Towards understanding molecular modes of probiotic action

The possibility that certain microorganisms might be beneficial to human health is highlighted by the numerous consumer products containing probiotic bacteria. Probiotics are typically administered in food that, following entry into the gastro-intestinal tract, results in measurable health-promoting effects. Although there is a growing list of health benefits provided by the consumption of probiotics, their precise mechanisms of action remain largely unknown. Recent molecular- and genomics-based studies are starting to provide insight into the ways probiotic bacteria sense and adapt to the gastro-intestinal tract environment. Complementary approaches using host cell in vitro systems together with animal models and human volunteers are revealing specific intestinal cell responses to probiotics. These studies should ultimately disclose the molecular mechanisms and pinpoint the bacterial and host effector molecules and pathways by which probiotics are able to modulate human health.     

Juvenile hormone action: a 2007 perspective

Juvenile hormone (JH) is a key hormone in regulation of the insect's life history, both in maintaining the larval state during molts and in directing reproductive maturation. This short review highlights the recent papers of the past year that lend new insight into the role of this hormone in the larva and the mechanisms whereby it achieves this role.     

保幼激素的分子作用机制

蜕皮激素(ecdysteroids, Ecd)和保幼激素(juvenile hormone, JH)是调控昆虫发育和变态的两种最为重要的昆虫激素。尽管Ecd的分子作用机制已经相当明了,但是,因为迄今为止还没有成功地鉴定出JH受体,人们对JH的分子作用机制还了解甚少。本文从三个方面较为详尽地介绍了近年来JH分子作用机制的相关研究进展：1) JH和Ecd在分子水平上相互作用, JH可以通过改变或者抑制Ecd信号来调控昆虫的发育和变态;2) JH核受体的两个候选基因为Met和USP;3) JH还可以通过膜受体和蛋白激酶C传导信号。     

保幼激素的分子作用机制研究

保幼激素(juvenile hormone,JH)和蜕皮激素(20-hydroxyecdysone,20E)是协同调控昆虫发育、变态与生殖的两个重要激素。由于20E的主要分子作用机制已经比较明了,揭示JH的分子作用机制成为过去20多年来昆虫学领域研究的一个重点和难点。国内外多个研究团队利用赤拟谷盗Tribolium castaneum、果蝇Drosophilamelanogaster、烟草天蛾Manduca sexta等为模式,在JH受体的鉴定、JH在昆虫发育变态和生殖中的分子调控机制以及JH与20E在分子水平上的交互作用等方面开展了大量的研究工作,本文就近几年在这些方面取得的主要研究进展作一个综述。     

Molecular mechanism of the juvenile hormone action.

Structures, physiological role and level regulation of the juvenile hormones are described. A scheme of juvenile hormone mode of action at the molecular level, which includes transport of hormone via its binding protein, is presented.     

Mutant analysis as an experimental approach towards understanding plant hormone action

A variety of mutants altered in hormone biosynthesis and response pathways have been described. Genetic and molecular analyses of these mutants have contributed information on the control of plant developmental processes and on mechanisms of signal transduction. A sampling of hormone mutants and their role in elucidating modes of hormone action in higher plants is described.     

Use of Chlorella as a test system for understanding the mode of action of a chemical hybridising agent

Studies were made on Chlorella zofingiensis to elucidate the mode of action of a chemical hybridising agent (gametocide) for small grain cereals, azetidine-3-carboxylic acid (A3C). The compound reduced the rate of cell division, but undivided cells continued to grow in size up to 26 times the volume of the largest cells in the control medium. The total biomass of cultures was not affected. Electron microscopy revealed that cells treated with A3C contained no autospores. When samples were returned to control medium, however, division took place rapidly. The results led to the conclusion that A3C blocks cell division prior to the formation of partition membranes and walls. A hypothesis was erected to explain its effects on pollen and was subsequently tested. The test system described is suitable for screening other potential gametocides and assessing their modes of action.     

Stress-reactivity of a Drosophila melanogaster strain with impaired juvenile hormone action

Met(27) is a null allele of the Methoprene-tolerant gene of D. melanogaster that shows resistance to the toxic effects of both juvenile hormone (JH) and a JH analog, methoprene. The mechanism of resistance appears to be altered JH reception. We measured fertility, JH-hydrolyzing activity, and dopamine (DA) levels in Met(27) and Met(+) flies under normal (25 degrees C) and heat-stress (38 degrees C) conditions. We show that under normal conditions Met(27) females have JH-hydrolyzing activity and fertility lower than Met(+), but DA content did not differ between the two strains. At 38 degrees C Met(27) flies show no impairment in JH-hydrolyzing activity in response to stress, but they do show lower DA levels and impaired reproduction. The results with Met(27) are consistent with the previous hypothesis that the alteration in fertility that follows heat stress in D. melanogaster could result from alteration in the JH endocrine system.     

The action of a juvenile hormone synthetic analog--ether-farnesol on Xenopsylla cheopis fleas

The authors tested the effect of the analogue of juvenile hormone (AJH), ether-farnesol, synthesized at the Institute of Chemistry of the Academy of Sciences of ESSR (the city of Tallin), on immature stages of X. cheopis. When testing the following doses of AJH (0.0025, 0.00025, 0.000025, 0.0000025 ml per lg of substrate, consisting of sand and dry bull blood, for feeding larvae) it was established that the dose of 0.0025 ml/g causes 100% mortality of insects on the 2nd day of the experiment. 10 fold decrease in ether-farnesol dose causes the mortality of the majority of larvae. Only 13.6% of them formed cocoons, which failed to produce mature individuals. A repeated 10 and 100 fold decrease of AJH caused mortality in 50.0 and 46.7% of insects, respectively. The effect of the preparation stipulated the disturbance of the normal course of metamorphosis (changes in the data of phases replacement and the mortality of insects during their moulting).     

Trial of the action of synthetic juvenile hormone analogs on fleas

H-chlorphenyl ether of geraniol (1.5 to 0.05 %) caused the mortality of all preimaginal stages of fleas of the species C. anisus. 0.025 % dose of the preparation broke the normal course of metamorphosis that resulted in the delay of moulting periods of larvae, the formation of cocoons and hatching of imagos. All these deviations reduced the emergence of adults by to 90 %.     

Effect of juvenile hormone analog in a natural host-parasite system

Hormones mediate the physiological responses of animals to environmental changes. Consequently, hormones can be responsible of trade offs between different life history traits. Juvenile hormone (JH) is thought to mediate resource allocation in insects: specifically, it is thought to enhance the expression of condition-related traits like sexual signals, whilst reducing immune responsiveness. Here, we experimentally test whether a JH analog (JHa) had an effect on immunity of male dragonflies Celithemis eponina, and if such effects are translated into faster growth or development of a natural parasite (water mite). We also tested the effects of JHa on host condition (muscular mass and fat reserves) of mature male dragonflies. Mites from JHa treated dragonflies grew faster than mites from control dragonflies receiving just an acetone carrier. However, there was no effect of JHa on measures of host immune response (melanization of a nylon implant) or condition of mature males. We suggest that better parasite growth in JHa treated males does not result from the JH immunosuppressive function, but instead it appears that parasites receive hormone signals from the host and alter their development without affecting host condition measurably. Our work highlights the importance of measuring both immune parameters and response to real parasites when studying evolutionary trade offs.     

Structural basis for juvenile hormone biosynthesis by the juvenile hormone acid methyltransferase

Juvenile hormone (JH) acid methyltransferase (JHAMT) is a rate-limiting enzyme that converts JH acids or inactive precursors of JHs to active JHs at the final step of JH biosynthesis in insects and thus presents an excellent target for the development of insect growth regulators or insecticides. However, the three-dimensional properties and catalytic mechanism of this enzyme are not known. Herein, we report the crystal structure of the JHAMT apoenzyme, the three-dimensional holoprotein in binary complex with its cofactor S-adenosyl-l-homocysteine, and the ternary complex with S-adenosyl-l-homocysteine and its substrate methyl farnesoate. These structures reveal the ultrafine definition of the binding patterns for JHAMT with its substrate/cofactor. Comparative structural analyses led to novel findings concerning the structural specificity of the progressive conformational changes required for binding interactions that are induced in the presence of cofactor and substrate. Importantly, structural and biochemical analyses enabled identification of one strictly conserved catalytic Gln/His pair within JHAMTs required for catalysis and further provide a molecular basis for substrate recognition and the catalytic mechanism of JHAMTs. These findings lay the foundation for the mechanistic understanding of JH biosynthesis by JHAMTs and provide a rational framework for the discovery and development of specific JHAMT inhibitors as insect growth regulators or insecticides.     

Effect of juvenile hormone on the autonomic nervous system

The effects of juvenoids on the autonomic nervous system, which controls certain physiological functions by means of special extracardiac pulsations in hemolymph pressure, have been studied in pupae of the mealworm, Tenebrio molitor. Prolonged tensiometric monitoring of the hemolymph pressure changes revealed that the extra-pupal development that had been caused by juvenoids evoked unusual reappearance of prepupal pattern of extracardiac pulsations. By contrast, the specific pharate adult types of the pulsations completely disappeared. The functioning of the autonomic nervous system was aberrant or incomplete during an unsuccessful extra-pupal ecdysis. It is suggested that malfunction of the autonomic nervous system in the extra-pupal instars may be the reason for the ecdysial failures associated with the use of juvenoids.     

A mathematical model for the regulation of juvenile hormone titers

The titer of juvenile hormone (JH) is determined by three factors: its rate of synthesis, its rate of degradation, and the degree to which JH is protected from degradation by binding to a diversity of JH-binding proteins. All three of these factors vary throughout the life history of an insect and contribute to variation in the JH titer. The relative importance of each of these factors in determining variation in the JH titer is not known and can, presumably, differ in different life stages and different species. Here we develop a mathematical model for JH synthesis, degradation, and sequestration that allows us to describe quantitatively how each of these contribute to the titer of total JH and free JH in the hemolymph. Our model allows for a diversity of JH-binding proteins with different dissociation constants, and also for a number of different modes of degradation and inactivation. The model can be used to analyze whether data on synthesis and degradation are compatible with the observed titer data. We use the model to analyze two data sets, from Manduca and Gryllus, and show that in both cases, the known data on synthesis and degradation cannot account for the observed JH titers because the role of JH sequestration by binding proteins is greatly underestimated, and/or the in vivo rate of JH degradation is greatly overestimated. These analyses suggest that there is a critical need to develop a better understanding of the in vivo role of synthesis, sequestration and degradation in JH titer regulation.     

Cnidarians as a model system for understanding evolution and regeneration

Hydra and Podocolyne are two cnidarian animals which provide complementary advantages for analysing developmental mechanisms possibly reflecting the basic developmental processes shared by most bilaterians. Interestingly, these mechanisms remain accessible all along the life of these animals, which bud and regenerate, whatever their age. The Hydra polyp permits a direct study of the molecular cascades linking amputation to regeneration. Podocoryne displays a complete life cycle, polyp and medusa stages with a fast and inducible sexual cycle and an unparalleled In vitro transdifferentiation potential. In both cases, a large number of evolutionarily conserved molecular markers are available, and analysis of their regulation highlights the molecular mechanisms which underly pattern formation in these two species.     

Inhibition of insect development by simultaneous action of prothoracic gland hormone and juvenile hormone

Injections of large amounts (around 20 μg) of ecdysterone and of Cecropia C₁₇-juvenile hormone or its bio-analogue 6,7-epoxygeranyl 3,4-methylendioxyphenyl ether into a pupa of Tenebrio molitor may inhibit the apolysis and cuticle secretion. Some of the treated insects secrete a new cuticle only locally, others do not even undergo apolysis although they survive until the controls ecdyse as adults. The development of internal organs is also hindered. These results are tentatively explained in terms of a mutually antagonistic action of the two hormones on nucleic acid synthesis.