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      Decreased Glomerular Expression of Agrin in Diabetic Nephropathy and Podocytes, Cultured in High Glucose Medium

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          Aim: A decrease in glomerular heparan sulfate (HS) proteoglycan (PG), without apparent decrease in HSPG core protein expression, has been reported to occur in diabetic nephropathy (DN). In most studies however, agrin, the major HSPG core protein in the glomerular basement membrane, has not been studied. This prompted us to study the glomerular expression of agrin in parallel to the expression of HS-glycosaminoglycans (GAG) in biopsies of patients with DN. Furthermore, the influence of glucose on agrin production in cultured podocytes and the expression of agrin in fetal kidneys was investigated. Methods: Cryostat sections of renal biopsies from patients with DN (n = 8) and healthy controls (HC, n = 8), were stained for agrin and HS-GAG. Sections of fetal kidneys were double stained for agrin and CD35 or CD31. Stainings were performed by indirect immunofluorescence (IIF). The production of agrin by cultured human podocytes was tested by ELISA and IIF. Results: The expression of agrin, detected by AS46, was significantly reduced in biopsies from patients with DN compared to HC (p < 0.01). Similar findings were observed when monoclonal antibody JM72 was used (p < 0.05). In addition, a significant reduction in the glomerular expression of HS-GAG was detected with JM403 in these patients (p < 0.01). Agrin is expressed in cultured podocytes, the expression hereof was reduced when the cells were cultured in the presence of 25 m M D-glucose (p < 0.01). In biopsies of human fetal kidneys, glomerular expression of agrin coincided with the expression of CD31. In early stages of glomerular differentiation there was a strong staining for agrin and CD31 while CD35 was only slightly positive. Conclusions: Our data argue against a selective dysregulation in HSPG sulfation in DN, but suggest a pivotal role for hyperglycemia in the downregulation of agrin core protein production.

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          Most cited references 5

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          Perlecan, basal lamina proteoglycan, promotes basic fibroblast growth factor-receptor binding, mitogenesis, and angiogenesis.

          A survey of defined species of cell surface and extracellular matrix heparan sulfate proteoglycans (HSPG) was performed in a search for cellular proteoglycans that can promote bFGF receptor binding and biological activity. Of the various affinity-purified HSPGs tested, perlecan, the large basement membrane HSPG, is found to induce high affinity binding of bFGF both to cells deficient in HS and to soluble FGF receptors. Heparin-dependent mitogenic activity of bFGF is strongly augmented by perlecan. Monoclonal antibodies to perlecan extract the receptor binding promoting activity from active HSPG preparations. In a rabbit ear model for in vivo angiogenesis, perlecan is a potent inducer of bFGF-mediated neovascularization. These results identify perlecan as a major candidate for a bFGF low affinity, accessory receptor and an angiogenic modulator.
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            Agrin Binds to the Nerve–Muscle Basal Lamina via Laminin

            Agrin is a heparan sulfate proteoglycan that is required for the formation and maintenance of neuromuscular junctions. During development, agrin is secreted from motor neurons to trigger the local aggregation of acetylcholine receptors (AChRs) and other proteins in the muscle fiber, which together compose the postsynaptic apparatus. After release from the motor neuron, agrin binds to the developing muscle basal lamina and remains associated with the synaptic portion throughout adulthood. We have recently shown that full-length chick agrin binds to a basement membrane-like preparation called Matrigel™. The first 130 amino acids from the NH2 terminus are necessary for the binding, and they are the reason why, on cultured chick myotubes, AChR clusters induced by full-length agrin are small. In the current report we show that an NH2-terminal fragment of agrin containing these 130 amino acids is sufficient to bind to Matrigel™ and that the binding to this preparation is mediated by laminin-1. The fragment also binds to laminin-2 and -4, the predominant laminin isoforms of the muscle fiber basal lamina. On cultured myotubes, it colocalizes with laminin and is enriched in AChR aggregates. In addition, we show that the effect of full-length agrin on the size of AChR clusters is reversed in the presence of the NH2-terminal agrin fragment. These data strongly suggest that binding of agrin to laminin provides the basis of its localization to synaptic basal lamina and other basement membranes.
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              Differential expression of agrin in renal basement membranes as revealed by domain-specific antibodies.

              We determined the specificity of two hamster monoclonal antibodies and a sheep polyclonal antiserum against heparan sulfate proteoglycan isolated from rat glomerular basement membrane. The antibodies were characterized by enzyme-linked immunosorbent assay on various basement membrane components and immunoprecipitation with heparan sulfate proteoglycan with or without heparitinase pre-treatment. These experiments showed that the antibodies specifically recognize approximately 150-, 105-, and 70-kDa core proteins of rat glomerular basement membrane heparan sulfate proteoglycan. Recently, we showed that agrin is a major heparan sulfate proteoglycan in the glomerular basement membrane (Groffen, A. J. A., Ruegg, M. A., Dijkman, H. B. P. M., Van der Velden, T. J., Buskens, C. A., van den Born, J., Assmann, K. J. M., Monnens, L. A. H., Veerkamp, J. H., and van den Heuvel, L. P. W. J. (1998) J. Histochem. Cytochem. 46, 19-27). Therefore, we tested whether our antibodies recognize agrin. To this end, we evaluated staining of Chinese hamster ovary cells transfected with constructs encoding full-length or the C-terminal half of rat agrin by analysis on a fluorescence-activated cell sorter. Both hamster monoclonals and the sheep antiserum clearly stained cells transfected with the construct encoding full-length agrin, whereas wild type cells and cells transfected with the construct encoding the C-terminal part of agrin were not recognized. A panel of previously characterized monoclonals, directed against C-terminal agrin, clearly stained cells transfected with either of the constructs but not wild type cells. This indicates that both hamster monoclonals and the sheep antiserum recognize epitopes on the N-terminal half of agrin. By immunohistochemistry on rat renal tissue, we compared distribution of N-terminal agrin with that of C-terminal agrin. The monoclonal antibodies against C-terminal agrin stained almost exclusively the glomerular basement membrane, whereas the anti-N-terminal agrin antibodies recognized all renal basement membranes, including tubular basement membranes. Based on these results, we hypothesize that full-length agrin is predominantly expressed in the glomerular basement membrane, whereas in most other renal basement membranes a truncated isoform of agrin is predominantly found that misses (part of) the C terminus, which might be due to alternative splicing and/or posttranslational processing. The possible significance of this finding is discussed.

                Author and article information

                Nephron Exp Nephrol
                Cardiorenal Medicine
                S. Karger AG
                June 2001
                23 April 2001
                : 9
                : 3
                : 214-222
                aV. Medizinische Universitätsklinik, Klinikum Mannheim, University of Heidelberg; bFachhochschule Mannheim; cGemeinschaftspraxis für Pathologie, Heidelberg, Germany; Departments of dPediatrics and eNephrology, University Hospital, Nijmegen, The Netherlands and fMax Planck Institut für Hirnforschung, Frankfurt, Germany
                52614 Exp Nephrol 2001;9:214–222
                © 2001 S. Karger AG, Basel

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                Page count
                Figures: 5, Tables: 1, References: 44, Pages: 9
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                Original Paper

                Cardiovascular Medicine, Nephrology

                Agrin, Diabetic nephropathy, Heparan sulfate proteoglycans


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