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      Call for Papers: Green Renal Replacement Therapy: Caring for the Environment

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      Rac1 Contributes to Actin Organization in Glomerular Podocytes

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          Abstract

          Background/Aims: The function of glomerular podocytes is closely associated with the actin cytoskeleton. In this study, we studied the role of the small Rho-GTPase, Rac1, in actin organization in podocytes. Methods: Conditionally immortalized mouse podocytes (MP) stably expressing nephrin or control plasmid were used. Results: In MP, Rac1 activity increased significantly at 1 week of differentiation. MP stably expressing nephrin showed Rac1 activity significantly higher and more sustained than vector-expressing control cells. Antibody-mediated cross-linking of nephrin also activated Rac1. Differentiated MP showed more distinct lamellipodia/cellular processes, as compared with undifferentiated cells, which was further augmented by nephrin expression. Transient transfection of constitutively active Rac1 markedly increased the number of lamellipodia/cellular processes in undifferentiated MP, while the Rac1 inhibitor caused actin cytoskeleton derangement in differentiating MP. In the rat model of puromycin aminonucleoside nephrosis, RhoA activity was increased at Day 7 (at the peak of proteinuria), while Rac1 activity increased significantly only at Day 14, when the recovery process had started. Conclusion: Rac1 is activated in differentiating MP and nephrin potentiates Rac1 activation. Rac1 likely contributes to lamellipodia formation in differentiating MP and may contribute to process formation in podocytes recovering from injuries.

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          Rearrangements of the cytoskeleton and cell contacts induce process formation during differentiation of conditionally immortalized mouse podocyte cell lines.

          P Mundel, J Reiser, A Zúñiga Mejía Borja … (1997)
          Mature podocytes are among the most complex differentiated cells and possess a highly branched array of foot processes that are essential to glomerular filtration in the kidney. Such differentiated podocytes are unable to replicate and culturing of primary podocytes results in rapid growth arrest. Therefore, conditionally immortalized mouse podocyte clones (MPC) were established, which are highly proliferative when cultured under permissive conditions. Nonpermissive conditions render the majority of MPC cells growth arrested within 6 days and induce many characteristics of differentiated podocytes. Both proliferating and differentiating MPC cells express the WT-1 protein and an ordered array of actin fibers and microtubules extends into the forming cellular processes during differentiation, reminiscent of podocyte processes in vivo. These cytoskeletal rearrangements and process formation are accompanied by the onset of synaptopodin synthesis, an actin-associated protein marking specifically differentiated podocytes. In addition, focal contacts are rearranged into an ordered pattern in differentiating MPC cells. Most importantly, electrophysiological studies demonstrate that differentiated MPC cells respond to the vasoactive peptide bradykinin by changes in intracellular calcium concentration. These results suggest a regulatory role of podocytes in glomerular filtration. Taken together, these studies establish that conditionally immortalized MPC cells retain a differentiation potential similar to podocytes in vivo. Therefore, the determinative steps of podocyte differentiation and process formation are studied for the first time using an inducible in vitro model.
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            Nck adaptor proteins link nephrin to the actin cytoskeleton of kidney podocytes.

            Nina Jones, Louise Larose, Ivan M Blasutig … (2006)
            The glomerular filtration barrier in the kidney is formed in part by a specialized intercellular junction known as the slit diaphragm, which connects adjacent actin-based foot processes of kidney epithelial cells (podocytes). Mutations affecting a number of slit diaphragm proteins, including nephrin (encoded by NPHS1), lead to renal disease owing to disruption of the filtration barrier and rearrangement of the actin cytoskeleton, although the molecular basis for this is unclear. Here we show that nephrin selectively binds the Src homology 2 (SH2)/SH3 domain-containing Nck adaptor proteins, which in turn control the podocyte cytoskeleton in vivo. The cytoplasmic tail of nephrin has multiple YDxV sites that form preferred binding motifs for the Nck SH2 domain once phosphorylated by Src-family kinases. We show that this Nck-nephrin interaction is required for nephrin-dependent actin reorganization. Selective deletion of Nck from podocytes of transgenic mice results in defects in the formation of foot processes and in congenital nephrotic syndrome. Together, these findings identify a physiological signalling pathway in which nephrin is linked through phosphotyrosine-based interactions to Nck adaptors, and thus to the underlying actin cytoskeleton in podocytes. Simple and widely expressed SH2/SH3 adaptor proteins can therefore direct the formation of a specialized cellular morphology in vivo.
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              Synaptopodin orchestrates actin organization and cell motility via regulation of RhoA signalling.

              Peter Mundel, Christian Faul, Etsuko Yanagida-Asanuma … (2006)
              The Rho family of small GTPases (RhoA, Rac1 and Cdc42) controls signal-transduction pathways that influence many aspects of cell behaviour, including cytoskeletal dynamics. At the leading edge, Rac1 and Cdc42 promote cell motility through the formation of lamellipodia and filopodia, respectively. On the contrary, RhoA promotes the formation of contractile actin-myosin-containing stress fibres in the cell body and at the rear. Here, we identify synaptopodin, an actin-associated protein, as a novel regulator of RhoA signalling and cell migration in kidney podocytes. We show that synaptopodin induces stress fibres by competitive blocking of Smurf1-mediated ubiquitination of RhoA, thereby preventing the targeting of RhoA for proteasomal degradation. Gene silencing of synaptopodin in kidney podocytes causes the loss of stress fibres and the formation of aberrant non-polarized filopodia and impairment of cell migration. Together, these data show that synaptopodin is essential for the integrity of the podocyte actin cytoskeleton and for the regulation of podocyte cell migration.
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                Author and article information

                Journal
                Journal ID (publisher-id): NEE
                Journal ID (nlm-ta): Nephron Exp Nephrol
                Journal ID (doi): 10.1159/issn.1660-2129
                Title: Cardiorenal Medicine
                Publisher: S. Karger AG
                ISSN (Electronic): 1660-2129
                Publication date Subscription-year: 2010
                Publication date (Print): February 2010
                Publication date (Electronic): 02 December 2009
                Volume: 114
                Issue: 3
                Pages: e93-e106
                Affiliations
                aDepartment of Medicine, McGill University Health Centre, Montreal, Que., Canada; bDepartment of Cell Biology, Institute of Nephrology, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
                Article
                Publisher ID: 262317 Other ID: Nephron Exp Nephrol 2010;114:e93–e106
                DOI: 10.1159/000262317
                PubMed ID: 19955829
                SO-VID: 041c090d-67cb-4177-b495-2aad811c217b
                Copyright © © 2009 S. Karger AG, Basel
                License:

                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.

                History
                Date received : 18 June 2009
                Date accepted : 27 August 2009
                Page count
                Figures: 8, References: 26, Pages: 1
                Categories
                Subject: Original Paper

                ScienceOpen disciplines: Cardiovascular Medicine,Nephrology
                Keywords: Rho-GTPases,Glomerular podocytes,Rac1
                Data availability:
                ScienceOpen disciplines: Cardiovascular Medicine, Nephrology
                Keywords: Rho-GTPases, Glomerular podocytes, Rac1

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