Activation of classical pathway of complement cascade by soluble oligomers of prion

Study of the complement activation by amyloid aggregates of smooth muscle titin in vitro

The giant muscle protein, titin, is the third most abundant protein in muscle (after myosin and actin). It was shown previously that smooth muscle titin (SMT) with a molecular mass of 500 kDa can form in vitro amorphous amyloid aggregates in two conditions: in solution of low ionic strength (0.15 M Glycine-KOH, pH 7.0) (SMT(Gly) aggregates) and in solution with ionic strength in the physiological range (0.2 M KCl, 20 mM imidazole, pH 7.2-7.4) (SMT(KCl) aggregates). Such aggregation in vivo, which may play a pathological or functional role, is not excluded. In view of the fact that some pathological amyloids can activate the classical and alternative pathways of complement system, we investigated the binding of complement component C1q and C3b to smooth muscle titin amyloid aggregates. The binding of С1q and C3b to SMT aggregates was not observed with ELISA assay. Since SMT aggregates do not activate the complement system, they are hardly implicated in the inflammatory process caused by muscle damage in amyloidoses.Abbreviations: SMT: smooth muscle titin; SMT(KCl) aggregates: SMT aggregates in solution containing 0.2 M KCl, 10 mM imidazole, pH 7.0; SMT(Gly) aggregates: SMT aggregates in solution containing 0.15 M glycine-KOH, pH 7.2-7.4; MAC: membrane attack complex; DLS: dynamic light scattering; NHS: Normal Human Serum.

Let's make microglia great again in neurodegenerative disorders

All of the common neurodegenerative disorders-Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and prion diseases-are characterized by accumulation of misfolded proteins that trigger activation of microglia; brain-resident mononuclear phagocytes. This chronic form of neuroinflammation is earmarked by increased release of myriad cytokines and chemokines in patient brains and biofluids. Microglial phagocytosis is compromised early in the disease process, obfuscating clearance of abnormal proteins. This review identifies immune pathologies shared by the major neurodegenerative disorders. The overarching concept is that aberrant innate immune pathways can be targeted for return to homeostasis in hopes of coaxing microglia into clearing neurotoxic misfolded proteins.

Complement factors alter the amount of PrP(Sc) in primary-cultured mouse cortical neurons associated with increased membrane permeability

We examined the effects of complement factors on primary-cultured neurons infected with prions. The amount of protease K (PK)-resistant abnormal form of prion protein (PrP(Sc)) reached a maximum level at 12 and 16 days post exposure (dpe) in 22L- and Chandler-infected neurons, respectively. In Chandler-infected neurons, the reaction of complement factors C1q, C3 and C9 significantly increased membrane permeability. This was followed by a decrease of PK-resistant PrP(Sc) at 16 and 20dpe. In contrast, in 22L-infected neurons, the effects of complement factors were observed at 12 and 16dpe, but not at 20dpe. Membrane permeability also increased in 22L-infected neurons by reaction of complement factor C3, but interestingly, the amount of PK-resistant PrP(Sc) initially decreased, and then increased. These results suggest that the reactivity of complement factors in prion-infected neurons depends on the amount of PrP(Sc) and the prion strain.

The human c1q globular domain: structure and recognition of non-immune self ligands

C1q, the ligand-binding unit of the C1 complex of complement, is a pattern recognition molecule with the unique ability to sense an amazing variety of targets, including a number of altered structures from self, such as apoptotic cells. The three-dimensional structure of its C-terminal globular domain, responsible for its recognition function, has been solved by X-ray crystallography, revealing a tightly packed heterotrimeric assembly with marked differences in the surface patterns of the subunits, and yielding insights into its versatile binding properties. In conjunction with other approaches, this same technique has been used recently to decipher the mechanisms that allow this domain to interact with various non-immune self ligands, including molecules known to provide eat-me signals on apoptotic cells, such as phosphatidylserine and DNA. These investigations provide evidence for a common binding area for these ligands located in subunit C of the C1q globular domain, and suggest that ligand recognition through this area down-regulates C1 activation, hence contributing to the control of the inflammatory reaction. The purpose of this article is to give an overview of these advances which represent a first step toward understanding the recognition mechanisms of C1q and their biological implications.

Reaction of complement factors varies with prion strains in vitro and in vivo

Roles of complement factors in prion infection of the central nervous system remain unclear. In this study, we assessed the strain-dependent reactivity of complement factors in prion infections of Neuro2a (N2a) cells and mouse brains. N2a cells persistently infected with either Chandler or 22L scrapie strains were cultured in the presence of normal mouse serum (NMS), followed by staining with phosphatidylserine binding protein and early apoptosis marker Annexin V. The proportion of Annexin V positive cells was increased both in Chandler- and 22L-infected cells. Preincubation of NMS with anti-C1q, C3 and/or C9 antibodies reduced Annexin V positive cells in Chandler-infected cells, while only anti-C3 antibodies were effective on 22L-infected cells. The immunohistochemistry showed that deposition of C1q and C3 was different between Chandler- and 22L-infected mouse brains. These results indicate that the reactivity of complement factors differs between prion strains both in vitro and in vivo.

Type III hereditary angio-oedema: clinical and biological features in a French cohort

BACKGROUND

Hereditary angio-oedema (HAE) has been associated with C1inhibitor deficiency. The first cases of type III HAE were described in patients with normal C1Inh antigenic protein level and function and normal C4 levels in 2000. This finding has been reported mostly in women with a family history and may be influenced by exogenous oestrogen exposure.

OBJECTIVES

The purpose of this article is to describe the clinical, biological and genetic characteristics of a French population suffering from type III HAE.

PATIENTS AND METHODS

We conducted a retrospective analysis of angio-oedema (AE) cases seen in the National Reference Centre of AE between 2000 and 2009.

RESULTS

We found 26 patients (from 15 unrelated families) with type III HAE. All but four were women and presented with typical AE attacks, exacerbated by pregnancy or oral contraceptives containing oestrogens (OC). We also found that 54.5% of women were worsened with oestrogen and 23% were oestrogen dependent. All patients improved on long-term prophylactic tranexamic acid treatment; some acute attacks improved with C1Inh concentrate infusion. All of the patients had normal C1Inh and C4 levels. C1Inh function was also normal, except in women receiving OC or during a pregnancy: transient, moderately low levels (32-74% of the normal range) were found in 18 patients tested (67%). No SERPING1 gene mutation was found. Six patients from three unrelated families were heterozygous for an F12 gene variant.

CONCLUSION

Diagnosis of type III HAE should be based on clinical (typical attacks, often hormonally influenced), laboratory (normal C1Inh antigenic protein) and genetic (F12 gene mutation) evidence.

Complement protein C1q forms a complex with cytotoxic prion protein oligomers

A growing number of studies have investigated the interaction between C1q and PrP, but the oligomeric form of PrP involved in this interaction remains to be determined. Aggregation of recombinant full-length murine PrP in the presence of 100 mm NaCl allowed us to isolate three different types of oligomers by size-exclusion chromatography. In contrast to PrP monomers and fibrils, these oligomers activate the classical complement pathway, the smallest species containing 8-15 PrP protomers being the most efficient. We used Thioflavine T fluorescence to monitor PrP aggregation and showed that, when added to the reaction, C1q has a cooperative effect on PrP aggregation and leads to the formation of C1q-PrP complexes. In these complexes, C1q interacts through its globular domains preferentially with the smallest oligomers, as shown by electron microscopy, and retains the ability to activate the classical complement pathway. Using two cell lines, we also provide evidence that C1q inhibits the cytotoxicity induced by the smallest PrP oligomers. The cooperative interaction between C1q and PrP could represent an early step in the disease, where it prevents elimination of the prion seed, leading to further aggregation.

Scrapie pathogenesis: the role of complement C1q in scrapie agent uptake by conventional dendritic cells

Mice lacking complement components show delayed development of prion disease following peripheral inoculation. The delay could relate to reduced scrapie prion protein (PrP(Sc)) accumulation on follicular dendritic cells (DCs). However conventional DCs (cDCs) play a crucial role in the early pathogenesis of prion diseases and complement deficiency could result in decreased PrP(Sc) uptake by cDCs in the periphery. To explore this possibility, we cultured murine splenic or gut-associated lymph node cDCs with scrapie-infected whole brain homogenate in the presence or absence of complement. Uptake decreased significantly if the serum in the cultures was heat-inactivated. Because heat inactivation primarily denatures C1q, we used serum from C1q(-/-) mice and showed that PrP(Sc) uptake was markedly decreased. PrP(Sc) internalization was saturable and temperature-dependent, suggesting receptor-mediated uptake. Furthermore, uptake characteristics differed from fluid-phase endocytosis. Immunofluorescence showed colocalization of C1q and PrP(Sc), suggesting interaction between these molecules. We evaluated the expression of several complement receptors on cDCs and confirmed that cDCs that take up PrP(Sc) express one of the C1q receptors, calreticulin. Our results show that C1q participates in PrP(Sc) uptake by cDCs, revealing a critical role for cDCs in initial prion capture, an event that takes place before the PrP(Sc) accumulation within the follicular DC network.

Aspergillus conidia activate the complement by the mannan-binding lectin C2 bypass mechanism

Innate immunity is the major host defense against invasive aspergillosis. To determine whether the collectin mannan-binding lectin (MBL) is involved in the initial protective immunity through complement activation against opportunistic fungal infections caused by Aspergillus, we performed in vitro studies on 29 different strains of Aspergillus conidia from five different species. Incubation of Aspergillus conidia in human normal serum leads to activation of the alternative pathway, whereas neither the classical nor the lectin pathways through C4 and C2 cleavage are activated. Complement response to conidia was investigated using a MBL-deficient serum and reconstitution experiments were conducted with MBL/MASPs complexes. We found that MBL can directly support C3 activation by a C2 bypass mechanism. Finally, a stronger activation of the alternative pathway was observed for the clinical strains isolated from patients with invasive aspergillosis, compared with the environmental strains.

PrP N-terminal domain triggers PrP(Sc)-like aggregation of Dpl

Transmissible spongiform encephalopathies are fatal neurodegenerative disorders thought to be transmitted by self-perpetuating conformational conversion of a neuronal membrane glycoprotein (PrP(C), for "cellular prion protein") into an abnormal state (PrP(Sc), for "scrapie prion protein"). Doppel (Dpl) is a protein that shares significant biochemical and structural homology with PrP(C). In contrast to its homologue PrP(C), Dpl is unable to participate in prion disease progression or to achieve an abnormal PrP(Sc)-like state. We have constructed a chimeric mouse protein, composed of the N-terminal domain of PrP(C) (residues 23-125) and the C-terminal part of Dpl (residues 58-157). This chimeric protein displays PrP-like biochemical and structural features; when incubated in presence of NaCl, the alpha-helical monomer forms soluble beta-sheet-rich oligomers which acquire partial resistance to pepsin proteolysis in vitro, as do PrP oligomers. Moreover, the presence of aggregates akin to protofibrils is observed in soluble oligomeric species by electron microscopy.