Hans-Ulrich Demuth

Crystal structure of the incretin-bound extracellular domain of a G protein-coupled receptor

Incretins, endogenous polypeptide hormones released in response to food intake, potentiate insulin secretion from pancreatic β cells after oral glucose ingestion (the incretin effect). This response is signaled by the two peptide hormones glucose-dependent insulinotropic polypeptide (GIP) (also known as gastric inhibitory polypeptide) and glucagon-like peptide 1 through binding and activation of their cognate class 2 G protein-coupled receptors (GPCRs). Because the incretin effect is lost or significantly reduced in patients with type 2 diabetes mellitus, glucagon-like peptide 1 and GIP have attracted considerable attention for their potential in antidiabetic therapy. A paucity of structural information precludes a detailed understanding of the processes of hormone binding and receptor activation, hampering efforts to develop novel pharmaceuticals. Here we report the crystal structure of the complex of human GIP receptor extracellular domain (ECD) with its agonist, the incretin GIP1–42. The hormone binds in an α-helical conformation in a surface groove of the ECD largely through hydrophobic interactions. The N-terminal ligand residues would remain free to interact with other parts of the receptor. Thermodynamic data suggest that binding is concomitant with structural organization of the hormone, resulting in a complex mode of receptor–ligand recognition. The presentation of a well structured, α-helical ligand by the ECD is expected to be conserved among other hormone receptors of this class.

Improved glucose tolerance in Zucker fatty rats by oral administration of the dipeptidyl peptidase IV inhibitor isoleucine thiazolidide.

The hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (GLP)-1 act on the pancreas to potentiate glucose-induced insulin secretion (enteroinsular axis). These hormones (incretins) are rapidly hydrolyzed by the circulating enzyme dipeptidyl peptidase IV (DP IV) into biologically inactive NHg-terminally truncated fragments. This study describes the effect of inhibiting endogenous DP IV with a specific DP IV inhibitor, isoleucine thiazolidide (Ile-thiazolidide), on glucose tolerance and insulin secretion in the obese Zucker rat. In initial studies, the specificity of Ile-thiazolidide as an inhibitor of incretin degradation was determined using matrix-assisted laser desorption7sol;ionization-time of flight mass spectrometry. These results showed that inhibiting DP IV activity with Ile-thiazolidide blocked the formation of NH2-terminally truncated GIP and GLP-1. Oral administration of Ile-thiazolidide resulted in rapid inhibition of circulating DP IV levels by 65% in obese and lean Zucker rats. Suppression of DP IV levels enhanced insulin secretion in both phenotypes with the most dramatic effect occurring in obese animals (150% increase in integrated insulin response vs. 27% increase in lean animals). Ile-thiazolidide treatment improved glucose tolerance in both phenotypes and restored glucose tolerance to near-normal levels in obese animals. This was attributed to the glucose-lowering actions of increasing the circulating half-lives of the endogenously released incretins GIP and, particularly, GLP-1. This study suggests that drug manipulation of plasma incretin activity by inhibiting the enzyme DP IV is a valid therapeutic approach for lowering glucose levels in NIDDM and other disorders involving glucose intolerance.

Glutaminyl cyclase inhibition attenuates pyroglutamate Aβ and Alzheimer's disease–like pathology

Because of their abundance, resistance to proteolysis, rapid aggregation and neurotoxicity, N-terminally truncated and, in particular, pyroglutamate (pE)-modified Aβ peptides have been suggested as being important in the initiation of pathological cascades resulting in the development of Alzheimers disease. We found that the N-terminal pE-formation is catalyzed by glutaminyl cyclase in vivo. Glutaminyl cyclase expression was upregulated in the cortices of individuals with Alzheimers disease and correlated with the appearance of pE-modified Aβ. Oral application of a glutaminyl cyclase inhibitor resulted in reduced Aβ3(pE)–42 burden in two different transgenic mouse models of Alzheimers disease and in a new Drosophila model. Treatment of mice was accompanied by reductions in Aβx–40/42, diminished plaque formation and gliosis and improved performance in context memory and spatial learning tests. These observations are consistent with the hypothesis that Aβ3(pE)–42 acts as a seed for Aβ aggregation by self-aggregation and co-aggregation with Aβ1–40/42. Therefore, Aβ3(pE)–40/42 peptides seem to represent Aβ forms with exceptional potency for disturbing neuronal function. The reduction of brain pE-Aβ by inhibition of glutaminyl cyclase offers a new therapeutic option for the treatment of Alzheimers disease and provides implications for other amyloidoses, such as familial Danish dementia.

The Crystal Structure of Dipeptidyl Peptidase IV (CD26) Reveals its Functional Regulation and Enzymatic Mechanism

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 β-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 β-propeller with open Velcro topology and the C-terminal α/β-hydrolase domain. Analogy with the structurally related POP and tricorn protease suggests that substrates access the buried active site through the β-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 P2-carbonyl oxygen necessary for efficient postproline cleavage. We discuss active and nonactive site-directed inhibition strategies of this pharmaceutical target protein.

Prion-like behaviour and tau-dependent cytotoxicity of pyroglutamylated amyloid-β

Extracellular plaques of amyloid-β and intraneuronal neurofibrillary tangles made from tau are the histopathological signatures of Alzheimer’s disease. Plaques comprise amyloid-β fibrils that assemble from monomeric and oligomeric intermediates, and are prognostic indicators of Alzheimer’s disease. Despite the importance of plaques to Alzheimer’s disease, oligomers are considered to be the principal toxic forms of amyloid-β. Interestingly, many adverse responses to amyloid-β, such as cytotoxicity, microtubule loss, impaired memory and learning, and neuritic degeneration, are greatly amplified by tau expression. Amino-terminally truncated, pyroglutamylated (pE) forms of amyloid-β are strongly associated with Alzheimer’s disease, are more toxic than amyloid-β, residues 1–42 (Aβ1–42) and Aβ1–40, and have been proposed as initiators of Alzheimer’s disease pathogenesis. Here we report a mechanism by which pE-Aβ may trigger Alzheimer’s disease. Aβ3(pE)–42 co-oligomerizes with excess Aβ1–42 to form metastable low-n oligomers (LNOs) that are structurally distinct and far more cytotoxic to cultured neurons than comparable LNOs made from Aβ1–42 alone. Tau is required for cytotoxicity, and LNOs comprising 5% Aβ3(pE)–42 plus 95% Aβ1–42 (5% pE-Aβ) seed new cytotoxic LNOs through multiple serial dilutions into Aβ1–42 monomers in the absence of additional Aβ3(pE)–42. LNOs isolated from human Alzheimer’s disease brain contained Aβ3(pE)–42, and enhanced Aβ3(pE)–42 formation in mice triggered neuron loss and gliosis at 3 months, but not in a tau-null background. We conclude that Aβ3(pE)–42 confers tau-dependent neuronal death and causes template-induced misfolding of Aβ1–42 into structurally distinct LNOs that propagate by a prion-like mechanism. Our results raise the possibility that Aβ3(pE)–42 acts similarly at a primary step in Alzheimer’s disease pathogenesis.

Dipeptidyl peptidase IV inhibitors: how do they work as new antidiabetic agents?

A number of new approaches to diabetes therapy are currently undergoing clinical trials, including those involving stimulation of the pancreatic beta-cell with the gut-derived insulinotropic hormones (incretins), GIP and GLP-1. The current review focuses on an approach based on the inhibition of dipeptidyl peptidase IV (DP IV), the major enzyme responsible for degrading the incretins in vivo. The rationale for this approach was that blockade of incretin degradation would increase their physiological actions, including the stimulation of insulin secretion and inhibition of gastric emptying. It is now clear that both GIP and GLP-1 also have powerful effects on beta-cell differentation, mitogenesis and survival. By potentiating these pleiotropic actions of the incretins, DP IV inhibition can therefore preserve beta-cell mass and improve secretory function in diabetics.

Glutaminyl cyclases unfold glutamyl cyclase activity under mild acid conditions

N‐terminal pyroglutamate (pGlu) formation from glutaminyl precursors is a posttranslational event in the processing of bioactive neuropeptides such as thyrotropin‐releasing hormone and neurotensin during their maturation in the secretory pathway. The reaction is facilitated by glutaminyl cyclase (QC), an enzyme highly abundant in mammalian brain. Here, we describe for the first time that human and papaya QC also catalyze N‐terminal glutamate cyclization. Surprisingly, the enzymatic Glu1 conversion is favored at pH 6.0 while Gln1 conversion occurs with an optimum at pH 8.0. This unexpected finding might be of importance for deciphering the events leading to deposition of highly toxic pyroglutamyl peptides in amyloidotic diseases.

Intraneuronal pyroglutamate-Abeta 3–42 triggers neurodegeneration and lethal neurological deficits in a transgenic mouse model

It is well established that only a fraction of Aβ peptides in the brain of Alzheimer’s disease (AD) patients start with N-terminal aspartate (Aβ1D) which is generated by proteolytic processing of amyloid precursor protein (APP) by BACE. N-terminally truncated and pyroglutamate modified Aβ starting at position 3 and ending with amino acid 42 [Aβ3(pE)–42] have been previously shown to represent a major species in the brain of AD patients. When compared with Aβ1–42, this peptide has stronger aggregation propensity and increased toxicity in vitro. Although it is unknown which peptidases remove the first two N-terminal amino acids, the cyclization of Aβ at N-terminal glutamate can be catalyzed in vitro. Here, we show that Aβ3(pE)–42 induces neurodegeneration and concomitant neurological deficits in a novel mouse model (TBA2 transgenic mice). Although TBA2 transgenic mice exhibit a strong neuronal expression of Aβ3–42 predominantly in hippocampus and cerebellum, few plaques were found in the cortex, cerebellum, brain stem and thalamus. The levels of converted Aβ3(pE)-42 in TBA2 mice were comparable to the APP/PS1KI mouse model with robust neuron loss and associated behavioral deficits. Eight weeks after birth TBA2 mice developed massive neurological impairments together with abundant loss of Purkinje cells. Although the TBA2 model lacks important AD-typical neuropathological features like tangles and hippocampal degeneration, it clearly demonstrates that intraneuronal Aβ3(pE)–42 is neurotoxic in vivo.

Pyroglutamate Formation Influences Solubility and Amyloidogenicity of Amyloid Peptides

N-Terminally truncated and pyroglutamate (pGlu) modified amyloid beta (Abeta) peptides are major constituents of amyloid deposits in sporadic and inherited Alzheimers disease (AD). Formation of pGlu at the N-terminus confers resistance against cleavage by most aminopeptidases, increases toxicity of the peptides, and may seed Abeta aggregate formation. Similarly, the deposited amyloid peptides ABri and ADan, which cause a very similar histopathology in familial British dementia (FBD) and familial Danish dementia (FDD), are N-terminally blocked by pGlu. Triggered by the coincidence of pGlu-modified amyloid peptides and similar pathology in AD, FBD, and FDD, we investigated the impact of N-terminal pGlu on biochemical and biophysical properties of Abeta, ABri, and ADan. N-Terminal pGlu increases the hydrophobicity and changes the pH-dependent solubility profile, rendering the pGlu-modified peptides less soluble in the basic pH range. The pGlu residue increases the aggregation propensity of all amyloid peptides as evidenced by ThT fluorescence assays and dynamic light scattering. The far-UV CD spectroscopic analysis points toward an enhanced beta-sheet structure of the pGlu-Abeta. Importantly, changes in fibril morphology are clearly caused by the N-terminal pGlu, resulting in the formation of short fibers, which are frequently arranged in bundles. The effect of pGlu on the morphology is virtually indistinguishable between ABri, ADan, and Abeta. The data provide evidence for a comparable influence of the pGlu modification on the aggregation process of structurally different amyloid peptides, thus likely contributing to the molecularly distinct neurodegenerative diseases AD, FBD, and FDD. The main driving force for the aggregation is apparently an increase in the hydrophobicity and thus an accelerated seed formation.

Subcellular localization suggests novel functions for prolyl endopeptidase in protein secretion

For a long time, prolyl endopeptidase (PEP) was believed to inactivate neuropeptides in the extracellular space. However, reports on the intracellular activity of PEP suggest additional, as yet unidentified, physiological functions for this enzyme. Here, we demonstrate using biochemical methods of subcellular fractionation, immunocytochemical double‐labelling procedures and localization of PEP–enhanced green fluorescent protein fusion proteins that PEP is mainly localized to the perinuclear space, and is associated with the microtubulin cytoskeleton in human neuroblastoma and glioma cell lines. Disassembly of the microtubules by nocodazole treatment disrupts both the fibrillar tubulin and PEP labelling. Furthermore, in a two‐hybrid screen, PEP was identified as binding partner of tubulin. These findings indicate novel functions for PEP in axonal transport and/or protein secretion. Indeed, a metabolic labelling approach revealed that both PEP inhibition and PEP antisense mRNA expression result in enhanced peptide/protein secretion from human U‐343 glioma cells. Because disturbances in intracellular transport and protein secretion mechanisms are associated with a number of ageing‐associated neurodegenerative diseases, cell‐permeable PEP inhibitors may be useful for the application in a variety of related clinical conditions.