The inner world of cell adhesion: integrin cytoplasmic domains

Polymer brush-induced depletion interactions and clustering of membrane proteins

We investigate the effect of mobile polymer brushes on proteins embedded in biological membranes by employing both Asakura-Oosawa type of theoretical model and coarse-grained molecular dynamics simulations. The brush polymer-induced depletion attraction between proteins changes non-monotonically with the size of brush. The depletion interaction, which is determined by the ratio of the protein size to the grafting distance between brush polymers, increases linearly with the brush size as long as the polymer brush height is shorter than the protein size. When the brush height exceeds the protein size, however, the depletion attraction among proteins is slightly reduced. We also explore the possibility of the brush polymer-induced assembly of a large protein cluster, which can be related to one of many molecular mechanisms underlying recent experimental observations of integrin nanocluster formation and signaling.

Efficient Isolation and Enrichment of Mesenchymal Stem Cells from Human Embryonic Stem Cells by Utilizing the Interaction between Integrin α5β1 and Fibronectin

Human pluripotent stem cells (hPSCs) are a potent source of clinically relevant mesenchymal stem cells (MSCs) that confer functional and structural benefits in cell therapy and tissue regeneration. Obtaining sufficient numbers of MSCs in a short period of time and enhancing the differentiation potential of MSCs can be offered the potential to improve the regenerative activity of MSCs therapy. In addition, the underlying processes in the isolation and derivation of MSCs from hPSCs are still poorly understood and controlled. To overcome these clinical needs, an efficient and simplified technique on the isolation of MSCs from spontaneously differentiated human embryonic stem cells (hESCs) via integrin α5β1 (fibronectin (FN) receptor)-to-FN interactions (hESC-FN-MSCs) is successfully developed. It is demonstrated that hESC-FN-MSCs exhibit a typical MSC surface phenotype, cellular morphology, with the whole transcriptome similar to conventional adult MSCs; but show higher proliferative capacity, more efficient trilineage differentiation, enhanced cytokine secretion, and attenuated cellular senescence. In addition, the therapeutic potential and regenerative capacity of the isolated hESC-FN-MSCs are confirmed by in vitro and in vivo multilineage differentiation. This novel method will be useful in the generation of abundant amounts of clinically relevant MSCs for stem cell therapeutics and regenerative medicine.

Calreticulin and integrin alpha dissociation induces anti-inflammatory programming in animal models of inflammatory bowel disease

Inflammatory bowel disease (IBD), including ulcerative colitis and Crohn’s disease, is a chronic intestinal inflammatory condition initiated by integrins-mediated leukocyte adhesion to the activated colonic microvascular endothelium. Calreticulin (CRT), a calcium-binding chaperone, is known as a partner in the activation of integrin α subunits (ITGAs). The relationship between their interaction and the pathogenesis of IBD is largely unknown. Here we show that a small molecule, orally active ER-464195-01, inhibits the CRT binding to ITGAs, which suppresses the adhesiveness of both T cells and neutrophils. Transcriptome analysis on colon samples from dextran sodium sulfate-induced colitis mice reveals that the increased expression of pro-inflammatory genes is downregulated by ER-464195-01. Its prophylactic and therapeutic administration to IBD mouse models ameliorates the severity of their diseases. We propose that leukocytes infiltration via the binding of CRT to ITGAs is necessary for the onset and development of the colitis and the inhibition of this interaction may be a novel therapeutic strategy for the treatment of IBD.

Inflammatory bowel disease (IBD) is initiated by integrins-mediated leukocyte adhesion to the activated colonic microvascular endothelium. Here, the authors show that inhibition of the calreticulin binding to integrin α subunits ameliorates the severity of IBD in animal models.

Peptides derived from central turn motifs within integrin αIIb and αV cytoplasmic tails inhibit integrin activation

We previously found that peptides derived from the full length of integrin αIIb and αV cytoplasmic tails inhibited their parent integrin activation, respectively. Here we showed that the cell-permeable peptides corresponding to the conserved central turn motif within αIIb and αV cytoplasmic tails, myr-KRNRPPLEED (αIIb peptide) and myr-KRVRPPQEEQ (αV peptide), similarly inhibited both αIIb and αV integrin activation. Pre-treatment with αIIb or αV peptides inhibited Mn(2+)-activated αIIbβ3 binding to soluble fibrinogen as well as the binding of αIIbβ3-expressing Chinese Hamster Ovary cells to immobilized fibrinogen. Our turn peptides also inhibited adhesion of two breast cancer cell lines (MDA-MB-435 and MCF7) to αV ligand vitronectin. These results suggest that αIIb and αV peptides share a same mechanism in regulating integrin function. Using αIIb peptide as a model, we found that replacement of RPP with AAA significantly attenuated the inhibitory activity of αIIb peptide. Furthermore, we found that αIIb peptide specifically bound to β-tubulin in cells. Our work suggests that the central motif of α tails is an anchoring point for cytoskeletons during integrin activation and integrin-mediated cell adhesion, and its function depends on the turn structure at RPP. However, post-treatment of peptides derived from the full-length tail or from the turn motif did not reverse αIIb and αV integrin activation.

Talin-driven inside-out activation mechanism of platelet αIIbβ3 integrin probed by multimicrosecond, all-atom molecular dynamics simulations

Platelet aggregation is the consequence of the binding of extracellular bivalent ligands such as fibrinogen and von Willebrand factor to the high affinity, active state of integrin αIIbβ3. This state is achieved through a so-called "inside-out" mechanism characterized by the membrane-assisted formation of a complex between the F2 and F3 subdomains of intracellular protein talin and the integrin β3 tail. Here, we present the results of multi-microsecond, all-atom molecular dynamics simulations carried on the complete transmembrane (TM) and C-terminal (CT) domains of αIIbβ3 integrin in an explicit lipid-water environment, and in the presence or absence of the talin-1 F2 and F3 subdomains. These large-scale simulations provide unprecedented molecular-level insights into the talin-driven inside-out activation of αIIbβ3 integrin. Specifically, they suggest a preferred conformation of the complete αIIbβ3 TM/CT domains in a lipid-water environment, and testable hypotheses of key intermolecular interactions between αIIbβ3 integrin and the F2/F3 domains of talin-1. Notably, not only do these simulations give support to a stable left-handed reverse turn conformation of the αIIb juxtamembrane motif rather than a helical turn, but they raise the question as to whether TM helix separation is required for talin-driven integrin activation.

Conformational changes in talin on binding to anionic phospholipid membranes facilitate signaling by integrin transmembrane helices

Integrins are heterodimeric (αβ) cell surface receptors that are activated to a high affinity state by the formation of a complex involving the α/β integrin transmembrane helix dimer, the head domain of talin (a cytoplasmic protein that links integrins to actin), and the membrane. The talin head domain contains four sub-domains (F0, F1, F2 and F3) with a long cationic loop inserted in the F1 domain. Here, we model the binding and interactions of the complete talin head domain with a phospholipid bilayer, using multiscale molecular dynamics simulations. The role of the inserted F1 loop, which is missing from the crystal structure of the talin head, PDB:3IVF, is explored. The results show that the talin head domain binds to the membrane predominantly via cationic regions on the F2 and F3 subdomains and the F1 loop. Upon binding, the intact talin head adopts a novel V-shaped conformation which optimizes its interactions with the membrane. Simulations of the complex of talin with the integrin α/β TM helix dimer in a membrane, show how this complex promotes a rearrangement, and eventual dissociation of, the integrin α and β transmembrane helices. A model for the talin-mediated integrin activation is proposed which describes how the mutual interplay of interactions between transmembrane helices, the cytoplasmic talin protein, and the lipid bilayer promotes integrin inside-out activation.

Transmission of signals across the cell membrane is an essential process for all living organisms. Integrins are one example of cell surface receptors (αβ) which, uniquely, form a bidirectional signalling pathway across the membrane. Integrins are crucial for many cellular processes and play key roles in pathological defects such as cardiovascular diseases and cancer. They are activated to a high affinity state by the intracellular protein talin in a process known as ‘inside-out activation’. Despite their importance and the existence of functional and structural data, the mechanism by which talin activates integrin remains elusive. In this study we use a multi-scale computational approach, which combines coarse-grained and atomistic molecular dynamics simulations, to suggest how the formation of the complex between the talin head domain, the cell membrane and the integrin moves the integrin equilibrium towards an active state. Our results show that conformational changes within the talin head domains optimize its interactions with the cell membrane. Upon binding to the integrin, talin facilitates rearrangement of the integrin TM region thus promoting integrin activation. This study also provides a demonstration of the strengths of a computational multi-scale approach in studies of membrane interactions and receptor conformational changes and associated proteins that enable transmembrane signaling.

Talin activates integrins by altering the topology of the β transmembrane domain

Talin binding to the integrin β tail alters the β transmembrane domain’s topology, resulting in integrin activation.

Talin binding to integrin β tails increases ligand binding affinity (activation). Changes in β transmembrane domain (TMD) topology that disrupt α–β TMD interactions are proposed to mediate integrin activation. In this paper, we used membrane-embedded integrin β3 TMDs bearing environmentally sensitive fluorophores at inner or outer membrane water interfaces to monitor talin-induced β3 TMD motion in model membranes. Talin binding to the β3 cytoplasmic domain increased amino acid side chain embedding at the inner and outer borders of the β3 TMD, indicating altered topology of the β3 TMD. Talin’s capacity to effect this change depended on its ability to bind to both the integrin β tail and the membrane. Introduction of a flexible hinge at the midpoint of the β3 TMD decoupled the talin-induced change in intracellular TMD topology from the extracellular side and blocked talin-induced activation of integrin αIIbβ3. Thus, we show that talin binding to the integrin β TMD alters the topology of the TMD, resulting in integrin activation.

Integrin αIIbβ3 inside-out activation: an in situ conformational analysis reveals a new mechanism

Integrins are a family of heterodimeric adhesion receptors that transmit signals bi-directionally across the plasma membranes. The transmembrane domain (TM) of integrin plays a critical role in mediating transition of the receptor from the default inactive to the active state on the cell surfaces. In this study, we successfully applied the substituted cysteine scanning accessibility method to determine the intracellular border of the integrin α(IIb)β(3) TM in the inactive and active states in living cells. We examined the aqueous accessibility of 75 substituted cysteines comprising the C terminus of both α(IIb) and β(3) TMs, the intracellular membrane-proximal regions, and the whole cytoplasmic tails, to the labeling of a membrane-permeable, cysteine-specific chemical biotin maleimide (BM). The active state of integrin α(IIb)β(3) heterodimer was generated by co-expression of activating partners with the cysteine-substituted constructs. Our data revealed that, in the inactive state, the intracellular lipid/aqueous border of α(IIb) TM was at Lys(994) and β(3) TM was at Phe(727) respectively; in the active state, the border of α(IIb) TM shifted to Pro(998), whereas the border of β(3) TM remained unchanged, suggesting that complex conformational changes occurred in the TMs upon α(IIb)β(3) inside-out activation. On the basis of the results, we propose a new inside-out activation mechanism for integrin α(IIb)β(3) and by inference, all of the integrins in their native cellular environment.

Talin-1 and kindlin-3 regulate alpha4beta1 integrin-mediated adhesion stabilization, but not G protein-coupled receptor-induced affinity upregulation

Chemokine/chemoattractant G protein-coupled receptors trigger an inside-out signaling network that rapidly activates integrins, a key step in inflammatory leukocyte recruitment. Integrins mediate leukocyte arrest and adhesion to endothelium through multivalent binding, and they transmit outside-in signals to stabilize adhesion and coordinate cell spreading and migration. In the present study, we used RNA interference in the U937 monocytic cell line to investigate the role of talin-1, kindlin-3, and α-actinin-1 in the fMLF- and SDF-1α-induced upregulation of α(4)β(1) integrin affinity and consequent adhesive events. Affinity upregulation of α(4)β(1) integrin was not impaired by small interfering RNA knockdown of talin-1, kindlin-3, or α-actinin-1. Only kindlin-3 knockdown increased flow-induced detachment from VCAM-1-coated surfaces in response to fluid flow, whereas knockdown of either talin-1 or kindlin-3 increased detachment from ICAM-1-coated surfaces. Biochemical analyses revealed that α(4)β(1) expression was highly enriched in U937 cell microridges and murine lymphocyte microvilli. Kindlin-3 was present throughout the cell, whereas talin-1 was largely excluded from microridges/microvilli. The subcellular colocalization of α(4)β(1) and kindlin-3 in microridges may explain why kindlin-3 rapidly associates with α(4)β(1) after G protein-coupled receptor signaling and contributes to adhesion strengthening. Talin-1 contributed to α(4)β(1)-dependent chemotaxis, suggesting that it participates in a later stage of the leukocyte adhesion cascade when the leukocyte cytoskeleton undergoes dramatic rearrangement.

Multiscale simulations suggest a mechanism for integrin inside-out activation

Integrins are large cell-surface adhesion receptors that can be activated to a high affinity state by the formation of an intracellular complex between the integrin β-subunit tail, the membrane, and talin. The F2 and F3 subdomains of the talin head play a key role in formation of this complex. Here, activation of the integrin αIIb/β3 dimer by the talin head domain was probed using multiscale molecular dynamics simulations. A number of novel insights emerge from these studies, including (i) the importance of the integrin αIIb subunit F992 and F993 residues in stabilizing the "off" state of the αIIb/β3 dimer, (ii) a crucial role for negatively charged groups in the F2-F3/membrane interaction, (iii) binding of the talin F2-F3 domain to negatively charged lipid headgroups in the membrane induces a reorientation of the β transmembrane (TM) domain, (iv) an increase in the tilt angle of the β TM domain relative to the bilayer normal helps to destabilize the α/β TM interaction and promote a scissor-like movement of the integrin TM helices. These results, combined with various published experimental observations, suggest a model for the mechanism of inside-out activation of integrins by talin.

Regulation of integrin activation

Regulation of cell-cell and cell-matrix interaction is essential for the normal physiology of metazoans and is important in many diseases. Integrin adhesion receptors can rapidly increase their affinity (integrin activation) in response to intracellular signaling events in a process termed inside-out signaling. The transmembrane domains of integrins and their interactions with the membrane are important in inside-out signaling. Moreover, integrin activation is tightly regulated by a complex network of signaling pathways. Here, we review recent progress in understanding how the membrane environment can, in cooperation with integrin-binding proteins, regulate integrin activation.

Participation of transmembrane domain 1 of presenilin 1 in the catalytic pore structure of the γ-secretase

γ-Secretase is an intramembrane-cleaving protease that is responsible for the generation of amyloid-β peptides linked to the pathogenesis of Alzheimer's disease. Using a substituted cysteine accessibility method, we have previously shown that the hydrophilic "catalytic pore" structure of γ-secretase is formed by the transmembrane domains (TMDs) 6, 7, and 9 of presenilin 1 (PS1), the catalytic subunit of γ-secretase, within the membrane. Here, we analyzed the structure in and around the first hydrophobic region, the putative TMD1, of PS1, of which the precise function as well as three-dimensional location within γ-secretase remained unknown. We found that TMD1 is located in proximity to the catalytic GxGD and PAL motifs within the C-terminal fragment of PS1, facing directly the catalytic pore. Competition experiments using known γ-secretase inhibitors suggested that the N-terminal region of TMD1 functions as a subsite during proteolytic action of the γ-secretase. Intriguingly, binding of inhibitors affected water accessibility of residues at the membrane border of TMD1, suggesting the possibility of a dynamic motion of TMD1 during the catalytic process. Our results provide mechanistic insights into the functional role of TMD1 of PS1 in the intramembrane-cleaving activity of the γ-secretase.

The final steps of integrin activation: the end game

Cell-directed changes in the ligand-binding affinity ('activation') of integrins regulate cell adhesion and migration, extracellular matrix assembly and mechanotransduction, thereby contributing to embryonic development and diseases such as atherothrombosis and cancer. Integrin activation comprises triggering events, intermediate signalling events and, finally, the interaction of integrins with cytoplasmic regulators, which changes an integrin's affinity for its ligands. The first two events involve diverse interacting signalling pathways, whereas the final steps are immediately proximal to integrins, thus enabling integrin-focused therapeutic strategies. Recent progress provides insight into the structure of integrin transmembrane domains, and reveals how the final steps of integrin activation are mediated by integrin-binding proteins such as talins and kindlins.

Integrins mediate their unconventional, mechanical-stress-induced secretion via RhoA and PINCH in Drosophila

During the epithelium remodelling such as the flattening of the Drosophila follicular epithelium, the alpha-integrin subunits are unconventionally secreted through a dGRASP-dependent route that is built de novo. The biogenetic process starts with the upregulation of a small subset of targeted mRNAs, including dgrasp. Here, we show that dgrasp mRNA upregulation is triggered by the tension of the underlying oocyte and by applied external forces at the basal side of the follicular epithelium. We show that integrins are also involved in dgrasp mRNA upregulation and the epithelium remodelling. Tension leads to the recruitment of RhoA to the plasma membrane, where it participates in its remodelling. The LIM protein PINCH can cycle to the nucleus and is involved in dgrasp mRNA upregulation. We propose that integrins are involved in triggering the biogenesis of their own unconventional secretion route that they use to strengthen adhesion and ensure epithelial integrity at the next stages of development, perhaps by acting as mechanosensors of the underlying tension through RhoA and PINCH.

Interactions of platelet integrin alphaIIb and beta3 transmembrane domains in mammalian cell membranes and their role in integrin activation

Clustering and occupancy of platelet integrin alpha(IIb)beta(3) (GPIIb-IIIa) generate biologically important signals: conversely, intracellular signals increase the integrins' affinity, leading to integrin activation; both forms of integrin signaling play important roles in hemostasis and thrombosis. Indirect evidence implicates interactions between integrin alpha and beta transmembrane domains (TMDs) and cytoplasmic domains in integrin signaling; however, efforts to directly identify these associations have met with varying and controversial results. In this study, we develop mini-integrin affinity capture and use it in combination with nuclear magnetic resonance spectroscopy to show preferential heterodimeric association of integrin alpha(IIb)beta(3) TMD tails via specific TMD interactions in mammalian cell membranes in lipid bicelles. Furthermore, charge reversal mutations at alpha(IIb)(R995)beta(3)(D723) confirm a proposed salt bridge and show that it stabilizes the TMD-tail association; talin binding to the beta(3) tail, which activates the integrin, disrupts this association. These studies establish the preferential heterodimeric interactions of integrin alpha(IIb)beta(3) TMD tails in mammalian cell membranes and document their role in integrin signaling.

Integrin activation

Agonist stimulation of integrin receptors, composed of transmembrane alpha and beta subunits, leads cells to regulate integrin affinity ('activation'), a process that controls cell adhesion and migration, and extracellular matrix assembly. A final step in integrin activation is the binding of talin to integrin beta cytoplasmic domains. We used forward, reverse and synthetic genetics to engineer and order integrin activation pathways of a prototypic integrin, platelet alphaIIbbeta3. PMA activated alphaIIbbeta3 only after expression of both PKCalpha (protein kinase Calpha) and talin at levels approximating those in platelets. Inhibition of Rap1 GTPase reduced alphaIIbbeta3 activation, whereas expression of constitutively active Rap1A(G12V) bypassed the requirement for PKCalpha. Overexpression of a Rap effector, RIAM (Rap1-GTP-interacting adaptor molecule), activated alphaIIbbeta3 and bypassed the requirement for PKCalpha and Rap1. In addition, shRNA (short hairpin RNA)-mediated knockdown of RIAM blocked talin interaction with and activation of integrin alphaIIbbeta3. Rap1 activation caused the formation of an 'activation complex' containing talin and RIAM that redistributed to the plasma membrane and activated alphaIIbbeta3. The central finding was that this Rap1-induced formation of an 'integrin activation complex' leads to the unmasking of the integrin-binding site on talin, resulting in integrin activation.

Transmembrane and cytoplasmic domains in integrin activation and protein-protein interactions (review)

Integrins are heterodimeric membrane-spanning adhesion receptors that are essential for a wide range of biological functions. Control of integrin conformational states is required for bidirectional signalling across the membrane. Key components of this control mechanism are the transmembrane and cytoplasmic domains of the alpha and beta subunits. These domains are believed to interact, holding the integrin in the inactive state, while inside-out integrin activation is accompanied by domain separation. Although there are strong indications for domain interactions, the majority of evidence is insufficient to precisely define the interaction interface. The current best model of the complex, derived from computational calculations with experimental restraints, suggests that integrin activation by the cytoplasmic protein talin is accomplished by steric disruption of the alpha/beta interface. Better atomic-level resolution structures of the alpha/beta transmembrane/cytoplasmic domain complex are still required for the resting state integrin to corroborate this. Integrin activation is also controlled by competitive interactions involving the cytoplasmic domains, particularly the beta-tails. The concept of the beta integrin tail as a focal adhesion interaction 'hub' for interactions and regulation is discussed. Current efforts to define the structure and affinity of the various complexes formed by integrin tails, and how these interactions are controlled, e.g. by phosphorylation and localization, are described.

The binding of Mss4 to alpha-integrin subunits regulates matrix metalloproteinase activation and fibronectin remodeling

In four independent yeast two-hybrid screens with the integrin alpha-subunits alpha3A, alpha6A, alpha7A, and alpha7B, we identified the Mss4 protein, a nucleotide exchange factor for exocytic Rab GTPases, as a novel integrin interacting protein. We have previously shown that it binds to the conserved KXGFFKR region of integrin alpha-subunits located directly beneath the cell membrane. Here we show that the binding site for integrins on Mss4 is overlapping with those for Rab GTPases. Functional analysis of the Mss4/integrin interaction revealed its importance for activation of matrix metalloproteinases (MMPs) and remodeling of secreted extracellular matrix (ECM) proteins. The exocytosis of all the proteins analyzed, however, was unaffected. Furthermore, our data suggest that Mss4 drives the coordinated action of the MT1-MMP/integrin protein complex, thus regulating the presence and activation of MT1-MMP at newly formed filopodia and lamellipodia. This in turn facilitates the conversion of pro-MMPs to MMPs, resulting in cleavage and remodeling of ECM proteins. C2C12 myoblasts with stably down-regulated Mss4 showed a disturbed fibronectin remodeling during differentiation, resulting in malfunctioned myotube formation.

New insights into molecular mechanisms of phagocytosis in Entamoeba histolytica by proteomic analysis

The protozoan parasite Entamoeba histolytica ingests microorganisms and mammalian cells. Phagocytosis is essential for cell growth and is implicated in pathogenesis of E. histolytica. Phagocytosis consists of a number of steps including recognition of and binding to ligands on the target cells via a galactose/N-acetylgalactosamine-specific lectin, activation of a signaling pathway leading to cytoskeletal reorganization, and vesicle trafficking, all of which play distinct but coordinated roles in phagocytosis. Recent studies of proteomic analysis of purified phagosomes or affinity-purified Gal/GalNAc-binding proteins using reversed phase capillary liquid chromatography and ion trap tandem mass spectrometry enabled high throughput identification of proteins involved in phagosome biogenesis. These studies provided a list of proteins involved in the pathway and also shed light on the dynamic process of phagosome maturation. These approaches should provide significant insights into molecular mechanisms of phagosome biogenesis and help to elucidate the pathogenesis of this important parasite.

Displacement of the {beta} cytoplasmic domain recovers focal adhesion formation, cytoskeletal organization and motility in swapped integrin chimeras

Integrin-mediated ;outside-in' signaling requires the transmission of a conformational change from the extracellular domains to the cytoplasmic domains. Although one component of this conformational change is the separation of the alpha and beta cytoplasmic domains, it is not clear how this separation could result in the initiation of downstream signals necessary for focal adhesion (FA) formation. To address this question, we used a swapped integrin heterodimer, in which the extracellular domains of the alpha and beta chains were attached to their opposing transmembrane and cytoplasmic domains. This receptor was able to bind ligand normally, but could not promote FA formation. We then displaced the beta cytoplasmic domain with either a duplication of its membrane-proximal region or an unrelated alpha-helical spacer. This displacement partially restored FA formation in these swapped receptors and rescued other aspects of integrin-mediated signaling, including cytoskeletal organization, motility and several tyrosine-phosphorylation-dependent signals. We suggest that separation of the cytoplasmic domains leads to alteration of the secondary structure of the distal beta tail, which initiates downstream signals leading to cytoskeletal reorganization.

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.

Determination of N- and C-terminal borders of the transmembrane domain of integrin subunits

Previous studies on the membrane-cytoplasm interphase of human integrin subunits have shown that a conserved lysine in subunits alpha(2), alpha(5), beta(1), and beta(2) is embedded in the plasma membrane in the absence of interacting proteins (Armulik, A., Nilsson, I., von Heijne, G., and Johansson, S. (1999) in J. Biol. Chem. 274, 37030-37034). Using a glycosylation mapping technique, we here show that alpha(10) and beta(8), two subunits that deviate significantly from the integrin consensus sequences in the membrane-proximal region, were found to have the conserved lysine at a similar position in the lipid bilayer. Thus, this organization at the C-terminal end of the transmembrane (TM) domain seems likely to be general for all 24 integrin subunits. Furthermore, we have determined the N-terminal border of the TM domains of the alpha(2), alpha(5), alpha(10), beta(1), and beta(8) subunits. The TM domain of subunit beta(8) is found to be 22 amino acids long, with a second basic residue (Arg(684)) positioned just inside the membrane at the exoplasmic side, whereas the lipidembedded domains of the other subunits are longer, varying from 25 (alpha(2)) to 29 amino acids (alpha(10)). These numbers implicate that the TM region of the analyzed integrins (except beta(8)) would be tilted or bent in the membrane. Integrin signaling by transmembrane conformational change may involve alteration of the position of the segment adjacent to the conserved lysine. To test the proposed "piston" model for signaling, we forced this region at the C-terminal end of the alpha(5) and beta(1) TM domains out of the membrane into the cytosol by replacing Lys-Leu with Lys-Lys. The mutation was found to not alter the position of the N-terminal end of the TM domain in the membrane, indicating that the TM domain is not moving as a piston. Instead the shift results in a shorter and therefore less tilted or bent TM alpha-helix.

An unraveling tale of how integrins are activated from within

Integrin cytoplasmic tail domains are short, but are essential for normal receptor function because of their key role in relaying bidirectional signals across the plasma membrane. Although it is well established that the cytoplasmic tails both initiate signalling pathways inside the cell and control the transition of integrins from a resting to a ligand-binding competent state, until recently the structural basis of these changes has been unclear. In the past year, however, a series of structural studies has revealed certain features of cytoplasmic domain function, and in this review we focus on how these advances have enlightened our understanding of integrin tail structure and function.

Integrins: bidirectional, allosteric signaling machines

In their roles as major adhesion receptors, integrins signal across the plasma membrane in both directions. Recent structural and cell biological data suggest models for how integrins transmit signals between their extracellular ligand binding adhesion sites and their cytoplasmic domains, which link to the cytoskeleton and to signal transduction pathways. Long-range conformational changes couple these functions via allosteric equilibria.

Integrin activation takes shape

Integrins are cell surface adhesion receptors that are essential for the development and function of multicellular animals. Here we summarize recent findings on the regulation of integrin affinity for ligand (activation), one mechanism by which cells modulate integrin function. The focus is on the structural basis of integrin activation, the role of the cytoplasmic domain in integrin affinity regulation, and potential mechanisms by which activation signals are propagated from integrin cytoplasmic domains to the extracellular ligand-binding domain.

Decreased expression of alpha3 and beta1 integrin subunits is responsible for differentiation-associated changes in cells behavior in terminally differentiated human oral keratinocytes

Primary normal human oral keratinocytes (NHOKs) terminally differentiate in serial subculture. To investigate whether this subculture-induced differentiation of NHOKs affects integrin expression and cell-matrix interaction, we studied the expression levels of integrin subunits and cellular response to the extracellular matrix (ECM) proteins in NHOKs at different population doublings. The phosphorylation statuses of focal adhesion kinase (FAK), extracellular signal regulated kinase (ERK), p38, and c-Jun amino-terminal kinase (JNK) were also determined in NHOK cells cultured on ECM proteins, to evaluate the functions of integrins with respect to cellular responses to ECM proteins. The expression levels of alpha3 and beta1 integrin subunits progressively decreased in NHOKs undergoing terminal differentiation. The ability of NHOKs to spread upon laminin and type I collagen significantly decreased in terminally differentiated oral keratinocytes. Keratinocyte migration was significantly increased on type I collagen for terminally differentiated NHOKs. Similar results were seen following preincubation of rapidly proliferating NHOKs with function-blocking antibodies to alpha3 or beta1 integrin subunit. In contrast, fibronectin had no effect on cellular responses in NHOKs, which were almost negligible in the expression levels of alpha5 integrin subunits. The extent of FAK phosphorylation in terminally differentiated NHOKs was notably lower than that of rapidly proliferating cells, but was enhanced in terminally differentiated cells that were cultured on type I collagen. Our results indicate that decreased expression of alpha3 and beta1 integrin subunits is responsible for differentiation-associated changes in cells behavior in terminally differentiated oral keratinocytes. Our data also show that the abrogation of the alpha5beta1 integrin function caused by omitting alpha5 subunit is linked to the loss of a cell-fibronectin interaction in human oral keratinocytes.

Solution structures of the cytoplasmic tail complex from platelet integrin alpha IIb- and beta 3-subunits

Integrin adhesion receptors constitute a cell-signaling system whereby interactions in the small cytoplasmic domains of the heterodimeric alpha- and beta-subunits provoke major functional alterations in the large extracellular domains. With two-dimensional NMR spectroscopy, we examined two synthetic peptides [alphaIIb((987)MWKVGFFKRNR) and beta3((716)KLLITIHDRKEFAKFEEERARAKWD)] encompassing the membrane-proximal regions of the cytoplasmic domain motifs from the platelet integrin complex alphaIotaIotabbeta3. These membrane-proximal regions contain two conserved motifs, represented by (989)KVGFFKR in the alphaIIb-subunit, and (716)KLLITIHDR in the beta3-subunit. The dimer interaction consists of two adjacent helices with residues V990 and F993 of the alphaIotaIotab-subunit heavily implicated in the dimer interfacial region, as is I719 of beta3. These residues are situated within the conserved motifs of their respective proteins. Further structural analysis of this unique peptide heterodimer suggests that two distinct conformers are present. The major structural difference between the two conformers is a bend in the beta3-peptide between D723 and A728, whereas the helical character in the other regions remains intact. Earlier mutational analysis has shown that a salt bridge between the side chains of alphaIotaIotab(R955) and beta3(D723) is formed. When this ion pair was modeled into both conformers, increased nuclear Overhauser effect violations suggested that the more bent structure was less able to accommodate this interaction. These results provide a molecular level rationalization for previously reported biochemical studies, as well as a basis for an atomic level understanding of the intermolecular interactions that regulate integrin activity.

A membrane-distal segment of the integrin alpha IIb cytoplasmic domain regulates integrin activation

Previous evidence suggests that interactions between integrin cytoplasmic domains regulate integrin activation. We have constructed and validated recombinant structural mimics of the heterodimeric alpha(IIb)beta(3) cytoplasmic domain. The mimics elicited polyclonal antibodies that recognize a combinatorial epitope(s) formed in mixtures of the alpha(IIb) and beta(3) cytoplasmic domains but not present in either isolated tail. This epitope(s) is present within intact alpha(IIb)beta(3), indicating that interaction between the tails can occur in the native integrin. Furthermore, the combinatorial epitope(s) is also formed by introducing the activation-blocking beta(3)(Y747A) mutation into the beta(3) tail. A membrane-distal heptapeptide sequence in the alpha(IIb) tail ((997)RPPLEED) is responsible for this effect on beta(3). Membrane-permeant palmitoylated peptides, containing this alpha(IIb) sequence, specifically blocked alpha(IIb)beta(3) activation in platelets. Thus, this region of the alpha(IIb) tail causes the beta(3) tail to resemble that of beta(3)(Y747A) and suppresses activation of the integrin.

Regulation of MCP-3 and BRCA2 mRNA expression levels by beta(1) integrins

The integrin cytoplasmic domain has been shown to modulate several cellular functions, including cell proliferation, adhesion, migration, and intracellular signaling. The beta(1) integrin subunits beta(1C) and beta(1A), which contain variant cytoplasmic domains, differentially affect cancer and normal cell functions. To identify target genes selectively regulated by these beta(1) cytoplasmic variants, stable cell transfectants expressing either beta(1A) or beta(1C) under the control of a doxycycline-inducible promoter were obtained using murine beta(1)-deficient GD25 cells. Screening of 1176 murine cDNAs using first-strand cDNA of mRNA isolated from either beta(1C)- or beta(1A)-expressing cells showed a striking differential expression of few genes. The differential expression of two genes, MCP-3 and BRCA2 (monocyte chemoattractant protein-3 and breast cancer susceptibility gene 2, respectively), whose products are involved, respectively, in chemotaxis and embryonic proliferation, was confirmed by Northern blot analysis. Increased MCP-3 and decreased BRCA2 mRNA levels in cells expressing beta(1C) compared to those in cells expressing beta(1A) were observed. Since beta(1C) and beta(1A) stable cell transfectants showed comparable adhesion to fibronectin, upregulation of MCP-3 and downregulation of BRCA2 mRNA levels did not appear to be due to a differential ability of the beta(1C) cells to adhere to the beta(1) ligand fibronectin. Overall, our data show that beta(1) integrin cytoplasmic domain variants control expression of downstream target genes in a differential manner without affecting cell adhesion.

A novel syndrome of variant leukocyte adhesion deficiency involving defects in adhesion mediated by beta1 and beta2 integrins

Leukocyte adhesion deficiency type I (LAD-1) is a disorder associated with severe and recurrent bacterial infections, impaired extravascular targeting and accumulation of myeloid leukocytes, altered wound healing, and significant morbidity that is caused by absent or greatly diminished surface expression of integrins of the beta2 class. We report clinical features and analysis of functions of cells from a patient with a myelodysplastic syndrome and infectious complications similar to those in the severe form of LAD-1, but whose circulating neutrophils displayed normal levels of beta2 integrins. Analysis of adhesion of these cells to immobilized ligands and to endothelial cells and assays of cell-cell aggregation and chemotaxis demonstrated a profound defect in adhesion mediated by beta2 integrins indicative of a variant form of LAD-1. A novel cell line established from Epstein-Barr virus-transformed lymphoblasts from the subject demonstrated deficient beta2 integrin-dependent adhesive function similar to that of the primary leukocytes. In addition, these cells had markedly impaired beta1 integrin-dependent adhesion. Sequence analysis and electrophoretic mobility of beta1 and beta2 proteins from the cell line demonstrated that the defects were not a result of structural abnormalities in the integrin subunit chains themselves and suggest that the adhesive phenotype of these cells is due to one or more abnormalities of inside-out signaling mechanisms that regulate the activity of integrins of these classes. These features define a unique LAD-1 variant syndrome that may reveal important insights that are generally relevant to inside-out signaling of integrins, a molecular process that is as yet incompletely understood.

The cytoplasmic domain of the alpha1 integrin subunit influences stress fiber formation via the conserved GFFKR motif

Integrins are heterodimeric transmembrane proteins that mediate substrate adhesion and migration but also the bidirectional transfer of information across the plasma membrane via their cytoplasmic domains. We addressed the question of whether the very short cytoplasmic tail of the alpha1 integrin subunit of alpha1beta1 integrin is required for alpha1beta1-specific adhesion, spreading, and migration. For this purpose we transfected the alpha1 integrin subunit and two cytoplasmically truncated alpha1 subunits into Chinese hamster ovary (CHO) cells. Elimination of the entire cytoplasmic domain of the alpha1 subunit does not affect adhesion but leads to inhibition of spreading and stress fiber formation. The defect in spreading could not be rescued by lysophosphatidic acid, which has been reported to stimulate actin stress fiber formation via Rho. Additionally, deletion of the entire cytoplasmic domain of the alpha1 subunit abolishes migration toward alpha1beta1-specific substrates. Migration and stress fiber formation are similar in CHO-alpha1 cells and CHO cells carrying an alpha1 subunit still containing the conserved GFFKR motif. So, the GFFKR motif of the alpha1 subunit is essential and sufficient for these processes.

Structures of the platelet calcium- and integrin-binding protein and the alphaIIb-integrin cytoplasmic domain suggest a mechanism for calcium-regulated recognition; homology modelling and NMR studies

Calcium- and integrin-binding protein (CIB) binds to the 20-residue alphaIIb cytoplasmic domain of platelet alphaIIbbeta3 integrin. Amino acid sequence similarities with calmodulin (CaM) and calcineurin B (CnB) allowed the construction of homology-based models of calcium-saturated CIB as well as apo-CIB. In addition, the solution structure of the alphaIIb cytoplasmic domain in 45% aqueous trifluoroethanol was solved by conventional two-dimensional NMR methods. The models indicate that the N-terminal domain of CIB possesses a number of positively charged residues in its binding site that could interact with the acidic carboxy-terminal LEEDDEEGE sequence of alphaIIb. The C-terminal domain of CIB seems well-suited to bind the sequence WKVGFFKR, which forms a well-structured alpha helix; this is analogous to calmodulin and calcineurin B, which also bind alpha helices. Similarities between the C-terminal domains of CIB and calmodulin suggest that binding of CIB to the cytoplasmic domain of alphaIIb may be affected by fluctuations in the intracellular calcium concentration.

Bi-directional signal transduction by integrin receptors

The integrin family of cell surface glycoproteins functions primarily as receptors for extracellular matrix ligands. There are now many well characterized integrin-ligand interactions which are known to influence many aspects of cell behaviour including cell morphology, cell adhesion, cell migration as well as cellular proliferation and differentiation. However, in fulfilling these functions, integrins are not simple adhesion receptors that physically mediate connections across the plasma membrane. Rather, integrin function itself is highly regulated, largely through the formation of specific associations with both structural and regulatory components within cells. It is these intracellular interactions which allow integrin function to effect many biochemical signalling pathways and therefore to impinge upon complex cellular activities. Recently, much research has focused on elucidating the molecular mechanisms which control integrin function and the molecular processes which transduce integrin-mediated signalling events. In this review, we discuss progress in the field of integrin signal transduction including, where applicable, potential therapeutic applications arising from the research.

beta1 integrins regulate keratinocyte adhesion and differentiation by distinct mechanisms

In keratinocytes, the beta1 integrins mediate adhesion to the extracellular matrix and also regulate the initiation of terminal differentiation. To explore the relationship between these functions, we stably infected primary human epidermal keratinocytes and an undifferentiated squamous cell carcinoma line, SCC4, with retroviruses encoding wild-type and mutant chick beta1 integrin subunits. We examined the ability of adhesion-blocking chick beta1-specific antibodies to inhibit suspension-induced terminal differentiation of primary human keratinocytes and the ability of the chick beta1 subunit to promote spontaneous differentiation of SCC4. A D154A point mutant clustered in focal adhesions but was inactive in the differentiation assays, showing that differentiation regulation required a functional ligand-binding domain. The signal transduced by beta1 integrins in normal keratinocytes was "do not differentiate" (transduced by ligand-occupied receptors) as opposed to "do differentiate" (transduced by unoccupied receptors), and the signal depended on the absolute number, rather than on the proportion, of occupied receptors. Single and double point mutations in cyto-2 and -3, the NPXY motifs, prevented focal adhesion targeting without inhibiting differentiation control. However, deletions in the proximal part of the cytoplasmic domain, affecting cyto-1, abolished the differentiation-regulatory ability of the beta1 subunit. We conclude that distinct signaling pathways are involved in beta1 integrin-mediated adhesion and differentiation control in keratinocytes.

Determination of the border between the transmembrane and cytoplasmic domains of human integrin subunits

In this study we have determined the position of the C-terminal end of the transmembrane domains of human integrin subunits (alpha2, alpha5, beta1, beta2) in microsomal membranes using the glycosylation mapping technique. In contrast to the common view, the transmembrane helices were found to extend roughly to Phe(1129) in alpha2, to Phe(1026) in alpha5, to Ile(757) in beta1, and to His(728) in beta2. The alpha-carbon of the conserved lysine present near the C-terminal end of the transmembrane helix (Lys(1125) in alpha2, Lys(1022) in alpha5, Lys(752) in beta1, and Lys(724) in beta2) is buried in the plasma membrane, and the charged amino group most likely reaches into the polar head-group region of the lipid bilayer. A possible role for the conserved lysine in integrin function is discussed.

Binding of paxillin to alpha4 integrins modifies integrin-dependent biological responses

The alpha4 integrins are indispensable for embryogenesis, haematopoiesis and immune responses, possibly because alpha4 regulates cellular functions differently from other integrins through its cytoplasmic tail. We used novel mimics of the alpha4 tail to identify molecules that could account for alpha4-specific signalling. Here we report that the alpha4 tail, but not several other alpha-subunit tails, binds tightly to the signalling adaptor paxillin. Paxillin physically associated with alpha4 integrins in Jurkat T cells at high stoichiometry, and joining the alpha4 tail to alphaIIb resulted in a complex of integrin alphaIIbbeta3 with paxillin. This association markedly enhanced the rates of alphaIIbbeta3-dependent phosphorylation of focal adhesion kinase and cell migration. It also reduced cell spreading, focal adhesion and stress fibre formation. A point mutation within the alpha4 tail that disrupts paxillin binding reversed all of these effects. Furthermore, alpha4beta1-dependent adhesion to VCAM-1 led to spreading of mouse embryonic fibroblasts derived from paxillin-null but not from wild-type mice. Thus, the tight association of paxillin with the alpha4 tail leads to distinct biochemical and biological responses to integrin-mediated cell adhesion.

Calpain cleavage of integrin beta cytoplasmic domains

We showed previously that the calcium-dependent protease, calpain, cleaves the cytoplasmic domain of the integrin beta3 subunit. To investigate whether susceptibility to calpain is a common feature of all integrin beta subunits, and to map calpain cleavage sites in different integrin beta tails, we treated recombinant cytoplasmic domains of integrin beta1A, beta1D, beta2, beta3 and beta7 subunits with purified calpain in vitro. We found that the cytoplasmic domains of all these integrin chains were cleaved by calpain. HPLC followed by mass spectrometry was used to identify calpain cleavage sites. These sites were clustered in the C-terminal half of the integrin beta cytoplasmic domains in regions flanking the two NXXY motifs, suggesting the possibility that the structural framework provided by these motifs is recognized by calpain. We used the knowledge of these cleavage sites to develop cleavage site-specific antibodies and to demonstrate cleavage of the beta1A cytoplasmic domain in intact platelets stimulated with calcium ionophore or thrombin. Thus susceptibility to calpain cleavage is common to integrin beta subunits, can be induced in intact cells, and appears to favor regions surrounding two conserved NXXY motifs.

Integrin cytoplasmic tyrosine motif is required for outside-in alphaIIbbeta3 signalling and platelet function

Integrins not only bind adhesive ligands, they also act as signalling receptors. Both functions allow the integrin alphaIIbbeta3 to mediate platelet aggregation. Platelet agonists activate alphaIIbbeta3 (inside-out signalling) to allow the binding of soluble fibrinogen. Subsequent platelet aggregation leads to outside-in alphaIIbbeta3 signalling, which results in calcium mobilization, tyrosine phosphorylation of numerous proteins including beta3 itself, increased cytoskeletal reorganisation and further activation of alphaIIbbeta3. Thus, outside-in signals enhance aggregation, although the mechanisms and functional consequences of specific signalling events remain unclear. Here we describe a mouse that expresses an alphaIIbbeta3 in which the tyrosines in the integrin cytoplasmic tyrosine motif have been mutated to phenylalanines. These mice are selectively impaired in outside-in alphaIIbbeta3 signalling, with defective aggregation and clot-retraction responses in vitro, and an in vivo bleeding defect which is characterized by a pronounced tendency to rebleed. These data provide evidence for an important role of outside-in signalling in platelet physiology. Furthermore, they identify the integrin cytoplasmic tyrosine motif as a key mediator of beta-integrin signals and a potential target for new therapeutic agents.

Ultrastructural localization of full-length trkB immunoreactivity in rat hippocampus suggests multiple roles in modulating activity-dependent synaptic plasticity

Neurotrophins acting at the trkB receptor have been shown to be important modulators of activity-dependent plasticity in the hippocampus, but the mechanisms underlying these effects are not yet well understood. To identify the cellular and subcellular targets of trkB ligands in the adult rat hippocampal formation, full-length trkB receptor immunoreactivity (trkB-IR) was localized using electron microscopy. trkB-IR was present in the glutamatergic pyramidal and granule cells. Labeling in these neurons appeared as discrete clusters and was primarily in axons, excitatory-type axon terminals, and dendritic spines and to a lesser extent in somata and dendritic shafts. trkB-IR was commonly found on the plasma membrane of dendritic spines, whereas in other subcellular regions trkB-IR was often intracellular. Labeling was strikingly dense within axon initial segments, suggesting extensive receptor trafficking. trkB-IR was not confined to pyramidal and granule cells. Dense trkB-IR was found in occasional interneuron axon initial segments, some axon terminals forming inhibitory-type synapses onto somata and dendritic shafts, and excitatory-type terminals likely to originate extrahippocampally. This suggests that trkB is contained in some GABAergic interneurons, neuromodulatory (e.g., cholinergic, dopaminergic, and noradrenergic) afferents, and/or glutamatergic afferents. These data indicate that full-length trkB receptor activation may modulate glutamatergic pathways of the trisynaptic circuit both presynaptically at axon terminals and initial segments and postsynaptically at dendritic spines and shafts. Signaling via catalytic trkB may also presynaptically affect inhibitory and modulatory neurons. A pan-trkB antibody labeled the same neuronal populations as the full-length-specific trkB antiserum, but the labels differed in density at various subcellular sites. These findings provide an ultrastructural foundation for further examining the mechanisms through which neurotrophins acting at trkB receptors contribute to synaptic plasticity.

Integrin alpha6beta1 plays a significant role in the attachment of hepatoma cells to laminin

BACKGROUND/AIMS

Tumor invasion and metastasis consist of a series of complex events. During this process, the ability of tumor cells to adhere to laminin, a major component of basement membranes, is required at various steps. The expression of laminin-binding integrins and the extent of tumor metastasis and progression appear to be related. In hepatocellular carcinoma, increased expression of laminin-binding integrins is observed. However, little is known concerning the possible functional interactions between laminin-binding integrins and laminin. Therefore, we investigated the participation of laminin-binding integrins in the attachment of hepatoma cells to laminin.

METHODS

Human hepatoma cell lines (KIM-1, KYN-1, 2) were used. We investigated the expression of integrin alpha1, alpha2, alpha3, alpha6, beta1, and beta4 subunits on hepatoma cells by immunocytochemical and flow cytometric analysis. Participation of these integrin subunits in the attachment of hepatoma cells to laminin was evaluated by an inhibition of cell adhesion assay.

RESULTS

Integrin alpha1, alpha2, alpha3, alpha6 and beta1 subunits were expressed at the marginal areas of hepatoma cells, while the integrin beta4 subunit was scarcely detected. Laminin promoted the attachment of hepatoma cells in a dose-dependent manner. Although anti-integrin alpha1, alpha2, beta3 and beta4 subunit antibodies did not inhibit cell attachment to laminin, anti-integrin alpha6 and beta1 subunit antibodies inhibited the attachment by 50% or more.

CONCLUSIONS

These findings indicate that integrin alpha6beta1 is very important in the attachment of hepatoma cells to laminin, suggesting the participation of this integrin in metastasis and invasion of hepatoma cells.

Cloning, sequence analysis, and chromosomal localization of the novel human integrin alpha11 subunit (ITGA11)

The integrins are a large family of cell adhesion molecules consisting of noncovalently associated alphabeta heterodimers. We have cloned and sequenced the cDNA of a novel human integrin alpha-subunit, designated alpha11. The alpha11 cDNA encodes a mature protein with a large 1120-residue extracellular domain that contains an I-domain of 207 residues and is linked by a transmembrane domain to a short cytoplasmic domain of 24 amino acids. The deduced alpha11 protein shows the typical structural features of integrin alpha-subunits and is similar to a distinct group of alpha-subunits from collagen-binding integrins. However, it differs from most integrin alpha-chains by an incompletely preserved cytoplasmic GFFKR motif. The human ITGA11 gene was localized to bands q22.3-q23 on chromosome 15, and its transcripts were found in a variety of tissues, but predominantly in bone, cartilage, cardiac muscle, and skeletal muscle. Expression of a 5.5-kb alpha11 mRNA was detectable in small intestine.

Integrin signaling

Cells reside in a protein network, the extracellular matrix (ECM), which they secrete and mold into the intercellular space. The ECM exerts profound control over cells. The effects of the matrix are primarily mediated by integrins, a family of cell surface receptors that attach cells to the matrix and mediate mechanical and chemical signals from it. These signals regulate the activities of cytoplasmic kinases, growth factor receptors, and ion channels and control the organization of the intracellular actin cytoskeleton. Many integrin signals converge on cell cycle regulation, directing cells to live or die, to proliferate, or to exit the cell cycle and differentiate.

Induction of apoptotic cell death in vascular endothelial cells cultured in three-dimensional collagen lattice

Endothelial cells derived from fetal bovine aorta (BAECs) undergo apoptosis in three-dimensional (3-D) type I collagen lattice in the absence of specific angiogenic factor. In the presence of angiogenic factor, BAECs survive and form a capillary-like tube structure in 3-D culture. In the present study we elucidate the mechanisms of BAECs apoptosis or survival and tube formation in 3-D culture. When BAECs embedded in collagen lattice were cultured with angiogenic factor (fibroblast growth factor-2 (FGF-2) or 4beta-phorbol 12-myristate 13-acetate (PMA)) in the presence of PD98059, a specific inhibitor of mitogen-activated protein kinase kinase, BAECs did not form tube structures and underwent apoptosis in collagen lattice. Function-blocking antibody against alphavbeta3 integrin also inhibited tube formation and induced apoptosis in 3-D culture in the presence of angiogenic factors. Exposure of BAECs to FGF-2 and PMA had no effect on the alphavbeta3 integrin expression but induced the activation of alphavbeta3 integrin. PD98059 attenuated alphavbeta3 integrin activation in response to angiogenic factor. KB-R8301, a hydroxamic acid-based matrix metalloproteinase (MMP) inhibitor, prevented apoptotic cell death in the absence of angiogenic factor in 3-D culture and enhanced capillary-like tube formation in the presence of angiogenic factor, which was not inhibited by the anti-alphavbeta3 integrin antibody. The results suggest that angiogenic factor-induced alphavbeta3 integrin activation through the MEK-ERK pathway regulates the BAEC fate between apoptosis and angiogenesis in collagen lattice. MMP derived from BAECs seems to play a key role in the release of cryptic ligands for alphavbeta3 integrin from intact collagen.