Association between dietary phosphate intake and skeletal muscle energetics in adults without cardiovascular disease

Highly bioavailable inorganic phosphate (Pi) is present in large quantities in the typical Western diet and represents a large fraction of total phosphate intake. Dietary Pi excess induces exercise intolerance and skeletal muscle mitochondrial dysfunction in normal mice. However, the relevance of this to humans remains unknown. The study was conducted on 13 individuals without a history of cardiopulmonary disease (46% female, 15% Black participants) enrolled in the pilot-phase of the Dallas Heart and Mind Study. Total dietary phosphate was estimated from 24-h dietary recall (ASA24). Muscle ATP synthesis was measured at rest, and phosphocreatinine (PCr) dynamics was measured during plantar flexion exercise using 7-T 31P magnetic resonance (MR) spectroscopy in the calf muscle. Correlation was assessed between dietary phosphate intake normalized to total caloric intake, resting ATP synthesis, and PCr depletion during exercise. Higher dietary phosphate intake was associated with lower resting ATP synthesis (r = -0.62, P = 0.03), and with higher levels of PCr depletion during plantar flexion exercise relative to the resting period (r = -0.72; P = 0.004). These associations remain significant after adjustment for age and estimated glomerular filtration rate (both P < 0.05). High dietary phosphate intake was also associated with lower serum Klotho levels, and Klotho levels are in turn associated with PCr depletion and higher ADP accumulation post exercise. Our study suggests that higher dietary phosphate is associated with reduced skeletal muscle mitochondrial function at rest and exercise in humans providing new insight into potential mechanisms linking the Western diet to impaired energy metabolism.NEW & NOTEWORTHY This is the first translational research study directly demonstrating the adverse effects of dietary phosphate on muscle energy metabolism in humans. Importantly, our data show that dietary phosphate is associated with impaired muscle ATP synthesis at rest and during exercise, independent of age and renal function. This is a new biologic paradigm with significant clinical dietary implications.

C-terminal fragment of fibroblast growth factor 23 improves heart function in murine models of high intact fibroblast growth factor 23

Cardiovascular disease (CVD) is the major cause of death in chronic kidney disease (CKD) and is associated with high circulating fibroblast growth factor (FGF)23 levels. It is unresolved whether high circulating FGF23 is a mere biomarker or pathogenically contributes to cardiomyopathy. It is also unknown whether the C-terminal FGF23 peptide (cFGF23), a natural FGF23 antagonist proteolyzed from intact FGF23 (iFGF23), retards CKD progression and improves cardiomyopathy. We addressed these questions in three murine models with high endogenous FGF23 and cardiomyopathy. First, we examined wild-type (WT) mice with CKD induced by unilateral ischemia-reperfusion and contralateral nephrectomy followed by a high-phosphate diet. These mice were continuously treated with intraperitoneal implanted osmotic minipumps containing either iFGF23 protein to further escalate FGF23 bioactivity, cFGF23 peptide to block FGF23 signaling, vehicle, or scrambled peptide as negative controls. Exogenous iFGF23 protein given to CKD mice exacerbated pathological cardiac remodeling and CKD progression, whereas cFGF23 treatment improved heart and kidney function, attenuated fibrosis, and increased circulating soluble Klotho. WT mice without renal insult placed on a high-phosphate diet and homozygous Klotho hypomorphic mice, both of whom develop moderate CKD and clear cardiomyopathy, were treated with cFGF23 or vehicle. Mice treated with cFGF23 in both models had improved heart and kidney function and histopathology. Taken together, these data indicate high endogenous iFGF23 is not just a mere biomarker but pathogenically deleterious in CKD and cardiomyopathy. Furthermore, attenuation of FGF23 bioactivity by cFGF23 peptide is a promising therapeutic strategy to protect the kidney and heart from high FGF23 activity.NEW & NOTEWORTHY There is a strong correlation between cardiovascular morbidity and high circulating fibroblast growth factor 23 (FGF23) levels, but causality was never proven. We used a murine chronic kidney disease (CKD) model to show that intact FGF23 (iFGF23) is pathogenic and contributes to both CKD progression and cardiomyopathy. Blockade of FGF23 signaling with a natural proteolytic product of iFGF23, C-terminal FGF23, alleviated kidney and cardiac histology, and function in three separate murine models of high endogenous FGF23.

Phosphate in Cardiovascular Disease: From New Insights Into Molecular Mechanisms to Clinical Implications

Hyperphosphatemia is a common feature in patients with impaired kidney function and is associated with increased risk of cardiovascular disease. This phenomenon extends to the general population, whereby elevations of serum phosphate within the normal range increase risk; however, the mechanism by which this occurs is multifaceted, and many aspects are poorly understood. Less than 1% of total body phosphate is found in the circulation and extracellular space, and its regulation involves multiple organ cross talk and hormones to coordinate absorption from the small intestine and excretion by the kidneys. For phosphate to be regulated, it must be sensed. While mostly enigmatic, various phosphate sensors have been elucidated in recent years. Phosphate in the circulation can be buffered, either through regulated exchange between extracellular and cellular spaces or through chelation by circulating proteins (ie, fetuin-A) to form calciprotein particles, which in themselves serve a function for bulk mineral transport and signaling. Either through direct signaling or through mediators like hormones, calciprotein particles, or calcifying extracellular vesicles, phosphate can induce various cardiovascular disease pathologies: most notably, ectopic cardiovascular calcification but also left ventricular hypertrophy, as well as bone and kidney diseases, which then propagate phosphate dysregulation further. Therapies targeting phosphate have mostly focused on intestinal binding, of which appreciation and understanding of paracellular transport has greatly advanced the field. However, pharmacotherapies that target cardiovascular consequences of phosphate directly, such as vascular calcification, are still an area of great unmet medical need.

Constitutive Plasma Membrane Turnover in T-REx293 cells via Ordered Membrane Domain Endocytosis under Mitochondrial Control

Clathrin/dynamin-independent endocytosis of ordered plasma membrane domains (ordered membrane domain endocytosis, OMDE) can become massive in response to cytoplasmic Ca elevations, G protein activation by non-hydrolyzable GTP analogs, and enhanced oxidative metabolism. In patch-clamped murine bone marrow macrophages (BMMs), cytoplasmic succinate and pyruvate, but not β-hydroxybutyrate, induce OMDE of 75% of the plasma membrane within 2 min. The responses require palmitoylation of membrane proteins, being decreased by 70% in BMMs lacking the acyltransferase, DHHC5, by treatment with carnitine to shift long-chain acyl groups from cytoplasmic to mitochondrial acyl-CoAs, by bromopalmitate/albumin complexes to block DHHCs, and by the mitochondria-specific cyclosporin, NIM811, to block permeability transition pores that may release mitochondrial coenzyme A into the cytoplasm. Using T-REx293 cells, OMDE amounts to 40% with succinate, pyruvate, or GTPγS, and it is inhibited by actin cytoskeleton disruption. Pyruvate-induced OMDE is blocked by the hydrophobic antioxidant, edaravone, which prevents permeability transition pore openings. Using fluorescent 3kD dextrans to monitor endocytosis, OMDE appears to be constitutively active in T-REx293 cells but not in BMMs. After 1 h without substrates or bicarbonate, pyruvate and hydroxybutyrate inhibit constitutive OMDE, as expected for a shift of CoA from long-chain acyl-CoAs to other CoA metabolites. In the presence of bicarbonate, pyruvate strongly enhances OMDE, which is then blocked by β-hydroxybutyrate, bromopalmitate/albumin complexes, cyclosporines, or edaravone. After pyruvate responses, T-REx293 cells grow normally with no evidence for apoptosis. Fatty acid-free albumin (15 μM) inhibits basal OMDE in T-REx293 cells, as do cyclosporines, carnitine, and RhoA blockade. Surprisingly, OMDE in the absence of substrates and bicarbonate is not inhibited by siRNA knockdown of the acyltransferases, DHHC5 or DHHC2, which are required for activated OMDE in patch clamp experiments. We verify biochemically that small CoA metabolites decrease long-chain acyl-CoAs. We verify also that palmitoylations of many PM-associated proteins decrease and increase when OMDE is inhibited and stimulated, respectively, by different metabolites. STED microscopy reveals that vesicles formed during constitutive OMDE in T-REX293 cells have 90 to 130 nm diameters. In summary, OMDE is likely a major G-protein-dependent endocytic mechanism that can be constitutively active in some cell types, albeit not BMMs. OMDE depends on different DHHC acyltransferases in different circumstances and can be limited by local supplies of fatty acids, CoA, and long-chain acyl-CoAs.

The Effects of Acid on Calcium and Phosphate Metabolism

A variety of changes in mineral metabolism aiming to restore acid-base balance occur in acid loading and metabolic acidosis. Phosphate plays a key role in defense against metabolic acidosis, both as an intracellular and extracellular buffer, as well as in the renal excretion of excess acid in the form of urinary titratable acid. The skeleton acts as an extracellular buffer in states of metabolic acidosis, as the bone matrix demineralizes, leading to bone apatite dissolution and the release of phosphate, calcium, carbonate, and citrate into the circulation. The renal handling of calcium, phosphate and citrate is also affected, with resultant hypercalciuria, hyperphosphaturia and hypocitraturia.

Potassium Magnesium Citrate Is Superior to Potassium Chloride in Reversing Metabolic Side Effects of Chlorthalidone

BACKGROUND

Thiazide diuretics (TD) are the first-line treatment of hypertension because of its consistent benefit in lowering blood pressure and cardiovascular risk. TD is also known to cause an excess risk of diabetes, which may limit long-term use. Although potassium (K) depletion was thought to be the main mechanism of TD-induced hyperglycemia, TD also triggers magnesium (Mg) depletion. However, the role of Mg supplementation in modulating metabolic side effects of TD has not been investigated. Therefore, we aim to determine the effect of potassium magnesium citrate (KMgCit) on fasting plasma glucose and liver fat by magnetic resonance imaging during TD therapy.

METHODS

Accordingly, we conducted a double-blinded RCT in 60 nondiabetic hypertension patients to compare the effects of KCl versus KMgCit during chlorthalidone treatment. Each patient received chlorthalidone alone for 3 weeks before randomization. Primary end point was the change in fasting plasma glucose after 16 weeks of KCl or KMgCit supplementation from chlorthalidone alone.

RESULTS

The mean age of subjects was 59±11 years (30% Black participants). Chlorthalidone alone induced a significant rise in fasting plasma glucose, and a significant fall in serum K, serum Mg, and 24-hour urinary citrate excretion (all P<0.05). KMgCit attenuated the rise in fasting plasma glucose by 7.9 mg/dL versus KCl (P<0.05), which was not observed with KCl. There were no significant differences in liver fat between the 2 groups.

CONCLUSIONS

KMgCit is superior to KCl, the common form of K supplement used in clinical practice, in preventing TD-induced hyperglycemia. This action may improve tolerability and cardiovascular safety in patients with hypertension treated with this drug class.

Endogenous renal adiponectin drives gluconeogenesis through enhancing pyruvate and fatty acid utilization

Adiponectin is a secretory protein, primarily produced in adipocytes. However, low but detectable expression of adiponectin can be observed in cell types beyond adipocytes, particularly in kidney tubular cells, but its local renal role is unknown. We assessed the impact of renal adiponectin by utilizing male inducible kidney tubular cell-specific adiponectin overexpression or knockout mice. Kidney-specific adiponectin overexpression induces a doubling of phosphoenolpyruvate carboxylase expression and enhanced pyruvate-mediated glucose production, tricarboxylic acid cycle intermediates and an upregulation of fatty acid oxidation (FAO). Inhibition of FAO reduces the adiponectin-induced enhancement of glucose production, highlighting the role of FAO in the induction of renal gluconeogenesis. In contrast, mice lacking adiponectin in the kidney exhibit enhanced glucose tolerance, lower utilization and greater accumulation of lipid species. Hence, renal adiponectin is an inducer of gluconeogenesis by driving enhanced local FAO and further underlines the important systemic contribution of renal gluconeogenesis.

Bone Dysregulation in Acute Kidney Injury

Acute kidney injury (AKI) is a highly prevalent condition with multiple acute and chronic consequences. Survivors of AKI are at risk of AKI-to-chronic kidney disease (CKD) transition, which carries significant morbidity and mortality. One retrospective analysis showed increased risk of bone fracture post-AKI in humans, which was independent of CKD development. While there are several theoretical reasons for late disturbances of bone health post-AKI, no definitive data are available to date. An important question is whether there are bone sequelae from AKI that are independent of CKD, meaning bone disease prior to the onset, or in the absence of CKD - a form of "post-AKI osteopathy." While preclinical studies examining bone health after acute stressors have focused mostly on sepsis models, multiple experimental AKI models are readily available for longitudinal bone health interrogation. Future research should be tailored to define whether AKI is a risk factor, independent of CKD, for bone disease and if present, the time course and type of bone disease. This review summarizes a fraction of the existing data to provide some guidance in future research efforts.

Urine pH and Citrate as Predictors of Calcium Phosphate Stone Formation

Key Points

The occurrence of calcium phosphate stones has increased over the past five decades, and this is most notable in female stone formers. High urine pH and hypocitraturia are the most discriminatory urine parameters between calcium phosphate and calcium oxalate stone formers. High urine pH in calcium phosphate stone formers is independent of the effect of dietary alkali and acid.

Background

Urinary parameters, including urine pH and citrate, are recognized as critical in the pathophysiology of calcium-based stones. The factors contributing to variation in these parameters between calcium oxalate (CaOx) and calcium phosphate (CaP) stone formers (SFs) are, however, not well-understood. In this study, using readily available laboratory data, we explore these differences to delineate the odds of forming CaP versus CaOx stones.

Methods

In this single-center retrospective study, we compared serum and urinary parameters between adult CaP SFs, CaOx SFs, and non–stone formers.

Results

Urine pH was higher and urine citrate lower in CaP SFs compared with same-sex CaOx SFs and non–stone formers. In CaP SFs, higher urine pH and lower citrate were independent of markers of dietary acid intake and gastrointestinal alkali absorption, suggesting abnormal renal citrate handling and urinary alkali excretion. In a multivariable model, urine pH and urine citrate were most discriminatory between CaP SFs and CaOx SFs (receiver-operating characteristic area under the curve of 0.73 and 0.65, respectively). An increase in urine pH by 0.35, a decrease in urine citrate by 220 mg/d, a doubling of urine calcium, and female sex all independently doubled the risk of CaP stone formation compared with CaOx stones.

Conclusions

High urine pH and hypocitraturia are two clinical parameters that distinguish the urine phenotype of CaP SFs from CaOx SFs. Alkalinuria is due to intrinsic differences in the kidney independent of intestinal alkali absorption and is accentuated in the female sex.

Serum Magnesium Levels and Cardiovascular Outcomes in Systolic Blood Pressure Intervention Trial Participants

Rationale and Objective

Serum magnesium levels have been inversely yet inconsistently associated with cardiovascular (CV) outcomes. In this study, we examined the association of serum magnesium levels with CV outcomes in the Systolic Blood Pressure Intervention Trial (SPRINT) participants.

Study Design

Case-control post hoc analysis of SPRINT.

Setting & Participants

A total of 2,040 SPRINT participants with available serum samples at baseline level were included in this study. Case participants (n = 510) who experienced a CV event during the SPRINT observation period (median follow-up of 3.2 years) and control participants (n = 1,530) without CV events were sampled in a 1:3 ratio for measurements of serum magnesium level at baseline and 2-year follow-up.

Predictors

Baseline serum magnesium levels and 2-year percentage change in serum magnesium levels (ΔSMg).

Outcome

SPRINT primary composite CV outcome.

Analytical Approach

Multivariable conditional logistic regression analysis, accounting for matching factors, was used to evaluate the association of baseline and ΔSMg with CV outcomes. Individual matching of cases and controls was based on the SPRINT treatment arm allocation (standard vs intensive) and prevalence of chronic kidney disease (CKD).

Results

The median serum magnesium level at baseline was similar among the case and control groups. In a fully adjusted model, each standard deviation (SD) (0.18 mg/dL) higher of the baseline serum magnesium level was independently associated with a lower risk for composite CV outcomes in all study participants (adjusted odds ratio 95% CI, 0.79 [0.70-0.89]). This association was similar when serum magnesium levels were analyzed in quartiles but dissipated in the standard (vs intensive) arm of SPRINT (0.88 [0.76-1.02] vs 0.65 [0.53-0.79], respectively; Pinteraction = 0.06). The presence or absence of CKD at baseline did not modify this association. ΔSMg was not independently associated with CV outcomes occurring after 2 years.

Limitations

ΔSMg was small in magnitude, limiting effect size.

Conclusions

Higher baseline serum magnesium levels were independently associated with reduced risk for CV outcomes in all study participants, but ΔSMg was not associated with CV outcomes.

Uric Acid Transport, Transporters, and their Pharmacological Targeting

Knowledge of uric acid crystallopathies preceded the identification of this compound. How the body handles and transports uric acid proved even more elusive. Over several decades, advances in molecular phenotyping have illuminated this hitherto nebulous field. Closely parallel to the characterization of the transport mechanisms of uric acid in the body was the development of drugs designed to manipulate uric acid levels. In this review, we highlight the study of uric acid transport and transporters. This is an evolving field, and we expect our knowledge of the transport mechanisms to both widen and deepen further. We focus on the best-characterized transporters rather than an exhaustive catalog of all suspected transporters. We review the established and novel compounds that modulate uric acid transport.

VEGFR2 insufficiency enhances phosphotoxicity and undermines Klotho's protection against peritubular capillary rarefaction and kidney fibrosis

Vascular endothelial growth factor (VEGF) and its cognate receptor (VEGFR2) system are crucial for cell functions associated with angiogenesis and vasculogenesis. Klotho contributes to vascular health maintenance in the kidney and other organs in mammals, but it is unknown whether renoprotection by Klotho is dependent on VEGF/VEGFR2 signaling. We used heterozygous VEGFR2-haploinsufficient (VEGFR2+/-) mice resulting from heterozygous knockin of green fluorescent protein in the locus of fetal liver kinase 1 encoding VEGFR2 to test the interplay of Klotho, phosphate, and VEGFR2 in kidney function, the vasculature, and fibrosis. VEGFR2+/- mice displayed downregulated VEGF/VEGFR2 signaling in the kidney, lower density of peritubular capillaries, and accelerated kidney fibrosis, all of which were also found in the homozygous Klotho hypomorphic mice. High dietary phosphate induced higher plasma phosphate, greater peritubular capillary rarefaction, and more kidney fibrosis in VEGFR2+/- mice compared with wild-type mice. Genetic overexpression of Klotho significantly attenuated the elevated plasma phosphate, kidney dysfunction, peritubular capillary rarefaction, and kidney fibrosis induced by a high-phosphate diet in wild-type mice but only modestly ameliorated these changes in the VEGFR2+/- background. In cultured endothelial cells, VEGFR2 inhibition reduced free VEGFR2 but enhanced its costaining of an endothelial marker (CD31) and exacerbated phosphotoxicity. Klotho protein maintained VEGFR2 expression and attenuated high phosphate-induced cell injury, which was reduced by VEGFR2 inhibition. In conclusion, normal VEGFR2 function is required for vascular integrity and for Klotho to exert vascular protective and antifibrotic actions in the kidney partially through the regulation of VEGFR2 function.NEW & NOTEWORTHY This research paper studied the interplay of vascular endothelial growth factor receptor type 2 (VEGFR2), high dietary phosphate, and Klotho, an antiaging protein, in peritubular structure and kidney fibrosis. Klotho protein was shown to maintain VEGFR2 expression in the kidney and reduce high phosphate-induced cell injury. However, Klotho cytoprotection was attenuated by VEGFR2 inhibition. Thus, normal VEGFR2 function is required for vascular integrity and Klotho to exert vascular protective and antifibrotic actions in the kidney.

Prediction of Mortality and Major Adverse Kidney Events in Critically Ill Patients With Acute Kidney Injury

RATIONALE & OBJECTIVE

Risk prediction tools for assisting acute kidney injury (AKI) management have focused on AKI onset but have infrequently addressed kidney recovery. We developed clinical models for risk stratification of mortality and major adverse kidney events (MAKE) in critically ill patients with incident AKI.

STUDY DESIGN

Multicenter cohort study.

SETTING & PARTICIPANTS

9,587 adult patients admitted to heterogeneous intensive care units (ICUs; March 2009 to February 2017) who experienced AKI within the first 3 days of their ICU stays.

PREDICTORS

Multimodal clinical data consisting of 71 features collected in the first 3 days of ICU stay.

OUTCOMES

(1) Hospital mortality and (2) MAKE, defined as the composite of death during hospitalization or within 120 days of discharge, receipt of kidney replacement therapy in the last 48 hours of hospital stay, initiation of maintenance kidney replacement therapy within 120 days, or a ≥50% decrease in estimated glomerular filtration rate from baseline to 120 days from hospital discharge.

ANALYTICAL APPROACH

Four machine-learning algorithms (logistic regression, random forest, support vector machine, and extreme gradient boosting) and the SHAP (Shapley Additive Explanations) framework were used for feature selection and interpretation. Model performance was evaluated by 10-fold cross-validation and external validation.

RESULTS

One developed model including 15 features outperformed the SOFA (Sequential Organ Failure Assessment) score for the prediction of hospital mortality, with areas under the curve of 0.79 (95% CI, 0.79-0.80) and 0.71 (95% CI, 0.71-0.71) in the development cohort and 0.74 (95% CI, 0.73-0.74) and 0.71 (95% CI, 0.71-0.71) in the validation cohort (P < 0.001 for both). A second developed model including 14 features outperformed KDIGO (Kidney Disease: Improving Global Outcomes) AKI severity staging for the prediction of MAKE: 0.78 (95% CI, 0.78-0.78) versus 0.66 (95% CI, 0.66-0.66) in the development cohort and 0.73 (95% CI, 0.72-0.74) versus 0.67 (95% CI, 0.67-0.67) in the validation cohort (P < 0.001 for both).

LIMITATIONS

The models are applicable only to critically ill adult patients with incident AKI within the first 3 days of an ICU stay.

CONCLUSIONS

The reported clinical models exhibited better performance for mortality and kidney recovery prediction than standard scoring tools commonly used in critically ill patients with AKI in the ICU. Additional validation is needed to support the utility and implementation of these models.

PLAIN-LANGUAGE SUMMARY

Acute kidney injury (AKI) occurs commonly in critically ill patients admitted to the intensive care unit (ICU) and is associated with high morbidity and mortality rates. Prediction of mortality and recovery after an episode of AKI may assist bedside decision making. In this report, we describe the development and validation of a clinical model using data from the first 3 days of an ICU stay to predict hospital mortality and major adverse kidney events occurring as long as 120 days after hospital discharge among critically ill adult patients who experienced AKI within the first 3 days of an ICU stay. The proposed clinical models exhibited good performance for outcome prediction and, if further validated, could enable risk stratification for timely interventions that promote kidney recovery.

Strategies to lower fibroblast growth factor 23 bioactivity

Fibroblast growth factor 23 (FGF23) is a circulating hormone derived from the bone whose release is controlled by many factors and exerts a multitude of systemic actions. There are congenital and acquired disorders of increased and decreased FGF23 levels. In chronic kidney disease (CKD), elevations of FGF23 levels can be 1000-fold above the upper physiological limit. It is still debated whether this high FGF23 in CKD is a biomarker or causally related to morbidity and mortality. Data from human association studies support pathogenicity, while experimental data are less robust. Knowledge of the biology and pathobiology of FGF23 has generated a plethora of means to reduce FGF23 bioactivity at many levels that will be useful for therapeutic translations. This article summarizes these approaches and addresses several critical questions that still need to be answered.

Preclinical and Clinical Evidence of Effect of Acid on Bone Health

Acid can have ill effect on bone health in the absence of frank clinical acidosis but affecting the bone mioneral matrix and bone cells via complex pathways botyh ascute;y and chronically. While the reaction of bone to an acid load is conserved in evolution and is adaptive, the capacity can be overwhelmed resulting in dire consequences. The preclinical an clincl evidence of the acdi effect on bone is very convincing and the clinical evidence in both association and interventiopn studies are also quite credible, The adverse effects of acid on bone is underappreoicated, under-investigated, and the potential benefits of alkali therapy is not generrally known.

Renal Clearance of Fibroblast Growth Factor-23 (FGF23) and its Fragments in Humans

Relative abundance of fibroblast growth factor-23 (FGF23) measured by the C-terminal (cFGF23, which measures both intact FGF23 and C-terminal fragments) versus intact (iFGF23, measures only intact hormone) assays varies by kidney function in humans. Differential kidney clearance may explain this finding. We measured cFGF23 and iFGF23 in the aorta and bilateral renal veins of 162 patients with essential hypertension undergoing renal angiography. Using multivariable linear regression, we examined factors associated with aorta to renal vein reduction of FGF23 using both assays. Similar parameters and with addition of urine concentrations of cFGF23 and iFGF23 were measured in six Wistar rats. Mean ± standard deviation (SD) age was 54 ± 12 years, 54% were women, and mean creatinine clearance was 72 ± 48 mL/min/100 g. The human kidney reduced the concentrations of both cFGF23 (16% ± 12%) and iFGF23 (21% ± 16%), but reduction was higher for iFGF23. Greater kidney creatinine and PTH reductions were each independently associated with greater reductions of both cFGF23 and iFGF23. The greater kidney reduction of iFGF23 compared to cFGF23 appeared stable and consistent across the range of creatinine clearance evaluated. Kidney clearance was similar, and urine concentrations of both assays were low in the rat models, suggesting kidney metabolism of both cFGF23 and iFGF23. Renal reduction of iFGF23 is higher than that of creatinine and cFGF23. Our data suggest that FGF23 is metabolized by the kidney. However, the major cell types involved in metabolization of FGF23 requires future study. Kidney clearance of FGF23 does not explain differences in C-terminal and intact moieties across the range of kidney function. © 2022 American Society for Bone and Mineral Research (ASBMR).

Serum IL-17 levels are higher in critically ill patients with AKI and associated with worse outcomes

Background

Interleukin-17 (IL-17) antagonism in rats reduces the severity and progression of AKI. IL-17-producing circulating T helper-17 (TH17) cells is increased in critically ill patients with AKI indicating that this pathway is also activated in humans. We aim to compare serum IL-17A levels in critically ill patients with versus without AKI and to examine their relationship with mortality and major adverse kidney events (MAKE).

Methods

Multicenter, prospective study of ICU patients with AKI stage 2 or 3 and without AKI. Samples were collected at 24–48 h after AKI diagnosis or ICU admission (in those without AKI) [timepoint 1, T1] and 5–7 days later [timepoint 2, T2]. MAKE was defined as the composite of death, dependence on kidney replacement therapy or a reduction in eGFR of ≥ 30% from baseline up to 90 days following hospital discharge.

Results

A total of 299 patients were evaluated. Patients in the highest IL-17A tertile (versus lower tertiles) at T1 had higher acuity of illness and comorbidity scores. Patients with AKI had higher levels of IL-17A than those without AKI: T1 1918.6 fg/ml (692.0–5860.9) versus 623.1 fg/ml (331.7–1503.4), p < 0.001; T2 2167.7 fg/ml (839.9–4618.9) versus 1193.5 fg/ml (523.8–2198.7), p = 0.006. Every onefold higher serum IL-17A at T1 was independently associated with increased risk of hospital mortality (aOR 1.35, 95% CI: 1.06–1.73) and MAKE (aOR 1.26, 95% CI: 1.02–1.55). The highest tertile of IL-17A (vs. the lowest tertile) was also independently associated with higher risk of MAKE (aOR 3.03, 95% CI: 1.34–6.87). There was no effect modification of these associations by AKI status. IL-17A levels remained significantly elevated at T2 in patients that died or developed MAKE.

Conclusions

Serum IL-17A levels measured by the time of AKI diagnosis or ICU admission were differentially elevated in critically ill patients with AKI when compared to those without AKI and were independently associated with hospital mortality and MAKE.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-022-03976-4.

“Ideal” parathyroid hormone in erythropoietin‐stimulating agents‐resistant anemia

Erythropoietin‐stimulating agents (ESAs) have revolutionized anemia treatment in end‐stage renal disease (ESRD), but ESA resistance is increasingly identified. Secondary hyperparathyroidism (SHP) is one cause of ESA resistance. We describe a patient with ESA‐resistant, transfusion‐dependent anemia and mild SHP with remodeling and reticulin fibrosis on bone marrow biopsy, all of which resolved with stricter SHP management. We identified 64 patients with anemia, ESRD, and bone marrow biopsy. The parathyroid hormone (PTH) range for bony remodeling was 183–16,161.9 pg/ml versus 90.8–3283 pg/ml. The PTH range for fibrotic changes was 183–2487 pg/ml versus 90.8–16,161.9 pg/ml. We found no clear PTH range predictive for bone marrow changes.

Phosphate and Cellular Senescence

Cellular senescence is one type of permeant arrest of cell growth and one of increasingly recognized contributor to aging and age-associated disease. High phosphate and low Klotho individually and synergistically lead to age-related degeneration in multiple organs. Substantial evidence supports the causality of high phosphate in cellular senescence, and potential contribution to human aging, cancer, cardiovascular, kidney, neurodegenerative, and musculoskeletal diseases. Phosphate can induce cellular senescence both by direct phosphotoxicity, and indirectly through downregulation of Klotho and upregulation of plasminogen activator inhibitor-1. Restriction of dietary phosphate intake and blockage of intestinal absorption of phosphate help suppress cellular senescence. Supplementation of Klotho protein, cellular senescence inhibitor, and removal of senescent cells with senolytic agents are potential novel strategies to attenuate phosphate-induced cellular senescence, retard aging, and ameliorate age-associated, and phosphate-induced disorders.

In vivo evidence for therapeutic applications of beclin 1 to promote recovery and inhibit fibrosis after acute kidney injury

Autophagy regulator beclin 1 activity determines the severity of kidney damage induced by ischemia reperfusion injury, but its role in kidney recovery and fibrosis are unknown and its therapeutic potentials have not been tested. Here, we explored beclin 1 effects on kidney fibrosis in three models of acute kidney injury (AKI)-ischemia reperfusion injury, cisplatin kidney toxicity, and unilateral ureteric obstruction in mouse strains with three levels of beclin 1 function: normal (wild type), low (heterozygous global deletion of beclin 1, Becn1^+/-), and high beclin 1 activity (knockin gain-of-function mutant Becn1, Becn1^FA). Fourteen days after AKI induction, heterozygous mice had more, but knockin mice had less kidney fibrosis than wild-type mice did. One day after ischemia reperfusion injury, heterozygous pan-kidney tubular Becn1 null mice had more severe kidney damage than homozygous distal tubular Becn1 null mice did, which was similar to the wild-type mice, implying that proximal tubular beclin 1 protects the kidney against ischemia reperfusion injury. By 14 days, both pan-kidney heterozygous Becn1 null and distal tubular homozygous Becn1 null mice had poorer kidney recovery than wild-type mice did. Injection of beclin 1 peptides increased cell proliferation in kidney tubules in normal mice. Beclin 1 peptides injection either before or after (2-5 days) ischemia reperfusion injury protected the kidney from injury and suppressed kidney fibrosis. Thus, both endogenous beclin 1 protein expression in kidney tubules and exogenous beclin 1 peptides are kidney protective via attenuation of acute kidney damage, promotion of cell proliferation, and inhibition of kidney fibrosis, consequently improving kidney recovery post-AKI. Hence, exogenous beclin 1 peptide may be a potential new therapy for AKI.

Physiologic Regulation of Systemic Klotho Levels by Renal CaSR Signaling in Response to CaSR Ligands and pH o

BACKGROUND

The kidney is the source of sKlotho and kidney-specific loss of Klotho leads to a phenotype resembling the premature multiorgan failure phenotype in Klotho-hypomorphic mice ( kl/kl mice). Klotho and the Ca-sensing receptor (CaSR) are highly expressed in the distal convoluted tubule (DCT). The physiologic mechanisms that regulate sKlotho levels are unknown.

METHODS

We measured sKlotho in WT and tubule-specific CaSR -/- (TS-CaSR -/- ) mice treated with calcimimetics, alkali, or acid, and Klotho shed from minced mouse kidneys, and from HEK-293 cells expressing the CaSR and Klotho, in response to calcimimetics, calcilytics, alkalotic and acidic pH, and ADAM protease inhibitors. The CaSR, Klotho, and ADAM10 were imaged in mouse kidneys and cell expression systems using confocal microscopy.

RESULTS

The CaSR, Klotho, and ADAM10 colocalize on the basolateral membrane of the DCT. Calcimimetics and HCO 3 increase serum sKlotho levels in WT but not in CaSR -/- mice, and acidic pH suppresses sKlotho levels in WT mice. In minced kidneys and cultured cells, CaSR activation with high Ca, calcimimetics, or alkali increase shed Klotho levels via ADAM10, as demonstrated using the ADAM10 inhibitor GI254023X and siRNA. In cultured cells, the CaSR, Klotho, and ADAM10 form cell surface aggregates that disperse after CaSR activation.

CONCLUSIONS

We identify a novel physiologic mechanism for regulation of sKlotho levels by the renal CaSR-ADAM10-Klotho pathway. We show that CaSR activators, including alkali, increase renal CaSR-stimulated Klotho shedding and predict that this mechanism is relevant to the effects of acidosis and alkali therapy on CKD progression.

Hypophosphatemia in acute liver failure of a broad range of etiologies is associated with phosphaturia without kidney damage or phosphatonin elevation

Hypophosphatemia is a common and dangerous complication of acute liver failure (ALF) of various etiologies. While various mechanisms for ALF-associated hypophosphatemia have been proposed including high phosphate uptake into regenerating hepatocytes, acetaminophen (APAP)-associated hypophosphatemia was linked to renal phosphate wasting, and APAP-induced renal tubular injury was proposed as underlying mechanism. We studied 30 normophosphatemic and 46 hypophosphatemic (serum phosphate < 2.5 mg/dL) patients from the Acute Liver Failure Study Group registry with APAP- or non-APAP-induced ALF. Since kidney injury affects phosphate excretion, patients with elevated serum creatinine (>1.2 mg/dL) were excluded. Maximal amount of renal tubular phosphate reabsorption per filtered volume (TmP/GFR) was calculated from simultaneous serum and urine phosphate and creatinine levels to assess renal phosphate handling. Instead of enhanced renal phosphate reabsorption as would be expected during hypophosphatemia of non-renal causes, serum phosphate was positively correlated with TmP/GFR in both APAP- and non-APAP-induced ALF patients (R2 = 0.66 and 0.46, respectively; both P < 0.0001), indicating renal phosphate wasting. Surprisingly, there was no evidence of kidney damage based on urinary markers including neutrophil gelatinase-associated lipocalin and cystatin C even in the APAP group. Additionally, there was no evidence that the known serum phosphatonins parathyroid hormone, fibroblast growth factor 23, and α-Klotho contribute to the observed hypophosphatemia. We conclude that the observed hypophosphatemia with renal phosphate wasting in both APAP- and non-APAP-mediated ALF is likely the result of renal tubular phosphate leak from yet-to-be identified factor(s) with no evidence for proximal tubular damage or contribution of known phosphatonins.

A generic crystallopathic model for chronic kidney disease progression

Chronic kidney disease (CKD) has reached epidemic proportions globally. The natural course of chronic kidney disease is almost uniformly progressive, albeit at different rates in different individuals. The downhill course appears to pervade kidney diseases of all etiologies and seems to spiral down a self-perpetuating vortex, even if the original insult is ameliorated or controlled. In this issue of the JCI, Shiizaki, Tsubouchi, and colleagues proposed a model of renal tubule luminal calcium phosphate crystallopathy that accounts for renal function demise. Calcium phosphate crystals attached to TLR4 and underwent endocytosis at the brush border, triggering inflammation and fibrosis. This mechanism might operate in different kinds of kidney disease, with a theoretical phosphate concentration threshold in the proximal tubular lumen, beyond which is triggered undesirable downstream effects that eventuate in loss of renal function. If this model parallels human CKD, clinicians may focus efforts on determining phosphate exposure in the proximal tubular lumen.

Serum renin and major adverse kidney events in critically ill patients: a multicenter prospective study

BACKGROUND

Preliminary studies have suggested that the renin-angiotensin system is activated in critical illness and associated with mortality and kidney outcomes. We sought to assess in a larger, multicenter study the relationship between serum renin and Major Adverse Kidney Events (MAKE) in intensive care unit (ICU) patients.

METHODS

Prospective, multicenter study at two institutions of patients with and without acute kidney injury (AKI). Blood samples were collected for renin measurement a median of 2 days into the index ICU admission and 5-7 days later. The primary outcome was MAKE at hospital discharge, a composite of mortality, kidney replacement therapy, or reduced estimated glomerular filtration rate to ≤ 75% of baseline.

RESULTS

Patients in the highest renin tertile were more severely ill overall, including more AKI, vasopressor-dependence, and severity of illness. MAKE were significantly greater in the highest renin tertile compared to the first and second tertiles. In multivariable logistic regression, this initial measurement of renin remained significantly associated with both MAKE as well as the individual component of mortality. The association of renin with MAKE in survivors was not statistically significant. Renin measurements at the second time point were also higher in patients with MAKE. The trajectory of the renin measurements between time 1 and 2 was distinct when comparing death versus survival, but not when comparing MAKE versus those without.

CONCLUSIONS

In a broad cohort of critically ill patients, serum renin measured early in the ICU admission is associated with MAKE at discharge, particularly mortality.

Constitutive transgenic alpha-Klotho overexpression enhances resilience to and recovery from murine acute lung injury

AIMS

Normal lungs do not express alpha-Klotho (Klotho) protein but derive cytoprotection from circulating soluble Klotho. It is unclear whether chronic supranormal Klotho levels confer additional benefit. To address this, we tested the age-related effects of Klotho overexpression on acute lung injury (ALI) and recovery.

METHODS

Transgenic Klotho-overexpressing (Tg-Kl) and wild-type (WT) mice (2 and 6 months old) were exposed to hyperoxia (95% O₂; 72 h) then returned to normoxia (21% O₂; 24 h) (Hx-R). Control mice were kept in normoxia. Renal and serum Klotho, lung histology, and bronchoalveolar lavage fluid oxidative damage markers were assessed. Effects of hyperoxia were tested in human embryonic kidney cells stably expressing Klotho. A549 lung epithelial cells transfected with Klotho cDNA or vector were exposed to cigarette smoke; lactate dehydrogenase and double-strand DNA breaks were measured.

RESULTS

Serum Klotho decreased with age. Hyperoxia suppressed renal Klotho at both ages and serum Klotho at 2-months of age. Tg-Kl mice at both ages and 2-months-old WT mice survived Hx-R; 6-months-old Tg-Kl mice showed lower lung damage than age-matched WT mice. Hyperoxia directly inhibited Klotho expression and release in vitro; Klotho transfection attenuated cigarette smoke-induced cytotoxicity and DNA double-strand breaks in lung epithelial cells.

CONCLUSIONS

Young animals with chronic high baseline Klotho expression are more resistant to ALI. Chronic constitutive Klotho overexpression in older Tg-Kl animals attenuates hyperoxia-induced lung damage and improves survival and short-term recovery despite an acute reduction in serum Klotho level during injury. We conclude that chronic enhancement of Klotho expression increases resilience to ALI.

Evidence for abnormal linkage between urine oxalate and citrate excretion in human kidney stone formers

BACKGROUND

Animal models have demonstrated an interactive relationship between the epithelial anion exchanger SLC26A6 and transporter NaDC-1 that regulates citrate and oxalate homeostasis. This relationship is a potential mechanism to protect against kidney stones as higher urine oxalate is accompanied by higher urine citrate but it has not been explored in humans.

METHODS

We examined 24-h urine data on 13,155 kidney stone forming patients (SF) from separate datasets at the University of Chicago and Litholink, a national laboratory, and 143 non-kidney stone forming participants (NSF) to examine this relationship in humans. We used multivariate linear regression models to examine the association between oxalate and citrate in all study participants and separately in SF and NSF.

RESULTS

Higher urinary oxalate was associated with higher urinary citrate in both SF and NSF. In NSF, the multivariate adjusted urine citrate excretion was 3.0 (1.5-4.6) (mmol)/creatinine (mmol) per oxalate (mmol)/creatinine (mmol). In SF, the multivariate adjusted urine citrate excretion was 0.3 (0.2-0.4) (mmol)/creatinine (mmol) per oxalate (mmol)/creatinine (mmol).

CONCLUSIONS

Higher urinary oxalate excretion was associated with higher urinary citrate excretion and this effect was larger in non-kidney stone forming participants compared with those who form kidney stones.

Sepsis-Associated Acute Kidney Disease and Long-term Kidney Outcomes

RATIONALE & OBJECTIVE

Sepsis-associated acute kidney injury often leads to acute kidney disease (AKD), predisposing patients to long-term complications such as chronic kidney disease (CKD), kidney failure with replacement therapy (KFRT), or mortality. Risk stratification of patients with AKD represents an opportunity to assist with prognostication of long-term kidney complications.

STUDY DESIGN

Single-center retrospective cohort.

SETTING & PARTICIPANTS

6,290 critically ill patients admitted to the intensive care unit with severe sepsis or septic shock. Patients were separated into cohorts based on incident acute kidney injury or not, and survivors identified who were alive and free of KFRT up to 90 days.

PREDICTORS

AKD stage (0A, 0C, or ≥1) using the last serum creatinine concentration available by discharge or up to 90 days postdischarge.

OUTCOME

Time to development of incident CKD, progression of CKD, KFRT, or death.

ANALYTICAL APPROACH

Multivariable Cox proportional hazards models.

RESULTS

Patients surviving kidney injury associated with sepsis often fail to return to baseline kidney function by discharge: 577/1,231 (46.9%) with stage 0C or 1 or greater AKD. AKD stage was significantly associated with the composite primary outcome. Stages 0C AKD and 1 or greater AKD were significantly and progressively associated with the primary outcome when compared with stage 0A AKD (adjusted HR [aHR], 1.74; 95% CI, 1.32-2.29, and aHR, 3.25; 95% CI, 2.52-4.20, respectively). Additionally, stage 1 or greater AKD conferred higher risk above stage 0C AKD (aHR, 1.87; 95% CI, 1.44-2.43). CKD incidence or progression and KFRT, more so than mortality, occurred with greater frequency in higher stages of AKD.

LIMITATIONS

Retrospective design, single center, exclusion of patients with KFRT within 90 days of discharge, potential ascertainment bias, and inability to subclassify above AKD stage 1.

CONCLUSIONS

Risk stratification using recommended AKD stages at hospital discharge or shortly thereafter associates with the development of long-term kidney outcomes following sepsis-associated acute kidney injury.

In search of alternatively spliced alpha-Klotho Kl1 protein in mouse brain

Alpha‐Klotho is a multi‐functional protein essential for maintenance of a myriad of cell functions. αKlotho is a single transmembrane protein with a large extracellular segment consisting of two domains (termed Kl1 and Kl2) which is shed into the extracellular fluid by proteolytic cleavage to furnish circulating soluble αKlotho. Based on cDNA sequence, an alternatively spliced mRNA is predicted to translate to a putative soluble αKlotho protein in mouse and human with only the Kl1 domain that represents a “spliced αKlotho Kl1” (spKl1) and is released from the cell without membrane targeting or cleavage. The existence of this protein remains in silico for two decades. We generated a novel antibody (anti‐spE15) against the 15 amino acid epitope (E15; VSPLTKPSVGLLLPH) which is not present in Kl1 or full‐length αKlotho and validated its specific reactivity against spKl1 in vitro. Using anti‐spE15 and two well‐established anti‐αKlotho monoclonal antibodies, we performed immunoblots, immunoprecipitation, and immunohistochemistry to investigate for expression of spKl1 in the mouse brain. We found anti‐spE15 labeling in mouse brain but were not able to see co‐labelling of Kl1 and spE15 epitopes on the same protein, which is the pre‐requisite for the existence of a spKl1 polypeptide, indicating that anti‐spE15 likely binds to another protein other than the putative spKl1. In isolated choroid plexus from mouse brain, we found strong staining with anti‐spE15, but did not find the spliced αKlotho transcript. We conclude that using reliable reagents and inclusion of proper controls, there is no evidence of the spKl1 protein in the mouse brain.

High Dietary Phosphate Exacerbates and Acts Independently of Low Autophagy Activity in Pathological Cardiac Remodeling and Dysfunction

High phosphate contributes to uremic cardiomyopathy. Abnormal autophagy is associated with the development and progression of heart disease. What is unknown is the effects of phosphate on autophagy and whether the ill effects of phosphate on cardiomyocytes are mediated by low autophagy. High (2.0% w/w)-phosphate diet reduced LC3 puncta in cardiomyocytes and ratio of LC3 II/I and increased p62 protein, indicating that autophagy activity was suppressed. Mice with cardiomyocyte-specific deletion of autophagy-related protein 5 (H-atg5-/-) had reduced autophagy only in the heart, developed cardiac dysfunction with hypertrophy and fibrosis, and had a short lifespan. When H-atg5-/- mice were fed a high-phosphate diet, they developed more apoptosis in cardiomyocytes, more severe cardiac remodeling, and shorter lifespan than normal phosphate-fed H-atg5-/- mice, indicating that cardiac phosphotoxicity is imparted independently of atg5. In conclusion, although high phosphate suppresses autophagy, high phosphate and low autophagy independently trigger and additionally amplify cardiac remodeling and dysfunction.

Assessment of a modified renal angina index for AKI prediction in critically ill adults

BACKGROUND

The renal angina index (RAI) is a useful tool for risk-stratification of acute kidney injury (AKI) in critically ill children. We evaluated the performance of a modified adult RAI (mRAI) for the risk-stratification of AKI in critically ill adults.

METHODS

We used two independent ICU cohorts: 13,965 adult patients from the University of Kentucky (UKY) and 4,789 from UT Southwestern (UTSW). The mRAI included: diabetes, presence of sepsis, mechanical ventilation, pressor/inotrope use, percentage change in SCr in reference to admission SCr (ΔSCr), and fluid overload percentage within the first day of ICU admission. The primary outcome was AKI stage ≥2 at Day 2-7. Performance and reclassification metrics were determined for the mRAI score compared to ΔSCr alone.

RESULTS

The mRAI score outperformed ΔSCr and readjusted probabilities to predict AKI stage ≥2 at Day 2-7: C-statistic: UKY 0.781 vs. 0.708 (IDI 2.2%) and UTSW 0.766 vs. 0.696 (IDI 1.8%), p<0.001 for both. In the UKY cohort, only 3.3% of patients with mRAI score <10 had the AKI event, while 16.4% of patients with mRAI score of ≥ 10 had the AKI event (NPV 96.8%). Similar findings were observed in the UTSW cohort as part of external validation.

CONCLUSIONS

In critically ill adults, the adult mRAI score determined within the first day of ICU admission outperformed changes in SCr for the prediction of AKI stage ≥2 at Day 2-7 of ICU stay. The modified adult RAI is a feasible tool for AKI risk-stratification in adult patients in the ICU.

Kidney Biomarkers and Major Adverse Kidney Events in Critically Ill Patients

Background

Several biomarkers of AKI have been examined for their ability to predict AKI before serum creatinine. Few studies have focused on using kidney biomarkers to better predict major adverse kidney events (MAKE), an increasingly used composite outcome in critical care nephrology research.

Methods

Single-center prospective study collecting blood and urine samples from critically ill patients with AKI Kidney Disease Improving Global Outcomes stage 2 or above, and matched controls from a single, tertiary care intensive care unit (ICU). Samples were collected at 24-48 hours after AKI diagnosis (patients) or ICU admission (controls), 5-7 days later, and 4-6 weeks after discharge for patients with AKI. The primary outcome of interest was MAKE at hospital discharge (MAKE-DC), consisting of the composite end point of death, RRT dependence, or a decrease in estimated glomerular filtration to <75% of baseline.

Results

Serum/urinary neutrophil gelatinase-associated lipocalin (NGAL), serum/urinary cystatin C, and urinary kidney injury molecule-1 early in the AKI or ICU course were all significantly higher in patients with MAKE-DC compared with those not experiencing MAKE-DC. Additionally, serum/urinary NGAL and serum cystatin C measurements at the first time point remained significantly associated with MAKE events at 3, 6, and 12 months. Serum cystatin C, and to a lesser extent serum NGAL, significantly improved upon a logistic regression clinical prediction model of MAKE-DC (AUROC 0.94 and 0.87 versus 0.83; P=0.001 and P=0.02, respectively). Patients without MAKE-DC experienced a greater decline in serum NGAL from first to second measurement than those patients experiencing MAKE-DC.

Conclusions

Early measures of kidney biomarkers in patients who are critically ill are associated with MAKE-DC. This relationship appears to be greatest with serum NGAL and cystatin C, which display additive utility to a clinical prediction model. Trending serum NGAL may also have utility in predicting MAKE-DC.

Klotho in Clinical Nephrology: Diagnostic and Therapeutic Implications

αKlotho (called Klotho here) is a membrane protein that serves as the coreceptor for the circulating hormone fibroblast growth factor 23 (FGF23). Klotho is also cleaved and released as a circulating substance originating primarily from the kidney and exerts a myriad of housekeeping functions in just about every organ. The vital role of Klotho is shown by the multiorgan failure with genetic deletion in rodents, with certain features reminiscent of human disease. The most common causes of systemic Klotho deficiency are AKI and CKD. Preclinical data on Klotho biology have advanced considerably and demonstrated its potential diagnostic and therapeutic value; however, multiple knowledge gaps exist in the regulation of Klotho expression, release, and metabolism; its target organs; and mechanisms of action. In the translational and clinical fronts, progress has been more modest. Nonetheless, Klotho has potential clinical applications in the diagnosis of AKI and CKD, in prognosis of progression and extrarenal complications, and finally, as replacement therapy for systemic Klotho deficiency. The overall effect of Klotho in clinical nephrology requires further technical advances and additional large prospective human studies.

Uric Acid and Urate in Urolithiasis: The Innocent Bystander, Instigator, and Perpetrator

Uric acid is an end product of purine metabolism in human beings. An unusual and still unexplained phenomenon is that higher primates have relatively high uric acid levels in body fluids owing to a combination of absence of degradation and renal retention. The physiologic purpose of high uric acid levels still is enigmatic, but the pathobiologic burden is a variety of crystallopathies owing to the low aqueous solubility of uric acid such as gouty arthritis and acute uric acid nephropathy. In the urinary space, three distinct conditions result from chronic uric acid and/or urate precipitation. The first and most common variety is uric acid urolithiasis. In this condition, urate is a victim of a systemic metabolic disease in which increased acid load to the kidney is coupled with diminished urinary buffer capacity owing to defective ammonium excretion, resulting in titration of urate to its sparingly soluble protonated counterpart, uric acid, and the formation of stones. Uric acid is the innocent bystander of the crime. The second variety is hyperuricosuric calcium urolithiasis, in which uric acid confers lithogenicity via promotion of calcium oxalate precipitation by multiple mechanisms involving soluble, colloidal, and crystalline urate salts. Uric acid is the instigator of the crime. The third and least common condition involves urate as an integral part of the urolith as an ammonium salt driven by high ammonium and high urate concentrations in urine. Here, uric acid is one of the perpetrators of the crime. Both known and postulated pathogenesis of these three types of urolithiasis are reviewed and summarized.

Dietary vitamin D interacts with high phosphate-induced cardiac remodeling in rats with normal renal function

BACKGROUND

Vitamin D (VD) and phosphate (Pi) load are considered as contributors to cardiovascular disease in chronic kidney disease and the general population, but interactive effects of VD and Pi intake on the heart are not clearly illustrated.

METHODS

We fed normal male rats with three levels of dietary VD (100, 1100 or 5000 IU/kg chow) and Pi (0.2, 0.6 or 1.6%) (3X3 design) for 8 weeks and examined renal and cardiac function and histology.

RESULTS

High dietary Pi decreased plasma and renal Klotho and plasma 25-hydroxyvitamin D, and increased plasma Pi, fibroblast growth factor 23 and parathyroid hormone without affecting renal function, while low Pi increased plasma and renal Klotho. Both low and high VD diets enhanced high Pi-reduced Klotho expression. Low dietary VD reduced-plasma Klotho was rescued by a low Pi diet. High dietary Pi reduced-cardiac ejection fraction was not modified by a low or high VD diet, but the dietary VD effects on cardiac pathologic changes were more complex. High dietary Pi-induced cardiac hypertrophy was attenuated by a low VD and exacerbated by a high VD diet. In contrast, high dietary Pi -induced cardiac fibrosis was magnified by a low VD and attenuated by a high VD diet.

CONCLUSIONS

High Pi diet induces hypertrophy and fibrosis in left ventricles, a low VD diet accelerates high Pi-induced fibrosis, and a high VD diet exacerbated high Pi -induced hypertrophy. Therefore, cardiac phosphotoxicity is exacerbated by either high or low dietary VD in rats with normal kidney function.

Fibroblast Growth Factor 23 and αKlotho in Acute Kidney Injury: Current Status in Diagnostic and Therapeutic Applications

Fibroblast growth factor (FGF) 23 and αKlotho are circulating mineral regulatory substances that also have a very diverse range of actions. Acute kidney injury (AKI) is a state of high FGF23 and low αKlotho. Clinical association data for FGF23 are strong, but the basic pathobiology of FGF23 in AKI is rather sparse. Conversely, preclinical data supporting a pathogenic role of αKlotho in AKI are strong, but the human data are still being generated. This pair of substances can potentially serve as diagnostic and prognostic biomarkers. FGF23 blockade and αKlotho restoration can have prophylactic and therapeutic utility in AKI. The literature to date is briefly reviewed in this article.

High Phosphate Induces and Klotho Attenuates Kidney Epithelial Senescence and Fibrosis

Cellular senescence is an irreversible cell growth arrest and is associated with aging and age-related diseases. High plasma phosphate (Pi) and deficiency of Klotho contribute to aging and kidney fibrosis, a pathological feature in the aging kidney and chronic kidney disease. This study examined the interactive role of Pi and Klotho in kidney senescence and fibrosis. Homozygous Klotho hypomorphic mice had high plasma Pi, undetectable Klotho in plasma and kidney, high senescence with massive collagen accumulation in kidney tubules, and fibrin deposits in peritubular capillaries. To examine the Pi effect on kidney senescence, a high (2%) Pi diet was given to wild-type mice. One week of high dietary Pi mildly increased plasma Pi, and upregulated kidney p16/p21 expression, but did not significantly decrease Klotho. Two weeks of high Pi intake led to increase in plasminogen activator inhibitor (PAI)-1, and decrease in kidney Klotho, but still without detectable increase in kidney fibrosis. More prolonged dietary Pi for 12 weeks exacerbated kidney senescence and fibrosis; more so in heterozygous Klotho hypomorphic mice compared to wild-type mice, and in mice with chronic kidney disease (CKD) on high Pi diet compared to CKD mice fed a normal Pi diet. In cultured kidney tubular cells, high Pi directly induced cellular senescence, injury and epithelial-mesenchymal transition, and enhanced H₂O₂-induced cellular senescence and injury, which were abrogated by Klotho. Fucoidan, a bioactive molecule with multiple biologic functions including senescence inhibition, blunted Pi-induced cellular senescence, oxidation, injury, epithelial-mesenchymal transition, and senescence-associated secretary phenotype. In conclusion, high Pi activates senescence through distinct but interconnected mechanisms: upregulating p16/p21 (early), and elevating plasminogen activator inhibitor-1 and downregulating Klotho (late). Klotho may be a promising agent to attenuate senescence and ameliorate age-associated, and Pi-induced kidney degeneration such as kidney fibrosis.

Novel Human Polymorphisms Define a Key Role for the SLC26A6-STAS Domain in Protection From Ca2+-Oxalate Lithogenesis

Impaired homeostasis of the carboxylic acids oxalate and citrate, dramatically increases the risk for the formation of Ca²⁺-oxalate kidney stones, which is the most common form of kidney stones in humans. Renal homeostasis of oxalate and citrate is controlled by complex mechanisms including epithelial transport proteins such as the oxalate transporter, SLC26A6, and the citrate transporters, the SLC13’s. These transporters interact via the SLC26A6-STAS domain in vitro, however, the role of the Sulfate Transporter and Anti-Sigma factor antagonist (STAS) domain in Ca²⁺-oxalate stone formation was not investigated in humans. Here, we report two novel human SLC26A6 polymorphisms identified in the STAS domain of SLC26A6 in two heterozygous carriers. Intriguingly, these individuals have low urinary citrate, but different clinical manifestations. Our in vitro experiments indicate that the homolog mutations of SLC26A6(D23H/D673N) and SLC26A6(D673N) alone abolished the expression and function of SLC26A6, and impaired the regulation of SLC13-mediated citrate transport by SLC26A6. On the other hand, the SLC26A6(R621G) variant showed reduced SLC26A6 protein expression and membrane trafficking, retained full transport activity, but impaired the regulation of the citrate transporter. Accordingly, the human SLC26A6(D23H/D673N) carrier showed a dramatic reduction in urinary citrate concentrations which resulted in Ca²⁺-oxalate stones formation, as opposed to the carrier of SLC26A6(R621G). Our findings indicate that the human SLC26A6-STAS domain mutations differentially impair SLC26A6 expression, function, and regulation of citrate transporters. This interferes with citrate and oxalate homeostasis thus potentially predisposes to Ca²⁺-oxalate kidney stones.

Parathyroid Hormone and Plasma Phosphate Are Predictors of Soluble α-Klotho Levels in Adults of European Descent

CONTEXT

α-klotho is an integral membrane protein that serves as a coreceptor for fibroblast growth factor 23 (FGF23) in conjunction with cognate fibroblast growth factor receptors. Proteolytic cleavage sheds the ectodomain of α-klotho (soluble α-klotho) as an endocrine substance into blood, urine, and cerebrospinal fluid.

OBJECTIVE

To study the relationship of soluble α-klotho to mineral metabolism in the general population with mainly preserved kidney function.

DESIGN

Cross-sectional analysis of the associations between soluble α-klotho with laboratory markers of markers of mineral metabolism in a population-based cohort.

SETTING

Three centers in Switzerland including 1128 participants.

MEASURES

Soluble full-length α-klotho levels by a specific immunoassay and markers of mineral metabolism.

RESULTS

The median serum level of soluble α-klotho was 15.0 pmol/L. Multivariable analyses using α-klotho as the outcome variable revealed a sex-by-PTH interaction: In men, PTH was positively associated with α-klotho levels, whereas this association was negative in women. Plasma phosphate associated with soluble α-klotho levels in an age-dependent manner, changing from a positive association in young adults gradually to a negative association in the elderly. The decline of 1,25 (OH)2 vitamin D3 levels in parallel to the gradual impairment of kidney function was greatly attenuated in the setting of high circulating soluble α-klotho levels.

CONCLUSIONS

Soluble α-klotho level is associated with plasma phosphate in an age-dependent manner and with PTH in a sex-dependent manner. Furthermore, our data reveal soluble α-klotho as a modulator of 1,25 (OH)2 vitamin D3 levels in individuals with preserved renal function.

Acute Kidney Injury After Burn: A Cohort Study From the Parkland Burn Intensive Care Unit

Acute kidney injury (AKI) is a common and morbid complication in patients with severe burn. The reported incidence of AKI and mortality in this population varies widely due to inconsistent and changing definitions. They aimed to examine the incidence, severity, and hospital mortality of patients with AKI after burn using consensus criteria. This is a retrospective cohort study of adults with thermal injury admitted to the Parkland burn intensive care unit (ICU) from 2008 to 2015. One thousand forty adult patients with burn were admitted to the burn ICU. AKI was defined by KDIGO serum creatinine criteria. Primary outcome includes hospital death and secondary outcome includes length of mechanical ventilation, ICU, and hospital stay. All available serum creatinine measurements were used to determine the occurrence of AKI during the hospitalization. All relevant clinical data were collected. The median total body surface area (TBSA) of burn was 16% (IQR: 6%-29%). AKI occurred in 601 patients (58%; AKI stage 1, 60%; stage 2, 19.8%; stage 3, 10.5%; and stage 3 requiring renal replacement therapy [3-RRT], 9.7%). Patients with AKI had larger TBSA burn (median 20.5% vs 11.0%; P < .001) and more mechanical ventilation and hospitalization days than patients without AKI. The hospital death rate was higher in those with AKI vs those without AKI (19.7% vs 3.9%; P < .001) and increased by each AKI severity stage (P trend < .001). AKI severity was independently associated with hospital mortality in the small burn group (for TBSA ≤ 10%: stage 1 adjusted OR 9.3; 95% CI, 2.6-33.0; stage 2-3 OR, 35.0; 95% CI, 9.0-136.8; stage 3-RRT OR, 30.7; 95% CI, 4.2-226.4) and medium burn group (TBSA 10%-40%: stage 2-3 OR, 6.5; 95% CI, 1.9-22.1; stage 3-RRT OR, 35.1; 95% CI, 8.2-150.3). AKI was not independently associated with hospital death in the large burn group (TBSA > 40%). Urine output data were unavailable. AKI occurs frequently in patients after burn. Presence of and increasing severity of AKI are associated with increased hospital mortality. AKI appears to be independently and strongly associated with mortality in patients with TBSA ≤ 40%. Further investigation to develop risk-stratification tools tailoring this susceptible population is direly needed.

Beclin 1/Bcl-2 complex-dependent autophagy activity modulates renal susceptibility to ischemia-reperfusion injury and mediates renoprotection by Klotho

Klotho- and beclin 1-driven autophagy extends life. We examined the role of beclin 1 in modifying acute kidney injury (AKI) and whether beclin 1 mediates Klotho's known renoprotective action in AKI. AKI was induced by ischemia-reperfusion injury in mice with different levels of autophagy activity by genetic manipulation: wild-type (WT) mice with normal beclin 1 expression and function, mice with normal beclin 1 levels but high activity through knockin of gain-of-function mutant beclin 1 (Becn1^F121A), mice with low beclin 1 levels and activity caused by heterozygous global deletion of beclin 1 (Becn1^+/-), or mice with extremely low beclin 1 activity from knockin of the mutant constitutively active beclin 1 inhibitor Bcl-2 (Bcl2^AAA). Klotho was increased by transgenic overexpression (Tg-Kl) or recombinant Klotho protein administration. After ischemia-reperfusion injury, Becn1^F121A mice (high autophagy) had milder AKI and Becn1^+/- and Bcl2^AAA mice (low autophagy) had more severe AKI than WT mice. Tg-Kl mice had milder AKI, but its renoprotection was partially attenuated in Becn1^+/-;Tg-Kl mice and was significantly reduced, although not completely abolished, in Bcl2^AAA;Tg-Kl mice. Recombinant Klotho protein conferred more renoprotection from AKI in WT mice than in Becn1^+/- or Bcl2^AAA mice. Klotho reduced beclin 1/Bcl-2 protein complexes and increased autophagy activity, but this effect was less prominent in mice or cells with Bcl2^AAA. Transfected Bcl2^AAA or Becn1^F123A decreased or increased autophagy activity and rendered cells more susceptible or more resistant to oxidative cytotoxicity, respectively. In conclusion, beclin 1 confers renoprotection by activating autophagy. Klotho protects the kidney partially via disruption of beclin 1/Bcl-2 interactions and enhancement of autophagy activity.

The tripartite interaction of phosphate, autophagy, and αKlotho in health maintenance

Aging-related organ degeneration is driven by multiple factors including the cell maintenance mechanisms of autophagy, the cytoprotective protein αKlotho, and the lesser known effects of excess phosphate (Pi), or phosphotoxicity. To examine the interplay between Pi, autophagy, and αKlotho, we used the BK/BK mouse (homozygous for mutant Becn1^F121A ) with increased autophagic flux, and αKlotho-hypomorphic mouse (kl/kl) with impaired urinary Pi excretion, low autophagy, and premature organ dysfunction. BK/BK mice live longer than WT littermates, and have heightened phosphaturia from downregulation of two key NaPi cotransporters in the kidney. The multi-organ failure in kl/kl mice was rescued in the double-mutant BK/BK;kl/kl mice exhibiting lower plasma Pi, improved weight gain, restored plasma and renal αKlotho levels, decreased pathology of multiple organs, and improved fertility compared to kl/kl mice. The beneficial effects of heightened autophagy from Becn1^F121A was abolished by chronic high-Pi diet which also shortened life span in the BK/BK;kl/kl mice. Pi promoted beclin 1 binding to its negative regulator BCL2, which impairs autophagy flux. Pi downregulated αKlotho, which also independently impaired autophagy. In conclusion, Pi, αKlotho, and autophagy interact intricately to affect each other. Both autophagy and αKlotho antagonizes phosphotoxicity. In concert, this tripartite system jointly determines longevity and life span.

High-Phosphate Diet Induces Exercise Intolerance and Impairs Fatty Acid Metabolism in Mice

BACKGROUND

Inorganic phosphate (Pi) is used extensively as a preservative and a flavor enhancer in the Western diet. Physical inactivity, a common feature of Western societies, is associated with increased cardiovascular morbidity and mortality. It is unknown whether dietary Pi excess contributes to exercise intolerance and physical inactivity.

METHODS

To determine an association between Pi excess and physical activity in humans, we assessed the relationship between serum Pi and actigraphy-determined physical activity level, as well as left ventricular function by cardiac magnetic resonance imaging, in DHS-2 (Dallas Heart Study phase 2) participants after adjusting for relevant variables. To determine direct effects of dietary Pi on exercise capacity, oxygen uptake, serum nonesterified fatty acid, and glucose were measured during exercise treadmill test in C57/BL6 mice fed either a high-Pi (2%) or normal-Pi (0.6%) diet for 12 weeks. To determine the direct effect of Pi on muscle metabolism and expression of genes involved in fatty acid metabolism, additional studies in differentiated C2C12 myotubes were conducted after subjecting to media containing 1 to 3 mmol/L Pi (pH 7.0) to simulate in vivo phosphate conditions.

RESULTS

In participants of the DHS-2 (n=1603), higher serum Pi was independently associated with reduced time spent in moderate to vigorous physical activity ( P=0.01) and increased sedentary time ( P=0.004). There was no association between serum Pi and left ventricular ejection fraction or volumes. In animal studies, compared with the control diet, consumption of high-Pi diet for 12 weeks did not alter body weight or left ventricular function but reduced maximal oxygen uptake, treadmill duration, spontaneous locomotor activity, fat oxidation, and fatty acid levels and led to downregulation of genes involved in fatty acid synthesis, release, and oxidation, including Fabp4, Hsl, Fasn, and Pparγ, in muscle. Similar results were recapitulated in vitro by incubating C2C12 myotubes with high-Pi media.

CONCLUSIONS

Our data demonstrate a detrimental effect of dietary Pi excess on skeletal muscle fatty acid metabolism and exercise capacity that is independent of obesity and cardiac contractile function. Dietary Pi may represent a novel and modifiable target to reduce physical inactivity associated with the Western diet.

Alpha-Klotho, a critical protein for lung health, is not expressed in normal lung

Alpha-Klotho (αKlotho), produced by the kidney and selected organs, is essential for tissue maintenance and protection. Homozygous αKlotho-deficiency leads to premature multi-organ degeneration and death; heterozygous insufficiency leads to apoptosis, oxidative stress, and increased injury susceptibility. There is inconsistent data in the literature regarding whether αKlotho is produced locally in the lung or derived from circulation. We probed murine and human lung by immunohistochemistry (IHC) and immunoblot (IB) using two monoclonal (anti-αKlotho Kl1 and Kl2 domains) and three other common commercial antibodies. Monoclonal anti-Kl1 and anti-Kl2 yielded no labeling in lung on IHC or IB; specific labeling was observed in kidney (positive control) and also murine lungs following tracheal delivery of αKlotho cDNA, demonstrating specificity and ability to detect artificial pulmonary expression. Other commercial antibodies labeled numerous lung structures (IHC) and multiple bands (IB) incompatible with known αKlotho mobility; labeling was not abolished by blocking with purified αKlotho or using lungs from hypomorphic αKlotho-deficient mice, indicating nonspecificity. Results highlight the need for rigorous validation of reagents. The lung lacks native αKlotho expression and derives full-length αKlotho from circulation; findings could explain susceptibility to lung injury in extrapulmonary pathology associated with reduced circulating αKlotho levels, for example, renal failure. Conversely, αKlotho may be artificially expressed in the lung, suggesting therapeutic opportunities.

Association of serum magnesium with all-cause mortality in patients with and without chronic kidney disease in the Dallas Heart Study

Background

Low serum magnesium (SMg) has been linked to increased mortality and cardiovascular disease (CVD) in the general population. We examined whether this association is similar in participants with versus without prevalent chronic kidney disease (CKD) in the multiethnic Dallas Heart Study (DHS) cohort.

Methods

SMg was analyzed as a continuous variable and divided into tertiles. Study outcomes were all-cause death, cardiovascular (CV) death or event, and CVD surrogate markers, evaluated using multivariable Cox regression models adjusted for demographics, comorbidity, anthropometric and biochemical parameters including albumin, phosphorus and parathyroid hormone, and diuretic use. Median follow-up was 12.3 years (11.9-12.8, 25th percentile-75th percentile).

Results

Among 3551 participants, 306 (8.6%) had prevalent CKD. Mean SMg was 2.08 ± 0.19 mg/dL (0.85 ± 0.08 mM, mean ± SD) in the CKD and 2.07 ± 0.18 mg/dL (0.85 ± 0.07 mM) in the non-CKD subgroups. During the follow-up period, 329 all-cause deaths and 306 CV deaths or events occurred. In a fully adjusted model, every 0.2 mg/dL decrease in SMg was associated with ∼20-40% increased hazard for all-cause death in both CKD and non-CKD subgroups. In CKD participants, the lowest SMg tertile was also independently associated with all-cause death (adjusted hazard ratio 2.31; 95% confidence interval 1.23-4.36 versus 1.15; 0.55-2.41; for low versus high tertile, respectively).

Conclusions

Low SMg levels (1.4-1.9 mg/dL; 0.58-0.78 mM) were independently associated with all-cause death in patients with prevalent CKD in the DHS cohort. Randomized clinical trials are important to determine whether Mg supplementation affects survival in CKD patients.

Hypertrophy of human embryonic stem cell-derived cardiomyocytes supported by positive feedback between Ca2+ and diacylglycerol signals

Human embryonic stem cell-derived cardiomyocytes develop pronounced hypertrophy in response to angiotensin-2, endothelin-1, and a selected mix of three fatty acids. All three of these responses are accompanied by increases in both basal cytoplasmic Ca2+ and diacylglycerol, quantified with the Ca2+ sensor Fluo-4 and a FRET-based diacylglycerol sensor expressed in these cardiomyocytes. The heart glycoside, ouabain (30 nM), and a recently developed inhibitor of diacylglycerol lipases, DO34 (1 μM), cause similar hypertrophy responses, and both responses are accompanied by equivalent increases of basal Ca2+ and diacylglycerol. These results together suggest that basal Ca2+ and diacylglycerol form a positive feedback signaling loop that promotes execution of cardiac growth programs in these human myocytes. Given that basal Ca2+ in myocytes depends strongly on the Na+ gradient, we also tested whether nanomolar ouabain concentrations might stimulate Na+/K+ pumps, as described by others, and thereby prevent hypertrophy. However, stimulatory effects of nanomolar ouabain (1.5 nM) were not verified on Na+/K+ pump currents in stem cell-derived myocytes, nor did nanomolar ouabain block hypertrophy induced by endothelin-1. Thus, low-dose ouabain is not a "protective" intervention under the conditions of these experiments in this human myocyte model. To summarize, the major aim of this study has been to characterize the progression of hypertrophy in human embryonic stem cell-derived cardiac myocytes in dependence on diacylglycerol and Na+ gradient changes, developing a case that positive feedback coupling between these mechanisms plays an important role in the initiation of hypertrophy programs.

Renal tubular cell spliced X-box binding protein 1 (Xbp1s) has a unique role in sepsis-induced acute kidney injury and inflammation

Sepsis is a systemic inflammatory state in response to infection, and concomitant acute kidney injury (AKI) increases mortality significantly. Endoplasmic reticulum stress is activated in many cell types upon microbial infection and modulates inflammation. The role of endoplasmic reticulum signaling in the kidney during septic AKI is unknown. Here we tested the role of the spliced X-box binding protein 1 (Xbp1s), a key component of the endoplasmic reticulum stress-activated pathways, in the renal response to sepsis in the lipopolysaccharide (LPS) model. Xbp1s was increased in the kidneys of mice treated with LPS but not in other models of AKI, or several chronic kidney disease models. The functional significance of Xbp1s induction was examined by genetic manipulation in renal tubules. Renal tubule-specific overexpression of Xbp1s caused severe tubule dilation and vacuolation with expression of the injury markers Kim1 and Ngal, the pro-inflammatory molecules interleukin-6 (Il6) and Toll-like receptor 4 (Tlr4), decreased kidney function and 50% mortality in five days. Renal tubule-specific genetic ablation of Xbp1 had no phenotype at baseline. However, after LPS, Xbp1 knockdown mice displayed lower renal NGAL, pro-apoptotic factor CHOP, serum creatinine levels, and a tendency towards lower Tlr4 compared to LPS-treated mice with intact Xbp1s. LPS treatment in Xbp1s-overexpressing mice caused a mild increase in NGAL and CHOP compared to LPS-treated mice without genetic Xbp1s overexpression. Thus, increased Xbp1s signaling in renal tubules is unique to sepsis-induced AKI and contributes to renal inflammation and injury. Inhibition of this pathway may be a potential portal to alleviate injury.

Performance of soluble Klotho assays in clinical samples of kidney disease

Background

Soluble Klotho has multiple systemic salutary effects. In animals, both acute and chronic kidney disease models display systemic Klotho deficiency. As such, there is considerable interest in investigating soluble Klotho as a biomarker in patients with different types and severity of kidney diseases. Unfortunately, there remains uncertainty regarding the best method to measure soluble Klotho in human serum samples.

Methods

Using human serum samples obtained from several clinical cohorts with a wide range of kidney function, we measured soluble Klotho using a commercial enzyme-linked immunosorbent assay (ELISA) as well as with an immunoprecipitation-immunoblot (IP-IB) assay utilizing a synthetic antibody with high affinity and specificity for Klotho. Recovery of spiking with a known amount of exogenous Klotho was tested. A subset of samples was analyzed with and without the addition of a protease inhibitor cocktail at the time of collection or after the first freeze-thaw cycle to determine if these maneuvers influenced performance.

Results

The IP-IB assay was superior to the ELISA at recovery of exogenous Klotho (81-115% versus 60-81%) across the spectrum of kidney function. Klotho measurements by IP-IB were highly correlated with estimated glomerular filtration rate (eGFR) (R = 0.80, P < 0.001) in comparison with the commercial ELISA, which exhibited minimal correlation with eGFR (R = 0.18, P = 0.12). Use of a protease inhibitor cocktail neither improved nor impaired performance of the IP-IB assay; however, subsequent freeze-thaw cycle resulted in a significant reduction in Klotho recovery and dissipated the correlation between Klotho levels and eGFR. With the ELISA, the use of protease inhibitor cocktail resulted in an increase in intrasubject variability.

Conclusions

The IP-IB assay is preferable to the commercial ELISA to measure soluble Klotho concentrations in never-thawed serum samples of humans with varying severity of kidney disease. However, due to the labor-intensive nature of the IP-IB assay, further research is needed to secure an assay suitable for high-throughput work.

Kidney Tubular Damage and Functional Biomarkers in Acute Kidney Injury Following Cardiac Surgery

Background

Cardiac surgery–associated acute kidney injury (AKI) is associated with increased morbidity and mortality. We examined the utility of combining biomarkers of kidney function loss (serum cystatin C) and kidney tubular damage (urine neutrophil gelatinase-associated lipocalin [NGAL] and Kidney Injury Molecule-1 [KIM-1]) for the prediction of post–cardiac surgery AKI.

Methods

Single-center prospective cohort study of 106 adults undergoing coronary artery bypass grafting and/or valve surgery with cardiopulmonary bypass (CPB). Primary outcome was postoperative in-hospital AKI defined by serum creatinine (SCr)–Kidney Disease: Improving Global Outcomes criteria. Biomarkers were measured preoperatively, 6 hours after CPB and on postoperative days (PODs) 1 to 4.

Results

A total of 23 subjects (21.7%) developed AKI. After adjusting for preoperative left ventricular ejection fraction, body mass index >30 kg/m², and estimated glomerular filtration rate (eGFR) <60 ml/min per 1.73 m², the combination of peak serum cystatin C and peak urine KIM-1/creatinine (Cr) (6 hours post-CPB to POD 1) above optimal cutoff significantly associated with postoperative AKI (odds ratio [OR]: 5.32; 95% confidence interval [CI]: 1.31–21.67; P = 0.020). This biomarker combination significantly improved the performance of the clinical model for the prediction of postoperative AKI (area under the curve [AUC]: 0.77, 95% CI: 0.65–0.90 for the clinical model alone versus 0.83, 95% CI: 0.73–0.93 for the clinical model with the addition of biomarker data, P = 0.049).

Conclusions

Combining biomarkers of postoperative kidney function loss and postoperative kidney tubular damage significantly improved prediction of in-hospital AKI following cardiac surgery. Future large, multicenter studies are warranted to assess whether panels of biomarkers reflecting distinct pathobiology can be used to guide interventions and improve short- and long-term outcomes in patients undergoing cardiac surgery.