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The Sez6 Family Inhibits Complement by Facilitating Factor I Cleavage of C3b and Accelerating the Decay of C3 Convertases.

ABSTRACT: The Sez6 family consists of Sez6, Sez6L, and Sez6L2. Its members are expressed throughout the brain and have been shown to influence synapse numbers and dendritic morphology. They are also linked to various neurological and psychiatric disorders. All Sez6 family members contain 2-3 CUB domains and 5 complement control protein (CCP) domains, suggesting that they may be involved in complement regulation. We show that Sez6 family members inhibit C3b/iC3b opsonization by the classical and alternative pathways with varying degrees of efficacy. For the classical pathway, Sez6 is a strong inhibitor, Sez6L2 is a moderate inhibitor, and Sez6L is a weak inhibitor. For the alternative pathway, the complement inhibitory activity of Sez6, Sez6L, and Sez6L2 all equaled or exceeded the activity of the known complement regulator MCP. Using Sez6L2 as the representative family member, we show that it specifically accelerates the dissociation of C3 convertases. Sez6L2 also functions as a cofactor for Factor I to facilitate the cleavage of C3b; however, Sez6L2 has no cofactor activity toward C4b. In summary, the Sez6 family are novel complement regulators that inhibit C3 convertases and promote C3b degradation.

SUBMITTER: Qiu WQ

PROVIDER: S-EPMC8081827 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Neisseria meningitidis NalP cleaves human complement C3, facilitating degradation of C3b and survival in human serum.

Project description:The complement system is a crucial component of the innate immune response against invading bacterial pathogens. The human pathogen Neisseria meningitidis (Nm) is known to possess several mechanisms to evade the complement system, including binding to complement inhibitors. In this study, we describe an additional mechanism used by Nm to evade the complement system and survive in human blood. Using an isogenic NalP deletion mutant and NalP complementing strains, we show that the autotransporter protease NalP cleaves C3, the central component of the complement cascade. The cleavage occurs 4 aa upstream from the natural C3 cleavage site and produces shorter C3a-like and longer C3b-like fragments. The C3b-like fragment is degraded in the presence of the complement regulators (factors H and I), and this degradation results in lower deposition of C3b on the bacterial surface. We conclude that NalP is an important factor to increase the survival of Nm in human serum.

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Project description:The control of complement activation in the embryo-maternal environment has been demonstrated to be critical for embryo survival. Complement proteins are expressed in the human endometrium; however, the modulation of this expression by embryo signals has not been explored. To assess the expression of complement proteins in response to human chorionic gonadotropin (hCG), we designed an experimental study using in vivo and in vitro models. Twelve fertile women were treated with hCG or left untreated during the mid-luteal phase, and an endometrial biopsy was performed 24 hours later. The localizations of C3, membrane cofactor protein (MCP; CD46), decay-accelerating factor (DAF; CD55), and protectin (CD59) were assessed by immunohistochemistry, and the messenger RNA (mRNA) levels of these proteins were quantified by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) in cells harvested from endometrial compartments using laser capture microdissection. Endometrial explants were cultured with or without hCG for 24 hours, and the C3 and DAF protein levels were measured by Western blotting. Elevated C3 mRNA levels in stromal cells and elevated DAF levels in epithelial luminal cells were detected after hCG treatment. In the endometrial explant model, the progesterone receptor antagonist RU486 inhibited the increases in the levels of C3 and DAF in response to hCG. The findings of this study indicate that hCG plays a role in embryo-endometrium communication and affects the expression of complement proteins in endometrial compartments during the implantation window.

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Complement Decay-Accelerating Factor is a modulator of influenza A virus lung immunopathology.

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A revised mechanism for the activation of complement C3 to C3b: a molecular explanation of a disease-associated polymorphism.

Project description:The solution structure of complement C3b is crucial for the understanding of complement activation and regulation. C3b is generated by the removal of C3a from C3. Hydrolysis of the C3 thioester produces C3u, an analog of C3b. C3b cleavage results in C3c and C3d (thioester-containing domain; TED). To resolve functional questions in relation to C3b and C3u, analytical ultracentrifugation and x-ray and neutron scattering studies were used with C3, C3b, C3u, C3c, and C3d, using the wild-type allotype with Arg(102). In 50 mm NaCl buffer, atomistic scattering modeling showed that both C3b and C3u adopted a compact structure, similar to the C3b crystal structure in which its TED and macroglobulin 1 (MG1) domains were connected through the Arg(102)-Glu(1032) salt bridge. In physiological 137 mm NaCl, scattering modeling showed that C3b and C3u were both extended in structure, with the TED and MG1 domains now separated by up to 6 nm. The importance of the Arg(102)-Glu(1032) salt bridge was determined using surface plasmon resonance to monitor the binding of wild-type C3d(E1032) and mutant C3d(A1032) to immobilized C3c. The mutant did not bind, whereas the wild-type form did. The high conformational variability of TED in C3b in physiological buffer showed that C3b is more reactive than previously thought. Because the Arg(102)-Glu(1032) salt bridge is essential for the C3b-Factor H complex during the regulatory control of C3b, the known clinical associations of the major C3S (Arg(102)) and disease-linked C3F (Gly(102)) allotypes of C3b were experimentally explained for the first time.

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Project description:To investigate whether the presence of decay-accelerating factor (or CD55), an intrinsic complement regulator, protects against the development of vascular disease, given that complement activation can affect leukocytes and platelets.Leukocyte-platelet complexes are critical for the initiation and progression of atherosclerosis and restenosis; however, the mechanism by which these processes promote vascular injury is incompletely defined. We performed femoral artery wire injury in Daf1(-/-) mice and their wild-type controls. Leukocyte accumulation, cellular proliferation, and neointimal thickening were enhanced in Daf1(-/-) mice versus wild-type mice. Deficiency of either the C3a or the C5a receptor, respectively, reversed the increased vascular inflammation, cellular proliferation, and neointimal formation in Daf1(-/-) mice.Decay-accelerating factor control of C3a and C5a generation and prevention of the binding of these activation fragments to the C3a and C5a receptors are critical for the biological response to vascular injury. Targeting the C3a and C5a receptors may be useful for the prevention of neointimal hyperplasia.

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Disease-associated N-terminal complement factor H mutations perturb cofactor and decay-accelerating activities.

Project description:Many mutations associated with atypical hemolytic uremic syndrome (aHUS) lie within complement control protein modules 19-20 at the C terminus of the complement regulator factor H (FH). This region mediates preferential action of FH on self, as opposed to foreign, membranes and surfaces. Hence, speculation on disease mechanisms has focused on deficiencies in regulation of complement activation on glomerular capillary beds. Here, we investigate the consequences of aHUS-linked mutations (R53H and R78G) within the FH N-terminal complement control protein module that also carries the I62V variation linked to dense-deposit disease and age-related macular degeneration. This module contributes to a four-module C3b-binding site (FH1-4) needed for complement regulation and sufficient for fluid-phase regulatory activity. Recombinant FH1-4(V62) and FH1-4(I62) bind immobilized C3b with similar affinities (K(D) = 10-14 ?M), whereas FH1-4(I62) is slightly more effective than FH1-4(V62) as cofactor for factor I-mediated cleavage of C3b. The mutant (R53H)FH1-4(V62) binds to C3b with comparable affinity (K(D) ?12 ?M) yet has decreased cofactor activities both in fluid phase and on surface-bound C3b, and exhibits only weak decay-accelerating activity for C3 convertase (C3bBb). The other mutant, (R78G)FH1-4(V62), binds poorly to immobilized C3b (K(D) >35 ?M) and is severely functionally compromised, having decreased cofactor and decay-accelerating activities. Our data support causal links between these mutations and disease; they demonstrate that mutations affecting the N-terminal activities of FH, not just those in the C terminus, can predispose to aHUS. These observations reinforce the notion that deficiency in any one of several FH functional properties can contribute to the pathogenesis of this disease.

| S-EPMC3064162 | biostudies-literature

Angiopoietin-like protein 3 inhibits lipoprotein lipase activity through enhancing its cleavage by proprotein convertases.

Project description:Lipoprotein lipase (LPL)-mediated lipolysis of triglycerides is the first and rate-limiting step in chylomicron/very low density lipoprotein clearance at the luminal surface of the capillaries. Angiopoietin-like protein 3 (ANGPTL3) is shown to inhibit LPL activity and plays important roles in modulating lipoprotein metabolism in vivo. However, the mechanism by which it inhibits LPL activity remains poorly understood. Using cell-based analysis of the interaction between ANGPTL3, furin, proprotein convertase subtilisin/kexin type 5 (PCSK5), paired amino acid converting enzyme-4 (PACE4), and LPL, we demonstrated that the cleavage of LPL by proprotein convertases is an inactivation process, similar to that seen for endothelial lipase cleavage. At physiological concentrations and in the presence of cells, ANGPTL3 is a potent inhibitor of LPL. This action is due to the fact that ANGPTL3 can enhance LPL cleavage by endogenous furin and PACE4 but not by PCSK5. This effect is specific to LPL but not endothelial lipase. Both N- and C-terminal domains of LPL are required for ANGPTL3-enhanced cleavage, and the N-terminal domain of ANGPTL3 is sufficient to exert its effect on LPL cleavage. Moreover, ANGPTL3 enhances LPL cleavage in the presence of either heparan sulfate proteoglycans or glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1). By enhancing LPL cleavage, ANGPTL3 dissociates LPL from the cell surface, inhibiting both the catalytic and noncatalytic functions of LPL. Taken together, our data provide a molecular connection between ANGPTL3, LPL, and proprotein convertases, which may represent a rapid signal communication among different metabolically active tissues to maintain energy homeostasis. These novel findings provide a new paradigm of specific protease-substrate interaction and further improve our knowledge of LPL biology.

| S-EPMC2934623 | biostudies-literature

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