cDNA isolation and partial gene structure of the human alpha 4(IV) collagen chain

Immunodominant epitopes of α3(IV)NC1 induce autoimmune glomerulonephritis in rats

BACKGROUND

The major Goodpasture antibody binding epitopes have been localized to the amino-terminal third of the noncollagenous domain (NC1) of the alpha3 chain of type IV collagen [alpha3(IV)NC1]. The present study determined whether the same epitopes induce glomerulonephritis in rats.

METHODS

We immunized Wistar Kyoto (WKY) rats with human alpha3(IV)/alpha1(IV)NC1 chimeric proteins or full-length recombinant alpha3(IV)NC1 (alpha3732). Chimeric protein constructs were thirds of alpha3(IV)NC1 (CP333) replaced by corresponding sequences of homologous nonreactive alpha1(IV)NC1 (CP111). All chimeric proteins contained 30 amino acids of type X collagen at the amino terminus except alpha3732. Two other constructs, T195 EA (EA) and T194 EB (EB), were entirely alpha1(IV)NC1, except for antibody-immunodominant amino acids from the first and second thirds of alpha3(IV)NC1.

RESULTS

Construct immunized animals developed specific antibody responses to recombinant proteins and native human, bovine and rat NC1. CP311 immunized rats, as well as alpha3732 rats, had glomerular IgG, fibrin, and glomerulonephritis with proteinuria by 3 weeks. CP331 produced more severe disease, comparable to positive controls. CP111 produced no disease. EA, but not EB, induced severe glomerulonephritis. Half-dose each of EA plus EB induced disease identical to full-dose EA alone.

CONCLUSION

The amino third of alpha3(IV)NC1 which contains the major epitope for Goodpasture antibody binding, also induces glomerulonephritis in rats. The middle third of alpha3(IV)NC1 does not induce glomerulonephritis but appears to enhance disease with the amino terminal third. Finally, the presence of the collagen X leader sequence appears to convey greater nephritogenicity. These studies suggest that not only the nephritogenic epitope itself, but flanking sequences and the conformational context of the nephritogenic epitope may influence its ability to cause glomerulonephritis.

The clinical spectrum of type IV collagen mutations

Clinical manifestations of type IV collagen mutations can vary from the severe, clinically and genetically heterogeneous renal disorder, Alport syndrome, to autosomal dominant familial benign hematuria. The predominant form of Alport syndrome is X-linked; more than 160 different mutations have yet been identified in the type IV collagen alpha 5 chain (COL4A5) gene, located at Xq22-24 head to head to the COL4A6 gene. The autosomal recessive form of Alport syndrome is caused by mutations in the COL4A3 and COL4A4 genes, located at 2q35-37. Recently, the first mutation in the COL4A4 gene was identified in familial benign hematuria. This paper presents an overview of type IV collagen mutations, including eight novel COL4A5 mutations from our own group in patients with Alport syndrome. The spectrum of mutations is broad and provides insight into the clinical heterogeneity of Alport syndrome with respect to age at renal failure and accompanying features such as deafness, leiomyomatosis, and anti-GBM nephritis.

Determination of the genomic structure of the COL4A4 gene and of novel mutations causing autosomal recessive Alport syndrome

Autosomal recessive Alport syndrome is a progressive hematuric glomerulonephritis characterized by glomerular basement membrane abnormalities and associated with mutations in either the COL4A3 or the COL4A4 gene, which encode the alpha3 and alpha4 type IV collagen chains, respectively. To date, mutation screening in the two genes has been hampered by the lack of genomic structure information. We report here the complete characterization of the 48 exons of the COL4A4 gene, a comprehensive gene screen, and the subsequent detection of 10 novel mutations in eight patients diagnosed with autosomal recessive Alport syndrome. Furthermore, we identified a glycine to alanine substitution in the collagenous domain that is apparently silent in the heterozygous carriers, in 11.5% of all control individuals, and in one control individual homozygous for this glycine substitution. There has been no previous finding of a glycine substitution that is not associated with any obvious phenotype in homozygous individuals.

Expression of the alpha1-alpha6 collagen IV chains in the dermoepidermal junction during human foetal skin development: temporal and spatial expression of the alpha4 collagen IV chain in an early stage of development

To study the expression of the alpha1-alpha6 chains of type IV collagen in the dermoepidermal junction (DEJ) during human foetal skin development, human foetal (10 and 20 weeks of gestation) and adult skin was immunostained with specific monoclonal antibodies to the alpha1-alpha6 chains of type IV collagen. Intense expression of the alpha4 chain and weak expression of the alpha2 and alpha6 chains were observed in the DEJ of 10 weeks gestational skin. In contrast, the alpha1, alpha2, alpha5 and alpha6 chains were detected in the DEJ of 20 weeks gestational and adult skin. Preferential expression of alpha4 during the early phase of development (10 weeks of gestation) may suggest a chain-specific regulatory mechanism for type IV collagen expression and its potential role in DEJ formation during development.

Recombinant alpha-chains of type IV collagen demonstrate that the amino terminal of the Goodpasture autoantigen is crucial for antibody recognition

Goodpasture's disease, an autoimmune disorder causing severe glomerulonephritis and pulmonary haemorrhage, is characterized by antibodies to the glomerular basement membrane (GBM). The principal target antigen has been identified as the carboxyl terminal non-collagenous (NC1) domain of the alpha3-chain of type IV collagen. Anti-GBM antibodies appear to recognize one major epitope that is common to all patients, and is largely conformational. We have analysed antibody binding to recombinant alpha(IV)NC1 domains using a construct and expression system shown to produce correctly folded antigen that is strongly recognized by autoantibodies. In this system, as with the native antigen, alpha3(IV)NC1 was bound strongly by antibodies from all patients, whereas the closely related alpha1(IV) and alpha5(IV)NC1 domains, similarly expressed, showed no such binding. A series of chimeric NC1 domains, between human alpha3(IV) and alpha1(IV), and between human and rat alpha3(IV), were expressed as recombinant molecules, and were recognized by autoantibodies to varying degrees. Strong binding required the presence of human alpha3(IV) sequence in the amino terminal region of both sets of chimeric molecules. This work strongly suggests that the amino terminal of alpha3(IV)NC1 is critical for antibody recognition, whereas the carboxyl terminal end of alpha3(IV)NC1 has a less important role.

Glomerular basement membrane. Identification of a novel disulfide-cross-linked network of alpha3, alpha4, and alpha5 chains of type IV collagen and its implications for the pathogenesis of Alport syndrome

Glomerular basement membrane (GBM) plays a crucial function in the ultrafiltration of blood plasma by the kidney. This function is impaired in Alport syndrome, a hereditary disorder that is caused by mutations in the gene encoding type IV collagen, but it is not known how the mutations lead to a defective GBM. In the present study, the supramolecular organization of type IV collagen of GBM was investigated. This was accomplished by using pseudolysin (EC 3.4.24.26) digestion to excise truncated triple-helical protomers for structural studies. Two distinct sets of truncated protomers were solubilized, one at 4 degrees C and the other at 25 degrees C, and their chain composition was determined by use of monoclonal antibodies. The 4 degrees C protomers comprise the alpha1(IV) and alpha2(IV) chains, whereas the 25 degrees C protomers comprised mainly alpha3(IV), alpha4(IV), and alpha5(IV) chains along with some alpha1(IV) and alpha2(IV) chains. The structure of the 25 degrees C protomers was examined by electron microscopy and was found to be characterized by a network containing loops and supercoiled triple helices, which are stabilized by disulfide cross-links between alpha3(IV), alpha4(IV), and alpha5(IV) chains. These results establish a conceptual framework to explain several features of the GBM abnormalities of Alport syndrome. In particular, the alpha3(IV). alpha4(IV).alpha5(IV) network, involving a covalent linkage between these chains, suggests a molecular basis for the conundrum in which mutations in the gene encoding the alpha5(IV) chain cause defective assembly of not only alpha5(IV) chain but also the alpha3(IV) and alpha4(IV) chains in the GBM of patients with Alport syndrome.

Two genes, COL4A3 and COL4A4 coding for the human alpha3(IV) and alpha4(IV) collagen chains are arranged head-to-head on chromosome 2q36

We first isolated and characterized genomic DNA fragments that cover the 5' flanking sequences of COL4A3 and COL4A4 encoding the human basement membrane alpha3(IV) and alpha4(IV) collagen chains, respectively. Nucleotide sequence analysis indicated that the two genes are arranged head-to-head. To determine transcription start site for COL4A4 gene, we performed RACE and RNase protection assays, indicating that there are two alternative transcripts presumably derived from two different promoters. Interestingly, one transcription start site (from exon 1') of COL4A4 is only 5 bp away from the reported transcription start site of COL4A3, whereas the other transcript (from exon 1) starts 373 nucleotides downstream from the first one, generating the two kinds of transcripts that differ in the 5' UTR regions. Expression of these two transcripts appears tissue-specific; exon 1 transcript was expressed predominantly in epithelial cells, while exon 1' transcript showed rather ubiquitous and low expression. The nucleotide sequence of the promoter region is composed of dense CpG dinucleotides, GC boxes, CTC boxes and a CCAAT box but no TATA box. These results provide information to delineate the promoter activity for the tissue-specific expression of the six type IV collagen genes and basement membrane assembly in different tissues and organs.

Seminiferous tubule basement membrane. Composition and organization of type IV collagen chains, and the linkage of alpha3(IV) and alpha5(IV) chains

Seminiferous tubule basement membrane (STBM) plays an important role in spermatogenesis. In the present study, the composition and structural organization of type IV collagen of bovine STBM was investigated. STBM was found to be composed of all six alpha-chains of type IV collagen based upon immunocytochemical and biochemical analysis. The content of alpha3(IV) chain (40%) and the alpha4(IV) chain (18%) was substantially higher than in any other basement membrane collagen. The supramolecular structure of the six alpha(IV) chains was investigated using pseudolysin (EC 3.4.24.26) digestion to excise triple-helical molecules, subsequent collagenase digestion to produce NC1 hexamers and antibody affinity chromatography to resolve populations of NC1 hexamers. The hexamers, which reflect specific arrangements of alpha(IV) chains, were characterized for their alpha(IV) chain composition using high performance liquid chromatography, two-dimensional electrophoresis, and immunoblotting with alpha(IV) chain-specific antibodies. Three major hexamer populations were found that represent the classical network of the alpha1(IV) and alpha2(IV) chains and two novel networks, one composed of the alpha1(IV)-alpha6(IV) chains and the other composed of the alpha3(IV)-alpha6(IV) chains. The results establish a structural linkage between the alpha3(IV) and alpha5(IV) chains, suggesting a molecular basis for the conundrum in which mutations in the gene encoding the alpha5(IV) chain cause defective assembly of the alpha3(IV) chain in the glomerular basement membrane of patients with Alport syndrome.

Differential expression of alpha 1(IV), alpha 2(IV), alpha 5(IV) and alpha 6(IV) collagen chains in the basement membrane of basal cell carcinoma

Type IV collagen, the major component of basement membrane, consists primarily of alpha 1(IV) and alpha 2(IV) chains. Recently, other types of collagen IV chains, i.e. alpha 3(IV), alpha 4(IV), alpha 5(IV) and alpha 6(IV) chains, have been identified by protein chemistry and molecular cloning. We have examined the diversity of the assembly of alpha (IV) chains of the basement membrane surrounding tumour nests of basal cell carcinomas, in tissues from 11 patients, by immunohistochemical analysis using specific monoclonal antibodies to six alpha (IV) chain. The immunostaining profile of each chain differed with respect to the histological subtypes of basal cell carcinoma. In the morphea-like subtype, which was more invasive, alpha 1(IV) and alpha 2(IV) chains were discontinuously stained, and alpha 5(IV) and alpha 6(IV) chains were entirely absent. However, in the superficial subtype, which was non-aggressive, alpha 1(IV), alpha 2(IV), alpha 5(IV) and alpha 6(IV) chains were well stained compared with the other subtypes of basal cell carcinoma. In addition, in the solid subtype, which showed slow growth and ulceration, alpha 1(IV) and alpha 2(IV) chains were continuously stained, and alpha 5(IV) and alpha 6(IV) chains were discontinuous or absent. The assembly of alpha 5(IV) and alpha 6(IV) chains into the basement membrane was inhibited in the solid and morphea subtypes of BCC. This differential expression of type IV collagen chains seems to be associated with the invasive potential of basal cell carcinoma.

Coordinate gene expression of the alpha3, alpha4, and alpha5 chains of collagen type IV. Evidence from a canine model of X-linked nephritis with a COL4A5 gene mutation

Canine X-linked hereditary nephritis is an animal model for human X-linked hereditary nephritis with a premature stop codon in the alpha5(IV) gene of collagen type IV. We used this model to examine the other alpha(IV) chains at the mRNA and protein level in the kidney, since in human X-linked hereditary nephritis, the alpha3(IV) and alpha4(IV) chains are often absent from the glomerular basement membrane, although both are encoded by autosomal genes. cDNA probes for the alpha1(IV)-alpha6(IV) chains were generated from normal dog kidney using the polymerase chain reaction. Sequences were >/=88% identical at the DNA level and >/=92% identical at the protein level to the respective human alpha(IV) chains. By Northern analysis, transcripts for the alpha1(IV), alpha2(IV), and alpha6(IV) chains were detected at comparable levels in both normal and affected male dog kidney RNA. As previously shown, the transcript for the alpha5(IV) chain was reduced to approximately 10% of normal. Unexpectedly, the alpha3(IV) and alpha4(IV) transcripts were both decreased >/=77% in affected male dog kidney, suggesting a mechanism coordinating the expression of these three basement membrane components. The NC1 domain of collagen type IV isolated from normal dog glomeruli was positive for the alpha3(IV), alpha4(IV), and alpha5(IV) chains by Western blotting. In contrast, in the NC1 domain isolated from affected dog glomeruli, these three chains were not detectable, except for a trace of alpha3(IV) dimer. In X-linked hereditary nephritis, the absence of the alpha3(IV) and alpha4(IV) chains from glomerular basement membrane may reflect factors acting at the transcriptional and/or translational level in addition to the protein assembly level.

Hsp47 and other ER-resident molecular chaperones form heterocomplexes with each other and with collagen type IV chains

Collagen type IV is a genetically distinct secretory protein that constitutes a major component of the basement membranes. Despite the differences between collagen types I and IV, it appears that collagen IV alpha-chain translation/translocation through the ER should follow similar pathways to those established for procollagen I. Using a collagen IV-producing mouse teratocarcinoma cell line, we sought to determine if evolving nascent chains of collagen IV are associated with Hsp47 and other ER-resident molecular chaperones. We show that Hsp47, Grp78 and Grp94 appear to form heteromeric complexes associated with each other and with nascent chains of collagen IV. In addition, we investigated whether ATP depletion, a condition known to promote stable association of Grp78 and Grp94 proteins with their chaperone substrates, extended this response to include a chaperone independent of ATP, Hsp47. These studies reveal that ATP depletion increased the levels of newly synthesized collagen IV associated with Hsp47, Grp78, Grp94 and with each other. These results indicate that Hsp47, Grp78 and Grp94 exist as oligomers and form heterocomplexes during the maturation of newly synthesized collagen IV chains.

Isolation and structure of the COL4A6 gene encoding the human alpha 6(IV) collagen chain and comparison with other type IV collagen genes

The genes COL4A5 and COL4A6, coding for the basement membrane collagen chains, alpha 5(IV) and alpha 6(IV), respectively, are located head-to-head in close proximity on human chromosome Xq22, and COL4A6 is transcribed from two alternative promoters in a tissue-specific fashion (Sugimoto M., Oohashi T., and Ninomiya Y. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 11679-11683). Immunofluorescence studies using alpha chain-specific antibodies demonstrated that the two genes are expressed in a tissue-specific manner (Ninomiya, Y., Kagawa, M., Iyama, K., Naito, L., Kishiro, Y., Seyer, J. M., Sugimoto, M., Oohashi, T., and Sado, Y. (1995) J. Cell Biol. 130, 1219-1229). We report here for the first time the isolation and the structural organization of the human COL4A6 gene. The entire gene presumably exceeds 200 kilobase pairs and contains 46 exons. Exons 1' and 1 encode the two different 5'-UTRs and the two amino-terminal parts of of the signal peptide. The carboxyl part of the signal peptide and the 7 S domain are coded for by the following 6 different exons, 2-7, whereas the exons 7-42 encode the central COL 1 domain, which contains the Gly-X-Y repeats. The last three exons, 43-45, encode the carboxyl-terminal NC1 domain. Sizes of more than a half of the exons of the gene are the same as those of Col4a2 but quite different from those of COL4A5. Within the COL4A6 gene we found three CA repeat markers that can be used for allele detection. The detailed structure of the COL4A6 gene and the high heterozygosity microsatellite markers located within the gene will be useful for linkage analysis and familial diagnosis of diseases caused by mutations of this gene.

Comparative distribution of the alpha 1(IV), alpha 5(IV), and alpha 6(IV) collagen chains in normal human adult and fetal tissues and in kidneys from X-linked Alport syndrome patients

We have shown previously that the 5' ends of the genes for the alpha 5(IV) and alpha 6(IV) collagen chains lie head-to-head on Xq22 and are deleted in patients with Alport syndrome (AS)-associated diffuse leiomyomatosis. In this study, we raised a rabbit anti-human alpha 6(IV)chain antibody, demonstrated its specificity by the analysis of recombinant NC1 domains af all six type IV chains, and studied the distribution of the alpha 6(IV) chain in relation to the alpha 1(IV) and alpha 5(IV) chains in human adult and fetal tissues involved in AS and diffuse leiomyomatosis. The alpha 6(IV) chain colocalizes with the alpha 5(IV) chain in basement membranes (BMs) of many tissues, but not in glomerular BM. These data exclude the alpha 6(IV) chain as a site for AS mutations. The head-to-head genomic pairing of the alpha 5(IV) and alpha 6 (IV) genes implies coordinate transcription of the two genes. Differential localization of the alpha 5(IV) and alpha 6(IV) chains shows that the two chains are not always coordinately regulated. The alpha 6(IV) chain, together with the alpha 3(IV)-alpha 5(IV) chains, was absent from all renal BMs in eight patients with X-linked AS while the alpha 1(IV) and alpha 2(IV) chains were increased. The data support the existence of two independent collagen networks, one for the alpha 3(IV)-alpha 6(IV) chains and one for the alpha 1(IV) and alpha 2(IV) chains.

Differential expression of two basement membrane collagen genes, COL4A6 and COL4A5, demonstrated by immunofluorescence staining using peptide-specific monoclonal antibodies

Genes for the human alpha 5(IV) and alpha 6(IV) collagen chains have a unique arrangement in that they are colocalized on chromosome Xq22 in a head-to-head fashion and appear to share a common bidirectional promoter. In addition we reported a novel observation that the COL4A6 gene is transcribed from two alternative promoters in a tissue-specific manner (Sugimoto, M., T. Oohashi, and Y. Ninomiya. 1994. Proc. Natl. Acad. Sci. USA. 91:11679-11683). To know whether the translation products of both genes are colocalized in various tissues, we raised alpha 5(IV) and alpha 6(IV) chain-specific rat monoclonal antibodies against synthetic peptides reflecting sequences near the carboxy terminus of each noncollagenous (NC)1 domain. By Western blotting alpha 6(IV) chain-specific antibody recognized 27-kD monomers and associated dimers of the human type IV collagen NC1 domain, which is the first demonstration of the presence in tissues of the alpha 6(IV) polypeptide as predicted from its cDNA sequence. Immunofluorescence studies using anti-alpha 6(IV) antibody demonstrated that in human adult kidney the alpha 6(IV) chain was never detected in the glomerular basement membrane, whereas the basement membranes of the Bowman's capsules and distal tubules were positive. The staining pattern of the glomerular basement membrane was quite different from that obtained with the anti-alpha 5(IV) peptide antibody. The alpha 5(IV) and alpha 6(IV) chains were colocalized in the basement membrane in the skin, smooth muscle cells, and adipocytes; however, little if any reaction was seen in basement membranes of cardiac muscles and hepatic sinusoidal endothelial cells. Thus, both genes are expressed in a tissue-specific manner, perhaps due to the unique function of the bidirectional promoter for both genes, which is presumably different from that for COL4A1 and COL4A2.

Analysis of T cell responses to the autoantigen in Goodpasture's disease

Goodpasture's disease is a rare form of glomerulonephritis characterized by the production of autoantibodies to the glomerular basement membrane (GBM). In order to understand the development of autoimmunity to the GBM, it is important to examine mechanisms underlying T cell responses to the autoantigen. A MoAb P1, with the same specificity as patients' autoantibodies, was used to affinity-purify the antigen from collagenase-digested human GBM. This material was enriched in the NC1 domain of the alpha 3 chain of type IV collagen (alpha 3(IV)NC1), known to be the principal target of anti-GBM antibodies, but also contained lower quantities of alpha 4(IV)NC1. In proliferation assays, T cells from 11/14 patients with Goodpasture's disease showed significant responses (SI > or = 2.0) to affinity-purified human GBM. Peak responses were demonstrated at 7 or 10 days at antigen concentrations of 10-30 micrograms/ml. As in other autoimmune disorders, the presence of autoantigen-reactive T cells was also demonstrated in 5/10 healthy volunteers. Tissue typing revealed that all patients possessed HLA-DR2 and/or -DR4 alleles, while normal individuals whose T cells responded possessed DR2 and/or DR7 alleles. The specificity of the T cell response in Goodpasture's disease was further investigated using monomeric components of human GBM purified by gel filtration and reverse phase high performance liquid chromatography (HPLC). Two antigenic monomer pools were obtained, which were shown by amino-terminal sequence analysis to contain alpha 3(IV)NC1 and alpha 4(IV)NC1, respectively. In all patients tested, significant T cell proliferation was observed in response to one or both of these alpha (IV)NC1 domains. These results demonstrate that patients with Goodpasture's disease possess T cells reactive with autoantigens known to be recognized by anti-GBM antibodies.

Basal lamina assembly

From studies of the 'classical' components, models for the assembly and structure of an idealized basal lamina have been developed. In particular, the evidence supports the concept of enmeshed collagen and laminin polymers, in which nidogen/entactin acts as a bridge between these molecules and provides anchorage for diverse matrix components. Different basement membranes, however, possess different members of the basic basal lamina families, such as the newly described alpha 6 (IV) collagen, alpha 2 (merosin) laminin, and beta 3 laminin (in kalinin/nicein) chains. Even though these members share homologous domains and sequences, and are likely to share certain functions, they also possess unique characteristics that are expected to provide for basal lamina heterogeneity. A combination of genetic, recombinant and biochemical approaches are now being applied to elucidate the special roles of both old and new components.

Identification of mutations in the alpha 3(IV) and alpha 4(IV) collagen genes in autosomal recessive Alport syndrome

Alport syndrome (AS) is an hereditary disease of basement membranes characterized by progressive renal failure and deafness. Changes in the glomerular basement membrane (GBM) in AS suggest that the type IV collagen matrix, the major structural component of GBM, is disrupted. We recently isolated the genes for two type IV collagens, alpha 3(IV) and alpha 4(IV), that are encoded head-to-head on human chromosome 2. These chains are abundant in normal GBM but are sometimes absent in AS. We screened for mutations in families in which consanguinity suggested autosomal recessive inheritance. Homozygous mutations were found in alpha 3(IV) in two families and in alpha 4(IV) in two others, demonstrating that these chains are important in the structural integrity of the GBM and that there is an autosomal form of AS in addition to the previously-defined X-linked form.