SummaryCD26/DPP4 (dipeptidyl peptidase 4/DP4/DPPIV) is a surface T cell activation antigen and has been shown to have DPP4 enzymatic activity, cleaving-off amino-terminal dipeptides with either L-proline or L-alanine at the penultimate position. It plays a major role in glucose metabolism by Nterminal truncation and inactivation of the incretins glucagon-like peptide-1 (GLP) and gastric inhibitory protein (GIP). In 2006, DPP4 inhibitors have been introduced to clinics and have been demonstrated to efficiently enhance the endogenous insulin secretion via prolongation of the half-life of GLP-1 and GIP in patients. However, a large number of studies demonstrate clearly that CD26/DPP4 also plays an integral role in the immune system, particularly in T cell activation. Therefore, inhibition of DPP4 might represent a double-edged sword. Apart from the metabolic benefit, the associated immunological effects of long term DPP4 inhibition on regulatory processes such as T cell homeostasis, maturation and activation are not understood fully at this stage. The current data point to an important role for CD26/DPP4 in maintaining lymphocyte composition and function, T cell activation and co-stimulation, memory T cell generation and thymic emigration patterns during immune-senescence. In rodents, critical immune changes occur at baseline levels as well as after in-vitro and in-vivo challenge. In patients receiving DPP4 inhibitors, evidence of immunological side effects also became apparent. The scope of this review is to recapitulate the role of CD26/DPP4 in the immune system regarding its pharmacological inhibition and T cell-dependent immune regulation.Keywords: autoimmunity, B cell, cell activation, chemokines, T cells Structure and characterization of CD26Originally described 50 years ago [1], the lymphocyte cell surface protein CD26 possess a dipeptidyl peptidase-4 (DPP4) activity. It cleaves dipeptides from the N-termini of oligopeptides and smaller peptides with proline or alanine at the penultimate position, as illustrated in Fig. 1b [International Union of Biochemistry and Molecular Biology (IUBMB) Enzyme Nomenclature EC 3.4.14.5].CD26/DPP4 is a homodimer and an integral type II glycoprotein anchored to the membrane by its signal peptide. The primary structure consists of a short six amino acid cytoplasmic tail, a 22 amino acid transmembrane, a 738 amino acid extracellular portion comprised of a flexible stalk, glycosylation-rich region, cysteine-rich region and catalytic region with the catalytic triad Ser 630 , Asp 708 and His 740 (Fig. 1e). Recent studies have revealed that the transmembrane region contributes to enzyme activity and quaternary structure by dimerization [2]. The crystal structure of human CD26/DPP4 has been elucidated to reveal two domains: an eight-bladed propeller and an a/b-hydrolase domain. The propeller is open and consists of two subdomains made up of blades II-V and VI-VIII for the glycosylation-rich and cysteine-rich regions, respectively (Fig. 1d). Most monoclonal anti-CD26/DPP4 antibodies,...

The membrane-bound glycoprotein dipeptidyl peptidase IV (DP IV, CD26) is a unique multifunctional protein, acting as receptor, binding and proteolytic molecule. We have determined the sequence and 1.8 Å crystal structure of native DP IV prepared from porcine kidney. The crystal structure reveals a 2-2-2 symmetric tetrameric assembly which depends on the natively glycosylated ␤-propeller blade IV. The crystal structure indicates that tetramerization of DP IV is a key mechanism to regulate its interaction with other components. Each subunit comprises two structural domains, the N-terminal eight-bladed ␤-propeller with open Velcro topology and the C-terminal ␣͞␤-hydrolase domain. Analogy with the structurally related POP and tricorn protease suggests that substrates access the buried active site through the ␤-propeller tunnel while products leave the active site through a separate side exit. A dipeptide mimicking inhibitor complexed to the active site discloses key determinants for substrate recognition, including a Glu-Glu motif that distinguishes DP IV as an aminopeptidase and an oxyanion trap that binds and activates the P 2-carbonyl oxygen necessary for efficient postproline cleavage. We discuss active and nonactive site-directed inhibition strategies of this pharmaceutical target protein.serine protease ͉ oxyanion hole ͉ substrate channeling ͉ drug design ͉ diabetes mellitus

SummaryDipeptidyl peptidase (DPP) 4 (CD26, DPP4) is a multi-functional protein involved in T cell activation by co-stimulation via its association with adenosine deaminase (ADA), caveolin-1, CARMA-1, CD45, mannose-6-phosphate/insulin growth factor-II receptor (M6P/IGFII-R) and C-X-C motif receptor 4 (CXC-R4). The proline-specific dipeptidyl peptidase also modulates the bioactivity of several chemokines. However, a number of enzymes displaying either DPP4-like activities or representing structural homologues have been discovered in the past two decades and are referred to as DPP4 activity and/or structure homologue (DASH) proteins. Apart from DPP4, DASH proteins include fibroblast activation protein alpha (FAP), DPP8, DPP9, DPP4-like protein 1 (DPL1, DPP6, DPPX L, DPPX S), DPP4-like protein 2 (DPL2, DPP10) from the DPP4-gene family S9b and structurally unrelated enzyme DPP2, displaying DPP4-like activity. In contrast, DPP6 and DPP10 lack enzymatic DPP4-like activity. These DASH proteins play important roles in the immune system involving quiescence (DPP2), proliferation (DPP8/DPP9), antigen-presenting (DPP9), costimulation (DPP4), T cell activation (DPP4), signal transduction (DPP4, DPP8 and DPP9), differentiation (DPP4, DPP8) and tissue remodelling (DPP4, FAP). Thus, they are involved in many pathophysiological processes and have therefore been proposed for potential biomarkers or even drug targets in various cancers (DPP4 and FAP) and inflammatory diseases (DPP4, DPP8/DPP9). However, they also pose the challenge of drug selectivity concerning other DASH members for better efficacy and/or avoidance of unwanted side effects. Therefore, this review unravels the complex roles of DASH proteins in immunology.Keywords: antigen presentation/processing, CD26, co-stimulation, DPP4 activity and/or structure homologue proteins (DASH), signal transductionThe dipeptidyl peptidase (DPP)4 family of DPP4 activity and/or structure homologue (DASH) proteins Within the last two decades a number of enzymes have been discovered to also display DPP4-like activity or are structural homologues to DPP4. These enzymes/proteins are referred to as DPP4 activity and/or structure homologue (DASH) proteins, and can be grouped into the DPP4 gene family and non-related DPP4-like enzymes. Except for DPL1 and DPL2, all members display DPP4-like activity with neutral to basic pH optima and similar inhibition profiles [6,7]. DPP4 (CD26). DPP4 (CD26) is the best-known DASH protein and has been described in detail elsewhere [7][8][9].

Chemokines mediate the recruitment of leukocytes to the sites of inflammation. N-terminal truncation of chemokines by the protease dipeptidyl peptidase IV (DPPIV) potentially restricts their activity during inflammatory processes such as allergic reactions, but direct evidence in vivo is very rare. After demonstrating that N-terminal truncation of the chemokine CCL11/eotaxin by DPPIV results in a loss of CCR3-mediated intracellular calcium mobilization and CCR3 internalization in human eosinophils, we focused on the in vivo role of CCL11 and provide direct evidence for specific kinetic and rate-determining effects by DPPIV-like enzymatic activity on CCL11-mediated responses of eosinophils. Namely, it is demonstrated that i.v. administration of CCL11 in wild-type F344 rats leads to mobilization of eosinophils into the blood, peaking at 30 min. This mobilization is significantly increased in DPPIV-deficient F344 rats. Intradermal administration of CCL11 is followed by a dose-dependent recruitment of eosinophils into the skin and is significantly more effective in DPPIV-deficient F344 mutants as well as after pharmacological inhibition of DPPIV. Interestingly, CCL11 application leads to an up-regulation of DPPIV, which is not associated with negative feedback inhibition via DPPIV-cleaved CCL11(3–74). These findings demonstrate regulatory effects of DPPIV for the recruitment of eosinophils. Furthermore, they illustrate that inhibitors of DPPIV have the potential to interfere with chemokine-mediated effects in vivo including but not limited to allergy.

No abstract

S U M M A R Y The expression of dipeptidyl peptidase 4 (DP4, CD26) affects T-cell recruitment to lungs in an experimental rat asthma model. Furthermore, the gene of the structural homologous DP10 represents a susceptibility locus for asthma in humans, and the functional homologous DP8/9 are expressed in human leukocytes. Thus, although several mechanisms may account for a role of DP4-like peptidases in asthma, detailed information on their anatomical sites of expression and function in lungs is lacking. Therefore, bronchi and lung parenchyma were evaluated using immunohistochemistry and histochemical/enzymatic activity assays, as well as quantitative real-time PCR for this family of peptidases in naïve and asthmatic rat lungs derived from wild-type F344 and DP4-deficient F344 rat strains. Surprisingly, results show not only that the induction of experimental asthma increases DP4 enzymatic activity in the bronchoalveolar lavage fluid and parenchyma, but also that DP8/9 enzymatic activity is regulated and, as well as the expression of DP10, primarily found in the bronchial epithelium of the airways. This is the first report showing a differential and sitespecific DP4-like expression and function in the lungs, suggesting a pathophysiologically significant role in asthma. (J Histochem Cytochem 56:147-155, 2008)

The bioactivity of neuropeptide Y (NPY) is either N-terminally modulated with respect to receptor selectivity by dipeptidyl peptidase 4 (DP4)-like enzymes or proteolytic degraded by neprilysin or meprins, thereby abrogating signal transduction. However, neither the subcellular nor the compartmental differentiation of these regulatory mechanisms is fully understood. Using mass spectrometry, selective inhibitors and histochemistry, studies across various cell types, body fluids, and tissues revealed that most frequently DP4-like enzymes, aminopeptidases P, secreted meprin-A (Mep-A), and cathepsin D (CTSD) rapidly hydrolyze NPY, depending on the cell DP4/8/9/2, dipeptidyl peptidases 4/8/9/2; EC, endothelial cell; FAP, fibroblast activating protein alpha; GEMSA, 2-Guanidinoethylmercaptosuccinic acid; hK1, tissue kallikrein; Kal, plasma kallikrein; MEE, external zone of median eminence; MEI, internal zone of median eminence; ME, median eminence; Mep-A/B, meprin A/B; MGTA, 2-mercaptomethyl-3-guanidinoethylthiopropanoic acid; NEP, neprilysin; NPY, neuropeptide Y; P32/98, Isoleucine thiazolidide hemifumarate; PEP, prolyl oligopeptidase; Pl, plasmin; sAmpP/mAmpP, aminopeptidases P1/P2; TOP, thimet oligopeptidase; uPA, urokinase; Y 1/2/5 -R, Y 1/2/5 -receptor. Ganong 2000). Apart from being involved in cardiovascular, body-fluid, food-intake, immunology, and temperature regulation, they are also associated with energy metabolism and stress-response as the median eminence is part of the hypothalamic-pituitary-adrenal axis, whereas the area postrema is linked to the neuro-autonomal-sympathetic stress axis via the nucleus tractus solitary. Although EM and AP lack the BBB, they tightly control transport/signals from blood/CSF to CNS and vice versa via tanycytes, membrane transporter systems and receptors such as Y2-R (Rodriguez et al.

Inherited Metabolic Diseases (IMDs) are rare diseases caused by genetic defects in biochemical pathways. Earlier diagnosis and advances in treatment have improved the life expectancy of IMD patients over the last decades, with the majority of patients now surviving beyond the age of 20. This has created a new challenge: as they grow up, the care of IMD patients' needs to be transferred from metabolic pediatricians to metabolic physicians specialized in treating adults, through a process called “transition.” The purpose of this study was to assess how this transition is managed in Europe: a survey was sent to all 77 centers of the European Reference Network for Hereditary Metabolic Disorders (MetabERN) to collect information and to identify unmet needs regarding the transition process. Data was collected from 63/77 (81%) healthcare providers (HCPs) from 20 EU countries. Responders were mostly metabolic pediatricians; of these, only ~40% have received appropriate training in health issues of adolescent metabolic patients. In most centers (~67%) there is no designated transition coordinator. About 50% of centers provide a written individualized transition protocol, which is standardized in just ~20% of cases. In 77% of centers, pediatricians share a medical summary, transition letter and emergency plan with the adult team and the patient. According to our responders, 11% of patients remain under pediatric care throughout their life. The main challenges identified by HCPs in managing transition are lack of time and shortage of adult metabolic physician positions, while the implementations that are most required for a successful transition include: medical staff dedicated to transition, a transition coordinator, and specific metabolic training for adult physicians. Our study shows that the transition process of IMD patients in Europe is far from standardized and in most cases is inadequate or non-existent. A transition coordinator to facilitate collaboration between the pediatric and adult healthcare teams should be central to any transition program. Standardized operating procedures, together with adequate financial resources and specific training for adult physicians focused on IMDs are the key aspects that must be improved in the rare metabolic field to establish successful transition processes in Europe.