Synthesis, Biological Profiling and Determination of the Tubulin-Bound Conformation of 12-Aza-Epothilones (Azathilones).

12-Aza-epothilones (azathilones) incorporating quinoline side chains and bearing different N12-substituents have been synthesized via highly efficient RCM-based macrocyclizations. Quinoline-based azathilones with the side chain N-atom in the meta-position to the C15 atom in the macrocycle are highly potent inhibitors of cancer cell growth in vitro. In contrast, shifting the quinoline nitrogen to the position para to C15 leads to a ca. 1000-fold loss in potency. Likewise, the desaturation of the C9-C10 bond in the macrocycle to an E double bond produces a substantial reduction in antiproliferative activity. This is in stark contrast to the effect exerted by the same modification in the natural epothilone macrocycle. The conformation of a representative azathilone bound to α/ß-tubulin heterodimers was determined based on TR-NOE measurements and a model for the posture of the compound in its binding site on ß-tubulin was deduced through a combination of STD measurements and CORCEMA-ST calculations. The tubulin-bound, bioactive conformation of azathilones was found to be overall similar to that of epothilones A and B.

Pressure adaptation is linked to thermal adaptation in salt-saturated marine habitats.

The present study provides a deeper view of protein functionality as a function of temperature, salt and pressure in deep-sea habitats. A set of eight different enzymes from five distinct deep-sea (3040-4908 m depth), moderately warm (14.0-16.5°C) biotopes, characterized by a wide range of salinities (39-348 practical salinity units), were investigated for this purpose. An enzyme from a 'superficial' marine hydrothermal habitat (65°C) was isolated and characterized for comparative purposes. We report here the first experimental evidence suggesting that in salt-saturated deep-sea habitats, the adaptation to high pressure is linked to high thermal resistance (P value = 0.0036). Salinity might therefore increase the temperature window for enzyme activity, and possibly microbial growth, in deep-sea habitats. As an example, Lake Medee, the largest hypersaline deep-sea anoxic lake of the Eastern Mediterranean Sea, where the water temperature is never higher than 16°C, was shown to contain halopiezophilic-like enzymes that are most active at 70°C and with denaturing temperatures of 71.4°C. The determination of the crystal structures of five proteins revealed unknown molecular mechanisms involved in protein adaptation to poly-extremes as well as distinct active site architectures and substrate preferences relative to other structurally characterized enzymes.

Identification and characterization of carboxyl esterases of gill chamber-associated microbiota in the deep-sea shrimp Rimicaris exoculata by using functional metagenomics.

The shrimp Rimicaris exoculata dominates the fauna in deep-sea hydrothermal vent sites along the Mid-Atlantic Ridge (depth, 2,320 m). Here, we identified and biochemically characterized three carboxyl esterases from microbial communities inhabiting the R. exoculata gill that were isolated by naive screens of a gill chamber metagenomic library. These proteins exhibit low to moderate identity to known esterase sequences (≤52%) and to each other (11.9 to 63.7%) and appear to have originated from unknown species or from genera of Proteobacteria related to Thiothrix/Leucothrix (MGS-RG1/RG2) and to the Rhodobacteraceae group (MGS-RG3). A library of 131 esters and 31 additional esterase/lipase preparations was used to evaluate the activity profiles of these enzymes. All 3 of these enzymes had greater esterase than lipase activity and exhibited specific activities with ester substrates (≤356 U mg(-1)) in the range of similar enzymes. MGS-RG3 was inhibited by salts and pressure and had a low optimal temperature (30°C), and its substrate profile clustered within a group of low-activity and substrate-restricted marine enzymes. In contrast, MGS-RG1 and MGS-RG2 were most active at 45 to 50°C and were salt activated and barotolerant. They also exhibited wider substrate profiles that were close to those of highly active promiscuous enzymes from a marine hydrothermal vent (MGS-RG2) and from a cold brackish lake (MGS-RG1). The data presented are discussed in the context of promoting the examination of enzyme activities of taxa found in habitats that have been neglected for enzyme prospecting; the enzymes found in these taxa may reflect distinct habitat-specific adaptations and may constitute new sources of rare reaction specificities.

Molecular adaptation of Lactobacillus plantarum WCFS1 to gallic acid revealed by genome-scale transcriptomic signature and physiological analysis.

BACKGROUND: Gallic acid (GA) is a model hydroxybenzoic acid that occurs esterified in the lignocellulosic biomass of higher plants. GA displays relevant biological activities including anticancer properties. Owing to its antimicrobial and cellulase-inhibiting activities, GA also imposes constraints to the fermentability of lignocellulosic hydrolysates. In depth-knowledge of the mechanisms used by tolerant microorganisms to adapt to hydroxybenzoic acids would be a step forward to improve the bioavailability of GA or select/engineer production hosts with improved metabolic traits for the bioconversion of pretreated lignocellulosic biomass. RESULTS: Whole genome transcriptional profiling using DNA microarrays was used to characterize the molecular response of Lactobacillus plantarum WCFS1 to GA. Expression levels of 14 and 40 genes were differentially regulated at 1.5 and 15 mM GA, respectively. The transcriptomic analysis identified a marked induction of genes with confirmed or related roles to gastrointestinal survival, the repression of genes coding for certain ABC-type transporters and modulation of genes involved in the control of intracellular ammonia levels, among other responses. Most notably, a core set of genes dedicated to produce GA from polyphenols (tanB Lp ), decarboxylate GA to pyrogallol (lpdB, lpdC and lpdD) and transport functions (lp_2943) was highly overexpressed at both GA concentrations. Correspondingly, resting cells of strain WCFS1 induced by GA, but not their non-induced controls, produced pyrogallol. Gene expression and organization of genes involved in GA metabolism suggested a chemiosmotic mechanism of energy generation. Resting cells of L. plantarum induced by GA generated a membrane potential and a pH gradient across the membrane immediately upon addition of GA. Altogether, transcriptome profiling correlated with physiological observations indicating that a proton motive force could be generated during GA metabolism as a result of electrogenic GA uptake coupled with proton consumption by the intracellular gallate decarboxylase. CONCLUSIONS: The combination of transcriptome and physiological analyses revealed versatile molecular mechanisms involved in the adaptation of L. plantarum to GA. These data provide a platform to improve the survival of Lactobacillus in the gut. Our data may also guide the selection/engineering of microorganisms that better tolerate phenolic inhibitors present in pretreated lignocellulosic feedstocks.

Taxanes with high potency inducing tubulin assembly overcome tumoural cell resistances.

We have found that four taxanes with chemical modifications at positions C10 and C13 were active against all types of taxane resistant cell lines, resistant by P-gp overexpression, by mutations in the ß-tubulin binding site or by overexpression of the highly dynamic ßIII-tubulin isotype. We have characterized the interaction of taxanes with high activity on chemotherapy resistant tumoural cells with microtubules, and also studied their cellular effects. The biochemical property enhanced in comparison with other taxanes is their potency at inducing tubulin assembly, despite the fact that their interactions with the microtubule binding sites (pore and luminal) are similar as studied by NMR and SAXS. A differential interaction with the S7-S9 loop (M-loop) is responsible for their enhanced assembly induction properties. The chemical changes in the structure also induce changes in the thermodynamic properties of the interaction, indicating a higher hydrophilicity and also explaining their properties on P-gp and ßIII overexpressing cells and on mutant cells. The effect of the compounds on the microtubular network is different from those observed with the classical (docetaxel and paclitaxel) taxanes, inducing different bundling in cells with microtubules being very short, indicating a very fast nucleation effect and reflecting their high assembly induction power.

PM534, an Optimized Target-Protein Interaction Strategy through the Colchicine Site of Tubulin.

Targeting microtubules is the most effective wide-spectrum pharmacological strategy in antitumoral chemotherapy, and current research focuses on reducing main drawbacks: neurotoxicity and resistance. PM534 is a novel synthetic compound derived from the Structure-Activity-Relationship study on the natural molecule PM742, isolated from the sponge of the order Lithistida, family Theonellidae, genus Discodermia (du Bocage 1869). PM534 targets the entire colchicine binding domain of tubulin, covering four of the five centers of the pharmacophore model. Its nanomolar affinity and high retention time modulate a strikingly high antitumor activity that efficiently overrides two resistance mechanisms in cells (detoxification pumps and tubulin ßIII isotype overexpression). Furthermore, PM534 induces significant inhibition of tumor growth in mouse xenograft models of human non-small cell lung cancer. Our results present PM534, a highly effective new compound in the preclinical evaluation that is currently in its first human Phase I clinical trial.

A structure-based design of new C2- and C13-substituted taxanes: tubulin binding affinities and extended quantitative structure-activity relationships using comparative binding energy (COMBINE) analysis.

Ten novel taxanes bearing modifications at the C2 and C13 positions of the baccatin core have been synthesized and their binding affinities for mammalian tubulin have been experimentally measured. The design strategy was guided by (i) calculation of interaction energy maps with carbon, nitrogen and oxygen probes within the taxane-binding site of ß-tubulin, and (ii) the prospective use of a structure-based QSAR (COMBINE) model derived from an earlier series comprising 47 congeneric taxanes. The tubulin-binding affinity displayed by one of the new compounds (CTX63) proved to be higher than that of docetaxel, and an updated COMBINE model provided a good correlation between the experimental binding free energies and a set of weighted residue-based ligand-receptor interaction energies for 54 out of the 57 compounds studied. The remaining three outliers from the original training series have in common a large unfavourable entropic contribution to the binding free energy that we attribute to taxane preorganization in aqueous solution in a conformation different from that compatible with tubulin binding. Support for this proposal was obtained from solution NMR experiments and molecular dynamics simulations in explicit water. Our results shed additional light on the determinants of tubulin-binding affinity for this important class of antitumour agents and pave the way for further rational structural modifications.

Enzyme adaptation to habitat thermal legacy shapes the thermal plasticity of marine microbiomes.

Microbial communities respond to temperature with physiological adaptation and compositional turnover. Whether thermal selection of enzymes explains marine microbiome plasticity in response to temperature remains unresolved. By quantifying the thermal behaviour of seven functionally-independent enzyme classes (esterase, extradiol dioxygenase, phosphatase, beta-galactosidase, nuclease, transaminase, and aldo-keto reductase) in native proteomes of marine sediment microbiomes from the Irish Sea to the southern Red Sea, we record a significant effect of the mean annual temperature (MAT) on enzyme response in all cases. Activity and stability profiles of 228 esterases and 5 extradiol dioxygenases from sediment and seawater across 70 locations worldwide validate this thermal pattern. Modelling the esterase phase transition temperature as a measure of structural flexibility confirms the observed relationship with MAT. Furthermore, when considering temperature variability in sites with non-significantly different MATs, the broadest range of enzyme thermal behaviour and the highest growth plasticity of the enriched heterotrophic bacteria occur in samples with the widest annual thermal variability. These results indicate that temperature-driven enzyme selection shapes microbiome thermal plasticity and that thermal variability finely tunes such processes and should be considered alongside MAT in forecasting microbial community thermal response.

Modulation of taxane binding to tubulin curved and straight conformations by systematic 3'N modification provides for improved microtubule binding, persistent cytotoxicity and in vivo potency.

The taxane class of microtubule stabilizers are some of the most effective and widely used chemotherapeutics. The anticancer activity of taxanes arises from their ability to induce tubulin assembly by selectively recognizing the curved (c-) conformation in unassembled tubulin as compared to the straight (s-) conformation in assembled tubulin. We first designed and synthesized a series of 3'N-modified taxanes bearing covalent groups. Instead of discovering covalent taxanes, we found a series of non-covalent taxanes 2, in which the 3'N side chain was found to be essential for cytotoxicity due to its role in locking tubulin in the s-conformation. A representative compound bearing an acrylamide moiety (2h) exhibited increased binding affinity to the unassembled tubulin c-conformation and less cytotoxicity than paclitaxel. Further exploration of chemical space around 2h afforded a new series 3, in which derivatives such as 3l bind more tightly to both the s- and c-conformations of tubulin compared to paclitaxel, leading to more efficient promotion of tubulin polymerization and a greater persistence of in vitro efficacy against breast cancer cells after drug washout. Although 3l also had improved in vivo potency as compared to paclitaxel, it was also associated with increased systemic toxicity that required localized, intratumoral injection to observe potent and prolonged antitumor efficacy.

Cyclostreptin derivatives specifically target cellular tubulin and further map the paclitaxel site.

Cyclostreptin is the first microtubule-stabilizing agent whose mechanism of action was discovered to involve formation of a covalent bond with tubulin. Treatment of cells with cyclostreptin irreversibly stabilizes their microtubules because cyclostreptin forms a covalent bond to ß-tubulin at either the T220 or the N228 residue, located at the microtubule pore or luminal taxoid binding site, respectively. Because of its unique mechanism of action, cyclostreptin overcomes P-glycoprotein-mediated multidrug resistance in tumor cells. We used a series of reactive cyclostreptin analogues, 6-chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and [(14)C-acetyl]-8-acetyl-cyclostreptin, to characterize the cellular target of the compound and to map the binding site. The three analogues were cytotoxic and stabilized microtubules in both sensitive and multidrug resistant tumor cells. In both types of cells, we identified ß-tubulin as the only or the predominantly labeled cellular protein, indicating that covalent binding to microtubules is sufficient to prevent drug efflux mediated by P-glycoprotein. 6-Chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and 8-acetyl-cyclostreptin labeled both microtubules and unassembled tubulin at a single residue of the same tryptic peptide of ß-tubulin as was labeled by cyclostreptin (219-LTTPTYGDLNHLVSATMSGVTTCLR-243), but labeling with the analogues occurred at different positions of the peptide. 8-Acetyl-cyclostreptin reacted with either T220 or N228, as did the natural product, while 8-chloroacetyl-cyclostreptin formed a cross-link to C241. Finally, 6-chloroacetyl-cyclostreptin reacted with any of the three residues, thus labeling the pathway for cyclostreptin-like compounds, leading from the pore where these compounds enter the microtubule to the luminal binding pocket.

Multiple keys for a single lock: the unusual structural plasticity of the nucleotidyltransferase (4')/kanamycin complex.

The most common mode of bacterial resistance to aminoglycoside antibiotics is the enzyme-catalysed chemical modification of the drug. Over the last two decades, significant efforts in medicinal chemistry have been focused on the design of non- inactivable antibiotics. Unfortunately, this strategy has met with limited success on account of the remarkably wide substrate specificity of aminoglycoside-modifying enzymes. To understand the mechanisms behind substrate promiscuity, we have performed a comprehensive experimental and theoretical analysis of the molecular-recognition processes that lead to antibiotic inactivation by Staphylococcus aureus nucleotidyltransferase 4'(ANT(4')), a clinically relevant protein. According to our results, the ability of this enzyme to inactivate structurally diverse polycationic molecules relies on three specific features of the catalytic region. First, the dominant role of electrostatics in aminoglycoside recognition, in combination with the significant extension of the enzyme anionic regions, confers to the protein/antibiotic complex a highly dynamic character. The motion deduced for the bound antibiotic seem to be essential for the enzyme action and probably provide a mechanism to explore alternative drug inactivation modes. Second, the nucleotide recognition is exclusively mediated by the inorganic fragment. In fact, even inorganic triphosphate can be employed as a substrate. Third, ANT(4') seems to be equipped with a duplicated basic catalyst that is able to promote drug inactivation through different reactive geometries. This particular combination of features explains the enzyme versatility and renders the design of non-inactivable derivatives a challenging task.

Modulation of microtubule interprotofilament interactions by modified taxanes.

Microtubules assembled with paclitaxel and docetaxel differ in their numbers of protofilaments, reflecting modification of the lateral association between αß-tubulin molecules in the microtubule wall. These modifications of microtubule structure, through a not-yet-characterized mechanism, are most likely related to the changes in tubulin-tubulin interactions responsible for microtubule stabilization by these antitumor compounds. We have used a set of modified taxanes to study the structural mechanism of microtubule stabilization by these ligands. Using small-angle x-ray scattering, we have determined how modifications in the shape and size of the taxane substituents result in changes in the interprotofilament angles and in their number. The observed effects have been explained using NMR-aided docking and molecular dynamic simulations of taxane binding at the microtubule pore and luminal sites. Modeling results indicate that modification of the size of substituents at positions C7 and C10 of the taxane core influence the conformation of three key elements in microtubule lateral interactions (the M-loop, the S3 ß-strand, and the H3 helix) that modulate the contacts between adjacent protofilaments. In addition, modifications of the substituents at position C2 slightly rearrange the ligand in the binding site, modifying the interaction of the C7 substituent with the M-loop.

Harnessing DNA for nanothermometry.

Temperature measurement at the nanoscale has brought insight to a wide array of research interests in modern chemistry, physics, and biology. These measurements have been enabled by the advent of nanothermometers, which relay nanoscale temperature information through the analysis of their intrinsic photophysical behavior. In the past decade, several nanothermometers have been developed including dyes, nanodiamonds, fluorescent proteins, nucleotides, and nanoparticles. However, temperature measurement using intact DNA has not yet been achieved. Here, we present a method to study the temperature sensitivity of the DNA molecule within a physiologic temperature range when complexed with fluorescent dye. We theoretically and experimentally report the temperature sensitivity of the DNA-Hoechst 33342 complex in different sizes of double-stranded oligonucleotides and plasmids, showing its potential use as a nanothermometer. These findings allow for extending the thermal study of DNA to several research fields including DNA nanotechnology, optical tweezers, and DNA nanoparticles.

Epothilone analogues with benzimidazole and quinoline side chains: chemical synthesis, antiproliferative activity, and interactions with tubulin.

A series of epothilone B and D analogues bearing isomeric quinoline or functionalized benzimidazole side chains has been prepared by chemical synthesis in a highly convergent manner. All analogues have been found to interact with the tubulin/microtubule system and to inhibit human cancer cell proliferation in vitro, albeit with different potencies (IC(50) values between 1 and 150 nM). The affinity of quinoline-based epothilone B and D analogues for stabilized microtubules clearly depends on the position of the N-atom in the quinoline system, while the induction of tubulin polymerization in vitro appears to be less sensitive to N-positioning. The potent inhibition of human cancer cell growth by epothilone analogues bearing functionalized benzimidazole side chains suggests that these systems might be conjugated with tumor-targeting moieties to form tumor-targeted prodrugs.

Optimization of taxane binding to microtubules: binding affinity dissection and incremental construction of a high-affinity analog of paclitaxel.

The microtubule binding affinities of a series of synthetic taxanes have been measured with the aims of dissecting individual group contributions and obtaining a rationale for the design of novel compounds with the ability to overcome drug resistance. As previously observed for epothilones, the positive and negative contributions of the different substituents to the binding free energies are cumulative. By combining the most favorable substitutions we increased the binding affinity of paclitaxel 500-fold. Insight into the structural basis for this improvement was gained with molecular modeling and NMR data obtained for microtubule-bound docetaxel. Taxanes with affinities for microtubules well above their affinities for P-glycoprotein are shown not to be affected by multidrug resistance. This finding strongly indicates that optimization of the ligand-target interaction is a good strategy to overcome multidrug resistance mediated by efflux pumps.

Synthesis and biological activities of high affinity taxane-based fluorescent probes.

Three fluorescent probes 3a,3b, and 4 have been synthesized through conjugation of fluorescein and difluorescein groups to the 7-OH of C-2 modified paclitaxel and cephalomannine derivatives with very high affinity to microtubules. All these probes exhibited potent tubulin assembly promotion and tumor cell killing activities, thus may be useful as tools for the determination of thermodynamic parameters and exploration of ligand-microtubule interactions.

Arylthioindole inhibitors of tubulin polymerization. 3. Biological evaluation, structure-activity relationships and molecular modeling studies.

The new arylthioindole (ATI) derivatives 10, 14-18, and 21-24, which bear a halogen atom or a small size ether group at position 5 of the indole moiety, were compared with the reference compounds colchicine and combretastatin A-4 for biological activity. Derivatives 10, 11, 16, and 21-24 inhibited MCF-7 cell growth with IC50 values <50 nM. A halogen atom (14-17) at position 5 caused a significant reduction in the free energy of binding of compound to tubulin, with a concomitant reduction in cytotoxicity. In contrast, methyl (21) and methoxy (22) substituents at position 5 caused an increase in cytotoxicity. Compound 16, the most potent antitubulin agent, led to a large increase (56%) in HeLa cells in the G2/M phase at 24 h, and at 48 h, 26% of the cells were hyperploid. Molecular modeling studies showed that, despite the absence of the ester moiety present in the previously examined analogues, most of the compounds bind in the colchicine site in the same orientation as the previously studied ATIs. Binding to beta-tubulin involved formation of a hydrogen bond between the indole and Thr179 and positioning of the trimethoxy phenyl group in a hydrophobic pocket near Cys241.

Triazolopyrimidines Are Microtubule-Stabilizing Agents that Bind the Vinca Inhibitor Site of Tubulin.

Microtubule-targeting agents (MTAs) are some of the clinically most successful anti-cancer drugs. Unfortunately, instances of multidrug resistances to MTA have been reported, which highlights the need for developing MTAs with different mechanistic properties. One less explored class of MTAs are [1,2,4]triazolo[1,5-a]pyrimidines (TPs). These cytotoxic compounds are microtubule-stabilizing agents that inexplicably bind to vinblastine binding site on tubulin, which is typically targeted by microtubule-destabilizing agents. Here we used cellular, biochemical, and structural biology approaches to address this apparent discrepancy. Our results establish TPs as vinca-site microtubule-stabilizing agents that promote longitudinal tubulin contacts in microtubules, in contrast to classical microtubule-stabilizing agents that primarily promote lateral contacts. Additionally we observe that TPs studied here are not affected by p-glycoprotein overexpression, and suggest that TPs are promising ligands against multidrug-resistant cancer cells.

New interfacial microtubule inhibitors of marine origin, PM050489/PM060184, with potent antitumor activity and a distinct mechanism.

We have investigated the target and mechanism of action of a new family of cytotoxic small molecules of marine origin. PM050489 and its dechlorinated analogue PM060184 inhibit the growth of relevant cancer cell lines at subnanomolar concentrations. We found that they are highly potent microtubule inhibitors that impair mitosis with a distinct molecular mechanism. They bind with nanomolar affinity to unassembled αß-tubulin dimers, and PM050489 binding is inhibited by known Vinca domain ligands. NMR TR-NOESY data indicated that a hydroxyl-containing analogue, PM060327, binds in an extended conformation, and STD results define its binding epitopes. Distinctly from vinblastine, these ligands only weakly induce tubulin self-association, in a manner more reminiscent of isohomohalichondrin B than of eribulin. PM050489, possibly acting like a hinge at the association interface between tubulin heterodimers, reshapes Mg(2+)-induced 42 S tubulin double rings into smaller 19 S single rings made of 7 ± 1 αß-tubulin dimers. PM060184-resistant mutants of Aspergillus nidulans map to ß-tubulin Asn100, suggesting a new binding site different from that of vinblastine at the associating ß-tubulin end. Inhibition of assembly dynamics by a few ligand molecules at the microtubule plus end would explain the antitumor activity of these compounds, of which PM060184 is undergoing clinical trials.

Zampanolide, a potent new microtubule-stabilizing agent, covalently reacts with the taxane luminal site in tubulin α,ß-heterodimers and microtubules.

Zampanolide and its less active analog dactylolide compete with paclitaxel for binding to microtubules and represent a new class of microtubule-stabilizing agent (MSA). Mass spectrometry demonstrated that the mechanism of action of both compounds involved covalent binding to ß-tubulin at residues N228 and H229 in the taxane site of the microtubule. Alkylation of N228 and H229 was also detected in α,ß-tubulin dimers. However, unlike cyclostreptin, the other known MSA that alkylates ß-tubulin, zampanolide was a strong MSA. Modeling the structure of the adducts, using the NMR-derived dactylolide conformation, indicated that the stabilizing activity of zampanolide is likely due to interactions with the M-loop. Our results strongly support the existence of the luminal taxane site of microtubules in tubulin dimers and suggest that microtubule nucleation induction by MSAs may proceed through an allosteric mechanism.