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      Ontogenetic Development of the Filtration Barrier

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          Abstract

          The development of the filtration barrier is part of a complex sequence of steps proceeding from the early nephron anlage (renal vesicle) via the comma- and S-shaped body to the capillary loop stage and mature glomerulus. The main players are the podocytes (already in the stage of presumptive podocytes), which hold the commander function in this process, and the endothelial and the mesangial cells. A decisive role is also played by the GBM; its change in composition during the developmental process is a precondition for the final maturation of the podocytes, i.e. for the formation of the foot processes and, clearly subsequent, the slit membrane. Failure in the consecutive developmental stages due to genetic mutations or manipulations leads to characteristic hereditary diseases of increasing severity. The last step in this development, the formation of the slit membrane, marks a caesura between diseases with early and late onset; all disorders without a properly developed slit membrane start prenatally or at birth.

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          Most cited references 27

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          NPHS2, encoding the glomerular protein podocin, is mutated in autosomal recessive steroid-resistant nephrotic syndrome.

          Familial idiopathic nephrotic syndromes represent a heterogeneous group of kidney disorders, and include autosomal recessive steroid-resistant nephrotic syndrome, which is characterized by early childhood onset of proteinuria, rapid progression to end-stage renal disease and focal segmental glomerulosclerosis. A causative gene for this disease, NPHS2, was mapped to 1q25-31 and we report here its identification by positional cloning. NPHS2 is almost exclusively expressed in the podocytes of fetal and mature kidney glomeruli, and encodes a new integral membrane protein, podocin, belonging to the stomatin protein family. We found ten different NPHS2 mutations, comprising nonsense, frameshift and missense mutations, to segregate with the disease, demonstrating a crucial role for podocin in the function of the glomerular filtration barrier.
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            Mutations in ACTN4, encoding alpha-actinin-4, cause familial focal segmental glomerulosclerosis.

            Focal and segmental glomerulosclerosis (FSGS) is a common, non-specific renal lesion. Although it is often secondary to other disorders, including HIV infection, obesity, hypertension and diabetes, FSGS also appears as an isolated, idiopathic condition. FSGS is characterized by increased urinary protein excretion and decreasing kidney function. Often, renal insufficiency in affected patients progresses to end-stage renal failure, a highly morbid state requiring either dialysis therapy or kidney transplantation. Here we present evidence implicating mutations in the gene encoding alpha-actinin-4 (ACTN4; ref. 2), an actin-filament crosslinking protein, as the cause of disease in three families with an autosomal dominant form of FSGS. In vitro, mutant alpha-actinin-4 binds filamentous actin (F-actin) more strongly than does wild-type alpha-actinin-4. Regulation of the actin cytoskeleton of glomerular podocytes may be altered in this group of patients. Our results have implications for understanding the role of the cytoskeleton in the pathophysiology of kidney disease and may lead to a better understanding of the genetic basis of susceptibility to kidney damage.
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              WT-1 is required for early kidney development.

              In humans, germline mutations of the WT-1 tumor suppressor gene are associated with both Wilms' tumors and urogenital malformations. To develop a model system for the molecular analysis of urogenital development, we introduced a mutation into the murine WT-1 tumor suppressor gene by gene targeting in embryonic stem cells. The mutation resulted in embryonic lethality in homozygotes, and examination of mutant embryos revealed a failure of kidney and gonad development. Specifically, at day 11 of gestation, the cells of the metanephric blastema underwent apoptosis, the ureteric bud failed to grow out from the Wolffian duct, and the inductive events that lead to formation of the metanephric kidney did not occur. In addition, the mutation caused abnormal development of the mesothelium, heart, and lungs. Our results establish a crucial role for WT-1 in early urogenital development.
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                Author and article information

                Journal
                NEE
                Nephron Exp Nephrol
                10.1159/issn.1660-2129
                Cardiorenal Medicine
                S. Karger AG
                978-3-8055-8311-4
                978-3-318-01479-2
                1660-2129
                2007
                June 2007
                06 June 2007
                : 106
                : 2
                : e44-e50
                Affiliations
                Anatomy and Cell Biology, Medical Faculty Mannheim, University of Heidelberg, Heidelberg, Germany
                Article
                101792 Nephron Exp Nephrol 2007;106:e44–e50
                10.1159/000101792
                17570939
                © 2007 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 1, References: 48, Pages: 1
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