Application of electron microscopic immunocytochemistry to the human kidney: distribution of type IV and type VI collagen in normal human kidney

Hyperglycaemia and the risk of post-surgical adhesion

Post-surgical adhesions are a major complication leading to organ dysfunctions, pain, intestinal obstruction, and infertility. The incidence of post-surgical adhesion is really high. The factors involved in the pathogenesis of post-surgical fibrosis, are largely unknown, for example why two patients with similar abdominal operation have a different risks of adhesion severity? High secretion of pro-inflammatory cytokines and growth factors, includes tumour necrosis factor α (TNF-α), interleukin 6 (IL6), and transforming growth factor β (TGF-β) by persistent recruitment of immune cells and the inappropriate proliferated fibroblast/mesothelial cells can stimulate signalling pathways particularly TGF-β leads to the up-regulation of some pro-fibrotic genes that impair fibrinolytic activity and promote extracellular matrix (ECM) accumulation. In this review, we focus on the role of diabetes and hyperglycaemia on post-surgical fibrosis, including the molecular mechanisms affected by hyperglycaemia that cause inflammation, oxidative stress, and increase the expression of pro-fibrotic molecules.

Biochemical composition of the glomerular extracellular matrix in patients with diabetic kidney disease

In the glomeruli, mesangial cells produce mesangial matrix while podocytes wrap glomerular capillaries with cellular extensions named foot processes and tether the glomerular basement membrane (GBM). The turnover of the mature GBM and the ability of adult podocytes to repair injured GBM are unclear. The actin cytoskeleton is a major cytoplasmic component of podocyte foot processes and links the cell to the GBM. Predominant components of the normal glomerular extracellular matrix (ECM) include glycosaminoglycans, proteoglycans, laminins, fibronectin-1, and several types of collagen. In patients with diabetes, multiorgan composition of extracellular tissues is anomalous, including the kidney, so that the constitution and arrangement of glomerular ECM is profoundly altered. In patients with diabetic kidney disease (DKD), the global quantity of glomerular ECM is increased. The level of sulfated proteoglycans is reduced while hyaluronic acid is augmented, compared to control subjects. The concentration of mesangial fibronectin-1 varies depending on the stage of DKD. Mesangial type III collagen is abundant in patients with DKD, unlike normal kidneys. The amount of type V and type VI collagens is higher in DKD and increases with the progression of the disease. The GBM contains lower amount of type IV collagen in DKD compared to normal tissue. Further, genetic variants in the α3 chain of type IV collagen may modulate susceptibility to DKD and end-stage kidney disease. Human cellular models of glomerular cells, analyses of human glomerular proteome, and improved microscopy procedures have been developed to investigate the molecular composition and organization of the human glomerular ECM.

Collagen turnover profiles in chronic kidney disease

Renal fibrosis is a hallmark of chronic kidney disease (CKD) caused by an imbalance between formation and degradation of extracellular matrix proteins. We investigated the collagen turnover profile of 81 non-dialysis CKD stage 2-5 patients by measuring peptides reflecting formation and degradation of collagen type (COL) I, III, IV, and VI. Based on the collagen turnover profile, we identified four clusters of patients. Cluster 1 contained one patient with prostate cancer, who had a distinct collagen turnover. The other clusters generally had severe (Cluster 2), moderate (Cluster 4), or mild CKD (Cluster 3). Cluster 4 patients were characterized by higher levels of COL III, COL IV, and COL VI (all p < 0.001) degradation fragments in plasma, while patients in Clusters 2 and 4 had higher levels of COL VI formation (p < 0.05). COL IV fragments in plasma were lower in Cluster 2 (p < 0.01). Urinary COL III fragments decreased from Cluster 3 to 4, and from Cluster 4 to 2 (both p < 0.001). We show that patients with similar kidney function have a different collagen remodeling profile, suggesting that different phenotypes exist with different disease activity and potentially disease progression. Biomarkers of collagen remodeling could provide additional information to traditional markers of renal function.

COL6A1 mutation leading to Bethlem myopathy with recurrent hematuria: a case report

Background

Collagen VI-related myopathies are a spectrum of muscular diseases with features of muscle weakness and atrophy, multiple contractures of joints, distal hyperextensibility, severe respiratory dysfunction and cutaneous alterations, attributable to mutations in the COL6A1, COL6A2, and COL6A3 genes. However, no case of collagen VI mutations with hematuria has been reported. We report a 14-year-old boy who had both Bethlem myopathy and recurrent hematuria and who carried a known de novo COL6A1 missense mutation c.877G > A (p.G293R).

Case presentation

The patient was a 14-year-old boy presenting with muscle weakness from 3 years of age without any family history. Six months before admission, he developed recurrent gross hematuria, three bouts in total, with the presence of blood clots in the urine. Next-generation sequencing of his whole-exome was performed. The result of sequencing revealed a de novo heterozygous G-to-A nucleotide substitution at position 877 in exon 10 of the COL6A1 gene. After treatment, the hematuria healed, but the muscle weakness failed to improve.

Conclusions

Hematuria in Bethlem myopathy can be caused by COL6 mutations, which may be related to the aberrant connection between collagen VI and collagen IV.

Electronic supplementary material

The online version of this article (10.1186/s12883-019-1263-0) contains supplementary material, which is available to authorized users.

The Ultrastructural Localization of Type II, IV, and VI Collagens at the Vitreoretinal Interface

Background

The vitreoretinal interface is the border of the cortical vitreous and the inner surface of the retina. The adhesion of the cortical vitreous to the ILM, namely vitreoretinal adhesion, involves a series of complex molecular adhesion mechanisms and has been considered as an important pathogenic factor in many vitreoretinal diseases. The presence of type VI collagen at the vitreoretinal interface and its possible interaction with collagens and glycoproteins indicates that type VI collagen may contribute to the vitreoretinal adhesion.

Purpose

To clarify the ultrastructural location of type VI collagen and its relationship to type II and IV collagens at the vitreoretinal interface.

Methods

The ultrastructural localization of type II, IV and VI collagens in the adult human vitreoretinal interface of five donor eyes was evaluated by transmission electron microscopy using immunogold labeling.

Results

In the pre-equatorial region, we observed densely packed vitreous lamellae with a partly intraretinal course containing type II and VI collagens, reticular structures containing type IV and VI collagens and a thin inner limiting membrane (ILM) containing type IV and VI collagens in a linear distribution pattern. From the anterior to the posterior retina, the linear pattern of type IV and VI collagen labeling gradually became more diffusely present throughout the entire thickness of the ILM.

Conclusions

The presence of type VI collagen in vitreous lamellae penetrating the ILM into the superficial retina suggests that type VI collagen may be involved in the organization of vitreous fibers into lamellae and in the adhesion of the vitreous fibers to the retina. The close relation of type VI to type IV collagen in the ILM suggests that type VI collagen is an important collagen type in the ILM. The topographic variations of type IV and VI collagens in the different regions of the ILM suggest a regional heterogeneity of the ILM. The reticular labeling pattern of type IV and VI collagens observed in the anterior vitreous are highly similar to labeling patterns of blood vessel walls. In the anterior vitreous, they may represent remnants of the regressed embryonic hyaloid blood vessel system. Their presence is in support of the theory on interactive remodeling of the developing vitreous as opposed to the main stream theory of displacement and compression of the primary by the secondary vitreous.

Diabetes-induced alterations in the extracellular matrix and their impact on myocardial function

Diabetes is an increasing public health problem that is expected to escalate in the future due to the growing incidence of obesity in the western world. While this disease is well known for its devastating effects on the kidneys and vascular system, diabetic individuals can develop cardiac dysfunction, termed diabetic cardiomyopathy, in the absence of other cardiovascular risk factors such as hypertension or atherosclerosis. While much effort has gone into understanding the effects of elevated glucose or altered insulin sensitivity on cellular components within the heart, significant changes in the cardiac extracellular matrix (ECM) have also been noted. In this review article we highlight what is currently known regarding the effects diabetes has on both the expression and chemical modification of proteins within the ECM and how the fibrotic response often observed as a consequence of this disease can contribute to reduced cardiac function.

Collagen VI is a basement membrane component that regulates epithelial cell-fibronectin interactions

Collagen VI is a heterotrimer composed of three α chains (α1, α2, α3) widely expressed throughout various interstitial matrices. Collagen VI is also found near the basement membranes of many tissues where it serves as an anchoring meshwork. The aim of this study was to investigate the distribution and role of collagen VI at the epithelial-stromal interface in the intestine. Results showed that collagen VI is a bona fide epithelial basal lamina component and constitutes the major collagen type of epithelial origin in this organ. In vitro, collagen VI co-distributes with fibronectin. Targeted knockdown of collagen VI expression in intestinal epithelial cells was used to investigate its function. Depletion of collagen VI from the matrix led to a significant increase in cell spreading and fibrillar adhesion formation coinciding with an upregulation of fibronectin expression, deposition and organization as well as activation of myosin light chain phosphorylation by the myosin light chain kinase and Rho kinase dependent mechanisms. Plating cells deficient for collagen VI on collagen VI rescued the phenotype. Taken together, these data demonstrate that collagen VI is an important basal lamina component involved in the regulation of epithelial cell behavior most notably as a regulator of epithelial cell-fibronectin interactions.

Compositional differences between infant and adult human corneal basement membranes

PURPOSE

Adult human corneal epithelial basement membrane (EBM) and Descemet's membrane (DM) components exhibit heterogeneous distribution. The purpose of the study was to identify changes of these components during postnatal corneal development.

METHODS

Thirty healthy adult corneas and 10 corneas from 12-day- to 3-year-old children were studied by immunofluorescence with antibodies against BM components.

RESULTS

Type IV collagen composition of infant corneal central EBM over Bowman's layer changed from alpha1-alpha2 to alpha3-alpha4 chains after 3 years of life; in the adult, alpha1-alpha2 chains were retained only in the limbal BM. Laminin alpha2 and beta2 chains were present in the adult limbal BM where epithelial stem cells are located. By 3 years of age, beta2 chain appeared in the limbal BM. In all corneas, limbal BM contained laminin gamma3 chain. In the infant DM, type IV collagen alpha1-alpha6 chains, perlecan, nidogen-1, nidogen-2, and netrin-4 were found on both faces, but they remained only on the endothelial face of the adult DM. The stromal face of the infant but not the adult DM was positive for tenascin-C, fibrillin-1, SPARC, and laminin-332. Type VIII collagen shifted from the endothelial face of infant DM to its stromal face in the adult. Matrilin-4 largely disappeared after the age of 3 years.

CONCLUSIONS

The distribution of laminin gamma3 chain, nidogen-2, netrin-4, matrilin-2, and matrilin-4 is described in the cornea for the first time. The observed differences between adult and infant corneal BMs may relate to changes in their mechanical strength, corneal cell adhesion and differentiation in the process of postnatal corneal maturation.

The pathogenesis of myocardial fibrosis in the setting of diabetic cardiomyopathy

Diabetes has emerged as a major threat to worldwide health. The increasing incidence of diabetes in young individuals is particularly worrisome given that the disease is likely to evolve over a period of years. In 1972, the existence of a diabetic cardiomyopathy was proposed based on the experience with four adult diabetic patients who suffered from congestive heart failure in the absence of discernible coronary artery disease, valvular or congenital heart disease, hypertension, or alcoholism. The exact mechanisms underlying the disease are unknown; however, an important component of the pathological alterations observed in these hearts includes the accumulation of extracellular matrix (ECM) proteins, in particular collagens. The excess deposition of ECM in the heart mirrors what occurs in other organs such as the kidney and peritoneum of diabetics. Mechanisms responsible for these alterations may include the excess production, reduced degradation, and/or chemical modification of ECM proteins. These effects may be the result of direct or indirect actions of high glucose concentrations. This article reviews our state of knowledge on the effects that diabetes-like conditions exert on the cells responsible for ECM production as well as relevant experimental and clinical data.

Extracellular matrix metabolism in diabetic nephropathy

Diabetic nephropathy is characterized by excessive deposition of extracellular matrix proteins in the mesangium and basement membrane of the glomerulus and in the renal tubulointerstitium. This review summarizes the main changes in protein composition of the glomerular mesangium and basement membrane and the evidence that, in the mesangium, these are initiated by changes in glucose metabolism and the formation of advanced glycation end products. Both processes generate reactive oxygen species (ROS). The review includes discussion of how ROS may activate intracellular signaling pathways leading to the activation of redox-sensitive transcription factors. This in turn leads to change in the expression of genes encoding extracellular matrix proteins and the protease systems responsible for their turnover.

The role of determining the levels of serum collagen type IV in diagnosing early diabetic nephropathy

Two molecules of type IV collagen are linked via the COOH-terminal non-collagenous domain (NCI domain). In this way, type IV collagen forms the main framework of basement membrane. We have developed sandwich Enzyme-Linked Immunosorbent Assay (ELISA) for human serum type IV collage peptide monoclonals for CO5 and CO6 recognizing two different determinants on the central triplex of human collagen molecules. Using this, we explored the possibility that serum levels of type IV collagen (Col IV) are an indicator of early diabetic nephropathy (DN). Serum type IV collagen levels were determined by ELISA in 36 patients with diabetic nephropathy. Serum hemoglobin A1c (HbA1c) and in twenty-four hour urinary albumin excretion rates (UAER) were measured at the same time. A relationship was found between serum levels of this collagen IV and the presence of diabetic nephropathy. That is, serum levels of type IV collagen in diabetic nephropathy were significantly higher than those found in non-insulin-dependent diabetic (NIDDM) patients without nephropathy and in healthy subjects (HS). It is possible that an elevated serum collagen type IV level reflects increased synthetic activity in basement membrane collagen type IV. Serum type IV collagen may be a useful marker for monitoring the dynamics of basement membrane metabolism and in this way serve as a clinical indicator of the progression or the activity of diabetic nephropathy and a marker for the assessment of therapeutic effects.

In human IgA nephropathy uteroglobin does not play the role inferred from transgenic mice

BACKGROUND

Uteroglobin (UG)-knockout and UG-antisense transgenic mice develop clinical and pathological features of immunoglobulin A (IgA) nephropathy with heavy proteinuria. These models suggested that UG, an anti-inflammatory protein with high affinity for fibronectin (Fn), prevents the formation of IgA-Fn complexes and mesangial deposits in mice. We aim to elucidate whether similar mechanisms underlie the development and severity of human IgA nephropathy.

METHODS

Specific enzyme-linked immunosorbent assays were devised to detect serum levels of UG binding to Fn or incorporated into IgA-Fn complexes and IgA binding to Fn or collagen IV. Sera from 75 patients with IgA nephropathy with normal renal function and various degrees of proteinuria (0.2 to 5 g/d of protein) stable over the previous 3 months without therapy were investigated and compared with healthy controls.

RESULTS

Levels of UG binding to Fn were similar in patients with IgA nephropathy and healthy controls. UG incorporated into circulating IgA-Fn complexes, as well as levels of IgA-Fn complexes and IgA binding Fn and collagen IV, were significantly greater in patients than healthy controls. Greater amounts of UG incorporated into IgA-Fn complexes reduced the risk for proteinuria with protein greater than 1 g/d (odds ratio = 0.67; P < 0.001). Logistic regression analysis assigned a predictive value for proteinuria persistently greater than 1 g/d of protein to lower amounts of UG incorporated into IgA-Fn complexes (R = -0.267; P = 0.008) and increased binding of IgA to collagen IV (R = 0.214; P = 0.0003).

CONCLUSION

This first report of human IgA nephropathy after the publication of the mouse model shows that UG is not reduced in circulation and is even increased in IgA-Fn complexes. Because aberrant IgA1 glycosylation is the event initiating IgA nephropathy in humans, we speculate that the enhanced incorporation of UG into IgA-Fn complexes might represent feedback to reduce the formation of macromolecular aggregates.

Differential expression of type IV collagen isoforms in rat glomerular endothelial and mesangial cells

Type IV collagen, which is encoded by six genetically distinct alpha-chains (alpha 1-alpha 6), is a major component of the kidney glomerulus. The alpha 1(IV) and alpha 2(IV) chains are present predominantly in the mesangial matrix, whereas the alpha 3(IV), alpha 4(IV), and alpha 5(IV) chains are localized almost exclusively to the glomerular basement membrane (GBM). Thickening of the GBM and expansion of the mesangial matrix are believed to contribute to the pathogenesis of diabetic nephropathy. In the present study, we evaluated the expression of alpha 1(IV), alpha 3(IV), and alpha 5(IV) chains in rat glomerular endothelial (GEndC) and mesangial cells (GMC). Under physiological concentrations of glucose (5 mM), alpha 1(IV) and alpha 5(IV) chains were detectable in GMCs, with an obvious absence of alpha 3(IV) chain. All three isoforms tested were present in GEndCs. At diabetic concentrations of glucose (25 mM), alpha 1(IV) was up-regulated in GMCs, whereas expression level of alpha 1(IV) remained unaltered in GEndCs. The alpha 3(IV) and alpha 5(IV) chains were up-regulated in GEndCs, but remained unchanged in GMCs under diabetic glucose concentrations (25 mM). Collectively, our results demonstrate that GMC might contribute to mesangial matrix expansion, mediated by alpha 1(IV) collagen, while GEndC might contribute to thickening of GBM, mediated by alpha 3(IV) collagen, in patients with diabetic nephropathy.

Antibody response against perlecan and collagen types IV and VI in chronic renal allograft rejection in the rat

Chronic rejection is the leading cause of late renal transplant failure. Various structural lesions are observed in grafts undergoing chronic rejection including glomerular basement membrane (GBM) duplications. The well-established Fisher (F344) to Lewis (LEW) rat renal transplant model for chronic rejection was used to assess the presence and role of the humoral immune response against graft antigens during chronic rejection. LEW recipients of F344 allografts develop transplant glomerulopathy and produce IgG1 antibodies directed against F344 GBM preparations that are detectable 3 weeks after transplantation. Glomerular IgG1 deposition was observed that in vitro co-localized with a rabbit anti-rat GBM antiserum in rejecting F344 grafts; elution experiments of isolated glomeruli yielded IgG1 antibodies reactive in vitro with F344 GBM, but not LEW GBM. Prevention of acute rejection by transient treatment of the recipients with cyclosporin A completely abrogated the production of anti-GBM antibodies. Using proteomic techniques we identified the antigens recognized by the LEW posttransplant sera as being the heparan sulfate proteoglycan perlecan and the alpha1 chain of collagen type VI in association with the alpha5 chain of collagen type IV. In conclusion, LEW recipients of F344 kidney grafts produce IgG1 antibodies against donor type perlecan and alpha1(VI)/alpha5(IV) collagen and develop transplant glomerulopathy. These data implicate an important role for the humoral immune response in the development of glomerulopathy during chronic rejection.

Quantitative immunoelectron microscopy of type VI collagen in glomeruli in type I diabetic patients

BACKGROUND

The nature of the extracellular matrix (ECM) accumulating in the glomerular basement membrane (GBM) and mesangium (Mes) in diabetes is unknown. Type IV collagen (Col IV) as estimated by quantitative immunoelectron microscopy was reduced in type I diabetic patients (D) with rapid ("fast-track") compared with slow ("slow-track") development of diabetic nephropathy (DN) lesions and controls (C). Col VI is another ECM component suggested to account for Mes matrix (MM) expansion in DN.

METHODS

Col VI ECM density was evaluated in eight "slow-track" {Mes fractional volume [Vv(Mes/glom)] <0.32, duration> 20 years} and seven "fast-track" patients [Vv(Mes/glom)> 0.37, duration <20 years diabetes] and in eight C. Quantitative immunoelectron microscopy was performed using polyclonal antibodies to Col VI. Gold particle density (PDG) in MM and the inner layer (IL) of the GBM was measured using stereologic methods.

RESULTS

GBM IL PDG was decreased in both fast-track (1.7 +/- 1.6/microm2, mean +/- SD, P < 0.002) and slow-track (3.9 +/- 2.4, P < 0.02) D versus C (10.8 +/- 7.9). GBM IL PDG was also lower in the fast-track versus slow-track D (P < 0.04). Mes matrix PDG/microm2 was decreased in fast-track D (3.2 +/- 3.6) versus C (14.1 +/- 14.6, P < 0.02); a similar trend was seen in slow-track D (5.7 +/- 5.6, P < 0.1). There was no significant difference in MM PDG between the slow-track and fast-track D.

CONCLUSION

Col VI density in MM and GBM is decreased in diabetic patients with slowly and rapidly developing renal lesions. This leaves the nature of ECM accumulation in DN unexplained. At least in part, glomerular ECM compositional change is related to diabetes per se and may be independent of the severity of lesions.

Ultrastructural triple localization of laminin-1, nidogen-1, and collagen type IV helps elucidate basement membrane structure in vivo

The basement membrane models which have been proposed to date are generally based on biochemical data, mainly binding studies and artificially synthesized polymers in vitro. Basically these have led to models proposing two three-dimensional laminin-1 and collagen type IV networks interconnected by nidogen-1. Whether they reflect the in vivo basement membrane structure is still not clear. We localized laminin-1, nidogen-1, and collagen type IV ultrastructurally in adult and fetal mouse kidney basement membranes with the help of immunogold-histochemistry performing double and triple localization to try to elucidate the molecular organization of basement membranes in vivo. We found laminin-1, nidogen-1, and collagen type IV distributed over the entire basement membranes in adult and fetal kidneys. This contradicts earlier studies ascribing laminin-1 to the lamina lucida and collagen type IV to the lamina densa. In addition, various basement membrane segments exhibited an organized labeling pattern for the BM components. Double-labeling revealed co-localization of laminin-1 and nidogen-1. We conclude that the combination of laminin-1 with collagen type IV as double-network basement membrane partially interconnected by nidogen-1 is found already in the early fetal kidney in vivo. However, our data cannot exclude the possibility of other variants of basement membrane assemblages. This is also indicated by a changing structure even in individual segments of one basement membrane type which renders a more flexible basement membrane architecture plausible.

Differential expression of collagen IV isoforms in experimental glomerulosclerosis

Expansion of the glomerular mesangial matrix (MM), thickening of the glomerular basement membrane (GBM), and eventually the development of glomerulosclerosis are often seen in immunologically mediated kidney diseases. In addition to quantitative changes in the extracellular matrix (ECM), qualitative changes in ECM molecules may contribute to alterations in the composition of the glomerular matrix. The expression of collagen IV, alpha 1-5(IV) mRNA, and polypeptides was therefore investigated during the development of chronic graft-versus-host disease (GvHD) in mice, a model for lupus nephritis, and in chronic serum sickness (CSS) in rats, a model for membranous nephropathy. Immunohistochemical studies showed increased mesangial expression of alpha 1 and alpha 2 early in the disease, but only late in the GBM. In contrast, alpha 3 and alpha 4 increased in the GBM during disease, but not in the MM. The mRNA levels for all collagen IV chains were increased in isolated glomeruli before morphological alterations were detectable. The mRNA increase was earlier and more profound for alpha 3, alpha 4 and alpha 5 than for alpha 1 and alpha 2. Expression of alpha 3(IV) was greatest in GvHD, whereas expression of alpha 4 was greatest in CSS. As determined by in situ hybridization (ISH), alpha 1 mRNA was observed dispersed in the glomerulus, but alpha 3, alpha 4, and alpha 5 mRNAs were mainly located in cells at the periphery of the glomerular tuft. The changes in the relative abundance of collagen IV mRNA in disease states may perturb the collagen IV network, altering glomerular structure and function, and may thereby play a central role in the development of glomerulonephritis and glomerulosclerosis.

Type VI collagen in glomeruli of short- and long-term experimental diabetic rats

Type VI collagen was revealed by high-resolution immunocytochemistry in renal glomeruli from short- and long-term streptozotocin-injected hyperglycaemic rats and from their age-matched normoglycaemic controls. The labellings obtained over the glomerular basement membrane and the mesangial matrix were assessed by quantitative evaluations. The labellings over the glomerular basement membrane were low and sparse in the young normoglycaemic animals but became consistent and increased in intensity with age and in both the short- and long-term diabetic animals. For the mesangial matrix, this was labelled more systematically, and its intensity increased with age and in the short-term hyperglycaemic animals. For the long-term hyperglycaemic animals, the intensities of labelling resembled those of their age-matched controls. These results indicate that type VI collagen appears to be a minor constituent of the extracellular matrix of the rat glomeruli, rather concentrated in the mesangial area in the young control animal. Concomitant with the general modifications of the extracellular matrix occurring with age and diabetes, this component increases, but apparently not with the length of the hyperglycaemic state.

Immunohistochemical and ultrastructural localization of MP78/70 (betaig-h3) in extracellular matrix of developing and mature bovine tissues

MP78/70 is a matrix protein, with 78-kD and 70-kD isoforms, which was initially identified in bovine tissue extracts designed to solubilize elastin-associated microfibrils. Peptide analysis has shown that MP78/70 is closely related to the human protein, betaig-h3. In the present study an antibody raised to a synthetic betaig-h3 peptide was shown specifically to identify MP78/70 in purified form and in bovine tissue extracts. This is consistent with MP78/70 and betaig-h3 being the bovine and human forms, respectively, of the same protein. The antibody was further affinity-purified on MP78/70 bound to Sepharose and used to localize the protein in a range of bovine tissues. Immunofluorescence showed that MP78/70 was localized to collagen fibers in tissues such as developing nuchal ligament, aorta and lung, and mature cornea; to reticular fibers in fetal spleen; and to capsule and tubule basement membranes in developing kidney. No general localization to elastic fibers was observed. The staining pattern in most tissues more closely resembled that of Type VI collagen, which occurs as collagen fiber-associated microfibrils, than that of fibrillin-1, a component of elastin-associated microfibrils. However, MP78/70 appeared to be less widely distributed than Type VI collagen. Immunoelectron microscopy showed that MP78/70 was predominantly found in loose association with collagen fibers in most tissues examined and was also located on the surface of the capsule basement membrane in developing kidney. Double labeling experiments indicated that MP78/70 is co-distributed with Type VI collagen microfibrils located in these regions. In some elastic tissues significant immunolabel was detected in regions of interface between collagen fibers and fibrillin-containing microfibrils of adjacent elastic fibers, and at the outer margins of the latter structures. Overall, the evidence points to MP78/70 having a bridging function, perhaps in association with Type VI collagen microfibrils, linking or stabilizing the interaction between interstitial collagen fibrils and other matrix structures, including some basement membranes and elastin-associated microfibrils.

Type VI collagen anchors endothelial basement membranes by interacting with type IV collagen

Type VI collagen filaments are found associated with interstitial collagen fibers, around cells, and in contact with endothelial basement membranes. To identify type VI collagen binding proteins, the amino-terminal domains of the alpha1(VI) and alpha2(VI) chains and a part of the carboxyl-terminal domain of the alpha3(VI) chain were used as bait in a yeast two-hybrid system to screen a human placenta library. Eight persistently positive clones were identified, two coding the known matrix proteins fibronectin and basement membrane type IV collagen and the rest coding new proteins. The amino-terminal domain of alpha1(VI) was shown to interact with the carboxyl-terminal globular domain of type IV collagen. The specificity of this interaction was further studied using the yeast two-hybrid system in a one-on-one format and confirmed by using isolated protein domains in immunoprecipitation, affinity blots, and enzyme-linked immunosorbent assay-based binding studies. Co-distribution of type VI and type IV collagens in human muscle was demonstrated using double labeling immunofluorescent microscopy and immunoelectron microscopy. The strong interaction of type VI collagen filaments with basement membrane collagen provided a possible molecular pathogenesis for the heritable disorder Bethlem myopathy.

Microfibril-associated glycoprotein-1 (MAGP-1) binds to the pepsin-resistant domain of the alpha3(VI) chain of type VI collagen

The interactions of type VI collagen have been investigated, using solid phase binding assays, with two components of the fibrillin-containing microfibrils, the elastin-binding protein, MAGP-1 and its structural relative MAGP-2. Both native and pepsin-treated forms of type VI collagen specifically bound to MAGP-1 but not to MAGP-2. Pepsin type VI collagen was shown to block the binding of MAGP-1 to native type VI collagen indicating that the major MAGP-1-binding site was in the triple-helical region of the molecule. MAGP-1 was found not to bind to collagens I, III, and V. Affinity blotting of pepsin-treated type VI collagen showed that MAGP-1 binding was specific for the collagenous domain of the alpha3(VI) chain. Decorin and biglycan were found not to inhibit the interaction of pepsin-treated type VI collagen with MAGP-1, indicating that its binding site on the collagen is not close to that for the proteoglycans. Reduction and alkylation of disulfide bonds in MAGP-1 did not destroy its type VI collagen-binding properties, indicating that the binding site was likely to be in the cysteine-free, N-terminal domain of MAGP-1. Interestingly, the interaction of MAGP-1 with type VI collagen was inhibited by tropoelastin, suggesting that the binding sites for tropoelastin and type VI collagen may be in the same domain of MAGP-1. A peptide, corresponding to amino acids 29-38 of MAGP-1, was found to inhibit the interactions of MAGP-1 with type VI collagen and tropoelastin. The results suggest that the peptide may contain the binding sequences for both type VI collagen and tropoelastin, and thus that these two proteins may share the same binding site on MAGP-1. The interactions of MAGP-1 with type VI collagen and tropoelastin were both determined to be of moderately high affinity, with Kd values of 5.6 x 10(-7) M and 2.6 x 10(-7) M, respectively. The findings indicate that MAGP-1 may mediate a molecular interaction between type VI collagen microfibrils and fibrillin-containing microfibrils, structures which are often found in close proximity to each other in a wide range of extracellular matrices.

Quantitative changes in the glomerular basement membrane components in human membranous nephropathy

In membranous nephropathy (MN), the glomerular basement membrane (GBM) is thickened due to accumulation of GBM material between and around the subepithelial immune deposits. Alterations in the GBM components in relation to subepithelial deposits and GBM thickening are not clearly defined. The GBM distribution of classical and novel [alpha 4(IV)] chains of type IV collagen, laminin, and fibronectin have been studied in seven patients with MN and in three normal controls by a quantitative immunogold technique. In normal kidneys, the labelling of type IV collagen or fibronectin was distributed predominantly along the endothelial side of the GBM; alpha 4(IV) was found in the lamina densa; and laminin was concentrated in the epithelial zone of the GBM (P < 0.01). In MN, there were increased immunogold densities for classical and novel type IV collagen chains, laminin, and fibronectin in the spikes of MN patients compared with controls (P < 0.05). Furthermore, gold particle labelling for the alpha 4(IV) collagen chain was increased in the middle zone (P < 0.01) and that for fibronectin was increased in the endothelial and middle zones of the GBM (P < 0.05) compared with normal controls. These findings suggest that subepithelial immune deposits stimulate glomerular epithelial cells (GEC), resulting in enhanced secretion of classical and novel type IV collagen chains, laminin, and fibronectin, forming spikes in MN; of these newly formed components, only novel type IV collagen appears to migrate towards the middle zone of the GBM, contributing to thickening of this zone. The results also suggest that fibronectin, possibly derived from the circulation, is related to thickening of the endothelial zone of the GBM, which in turn might be related to progressive glomerulosclerosis.

Ultrastructural immunocytochemistry of collagenous and non-collagenous proteins in fast-frozen, freeze-substituted, and low-temperature-embedded renal tissue in Alport syndrome

This paper describes the ultrastructural immunolocalization of the alpha 2 chain of collagen IV, laminin, and the amino terminal propeptide of collagen I (N-Pro I) in glomeruli of rapidly frozen, freeze-substituted, and low-temperature-embedded renal biopsy specimens from two cases of Alport disease and from normal kidneys. The alpha 2 chain of collagen IV is present in the whole thickness of the basement membrane in glomeruli of Alport patients, while it is limited to the subendothelial portion of the basement membrane of normal glomeruli. Laminin has the same distribution in both normal and Alport glomeruli, but is apparently more concentrated along the basement membrane of normal glomeruli. N-Pro I is localized in mesangial areas and in the basement membrane in Alport cases, while it is not detected in normal glomeruli. These data suggest complex rearrangements of major constituents of the glomerular basement membrane network and demonstrate early deposition of fibrillary collagen proteins in the matrix before the appearance of banded collagen fibres. This finding could be an indicator of early evolution towards glomerulosclerosis.

Collagen distribution in focal and segmental glomerulosclerosis: an immunofluorescence and ultrastructural immunogold study

Focal and segmental glomerulosclerosis (FSGS) is a non-specific scarring process of the glomerulus, initially described in idiopathic nephrotic syndrome. The distribution of types I, III, IV, V, and VI collagen and of the alpha 1, alpha 3, alpha 4, alpha 5, and alpha 6 chains of type IV collagen was studied by immunohistochemistry in sclerotic lesions of nine nephrotic children. Dual immunofluorescence and high-resolution immunogold labelling were used to determine the precise distribution of the antigens. No changes were detected in normal glomeruli of patients compared with controls. In FSGS, type IV collagen [alpha 1(IV)2 alpha 2(IV)], and to a lesser degree type VI, accumulates in the two components of the lesion: the enlarged mesangial matrix and the material deposited between the pushed-out podocytes and the alpha 3-alpha 5(IV)-positive glomerular basement membrane. Staining for alpha 6(IV) and types I, III, and V collagen was practically negative. These results suggest that the matrix components of the sclerotic lesion are produced solely by glomerular cells. Changes in the relative distribution of type IV collagen chains, characterized by the presence of collagen [alpha 1(IV)2 alpha 2(IV)] in close contact with the podocytes, strongly suggest a switch in the podocyte programme of collagen synthesis.

Immunohistochemical distribution of type VI collagen in developing human kidney

The distribution of type VI collagen was investigated immunohistochemically in the developing human kidney from 15 to 32 weeks gestational age and it was compared with that observed in the normal infantile and adult human kidney. In fetal kidney, type VI collagen was widely distributed as a fibrillar network in the subcapsularly undifferentiated mesenchyme and intertubular interstitium, and as a basement membrane-like structure around the ureteral bud branches, tubules, and collecting ducts. During nephrogenesis, type VI collagen disappeared from the induced mesenchyme close to the tips of ureteral branches, while it formed a distinct basement membrane-like structure around the early stages of nephron differentiation (comma-shaped and S-shaped bodies) and later along Bowman's capsule of capillary loop and maturing glomeruli A strong immureactivity for type VI collagen was also found in the glomerular basement membrane and mesangial areas of capillary loop and maturing glomeruli. In infantile kidney, type VI collagen showed a distribution pattern similar to that observed during the fetal period. In adult human kidney, glomerular basement membrane showed a weak positivity for type VI collagen and the basement membrane-like staining around Bowman's capsule, tubules, and collecting ducts was less evident than in fetal and infantile kidney. Our immunohistochemical findings suggest that type VI collagen is a normal component of the glomerular and extraglomerular extracellular matrix of developing human kidney and that it undergoes changes in the expression during maturation.

Alterations of glomerular basement membrane charge and structure in diabetic nephropathy

We examined glomerular basement membrane anionic site distribution identified by cationic gold in seven patients with insulin-dependent and four patients with non-insulin-dependent diabetes mellitus, presenting a spectrum of clinical and glomerular changes. Anionic sites were investigated by pretreatment of tissue with glycosaminoglycan-degrading enzymes prior to cationic gold staining. The distribution of chondroitin sulphate proteoglycans--a previously unrecognized glomerular basement membrane component--and type IV collagen was examined by immunoelectron microscopy to identify structural changes in the basement membrane. Findings were compared with those of non-diabetic patients showing minor proteinuria and morphologically normal glomerular basement membranes. Two patients, originally diagnosed as having diabetic nephropathy were also examined at 19 weeks and 5 years after renal transplantation. Characteristic redistribution of type IV collagen and chondroitin sulphate proteoglycans was noted in thickened glomerular basement membrane segments (> 400 nm) of diabetic patients and those with renal transplants. Extension of anionic sites deep into the glomerular basement membrane at pH 2.5, together with loss of interna sites at pH 5.8 is unique to diabetic nephropathy. Reduced charge density was apparent in some patients due to thickening of the glomerular basement membrane, although the number of anionic sites per unit length of membrane was actually increased. Thus, charge aberration in diabetic nephropathy is due to displacement rather than loss of anionic sites. Removal of more than 90% of these sites by heparitinase, confirms their association with heparan sulphate proteoglycans. Similar derangement of anionic sites in all patients with diabetic nephropathy irrespective of the degree of proteinuria, suggests that a heparan sulphate proteoglycan-related charge barrier plays a minor role in controlling permeability of the diabetic glomerular basement membrane.

Distribution of extracellular matrix glycoproteins in the human mesonephros

We analyzed the expression and distribution of collagen types IV and VI, laminin and fibronectin during the development and regression of the mesonephros in human embryos and fetuses ranging from 6 to 12 weeks of gestation by indirect immunoperoxidase methods. Type IV collagen, laminin and fibronectin were detected along the glomerular, tubular and capsular basement membranes of developing and mature nephrons. Only type IV collagen and fibronectin were found in the mesangium. Type VI collagen formed a delicate interstitial fibrillar network and a continuous basement membrane-like structure along the mesonephric nephrons. Basement membranes (GBM) of developing and mature glomeruli showed a distinct continuous staining for this collagen. The mesangial matrix was rich in type VI collagen. Mesonephric involution started during the 8th week of gestation and coincided with a moderate expansion of mesangial matrix and progressive collapse of the capillary walls, while the tubules became thinner and shorter. Staining for all extracellular matrix glycoproteins studied showed GBM wrinkling, gradual disintegration of some capillary loops and glomerulosclerosis. The sclerotic glomeruli were strongly positive for type IV collagen and less positive for type VI collagen and fibronectin. Laminin was absent. Our results indicate that collagen types IV, VI, laminin and fibronectin may be involved in the development and regression of the human mesonephros.