The human lectin complement pathway involves circulating complexes consisting of mannose-binding lectin (MBL) or three ficolins (ficolin-1, -2, and -3) in association with three MBL/ ficolin-associated serine proteases (MASP) (MASP-1, -2, and -3) and a nonenzymatic sMAP. MASP-1 and MASP-3 (MASP1 isoforms 1 and 2, respectively) are splice variants of the MASP1 gene, whereas MASP-2 and sMAP are splice variants of the MASP2 gene. We have identified a novel serum protein of 45 kDa that is associated with MBL and the ficolins. This protein is named MBL/ficolin-associated protein 1 (MAP-1 corresponding to MASP1 isoform 3). The transcript generating MAP-1 (MASP1_v3) contains exons 1-8 and a novel exon encoding an in-frame stop codon. The corresponding protein lacks the serine protease domains but contains most of the common heavy chain of MASP-1 and MASP-3. Additionally MAP-1 contains 17 unique C-terminal amino acids. By use of quantitative PCR and MAP-1-specific immunohistochemistry, we found that MAP-1 is highly expressed in myocardial and skeletal muscle tissues as well as in liver hepatocytes with a different expression profile than that observed for MASP-1 and MASP-3. MAP-1 co-precipitated from human serum with MBL, ficolin-2, and ficolin-3, and recombinant MAP-1 was able to inhibit complement C4 deposition via both the ficolin-3 and MBL pathway. In conclusion we have identified a novel 45-kDa serum protein derived from the MASP1 gene, which is highly expressed in striated muscle tissues. It is found in complex with MBL and ficolins and may function as a potent inhibitor of the complement system in vivo.Activation of the complement system is accomplished via three different initiation pathways: the alternative pathway, the classical pathway, and the lectin pathway (1). In humans, four recognition molecules of the lectin pathway have been described: mannose-binding lectin (MBL), 3 ficolin-1 (also called M-ficolin), ficolin-2 (also called L-ficolin), and ficolin-3 (also called H-ficolin or Hakata antigen) (2). MBL and the ficolins bind structures on different classes of microorganisms and are involved in sequestration and removal of dying host cells (2, 3). Three MBL/ficolin-associated serine proteases have been described so far (MASP-1, MASP-2, and MASP-3), as well as a protein lacking a serine protease domain named sMAP or MAp19 (4). Present consensus places MASP-2 as the main initiator of the lectin complement pathway by cleaving C4 and C2 to form the C4b2a complex leading to further downstream complement activation (5). Although the functions of the other MASPs are poorly understood, MASP-1 appears to play a role as an amplifier of complement activation (6, 7). Additionally, MASP-1 is able to cleave fibrinogen to fibrin, whereas MASP-2 is able to generate active thrombin by cleavage of prothrombin (8,9). No conclusive biological function has yet been attributed to MASP-3 and sMAP. MASP-1 and MASP-2 were originally named after their association with MBL (5, 10). Subsequently, MASP-3 and sMAP (also named Map19) we...

The complement system is an important part of the innate immune system. The complement cascade may be initiated downstream of the lectin activation pathway upon binding of mannan-binding lectin, ficolins, or collectin kidney 1 (CL-K1, alias CL-11) to suitable microbial patterns consisting of carbohydrates or acetylated molecules. During purification and characterization of native CL-K1 from plasma, we observed that collectin liver 1 (CL-L1) was copurified. Based on deglycosylation and nonreduced/reduced two-dimensional SDS-PAGE, we detected CL-K1 and CL-L1 in disulfide bridge-stabilized complexes. Heteromeric complex formation in plasma was further shown by ELISA and transient coexpression. Judging from the migration pattern on two-dimensional SDS-PAGE, the majority of plasma CL-K1 was found in complex with CL-L1. The ratio of this complex was in favor of CL-K1, suggesting that a heteromeric subunit is composed of one CL-L1 and two CL-K1 polypeptide chains. We found that the complex bound to mannan-binding lectin–associated serine proteases (MASPs) with affinities in the nM range in vitro and was associated with both MASP-1/-3 and MASP-2 in plasma. Upon binding to mannan or DNA in the presence of MASP-2, the CL-L1–CL-K1 complex mediated deposition of C4b. In favor of large oligomers, the activity of the complex was partly determined by the oligomeric size, which may be influenced by an alternatively spliced variant of CL-K1. The activity of the native heteromeric complexes was superior to that of recombinant CL-K1. We conclude that CL-K1 exists in circulation in the form of heteromeric complexes with CL-L1 that interact with MASPs and can mediate complement activation.

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Background and objectives C3 glomerulopathy (C3G) applies to a group of renal diseases defined by a specific renal biopsy finding: a dominant pattern of C3 fragment deposition on immunofluorescence. The primary pathogenic mechanism involves abnormal control of the alternative complement pathway, although a full description of the disease spectrum remains to be determined. This study sought to validate and define the association of complement dysregulation with C3G and to determine whether specific complement pathway abnormalities could inform disease definition.Design, setting, participants, & measurements This study included 34 patients with C3G (17 with C3 glomerulonephritis [C3GN] and 17 with dense deposit disease [DDD]) diagnosed between 2008 and 2013 selected from the C3G Registry. Control samples (n=100) were recruited from regional blood drives. Nineteen complement biomarkers were assayed on all samples. Results were compared between C3G disease categories and with normal controls.Results Assessment of the alternative complement pathway showed that compared with controls, patients with C3G had lower levels of serum C3 (P,0.001 for both DDD and C3GN) and factor B (P,0.001 for both DDD and C3GN) as well as higher levels of complement breakdown products including C3d (P,0.001 for both DDD and C3GN) and Bb (P,0.001 for both DDD and C3GN). A comparison of terminal complement pathway proteins showed that although C5 levels were significantly suppressed (P,0.001 for both DDD and C3GN) its breakdown product C5a was significantly higher only in patients with C3GN (P,0.05). Of the other terminal pathway components (C6-C9), the only significant difference was in C7 levels between patients with C3GN and controls (P,0.01). Soluble C5b-9 was elevated in both diseases but only the difference between patients with C3GN and controls reached statistical significance (P,0.001). Levels of C3 nephritic factor activity were qualitatively higher in patients with DDD compared with patients with C3GN.Conclusions Complement biomarkers are significantly abnormal in patients with C3G compared with controls. These data substantiate the link between complement dysregulation and C3G and identify C3G interdisease differences.

Svenningsen P, Uhrenholt TR, Palarasah Y, Skjødt K, Jensen BL, Skøtt O. Prostasin-dependent activation of epithelial Na ϩ channels by low plasmin concentrations.