arXiv subjects

Masayuki Imai

Publications and source records attributed to Masayuki Imai.

11 recordsLinked to original sources

arXiv2026

Vesicle-surface-templated catalytic polymers drive differential growth in synthetic minimal cell variants

Understanding how life-like behaviors can emerge from simple molecular assemblies and primitive compartments remains a central challenge in origins-of-life research. Synthetic minimal cells provide a bottom-up platform for investigating, from scratch, the minimal physicochemical principles underlying compartment growth, reproduction, and evolution. Previously, we developed a vesicle/polymer-based compartment system in which the vesicle membranes template the formation of a catalytic polymer. This polymer promotes selective incorporation of amphiphiles into the vesicle membrane, driving vesicle growth while maintaining the compositional identity and enabling spontaneous deformation and division over several generations. Here, we report about experiments in which we advanced this system beyond reproduction by systematically constructing eight synthetic minimal cell variants from combinations of two template vesicles, two catalytic polymers, and two supplied amphiphiles. The variants exhibited distinct, composition-dependent vesicle growth responses, ranging from pronounced growth to suppressed growth or vesicle shrinkage. These growth responses were described by the Hill kinetics and characterized by three parameters, revealing a multi-dimensional fitness landscape shaped by environmental conditions, in which the relative advantage of each variant depends on both composition and amphiphile availability. This framework links molecular recognition, compositional inheritance, and differential growth, providing a physicochemical route toward evolvable synthetic minimal cells.

cond-mat.soft↗

arXiv2025

Protocellular energetics: Free energy estimates for all metabolic, self-assembly and vesicle fission processes

As minimal cells or protocells are dramatically simpler than modern unicells it is possible to quantitatively estimate free energy changes for every process in the lifecycle of a protocell and compare these with estimates of the free energy changes for lifecycles in modern unicells. We present quantitative estimates of all metabolic changes in part by new density function theory (DFT) estimations, in part by compiling previously measured or estimated free energy changes, and in part by new thermodynamic calculations for all self-assembly, vesicle bending, and fission energies.

physics.bio-ph↗

arXiv2023

Translocation of a daughter vesicle in a model system of self-reproducing vesicles

Translocation of a daughter vesicle from a mother vesicle through a pore is experimentally studied by many groups using a model system of self-reproducing vesicles. However, the theoretical formulation of the problem is not fully understood. In the present study, we present a theoretical formulation of the process based on our previous work [P. Khunpetch et al., Phys. Fluids 33, 077103 (2021)]. In our previous work, we considered the daughter vesicle as a rigid body. In the present work, however, we allow the daughter vesicle to deform during the expulsion process. We thus derive the free energy constituting of the elastic moduli of both the mother and daughter vesicles, and of pressure-driven contribution. The minimum energy path of the translocation is searched by using the string method. Our improved model successfully suggests the disappearance of the energy barrier where all the elastic moduli are in agreement with the experimental reports, while the previous work is unsuccessful to do so. The equations of motion of the daughter vesicle have been derived within the framework of the Onsager principle. We found that the translocation time of the daughter vesicle can be reduced when the pressure inside the mother vesicle increases, or the initial size of the daughter vesicle decreases.

cond-mat.soft↗

arXiv2021

Birthing of a daughter vesicle in a model system for self-reproduction vesicles

Sakuma and Imai [Phys. Rev. Lett. 107, 198101 (2011)] established a temperature-controlled cyclic process for a model system of self-reproducing vesicles without feeding. The vesicle generates a smaller inclusion vesicle called "daughter vesicle" inside the original vesicle (we call this "mother vesicle") and then the daughter vesicle is expelled through a small pore on the mother vesicle. This self-reproducing process is called birthing. In the present study we present theoretical model on the birthing process of a single, rigid daughter vesicle through a pore. By using a simple geometric picture, we derive the free energy constituting the material properties of the bending, stretching and line tension moduli of the mother vesicle, as a function of the distance between the centers of the daughter and mother vesicles, and the size of the daughter vesicle. We see clearly the disappearance of the energy barrier by selecting appropriate moduli. The dynamics of the system is studied by employing the Onsager principle. The results indicate that translocation time decreases as the friction parameter decreases, or the initial size of the daughter vesicle decreases.

cond-mat.soft↗

arXiv2019

Hydrodynamic synchronization and collective dynamics of colloidal particles driven along a circular path

We study theoretically the collective dynamics of particles driven by an optical vortex along a circular path. Phase equations of N particles are derived by taking into account both hydrodynamic and repulsive interactions between them. For N = 2, the particles attract with each other and synchronize, forming a doublet that moves faster than a singlet. For N = 3 and 5, we find periodic rearrangement of doublets and a singlet. For N = 4 and 6, the system exhibits either a periodic oscillating state or a stable synchronized state depending on the initial conditions. These results reproduce main features of previous experimental findings. We quantitatively discuss the mechanisms governing the non-trivial collective dynamics.

nlin.AO↗

arXiv2018

pH Sensing by Lipids in Membranes: The Fundamentals of pH-driven Migration, Polarization and Deformations of Lipid Bilayer Assemblies

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described.

physics.bio-ph↗

arXiv2014

Shape transformations of toroidal vesicles

Morphologies of genus-1 and 2 toroidal vesicles are studied numerically by dynamically triangulated membrane models and experimentally by confocal laser microscopy. Our simulation results reproduce shape transformations observed in our experiments well. At large reduced volumes of the genus-1 vesicles, obtained vesicle shapes agree with the previous theoretical prediction, in which axisymmetric shapes are assumed: double-necked stomatocyte, discoidal toroid, and circular toroid. However, for small reduced volumes, it is revealed that a non-axisymmetric discoidal toroid and handled discocyte exist in thermal equilibrium in the parameter range, in which the previous theory predicts axisymmetric discoidal shapes. Polygonal toroidal vesicles and subsequent budding transitions are also found. The entropy caused by shape fluctuations slightly modifies the stability of the vesicle shapes.

cond-mat.soft↗

arXiv2013

Confinement-induced shape transitions in multilamellar vesicles

Morphologies of a vesicle confined in a spherical vesicle were explored experimentally by fast confocal laser microscopy and numerically by a dynamically-triangulated membrane model with area-difference elasticity. The confinement was found to induce several novel shapes of the inner vesicles, that had been never observed in unilamellar vesicles: double and quadruple stomatocytes, slit vesicle, and vesicles of two or three compartments with various shapes. The simulations reproduced the experimental results very well and some of the shape transitions can be understood by a simple theoretical model for axisymmetric shapes.

cond-mat.soft↗

arXiv2012

Lipid membrane deformation in response to a local pH modification: theory and experiments

We study the deformation of a lipid membrane in response to a local pH modification. Experimentally, a basic solution is microinjected close to a giant unilamellar vesicle. A local deformation appears in the zone of the membrane that is closest to the micropipette, and relaxes when the injection is stopped. A theoretical description of this phenomenon is provided. It takes fully into account the spatiotemporal evolution of the concentration of hydroxide ions during and after the microinjection, as well as the linear dynamics of the membrane. This description applies to a local injection of any substance that reacts reversibly with the membrane lipids. We compare experimental data obtained in the domain of small deformations to the results of our linear description, and we obtain a good agreement between theory and experiments. In addition, we present direct experimental observations of the pH profile on the membrane during and after the microinjection, using pH-sensitive fluorescent lipids.

cond-mat.soft↗

arXiv2010

Hydrodynamic effects on concentration fluctuations in multicomponent membranes

We investigate the hydrodynamic effects on the dynamics of critical concentration fluctuations in multicomponent fluid membranes. Two geometrical cases are considered; (i) confined membrane case and (ii) supported membrane case. We numerically calculate the wavenumber dependence of the effective diffusion coefficient by changing the temperature and/or the thickness of the bulk fluid. For some limiting cases, the result is compared with the previously obtained analytical expression. An analogy of the multicomponent membrane to 2D microemulsion is explored for the confined membrane geometry.

cond-mat.soft↗

arXiv2010

Drag coefficient of a liquid domain in a two-dimensional membrane

Using a hydrodynamic theory that incorporates a momentum decay mechanism, we calculate the drag coefficient of a circular liquid domain of finite viscosity moving in a two-dimensional membrane. We derive an analytical expression for the drag coefficient which covers the whole range of domain sizes. Several limiting expressions are discussed. The obtained drag coefficient decreases as the domain viscosity becomes smaller with respect to the outer membrane viscosity. This is because the flow induced in the domain acts to transport the fluid in the surrounding matrix more efficiently.

cond-mat.soft↗