Rationale & Objective:Quantifying the change in proteinuria after acute kidney injury (AKI) may shed light on the pathway through which AKI contributes to kidney disease progression. Study Design:Prospective cohort study of patients with chronic kidney disease (CKD). Setting & Participants:3,197 participants who were enrolled in the multicenter, prospective Chronic Renal Insufficiency Cohort between January 7, 2013, and January 12, 2021. Exposure:AKI was defined as ≥1.5 peak to nadir inpatient serum creatinine ascertained during a hospital admission. Outcomes:Natural log-transformed urinary protein-creatinine ratio (UPCR) ascertained at annual Chronic Renal Insufficiency Cohort research study visits. Analytical Approach:Mixed-effects regression. Results:Among 3,197 participants, the mean age was 65 years, 44% were women, and 42% self-identified as non-Hispanic Black. The median baseline estimated glomerular filtration rate was 52 mL/min/1.73 m2 and UPCR was 0.14 g/g. 560 patients experienced 861 episodes of AKI over a median period of 6 years, with most being stage 1 (68%). In the multivariable model that adjusted for blood pressure and renin-angiotensin system inhibitor use, each AKI episode was associated with a 3% increase in UPCR (relative change ratio, 1.03; 95% CI, 1.01-1.05; P < 0.01). Limitations:Hospitalized AKI with baseline CKD only, predominance of stage 1 AKI, timing of proteinuria collection was limited to annual visits, residual confounding. Conclusions:Among CKD patients, AKI was independently associated with worsening proteinuria (independent of changes in blood pressure or renin-angiotensin system inhibitor use) and may reflect residual structural damage to the kidneys. However, the increase in proteinuria among patients with CKD after AKI was small.

Introduction:Individuals with chronic kidney disease (CKD) experience substantial impairments in physical function that are linked to disability and mortality, yet the trajectory of functional decline across stages of CKD remains poorly characterized. We examined baseline levels and longitudinal changes in objective physical function in a large prospective CKD cohort. Methods:At 2-to-3-year intervals, the Chronic Renal Insufficiency Cohort (CRIC) Study measured grip strength (kg) and the Short Physical Performance Battery (SPPB; range: 0-12), including gait speed (m/s), 5-repetition chair stand time (s), and balance. Linear mixed-effects models were used to estimate annual changes in physical function adjusted for age, sex, estimated glomerular filtration rate (eGFR), body mass index, diabetes, cardiovascular disease, education, hemoglobin, physical activity, and depressive symptoms. Baseline physical function was compared with age-matched population reference values and categorized using published impairment thresholds. Results:Among 3955 participants (mean age: 62 ± 10 years, mean eGFR: 47 ± 20 ml/min per 1.73 m2, 43% female), baseline impairment was common: 33% gait speed, 70% chair stands, 43% grip strength, and 50% SPPB. Compared with normative values, physical function was up to 75% worse depending on age and test. Over a median follow-up of 8.5 years, older age and lower eGFR were associated with steeper declines across measures. After full adjustment, each 15 ml/min per 1.73 m2 lower eGFR was associated with declines equivalent to approximately 3 to 5 additional years of aging. Conclusion:Physical function impairment is common and progressive in moderate-to-severe CKD, with kidney dysfunction contributing substantially to functional decline beyond chronological aging. Although per-unit changes are modest, their cumulative impact over time may be clinical meaningful. These findings support the evaluation of objective physical function measures in CKD and the development and testing of interventions to preserve function and independence.

Abnormal phosphate homeostasis is associated with inflammation, vascular calcification, and endothelial dysfunction in chronic kidney disease (CKD). Apparent treatment-resistant hypertension (ATRH) is common in CKD and is associated with increased cardiovascular mortality. We examined the associations between phosphate homeostasis-related biomarkers and ATRH in patients with CKD. The Chronic Renal Insufficiency Cohort (CRIC) Study enrolled 3939 participants with CKD in the United States between 2003 and 2008. After excluding those with missing ATRH data, 3752 were analyzed. ATRH was defined as blood pressure (BP) ≥ 140/90 mm Hg while taking ≥3 antihypertensive medications, or BP < 140/90 mm Hg while taking ≥4 medications. In addition to phosphate homeostasis-related biomarkers, we calculated the phosphate burden index as the combined effect of phosphate retention on circulation and bone metabolism: index = serum phosphate × parathyroid hormone. Elastic Net and multivariable regression models assessed associations between phosphate biomarkers and ATRH. Participants with ATRH were older and more often male and Black. The adjusted odds ratios (95% confidence intervals) for the highest quartiles compared with the lowest quartile were 1.42 (1.09-1.86) for serum phosphate, 1.48 (1.15-1.91) for the urine phosphate-to-creatinine ratio, and 2.30 (1.72-3.08) for phosphate burden index. These associations were also significant in linear regression models for the urine phosphate-to-creatinine ratio and the phosphate burden index. We conclude that higher levels of phosphate-related biomarkers are independently associated with ATRH. Future studies should investigate whether targeting abnormal phosphate homeostasis can reduce ATRH risk in patients with CKD.

Chronic kidney disease (CKD) imposes a substantial health burden globally, with emerging evidence pointing to the significance of metabolic acidosis and low urinary NH4+ excretion resulting in poor CKD outcomes. The present study aims to identify in CKD patients, loss of function mutations in RhBG, one of the NH3/NH4+ transporters in the collecting duct, and to show that NH3/NH4+ transport is impaired by these mutations. Single nucleotide polymorphisms of RhBG associated with CKD occurrence were identified using ancestry-stratified data from the Chronic Renal Insufficiency Cohort (CRIC) study. Functional analysis of NH3/NH4+ transport was conducted in Xenopus oocytes expressing RhBG protein or mutants. NH3 and NH4+ transport was evaluated by electrophysiological measurements, including whole cell current, surface pH and intracellular pH. Our study identified six critical RhBG mutations associated with CKD. G86S and G86C inhibited the transport of NH3; mutations G148R and G148W completely blocked transport of NH3 and NH4+, whereas T250A and T250S only inhibited NH3 transport. Mutation T250M completely inhibited transport of both NH3 and NH4+. Our study identified critical rare non-synonymous single nucleotide polymorphisms in RhBG associated with CKD and elucidated the impact of these variants on NH3/NH4+ transport. These data are crucial to our understanding of how mutations can disrupt NH3/NH4+ transport, potentially affecting kidney function in CKD patients susceptible to acidosis.Key points Acidosis and low urinary ammonium excretion contribute to poor outcomes in chronic kidney disease (CKD). This study investigates how the function of an ammonia transporter in renal collecting duct (RhBG) may contribute to CKD. Here, we report six rare RhBG mutations associated with CKD, identified using data from the Chronic Renal Insufficiency Cohort (CRIC) study. Using electrophysiological measurements, functional analysis in Xenopus oocytes showed that these RhBG mutations disrupt ammonia transport, with some mutations affecting only NH3 transport, whereas others affect both NH3 and NH4+ transport. The results suggest that impaired ammonia transport by RhBG contributes to CKD, highlighting the need to understand mechanisms that link function (NH3/NH4+ and acid-base regulation) and genetic predisposition to CKD.

Rationale & Objective:Patients with chronic kidney disease (CKD) have an increased risk of peripheral artery disease (PAD), yet no PAD risk-prediction models currently exist for this population. We developed and internally validated 5-year PAD risk-prediction models for individuals with CKD. Study Design:Prospective cohort study. Setting & Participants:3,076 patients with CKD without PAD from the Chronic Renal Insufficiency Cohort (CRIC) study. Exposure:Clinically available variables, ankle-brachial index (ABI), and non-routinely measured cardiovascular disease biomarkers assessed at baseline. Outcomes:New-onset adjudicated clinical PAD event or an ABI ≤0.9 at an annual follow-up visit. Analytical Approach:Cox proportional hazards models were applied to estimate 5-year risk of incident PAD from baseline. Model performance was assessed by discrimination, calibration, and net reclassification improvement. All models were internally validated using Monte Carlo cross-validation. Results:Participants had a mean age of 57 years; 55% were male and 40% were Black. Over 5 years, 512 developed PAD. Compared with an ABI-only model that included ABI, age, and sex (area under the receiver operating characteristic curve [AUC], 0.697; 95% CI, 0.688-0.713), a clinical model using routine variables showed similar discrimination (AUC, 0.682; 95% CI, 0.669-0.691; P = 0.09). Adding ABI to the clinical model improved discrimination (AUC, 0.721; 95% CI, 0.709-0.736; P <0.001). The biomarker-enhanced model achieved an AUC of 0.724 (95% CI, 0.711-0.745), which was not significantly different from the clinical model with ABI. Both the clinical model with ABI and the biomarker-enhanced model were well calibrated and improved reclassification of non-events compared with the ABI model (19.7%; 95% CI, 17.5-22.0% and 22.2%; 95% CI, 19.8-24.5%, respectively). Limitations:Lack of external validation. Conclusions:A PAD risk prediction model that combines clinical variables with ABI improves the identification of patients with CKD at high risk for PAD better than ABI alone.

Key PointsQuantifying albuminuria 3 months after AKI predicts future risk of kidney disease progression but is often not completed.In a cohort of 554 patients with CKD, urine protein-to-creatinine ratio measured within 1 year after AKI had robust prognostic value.Testing of total proteinuria beyond 3 months after AKI among patients with CKD identified those at risk of kidney disease progression.BackgroundExpert consensus recommends quantifying albuminuria 3 months after hospitalized AKI to identify patients at risk for kidney disease progression, but testing is often not completed in clinical practice. We evaluated the utility of testing total proteinuria (which is less expensive than albuminuria) and on a less rigid time schedule after AKI to facilitate its adoption into clinical practice.MethodsProspective multicenter cohort study of patients with CKD who were hospitalized with AKI between 2013 and 2021. Urine protein-to-creatinine ratio (PCR) was assessed per research protocol within 1 year of hospital discharge. Cox proportional hazard models were applied with a focus on C-statistics as the primary metric regarding post-AKI PCR's ability to discriminate risk of kidney disease progression as defined by halving of eGFR or ESKD.ResultsFive hundred and fifty-four Chronic Renal Insufficiency Cohort participants (43% female, 51% non-Hispanic Black, mean eGFR 43 ml/min per 1.73 m2) had PCR measurements within a year of AKI hospital discharge (82% stage 1 AKI). The median PCR, obtained 147 median (interquartile range, 79-233) days after AKI, was 0.36 mg/g (interquartile range, 0.12-1.37 mg/g). Over the mean follow-up of 2.6 years, 124 participants had kidney disease progression. Higher post-AKI PCR was associated with increased risk of kidney disease progression (hazard ratio, 2.99 for each doubling of proteinuria; 95% confidence interval, 2.54 to 3.52; C-statistic, 0.80). An adjusted model including demographic and clinical risk factors had high discrimination for kidney disease progression (C-statistic, 0.86). Renin-angiotensin system inhibitors use decreased after AKI, especially among patients with higher AKI stage and lower eGFR.ConclusionsProteinuria evaluation by PCR within 1 year after AKI hospital discharge may be a feasible and flexible option for risk-stratifying survivors at higher risk of subsequent kidney function loss. We also identified opportunities to optimize management of proteinuria after AKI.

KEY POINTS:Quantifying albuminuria 3 months after AKI predicts future risk of kidney disease progression but is often not completed. In a cohort of 554 patients with CKD, urine protein-to-creatinine ratio measured within 1 year after AKI had robust prognostic value. Testing of total proteinuria beyond 3 months after AKI among patients with CKD identified those at risk of kidney disease progression. BACKGROUND:Expert consensus recommends quantifying albuminuria 3 months after hospitalized AKI to identify patients at risk for kidney disease progression, but testing is often not completed in clinical practice. We evaluated the utility of testing total proteinuria (which is less expensive than albuminuria) and on a less rigid time schedule after AKI to facilitate its adoption into clinical practice. METHODS:Prospective multicenter cohort study of patients with CKD who were hospitalized with AKI between 2013 and 2021. Urine protein-to-creatinine ratio (PCR) was assessed per research protocol within 1 year of hospital discharge. Cox proportional hazard models were applied with a focus on C-statistics as the primary metric regarding post-AKI PCR's ability to discriminate risk of kidney disease progression as defined by halving of eGFR or ESKD. RESULTS:Five hundred and fifty-four Chronic Renal Insufficiency Cohort participants (43% female, 51% non-Hispanic Black, mean eGFR 43 ml/min per 1.73 m2) had PCR measurements within a year of AKI hospital discharge (82% stage 1 AKI). The median PCR, obtained 147 median (interquartile range, 79-233) days after AKI, was 0.36 mg/g (interquartile range, 0.12-1.37 mg/g). Over the mean follow-up of 2.6 years, 124 participants had kidney disease progression. Higher post-AKI PCR was associated with increased risk of kidney disease progression (hazard ratio, 2.99 for each doubling of proteinuria; 95% confidence interval, 2.54 to 3.52; C-statistic, 0.80). An adjusted model including demographic and clinical risk factors had high discrimination for kidney disease progression (C-statistic, 0.86). Renin-angiotensin system inhibitors use decreased after AKI, especially among patients with higher AKI stage and lower eGFR. CONCLUSIONS:Proteinuria evaluation by PCR within 1 year after AKI hospital discharge may be a feasible and flexible option for risk-stratifying survivors at higher risk of subsequent kidney function loss. We also identified opportunities to optimize management of proteinuria after AKI.

Renal Rhesus type B glycoprotein (Rhbg) is a glycosylated mammalian NH3/NH4+ transporter expressed in α-intercalated cells of the collecting duct. Carbonic anhydrase-IV (CA-IV) is also expressed in the mammalian kidney, where it catalyzes the reversible hydration of CO2. This study aims to demonstrate: 1) whether Rhbg and CA-IV proteins physically interact; and 2) if this interaction functionally affects transport of NH3/NH4+ and possibly CO2. We measured transport of NH4+, NH3, and CO2 in four groups of Xenopus oocytes. In the first group, we coexpressed Rhbg with CA-IV and compared the measurements to three groups of oocytes expressing either Rhbg or CA-IV or injected with H2O. We used ion-selective microelectrodes to measure surface pH, to monitor NH3 transport, and intracellular pH to monitor NH4+ and CO2 transport. We also used a two-electrode voltage clamp to measure current changes caused by electrogenic NH4+ transport. These parameters measured NH3/NH4+ and CO2 transport in oocytes expressing Rhbg and/or CA. Our results indicate that: 1) Rhbg and CA-IV were coimmunoprecipitated, suggesting a physical interaction; and 2) coexpressing CA-IV with Rhbg: i) inhibited electrogenic NH4+ transport by Rhbg in the presence and absence of CO2; ii) reduced NH3 transport by Rhbg only in the presence of CO2; and iii) had no detectable effect on CO2 transport by Rhbg. We demonstrated for the first time that Rhbg and CA-IV physically interact, and this interaction has inhibitory effects on Rhbg function but not CA-IV. The interaction of Rhbg and CA-IV is important to explain their role in renal acid-base homeostasis.NEW & NOTEWORTHY Our study revealed the complex regulation of NH3/NH4+ transport, highlighting the roles of Rhbg, CA-IV, and environmental factors such as CO2 concentration. These interactions are critical to our understanding of NH3/NH4+ transport and regulation. Our findings lay a strong foundation for future investigations into the molecular dynamics among these transport proteins and their physiological significance. These studies are essential to fully understand how these mechanisms influence renal ammonia handling, urinary acidification, and systemic pH balance.

Metabolic acidosis is a major complication of CKD (chronic kidney disease) and DKD (diabetic kidney disease). The kidney responds to metabolic acidosis by eliminating acid, primarily by producing and excreting NH4+ in the urine. Two Rh glycoproteins, RhBG and RhCG, expressed in the collecting duct are directly involved in NH3/NH4+transport. We previously determined that RhBG transports both NH4+ and NH3 across the basolateral membrane of intercalated cells. Impaired NH3/NH4+ production and/or transport may be a factor in increasing acid load and worsening kidney disease. In type 2 diabetes, net endogenous acid production is significantly elevated, and the proportion of acid excreted as NH4+ is decreased even after accounting for diet and renal function. This is significant given that the association between acidosis and faster decline in GFR is now well established. In DKD, impaired NH4+ excretion as a potential risk factor for progression of kidney disease remains poorly understood. Specifically, the genetic risk factors contributing to DKD are unclear. Hypothesis: Defined RhBG variant(s) linked to DKD cause impaired NH3/NH4+ transport that may contribute to DKD development and progression. Methods: We conducted whole-exome sequencing (WES) analysis using data from A therosclerosis R isk i n C ommunities (ARIC) study and the C hronic R enal I nsuffciency C ohort (CRIC) study. We selected DKD patients with the most rapid GFR decline and compared them with: 1) subjects without kidney disease (healthy control); 2) diabetic patients without CKD (DM control); 3) CKD patients without diabetes (CKD control). This approach will identify single nucleotide variants (SNVs) of RhBG associated with DKD. This will also determine if the detected variants contribute to the development of kidney disease per se and whether they contribute to the development of diabetes and/or fast GFR decline. To study function, we expressed RhBG variants (and wild-type RhBG) in Xenopus oocytes by microinjecting oocytes with the respective mRNA. The effect of defined SNV of RhBG on NH3/NH4+ transport was assessed from measurements of changes in intracellular pH (pHi), membrane potential (Vm), whole cell currents (I) or surface pH (pHs) induced by exposing the oocytes to bath solutions containing NH4Cl or methyl ammonium chloride (MA/MA+). Results: We identified an SNV, rs3748569 (minor allele frequency MAF > 0.05), that was significantly associated with DKD. Significance was determined compared to DM control (OR=1.38, p=0.0026) and healthy control (OR=1.26, p=0.01). This variant has also been predicted to be deleterious by 4 bioinformatic tools. Our electrophysiological measurements showed that the resultant amino acid switch, G315R, completely inhibited transport of NH3 and NH4+ (p<0.001) and the transport of MA and MA+ (p<0.0001), compared with RhBG-WT. Conclusions: WES results suggest that this naturally occurring RhBG mutation, G315R, contributes to DKD occurrence. This effect is through the development of kidney disease and less likely due to development of diabetes. It is therefore likely a general CKD risk factor. Inhibition of NH3/NH4+ transport by this mutation is a contributing factor to its deleterious effect. This work provides experimental validation of computational predictions concerning CKD and diabetes. NIH-U54 GM104940, Paul Teschan Research, NIH-R01, Carol Lavin Bernick, Tulane institutional. 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.