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Protein Science : a Publication of the... Feb 1997The covalent binding of complement components C3 and C4 is critical for their activities. This reaction is made possible by the presence of an internal thioester in the... (Review)
Review
The covalent binding of complement components C3 and C4 is critical for their activities. This reaction is made possible by the presence of an internal thioester in the native protein. Upon activation, which involves a conformational change initiated by the cleavage of a single peptide bond, the thioester becomes available to react with molecules with nucleophilic groups. This description is probably sufficient to account for the binding of the C4A isotype of human C4 to amino nucleophiles. The binding of the C4B isotype, and most likely C3, to hydroxyl nucleophiles, however, involves a histidine residue, which attacks the thioester to form an intramolecular acyl-imidazole bond. The released thiolate anion then acts as a base to catalyze the binding of hydroxyl nucleophiles, including water, to the acyl function. This mechanism allows the complement proteins to bind to the hydroxyl groups of carbohydrates found on all biological surfaces, including the components of bacterial cell walls. In addition, the fast hydrolysis of the thioester provides a means to contain this very damaging reaction to the immediate proximity of the site of activation.
Topics: Complement C3; Complement C4; Esters; Humans; Protein Binding; Sulfhydryl Compounds
PubMed: 9041627
DOI: 10.1002/pro.5560060201 -
International Journal of Molecular... Jul 2021In recent years, accumulating evidence has shown that the innate immune complement system is involved in several aspects of normal brain development and in...
In recent years, accumulating evidence has shown that the innate immune complement system is involved in several aspects of normal brain development and in neurodevelopmental disorders, including autism spectrum disorder (ASD). Although abnormal expression of complement components was observed in post-mortem brain samples from individuals with ASD, little is known about the expression patterns of complement molecules in distinct cell types in the developing autistic brain. In the present study, we characterized the mRNA and protein expression profiles of a wide range of complement system components, receptors and regulators in induced pluripotent stem cell (iPSC)-derived neural progenitor cells, neurons and astrocytes of individuals with ASD and neurotypical controls, which constitute in vitro cellular models that recapitulate certain features of both human brain development and ASD pathophysiology. We observed that all the analyzed cell lines constitutively express several key complement molecules. Interestingly, using different quantification strategies, we found that complement C4 mRNA and protein are expressed in significantly lower levels by astrocytes derived from ASD individuals compared to control astrocytes. As astrocytes participate in synapse elimination, and diminished C4 levels have been linked to defective synaptic pruning, our findings may contribute to an increased understanding of the atypically enhanced brain connectivity in ASD.
Topics: Astrocytes; Autism Spectrum Disorder; Cells, Cultured; Complement C4; Humans; Induced Pluripotent Stem Cells; Neural Stem Cells; Neuronal Plasticity; Neurons
PubMed: 34299197
DOI: 10.3390/ijms22147579 -
Clinics (Sao Paulo, Brazil) Mar 2015To perform a molecular characterization of the C1q, C2 and C4 genes in patients with juvenile systemic lupus erythematosus.
OBJECTIVE
To perform a molecular characterization of the C1q, C2 and C4 genes in patients with juvenile systemic lupus erythematosus.
METHODS
Patient 1 (P1) had undetectable C1q, patient 2 (P2) and patient 3 (P3) had decreased C2 and patient 4 (P4) had decreased C4 levels. All exons and non-coding regions of the C1q and C2 genes were sequenced. Mononuclear cells were cultured and stimulated with interferon gamma to evaluate C1q, C2 and C4 mRNA expression by quantitative real-time polymerase chain reaction.
RESULTS
C1q sequencing revealed heterozygous silent mutations in the A (c.276 A>G Gly) and C (c.126 C>T Pro) chains, as well as a homozygous single-base change in the 3' non-coding region of the B chain (c*78 A>G). C1qA mRNA expression without interferon was decreased compared with that of healthy controls (p<0.05) and was decreased after stimulation compared with that of non-treated cells. C1qB mRNA expression was decreased compared with that of controls and did not change with stimulation. C1qC mRNA expression was increased compared with that of controls and was even higher after stimulation. P2 and P3 had Type I C2 deficiency (heterozygous 28 bp deletion at exon 6). The C2 mRNA expression in P3 was 23 times lower compared with that of controls and did not change after stimulation. The C4B mRNA expression of P4 was decreased compared with that of controls and increased after stimulation.
CONCLUSIONS
Silent mutations and single-base changes in the 3' non-coding regions may modify mRNA transcription and C1q production. Type I C2 deficiency should be evaluated in JSLE patients with decreased C2 serum levels. Further studies are needed to clarify the role of decreased C4B mRNA expression in JSLE pathogenesis.
Topics: Adolescent; Base Sequence; Brazil; Case-Control Studies; Child; Complement C1q; Complement C2; Complement C4; Female; Gene Expression; Genetic Association Studies; Genetic Predisposition to Disease; Humans; Lupus Erythematosus, Systemic; Mothers; Mutation; Real-Time Polymerase Chain Reaction; Reference Values; Risk Factors
PubMed: 26017655
DOI: 10.6061/clinics/2015(03)12 -
Journal of the American Society of... Aug 2021Genetic variants in complement genes have been associated with a wide range of human disease states, but well-powered genetic association studies of complement...
BACKGROUND
Genetic variants in complement genes have been associated with a wide range of human disease states, but well-powered genetic association studies of complement activation have not been performed in large multiethnic cohorts.
METHODS
We performed medical records-based genome-wide and phenome-wide association studies for plasma C3 and C4 levels among participants of the Electronic Medical Records and Genomics (eMERGE) network.
RESULTS
In a GWAS for C3 levels in 3949 individuals, we detected two genome-wide significant loci: chr.1q31.3 (CFH locus; rs3753396-A; =0.20; 95% CI, 0.14 to 0.25; =1.52x10) and chr.19p13.3 (C3 locus; rs11569470-G; =0.19; 95% CI, 0.13 to 0.24; =1.29x10). These two loci explained approximately 2% of variance in C3 levels. GWAS for C4 levels involved 3998 individuals and revealed a genome-wide significant locus at chr.6p21.32 (C4 locus; rs3135353-C; =0.40; 95% CI, 0.34 to 0.45; =4.58x10). This locus explained approximately 13% of variance in C4 levels. The multiallelic copy number variant analysis defined two structural genomic C4 variants with large effect on blood C4 levels: C4-BS (=-0.36; 95% CI, -0.42 to -0.30; =2.98x10) and C4-AL-BS (=0.25; 95% CI, 0.21 to 0.29; =8.11x10). Overall, C4 levels were strongly correlated with copy numbers of C4A and C4B genes. In comprehensive phenome-wide association studies involving 102,138 eMERGE participants, we cataloged a full spectrum of autoimmune, cardiometabolic, and kidney diseases genetically related to systemic complement activation.
CONCLUSIONS
We discovered genetic determinants of plasma C3 and C4 levels using eMERGE genomic data linked to electronic medical records. Genetic variants regulating C3 and C4 levels have large effects and multiple clinical correlations across the spectrum of complement-related diseases in humans.
Topics: Adult; Aged; Alleles; Complement Activation; Complement C3; Complement C4; Databases, Genetic; Epidemiologic Studies; Female; Gene Dosage; Genetic Loci; Genetic Variation; Genome-Wide Association Study; Humans; Male; Medical Record Linkage; Medical Records; Middle Aged; Young Adult
PubMed: 33941608
DOI: 10.1681/ASN.2020091371 -
The Journal of Experimental Medicine Nov 2000The complement system enhances antibody responses to T-dependent antigens, but paradoxically, deficiencies in C1 and C4 are strongly linked to autoantibody production in...
The complement system enhances antibody responses to T-dependent antigens, but paradoxically, deficiencies in C1 and C4 are strongly linked to autoantibody production in humans. In mice, disruption of the C1qa gene also results in spontaneous autoimmunity. Moreover, deficiencies in C4 or complement receptors 1 and 2 (CR1/CR2) lead to reduced selection against autoreactive B cells and impaired humoral responses. These observations suggest that C1 and C4 act through CR1/CR2 to enhance humoral immunity and somehow suppress autoimmunity. Here we report high titers of spontaneous antinuclear antibody (ANA) in C4(-/)- mice. This systemic lupus erythematosus-like autoimmunity is highly penetrant; by 10 mo of age, all C4(-)(/)- females and most males produced ANA. In contrast, titers and frequencies of ANA in Cr2(-)(/)- mice, which are deficient in CR1 and CR2, never rose significantly above those in normal controls. Glomerular deposition of immune complexes (ICs), glomerulonephritis, and splenomegaly were observed in C4(-)(/)- but not Cr2(-)(/)- mice. C4(-)(/)-, but not Cr2(-)(/)-, mice accumulate activated T and B cells. Clearance of circulating ICs is impaired in preautoimmune C4(-)(/)-, but not Cr2(-)(/)-, mice. C4 deficiency causes spontaneous, lupus-like autoimmunity through a mechanism that is independent of CR1/CR2.
Topics: Animals; Antibodies, Antinuclear; Antigen-Antibody Complex; Autoimmunity; Cells, Cultured; Complement C4; Disease Models, Animal; Enzyme-Linked Immunosorbent Assay; Female; Gene Deletion; Histocytochemistry; Kidney; Lupus Erythematosus, Systemic; Lymphocyte Activation; Male; Mice; Mice, Inbred Strains; Mice, Knockout; Receptors, Complement 3b; Receptors, Complement 3d; Spleen; Splenomegaly
PubMed: 11067882
DOI: 10.1084/jem.192.9.1339 -
Lupus Science & Medicine Apr 2020To evaluate the association between lupus severity and cell-bound complement activation products (CB-CAPs) or low complement proteins C3 and C4.
OBJECTIVES
To evaluate the association between lupus severity and cell-bound complement activation products (CB-CAPs) or low complement proteins C3 and C4.
METHODS
All subjects (n=495) fulfilled the American College of Rheumatology (ACR) classification criteria for SLE. Abnormal CB-CAPs (erythrocyte-bound C4d or B-lymphocyte-bound C4d levels >99th percentile of healthy) and complement proteins C3 and C4 were determined using flow cytometry and turbidimetry, respectively. Lupus severity was estimated using the Lupus Severity Index (LSI). Statistical analysis consisted of multivariable linear regression and groups comparisons.
RESULTS
Abnormal CB-CAPs were more prevalent than low complement values irrespective of LSI levels (62% vs 38%, respectively, p<0.0001). LSI was low (median 5.44, IQR: 4.77-6.93) in patients with no complement abnormality, intermediate in patients with abnormal CB-CAPs (median 6.09, IQR: 5.31-8.20) and high in the group presenting with both abnormal CB-CAPs and low C3 and/or C4 (median 7.85, IQR: 5.51-8.37). Odds of immunosuppressant use was higher in subjects with LSI ≥5.95 compared with subjects with LSI <5.95 (1.60 vs 0.53, p<0.0001 for both). Multivariable regression analysis revealed that higher LSI scores associated with abnormal CB-CAPs-but not low C3/C4-after adjusting for younger age, race and longer disease duration (p=0.0001), which were also independent predictors of disease severity (global R=0.145).
CONCLUSION
Abnormalities in complement activation as measured by CB-CAPs are associated with increased LSI.
Topics: Adolescent; Adult; B-Lymphocytes; Case-Control Studies; Complement Activation; Complement C3; Complement C4; Cross-Sectional Studies; Erythrocytes; Ethnicity; Female; Flow Cytometry; Humans; Immunosuppressive Agents; Lupus Erythematosus, Systemic; Male; Middle Aged; Severity of Illness Index; Young Adult
PubMed: 32371480
DOI: 10.1136/lupus-2019-000377 -
Clinical and Experimental Immunology Mar 2005We present a consensus document on the diagnosis and management of C1 inhibitor deficiency, a syndrome characterized clinically by recurrent episodes of angio-oedema. In... (Review)
Review
We present a consensus document on the diagnosis and management of C1 inhibitor deficiency, a syndrome characterized clinically by recurrent episodes of angio-oedema. In hereditary angio-oedema, a rare autosomal dominant condition, C1 inhibitor function is reduced due to impaired transcription or production of non-functional protein. The diagnosis is confirmed by the presence of a low serum C4 and absent or greatly reduced C1 inhibitor level or function. The condition can cause fatal laryngeal oedema and features indistinguishable from gastrointestinal tract obstruction. Attacks can be precipitated by trauma, infection and other stimulants. Treatment is graded according to response and the clinical site of swelling. Acute treatment for severe attack is by infusion of C1 inhibitor concentrate and for minor attack attenuated androgens and/or tranexamic acid. Prophylactic treatment is by attenuated androgens and/or tranexamic acid. There are a number of new products in trial, including genetically engineered C1 esterase inhibitor, kallikrein inhibitor and bradykinin B2 receptor antagonist. Individual sections provide special advice with respect to diagnosis, management (prophylaxis and emergency care), special situations (childhood, pregnancy, contraception, travel and dental care) and service specification.
Topics: Adolescent; Adult; Angioedema; Animals; Child; Complement C1 Inactivator Proteins; Complement C4; Emergencies; Female; Humans; Male; Pregnancy; Pregnancy Complications, Hematologic; Syndrome
PubMed: 15730382
DOI: 10.1111/j.1365-2249.2005.02726.x -
Annals of the Rheumatic Diseases Sep 2016Complement-mediated vasculopathy of muscle and skin are clinical features of juvenile dermatomyositis (JDM). We assess gene copy-number variations (CNVs) for complement...
OBJECTIVE
Complement-mediated vasculopathy of muscle and skin are clinical features of juvenile dermatomyositis (JDM). We assess gene copy-number variations (CNVs) for complement C4 and its isotypes, C4A and C4B, in genetic risks and pathogenesis of JDM.
METHODS
The study population included 105 patients with JDM and 500 healthy European Americans. Gene copy-numbers (GCNs) for total C4, C4A, C4B and HLA-DRB1 genotypes were determined by Southern blots and qPCRs. Processed activation product C4d bound to erythrocytes (E-C4d) was measured by flow cytometry. Global gene-expression microarrays were performed in 19 patients with JDM and seven controls using PAXgene-blood RNA. Differential expression levels for selected genes were validated by qPCR.
RESULTS
Significantly lower GCNs and differences in distribution of GCN groups for total C4 and C4A were observed in JDM versus controls. Lower GCN of C4A in JDM remained among HLA DR3-positive subjects (p=0.015). Homozygous or heterozygous C4A-deficiency was present in 40.0% of patients with JDM compared with 18.2% of controls (OR=3.00 (1.87 to 4.79), p=8.2×10(-6)). Patients with JDM had higher levels of E-C4d than controls (p=0.004). In JDM, C4A-deficient subjects had higher levels of E-C4d (p=0.0003) and higher frequency of elevated levels of multiple serum muscle enzymes at diagnosis (p=0.0025). Microarray profiling of blood RNA revealed upregulation of type I interferon-stimulated genes and lower abundance of transcripts for T-cell and chemokine function genes in JDM, but this was less prominent among C4A-deficient or DR3-positive patients.
CONCLUSIONS
Complement C4A deficiency appears to be an important factor for the genetic risk and pathogenesis of JDM, particularly in patients with a DR3-positive background.
Topics: Adolescent; Case-Control Studies; Child; Child, Preschool; Complement C4; Complement C4a; Complement C4b; DNA Copy Number Variations; Dermatomyositis; Female; Genetic Predisposition to Disease; Genotype; HLA-DRB1 Chains; Hereditary Complement Deficiency Diseases; Humans; Immunologic Deficiency Syndromes; Male; Receptors, Tumor Necrosis Factor, Member 25; Risk Factors; White People
PubMed: 26493816
DOI: 10.1136/annrheumdis-2015-207762 -
Arthritis & Rheumatology (Hoboken, N.J.) Jun 2016Human complement C4 is complex, with multiple layers of diversity. The aims of this study were to elucidate the copy number variations (CNVs) of C4A and C4B in relation... (Comparative Study)
Comparative Study
Effects of Complement C4 Gene Copy Number Variations, Size Dichotomy, and C4A Deficiency on Genetic Risk and Clinical Presentation of Systemic Lupus Erythematosus in East Asian Populations.
OBJECTIVE
Human complement C4 is complex, with multiple layers of diversity. The aims of this study were to elucidate the copy number variations (CNVs) of C4A and C4B in relation to disease risk in systemic lupus erythematosus (SLE), and to compare the basis of race-specific C4A deficiency between East Asians and individuals of European descent.
METHODS
The East Asian study population included 999 SLE patients and 1,347 healthy subjects. Variations in gene copy numbers (GCNs) of total C4, C4A, and C4B, as well as C4-Long and C4-Short genes, were determined and validated using independent genotyping technologies. Genomic regions with C4B96 were investigated to determine the basis of the most basic C4B protein occurring concurrently with C4A deficiency.
RESULTS
In East Asians, high GCNs of total C4 and C4A were strongly protective against SLE, whereas low and medium GCNs of total C4 and C4A, and the absence of C4-Short genes, were risk factors for SLE. Homozygous C4A deficiency was infrequent in East Asian subjects, but had an odds ratio (OR) of 12.4 (P = 0.0015) for SLE disease susceptibility. Low serum complement levels were strongly associated with low GCNs of total C4 (OR 3.19, P = 7.3 × 10(-7) ) and C4B (OR 2.53, P = 2.5 × 10(-5) ). Patients with low serum complement levels had high frequencies of anti-double-stranded DNA antibodies (OR 4.96, P = 9.7 × 10(-17) ), hemolytic anemia (OR 3.89, P = 3.6 × 10(-10) ), and renal disease (OR 2.18, P = 8.5 × 10(-6) ). The monomodular-Short haplotype found to be prevalent in European Americans with C4A deficiency, which was in linkage disequilibrium with HLA-DRB1*0301, was scarce in East Asians. Instead, most East Asian subjects with C4A deficiency were found to have a recombinant haplotype with bimodular C4-Long and C4-Short genes, encoding C4B1 and C4B96, which was linked to HLA-DRB1*1501. DNA sequencing revealed an E920K polymorphism in C4B96.
CONCLUSION
C4 CNVs and deficiency of C4A both play an important role in the risk and manifestations of SLE in East Asian and European populations.
Topics: Adult; Asian People; Complement C4a; Complement C4b; DNA Copy Number Variations; Female; Hereditary Complement Deficiency Diseases; Humans; Immunologic Deficiency Syndromes; Lupus Erythematosus, Systemic; Male; Risk Assessment; Risk Factors; White People
PubMed: 26814708
DOI: 10.1002/art.39589 -
The Journal of Investigative Dermatology Dec 1992In view of evidence suggesting vitiligo is an autoimmune disease, we investigated whether vitiligo is associated with inherited deficiencies of the fourth (C4) and...
In view of evidence suggesting vitiligo is an autoimmune disease, we investigated whether vitiligo is associated with inherited deficiencies of the fourth (C4) and second (C2) component of complement and with certain human leukocyte antigens (HLA). Analysis of functional activities of C4 and C2 in sera of patients with vitiligo (n = 42) showed that 17% of them had a heterozygous C4 deficiency and 5% had a heterozygous C2 deficiency. In the normal control group (n = 30), 3% had a heterozygous C4 deficiency and none had a C2 deficiency. C4 typing by Western blot analysis showed the frequency of the C4A*Q0 allele in the vitiligo patient group to be close to normal. However, the frequency of one C4B*Q0 allele was three times higher, and that of two C4B*Q0 alleles five times higher in the vitiligo patient group than the reported frequencies in normal control groups. Southern blot analysis of Taq1 digests of DNA using C4 and 21-hydroxylase probes showed that two patients with two C4B*Q0 alleles had a deletion of a 21-OHA-C4B segment. In the other patients, having one or two C4B*Q0 alleles, these null alleles probably occurred due to a loss of C4 gene expression. HLA analysis did not show any allelic association of C4A*Q0 or C4B*Q0 with any HLA antigen in vitiligo, but confirmed the previous findings of a negative association with HLA-DR3 and a positive association with HLA-DR4. These results suggest that abnormalities of the C4B gene and the above-mentioned associations with HLA antigens may be some of the risk factors in vitiligo.
Topics: Adrenal Hyperplasia, Congenital; Alleles; Complement C2; Complement C4; Family Health; Female; Gene Deletion; HLA Antigens; Histocompatibility Testing; Homozygote; Humans; Male; Pedigree; Proteins; Steroid 21-Hydroxylase; Vitiligo
PubMed: 1469300
DOI: 10.1111/1523-1747.ep12614826