Cadmium toxicity mediated by the inhibition of SLC2A4 expression in human proximal Tubule cells

Cadmium (Cd) is an environmental contaminant that causes renal toxicity. We have previously demonstrated that Cd induces renal toxicity by altering transcriptional activities. In this study, we show that Cd markedly inhibited the activity of transcription factor MEF2A in HK-2 human proximal tubule cells, which generated significant cytotoxicity in the cells. This reduction in the nuclear levels of MEF2A protein may be involved in the Cd-induced inhibition of MEF2A activity. We also demonstrate that one of the glucose transporters, GLUT4, was downregulated not only by Cd treatment but also by MEF2A knockdown. Knockdown of SLC2A4, encoding GLUT4, eliminated both cell viability and Cd toxicity. Cd treatment or SLC2A4 deficiency reduced the cellular concentration of glucose. Therefore, the suppression of SLC2A4 expression, which mediates the reduction in cellular glucose, is involved in Cd toxicity. The Cd toxicity induced by the reduction in GLUT4 may be associated with a reduction of cellular ATP levels in HK-2 cells. The levels of Slc2a4 mRNA in the kidney of mice exposed to Cd for 6 or 12 months were significantly lower than those in the control group. These results demonstrate that Cd exerts its cytotoxicity through the suppression in SLC2A4 expression and the subsequent inhibition of MEF2A transcriptional activity. Cd-induced suppression of SLC2A4 expression also reduces cellular ATP levels, partly by reducing glucose levels. This study suggests that the glucose transporter plays an important role in the renal toxicity of Cd, and provides a crucial breakthrough in our understanding of the mechanism of Cd toxicity.

Krüppel-like factor 6-mediated loss of BCAA catabolism contributes to kidney injury in mice and humans

Altered cellular metabolism in kidney proximal tubule (PT) cells plays a critical role in acute kidney injury (AKI). The transcription factor Krüppel-like factor 6 (KLF6) is rapidly and robustly induced early in the PT after AKI. We found that PT-specific Klf6 knockdown (Klf6 ^PTKD) is protective against AKI and kidney fibrosis in mice. Combined RNA and chromatin immunoprecipitation sequencing analysis demonstrated that expression of genes encoding branched-chain amino acid (BCAA) catabolic enzymes was preserved in Klf6 ^PTKD mice, with KLF6 occupying the promoter region of these genes. Conversely, inducible KLF6 overexpression suppressed expression of BCAA genes and exacerbated kidney injury and fibrosis in mice. In vitro, injured cells overexpressing KLF6 had similar decreases in BCAA catabolic gene expression and were less able to utilize BCAA. Furthermore, knockdown of BCKDHB, which encodes one subunit of the rate-limiting enzyme in BCAA catabolism, resulted in reduced ATP production, while treatment with BCAA catabolism enhancer BT2 increased metabolism. Analysis of kidney function, KLF6, and BCAA gene expression in human chronic kidney disease patients showed significant inverse correlations between KLF6 and both kidney function and BCAA expression. Thus, targeting KLF6-mediated suppression of BCAA catabolism may serve as a key therapeutic target in AKI and kidney fibrosis.

Defects in KCNJ16 Cause a Novel Tubulopathy with Hypokalemia, Salt Wasting, Disturbed Acid-Base Homeostasis, and Sensorineural Deafness

BACKGROUND

The transepithelial transport of electrolytes, solutes, and water in the kidney is a well-orchestrated process involving numerous membrane transport systems. Basolateral potassium channels in tubular cells not only mediate potassium recycling for proper Na⁺,K⁺-ATPase function but are also involved in potassium and pH sensing. Genetic defects in KCNJ10 cause EAST/SeSAME syndrome, characterized by renal salt wasting with hypokalemic alkalosis associated with epilepsy, ataxia, and sensorineural deafness.

METHODS

A candidate gene approach and whole-exome sequencing determined the underlying genetic defect in eight patients with a novel disease phenotype comprising a hypokalemic tubulopathy with renal salt wasting, disturbed acid-base homeostasis, and sensorineural deafness. Electrophysiologic studies and surface expression experiments investigated the functional consequences of newly identified gene variants.

RESULTS

We identified mutations in the KCNJ16 gene encoding KCNJ16, which along with KCNJ15 and KCNJ10, constitutes the major basolateral potassium channel of the proximal and distal tubules, respectively. Coexpression of mutant KCNJ16 together with KCNJ15 or KCNJ10 in Xenopus oocytes significantly reduced currents.

CONCLUSIONS

Biallelic variants in KCNJ16 were identified in patients with a novel disease phenotype comprising a variable proximal and distal tubulopathy associated with deafness. Variants affect the function of heteromeric potassium channels, disturbing proximal tubular bicarbonate handling as well as distal tubular salt reabsorption.

Local and downstream actions of proximal tubule angiotensin II signaling on Na+ transporters in the mouse nephron

The renal nephron consists of a series of distinct cell types that function in concert to maintain fluid and electrolyte balance and blood pressure. The renin-angiotensin system (RAS) is central to Na⁺ and volume balance. We aimed to determine how loss of angiotensin II signaling in the proximal tubule (PT), which reabsorbs the bulk of filtered Na⁺ and volume, impacts solute transport throughout the nephron. We hypothesized that PT renin-angiotensin system disruption would not only depress PT Na⁺ transporters but also impact downstream Na⁺ transporters. Using a mouse model in which the angiotensin type 1a receptor (AT_1aR) is deleted specifically within the PT (AT_1aR PTKO), we profiled the abundance of Na⁺ transporters, channels, and claudins along the nephron. Absence of PT AT_1aR signaling was associated with lower abundance of PT transporters (Na⁺/H⁺ exchanger isoform 3, electrogenic Na⁺-bicarbonate cotransporter 1, and claudin 2) as well as lower abundance of downstream transporters (total and phosphorylated Na⁺-K⁺-2Cl^- cotransporter, medullary Na⁺-K⁺-ATPase, phosphorylated NaCl cotransporter, and claudin 7) versus controls. However, transport activities of Na⁺-K⁺-2Cl^- cotransporter and NaCl cotransporter (assessed with diuretics) were similar between groups in order to maintain electrolyte balance. Together, these results demonstrate the primary impact of angiotensin II regulation on Na⁺ reabsorption in the PT at baseline and the associated influence on downstream Na⁺ transporters, highlighting the ability of the nephron to integrate Na⁺ transport along the nephron to maintain homeostasis.NEW & NOTEWORTHY Our study defines a novel role for proximal tubule angiotensin receptors in regulating the abundance of Na⁺ transporters throughout the nephron, thereby contributing to the integrated control of fluid balance in vivo.

Sugar or Fat? Renal Tubular Metabolism Reviewed in Health and Disease

The kidney is a highly metabolically active organ that relies on specialized epithelial cells comprising the renal tubules to reabsorb most of the filtered water and solutes. Most of this reabsorption is mediated by the proximal tubules, and high amounts of energy are needed to facilitate solute movement. Thus, proximal tubules use fatty acid oxidation, which generates more adenosine triphosphate (ATP) than glucose metabolism, as its preferred metabolic pathway. After kidney injury, metabolism is altered, leading to decreased fatty acid oxidation and increased lactic acid generation. This review discusses how metabolism differs between the proximal and more distal tubular segments of the healthy nephron. In addition, metabolic changes in acute kidney injury and chronic kidney disease are discussed, as well as how these changes in metabolism may impact tubule repair and chronic kidney disease progression.

KIM-1 mediates fatty acid uptake by renal tubular cells to promote progressive diabetic kidney disease

Tubulointerstitial abnormalities are predictive of the progression of diabetic kidney disease (DKD), and their targeting may be an effective means for prevention. Proximal tubular (PT) expression of kidney injury molecule (KIM)-1, as well as blood and urinary levels, are increased early in human diabetes and can predict the rate of disease progression. Here, we report that KIM-1 mediates PT uptake of palmitic acid (PA)-bound albumin, leading to enhanced tubule injury with DNA damage, PT cell-cycle arrest, interstitial inflammation and fibrosis, and secondary glomerulosclerosis. Such injury can be ameliorated by genetic ablation of the KIM-1 mucin domain in a high-fat-fed streptozotocin mouse model of DKD. We also identified TW-37 as a small molecule inhibitor of KIM-1-mediated PA-albumin uptake and showed in vivo in a kidney injury model in mice that it ameliorates renal inflammation and fibrosis. Together, our findings support KIM-1 as a new therapeutic target for DKD.

Pathophysiology of AKI to CKD progression

Acute kidney injury (AKI), defined as a rapid decrease in glomerular filtration rate, is a common and devastating pathologic condition. AKI is associated with significant morbidity and subsequent chronic kidney disease (CKD) development. Regardless of the initial insult, CKD progression after AKI involves multiple types of cells, including proximal tubular cells, fibroblasts, and immune cells. Although the mechanisms underlying this AKI to CKD progression have been investigated extensively over the past decade, therapeutic strategies still are lacking. One of the reasons for this stems from the fact that AKI and its progression toward CKD is multifactorial and variable because it is dependent on patient background. In this review, we describe the current understanding of AKI and its maladaptive repair with a focus on proximal tubules and resident fibroblasts. Subsequently, we discuss the unique pathophysiology of AKI in the elderly, highlighting our recent finding of age-dependent tertiary lymphoid tissues.

The Proximal Tubule Toxicity of Immunoglobulin Light Chains

Plasma and B cells dyscrasias that overproduce monoclonal immunoglobulin free light chains (FLCs) affect the kidney frequently in various ways. The hematologic dyscrasia responsible for the production of FLCs may or may not meet the criteria for cancer, such as multiple myeloma (MM) or lymphoma, or may remain subclinical. If there is overt malignancy, the accompanying kidney disorder is called myeloma- or lymphoma-associated. If the dyscrasia is subclinical, the associated kidney disorders are grouped as monoclonal gammopathy of renal significance. Glomeruli and tubules may both be involved. The proximal tubule disorders comprise a spectrum of interesting syndromes, which range in severity. This review focuses on the recent insights gained into the patterns and the mechanisms of proximal tubule toxicity of FLCs, including subtle transport disorders, such as proximal tubule acidosis, partial or complete Fanconi syndrome, or severe acute or chronic renal failure. Histologically, there may be crystal deposition in the proximal tubule cells, acute tubule injury, interstitial inflammation, fibrosis, and tubule atrophy. Specific structural alterations in the V domain of FLCs caused by somatic hypermutations are responsible for crystal formation as well as partial or complete Fanconi syndrome. Besides crystal formation, tubulointerstitial inflammation and proximal tubulopathy can be mediated by direct activation of inflammatory pathways through cytokines and Toll-like receptors due to cell stress responses induced by excessive FLC endocytosis into the proximal tubule cells. Therapy directed against the clonal source of the toxic light chain can prevent progression to more severe lesions and may help preserve kidney function.

Renal Sodium Handling in Relation to Environmental and Genetic Factors in Untreated Chinese

BACKGROUND

We investigated proximal and distal renal tubular sodium handling, as assessed by fractional excretion of lithium (FELi) and fractional distal reabsorption rate of sodium (FDRNa), in relation to environmental and genetic factors in untreated patients.

METHODS

Our study participants were suspected hypertensive patients being off antihypertensive medication for ≥2 weeks and referred for 24-hour ambulatory blood pressure monitoring. We collected serum and 24-hour urine for measurement of sodium, creatinine, and lithium concentration, and calculated FELi and FDRNa. We genotyped 19 single-nucleotide polymorphisms associated with renal sodium handling or blood pressure using the ABI SNapShot method.

RESULTS

The 1,409 participants (664 men, 47.1%) had a mean (±SD) age of 51.0 ± 10.5 years. After adjustment for host factors, both FELi and FDRNa were significantly (P ≤ 0.01) associated with season and humidity, explaining ~1.3% and ~3.5% of the variance, respectively. FELi was highest in autumn and lowest in summer and intermediate in spring and winter (P = 0.007). FDRNa was also highest in autumn but lowest in winter and intermediate in spring and summer (P < 0.001). Neither FELi nor FDRNa was associated with outdoor temperature or atmospheric pressure (P ≥ 0.13). After adjustment for host and environmental factors and Bonferroni multiple testing, among the 19 studied genetic variants, only rs12513375 was significantly associated with FELi and FDRNa (P ≤ 0.004) and explained about 1.7% of the variance.

CONCLUSIONS

Renal sodium handling as measured by endogenous lithium clearance was sensitive to major environmental and genetic factors. Our finding is toward the use of these indexes for the definition of renal tubular dysfunction.

Urine Metabolite Levels, Adverse Kidney Outcomes, and Mortality in CKD Patients: A Metabolome-wide Association Study

RATIONALE & OBJECTIVE

Mechanisms underlying the variable course of disease progression in patients with chronic kidney disease (CKD) are incompletely understood. The aim of this study was to identify novel biomarkers of adverse kidney outcomes and overall mortality, which may offer insights into pathophysiologic mechanisms.

STUDY DESIGN

Metabolome-wide association study.

SETTING & PARTICIPANTS

5,087 patients with CKD enrolled in the observational German Chronic Kidney Disease Study.

EXPOSURES

Measurements of 1,487 metabolites in urine.

OUTCOMES

End points of interest were time to kidney failure (KF), a combined end point of KF and acute kidney injury (KF+AKI), and overall mortality.

ANALYTICAL APPROACH

Statistical analysis was based on a discovery-replication design (ratio 2:1) and multivariable-adjusted Cox regression models.

RESULTS

After a median follow-up of 4 years, 362 patients died, 241 experienced KF, and 382 experienced KF+AKI. Overall, we identified 55 urine metabolites whose levels were significantly associated with adverse kidney outcomes and/or mortality. Higher levels of C-glycosyltryptophan were consistently associated with all 3 main end points (hazard ratios of 1.43 [95% CI, 1.27-1.61] for KF, 1.40 [95% CI, 1.27-1.55] for KF+AKI, and 1.47 [95% CI, 1.33-1.63] for death). Metabolites belonging to the phosphatidylcholine pathway showed significant enrichment. Members of this pathway contributed to the improvement of the prediction performance for KF observed when multiple metabolites were added to the well-established Kidney Failure Risk Equation.

LIMITATIONS

Findings among patients of European ancestry with CKD may not be generalizable to the general population.

CONCLUSIONS

Our comprehensive screen of the association between urine metabolite levels and adverse kidney outcomes and mortality identifies metabolites that predict KF and represents a valuable resource for future studies of biomarkers of CKD progression.

Proteinuria and Clinical Outcomes in Hospitalized COVID-19 Patients: A Retrospective Single-Center Study

BACKGROUND AND OBJECTIVES

Kidney involvement is frequent among patients with coronavirus disease 2019 (COVID-19), and occurrence of AKI is associated with higher mortality in this population. The objective of this study was to describe occurrence and significance of proteinuria in this setting.

DESIGN , SETTING, PARTICIPANTS MEASUREMENTS

We conducted a single-center retrospective study to describe the characteristic features of proteinuria measured within 48 hours following admission among patients with COVID-19 admitted in a tertiary care hospital in France, and to evaluate its association with initiation of dialysis, intensive care unit admission, and death.

RESULTS

Among 200 patients with available data, urine protein-creatinine ratio at admission was ≥1 g/g for 84 (42%), although kidney function was normal in most patients, with a median serum creatinine of 0.94 mg/dl (interquartile range, 0.75-1.21). Median urine albumin-creatinine ratio was 110 mg/g (interquartile range, 50-410), with a urine albumin-protein ratio <50% in 92% of patients. Urine retinol binding protein concentrations, available for 85 patients, were ≥0.03 mg/mmol in 62% of patients. Urine protein-creatinine ratio ≥1 g/g was associated with initiation of dialysis (odds ratio, 4.87; 95% confidence interval, 2.03 to 13.0; P <0.001), admission to the intensive care unit (odds ratio, 3.55; 95% confidence interval, 1.93 to 6.71; P <0.001), and death (odds ratio, 3.56; 95% confidence interval, 1.90 to 6.54; P <0.001).

CONCLUSIONS

Proteinuria is very frequent among patients admitted for COVID-19 and may precede AKI. Low levels of albuminuria suggest a predominant tubular origin, confirmed by the elevated levels of urine retinol binding protein. Urine protein-creatinine ratio ≥1 g/g at admission is strongly associated with poor kidney and patient outcome.

Enhanced MCP-1 Release in Early Autosomal Dominant Polycystic Kidney Disease

Introduction

Autosomal dominant polycystic kidney disease (ADPKD) causes kidney failure typically in adulthood, but the disease starts in utero. Copeptin, epidermal growth factor (EGF), and monocyte chemoattractant protein-1 (MCP-1) are associated with severity and hold prognostic value in adults but remain unstudied in the early disease stage. Kidneys from adults with ADPKD exhibit macrophage infiltration, and a prominent role of MCP-1 secretion by tubular epithelial cells is suggested from rodent models.

Methods

In a cross-sectional study, plasma copeptin, urinary EGF, and urinary MCP-1 were evaluated in a pediatric ADPKD cohort and compared with age-, sex-, and body mass index (BMI)-matched healthy controls. MCP-1 was studied in mouse collecting duct cells, human proximal tubular cells, and fetal kidney tissue.

Results

Fifty-three genotyped ADPKD patients and 53 controls were included. The mean (SD) age was 10.4 (5.9) versus 10.5 (6.1) years (P = 0.543), and the estimated glomerular filtration rate (eGFR) was 122.7 (39.8) versus 114.5 (23.1) ml/min per 1.73 m² (P = 0.177) in patients versus controls, respectively. Plasma copeptin and EGF secretion were comparable between groups. The median (interquartile range) urinary MCP-1 (pg/mg creatinine) was significantly higher in ADPKD patients (185.4 [213.8]) compared with controls (154.7 [98.0], P = 0.010). Human proximal tubular cells with a heterozygous PKD1 mutation and mouse collecting duct cells with a PKD1 knockout exhibited increased MCP-1 secretion. Human fetal ADPKD kidneys displayed prominent MCP-1 immunoreactivity and M2 macrophage infiltration.

Conclusion

An increase in tubular MCP-1 secretion is an early event in ADPKD. MCP-1 is an early disease severity marker and a potential treatment target.

A key role for the transporter OAT1 in systemic lipid metabolism

Organic anion transporter 1 (OAT1/SLC22A6) is a drug transporter with numerous xenobiotic and endogenous substrates. The Remote Sensing and Signaling Theory suggests that drug transporters with compatible ligand preferences can play a role in “organ crosstalk,” mediating overall organismal communication. Other drug transporters are well known to transport lipids, but surprisingly little is known about the role of OAT1 in lipid metabolism. To explore this subject, we constructed a genome-scale metabolic model using omics data from the Oat1 knockout mouse. The model implicated OAT1 in the regulation of many classes of lipids, including fatty acids, bile acids, and prostaglandins. Accordingly, serum metabolomics of Oat1 knockout mice revealed increased polyunsaturated fatty acids, diacylglycerols, and long-chain fatty acids and decreased ceramides and bile acids when compared with wildtype controls. Some aged knockout mice also displayed increased lipid droplets in the liver when compared with wildtype mice. Chemoinformatics and machine learning analyses of these altered lipids defined molecular properties that form the structural basis for lipid-transporter interactions, including the number of rings, positive charge/volume, and complexity of the lipids. Finally, we obtained targeted serum metabolomics data after short-term treatment of rodents with the OAT-inhibiting drug probenecid to identify potential drug–metabolite interactions. The treatment resulted in alterations in eicosanoids and fatty acids, further supporting our metabolic reconstruction predictions. Consistent with the Remote Sensing and Signaling Theory, the data support a role of OAT1 in systemic lipid metabolism.

Imaging the kidney: from light to super-resolution microscopy

The important achievements in kidney physiological and pathophysiological mechanisms can largely be ascribed to progress in the technology of microscopy. Much of what we know about the architecture of the kidney is based on the fundamental descriptions of anatomic microscopists using light microscopy and later by ultrastructural analysis provided by electron microscopy. These two techniques were used for the first classification systems of kidney diseases and for their constant updates. More recently, a series of novel imaging techniques added the analysis in further dimensions of time and space. Confocal microscopy allowed us to sequentially visualize optical sections along the z-axis and the availability of specific analysis software provided a three-dimensional rendering of thicker tissue specimens. Multiphoton microscopy permitted us to simultaneously investigate kidney function and structure in real time. Fluorescence-lifetime imaging microscopy allowed to study the spatial distribution of metabolites. Super-resolution microscopy increased sensitivity and resolution up to nanoscale levels. With cryo-electron microscopy, researchers could visualize the individual biomolecules at atomic levels directly in the tissues and understand their interaction at subcellular levels. Finally, matrix-assisted laser desorption/ionization imaging mass spectrometry permitted the measuring of hundreds of different molecules at the same time on tissue sections at high resolution. This review provides an overview of available kidney imaging strategies, with a focus on the possible impact of the most recent technical improvements.

Modulation of Tubular pH by Acetazolamide in a Ca2+ Transport Deficient Mice Facilitates Calcium Nephrolithiasis

Proximal tubular (PT) acidosis, which alkalinizes the urinary filtrate, together with Ca²⁺ supersaturation in PT can induce luminal calcium phosphate (CaP) crystal formation. While such CaP crystals are known to act as a nidus for CaP/calcium oxalate (CaOx) mixed stone formation, the regulation of PT luminal Ca²⁺ concentration ([Ca²⁺]) under elevated pH and/or high [Ca²⁺] conditions are unknown. Since we found that transient receptor potential canonical 3 (TRPC3) knockout (KO; -/-) mice could produce mild hypercalciuria with CaP urine crystals, we alkalinized the tubular pH in TRPC3-/- mice by oral acetazolamide (0.08%) to develop mixed urinary crystals akin to clinical signs of calcium nephrolithiasis (CaNL). Our ratiometric (λ340/380) intracellular [Ca²⁺] measurements reveal that such alkalization not only upsurges Ca²⁺ influx into PT cells, but the mode of Ca²⁺ entry switches from receptor-operated to store-operated pathway. Electrophysiological experiments show enhanced bicarbonate related current activity in treated PT cells which may determine the stone-forming phenotypes (CaP or CaP/CaOx). Moreover, such alkalization promotes reactive oxygen species generation, and upregulation of calcification, inflammation, fibrosis, and apoptosis in PT cells, which were exacerbated in absence of TRPC3. Altogether, the pH-induced alteration of the Ca²⁺ signaling signature in PT cells from TRPC3 ablated mice exacerbated the pathophysiology of mixed urinary stone formation, which may aid in uncovering the downstream mechanism of CaNL.

Long noncoding RNAs may impact podocytes and proximal tubule function through modulating miRNAs expression in Early Diabetic Kidney Disease of Type 2 Diabetes Mellitus patients

Aims: Long noncoding RNAs (lncRNAs) play key roles in the pathophysiology of DKD involving actions of microRNAs (miRNAs). The aims of the study were to establish the involvement of selected lncRNAs in the epigenetic mechanisms of podocyte damage and tubular injury in DKD of type 2 diabetes mellitus (DM) patients in relation to a particular miRNAs profile. Methods: A total of 136 patients with type 2 DM and 25 healthy subjects were assessed in a cross-sectional study concerning urinary albumin: creatinine ratio (UACR), eGFR, biomarkers of podocyte damage (synaptopodin, podocalyxin) and of proximal tubule (PT) dysfunction (Kidney injury molecule-1-KIM-1, N-acetyl-D-glucosaminidase-NAG), urinary lncRNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1), nuclear-enriched abundant transcript 1 (NEAT1), myocardial infarction-associated transcript (MIAT), taurine-upregulated gene 1 (TUG1), urinary miRNA21, 124, 93, 29a. Results: Multivariable regression analysis showed that urinary lncMALAT1 correlated directly with urinary synaptopodin, podocalyxin, KIM-1, NAG, miRNA21, 124, UACR, and negatively with eGFR, miRNA93, 29a (p<0.0001; R2=0.727); urinary lncNEAT1 correlated directly with synaptopodin, KIM-1, NAG, miRNA21, 124, and negatively with eGFR, miRNA93, 29a (p<0.0001; R2=0.702); urinary lncMIAT correlated directly with miRNA93 and 29a, eGFR (p<0.0001; R2=0.671) and negatively with synaptopodin, KIM-1, NAG, UACR, miRNA21, 124 (p<0.0001; R2=0.654); urinary lncTUG1 correlated directly with eGFR, miRNA93, 29a, and negatively with synaptopodin, podocalyxin, NAG, miRNA21, 124 (p<0.0001; R2=0.748). Conclusions: In patients with type 2 DM lncRNAs exert either deleterious or protective functions within glomeruli and PT. LncRNAs may contribute to DKD through modulating miRNAs expression and activities. This observation holds true independently of albuminuria and DKD stage.

Aligned Silk Sponge Fabrication and Perfusion Culture for Scalable Proximal Tubule Tissue Engineering

Chronic kidney disease and kidney failure are on the rise globally, yet there has not been a corresponding improvement in available therapies. A key challenge in a biological approach to developing kidney tissue is to identify scaffolding materials that support cell growth both in vitro and in vivo to facilitate translational goals. Scaffolds composed of silk fibroin protein possess the biocompatibility, mechanical robustness, and stability required for tissue engineering. Here, we use a silk sponge system to support kidney cells in a perfused bioreactor system. Silk fibroin protein underwent directional freezing to form parallel porous structures that mimic the native kidney structure of aligned tubules and are able to support more cells than nonaligned silk sponges. Adult immortalized renal proximal tubule epithelial cells were seeded into the sponges and cultured under static conditions for 1 week, then grown statically or with perfusion with culture media flowing through the sponge to enhance cell alignment and maturation. The sponges were imaged with confocal and scanning electron microscopies to analyze and quantify cell attachment, alignment, and expression of proteins important to proximal tubule differentiation and function. The perfused tissue constructs showed higher number of cells that are more evenly distributed through the construct and increased gene expression of several key markers of proximal tubule epithelial cell function compared to sponges grown under static conditions. These perfused tissue constructs represent a step toward a scalable approach to engineering proximal tubule structures with the potential to be used as in vitro models or as in vivo implantable tissues to supplement or replace impaired kidney function.

Urinary DPP4 correlates with renal dysfunction, and DPP4 inhibition protects against the reduction in megalin and podocin expression in experimental CKD

This study investigated the molecular mechanisms underlying the antiproteinuric effect of DPP4 inhibition in 5/6 renal ablation rats and tested the hypothesis that the urinary activity of DPP4 correlates with chronic kidney disease (CKD) progression. Experiments were conducted in male Wistar rats who underwent 5/6 nephrectomy (Nx) or sham operation followed by 8 wk of treatment with the DPP4 inhibitor (DPP4i) sitagliptin or vehicle. Proteinuria increased progressively in Nx rats throughout the observation period. This increase was remarkably mitigated by sitagliptin. Higher levels of proteinuria in Nx rats compared to control rats were accompanied by higher urinary excretion of retinol-binding protein 4, a marker of tubular proteinuria, as well as higher urinary levels of podocin, a marker of glomerular proteinuria. Retinol-binding protein 4 and podocin were not detected in the urine of Nx + DPP4i rats. Tubular and glomerular proteinuria was associated with the reduced expression of megalin and podocin in the renal cortex of Nx rats. Sitagliptin treatment partially prevented this decrease. Besides, the angiotensin II renal content was significantly reduced in the Nx rats that received sitagliptin compared to vehicle-treated Nx rats. Interestingly, both urinary DPP4 activity and abundance increased progressively in Nx rats. Additionally, urinary DPP4 activity correlated positively with serum creatinine levels, proteinuria, and blood pressure. Collectively, these results suggest that DPP4 inhibition ameliorated both tubular and glomerular proteinuria and prevented the reduction of megalin and podocin expression in CKD rats. Furthermore, these findings suggest that urinary DPP4 activity may serve as a biomarker of renal disease and progression.

Changes in NAD and Lipid Metabolism Drive Acidosis-Induced Acute Kidney Injury

BACKGROUND

The kidney plays an important role in maintaining normal blood pH. Metabolic acidosis (MA) upregulates the pathway that mitochondria in the proximal tubule (PT) use to produce ammonia and bicarbonate from glutamine, and is associated with AKI. However, the extent to which MA causes AKI, and thus whether treating MA would be beneficial, is unclear.

METHODS

Gavage with ammonium chloride induced acute MA. Multiphoton imaging of mitochondria (NADH/membrane potential) and transport function (dextran/albumin uptake), oxygen consumption rate (OCR) measurements in isolated tubules, histologic analysis, and electron microscopy in fixed tissue, and urinary biomarkers (KIM-1/clara cell 16) assessed tubular cell structure and function in mouse kidney cortex.

RESULTS

MA induces an acute change in NAD redox state (toward oxidation) in PT mitochondria, without changing the mitochondrial energization state. This change is associated with a switch toward complex I activity and decreased maximal OCR, and a major alteration in normal lipid metabolism, resulting in marked lipid accumulation in PTs and the formation of large multilamellar bodies. These changes, in turn, lead to acute tubular damage and a severe defect in solute uptake. Increasing blood pH with intravenous bicarbonate substantially improves tubular function, whereas preinjection with the NAD precursor nicotinamide (NAM) is highly protective.

CONCLUSIONS

MA induces AKI via changes in PT NAD and lipid metabolism, which can be reversed or prevented by treatment strategies that are viable in humans. These findings might also help to explain why MA accelerates decline in function in CKD.

Interferon-regulated genetic programs and JAK/STAT pathway activate the intronic promoter of the short ACE2 isoform in renal proximal tubules

Recently, a short, interferon-inducible isoform of Angiotensin-Converting Enzyme 2 (ACE2), dACE2 was identified. ACE2 is a SARS-Cov-2 receptor and changes in its renal expression have been linked to several human nephropathies. These changes were never analyzed in context of dACE2, as its expression was not investigated in the kidney. We used Human Primary Proximal Tubule (HPPT) cells to show genome-wide gene expression patterns after cytokine stimulation, with emphasis on the ACE2/dACE2 locus. Putative regulatory elements controlling dACE2 expression were identified using ChIP-seq and RNA-seq. qRT-PCR differentiating between ACE2 and dACE2 revealed 300- and 600-fold upregulation of dACE2 by IFNα and IFNβ, respectively, while full length ACE2 expression was almost unchanged. JAK inhibitor ruxolitinib ablated STAT1 and dACE2 expression after interferon treatment. Finally, with RNA-seq, we identified a set of genes, largely immune-related, induced by cytokine treatment. These gene expression profiles provide new insights into cytokine response of proximal tubule cells.

Association Between Kidney Clearance of Secretory Solutes and Cardiovascular Events: The Chronic Renal Insufficiency Cohort (CRIC) Study

RATIONALE & OBJECTIVE

The clearance of protein-bound solutes by the proximal tubules is an innate kidney mechanism for removing putative uremic toxins that could exert cardiovascular toxicity in humans. However, potential associations between impaired kidney clearances of secretory solutes and cardiovascular events among patients with chronic kidney disease (CKD) remains uncertain.

STUDY DESIGN

A multicenter, prospective, cohort study.

SETTING & PARTICIPANTS

We evaluated 3,407 participants from the Chronic Renal Insufficiency Cohort (CRIC) study.

EXPOSURES

Baseline kidney clearances of 8 secretory solutes. We measured concentrations of secretory solutes in plasma and paired 24-hour urine specimens using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

OUTCOMES

Incident heart failure, myocardial infarction, and stroke events.

ANALYTICAL APPROACH

We used Cox regression to evaluate associations of baseline secretory solute clearances with incident study outcomes adjusting for estimated GFR (eGFR) and other confounders.

RESULTS

Participants had a mean age of 56 years; 45% were women; 41% were Black; and the median estimated glomerular filtration rate (eGFR) was 43 mL/min/1.73 m². Lower 24-hour kidney clearance of secretory solutes were associated with incident heart failure and myocardial infarction but not incident stroke over long-term follow-up after controlling for demographics and traditional risk factors. However, these associations were attenuated and not statistically significant after adjustment for eGFR.

LIMITATIONS

Exclusion of patients with severely reduced eGFR at baseline; measurement variability in secretory solutes clearances.

CONCLUSIONS

In a national cohort study of CKD, no clinically or statistically relevant associations were observed between the kidney clearances of endogenous secretory solutes and incident heart failure, myocardial infarction, or stroke after adjustment for eGFR. These findings suggest that tubular secretory clearance provides little additional information about the development of cardiovascular disease events beyond glomerular measures of GFR and albuminuria among patients with mild-to-moderate CKD.

Mechanism of how carbamylation reduces albumin binding to FcRn contributing to increased vascular clearance

Chronic kidney disease results in high serum urea concentrations leading to excessive protein carbamylation, primarily albumin. This is associated with increased cardiovascular disease and mortality. Multiple methods were used to address whether carbamylation alters albumin metabolism. Intravital two-photon imaging of the Munich Wistar Frömter (MWF) rat kidney and liver allowed us to characterize filtration and proximal tubule uptake and liver uptake. Microscale thermophoresis enabled quantification of cubilin (CUB7,8 domain) and FcRn binding. Finally, multiple biophysical methods including dynamic light scattering, small-angle X-ray scattering, LC-MS/MS and in silico analyses were used to identify the critical structural alterations and amino acid modifications of rat albumin. Carbamylation of albumin reduced binding to CUB7,8 and FcRn in a dose-dependent fashion. Carbamylation markedly increased vascular clearance of carbamylated rat serum albumin (cRSA) and altered distribution of cRSA in both the kidney and liver at 16 h post intravenous injection. By evaluating the time course of carbamylation and associated charge, size, shape, and binding parameters in combination with in silico analysis and mass spectrometry, the critical binding interaction impacting carbamylated albumin's reduced FcRn binding was identified as K524. Carbamylation of RSA had no effect on glomerular filtration or proximal tubule uptake. These data indicate urea-mediated time-dependent carbamylation of albumin lysine K524 resulted in reduced binding to CUB7,8 and FcRn that contribute to altered albumin transport, leading to increased vascular clearance and increased liver and endothelial tissue accumulation.

Sex differences in renal ammonia metabolism

Sexual dimorphic variations are present in many aspects of biology and involve the structure and/or function of nearly every organ system. Acid-base homeostasis is critical for optimal health, and renal ammonia metabolism has a major role in the maintenance of acid-base homeostasis. Recent studies have shown sex-dependent differences in renal ammonia metabolism with regard to both basal ammonia excretion and the response to an exogenous acid load. These sexual dimorphisms are associated with structural changes in the proximal tubule and the collecting duct and variations in the expression of multiple proteins involved in ammonia metabolism and transport. Studies using orchiectomy-induced testosterone deficiency and physiological testosterone replacement have shown that testosterone underlies much of the sex-dependent differences in the proximal tubule. This parallels the finding that the canonical testosterone target receptor, androgen receptor (AR), is present exclusively in the proximal tubule. Thus testosterone, possibly acting through AR activation, regulates multiple components of renal structure and ammonia metabolism. The lack of detectable AR in the remainder of the nephron and collecting duct suggests that some dimorphisms in renal structure and ammonia transporter expression are mediated through mechanisms other than direct testosterone-dependent AR activation. A better understanding of the mechanism and biological implications of sex's effect on renal structure and ammonia metabolism is critical for optimizing our ability to care for both men and women with acid-base disturbances.

Transcriptomes of Major Proximal Tubule Cell Culture Models

BACKGROUND

Cultured cell lines are widely used for research in the physiology, pathophysiology, toxicology, and pharmacology of the renal proximal tubule. The lines that are most appropriate for a given use depend upon the genes expressed. New tools for transcriptomic profiling using RNA sequencing (RNA-Seq) make it possible to catalog expressed genes in each cell line.

METHODS

Fourteen different proximal tubule cell lines, representing six species, were grown on permeable supports under conditions specific for the respective lines. RNA-Seq followed standard procedures.

RESULTS

Transcripts expressed in cell lines variably matched transcripts selectively expressed in native proximal tubule. Opossum kidney (OK) cells displayed the highest percentage match (45% of proximal marker genes [TPM threshold =15]), with pig kidney cells (LLC-PK1) close behind (39%). Lower-percentage matches were seen for various human lines, including HK-2 (26%), and lines from rodent kidneys, such as NRK-52E (23%). Nominally, identical OK cells from different sources differed substantially in expression of proximal tubule markers. Mapping cell line transcriptomes to gene sets for various proximal tubule functions (sodium and water transport, protein transport, metabolic functions, endocrine functions) showed that different lines may be optimal for experimentally modeling each function. An online resource (https://esbl.nhlbi.nih.gov/JBrowse/KCT/) has been created to interrogate cell line transcriptome data. Proteomic analysis of NRK-52E cells confirmed low expression of many proximal tubule marker proteins.

CONCLUSIONS

No cell line fully matched the transcriptome of native proximal tubule cells. However, some of the lines tested are suitable for the study of particular metabolic and transport processes seen in the proximal tubule.

A well-developed endolysosomal system reflects protein reabsorption in segment 1 and 2 of rat proximal tubules

Proteinuria is a well-established marker and predictor of kidney disease. The receptors megalin and cubilin reabsorb filtered proteins and thereby proteinuria is avoided. It is unknown if all segments of the proximal tubule are involved in clearing the filtrate or if there exists a reserve capacity in case of increased glomerular protein filtration. To determine this, we performed serial sectioning of rat kidney and used stereology to quantify the endolysosomal system of the three segments of cortical and juxtamedullary nephrons by electron microscopy. Immunohistochemistry was applied to analyze the adaptor protein Dab2, which assists in megalin mediated endocytosis, megalin, and endocytic uptake of two endogenous megalin ligands; retinol binding protein and β2-microglobulin at exact tubular positions. Proteinuric rats (puromycin-treated) and mice (podocin knock-out) were analyzed to clarify the response of the tubule to increased protein filtration. We found that the endolysosomal system was most prominent in segment 1 and 2, whereas segment 3 was less developed. The depth of ligand uptake varied among nephrons, but it descended into segment 2 although uptake was lower than in segment 1 and it was never observed in segment 3. This was supported by prominent expression of Dab2 in segment 1 and 2. When protein filtration increased, segment 3 was included in the reabsorption process in proteinuric animals. Thus, segment 1 and 2 are responsible for clearing the filtrate for protein during normal physiological conditions, but the tubule exhibits plasticity and is able to include segment 3 under proteinuric stress.