Plasma Soluble Urokinase Plasminogen Activator Receptor (suPAR) and CKD Progression in Children

Time to Change Our Viewpoints to Assess Renal Risks in Patients with Solitary Kidneys beyond Traditional Approaches?

Solitary functioning kidney (SFK) can be defined as the absence or hypofunction of a kidney due to acquired or congenital reasons. A congenital solitary functioning kidney (cSFK) is more common than is an acquired one (aSFK) and is characterized by the anatomical absence (agenesis) or hypofunction (hypoplasia; hypodysplasia) of one kidney from birth. Among the acquired causes, the most important is nephrectomy (Nx) (due to the donor, trauma or mass resection). Patients with SFK are at risk for the development of chronic kidney disease (CKD) in the long term. This risk potential is also significantly affected by hypertension. The relationship between hypertension and subclinical chronic inflammation is a connection that has not yet been fully clarified pathogenetically, but there are many studies highlighting this association. In recent years, studies examining different fibrosis and inflammation biomarkers in terms of the evaluation and prediction of renal risks have become increasingly popular in the literature. Oxidative stress is known to play an important role in homocysteine-induced endothelial dysfunction and has been associated with hypertension. In our study, we aimed to investigate the relationship between ambulatory blood pressure monitoring (ABPM) and urinary/serum fibrosis and inflammatory markers in patients with SFK. We prospectively investigated the relationship between ABPM results and soluble urokinase plasminogen activator receptor (suPAR), procollagen type III N-terminal peptide (PIIINP), homocysteine and other variables in 85 patients with SFK and compared them between cSFK and aSFK groups. In the etiology of SFK, a congenital or acquired origin may differ in terms of the significance of biomarkers. In particular, the serum homocysteine level may be associated with different clinical outcomes in patients with cSFK and aSFK.

Kidney fibrosis: Emerging diagnostic and therapeutic strategies

An increasing number of patients worldwide suffers from chronic kidney disease (CKD). CKD is accompanied by kidney fibrosis, which affects all compartments of the kidney, i.e., the glomeruli, tubulointerstitium, and vasculature. Fibrosis is the best predictor of progression of kidney diseases. Currently, there is no specific anti-fibrotic therapy for kidney patients and invasive renal biopsy remains the only option for specific detection and quantification of kidney fibrosis. Here we review emerging diagnostic approaches and potential therapeutic options for fibrosis. We discuss how translational research could help to establish fibrosis-specific endpoints for clinical trials, leading to improved patient stratification and potentially companion diagnostics, and facilitating and optimizing development of novel anti-fibrotic therapies for kidney patients.

Protein Biomarkers in Chronic Kidney Disease in Children-What Do We Know So Far?

Chronic kidney disease (CKD) in children is a major concern of medical care and public health as it is related to high morbidity and mortality due to progression to end-stage kidney disease (ESKD). It is essential to identify patients with a risk of developing CKD to implement therapeutic interventions. Unfortunately, conventional markers of CKD, such as serum creatinine, glomerular filtration rate (GFR) and proteinuria, have many limitations in serving as an early and specific diagnostic tool for this condition. Despite the above, they are still the most frequently utilized as we do not have better. Studies from the last decade identified multiple CKD blood and urine protein biomarkers but mostly assessed the adult population. This article outlines some recent achievements and new perspectives in finding a set of protein biomarkers that might improve our ability to prognose CKD progression in children, monitor the response to treatment, or even become a potential therapeutic target.

Current and Novel Biomarkers of Progression Risk in Children with Chronic Kidney Disease

BACKGROUND

Due to the complexity of chronic kidney disease (CKD) pathophysiology, biomarkers representing different mechanistic pathways have been targeted for the study and development of novel biomarkers. The discovery of clinically useful CKD biomarkers would allow for the identification of those children at the highest risk of kidney function decline for timely interventions and enrollment in clinical trials.

SUMMARY

Glomerular filtration rate and proteinuria are traditional biomarkers to classify and prognosticate CKD progression in clinical practice but have several limitations. Over the recent decades, novel biomarkers have been identified from blood or urine with metabolomic screening studies, proteomic screening studies, and an improved knowledge of CKD pathophysiology. This review highlights promising biomarkers associated with the progression of CKD that could potentially serve as future prognostic markers in children with CKD.

KEY MESSAGES

Further studies are needed in children with CKD to validate putative biomarkers, particularly candidate proteins and metabolites, for improving clinical management.

Prevention of cardiorenal damage: importance of albuminuria

Chronic kidney disease (CKD) is projected to become a leading global cause of death by 2040, and its early detection is critical for effective and timely management. The current definition of CKD identifies only advanced stages, when kidney injury has already destroyed >50% of functioning kidney mass as reflected by an estimated glomerular filtration rate <60 mL/min/1.73 m2 or a urinary albumin/creatinine ratio >six-fold higher than physiological levels (i.e. > 30 mg/g). An elevated urinary albumin-excretion rate is a known early predictor of future cardiovascular events. There is thus a ‘blind spot’ in the detection of CKD, when kidney injury is present but is undetectable by current diagnostic criteria, and no intervention is made before renal and cardiovascular damage occurs. The present review discusses the CKD ‘blind spot’ concept and how it may facilitate a holistic approach to CKD and cardiovascular disease prevention and implement the call for albuminuria screening implicit in current guidelines. Cardiorenal risk associated with albuminuria in the high-normal range, novel genetic and biochemical markers of elevated cardiorenal risk, and the role of heart and kidney protective drugs evaluated in recent clinical trials are also discussed. As albuminuria is a major risk factor for cardiovascular and renal disease, starting from levels not yet considered in the definition of CKD, the implementation of opportunistic or systematic albuminuria screening and therapy, possibly complemented with novel early biomarkers, has the potential to improve cardiorenal outcomes and mitigate the dismal 2040 projections for CKD and related cardiovascular burden.

The Perspective of Vitamin D on suPAR-Related AKI in COVID-19

The coronavirus disease 2019 (COVID-19) pandemic has claimed the lives of millions of people around the world. Severe vitamin D deficiency can increase the risk of death in people with COVID-19. There is growing evidence that acute kidney injury (AKI) is common in COVID-19 patients and is associated with poorer clinical outcomes. The kidney effects of SARS-CoV-2 are directly mediated by angiotensin 2-converting enzyme (ACE2) receptors. AKI is also caused by indirect causes such as the hypercoagulable state and microvascular thrombosis. The increased release of soluble urokinase-type plasminogen activator receptor (suPAR) from immature myeloid cells reduces plasminogen activation by the competitive inhibition of urokinase-type plasminogen activator, which results in low plasmin levels and a fibrinolytic state in COVID-19. Frequent hypercoagulability in critically ill patients with COVID-19 may exacerbate the severity of thrombosis. Versican expression in proximal tubular cells leads to the proliferation of interstitial fibroblasts through the C3a and suPAR pathways. Vitamin D attenuates the local expression of podocyte uPAR and decreases elevated circulating suPAR levels caused by systemic inflammation. This decrease preserves the function and structure of the glomerular barrier, thereby maintaining renal function. The attenuated hyperinflammatory state reduces complement activation, resulting in lower serum C3a levels. Vitamin D can also protect against COVID-19 by modulating innate and adaptive immunity, increasing ACE2 expression, and inhibiting the renin–angiotensin–aldosterone system. We hypothesized that by reducing suPAR levels, appropriate vitamin D supplementation could prevent the progression and reduce the severity of AKI in COVID-19 patients, although the data available require further elucidation.

Soluble urokinase plasminogen activator receptor (suPAR) and glomerular disease in children: a narrative review

Background

Focal segmental glomerulosclerosis (FSGS) is a chronic glomerular disease that responds poorly to treatment, with a large proportion of patients progressing to end-stage renal disease in spite of initial aggressive treatment. It is worth emphasizing that the FSGS group has still a high recurrence rate after kidney transplantation. Therefore, understanding the factors involved in the pathogenesis of FSGS will help nephrologists better understand the pathogenesis as well as find out specific targeted therapies. Circulating immune factors have long been implicated in the pathogenesis of FSGS, and recent studies have suggested that soluble urokinase plasminogen activator receptor (suPAR) is one of the good candidates for this hypothesis. The aim of this review study was to analyze the value of suPAR in glomerular disease, especially in clinical studies.

Methods

In this review study, the PubMed database was searched using relevant keywords (suPAR, circulating permeability factors Children, FSGS, and children). Descriptive and cross-sectional studies were reviewed in the current study with the main focuses on the role of suPAR in FSGS, nephrotic syndrome, and the relation to progression of renal failure, especially the research in children.

Results

Overall, 32 studies from different countries were selected. These clinical studies on suPAR have shown the following: (i) the role of suPAR in the diagnosis of FSGS has not yet been confirmed, and (ii) there is strong evidence demonstrating a significant relationship between suPAR and the severity of kidney disease as well as a high value of suPAR in predicting the steroid responsiveness of nephrotic syndrome.

Conclusion

Researching on circulating permeability factors in FSGS is a current trend, which opens new avenues in targeted diagnosis and treatment. suPAR is a promising candidate, and urinary suPAR has also shown advantages over serum suPAR; therefore, more research on this issue is needed in the future.

suPAR: An Inflammatory Mediator for Kidneys

Background

Inflammation is a common feature of many kidney diseases. The implicated inflammatory mediators and their underlying molecular mechanisms however are often not clear.

Summary

suPAR is the soluble form of urokinase-type plasminogen activator receptor (uPAR), associated with inflammation and immune activation. It has evolved into a unique circulating kidney disease factor over the last 10 years. In particular, suPAR has multiple looks due to enzymatic cleavage and alternative transcriptional splicing of the uPAR gene. Most recently, suPAR has emerged as a systemic mediator for COVID-19 infection, associated with lung as well as kidney dysfunction. Like membrane-bound uPAR, suPAR could interact with integrins (e.g., αvβ3 integrin) on podocytes, providing the molecular basis for some glomerular kidney diseases. In addition, there have been numerous studies suggesting that suPAR connects acute kidney injury to chronic kidney disease as a special kidney risk factor. Moreover, the implication of circulating suPAR levels in kidney transplantation and plasmapheresis not only indicates its relevance in monitoring for recurrence but also implies suPAR as a possible therapeutic target. In fact, the therapeutic concept of manipulating suPAR function has been evidenced in several kidney disease experimental models.

Key Messages

The last 10 years of research has established suPAR as a unique inflammatory mediator for kidneys. While open questions remain and deserve additional studies, modulating suPAR function may represent a promising novel therapeutic strategy for kidney disease.

Variability in CKD Biomarker Studies: Soluble Urokinase Plasminogen Activator Receptor (suPAR) and Kidney Disease Progression in the Chronic Kidney Disease in Children (CKiD) Study

Rationale & Objective

Biomarker studies are important for generating mechanistic insight and providing clinically useful predictors of chronic kidney disease (CKD) progression. However, variability across studies can often muddy the evidence waters. Here we evaluated real-world variability in biomarker studies using two published studies, independently conducted, of the novel plasma marker soluble urokinase-type plasminogen activator receptor (suPAR) for predicting CKD progression in children with CKD.

Study Design

A comparison of 2 prospective cohort studies.

Setting & Participants

541 children from the Chronic Kidney Disease in Children (CKiD) study, median age 12 years, median glomerular filtration rate (GFR) of 54 mL/min/1.73m².

Outcome

The first occurrence of either a 50% decline in GFR from baseline or incident end-stage kidney disease.

Analytical Approach

The suPAR plasma marker was measured using the Quantikine ELISA immunoassay in the first study and Meso Scale Discovery (MSD) platform in the second. The analytical approaches varied. We used suPAR data from the 2 assays and mimicked each analytical approach in an overlapping subset.

Results

We found that switching assays had the greatest impact on inferences, resulting in a 38% to 66% change in the magnitude of the effect estimates. Covariate and modeling choices resulted in an additional 8% to 40% variability in the effect estimate. The cumulative variability led to different inferences despite using a similar sample of CKiD participants and addressing the same question.

Limitations

The estimated variability does not represent optimal repeatability but instead illustrates real-world variability that may be present in the CKD biomarker literature.

Conclusions

Our results highlight the importance of validation, avoiding conclusions based on P value thresholds, and providing comparable metrics. Further transparency of data and equal weighting of negative and positive findings in explorations of novel biomarkers will allow investigators to more quickly weed out less promising biomarkers.

Plasma and Urine Biomarkers of CKD: A Review of Findings in the CKiD Study

Serum creatinine and level of proteinuria, as biomarkers of chronic kidney disease (CKD) progression, inadequately explain the variability of glomerular filtration rate decline, and are late markers of glomerular filtration rate decline. Recent studies have identified plasma and urine biomarkers at higher levels in children with CKD and also associate independently with CKD progression, even after adjustment for serum creatinine and proteinuria. These novel biomarkers represent diverse biologic pathways of tubular injury, tubular dysfunction, inflammation, and tubular health, and can be used as a liquid biopsy to better characterize CKD in children. In this review, we highlight the biomarker findings from the Chronic Kidney Disease in Children cohort, a large longitudinal study of children with CKD, and compare results with those from other pediatric CKD cohorts. The biomarkers in focus in this review include plasma kidney injury molecule-1, monocyte chemoattractant protein-1, fibroblast growth factor-23, tumor necrosis factor receptor-1, tumor necrosis factor receptor-2, soluble urokinase plasminogen activator receptor, and chitinase-3-like protein 1, as well as urine epidermal growth factor, α-1 microglobulin, kidney injury molecule-1, monocyte chemoattractant protein-1, and chitinase-3-like protein 1. Blood and urine biomarkers improve our ability to prognosticate CKD progression and may improve our understanding of CKD pathophysiology. Further research is required to establish how these biomarkers can be used in the clinical setting to improve the clinical management of CKD.

Plasma Biomarkers of Tubular Injury and Inflammation Are Associated with CKD Progression in Children

BACKGROUND

After accounting for known risk factors for CKD progression in children, clinical outcomes among children with CKD still vary substantially. Biomarkers of tubular injury (such as KIM-1), repair (such as YKL-40), or inflammation (such as MCP-1, suPAR, TNF receptor-1 [TNFR-1], and TNFR-2) may identify children with CKD at risk for GFR decline.

METHODS

We investigated whether plasma KIM-1, YKL-40, MCP-1, suPAR, TNFR-1, and TNFR-2 are associated with GFR decline in children with CKD and in subgroups defined by glomerular versus nonglomerular cause of CKD. We studied participants of the prospective CKiD Cohort Study which enrolled children with an eGFR of 30-90 ml/min per 1.73 m² and then assessed eGFR annually. Biomarkers were measured in plasma collected 5 months after study enrollment. The primary endpoint was CKD progression, defined as a composite of a 50% decline in eGFR or incident ESKD.

RESULTS

Of the 651 children evaluated (median age 11 years; median baseline eGFR of 53 ml/min per 1.73 m²), 195 (30%) had a glomerular cause of CKD. Over a median follow-up of 5.7 years, 223 children (34%) experienced CKD progression to the composite endpoint. After multivariable adjustment, children with a plasma KIM-1, TNFR-1, or TNFR-2 concentration in the highest quartile were at significantly higher risk of CKD progression compared with children with a concentration for the respective biomarker in the lowest quartile (a 4-fold higher risk for KIM-1 and TNFR-1 and a 2-fold higher risk for TNFR-2). Plasma MCP-1, suPAR, and YKL-40 were not independently associated with progression. When stratified by glomerular versus nonglomerular etiology of CKD, effect estimates did not differ significantly.

CONCLUSIONS

Higher plasma KIM-1, TNFR-1, and TNFR-2 are independently associated with CKD progression in children.