Unusual dissociative behavior of the noncollagenous domain (hexamer) of basement membrane collagen during electrophoresis and chromatofocusing

Collagen IV of basement membranes: I. Origin and diversification of COL4 genes enabling animal evolution

Collagen IV is a primordial component of basement membranes, a specialized form of extracellular matrix that enabled multi-cellular epithelial tissues. In mammals, collagen IV assembles from a family of six α-chains (α1 to α6), encoded by six genes (COL4A1 to COL4A6), into three distinct scaffolds: the α121, the α345 and a mixed scaffold containing both α121 and α565. The six mammalian COL4A genes occur in pairs that occur in a head-to-head arrangement on three distinct chromosomes. In Alport syndrome, variants in the COL4A3, 4 or 5 genes cause either loss or defective assembly of the collagen IV α345 scaffold which results in a dysfunctional glomerular basement membrane, proteinuria and progression to renal failure in millions of people worldwide. Here, we determine the evolutionary emergence and diversification of the COL4A genes using comparative genomics and biochemical analyses. Using syntenic relationships to genes closely linked to the COL4A genes, we determine that the COL4A3 and COL4A4 gene pair appeared in cyclostomes (hagfish and lampreys) while the COL4A5 and COL4A6 gene pair emerged in gnathostomes, jawed vertebrates. The more basal chordate species, lancelets and tunicates, do not have discrete kidneys and have a single COL4A gene pair, though often with single isolated COL4 genes similar to those found in C elegans . Remarkably, while the six COL4A genes are conserved in vertebrates, amphibians have lost the COL4A3 and COL4A4 genes. Our findings of the evolutionary emergence of these genes, together with the amphibian double-knockout, opens an experimental window to gain insights into functionality of the Col IV α345 scaffold.

Vitamin a deficiency and alterations in the extracellular matrix

Vitamin A or retinol which is the natural precursor of several biologically active metabolites can be considered the most multifunctional vitamin in mammals. Its deficiency is currently, along with protein malnutrition, the most serious and common nutritional disorder worldwide. It is necessary for normal embryonic development and postnatal tissue homeostasis, and exerts important effects on cell proliferation, differentiation and apoptosis. These actions are produced mainly by regulating the expression of a variety of proteins through transcriptional and non-transcriptional mechanisms. Extracellular matrix proteins are among those whose synthesis is known to be modulated by vitamin A. Retinoic acid, the main biologically active form of vitamin A, influences the expression of collagens, laminins, entactin, fibronectin, elastin and proteoglycans, which are the major components of the extracellular matrix. Consequently, the structure and macromolecular composition of this extracellular compartment is profoundly altered as a result of vitamin A deficiency. As cell behavior, differentiation and apoptosis, and tissue mechanics are influenced by the extracellular matrix, its modifications potentially compromise organ function and may lead to disease. This review focuses on the effects of lack of vitamin A in the extracellular matrix of several organs and discusses possible molecular mechanisms and pathologic implications.

Quaternary organization of the goodpasture autoantigen, the alpha 3(IV) collagen chain. Sequestration of two cryptic autoepitopes by intrapromoter interactions with the alpha4 and alpha5 NC1 domains

Goodpasture's (GP) disease is caused by autoantibodies that target the alpha3(IV) collagen chain in the glomerular basement membrane (GBM). Goodpasture autoantibodies bind two conformational epitopes (E(A) and E(B)) located within the non-collagenous (NC1) domain of this chain, which are sequestered within the NC1 hexamer of the type IV collagen network containing the alpha3(IV), alpha4(IV), and alpha5(IV) chains. In this study, the quaternary organization of these chains and the molecular basis for the sequestration of the epitopes were investigated. This was accomplished by physicochemical and immunochemical characterization of the NC1 hexamers using chain-specific antibodies. The hexamers were found to have a molecular composition of (alpha3)(2)(alpha4)(2)(alpha5)(2) and to contain cross-linked alpha3-alpha5 heterodimers and alpha4-alpha4 homodimers. Together with association studies of individual NC1 domains, these findings indicate that the alpha3, alpha4, and alpha5 chains occur together in the same triple-helical protomer. In the GBM, this protomer dimerizes through NC1-NC1 domain interactions such that the alpha3, alpha4, and alpha5 chains of one protomer connect with the alpha5, alpha4, and alpha3 chains of the opposite protomer, respectively. The immunodominant Goodpasture autoepitope, located within the E(A) region, is sequestered within the alpha3alpha4alpha5 protomer near the triple-helical junction, at the interface between the alpha3NC1 and alpha5NC1 domains, whereas the E(B) epitope is sequestered at the interface between the alpha3NC1 and alpha4NC1 domains. The results also reveal the network distribution of the six chains of collagen IV in the renal glomerulus and provide a molecular explanation for the absence of the alpha3, alpha4, alpha5, and alpha6 chains in Alport syndrome.

Type IV collagen of the glomerular basement membrane. Evidence that the chain specificity of network assembly is encoded by the noncollagenous NC1 domains

The ultrafiltration function of the glomerular basement membrane (GBM) of the kidney is impaired in genetic and acquired diseases that affect type IV collagen. The GBM is composed of five (alpha1 to alpha5) of the six chains of type IV collagen, organized into an alpha1.alpha2(IV) and an alpha3.alpha4.alpha5(IV) network. In Alport syndrome, mutations in any of the genes encoding the alpha3(IV), alpha4(IV), and alpha5(IV) chains cause the absence of the alpha3. alpha4.alpha5 network, which leads to progressive renal failure. In the present study, the molecular mechanism underlying the network defect was explored by further characterization of the chain organization and elucidation of the discriminatory interactions that govern network assembly. The existence of the two networks was further established by analysis of the hexameric complex of the noncollagenous (NC1) domains, and the alpha5 chain was shown to be linked to the alpha3 and alpha4 chains by interaction through their respective NC1 domains. The potential recognition function of the NC1 domains in network assembly was investigated by comparing the composition of native NC1 hexamers with hexamers that were dissociated and reconstituted in vitro and with hexamers assembled in vitro from purified alpha1-alpha5(IV) NC1 monomers. The results showed that NC1 monomers associate to form native-like hexamers characterized by two distinct populations, an alpha1.alpha2 and alpha3.alpha4.alpha5 heterohexamer. These findings indicate that the NC1 monomers contain recognition sequences for selection of chains and protomers that are sufficient to encode the assembly of the alpha1.alpha2 and alpha3.alpha4.alpha5 networks of GBM. Moreover, hexamer formation from the alpha3, alpha4, and alpha5 NC1 monomers required co-assembly of all three monomers, suggesting that mutations in the NC1 domain in Alport syndrome may disrupt the assembly of the alpha3.alpha4.alpha5 network by interfering with the assembly of the alpha3.alpha4.alpha5 NC1 hexamer.

Comparative analysis of the noncollagenous NC1 domain of type IV collagen: identification of structural features important for assembly, function, and pathogenesis

Type IV collagen alpha1-alpha6 chains have important roles in the assembly of basement membranes and are implicated in the pathogenesis of Goodpasture syndrome, an autoimmune disorder, and Alport syndrome, a hereditary renal disease. We report comparative sequence analyses and structural predictions of the noncollagenous C-terminal globular NC1 domain (28 sequences). The inferred tree verified that type IV collagen sequences fall into two groups, alpha1-like and alpha2-like, and suggested that vertebrate alpha3/alpha4 sequences evolved before alpha1/alpha2 and alpha5/alpha6. About one fifth of NC1 residues were identified to confer either the alpha1 or alpha2 group-specificity. These residues accumulate opposite charge in subdomain B of alpha1 (positive) and alpha2 (negative) sequences and may play a role in the stoichiometric chain selection upon type IV collagen assembly. Neural network secondary structure prediction on multiple aligned sequences revealed a subdomain core structure consisting of six hydrophobic beta-strands and one short alpha-helix with a significant hydrophobic moment. The existence of opposite charges in the alpha-helices may carry implications for intersubdomain interactions. The results provide a rationale for defining the epitope that binds Goodpasture autoantibodies and a framework for understanding how certain NC1 mutations may lead to Alport syndrome. A search algorithm, based entirely on amino acid properties, yielded a possible similarity of NC1 to tissue inhibitor of metalloproteinases (TIMP) and prompted an investigation of a possible functional relationship. The results indicate that NC1 preparations decrease the activity of matrix metalloproteinases 2 and 3 (MMP-2, MMP-3) toward a peptide substrate, though not to [14C]-gelatin. We suggest that an ancestral NC1 may have been incorporated into type IV collagen as an evolutionarily mobile domain carrying proteinase inhibitor function.

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.

Nonenzymatic glycation of type IV collagen and matrix metalloproteinase susceptibility

The glomerular basement membrane (GBM) is damaged in diabetes through complex mechanisms that are not fully understood. Prominent among them is nonenzymatic protein glycation leading to the formation of so-called advanced glycation end products (AGEs). We examined the effects of in vitro glycation of intact collagen type IV in bovine lens capsule (LBM) and kidney glomerular (GBM) basement membranes on their susceptibility to matrix metalloproteinases, using stromelysin 1 (MMP-3) and gelatinase B (MMP-9). Sites of cleavage of unmodified LBM collagen were located in the triple helical region. In vitro glycation by glucose severely inhibited the release of soluble collagen cleavage peptides by MMP-3 and MMP-9. The distribution of AGEs within the three domains of collagen IV (7S, triple helical, and noncollagenous NC1) were compared for LBM glycation using AGE fluorescence, pentosidine quantitation, and immunoreactivity towards anti-AGE antibodies that recognize the AGE carboxymethyllysine (CML). Marked asymmetry was observed, with the flexible triple helical domain having the most pentosidine and fluorescent AGEs but the least CML. The in vivo relevance of these findings is supported by preliminary studies of AGE distribution in renal basement membrane (RBM) collagen IV domains from human kidneys of two insulin-dependent diabetics and one normal subject. Pentosidine and fluorescent AGE distributions of diabetic RBM were similar to LBM, but the CML AGE in diabetic kidney was less in the triple helical domain than in NC1. Our results support the hypothesis that nonenzymatic glycation of collagen IV contributes to the thickening of basement membranes, a hallmark of diabetic nephropathy.

Isoform switching of type IV collagen is developmentally arrested in X-linked Alport syndrome leading to increased susceptibility of renal basement membranes to endoproteolysis

Normal glomerular capillaries filter plasma through a basement membrane (GBM) rich in alpha3(IV), alpha4(IV), and alpha5(IV) chains of type IV collagen. We now show that these latter isoforms are absent biochemically from the glomeruli in patients with X-linked Alport syndrome (XAS). Their GBM instead retain a fetal distribution of alpha1(IV) and alpha2(IV) isoforms because they fail to developmentally switch their alpha-chain use. The anomalous persistence of these fetal isoforms of type IV collagen in the GBM in XAS also confers an unexpected increase in susceptibility to proteolytic attack by collagenases and cathepsins. The incorporation of cysteine-rich alpha3(IV), alpha4(IV), and alpha5(IV) chains into specialized basement membranes like the GBM may have normally evolved to protectively enhance their resistance to proteolytic degradation at the site of glomerular filtration. The relative absence of these potentially protective collagen IV isoforms in GBM from XAS may explain the progressive basement membrane splitting and increased damage as these kidneys deteriorate.

A COL4A3 gene mutation and post-transplant anti-alpha 3(IV) collagen alloantibodies in Alport syndrome

The X-linked Alport syndrome is associated with mutations and deletions in COL4A5 gene, one of six genes which constitute the alpha-chains of type IV collagen in basement membranes. The autosomal recessive form of Alport syndrome is characterized by mutations and deletions in the COL4A3 and COL4A4 genes. A fraction of Alport patients who undergo renal transplantation develop anti-glomerular basement membrane (GBM) nephritis, which results in loss of the renal allograft function. Recently, the target for alloantibodies from an X-linked Alport patient with complete COL4A5 gene deletion was determined to be the alpha 3 chain of type IV collagen. The present study characterized the post-transplant alloantibodies from an autosomal recessive Alport patient with anti-GBM glomerulonephritis and a COL4A3 gene mutation which predicted a loss of 85% of the alpha 3(IV) NC1 domain. The specificity of these new antibodies were studied using glomerular basement membrane constituents and recombinant type IV collagen domains. The results establish the target for the alloantibodies from an autosomal recessive Alport patient with COL4A3 deletion as principally the alpha 3(IV) collagen chain, similar to the post-transplant alloantibodies from X-linked Alport patients with COL4A5 gene deletions. The absence of alpha 3(IV) chain in the GBM of patients with both these forms of Alport syndrome, due either to a failure of synthesis or a failure of assembly, presumably leads to a loss of immunologic tolerance for the alpha 3(IV) NC1 domain in transplanted allografts.