A mechanism for hereditary angioedema caused by a methionine-379-to-lysine substitution in kininogens

ABSTRACT

Hereditary angioedema (HAE) is associated with episodic kinin-induced swelling of the skin and mucosal membranes. Most patients with HAE have low plasma C1-inhibitor activity, leading to increased generation of the protease plasma kallikrein (PKa) and excessive release of the nanopeptide bradykinin from high-molecular-weight kininogen (HK). However, disease-causing mutations in at least 10% of patients with HAE appear to involve genes for proteins other than C1-inhibitor. A point mutation in the Kng1 gene encoding HK and low-molecular weight kininogen (LK) was identified recently in a family with HAE. The mutation changes a methionine (Met379) to lysine (Lys379) in both proteins. Met379 is adjacent to the Lys380-Arg381 cleavage site at the N-terminus of the bradykinin peptide. Recombinant wild-type (Met379) and variant (Lys379) versions of HK and LK were expressed in HEK293 cells. PKa-catalyzed kinin release from HK and LK was not affected by the Lys379 substitutions. However, kinin release from HK-Lys379 and LK-Lys379 catalyzed by the fibrinolytic protease plasmin was substantially greater than from wild-type HK-Met379 and LK-Met379. Increased kinin release was evident when fibrinolysis was induced in plasma containing HK-Lys379 or LK-Lys379 compared with plasma containing wild-type HK or LK. Mass spectrometry revealed that the kinin released from wild-type and variant kininogens by PKa is bradykinin. Plasmin also released bradykinin from wild-type kininogens but cleaved HK-Lys379 and LK-Lys379 after Lys379 rather than Lys380, releasing the decapeptide Lys-bradykinin (kallidin). The Met379Lys substitutions make HK and LK better plasmin substrates, reinforcing the relationship between fibrinolysis and kinin generation.

Factor XII Structure-Function Relationships

Factor XII (FXII), the zymogen of the protease FXIIa, contributes to pathologic processes such as bradykinin-dependent angioedema and thrombosis through its capacity to convert the homologs prekallikrein and factor XI to the proteases plasma kallikrein and factor XIa. FXII activation and FXIIa activity are enhanced when the protein binds to a surface. Here, we review recent work on the structure and enzymology of FXII with an emphasis on how they relate to pathology. FXII is a homolog of pro-hepatocyte growth factor activator (pro-HGFA). We prepared a panel of FXII molecules in which individual domains were replaced with corresponding pro-HGFA domains and tested them in FXII activation and activity assays. When in fluid phase (not surface bound), FXII and prekallikrein undergo reciprocal activation. The FXII heavy chain restricts reciprocal activation, setting limits on the rate of this process. Pro-HGFA replacements for the FXII fibronectin type 2 or kringle domains markedly accelerate reciprocal activation, indicating disruption of the normal regulatory function of the heavy chain. Surface binding also enhances FXII activation and activity. This effect is lost if the FXII first epidermal growth factor (EGF1) domain is replaced with pro-HGFA EGF1. These results suggest that FXII circulates in blood in a "closed" form that is resistant to activation. Intramolecular interactions involving the fibronectin type 2 and kringle domains maintain the closed form. FXII binding to a surface through the EGF1 domain disrupts these interactions, resulting in an open conformation that facilitates FXII activation. These observations have implications for understanding FXII contributions to diseases such as hereditary angioedema and surface-triggered thrombosis, and for developing treatments for thrombo-inflammatory disorders.

Mechanisms involved in hereditary angioedema with normal C1-inhibitor activity

Patients with the inherited disorder hereditary angioedema (HAE) suffer from episodes of soft tissue swelling due to excessive bradykinin production. In most cases, dysregulation of the plasma kallikrein-kinin system due to deficiency of plasma C1 inhibitor is the underlying cause. However, at least 10% of HAE patients have normal plasma C1 inhibitor activity levels, indicating their syndrome is the result of other causes. Two mutations in plasma protease zymogens that appear causative for HAE with normal C1 inhibitor activity have been identified in multiple families. Both appear to alter protease activity in a gain-of-function manner. Lysine or arginine substitutions for threonine 309 in factor XII introduces a new protease cleavage site that results in formation of a truncated factor XII protein (Δ-factor XII) that accelerates kallikrein-kinin system activity. A glutamic acid substitution for lysine 311 in the fibrinolytic protein plasminogen creates a consensus binding site for lysine/arginine side chains. The plasmin form of the variant plasminogen cleaves plasma kininogens to release bradykinin directly, bypassing the kallikrein-kinin system. Here we review work on the mechanisms of action of the FXII-Lys/Arg309 and Plasminogen-Glu311 variants, and discuss the clinical implications of these mechanisms.

A site on factor XII required for productive interactions with polyphosphate

BACKGROUND

During plasma contact activation, factor XII (FXII) binds to surfaces through its heavy chain and undergoes conversion to the protease FXIIa. FXIIa activates prekallikrein and factor XI (FXI). Recently, we showed that the FXII first epidermal growth factor-1 (EGF1) domain is required for normal activity when polyphosphate is used as a surface.

OBJECTIVES

The aim of this study was to identify amino acids in the FXII EGF1 domain required for polyphosphate-dependent FXII functions.

METHODS

FXII with alanine substitutions for basic residues in the EGF1 domain were expressed in HEK293 fibroblasts. Wild-type FXII (FXII-WT) and FXII containing the EGF1 domain from the related protein Pro-HGFA (FXII-EGF1) were positive and negative controls. Proteins were tested for their capacity to be activated, and to activate prekallikrein and FXI, with or without polyphosphate, and to replace FXII-WT in plasma clotting assays and a mouse thrombosis model.

RESULTS

FXII and all FXII variants were activated similarly by kallikrein in the absence of polyphosphate. However, FXII with alanine replacing Lys⁷³, Lys⁷⁴, and Lys⁷⁶ (FXII-Ala^73,74,76) or Lys⁷⁶, His⁷⁸, and Lys⁸¹ (FXII-Ala^76,78,81) were activated poorly in the presence of polyphosphate. Both have <5% of normal FXII activity in silica-triggered plasma clotting assays and have reduced binding affinity for polyphosphate. Activated FXIIa-Ala^73,74,76 displayed profound defects in surface-dependent FXI activation in purified and plasma systems. FXIIa-Ala^73,74,76 reconstituted FXII-deficient mice poorly in an arterial thrombosis model.

CONCLUSION

FXII Lys⁷³, Lys⁷⁴, Lys⁷⁶, and Lys⁸¹ form a binding site for polyanionic substances such as polyphosphate that is required for surface-dependent FXII function.

A mechanism for hereditary angioedema caused by a lysine 311-to-glutamic acid substitution in plasminogen

Patients with hereditary angioedema (HAE) experience episodes of bradykinin (BK)-induced swelling of skin and mucosal membranes. The most common cause is reduced plasma activity of C1 inhibitor, the main regulator of the proteases plasma kallikrein (PKa) and factor XIIa (FXIIa). Recently, patients with HAE were described with a Lys311 to glutamic acid substitution in plasminogen (Plg), the zymogen of the protease plasmin (Plm). Adding tissue plasminogen activator to plasma containing Plg-Glu311 vs plasma containing wild-type Plg (Plg-Lys311) results in greater BK generation. Similar results were obtained in plasma lacking prekallikrein or FXII (the zymogens of PKa and FXIIa) and in normal plasma treated with a PKa inhibitor, indicating Plg-Glu311 induces BK generation independently of PKa and FXIIa. Plm-Glu311 cleaves high and low molecular weight kininogens (HK and LK, respectively), releasing BK more efficiently than Plm-Lys311. Based on the plasma concentrations of HK and LK, the latter may be the source of most of the BK generated by Plm-Glu311. The lysine analog ε-aminocaproic acid blocks Plm-catalyzed BK generation. The Glu311 substitution introduces a lysine-binding site into the Plg kringle 3 domain, perhaps altering binding to kininogens. Plg residue 311 is glutamic acid in most mammals. Glu311 in patients with HAE, therefore, represents reversion to the ancestral condition. Substantial BK generation occurs during Plm-Glu311 cleavage of human HK, but not mouse HK. Furthermore, mouse Plm, which has Glu311, did not liberate BK from human kininogens more rapidly than human Plg-Lys311. This indicates Glu311 is pathogenic in the context of human Plm when human kininogens are the substrates.

A Common Missense Variant Causing Factor XI Deficiency and Increased Bleeding Tendency in Maine Coon Cats

Hereditary factor XI (FXI) deficiency is characterized as an autosomal mild to moderate coagulopathy in humans and domestic animals. Coagulation testing revealed FXI deficiency in a core family of Maine Coon cats (MCCs) in the United States. Factor XI-deficient MCCs were homozygous for a guanine to adenine transition resulting in a methionine substitution for the highly conserved valine-516 in the FXI catalytic domain. Immunoblots detected FXI of normal size and quantity in plasmas of MCCs homozygous for V516M. Some FXI-deficient MCCs experienced excessive post-operative/traumatic bleeding. Screening of 263 MCCs in Europe revealed a mutant allele frequency of 0.232 (23.2%). However, V516M was not found among 100 cats of other breeds. Recombinant feline FXI-M516 (fFXI-M516) expressed ~4% of the activity of wild-type fFXI-V516 in plasma clotting assays. Furthermore, fFXIa-M516 cleaved the chromogenic substrate S-2366 with ~4.3-fold lower catalytic efficacy (kcat/Km) than fFXIa-V516, supporting a conformational alteration of the protease active site. The rate of FIX activation by fFXIa-M516 was reduced >3-fold compared with fFXIa-V516. The common missense variant FXI-V516M causes a cross-reactive material positive FXI deficiency in MCCs that is associated with mild-moderate bleeding tendencies. Given the prevalence of the variant in MCCs, genotyping is recommended prior to invasive procedures or breeding.

The evolution of factor XI and the kallikrein-kinin system

Factor XI (FXI) is the zymogen of a plasma protease (FXIa) that contributes to hemostasis by activating factor IX (FIX). In the original cascade model of coagulation, FXI is converted to FXIa by factor XIIa (FXIIa), a component, along with prekallikrein and high-molecular-weight kininogen (HK), of the plasma kallikrein-kinin system (KKS). More recent coagulation models emphasize thrombin as a FXI activator, bypassing the need for FXIIa and the KKS. We took an evolutionary approach to better understand the relationship of FXI to the KKS and thrombin generation. BLAST searches were conducted for FXI, FXII, prekallikrein, and HK using genomes for multiple vertebrate species. The analysis shows the KKS appeared in lobe-finned fish, the ancestors of all land vertebrates. FXI arose later from a duplication of the prekallikrein gene early in mammalian evolution. Features of FXI that facilitate efficient FIX activation are present in all living mammals, including primitive egg-laying monotremes, and may represent enhancement of FIX-activating activity inherent in prekallikrein. FXI activation by thrombin is a more recent acquisition, appearing in placental mammals. These findings suggest FXI activation by FXIIa may be more important to hemostasis in primitive mammals than in placental mammals. FXI activation by thrombin places FXI partially under control of the vitamin K-dependent coagulation mechanism, reducing the importance of the KKS in blood coagulation. This would explain why humans with FXI deficiency have a bleeding abnormality, whereas those lacking components of the KKS do not.

Protease activity in single-chain prekallikrein

Prekallikrein (PK) is the precursor of the trypsin-like plasma protease kallikrein (PKa), which cleaves kininogens to release bradykinin and converts the protease precursor factor XII (FXII) to the enzyme FXIIa. PK and FXII undergo reciprocal conversion to their active forms (PKa and FXIIa) by a process that is accelerated by a variety of biological and artificial surfaces. The surface-mediated process is referred to as contact activation. Previously, we showed that FXII expresses a low level of proteolytic activity (independently of FXIIa) that may initiate reciprocal activation with PK. The current study was undertaken to determine whether PK expresses similar activity. Recombinant PK that cannot be converted to PKa was prepared by replacing Arg371 with alanine at the activation cleavage site (PK-R371A, or single-chain PK). Despite being constrained to the single-chain precursor form, PK-R371A cleaves high-molecular-weight kininogen (HK) to release bradykinin with a catalytic efficiency ∼1500-fold lower than that of kallikrein cleavage of HK. In the presence of a surface, PK-R371A converts FXII to FXIIa with a specific activity ∼4 orders of magnitude lower than for PKa cleavage of FXII. These results support the notion that activity intrinsic to PK and FXII can initiate reciprocal activation of FXII and PK in solution or on a surface. The findings are consistent with the hypothesis that the putative zymogens of many trypsin-like proteases are actually active proteases, explaining their capacity to undergo processes such as autoactivation and to initiate enzyme cascades.

Perlecan Domain V Inhibits Amyloid-β Induced Activation of the α2β1 Integrin-Mediated Neurotoxic Signaling Cascade

Alzheimer's disease (AD) is characterized by neuronal death, neurofibrillary tangles, and senile plaques. Amyloid-beta (Aβ) is the major component of plaques and consists of two prominent isoforms, Aβ40 and Aβ42. As many risk factors for AD are vascular in origin and blood vessel defects in clearing Aβ from the brain are a potential key component of AD pathology, we have focused on the neuron-blood vessel interface, and in particular, the vascular basement membrane, which coats blood vessels and physically separates them from neurons. A prominent component of the vascular basement membrane is the extracellular matrix proteoglycan perlecan. Domain V (DV) is the C-terminal domain and is generated by perlecan proteolysis. DV interacts with the α2 integrin and Aβ is a ligand for both α2β1 and αvβ1. Due to the known interaction of DV with α2β1 and α2β1's requirement for Aβ deposition and neurotoxicity, we hypothesized that DV and/or its C-terminal domain, LG3, might alter neurotoxic signaling pathways by directly blocking or otherwise interfering with α2β1 binding by Aβ. Our study suggests that α2β1 mediates Aβ-induced activation of c-Jun and caspase-3, key components of the neurotoxic pathway, in primary cortical and hippocampal neurons. We further demonstrate that DV and/or LG3 may therapeutically modulate these α2β1 mediated neurotoxic effects suggesting that they or other α2β1 integrin modulators could represent a novel approach to treat AD. Finally, our results suggest different neurotoxicity susceptibilities between cortical and hippocampal neurons to Aβ40 and Aβ42 as further underscored by differing neuroprotective potencies of LG3 in each cell type.

An update on factor XI structure and function

Factor XI (FXI) is the zymogen of a plasma protease, factor XIa (FXIa), that contributes to thrombin generation during blood coagulation by proteolytic activation of several coagulation factors, most notably factor IX (FIX). FXI is a homolog of prekallikrein (PK), a component of the plasma kallikrein-kinin system. While sharing structural and functional features with PK, FXI has undergone adaptive changes that allow it to contribute to blood coagulation. Here we review current understanding of the biology and enzymology of FXI, with an emphasis on structural features of the protein as they relate to protease function.

Nucleic acids as cofactors for factor XI and prekallikrein activation: Different roles for high-molecular-weight kininogen

The plasma zymogens factor XI (fXI) and prekallikrein (PK) are activated by factor XIIa (fXIIa) during contact activation. Polyanions such as DNA and RNA may contribute to thrombosis and inflammation partly by enhancing PK and fXI activation. We examined PK and fXI activation in the presence of nucleic acids, and determine the effects of the cofactor high molecular weight kininogen (HK) on the reactions. In the absence of HK, DNA and RNA induced fXI autoactivation. Proteases known to activate fXI (fXIIa and thrombin) did not enhance this process appreciably. Nucleic acids had little effect on PK activation by fXIIa in the absence of HK. HK had significant but opposite effects on PK and fXI activation. HK enhanced fXIIa activation of PK in the presence of nucleic acids, but blocked fXI autoactivation. Thrombin and fXIIa could overcome the HK inhibitory effect on autoactivation, indicating these proteases are necessary for nucleic acid-induced fXI activation in an HK-rich environment such as plasma. In contrast to PK, which requires HK for optimal activation, fXI activation in the presence of nucleic acids depends on anion binding sites on the fXI molecule. The corresponding sites on PK are not necessary for PK activation. Our results indicate that HK functions as a cofactor for PK activation in the presence of nucleic acids in a manner consistent with classic models of contact activation. However, HK has, on balance, an inhibitory effect on nucleic acid-supported fXI activation and may function as a negative regulator of fXI activation.

A comparison of the effects of factor XII deficiency and prekallikrein deficiency on thrombus formation

Studies with animal models implicate the plasma proteases factor XIIa (FXIIa) and α-kallikrein in arterial and venous thrombosis. As congenital deficiencies of factor XII (FXII) or prekallikrein (PK), the zymogens of FXIIa and α-kallikrein respectively, do not cause bleeding disorders, inhibition of these enzymes may have therapeutic benefit without compromising hemostasis. The relative contributions of FXIIa and α-kallikrein to thrombosis in animal models are not clear. We compared mice lacking FXII or PK to wild type mice in established models of arterial thrombosis. Wild type mice developed carotid artery occlusion when the vessel was exposed to a 3.5% solution of ferric chloride (FeCl3). FXII-deficient mice were resistant to occlusion at 5% FeCl3 and partially resistant at 10% FeCl3. PK-deficient mice were resistant at 3.5% FeCl3 and partially resistant at 5% FeCl3. Mice lacking high molecular weight kininogen, a cofactor for PK activation and activity, were also partially resistant to thrombosis at 5% FeCl3. Induction of carotid artery thrombosis with Rose Bengal was delayed in FXII-deficient mice compared to wild type or PK-deficient animals. In human plasma supplemented with silica, DNA or collagen to induce contact activation, an antibody to the FXIIa active site was more effective at preventing thrombin generation than an antibody to the α-kallikrein active site. Similarly, the FXIIa antibody was more effective at reducing fibrin formation in human blood flowing through collagen coated-tubes. The findings suggest that inhibitors of FXIIa will have more potent anti-thrombotic effects than inhibitors of α-kallikrein.

α2β1 integrin, GPVI receptor, and common FcRγ chain on mouse platelets mediate distinct responses to collagen in models of thrombosis

Objective

Platelets express the α2β1 integrin and the glycoprotein VI (GPVI)/FcRγ complex, both collagen receptors. Understanding platelet-collagen receptor function has been enhanced through use of genetically modified mouse models. Previous studies of GPVI/FcRγ-mediated collagen-induced platelet activation were perfomed with mice in which the FcRγ subunit was genetically deleted (FcRγ^−/−) or the complex was depleted. The development of α2β1^−/− and GPVI^−/− mice permits side-by-side comparison to address contributions of these collagen receptors in vivo and in vitro.

Approach and Results

To understand the different roles played by the α2β1 integrin, the GPVI receptor or FcRγ subunit in collagen-stimulated hemostasis and thrombosis, we compared α2β1^−/−, FcRγ^−/−, and GPVI^−/− mice in models of endothelial injury and intravascular thrombosis in vivo and their platelets in collagen-stimulated activation in vitro. We demonstrate that both the α2β1 integrin and the GPVI receptor, but not the FcRγ subunit influence carotid artery occlusion in vivo. In contrast, the GPVI receptor and the FcRγ chain, but not the α2β1 integrin, play similar roles in intravascular thrombosis in response to soluble Type I collagen. FcRγ^−/− platelets showed less attenuation of tyrosine phosphorylation of several proteins including RhoGDI when compared to GPVI^−/− and wild type platelets. The difference between FcRγ^−/− and GPVI^−/− platelet phosphotyrosine levels correlated with the in vivo thrombosis findings.

Conclusion

Our data demonstrate that genetic deletion of GPVI receptor, FcRγ chain, or the α2β1 integrin changes the thrombotic potentials of these platelets to collagen dependent on the stimulus mechanism. The data suggest that the FcRγ chain may provide a dominant negative effect through modulating signaling pathways in platelets involving several tyrosine phosphorylated proteins such as RhoGDI. In addition, these findings suggest a more complex signaling network downstream of the platelet collagen receptors than previously appreciated.

Mitigation of oxygen-induced retinopathy in α2β1 integrin-deficient mice

PURPOSE

The α2β1 integrin plays an important but complex role in angiogenesis and vasculopathies. Published GWAS studies established a correlation between genetic polymorphisms of the α2β1 integrin gene and incidence of diabetic retinopathy. Recent studies indicated that α2-null mice demonstrate superior vascularization in both the wound and diabetic microenvironments. The goal of this study was to determine whether the vasculoprotective effects of α2-integrin deficiency extended to the retina, using the oxygen-induced retinopathy (OIR) model for retinopathy of prematurity (ROP).

METHODS

In the OIR model, wild-type (WT) and α2-null mice were exposed to 75% oxygen for 5 days (postnatal day [P] 7 to P12) and subsequently returned to room air for 6 days (P12-P18). Retinas were collected at postnatal day 7, day 13, and day 18 and examined via hematoxylin and eosin and Lectin staining. Retinas were analyzed for retinal vascular area, neovascularization, VEGF expression, and Müller cell activation. Primary Müller cell cultures from WT and α2-null mice were isolated and analyzed for hypoxia-induced VEGF-A expression.

RESULTS

In the retina, the α2β1 integrin was minimally expressed in endothelial cells and strongly expressed in activated Müller cells. Isolated α2-null primary Müller cells demonstrated decreased hypoxia-induced VEGF-A expression. In the OIR model, α2-null mice displayed reduced hyperoxia-induced vaso-attenuation, reduced pathological retinal neovascularization, and decreased VEGF expression as compared to WT counterparts.

CONCLUSIONS

Our data suggest that the α2β1 integrin contributes to the pathogenesis of retinopathy. We describe a newly identified role for α2β1 integrin in mediating hypoxia-induced Müller cell VEGF-A production.

Suboptimal activation of protease-activated receptors enhances alpha2beta1 integrin-mediated platelet adhesion to collagen

Thrombin and fibrillar collagen are potent activators of platelets at sites of vascular injury. Both agonists cause platelet shape change, granule secretion, and aggregation to form the primary hemostatic plug. Human platelets express two thrombin receptors, protease-activated receptors 1 and 4 (PAR1 and PAR4) and two collagen receptors, the alpha(2)beta(1) integrin (alpha(2)beta(1)) and the glycoprotein VI (GPVI)/FcRgamma chain complex. Although these receptors and their signaling mechanisms have been intensely studied, it is not known whether and how these receptors cooperate in the hemostatic function of platelets. This study examined cooperation between the thrombin and collagen receptors in platelet adhesion by utilizing a collagen-related peptide (alpha2-CRP) containing the alpha(2)beta(1)-specific binding motif, GFOGER, in conjunction with PAR-activating peptides. We demonstrate that platelet adhesion to alpha2-CRP is substantially enhanced by suboptimal PAR activation (agonist concentrations that do not stimulate platelet aggregation) using the PAR4 agonist peptide and thrombin. The enhanced adhesion induced by suboptimal PAR4 activation was alpha(2)beta(1)-dependent and GPVI/FcRgamma-independent as revealed in experiments with alpha(2)beta(1)- or FcRgamma-deficient mouse platelets. We further show that suboptimal activation of other platelet G(q)-linked G protein-coupled receptors (GPCRs) produces enhanced platelet adhesion to alpha2-CRP. The enhanced alpha(2)beta(1)-mediated platelet adhesion is controlled by phospholipase C (PLC), but is not dependent on granule secretion, activation of alpha(IIb)beta(3) integrin, or on phosphoinositol-3 kinase (PI3K) activity. In conclusion, we demonstrate a platelet priming mechanism initiated by suboptimal activation of PAR4 or other platelet G(q)-linked GPCRs through a PLC-dependent signaling cascade that promotes enhanced alpha(2)beta(1) binding to collagens containing GFOGER sites.

Beta1 integrin mediates internalization of mammalian reovirus

Reovirus infection is initiated by interactions between the attachment protein sigma1 and cell surface carbohydrate and junctional adhesion molecule A (JAM-A). Expression of a JAM-A mutant lacking a cytoplasmic tail in nonpermissive cells conferred full susceptibility to reovirus infection, suggesting that cell surface molecules other than JAM-A mediate viral internalization following attachment. The presence of integrin-binding sequences in reovirus outer capsid protein lambda2, which serves as the structural base for sigma1, suggests that integrins mediate reovirus endocytosis. A beta1 integrin-specific antibody, but not antibodies specific for other integrin subunits, inhibited reovirus infection of HeLa cells. Expression of a beta1 integrin cDNA, along with a cDNA encoding JAM-A, in nonpermissive chicken embryo fibroblasts conferred susceptibility to reovirus infection. Infectivity of reovirus was significantly reduced in beta1-deficient mouse embryonic stem cells in comparison to isogenic cells expressing beta1. However, reovirus bound equivalently to cells that differed in levels of beta1 expression, suggesting that beta1 integrins are involved in a postattachment entry step. Concordantly, uptake of reovirus virions into beta1-deficient cells was substantially diminished in comparison to viral uptake into beta1-expressing cells. These data provide evidence that beta1 integrin facilitates reovirus internalization and suggest that viral entry occurs by interactions of reovirus virions with independent attachment and entry receptors on the cell surface.

Novel collectin/C1q receptor mediates mast cell activation and innate immunity

Mast cells play a critical role in innate immunity, allergy, and autoimmune diseases. The receptor/ligand interactions that mediate mast cell activation are poorly defined. The alpha2beta1 integrin, a receptor for collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1 (MMP-1), endorepellin, and several viruses, has been implicated in normal developmental, inflammatory, and oncogenic processes. We recently reported that alpha2 integrin subunit-deficient mice exhibited markedly diminished neutrophil and IL-6 responses during Listeria monocytogenes- and zymosan-induced peritonitis. Peritoneal mast cells require alpha2beta1 integrin expression for activation in response to pathogens, yet the ligand and molecular mechanisms by which the alpha2beta1 integrin induces activation and cytokine secretion remain unknown. We now report that the alpha2beta1 integrin is a novel receptor for multiple collectins and the C1q complement protein. We demonstrate that the alpha2beta1 integrin provides a costimulatory function required for mast cell activation and cytokine secretion. This finding suggests that the alpha2beta1 integrin is not only important for innate immunity but may serve as a critical target for the regulation of autoimmune/allergic disorders.

Structural Analysis of the α2 Integrin I Domain/Procollagenase-1 (Matrix Metalloproteinase-1) Interaction

Previous studies have established that ligation of keratinocyte alpha(2)beta(1) integrin by type I collagen induces expression of matrix metalloproteinase-1 (MMP-1) and that MMP-1 activity is required for the alpha(2)beta(1) integrin-dependent migration of primary keratinocytes across collagenous matrices. We now present evidence that MMP-1 binds the alpha(2)beta(1) integrin via the I domain of the alpha(2) integrin subunit. Using an enzyme-linked immunosorbent assay with purified human MMP-1 and recombinant alpha(2) integrin I domain, we showed that the alpha(2) integrin I domain specifically bound in a divalent cation-dependent manner to both the pro and active forms of MMP-1, but not to MMP-3 or MMP-13. Although both the I domain and MMP-1 bind divalent cations, MMP-1 bound, in a divalent cation-dependent manner, to alpha(2) integrin I domains containing metal ion-dependent adhesion sites motif mutations that prevent divalent cation binding to the I domain, demonstrating that the metal ion dependence is a function of MMP-1. Using a series of MMP-1-MMP-3 and MMP-1-MMP-13 chimeras, we determined that both the linker domain and the hemopexin-like domain of MMP-1 were required for optimal binding to the I domain. The alpha(2) integrin/MMP-1 interaction described here extends an emerging paradigm in matrix biology involving anchoring of proteinases to the cell surface to regulate their biological activities.

Pro-collagenase-1 (Matrix Metalloproteinase-1) Binds the α2β1 Integrin upon Release from Keratinocytes Migrating on Type I Collagen

In injured skin, collagenase-1 (matrix metalloproteinase-1 (MMP-1)) is induced in migrating keratinocytes. This site-specific expression is regulated by binding of the alpha(2)beta(1) integrin with dermal type I collagen, and the catalytic activity of MMP-1 is required for keratinocyte migration. Because of this functional association among substrate/ligand, receptor, and proteinase, we assessed whether the integrin also directs the compartmentalization of MMP-1 to its matrix target. Indeed, pro-MMP-1 co-localized to sites of alpha(2)beta(1) contacts in migrating keratinocytes. Furthermore, pro-MMP-1 co-immunoprecipitated with alpha(2)beta(1) from keratinocytes, and alpha(2)beta(1) co-immunoprecipitated with pro-MMP-1. No other MMPs bound alpha(2)beta(1), and no other integrins interacted with MMP-1. Pro-MMP-1 also provided a substrate for alpha(2)beta(1)-dependent adhesion of platelets. Complex formation on keratinocytes was most efficient on native type I collagen and reduced or ablated on denatured or cleaved collagen. Competition studies suggested that the alpha(2) I domain interacts with the linker and hemopexin domains of pro-MMP-1, not with the pro-domain. These data indicate that the interaction of pro-MMP-1 with alpha(2)beta(1) confines this proteinase to points of cell contact with collagen and that the ternary complex of integrin, enzyme, and substrate function together to drive and regulate keratinocyte migration.

Echoviruses 1 and 8 are closely related genetically, and bind to similar determinants within the VLA-2 I domain

Echoviruses (EV) 1 and 8 were originally considered to be distinct serotypes, but more recently have been considered strains of the same virus. In experiments with chimeric recombinant fusion proteins, both viruses bound to the I domain of the integrin VLA-2, and both required the same receptor residues for attachment. A full-length, infectious cDNA clone encoding EV1 was obtained; its nucleotide sequence was determined, as were the sequences encoding the EV8 capsid. EV1 and 8 show 94% amino acid identity within the capsid region and are more similar to each other than to any other human picornavirus.

Extracellular FGF-1 inhibits cytoskeletal organization and promotes fibroblast motility

Previous efforts from this laboratory have established that acidic fibroblast growth factor (FGF-1), either added exogenously or secreted as a biologically active protein, induces a transformed phenotype in primary murine fibroblasts. Experimental studies described here demonstrate that constitutive exposure to extracellular FGF-I results in reduced cell attachment to multiple ligands, inhibition of cytoskeletal organization, and reduced collagen contraction, despite no detectable change in integrin cell surface expression. In addition, FGF-1-transduced fibroblasts demonstrated a > 10-fold increase in migration, an observation correlated with increased tyrosine phosphorylation of p125FAK and p130CAS. Collectively, these results suggest that FGF-1-induced fibroblast transformation includes the involvement of specific FGF receptor-mediated signal transduction cascades targeted to cytoskeletal and focal adhesion structures.

Determinants of ligand binding specificity of the alpha(1)beta(1) and alpha(2)beta(1) integrins

The alpha(1)beta(1) and alpha(2)beta(1) integrins are cell surface collagen receptors. Cells expressing the alpha(1)beta(1) integrin preferentially adhere to collagen IV, whereas cells expressing the alpha(2)beta(1) integrin preferentially adhere to collagen I. Recombinant alpha(1) and alpha(2) integrin I domains exhibit the same collagen type preferences as the intact integrins. In addition, the alpha(2) integrin I domain binds echovirus 1; the alpha(1) I domain does not. To identify the structural components of the I domains responsible for the varying ligand specificities, we have engineered several alpha(1)/alpha(2) integrin I domain chimeras and evaluated their virus and collagen binding activities. Initially, large secondary structural components of the alpha(2) I domain were replaced with corresponding regions of the alpha(1) I domain. Following analysis in echovirus 1 and collagen binding assays, chimeras with successively smaller regions of alpha(1) I were constructed and analyzed. The chimeras were analyzed by ELISA with several different alpha(2) integrin monoclonal antibodies to assess their proper folding. Three different regions of the alpha(1) I domain, when present in the alpha(2) I domain, conferred enhanced collagen IV binding activity upon the alpha(2) I domain. These include the alpha3 and alpha5 helices and a portion of the alpha6 helix. Echovirus 1 binding was lost in a chimera containing the alphaC-alpha6 loop; higher resolution mapping identified Asn(289) as playing a critical role in echovirus 1 binding. Asn(289) had not been implicated in previous echovirus 1 binding studies. Taken together, these data reveal the existence of multiple determinants of ligand binding specificities within the alpha(1) and alpha(2) integrin I domains.

Collagen receptor control of epithelial morphogenesis and cell cycle progression

To define the unique contributions of the alpha subunit cytoplasmic tails of the alpha(1)beta(1) and alpha(2)beta(1) integrin to epithelial differentiation and branching morphogenesis, a variant NMuMG cell line lacking alpha(1)beta(1) and alpha(2)beta(1) integrin expression was stably transfected with the full-length alpha(2) integrin subunit cDNA (X2C2), chimeric cDNA consisting of the extracellular and transmembrane domains of the alpha(2) subunit and the cytoplasmic domain of the alpha(1) subunit (X2C1), or alpha(2) cDNA truncated after the GFFKR sequence (X2C0). The X2C2 and X2C1 transfectants effectively adhered, spread, and formed focal adhesion complexes on type I collagen matrices. The X2C0 transfectants were less adherent to low concentrations of type I collagen, spread less well, and formed poorly defined focal adhesion complexes in comparison to the X2C2 and X2C1 transfectants. The X2C2 and X2C1 transfectants but not the X2C0 transfectants proliferated on collagen substrates. Only the X2C2 transfectants developed elongate branches and tubules in three-dimensional collagen gels and migrated on type I collagen. These findings suggest a unique role for the alpha(2) integrin cytoplasmic domain in postligand binding events and cooperative interactions with growth factors that mediate epithelial differentiation and branching morphogenesis. Either intact alpha(1) or alpha(2) integrin subunit cytoplasmic domain can promote cell cycle progression.

Ligand binding results in divalent cation displacement from the alpha 2 beta 1 integrin I domain: evidence from terbium luminescence spectroscopy

The alpha 2 beta 1 integrin serves as a cell surface collagen or collagen/laminin receptor. Binding of the integrin to its ligands is largely mediated by the alpha 2 subunit I domain and requires the presence of divalent cations. Terbium ion (Tb3+), a fluorescent trivalent cation that often binds divalent cation-binding sites on proteins, supported binding of the I domain to collagen with half-maximal binding occurring at 5.2 +/- 1.7 microM Tb3+. By fluorescence resonance energy transfer spectroscopy, Tb3+ showed specific and saturable binding to the recombinant I domain with a Kd of 27 +/- 4 microM. Although both Mg2+ and Mn2+ were capable of quenching Tb3+ fluorescence, Mn2+ was much more effective than Mg2+. The alpha 2 beta 1 integrin also binds the pro-alpha 1(I) collagen carboxyl-terminal propeptide in a Mg2+-dependent manner via the I domain. Recombinant propeptide was used to examine the effect of ligand on the Tb3+ binding properties of the alpha 2 integrin I domain. As propeptide bound to the I domain, Tb3+ fluorescence progressively diminished suggesting that as ligand binds to the I domain, either Tb3+ is displaced or its fluorescence is quenched. Consistent with the former possibility, little dissociation of collagen-bound I domain occurred upon the addition of EDTA and subsequent incubation. These data support a model in which (1) the divalent cation is required for initial ligand-binding activity of the I domain and (2) ligand binding results in subsequent metal ion displacement to generate a metal-free I domain-ligand complex.

Alpha2beta1 integrin recognition of the carboxyl-terminal propeptide of type I procollagen: integrin recognition and feed-back regulation of matrix biosynthesis are mediated by distinct sequences

It has recently been established that the carboxyl-terminal propeptide of type I collagen exert a feedback regulatory effect on extracellular matrix biosynthesis and that the propeptide bind to the alpha2beta1 integrin. This raises the intriguing hypothesis that the regulatory propeptide sequences exert their effects as a consequence of binding to the integrin. We show that recombinant alpha1(I) collagen chain C-terminal propeptide contains a binding site for the intact alpha2beta1 integrin and for a recombinant alpha2 integrin I domain, but not for the alpha1beta1 integrin, a structurally and functionally related collagen/laminin receptor. Additional studies employing a series of recombinant N-terminal and C-terminal deletion mutants, internal fragments of the propeptide, synthetic peptides, recombinant alpha2 integrin I domain and inhibitory monoclonal antibodies established that the previously identified sequences within the alpha1(I) C-terminal propeptide that mediate regulation of matrix biosynthesis are neither necessary nor sufficient for alpha2beta1 integrin binding. In contrast, the integrin recognition site is composed of a conformationally complex determinant located within a structurally distinct 115 amino acid region of the propeptide.

Ligand recognition by the I domain-containing integrins

Seven of the integrin alpha subunits described to date, alpha 1, alpha 2, alpha L, alpha X, alpha d, alpha M and alpha E, contain a highly conserved I (or A) domain of approximately 200 amino acid residues inserted near the amino-terminus of the subunit. As the result of a variety of independent experimental approaches, a large body of data has recently accumulated that indicates that the I domains are independent, autonomously folding domains capable of directly binding ligands that play a necessary and important role in ligand binding by the intact integrins. Recent crystallographic studies have elucidated the structures of recombinant alpha M and alpha L I domains and also delineated a novel divalent cation-binding motif within the I domains (metal ion-dependent adhesion site, MIDAS) that appears to mediate the divalent cation binding of the I domains and the I domain-containing integrins to their ligands.

Binding of the alpha 2 integrin I domain to extracellular matrix ligands: structural and mechanistic differences between collagen and laminin binding

The alpha 2 beta 1 integrin functions as a cell surface receptor for collagen on some cells and as both a collagen and laminin receptor on a more restricted subset of cell types including endothelial and epithelial cells. The alpha 2 integrin subunit I domain binds collagen in a divalent cation-dependent manner. In contrast, I domain binding to laminin occurs via both divalent cation-dependent and -independent mechanisms. Saturable binding was observed in the presence of either Mn2+ or EDTA, although the extent of binding in Mn2+ was twice that observed in EDTA. Half-maximal binding occurred at about 22 nM I domain in either case. Whereas laminin binding was significantly enhanced by Mn2+, with half-maximal binding occurring at 1.9 mM Mn2+, Mg2+ was much less effective. Deletion of the N-terminal 35 residues of the I domain, including the DXSXS portion of the MIDAS motif, caused a significant diminution of laminin binding activity. Laminin binding by the I domain was significantly inhibited by the alpha 2 beta 1 function-blocking antibody 6F1 in the presence of either EDTA or Mn2+. The non-function-blocking antibody 12F1 had no effect. In contrast to the binding of the alpha 2 integrin I domain to collagen, the laminin binding activity of the I domain was not enhanced by the addition of the first EF hand motif of the integrin.

Contributions of the I and EF hand domains to the divalent cation-dependent collagen binding activity of the alpha2beta1 integrin

The alpha2beta1 integrin binds collagen in a Mg2+-dependent manner that is inhibited by Ca2+. Like the intact integrin, purified recombinant proteins containing the alpha2 integrin I domain, either alone or with variable numbers of alpha2 integrin EF hand metal binding sites, bound collagen in a Mg2+-dependent manner, and Ca2+ did not support binding. However, unlike the intact integrin, Ca2+ did not inhibit the Mg2+-dependent binding of any of the fusion proteins to collagen. Binding to collagen was saturable and blocked by the alpha2beta1 function blocking antibody 6F1. Deletional analysis demonstrated that residues present within the amino-terminal 35 amino acids contribute to the 6F1 epitope and are required for Mg2+-dependent collagen binding. The results indicate that the I domain contains a Mg2+ binding site that is essential for collagen binding and that the I domain alone is sufficient for collagen binding. Binding is markedly enhanced in a divalent cation-dependent manner by the addition of the first EF hand motif. Mutation of the EF hand to an inactive form completely abrogated the effect. The sites necessary for Ca2+ inhibition are not present within the I domain or the adjacent region containing the three EF hand sites.

Sequence of a human cDNA encoding phosphatidylinositol transfer protein and occurrence of a related sequence in widely divergent eukaryotes

Phosphatidylinositol (PtdIns) transfer protein (PtdInsTP) is a phospholipid transfer protein that has been detected in all mammalian tissues examined. It catalyzes the transfer in vitro of PtdIns and phosphatidylcholine between membranes in a number of natural and artificial membrane systems and may be involved in secretion in vivo. In previous studies, we isolated and sequenced a cDNA encoding a rat PtdInsTP. A rat cDNA probe was used to isolate clones from a lambda gt11 human testis cDNA library which encoded full-length human PtdInsTP. The cDNA sequence defines a 270-amino-acid, 31.8-kDa protein whose sequence shares 98.9% identity to that of rat, making it one of the most conserved proteins known between the two species. DNA blot hybridization studies suggest that there may be more than one gene encoding this protein in humans. A comparison of rat and human PtdInsTP cDNAs revealed strong sequence similarity (88 and 84%) in portions of the corresponding 5'- and 3'-untranslated regions (UTR) of the rat and human mRNAs.

Isolation and sequence of cDNA clones encoding rat phosphatidylinositol transfer protein

Phosphatidylinositol (PtdIns) transfer protein is a cytosolic protein that catalyzes the transfer of PtdIns between membranes. It is expressed in organisms from yeast to man, and activity has been found in all animal tissues examined. Using antibodies prepared against bovine brain PtdIns transfer protein, lambda gt11 rat brain cDNA libraries were screened and several clones isolated. DNA sequence analysis showed that the cDNAs encoded a polypeptide of 271 amino acids with a mass of 31,911 Da. Comparison of the deduced amino acid sequence with N-terminal sequence data obtained for the intact purified bovine brain protein and rat lung phospholipid transfer protein verified that the cDNAs were PtdIns transfer protein clones. The predicted protein shows no significant sequence similarity to other known (phospholipid)-binding proteins. DNA blot hybridization suggests that the rat genome may contain more than one gene encoding PtdIns transfer protein. RNA blot hybridization reveals that the PtdIns transfer protein gene is expressed at low levels in a wide variety of rat tissues; all tissues examined showed a major mRNA component of 1.9 kilobases and a minor component of 3.4 kilobases. The isolation of clones encoding rat PtdIns transfer protein will greatly facilitate studies of the structure and function of PtdIns transfer proteins and their role in lipid metabolism.