Leukocyte adhesion deficiency-III is caused by mutations in KINDLIN3 affecting integrin activation

Spatial coordination of kindlin-2 with talin head domain in interaction with integrin β cytoplasmic tails

Both talin head domain and kindlin-2 interact with integrin β cytoplasmic tails, and they function in concert to induce integrin activation. Binding of talin head domain to β cytoplasmic tails has been characterized extensively, but information on the interaction of kindin-2 with this integrin segment is limited. In this study, we systematically examine the interactions of kindlin-2 with integrin β tails. Kindlin-2 interacted well with β(1) and β(3) tails but poorly with the β(2) cytoplasmic tail. This binding selectivity was determined by the non-conserved residues, primarily the three amino acids at the extreme C terminus of the β(3) tail, and the sequence in β(2) was non-permissive. The region at the C termini of integrin β(1) and β(3) tails recognized by kindlin-2 was a binding core of 12 amino acids. Kindlin-2 and talin head do not interact with one another but can bind simultaneously to the integrin β(3) tail without enhancing or inhibiting the interaction of the other binding partner. Kindlin-2 itself failed to directly unclasp integrin α/β tail complex, indicating that kindlin-2 must cooperate with talin to support the integrin activation mechanism.

Chemokines, selectins and intracellular calcium flux: temporal and spatial cues for leukocyte arrest

Leukocyte trafficking to acute sites of injury or infection requires spatial and temporal cues that fine tune precise sites of firm adhesion and guide migration to endothelial junctions where they undergo diapedesis to sites of insult. Many detailed studies on the location and gradient of chemokines such as IL-8 and other CXCR ligands reveal that their recognition shortly after selectin-mediated capture and rolling exerts acute effects on integrin activation and subsequent binding to their ligands on the endothelium, which directs firm adhesion, adhesion strengthening, and downstream migration. In this process, G-protein coupled receptor (GPCR) signaling has been found to play an integral role in activating and mobilizing intracellular stores of calcium, GTPases such as Rap-1 and Rho and cytokeletal proteins such as Talin and F-actin to facilitate cell polarity and directional pseudopod formation. A critical question remaining is how intracellular Ca(2+) flux from CRAC channels such as Orai1 synergizes with cytosolic stores to mediate a rapid flux which is critical to the onset of PMN arrest and polarization. Our review will highlight a specific role for calcium as a signaling messenger in activating focal clusters of integrins bound to the cytoskeleton which allows the cell to attain a migratory phenotype. The precise interplay between chemokines, selectins, and integrins binding under the ubiquitous presence of shear stress from blood flow provides an essential cooperative signaling mechanism for effective leukocyte recruitment.

Protein kinase D isoforms are dispensable for integrin-mediated lymphocyte adhesion and homing to lymphoid tissues

Leukocyte function-associated antigen-1 (LFA-1) and very late antigen-4 (VLA-4) integrins are essential for lymphocyte adhesion, trafficking and effector functions. Protein kinase D (PKD) has previously been implicated in lymphocyte integrin regulation through regulation of Rap1 activity. However, the true role of PKD in integrin regulation in primary lymphocytes has not previously been investigated. The major PKD isoform in lymphocytes is PKD2. Here we employed PKD2-deficient mice, a specific PKD kinase inhibitor, as well as PKD-null DT40 B cells to investigate the role of PKD in integrin regulation in lymphocytes. We report that PKD2-deficient lymphocytes bound normally to integrin ligands in static and shear flow adhesion assays. They also homed normally to lymphoid organs after adoptive transfer into wild-type mice. DT40 B cells devoid of any PKD isoforms and primary lymphocytes pretreated with a specific PKD inhibitor bound normally to integrin ligands, indicating that multiple PKD isoforms do not redundantly regulate lymphocyte integrins. In addition, PKD2-deficient lymphocytes, as well as DT40 cells devoid of any PKD isoforms, could activate Rap1 in response to B-cell receptor ligation or phorbol ester treatment. Together, these results show that the PKD family does not play a critical role in lymphocyte integrin-mediated cell adhesion or lymphocyte trafficking in vivo.

Chemokine-triggered leukocyte arrest: force-regulated bi-directional integrin activation in quantal adhesive contacts

The arrest of rolling leukocytes on target vascular beds is mediated by specialized leukocyte integrins and their endothelial ligands. In the circulation, these integrins are generally maintained as inactive 'clasped' heterodimers. Encounter by leukocytes of specialized endothelial-presented chemoattractants termed arrest chemokines drive these integrins to undergo force-regulated biochemical conformational changes in response to signals from chemokine-stimulated Gi-protein coupled receptors (GPCRs) and actin remodeling Rho GTPases. To arrest rolling leukocytes, integrin:ligand bonds must undergo stabilization by several orders of magnitude within quantal submicron contacts that consist of discrete integrin:ligand bonds. We present a unifying three step model for rapid integrin activation by chemokines in the quantal arrest unit, the smallest firm adhesive contact formed by a rolling or a captured leukocyte: integrin extension triggered by talin, integrin headpiece opening driven by surface-immobilized ligand and stabilized by low force, and full heterodimer unclasping requiring integrin tail associations with actin-connected talin and Kindlin-3. Specialized GPCRs and their Gi-protein signaling assemblies drive these and other adaptors to specifically bind integrin cytoplasmic tails possibly in conjunction with de novo actin remodeling, thereby optimizing bi-directional activation of ligand-occupied integrins.

Novel aspects of Kindlin-3 function in humans based on a new case of leukocyte adhesion deficiency III

BACKGROUND

Kindlin-3 is a novel integrin activator in hematopoietic cells, and its deficiency leads to immune problems and severe bleeding, known as leukocyte adhesion deficiency III (LAD-III). Our current understanding of Kindlin-3 function primarily relies on analysis of animal models or cell lines.

OBJECTIVES

To understand the functions of Kindlin-3 in human primary blood cells.

PATIENTS/METHODS

We analyzed primary and immortalized hematopoietic cells obtained from a new LAD-III patient with immune problems, bleeding, a history of anemia, and abnormally shaped red blood cells.

RESULTS

The patient's white blood cells (WBCs) and platelets showed defects in agonist-induced integrin activation and botrocetin-induced platelet agglutination. Primary leukocytes from this patient exhibited abnormal activation of β(1) integrin. Integrin activation defects were responsible for the observed deficiency in the botrocetin-induced platelet response. Analysis of patient genomic DNA revealed a novel mutation in the Kindlin3 gene. The mutation abolished Kindlin-3 expression in primary WBCs and platelets, owing to abnormal splicing. Kindlin-3 is expressed in red blood cells (RBCs), and its deficiency is proposed to lead to abnormally shaped RBCs. Immortalized patient WBCs expressed a truncated form of Kindlin-3 that was not sufficient to support integrin activation. Expression of Kindlin-3 cDNA in immortalized patient WBCs rescued integrin activation defects, whereas overexpression of the truncated form did not.

CONCLUSIONS

Kindlin-3 deficiency impairs integrin function, including activation of β(1) integrin. Abnormalities in glycoprotein Ib-IX function in Kindlin-3-deficient platelets are secondary to integrin defects. The region of Kindlin-3 encoded by exon 11 is crucial for its ability to activate integrins in humans.

Neutrophil arrest by LFA-1 activation

Lymphocyte function-associated antigen-1 (LFA-1) is a heterodimeric integrin consisting of α_L (gene name, Itgal) and β₂ (gene name, Itgb2) subunits expressed in all leukocytes. LFA-1 is essential for neutrophil recruitment to inflamed tissue. Activation of LFA-1 by chemokines allows neutrophils and other leukocytes to undergo arrest, resulting in firm adhesion on endothelia expressing intercellular adhesion molecules (ICAMs). In mice, CXCR2 is the primary chemokine receptor involved in triggering neutrophil arrest, and it does so through “inside-out” activation of LFA-1. CXCR2 signaling induces changes in LFA-1 conformation that are coupled to affinity upregulation of the ligand-binding headpiece (extended with open I domain). Unlike naïve lymphocytes, engagement of P-selectin glycoprotein ligand-1 (PSGL-1) on neutrophils stimulates a slow rolling behavior that is mediated by LFA-1 in a distinct activation state (extended with closed I domain). How inside-out signaling cascades regulate the structure and function of LFA-1 is being studied using flow chambers, intravital microscopy, and flow cytometry for ligand and reporter antibody binding. Here, we review how LFA-1 activation is regulated by cellular signaling and ligand binding. Two FERM domain-containing proteins, talin-1 and Kindlin-3, are critical integrin co-activators and have distinct roles in the induction of LFA-1 conformational rearrangements. This review integrates these new results into existing models of LFA-1 activation.

Distinct roles for talin-1 and kindlin-3 in LFA-1 extension and affinity regulation

In inflammation, neutrophils and other leukocytes roll along the microvascular endothelium before arresting and transmigrating into inflamed tissues. Arrest requires conformational activation of the integrin lymphocyte function-associated antigen-1 (LFA-1). Mutations of the FERMT3 gene encoding kindlin-3 underlie the human immune deficiency known as leukocyte adhesion deficiency-III. Both kindlin-3 and talin-1, another FERM domain-containing cytoskeletal protein, are required for integrin activation, but their individual roles in the induction of specific integrin conformers are unclear. Here, we induce differential LFA-1 activation in neutrophils through engagement of the selectin ligand P-selectin glycoprotein ligand-1 or the chemokine receptor CXCR2. We find that talin-1 is required for inducing LFA-1 extension, which corresponds to intermediate affinity and induces neutrophil slow rolling, whereas both talin-1 and kindlin-3 are required for induction of the high-affinity conformation of LFA-1 with an open headpiece, which results in neutrophil arrest. In vivo, both slow rolling and arrest are defective in talin-1-deficient neutrophils, whereas only arrest is defective in kindlin-3-deficient neutrophils. We conclude that talin-1 and kindlin-3 serve distinct functions in LFA-1 activation.

Integrin signalling and function in immune cells

Integrins not only mediate cell-cell and cell-extracellular matrix adhesion, but also affect the multitude of signal transduction cascades in control of cell survival, proliferation, differentiation and organ development. Mutations in integrins or the major effectors of integrin signalling pathways cause defective organ development, immunodeficiency, cancer or autoimmune disease. Understanding of the signalling events that drive integrin activation and signalling is therefore crucial to uncover the molecular mechanisms of these diseases. This review discusses the key signalling complexes regulating integrin activation and function in both 'inside-out' and 'outside-in' pathways in T lymphocytes, including kinases, SLP-76, VAV1, ADAP, SKAP-55, RapL, RIAM, Rap1, Talin and Kindlin.

Human deficiencies of fucosylation and sialylation affecting selectin ligands

Selectins are carbohydrate-binding adhesion molecules that are required for leukocyte trafficking to secondary lymphoid organs and to sites of infection. They interact with fucosylated and sialylated ligands bearing sialyl-Lewis X as a minimal carbohydrate structure. With this in mind, it should be expected that individuals with deficient fucosylation or sialylation show immunodeficiency. However, as this review shows, the picture appears to be more complex and more interesting. Although there are only few patients with such glycosylation defects, they have turned out to be very instructive for our understanding of the functions of fucosylation and sialylation in immunity, development and hemostasis.

Membrane binding of the N-terminal ubiquitin-like domain of kindlin-2 is crucial for its regulation of integrin activation

Kindlin-2 belongs to an emerging class of regulators for heterodimeric (α/β) integrin adhesion receptors. By binding to integrin β cytoplasmic tail via its C-terminal FERM-like domain, kindlin-2 promotes integrin activation. Intriguingly, this activation process depends on the N terminus of kindlin-2 (K2-N) that precedes the FERM domain. The molecular function of K2-N is unclear. We present the solution structure of K2-N, which displays a ubiquitin fold similar to that observed in kindlin-1. Using chemical shift mapping and mutagenesis, we found that K2-N contains a conserved positively charged surface that binds to membrane enriched with negatively charged phosphatidylinositol-(4,5)-bisphosphate. We show that while wild-type kindlin-2 is capable of promoting integrin activation, such ability is significantly reduced for its membrane-binding defective mutant. These data suggest a membrane-binding function of the ubiquitin-like domain of kindlin-2, which is likely common for all kindlins to promote their localization to the plasma membrane and control integrin activation.

Leukocyte adhesion deficiency-I variant syndrome (LAD-Iv, LAD-III): molecular characterization of the defect in an index family

Leukocyte adhesion deficiencies are rare clinical syndromes of impaired host defense that provide novel insights into regulation of immune and inflammatory responses. Leukocyte adhesion deficiency (LAD)-I variant (LAD-Iv), also called LAD-III, is a unique disorder in which inside-out signaling of β₁, β₂, and β₃ integrins on leukocytes and platelets is disrupted, leading to impaired cellular adhesion, recurrent infections, and bleeding. We originally reported the second patient with this disorder to be identified and characterized the adhesive deficiencies and functional phenotype of this subject's leukocytes. Here, we show that the molecular defect in this index subject is a new mutation in FERMT3 (KINDLIN-3) which encodes KINDLIN-3, a cytoskeletal protein that interacts with the cytoplasmic tails of β₁, β₂, and β₃ integrins and is required for inside-out and outside-in signaling of these heterodimers. We also report clinical features and previously unrecognized defects in cells from a new patient, a sibling of the original subject that we described who carries the same FERMT3 mutation. Mutations in FERMT3 have now been shown to be the basis for LAD-Iv/LAD-III in each of the four original patients or families that established this syndrome, including the family that we describe.

Kindlin-3 mediates integrin αLβ2 outside-in signaling, and it interacts with scaffold protein receptor for activated-C kinase 1 (RACK1)

Integrins are heterodimeric type I membrane cell adhesion molecules that are involved in many biological processes. Integrins are bidirectional signal transducers because their cytoplasmic tails are docking sites for cytoskeletal and signaling molecules. Kindlins are cytoplasmic molecules that mediate inside-out signaling and activation of the integrins. The three kindlin paralogs in humans are kindlin-1, -2, and -3. Each of these contains a 4.1-ezrin-radixin-moesin (FERM) domain and a pleckstrin homology domain. Kindlin-3 is expressed in platelets, hematopoietic cells, and endothelial cells. Here we show that kindlin-3 is involved in integrin αLβ2 outside-in signaling. It also promotes micro-clustering of integrin αLβ2. We provide evidence that kindlin-3 interacts with the receptor for activated-C kinase 1 (RACK1), a scaffold protein that folds into a seven-blade propeller. This interaction involves the pleckstrin homology domain of kindlin-3 and blades 5-7 of RACK1. Using the SKW3 human T lymphoma cells, we show that integrin αLβ2 engagement by its ligand ICAM-1 promotes the association of kindlin-3 with RACK1. We also show that kindlin-3 co-localizes with RACK1 in polarized SKW3 cells and human T lymphoblasts. Our findings suggest that kindlin-3 plays an important role in integrin αLβ2 outside-in signaling.

Platelet signaling-a primer

OBJECTIVE

To review the receptors and signal transduction pathways involved in platelet plug formation and to highlight links between platelets, leukocytes, endothelium, and the coagulation system.

DATA SOURCES

Original studies, review articles, and book chapters in the human and veterinary medical fields.

DATA SYNTHESIS

Platelets express numerous surface receptors. Critical among these are glycoprotein VI, the glycoprotein Ib-IX-V complex, integrin α(IIb) β(3) , and the G-protein-coupled receptors for thrombin, ADP, and thromboxane. Activation of these receptors leads to various important functional events, in particular activation of the principal adhesion receptor α(IIb) β(3) . Integrin activation allows binding of ligands such as fibrinogen, mediating platelet-platelet interaction in the process of aggregation. Signals activated by these receptors also couple to 3 other important functional events, secretion of granule contents, change in cell shape through cytoskeletal rearrangement, and procoagulant membrane expression. These processes generate a stable thrombus to limit blood loss and promote restoration of endothelial integrity.

CONCLUSIONS

Improvements in our understanding of how platelets operate through their signaling networks are critical for diagnosis of unusual primary hemostatic disorders and for rational antithrombotic drug design.

Thirty years of primary immunodeficiencies in Turkey

Turkey, with its population of some 75 million, has a high rate of consanguineous marriages. Because the majority of the primary immunodeficiencies (PIDs) are inherited as autosomal recessive (AR) forms, the high consanguinity rate leads to a high prevalence of PID diseases in Turkey. The first pediatric immunology division was established in 1972, since then over 10 other immunology divisions have been established in different cities. Approximately 4,000 patients with possible PID are referred to these centers annually. The percentages of some of the major immunodeficiency groups and individual disease numbers among these patients differ somewhat in comparison with Western countries, likely because the relative incidences of PIDs with AR inheritance and of rare diseases are higher. These characteristics of the patient population, and our determination of differences in disease presentation and unusual features, have led us to undertake studies in collaboration with various centers in Western countries. These collaborations have contributed to the identification of the genes responsible for some rare immunodeficiencies, to the resolution of the genetic heterogeneity underlying conventional phenotypes, and to the description of new clinical phenotypes.

Leukocyte adhesion deficiencies

Leukocyte trafficking from the blood stream to tissues is essential for continuous surveillance of foreign antigens. This dynamic process, designated as the leukocyte adhesion cascade, involves distinct steps. In leukocyte adhesion deficiency (LAD) I the firm adhesion of leukocyte to the endothelium is defective, due to mutations in the beta 2 integrin gene. LAD II is caused by mutations in the fucose transporter specific to the Golgi apparatus, leading to the absence of Sialyl Lewis X-the fucosylated ligand for the selectins-thus affecting the rolling phase, the first phase of the cascade. In LAD III, a primary activation defect occurs in beta integrins 1, 2, and 3. Recently, the genetic basis for LAD III has been revealed to involve mutations in kindlin-3, a newly recognized essential component of integrin activation-the second phase of the adhesion cascade. Until now, no human or animal models of defect in transmigration-the fourth and last phase of the cascade-has been described.

Inherited platelet disorders

OBJECTIVE

To present the latest information on inherited platelet disorders in domestic animals.

DATA SOURCES

Research articles and reviews spanning 40 years available on PubMed.

HUMAN DATA SYNTHESIS

Information regarding inherited platelet disorders in people is plentiful and often descriptions of human conditions have led to the identification of similar disorders in veterinary species. There are exceptions, however, in which specific inherited platelet disorders were first described in animals with subsequent identification in people.

VETERINARY DATA SYNTHESIS

Many inherited platelet disorders have been documented in animals at the functional and molecular level and that information is presented in this review.

CONCLUSIONS

Much progress has been made in the past 20 years in the characterization of inherited platelet disorders in animals at the functional, biochemical, and molecular level. The study of inherited platelet disorders has greatly enhanced the understanding of platelet physiology and has led in some instances to the development of platelet inhibitory medications. Characterization of inherited disorders at the molecular level greatly facilitates diagnosis and identification of affected and heterozygous animals thus avoiding propagation of the defect by breeders. When used with available functional and biochemical diagnostic tests, it significantly enhances the quality of care and case management.

A conserved lipid-binding loop in the kindlin FERM F1 domain is required for kindlin-mediated αIIbβ3 integrin coactivation

The activation of heterodimeric integrin adhesion receptors from low to high affinity states occurs in response to intracellular signals that act on the short cytoplasmic tails of integrin β subunits. Binding of the talin FERM (four-point-one, ezrin, radixin, moesin) domain to the integrin β tail provides one key activation signal, but recent data indicate that the kindlin family of FERM domain proteins also play a central role. Kindlins directly bind integrin β subunit cytoplasmic domains at a site distinct from the talin-binding site, and target to focal adhesions in adherent cells. However, the mechanisms by which kindlins impact integrin activation remain largely unknown. A notable feature of kindlins is their similarity to the integrin-binding and activating talin FERM domain. Drawing on this similarity, here we report the identification of an unstructured insert in the kindlin F1 FERM domain, and provide evidence that a highly conserved polylysine motif in this loop supports binding to negatively charged phospholipid head groups. We further show that the F1 loop and its membrane-binding motif are required for kindlin-1 targeting to focal adhesions, and for the cooperation between kindlin-1 and -2 and the talin head in αIIbβ3 integrin activation, but not for kindlin binding to integrin β tails. These studies highlight the structural and functional similarities between kindlins and the talin head and indicate that as for talin, FERM domain interactions with acidic membrane phospholipids as well β-integrin tails contribute to the ability of kindlins to activate integrins.

Overview of integrin signaling in the immune system

It has been well established that integrins mediate cell-cell and cell-matrix adhesion and play crucial roles in the immune system such as leukocyte-endothelium interactions, immune synapse formation, and effector functions. Since the discovery that integrins undergo dynamic changes of adhesive activities in response to external stimuli, intensive studies have been conducted to elucidate the signaling events that control the activation of integrins (inside-out signaling) and signaling events from the induced integrin-dependent adhesion (outside-in signaling). The molecular characterization of these signaling pathways highlights the importance of integrins as bidirectional signaling receptors. The characteristics of integrin signaling are best exemplified in the immune system. This chapter highlights the recent studies of intracellular signaling pathways that regulate integrins in immunological contexts.

Lentiviral gene transfer method to study integrin function in T lymphocytes

Integrins play critical roles in adhesion and migration of T cells during an immune response and inflammation. It is of great importance to understand the molecular pathways that regulate integrin function in T cells. Lentiviral vector-based gene transfer method has emerged in the past decade as an efficient means of transferring genes into both resting and activated hard-to-transfect cells, including T cells to knockdown gene expression. Therefore, this technology could be utilized effectively to study different aspects of integrin function or even to perform genome-wide RNAi screens to look globally for regulators of integrin function in T cells. In this chapter, we provide the simplest protocol to infect activated CD4(+) human T cells with high efficiency.

Hematologically important mutations: leukocyte adhesion deficiency (first update)

Leukocyte adhesion deficiency (LAD) is an immunodeficiency caused by defects in the adhesion of leukocytes (especially neutrophils) to the blood vessel wall. As a result, patients with LAD suffer from severe bacterial infections and impaired wound healing, accompanied by neutrophilia. In LAD-I, mutations are found in ITGB2, the gene that encodes the β subunit of the β(2) integrins. This syndrome is characterized directly after birth by delayed separation of the umbilical cord. In the rare LAD-II disease, the fucosylation of selectin ligands is disturbed, caused by mutations in SLC35C1, the gene that encodes a GDP-fucose transporter of the Golgi system. LAD-II patients lack the H and Lewis Le(a) and Le(b) blood group antigens. Finally, in LAD-III (also called LAD-I/variant) the conformational activation of the hematopoietically expressed β integrins is disturbed, leading to leukocyte and platelet dysfunction. This last syndrome is caused by mutations in FERMT3, encoding the kindlin-3 protein in all blood cells that is involved in the regulation of β integrin conformation.

Defects in Glanzmann thrombasthenia and LAD-III (LAD-1/v) syndrome: the role of integrin β1 and β3 in platelet adhesion to collagen

Patients with Glanzmann thrombasthenia or Leukocyte Adhesion Deficiency-III syndrome (LAD-III or LAD-1/variant) present with increased bleeding tendency because of the lack or dysfunction of the fibrinogen receptor GPIIb/IIIa (integrin αIIbβ3), respectively. Although the bleeding disorder is more severe in LAD-III patients, classic aggregometry or perfusion of Glanzmann or LAD-III platelets over collagen-coated slides under physiologic shear rate does not discriminate between these 2 conditions. However, in a novel flow cytometry-based aggregation assay, Glanzmann platelets were still capable of forming small aggregates upon collagen stimulation, whereas LAD-III platelets were not. These aggregates required functional GPIa/IIa (integrin α2β1) instead of integrin αIIbβ3, thus explaining the clinically more severe bleeding manifestations in LAD-III patients, in which all platelet integrins are functionally defective. These findings provide genetic evidence for the differential requirements of platelet integrins in thrombus formation and demonstrate that correct integrin function assessment can be achieved with a combination of diagnostic methods.

Structures and interaction analyses of integrin αMβ2 cytoplasmic tails

Integrins are heterodimeric (α and β subunits) signal transducer proteins involved in cell adhesions and migrations. The cytosolic tails of integrins are essential for transmitting bidirectional signaling and also implicated in maintaining the resting states of the receptors. In addition, cytosolic tails of integrins often undergo post-translation modifications like phosphorylation. However, the consequences of phosphorylation on the structures and interactions are not clear. The leukocyte-specific integrin αMβ2 is essential for myeloid cell adhesion, phagocytosis, and degranulation. In this work, we determined solution structures of the myristoylated cytosolic tail of αM and a Ser phosphorylated variant in dodecylphosphocholine micelles by NMR spectroscopy. Furthermore, the interactions between non-phosphorylated and phosphorylated αM tails with β2 tail were investigated by NMR and fluorescence resonance energy transfer (FRET). The three-dimensional structures of the 24-residue cytosolic tail of αM or phosphorylated αM are characterized by an N-terminal amphipathic helix and a loop at the C terminus. The residues at the loop are involved in packing interactions with the hydrophobic face of the helix. 15N-1H heteronuclear single quantum coherence experiments identified residues of αM and β2 tails that may be involved in the formation of a tail-tail heterocomplex. We further examined interactions between myristoylated β2 tail in dodecylphosphocholine micelles with dansylated αM tail peptides by FRET. These studies revealed enhanced interactions between αM or phosphorylated αM tails with β2 tail with Kd values ∼5.2±0.6 and ∼4.4±0.7 μm, respectively. Docked structures of tail-tail complexes delineated that the αM/β2 interface at the cytosolic region could be sustained by a network of polar interactions, ionic interactions, and/or hydrogen bonds.

Structural basis of phosphoinositide binding to kindlin-2 protein pleckstrin homology domain in regulating integrin activation

Kindlins are a subclass of FERM-containing proteins that have recently emerged as key regulators of integrin receptor activation and signaling. As compared with the conventional FERM domain, the kindlin FERM domain contains an inserted pleckstrin homology (PH) domain that recognizes membrane phosphoinositides, including phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 3,4,5-trisphosphate (PIP3). Using NMR spectroscopy, we show that PIP3 site-specifically binds to kindlin-2 PH with substantial chemical shift changes that are much larger than PIP2. This suggests an enhanced association of kindlin-2 with membrane as mediated by PIP3 upon its conversion from PIP2 by phosphoinositide-3 kinase, a known regulator of integrin activation. We determined the NMR structure of the kindlin-2 PH domain bound to the head group of PIP3, inositol 1,3,4,5-tetraphosphate (IP4). The structure reveals a canonical PH domain fold, yet with a distinct IP4 binding pocket that appears highly conserved for the kindlin family members. Functional experiments demonstrate that although wild type kindlin-2 is capable of cooperating with integrin activator talin to induce synergistic integrin α(IIb)β(3) activation, this ability is significantly impaired for a phosphoinositide binding-defective kindlin-2 mutant. These results define a specific PIP3 recognition mode for the kindlin PH domain. Moreover, they shed light upon a mechanism as to how the PH domain mediates membrane engagement of kindlin-2 to promote its binding to integrin and cooperation with talin for regulation of integrin activation.

Reconstruction of integrin activation

Integrins are integral membrane proteins that mediate cell-matrix and cell-cell adhesion. They are important for vascular development and hematopoiesis, immune and inflammatory responses, and hemostasis. Integrins are also signaling receptors that can transmit information bidirectionally across plasma membranes. Research in the past 2 decades has made progress in unraveling the mechanisms of integrin signaling and brings the field to the moment of attempting synthetic reconstruction of the signaling pathways in vitro. Reconstruction of biologic processes provides stringent tests of our understanding of the process, as evidenced by studies of other biologic machines, such as ATP synthase, lactose permease, and G-protein-coupled receptors. Here, we review recent progress in reconstructing integrin signaling and the insights that we have gained through these experiments.

Glanzmann thrombasthenia: a review of ITGA2B and ITGB3 defects with emphasis on variants, phenotypic variability, and mouse models

Characterized by mucocutaneous bleeding arising from a lack of platelet aggregation to physiologic stimuli, Glanzmann thrombasthenia (GT) is the archetype-inherited disorder of platelets. Transmitted by autosomal recessive inheritance, platelets in GT have quantitative or qualitative deficiencies of the fibrinogen receptor, αIIbβ3, an integrin coded by the ITGA2B and ITGB3 genes. Despite advances in our understanding of the disease, extensive phenotypic variability with respect to severity and intensity of bleeding remains poorly understood. Importantly, genetic defects of ITGB3 also potentially affect other tissues, for β3 has a wide tissue distribution when present as αvβ3 (the vitronectin receptor). We now look at the repertoire of ITGA2B and ITGB3 gene defects, reexamine the relationship between phenotype and genotype, and review integrin structure in the many variant forms. Evidence for modifications in platelet production is assessed, as is the multifactorial etiology of the clinical expression of the disease. Reports of cardiovascular disease and deep vein thrombosis, cancer, brain disease, bone disorders, and pregnancy defects in GT are discussed in the context of the results obtained for mouse models where nonhemostatic defects of β3-deficiency or nonfunction are being increasingly described.

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.

Leukocyte integrin activation mediates transient neutropenia after G-CSF administration

After administration of granulocyte colony-stimulating factor (G-CSF), there is a marked, albeit transient, drop in circulating neutrophils. To determine the role of leukocyte integrins in this disappearance, a dog having canine leukocyte adhesion deficiency (CLAD) or CLAD dogs who had undergone gene correction either by matched littermate allogeneic transplant or autologous gene therapy were evaluated. Shortly after G-CSF administration, a dramatic, yet transient, neutropenia was observed in the control littermates. This neutropenia was not as marked in the CLAD dogs. In all instances, it was CD18(+) neutrophils that preferentially egressed from the circulation. The association of CD18 with this rapid loss suggested leukocyte integrin activation after G-CSF administration. To determine the activation status of the integrin, a monoclonal antibody recognizing the activated α-subunit cation binding domain (mAb24) was used to evaluate human leukocytes after G-CSF administration. Mirroring the dramatic decrease in circulating neutrophil numbers, there was a dramatic and specific increase in the activation of the α-subunit after G-CSF expression on polymorphonuclear leukocytes. This activation, like the drop in neutrophil count, was transient. These results demonstrate that the leukocyte integrin on circulating neutrophils is transiently activated after G-CSF administration and mediates the transient neutropenia observed after G-CSF administration.

Molecular mechanism of inside-out integrin regulation

Integrins are cell surface adhesion and signaling receptors important for cell adhesion, survival and migration. Integrins are known to be regulated by signals from inside the cells. Such inside-out regulation modulates affinities of integrins for their extracellular matrix ligand and is critical for thrombosis, haemostasis and immune response. Talin and kindlin, two integrin binding proteins, have been shown to be important regulators of integrin function. In this review, we will focus on the molecular mechanism of integrin regulation that has emerged from recent structural, biochemical and genetic studies.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

Platelet adhesion and activation mechanisms in arterial thrombosis and ischaemic stroke

Platelet adhesion, activation and aggregation on the exposed subendothelial extracellular matrix (ECM) are essential for haemostasis, but may also lead to occlusion of diseased vessels. Binding of the glycoprotein (GP)Ib-V-IX complex to immobilised von Willebrand factor (VWF) initiates adhesion of flowing platelets to the ECM, and thereby enables the collagen receptor GPVI to interact with its ligand and to mediate platelet activation. This process is reinforced by locally produced thrombin and platelet-derived secondary mediators, such as adenosine diphosphate (ADP) and thromboxane A(2) (TxA(2)). Together, these events promote a shift of β1 and β3 integrins from a low to a high affinity state for their ligands through 'inside-out' signalling allowing firm platelet adhesion and aggregation. Formed platelet aggregates are stabilised by fibrin formation and signalling events between adjacent platelets involving multiple platelet receptors, such as the newly discovered C-type lectin-like receptor 2 (CLEC-2). While occlusive thrombus formation is the principal pathogenic event in myocardial infarction, the situation is more complex in ischaemic stroke where infarct development often progresses despite sustained early reperfusion of previously occluded major intracranial arteries, a process referred to as 'reperfusion injury'. Increasing experimental evidence now suggests that early platelet adhesion and activation events, orchestrate a 'thrombo-inflammatory' cascade in this setting, whereas platelet aggregation and thrombus formation are not required. This review summarises recent developments in understanding the principal platelet adhesion receptor systems with a focus on their involvement in arterial thrombosis and ischaemic stroke models.

Advances in our understanding of the molecular basis of disorders of platelet function

Genetic defects of platelet function give rise to mucocutaneous bleeding of varying severity because platelets fail to fulfil their haemostatic role after vessel injury. Abnormalities of pathways involving glycoprotein (GP) mediators of adhesion (Bernard-Soulier syndrome, platelet-type von Willebrand disease) and aggregation (Glanzmann thrombasthenia) are the most studied and affect the GPIb-IX-V complex and integrin αIIbβ3, respectively. Leukocyte adhesion deficiency-III combines Glanzmann thrombasthenia with infections and defects of kindlin-3, a mediator of integrin activation. Agonist-specific deficiencies in platelet aggregation relate to mutations of primary receptors for ADP (P2Y(12)), thromboxane A(2) (TXA2R) and collagen (GPVI); however, selective abnormalities of intracellular signalling pathways remain better understood in mouse models. Defects of secretion from δ-granules are accompanied by pigment defects in the Hermansky-Pudlak and Chediak-Higashi syndromes; they concern multiple genes and protein complexes involved in secretory organelle biogenesis and function. Quebec syndrome is linked to a tandem duplication of the urokinase plasminogen activator (PLAU) gene while locus assignment to chromosome 3p has advanced the search for the gene(s) responsible for α-granule deficiency in the gray platelet syndrome. Defects of α-granule biosynthesis also involve germline VPS33B mutations in the ARC (arthrogryposis, renal dysfunction and cholestasis) syndrome. A mutation in transmembrane protein 16F (TMEM16F) has been linked to a defective procoagulant activity and phosphatidylserine expression in the Scott syndrome. Cytoskeletal dysfunction (with platelet anisotrophy) occurs not only in the Wiskott-Aldrich syndrome but also in filamin A deficiency or MYH9-related disease while GATA1 mutations or RUNX1 haploinsufficiency can affect expression of multiple platelet proteins.

Kindlin-3 is required for the stabilization of TCR-stimulated LFA-1:ICAM-1 bonds critical for lymphocyte arrest and spreading on dendritic cells

Kindlin-3 is a key lymphocyte function–associated antigen-1 (LFA-1) coactivator deleted in leukocyte adhesion deficiency-III (LAD-III). In the present study, we investigated the involvement of this adaptor in lymphocyte motility and TCR-triggered arrest on ICAM-1 or on dendritic cells (DCs). Kindlin-3–null primary T cells from a LAD-III patient migrated normally on the major lymph node chemokine CCL21 and engaged in normal TCR signaling. However, TCR activation of Kindlin-3–null T lymphocytes failed to trigger the robust LFA-1–mediated T-cell spreading on ICAM-1 and ICAM-1–expressing DCs that is observed in normal lymphocytes. Kindlin-3 was also essential for cytoskeletal anchorage of the LFA-1 heterodimer and for microclustering of LFA-1 within ventral focal dots of TCR-stimulated lymphocytes spread on ICAM-1. Surprisingly, LFA-1 on Kindlin-3–null lymphocytes migrating over CCL21 acquired normal expression of an epitope associated with the conformational activation of the key headpiece domain, β I. This activated LFA-1 was highly responsive to TCR-triggered ICAM-1–driven stop signals in normal T cells locomoting on CCL21, but not in their Kindlin-3–null T-cell counterparts. We suggest that Kindlin-3 selectively contributes to a final TCR-triggered outside-in stabilization of bonds generated between chemokine-primed LFA-1 molecules and cell-surface ICAM-1.

How has neutrophil research improved our understanding of periodontal pathogenesis?

BACKGROUND

Neutrophils are the predominant cells responsible for host defence against bacterial infection. Loss of neutrophil defence, due either to deficient number or function, strongly predisposes to bacterial infections such as periodontitis. Yet, the neutrophil oxidative and proteolytic arsenal has also been implicated in perpetrating periodontal tissue damage in periodontitis.

AIM

In this review, we focus on recent developments that shed light on these two aspects of neutrophil function in periodontitis.

METHODS

Primary search: using PubMed search for "neutophil", "periodontal", and "periodontitis". Secondary search: using references from the articles found in the first stage.

RESULTS

Early histological studies showed that infiltrating neutrophils form a wall of cells abutting the junctional epithelium in periodontal inflammatory lesions. The chronic standoff between these neutrophils and the bacterial community suggests that bacterial evasion of neutrophil clearance is a major characteristic of periodontitis. Indeed, not all functional neutrophil deficiencies increase the risk of periodontitis, an observation that points the way towards identification of particular anti-bacterial pathways essential for protection against periodontal pathogens. The net result in the majority of periodontitis patients who exhibit normal neutrophil number and function, is that neutrophils accumulate in the periodontal tissue where they are available to participate in tissue destruction. Diminished neutrophil clearance further contributes to the persistence of activated neutrophils in the periodontal tissue.

CONCLUSIONS

Data on the role of neutrophils in the pathogenesis of periodontitis are mixed. Neutrophils are a critical arm of the defence against periodontitis, but bacterial evasion of the neutrophil microbicidal machinery coupled with delayed neutrophil apoptosis may transform the neutrophil from defender to perpetrator. At this stage of knowledge, attempts to induce host modulation through neutrophil suppression or activation are premature.

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.

Chemokine triggered integrin activation and actin remodeling events guiding lymphocyte migration across vascular barriers

Chemokine signals activate leukocyte integrins and actin remodeling machineries critical for leukocyte adhesion and motility across vascular barriers. The arrest of leukocytes at target blood vessel sites depends on rapid conformational activation of their α4 and β2 integrins by the binding of endothelial-displayed chemokines to leukocyte Gi-protein coupled receptors (GPCRs). A universal regulator of this event is the integrin-actin adaptor, talin1. Chemokine-stimulated GPCRs can transmit within fractions of seconds signals via multiple Rho GTPases, which locally raise plasma membrane levels of the talin activating phosphatidyl inositol, PtdIns(4,5)P2 (PIP2). Additional pools of GPCR stimulated Rac-1 and Rap-1 GTPases together with GPCR stimulated PLC and PI3K family members regulate the turnover of focal contacts of leukocyte integrins, induce the collapse of leukocyte microvilli, and promote polarized leukocyte crawling in search of exit cues. Concomitantly, other leukocyte GTPases trigger invasive protrusions into and between endothelial cells in search of basolateral chemokine exit cues. We will review here major findings and open questions related to these sequential guiding activities of endothelial presented chemokines, focusing mainly on lymphocyte-endothelial interactions as a paradigm for other leukocytes.

Molecular architecture and function of matrix adhesions

Cell adhesions mediate important bidirectional interactions between cells and the extracellular matrix. They provide an interactive interface between the extracellular chemical and physical environment and the cellular scaffolding and signaling machinery. This dynamic, reciprocal regulation of intracellular processes and the matrix is mediated by membrane receptors such as the integrins, as well as many other components that comprise the adhesome. Adhesome constituents assemble themselves into different types of cell adhesion structures that vary in molecular complexity and change over time. These cell adhesions play crucial roles in cell migration, proliferation, and determination of cell fate.

A novel leukocyte adhesion deficiency III variant: kindlin-3 deficiency results in integrin- and nonintegrin-related defects in different steps of leukocyte adhesion

Leukocyte adhesion deficiency type III is a recently described condition involving a Glanzmann-type bleeding syndrome and leukocyte adhesion deficiency. This was ascribed to a defect of the FERMT3 gene resulting in abnormal expression of kindlin-3, a protein expressed in hematopoietic cells with a major role in the regulation of integrin activation. In this article, we describe a patient with a new mutation of FERMT3 and lack of kindlin-3 expression in platelets and leukocytes. We assayed quantitatively the first steps of kindlin-3-defective leukocyte adhesion, namely, initial bond formation, bond strengthening, and early spreading. Initial bond formation was readily stimulated with neutrophils stimulated by fMLF, and neutrophils and lymphocytes stimulated by a phorbol ester or Mn(2+). In contrast, attachment strengthening was defective in the patient's lymphocytes treated with PMA or Mn(2+), or fMLF-stimulated neutrophils. However, attachment strengthening was normal in patient's neutrophils treated with phorbol ester or Mn(2+). In addition, the patient's T lymphocytes displayed defective integrin-mediated spreading and a moderate but significant decrease of spreading on anti-CD3-coated surfaces. Patient's neutrophils displayed a drastic alteration of integrin-mediated spreading after fMLF or PMA stimulation, whereas signaling-independent Mn(2+) allowed significant spreading. In conclusion, the consequences of kindlin-3 deficiency on β(2) integrin function depend on both cell type and the stimulus used for integrin activation. Our results suggest looking for a possible kindlin-3 involvement in membrane dynamical event independent of integrin-mediated adhesion.

Phylogenetic analysis of kindlins suggests subfunctionalization of an ancestral unduplicated kindlin into three paralogs in vertebrates

Kindlin proteins represent a newly discovered family of evolutionarily conserved FERM domain-containing proteins. This family includes three highly conserved proteins: Kindlin-1, Kindlin-2 and Kindlin-3. All three Kindlin proteins are associated with focal adhesions and are involved in integrin activation. The FERM domain of each Kindlin is bipartite and plays a key role in integrin activation. We herein explore for the first time the evolutionary history of these proteins. The phylogeny of the Kindlins suggests a single ancestral Kindlin protein present in even the earliest metazoan ie, hydra. This protein then underwent duplication events in insects and also experienced genome duplication in vertebrates, leading to the Kindlin family. A comparative study of the Kindlin paralogs showed that Kindlin-2 is the slowest evolving protein among the three family members. The analysis of synonymous and non-synonymous substitutions in orthologous Kindlin sequences in different species showed that all three Kindlins have been evolving under the influence of purifying selection. The expression pattern of Kindlins along with phylogenetic studies supports the subfunctionalization model of gene duplication.

The integrin coactivator kindlin-2 plays a critical role in angiogenesis in mice and zebrafish

Kindlin-2, a widely distributed cytoskeletal protein, has been implicated in integrin activation, and its absence is embryonically lethal in mice and causes severe developmental defects in zebrafish. Knockdown of kindlin-2 levels in endothelial cells resulted in defective adhesive and migratory responses, suggesting that angiogenesis might be aberrant even with partial reduction of kindlin-2. This hypothesis has now been tested in the kindlin-2(+/-) mice. RM1 prostate tumors grown in kindlin-2(+/-) mice had fewer blood vessels, which were thinner and shorter and supported less tumor growth compared with wild-type littermates. The vessels that did form in the kindlin-2(+/-) mice lacked smooth muscle cells and pericytes and had thinner basement membranes, indicative of immature vessels. VEGF-induced angiogenesis in matrigel implants was also abnormal in the kindlin-2(+/-) mice. Vessels in the kindlin-2(+/-) mice were leaky, and BM transplantation from kindlin-2(+/-) to WT mice did not correct this defect. Endothelial cells derived from kindlin-2(+/-) mice had integrin expression levels similar to WT mice but reduced αVβ3-dependent signaling, migration, adhesion, spreading, and tube formation. Developmental angiogenesis was markedly impaired by kindlin-2 morpholinos in zebrafish. Taken together, kindlin-2 plays an important role in pathologic and developmental angiogenesis, which arises from defective activation of integrin αVβ3.

Kindlin-3-mediated signaling from multiple integrin classes is required for osteoclast-mediated bone resorption

Loss of kindlin-3 impairs activation of β1, β2, and β3 integrin classes, resulting in osteopetrotic defects in osteoclast adhesion and spreading.

The blood cell–specific kindlin-3 protein is required to activate leukocyte and platelet integrins. In line with this function, mutations in the KINDLIN-3 gene in man cause immunodeficiency and severe bleeding. Some patients also suffer from osteopetrosis, but the underlying mechanism leading to abnormal bone turnover is unknown. Here we show that kindlin-3–deficient mice develop severe osteopetrosis because of profound adhesion and spreading defects in bone-resorbing osteoclasts. Mechanistically, loss of kindlin-3 impairs the activation of β1, β2, and β3 integrin classes expressed on osteoclasts, which in turn abrogates the formation of podosomes and sealing zones required for bone resorption. In agreement with these findings, genetic ablation of all integrin classes abolishes the development of podosomes, mimicking kindlin-3 deficiency. Although loss of single integrin classes gives rise to podosomes, their resorptive activity is impaired. These findings show that osteoclasts require their entire integrin repertoire to be regulated by kindlin-3 to orchestrate bone homeostasis.

Genetic analyses of integrin signaling

The development of multicellular organisms, as well as maintenance of organ architecture and function, requires robust regulation of cell fates. This is in part achieved by conserved signaling pathways through which cells process extracellular information and translate this information into changes in proliferation, differentiation, migration, and cell shape. Gene deletion studies in higher eukaryotes have assigned critical roles for components of the extracellular matrix (ECM) and their cellular receptors in a vast number of developmental processes, indicating that a large proportion of this signaling is regulated by cell-ECM interactions. In addition, genetic alterations in components of this signaling axis play causative roles in several human diseases. This review will discuss what genetic analyses in mice and lower organisms have taught us about adhesion signaling in development and disease.