Publications by authors named "Felix Knauf"

33 Publications

Author Reply to Comment on "Assessment of Plasma Oxalate Concentration in Patients With CKD" by Oka

Kidney Int Rep 2021 Apr 5;6(4):1194-1195. Epub 2021 Mar 5.

Department of Nephrology and Medical Intensive Care, Charité - Universitätsmedizin Berlin, Berlin, Germany.

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http://dx.doi.org/10.1016/j.ekir.2021.02.027DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8071653PMC
April 2021

Pilot study of reloxaliase in patients with severe enteric hyperoxaluria and hyperoxalemia.

Nephrol Dial Transplant 2021 Apr;36(5):945-948

Department of Nephrology and Medical Intensive Care, Charité Universitätsmedizin Berlin, Berlin, Germany.

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http://dx.doi.org/10.1093/ndt/gfaa379DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8075375PMC
April 2021

Assessment of Plasma Oxalate Concentration in Patients With CKD.

Kidney Int Rep 2020 Nov 2;5(11):2013-2020. Epub 2020 Sep 2.

Department of Nephrology and Medical Intensive Care, Charité - Universitätsmedizin Berlin, Berlin, Germany.

Introduction: Alterations in oxalate homeostasis are associated with kidney stone disease and progression of chronic kidney disease (CKD). However, accurate measurement of plasma oxalate (P) concentrations in large patient cohorts is challenging as prompt acidification of samples has been deemed necessary. In the present study, we investigated the effects of variations in sample handling on P results and examined an alternative strategy to the established preanalytical procedures.

Methods: The effect of storage time at room temperature (RT) and maintenance of samples at -80°C was tested. P was measured in 1826 patients enrolled in the German Chronic Kidney Disease (GCKD) study, an ongoing multicenter, prospective, observational cohort study.

Results: We demonstrate that P concentrations increased rapidly when samples were maintained at RT. This was most relevant for P <10 μM, as concentrations more than doubled within a few hours. Immediate freezing on dry ice and storage at -80°C provided stable results and allowed postponement of acidification for >1 year. In the patients of the lowest estimated glomerular filtration rate (eGFR) quartile, median P was 2.7 μM (interquartile range [IQR] <2.0-4.2) with a median eGFR of 25.1 ml/min per 1.73 m (IQR 20.3-28.1).

Conclusion: We conclude that immediate freezing and maintenance of plasma samples at -80°C facilitates the sample collection process and allows accurate P assessment in large cohorts. The present study may serve as a reference for sample handling to assess P in clinical trials and to determine its role in CKD progression.
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http://dx.doi.org/10.1016/j.ekir.2020.08.029DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7609998PMC
November 2020

Pathophysiology and Treatment of Enteric Hyperoxaluria.

Clin J Am Soc Nephrol 2021 Mar 8;16(3):487-495. Epub 2020 Sep 8.

Allena Pharmaceuticals, Inc., Newton, Massachusetts

Enteric hyperoxaluria is a distinct entity that can occur as a result of a diverse set of gastrointestinal disorders that promote fat malabsorption. This, in turn, leads to excess absorption of dietary oxalate and increased urinary oxalate excretion. Hyperoxaluria increases the risk of kidney stones and, in more severe cases, CKD and even kidney failure. The prevalence of enteric hyperoxaluria has increased over recent decades, largely because of the increased use of malabsorptive bariatric surgical procedures for medically complicated obesity. This systematic review of enteric hyperoxaluria was completed as part of a Kidney Health Initiative-sponsored project to describe enteric hyperoxaluria pathophysiology, causes, outcomes, and therapies. Current therapeutic options are limited to correcting the underlying gastrointestinal disorder, intensive dietary modifications, and use of calcium salts to bind oxalate in the gut. Evidence for the effect of these treatments on clinically significant outcomes, including kidney stone events or CKD, is currently lacking. Thus, further research is needed to better define the precise factors that influence risk of adverse outcomes, the long-term efficacy of available treatment strategies, and to develop new therapeutic approaches.
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http://dx.doi.org/10.2215/CJN.08000520DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8011014PMC
March 2021

Statins, obesity, and the microbiome: a potential mechanism for the pleiotropic effects of statin therapy.

Kidney Int 2021 03 19;99(3):531-533. Epub 2020 Aug 19.

Department of Nephrology, Charité Universitätsmedizin Berlin, Berlin, Germany; Department of Internal Medicine, Section of Nephrology, Yale University School of Medicine, New Haven, USA. Electronic address:

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http://dx.doi.org/10.1016/j.kint.2020.07.038DOI Listing
March 2021

Enteric Oxalate Secretion Mediated by Slc26a6 Defends against Hyperoxalemia in Murine Models of Chronic Kidney Disease.

J Am Soc Nephrol 2020 09 13;31(9):1987-1995. Epub 2020 Jul 13.

Department of Internal Medicine, Section of Nephrology, Yale University School of Medicine, New Haven, Connecticut

Background: A state of oxalate homeostasis is maintained in patients with healthy kidney function. However, as GFR declines, plasma oxalate (P) concentrations start to rise. Several groups of researchers have described augmentation of oxalate secretion in the colon in models of CKD, but the oxalate transporters remain unidentified. The oxalate transporter Slc26a6 is a candidate for contributing to the extrarenal clearance of oxalate the gut in CKD.

Methods: Feeding a diet high in soluble oxalate or weekly injections of aristolochic acid induced CKD in age- and sex-matched wild-type and mice. qPCR, immunohistochemistry, and western blot analysis assessed intestinal expression. An oxalate oxidase assay measured fecal and P concentrations.

Results: Fecal oxalate excretion was enhanced in wild-type mice with CKD. This increase was abrogated in mice associated with a significant elevation in plasma oxalate concentration. mRNA and protein expression were greatly increased in the intestine of mice with CKD. Raising P without inducing kidney injury did not alter intestinal expression, suggesting that changes associated with CKD regulate transporter expression rather than elevations in P.

Conclusions: Slc26a6-mediated enteric oxalate secretion is critical in decreasing the body burden of oxalate in murine CKD models. Future studies are needed to address whether similar mechanisms contribute to intestinal oxalate elimination in humans to enhance extrarenal oxalate clearance.
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http://dx.doi.org/10.1681/ASN.2020010105DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7461683PMC
September 2020

Stiripentol fails to lower plasma oxalate in a dialysis-dependent PH1 patient.

Pediatr Nephrol 2020 09 16;35(9):1787-1789. Epub 2020 May 16.

Departments of Pediatric Gastroenterology, Nephrology and Metabolic Diseases, Charité University Medicine, Berlin, Germany.

Background: Primary hyperoxaluria type 1 (PH1) is a multisystemic metabolic disorder caused by an excessive production of oxalate by the liver. The majority of patients presenting in early infancy have end-stage renal disease (ESRD). While awaiting the results of sRNAi trials, the current standard treatment is combined liver-kidney transplantation. Recently, Stiripentol has been reported as a promising drug in the treatment of primary hyperoxaluria by reducing urinary oxalate (U). Stiripentol is an anti-convulsive drug used in the treatment of children suffering from Dravet syndrome. It causes blockage of the last step in oxalate production by inhibition of hepatic lactate dehydrogenase 5 (LDH5).

Case: We administered Stiripentol as compassionate use in an anuric infant with dialysis-dependent PH1 over a period of 4 months. Although achieving plasma concentrations of Stiripentol that were recently reported to lower U excretion, we did not observe significant reduction to plasma oxalate concentrations (P).

Conclusion: We conclude that Stiripentol may not be useful to reduce P in PH patients with advanced chronic kidney disease (CKD), but larger studies are needed to confirm this finding.
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http://dx.doi.org/10.1007/s00467-020-04585-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7385015PMC
September 2020

P2X7 Receptor Stimulation Is Not Required for Oxalate Crystal-Induced Kidney Injury.

Sci Rep 2019 12 27;9(1):20086. Epub 2019 Dec 27.

Department of Nephrology and Medical Intensive Care, Charité - Universitätsmedizin Berlin, Berlin, Germany.

Oxalate crystal-induced renal inflammation is associated with progressive kidney failure due to activation of the NLRP3/CASP-1 inflammasome. It has been suggested previously that purinergic P2X7 receptor signaling is critical for crystal-induced inflammasome activation and renal injury. Therefore, we investigated the role of the P2X7 receptor in response to crystal-induced cytokine release, inflammation, and kidney failure using in vitro and in vivo models. Dendritic cells and macrophages derived from murine bone marrow and human peripheral blood mononucleated cells stimulated with calcium-oxalate crystals, monosodium urate crystals, or ATP lead to the robust release of interleukin-1beta (IL-1ß). Treatment with the P2X7 inhibitor A740003 or the depletion of ATP by apyrase selectively abrogated ATP-induced, but not oxalate and urate crystal-induced IL-1ß release. In line with this finding, dendritic cells derived from bone marrow (BMDCs) from P2X7 mice released reduced amounts of IL-1ß following stimulation with ATP, while oxalate and urate crystal-induced IL-1ß release was unaffected. In sharp contrast, BMDCs from Casp1 mice exhibited reduced IL-1ß release following either of the three stimulants. In addition, P2X7 mice demonstrated similar degrees of crystal deposition, tubular damage and inflammation when compared with WT mice. In line with these findings, increases in plasma creatinine were no different between WT and P2X7 mice. In contrast to previous reports, our results indicate that P2X7 receptor is not required for crystal-induced CKD and it is unlikely to be a suitable therapeutic target for crystal-induced progressive kidney disease.
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http://dx.doi.org/10.1038/s41598-019-56560-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6934555PMC
December 2019

Tumor necrosis factor stimulates fibroblast growth factor 23 levels in chronic kidney disease and non-renal inflammation.

Kidney Int 2019 10 14;96(4):890-905. Epub 2019 May 14.

Institute of Physiology, University of Zurich, Zurich, Switzerland; Swiss National Center of Competence in Research NCCR-Kidney.CH, University of Zurich, Zurich, Switzerland. Electronic address:

Fibroblast growth factor 23 (FGF23) regulates phosphate homeostasis, and its early rise in patients with chronic kidney disease is independently associated with all-cause mortality. Since inflammation is characteristic of chronic kidney disease and associates with increased plasma FGF23 we examined whether inflammation directly stimulates FGF23. In a population-based cohort, plasma tumor necrosis factor (TNF) was the only inflammatory cytokine that independently and positively correlated with plasma FGF23. Mouse models of chronic kidney disease showed signs of renal inflammation, renal FGF23 expression and elevated systemic FGF23 levels. Renal FGF23 expression coincided with expression of the orphan nuclear receptor Nurr1 regulating FGF23 in other organs. Antibody-mediated neutralization of TNF normalized plasma FGF23 and suppressed ectopic renal Fgf23 expression. Conversely, TNF administration to control mice increased plasma FGF23 without altering plasma phosphate. Moreover, in Il10-deficient mice with inflammatory bowel disease and normal kidney function, plasma FGF23 was elevated and normalized upon TNF neutralization. Thus, the inflammatory cytokine TNF contributes to elevated systemic FGF23 levels and also triggers ectopic renal Fgf23 expression in animal models of chronic kidney disease.
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http://dx.doi.org/10.1016/j.kint.2019.04.009DOI Listing
October 2019

A preliminary survey of practice patterns across several European kidney stone centers and a call for action in developing shared practice.

Urolithiasis 2019 Jun 8;47(3):219-224. Epub 2019 Mar 8.

U.O.C. Nefrologia, Fondazione Policlinico Universitario A. Gemelli IRCCS, Via G. Moscati 31, 00168, Roma, Italy.

Currently an evidence-based approach to nephrolithiasis is hampered by a lack of randomized controlled trials. Thus, there is a need for common platforms for data sharing and recruitment of patients to interventional studies. A first step in achieving this objective would be to share practice methods and protocols for subsequent standardization in what is still a heterogeneous clinical field. Here, we present the results of a pilot survey performed across 24 European clinical kidney stone centers. The survey was distributed by a voluntary online questionnaire circulated between June 2017 and January 2018. About 46% of centers reported seeing on average 20 or more patients per month. Only 21% adopted any formal referral criteria. Centers were relatively heterogeneous in respect of the definition of an incident stone event. The majority (71%) adopted a formal follow-up scheme; of these, 65% included a follow-up visit at 3 and 12 months, and 41% more than 12 months. In 79% of centers some kind of imaging was performed systematically. 75% of all centers performed laboratory analyses on blood samples at baseline and during follow-up. All centers performed laboratory analyses on 24-h urine samples, the majority (96%) at baseline and during follow-up. There was good correspondence across centers for analyses performed on 24-h urine samples, although the methods of 24-h urine collection and analysis were relatively heterogeneous. Our survey among 24 European stone centers highlights areas of homogeneity and heterogeneity that will be investigated further. Our aim is the creation of a European network of stone centers sharing practice patterns and hosting a common database for research and guidance in clinical care.
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http://dx.doi.org/10.1007/s00240-019-01119-zDOI Listing
June 2019

Challenges to hypertension and diabetes management in rural Uganda: a qualitative study with patients, village health team members, and health care professionals.

Int J Equity Health 2019 02 28;18(1):38. Epub 2019 Feb 28.

Department of Internal Medicine, Yale University School of Medicine, New Haven, CT, USA.

Background: The prevalence of hypertension and diabetes are expected to increase in sub-Saharan Africa over the next decade. Some studies have documented that lifestyle factors and lack of awareness are directly influencing the control of these diseases. Yet, few studies have attempted to understand the barriers to control of these conditions in rural settings. The main objective of this study was to understand the challenges to hypertension and diabetes care in rural Uganda.

Methods: We conducted semi-structured interviews with 24 patients with hypertension and/or diabetes, 11 health care professionals (HCPs), and 12 community health workers (known as village health team members [VHTs]) in Nakaseke District, Uganda. Data were coded using NVivo software and analyzed using a thematic approach.

Results: The results replicated several findings from other settings, and identified some previously undocumented challenges including patients' knowledge gaps regarding the preventable aspects of HTN and DM, patients' mistrust in the Ugandan health care system rather than in individual HCPs, and skepticism from both HCPs and patients regarding a potential role for VHTs in HTN and DM management.

Conclusions: In order to improve hypertension and diabetes management in this setting, we recommend taking actions to help patients to understand NCDs as preventable, for HCPs and patients to advocate together for health system reform regarding medication accessibility, and for promoting education, screening, and monitoring activities to be conducted on a community level in collaboration with village health team members.
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http://dx.doi.org/10.1186/s12939-019-0934-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6394065PMC
February 2019

Immunity, microbiota and kidney disease.

Nat Rev Nephrol 2019 05;15(5):263-274

Department of Immunobiology, Yale University School of Medicine, New Haven, CT, USA.

The recognition that intestinal microbiota exert profound effects on human health has led to major advances in our understanding of disease processes. Studies over the past 20 years have shown that host components, including components of the host immune system, shape the microbial community. Pathogenic alterations in commensal microorganisms contribute to disease manifestations that are generally considered to be noncommunicable, such as inflammatory bowel disease, diabetes mellitus and liver disease, through a variety of mechanisms, including effects on host immunity. More recent studies have shed new light on how the immune system and microbiota might also drive the pathogenesis of renal disorders. In this Review, we discuss the latest insights into the mechanisms regulating the microbiome composition, with a focus both on genetics and environmental factors, and describe how commensal microorganisms calibrate innate and adaptive immune responses to affect the activation threshold for pathogenic stimulations. We discuss the mechanisms that lead to intestinal epithelial barrier inflammation and the relevance of certain bacteria to the pathogenesis of two common kidney-based disorders: hypertension and renal stone disease. Limitations of current approaches to microbiota research are also highlighted, emphasizing the need to move beyond studies of correlation to causation.
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http://dx.doi.org/10.1038/s41581-019-0118-7DOI Listing
May 2019

Characterization of renal NaCl and oxalate transport in Slc26a6 mice.

Am J Physiol Renal Physiol 2019 01 14;316(1):F128-F133. Epub 2018 Nov 14.

Department of Internal Medicine, Yale University School of Medicine , New Haven, Connecticut.

The apical membrane Cl/oxalate exchanger SLC26A6 has been demonstrated to play a role in proximal tubule NaCl transport based on studies in microperfused tubules. The present study is directed at characterizing the role of SLC26A6 in NaCl homeostasis in vivo under physiological conditions. Free-flow micropuncture studies revealed that volume and Cl absorption were similar in surface proximal tubules of wild-type and Slc26a6 mice. Moreover, the increments in urine flow rate and sodium excretion following thiazide and furosemide infusion were identical in wild-type and Slc26a6 mice, indicating no difference in NaCl delivery out of the proximal tubule. The absence of an effect of deletion of SLC26A6 on NaCl homeostasis was further supported by the absence of lower blood pressure in Slc26a6 compared with wild-type mice on normal or low-salt diets. Moreover, raising plasma and urine oxalate by feeding mice a diet enriched in soluble oxalate did not affect mean blood pressure. In contrast to the lack of effect of SLC26A6 deletion on NaCl homeostasis, fractional excretion of oxalate was reduced from 1.6 in wild-type mice to 0.7 in Slc26a6 mice. We conclude that, although SLC26A6 is dispensable for renal NaCl homeostasis, it is required for net renal secretion of oxalate.
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http://dx.doi.org/10.1152/ajprenal.00309.2018DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6383200PMC
January 2019

A Successful Approach to Kidney Transplantation in Patients With Enteric (Secondary) Hyperoxaluria.

Transplant Direct 2017 Dec 8;3(12):e331. Epub 2017 Nov 8.

Division of Nephrology and Transplantation, Department of Internal Medicine, Erasmus Medical Center, Rotterdam, The Netherlands.

Background: Enteric hyperoxaluria due to malabsorption may cause chronic oxalate nephropathy and lead to end-stage renal disease. Kidney transplantation is challenging given the risk of recurrent calcium-oxalate deposition and nephrolithiasis.

Methods: We established a protocol to reduce plasma oxalic acid levels peritransplantation based on reduced intake and increased removal of oxalate. The outcomes of 10 kidney transplantation patients using this protocol are reported.

Results: Five patients received a living donor kidney and had immediate graft function. Five received a deceased donor kidney and had immediate (n = 1) or delayed graft function (n = 4). In patients with delayed graft function, the protocol was prolonged after transplantation. In 3 patients, our protocol was reinstituted because of late complications affecting graft function. One patient with high-output stoma and relatively low oxalate levels had lost her first kidney transplant because of recurrent oxalate depositions but now receives intravenous fluid at home on a routine basis 3 times per week to prevent dehydration. Patients are currently between 3 and 32 months after transplantation and all have a stable estimated glomerular filtration rate (mean, 51 ± 21 mL/min per 1.73 m). In 4 of 8 patients who underwent for cause biopsies after transplantation oxalate depositions were found.

Conclusions: This is the first systematic description of kidney transplantation in a cohort of patients with enteric hyperoxaluria. Common complications after kidney transplantation impact long-term transplant function in these patients. With our protocol, kidney transplantation outcomes were favorable in this population with unfavorable transplantation prospects and even previous unsuccessful transplants.
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http://dx.doi.org/10.1097/TXD.0000000000000748DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5828694PMC
December 2017

Impact of Regular or Extended Hemodialysis and Hemodialfiltration on Plasma Oxalate Concentrations in Patients With End-Stage Renal Disease.

Kidney Int Rep 2017 Nov 8;2(6):1050-1058. Epub 2017 Jun 8.

Department of Nephrology and Hypertension, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.

Introduction: Calcium oxalate supersaturation is regularly exceeded in the plasma of patients with end-stage renal disease (ESRD). Previous reports have indicated that hemodialfiltration (HDF) lowers elevated plasma oxalate (P) concentrations more effectively compared with hemodialysis (HD). We reevaluate the therapeutic strategy for optimized P reduction with advanced dialysis equipment and provide data on the effect of extended treatment time on dialytic oxalate kinetics.

Methods: Fourteen patients with ESRD who underwent HDF 3 times a week for 4 to 4.5 hours (regular HDF; n = 8) or 7 to 7.5 hours (extended HDF; n = 6) were changed to HD for 2 weeks and then back to HDF for another 2 weeks. P was measured at baseline, pre-, mid-, and postdialysis, and 2 hours after completion of the treatment session.

Results: Baseline P for all patients averaged 28.0 ± 7.0 μmol/l. Intradialytic P reduction was approximately 90% and was not significantly different between groups or treatment modes [F(1) = 0.63;  = 0.44]. Mean postdialysis P concentrations were 3.3 ± 1.8 μmol/l. A rebound of 2.1 ± 1.9 μmol/l was observed within 2 hours after dialysis. After receiving 2 weeks of the respective treatment, predialysis P concentrations on HD did not differ significantly from those on HDF [F(1) = 0.21;  = 0.66]. Extended treatment time did not provide any added benefit [F(1) = 0.76;  = 0.40].

Discussion: In contrast to earlier observations, our data did not support a benefit of HDF over HD for P reduction. With new technologies evolving, our results emphasized the need to carefully reevaluate and update traditional therapeutic regimens for optimized uremic toxin removal, including those used for oxalate.
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http://dx.doi.org/10.1016/j.ekir.2017.06.002DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5733827PMC
November 2017

Implementation of Patient-Centered Education for Chronic-Disease Management in Uganda: An Effectiveness Study.

PLoS One 2016 16;11(11):e0166411. Epub 2016 Nov 16.

Department of Medicine, Yale School of Medicine, Yale University, New Haven, Connecticut, United States of America.

Background: The majority of non-communicable disease related deaths occur in low- and middle-income countries. Patient-centered care is an essential component of chronic disease management in high income settings.

Objective: To examine feasibility of implementation of a validated patient-centered education tool among patients with heart failure in Uganda.

Design: Mixed-methods, prospective cohort.

Settings: A private and public cardiology clinic in Mulago National Referral and Teaching Hospital, Kampala, Uganda.

Participants: Adults with a primary diagnosis of heart failure.

Interventions: PocketDoktor Educational Booklets with patient-centered health education.

Main Measures: The primary outcomes were the change in Patient Activation Measure (PAM-13), as well as the acceptability of the PocketDoktor intervention, and feasibility of implementing patient-centered education in outpatient clinical settings. Secondary outcomes included the change in satisfaction with overall clinical care and doctor-patient communication.

Key Results: A total of 105 participants were enrolled at two different clinics: the Mulago Outpatient Department (public) and the Uganda Heart Institute (private). 93 participants completed follow up at 3 months and were included in analysis. The primary analysis showed improved patient activation measure scores regarding disease-specific knowledge, treatment options and prevention of exacerbations among both groups (mean change 0.94 [SD = 1.01], 1.02 [SD = 1.15], and 0.92 [SD = 0.89] among private paying patients and 1.98 [SD = 0.98], 1.93 [SD = 1.02], and 1.45 [SD = 1.02] among public paying patients, p<0.001 for all values) after exposure to the intervention; this effect was significantly larger among indigent patients. Participants reported that materials were easy to read, that they had improved knowledge of disease, and stated improved communication with physicians.

Conclusions: Patient-centered medical education can improve confidence in self-management as well as satisfaction with doctor-patient communication and overall care in Uganda. Our results show that printed booklets are locally appropriate, highly acceptable and feasible to implement in an LMIC outpatient setting across socioeconomic groups.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0166411PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5112982PMC
June 2017

Update on Nephrolithiasis: Core Curriculum 2016.

Am J Kidney Dis 2016 12 3;68(6):973-985. Epub 2016 Aug 3.

Department of Nephrology und Hypertension, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany; Department of Internal Medicine, Yale University School of Medicine, New Haven, CT. Electronic address:

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http://dx.doi.org/10.1053/j.ajkd.2016.05.016DOI Listing
December 2016

Loss of Cystic Fibrosis Transmembrane Regulator Impairs Intestinal Oxalate Secretion.

J Am Soc Nephrol 2017 Jan 16;28(1):242-249. Epub 2016 Jun 16.

Departments of Internal Medicine,

Patients with cystic fibrosis have an increased incidence of hyperoxaluria and calcium oxalate nephrolithiasis. Net intestinal absorption of dietary oxalate results from passive paracellular oxalate absorption as modified by oxalate back secretion mediated by the SLC26A6 oxalate transporter. We used mice deficient in the cystic fibrosis transmembrane conductance regulator gene (Cftr) to test the hypothesis that SLC26A6-mediated oxalate secretion is defective in cystic fibrosis. We mounted isolated intestinal tissue from C57BL/6 (wild-type) and Cftr mice in Ussing chambers and measured transcellular secretion of [C]oxalate. Intestinal tissue isolated from Cftr mice exhibited significantly less transcellular oxalate secretion than intestinal tissue of wild-type mice. However, glucose absorption, another representative intestinal transport process, did not differ in Cftr tissue. Compared with wild-type mice, Cftr mice showed reduced expression of SLC26A6 in duodenum by immunofluorescence and Western blot analysis. Furthermore, coexpression of CFTR stimulated SLC26A6-mediated Cl-oxalate exchange in Xenopus oocytes. In association with the profound defect in intestinal oxalate secretion, Cftr mice had serum and urine oxalate levels 2.5-fold greater than those of wild-type mice. We conclude that defective intestinal oxalate secretion mediated by SLC26A6 may contribute to the hyperoxaluria observed in this mouse model of cystic fibrosis. Future studies are needed to address whether similar mechanisms contribute to the increased risk for calcium oxalate stone formation observed in patients with cystic fibrosis.
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http://dx.doi.org/10.1681/ASN.2016030279DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5198290PMC
January 2017

Oxalate, inflammasome, and progression of kidney disease.

Curr Opin Nephrol Hypertens 2016 07;25(4):363-71

aDepartment of Nephrology und Hypertension, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany bDepartment of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut, USA.

Purpose Of Review: Oxalate is an end product of metabolism excreted via the kidney. Excess urinary oxalate, whether from primary or enteric hyperoxaluria, can lead to oxalate deposition in the kidney. Oxalate crystals are associated with renal inflammation, fibrosis, and progressive renal failure. It has long been known that as the glomerular filtration rate becomes reduced in chronic kidney disease (CKD), there is striking elevation of plasma oxalate. Taken together, these findings raise the possibility that elevation of plasma oxalate in CKD may promote renal inflammation and more rapid progression of CKD independent of primary cause.

Recent Findings: The inflammasome has recently been identified to play a critical role in oxalate-induced renal inflammation. Oxalate crystals have been shown to activate the NOD-like receptor family, pyrin domain containing 3 inflammasome (also known as NALP3, NLRP3, or cryopyrin), resulting in release of IL-1β and macrophage infiltration. Deletion of inflammasome proteins in mice protects from oxalate-induced renal inflammation and progressive renal failure.

Summary: The findings reviewed in this article expand our understanding of the relevance of elevated plasma oxalate levels leading to inflammasome activation. We propose that inhibiting oxalate-induced inflammasome activation, or lowering plasma oxalate, may prevent or mitigate progressive renal damage in CKD, and warrants clinical trials.
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http://dx.doi.org/10.1097/MNH.0000000000000229DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4891250PMC
July 2016

Oxalate-induced chronic kidney disease with its uremic and cardiovascular complications in C57BL/6 mice.

Am J Physiol Renal Physiol 2016 04 13;310(8):F785-F795. Epub 2016 Jan 13.

Department of Nephrology and Hypertension, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany;

Chronic kidney disease (CKD) research is limited by the lack of convenient inducible models mimicking human CKD and its complications in experimental animals. We demonstrate that a soluble oxalate-rich diet induces stable stages of CKD in male and female C57BL/6 mice. Renal histology is characterized by tubular damage, remnant atubular glomeruli, interstitial inflammation, and fibrosis, with the extent of tissue involvement depending on the duration of oxalate feeding. Expression profiling of markers and magnetic resonance imaging findings established to reflect inflammation and fibrosis parallel the histological changes. Within 3 wk, the mice reproducibly develop normochromic anemia, metabolic acidosis, hyperkalemia, FGF23 activation, hyperphosphatemia, and hyperparathyroidism. In addition, the model is characterized by profound arterial hypertension as well as cardiac fibrosis that persist following the switch to a control diet. Together, this new model of inducible CKD overcomes a number of previous experimental limitations and should serve useful in research related to CKD and its complications.
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http://dx.doi.org/10.1152/ajprenal.00488.2015DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5504458PMC
April 2016

Prevention of diet-induced hepatic steatosis and hepatic insulin resistance by second generation antisense oligonucleotides targeted to the longevity gene mIndy (Slc13a5).

Aging (Albany NY) 2015 Dec;7(12):1086-93

Section of Metabolic Vascular Medicine, Medical Clinic III and Paul Langerhans Institute Dresden (PLID), TU Dresden, Germany.

Reducing the expression of the Indy (I'm Not Dead Yet) gene in lower organisms extends life span by mechanisms resembling caloric restriction. Similarly, deletion of the mammalian homolog, mIndy (Slc13a5), encoding for a plasma membrane tricarboxylate transporter, protects from aging- and diet-induced adiposity and insulin resistance in mice. The organ specific contribution to this phenotype is unknown. We examined the impact of selective inducible hepatic knockdown of mIndy on whole body lipid and glucose metabolism using 2'-O-methoxyethyl chimeric anti-sense oligonucleotides (ASOs) in high-fat fed rats. 4-week treatment with 2'-O-methoxyethyl chimeric ASO reduced mIndy mRNA expression by 91% (P=0.001) compared to control ASO. Besides similar body weights between both groups, mIndy-ASO treatment lead to a 74% reduction in fasting plasma insulin concentrations as well as a 35% reduction in plasma triglycerides. Moreover, hepatic triglyceride content was significantly reduced by the knockdown of mIndy, likely mediating a trend to decreased basal rates of endogenous glucose production as well as an increased suppression of hepatic glucose production by 25% during a hyperinsulinemic-euglycemic clamp. Together, these data suggest that inducible liver-selective reduction of mIndy in rats is able to ameliorate hepatic steatosis and insulin resistance, conditions occurring with high calorie diets and during aging.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4712334PMC
http://dx.doi.org/10.18632/aging.100854DOI Listing
December 2015

An update on the role of the inflammasomes in the pathogenesis of kidney diseases.

Pediatr Nephrol 2016 Apr 16;31(4):535-44. Epub 2015 Jul 16.

Department of Nephrology and Hypertension, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Schwabachanlage 12, 91054, Erlangen, Germany.

Innate immune response pathways play a critical role as the first line of defense. Initiation of an immune response requires sensors that can detect noxious stimuli within the cellular microenvironment. Inflammasomes are signaling platforms that are assembled in response to both microbe-specific and nonmicrobial antigens. Upon activation, proinflammatory cytokines are released to engage immune defenses and to trigger an inflammatory cell death referred to as pyroptosis. The aim of this review is to provide an overview of the current knowledge of the role of the inflammasomes in the pathogenesis of kidney diseases. As crystal deposition in the kidney is a frequent cause of acute kidney injury and chronic kidney disease in children, recent insights into mechanisms of inflammasome activation by renal crystals are highlighted. This may be of particular interest to pediatric patients and nephrologists in need of new therapeutic approaches. Lastly, current data findings that inflammasomes are not only of major importance in host defense but are also a key regulator of the intestinal microbiota and the progression of systemic diseases are reviewed.
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http://dx.doi.org/10.1007/s00467-015-3153-zDOI Listing
April 2016

Dabigatran and kidney disease: a bad combination.

Clin J Am Soc Nephrol 2013 Sep 18;8(9):1591-7. Epub 2013 Jul 18.

Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut, USA.

Dabigatran is an oral direct thrombin inhibitor widely used to prevent and treat various thromboembolic complications. An advantage of this agent over other anticoagulants is that routine laboratory monitoring and related dose adjustments are considered unnecessary. A major disadvantage is the absence of a reliable means of reversing its anticoagulant effect. After U.S. Food and Drug Administration approval, recently emerged data suggest a higher bleeding risk with dabigatran, especially in the elderly. Clinicians are thus faced with caring for patients with serious bleeding events without readily available tests to measure drug levels or the anticoagulant effects of dabigatran and without effective antidotes to rapidly reverse the anticoagulant effect. On the basis of dabigatran's pharmacokinetic profile, hemodialysis and continuous renal replacement therapy have been used to remove dabigatran with the hope, still unproven, that this would rapidly reverse the anticoagulant effect and reduce bleeding in patients with normal and those with reduced kidney function. However, the best clinical approach to the patient with serious bleeding is not known, and the risks of placing a hemodialysis catheter in an anticoagulated patient can be substantial. This article reviews this issue, addressing clinical indications, drug pharmacokinetics, clinical and laboratory monitoring tests, and dialytic and nondialytic approaches to reduce bleeding in dabigatran-treated patients.
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http://dx.doi.org/10.2215/CJN.01260213DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3805067PMC
September 2013

NALP3-mediated inflammation is a principal cause of progressive renal failure in oxalate nephropathy.

Kidney Int 2013 Nov 5;84(5):895-901. Epub 2013 Jun 5.

Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut, USA.

Oxalate nephropathy with renal failure is caused by multiple disorders leading to hyperoxaluria due to either overproduction of oxalate (primary hyperoxaluria) or excessive absorption of dietary oxalate (enteric hyperoxaluria). To study the etiology of renal failure in crystal-induced kidney disease, we created a model of progressive oxalate nephropathy by feeding mice a diet high in soluble oxalate (high oxalate in the absence of dietary calcium). Renal histology was characterized by intratubular calcium-oxalate crystal deposition with an inflammatory response in the surrounding interstitium. Oxalate nephropathy was not found in mice fed a high oxalate diet that also contained calcium. NALP3, also known as cryopyrin, has been implicated in crystal-associated diseases such as gout and silicosis. Mice fed the diet high in soluble oxalate demonstrated increased NALP3 expression in the kidney. Nalp3-null mice were completely protected from the progressive renal failure and death that occurred in wild-type mice fed the diet high in soluble oxalate. NALP3 deficiency did not affect oxalate homeostasis, thereby excluding differences in intestinal oxalate handling to explain the observed phenotype. Thus, progressive renal failure in oxalate nephropathy results primarily from NALP3-mediated inflammation.
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http://dx.doi.org/10.1038/ki.2013.207DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3772982PMC
November 2013

Sat1 is dispensable for active oxalate secretion in mouse duodenum.

Am J Physiol Cell Physiol 2012 Jul 18;303(1):C52-7. Epub 2012 Apr 18.

Section of Nephrology, Dept. of Internal Medicine, Yale School of Medicine, New Haven, CT 06520-8029, USA.

Mice deficient for the apical membrane oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium oxalate stones due to a defect in intestinal oxalate secretion. However, the nature of the basolateral membrane oxalate transport process that operates in series with SLC26A6 to mediate active oxalate secretion in the intestine remains unknown. Sulfate anion transporter-1 (Sat1 or SLC26A1) is a basolateral membrane anion exchanger that mediates intestinal oxalate transport. Moreover, Sat1-deficient mice also have a phenotype of hyperoxalemia, hyperoxaluria, and calcium oxalate stones. We, therefore, tested the role of Sat1 in mouse duodenum, a tissue with Sat1 expression and SLC26A6-dependent oxalate secretion. Although the active secretory flux of oxalate across mouse duodenum was strongly inhibited (>90%) by addition of the disulfonic stilbene DIDS to the basolateral solution, secretion was unaffected by changes in medium concentrations of sulfate and bicarbonate, key substrates for Sat1-mediated anion exchange. Inhibition of intracellular bicarbonate production by acetazolamide and complete removal of bicarbonate from the buffer also produced no change in oxalate secretion. Finally, active oxalate secretion was not reduced in Sat1-null mice. We conclude that a DIDS-sensitive basolateral transporter is involved in mediating oxalate secretion across mouse duodenum, but Sat1 itself is dispensable for this process.
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http://dx.doi.org/10.1152/ajpcell.00385.2011DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3404526PMC
July 2012

Net intestinal transport of oxalate reflects passive absorption and SLC26A6-mediated secretion.

J Am Soc Nephrol 2011 Dec 21;22(12):2247-55. Epub 2011 Oct 21.

Section of Nephrology., Department of Internal Medicine, Yale University School of Medicine, P.O. Box 208029, One Gilbert Street, TAC S-255, New Haven, Connecticut 06520-8029, USA.

Mice lacking the oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium-oxalate stones as a result of a defect in intestinal oxalate secretion, but what accounts for the absorptive oxalate flux remains unknown. We measured transepithelial absorption of [(14)C]oxalate simultaneously with the flux of [(3)H]mannitol, a marker of the paracellular pathway, across intestine from wild-type and Slc26a6-null mice. We used the anion transport inhibitor DIDS to investigate other members of the SLC26 family that may mediate transcellular oxalate absorption. Absorptive flux of oxalate in duodenum was similar to mannitol, insensitive to DIDS, and nonsaturable, indicating that it is predominantly passive and paracellular. In contrast, in wild-type mice, secretory flux of oxalate in duodenum exceeded that of mannitol, was sensitive to DIDS, and saturable, indicating transcellular secretion of oxalate. In Slc26a6-null mice, secretory flux of oxalate was similar to mannitol, and no net flux of oxalate occurred. Absorptive fluxes of both oxalate and mannitol varied in parallel in different segments of small and large intestine. In epithelial cell lines, modulation of the charge selectivity of the claudin-based pore pathway did not affect oxalate permeability, but knockdown of the tight-junction protein ZO-1 enhanced permeability to oxalate and mannitol in parallel. Moreover, formation of soluble complexes with cations did not affect oxalate absorption. In conclusion, absorptive oxalate flux occurs through the paracellular "leak" pathway, and net absorption of dietary oxalate depends on the relative balance between absorption and SLC26A6-dependent transcellular secretion.
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http://dx.doi.org/10.1681/ASN.2011040433DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3250206PMC
December 2011

Deletion of the mammalian INDY homolog mimics aspects of dietary restriction and protects against adiposity and insulin resistance in mice.

Cell Metab 2011 Aug;14(2):184-95

Howard Hughes Medical Institute, Yale School of Medicine, New Haven, CT 06520, USA.

Reduced expression of the Indy (I'm Not Dead, Yet) gene in D. melanogaster and its homolog in C. elegans prolongs life span and in D. melanogaster augments mitochondrial biogenesis in a manner akin to caloric restriction. However, the cellular mechanism by which Indy does this is unknown. Here, we report on the knockout mouse model of the mammalian Indy (mIndy) homolog, SLC13A5. Deletion of mIndy in mice (mINDY(-/-) mice) reduces hepatocellular ATP/ADP ratio, activates hepatic AMPK, induces PGC-1α, inhibits ACC-2, and reduces SREBP-1c levels. This signaling network promotes hepatic mitochondrial biogenesis, lipid oxidation, and energy expenditure and attenuates hepatic de novo lipogenesis. Together, these traits protect mINDY(-/-) mice from the adiposity and insulin resistance that evolve with high-fat feeding and aging. Our studies demonstrate a profound effect of mIndy on mammalian energy metabolism and suggest that mINDY might be a therapeutic target for the treatment of obesity and type 2 diabetes.
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http://dx.doi.org/10.1016/j.cmet.2011.06.009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3163140PMC
August 2011

Drosophila: a fruitful model for calcium oxalate nephrolithiasis?

Kidney Int 2011 Aug;80(4):327-9

Section of Nephrology, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520-8029, USA.

Even though the prevalence of nephrolithiasis is increasing, our understanding of the pathophysiology has not kept pace and new therapeutic approaches have not emerged. The potential of a new physiological model (the fruitfly) is exciting. The model has strengths, namely the low cost of maintaining colonies and rapid deployment of new transgenic lines, but also weaknesses that may ultimately limit its usefulness, such as the mechanism of tubular fluid formation and difficulties in following plasma and urine biochemistries.
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http://dx.doi.org/10.1038/ki.2011.166DOI Listing
August 2011

ESRD as a window into America's cost crisis in health care.

J Am Soc Nephrol 2009 Oct 3;20(10):2093-7. Epub 2009 Sep 3.

Section of Nephrology, Department of Internal Medicine, Yale University School of Medicine, TAC S-255, New Haven, CT 06520-8029, USA.

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http://dx.doi.org/10.1681/ASN.2009070715DOI Listing
October 2009

A case of extreme hemodynamic lability and hypocalcemia.

Am J Med Sci 2009 Sep;338(3):241-4

Section of Nephrology, Yale University School of Medicine, New Haven, Connecticut 06520-8029, USA.

An epinephrine-secreting pheochromocytoma can present with a confusing picture of hemodynamic lability with rapid fluctuations between hypo- and hypertension. They are challenging to manage and diagnose. We herein present the case of a 44-year-old patient who presented with extreme hemodynamic lability due to an epinephrine-secreting tumor. We discuss the initial management, diagnosis, and definitive therapy of this relatively rare type of pheochromocytoma.
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http://dx.doi.org/10.1097/MAJ.0b013e3181a66af1DOI Listing
September 2009