Danger Signaling by Glomerular Podocytes Defines a Novel Function of Inducible B7-1 in the Pathogenesis of Nephrotic Syndrome
Jochen Reiser* and
Peter Mundel
*Renal Unit, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts; and Departments of Medicine and Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York
Correspondence to Dr. Peter Mundel, Division of Nephrology, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461. Phone: 718-430-3219; Fax: 718-430-8963; E-mail: mundel{at}aecom.yu.edu
Podocyte foot processes (FP) and the interposed slit diaphragms(SD) form the final barrier to protein loss, explaining whypodocyte injury is typically associated with marked proteinuria(1). The highly dynamic FP actin cytoskeleton is linked to theSD and proteins that regulate podocyte actin dynamics thereforeare of critical importance for structural maintenance and sustainedfunction of the glomerular filter. We recently made the novelfinding that under pathologic conditions, with FP effacementand proteinuria, podocytes upregulate B7-1 (2). B7-1 (also termedCD80) is a transmembrane protein expressed on the surface ofB cells and other antigen-presenting cells (APC). On B cellsand other APC, B7-1 provides a co-stimulatory signal for T cellsthrough binding to its receptors CD28 and CTLA-4 (3–5).The immune function of B7-1 has been well described (3–5).To explore the function of B7-1 in podocytes, we developed anovel, unique murine model of LPS-induced, B7-1-dependent transientnephrotic syndrome that shares several key features of humanminimal-change disease (MCD) (2). On the basis of our observations,we propose that LPS induces transient B7-1-dependent nephroticsyndrome through the reorganization of the podocyte FP actincytoskeleton and disruption of the SD (2). Our findings suggesta novel function for B7-1 in danger signaling by podocytes (2).
Podocytes Are Important for Glomerular Homeostasis
Renal podocytes are highly differentiated cells with a complexcellular morphology. They are located inside the kidney glomerulus,a twisted corpuscle of capillaries through which the blood isfiltered hydrostatically through a high-volume/high-discriminationfilter (1,6). Podocyte FP are anchored to the glomerular basementmembrane (GBM) via31 integrin (7) and -/-dystroglycans (8,9).Neighboring FP are connected by a specialized cell-cell junction,the glomerular SD, which represents the main size selectivefilter barrier in the kidney (6,10,11). The SD is thought tobe a modified adherens junction (12) and is composed of a growingnumber of proteins, including P-cadherin (12), nephrin (13–15),FAT (16,17), podocin (18), and Neph1 (19–21). Podocytesare injured in many forms of human and experimental glomerulardisease, including MCD, focal segmental glomerulosclerosis,membranous glomerulopathy, diabetes, and lupus nephritis (6,22).Independent of the underlying disease, the early events arecharacterized by molecular alterations of the SD without visiblemorphologic changes or, more obvious, by a reorganization ofthe FP structure with apical displacement of the SD (6,23,24).From a clinical standpoint, it is important to recognize thatearly structural changes in podocyte morphology, such as FPeffacement, have to be reversed within a certain period of timeto prevent development of severe and progressive glomerulardamage (23,25).
The role of B7-1 in T cell co-stimulation is well established(3–5). Much less is known about B7-1-mediated signalingin B cells and other APC. Two regions in the cytoplasmic tailof B7-1 are thought to be important for B7-1 membrane distributionand T cell co-stimulation in APC (26,27). Another report showedB7-1 mediated outside-in signaling in B cells through tyrosinekinases (28). Cross-linking of B7-1 on B lymphoma Raji cellsinduced tyrosine phosphorylation of 160-, 120-, 55-, 46-, and44-kD proteins, which was inhibited by genistein, a tyrosinekinase inhibitor. B7-1-mediated signaling blocked DNA synthesisand induced cell spreading in a fibroblast-like manner, whichwas blocked by genistein (28). These results suggest that B7-1is involved in transmembrane outside-in signaling in B cells,and its biologic effects seem to be mediated by tyrosine kinases(28). Further support for a role of B7-1 in outside-in signalingcomes from a recent report showing that CTLA-4-Ig-mediated ligationof B7-1 regulates tryptophan catabolism in dendritic cells (29).
Novel Function for B7-1 in Danger Signaling by Podocytes
Podocytes of 3–/– mice are unable to maintain normalpodocyte structure, including the elaboration of mature FP alongthe GBM (30). To identify genes that are critically involvedin the development of FP effacement and proteinuria, we performeda genomic screen comparing wild-type and 3–/– podocytes.With this approach, we uncovered an unanticipated novel rolefor B7-1 in podocytes as inducible modifier of glomerular permselectivity(2). B7-1 in podocytes was found in genetic, drug-induced, immune-mediated,and bacterial toxin-induced experimental kidney diseases withnephrotic syndrome (2). The clinical significance of our resultsis underscored by the observation that podocyte expression ofB7-1 correlated with the severity of human lupus nephritis.LPS signaling through TLR-4 reorganized the podocyte actin cytoskeletonin vitro, and activation of B7-1 in cultured podocytes led toreorganization of vital slit diaphragm proteins (2). In vivo,LPS rapidly upregulated B7-1 in podocytes of wild-type and SCIDmice, leading to nephrotic-range proteinuria. Mice lacking B7-1were protected from LPS-induced nephrotic syndrome. Taken together,these data established a causal link between podocyte B7-1 expressionand urinary protein loss that is independent of lymphocyte infiltrationor activation (2). It has not escaped our attention that LPS-inducednephrotic syndrome shares several key features of human MCDin that the FP effacement and proteinuria are transient andpresent without signs of glomerular inflammation.
Separating the Good and the Bad in Nephrotic Syndrome
The upregulation of B7-1 in podocytes suits well to providea unifying molecular explanation for the pathogenesis of proteinuriain nephrotic syndrome. However, it is difficult to believe thatthe sole (and harmful) function of podocyte B7-1 should be todisassemble the glomerular filter. Hence, it is intriguing tospeculate that transient B7-1-dependent proteinuria may be aphysiologic response that is desirable under certain conditions,e.g., in patients with Gram-negative infection. The disruptionof the glomerular filter by TLR-induced B7-1 may help the innateimmune system in clearing the circulation from harmful agentsby dumping them into the urine. In this scenario, the transienturinary protein loss would be the price that we pay for therapid decay of the harmful agent. Consistent with this idea,the development of transient proteinuria has been found duringthe course of gram-negative sepsis (31). Moreover, a prospectivestudy showed that in postoperative septic patients, microalbuminuriais an early indicator of increased glomerular permeability butnot in nonseptic patients (32). Clearly, this is highly speculative,and future studies will be required to explore this hypothesisin detail. In MCD, the onset of nephrotic syndrome is oftenpreceded by an infection or allergic reaction (33). Some childrenpresent with fever and bacteremia. In a number of these patients,the peritoneal cavity is the site of the infection, and Streptococcuspneumoniae, Staphylococcus aureus, and Escherichia coli arecommonly isolated (33). A urinary tract infection is occasionallypresent, and a history of upper respiratory tract infectionimmediately preceding the first clinical signs of nephroticsyndrome may exist and stimulate the activation of podocytedanger signaling pathways, thereby leading to B7-1-induced nephroticsyndrome. This raises the intriguing possibility that prolongednephrotic syndrome in MCD may result from the persistence ofa normally beneficial response caused by genetic defects inthe B7-1 pathway or by the persistence of other unphysiologicactivators of podocyte B7-1. The sustained B7-1 activation inpodocytes, in turn, may result in the continuation or recurrenceof nephrotic syndrome in this setting.
The activation of the innate immune system by LPS (2) (and potentiallyother pathogen-associated particles) leads to the upregulationof podocyte B7-1. In light of these results, it is temptingto speculate that podocyte B7-1 may not only act to reorganizethe slit diaphragm but also participate in the activation ofcirculating T cells. Under normal conditions, T cells are unableto enter the glomerulus, but in advanced stages of nephroticsyndrome, it may be facilitated for T cells to enter Bowman’sspace and directly interact with podocytes. A podocyte-T cellinteraction may result in stimulation and activation of T cells.In inflammatory glomerulonephritis, T cells can be found inthe glomerulus in Bowman’s space, and B7-1 as well asB7-2 regulate crescentic glomerulonephritis (34). Therefore,podocyte B7-1 may also modulate T cell-mediated cytotoxicityin immune and nonimmune kidney diseases with proteinuria. Theexpression of B7-1 by podocytes in glomerulonephritic situationsmay serve to recruit T cells to sites of GBM damage and breakdownand promote further inflammation.
It has been known for a long time that T cell dysfunction isthought to occur in MCD, and T cell activation was proposedto play a pathogenic role in this disease (35,36). In the contextof our results that LPS-induced nephrotic syndrome can occurin SCID mice (2), the role of T cells in MCD may be reinterpretedin the following way. Rather than causing disease, the activationof T cells may reflect the activation of the immune system,including the upregulation of podocyte B7-1 in response to anyof the above discussed stress (e.g., infection, allergens).Clearly, this concept is highly speculative and converse tothe classical idea that T cell activation precedes glomerulardysfunction, and future studies will be necessary to confirmor refute this hypothesis.
Genomic profiling identified B7-1 expression in podocytes underconditions of FP effacement and proteinuria (2). The biologicwork-up of podocyte B7-1 expression unraveled a novel functionof B7-1 as an inducible modifier of glomerular permselectivity(2). The induction of B7-1 in podocytes by LPS through TLR-4signaling suggests that the podocyte is a novel component ofthe innate immune system that is equipped with a danger signalingmachinery (37). In summary, upregulation of B7-1 in podocytesmay contribute to the pathogenesis of proteinuria by disruptingthe glomerular filter and provides a novel molecular targetto tackle proteinuric kidney diseases (2).
Acknowledgments
This study was supported by the Lupus Research Institute, NationalInstitutes of Health Grants DK57683 and DK062472, and the GeorgeM. O’Brien Kidney Center (DK064236).
This review is based on the Young Investigator Address givenby P.M. at the 2003 annual meeting of the American Society ofNephrology.
Mundel P, Shankland SJ: Podocyte biology and response to injury. J Am Soc Nephrol 13: 3005–3015, 2002[Free Full Text]
Reiser J, von Gersdorff G, Loos M, Oh J, Asanuma K, Giardino L, Rastaldi MP, Calvaresi N, Watanabe H, Schwarz K, Faul C, Kretzler M, Davidson A, Sugimoto H, Kalluri R, Sharpe AH, Kreidberg JA, Mundel P: Induction of B7-1 in podocytes is associated with nephrotic syndrome. J Clin Invest 113: 1390–1397, 2004[CrossRef][Medline]
Henry J, Miller MM, Pontarotti P: Structure and evolution of the extended B7 family. Immunol Today 20: 285–288, 1999[CrossRef][Medline]
Chambers CA, Allison JP: Costimulatory regulation of T cell function. Curr Opin Cell Biol 11: 203–210, 1999[CrossRef][Medline]
Abbas AK, Sharpe AH: T-cell stimulation: An abundance of B7s. Nat Med 5: 1345–1346, 1999[CrossRef][Medline]
Somlo S, Mundel P: Getting a foothold in nephrotic syndrome. Nat Genet 24: 333–335, 2000[CrossRef][Medline]
Adler S: Characterization of glomerular epithelial cell matrix receptors. Am J Pathol 141: 571–578, 1992[Abstract]
Regele HM, Fillipovic E, Langer B, Poczewki H, Kraxberger I, Bittner RE, Kerjaschki D: Glomerular expression of dystroglycans is reduced in minimal change nephrosis but not in focal segmental glomerulosclerosis. J Am Soc Nephrol 11: 403–412, 2000[Abstract/Free Full Text]
Raats CJ, van Den Born J, Bakker MA, Oppers-Walgreen B, Pisa BJ, Dijkman HB, Assmann KJ, Berden JH: Expression of agrin, dystroglycan, and utrophin in normal renal tissue and in experimental glomerulopathies. Am J Pathol 156: 1749–1765, 2000[Abstract/Free Full Text]
Reiser J, Kriz W, Kretzler M, Mundel P: The glomerular slit diaphragm is a modified adherens junction. J Am Soc Nephrol 11: 1–8, 2000[Abstract/Free Full Text]
Ruotsalainen V, Ljungberg P, Wartiovaara J, Lenkkeri U, Kestil M, Jalanko H, Holmberg C, Tryggvason K: Nephrin is specifically located at the slit diaphragm of glomerular podocytes. Proc Natl Acad Sci U S A 96: 7962–7967, 1999[Abstract/Free Full Text]
Holthofer H, Ahola H, Solin ML, Wang S, Palmen T, Luimula P, Miettinen A, Kerjaschki D: Nephrin localizes at the podocyte filtration slit area and is characteristically spliced in the human kidney. Am J Pathol 155: 1681–1687, 1999[Abstract/Free Full Text]
Holzman LB, John PL, Kovari IA, Verma R, Holthofer H, Abrahamson DR: Nephrin localizes to the slit pore of the glomerular epithelial cell. Kidney Int 56: 1481–1491, 1999[CrossRef][Medline]
Inoue T, Yaoita E, Kurihara H, Shimizu F, Sakai T, Kobayashi T, Ohshiro K, Kawachi H, Okada H, Suzuki H, Kihara I, Yamamoto T: FAT is a component of glomerular slit diaphragms. Kidney Int 59: 1003–1012, 2001[CrossRef][Medline]
Ciani L, Patel A, Allen ND, Ffrench-Constant C: Mice lacking the giant protocadherin mFAT1 exhibit renal slit junction abnormalities and a partially penetrant cyclopia and anophthalmia phenotype. Mol Cell Biol 23: 3575–3582, 2003[Abstract/Free Full Text]
Schwarz K, Simons M, Reiser J, Saleem MA, Faul C, Kriz W, Shaw AS, Holzman LB, Mundel P: Podocin, a raft-associated component of the glomerular slit diaphragm, interacts with CD2AP and nephrin. J Clin Invest 108: 1621–1629, 2001[CrossRef][Medline]
Donoviel DB, Freed DD, Vogel H, Potter DG, Hawkins E, Barrish JP, Mathur BN, Turner CA, Geske R, Montgomery CA, Starbuck M, Brandt M, Gupta A, Ramirez-Solis R, Zambrowicz BP, Powell DR: Proteinuria and perinatal lethality in mice lacking neph1, a novel protein with homology to nephrin. Mol Cell Biol 21: 4829–4836, 2001[Abstract/Free Full Text]
Barletta GM, Kovari IA, Verma RK, Kerjaschki D, Holzman LB: Nephrin and Neph1 co-localize at the podocyte foot process intercellular junction and form cis hetero-oligomers. J Biol Chem 278: 19266–19271, 2003[Abstract/Free Full Text]
Gerke P, Huber TB, Sellin L, Benzing T, Walz G: Homodimerization and heterodimerization of the glomerular podocyte proteins nephrin and NEPH1. J Am Soc Nephrol 14: 918–926, 2003[Abstract/Free Full Text]
Kerjaschki D: Caught flat-footed: Podocyte damage and the molecular bases of focal glomerulosclerosis. J Clin Invest 108: 1583–1587, 2001[CrossRef][Medline]
Smoyer WE, Mundel P: Regulation of podocyte structure during the development of nephrotic syndrome. J Mol Med 76: 172–183, 1998[CrossRef][Medline]
Reiser J, Von Gersdorff G, Simons M, Schwarz K, Faul C, Giardino L, Heider T, Loos M, Mundel P: Novel concepts in understanding and management of glomerular proteinuria. Nephrol Dial Transplant 17: 951–955, 2002[Free Full Text]
Mundel P, Shankland SJ: Glomerular podocytes and adhesive interaction with glomerular basement membrane. Exp Nephrol 7: 160–166, 1999[CrossRef][Medline]
Doty RT, Clark EA: Subcellular localization of CD80 receptors is dependent on an intact cytoplasmic tail and is required for CD28-dependent T cell costimulation. J Immunol 157: 3270–3279, 1996[Abstract]
Doty RT, Clark EA: Two regions in the CD80 cytoplasmic tail regulate CD80 redistribution and T cell costimulation. J Immunol 161: 2700–2707, 1998[Abstract/Free Full Text]
Hirokawa M, Kuroki J, Kitabayashi A, Miura AB: Transmembrane signaling through CD80 (B7-1) induces growth arrest and cell spreading of human B lymphocytes accompanied by protein tyrosine phosphorylation. Immunol Lett 50: 95–98, 1996[CrossRef][Medline]
Kreidberg JA, Donovan MJ, Goldstein SL, Rennke H, Shepherd K, Jones RC, Jaenisch R: Alpha 3 beta 1 integrin has a crucial role in kidney and lung organogenesis. Development 122: 3537–3547, 1996[Abstract]
Pacquement H, Sinnassamy P, Quintana E, Thomas C, Bensman A, Zucker JM: [Nephrotic syndrome and B leukemia]. Arch Fr Pediatr 46: 741–742, 1989[Medline]
De Gaudio AR, Adembri C, Grechi S, Novelli GP: Microalbuminuria as an early index of impairment of glomerular permeability in postoperative septic patients. Intensive Care Med 26: 1364–1368, 2000[CrossRef][Medline]
Eddy AA, Schnaper HW: The nephrotic syndrome: From the simple to the complex. Semin Nephrol 18: 304–316, 1998[Medline]
Kinoshita K, Tesch G, Schwarting A, Maron R, Sharpe AH, Kelley VR: Costimulation by B7-1 and B7-2 is required for autoimmune disease in MRL-faslpr mice. J Immunol 164: 6046–6056, 2000[Abstract/Free Full Text]
Sewell RF, Short CD: Minimal-change nephropathy: How does the immune system affect the glomerulus? Nephrol Dial Transplant 8: 108–112, 1993[Free Full Text]
Matzinger P: An innate sense of danger. Semin Immunol 10: 399–415, 1998[CrossRef][Medline]
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Introduction
Podocytes Are Important for...
3
1 integrin (7) and 
