Stringent response activates quorum sensing and modulates cell density-dependent gene expression in Pseudomonas aeruginosa.

During nutrient starvation, Escherichia coli elicits a stringent response involving the ribosome-associated protein RelA. Activation of RelA results in a global change in the cellular metabolism including enhanced expression of the stationary-phase sigma factor RpoS. In the human pathogen Pseudomonas aeruginosa, a complex quorum-sensing circuitry, linked to RpoS expression, is required for cell density-dependent production of many secreted virulence factors, including LasB elastase. Quorum sensing relies on the activation of specific transcriptional regulators (LasR and RhlR) by their corresponding autoinducers (3-oxo-C(12)-homoserine lactone [HSL] and C(4)-HSL), which function as intercellular signals. We found that overexpression of relA activated the expression of rpoS in P. aeruginosa and led to premature, cell density-independent LasB elastase production. We therefore investigated the effects of the stringent response on quorum sensing. Both lasR and rhlR gene expression and autoinducer synthesis were prematurely activated during the stringent response induced by overexpression of relA. Premature expression of lasR and rhlR was also observed when relA was overexpressed in a PAO1 rpoS mutant. The stringent response induced by the amino acid analogue serine hydroxamate (SHX) also led to premature production of the 3-oxo-C(12)-HSL autoinducer. This response to SHX was absent in a PAO1 relA mutant. These findings suggest that the stringent response can activate the two quorum-sensing systems of P. aeruginosa independently of cell density.

A dynamic analysis of knee ligament injuries in alpine skiing.

The aim of this study is to understand better the dynamic mechanisms of anterior cruciate ligament (ACL) rupture in alpine skiing. As a result of these findings, improvements in the boot-binding release systems are proposed. Six patients who sustained ACL rupture, aged 19 to 43 years, were studied. A step by step reconstruction of the accident was made using a questionnaire followed by an interview. The skiing equipment was tested in a specialized laboratory and the release levels of the boot-binding systems were measured in twist and in forward lean. ACL injury was confirmed by arthroscopy or surgery. Most patients had recent skiing equipment; their binding systems were correctly mounted and in perfect working order. Release levels of the bindings were equal to or slightly above those suggested by the ISO standards. The mechanisms of injury were valgus-external rotation in knee flexion of more than 90 degrees, internal rotation in knee flexion of more than 120 degrees with anterior drawer loads, internal rotation-valgus in knee flexion of less than 30 degrees with anterior drawer loads, external rotation-valgus in knee flexion of more than 120 degrees with anterior drawer loads, external rotation in knee flexion of about 100 degrees with anterior drawer loads and internal rotation-valgus. Even with a modern boot-binding system, properly adjusted, ACL injury remains possible. The improvements in the current equipment should be concerned with the behavior when confronted with combined loads producing friction in the system. We concluded that for lateral release at the toe, there should be a better compensation of the vertical parasite forces and an improvement of the antifriction devices. In order to prevent excessive anterior drawer loads on the ACL an additional release direction of the binding system, especially backwards, could, if properly adjusted, enhance the opportunities of early release in a dangerous backward lean "out of control" position.

[Experimental study of external fixation of femoral fractures. Mechanical properties of different kinds of fixation. Clinical implications]. / Analyse expérimentale de la fixation externe des fracas du fémur. Propriétés mécaniques de différents types de montage. Implications cliniques.

An in vitro assessment of the mechanical performance of 10 types of external femoral stabilization allowed the amplitudes of fracture site displacements during the initial loading phase of a comminuted fracture treated by external fixation to be established. The largest displacement to load ratios were observed in the sagittal plane for all tested configurations. The highest fixator stiffnesses were obtained by using large diameter tubes and pins. The most rigid fixators have a derigidification system which creates the problem of choosing the ideal time to change the rigidity.

Species specificity in cell-substrate interactions in medusae.

A new system is described for the study of ECM-tissue interactions, using the ECM (called mesogloea) of various cnidarians and isolated striated muscle and endodermal tissue of jellyfish. The mesogloea consists mainly of water and collagen. It is present in all cnidarians and can be isolated without enzyme treatment. It can be used as a substrate to which cells and tissues adhere and on which they spread and migrate. Tissues of striated muscle and endoderm adhere and spread not only on mesogloea from regions they normally cover, but also from other regions of the animal. However, adhesion and spreading are highly species-specific. Species-specific adhesion is found throughout the whole mass of mesogloea even at regions where cells do not occur naturally. The cell adhesion factor can be extracted from the mesogloea so that the mesogloea no longer shows any cell adhesion properties. The extract consists mainly of a cysteine-containing collagen.

The jellyfish and its polyp: a comparative study of gene expression monitored by the protein patterns, using two-dimensional gels with double-label autoradiography.

The life cycle of Podocoryne carnea (Coelenterata, Anthomedusae) shows several distinct stages which differ considerably in terms of their ecology, morphology, cellular composition, and ultrastructure. Previously these stages had even been described as separate species. Using two-dimensional gel electrophoresis and a new method of double-label autoradiography, we show here for the first time for metagenic hydrozoans that only minor differences in gene expression exist between the various life cycle stages. Our results demonstrate the high resolution power of these techniques and show that the different life stages of P. carnea remain rather similar on the protein level. Most of the prominent spots of the two-dimensional gel protein patterns are common to all stages studied. These data show that the hydrozoan life cycle and development are regulated by only minor distinctions in gene expression which possibly explains the great morphogenetic repertoire of these animals described in many studies.