Subclinical Acute Kidney Injury in COVID-19: Possible Mechanisms and Future Perspectives.

Since the outbreak of COVID-19 disease, a bidirectional interaction between kidney disease and the progression of COVID-19 has been demonstrated. Kidney disease is an independent risk factor for mortality of patients with COVID-19 as well as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection leading to the development of acute kidney injury (AKI) and chronic kidney disease (CKD) in patients with COVID-19. However, the detection of kidney damage in patients with COVID-19 may not occur until an advanced stage based on the current clinical blood and urinary examinations. Some studies have pointed out the development of subclinical acute kidney injury (subAKI) syndrome with COVID-19. This syndrome is characterized by significant tubule interstitial injury without changes in the estimated glomerular filtration rate. Despite the complexity of the mechanism(s) underlying the development of subAKI, the involvement of changes in the protein endocytosis machinery in proximal tubule (PT) epithelial cells (PTECs) has been proposed. This paper focuses on the data relating to subAKI and COVID-19 and the role of PTECs and their protein endocytosis machinery in its pathogenesis.

HR-pQCT detects alterations in bone microstructure in men with CKD stages 3 and 4, which are influenced by hormonal changes and body composition
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INTRODUCTION: Factors associated with osteodystrophy in predialysis patients are poorly understood. In the present study, we attempted to evaluate the impact of body composition and hormonal regulatory factors on the bone microstructure in a group of men with chronic kidney disease (CKD) stages 3 and 4. MATERIALS AND METHODS: 46 men, aged 50 - 75 years, with previously unrecognized CKD were evaluated by high-resolution peripheral quantitative computed tomography (HR-pQCT), and dual-energy X-ray absorptiometry (DXA). HR-pQCT parameters were correlated with estimated glomerular filtration rate (eGFR), age, body mass index (BMI), muscle mass index (MMI), and biochemistry. RESULTS: As compared to patients in stage 3 CKD, those with stage 4 CKD showed lower serum 25-hydroxyvitamin D (25(OH)D) and bicarbonate levels, and higher serum fibroblast growth factor 23 (FGF-23) and parathyroid hormone (PTH) levels. They also exhibited lower total, trabecular, and cortical volumetric bone mineral density, lower trabecular bone volume/tissue volume, trabecular number, trabecular and cortical thickness, and increased heterogeneity of the trabecular network. In the whole cohort, cortical bone density and thickness were negatively associated with age, PTH, and FGF-23, and positively with BMI. Trabecular bone parameters were positively associated with MMI and 25(OH)D. After simultaneously adjusting for age and eGFR, BMI, and MMI remained significantly associated with bone microstructural variables. CONCLUSION: HR-pQCT showed significant differences in bone microstructure in stage 4 vs. stage 3 CKD patients. Increased BMI, probably due to increased muscle mass, may favorably affect bone architecture in predialysis CKD patients.
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Tubular dysfunction in renal transplant patients using sirolimus or tacrolimus.

BACKGROUND: Tubular dysfunction is prevalent among kidney transplant patients using calcineurin inhibitors, but our knowledge of the tubular effects of mTOR inhibitors is more limited. METHODS: 60 kidney transplant outpatients using either the calcineurin inhibitor tacrolimus or the mTOR inhibitor sirolimus were investigated for renal tubular dysfunction. Proximal tubule function was assessed by quantification of albumin and ß2-microglobulin, tubular reabsorption of phosphate and fractional excretion of bicarbonate. Distal tubular function was evaluated by water deprivation test and by urinary acidification test using furosemide and fludrocortisone for pH, ammonium and titratable acidity measurements. RESULTS: The prevalence of distal renal tubular acidosis (dRTA) was 17% for both treatment groups. 70% of patients treated with sirolimus and 94% using tacrolimus presented with urine concentrating defect (p=0.04). CONCLUSION: Distal RTA and urine concentrating defect were highly prevalent after kidney transplantation both in the sirolimus and tacrolimus treated patients. Acidification test was essential for the appropriate diagnosis of dRTA while dipstick urine specific gravity test was able to detect urine concentrating defect in this population.

Inhibition of SGLT2 co-transporter by dapagliflozin ameliorates tubular proteinuria and tubule-interstitial injury at the early stage of diabetic kidney disease.

Diabetic kidney disease (DKD) is characterized by progressive impairment of kidney function. It has been postulated that tubule-interstitial injury, associated with tubular albuminuria, precedes glomerular damage in the early stage of DKD. Here, we wanted to determine if the development of tubule-interstitial injury at the early stage of DKD implies modulation of megalin-mediated protein reabsorption in proximal tubule epithelial cells (PTECs) by SGLT2-dependent high glucose influx. Rats with streptozotocin (STZ)-induced diabetes were treated or not with dapagliflozin (DAPA) for 8 weeks. Four experimental groups were generated: (1) CONT, control; (2) DAPA, rats treated with DAPA; (3) STZ, diabetic rats; (4) STZ + DAPA, diabetic rats treated with DAPA. No changes in glomerular structure and function were observed. The STZ group presented proteinuria and albuminuria associated with an increase in the fractional excretion of proteins. A positive correlation between glycemia and proteinuria was found. These phenomena were linked to a decrease in luminal and total megalin expression and, consequently, in albumin reabsorption in PTECs. We also observed tubule-interstitial injury characterized by an increase in urinary tubular injury biomarkers and changes in tubular histomorphometry parameters. In addition, inverse correlations were found between cortical albumin uptake and tubule-interstitial injury or glycemia. All these modifications were attenuated in the STZ + DAPA group. These results suggest that SGLT2-dependent high glucose influx into PTECs promotes a harmful effect on the PTECs, leading to the development of tubular albuminuria and tubule-interstitial injury preceding glomerular damage. These results expand current knowledge on the renoprotective effects of gliflozins.

Rapamycin treatment induces tubular proteinuria: role of megalin-mediated protein reabsorption.

Introduction: Rapamycin is an immunosuppressor that acts by inhibiting the serine/threonine kinase mechanistic target of rapamycin complex 1. Therapeutic use of rapamycin is limited by its adverse effects. Proteinuria is an important marker of kidney damage and a risk factor for kidney diseases progression and has been reported in patients and animal models treated with rapamycin. However, the mechanism underlying proteinuria induced by rapamycin is still an open matter. In this work, we investigated the effects of rapamycin on parameters of renal function and structure and on protein handling by proximal tubule epithelial cells (PTECs). Methods: Healthy BALB/c mice were treated with 1.5 mg/kg rapamycin by oral gavage for 1, 3, or 7 days. At the end of each treatment, the animals were kept in metabolic cages and renal function and structural parameters were analyzed. LLC-PK1 cell line was used as a model of PTECs to test specific effect of rapamycin. Results: Rapamycin treatment did not change parameters of glomerular structure and function. Conversely, there was a transient increase in 24-h proteinuria, urinary protein to creatinine ratio (UPCr), and albuminuria in the groups treated with rapamycin. In accordance with these findings, rapamycin treatment decreased albumin-fluorescein isothiocyanate uptake in the renal cortex. This effect was associated with reduced brush border expression and impaired subcellular distribution of megalin in PTECs. The effect of rapamycin seems to be specific for albumin endocytosis machinery because it did not modify renal sodium handling or (Na++K+)ATPase activity in BALB/c mice and in the LLC-PK1 cell line. A positive Pearson correlation was found between megalin expression and albumin uptake while an inverse correlation was shown between albumin uptake and UPCr or 24-h proteinuria. Despite its effect on albumin handling in PTECs, rapamycin treatment did not induce tubular injury measured by interstitial space and collagen deposition. Conclusion: These findings suggest that proteinuria induced by rapamycin could have a tubular rather than a glomerular origin. This effect involves a specific change in protein endocytosis machinery. Our results open new perspectives on understanding the undesired effect of proteinuria generated by rapamycin.

SARS-CoV-2 spike protein inhibits megalin-mediated albumin endocytosis in proximal tubule epithelial cells.

Patients with COVID-19 have high prevalence of albuminuria which is used as a marker of progression of renal disease and is associated with severe COVID-19. We hypothesized that SARS-CoV-2 spike protein (S protein) could modulate albumin handling in proximal tubule epithelial cells (PTECs) and, consequently contribute to the albuminuria observed in patients with COVID-19. In this context, the possible effect of S protein on albumin endocytosis in PTECs was investigated. Two PTEC lines were used: HEK-293A and LLC-PK1. Incubation of both cell types with S protein for 16 h inhibited albumin uptake at the same magnitude. This effect was associated with canonical megalin-mediated albumin endocytosis because: (1) DQ-albumin uptake, a marker of the lysosomal degradation pathway, was reduced at a similar level compared with fluorescein isothiocyanate (FITC)-albumin uptake; (2) dextran-FITC uptake, a marker of fluid-phase endocytosis, was not changed; (3) cell viability and proliferation were not changed. The inhibitory effect of S protein on albumin uptake was only observed when it was added at the luminal membrane, and it did not involve the ACE2/Ang II/AT1R axis. Although both cells uptake S protein, it does not seem to be required for modulation of albumin endocytosis. The mechanism underlying the inhibition of albumin uptake by S protein encompasses a decrease in megalin expression without changes in megalin trafficking and stability. These results reveal a possible mechanism to explain the albuminuria observed in patients with COVID-19.

Aminotransferase-to-platelet ratio index and Fibrosis-4 index score predict hepatic fibrosis evaluated by transient hepatic elastography in hepatitis C virus-infected hemodialysis patients.

OBJECTIVE: This study aimed to evaluate the performance of aminotransferase-to-platelet ratio index (APRI) and Fibrosis-4 index (FIB-4) in chronic kidney disease stage 5D HCV-infected patients compared to transient hepatic elastography (TE) as the gold standard. METHODS: Hemodialysis HCV-infected patients submitted to TE (FibroScan, Echosens, Paris, France) had APRI and FIB-4 calculated. Based on the best area under receiver operating characteristic curve (AUROC) for significant fibrosis and cirrhosis, APRI and FIB-4 cutoffs were determined and their performances were compared. RESULTS: Seventy patients were included. Both APRI and FIB-4 showed good performance for identifying significant fibrosis [AUROC = 0.73, 95% confidence interval (CI) 0.61-0.83 and 0.79, 95% CI 0.68-0.88; P < 0.05] and cirrhosis [AUROC = 0.82, 95% CI 0.71-0.90 and 0.85, 95% CI 0.75-0.93; P < 0.05]. APRI ≤ 0.25 excluded significant fibrosis with negative predictive value (NPV) of 81.8% and APRI > 0.61 confirmed it with a positive predictive value (PPV) of 81.8%. Similarly, NPV for FIB-4 ≤ 0.60 regarding significant fibrosis was 90.9%. NPV for cirrhosis for APRI ≤ 0.42 or FIB-4 ≤ 1.40 was 97%. However, APRI > 0.73 or FIB-4 > 2.22 showed a modest PPV of 60 and 70% to confirm cirrhosis, respectively. CONCLUSION: APRI and FIB-4 are simple, non-expensive scoring systems with good accuracy to assess fibrosis in HCV-infected hemodialysis patients, mainly excluding both significant fibrosis or cirrhosis and may be an alternative to TE in the evaluation of this population.

Is it possible to use modification of diet in renal disease (MDRD) equation in a Brazilian population?

BACKGROUND: Accurate assessment of kidney function level is the key to the identification and management of chronic kidney disease (CKD). Glomerular filtration rate (GFR) is the best measure of overall kidney function in health and disease. There is no consensus about the method to be used routinely to measure and/or estimate GFR. The objectives of this study were to assess which method correlates better with creatinine (Cr) clearance, extensively used in medical practice, as well as assessing the efficacy of the modification of diet in renal disease (MDRD) equation, in our population. METHODS: We studied 262 adult out-patients with stable CKD on conservative treatment. GFR was evaluated by Cr clearance, Cockcroft-Gault (CG) formula, the mean of urea and Cr clearances (total clearance (TCl)), the MDRD study equation, with and without the variable for African-Americans (MDRD1) and the simplified one (MDRDs). Data were analyzed by Pearson's correlation coefficient (r) and Bland & Altman plot analysis. RESULTS: Pearson's correlation showed that all methods where similar when compared to Cr clearance. A high correlation was observed between CG and MDRD equations, and TCl and MDRD equations showed the worst correlation. Among the MDRD equations, no differences were found. Bland-Altman plot analysis indicated a concordance among the studied methods. CONCLUSION: The CG formula could replace Cr clearance in our population, being simpler than and equally as sensitive as the MDRD equation.