Complete Intron/Exon Organization of DNA encoding the α' Chain of Human C3

Additional organizational features of the murine γ-glutamyl hydrolase gene

Analysis of independently isolated clones from a mouse liver cDNA library identified a splice variant of gamma-GH mRNA with novel nucleotide sequence at the 5' end. Genomic sequencing now shows that this variant (variant II) incorporates two new alternates (exons Bla and Blb) of exon 1 in the murine gamma-GH gene remotely situated with respect to the rest of the gene. Further analysis of this variant also showed that it incorporates a small segment at the 3' end of exon A1, revealing that the previously described exon 1 consists of two individual exons (Ala and Alb) joined at a cryptic splice site. The 5' UTR and a segment of the ORF of variant II results from splicing of exon Bla to exon Blb which in turn is spliced to exon Alb and through this splicing to the rest of the exons within this gene. Remarkably, this splicing occurs even though exons Bla and Blb are located >45 Kb upstream of exons Ala and Alb. Our results also show that transcription starting at exons Bla and Blb is under the control of a separate and bidirectional promoter (promoter B). Exons Bla and Blb are located on the sense DNA strand within the complement C3 gene locus which is encoded on the antisense strand. This promoter is less efficient than the downstream promoter (promoter A) in regulating transcription at least in the context of reporter gene and primer extension assays. However, in these same contexts, this region of DNA sequence in the reverse orientation is markedly more efficient in driving transcription of an unidentified gene. Deletion of specific regions of sequence within this promoter have different effects depending upon the orientation (forward or reverse) within the reporter gene construct.

cDNA sequence coding for the alpha'-chain of the third complement component in the African lungfish

cDNA clones coding for almost the entire C3 alpha-chain of the African lungfish (Protopterus aethiopicus), a representative of the Sarcopterygii (lobe-finned fishes), were sequenced and characterized. From the sequence it is deduced that the lungfish C3 molecule is probably a disulphide-bonded alpha:beta dimer similar to that of the C3 components of other jawed vertebrates. The deduced sequence contains conserved sites presumably recognized by proteolytic enzymes (e.g. factor I) involved in the activation and inactivation of the component. It also contains the conserved thioester region and the putative site for binding properdin. However, the site for the interaction with complement receptor 2 and factor H are poorly conserved. Either complement receptor 2 and factor H are not present in the lungfish or they bind to different residues at the same or a different site than mammalian complement receptor 2 and factor H. The C3 alpha-chain sequences faithfully reflect the phylogenetic relationships among vertebrate classes and can therefore be used to help to resolve the long-standing controversy concerning the origin of the tetrapods.

Regulation of C3 and factor H synthesis of human glomerular mesangial cells by IL-1 and interferon-gamma

Previous reports have shown production of complement components C4, C2 and factor B by renal tissue. We have shown recently that human proximal tubular epithelial cells (PTEC) synthesize C3 in vitro, and that IL-2 enhances this production. In the present study we demonstrate that human mesangial cells (MC) in culture produce factor H and that supernatants of activated peripheral blood mononuclear cells (T cell growth factor (TCGF)) induce C3 production and enhance factor H synthesis in both a time- and dose-dependent manner. To investigate whether certain defined cytokines from TCGF were responsible for the observed effect, we tested various cytokines for their effect on complement production by MC. It is shown that IL-1 induces C3 synthesis whereas factor H production is up-regulated by IFN-gamma, in both a dose- and time-dependent manner. Antibody blocking experiments revealed that C3 synthesis induced by both TCGF and IL-1 could be blocked with antibodies specific for IL-1, and also that TCGF and IFN-gamma enhanced factor H synthesis could both be blocked with antibodies specific for IFN-gamma. Cycloheximide was able to inhibit C3 and factor H production, suggesting de novo synthesis of the proteins. mRNA-polymerase chain reaction (PCR) analysis revealed mRNA encoding for C3 after stimulation with TCGF and IL-1. Factor H genes are constitutively expressed in cultured mesangial cells and its expression is up-regulated by TCGF and IFN-gamma. Northern blot analysis with specific probes for C3 and factor H revealed bands which support the results obtained by PCR analysis.

Hereditary deficiency of C3 in animals and humans

Inherited deficiency of complement C3 has been described in guinea pigs, dogs and 20 humans. Homozygous deficiency of C3 is associated with recurrent pyogenic infections by encapsulated bacteria, especially H. influenzae, S. pneumoniae and N. meningitidis. In dogs and humans there is also an association with development of glomerulonephritis of the mesangiocapillary type. Some patients also develop transient erythematous rashes in association with pyogenic infections, with histology showing predominantly neutrophil infiltration and small vessel vasculitis. Studies of antibody responses, mainly in experimental animals have shown impaired primary and secondary responses to both thymus-dependent and -independent antigens at low immunizing doses, with a reduced switch from IgM to IgG production. The molecular basis of C3 deficiency has been established in two humans with C3 deficiency. In one it was due to a splice junction mutation and in another, to a partial gene deletion. These mutations are not compatible with the production of functional C3 in any tissue. Such patients with absolute C3 deficiency are a valid model for understanding the physiological role of C3 in vivo.

Molecular organization of the human cathepsin D gene

A 16-kb fragment of human DNA containing the cathepsin D (CATD) gene was isolated. Nucleotide sequencing, primer extension, protection from mung bean nuclease, and promoter activity assays were used to characterize the gene. The transcribed portion of the gene is about 11,000 bp and is organized into 9 exons analogous with the human pepsinogen A gene. Human pepsinogen A and CATD proteins have 42% sequence identity, while the two cDNAs are 55.7% identical. The positions of the splice junctions are fully conserved in these two genes. The noncoding sequences of the two genes are dissimilar. We report the nucleotide sequence of an Eco RI-Bam HI fragment that contains the transcription initiation site. The promoter region contains no TATA and CCAAT boxes, but five potential Sp1 binding sites (one of them in the first intron) and four AP-2 binding sites (two of them in the first intron). In COS-1 cells, the region containing the three proximal Sp1 sites possesses the bulk of the promoter activity of the 5'-flanking sequence. The transcription start site of the CATD gene is localized within a CpG cluster. In the interval -390 through +450, the content of CpG is 5.8 times above the average throughout the human genome.

Genomic organization of human complement component C3

Six overlapping clones, spanning the entire C3 gene and the 5' flanking region, were isolated from two genomic lambda libraries. Thirty exons, covering the complete beta chain and part of the alpha chain as far as the C3d region, were analyzed. The full exon-intron organization of the gene was deduced by combining our data with the reported organization of the alpha' chain (Barnum et al., 1989, J. Biol. Chem. 264: 8471-8474). The complete gene is 41 kb and consists of 41 exons. The C3 beta chain spans 13 kb from exon 1 to exon 16. Exon 16 encodes both alpha and beta chains. The alpha chain is 28 kb and contains 26 exons, including exon 16. The 5' flanking region was sequenced up to 514 bases upstream from the ATG start site. The major transcription initiation site was mapped to an adenine residue 61 bases upstream from the signal peptide by primer extension analysis of poly(A)+ mRNA from hepatocytes and U937 cells. The TATA box motif was assigned at position -85. Several putative binding sites for transcription factors were found in the 5' flanking region, suggesting possible pathways for the regulation of the C3 gene.

Isolation and analysis of the human MEKA gene encoding a retina-specific protein

We have isolated and sequenced a human gene encoding photoreceptor cell-specific MEKA protein. The protein coding region was separated into three exons, which encoded 246 amino acid residues with a molecular weight of 28,311. The coding region of the human gene exhibited the homologies of 90.7% nucleotides and 88.5% amino acids with those of the bovine cDNA. Southern blot analysis revealed that the human MEKA gene has a single copy number. However, the anti-bovine MEKA stained both rod and cone photoreceptor cells in the human retina.