The NC1 domain of collagen IV encodes a novel network composed of the alpha 1, alpha 2, alpha 5, and alpha 6 chains in smooth muscle basement membranes

Type IV collagen alpha chains of the basement membrane in the rat bronchioalveolar transitional segment

In the present study, we have analyzed the alpha(IV) chain distribution in the subepithelial basement membrane (BM) of the rat pulmonary airway from the bronchi to alveoli. We have furthermore analyzed the alpha(IV) chain distribution in the subepithelial BM of the bronchioalveolar duct junction (BADJ) using alpha(IV) chain specific monoclonal antibodies. Our results show that the BM of the bronchial and bronchiolar epithelium contains [alpha1(IV)]2alpha2(IV) and [alpha5(IV)]2alpha6(IV) molecules and confirmed that the alveolar BM consists of [alpha1(IV)]2alpha2(IV) and alpha3(IV) alpha4(IV)alpha5(IV) molecules. There are also small regions in BADJ consisting of only [alpha1(IV)]2alpha2(IV) molecules without alpha3(IV)alpha4(IV)alpha5(IV) and [alpha5(IV)]2alpha6(IV) molecules. Moreover, the bronchioalveolar stem cells (BASCs)-primordial cells for bronchiolar Clara cells and alveolar type II (AT2) cells - lie adjacent to such small regions. These findings suggest that [alpha1(IV)]2 alpha2(IV) may be important for the BASCs to self-renew or to self-maintain themselves and that microenvironments produced by alpha(IV) chains may be important for cell differentiation.

Developmental distribution of collagen IV isoforms and relevance to ocular diseases

Type IV collagens are the most abundant proteins in basement membranes. Distinct genes encode each of six isoforms, alpha1(IV) through alpha6(IV), which assemble into one of three characteristic heterotrimers. Disease-causing mutations in each of the six genes are identified in humans or mice and frequently include diverse ocular pathogenesis that encompass common congenital and progressive blinding diseases, such as optic nerve hypoplasia, glaucoma, and retinal degeneration. Understanding where and when collagen IV molecules are expressed is important because it defines limits for the location and timing of primary pathogenesis. Although localization of collagen IV isoforms in developed human eyes is known, the spatial and temporal distribution of type IV collagens throughout ocular development has not been determined in humans or in mice. Here, we use isoform-specific monoclonal antibodies to systematically reveal the localization of all six collagen IV isoforms in developing mouse eyes. We found that alpha1(IV) and alpha2(IV) always co-localized and were ubiquitously expressed throughout development. alpha3(IV) and alpha4(IV) also always co-localized but in a much more spatially and temporally specific manner than alpha1(IV) and alpha2(IV). alpha5(IV) co-localized both with alpha3(IV)/alpha4(IV), and with alpha6(IV), consistent with alpha5(IV) involvement in two distinct heterotrimers. alpha5(IV) was present in all basement membranes except those of the vasculature. alpha6(IV) was not detected in vasculature or in Bruch's membrane, indicating that alpha5(IV) in Bruch's membrane is part of the alpha3alpha4alpha5 heterotrimer. This comprehensive analysis defines the spatial and temporal distribution of type IV collagen isoforms in the developing eye, and will contribute to understanding the mechanisms underlying collagen IV-related ocular diseases that collectively lead to blindness in millions of people worldwide.

Identification of noncollagenous sites encoding specific interactions and quaternary assembly of alpha 3 alpha 4 alpha 5(IV) collagen: implications for Alport gene therapy

Defective assembly of alpha 3 alpha 4 alpha 5(IV) collagen in the glomerular basement membrane causes Alport syndrome, a hereditary glomerulonephritis progressing to end-stage kidney failure. Assembly of collagen IV chains into heterotrimeric molecules and networks is driven by their noncollagenous (NC1) domains, but the sites encoding the specificity of these interactions are not known. To identify the sites directing quaternary assembly of alpha 3 alpha 4 alpha 5(IV) collagen, correctly folded NC1 chimeras were produced, and their interactions with other NC1 monomers were evaluated. All alpha1/alpha 5 chimeras containing alpha 5 NC1 residues 188-227 replicated the ability of alpha 5 NC1 to bind to alpha3NC1 and co-assemble into NC1 hexamers. Conversely, substitution of alpha 5 NC1 residues 188-227 by alpha1NC1 abolished these quaternary interactions. The amino-terminal 58 residues of alpha3NC1 encoded binding to alpha 5 NC1, but this interaction was not sufficient for hexamer co-assembly. Because alpha 5 NC1 residues 188-227 are necessary and sufficient for assembly into alpha 3 alpha 4 alpha 5 NC1 hexamers, whereas the immunodominant alloantigenic sites of alpha 5 NC1 do not encode specific quaternary interactions, the findings provide a basis for the rational design of less immunogenic alpha 5(IV) collagen constructs for the gene therapy of X-linked Alport patients.

Inhibition of tumor angiogenesis by tumstatin: insights into signaling mechanisms and implications in cancer regression

Growing tumors develop additional new blood vessels to meet the demand for adequate nutrients and oxygen, a process called angiogenesis. Cancer is a highly complex disease promoted by excess angiogenesis; interfering with this process poses for an attractive approach for controlling tumor growth. This hypothesis led to the identification of endogenous angiogenesis inhibitors generated from type IV collagen, a major component of vascular basement membrane (VBM). Type IV collagen and the angiogenesis inhibitors derived from it are involved in complex roles, than just the molecular construction of basement membranes. Protease degradation of collagens in VBM occurs in various physiological and pathological conditions and produces several peptides. Some of these peptides are occupied in the regulation of functions conflicting from those of their original integral molecules. Tumstatin (alpha3(IV)NC1), a proteolytic C-terminal non-collagenous (NC1) domain from type IV collagen alpha3 chain has been highlighted recently because of its potential role in anti-angiogenesis, however its biological actions are not limited to these processes. alpha3(IV)NC1 inhibits proliferation by promoting endothelial cell apoptosis and suppresses diverse tumor angiogenesis, thus making it a potential candidate for future cancer therapy. The present review surveys the physiological functions of type IV collagen and discovery of alpha3(IV)NC1 as an antiangiogenic protein with a comprehensive overview of the knowledge gained by us towards understanding its signaling mechanisms.

Mammalian collagen IV

Four decades have passed since the first discovery of collagen IV by Kefalides in 1966. Since then collagen IV has been investigated extensively by a large number of research laboratories around the world. Advances in molecular genetics have resulted in identification of six evolutionary related mammalian genes encoding six different polypeptide chains of collagen IV. The genes are differentially expressed during the embryonic development, providing different tissues with specific collagen IV networks each having unique biochemical properties. Newly translated alpha-chains interact and assemble in the endoplasmic reticulum in a chain-specific fashion and form unique heterotrimers. Unlike most collagens, type IV collagen is an exclusive member of the basement membranes and through a complex inter- and intramolecular interactions form supramolecular networks that influence cell adhesion, migration, and differentiation. Collagen IV is directly involved in a number of genetic and acquired disease such as Alport's and Goodpasture's syndromes. Recent discoveries have also highlighted a new and direct role for collagen IV in the development of rare genetic diseases such as cerebral hemorrhage and porencephaly in infants and hemorrhagic stroke in adults. Years of intensive investigations have resulted in a vast body of information about the structure, function, and biology of collagen IV. In this review article, we will summarize essential findings on the structural and functional relationships of different collagen IV chains and their roles in health and disease.

The distributions of type IV collagen alpha chains in basement membranes of human epidermis and skin appendages

Distributions of type IV collagen alpha chains in the basement membrane (BM) of human skin and its appendages were analyzed by immunofluorescent microscopy using chain-specific monoclonal antibodies. The basement membrane beneath the epidermis contained [alpha1(IV)](2)alpha2(IV) and [alpha5(IV)](2)alpha6(IV) but no alpha3(IV)alpha4(IV)alpha5(IV); this held true for at the eccrine sweat glands and glandular ducts, sebaceous glands, hair follicles, and arrector muscles of hair. The secretary portion of the eccrine sweat glands was rich in [alpha1(IV)](2) alpha2(IV) and had less [alpha5(IV)](2)alpha6(IV), while [alpha5(IV)](2) alpha6(IV) was abundant in the ductal portion. In the subepidermal zone, alpha5(IV)/alpha6(IV) chain negative spots (1.9-15.0 microm) were frequently observed. Triple staining samples (Mel.2, alpha2(IV) and alpha5(IV) chains) showed that about 50% of epidermal melanocytes colocalized with such spots. Results suggest that these alpha5(IV)/alpha6(IV) chain negative spots of the subepidermal basement membrane have a particular relationship with melanocytes and are sites for certain interactions between the two.

Distinct target-derived signals organize formation, maturation, and maintenance of motor nerve terminals

Target-derived factors organize synaptogenesis by promoting differentiation of nerve terminals at synaptic sites. Several candidate organizing molecules have been identified based on their bioactivities in vitro, but little is known about their roles in vivo. Here, we show that three sets of organizers act sequentially to pattern motor nerve terminals: FGFs, beta2 laminins, and collagen alpha(IV) chains. FGFs of the 7/10/22 subfamily and broadly distributed collagen IV chains (alpha1/2) promote clustering of synaptic vesicles as nerve terminals form. beta2 laminins concentrated at synaptic sites are dispensable for embryonic development of nerve terminals but are required for their postnatal maturation. Synapse-specific collagen IV chains (alpha3-6) accumulate only after synapses are mature and are required for synaptic maintenance. Thus, multiple target-derived signals permit discrete control of the formation, maturation, and maintenance of presynaptic specializations.

Loss of assembly of the main basement membrane collagen, type IV, but not fibril-forming collagens and embryonic death in collagen prolyl 4-hydroxylase I null mice

Collagen prolyl 4-hydroxylases (C-P4Hs) catalyze the formation of the 4-hydroxyproline residues that are essential for the generation of triple helical collagen molecules. The vertebrate C-P4Hs I, II, and III are [alpha(I)]2beta2, [alpha(II)]2beta2, and [alpha(III)]2beta2 tetramers with identical beta subunits. We generated mice with targeted inactivation of the P4ha1 gene encoding the catalytic alpha subunit of C-P4H I to analyze its specific functions. The null mice died after E10.5, showing an overall developmental delay and a dilated endoplasmic reticulum in their cells. The capillary walls were frequently ruptured, but the capillary density remained unchanged. The C-P4H activity level in the null embryos and fibroblasts cultured from them was 20% of that in the wild type, being evidently due to the other two isoenzymes. Collagen IV immunofluorescence was almost absent in the basement membranes of the null embryos, and electron microscopy revealed disrupted basement membranes, while immunoelectron microscopy showed a lack of collagen IV in them. The amount of soluble collagen IV was increased in the null embryos and cultured null fibroblasts, indicating a lack of assembly of collagen IV molecules into insoluble structures, probably due to their underhydroxylation and hence abnormal conformation. In contrast, the null embryos had collagen I and III fibrils with a typical cross-striation pattern but slightly increased diameters, and the null fibroblasts secreted fibril-forming collagens, although less efficiently than wild-type cells. The primary cause of death of the null embryos was thus most likely an abnormal assembly of collagen IV.

Self-association of Collagen Triple Helic Peptides into Higher Order Structures

Interest in self-association of peptides and proteins is motivated by an interest in the mechanism of physiologically higher order assembly of proteins such as collagen as well as the mechanism of pathological aggregation such as beta-amyloid formation. The triple helical form of (Pro-Hyp-Gly)(10), a peptide that has proved a useful model for molecular features of collagen, was found to self-associate, and its association properties are reported here. Turbidity experiments indicate that the triple helical peptide self-assembles at neutral pH via a nucleation-growth mechanism, with a critical concentration near 1 mM. The associated form is more stable than individual molecules by about 25 degrees C, and the association is reversible. The rate of self-association increases with temperature, supporting an entropically favored process. After self-association, (Pro-Hyp-Gly)(10) forms branched filamentous structures, in contrast with the highly ordered axially periodic structure of collagen fibrils. Yet a number of characteristics of triple helix assembly for the peptide resemble those of collagen fibril formation. These include promotion of fibril formation by neutral pH and increasing temperature; inhibition by sugars; and a requirement for hydroxyproline. It is suggested that these similar features for peptide and collagen self-association are based on common lateral underlying interactions between triple helical molecules mediated by hydrogen-bonded hydration networks involving hydroxyproline.

Differential expression of basement membrane collagen-IV α1 to α6 chains during oral carcinogenesis

This study aimed to resolve if basement membrane (BM) collagen alpha chains undergo remodeling during oral carcinogenesis. Using immunohistochemistry and transmission electron microscopy, we found that BMs in oral epithelial dysplasias (OED: mild, n=10; moderate, n=10; severe, n=10) and carcinoma in situ (CIS) (n=10) differed from normal mucosa (n=6) and oral epithelial hyperplasia (n=5) in showing: (1) excessive lamina densa-like material ultrastructurally, and (2) stronger immunoexpression for alpha5(IV) than for alpha1(IV), alpha2(IV), and alpha6(IV) chains-findings that implicate these molecules' role as an adhesive template for the attachment and persistence of basal dysplastic cells. Incipient loss of BM integrity in CIS, where alpha5(IV)/alpha6(IV) chains were more frequently absent than alpha1(IV)/alpha2(IV) chains, suggests that alpha(IV) network disruption is crucial for progression of dysplastic cells into the extracellular compartment, marking transition into the invasive phase. In carcinomatous BM, the disappearance of alpha(IV) chains was more severe in poorly differentiated oral squamous cell carcinoma (OSCC) (n=10) than in well-differentiated OSCC (n=10). In all samples examined, alpha3(IV) and alpha4(IV) chains were absent. These findings taken together suggest that BM collagen-IV alpha chains undergo remodeling where selective increase and loss of these molecules are probably early and late events, respectively, during progression of oral dysplasia to cancer.

Sequential expression of type IV collagen networks: testis as a model and relevance to spermatogenesis

The six alpha chains of type IV collagen are organized into three networks: alpha1/alpha2, alpha3/alpha4/alpha5, and alpha1/alpha2/alpha5/alpha6. A shift from the alpha1/alpha2 to the alpha3/alpha4/alpha5 network occurs in the developing glomerular basement membrane, but how the alpha1/alpha2/alpha5/alpha6 network fits into this sequence is less clear, because the three networks do not colocalize. Here, we studied the seminiferous tubule basement membrane of normal canine testis where all three networks do colocalize: the alpha1/alpha2 network is expressed from birth, the alpha1/alpha2/alpha5/alpha6 network by 5-6 weeks of age, and the alpha3/alpha4/alpha5 network by 2 months of age. A canine model of Alport syndrome allowed study of the absence of alpha3/alpha4/alpha5 and alpha1/alpha2/alpha5/alpha6 networks in testis. In Alport dogs, the seminiferous tubule basement membrane was thinner than in controls. Spermatogenesis began at the same time as with normal dogs; however, the number of mature sperm was significantly reduced in Alport dogs. Thus, it would appear that alpha3/alpha4/alpha5 and alpha1/alpha2/alpha5/alpha6 networks are not essential for onset of spermatogenesis, but long-term function may be compromised by the loss of one or both networks. This situation is analogous to the glomerular basement membrane in Alport syndrome. In conclusion, testis can serve as a model system to study the sequence of type IV collagen network expression.

Glomerular injury is exacerbated in diabetic integrin α1-null mice

Excessive glomerular collagen IV and reactive oxygen species (ROS) production are key factors in the development of diabetic nephropathy. Integrin alpha1beta1, the major collagen IV receptor, dowregulates collagen IV and ROS production, suggesting this integrin might determine the severity of diabetic nephropathy. To test this possibility, wild-type and integrin alpha1-null mice were rendered diabetic with streptozotocin (STZ) (100 mg/kg single intraperitoneal injection), after which glomerular filtration rate (GFR), glomerular collagen deposition, and glomerular basement membrane (GBM) thickening were evaluated. In addition, ROS and collagen IV production by mesangial cells as well as their proliferation was measured in vitro. Diabetic alpha1-null mice developed worse renal disease than diabetic wild-type mice. A significant increase in GFR was evident in the alpha1-null mice at 6 weeks after the STZ injection; it started to decrease by week 24 and reached levels of non-diabetic mice by week 36. In contrast, GFR only increased in wild-type mice at week 12 and its elevation persisted throughout the study. Diabetic mutant mice also showed increased glomerular deposition of collagen IV and GBM thickening compared to diabetic wild-type mice. Primary alpha1-null mesangial cells exposed to high glucose produced more ROS than wild-type cells, which led to decreased proliferation and increased collagen IV synthesis, thus mimicking the in vivo finding. In conclusion, this study suggests that lack of integrin alpha1beta1 exacerbates the glomerular injury in a mouse model of diabetes by modulating GFR, ROS production, cell proliferation, and collagen deposition.

Gene expression in acute Stanford type A dissection: a comparative microarray study

Background

We compared gene expression profiles in acutely dissected aorta with those in normal control aorta.

Materials and methods

Ascending aorta specimen from patients with an acute Stanford A-dissection were taken during surgery and compared with those from normal ascending aorta from multiorgan donors using the BD Atlas™ Human1.2 Array I, BD Atlas™ Human Cardiovascular Array and the Affymetrix HG-U133A GeneChip^®. For analysis only genes with strong signals of more than 70 percent of the mean signal of all spots on the array were accepted as being expressed. Quantitative real-time polymerase chain reaction (RT-PCR) was used to confirm regulation of expression of a subset of 24 genes known to be involved in aortic structure and function.

Results

According to our definition expression profiling of aorta tissue specimens revealed an expression of 19.1% to 23.5% of the genes listed on the arrays. Of those 15.7% to 28.9% were differently expressed in dissected and control aorta specimens. Several genes that encode for extracellular matrix components such as collagen IV α2 and -α5, collagen VI α3, collagen XIV α1, collagen XVIII α1 and elastin were down-regulated in aortic dissection, whereas levels of matrix metalloproteinases-11, -14 and -19 were increased. Some genes coding for cell to cell adhesion, cell to matrix signaling (e.g., polycystin1 and -2), cytoskeleton, as well as several myofibrillar genes (e.g., α-actinin, tropomyosin, gelsolin) were found to be down-regulated. Not surprisingly, some genes associated with chronic inflammation such as interleukin -2, -6 and -8, were up-regulated in dissection.

Conclusion

Our results demonstrate the complexity of the dissecting process on a molecular level. Genes coding for the integrity and strength of the aortic wall were down-regulated whereas components of inflammatory response were up-regulated. Altered patterns of gene expression indicate a pre-existing structural failure, which is probably a consequence of insufficient remodeling of the aortic wall resulting in further aortic dissection.

Loss of alpha3/alpha4(IV) collagen from the glomerular basement membrane induces a strain-dependent isoform switch to alpha5alpha6(IV) collagen associated with longer renal survival in Col4a3-/- Alport mice

Mutations in COL4A3/4/5 genes that affect the normal assembly of the alpha3/4/5(IV) collagen network in the glomerular basement membrane (GBM) cause Alport syndrome. Patients progress to renal failure at variable rates that are determined by the underlying mutation and putative modifier genes. Col4a3(-/-) mice, a model for autosomal recessive Alport syndrome, progress to renal failure significantly slower on the C57BL/6 than on the 129X1/Sv background. Reported here is a novel strain-specific alternative collagen IV isoform switch that is associated with the differential renal survival in Col4a3(-/-) Alport mice. The downregulation or the absence of alpha3/4(IV) collagen chains in the GBM of Lmx1b(-/-) and Col4a3(-/-) mice was found to induce ectopic deposition of alpha5/6(IV) collagen. The GBM deposition of alpha5/6(IV) collagen was abundant in C57BL/6 Col4a3(-/-) mice but almost undetectable in 129X1/Sv Col4a3(-/-) mice. This strain difference was due to overall low expression of alpha6(IV) chain and alpha5/6(IV) protomers in the tissues of 129X1/SvJ mice, a natural Col4a6 knockdown. In (129 x B6)F1 Col4a3(-/-) mice, the amount of alpha5/6(IV) collagen in the GBM was inherited in a mother-to-son manner, suggesting that it is controlled by one or more X-linked loci, possibly Col4a6 itself. Importantly, high levels of ectopic alpha5/6(IV) collagen in the GBM were associated with approximately 46% longer renal survival. These findings suggest that alpha5/6(IV) collagen, the biologic role of which has been hitherto unknown, may partially substitute for alpha3/4/5(IV) collagen. Therapeutically induced GBM deposition of alpha5/6(IV) collagen may provide a novel strategy for delaying renal failure in patients with autosomal recessive Alport syndrome.

Conformational features of a natural break in the type IV collagen Gly-X-Y repeat

Fibrillar collagens have an absolute requirement for Gly as every 3rd residue, whereas breaks in the Gly-X-Y repeating pattern are found normally in the triple helix domains of non-fibrillar collagens, such as type IV collagen in basement membranes. In this study, a model 30-mer peptide is designed to include the interruption GPOGAAVMGPOGPO found in the alpha5 chain of type IV collagen. The GAAVM peptide forms a stable triple helix, with Tm= 29 degrees C. When compared with a control peptide with Gly as every 3rd residue, the GAAVM peptide has a marked decrease in the 225 nm maximum of its CD spectrum and a 10 degrees C drop in stability. A 50% decrease in calorimetric enthalpy is observed, which may result from disruption of ordered water structure anchored by regularly placed backbone carbonyls. NMR studies on specific 15N-labeled residues within the GAAVM peptide indicate a normal triple helical structure for Gly-Pro-Hyp residues flanking the break. The sequence within the break is not disordered but shows altered hydrogen exchange rates and an abnormal Val chemical shift. It was previously reported that a peptide designed to model a similar kind of interruption in the peptide (Pro-Hyp-Gly)10, (GPOGPOPOGPO), is unable to form a stable triple helix, and replacement of GAA by GPO or VM by PO within the GAAVM break decreases the stability. Thus, rigid imino acids are unfavorable within a break, despite their favorable stabilization of the triple helix itself. These results suggest some non-random structure typical of this category of breaks in the Gly-X-Y repeat of the triple helix.

Smooth muscle trans-membrane sarcoglycan complex in partial bladder outlet obstruction

The urinary bladder experiences both distension and contraction as a part of the normal filling and emptying cycle. To empty properly, tension generated intracellularly in a smooth muscle cell must be smoothly and efficiently transferred across its sarcolemma to the basement membrane, which mediates its binding to both the extracellular matrix and to other cells. As a consequence of urethral obstruction, the bladder cannot generate appropriate force to contract the organ, thereby leading to inefficient emptying and associated sequelae. In this study, an animal model of urethral obstruction was utilized to study the membrane-associated structures that transfer tension across the sarcolemma of bladder smooth muscle cells. Immunohistochemical localization of key components of the smooth muscle tension transfer apparatus (TTA) was performed utilizing specific antibodies against:(1) the alpha-chains of type IV collagen, a basement membrane component, and (2) beta-sarcoglycan, an integral membrane protein that is a participant in the physical linkage between the cytoskeleton and the basement membrane. We demonstrate, in obstructed animals, that there is a pronounced disruption of the TTA with a physical displacement of these two components that can be demonstrated at the level of the light microscope using scanning confocal microscopy. Electron microscopy further demonstrates significant increases in the size of the junctional plaques between smooth muscle cells.

Mechanism of chain selection in the assembly of collagen IV: a prominent role for the alpha2 chain

Collagens comprise a large superfamily of extracellular matrix proteins that play diverse roles in tissue function. The mechanism by which newly synthesized collagen chains recognize each other and assemble into specific triple-helical molecules is a fundamental question that remains unanswered. Emerging evidence suggests a role for the non-collagenous domain (NC1) located at the C-terminal end of each chain. In this study, we have investigated the molecular mechanism underlying chain selection in the assembly of collagen IV. Using surface plasmon resonance, we have determined the kinetics of interaction and assembly of the alpha1(IV) and alpha2(IV) NC1 domains. We show that the differential affinity of alpha2(IV) NC1 domain for dimer formation underlies the driving force in the mechanism of chain discrimination. Given its characteristic domain recognition and affinity for the alpha1(IV) NC1 domain, we conclude that the alpha2(IV) chain plays a regulatory role in directing chain composition in the assembly of (alpha1)(2)alpha2 triple-helical molecule. Detailed crystal structure analysis of the [(alpha1)(2)alpha2](2) NC1 hexamer and sequence alignments of the NC1 domains of all six alpha-chains from mammalian species revealed the residues involved in the molecular recognition of NC1 domains. We further identified a hypervariable region of 15 residues and a beta-hairpin structural motif of 13 residues as two prominent regions that mediate chain selection in the assembly of collagen IV. To our knowledge, this report is the first to combine kinetics and structural data to describe molecular basis for chain selection in the assembly of a collagen molecule.

The NC1 dimer of human placental basement membrane collagen IV: does a covalent crosslink exist?

Triple-helical collagen IV protomers associate through their N- and C-termini, forming a three-dimensional network that provides basement membranes with mechanical strength. Within this network, the C-terminal non-collagenous (NC1) domains form tight dimeric junctions. Crystallographic analyses of isolated NC1 domains show two trimeric cap-like structures interacting via a large interface. Previously, for NC1 from human placenta type-IV collagen we described covalent alpha1-alpha1 and alpha2-alpha2 crosslinks between Met93 and Lys211 of opposing alpha1(IV) and alpha2(IV) NC1-chains, which further stabilize this interface and explain the occurrence of reduction-insensitive NC1-chain dimers. However, their existence was recently questioned, and we therefore analyzed NC1-domain dimers in more detail by biochemical and protein crystallographic methods. Short-exposure diffraction data show a clear electron density cross-connecting the respective residues, which gradually disappears with prolonged crystal irradiation. Sequence analyses of isolated tryptic peptides derived from denatured NC1 monomers and dimers indicate that only the dimers, but not the monomers, yield these chemically labile cross-linked peptides. These data clearly demonstrate the presence of reduction-resistant, but chemically and radiation-sensitive covalent crosslinks between the side chains of Met93 and Lys211 in human placenta type-IV collagen.

Type IV Collagen Induces Podocytic Features in Bone Marrow Stromal Stem CellsIn Vitro

Bone marrow-derived stromal stem cells (BMSC) can differentiate along a variety of mesenchymal lines, including mesangial cells. For determining whether BMSC can be induced to differentiate along podocytic lines in vitro, canine BMSC were cultured on plastic, type I collagen, and NC1 hexamers of type IV collagen from normal and Alport canine glomerular basement membrane. Results were compared with a mouse podocyte cell line. In the case of the podocyte line, differentiation occurred on all three matrices as indicated by the expression of synaptopodin and CD2-associated protein (CD2AP) and organization of myosin heavy chain IIA into a linear pattern. BMSC proliferated equally well on all matrices, but cells that were grown on type IV collagen NC1 hexamers became larger and stellate. Evidence for podocytic differentiation occurred on all three collagen matrices as indicated by the redistribution of myosin IIA to a linear pattern and expression of synaptopodin, CD2AP, and alpha-actinin. A punctate distribution of CD2AP was seen only in cells that were grown on normal and Alport glomerular basement membrane NC1 hexamers. Differentiated podocytes expressed the alpha1, alpha2, and alpha5 chains of type IV collagen but at higher levels in cells that were grown on NC1 hexamers. Similar results were obtained in BMSC for the alpha1 and alpha2 chains only. The alpha3, alpha4, and alpha6 chains were never detected in the podocyte line or BMSC. These results indicate that BMSC undergo a degree of podocytic differentiation in vitro and greater when grown on type IV collagen NC1 hexamers than type I collagen. Alport and normal NC1 hexamers seem equally permissive to BMSC growth and differentiation, suggesting that these processes are not influenced specifically by the alpha3/alpha4/alpha5 network. BMSC may be useful in the development of stem cell-based reconstitution of glomeruli that are damaged by disease and for gene therapy of genetic glomerular diseases such as Alport syndrome.

Regulation of collagen type IV genes is organ-specific: Evidence from a canine model of Alport syndrome

BACKGROUND

Despite advances in knowledge about collagen type IV at the protein level, little is known about expression of its six alpha chains. X-linked Alport syndrome provides a system to study collagen type IV gene expression within a setting of disturbed protein synthesis. Mutations in the alpha5 chain result in loss of the alpha3/alpha4/alpha5 and alpha1/alpha2/alpha5/alpha6 networks from the kidney, with progressive renal disease.

METHODS

We used a canine model of Alport syndrome to measure expression of the six type IV collagen chains from 11 days to 7(1/2) months of age. We determined to what extent message levels in kidney change over time, and what correlation exists with clinical and pathologic changes in glomeruli, and the primary mutation. The latter was evaluated by examining testis, an organ normally containing the same collagen type IV networks but uninvolved by disease.

RESULTS

The alpha1 to alpha6 mRNAs were expressed at all time points in normal canine kidney. By comparison to normal, in Alport dog kidney, the alpha1 and alpha2 mRNAs were up-regulated after 2 months of age, alpha3 and alpha4 mRNAs were down-regulated by 2 months of age, and the alpha5 mRNA was almost undetectable at any time. In testis, all mRNAs were expressed at comparable levels in normal and affected dogs other than the alpha5 chain, which was not expressed in affected testis.

CONCLUSION

Normal expression of collagen type IV is under control mechanisms specific to each organ and to individual chains. The altered expression in canine Alport syndrome is not the direct result of the mutation, since these changes do not occur in all organs nor are they present from birth. Instead, collagen type IV expression is influenced by disease, with down-regulation of alpha3 and alpha4 chains temporally related to the onset of proteinuria, and up-regulation of alpha1 and alpha2 chains to glomerulosclerosis. This dysregulation of the alpha3 and alpha4 chains is unique to this Alport model, and suggests an unidentified mechanism linking pathology with down-regulation of expression of these two chains.

Tumstatin, the NC1 domain of alpha3 chain of type IV collagen, is an endogenous inhibitor of pathological angiogenesis and suppresses tumor growth

Angiogenesis, the formation of new blood vessels, is required for physiological development of vertebrates and repair of damaged tissue, but in the pathological setting contributes to progression of cancer. During tumor growth, angiogenesis is supported by up-regulation of angiogenic stimulators (pro-angiogenic) and down-regulation of angiogenic inhibitors (anti-angiogenic). The switch to the angiogenic phenotype (angiogenic switch) allows the tumors to grow and facilitate metastasis. The bioactive NC1 domain of type IV collagen alpha3 chain, called tumstatin, imparts anti-tumor activity by inducing apoptosis of proliferating endothelial cells. Tumstatin binds to alphaVbeta3 integrin via a mechanism independent of the RGD-sequence recognition and inhibits cap-dependent protein synthesis in the proliferating endothelial cells. The physiological level of tumstatin is controlled by matrix metalloproteinase-9, which most effectively cleaves it from the basement membrane and its physiological concentration in the circulation keeps pathological angiogenesis and tumor growth in check. These findings suggest that tumstatin functions as an endogenous inhibitor of pathological angiogenesis and functions as a novel suppressor of proliferating endothelial cells and growth of tumors.

Novel mutations in three families confirm a major role of COL4A1 in hereditary porencephaly

BACKGROUND

Porencephaly (cystic cavities of the brain) is caused by perinatal vascular accidents from various causes. Several familial cases have been described and autosomal dominant inheritance linked to chromosome 13q has been suggested. COL4A1 is an essential component in basal membrane stability. Mouse mutants bearing an in-frame deletion of exon 40 of Col4a1 either die from haemorrhage in the perinatal period or have porencephaly in survivors. A report of inherited mutations in COL4A1 in two families has shown that familial porencephaly may have the same cause in humans.

OBJECTIVE

To describe three novel COL4A1 mutations.

RESULTS

The three mutations occurred in three unrelated Dutch families. There were two missense mutations of glycine residues predicted to result in abnormal collagen IV assembly, and one mutation predicted to abolish the traditional COL4A1 start codon. The last mutation was also present in an asymptomatic obligate carrier with white matter abnormalities on brain magnetic resonance imaging.

CONCLUSIONS

This observation confirms COL4A1 as a major locus for genetic predisposition to perinatal cerebral haemorrhage and porencephaly and suggests variable expression of COL4A1 mutations.

Molecular packing in network-forming collagens

Different collagen types can vary considerably in length, molecular weight, chemical composition, and the way they interact with each other to form molecular aggregates. Collagen Types IV, VI, VIII, X, and dogfish egg case collagen make linear and lateral associations to form open networks rather than fibers. The roles played by these network-forming collagens are diverse: they can act as support and anchorage for cells and tissues, serve as molecular filters, and even provide protective permeable barriers for developing embryos. Their functional properties are intimately linked to their molecular organization. This Chapter reviews what is known about the molecular structure of this group of collagens, describes the ways the molecules interact to form networks, and-despite the large variations in molecular size-identifies common aggregation themes.

Immunolocalization and distribution patterns of type IV collagen alpha chains in oral mucosal melanoma

The basement membrane (BM) is mainly composed of type IV collagen, which is composed of triple combinations of six distinct alpha (alpha) chains in a tissue-specific manner. The six collagen chain-specific antibodies (alpha1-alpha6) were used to examine the BMs of the oral epithelium (OE) and tumor clusters in oral mucosal melanoma (OMM). Eight OMM cases were examined. Results showed that the alpha1 and alpha2 chains were constantly detected at the BM of the normal OE as well as at the OE with atypical melanocytic proliferation and in invasive melanoma with nodular nests. The alpha1 and alpha2 chains were intermittently detected in in situ OMM, early invasive OMM and advanced invasive OMM with sheet-like nests. Gradual loss of alpha5 and alpha6 from the OE with atypical melanocyte through in situ OMM and early invasive OMM was observed. These findings suggest that changes in the immunolocalization and distribution patterns of type IV collagen alpha chains are associated with the progression of OMM. The distribution pattern of type IV collagen alpha chain varies depending on the architecture of the nest.

Identification of S-hydroxylysyl-methionine as the covalent cross-link of the noncollagenous (NC1) hexamer of the alpha1alpha1alpha2 collagen IV network: a role for the post-translational modification of lysine 211 to hydroxylysine 211 in hexamer assembly

Collagen IV networks are present in all metazoans as components of basement membranes that underlie epithelia. They are assembled by the oligomerization of triple-helical protomers, composed of three alpha-chains. The trimeric noncollagenous domains (NC1) of each protomer interact forming a hexamer structure. Upon exposure to acidic pH or denaturants, the hexamer dissociates into monomer and dimer subunits, the latter reflect distinct interactions that reinforce/cross-link the quaternary structure of hexamer. Recently, the cross-link site of the alpha1alpha1alpha2 network was identified, on the basis of x-ray crystal structures at 1.9-A resolution, in which the side chains of Met93 and Lys211 were proposed to be connected by a novel thioether bond (Than, M. E., Henrich, S., Huber, R., Ries, A., Mann, K., Kuhn, K., Timpl, R., Bourenkov, G. P., Bartunik, H. D., and Bode, W. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 6607-6612); however, at the higher resolution of 1.5 A, we found no evidence for this cross-link (Vanacore, R. M., Shanmugasundararaj, S., Friedman, D. B., Bondar, O., Hudson, B. G., and Sundaramoorthy, M. (2004) J. Biol. Chem. 279, 44723-44730). Given this discrepancy in crystallographic findings, we sought chemical evidence for the location and nature of the reinforcement/cross-link site. Trypsin digestion of monomer and dimer subunits excised a approximately 5,000-Da complex that distinguished dimers from monomers; the complex was characterized by mass spectrometry, Edman degradation, and amino acid composition analyses. The tryptic complex, composed of two peptides of 44 residues derived from two alpha1 NC1 monomers, contained Met93 and Lys211 post-translationally modified to hydroxylysine (Hyl211). Truncation of the tryptic complex with post-proline endopeptidase reduced its size to 14 residues to facilitate characterization by tandem mass spectrometry, which revealed a covalent linkage between Met93 and Hyl211. The novel cross-link, termed S-hydroxylysyl-methionine, reflects at least two post-translational events in its formation: the hydroxylation of Lys211 to Hyl211 within the NC1 domain during the biosynthesis of alpha-chains and the connection of Hyl211 to Met93 between the trimeric NC1 domains of two adjoining triple-helical protomers, reinforcing the stability of collagen IV networks.

Various immunostaining patterns of CD31, CD34 and endoglin and their relationship with lymph node metastasis in oral squamous cell carcinomas

BACKGROUND

Intratumoral blood vessels are known to play an important role in cancer growth and metastasis. The discrepancy in previous reports using various endothelial markers individually suggested us to investigate both normal and various tumor areas with a wide panel of vascular markers.

METHODS

Here, we used a panel of three antibodies (CD31, CD34, and endoglin) as blood vessel markers to investigate the distribution and properties of blood vessels in normal oral tissues and squamous cell carcinomas.

RESULTS

Many microvessels with strong remodeling activity as well as undifferentiated tumoral vascular endothelial cells and immature endothelial cells were present in the cancer cell nest and marginal area of cancer infiltration. Our results showed different vascular distribution patterns using various immunostaining markers in normal and tumoral tissues.

CONCLUSION

Vascular distribution and properties of endothelial cells appear to be closely associated with metastasis.

High nephritogenicity of monoclonal antibodies belonging to IgG2a and IgG2b subclasses in rat anti-GBM nephritis

BACKGROUND

To examine a subclass-effect relationship and a dose-effect relationship of autoantibodies in the rat antiglomerular basement membrane (GBM) antibody-induced glomerulonephritis (anti-GBM nephritis) model, we injected homologous monoclonal antibodies against the NC1 domains of rat type IV collagen into inbred Wistar-Kyoto (WKY) rats.

METHODS

Eight different autoantibodies from each of the IgG1, IgG2a, and IgG2b subclasses were established and administered to groups of four WKY rats at a dose of 300 microg/rat. To examine the dose-effect relationship, we administered 0 to 300 microg/rat of autoantibodies from each subclass to rats.

RESULTS

All IgG1 antibodies induced mild nephritis, whereas the IgG2a and IgG2b antibodies induced moderate to severe nephritis. Some IgG2a and IgG2b antibodies induced pulmonary hemorrhage as well. These antibodies were reactive with alpha3(IV)NC1, alpha4(IV)NC1, or alpha5(IV)NC1. The minimum dose of antibody required to induce nephritis was 30 microg/rat for IgG1, 3 microg/rat for IgG2a, and 1 microg/rat for IgG2b. At doses of 30 microg/rat or less, antibody deposition was generally restricted to the GBM. At doses of 100 microg/rat or greater, antibody deposition extended to both the GBM and tubular basement membrane (TBM). Pulmonary hemorrhage was observed only when a large amount of pulmonary hemorrhagic antibody was administered.

CONCLUSION

The severity of nephritis was dependent on both subclass and dose of autoantibodies. It becomes clear that pulmonary hemorrhage in anti-GBM nephritis is induced by autoantibodies.

The NC1 domain of human collagen IV is necessary to initiate triple helix formation

Type IV collagen is a heterotrimeric molecule, which contains the N-terminal 7S, a central triple-helical domain, and the globular C-terminal NC1 domain. A zipper-like mechanism of triple helix formation, starting from the C-terminus, has been proposed for most collagens but for collagen type IV there has only been indirect evidence so far. In this study we expressed trimeric human collagen type IV to compare the effects of different structural variants on the formation of collagen IV molecules. Our data show that the NC1 but not 7S domain is essential for the chain association and initiation of triple helix formation. This strongly suggests an N-to-C terminal mechanism of triple helix formation. Additionally, we could show that the human alpha2(IV) chain can form chimeric alpha1.alpha1.alpha2(IV) heterotrimers with mouse subunits when expressed in PF-HR9 cells.

The alpha1.alpha2 network of collagen IV. Reinforced stabilization of the noncollagenous domain-1 by noncovalent forces and the absence of Met-Lys cross-links

Collagen IV networks are present in all metazoa and underlie epithelia as a component of basement membranes. The networks are essential for tissue function and are defective in disease. They are assembled by the oligomerization of triple-helical protomers that are linked end-to-end. At the C terminus, two protomers are linked head-to-head by interactions of their trimeric noncollagenous domains, forming a hexamer structure. This linkage in the alpha1.alpha2 network is stabilized by a putative covalent Met-Lys cross-link between the trimer-trimer interface (Than, M. E., Henrich, S., Huber, R., Ries, A., Mann, K., Kuhn, K., Timpl, R., Bourenkov, G. P., Bartunik, H. D., and Bode, W. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 6607-6612) forming a nonreducible dimer that connects the hexamer. In the present study, this cross-link was further investigated by: (a) comparing the 1.5-A resolution crystal structures of the alpha1.alpha2 hexamers from bovine placenta and lens capsule basement membranes, (b) mass spectrometric analysis of monomer and nonreducible dimer subunits of placenta basement membrane hexamers, and (c) hexamer dissociation/re-association studies. The findings rule out the novel Met-Lys cross-link, as well as other covalent cross-links, but establish that the nonreducible dimer is an inherent structural feature of a subpopulation of hexamers. The dimers reflect the reinforced stabilization, by noncovalent forces, of the connection between two adjoining protomers of a network. The reinforcement extends to other types of collagen IV networks, and it underlies the cryptic nature of a B-cell epitope of the alpha3.alpha4.alpha5 hexamer, implicating the stabilization event in the etiology and pathogenesis of Goodpasture autoimmune disease.

Immunohistochemical localization of type IV collagen alpha chains in the basement membrane of the pancreatic duct in human normal pancreas and pancreatic diseases

The type IV collagen (Col-IV) consists of 3 alpha chains. Six different alpha chains [alpha1(IV), alpha2(IV), alpha3(IV), alpha4(IV), alpha5(IV), and alpha6(IV)] have been identified, and their combination is considered to be organ specific. We investigated the immunohistochemical localization of alpha (IV) chains in the basement membrane (BM) of the pancreatic duct in human normal pancreas (NP) and pancreatic diseases. Fifty specimens [10 NP, 10 chronic pancreatitis (CP), 10 intraductal papillary mucinous tumor (IPMT), and 20 pancreatic adenocarcinoma (PAC)] were examined. Alpha 1(IV), alpha2(IV), alpha5(IV), and alpha6(IV) were linearly immunostained in NP, CP, and IPMT. In PAC, alpha(IV) and alpha2(IV) were immunostained, but alpha5(IV) and alpha6(IV) were not stained in 30% and 40% of the cases, respectively. In conclusion, immunohistochemically, the Col-IV of human normal pancreatic duct consisted of alpha1(IV), alpha2(IV), alpha5(IV), and alpha6(IV). alpha5(IV) and alpha6(IV) were frequently absent in PAC, and their absence might be related to the invasion of cancer cells.

Collagen IV is essential for basement membrane stability but dispensable for initiation of its assembly during early development

Basement membranes are specialized extracellular matrices consisting of tissue-specific organizations of multiple matrix molecules and serve as structural barriers as well as substrates for cellular interactions. The network of collagen IV is thought to define the scaffold integrating other components such as, laminins, nidogens or perlecan, into highly organized supramolecular architectures. To analyze the functional roles of the major collagen IV isoform alpha1(IV)(2)alpha2(IV) for basement membrane assembly and embryonic development, we generated a null allele of the Col4a1/2 locus in mice, thereby ablating both alpha-chains. Unexpectedly, embryos developed up to E9.5 at the expected Mendelian ratio and showed a variable degree of growth retardation. Basement membrane proteins were deposited and assembled at expected sites in mutant embryos, indicating that this isoform is dispensable for matrix deposition and assembly during early development. However, lethality occurred between E10.5-E11.5, because of structural deficiencies in the basement membranes and finally by failure of the integrity of Reichert's membrane. These data demonstrate for the first time that collagen IV is fundamental for the maintenance of integrity and function of basement membranes under conditions of increasing mechanical demands, but dispensable for deposition and initial assembly of components. Taken together with other basement membrane protein knockouts, these data suggest that laminin is sufficient for basement membrane-like matrices during early development, but at later stages the specific composition of components including collagen IV defines integrity, stability and functionality.

Detection of COL4A5 gene mutations in Chinese patients with Alport's syndrome

BACKGROUND

Mutations in the COL4A5 gene, encoding the alpha 5 chain of type IV collagen, are responsible for X-linked Alport's syndrome (XLAS), a progressive nephropathy characterized by glomerular basement membrane abnormalities and usually associated with progressive hearing loss and ocular lesions.

METHODS

In this study, we analysed all 51 exons of the COL4A5 gene in 20 Chinese patients with XLAS or suspected XLAS from 16 families by using polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) DNA sequencing.

RESULTS

Five gene mutations identified in five families were considered to be pathogenic, including one nonsense mutation in exon 1 (266C-->T, Gln22Term), two missense mutations in exons 31 (2757G-->T, Gly852Val) and 43 (4142C-->T, Pro1314Ser), and two splice site mutations in introns 1 and 25 just next to the 3' end of their respective exons (283+1G-->T, 2150+1G-->T). According to GenBank, these five mutations have not been reported previously. All male patients have typical clinical manifestations and pathological findings that closely correspond to the effects of the mutations. Furthermore, seven gene polymorphisms were detected in introns 18 and 10 and exons 20, 27, 29, 39 and 46. Only the substitution in intron 18 (1234+25G-->A) had a gene frequency significantly higher in patients than in normal individuals.

CONCLUSION

Our study demonstrated the critical role of COL4A5 gene mutations in the pathogenesis of XLAS. The linkage of the polymorphism to AS is still unknown.

A human-mouse chimera of the alpha3alpha4alpha5(IV) collagen protomer rescues the renal phenotype in Col4a3-/- Alport mice

Collagen IV is a major structural component of basement membranes. In the glomerular basement membrane (GBM) of the kidney, the alpha3, alpha4, and alpha5(IV) collagen chains form a distinct network that is essential for the long-term stability of the glomerular filtration barrier, and is absent in most patients affected with Alport syndrome, a progressive inherited nephropathy associated with mutation in COL4A3, COL4A4, or COL4A5 genes. To investigate, in vivo, the regulation of the expression, assembly, and function of the alpha3alpha4alpha5(IV) protomer, we have generated a yeast artificial chromosome transgenic line of mice carrying the human COL4A3-COL4A4 locus. Transgenic mice expressed the human alpha3 and alpha4(IV) chains in a tissue-specific manner. In the kidney, when expressed onto a Col4a3(-/-) background, the human alpha3(IV) chain restored the expression of and co-assembled with the mouse alpha4 and alpha5(IV) chains specifically at sites where the human alpha3(IV) was expressed, demonstrating that the expression of all three chains is required for network assembly. The co-assembly of the human and mouse chains into a hybrid network in the GBM restores a functional GBM and rescues the Alport phenotype, providing further evidence that defective assembly of the alpha3-alpha4-alpha5(IV) protomer, caused by mutations in any of the three chains, is the pathogenic mechanism responsible for the disease. This line of mice, humanized for the alpha3(IV) collagen chain, will also provide a valuable model for studying the pathogenesis of Goodpasture syndrome, an autoimmune disease caused by antibodies against this chain.

Basement membranes: structure, assembly and role in tumour angiogenesis

In recent years, the basement membrane (BM)--a specialized form of extracellular matrix (ECM)--has been recognized as an important regulator of cell behaviour, rather than just a structural feature of tissues. The BM mediates tissue compartmentalization and sends signals to epithelial cells about the external microenvironment. The BM is also an important structural and functional component of blood vessels, constituting an extracellular microenvironment sensor for endothelial cells and pericytes. Vascular BM components have recently been found to be involved in the regulation of tumour angiogenesis, making them attractive candidate targets for potential cancer therapies.

Transfer of the alpha 5(IV) collagen chain gene to smooth muscle restores in vivo expression of the alpha 6(IV) collagen chain in a canine model of Alport syndrome

X-linked Alport syndrome is a progressive renal disease caused by mutations in the COL4A5 gene, which encodes the alpha 5(IV) collagen chain. As an initial step toward gene therapy for Alport syndrome, we report on the expression of recombinant alpha 5(IV) collagen in vitro and in vivo. A full-length cDNA-encoding canine alpha 5(IV) collagen was cloned and expressed in vitro by transfection of HEK293 cells that synthesize the alpha1(IV) and alpha2(IV), but not the alpha 3(IV) to alpha 6(IV) collagen chains. By Northern blotting, an alpha 5(IV) mRNA transcript of 5.2 kb was expressed and the recombinant protein was detected by immunocytochemistry. The chain was secreted into the medium as a 190-kd monomer; no triple helical species were detected. Transfected cells synthesized an extracellular matrix containing the alpha1(IV) and alpha2(IV) chains but the recombinant alpha 5(IV) chain was not incorporated. These findings are consistent with the concept that the alpha 5(IV) chain requires one or more of the alpha 3(IV), alpha 4(IV), or alpha 6(IV) chains for triple helical assembly. In vivo studies were performed in dogs with X-linked Alport syndrome. An adenoviral vector containing the alpha 5(IV) transgene was injected into bladder smooth muscle that lacks both the alpha 5(IV) and alpha 6(IV) chains in these animals. At 5 weeks after injection, there was expression of both the alpha 5(IV) and alpha 6(IV) chains by smooth muscle cells at the injection site in a basement membrane distribution. Thus, this recombinant alpha 5(IV) chain is capable of restoring expression of a second alpha(IV) chain that requires the presence of the alpha 5(IV) chain for incorporation into collagen trimers. This vector will serve as a useful tool to further explore gene therapy for Alport syndrome.

Distribution of the collagen IV isoforms in human Bruch's membrane

AIMS

To determine the distribution of the alpha1 to alpha6 chains of type IV collagen in Bruch's membrane of the human posterior pole.

METHODS

Cryosections (10 micro m) from 18 human eyes (20 months to 83 years old) were acid treated, blocked with 10% normal goat serum, incubated for 1 hour with monoclonal antibodies against type IV collagen isoform specific peptides at 1:75 dilution, and visualised with an ABC staining kit.

RESULTS

In Bruch's membrane, the alpha1(IV) and alpha2(IV) chains were identified in retinal pigment epithelial (10/18 = 55%) and choriocapillaris basement membranes (18/18 = 100%); the alpha3(IV), alpha4(IV), and alpha5(IV) chains were also found in the retinal pigment epithelial basement membrane (13/18 = 72%). In the choroid, the alpha1(IV) and alpha2(IV) chains were detected in the blood vessels (18/18=100%). The alpha6(IV) chain was not identified in any sections.

CONCLUSION

The heterogeneous distribution of alpha1-2(IV) and alpha3-5(IV) in Bruch's membrane could give insights into the function of this structure in health, ageing, and diseases such as age related macular degeneration.

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.

Quantitative analysis of type IV collagen alpha chains in the basement membrane of human urogenital epithelium

Type IV collagen is a major component of the basement membrane (BM), which consists of six genetically distinct alpha(IV) chains. In this study the expression of these six alpha(IV) chains was demonstrated immunohistochemically. In addition, the alpha2(IV) and alpha5(IV) chains were analysed quantitatively by confocal laser scanning microscopy in human urogenital epithelial BM. The alpha1/alpha2(IV) and alpha5/alpha6(IV) chains were immunoreactive in the epithelial BM, whereas, alpha3/alpha4(IV) chains were not. The quantitative analysis revealed that the amount of alpha2(IV) and alpha5(IV) chains differed in each urogenital epithelial BM. The content of alpha5(IV) chains in the epithelial BM of the bladder was differentially high, and that of the foreskin was differentially low. It is concluded that the elasticity of epithelial BM of the bladder may be structurally related to the high content of alpha5/alpha6(IV) chains.

Crystal structure of NC1 domains. Structural basis for type IV collagen assembly in basement membranes

Type IV collagen, which is present in all metazoan, exists as a family of six homologous alpha(IV) chains, alpha1-alpha6, in mammals. The six chains assemble into three different triple helical protomers and self-associate as three distinct networks. The network underlies all epithelia as a component of basement membranes, which play important roles in cell adhesion, growth, differentiation, tissue repair and molecular ultrafiltration. The specificity of both protomer and network assembly is governed by amino acid sequences of the C-terminal noncollagenous (NC1) domain of each chain. In this study, the structural basis for protomer and network assembly was investigated by determining the crystal structure of the ubiquitous [(alpha1)(2).alpha2](2) NC1 hexamer of bovine lens capsule basement membrane at 2.0 A resolution. The NC1 monomer folds into a novel tertiary structure. The (alpha1)(2).alpha2 trimer is organized through the unique three-dimensional domain swapping interactions. The differences in the primary sequences of the hypervariable region manifest in different secondary structures, which determine the chain specificity at the monomer-monomer interfaces. The trimer-trimer interface is stabilized by the extensive hydrophobic and hydrophilic interactions without a need for disulfide cross-linking.

Basement membrane type IV collagen molecules in the choroid plexus, pia mater and capillaries in the mouse brain

We investigated the differential distribution of basement membrane type IV collagen a chains in the mouse brain by immunohistochemistry using a chain-specific monoclonal antibodies. Subendothelial basement membranes were found to contain alpha1 and alpha2 chains. Basement membranes surrounding smooth muscle cells on blood vascular walls were immunoreactive for alpha1 and alpha2 chains but not for alpha5 and alpha6 chains. Interestingly, the pia mater contained a thin basement membrane which was positive for alpha1, alpha2, alpha5, and alpha6 chains, suggesting that glia limitans superficialis coheres basement membranes containing [alpha1(IV)]2alpha2(IV) and [alpha5(IV)]2alpha6(IV) molecules. In contrast, capillaries always possessed thin basement membranes of [alpha1(IV)]2alpha2(IV) molecules. Cerebrospinal fluid is produced through filtration of blood at the choroid plexus, where two distinct basement membranes were detected by anti-al and anti-alpha2 antibodies. The subendothelial basement membrane appeared to consist of [alpha1(IV)]2alpha2(IV) molecules, whereas the subependymal basement membrane in the choroid plexus was strongly positive for alpha3, alpha4, and alpha5 chains, indicating that the filtering unit was composed of alpha3(IV)alpha4(IV)alpha5(IV) molecules. That the specific localizations of these molecules are shared by renal glomeruli and the choroid plexus leads us to hypothesize that the supramolecular network containing alpha3(IV) alpha4(IV)alpha5(IV) molecules may function as a permeability selective barrier.