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M Vallade

Publications and source records attributed to M Vallade.

8 recordsLinked to original sources
PubMed2004

Periodic adhesive fingers between contacting cells.

We study in detail the properties of fingers, a particular type of cell-cell adhesive structures appearing in adherens junctions. These periodic patterns break the symmetry of cell-cell contacts. We show that finger formation is driven by cadherin interactions and actin growth. A theoretical model is introduced in which the growth of fingers is limited by membrane tension. The steady shape and formation kinetics of fingers are experimentally measured and compared with the theoretical predictions.

Actin Cytoskeleton↗
PubMed2004

Membrane and acto-myosin tension promote clustering of adhesion proteins.

Physicists have studied the aggregation of adhesive proteins, giving a central role to the elastic properties of membranes, whereas cell biologists have put the emphasis on the cytoskeleton. However, there is a dramatic lack of experimental studies probing both contributions on cellular systems. Here, we tested both mechanisms on living cells. We compared, for the same cell line, the growth of cadherin-GFP patterns on recombinant cadherin-coated surfaces, with the growth of vinculin-GFP patterns on extracellular matrix protein-coated surfaces by using evanescent wave microscopy. In our setup, cadherins are not linked to actin, whereas vinculins are. This property allows us to compare formation of clusters with proteins linked or not to the cytoskeleton and thus study the role of membrane versus cytoskeleton in protein aggregation. Strikingly, the motifs we obtained on both surfaces share common features: they are both elongated and located at the cell edges. We showed that a local force application can impose this symmetry breaking in both cases. However, the origin of the force is different as demonstrated by drug treatment (butanedione monoxime) and hypotonic swelling. Cadherins aggregate when membrane tension is increased, whereas vinculins (cytoplasmic proteins of focal contacts) aggregate when acto-myosin stress fibers are pulling. We propose a mechanism by which membrane tension is localized at cell edges, imposing flattening of membrane and enabling aggregation of cadherins by diffusion. In contrast, cytoplasmic proteins of focal contacts aggregate by opening cryptic sites in focal contacts under acto-myosin contractility.

Actomyosin↗
PubMed2003

Clustering in neutral ecology.

The "neutral ecology" model assumes that all organisms in the same trophic level have the same death, duplication, migration and mutation rates and are subjected to a zero sum rule. We show by exact analytical methods that under the assumption of this model, organisms tend to aggregate and form clusters. At dimensions less than or equal to 2, cluster size grows in average and one specie will dominate the whole ecosystem if enough time is allocated. At dimension d=3 (or higher), an equilibrium is reached and cluster sizes are distributed as a power law.

Biodiversity↗
PubMed2003

Analytical solution of a neutral model of biodiversity.

The unified neutral model of biodiversity proposed by S. Hubbell is solved analytically: The distributions of species abundance in the metacommunity and in a local community are calculated exactly as a function of speciation and migration rates and of the size of the community. In the limit of large population sizes the densities of species of given relative abundance are found to be given by universal functions depending only on two parameters.

Biodiversity↗
PubMed1992

Expansion of insecticidal host range of Bacillus thuringiensis by in vivo genetic recombination.

We describe a novel approach for the insertion of an insecticidal toxin gene into a resident plasmid in Bacillus thuringiensis (Bt). A gene encoding a coleopteran-specific toxin was cloned within a fragment of IS232 and inserted into a plasmid thermosensitive for replication in Bt. The plasmid was used to transform a Bt strain toxic to lepidoptera, and the transformants were then selected at non-permissive temperature for clones in which the vector had integrated into a copy of IS232 present on a resident plasmid. A second recombination event was selected such that the vector was eliminated and the newly introduced toxin gene was conserved. The resulting strain contained only DNA of Bt origin, and displayed insecticidal activity against both lepidoptera and coleoptera.

Bacillus thuringiensis↗