Modulation of cytoskeletal organization of podocytes during the course of aminonucleoside nephrosis in rats

Phosphorylation of podocalyxin (Ser415) Prevents RhoA and ezrin activation and disrupts its interaction with the actin cytoskeleton

Podocalyxin (PC) is a polysialylated, anti-adhesin that is essential for maintaining foot process architecture and the integrity of the glomerular filtration barrier. We showed previously that PC is firmly attached to the actin cytoskeleton through ezrin, that in puromycin aminonucleoside (PAN)-mediated nephrosis the PC-ezrin-actin complex is disrupted, and that PC is uncoupled from actin. However, the precise mechanisms involved remained unknown. Here we show that detachment of PC from actin is regulated by phosphorylation of PC. PC is hyperphosphorylated at serines in PAN- and protamine sulfate (PS)-treated rat glomeruli. We determined that PC is a substrate of PKC and that the site of phosphorylation is Ser415, located within the juxtamembrane, ezrin-binding domain of the cytoplasmic tail of PC. Mutation of Ser415 to the phosphomimetic residues Glu (S415E) or Asp (S415D) interfered with direct binding of the PC cytoplasmic tail to ezrin in vitro. Moreover, stable expression of a phosphomimetic (S415E) PC mutant but not the WT or the phosphorylation-deficient (S415A) PC mutant, disrupted PC-ezrin-actin interaction, failed to activate RhoA, and the cytoskeletal linker, ezrin, remained inactive. Our data indicate that phosphorylation of PC at Ser415 prevents attachment of PC and ezrin to actin and highlights the strategic position of Ser415 and direct binding of PC to ezrin in regulating podocyte foot process architecture.

Increased expression of olfactomedin-1 and myocilin in podocytes during puromycin aminonucleoside nephrosis

BACKGROUND

The olfactomedin domain proteins Olfm-1 and myocilin are expressed in podocytes. Myocilin stimulates the formation of focal contacts and actin stress fibres in podocytes and other cell types, effects that are mediated through the Wnt signalling pathway. Here, we tested if the expression of both proteins is modified during puromycin aminonucleoside (PAN) nephrosis, which leads to structural changes in the actin cytoskeleton of podocytes.

METHODS

Rats were treated with PAN, and the effectiveness of treatment was analysed by electron microscopy of podocytes and protein detection in the urine. The expression of Olfm-1 and myocilin was studied by immunohistochemistry, western blot analysis of glomerular proteins and real-time RT-PCR of glomerular proteins. In parallel experiments, the expression of Olfm-1 was studied in cultured podocytes treated with dexamethasone, TGF-β, TNF-α and PAN.

RESULTS

Between Days 5 and 22 after treatment, the amounts of the BMZ and BMY splice variants of Olfm-1 and their mRNA were markedly elevated in proteins and mRNA from isolated glomeruli. Immunohistochemistry showed that the expression of Olfm-1 was confined to podocytes. Essentially, comparable results were obtained for myocilin. The BMZ variant of Olfm-1 appeared to be secreted from podocytes and was found in high amounts in urine of treated animals. Treatment of cultured podocytes with dexamethasone and PAN caused an increase in Olfm-1 expression, while treatment with recombinant Olfm-1 increased the formation of actin stress fibres.

CONCLUSIONS

Olfm-1 and myocilin are markedly induced in podocytes during PAN nephrosis and appear to be involved in the processes that govern the reorganization of the actin cytoskeleton during podocyte repair.

Inducible rodent models of acquired podocyte diseases

Glomerular diseases remain the leading cause of chronic and end-stage kidney disease. Significant advances in our understanding of human glomerular diseases have been enabled by the development and better characterization of animal models. Diseases of the glomerular epithelial cells (podocytes) account for the majority of proteinuric diseases. Rodents have been extensively used experimentally to better define mechanisms of disease induction and progression, as well as to identify potential targets and therapies. The development of podocyte-specific genetically modified mice has energized the research field to better understand which animal models are appropriate to study acquired podocyte diseases. In this review we discuss inducible experimental models of acquired nondiabetic podocyte diseases in rodents, namely, passive Heymann nephritis, puromycin aminonucleoside nephrosis, adriamycin nephrosis, liopolysaccharide, crescentic glomerulonephritis, and protein overload nephropathy models. Details are given on the model backgrounds, how to induce each model, the interpretations of the data, and the benefits and shortcomings of each. Genetic rodent models of podocyte injury are excluded.

Podocyte protection by darbepoetin: preservation of the cytoskeleton and nephrin expression

Podocyte injury is a significant contributor to proteinuria and glomerulosclerosis. Recent studies have shown a renoprotective effect of erythropoietin (EPO) during ischemic kidney disease. In this study, we examine mechanisms by which a long acting recombinant EPO analog, darbepoetin, may confer renoprotection in the puromycin aminonucleoside-induced model of nephrotic syndrome. Darbepoetin decreased the proteinuria of rats treated with puromycin. This protective effect was correlated with the immunohistochemical disappearance of the podocyte injury markers desmin and the immune costimulator molecule B7.1 with the reappearance of nephrin expression in the slit diaphragm. Podocyte foot process retraction and effacement along with actin filament rearrangement, determined by electron microscopy, were all reversed by darbepoetin treatment. The protective effects were confirmed in puromycin-induced nephrotic rats that had been hemodiluted to normal hematocrit levels. Furthermore, puromycin treatment of rat podocytes in culture caused actin cytoskeletal reorganization along with deranged nephrin distribution. All these effects in vitro were reversed by darbepoetin. Our study demonstrates that darbepoetin treatment ameliorates podocyte injury and decreases proteinuria by a direct effect on podocytes. This may be accomplished by maintenance of the actin cytoskeleton and nephrin expression.

Nestin expression in adult and developing human kidney

Nestin is considered a marker of neurogenic and myogenic precursor cells. Its arrangement is regulated by cyclin-dependent kinase 5 (CDK5), which is expressed in murine podocytes. We investigated nestin expression in human adult and fetal kidney as well as CDK5 presence in adult human podocytes. Confocal microscopy demonstrated that adult glomeruli display nestin immunoreactivity in vimentin-expressing cells with the podocyte morphology and not in cells bearing the endothelial marker CD31. Glomerular nestin-positive cells were CDK5 immunoreactive as well. Western blotting of the intermediate filament-enriched cytoskeletal fraction and coimmunoprecipitation of nestin with anti-CDK5 antibodies confirmed these results. Nestin was also detected in developing glomeruli within immature podocytes and a few other cells. Confocal microscopy of experiments conducted with antibodies against nestin and endothelial markers demonstrated that endothelial cells belonging to capillaries invading the lower cleft of S-shaped bodies and the immature glomeruli were nestin immunoreactive. Similar experiments carried out with antibodies raised against nestin and alpha-smooth muscle actin showed that the first mesangial cells that populate the developing glomeruli expressed nestin. In conclusion, nestin is expressed in the human kidney from the first steps of glomerulogenesis within podocytes, mesangial, and endothelial cells. This expression, restricted to podocytes in mature glomeruli, appears associated with CDK5.

Glucocorticoids protect and enhance recovery of cultured murine podocytes via actin filament stabilization

BACKGROUND

Nephrotic syndrome is a common kidney disease in both children and adults that is characterized by dramatic structural changes in the actin-rich foot processes of glomerular podocytes. Although glucocorticoids are the primary treatment for nephrotic syndrome, neither the target cell nor mechanism of action of glucocorticoids in nephrotic syndrome is known. For the last 30 years glucocorticoids have been presumed to act by reducing the release of soluble mediators of disease by circulating lymphocytes. In contrast, we hypothesized that glucocorticoids exert their beneficial effects in nephrotic syndrome by direct action on podocytes.

METHODS

Cultured murine podocytes were treated with glucocorticoids in the presence and absence of mifepristone (to inhibit glucocorticoid-induced transcriptional activation) and challenged using our previously reported in vitro model of puromycin aminonucleoside (PAN)-induced podocyte injury, as well as by direct disruption of actin filaments with latrunculin and cytochalasin. Cell viability, actin filament distribution, total polymerized actin content, and actin-regulating guanine triphosphatase (GTPase) activities were measured.

RESULTS

We demonstrated that treatment of cultured murine podocytes with the glucocorticoid dexamethasone both protected and enhanced recovery from PAN-induced injury. Dexamethasone also increased total cellular polymerized actin, stabilized actin filaments against disruption by PAN, latrunculin, or cytochalasin, and induced a significant increase in the activity of the actin-regulating GTPase RhoA.

CONCLUSION

These data suggest that, contrary to the current therapeutic paradigm, the beneficial effects of glucocorticoids in nephrotic syndrome may result, at least in part, from direct effects on podocytes leading to enhanced actin filament stability.

Vincristine treatment in steroid-dependent nephrotic syndrome

Treatment of children with steroid-dependent nephrotic syndrome (SDNS) continues to be a challenge when relapses recur after treatment with cyclophosphamide and side effects or non-compliance make steroids and cyclosporin unsatisfactory. We treated 12 patients with intravenous vincristine for SDNS in a regime of 1-1.5 mg/m2 weekly for 4 weeks then monthly for 4 months. Four of the 5 patients in relapse when commencing vincristine remitted within 2 doses. Comparing relapse frequency in the 12 months before and after vincristine, there was a reduction from 4 to 1.5 (p=0.004) relapses per year. Median sustained remission was 5 months, but 1 frequently relapsing patient remains in remission 4 years after vincristine. Vincristine was also successfully used in 1 or 2 doses at weekly intervals for subsequent relapses in 5 patients. Side effects were minimal in most cases. Abdominal pain occurred in 2 patients who commenced vincristine at 1.5 mg/m2, but resolved when continued at 1 mg/m2. We felt vincristine had a role in a subset of children with challenging SDNS administered as 1 mg/m2 weekly for 4 weeks then 1.5 mg/m2 monthly for 4 months. Vincristine allowed steroid- and cyclosporin-sparing, contributed to long-term remission in some patients, and was especially valuable in children with poor compliance with oral medication. Many patients expressed a preference for a few doses of vincristine rather than a standard course of oral prednisolone or cyclosporin.

Genetics, clinical and pathological features of glomerulonephritis associated with mutations of nonmuscle myosin IIA (Fechtner syndrome)

BACKGROUND

Fechtner syndrome (FTNS), also known as Alport-like syndrome, is a rare inherited condition characterized by progressive nephritis, macrothrombocytopenia, Döhle-like leukocyte inclusions, deafness, and cataract. Although it recently was shown that FTNS derives from mutation of MYH9, the gene for the heavy chain of nonmuscle myosin IIA (NMMHC-IIA), its pathophysiological characteristics remain unknown.

METHODS

We studied a large FTNS family in which 10 components carried a missense mutation of MYH9 determining the D1424H substitution.

RESULTS

All affected subjects presented with macrothrombocytopenia and leukocyte Döhle-like bodies consisting of macroaggregates of NMMHC-IIA, but only two subjects had major renal problems characterized by proteinuria and renal failure. Electron microscopy showed focal and segmental effacement of podocytes and loss of the interpodocyte slit diaphragm. Immunohistochemistry showed apical localization of NMMHC-IIA in tubular epithelia and less podocyte staining in the two patients, whereas it was diffuse in normal epithelia. Three patients presented with stable microhematuria, and another five patients had no renal lesions, although they carried the same mutation of MYH9. Therefore, MYH9 mutation per se was responsible for platelet and leukocyte abnormalities, whereas additional predisposing conditions and/or environmental factors are necessary for nephropathy, cataract, and deafness. Looking at podocyte components conferring permselectivity properties to the kidney, we characterized the haplotype of podocin and found cosegregation of one specific allele in the two patients with nephrotic syndrome, suggesting a relationship between podocin features and proteinuria.

CONCLUSION

Our study indicates a major role for the NMMHC-IIA abnormality in the pathogenesis of leukocyte, platelet, and kidney defects in FTNS. The basic feature in all cases is aggregation and compartmentation of NMMHC-IIA. However, proteinuria and podocyte lesions are the hallmark of nephropathy in patients who develop renal failure, and podocin may have some function in this setting.

Podocyte-associated molecules in puromycin aminonucleoside nephrosis of the rat

Molecules of central functional significance for the glomerular podocytes are rapidly emerging and have been shown to be distinctly involved in diseases with altered glomerular filtration barrier. Here we used the puromycin aminonucleoside (PA) nephrosis model in the rat to study some key proteins associated with the maintenance of the functional glomerular filtration barrier in vivo. The molecules studied included the filtration slit component nephrin, the hairpin-like membrane protein podocin, the basolateral adhesion molecules beta1 integrin and alpha-dystroglycan, and the cytoskeleton-linking intermediary beta-catenin and the actin-associated alpha-actinin-4. The results showed diminished protein levels of podocin and nephrin in the PA-treated group. beta-catenin showed distinct down-regulation at 3 days of induction, and the control level was reached at 10 days. beta1 integrin was markedly up-regulated during induction. alpha-actinin-4 was not changed at the studied time points. The results show distinct differences in the different domains of podocytes during PA-induced proteinuria.

Synaptopodin expression in idiopathic nephrotic syndrome of childhood

BACKGROUND

Synaptopodin is a proline-rich protein intimately associated with actin microfilaments present in the podocytes' foot processes. We investigated for synaptopodin expression in children with idiopathic nephrotic syndrome (INS), including minimal change disease (MCD), diffuse mesangial hypercellularity (DMH), and focal segmental glomerulosclerosis (FSGS); in children with congenital nephrotic syndrome of the Finnish type (CNF); and in normal kidney tissue. In particular, we examined whether an association exists between synaptopodin expression in podocyte cells and the response to steroids in INS, and whether synaptopodin expression can predict FSGS upon the initial kidney biopsy in children who progress from MCD or DMH to FSGS.

METHODS

Immunohistochemistry was performed for synaptopodin expression on renal tissues from MCD (N = 18), DMH (N = 7), FSGS (N = 13), CNF (N = 9), and normal children (N = 7). Synaptopodin expression in nonsclerosed glomeruli was quantitated by computerized image analysis on the Optimastrade mark software for both luminance (L) and percentage of glomerular area (A).

RESULTS

Synaptopodin expression was absent in areas of sclerosis. In nonsclerosed glomeruli, synaptopodin was significantly less expressed in all groups of INS and in CNF compared with normal (P < 0.0001 for both L and A, in each MCD, DMH, FSGS, and CNF). In INS, synaptopodin expression decreased in order from MCD to DMH to FSGS, reaching statistical significance between MCD and FSGS (P = 0.001 for L and P = 0.05 for A). Greater synaptopodin expression in podocytes was associated with a significantly better response to steroid therapy (P < 0.05 for both L and A). On the other hand, the expression of synaptopodin did not predict progression of MCD or DMH to FSGS.

CONCLUSION

We conclude that measurement of synaptopodin has the potential to be used as a marker to study the alteration in podocyte cell and response to therapy in INS.

Adaptation and distortion of podocytes in rat remnant kidney

A sequence of alterations of podocytes as well as the development of glomerulosclerosis was analyzed in a five-sixths nephrectomy rat model. Morphometric measurements showed a gradual enlargement in glomerular size without any increase in the number of podocytes in the glomerulus. Structural changes of podocytes appeared as early as 4 weeks after operation, and increased their degree and extent with time. In electron microscopic studies, changes of podocytes in the early stage consisted of foot process stretching and thinning, and cell body attenuation accompanied by pseudocyst formation. As weeks passed, these changes became prominent and were occasionally associated with podocyte detachment from the glomerular basement membrane followed by tuft adhesion and the development of segmental sclerosis. In parallel to the structural changes, immunohistochemical staining demonstrated a marked increase of desmin expression in the glomerulus with intense staining in podocytes. Podocyte alterations were identified as an adaptation to the enlargement of the glomerulus and, consequently, the podocytes seemed to fall to adapt to the structural changes of the glomerulus. The present study supports the hypothesis that podocyte defects are responsible for both the initiation and progression of glomerular damage. Furthermore, the findings may provide the hypothesis that the native disposition of podocytes in the glomerulus participates in the development of glomerular lesions in a focal and segmental fashion.

A quantitative immunofluorescence study of glomerular cell adhesion proteins in proteinuric states

Whenever there is heavy proteinuria, the glomerular epithelial cells, the podocytes, show dramatic morphological changes which clearly demonstrate changes in cell adhesion. However, there is little information on the types of cell adhesion molecules expressed in the normal human glomerulus. Assessments of changes in cell adhesion molecules in human proteinuria have been confined to semi-quantitative immunostaining for integrins, and the results have not been entirely consistent. This study sought first to define which cell adhesion molecules are present in the normal glomerulus, using indirect immunofluorescence and a panel of antibodies directed against transmembrane adhesion proteins and against several cytoplasmic proteins which are known to be involved in adhesion. A wide variety of integrins were detected, the dominant form being alpha 3 beta 1. The cytoplasmic focal adhesion proteins vinculin, talin, paxillin, p130CAS, and pp125FAK were detected, although vinculin appeared to be confined mainly to the mesangium. The only intercellular adhesion molecule detected in the vicinity of the slit diaphragm was ZO-1; the results imply that the slit diaphragm does not bear a close relationship to any other form of intercellular junction. Changes in these adhesion components were also studied in proteinuria, using 18 cases each of minimal change nephropathy, 'early' membranous nephropathy, and normal controls. Fluorescence intensity was measured by image capture using a low light video camera and subsequent digital image analysis, an approach which demonstrated acceptable reproducibility. The most striking changes were an increase in phosphotyrosine and p130CAS in the nephrotic patients. Contrary to previous reports, little change was found in the expression of the most abundant integrins, nor did overall glomerular staining for ZO-1 alter. These results imply a controlled alteration in glomerular cell adhesion in nephrotic states in man, probable representing increased turnover of cell adhesion structures rather than the decrease which has been reported in short-term animal models. This is the first report of increased glomerular phosphotyrosine in man, which is associated with less stable adhesions and may be related to the loss of foot processes. Using human biopsy material, it was not possible to determine which proteins were phosphorylated, but the probable relationships to changes in cytoskeletal structure and slit diaphragm permeability justify further study.

Limitations of podocyte adaptation for glomerular injury in puromycin aminonucleoside nephrosis

Glomerular synechiae that occurred in nephrotic rats with a single intraperitoneal injection of puromycin aminonucleoside were analyzed by immunohistochemistry, radiolabeled thymidine ([3H]-thymidine) autoradiography, as well as light, electron and immunoelectron microscopy. To discriminate podocytes from parietal epithelial cells (PEC) and monocytes, monoclonal antibodies (mAb) against podocalyxin and ED1 were used. The cell kinetics of glomerular epithelial cells were autoradiographically assessed with isotope labeling procedures before and during nephrosis (co-labeled), and a mAb against proliferating cell nuclear antigen (PCNA). All the cell types except the podocyte of normal kidneys were labelled with [3H]-thymidine at different rates. Detachment of degenerated podocytes from the outside of the glomerular basement membrane (GBM) is the first step of synechia, and detached sites are confronted by PEC that were hypertrophied and frequently radiolabeled. Evidence that podocytes in glomeruli of nephrotic rats can proliferate was shown by the presence of mitoses, [3H]-thymidine uptake in the co-labeled experiment, and by PCNA staining, but re-epithelialization over bare segments of the GBM with proliferated podocytes is doubtful. It was concluded that glomerular synechia resulted from the limits of podocyte adaptation to glomerular injuries.