Morphological alterations of the glomerular (visceral) epithelium in response to pathological and experimental situations

Prakash. Nephrin a biomarker of early glomerular injury in newly diagnosed untreated hypertensive subjects

Introduction and Aim: Hypertension and proteinuria is known to cause renal and cardiovascular disease and mortality in patients irrespective of diabetes. It is beneficial to identify proteinuria and probable glomerular injury early to take preventive measures from cardiovascular event.  In our study, we aimed to evaluate whether a biomarker such as nephrin can detect early glomerular injury in treatment naïve hypertensive subjects.   Materials and Methods: Forty newly diagnosed, treatment naïve hypertensive subjects were recruited for the study along with 40 normotensive controls after obtaining informed consent and procuring approval from. Institutional Ethics Committee. The hypertensive group was classified as diabetic and non-diabetic hypertensives and compared with apparently healthy controls (normotensive). Urine sample was analyzed for microalbumin, creatinine and nephrin. Blood sample was analyzed for glycated hemoglobin, urea, creatinine, sodium, and potassium.  Statistical analysis was performed using ANOVA to compare the groups for various parameters. Odds ratio was calculated.   Results: Hypertensives were sub-grouped based on amount of microalbumin excreted. Urine nephrin excretion was significantly higher in hypertensive subjects than normotensive subjects (nephrin cut-off: 0.09 mg/g of creatinine). Urine nephrin (mg/g) was found to be elevated (median 0.15; interquartile range, 0.12 and 0.17) in hypertensives with normoalbuminuria and it was significantly higher than normotensive subjects (median 0.07; interquartile range, 0.04 and 0.09).     Conclusion: Urine nephrin may be used as a biomarker of early glomerular injury in hypertensive subjects even before microalbuminuria is detected.

Contribution of Coiled-Coil Assembly to Ca2+/Calmodulin-Dependent Inactivation of TRPC6 Channel and its Impacts on FSGS-Associated Phenotypes

BACKGROUND

TRPC6 is a nonselective cation channel, and mutations of this gene are associated with FSGS. These mutations are associated with TRPC6 current amplitude amplification and/or delay of the channel inactivation (gain-of-function phenotype). However, the mechanism of the gain-of-function in TRPC6 activity has not yet been clearly solved.

METHODS

We performed electrophysiologic, biochemical, and biophysical experiments to elucidate the molecular mechanism underlying calmodulin (CaM)-mediated Ca²⁺-dependent inactivation (CDI) of TRPC6. To address the pathophysiologic contribution of CDI, we assessed the actin filament organization in cultured mouse podocytes.

RESULTS

Both lobes of CaM helped induce CDI. Moreover, CaM binding to the TRPC6 CaM-binding domain (CBD) was Ca²⁺-dependent and exhibited a 1:2 (CaM/CBD) stoichiometry. The TRPC6 coiled-coil assembly, which brought two CBDs into adequate proximity, was essential for CDI. Deletion of the coiled-coil slowed CDI of TRPC6, indicating that the coiled-coil assembly configures both lobes of CaM binding on two CBDs to induce normal CDI. The FSGS-associated TRPC6 mutations within the coiled-coil severely delayed CDI and often increased TRPC6 current amplitudes. In cultured mouse podocytes, FSGS-associated channels and CaM mutations led to sustained Ca²⁺ elevations and a disorganized cytoskeleton.

CONCLUSIONS

The gain-of-function mechanism found in FSGS-causing mutations in TRPC6 can be explained by impairments of the CDI, caused by disruptions of TRPC's coiled-coil assembly which is essential for CaM binding. The resulting excess Ca²⁺ may contribute to structural damage in the podocytes.

Intravital Imaging Reveals Angiotensin II-Induced Transcytosis of Albumin by Podocytes

Albuminuria is a hallmark of kidney disease of various etiologies and usually caused by deterioration of glomerular filtration barrier integrity. We recently showed that angiotensin II (Ang II) acutely increases albumin filtration in the healthy kidney. Here, we used intravital microscopy to assess the effects of Ang II on podocyte function in rats. Acute infusion of 30, 60, or 80 ng/kg per minute Ang II enhanced the endocytosis of albumin by activation of the type 1 Ang II receptor and resulted in an average (±SEM) of 3.7±2.2, 72.3±18.6 (P<0.001), and 239.4±34.6 µm(3) (P<0.001) albumin-containing vesicles per glomerulus, respectively, compared with none at baseline or 10 ng/kg per minute Ang II. Immunostaining of Ang II-infused kidneys confirmed the presence of albumin-containing vesicles, which colocalized with megalin, in podocin-positive cells. Furthermore, podocyte endocytosis of albumin was markedly reduced in the presence of gentamicin, a competitive inhibitor of megalin-dependent endocytosis. Ang II infusion increased the concentration of albumin in the subpodocyte space, a potential source for endocytic protein uptake, and gentamicin further increased this concentration. Some endocytic vesicles were acidified and colocalized with LysoTracker. Most vesicles migrated from the capillary to the apical aspect of the podocyte and were eventually released into the urinary space. This transcytosis accounted for approximately 10% of total albumin filtration. In summary, the transcellular transport of proteins across the podocyte constitutes a new pathway of glomerular protein filtration. Ang II enhances the endocytosis and transcytosis of plasma albumin by podocytes, which may eventually impair podocyte function.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Imaging of podocyte foot processes by fluorescence microscopy

Visualizing podocyte foot processes requires electron microscopy, a technique that depends on special equipment, requires immunogold for colabeling, and does not take advantage of the growing number of in vivo fluorophores available. To address these limitations, we developed a genetic strategy to allow detailed visualization of single podocytes and their foot processes by conventional fluorescence microscopy. We generated a transgenic mouse line expressing a GFP-Cre-ERT2 fusion protein under the control of the collagen α1(I) promoter with strong podocyte expression. Administration of submaximal tamoxifen allowed genetic labeling of single podocytes when crossed with a Cre-reporter line. Of three different reporter systems that we evaluated for the ability to reveal fine structural details of podocytes, bigenic Coll1α1GCE;Gt(ROSA)26Sor(tm9(CAG-tdTomato)) mice allowed podocyte labeling with a strong and homogeneous reporter signal that was easily observed by epifluorescence. We could easily detect anatomic features of podocytes down to tertiary foot processes, and we were able to visualize and quantitate ultrastructural changes to foot processes after podocyte injury. In summary, using this method of genetic labeling and conventional fluorescence microscopy to visualize podocyte foot processes will complement electron microscopy and facilitate the analysis of podocytes and their precursors in vivo.

Podocyte membrane vesicles in urine originate from tip vesiculation of podocyte microvilli

Podocyte injury is involved in both the onset and progression of glomerular diseases. Our previous studies revealed that apical cell membranes of podocyte are shed into urine sediment and that urinary podocalyxin is a useful biomarker of podocyte injury. In this study, we examined the origin of urinary podocalyxin. Urine samples and kidney specimens from healthy children (n = 126) and patients with glomerular diseases (n = 77) were analyzed by immunohistologic methods. Immunofluorescence studies demonstrated that urinary podocalyxin was shed as granular structures into both the urine sediment and supernatant. Large amounts of podocalyxin were shed into both the urine sediment (17.2 +/- 3.2 ng/mg creatinine) and the supernatant (172.6 +/- 24.6 ng/mg creatinine) of patients, compared with the small amounts of urinary podocalyxin in healthy controls (sediment, 0.5 +/- 0.1 ng/mg creatinine; supernatant, 24.3 +/- 3.5 ng/mg creatinine). Electron and immunoelectron microscopic examinations showed that podocalyxin-positive vesicles in the sediment (125.6 +/- 8.8 nm) and the supernatant (121.2 +/- 6.4 nm) were similar in size to podocyte microvilli in biopsy specimens (123.6 +/- 8.9 nm), differentiating them from the much smaller urine exosomes (30-80 nm in diameter). Urine podocalyxin-positive vesicles tested negative in immunofluorescence microscopy on both exosomal markers CD24 and CD63. Podocalyxin-positive vesicles also tested negative for cytoskeletal markers, and electron microscopic examination revealed tip vesiculation of microvilli. We conclude that human urinary apical cell membrane vesicles appear to originate not from podocyte exosomes but from tip vesiculation of glomerular podocyte microvilli.

Drug-induced Kidney Disease – Pathology and Current Concepts

The kidneys can be damaged by a large number of therapeutic agents. The aim of this article is to discuss the pathological features of drug-induced renal disease as diagnosed by kidney biopsy. The literature is reviewed and cases seen by the authors that have a known drug association are analysed. Mechanisms of injury are varied and all renal structures may be affected. The tubulointerstitial compartment is most frequently involved, but glomerular and vascular lesions are seen in a significant proportion of cases. Key words: Drug, Kidney, Nephrotoxicity, Pathology

Ischemic injury to kidney induces glomerular podocyte effacement and dissociation of slit diaphragm proteins Neph1 and ZO-1

Glomerular injury is often characterized by the effacement of podocytes, loss of slit diaphragms, and proteinuria. Renal ischemia or the loss of blood flow to the kidneys has been widely associated with tubular and endothelial injury but rarely has been shown to induce podocyte damage and disruption of the slit diaphragm. In this study, we have used an in vivo rat ischemic model to demonstrate that renal ischemia induces podocyte effacement with loss of slit diaphragm and proteinuria. Biochemical analysis of the ischemic glomerulus shows that ischemia induces rapid loss of interaction between slit diaphragm junctional proteins Neph1 and ZO-1. To further understand the effect of ischemia on molecular interactions between slit diaphragm proteins, a cell culture model was employed to study the binding between Neph1 and ZO-1. Under physiologic conditions, Neph1 co-localized with ZO-1 at cell-cell contacts in cultured human podocytes. Induction of injury by ATP depletion resulted in rapid loss of Neph1 and ZO-1 binding and redistribution of Neph1 and ZO-1 proteins from cell membrane to the cytoplasm. Recovery resulted in increased Neph1 tyrosine phosphorylation, restoring Neph1 and ZO-1 binding and their localization at the cell membrane. We further demonstrate that tyrosine phosphorylation of Neph1 mediated by Fyn results in significantly increased Neph1 and ZO-1 binding, suggesting a critical role for Neph1 tyrosine phosphorylation in reorganizing the Neph1-ZO-1 complex. This study documents that renal ischemia induces dynamic changes in the molecular interactions between slit diaphragm proteins, leading to podocyte damage and proteinuria.

A case of systemic lupus erythematosus showing invagination of the podocyte into the glomerular basement membrane: an electron microscopic observation of a repeated-renal biopsy

A case of systemic lupus erythematosus (SLE) showing invagination of glomerular epithelial cells into the glomerular basement membrane (GBM) has been reported. The patient was a 30-year-old woman who was diagnosed with SLE at the age of 25 and had been medicated with corticosteroid. At the age of 30, she was re-admitted into our hospital because of the relapse of lupus nephritis. Renal biopsy was performed twice: at the onset of SLE and the second admission. Morphologically, the results of the first and second renal biopsies were compatible with the classification of lupus nephritis class II. Immunofluorescent study revealed the mesangial deposition of IgG, IgA, C1q, C3 and membrane attack complex (MAC) in two renal biopsies. Especially, in the second renal biopsy, numerous vesicular structures composed of a unit membrane going into the epithelial site of the GBM were observed by electron microscopy (EM), but not in the first renal biopsy. Moreover, observations of serial sections of EM suggested that these vesicular structures were invaginated into the GBM and derived from a part of the podocytes.

Ezrin--a useful factor in the prognosis of nephrotic syndrome in children: an immunohistochemical approach

BACKGROUND

Minimal change disease (MCD) and diffuse mesangial proliferation (DMP) are the most common pathomorphological forms of nephrotic syndrome glomerulopathies in children. The clinical course of DMP can be characterised by either DMP-sensitivity (DMP-S) or DMP-resistance (DMP-R) to steroids, resulting in an unfavourable course of the glomerulopathy. Although the clinical processes of DMP-S and DMP-R are initially identical, resistance to steroids may be foreseen by the immunohistochemical expression of cytoskeleton-associated proteins in podocytes.

AIMS

To estimate the immunohistochemical expression of ezrin in children with MCD, DMP and focal segmental glomerulosclerosis (FSGS) and to evaluate its usefulness in predicting resistance to steroids.

MATERIALS AND METHODS

Renal biopsy specimens of patients with MCD (n = 15), DMP (n = 16) and FSGS (n = 6) were taken. The control tissue consisted of normal-appearing cortex taken from kidneys resected for localised neoplasms (n = 6). The indirect immunohistochemical protocol for the use of a monoclonal antibody directed against ezrin was used.

RESULTS

The immunohistochemical expression of ezrin in cases progressively reduced from MCD to DMP-S to DMP-R to FSGS. Except for DMP-R and FSGS (p>0.05), the difference in ezrin expression in podocytes was significant.

CONCLUSION

Ezrin can be a potent marker of podocyte injury (podocytopathy) and may help in the histological qualification of MCD, DMP and FSGS. The increased permeability of the filtration barrier in steroid-resistant and proteinuric glomerulopathies may be a consequence of subcellular changes in podocyte-associated proteins following decreased expression of ezrin.

Apical cell membranes are shed into urine from injured podocytes: a novel phenomenon of podocyte injury

Previously it was shown that urine from patients with nephritis contains podocytes and their fragments (podocalyxin [PCX]-positive granular structures [PPGS]), reflecting the degree of podocyte injury. The present study was designed to trace PPGS to their origin. Urine samples and renal biopsy specimens from 53 children with nephrotic syndrome and nephritis were examined immunohistochemically. Immunofluorescence studies of kidney sections using an anti-PCX antibody demonstrated that PPGS originated from the glomerulus and flowed into the tubular lumen. Electron microscopic examination revealed that PPGS originated from microvillous or vesicle-like structures on injured podocytes in the glomerulus. For examining the origin of the PPGS, apical, slit-diaphragmatic, and basal portions of the podocytes were specifically stained, revealing that PPGS are composed primarily of apical podocyte membranes. Several newly developed antibodies that are reactive with various segments of the PCX molecule were used to analyze more detailed membrane structures, and it was found that PPGS contained intact PCX molecules, indicating that cell membrane structures are excreted in urine. The quantification of PCX content and podocyte numbers revealed that urinary sediment PCX (u-sed-PCX) content per urinary podocyte was much higher than PCX content per podocyte from isolated glomeruli of normal controls, suggesting that u-sed-PCX are derived from sources other than just the cell debris of detached podocytes. Analysis of the correlation between u-sed-PCX and renal histology revealed that the presence of PPGS reflects acute glomerular injury. In conclusion, podocyte apical cell membranes are shed into the urine from injured podocytes, indicating a previously unrecognized manifestation of podocyte injury.

Estimation of podocyte number: a comparison of methods

BACKGROUND

The podocyte is the focus of much research into the mechanisms of renal disease progression, and the number of podocytes per glomerulus has thus become a parameter of much interest. When counting podocytes, the actual particle counted is the cell nucleus. The majority of published studies estimating podocyte number have used the method of Weibel and Gomez (1962). This makes assumptions about the shape and size of the cell nuclei and therefore has an inherent bias. In our studies we have used a more recent stereologic method--the disector/fractionator--that makes no assumptions about the shape or size of the cell nuclei and is therefore free of bias.

METHODS

We set out to compare the two methods, in both type 1 diabetic patients and normal controls, to determine whether eliminating bias and thus improving accuracy had any effect on the overall results. The Weibel-Gomez method estimates cell number from a single section through the glomerulus, whereas the disector/fractionator requires the glomerulus to be serially sectioned.

RESULTS

There was no significant difference between mean values obtained by the two methods, providing that the Weibel-Gomez estimate was performed on electron micrographs. However, the overall variance was high for all groups of patients, independent of the method employed.

CONCLUSION

Although the disector/fractionator is the theoretic gold standard method for podocyte number estimation, comparable estimates can be obtained by the Weibel-Gomez method provided they are made from electron micrographs. Thus the technical resources available may determine the choice of method employed. Investigators should be aware of the high degree of variability in the estimate, particularly when trying to detect small changes in podocyte number.

Glomerular dysfunction, independent of tubular dysfunction, induced by antineoplastic chemotherapy in children

BACKGROUND

For the purpose of studying renal side-effects induced by antineoplastic agents, the authors examined glomerular injury as well as tubular injury of patients with chemotherapy.

METHODS

Thirteen patients underwent a combined total of 64 courses of chemotherapy. Urinary albumin, beta2-microglobulin (beta2-MG), N-acetyl-beta-glucosaminidase (NAG) and urinary protein were measured before and serially after chemotherapy.

RESULTS

The values of albumin/creatinine (albumin/cre) ratio and beta2-MG/creatinine (beta2-MG/cre) ratio after chemotherapy were higher than those before chemotherapy (P <0.01). NAG/creatinine (NAG/cre) ratio and creatinine clearance (Ccr) were not different. These were also examined before the next course of chemotherapy and were compared with those of control children. Albumin/cre ratio was significantly different (P <0.01). beta2-MG/cre ratio and NAG/cre were not different. Furthermore, in patients with normal beta2-MG/cre, the albumin/cre ratio was significantly higher than in control children.

CONCLUSIONS

These results indicate that antineoplastic agents can not only induce tubular dysfunction but also glomerular dysfunction, which is persistent and independent of tubular dysfunction.

Pathobiochemistry of nephrotic syndrome

Nephrotic syndrome is a clinical and laboratory syndrome caused by the increased permeability of the glomerular capillary wall for macromolecules. Nephrotic syndrome is a potentially life-threatening state and persistent nephrotic syndrome has a poor prognosis with a high risk of progression to end-stage renal failure and a high risk of cardiovascular complications due to severe hyperlipidemia. Pathogenesis of increased glomerular permeability in different glomerular diseases has not been fully elucidated. Recently, identification of the mutated genes for some podocyte proteins (nephrin, podocin, alpha-actinin-4) in rare familial forms of nephrotic syndrome shed has new light on the molecular mechanisms of glomerular permselectivity. Gradually it becomes apparent that sporadic mutations of podocyte proteins (e.g., podocin) may be present even in some patients with acquired nephrotic syndrome. Expression of other podocyte proteins may change during the course of experimental nephrotic syndrome, possibly as a response to podocyte damage resulting either in apoptosis or stimulation of proliferation and some form of repair, including glomerular sclerosis. Better understanding of these mechanisms could clearly also have therapeutic implications. Glomerular permeability factors are believed to play a role in some noninflammatory glomerular diseases, mainly minimal change disease and focal segmental glomerulosclerosis, but their molecular identification remains elusive, possibly due to the nonhomogeneous nature of the underlying diseases. As an example, focal segmental glomerulosclerosis possibly can be caused by the sporadic mutation of some genes for podocyte proteins, increased production of glomerular permeability factor (possibly by T lymphocytes), or the loss of inhibitors of glomerular permeability factors in nephrotic urine. Clearly the factors causing increased glomerular permeability and factors perpetuating glomerular sclerosis are not necessarily the same. Proteinuria does not seem to be only the consequence of glomerular damage, but it may possibly cause tubular damage and initiate interstitial fibrosis and thus contribute to the progression of chronic renal failure in proteinuric renal diseases. Recent insights into the mechanisms of tubular protein reabsorption may give new tools for preventing the progression of chronic renal disease. Cubilin inhibitors could potentially ameliorate tubular and interstitial damage in patients with heavy proteinuria refractory to treatment. Nephrotic hyperlipidemia is accompanied with increased risk of cardiovascular complications and should be treated in all patients with persistent nephrotic syndrome. The putative positive effect of hypolipidemic drugs (namely statins) on the cardiovascular risk and potentially also on the rate of progression of chronic renal failure remains to be demonstrated in prospective controlled studies. Recent progress in understanding podocyte biology in rare inherited glomerular diseases gives the chance to understand in the near future the molecular pathogenesis of increased glomerular permeability in the much more common acquired forms of nephrotic syndrome.

Podocyte process effacement in vivo

Foot process effacement is the most characteristic change in podocyte structure under a wide variety of human and experimental glomerulopathies with heavy proteinuria. It consists of simplification and even total disappearance of the interdigitating foot process pattern, resulting in the formation of a diffuse cytoplasmic sheet along the glomerular basement membrane. Although abundant evidence related to structural changes in podocyte foot processes has been reported, cellular or molecular mechanisms that occur within podocytes during the development of foot process effacement remain unclear. This review summarizes recent advances concerning structural and functional aspects of foot process effacement in vivo. Following a description of the general morphology of foot process effacement, the role of the cytoskeleton and its related proteins in the effacement are discussed. Finally, the relevance of foot process effacement in glomerular function is considered.

Biology of kidney cells: ontogeny-recapitulating phylogeny

Biology of kidney cells can be used as a model for further understanding of ontogeny-recapitulating phylogeny. The common and species-specific structural and functional relationship between blood capillaries and the environment via a filtration barrier of nephrons is a biological phenomenon resulting from renal cell memory acquired through evolution. Genetically programmed development, a subsequent series of gene expression, and inductive interactions played a key role in differentiation and maintenance of specific activities of kidneys in birds and mammals. Various environmental factors may alter kidney development and specific activities at the levels of gene expression, repression, or derepression, and defensive mechanisms involved in reaction to risk factors are developed. Autoimmunity and cancerogenesis are closely dependent on a variety of environmental agents, such as antigens originating from infections with some viruses and toxins, or irradiation, advanced industrialization, and progress of civilization. As a result of gene mutation, delation, rearrangement, and/or susceptibility to different agents, renal cell memory is altered. Instead of cell-specific activities, the abilities for regeneration, and other genetically programmed activities, the genesis of kidney diseases are common. Balkan endemic nephropathy, as regional disease, is an important example of the role, of environmental agents, at the level of genes. Research programs on molecular genetics will contribute to our efforts both to prevent infections and to elucidate the genesis, diagnosis, prognosis, prevention, and therapy of kidney diseases.

The glomerular epithelial cell anti-adhesin podocalyxin associates with the actin cytoskeleton through interactions with ezrin

During development, renal glomerular epithelial cells (podocytes) undergo extensive morphologic changes necessary for creation of the glomerular filtration apparatus. These changes include formation of interdigitating foot processes, replacement of tight junctions with slit diaphragms, and the concomitant opening of intercellular urinary spaces. It was postulated previously and confirmed recently that podocalyxin, a sialomucin, plays a major role in maintaining the urinary space open by virtue of the physicochemical properties of its highly negatively charged ectodomain. This study examined whether the highly conserved cytoplasmic tail of podocalyxin also contributes to the unique organization of podocytes by interacting with the cytoskeletal network found in their cell bodies and foot processes. By immunocytochemistry, it was shown that podocalyxin and the actin binding protein ezrin are co-expressed in podocytes and co-localize along the apical plasma membrane, where they form a co-immunoprecipitable complex. Selective detergent extraction followed by differential centrifugation revealed that some of the podocalyxin cosediments with actin filaments. Moreover, its sedimentation is dependent on polymerized actin and is mediated by complex formation with ezrin. Once formed, podocalyxin/ezrin complexes are very stable, because they are insensitive to actin depolymerization or inactivation of Rho kinase, which is known to be necessary for regulation of ezrin and to mediate Rho-dependent actin organization. These data indicate that in podocytes, podocalyxin is complexed with ezrin, which mediates its link to the actin cytoskeleton. Thus, in addition to its ectodomain, the cytoplasmic tail of podocalyxin also likely contributes to maintaining the unique podocyte morphology.

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.

Aquaporin-6: An intracellular vesicle water channel protein in renal epithelia

All characterized mammalian aquaporins (AQPs) are localized to plasma membranes where they function chiefly to mediate water transport across cells. Here we show that AQP6 is localized exclusively in intracellular membranes in renal epithelia. By using a polyclonal antibody to the C terminus of AQP6, immunoblots revealed a major 30-kDa band in membranes from rat renal cortex and medulla. Endoglycosidase treatment demonstrated presence of an intracellular high mannose glycan on each subunit. Sequential ultracentrifugation of rat kidney homogenates confirmed that AQP6 resides predominantly in vesicular fractions, and immunohistochemical and immunoelectron microscopic studies confirmed that >98% of AQP6 is located in intracellular membrane vesicles. In glomeruli, AQP6 is present in membrane vesicles within podocyte cell bodies and foot processes. In proximal tubules, AQP6 is also abundant in membrane vesicles within the subapical compartment of segment 2 and segment 3 cells, but was not detected in the brush border or basolateral membranes. In collecting duct, AQP6 resides in intracellular membrane vesicles in apical, mid, and basolateral cytoplasm of type A intercalated cells, but was not observed in the plasma membrane. Unlike other members of the AQP family, the unique distribution in intracellular membrane vesicles in multiple types of renal epithelia indicates that AQP6 is not simply involved in transcellular fluid absorption. Moreover, our studies predict that AQP6 participates in distinct physiological functions such as glomerular filtration, tubular endocytosis, and acid-base metabolism.

Dynamic structure of glomerular capillary loop as revealed by an in vivo cryotechnique

Morphological studies using immersion or perfusion fixation methods do not reveal the ultrastructure of functioning kidneys with normal circulation. A simple apparatus was developed for freezing the kidneys in vivo without stopping the blood supply, and the ultrastructure of the glomerular capillary loops was examined under different haemodynamic conditions. Mouse kidneys were frozen under normal blood flow conditions; others were frozen in the same way after ligation of the abdominal aorta at a point caudal to the renal arteries. They were then processed for the freeze-substitution or deep-etching method. Good ultrastructural preservation was obtained within about 5 microM depth from the frozen tissue surface. Functioning glomeruli with normal blood flow possessed open capillary lumens, different shapes of foot processes and atypical basement membranes with low density. Moreover, heterogeneity in width between foot processes was identified on the replica membranes. Under the acute conditions used to increase blood supply into the kidneys, the spaces between the flat foot processes became more widely dilated and the basement membrane was seen to be three-layered. The ultrastructure of glomeruli in functioning kidneys has been demonstrated for the first time by this "in vivo cryotechnique."

Cell biology of kidney glomerulus

It has been accepted that some artifacts are inevitably produced by the conventional preparation steps for electron microscopy, including fixation, dehydration, embedding, ultrathin sectioning, and staining. Therefore, conventional ultrastructural findings on kidney glomeruli are hardly thought to be correlated with the physiological functions of kidneys in vivo. In this chapter, two preparation techniques, the quick-freezing and deep-etching (QF-DE) method or the quick-freezing and freeze-substitution (QF-FS) method, are presented and shown to be useful for clarifying the ultrastructures of kidney glomeruli more closely to structures in vivo with fewer artifacts. Moreover, the ultrastructures of glomerular capillary loops have been demonstrated by a new "in vivo cryotechnique," that shows that hemodynamic factors should be considered in the morphological study of glomerular functions.

Structure and function of podocytes: an update

Glomerular visceral epithelial cells, also termed podocytes, are highly specialized epithelial cells that cover the outer aspect of the glomerular basement membrane. Recent studies point to an important role of podocytes in the physiology and pathophysiology of the glomerulus. This review summarizes the structure-function relationships of podocytes. Following a description of the general morphology of podocytes, the technical problems associated with studying these cells are discussed. A survey of podocyte function forms the center of this review. Finally, selected aspects of podocyte development and cell division are discussed.

A role for podocytes to counteract capillary wall distension

In a previous study of the changes in glomerular structure in the isolated perfused kidney (IPK), perfusion at high pressures lead to an enlargement of the glomerular tuft and to the formation of giant capillaries. The present paper analyzes the morphological and dimensional changes of the peripheral glomerular capillary wall under these circumstances. The enlargement of glomerular capillaries at high pressure perfusion was accompanied by a considerable increase in the surface area of the glomerular basement membrane (GBM). The podocyte as well as the endothelial layer perfectly adapted to the acute challenge in covering increasing GBM area. The interdigitating foot process pattern showed up in an ideal arrangement. The capillary wall expansion was associated with a significant increase in total pericapillary slit area. Compared to the corresponding low pressure groups (65 mm Hg, without and with the application of vasodilators) the slit area increased in the high pressure groups (105 mm Hg, without and with vasodilator) by approximately 50 and 75%, respectively. This increase of the slit area was mainly due to an increase in slit length; the slit width remained fairly constant. These findings indicate that the pericapillary wall is distensible based on a distensibility of the GBM. We suggest that the contractile apparatus of podocyte foot processes regulates the expansion of the GBM.

Explaining glomerular pores with fiber matrices. A visualization study based on computer modeling

The extracellular space of the glomerular capillary wall is occupied by a complex meshwork of fibrous molecules. Little is understood about how the size, shape, and charge recognition properties of glomerular ultrafiltration arise from this space-filling fiber matrix. We studied the problem of size recognition by visualizing the void volume accessible to hard spheres in computer-generated three-dimensional homogeneous random fiber matrices. The spatial organization of the void volume followed a complex "blob-and-throat" pattern in which circumscribed cavities of free space within the matrix ("blobs") were joined to adjacent cavities by narrower throats of void space. For sufficiently small solutes, chains of blobs and throats traversed the matrix, providing pathways for trans-matrix permeation. The matrices showed threshold or gating properties with respect to permeation: solutes whose radius exceeded a critical value, at which a throat on the last connected trans-matrix pathway pinched off, could not cross, whereas smaller solutes had nonzero permeability. The thresholds may give the glomerular fiber matrix porelike response properties and explain why pore models have been such a useful means of treating permselectivity.

Scanning electron microscopic observations of human fetal kidney maturing in vivo and in serum-free organ culture

A serum-free model has been developed in our laboratory enabling us to maintain human fetal kidney in culture for periods of 5 days or more. In this totally defined system, morphological integrity of these explants was shown to be preserved at both the light and the electron microscopic levels. The present work was undertaken to validate our culture model via scanning electron microscopy, a technique allowing surface observation of micromorphological features overlooked by conventional microscopy. In uncultured kidney, different developmental stages of nephron formation were identified. A sparse population of short microvilli was present on most cell apical membranes. Cell outlines were polygonal and demarcated by longer and densely packed microvilli. In proximal tubules, these microvilli were in the process of forming a brush border. In the majority of cells, one or two cilia with twisted or hooked tips projected into the capsular space or tubule lumen. Microcraters and bleb-like structures characterized the luminal membrane of many cells. The urinary papilla epithelium was composed of some ciliated principal cells but mostly of intercalated cells with either apical microplicae, microvilli, or both. Micro-projections formed zipper-like intercellular junctions. In culture, ultrastructural features, including membrane pits and spherical vesicles, were similar to those in uncultured explants. In summary, these novel observations in cultured fetal kidney indicate that ultrastructural integrity is well preserved in serum-free medium and that the present model is a valuable tool to study human nephrogenesis.

Ultrastructural changes in glomerular epithelial cells in acute puromycin aminonucleoside nephrosis: a study by high-resolution scanning electron microscopy

Ultrastructural changes in the podocytes were studied during the development of, and recovery from, acute puromycin aminonucleoside (PAN) nephrosis using high-resolution scanning electron microscopy (hrSEM) and transmission electron microscopy (TEM). In the process of development of PAN nephrosis, four types of early structural changes were observed before total loss of foot processes: formation of cytoplasmic blebs, masking of filtration clefts, flattening of foot processes, and retraction of foot processes. The masking of filtration clefts visualized by hrSEM corresponded to the multiplication of slit diaphragms and adhesion of foot processes in the TEM findings, and preceded retraction of the foot processes. Changes of podocyte configuration were produced. Recovery from this change of podocyte configuration began as islands of podocyte interdigitation, and was proceeded by expansion of the islands. During recovery, the primary processes were re-established either by retraction or perforation of the thin cytoplasm after the formation of foot processes. We conclude that loss of foot processes begins with the masking of filtration clefts. Recovery from the change in podocyte configuration begins with the formation of new foot processes.

Microfilament disruption occurs very early in ischemic proximal tubule cell injury

Experimental ischemic acute renal failure results in disruption of proximal tubule apical membranes. Previous work utilizing immunofluorescence with an anti-actin antibody has demonstrated that the apical cytoskeleton of proximal tubule cells is disrupted during ischemic injury. In this study, using rhodamine-phalloidin which stains only filamentous actin, we demonstrate that graded durations of ischemia resulted in progressive disruption of proximal tubule apical microfilaments. Quantification using spectrofluorometry showed that 5, 15 and 50 minutes of ischemia resulted in 32.8 +/- 4%, 48.8 +/- 2.5%, and 58.4 +/- 2.6% decreases in apical F-actin relative to controls. Ischemia did not qualitatively affect either glomerular or distal tubule F-actin structure, though there were nonprogressive increases in glomerular fluorescence. In summary, rhodamine-phalloidin staining can be used to qualitatively and quantitatively assess proximal tubule microfilaments in vivo. We conclude that ischemia results in very early loss of proximal tubule apical microfilaments, with the majority of F-actin loss occurring within five minutes.

In vitro effects of puromycin aminonucleoside on the ultrastructure of rat glomerular podocytes

Puromycin aminonucleoside (PAN)-induced nephrosis in rats provides a model for studying the pathogenesis of severe proteinuric conditions, such as minimal change disease. The present study used scanning (SEM) and transmission (TEM) electron microscopy to investigate the in vitro effects of PAN on rat glomerular podocytes. Slices of rat kidney were incubated for up to 3 days in Medium 199 with Hanks' salts (control) or in medium with PAN. Semiquantitative SEM analysis of glomeruli on the upper surface of kidney slices indicated that incubation with PAN (100 micrograms/ml and 500 micrograms/ml) decreased the number of microvilli on podocyte cell bodies (days 1, 2 and 3), increased the number of glomeruli showing flattening of podocyte cell bodies and major processes (days 2 and 3), and increased the number of glomeruli showing surface membrane blebbing on podocyte foot processes (day 3) (p less than 0.001 in all cases). TEM morphometry revealed that incubation with 500 micrograms/ml PAN retarded significantly (p less than 0.001 at days 2 and 3) the loss of podocyte foot processes observed in control cultures. Whilst the SEM changes to podocyte ultrastructure largely mimic those seen in PAN nephrosis in vivo, the retardation of foot process loss runs counter to the major TEM change observed in vivo.

Ultrastructure of the pronephric kidney in upstream migrant sea lamprey, Petromyzon marinus L

The pronephric kidneys were examined in upstream migrant sea lampreys, Petromyzon marinus L., by transmission and scanning electron microscopy. Each pronephros consists of an enlarged renal corpuscle (glomus) and ciliated nephrostomes, but there are no renal tubules. The renal corpuscle contains an extensive mesangium, which consists of a highly fibrous extracellular matrix, numerous mesangial cells, granulocytes, and macrophages. The extracellular matrix contains microfibrils with a morphology similar to amyloid P microfibrils, fibrils with a periodicity similar to fibrin, and abundant collagen. Often these fibrillar components are aggregated in the region of the basement membrane, giving it a thickened appearance. Some podocytes of the visceral epithelium appear swollen, and their cytoplasm contains numerous vacuolar inclusions, and many have only primary major processes with only a few or no foot processes. The morphological features of the pronephric kidney of the lamprey at this time in the life cycle reflect the regression of this organ, but some features also resemble those seen in renal pathologies of higher vertebrates.