Association of Proximal Tubular Secretory Clearance with Long-Term Decline in Cognitive Function

BACKGROUND

People with chronic kidney disease (CKD) are at high risk for cognitive impairment and progressive cognitive decline. Retention of protein-bound organic solutes that are normally removed by tubular secretion is hypothesized to contribute to cognitive impairment in CKD.

METHODS

We followed 2362 participants who were initially free of cognitive impairment and stroke in the prospective Chronic Renal Insufficiency Cohort (CRIC) Study. We estimated tubular secretory clearance by the 24-hour kidney clearances of eight endogenous solutes that are primarily eliminated by tubular secretion. CRIC study investigators assessed participants' cognitive function annually using the Modified Mini-Mental State (3MS) Examination. Cognitive decline was defined as a sustained decrease of more than five points in the 3MS score from baseline. Using Cox regression models adjusted for potential confounders, we analyzed associations between secretory solute clearances, serum solute concentrations, and cognitive decline.

RESULTS

The median number of follow-up 3MS examinations was six per participant. There were 247 incident cognitive decline events over a median of 9.1 years of follow-up. Lower kidney clearances of five of the eight secretory solutes (cinnamoylglycine, isovalerylglycine, kynurenic acid, pyridoxic acid, and tiglylglycine) were associated with cognitive decline after adjustment for baseline eGFR, proteinuria, and other confounding variables. Effect sizes ranged from a 17% to a 34% higher risk of cognitive decline per 50% lower clearance. In contrast, serum concentrations of the solutes were not associated with cognitive decline.

CONCLUSIONS

Lower kidney clearances of secreted solutes are associated with incident global cognitive decline in a prospective study of CKD, independent of eGFR. Further work is needed to determine the domains of cognition most affected by decreased secretory clearance and the mechanisms of these associations.

Physiology assays in human kidney organoids

Kidney organoids derived from human pluripotent stem cells constitute a novel model of disease, development, and regenerative therapy. Organoids are human, experimentally accessible, high throughput, and enable reconstitution of tissue-scale biology in a petri dish. Although gene expression patterns in organoid cells have been analyzed extensively, less is known about the functionality of these structures. Here, we review assays of physiological function in human kidney organoids, including best practices for quality control, and future applications. Tubular structures in organoids accumulate specific molecules through active transport, including dextran and organic anions, and swell with fluid in response to cAMP stimulation. When engrafted into animal models in vivo, organoids form vascularized glomerulus-like structures capable of size-selective filtration. Organoids exhibit metabolic, endocrine, injury, and infection phenotypes, although their specificity is not yet fully clear. To properly interpret organoid physiology assays, it is important to incorporate appropriate negative and positive controls, statistical methods, data presentation, molecular mechanisms, and clinical data sets. Improvements in organoid perfusion, patterning, and maturation are needed to enable branching morphogenesis, urine production, and renal replacement. Reconstituting renal physiology with kidney organoids is a new field with potential to provide fresh insights into classical phenomena.

Serum Vitamin D: Correlates of Baseline Concentration and Response to Supplementation in VITAL-DKD

CONTEXT

The effect of daily vitamin D supplementation on the serum concentration of vitamin D (the parent compound) may offer insight into vitamin D disposition.

OBJECTIVE

To assess the total serum vitamin D response to vitamin D3 supplementation and whether it varies according to participant characteristics. To compare results with corresponding results for total serum 25-hydroxyvitamin D [25(OH)D], which is used clinically and measured in supplementation trials.

DESIGN

Exploratory study within a randomized trial.

INTERVENTION

2000 International Units of vitamin D3 per day (or matching placebo).

SETTING

Community-based.

PARTICIPANTS

161 adults (mean ± SD age 70 ± 6 years; 66% males) with type 2 diabetes.

MAIN OUTCOME MEASURES

Changes in total serum vitamin D and total serum 25(OH)D concentrations from baseline to year 2.

RESULTS

At baseline, there was a positive, nonlinear relation between total serum vitamin D and total serum 25(OH)D concentrations. Adjusted effects of supplementation were a 29.2 (95% CI: 24.3, 34.1) nmol/L increase in serum vitamin D and a 33.4 (95% CI: 27.7, 39.2) nmol/L increase in serum 25(OH)D. Among those with baseline 25(OH)D < 50 compared with ≥ 50 nmol/L, the serum vitamin D response to supplementation was attenuated (15.7 vs 31.2 nmol/L; interaction P-value = 0.02), whereas the serum 25(OH)D response was augmented (47.9 vs 30.7 nmol/L; interaction P-value = 0.05).

CONCLUSIONS

Vitamin D3 supplementation increases total serum vitamin D and 25(OH)D concentrations with variation according to baseline 25(OH)D, which suggests that 25-hydroxylation of vitamin D3 is more efficient when serum 25(OH)D concentration is low.

Prediction of Incident Atrial Fibrillation in Chronic Kidney Disease: The Chronic Renal Insufficiency Cohort Study

BACKGROUND AND OBJECTIVES

Atrial fibrillation (AF) is common in CKD and associated with poor kidney and cardiovascular outcomes. Prediction models developed using novel methods may be useful to identify patients with CKD at highest risk of incident AF. We compared a previously published prediction model with models developed using machine learning methods in a CKD population.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We studied 2766 participants in the Chronic Renal Insufficiency Cohort study without prior AF with complete cardiac biomarker (N-terminal pro-B-type natriuretic peptide and high-sensitivity troponin T) and clinical data. We evaluated the utility of machine learning methods as well as a previously validated clinical prediction model (Cohorts for Heart and Aging Research in Genomic Epidemiology [CHARGE]-AF, which included 11 predictors, using original and re-estimated coefficients) to predict incident AF. Discriminatory ability of each model was assessed using the ten-fold cross-validated C-index; calibration was evaluated graphically and with the Grønnesby and Borgan test.

RESULTS

Mean (SD) age of participants was 57 (11) years, 55% were men, 38% were Black, and mean (SD) eGFR was 45 (15) ml/min per 1.73 m2; 259 incident AF events occurred during a median of 8 years of follow-up. The CHARGE-AF prediction equation using original and re-estimated coefficients had C-indices of 0.67 (95% confidence interval, 0.64 to 0.71) and 0.67 (95% confidence interval, 0.64 to 0.70), respectively. A likelihood-based boosting model using clinical variables only had a C-index of 0.67 (95% confidence interval, 0.64 to 0.70); adding N-terminal pro-B-type natriuretic peptide, high-sensitivity troponin T, or both biomarkers improved the C-index by 0.04, 0.01, and 0.04, respectively. In addition to N-terminal pro-B-type natriuretic peptide and high-sensitivity troponin T, the final model included age, non-Hispanic Black race/ethnicity, Hispanic race/ethnicity, cardiovascular disease, chronic obstructive pulmonary disease, myocardial infarction, peripheral vascular disease, use of angiotensin-converting enzyme inhibitor/angiotensin receptor blockers, calcium channel blockers, diuretics, height, and weight.

CONCLUSIONS

Using machine learning algorithms, a model that included 12 standard clinical variables and cardiac-specific biomarkers N-terminal pro-B-type natriuretic peptide and high-sensitivity troponin T had moderate discrimination for incident AF in a CKD population.

Histopathologic and Clinical Features in Patients with Diabetes and Kidney Disease

BACKGROUND

The discovery of nondiabetic kidney disease (NDKD) in an individual patient with diabetes may have significant treatment implications. Extensive histopathologic data in this population are lacking, but they may provide insights into the complex pathogenesis of diabetic nephropathy (DN) and reveal specific phenotypes for the development of targeted therapies. This study seeks to elucidate the clinical and laboratory parameters associated with the spectrum of kidney histopathologic features in patients with diabetes.

METHODS

This study is a retrospective analysis of 399 kidney biopsies assessed from 2014 to 2016 at the University of Washington among patients with diabetes. More comprehensive clinical data were evaluated in a subset of 79 participants.

RESULTS

Of the 399 biopsies reviewed, 192 (48%) had a primary diagnosis of DN (including 26 with an additional diagnosis), and 207 (52%) had a primary diagnosis of NDKD (including 67 who also had DN). Retinopathy (sensitivity: 0.86; specificity: 0.81; OR, 27.1; 95% CI, 6.8 to 107.7) and higher levels of proteinuria (7.6 versus 4.1 g/d; P=0.004) were associated with DN, whereas a physician description of AKI was associated with a lower risk of DN (OR, 0.13; 95% CI, 0.04 to 0.38). The four most prevalent diagnoses in participants with NDKD were FSGS in 39, nephrosclerosis in 29, IgA nephropathy in 27, and acute tubular injury in 21.

CONCLUSIONS

Among patients with diabetes who undergo kidney biopsy in the Pacific Northwest, approximately half have DN, and half have NDKD. Retinopathy and more severe proteinuria were associated with DN, and AKI was a more common descriptor in NDKD.Podcast: This article contains a podcast at https://www.asnonline.org/media/podcast/K360/2020_11_25_KID0003962020.mp3.

Tubular Secretory Clearance Is Associated With Whole-Body Insulin Clearance

CONTEXT

The kidneys eliminate insulin via glomerular and peritubular mechanisms; consequently, the kidney contribution to insulin clearance may be underestimated by the glomerular filtration rate (GFR) alone.

OBJECTIVE

To determine associations of tubular secretory clearance with whole-body insulin clearance and sensitivity in a dedicated study of glucose and insulin metabolism.

DESIGN, SETTING, AND PARTICIPANTS

We performed an ancillary, cross-sectional study of tubular secretion in the Study of Glucose and Insulin in Renal Disease (SUGAR). Hyperinsulinemic-euglycemic clamps were performed in 57 nondiabetic persons with chronic kidney disease and 38 persons without kidney disease.

INTERVENTION

We measured plasma and 24-hour urine concentrations of endogenous solutes primarily eliminated by tubular secretion. Kidney clearances of secretory solutes were calculated as the amount of blood fully cleared of that solute per minute.

MAIN OUTCOME MEASURES

Whole-body insulin clearance, insulin sensitivity.

RESULTS

Mean whole-body insulin clearance was 924 ± 228 mL/min. After adjustment for age, sex, Black race, fat and fat-free mass, each 20% lower estimated GFR was associated with a 13 mL/min lower insulin clearance (95% confidence interval [CI], 2-24 mL/min lower). Each 20% lower clearance of isovalerylglycine and xanthosine were associated with a 16 mL/min lower (95% CI, 5-26 mL/min lower) and 19 mL/min lower (95% CI, 7-31 mL/min lower) insulin clearance, respectively. Neither estimated GFR nor secretory solute clearances were associated with insulin sensitivity after adjustment.

CONCLUSIONS

These results highlight the importance of tubular secretory pathways to insulin elimination but suggest that kidney functions in aggregate contribute only modestly to systemic insulin clearance.