Euglenatides, Potent Antiproliferative Cyclic Peptides Isolated from the Freshwater Photosynthetic Microalga Euglena gracilis.

By limiting the nitrogen source to glutamic acid, we isolated cyclic peptides from Euglena gracilis containing asparagine and non-proteinogenic amino acids. Structure elucidation was accomplished through spectroscopic methods, mass spectrometry and chemical degradation. The euglenatides potently inhibit pathogenic fungi and cancer cell lines e.g., euglenatide B exhibiting IC50 values of 4.3 µM in Aspergillus fumigatus and 0.29 µM in MCF-7 breast cancer cells. In an unprecedented convergence of non-ribosomal peptide synthetase and polyketide synthase assembly-line biosynthesis between unicellular species and the metazoan kingdom, euglenatides bear resemblance to nemamides from Caenorhabditis elegans and inhibited both producing organisms E. gracilis and C. elegans. By molecular network analysis, we detected over forty euglenatide-like metabolites in E. gracilis, E. sanguinea and E. mutabilis, suggesting an important biological role for these natural products.

Multiple effects of the herbicide glufosinate-ammonium and its main metabolite on neural stem cells from the subventricular zone of newborn mice.

The globally used herbicide glufosinate-ammonium (GLA) is structurally analogous to the excitatory neurotransmitter glutamate, and is known to interfere with cellular mechanisms involved in the glutamatergic system. In this report, we used an in vitro model of murine primary neural stem cell culture to investigate the neurotoxicity of GLA and its main metabolite, 4-methylphosphinico-2-oxobutanoic acid (PPO). We demonstrated that GLA and PPO disturb ependymal wall integrity in the ventricular-subventricular zone (V-SVZ) and alter the neuro-glial differentiation of neural stem cells. GLA and PPO impaired the formation of cilia, with reduced Celsr2 expression after PPO exposure. GLA promoted the differentiation of neuronal and oligodendroglial cells while PPO increased B1 cell population and impaired neuronal fate of neural stem cells. These results confirm our previous in vivo report that developmental exposure to GLA alters neurogenesis in the SVZ, and neuroblast migration along the rostral migratory stream. They also highlight the importance of investigating the toxicity of pesticide degradation products. Indeed, not only GLA, but also its metabolite PPO disrupts V-SVZ homeostasis and provides a novel cellular mechanism underlying GLA-induced neurodevelopmental toxicity. Furthermore, we were able to demonstrate a neurotoxic activity of a metabolite of GLA different from that of GLA active substance for the very first time.

Continuous High-Throughput Colorimetric Assays for α-Transaminases.

Transaminases are efficient tools for the stereoselective conversion of prochiral ketones into valuable chiral amines. Notably, the diversity of naturally occurring α-transaminases offers access to a wide range of L- and D-α-amino acids. We describe here two continuous colorimetric assays for the quantification of transamination activities between a keto acid and a standard donor substrate (L- or D-Glutamic acid or cysteine sulfinic acid). These assays are helpful for kinetic studies as well as for high-throughput screening of enzyme collections.

Stereoselective synthesis of γ-hydroxy-α-amino acids through aldolase-transaminase recycling cascades.

Efficient bi-enzymatic cascades combining aldolases and α-transaminases were designed for the synthesis of γ-hydroxy-α-amino acids. These recycling cascades provide high stereoselectivity, atom economy, and an equilibrium shift of the transamination. l-syn or anti-4-hydroxyglutamic acid and d-anti-4,5-dihydroxynorvaline were thus prepared in 83-95% yield in one step from simple substrates.

New 4-Functionalized Glutamate Analogues Are Selective Agonists at Metabotropic Glutamate Receptor Subtype 2 or Selective Agonists at Metabotropic Glutamate Receptor Group III.

The metabotropic glutamate (Glu) receptors (mGluRs) play key roles in modulating excitatory neurotransmission in the brain. In all, eight subtypes have been identified and divided into three groups, group I (mGlu1,5), group II (mGlu2,3), and group III (mGlu4,6-8). In this article, we present a L-2,4-syn-substituted Glu analogue, 1d, which displays selective agonist activity at mGlu2 over the remaining mGluR subtypes. A modeling study and redesign of the core scaffold led to the stereoselective synthesis of four new conformationally restricted Glu analogues, 2a-d. Most interestingly, 2a retained a selective agonist activity profile at mGlu2 (EC50 in the micromolar range), whereas 2c/2d were both selective agonists at group III, subtypes mGlu4,6,8. In general, 2d was 20-fold more potent than 2c and potently activated mGlu4,6,8 in the low-mid nanomolar range.

Continuous colorimetric screening assays for the detection of specific L- or D-α-amino acid transaminases in enzyme libraries.

In the course of a project devoted to the stereoselective synthesis of non-proteinogenic α-amino acids using α-transaminases (α-TA), we report the design and optimization of generic high-throughput continuous assays for the screening of α-TA libraries. These assays are based on the use of L- or D-cysteine sulfinic acid (CSA) as irreversible amino donor and subsequent sulfite titration by colorimetry. The assays' quality was assessed under screening conditions. Hit selection thresholds were accurately determined for every couple of substrates and a library of 232 putative transaminases expressed in Escherichia coli host cells was screened. The reported high throughput screening assays proved very sensitive allowing the detection with high confidence of activities as low as 10 µU (i.e., 0.01 nmol substrate converted per min). The assays were also evidenced to be stereochemically discriminant since L-CSA and D-CSA allowed the exclusive detection of L-TA and D-TA, respectively. These generic assays thus allow testing the stereoselective conversion of a wide range of α-keto acids into α-amino acids of interest. As a proof of principle, the use of 2-oxo-4-phenylbutyric acid as acceptor substrate led to the identification of 54 new α-TA offering an access to valuable L- or D-homophenylalanine.

Profiling substrate specificity of two series of phenethylamine analogs at monoamine oxidase A and B.

The membrane bound enzyme monoamine oxidase exist in two splice variants designated A and B (MAO-A and MAO-B) and are key players in the oxidative metabolism of monoamines in mammalians. Despite their importance and being a prevalent target for the development of inhibitors as drugs, no systematic study of substrate specificity has been reported. In this study we present a systematic study of the MAO-A and MAO-B substrate specificity profile by probing two series of phenethylamine analogs. Km and kcat values were determined for four N-alkyl analogs 2-5 and four aryl halide analogs 6-9 at MAO-A and MAO-B. A following in silico study disclosed a new adjacent compartment to the MAO-B substrate pocket defined by amino acids Tyr188, Tyr435, Tyr398, Thr399, Cys172 and Gly434. This new insight is important for the understanding of the substrate specificity of the MAO-B enzyme and will be relevant for future drug design within the field of monoamines.

Molecular recognition of two 2,4-syn-functionalized (S)-glutamate analogues by the kainate receptor GluK3 ligand binding domain.

The kainate receptors are the least studied subfamily of ionotropic glutamate receptors. These receptors are thought to have a neuromodulatory role and have been associated with a variety of disorders in the central nervous system. This makes kainate receptors interesting potential drug targets. Today, structures of the ligand binding domain (LBD) of the kainate receptor GluK3 are only known in complex with the endogenous agonist glutamate, the natural product kainate, and two synthetic agonists. Herein we report structures of GluK3 LBD in complex with two 2,4-syn-functionalized (S)-glutamate analogues to investigate their structural potential as chemical scaffolds. Similar binding affinities at GluK3 were determined for the 2-(methylcarbamoyl)ethyl analogue (Ki =4.0 µM) and the 2-(methoxycarbonyl)ethyl analogue (Ki =1.7 µM), in agreement with the similar positioning of the compounds within the binding pocket. As the binding affinity is similar to that of glutamate, this type of Cγ substituent could be used as a scaffold for introduction of even larger substituents reaching into unexplored binding site regions to achieve subtype selectivity.

Chemoenzymatic synthesis of new 2,4-syn-functionalized (S)-glutamate analogues and structure-activity relationship studies at ionotropic glutamate receptors and excitatory amino acid transporters.

In the mammalian central nervous system, (S)-glutamate (Glu) is released from the presynaptic neuron where it activates a plethora of pre- and postsynaptic Glu receptors. The fast acting ionotropic Glu receptors (iGluRs) are ligand gated ion channels and are believed to be involved in a vast number of neurological functions such as memory and learning, synaptic plasticity, and motor function. The synthesis of 14 enantiopure 2,4-syn-Glu analogues 2b-p is accessed by a short and efficient chemoenzymatic approach starting from readily available cyclohexanone 3. Pharmacological characterization at the iGluRs and EAAT1-3 subtypes revealed analogue 2i as a selective GluK1 ligand with low nanomolar affinity. Two X-ray crystal structures of the key analogue 2i in the ligand-binding domain (LBD) of GluA2 and GluK3 were determined. Partial domain closure was seen in the GluA2-LBD complex with 2i comparable to that induced by kainate. In contrast, full domain closure was observed in the GluK3-LBD complex with 2i, similar to that of GluK3-LBD with glutamate bound.

Pharmacological and structural characterization of conformationally restricted (S)-glutamate analogues at ionotropic glutamate receptors.

Conformationally restricted glutamate analogues have been pharmacologically characterized at AMPA and kainate receptors and the crystal structures have been solved of the ligand (2S,1'R,2'S)-2-(2'-carboxycyclobutyl)glycine (CBG-IV) in complex with the ligand binding domains of the AMPA receptor GluA2 and the kainate receptor GluK3. These structures show that CBG-IV interacts with the binding pocket in the same way as (S)-glutamate. The binding affinities reveal that CBG-IV has high affinity at the AMPA and kainate receptor subtypes. Appreciable binding affinity of CBG-IV was not observed at NMDA receptors, where the introduction of the carbocyclic ring is expected to lead to a steric clash with binding site residues. CBG-IV was demonstrated to be an agonist at both GluA2 and the kainate receptor GluK1. CBG-IV showed high affinity binding to GluK1 compared to GluA2, GluK2 and GluK3, which exhibited lower affinity for CBG-IV. The structure of GluA2 LBD and GluK3 LBD in complex with CBG-IV revealed similar binding site interactions to those of (S)-glutamate. No major conformational rearrangements compared to the (S)-glutamate bound conformation were found in GluK3 in order to accommodate CBG-IV, in contrast with GluA2 where a shift in lobe D2 binding site residues occurs, leading to an increased binding cavity volume compared to the (S)-glutamate bound structure.

Preparation of glutamate analogues by enzymatic transamination.

Aminotransferases are key enzymes of the metabolism of proteinogenic amino acids. These ubiquitous biocatalysts show high specific activities and relaxed substrate specificities making them valuable tools for the stereoselective synthesis of unnatural amino acids. We describe here the application of aspartate aminotransferase and branched chain aminotransferase from E. coli for the synthesis of various glutamate analogues, molecules of particular interest regarding the neuroactive properties of glutamic acid.

4,4-Dimethyl- and diastereomeric 4-hydroxy-4-methyl- (2S)-glutamate analogues display distinct pharmacological profiles at ionotropic glutamate receptors and excitatory amino acid transporters.

Subtype-selective ligands are of great interest to the scientific community, as they provide a tool for investigating the function of one receptor or transporter subtype when functioning in its native environment. Several 4-substituted (S)-glutamate (Glu) analogues were synthesized, and altogether this approach has provided important insight into the structure-activity relationships (SAR) for ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs), as well as the excitatory amino acid transporters (EAATs). In this work, three 4,4-disubstituted Glu analogues 1-3, which are hybrid structures of important 4-substituted Glu analogues 4-8, were investigated at iGluRs and EAATs. Collectively, their pharmacological profiles add new and valuable information to the SAR for the iGluRs and EAAT1-3.

MRI characterization of structural mouse brain changes in response to chronic exposure to the glufosinate ammonium herbicide.

Glufosinate ammonium (GLA) is the active component of herbicides widely used in agriculture, truck farming, or public domains. GLA acts by inhibiting the plant glutamine synthetase (GlnS). It also inhibits mammalian GlnS in vitro and ex vivo. In the central nervous system this enzyme is exclusively localized in glial cells. Whereas acute neurotoxic effects of GLA are well documented, long-term effects during chronic exposure at low doses remain largely undisclosed. In the present work, C57BL/6J mice were treated intraperitoneally with 2.5, 5, and 10 mg/kg of GLA three times a week during 10 weeks. Cerebral magnetic resonance imaging (MRI) experiments were performed at high field (9.4 T) and the images were analyzed with four texture analysis (TA) methods. TA highlighted structural changes in seven brain structures after chronic GLA treatments. Changes are dose dependent and can be seen at a dose as low as 2.5 mg/kg for two areas, namely hippocampus and somatosensorial cortex. Glial fibrillary acidic protein (GFAP) expression in the same seven brain structures and GlnS activity in the hippocampus and cortex areas were also studied. The number of GFAP-positive cells is modified in six out of the seven areas examined. GlnS activity was significantly increased in the hippocampus but not in the cortex. These results indicate some kind of suffering at the cerebral level after chronic GLA treatment. Changes in TA were compared with the modification of the number of GFAP-positive astrocytes in the studied brain areas after GLA treatment. We show that the noninvasive MRI-TA is a sensitive method and we suggest that it would be a very helpful tool that can efficiently contribute to the detection of cerebral alterations in vivo during chronic exposure to xenobiotics.

Pharmacological characterization of (4R)-alkyl glutamate analogues at the ionotropic glutamate receptors--focus on subtypes iGlu(5-7).

The kainic acid (kainate, KA) receptors belong to the class of ionotropic glutamate (iGlu) receptors in the central nervous system. Five subtypes have been identified, which have been termed KA(1,2) and iGlu(5-7). In the search for subtype selective ligands, alpha-amino-5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA), (4R)-methyl Glu (1a), and E-4-neopentylidene Glu (2f) have all previously been reported as selective agonists for the iGlu(5) receptor subtype. In this paper, we present the pharmacological evaluation of a five-compound series of (4R)-alkyl Glu analogs (1b-e,g) which may be envisaged as conformationally released designs of ATPA and 4-alkylidenes 2a-h. Most notable is the pharmacological profile for (4R)-isopentyl Glu (1g) which shows a 10-fold increase in binding affinity for the iGlu(5) receptor subtype (K(i)=20.5 nM) in comparison with its E-4-alkylidene structural isomer 2g. Furthermore, 1g displays high selectivity over other KA receptor subtypes (KA(1,2) and iGlu(6,7)), AMPA-, and NMDA receptors (2050 and >5000 fold, respectively).

Chemo-enzymatic synthesis of (2S,4R)-2-amino-4-(3-(2,2-diphenylethylamino)-3-oxopropyl)pentanedioic acid: a novel selective inhibitor of human excitatory amino acid transporter subtype 2.

In the mammalian central nervous system (CNS), the action of sodium dependent excitatory amino acid transporters (EAATs) is responsible for termination of glutamatergic neurotransmission by reuptake of ( S) -glutamate (Glu) from the synaptic cleft. Five EAAT subtypes have been identified, of which EAAT1-4 are present in the CNS, while EAAT5 is localized exclusively in the retina. In this study, we have used an enantioselective chemo-enzymatic strategy to synthesize 10 new Glu analogues 2a- k ( 2d is exempt) with different functionalities in the 4 R-position and characterized their pharmacological properties at the human EAAT1-3. In particular, one compound, 2k, displayed a significant preference as inhibitor of the EAAT2 subtype over EAAT1,3. The compound also displayed very low affinities toward ionotropic and metabotropic Glu receptors, making it the most selective EAAT2 inhibitor described so far.

Chemo-enzymatic synthesis of a series of 2,4-syn-functionalized (S)-glutamate analogues: new insight into the structure-activity relation of ionotropic glutamate receptor subtypes 5, 6, and 7.

( S)-Glutamic acid (Glu) is the major excitatory neurotransmitter in the central nervous system (CNS) activating the plethora of ionotropic Glu receptors (iGluRs) and metabotropic Glu receptors (mGluRs). In this paper, we present a chemo-enzymatic strategy for the enantioselective synthesis of five new Glu analogues 2a- f ( 2d is exempt) holding a functionalized substituent in the 4-position. Nine Glu analogues 2a- j are characterized pharmacologically at native 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA), kainic acid (KA), and N-methyl- d-aspartic acid (NMDA) receptors in rat synaptosomes as well as in binding assays at cloned rat iGluR5-7 subtypes. A detailed in silico study address as to why 2h is a high-affinity ligand at iGluR5-7 ( K i = 3.81, 123, 57.3 nM, respectively), while 2e is only a high affinity ligand at iGluR5 ( K i = 42.8 nM). Furthermore, a small series of commercially available iGluR ligands are characterized in iGluR5-7 binding.

Chronic exposure to glufosinate-ammonium induces spatial memory impairments, hippocampal MRI modifications and glutamine synthetase activation in mice.

Glufosinate-ammonium (GLA), the active compound of a worldwide-used herbicide, acts by inhibiting the plant glutamine synthetase (GS) leading to a lethal accumulation of ammonia. GS plays a pivotal role in the mammalian brain where it allows neurotransmitter glutamate recycling within astroglia. Clinical studies report that an acute GLA ingestion induces convulsions and memory impairment in humans. Toxicological studies performed at doses used for herbicidal activity showed that GLA is probably harmless at short or medium range periods. However, effects of low doses of GLA on chronically exposed subjects are not known. In our study, C57BL/6J mice were treated during 10 weeks three times a week with 2.5, 5 and 10mg/kg of GLA. Effects of this chronic treatment were assessed at behavioral, structural and metabolic levels by using tests of spatial memory, locomotor activity and anxiety, hippocampal magnetic resonance imaging (MRI) texture analysis, and hippocampal GS activity assay, respectively. Chronic GLA treatments have effects neither on anxiety nor on locomotor activity of mice but at 5 and 10mg/kg induce (1) mild memory impairments, (2) a modification of hippocampal texture and (3) a significant increase in hippocampal GS activity. It is suggested that these modifications may be causally linked one to another. Since glutamate is the main neurotransmitter in hippocampus where it plays a crucial role in spatial memory, hippocampal MRI texture and spatial memory alterations might be the consequences of hippocampal glutamate homeostasis modification revealed by increased GS activity in hippocampus. The present study provides the first data that show cerebral alterations after chronic exposure to GLA.

Irreversible inhibition of aldolase by a phosphorylated alpha-dicarbonyl compound.

The preparation of a phosphorylated alpha-dicarbonyl compound designed to specifically react with arginine residues of enzymes accepting phosphorylated compounds as effectors is reported, and shown to inhibit rabbit muscle aldolase in a time-dependent and irreversible manner. This irreversible inhibition occured in a buffer devoid of borate ions, suggesting that the presence of the phosphate moiety contributes in the stabilization of the adduct formed with arginine residues. Under the same conditions, the metalloenzyme iron superoxide dismutase, in which an arginine is known to be critical for the catalytic function, is not significantly inhibited.

Chemoenzymatic synthesis of glutamic acid analogues: substrate specificity and synthetic applications of branched chain aminotransferase from Escherichia coli.

A new route to alpha-keto acids is described, based on the ozonolysis of enol acetates obtained from alpha-substituted beta-keto esters. Escherichia coli branched chain aminotransferase (BCAT) activity toward a variety of substituted 2-oxoglutaric acids was demonstrated analytically. BCAT was shown to have a broad substrate spectrum, complementary to that of aspartate aminotransferase, and to offer access to a variety of glutamic acid analogues. The usefulness of BCAT was demonstrated through the synthesis of several 3- and 4-substituted derivatives.

Stereoselective chemoenzymatic synthesis of the four stereoisomers of l-2-(2-carboxycyclobutyl)glycine and pharmacological characterization at human excitatory amino acid transporter subtypes 1, 2, and 3.

The four stereoisomers of l-2-(2-carboxycyclobutyl)glycine, l-CBG-I, l-CBG-II, l-CBG-III, and l-CBG-IV, were synthesized in good yield and high enantiomeric excess, from the corresponding cis and trans-2-oxalylcyclobutanecarboxylic acids 5 and 6 using the enzymes aspartate aminotransferase (AAT) and branched chain aminotransferase (BCAT) from Escherichia coli. The four stereoisomeric compounds were evaluated as potential ligands for the human excitatory amino acid transporters, subtypes 1, 2, and 3 (EAAT1, EAAT2, and EAAT3) in the FLIPR membrane potential assay. While the one trans-stereoisomer, l-CBG-I, displayed weak substrate activity at all three transporters, EAAT1-3, we found a particular pharmacological profile for the other trans-stereoisomer, l-CBG-II, which displayed EAAT1 substrate activity and inhibitory activity at EAAT2 and EAAT3. Whereas l-CBG-III was found to be a weak inhibitor at all three EAAT subtypes, the other cis-stereoisomer l-CBG-IV was a moderately potent inhibitor with 20-30-fold preference for EAAT2/3 over EAAT1.