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Cells in tissues can organize into a broad spectrum of structures according to their function. Drastic changes of organization, such as epithelial–mesenchymal transitions or the formation of spheroidal aggregates, are often associated either to tissue morphogenesis or to cancer progression. Here, we study the organization of cell colonies by means of simulations of self-propelled particles with generic cell-like interactions. The interplay between cell softness, cell–cell adhesion, and contact inhibition of locomotion (CIL) yields structures and collective dynamics observed in several existing tissue phenotypes. These include regular distributions of cells, dynamic cell clusters, gel-like networks, collectively migrating monolayers, and 3D aggregates. We give analytical predictions for transitions between noncohesive, cohesive, and 3D cell arrangements. We explicitly show how CIL yields an effective repulsion that promotes cell dispersal, thereby hindering the formation of cohesive tissues. Yet, in continuous monolayers, CIL leads to collective cell motion, ensures tensile intercellular stresses, and opposes cell extrusion. Thus, our work highlights the prominent role of CIL in determining the emergent structures and dynamics of cell colonies.

Contact inhibition enables noncancerous cells to cease proliferation and growth when they contact each other. This characteristic is lost when cells undergo malignant transformation, leading to uncontrolled proliferation and solid tumor formation. Here we report that autophagy is compromised in contact-inhibited cells in 2D or 3D-soft extracellular matrix cultures. In such cells, YAP/TAZ fail to co-transcriptionally regulate the expression of myosin-II genes, resulting in the loss of F-actin stress fibers, which impairs autophagosome formation. The decreased proliferation resulting from contact inhibition is partly autophagy-dependent, as is their increased sensitivity to hypoxia and glucose starvation. These findings define how mechanically repressed YAP/TAZ activity impacts autophagy to contribute to core phenotypes resulting from high cell confluence that are lost in various cancers. At high cell density or when plated on soft matrix, YAP/TAZ are redistributed from the nucleus to the cytosol, becoming transcriptionally inactive. Here the authors show that at high cell density, autophagosome formation is impaired due to reduced YAP/TAZ-dependent transcription of actomyosin genes

Contact inhibition (CI) represents a crucial tumor-suppressive mechanism responsible for controlling the unbridled growth of cells, thus preventing the formation of cancerous tissues. CI can be further categorized into two distinct yet interrelated components: CI of locomotion (CIL) and CI of proliferation (CIP). These two components of CI have historically been viewed as separate processes, but emerging research suggests that they may be regulated by both distinct and shared pathways. Specifically, recent studies have indicated that both CIP and CIL utilize mechanotransduction pathways, a process that involves cells sensing and responding to mechanical forces. This review article describes the role of mechanotransduction in CI, shedding light on how mechanical forces regulate CIL and CIP. Emphasis is placed on filamin A (FLNA)-mediated mechanotransduction, elucidating how FLNA senses mechanical forces and translates them into crucial biochemical signals that regulate cell locomotion and proliferation. In addition to FLNA, trans-acting factors (TAFs), which are proteins or regulatory RNAs capable of directly or indirectly binding to specific DNA sequences in distant genes to regulate gene expression, emerge as sensitive players in both the mechanotransduction and signaling pathways of CI. This article presents methods for identifying these TAF proteins and profiling the associated changes in chromatin structure, offering valuable insights into CI and other biological functions mediated by mechanotransduction. Finally, it addresses unanswered research questions in these fields and delineates their possible future directions.

Cells organized in tissues exert forces on their neighbors and their environment. Those cellular forces determine tissue homeostasis as well as reorganization during embryonic development and wound healing. To understand how cellular forces are generated and how they can influence the tissue state, we develop a particle-based simulation model for adhesive cell clusters and monolayers. Cells are contractile, exert forces on their substrate and on each other, and interact through contact inhibition of locomotion (CIL), meaning that cell–cell contacts suppress force transduction to the substrate and propulsion forces align away from neighbors. Our model captures the traction force patterns of small clusters of nonmotile cells and larger sheets of motile Madin–Darby canine kidney (MDCK) cells. In agreement with observations in a spreading MDCK colony, the cell density in the center increases as cells divide and the tissue grows. A feedback between cell density, CIL, and cell–cell adhesion gives rise to a linear relationship between cell density and intercellular tensile stress and forces the tissue into a nonmotile state characterized by a broad distribution of traction forces. Our model also captures the experimentally observed tissue flow around circular obstacles, and CIL accounts for traction forces at the edge.

The Hippo signaling pathway inhibits cell growth and regulates organ size through a kinase cascade that leads to the phosphorylation and nuclear exclusion of the growth-promoting transcriptional coactivator Yes-associated protein (YAP)/Yorkie. It mediates contact inhibition of cell growth downstream of cadherin adhesion molecules and other cell surface proteins. Contact inhibition is often antagonized by mitogenic growth factor signaling. We report an important mechanism for this antagonism, inhibition of Hippo pathway signaling by mitogenic growth factors. EGF treatment of immortalized mammary cells triggers the rapid translocation of YAP into the nucleus along with YAP dephosphorylation, both of which depend on Lats, the terminal kinase in the Hippo pathway. A small-molecule inhibitor screen of downstream effector pathways shows that EGF receptor inhibits the Hippo pathway through activation of PI3-kinase (PI3K) and phosphoinositide-dependent kinase (PDK1), but independent of AKT activity. The PI3K-PDK1 pathway also mediates YAP nuclear translocation downstream of lysophosphatidic acid and serum as a result of constitutive oncogenic activation of PI3K. PDK1 associates with the core Hippo pathway-kinase complex through the scaffold protein Salvador. The entire Hippo core complex dissociates in response to EGF signaling in a PI3K-PDK1–dependent manner, leading to inactivation of Lats, dephosphorylation of YAP, and YAP nuclear accumulation and transcriptional activation of its target gene, CTGF . These findings show that an important activity of mitogenic signaling pathways is to inactivate the growth-inhibitory Hippo pathway and provide a mechanism for antagonism between contact inhibition and growth factor action.

Key Points Nectins and nectin-like molecules (Necls) have recently emerged as cell adhesion molecules that have a variety of cellular functions, including cell movement, proliferation, differentiation, polarization and survival, as well as cell–cell adhesion. The nectin–afadin complex that localizes at adherens junctions (AJs) has a crucial role in the formation of not only cadherin-based AJs but also claudin-based tight junctions (TJs) in epithelial cells. However, it remains unclear how nectins and afadin participate in the positioning of TJs, which are always formed at the apical side of AJs, in epithelial cells. The activation of integrin αvβ3 and its downstream signalling molecules is necessary for the nectin-induced formation of AJs; in turn, integrin αvβ3 is inactivated by the trans -interaction of nectins after the establishment of AJs, indicating that nectins and integrin αvβ3 are involved in the crosstalk between cell–cell and cell–matrix junctions during the formation of AJs. NECL-5, one of the members of the Necl family, significantly promotes cell movement and proliferation in cooperation with the PDGF receptor and integrin αvβ3, but NECL-5 is downregulated from the cell surface after NECL-5 interacts with nectin-3 at the primordial intercellular adhesion sites. This downregulation reduces cell movement and proliferation, indicating the primary involvement of NECL-5 in the contact inhibition of cell movement and proliferation. The expression of NECL-5 is upregulated in cancer cells and this upregulation is correlated with the increased metastatic ability of cancer cells. Nectins and nectin-like molecules (Necls) are transmembrane cell adhesion molecules that have recently been shown to have a variety of cellular functions. They have roles in cell–cell adhesion, differentiation, polarization and survival, as well as in contact inhibition of cell movement and proliferation. Nectins and nectin-like molecules (Necls) are immunoglobulin-like transmembrane cell adhesion molecules that are expressed in various cell types. Homophilic and heterophilic engagements between family members provide cells with molecular tools for intercellular communications. Nectins primarily regulate cell–cell adhesions, whereas Necls are involved in a greater variety of cellular functions. Recent studies have revealed that nectins and NECL-5, in cooperation with integrin αvβ3 and platelet-derived growth factor receptor, are crucial for the mechanisms that underlie contact inhibition of cell movement and proliferation; this has important implications for the development and tissue regeneration of multicellular organisms and the phenotypes of cancer cells.

Contact-dependent growth inhibition (CDI) systems function to deliver toxins into neighboring bacterial cells. CDI⁺ bacteria export filamentous CdiA effector proteins, which extend from the inhibitor-cell surface to interact with receptors on neighboring target bacteria. Upon binding its receptor, CdiA delivers a toxin derived from its C-terminal region. CdiA C-terminal (CdiA-CT) sequences are highly variable between bacteria, reflecting the multitude of CDI toxin activities. Here, we show that several CdiA-CT regions are composed of two domains, each with a distinct function during CDI. The C-terminal domain typically possesses toxic nuclease activity, whereas the N-terminal domain appears to control toxin transport into target bacteria. Using genetic approaches, we identifiedptsG, metI, rbsC, gltK/gltJ, yciB,andftsHmutations that confer resistance to specific CdiA-CTs. The resistance mutations all disrupt expression of inner-membrane proteins, suggesting that these proteins are exploited for toxin entry into target cells. Moreover, each mutation only protects against inhibition by a subset of CdiA-CTs that share similar N-terminal domains. We propose that, following delivery of CdiA-CTs into the periplasm, the N-terminal domains bind specific inner-membrane receptors for subsequent translocation into the cytoplasm. In accord with this model, we find that CDI nuclease domains are modular payloads that can be redirected through different import pathways when fused to heterologous N-terminal “translocation domains.” These results highlight the plasticity of CDI toxin delivery and suggest that the underlying translocation mechanisms could be harnessed to deliver other antimicrobial agents into Gram-negative bacteria.

Significance Normal microenvironments can restrict cancer development and progression. Inhibition of tumor cell growth and motility by normal fibroblasts is one measurable manifestation of this multicomponential control. Here we show that inhibition withstands formalin fixation and can be augmented by the addition of conditioned medium derived from live cultures of tumor cells confronting the stromal fibroblasts. We describe a number of molecules involved in this process. This study lays the foundation for further mechanistic studies of this important phenomenon and its contribution to possible dormancy and the tumor’s resistance to therapy. Normal human and murine fibroblasts can inhibit proliferation of tumor cells when cocultured in vitro. The inhibitory capacity varies depending on the donor and the site of origin of the fibroblast. We showed previously that effective inhibition requires formation of a morphologically intact fibroblast monolayer before seeding of the tumor cells. Here we show that inhibition is extended to motility of tumor cells and we dissect the factors responsible for these inhibitory functions. We find that inhibition is due to two different sets of molecules: ( i ) the extracellular matrix (ECM) and other surface proteins of the fibroblasts, which are responsible for contact-dependent inhibition of tumor cell proliferation; and ( ii ) soluble factors secreted by fibroblasts when confronted with tumor cells (confronted conditioned media, CCM) contribute to inhibition of tumor cell proliferation and motility. However, conditioned media (CM) obtained from fibroblasts alone (nonconfronted conditioned media, NCM) did not inhibit tumor cell proliferation and motility. In addition, quantitative PCR (Q-PCR) data show up-regulation of proinflammatory genes. Moreover, comparison of CCM and NCM with an antibody array for 507 different soluble human proteins revealed differential expression of growth differentiation factor 15, dickkopf-related protein 1, endothelial-monocyte-activating polypeptide II, ectodysplasin A2, Galectin-3, chemokine (C-X-C motif) ligand 2, Nidogen1, urokinase, and matrix metalloproteinase 3.

Contact toxins in bacteria Contact-dependent growth inhibition (CDI), first described in Escherichia coli five years ago, is a mechanism by which cell-to-cell contact inhibits the growth of bacterial cells that do not have this system. CDI is mediated by the two-partner secretion proteins CdiA and CdiB, and a small immunity protein CdiI gives protection against autoinhibition. The molecular basis for some of the interactions involved in CDI has now been elucidated; the toxic properties of CdiA are contained within the protein's carboxy-terminal end (CdiA-CT). A search across other E. coli strains and bacterial species shows the system to be widespread — a range of bacteria contain one or more CdiA homologues, with varied CdiA-CT toxin sequences. These findings suggest that CDI systems constitute an intricate immunity network with an important function in bacterial growth competition in the environment. Contact-dependent growth inhibition (CDI) through a two-component system was first described in Escherichia coli as a mechanism to inhibit growth of bacterial cells that do not possess this system. Now the widespread occurrence of CDI in bacteria and the molecular basis for some of these interactions have been elucidated. The data suggest that CDI is a common mechanism by which microbes compete with each other in the environment. Bacteria have developed mechanisms to communicate and compete with one another in diverse environments 1 . A new form of intercellular communication, contact-dependent growth inhibition (CDI), was discovered recently in Escherichia coli 2 . CDI is mediated by the CdiB/CdiA two-partner secretion (TPS) system. CdiB facilitates secretion of the CdiA ‘exoprotein’ onto the cell surface. An additional small immunity protein (CdiI) protects CDI + cells from autoinhibition 2 , 3 . The mechanisms by which CDI blocks cell growth and by which CdiI counteracts this growth arrest are unknown. Moreover, the existence of CDI activity in other bacteria has not been explored. Here we show that the CDI growth inhibitory activity resides within the carboxy-terminal region of CdiA (CdiA-CT), and that CdiI binds and inactivates cognate CdiA-CT, but not heterologous CdiA-CT. Bioinformatic and experimental analyses show that multiple bacterial species encode functional CDI systems with high sequence variability in the CdiA-CT and CdiI coding regions. CdiA-CT heterogeneity implies that a range of toxic activities are used during CDI. Indeed, CdiA-CTs from uropathogenic E. coli and the plant pathogen Dickeya dadantii have different nuclease activities, each providing a distinct mechanism of growth inhibition. Finally, we show that bacteria lacking the CdiA-CT and CdiI coding regions are unable to compete with isogenic wild-type CDI + cells both in laboratory media and on a eukaryotic host. Taken together, these results suggest that CDI systems constitute an intricate immunity network with an important function in bacterial competition.

In the rat model of amyotrophic lateral sclerosis expressing the G93A superoxide dismutase-1 mutation, motor neuron death and rapid paralysis progression are associated with the emergence of a population of aberrant glial cells (AbAs) that proliferate in the degenerating spinal cord. Targeting of AbAs with anti-neoplasic drugs reduced paralysis progression, suggesting a pathogenic potential contribution of these cells accelerating paralysis progression. In the present study, analyze the cellular and ultrastructural features of AbAs following their isolation and establishment in culture during several passages. We found that AbAs exhibit permanent loss of contact inhibition, absence of intermediate filaments and abundance of microtubules, together with an important production of extracellular matrix components. Remarkably, AbAs also exhibited exacerbated ER stress together with a significant abundance of lipid droplets, as well as autophagic and secretory vesicles, all characteristic features of cellular stress and inflammatory activation. Taken together, the present data show AbA cells as a unique aberrant phenotype for a glial cell that might explain their pathogenic and neurotoxic effects.