Clustering of integrin β cytoplasmic domains triggers nascent adhesion formation and reveals a protozoan origin of the integrin-talin interaction

A flexible loop in the paxillin LIM3 domain mediates its direct binding to integrin β subunits

Integrins are fundamental for cell adhesion and the formation of focal adhesions (FA). Accordingly, these receptors guide embryonic development, tissue maintenance, and haemostasis but are also involved in cancer invasion and metastasis. A detailed understanding of the molecular interactions that drive integrin activation, FA assembly, and downstream signalling cascades is critical. Here, we reveal a direct association of paxillin, a marker protein of FA sites, with the cytoplasmic tails of the integrin β1 and β3 subunits. The binding interface resides in paxillin's LIM3 domain, where based on the NMR structure and functional analyses, a flexible, 7-amino acid loop engages the unstructured part of the integrin cytoplasmic tail. Genetic manipulation of the involved residues in either paxillin or integrin β3 compromises cell adhesion and motility of murine fibroblasts. This direct interaction between paxillin and the integrin cytoplasmic domain identifies an alternative, kindlin-independent mode of integrin outside-in signalling particularly important for integrin β3 function.

Producing highly effective extracellular vesicles using IBAR and talin F3 domain fusion

Extracellular vesicles (EVs), transporting diverse cellular components, play a crucial role in intercellular communication in numerous physiological and pathological processes. EVs have also been recognized as a drug delivery platform for therapeutic purposes and cell-free regenerative medicine. While various approaches have focused on increasing EV production for efficient use therapeutic use of EVs, enhancing the quality of EVs, such as ensuring efficient uptake by their target cells, has not been widely explored. In this study, we linked a negative membrane curvature-forming inverse BAR (IBAR) domain with an integrin β tail-binding talin F3 domain to create the IBAR-F3 fusion protein. We observed that IBAR-F3 can trigger filopodia-like membrane protrusions and attract integrins to those protrusion-rich regions, when expressed in Chinese hamster ovary cells expressing integrin αIIbβ3. Surprisingly, the expression of IBAR-F3 also induced a robust production of EVs, which were then efficiently taken up by nearby cells in an integrin-dependent manner. Moreover, IBAR triggered integrin activation, presumably by inducing negative membrane curvature that likely disrupts the interaction between the integrin α and β transmembrane domain. Therefore, we suggest that IBAR-F3 should be utilized to promote both EV production and efficient uptake mediated by integrins. Furthermore, the negative curvature-inducing integrin activation suggests that integrins on EVs can be activated by the nanoscale change in the curvature of the EV without the need for conventional machinery to activate integrin inside the EVs.

Ihog proteins contribute to integrin-mediated focal adhesions

Integrin expression forms focal adhesions, but how this process is physiologically regulated is unclear. Ihog proteins are evolutionarily conserved, playing roles in Hedgehog signaling and serving as trans-homophilic adhesion molecules to mediate cell-cell interactions. Whether these proteins are also engaged in other cell adhesion processes remains unknown. Here, we report that Drosophila Ihog proteins function in the integrin-mediated adhesions. Removal of Ihog proteins causes blister and spheroidal muscle in wings and embryos, respectively. We demonstrate that Ihog proteins interact with integrin via the extracellular portion and that their removal perturbs integrin distribution. Finally, we show that Boc, a mammalian Ihog protein, rescues the embryonic defects caused by removing its Drosophila homologs. We thus propose that Ihog proteins contribute to integrin-mediated focal adhesions.

Unravelling cell migration: defining movement from the cell surface

Cell motility is essential for life and development. Unfortunately, cell migration is also linked to several pathological processes, such as cancer metastasis. Cells' ability to migrate relies on many actors. Cells change their migratory strategy based on their phenotype and the properties of the surrounding microenvironment. Cell migration is, therefore, an extremely complex phenomenon. Researchers have investigated cell motility for more than a century. Recent discoveries have uncovered some of the mysteries associated with the mechanisms involved in cell migration, such as intracellular signaling and cell mechanics. These findings involve different players, including transmembrane receptors, adhesive complexes, cytoskeletal components , the nucleus, and the extracellular matrix. This review aims to give a global overview of our current understanding of cell migration.

An evolutionary and physiological perspective on cell-substrate adhesion machinery for cell migration

Cell-substrate adhesion is a critical aspect of many forms of cell migration. Cell adhesion to an extracellular matrix (ECM) generates traction forces necessary for efficient migration. One of the most well-studied structures cells use to adhere to the ECM is focal adhesions, which are composed of a multilayered protein complex physically linking the ECM to the intracellular actin cytoskeleton. Much of our understanding of focal adhesions, however, is primarily derived from in vitro studies in Metazoan systems. Though these studies provide a valuable foundation to the cell-substrate adhesion field, the evolution of cell-substrate adhesion machinery across evolutionary space and the role of focal adhesions in vivo are largely understudied within the field. Furthering investigation in these areas is necessary to bolster our understanding of the role cell-substrate adhesion machinery across Eukaryotes plays during cell migration in physiological contexts such as cancer and pathogenesis. In this review, we review studies of cell-substrate adhesion machinery in organisms evolutionary distant from Metazoa and cover the current understanding and ongoing work on how focal adhesions function in single and collective cell migration in an in vivo environment, with an emphasis on work that directly visualizes cell-substrate adhesions. Finally, we discuss nuances that ought to be considered moving forward and the importance of future investigation in these emerging fields for application in other fields pertinent to adhesion-based processes.

Mechanism of integrin activation by talin and its cooperation with kindlin

Talin-induced integrin binding to extracellular matrix ligands (integrin activation) is the key step to trigger many fundamental cellular processes including cell adhesion, cell migration, and spreading. Talin is widely known to use its N-terminal head domain (talin-H) to bind and activate integrin, but how talin-H operates in the context of full-length talin and its surrounding remains unknown. Here we show that while being capable of inducing integrin activation, talin-H alone exhibits unexpectedly low potency versus a constitutively activated full-length talin. We find that the large C-terminal rod domain of talin (talin-R), which otherwise masks the integrin binding site on talin-H in inactive talin, dramatically enhances the talin-H potency by dimerizing activated talin and bridging it to the integrin co-activator kindlin-2 via the adaptor protein paxillin. These data provide crucial insight into the mechanism of talin and its cooperation with kindlin to promote potent integrin activation, cell adhesion, and signaling.

Mice lacking PECAM-1 and Ceacam1 have an aberrant platelet and thrombus phenotype

The immunoglobulin (Ig)-immunoreceptor tyrosine-based inhibitory motif (ITIM) bearing receptors, PECAM-1 and CEACAM1 have been shown net negative regulators of platelet-collagen interactions and hemi-ITAM signalling pathways. In this study, a double knockout (DKO) mouse was developed with deleted PECAM-1 and CEACAM1 to study their combined contribution in platelet activation by glycoprotein VI, C-type lectin-like receptor 2 (CLEC-2), protease activated receptor PAR-4, ADP purinergic receptors and thromboxane receptor TP A2 pathways. Additionally, their collective contribution was examined in thrombus formation under high shear and microvascular thrombosis using in vivo models. DKO platelets responded normally to ADP purinergic receptors and TP A2 pathway. However, DKO platelets released significantly higher amounts of P-selectin compared to hyper-responsive Pecam-1-/- or Ceacam1-/- versus wild-type (WT) upon stimulation with collagen related peptide or rhodocytin. Contrastingly, DKO platelets released increased amounts of P-selectin upon stimulation with PAR-4 agonist peptide or thrombin but not Pecam-1-/-, Ceacam1-/- or WT platelets. Blockade of phospholipase C (PLC) or Rho A kinase revealed that DKO platelets enhanced alpha granule release via PAR-4/Gαq/PLC signalling without crosstalk with Src/Syk or G12/13 signalling pathways. This DKO model showed a significant increase in thrombus formation compared to the hyper-responsive Ceacam1-/- or Pecam-1-/- versus WT phenotype. DKO platelets have similar glycoprotein surface expression compared to Pecam-1-/-, Ceacam1-/- and WT platelets. PECAM-1 and CEACAM1 work in concert to negatively regulate hemiITAM signalling, platelet-collagen interactions and PAR-4 Gαq protein coupled signalling pathways. Both PECAM-1 and CEACAM1 are required for negative regulation of platelet activation and microvascular thrombosis in vivo.

Vimentin regulates the assembly and function of matrix adhesions

The intermediate filament protein vimentin is a widely used phenotypic marker for identifying cells of the mesenchymal linkage such as fibroblasts and myofibroblasts, but the full repertoire of vimentin's functional attributes has not been fully explored. Here we consider how vimentin, in addition to its contributions to mechanical stabilization of cell structure, also helps to control the assembly of cell adhesions and migration through collagen matrices. While the assembly and function of matrix adhesions are critical for the differentiation of myofibroblasts and many other types of adherent cells, a potential mechanism that explains how vimentin affects the recruitment and abundance of centrally important proteins in cell adhesions has been elusive. Here we review recent data indicating that vimentin plays a central regulatory role in the assembly of focal adhesions which form in response to the attachment to collagen. We show that in particular, vimentin is a key organizer of the β1 integrin adhesive machinery, which affects cell migration through collagen. This review provides a comprehensive picture of the surprisingly broad array of processes and molecules with which vimentin interacts to affect cell function in the context of fibroblast and myofibroblast adhesion and migration on collagen.

Vimentin tunes cell migration on collagen by controlling β1 integrin activation and clustering

Vimentin is a structural protein that is required for mesenchymal cell migration and directly interacts with actin, β1 integrin and paxillin. We examined how these interactions enable vimentin to regulate cell migration on collagen. In fibroblasts, depletion of vimentin increased talin-dependent activation of β1 integrin by more than 2-fold. Loss of vimentin was associated with reduction of β1 integrin clustering by 50% and inhibition of paxillin recruitment to focal adhesions by more than 60%, which was restored by vimentin expression. This reduction of paxillin was associated with 65% lower Cdc42 activation, a 60% reduction of cell extension formation and a greater than 35% decrease in cell migration on collagen. The activation of PAK1, a downstream effector of Cdc42, was required for vimentin phosphorylation and filament maturation. We propose that vimentin tunes cell migration through collagen by acting as an adaptor protein for focal adhesion proteins, thereby regulating β1 integrin activation, resulting in well-organized, mature integrin clusters.This article has an associated First Person interview with the first author of the paper.

Manipulation of Focal Adhesion Signaling by Pathogenic Microbes

Focal adhesions (FAs) serve as dynamic signaling hubs within the cell. They connect intracellular actin to the extracellular matrix (ECM) and respond to environmental cues. In doing so, these structures facilitate important processes such as cell-ECM adhesion and migration. Pathogenic microbes often modify the host cell actin cytoskeleton in their pursuit of an ideal replicative niche or during invasion to facilitate uptake. As actin-interfacing structures, FA dynamics are also intimately tied to actin cytoskeletal organization. Indeed, exploitation of FAs is another avenue by which pathogenic microbes ensure their uptake, survival and dissemination. This is often achieved through the secretion of effector proteins which target specific protein components within the FA. Molecular mimicry of the leucine-aspartic acid (LD) motif or vinculin-binding domains (VBDs) commonly found within FA proteins is a common microbial strategy. Other effectors may induce post-translational modifications to FA proteins through the regulation of phosphorylation sites or proteolytic cleavage. In this review, we present an overview of the regulatory mechanisms governing host cell FAs, and provide examples of how pathogenic microbes have evolved to co-opt them to their own advantage. Recent technological advances pose exciting opportunities for delving deeper into the mechanistic details by which pathogenic microbes modify FAs.

Sialyltransferase Inhibitors Suppress Breast Cancer Metastasis

We report the synthesis and evaluation of a series of cell-permeable and N- versus O-selective sialyltransferase inhibitors. Inhibitor design entailed the functionalization of lithocholic acid at C(3) and at the cyclopentane ring side chain. Among the series, FCW34 and FCW66 were shown to inhibit MDA-MB-231 cell migration as effectively as ST3GALIII-gene knockdown did. FCW34 was shown to inhibit tumor growth, reduce angiogenesis, and delay cancer cell metastasis in animal models. Furthermore, FCW34 inhibited vessel development and suppressed angiogenic activity in transgenic zebrafish models. Our results provide clear evidence that FCW34-induced sialyltransferase inhibition reduces cancer cell metastasis by decreasing N-glycan sialylation, thus altering the regulation of talin/integrin/FAK/paxillin and integrin/NFκB signaling pathways.

PPM1F controls integrin activity via a conserved phospho-switch

Control of integrin activity is vital during development and tissue homeostasis, while derailment of integrin function contributes to pathophysiological processes. Phosphorylation of a conserved threonine motif (T788/T789) in the integrin β cytoplasmic domain increases integrin activity. Here, we report that T788/T789 functions as a phospho-switch, which determines the association with either talin and kindlin-2, the major integrin activators, or filaminA, an integrin activity suppressor. A genetic screen identifies the phosphatase PPM1F as the critical enzyme, which selectively and directly dephosphorylates the T788/T789 motif. PPM1F-deficient cell lines show constitutive integrin phosphorylation, exaggerated talin binding, increased integrin activity, and enhanced cell adhesion. These gain-of-function phenotypes are reverted by reexpression of active PPM1F, but not a phosphatase-dead mutant. Disruption of the ppm1f gene in mice results in early embryonic death at day E10.5. Together, PPM1F controls the T788/T789 phospho-switch in the integrin β1 cytoplasmic tail and constitutes a novel target to modulate integrin activity.

Integrins meet complement: The evolutionary tip of an iceberg orchestrating metabolism and immunity

Immunologists have recently realized that there is more to the classic innate immune sensor systems than just mere protection against invading pathogens. It is becoming increasingly clear that such sensors, including the inflammasomes, toll-like receptors, and the complement system, are heavily involved in the regulation of basic cell physiological processes and particularly those of metabolic nature. In fact, their "non-canonical" activities make sense as no system directing immune cell activity can perform such task without the need for energy. Further, many of these ancient immune sensors appeared early and concurrently during evolution, particularly during the developmental leap from the single-cell organisms to multicellularity, and therefore crosstalk heavily with each other. Here, we will review the current knowledge about the emerging cooperation between the major inter-cell communicators, integrins, and the cell-autonomous intracellularly and autocrine-active complement, the complosome, during the regulation of single-cell metabolism. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

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.