Model for ring closure in ER tubular network dynamics.

Tubular networks of the endoplasmic reticulum (ER) are dynamic structures whose steady-state conformations are maintained by a balance between the persistent generation and vanishing of the network elements. While factors producing the ER tubules and intertubular junctions have been investigated, the mechanisms behind their elimination remained unknown. Here, we addressed the ER ring closure, the process resulting in the tubule and junction removal through constriction of the network unit cells into junctional knots followed by the knot remodeling into regular junctions. We considered the ring closure to be driven by the tension existing in ER membranes. We based our consideration on the notion of Gibbs' thermodynamic tension and reviewed its relationship to other tension definitions used in the literature. We modeled, computationally, the structures of the junctional knots containing internal nanopores and analyzed their tension dependence. We analyzed the process of the pore sealing through membrane fission resulting in the formation of regular junctions. Considering the hemi-fission as the rate-limiting stage of the fission reaction, we evaluated the membrane tensions guaranteeing the spontaneous character of the pore sealing. We concluded that feasible membrane tensions explain all stages of the ER ring closure.

Molecular mechanics underlying flat-to-round membrane budding in live secretory cells.

Membrane budding entails forces to transform flat membrane into vesicles essential for cell survival. Accumulated studies have identified coat-proteins (e.g., clathrin) as potential budding factors. However, forces mediating many non-coated membrane buddings remain unclear. By visualizing proteins in mediating endocytic budding in live neuroendocrine cells, performing in vitro protein reconstitution and physical modeling, we discovered how non-coated-membrane budding is mediated: actin filaments and dynamin generate a pulling force transforming flat membrane into Λ-shape; subsequently, dynamin helices surround and constrict Λ-profile's base, transforming Λ- to Ω-profile, and then constrict Ω-profile's pore, converting Ω-profiles to vesicles. These mechanisms control budding speed, vesicle size and number, generating diverse endocytic modes differing in these parameters. Their impact is widespread beyond secretory cells, as the unexpectedly powerful functions of dynamin and actin, previously thought to mediate fission and overcome tension, respectively, may contribute to many dynamin/actin-dependent non-coated-membrane buddings, coated-membrane buddings, and other membrane remodeling processes.

The role of size, charge, and cholesterol of cell membrane models in interactions with graphene oxide.

The growing in manufacturing and applications of graphene oxide (GO), a two-dimensional nanomaterial, highlights the need for a better understanding of its environmental impact and toxicity. This work investigates the interaction of GO with cell membrane models as an indication for GO's potential harmfulness. A wide range of biologically-relevant membrane parameters (size, charge and, cholesterol content) and simple optical techniques were used to evaluate the outcome of interactions of vesicular cell membrane models with GO. Loss of membrane integrity was found to be positively correlated with electrostatic attraction and negatively correlated with cholesterol content. The size of vesicle-GO aggregates increased as a function of initial vesicle size, while cholesterol content was found to have a negligible effect on aggregation. Interestingly, charged vesicles reduced vesicle-GO aggregate size either by electrostatic repulsion of negatively charge vesicles or by GO folding following attachment of positively charge vesicles. Overall, by examining how key biologically-relevant parameters of membrane models affect interactions with GO, we have augmented the understanding of the potential threats of GO towards biological cell and to the environment.

Mechanism of shaping membrane nanostructures of endoplasmic reticulum.

Recent advances in super-resolution microscopy revealed the previously unknown nanoscopic level of organization of endoplasmic reticulum (ER), one of the most vital intracellular organelles. Membrane nanostructures of 10- to 100-nm intrinsic length scales, which include ER tubular matrices, ER sheet nanoholes, internal membranes of ER exit sites (ERES), and ER transport intermediates, were discovered and imaged in considerable detail, but the physical factors determining their unique geometrical features remained unknown. Here, we proposed and computationally substantiated a common concept for mechanisms of all ER nanostructures based on the membrane intrinsic curvature as a primary factor shaping the membrane and ultra-low membrane tensions as modulators of the membrane configurations. We computationally revealed a common structural motif underlying most of the nanostructures. We predicted the existence of a discrete series of equilibrium configurations of ER tubular matrices and recovered the one corresponding to the observations and favored by ultra-low tensions. We modeled the nanohole formation as resulting from a spontaneous collapse of elements of the ER tubular network adjacent to the ER sheet edge and calculated the nanohole dimensions. We proposed the ERES membrane to have a shape of a super flexible membrane bead chain, which acquires random walk configurations unless an ultra-low tension converts it into a straight conformation of a transport intermediate. The adequacy of the proposed concept is supported by a close qualitative and quantitative similarity between the predicted and observed configurations of all four ER nanostructures.

Publisher Correction: Migrasome formation is mediated by assembly of micron-scale tetraspanin macrodomains.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Migrasome formation is mediated by assembly of micron-scale tetraspanin macrodomains.

Migrasomes are recently discovered cellular organelles that form as large vesicle-like structures on retraction fibres of migrating cells. While the process of migrasome formation has been described before, the molecular mechanism underlying migrasome biogenesis remains unclear. Here, we propose that the mechanism of migrasome formation consists of the assembly of tetraspanin- and cholesterol-enriched membrane microdomains into micron-scale macrodomains, which swell into migrasomes. The major finding underlying the mechanism is that tetraspanins and cholesterol are necessary and sufficient for migrasome formation. We demonstrate the necessity of tetraspanins and cholesterol via live-cell experiments, and their sufficiency by generating migrasome-like structures in reconstituted membrane systems. We substantiate the mechanism by a theoretical model proposing that the key factor driving migrasome formation is the elevated membrane stiffness of the tetraspanin- and cholesterol-enriched macrodomains. Finally, the theoretical model was quantitatively validated by experimental demonstration of the membrane-stiffening effect of tetraspanin 4 and cholesterol.