This weekly seminar aims at gathering researcher form different thematics (physicists, biologists and chemists) and from different institutes in the center of Paris. The objective is to cover an interface between physics, chemistry and biology as broad as possible, with experimental, numerical and/or theoretical approaches. To describe life sciences all scales are needed, from single molecules, cell biology, organisms, population dynamics. That's why the range of our seminar is quite broad form embryonic development, genetic regulation, evolution, mechanics and cell migration, immunology, microbiology, synthetic biology, etc.
16 November 2018, 1pm - Guillaume Achaz (MNHN)
23 November 2018, 1pm - Silvia Grigolon (Crick Institute)
The overdamped Langevin equation describes the inertialess motion of a particle under deterministic drift and thermal noise. The deterministic drift is the result of the combined action of active forces and the diffusivity gradient (the “spurious” force). For biological applications, it is important to distinguish between the two components, because the former indicates specific interactions, while the latter is due to a heterogeneous environment, in which these interactions take place. The spurious force is always proportional to the diffusivity gradient, but the proportionality coefficient is only known for equilibrium systems. This leads to a range of possible spurious force values in out-of-equilibrium systems and leads to ambiguity in the interpretation of the observed drift. This ambiguity is known as the Itô-Stratonovich dilemma. In this work, we do not try to resolve the dilemma, but analyze the information that can be extracted about the active forces in an a priori unknown out-of-equilibrium system. To this end, we propose a Bayesian method that marginalizes over all possible values of the spurious force and allows robust identification of active forces in both equilibrium and out-of-equilibrium setups. Under certain assumptions, the main result can be obtained in a closed form. The method has a significantly decreased false positive rate of active force detection as compared to conventional approaches. We illustrate the practical value of the method by integrating it into the open-source software project TRamWAy and applying it to both numerical trajectories and experimental single-biomolecule tracks (HIV capsids assembly) recorded on the cell membrane.
Spatially growing populations are ubiquitous across scales, ranging from the human migration out of Africa to the spreading of diseases. In contrast to well-mixed populations where an individual’s chance to survival is only determined by its fitness, in spatially growing populations the physical location of an individual is determinant for its survival: the individuals at the edge of the expanding front benefit from having access to virgin territory and giving their offspring the same advantage. The emerging population dynamics results in an evolutionary dynamics dominated by noise, with extreme consequences such as the accumulation of deleterious mutations at the front of the population. To explore experimentally how spatial constraints affect evolutionary dynamics, we employ bacteriophage T7, an E. coli virus that allows to cover many generations in short periods of times while controlling the underlying resource constraints, i.e., the bacterial host. In an evolutionary experiment lasting only 7 days, we were able to evolve a T7 strain that more than doubled its spreading speed on a bacterial lawn compared to its ancestor. The results from the experiments pointed out specific properties that are under strong selection in viral expansions and uncovered new remarkable properties of phage spreading dynamics that we believe are shared across viruses and pathogens in general.
Understanding how groups of species diversified, and how species phenotypes evolved during evolutionary history, is key to our understanding patterns of biodiversity as we see them around us today. Phylogenetic comparative methods have been developed to study diversification and phenotypic evolution from present-day data. I will present recent developments that allow testing the effect of past environmental changes on rates of speciation, extinction and phenotypic evolution, as well as models that allow testing the role of species interactions -- such as competitive, mutualistic, and antagonistic interactions – on phenotypic evolution. Empirical applications of these new phylogenetic comparative methods to large empirical datasets demonstrate the pervasive effect of past environmental changes on evolutionary rates across diverse clades spanning micro and macroorganims. They also demonstrate a distinct effect of interspecific competition in traits involved in resource use versus social signaling. Past environmental changes and species interactions have been key in driving the heterogeneity of evolution rates repeatedly observed across the tree of life.
The migration of immune cells is guided by specific chemical signals, such as chemokine gradients. Their trajectories can also be diverted by physical cues and obstacles imposed by the cellular environment, such as topography, rigidity, adhesion, or hydraulic resistance. On the example of hydraulic resistance, it was shown that neutrophils preferentially follow paths of least resistance, a phenomenon referred to as barotaxis. We here combined quantitative imaging and physical modeling to show that barotaxis results from a force imbalance at the scale of the cell, which is amplified by the actomyosin intrinsic polarization capacity. Strikingly, we found that macropinocytosis specifically confers to immature dendritic cells a unique capacity to overcome this physical bias by facilitating external fluid transport across the cell, thereby enhancing their space exploration capacity and promoting their tissue-patrolling function both in silicoand in vivo. Conversely, mature dendritic cells, which down-regulate macropinocytosis, were found to be sensitive to hydraulic resistance. Theoretical modeling suggested that barotaxis, which helps them avoid dead-ends, might accelerate their migration to lymph nodes, where they initiate adaptive immune responses. We conclude that the physical properties of the microenvironment of moving cells can introduce biases in their migratory behaviors but that specific active mechanisms such as macropinocytosis have emerged to diminish the influence of these biases, allowing motile cells to reach their final destination and efficiently fulfill their functions.
My colleague Sunil Laxman has observed the spontaneous emergence of subpopulations of cells in different metabolic states in growing populations of budding yeast. I'll talk about two situations - one where the yeast is in a well-mixed chemostat, and the other where it grows on agar plates. The chemostat produces incredibly regular oscillations between quiescence and proliferation which can sustain for days. I'll spend most time in the talk discussing a simple mathematical model that we think captures many aspects of this oscillation. The model helped us deduce that the the key metabolites triggering the switch from quiescence to proliferation are probably Acetyl-CoA and NADPH. If there is time, I'll also discuss the results of experiments and modelling of the spatial colonies where cells in two different metabolic states self-organize into a complex intermingled spatial pattern, with one state dependent on the other for metabolic raw material.
Our understanding of bacterial cell size control is based mainly on stress-free growth conditions in the laboratory. In the real-world however, bacteria are routinely faced with stresses that produce long filamentous cell morphologies. E. coli is observed to filament in response to DNA damage, antibiotic treatment, host immune systems, temperature, starvation, and more; conditions which are relevant to clinical settings and food preservation. This shape plasticity is considered a survival strategy. Size control in this regime remains largely unexplored. Here we report that E. coli cells use a dynamic size ruler to determine division locations combined with an adder-like mechanism to trigger divisions. As filamentous cells increase in size due to growth, or decrease in size due to divisions, its multiple Fts division rings abruptly reorganize to remain one characteristic cell length away from the cell pole, and two such length units away from each other. These rules can be explained by spatio-temporal oscillations of Min proteins. Upon removal of filamentation stress, the cells undergo a sequence of division events, randomly at one of the possible division sites, on average after the time required to grow one characteristic cell size. These results indicate that E. coli cells continuously keep track of absolute length to control size, suggest a wider relevance for the adder principle beyond the control of normally sized cells, and provide a new perspective on the function of the Fts and Min systems.
Cells exhibit rich repertoire of dynamical behaviors, which depend on cell phenotype and microenvironment. Collective dynamics is not just a product of many individually moving units. Instead, cells coordinate their movements and move as collective entities. Various biological processes, including morphogenesis, cancer progression and tissue remodeling, largely depends on the coordinated behavior of cells. Using in vitro experiments, I will show that very different cell types, such as mesenchymal cells and epithelial cells, which look very unlike in their dynamics and morphology, actually move and self-organize according same physical principles. Nematodynamics - theory of active non-polar gels allows to formulate unified description of collective motion of cells, where different modes of motion determined by small set of parameters (activity, elastic constant, viscosity and friction). I will present examples of (1) spontaneous left-right symmetry breaking and emergence of collective shear flows of spindle-shaped cells when they confined in adhesive stripes; (2) chaotic vorticity in human bronchial epithelial cell monolayers; and (3) critical dimensions required for collective invasion of fibrosarcoma cells. I will discuss the mechanism behind these phenomena and their effect on cell organization and function.
Actin is a major component of biological cells and forms filaments that constantly polymerize and depolymerize. This activity is used by the cell to generate forces in order to crawl on a surface or to pinch part of its membrane during endocytosis. The cortex, a dense actin network, endows its viscoelastic properties to the cell and protects it from unwanted deformations. The in vitro reconstitution of dense actin networks on micron-size spheres was a seminal step to understand these processes. Here I will describe two experiments where we reconstituted actin networks around non-spherical micro-objects. In the first one, actin grows from the faces of magnetic cylinders arranged in a string. A load is applied by the attraction of neighboring cylinders to monitor the actin network’s growth under stress and measure its elasticity. We evidenced a peculiar non-linear elastic behavior of the network and a direct interaction between growth velocity and viscoelastic behavior. In the second experiment, we revisit the problem of the symmetry breaking occurring in an actin gel growing from a spherical object by growing the actin from star and diamond-shaped prisms. I will show unexpected results on where the fracture arises that downplay the role of mechanical stress in this phenomenon.
Older seminars can be found here.