Guidance by followers ensures long-range coordination of cell migration through α-catenin mechanoperception

Photoactivatable substrates show diverse phenotypes of leader cells in collective migration when moving along different extracellular matrix proteins

In cancer metastasis, collectively migrating clusters are discriminated into leader and follower cells that move through extracellular matrices (ECMs) with different characteristics. The impact of changes in ECM protein types on leader cells and migrating clusters is unknown. To address this, we investigated the response of leader cells and migrating clusters upon moving from one ECM protein to another using a photoactivatable substrate bearing photocleavable PEG (PCP), whose surface changes from protein-repellent to protein-adhesive in response to light. We chose laminin and collagen I for our study since they are abundant in two distinct regions in living tissues, namely basement membrane and connective tissue. Using the photoactivatable substrates, the precise deposition of the first ECM protein in the irradiated areas was achieved, followed by creating well-defined cellular confinements. Secondary irradiation enabled the deposition of the second ECM protein in the new irradiated regions, resulting in region-selective heterogeneous and homogenous ECM protein-coated surfaces. Different tendencies in leader cell formation from laminin into laminin compared to those migrating from laminin into collagen were observed. The formation of focal adhesion and actin structures for cells within the same cluster in the ECM proteins responded according to the underlying ECM protein type. Finally, integrin β1 was crucial for the appearance of leader cells for clusters migrating from laminin into collagen. However, when it came to laminin into laminin, integrin β1 was not responsible. This highlights the correlation between leader cells in collective migration and the biochemical signals that arise from underlying extracellular matrix proteins.

A Hierarchical Mechanotransduction System: From Macro to Micro

Mechanotransduction is a strictly regulated process whereby mechanical stimuli, including mechanical forces and properties, are sensed and translated into biochemical signals. Increasing data demonstrate that mechanotransduction is crucial for regulating macroscopic and microscopic dynamics and functionalities. However, the actions and mechanisms of mechanotransduction across multiple hierarchies, from molecules, subcellular structures, cells, tissues/organs, to the whole-body level, have not been yet comprehensively documented. Herein, the biological roles and operational mechanisms of mechanotransduction from macro to micro are revisited, with a focus on the orchestrations across diverse hierarchies. The implications, applications, and challenges of mechanotransduction in human diseases are also summarized and discussed. Together, this knowledge from a hierarchical perspective has the potential to refresh insights into mechanotransduction regulation and disease pathogenesis and therapy, and ultimately revolutionize the prevention, diagnosis, and treatment of human diseases.

Inactivating negative regulators of cortical branched actin enhances persistence of single cell migration

The Rac1-WAVE-Arp2/3 pathway pushes the plasma membrane by polymerizing branched actin, thereby powering membrane protrusions that mediate cell migration. Here, using knockdown (KD) or knockout (KO), we combine the inactivation of the Arp2/3 inhibitory protein arpin, the Arp2/3 subunit ARPC1A and the WAVE complex subunit CYFIP2, all of which enhance the polymerization of cortical branched actin. Inactivation of the three negative regulators of cortical branched actin increases migration persistence of human breast MCF10A cells and of endodermal cells in the zebrafish embryo, significantly more than any single or double inactivation. In the triple KO cells, but not in triple KD cells, the 'super-migrator' phenotype was associated with a heterogenous downregulation of vimentin (VIM) expression and a lack of coordination in collective behaviors, such as wound healing and acinus morphogenesis. Re-expression of vimentin in triple KO cells largely restored normal persistence of single cell migration, suggesting that vimentin downregulation contributes to the maintenance of the super-migrator phenotype in triple KO cells. Constant excessive production of branched actin at the cell cortex thus commits cells into a motile state through changes in gene expression.

Chromatically Corrected Multicolor Multiphoton Microscopy

Simultaneous imaging of multiple labels in tissues is key to studying complex biological processes. Although strategies for color multiphoton excitation have been established, chromatic aberration remains a major problem when multiple excitation wavelengths are used in a scanning microscope. Chromatic aberration introduces a spatial shift between the foci of beams of different wavelengths that varies across the field of view, severely degrading the performance of color imaging. In this work, we propose an adaptive correction strategy that solves this problem in two-beam microscopy techniques. Axial chromatic aberration is corrected by a refractive phase mask that introduces pure defocus into one beam, while lateral chromatic aberration is corrected by a piezoelectric mirror that dynamically compensates for lateral shifts during scanning. We show that this light-efficient approach allows seamless chromatic correction over the entire field of view of different multiphoton objectives without compromising spatial and temporal resolution and that the effective area for beam-mixing processes can be increased by more than 1 order of magnitude. We illustrate this approach with simultaneous three-color, two-photon imaging of developing zebrafish embryos and fixed Brainbow mouse brain slices over large areas. These results establish a robust and efficient method for chromatically corrected multiphoton imaging.

Organizing collective cell migration through guidance by followers

Morphogenesis, wound healing, and some cancer metastases rely on the collective migration of groups of cells. In these processes, guidance and coordination between cells and tissues are critical. While strongly adherent epithelial cells have to move collectively, loosely organized mesenchymal cells can migrate as individual cells. Nevertheless, many of them migrate collectively. This article summarizes how migratory reactions to cell-cell contacts, also called "contact regulation of locomotion" behaviors, organize mesenchymal collective cell migration. It focuses on one recently discovered mechanism called "guidance by followers", through which a cell is oriented by its immediate followers. In the gastrulating zebrafish embryo, during embryonic axis elongation, this phenomenon is responsible for the collective migration of the leading tissue, the polster, and its guidance by the following posterior axial mesoderm. Such guidance of migrating cells by followers ensures long-range coordination of movements and developmental robustness. Along with other "contact regulation of locomotion" behaviors, this mechanism contributes to organizing collective migration of loose populations of cells.

FitMultiCell: simulating and parameterizing computational models of multi-scale and multi-cellular processes

MOTIVATION

Biological tissues are dynamic and highly organized. Multi-scale models are helpful tools to analyse and understand the processes determining tissue dynamics. These models usually depend on parameters that need to be inferred from experimental data to achieve a quantitative understanding, to predict the response to perturbations, and to evaluate competing hypotheses. However, even advanced inference approaches such as approximate Bayesian computation (ABC) are difficult to apply due to the computational complexity of the simulation of multi-scale models. Thus, there is a need for a scalable pipeline for modeling, simulating, and parameterizing multi-scale models of multi-cellular processes.

RESULTS

Here, we present FitMultiCell, a computationally efficient and user-friendly open-source pipeline that can handle the full workflow of modeling, simulating, and parameterizing for multi-scale models of multi-cellular processes. The pipeline is modular and integrates the modeling and simulation tool Morpheus and the statistical inference tool pyABC. The easy integration of high-performance infrastructure allows to scale to computationally expensive problems. The introduction of a novel standard for the formulation of parameter inference problems for multi-scale models additionally ensures reproducibility and reusability. By applying the pipeline to multiple biological problems, we demonstrate its broad applicability, which will benefit in particular image-based systems biology.

AVAILABILITY AND IMPLEMENTATION

FitMultiCell is available open-source at https://gitlab.com/fitmulticell/fit.

Expect the unexpected: conventional and unconventional roles for cadherins in collective cell migration

Migrating cell collectives navigate complex tissue environments both during normal development and in pathological contexts such as tumor invasion and metastasis. To do this, cells in collectives must stay together but also communicate information across the group. The cadherin superfamily of proteins mediates junctional adhesions between cells, but also serve many essential functions in collective cell migration. Besides keeping migrating cell collectives cohesive, cadherins help follower cells maintain their attachment to leader cells, transfer information about front-rear polarity among the cohort, sense and respond to changes in the tissue environment, and promote intracellular signaling, in addition to other cellular behaviors. In this review, we highlight recent studies that reveal diverse but critical roles for both classical and atypical cadherins in collective cell migration, specifically focusing on four in vivo model systems in development: the Drosophila border cells, zebrafish mesendodermal cells, Drosophila follicle rotation, and Xenopus neural crest cells.

Who's really in charge: Diverse follower cell behaviors in collective cell migration

Collective cell migrations drive morphogenesis, wound healing, and cancer dissemination. Cells located at the front are considered leaders while those behind them are defined topologically as followers. Leader cell behaviors, including chemotaxis and their coupling to followers, have been well-studied and reviewed. However, the contributions of follower cells to collective cell migration represent an emerging area of interest. In this perspective, we highlight recent research into the broadening array of follower cell behaviors found in moving collectives. We describe examples of follower cells that possess cryptic leadership potential and followers that lack that potential but contribute in diverse and sometimes surprising ways to collective movement, even steering from behind. We highlight collectives in which all cells both lead and follow, and a few passive passengers. The molecular mechanisms controlling follower cell function and behavior are just emerging and represent an exciting frontier in collective cell migration research.

Receptors, enzymes and self-attraction as autocrine generators and amplifiers of chemotaxis and cell steering

Cells create their own steering cues, or modify cues from their outside, for a number of reasons. These include generating optimal, legible directional information; probing their environments for information to help decide an optimal route; symmetry breaking; generating new patterns and complexity; and bringing together collectives such as neutrophil swarms. Recent advances include more mechanisms of self-steering, in particular by using cell-generated mechanical cues, and gradients of respired oxygen. An increasing number of cell types are being found to use self-steering, in particular immune cells responding to chemokines and mesodermal cells during gastrulation. Finally, receptor modification has emerged as an important limit on the range of neutrophil swarming, allowing cells to monitor other areas as well as coming together. Self-steering is thus emerging as a dominant feature of cell motility.

The α-Catenin mechanosensing M region is required for cell adhesion during tissue morphogenesis

α-Catenin couples the cadherin-catenin complex to the actin cytoskeleton. The mechanosensitive α-Catenin M region undergoes conformational changes upon application of force to recruit interaction partners. Here, we took advantage of the tension landscape in the Drosophila embryo to define three different states of α-Catenin mechanosensing in support of cell adhesion. Low-, medium-, and high-tension contacts showed a corresponding recruitment of Vinculin and Ajuba, which was dependent on the α-Catenin M region. In contrast, the Afadin homolog Canoe acts in parallel to α-Catenin at bicellular low- and medium-tension junctions but requires an interaction with α-Catenin for its tension-sensitive enrichment at high-tension tricellular junctions. Individual M region domains make complex contributions to cell adhesion through their impact on interaction partner recruitment, and redundancies with the function of Canoe. Our data argue that α-Catenin and its interaction partners are part of a cooperative and partially redundant mechanoresponsive network that supports AJs remodeling during morphogenesis.