in the amount of TCR/ CD3 present in DIMs, which directly involves the engaged receptor complexes. TCR/CD3 recruitment is accompanied by the accumulation of a series of prominent tyrosine-phosphorylated substrates and by an increase of the Lck activity in DIMs. Upon TCR stimulation, the DIM-associated receptor complexes are highly enriched in the hyperphosphorylated p23 ζ chains, contain most of the TCR/CD3-associated, phosphorylation-activated ZAP-70 kinases and seem to integrate into higher order, multiple tyrosine-phosphorylated substrate-containing protein complexes. The TCR/CD3 recruitment was found to depend on the activity of Src family kinases. We thus provide the first demonstration of recuitment of TCR/CD3 to DIMs upon receptor stimulation and propose it as a mechanism whereby TCR engagement is coupled to downstream signaling cascades.

A subset of prolyl oligopeptidases, including dipeptidyl-peptidase IV (DPP IV or CD26, EC 3.4.14.5), specifically cleave off N-terminal dipeptides from substrates having proline or alanine in amino acid position 2. This enzyme activity has been implicated in the regulation of the biological activity of multiple hormones and chemokines, including the insulinotropic peptides glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Targeted inactivation of the CD26 gene yielded healthy mice that have normal blood glucose levels in the fasted state, but reduced glycemic excursion after a glucose challenge. Levels of glucosestimulated circulating insulin and the intact insulinotropic form of GLP-1 are increased in CD26 ؊/؊ mice. A pharmacological inhibitor of DPP IV enzymatic activity improved glucose tolerance in wildtype, but not in CD26 ؊/؊ , mice. This inhibitor also improved glucose tolerance in GLP-1 receptor ؊/؊ mice, indicating that CD26 contributes to blood glucose regulation by controlling the activity of GLP-1 as well as additional substrates. These data reveal a critical role for CD26 in physiological glucose homeostasis, and establish it as a potential target for therapy in type II diabetes.