Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi


Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101691


Historical Background

In the late 1980s, a new activation pathway of the complement system, the lectin pathway, was discovered. The complement system is a proteolytic cascade system which forms an effector arm of innate immunity (Ricklin et al. 2010). The complement system is capable of labeling and eliminating invading pathogens (e.g., bacteria, fungi) and dangerously altered host cells (e.g., cancer cells, apoptotic cells). In this way, the complement system contributes to the protection against infection and to the maintenance of the homeostasis of the body. There are three ways through which the complement cascade can be activated. The classical and the alternative pathways of complement activation had been known for decades, when (in 1987) Ikeda showed that a blood-borne lectin (mannose binding lectin = MBL) is capable of initiating the complement activation (Ikeda et al. 1987). First it was assumed that a complex consisting of MBL and the proteases of the classical pathway (C1r and C1s) is responsible for the cleavage of C4 and C2, the components of the classical pathway C3 convertase (C4bC2a). Later, it was discovered that MBL is associated in serum with a novel protease: MBL-associated serine protease (MASP). It became gradually evident that the MASP fraction is not homogeneous, it contains at least two components, and the minor component called MASP-2 is the enzyme which has C1s-like activity through cleaving C4 and C2. The formerly discovered major protease component was then renamed as MASP-1. The physiological role of MASP-1 was a debated issue for a long time. Unlike MASP-2, MASP-1 cannot cleave C4; consequently, it cannot initiate the complement cascade alone. Later, a third protease component of the MBL-MASPs complexes (MASP-3) has also been discovered. Besides the protease components, there are two noncatalytic proteins associated with MBL and with the other pattern recognition molecules: MAp19 (aka MAP-2, sMAP) and MAp44 (aka MAP-1).

Gene Structure and Domain Organization

The proteases of the classical pathway (C1r and C1s) and that of the lectin pathway (MASP-1, MASP-2, and MASP-3) form a family of serine proteases with identical domain organization (Gál et al. 2007). Five N-terminal noncatalytic domains precede the C-terminal serine protease domain (Fig. 1). At the N-terminus, there is a CUB (C1r/C1s, sea urchin Uegf and bone morphogenetic protein-1) domain followed by an EGF (epidermal growth factor)-like and a second CUB domain. The CUB1-EGF-CUB2 fragment is responsible for dimerization and for the binding to the pattern recognition molecules (MBL, ficolins, collectins) in a calcium-dependent manner. The catalytic region of these molecules consists of two CCP (complement control protein) and an SP (serine protease) domain. The serine protease domain, like the proteases of other plasma cascade systems (e.g., blood coagulation, fibrinolysis), is a chymotrypsin-like domain belonging to S1 family of the serine proteases (clan PA). The enzymatic properties of the CCP1-CCP2-SP fragment are equivalent to that of the full-length molecule. The SP domain contains the active center and the substrate-binding cleft, while the CCP modules act like spacers and contain exosites for substrates. In addition to the full-length proteases, there are two truncated gene products: MAp19 and MAp44. MAp19 consists of the first two domains of MASP-2 (CUB1-EGF) plus a unique C-terminus of four amino acids. MAp44 represents the first four domains of MASP-1/3 (CUB1-EGF-CUB2-CCP1) and a 17-amino acid-long C-terminus. The MASPs and MAps are the alternative splice products of two genes: MASP-1, MASP-3, and MAp44 are encoded by the MASP1 gene on chromosome 3q27-q28 (Fig. 1). MASP-1 and MASP-3 have identical noncatalytic region but their SP domains are different. MASP-2 and MAp19 are alternative splice products of the MASP-2 gene on chromosome 1p36.2-3.
MASP-1, Fig. 1

Exon organization of the MASP1 gene and domain structures of the encoded proteins. The MASP1 gene is composed of 18 exons. Exon 1 encodes the signal peptide and exons 2–18 encode the mature proteins. Three proteins are derived from this gene by alternative splicing: MASP-1, MASP-3, and MAp44. Respective domains and the exons encoding them are indicated by identical colors. The activation sites of MASP-1 and MASP-3 are indicated by arrows

The Role of MASP-1 in the Activation of the Complement System

The proteases of the lectin pathway do not act alone, but they form complexes with pattern recognition molecules (PRMs) (Fig. 2). MBL, ficolin-1,-2,-3, collectin liver 1 (CL-L1), and collectin kidney 1 (CL-K1) are the known PRMs of the lectin pathway. They recognize and bind to the surface of invading microorganisms (PAMP = pathogen-associated molecular pattern) or to altered self structures (DAMP = damage/danger-associated molecular pattern). The serine proteases are synthesized as zymogens and they become activated by limited proteolysis upon the PRMs bind to the target surface. The first enzymatic step in the complement cascade is usually an autoactivation reaction. The exact role of MASP-1 in the complement activation was a subject of intense debate for decades. It was shown that MASP-1 can autoactivate and cleave C2 but not C4. Since MASP-2 also can autoactivate and cleave both C4 and C2, it was suggested that MASP-2 is the autonomous activator of the lectin pathway. Although the serum concentration of MASP-1 (143 nM) is much higher than that of MASP-2 (6 nM), only a supporting role was suggested for MASP-1. Using specific inhibitors against MASP-1 and MASP-2, the exact role of these proteases in the lectin pathway activation was clarified (Héja et al. 2012). Inhibition of MASP-1 in normal human serum completely blocked lectin pathway activation including activation of MASP-2. Hence, under physiological circumstances, MASP-1 activates MASP-2 (Fig. 3). The first enzymatic step in the lectin pathway activation is the autoactivation of MASP-1. In the next step, active MASP-1 activates zymogen MASP-2. Active MASP-2 then cleaves C4 while C2 are cleaved by both MASP-1 and MASP-2. Using the specific inhibitors, it was also established that MASP-1 generates the majority (about 60%) of C2a responsible for C3 cleavage in the C4b2a enzyme complex. This mechanism of lectin pathway activation was also confirmed by measuring the kinetic rate constants of the proteolytic reactions (Megyeri et al. 2013) and by using serum of MASP-1/-3-deficient individual (Degn et al. 2012). Recently, it was proposed that the lectin pathway activation occurs through PRM-driven juxtaposition of MASP-1 and MASP-2 on the ligand surface. The MASP-1 and MASP-2 containing complexes form a cluster on the surface and the proteolytic cleavage of MASP-2 by MASP-1 takes place between the complexes (Degn et al. 2014). It must be noted that this clustering-based mechanism is fundamentally different from the conformational model proposed for the classical pathway activation.
MASP-1, Fig. 2

Schematic structure of a typical MASP-1/MBL complex. The initiator complexes of the lectin pathway are usually composed of a pattern recognition molecule (PRM) and a dimeric MASP molecule. The figure depicts the most abundant tetrameric form of MBL complexed with a MASP-1 homodimer. Higher oligomeric forms of PRMs might contain two MASP dimers. Some PRMs might contain MASP heterodimers

MASP-1, Fig. 3

The role of MASPs in complement activation. MASP-1 is the main initiator during lectin pathway activation. It autoactivates first and then it cleaves MASP-2. MASP-2 then cleaves C4 and both MASP-1 and MASP-2 cleave C2 to form C4b2a, the common C3 convertase of the lectin and the classical pathways. Involvement of MASP-1 in alternative pathway activation is also a plausible, possibly through C3 cleavage, whereas MASP-3 is responsible for pro-FD activation in resting blood, hence linking the lectin and the alternative pathways. Red arrows indicate proteolytic cleavage reactions pointing from the enzyme to the substrate. Dashed arrows indicate cleavage reactions that were demonstrated in vitro but their in vivo relevance is not firmly established. Black arrows indicate conversion

It was also demonstrated that MASP-1 is capable of activating zymogen MASP-3 (Megyeri et al. 2013). Recently, it was shown that MASP-3 is responsible for activation of profactor D in resting blood (Dobó et al. 2016). In this way, a crucial link was revealed between the lectin and alternative pathway activation (Fig. 3). Although there is no direct proof that MASP-1 is the physiological activator of MASP-3, we cannot exclude this possibility.

One of the most debated functions of MASP-1 is its ability to cleave C3. Early after its discovery, it was shown that MASP-1 can directly cleave C3 (Matsushita and Fujita 1995). It was suggested that this activity of MASP-1 bypasses the formation of the C3 convertase complexes and might have physiological relevance. However, measuring the catalytic efficiency (kcat/KM value) of the C3 cleavage revealed that MASP-1 is much less efficient in cleaving C3 than a bona fide C3 convertase (C3bBb) (Ambrus et al. 2003). There is three orders of magnitude difference between the corresponding kcat/KM values. Surprisingly, MASP-1 showed 30-fold higher activity on C3(NH3), a molecule which resembles hydrolyzed C3, than on intact C3. We do not know the physiological relevance of these reactions, but we cannot exclude that the weak C3 cleaving ability of MASP-1 is enough for triggering the alternative pathway activation in certain pathophysiological situations.

The Role of MASP-1 in Blood Coagulation

The serine proteases of the complement system have usually very narrow substrate specificity cleaving only one or two physiological substrate during complement activation. MASP-1 is an atypical complement serine protease in this respect (Dobó et al. 2009). Besides the complement proteins (MASP-2, MASP-3, C2, C3), MASP-1 has substrates outside the complement system. It was shown that MASP-1 can initiate and/or enhance blood coagulation. MASP-1 exhibits thrombin-like properties (Fig. 4). First, it was shown that MASP-1 can cleave fibrinogen and Factor XIII (plasma transglutaminase) generating cross-linked fibrin polymer (Hajela et al. 2002). The proteolysis of fibrinogen by MASP-1 liberates fibrinopeptide B, a chemoattractant for neutrophils. The fact that MASP-1 cannot generate fibrinopeptide A indicates that MASP-1 and thrombin have different cleavage sites on the α-chain of fibrinogen. MASP-1 cleaves Factor XIII at the same cleavage site where thrombin cleaves; however, the catalytic efficiency of MASP-1 is much lower on this substrate than that of thrombin. Another interesting difference between MASP-1 and thrombin is that they show different preference towards the polymorphic variants of Factor XIII. MASP-1 prefers the Val34 (P4 residue) polymorphic variant of Factor XIII to the Leu 34 variant (Hess et al. 2012). This phenomenon may have pathophysiological relevance since the cleavage of Factor XIII Val 34 variant may contribute to higher prevalence of thrombotic events. MASP-1, like thrombin, also cleaves thrombin-activatable fibrinolysis inhibitor (TAFI). TAFI prevents fibrinolysis and, interestingly, it also inactivates complement-derived anaphylatoxins (C3a, C5a) with high efficiency. Recently, it was demonstrated that MASP-1 cleaves and activates prothrombin in whole blood. MASP-1 cleaves prothrombin somewhat differently than Factor Xa and thrombin but it can generate thrombin molecule that is active in coagulation (Jenny et al. 2015). The procoagulant activity of MASP-1 may represent an ancient type of innate immune response. Preventing the spread of invading pathogenic microorganisms by engulfing them in a protein mesh is a general protective mechanism in lower animals (e.g., horseshoe crab). The procoagulant activity of MASP-1 was also demonstrated in vivo using mouse models. MASP-1-knockout animals have prolonged bleeding time upon tail tip excision, compared to the wild-type mice (Takahashi et al. 2011). MASP-1-knockout mice are largely protected from ferric chloride-induced occlusive arterial thrombogenesis (La Bonte et al. 2012). These observations reflect that the proteolytic cascade systems in the blood are evolutionary and functionally related, and MASP-1 forms a link between the complement system and coagulation.
MASP-1, Fig. 4

The possible involvement of MASP-1 in blood coagulation. MASP-1 was shown to have a thrombin-like specificity. The action of MASP-1 on several thrombin substrates was demonstrated in vitro. Latest results indicate that the procoagulant activity of MASP-1 depends on the presence of prothrombin. Red arrows indicate proteolytic cleavage reactions pointing from the enzyme to the substrate. Dashed arrows indicate cleavage reactions that were demonstrated in vitro but their in vivo relevance is not firmly established. Black arrows indicate conversion

Proinflammatory Effects of MASP-1 Activation

One of the most important functions of the complement system is the initiation of inflammation. The proteolytic fragments released during complement activation bind to their receptors on leukocytes and endothelial cells triggering proinflammatory responses. There is another way through which proteases can activate cells. It was shown that MASP-1, like thrombin, can cleave protease-activated receptors (PARs) on the endothelial cells (Fig. 5) (Megyeri et al. 2009). In in vitro experiments, MASP-1 was able to cleave PAR1, PAR2, and PAR4. MASP-1 cleaves PAR4 almost as efficiently as thrombin but it cleaves PAR1with lower efficiency. An important difference between thrombin and MASP-1 is that PAR2 can be cleaved only by MASP-1. Treatment of human umbilical vein endothelial cells (HUVEC) with MASP-1 triggered the NF-κB, p38MAPK, pCREB, and JNK pathways and elicited Ca signaling. The MASP-1-treated endothelial cells secreted IL-6 and IL-8 (Jani et al. 2014). The cytokine production was regulated largely by the p38-MAPK pathway. The secreted cytokines induced chemotaxis of neutrophil granulocytes. Moreover, the stimulated endothelial cells increased their E-selectin expression while the amount of ICAM-2 was decreased (Jani et al. 2016). The altered adhesion molecule pattern in HUVECs resulted in increased adherence between differentiated neutrophil granulocyte model cells (PLB-985 cells) and the endothelial cells. These results suggest that MASP-1 accelerates the recruitment of neutrophils at the scene of infection or tissue damage and boosts neutrophil functions. Only activated MASP-1 was able to exert the direct cell-activating function, whereas the zymogen and the active-center mutant forms could not trigger the cells.
MASP-1, Fig. 5

The possible proinflammatory and cell activation roles of MASP-1. MASP-1 was shown to activate endothelial cells through the cleavage protease activated receptors (PARs) resulting in the release of chemotactic and proinflammatory cytokines and upregulating E-selectin expression. MASP-1 can also cleave high-molecular-weight (HMW) kininogen to release the potent proinflammatory peptide bradykinin. Dashed red arrows indicate proteolytic cleavage reactions pointing from the enzyme to the substrate. These reactions were demonstrated in vitro but their in vivo relevance is not firmly established yet. Black arrows indicate conversion or the transduction of a signal

Bradykinin is a potent inflammatory mediator peptide which is liberated from kininogens by the action of proteases. In the blood, high-molecular-weight kininogen is digested by plasma kallikrein during contact system activation resulting in the production of bradykinin. It was shown that MASP-1 also can cleave high-molecular-weight kininogen and liberate bradykinin (Fig. 5) (Dobó et al. 2011). The efficiency of this reaction is lower than the efficiency of the plasma kallikrein-mediated cleavage. However, on the site of infection where the local concentration of complement proteins is high, the MASP-1-mediated bradykinin release can boost the proinflammatory activity of the complement system.

Regulation of MASP-1 Activity

The activity of the early complement proteases in the blood is mainly regulated by serpins (serine protease inhibitors). C1-inhibitor is a serpin which regulates the complement, the coagulation and the kallikrein-kinin systems. It inhibits all the members of the C1r/C1s/MASPs family except MASP-3. The presence of heparin, a naturally occurring glycosaminoglycan, facilitates the interaction between serpins and proteases. It was demonstrated that antithrombin, a serpin which inhibits thrombin, is a very efficient inhibitor of MASP-1 in the presence of heparin (Paréj et al. 2013). It seems that both C1-inhibitor and antithrombin are equally important physiological inhibitors of MASP-1. Another potential inhibitor which regulates plasma proteases is α2-macroglobulin. The role of α2-macroglobulin in the regulation of the lectin pathway is controversial. It was shown that MASP-1 cleaves the bait region of α2-macroglobulin in vitro and a complex is formed between the protease and the inhibitor in fluid phase. However, α2-macroglobulin was unable to prevent complement activation on activating surfaces using normal human serum.


MASP-1 is an abundant protease of the lectin pathway of complement system in the blood. The main function of MASP-1 is the initiation of the lectin pathway activation. MASP-1 is the exclusive activator of MASP-2 in normal human serum and it also contributes to C3 convertase formation by C2 cleavage. MASP-1 has a high autoactivating potential, it can cleave MASP-3 and, to a much lesser extent, C3. MASP-1 is an atypical, promiscuous complement protease, since it has many substrates outside the complement system. MASP-1 can induce coagulation by activating prothrombin and cleaving fibrinogen, Factor XIII, and TAFI. MASP-1 can directly activate endothelial cells through cleaving protease-activated receptors on the surface of the cells. The activated endothelial cells produce cytokines and adhesion molecules which facilitate recruitment and activation of neutrophils, the major cellular elements of innate antimicrobial immunity. MASP-1 also can cleave high-molecular-weight kininogen and liberate bradykinin, a highly inflammatory vasoactive peptide. Taken together, MASP-1 is a versatile protease which initiates the activation of the lectin pathway of the complement system and its activity on noncomplement substrates contributes to mount an even more powerful immune response.


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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Institute of Enzymology, Research Centre for Natural SciencesHungarian Academy of SciencesBudapestHungary