Immunopurification and protease inhibitory properties of protease nexin-2/amyloid beta-protein precursor

Abnormal clotting of the intrinsic/contact pathway in Alzheimer disease patients is related to cognitive ability

Alzheimer disease (AD) is a neurodegenerative disorder characterized by extracellular β-amyloid (Aβ) deposition. Although peripheral inflammation and cerebrovascular pathology are reported in AD, there is a lack of plasma biomarkers in this field. Because the contact system is triggered in patient plasma, we hypothesized that the hemostasis profile could be a novel biomarker in AD. Here, we assessed the clotting profile in plasma from AD patients and age-matched controls. Utilizing clinically relevant assays, thromboelastography and activated partial thromboplastin time, we found impaired clot initiation and formation rate in AD patient plasma. These coagulation end points correlated with cerebrospinal fluid neurofilament-light levels and cognition and were more profound in younger AD patients. Ex vivo intrinsic clotting of plasma from AD mice expressing human amyloid precursor protein (APP) was also delayed in an age-dependent manner, suggesting that this phenotype is related to APP, the parent protein of Aβ. Further analysis of coagulation factors in human plasma indicated that endogenous inhibitor(s) of factors XII and XI in AD plasma contribute to this delayed clotting. Together, these data suggest that delayed clotting in young AD patients is a novel biomarker and that therapies aimed to correct this phenotype might be beneficial in this patient population. Follow-up studies in additional AD patient cohorts are warranted to further evaluate these findings.

The intact Kunitz domain protects the amyloid precursor protein from being processed by matriptase-2

Proteolytic processing of the amyloid precursor protein (APP) leads to amyloid-β (Aβ) peptides. So far, the mechanism of APP processing is insufficiently characterized at the molecular level. Whereas the knowledge of Aβ generation by several proteases has been expanded, the contribution of the Kunitz-type protease inhibitor domain (KPI) present in two major APP isoforms to the complex proteolytic processing of APP is poorly understood. In this study, we have identified KPI-containing APP as a very potent, slow-binding inhibitor for the membrane-bound proteolytic regulator of iron homeostasis matriptase-2 by forming stable complexes with its target protease in HEK cells. Inhibition and complex formation depend on the intact KPI domain. By inhibiting matriptase-2, KPI-containing APP is protected from matriptase-2-mediated proteolysis within the Aβ region, thus preventing the generation of N-terminally truncated Aβ.

Mutation of the Kunitz-type proteinase inhibitor domain in the amyloid β-protein precursor abolishes its anti-thrombotic properties in vivo

INTRODUCTION

Kunitz proteinase inhibitor (KPI) domain-containing forms of the amyloid β-protein precursor (AβPP) inhibit cerebral thrombosis. KPI domain-lacking forms of AβPP are abundant in brain. Regions of AβPP other than the KPI domain may also be involved with regulating cerebral thrombosis. To determine the contribution of the KPI domain to the overall function of AβPP in regulating cerebral thrombosis we generated a reactive center mutant that was devoid of anti-thrombotic activity and studied its anti-thrombotic function in vitro and in vivo.

METHODS

To determine the extent of KPI function of AβPP in regulating cerebral thrombosis we generated a recombinant reactive center KPI^R13I mutant devoid of anti-thrombotic activity. The anti-proteolytic and anti-coagulant properties of wild-type and R13I mutant KPI were investigated in vitro. Cerebral thrombosis of wild-type, AβPP knock out and AβPP/KPI^R13I mutant mice was evaluated in experimental models of carotid artery thrombosis and intracerebral hemorrhage.

RESULTS

Recombinant mutant KPI^R13I domain was ineffective in the inhibition of pro-thrombotic proteinases and did not inhibit the clotting of plasma in vitro. AβPP/KPI^R13I mutant mice were similarly deficient as AβPP knock out mice in regulating cerebral thrombosis in experimental models of carotid artery thrombosis and intracerebral hemorrhage.

CONCLUSIONS

We demonstrate that the anti-thrombotic function of AβPP primarily resides in the KPI activity of the protein.

Cerebral haemostasis and antiphospholipid antibodies

One of the major clinical concerns of the antiphospholipid syndrome (APS) is the propensity of antiphospholipid(aPL) antibodiesto cause thrombosis in both the large and small vessels of the brain. In this article, we review the current understandingof haemostasis in cerebral circulation and discuss this in the context of antiphospholipidantibodies. The systemic-defect-local-phenotypeparadox is of particular importance in this discussion. In this paradigm, a systemic defect in thrombosis and haemostasis leads to a localized pattern of thrombotic disease because the regional physiological variations in the several prothromboticand anticoagulantfactors and the defect interact so as to favour thrombosis at a particular site. One possible mechanism of initiation of thrombosis in APS is the activation of endothelialcells by aPL that could occur in the cerebral vessels and provoke thrombosis. We review the evidence from gene knockout mice, other animal models and human postmortem examination studies as to which pro- and antithrombotic mechanisms are effecting haemostasis in the cerebral circulation. We conclude that there are large deficits in the understanding of the regulation of haemostasis in the human brain. As a consequencethere is a lack of knowledgeabout the effect of aPL on cerebral endothelium and thrombosis. Recent developments in gene expression profiling may have an impact on our understandingof endothelialfunctionin the brain. More research is required.

The Alzheimer's disease peptide β-amyloid promotes thrombin generation through activation of coagulation factor XII

Essentials How the Alzheimer's disease (AD) peptide β-amyloid (Aβ) disrupts neuronal function in the disease is unclear. Factor (F) XII initiates blood clotting via FXI, and thrombosis has been implicated in AD. Aβ triggers FXII-dependent FXI and thrombin activation, evidence of which is seen in AD plasma. Aβ-triggered clotting could contribute to neuronal dysfunction in AD and be a novel therapeutic target.

SUMMARY

Background β-Amyloid (Aβ) is a key pathologic element in Alzheimer's disease (AD), but the mechanisms by which it disrupts neuronal function in vivo are not completely understood. AD is characterized by a prothrombotic state, which could contribute to neuronal dysfunction by affecting cerebral blood flow and inducing inflammation. The plasma protein factor XII triggers clot formation via the intrinsic coagulation cascade, and has been implicated in thrombosis. Objectives To investigate the potential for Aβ to contribute to a prothrombotic state. Methods and results We show that Aβ activates FXII, resulting in FXI activation and thrombin generation in human plasma, thereby establishing Aβ as a possible driver of prothrombotic states. We provide evidence for this process in AD by demonstrating decreased levels of FXI and its inhibitor C1 esterase inhibitor in AD patient plasma, suggesting chronic activation, inhibition and clearance of FXI in AD. Activation of the intrinsic coagulation pathway in AD is further supported by elevated fibrin levels in AD patient plasma. Conclusions The ability of Aβ to promote coagulation via the FXII-driven contact system identifies new mechanisms by which it could contribute to neuronal dysfunction and suggests potential new therapeutic targets in AD.

A multi-factorial analysis of response to warfarin in a UK prospective cohort

Background

Warfarin is the most widely used oral anticoagulant worldwide, but it has a narrow therapeutic index which necessitates constant monitoring of anticoagulation response. Previous genome-wide studies have focused on identifying factors explaining variance in stable dose, but have not explored the initial patient response to warfarin, and a wider range of clinical and biochemical factors affecting both initial and stable dosing with warfarin.

Methods

A prospective cohort of 711 patients starting warfarin was followed up for 6 months with analyses focusing on both non-genetic and genetic factors. The outcome measures used were mean weekly warfarin dose (MWD), stable mean weekly dose (SMWD) and international normalised ratio (INR) > 4 during the first week. Samples were genotyped on the Illumina Human610-Quad chip. Statistical analyses were performed using Plink and R.

Results

VKORC1 and CYP2C9 were the major genetic determinants of warfarin MWD and SMWD, with CYP4F2 having a smaller effect. Age, height, weight, cigarette smoking and interacting medications accounted for less than 20 % of the variance. Our multifactorial analysis explained 57.89 % and 56.97 % of the variation for MWD and SMWD, respectively. Genotypes for VKORC1 and CYP2C9*3, age, height and weight, as well as other clinical factors such as alcohol consumption, loading dose and concomitant drugs were important for the initial INR response to warfarin. In a small subset of patients for whom data were available, levels of the coagulation factors VII and IX (highly correlated) also played a role.

Conclusion

Our multifactorial analysis in a prospectively recruited cohort has shown that multiple factors, genetic and clinical, are important in determining the response to warfarin. VKORC1 and CYP2C9 genetic polymorphisms are the most important determinants of warfarin dosing, and it is highly unlikely that other common variants of clinical importance influencing warfarin dosage will be found. Both VKORC1 and CYP2C9*3 are important determinants of the initial INR response to warfarin. Other novel variants, which did not reach genome-wide significance, were identified for the different outcome measures, but need replication.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-015-0255-y) contains supplementary material, which is available to authorized users.

The influence of the amyloid ß-protein and its precursor in modulating cerebral hemostasis

Ischemic and hemorrhagic strokes are a significant cause of brain injury leading to vascular cognitive impairment and dementia (VCID). These deleterious events largely result from disruption of cerebral hemostasis, a well-controlled and delicate balance between thrombotic and fibrinolytic pathways in cerebral blood vessels and surrounding brain tissue. Ischemia and hemorrhage are both commonly associated with cerebrovascular deposition of amyloid ß-protein (Aß). In this regard, Aß directly and indirectly modulates cerebral thrombosis and fibrinolysis. Further, major isoforms of the Aß precursor protein (AßPP) function as a potent inhibitor of pro-thrombotic proteinases. The purpose of this review article is to summarize recent research on how cerebral vascular Aß and AßPP influence cerebral hemostasis. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia, edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Mesotrypsin Has Evolved Four Unique Residues to Cleave Trypsin Inhibitors as Substrates

Human mesotrypsin is highly homologous to other mammalian trypsins, and yet it is functionally unique in possessing resistance to inhibition by canonical serine protease inhibitors and in cleaving these inhibitors as preferred substrates. Arg-193 and Ser-39 have been identified as contributors to the inhibitor resistance and cleavage capability of mesotrypsin, but it is not known whether these residues fully account for the unusual properties of mesotrypsin. Here, we use human cationic trypsin as a template for engineering a gain of catalytic function, assessing mutants containing mesotrypsin-like mutations for resistance to inhibition by bovine pancreatic trypsin inhibitor (BPTI) and amyloid precursor protein Kunitz protease inhibitor (APPI), and for the ability to hydrolyze these inhibitors as substrates. We find that Arg-193 and Ser-39 are sufficient to confer mesotrypsin-like resistance to inhibition; however, compared with mesotrypsin, the trypsin-Y39S/G193R double mutant remains 10-fold slower at hydrolyzing BPTI and 2.5-fold slower at hydrolyzing APPI. We identify two additional residues in mesotrypsin, Lys-74 and Asp-97, which in concert with Arg-193 and Ser-39 confer the full catalytic capability of mesotrypsin for proteolysis of BPTI and APPI. Novel crystal structures of trypsin mutants in complex with BPTI suggest that these four residues function cooperatively to favor conformational dynamics that assist in dissociation of cleaved inhibitors. Our results reveal that efficient inhibitor cleavage is a complex capability to which at least four spatially separated residues of mesotrypsin contribute. These findings suggest that inhibitor cleavage represents a functional adaptation of mesotrypsin that may have evolved in response to positive selection pressure.

Fasxiator, a novel factor XIa inhibitor from snake venom, and its site-specific mutagenesis to improve potency and selectivity

BACKGROUND

Bleeding remains a major limitation of standard anticoagulant drugs that target the extrinsic and common coagulation pathways. Recently, intrinsic coagulation factors are increasingly being investigated as alternative targets for developing anticoagulant drugs with lower bleeding risk.

OBJECTIVES

Goals were to (i) identify novel anticoagulants selectively targeting intrinsic coagulation pathway and (ii) characterize and further improve the properties of the identified anticoagulants.

METHODS AND RESULTS

We have isolated and sequenced a specific factor XIa (FXIa) inhibitor, henceforth named Fasxiator, from the venom of the banded krait snake, Bungarus fasciatus. It is a Kunitz-type protease inhibitor that prolonged activated partial thromboplastin time without significant effects on prothrombin time. Fasxiator was recombinantly expressed (rFasxiator), purified, and characterized to be a slow-type inhibitor of FXIa that exerts its anticoagulant activities (doubled activated partial thromboplastin time at ~ 3 μmol L(-1) ) by selectively inhibiting human FXIa in in vitro assays. A series of mutants were subsequently generated to improve the potency and selectivity of recombinant rFasxiator. rFasxiatorN17R,L19E showed the best balance between potency (IC50 ~ 1 nmol L(-1) ) and selectivity (> 100 times). rFasxiatorN17R,L19E is a competitive slow-type inhibitor of FXIa (Ki  = 0.86 nmol L(-1) ), possesses anticoagulant activity that is ~ 10 times stronger in human plasma than in murine plasma, and prolonged the occlusion time of mice carotid artery in FeCl3 -induced thrombosis models.

CONCLUSION

We have isolated an exogenous FXIa specific inhibitor, engineered it to improve its potency by ~ 1000 times and demonstrated its in vitro and in vivo efficacy. These proof-of-principle data supported the further development of Fasxiator as a novel anticoagulant candidate.

Sequence and conformational specificity in substrate recognition: several human Kunitz protease inhibitor domains are specific substrates of mesotrypsin

Mesotrypsin is an isoform of trypsin that is uniquely resistant to polypeptide trypsin inhibitors and can cleave some inhibitors rapidly. Previous studies have shown that the amyloid precursor protein Kunitz protease inhibitor domain (APPI) is a specific substrate of mesotrypsin and that stabilization of the APPI cleavage site in a canonical conformation contributes to recognition by mesotrypsin. We hypothesized that other proteins possessing potential cleavage sites stabilized in a similar conformation might also be mesotrypsin substrates. Here we evaluated a series of candidate substrates, including human Kunitz protease inhibitor domains from amyloid precursor-like protein 2 (APLP2), bikunin, hepatocyte growth factor activator inhibitor type 2 (HAI2), tissue factor pathway inhibitor-1 (TFPI1), and tissue factor pathway inhibitor-2 (TFPI2), as well as E-selectin, an unrelated protein possessing a potential cleavage site displaying canonical conformation. We find that Kunitz domains within APLP2, bikunin, and HAI2 are cleaved by mesotrypsin with kinetic profiles of specific substrates. TFPI1 and TFPI2 Kunitz domains are cleaved less efficiently by mesotrypsin, and E-selectin is not cleaved at the anticipated site. Cocrystal structures of mesotrypsin with HAI2 and bikunin Kunitz domains reveal the mode of mesotrypsin interaction with its canonical substrates. Our data suggest that major determinants of mesotrypsin substrate specificity include sequence preferences at the P1 and P'2 positions along with conformational stabilization of the cleavage site in the canonical conformation. Mesotrypsin up-regulation has been implicated previously in cancer progression, and proteolytic clearance of Kunitz protease inhibitors offers potential mechanisms by which mesotrypsin may mediate pathological effects in cancer.

P1 and P2' site mutations convert protease nexin-2 from a factor XIa inhibitor to a plasmin inhibitor

The kunitz protease inhibitor domain of PN2 (PN2KPI) is a potent and specific inhibitor (K(i) 0.5-2 nM) of factor XIa (FXIa) and inhibits cerebrovascular thrombosis in mice. To determine whether the antithrombotic properties of PN2KPI arise from its FXIa-inhibitory activity, we have now prepared mutant forms of PN2KPI. Mutations at the P1 (Arg(15)) site in combination with P2' (Met(17)) mutations profoundly affect inhibition of FXIa, plasmin, kallikrein, factor Xa and thrombin. The mutant proteins PN2KPI-R(15)K, -M(17)K, -R(15)K,M(17)K and -R(15)K,M(17)R lost inhibitory activity against FXIa (K(i) 34, 94, 3081 and 707 nM, respectively) and kallikrein (no inhibition) and gained inhibitory activity against plasmin (K(i) 108, 7, 8 and 8 nM, respectively). The intravenous administration of rPN2KPI into mice dramatically decreased thrombus formation in a murine model of FeCl(3)-induced carotid injury, whereas rPN2KPI-R(15)K,M(17)K failed to inhibit thrombus formation. Molecular modelling studies showed that fine structural variations explain the observed functional differences in FXIa and plasmin inhibition. PN2KPI has potent antithrombotic activity due to its specific FXIa anticoagulant activity, whereas PN2KPI-R(15)K,M(17)K and PN2KPI-R(15)K,M(17)R have potent antifibrinolytic (antiplasmin) activity without anticoagulant or antithrombotic activity.

The kunitz protease inhibitor domain of protease nexin-2 inhibits factor XIa and murine carotid artery and middle cerebral artery thrombosis

Coagulation factor XI (FXI) plays an important part in both venous and arterial thrombosis, rendering FXIa a potential target for the development of antithrombotic therapy. The kunitz protease inhibitor (KPI) domain of protease nexin-2 (PN2) is a potent, highly specific inhibitor of FXIa, suggesting its possible role in the inhibition of FXI-dependent thrombosis in vivo. Therefore, we examined the effect of PN2KPI on thrombosis in the murine carotid artery and the middle cerebral artery. Intravenous administration of PN2KPI prolonged the clotting time of both human and murine plasma, and PN2KPI inhibited FXIa activity in both human and murine plasma in vitro. The intravenous administration of PN2KPI into WT mice dramatically decreased the progress of FeCl(3)-induced thrombus formation in the carotid artery. After a similar initial rate of thrombus formation with and without PN2KPI treatment, the propagation of thrombus formation after 10 minutes and the amount of thrombus formed were significantly decreased in mice treated with PN2KPI injection compared with untreated mice. In the middle cerebral artery occlusion model, the volume and fraction of ischemic brain tissue were significantly decreased in PN2KPI-treated compared with untreated mice. Thus, inhibition of FXIa by PN2KPI is a promising approach to antithrombotic therapy.

Chemical characterization of pro-inflammatory amyloid-beta peptides in human atherosclerotic lesions and platelets

Amyloid-β (Aβ) peptides are intimately involved in the inflammatory pathology of atherosclerotic vascular disease (AVD) and Alzheimer's disease (AD). Although substantial amounts of these peptides are produced in the periphery, their role and significance to vascular disease outside the brain requires further investigation. Amyloid-β peptides present in the walls of human aorta atherosclerotic lesions as well as activated and non-activated human platelets were isolated using sequential size-exclusion columns and HPLC reverse-phase methods. The Aβ peptide isolates were quantified by ELISA and structurally analyzed using MALDI-TOF mass spectrometry procedures. Our experiments revealed that both aorta and platelets contained Aβ peptides, predominately Aβ40. The source of the Aβ pool in aortic atherosclerosis lesions is probably the activated platelets and/or vascular wall cells expressing APP/PN2. Significant levels of Aβ42 are present in the plasma, suggesting that this reservoir makes a minor contribution to atherosclerotic plaques. Our data reveal that although aortic atherosclerosis and AD cerebrovascular amyloidosis exhibit clearly divergent end-stage manifestations, both vascular diseases share some key pathophysiological promoting elements and pathways. Whether they happen to be deposited in vessels of the central nervous system or atherosclerotic plaques in the periphery, Aβ peptides may promote and perhaps synergize chronic inflammatory processes which culminate in the degeneration, malfunction and ultimate destruction of arterial walls.

Complex assemblies of factors IX and X regulate the initiation, maintenance, and shutdown of blood coagulation

Blood hemostasis is accomplished by a complex network of (anti-)coagulatory and fibrinolytic processes. These physiological processes are implemented by the assembly of multiprotein complexes involving both humoral and cellular components. Coagulation factor X, and particularly, factor IX, exemplify the dramatic enhancement that is obtained by the synergistic interaction of cell surface, inorganic and protein cofactors, protease, and substrate. With a focus on structure-function relationship, we review the current knowledge of activity modulation principles in the coagulation proteases factors IX and X and indicate future challenges for hemostasis research. This chapter is organized by describing the principles of hierarchical activation of blood coagulation proteases, including endogenous and exogenous protease activators, cofactor binding, substrate specificities, and protein inhibitors. We conclude by outlining pharmaceutical opportunities for unmet needs in hemophilia and thrombosis.

Determinants of affinity and proteolytic stability in interactions of Kunitz family protease inhibitors with mesotrypsin

An important functional property of protein protease inhibitors is their stability to proteolysis. Mesotrypsin is a human trypsin that has been implicated in the proteolytic inactivation of several protein protease inhibitors. We have found that bovine pancreatic trypsin inhibitor (BPTI), a Kunitz protease inhibitor, inhibits mesotrypsin very weakly and is slowly proteolyzed, whereas, despite close sequence and structural homology, the Kunitz protease inhibitor domain of the amyloid precursor protein (APPI) binds to mesotrypsin 100 times more tightly and is cleaved 300 times more rapidly. To define features responsible for these differences, we have assessed the binding and cleavage by mesotrypsin of APPI and BPTI reciprocally mutated at two nonidentical residues that make direct contact with the enzyme. We find that Arg at P(1) (versus Lys) favors both tighter binding and more rapid cleavage, whereas Met (versus Arg) at P'(2) favors tighter binding but has minimal effect on cleavage. Surprisingly, we find that the APPI scaffold greatly enhances proteolytic cleavage rates, independently of the binding loop. We draw thermodynamic additivity cycles analyzing the interdependence of P(1) and P'(2) substitutions and scaffold differences, finding multiple instances in which the contributions of these features are nonadditive. We also report the crystal structure of the mesotrypsin·APPI complex, in which we find that the binding loop of APPI displays evidence of increased mobility compared with BPTI. Our data suggest that the enhanced vulnerability of APPI to mesotrypsin cleavage may derive from sequence differences in the scaffold that propagate increased flexibility and mobility to the binding loop.

Mechanisms and specificity of factor XIa and trypsin inhibition by protease nexin 2 and basic pancreatic trypsin inhibitor

Factor XIa (FXIa) inhibition by protease nexin-2 (PN2KPI) was compared with trypsin inhibition by basic pancreatic trypsin inhibitor (BPTI). PN2KPI was a potent inhibitor of FXIa (K(i) ∼ 0.81 nM) and trypsin (K(i) ∼ 0.03 nM), but not of other coagulation proteases (thrombin, FVIIa, FIXa, FXa, FXIIa, plasmin, kallikrein, K(i) > 185 nM). PN2KPI was ∼775-fold more potent than BPTI in FXIa inhibition, but both exhibited similar potencies against trypsin. Studies of FXIa and trypsin inhibition by PN2KPI and BPTI and P1 site swap mutants (PN2KPI-R15 K, BPTI-K15 R) demonstrated that FXIa inhibition by PN2KPI and P1 site swap mutants and trypsin inhibition by PN2KPI and BPTI conform to a single-step, slow equilibration inhibitory mechanism, whereas FXIa-inhibition by BPTI follows a classical, competitive inhibitory mechanism. Mutation of P1 impaired FXIa inhibition by PN2KPI-R15 K ∼14-fold, enhanced FXIa inhibition by BPTI-K15 R ∼150-fold, and had no effect on trypsin inhibition. Arginine at the P1 site of either PN2KPI or BPTI confers high affinity and specificity for FXIa, whereas either arginine or lysine suffices for trypsin inhibition. Thus, PN2KPI is a highly specific inhibitor of FXIa among coagulation enzymes, but the flexibility of trypsin renders it susceptible to inhibition by both wild-type and mutant forms of PN2KPI and BPTI.

The amyloid precursor protein/protease nexin 2 Kunitz inhibitor domain is a highly specific substrate of mesotrypsin

The amyloid precursor protein (APP) is a ubiquitously expressed transmembrane adhesion protein and the progenitor of amyloid-beta peptides. The major splice isoforms of APP expressed by most tissues contain a Kunitz protease inhibitor domain; secreted APP containing this domain is also known as protease nexin 2 and potently inhibits serine proteases, including trypsin and coagulation factors. The atypical human trypsin isoform mesotrypsin is resistant to inhibition by most protein protease inhibitors and cleaves some inhibitors at a substantially accelerated rate. Here, in a proteomic screen to identify potential physiological substrates of mesotrypsin, we find that APP/protease nexin 2 is selectively cleaved by mesotrypsin within the Kunitz protease inhibitor domain. In studies employing the recombinant Kunitz domain of APP (APPI), we show that mesotrypsin cleaves selectively at the Arg(15)-Ala(16) reactive site bond, with kinetic constants approaching those of other proteases toward highly specific protein substrates. Finally, we show that cleavage of APPI compromises its inhibition of other serine proteases, including cationic trypsin and factor XIa, by 2 orders of magnitude. Because APP/protease nexin 2 and mesotrypsin are coexpressed in a number of tissues, we suggest that processing by mesotrypsin may ablate the protease inhibitory function of APP/protease nexin 2 in vivo and may also modulate other activities of APP/protease nexin 2 that involve the Kunitz domain.

Multifunctional roles of enolase in Alzheimer's disease brain: beyond altered glucose metabolism

Enolase enzymes are abundantly expressed, cytosolic carbon-oxygen lyases known for their role in glucose metabolism. Recently, enolase has been shown to possess a variety of different regulatory functions, beyond glycolysis and gluconeogenesis, associated with hypoxia, ischemia, and Alzheimer's disease (AD). AD is an age-associated neurodegenerative disorder characterized pathologically by elevated oxidative stress and subsequent damage to proteins, lipids, and nucleic acids, appearance of neurofibrillary tangles and senile plaques, and loss of synapse and neuronal cells. It is unclear if development of a hypometabolic environment is a consequence of or contributes to AD pathology, as there is not only a significant decline in brain glucose levels in AD, but also there is an increase in proteomics identified oxidatively modified glycolytic enzymes that are rendered inactive, including enolase. Previously, our laboratory identified alpha-enolase as one the most frequently up-regulated and oxidatively modified proteins in amnestic mild cognitive impairment (MCI), early-onset AD, and AD. However, the glycolytic conversion of 2-phosphoglycerate to phosphoenolpyruvate catalyzed by enolase does not directly produce ATP or NADH; therefore it is surprising that, among all glycolytic enzymes, alpha-enolase was one of only two glycolytic enzymes consistently up-regulated from MCI to AD. These findings suggest enolase is involved with more than glucose metabolism in AD brain, but may possess other functions, normally necessary to preserve brain function. This review examines potential altered function(s) of brain enolase in MCI, early-onset AD, and AD, alterations that may contribute to the biochemical, pathological, clinical characteristics, and progression of this dementing disorder.

Comparison of Composer and ORCHESTRAR

Although the number of known protein structures is increasing, the number of protein sequences without determined structures is still much larger. Three-dimensional (3D) protein structure information helps in the understanding of functional mechanisms, but solving structures by X-ray crystallography or NMR is often a lengthy and difficult process. A relatively fast way of determining a protein's 3D structure is to construct a computer model using homologous sequence and structure information. Much work has gone into algorithms that comprise the ORCHESTRAR homology modeling program in the SYBYL software package. This novel homology modeling tool combines algorithms for modeling conserved cores, variable regions, and side chains. The paradigm of using existing knowledge from multiple templates and the underlying protein environment knowledgebase is used in all of these algorithms, and will become even more powerful as the number of experimentally derived protein structures increases. To determine how ORCHESTRAR compares to Composer (a broadly used, but an older tool), homology models of 18 proteins were constructed using each program so that a detailed comparison of each step in the modeling process could be carried out. Proteins modeled include kinases, dihydrofolate reductase, HIV protease, and factor Xa. In almost all cases ORCHESTRAR produces models with lower root-mean-squared deviation (RMSD) values when compared with structures determined by X-ray crystallography or NMR. Moreover, ORCHESTRAR produced a homology model for three target sequences where Composer failed to produce any. Data for RMSD comparisons between structurally conserved cores, structurally variable regions, side-chain conformations are presented, as well as analyses of active site and protein-protein interface configurations.

Functional interactions of APP with the apoE receptor family

The beta-amyloid precursor protein (APP) is central to the pathogenesis of Alzheimer's disease, but its normal functions in the brain are poorly understood. A number of APP-interacting proteins have been identified: intracellularly, APP interacts with adaptor proteins through its conserved NPXY domain; extracellularly, APP interacts with a component of the extracellular matrix, F-spondin. Interestingly, many of these APP-interacting proteins also interact with the family of receptors for apolipoprotein E (apoE), the Alzheimer's disease risk factor. apoE receptors also share with APP the fact that they are cleaved by the same secretase activities. apoE receptors are shed from the cell surface, a cleavage that is regulated by receptor-ligand interactions, and C-terminal fragments of apoE receptors are cleaved by gamma-secretase. Functionally, both APP and apoE receptors affect neuronal migration and synapse formation in the brain. This review summarizes these numerous interactions between APP and apoE receptors, which provide clues about the normal functions of APP.

Reduced incidence of ischemic stroke in patients with severe factor XI deficiency

Inherited disorders of hemostasis are natural models for investigating mechanisms of thrombosis and development of antithrombotic therapy. Because mice with total factor XI deficiency are protected against ischemic stroke and do not manifest excessive bleeding, we investigated the incidence of ischemic stroke in patients with severe inherited factor XI deficiency. Incidence of ischemic stroke in 115 patients aged 45 years or more with severe factor XI deficiency (activity less than 15 U/dL) was compared with incidence in the Israeli population as estimated from a stroke survey of 1528 patients. Adjustment for major risk factors of stroke (hypertension, diabetes mellitus, hypercholesterolemia, current smoking) was based on comparison of their prevalence in the stroke survey to an Israeli health survey of 9509 subjects. Incidence of myocardial infarction in the factor XI cohort was also recorded. After adjustment for the 4 major risk factors of ischemic stroke, the expected incidence of ischemic stroke was 8.56 compared with one observed (P = .003). The reduced 1:115 incidence of ischemic stroke contrasted with a 19:115 incidence of myocardial infarction, similar to the expected incidence. Thus, severe factor XI deficiency probably is protective against ischemic stroke but not against acute myocardial infarction.

Increased severity of hemorrhage in transgenic mice expressing cerebral protease nexin-2/amyloid beta-protein precursor

BACKGROUND AND PURPOSE

Secreted isoforms of amyloid beta-protein precursor (AbetaPP) that contain the Kunitz proteinase inhibitor domain, also known as protease nexin-2 (PN2), are enriched in brain. Although little is known of its physiological function, the potent inhibition of certain prothrombotic proteinases by PN2/AbetaPP suggests that it may function to regulate cerebral thrombosis during vascular injury events.

METHODS

To examine the antithrombotic function of cerebral PN2/AbetaPP in vivo, we performed measurements of carotid artery thrombosis and experimental intracerebral hemorrhage in transgenic mice with specific and modest overexpression of PN2/AbetaPP in brain. Comparisons were made with wild-type mice and Tg-rPF4/APP mice, a model that possesses specific and modest overexpression of PN2/AbetaPP in platelets and exhibits reduced thrombosis in vivo.

RESULTS

Modest overexpression of PN2/AbetaPP in transgenic mouse brain had no effect on intraluminal carotid arterial thrombosis but resulted in larger hematoma volumes and hemoglobin levels (23.1+/-2.7 mm(3) [n=6; P<0.01] and 1411+/-202 microg/hemisphere [n=12; P<0.01], respectively), compared with wild-type mice (15.9+/-2.2 mm(3) [n=6] and 935+/-418 microg/hemisphere [n=12], respectively).

CONCLUSIONS

These findings indicate that cerebral PN2/AbetaPP plays a significant role in regulating thrombosis in brain and that modest age-related increases in the cerebral levels of this protein could markedly enhance the extent of cerebral hemorrhage.

Feedback activation of factor XI by thrombin does not occur in plasma

In this study, we tested the hypothesis that factor XI (FXI) activation occurs in plasma following activation of the extrinsic pathway by thrombin-mediated feedback activation. We used two different assays: (i) a direct measurement of activated FXI by ELISA and (ii) a functional assay that follows the activation of the coagulation cascade in the presence or absence of a FXI inhibiting antibody by monitoring thrombin activity. We failed to detect any FXI activation or functional contribution to the activation of the coagulation cascade in platelet poor or platelet-rich plasma, when activation was initiated by thrombin or tissue factor. Additionally, we found that, in the absence of a contact system inhibitor during blood draw, contact activation of FXI can mistakenly appear as thrombin- or tissue-factor-dependent activation. Thus, activation of FXI by thrombin in solution or on the surface of activated platelets does not appear to play a significant role in a plasma environment. These results call for reevaluation of the physiological role of the contact activation system in blood coagulation.

Heparin modulates the 99-loop of factor IXa: effects on reactivity with isolated Kunitz-type inhibitor domains

Reactivity of factor IXa with basic pancreatic trypsin inhibitor is enhanced by low molecular weight heparin (enoxaparin). Previous studies by us have suggested that this effect involves allosteric modulation of factor IXa. We examined the reactivity of factor IXa with several isolated Kunitz-type inhibitor domains: basic pancreatic trypsin inhibitor, the Kunitz inhibitor domain of protease Nexin-2, and the first two inhibitor domains of tissue factor pathway inhibitor. We find that enhancement of factor IXa reactivity by enoxaparin is greatest for basic pancreatic trypsin inhibitor (>10-fold), followed by the second tissue factor pathway inhibitor domain (1.7-fold) and the Kunitz inhibitor domain of protease Nexin-2 (1.4-fold). Modeling studies of factor IXa with basic pancreatic trypsin inhibitor suggest that binding of this inhibitor is sterically hindered by the 99-loop of factor IXa, specifically residue Lys(98). Slow-binding kinetic studies support the formation of a weak initial enzyme-inhibitor complex between factor IXa and basic pancreatic trypsin inhibitor that is facilitated by enoxaparin binding. Mutation of Lys(98) to Ala in factor IXa results in enhanced reactivity with all inhibitors examined, whereas almost completely abrogating the enhancing effects of enoxaparin. The results implicate Lys(98) and the 99-loop of factor IXa in defining enzyme inhibitor specificity. More importantly, these results demonstrate the ability of factor IXa to be allosterically modulated by occupation of the heparin-binding exosite.

Visualization of APP dimerization and APP-Notch2 heterodimerization in living cells using bimolecular fluorescence complementation

We previously demonstrated that the amyloid precursor protein (APP) interacts with Notch receptors. Here, we confirmed the APP/Notch1 endogenous interaction in embryonic day 17 rat brain tissue, suggesting the interaction was not as a result of over-expression artifacts. To investigate potential homodimeric and heterodimeric interactions of APP and Notch2 (N2), we have visualized the subcellular localization of the APP/N2 complexes formed in living cells using bimolecular fluorescence complementation (BiFC) analysis. BiFC was accomplished by fusing the N-terminal fragment or the C-terminal fragment of yellow fluorescent protein (YFP) to APP, N2, and a C-terminally truncated form of N2. When expressed in COS-7 cells, these tagged proteins alone did not produce a fluorescent signal. The tagged APP homodimer produced a weak fluorescent signal, while neither full-length N2, nor a truncated N2 alone, produced a visible signal, suggesting that N2 receptors do not form homodimers. The strongest fluorescent signal was obtained with co-expression of the C-terminal fragment of YFP fused to APP and the N-terminal fragment of YFP fused to the truncated form of N2. This heterodimer localized to plasma membrane, endoplasmic reticulum (ER), Golgi and other compartments. The results were confirmed and quantified by flow cytometry. The BiFC method of specifically visualizing APP/Notch interactions can be applied to study APP and Notch signaling during development, aging and neurodegeneration.

Amyloid precursor protein interacts with notch receptors

The amyloid precursor protein (APP) must fulfill important roles based on its sequence conservation from fly to human. Although multiple functions for APP have been proposed, the best-known role for this protein is as the precursor of Abeta peptide, a neurotoxic 39-43-amino acid peptide crucial to the pathogenesis of Alzheimer's disease. To investigate additional roles for APP with an eye toward understanding the molecular basis of the pleiotropic effects ascribed to APP, we isolated proteins that interacted with the plasma membrane isoform of APP. We employed a membrane-impermeable crosslinker to immobilize proteins binding to transmembrane APP in human embryonic kidney (HEK)293 cells expressing APP751 (HEK275) or rat embryonic day 18 primary neurons infected with a virus expressing APP. Notch2 was identified as a potential APP binding partner based on mass spectrometry analysis of APP complexes immunopurified from neurons. To confirm the interaction between Notch2 and APP, we carried out immunoprecipitation studies in HEK275 cells transiently expressing full-length Notch2 using Notch2 antibodies. The results indicated that APP and Notch2 interact in mammalian cells, and confirmed our initial findings. Interestingly, Notch1 also coimmunoprecipitated with APP, suggesting that APP and Notch family members may engage in intermolecular cross talk to modulate cell function. Finally, cotransfection of APP/CFP and Notch2/YFP into COS cells revealed that these two proteins colocalize on the plasma membrane. Intracellularly, however, although some APP and Notch molecules colocalize, others reside in distinct locations. The discovery of proteins that interact with APP may aid in the identification of new functions for APP.

Protease nexin-2/amyloid beta-protein precursor limits cerebral thrombosis

The amyloid beta-protein precursor (AbetaPP) is best known as the parent molecule to the amyloid beta-peptide that accumulates in the brains of patients with Alzheimer's disease. Secreted isoforms of AbetaPP that contain the Kunitz proteinase inhibitor domain are analogous to the previously identified cell-secreted proteinase inhibitor known as protease nexin-2 (PN2). Although PN2/AbetaPP is enriched in brain and in circulating blood platelets, little is understood of its physiological function and potential role in disease processes outside of amyloid beta-peptide generation. We hypothesized that the potent inhibition of certain procoagulant proteinases by PN2/AbetaPP, coupled with its abundance in platelets and brain, indicate that it may function to regulate cerebral thrombosis. Here we show that specific and modest 2-fold overexpression of PN2/AbetaPP in circulating platelets of transgenic mice caused a marked inhibition of thrombosis in vivo. In contrast, deletion of PN2/AbetaPP in AbetaPP gene knockout mice resulted in a significant increase in thrombosis. Similarly, platelet PN2/AbetaPP transgenic mice developed larger hematomas in experimental intracerebral hemorrhage, whereas AbetaPP gene knockout mice exhibited reduced hemorrhage size. These findings indicate that PN2/AbetaPP plays a significant role in regulating cerebral thrombosis and that modest increases in this protein can profoundly enhance cerebral hemorrhage.

Amyloid precursor protein expression in the rat hippocampal dentate gyrus modulates during memory consolidation

Despite advances in our understanding of the basic biology of amyloid precursor protein (APP), the normal physiological function(s) of APP in learning and memory remains unclear. Here we show increased APP degradation in the hippocampus to be associated with the consolidation of a passive avoidance response. Neurone-specific APP695 expression became transiently reduced 2-4 h post-training through association with endosomal adaptin proteins and enhanced internalization. By contrast, internalization of glial-associated APP containing a Kunitz protease inhibitor-like domain (APP-KPI) was dependent on the low-density lipoprotein receptor-related protein (LRP). In addition, LRP expression and association with apolipoprotein E increased in the 2-4 h post-training period. The LRP antagonist receptor-associated protein prevented the APP-KPI internalization and LRP-apolipoprotein E association and this resulted in amnesia. Degradation of APP695 and APP-KPI did not appear to be related to alpha-secretase activity, as no learning-associated increase of secreted APP was observed in the CSF. Moreover, as internalization of APP isoforms was observed only in dentate gyrus, it probably relates to the learning-associated restructuring of the perforant path terminals. Memory-associated APP processing in both neuronal and glial compartments points to a role for glial unsheathing of synaptic connections, an event required for the synaptic restructuring that accompanies memory consolidation. These observations may have a direct relevance to understanding the pathophysiology of Alzheimer's disease as beta/gamma-secretase-derived beta-amyloid is formed following internalization of cell surface APP into the endosomal compartment.

Structural and mutational analyses of the molecular interactions between the catalytic domain of factor XIa and the Kunitz protease inhibitor domain of protease nexin 2

Factor XIa (FXIa) is a serine protease important for initiating the intrinsic pathway of blood coagulation. Protease nexin 2 (PN2) is a Kunitz-type protease inhibitor secreted by activated platelets and a physiologically important inhibitor of FXIa. Inhibition of FXIa by PN2 requires interactions between the two proteins that are confined to the catalytic domain of the enzyme and the Kunitz protease inhibitor (KPI) domain of PN2. Recombinant PN2KPI and a mutant form of the FXI catalytic domain (FXIac) were expressed in yeast, purified to homogeneity, co-crystallized, and the structure of the complex was solved at 2.6 angstroms (Protein Data Bank code 1ZJD). In this complex, PN2KPI has a characteristic, disulfide-stabilized double loop structure that fits into the FXIac active site. To determine the contributions of residues within PN2KPI to its inhibitory activity, selected point mutations in PN2KPI loop 1 11TGPCRAMISR20 and loop 2 34FYGGC38 were tested for their ability to inhibit FXIa. The P1 site mutation R15A completely abolished its ability to inhibit FXIa. IC50 values for the wild type protein and the remaining mutants were as follows: PN2KPI WT, 1.28 nM; P13A, 5.92 nM; M17A, 1.62 nM; S19A, 1.86 nM; R20A, 5.67 nM; F34A, 9.85 nM. The IC50 values for the M17A and S19A mutants were not significantly different from those obtained with wild type PN2KPI. These functional studies and activated partial thromboplastin time analysis validate predictions made from the PN2KPI-FXIac co-crystal structure and implicate PN2KPI residues, in descending order of importance, Arg15, Phe34, Pro13, and Arg20 in FXIa inhibition by PN2KPI.

Fusion proteins comprising annexin V and Kunitz protease inhibitors are highly potent thrombogenic site-directed anticoagulants

The anionic phospholipid, phosphatidyl-L-serine (PS), is sequestered in the inner layer of the plasma membrane in normal cells. Upon injury, activation, and apoptosis, PS becomes exposed on the surfaces of cells and sheds microparticles, which are procoagulant. Coagulation is initiated by formation of a tissue factor/factor VIIa complex on PS-exposed membranes and propagated through the assembly of intrinsic tenase (factor VIIIa/factor IXa), prothrombinase (factor Va/factor Xa), and factor XIa complexes on PS-exposed activated platelets. We constructed a novel series of recombinant anticoagulant fusion proteins by linking annexin V (ANV), a PS-binding protein, to the Kunitz-type protease inhibitor (KPI) domain of tick anticoagulant protein, an aprotinin mutant (6L15), amyloid beta-protein precursor, or tissue factor pathway inhibitor. The resulting ANV-KPI fusion proteins were 6- to 86-fold more active than recombinant tissue factor pathway inhibitor and tick anticoagulant protein in an in vitro tissue factor-initiated clotting assay. The in vivo antithrombotic activities of the most active constructs were 3- to 10-fold higher than that of ANV in a mouse arterial thrombosis model. ANV-KPI fusion proteins represent a new class of anticoagulants that specifically target the anionic membrane-associated coagulation enzyme complexes present at sites of thrombogenesis and are potentially useful as antithrombotic agents.

A fusion protein system for the recombinant production of short disulfide bond rich cystine knot peptides using barnase as a purification handle

The inhibitor cystine knot (ICK) structural motif has been found in several small proteins and peptides from plants, insects, marine molluscs, and also in human. It is defined by a triple beta-sheet that is held together by three intramolecular disulfide bonds built by six conserved cysteine residues that generate a highly rigid and stable fold. We describe a procedure for the production of ICK peptides with correct disulfide bond connectivities via expression in Escherichia coli as fusion proteins with an enzymatically inactive variant of the Bacillus amyloliquefaciens RNAse barnase. Barnase directs the fused peptide to the culture medium and the fusion protein can be isolated by combined cation exchange/reverse-phase chromatography. The ICK peptides are released from the barnase expression and purification handle either by cyanogen bromide or by protease cleavage to give pure and correctly folded cystine knot peptides.

Upregulation of amyloid precursor protein isoforms containing Kunitz protease inhibitor in dementia with Lewy bodies

Amyloid precursor protein (APP) is involved in the accumulation of alpha-synuclein, the main component of Lewy bodies. It is currently unknown, however, whether any of the APP isoforms is instrumental in alpha-synuclein deposition in dementia with Lewy bodies (DLB). Using real-time RT-PCR, we have studied relative mRNA expression levels of APP isoforms in frozen postmortem frontal cortices of DLB patients, Alzheimer disease (AD) patients, and control subjects. Of the three main APP isoforms, the two with a Kunitz protease inhibitory (KPI) motif (APP770 and APP751) were found to be specifically overexpressed in the frontal cortices of DLB patients when compared with controls and AD patients. These findings suggest a specific role of APP isoforms containing Kunitz protease inhibitor in DLB pathogenesis.

Allosteric Modification of Factor XIa Functional Activity upon Binding to Polyanions

The effects of several polyanions on the hydrolysis of the chromogenic substrate L-pyroglutamyl-L-prolyl-L-arginyl-p-nitroaniline (S-2366) and on the activation of factor IX by factor XIa have been investigated. Two forms of dextran sulfate (M(r) approximately 500000 and M(r) approximately 10000, DX10) and two forms of heparin (64 disaccharide units, M(r) approximately 14000, and hypersulfated heparin, S-Hep, M(r) approximately 12000) inhibited both factor XIa amidolytic activity and factor IX activation in a concentration-dependent manner. The inhibitory effect was not due to binding of either substrate by the polyanions since only a decrease in V(max) without any effect on K(m) was observed in kinetic assays. Steric inhibition is unlikely since the concentrations of polyanions required for inhibition of small peptide hydrolysis were lower than those required for macromolecular substrate cleavage. In contrast, an allosteric inhibitory mechanism was supported by an enhancement of the dansyl fluorescence of 5-(dimethylamino)-1-(naphthalenesulfonyl)glutamylglycylarginyl- (DEGR-) factor XIa observed when the fluorophore was in complex with either DX10 or S-Hep. Moreover, in the presence of a polyanion the fluorophore was far more resistant to quenching by acrylamide. These results provide compelling evidence that factor XIa binding to the polyanions, dextran sulfate and heparin, results in inhibition of the enzyme by an allosteric mechanism.

Cerebrovascular requirement for sealant, anti-coagulant and remodeling molecules that allow for the maintenance of vascular integrity and blood supply

The integrity of the vasculature and the maintenance of the blood supply to the brain are crucial for the survival of higher vertebrates. However, peripheral mechanisms of sealing the vasculature that rely on the clotting of blood and platelet aggregation around the site of a 'leak' would lead to decreased cerebral perfusion and compromise the viability of terminally differentiated and irreplaceable neurons. Therefore, in higher organisms it is likely that a sealant/anti-coagulant system that maintains vascular supply has evolved as a necessity to life. We propose that one such system involves the amyloid-beta precursor protein (AbetaPP) and its cleavage product Abeta since (1) both AbetaPP/Abeta are known to deposit in the media of the cerebrovasculature wall following localized injury, (2) Abeta is generated from AbetaPP, a known acute phase reactant, (3) Abeta's physiochemical properties allow it to span between the extracellular matrix and the (endothelial) cell membrane and under inflammatory conditions aggregate to form an intracranial 'scab', thereby maintaining structural integrity of the blood brain barrier, (4) AbetaPP/Abeta together act as an anti-coagulant, (5) Abeta promotes vascular/neuronal remodeling, and (6) Abeta deposits resolve after injury. These properties are consistent with the acute phase generation and rapid cortical deposition of AbetaPP/Abeta following injury (either sustained by trauma or stresses associated with aging) that would be an important compensatory response aimed at limiting the loss of terminally differentiated neurons. Such a system would allow the maintenance of blood supply to the brain by sealing vascular lesions, preventing hemorrhagic stroke while at the same time inhibiting the coagulation cascade from blocking capillaries. Obviously, strategies to remove Abeta would have serious consequences for the integrity of the blood-brain barrier. Indeed, recent in vivo evidence demonstrates that the removal of deposited Abeta from the vasculature leads to increased cerebral microhemorrhage and strongly support the above mentioned functions of AbetaPP/Abeta. These insights also explain the root cause of the encephalitis and meningitis suffered by individuals in immunotherapy trials as being directly associated with the removal of Abeta from the vasculature, i.e. immunological responses to Abeta vaccination do not discriminate between physiologically purposive deposits of Abeta (vascular deposits) and pathological deposits of Abeta (senile plaques).

Solution studies and structural model of the extracellular domain of the human amyloid precursor protein

The amyloid precursor protein (APP) is the precursor of the beta-amyloid peptide (Abeta), which is centrally related to the genesis of Alzheimer's disease (AD). In addition, APP has been suggested to mediate and/or participate in events that lead to neuronal degeneration in AD. Despite the fact that various aspects of the cell biology of APP have been investigated, little information on the structure of this protein is available. In this work, the solution structure of the soluble extracellular domain of APP (sAPP, composing 89% of the amino acid residues of the whole protein) has been investigated through a combination of size-exclusion chromatography, circular dichroism, and synchrotron radiation small-angle x-ray scattering (SAXS) studies. sAPP is monomeric in solution (65 kDa obtained from SAXS measurements) and exhibits an anisometric molecular shape, with a Stokes radius of 39 or 51 A calculated from SAXS or chromatographic data, respectively. The radius of gyration and the maximum molecular length obtained by SAXS were 38 A and 130 A, respectively. Analysis of SAXS data further allowed building a structural model for sAPP in solution. Circular dichroism data and secondary structure predictions based on the amino acid sequence of APP suggested that a significant fraction of APP (30% of the amino acid residues) is not involved in standard secondary structure elements, which may explain the elongated shape of the molecule recovered in our structural model. Possible implications of the structure of APP in ligand binding and molecular recognition events involved in the biological functions of this protein are discussed.

Age-dependent differential regulation of genes encoding APP and alpha-synuclein in hippocampal synaptic plasticity

We investigated the modulation of the messenger RNA encoding the amyloid precursor protein (APP) and alpha-synuclein following induction of long-term potentiation (LTP) in the dentate gyrus of young and aged rats. Three hours after tetanic stimulation, LTP induced in the young rats was maintained; the aged rats, however, fell into two subgroups: those in which LTP was maintained, and those in which LTP had declined to basal levels. In young rats, the global expression of mRNAs of all isoforms of APP and in particular that of the isoform lacking the KPI domain were significantly upregulated. In aged rats, the global expression of mRNAs of all isoforms of APP was not modified, regardless of whether LTP was maintained or not. The level of mRNA encoding the Kunitz protease-inhibitory (KPI)-minus isoform of APP, however, was increased in aged rats in which LTP was maintained, suggesting that the gene of this isoform may be more specifically regulated by synaptic plasticity. In contrast, we found that the gene encoding alpha-synuclein showed a trend towards being downregulated at the mRNA level in young rats following LTP, and significantly so in aged rats in which LTP was maintained, whereas it was not downregulated in aged rats with decremental LTP. These data suggest that the regulated expression of APP isoforms is part of the tanscriptional response associated with the enduring forms of synaptic plasticity and is altered with age. Whereas the level of alpha-synuclein mRNA is not apparently modified in normal LTP, it may reflect a mechanism of apoptotic cell death in aging that is in part responsible for decremental synaptic plasticity.

Localization of a fibrillar amyloid beta-protein binding domain on its precursor

Deposition of fibrillar amyloid-beta protein (Abeta) in senile plaques and in the walls of cerebral blood vessels is a key pathological feature of Alzheimer's disease and certain related disorders. Fibrillar Abeta deposition is intimately associated with neuronal and cerebrovascular cell death both in vivo and in vitro. Similarly, accumulation of the Abeta protein precursor (AbetaPP) is also observed at sites of fibrillar Abeta deposition. Recently, we reported that fibrillar Abeta, but not unassembled Abeta, promotes the specific binding of AbetaPP through its cysteine-rich, amino-terminal region (Melchor, J. P., and Van Nostrand, W. E. (2000) J. Biol. Chem. 275, 9782-9791). In the present study we sought to determine the precise site on AbetaPP that facilitates its binding to fibrillar Abeta. A series of synthesized overlapping peptides spanning the cysteine-rich, amino-terminal region of AbetaPP were used as competitors for AbetaPP binding to fibrillar Abeta. A peptide spanning residues 105-119 of AbetaPP competitively inhibited AbetaPP binding to fibrillar Abeta in a solid-phase binding assay and on the surface of cultured human cerebrovascular smooth muscle cells. Alanine-scanning mutagenesis of residues 105-117 within glutathione S-transferase (GST)-AbetaPP-(18-119) revealed that His(110), Val(112), and Ile(113) are key residues that facilitate AbetaPP binding to fibrillar Abeta. These specific residues belong to a common beta-strand within this region of AbetaPP. Wild-type GST-AbetaPP-(18-119) protected cultured human cerebrovascular smooth muscle cells from Abeta-induced toxicity whereas H110A mutant GST-AbetaPP-(18-119) did not. Wild-type GST-AbetaPP-(18-119) bound to different isoforms of fibrillar Abeta and fibrillar amylin peptides whereas H110A mutant and I113A mutant GST-AbetaPP-(18-119) were substantially less efficient binding to each fibrillar peptide. We conclude that His(110), Val(112), and Ile(113), residing in a common beta-strand region within AbetaPP-(18-119), comprise a domain that mediates the binding of AbetaPP to fibrillar peptides.

Secreted forms of the amyloid-beta precursor protein are ligands for the class A scavenger receptor

Upon activation, platelets secrete a 120-kDa protein that competes for the binding and internalization of acetyl low density lipoproteins (AcLDL) by macrophages. From the amino-terminal amino acid sequence, amino acid composition, and immunoblot analysis, we identified the active factor in platelet secretion products as sAPP, an alpha-secretase cleavage product of the beta-amyloid precursor protein (APP), that contains a Kunitz-type protease inhibitor (KPI) domain. We showed that both sAPP751 (also called Nexin II) and sAPP695, which does not contain a KPI domain, are ligands for the class A scavenger receptor (SR-A). Chinese hamster ovary cells stably transfected to express the SR-A bound and internalized 4-fold more human platelet-derived sAPP than control cells. The binding and internalization of sAPP were inhibited by the SR-A antagonist fucoidin. In addition, sAPP competed as effectively as fucoidin for SR-A-mediated cell association and degradation of (125)I-AcLDL. To determine if the KPI domain is required for the binding of sAPP to the SR-A, APP751 and APP695 were expressed in Chinese hamster ovary cells, and sAPP751 and sAPP695 purified from the medium were tested for their binding to the SR-A. sAPP751 and sAPP695 were equally effective in competing for the cell association of (125)I-AcLDL by SR-A-expressing cells, demonstrating that the KPI domain is not essential for binding. We also found that sAPP751 is present in extracts of atherosclerotic lesions and that sAPP competes for the SR-A-mediated cell association of oxidized low density lipoprotein. Deletion mutagenesis indicated that a negatively charged region of APP (residues 191-264) contributes to binding to the SR-A. These results suggest that the SR-A contributes to the clearance of sAPP and that sAPP competes for the cell association of other SR-A ligands.

Localization of a heparin binding site in the catalytic domain of factor XIa

Inhibition of factor XIa by protease nexin II (K(i) approximately 450 pM) is potentiated by heparin (K(I) approximately 30 pM). The inhibition of the isolated catalytic domain of factor XIa demonstrates a similar potentiation by heparin (K(i) decreasing from 436 +/- 62 to 88 +/- 10 pM) and also binds to heparin on surface plasmon resonance (K(d) 11.2 +/- 3.2 nM vs K(d) 8.63 +/- 1.06 nM for factor XIa). The factor XIa catalytic domain contains a cysteine-constrained alpha-helix-containing loop: (527)CQKRYRGHKITHKMIC(542), identified as a heparin-binding region in other coagulation proteins. Heparin-binding studies of coagulation proteases allowed a grouping of these proteins into three categories: group A (binding within a cysteine-constrained loop or a C-terminal heparin-binding region), factors XIa, IXa, Xa, and thrombin; group B (binding by a different mechanism), factor XIIa and activated protein C; and group C (no binding), factor VIIa and kallikrein. Synthesized peptides representative of the factor XIa catalytic domain loop were used as competitors in factor XIa binding and inhibition studies. A native sequence peptide binds to heparin with a K(d) = 86 +/- 15 nM and competes with factor XIa in binding to heparin, K(i) = 241 +/- 37 nM. A peptide with alanine substitutions at (534)H, (535)K, (538)H, and (539)K binds and competes with factor XIa for heparin-binding in a manner nearly identical to that of the native peptide, whereas a scrambled peptide is approximately 10-fold less effective, and alanine substitutions at residues (529)K, (530)R, and (532)R result in loss of virtually all activity. We conclude that residues (529)K, (530)R, and (532)R comprise a high-affinity heparin-binding site in the factor XIa catalytic domain.

A genetically engineered human Kunitz protease inhibitor with increased kallikrein inhibition in an ovine model of cardiopulmonary bypass

A recombinant human serine protease inhibitor known as Kunitz protease inhibitor (KPI) wild type has functional similarities to the bovine Kunitz inhibitor, aprotinin, and had shown a potential to reduce bleeding in an ovine model of cardiopulmonary bypass (CPB). The aim of this study was to assess KPI-185, a modification of KPI-wild type that differs from KPI-wild type in two amino acid residues and which enhances anti-kallikrein activity in a further double-blind, randomized study in an ovine model of CPB, and to compare with our previous study of KPI-wild type and aprotinin in the same ovine model. Post-operative drain losses and subjective assessment of wound 'dryness' showed no significant differences between KPI-185 and KPI-wild type, despite the significant enhancement of kallikrein inhibition using KPI-185 seen in serial kallikrein inhibition assays. These preliminary findings support the hypothesis that kallikrein inhibition is not the major mechanism by which Kunitz inhibitors such as aprotinin reduce perioperative bleeding.

SERPIN regulation of factor XIa. The novel observation that protease nexin 1 in the presence of heparin is a more potent inhibitor of factor XIa than C1 inhibitor

In the present studies we have made the novel observation that protease nexin 1 (PN1), a member of the serine protease inhibitor (SERPIN) superfamily, is a potent inhibitor of the blood coagulation Factor XIa (FXIa). The inhibitory complexes formed between PN1 and FXIa are stable when subjected to reducing agents, SDS, and boiling, a characteristic of the acyl linkage formed between SERPINs and their cognate proteases. Using a sensitive fluorescence-quenched peptide substrate, the K(assoc) of PN1 for FXIa was determined to be 7.9 x 10(4) m(-)(1) s(-)(1) in the absence of heparin. In the presence of heparin, this rate was accelerated to 1.7 x 10(6), M(-)(1) s(-)(1), making PN1 a far better inhibitor of FXIa than C1 inhibitor, which is the only other SERPIN known to significantly inhibit FXIa. FXIa-PN1 complexes are shown to be internalized and degraded by human fibroblasts, most likely via the low density lipoprotein receptor-related protein (LRP), since degradation was strongly inhibited by the LRP agonist, receptor-associated protein. Since FXIa proteolytically modifies the amyloid precursor protein, this observation may suggest an accessory role for PN1 in the pathobiogenesis of Alzheimer's disease.

Localization of a novel type trypsin-like serine protease, neurosin, in brain tissues of Alzheimer's disease and Parkinson's disease

Neurosin, a novel type of trypsin-like serine protease, has been shown to be preferentially expressed in human brain by northern blotting. We examined neurosin immunolabeling in the brains of neurologically normal persons and patients with Alzheimer's disease (AD) and with Parkinson's disease. We also identified the expression of the mRNA for neurosin by in situ hybridization histochemistry and reverse transcription-polymerase chain reaction (RT-PCR). The neurosin antibody stained all of the nuclei of various cell types. In neurons, there was also staining of neuronal cytoplasm, nucleoli and their processes. In AD, staining of neurons with processes was rare in the damaged areas. Some senile plaques, extracellular tangles and Lewy bodies were also positive for neurosin. Expression of the mRNA for neurosin was seen in neurons in the gray matter, and in microglial cells in the white matter. In AD, the intensity of the signal for neurosin mRNA in the gray matter was decreased compared with normal control brains. The relative levels of neurosin mRNA in AD brains, measured by RT-PCR, were lower than those in controls. These results suggest that in human brain neurosin plays various physiological roles, and that in AD this molecule, like other serine proteases, may have a role in the degradation of such substances as beta-amyloid protein.

Protease nexin-2/Amyloid beta-protein precursor regulates factor VIIa and the factor VIIa-tissue factor complex

Protease nexin-2/amyloid beta-protein precursor (PN-2/AbetaPP) and its Kunitz protease inhibitory (KPI) domain were characterized as inhibitors of factor VIIa (FVIIa) and factor VIIa-tissue factor complex (FVIIa-TF). PN-2/AbetaPP and KPI domain inhibited FVIIa with an apparent K(i) of 1.1+/-0.2x 10(-7) M and 1.5+/-0.1x10(-7) M, respectively. When soluble tissue factor (TF(1-219)) was present, there was increased FVIIa inhibition by PN-2/AbetaPP or KPI domain (K(i)=7.8+/-0.3x10(-8) M and 6.8+/-0.6x10(-8) M, respectively). When relipidated tissue factor (TF(1-243)) was present, the K(i) of FVIIa inhibition by PN-2/AbetaPP increased 4.7-fold further. PN-2/AbetaPP complexed with FVIIa, as shown on gel filtration and solid phase binding assay. The apparent second-order rate constant of inhibition of FVIIa by PN-2/AbetaPP in the absence of TF(1-219) was less than that of the FVIIa-TF(1-219) complex. Antithrombin in the absence of TF(1-219) also had a lower apparent second-order rate constant of inhibition than in its presence. In a mixture that included FVIIa, relipidated TF(1-243) and factor X, PN-2/AbetaPP or KPI domain had an IC(50) at 65 and 250 nM, respectively; antithrombin and heparin (1 U/mL) had an IC(50) of 12.8 nM. These data indicate that tissue factor promoted the inhibition of FVIIa by PN-2/AbetaPP or KPI domain, but antithrombin was a better inhibitor of soluble FVIIa-TF in extrinsic tenase.

Fibrillar amyloid beta-protein binds protease nexin-2/amyloid beta-protein precursor: stimulation of its inhibition of coagulation factor XIa

Cerebrovascular deposition of fibrillar 39-42 amino acid amyloid beta-protein (Abeta), a condition known as cerebral amyloid angiopathy (CAA), is a key pathological feature of Alzheimer's disease and related disorders including hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D). Severe cases of CAA, particularly in HCHWA-D, lead to recurrent and often fatal hemorrhagic strokes. Although the reasons for this pathological consequence remain unclear, alterations in proteolytic hemostasis mechanisms have been implicated. For example, the Abeta parent molecule protease nexin-2/amyloid beta-protein precursor (PN-2/AbetaPP), which is elevated in HCHWA-D cerebral vessels with Abeta deposits, is a potent inhibitor of coagulation factor XIa (FXIa). Here we show that fibrillar HCHWA-D Abeta binds PN-2/AbetaPP, but not its isolated Kunitz-type proteinase inhibitor (KPI) domain, in a saturable, dose-dependent manner with a K(d) of approximately 28 nM. Neither PN-2/AbetaPP nor its KPI domain bound to nonfibrillar HCHWA-D Abeta. The fibrillar Abeta binding domain on PN-2/AbetaPP was localized to residues 18-119. PN-2/AbetaPP that bound to fibrillar HCHWA-D Abeta immobilized either in plastic wells or on the surface of cultured cerebrovascular smooth muscle cells was active in inhibiting FXIa. Quantitative kinetic measurements revealed that fibrillar HCHWA-D Abeta caused a >5-fold enhancement of FXIa inhibition by PN-2/AbetaPP. Similar stimulatory effects on FXIa inhibition by PN-2/AbetaPP were also observed with fibrillar wild-type Abeta. However, fibrillar Abeta had no effect on the inhibition of trypsin by PN-2/AbetaPP. These findings suggest that fibrillar Abeta deposits in cerebral vessels can effectively localize and enhance the anticoagulant functions of PN-2/AbetaPP, thereby contributing to a microenvironment conducive to hemorrhaging.

Protease nexin II interactions with coagulation factor XIa are contained within the Kunitz protease inhibitor domain of protease nexin II and the factor XIa catalytic domain

Protease nexin II, a platelet-secreted protein containing a Kunitz-type domain, is a potent inhibitor of factor XIa with an inhibition constant of 250-400 pM. The present study examined the protein interactions responsible for this inhibition. The isolated catalytic domain of factor XIa is inhibited by protease nexin II with an inhibition constant of 437 +/- 62 pM, compared to 229 +/- 40 pM for the intact protein. Factor XIa is inhibited by a recombinant Kunitz domain with an inhibition constant of 344 +/- 37 pM versus 422 +/- 33 pM for the catalytic domain. Kinetic rate constants were determined by progress curve analysis. The association rate constants for inhibition of factor XIa by protease nexin II [(3.35 +/- 0.35) x 10(6) M(-1) s(-1)] and catalytic domain [(2.27 +/- 0. 25) x 10(6) M(-1) s(-1)] are nearly identical. The dissociation rate constants are very similar, (9.17 +/- 0.71) x 10(-4) and (7.97 +/- 1.1) x 10(-4) s(-1), respectively. The rate constants for factor XIa and catalytic domain inhibition by recombinant Kunitz domain are also very similar: association constants of (3.19 +/- 0.29) x 10(6) and (3.25 +/- 0.44) x 10(6) M(-1) s(-1), respectively; dissociation constants of (10.73 +/- 0.84) x 10(-4) and (10.36 +/- 1.3) x 10(-4) s(-1). The inhibition constant (K(i)) values calculated from these kinetic parameters are in close agreement with those measured from equilibrium binding experiments. These results suggest that the major interactions required for factor XIa inhibition by protease nexin II are localized to the catalytic domain of factor XIa and the Kunitz domain of protease nexin II.

Cerebrovascular smooth muscle cell surface fibrillar A beta. Alteration of the proteolytic environment in the cerebral vessel wall

Cerebrovascular deposition of the amyloid beta-protein (A beta) is a common pathologic event in patients with Alzheimer's disease (AD) and certain related disorders including hereditary cerebral hemorrhage with amyloidosis Dutch-type (HCHWA-D). A beta deposition occurs primarily in the medial layer of the cerebral vessel wall in an assembled fibrillar state. These deposits are associated with several pathological responses including degeneration of the smooth muscle cells in the cerebral vessel wall. Severe cases of cerebrovascular A beta deposition are also accompanied by loss of vessel wall integrity and hemorrhagic stroke. Although the reasons for this pathological consequence are unclear, altered proteolytic mechanisms within the cerebral vessel wall may be involved. Recent studies from our laboratory have shown that cell-surface assembly of A beta into fibrillar structures causes cellular degeneration via an apoptotic pathway and creates an altered proteolytic microenvironment on the cell surface of human cerebrovascular smooth muscle cells (HCSM cells). For example, HCSM cell-surface A beta fibrils serve as a site for tight binding of cell-secreted amyloid beta-precursor protein (A beta PP). Since A beta PP is a potent inhibitor of key proteinases of coagulation cascade, its enhanced localization on the A beta fibrils would provide an strong anticoagulant environment. In addition, HCSM cell-surface A beta fibrils are potent stimulators of tissue plasminogen activator (tPA) creating a profibrinolytic milieu. Our findings indicate that A beta fibril assembly on the HCSM cell surface causes cellular degeneration and results in both a strong anticoagulant and fibrinolytic environment. Together, these altered proteolytic events could create a setting that is conducive to loss of vessel wall integrity and hemorrhagic stroke.