Talin autoinhibition is required for morphogenesis

High-resolution snapshots of the talin auto-inhibitory states suggest roles in cell adhesion and signaling

Talin regulates crucial cellular functions, including cell adhesion and motility, and affects human diseases. Triggered by mechanical forces, talin plays crucial roles in facilitating the formation of focal adhesions and recruiting essential focal adhesion regulatory elements such as vinculin. The structural flexibility allows talin to fine-tune its signaling responses. This study presents our 2.7 Å cryoEM structures of talin, which surprisingly uncovers several auto-inhibitory states. Contrary to previous suggestions, our structures reveal that (1) the first and last three domains are not involved in maintaining talin in its closed state and are mobile, (2) the talin F-actin and membrane binding domain are loosely attached and thus available for binding, and (3) the main force-sensing domain is oriented with its vinculin binding sites ready for release. These structural snapshots offer insights and advancements in understanding the dynamic talin activation mechanism, which is crucial for mediating cell adhesion.

The Id protein Extramacrochaetae restrains the E protein Daughterless to regulate Notch, Rap1, and Sevenless within the R7 equivalence group of the Drosophila eye

The Drosophila Id gene extramacrochaetae (emc) is required during Drosophila eye development for proper cell fate specification within the R7 equivalence group. Without emc, R7 cells develop like R1/6 cells, and there are delays and deficits in differentiation of non-neuronal cone cells. Although emc encodes an Inhibitor of DNA-binding (Id) protein that is known to antagonize proneural bHLH protein function, no proneural gene is known for R7 or cone cell fates. These fates are also independent of daughterless (da), which encodes the ubiquitous E protein heterodimer partner of proneural bHLH proteins. We report here that the effects of emc mutations disappear in the absence of da, and are partially mimicked by forced expression of Da dimers, indicating that emc normally restrains da from interfering with R7 and cone cell specification, as occurs in emc mutants. emc, and da, regulate three known contributors to R7 fate, which are Notch signaling, Rap1, and Sevenless. R7 specification is partially restored to emc mutant cells by mutation of RapGap1, confirming that Rap1 activity, in addition to Notch activity, is a critical target of emc. These findings exemplify how mutations of an Id protein gene can affect processes that do not require any bHLH protein, by restraining Da activity within physiological bounds.

The actin binding sites of talin have both distinct and complementary roles in cell-ECM adhesion

Cell adhesion requires linkage of transmembrane receptors to the cytoskeleton through intermediary linker proteins. Integrin-based adhesion to the extracellular matrix (ECM) involves large adhesion complexes that contain multiple cytoskeletal adapters that connect to the actin cytoskeleton. Many of these adapters, including the essential cytoskeletal linker Talin, have been shown to contain multiple actin-binding sites (ABSs) within a single protein. To investigate the possible role of having such a variety of ways of linking integrins to the cytoskeleton, we generated mutations in multiple actin binding sites in Drosophila talin. Using this approach, we have been able to show that different actin-binding sites in talin have both unique and complementary roles in integrin-mediated adhesion. Specifically, mutations in either the C-terminal ABS3 or the centrally located ABS2 result in lethality showing that they have unique and non-redundant function in some contexts. On the other hand, flies simultaneously expressing both the ABS2 and ABS3 mutants exhibit a milder phenotype than either mutant by itself, suggesting overlap in function in other contexts. Detailed phenotypic analysis of ABS mutants elucidated the unique roles of the talin ABSs during embryonic development as well as provided support for the hypothesis that talin acts as a dimer in in vivo contexts. Overall, our work highlights how the ability of adhesion complexes to link to the cytoskeleton in multiple ways provides redundancy, and consequently robustness, but also allows a capacity for functional specialization.

Rap1 coordinates cell-cell adhesion and cytoskeletal reorganization to drive collective cell migration in vivo

Collective cell movements contribute to tissue development and repair and spread metastatic disease. In epithelia, cohesive cell movements require reorganization of adherens junctions and the actomyosin cytoskeleton. However, the mechanisms that coordinate cell-cell adhesion and cytoskeletal remodeling during collective cell migration in vivo are unclear. We investigated the mechanisms of collective cell migration during epidermal wound healing in Drosophila embryos. Upon wounding, the cells adjacent to the wound internalize cell-cell adhesion molecules and polarize actin and the motor protein non-muscle myosin II to form a supracellular cable around the wound that coordinates cell movements. The cable anchors at former tricellular junctions (TCJs) along the wound edge, and TCJs are reinforced during wound closure. We found that the small GTPase Rap1 was necessary and sufficient for rapid wound repair. Rap1 promoted myosin polarization to the wound edge and E-cadherin accumulation at TCJs. Using embryos expressing a mutant form of the Rap1 effector Canoe/Afadin that cannot bind Rap1, we found that Rap1 signals through Canoe for adherens junction remodeling, but not for actomyosin cable assembly. Instead, Rap1 was necessary and sufficient for RhoA/Rho1 activation at the wound edge. The RhoGEF Ephexin localized to the wound edge in a Rap1-dependent manner, and Ephexin was necessary for myosin polarization and rapid wound repair, but not for E-cadherin redistribution. Together, our data show that Rap1 coordinates the molecular rearrangements that drive embryonic wound healing, promoting actomyosin cable assembly through Ephexin-Rho1, and E-cadherin redistribution through Canoe, thus enabling rapid collective cell migration in vivo.

Specialized cells that sense tissue mechanics to regulate Drosophila morphogenesis

Shaping of developing organs requires dynamic regulation of force and resistance to achieve precise outcomes, but how organs monitor tissue mechanical properties is poorly understood. We show that in developing Drosophila follicles (egg chambers), a single pair of cells performs such monitoring to drive organ shaping. These anterior polar cells secrete a matrix metalloproteinase (MMP) that specifies the appropriate degree of tissue elongation, rather than hyper- or hypo-elongated organs. MMP production is negatively regulated by basement membrane (BM) mechanical properties, which are sensed through focal adhesion signaling and autonomous contractile activity; MMP then reciprocally regulates BM remodeling, particularly at the anterior region. Changing BM properties at remote locations alone is sufficient to induce a remodeling response in polar cells. We propose that this small group of cells senses both local and distant stiffness cues to produce factors that pattern the organ's BM mechanics, ensuring proper tissue shape and reproductive success.

Requirement for PINCH in skeletal myoblast differentiation

PINCH, an adaptor of focal adhesion complex, plays essential roles in multiple cellular processes and organogenesis. Here, we ablated PINCH1 or both of PINCH1 and PINCH2 in skeletal muscle progenitors using MyoD-Cre. Double ablation of PINCH1 and PINCH2 resulted in early postnatal lethality with reduced size of skeletal muscles and detachment of diaphragm muscles from the body wall. PINCH mutant myofibers failed to undergo multinucleation and exhibited disrupted sarcomere structures. The mutant myoblasts in culture were able to adhere to newly formed myotubes but impeded in cell fusion and subsequent sarcomere genesis and cytoskeleton organization. Consistent with this, expression of integrin β1 and some cytoskeleton proteins and phosphorylation of ERK and AKT were significantly reduced in PINCH mutants. However, N-cadherin was correctly expressed at cell adhesion sites in PINCH mutant cells, suggesting that PINCH may play a direct role in myoblast fusion. Expression of MRF4, the most highly expressed myogenic factor at late stages of myogenesis, was abolished in PINCH mutants that could contribute to observed phenotypes. In addition, mice with PINCH1 being ablated in myogenic progenitors exhibited only mild centronuclear myopathic changes, suggesting a compensatory role of PINCH2 in myogenic differentiation. Our results revealed a critical role of PINCH proteins in myogenic differentiation.

Identification of TLN1 as a prognostic biomarker to effect cell proliferation and differentiation in acute myeloid leukemia

The protein Talin1 encoded by the TLN1 gene is a focal adhesion-related protein that binds to various cytoskeletal proteins and plays an important role in cell adhesion and movement. Recent studies have shown that it is overexpressed in prostate cancer, liver cancer, and oral squamous cell carcinoma, and is closely related to tumor progression and metastasis. This study integrated bioinformatics and functional analysis to reveal the prognosis and potential functions of TLN1 in AML. The results showed that the expression level of TLN1 was abnormally increased in AML and localized in the cell membrane and cytoplasm, and TLN1 is a significant prognostic indicator of overall survival (OS). Enrichment analysis of related genes showed that TLN1 is related to neutrophil mediated immunity, neutrophil activation and may regulate important signal pathways in hematological tumors including tyrosine kinase receptor, FLT3 and PIK3/AKT. The PPI network shows that TLN1 and MYH9 may be involved in the process of AML tumors together with PIP5K1C, ROCK1, S100A4, MY01A and WAC. Immune infiltration analysis explains that TLN1 is associated with multiple immune cells and may be an important immune marker in AML. Furthermore, molecular biology experiments confirmed that TLN1 is related to the proliferation, differentiation and cycle of AML cells. Silencing TLN1 can inhibit the proliferation of AML cells and promote differentiation through the Talin1/P-AKT/CREB signaling pathway.

Talin variant P229S compromises integrin activation and associates with multifaceted clinical symptoms

Adhesion of cells to the extracellular matrix (ECM) must be exquisitely coordinated to enable development and tissue homeostasis. Cell-ECM interactions are regulated by multiple signalling pathways that coordinate the activation state of the integrin family of ECM receptors. The protein talin is pivotal in this process, and talin's simultaneous interactions with the cytoplasmic tails of the integrins and the plasma membrane are essential to enable robust, dynamic control of integrin activation and cell-ECM adhesion. Here, we report the identification of a de novo heterozygous c.685C>T (p.Pro229Ser) variant in the TLN1 gene from a patient with a complex phenotype. The mutation is located in the talin head region at the interface between the F2 and F3 domains. The characterization of this novel p.P229S talin variant reveals the disruption of adhesion dynamics that result from disturbance of the F2-F3 domain interface in the talin head. Using biophysical, computational and cell biological techniques, we find that the variant perturbs the synergy between the integrin-binding F3 and the membrane-binding F2 domains, compromising integrin activation, adhesion and cell migration. Whilst this remains a variant of uncertain significance, it is probable that the dysregulation of adhesion dynamics we observe in cells contributes to the multifaceted clinical symptoms of the patient and may provide insight into the multitude of cellular processes dependent on talin-mediated adhesion dynamics.

Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

The F1 loop of the talin head domain acts as a gatekeeper in integrin activation and clustering

Integrin activation and clustering by talin are early steps of cell adhesion. Membrane-bound talin head domain and kindlin bind to the β integrin cytoplasmic tail, cooperating to activate the heterodimeric integrin, and the talin head domain induces integrin clustering in the presence of Mn2+ Here we show that kindlin-1 can replace Mn2+ to mediate β3 integrin clustering induced by the talin head, but not that induced by the F2-F3 fragment of talin. Integrin clustering mediated by kindlin-1 and the talin head was lost upon deletion of the flexible loop within the talin head F1 subdomain. Further mutagenesis identified hydrophobic and acidic motifs in the F1 loop responsible for β3 integrin clustering. Modeling, computational and cysteine crosslinking studies showed direct and catalytic interactions of the acidic F1 loop motif with the juxtamembrane domains of α- and β3-integrins, in order to activate the β3 integrin heterodimer, further detailing the mechanism by which the talin-kindlin complex activates and clusters integrins. Moreover, the F1 loop interaction with the β3 integrin tail required the newly identified compact FERM fold of the talin head, which positions the F1 loop next to the inner membrane clasp of the talin-bound integrin heterodimer.This article has an associated First Person interview with the first author of the paper.

Precise coordination of cell-ECM adhesion is essential for efficient melanoblast migration during development

Melanoblasts disperse throughout the skin and populate hair follicles through long-range cell migration. During migration, cells undergo cycles of coordinated attachment and detachment from the extracellular matrix (ECM). Embryonic migration processes that require cell-ECM attachment are dependent on the integrin family of adhesion receptors. Precise regulation of integrin-mediated adhesion is important for many developmental migration events. However, the mechanisms that regulate integrin-mediated adhesion in vivo in melanoblasts are not well understood. Here, we show that autoinhibitory regulation of the integrin-associated adapter protein talin coordinates cell-ECM adhesion during melanoblast migration in vivo Specifically, an autoinhibition-defective talin mutant strengthens and stabilizes integrin-based adhesions in melanocytes, which impinges on their ability to migrate. Mice with defective talin autoinhibition exhibit delays in melanoblast migration and pigmentation defects. Our results show that coordinated integrin-mediated cell-ECM attachment is essential for melanoblast migration and that talin autoinhibition is an important mechanism for fine-tuning cell-ECM adhesion during cell migration in development.

Integrins Modulate Extracellular Matrix Organization to Control Cell Signaling during Hematopoiesis

During hematopoiesis, progenitor cells receive and interpret a diverse array of regulatory signals from their environment. These signals control the maintenance of the progenitors and regulate the production of mature blood cells. Integrins are well known in vertebrates for their roles in hematopoiesis, particularly in assisting in the migration to, as well as the physical attachment of, progenitors to the niche. However, whether and how integrins are also involved in the signaling mechanisms that control hematopoiesis remains to be resolved. Here, we show that integrins play a key role during fly hematopoiesis in regulating cell signals that control the behavior of hematopoietic progenitors. Integrins can regulate hematopoiesis directly, via focal adhesion kinase (FAK) signaling, and indirectly, by directing extracellular matrix (ECM) assembly and/or maintenance. ECM organization and density controls blood progenitor behavior by modulating multiple signaling pathways, including bone morphogenetic protein (BMP) and Hedgehog (Hh). Furthermore, we show that integrins and the ECM are reduced following infection, which may assist in activating the immune response. Our results provide mechanistic insight into how integrins can shape the signaling environment around hematopoietic progenitors.

The Architecture of Talin1 Reveals an Autoinhibition Mechanism

Focal adhesions (FAs) are protein machineries essential for cell adhesion, migration, and differentiation. Talin is an integrin-activating and tension-sensing FA component directly connecting integrins in the plasma membrane with the actomyosin cytoskeleton. To understand how talin function is regulated, we determined a cryoelectron microscopy (cryo-EM) structure of full-length talin1 revealing a two-way mode of autoinhibition. The actin-binding rod domains fold into a 15-nm globular arrangement that is interlocked by the integrin-binding FERM head. In turn, the rod domains R9 and R12 shield access of the FERM domain to integrin and the phospholipid PIP₂ at the membrane. This mechanism likely ensures synchronous inhibition of integrin, membrane, and cytoskeleton binding. We also demonstrate that compacted talin1 reversibly unfolds to an ∼60-nm string-like conformation, revealing interaction sites for vinculin and actin. Our data explain how fast switching between active and inactive conformations of talin could regulate FA turnover, a process critical for cell adhesion and signaling.

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The structure of the autoinhibited human full-length talin1 was analyzed by cryo-EM

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Talin1 reversibly changes between a 15-nm closed and a ∼60-nm open conformation

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Rod R9/R12 and FERM domains synchronously shield membrane and cytoskeleton binding

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F-Actin and vinculin binding to talin is regulated by the opening of talin

Chapter 22: Structural and signaling functions of integrins

The integrin family of transmembrane adhesion receptors is essential for sensing and adhering to the extracellular environment. Integrins are heterodimers composed of non-covalently associated α and β subunits that engage extracellular matrix proteins and couple to intracellular signaling and cytoskeletal complexes. Humans have 24 different integrin heterodimers with differing ligand binding specificities and non-redundant functions. Complex structural rearrangements control the ability of integrins to engage ligands and to activate diverse downstream signaling networks, modulating cell adhesion and dynamics, processes which are crucial for metazoan life and development. Here we review the structural and signaling functions of integrins focusing on recent advances which have enhanced our understanding of how integrins are activated and regulated, and the cytoplasmic signaling networks downstream of integrins.

Conformationally active integrin endocytosis and traffic: why, where, when and how?

Spatiotemporal control of integrin-mediated cell adhesion to the extracellular matrix (ECM) is critical for physiological and pathological events in multicellular organisms, such as embryonic development, angiogenesis, platelet aggregation, leukocytes extravasation, and cancer cell metastatic dissemination. Regulation of integrin adhesive function and signaling relies on the modulation of both conformation and traffic. Indeed, integrins exist in a dynamic equilibrium between a bent/closed (inactive) and an extended/open (active) conformation, respectively endowed with low and high affinity for ECM ligands. Increasing evidence proves that, differently to what hypothesized in the past, detachment from the ECM and conformational inactivation are not mandatory for integrin to get endocytosed and trafficked. Specific transmembrane and cytosolic proteins involved in the control of ECM proteolytic fragment-bound active integrin internalization and recycling exist. In the complex masterplan that governs cell behavior, active integrin traffic is key to the turnover of ECM polymers and adhesion sites, the polarized secretion of endogenous ECM proteins and modifying enzymes, the propagation of motility and survival endosomal signals, and the control of cell metabolism.

Adhesions Assemble!-Autoinhibition as a Major Regulatory Mechanism of Integrin-Mediated Adhesion

The advent of cell-cell and cell-extracellular adhesion enabled cells to interact in a coherent manner, forming larger structures and giving rise to the development of tissues, organs and complex multicellular life forms. The development of such organisms required tight regulation of dynamic adhesive structures by signaling pathways that coordinate cell attachment. Integrin-mediated adhesion to the extracellular matrix provides cells with support, survival signals and context-dependent cues that enable cells to run different cellular programs. One mysterious aspect of the process is how hundreds of proteins assemble seemingly spontaneously onto the activated integrin. An emerging concept is that adhesion assembly is regulated by autoinhibition of key proteins, a highly dynamic event that is modulated by a variety of signaling events. By enabling precise control of the activation state of proteins, autoinhibition enables localization of inactive proteins and the formation of pre-complexes. In response to the correct signals, these proteins become active and interact with other proteins, ultimately leading to development of cell-matrix junctions. Autoinhibition of key components of such adhesion complexes—including core components integrin, talin, vinculin, and FAK and important peripheral regulators such as RIAM, Src, and DLC1—leads to a view that the majority of proteins involved in complex assembly might be regulated by intramolecular interactions. Autoinhibition is relieved via multiple different signals including post-translation modification and proteolysis. More recently, mechanical forces have been shown to stabilize and increase the lifetimes of active conformations, identifying autoinhibition as a means of encoding mechanosensitivity. The complexity and scope for nuanced adhesion dynamics facilitated via autoinhibition provides numerous points of regulation. In this review, we discuss what is known about this mode of regulation and how it leads to rapid and tightly controlled assembly and disassembly of cell-matrix adhesion.

Talin Autoinhibition Regulates Cell-ECM Adhesion Dynamics and Wound Healing In Vivo

Cells in multicellular organisms are arranged in complex three-dimensional patterns. This requires both transient and stable adhesions with the extracellular matrix (ECM). Integrin adhesion receptors bind ECM ligands outside the cell and then, by binding the protein talin inside the cell, assemble an adhesion complex connecting to the cytoskeleton. The activity of talin is controlled by several mechanisms, but these have not been well studied in vivo. By generating mice containing the activating point mutation E1770A in talin (Tln1), which disrupts autoinhibition, we show that talin autoinhibition controls cell-ECM adhesion, cell migration, and wound healing in vivo. In particular, blocking autoinhibition gives rise to more mature, stable focal adhesions that exhibit increased integrin activation. Mutant cells also show stronger attachment to ECM and decreased traction force. Overall, these results demonstrate that modulating talin function via autoinhibition is an important mechanism for regulating multiple aspects of integrin-mediated cell-ECM adhesion in vivo.

Slik phosphorylation of Talin T152 is crucial for proper Talin recruitment and maintenance of muscle attachment in Drosophila

Talin is the major scaffold protein linking integrin receptors with the actin cytoskeleton. In Drosophila, extended Talin generates a stable link between the sarcomeric cytoskeleton and the tendon matrix at muscle attachment sites. Here, we identify phosphorylation sites on Drosophila Talin by mass spectrometry. Talin is phosphorylated in late embryogenesis when muscles differentiate, especially on T152 in the exposed loop of the F1 domain of the Talin head. Localization of a mutated version of Talin (Talin-T150/T152A) is reduced at muscle attachment sites and can only partially rescue muscle attachment compared with wild-type Talin. We also identify Slik as the kinase phosphorylating Talin at T152. Slik localizes to muscle attachment sites, and the absence of Slik reduces the localization of Talin at muscle attachment sites causing phenotypes similar to Talin-T150/T152A. Thus, our results demonstrate that Talin phosphorylation by Slik plays an important role in fine-tuning Talin recruitment to integrin adhesion sites and maintaining muscle attachment.

Direct binding of Talin to Rap1 is required for cell-ECM adhesion in Drosophila

Attachment of cells to the extracellular matrix (ECM) via integrins is essential for animal development and tissue maintenance. The cytoplasmic protein Talin (encoded by rhea in flies) is necessary for linking integrins to the cytoskeleton, and its recruitment is a key step in the assembly of the adhesion complex. However, the mechanisms that regulate Talin recruitment to sites of adhesion in vivo are still not well understood. Here, we show that Talin recruitment to, and maintenance at, sites of integrin-mediated adhesion requires a direct interaction between Talin and the GTPase Rap1. A mutation that blocks the direct binding of Talin to Rap1 abolished Talin recruitment to sites of adhesion and the resulting phenotype phenocopies that seen with null alleles of Talin. Moreover, we show that Rap1 activity modulates Talin recruitment to sites of adhesion via its direct binding to Talin. These results identify the direct Talin-Rap1 interaction as a key in vivo mechanism for controlling integrin-mediated cell-ECM adhesion.

Talin as a mechanosensitive signaling hub

Cell adhesion to the extracellular matrix (ECM), mediated by transmembrane receptors of the integrin family, is exquisitely sensitive to biochemical, structural, and mechanical features of the ECM. Talin is a cytoplasmic protein consisting of a globular head domain and a series of α-helical bundles that form its long rod domain. Talin binds to the cytoplasmic domain of integrin β-subunits, activates integrins, couples them to the actin cytoskeleton, and regulates integrin signaling. Recent evidence suggests switch-like behavior of the helix bundles that make up the talin rod domains, where individual domains open at different tension levels, exerting positive or negative effects on different protein interactions. These results lead us to propose that talin functions as a mechanosensitive signaling hub that integrates multiple extracellular and intracellular inputs to define a major axis of adhesion signaling.

The tale of two talins - two isoforms to fine-tune integrin signalling

Talins are cytoplasmic adapter proteins essential for integrin-mediated cell adhesion to the extracellular matrix. Talins control the activation state of integrins, link integrins to cytoskeletal actin, recruit numerous signalling molecules that mediate integrin signalling and coordinate recruitment of microtubules to adhesion sites via interaction with KANK (kidney ankyrin repeat-containing) proteins. Vertebrates have two talin genes, TLN1 and TLN2. Although talin1 and talin2 share 76% protein sequence identity (88% similarity), they are not functionally redundant, and the differences between the two isoforms are not fully understood. In this Review, we focus on the similarities and differences between the two talins in terms of structure, biochemistry and function, which hint at subtle differences in fine-tuning adhesion signalling.

Mechanical stress regulates insulin sensitivity through integrin-dependent control of insulin receptor localization

Kim et al. show that insulin signaling in Drosophila adipocytes is abolished in the absence of physical activity and mechanical stress. The insulin receptor and downstream components are recruited to the plasma membrane upon stress sensing mediated by integrins.

Insulin resistance, the failure to activate insulin signaling in the presence of ligand, leads to metabolic diseases, including type 2 diabetes. Physical activity and mechanical stress have been shown to protect against insulin resistance, but the molecular mechanisms remain unclear. Here, we address this relationship in the Drosophila larval fat body, an insulin-sensitive organ analogous to vertebrate adipose tissue and livers. We found that insulin signaling in Drosophila fat body cells is abolished in the absence of physical activity and mechanical stress even when excess insulin is present. Physical movement is required for insulin sensitivity in both intact larvae and fat bodies cultured ex vivo. Interestingly, the insulin receptor and other downstream components are recruited to the plasma membrane in response to mechanical stress, and this membrane localization is rapidly lost upon disruption of larval or tissue movement. Sensing of mechanical stimuli is mediated in part by integrins, whose activation is necessary and sufficient for mechanical stress-dependent insulin signaling. Insulin resistance develops naturally during the transition from the active larval stage to the immotile pupal stage, suggesting that regulation of insulin sensitivity by mechanical stress may help coordinate developmental programming with metabolism.

Rap1 acts via multiple mechanisms to position Canoe and adherens junctions and mediate apical-basal polarity establishment

Epithelial apical-basal polarity drives assembly and function of most animal tissues. Polarity initiation requires cell-cell adherens junction assembly at the apical-basolateral boundary. Defining the mechanisms underlying polarity establishment remains a key issue. Drosophila embryos provide an ideal model, as 6000 polarized cells assemble simultaneously. Current data place the actin-junctional linker Canoe (fly homolog of Afadin) at the top of the polarity hierarchy, where it directs Bazooka/Par3 and adherens junction positioning. Here we define mechanisms regulating Canoe localization/function. Spatial organization of Canoe is multifaceted, involving membrane localization, recruitment to nascent junctions and macromolecular assembly at tricellular junctions. Our data suggest apical activation of the small GTPase Rap1 regulates all three events, but support multiple modes of regulation. The Rap1GEF Dizzy (PDZ-GEF) is crucial for Canoe tricellular junction enrichment but not apical retention. The Rap1-interacting RA domains of Canoe mediate adherens junction and tricellular junction recruitment but are dispensable for membrane localization. Our data also support a role for Canoe multimerization. These multifactorial inputs shape Canoe localization, correct Bazooka and adherens junction positioning, and thus apical-basal polarity. We integrate the existing data into a new polarity establishment model.

Talin - the master of integrin adhesions

Talin has emerged as the key cytoplasmic protein that mediates integrin adhesion to the extracellular matrix. In this Review, we draw on experiments performed in mammalian cells in culture and Drosophila to present evidence that talin is the most important component of integrin adhesion complexes. We describe how the properties of this adaptor protein enable it to orchestrate integrin adhesions. Talin forms the core of integrin adhesion complexes by linking integrins directly to actin, increasing the affinity of integrin for ligands (integrin activation) and recruiting numerous proteins. It regulates the strength of integrin adhesion, senses matrix rigidity, increases focal adhesion size in response to force and serves as a platform for the building of the adhesion structure. Finally, the mechano-sensitive structure of talin provides a paradigm for how proteins transduce mechanical signals to chemical signals.

Basal Cell-Extracellular Matrix Adhesion Regulates Force Transmission during Tissue Morphogenesis

Tissue morphogenesis requires force-generating mechanisms to organize cells into complex structures. Although many such mechanisms have been characterized, we know little about how forces are integrated across developing tissues. We provide evidence that integrin-mediated cell-extracellular matrix (ECM) adhesion modulates the transmission of apically generated tension during dorsal closure (DC) in Drosophila. Integrin-containing adhesive structures resembling focal adhesions were identified on the basal surface of the amnioserosa (AS), an extraembryonic epithelium essential for DC. Genetic modulation of integrin-mediated adhesion results in defective DC. Quantitative image analysis and laser ablation experiments reveal that basal cell-ECM adhesions provide resistance to apical cell displacements and force transmission between neighboring cells in the AS. Finally, we provide evidence for integrin-dependent force transmission to the AS substrate. Overall, we find that integrins regulate force transmission within and between cells, thereby playing an essential role in transmitting tension in developing tissues.

Cell-cell and cell-extracellular matrix adhesions cooperate to organize actomyosin networks and maintain force transmission during dorsal closure

Cell–extracellular matrix (ECM) and cell–cell adhesion are interdependent during dorsal closure in the fly. Cell–ECM adhesion is required for normal myosin dynamics and organization of both cell–cell adhesions and actin networks during dorsal closure. Loss of cell–cell adhesion affects cell–ECM adhesion and tissue biomechanics.

Tissue morphogenesis relies on the coordinated action of actin networks, cell–cell adhesions, and cell–extracellular matrix (ECM) adhesions. Such coordination can be achieved through cross-talk between cell–cell and cell–ECM adhesions. Drosophila dorsal closure (DC), a morphogenetic process in which an extraembryonic tissue called the amnioserosa contracts and ingresses to close a discontinuity in the dorsal epidermis of the embryo, requires both cell–cell and cell–ECM adhesions. However, whether the functions of these two types of adhesions are coordinated during DC is not known. Here we analyzed possible interdependence between cell–cell and cell–ECM adhesions during DC and its effect on the actomyosin network. We find that loss of cell–ECM adhesion results in aberrant distributions of cadherin-mediated adhesions and actin networks in the amnioserosa and subsequent disruption of myosin recruitment and dynamics. Moreover, loss of cell–cell adhesion caused up-regulation of cell–ECM adhesion, leading to reduced cell deformation and force transmission across amnioserosa cells. Our results show how interdependence between cell–cell and cell–ECM adhesions is important in regulating cell behaviors, force generation, and force transmission critical for tissue morphogenesis.

α-Catenin stabilises Cadherin-Catenin complexes and modulates actomyosin dynamics to allow pulsatile apical contraction

We have investigated how cell contractility and adhesion are functionally integrated during epithelial morphogenesis. To this end, we have analysed the role of α-Catenin, a key molecule linking E-Cadherin-based adhesion and the actomyosin cytoskeleton, during Drosophila embryonic dorsal closure, by studying a newly developed allelic series. We find that α-Catenin regulates pulsatile apical contraction in the amnioserosa, the main force-generating tissue driving closure of the embryonic epidermis. α-Catenin controls actomyosin dynamics by stabilising and promoting the formation of actomyosin foci, and also stabilises DE-Cadherin (Drosophila E-Cadherin, also known as Shotgun) at the cell membrane, suggesting that medioapical actomyosin contractility regulates junction stability. Furthermore, we uncover a genetic interaction between α-Catenin and Vinculin, and a tension-dependent recruitment of Vinculin to amniosersoa apical cell membranes, suggesting the existence of a mechano-sensitive module operating in this tissue.

Drosophila vinculin is more harmful when hyperactive than absent, and can circumvent integrin to form adhesion complexes

Vinculin is a highly conserved protein involved in cell adhesion and mechanotransduction, and both gain and loss of its activity causes defective cell behaviour. Here, we examine how altering vinculin activity perturbs integrin function within the context of Drosophila development. Whereas loss of vinculin produced relatively minor phenotypes, gain of vinculin activity, through a loss of head–tail autoinhibition, caused lethality. The minimal domain capable of inducing lethality is the talin-binding D1 domain, and this appears to require talin-binding activity, as lethality was suppressed by competition with single vinculin-binding sites from talin. Activated Drosophila vinculin triggered the formation of cytoplasmic adhesion complexes through the rod of talin, but independently of integrin. These complexes contain a subset of adhesion proteins but no longer link the membrane to actin. The negative effects of hyperactive vinculin were segregated into morphogenetic defects caused by its whole head domain and lethality caused by its D1 domain. These findings demonstrate the crucial importance of the tight control of the activity of vinculin.

Summary: Development is more sensitive to gain of vinculin activity than its loss, and vinculin can promote cytoplasmic adhesion complexes independently of the usual integrin cue.

Cell Boundary Elongation by Non-autonomous Contractility in Cell Oscillation

Throughout development, tissues exhibit dynamic cell deformation, which is characterized by the integration of cell boundary contraction and/or elongation. Such changes ultimately establish tissue morphology and function [1-5]. In comparison to cell boundary contraction, which is predominantly driven by non-muscle myosin II (MyoII)-dependent contraction [6-9], the mechanisms of cell boundary elongation remain elusive. We explored the dynamics of the amnioserosa, which is known to exhibit cell shape oscillation [10-15], as a model system to study the subcellular-level mechanics that spatiotemporally evolve during Drosophila dorsal closure. Here we show that cell boundary elongation occurs through a combination of a non-autonomous active process and an autonomous process. The former is driven by a transient change in the level of MyoII in the neighboring cells that pull the vertices, whereas the latter is governed by the relaxation of junctional tension. By monitoring cell boundary deformation during live imaging, junctional tension at the specific phase of cell boundary oscillation, e.g., contraction or elongation, was probed by laser ablation. Junctional tension during boundary elongation is lower than during the other phase of oscillation. We extended our tension measurements to non-invasively estimate a tension map across the tissue, and found a correlation between junctional tension and vinculin dynamics at the cell junction. We propose that the medial actomyosin network is used as an entity to both contract and elongate the cell boundary. Moreover, our findings raise a possibility that the level of vinculin at the cell boundary could be used to approximate junctional tension in vivo.

The mechanical response of talin

Talin, a force-bearing cytoplasmic adapter essential for integrin-mediated cell adhesion, links the actin cytoskeleton to integrin-based cell-extracellular matrix adhesions at the plasma membrane. Its C-terminal rod domain, which contains 13 helical bundles, plays important roles in mechanosensing during cell adhesion and spreading. However, how the structural stability and transition kinetics of the 13 helical bundles of talin are utilized in the diverse talin-dependent mechanosensing processes remains poorly understood. Here we report the force-dependent unfolding and refolding kinetics of all talin rod domains. Using experimentally determined kinetics parameters, we determined the dynamics of force fluctuation during stretching of talin under physiologically relevant pulling speeds and experimentally measured extension fluctuation trajectories. Our results reveal that force-dependent stochastic unfolding and refolding of talin rod domains make talin a very effective force buffer that sets a physiological force range of only a few pNs in the talin-mediated force transmission pathway.

In vivo regulation of integrin turnover by outside-in activation

The development of three-dimensional tissue architecture requires precise control over the attachment of cells to the extracellular matrix (ECM). Integrins, the main ECM-binding receptors in animals, are regulated in multiple ways to modulate cell-ECM adhesion. One example is the conformational activation of integrins by extracellular signals ('outside-in activation') or by intracellular signals ('inside-out activation'), whereas another is the modulation of integrin turnover. We demonstrate that outside-in activation regulates integrin turnover to stabilize tissue architecture in vivo Treating Drosophila embryos with Mg(2+) and Mn(2+), known to induce outside-in activation, resulted in decreased integrin turnover. Mathematical modeling combined with mutational analysis provides mechanistic insight into the stabilization of integrins at the membrane. We show that as tissues mature, outside-in activation is crucial for regulating the stabilization of integrin-mediated adhesions. This data identifies a new in vivo role for outside-in activation and sheds light on the key transition between tissue morphogenesis and maintenance.

An integrated platform for large-scale data collection and precise perturbation of live Drosophila embryos

Understanding the fundamental principles governing embryogenesis is a key goal of developmental biology. Direct observation of embryogenesis via in vivo live imaging is vital to understanding embryogenesis; yet, tedious sample preparation makes it difficult to acquire large-scale imaging data that is often required to overcome experimental and biological noises for quantitative studies. Furthermore, it is often difficult, and sometimes impossible, to incorporate environmental perturbation for understanding developmental responses to external stimuli. To address this issue, we have developed a method for high-throughput imaging of live embryos, delivering precise environmental perturbations, and unbiased data extraction. This platform includes an optimized microfluidic device specifically for live embryos and also for precise perturbations in the microenvironment of the developing embryos. In addition, we developed software for simple, yet accurate, automated segmentation of fluorescent images, and automated data extraction. Using a quantitative assessment we find that embryos develop normally within the microfluidic device. Finally, we show an application of the high-throughput assay for monitoring developmental responses to external stimuli: anoxia-induced developmental arrest in Drosophila embryos. With slight modifications, the method developed in this work can be applied to many other models of development and other stimulus-response behaviors during development.

In vivo quantitative analysis of Talin turnover in response to force

Cell–ECM adhesion is regulated by mechanical force. Quantitative imaging and mathematical modeling are used to elucidate how the intracellular adhesion complex of integrin-based adhesions responds to force, revealing the molecular mechanisms that allow the adhesion complex to respond to force to stabilize cell–ECM adhesion over development.

Cell adhesion to the extracellular matrix (ECM) allows cells to form and maintain three-dimensional tissue architecture. Cell–ECM adhesions are stabilized upon exposure to mechanical force. In this study, we used quantitative imaging and mathematical modeling to gain mechanistic insight into how integrin-based adhesions respond to increased and decreased mechanical forces. A critical means of regulating integrin-based adhesion is provided by modulating the turnover of integrin and its adhesion complex (integrin adhesion complex [IAC]). The turnover of the IAC component Talin, a known mechanosensor, was analyzed using fluorescence recovery after photobleaching. Experiments were carried out in live, intact flies in genetic backgrounds that increased or decreased the force applied on sites of adhesion. This analysis showed that when force is elevated, the rate of assembly of new adhesions increases such that cell–ECM adhesion is stabilized. Moreover, under conditions of decreased force, the overall rate of turnover, but not the proportion of adhesion complex components undergoing turnover, increases. Using point mutations, we identify the key functional domains of Talin that mediate its response to force. Finally, by fitting a mathematical model to the data, we uncover the mechanisms that mediate the stabilization of ECM-based adhesion during development.

Polarization of the epithelial layer and apical localization of integrins are required for engulfment of apoptotic cells in the Drosophila ovary

Inefficient clearance of dead cells or debris by epithelial cells can lead to or exacerbate debilitating conditions such as retinitis pigmentosa, macular degeneration, chronic obstructive pulmonary disease and asthma. Despite the importance of engulfment by epithelial cells, little is known about the molecular changes that are required within these cells. The misregulation of integrins has previously been associated with disease states, suggesting that a better understanding of the regulation of receptor trafficking could be key to treating diseases caused by defects in phagocytosis. Here, we demonstrate that the integrin heterodimer αPS3/βPS becomes apically enriched and is required for engulfment by the epithelial follicle cells of the Drosophila ovary. We found that integrin heterodimer localization and function is largely directed by the α-subunit. Moreover, proper cell polarity promotes asymmetric integrin enrichment, suggesting that αPS3/βPS trafficking occurs in a polarized fashion. We show that several genes previously known for their roles in trafficking and cell migration are also required for engulfment. Moreover, as in mammals, the same α-integrin subunit is required by professional and non-professional phagocytes and migrating cells in Drosophila. Our findings suggest that migrating and engulfing cells use common machinery, and demonstrate a crucial role for integrin function and polarized trafficking of integrin subunits during engulfment. This study also establishes the epithelial follicle cells of the Drosophila ovary as a powerful model for understanding the molecular changes required for engulfment by a polarized epithelium.

Summary: Apical integrin localization, mediated by polarized and directed trafficking, is crucial for proper engulfment by epithelial cells.

Structural basis of blocking integrin activation and deactivation for anti-inflammation

Integrins mediate leukocyte accumulation to the sites of inflammation, thereby enhancing their potential as an important therapeutic target for inflammatory disorders. Integrin activation triggered by inflammatory mediators or signaling pathway is a key step to initiate leukocyte migration to inflamed tissues; however, an appropriately regulated integrin deactivation is indispensable for maintaining productive leukocyte migration. While typical integrin antagonists that block integrin activation target the initiation of leukocyte migration, a novel class of experimental compounds has been designed to block integrin deactivation, thereby perturbing the progression of cell migration. Current review discusses the mechanisms by which integrin is activated and subsequently deactivated by focusing on its structure-function relationship.

The talin head domain reinforces integrin-mediated adhesion by promoting adhesion complex stability and clustering

Talin serves an essential function during integrin-mediated adhesion in linking integrins to actin via the intracellular adhesion complex. In addition, the N-terminal head domain of talin regulates the affinity of integrins for their ECM-ligands, a process known as inside-out activation. We previously showed that in Drosophila, mutating the integrin binding site in the talin head domain resulted in weakened adhesion to the ECM. Intriguingly, subsequent studies showed that canonical inside-out activation of integrin might not take place in flies. Consistent with this, a mutation in talin that specifically blocks its ability to activate mammalian integrins does not significantly impinge on talin function during fly development. Here, we describe results suggesting that the talin head domain reinforces and stabilizes the integrin adhesion complex by promoting integrin clustering distinct from its ability to support inside-out activation. Specifically, we show that an allele of talin containing a mutation that disrupts intramolecular interactions within the talin head attenuates the assembly and reinforcement of the integrin adhesion complex. Importantly, we provide evidence that this mutation blocks integrin clustering in vivo. We propose that the talin head domain is essential for regulating integrin avidity in Drosophila and that this is crucial for integrin-mediated adhesion during animal development.

Cells are the building blocks of our bodies. How do cells rearrange to form three-dimensional body plans and maintain specific tissue structures? Specialized adhesion molecules on the cell surface mediate attachment between cells and their surrounding environment to hold tissues together. Our work uses the developing fruit fly embryo to demonstrate how such connections are regulated during tissue growth. Since the genes and molecules involved in this process are highly similar between flies and humans, we can also apply our findings to our understanding of how human tissues form and are maintained. We observe that, in late developing muscles, clusters of cell adhesion molecules concentrate together to create stronger attachments between muscle cells and tendon cells. This strengthening mechanism allows the fruit fly to accommodate increasing amounts of force imposed by larger, more active muscles. We identify specific genetic mutations that disrupt these strengthening mechanisms and lead to severe developmental defects during fly development. Our results illustrate how subtle fine-tuning of the connections between cells and their surrounding environment is important to form and maintain normal tissue structure across the animal kingdom.

Mechanical activation of vinculin binding to talin locks talin in an unfolded conformation

The force-dependent interaction between talin and vinculin plays a crucial role in the initiation and growth of focal adhesions. Here we use magnetic tweezers to characterise the mechano-sensitive compact N-terminal region of the talin rod, and show that the three helical bundles R1–R3 in this region unfold in three distinct steps consistent with the domains unfolding independently. Mechanical stretching of talin R1–R3 enhances its binding to vinculin and vinculin binding inhibits talin refolding after force is released. Mutations that stabilize R3 identify it as the initial mechano-sensing domain in talin, unfolding at ∼5 pN, suggesting that 5 pN is the force threshold for vinculin binding and adhesion progression.

Protein conformation as a regulator of cell–matrix adhesion

Conformational changes within proteins play key roles in the regulation of cell–matrix adhesion. We discuss the mechanisms involved in conformational regulation, including mechanical signals, posttranslational modifications and intrinsically disordered proteins.