Knock-in mutagenesis in Drosophila Rdl underscores the critical role of the conserved M3 glycine in mediating the actions of broflanilide and isocycloseram on GABA receptors

γ-Aminobutyric acid receptors (GABARs) are crucial targets for pest control chemicals, including meta-diamide and isoxazoline insecticides, which act as negative allosteric modulators of insect GABARs. Previous cell-based assays have indicated that amino acid residues in the transmembrane cavity between adjacent subunits of Drosophila RDL GABAR (i.e., Ile276, Leu280, and Gly335) are involved in mediating the action of meta-diamides. In this study, to confirm this result at the organismal level, we employed CRISPR/Cas9-mediated genome editing, generated six transgenic Drosophila strains carrying substitutions in these amino acid residues, and investigated their sensitivity to broflanilide and isocycloseram. Flies homozygous for the I276F mutation did not exhibit any change in sensitivity to the tested insecticides compared to the control flies. Conversely, I276C homozygosity was lethal, and heterozygous flies exhibited ∼2-fold lower sensitivity to broflanilide than the control flies. Flies homozygous for the L280C mutation survived into adulthood but exhibited infertility. Both heterozygous and homozygous L280C flies exhibited ∼3- and ∼20-fold lower sensitivities to broflanilide and isocycloseram, respectively, than the control flies. The reduction in sensitivity to isocycloseram in L280C flies diminished to ∼3-fold when treated with piperonyl butoxide. Flies homozygous for the G335A mutation reached the adult stage. However, they were sterile, had small bodies, and exhibited reduced locomotion, indicating the critical role of Gly335 in RDL function. These flies exhibited markedly increased tolerance to topically applied broflanilide and isocycloseram, demonstrating that the conserved Gly335 is the target of the insecticidal actions of broflanilide and isocycloseram. Considering the significant fitness costs, the Gly335 mutation may not pose a serious risk for the development of resistance in field populations of insect pests. However, more careful studies using insect pests are needed to investigate whether our perspective applies to resistance development under field conditions.

Controlled expression of nicotinic acetylcholine receptor-encoding genes in insects uncovers distinct mechanisms of action of the neonicotinoid insecticide dinotefuran

Dinotefuran, a neonicotinoid, is a unique insecticide owing to its structure and action. We took two approaches that employed insects with controlled expression of nicotinic acetylcholine receptor (nAChR)-encoding genes to gain insight into the uniqueness of dinotefuran. First, we examined the insecticidal activity of dinotefuran and imidacloprid against brown planthoppers (Nilaparvata lugens), in which the expression of eight (of 13) individual subunit-encoding genes was specifically reduced using RNA interference. Knockdown of the tested gene, except one, resulted in a decrease in sensitivity to imidacloprid, whereas the sensitivity of N. lugens to dinotefuran decreased only when two of the eight genes were knocked down. These findings imply that a major dinotefuran-targeted nAChR subtype may contain specific subunits although imidacloprid acts on a broad range of receptor subtypes. Next, we examined the effects of knockout of Drosophila α1 subunit-encoding gene (Dα1) on the insecticidal effects of dinotefuran and imidacloprid. Dα1-deficient flies (Dα1^KO) demonstrated the same sensitivity to dinotefuran as control flies, but a decreased sensitivity to imidacloprid. This difference was attributed to a reduction in imidacloprid-binding sites in Dα1^KO flies, whereas the binding of dinotefuran remained unchanged. RNA sequencing analysis indicated that Dα2 expression was specifically enhanced in Dα1^KO flies. These findings suggest that changes in Dα1 and Dα2 expression contribute to the differences in the insecticidal activity of dinotefuran and imidacloprid in Dα1^KO flies. Overall, our findings suggest that dinotefuran acts on distinct nAChR subtypes.

Novel fungicide quinofumelin shows selectivity for fungal dihydroorotate dehydrogenase over the corresponding human enzyme

The species selectivity of class 2 dihydroorotate dehydrogenase (DHODH), a target enzyme for quinofumelin, was examined. The Homo sapiens DHODH (HsDHODH) assay system was developed to compare the selectivity of quinofumelin for fungi with that for mammals. The IC₅₀ values of quinofumelin for Pyricularia oryzae DHODH (PoDHODH) and HsDHODH were 2.8 nM and >100 µM, respectively. Quinofumelin was highly selective for fungal over human DHODH. Additionally, we constructed recombinant P. oryzae mutants where PoDHODH (PoPYR4) or HsDHODH was inserted into the PoPYR4 disruption mutant. At quinofumelin concentration of 0.01-1 ppm, the PoPYR4 insertion mutants could not grow, but the HsDHODH gene-insertion mutants thrived. This indicates that HsDHODH is a substitute for PoDHODH, and quinofumelin could not inhibit HsDHODH as in the HsDHODH enzyme assay. Comparing the amino acid sequences of human and fungal DHODHs indicates that the significant difference at the ubiquinone-binding site contributes to the species selectivity of quinofumelin.

Design and biological activity of a novel fungicide, quinofumelin

Developed by Mitsui Chemicals Agro, Inc. (Tokyo, Japan), quinofumelin is a novel fungicide with a distinct chemical structure including 3-(isoquinolin-1-yl) quinoline, demonstrating fungicidal activity against a variety of fungi, including rice blast and gray mold. We screened our compound library to identify curative compounds for rice blast and evaluated the effect of fungicide-resistant strains of gray mold. Our research demonstrated that quinofumelin has curative effects against rice blast and is not cross-resistant to existing fungicides. Accordingly, the use of quinofumelin can be considered a novel approach for disease control in agricultural production. In this report, the discovery of quinofumelin from the initial compound is described in detail.

The target site of the novel fungicide quinofumelin, Pyricularia oryzae class II dihydroorotate dehydrogenase

The target site of the novel fungicide quinofumelin was investigated in the rice blast fungus Pyricularia oryzae. Quinofumelin-induced mycelial growth inhibition was reversed by orotate but not by dihydroorotate. Recovery tests suggested that the target site of quinofumelin was dihydroorotate dehydrogenase (DHODH), which catalyzes the oxidation of dihydroorotate to orotate. Quinofumelin strongly inhibited P. oryzae class 2 DHODH (DHODH II) (IC₅₀: 2.8 nM). The inhibitory activities of mycelial growth and DHODH II were strongly positively correlated, indicating that DHODH II inhibition by quinofumelin lead to antifungal activity. A P. oryzae DHODH II gene (PoPYR4) disruption mutant (ΔPopyr4), showing the same tendency as the quinofumelin-treated wild strain in recovery tests, was constructed, and disease symptoms were not observed in rice plants infected by ΔPopyr4. Thus, DHODH II, which plays an important role in pathogenicity and mycelial growth, is found to be the target site of quinofumelin.

Application of computational methods in the analysis of pesticide target-site and resistance mechanisms

Meta-diamide insecticides including broflanilide have a high insecticidal activity by acting on RDL GABA receptors. Both membrane potential assays and docking studies suggest that the target site of meta-diamides is different from that of conventional noncompetitive inhibitors, such as fipronil. In fact, meta-diamides are effective against cyclodiene- and fipronil-resistant pests that carry target-site mutations. Dinotefuran uniquely possesses a tetrahydrofuran ring, whereas other neonicotinoids possess aromatic rings. Moreover, dinotefuran has been reported to be effective against imidacloprid-resistant strains. A docking study predicted the weak binding of dinotefuran to cytochrome P450s which are associated with imidacloprid resistance. Metabolic assays revealed that dinotefuran was not metabolized by these cytochrome P450s. These findings suggest that the lack of metabolic activity of P450s against dinotefuran causes a low level of cross-resistance.

Mechanisms underlying the selectivity of meta-diamides between insect resistance to dieldrin (RDL) and human γ-aminobutyric acid (GABA) and glycine receptors

BACKGROUND

Meta-diamides [3-benzamido-N-(4-(perfluoropropan-2-yl)phenyl)benzamides] show high insecticide activity by acting as antagonists to the insect resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors. In contrast, low-level antagonist activities of meta-diamides have been demonstrated against the human GABA type A receptor (GABA_A R) α1β2γ2S, mammalian GABA_A R α1β3γ2S, and the human glycine receptor (GlyR) α1β. Glycine residue 336 in the membrane-spanning region M3 of the Drosophila RDL GABA receptor is essential for its high sensitivity to meta-diamide 7, [3-benzamido-N-(2-bromo-4-(perfluoropropan-2-yl)-6-(trifluoromethyl)phenyl)-2-fluorobenzamide].

RESULTS

We examined the effects of an equivalent mutation (M288G) in spontaneously opened human GABA_A R β3 homomers using membrane potential assay. Picrotoxin and fipronil blocked spontaneously opened human GABA_A Rs β3 and β3-M286G in a concentration-dependent manner. In contrast, meta-diamide 7 did not block spontaneously opened GABA_A R β3 homomers, although meta-diamide 7 blocked spontaneously opened GABA_A R β3-M286G homomers. In addition, inhibitory potency of meta-diamide 7 for GABA-induced membrane potential change in cells expressing GABA_A R α1β3-M286G was much higher than that in cells expressing GABA_A R α1β3. In the same way, the equivalent mutation (A288G) in GlyR α1 increased the inhibitory potency of meta-diamide 7 for GlyRs α1 and α1β.

CONCLUSION

Studies substituting an equivalent mutation (M288G) in spontaneously opening human GABA_A R β3 homomers and human GABA_A Rs α1β3 heteromers suggest that M286 in human GABA_A R β3 is important for the low sensitivity to meta-diamide 7. In this study, we summarize the mechanisms underlying the selectivity of meta-diamides between insect RDL and human GABA and glycine receptors. © 2020 Society of Chemical Industry.

Important amino acids for function of the insect Rdl GABA receptor

BACKGROUND

Insect Rdl GABA receptor is an important insecticide target. To design a novel insecticide, studies on the structures of homologous pentameric ligand-gated ion channels provide information about important amino acids that are necessary for the function of insect Rdl GABA receptors.

RESULTS

L9'A, T12'A, T13'A, T13'S, M15'S, and M15'N mutations in the Drosophila Rdl GABA receptor subunit caused the protein to spontaneously adopt the open state conformation. In contrast, the S16'A, S16'T, S17'A, and S17'H mutant homomers showed the same levels of agonist and antagonist sensitivity as the wild-type receptor. The G336M mutation in the Drosophila Rdl GABA receptor abolished the agonist activities of ivermectin and milbemectin, but the F339M mutation did not. Additionally, the F339M mutation caused spontaneous opening of the receptor. In the Drosophila Rdl model, the hydrophobic girdle plays an important role in stabilization of the closed state. Mutations which decrease hydrophobic interactions resulted in spontaneous opening, supporting the importance of the hydrophobic girdle for keeping the channel closed. Through a mutational study of transmembrane 3 (TM3) cytoplasmic domain and Rdl GABA receptor modeling, hydrophobic interactions between TM3 and TM4 and intersubunit interaction were demonstrated to be important for channel gating. Alternatively, the intrasubunit interaction between TM2 and TM3 domains were less important for channel gating in case of Drosophila Rdl GABA receptor.

CONCLUSION

This study demonstrates important amino acids critical to the function of the Drosophila Rdl GABA receptor based on the mutational studies and Drosophila Rdl GABA receptor modeling approach. © 2020 Society of Chemical Industry.

Experimental and theoretical investigation of the reaction of a 3-amidothiophene derivative with various carbonyl compounds

The reactions of a 3-amidothiophene derivative, which is a partial structure of penthiopyrad, with various carbonyl compounds were investigated. Depending on the carbonyl compound that was used as a reactant, different products (alkenes and bis-products) were obtained from the attack of the carbon at the 2-position of the 3-amidothiophene on the carbonyl compounds. Density functional theory (DFT) calculations revealed that dehydration conditions were important for the first carbonyl addition to shift the reaction toward the product, as the products are more unstable than reactants other than aldehyde. The DFT calculations also suggested that the relative stability of the alkenyl state determined whether the second bis-product formation would proceed; i.e., the relatively unstable disubstituted alkene led to bis-products, and the stable trisubstituted or conjugated alkene yielded alkenyl products.

Synthesis and fungicidal activities of positional isomers of the N-thienylcarboxamide

To investigate the effects of bioisosteric replacement of the phenyl group with the thienyl group, N-phenylcarboxamide and three regioisomers of N-(substituted-thienyl)carboxamide were synthesized. The inhibitory activity on the succinate dehydrogenase prepared from the gray mold Botrytis cinerea as well as the fungicidal activity against B. cinerea were evaluated. Two isomers, N-(2-substituted-3-thienyl)carboxamide and N-(4-substituted-3-thienyl)carboxamide exhibited the same level of activity as the phenyl derivative, whereas N-(3-substituted-2-thienyl)carboxamide exhibited lower activity than the phenyl derivative, suggesting that the 2-substituted-3-thienyl and 4-substituted-3-thienyl groups functioned as bioisosteres of the phenyl group in N-phenylcarboxamide, but the other did not.

In vivo and in vitro evidence for the inhibition of homogentisate solanesyltransferase by cyclopyrimorate

BACKGROUND

Cyclopyrimorate is a highly effective bleaching herbicide discovered by Mitsui Chemicals Agro, Inc. The target site was recently reported to be homogentisate solanesyltransferase (HST) in the plastoquinone (PQ) biosynthesis pathway on the basis of the number of intermediates in cyclopyrimorate-treated plants and in vitro HST assays. Here, the target site of cyclopyrimorate was further explored using both in vivo and in vitro experiments.

RESULTS

The cyclopyrimorate-dependent bleaching effect on Arabidopsis thaliana was reversed by decyl PQ, suggesting that this symptom is attributable to the inhibition of PQ biosynthesis. Furthermore, homogentisate (HGA), a substrate of HST, weakly reversed the bleaching effect of cyclopyrimorate in a dose-dependent manner. We expected that the weak reversal by HGA was due to competitive inhibition by cyclopyrimorate or des-morpholinocarbonyl cyclopyrimorate (DMC), a metabolite of cyclopyrimorate in plants that exhibit higher HST-inhibitory activity as compared to cyclopyrimorate. Kinetic analysis was therefore conducted using DMC. DMC inhibited HST competitively with respect to HGA, and was a mixed non-competitive inhibitor with respect to the other substrate, farnesyl diphosphate. Moreover, neither cyclopyrimorate nor DMC inhibited 2-methyl-6-phytyl-1,4-benzoquinone/2-methyl-6-solanesyl-1,4-benzoquinone methyltransferase, which is located downstream of HST in the PQ biosynthesis pathway.

CONCLUSIONS

The target site of cyclopyrimorate and DMC is HST, which is a novel target site for commercial herbicides. © 2019 Society of Chemical Industry.

Insecticides, biologics and nematicides: Updates to IRAC's mode of action classification - a tool for resistance management

Insecticide resistance has been and continues to be a significant problem for invertebrate pest control. As such, effective insecticide resistance management (IRM) is critical to maintain the efficacy of current and future insecticides. A technical group within CropLife International, the Insecticide Resistance Action Committee (IRAC) was established 35 years ago (1984) as an international association of crop protection companies that today spans the globe. IRAC's focus is on preserving the long-term utility of insect, mite, and most recently nematode control products through effective resistance management to promote sustainable agriculture and improved public health. A central task of IRAC has been the continual development and documentation of the Mode of Action (MoA) Classification scheme, which serves as an important tool for implementing IRM strategies focused on compound rotation / alternations. Updates to the IRAC MoA Classification scheme provide the latest information on the MoA of current and new insecticides and acaricides, and now includes information on biologics and nematicides. Details for these new changes and additions are reviewed herein.

Further characterization of distinct high-affinity binding sites for dinotefuran in the abdominal nerve cord of the American cockroach Periplaneta americana (Blattodea)

Dinotefuran (DTF) is a systemic neonicotinoid insecticide characterized by a tetrahydrofuran ring. In the present study, we examined the characteristics of DTF binding to native nicotinic acetylcholine receptors (nAChRs) expressed in the American cockroach Periplaneta americana using radioligand-binding methods. The Scatchard analysis, using [³H]imidacloprid (IMI), indicated that IMI has a single class of high-affinity binding sites in the P. americana nerve cord. In contrast, the Scatchard analysis using [³H]DTF indicated that DTF has two different classes of binding sites. Both DTF and IMI were found to bind to one of the classes, for which DTF showed low affinity. The other class, for which DTF showed high affinity, was localized in the abdominal nerve cord but not in the thoracic nerve cord. IMI showed low affinity for the high-affinity DTF binding sites. Our data suggest that DTF binds with high affinity to a nAChR subtype distinct from the high-affinity subtype for IMI. This difference might be responsible, at least in part, for the difference in resistance development to DTF and IMI in P. americana.

Differential metabolism of neonicotinoids by brown planthopper, Nilaparvata lugens, CYP6ER1 variants

Imidacloprid is very effective in controlling Nilaparvata lugens Stål, which severely damages rice plants. Following heavy imidacloprid use, imidacloprid-resistant N. lugens, which showed cross-resistance to other neonicotinoids, appeared. We used the baculovirus/Sf9 expression system to express CYP6ER1 variants carrying A375del + A376G (del3) mutations, either with or without T318S mutation, which confer imidacloprid resistance in N. lugens. These CYP6ER1 variants metabolized imidacloprid but did not metabolize dinotefuran. Moreover, Drosophila expressing a CYP6ER1 variant carrying T318S + del3 mutations were resistant to imidacloprid, with a resistance ratio of 288.7, whereas the resistance ratio to dinotefuran was 3.6. These findings indicate that N. lugens has a low level of resistance to dinotefuran, and the increase of resistance is slow. We also studied the metabolism of other neonicotinoids, as well as sulfoxaflor and flupyradifurone, by CYP6ER1 variants carrying del3 mutations, either with or without the T318S mutation. Sulfoxaflor, was not metabolized by either CYP6ER1-del3 or CYP6ER1-T318Sdel3 variants. However, these variants did metabolize flupyradifurone. This study sheds light on the substrate selectivity of CYP6ER1 variants.

Differential metabolism of imidacloprid and dinotefuran by Bemisia tabaci CYP6CM1 variants

Imidacloprid has been used to control one of most serious pests, Bemisia tabaci. However, B. tabaci has developed imidacloprid resistance mainly by over-expressing CYP6CM1. It was reported that imidacloprid-resistant B. tabaci showed no or low level of cross-resistance against dinotefuran. Here, we expressed CYP6CM1 variants using Sf9/baculovirus and/or Drosophila S2 cells and showed that CYP6CM1 variants metabolized imidacloprid but not dinotefuran. In addition, we demonstrated that imidacloprid and pymetrozine competed for a CYP6CM1 variant more efficiently than dinotefuran, using a luminescent substrate competition assay. These results suggest that lack of metabolic activity of CYP6CM1 variants against dinotefuran caused no or low level of cross-resistance.

Differential metabolism of neonicotinoids by Myzus persicae CYP6CY3 stably expressed in Drosophila S2 cells

The peach-potato aphid, Myzus persicae, is a serious crop pest that has developed imidacloprid resistance, mainly through overexpression of CYP6CY3. Here, we established a metabolic assay using Drosophila S2 cells that stably expressed CYP6CY3. We found that CYP6CY3 showed metabolic activity against imidacloprid, as well as acetamiprid, clothianidin, and thiacloprid, but had no activity against dinotefuran. Our study suggested that stable gene expression in Drosophila S2 cells is useful for examining which insecticide is metabolized by P450 monooxygenases.

Discovery of broflanilide, a novel insecticide

Broflanilide (1), discovered by Mitsui Chemicals Agro, Inc., has a unique chemical structure characterized as a meta-diamide and exhibits high activity against various pests, including Lepidopteran, Coleopteran, and Thysanopteran pests. Because broflanilide has a novel mode of action, the Insecticide Resistance Action Committee (IRAC) categorized it as a member of a new group: Group 30. The meta-diamide structure was generated via drastic structural modification of a lead compound, flubendiamide (2), and the subsequent structural optimization of meta-diamides on each of its three benzene rings led to the discovery of broflanilide. In the present study, the details of the generation of meta-diamides from the lead compound and the structural optimization of meta-diamides are described.

Action mechanism of bleaching herbicide cyclopyrimorate, a novel homogentisate solanesyltransferase inhibitor

The action mechanism of cyclopyrimorate, a novel herbicide for weed control in rice fields, was investigated. Cyclopyrimorate caused bleaching symptoms in Arabidopsis thaliana similar to those caused by existing carotenoid biosynthesis inhibitors, mesotrione and norflurazon. However, cyclopyrimorate treatment resulted in significant accumulation of homogentisate and a reduction in the level of plastoquinone. A metabolite of cyclopyrimorate, des-morpholinocarbonyl cyclopyrimorate (DMC), was detected in plants. These data suggested that cyclopyrimorate and/or DMC inhibit homogentisate solanesyltransferase (HST), a downstream enzyme of 4-hydroxyphenylpyruvate dioxygenase in the plastoquinone biosynthesis pathway. In vitro assays showed that A. thaliana HST was strongly inhibited by DMC and weakly by cyclopyrimorate, whereas other commercial bleaching herbicides did not inhibit HST. DMC derivatives showed a positive correlation between HST inhibition and in vivo bleaching activities. These results indicate that the target site of cyclopyrimorate and DMC is HST, a novel target site of commercial herbicides.

Synthesis and activities of tolprocarb derivatives against Pyricularia oryzae: relationships among the activities for polyketide synthase, melanin biosynthesis, and rice blast

The target site of tolprocarb has been reported to be polyketide synthase (PKS). Here, we evaluated the activities for Pyricularia oryzae PKS and melanin biosynthesis as well as the control efficacy of rice blast using a series of tolprocarb derivatives. A comparison of the inhibitory activities of PKS and melanin biosynthesis revealed a linear relationship (r 2=0.90), confirming PKS as the target site of tolprocarb. A compound beyond this relationship was metabolized by P. oryzae to an inactive compound. The control efficacy of rice blast was explained using the melting point and either the inhibitory activity of PKS or melanin biosynthesis. Structure-activity analysis revealed that both end parts of tolprocarb preferred hydrophobic groups, and the chirality of the substituent in the middle part significantly influenced the activities. These relationships will provide useful information for the development of novel PKS inhibitors.

Minireview: Mode of action of meta-diamide insecticides

Meta-diamides [3-benzamido-N-(4-(perfluoropropan-2-yl)phenyl)benzamides] are a distinct class of RDL GABA receptor noncompetitive antagonists showing high insecticidal activity against Spodoptera litura. The mode of action of the meta-diamides was demonstrated to be distinct from that of conventional noncompetitive antagonists (NCAs) such as fipronil, picrotoxin, lindane, dieldrin, and α-endosulfan. It was suggested that meta-diamides act at or near G336 in the M3 region of the Drosophila RDL GABA receptor. Although the site of action of the meta-diamides appears to overlap with that of macrocyclic lactones including avermectins and milbemycins, differential effects of mutations on the actions of the meta-diamides and the macrocyclic lactones were observed. Molecular modeling studies revealed that the meta-diamides may bind to an inter-subunit pocket near G336 in the Drosophila RDL GABA receptor better when in the closed state, which is distinct from the NCA-binding site, which is in a channel formed by M2s. In contrast, the macrocyclic lactones were suggested to bind to an inter-subunit pocket near G336 in the Drosophila RDL GABA receptor when in the open state. Furthermore, mechanisms underlying the high selectivity of meta-diamides are discussed. This minireview highlights the unique features of novel meta-diamide insecticides and demonstrates why meta-diamides are anticipated to become prominent insecticides that are effective against pests resistant to cyclodienes and fipronil.

Comparison between the modes of action of novel meta-diamide and macrocyclic lactone insecticides on the RDL GABA receptor

Macrocyclic lactones, avermectins, and milbemycins are widely used to control arthropods, nematodes, and endo- and ectoparasites in livestock and pets. Their main targets are glutamate-gated chloride channels. Furthermore, macrocyclic lactones reportedly interact with insect RDL γ-aminobutyric acid (GABA) receptors, but their modes of action on insect RDL GABA receptors remain unknown. In this study, we attempted to better understand the modes of action of macrocyclic lactones on RDL GABA receptors. We observed that ivermectin and milbemectin behaved as allosteric agonists of the Drosophila RDL GABA receptor. G336A, G336S, and G336T mutations had profound effects on the activities of ivermectin and milbemectin, and a G336M mutation abolished the allosteric agonist and antagonist activities of these macrocyclic lactones. These results suggest that G336 in TM3 of the Drosophila RDL GABA receptor is important for the binding of macrocyclic lactones. Recently, it has been suggested that a novel RDL GABA receptor antagonist, 3-benzamido-N-(2-bromo-4-perfluoroisopropyl-6-(trifluoromethyl)phenyl)-2-fluorobenzamide (meta-diamide 7), binds to the transmembrane intersubunit pocket near G336 in the Drosophila RDL GABA receptor. Thus, we compared the effects of mutations around G336 and A302 mutations in TM2 on the activities of macrocyclic lactone and meta-diamide 7. The effects of L281C, V340Q, V340N, A302S, and A302N mutations on the activity of meta-diamide 7 differed from those on ivermectin and milbemectin. Molecular modeling studies showed that macrocyclic lactones docked in the intersubunit pocket near G336 in the Drosophila RDL GABA receptor in the open state. In contrast, meta-diamide 7 docked into the Drosophila RDL GABA receptor in the closed state. This suggests that the modes of action of macrocyclic lactone binding to the wild-type Drosophila RDL GABA receptor differ from those of meta-diamide binding.

Novel 3,3a,5,9b-tetrahydro-2H-furo[3,2-c][2] benzopyran derivatives: synthesis of chiral glycol benzyl ether herbicides

Novel tricyclic 3,3a,5,9b-tetrahydro-2H-furo[3,2-c][2]benzopyran (TFB) derivatives were synthesized, and their herbicidal activities were elucidated. They were synthesized from D-glucose as a natural chiral source. The formation of the TFB skeleton was achieved by a Friedel-Crafts type intramolecular cyclization of methyl 5-deoxy-2,3-O-dibenzyl-5-C-methyl-D-xylofranosides. The intramolecular cyclization was dependent upon the electronic effects of the substituents at the C-2 benzyloxy group of methyl xylofranosides. Some TFBs exhibited a remarkable herbicidal activity to annual paddy weeds, such as Echinochloa sp, without injury to the rice.

The binding between the stem regions of human growth hormone (GH) receptor compensates for the weaker site 1 binding of 20-kDa human GH (hGH) than that of 22-kDa hGH

Despite the lower site 1 affinity of the 20-kDa human growth hormone (20K-hGH) for the hGH receptor (hGHR), 20K-hGH has the same hGHR-mediated activity as 22-kDa human GH (22K-hGH) at low hGH concentration and even higher activity at high hGH concentration. This study was performed to elucidate the reason why 20K-hGH can activate hGHR to the same level as 22K-hGH. To answer the question, we hypothesized that the binding between the stem regions of hGHR could compensate for the weaker site 1 binding of 20K-hGH than that of 22K-hGH in the sequential binding with hGHR. To demonstrate it, we prepared 15 types of alanine-substituted hGHR gene at the stem region and stably transfected them into Ba/F3 cells. Using these cells, we measured and compared the cell proliferation activities between 20K- and 22K-hGH. As a result, the activity of 20K-hGH was markedly reduced than that of 22K-hGH in three types of mutant hGHR (T147A, H150A, and Y200A). Regarding these mutants, the dissociation constant of hGH at the first and second step (KD1 and KD2) in the sequential binding with two hGHRs was predicted based on the mathematical cell proliferation model and computational simulation. Consequently, it was revealed that the reduction of the activity in 20K-hGH was attributed to the change of not KD1 but KD2. In conclusion, these findings support our hypothesis, which can account for the same potencies for activating hGHR between 20K- and 22K-hGH, although the site 1 affinity of 20K-hGH is lower than that of 22K-hGH.

Analysis of binding properties between 20 kDa human growth hormone (hGH) and hGH receptor (hGHR): the binding affinity for hGHR extracellular domain and mode of receptor dimerization

It has recently been shown that 20 kDa human growth hormone (hGH) forms the 1:2 hGH:hGH receptor (hGHR) complex and expresses full agonistic activity, although it hardly forms the 1:1 GH:GHR complex as compared with 22 kDa hGH. To clarify this mechanism, we analyzed the mode of receptor dimerization of 20 kDa hGH using the intact form and mutants. Complex formation analysis between hGHR extracellular domain (hGHBP) and either site1 mutant (K157A) or site2 mutant (G105R) by gel-filtration showed that the site1 mutant apparently formed no 1:1 complex and that the site2 mutant formed only the 1:1 complex. Cell proliferation analysis revealed that the activity curve (vs ligand concentration) of 20 kDa hGH showed a bell-shaped pattern. This indicates that the receptor dimerization of 20 kDa hGH proceeds in a sequential manner. Based on this sequential binding we have produced a mathematical model for receptor dimerization as a function of [hGH], [hGHBP], K(d) values for the first hGHBP binding (K(d1)) and the second hGHBP binding (K(d2)). The result of 20 kDa hGH binding to (S201C) hGHBP immobilized on biosensor tip showed that the K(d1) value was 1. 6x10(-8) M. Adopting this value as a constant in the function described above, we have obtained calculative hGHR dimerization curves vs hGH concentration. Since the K(d2) value could not be experimentally determined, the curves were simulatively obtained with varied K(d2) values. The simulated curve pattern coincided with the experimental result of the cell proliferation in Ba/F3-hGHR when the value 2.5x10(-10) M was adopted as K(d2). In conclusion, although the affinity of 20 kDa hGH for the first hGHR binding is reduced to one-tenth, that for the second binding is increased ten-fold in comparison with those of 22 kDa hGH, indicating that 20 kDa hGH can be an effective hGH isoform in the presence of hGHBP.

New platelet fibrinogen receptor glycoprotein IIb-IIIa antagonists: orally active series of N-alkylated amidines with a 6,6-bicyclic template

The design, synthesis, and pharmacological evaluation of (S)-(-)-ethyl [6-[4-(morpholinoformimidoyl)benzamido]-3, 4-dihydro-2H-1-benzopyran-3-yl]acetate hydrochloride ((S)-4.HCl, MS-180), an orally active glycoprotein IIb-IIIa (GPIIb-IIIa) antagonist, are reported. Pharmacophore mapping of amidino and carboxyl groups of already known GPIIb-IIIa antagonists led to the synthesis of nine amidino acids containing 6,6-bicyclic ring skeletons (10a-i). Among them, the compounds 10a,c,e having an amide bond and 1,2,3,4-tetrahydronaphthalene or 3, 4-dihydro-2H-1-benzopyran skeleton showed marked inhibitions with IC50 values of 46-57 nM in human platelet aggregation assay in vitro, but low oral activities. N-Alkylation of the amidino group coupled with the ester prodrug approach afforded MS-180 ((S)-4.HCl), which generates in vivo the corresponding carboxylic acid (S)-3 as an active species. In vitro, (S)-3 inhibited ADP-induced aggregation of guinea pig, dog, and human platelets (IC50 = 110, 253, and 35 nM, respectively) and inhibited the binding of fibrinogen to immobilized GPIIb-IIIa of human platelets (IC50 = 0.12 nM). After oral administration of MS-180 ((S)-4.HCl) to fasted beagle dog, ex vivo inhibition of platelet aggregation was observed. The maximal inhibitions were observed 2-4 h after dosing with dose dependency (60% inhibition at a dose of 1 mg/kg, 85% at 3 mg/kg, and 100% at 10 mg/kg, respectively) and the extent of the inhibitions paralleled the plasma concentration of the active species (S)-3. On the basis of these studies, we selected MS-180 ((S)-4.HCl) as a candidate for clinical evaluation as a drug for the treatment and prevention of thrombosis in patients.

The 20-kilodalton (kDa) human growth hormone (hGH) differs from the 22-kDa hGH in the complex formation with cell surface hGH receptor and hGH-binding protein circulating in human plasma

In spite of recent advance in understanding of the stoichiometry of 22-kDa human GH (22K-hGH) with cell surface hGH receptor (hGHR) and hGH-binding protein (hGH-BP) circulating in human plasma, that of 20-kDa hGH (20K-hGH) is poorly understood. To clarify this, mouse pro-B Ba/F3 cells stably expressing the full-length hGHR (Ba/F3-hGHR) and both recombinant and native hGH-BP were used in this study. Cell proliferation assay revealed that the two hGH isoforms increased Ba/F3-hGHR cells to the same extent in a dose-dependent manner at 0.1 pM-10 nM. However, the self-inhibition observed in 20K-hGH at 5 microM was significantly less than that in 22K-hGH. Furthermore, addition of 1 and 10 nM recombinant hGH-BP caused a slight inhibition in 20K-hGH, but a drastic inhibition in 22K-hGH. Gel filtration chromatography of mixtures of 20K-hGH with recombinant hGH-BP clearly demonstrated that 20K-hGH formed a 1:2 (hGH:hGH-BP) complex efficiently but no detectable 1:1 complex in any conditions. Supporting data were also obtained with native hGH-BP. Computer-aided homology modeling of 20K-hGH has provided speculative data that the conformational change caused by deletion of 15 residues may occur only in the loop between helix 1 and helix 2, resulting in the reduction of its site 1 affinity. In conclusion, 20K-hGH possesses a unique property for forming a 1:2 complex to the same extent as 22K-hGH but has difficulty in forming a 1:1 complex, which might be attributed to the conformational change restricted to its site 1 region.