2D map projections for visualization and quantitative analysis of 3D fluorescence micrographs

Hyperstabilization of T cell microvilli contacts by chimeric antigen receptors

T cells typically recognize their ligands using a defined cell biology-the scanning of their membrane microvilli (MV) to palpate their environment-while that same membrane scaffolds T cell receptors (TCRs) that can signal upon ligand binding. Chimeric antigen receptors (CARs) present both a therapeutic promise and a tractable means to study the interplay between receptor affinity, MV dynamics and T cell function. CARs are often built using single-chain variable fragments (scFvs) with far greater affinity than that of natural TCRs. We used high-resolution lattice lightsheet (LLS) and total internal reflection fluorescence (TIRF) imaging to visualize MV scanning in the context of variations in CAR design. This demonstrated that conventional CARs hyper-stabilized microvillar contacts relative to TCRs. Reducing receptor affinity, antigen density, and/or multiplicity of receptor binding sites normalized microvillar dynamics and synapse resolution, and effector functions improved with reduced affinity and/or antigen density, highlighting the importance of understanding the underlying cell biology when designing receptors for optimal antigen engagement.

Activity-induced polar patterns of filaments gliding on a sphere

Active matter systems feature the ability to form collective patterns as observed in a plethora of living systems, from schools of fish to swimming bacteria. While many of these systems move in a wide, three-dimensional environment, several biological systems are confined by a curved topology. The role played by a non-Euclidean geometry on the self-organization of active systems is not yet fully understood, and few experimental systems are available to study it. Here, we introduce an experimental setup in which actin filaments glide on the inner surface of a spherical lipid vesicle, thus embedding them in a curved geometry. We show that filaments self-assemble into polar, elongated structures and that, when these match the size of the spherical geometry, both confinement and topological constraints become relevant for the emergent patterns, leading to the formation of polar vortices and jammed states. These results experimentally demonstrate that activity-induced complex patterns can be shaped by spherical confinement and topology.

Left-right symmetry of zebrafish embryos requires somite surface tension

The body axis of vertebrate embryos is periodically segmented into bilaterally symmetric pairs of somites^1,2. The anteroposterior length of somites, their position and left-right symmetry are thought to be molecularly determined before somite morphogenesis^3,4. Here we show that, in zebrafish embryos, initial somite anteroposterior lengths and positions are imprecise and, consequently, many somite pairs form left-right asymmetrically. Notably, these imprecisions are not left unchecked and we find that anteroposterior lengths adjust within an hour after somite formation, thereby increasing morphological symmetry. We find that anteroposterior length adjustments result entirely from changes in somite shape without change in somite volume, with changes in anteroposterior length being compensated by corresponding changes in mediolateral length. The anteroposterior adjustment mechanism is facilitated by somite surface tension, which we show by comparing in vivo experiments and in vitro single-somite explant cultures using a mechanical model. Length adjustment is inhibited by perturbation of molecules involved in surface tension, such as integrin and fibronectin. By contrast, the adjustment mechanism is unaffected by perturbations to the segmentation clock, therefore revealing a distinct process that influences morphological segment lengths. We propose that tissue surface tension provides a general mechanism to adjust shapes and ensure precision and symmetry of tissues in developing embryos.

Giant Unilamellar Vesicle Electroformation: What to Use, What to Avoid, and How to Quantify the Results

Since its inception more than thirty years ago, electroformation has become the most commonly used method for growing giant unilamellar vesicles (GUVs). Although the method seems quite straightforward at first, researchers must consider the interplay of a large number of parameters, different lipid compositions, and internal solutions in order to avoid artifactual results or reproducibility problems. These issues motivated us to write a short review of the most recent methodological developments and possible pitfalls. Additionally, since traditional manual analysis can lead to biased results, we have included a discussion on methods for automatic analysis of GUVs. Finally, we discuss possible improvements in the preparation of GUVs containing high cholesterol contents in order to avoid the formation of artifactual cholesterol crystals. We intend this review to be a reference for those trying to decide what parameters to use as well as an overview providing insight into problems not yet addressed or solved.

A DNA Aptameric Ligand of Human Transferrin Receptor Generated by Cell-SELEX

General cancer-targeted ligands that can deliver drugs to cells have been given considerable attention. In this paper, a high-affinity DNA aptamer (HG1) generally binding to human tumor cells was evolved by cell-SELEX, and was further optimized to have 35 deoxynucleotides (HG1-9). Aptamer HG1-9 could be taken up by live cells, and its target protein on a cell was identified to be human transferrin receptor (TfR). As a man-made ligand of TfR, aptamer HG1-9 was demonstrated to bind at the same site of human TfR as transferrin with comparable binding affinity, and was proved to cross the epithelium barrier through transferrin receptor-mediated transcytosis. These results suggest that aptamer HG1-9 holds potential as a promising ligand to develop general cancer-targeted diagnostics and therapeutics.

A self-organized synthetic morphogenic liposome responds with shape changes to local light cues

Reconstituting artificial proto-cells capable of transducing extracellular signals into cytoskeletal changes can reveal fundamental principles of how non-equilibrium phenomena in cellular signal transduction affect morphogenesis. Here, we generated a Synthetic Morphogenic Membrane System (SynMMS) by encapsulating a dynamic microtubule (MT) aster and a light-inducible signaling system driven by GTP/ATP chemical potential into cell-sized liposomes. Responding to light cues in analogy to morphogens, this biomimetic design embodies basic principles of localized Rho-GTPase signal transduction that generate an intracellular MT-regulator signaling gradient. Light-induced signaling promotes membrane-deforming growth of MT-filaments by dynamically elevating the membrane-proximal tubulin concentration. The resulting membrane deformations enable recursive coupling of the MT-aster with the signaling system, which generates global self-organized morphologies that reorganize towards local external cues in dependence on prior shape. SynMMS thereby signifies a step towards bio-inspired engineering of self-organized cellular morphogenesis.

The authors generated a Synthetic Morphogenic Membrane System by encapsulating a dynamic microtubule aster and a light-inducible signaling system driven by GTP/ATP chemical potential into cell-sized liposomes. This reconstitution of artificial proto-cells reveals how non-equilibrium phenomena affect cellular information processing in morphogenesis.

GUV-AP: multifunctional FIJI-based tool for quantitative image analysis of Giant Unilamellar Vesicles

MOTIVATION

Giant Unilamellar Vesicles (GUVs) are widely used synthetic membrane systems that mimic native membranes and cellular processes. Various fluorescence imaging techniques can be employed for their characterization. In order to guarantee a fast and unbiased analysis of imaging data, the development of automated recognition and processing steps is required.

RESULTS

We developed a fast and versatile Fiji-based macro for the analysis of digital microscopy images of GUVs. This macro was designed to investigate membrane dye incorporation and protein binding to membranes. Moreover, we propose a fluorescence intensity-based method to quantitatively assess protein binding.

AVAILABILITY AND IMPLEMENTATION

The ImageJ distribution package FIJI is freely available online: https://imagej.net/Fiji. The macro file GUV-AP.ijm is available at https://github.com/AG-Roemer/GUV-AP.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Vimentin filaments interact with the actin cortex in mitosis allowing normal cell division

The vimentin network displays remarkable plasticity to support basic cellular functions and reorganizes during cell division. Here, we show that in several cell types vimentin filaments redistribute to the cell cortex during mitosis, forming a robust framework interwoven with cortical actin and affecting its organization. Importantly, the intrinsically disordered tail domain of vimentin is essential for this redistribution, which allows normal mitotic progression. A tailless vimentin mutant forms curly bundles, which remain entangled with dividing chromosomes leading to mitotic catastrophes or asymmetric partitions. Serial deletions of vimentin tail domain gradually impair cortical association and mitosis progression. Disruption of f-actin, but not of microtubules, causes vimentin bundling near the chromosomes. Pathophysiological stimuli, including HIV-protease and lipoxidation, induce similar alterations. Interestingly, full filament formation is dispensable for cortical association, which also occurs in vimentin particles. These results unveil implications of vimentin dynamics in cell division through its interplay with the actin cortex.

The intermediate filament vimentin reorganizes during mitosis, but its molecular regulation and impact on the cell during cell division is unclear. Here, the authors show that vimentin filaments redistribute to the cell cortex during mitosis intertwining with and affecting actin organization.

Visualization of lipid directed dynamics of perilipin 1 in human primary adipocytes

Perilipin 1 is a lipid droplet coating protein known to regulate lipid metabolism in adipocytes by serving as a physical barrier as well as a recruitment site for lipases to the lipid droplet. Phosphorylation of perilipin 1 by protein kinase A rapidly initiates lipolysis, but the detailed mechanism on how perilipin 1 controls lipolysis is unknown. Here, we identify specific lipid binding properties of perilipin 1 that regulate the dynamics of lipolysis in human primary adipocytes. Cellular imaging combined with biochemical and biophysical analyses demonstrate that perilipin 1 specifically binds to cholesteryl esters, and that their dynamic properties direct segregation of perilipin 1 into topologically distinct micro domains on the lipid droplet. Together, our data points to a simple unifying mechanism that lipid assembly and segregation control lipolysis in human primary adipocytes.