Markus Pietsch

Calpains: Attractive Targets for the Development of Synthetic Inhibitors

The physiological roles of calpains are discussed, as are the associated pathological disorders that result from their over-activation. We also present practical information for establishing functional inhibition assays and an overview of X-ray crystal structures of calpain-inhibitor complexes to aid inhibitor design. These structures reveal the expected extended beta-strand conformation for the inhibitor backbone, a geometry that has been engineered into inhibitors with the introduction of either an N-terminal heterocycle or a macrocycle that links the P(1) and P(3) residues. The structure and function of all the main classes of inhibitors are reviewed, with most examples being classified according to the nature of the C-terminal reactive warhead group that reacts with the active site cysteine of calpains. These inhibitor classes include epoxysuccinate derivatives, aldehydes, aldehyde prodrugs (hemiacetals) and alpha-keto carbonyl compounds. Inhibitors derived from the endogenous inhibitor calpastatin and examples lacking a warhead, are now known and these are also discussed.

Activated microglia/macrophage whey acidic protein (AMWAP) inhibits NFκB signaling and induces a neuroprotective phenotype in microglia

BackgroundMicroglia reactivity is a hallmark of neurodegenerative diseases. We have previously identified activated microglia/macrophage whey acidic protein (AMWAP) as a counter-regulator of pro-inflammatory response. Here, we studied its mechanisms of action with a focus on toll-like receptor (TLR) and nuclear factor κB (NFκB) signaling.MethodsRecombinant AMWAP was produced in Escherichia coli and HEK293 EBNA cells and purified by affinity chromatography. AMWAP uptake was identified by fluorescent labeling, and pro-inflammatory microglia markers were measured by qRT-PCR after stimulation with TLR ligands. NFκB pathway proteins were assessed by immunocytochemistry, Western blot, and immunoprecipitation. A 20S proteasome activity assay was used to investigate the anti-peptidase activity of AMWAP. Microglial neurotoxicity was estimated by nitrite measurement and quantification of caspase 3/7 levels in 661W photoreceptors cultured in the presence of microglia-conditioned medium. Microglial proliferation was investigated using flow cytometry, and their phagocytosis was monitored by the uptake of 661W photoreceptor debris.ResultsAMWAP was secreted from lipopolysaccharide (LPS)-activated microglia and recombinant AMWAP reduced gene transcription of IL6, iNOS, CCL2, CASP11, and TNFα in BV-2 microglia treated with LPS as TLR4 ligand. This effect was replicated with murine embryonic stem cell-derived microglia (ESdM) and primary brain microglia. AMWAP also diminished pro-inflammatory markers in microglia activated with the TLR2 ligand zymosan but had no effects on IL6, iNOS, and CCL2 transcription in cells treated with CpG oligodeoxynucleotides as TLR9 ligand. Microglial uptake of AMWAP effectively inhibited TLR4-dependent NFκB activation by preventing IRAK-1 and IκBα proteolysis. No inhibition of IκBα phosphorylation or ubiquitination and no influence on overall 20S proteasome activity were observed. Functionally, both microglial nitric oxide (NO) secretion and 661W photoreceptor apoptosis were significantly reduced after AMWAP treatment. AMWAP promoted the filopodia formation of microglia and increased the phagocytic uptake of apoptotic 661W photoreceptor cells.ConclusionsAMWAP is secreted from reactive microglia and acts in a paracrine fashion to counter-balance TLR2/TLR4-induced reactivity through NFκB inhibition. AMWAP also induces a neuroprotective microglial phenotype with reduced neurotoxicity and increased phagocytosis. We therefore hypothesize that anti-inflammatory whey acidic proteins could have a therapeutic potential in neurodegenerative diseases of the brain and the retina.

THE INTRACELLULAR CA2+-HOMEOSTASIS INFLUENCES THE FREQUENCY-DEPENDENT FORCE-GENERATION IN MAN

Abstract The present study invesitgates the effect of stimulation frequency and external Ca2+-concentration on intracellular systolic and diastolic Ca2+ as well as on the force-frequency relationship (FFR, 0.5 to 3.0 Hz, 1.0 mmol/l extracellular Ca2+) in human myocardium using fura-2 AM loaded electrically stimulated right atrial muscle strips (coronary bypass surgery, n = 15, age: 60.0 ± 1.9 years). The FFR was positive (3.0 vs. 0.5 Hz: 184 ± 43 % of basal value) and linked to an increase in peak systolic (R340/380 sys, 119 ± 7 %) as well as diastolic Ca2+ (R340/380ED, Δ fura-2 ratio +0.20 ± 0.02). After elevating the extracellular Ca2+ concentration from 1.0 to 2.4 mmol/l, force of contraction (FOC) increased from 0.5 up to 1.0 Hz (128 ± 8 %) and declined after further augmentation of stimulation frequency (3.0 Hz: 87 ± 15 %). However, this decrease in FOC was accompanied by an increase in diastolic Ca2+ (Δ fura-2 ratio +0.45 ± 0.08), while systolic Ca2+ declined at high stimulation frequencies.In conclusion, the frequency-dependent force generation is accompanied by an increase in both systolic and diastolic Ca2+ levels. Thus, especially at high stimulation frequencies the Ca2+-lowering mechanisms may become crucial and may be responsible for the blunted force-frequency relationship in failing human myocardium.

2,4,5-Triphenylisothiazol-3(2H)-one 1,1-dioxides as inhibitors of human leukocyte elastase

A series of substituted 2,4,5-triphenylisothiazol-3(2H)-one 1,1-dioxides 9 was synthesized and investigated as inhibitors of human leukocyte elastase (HLE). All compounds were found to inhibit HLE in a time-dependent manner and most of them exhibited kobs/[I] values > 300 M− 1s− 1. The most potent 3-oxosultam of this series was 9l (kobs/[I] = 2440 M− 1s− 1). Kinetic investigations performed with 9g and different substrate concentrations did not allow to clearly distinguish between a competitive or noncompetitive mode of inhibition. A more complex interaction is supported by the failure of a linear dependency of kobs values on the inhibitor concentration.

Cholesterol esterase action on human high density lipoproteins and inhibition studies: detection by MALDI-TOF MS.

The modification of lipoproteins by lipolytic enzymes such as cholesterol esterase (CEase) is assumed to play an important role in the pathogenesis of atherosclerosis. However, details of the activation and inhibition of CEase are still unknown. In this study, matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to investigate the extracts of human lipoproteins after treatment with CEase and to monitor the effects of the inhibitor 2-(diethylamino)-6,7-dihydro-4H,5H-cyclopenta[4,5] thieno[2,3-d][1,3]oxazin-4-one (DOT-3). This approach has the advantage that all lipid classes can be independently detected; therefore, conclusions on the mechanism of the applied enzyme are possible. Besides the expected decrease of cholesteryl esters (CEs) in HDL, a significantly enhanced content of lysophosphatidylcholine (LPC) was also detected, confirming the broad substrate specificity of CEase. It was also demonstrated that DOT-3 significantly inhibited the CEase-catalyzed cleavage of CEs in HDL. Phospholipid (PL) vesicles prepared from phosphatidylcholine (PC) or PC and cholesteryl linoleate were treated with CEase, and the changes in lipid composition were investigated. From the analysis of the generated LPC species in HDL and in the isolated lipid mixtures, it is evident that CEase catalyzes the cleavage of the fatty acid residues in both the sn-1 and sn-2 positions of the PLs. These effects are obvious in the absence as well as in the presence of detergents.

Kinetics of inhibition of acetylcholinesterase in the presence of acetonitrile

The hydrolysis of acetylthiocholine by acetylcholinesterase from Electrophorus electricus was investigated in the presence of the inhibitors tacrine, gallamine and compound 1. The interaction of the enzyme with the substrate and the inhibitors was characterized by the parameters KI, α′, b or β, Km and Vmax, which were determined directly and simultaneously from nonlinear Michaelis–Menten plots. Tacrine was shown to act as a mixed‐type inhibitor with a strong noncompetitive component (α′ ≈ 1) and to completely block deacylation of the acyl‐enzyme. In contrast, acetylcholinesterase inhibition by gallamine followed the ‘steric blockade hypothesis’, i.e. only substrate association to as well as substrate/product dissociation from the active site were reduced in the presence of the inhibitor. The relative efficiency of the acetylcholinesterase–gallamine complex for the catalysis of substrate conversion was determined to be 1.7–25% of that of the free enzyme. Substrate hydrolysis and the inhibition of acetylcholinesterase were also investigated in the presence of 6% acetonitrile, and a competitive pseudo‐inhibition was observed for acetonitrile (KI = 0.25 m). The interaction of acetylcholinesterase with acetonitrile and tacrine or gallamine resulted in a seven‐ to 10‐fold increase in the KI values, whereas the principal mode of inhibition was not affected by the organic solvent. The determination of the inhibitory parameters of compound 1 in the presence of acetonitrile revealed that the substance acts as a hyperbolic mixed‐type inhibitor of acetylcholinesterase. The complex formed by the enzyme and the inhibitor still catalysed product formation with 8.7–9.6% relative efficiency.

Synthesis and extended activity of triazole-containing macrocyclic protease inhibitors.

Peptide-derived protease inhibitors are an important class of compounds with the potential to treat a wide range of diseases. Herein, we describe the synthesis of a series of triazole-containing macrocyclic protease inhibitors pre-organized into a β-strand conformation and an evaluation of their activity against a panel of proteases. Acyclic azido-alkyne-based aldehydes are also evaluated for comparison. The macrocyclic peptidomimetics showed considerable activity towards calpain II, cathepsin L and S, and the 20S proteasome chymotrypsin-like activity. Some of the first examples of highly potent macrocyclic inhibitors of cathepsin S were identified. These adopt a well-defined β-strand geometry as shown by NMR spectroscopy, X-ray analysis, and molecular docking studies.

Macrocyclic Protease Inhibitors with Reduced Peptide Character

There is a real need for simple structures that define a β-strand conformation, a secondary structure that is central to peptide-protein interactions. For example, protease substrates and inhibitors almost universally adopt this geometry on active site binding. A planar pyrrole is used to replace two amino acids of a peptide backbone to generate a simple macrocycle that retains the required geometry for active site binding. The resulting β-strand templates have reduced peptide character and provide potent protease inhibitors with the attachment of an appropriate amino aldehyde to the C-terminus. Picomolar inhibitors of cathepsin L and S are reported and the mode of binding of one example to the model protease chymotrypsin is defined by X-ray crystallography.

Regioselective sulfonylation and N- to O-sulfonyl migration of quinazolin-4(3H)-ones and analogous thienopyrimidin-4(3H)-ones.

The sulfonylation of quinazolin-4(3H)-ones and related tetrahydrobenzothieno[2,3-d]pyrimidin-4(3H)-ones with mesyl, tosyl, and p-cyanobenzenesulfonyl chloride was studied. A hydrogen substituent at 2-position directed the sulfonyl group to the N-3 position, while alkylsulfanyl or amino substituents led to sulfonylation of the carbonyl oxygen. The latter effect was attributed to steric influence and the positive mesomeric effect of the 2-substituent. An access to N-sulfonylated 2-substituted regioisomers was established. An unexpected 1,3-sulfonyl migration was observed and further analyzed. This process occurred as an intramolecular N- to O-shift as verified by kinetic and crossover experiments.

Inhibition of cathepsin G by 2-amino-3,1-benzoxazin-4-ones: Kinetic investigations and docking studies

A series of benzoxazinones was used to investigate the interaction of human cathepsin G with acyl-enzyme inhibitors. With respect to the primary specificity of cathepsin G, inhibitors with hydrophobic or basic residues at position 2 were included in the study. Parameters of the enzyme acylation and deacylation were determined by slow-binding kinetics in the presence of a chromogenic substrate. For selected inhibitors, the time course of the enzyme-catalyzed conversion of the inhibitors was followed. This approach was suitable to elucidate a rate-determining deacylation step. Docking simulations of the noncovalent enzyme-inhibitor complexes were performed and several clusters were analyzed for each inhibitor. The amino acids of the active site that participate in the binding of the inhibitors were determined. The arrangements in several clusters of an inhibitor were not uniform with respect to the orientation by which the inhibitor was bound in the S(1) pocket. Docking of the basic piperazino derivatives 6 and 10 indicated an interaction with Glu 226 at the bottom of the S(1) specificity pocket. The (N-methyl)benzylamino derivative 1 showed the strongest acylation rate (k(on)=1200 M(-1) s(-1)), which was attributed to a high extent of pseudo-productive orientations of the noncovalent preassociation complex.