Domain-specific N-glycosylation of the membrane glycoprotein dipeptidylpeptidase IV (CD26) influences its subcellular trafficking, biological stability, enzyme activity and protein folding

N-Glycans on the extracellular domain of the Notch1 receptor control Jagged-1 induced Notch signalling and myogenic differentiation of S100β resident vascular stem cells

Notch signalling, critical for development and postnatal homeostasis of the vascular system, is highly regulated by several mechanisms including glycosylation. While the importance of O-linked glycosylation is widely accepted, the structure and function of N-glycans has yet to be defined. Here, we take advantage of lectin binding assays in combination with pharmacological, molecular, and site-directed mutagenetic approaches to study N-glycosylation of the Notch1 receptor. We find that several key oligosaccharides containing bisecting or core fucosylated structures decorate the receptor, control expression and receptor trafficking, and dictate Jagged-1 activation of Notch target genes and myogenic differentiation of multipotent S100β vascular stem cells. N-glycans at asparagine (N) 1241 and 1587 protect the receptor from accelerated degradation, while the oligosaccharide at N888 directly affects signal transduction. Conversely, N-linked glycans at N959, N1179, N1489 do not impact canonical signalling but inhibit differentiation. Our work highlights a novel functional role for N-glycans in controlling Notch1 signalling and differentiation of vascular stem cells.

A CRISPR screen in intestinal epithelial cells identifies novel factors for polarity and apical transport

Epithelial polarization and polarized cargo transport are highly coordinated and interdependent processes. In our search for novel regulators of epithelial polarization and protein secretion, we used a genome-wide CRISPR/Cas9 screen and combined it with an assay based on fluorescence-activated cell sorting (FACS) to measure the secretion of the apical brush-border hydrolase dipeptidyl peptidase 4 (DPP4). In this way, we performed the first CRISPR screen to date in human polarized epithelial cells. Using high-resolution microscopy, we detected polarization defects and mislocalization of DPP4 to late endosomes/lysosomes after knockout of TM9SF4, anoctamin 8, and ARHGAP33, confirming the identification of novel factors for epithelial polarization and apical cargo secretion. Thus, we provide a powerful tool suitable for studying polarization and cargo secretion in epithelial cells. In addition, we provide a dataset that serves as a resource for the study of novel mechanisms for epithelial polarization and polarized transport and facilitates the investigation of novel congenital diseases associated with these processes.

Dipeptidyl Peptidase 4 (DPP4) as A Novel Adipokine: Role in Metabolism and Fat Homeostasis

Dipeptidyl peptidase 4 (DPP4) is a molecule implicated in the regulation of metabolic homeostasis and inflammatory processes, and it exerts its main action through its enzymatic activity. DPP4 represents the enzyme most involved in the catabolism of incretin hormones; thus, its activity impacts appetite, energy balance, and the fine regulation of glucose homeostasis. Indeed, DPP4 inhibitors represent a class of antidiabetic agents widely used for the treatment of Type 2 diabetes mellitus (T2DM). DPP4 also acts as an adipokine and is mainly secreted by the adipose tissue, mostly from mature adipocytes of the visceral compartment, where it exerts autocrine and paracrine activities. DPP4 can disrupt insulin signaling within the adipocyte and in other target cells and tissues, where it also favors the development of a proinflammatory environment. This is likely at the basis of the presence of elevated circulating DPP4 levels in several metabolic diseases. In this review, we summarize the most recent evidence of the role of the DPP4 as an adipokine-regulating glucose/insulin metabolism and fat homeostasis, with a particular focus on clinical outcomes associated with its increased secretion in the presence of adipose tissue accumulation and dysfunction.

Role of Dipeptidyl Peptidase-4 (DPP4) on COVID-19 Physiopathology

DPP4/CD26 is a single-pass transmembrane protein with multiple functions on glycemic control, cell migration and proliferation, and the immune system, among others. It has recently acquired an especial relevance due to the possibility to act as a receptor or co-receptor for SARS-CoV-2, as it has been already demonstrated for other coronaviruses. In this review, we analyze the evidence for the role of DPP4 on COVID-19 risk and clinical outcome, and its contribution to COVID-19 physiopathology. Due to the pathogenetic links between COVID-19 and diabetes mellitus and the hyperinflammatory response, with the hallmark cytokine storm developed very often during the disease, we dive deep into the functions of DPP4 on carbohydrate metabolism and immune system regulation. We show that the broad spectrum of functions regulated by DPP4 is performed both as a protease enzyme, as well as an interacting partner of other molecules on the cell surface. In addition, we provide an update of the DPP4 inhibitors approved by the EMA and/or the FDA, together with the newfangled approval of generic drugs (in 2021 and 2022). This review will also cover the effects of DPP4 inhibitors (i.e., gliptins) on the progression of SARS-CoV-2 infection, showing the role of DPP4 in this disturbing disease.

Dipeptidyl Peptidase-4 at the Interface Between Inflammation and Metabolism

Dipeptidyl peptidase-4 (DPP4) is a serine protease that rapidly inactivates the incretin peptides, glucagon-like peptide-1, and glucose-dependent insulinotropic polypeptide to modulate postprandial islet hormone secretion and glycemia. Dipeptidyl peptidase-4 also has nonglycemic effects by controlling the progression of inflammation, which may be mediated more through direct protein-protein interactions than catalytic activity in the context of nonalcoholic fatty liver disease (NAFLD), obesity, and type 2 diabetes (T2D). Failure to resolve inflammation resulting in chronic subclinical activation of the immune system may influence the development of metabolic dysregulation. Thus, through both its cleavage and regulation of the bioactivity of peptide hormones and its influence on inflammation, DPP4 exhibits a diverse array of effects that can influence the progression of metabolic disease. Here, we highlight our current understanding of the complex biology of DPP4 at the intersection of inflammation, obesity, T2D, and NAFLD. We compare and review new mechanisms identified in basic laboratory and clinical studies, which may have therapeutic application and relevance to the pathogenesis of obesity and T2D.

Dynamic Phosphorylation and Dephosphorylation of Cyclase-Associated Protein 1 by Antagonistic Signaling through Cyclin-Dependent Kinase 5 and cAMP Are Critical for the Protein Functions in Actin Filament Disassembly and Cell Adhesion

Cyclase-associated protein 1 (CAP1) is a conserved actin-regulating protein that enhances actin filament dynamics and also regulates adhesion in mammalian cells. We previously found that phosphorylation at the Ser307/Ser309 tandem site controls its association with cofilin and actin and is important for CAP1 to regulate the actin cytoskeleton. Here, we report that transient Ser307/Ser309 phosphorylation is required for CAP1 function in both actin filament disassembly and cell adhesion. Both the phosphomimetic and the nonphosphorylatable CAP1 mutant, which resist transition between phosphorylated and dephosphorylated forms, had defects in rescuing the reduced rate of actin filament disassembly in the CAP1 knockdown HeLa cells. The phosphorylation mutants also had defects in alleviating the elevated focal adhesion kinase (FAK) activity and the enhanced focal adhesions in the knockdown cells. In dissecting further phosphoregulatory cell signals for CAP1, we found that cyclin-dependent kinase 5 (CDK5) phosphorylates both Ser307 and Ser309 residues, whereas cAMP signaling induces dephosphorylation at the tandem site, through its effectors protein kinase A (PKA) and exchange proteins directly activated by cAMP (Epac). No evidence supports an involvement of activated protein phosphatase in executing the dephosphorylation downstream from cAMP, whereas preventing CAP1 from accessing its kinase CDK5 appears to underlie CAP1 dephosphorylation induced by cAMP. Therefore, this study provides direct cellular evidence that transient phosphorylation is required for CAP1 functions in both actin filament turnover and adhesion, and the novel mechanistic insights substantially extend our knowledge of the cell signals that function in concert to regulate CAP1 by facilitating its transient phosphorylation.

Characterization of Cell Glycocalyx with Mass Spectrometry Methods

The cell membrane plays an important role in protecting the cell from its extracellular environment. As such, extensive work has been devoted to studying its structure and function. Crucial intercellular processes, such as signal transduction and immune protection, are mediated by cell surface glycosylation, which is comprised of large biomolecules, including glycoproteins and glycosphingolipids. Because perturbations in glycosylation could result in dysfunction of cells and are related to diseases, the analysis of surface glycosylation is critical for understanding pathogenic mechanisms and can further lead to biomarker discovery. Different mass spectrometry-based techniques have been developed for glycan analysis, ranging from highly specific, targeted approaches to more comprehensive profiling studies. In this review, we summarized the work conducted for extensive analysis of cell membrane glycosylation, particularly those employing liquid chromatography with mass spectrometry (LC-MS) in combination with various sample preparation techniques.

Functional similarities between the dictyostelium protein AprA and the human protein dipeptidyl-peptidase IV

Autocrine proliferation repressor protein A (AprA) is a protein secreted by Dictyostelium discoideum cells. Although there is very little sequence similarity between AprA and any human protein, AprA has a predicted structural similarity to the human protein dipeptidyl peptidase IV (DPPIV). AprA is a chemorepellent for Dictyostelium cells, and DPPIV is a chemorepellent for neutrophils. This led us to investigate if AprA and DPPIV have additional functional similarities. We find that like AprA, DPPIV is a chemorepellent for, and inhibits the proliferation of, D. discoideum cells, and that AprA binds some DPPIV binding partners such as fibronectin. Conversely, rAprA has DPPIV-like protease activity. These results indicate a functional similarity between two eukaryotic chemorepellent proteins with very little sequence similarity, and emphasize the usefulness of using a predicted protein structure to search a protein structure database, in addition to searching for proteins with similar sequences.

Dendritic cells and adipose tissue

Visceral adipose tissue inflammation in obesity is an established risk factor for metabolic syndrome, which can include insulin resistance, type 2 diabetes, hypertension and cardiovascular diseases. With obesity and related metabolic disorders reaching epidemic proportions globally, an understanding of the mechanisms of adipose tissue inflammation is crucial. Within the immune cell cohort, dendritic cells (DC) play a key role in balancing tolerance and immunity. Despite decades of research into the characterization of DC in lymphoid and non-lymphoid organs, their role in adipose tissue function is poorly understood. There is now an increasing interest in identification and characterization of DC in adipose tissue and understanding their function in regulating tissue metabolic homeostasis. This review provides an overview of the study of DC in adipose tissue, focusing on possible mechanisms by which DC may contribute to adipose tissue homeostasis.

Cut to the chase: a review of CD26/dipeptidyl peptidase-4's (DPP4) entanglement in the immune system

CD26/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 N-terminal 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.

Relative Quantification and Higher-Order Modeling of the Plasma Glycan Cancer Burden Ratio in Ovarian Cancer Case-Control Samples

An early-stage, population-wide biomarker for ovarian cancer (OVC) is essential to reverse its high mortality rate. Aberrant glycosylation by OVC has been reported, but studies have yet to identify an N-glycan with sufficiently high specificity. We curated a human biorepository of 82 case-control plasma samples, with 27%, 12%, 46%, and 15% falling across stages I-IV, respectively. For relative quantitation, glycans were analyzed by the individuality normalization when labeling with glycan hydrazide tags (INLIGHT) strategy for enhanced electrospray ionization, MS/MS analysis. Sixty-three glycan cancer burden ratios (GBRs), defined as the log10 ratio of the case-control extracted ion chromatogram abundances, were calculated above the limit of detection. The final GBR models, built using stepwise forward regression, included three significant terms: OVC stage, normalized mean GBR, and tag chemical purity; glycan class, fucosylation, or sialylation were not significant variables. After Bonferroni correction, seven N-glycans were identified as significant (p < 0.05), and after false discovery rate correction, an additional four glycans were determined to be significant (p < 0.05), with one borderline (p = 0.05). For all N-glycans, the vectors of the effects from stages II-IV were sequentially reversed, suggesting potential biological changes in OVC morphology or in host response.

DPP4 in Diabetes

Dipeptidyl-peptidase 4 (DPP4) is a glycoprotein of 110 kDa, which is ubiquitously expressed on the surface of a variety of cells. This exopeptidase selectively cleaves N-terminal dipeptides from a variety of substrates, including cytokines, growth factors, neuropeptides, and the incretin hormones. Expression of DPP4 is substantially dysregulated in a variety of disease states including inflammation, cancer, obesity, and diabetes. Since the incretin hormones, glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide (GIP), are major regulators of post-prandial insulin secretion, inhibition of DPP4 by the gliptin family of drugs has gained considerable interest for the therapy of type 2 diabetic patients. In this review, we summarize the current knowledge on the DPP4–incretin axis and evaluate most recent findings on DPP4 inhibitors. Furthermore, DPP4 as a type II transmembrane protein is also known to be cleaved from the cell membrane involving different metalloproteases in a cell-type-specific manner. Circulating, soluble DPP4 has been identified as a new adipokine, which exerts both para- and endocrine effects. Recently, a novel receptor for soluble DPP4 has been identified, and data are accumulating that the adipokine-related effects of DPP4 may play an important role in the pathogenesis of cardiovascular disease. Importantly, circulating DPP4 is augmented in obese and type 2 diabetic subjects, and it may represent a molecular link between obesity and vascular dysfunction. A critical evaluation of the impact of circulating DPP4 is presented, and the potential role of DPP4 inhibition at this level is also discussed.

Definitive Screening Design Optimization of Mass Spectrometry Parameters for Sensitive Comparison of Filter and Solid Phase Extraction Purified, INLIGHT Plasma N-Glycans

High-throughput, quantitative processing of N-linked glycans would facilitate large-scale studies correlating the glycome with disease and open the field to basic and applied researchers. We sought to meet these goals by coupling filter-aided-N-glycan separation (FANGS) to the individuality normalization when labeling with glycan hydrazide tags (INLIGHT) for analysis of plasma. A quantitative comparison of this method was conducted against solid phase extraction (SPE), a ubiquitous and trusted method for glycan purification. We demonstrate that FANGS-INLIGHT purification was not significantly different from SPE in terms of glycan abundances, variability, functional classes, or molecular weight distributions. Furthermore, to increase the depth of glycome coverage, we executed a definitive screening design of experiments (DOE) to optimize the MS parameters for glycan analyses. We optimized MS parameters across five N-glycan responses using a standard glycan mixture, translated these to plasma and achieved up to a 3-fold increase in ion abundances.

DPP4 in cardiometabolic disease: recent insights from the laboratory and clinical trials of DPP4 inhibition

The discovery of incretin-based medications represents a major therapeutic advance in the pharmacological management of type 2 diabetes mellitus (T2DM), as these agents avoid hypoglycemia, weight gain, and simplify the management of T2DM. Dipeptidyl peptidase-4 (CD26, DPP4) inhibitors are the most widely used incretin-based therapy for the treatment of T2DM globally. DPP4 inhibitors are modestly effective in reducing HbA1c (glycated hemoglobin) (≈0.5%) and while these agents were synthesized with the understanding of the role that DPP4 plays in prolonging the half-life of incretins such as glucagon-like peptide-1 and gastric inhibitory peptide, it is now recognized that incretins are only one of many targets of DPP4. The widespread expression of DPP4 on blood vessels, myocardium, and myeloid cells and the nonenzymatic function of CD26 as a signaling and binding protein, across a wide range of species, suggest a teleological role in cardiovascular regulation and inflammation. Indeed, DPP4 is upregulated in proinflammatory states including obesity, T2DM, and atherosclerosis. Consistent with this maladaptive role, the effects of DPP4 inhibition seem to exert a protective role in cardiovascular disease at least in preclinical animal models. Although 2 large clinical trials suggest a neutral effect on cardiovascular end points, current limitations of performing trials in T2DM over a limited time horizon on top of maximal medical therapy must be acknowledged before rendering judgment on the cardiovascular efficacy of these agents. This review will critically review the science of DPP4 and the effects of DPP4 inhibitors on the cardiovascular system.

Glycosylation patterns of kidney proteins differ in rat diabetic nephropathy

Diabetic nephropathy often progresses to end-stage kidney disease and, ultimately, to renal replacement therapy. Hyperglycemia per se is expected to have a direct impact on the biosynthesis of N- and O-linked glycoproteins. This study aims to establish the link between protein glycosylation and progression of experimental diabetic kidney disease using orthogonal methods. Kidneys of streptozotocin-diabetic and control rats were harvested at three different time points post streptozotocin injection. A panel of 12 plant lectins was used in the screening of lectin blots. The lectins UEAI, PHA-E, GSI, PNA, and RCA identified remarkable disease-associated differences in glycoprotein expression. Lectin affinity chromatography followed by mass spectrometric analyses led to the identification of several glycoproteins involved in salt-handling, angiogenesis, and extracellular matrix degradation. Our data confirm a substantial link between glycosylation signature and diabetes progression. Furthermore, as suggested by our findings on dipeptidyl peptidase-IV, altered protein glycosylation may reflect changes in biochemical properties such as enzymatic activity. Thus, our study demonstrates the unexplored potential of protein glycosylation analysis in the discovery of molecules linked to diabetic kidney disease.

Pharmacology, physiology, and mechanisms of action of dipeptidyl peptidase-4 inhibitors

Dipeptidyl peptidase-4 (DPP4) is a widely expressed enzyme transducing actions through an anchored transmembrane molecule and a soluble circulating protein. Both membrane-associated and soluble DPP4 exert catalytic activity, cleaving proteins containing a position 2 alanine or proline. DPP4-mediated enzymatic cleavage alternatively inactivates peptides or generates new bioactive moieties that may exert competing or novel activities. The widespread use of selective DPP4 inhibitors for the treatment of type 2 diabetes has heightened interest in the molecular mechanisms through which DPP4 inhibitors exert their pleiotropic actions. Here we review the biology of DPP4 with a focus on: 1) identification of pharmacological vs physiological DPP4 substrates; and 2) elucidation of mechanisms of actions of DPP4 in studies employing genetic elimination or chemical reduction of DPP4 activity. We review data identifying the roles of key DPP4 substrates in transducing the glucoregulatory, anti-inflammatory, and cardiometabolic actions of DPP4 inhibitors in both preclinical and clinical studies. Finally, we highlight experimental pitfalls and technical challenges encountered in studies designed to understand the mechanisms of action and downstream targets activated by inhibition of DPP4.

CD26/DPPIV down-regulation in endometrial stromal cell migration in endometriosis

OBJECTIVE

To test the hypothesis that endometrial stromal cells (ESCs) in endometriosis exhibit increased cell motility under hypoxia.

DESIGN

Prospective case-control study.

SETTING

University research laboratory.

PATIENT(S)

Women with endometriosis (n = 18) or benign gynecological disease (n=19).

INTERVENTION(S)

Eutopic ESCs were cultured under normoxia (20% O2) or hypoxia (6.5% O2), and migration and invasion capacity assayed, with pathway-focused polymerase chain reaction (PCR) array and ELISAs performed. CD26/dipeptidyl peptidase IV (DPPIV) expression was determined by flow cytometric analysis and enzymatic activity assay. The ESCs supplemented with Diprotin A (CD26 inhibitor), stromal cell-derived factor-1α, or AMD3100 (C-X-C motif receptor 4; CXCR4 blocker) were assayed for their migratory potential.

MAIN OUTCOME MEASURE(S)

Endometrial stromal cell migration and invasion under hypoxia.

RESULT(S)

Endometriotic ESCs showed significantly higher migration and invasion through collagen gels under hypoxia compared with nonendometriotic ESCs. The PCR array revealed down-regulation of the migration inhibitor CD26/DPPIV and up-regulation of angiogenic factors (vascular endothelial growth factor A, C-X-C motif Ligand 6; CXCL6) in endometriotic ESCs under hypoxia. The CD26/DPPIV surface expression and activity as well as angiogenic protein secretions suggested that the molecular mechanisms underlying aberrant migratory and angiogenic behavior in endometriotic ESCs. A combinatorial treatment with diprotin A and stromal cell-derived factor-1α effectively enhanced migration and invasion preferentially in endometriotic ESCs cultured hypoxically.

CONCLUSION(S)

Loss of CD26/DPPIV under hypoxia and the subsequent increase in migratory and angiogenic factors may favor conditions for lesion development in endometriosis.

The determinants of hydrophobic mismatch response for transmembrane helices

Hydrophobic mismatch arises from a difference in the hydrophobic thickness of a lipid membrane and a transmembrane protein segment, and is thought to play an important role in the folding, stability and function of membrane proteins. We have investigated the possible adaptations that lipid bilayers and transmembrane α-helices undergo in response to mismatch, using fully-atomistic molecular dynamics simulations totaling 1.4 μs. We have created 25 different tryptophan-alanine-leucine transmembrane α-helical peptide systems, each composed of a hydrophobic alanine-leucine stretch, flanked by 1-4 tryptophan side chains, as well as the β-helical peptide dimer, gramicidin A. Membrane responses to mismatch include changes in local bilayer thickness and lipid order, varying systematically with peptide length. Adding more flanking tryptophan side chains led to an increase in bilayer thinning for negatively mismatched peptides, though it was also associated with a spreading of the bilayer interface. Peptide tilting, bending and stretching were systematic, with tilting dominating the responses, with values of up to ~45° for the most positively mismatched peptides. Peptide responses were modulated by the number of tryptophan side chains due to their anchoring roles and distributions around the helices. Potential of mean force calculations for local membrane thickness changes, helix tilting, bending and stretching revealed that membrane deformation is the least energetically costly of all mismatch responses, except for positively mismatched peptides where helix tilting also contributes substantially. This comparison of energetic driving forces of mismatch responses allows for deeper insight into protein stability and conformational changes in lipid membranes.

Functional regulation of sugar assimilation by N-glycan-specific interaction of pancreatic α-amylase with glycoproteins of duodenal brush border membrane

Porcine pancreatic α-amylase (PPA) binds to N-linked glycans of glycoproteins (Matsushita, H., Takenaka, M., and Ogawa, H. (2002) J. Biol Chem., 277, 4680-4686). Immunostaining revealed that PPA is located at the brush-border membrane (BBM) of enterocytes in the duodenum and that the binding is inhibited by mannan but not galactan, indicating that PPA binds carbohydrate-specifically to BBM. The ligands for PPA in BBM were identified as glycoprotein N-glycans that are significantly involved in the assimilation of glucose, including sucrase-isomaltase (SI) and Na(+)/Glc cotransporter 1 (SGLT1). Binding of SI and SGLT1 in BBM to PPA was dose-dependent and inhibited by mannan. Using BBM vesicles, we found functional changes in PPA and its ligands in BBM due to the N-glycan-specific interaction. The starch-degrading activity of PPA and maltose-degrading activity of SI were enhanced to 240 and 175%, respectively, while Glc uptake by SGLT1 was markedly inhibited by PPA at high but physiologically possible concentrations, and the binding was attenuated by the addition of mannose-specific lectins, especially from Galanthus nivalis. Additionally, recombinant human pancreatic α-amylases expressed in yeast and purified by single-step affinity chromatography exhibited the same carbohydrate binding specificity as PPA in binding assays with sugar-biotinyl polymer probes. The results indicate that mammalian pancreatic α-amylases share a common carbohydrate binding activity and specifically bind to the intestinal BBM. Interaction with N-glycans in the BBM activated PPA and SI to produce much Glc on the one hand and to inhibit Glc absorption by enterocytes via SGLT1 in order to prevent a rapid increase in blood sugar on the other.

Chitosan-based therapeutic nanoparticles for combination gene therapy and gene silencing of in vitro cell lines relevant to type 2 diabetes

Glucagon like peptide 1 (GLP-1), a blood glucose homeostasis modulating incretin, has been proposed for the treatment of type 2 diabetes mellitus (T2DM). However, native GLP-1 pharmacokinetics reveals low bioavailability due to degradation by the ubiquitous dipeptydil peptidase IV (DPP-IV) endoprotease. In this study, the glucosamine-based polymer chitosan was used as a cationic polymer-based in vitro delivery system for GLP-1, DPP-IV resistant GLP-1 analogues and siRNA targeting DPP-IV mRNA. We found chitosans to form spherical nanocomplexes with these nucleic acids, generating two distinct non-overlapping size ranges of 141-283 nm and 68-129 nm for plasmid and siRNA, respectively. The low molecular weight high DDA chitosan 92-10-5 (degree of deacetylation, molecular weight and N:P ratio (DDA-Mn-N:P)) showed the highest plasmid DNA transfection efficiency in HepG2 and Caco-2 cell lines when compared to 80-10-10 and 80-80-5 chitosans. Recombinant native GLP-1 protein levels in media of transfected cells reached 23 ng/L while our DPP-IV resistant analogues resulted in a fivefold increase of GLP-1 protein levels (115 ng/L) relative to native GLP-1, and equivalent to the Lipofectamine positive control. We also found that all chitosan-DPP-IV siRNA nanocomplexes were capable of DPP-IV silencing, with 92-10-5 being significantly more effective in abrogating enzymatic activity of DPP-IV in media of silenced cells, and with no apparent cytotoxicity. These results indicate that specific chitosan formulations may be effectively used for the delivery of plasmid DNA and siRNA in a combination therapy of type 2 diabetes.

Molecular mechanism and structural basis of interactions of dipeptidyl peptidase IV with adenosine deaminase and human immunodeficiency virus type-1 transcription transactivator

Dipeptidyl peptidase IV (DPPIV or CD26) is a multifunctional membrane glycoprotein. As an exopeptidase it regulates the activity of a series of biologically important peptides. Through its interaction with specific proteins and peptides, DPPIV is also involved in a wide range of biologically relevant processes such as cell adhesion, T cell activation and apoptosis. In this paper, we review our recent studies on the interactions of DPPIV with adenosine deaminase (ADA) and the transcription transactivator of the human immunodeficiency virus type-1 (HIV-1 Tat) as revealed by three-dimensional structure reconstructed by single particle analysis of cryo-electron microscopy (EM) and crystal structures of the human DPPIV-bovine ADA complex as well as the crystal structures of DPPIV in complex with HIV-1 Tat-derived nonapeptides. These results contribute importantly to the clarification of the molecular mechanisms of this multifunctional protein. The biological relevance of these interactions is discussed.

Upregulation of CD26 expression in epithelial cells and stromal cells during wound-induced skin tumour formation

We have previously described InvEE transgenic mice in which non-dividing, differentiating epidermal cells express oncogenically activated MAPK kinase 1 (MEK1). Skin wounding triggers tumour formation in InvEE mice via a mechanism that involves epidermal release of IL-1α and attraction of a pro-tumorigenic inflammatory infiltrate. To look for potential effects on the underlying connective tissue, we screened InvEE and wild-type epidermis for differential expression of cytokines and immune modulators. We identified a single protein, CD26 (dipeptidyl peptidase-4). CD26 serum levels were not increased in InvEE mice. In contrast, CD26 was upregulated in keratinocytes expressing mutant MEK1 and in the epithelial compartment of InvEE tumours, where it accumulated at cell-cell borders. CD26 expression was increased in dermal fibroblasts following skin wounding but was downregulated in tumour stroma. CD26 activity was stimulated by calcium-induced intercellular adhesion in keratinocytes, suggesting that the upregulation of CD26 in InvEE epidermis is due to expansion of the differentiated cell layers. IL-1α treatment of dermal fibroblasts stimulated CD26 activity, and therefore epidermal IL-1α release may contribute to the upregulation of CD26 expression in wounded dermis. Pharmacological blockade of CD26, via Sitagliptin, reduced growth of InvEE tumours, while combined inhibition of IL-1α and CD26 delayed tumour onset and reduced tumour incidence. Our results demonstrate that inappropriate activation of MEK1 in the epidermis leads to changes in dermal fibroblasts that, like the skin inflammatory infiltrate, contribute to tumour formation.

Isolation of dipeptidyl peptidase IV (DP 4) isoforms from porcine kidney by preparative isoelectric focusing to improve crystallization

In the present studies we resolved the post-translational microheterogeneity of purified porcine dipeptidyl peptidase IV (DP 4) from kidney cortex. Applying SDS-homogeneous DP 4 onto an analytical agarose isoelectric focusing (IEF) gel, pH 4–6, activity staining resulted in at least 17 isoforms between pH 4.8–6.0. These could be separated into fractions with only two to six isoforms by means of preparative liquid-phase IEF, using a Rotofor cell. Starting off with three parallel Rotofor runs under the same conditions at pH 5–6, the fractions were pooled according to the specific activity of DP 4, pH and analytical IEF profile, and further refractionated without any additional ampholytes. Since excessive dilution of ampholytes and proteins was kept to the minimum, a second refractionation step could be introduced, resulting in pH gradients between 0.022 and 0.028 pH increments per fraction. By performing two consecutive refractionation steps, the high resolution necessary for the separation of DP 4 isoforms could be achieved. This represents an alternative method if isolation of isoforms with similar pI's results in precipitation and denaturation in presence of a narrow pH range. Furthermore, it demonstrates that preparative IEF is a powerful tool to resolve post-translational microheterogeneity of a purified protein required for crystallization processing.

Involvement of sialic acid in the regulation of γ--aminobutyric acid uptake activity of γ-aminobutyric acid transporter 1

The γ-aminobutyric acid (GABA) transporters (GATs) have long been recognized for their key role in the uptake of neurotransmitters. The GAT1 belongs to the family of Na(+)- and Cl(-)-coupled transport proteins, which possess 12 putative transmembrane (TM) domains and three N-glycosylation sites on the extracellular loop between TM domains 3 and 4. Previously, we demonstrated that terminal trimming of N-glycans is important for the GABA uptake activity of GAT1. In this work, we examined the effect of deficiency, removal or oxidation of surface sialic acid residues on GABA uptake activity to investigate their role in the GABA uptake of GAT1. We found that the reduced concentration of sialic acid on N-glycans was paralleled by a decreased GABA uptake activity of GAT1 in Chinese hamster ovary (CHO) Lec3 cells (mutant defective in sialic acid biosynthesis) in comparison to CHO cells. Likewise, either enzymatic removal or chemical oxidation of terminal sialic acids using sialidase or sodium periodate, respectively, resulted in a strong reduction in GAT1 activity. Kinetic analysis revealed that deficiency, removal or oxidation of terminal sialic acids did not affect the K(m) GABA values. However, deficiency and removal of terminal sialic acids of GAT1 reduced the V(max) GABA values with a reduced apparent affinity for extracellular Na(+). Oxidation of cell surface sialic acids also strongly reduced V(max) without affecting both affinities of GAT1 for GABA and Na(+), respectively. These results demonstrated for the first time that the terminal sialic acid of N-linked oligosaccharides of GAT1 plays a crucial role in the GABA transport process.

The effect of individual N-glycans on enzyme activity

In a series of investigations, N-glycosylation has proven to be a key determinant of enzyme secretion, activity, binding affinity and substrate specificity, enabling a protein to fine-tune its activity. In the majority of cases elimination of all putative N-glycosylation sites of an enzyme results in significantly reduced protein secretion levels, while removal of individual N-glycosylation sites often leads to the expression of active enzymes showing markedly reduced catalytic activity, with the decreased activity often commensurate with the number of glycosylation sites available, and the fully deglycosylated enzymes showing only minimal activity relative to their glycosylated counterparts. On the other hand, several cases have also recently emerged where deglycosylation of an enzyme results in significantly increased catalytic activity, binding affinity and altered substrate specificity, highlighting the very unique and diverse roles that individual N-glycans play in regulating enzyme function.

Corin is co-expressed with pro-ANP and localized on the cardiomyocyte surface in both zymogen and catalytically active forms

The multi-domain transmembrane serine protease corin cleaves pro-atrial natriuretic peptide (pro-ANP) in vitro to generate an active hormone, ANP. Corin may also contribute to the regulation of the natriuretic peptide system in vivo, and might be an attractive target for treatment of cardiovascular diseases. In order for corin to cleave its substrate pro-ANP, it should be catalytically active and located proximally. However, because knowledge of native corin is limited, we examined the expression, cardiac localization and molecular forms of the native corin protein. Immunofluorescence studies using a series of anti-corin antibodies directed against the stem and protease domains reveal that corin is present on the cell-surface of rat neonatal cardiomyocytes and murine HL-1 cardiomyocyte-like cells. Furthermore, we immunolocalized native corin in pro-ANP expressing cardiomyocytes. Immunoprecipitation of the membrane fraction of mouse heart extract showed that native corin had a relative mass of 205-210 kDa. Under reducing conditions native corin migrates as several different molecular weight forms corresponding to zymogen (uncleaved) and active (cleaved) forms. Studies using a FITC-tagged chloromethyl ketone that mimics the corin cleavage sequence in pro-ANP, suggest that an enzymatically active form of corin is localized to the cell surface of myocardial cells in vivo. Additionally, we showed that the 205-210 kDa form of corin is a glycosylated protein. Treatment of HL-1 cells with tunicamycin reduced the relative mass of expressed corin. We conclude that native corin is a glycosylated protease that is localized on the cell surface of pro-ANP-expressing cardiomyocytes in both zymogen and catalytically active forms.

Importance of N-linked glycosylation in the functional expression of murine CD1d1

The mouse CD1d1 glycoprotein is specialized in presenting lipid antigens to a novel class of T cells called natural killer T (NKT) cells. CD1d1 is predicted to contain five potential N-linked glycosylation sites (asparagine residues at positions 25, 38, 60, 128, and 183). Glycosylation has been shown to invariably affect the molecular and functional properties of various glycoproteins, and in the current report it was found that a conservative change of the individual endogenous asparagine residues in CD1d1 to glutamine differentially affected its functional expression. Although the maturation rate of the glycosylation mutants was comparable to that of wild type, they differed in their relative levels of surface expression and in their ability to stimulate NKT cells. Mutating all five glycosylation residues resulted in the absence of detectable CD1d1 expression, with a concomitant lack of NKT cell activation. Therefore, these results demonstrate that glycosylation plays a significant role in the functional expression of CD1d1.

Energetics of membrane protein folding and stability

The critical role of membrane proteins in a myriad of biological and physiological functions has spawned numerous investigations over the past several decades with the long-term goal of identifying the molecular origins and energetic forces that stabilize these proteins within the membrane. Parallel structural and thermodynamics studies on several systems have provided significant insight regarding the driving forces governing folding, assembly, insertion, and translocation of membrane proteins. The present review surveys families of membrane-associated proteins including alpha-helical and beta-barrel structures, viral surface receptors, and pore-forming toxins, citing representative proteins within each of these classes for further scrutiny in terms of structure-function relationships and global conformational stability. This overview presents seminal findings from pioneering studies on the energetics of membrane protein folding and stability to modern techniques that are exploiting the use of molecular genetics and single molecule studies. An overall consensus regarding the molecular origins of membrane protein stability is that a number of intrinsic properties resemble features of soluble proteins, yet there are distinct energetic differences arising from specific intra- and intermolecular interactions within the membrane. The combined efforts from structural, energetics, and dynamics approaches offer unique insights and improve our fundamental understanding of the driving forces dictating membrane protein folding and stability.

Insulin-dependent phosphorylation of DPP IV in liver. Evidence for a role of compartmentalized c-Src

Dipeptidyl peptidase IV (DPP IV, CD26, EC 3.4.14.5) serves as a model aimed at elucidating protein sorting signals. We identify here, by MS, several tyrosine-phosphorylated proteins in a rat liver Golgi/endosome (G/E) fraction including DPP IV. We show that a pool of DPP IV is tyrosine-phosphorylated. Maximal phosphorylation was observed after 2 min following intravenous insulin injection. DPP IV coimmunoprecipitated with the cellular tyrosine kinase Src (c-Src) with maximal association also observed after 2 min following insulin injection. DPP IV was found phosphorylated after incubation of nonsolubilized G/E membranes with [gamma-32P]ATP. The c-Src inhibitor PP2 inhibited DPP IV phosphorylation. Oriented proteolysis experiments indicate that a large pool of c-Src is protected in G/E fractions. Following injection of the protein-tyrosine phosphatase inhibitor bpV(phen), DPP IV levels markedly decreased by 40% both in plasma membrane and G/E fractions. In the fraction designated Lh, DPP IV levels decreased by 50% 15 min following insulin injection. Therefore, a pool of DPP IV is tyrosine-phosphorylated in an insulin-dependent manner. The results suggest the presence of a yet to be characterized signalling mechanism whereby DPP IV has access to c-Src-containing signalling platforms.

The role of N-glycosylation in the stability, trafficking and GABA-uptake of GABA-transporter 1. Terminal N-glycans facilitate efficient GABA-uptake activity of the GABA transporter

Neurotransmitter transporters play a major role in achieving low concentrations of their respective transmitter in the synaptic cleft. The GABA transporter GAT1 belongs to the family of Na(+)- and Cl(-)-coupled transport proteins which possess 12 putative transmembrane domains and three N-glycosylation sites in the extracellular loop between transmembrane domain 3 and 4. To study the significance of N-glycosylation, green fluorescence protein (GFP)-tagged wild type GAT1 (NNN) and N-glycosylation defective mutants (DDQ, DGN, DDN and DDG) were expressed in CHO cells. Compared with the wild type, all N-glycosylation mutants showed strongly reduced protein stability and trafficking to the plasma membrane, which however were not affected by 1-deoxymannojirimycin (dMM). This indicates that N-glycosylation, but not terminal trimming of the N-glycans is involved in the attainment of a correctly folded and stable conformation of GAT1. All N-glycosylation mutants were expressed on the plasma membrane, but they displayed markedly reduced GABA-uptake activity. Also, inhibition of oligosaccharide processing by dMM led to reduction of this activity. Further experiments showed that both N-glycosylation mutations and dMM reduced the V(max) value, while not increasing the K(m) value for GABA uptake. Electrical measurements revealed that the reduced transport activity can be partially attributed to a reduced apparent affinity for extracellular Na+ and slowed kinetics of the transport cycle. This indicates that N-glycans, in particular their terminal trimming, are important for the GABA-uptake activity of GAT1. They play a regulatory role in the GABA translocation by affecting the affinity and the reaction steps associated with the sodium ion binding.

Elevation of the level and activity of acid ceramidase in Alzheimer's disease brain

Protein glycosylation modifies the processing of several key proteins involved in the molecular pathogenesis of Alzheimer's disease (AD). Aberrant glycosylation of tau and down-regulation of sialyltransferase in AD brain suggest a possible dysregulation of protein glycosylation that may play a role in AD. We therefore isolated major glycoproteins from AD brain by using lectin-affinity chromatographies and ion-exchange chromatography and further separated them using SDS-polyacylamide gel electrophoresis. Mass spectrometry analysis of 11 isolated glycoproteins led to their identification as: neuronal cell adhesion molecule, beta-globin, IgM heavy chain VH1 region precursor, contactin precursor, dipeptidylpeptidase VI, CD81 partner 3, prenylcysteine lyase, adipocyte plasma-associated protein, acid ceramidase and two novel proteins. We found that the level and activity of acid ceramidase (AC), one of the major identified human brain glycoproteins, were significantly elevated in AD brain. Immunohistochemical staining indicated that AC was located mainly in the cell bodies of neurons and colocalized with neurofibrillary tangles. Our findings suggest that AC might play a role in controlling neuronal apoptosis and that AC-mediated signalling pathways might be involved in the molecular mechanism of AD.

Modulation of Kv4.2 channel expression and gating by dipeptidyl peptidase 10 (DPP10)

The dipeptidyl aminopeptidase-like protein DPPX (DPP6) associates with Kv4 potassium channels, increasing surface trafficking and reconstituting native neuronal ISA-like properties. Dipeptidyl peptidase 10 (DPP10) shares with DPP6 a high amino acid identity, lack of enzymatic activity, and expression predominantly in the brain. We used a two-electrode voltage-clamp and oocyte expression system to determine if DPP10 also interacts with Kv4 channels and modulates their expression and function. Kv4.2 coimmunoprecipitated with HA/DPP10 from extracts of oocytes heterologously expressing both proteins. Coexpression with DPP10 and HA/DPP10 enhanced Kv4.2 current by approximately fivefold without increasing protein level. DPP10 also remodeled Kv4.2 kinetic and steady-state properties by accelerating time courses of inactivation and recovery (taurec: WT = 200 ms, +DPP10 = 78 ms). Furthermore, DPP10 introduced hyperpolarizing shifts in the conductance-voltage relationship (approximately 19 mV) as well as steady-state inactivation (approximately 7 mV). The effects of DPP10 on Kv4.1 were similar to Kv4.2; however, distinct biophysical differences were observed. Additional experiments suggested that the cytoplasmic N-terminal domain of DPP10 determines the acceleration of inactivation. In summary, DPP10 is a potent modulator of Kv4 expression and biophysical properties and may be a critical component of somatodendritic ISA channels in the brain.

N-linked glycosylation of dipeptidyl peptidase IV (CD26): effects on enzyme activity, homodimer formation, and adenosine deaminase binding

The type II transmembrane serine protease dipeptidyl peptidase IV (DPPIV), also known as CD26 or adenosine deaminase binding protein, is a major regulator of various physiological processes, including immune, inflammatory, nervous, and endocrine functions. It has been generally accepted that glycosylation of DPPIV and of other transmembrane dipeptidyl peptidases is a prerequisite for enzyme activity and correct protein folding. Crystallographic studies on DPPIV reveal clear N-linked glycosylation of nine Asn residues in DPPIV. However, the importance of each glycosylation site on physiologically relevant reactions such as dipeptide cleavage, dimer formation, and adenosine deaminase (ADA) binding remains obscure. Individual Asn-->Ala point mutants were introduced at the nine glycosylation sites in the extracellular domain of DPPIV (residues 39-766). Crystallographic and biochemical data demonstrate that N-linked glycosylation of DPPIV does not contribute significantly to its peptidase activity. The kinetic parameters of dipeptidyl peptidase cleavage of wild-type DPPIV and the N-glycosylation site mutants were determined by using Ala-Pro-AFC and Gly-Pro-pNA as substrates and varied by <50%. DPPIV is active as a homodimer. Size-exclusion chromatographic analysis showed that the glycosylation site mutants do not affect dimerization. ADA binds to the highly glycosylated beta-propeller domain of DPPIV, but the impact of glycosylation on binding had not previously been determined. Our studies indicate that glycosylation of DPPIV is not required for ADA binding. Taken together, these data indicate that in contrast to the generally accepted view, glycosylation of DPPIV is not a prerequisite for catalysis, dimerization, or ADA binding.

CD38 is expressed as noncovalently associated homodimers on the surface of murine B lymphocytes

CD38 is a transmembrane glycoprotein that functions as an ectoenzyme and as a receptor. Based on the structural similarity between CD38 and ADP-ribosyl cyclase from Aplysia californica, it was hypothesized that CD38 is expressed as a homodimer on the surface of cells. Indeed, CD38 dimers have been reported, however, the structural requirements for their stabilization on the plasma membrane are unknown. We demonstrate that the majority of CD38 is assembled as noncovalently associated homodimers on the surface of B cells. Analysis of CD38 mutants, expressed in Ba/F3 cells, revealed that truncation of the cytoplasmic region or mutation of a single amino acid within the alpha1-helix of CD38 decreased the stability of the CD38 homodimers when solubilized in detergent. Cells expressing the unstable CD38 homodimers had diminished expression of CD38 on the plasma membrane and the half-lives of these CD38 mutant proteins on the plasma membrane were significantly reduced. Together, these results show that CD38 is expressed as noncovalently associated homodimers on the surface of murine B cells and suggest that appropriate assembly of CD38 homodimers may play an important role in stabilizing CD38 on the plasma membrane of B cells.

Glucocorticoid agonistic and antagonistic effects of mifepristone and onapristone on thymocyte subset composition and CD26/dipeptidyl peptidase IV activity in infant male rats

Antiglucocorticoid activities of two antigestagens-antiglucocorticoids (AGs)-mifepristone and onapristone-were tested in hydrocortisone-treated suckling male rats. Hydrocortisone (HC) treatment in vivo resulted in (1) reduction of the relative thymus weight and absolute thymocyte counts; (2) relative decrease of the CD4(+)CD8(+) thymocyte proportion accompanied by an increase of single-positive and double negative thymocyte populations, the latter of which contained large CD3-negative cells expressing a high level of CD26 on their surface; (3) increase of specific dipeptidyl peptidase IV (DPP IV) activity in thymocyte homogenates. Both AGs suppressed the systems (1) and (2) to a comparable extent. When administered alone, mifepristone and onapristone at higher doses exhibited a slight thymolytic effect as revealed by the reduction of the relative thymus weight and thymocyte counts, accompanied by some reduction of the numbers of cycling thymocytes. These effects were limited to the early postnatal period (days 12-17). A comparable agonistic effect of AGs was not observed in systems (2) and (3). Neither HC nor AGs influenced the sialylation pattern of thymocyte membrane bound CD26/DPP IV, which was exclusively of alpha2,6-type, as demonstrated by analytical isoelectric focusing (IEF) and PAGE analysis in combination with the application of neuraminidases, specific lectins and histochemical staining for DPP IV activity in the gels.

Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV

Dipeptidyl-peptidase IV/CD26 (DPP IV) is a cell-surface protease belonging to the prolyloligopeptidase family. It selectively removes the N-terminal dipeptide from peptides with proline or alanine in the second position. Apart from its catalytic activity, it interacts with several proteins, for instance, adenosine deaminase, the HIV gp120 protein, fibronectin, collagen, the chemokine receptor CXCR4, and the tyrosine phosphatase CD45. DPP IV is expressed on a specific set of T lymphocytes, where it is up-regulated after activation. It is also expressed in a variety of tissues, primarily on endothelial and epithelial cells. A soluble form is present in plasma and other body fluids. DPP IV has been proposed as a diagnostic or prognostic marker for various tumors, hematological malignancies, immunological, inflammatory, psychoneuroendocrine disorders, and viral infections. DPP IV truncates many bioactive peptides of medical importance. It plays a role in glucose homeostasis through proteolytic inactivation of the incretins. DPP IV inhibitors improve glucose tolerance and pancreatic islet cell function in animal models of type 2 diabetes and in diabetic patients. The role of DPP IV/ CD26 within the immune system is a combination of its exopeptidase activity and its interactions with different molecules. This enables DPP IV/CD26 to serve as a co-stimulatory molecule to influence T cell activity and to modulate chemotaxis. DPP IV is also implicated in HIV-1 entry, malignant transformation, and tumor invasion.

Dipeptidyl peptidase IV (CD26) activity in the hematopoietic system: differences between the membrane-anchored and the released enzyme activity

Dipeptidyl peptidase IV (DPP-IV; CD26) (EC 3.4.14.5) is a membrane-anchored ectoenzyme with N-terminal exopeptidase activity that preferentially cleaves X-Pro-dipeptides. It can also be spontaneously released to act in the extracellular environment or associated with the extracellular matrix. Many hematopoietic cytokines and chemokines contain DPP-IV-susceptible N-terminal sequences. We monitored DPP-IV expression and activity in murine bone marrow and liver stroma cells which sustain hematopoiesis, myeloid precursors, skin fibroblasts, and myoblasts. RT-PCR analysis showed that all these cells produced mRNA for DPP-IV. Partially purified protein reacted with a commercial antibody to CD26. The K M values for Gly-Pro-p-nitroanilide ranged from 0.43 to 0.98 mM for the membrane-associated enzyme of connective tissue stromas, and from 6.76 to 8.86 mM for the enzyme released from the membrane, corresponding to a ten-fold difference, but only a two-fold difference in K M was found in myoblasts. K M of the released soluble enzyme decreased in the presence of glycosaminoglycans, nonsulfated polysaccharide polymers (0.8-10 micro g/ml) or simple sugars (320-350 micro g/ml). Purified membrane lipid rafts contained nearly 3/4 of the total cell enzyme activity, whose K M was three-fold decreased as compared to the total cell membrane pool, indicating that, in the hematopoietic environment, DPP-IV activity is essentially located in the lipid rafts. This is compatible with membrane-associated events and direct cell-cell interactions, whilst the long-range activity depending upon soluble enzyme is less probable in view of the low affinity of this form.

The 3D structure of rat DPPIV/CD26 as obtained by cryo-TEM and single particle analysis

We present the three-dimensional structure of rat DPPIV/CD26, as determined by cryo-TEM and single particle analysis at a resolution of approximately 14A. The reconstruction confirms that the protein exists as a dimer, as predicted earlier. Since there are structural analogies to the serine peptidase POP, docking calculations of the two structures were performed. Although the docking showed a similar spatial organization (catalytic domain, beta-propeller, distal opening, central cavity), the detailed comparison revealed clear discrepancies. The most marked difference is a second (lateral) opening in DPPIV/CD26, which would enable direct access to the catalytic site. We therefore assume that substrate selectivity and binding rate are most probably driven by different mechanisms in DPPIV/CD26 and POP.

Characterisation of Human Dipeptidyl Peptidase IV Expressed in Pichia pastoris. A Structural and Mechanistic Comparison between the Recombinant Human and the Purified Porcine Enzyme

Dipeptidyl peptidase IV/CD26 (DP IV) is a multifunctional serine protease cleaving off dipeptides from the N-terminus of peptides. The enzyme is expressed on the surface of epithelial and endothelial cells as a type II transmembrane protein. However, a soluble form of DP IV is also present in body fluids. Large scale expression of soluble human recombinant His(6)-37-766 DP IV, using the methylotrophic yeast Pichia pastoris, yielded 1.7 mg DP IV protein per litre of fermentation supernatant. The characterisation of recombinant DP IV confirmed proper folding and glycosylation similar to DP IV purified from porcine kidney. Kinetic comparison of both proteins using short synthetic substrates and inhibitors revealed similar characteristics. However, interaction analysis of both proteins with the gastrointestinal hormone GLP-1(7-36) resulted in significantly different binding constants for the human and the porcine enzyme (Kd = 153.0 +/- 17.0 microM and Kd = 33.4 +/- 2.2 microM, respectively). In contrast, the enzyme adenosine deaminase binds stronger to human than to porcine DP IV (Kd = 2.15 +/- 0.18 nM and Kd = 7.38 +/- 0.54 nM, respectively). Even though the sequence of porcine DP IV, amplified by RT-PCR, revealed 88% identity between both enzymes, the species-specific variations between amino acids 328 to 341 are likely to be responsible for the differences in ADA-binding.

Expression, purification, and characterization of human dipeptidyl peptidase IV/CD26 in Sf9 insect cells

The human dipeptidyl peptidase IV/CD26 (DPPIV/CD26) is a multifunctional type-II membrane bound glycoprotein. As a receptor of collagen I and fibronectin it mediates cell-cell and cell-matrix adhesion, and by interacting with extracellular adenosine deaminase and CD45 it is involved in regulatory and costimulatory events in the immune system. DPPIV/CD26 has a very distinct substrate specificity, and is potentially capable of truncating many cytokines, chemokines, and peptide hormones. In this study, we describe the overexpression, purification, and characterization of human DPPIV/CD26 in Spodoptera frugiperda (Sf9) cells, using the baculovirus system. Overexpression of DPPIV/CD26 was confirmed by measurement of its peptidase specificity, SDS-PAGE, and Western blot analyses. Expression rates were between 6.4 and 17.6 mg protein per liter suspension culture (1.5 x 10(9)cells). The N-linked oligosaccharide composition was examined and compared with that of mammalian cell-expressed DPPIV/CD26. Two-step purification by immunoaffinity chromatography and size-exclusion fast protein liquid chromatography (SE-FPLC) led to highly stable protein with significant peptidase activity. A subsequent gel filtration step on a Superdex 200 column yielded 2mg homogeneous dimeric DPPIV/CD26 (per liter insect cell culture) for crystallographic studies. Protein homogeneity was confirmed by silver staining of non-denaturating PAGE gels and by MALDI-TOF analysis of tryptic peptides.