Perturbation calculation of thermodynamic density of states.

The density of states g (ε) is frequently used to calculate the temperature-dependent properties of a thermodynamic system. Here a derivation is given for calculating the warped density of states g*(ε) resulting from the addition of a perturbation. The method is validated for a classical Heisenberg model of bcc Fe and the errors in the free energy are shown to be second order in the perturbation. Taking the perturbation to be the difference between a first-principles quantum-mechanical energy and a corresponding classical energy, this method can significantly reduce the computational effort required to calculate g(ε) for quantum systems using the Wang-Landau approach.

Convergence for the Wang-Landau density of states.

The Wang-Landau method of estimating the density of states g(E) has become a powerful tool in statistical mechanics. Here it is shown that the distribution of random walkers sampled using an estimated density of states can always be used to improve the estimate. Specifically, this can be done without resorting to an auxiliary modification factor f, which previously has been used to find g(E) self-consistently through a procedure that reduces f incrementally toward unity. This straightforward approach is validated for multiple, independent random walkers.

The CXCR4 antagonist AMD3100 impairs survival of human AML cells and induces their differentiation.

The chemokine stromal cell-derived factor-1 (SDF-1) and its receptor, CXCR4, participate in the retention of acute myeloblastic leukemia (AML) cells within the bone marrow microenvironment and their release into the circulation. AML cells also constitutively express SDF-1-dependent elastase, which regulates their migration and proliferation. To study the molecular events and genes regulated by the SDF-1/CXCR4 axis and elastase in AML cells, we examined gene expression profiles of the AML cell line, U937, under treatment with a neutralizing anti-CXCR4 antibody or elastase inhibitor, as compared with non-treated cells, using DNA microarray technology. Unsupervised hierarchical clustering analysis demonstrated similar gene expression profiles of anti-CXCR4 antibody or elastase inhibitor-treated cells, as compared with control. Pathway and functional analysis showed a greater tendency toward differentiation in cells under either one of both treatment modalities. Thus given, we further analyzed the effects of CXCR4 inhibition on AML cell growth and differentiation using the antagonist AMD3100. AMD3100 arrested proliferation in AML cell lines and triggered changes that mimicked differentiation, including morphological changes and the expression of myeloid differentiation antigens. Inhibition of elastase also triggered the differentiation of AML cells. Our study defines a new role for the SDF-1/CXCR4 axis in the regulation of leukemic cell survival and differentiation.

The sperm chemoattractant secreted from human cumulus cells is progesterone.

BACKGROUND: Human spermatozoa appear to be guided by chemotaxis to the oocyte in the female genital tract. While one of the sources of sperm chemoattractants is the cumulus cells that surround the oocyte, the identity of the chemoattractant secreted from them is unknown. Progesterone, recognized to be secreted from cumulus cells, was demonstrated, at the pM concentration range, to be a chemoattractant for human spermatozoa. Here, we examined whether this steroid is the cumulus-originated chemoattractant for human spermatozoa. METHODS: Human cumulus cells were cultured, and the cultured medium was demonstrated to be chemotactically active. Progesterone was then eliminated from the medium by a specific anti-progesterone antibody, and the residual chemotactic activity was assessed. RESULTS: The rate of progesterone secretion from the cells decreased with time. Removal of progesterone from the cumulus-cultured medium resulted in total loss of the chemotactic activity of the medium. Furthermore, the cumulus-cultured medium could substitute for progesterone in stimulating changes in the intracellular Ca(2+) concentration in the spermatozoa, and the changes were very similar to those caused by measured progesterone concentrations in the medium. CONCLUSIONS: Taken together, the results suggest that progesterone is the main, if not the sole, chemoattractant secreted by human cumulus cells.

Epitaxial stabilization of ferromagnetism in the nanophase of FeGe.

Epitaxial nanocrystals of FeGe have been stabilized on Ge(111). The nanocrystals assume a quasi-one-dimensional shape as they grow exclusively along the <110> direction of the Ge(111) substrate, culminating in a compressed monoclinic modification of FeGe. Whereas monoclinic FeGe is antiferromagnetic in the bulk, the nanowires are surprisingly strong ferromagnets below approximately 200 K with an average magnetic moment of 0.8 microB per Fe atom. Density functional calculations indicate an unusual stabilization mechanism for the observed ferromagnetism: lattice compression destabilizes the antiferromagnetic Peierls-like ground state observed in the bulk while increased p-d hybridization suppresses the magnetic moments and stabilizes ferromagnetism.

Changing the direction of flagellar rotation in bacteria by modulating the ratio between the rotational states of the switch protein FliM.

One of the major questions in bacterial chemotaxis is how the switch, which controls the direction of flagellar rotation, functions. It is well established that binding of the signaling molecule CheY to the switch protein FliM shifts the rotation from the default direction, counterclockwise, to clockwise. How this shift is done is still a mystery. Our aim in this study was to determine the correlation between the fraction of FliM molecules in the clockwise state (i.e. occupied by CheY) and the probability of clockwise rotation. For this purpose we gradually expressed, from a plasmid, a clockwise FliM mutant protein in cells that express, from the chromosome, wild-type FliM but no chemotaxis proteins. We verified that plasmid-borne FliM exchanges chromosomal FliM in the switch. Surprisingly, a substantial clockwise probability was not obtained before the large majority of the FliM molecules in the switch were clockwise molecules. Thereafter, the rise in clockwise probability was very steep. These results suggest that an increase in the clockwise probability requires a high level of FliM occupancy by CheY approximately P. They further suggest that the steep increase in clockwise rotation upon increasing CheY levels, reported in several studies, is due, at least in part, to cooperativity of post-binding interactions within the switch. We also carried out the inverse experiment, in which wild-type FliM was gradually expressed in a background of a clockwise fliM mutant. In this case, the level of the clockwise mutant protein, required for establishing a certain clockwise probability, was lower than in the original experiment. If our system (in which the ratio between the rotational states of FliM in the switch is established by slow exchange) and the native system (in which the ratio is established by fast changes in FliM occupancy) are comparable, the results suggest that hysteresis is involved in the switch function. Such a situation might reflect a damping mechanism, which prevents a situation in which fluctuations in the phosphorylation level of CheY throw the switch from one direction of rotation to the other.

Acetylation of the response regulator, CheY, is involved in bacterial chemotaxis.

It is well established that the response regulator of the chemotaxis system of Escherichia coli, CheY, can undergo acetylation at lysine residues 92 and 109 via a reaction mediated by acetyl-CoA synthetase (Acs). The outcome is activation of CheY, which results in increased clockwise rotation. Nevertheless, it has not been known whether CheY acetylation is involved in chemotaxis. To address this question, we examined the chemotactic behaviour of two mutants, one lacking the acetylating enzyme Acs, and the other having an arginine-for-lysine substitution at residue 92 of CheY - one of the acetylation sites. The Deltaacs mutant exhibited much reduced sensitivity to chemotactic stimuli (both attractants and repellents) in tethering assays and greatly reduced responses in ring-forming, plug and capillary assays. Likewise, the cheY(92KR) mutant had reduced sensitivity to repellents in tethering assays and a reduced response in capillary assays. However, its response to the addition or removal of attractants was normal. These observations suggest that Acs-mediated acetylation of CheY is involved in chemotaxis and that the acetylation site Lys-92 is only involved in the response to repellents. The observation that, in the cheY(92KR) mutant, the addition of a repellent was not chemotactically equivalent to the removal of an attractant also suggests that there are different signalling pathways for attractants and repellents in E. coli.

Mammalian sperm chemotaxis and its association with capacitation.

Much progress has been made in recent years in establishing mammalian sperm chemotaxis and understanding sperm capacitation. Thus far, chemotaxis to follicular fluid has been established by a variety of means in human and mouse spermatozoa. It was found that only a small fraction of a given sperm population (averaging around 10%) is chemotactically responsive and that this fraction constitutes capacitated (ripe) spermatozoa. Both the chemotactic responsiveness and the capacitated state are transient (with a lifetime of 50 min to 4 h) and they occur only once in the sperm's lifetime. It has been proposed that the role of sperm chemotaxis in mammals (at least in humans) is selective recruitment of capacitated spermatozoa for fertilizing the egg, and that the role of the continuous replacement of chemotactic/capacitated spermatozoa is to prolong the time during which capacitated spermatozoa are available in the female reproductive tract. The sperm chemoattractants have not been identified, but they appear to be heat-stable peptides. Although the molecular mechanism and the in vivo location of sperm chemotaxis are not known, a number of possible mechanisms and locations are discussed.

Identification of the binding interfaces on CheY for two of its targets, the phosphatase CheZ and the flagellar switch protein fliM.

CheY is the response regulator protein serving as a phosphorylation-dependent switch in the bacterial chemotaxis signal transduction pathway. CheY has a number of proteins with which it interacts during the course of the signal transduction pathway. In the phosphorylated state, it interacts strongly with the phosphatase CheZ, and also the components of the flagellar motor switch complex, specifically with FliM. Previous work has characterized peptides consisting of small regions of CheZ and FliM which interact specifically with CheY. We have quantitatively measured the binding of these peptides to both unphosphorylated and phosphorylated CheY using fluorescence spectroscopy. There is a significant enhancement of the binding of these peptides to the phosphorylated form of CheY, suggesting that these peptides share much of the binding specificity of the intact targets of the phosphorylated form of CheY. We also have used modern nuclear magnetic resonance methods to characterize the sites of interaction of these peptides on CheY. We have found that the binding sites are overlapping and primarily consist of residues in the C-terminal portion of CheY. Both peptides affect the resonances of residues at the active site, indicating that the peptides may either bind directly at the active site or exert conformational influences that reach to the active site. The binding sites for the CheZ and FliM peptides also overlap with the previously characterized CheA binding interface. These results suggest that interaction with these three proteins of the signal transduction pathway are mutually exclusive. In addition, since these three proteins are sensitive to the phosphorylation state of CheY, it may be that the C-terminal region of CheY is most sensitive for the conformational changes occurring upon phosphorylation.

Detection of partial and complete acrosome reaction in human spermatozoa: which inducers and probes to use?

The acrosome reaction (AR), an essential step for achieving mammalian fertilization, was recently introduced as a means of clinical evaluation of male fertility. However, most of the available techniques for acrosomal status assessment (except those employing electron microscopy) do not define whether the measurements represent partial or complete AR. We, therefore, performed a crossover investigation of the types of inducers and probes required for detecting partial or complete AR in human spermatozoa. The acrosomal status before and after stimulation with four AR inducers was evaluated after incubation for 3 h in capacitating conditions. We used a fluorescence-activated cell sorter with fluorescein isothiocyanate-conjugated monoclonal antibody CD46 (FITC-CD46) targeting the inner acrosomal membrane for detecting a complete AR, and fluorescein isothiocyanate-Pisum sativum agglutinin (FITC-PSA) targeting the acrosomal content for detection of both partial and complete AR. Without stimulation or following stimulation with progesterone, follicular fluid (FF) or phorbol myristate ester (PMA), the AR could be detected with FITC-PSA but not with FITC-CD46. Following stimulation with the calcium ionophore A23187, the AR could be detected by both FITC-PSA and FITC-CD46. These results suggest that spontaneous AR as well as AR induced by progesterone, PMA and FF are partial. In contrast, the AR induced by A23187 is total, i.e. both partial and complete. These findings are valuable for both research and clinical purposes and are a step towards an international agreement on a standard test for human sperm AR, for which there is an urgent need.

Do human eggs attract spermatozoa?

A key process in human fertilization is bringing the two gametes together, so that the complex molecular events involved in sperm and egg interaction can begin. Does nature allow fertilization to occur only as a consequence of a chance collision, or is there a precontact sperm-egg communication? This review summarizes the bioassays used in testing human spermatozoa for chemotaxis, emphasizing the necessity to distinguish between chemotaxis and other accumulation-causing processes, and the results obtained. It demonstrates that human sperm chemotaxis to a follicular factor(s) does occur, at least in vitro, and that only capacitated spermatozoa are chemotactically responsive. Substances that have been proposed as attractants for human spermatozoa are reassessed. The potential role of sperm chemotaxis in vivo is discussed. Faulty precontact sperm-egg communication may be one of the causes of male infertility, female infertility, or both. On the other hand, interfering with human sperm chemotaxis may represent an exciting new approach to contraception.

Human sperm chemotaxis: is progesterone a chemoattractant?

Follicular fluid (FF) induces sperm chemotaxis in human spermatozoa. Progesterone also causes sperm accumulation. However, sperm accumulation can be caused by chemotaxis, chemokinesis, and trapping of various kinds. It has been suggested that progesterone also induces chemotaxis in human spermatozoa. In view of the physiological significance of sperm chemotaxis in human fertilization and its potential clinical implications, it is important to determine unequivocally whether chemotaxis is induced by progesterone and, if so, whether progesterone in FF is the chemoattractant. To resolve these questions we looked for characteristic changes in the direction of sperm swimming toward pure progesterone as well as toward FF before and after progesterone removal. Progesterone caused sperm accumulation and hyperactivation-like motility, but it caused very few changes in the direction of sperm swimming that are characteristic of chemotaxis. Removal of progesterone (and other steroids) from FF by charcoal treatment abolished the sperm hyperactivation-like motility but not sperm chemotaxis. These results suggest that while progesterone might be a weak chemoattractant, it is not the major chemoattractant in FF. Progesterone probably causes human sperm accumulation mainly by inducing hyperactivation-like motility and, as a consequence, sperm trapping.

Chemotactic-like response of Escherichia coli cells lacking the known chemotaxis machinery but containing overexpressed CheY.

We describe a chemotactic-like response of Escherichia coli strains lacking most of the known chemotaxis machinery but containing high levels of the response regulator CheY. The bacteria accumulated in aspartate-containing capillaries, they formed rings on tryptone-containing semisolid agar, and the probability of counterclockwise flagellar rotation transiently increased in response to stimulation with aspartate (10(-10)-10(-5) M; the response was inverted at > 10(-4) M). The temporal response was partial and delayed, as was the response of a control wild-type strain having a high CheY level. alpha-Methyl-DL-aspartate, a non-metabolizable analogue of aspartate as well as other known attractants of E. Coli, glucose and, to a lesser extent, galactose, maltose and serine caused a similar response. So did low concentrations of acetate and benzoate (which, at higher concentrations, act as repellents for wild-type E. coli). Other tested repellents such as indole, Ni2+ and CO2+ increased the clockwise bias. These observations raise the possibility that, at least when the conventional signal transduction components are missing, a non-conventional chemotactic signal transduction pathway might be functional in E. coli. Potential molecular mechanisms are discussed.

Sperm chemotaxis.

Communication between spermatozoa and egg before contact by chemotaxis appears to be prevalent throughout the animal kingdom. In non-mammalian species, sperm chemotaxis to factors secreted from the egg is well documented. In mammals, sperm chemotaxis to follicular factors in vitro has been established in humans and mice. The attractants of female origin in non-mammalian species are heat-stable peptides or proteins of various sizes, or other small molecules, depending on the species. Species specificity of the attractants in non-mammalian species may vary from high species specificity, through specificity to families with no specificity within a family, to absence of specificity. The mammalian sperm attractants have not been identified but they appear to be heat-stable peptides. The claim that progesterone is the attractant for human spermatozoa has failed to be substantiated, neither have claims for other mammalian sperm attractants been verified. The molecular mechanism of sperm chemotaxis is not known. Models involving modulation of the intracellular Ca2+ concentration have been proposed for both mammalian and non-mammalian sperm chemotaxis. The physiological role of sperm chemotaxis in non-mammalian species appears to differ from that in mammals. In non-mammalian species, sperm chemotaxis strives to bring as many spermatozoa as possible to the egg. However, in mammals, the role appears to be recruitment of a selective population of capacitated ('ripe') spermatozoa to fertilize the egg.

Regulation of phosphatase activity in bacterial chemotaxis.

Bacterial chemotaxis is the most studied model system for signaling by the widely spread family of two-component regulatory systems. It is controlled by changes in the phosphorylation level of the chemotactic response regulator, CheY, mediated by a histidine kinase (CheA) and a specific phosphatase (CheZ). While it is known that CheA activity is regulated, via the receptors, by chemotactic stimuli, the input that may regulate CheY dephosphorylation by CheZ has not been found. We measured, by using stopped-flow fluorometry, the kinetics of CheZ-mediated dephosphorylation of CheY. The onset of dephosphorylation was delayed by approximately 50 ms after mixing phosphorylated CheY (CheY approximately P) with CheZ, and a distinct overshoot was observed in the approach to the new steady state of CheY approximately P. The delay and overshoot were not observed in a hyperactive mutant CheZ protein (CheZ54RC) that does not support chemotaxis in vivo and appears to be constitutively active. CheZ activity was cooperative with respect to CheY approximately P, with a Hill-coefficient of 2.5. The observed delayed modulation of CheZ activity and its cooperativity suggest that the phosphatase activity is regulated at the level of CheY approximately P-CheZ interaction. This novel kind of interplay between a response regulator and its phosphatase may be involved in signal tuning and in adaptation to chemotactic signals.

Bacterial chemotaxis: unsolved mystery of the flagellar switch.

Impressive progress has been made in understanding the mechanism of bacterial chemotaxis and function of the flagellar motor, but how the direction of rotation is reversed by the 'flagellar switch'--a central step in chemotaxis--remains obscure and calls for new experimental approaches.

Fumarate modulates bacterial flagellar rotation by lowering the free energy difference between the clockwise and counterclockwise states of the motor.

Switching flagellar rotation from one direction to another is an essential part of bacterial chemotaxis. Fumarate has been shown to possess the capacity to restore to flagella of cytoplasm-free, CheY-containing bacterial envelopes the ability to switch directions and to increase the probability of reversal in intact cells. Neither the target of fumarate action nor the mechanism of function is known. To distinguish between the two potential targets of fumarate, the response regulator CheY and the flagellar switch-motor complex, we compared flagellar rotation between isogenic strains that lacked CheY and had either low or high levels of fumarate. The difference in the fumarate levels was due to a deletion of the genes encoding the enzymes that synthesize and metabolize fumarate; succinate dehydrogenase and fumarase, respectively. The strains were in a gutted background (i.e. a background deleted for the cytoplasmic chemotaxis proteins and some of the receptors), and switching was achieved by carrying out the measurements at 2.5 degreesC, where it has been demonstrated that gutted cells switch spontaneously. The flagellar rotation of the strain with the highest level of fumarate was the most clockwise-biased and had the highest reversal frequency, indicating that fumarate is effective even in the absence of CheY. Fumarate reduced the free energy difference of the counterclockwise-to-clockwise transition and had no appreciable effect on the activation energy of this transition. Similar observations were made at room temperature, provided that intracellular CheY was present. In a wild-type background, both mutants made rings on semi-solid agar typical of normal chemotaxis. Taken together, the results suggest that the target of fumarate is the switch-motor complex, that fumarate acts by increasing the probability of the clockwise state, and that a fumarate level as low as that found in succinate dehydrogenase mutants is sufficient for normal chemotaxis.

The N terminus of the flagellar switch protein, FliM, is the binding domain for the chemotactic response regulator, CheY.

A key event in signal transduction during chemotaxis of Salmonella typhimurium and related bacterial species is the interaction between the phosphorylated form of the response regulator CheY (CheY approximately P) and the switch of the flagellar motor, located at its base. The consequence of this interaction is a shift in the direction of flagellar rotation from the default, counterclockwise, to clockwise. The docking site of CheY approximately P at the switch is the protein FliM. The purpose of this study was to identify the CheY-binding domain of FliM. We cloned 17 fliM mutants, each defective in switching and having a point mutation at a different location, and then overexpressed and purified their products. The CheY-binding ability of each of the FliM mutant proteins was determined by chemical crosslinking. All the mutant proteins with an amino acid substitution at the N terminus, FliM6LI, FliM7SY and FliM10EG, bound CheY approximately P to a much lesser extent than did wild-type FliM. CheY approximately P-binding of the other mutant proteins was similar to wild-type FliM. To investigate whether the FliM domain that includes these three mutations is indeed the CheY-binding domain, we synthesized a peptide composed of the first 16 amino acid residues of FliM, including a highly conserved region of FliM (residues 6 to 15). The peptide bound CheY and, to a larger extent, CheY approximately P. It also competed with full-length FliM on CheY approximately P. These results indicate that the CheY-binding domain of FliM is located at the N terminus, within residues 1 to 16, and suggest that FliM monomers can form a complete site for CheY binding.

Regulation of switching frequency and bias of the bacterial flagellar motor by CheY and fumarate.

The effect of CheY and fumarate on switching frequency and rotational bias of the bacterial flagellar motor was analyzed by computer-aided tracking of tethered Escherichia coli. Plots of cells overexpressing CheY in a gutted background showed a bell-shaped correlation curve of Switching frequency and bias centering at about 50% clockwise rotation. Gutted cells (i.e., with cheA to cheZ deleted) with a low CheY level but a high cytoplasmic fumarate concentration displayed the same correlation of switching frequency and bias as cells overexpressing CheY at the wild-type fumarate level. Hence, a high fumarate level can phenotypically mimic CheY overexpression by simultaneously changing the switching frequency and the bias. A linear correlation of cytoplasmic fumarate concentration and clockwise rotation bias was found and predicts exclusively counter-clockwise rotation without switching when fumarate is absent. This suggests that (i) fumarate is essential for clockwise rotation in vivo and (ii) any metabolically induced fluctuation of its cytoplasmic concentration will result in a transient change in bias and switching probability. A high fumarate level resulted in a dose-response curve linking bias and cytoplasmic CheY concentration that was offset but with a slope similar to that for a low fumarate level. It is concluded that fumarate and CheY act additively presumably at different reaction steps in the conformational transition of the switch complex from counterclockwise to clockwise motor rotation.