Modulation of integrin activity is vital for morphogenesis

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.

LanB1 Cooperates With Kon-Tiki During Embryonic Muscle Migration in Drosophila

Muscle development is a multistep process that involves cell specification, myoblast fusion, myotube migration, and attachment to the tendons. In spite of great efforts trying to understand the basis of these events, little is known about the molecular mechanisms underlying myotube migration. Knowledge of the few molecular cues that guide this migration comes mainly from studies in Drosophila. The migratory process of Drosophila embryonic muscles involves a first phase of migration, where muscle progenitors migrate relative to each other, and a second phase, where myotubes migrate searching for their future attachment sites. During this phase, myotubes form extensive filopodia at their ends oriented preferentially toward their attachment sites. This myotube migration and the subsequent muscle attachment establishment are regulated by cell adhesion receptors, such as the conserved proteoglycan Kon-tiki/Perdido. Laminins have been shown to regulate the migratory behavior of many cell populations, but their role in myotube migration remains largely unexplored. Here, we show that laminins, previously implicated in muscle attachment, are indeed required for muscle migration to tendon cells. Furthermore, we find that laminins genetically interact with kon-tiki/perdido to control both myotube migration and attachment. All together, our results uncover a new role for the interaction between laminins and Kon-tiki/Perdido during Drosophila myogenesis. The identification of new players and molecular interactions underlying myotube migration broadens our understanding of muscle development and disease.

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.

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.

Tip cells act as dynamic cellular anchors in the morphogenesis of looped renal tubules in Drosophila

Tissue morphogenesis involves both the sculpting of tissue shape and the positioning of tissues relative to one another in the body. Using the renal tubules of Drosophila, we show that a specific distal tubule cell regulates both tissue architecture and position in the body cavity. Focusing on the anterior tubules, we demonstrate that tip cells make transient contacts with alary muscles at abdominal segment boundaries, moving progressively forward as convergent extension movements lengthen the tubule. Tip cell anchorage antagonizes forward-directed, TGF-β-guided tubule elongation, thereby ensuring the looped morphology characteristic of renal tubules from worms to humans. Distinctive tip cell exploratory behavior, adhesion, and basement membrane clearing underlie target recognition and dynamic interactions. Defects in these features obliterate tip cell anchorage, producing misshapen and misplaced tubules with impaired physiological function.

•

Tip cells stabilize elongating renal tubules by binding to specific muscle targets

•

Tip cell anchorage antagonizes migration, producing excretory tubule looping

•

Tip cell exploration and adhesion dynamics underpin regulated tubule morphogenesis

•

Lack of continued anchorage results in deformed tubules with impaired function

Extracellular matrix is required for muscle differentiation in primary cell cultures of larval Mytilus trossulus (Mollusca: Bivalvia)

Components of the extracellular matrix may modulate the growth factor effects that play important roles in the proliferation and differentiation of precursor cells. We developed an in vitro cultivation protocol for cells of the larval marine bivalve Mytilus trossulus to study the role that extracellular matrix components may play in myodifferentiation and replication-mediated DNA synthesis using immunofluorescence and confocal laser scanning microscopy. Here, we demonstrate that the extracellular matrix regulates the expression of muscle proteins, leading to their assembly and the terminal muscle differentiation of larval cells during cultivation. We further show that the myogenesis process progresses in cells cultivated on fibronectin, carbon or poly-L-lysine but is inhibited in cells grown on a collagen carpet. Consistent with a decrease in muscle protein expression in cells cultivated on collagen, we demonstrate an increase in the number of BrdU-positive cells in comparison with cells cultured on other substrates during the entire cultivation period. Moreover, we demonstrate that the matrix-dependent myogenic differentiation of larval mussel cells is reversible. Round-shaped cells cultivated on collagen were able to differentiate into muscle cells after reseeding on fibronectin, carbon or poly-L-lysine. In addition, cells cultured on collagen and then transplanted to fibronectin exhibited distinct cross-striation and contractile activity. Taken together, our data suggest that the extracellular matrix participates in the regulation of the proliferation and myodifferentiation of mussel trochophore progenitor cells and validate novel approaches for successfully culturing cells from bivalves over extended periods.

Zasp regulates integrin activation

Integrins are heterodimeric adhesion receptors that link the extracellular matrix (ECM) to the cytoskeleton. Binding of the scaffold protein, talin, to the cytoplasmic tail of β-integrin causes a conformational change of the extracellular domains of the integrin heterodimer, thus allowing high-affinity binding of ECM ligands. This essential process is called integrin activation. Here we report that the Z-band alternatively spliced PDZ-motif-containing protein (Zasp) cooperates with talin to activate α5β1 integrins in mammalian tissue culture and αPS2βPS integrins in Drosophila. Zasp is a PDZ-LIM-domain-containing protein mutated in human cardiomyopathies previously thought to function primarily in assembly and maintenance of the muscle contractile machinery. Notably, Zasp is the first protein shown to co-activate α5β1 integrins with talin and appears to do so in a manner distinct from known αIIbβ3 integrin co-activators.

Integrins are required for cardioblast polarisation in Drosophila

BACKGROUND

The formation of a tubular organ, such as the heart, requires the communication of positional and polarity signals between migratory cells. Key to this process is the establishment of a new luminal domain on the cell surface, generally from the apical domain of a migratory cell. This domain will also acquire basal properties, as it will produce a luminal extracellular matrix. Integrin receptors are the primary means of cell adhesion and adhesion signaling with the extracellular matrix. Here we characterise the requirement of Integrins in a genetic model of vasculogenesis, the formation of the heart in Drosophila.

RESULTS

As with vertebrates, the Drosophila heart arises from lateral mesoderm that migrates medially to meet their contralateral partners, to then assemble a midline vessel. During migration, Integrins are among the first proteins restricted to the presumptive luminal domain of cardioblasts. Integrins are required for normal levels of leading edge membrane motility. Apical accumulation of Integrins is enhanced by Robo, and reciprocally, apicalisation of luminal factors like Slit and Robo requires Integrin function. Integrins may provide a template for the formation of a lumen by stabilising lumen factors like Robo. Subsequent to migration, Integrin is required for normal cardioblast alignment and lumen formation. This phenotype is most readily modified by other mutations that affect adhesion, such as Talin and extracellular matrix ligands.

CONCLUSION

Our findings reveal an instructive role for Integrins in communicating polarising information to cells during migration, and during transition to an epithelial tube structure.

PS integrins and laminins: key regulators of cell migration during Drosophila embryogenesis

During embryonic development, there are numerous cases where organ or tissue formation depends upon the migration of primordial cells. In the Drosophila embryo, the visceral mesoderm (vm) acts as a substrate for the migration of several cell populations of epithelial origin, including the endoderm, the trachea and the salivary glands. These migratory processes require both integrins and laminins. The current model is that αPS1βPS (PS1) and/or αPS3βPS (PS3) integrins are required in migrating cells, whereas αPS2βPS (PS2) integrin is required in the vm, where it performs an as yet unidentified function. Here, we show that PS1 integrins are also required for the migration over the vm of cells of mesodermal origin, the caudal visceral mesoderm (CVM). These results support a model in which PS1 might have evolved to acquire the migratory function of integrins, irrespective of the origin of the tissue. This integrin function is highly specific and its specificity resides mainly in the extracellular domain. In addition, we have identified the Laminin α1,2 trimer, as the key extracellular matrix (ECM) component regulating CVM migration. Furthermore, we show that, as it is the case in vertebrates, integrins, and specifically PS2, contributes to CVM movement by participating in the correct assembly of the ECM that serves as tracks for migration.

Identification of integrin beta subunit mutations that alter affinity for extracellular matrix ligand

We examined over 50 mutations in the Drosophila βPS integrin subunit that alter integrin function in situ for their ability to bind a soluble monovalent ligand, TWOW-1. Surprisingly, very few of the mutations, which were selected for conditional lethality in the fly, reduce the ligand binding ability of the integrin. The most prevalent class of mutations activates the integrin heterodimer. These findings emphasize the importance of integrin affinity regulation and point out how molecular interactions throughout the integrin molecule are important in keeping the integrin in a low affinity state. Mutations strongly support the controversial deadbolt hypothesis, where the CD loop in the β tail domain acts to restrain the I domain in the inactive, bent conformation. Site-directed mutations in the cytoplasmic domains of βPS and αPS2C reveal different effects on ligand binding from those observed for αIIbβ3 integrins and identify for the first time a cytoplasmic cysteine residue, conserved in three human integrins, as being important in affinity regulation. In the fly, we find that genetic interactions of the βPS mutations with reduction in talin function are consistent with the integrin affinity differences measured in cells. Additionally, these genetic interactions report on increased and decreased integrin functions that do not result in affinity changes in the PS2C integrin measured in cultured cells.

Vestigial is required during late-stage muscle differentiation in Drosophila melanogaster embryos

The Drosophila member of the vestigial-like gene family (vestigial) is known primarily as a transcriptional activator that defines cell identity during Drosophila wing differentiation. We show that during embryo development Vestigial also has a role during specification of muscle–muscle attachments in ventral longitudinal muscles.

The somatic muscles of Drosophila develop in a complex pattern that is repeated in each embryonic hemi-segment. During early development, progenitor cells fuse to form a syncytial muscle, which further differentiates via expression of muscle-specific factors that induce specific responses to external signals to regulate late-stage processes such as migration and attachment. Initial communication between somatic muscles and the epidermal tendon cells is critical for both of these processes. However, later establishment of attachments between longitudinal muscles at the segmental borders is largely independent of the muscle–epidermal attachment signals, and relatively little is known about how this event is regulated. Using a combination of null mutations and a truncated version of Sd that binds Vg but not DNA, we show that Vestigial (Vg) is required in ventral longitudinal muscles to induce formation of stable intermuscular attachments. In several muscles, this activity may be independent of Sd. Furthermore, the cell-specific differentiation events induced by Vg in two cells fated to form attachments are coordinated by Drosophila epidermal growth factor signaling. Thus, Vg is a key factor to induce specific changes in ventral longitudinal muscles 1–4 identity and is required for these cells to be competent to form stable intermuscular attachments with each other.

The role of natalizumab in hematopoietic stem cell mobilization

The humanized monoclonal very late antigen 4 (VLA-4) antibody natalizumab is FDA approved for the treatment of patients with multiple sclerosis and Crohn's disease. In this review we focus on its role in the context of hematopoietic stem cell transplantation and stem cell diseases. The use of natalizumab alone or in combination with either cytotoxic drugs or other antibodies might be a new modality for stem cell mobilization and a therapeutic option for patients with hematologic malignancies.

Differences in regulation of Drosophila and vertebrate integrin affinity by talin

Integrin-mediated cell adhesion is essential for development of multicellular organisms. In worms, flies, and vertebrates, talin forms a physical link between integrin cytoplasmic domains and the actin cytoskeleton. Loss of either integrins or talin leads to similar phenotypes. In vertebrates, talin is also a key regulator of integrin affinity. We used a ligand-mimetic Fab fragment, TWOW-1, to assess talin's role in regulating Drosophila alphaPS2 betaPS affinity. Depletion of cellular metabolic energy reduced TWOW-1 binding, suggesting alphaPS2 betaPS affinity is an active process as it is for vertebrate integrins. In contrast to vertebrate integrins, neither talin knockdown by RNA interference nor talin head overexpression had a significant effect on TWOW-1 binding. Furthermore, replacement of the transmembrane or talin-binding cytoplasmic domains of alphaPS2 betaPS with those of human alphaIIb beta3 failed to enable talin regulation of TWOW-1 binding. However, substitution of the extracellular and transmembrane domains of alphaPS2 betaPS with those of alphaIIb beta3 resulted in a constitutively active integrin whose affinity was reduced by talin knockdown. Furthermore, wild-type alphaIIb beta3 was activated by overexpression of Drosophila talin head domain. Thus, despite evolutionary conservation of talin's integrin/cytoskeleton linkage function, talin is not sufficient to regulate Drosophila alphaPS2 betaPS affinity because of structural features inherent in the alphaPS2 betaPS extracellular and/or transmembrane domains.

The N-terminal domains of talin cooperate with the phosphotyrosine binding-like domain to activate beta1 and beta3 integrins

The activation of integrin adhesion receptors from low to high affinity in response to intracellular cues controls cell adhesion and signaling. Binding of the cytoskeletal protein talin to the beta3 integrin cytoplasmic tail is required for beta3 activation, and the integrin-binding PTB-like F3 domain of talin is sufficient to activate beta3 integrins. Here we report that, whereas the conserved talin-integrin interaction is also required for beta1 activation, and talin F3 binds beta1 and beta3 integrins with comparable affinity, expression of the talin F3 domain is not sufficient to activate beta1 integrins. beta1 integrin activation could, however, be detected following expression of larger talin fragments that included the N-terminal and F1 domains, and mutagenesis indicates that these domains cooperate with talin F3 to mediate beta1 activation. This effect is not due to increased affinity for the integrin beta tail and we hypothesize that the N-terminal domains function by targeting or orienting talin in such a way as to optimize the interaction with the integrin tail. Analysis of beta3 integrin activation indicates that inclusion of the N-terminal and F1 domains also enhances F3-mediated beta3 activation. Our results therefore reveal a role for the N-terminal and F1 domains of talin during integrin activation and highlight differences in talin-mediated activation of beta1 and beta3 integrins.

Loss of talin1 in platelets abrogates integrin activation, platelet aggregation, and thrombus formation in vitro and in vivo

Platelet adhesion and aggregation at sites of vascular injury are essential for normal hemostasis but may also lead to pathological thrombus formation, causing diseases such as myocardial infarction or stroke. Heterodimeric receptors of the integrin family play a central role in the adhesion and aggregation of platelets. In resting platelets, integrins exhibit a low affinity state for their ligands, and they shift to a high affinity state at sites of vascular injury. It has been proposed that direct binding of the cytoskeletal protein talin1 to the cytoplasmic domain of the integrin β subunits is necessary and sufficient to trigger the activation of integrins to this high affinity state, but direct in vivo evidence in support of this hypothesis is still lacking. Here, we show that platelets from mice lacking talin1 are unable to activate integrins in response to all known major platelet agonists while other cellular functions are still preserved. As a consequence, mice with talin-deficient platelets display a severe hemostatic defect and are completely resistant to arterial thrombosis. Collectively, these experiments demonstrate that talin is required for inside-out activation of platelet integrins in hemostasis and thrombosis.

An interaction between integrin and the talin FERM domain mediates integrin activation but not linkage to the cytoskeleton

Transmembrane adhesion receptors, such as integrins, mediate cell adhesion by interacting with intracellular proteins that connect to the cytoskeleton. Talin, one such linker protein, is thought to have two roles: mediating inside-out activation of integrins, and connecting extracellular matrix (ECM)-bound integrins to the cytoskeleton. Talin's amino-terminal head, which consists of a FERM domain, binds an NPxY motif within the cytoplasmic tail of most integrin beta subunits. This is consistent with the role of FERM domains in recruiting other proteins to the plasma membrane. We tested the role of the talin-head-NPxY interaction in integrin function in Drosophila. We found that introduction of a mutation that perturbs this binding in vitro into the isolated talin head disrupts its recruitment by integrins in vivo. Surprisingly, when engineered into the full-length talin, this mutation did not disrupt talin recruitment by integrins nor its ability to connect integrins to the cytoskeleton. However, it reduced the ability of talin to strengthen integrin adhesion to the ECM, indicating that the function of the talin-head-NPxY interaction is solely to regulate integrin adhesion.

Multiple factors contribute to integrin-talin interactions in vivo

The cytoplasmic protein talin is an essential part of the integrin-cytoskeleton link. We characterized the interaction between integrin and two conserved regions of talin, the N-terminal ;head' domain and the C-terminus, which includes the I/LWEQ domain, within the living organism. Green-fluorescent-protein-tagged head and C-terminal domains were recruited to integrin adhesion sites. Both required integrins for recruitment, but the C-terminal domain also required endogenous talin, showing it was not recruited directly by integrins. We used chimeric transmembrane proteins containing the cytoplasmic domain of the integrin beta subunit to examine the integrin-talin head interaction. Monomeric chimeric proteins did not recruit talin head, whereas dimeric chimeras efficiently recruited it and caused a strong inhibition of integrin-mediated adhesion. These chimeras recruited surprisingly few integrin-associated proteins, indicating that recruitment of talin did not initiate a cascade of recruitment. Mutagenesis of the integrin cytoplasmic domain, within the chimera, showed the dominant-negative inhibition was not due to talin sequestration alone and that additional interactions are required.

Amino acid changes in Drosophila alphaPS2betaPS integrins that affect ligand affinity

We developed a ligand-mimetic antibody Fab fragment specific for Drosophila alphaPS2betaPS integrins to probe the ligand binding affinities of these invertebrate receptors. TWOW-1 was constructed by inserting a fragment of the extracellular matrix protein Tiggrin into the H-CDR3 of the alphavbeta3 ligand-mimetic antibody WOW-1. The specificity of alphaPS2betaPS binding to TWOW-1 was demonstrated by numerous tests used for other integrin-ligand interactions. Binding was decreased in the presence of EDTA or RGD peptides and by mutation of the TWOW-1 RGD sequence or the betaPS metal ion-dependent adhesion site (MIDAS) motif. TWOW-1 binding was increased by mutations in the alphaPS2 membrane-proximal cytoplasmic GFFNR sequence or by exposure to Mn2+. Although Mn2+ is sometimes assumed to promote maximal integrin activity, TWOW-1 binding in Mn2+ could be increased further by the alphaPS2 GFFNR --> GFANA mutation. A mutation in the betaPS I domain (betaPS-b58; V409D) greatly increased ligand binding affinity, explaining the increased cell spreading mediated by alphaPS2betaPS-b58. Further mutagenesis of this residue suggested that Val-409 normally stabilizes the closed head conformation. Mutations that potentially reduce interaction of the integrin beta subunit plexin-semaphorin-integrin (PSI) and stalk domains have been shown to have activating properties. We found that complete deletion of the betaPS PSI domain enhanced TWOW-1 binding. Moreover the PSI domain is dispensable for at least some other integrin functions because betaPS-DeltaPSI displayed an enhanced ability to mediate cell spreading. These studies establish a means to evaluate mechanisms and consequences of integrin affinity modulation in a tractable model genetic system.

The integrin alpha5beta1 regulates alphavbeta3-mediated extracellular signal-regulated kinase activation

BACKGROUND

Integrin-mediated cell migration is essential for wound repair. Previous studies have shown that the interaction between integrins and the extracellular matrix (ECM) can initiate intracellular signaling pathways to regulate cell movement. Both the focal adhesion kinase (FAK) and the extracellular signal-regulated kinase/activated mitogen-activated protein kinase (ERK/MAPK) signaling pathways are required for efficient cell migration. Our previous work has shown that co-expression of the integrin alpha5beta1 inhibits alphavbeta3-mediated cell migration. We hypothesized that alpha5beta1 may regulate cell migration by modulating these alphavbeta3-mediated intracellular signaling events.

METHODS

CHO B3 (alphavbeta3+) and B3C5 (alphavbeta3+/alpha5beta1+) cells were monitored by flow cytometry to determine integrin expression. Cells were allowed to migrate on fibrinogen (FBG)-coated transwells, with or without PD98059, an inhibitor of the ERK activator, mitogen-activated protein kinase kinase (MEK). Fixation, staining, and cell counting were used to quantify cell migration. Cells adherent to FBG were lysed and analyzed for FAK and ERK/MAPK activation by immunoblotting followed by image analysis densitometry. All experiments were repeated in triplicate.

RESULTS

Treatment with PD98059 significantly decreased alphavbeta3-mediated cell migration on FBG (P = 0.0001) to a level comparable to untreated B3C5 cells. Following adhesion to FBG, B3 cells demonstrated a marked increase in ERK/MAPK activation compared to B3C5 cells. However, no significant difference was detected in FAK activation.

CONCLUSION

Signaling through the ERK/MAPK pathway is required for efficient alphavbeta3-mediated migration on FBG. Inhibition of alphavbeta3-mediated migration by the integrin alpha5beta1 correlates with altered intensity and duration of ERK/MAPK activation, but not FAK activation, in response to adhesion. This suggests a mechanism for the regulatory effect of alpha5beta1 on alphavbeta3-mediated cell migration.

Phosphodiesterase-I alpha/autotaxin controls cytoskeletal organization and FAK phosphorylation during myelination

Myelination within the central nervous system (CNS) involves substantial morphogenesis of oligodendrocytes requiring plastic changes in oligodendrocyte-extracellular matrix (ECM) interactions, that is, adhesion. Our previous studies indicated that a regulator of such adhesive plasticity is oligodendrocyte-released phosphodiesterase-I alpha/autotaxin (PD-I alpha/ATX). We report here, that PD-I alpha/ATX's adhesion antagonism is mediated by a protein fragment different from the one that stimulates tumor cell motility. Furthermore, PD-I alpha/ATX's adhesion-antagonizing fragment causes a reorganized distribution of the focal adhesion components vinculin and paxillin and an integrin-dependent reduction in focal adhesion kinase (FAK) phosphorylation at tyrosine residue 925 (pFAK-925). In vivo, a similar reduction in pFAK-925 occurs at the onset of myelination when PD-I alpha/ATX expression is significantly upregulated. Most importantly, it can also be induced by the application of exogenous PD-I alpha/ATX. Our data, therefore, suggest that PD-I alpha/ATX participates in the regulation of myelination via a novel signaling pathway leading to changes in integrin-dependent focal adhesion assembly and consequently oligodendrocyte-ECM interactions.

Integrin activation

The ability of cells to regulate dynamically their adhesion to one another and to the extracellular matrix (ECM) that surrounds them is essential in multicellular organisms. The integrin family of transmembrane adhesion receptors mediates both cell-cell and cell-ECM adhesion. One important, rapid and reversible mechanism for regulating adhesion is by increasing the affinity of integrin receptors for their extracellular ligands (integrin activation). This is controlled by intracellular signals that, through their action on integrin cytoplasmic domains, induce conformational changes in integrin extracellular domains that result in increased affinity for ligand. Recent studies have shed light on the final intracellular steps in this process and have revealed a vital role for the cytoskeletal protein talin.

Novel activating and inactivating mutations in the integrin beta1 subunit A domain

The ligand-binding activity of integrins is regulated by shape changes that convert these receptors from a resting (or inactive) state to an active state. However, the precise conformational changes that take place in head region of integrins (the site of ligand binding) during activation are not well understood. The portion of the integrin beta subunit involved in ligand recognition contains a von Willebrand factor type A domain, which comprises a central beta-sheet surrounded by seven alpha helices (alpha1-alpha7). Using site-directed mutagenesis, we show here that point mutation of hydrophobic residues in the alpha1 and alpha7 helices (which would be predicted to increase the mobility of these helices) markedly increases the ligand-binding activity of both integrins alpha5beta1 and alpha4beta1. In contrast, mutation of a hydrophilic residue near the base of the alpha1 helix decreases activity and also suppresses exposure of activation epitopes on the underlying hybrid domain. Our results provide new evidence that shifts of the alpha1 and alpha7 helices are involved in activation of the A domain. Although these changes are grossly similar to those defined in the A domains found in some integrin alpha subunits, movement of the alpha1 helix appears to play a more prominent role in betaA domain activation.

Identification of integrin beta subunit mutations that alter heterodimer function in situ

We conducted a genetic screen for mutations in myospheroid, the gene encoding the Drosophila betaPS integrin subunit, and identified point mutants in all of the structural domains of the protein. Surprisingly, we find that mutations in very strongly conserved residues will often allow sufficient integrin function to support the development of adult animals, including mutations in the ADMIDAS site and in a cytoplasmic NPXY motif. Many mutations in the I-like domain reduce integrin expression specifically when betaPS is combined with activating alphaPS2 cytoplasmic mutations, indicating that integrins in the extended conformation are unstable relative to the inactive, bent heterodimers. Interestingly, the screen has identified alleles that show gain-of-function characteristics in cell culture, but have negative effects on animal development or viability. This is illustrated by the allele mys(b58); available structural models suggest that the molecular lesion of mys(b58), V409>D, should promote the "open" conformation of the beta subunit I-like domain. This expectation is supported by the finding that alphaPS2betaPS (V409>D) promotes adhesion and spreading of S2 cells more effectively than does wild-type alphaPS2betaPS, even when betaPS is paired with alphaPS2 containing activating cytoplasmic mutations. Finally, comparisons with the sequence of human beta8 suggest that evolution has targeted the "mys(b58)" residue as a means of affecting integrin activity.

Platelets with wings: the maturation of Drosophila integrin biology

The integrin family of cell surface receptors is strongly conserved in metazoans, making simple invertebrate genetic systems valuable contributors to understanding integrin function. The Drosophila integrins have long served as a paradigm for genetic studies of adhesion proteins during development. Currently, Drosophila experiments are exploring more general aspects of integrin biology. Genetic screens are identifying proteins involved in integrin adhesion complexes and signaling, and structures such as embryonic muscle attachments can be manipulated experimentally to dissect the functions of cytoplasmic components of integrin adhesion sites in whole animals. Drosophila also is beginning to yield some insights into integrin heterodimer structure and function.

Competition for talin results in trans-dominant inhibition of integrin activation

The ability of integrin adhesion receptors to undergo rapid changes in affinity for their extracellular ligands (integrin activation) is essential for the development and function of multicellular animals and is dependent on interactions between the integrin beta subunit-cytoplasmic tail and the cytoskeletal protein talin. Cross-talk among different integrins and between integrins and other receptors impacts many cellular processes including adhesion, spreading, migration, clot retraction, proliferation, and differentiation. One form of integrin cross-talk, transdominant inhibition of integrin activation, occurs when ligand binding to one integrin inhibits the activation of a second integrin. This may be relevant clinically in a number of settings such as during platelet adhesion, leukocyte trans-migration, and angiogenesis. Here we report that competition for talin underlies the trans-dominant inhibition of integrin activation. This conclusion is based on our observations that (i). beta tails selectively defective in talin binding are unable to mediate trans-dominant inhibition, (ii). trans-dominant inhibition can be reversed by overexpression of integrin binding and activating fragments of talin, and (iii). expression of another non-integrin talin-binding protein, phosphatidylinositol phosphate kinase type Igamma-90, also inhibits integrin activation. Thus, the sequestration of talin by the suppressive species is both necessary and sufficient for trans-dominant inhibition of integrin activation.

Class 3 semaphorins control vascular morphogenesis by inhibiting integrin function

The motility and morphogenesis of endothelial cells is controlled by spatio-temporally regulated activation of integrin adhesion receptors, and integrin activation is stimulated by major determinants of vascular remodelling. In order for endothelial cells to be responsive to changes in activator gradients, the adhesiveness of these cells to the extracellular matrix must be dynamic, and negative regulators of integrins could be required. Here we show that during vascular development and experimental angiogenesis, endothelial cells generate autocrine chemorepulsive signals of class 3 semaphorins (SEMA3 proteins) that localize at nascent adhesive sites in spreading endothelial cells. Disrupting endogenous SEMA3 function in endothelial cells stimulates integrin-mediated adhesion and migration to extracellular matrices, whereas exogenous SEMA3 proteins antagonize integrin activation. Misexpression of dominant negative SEMA3 receptors in chick embryo endothelial cells locks integrins in an active conformation, and severely impairs vascular remodelling. Sema3a null mice show vascular defects as well. Thus during angiogenesis endothelial SEMA3 proteins endow the vascular system with the plasticity required for its reshaping by controlling integrin function.

Integrin-ECM interactions regulate cadherin-dependent cell adhesion and are required for convergent extension in Xenopus

BACKGROUND

Convergence extension movements are conserved tissue rearrangements implicated in multiple morphogenetic events. While many of the cell behaviors involved in convergent extension are known, the molecular interactions required for this process remain elusive. However, past evidence suggests that regulation of cell adhesion molecule function is a key step in the progression of these behaviors.

RESULTS

Antibody blocking of fibronectin (FN) adhesion or dominant-negative inhibition of integrin beta 1 function alters cadherin-mediated cell adhesion, promotes cell-sorting behaviors in reaggregation assays, and inhibits medial-lateral cell intercalation and axial extension in gastrulating embryos and explants. Embryo explants were used to demonstrate that normal integrin signaling is required for morphogenetic movements within defined regions but not for cell fate specification. The binding of soluble RGD-containing fragments of fibronectin to integrins promotes the reintegration of dissociated single cells into intact tissues. The changes in adhesion observed are independent of cadherin or integrin expression levels.

CONCLUSIONS

We conclude that integrin modulation of cadherin adhesion influences cell intercalation behaviors within boundaries defined by extracellular matrix. We propose that this represents a fundamental mechanism promoting localized cell rearrangements throughout development.

De novo expression of the integrin alpha5beta1 regulates alphavbeta3-mediated adhesion and migration on fibrinogen

Recent evidence demonstrates that interactions between different integrins that are present on the cell surface can strongly influence the adhesive function of individual receptors. In this report, we show that Chinese hamster ovary cells that express the integrin alphavbeta3 in the absence of alpha5beta1 demonstrate increased adhesion and migration on fibrinogen. Furthermore, alphavbeta3-mediated adhesion to fibrinogen is not augmented by the soluble agonist, MnCl2, suggesting that alphavbeta3 exists in a higher affinity state in these cells. De novo expression of wild-type alpha5beta1 negatively regulates alphavbeta3-mediated adhesion and migration. This effect is not seen with expression of a chimeric alpha5beta1 integrin in which the cytoplasmic portion of the alpha5 integrin subunit is replaced by the cytoplasmic portion of the alpha4 integrin. In addition, it does not require ligation of alpha5beta1 by fibronectin. Cells that express a constitutively active beta3 integrin that contains a point mutation in the conserved membrane proximal region of the cytoplasmic tail, D723R, are resistant to the effect of alpha5beta1 expression. These data provide additional evidence of "cross-talk" between the integrins alpha5beta1 and alphavbeta3, and support the idea that alpha5beta1 regulates alphavbeta3-mediated ligand binding. This provides a relevant biological mechanism whereby variations in alpha5beta1 expression in vivo may modulate activation of alphavbeta3 to influence its adhesive function.

Talin is essential for integrin function in Drosophila

We show that the Drosophila gene rhea, isolated because its wing blister phenotype is typical of mutants affecting integrin function, encodes talin. Embryos deficient in talin have very similar phenotypes to integrin (betaPS) null embryos, including failure in germ band retraction and muscle detachment. We demonstrate that talin is not required for the presence of integrins on the cell surface or their localization at muscle termini. However, talin is required for formation of focal adhesion-like clusters of integrins on the basal surface of imaginal disc epithelia and junctional plaques between muscle and tendon cells. These results indicate that talin is essential for integrin function and acts by stably linking clusters of ECM-linked integrins to the cytoskeleton.

Genetic Interaction Between Integrins and moleskin, a Gene Encoding a Drosophila Homolog of Importin-7

The Drosophila PS1 and PS2 integrins are required to maintain the connection between the dorsal and ventral wing epithelia. If αPS subunits are inappropriately expressed during early pupariation, the epithelia separate, causing a wing blister. Two lines of evidence indicate that this apparent loss-of-function phenotype is not a dominant negative effect, but is due to inappropriate expression of functional integrins: wing blisters are not generated efficiently by misexpression of loss-of-function αPS2 subunits with mutations that inhibit ligand binding, and gain-of-function, hyperactivated mutant αPS2 proteins cause blistering at expression levels well below those required by wild-type proteins. A genetic screen for dominant suppressors of wing blisters generated null alleles of a gene named moleskin, which encodes the protein DIM-7. DIM-7, a Drosophila homolog of vertebrate importin-7, has recently been shown to bind the SHP-2 tyrosine phosphatase homolog Corkscrew and to be important in the nuclear translocation of activated D-ERK. Consistent with this latter finding, homozygous mutant clones of moleskin fail to grow in the wing. Genetic tests suggest that the moleskin suppression of wing blisters is not directly related to inhibition of D-ERK nuclear import. These data are discussed with respect to the possible regulation of integrin function by cytoplasmic ERK.

Role of FGFs in the control of programmed cell death during limb development

We have investigated the role of FGFs in the control of programmed cell death during limb development by analyzing the effects of increasing and blocking FGF signaling in the avian limb bud. BMPs are currently considered as the signals responsible for cell death. Here we show that FGF signaling is also necessary for apoptosis and that the establishment of the areas of cell death is regulated by the convergence of FGF- and BMP-mediated signaling pathways. As previously demonstrated, cell death is inhibited for short intervals (12 hours) after administration of FGFs. However, this initial inhibition is followed (24 hours) by a dramatic increase in cell death, which can be abolished by treatments with a BMP antagonist (Noggin or Gremlin). Conversely, blockage of FGF signaling by applying a specific FGF-inhibitor (SU5402) into the interdigital regions inhibits both physiological cell death and that mediated by exogenous BMPs. Furthermore, FGF receptors 1, 2 and 3 are expressed in the autopodial mesoderm during the regression of the interdigital tissue, and the expression of FGFR3 in the interdigital regions is regulated by FGFs and BMPs in the same fashion as apopotosis. Together our findings indicate that, in the absence of FGF signaling BMPs are not sufficient to trigger apoptosis in the developing limb. Although we provide evidence for a positive influence of FGFs on BMP gene expression, the physiological implication of FGFs in apoptosis appears to result from their requirement for the expression of genes of the apoptotic cascade. We have identified MSX2 and Snail as candidate genes associated with apoptosis the expression of which requires the combined action of FGFs and BMPs.

A redox site involved in integrin activation

Integrin adhesion receptors contain an on/off switch that regulates ligand binding affinity and cell adhesion. The switch from "off" to "on" is commonly referred to as integrin activation. The objective of this study was to gain insight into the nature of the on/off switch in platelet integrin alpha(IIb)beta(3). Here, we show that a select group of the cysteines, located within the extracellular cysteine-rich domain of the beta subunit, remain unpaired. These unpaired cysteine residues exhibit the properties of a redox site involved in integrin activation. Alterations to the redox site prevent the inter-conversion between resting and active integrin. Altogether, the study establishes integrin as a direct target for redox modulation, revealing an unappreciated link between cell adhesion and redox biology.

The localized assembly of extracellular matrix integrin ligands requires cell-cell contact

The assembly of an organism requires the interaction between different layers of cells, in many cases via an extracellular matrix. In the developing Drosophila larva, muscles attach in an integrin-dependent manner to the epidermis, via a specialized extracellular matrix called tendon matrix. Tiggrin, a tendon matrix integrin ligand, is primarily synthesized by cells distant to the muscle attachment sites, yet it accumulates specifically at these sites. Previous work has shown that the PS integrins are not required for tiggrin localization, suggesting that there is redundancy among tiggrin receptors. We have examined this by testing whether the PS2 integrin can recruit tiggrin to ectopic locations within the Drosophila embryo. We found that neither the wild type nor modified forms of the PS2 integrin, which have higher affinity for tiggrin, can recruit tiggrin to new cellular contexts. Next, we genetically manipulated the fate of the muscles and the epidermal muscle attachment cells, which demonstrated that muscles have the primary role in recruiting tiggrin to the tendon matrix and that cell-cell contact is necessary for this recruitment. Thus we propose that the inherent polarity of the muscle cells leads to a molecular specialization of their ends, and interactions between the ends produces an integrin-independent tiggrin receptor. Thus, interaction between cells generates an extracellular environment capable of nucleating extracellular matrix assembly.

The role of cytokines and adhesion molecules for mobilization of peripheral blood stem cells

CD34(+) hematopoietic stem cells from peripheral blood are commonly used for autologous or allogeneic transplantation following high-dose therapy in malignant diseases. The introduction of hematopoietic growth factors such as G-CSF has greatly facilitated the mobilization of CD34(+) cells. The mechanism of stem cell mobilization is not yet clear. It seems to be a multistep process with a crosstalk between cytokines and adhesion molecules. In this review, the role of hematopoietic growth factors, chemokines, and adhesion molecules for mobilization and homing of CD34(+) cells is summarized. In addition, factors influencing the cytokine-induced mobilization in patients and healthy donors are described. The review closes with an overview of new classes of mobilizing drugs such as monoclonal antibodies, specific peptides, or antisense oligonucleotides targeting adhesion molecules.

Mobilization of CD34+ haematopoietic stem cells is associated with a functional inactivation of the integrin very late antigen 4

The beta1 integrin very late antigen 4 (VLA-4) plays a central role in mobilization and homing of CD34+ cells. In this study, we examined the activation state of VLA-4 on CD34+ cells from bone marrow (BM) and peripheral blood (PB) by flow cytometry using a vascular cell adhesion molecule I-immunoglobulin (VCAM-I/IgG) fusion protein as soluble ligand. In an intraindividual analysis, we found a significantly reduced affinity and avidity of the VLA-4 receptor on CD34+ cells from PB during granulocyte colony-stimulating factor (G-CSF)-enhanced marrow recovery in comparison with steady-state BM. Moreover, the amount of circulating CD34+ cells during marrow recovery was inversely related to the activation state but not to the expression level of VLA-4, suggesting that a modulation of the functional state of VLA-4 is involved in the mobilization of CD34+ cells. Moreover, VLA-4 function on CD34+ cells from BM was associated with the maturation state of CD34+ cells as high-affinity VLA-4 receptors were observed on the vast majority of more primitive CD34+ cells. In addition, we found that Mg2+ ions as well as co-incubation of CD34+ cells with endothelial cells resulted in an activation of the VLA-4 receptor. In conclusion, modulation of the functional state of VLA-4 appears to be of relevance for the mobilization and homing of CD34+ haematopoietic stem cells.

Integrins modulate the Egfr signaling pathway to regulate tendon cell differentiation in the Drosophila embryo

Changes in the extracellular matrix (ECM) govern the differentiation of many cell types during embryogenesis. Integrins are cell matrix receptors that play a major role in cell-ECM adhesion and in transmitting signals from the ECM inside the cell to regulate gene expression. In this paper, it is shown that the PS integrins are required at the muscle attachment sites of the Drosophila embryo to regulate tendon cell differentiation. The analysis of the requirements of the individual alpha subunits, alphaPS1 and alphaPS2, demonstrates that both PS1 and PS2 integrins are involved in this process. In the absence of PS integrin function, the expression of tendon cell-specific genes such as stripe and beta1 tubulin is not maintained. In addition, embryos lacking the PS integrins also exhibit reduced levels of activated MAPK. This reduction is probably due to a downregulation of the Epidermal Growth Factor receptor (Egfr) pathway, since an activated form of the Egfr can rescue the phenotype of embryos mutant for the PS integrins. Furthermore, the levels of the Egfr ligand Vein at the muscle attachment sites are reduced in PS mutant embryos. Altogether, these results lead to a model in which integrin-mediated adhesion plays a role in regulating tendon cell differentiation by modulating the activity of the Egfr pathway at the level of its ligand Vein.

Probing chemical and conformational differences in the resting and active conformers of platelet integrin alpha(IIb)beta(3)

Integrin alpha(IIb)beta(3) is the fibrinogen receptor that mediates platelet adhesion and aggregation. The ligand binding function of alpha(IIb)beta(3) is "activated" on the platelet surface by physiologic stimuli. Two forms of alpha(IIb)beta(3) can be purified from platelet lysates. These forms are facsimiles of the resting (Activation State-1 or AS-1) and the active (Activation State-2 or AS-2) conformations of the integrin found on the platelet surface. Here, the differences between purified AS-1 and AS-2 were examined to gain insight into the mechanism of activation. Four major findings are put forth. 1) The association rate (k(1)) between fibrinogen and the integrin is a key difference between AS-1 and AS-2. 2) Although the divalent ion Mn(2+) enhances the ligand binding function of AS-1, this ion is unable to convert AS-1 to AS-2. Therefore, its effect on integrin is unrelated to activation. 3) Peptide mass fingerprints indicate that the chemical structure of AS-1 and AS-2 are virtually identical, calling into question the idea that post-translational modifications are necessary for activation. 4) The two forms of alpha(IIb)beta(3) have significant conformational differences at three positions. These include the junction of the heavy and light chain of alpha(IIb), the divalent ion binding sites on alpha(IIb), and at a disulfide-bonded knot linking the amino terminus of beta(3) to the cysteine-rich domain. These observations indicate that integrin is activated by a series of specific conformational rearrangements in the ectodomain that increase the rate of ligand association.

CD9 is expressed on human endometrial epithelial cells in association with integrins alpha(6), alpha(3) and beta(1)

Recently we reported that CD9 is involved in the invasion of a trophoblast-like choriocarcinoma cell line, BeWo, probably through the regulation of integrin functions. Integrins have also been reported to be expressed in the human endometrium and it has been suggested that they play important roles in blastocyst implantation. This study used immunohistochemistry to investigate the expression of CD9 in the endometrium during the menstrual cycle. CD9 was found to be intensely expressed on the cell surface of the glandular epithelium throughout the menstrual cycle without any apparent differences in staining intensity. In addition, Western blotting analysis of the affinity-purified proteins confirmed that CD9 was associated with integrins beta(1), alpha(3) and alpha(6) in the human endometrium. Therefore it can be concluded that CD9, in association with integrins alpha(6), alpha(3) and beta(1), is a constitutive molecule of the endometrial glandular epithelium. These results also suggest that CD9 may be an important regulator of these integrins in the human endometrium.

Class- and splice variant-specific association of CD98 with integrin beta cytoplasmic domains

CD98 is a type II transmembrane protein involved in neutral and basic amino acid transport and in cell fusion events. CD98 was implicated in the function of integrin adhesion receptors by its capacity to reverse suppression of integrin activation by isolated integrin beta(1A) domains. Here we report that CD98 associates with integrin beta cytoplasmic domains with a unique integrin class and splice variant specificity. In particular, CD98 interacted with the ubiquitous beta(1A) but not the muscle-specific splice variant, beta(1D), or leukocyte-specific beta(7) cytoplasmic domains. The ability of CD98 to associate with integrin cytoplasmic domains correlated with its capacity to reverse suppression of integrin activation. The association of CD98 with integrin beta(1A) cytoplasmic domains may regulate the function and localization of these membrane proteins.

Beta3 integrin activation improves alphavbeta3-mediated retraction of fibrin matrices

BACKGROUND

Integrins are heterodimeric transmembrane glycoproteins that mediate cell interactions with the extracellular matrix. In vivo, integrin affinity can be modulated by intracellular signaling events. This can be simulated by a point mutation (D723R) in the cytoplasmic tail of the beta3 integrin subunit which results in constitutive activation. The effects of beta3 integrin activation on the function of alphavbeta3, an integrin which is important to the adhesive events of multiple cell types, were addressed using Chinese hamster ovary cells expressing either the wild-type alphavbeta3 integrin or the mutant alphavbeta3(D723R). The interactions of these cell lines with fibrin matrices were compared.

METHODS

Receptor expression levels were confirmed by FACS analyses using a monoclonal anti-alphavbeta3 antibody. Cell attachment to fibrin-coated dishes was determined after 1 h by fixation and crystal violet staining followed by elution of the dye and OD measurement. Fibrin clot retraction was measured by culturing cells in fibrin clots for 24 h. The clots were detached from the dish and the surface area was calculated at individual time points.

RESULTS

CHO alphavbeta3(D723R) cells displayed a greater than twofold increase in attachment to fibrinogen or to fibrin matrices when compared to wild-type transfectants. Further, CHO alphavbeta3(D723R) cell retraction of fibrin matrices was significantly greater at nearly all time points.

CONCLUSION

Activation of the beta3 integrin subunit significantly improves the interaction of alphavbeta3 with fibrin and may play a role in the integrin-mediated signaling events which occur following vascular injury.

Evidence that distinct states of the integrin alpha6beta1 interact with laminin and an ADAM

Integrins can exist in different functional states with low or high binding capacity for particular ligands. We previously provided evidence that the integrin alpha6beta1, on mouse eggs and on alpha6-transfected cells, interacted with the disintegrin domain of the sperm surface protein ADAM 2 (fertilin beta). In the present study we tested the hypothesis that different states of alpha6beta1 interact with fertilin and laminin, an extracellular matrix ligand for alpha6beta1. Using alpha6-transfected cells we found that treatments (e.g., with phorbol myristate acetate or MnCl2) that increased adhesion to laminin inhibited sperm binding. Conversely, treatments that inhibited laminin adhesion increased sperm binding. Next, we compared the ability of fluorescent beads coated with either fertilin beta or with the laminin E8 fragment to bind to eggs. In Ca2+-containing media, fertilin beta beads bound to eggs via an interaction mediated by the disintegrin loop of fertilin beta and by the alpha6 integrin subunit. In Ca2+-containing media, laminin E8 beads did not bind to eggs. Treatment of eggs with phorbol myristate acetate or with the actin disrupting agent, latrunculin A, inhibited fertilin bead binding, but did not induce laminin E8 bead binding. Treatment of eggs with Mn2+ dramatically increased laminin E8 bead binding, and inhibited fertilin bead binding. Our results provide the first evidence that different states of an integrin (alpha6beta1) can interact with an extracellular matrix ligand (laminin) or a membrane-anchored cell surface ligand (ADAM 2).

The death effector domain of PEA-15 is involved in its regulation of integrin activation

Increased integrin ligand binding affinity (activation) is triggered by intracellular signaling events. A Ras-initiated mitogen-activated protein kinase pathway suppresses integrin activation in fibroblasts. We used expression cloning to isolate cDNAs that prevent Ras suppression of integrin activation. Here, we report that PEA-15, a small death effector domain (DED)-containing protein, blocks Ras suppression. PEA-15 does not block the capacity of Ras to activate the ERK mitogen-activated protein kinase pathway. Instead, it inhibits suppression via a pathway blocked by a dominant-negative form of the distinct small GTPase, R-Ras. Heretofore, all known DEDs functioned in the regulation of apoptosis. In contrast, the DED of PEA-15 is essential for its capacity to reverse suppression of integrin activation. Thus, certain DED-containing proteins can regulate integrin activation as opposed to apoptotic protease cascades.

Integrin affinity modulation

Integrin cell-adhesion receptors mediate interactions between cells and the extracellular matrix. Dynamic regulation of integrin adhesive function is termed 'activation' or 'inside-out' signalling. Activation is key to integrin function in processes as diverse as cell migration, the organization of the extracellular matrix and platelet aggregation. Consequently, there has been an intense effort to elucidate the molecular mechanism of integrin activation. This has resulted in the recent identification of novel cytoplasmic partners for integrins and the emerging characterization of the signal-transduction pathways that regulate integrin 'inside-out' signalling. Here, the authors review the recent developments that have provided us with an increased understanding of the basis of integrin activation.