Through the analysis of a simple two-level model, we show how and to what extent structures posse... more Through the analysis of a simple two-level model, we show how and to what extent structures possessing vectorial symmetry act as pumps in the presence of dissipation. The relevance to new separation concepts and protein motor assemblies is discussed.
The hair bundle of sensory cells in the vertebrate ear provides an example of a noisy oscillator ... more The hair bundle of sensory cells in the vertebrate ear provides an example of a noisy oscillator close to a Hopf bifurcation. The analysis of the data from both spontaneous and forced oscillations shows a strong violation of the fluctuation-dissipation theorem, revealing the presence of an underlying active process that keeps the system out of equilibrium. Nevertheless, we show that a generalized fluctuation-dissipation theorem, valid for non-equilibrium steady states, is fulfilled within the limits of our experimental accuracy and computational approximations, when the adequate conjugate degrees of freedom are chosen.
The fluctuation spectrum of giant unilamellar vesicles is measured using a high-resolution contou... more The fluctuation spectrum of giant unilamellar vesicles is measured using a high-resolution contour detection technique. An analysis at higher q vectors than previously achievable is now possible due to technical improvements of the experimental setup and of the detection algorithm. The global fluctuation spectrum is directly fitted to deduce the membrane tension and the bending modulus of lipid membranes. Moreover, we show that the planar analysis of fluctuations is valid for spherical objects, even at low wave vectors. Corrections due to the integration time of the video camera and to the section of a 3D object by the observation plane are introduced. A precise calculation of the error bars has been done in order to provide reliable error estimate. Eventually, using this technique, we have measured bending moduli for EPC, SOPC and SOPC : CHOL membranes confirming previously published values. An interesting application of this technique can be the measurement of the fluctuation spectra for non-equilibrium membranes, such as "active membranes".
Our treatment includes the permeation process hitherto omitted in non-equilibrium structures. Com... more Our treatment includes the permeation process hitherto omitted in non-equilibrium structures. Comparison with experimental rosults for the shear inatability allows the phase diffusion coefficients to be estimated. Climb friction coefficients are shown to depcnd on velocity.
mechanistic insight into the gating of this channel and validates the use of VCF in this channel.... more mechanistic insight into the gating of this channel and validates the use of VCF in this channel. Co-expression of KCNE1 dramatically separates the voltage dependence of fluorescence and current signals, suggesting multiple independent voltage sensor movements are now required for channel opening. This prediction is further supported by measurement of a Cole-Moore effect in KCNQ1/KCNE1 but not KCNQ1 alone, and by computer-based Markov modeling of channel kinetics. Taken together, our results indicate a KCNE1-dependent change in the coupling between the voltage sensor and the channel gate. This represents the first measurements of KCNQ1 voltage sensor using VCF and elucidates a novel role for a potassium channel beta-subunit: to alter the relationship between voltage sensor movement and channel opening.
We propose a mechanism for tumor growth emphasizing the role of homeostatic regulation and tissue... more We propose a mechanism for tumor growth emphasizing the role of homeostatic regulation and tissue stability. We show that competition between surface and bulk effects leads to the existence of a critical size that must be overcome by metastases to reach macroscopic sizes. This property can qualitatively explain the observed size distributions of metastases, while size-independent growth rates cannot account for clinical and experimental data. In addition, it potentially explains the observed preferential growth of metastases on tissue surfaces and membranes such as the pleural and peritoneal layers, suggests a mechanism underlying the seed and soil hypothesis introduced by Stephen Paget in 1889 and yields realistic values for metastatic inefficiency. We propose a number of key experiments to test these concepts. The homeostatic pressure as introduced in this work could constitute a quantitative, experimentally accessible measure for the metastatic potential of early malignant growths. arXiv:0902.1730v1 [q-bio.TO]
We show that the synchronization transition of a large number of noisy coupled oscillators is an ... more We show that the synchronization transition of a large number of noisy coupled oscillators is an example for a dynamic critical point far from thermodynamic equilibrium. The universal behaviors of such critical oscillators, arranged on a lattice in a d-dimensional space and coupled by nearest neighbors interactions, can be studied using field theoretical methods. The field theory associated with the critical point of a homogeneous oscillatory instability (or Hopf bifurcation of coupled oscillators) is the complex Ginzburg-Landau equation with additive noise. We perform a perturbative renormalization group (RG) study in a 4− dimensional space. We develop an RG scheme that eliminates the phase and frequency of the oscillations using a scale-dependent oscillating reference frame. Within a Callan-Symanzik RG scheme to two-loop order in perturbation theory, we find that the RG fixed point is formally related to the one of the model A dynamics of the real Ginzburg-Landau theory with an O(2) symmetry of the order parameter. Therefore, the dominant critical exponents for coupled oscillators are the same as for this equilibrium field theory. This formal connection with an equilibrium critical point imposes a relation between the correlation and response functions of coupled oscillators in the critical regime. Since the system operates far from thermodynamic equilibrium, a strong violation of the fluctuation-dissipation relation occurs and is characterized by a universal divergence of an effective temperature. The formal relation between critical oscillators and equilibrium critical points suggests that long-range phase order exists in critical oscillators above two dimensions. arXiv:0902.1334v3 [cond-mat.stat-mech]
Treating the epithelium as an incompressible fluid adjacent to a viscoelastic stroma, we find a n... more Treating the epithelium as an incompressible fluid adjacent to a viscoelastic stroma, we find a novel hydrodynamic instability that leads to the formation of protrusions of the epithelium into the stroma. This instability is a candidate for epithelial fingering observed in vivo. It occurs for sufficiently large viscosity, cell-division rate and thickness of the dividing region in the epithelium. Our work provides physical insight into a potential mechanism by which interfaces between epithelia and stromas undulate, and potentially by which tissue dysplasia leads to cancerous invasion.
The European physical journal. E, Soft matter, 2015
We use the theory of active gels to study theoretically the merging and separation of two actin d... more We use the theory of active gels to study theoretically the merging and separation of two actin dense layers akin to cortical layers of animal cells. The layers bind at a distance equal to twice the thickness of a free layer, thus forming a single dense layer, similar in this sense to a lamellipodium. When that unique layer is stretched apart, it is resilient to break apart up to a critical length larger than twice the thickness of a free layer. We show that this behavior can result from the high contractile properties of the actomyosin gel due to the activity of myosin molecular motors. Furthermore, we establish that the stability of the stretched single layer is highly dependent on the properties of the gel. Indeed, the nematic order of the actin filaments along the polymerizing membranes is a destabilizing factor.
Dynamic light scattering measurements on oriented samples of dilute lyotropic smectics have been ... more Dynamic light scattering measurements on oriented samples of dilute lyotropic smectics have been performed. The hydrodynamics of a two-component smectic A is reviewed and applied to describe the fluctuation spectrum in our multi-component samples. The layer compressibility modulus (at constant chemical potential) [`</font >(B)]\bar B is extracted from the anisotropic dispersion relation of the so-called slip mode, which arises from
2014 Des mesures de diffusion dynamique de la lumière ont été effectuées sur des échantillons ori... more 2014 Des mesures de diffusion dynamique de la lumière ont été effectuées sur des échantillons orientés de phases smectiques lyotropes diluées. L'hydrodynamique des smectiques A binaires est utilisée pour décrire le spectre des fluctuations dans les echantillons. Le seul mode hydrodynamique experimentalement accessible est le mode barocline (ainsi que son cas limite : le mode d'ondulation), dont l'origine est le couplage entre fluctuations de concentration et fluctuations de deplacement des couches. Sa relation de dispersion anisotrope est fonction notamment de deux constantes élastiques : le module de compression des couches (à potentiel chimique constant) B ; le module de courbure des couches K. L'étude de la flexibility des membranes (K) et des interactions intermembranaires (B) a lieu sur une droite de dilution le long de laquelle le pas du smectique varie de 4 à 35 nm. Les résultats confortent la description en termes de membranes flexibles (flexibilité proche de kB T) soumises à l'interaction stérique de Helfrich.
Through the analysis of a simple two-level model, we show how and to what extent structures posse... more Through the analysis of a simple two-level model, we show how and to what extent structures possessing vectorial symmetry act as pumps in the presence of dissipation. The relevance to new separation concepts and protein motor assemblies is discussed.
The hair bundle of sensory cells in the vertebrate ear provides an example of a noisy oscillator ... more The hair bundle of sensory cells in the vertebrate ear provides an example of a noisy oscillator close to a Hopf bifurcation. The analysis of the data from both spontaneous and forced oscillations shows a strong violation of the fluctuation-dissipation theorem, revealing the presence of an underlying active process that keeps the system out of equilibrium. Nevertheless, we show that a generalized fluctuation-dissipation theorem, valid for non-equilibrium steady states, is fulfilled within the limits of our experimental accuracy and computational approximations, when the adequate conjugate degrees of freedom are chosen.
The fluctuation spectrum of giant unilamellar vesicles is measured using a high-resolution contou... more The fluctuation spectrum of giant unilamellar vesicles is measured using a high-resolution contour detection technique. An analysis at higher q vectors than previously achievable is now possible due to technical improvements of the experimental setup and of the detection algorithm. The global fluctuation spectrum is directly fitted to deduce the membrane tension and the bending modulus of lipid membranes. Moreover, we show that the planar analysis of fluctuations is valid for spherical objects, even at low wave vectors. Corrections due to the integration time of the video camera and to the section of a 3D object by the observation plane are introduced. A precise calculation of the error bars has been done in order to provide reliable error estimate. Eventually, using this technique, we have measured bending moduli for EPC, SOPC and SOPC : CHOL membranes confirming previously published values. An interesting application of this technique can be the measurement of the fluctuation spectra for non-equilibrium membranes, such as "active membranes".
Our treatment includes the permeation process hitherto omitted in non-equilibrium structures. Com... more Our treatment includes the permeation process hitherto omitted in non-equilibrium structures. Comparison with experimental rosults for the shear inatability allows the phase diffusion coefficients to be estimated. Climb friction coefficients are shown to depcnd on velocity.
mechanistic insight into the gating of this channel and validates the use of VCF in this channel.... more mechanistic insight into the gating of this channel and validates the use of VCF in this channel. Co-expression of KCNE1 dramatically separates the voltage dependence of fluorescence and current signals, suggesting multiple independent voltage sensor movements are now required for channel opening. This prediction is further supported by measurement of a Cole-Moore effect in KCNQ1/KCNE1 but not KCNQ1 alone, and by computer-based Markov modeling of channel kinetics. Taken together, our results indicate a KCNE1-dependent change in the coupling between the voltage sensor and the channel gate. This represents the first measurements of KCNQ1 voltage sensor using VCF and elucidates a novel role for a potassium channel beta-subunit: to alter the relationship between voltage sensor movement and channel opening.
We propose a mechanism for tumor growth emphasizing the role of homeostatic regulation and tissue... more We propose a mechanism for tumor growth emphasizing the role of homeostatic regulation and tissue stability. We show that competition between surface and bulk effects leads to the existence of a critical size that must be overcome by metastases to reach macroscopic sizes. This property can qualitatively explain the observed size distributions of metastases, while size-independent growth rates cannot account for clinical and experimental data. In addition, it potentially explains the observed preferential growth of metastases on tissue surfaces and membranes such as the pleural and peritoneal layers, suggests a mechanism underlying the seed and soil hypothesis introduced by Stephen Paget in 1889 and yields realistic values for metastatic inefficiency. We propose a number of key experiments to test these concepts. The homeostatic pressure as introduced in this work could constitute a quantitative, experimentally accessible measure for the metastatic potential of early malignant growths. arXiv:0902.1730v1 [q-bio.TO]
We show that the synchronization transition of a large number of noisy coupled oscillators is an ... more We show that the synchronization transition of a large number of noisy coupled oscillators is an example for a dynamic critical point far from thermodynamic equilibrium. The universal behaviors of such critical oscillators, arranged on a lattice in a d-dimensional space and coupled by nearest neighbors interactions, can be studied using field theoretical methods. The field theory associated with the critical point of a homogeneous oscillatory instability (or Hopf bifurcation of coupled oscillators) is the complex Ginzburg-Landau equation with additive noise. We perform a perturbative renormalization group (RG) study in a 4− dimensional space. We develop an RG scheme that eliminates the phase and frequency of the oscillations using a scale-dependent oscillating reference frame. Within a Callan-Symanzik RG scheme to two-loop order in perturbation theory, we find that the RG fixed point is formally related to the one of the model A dynamics of the real Ginzburg-Landau theory with an O(2) symmetry of the order parameter. Therefore, the dominant critical exponents for coupled oscillators are the same as for this equilibrium field theory. This formal connection with an equilibrium critical point imposes a relation between the correlation and response functions of coupled oscillators in the critical regime. Since the system operates far from thermodynamic equilibrium, a strong violation of the fluctuation-dissipation relation occurs and is characterized by a universal divergence of an effective temperature. The formal relation between critical oscillators and equilibrium critical points suggests that long-range phase order exists in critical oscillators above two dimensions. arXiv:0902.1334v3 [cond-mat.stat-mech]
Treating the epithelium as an incompressible fluid adjacent to a viscoelastic stroma, we find a n... more Treating the epithelium as an incompressible fluid adjacent to a viscoelastic stroma, we find a novel hydrodynamic instability that leads to the formation of protrusions of the epithelium into the stroma. This instability is a candidate for epithelial fingering observed in vivo. It occurs for sufficiently large viscosity, cell-division rate and thickness of the dividing region in the epithelium. Our work provides physical insight into a potential mechanism by which interfaces between epithelia and stromas undulate, and potentially by which tissue dysplasia leads to cancerous invasion.
The European physical journal. E, Soft matter, 2015
We use the theory of active gels to study theoretically the merging and separation of two actin d... more We use the theory of active gels to study theoretically the merging and separation of two actin dense layers akin to cortical layers of animal cells. The layers bind at a distance equal to twice the thickness of a free layer, thus forming a single dense layer, similar in this sense to a lamellipodium. When that unique layer is stretched apart, it is resilient to break apart up to a critical length larger than twice the thickness of a free layer. We show that this behavior can result from the high contractile properties of the actomyosin gel due to the activity of myosin molecular motors. Furthermore, we establish that the stability of the stretched single layer is highly dependent on the properties of the gel. Indeed, the nematic order of the actin filaments along the polymerizing membranes is a destabilizing factor.
Dynamic light scattering measurements on oriented samples of dilute lyotropic smectics have been ... more Dynamic light scattering measurements on oriented samples of dilute lyotropic smectics have been performed. The hydrodynamics of a two-component smectic A is reviewed and applied to describe the fluctuation spectrum in our multi-component samples. The layer compressibility modulus (at constant chemical potential) [`</font >(B)]\bar B is extracted from the anisotropic dispersion relation of the so-called slip mode, which arises from
2014 Des mesures de diffusion dynamique de la lumière ont été effectuées sur des échantillons ori... more 2014 Des mesures de diffusion dynamique de la lumière ont été effectuées sur des échantillons orientés de phases smectiques lyotropes diluées. L'hydrodynamique des smectiques A binaires est utilisée pour décrire le spectre des fluctuations dans les echantillons. Le seul mode hydrodynamique experimentalement accessible est le mode barocline (ainsi que son cas limite : le mode d'ondulation), dont l'origine est le couplage entre fluctuations de concentration et fluctuations de deplacement des couches. Sa relation de dispersion anisotrope est fonction notamment de deux constantes élastiques : le module de compression des couches (à potentiel chimique constant) B ; le module de courbure des couches K. L'étude de la flexibility des membranes (K) et des interactions intermembranaires (B) a lieu sur une droite de dilution le long de laquelle le pas du smectique varie de 4 à 35 nm. Les résultats confortent la description en termes de membranes flexibles (flexibilité proche de kB T) soumises à l'interaction stérique de Helfrich.
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