Claudin 1 and nephrin label cellular crescents in diabetic glomerulosclerosis

Association of the podocyte phenotype with extracapillary hypercellularity in patients with diabetic kidney disease

BACKGROUND

Extracapillary hypercellularity was recently identified as a poor prognostic factor for diabetic kidney disease (DKD), but its nature, pathogenesis, and relationship with glomerular sclerosis are still unclear.

METHODS

We retrospectively studied 107 patients with biopsy-proven DKD, recruited from January 2018 through December 2020. We compared the clinicopathologic characteristics of 25 patients with extracapillary hypercellularity lesions (the extracapillary hypercellularity group) to those of 82 patients without extracapillary hypercellularity (the control group). Multiple cell-specific markers were used for immunohistochemical staining to analyse the types of cells that exhibited extracapillary hypercellularity. Podocyte phenotype changes were evaluated via immunohistochemical staining for Synaptopodin and Nephrin, and foot process width was measured via transmission electron microscopy.

RESULTS

Patients with extracapillary hypercellularity lesions had more severe clinical features than patients without extracapillary hypercellularity in DKD, as indicated by elevated proteinuria and serum creatinine levels, and decreased serum albumin. Pathologically, extracapillary hypercellularity was accompanied by increased mesangial hyperplasia and interstitial fibrosis. Severe obliterative microvascular disease was observed more frequently in the extracapillary hypercellularity group than in the control group. At cell type analysis, 25 patients in the DKD-extracapillary hypercellularity group showed that a mixture of cells expressed either Wilm's tumor-1 or paired box protein 2. Furthermore, DKD-extracapillary hypercellularity patients had significant loss of podocyte phenotype and severe foot process effacement. Cells in extracapillary hypercellularity had increased hypoxia-induced factor-1 alpha expression.

CONCLUSIONS

Extracapillary hypercellularity is associated with severe renal dysfunction and renal sclerosis. Vascular damage is closely related to severe podocyte hypoxia injury and requires additional attention in future research.

Glomerular crescents are associated with the risk of type 2 diabetic kidney disease progression: a retrospective cohort study

BACKGROUND

Diabetic kidney disease (DKD) stands as the predominant cause of chronic kidney disease and end-stage kidney disease. Its diverse range of manifestations complicates the treatment approach for patients. Although kidney biopsy is considered the gold standard for diagnosis, it lacks precision in predicting the progression of kidney dysfunction. Herein, we addressed whether the presence of glomerular crescents is linked to the outcomes in patients with biopsy-confirmed type 2 DKD.

METHODS

We performed a retrospective evaluation, involving 327 patients diagnosed with biopsy-confirmed DKD in the context of type 2 diabetes, excluding cases with other glomerular diseases, from nine tertiary hospitals. Hazard ratios (HRs) were calculated using a Cox regression model to assess the risk of kidney disease progression, defined as either ≥ 50% decrease in estimated glomerular filtration rates or the development of end-stage kidney disease, based on the presence of glomerular crescents.

RESULTS

Out of the 327 patients selected, ten patients had glomerular crescents observed in their biopsied tissues. Over the follow-up period (median of 19 months, with a maximum of 18 years), the crescent group exhibited a higher risk of kidney disease progression than the no crescent group, with an adjusted HR of 2.82 (1.32-6.06) (P = 0.008). The presence of heavy proteinuria was associated with an increased risk of developing glomerular crescents.

CONCLUSION

The presence of glomerular crescents is indeed linked to the progression of type 2 DKD. Therefore, it is important to determine whether there is an additional immune-mediated glomerulonephritis requiring immunomodulation, and it may be prudent to monitor the histology and repeat a biopsy.

[Non-Classical Clinical Types and Pathological Changes of Diabetic Kidney Disease: A Review]

Diabetic kidney disease (DKD) is a common complication of diabetes mellitus and approximately 1/3 of diabetic patients may progress to DKD. A typical early clinical manifestation of DKD is microalbuminuria and patients may present with macroproteinuria accompanied by a decrease in renal function condition as the disease progresses. It is generally believed that the likelihood of a reversal of the disease is reduced after the development of macroproteinuria in patients with DKD, and that eventually some patients' condition may develop into end-stage renal disease (ESRD). Moreover, the thickening of the glomerular basement membrane, mesangial matrix expansion, Kimmelstiel-Wilson (K-W) nodules, and glomerulosclerosis in end-stage diabetes mellitus are typical pathologic changes of DKD. However, some DKD patients, especially those with type 2 diabetes mellitus (T2DM) combined with DKD, may have diverse clinical manifestations, showing variations in disease progression and regression, and manifesting as non-classical types of DKD, such as normoalbuminuric DKD, proteinuria-reduced DKD, and DKD with rapid decline in renal function. In addition, the formation of crescents, a special pathological change, is observed in renal biopsy. However, this issue is currently under-recognized by clinicians and therefore deserves more attention. In order to improve clinicians' understanding of the presentations and pathological changes of non-classical DKD and the level of DKD prevention and treatment in China, we present a preliminary introduction to the clinical phenotypes and pathological changes of non-classical types of DKD in this paper by summarizing the findings of our prior studies as well as domestic and international literature.

Sex Differences in Glomerular Lesions, in Atherosclerosis Progression, and in the Response to Angiotensin-Converting Enzyme Inhibitors in the ApoE-/- Mice Model

This study analyzes sex-based differences in renal structure and the response to the Angiotensin-Converting Enzyme (ACE) inhibitor enalapril in a mouse model of atherosclerosis. Eight weeks old ApoE-/- mice received enalapril (5 mg/kg/day, subcutaneous) or PBS (control) for an additional 14 weeks. Each group consisted of six males and six females. Females exhibited elevated LDL-cholesterol levels, while males presented higher creatinine levels and proteinuria. Enalapril effectively reduced blood pressure in both groups, but proteinuria decreased significantly only in females. Plaque size analysis and assessment of kidney inflammation revealed no significant sex-based differences. However, males displayed more severe glomerular injury, with increased mesangial expansion, mesangiolysis, glomerular foam cells, and activated parietal epithelial cells (PECs). Enalapril mitigated mesangial expansion, glomerular inflammation (particularly in the female group), and hypertrophy of the PECs in males. This study demonstrates sex-based differences in the response to enalapril in a mouse model of atherosclerosis. Males exhibited more severe glomerular injury, while enalapril provided renal protection, particularly in females. These findings suggest potential sex-specific considerations for ACE inhibitor therapy in chronic kidney disease and atherosclerosis cardiovascular disease. Further research is needed to elucidate the underlying mechanism behind these observations.

Crescents, an Independent Risk Factor for the Progression of Type 2 Diabetic Kidney Disease

CONTEXT

Crescents have been noticed in pathologic changes in patients with diabetic kidney disease (DKD). However, the clinical significance of crescents is still not well recognized.

OBJECTIVE

The main objective was to investigate the association between crescents and the prognoses of type 2 DKD (T2DKD) patients, and, secondly, to analyze the relationship between crescents and clinicopathologic features.

METHODS

A retrospective cohort study of 155 patients with T2DKD diagnosed by renal biopsy was carried out in a single center. Clinicopathologic features of patients with or without crescents were analyzed. Cox regression models and meta-analysis were used to determine the prognostic values of crescents for T2DKD. A nomogram was constructed to provide a simple estimation method of 1, 3, and 5-year renal survival for patients with T2DKD.

RESULTS

Compared with T2DKD patients without crescents, patients with crescents had higher 24-hour proteinuria and serum creatinine levels, as well as more severe Kimmelstiel-Wilson (K-W) nodules, segmental sclerosis (SS), and mesangiolysis (all P < .05). Furthermore, the crescents were positively correlated with serum creatinine, 24-hour proteinuria, K-W nodules, SS, mesangiolysis, and complement 3 deposition. Multivariate Cox models showed that crescents were an independent prognostic risk factor for renal survival (hazard ratio [HR] 2.68, 95% CI 1.27-5.64). The meta-analyzed results of 4 studies on crescents in T2DKD confirmed that patients with crescents had a significantly higher HR for renal progression.

CONCLUSION

Patients with crescents in T2DKD have more severe clinicopathologic changes and worse prognoses. The crescent can serve as an independent risk factor for T2DKD progression.

Cell Hypertrophy: A “Biophysical Roadblock” to Reversing Kidney Injury

In anamniotes cell loss can typically be compensated for through proliferation, but in amniotes, this capacity has been significantly diminished to accommodate tissue complexity. In order to cope with the increased workload that results from cell death, instead of proliferation highly specialised post-mitotic cells undergo polyploidisation and hypertrophy. Although compensatory hypertrophy is the main strategy of repair/regeneration in various parenchymal tissues, the long-term benefits and its capacity to sustain complete recovery of the kidney has not been addressed sufficiently. In this perspective article we integrate basic principles from biophysics and biology to examine whether renal cell hypertrophy is a sustainable adaptation that can efficiently regenerate tissue mass and restore organ function, or a maladaptive detrimental response.

IL-6/STAT3 signaling activation exacerbates high fructose-induced podocyte hypertrophy by ketohexokinase-A-mediated tristetraprolin down-regulation

Glomerular hypertrophy is a crucial factor of severe podocyte damage and proteinuria. Our previous study showed that high fructose induced podocyte injury. The current study aimed to explore a novel molecular mechanism underlying podocyte hypertrophy induced by high fructose. Here we demonstrated for the first time that high fructose significantly initiated the hypertrophy in rat glomeruli and differentiated human podocytes (HPCs). Consistently, it induced inflammatory response with the down-regulation of anti-inflammatory factor zinc-finger protein tristetraprolin (TTP) and the activation of interleukin-6 (IL-6)/signal transducer and activator of transcription 3 (STAT3) signaling in these animal and cell models. Subsequently, high-expression of microRNA-92a-3p (miR-92a-3p) and its target protein cyclin-dependent kinase inhibitor p57 (P57) down-regulation, representing abnormal proliferation and apoptosis, were observed in vivo and in vitro. Moreover, high fructose increased ketohexokinase-A (KHK-A) expression in rat glomeruli and differentiated HPCs. Exogenous IL-6 stimulation up-regulated IL-6/STAT3 signaling and miR-92a-3p, reduced P57 expression and promoted podocyte proliferation, apoptosis and hypertrophy in vitro. The data from anti-inflammatory agent maslinic acid treatment or TTP siRNA transfection showed that high fructose may decrease TTP to activate IL-6/STAT3 signaling in podocyte overproliferation and apoptosis, causing podocyte hypertrophy. Whereas, KHK-A siRNA transfection remarkably restored high fructose-induced TTP down-regulation, IL-6/STAT3 signaling activation, podocyte overproliferation, apoptosis and hypertrophy in differentiated HPCs. Taken together, these results suggested that high fructose possibly increased KHK-A expression to down-regulate TTP, subsequently activated IL-6/STAT3 signaling to interfere with podocyte proliferation and apoptosis by up-regulating miR-92a-3p to suppress P57 expression, causing podocyte hypertrophy. Therefore, the inactivation of IL-6/STAT3 to relieve podocyte hypertrophy mediated by inhibiting KHK-A to increase TTP may be a novel strategy for high fructose diet-associated podocyte injury and proteinuria.

Diabetic condition induces hypertrophy and vacuolization in glomerular parietal epithelial cells

Diabetic nephropathy (DN) is accompanied by characteristic changes in the glomerulus, but little is known about the effect of diabetes on parietal epithelial cells (PECs). In this study, a descriptive analysis of PECs was undertaken in diabetic db/db mice and in diabetic patients. PEC hypertrophy was significantly more prominent in diabetic mice than in nondiabetic mice, and this was evident even at the early stage. Additionally, the number of vacuoles in PECs was markedly increased in diabetic mice, suggesting the presence of cellular injury in PECs in DN. Although rare, binuclear cells were observed in mice with early diabetes. In cultured PECs, a high glucose condition, compared with normal glucose condition, induced cellular hypertrophy and apoptosis. Flow cytometry showed that some PECs in the G0 phase reentered the cell cycle but got arrested in the S phase. Finally, in human diabetic subjects, hypertrophy and vacuolization were observed in the PECs. Our data showed that PECs undergo substantial changes in DN and may participate in rearrangement for differentiation into podocytes.

Podocyte-specific knockout of the neonatal Fc receptor (FcRn) results in differential protection depending on the model of glomerulonephritis

Podocytes have been proposed to be antigen presenting cells (APCs). In traditional APCs, the neonatal Fc receptor (FcRn) is required for antigen presentation and global knockout of FcRn protects against glomerulonephritis. Since podocytes express FcRn, we sought to determine whether the absence of podocyte FcRn ameliorates immune-mediated disease. We examined MHCII and costimulatory markers expression in cultured wild type (WT) and FcRn knockout (KO) podocytes. Interferon gamma (IFNγ) induced MHCII expression in both WT and KO podocytes but did not change CD80 expression. Neither WT nor KO expressed CD86 or inducible costimulatory ligand (ICOSL) at baseline or with IFNγ. Using an antigen presentation assay, WT podocytes but not KO treated with immune complexes induced a modest increase in IL-2. Induction of the anti-glomerular basement membrane (anti-GBM) model resulted in a significant decrease in glomerular crescents in podocyte-specific FcRn knockout mouse (podFcRn KO) versus controls but the overall percentage of crescents was low. To examine the effects of the podocyte-specific FcRn knockout in a model with a longer autologous phase, we used the nephrotoxic serum nephritis (NTS) model. We found that the podFcRn KO mice had significantly reduced crescent formation and glomerulosclerosis compared to control mice. This study demonstrates that lack of podocyte FcRn is protective in immune mediated kidney disease that is dependent on an autologous phase. This study also highlights the difference between the anti-GBM model and NTS model of disease.

Crescent-Like Lesions as an Early Signature of Nephropathy in a Rat Model of Prediabetes Induced by a Hypercaloric Diet

Diabetic nephropathy (DN) is a major microvascular complication of diabetes. Obesity and hyperlipidemia, fueled by unhealthy food habits, are risk factors to glomerular filtration rate (GFR) decline and DN progression. Several studies recommend that diabetic patients should be screened early (in prediabetes) for kidney disease, in order to prevent advanced stages, for whom the current interventions are clearly inefficient. This ambition greatly depends on the existence of accurate early biomarkers and novel molecular targets, which only may arise with a more thorough knowledge of disease pathophysiology. We used a rat model of prediabetes induced by 23 weeks of high-sugar/high-fat (HSuHF) diet to characterize the phenotype of early renal dysfunction and injury. When compared with the control animals, HSuHF-treated rats displayed a metabolic phenotype compatible with obese prediabetes, displaying impaired glucose tolerance and insulin sensitivity, along with hypertriglyceridemia, and lipid peroxidation. Despite unchanged creatinine levels, the prediabetic animals presented glomerular crescent-like lesions, accompanied by increased kidney Oil-Red-O staining, triglycerides content and mRNA expression of IL-6 and iNOS. This model of HSuHF-induced prediabetes can be a useful tool to study early features of DN, namely crescent-like lesions, an early signature that deserves in-depth elucidation.

Clinicopathological features and outcomes of diabetic kidney disease with extracapillary hypercellularity: a Japanese single-center experience

BACKGROUND

The prognostic significance of glomerular extracapillary hypercellularity (EXHC) in diabetic kidney disease (DKD) is unclear. The aim of this study was to investigate the clinicopathological features and outcomes of DKD patients with EXHC.

METHODS

We studied 70 cases of renal biopsy-confirmed type 2 DKD that were diagnosed between 2004 and 2014 and compared the clinicopathological features and outcomes of 22 patients with EXHC (EXHC group) with those of 48 patients without EXHC (control group). All of the patients were Japanese. We assessed the renal biopsy specimens based on the Renal Pathology Society classification system. Clinical and laboratory data were collected at the time of the renal biopsy, and renal outcomes were assessed based on progression to end-stage renal disease (ESRD) requiring renal replacement therapy. The median duration of the observation period was 3 years.

RESULTS

In pathological features, nodular sclerosis (Kimmelstiel-Wilson lesions) was observed more frequently in the EXHC group than in the control group (63.6% vs. 35.4%, P = 0.027). There were no significant intergroup differences in clinical features or renal outcomes. Univariate and multivariate Cox regression analyses of all patients showed that a high level of proteinuria, a low initial eGFR, and severe interstitial inflammation were poor prognostic factors.

CONCLUSIONS

EXHC is related to nodular sclerosis, which is a known risk factor for ESRD. Careful observation is needed during the follow-up of DKD patients with EXHC, although there were no significant differences in renal outcomes between the EXHC and control groups.

Graft Growth and Podocyte Dedifferentiation in Donor-Recipient Size Mismatch Kidney Transplants

Background

Kidney transplantation is the treatment choice for patients with end-stage renal diseases. Because of good long-term outcome, pediatric kidney grafts are also accepted for transplantation in adult recipients despite a significant mismatch in body size and age between donor and recipient. These grafts show a remarkable ability of adaptation to the recipient body and increase in size in a very short period, presumably as an adaptation to hyperfiltration.

Methods

We investigated renal graft growth as well as glomerular proliferation and differentiation markers Kiel-67, paired box gene 2 and Wilms tumor protein (WT1) expression in control biopsies from different transplant constellations: infant donor for infant recipient, infant donor for child recipient, infant donor for adult recipient, child donor for child recipient, child donor for adult recipient, and adult donor for an adult recipient.

Results

We detected a significant increase in kidney graft size after transplantation in all conditions with a body size mismatch, which was most prominent when an infant donated for a child. Podocyte WT1 expression was comparable in different transplant conditions, whereas a significant increase in WT1 expression could be detected in parietal epithelial cells, when a kidney graft from a child was transplanted into an adult. In kidney grafts that were relatively small for the recipients, we could detect reexpression of podocyte paired box gene 2. Moreover, the proliferation marker Kiel-67 was expressed in glomerular cells in grafts that increased in size after transplantation.

Conclusions

Kidney grafts rapidly adapt to the recipient size after transplantation if they are transplanted in a body size mismatch constellation. The increase in transplant size is accompanied by an upregulation of proliferation and dedifferentiation markers in podocytes. The different examined conditions exclude hormonal factors as the key trigger for this growth so that most likely hyperfiltration is the key trigger inducing the rapid growth response.

Therapeutic Mechanism of Glucocorticoids on Cellular Crescent Formation in Patients With Antiglomerular Basement Membrane Disease

BACKGROUND

This study aimed to explore the therapeutic mechanism of glucocorticoids (GCs) in antiglomerular basement membrane disease.

MATERIALS AND METHODS

Thirty-four patients with biopsy-proven antiglomerular basement membrane nephritis were divided into the following 2 groups: group 1 (patients treated with GCs, n = 22) and group 2 (patients who were not treated with GCs, n = 12). The expression of parietal epithelial cells (PECs), activated PECs and glucocorticoid receptors (GRs) was examined quantitatively and compared between the 2 groups. Correlations between GR expression in glomeruli and patients' clinicopathological indices were also analyzed.

RESULTS

Compared with patients in group 2, patients in group 1 showed lower levels of serum creatinine (SCr) (P = 0.03), average cellular crescent percentage (P = 0.005) and macrophages infiltrating in renal interstitium (P = 0.03). PECs (P = 0.007) and activated PECs (P = 0.03) were strongly detected in the cellular components of classic crescents, and both were significantly reduced in group 1 compared to group 2. GR expression either in glomeruli (P = 0.01) or interstitium (P = 0.009) was lower in group 1 after GCs treatment than in group 2. Additionally, GR expression in glomeruli was strongly correlated with renal function (SCr: r = 0.45, P = 0.009; eGFR: r = -0.35, P = 0.046), the proportion of cellular crescents (r = 0.67, P < 0.001), PECs (r = 0.64, P < 0.001) and activated PECs (r = 0.72, P < 0.001), and the degree of interstitial (r = 0.50, P = 0.004) and glomerular (r = 0.49, P = 0.007) macrophage infiltration.

CONCLUSIONS

GCs might exert their therapeutic effects via inhibiting the activation and proliferation of PECs, as well as macrophage infiltration, which could contribute to crescent formation and determine renal survival. GRs are involved in this process as well.

Podocyte and Parietal Epithelial Cell Interactions in Health and Disease

The glomerulus has 3 resident cells namely mesangial cells that produce the mesangial matrix, endothelial cells that line the glomerular capillaries, and podocytes that cover the outer surface of the glomerular basement membrane. Parietal epithelial cells (PrECs), which line the Bowman's capsule are not part of the glomerular tuft but may have an important role in the normal function of the glomerulus. A significant progress has been made in recent years regarding our understanding of the role and function of these cells in normal kidney and in kidneys with various types of glomerulopathy. In crescentic glomerulonephritis necrotizing injury of the glomerular tuft results in activation and leakage of fibrinogen which provides the trigger for excessive proliferation of PrECs giving rise to glomerular crescents. In cases of collapsing glomerulopathy, podocyte injury causes collapse of the glomerular capillaries and activation and proliferation of PrECs, which accumulate within the urinary space in the form of pseudocrescents. Many of the noninflammatory glomerular lesions such as focal segmental glomerulosclerosis and global glomerulosclerosis also result from podocyte injury which causes variable loss of podocytes. In these cases podocyte injury leads to activation of PrECs that extend on to the glomerular tuft where they cause segmental and/or global sclerosis by producing excess matrix, resulting in obliteration of the capillary lumina. In diabetic nephropathy, in addition to increased matrix production in the mesangium and glomerular basement membranes, increased loss of podocytes is an important determinant of long-term prognosis. Contrary to prior belief there is no convincing evidence for an active podocyte proliferation in any of the above mentioned glomerulopathies.

Association of kidney structure-related gene variants with type 2 diabetes-attributed end-stage kidney disease in African Americans

African Americans (AAs) are at higher risk for developing end-stage kidney disease (ESKD) compared to European Americans. Genome-wide association studies have identified variants associated with diabetic and non-diabetic kidney diseases. Nephropathy loci, including SLC7A9, UMOD, and SHROOM3, have been implicated in the maintenance of normal glomerular and renal tubular structure and function. Herein, 47 genes important in podocyte, glomerular basement membrane, mesangial cell, mesangial matrix, renal tubular cell, and renal interstitium structure were examined for association with type 2 diabetes (T2D)-attributed ESKD in AAs. Single-variant association analysis was performed in the discovery stage, including 2041 T2D-ESKD cases and 1140 controls (non-diabetic, non-nephropathy). Discrimination analyses in 667 T2D cases-lacking nephropathy excluded T2D-associated SNPs. Nominal associations were tested in an additional 483 T2D-ESKD cases and 554 controls in the replication stage. Meta-analysis of 4218 discovery and replication samples revealed three significant associations with T2D-ESKD at CD2AP and MMP2 (P _corr < 0.05 corrected for effective number of SNPs in each locus). Removal of APOL1 renal-risk genotype carriers revealed additional association at five loci, TTC21B, COL4A3, NPHP3-ACAD11, CLDN8, and ARHGAP24 (P _corr < 0.05). Genetic variants at COL4A3, CLDN8, and ARHGAP24 were potentially pathogenic. Gene-based associations revealed suggestive significant aggregate effects of coding variants at four genes. Our findings suggest that genetic variation in kidney structure-related genes may contribute to T2D-attributed ESKD in the AA population.

Podocyte injury and repair mechanisms

PURPOSE OF REVIEW

Podocytes are the main gatekeeper of protein filtration in the glomerulus. When podocytes work less efficiently, this translates to the appearance of proteins in the urine, a condition that, if not promptly treated, leads to progression of glomerular damage and renal failure.

RECENT FINDINGS

Novel gene mutations have been uncovered in patients with nephrotic syndrome combined with a better definition of the role of podocin mutations. Although the importance of the inflammasome pathway and of the mechanisms of autophagy in podocyte health and disease have been increasingly recognized, a precise relationship between these processes still needs to be assessed. Numerous potential therapeutic targets have been identified and numerous data support the possibility of boosting podocyte regeneration. However, translation of experimental results into the clinic could largely depend on the avoidance of undesired side-effects; nanomedicine could provide the means to target old and novel drugs specifically to the podocytes.

SUMMARY

Podocytes are key cells in the glomerulus, and their damage inevitably leads to proteinuria and glomerular dysfunction. The more is known about the causes and mechanisms of podocyte damage, the more it will be possible to find new cures for glomerular diseases of the kidney.

The phenotypes of podocytes and parietal epithelial cells may overlap in diabetic nephropathy

Reversal of diabetic nephropathy (DN) has been achieved in humans and mice, but only rarely and under special circumstances. Since progression of DN is related to podocyte loss, reversal of DN requires restoration of podocytes. Here we identified and quantified potential glomerular progenitor cells that could be a source for restored podocytes. DN was identified in 31 human renal biopsy cases and separated into morphologically early or advanced lesions. Markers of podocytes (WT-1, p57), parietal epithelial cells (claudin-1) and cell proliferation (Ki-67) were identified by immunohistochemistry. Podocyte density was progressively reduced with DN. Cells marking as podocytes (p57) were present infrequently on Bowman's capsule in controls, but significantly increased in histologically early DN. Ki-67 expressing cells were identified on the glomerular tuft and Bowman's capsule in DN, but rarely in controls. Cells marking as PECs were present on the glomerular tuft, particularly in morphologically advanced DN. These findings show evidence of phenotypic plasticity in podocyte and PEC populations and are consistent with studies in the BTBR ob/ob murine model in which reversibility of DN occurs with podocytes potentially regenerating from PEC precursors. Thus, our findings support, but do not prove, that podocytes may regenerate from PEC progenitors in human DN. If so, progression of DN may represent a modifiable net balance between podocyte loss and regeneration.

New insights into glomerular parietal epithelial cell activation and its signaling pathways in glomerular diseases

The glomerular parietal epithelial cells (PECs) have aroused an increasing attention recently. The proliferation of PECs is the main feature of crescentic glomerulonephritis; besides that, in the past decade, PEC activation has been identified in several types of noninflammatory glomerulonephropathies, such as focal segmental glomerulosclerosis, diabetic glomerulopathy, and membranous nephropathy. The pathogenesis of PEC activation is poorly understood; however, a few studies delicately elucidate the potential mechanisms and signaling pathways implicated in these processes. In this review we will focus on the latest observations and concepts about PEC activation in glomerular diseases and the newest identified signaling pathways in PEC activation.

Glomerular parietal epithelial cell activation induces collagen secretion and thickening of Bowman's capsule in diabetes

The metabolic and hemodynamic alterations in diabetes activate podocytes to increase extracellular matrix (ECM) production, leading to thickening of the glomerular basement membrane (GBM). We hypothesized that diabetes would activate parietal epithelial cells (PECs) in a similar manner and cause thickening of Bowman's capsules. Periodic acid Schiff staining of human kidney biopsies of 30 patients with diabetic nephropathy (DN) revealed a significantly thicker Bowman's capsule as compared with 20 non-diabetic controls. The average thickness was 4.55±0.21 μm in the group of patients with DN compared with 2.92±0.21 μm in the group of non-diabetic controls (P<0.001). Transmission electron microscopy confirmed this finding. In vitro, short-term exposure of human PECs to hyperglycemic conditions (30 mM glucose) advanced glycation end products (100 μg/ml) or transforming growth factor-β1 (TGF-β1; 5 ng/ml) increased the mRNA expression of collagen type I α-1, collagen type IV (all six α-chains), bamacan, nidogen 1, laminin α-1, and perlecan. Western blot and colorimetric collagen assays confirmed these results for collagen type IV at the protein level. The production and secretion of TGF-β1 as a possible positive feedback loop was excluded as a mechanism for the autocrine activation of human PECs. To validate these findings in vivo, activation of the PECs was assessed by immunohistochemical staining for CD44 of 12 human biopsy cases with DN. Thickening of the Bowman's capsule showed strong association with CD44-positive PECs. In summary, metabolic alterations in diabetes activate PECs to increase the expression and secretion of Bowman's capsule proteins. This process may contribute to the thickening of the Bowman's capsule, similar to the thickening of the GBM that is driven by activated podocytes. These data may also imply that activated PECs contribute to ECM production once they migrate to the glomerular tuft, a process resulting in glomerular scaring, for example, in diabetic glomerulosclerosis.