A feedback loop between lamellipodial extension and HGF-ERK signaling specifies leader cells during collective cell migration

The transcriptional regulator Sin3a activates CD44 and promotes collective luminal breast cancer cell migration

Luminal type breast cancer (BrCa) cells invade into the surrounding tissues as collective strands, making them more metastatic than single cells. We have previously reported that the leading subpopulation of collective cells expressed high levels of CD44, which was associated with enhanced migratory and invasive potential of BrCa. It is crucial to elucidate how CD44 becomes enriched in leader cells and contributes to collective migration. In this study, we aimed to uncover the mechanisms responsible for CD44 upregulation in this context. First, we demonstrated that CD44 could facilitate dynamic lamellipodia formation by interacting with cytoskeletal proteins through its cytoplasmic domain. Then, we identified that a transcriptional regulator, Sin3a, was remarkably upregulated at the front edge of collectively migrating cells, exhibiting a correlation with enhanced CD44 expression. Notably, the knockdown of Sin3a effectively suppressed CD44 enrichment and lamellipodia outgrowth in leader cells, resulting in a significantly decreased cohesive movement of BrCa cells in vitro and in vivo. Our findings suggested that Sin3a was a novel regulator in CD44-facilitated lamellipodia formation and subsequent collective migration. This study elucidated the molecular mechanism underlying CD44 upregulation during collective migration of luminal type BrCa cells, providing potential therapeutic targets to prevent cancer metastasis.

Morphological control of merlin-Rac antagonism in proliferation-promoting signaling

The extension of lamellipodia, which are thin, fanlike projections at the cell periphery, requires the assembly of branched actin networks under the control of the small GTPase Rac1. In melanoma, a hyperactive P29S Rac1 mutant is associated with resistance to inhibitors that target the kinases BRAF and MAPK and with more aggressive disease because it sequesters and inactivates the tumor suppressor merlin (encoded by NF2) inside abnormally large lamellipodia. Here, we investigated how these merlin-inactivating lamellipodia are maintained using quantitative, live cell imaging of cell morphology and signaling dynamics. We showed that Rac1 and merlin activity were regulated in spatially confined regions or microdomains within the lamellipodium. The role of merlin as a proliferation-limiting tumor suppressor required its ability to inhibit lamellipodial extension and to locally inhibit Rac1 signaling. Conversely, local inactivation of merlin in lamellipodia released these restraints on morphology and signaling, leading to enhanced proliferation. Merlin and Rac1 are thus in a morphologically and dynamically regulated double-negative feedback loop, a signaling motif that can amplify and stabilize modest stimuli of lamellipodia extensions that enable melanoma to sustain mitogenic signaling under growth challenge. This represents an example of how acute oncogenicity is promoted by collaborations between cell morphological programs and biochemical signaling.

Gossiping about death: Apoptosis-induced ERK waves as coordinators of multicellular fate decisions

Apoptosis is now recognized as a highly dynamic process that involves the release of a large set of signaling molecules that convey information to cells neighboring an apoptotic site. Recent studies in epithelial systems have discovered that apoptotic cells trigger waves of pulses of mitogen-activated protein kinase (MAPK) / extracellular signal-regulated kinase (ERK) pathway activity in their neighbors. At the single-cell level, the ERK pulses emerge from the MAPK pathway's excitable network properties, such as ultrasensitivity and adaptation. At the cell population level, apoptosis-induced ERK waves (AiEWs) emerge from propagation of ERK pulses across cells via a mechanism that involves mechanical inputs and paracrine signaling. AiEWs enable cell populations to dynamically coordinate fate decision signaling during tissue homeostasis and development. This spatio-temporal signaling mechanism can be hijacked by cancer cells to induce drug-tolerant persister states when apoptosis is triggered by cytotoxic or targeted therapies, undermining treatment efficacy. In this review, we summarize our current understanding of AiEWs, including their initiation, propagation, and coordination of fate decision signaling within a population. We discuss how the relatively simple properties of single cells, and their interactions within a collective coordinate these dynamic signaling patterns. We highlight their implication in resistance to cancer therapy and explore potential strategies to target these waves to re-sensitize cancer cells. Finally, we discuss emerging technologies and future directions to expand the study of this biological phenomenon.

Internal cohesion gradient as a novel mechanism of collective cell migration

Experiments demonstrate that individual cells that wander stochastically can migrate persistently as a cluster. We show by simulating cells and their interactions that collective migration by omnidirectional cells is a generic phenomenon that can be expected to arise whenever (a) leading and trailing cells migrate randomly, and (b) leading cells are more closely packed than trailing neighbors. The first condition implies that noise is essential to cluster motion, while the second implies that an internal cohesion gradient can drive external motion of a cluster. Unlike other swarming phenomena, we find that this effect is driven by cohesion asymmetry near the leading cell, and motion of interior cells contribute minimally - and in fact interfere with - a cluster's persistent migration.

Epithelial tubule interconnection driven by HGF-Met signaling in the kidney

The formation of functional epithelial tubules is critical for the development and maintenance of many organ systems. While the mechanisms of tubule formation by epithelial cells are well studied, the process of tubule anastomosis-where tubules connect to form a continuous network-remains poorly understood. In this study, we utilized single-cell RNA sequencing to analyze embryonic mouse kidney tubules undergoing anastomosis. Our analysis identified hepatocyte growth factor (HGF) as a key potential mediator of this process. To investigate this further, we developed an assay using epithelial spheroids with fluorescently tagged apical surfaces, allowing us to visualize and quantify tubule-tubule connections. Our results demonstrate that HGF promotes tubule anastomosis, and it does so through the MAPK signaling pathway and MMPs, independently of cell proliferation. Remarkably, treatment with HGF and collagenase was sufficient to induce tubule anastomosis in embryonic mouse kidneys. These findings provide a foundational understanding of how to enhance the formation of functional tubular networks. This has significant clinical implications for the use of in vitro-grown kidney tissues in transplant medicine, potentially improving the success and integration of transplanted tissues.

pERK transition-induced directional mode switching promotes epithelial tumor cell migration

Increasing evidence suggests that tumor cells exhibit extreme plasticity in migration modes in order to adapt to microenvironments. However, the underlying mechanism for governing the migration mode switching is still unclear. Here, we revealed that epithelial tumor cells could develop a stable directional mode driven by hyperactivated ERK activity. This highly activated and dynamically changing ERK activity, called pERK transition, is crucial for inducing the switch from pauses state to directional movement and is also necessary for maintaining epithelial tumor cells in the directional mode. PERK transition integrated pERK surf, the dynamic and localized ERK activity at the leading edge. The sequential activation of RhoA and Rac1 by pERK transition played critical roles in generation of pERK surf activity through a movement feedback mechanism. PERK transition activity converted the orderly collective migration into the disordered dispersal movement, enhanced the invasiveness of epithelial tumor cells, and promoted their metastasis in immune-deficient mice. These findings revealed that the exquisite spatiotemporal organization of ERK activity orchestrates migration and invasion of tumor cells and provide evidence for the mechanism underlying migration mode switching in epithelial tumor cells.

Low-affinity ligands of the epidermal growth factor receptor are long-range signal transmitters in collective cell migration of epithelial cells

Canonical epidermal growth factor (EGF) receptor (EGFR) activation involves the binding of seven EGFR ligands (EGFRLs); however, their extracellular dynamics remain elusive. Here, employing fluorescent probes and a tool for triggering ectodomain shedding, we show that epiregulin (EREG), a low-affinity EGFRL, rapidly and efficiently activates EGFR in Madin-Darby canine kidney (MDCK) epithelial cells and mouse epidermis. During collective cell migration, EGFR and extracellular signal-regulated kinase (ERK) activation waves propagate in an a disintegrin and metalloprotease 17 (ADAM17) sheddase- and EGFRL-dependent manner. Upon induced EGFRL shedding, low-affinity ligands EREG and amphiregulin (AREG) mediate faster and broader ERK waves than high-affinity ligands. Tight/adherens junction integrity is essential for ERK activation propagation, suggesting that tight intercellular spaces prefer the low-affinity EGFRLs for efficient signal transmission. In EREG-deficient mice, ERK wave propagation and cell migration were impaired during skin wound repair. We additionally show that heparin-binding EGF-like growth factor (HBEGF) primarily promotes surrounding cell motility. Our findings underscore the pivotal role of low-affinity EGFRLs in rapid intercellular signal transmission.

Low-affinity ligands of the epidermal growth factor receptor are long-range signal transmitters during collective cell migration of epithelial cells

Epidermal growth factor receptor ligands (EGFRLs) consist of seven proteins. In stark contrast to the amassed knowledge concerning the epidermal growth factor receptors themselves, the extracellular dynamics of individual EGFRLs remain elusive. Here, employing fluorescent probes and a tool for triggering ectodomain shedding of EGFRLs, we show that EREG, a low-affinity EGFRL, exhibits the most rapid and efficient activation of EGFR in confluent epithelial cells and mouse epidermis. In Madin-Darby canine kidney (MDCK) renal epithelial cells, EGFR- and ERK-activation waves propagate during collective cell migration in an ADAM17 sheddase- and EGFRL-dependent manner. Upon induction of EGFRL shedding, radial ERK activation waves were observed in the surrounding receiver cells. Notably, the low-affinity ligands EREG and AREG mediated faster and broader ERK waves than the high-affinity ligands. The integrity of tight/adherens junctions was essential for the propagation of ERK activation, implying that the tight intercellular spaces prefer the low-affinity EGFRL to the high-affinity ligands for efficient signal transmission. To validate this observation in vivo , we generated EREG-deficient mice expressing the ERK biosensor and found that ERK wave propagation and cell migration were impaired during skin wound repair. In conclusion, we have quantitatively demonstrated the distinctions among EGFRLs in shedding, diffusion, and target cell activation in physiological contexts. Our findings underscore the pivotal role of low-affinity EGFRLs in rapid intercellular signal transmission.

Epithelial tubule interconnection driven by HGF-Met signaling in the kidney

The formation of functional epithelial tubules is a central feature of many organ systems. Although the process of tubule formation by epithelial cells is well-studied, the way in which tubules connect with each other (i.e. anastomose) to form functional networks both in vivo and in vitro is not well understood. A key, unanswered question in the kidney is how the renal vesicles of the embryonic kidney connect with the nascent collecting ducts to form a continuous urinary system. We performed a ligand-receptor pair analysis on single cell RNA-seq data from embryonic mouse kidney tubules undergoing anastomosis to select candidates that might mediate this process in vivo. This analysis identified hepatocyte growth factor (HGF), which has known roles in cell proliferation, migration, and tubulogenesis, as one of several possible candidates. To test this possibility, we designed a novel assay to quantitatively examine epithelial tubule anastomosis in vitro using epithelial spheroids with fluorescently-tagged apical surfaces to enable direct visualization of anastomosis. This revealed that HGF is a potent inducer of tubule anastomosis. Tubule anastomosis occurs through a proliferation-independent mechanism that acts through the MAPK signaling cascade and matrix metalloproteinases (MMPs), the latter suggestive of a role in extracellular matrix turnover. Accordingly, treatment of explanted embryonic mouse kidneys with HGF and collagenase was sufficient to induce kidney tubule anastomosis. These results lay the groundwork for investigating how to promote functional interconnections between tubular epithelia, which have important clinical implications for utilizing in vitro grown kidney tissue in transplant medicine.

The mitogen-activated protein kinase network, wired to dynamically function at multiple scales

The mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling network is a key transducer of signals from various receptors, including receptor tyrosine kinases (RTKs). It controls cell-cycle entry, survival, motility, differentiation, as well as other fates. After four decades of studying this pathway with biochemical methods, the use of fluorescent biosensors has revealed dynamic behaviors such as ERK pulsing, oscillations, and amplitude-modulated activity. Different RTKs equip the MAPK network with specific feedback mechanisms to encode these different ERK dynamics, which are then subsequently decoded into cytoskeletal events and transcriptional programs, actuating cellular fates. Recently, collective ERK wave behaviors have been observed in multiple systems to coordinate cytoskeletal dynamics with fate decisions within cell collectives. This emphasizes that a correct understanding of this pathway requires studying it at multiple scales.

Deficiency of the HGF/Met pathway leads to thyroid dysgenesis by impeding late thyroid expansion

The mechanisms of bifurcation, a key step in thyroid development, are largely unknown. Here we find three zebrafish lines from a forward genetic screening with similar thyroid dysgenesis phenotypes and identify a stop-gain mutation in hgfa and two missense mutations in met by positional cloning from these zebrafish lines. The elongation of the thyroid primordium along the pharyngeal midline was dramatically disrupted in these zebrafish lines carrying a mutation in hgfa or met. Further studies show that MAPK inhibitor U0126 could mimic thyroid dysgenesis in zebrafish, and the phenotypes are rescued by overexpression of constitutively active MEK or Snail, downstream molecules of the HGF/Met pathway, in thyrocytes. Moreover, HGF promotes thyrocyte migration, which is probably mediated by downregulation of E-cadherin expression. The delayed bifurcation of the thyroid primordium is also observed in thyroid-specific Met knockout mice. Together, our findings reveal that HGF/Met is indispensable for the bifurcation of the thyroid primordium during thyroid development mediated by downregulation of E-cadherin in thyrocytes via MAPK-snail pathway.

Staying away from the breaking point: Probing the limits of epithelial cell elimination

Epithelial tissues are dramatically remodelled during embryogenesis and tissue homeostasis and yet need to maintain their sealing properties to sustain their barrier functions at any time. Part of these remodellings involve the elimination of a large proportion of cells through apoptosis. Cell extrusion, the remodelling steps leading to seamless dying cell expulsion, helps to maintain tissue cohesion. However, there is an intrinsic limit in the system that can only accommodate a certain proportion/rate of cell elimination as well as certain spatiotemporal distributions. What are then the critical conditions leading to epithelial rupture/tear/sealing defects upon cell elimination and which mechanisms ensure that such limits are never reached? In this short review, I document the conditions in which epithelial rupture has been observed, including in the contexts of epithelial cell death, and the mechanical parameters influencing tissue rupture, and review feedback mechanisms which help to keep the epithelia away from the breaking point.

Emerging roles and mechanisms of ERK pathway mechanosensing

The coupling between mechanical forces and modulation of cell signalling pathways is essential for tissue plasticity and their adaptation to changing environments. Whilst the number of physiological and pathological relevant roles of mechanotransduction has been rapidly expanding over the last decade, studies have been mostly focussing on a limited number of mechanosensitive pathways, which include for instance Hippo/YAP/TAZ pathway, Wnt/β-catenin or the stretch-activated channel Piezo. However, the recent development and spreading of new live sensors has provided new insights into the contribution of ERK pathway in mechanosensing in various systems, which emerges now as a fast and modular mechanosensitive pathway. In this review, we will document key in vivo and in vitro examples that have established a clear link between cell deformation, mechanical stress and modulation of ERK signalling, comparing the relevant timescale and mechanical stress. We will then discuss different molecular mechanisms that have been proposed so far, focussing on the epistatic link between mechanics and ERK and discussing the relevant cellular parameters affecting ERK signalling. We will finish by discussing the physiological and the pathological consequences of the link between ERK and mechanics, outlining how this interplay is instrumental for self-organisation and long-range cell-cell coordination.

Fluorescent biosensors illuminate the spatial regulation of cell signaling across scales

As cell signaling research has advanced, it has become clearer that signal transduction has complex spatiotemporal regulation that goes beyond foundational linear transduction models. Several technologies have enabled these discoveries, including fluorescent biosensors designed to report live biochemical signaling events. As genetically encoded and live-cell compatible tools, fluorescent biosensors are well suited to address diverse cell signaling questions across different spatial scales of regulation. In this review, methods of examining spatial signaling regulation and the design of fluorescent biosensors are introduced. Then, recent biosensor developments that illuminate the importance of spatial regulation in cell signaling are highlighted at several scales, including membranes and organelles, molecular assemblies, and cell/tissue heterogeneity. In closing, perspectives on how fluorescent biosensors will continue enhancing cell signaling research are discussed.

Increased collective migration correlates with germline stem cell competition in a basal chordate

Cell competition is a process that compares the relative fitness of progenitor cells, resulting in winners, which contribute further to development, and losers, which are excluded, and is likely a universal quality control process that contributes to the fitness of an individual. Cell competition also has pathological consequences, and can create super-competitor cells responsible for tumor progression. We are studying cell competition during germline regeneration in the colonial ascidian, Botryllus schlosseri. Germline regeneration is due to the presence of germline stem cells (GSCs) which have a unique property: a competitive phenotype. When GSCs from one individual are transplanted into another, the donor and recipient cells compete for germline development. Often the donor GSCs win, and completely replace the gametes of the recipient- a process called germ cell parasitism (gcp). gcp is a heritable trait, and winner and loser genotypes can be found in nature and reared in the lab. However, the molecular and cellular mechanisms underlying gcp are unknown. Using an ex vivo migration assay, we show that GSCs isolated from winner genotypes migrate faster and in larger clusters than losers, and that cluster size correlates with expression of the Notch ligand, Jagged. Both cluster size and jagged expression can be manipulated simultaneously in a genotype dependent manner: treatment of loser GSCs with hepatocyte growth factor increases both jagged expression and cluster size, while inhibitors of the MAPK pathway decrease jagged expression and cluster size in winner GSCs. Live imaging in individuals transplanted with labeled winner and loser GSCs reveal that they migrate to the niche, some as small clusters, with the winners having a slight advantage in niche occupancy. Together, this suggests that the basis of GSC competition resides in a combination in homing ability and niche occupancy, and may be controlled by differential utilization of the Notch pathway.

Stretching the limits of extracellular signal-related kinase (ERK) signaling - Cell mechanosensing to ERK activation

Extracellular signal-regulated kinase (ERK) has been recognized as a critical regulator in various physiological and pathological processes. Extensive research has elucidated the signaling mechanisms governing ERK activation via biochemical regulations with upstream molecules, particularly receptor tyrosine kinases (RTKs). However, recent advances have highlighted the role of mechanical forces in activating the RTK-ERK signaling pathways, thereby opening new avenues of research into mechanochemical interplay in multicellular tissues. Here, we review the force-induced ERK activation in cells and propose possible mechanosensing mechanisms underlying the mechanoresponsive ERK activation. We conclude that mechanical forces are not merely passive factors shaping cells and tissues but also active regulators of cellular signaling pathways controlling collective cell behaviors.

Cell-stretching devices: advances and challenges in biomedical research and live-cell imaging

Basic human functions such as breathing and digestion require mechanical stretching of cells and tissues. However, when it comes to laboratory experiments, the mechanical stretching that cells experience in the body is not often replicated, limiting the biomimetic nature of the studies and the relevance of results. Herein, we establish the importance of mechanical stretching during in vitro investigations by reviewing seminal works performed using cell-stretching platforms, highlighting important outcomes of these works as well as the engineering characteristics of the platforms used. Emphasis is placed on the compatibility of cell-stretching devices (CSDs) with live-cell imaging as well as their limitations and on the research advancements that could arise from live-cell imaging performed during cell stretching.

Hemocytes of Yesso scallop characterized by cytological, molecular marker, and functional analyses

Bivalve hemocytes have pivotal role as cellular biodefense. However, no information is available for cytological parameters, marker gene and function of the hemocytes in Yesso scallop, a commercially important aquaculture species worldwide. Due to their extremely strong cell aggregation ability, the scallop hemocytes were not able to assess as a single cell so far. In the present study, we established methodologies for studying the hemocytes of Yesso scallop, assessed cell morphology, measured seasonal fluctuation, and analyzed transcriptomes and cellular behavior during the immune response. Our results showed that the Yesso scallop possesses a single type of leukocyte-type hemocytes similar to other bivalve granulocytes circulating at an average of 1 × 10⁷ cells/ml throughout the year. In addition, we identified five molecular marker genes specific to the scallop hemocytes. These hemocyte markers enabled us to precisely detect the hemocyte localization. Using these markers, we confirmed that tissue transplantation can experimentally induce an immune response, leading to the mobilization of circulating hemocytes for encapsulation. This study provides a comprehensive understanding of scallop hemocytes and their role in the cellular biodefense system of bivalves and various methods for cytological analysis.

Exploring membrane mechanics: The role of membrane-cortex attachment in cell dynamics

The role of plasma membrane (PM) tension in cell dynamics has gained increasing interest in recent years to understand the mechanism by which individual cells regulate their dynamic behavior. Membrane-to-cortex attachment (MCA) is a component of apparent PM tension, and its assembly and disassembly determine the direction of cell motility, controlling the driving forces of migration. There is also evidence that membrane tension plays a role in malignant cancer cell metastasis and stem cell differentiation. Here, we review recent important discoveries that explore the role of membrane tension in the regulation of diverse cellular processes, and discuss the mechanisms of cell dynamics regulated by this physical parameter.