Milbemycins, a new family of macrolide antibiotics: fermentation, isolation and physico-chemical properties

Design, Optimization, Manufacture and Characterization of Milbemycin Oxime Nanoemulsions

Background: Despite the rapid development of nanoemulsions in recent years, no method has been established for the preparation of milbemycin oxime nanoemulsions. Milbemycin oxime is a widely used macrolide antibiotic in veterinary medicine, particularly for treating parasitic infections in animals such as dogs. However, its poor solubility in water limits its bioavailability and therapeutic efficacy. Developing a nanoemulsion formulation can enhance its solubility, stability, and bioavailability, offering a more effective treatment option. Methods: In this experiment, oil-in-water (O/W) milbemycin oxime nanoemulsions were successfully prepared by the phase inversion composition (PIC) method using ethyl butyrate as the oil phase, Tween-80 as the surfactant, and anhydrous ethanol as the co-surfactant. The region of O/W nanoemulsions was identified by constructing a pseudo-ternary phase diagram and, based on this, was screened by determining the droplet size, polydispersity coefficient, and zeta potential of each preparation. Results and Conclusions: The finalized formulation had a 2:1 ratio of surfactant to co-surfactant and a 7:3 ratio of mixed surfactant to oil, and its droplet size, polydispersity index (PDI), and zeta potential were 12.140 ± 0.128 nm, 0.155 ± 0.015, and -4.947 ± 0.768 mV, respectively. Transmission electron microscopy confirmed the spherical uniform distribution of droplets, and the nanoemulsions passed thermodynamic stability tests. The in vitro release of milbemycin oxime nanoemulsions followed first-order kinetic equations. In conclusion, nanoemulsions are an interesting option for the delivery of poorly water-soluble molecules such as milbemycin oxime.

Isolation and identification of three new isomer impurities in milbemycin oxime drug substance

In an effort to identification of the unknown impurities in milbemycin oxime (MO) bulk drug, three impurities 1, 2 and 3 were isolated by two-dimensional (2 D) preparation method (Agilent Zorbax-C3 preparative column and Sepax Amethyst C18-H preparative column). Based on the extensive NMR analysis and ESIMS data, the structures of the three impurities were established as 14-desmethyl-14-ethyl-MO A4 (1), 24-desmethyl-24-ethyl-MO A4 (2) and 12-desmethyl-12-ethyl-MO A4 (3), respectively. They are the new isomer impurities of MO D and most likely originate from the oxidation and oximation of natural milbemycin homologs present in the original fermentation broth.

Chemical case studies from natural products of recent interest in the crop protection industry

Covering: up to 2024This review showcases selected natural products, which are of high relevance to the craft of crop protection, including in its most recent aspects such as their non-cidal use as biostimulants in plant health. Focussing on the chemistry and associated structure-activity relationships that were disclosed, the review presents case studies from the recent chemical development of important natural products and compounds inspired by them for their use in the crop protection industry.

Structural perspective into thiazoles and other azole derivatives as anthelmintic agents

Helminths or worm-like parasites are common infectious pathogens that cause significant diseases and mortality in animals and humans. To be noted, nematodes or soil-transmitted helminths (STH) have been infected about 1.5 billion people according to the recent reports of world health organization (WHO). Helminths-based infections are a significant type of neglected tropical diseases (NTDs) that show higher occurrence in deprived regions of tropical and subtropical areas. Low-level sanitation and limited facilities increase the risk of affliction with these infectious diseases. Helminths infections are known to be associated with non-significant symptoms to heavier ones such as gastrointestinal (GI) manifestations, weakness, impaired growth, disturbed physical development and death on the basis of involved worm population. Despite the implementations of preventive and mass-treatment strategies, incomplete curative effects and drug-resistance potentiality inspires the discovery of new and potent anthelmintic agents. In this context, several efforts have been focused on the synthesis and biological evaluation of various heterocycles, and in particular azoles, as anthelmintic agents. Structural elucidations of the emerged anti-infective agents from the medicinal chemistry perspective is a rational way to tackle the burden of infections throughout development of new anthelmintic medications. In continuation to our previous contributions, privileged azole-based anthelmintic compounds that have been documented in the 2012-2023 period and their structure activity relationship (SAR), are going to be reviewed in this study.

A phenotypic screen of the Global Health Priority Box identifies an insecticide with anthelmintic activity

BACKGROUND

Infection with parasitic nematodes (helminths), particularly those of the order Strongylida (such as Haemonchus contortus), can cause significant and burdensome diseases in humans and animals. Widespread drug (anthelmintic) resistance in livestock parasites, the absence of vaccines against most of these nematodes, and a lack of new and effective chemical entities on the commercial market demands the discovery of new anthelmintics. In the present study, we searched the Global Health Priority Box (Medicines for Malaria Venture) for new candidates for anthelmintic development.

METHODS

We employed a whole-organism, motility-based phenotypic screening assay to identify compounds from the Global Health Priority Box with activity against larvae of the model parasite H. contortus, and the free-living comparator nematode Caenorhabditis elegans. Hit compounds were further validated via dose-response assays, with lead candidates then assessed for nematocidal activity against H. contortus adult worms, and additionally, for cytotoxic and mitotoxic effects on human hepatoma (HepG2) cells.

RESULTS

The primary screen against H. contortus and C. elegans revealed or reidentified 16 hit compounds; further validation established MMV1794206, otherwise known as 'flufenerim', as a significant inhibitor of H. contortus larval motility (half-maximal inhibitory concentration [IC50] = 18 μM) and development (IC50 = 1.2 μM), H. contortus adult female motility (100% after 12 h of incubation) and C. elegans larval motility (IC50 = 0.22 μM). Further testing on a mammalian cell line (human hepatoma HepG2 cells), however, identified flufenerim to be both cytotoxic (half-maximal cytotoxic concentration [CC50] < 0.7 μM) and mitotoxic (half-maximal mitotoxic concentration [MC50] < 0.7 μM).

CONCLUSIONS

The in vitro efficacy of MMV1794206 against the most pathogenic stages of H. contortus, as well as the free-living C. elegans, suggests the potential for development as a broad-spectrum anthelmintic compound; however, the high toxicity towards mammalian cells presents a significant hindrance. Further work should seek to establish the protein-drug interactions of MMV1794206 in a nematode model, to unravel the mechanism of action, in addition to an advanced structure-activity relationship investigation to optimise anthelmintic activity and eliminate mammalian cell toxicity.

An Overview of Julia-lythgoe Olefination

Julia-Lythgoe olefination (or simply Julia olefination) is an olefination process between phenyl sulfones and aldehydes (or ketones) to give alkenes after alcohol functionalization and reductive elimination using sodium amalgam or SmI2. It is mainly used to synthesize E-alkenes and is a key step in numerous total syntheses of many natural products. This review exclusively deals with the Julia-Lythgoe olefination and concentrates mainly on the applications of this reaction in natural product synthesis covering literature up to 2021.

Natural products as anthelmintics: safeguarding animal health

Covering literature to December 2022This review provides a comprehensive account of all natural products (500 compounds, including 17 semi-synthetic derivatives) described in the primary literature up to December 2022, reported to be capable of inhibiting the egg hatching, motility, larval development and/or the survival of helminths (i.e., nematodes, flukes and tapeworms). These parasitic worms infect and compromise the health and welfare, productivity and lives of commercial livestock (i.e., sheep, cattle, horses, pigs, poultry and fish), companion animals (i.e., dogs and cats) and other high value, endangered and/or exotic animals. Attention is given to chemical structures, as well as source organisms and anthelmintic properties, including the nature of bioassay target species, in vivo animal hosts, and measures of potency.

Effects of the Coculture Initiation Method on the Production of Secondary Metabolites in Bioreactor Cocultures of Penicillium rubens and Streptomyces rimosus

Bioreactor cocultures involving Penicillium rubens and Streptomyces rimosus were investigated with regard to secondary metabolite production, morphological development, dissolved oxygen levels, and carbon substrate utilization. The production profiles of 22 secondary metabolites were analyzed, including penicillin G and oxytetracycline. Three inoculation approaches were tested, i.e., the simultaneous inoculation of P. rubens with S. rimosus and the inoculation of S. rimosus delayed by 24 or 48 h relative to P. rubens. The delayed inoculation of S. rimosus into the P. rubens culture did not prevent the actinomycete from proliferating and displaying its biosynthetic repertoire. Although a period of prolonged adaptation was needed, S. rimosus exhibited growth and the production of secondary metabolites regardless of the chosen delay period (24 or 48 h). This promising method of coculture initiation resulted in increased levels of metabolites tentatively identified as rimocidin B, 2-methylthio-cis-zeatin, chrysogine, benzylpenicilloic acid, and preaustinoid D relative to the values recorded for the monocultures. This study demonstrates the usefulness of the delayed inoculation approach in uncovering the metabolic landscape of filamentous microorganisms and altering the levels of secondary metabolites.

Synthetic Routes to Approved Drugs Containing a Spirocycle

The use of spirocycles in drug discovery and medicinal chemistry has been booming in the last two decades. This has clearly translated into the landscape of approved drugs. Among two dozen clinically used medicines containing a spirocycle, 50% have been approved in the 21st century. The present review focuses on the notable synthetic routes to such drugs invented in industry and academia, and is intended to serve as a useful reference source of synthetic as well as general drug information for researchers engaging in the design of new spirocyclic scaffolds for medicinal use or embarking upon analog syntheses inspired by the existing approved drugs.

Modular polyketide synthase-derived insecticidal agents: from biosynthesis and metabolic engineering to combinatorial biosynthesis for their production

Covering: up to 2022Polyketides derived from actinomycetes are a valuable source of eco-friendly biochemical insecticides. The development of new insecticides is urgently required, as the number of insects resistant to more than one drug is rapidly increasing. Moreover, significant enhancement of the production of such biochemical insecticides is required for economical production. There has been considerable improvement in polyketide insecticidal agent production and development of new insecticides. However, most commercially important biochemical insecticides are synthesized by modular type I polyketide synthases (PKSs), and their structural complexities make chemical modification challenging. A detailed understanding of the biosynthetic mechanisms of potent polyketide insecticides and the structure-activity relationships of their analogs will provide insight into the comprehensive design of new insecticides with improved efficacies. Further metabolic engineering and combinatorial biosynthesis efforts, reinvigorated by synthetic biology, can eventually produce designed analogs in large quantities. This highlight reviews the biosynthesis of representative insecticides produced by modular type I PKSs, such as avermectin, spinosyn, and spectinabilin, and their insecticidal properties. Metabolic engineering and combinatorial biosynthetic strategies for the development of high-yield strains and analogs with insecticidal activities are emphasized, proposing a way to develop a next-generation insecticide.

The equine ascarids: resuscitating historic model organisms for modern purposes

The equine ascarids, Parascaris spp., are important nematode parasites of juvenile horses and were historically model organisms in the field of cell biology, leading to many important discoveries, and are used for the study of chromatin diminution. In veterinary parasitology, Parascaris spp. are important not only because they can cause clinical disease in young horses but also because they are the only ascarid parasites to have developed widespread anthelmintic resistance. Despite this, much of the general biology and mechanisms of anthelmintic resistance are poorly understood. This review condenses known basic biological information and knowledge on the mechanisms of anthelmintic resistance in Parascaris spp., highlighting the importance of foundational research programs. Although two variants of this parasite were recognized based on the number of chromosomes in the 1870s and suggested to be two species in 1890, one of these, P. univalens, appears to have been largely forgotten in the veterinary scientific literature over the past 100 years. We describe how this omission has had a century-long effect on nomenclature and data analysis in the field, highlighting the importance of proper specimen identification in public repositories. A summary of important basic biology, including life cycle, in vitro maintenance, and immunology, is given, and areas of future research for the improvement of knowledge and development of new systems are given. Finally, the limited knowledge regarding anthelmintic resistance in Parascaris spp. is summarized, along with caution regarding assumptions that resistance mechanisms can be applied across clades.

Structural elucidation of major degradation products of milbemycin oxime drug substance using LC-MS and NMR

Milbemycin oxime (MO) drug substance is a 16-membered macrocyclic lactone that exhibits a broad spectrum of biological activity and high potency towards parasites. In this study, a comprehensive forced degradation study was carried out on MO drug substance to identify and characterize its major degradation products (DPs). MO drug substance was subjected to acid, base, oxidation (H₂O₂), heat (solid and solution state), and photolytic (solid and solution state) stress degradation as per the ICH guidelines. Chromatographic separation of the drug substance (MO A₃ and MO A₄) and its DPs was achieved using a gradient elution on a HALO C18 column (100 × 4.6 mm, 2.7 µm). Mobile phase A consisted of water/acetonitrile (60/40, v/v) and mobile phase B consisted of ethanol/isopropanol (50/50, v/v). A total of twelve major DPs were observed for MO drug substance under various stress conditions. These DPs were further identified and characterized using liquid chromatography-high resolution mass spectrometry and comparison of their fragmentation profile with MO A₄ and MO A₃ using tandem mass spectrometry. Of these, H₂O₂ induced oxidative degradation product (3,4-dihydroperoxide MO A₄) was isolated using semi-preparative HPLC and characterized by comparison of its nuclear magnetic resonance spectroscopy data with MO A_4. The proposed structures of the DPs have been rationalized by appropriate degradation pathways for MO A₄ and MO A₃.

The Pattern of Blood-Milk Exchange for Antiparasitic Drugs in Dairy Ruminants

Simple Summary

This review article is focused on the description of the plasma–milk partition coefficients for different antiparasitic drug classes in dairy ruminants, and it contributes to rational pharmaco-therapy in lactating dairy animals, which is critical to understand the pattern of drug excretion in milk as well as the residual concentration patterns in dairy products elaborated by processing milk from drug-treated animals.

Abstract

The prolonged persistence of milk residual concentration of different antiparasitic drugs in lactating dairy animals should be considered before recommending their use (label or extra-label) for parasite control in dairy animals. The partition blood-to-milk ratio for different antiparasitic compounds depends on their ability to diffuse across the mammary gland epithelium. The high lipophilicity of some of the most widely used antiparasitic drugs explains their high partition into milk and the extended persistence of high residual concentrations in milk after treatment. Most of the antiparasitic drug compounds studied were shown to be stable in various milk-related industrial processes. Thus, the levels of residues detected in raw milk can be directly applicable to estimating consumer exposure and dietary intake calculations when consuming heat-processed fluid milk. However, after milk is processed to obtain milk products such as cheese, yogurt, ricotta, and butter, the residues of lipophilic antiparasitic drugs are higher than those measured in the milk used for their elaboration. This review article contributes pharmacokinetics-based information, which is useful to understand the relevance of rational drug-based parasite control in lactating dairy ruminants to avoid undesirable consequences of residual drug concentrations in milk and derived products intended for human consumption.

Activation of cryptic milbemycin A4 production in Streptomyces sp. BB47 by the introduction of a functional bldA gene

Streptomycetes are characterized by their ability to produce structurally diverse compounds as secondary metabolites and by their complex developmental life cycle, which includes aerial mycelium formation and sporulation. The production of secondary metabolites is growth-stage dependent, and generally coincides with morphological development on a solid culture. Streptomyces sp. BB47 produces several types of bioactive compounds and displays a bald phenotype that is devoid of an aerial mycelium and spores. Here, we demonstrated by genome analysis and gene complementation experiments that the bald phenotype arises from the bldA gene, which is predicted to encode the Leu-tRNA^UUA molecule. Unlike the wild-type strain producing jomthonic acid A (1) and antarlide A (2), the strain complemented with a functional bldA gene newly produced milbemycin (3). The chemical structure of compound 3 was elucidated on the basis of various spectroscopic analyses, and was identified as milbemycin A₄, which is an insecticidal/acaricidal antibiotic. These results indicate that genetic manipulation of genes involved in morphological development in streptomycetes is a valuable way to activate cryptic biosynthetic pathways.

Heartworm disease - Overview, intervention, and industry perspective

Dirofilaria immitis, also known as heartworm, is a major parasitic threat for dogs and cats around the world. Because of its impact on the health and welfare of companion animals, heartworm disease is of huge veterinary and economic importance especially in North America, Europe, Asia and Australia. Within the animal health market many different heartworm preventive products are available, all of which contain active components of the same drug class, the macrocyclic lactones. In addition to compliance issues, such as under-dosing or irregular treatment intervals, the occurrence of drug-resistant heartworms within the populations in the Mississippi River areas adds to the failure of preventive treatments. The objective of this review is to provide an overview of the disease, summarize the current disease control measures and highlight potential new avenues and best practices for treatment and prevention.

Two new milbemycin derivatives from a genetically engineered strain Streptomyces bingchenggensis

Two new milbemycin derivatives, milbemycin M (1) and milbemycin N (2), were isolated from the culture of a genetically engineered strain Streptomyces bingchenggensis BCJ60. Their structures were elucidated through the interpretation of NMR and HR-ESI-MS spectroscopic data, as well as comparison with previous reports. The acaricidal and nematicidal activities of them against Tetranychus cinnabarinus and Bursaphelenchus xylophilus were tested. The results showed that compounds 1-2 possessed potent acaricidal and nematocidal activities.

Effects of verapamil on the pharmacokinetics of ivermectin in rabbits

This study was aimed to investigate the influence of verapamil-mediated inhibition of P-glycoprotein (P-gp) on the pharmacokinetics of ivermectin (IVM) given orally and subcutaneously (SC) to rabbits. Twenty New Zealand rabbits were allotted to 4 groups (n = 5) and received IVM either orally or SC (0.4 mg/kg) alone or co-administered with verapamil (2 mg/kg SC, 3 times at a 12-hr interval). Plasma, fecal, and urine samples were collected over 30 days after medication to assess IVM concentrations in these samples. No significant differences were observed in the pharmacokinetic parameters of IVM between oral and SC administrations. The area under the plasma concentration-time curve was higher (p < .05) after IVM (oral)/verapamil treatment, compared with oral IVM alone. Moreover, the time to the C_max of IVM was shorter (p < .05), whereas the elimination half-life and the mean residence time were longer (p < .05) in the presence of verapamil. The IVM/verapamil combination administered orally or SC reduced fecal IVM concentrations, compared with IVM alone. In conclusion, the significant changes by verapamil on the pharmacokinetics of IVM, likely due to the inhibition of a P-gp-mediated intestinal secretion, may change IVM's antinematodal activity.

Mining and fine-tuning sugar uptake system for titer improvement of milbemycins in Streptomyces bingchenggensis

Dramatic decrease of sugar uptake is a general phenomenon in Streptomyces at stationary phase, when antibiotics are extensively produced. Milbemycins produced by Streptomyces bingchenggensis are a group of valuable macrolide biopesticides, while the low yield and titer impede their broad applications in agricultural field. Considering that inadequate sugar uptake generally hinders titer improvement of desired products, we mined the underlying sugar uptake systems and fine-tuned their expression in this work. First, we screened the candidates at both genomic and transcriptomic level in S. bingchenggensis. Then, two ATP-binding cassette transporters named TP2 and TP5 were characterized to improve milbemycin titer and yield significantly. Next, the appropriate native temporal promoters were selected and used to tune the expression of TP2 and TP5, resulting in a maximal milbemycin A3/A4 titer increase by 36.9% to 3321 mg/L. Finally, TP2 and TP5 were broadly fine-tuned in another two macrolide biopesticide producers Streptomyces avermitilis and Streptomyces cyaneogriseus, leading to a maximal titer improvement of 34.1% and 52.6% for avermectin B_1a and nemadectin, respectively. This work provides useful transporter tools and corresponding engineering strategy for Streptomyces.

Molecular Design and Synthesis of Ivermectin Hybrids Targeting Hepatic and Erythrocytic Stages of Plasmodium Parasites

Ivermectin is a powerful endectocide, which reduces the incidence of vector-borne diseases. Besides its strong insecticidal effect on mosquito vectors of the disease, ivermectin inhibits Plasmodium falciparum sporogonic and blood stage development and impairs Plasmodium berghei development inside hepatocytes, both in vitro and in vivo. Herein, we present the first report on structural modification of ivermectin to produce dual-action molecular hybrids with good structure-dependent in vitro activity against both the hepatic and erythrocytic stages of P. berghei and P. falciparum infection, suggesting inclusion of ivermectin antimalarial hybrids in malaria control strategies. The most active hybrid displayed over threefold and 10-fold higher in vitro activity than ivermectin against hepatic and blood stage infections, respectively. Although an overwhelming insecticidal effect against Anopheles stephensi mosquitoes in laboratory conditions was not noticed, in silico docking analysis supports allosteric binding to glutamate-gated chloride channels similar to ivermectin.

Improved milbemycin production by engineering two Cytochromes P450 in Streptomyces bingchenggensis

Milbemycins and their semisynthetic derivatives are recognized as effective and eco-friendly pesticides, whereas the high price limits their widespread applications in agriculture. One of the pivotal questions is the accumulation of milbemycin-like by-products, which not only reduces the yield of the target products milbemycin A3/A4, but also brings difficulty to the purification. With other analogous by-products abolished, α9/α10 and β-family milbemycins remain to be eliminated. Herein, we solved these issues by engineering of post-modification steps. First, Cyp41, a CYP268 family cytochrome P450, was identified to participate in α9/α10 biosynthesis. By deleting cyp41, milbemycin α9/α10 was eliminated with an increase of milbemycin A3/A4 titer from 2382.5 ± 55.7 mg/L to 2625.6 ± 64.5 mg/L. Then, MilE, a CYP171 family cytochrome P450, was determined to be responsible for the generation of the furan ring between C6 and C8a of milbemycins. By further overexpression of milE, the production of β-family milbemycins was reduced by 77.2%. Finally, the titer of milbemycin A3/A4 was increased by 53.1% to 3646.9 ± 69.9 mg/L. Interestingly, overexpression of milE resulted in increased transcriptional levels of milbemycin biosynthetic genes and production of total milbemycins, which implied that the insufficient function of MilE was a limiting factor to milbemycin biosynthesis. Our research not only provides an efficient engineering strategy to improve the production of a commercially important product milbemycins, but also offers the clues for future study about transcriptional regulation of milbemycin biosynthesis.

Perspectives on the utility of moxidectin for the control of parasitic nematodes in the face of developing anthelmintic resistance

Macrocyclic lactone (ML) anthelmintics are the most important class of anthelmintics because of our high dependence on them for the control of nematode parasites and some ectoparasites in livestock, companion animals and in humans. However, resistance to MLs is of increasing concern. Resistance is commonplace throughout the world in nematode parasites of small ruminants and is of increasing concern in horses, cattle, dogs and other animals. It is suspected in Onchocerca volvulus in humans. In most animals, resistance first arose to the avermectins, such as ivermectin (IVM), and subsequently to moxidectin (MOX). Usually when parasite populations are ML-resistant, MOX is more effective than avermectins. MOX may have higher intrinsic potency against some parasites, especially filarial nematodes, than the avermectins. However, it clearly has a significantly different pharmacokinetic profile. It is highly distributed to lipid tissues, less likely to be removed by ABC efflux transporters, is poorly metabolized and has a long half-life. This results in effective concentrations persisting for longer in target hosts. It also has a high safety index. Limited data suggest that anthelmintic resistance may be overcome, at least temporarily, if a high concentration can be maintained at the site of the parasites for a prolonged period of time. Because of the properties of MOX, there are reasonable prospects that strains of parasites that are resistant to avermectins at currently recommended doses will be controlled by MOX if it can be administered at sufficiently high doses and in formulations that enhance its persistence in the host. This review examines the properties of MOX that support this contention and compares them with the properties of other MLs. The case for using MOX to better control ML-resistant parasites is summarised and some outstanding research questions are presented.

Acaricides of Natural Origin. Part 2. Review of the Literature (2002–2006)

Acari are responsible for millions of dollars worth of damage each year as a result of infestations of animals, plants and man. They directly affect our health and prosperity as animal and plant parasites, vectors of disease, and producers of allergens. The indiscriminate use of pesticides has quickly induced resistance in many parasites. At present, the control of acarid parasitic diseases in agriculture, human and veterinary medicine is mainly based on the use of drugs; and for this reason the lack of effective drugs often prevents the control of some parasitic diseases, making them more serious and important. The use of commercial drugs involves many problems, besides the drug-resistance shown by the most important parasites. Environmental damage and the toxicity of many synthetic drugs, represent the main problems that strongly limit drug use. In addition, drug residues in plant and animal food products are important reasons for further economic losses for farmers and must be regarded as potentially hazardous to man and the environment. Plant-derived compounds are generally more easily degradable and could show a smaller negative environmental impact with respect to synthetic drugs. For these reasons, the evaluation of the antiacarid activity of plant extracts is increasingly being investigated in order to obtain new leads, as demonstrated by recent studies that have evaluated and confirmed the effectiveness of many plant compounds on bacteria, fungi, protozoa, helminths and arthropods. This review will be limited to the class Arachnida, sub-class Acaridi, particularly to their control in agriculture, veterinary and human medicine using natural methods.

aMSGE: advanced multiplex site-specific genome engineering with orthogonal modular recombinases in actinomycetes

Chromosomal integration of genes and pathways is of particular importance for large-scale and long-term fermentation in industrial biotechnology. However, stable, multi-copy integration of long DNA segments (e.g., large gene clusters) remains challenging. Here, we describe a plug-and-play toolkit that allows for high-efficiency, single-step, multi-locus integration of natural product (NP) biosynthetic gene clusters (BGCs) in actinomycetes, based on the innovative concept of "multiple integrases-multiple attB sites". This toolkit consists of 27 synthetic modular plasmids, which contain single- or multi-integration modules (from two to four) derived from five orthogonal site-specific recombination (SSR) systems. The multi-integration modules can be readily ligated into plasmids containing large BGCs by Gibson assembly, which can be simultaneously inserted into multiple native attB sites in a single step. We demonstrated the applicability of this toolkit by performing stabilized amplification of acetyl-CoA carboxylase genes to facilitate actinorhodin biosynthesis in Streptomyces coelicolor. Furthermore, using this toolkit, we achieved a 185.6% increase in 5-oxomilbemycin titers (from 2.23 to 6.37 g/L) in Streptomyces hygroscopicus via the multi-locus integration of the entire 5-oxomilbemycin BGC (72 kb) (up to four copies). Compared with previously reported methods, the advanced multiplex site-specific genome engineering (aMSGE) method does not require the introduction of any modifications into host genomes before the amplification of target genes or BGCs, which will drastically simplify and accelerate efforts to improve NP production. Considering that SSR systems are widely distributed in a variety of industrial microbes, this novel technique also promises to be a valuable tool for the enhanced biosynthesis of other high-value bioproducts.

Anthelmintic drugs used in equine species

Internal parasites of horses comprise an intractable problem conferring disease, production and performance losses. Parasitism can rarely be controlled in grazing horses by management alone and anthelmintic drugs have formed the basis of therapy and prophylaxis for the last sixty years. The pharmacology of the anthelmintic drugs available dictate their spectrum of activity and degree of efficacy, their optimal routes of administration and characteristics which prevent some routes of administration, their safety tolerance and potential toxicities and as a consequence of their persistence in the body at effective concentrations their use in epidemiological control programmes. Their use has also resulted in the selection of parasites with genetically controlled characteristics which reduce their susceptibility to treatment, characteristics which are often common to whole chemical classes of anthelmintics. Pharmacological properties also confer compatibility in terms of safety and persistence with other anthelmintic drugs and thus the potential of combinations to treat parasites from different phylogenetic groups such as nematodes, cestodes and trematodes and also the potential by agency of their different molecular mechanisms of action to delay the selection of resistant genes. The major groups of anthelmintics now available, the benzimidazoles (BZD), macrocyclic lactones (MLs) and tetrahydropyrimidines are all highly effective against their targeted parasites (primarily nematodes for BZD's and ML's and cestodes for tetrahydropyrimidines) easily administered orally to horses and are well tolerated with wide margins of safety. Nevertheless, some parasitic stages are inherently less susceptible such as hypobiotic stages of the small strongyles (cyathostomins) and for some such as the adult stages of cyathostomins resistance has developed. Furthermore, for some less common parasites such as the liver fluke unlicensed drugs such as the salicylanilide, closantel have been used. A deep understanding of the pharmacology of anthelmintic drugs is essential to their optimal use in equine species.

C. elegans avoids toxin-producing Streptomyces using a seven transmembrane domain chemosensory receptor

Predators and prey co-evolve, each maximizing their own fitness, but the effects of predator–prey interactions on cellular and molecular machinery are poorly understood. Here, we study this process using the predator Caenorhabditis elegans and the bacterial prey Streptomyces, which have evolved a powerful defense: the production of nematicides. We demonstrate that upon exposure to Streptomyces at their head or tail, nematodes display an escape response that is mediated by bacterially produced cues. Avoidance requires a predicted G-protein-coupled receptor, SRB-6, which is expressed in five types of amphid and phasmid chemosensory neurons. We establish that species of Streptomyces secrete dodecanoic acid, which is sensed by SRB-6. This behavioral adaptation represents an important strategy for the nematode, which utilizes specialized sensory organs and a chemoreceptor that is tuned to recognize the bacteria. These findings provide a window into the molecules and organs used in the coevolutionary arms race between predator and potential prey.

A BRIEF REVIEW ON THE MODE OF ACTION OF ANTINEMATODAL DRUGS

Anthelmintics are some of the most widely used drugs in veterinary medicine. Here we review the mechanism of action of these compounds on nematode parasites. Included are the older classes of compounds; the benzimidazoles, cholinergic agonists and macrocyclic lactones. We also consider newer anthelmintics, including emodepside, derquantel and tribendimidine. In the absence of vaccines for most parasite species, control of nematode parasites will continue to rely on anthelmintic drugs. As a consequence, vigilance in detecting drug resistance in parasite populations is required. Since resistance development appears almost inevitable, there is a continued and pressing need to fully understand the mode of action of these compounds. It is also necessary to identify new drug targets and drugs for the continued effective control of nematode parasites.

Engineered biosynthesis of milbemycins in the avermectin high-producing strain Streptomyces avermitilis

BACKGROUND

Milbemycins, produced from Streptomyces hygroscopicus subsp. aureolacrimosus and Streptomyces bingchenggensis, are 16-membered macrolides that share structural similarity with avermectin produced from Streptomyces avermitilis. Milbemycins possess strong acaricidal, insecticidal, and anthelmintic activities but low toxicity. Due to the high commercial value of the milbemycins and increasing resistance to the avermectins and their derivatives, it is imperative to develop an efficient combinatorial biosynthesis system exploiting an overproduction host strain to produce the milbemycins and novel analogs in large quantities.

RESULTS

The respective replacement of AveA1 and AveA3 (or module 7 in AveA3) of the avermectin polyketide synthase (PKS) in the avermectin high-producing strain S. avermitilis SA-01 with MilA1 and MilA3 (or module 7 in MilA3) of the milbemycin PKS resulted in the production of milbemycins A3, A4, and D in small amounts and their respective C5-O-methylated congener milbemycins B2, B3, and G as major products with total titers of approximately 292 mg/l. Subsequent inactivation of the C5-O-methyltransferase AveD led to a production of milbemycins A3/A4 (the main components of the commercial product milbemectin) in approximately 225 and 377 mg/l in the flask and 5 l fermenter culture, respectively, along with trace amounts of milbemycin D.

CONCLUSIONS

We demonstrated that milbemycin biosynthesis can be engineered in the avermectin-producing S. avermitilis by combinatorial biosynthesis with only a slight decrease in its production level. Application of a similar strategy utilizing higher producing industrial strains will provide a more efficient combinatorial biosynthesis system based on S. avermitilis for further enhanced production of the milbemycins and their novel analogs with improved insecticidal potential.

Characterization of a pathway-specific activator of milbemycin biosynthesis and improved milbemycin production by its overexpression in Streptomyces bingchenggensis

BACKGROUND

Milbemycins, a group of 16-membered macrolides with potent anthelminthic and insecticidal activity, are produced by several Streptomyces and used widely in agricultural, medical and veterinary fields. Milbemycin A3 and A4, the main components produced by Streptomyces bingchenggensis, have been developed as an acaricide to control mites. The subsequent structural modification of milbemycin A3/A4 led to other commercial products, such as milbemycin oxime, lepimectin and latidectin. Despite its importance, little is known about the regulation of milbemycin biosynthesis, which has hampered efforts to enhance milbemycin production via engineering regulatory genes.

RESULTS

milR, a regulatory gene in the milbemycin (mil) biosynthetic gene cluster of S. bingchenggensis, encodes a large ATP-binding regulator of the LuxR family (LAL family), which contains an ATPase domain at its N-terminus and a LuxR-like DNA-binding domain at the C-terminus. Gene disruption and genetic complementation revealed that milR plays an important role in the biosynthesis of milbemycin. β-glucuronidase assays and transcriptional analysis showed that MilR activates the expression of the milA4-E operon and milF directly, and activates the other mil genes indirectly. Site-directed mutagenesis confirmed that the ATPase domain is indispensable for MilR's function, and particularly mutation of the conserved amino acids K37A, D122A and D123A, led to the loss of MilR function for milbemycin biosynthesis. Overexpression of an extra copy of milR under the control of its native promoter significantly increased production of milbemycin A3/A4 in a high-producing industrial strain S. bingchenggensis BC04.

CONCLUSIONS

A LAL regulator, MilR, was characterized in the mil gene cluster of S. bingchenggensis BC04. MilR could activate milbemycin biosynthesis through direct interaction with the promoter of the milA4-E operon and that of milF. Overexpression of milR increased milbemycin A3/A4 production by 38 % compared with the parental strain BC04, suggesting that genetic manipulation of this activator gene could enhance the yield of antibiotics.

The Effects of Ivermectin on Brugia malayi Females In Vitro: A Transcriptomic Approach

BACKGROUND

Lymphatic filariasis and onchocerciasis are disabling and disfiguring neglected tropical diseases of major importance in developing countries. Ivermectin is the drug of choice for mass drug administration programs for the control of onchocerciasis and lymphatic filariasis in areas where the diseases are co-endemic. Although ivermectin paralyzes somatic and pharyngeal muscles in many nematodes, these actions are poorly characterized in adult filariae. We hypothesize that paralysis of pharyngeal pumping by ivermectin in filariae could result in deprivation of essential nutrients, especially iron, inducing a wide range of responses evidenced by altered gene expression, changes in metabolic pathways, and altered developmental states in embryos. Previous studies have shown that ivermectin treatment significantly reduces microfilariae release from females within four days of exposure in vivo, while not markedly affecting adult worms. However, the mechanisms responsible for reduced production of microfilariae are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed transcriptomic profiles from Brugia malayi adult females, an important model for other filariae, using RNAseq technology after exposure in culture to ivermectin at various concentrations (100 nM, 300 nM and 1 μM) and time points (24, 48, 72 h, and 5 days). Our analysis revealed drug-related changes in expression of genes involved in meiosis, as well as oxidative phosphorylation, which were significantly down-regulated as early as 24 h post-exposure. RNA interference phenotypes of the orthologs of these down-regulated genes in C. elegans include "maternal sterile", "embryonic lethal", "larval arrest", "larval lethal" and "sick".

CONCLUSION/SIGNIFICANCE

These changes provide insight into the mechanisms involved in ivermectin-induced reduction in microfilaria output and impaired fertility, embryogenesis, and larval development.

Three new milbemycins from a genetically engineered strain S. avermitilis MHJ1011

Three new β-class milbemycins, 13α-hydroxy-4-ethy1 milbemycin β3 (1), 13α-hydroxy-25-ethy1 milbemycin β3 (2), 13α-hydroxy milbemycin β3 (3), were isolated from the broth of the genetically engineered strains Streptomyces avermitilis MHJ1011, whose aveA1 gene was replaced by milA1 gene seamlessly. Their structures were determined on the basis of extensive spectroscopic analysis and comparison with data from the literature. These three compounds, especially compound 1, exhibited potent acaricidal activity.

Gene Replacement for the Generation of Designed Novel Avermectin Derivatives with Enhanced Acaricidal and Nematicidal Activities

Avermectin (AVM) and ivermectin (IVM) are potent pesticides and acaricides which have been widely used during the past 30 years. As insect resistance to AVM and IVM is greatly increasing, alternatives are urgently needed. Here, we report two novel AVM derivatives, tenvermectin A (TVM A) and TVM B, which are considered a potential new generation of agricultural and veterinary drugs. The molecules of the TVMs were designed based on structure and pharmacological property comparisons among AVM, IVM, and milbemycin (MBM). To produce TVMs, a genetically engineered strain, MHJ1011, was constructed from Streptomyces avermitilis G8-17, an AVM industrial strain. In MHJ1011, the native aveA1 gene was seamlessly replaced with milA1 from Streptomyces hygroscopicus. The total titer of the two TVMs produced by MHJ1011 reached 3,400 mg/liter. Insecticidal tests proved that TVM had enhanced activities against Tetranychus cinnabarinus and Bursaphelenchus xylophilus, as desired. This study provides a typical example of exploration for novel active compounds through a new method of polyketide synthase (PKS) reassembly for gene replacement. The results of the insecticidal tests may be of use in elucidating the structure-activity relationship of AVMs and MBMs.

A review on the effect of macrocyclic lactones on dung-dwelling insects: Toxicity of macrocyclic lactones to dung beetles

Avermectins and milbemycins are commonly used in agro-ecosystems for the control of parasites in domestic livestock. As integral members of agro-ecosystems with importance in maintaining pasture health through dung burial behaviour, dung beetles are an excellent nontarget bio-indicator taxon for examining potential detrimental effects of pesticide application. The current review focuses on the relative toxicity of four different anthelmintics (ivermectin, eprinomectin, doramectin and moxidectin) in dung residues using dung beetles as a bioindicator species. One of the implications of this review is that there could be an effect that extends to the entire natural assemblage of insects inhabiting and feeding on the dung of cattle treated with avermectin or milbemycin products. Over time, reduced reproductive rate would result in decreased dung beetle populations and ultimately, a decrease in the rate of dung degradation and dung burial.

Evaluation of Bacillus cereus and Bacillus pumilus metabolites for anthelmintic activity

Objective:

To assess the anthelmintic acivity of Bacillus cereus and Bacillus pumilus metabolites.

Materials and Methods:

The successive solvent extractions with petroleum ether, ethyl acetate and methanol. The solvent extracts were tested for anthelmintic activity against Pheretima posthuma at 20 mg/ml concentration. The time of paralysis and time of death of the worms was determined for all the extracts. Albendazole was taken as a standard reference and sterile water as a control.

Results:

All the sample extracts showed significant anthelmintic activity in paralyzing the worms comparable with that of the standard drug. The time of death exhibited by BP metabolites was close to the time exhibited by standard.

Conclusion:

The study indicates both bacteria Bacillus cereus and Bacillus pumilus have anthelmintic activity indicating potential metabolites in them.

Combined application of plasma mutagenesis and gene engineering leads to 5-oxomilbemycins A3/A4 as main components from Streptomyces bingchenggensis

Milbemycin oxime has been commercialized as effective anthelmintics in the fields of animal health, agriculture, and human infections. Currently, milbemycin oxime is synthesized by a two-step chemical reaction, which involves the ketonization of milbemycins A3/A4 to yield the intermediates 5-oxomilbemycins A3/A4 using CrO3 as catalyst. Due to the low efficiency and environmental unfriendliness of the ketonization of milbemycins A3/A4, it is imperative to develop alternative strategies to produce 5-oxomilbemycins A3/A4. In this study, the atmospheric and room temperature plasma (ARTP) mutation system was first employed to treat milbemycin-producing strain Streptomyces bingchenggensis, and a mutant strain BC-120-4 producing milbemycins A3, A4, B2, and B3 as main components was obtained, which favors the construction of genetically engineered strains producing 5-oxomilbemycins. Importantly, the milbemycins A3/A4 yield of BC-120-4 reached 3,890 ± 52 g/l, which was approximately two times higher than that of the initial strain BC-109-6 (1,326 ± 37 g/l). The subsequent interruption of the gene milF encoding a C5-ketoreductase responsible for the ketonization of milbemycins led to strain BCJ60 (∆milF) with the production of 5-oxomilbemycins A3/A4 and the elimination of milbemycins A3, A4, B2, and B3. The high 5-oxomilbemycins A3/A4 yield (3,470 ± 147 g/l) and genetic stability of BCJ60 implied the potential use in industry to prepare 5-oxomilbemycins A3/A4 for the semisynthesis of milbemycins oxime.

Genome mining of the Streptomyces avermitilis genome and development of genome-minimized hosts for heterologous expression of biosynthetic gene clusters

To date, several actinomycete genomes have been completed and annotated. Among them, Streptomyces microorganisms are of major pharmaceutical interest because they are a rich source of numerous secondary metabolites. S. avermitilis is an industrial microorganism used for the production of an anthelmintic agent, avermectin, which is a commercially important antiparasitic agent in human and veterinary medicine, and agricultural pesticides. Genome analysis of S. avermitilis provides significant information for not only industrial applications but also understanding the features of this genus. On genome mining of S. avermitilis, the microorganism has been found to harbor at least 38 secondary metabolic gene clusters and 46 insertion sequence (IS)-like sequences on the genome, which have not been searched so far. A significant use of the genome data of Streptomyces microorganisms is the construction of a versatile host for heterologous expression of exogenous biosynthetic gene clusters by genetic engineering. Since S. avermitilis is used as an industrial microorganism, the microorganism is already optimized for the efficient supply of primary metabolic precursors and biochemical energy to support multistep biosynthesis. The feasibility of large-deletion mutants of S. avermitilis has been confirmed by heterologous expression of more than 20 exogenous biosynthetic gene clusters.

Genetic engineering of Streptomyces bingchenggensis to produce milbemycins A3/A4 as main components and eliminate the biosynthesis of nanchangmycin

Milbemycins A3/A4 are important 16-membered macrolides which have been commercialized and widely used as pesticide and veterinary medicine. However, similar to other milbemycin producers, the production of milbemycins A3/A4 in Streptomyces bingchenggensis is usually accompanied with undesired by-products such as C5-O - methylmilbemycins B2/B3 (α-class) and β1/β2 (β-class) together with nanchangmycin. In order to obtain high yield milbemycins A3/A4-producing strains that produce milbemycins A3/A4 as main components, milD, a putative C5-O-methyltransferase gene of S. bingchenggensis , was biofunctionally investigated by heterologous expression in Escherichia coli . Enzymatic analysis indicated that MilD can catalyze both α-class (A3/A4) and β-class milbemycins (β11) into C5-O-methylmilbemycins B2/B3 and β1, respectively, suggesting little effect of furan ring formed between C6 and C8a on the C5-O-methylation catalyzed by MilD. Deletion of milD gene resulted in the elimination of C5-Omethylmilbemycins B2/B3 and β1/β2 together with an increased yield of milbemycins A3/A4 in disruption strain BCJ13. Further disruption of the gene nanLD encoding loading module of polyketide synthase responsible for the biosynthesis of nanchangmycin led to strain BCJ36 that abolished the production of nanchangmycin. Importantly, mutant strain BCJ36 (ΔmilDΔnanLD) produced milbemycins A3/A4 as main secondary metabolites with a yield of 2312 ± 47 μg/ml, which was approximately 74 % higher than that of the initial strain S. bingchenggensis BC-109-6 (1326 ± 37 μg/ml).

Amino acid residues of both the extracellular and transmembrane domains influence binding of the antiparasitic agent milbemycin to Haemonchus contortus AVR-14B glutamate-gated chloride channels

Glutamate-gated chloride (GluCl) channels are pentameric receptors for the inhibitory neurotransmitter glutamate in invertebrates and are a major target for macrolide anthelmintics. Three amino acids in GluCl channels are reported to render macrolide resistance in nematodes and insects. To examine whether these three amino acids are involved in binding of the antiparasitic agent milbemycin (MLM) to the GluCl channels of the nematode parasite Haemonchus contortus, the equivalent amino acids (L256, P316, and G329) of the Hco-AVR-14B subunit were substituted with various amino acids. cDNAs encoding the wild type and mutants of this subunit were transfected into COS-1 cells for transient expression and analysis of GluCl channels. The abilities of these mutant channels to bind [(3)H]MLM A(4) were remarkably decreased when compared with the wild-type channel. In patch clamp analysis, L256F and P316S mutant channels were 37- and 100-fold less sensitive to MLM A(4) when compared with the wild-type channel, respectively. These findings indicate that amino acid changes in the β10 strand, the M2-M3 linker, and the M3 region influence MLM A(4) binding to the channel. Homology modeling and ligand docking studies suggest the presence of two potential binding sites for MLM A(4).

Comparative evaluation of systemic drugs for their effects against Anopheles gambiae

Laboratory and field studies have shown that ivermectin, a drug that targets invertebrate ligand-gated ion channels (LGICs), is potently active against Anopheles spp. mosquitoes at concentrations present in human blood after standard drug administrations; thus ivermectin holds promise as a mass human-administered endectocide that could help suppress malaria parasite transmission. We evaluated other systemic LGIC-targeting drugs for their activities against the African malaria vector Anopheles gambiae using in vitro blood feeding assays. Eprinomectin, selamectin, moxidectin, and N-tert-butyl nodulisporamide were evaluated as potentially systemic drugs having similar modes of action to ivermectin; all primarily are agonists of invertebrate glutamate-gated chloride ion channels. Additionally, nitenpyram and spinosad were evaluated as systemic drugs that primarily work as agonists of nicotinic acetylcholine receptor channels. Only eprinomectin killed An. gambiae at concentrations that were comparable to ivermectin. At sub-lethal doses, nitenpyram and moxidectin marginally affected mosquito re-blood feeding ability. The macrocyclic lactones, particularly eprinomectin, caused significantly increased knockdown and significantly inhibited recovery in blood fed females. These data are a first step in evaluating drugs that might be eventually combined with, or substituted for ivermectin for future malaria parasite transmission control.

De novo synthesis of natural products via the asymmetric hydration of polyenes

For the last ten years our group has been working toward the development of an asymmetric hydration approach to polyketide natural products based on the regioselective hydration of di- and tri-enoates. Key to the success of this approach is the recognition that both high regiocontrol and asymmetric induction could be obtained by the use of a Sharpless asymmetric dihydroxylation reaction. Herein we describe the development of the method and its application to natural product total synthesis.

Modeling with a view to target identification in metabolic engineering: a critical evaluation of the available tools

The state of the art tools for modeling metabolism, typically used in the domain of metabolic engineering, were reviewed. The tools considered are stoichiometric network analysis (elementary modes and extreme pathways), stoichiometric modeling (metabolic flux analysis, flux balance analysis, and carbon modeling), mechanistic and approximative modeling, cybernetic modeling, and multivariate statistics. In the context of metabolic engineering, one should be aware that the usefulness of these tools to optimize microbial metabolism for overproducing a target compound depends predominantly on the characteristic properties of that compound. Because of their shortcomings not all tools are suitable for every kind of optimization; issues like the dependence of the target compound's synthesis on severe (redox) constraints, the characteristics of its formation pathway, and the achievable/desired flux towards the target compound should play a role when choosing the optimization strategy.

Plasma dispositions of ivermectin, doramectin and moxidectin following subcutaneous administration in rabbits

This study evaluated the comparative plasma dispositions of ivermectin (IVM), doramectin (DRM) and moxidectin (MXD) following subcutaneous administration in rabbits. Fifteen New Zealand white rabbits were allocated into three groups of five animals each. The animals in each group received IVM, DRM or MXD by subcutaneous injection at a single dose of 0.3 mg/kg. Blood samples were collected at various times between 1 h and 40 days after treatment and the plasma samples were analysed by high-performance liquid chromatography using fluorescence detection. Moxidectin was absorbed faster from the injection site and reached the peak plasma concentration ( Cmax) significantly earlier than IVM and DRM. There was no significant difference in Cmax values among the three molecules, whereas the area under the concentration–time curves of DRM (258.40 ng.d/mL) and IVM (191.62 ng.d/mL) was significantly higher than that of MXD (83.17 ng.d/mL). The mean plasma residence time and terminal half-life ( t1/2λ z) were longer for DRM (7.52 and 4.48 days, respectively) and MXD (8.97 and 8.16 days, respectively) compared with IVM (4.73 and 2.75 days, respectively). Considering the pharmacokinetic parameters for the parent molecules, the persistence of DRM and MXD are significantly longer than IVM and this may have a positive effect on their efficacy in rabbits following subcutaneous administration or utility relating to interdosing interval.