title: MAGI-1 Interacts with Nephrin to Maintain Slit Diaphragm Structure through Enhanced Rap1 Activation in Podocytes.
abstract: MAGI-1 is a multidomain cytosolic scaffolding protein that in the kidney is specifically located at the podocyte slit diaphragm, a specialized junction that is universally injured in proteinuric diseases. There it interacts with several essential molecules, including nephrin and neph1, which are required for slit diaphragm formation and as an intracellular signaling hub. Here, we show that diminished MAGI-1 expression in cultured podocytes reduced nephrin and neph1 membrane localization and weakened tight junction integrity. Global magi1 knock-out mice, however, demonstrated normal glomerular histology and function into adulthood. We hypothesized that a second mild but complementary genetic insult might induce glomerular disease susceptibility in these mice. To identify such a gene, we utilized the developing fly eye to test for functional complementation between MAGI and its binding partners. In this way, we identified diminished expression of fly Hibris (nephrin) or Roughest (neph1) as dramatically exacerbating the effects of MAGI depletion. Indeed, when these combinations were studied in mice, the addition of nephrin, but not neph1, heterozygosity to homozygous deletion of MAGI-1 resulted in spontaneous glomerulosclerosis. In cultured podocytes, MAGI-1 depletion reduced intercellular contact-induced Rap1 activation, a pathway critical for proper podocyte function. Similarly, magi1 knock-out mice showed diminished glomerular Rap1 activation, an effect dramatically enhanced by concomitant nephrin haploinsufficiency. Finally, combined overexpression of MAGI-1 and nephrin increased Rap1 activation, but not when substituting a mutant MAGI-1 that cannot bind nephrin. We conclude that the interaction between nephrin and MAGI-1 regulates Rap1 activation in podocytes to maintain long term slit diaphragm structure.
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029,; the Division of Nephrology, First Affiliated Hospital of Harbin Medical University, Harbin, China 150001.
authors.forename: Jie
authors.initials: J
authors.lastname: Ni
authors.affiliation: the Saint James School of Medicine, Saint Vincent and the Grenadines.
authors.forename: Sujin
authors.initials: S
authors.lastname: Bao
authors.affiliation: the Biology Department, Wesleyan University, Middletown, Connecticut, 06459.
authors.forename: Ruth I
authors.initials: RI
authors.lastname: Johnson
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029,; the Department of Nephrology, Laboratory of Renal Disease, Putuo Hospital, Shanghai University of Traditional Chinese Medicine, Shanghai, China 200062.
authors.forename: Bingbing
authors.initials: B
authors.lastname: Zhu
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029.
authors.forename: Jianhua
authors.initials: J
authors.lastname: Li
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029.
authors.forename: Justin
authors.initials: J
authors.lastname: Vadaparampil
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Smith
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029.
authors.forename: Kirk N
authors.initials: KN
authors.lastname: Campbell
authors.affiliation: the Department of Medicine IV, Medical Center-University of Freiburg, Faculty of Medicine, University of Freiburg, 79106 Freiburg, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Grahammer
authors.affiliation: the Department of Medicine IV, Medical Center-University of Freiburg, Faculty of Medicine, University of Freiburg, 79106 Freiburg, Germany,; the BIOSS Center for Biological Signaling Studies, Albert-Ludwigs-University Freiburg, 79104 Freiburg, Germany,; FRIAS, Freiburg Institute for Advanced Studies and Center for Systems Biology (ZBSA), Albert-Ludwigs-University, 79104 Freiburg, Germany.
authors.forename: Tobias B
authors.initials: TB
authors.lastname: Huber
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029.
authors.forename: John C
authors.initials: JC
authors.lastname: He
authors.affiliation: the Department of Pathology, Columbia University Medical Center, New York, New York 10032, and.
authors.forename: Vivette D
authors.initials: VD
authors.lastname: D'Agati
authors.affiliation: the School of Biomedical Sciences, Chinese University of Hong Kong, Hong Kong, China.
authors.forename: Andrew
authors.initials: A
authors.lastname: Chan
authors.affiliation: From the Division of Nephrology, Icahn School of Medicine at Mount Sinai, New York, New York 10029,. Electronic address: lewis.kaufman@mssm.edu.
authors.forename: Lewis
authors.initials: L
authors.lastname: Kaufman
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D015815:Cell Adhesion Molecules; D004789:Enzyme Activation; D005923:Glomerulosclerosis, Focal Segmental / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D051528:Guanylate Kinases; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D050199:Podocytes / Q000378:metabolism*; D020827:rap1 GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D015815:Cell Adhesion Molecules; C486606:Kirrel1 protein, mouse; D008565:Membrane Proteins; D051528:Guanylate Kinases; C493280:Magi1 protein, mouse; D020827:rap1 GTP-Binding Proteins
keywords: Ras-related protein 1 (Rap1); mouse genetics; nephrology; podocyte; proteinuria