Kidney tubule health, mineral metabolism, and adverse events in persons with CKD in SPRINT

Background Measures of kidney tubule health are risk markers for acute kidney injury (AKI) in persons with chronic kidney disease (CKD) during hypertension treatment, but their associations with other adverse events (AEs) are unknown. Methods Among 2,377 Systolic Blood Pressure Intervention Trial (SPRINT) participants with CKD, we measured at baseline eight urine biomarkers of kidney tubule health and two serum biomarkers of mineral metabolism pathways that act on the kidney tubules. Cox proportional hazards models were used to evaluate biomarker associations with risk of a composite of pre-specified serious AEs (hypotension, syncope, electrolyte abnormalities, AKI, bradycardia, and injurious falls) and outpatient AEs (hyperkalemia and hypokalemia). Results At baseline, the mean age was 73 ±9 years and mean eGFR was 46 ±11 ml/min/1.73m2. During a median follow-up of 3.8 years, 716 (30%) participants experienced the composite AE. Higher urine interleukin-18, kidney injury molecule-1, neutrophil gelatinase-associated lipocalin (NGAL), and monocyte chemoattractant protein-1 (MCP-1), lower urine uromodulin (UMOD), and higher serum fibroblast growth factor-23 were individually associated with higher risk of the composite AE outcome in multivariable-adjusted models including eGFR and albuminuria. When modeling biomarkers in combination, higher NGAL (HR: 1.08 per 2-fold higher biomarker level, 95% CI: 1.03, 1.13), higher MCP-1 (HR: 1.11, 95% CI: 1.03, 1.19), and lower UMOD (HR: 0.91, 95% CI: 0.85, 0.97) were each associated with higher composite AE risk. Biomarker associations did not vary by intervention arm (P >0.10 for all interactions). Conclusions Among persons with CKD, several kidney tubule biomarkers are associated with higher risk of AEs during hypertension treatment, independent of eGFR and albuminuria.

Embryonic and aglomerular kidney development in the bay pipefish, Syngnathus leptorhynchus

In this report we describe the embryogenesis of the bay pipefish, Syngnathus leptorhynchus, and the organogenesis of its aglomerular kidney. Early development was analyzed via a series of montages and images documenting embryos collected from the brood pouches of pregnant males. Despite differences in terminal morphology between pipefish and common teleost models such as medaka and zebrafish, the embryogenesis of these highly advanced fishes is very similar to that of other species. One of the unique features of these fishes is their utilization of an aglomerular kidney. Histological analysis revealed a single long, unbranched kidney tubule in late embryos. The development and structure of this organ was further investigated by cloning the sodium potassium ATPase alpha subunit, atp1a, from S. leptorhynchus and developing whole mount fluorescent in situ hybridization protocols for embryos of this species. Fluorescent stereoscopic and confocal visualization techniques were then used to characterize the 3D morphology of aglomerular kidneys in intact embryos. In all embryonic stages characterized, the aglomerular kidney is a single unbranched tube extending from posterior to the head to the cloaca.

Deletion of renal Nedd4-2 abolishes the effect of high K+ intake (HK) on Kir4.1/Kir5.1 and NCC activity in the distal convoluted tubule.

High-dietary K+ (HK) intake inhibits the basolateral Kir4.1/Kir5.1 activity in the distal convoluted tubule (DCT) and HK-induced inhibition of Kir4.1/Kir5.1 is essential for HK-induced inhibition of Na-Cl cotransporter (NCC). We now examine whether Nedd4-2-deletion compromises the effect of HK on basolateral Kir4.1/Kir5.1 and NCC in the DCT. Single- channel-recording and whole-cell-recording showed that neither HK decreased nor low-dietary-K+ (LK) increased the basolateral Kir4.1/Kir5.1 activity of the DCT in kidney-tubule-specific Nedd4-2 knockout (Ks-Nedd4-2 KO) mice. In contrast, HK inhibited and LK increased Kir4.1/Kir5.1 activity in the control mice (Nedd4lflox/flox). Also, HK-intake decreased the negativity of K+-current (IK) reversal potential in the DCT (depolarization) only in the control mice but not in Ks-Nedd4-2 KO mice. Renal clearance experiments showed that HK-intake decreased while LK intake increased hydrochlorothiazide (HCTZ)-induced renal Na+ excretion only in the control mice but this effect was absent in Ks-Nedd4-2 KO mice. Western blot also demonstrated that HK-induced inhibition of phosphor-NCC (pNCC at Thr53) and total NCC (tNCC) was observed only in the control but not in Ks-Nedd4-2 KO mice. Furthermore, the expression of all three subunits of epithelia-Na+-channel (ENaC) in the Ks-Nedd4-2 KO mice on HK was higher than in the control mice. Thus, plasma K+ concentrations were similar between Nedd4lflox/flox and Ks-Nedd4-2 KO mice on HK for 7 days despite high NCC expression. We conclude that Nedd4-2 plays a role in regulating HK-induced inhibition of Kir4.1/Kir5.1 and NCC in the DCT.

Mapping kidney tubule diameter ex vivo by diffusion MRI

Tubular pathologies are a common feature of kidney disease. Current metrics to assess kidney health, in vivo or in transplant, are generally based on urinary or serum biomarkers and pathologic findings from kidney biopsies. Biopsies, usually taken from kidney cortex, are invasive and prone to sampling error. Tools to directly and non-invasively measure tubular pathology could provide a new approach to assess kidney health. This study uses diffusion magnetic resonance imaging (dMRI) as a non-invasive tool to measure the size of the tubular lumen in ex vivo, perfused kidneys. We first used Monte-Carlo simulations to demonstrate that dMRI is sensitive to restricted tissue water diffusion at the scale of the kidney tubule. We applied dMRI and biophysical modeling to examine the distribution of tubular diameters in ex vivo, fixed kidneys from mice, rats, and a human donor. The biophysical model to fit the dMRI signal was based on a superposition of freely diffusing water and water diffusing inside infinitely long cylinders of different diameters. Tubular diameters measured by MRI were within 10% of those measured by histology within the same tissue. Finally, we applied dMRI to investigate kidney pathology in a mouse model of folic-acid induced acute kidney injury (AKI). dMRI detected heterogeneity in the distribution of tubules within kidney cortex of AKI mice compared to the control group. We conclude that dMRI can be used to measure the distribution of tubule diameters in the kidney cortex ex vivo and dMRI may provide a new non-invasive biomarker of tubular pathology.