Actin-binding marine macrolides: total synthesis and biological importance

Marine organisms produce a fascinating range of structurally diverse secondary metabolites, which often possess unusual and sometimes unexpected biological activities. This structural diversity makes these marine natural products excellent molecular probes for the investigation of biochemical pathways. Recently, a number of novel and stereochemically complex macrolides, having a large macrolactone (22- to 44-membered) ring, that interact with the actin cycloskeleton have been isolated from different marine sources. Actin, like tubulin, is a major component of the cytoskeleton and has important cellular functions. Although the details of these interactions are still under investigation, these marine macrolides are becoming increasingly important as novel molecular probes to help elucidate the cellular functions of actin. Owing to their potent antitumor activities, these compounds, for example the aplyronines, also have potential for preclinical development in cancer chemotherapy. Their appealing molecular structures, with an abundance of stereochemistry, and biological significance, coupled with the extremely limited availability from the marine sources, have stimulated enormous interest in the synthesis of these compounds. This review summarizes the biological properties of these unusual marine natural products and features the recently completed total syntheses of swinholide A, scytophycin C, aplyronine A, mycalolide A--all of these being potent cytotoxic agents that target actin--and a diastereoisomer of ulapualide A. Rather than detailing each individual step of these multistep total syntheses, the different synthetic strategies, key reactions, and methods adopted for controlling the stereochemistry are compared.     

Actin-binding toxin "tail" wags the dog

Actin-binding marine macrolides exhibit significant structural and functional diversity. In this issue, Perrins et al. demonstrate that a long stereochemically conserved aliphatic side chain, known as the "tail", found in many of these compounds is the functional determinant of cytotoxicity (Perrins et al., 2008).     

The effects of new cytochalasins from Phomopsis sp. and the derivatives on cellular structure and actin polymerization

The effects of ten 10-phenyl-[11]cytochalasins produced by Phomopsis sp. including novel compounds having 5,7- or 6,7-glycol structures and their derivatives, on the cell morphology and actin distribution in C3H-2K cells, as well as on lymphocyte capping and actin polymerization, were examined. The structure-activity relationship reported in the previous papers has been confirmed. The novel glycol type compounds showed little or no activity, suggesting the importance of the perhydroisoindol-1-one nucleus for the manifestation of the cytochalasin actions.     

Structural basis of swinholide A binding to actin

Marine toxins targeting the actin cytoskeleton represent a new and promising class of anti-cancer compounds. Here we present a 2.0 A resolution structure of swinholide A, a marine macrolide, bound to two actin molecules. The structure demonstrates that the actin dimer in the complex does not represent a physiologically relevant entity, for the two actin molecules do not interact with each other. The swinholide A actin binding site is the same as that targeted by toxins of the trisoxazole family and numerous actin binding proteins, highlighting the importance of this site in actin polymerization. The observed structure reveals the mechanism of action of swinholide A and provides a structural framework about which to design new agents directed at the cytoskeleton.     

Molecular dynamics and high throughput binding free energy calculation of anti-actin anticancer drugs—New insights for better design

Actin cytoskeleton plays an important role in cancerous cell progression. Till date many anticancer toxins are discovered that binds to different sites of actin. Mechanism of action of these toxins varies with respect to the site where they bind to actin. Latrunculin A (LAT) binds closely to nucleotide binding site and Reidispongiolide binds to the barbed end of actin. LAT is reported to reduce the displacement of domain 2 with respect to domain 1 and allosterically modulate nucleotide exchange. On the other hand Reidispongiolide binds with the higher affinity to actin and competes with the DNaseI binding loop once the inter-monomer interaction has been formed. Evolving better actin binders being the aim of this study we conducted a comparative molecular dynamics of these two actin-drug complexes and actin complexed with ATP alone, 50 ns each. High throughput binding free energy calculations in conjugation with the high-throughput MD simulations was used to predict modifications in these two renowned anti-actin anticancer drugs for better design. Per residue energy profiling that contribute to free energy of binding shows that there is an unfavourable energy at the site where Asp157 interacts with 2-thiazolidinone moiety of LAT. Similarly, unfavourable energies are reported near macrocyclic region of Reidispongiolide specifically near carbons 7, 11 & 25 and tail region carbons 27 & 30. These predicted sites can be used for modifications and few of these are discussed in this work based on the interactions with the binding site residues. The study reveals specific interactions that are involved in the allosteric modulation of ATP by these two compounds. Glu207 closely interacting with LAT A initiates the allosteric effect on ATP binding site specifically affecting residues Asp184, Lys215 and Lys336. RGA bound actin shows high anti-correlated motions between sub domain 3 and 4. Unlike LAT A, Reidispongiolide induces a flat structure of actin which definitely should affect actin polymerisation and lead to disassembly of actin filaments.     

Recent Advances and Challenges in the Design of Organic Photoacid and Photobase Generators for Polymerizations

Photopolymerization, or the use of light to trigger polymerization, is one of the most exciting technologies for advanced manufacturing of polymers. One of the key components in the photopolymerization processes is the photoactive compound that absorbs the light, generating the active species that promotes the polymerization and largely determines the final properties of the material. The field of photopolymerization has been dominated by photoradical generators to mediate radical reactions. In the last decade, to expand the number of polymers that can be prepared by photopolymerization, intensive research has been devoted to the synthesis and utilization of photoactive molecules that are able to generate a base or an acid upon irradiation. These organic compounds are known to promote not only the ring‐opening polymerization of various heterocyclic monomers such as lactones, carbonates, or epoxides but also to trigger the step‐growth synthesis of polyurethanes. This Minireview highlights the recent advances in the development of organic photobase and photoacid generators, with the aim of encouraging the wider application of these photoactive compounds in the photopolymerization area and to expand the use of these polymers in advanced manufacturing processes.     

Molecular docking and QSAR of aplyronine A and analogues: potent inhibitors of actin

Actin-binding natural products have been identified as a potential basis for the design of cancer therapeutic agents. We report flexible docking and QSAR studies on aplyronine A analogues. Our findings show the macrolide ‘tail’ to be fundamental for the depolymerisation effect of actin-binding macrolides and that it is the tail which forms the initial interaction with the actin rather than the macrocycle, as previously believed. Docking energy scores for the compounds were highly correlated with actin depolymerisation activity. The 3D-QSAR models were predictive, with the best model giving a q ² value of 0.85 and a r ² of 0.94. Results from the docking simulations and the interpretation from QSAR “coeff*stdev” contour maps provide insight into the binding mechanism of each analogue and highlight key features that influence depolymerisation activity. The results herein may aid the design of a putative set of analogues that can help produce efficacious and tolerable anti-tumour agents. Finally, using the best QSAR model, we have also made genuine predictions for an independent set of recently reported aplyronine analogues.     

Kinetic Insights into the Elongation Reaction of Actin Filaments as a Function of Temperature,Pressure, and Macromolecular Crowding

Actin polymerization is an essential process in eukaryotic cells that provides a driving force for motility and mechanical resistance for cell shape. By using preformed gelsolin–actin nuclei and applying stopped‐flow methodology, we quantitatively studied the elongation kinetics of actin filaments as a function of temperature and pressure in the presence of synthetic and protein crowding agents. We show that the association of actin monomers to the pointed end of double‐stranded helical actin filaments (F‐actin) proceeds via a transition state that requires an activation energy of 56 kJ mol^?1 for conformational and hydration rearrangements, but exhibits a negligible activation volume, pointing to a compact transition state that is devoid of packing defects. Macromolecular crowding causes acceleration of the F‐actin elongation rate and counteracts the deteriorating effect of pressure. The results shed new light on the combined effect of these parameters on the polymerization process of actin, and help us understand the temperature and pressure sensitivity of actin polymerization under extreme conditions.     

A library of spirooxindoles based on a stereoselective three-component coupling reaction

A collection of structurally complex and chemically diverse small molecules is a useful tool to explore cell circuitry. In this article, we report the split-pool synthesis of more than 3000 spirooxindoles on high capacity macrobeads. The key reaction to assemble the spirooxindole core stereoselectively is a Lewis acid variant of the Williams' three-component coupling. After formation, the skeleton was elaborated using Sonogashira couplings, amide forming reactions, and N-acylations of gamma-lactams. The final library was analyzed by sampling individual macrobeads and by using binomial confidence limits. It was determined that at least 82% of the library compounds should have better than 80% purity. To demonstrate the utility of our discovery process, a high-throughput chemical genetic modifier screen was performed using stock solutions of the resultant products. A number of positives were identified as enhancers of the cellular actions of latrunculin B, an actin polymerization inhibitor. Through resynthesis, we confirmed one of the positives and demonstrated that, in yeast cells, it has an EC50 in the sub-micromolar range.     

Mechanism of actin polymerization by myosin subfragment-1 probed by dynamic light scattering

Monomeric actin (G-actin) polymerizes spontaneously into helical filaments in the presence of inorganic salts. The slowest, rate-limiting step of the polymerization process is formation of actin trimers, the smallest oligomers that serve as nuclei for fast filament growth (filament elongation) by monomer addition at the filament ends. In low ionic-strength solutions, actin can be polymerized by myosin subfragment-1 (S1). In early works it has been suggested that G-actin-S1 1:1 complexes (GS) assemble into filaments according to the nucleation-filament elongation scheme. Subsequent studies indicated that one S1 molecule can bind two actin monomers, and that oligomerization of the initial complexes is a fast reaction. This has led to suggest an alternative mechanism, with a ternary G(2)S complex and its oligomers being predominant intermediates of S1-induced assembly of G-actin into filaments. We used dynamic light scattering to analyze the initial steps of S1-induced polymerization of actin. Our results suggest formation of GS complexes and their oligomers in the presence of S1 equimolar to or in excess over actin. We confirm formation of G(2)S complexes as intermediates of S1-induced polymerization in the presence of actin in excess over S1.     

Deoxyelephantopin and Its Isomer Isodeoxyelephantopin: Anti-Cancer Natural Products with Multiple Modes of Action

Cancer is a leading cause of morbidity and mortality worldwide. The development of cancer involves aberrations in multiple pathways, representing promising targets for anti-cancer drug discovery. Natural products are regarded as a rich source for developing anti-cancer therapies due to their unique structures and favorable pharmacology and toxicology profiles. Deoxyelephantopin and isodeoxyelephantopin, sesquiterpene lactone compounds, are major components of Elephantopus scaber and Elephantopus carolinianus, which have long been used as traditional medicines to treat multiple ailments, including liver diseases, diabetes, bronchitis, fever, diarrhea, dysentery, cancer, renal disorders, and inflammation-associated diseases. Recently, deoxyelephantopin and isodeoxyelephantopin have been extensively explored for their anti-cancer activities. This review summarizes and discusses the anti-cancer activities of deoxyelephantopin and isodeoxyelephantopin, with an emphasis on their modes of action and molecular targets. Both compounds disrupt several processes involved in cancer progression by targeting multiple signaling pathways deregulated in cancers, including cell cycle and proliferation, cell survival, autophagy, and invasion pathways. Future directions of research on these two compounds towards anti-cancer drug development are discussed.     

Fast-atom bombardment and tandem mass spectrometry of macrolide antibiotics

Molecular weights of macrolide antibiotics can be determined from either (M + H)⁺ or (M + Met)⁺, the latter desorbed from alkali metal salt-saturated matrices. The ion chemistry of macrolides, as determined by tandem mass spectrometry (MS/MS), is different for ions produced as metallated than those formed as (M + H)⁺ species. An explanation for these differences is the location of the charge. For protonated species, the charge is most likely situated on a functional group with high proton affinity, such as the dimethylamino group of the ammo sugar. The alkali metal ion, however, is bonded to the highly oxygenated aglycone. As a result, the collision-activated dissociation spectra of protonated macrolides are simple with readily identifiable fragment ions in both the high and low mass regions but no fragments in the middle mass range. In contrast, the cationized species give complex spectra with many abundant ions, most of which are located in the high mass range. The complementary nature of the fragmentation of these two species recommends the study of both by MS/MS when determining the structure or confirming the identity of these biomaterials.     

Polymers from non‐homopolymerizable monomers

There are many inorganic and organic compounds known which are not able to homopolymerize either with well‐known polymerizable monomers or even with other non‐homopolymerizable compounds. The participation of non‐homopolymerizable comonomers with reactivity ratios close to 0 results in copolymers with more or less alternating structure, whereas for a strictly alternating copolymer, both reactivity ratios must be 0. Binary copolymerizations of non‐homopolymerizable and homopolymerizable monomers can give information on the topochemistry, and also on the kinetics of such processes, as in these cases the number of propagating steps is remarkably reduced. Up to now, very little is known on the terpolymerization of three non‐homopolymerizable comonomers. Experimental investigations have shown that only combinations of two monomers with electron donor and one monomer with electron acceptor properties or vice versa yield terpolymers, whereas from three monomers of similar electronic behavior, no terpolymers are obtained. All such terpolymers are of alternating structure where a donor unit is succeeded by an acceptor unit. For copolymerizations of two or three non‐homopolymerizable monomers, two different mechanisms must be considered: the so‐called complex model postulates the incorporation of donor‐acceptor complexes of the monomers into the growing chain, whereas with the terminal or penultimate model the addition of free monomers to growing macroradicals is described. Measurements of the rate of polymerization in combination with determinations of the complex constants of the involved donor and acceptor monomer pairs together with a new kinetic scheme allow us to distinguish between the simultaneous participation of free monomers and complexes in the polymerization process.     

Metabolomics applied in the study of emerging arboviruses caused by Aedes aegypti mosquitoes: A review

Arboviruses, such as chikungunya, dengue, yellow fever, and zika, caused by the bite of the Aedes aegypti mosquito, have been a frequent public health problem, with a high incidence of outbreaks in tropical and subtropical countries. These diseases are easily confused with a flu-like illness and present very similar symptoms, difficult to distinguish, and treat appropriately. The effects that these infections cause in the organism are fundamentally derived from complex metabolic processes. A prominent area of science that investigates the changes in the metabolism of complex organisms is the metabolomics. Metabolomics measures the metabolites produced or altered in biological organisms, through the use of robust analytical platforms, such as separation techniques hyphenated with mass spectrometry, combined with bioinformatics. This review article presents an overview of the basic concepts of metabolomics workflow and advances in this field, and compiles research articles that use this omic approach to study these arboviruses. In this context, the metabolomics is applied to search new therapies, understand the viral replication mechanisms, and access the host-virus interactions.     

Guidance of actin filament elongation on filament-binding tracks

Biomolecular motors, which convert chemical energy into mechanical work in intracellular processes, have high potential in bionanotechnology in vitro as molecular shuttles or nanoscale actuators. In this context, guided elongation of actin filaments in vitro could be used to lay tracks for myosin motor-based shuttles or to direct nanoscale actuators based on actin filament end-tracking motors. To guide the direction of filament polymerization on surfaces, microcontact printing was used to create tracks of chemically modified myosin, which binds to, but cannot exert force on, filaments. These filament-binding tracks captured nascent filaments from solution and guided the direction of their subsequent elongation. The effect of track width and protein surface density on filament alignment and elongation rate was quantified. These results indicate that microcontact printing is a useful method for guiding actin filament polymerization in vitro for biomolecular motor-based applications.     

Organoboron Compounds: Effective Antibacterial and Antiparasitic Agents

The unique electron deficiency and coordination property of boron led to a wide range of applications in chemistry, energy research, materials science and the life sciences. The use of boron-containing compounds as pharmaceutical agents has a long history, and recent developments have produced encouraging strides. Boron agents have been used for both radiotherapy and chemotherapy. In radiotherapy, boron neutron capture therapy (BNCT) has been investigated to treat various types of tumors, such as glioblastoma multiforme (GBM) of brain, head and neck tumors, etc. Boron agents playing essential roles in such treatments and other well-established areas have been discussed elsewhere. Organoboron compounds used to treat various diseases besides tumor treatments through BNCT technology have also marked an important milestone. Following the clinical introduction of bortezomib as an anti-cancer agent, benzoxaborole drugs, tavaborole and crisaborole, have been approved for clinical use in the treatments of onychomycosis and atopic dermatitis. Some heterocyclic organoboron compounds represent potentially promising candidates for anti-infective drugs. This review highlights the clinical applications and perspectives of organoboron compounds with the natural boron atoms in disease treatments without neutron irradiation. The main topic focuses on the therapeutic applications of organoboron compounds in the diseases of tuberculosis and antifungal activity, malaria, neglected tropical diseases and cryptosporidiosis and toxoplasmosis.     

Hybrids of the hemiasterlin analogue taltobulin and the dolastatins are potent antimicrotubule agents

The targeting of microtubules is an important mechanism for cancer chemotherapy. However, there is still a need for improved antimicrotubule agents. A number of seemingly structurally disparate peptidic natural products inhibit tubulin polymerization by binding to a region of the tubulin heterodimer close to the vinca binding site. An analogue of the naturally occurring tripeptide hemiasterlin, taltobulin (HTI-286, 3), has advanced to clinical trials. Structure-activity relationship studies of 3 have revealed critical structural elements necessary for antimicrotubule activity that correspond to comparable groups in the amino terminus tripeptide region of the dolastatins. To investigate the structural relationship between the hemiasterlins and the more complex dolastatins, hybrid compounds composed of 3 and the carboxy terminus dipeptides of dolastatin 10, or the dolastatin 15 analogue cemadotin, were synthesized. The resulting hybrid compounds were potent antimicrotubule agents, thus establishing a structural relationship between the hemiasterlins and the dolastatins. This relationship may be useful in the design of analogues having improved activity in resistant cell lines expressing the P-glycoprotein transporter, for establishing structural relationships with other classes of peptidic antimicrotubule agents, or for modeling studies of the tubulin binding site of these agents.