MASP-1 Induced Clotting--The First Model of Prothrombin Activation by MASP-1

Increased risk of venous thromboembolism in young and middle-aged individuals with hereditary angioedema: a family study

ABSTRACT

Hereditary angioedema (HAE), caused by C1 inhibitor protein deficiency, was recently shown to be associated with an increased risk for venous thromboembolism (VTE). To our knowledge, this is the first national family study of HAE, which aimed to determine the familial risk of VTE. The Swedish Multi-Generation Register was linked to the Swedish National Patient Register for the period of 1964 to 2018. Only patients with HAE with a validated diagnosis were included in the study and were linked to their family members. Hazard ratios (HRs) and 95% confidence intervals (CIs) for VTE were calculated for patients with HAE in comparison with relatives without HAE. Among 2006 individuals (from 276 pedigrees of 365 patients with HAE), 103 individuals were affected by VTE. In total, 35 (9.6%) patients with HAE were affected by VTE, whereas 68 (4.1%) non-HAE relatives were affected (P < .001). The adjusted HR for VTE among patients with HAE was 2.51 (95% CI, 1.67-3.77). Patients with HAE were younger at the first VTE than their non-HAE relatives (mean age, 51 years vs 63 years; P < .001). Before the age of 70 years, the HR for VTE among patients with HAE was 3.62 (95% CI, 2.26-5.80). The HR for VTE for patients with HAE born after 1964 was 8.29 (95% CI, 2.90-23.71). The HR for VTE for patients with HAE who were born in 1964 or earlier was 1.82 (95% CI, 1.14-2.91). HAE is associated with VTE among young and middle-aged individuals in Swedish families with HAE. The effect size of the association is in the order of other thrombophilias. We suggest that HAE may be considered a new rare thrombophilia.

Clinical exome sequencing elucidates underlying cause of death in sudden unexpected death of infants: two case reports

Sudden unexpected death in infants (SUDI) is a traumatic event for families, and unfortunately its occurrence remains high in many parts of the world. Whilst cause of death is resolved for most cases, others remain undetermined following postmortem investigations. There has been a recognition of the role of genetic testing in unexplained cases, where previous studies have demonstrated the resolution of cases through DNA analyses. Here we present two case reports of SUDI cases admitted to Salt River Mortuary, South Africa, and show that underlying causes of death were determined for both infants using clinical exome sequencing. The first infant was heterozygous for a variant (rs148175795) in COL6A3, which suggested a bronchopulmonary dysplasia phenotype. This hypothesis led to finding of a second candidate variant in DMP1 (rs142880465), which may contribute towards a digenic/polygenic mechanism of a more severe phenotype. Histological analysis of retained tissue sections showed an asphyxial mechanism of death, where bronchiolar muscle weakness from an underlying bronchopulmonary dysplasia may have contributed to the asphyxia by affecting respiration. In the second infant, a homozygous variant (rs201340753) was identified in MASP1, which was heterozygous in each parent, highlighting the value of including parental DNA in genetic studies. Whilst mannose-binding lectin deficiency could not be assessed, it is plausible that this variant may have acted in combination with other risk factors within the triple-risk model to result in sudden death. These results may have genetic implications for family members, and represent possible new candidate variants for molecular autopsies.

Serum Olink Proteomics-Based Identification of Protein Biomarkers Associated with the Immune Response in Ischemic Stroke

The immune response is considered essential for pathology of ischemic stroke (IS), but it remains unclear which immune response-related proteins exhibit altered expression in IS patients. Here, we used Olink proteomics to examine the expression levels of 92 immune response-related proteins in the sera of IS patients (n = 88) and controls (n = 88), and we found that 59 of these proteins were differentially expressed. Feature variables were screened from the differentially expressed proteins by the least absolute shrinkage and selection operator (LASSO) and the random forest and by determining whether their proteins had an area under the curve (AUC) greater than 0.8. Ultimately, we identified six potential protein biomarkers of IS, namely, MASP1, STC1, HCLS1, CLEC4D, PTH1R, and PIK3AP1, and established a logistic regression model that used these proteins to diagnose IS. The AUCs of the models in the internal validation and the test set were 0.962 (95% confidence interval (CI): 0.895-1.000) and 0.954 (95% CI: 0.884-1.000), respectively, and the same protein detection method was performed in an external independent validation set (AUC: 0.857 (95% CI: 0.801-0.913)). These proteins may play a role in immune regulation via the C-type lectin receptor signaling pathway, the PI3K-AKT signaling pathway, and the B-cell receptor signaling pathway.

The Lectin Pathway of the Complement System-Activation, Regulation, Disease Connections and Interplay with Other (Proteolytic) Systems

The complement system is the other major proteolytic cascade in the blood of vertebrates besides the coagulation-fibrinolytic system. Among the three main activation routes of complement, the lectin pathway (LP) has been discovered the latest, and it is still the subject of intense research. Mannose-binding lectin (MBL), other collectins, and ficolins are collectively termed as the pattern recognition molecules (PRMs) of the LP, and they are responsible for targeting LP activation to molecular patterns, e.g., on bacteria. MBL-associated serine proteases (MASPs) are the effectors, while MBL-associated proteins (MAps) have regulatory functions. Two serine protease components, MASP-1 and MASP-2, trigger the LP activation, while the third component, MASP-3, is involved in the function of the alternative pathway (AP) of complement. Besides their functions within the complement system, certain LP components have secondary ("moonlighting") functions, e.g., in embryonic development. They also contribute to blood coagulation, and some might have tumor suppressing roles. Uncontrolled complement activation can contribute to the progression of many diseases (e.g., stroke, kidney diseases, thrombotic complications, and COVID-19). In most cases, the lectin pathway has also been implicated. In this review, we summarize the history of the lectin pathway, introduce their components, describe its activation and regulation, its roles within the complement cascade, its connections to blood coagulation, and its direct cellular effects. Special emphasis is placed on disease connections and the non-canonical functions of LP components.

Complement inhibition can decrease the haemostatic response in a microvascular bleeding model at multiple levels

Background

Haemostasis is a crucial process by which the body stops bleeding. It is achieved by the formation of a platelet plug, which is strengthened by formation of a fibrin mesh mediated by the coagulation cascade. In proinflammatory and prothrombotic conditions, multiple interactions of the complement system and the coagulation cascade are known to aggravate thromboinflammatory processes and increase the risk of arterial and venous thrombosis. Whether those interactions also play a relevant role during the physiological process of haemostasis is not yet completely understood. The aim of this study was to investigate the potential role of complement components and activation during the haemostatic response to mechanical vessel injury.

Methods

We used a microvascular bleeding model that simulates a blood vessel, featuring human endothelial cells, perfusion with fresh human whole blood, and an inducible mechanical injury to the vessel. We studied the effects of complement inhibitors against components of the lectin (MASP-1, MASP-2), classical (C1s), alternative (FD) and common pathways (C3, C5), as well as a novel triple fusion inhibitor of all three complement pathways (TriFu). Effects on clot formation were analysed by recording of fibrin deposition and the platelet activation marker CD62P at the injury site in real time using a confocal microscope.

Results

With the inhibitors targeting MASP-2 or C1s, no significant reduction of fibrin formation was observed, while platelet activation was significantly reduced in the presence of the FD inhibitor. Both common pathway inhibitors targeting C3 or C5, respectively, were associated with a substantial reduction of fibrin formation, and platelet activation was also reduced in the presence of the C3 inhibitor. Triple inhibition of all three activation pathways at the C3-convertase level by TriFu reduced both fibrin formation and platelet activation. When several complement inhibitors were directly compared in two individual donors, TriFu and the inhibitors of MASP-1 and C3 had the strongest effects on clot formation.

Conclusion

The observed impact of complement inhibition on reducing fibrin clot formation and platelet activation suggests a role of the complement system in haemostasis, with modulators of complement initiation, amplification or effector functions showing distinct profiles. While the interactions between complement and coagulation might have evolved to support haemostasis and protect against bleeding in case of vessel injury, they can turn harmful in pathological conditions when aggravating thromboinflammation and promoting thrombosis.

Inter-α-inhibitor heavy chain H4 and sepsis-related coagulation disturbances: Another link between innate immunity and coagulation

Background

The protease inhibitor inter-α-inhibitor heavy chain H4 (ITIH4) has been described as an acute-phase reactant and could potentially aid in sepsis monitoring and prognostication.

Objectives

To investigate ITIH4 plasma levels in sepsis patients compared with healthy controls and to examine the association between ITIH4 and acute-phase response markers, blood coagulation, and organ dysfunction in sepsis.

Methods

We performed a post hoc study to a prospective cohort study. Patients with septic shock (n = 39) were enrolled upon intensive care unit admission. ITIH4 was analyzed using an in-house immunoassay. Standard coagulation parameters, thrombin generation, fibrin formation and lysis, C-reactive protein, organ dysfunction markers, Sequential Organ Failure Assessment score, and disseminated intravascular coagulation (DIC) score were registered. ITIH4 levels were also investigated in a murine Escherichia coli sepsis model.

Results

ITIH4 did not display acute-phase behavior as mean ITIH4 levels were not increased in patients with septic shock or in E. coli-infected mice. However, ITIH4 exhibited large interindividual variation in patients with septic shock compared with healthy controls. Low ITIH4 was associated with sepsis-related coagulopathy, including a high DIC score (mean ITIH4: DIC, 203 μg/mL vs non-DIC, 267 μg/mL, P = .01), low antithrombin (r = 0.70, P < .0001) and decreased thrombin generation (mean ITIH4: first peak thrombin tertile, 210 μg/mL vs third peak thrombin tertile, 303 μg/mL, P = .01). ITIH4 showed moderate correlation with arterial blood lactate (ρ = -0.50, P < .001) but only weak correlations with C-reactive protein, alanine transaminase, bilirubin, and Sequential Organ Failure Assessment score (all, ρ < 0.26, P > .05).

Conclusion

ITIH4 is associated with sepsis-related coagulopathy but is not an acute-phase reactant during septic shock.

Identification of substrates of MBL Associated Serine Protease-1 (MASP-1) from human plasma using N-terminomics strategy

MBL Associated Serine Protease-1 (MASP-1) is an abundant enzyme of the lectin complement pathway. MASP-1 cleaves numerous substrates like MASP-2, MASP-3, C2, C3i, fibrinogen, FXIII and prothrombin. It has thrombin-like specificity and can cleave thrombin substrates. Owing to its high concentration and relaxed substrate specificity, MASP-1 has substrates outside the complement system and can influence other proteolytic cascades and physiological processes. The unidentified substrates may assist us to ascertain the role(s) of MASP-1. In this study, we used a high-throughput N-terminomics method to identify substrates of MASP-1 from human plasma. We have identified 35 putative substrates of MASP-1. Among the identified proteins, alpha 2-antiplasmin, alpha-1-acid glycoprotein, antithrombin III, and siglec-6 were demonstrated to be cleaved by MASP-1. We have discussed the physiological relevance of cleavage of these substrates by MASP-1. The expression of Siglec-6 and MASP-1 has been reported in the B cells. Alpha-1-acid glycoprotein cleavage by MASP-1 may occur in the acute phase as it is known to be an inhibitor of platelet aggregation, whereas MASP-1 triggers platelet aggregation. The cleavage alpha2 antiplasmin by MASP-1 implies that MASP-1 may be promoting plasmin-mediated fibrinolysis. Our study supports that MASP-1 may be implicated in thrombosis as well as thrombolysis.

Complement lectin pathway components MBL and MASP-1 promote haemostasis upon vessel injury in a microvascular bleeding model

Background

Complement lectin pathway components, in particular mannan-binding lectin (MBL) and MBL-associated serine proteases (MASPs) have been shown to interact with coagulation factors and contribute to clot formation. Here we investigated the role of MBL and MASP-1 in the haemostatic response following mechanical vessel injury in a human microfluidic bleeding model.

Methods

We studied haemostasis in a microvascular bleeding model in the presence of human endothelial cells and human whole blood under flow conditions. We monitored incorporation of proteins into the clot with fluorescently labelled antibodies and studied their effects on clot formation, platelet activation, and bleeding time with specific inhibitors. Platelet activation was also studied by flow cytometry.

Results

Upon vessel injury, MBL accumulated at the injury site in a well-defined wall-like structure. MBL showed partial colocalisation with fibrin, and strong colocalisation with von Willebrand factor and (activated) platelets. Flow cytometry ruled out direct binding of MBL to platelets, but confirmed a PAR4- and thrombin-dependent platelet-activating function of MASP-1. Inhibiting MBL during haemostasis reduced platelet activation, while inhibiting MASP-1 reduced platelet activation, fibrin deposition and prolonged bleeding time.

Conclusion

We show in a microvascular human bleeding model that MBL and MASP-1 have important roles in the haemostatic response triggered by mechanical vessel injury: MBL recognises the injury site, while MASP-1 increases fibrin formation, platelet activation and shortens bleeding time. While the complement lectin pathway may be harmful in the context of pathological thrombosis, it appears to be beneficial during the physiological coagulation response by supporting the crucial haemostatic system.

High Levels of Complement Activating Enzyme MASP-2 Are Associated With the Risk of Future Incident Venous Thromboembolism

BACKGROUND

Experimental studies have shown that the complement activating enzyme MASP-2 (mannose-binding lectin associated serine protease 2) exhibits a thrombin-like activity and that inhibition of MASP-2 protects against thrombosis. In this study, we investigated whether plasma MASP-2 levels were associated with risk of future venous thromboembolism (VTE) and whether genetic variants linked to MASP-2 levels were associated with VTE risk.

METHODS

We conducted a population-based nested case-control study involving 410 VTE patients and 842 age- and sex-matched controls derived from the Norwegian Tromsø Study. Logistic regression was used to estimate odds ratios (ORs) of VTE across MASP-2 quartiles. Whole-exome sequencing and protein quantitative trait loci analyses were performed to assess genetic variants associated with MASP-2 levels. A 2-sample Mendelian randomization study, also including data from the INVENT consortium (International Network of Venous Thrombosis), was performed to assess causality.

RESULTS

Subjects with plasma MASP-2 in the highest quartile had a 48% higher OR of VTE (OR, 1.48 [95% CI, 1.06-2.06]) and 83% higher OR of deep vein thrombosis (OR, 1.83 [95% CI, 1.23-2.73]) compared with those with MASP-2 levels in the lowest quartile. The protein quantitative trait loci analysis revealed that 3 previously described gene variants, rs12711521 (minor allele frequency, 0.153), rs72550870 (minor allele frequency, 0.045; missense variants in the MASP2 gene), and rs2275527 (minor allele frequency, 0.220; exon variant in the adjacent MTOR gene) explained 39% of the variation of MASP-2 plasma concentration. The OR of VTE per 1 SD increase in genetically predicted MASP-2 was 1.03 ([95% CI, 1.01-1.05] P=0.0011).

CONCLUSIONS

Our findings suggest that high plasma MASP-2 levels are causally associated with risk of future VTE.

Proprotein Convertases and the Complement System

Proteins destined for secretion - after removal of the signal sequence - often undergo further proteolytic processing by proprotein convertases (PCs). Prohormones are typically processed in the regulated secretory pathway, while most plasma proteins travel though the constitutive pathway. The complement system is a major proteolytic cascade in the blood, serving as a first line of defense against microbes and also contributing to the immune homeostasis. Several complement components, namely C3, C4, C5 and factor I (FI), are multi-chain proteins that are apparently processed by PCs intracellularly. Cleavage occurs at consecutive basic residues and probably also involves the action of carboxypeptidases. The most likely candidate for the intracellular processing of complement proteins is furin, however, because of the overlapping specificities of basic amino acid residue-specific proprotein convertases, other PCs might be involved. To our surprise, we have recently discovered that processing of another complement protein, mannan-binding lectin-associated serine protease-3 (MASP-3) occurs in the blood by PCSK6 (PACE4). A similar mechanism had been described for the membrane protease corin, which is also activated extracellularly by PCSK6. In this review we intend to point out that the proper functioning of the complement system intimately depends on the action of proprotein convertases. In addition to the non-enzymatic components (C3, C4, C5), two constitutively active complement proteases are directly activated by PCs either intracellularly (FI), or extracellularly (MASP-3), moreover indirectly, through the constitutive activation of pro-factor D by MASP-3, the activity of the alternative pathway also depends on a PC present in the blood.

Thrombin: A Pivotal Player in Hemostasis and Beyond

The serine protease thrombin, a naturally derived enzyme, plays a key role in hemostasis by converting fibrinogen to fibrin and activating coagulation factor XIII whereby the fibrin clot is stabilized. Furthermore, thrombin activates platelets through protease-activated receptors on the platelet surface. Conversely, thrombin also exerts anticoagulant effects, enhancing the protein C activity while complexed with thrombomodulin. During recent years, it has become evident that thrombin has significant effects beyond hemostasis, as it contributes also to modulation of the endothelium, promotes inflammation and angiogenesis, and plays a role in tumor progression. Yet, due to the very short half-life and almost immediate inhibition in fluid phase by antithrombin, thrombin itself remains elusive, and only indirect measurement of thrombin generation is possible. This review provides a description of structure and mechanisms of action of thrombin both in physiological and pathological processes. Furthermore, it summarizes laboratory tests that measure in vivo or ex vivo thrombin generation, and presents knowledge on the value of these biomarkers in bleeding disorders, cardiopulmonary bypass surgery, and thromboembolic risk assessment in different patient populations. Finally, this review outlines further perspectives on using thrombin generation biomarkers for research purposes and in clinical practice.

MASP-1 of the complement system alters fibrinolytic behaviour of blood clots

BACKGROUND

The lectin pathway serine protease mannan-binding lectin-associated serine protease 1 (MASP-1) has been demonstrated to be a major link between complement and coagulation, yet little is known about its interactions with the fibrinolytic system. The aim of this work was to assess the effects of MASP-1 on fibrin clot lysis in different experimental settings.

METHODS

Rotational thrombelastometry was used to evaluate the effect of MASP-1 on the lysis of clots formed in whole blood under static conditions. Whole blood clots were also formed in the presence and absence of MASP-1 under flow conditions in the Chandler loop and their lysis was analysed separately by fluorescence release of incorporated labelled fibrin. Real-time observation by laser scanning confocal microscopy was used to investigate the lysis of plasma clots where MASP-1 was present either during clot formation or lysis. Cleavage of tPA or plasminogen by MASP-1 was analysed by gel electrophoresis. We performed a turbidimetric clot lysis assay in the presence and absence of the MASP-1 inhibitor SGMI-1 (Schistocerca gregaria protease inhibitor (SGPI)-based MASP inhibitor-1) to evaluate the effect of endogenous MASP-1 in normal plasma and plasma samples from sepsis patients.

RESULTS

In the thrombelastometric experiments, where MASP-1 was present during the entire clotting and lysis process, MASP-1 had a significant profibrinolytic effect and accelerated clot lysis. When clots were formed in the presence of MASP-1 under flow in the Chandler loop, the effects on fibrinolysis were heterogenous with impaired fibrinolysis in some individuals (n = 5) and no (n = 3) or even the opposite effect (n = 2) in others. In plasma clot lysis observed by confocal microscopy, lysis was prolonged when MASP-1 was added to the lysis solution, yet there was no difference in lysis time when MASP-1 was present during clot formation. When MASP-1 was incubated with tPA or plasminogen, respectively, cleavage of single-chain tPA into two-chain tPA and a slight reduction of plasminogen were observed. SGMI-1 significantly prolonged clot lysis in the turbidimetric clot lysis assay suggesting that MASP-1 accelerated lysis in plasma samples.

CONCLUSION

MASP-1 is able to alter the susceptibility of blood clots to the fibrinolytic system. MASP-1 has complex, mostly promoting effects on fibrinolysis with high inter-individual variation. Interactions of MASP-1 with the fibrinolytic system may be relevant in the development and therapy of cardiovascular and thrombotic diseases.

MASP-1 Increases Endothelial Permeability

Pathologically increased vascular permeability is an important dysfunction in the pathomechanism of life-threatening conditions, such as sepsis, ischemia/reperfusion, or hereditary angioedema (HAE), diseases accompanied by uncontrolled activation of the complement system. HAE for example is caused by the deficiency of C1-inhibitor (the main regulator of early complement activation), which leads to edematous attacks threatening with circulatory collapse. We have previously reported that endothelial cells become activated during HAE attacks. A natural target of C1-inhibitor is mannan-binding lectin-associated serine protease-1 (MASP-1), a multifunctional serine protease, which plays a key role in the activation of complement lectin pathway. We have previously shown that MASP-1 induces the pro-inflammatory activation of endothelial cells and in this study we investigated whether MASP-1 can directly affect endothelial permeability. All experiments were performed on human umbilical vein endothelial cells (HUVECs). Real-time micro electric sensing revealed that MASP-1 decreases the impedance of HUVEC monolayers and in a recently developed permeability test (XperT), MASP-1 dose-dependently increased endothelial paracellular transport. We show that protease activated receptor-1 mediated intracellular Ca2+-mobilization, Rho-kinase activation dependent myosin light chain (MLC) phosphorylation, cytoskeletal actin rearrangement, and disruption of interendothelial junctions are underlying this phenomenon. Furthermore, in a whole-transcriptome microarray analysis MASP-1 significantly changed the expression of 25 permeability-related genes in HUVECs-for example it up-regulated bradykinin B2 receptor expression. According to our results, MASP-1 has potent permeability increasing effects. During infections or injuries MASP-1 may help eliminate the microbes and/or tissue debris by enhancing the extravasation of soluble and cellular components of the immune system, however, it may also play a role in the pathomechanism of diseases, where edema formation and complement lectin pathway activation are simultaneously present. Our findings also raise the possibility that MASP-1 may be a promising target of anti-edema drug development.

Be on Target: Strategies of Targeting Alternative and Lectin Pathway Components in Complement-Mediated Diseases

The complement system has moved into the focus of drug development efforts in the last decade, since its inappropriate or uncontrolled activation has been recognized in many diseases. Some of them are primarily complement-mediated rare diseases, such as paroxysmal nocturnal hemoglobinuria, C3 glomerulonephritis, and atypical hemolytic uremic syndrome. Complement also plays a role in various multifactorial diseases that affect millions of people worldwide, such as ischemia reperfusion injury (myocardial infarction, stroke), age-related macular degeneration, and several neurodegenerative disorders. In this review, we summarize the potential advantages of targeting various complement proteins with special emphasis on the components of the lectin (LP) and the alternative pathways (AP). The serine proteases (MASP-1/2/3, factor D, factor B), which are responsible for the activation of the cascade, are straightforward targets of inhibition, but the pattern recognition molecules (mannose-binding lectin, other collectins, and ficolins), the regulatory components (factor H, factor I, properdin), and C3 are also subjects of drug development. Recent discoveries about cross-talks between the LP and AP offer new approaches for clinical intervention. Mannan-binding lectin-associated serine proteases (MASPs) are not just responsible for LP activation, but they are also indispensable for efficient AP activation. Activated MASP-3 has recently been shown to be the enzyme that continuously supplies factor D (FD) for the AP by cleaving pro-factor D (pro-FD). In this aspect, MASP-3 emerges as a novel feasible target for the regulation of AP activity. MASP-1 was shown to be required for AP activity on various surfaces, first of all on LPS of Gram-negative bacteria.

MASP-1 of the complement system enhances clot formation in a microvascular whole blood flow model

The complement and coagulation systems closely interact with each other. These interactions are believed to contribute to the proinflammatory and prothrombotic environment involved in the development of thrombotic complications in many diseases. Complement MASP-1 (mannan-binding lectin-associated serine protease-1) activates coagulation factors and promotes clot formation. However, this was mainly shown in purified or plasma-based static systems. Here we describe the role of MASP-1 and complement activation in fibrin clot formation in a microvascular, whole blood flow model. This microfluidic system simulates blood flow through microvessels at physiological flow and shear rates and represents the closest model system to human physiology so far. It features parallel microchannels cultured with endothelial cells in a transparent microfluidic chip allowing real-time evaluation of clot formation by confocal microscopy. To test their effects on clot formation, we added the following activators or inhibitors (individually or in combination) to whole blood and performed perfusion experiments: rMASP-1cf (recombinant active form of MASP-1), complement activator zymosan, selective MASP-1 inhibitor SGMI-1 (based on the Schistocerca gregaria protease inhibitor scaffold), classical pathway inhibitor rSALO (recombinant salivary anti-complement from Lutzomyia longipalpis). Addition of rMASP-1cf resulted in accelerated fibrin clot formation while addition of SGMI-1 delayed it. Complement activation by zymosan led to increased clot formation and this effect was partially reversed by addition of rSALO and almost abolished in combination with SGMI-1. We show for the first time a strong influence of MASP-1, complement activation and pathway-specific inhibition on coagulation in a microvascular flow system that is closest to human physiology, further underpinning the in vivo relevance of coagulation and complement interactions.

The emerging roles of mannose-binding lectin-associated serine proteases (MASPs) in the lectin pathway of complement and beyond

Mannose-binding lectin (MBL)-associated serine proteases (MASPs) are the enzymatic constituents of the lectin pathway of the complement system. They are complexed with large pattern recognition molecules (PRMs) such as MBL, other collectins, and ficolins. The main function of two of the three MASPs has crystallized lately: MASP-1 autoactivates first, then it activates MASP-2, and finally both participate in the formation of the C4b2a convertase. In addition to this, both enzymes are involved in several other processes which are subject to intense research nowadays. Notably, MASP-1, as a promiscuous enzyme, has been implicated in the coagulation cascade, in the kinin generating contact system, and in cellular activation through protease-activated receptor (PAR) cleavage on endothelial cells. The third protease MASP-3 has emerged recently as the protease responsible for pro-factor D activation in resting blood, providing a fundamental link between two complement pathways. At present all three MASPs have at least one well-defined role and several other possible functions were implicated. Defect or more likely over-activation of MASPs may culminate into diseases such as ischemia-reperfusion injury (IRI); hence, MASPs are all potential targets of drug development.