Effect of chlorpromazine on kinetics of injected monoclonal antibody in MoAb-induced glomerular injury

Role of podocyte slit diaphragm as a filtration barrier (Review Article)

Although the role of glomerular basement membrane has been emphasised as the barrier for retaining plasma proteins in the past three decades, some recent studies have demonstrated that the slit diaphragm of the glomerular epithelial cell (podocyte) is the structure likely to be the barrier in the glomerular capillary wall. Nephrin and podocin were identified as gene products mutated in Finnish-type congenital nephrotic syndrome and autosomal recessive steroid-resistant nephrotic syndrome, respectively. Nephrin s located at the outer leaflet of plasma membranes of the slit diaphragm. Podocin is reported to have an interaction with nephrin. The anti-nephrin antibody is capable of inducing massive proteinuria, which indicates that nephrin is a key functional molecule in the slit diaphragm. The expression of nephrin and podocin was reduced in glomeruli of minimal change nephrotic syndrome, which suggested that the altered expression of these molecules contributes to the development of proteinuria also in acquired diseases. Some recent studies demonstrated that CD2-associated protein (CD2AP) is also a functional molecule in the slit diaphragm, and its expression is altered in membranous nephropathy. These observations suggested that alteration of the molecular arrangement in the slit diaphragm is involved in the development of proteinuria in several kinds of glomerular diseases.

Transient myofibroblast differentiation of interstitial fibroblastic cells relevant to tubular dilatation in uranyl acetate-induced acute renal failure in rats

To investigate the mechanisms of myofibroblast differentiation of interstitial fibroblastic cells (FCs) in rats with uranyl acetate-induced acute renal failure (ARF), we examined the relationship between the expression of alpha-smooth muscle actin (alpha-SMA), myofibroblast phenotype and tubular dilatation as well as cell shape and adhesion of FCs. Peritubular alpha-SMA-positive myofibroblasts appeared after induction of ARF and extended along the damaged, dilated proximal tubules and then almost disappeared after proximal tubular recovery. The perimeter of proximal tubules correlated with fractional areas stained for alpha-SMA (P<0.001). Most alpha-SMA-positive cells did not incorporate [3H]-thymidine, indicating a low proliferative activity. Transmission electron microscopy showed that FCs increasingly attached to the tubular basement membrane by elongated cytoplasm-containing microfilament bundles, which formed abundant adherens and gap junctions from day 4 to day 7. Scanning electron microscopy showed hypertrophic FCs covering large areas of tubules after induction of ARF. Administration of chlorpromazine, which can inhibit cytoskeletal movement, after induction of ARF partially inhibited myofibroblast differentiation of FCs immunohistochemically and morphologically and resulted in more dilated proximal tubules in concert with aggravation of renal dysfunction and inhibition of regenerative repair at day 4 than vehicle-administered rats. Our results indicate that mechanical tension, judged by tubular dilatation, may contribute to the induction of alpha-SMA phenotype with increased stress fiber formation and intercellular junctions in FCs to support damaged nephron structures by adjusting tensional homeostasis in rats with uranyl acetate-induced ARF.

mAb 5-1-6 nephropathy and nephrin

It is well established that the glomerular capillary wall consists of three layers: endothelial cell, glomerular basement membrane, and the slit diaphragm bridging foot processes of glomerular epithelial cell. Which structure in the glomerular capillary wall represents the primary filter for retaining plasma proteins is not clearly elucidated. An anti-slit diaphragm monoclonal antibody (mAb) 5-1-6 causes massive proteinuria in rats by single intravenous injection, which clearly indicates that the slit diaphragm plays a critical role for maintaining the barrier function of the glomerular capillary wall. Recently, we concluded that mAb 5-1-6 recognized a rat homolog of nephrin, a gene product of NPHS1. The expression of nephrin decreased in puromycin aminonucleoside nephropathy and adriamycin nephropathy as well as mAb 5-1-6-induced nephropathy, which suggested that nephrin was involved in the development of proteinuria in these proteinuric states. In mAb 5-1-6 nephropathy, the slit diaphragm was maintained morphologically normal, although nephrin expression dramatically decreased. The finding suggested that nephrin was not a sole component of the slit diaphragm. To better understand the structure of the slit diaphragm, it is particularly important to identify other components that build up the structure of the slit diaphragm together with nephrin.

Cloning of rat nephrin: expression in developing glomeruli and in proteinuric states

BACKGROUND

Nephrin is identified as a product of the gene mutated in a patient with congenital nephrotic syndrome of the Finnish type. However, its precise localization and function are not yet fully clarified.

METHODS

To clone the rat homologue of nephrin, polymerase chain reaction (PCR) was employed. To elucidate the localization and expression of nephrin, immunohistological analysis with a specific antirat nephrin antibody, reverse transcription-PCR, and RNase protection assay were performed.

RESULTS

Amino acid sequences of rat and human nephrin are highly homologous (82.2% identity). The domain structure of nephrin is also highly conserved between rats and humans. The rat nephrin was detected only in kidney glomeruli along glomerular capillary walls, and its localization was always identical to that of the anti-slit diaphragm monoclonal antibody (mAb) 5-1-6-recognized antigen in normal matured and fetal rat glomeruli and in the glomeruli of proteinuric states. The nephrin staining pattern was clearly distinguished from that of zonula occludens-1 (ZO-1), alpha3-integrin, or podocalyxin. mRNA expression for nephrin was first detected in the fetal rat kidneys at 18.5 embryonic days. Nephrin mRNA expression decreased just after injection of mAb 5-1-6 (47.4%) or puromycin aminonucleoside (51.2%), and the staining pattern of nephrin shifted from a linear to a granular pattern in both proteinuric states.

CONCLUSIONS

Nephrin is localized in slit diaphragm in the matured glomeruli and is identical with mAb 5-1-6 antigen. Nephrin is involved in the development of proteinuria not only in mAb 5-1-6 nephropathy, but also in puromycin aminonucleoside nephropathy.

Structural continuity of filtration slit (slit diaphragm) to plasma membrane of podocyte

Murine monoclonal antibody 5-1-6 was reported to bind to the slit membrane and closely related structures in rat renal glomeruli; it induced heavy, reversible proteinuria and appeared to redistribute onto the plasma membrane of epithelial cells after binding at the original target sites. This phenomenon of antigenic movement has not been analyzed in detail to date. In addition to normal kidneys we also studied localization of the antigen recognized by monoclonal antibody 5-1-6 in protamine sulfate-perfused rat kidneys, in which slit diaphragms are known to be functionally modified. Isolated glomeruli as well as ultrathin kidney cryosections were labeled by the immunogold technique to clarify the relation between this antigen and the slit diaphragm. Sequential localization of injected monoclonal antibody was visualized using a post-embedding immunogold method in rats 2 hours to 12 days after injection of antibody. Ultrastructural immunogold labeling demonstrated that under normal conditions antigenic molecules were expressed mainly in the area beneath the slit diaphragms. Occasionally labeling was found at the base of the foot process, facing the glomerular basement membrane. After protamine sulfate treatment antigenic sites were dislocated due to the lifting and disruption of slit diaphragms, indicating that this antigen is associated with slit diaphragms. Injected antibody was localized at the filtration slits at 2 hours, and by 12 hours it had moved onto the apical plasma cell membrane of foot process. In addition, from 3 days onwards patch or cap-like formation on the plasma cell membrane of podocytes was seen. Possible shedding of antibody from podocyte cell surface membrane was occasionally encountered, but internalization of antibody was a minor event. Elution experiments in isolated glomeruli at day 3 indicated that antigen and antibody were both localized on the podocyte cell surface membrane, suggesting redistribution of immune complexes. In conclusion, filtration slits (slit diaphragms) and the apical membrane of foot process of podocytes demonstrate structural continuity, as revealed by the movement of the antigen recognized by monoclonal antibody 5-1-6 as antigen-antibody complexes.

Developmental expression of the nephritogenic antigen of monoclonal antibody 5-1-6

The biogenesis of p51, the target of nephritogenic monoclonal antibody 5-1-6, was studied in the developing glomerulus by immunolocalization and metabolic labeling. The localization of p51 was compared with that of ZO-1, a component of the cytoplasmic face of the epithelial slit diaphragm, and with that of podocalyxin, and apical marker of the podocyte. p51 first became faintly, but clearly, detectable on the basal and lateral sides of the developing podocytes at the S-shaped body stage. Staining intensity increased with further maturation and was restricted to the visceral epithelial cells. On immunoelectron microscopy, the antigen was seen along the basal and lateral surfaces below occluding junction at the early capillary loop stage and later, with the interdigitation of foot processes, became concentrated in the slit pores. At no stage was p51 seen on the apical surface. p51 and ZO-1 were closely localized in the mature glomerulus but arrived at their final positions from opposite directions. p51 was on basal and podocalyxin was on apical sides of the glomerular epithelium from the S-shaped body stage onwards. Metabolic labeling studies showed that p51 is actively synthesized during initial glomerular development and that the rate of synthesis declines substantially with maturation. We conclude that p51 is primarily synthesized during the initial glomerular development, becomes concentrated in the slit pores of mature podocytes, and serves as a basal differentiation marker for podocytes.

Strain variation in susceptibility to the development of monoclonal antibody 5-1-6-induced proteinuria in rats

Susceptibility to the development of MoAb 5-1-6-induced proteinuria was investigated in four different rat strains, i.e. Brown-Norway (BN), Lewis (LEW), Sprague-Dawley (SD) and Wistar. An intravenous injection of 5 mg of MoAb 5-1-6 to female 7-week-old rats of a given strain induced massive proteinuria in BN, LEW and Wistar rats. However, SD rats developed almost no proteinuria. A similar tendency was observed in the second experiment, in which the injected dose of MoAb was adjusted according to the body weight of each rat (3 mg/100 g body weight). Immunofluorescence (IF) and immunoelectron microscopy (IEM) revealed no differences between the binding patterns of the MoAbs to normal rat kidneys derived from each strain. Quantitative study using 125I-labelled MoAb showed that there was no significant difference in the amount of antibody bound to the kidney 1 h and 5 days after injection between two rat strains, LEW and SD. Localization of 5-1-6 in vivo and its kinetics were investigated. In IF a linear-like pattern along capillary walls was observed 2 h after injection in both LEW and SD strains. This linear-like pattern was shifted to a granular pattern in proteinuric LEW rats 6 days after injection, whereas it remained linear-like in non-proteinuric SD rats. IEM confirmed this difference in the localization of injected MoAb 6 days after injection to LEW and SD rats also at the ultrastructural level. We conclude that there is a clear-cut strain difference in the development of proteinuria induced by MoAb 5-1-6. SD rats were less susceptible to MoAb-induced glomerular injury than BN, LEW and Wistar rats. Although the exact reason for strain variation in susceptibility to MoAb-induced proteinuria remains to be clarified, the movement of bound MoAb, presumably together with corresponding antigenic molecule along the glomerular epithelial cell surface followed by endocytosis into the epithelial cell, seems to be closely related to the induction of proteinuria.

Divalency of the monoclonal antibody 5-1-6 is required for induction of proteinuria in rats

A single i.v. injection of 3 mg of the F(ab')2 fragment of MoAb 5-1-6 into rats induced immediate proteinuria (128.1 +/- 80.7 mg/24 h on day 1) which lasted 1-2 days. In contrast, rats administered 10 mg of the corresponding Fab fragment did not develop abnormal proteinuria even though an equivalent dose of the intact MoAb 5-1-6 far exceeded the nephritogenic dose. The total kidney binding of 125I-Fab fragment was 209.5 +/- 34.3 micrograms/2 kidneys. This exceeded that obtained by injection of 3 mg MoAb 5-1-6 IgG1 (58.9 +/- 12.5 micrograms/2 kidneys at 1 h) and was similar to that obtained following injection of 3 mg F(ab')2 fragment (235.3 +/- 16.9 micrograms/2 kidneys). Immunofluorescence (IF) showed a linear pattern along the glomerular capillary wall at 1 h after the administration of MoAb 5-1-6 IgG1, F(ab')2 or Fab fragment. On day 5, fine to coarse granules were observed scattered in F(ab')2-injected rat glomeruli, whereas granules were densely localized in Fab-injected rat glomeruli. Complement-depleted rats injected with 3 mg of MoAb 5-1-6 IgG1 developed proteinuria with the same time course as non-depleted rats. This observation, together with the ability of F(ab')2 to induce proteinuria, indicates that proteinuria induced by MoAb 5-1-6 is complement-independent. This study suggests that MoAb 5-1-6-induced proteinuria is initiated by cross-linking of the epitopes by divalent MoAb 5-1-6 and is independent of complement activity.