Experimental realization of crossover in shape and director field of nematic tactoids

The actin cytoskeleton is a critical regulator of cytoplasmic architecture and mechanics, essential in a myriad of physiological processes. Here we demonstrate a liquid phase of actin filaments in the presence of the physiological cross-linker, filamin. Filamin condenses short actin filaments into spindle-shaped droplets, or tactoids, with shape dynamics consistent with a continuum model of anisotropic liquids. We find that cross-linker density controls the droplet shape and deformation timescales, consistent with a variable interfacial tension and viscosity. Near the liquid-solid transition, cross-linked actin bundles show behaviors reminiscent of fluid threads, including capillary instabilities and contraction. These data reveal a liquid droplet phase of actin, demixed from the surrounding solution and dominated by interfacial tension. These results suggest a mechanism to control organization, morphology, and dynamics of the actin cytoskeleton.actin | phase separation | liquid crystal | cytoskeleton T he cellular cytoplasm is a hierarchical array of diverse, soft materials assembled from biological molecules that work in concert to support cell physiology (1). The actin cytoskeleton constitutes a spectrum of materials constructed from the semiflexible polymer actin (F-actin) that are crucial in diverse physical processes ranging from cell division and migration to tissue morphogenesis (2, 3). Cross-linking and regulatory proteins assemble actin filaments into bundles and networks with varied composition, mechanics, and physiological function (4). The mechanical properties of actin assemblies regulate force generation and transmission to dynamically control morphogenic processes from the subcellular to tissue length scales (5, 6).A mechanistic understanding of cytoplasmic mechanics is obscured by the rich complexity of in vivo cytoskeletal assemblies (7) and has been investigated via in vitro model systems (8, 9). Vastly different material properties have been accessed through varying filament length, concentration, and cross-linking. For semidilute concentrations of long actin filaments (>1 μm), the mean spacing between actin filaments, or mesh size, is much smaller than the filament length. In this case, cross-linking proteins mechanically constrain actin filaments to result in space-spanning networks that are viscoelastic gels (10). The structure of cross-linked actin networks is kinetically determined, reflecting a metastable state (11, 12) that requires motor-driven stresses for significant shape changes (13). In contrast, highly concentrated solutions of short actin filaments (<1 μm) align due to entropic effects and form equilibrium liquid crystal phases (14). Liquid crystal theory has been introduced as a framework to understand actin cortex mechanics and mitotic spindle shape (5, 15), but the existence of liquid crystal-like phases at physiological conditions is uncertain.Liquid-like phases of proteins and nucleic acids have been found within the cytoplasm and are thought to be important in subcellular...

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“…3A, open squares). This is consistent with theory and experiments on tactoids with homogeneous nematic alignment (26,27) …”

Section: Significancesupporting

confidence: 91%

Liquid behavior of cross-linked actin bundles

Weirich

Banerjee

Dasbiswas

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

121

124

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“…These nematic droplets have characteristic shapes that have already been the subject of several theoretical studies [2][3][4][5][6][7][8] and experiments on systems such as vanadium pentoxide (V 2 O 5 ) [2,9], carbon nanotubes [10,11], rodlike viruses [12,13], F-actin in cells [14], chromonic liquid crystals [15], and cellulose nano-crystals [16][17][18]. These investigations showed that the typical tactoid shapes, for example the so-called spindle-like shape with a bipolar director field (see the right inset in Fig.…”

Section: Introductionmentioning

confidence: 94%

Electric-field-induced shape transition of nematic tactoids

Metselaar¹,

Dozov

Antonova

et al. 2017

Phys. Rev. E

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The occurrence of new textures of liquid crystals is an important factor in tuning their optical and photonics properties. Here, we show, both experimentally and by numerical computation, that under an electric field chitin tactoids (i.e. nematic droplets) can stretch to aspect ratios of more than 15, leading to a transition from a spindle-like to a cigar-like shape. We argue that the large extensions occur because the elastic contribution to the free energy is dominated by the anchoring. We demonstrate that the elongation involves hydrodynamic flow and is reversible, the tactoids return to their original shapes upon removing the field.

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“…Indeed, they eventually coalesce to minimize the interface between the isotropic and the nematic phases to achieve the thermodynamically stable state: two homogeneous phases, the isotropic and nematic phase separated by a single interface. This transition has been observed in many experimental systems, tobacco mosaic viruses (TMV)14, fd–viruses15, F–actin16, bohemite rods17, carbon nanotubes181920, liquid crystal molecules2122 and computer simulations2324. This transition is purely entropic and counterintuitive25: how can a hard-rod system gain entropy by going from a disordered fluid phase to an orientationally ordered phase?…”

mentioning

confidence: 94%

Condensation and dissolution of nematic droplets in dispersions of colloidal rods with thermo–sensitive depletants

Modlińska

Alsayed²,

Gibaud

2015

Sci Rep

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Nematic droplets are droplets composed of elongated molecules that tend to point in the same direction but do not have any positional order. Such droplets are well known to adopt a spindle shape called tactoid. How such droplets condensate or melt and how the orientational symmetry is broken remains however unclear. Here we use a colloidal system composed of filamentous viruses as model rod–like colloids and pnipam microgel particles to induce thermo–sensitive depletion attraction between the rods. Microscopy experiments coupled to particle tracking reveal that the condensation of a nematic droplet is preceded by the formation of a new phase, an isotropic droplet. As the viruses constitute an excellent experimental realization of hard rods, it follows that the phenomenology we describe should be relevant to diverse micro- and nano-sized rods that interact through excluded volume interactions. This transition between isotropic and nematic droplets provides a new and reversible pathway to break the symmetry and order colloidal rods within a droplet with an external stimulus, and could constitute a benchmark experiment for a variety of technologies relying on reconfigurable control of rods.

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Experimental realization of crossover in shape and director field of nematic tactoids

Cited by 64 publications

References 42 publications

Liquid behavior of cross-linked actin bundles

Liquid behavior of cross-linked actin bundles

Electric-field-induced shape transition of nematic tactoids

Condensation and dissolution of nematic droplets in dispersions of colloidal rods with thermo–sensitive depletants

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