Showing papers by "Université libre de Bruxelles published in 1998"

AntNet: distributed stigmergetic control for communications networks

Abstract: This paper introduces AntNet, a novel approach to the adaptive learning of routing tables in communications networks. AntNet is a distributed, mobile agents based Monte Carlo system that was inspired by recent work on the ant colony metaphor for solving optimization problems. AntNet's agents concurrently explore the network and exchange collected information. The communication among the agents is indirect and asynchronous, mediated by the network itself. This form of communication is typical of social insects and is called stigmergy. We compare our algorithm with six state-of-the-art routing algorithms coming from the telecommunications and machine learning fields. The algorithms' performance is evaluated over a set of realistic testbeds. We run many experiments over real and artificial IP datagram networks with increasing number of nodes and under several paradigmatic spatial and temporal traffic distributions. Results are very encouraging. AntNet showed superior performance under all the experimental conditions with respect to its competitors. We analyze the main characteristics of the algorithm and try to explain the reasons for its superiority.

Carbon and carbonate metabolism in coastal aquatic ecosystems

Abstract: The coastal zone is where land, ocean, and atmosphere interact. It exhibits a wide diversity of geomorphological types and ecosystems, each one displaying great variability in terms of physical and biogeochemical forcings. Despite its relatively modest surface area, the coastal zone plays a considerable role in the biogeochemical cycles because it receives massive inputs of terrestrial organic matter and nutrients, is among the most geochemically and biologically active areas of the biosphere, and exchanges large amounts of matter and energy with the open ocean. Coastal ecosystems have therefore attracted much attention recently and are the focus of several current national and international research programs (e.g. LOICZ, ELOISE). The primary production, respiration, calcification, carbon burial and exchange with adjacent systems, including the atmosphere, are reviewed for the major coastal ecosystems (estuaries, macrophyte communities, mangroves, coral reefs, and the remaining continental shelf ). All ecosystems

Blocks, liquidity and corporate control

Abstract: The paper develops a simple model of corporate ownership structure in which costs and benefits of ownership concentration are analyzed The model cornpares the liquidity benefits obtained through dispersed corporate ownership with the benefits from efficient management control achieved by some degree of ownership concentration The paper reexamines the free-rider problem in corporate control in the presence of liquidity trading, derives predictions for the trade and pricing of blocks, and provides criteria for the optimal choice of ownership structure THE RECENT INCOMPLETE CONTRACTING approach in corporate finance has considerably improved our understanding of how small firms determine their capital structure The basic setting considered in this line of research is one where a founder-manager seeks funding from one or several financiers The main premise is that the founder-manager, in her dealings with the financiers, is primarily concerned with maintaining her private benefits of control For small firms these are often quite large relative to the financial returns Thus, for a small firm the problem of determining the financial structure often reduces to the problem of how to obtain fundingr while giving away as little control as possible to the financiers Of course, most financiers insist on some form of protection, so that the final compromise reached in most financial contracts for small firms is one resembling a debt contract (or a venture capital contract), which protects the founder-manager's control as long as the firm is performing adequately' This perspective for small firms does not extend naturally to large firms because the private benefits of control of the managers for large firms are likely to be small relative to the firm's monetary returns, so that protection of these benefits is not an overriding consideration Moreover, large firms

Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans

Abstract: 1. The adaptations of the ankle dorsiflexor muscles and the behaviour of single motor units in the tibialis anterior in response to 12 weeks of dynamic training were studied in five human subjects. In each training session ten series of ten fast dorsiflexions were performed 5 days a week, against a load of 30-40% of the maximal muscle strength. 2. Training led to an enhancement of maximal voluntary muscle contraction (MVC) and the speed of voluntary ballistic contraction. This last enhancement was mainly related to neural adaptations since the time course of the muscle twitch induced by electrical stimulation remained unaffected. 3. The motor unit torque, recorded by the spike-triggered averaging method, increased without any change in its time to peak. The orderly motor unit recruitment (size principle) was preserved during slow ramp contraction after training but the units were activated earlier and had a greater maximal firing frequency during voluntary ballistic contractions. In addition, the high frequency firing rate observed at the onset of the contractions was maintained during the subsequent spikes after training. 4. Dynamic training induced brief (2-5 ms) motor unit interspike intervals, or 'doublets'. These doublets appeared to be different from the closely spaced (+/-10 ms) discharges usually observed at the onset of the ballistic contractions. Motor units with different recruitment thresholds showed doublet discharges and the percentage of the sample of units firing doublets was increased by training from 5.2 to 32.7%. The presence of these discharges was observed not only at the onset of the series of spikes but also later in the electromyographic (EMG) burst. 5. It is likely that earlier motor unit activation, extra doublets and enhanced maximal firing rate contribute to the increase in the speed of voluntary muscle contraction after dynamic training.

Corporate strategies and environmental regulations: an organizing framework

Abstract: An emerging subfield of strategic management is that dealing with the natural environment as it affects corporate strategy. To analyze this we organize the literature on environmental regulations and corporate strategy into a new managerial framework. Next we develop a resource-based view of the interaction between firm-level competitiveness and environmental regulations, including the conditions for the use of green capabilities. Finally, we analyze the green capabilities of multinational enterprises within a standard international business model, using firm-specific advantages (FSAs) and country-specific advantages (CSAs). We then use this FSA/CSA configuration to explore hypotheses on environmental regulations, competitiveness, and corporate strategy. © 1998 John Wiley & Sons, Ltd.

Measurements of the NO2 absorption cross-section from 42 000 cm−1 to 10 000 cm−1 (238–1000 nm) at 220 K and 294 K

Abstract: The NO2 absorption cross-section has been measured from 42 000 to 10 000 cm−1 (238–1000 nm) with a Fourier transform spectrometer (at the resolution of 2 cm−1, 0.01 nm at 240 nm to 0.2 nm at 1000 nm) and a 5 m temperature controlled multiple reflection cell. The uncertainty on the cross-section is estimated to be less than 3% below 40 000 cm−1 (λ > 250 nm) at 294 K, 3% below 30 000 cm−1 (λ > 333 nm) at 220 K, but reaches 10% for higher wavenumbers. Temperature and pressure effects have been observed. Comparison with data from the literature generally shows a good agreement for wavenumbers between 37 500 and 20 000 cm−1 (267–500 nm). Outside these limits, the difference can reach several percent.

Chaos, Scattering and Statistical Mechanics

Abstract: 1. Dynamical systems and their linear stability 2. Topological chaos 3. Liouvillian dynamics 4. Probabalistic chaos 5. Chaotic scattering 6. Scattering theory of transport 7. Hydrodynamic modes of diffusion 8. Systems maintained out of equilibrium 9. Noises as microscopic chaos.

Let's Get Real: A Factor Analytical Approach to Disaggregated Business Cycle Dynamics

Abstract: This paper develops a method for analysing the dynamics of large cross-sections based on a factor analytic model. We use "law of large numbers" arguments to show that the number of common factors can be determined by a principal components method, the economy-wide shocks can be identified by means of simple structural VAR techniques and that the parameters of the unobserved factor model can be estimated consistently by applying OLS equation by equation. We distinguish between a technological and a non-technological shock. Identification is obtained by minimizing the negative realizations of the technology shock. Empirical results on 4-digit industrial output and productivity for the U.S. economy from 1958 to 1986 show that: (1) at least two economy-wide shocks, both having a long-run effect on sectoral output, are needed to explain the common dynamics; (2) although the technological shock accounts for at least 50% of the aggregate dynamics of output, it cannot by itself explain dynamics at business cycle frequencies; (3) sector-specific shocks explain the main bulk of total variance but generate mainly high frequency dynamics; (4) both the technological and the non-technological component of output show a peak for positive sectoral comovements of output at business cycle frequencies; (5) technological shocks are strongly correlated with the growth rates of the investment in machinery and equipment sectors and their inputs. Many interesting questions about cyclical fluctuations and economic growth can be answered only by studying the dynamic behaviour of sectoral variables. When data contain information on time for a large cross-section of sectors, traditional econometric techniques used in the macroeconomic literature such as Vector Autoregressive (VAR) and Vector Autoregressive Moving Average (VARMA) models are not appropriate since they require the estimation of too many parameters. This is why new methods which allow for the reduction of the parameter space need to be developed. The objective of this paper is both methodological and descriptive. At the methodological level we develop a simple framework for the dynamic analysis of large cross-sections. The basic model is a dynamic factor analytic model as in Sargent and Sims (1977). The sectoral variables are decomposed into two unobservable components: a common component, driven by macroeconomic shocks, and a purely sectoral component. When

Analysis of zinc binding sites in protein crystal structures.

Abstract: The geometrical properties of zinc binding sites in a dataset of high quality protein crystal structures deposited in the Protein Data Bank have been examined to identify important differences between zinc sites that are directly involved in catalysis and those that play a structural role. Coordination angles in the zinc primary coordination sphere are compared with ideal values for each coordination geometry, and zinc coordination distances are compared with those in small zinc complexes from the Cambridge Structural Database as a guide of expected trends. We find that distances and angles in the primary coordination sphere are in general close to the expected (or ideal) values. Deviations occur primarily for oxygen coordinating atoms and are found to be mainly due to H-bonding of the oxygen coordinating ligand to protein residues, bidentate binding arrangements, and multi-zinc sites. We find that H-bonding of oxygen containing residues (or water) to zinc bound histidines is almost universal in our dataset and defines the elec-His-Zn motif. Analysis of the stereochemistry shows that carboxyl elec-His-Zn motifs are geometrically rigid, while water elec-His-Zn motifs show the most geometrical variation. As catalytic motifs have a higher proportion of carboxyl elec atoms than structural motifs, they provide a more rigid framework for zinc binding. This is understood biologically, as a small distortion in the zinc position in an enzyme can have serious consequences on the enzymatic reaction. We also analyze the sequence pattern of the zinc ligands and residues that provide elecs, and identify conserved hydrophobic residues in the endopeptidases that also appear to contribute to stabilizing the catalytic zinc site. A zinc binding template in protein crystal structures is derived from these observations.

Response threshold reinforcements and division of labour in insect societies

Abstract: A model of division of labour in insect societies, based on variable response thresholds is introduced. Response thresholds refer to the likelihood of reacting to task–associated stimuli. Low–threshold individuals perform tasks at a lower level of stimulus than high–threshold individuals. Within individual workers, performing a given task induces a decrease in the corresponding threshold, and not performing the task induces an increase in the threshold. This combined reinforcement process leads to the emergence of specialized workers, i.e. workers that are more responsive to stimuli associated with particular task requirements, from a group of initially identical individuals. Predictions of the dynamics of task specialization resulting from this model are presented. Predictions are also made as to what should be observed when specialists of a given task are removed from the colony and reintroduced after a varying amount of time: the colony does not recover the same state as that prior to the perturbation, and the difference between before and after the perturbation is more strongly marked as the time between separation and reintroduction increases.

Current use of bisphosphonates in oncology. International Bone and Cancer Study Group.

Abstract: PURPOSEThe purpose of this article is to review the recent data on bisphosphonate use in oncology and to provide some guidelines on the indications for their use in cancer patients.DESIGNThe group consensus reached by experts on the rationale for the use of bisphosphonates in cancer patients and their current indications for the treatment of tumor-induced hypercalcemia and metastatic bone pain in advanced disease and for the prevention of the complications of multiple myeloma and of metastatic bone disease are reviewed.RESULTSBisphosphonates are potent inhibitors of tumor-induced osteoclast-mediated bone resorption. They now constitute the standard treatment for cancer hypercalcemia, for which we recommend a dose of 1,500 mg of clodronate or 90 mg of pamidronate; the latter compound is more potent and has a longer lasting effect. Intravenous bisphosphonates exert clinically relevant analgesic effects in patients with metastatic bone pain. Regular pamidronate infusions can also achieve a partial objective ...

A Model for Circadian Rhythms in Drosophila Incorporating the Formation of a Complex between the PER and TIM Proteins

Abstract: The authors present a model for circadian oscillations of the Period (PER) and Timeless (TIM) proteins in Drosophila. The model for the circadian clock is based on multiple phosphorylation of PER and TIM and on the negative feedback exerted by a nuclear PER-TIM complex on the transcription of the per and tim genes. Periodic behavior occurs in a large domain of parameter space in the form of limit cycle oscillations. These sustained oscillations occur in condi- tions corresponding to continuous darkness or to entrainment by light-dark cycles and are in good agreement with experimental observations on the tempo- ral variations of PER and TIM and of per and tim mRNAs. Birhythmicity (coex- istence of two periodic regimes) and aperiodic oscillations (chaos) occur in a restricted range of parameter values. The results are compared to the predictions of a model based on the sole regulation by PER. Both the formation of a complex between PER and TIM and protein phosphorylation are found to favor oscilla- tory behavior. Determining how the period depends on several key parameters allows us to test possible molecular explanations proposed for the altered period in the per l and per s mutants. The extended model further allows the construction of phase-response curves based on the light-induced triggering of TIM degrada- tion. These curves, established as a function of both the duration and magnitude of the effect of a light pulse, match the phase-response curves obtained experi- mentally in the wild type and per s mutant of Drosophila.

Possibility of spontaneous parity violation in hot QCD

Abstract: We argue that for QCD in the limit of a large number of colors, the axial $\mathrm{U}(1)$ symmetry of massless quarks is effectively restored at the deconfining phase transition. If this transition is of second order, metastable states in which parity is spontaneously broken can appear in the hadronic phase. These metastable states have dramatic signatures, including enhanced production of $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$ mesons, which can decay through parity violating decay processes such as $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$, and global parity odd asymmetries for charged pions.

The Transferable Belief Model for Quantified Belief Representation

Abstract: We present the transferable belief model (TBM), a model for the representation of quantified beliefs The model aims in representing the same concept as the Bayesian model, ie, the graded dispositions that guide ‘our’ behaviour We use the word ‘belief’ in a broad sense It could be replaced by quantified credibility, subjective support, strength of opinion These beliefs are not categorical as in modal logic, but admits degrees as in probability theory Our approach is normative The beliefs are held by an idealized rational agent, denoted by You This ‘You’ can be a human, but also a robot, a computer program

Familial Gestational Hyperthyroidism Caused by a Mutant Thyrotropin Receptor Hypersensitive to Human Chorionic Gonadotropin

Abstract: Some degree of stimulation of the thyroid gland by human chorionic gonadotropin is common during early pregnancy1–3 When serum chorionic gonadotropin concentrations are abnormally high — for example, in women with molar pregnancies — overt hyperthyroidism may ensue The pathophysiologic mechanism is believed to be promiscuous stimulation of the thyrotropin receptor by the excess chorionic gonadotropin4,5 The explanation for this stimulation is the close structural relations between chorionic gonadotropin and thyrotropin and between their receptors6 Hyperemesis gravidarum is characterized by excessive vomiting in early pregnancy, leading to the loss of 5 percent or more of body weight4 It

Degradation of carboxy-terminal-tagged cytoplasmic proteins by the Escherichia coli protease HflB (FtsH)

Abstract: Proteins with short nonpolar carboxyl termini are unstable in Escherichia coli. This proteolytic pathway is used to dispose of polypeptides synthesized from truncated mRNA molecules. Such proteins are tagged with an 11-amino-acid nonpolar destabilizing tail via a mechanism involving the 10Sa (SsrA) stable RNA and then degraded. We show here that the ATP-dependent zinc protease HflB (FtsH) is involved in the degradation of four unstable derivatives of the amino-terminal domain of the lcI repressor: three with nonpolar pentapeptide tails (cI104, cI105, cI108) and one with the SsrA tag (cI‐SsrA). cI105 and cI-SsrA are also degraded by the ClpP-dependent proteases. Loss of ClpP can be compensated for by overproducing HflB. In an in vitro system, cI108 and cI‐SsrA are degraded by HflB in an energy-dependent reaction, indicating that HflB itself recognizes the carboxyl terminus. These results establish a tail-specific pathway for removing abnormal cytoplasmic proteins via the HflB and Clp proteases.

Haematopoietic stem cell transplantation for sickle cell anaemia: the first 50 patients transplanted in Belgium.

Abstract: Fifty patients affected by sickle cell anaemia underwent transplantation of HLA-identical haematopoietic stem cells (bone marrow, 48; cord blood, 2). Two groups of patients were considered for transplantation. Group 1 included 36 permanent residents of a European country who, retrospectively, met the inclusion criteria accepted at a consensus conference held in Seattle in 1990, wherein children were selected because they already had evidence of a morbid course. Group 2 included 14 patients who were transplanted earlier, had not received more than three blood transfusions and were transplanted because they had decided to return to their country of origin. Kaplan-Meier estimates of overall survival, event-free survival and disease-free survival at 11 years of the whole grafted population are 93, 82 and 85%, respectively. In group 1, overall survival, EFS and DFS were 88, 76 and 80% and in group 2, 100, 93 and 93%, respectively. Clinical manifestations of the disease, as well as disease associated haemolytic anaemia, disappeared in all successfully treated patients. Recovery of spleen function was present in seven out of 10 evaluated patients. Adverse events (death, absence of engraftment, mixed chimerism and relapse) occurred more frequently in group 1 than in group 2 (25% vs 7%, P< 0.001). Acute graft-versus-host disease (GVHD) was present in 20 patients (grade I or II, 19; grade III, 1), chronic GVHD in 10 (limited, 7; extensive, 3). One patient developed an acute myeloid leukaemia. Gonadal dysfunction was present in all patients (six boys and eight girls) transplanted close to or after puberty, although transient in one adolescent girl.

The Δccr5 Mutation Conferring Protection Against HIV-1 in Caucasian Populations Has a Single and Recent Origin in Northeastern Europe

Abstract: The chemokine receptor CCR5 is encoded by the CMKBR5 gene located on the p21.3 region of human chromosome 3, and constitutes the major co-receptor for the macrophage-tropic strains of HIV-1. A mutant allele of the CCR5 gene, Delta ccr5 , was shown to provide to homozygotes with a strong resistance against infection by HIV. The frequency of the Delta ccr5 allele was investigated in 18 European populations. A North to South gradient was found, with the highest allele frequencies in Finnish and Mordvinian populations (16%), and the lowest in Sardinia (4%). Highly polymorphic microsatellites (IRI3.1, D3S4579 and IRI3.2, D3S4580 ) located respectively 11 kb upstream and 68 kb downstream of the CCR5 gene deletion were used to determine the haplotype of the chromosomes carrying the Delta ccr5 variant. A strong linkage disequilibrium was found between Delta ccr5 and specific alleles of the IRI3.1 and IRI3.2 microsatellites: >95% of the Delta ccr5 chromosomes carried the IRI3.1-0 allele, while 88% carried the IRI3.2-0 allele. These alleles were found respectively in only 2 or 1.5% of the chromosomes carrying a wild-type CCR5 gene. From these data, it was inferred that most, if not all Delta ccr5 alleles originate from a single mutation event, and that this mutation event probably took place a few thousand years ago in Northeastern Europe. The high frequency of the Delta ccr5 allele in Caucasian populations cannot be explained easily by random genetic drift, suggesting that a selection advantage is or has been associated with homo- or heterozygous carriers of the Delta ccr5 allele.

ChemR23, a putative chemoattractant receptor, is expressed in monocyte-derived dendritic cells and macrophages and is a coreceptor for SIV and some primary HIV-1 strains.

Abstract: Leukocyte chemoattractants act through a rapidly growing subfamily of G protein-coupled receptors. We report the cloning of a novel human gene encoding an orphan receptor (ChemR23) related to the C3a, C5a and formyl Met-Leu-Phe receptors, and more distantly to the subfamilies of chemokine receptors. ChemR23 transcripts were found to be abundant in monocyte-derived dendritic cells and macrophages, treated or not with LPS. Low expression could also be detected by reverse transcription-PCR in CD4+ T lymphocytes. The gene encoding ChemR23 was assigned by radiation hybrid mapping to the q21.2-21.3 region of human chromosome 12, outside the gene clusters identified so far for chemoattractant receptors. Given the increasing number of chemoattractant receptors used by HIV-1, HIV-2 and SIV as coreceptors, ChemR23 was tested in fusion assays for potential coreceptor activity by a range of viral strains. None of the tested HIV-2 strains made use of ChemR23 as a coreceptor, but several SIV strains (SIVmac316, SIVmac239, SIVmacl7E-Fr and SIVsm62A), as well as a primary HIV-1 strain (92UG024-2) used it efficiently. ChemR23 therefore appears as a coreceptor for immunodeficiency viruses that does not belong to the chemokine receptor family. It is also a putative chemoattractant receptor relatively specific for antigen-presenting cells, and it could play an important role in the recruitment or trafficking of these cell populations. Future work will be required to identify the ligand(s) of this new G protein-coupled receptor and to define its precise role in the physiology of dendritic cells and macrophages.