Clathrin senses membrane curvature.

The ability of proteins to assemble at sites of high membrane curvature is essential to diverse membrane remodeling processes, including clathrin-mediated endocytosis. Multiple adaptor proteins within the clathrin pathway have been shown to sense regions of high membrane curvature, leading to local recruitment of the clathrin coat. Because clathrin triskelia do not bind to the membrane directly, it has remained unclear whether the clathrin coat plays an active role in sensing membrane curvature or is passively recruited by adaptor proteins. Using a synthetic tag to assemble clathrin directly on membrane surfaces, here we show that clathrin is a strong sensor of membrane curvature, comparable with previously studied adaptor proteins. Interestingly, this sensitivity arises from clathrin assembly rather than from the properties of unassembled triskelia, suggesting that triskelia have preferred angles of interaction, as predicted by earlier structural data. Furthermore, when clathrin is recruited by adaptors, its curvature sensitivity is amplified by 2- to 10-fold, such that the resulting protein complex is up to 100 times more likely to assemble on a highly curved surface compared with a flatter one. This exquisite sensitivity points to a synergistic relationship between the coat and its adaptor proteins, which enables clathrin to pinpoint sites of high membrane curvature, an essential step in ensuring robust membrane traffic. More broadly, these findings suggest that protein networks, rather than individual protein domains, are likely the most potent drivers of membrane curvature sensing.

Membrane fission by protein crowding.

Membrane fission, which facilitates compartmentalization of biological processes into discrete, membrane-bound volumes, is essential for cellular life. Proteins with specific structural features including constricting rings, helical scaffolds, and hydrophobic membrane insertions are thought to be the primary drivers of fission. In contrast, here we report a mechanism of fission that is independent of protein structure-steric pressure among membrane-bound proteins. In particular, random collisions among crowded proteins generate substantial pressure, which if unbalanced on the opposite membrane surface can dramatically increase membrane curvature, leading to fission. Using the endocytic protein epsin1 N-terminal homology domain (ENTH), previously thought to drive fission by hydrophobic insertion, our results show that membrane coverage correlates equally with fission regardless of the hydrophobicity of insertions. Specifically, combining FRET-based measurements of membrane coverage with multiple, independent measurements of membrane vesiculation revealed that fission became spontaneous as steric pressure increased. Further, fission efficiency remained equally potent when helices were replaced by synthetic membrane-binding motifs. These data challenge the view that hydrophobic insertions drive membrane fission, suggesting instead that the role of insertions is to anchor proteins strongly to membrane surfaces, amplifying steric pressure. In line with these conclusions, even green fluorescent protein (GFP) was able to drive fission efficiently when bound to the membrane at high coverage. Our conclusions are further strengthened by the finding that intrinsically disordered proteins, which have large hydrodynamic radii yet lack a defined structure, drove fission with substantially greater potency than smaller, structured proteins.

The Display of Single-Domain Antibodies on the Surfaces of Connectosomes Enables Gap Junction-Mediated Drug Delivery to Specific Cell Populations.

Gap junctions, transmembrane protein channels that directly connect the cytoplasm of neighboring cells and enable the exchange of molecules between cells, are a promising new frontier for therapeutic delivery. Specifically, cell-derived lipid vesicles that contain functional gap junction channels, termed Connectosomes, have recently been demonstrated to substantially increase the effectiveness of small molecule chemotherapeutics. However, because gap junctions are present in nearly all tissues, Connectosomes have no intrinsic ability to target specific cell types, which potentially limits their therapeutic effectiveness. To address this challenge, here we display targeting ligands consisting of single-domain antibodies on the surfaces of Connectosomes. We demonstrate that these targeted Connectosomes selectively interact with cells that express a model receptor, promoting the selective delivery of the chemotherapeutic doxorubicin to this target cell population. More generally, our approach has the potential to boost cytoplasmic delivery of diverse therapeutic molecules to specific cell populations while protecting off-target cells, a critical step toward realizing the therapeutic potential of gap junctions.

Connectosomes for Direct Molecular Delivery to the Cellular Cytoplasm.

Transport of biomolecules, drugs, and other reagents across the cell's plasma membrane barrier is an inefficient and poorly controlled process, despite its fundamental importance to biotechnology, cell biology, and pharmaceutics. In particular, insufficient membrane permeability frequently limits the accumulation of drugs and reagents in the cytoplasm, undermining their efficacy. While encapsulating drugs in particles increases uptake by cells, inefficient release of drugs from these particles into the cytoplasm ultimately limits drug efficacy. In contrast, gap junctions provide a direct route to the cytoplasm that bypasses the plasma membrane. As transmembrane channels that physically connect the cytoplasm of adjacent cells, gap junctions permit transport of a diverse range of molecules, from ions and metabolites to siRNA, peptides, and chemotherapeutics. To utilize gap junctions for molecular delivery we have developed Connectosomes, cell-derived lipid vesicles that contain functional gap junction channels and encapsulate molecular cargos. Here we show that these vesicles form gap junction channels with cells, opening a direct and efficient route for the delivery of molecular cargo to the cellular cytoplasm. Specifically, we demonstrate that using gap junctions to deliver the chemotherapeutic doxorubicin reduces the therapeutically effective dose of the drug by more than an order of magnitude. Delivering drugs through gap junctions has the potential to boost the effectiveness of existing drugs such as chemotherapeutics, while simultaneously enabling the delivery of membrane-impermeable drugs and reagents.

Maximizing peptide identification events in proteomic workflows using data-dependent acquisition (DDA).

Current analytical strategies for collecting proteomic data using data-dependent acquisition (DDA) are limited by the low analytical reproducibility of the method. Proteomic discovery efforts that exploit the benefits of DDA, such as providing peptide sequence information, but that enable improved analytical reproducibility, represent an ideal scenario for maximizing measureable peptide identifications in "shotgun"-type proteomic studies. Therefore, we propose an analytical workflow combining DDA with retention time aligned extracted ion chromatogram (XIC) areas obtained from high mass accuracy MS1 data acquired in parallel. We applied this workflow to the analyses of sample matrixes prepared from mouse blood plasma and brain tissues and observed increases in peptide detection of up to 30.5% due to the comparison of peptide MS1 XIC areas following retention time alignment of co-identified peptides. Furthermore, we show that the approach is quantitative using peptide standards diluted into a complex matrix. These data revealed that peptide MS1 XIC areas provide linear response of over three orders of magnitude down to low femtomole (fmol) levels. These findings argue that augmenting "shotgun" proteomic workflows with retention time alignment of peptide identifications and comparative analyses of corresponding peptide MS1 XIC areas improve the analytical performance of global proteomic discovery methods using DDA.

Designing lipids for selective partitioning into liquid ordered membrane domains.

Self-organization of lipid molecules into specific membrane phases is key to the development of hierarchical molecular assemblies that mimic cellular structures. While the packing interaction of the lipid tails should provide the major driving force to direct lipid partitioning to ordered or disordered membrane domains, numerous examples show that the headgroup and spacer play important but undefined roles. We report here the development of several new biotinylated lipids that examine the role of spacer chemistry and structure on membrane phase partitioning. The new lipids were prepared with varying lengths of low molecular weight polyethylene glycol (EGn) spacers to examine how spacer hydrophilicity and length influence their partitioning behavior following binding with FITC-labeled streptavidin in liquid ordered (Lo) and liquid disordered (Ld) phase coexisting membranes. Partitioning coefficients (Kp Lo/Ld) of the biotinylated lipids were determined using fluorescence measurements in studies with giant unilamellar vesicles (GUVs). Compared against DPPE-biotin, DPPE-cap-biotin, and DSPE-PEG2000-biotin lipids, the new dipalmityl-EGn-biotin lipids exhibited markedly enhanced partitioning into liquid ordered domains, achieving Kp of up to 7.3 with a decaethylene glycol spacer (DP-EG10-biotin). We further demonstrated biological relevance of the lipids with selective partitioning to lipid raft-like domains observed in giant plasma membrane vesicles (GPMVs) derived from mammalian cells. Our results found that the spacer group not only plays a pivotal role for designing lipids with phase selectivity but may also influence the structural order of the domain assemblies.

Connexin membrane materials as potent inhibitors of breast cancer cell migration.

Gap junction (GJ) channels facilitate cell-cell communication through the exchange of chemical and mechanical signals, ensuring proper tissue development and homeostasis. The complex, disease stage-dependent role of connexins in breast cancer progression has been extensively studied over the past two decades. In the early stages of breast cancer, substantial evidence supports the role of GJ channels, formed by connexins at the interfaces between neighbouring cells, as suppressors of cell migration and proliferation. These findings suggest that materials that reintroduce connexins into the tumour cell environment have the potential to inhibit cell migration. Here, we report that exposure of highly metastatic MDA-MB-231 breast tumour cells to connexin-rich biovesicle materials potently suppresses cell migration. Specifically, these biovesicles, which can form GJ interfaces with cells, were extracted from the plasma membrane of donor cells engineered to express a high concentration of functional connexin 43 channels. These connexin-rich membrane materials dramatically reduced cell migration in both a transwell migration assay and a scratch closure assay. Collectively, these results suggest that using membrane materials to reintroduce connexins into the tumour cell environment provides a novel approach for combating cell migration and invasion.

Phase-Separated Liposomes Enhance the Efficiency of Macromolecular Delivery to the Cellular Cytoplasm.

INTRODUCTION: From viruses to organelles, fusion of biological membranes is used by diverse biological systems to deliver macromolecules across membrane barriers. Membrane fusion is also a potentially efficient mechanism for the delivery of macromolecular therapeutics to the cellular cytoplasm. However, a key shortcoming of existing fusogenic liposomal systems is that they are inefficient, requiring a high concentration of fusion-promoting lipids in order to cross cellular membrane barriers. OBJECTIVES: Toward addressing this limitation, our experiments explore the extent to which membrane fusion can be amplified by using the process of lipid membrane phase separation to concentrate fusion-promoting lipids within distinct regions of the membrane surface. METHODS: We used confocal fluorescence microscopy to investigate the integration of fusion-promoting lipids into a ternary lipid membrane system that separated into liquid-ordered and liquid-disordered membrane phases. Additionally, we quantified the impact of membrane phase separation on the efficiency with which liposomes transferred lipids and encapsulated macromolecules to cells, using a combination of confocal fluorescence imaging and flow cytometry. RESULTS: Here we report that concentrating fusion-promoting lipids within phase-separated lipid domains on the surfaces of liposomes significantly increases the efficiency of liposome fusion with model membranes and cells. In particular, membrane phase separation enhanced the delivery of lipids and model macromolecules to the cytoplasm of tumor cells by at least 4-fold in comparison to homogenous liposomes. CONCLUSIONS: Our findings demonstrate that phase separation can enhance membrane fusion by locally concentrating fusion-promoting lipids on the surface of liposomes. This work represents the first application of lipid membrane phase separation in the design of biomaterials-based delivery systems. Additionally, these results lay the ground work for developing fusogenic liposomes that are triggered by physical and molecular cues associated with target cells.

Erratum to: Phase-Separated Liposomes Enhance the Efficiency of Macromolecular Delivery to the Cellular Cytoplasm.

[This corrects the article DOI: 10.1007/s12195-017-0489-4.].