Impairment in renal medulla development underlies salt wasting in Clc-k2 channel deficiency

Postnatal renal tubule development: roles of tubular flow and flux

PURPOSE OF REVIEW

Postnatal renal tubule development is critical to adult kidney function. Several postnatal changes regulate the differentiation and proliferation of renal tubular cells. Here, we review the literature and our efforts on thick ascending limb (TAL) development in Bartter syndrome (BS).

RECENT FINDINGS

Glomerular filtrate quickly increases after birth, imposing fluid shear stress and circumferential stretch on immature renal tubules. Recent studies showed that kidney organoids under flow (superfusion) have better development of tubular structures and the expression of cilia and solute transporters. These effects are likely mediated by mechanosensors, such as cilia and the piezo1 channel. Improved renal oxygenation and sodium pump-dependent active transport can stimulate mitochondrial respiration and biogenesis. The functional coupling between transport and mitochondria ensures ATP supply for energy-demanding reactions in tubular cells, including cell cycle progression and proliferation. We recently discovered that postnatal renal medulla maturation and TAL elongation are impaired in Clc-k2-deficient BS mice. Primary cultured Clc-k2-deficient TAL cells have G1-S transition and proliferation delay. These developmental defects could be part of the early pathogenesis of BS and worsen the phenotype.

SUMMARY

Understanding how tubular flow and transepithelial ion fluxes regulate renal tubule development may improve the treatment of congenital renal tubulopathies.

Transport activity regulates mitochondrial bioenergetics and biogenesis in renal tubules

Renal tubules are featured with copious mitochondria and robust transport activity. Mutations in mitochondrial genes cause congenital renal tubulopathies, and changes in transport activity affect mitochondrial morphology, suggesting mitochondrial function and transport activity are tightly coupled. Current methods of using bulk kidney tissues or cultured cells to study mitochondrial bioenergetics are limited. Here, we optimized an extracellular flux analysis (EFA) to study mitochondrial respiration and energy metabolism using microdissected mouse renal tubule segments. EFA detects mitochondrial respiration and glycolysis by measuring oxygen consumption and extracellular acidification rates, respectively. We show that both measurements positively correlate with sample sizes of a few centimeter-length renal tubules. The thick ascending limbs (TALs) and distal convoluted tubules (DCTs) critically utilize glucose/pyruvate as energy substrates, whereas proximal tubules (PTs) are significantly much less so. Acute inhibition of TALs' transport activity by ouabain treatment reduces basal and ATP-linked mitochondrial respiration. Chronic inhibition of transport activity by 2-week furosemide treatment or deletion of with-no-lysine kinase 4 (Wnk4) decreases maximal mitochondrial capacity. In addition, chronic inhibition downregulates mitochondrial DNA mass and mitochondrial length/density in TALs and DCTs. Conversely, gain-of-function Wnk4 mutation increases maximal mitochondrial capacity and mitochondrial length/density without increasing mitochondrial DNA mass. In conclusion, EFA is a sensitive and reliable method to investigate mitochondrial functions in isolated renal tubules. Transport activity tightly regulates mitochondrial bioenergetics and biogenesis to meet the energy demand in renal tubules. The system allows future investigation into whether and how mitochondria contribute to tubular remodeling adapted to changes in transport activity.

Transport activity regulates mitochondrial bioenergetics and biogenesis in renal tubules

Renal tubules are featured with copious mitochondria and robust transport activity. Mutations in mitochondrial genes cause congenital renal tubulopathies, and changes in transport activity affect mitochondrial morphology, suggesting mitochondrial function and transport activity are tightly coupled. Current methods of using bulk kidney tissues or cultured cells to study mitochondrial bioenergetics are limited. Here, we optimized an extracellular flux analysis (EFA) to study mitochondrial respiration and energy metabolism using microdissected mouse renal tubule segments. EFA detects mitochondrial respiration and glycolysis by measuring oxygen consumption and extracellular acidification rates, respectively. We show that both measurements positively correlate with sample sizes of a few centimeter-length renal tubules. The thick ascending limbs (TALs) and distal convoluted tubules (DCTs) predominantly utilize glucose/pyruvate as energy substrates, whereas proximal tubules (PTs) are significantly much less so. Acute inhibition of TALs' transport activity by ouabain treatment reduces basal and ATP-linked mitochondrial respiration. Chronic inhibition of transport activity by 2-week furosemide treatment or deletion of with-no-lysine kinase 4 (Wnk4) decreases maximal mitochondrial capacity. In addition, chronic inhibition downregulates mitochondrial DNA mass and mitochondrial length/density in TALs and DCTs. Conversely, gain-of-function Wnk4 mutation increases maximal mitochondrial capacity and mitochondrial length/density without increasing mitochondrial DNA mass. In conclusion, EFA is a sensitive and reliable method to investigate mitochondrial functions in isolated renal tubules. Transport activity tightly regulates mitochondrial bioenergetics and biogenesis to meet the energy demand in renal tubules. The system allows future investigation into whether and how mitochondria contribute to tubular remodeling adapted to changes in transport activity.

Key points

A positive correlation between salt reabsorption and oxygen consumption in mammalian kidneys hints at a potential interaction between transport activity and mitochondrial respiration in renal tubules.Renal tubules are heterogeneous in transport activity and mitochondrial metabolism, and traditional assays using bulk kidney tissues cannot provide segment-specific information.Here, we applied an extracellular flux analysis to investigate mitochondrial respiration and energy metabolism in isolated renal tubules. This assay is sensitive in detecting oxygen consumption and acid production in centimeter-length renal tubules and reliably recapitulates segment-specific metabolic features.Acute inhibition of transport activity reduces basal and ATP-linked mitochondrial respirations without changing maximal mitochondrial respiratory capacity. Chronic alterations of transport activity further adjust maximal mitochondrial respiratory capacity via regulating mitochondrial biogenesis or non-transcriptional mechanisms.Our findings support the concept that renal tubular cells finely adjust mitochondrial bioenergetics and biogenesis to match the new steady state of transport activity.

ClC-K Kidney Chloride Channels: From Structure to Pathology

The molecular basis of chloride transport varies all along the nephron depending on the tubular segments especially in the apical entry of the cell. The major chloride exit pathway during reabsorption is provided by two kidney-specific ClC chloride channels ClC-Ka and ClC-Kb (encoded by CLCNKA and CLCNKB gene, respectively) corresponding to rodent ClC-K1 and ClC-K2 (encoded by Clcnk1 and Clcnk2). These channels function as dimers and their trafficking to the plasma membrane requires the ancillary protein Barttin (encoded by BSND gene). Genetic inactivating variants of the aforementioned genes lead to renal salt-losing nephropathies with or without deafness highlighting the crucial role of ClC-Ka, ClC-Kb, and Barttin in the renal and inner ear chloride handling. The purpose of this chapter is to summarize the latest knowledge on renal chloride structure peculiarity and to provide some insight on the functional expression on the segments of the nephrons and on the related pathological effects.

Intravital Imaging with Two-Photon Microscopy: A Look into the Kidney

Fluorescence microscopy has represented a crucial technique to explore the cellular and molecular mechanisms in the field of biomedicine. However, the conventional one-photon microscopy exhibits many limitations when living samples are imaged. The new technologies, including two-photon microscopy (2PM), have considerably improved the in vivo study of pathophysiological processes, allowing the investigators to overcome the limits displayed by previous techniques. 2PM enables the real-time intravital imaging of the biological functions in different organs at cellular and subcellular resolution thanks to its improved laser penetration and less phototoxicity. The development of more sensitive detectors and long-wavelength fluorescent dyes as well as the implementation of semi-automatic software for data analysis allowed to gain insights in essential physiological functions, expanding the frontiers of cellular and molecular imaging. The future applications of 2PM are promising to push the intravital microscopy beyond the existing limits. In this review, we provide an overview of the current state-of-the-art methods of intravital microscopy, focusing on the most recent applications of 2PM in kidney physiology.