Comparison of Ultrastructural Features Between Patients with Mercury-associated Membranous Nephropathy and Idiopathic Membranous Nephropathy

BACKGROUND

Prolonged exposure to mercury can cause membranous nephropathy. Mercury-associated membranous nephropathy (M-MN) and idiopathic membranous nephropathy (I-MN) have similar clinical manifestations, making misdiagnoses likely. We compared the clinicopathological and ultrastructural features of M-MN and I-MN.

METHODS

We retrospectively analyzed the clinicopathological data of 13 M-MN patients and 13 I-MN patients. Electron micrographs of glomerular capillaries were taken, and foot process width (FPW) and the number of foot processes per 10 μm glomerular basement membrane (GBM) were calculated. The presence and location of electron-dense deposits were recorded.

RESULTS

Compared with I-MN patients, M-MN patients were younger (38.7 ± 8.5 versus 45.8 ± 5.7 years, P = 0.020), achieved complete remission more quickly (9.0 ± 6.1 versus 20.3 ± 9.8 months, P = 0.004), and had a lower relapse rate (0 versus 45.5%, P = 0.014). Patients with M-MN also had lower FPW (974.3 [interquartile range or IQR, 791.2-1504.4] nm versus 2370.6 [IQR, 2219.4-2559.1] nm, P = 0.001), more foot processes per 10 μm GBM (8.1 [IQR, 5.2-10.0] versus 3.3 [IQR, 3.1-3.5], P = 0.001), and a higher rate of mesangial electron-dense deposits (41.7% versus 0, P = 0.015). A cut-off FPW of <1654 nm differentiated M-MN from I-MN with high sensitivity (92.3%) and specificity (83.3%).

CONCLUSIONS

Foot process effacement was less severe in M-MN than in I-MN. In patients with mercury toxic exposure, MN with less severe foot processes effacement suggested mercury could be the cause. Better prognosis in patients with M-MN may be associated with minor podocyte damage.

Unveiling the Features of Mercury-Associated Minimal Change Disease: Comparison with Primary Minimal Change Disease

Introduction

Long-term exposure to mercury can cause minimal change disease. However, the current understanding of mercury-associated minimal change disease (M-MCD) is inadequate. To improve the understanding of M-MCD, this study retrospectively analyzed the clinicopathological, ultrastructural, and prognostic features of M-MCD, in comparison with primary minimal change disease (P-MCD).

Methods

We retrospectively analyzed the clinicopathological data of 21 M-MCD patients and 21 P-MCD patients. Electron micrographs of glomerular capillaries were taken, and the foot process width (FPW) was measured. A receiver operating characteristics (ROC) curve analysis was performed to determine the optimum cutoff value of FPW that can differentiate the M-MCD from P-MCD.

Results

M-MCD patients presented similar clinical and routine pathological characteristics with P-MCD patients but had lower levels of FPW (935.0 [interquartile range (IQR) 853.7-1,176.7] nm vs. 1,403.2 [IQR 1,089.2-1,841.8] nm, p = 0.002). ROC curve analysis showed that FPW value below 1,385 nm might help to differentiate M-MCD from P-MCD (area under the curve of 0.787, sensitivity of 94.7%, and specificity of 52.4%). For patients with M-MCD, 77.8% achieved complete remission after mercury detoxification monotherapy. Patients with M-MCD had a lower relapse rate than patients with P-MCD (0 vs. 47.1%, p = 0.003). In addition, there was no significant difference in remission time between M-MCD patients treated with mercury detoxification monotherapy and those initially treated with immunosuppressive therapy (2.0 [IQR 1.0-6.0] months vs. 2.0 [IQR 1.5-2.5] months, p = 0.606).

Conclusions

M-MCD patients showed similar clinicopathological features with P-MCD patients, but with less severe foot process effacement, suggesting different pathogenesis of these 2 disease entities. The treatment of mercury detoxification was highly effective for patients with M-MCD and can be considered as a primary choice in clinical practice.

Autoantibodies against podocytic UCHL1 are associated with idiopathic nephrotic syndrome relapses and induce proteinuria in mice

Idiopathic steroid sensitive nephrotic syndrome (INS), the most frequent childhood nephropathy, is thought to be mediated by a circulating soluble factor that reversibly affects the renal protein sieving. The efficiency of rituximab therapy recently highlighted the involvement of B cells. Here we studied the involvement of a specific immunoglobulin G (IgG) in the disease. After plasma fractionation by size exclusion chromatography, a detachment of cultured podocyte was observed with one IgG-containing fraction from 47% patients in relapse, 9% of patients in remission and 0% of controls. Podocyte protein lysates were immunoprecipitated by IgG from those plasma fractions identifying a list of 41 podocyte proteins after proteomic analysis. Five podocyte targets were selected on statistical and biological criteria. Specific antibodies were tested and only anti-Ubiquitin Carboxyl-Terminal Hydrolase L1 (UCHL1) IgG led to podocyte detachment. UCHL1 was mainly found inside the podocyte but also weakly expressed on podocyte cell surface. Incubation of either anti-UCHL1 IgG or plasma fractions with recombinant UCHL1 prevented podocyte detachment. Plasma levels of anti-UCHL1 IgG were significantly increased in relapsing INS patients compared to patients in remission and controls. Proteinuria correlated with anti-UCHL1 IgG level at various stages of the disease. Purified patient anti-UCHL1 antibodies induced proteinuria and podocyte foot effacement in mice. Altogether, these results identified UCHL1 as a target podocyte protein of autoantibodies in a set of relapsing patients and support a causative role of anti-UCHL1 autoantibodies in the development of INS.

Mechanical challenges to the glomerular filtration barrier: adaptations and pathway to sclerosis

Podocytes are lost as viable cells by detachment from the glomerular basement membrane (GBM), possibly due to factors such as pressure and filtrate flow. Distension of glomerular capillaries in response to increased pressure is limited by the elastic resistance of the GBM. The endothelium and podocytes adapt to changes in GBM area. The slit diaphragm (SD) seems to adjust by shuttling SD components between the SD and the adjacent foot processes (FPs), resulting in changes in SD area that parallel those in perfusion pressure.Filtrate flow tends to drag podocytes towards the urinary orifice by shear forces, which are highest within the filtration slits. The SD represents an atypical adherens junction, mechanically interconnecting the cytoskeleton of opposing FPs and tending to balance the shear forces.If under pathological conditions, increased filtrate flows locally overtax the attachment of FPs, the SDs are replaced by occluding junctions that seal the slits and the attachment of podocytes to the GBM is reinforced by FP effacement. Failure of these temporary adaptive mechanisms results in a steady process of podocyte detachment due to uncontrolled filtrate flows through bare areas of the GBM and, subsequently, the labyrinthine subpodocyte spaces, presenting as pseudocysts. In our view, shear stress due to filtrate flow-not capillary hydrostatic pressure-is the major challenge to the attachment of podocytes to the GBM.

Podocyte directed therapy of nephrotic syndrome-can we bring the inside out?

Several of the drugs currently used for the treatment of glomerular diseases are prescribed for their immunotherapeutic or anti-inflammatory properties, based on the current understanding that glomerular diseases are mediated by immune responses. In recent years our understanding of podocytic signalling pathways and the crucial role of genetic predispositions in the pathology of glomerular diseases has broadened. Delineation of those signalling pathways supports the hypothesis that several of the medications and immunosuppressive agents used to treat glomerular diseases directly target glomerular podocytes. Several central downstream signalling pathways merge into regulatory pathways of the podocytic actin cytoskeleton and its connection to the slit diaphragm. The slit diaphragm and the cytoskeleton of the foot process represent a functional unit. A breakdown of the cytoskeletal backbone of the foot processes leads to internalization of slit diaphragm molecules, and internalization of slit diaphragm components in turn negatively affects cytoskeletal signalling pathways. Podocytes display a remarkable ability to recover from complete effacement and to re-form interdigitating foot processes and intact slit diaphragms after pharmacological intervention. This ability indicates an active inside-out signalling machinery which stabilizes integrin complex formations and triggers the recycling of slit diaphragm molecules from intracellular compartments to the cell surface. In this review we summarize current evidence from patient studies and model organisms on the direct impact of immunosuppressive and supportive drugs on podocyte signalling pathways. We highlight new therapeutic targets that may open novel opportunities to enhance and stabilize inside-out pathways in podocytes.

Surges in proteinuria are associated with plasma GL-3 elevations in a young patient with classic Fabry disease

Fabry disease is an X-linked glycosphingolipidosis caused by deficient synthesis of the enzyme α-galactosidase A, which results in accumulations of globotriaosylceramide (GL-3) in systemic tissues. Nephropathy is a dominant feature of Fabry disease. It still remains unclear how the nephropathy progresses. Recombinant agalsidase replacement therapy is currently the only approved, specific therapy for Fabry disease. The optimal dose of replacement enzyme also still remains unclear. The worldwide shortage of agalsidase-β in 2009 forced dose reduction of administration. It showed that the proteinuria emerged like surges, followed by temporary plasma GL-3 elevations in the early stages of classic Fabry disease. Additionally, it also showed that 1 mg/kg of agalsidase-β every other week could clear the GL-3 accumulations from podocytes and was required to maintain negative proteinuria and normal plasma GL-3 levels.

CONCLUSION

This observation of a young patient with classic Fabry disease about 5 years reveals that the long-term, low-dose agalsidase-β caused proteinuria surges, but not persistent proteinuria, followed by temporary plasma GL-3 elevations, and agalsidase-β at 1 mg/kg every other week could clear accumulated GL-3 from podocytes and was required to maintain normal urinalysis and plasma GL-3 levels.

Podocyte and endothelial injury in focal segmental glomerulosclerosis: an ultrastructural analysis

Podocyte injury contributes to the development of focal segmental glomerulosclerosis (FSGS). Endocapillary hypercellularity, which is one of the pathological characteristics of FSGS, suggests that glomerular endothelial injury may also be involved in the pathogenesis of FSGS. In electron micrographs of patients with FSGS (n = 43), we conducted morphometric measurements of foot process width (FPW) and podocyte detachment (PD) as markers of podocyte injury and subendothelial widening (SW) of the glomerular basement membrane as a marker of endothelial injury and compared them to those in patients with minimal change nephrotic syndrome (MCNS; n = 11) and control kidney donors (n = 5). Associations between ultrastructural and clinical parameters were analyzed according to the FSGS variants defined by the Columbia classification. FPW was significantly higher in the FSGS group than that in the MCNS and control groups, particularly in the collapsing, tip, and cellular variants of FSGS. Percentage of glomerular basement membrane (GBM) length showing PD and SW was significantly increased in the FSGS group, especially in the collapsing, cellular, and not otherwise specified variants. In FSGS, FPW was inversely correlated with disease duration, but not with proteinuria. Finally, the percentage of GBM length with SW significantly correlated with clinical parameters indicative of poor prognosis, such as lower remission rate and lower estimated glomerular filtration rate at the final observation. Quantitative measurement of podocyte and endothelial injury by electron microscopy might be useful for evaluating histological activity and predicting prognosis in FSGS.

Correlation of proteinuria with podocyte foot process effacement in IgA nephropathy: an ultrastructural study

BACKGROUND

Proteinuria is an uncommon clinical manifestation of IgA nephropathy and is usually seen in cases with severe lesions like endocapillary proliferation. However, it is occasionally seen even with cases with mild glomerular manifestations and may even be of nephrotic range.

PREDICTOR

Podocyte foot process effacement.

OUTCOME

Severity of proteinuria.

MEASUREMENTS

Podocyte foot process effacement was measured. Morphometric analysis was performed on transmission electron microscope images using a computerized digital photomicrograph system (BioWizard 4.2 Image analysis software, New Delhi, India). Proteinuria was measured quantitatively assigned into five grades.

RESULTS

It was found that as the extent of proteinuria increased, the effacement ratio also increased, and this was most significant between "no" proteinuria and the rest of the categories.

CONCLUSION

Nephrotic presentation in IgA nephropathy is a known phenomenon and in certain cases may show near normal glomerular morphology with severe foot process effacement on EM being the only significant finding to explain the proteinuria. Proteinuria in these cases shows a significant correlation with degree of foot process effacement. Renal biopsy is important in these cases because they are known to have a better prognosis and are usually steroid responsive.