Tissue self-organization based on collective cell migration by contact activation of locomotion and chemotaxis

Dynamics of multicellular swirling on micropatterned substrates

Chirality plays a crucial role in biology, as it is highly conserved and fundamentally important in the developmental process. To better understand the relationship between the chirality of individual cells and that of tissues and organisms, we develop a generalized mechanics model of chiral polarized particles to investigate the swirling dynamics of cell populations on substrates. Our analysis reveals that cells with the same chirality can form distinct chiral patterns on ring-shaped or rectangular substrates. Interestingly, our studies indicate that an excessively strong or weak individual cellular chirality hinders the formation of such chiral patterns. Our studies also indicate that there exists the influence distance of substrate boundaries in chiral patterns. Smaller influence distances are observed when cell-cell interactions are weaker. Conversely, when cell-cell interactions are too strong, multiple cells tend to be stacked together, preventing the formation of chiral patterns on substrates in our analysis. Additionally, we demonstrate that the interaction between cells and substrate boundaries effectively controls the chiral distribution of cellular orientations on ring-shaped substrates. This research highlights the significance of coordinating boundary features, individual cellular chirality, and cell-cell interactions in governing the chiral movement of cell populations and provides valuable mechanics insights into comprehending the intricate connection between the chirality of single cells and that of tissues and organisms.

Prestalk-like positioning of de-differentiated cells in the social amoeba Dictyostelium discoideum

The social amoeba Dictyostelium discoideum switches between solitary growth and social fruitification depending on nutrient availability. Under starvation, cells aggregate and form fruiting bodies consisting of spores and altruistic stalk cells. Once cells socially committed, they complete fruitification, even if a new source of nutrients becomes available. This social commitment is puzzling because it hinders individual cells from resuming solitary growth quickly. One idea posits that traits that facilitate premature de-commitment are hindered from being selected. We studied outcomes of the premature de-commitment through forced refeeding. Our results show that when refed cells interacted with non-refed cells, some of them became solitary, whereas a fraction was redirected to the altruistic stalk, regardless of their original fate. The refed cells exhibited reduced cohesiveness and were sorted out during morphogenesis. Our findings provide an insight into a division of labor of the social amoeba, in which less cohesive individuals become altruists.

Structure formation induced by non-reciprocal cell-cell interactions in a multicellular system

Collective cellular behavior plays a crucial role in various biological processes, ranging from developmental morphogenesis to pathological processes such as cancer metastasis. Our previous research has revealed that a mutant cell of Dictyostelium discoideum exhibits collective cell migration, including chain migration and traveling band formation, driven by a unique tail-following behavior at contact sites, which we term "contact following locomotion" (CFL). Here, we uncover an imbalance of forces between the front and rear cells within cell chains, leading to an additional propulsion force in the rear cells. Drawing inspiration from this observation, we introduce a theoretical model that incorporates non-reciprocal cell-cell interactions. Our findings highlight that the non-reciprocal interaction, in conjunction with self-alignment interactions, significantly contributes to the emergence of the observed collective cell migrations. Furthermore, we present a comprehensive phase diagram, showing distinct phases at both low and intermediate cell densities. This phase diagram elucidates a specific regime that corresponds to the experimental system.

Differential cell motion: A mathematical model of anterior posterior sorting

Here, we investigate how a subpopulation of cells can move through an aggregate of cells. Using a stochastic force-based model of Dictyostelium discoideum when the population is forming a slug, we simulate different strategies for prestalk cells to reliably move to the front of the slug while omitting interaction with the substrate thus ignoring the overall motion of the slug. Of the mechanisms that we simulated, prestalk cells being more directed is the best strategy followed by increased asymmetric motive forces for prestalk cells. The lifetime of the cell adhesion molecules, while not enough to produce differential motion, did modulate the results of the strategies employed. Finally, understanding and simulating the appropriate boundary conditions are essential to correctly predict the motion.

Random walk and cell morphology dynamics in Naegleria gruberi

Amoeboid cell movement and migration are wide-spread across various cell types and species. Microscopy-based analysis of the model systems Dictyostelium and neutrophils over the years have uncovered generality in their overall cell movement pattern. Under no directional cues, the centroid movement can be quantitatively characterized by their persistence to move in a straight line and the frequency of re-orientation. Mathematically, the cells essentially behave as a persistent random walker with memory of two characteristic time-scale. Such quantitative characterization is important from a cellular-level ethology point of view as it has direct connotation to their exploratory and foraging strategies. Interestingly, outside the amoebozoa and metazoa, there are largely uncharacterized species in the excavate taxon Heterolobosea including amoeboflagellate Naegleria. While classical works have shown that these cells indeed show typical amoeboid locomotion on an attached surface, their quantitative features are so far unexplored. Here, we analyzed the cell movement of Naegleria gruberi by employing long-time phase contrast imaging that automatically tracks individual cells. We show that the cells move as a persistent random walker with two time-scales that are close to those known in Dictyostelium and neutrophils. Similarities were also found in the shape dynamics which are characterized by the appearance, splitting and annihilation of the curvature waves along the cell edge. Our analysis based on the Fourier descriptor and a neural network classifier point to importance of morphology features unique to Naegleria including complex protrusions and the transient bipolar dumbbell morphologies.

Closing the loop on morphogenesis: a mathematical model of morphogenesis by closed-loop reaction-diffusion

Morphogenesis, the establishment and repair of emergent complex anatomy by groups of cells, is a fascinating and biomedically-relevant problem. One of its most fascinating aspects is that a developing embryo can reliably recover from disturbances, such as splitting into twins. While this reliability implies some type of goal-seeking error minimization over a morphogenic field, there are many gaps with respect to detailed, constructive models of such a process. A common way to achieve reliability is negative feedback, which requires characterizing the existing body shape to create an error signal-but measuring properties of a shape may not be simple. We show how cells communicating in a wave-like pattern could analyze properties of the current body shape. We then describe a closed-loop negative-feedback system for creating reaction-diffusion (RD) patterns with high reliability. Specifically, we use a wave to count the number of peaks in a RD pattern, letting us use a negative-feedback controller to create a pattern with N repetitions, where N can be altered over a wide range. Furthermore, the individual repetitions of the RD pattern can be easily stretched or shrunk under genetic control to create, e.g., some morphological features larger than others. This work contributes to the exciting effort of understanding design principles of morphological computation, which can be used to understand evolved developmental mechanisms, manipulate them in regenerative-medicine settings, or engineer novel synthetic morphology constructs with desired robust behavior.

ATP levels influence cell movement during the mound phase in Dictyostelium discoideum as revealed by ATP visualization and simulation

Cell migration plays an important role in multicellular organism development. The cellular slime mold Dictyostelium discoideum is a useful model organism for the study of cell migration during development. Although cellular ATP levels are known to determine cell fate during development, the underlying mechanism remains unclear. Here, we report that ATP-rich cells efficiently move to the central tip region of the mound against rotational movement during the mound phase. A simulation analysis based on an agent-based model reproduces the movement of ATP-rich cells observed in the experiments. These findings indicate that ATP-rich cells have the ability to move against the bulk flow of cells, suggesting a mechanism by which high ATP levels determine the cell fate of differentiation.

Is cell segregation like oil and water: Asymptotic versus transitory regime

Understanding the segregation of cells is crucial to answer questions about tissue formation in embryos or tumor progression. Steinberg proposed that separation of cells can be compared to the separation of two liquids. Such a separation is well described by the Cahn-Hilliard (CH) equations and the segregation indices exhibit an algebraic decay with exponent 1/3 with respect to time. Similar exponents are also observed in cell-based models. However, the scaling behavior in these numerical models is usually only examined in the asymptotic regime and these models have not been directly applied to actual cell segregation data. In contrast, experimental data also reveals other scaling exponents and even slow logarithmic scaling laws. These discrepancies are commonly attributed to the effects of collective motion or velocity-dependent interactions. By calibrating a 2D cellular automaton (CA) model which efficiently implements a dynamic variant of the differential adhesion hypothesis to 2D experimental data from Méhes et al., we reproduce the biological cell segregation experiments with just adhesive forces. The segregation in the cellular automaton model follows a logarithmic scaling initially, which is in contrast to the proposed algebraic scaling with exponent 1/3. However, within the less than two orders of magnitudes in time which are observable in the experiments, a logarithmic scaling may appear as a pseudo-algebraic scaling. In particular, we demonstrate that the cellular automaton model can exhibit a range of exponents ≤1/3 for such a pseudo-algebraic scaling. Moreover, the time span of the experiment falls into the transitory regime of the cellular automaton rather than the asymptotic one. We additionally develop a method for the calibration of the 2D Cahn-Hilliard model and find a match with experimental data within the transitory regime of the Cahn-Hilliard model with exponent 1/4. On the one hand this demonstrates that the transitory behavior is relevant for the experiment rather than the asymptotic one. On the other hand this corroborates the ambiguity of the scaling behavior, when segregation processes can be only observed on short time spans.

Segregation of different cell types is a crucial process for the pattern formation in tissues, in particular during embryogenesis. Since the involved cell interactions are complex and difficult to measure individually in experiments, mathematical modelling plays an increasingly important role to unravel the mechanisms governing segregation. The analysis of these theoretical models focuses mainly on the asymptotic behavior at large times, in a steady regime and for large numbers of cells. Most famously, cell-segregation models based on the minimization of the total surface energy, a mechanism also driving the demixing of immiscible fluids, are known to exhibit asymptotically a particular algebraic scaling behavior. However, it is not clear, whether the asymptotic regime of the numerical models is relevant at the spatio-temporal scales of actual biological processes and in-vitro experiments. By developing a mapping between 2D cell-based models and experimental settings, we are able to directly compare previous experimental data to numerical simulations of cell segregation quantitatively. We demonstrate that the experiments are reproduced by the transitory regime of the models rather than the asymptotic one. Our work puts a new perspective on previous model-driven conclusions on cell segregation mechanisms.

Evolving social behaviour through selection of single-cell adhesion in Dictyostelium discoideum

The social amoeba Dictyostelium discoideum commonly forms chimeric fruiting bodies. Genetic variants that produce a higher proportion of spores are predicted to undercut multicellular organization unless cooperators assort positively. Cell adhesion is considered a primary factor driving such assortment, but evolution of adhesion has not been experimentally connected to changes in social performance. We modified by experimental evolution the efficiency of individual cells in attaching to a surface. Surprisingly, evolution appears to have produced social cooperators irrespective of whether stronger or weaker adhesion was selected. Quantification of reproductive success, cell-cell adhesion, and developmental patterns, however, revealed two distinct social behaviors, as captured when the classical metric for social success is generalized by considering clonal spore production. Our work shows that cell mechanical interactions can constrain the evolution of development and sociality in chimeras and that elucidation of proximate mechanisms is necessary to understand the ultimate emergence of multicellular organization.

•

Cooperative behavior evolved as a pleiotropic effect of selection for surface adhesion

•

Multicellular development of evolved lines with the ancestor follows two different paths

•

A metric of social behavior including clonal development differentiates these two paths

Calcium responses to external mechanical stimuli in the multicellular stage of Dictyostelium discoideum

Calcium acts as a second messenger to regulate many cellular functions, including cell motility. In Dictyostelium discoideum, the cytosolic calcium level oscillates synchronously, and calcium waves propagate through the cell population during the early stages of development, including aggregation. In the unicellular phase, the calcium response through Piezo channels also functions in mechanosensing. However, calcium dynamics during multicellular morphogenesis are still unclear. Here, live imaging of cytosolic calcium revealed that calcium wave propagation, depending on cAMP relay, disappeared at the onset of multicellular body (slug) formation. Later, other forms of occasional calcium bursts and their propagation were observed in both anterior and posterior regions of migrating slugs. This calcium signaling also occurred in response to mechanical stimuli. Two pathways—calcium release from the endoplasmic reticulum via IP3 receptor and calcium influx from outside the cell—were involved in calcium signals induced by mechanical stimuli. These data suggest that calcium signaling is involved in mechanosensing in both the unicellular and multicellular phases of Dictyostelium development using different molecular mechanisms.

Dynamic self-organization of migrating cells under constraints by spatial confinement and epithelial integrity

Understanding how migrating cells can establish both dynamic structures and coherent dynamics may provide mechanistic insights to study how living systems acquire complex structures and functions. Recent studies revealed that intercellular contact communication plays a crucial role for establishing cellular dynamic self-organization (DSO) and provided a theoretical model of DSO for migrating solitary cells in a free space. However, to apply those understanding to situations in living organisms, we need to know the role of cell-cell communication for tissue dynamics under spatial confinements and epithelial integrity. Here, we expand the previous numerical studies on DSO to migrating cells subjected spatial confinement and/or epithelial integrity. An epithelial monolayer is simulated by combining the model of cellular DSO and the cellular vertex model in two dimensions for apical integrity. Under confinement to a small space, theoretical models of both solitary and epithelial cells exhibit characteristic coherent dynamics, including apparent swirling. We also find that such coherent dynamics can allow the cells to overcome the strong constraint due to spatial confinement and epithelial integrity. Furthermore, we demonstrate how epithelial cell clusters behave without spatial confinement and find various cluster dynamics, including spinning, migration and elongation.

Growth kinetics and power laws indicate distinct mechanisms of cell-cell interactions in the aggregation process

Cellular aggregation is a complex process orchestrated by various kinds of interactions depending on the environment. Different interactions give rise to different pathways of cellular rearrangement and the development of specialized tissues. To distinguish the underlying mechanisms, in this theoretical work, we investigate the spontaneous emergence of tissue patterns from an ensemble of single cells on a substrate following three leading pathways of cell-cell interactions, namely, direct cell adhesion contacts, matrix-mediated mechanical interaction, and chemical signaling. Our analysis shows that the growth kinetics of the aggregation process are distinctly different for each pathway and bear the signature of the specific cell-cell interactions. Interestingly, we find that the average domain size and the mass of the clusters exhibit a power law growth in time under certain interaction mechanisms hitherto unexplored. Further, as observed in experiments, the cluster size distribution can be characterized by stretched exponential functions showing distinct cellular organization processes.

Microtopographical guidance of macropinocytic signaling patches

Morphologies of amoebae and immune cells are highly deformable and dynamic, which facilitates migration in various terrains, as well as ingestion of extracellular solutes and particles. It remains largely unexplored whether and how the underlying membrane protrusions are triggered and guided by the geometry of the surface in contact. In this study, we show that in Dictyostelium, the precursor of a structure called macropinocytic cup, which has been thought to be a constitutive process for the uptake of extracellular fluid, is triggered by micrometer-scale surface features. Imaging analysis and computational simulations demonstrate how the topographical dependence of the self-organizing dynamics supports efficient guidance and capturing of the membrane protrusion and hence movement of an entire cell along such surface features.

In fast-moving cells such as amoeba and immune cells, dendritic actin filaments are spatiotemporally regulated to shape large-scale plasma membrane protrusions. Despite their importance in migration, as well as in particle and liquid ingestion, how their dynamics are affected by micrometer-scale features of the contact surface is still poorly understood. Here, through quantitative image analysis of Dictyostelium on microfabricated surfaces, we show that there is a distinct mode of topographical guidance directed by the macropinocytic membrane cup. Unlike other topographical guidance known to date that depends on nanometer-scale curvature sensing protein or stress fibers, the macropinocytic membrane cup is driven by the Ras/PI3K/F-actin signaling patch and its dependency on the micrometer-scale topographical features, namely PI3K/F-actin–independent accumulation of Ras-GTP at the convex curved surface, PI3K-dependent patch propagation along the convex edge, and its actomyosin-dependent constriction at the concave edge. Mathematical model simulations demonstrate that the topographically dependent initiation, in combination with the mutually defining patch patterning and the membrane deformation, gives rise to the topographical guidance. Our results suggest that the macropinocytic cup is a self-enclosing structure that can support liquid ingestion by default; however, in the presence of structured surfaces, it is directed to faithfully trace bent and bifurcating ridges for particle ingestion and cell guidance.

Topographic Cues Guiding Cell Polarization via Distinct Cellular Mechanosensing Pathways

Cell polarization exists in a variety of tissues to regulate cell behaviors and functions. Space constraint (spatially limiting cell extension) and adhesion induction (guiding adhesome growth) are two main ways to induce cell polarization according to the microenvironment topographies. However, the mechanism of cell polarization induced by these two ways and the downstream effects on cell functions are yet to be understood. Here, space constraint and adhesion induction guiding cell polarization are achieved by substrate groove arrays in micro and nano size, respectively. Although the morphology of polarized cells is similar on both structures, the signaling pathways to induce the cell polarization and the downstream functions are distinctly different. The adhesion induction (nano-groove) leads to the formation of focal adhesions and activates the RhoA/ROCK pathway to enhance the myosin-based intracellular force, while the space constraint (micro-groove) only activates the formation of pseudopodia. The enhanced intracellular force caused by adhesion induction inhibits the chromatin condensation, which promotes the osteogenic differentiation of stem cells. This study presents an overview of cell polarization and mechanosensing at biointerface to aid in the design of novel biomaterials.

Moving the Research Forward: The Best of British Biology Using the Tractable Model System Dictyostelium discoideum

The social amoeba Dictyostelium discoideum provides an excellent model for research across a broad range of disciplines within biology. The organism diverged from the plant, yeast, fungi and animal kingdoms around 1 billion years ago but retains common aspects found in these kingdoms. Dictyostelium has a low level of genetic complexity and provides a range of molecular, cellular, biochemical and developmental biology experimental techniques, enabling multidisciplinary studies to be carried out in a wide range of areas, leading to research breakthroughs. Numerous laboratories within the United Kingdom employ Dictyostelium as their core research model. This review introduces Dictyostelium and then highlights research from several leading British research laboratories, covering their distinct areas of research, the benefits of using the model, and the breakthroughs that have arisen due to the use of Dictyostelium as a tractable model system.

Collective cell migration driven by filopodia-New insights from the social behavior of myotubes

Collective migration is a key process that is critical during development, as well as in physiological and pathophysiological processes including tissue repair, wound healing and cancer. Studies in genetic model organisms have made important contributions to our current understanding of the mechanisms that shape cells into different tissues during morphogenesis. Recent advances in high-resolution and live-cell-imaging techniques provided new insights into the social behavior of cells based on careful visual observations within the context of a living tissue. In this review, we will compare Drosophila testis nascent myotube migration with established in vivo model systems, elucidate similarities, new features and principles in collective cell migration.

Cyclic AMP is dispensable for allorecognition in Dictyostelium cells overexpressing PKA-C

Allorecognition and tissue formation are interconnected processes that require signaling between matching pairs of the polymorphic transmembrane proteins TgrB1 and TgrC1 in Dictyostelium. Extracellular and intracellular cAMP signaling are essential to many developmental processes. The three adenylate cyclase genes, acaA, acrA and acgA are required for aggregation, culmination and spore dormancy, respectively, and some of their functions can be suppressed by activation of the cAMP-dependent protein kinase PKA. Previous studies have suggested that cAMP signaling might be dispensable for allorecognition and tissue formation, while others have argued that it is essential throughout development. Here, we show that allorecognition and tissue formation do not require cAMP production as long as PKA is active. We eliminated cAMP production by deleting the three adenylate cyclases and overexpressed PKA-C to enable aggregation. The cells exhibited cell polarization, tissue formation and cooperation with allotype-compatible wild-type cells, but not with incompatible cells. Therefore, TgrB1-TgrC1 signaling controls allorecognition and tissue formation, while cAMP is dispensable as long as PKA-C is overexpressed.

Learning the dynamics of cell-cell interactions in confined cell migration

The migratory dynamics of cells in physiological processes, ranging from wound healing to cancer metastasis, rely on contact-mediated cell-cell interactions. These interactions play a key role in shaping the stochastic trajectories of migrating cells. While data-driven physical formalisms for the stochastic migration dynamics of single cells have been developed, such a framework for the behavioral dynamics of interacting cells still remains elusive. Here, we monitor stochastic cell trajectories in a minimal experimental cell collider: a dumbbell-shaped micropattern on which pairs of cells perform repeated cellular collisions. We observe different characteristic behaviors, including cells reversing, following, and sliding past each other upon collision. Capitalizing on this large experimental dataset of coupled cell trajectories, we infer an interacting stochastic equation of motion that accurately predicts the observed interaction behaviors. Our approach reveals that interacting noncancerous MCF10A cells can be described by repulsion and friction interactions. In contrast, cancerous MDA-MB-231 cells exhibit attraction and antifriction interactions, promoting the predominant relative sliding behavior observed for these cells. Based on these experimentally inferred interactions, we show how this framework may generalize to provide a unifying theoretical description of the diverse cellular interaction behaviors of distinct cell types.

Dynamic Self-Organization of Idealized Migrating Cells by Contact Communication

This Letter investigates what forms of cellular dynamic self-organization are caused through intercellular contact communication based on a theoretical model in which migrating cells perform contact following and contact inhibition and attraction of locomotion. Tuning those strengths causes varieties of dynamic patterns. This further includes a novel form of collective migration, snakelike dynamic assembly. Scrutinizing this pattern reveals that cells in this state can accurately respond to an external directional cue but have no spontaneous global polar order.

Molecular Regulators of Cellular Mechanoadaptation at Cell-Material Interfaces

Diverse essential cellular behaviors are determined by extracellular physical cues that are detected by highly orchestrated subcellular interactions with the extracellular microenvironment. To maintain the reciprocity of cellular responses and mechanical properties of the extracellular matrix, cells utilize a variety of signaling pathways that transduce biophysical stimuli to biochemical reactions. Recent advances in the micromanipulation of individual cells have shown that cellular responses to distinct physical and chemical features of the material are fundamental determinants of cellular mechanosensation and mechanotransduction. In the process of outside-in signal transduction, transmembrane protein integrins facilitate the formation of focal adhesion protein clusters that are connected to the cytoskeletal architecture and anchor the cell to the substrate. The linkers of nucleoskeleton and cytoskeleton molecular complexes, collectively termed LINC, are critical signal transducers that relay biophysical signals between the extranuclear cytoplasmic region and intranuclear nucleoplasmic region. Mechanical signals that involve cytoskeletal remodeling ultimately propagate into the nuclear envelope comprising the nuclear lamina in assistance with various nuclear membrane proteins, where nuclear mechanics play a key role in the subsequent alteration of gene expression and epigenetic modification. These intracellular mechanical signaling cues adjust cellular behaviors directly associated with mechanohomeostasis. Diverse strategies to modulate cell-material interfaces, including alteration of surface rigidity, confinement of cell adhesive region, and changes in surface topology, have been proposed to identify cellular signal transduction at the cellular and subcellular levels. In this review, we will discuss how a diversity of alterations in the physical properties of materials induce distinct cellular responses such as adhesion, migration, proliferation, differentiation, and chromosomal organization. Furthermore, the pathological relevance of misregulated cellular mechanosensation and mechanotransduction in the progression of devastating human diseases, including cardiovascular diseases, cancer, and aging, will be extensively reviewed. Understanding cellular responses to various extracellular forces is expected to provide new insights into how cellular mechanoadaptation is modulated by manipulating the mechanics of extracellular matrix and the application of these materials in clinical aspects.

Cell Migration Guided by Cell-Cell Contacts in Innate Immunity

The directed migration of leukocytes to sites of damage or infection is necessary for a productive immune response. There is substantial evidence supporting a key role for chemoattractants in directed migration, however, less is known about how cell-cell contacts affect the migratory behavior of leukocytes in innate immunity. Here, we explore how cell-cell contacts can affect the directed migration of innate immune cells, including their role in attracting, repelling, or stopping cell motility. Further investigation of cell contact dynamics as guidance cues may yield new insights into the regulation of innate immunity.

Development of an Enhanced-Throughput Radial Cell Migration Device

It is often desirable to evaluate the ability of cells to move in an unrestricted manner in multiple directions without chemical gradients. By combining the standard radial migration assay with injection-molded gaskets and a rigid fixture, we have developed a highly reliable and sensitive method for observing and measuring radial cell migration. This method is adapted for use on high-throughput automated imaging systems. The use of injection-molded gaskets enables low-cost replacement of cell-wetted components. Moreover, the design enables secondary placement of attractants and co-cultures. This device and its enhanced throughput permit the use of therapeutic screening to evaluate phenotypic responses, for example, cancer cell migration response due to drugs or chemical signals. This approach is orthogonal to other 2D cell migration applications, such as scratch wound assays, although here we offer a noninvasive, enhanced-throughput device, which currently is not commercially available but is easily constructed. The proposed device is a systematic, reliable, rapid application to monitor phenotypic responses to chemotherapeutic screens, genetic alterations (e.g., RNAi and CRISPR), supplemental regimens, and other approaches, offering a reliable methodology to survey unbiased and noninvasive cell migration.

Interplay of mesoscale physics and agent-like behaviors in the parallel evolution of aggregative multicellularity

Myxobacteria and dictyostelids are prokaryotic and eukaryotic multicellular lineages, respectively, that after nutrient depletion aggregate and develop into structures called fruiting bodies. The developmental processes and resulting morphological outcomes resemble one another to a remarkable extent despite their independent origins, the evolutionary distance between them and the lack of traceable homology in molecular mechanisms. We hypothesize that the morphological parallelism between the two lineages arises as the consequence of the interplay within multicellular aggregates between generic processes, physical and physicochemical processes operating similarly in living and non-living matter at the mesoscale (~10^-3-10^-1 m) and agent-like behaviors, unique to living systems and characteristic of the constituent cells, considered as autonomous entities acting according to internal rules in a shared environment. Here, we analyze the contributions of generic and agent-like determinants in myxobacteria and dictyostelid development and their roles in the generation of their common traits. Consequent to aggregation, collective cell-cell contacts mediate the emergence of liquid-like properties, making nascent multicellular masses subject to novel patterning and morphogenetic processes. In both lineages, this leads to behaviors such as streaming, rippling, and rounding-up, as seen in non-living fluids. Later the aggregates solidify, leading them to exhibit additional generic properties and motifs. Computational models suggest that the morphological phenotypes of the multicellular masses deviate from the predictions of generic physics due to the contribution of agent-like behaviors of cells such as directed migration, quiescence, and oscillatory signal transduction mediated by responses to external cues. These employ signaling mechanisms that reflect the evolutionary histories of the respective organisms. We propose that the similar developmental trajectories of myxobacteria and dictyostelids are more due to shared generic physical processes in coordination with analogous agent-type behaviors than to convergent evolution under parallel selection regimes. Insights from the biology of these aggregative forms may enable a unified understanding of developmental evolution, including that of animals and plants.

Rules of collective migration: from the wildebeest to the neural crest

Collective migration, the movement of groups in which individuals affect the behaviour of one another, occurs at practically every scale, from bacteria up to whole species' populations. Universal principles of collective movement can be applied at all levels. In this review, we will describe the rules governing collective motility, with a specific focus on the neural crest, an embryonic stem cell population that undergoes extensive collective migration during development. We will discuss how the underlying principles of individual cell behaviour, and those that emerge from a supracellular scale, can explain collective migration. This article is part of the theme issue 'Multi-scale analysis and modelling of collective migration in biological systems'.

Polar pattern formation induced by contact following locomotion in a multicellular system

Biophysical mechanisms underlying collective cell migration of eukaryotic cells have been studied extensively in recent years. One mechanism that induces cells to correlate their motions is contact inhibition of locomotion, by which cells migrating away from the contact site. Here, we report that tail-following behavior at the contact site, termed contact following locomotion (CFL), can induce a non-trivial collective behavior in migrating cells. We show the emergence of a traveling band showing polar order in a mutant Dictyostelium cell that lacks chemotactic activity. We find that CFL is the cell-cell interaction underlying this phenomenon, enabling a theoretical description of how this traveling band forms. We further show that the polar order phase consists of subpopulations that exhibit characteristic transversal motions with respect to the direction of band propagation. These findings describe a novel mechanism of collective cell migration involving cell-cell interactions capable of inducing traveling band with polar order.

Intracellular ATP levels influence cell fates in Dictyostelium discoideum differentiation

Multicellular organisms contain various differentiated cells. Fate determination of these cells remains a fundamental issue. The cellular slime mold Dictyostelium discoideum is a useful model organism for studying differentiation; it proliferates as single cells in nutrient-rich conditions, which aggregate into a multicellular body upon starvation, subsequently differentiating into stalk cells or spores. The fates of these cells can be predicted in the vegetative phase: Cells expressing higher and lower levels of omt12 differentiate into stalk cells and spores, respectively. However, omt12 is merely a marker gene and changes in its expression do not influence the cell fate, and determinant factors remain unknown. In this study, we analyzed cell fate determinants in the stalk-destined and spore-destined cells that were sorted based on omt12 expression. Luciferase assay demonstrated higher levels of intracellular ATP in the stalk-destined cells than in the spore-destined cells. Live-cell observation during development using ATP sensor probes revealed that cells with higher ATP levels differentiated into stalk cells. Furthermore, reducing the ATP level by treating with an inhibitor of ATP production changed the differentiation fates of the stalk-destined cells to spores. These results suggest that intracellular ATP levels influence cell fates in D. discoideum differentiation.

Talin B regulates collective cell migration via PI3K signaling in Dictyostelium discoideum mounds

Collective cell migration is a key process during the development of multicellular organisms, in which the migrations of individual cells are coordinated through chemical guidance and physical contact between cells. Talin has been implicated in mechanical linkage between actin-based motile machinery and adhesion molecules, but how talin contributes to collective cell migration is unclear. Here we show that talin B is involved in chemical coordination between cells for collective cell migration at the multicellular mound stage in the development of Dictyostelium discoideum. From early aggregation to the mound formation, talB-null cells exhibited collective migration normally with cAMP relay. Subsequently, talB-null cells showed developmental arrest at the mound stage, and at the same time, they had impaired collective migration and cAMP relay, while wild-type cells exhibited rotational cell migration continuously in concert with cAMP relay during the mound stage. Genetic suppression of PI3K activity partially restored talB-null phenotypes in collective cell migration and cAMP relay. Overall, our observations suggest that talin B regulates chemical coordination via PI3K-mediated signaling in a stage-specific manner for the multicellular development of Dictyostelium cells.

Cell interactions in collective cell migration

Collective cell migration is the coordinated movement of a physically connected group of cells and is a prominent driver of development and metastasis. Interactions between cells within migrating collectives, and between migrating cells and other cells in the environment, play key roles in stimulating motility, steering and sometimes promoting cell survival. Similarly, diverse heterotypic interactions and collective behaviors likely contribute to tumor metastasis. Here, we describe a sampling of cells that migrate collectively in vivo, including well-established and newer examples. We focus on the under-appreciated property that many - perhaps most - collectively migrating cells move as cooperating groups of distinct cell types.

Live cell imaging of cell movement and transdifferentiation during regeneration of an amputated multicellular body of the social amoeba Dictyostelium discoideum

The regeneration of lost body parts is a fascinating phenomenon exhibited by some multicellular organisms. In social amoebae, such as Dictyostelium discoideum, the pseudoplasmodium is a temporary migratory multicellular structure with high regeneration ability. It consists of future stalk cells (prestalk cells) at the anterior end and future spore cells (prespore cells) at the posterior end, and if amputated, the remaining cells can rapidly regenerate the lost portion within several hours. Details of this regeneration event have been extensively documented; however, little is known about the behavior of individual cells involved in this process. In this study, we performed live cell imaging of cell behavior during regeneration of the excised anterior prestalk region. We used cells that specifically express GFP in the prestalk cell lineage to examine how the prestalk region is regenerated after this region is excised. The current model of prestalk regeneration suggests that the progenitors of prestalk cells, known as anterior-like cells (ALCs), which are sparsely distributed in the prespore region, are redistributed to form the new prestalk region. However, we found that the regenerated prestalk region was formed mainly by the transdifferentiation of prespore cells surrounding the excised anterior end, with little clustering of pre-existing ALCs. Furthermore, the movement of randomly distributed labeled cells during regeneration revealed that although the posterior end was deformed and rounded in shape, the relative position of cells along the anterior-posterior axis remained largely unchanged. These results suggest that the original anterior-posterior axis is maintained in posterior bodies and that prespore cells at the anterior side transdifferentiate and regenerate the prestalk region.

Building Complex Life Through Self-Organization

Cells are inherently conferred with the ability to self-organize into the tissues and organs comprising the human body. Self-organization can be recapitulated in vitro and recent advances in the organoid field are just one example of how we can generate small functioning elements of organs. Tissue engineers can benefit from the power of self-organization and should consider how they can harness and enhance the process with their constructs. For example, aggregates of stem cells and tissue-specific cells benefit from the input of carefully selected biomolecules to guide their differentiation toward a mature phenotype. This can be further enhanced by the use of technologies to provide a physiological microenvironment for self-organization, enhance the size of the constructs, and enable the long-term culture of self-organized structures. Of importance, conducting self-organization should be limited to fine-tuning and should avoid over-engineering that could counteract the power of inherent cellular self-organization. Impact Statement Self-organization is a powerful innate feature of cells that can be fine-tuned but not over-engineered to create new tissues and organs.

Cellular allorecognition and its roles in Dictyostelium development and social evolution

The social amoeba Dictyostelium discoideum is a tractable model organism to study cellular allorecognition, which is the ability of a cell to distinguish itself and its genetically similar relatives from more distantly related organisms. Cellular allorecognition is ubiquitous across the tree of life and affects many biological processes. Depending on the biological context, these versatile systems operate both within and between individual organisms, and both promote and constrain functional heterogeneity. Some of the most notable allorecognition systems mediate neural self-avoidance in flies and adaptive immunity in vertebrates. D. discoideum's allorecognition system shares several structures and functions with other allorecognition systems. Structurally, its key regulators reside at a single genomic locus that encodes two highly polymorphic proteins, a transmembrane ligand called TgrC1 and its receptor TgrB1. These proteins exhibit isoform-specific, heterophilic binding across cells. Functionally, this interaction determines the extent to which co-developing D. discoideum strains co-aggregate or segregate during the aggregation phase of multicellular development. The allorecognition system thus affects both development and social evolution, as available evidence suggests that the threat of developmental cheating represents a primary selective force acting on it. Other significant characteristics that may inform the study of allorecognition in general include that D. discoideum's allorecognition system is a continuous and inclusive trait, it is pleiotropic, and it is temporally regulated.