Discovery of dipeptidyl-peptidase IV--a 40 year journey from bench to patient

Regulation of Chemokine Activity - A Focus on the Role of Dipeptidyl Peptidase IV/CD26

Chemokines are small, chemotactic proteins that play a crucial role in leukocyte migration and are, therefore, essential for proper functioning of the immune system. Chemokines exert their chemotactic effect by activation of chemokine receptors, which are G protein-coupled receptors (GPCRs), and interaction with glycosaminoglycans (GAGs). Furthermore, the exact chemokine function is modulated at the level of posttranslational modifications. Among the different types of posttranslational modifications that were found to occur in vitro and in vivo, i.e., proteolysis, citrullination, glycosylation, and nitration, NH₂-terminal proteolysis of chemokines has been described most intensively. Since the NH₂-terminal chemokine domain mediates receptor interaction, NH₂-terminal modification by limited proteolysis or amino acid side chain modification can drastically affect their biological activity. An enzyme that has been shown to provoke NH₂-terminal proteolysis of various chemokines is dipeptidyl peptidase IV or CD26. This multifunctional protein is a serine protease that preferably cleaves dipeptides from the NH₂-terminal region of peptides and proteins with a proline or alanine residue in the penultimate position. Various chemokines possess such a proline or alanine residue, and CD26-truncated forms of these chemokines have been identified in cell culture supernatant as well as in body fluids. The effects of CD26-mediated proteolysis in the context of chemokines turned out to be highly complex. Depending on the chemokine ligand, loss of these two NH₂-terminal amino acids can result in either an increased or a decreased biological activity, enhanced receptor specificity, inactivation of the chemokine ligand, or generation of receptor antagonists. Since chemokines direct leukocyte migration in homeostatic as well as pathophysiologic conditions, CD26-mediated proteolytic processing of these chemotactic proteins may have significant consequences for appropriate functioning of the immune system. After introducing the chemokine family together with the GPCRs and GAGs, as main interaction partners of chemokines, and discussing the different forms of posttranslational modifications, this review will focus on the intriguing relationship of chemokines with the serine protease CD26.

Mechanisms underlying the rapid degradation and elimination of the incretin hormones GLP-1 and GIP

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP, gastric inhibitory peptide) are secreted from intestinal L and K cells and stimulate insulin secretion from pancreatic beta cells. However, they are immediately inactivated mainly via N-terminal degradation by dipeptidyl peptidase IV (DPP IV, CD26), a specialised enzyme located on the cell surface enzyme of endothelial, epithelial and some other cell types. Cleavage by neprilysin (neutral endopeptidase) is a minor degradation route, and renal clearance eliminates incretin/fragments, but appears of less importance for regulating incretin bioactivities. Based on these observations two novel types of drugs for the treatment of type 2 diabetes have been developed: DPP IV inhibitors and DPP IV-resistant incretin analogues. Both have distinct advantages and disadvantages. Potential side effects of DPP IV inhibitors may result from affecting the bioactivity of other hormones, neuropeptides or chemokines and also by their cross-reactivity with DPP IV-related enzymes.

CD26/dipeptidyl peptidase 4-deficiency alters thymic emigration patterns and leukcocyte subsets in F344-rats age-dependently

As CD26 (dipeptidyl peptidase 4/DPP4) rapidly truncates incretins N-terminally, including glucagon-like peptide-1, DPP4-inhibitors have been developed for treatment of diabetes type 2. To some extent this is surprising, as CD26/DPP4 is also deeply involved in immune regulation. Long-term pharmacological studies are hampered by off-target inhibition of DPP4-homologues. Therefore, we studied the effects of genetic CD26/DPP4-deficiency by investigating blood, spleen and thymus leucocyte subpopulations of wild-type and CD26-deficient F344-rats at different ages. In young animals at 1 and 3 months of age, there were no differences in leucocyte subsets, while in older animals the T cell composition was changed significantly. From the age of 6 months onwards, reduced numbers of recent thymic emigrants and memory T cells, and consequently an increased amount of naive T cells were observed in CD26-deficient rats. In addition, the architecture of the thymus was altered, as observed by a reduced density of lymphocytes in the medulla. Furthermore, the number of proliferating cells in the thymus was decreased in CD26-deficient rats at a higher age. Moreover, CD26-deficiency resulted in markedly reduced numbers of B cells in later life. Additionally, an age- but not CD26-dependent increase of regulatory T cells and a decrease of natural killer cell numbers were detected in the blood and spleen. Our findings indicate an important role of CD26 in maintaining lymphocyte composition, memory T cell generation and thymic emigration patterns during immunosenescence, with possible implications for using DPP4-inhibitors.