DHA-containing phospholipids control membrane fusion and transcellular tunnel dynamics.

Metabolic studies and animal knockout models point to the critical role of polyunsaturated docosahexaenoic acid (22:6, DHA)-containing phospholipids (DHA-PLs) in physiology. Here, we investigated the impact of DHA-PLs on the dynamics of transendothelial cell macroapertures (TEMs) triggered by RhoA inhibition-associated cell spreading. Lipidomic analyses showed that human umbilical vein endothelial cells (HUVECs) subjected to a DHA diet undergo a 6-fold enrichment in DHA-PLs at the plasma membrane (PM) at the expense of monounsaturated oleic acid-containing PLs (OA-PLs). Consequently, DHA-PL enrichment at the PM induces a reduction in cell thickness and shifts cellular membranes towards a permissive mode of membrane fusion for transcellular tunnel initiation. We provide evidence that a global homeostatic control of membrane tension and cell cortex rigidity minimizes overall changes of TEM area through a decrease of TEM size and lifetime. Conversely, low DHA-PL levels at the PM lead to the opening of unstable and wider TEMs. Together, this provides evidence that variations of DHA-PL levels in membranes affect cell biomechanical properties.

Stacked or Folded? Impact of Chelate Cooperativity on the Self-Assembly Pathway to Helical Nanotubes from Dinucleobase Monomers.

Self-assembled nanotubes exhibit impressive biological functions that have always inspired supramolecular scientists in their efforts to develop strategies to build such structures from small molecules through a bottom-up approach. One of these strategies employs molecules endowed with self-recognizing motifs at the edges, which can undergo either cyclization-stacking or folding-polymerization processes that lead to tubular architectures. Which of these self-assembly pathways is ultimately selected by these molecules is, however, often difficult to predict and even to evaluate experimentally. We show here a unique example of two structurally related molecules substituted with complementary nucleobases at the edges (i.e., G:C and A:U) for which the supramolecular pathway taken is determined by chelate cooperativity, that is, by their propensity to assemble in specific cyclic structures through Watson-Crick pairing. Because of chelate cooperativities that differ in several orders of magnitude, these molecules exhibit distinct supramolecular scenarios prior to their polymerization that generate self-assembled nanotubes with different internal monomer arrangements, either stacked or coiled, which lead at the same time to opposite helicities and chiroptical properties.

A Modular and Convergent Synthetic Route to Supramolecular Cyclic Dimers Based on Amidinium-Carboxylate Interactions.

We describe herein the optimized design and modular synthetic approach towards supramolecularly programmed monomers that can form discrete macrocyclic species of controllable size and shape through amidinium-carboxylate interactions in apolar and polar media.

Effect of Solvent on Convectively Driven Silica Particle Assembly: Decoupling Surface Tension, Viscosity, and Evaporation Rate.

The process of convectively self-assembling particles in films suffers from low reproducibility due to its high dependency on particle concentration, as well as a variety of interactions and physical parameters. Inhomogeneities in flow rates and instabilities at the air-liquid interface are mostly responsible for reproducibility issues. These problems are aggravated by adding multiple components to the dispersion, such as binary solvent mixtures or surfactant/polymer additives, both common approaches to control stick-slip behavior. When an additive is used, not only does it change the surface tension, but also the viscosity and the evaporation rate. Worse yet, gradients in these three properties can form, which then lead to Marangoni currents. Here, we use a series of alcohols to study the role of viscosity independently of other solvent properties, to show its impact on stick-slip behavior and interband distances. We show that mixtures of glycerol and alcohol or poly(acrylic acid) and alcohol lead to more complex patterning. Marangoni currents are not always observed in co-solvent systems, being dependent on the rate of solvent evaporation. To produce homogeneous particle assemblies and control stick-slip behavior, gradients must be avoided, and the surface tension and viscosity need both be carefully controlled.

Self-Sorting Governed by Chelate Cooperativity.

Self-sorting phenomena are the basis of manifold relevant (bio)chemical processes where a set of molecules is able to interact with no interference from other sets and are ruled by a number of codes that are programmed in molecular structures. In this work, we study, the relevance of chelate cooperativity as a code for achieving high self-sorting fidelities. In particular, we establish qualitative and quantitative relationships between the cooperativity of a cyclic system and the self-sorting fidelity when combined with other molecules that share identical geometry and/or binding interactions. We demonstrate that only systems displaying sufficiently strong chelate cooperativity can achieve quantitative narcissistic self-sorting fidelities either by dictating the distribution of cyclic species in complex mixtures or by ruling the competition between the intra- and intermolecular versions of a noncovalent interaction.

Fusion Dynamics of Hybrid Cell-Microparticle Aggregates: A Jelly Pearl Model.

We study the fusion of homogeneous cell aggregates and of hybrid aggregates combining cells and microparticles. In all cases, we find that the contact area does not vary linearly over time, as observed for liquid drops, but rather it follows a power law in t2/3. This result is interpreted by generalizing the fusion model of soft viscoelastic solid balls to viscoelastic liquid balls, akin to jelly pearls. We also explore the asymmetric fusion between a homogeneous aggregate and a hybrid aggregate. This latter experiment allows the determination of the self-diffusion coefficient of the cells in a tissue by following the spatial distribution of internalized particles in the cells.

Gas-liquid interface of a Lennard-Jones binary mixture controlled by differential activity: phase transition and interfacial stability.

We perform molecular dynamics simulations of a two-dimensional binary mixture of Lennard-Jones particles, characterized by some degree of "activity" inside. Starting from a base state that features a gas-liquid interface and a completely segregated system at thermodynamic equilibrium, we introduce differential scalar activity between the two species by prescribing two different effective temperatures. The differential activity is measured as the ratio of the two temperatures. Previous studies showed segregation in a homogeneously mixed system induced by high activity. In this study, we investigate the effect of activity on a pre-existing gas-liquid interface between two separated species. Whereas a high activity ratio induces the formation of new interfaces, we show that a low activity ratio destabilizes existing ones. Moreover, the combination of a pre-existent interface with differential activity leads to partial crystallization and thus to triple phase coexistence (solid, liquid and gas), which is observed over a wide range of moderate differential activities. Findings from this idealized system can guide our understanding of interfacial behaviors in certain biological systems.

Dinucleoside-Based Macrocycles Displaying Unusually Large Chelate Cooperativities.

High-fidelity production of a single self-assembled species in competition with others relies on achieving strong chelate cooperativities, which can be quantified by the effective molarity parameter. Therefore, supramolecular systems displaying very high effective molarities are reliably formed in a wide range of experimental conditions and exhibit "all-or-none" phenomena, meaning that the assembly is either fully formed or fully dissociated into the corresponding monomeric components. We summarize here our efforts in the study and characterization of one of these synthetic systems exhibiting record chelate cooperativities: the self-assembly of rod-like dinucleoside molecules into tetrameric macrocycles through hydrogen-bonding Watson-Crick interactions.

Dissipative non-equilibrium dynamics of self-assembled paramagnetic colloidal clusters.

We study experimentally and theoretically the dynamics of two-dimensional clusters of paramagnetic colloids under a time-varying magnetic field. These self-assembled clusters are a dissipative non-equilibrium system with shared features with aggregates of living matter. We investigate the dynamics of cluster rotation and develop a theoretical model to explain the emergence of collective viscoelastic properties. The model successfully captures the observed dependence on particle, cluster, and field characteristics, and it provides an estimate of cluster viscoelasticity. We also study the rapid cluster disassembly in response to a change in the external field. The experimentally observed disassembly dynamics are successfully described by a model, which also allows estimating the particle-substrate friction coefficient. Our study highlights physical mechanisms that may be at play in biological aggregates, where similar dynamical behaviors are observed.

Rotation dynamics and internal structure of self-assembled binary paramagnetic colloidal clusters.

We study experimentally and theoretically the dynamics of two-dimensional self-assembled binary clusters of paramagnetic colloids of two different sizes and magnetic susceptibilities under a time-varying magnetic field. Due to the continuous energy input by the rotating field, these clusters are at a state of dissipative nonequilibrium. Dissipative viscoelastic shear waves traveling around their interface enable the rotation of isotropic binary clusters. The angular velocity of a binary cluster is much slower than that of the magnetic field; it increases with the concentration of big particles, and it saturates at a concentration threshold. We generalize an earlier theoretical model to successfully account for the observed effect of cluster composition on cluster rotation. We also investigate the evolution of the internal distribution of the two particle types, reminiscent of segregation in a drop of two immiscible liquids, and the effect of this internal structure on rotation dynamics. The binary clusters exhibit short-range order, which rapidly vanishes at a larger scale, consistent with the clusters' viscoelastic liquid behavior.

Spontaneous migration of cellular aggregates from giant keratocytes to running spheroids.

Despite extensive knowledge on the mechanisms that drive single-cell migration, those governing the migration of cell clusters, as occurring during embryonic development and cancer metastasis, remain poorly understood. Here, we investigate the collective migration of cell on adhesive gels with variable rigidity, using 3D cellular aggregates as a model system. After initial adhesion to the substrate, aggregates spread by expanding outward a cell monolayer, whose dynamics is optimal in a narrow range of rigidities. Fast expansion gives rise to the accumulation of mechanical tension that leads to the rupture of cell-cell contacts and the nucleation of holes within the monolayer, which becomes unstable and undergoes dewetting like a liquid film. This leads to a symmetry breaking and causes the entire aggregate to move as a single entity. Varying the substrate rigidity modulates the extent of dewetting and induces different modes of aggregate motion: "giant keratocytes," where the lamellipodium is a cell monolayer that expands at the front and retracts at the back; "penguins," characterized by bipedal locomotion; and "running spheroids," for nonspreading aggregates. We characterize these diverse modes of collective migration by quantifying the flows and forces that drive them, and we unveil the fundamental physical principles that govern these behaviors, which underscore the biological predisposition of living material to migrate, independent of length scale.

Dual-Mode Chiral Self-Assembly of Cone-Shaped Subphthalocyanine Aromatics.

Columnar polymers and liquid crystals obtained from π-conjugated cone-shaped molecules are receiving increasing interest due to the possibility of obtaining unconventional polar organizations that show anisotropic charge transport and unique chiroptical properties. However, and in contrast to the more common planar discotics, the self-assembly of conic or pyramidic molecules in solution remains largely unexplored. Here, we show how a molecular geometry change, from flat to conic, can generate supramolecular landscapes where different self-assembled species, each of them being under thermodynamic equilibrium with the monomer, exist exclusively within distinct regimes. In particular, depending on the solvent nature-aromatic or aliphatic-cone-shaped C3-symmetric subphthalocyanine 1 can undergo self-assembly either as a tail-to-tail dimer, showing monomer-dimer sigmoidal transitions, or as a head-to-tail noncentrosymmetric columnar polymer, exhibiting a nucleation-elongation polymerization mechanism. Moreover, the experimental and theoretical comparison between racemic and enantiopure samples revealed that the two enantiomers (1M and 1P) tend to narcissistically self-sort in the dimer regime, each enantiomer showing a strong preference to associate with itself, but socially self-sort in the polymer regime, favoring an alternate stacking order along the columns.

Activity-modulated phase transition in a two-dimensional mixture of active and passive colloids.

We study a two-dimensional binary mixture of active and passive colloids as an idealized model of an hybrid aggregate of living cells and inert particles. We perform molecular dynamics simulations of this system using two different thermostats, and we systematically investigate the effect of varying these two effective temperatures on the system behavior, as characterized by its density, structure and thermoelastic properties. Our results indicate that the presence of active colloids shifts the mixture towards the liquid state and renders it more deformable. Such system softening and melting effects due to the addition of active particles are larger than expected from a linear combination of temperatures of the active and passive components. This heightened effect becomes more pronounced as the effective temperature difference between the two components becomes larger. The binary mixture remains homogeneous for moderate colloidal activity, but segregation arises for large effective temperature difference. Our results provide insights to guide future experimental hybrid aggregate studies with promising biomedical applications.

Dewetting: From Physics to the Biology of Intoxicated Cells.

Pathogenic bacteria colonize or disseminate into cells and tissues by inducing large-scale remodeling of host membranes. The physical phenomena underpinning these massive membrane extension and deformation are poorly understood. Invasive strategies of pathogens have been recently enriched by the description of a spectacular mode of opening of large transendothelial cell macroaperture (TEM) tunnels correlated to the dissemination of EDIN-producing strains of Staphylococcus aureus via a hematogenous route or to the induction of gelatinous edema triggered by the edema toxin from Bacillus anthracis. Remarkably, these highly dynamic tunnels close rapidly after they reach a maximal size. Opening and closure of TEMs in cells lasts for hours without inducing endothelial cell death. Multidisciplinary studies have started to provide a broader perspective of both the molecular determinants controlling cytoskeleton organization at newly curved membranes generated by the opening of TEMs and the physical processes controlling the dynamics of these tunnels. Here we discuss the analogy between the opening of TEM tunnels and the physical principles of dewetting, stemming from a parallel between membrane tension and surface tension. This analogy provides a broad framework to investigate biophysical constraints in cell membrane dynamics and their diversion by certain invasive microbial agents.

Nanostructured Micelle Nanotubes Self-Assembled from Dinucleobase Monomers in Water.

Despite the central importance of aqueous amphiphile assemblies in science and industry, the size and shape of these nano-objects is often difficult to control with accuracy owing to the non-directional nature of the hydrophobic interactions that sustain them. Here, using a bioinspired strategy that consists of programming an amphiphile with shielded directional Watson-Crick hydrogen-bonding functions, its self-assembly in water was guided toward a novel family of chiral micelle nanotubes with partially filled lipophilic pores of about 2 nm in diameter. Moreover, these tailored nanotubes are successfully demonstrated to extract and host molecules that are complementary in size and chemical affinity.

Elucidating Noncovalent Reaction Mechanisms: G-Quartet as an Intermediate in G-Quadruplex Assembly.

In analogy to covalent reactions, the understanding of noncovalent association pathways is fundamental to influence and control any supramolecular process. Following an approach that is reminiscent of covalent methodologies, we study here, for the first time, the mechanism of G-quadruplex formation in organic solvents. Our results support a reaction pathway in which the cation shifts the equilibrium towards a G-quartet transient intermediate, which then acts as a template in the formation of the G-quadruplex product.

Noncovalent Synthesis of Self-Assembled Nanotubes through Decoupled Hierarchical Cooperative Processes.

Because of their wide number of biological functions and potential applications, self-assembled nanotubes constitute highly relevant targets in noncovalent synthesis. Herein, we introduce a novel approach to produce supramolecular nanotubes with defined inner and outer diameters from rigid rod-like monomers programmed with complementary nucleobases through two distinct, decoupled cooperative processes of different hierarchy and acting in orthogonal directions: chelate cooperativity, responsible for the formation of robust Watson-Crick H-bonded cyclic tetramers, and nucleation-growth cooperative polymerization.

Impact of Conformational Effects on the Ring-Chain Equilibrium of Hydrogen-Bonded Dinucleosides.

Supramolecular ring-versus-chain equilibria are ubiquitous in biological and synthetic systems. Understanding the factors that decide whether a system will fall on one side or the other is crucial to the control of molecular self-assembly. This work reports results with two kinds of dinucleoside monomers, in which the balance between closed cycles and open polymers is found to depend on subtle factors that rule conformational equilibria, such as steric hindrance, intramolecular interactions, or π-conjugation pathways.

Permeability and viscoelastic fracture of a model tumor under interstitial flow.

Interstitial flow in tumors is a key mechanism leading to cancer metastasis. Tumor growth is accompanied by the development of a leaky vasculature, which increases intratumoral pressure and generates an outward interstitial flow. This flow promotes tumor cell migration away from the tumor. The nature of such interstitial flow depends on the coupling between hydrodynamic conditions and material properties of the tumor, such as porosity and deformability. Here we investigate this coupling by means of a microfluidic model of interstitial flow through a tumor, which is represented by a tumor cell aggregate. For a weak intratumoral pressure, the model tumor behaves as a viscoelastic material of low permeability, which we estimate by means of a newly developed microfluidic device. As intratumoral pressure is raised, the model tumor deforms and its permeability increases. For a high enough pressure, localized intratumoral fracture occurs, which creates preferential flow paths and causes tumor cell detachment. The energy required to fracture depends on the rate of variation of intratumoral pressure, as explained here by a theoretical model originally derived to describe polymer adhesion. Besides the well-established picture of individual tumor cells migrating away under interstitial flow, our findings suggest that intratumoral pressures observed in tumors can suffice to detach tumor fragments, which may thus be an important mechanism to release cancer cells and initiate metastasis.

How Large Can We Build a Cyclic Assembly? Impact of Ring Size on Chelate Cooperativity in Noncovalent Macrocyclizations.

Self-assembled systems rely on intramolecular cooperative effects to control their growth and regulate their shape, thus yielding discrete, well-defined structures. However, as the size of the system increases, cooperative effects tend to dissipate. We analyze here this situation by studying a set of oligomers of different lengths capped with guanosine and cytidine nucleosides, which associate in cyclic tetramers by complementary Watson-Crick H-bonding interactions. As the monomer length increases, and thus the number of C(sp)-C(sp2 ) σ-bonds in the π-conjugated skeleton, the macrocycle stability decreases due to a notable reduction in effective molarity (EM), which has a clear entropic origin. We determined the relationship between EM or ΔS and the number of σ-bonds, which allowed us to predict the maximum monomer lengths at which cyclic species would still assemble quantitatively, or whether the cyclic species would not able to compete at all with linear oligomers over the whole concentration range.