ELISA analysis of urinary nephrin and podocalyxin standardized by aquaporin-2 in adult patients with nephrotic syndrome

Urinary podocyte markers in kidney diseases

Podocytes play an important role in the maintenance of kidney function, and they are the primary focus of many kidney diseases. Podocyte injury results in the shedding of podocyte-derived cellular fragments and podocyte-specific molecular targets into the urine, which may serve as biomarkers of kidney diseases. Intact podocytes, either viable or dead, and podocyte-derived microvesicles could be quantified in the urine by various centrifugation, visualization and culture methods. Podocyte-specific protein targets from the nucleus, cytoplasm, slit-diaphragm, glomerular capillary basement membrane, and cytoskeleton, as well as their corresponding messenger RNA (mRNA), in the urine could be quantified by western blotting, ELISA, or quantitative polymerase chain reaction. Although some of these techniques may be expensive or labor-intensive at present, they may become widely available in the future because of the improvement in technology and automation. The application of urinary podocyte markers for the diagnosis and monitoring of various kidney diseases have been explored but the published data in this area are not sufficiently systematic and lack external validation. Further research should focus on standardizing, comparing, and automizing laboratory methods, as well as defining their added value to the routine clinical tests.

Evaluation of the urinary podocalyxin and nephrin excretion levels to determine a safe time interval between two sessions of SWL for renal stones: a non randomized exploratory study

OBJECTIVES

We aimed to evaluate the role of nephrin and podocalyxin in determining the intervals between shock wave lithotripsy (SWL) sessions and how soon the kidney damage was recovered.

METHODS

This work was a prospective study that included 30 patients with unilateral kidney stones. The patients' midflow urine samples were collected before SWL and 1 h, 1 day and 1 week after the procedure. Nephrin and podocalyxin levels in the urine samples were measured by the enzyme-linked immunosorbent assay method.

RESULTS

Among the 30 patients who underwent SWL, 19 were males and 11 were females. The mean age of the SWL group was 34.7 ± 13.2. Both biomarkers did not correlate with age, creatinine values, body mass index, stone side, stone size, energy, frequency and shock numbers. Nephrin and podocalyxin levels were significantly higher at the pre-SWL point (p < 0.05). After the procedure, a significant decrease was observed in both biomarker levels (p < 0.05). At the end of first day, these levels started to increase progressively up to the end of the first week (p  > 0.05).

CONCLUSIONS

Nephrin and podocalyxin may help to determine early period kidney damage associated with SWL. Post-SWL podocalyxin and nephrin values may be used to determine the interval between SWL sessions.

[Value of podocalyxin levels in urinary extracellular vesicles for diagnosis of diabetic nephropathy]

OBJECTIVE

To explore the value of detecting podocalyxin (PCX) level in urinary extracellular vesicles for the diagnosis of diabetic nephropathy.

METHODS

This study was conducted among 57 diabetic patients admitted during the period from March to September, 2017, including 34 with uncomplicated diabetics and 23 with diabetic nephropathy; 21 patients with other types of nephropathy and 11 healthy individuals were also included to serve as the controls. Transmission electron microscopy (TEM) and nanoparticle tracking analysis (NTA) were used to verify the separation of urinary extracellular vesicles. The molecular markers of extracellular vesicles (TSG101 and podocalyxin [PCX]) were detected using Western blotting. PCX levels in extracellular vesicles were also detected using ELISA.

RESULTS

TEM reveal the presence of numerous extracellular vesicles in the urine with intact morphology and different sizes, and most of them were below 300 nm in diameter as shown by NTA. TSG101 expression was detected in the samples from all the 4 groups. Positive expression of PCX was detected in the samples from patients with diabetic nephropathy but not in the other groups. In patients with diabetic nephropathy, the mean PCX levels (3.27±2.30 ng/μmol)was significantly higher than those in the healthy control group (1.22±0.36 ng/μmol), uncomplicated diabetes group (2.22±1.29 ng/μmol) and nephropathy group (1.24±0.45 ng/μmol).

CONCLUSIONS

PCX level in urinary extracellular vesicles is significantly increased in patients with diabetic nephropathy, suggesting the value of PCX as a potential marker for clinical diagnosis of diabetic nephropathy.

The first identified heterozygous nonsense mutations in podocalyxin offer new perspectives on the biology of podocytopathies

In the last two decades, our understanding of the genetic underpinnings of inherited podocytopathies has advanced immensely. By sequencing the genomes of a large pool of families affected by focal segmental glomerulosclerosis (FSGS), researchers have identified a common theme: familial podocytopathies are frequently caused by genes selectively expressed in podocytes. Podocalyxin is a podocyte-specific surface sialomucin that has long been known to play important roles in podocyte morphogenesis and function. Few studies, however, have shown a conclusive link between mutations in the gene and FSGS complemented by functional evidence. In a fascinating new paper published in Clinical Science, Lin et al. identify two unrelated pedigrees in which dominant loss-of-function mutations in PODXL lead to adult-onset FSGS. Nonsense-mediated decay of the mutated PODXL transcripts leads to protein insufficiency, which in turn cause podocyte dysfunction through defects in motility and cytoskeletal organization. This is the first study to date that demonstrates, mechanistically, how autosomal dominant mutations in podocalyxin can lead to FSGS and renal insufficiency. Here, we summarize the experimental findings of this manuscript and propose, perhaps, a more controversial hypothesis: down-regulation of podocalyxin protein expression from podocytes is a critical turning point in the progression of most podocytopathies and may be mechanistically relevant to glomerulopathies in which podocyte damage is not necessarily induced by genetic lesions.

Urine markers of podocyte dysfunction: a review of podocalyxin and nephrin in selected glomerular diseases

Urinary podocalyxin and nephrin are urine markers of podocyte dysfunction that may reflect the integrity of kidney's filtration barrier. Studies on their respective roles in glomerular diseases are still underway. However, the isolated and unsystematic manner in which they are being studied does not permit proper identification of their roles in each glomerular disease. As such, there is little or no appreciation of what research has already achieved and what remains to be achieved as the research direction is not clearly defined. We explored the recent studies and outlined the major findings regarding the value of both biomarkers in each of the three glomerular disease entities. Our review covered diabetic nephropathy, membranous nephropathy and IgA nephropathy.

Original Research: Potential of urinary nephrin as a biomarker reflecting podocyte dysfunction in various kidney disease models

Urinary nephrin is a potential non-invasive biomarker of disease. To date, however, most studies of urinary nephrin have been conducted in animal models of diabetic nephropathy, and correlations between urinary nephrin-to-creatinine ratio and other parameters have yet to be evaluated in animal models or patients of kidney disease with podocyte dysfunction. We hypothesized that urinary nephrin-to-creatinine ratio can be up-regulated and is negatively correlated with renal nephrin mRNA levels in animal models of kidney disease, and that increased urinary nephrin-to-creatinine ratio levels are attenuated following administration of glucocorticoids. In the present study, renal nephrin mRNA, urinary nephrin-to-creatinine ratio, urinary protein-to-creatinine ratio, and creatinine clearance ratio were measured in animal models of adriamycin nephropathy, puromycin aminonucleoside nephropathy, anti-glomerular basement membrane glomerulonephritis, and 5/6 nephrectomy. The effects of prednisolone on urinary nephrin-to-creatinine ratio and other parameters in puromycin aminonucleoside (single injection) nephropathy rats were also investigated. In all models tested, urinary nephrin-to-creatinine ratio and urinary protein-to-creatinine ratio increased, while renal nephrin mRNA and creatinine clearance ratio decreased. Urinary nephrin-to-creatinine ratio exhibited a significant negative correlation with renal nephrin mRNA in almost all models, as well as a significant positive correlation with urinary protein-to-creatinine ratio and a significant negative correlation with creatinine clearance ratio. Urinary protein-to-creatinine ratio exhibited a significant negative correlation with renal nephrin mRNA. Following the administration of prednisolone to puromycin aminonucleoside (single injection) nephropathy rats, urinary nephrin-to-creatinine ratio was significantly suppressed and exhibited a significant positive correlation with urinary protein-to-creatinine ratio. In addition, the decrease in number of glomerular Wilms tumor antigen-1-positive cells was attenuated, and urinary nephrin-to-creatinine ratio exhibited a significant negative correlation in these cells. In conclusion, these results suggest that urinary nephrin-to-creatinine ratio level is a useful and reliable biomarker for predicting the amelioration of podocyte dysfunction by candidate drugs in various kidney disease models with podocyte dysfunction. This suggestion will also be validated in a clinical setting in future studies.

An update: the role of Nephrin inside and outside the kidney

Nephrin is a key molecule in podocytes to maintain normal slit diaphragm structure. Nephin interacts with many other podocyte and slit diaphragm protein and also mediates important cell signaling pathways in podocytes. Loss of nephrin during the development leads to the congenital nephrotic syndrome in children. Reduction of nephrin expression is often observed in adult kidney diseases including diabetic nephropathy and HIV-associated nephropathy. The critical role of nephrin has been confirmed by different animal models with nephrin knockout and knockdown. Recent studies demonstrate that knockdown of nephrin expression in adult mice aggravates the progression of unilateral nephrectomy and Adriamycin-induced kidney disease. In addition to its critical role in maintaining normal glomerular filtration unit in the kidney, nephrin is also expressed in other organs. However, the exact role of nephrin in kidney and extra-renal organs has not been well characterized. Future studies are required to determine whether nephrin could be developed as a drug target to treat patients with kidney disease.

Urinary podocalyxin, the novel biomarker for detecting early renal change in obesity

BACKGROUND

The prevalence of obesity is increasing during the past decade along with obesity-related glomerulopathy (ORG), glomeruli injury due to the obesity. The major pathogenesis of ORG is the shedding of podocytes from the glomerular cell barrier into urine. Podocalyxin (PCX), a main surface antigen of podocyte, correlates well with glomerulosclerosis progression and glomerular injury severity, and might be a potential biomarker for early renal alteration in obesity. In addition, vascular endothelial growth factor (VEGF) and alpha-smooth muscle actin (α-SMA) also play a role in promoting glomerulosclerosis. The aim of this study was to explore whether obese subjects without other diseases excrete more PCX-positive (PCX+) cells than non-obese individuals, in comparison with urine protein-creatinine ratio (UPCR) and glomerular filtration rate (GFR) as traditional renal markers. Moreover, the effect of body mass index (BMI) on urinary VEGF, PCX or α-SMA positive cells was also investigated.

METHODS

Forty-eight obese and 13 non-obese adults were included. Exfoliated cells from fresh first void morning urine were harvested, stained with PCX, VEGF, and α-SMA antibody, and quantified by flow cytometry. Correlation between interested urinary biomarkers (cells positive for PCX, VEGF plus PCX and α-SMA), UPCR and GFR with BMI and metabolic risk factors were analyzed.

RESULTS

Obese patients had significantly higher PCX+ cells than non-obese [0.62 (0.00-13.13) vs. 0.15 (0.00-0.72) cells/ml × mg cr, p < 0.05]. There was no significant difference in GFR and UPCR between the groups. Of interest, BMI demonstrated a correlation with PCX+ cells (r = 0.343, p = 0.008) and cells positive for PCX plus VEGF (r = 0.374, p = 0.004).

CONCLUSION

Obese subjects without other diseases and with normal UPCR and GFR showed evidence of renal alteration through the detection of a higher number of PCX+ cells. Increasing BMI also resulted in higher number of PCX+ cells.