A novel metalloproteinase present in freshly isolated rat glomeruli

Identification of proteins that interact with podocin using the yeast 2-hybrid system

PURPOSE

As a membrane protein at the insertion site of the slit diaphragm (SD) complex in podocyte foot processes, podocin has been reported to act as a scaffolding protein required to maintain or regulate the structural integrity of the SD. In order to identify proteins that associate or interact with podocin, we screened a mouse kidney complementary DNA (cDNA) library using a yeast 2-hybrid system.

MATERIALS AND METHODS

1) The full-length cDNA of podocin from the mouse kidney was amplified by Polymerase Chain Reaction (PCR), 2) The PCR product was cloned into a pGBKT7 vector, pGBKT7-podocin, 3) After the pGBKT7-podocin was transformed into AH109, the AH109/pGBKT7-podocin product was obtained, 4) The mouse kidney cDNA library was transformed into the AH109/pGBKT7-podocin and screened by selection steps, 5) Next, twelve clones were cultured and isolated, 6) The yeast-purified plasmids were transformed into Escherichia coli (E. coli) by heat shock, and 7) To identify the activation domain (AD)/library inserts, we digested them with Him III, and the fragments were then sequenced.

RESULTS

12 positive clones that interacted with podocin were obtained by screening a mouse kidney cDNA library using pGBKT7-podocin. Among them, only 4 clones were found to function at the podocyte where podocin is present.

CONCLUSION

Additional studies are needed to clarify the role and interaction with podocin and candidates.

Metalloprotease meprin beta in rat kidney: glomerular localization and differential expression in glomerulonephritis

Meprin (EC 3.4.24.18) is an oligomeric metalloendopeptidase found in microvillar membranes of kidney proximal tubular epithelial cells. Here, we present the first report on the expression of meprin beta in rat glomerular epithelial cells and suggest a potential involvement in experimental glomerular disease. We detected meprin beta in glomeruli of immunostained rat kidney sections on the protein level and by quantitative RT-PCR of laser-capture microdissected glomeruli on the mRNA level. Using immuno-gold staining we identified the membrane of podocyte foot processes as the main site of meprin beta expression. The glomerular meprin beta expression pattern was altered in anti-Thy 1.1 and passive Heymann nephritis (PHN). In addition, the meprin beta staining pattern in the latter was reminiscent of immunostaining with the sheep anti-Fx1A antiserum, commonly used in PHN induction. Using Western blot and immunoprecipitation assays we demonstrated that meprin beta is recognized by Fx1A antiserum and may therefore represent an auto-antigen in PHN. In anti-Thy 1.1 glomerulonephritis we observed a striking redistribution of meprin beta in tubular epithelial cells from the apical to the basolateral side and the cytosol. This might point to an involvement of meprin beta in this form of glomerulonephritis.

Matrix metalloproteinases: a role in the contraction of vitreo-retinal scar tissue

The most common cause of failure of retinal reattachment surgery is formation of fibrocellular contractile membranes on both surfaces of the neuroretina. This intraocular fibrosis, known as proliferative vitreoretinopathy, results in a blinding tractional retinal detachment because of the contractile nature of the membrane. Contractility is a cell-mediated event that is thought to be dependent on locomotion and adhesion to the extracellular matrix. Interactions between cells and the extracellular matrix can be influenced by matrix metalloproteinases (MMPs) and we investigated the role of MMPs in two in vitro models (two- and three-dimensional) of human retinal pigment epithelial (RPE) cell-mediated contraction. MMP activity was detected using enzyme-linked immunosorbent assays and zymography techniques that revealed MMP-1, -2, -3, and -9 positivity during the collagen matrix contraction assays. RPE-populated collagen matrix contraction (three-dimensional) was inhibited using a cocktail of anti-MMP antibodies and with Galardin (a broad-spectrum MMP inhibitor). Galardin inhibition was dose-dependent, reversible, and dependent on cell number. MMP inhibitors had no effect on contraction when RPEs were seeded on two-dimensional collagen matrices or on cellular adhesion to collagen type I. Our results suggest that MMP activity may be required for three-dimensional but not two-dimensional RPE-collagen matrix contraction.

Balance between matrix synthesis and degradation: a determinant of glomerulosclerosis

In glomerular health and disease, the balance between extracellular matrix (ECM) protein synthesis and degradation determines the amount of matrix that accumulates locally. While cell and whole animal regulation of ECM synthesis has been the subject of ongoing study, attention has become focused on proteases that degrade matrix components only recently. Two major ECM protease systems have been defined. The plasminogen activators (PAs) are serine proteases that have matrix-degrading capability and also activate plasminogen to plasmin. Plasmin not only degrades ECM proteins, but also may activate members of the matrix metalloproteinase (MMP) family which comprise the second major matrix-degrading system. Specific biological antagonists of both the PAs and the MMPs tightly regulate proteolysis by these enzymes. All of these enzymes and inhibitors have been detected in the kidney, and their expression may be altered to facilitate ECM accumulation in conditions associated with matrix expansion, such as glomerulosclerosis. Work is in progress to determine how these systems are regulated in the kidney and to further define their contribution to the sclerotic process.

An old enzyme with a new function: purification and characterization of a distinct matrix-degrading metalloproteinase in rat kidney cortex and its identification as meprin

We have purified to homogeneity the enzyme in the kidney cortex which accounts for the vast majority of matrix-degrading activity at neutral pH. The purified enzyme has an apparent molecular mass of 350 kD by gel filtration and of 85 kD on SDS-PAGE under reducing conditions; and it degrades laminin, type IV collagen and fibronectin. The enzyme was inhibited by EDTA and 1,10-phenanthroline, but not by other proteinase inhibitors. The enzyme was not activated by organomercurials or by trypsin and was not inhibited by tissue inhibitors of metalloproteinases indicating that it is distinct from the other matrix-degrading metalloproteinases. Unexpectedly, the amino acid sequence of the NH2-terminal and two internal peptides of the enzyme showed complete homology to those alpha subunits of rat meprin, an enzyme previously shown to degrade azocasein and insulin B chain but not known to degrade extracellular matrix components. Immunoprecipitation studies, Western blot analyses and other biochemical properties of the purified enzyme confirm that the distinct matrix-degrading enzyme is indeed meprin. Our data also demonstrate that meprin is the major enzyme in the renal cortex capable of degrading components of the extracellular matrix. The demonstration of this hitherto unknown function of meprin suggests its potential role in renal pathophysiology.

Proteolytic machinery of glomerular epithelial cells against IgG

Accumulation of immune complexes in the subepithelial region of the glomerular basement membrane results in the lesion of membranous nephropathy. The inefficient handling of immune complexes by the glomerular epithelial cell was investigated by studying the mechanism of IgG proteolysis by the intracellular proteases of cultured epithelial cells. Radiolabelled IgG was incubated with extracts of cells and the digestion of IgG was monitored by SDS-PAGE analysis. Prolonged incubation of IgG with the cell extracts resulted in only partial degradation of the IgG. The enzyme responsible for the breakdown was determined to be the lysosomal cathepsin D based on the pH optimum and the presence of aspartate in the active site of the enzyme. SDS-PAGE analysis of the digestion fragments revealed that a large proportion of the incompletely degraded IgG was the (Fab)2 fragment, which was resistant to further proteolysis. This could be one of the possible explanations for the slow removal of IgG from the subepithelial location of the basement membrane.