Action of tolytoxin on cell morphology, cytoskeletal organization, and actin polymerization

A Review of the Antimicrobial Properties of Cyanobacterial Natural Products

The development of multiple-drug-resistant pathogens has prompted medical research toward the development of new and effective antimicrobial therapies. Much research into novel antibiotics has focused on bacterial and fungal compounds, and on chemical modification of existing compounds to increase their efficacy or reactivate their antimicrobial properties. In contrast, cyanobacteria have been relatively overlooked for antibiotic discovery, and much more work is required. This may be because some cyanobacterial species produce environmental toxins, leading to concerns about the safety of cyanobacterial compounds in therapy. Despite this, several cyanobacterial-derived compounds have been identified with noteworthy inhibitory activity against bacterial, fungal and protozoal growth, as well as viral replication. Additionally, many of these compounds have relatively low toxicity and are therefore relevant targets for drug development. Of particular note, several linear and heterocyclic peptides and depsipeptides with potent activity and good safety indexes have been identified and are undergoing development as antimicrobial chemotherapies. However, substantial further studies are required to identify and screen the myriad other cyanobacterial-derived compounds to evaluate their therapeutic potential. This study reviews the known phytochemistry of cyanobacteria, and where relevant, the effects of those compounds against bacterial, fungal, protozoal and viral pathogens, with the aim of highlighting gaps in the literature and focusing future studies in this field.

Cyanobacteria: A Promising Source of Antifungal Metabolites

Cyanobacteria are a rich source of secondary metabolites, and they have received a great deal of attention due to their applicability in different industrial sectors. Some of these substances are known for their notorious ability to inhibit fungal growth. Such metabolites are very chemically and biologically diverse. They can belong to different chemical classes, including peptides, fatty acids, alkaloids, polyketides, and macrolides. Moreover, they can also target different cell components. Filamentous cyanobacteria have been the main source of these compounds. This review aims to identify the key features of these antifungal agents, as well as the sources from which they are obtained, their major targets, and the environmental factors involved when they are being produced. For the preparation of this work, a total of 642 documents dating from 1980 to 2022 were consulted, including patents, original research, review articles, and theses.

Cyanotoxins and Other Bioactive Compounds from the Pasteur Cultures of Cyanobacteria (PCC)

In tribute to the bicentenary of the birth of Louis Pasteur, this report focuses on cyanotoxins, other natural products and bioactive compounds of cyanobacteria, a phylum of Gram-negative bacteria capable of carrying out oxygenic photosynthesis. These microbes have contributed to changes in the geochemistry and the biology of Earth as we know it today. Furthermore, some bloom-forming cyanobacterial species are also well known for their capacity to produce cyanotoxins. This phylum is preserved in live cultures of pure, monoclonal strains in the Pasteur Cultures of Cyanobacteria (PCC) collection. The collection has been used to classify organisms within the Cyanobacteria of the bacterial kingdom and to investigate several characteristics of these bacteria, such as their ultrastructure, gas vacuoles and complementary chromatic adaptation. Thanks to the ease of obtaining genetic and further genomic sequences, the diversity of the PCC strains has made it possible to reveal some main cyanotoxins and to highlight several genetic loci dedicated to completely unknown natural products. It is the multidisciplinary collaboration of microbiologists, biochemists and chemists and the use of the pure strains of this collection that has allowed the study of several biosynthetic pathways from genetic origins to the structures of natural products and, eventually, their bioactivity.

Polyphasic evaluation and cytotoxic investigation of isolated cyanobacteria with an emphasis on potent activities of a Scytonema strain

Cyanobacteria are phototrophic organisms widely found in most types of natural habitats in the tropical regions of the world. In this study, we isolated and identified cyanobacterial strains from paddy soil in Hanoi (Vietnam) and investigated their cytotoxic activities. Five isolated cyanobacterial strains showed distinctive profiles of gene sequences (rRNA 16S and rbcL), phylogenetic placements, and morphological characteristics. Based on the polyphasic evaluation, they were classified as Scytonema bilaspurense NK13, Hapalosiphon welwitschii MD2411, Aulosira sp. XN1103, Desikacharya sp. NS2000, and Desmonostoc sp. NK1813. The cytotoxic screening revealed that the extract of strain Scytonema bilaspurense NK13 exhibited potent cytotoxic activities against four human cell lines of HeLa cells, OVCAR-8 cells, HaCaT cells, and HEK-293T cells, with IC50 values of 3.8, 34.2, 21.6, and 0.6 μg/mL, respectively. This is the first time a well-classified Scytonema strain from tropical habitat in Southeast Asia has been recognized as a potential producer of cytotoxic compounds.

New Insights into Tolytoxin Effect in Human Cancer Cells: Apoptosis Induction and the Relevance of Hydroxyl Substitution of Its Macrolide Cycle on Compound Potency

Scytophycins, including tolytoxin, represent a class of actin disrupting macrolides with strong antiproliferative effects on human cells. Despite intense research, little attention has been paid to scytophycin-induced cell death or the structural features affecting its potency. We show that tolytoxin and its natural analogue, 7-O-methylscytophycin B, lacking the hydroxyl substitution in its macrolactone ring, differ substantially in their cytotoxic effect. Both compounds increase the level of caspases 3/7, which are the main executioner proteases during apoptosis, in HeLa wild-type (WT) cells. However, no caspase activity was detected in HeLa cells lacking Bax/Bak proteins crucial for caspase activation via the mitochondrial pathway. Obtained data strongly suggests that scytophycins are capable of inducing mitochondria-dependent apoptosis. These findings encourage further research in structure-activity relationships in scytophycins and highlight the potential of these compounds in targeted drug delivery.

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

Scytodecamide from the Cultured Scytonema sp. UIC 10036 Expands the Chemical and Genetic Diversity of Cyanobactins

Cyanobactins are a large family of cyanobacterial ribosomally synthesized and post-translationally modified peptides (RiPPs) often associated with biological activities, such as cytotoxicity, antiviral, and antimalarial activities. They are traditionally described as cyclic molecules containing heterocyclized amino acids. However, this definition has been recently challenged by the discovery of short, linear cyanobactins containing three to five amino acids as well as cyanobactins containing no heterocyclized residues. Herein we report the discovery of scytodecamide (1) from the freshwater cyanobacterium Scytonema sp. UIC 10036. Structural elucidation based on mass spectrometry, 1D and 2D NMR spectroscopy, and Marfey's method revealed 1 to be a linear decapeptide with an N-terminal N-methylation and a C-terminal amidation. The genome of Scytonema sp. UIC 10036 was sequenced, and bioinformatic analysis revealed a cyanobactin-like biosynthetic gene cluster consistent with the structure of 1. The discovery of 1 as a novel linear peptide containing an N-terminal N-methylation and a C-terminal amidation expands the chemical and genetic diversity of the cyanobactin family of compounds.

Effect of tolytoxin on tunneling nanotube formation and function

Tunneling nanotubes (TNTs) are actin-containing membrane protrusions that play an essential role in long-range intercellular communication. They are involved in development of various diseases by allowing transfer of pathogens or protein aggregates as well as organelles such as mitochondria. Increase in TNT formation has been linked to many pathological conditions. Here we show that nM concentrations of tolytoxin, a cyanobacterial macrolide that targets actin by inhibition of its polymerization, significantly decrease the number of TNT-connected cells, as well as transfer of mitochondria and α-synuclein fibrils in two different cell lines of neuronal (SH-SY5Y) and epithelial (SW13) origin. As the cytoskeleton of the tested cell remain preserved, this macrolide could serve as a valuable tool for future therapies against diseases propagated by TNTs.

Biological targets and mechanisms of action of natural products from marine cyanobacteria

Marine cyanobacteria are an ancient group of organisms and prolific producers of bioactive secondary metabolites. These compounds are presumably optimized by evolution over billions of years to exert high affinity for their intended biological target in the ecologically relevant organism but likely also possess activity in different biological contexts such as human cells. Screening of marine cyanobacterial extracts for bioactive natural products has largely focused on cancer cell viability; however, diversification of the screening platform led to the characterization of many new bioactive compounds. Targets of compounds have oftentimes been elusive if the compounds were discovered through phenotypic assays. Over the past few years, technology has advanced to determine mechanism of action (MOA) and targets through reverse chemical genetic and proteomic approaches, which has been applied to certain cyanobacterial compounds and will be discussed in this review. Some cyanobacterial molecules are the most-potent-in-class inhibitors and therefore may become valuable tools for chemical biology to probe protein function but also be templates for novel drugs, assuming in vitro potency translates into cellular and in vivo activity. Our review will focus on compounds for which the direct targets have been deciphered or which were found to target a novel pathway, and link them to disease states where target modulation may be beneficial.

Exploiting the cytoskeletal filaments of neoplastic cells to potentiate a novel therapeutic approach

Although cytoskeletal-directed agents have been a mainstay in chemotherapeutic protocols due to their ability to readily interfere with the rapid mitotic progression of neoplastic cells, they are all microtubule-based drugs, and there has yet to be any microfilament- or intermediate filament-directed agents approved for clinical use. There are many inherent differences between the cytoskeletal networks of malignant and normal cells, providing an ideal target to attain preferential damage. Further, numerous microfilament-directed agents, and an intermediate filament-directed agent of particular interest (withaferin A) have demonstrated in vitro and in vivo efficacy, suggesting that cytoskeletal filaments may be exploited to supplement chemotherapeutic approaches currently used in the clinical setting. Therefore, this review is intended to expose academics and clinicians to the tremendous variety of cytoskeletal filament-directed agents that are currently available for further chemotherapeutic evaluation. The mechanisms by which microfilament directed- and intermediate filament-directed agents damage malignant cells are discussed in detail in order to establish how the drugs can be used in combination with each other, or with currently approved chemotherapeutic agents to generate a substantial synergistic attack, potentially establishing a new paradigm of chemotherapeutic agents.

Cyanobacterial toxins: modes of actions, fate in aquatic and soil ecosystems, phytotoxicity and bioaccumulation in agricultural crops

The occurrence of harmful cyanobacterial blooms in surface waters is often accompanied by the production of a variety of cyanotoxins. These toxins are designed to target in humans and animals specific organs on which they act: hepatotoxins (liver), neurotoxins (nervous system), cytotoxic alkaloids, and dermatotoxins (skin), but they often have important side effects too. When introduced into the soil ecosystem by spray irrigation of crops they may affect the same molecular pathways in plants having identical or similar target organs, tissues, cells or biomolecules. There are also several indications that terrestrial plants, including food crop plants, can bioaccumulate cyanotoxins and present, therefore, potential health hazards for human and animals. The number of publications concerned with phytotoxic effects of cyanotoxins on agricultural plants has increased recently. In this review, we first examine different cyanotoxins and their modes of actions in humans and mammals and occurrence of target biomolecules in vegetable organisms. Then we present environmental concentrations of cyanotoxins in freshwaters and their fate in aquatic and soil ecosystems. Finally, we highlight bioaccumulation of cyanotoxins in plants used for feed and food and its consequences on animals and human health. Overall, our review shows that the information on the effects of cyanotoxins on non-target organisms in the terrestrial environment is particularly scarce, and that there are still serious gaps in the knowledge about the fate in the soil ecosystems and phytotoxicity of these toxins.

Cytotoxicity and secondary metabolites production in terrestrial Nostoc strains, originating from different climatic/geographic regions and habitats: is their cytotoxicity environmentally dependent?

Extensive selection of cyanobacterial strains (82 isolates) belonging to the genus Nostoc, isolated from different climatic regions and habitats, were screened for both their secondary metabolite content and their cytotoxic effects to mammalian cell lines. The overall occurrence of cytotoxicity was found to be 33%, which corresponds with previously published data. However, the frequency differs significantly among strains, which originate from different climatic regions and microsites (particular localities). A large fraction of intensely cytotoxic strains were found among symbiotic strains (60%) and temperate and continental climatic isolates (45%); compared with the less significant incidences in strains originating from cold regions (36%), deserts (14%), and tropical habitats (9%). The cytotoxic strains were not randomly distributed; microsites that clearly had a higher occurrence of cytotoxicity were observed. Apparently, certain natural conditions lead to the selection of cytotoxic strains, resulting in a high cytotoxicity occurrence, and vice versa. Moreover, in strains isolated from a particular microsite, the cytotoxic effects were caused by different compounds. This result supports our hypothesis for the environmental dependence of cytotoxicity. It also contradicts the hypothesis that clonality and lateral gene transfer could be the reason for this phenomenon. Enormous variability in the secondary metabolites was detected within the studied Nostoc extracts. According to their molecular masses, only 26% of these corresponded to any known structures; thus, pointing to the high potential for the use of many terrestrial cyanobacteria in both pharmacology and biotechnology.

Cell-morphology profiling of a natural product library identifies bisebromoamide and miuraenamide A as actin filament stabilizers

Natural products provide a rich source of biological tools, but elucidating their molecular targets remains challenging. Here we report a cell morphological profiling of a natural product library, which permitted the identification of bisebromoamide and miuraenamide A as actin filament stabilizers. Automated high-content image analysis showed that these two structurally distinct marine natural products induce morphological changes in HeLa cells similar to those induced by known actin-stabilizing compounds. Bisebromoamide and miuraenamide A stabilized actin filaments in vitro, and fluorescein-conjugated bisebromoamide localized specifically to actin filaments in cells. Cell morphological profiling was also used to identify actin-stabilizing or -destabilizing natural products from marine sponge extracts, leading to the isolation of pectenotoxin-2 and lyngbyabellin C. Overall, the results demonstrate that high-content imaging of nuclei and cell shapes offers a sensitive and convenient method for detecting and isolating molecules that target actin.

Toxins affecting actin filaments and microtubules

Actin and tubulin are the two major proteins of the cytoskeleton in eukaryotic cells and both display a common property to reversibly assemble into long and flexible polymers, actin filaments and microtubules, respectively. These proteins play important roles in a variety of cellular functions and are also involved in numbers of diseases. An emerging number of marine-derived cytotoxins have been found to bind either actin or tublin, resulting in either inhibition or enhancement of polymerization. Thus, these toxins are valuable molecular probes for solving complex mechanisms of biological processes. This chapter describes actin- and tubulin-targeting marine natural products and their modes of action, with reference to their use as research tools and their clinical applications.

Trisoxazole Macrolides from Hexabranchus Nudibranchs and Other Marine Invertebrates

Trisoxazole macrolides are cytotoxic and antifungal metabolites initially isolated from the egg-ribbons of the Hexabranchus nudibranch and later found in other marine invertebrates. They possess a characteristic macrolide portion, in which three contiguous oxazole units are integrated, and a side-chain with an N-methyl-vinylformamide terminus. The planar structures of the first members of this group, ulapualides and kabiramide C, were determined by interpretation of spectral data in conjunction with chemical degradation. Following these studies, the structures of approximately 35 congeners have been reported, including mycalolides from a marine sponge Mycale sp. The absolute stereochemistry of mycalolides was determined by chemical methods. Trisoxazole macrolides depolymerize F-actin and form a 1:1 complex with G-actin, thereby exhibiting potent toxicity toward eukaryotic cells. X-ray crystallography established the mode of binding of some of the members to G-actin and their absolute stereochemistry.

Amphidinolide H, a novel type of actin-stabilizing agent isolated from dinoflagellate

The effect of novel cytotoxic marine macrolide, amphidinolide H (Amp-H), on actin dynamics was investigated in vitro. Amp-H attenuated actin depolymerization induced by diluting F-actin. This effect remained after washing out of unbound Amp-H by filtration. In the presence of either Amp-H or phalloidin, lag phase, which is the rate-limiting step of actin polymerization, was shortened. Phalloidin decreased the polymerization-rate whereas Amp-H did not. Meanwhile, the effects of both compounds were the same when barbed end of actin was capped by cytochalasin D. Quartz crystal microbalance system revealed interaction of Amp-H with G-actin and F-actin. Amp-H also enhanced the binding of phalloidin to F-actin. We concluded that Amp-H stabilizes actin in a different manner from that of phalloidin and serves as a novel pharmacological tool for analyzing actin-mediated cell function.

Seaweed resistance to microbial attack: a targeted chemical defense against marine fungi

Pathogenic microbes can devastate populations of marine plants and animals. Yet, many sessile organisms such as seaweeds and sponges suffer remarkably low levels of microbial infection, despite lacking cell-based immune systems. Antimicrobial defenses of marine organisms are largely uncharacterized, although from a small number of studies it appears that chemical defenses may improve host resistance. In this study, we asked whether the common seaweed Lobophora variegata is chemically defended against potentially deleterious microorganisms. Using bioassay-guided fractionation, we isolated and characterized a 22-membered cyclic lactone, lobophorolide (1), of presumed polyketide origin, with sub-microM activity against pathogenic and saprophytic marine fungi. Deterrent concentrations of 1 were found in 46 of 51 samples collected from 10 locations in the Bahamas over a 4-year period. Lobophorolide (1) is structurally unprecedented, yet parts of the molecule are related to tolytoxin, the scytophycins, and the swinholides, macrolides previously isolated from terrestrial cyanobacteria and from marine sponges and gastropods. Until now, compounds of this structural class have not been associated with marine macrophytes. Our findings suggest that seaweeds use targeted antimicrobial chemical defense strategies and that secondary metabolites important in the ecological interactions between marine macroorganisms and microorganisms could be a promising source of novel bioactive compounds.

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.

New anti-actin drugs in the study of the organization and function of the actin cytoskeleton

The high degree of structural and molecular complexity of the actin-based cytoskeleton, combined with its ability to reorganize rapidly and locally in response to stimuli, and its force-generating properties, have made it difficult to assess how the different actin structures are assembled in cells, and how they regulate cell behavior. An obvious approach to study the relationships between actin organization, dynamics, and functions is the specific perturbation of actin structures using pharmacological means. Until recently there were only a few agents available that interfered with cellular activities by binding to actin and most of our knowledge concerning the involvement of actin in basic cellular processes was based on the extensive use of the cytochalasins. In recent years we have identified an increasing number of actin-targeted marine natural products, including the latrunculins, jasplakinolides (jaspamides), swinholide A, misakinolide A, halichondramides, and pectenotoxin II, which are discussed in this article. All these marine-sponge-derived compounds are unusual macrolides and can be classified into several major families, each with its own distinct chemical structures. We describe the current state of knowledge concerning the actin-binding properties of these compounds and show that each class of drugs alters the distribution patterns of actin in a unique way, and that even within a chemical class, structurally similar compounds can have different biochemical properties and cellular effects. We also discuss the effects of these new drugs on fenestrae formation in liver endothelial cells as an example of their usefulness as powerful tools to selectively unmask actin-mediated dynamic processes.

New actin mutants allow further characterization of the nucleotide binding cleft and drug binding sites

We have generated 9 site-specific mutations in Saccharomyces cerevisiae actin. These mutants display a variety of phenotypes when expressed in vivo, including slow actin filament turnover, slow fluid-phase endocytosis, and defects in actin organization. Actin mutation D157E confers resistance to the actin-sequestering drug, latrunculin A. Latrunculin A inhibits nucleotide exchange on wild-type yeast actin but not on D157E actin, suggesting that this residue is part of the latrunculin A binding site. We have refined our earlier map of the phalloidin binding site on actin, demonstrating a requirement for residue G158 in addition to D179 and R177. The nine new actin mutants as well as a large collection of existing actin mutants were also used to identify the putative binding site of another actin binding drug, tolytoxin, on actin. The actin alleles that result in decreased sensitivity to this drug cluster at a site near the nucleotide-binding pocket. Actin purified from one of these mutants has a reduced affinity for tolytoxin. In addition, tolytoxin causes a 2.4-fold increase in the t1/2 of ATP exchange, further suggesting that this drug binds near the nucleotide-binding pocket of actin. We note that the binding sites for latrunculin A, phalloidin, and tolytoxin all map close to the actin nucleotide binding pocket.

The multiple roles of Cyk1p in the assembly and function of the actomyosin ring in budding yeast

The budding yeast IQGAP-like protein Cyk1p/Iqg1p localizes to the mother-bud junction during anaphase and has been shown to be required for the completion of cytokinesis. In this study, video microscopy analysis of cells expressing green fluorescent protein-tagged Cyk1p/Iqg1p demonstrates that Cyk1p/Iqg1p is a dynamic component of the contractile ring during cytokinesis. Furthermore, in the absence of Cyk1p/Iqg1p, myosin II fails to undergo the contraction-like size change at the end of mitosis. To understand the mechanistic role of Cyk1p/Iqg1p in actomyosin ring assembly and dynamics, we have investigated the role of the structural domains that Cyk1p/Iqg1p shares with IQGAPs. An amino terminal portion containing the calponin homology domain binds to actin filaments and is required for the assembly of actin filaments to the ring. This result supports the hypothesis that Cyk1p/Iqg1p plays a direct role in F-actin recruitment. Deletion of the domain harboring the eight IQ motifs abolishes the localization of Cyk1p/Iqg1p to the bud neck, suggesting that Cyk1p/Iqg1p may be localized through interactions with a calmodulin-like protein. Interestingly, deletion of the COOH-terminal GTPase-activating protein-related domain does not affect Cyk1p/Iqg1p localization or actin recruitment to the ring but prevents actomyosin ring contraction. In vitro binding experiments show that Cyk1p/Iqg1p binds to calmodulin, Cmd1p, in a calcium-dependent manner, and to Tem1p, a small GTP-binding protein previously found to be required for the completion of anaphase. These results demonstrate the critical function of Cyk1p/Iqg1p in regulating various steps of actomyosin ring assembly and cytokinesis.

Poly(ADP-ribose) modulates the properties of MARCKS proteins

In mammalian cells, the formation of DNA strand breaks is accompanied by synthesis of poly(ADP-ribose). This nucleic acid-like homopolymer may modulate protein functions by covalent and/or noncovalent interactions. Here we show that poly(ADP-ribose) binds strongly to the proteins of the myristoylated alanine-rich C kinase substrate (MARCKS) family, MARCKS and MARCKS-related protein (also MacMARCKS or F52). MARCKS proteins are myristoylated proteins associated with membranes and the actin cytoskeleton. As targets for both protein kinase C (PKC) and calmodulin (CaM), MARCKS proteins are thought to mediate cross-talk between these two signal transduction pathways. Dot blot assays show that poly(ADP-ribose) binds to MARCKS proteins at the highly basic effector domain. Complex formation between MARCKS-related protein and CaM as well as phosphorylation of MARCKS-related protein by the catalytic subunit of PKC are strongly inhibited by equimolar amounts of poly(ADP-ribose), suggesting a high affinity of poly(ADP-ribose) for MARCKS-related protein. Binding of MARCKS-related protein to membranes is also inhibited by poly(ADP-ribose). Finally, poly(ADP-ribose) efficiently reverses the actin-filament bundling activity of a peptide corresponding to the effector domain and inhibits the formation of actin filaments in vitro. Our results suggest that MARCKS proteins and actin could be targets of the poly(ADP-ribose) DNA damage signal pathway.

Misakinolide A is a marine macrolide that caps but does not sever filamentous actin

We have investigated the biochemical properties of the marine natural product, misakinolide A, a 40-membered dimeric lactone macrolide that differs from swinholide A only in the size of the macrolide ring. Analytical ultracentrifugation and steady-state fluorescence experiments show that misakinolide A binds simultaneously to two actin subunits with virtually the same affinity as swinholide A, suggesting that the modification in the ring size does not change the actin-binding site. Sedimentation equilibrium experiments suggest that binding is independent at each binding site, with a Kd of approximately 50 nM. Remarkably, misakinolide A does not sever actin filaments like swinholide A; rather, it caps the barbed end of F-actin. When capped by misakinolide A, the elongation rate constant at the barbed end is reduced to zero; pointed end growth was affected only to the extent that the compound sequesters unpolymerized actin. Misakinolide A has essentially no effect on the off-rate of actin subunits leaving the barbed end. Energy-minimized models of misakinolide A and swinholide A are consistent with conservation of identical binding sites in both molecules, but a difference in orientation of one binding site relative to the other may explain why swinholide A has severing activity whereas misakinolide A only has capping activity.

A family of novel actin-inhibiting marine toxins

1. We have examined the effects of marine toxins with a macrolide structure on actin. These toxins include mycalolide B, aplyronine A and bistheonellide A. 2. Measuring actin polymerization by monitoring fluorescent intensity of pyrenyl-actin, mycalolide B did not accelerate the actin polymerization but quickly depolymerized F-actin. In contrast, cytochalasin D, which inhibits actin polymerization by binding to the barbed end of F-actin, accelerated actin nucleation and depolymerized F-actin at a slower rate. 3. Analysing the kinetics of depolymerization by monitoring the rate of spontaneous depolymerization of F-actin under the unpolymerizable state or the rate of polymerization where F-actin was seeded as a nucleus, mycalolide B was suggested to sever F-actin. 4. The relationship between the concentration of total actin and F-actin at different concentrations of mycalolide B suggests that mycalolide B forms a 1:1 complex with G-actin. Viscometry and electron microscopic observations further suggest that the actin filament was depolymerized by mycalolide B. Furthermore, mycalolide B suppressed actin-activated myosin Mg(2+)-ATPase activity, although cytochalasin D did not. Aplyronine A has similar effects on actin. 5. Bistheonellide A also depolymerized F-actin and sequestered G-actin by forming a 1:2 complex with G-actins, but, its severing effect was not detected. We conclude that mycalolide B, aplyronine A and bistheonellide A are novel types of actin-depolymerizing agents, the mechanism of action of which is different from that of cytochalasin D. These structurally related marine toxins may be categorized as 'actin depolymerizing macrolides' and may serve as novel pharmacological tools for analysing actin-mediated cell functions.

Regulation of scytophycin accumulation in cultures of Scytonema ocellatum. II. Nutrient requirements

The effects of optimal sources and concentrations of major nutrients (supplying N, S, P, K+, Na+, Ca2+, Mg2+, and inorganic carbon) and organic buffers on growth and secondary metabolite accumulation in Scytonema ocellatum strain FF-66-3 were determined. Nitrate, phosphate, magnesium, and sulfur had no specific stimulatory or inhibitory effects on scytophycin accumulation within the range of concentrations that supported optimal growth. Calcium concentrations greater than those required for growth (0.1 mM) stimulated scytophycin accumulation. Sodium carbonate concentrations in excess of 0.25 mM strongly inhibited growth. Ammonium (2.5 mM) inhibited both growth and product formation. 3-[N-Morpholino]propanesulfonic acid at 3-5 mM effectively controlled pH and facilitated both growth and product formation.

Swinholide A is a microfilament disrupting marine toxin that stabilizes actin dimers and severs actin filaments

Swinholide A, isolated from the marien sponge Theonella swinhoei, is a 44-carbon ring dimeric dilactone macrolide with a 2-fold axis of symmetry. Recent studies have elucidated its unusual structure and shown that it has potent cytotoxic activity. We now report that swinholide A disrupts the actin cytoskeleton of cells grown in culture, sequesters actin dimers in vitro in both polymerizing and non-polymerizing buffers with a binding stoichiometry of one swinholide A molecule per actin dimer, and rapidly severs F-actin in vitro with high cooperativity. These unique properties are sufficient to explain the cytotoxicity of swinholide A. They also suggest that swinholide A might be a model for studies of the mechanism of action of F-actin severing proteins and be therapeutically useful in conditions where filamentous actin contributes to pathologically high viscosities.

A sensitive assay for taxol and other microtubule-stabilizing agent

The ability of taxol to protect microtubules in cultured human ovarian carcinoma cells from drug- and cold-induced depolymerization was characterized as a functional assay for microtubule stabilizing agents. Treatment of the cells with concentrations of vinblastine or colchicine of 50 nM or greater, or incubation at 4 degrees C resulted in complete depolymerization of cytoplasmic microtubules. Pretreatment with taxol for 3 h enabled the cells to maintain substantial numbers of microtubules following the application of vinblastine or colchicine. This protective effect was easily observed at 50 nM taxol, whereas taxol-induced microtubule bundling was observed only at concentrations of 500 nM or greater. Concentrations of taxol as low as 10 nM stabilized microtubules against cold-induced depolymerization. Therefore, protection of microtubules from drug- and cold-induced depolymerization provides a sensitive functional assay for taxol. These systems should be similarly effective in identifying novel compounds which stabilize microtubules.

Scytophycin production by axenic cultures of the cyanobacterium Scytonema ocellatum

Nonaxenic clones prepared from the cyanobacterium Syctonema ocellatum Lyngbye ex Bornet et Flahault strain FF-66-3 exhibited a high degree of heterogeneity with respect to tolytoxin titer. Thirty-four of 114 clones (29.8%) isolated by fragmentation of Scytonema filaments did not produce detectable amounts of tolytoxin in culture. One clone (designated SO127) produced approximately twice as much tolytoxin as the parental culture and continued to produce tolytoxin after repeated subculture. Three axenic clones were prepared from SO127 by a combination of antibiotic treatment and mechanical separation. Although axenic cultures yielded slightly greater biomass, tolytoxin content did not significantly differ between axenic and nonaxenic cultures, indicating that bacteria do not play a role in tolytoxin biosynthesis. Bacterial cultures derived from S. ocellatum did not produce detectable amounts of tolytoxin.