Lipids hit a “sweet spot” to direct cellular membrane remodeling.

Lipid membrane reshaping is critical to many common cellular processes, including cargo trafficking, cell motility, and organelle biogenesis. The Rodal lab studies how dynamic membrane remodeling is achieved by the active interplay between lipids and proteins. Recent results, published in Cell Reports, demonstrate that for the membrane remodeling protein Nervous Wreck (Nwk), intramolecular autoregulation and membrane charge work together in surprising ways to restrict remodeling to a limited range of lipid compositions.

F-BAR (Fes/Cip4 homology Bin/Amphiphysin/Rvs) domain family proteins are important mediators of membrane remodeling events. The F-BAR domain forms a crescent-shaped α-helical dimer that interacts with and deforms negatively charged membrane phospholipids by assembling into higher-order scaffolds. In this paper, Kelley et al. have shown that the neuronal F-BAR protein Nwk is autoregulated by its C-terminal SH3 domains, which interact directly with the F-BAR domain to inhibit membrane binding. Until now, the dogma in the field has been that increasing concentrations of negatively charged lipids would increase Nwk membrane binding, and thus would induce membrane deformation.

Surprisingly, Kelley et al. found that autoregulation does not mediate this kind of simple “on-off” switch for membrane remodeling. Instead, increasing the concentration of negatively charged lipids increases membrane binding, but inhibits F-BAR membrane deforming activities (see below). Using a combination of in vitro assays and single particle electron microscopy, they found that the Nwk F-BAR domain efficiently assembles into higher-order structures and deforms membranes only within “sweet spot” of negative membrane charge, and that autoregulation elevates this range. The implication of this work is that autoregulation could either reduce membrane binding or promote higher-order assembly, depending on local cellular membrane composition. This study suggests a significant role for the regulation of membrane composition in remodeling.

Brandeis authors on the study included Molecular and Cell Biology graduate students Charlotte Kelley and Shiyu Wang, staff member Tania Eskin, and undergraduate Emily Messelaar ’13 from the Rodal lab; postdoctoral fellow Kangkang Song, Associate Professor of Biology Daniela Nicastro (currently at UT Southwestern), and Associate Professor of Physics Michael Hagan.

Kelley CF, Messelaar EM, Eskin TL, Wang S, Song K, Vishnia K, Becalska AN, Shupliakov O, Hagan MF, Danino D, Sokolova OS, Nicastro D, Rodal AA. Membrane Charge Directs the Outcome of F-BAR Domain Lipid Binding and Autoregulation. Cell reports. 2015;13(11):2597-609.

“Scientist of small things”

IMAGE: BMXIMAGE (from Forbes India)

IMAGE: BMXIMAGE (from Forbes India)

Forbes India recently named Brandeis post-doc alumna Prerna Sharma as one of India’s “30 under 30”. Sharma, who worked in Prof. Zvonimir Dogic’s group in Physics, is currently an Assistant Professor at the Indian Institute of Science (IISc), Bangalore.

Read the original at Prerna Sharma: The scientist of small things, or perhaps her 2014 Nature paper on Hierarchical organization of chiral rafts in colloidal membranes


Nervous Wreck forms zig-zags to induce membrane ridges and scallops.

Em:LM Nwk F-BAR S2

Merged LM/EM images of Drosophila S2 cells featuring Nwk F-BAR induced protrusions.

Sorting and processing of the proteins that span cell membranes requires extensive membrane remodeling , including budding, tubulation, and fission. F-BAR domains form crescent-shaped dimers that bind to and deform membranes. Until now, it was thought that proteins containing these F-BAR domains induced membrane tubulation by assembling in highly ordered helical coats on lipid bilayers.

A new paper in Molecular Biology of the Cell from the Rodal lab (in collaboration with the Nicastro lab and the Sokolova Lab at Lomonosov Moscow State University) describes a novel membrane deforming activity for Nervous Wreck (Nwk), an F-BAR protein that regulates trafficking of transmembrane growth signal receptors at the Drosophila neuromuscular junction. The authors found that Nwk assembles into zig-zags on lipid monolayers, unlike the canonical F-BAR protein CIP4 which forms long filaments, even though the two proteins are predicted to be very structurally similar.  Unlike other members of the F-BAR family that tubulate the membrane, Nwk can induce the formation of membrane ridges and scallops (see figure below). These deformations can lead to dramatic cellular remodeling in cooperation with the cytoskeleton (see figure above). The work done by the Rodal lab suggests that while basic self-assembly and membrane binding properties are likely conserved between F-BAR proteins, the higher-order organization of Nwk may account for differences in membrane remodeling and its specialized role in the cell.

Nwk Model

A new twist on interfacial tension

In a mixture of two molecular components, the surface tension is defined as the energetic cost per unit area of moving molecules from the bulk and bringing them to the interface. The higher the magnitude of the surface tension, the greater the tendency of two components to demix. Surface tension allows trees to carry nutrients from the roots out to the branches, and water striders to walk on the surface of water.

The interface between hydrophobic and hydrophilic components has very high interfacial tension. A common way to adjust the magnitude of surface tension is to add amphiphilic molecules (like soaps), which contain both hydrophilic and hydrophobic components. These amphiphilic molecules prefer to be at the interface between the two components, and effectively lower the interfacial tension, allowing the components to mix more easily. This is how detergent causes oily stains to dissolve in water.

In a recently published article in Nature, an interdisciplinary team of researchers at Brandeis headed by Zvonimir Dogic, and consisting of experimental, theoretical, and computational physicists as well as biologists, has demonstrated a new way of controlling interfacial tension using a molecular property called chirality, or lack of mirror symmetry. The study was performed on a model system of two-dimensional colloidal membranes composed of the rod-like bacteriophage virus fd, which are about one micrometer in length and 7 nanometers in diameter. The electrostatically repulsive virus particles are condensed into membranes through the depletion mechanism by adding non-adsorbing polymer to a virus suspension. Because the fd rods are chiral, they tend to twist by a small angle with respect to neighboring rods. However, the geometry of the membrane prevents twisting in the structure’s interior; only along the perimeter can the rods twist. Thus, increasing the strength of chirality of the rods both lowers the energy of the rods along the membrane’s edge and increases the frustration of untwisted rods in the bulk, lowering the interfacial tension. This contrasts the standard method of controlling interfacial tension using amphiphilic molecules, since the rod-like particles are completely homogenous, and do not contain any hydrophilic components.

The strength of chiral interactions in fd is temperature sensitive; the rods are achiral at 60o C, and the strength of chirality increases with decreasing temperature. By increasing the strength of chiral interactions in-situ, the team of researchers was able to dynamically vary the membrane’s interfacial tension in order to drive a dramatic transition from a membrane to several twisted ribbon structures (Movie 1). The twisted ribbons have much more interfacial area than the membranes, but are much “twistier” structures, and are therefore favored when the strength of chirality is relatively high. Additionally, the team was able to drive the same membrane-to-ribbon transition using optical tweezers, as shown in Movie 2. Membranes and ribbons are only two of a myriad of structures that were observed in the fd system. This work presents a powerful new method to control the assembly of materials by tuning interfacial tension with chirality.

Why nanorods assemble

In a recent paper in Phys. Rev. E, Brandeis postdoc Yasheng Yang and Assistant Professor of Physics Michael Hagan developed a theory to describe the assembly behavior of colloidal rods (i.e. nanorods) in the presence of inert polymer (which induces attractions between the nanorods). The nanorods assemble into several kinds of structures, including colloidal membranes, which  are two dimensional membrane-like structures composed of a one rod-length thick monolayer of aligned rods.  The theory shows that colloidal membranes are stabilized against stacking on top of each other by an entropic force arising from protrusions of rods from the membranes and that there is a critical aspect ratio (rod length/rod diameter) below which membranes are never stable. Understanding the forces that stabilize colloidal membranes is of practical importance since these structures could enable the manufacture of inexpensive and easily scalable optoelectronic devices. This work was part of a collaboration with the experimental lab of Zvonimir Dogic at Brandeis, where colloidal membranes are developed and studied.

Yang YS, Hagan MF. Theoretical calculation of the phase behavior of colloidal membranes. Phys Rev E. 2011;84(5).

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