Insect chitinase and chitinase-like proteins

Insect chitinases belong to family 18 glycosylhydrolases that hydrolyze chitin by an endo-type of cleavage while retaining the anomeric β-(1→4) configuration of products. There are multiple genes encoding chitinases and chitinase-like proteins in all insect species studied using bioinformatics searches. These chitinases differ in size, domain organization, physical, chemical and enzymatic properties, and in patterns of their expression during development. There are also differences in tissue specificity of expression. Based on a phylogenetic analysis, insect chitinases and chitinase-like proteins have been classified into several different groups. Results of RNA interference experiments demonstrate that at least some of these chitinases belonging to different groups serve non-redundant functions and are essential for insect survival, molting or development. Chitinases have been utilized for biological control of insect pests on transgenic plants either alone or in combination with other insecticidal proteins. Specific chitinases may prove to be useful as biocontrol agents and/or as vaccines.     

Novel insecticidal toxins from nematode-symbiotic bacteria

The current strategy of using transgenic crops expressing insecticidal protein toxins is placing increasing emphasis on the discovery of novel toxins, beyond those already derived from the bacterium Bacillus thuringiensis. Here we review the cloning of four insecticidal toxin complex (tc) encoding genes from a different bacterium Photorhabdus luminescens and of similar gene sequences from Xenorhabdus nematophilus. Both these bacteria occupy the gut of entomopathogenic nematodes and are released into the insect upon invasion by the nematode. In the insect the bacteria presumably secrete these insecticidal toxins, as well as a range of other antimicrobials, to establish the insect cadaver as a monocultural breeding ground for both bacteria and nematodes. In this review, the protein biochemistry and structure of the tc encoding loci are discussed in relation to their observed toxicity and histopathology. These toxins may prove useful as alternatives to those derived from B. thuringiensis for deployment in insect-resistant transgenic plants.     

Selectivity of action between pyrethroids and combined DDT-pyrethroid insecticides on Na+ influx into mammalian neuroblastoma

Summary Several of the most active synthetic pyrethroid insecticides in the presence of sea anemone toxin II, induced a dose related influx of sodium ion into the C9 mouse neuroblastoma. The influx of sodium ion into this mammalian cell did not take place with a DDT analogue, EDO and several new combined DDT-pyrethroid insecticides, although these have been reported to cause excess sodium influx into arthropod axons, related to their insecticidal activity. This difference between species in the action of the new insecticides at the nerve sodium channel explains their low mammalian toxicity.     

A new selective insecticidal uncoupler of oxidative phosphorylation

Summary A new aryl hydrazone structure with high insecticidal activity against the Australian sheep blowfly,Lucilia cuprina, was shown to have a higher activity as an uncoupler of oxidative phosphorylation in insect compared to mammalian mitochondrial preparations. This compound possesses the requirements of other uncouplers in its measured pKa and lipid solubility. However, when compared to a closely related structure with similar physicochemical properties, its insecticidal and insect mitochondrial uncoupling activities are greater and it exhibits decreased mammalian toxicity corresponding to this differential biochemical selectivity.Acknowledgment. We thank Mr K. Rihs for preparation of the hydrazones and Prof. Dr K.H. Büchel for supply of hydrazone III.     

Biology of the CAPA peptides in insects

CAPA peptides have been isolated from a broad range of insect species as well as an arachnid, and can be grouped into the periviscerokinin and pyrokinin peptide families. In insects, CAPA peptides are the characteristic and most abundant neuropeptides in the abdominal neurohemal system. In many species, CAPA peptides exert potent myotropic effects on different muscles such as the heart. In others, including blood-sucking insects able to transmit serious diseases, CAPA peptides have strong diuretic or anti-diuretic effects and thus are potentially of medical importance. CAPA peptides undergo cell-type-specific sorting and packaging, and are the first insect neuropeptides shown to be differentially processed. In this review, we discuss the current knowledge on the structure, distribution, receptors and physiological actions of the CAPA peptides. Received 28 April 2006; received after revision 5 June 2006; accepted 4 July 2006     

Cyclotides: macrocyclic peptides with applications in drug design and agriculture

Cyclotides are disulfide-rich peptides from plants that are exceptionally stable as a result of their unique cyclic cystine knot structural motif. Their natural role is thought to be as plant defence agents, most notably against insect pests, but they also have potential applications in drug design and agriculture. This article identifies gaps in current knowledge on cyclotides and suggests future directions for research into this fascinating family of ultra-stable mini-proteins.     

Proline-rich antimicrobial peptides: converging to a non-lytic mechanism of action

Proline-rich antimicrobial peptides are a group of cationic host defense peptides of vertebrates and invertebrates characterized by a high content of proline residues, often associated with arginine residues in repeated motifs. Those isolated from some mammalian and insect species, although not evolutionarily related, use a similar mechanism to selectively kill Gram-negative bacteria, with a low toxicity to animals. Unlike other types of antimicrobial peptides, their mode of action does not involve the lysis of bacterial membranes but entails penetration into susceptible cells, where they then act intracellularly. Some aspects of the transport system and cytoplasmic targets have been elucidated. These features make them attractive both as anti-infective lead compounds and as a new class of potential cell-penetrating peptides capable of internalising membrane-impermeant drugs into both bacterial and eukaryotic cells     

Growth inhibitory, insecticidal and antifeedant effects of some antileukemic and cytotoxic quassinoids on two species of agricultural pests

Several quassinoids, obtained by isolation and derivatization from Simaba multiflora and Soulamea soulameoides, were evaluated for growth inhibitory and insecticidal effects against the tobacco budworm (Heliothis virescens) and for antifeedant effects against H. virescens and the fall armyworm (Spodoptera frugiperda). The relative activity of the quassinoids as insect growth inhibitors generally paralleled their known relative potency as antileukemic and cytotoxic agents.     

Pharmacologically active spider peptide toxins

Advances in mass spectrometry and peptide biochemistry coupled to modern methods in electrophysiology have permitted the isolation and identification of numerous novel peptide toxins from animal venoms in recent years. These advances have also opened up the field of spider venom research, previously unexplored due to methodological limitations. Many peptide toxins from spider venoms share structural features, amino acid composition and consensus sequences that allow them to interact with related classes of cellular receptors. They have become increasingly useful agents for the study of voltage-sensitive and ligand-gated ion channels and the discrimination of their cellular subtypes. Spider peptide toxins have also been recognized as useful agents for their antimicrobial properties and the study of pore formation in cell membranes. Spider peptide toxins with nanomolar affinities for their receptors are thus promising pharmacological tools for understanding the physiological role of ion channels and as leads for the development of novel therapeutic agents and strategies for ion channel-related diseases. Their high insecticidal potency can also make them useful probes for the discovery of novel insecticide targets in the insect nervous system or for the development of genetically engineered microbial pesticides.Received 19 March 2003; received after revision 9 May 2003; accepted 16 May 2003     

Activation of stress signalling pathways enhances tolerance of fungi to chemical fungicides and antifungal proteins

Fungal disease is an increasing problem in both agriculture and human health. Treatment of human fungal disease involves the use of chemical fungicides, which generally target the integrity of the fungal plasma membrane or cell wall. Chemical fungicides used for the treatment of plant disease, have more diverse mechanisms of action including inhibition of sterol biosynthesis, microtubule assembly and the mitochondrial respiratory chain. However, these treatments have limitations, including toxicity and the emergence of resistance. This has led to increased interest in the use of antimicrobial peptides for the treatment of fungal disease in both plants and humans. Antimicrobial peptides are a diverse group of molecules with differing mechanisms of action, many of which remain poorly understood. Furthermore, it is becoming increasingly apparent that stress response pathways are involved in the tolerance of fungi to both chemical fungicides and antimicrobial peptides. These signalling pathways such as the cell wall integrity and high-osmolarity glycerol pathway are triggered by stimuli, such as cell wall instability, changes in osmolarity and production of reactive oxygen species. Here we review stress signalling induced by treatment of fungi with chemical fungicides and antifungal peptides. Study of these pathways gives insight into how these molecules exert their antifungal effect and also into the mechanisms used by fungi to tolerate sub-lethal treatment by these molecules. Inactivation of stress response pathways represents a potential method of increasing the efficacy of antifungal molecules.     

‘La Guerre aux Insectes’: Pest Control and Agricultural Reform in the French Enlightenment

This paper examines the entomological investigations carried out by the French naturalist Henri-Louis Duhamel du Monceau during a series of insect epidemics that ravaged France in the second half of the eighteenth century. It shows how a particularly fierce invasion of caterpillars in the Angoumois region in the 1760s sparked theoretical debates about the nature of animal generation between academic naturalists, farmers, provincial officials and amateur naturalists. As part of a wider effort to reform agricultural production in France, Duhamel du Monceau sought to eliminate vernacular understandings of insect generation and to reform local pest control techniques. In his attempt to develop a body of pest-control knowledge grounded in the systematic observation of insect generation, however, Duhamel du Monceau relied heavily on the efforts of amateur naturalists. The paper shows how he mobilized a nation-wide network of entomological observers, and collected specimens and observational reports from farmers, improving landlords and local officials throughout France. Some informants did not only act as ‘mere’ observers, but formulated their own causal claims about insect generation that sometimes contradicted those of their metropolitan counterpart. Finally, it demonstrates that a ‘patriotic’ discourse that joined agricultural improvement and civic virtue provided a powerful impetus for the formation of collaborative relationships between academic naturalists, state officials and enlightened agricultural improvers.     

Properties and mechanisms of action of naturally occurring antifungal peptides

Antimicrobial peptides are a vital component of the innate immune system of all eukaryotic organisms and many of these peptides have potent antifungal activity. They have potential application in the control of fungal pathogens that are a serious threat to both human health and food security. Development of antifungal peptides as therapeutics requires an understanding of their mechanism of action on fungal cells. To date, most research on antimicrobial peptides has focused on their activity against bacteria. Several antimicrobial peptides specifically target fungal cells and are not active against bacteria. Others with broader specificity often have different mechanisms of action against bacteria and fungi. This review focuses on the mechanism of action of naturally occurring antifungal peptides from a diverse range of sources including plants, mammals, amphibians, insects, crabs, spiders, and fungi. While antimicrobial peptides were originally proposed to act via membrane permeabilization, the mechanism of antifungal activity for these peptides is generally more complex and often involves entry of the peptide into the cell.     

Chemical ecology of bark beetles

Summary The purview of chemical ecology and the recent criticisms of improper application of theory to bark beetle phenomena is briefly discussed. Seven levels of research in chemical ecology are presented as well as their relationship to research on bark beetles. The biology and chemical ecology of several pest bark beetles from North America and Europe are discussed in regard to host tree selection theories of random landing on trees or attraction to semiochemicals. The diversity and similarities of pheromone components among species are presented in relation to their biosynthesis from host tree precursors and in relation to the ecological implications of de novo or precursor syntheses. Individual variation in biosynthesis of, response to, and release of pheromones is discussed. Olfactory perception of semiochemicals at both the electrophysiological and behavioral levels is presented. Orientation to semiochemicals during walking and flying is discussed with reference to the significance of dose-response curves for determining a compound's functionality in short- or long-range communication. The regulation of attack density, termination of the aggregation, mechanisms of attack spacing, and recognition of host suitability are presented in the context of an individual's avoidance of intra- and interspecific competition. Finally, a brief summary of topics where our understanding of the chemical ecology of bark beetles and their associates is poorly known is presented.     

Evolutionary diversification of the mammalian defensins

Defensins are cysteine-rich cationic peptides that function in antimicrobial defense in both invertebrates and vertebrates. Three main groups of animal defensins are known: insect defensins; mammalian alpha-defensins and vertebrate beta-defensins. It has been difficult to determine whether these molecules are homologous or have independently evolved similar features, but overall the evidence favors a distant relationship. The best evidence of this relationship is structural, particularly from their overall three-dimensional structure and from the spacing of half-cystine residues involved in intra-chain disulfide bonds. Some evidence favors a closer relationship between vertebrate beta-defensins and insect defensins than between the two groups of vertebrate defensins. Examination of nucleotide substitutions between recently duplicated mammalian defensin genes shows that the rate of nonsynonymous (amino-acid-altering) substitution exceeds that of synonymous substitution in the region of the gene encoding the mature defensin. This highly unusual pattern of nucleotide substitution is evidence that natural selection has acted to diversify defensins at the amino acid level. The resulting rapid evolution explains why it is difficult to reconstruct the evolutionary history of these molecules.     

Growth inhibitory,insecticidal and antifeedant effects of some antileukemic and cytotoxic quassinoids on two species of agricultural pests

Summary Several quassinoids, obtained by isolation and derivatization fromSimaba multiflora andSoulamea soulameoides, were evaluated for growth inhibitory and insecticidal effects against the tobacco budworm (Heliothis virescens) and for antifeedant effects againstH. virescens and the fall armyworm (Spodoptera frugiperda). The relative activity of the quassinoids as insect growth inhibitors generally paralleled their known relative potency as antileukemic and cytotoxic agents.Phytochemical aspects of this work were supported by Contract CM-97295 with the Division of Cancer Treatment, National Cancer Institute, National Institutes of Health, Bethesda, Md. Insects were kindly supplied by the agency of the United States Department of Agriculture at Brownsville, TX.     

The mode of antifungal action of plant, insect and human defensins

Defensins are small (~5 kDa), basic, cysteine-rich antimicrobial peptides that fulfill an important role in the innate immunity of their host by combating pathogenic invading micro-organisms. Defensins can inhibit the growth or virulence of microorganisms directly or can do so indirectly by enhancing the host's immune system. Because of their wide distribution in nature, defensins are believed to be ancient molecules with a common ancestor that arose more than a billion years ago. This review summarizes current knowledge concerning the mode of antifungal action of plant, insect and human defensins.     

Diuretic and antifeedant actions byManduca sexta diuretic hormone in lepidopteran larvae

Neuroendocrine disruption of water balance in insect larvae was evaluated as the basis for a new approach to pest insect control. Effects on water balance and food consumption were measured for larvae ofHeliothis virescens (cotton budworm) treated with syntheticManduca sexta diuretic neurohormone (MasDH). Synthetic MasDH (50–250 nM) caused dose-dependent increases in fluid secretion by larval Malpighian tubules in vitro; higher concentrations resulted in lower fluid secretion. Last instar larvae injected with 20–100 pmol MasDH exhibited increasing weight loss. Larvae injected with 500 pmol MasDH dramatically reduced both water excretion and food consumption by 70%. These latter results indicate that high titers of diuretic hormone can suppress feeding damage by larval insects by depressing water excretion and food consumption.     

(Z)-9-tetradecen-1-ol and (Z)-9-tetradecenyl acetate: A potent attractant system for maleSesamia cretica Led. (Lep., Noctuidae)

Summary Field studies have shown that a combination of (Z)-9-tetradecenol with (Z)-9-tetradecenyl acetate is an effective attractant for maleSesamia cretica Led., a pest of sorghum crops. These studies also indicated that the most effective composition is 7525 (alcohol:acetate).We thank Dr E. Berio for the insect determination.     

Polyoxin fungicides: Demonstration of insecticidal activity due to inhibition of chitin synthesis

Summary Polyoxin A, a selective inhibitor of chitin synthetase, was found to be insecticidal when injected into the abdomen of grasshoppers nymphs, with an LD₅₀ of 1.26±0.20 g per insect. Symptomatology and the absence of any toxicity toward adult insects indicate that toxicity is due to interference with cuticle deposition.Acknowledgment. I thank Henry Bork for his expert technical assistance.     

Cell-penetrating peptides: tools for intracellular delivery of therapeutics

The main problem of therapeutic efficiency lies in the crossing of cellular membranes. Therefore, significant effort is being made to develop agents which can cross these barriers and deliver therapeutic agents into cellular compartments. In recent years, a large amount of data on the use of peptides as delivery agents has accumulated. Several groups have published the first positive results using peptides for the delivery of therapeutic agents in relevant animal models. These peptides, called cell-penetrating peptides (CPPs), are short peptides (fewer than 30 residues) with a net positive charge and acting in a receptor- and energy-independent manner. Here, we give an extensive review of peptide-mediated delivery systems and discuss their applications, with particular focus on the mechanisms leading to cellular internalization.Received 14 March 2005; received after revision 25 April 2005; accepted 28 April 2005