Chronic Kidney Disease Phenotypes Research Articles

Assessment of Mitochondrial Function in a Mouse Model of Mild Chronic Kidney Disease Induced by High-Fat and High-Salt Diet

Intro: Kidneys are known for high energy demands involved in active transport and are thus highly oxidative organs, consuming the second-highest amount of oxygen per gram of tissue at rest (only exceeded by the heart). Chronic kidney disease (CKD) is a complex disorder that presents substantial challenges in understanding its pathophysiological mechanisms, including mitochondrial alterations. However, there are no established mouse models that recapitulate the CKD phenotype observed in humans.Hypothesis: We hypothesized that a high-fat, high-salt diet would induce a mild CKD phenotype and mitochondrial dysfunction in mice. Methods: We subjected C57Bl/6 mice to a high-fat (42%) and high-salt (8% NaCl) diet (HF/HNaCl) or standard chow diet for 16 weeks (n=4 per group). Additionally, we conducted comprehensive investigations into mitochondrial function, focusing on state 3 mitochondrial oxygen consumption (Oroboros O2K) and adenosine triphosphate (ATP) production (Horiba Fluoromax). Differences between groups were determined using Student’s t-tests. Results: HF/HNaCl feeding resulted in polyurea indicated by an increase in 24-hour urine output (Chow: 1.5 ± 0.2 mL; HF/HNaCl: 3.7 ± 0.8 mL; p=0.05) and a 30% reduction in glomerular filtration rate compared to chow-fed animals (Chow: 911.2 ± 107.8 μL/min/100g; Hf/HNaCl: 639.5 ± 25.9 μL/min/100g; p=0.08), indicative of the development of mild kidney disease. Although subtle changes were observed in mitochondrial oxygen consumption and ATP production, the alterations were not statistically significant. Discussion: Our findings suggest that the induced mild CKD state via the dietary regimen led to moderate renal impairment without substantial impacts on mitochondrial respiratory capacity and ATP synthesis. The establishment of this mouse model provides a valuable platform for further investigations into the complex interplay between renal dysfunction and mitochondrial dynamics in the context of mild CKD, offering insights that may pave the way for the development of targeted therapeutic interventions for this prevalent and debilitating condition. This project was supported by grants from the National Institutes of Health. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A Novel Mouse Model of Physical Inactivity to Study the Role of Mitochondria in Chronic Kidney Disease

Chronic kidney disease (CKD) is a progressive disorder marked by a decline in kidney function. It is thought that a decline in mitochondrial function likely contributes to the pathogenesis of CKD, but the detailed mechanisms remain unclear. This gap in knowledge is contributed by a lack of a murine CKD model that does not rely on injury, toxin, or gene deletion to induce reduced kidney function. In humans, obesity and sedentary behavior strongly contribute to the development of CKD. The aim of the study was to employ small mouse cage (SMC) housing, a novel mouse physical inactivity model that was recently developed in our lab, and test whether it can be used to reliably induce CKD in mice. Wildtype C57BL6J mice were given standard chow or Western high-fat diet (HFD) with or without SMC intervention for 24 weeks to examine the effects of HFD and SMC on glomerular filtration rate (GFR) and respiration on isolated mitochondria from kidney cortex. Although the GFR values of the HFD+SMC group trended higher than other groups, the kidney function was still considered to be normal. Neither HFD nor HFD+SMC group had developed CKD, but mitochondrial respiration of SMC+HFD (1512.64 ± 77.86 pmol·sec−1·mgprotein−1) was significantly greater than that of the HFD-only group (1307.82 ± 79.08 pmol·sec−1·mgprotein−1, p = 0.0017) or the standard chow group (1104.69 ±143.12 pmol·sec−1·mgprotein−1, p = 0.0314. One-way ANOVA followed by Tukey’s test for multiple comparisons). Increased mitochondrial respiration may suggest a hyperfiltration-induced stress response, as commonly observed in the early stages of CKD in humans, and/or an increase in respiration to compensate for reduced effciency of oxidative phosphorylation. We interpret these data to mean that HFD and HFD+SMC are beginning to show adaptations that are in an early phase of CKD development. However, this model requires further optimization to yield the CKD phenotype. Therefore, we intend to further adapt the intervention to induce symptomatic CKD. We envision that developing preclinical models of CKD will help better elucidate its molecular mechanism and identify therapeutic targets for treating CKD in human patients. DK107397, DK127979. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Cardiorenal Effects of Chronic High Phosphate Diet in Female Rats

Cardiorenal Syndrome Type 4 (CRS4) refers to the development of cardiovascular dysfunction as a result of chronic kidney disease. The accumulation of circulating phosphate in the later stages of chronic kidney disease (CKD) has been positively associated with cardiac hypertrophy and dysfunction. In our previous work, we found that male Sprague Dawley rats that underwent 5/6 nephrectomy (5/6Nx) to induce CKD phenotypes also developed cardiac pathology when serum phosphorus levels became elevated. Small RNA sequencing of left ventricle tissue revealed upregulated microRNA-21-5p and -3p expression in 5/6Nx rats, when compared to sham-operated controls, and suppression of miR-21-5p attenuated left ventricle pathology in male 5/6Nx rats. We hypothesize that hyperphosphatemia, independent of CKD, can impair cardiac function and that miR-21 may mediate these effects. It has been recently reported that females exhibit higher urinary phosphate excretion in response to high dietary phosphate intake than males. In this study, hyperphosphatemia was induced by placing 9-week old female wild-type (WT) Sprague Dawley and global miR-21 knockout (miR-21−/−; lacking both miR-21-5p and -3p rats) (n=4-9/group) on either high phosphate (2% Pi) or control diet (0.6% Pi) for 8 weeks. Echocardiography and renal ultrasound were performed under anesthesia to assess cardiac function and renal cross-sectional area, respectively, prior to tissue collection at endpoint. Rats of either genotype fed 2% Pi diet exhibited increased left kidney weight, left kidney/body weight, and left kidney cross-sectional area (2-way ANOVA, main effect of diet; p<0.05). In animals fed 2% Pi, miR-21−/− rats had significantly higher left ventricle fractional shortening than WT rats (57.06 ± 3.32 vs. 43.96 ± 1.34; 2-way ANOVA with Tukey multiple comparisons; p<0.05). This study allowed the cardiovascular effects of chronically elevated phosphate to be isolated from other uremic toxins elevated in CKD. We find that loss of miR-21 augments cardiac function, supporting a role for miR-21 in mediating phosphate-related pathology. Ongoing studies are focused on identifying specific molecular mechanisms driving these changes. This study was supported by T32HL007852 and R01HL128332. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Retinoic acid receptor α activity in proximal tubules prevents kidney injury and fibrosis

Chronic kidney disease (CKD) is characterized by a gradual loss of kidney function and affects ~13.4% of the global population. Progressive tubulointerstitial fibrosis, driven in part by proximal tubule (PT) damage, is a hallmark of late stages of CKD and contributes to the development of kidney failure, for which there are limited treatment options. Normal kidney development requires signaling by vitamin A (retinol), which is metabolized to retinoic acid (RA), an endogenous agonist for the RA receptors (RARα, β, γ). RARα levels are decreased in a mouse model of diabetic nephropathy and restored with RA administration; additionally, RA treatment reduced fibrosis. We developed a mouse model in which a spatiotemporal (tamoxifen-inducible) deletion of RARα in kidney PT cells of adult mice causes mitochondrial dysfunction, massive PT injury, and apoptosis without the use of additional nephrotoxic substances. Long-term effects (3 to 4.5 mo) of RARα deletion include increased PT secretion of transforming growth factor β1, inflammation, interstitial fibrosis, and decreased kidney function, all of which are major features of human CKD. Therefore, RARα's actions in PTs are crucial for PT homeostasis, and loss of RARα causes injury and a key CKD phenotype.

Phenome-wide analysis reveals epistatic associations between APOL1 variants and chronic kidney disease and multiple other disorders

APOL1 variants G1 and G2 are common in populations with recent African ancestry. They are associated with protection from African sleeping sickness, however homozygosity or compound heterozygosity for these variants is associated with chronic kidney disease (CKD) and related conditions. What is not clear is the extent of associations with non-kidney-related disorders, and whether there are clusters of diseases associated with individual APOL1 genotypes. Using a cohort of 7462 UK Biobank participants with recent African ancestry, we conducted a phenome-wide association study investigating associations between individual APOL1 genotypes and conditions identified by the International Classification of Disease phenotypes. We identified 27 potential associations between individual APOL1 genotypes and a diverse range of conditions. G1/G2 compound heterozygotes were specifically associated with 26 of these conditions (all deleteriously), with an over-representation of infectious diseases (including hospitalisation and death resulting from COVID-19). The analysis also exposed complexities in the relationship between APOL1 and CKD that are not evident when risk variants are grouped together: G1 homozygosity, G2 homozygosity, and G1/G2 compound heterozygosity were each shown to be associated with distinct CKD phenotypes. The multi-locus nature of the G1/G2 genotype means that its associations would go undetected in a standard genome-wide association study. Our findings have implications for understanding health risks and better-targeted detection, intervention, and therapeutic strategies, particularly in populations where APOL1 G1 and G2 are common such as in sub-Saharan Africa and its diaspora. This study was funded by the Wellcome Trust (209511/Z/17/Z) and H3Africa (H3A/18/004).

Renal-specific loss of ferroportin disrupts iron homeostasis and attenuates recovery from acute kidney injury.

Chronic kidney disease is increasing at an alarming rate and correlates with the increase in diabetes, obesity, and hypertension that disproportionately impact socioeconomically disadvantaged communities. Iron plays essential roles in many biological processes including oxygen transport, mitochondrial function, cell proliferation, and regeneration. However, excess iron induces the generation and propagation of reactive oxygen species, which lead to oxidative stress, cellular damage, and ferroptosis. Iron homeostasis is regulated in part by the kidney through iron resorption from the glomerular filtrate and exports into the plasma by ferroportin (FPN). Yet, the impact of iron overload in the kidney has not been addressed. To test more directly whether excess iron accumulation is toxic to kidneys, we generated a kidney proximal tubule-specific knockout of FPN. Despite significant intracellular iron accumulation in FPN mutant tubules, basal kidney function was not measurably different from wild type kidneys. However, upon induction of acute kidney injury (AKI), FPN mutant kidneys exhibited significantly more damage and failed recovery, evidence for ferroptosis, and increased fibrosis. Thus, disruption of iron export in proximal tubules, leading to iron overload, can significantly impair recovery from AKI and can contribute to progressive renal damage indicative of chronic kidney disease. Understanding the mechanisms that regulate iron homeostasis in the kidney may provide new therapeutic strategies for progressive kidney disease and other ferroptosis-associated disorders.NEW & NOTEWORTHY Physiological iron homeostasis depends in part on renal resorption and export into the plasma. We show that specific deletion of iron exporters in the proximal tubules sensitizes cells to injury and inhibits recovery. This can promote a chronic kidney disease phenotype. Our paper demonstrates the need for iron balance in the proximal tubules to maintain and promote healthy recovery after acute kidney injury.

Interplay Between Chronic Kidney Disease, Hypertension, and Stroke: Insights From a Multivariable Mendelian Randomization Analysis.

Chronic kidney disease (CKD) increases the risk of stroke, but the extent through which this association is mediated by hypertension is unknown. We leveraged large-scale genetic data to explore causal relationships between CKD, hypertension, and cerebrovascular disease phenotypes. We used data from genome-wide association studies of European ancestry to identify genetic proxies for kidney function (CKD diagnosis, estimated glomerular filtration rate [eGFR], and urinary albumin-to-creatinine ratio [UACR]), systolic blood pressure (SBP), and cerebrovascular disease (ischemic stroke and its subtypes and intracerebral hemorrhage). We then conducted univariable, multivariable, and mediation Mendelian randomization (MR) analyses to investigate the effect of kidney function on stroke risk and the proportion of this effect mediated through hypertension. Univariable MR revealed associations between genetically determined lower eGFR and risk of all stroke (odds ratio [OR] per 1-log decrement in eGFR, 1.77; 95% CI 1.31-2.40; p < 0.001), ischemic stroke (OR 1.81; 95% CI 1.31-2.51; p < 0.001), and most strongly with large artery stroke (LAS) (OR 3.00; 95% CI 1.33-6.75; p = 0.008). These associations remained significant in the multivariable MR analysis, controlling for SBP (OR 1.98; 95% CI 1.39-2.82; p < 0.001 for all stroke; OR 2.16; 95% CI 1.48-3.17; p < 0.001 for ischemic stroke; OR 4.35; 95% CI 1.84-10.27; p = 0.001 for LAS), with only a small proportion of the total effects mediated by SBP (6.5% [0.7%-16.8%], 6.6% [0.8%-18.3%], and 7.2% [0.5%-24.8%], respectively). Total, direct and indirect effect estimates were similar across a number of sensitivity analyses (weighted median, MR-Egger regression). Our results demonstrate an independent causal effect of impaired kidney function, as assessed by decreased eGFR, on stroke risk, particularly LAS, even when controlled for SBP. Targeted prevention of kidney disease could lower atherosclerotic stroke risk independent of hypertension.

Identification of Genetic Markers Linked to The Activity of Indoleamine 2,3-Dioxygenase and Kidney Function.

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-degrading enzyme belonging to the kynurenine pathway. IDO activity has been suggested as a potential biomarker for early diagnosis of chronic kidney disease (CKD). The aim of this study was to perform coincident association analysis to gain genetic insights into the correlation between IDO activity and CKD. This study evaluated the association between IDO activity and CKD using the Korea Association REsource (KARE) cohort. Logistic and linear regression were used to analyze CKD and quantitative phenotypes such as IDO and estimated glomerular filtration rate (eGFR). Our results identified 10 single nucleotide polymorphisms (SNPs) that were coincidently associated with both IDO and CKD (p < 0.001). Among them, rs6550842, rs77624055, and rs35651150 were selected as potential candidates after excluding SNPs with insufficient evidence for having an association with IDO or CKD. Further expression quantitative trait loci (eQTL) analysis for variants at selected loci showed that rs6550842 and rs35651150 significantly affected the expression of NKIRAS1 and SH2D4A genes in human tissues, respectively. Additionally, we highlighted that the NKIRAS1 and BMP6 genes were correlated with IDO activity and CKD through signaling pathways associated with inflammation. Our data suggest that NKIRAS1, SH2D4A, and BMP6 were potential causative genes affecting IDO activity and CKD through integrated analysis. Identifying these genes could aid in early detection and treatment by predicting the risk of CKD associated with IDO activity.

Association of polygenic scores with chronic kidney disease phenotypes in a longitudinal study of older adults

Risk of chronic kidney disease (CKD) is influenced by environmental and genetic factors and increases sharply in individuals 70 years and older. Polygenic scores (PGS) for kidney disease-related traits have shown promise but require validation in well-characterized cohorts. Here, we assessed the performance of recently developed PGSs for CKD-related traits in a longitudinal cohort of healthy older individuals enrolled in the Australian ASPREE randomized controlled trial of daily low-dose aspirin with CKD risk at baseline and longitudinally. Among 11,813 genotyped participants aged 70 years or more with baseline eGFR measures, we tested associations between PGSs and measured eGFR at baseline, clinical phenotype of CKD, and longitudinal rate of eGFR decline spanning up to six years of follow-up per participant. A PGS for eGFR was associated with baseline eGFR, with a significant decrease of 3.9 mL/min/1.73m2 (95% confidence interval -4.17 to -3.68) per standard deviation (SD) increase of the PGS. This PGS, as well as a PGS for CKD stage 3 were both associated with higher risk of baseline CKD stage 3 in cross-sectional analysis (Odds Ratio 1.75 per SD, 95% confidence interval 1.66-1.85, and Odds Ratio 1.51 per SD, 95% confidence interval 1.43-1.59, respectively). Longitudinally, two separate PGSs for eGFR slope were associated with significant kidney function decline during follow-up. Thus, our study demonstrates that kidney function has a considerable genetic component in older adults, and that new PGSs for kidney disease-related phenotypes may have potential utility for CKD risk prediction in advanced age.

A Novel Chronic Kidney Disease Phenotyping Algorithm Using Combined Electronic Health Record and Claims Data.

Because chronic kidney disease (CKD) is often under-coded as a diagnosis in claims data, we aimed to develop claims-based prediction models for CKD phenotypes determined by laboratory results in electronic health records (EHRs). We linked EHR from two networks (used as training and validation cohorts, respectively) with Medicare claims data. The study cohort included individuals ≥65 years with a valid serum creatinine result in the EHR from 2007 to 2017, excluding those with end-stage kidney disease or on dialysis. We used LASSO regression to select among 134 predictors for predicting continuous estimated glomerular filtration rate (eGFR). We assessed the model performance when predicting eGFR categories of <60, <45, <30 mL/min/1.73m2 in terms of area under the receiver operating curves (AUC). The model training cohort included 117,476 patients (mean age 74.8 years, female 58.2%) and the validation cohort included 56,744 patients (mean age 73.8 years, female 59.6%). In the validation cohort, the AUC of the primary model (with 113 predictors and an adjusted R2 of 0.35) for predicting eGFR <60, eGFR<45, and eGFR <30 mL/min/1.73m2 categories was 0.81, 0.88, and 0.92, respectively, and the corresponding positive predictive values for these 3 phenotypes were 0.80 (95% confidence interval: 0.79, 0.81), 0.79 (0.75, 0.84), and 0.38 (0.30, 0.45), respectively. We developed a claims-based model to determine clinical phenotypes of CKD stages defined by eGFR values. Researchers without access to laboratory results can use the model-predicted phenotypes as a proxy clinical endpoint or confounder and to enhance subgroup effect assessment.

Paradoxical reduction of plasma lipids and atherosclerosis in mice with adenine-induced chronic kidney disease and hypercholesterolemia.

Atherosclerotic cardiovascular disease is prevalent among patients with chronic kidney disease (CKD). In this study, we initially aimed to test whether vascular calcification associated with CKD can worsen atherosclerosis. However, a paradoxical finding emerged from attempting to test this hypothesis in a mouse model of adenine-induced CKD. We combined adenine-induced CKD and diet-induced atherosclerosis in mice with a mutation in the low-density lipoprotein receptor gene. In the first study, mice were co-treated with 0.2% adenine in a western diet for 8 weeks to induce CKD and atherosclerosis simultaneously. In the second study, mice were pre-treated with adenine in a regular diet for 8 weeks, followed by a western diet for another 8 weeks. Co-treatment with adenine and a western diet resulted in a reduction of plasma triglycerides and cholesterol, liver lipid contents, and atherosclerosis in co-treated mice when compared with the western-only group, despite a fully penetrant CKD phenotype developed in response to adenine. In the two-step model, renal tubulointerstitial damage and polyuria persisted after the discontinuation of adenine in the adenine-pre-treated mice. The mice, however, had similar plasma triglycerides, cholesterol, liver lipid contents, and aortic root atherosclerosis after being fed a western diet, irrespective of adenine pre-treatment. Unexpectedly, adenine pre-treated mice consumed twice the calories from the diet as those not pre-treated without showing an increase in body weight. The adenine-induced CKD model does not recapitulate accelerated atherosclerosis, limiting its use in pre-clinical studies. The results indicate that excessive adenine intake impacts lipid metabolism.

Abstract 15473: A New Metabolic Phenotype in a Mouse Model of Chronic Kidney Disease and Atherosclerosis

Introduction: Patients with chronic kidney disease (CKD) experience significant cardiovascular morbidity and mortality. Vascular calcification is a hallmark of CKD and a strong predictor of atherosclerotic cardiovascular disease. Hypothesis: We hypothesize that CKD-induced vascular calcification potentiates atherosclerosis. Methods: First, wished to conduct a pilot study to confirm that a co-treatment of the low-density protein receptor mutant mice (Ldlr whc ) with a 0.2% adenine in a western diet will produce accelerated atherosclerosis, consistent with a well-known phenotype in the 5/6 nephrectomy model on an ApoE or Ldlr knockout background. Then we developed a two-step non-surgical mouse model in which the induction of calcification by a 0.2% adenine treatment was followed by western diet-induced hyperlipidemia. Results: Contrary to expected, the adenine and western diet (W+A) co-treatment resulted in a striking reduction of plasma triglycerides (TG), aortic root atherosclerosis, and liver lipids despite a fully penetrant CKD phenotype with tubulointerstitial damage, increased blood urea nitrogen, hyperphosphatemia, polyuria, vascular calcification, and osteoporosis. The two-step induction of CKD and atherosclerosis demonstrated partially normalized but reduced plasma triglycerides and liver lipid content, polyurea, and polydipsia in CKD mice, and there was no difference in atherosclerotic burden or plaque calcification between the groups. Interestingly we observed an increase in food and calorie intake and 24-hour glucose excretion in the urine in the pre-treated group compared with mice on a western diet without pre-treatment, while plasma glucose levels were not different between the groups. Conclusion: The paradoxical lack of dyslipidemia in mice treated with adenine provides an insight into the potential effects of tubulointerstitial kidney disease and nephrogenic diabetes incipits on lipid metabolism and atherosclerosis. 1

Exome sequencing unravels genetic variants associated with chronic kidney disease in Saudi Arabian patients.

The use of genetic testing within nephrology is increasing and its diagnostic yield depends on the methods utilized, patient selection criteria, and population characteristics. We performed exome sequencing (ES) analysis on 102 chronic kidney disease (CKD) patients with likely genetic kidney disease. Patients had diverse CKD subtypes with/without consanguinity, positive family history, and possible hereditary renal syndrome with extra-renal abnormalities or progressive kidney disease of unknown etiology. The identified genetic variants associated with the observed kidney phenotypes were then confirmed and reported. End-stage kidney disease was reported in 51% of the cohort and a family history of kidney disease in 59%, while known consanguinity was reported in 54%. Pathogenic/likely pathogenic variants were identified in 43 patients with a diagnostic yield of 42%, and clinically associated variants of unknown significance (VUS) were identified in further 21 CKD patients (21%). A total of eight novel predicted pathogenic variants and eight VUS were detected. The clinical utility of ES within the nephrology clinic was demonstrated allowing patient management to be disease-specific. In this cohort, ES detected a diagnostic molecular abnormality in 42% of patients with CKD phenotypes. Positive family history and high rates of consanguinity likely contributed to this high diagnostic yield.

Abstract 013: Microbial Metabolites Induce Pro-inflammatory Mechanisms In Children With Chronic Kidney Disease

Introduction: Chronic inflammation is a major risk factor for cardiovascular disease in chronic kidney disease (CKD). The underlying mechanisms are incompletely understood, but may be linked to gut dysbiosis. We aim to describe the microbiota-immune interaction in a cohort of pediatric CKD, thus independent of confounding comorbidities frequently seen in adult patients. Methods: We analyzed the fecal microbiome, plasma metabolites and peripheral immune phenotypes in 48 children (normal kidney function (HC, n=10), CKD stage G3-G4 (CKD, n=12), G5 treated by hemodialysis (HD, n=11) or kidney transplantation (KT, n=15)) with a mean age of 10.6 ± 3.8 (mean±SD) years. Results: Children exhibit stage-dependently increased cardiovascular risk as seen by the presence of arterial hypertension despite anti-hypertensive medication. Serum TNF-α (2.87±1.10 (HC); 7.54±2.29 (CKD); 9.63±2.73 (HD); 5.70 ± 1.66 (KT) pg/mL) and sCD14 (1.83±0.28 (HC); 2.88±0.47 (CKD); 2.83±0.69 (HD); 2.22±0.34 (KT) μg/mL) were elevated, indicating inflammation, gut barrier dysfunction and endotoxemia. We observed compositional and functional alterations of the microbiome, including a diminished production of short-chain fatty acids (e.g. propionate 10.43±5.49 (HC); 8.95±4.66 (CKD); 2.75±3.84 (HD); 12.49±4.89 (KT) μM). Plasma metabolite analysis revealed a stage-dependent increase of tryptophan metabolites of bacterial origin (e.g. indoxylsulfate). Serum from HD patients activated the aryl hydrocarbon receptor and stimulated TNF production in monocytes (fold change over HC: 1.80±0.61), corresponding to a pro-inflammatory shift from classical to non-classical and intermediate monocytes. Moreover, unsupervised analysis (FlowSOM) of T cells revealed a loss of mucosa-associated invariant T (MAIT) cells and regulatory T cell subtypes (e.g. CCR6+ and CXCR3+ subsets) in HD patients. Conclusions: In conclusion, gut barrier dysfunction and microbial metabolite imbalance mediate the pro-inflammatory immune phenotype in CKD. These dysbiosis-driven immunological changes are already detectable in children with CKD, in whom comorbidities usually found in adults are absent, highlights the specificity of CKD-related microbiota-immune interaction.

Limited effects of systemic or renal lipoprotein lipase deficiency on renal physiology and diseases

Lipoprotein lipase (LPL) is an enzyme that catalyzes the hydrolysis of circulating triglyceride and the transport of fatty acids into cells. Its activity is positively regulated by insulin, and insulin resistance is associated with low LPL activity and subsequent hypertriglyceridemia. The involvement of hypertriglyceridemia in chronic kidney disease (CKD) is still under the debate in a clinical setting. Therefore, we aimed to study the role of hypertriglyceridemia in the disease using mice with systemic or renal-specific LPL deficiency. Systemic LPL deficiency was characterized by hypertriglyceridemia, but not renal injury or dyslipidemia-related conditions, such as fatty liver. Furthermore, the LPL deficiency-induced hypertriglyceridemia was not associated with a worsening of the CKD phenotype or atherosclerosis, even when CKD was induced by 5/6 nephrectomy. Next, because LPL-mediated fatty acid uptake may be important for energy metabolism in proximal tubular epithelial cells (PTECs), the role of renal LPL in renal physiology was studied by generating mice lacking LPL specifically in PTECs. These mice showed no abnormalities in their histology or renal reabsorption of micro molecules. These findings suggest that systemic and renal lipid abnormalities caused by LPL deficiency do not cause or worsen the development of renal injury, and provide novel insight regarding the potential role of lipotoxicity in the pathogenesis of obesity-related kidney injury.

Muscle fibrosis and maladaptation occur progressively in CKD and are rescued by dialysis.

BACKGROUNDSkeletal muscle maladaptation accompanies chronic kidney disease (CKD) and negatively affects physical function. Emphasis in CKD has historically been placed on muscle fiber–intrinsic deficits, such as altered protein metabolism and atrophy. However, targeted treatment of fiber-intrinsic dysfunction has produced limited improvement, whereas alterations within the fiber-extrinsic environment have scarcely been examined.METHODSWe investigated alterations to the skeletal muscle interstitial environment with deep cellular phenotyping of biopsies from patients with CKD and age-matched controls and performed transcriptome profiling to define the molecular underpinnings of CKD-associated muscle impairments. We examined changes in muscle maladaptation following initiation of dialysis therapy for kidney failure.RESULTSPatients with CKD exhibited a progressive fibrotic muscle phenotype, which was associated with impaired regenerative capacity and lower vascular density. The severity of these deficits was strongly associated with the degree of kidney dysfunction. Consistent with these profound deficits, CKD was associated with broad alterations to the muscle transcriptome, including altered ECM organization, downregulated angiogenesis, and altered expression of pathways related to stem cell self-renewal. Remarkably, despite the seemingly advanced nature of this fibrotic transformation, dialysis treatment rescued these deficits, restoring a healthier muscle phenotype. Furthermore, after accounting for muscle atrophy, strength and endurance improved after dialysis initiation.CONCLUSIONThese data identify a dialysis-responsive muscle fibrotic phenotype in CKD and suggest the early dialysis window presents a unique opportunity of improved muscle regenerative capacity during which targeted interventions may achieve maximal impact.TRIAL REGISTRATIONNCT01452412FUNDINGNIH, NIH Clinical and Translational Science Awards (CTSA), and Einstein-Mount Sinai Diabetes Research Center

The cell membrane repair protein MG53 modulates transcription factor NF-κB signaling to control kidney fibrosis

Kidney fibrosis is associated with the progression of acute kidney injury to chronic kidney disease. MG53, a cell membrane repair protein, has been shown to protect against injury to kidney epithelial cells and acute kidney injury. Here, we evaluated the role of MG53 in modulation of kidney fibrosis in aging mice and in mice with unilateral ureteral obstruction (UUO) a known model of progressive kidney fibrosis. Mice with ablation of MG53 developed more interstitial fibrosis with age than MG53-intact mice of the same age. Similarly, in the absence of MG53, kidney fibrosis was exaggerated compared to mice with intact MG53 in the obstructed kidney compared to the contralateral unobstructed kidney or the kidneys of sham operated mice. The ureteral obstructed kidneys from MG53 deficient mice also showed significantly more inflammation than ureteral obstructed kidneys from MG53 intact mice. Invitro experiments demonstrated that MG53 could enter the nuclei of proximal tubular epithelial cells and directly interact with the p65 component of transcription factor NF-κB, providing a possible explanation of enhanced inflammation in the absence of MG53. To test this, enhanced MG53 expression through engineered cells or direct recombinant protein delivery was given to mice subject to UUO. This reduced NF-κB activation and inflammation and attenuated kidney fibrosis. Thus, MG53 may have a therapeutic role in treating chronic kidney inflammation and thereby provide protection against fibrosis that leads to the chronic kidney disease phenotype.

Hyperphosphatemia Contributes to Skeletal Muscle Atrophy in Chronic Kidney Disease

Alterations in mineral metabolism involve changes in serum levels of phosphate as well as its key regulators, fibroblast growth factor 23 (FGF23), vitamin D, parathyroid hormone and klotho. Chronic Kidney Disease (CKD) is a public health epidemic that affects an estimated 37 million Americans, and is associated with elevated serum levels of phosphate (hyperphosphatemia) and FGF23 as well as various pathologies, such vascular calcification, cardiac hypertrophy, and skeletal muscle atrophy. We previously found that elevated FGF23 directly targets cardiac myocytes and contributes to cardiac hypertrophy. Furthermore, elevated phosphate has direct effects on smooth muscle cells and induces vascular calcification. In order to determine whether FGF23 and/or phosphate might also contribute to CKD-associated skeletal muscle atrophy, we tested if skeletal muscle cells can respond to elevations of FGF23 or phosphate. Since the skeletal muscle phenotype in CKD is not well characterized, we analyzed skeletal muscle on a functional, histological, and molecular level in two established mouse models of CKD. We studied the C2C12 skeletal muscle cell line during the differentiation process from myoblasts to myotubes as well as fully differentiated myotubes. Myoblast differentiation was induced with horse serum; cells were co-treated with 25 ng/ml FGF23 or 5 mM phosphate for 5 days, followed by the expression analysis of differentiation markers by qPCR. In a separate study, differentiated C2C12 myotubes were treated with FGF23 or phosphate for 24 hours, followed by the qPCR analysis of atrophy genes (atrogenes), including MT1, Murf1, and Fbox32. Furthermore, we studied mice with global deletion of collagen 4a3 (Col4a3-/-) and wildtype littermates at 10 weeks of age, as well as C57Bl/6 mice receiving an adenine-rich (0.2%) diet or regular chow for 12 weeks. In all mice we studied grip strength, hindlimb circumference by MRI, cross-sectional area of individual muscle fibers that were immuno-labeled with anti-laminin by fluorescence microscopy, and expression levels of atrogenes by qPCR. Phosphate but not FGF23 affected the differentiation process of C2C12 myoblasts and differentiated C2C12 myotubes. Phosphate treatments significantly reduced expression levels of MyoD and MyoG and increased expression of MT1. In both mouse models, grip strength and areas of hindlimb and myotubes were significantly reduced, and expression levels of MT1 and Murf1 were significantly elevated when compared to their controls. FGF23 does not target skeletal muscle cells suggesting that it cannot directly contribute to skeletal muscle atrophy. Elevated phosphate impairs myoblast differentiation and induces atrophy in vitro. We have characterized two CKD mouse models with associated skeletal muscle atrophy, where we will now determine the contribution of hyperphosphatemia to the injury process. Pharmacological approaches targeting phosphate uptake or excretion, or phosphate's direct actions on tissues might alleviate various CKD-associated pathologies.

Hypertension due to loss of water

In the current issue of Acta Physiologica, two papers are published investigating the development of arterial hypertension under conditions of pathological loss of free water in two disease models. Kovaric et al 1 studied rats with chronic renal failure, shedding excessive free water via their remnant kidney due to a hypostenuric concentrating defect. Wild et al. 2 investigated a transgenic mouse model for psoriasis, losing free water via the psoriatic skin lesions. In both cases the animals displayed hypertension, cutaneous vasoconstriction, and metabolic adaptations towards enhanced production of urea and organic osmolytes typical for a phylogenetically ancient water conservation response termed aestivation 3 . The international consortia of investigators spearheaded by Titze, Karbach, and Kitada therefore conclude that in both cases the hypertension was due to cutaneous vasoconstriction in an attempt to limit epidermal water loss. Because in either case the hypertension was associated with loss of water, and thus with hypo- rather than hypervolemia, the authors argue, that this concept of pathogenesis of hypertension is antipodal to the classic view that relies on hypervolemia due to sodium retention.

Medical records-based chronic kidney disease phenotype for clinical care and \u201cbig data\u201d observational and genetic studies

Chronic Kidney Disease (CKD) represents a slowly progressive disorder that is typically silent until late stages, but early intervention can significantly delay its progression. We designed a portable and scalable electronic CKD phenotype to facilitate early disease recognition and empower large-scale observational and genetic studies of kidney traits. The algorithm uses a combination of rule-based and machine-learning methods to automatically place patients on the staging grid of albuminuria by glomerular filtration rate (“A-by-G” grid). We manually validated the algorithm by 451 chart reviews across three medical systems, demonstrating overall positive predictive value of 95% for CKD cases and 97% for healthy controls. Independent case-control validation using 2350 patient records demonstrated diagnostic specificity of 97% and sensitivity of 87%. Application of the phenotype to 1.3 million patients demonstrated that over 80% of CKD cases are undetected using ICD codes alone. We also demonstrated several large-scale applications of the phenotype, including identifying stage-specific kidney disease comorbidities, in silico estimation of kidney trait heritability in thousands of pedigrees reconstructed from medical records, and biobank-based multicenter genome-wide and phenome-wide association studies.