Developmental expression of acid-base-related proteins in the rabbit kidney

Generation of Atp6v1g3-Cre mice for investigation of intercalated cells and the collecting duct

Kidney intercalated cells (ICs) maintain acid-base homeostasis and recent studies have demonstrated that they function in the kidney's innate defense. To study kidney innate immune function, ICs have been enriched using vacuolar ATPase (V-ATPase) B1 subunit (Atp6v1b1)-Cre (B1-Cre) mice. Although Atp6v1b1 is considered kidney specific, it is expressed in multiple organ systems, both in mice and humans, raising the possibility of off-target effects when using the Cre-lox system. We have recently shown using single-cell RNA sequencing that the gene that codes for the V-ATPase G3 subunit (mouse gene: Atp6v1g3; human gene: ATP6V1G3; protein abbreviation: G3) mRNA is selectively enriched in human kidney ICs. In this study, we generated Atp6v1g3-Cre (G3-Cre) reporter mice using CRISPR/CAS technology and crossed them with Tdtomatoflox/flox mice. The resultant G3-Cre+Tdt+ progeny was evaluated for kidney specificity in multiple tissues and found to be highly specific to kidney cells with minimal or no expression in other organs evaluated compared with B1-Cre mice. Tdt+ cells were flow sorted and were enriched for IC marker genes on RT-PCR analysis. Next, we crossed these mice to ihCD59 mice to generate an IC depletion mouse model (G3-Cre+ihCD59+/+). ICs were depleted in these mice using intermedilysin, which resulted in lower blood pH, suggestive of a distal renal tubular acidosis phenotype. The G3-Cre mice were healthy, bred normally, and produce regular-sized litter. Thus, this new "IC reporter" mice can be a useful tool to study ICs.NEW & NOTEWORTHY This study details the development, validation, and experimental use of a new mouse model to study the collecting duct and intercalated cells. Kidney intercalated cells are a cell type increasingly recognized to be important in several human diseases including kidney infections, acid-base disorders, and acute kidney injury.

Developmental changes in renal tubular transport-an overview

The adult kidney maintains a constant volume and composition of extracellular fluid despite changes in water and salt intake. The neonate is born with a kidney that has a small fraction of the glomerular filtration rate of the adult and immature tubules that function at a lower capacity than that of the mature animal. Nonetheless, the neonate is also able to maintain a constant extracellular fluid volume and composition. Postnatal renal tubular development was once thought to be due to an increase in the transporter abundance to meet the developmental increase in glomerular filtration rate. However, postnatal renal development of each nephron segment is quite complex. There are isoform changes of several transporters as well as developmental changes in signal transduction that affect the capacity of renal tubules to reabsorb solutes and water. This review will discuss neonatal tubular function with an emphasis on the differences that have been found between the neonate and adult. We will also discuss some of the factors that are responsible for the maturational changes in tubular transport that occur during postnatal renal development.

Development of pH regulatory transport in glomerular mesangial cells

Developmental changes in activity or expression of transporters may account for alterations in cell behavior as the nonpolarized cell matures. We sought to ascertain whether there is a maturational change in each of the major acid-base transporters in the developing mesangial cell (MC), the Na/H exchanger, Na-dependent Cl/HCO3 exchanger, and the Cl/HCO3 exchanger. Intracellular pH (pHi) was determined by the use of the fluorescent pH-sensitive dye, 2',7'-bis(2- carboxyethyl)-5(6)-carboxyfluorescein (BCECF). We assessed transporter activity by studying recovery from an acid load (NH4/NH3) in CO2/HCO3. In adult MCs, Na/H exchanger was responsible for 35.2 +/- 4.3% of steady-state pHi, whereas the Na-dependent Cl/HCO3 exchanger contributed 58.7 +/- 6.1 (n = 14). In term MCs (tMCs), from days 1-3 after birth, the Na/H exchanger contributes 62.9 +/- 7.8% (n = 11, P < 0.001 vs. adult), whereas the Na-dependent Cl/HCO3 exchanger contributes 34.0 +/- 5.7% (n = 12, P < 0.001 vs. adult), to the rate of recovery from an acid load in these cells. However, in tMCs (days 4-6), the Na/H contributes 47.2 +/- 5.9% (n = 8, P < 0.05 vs. adult), whereas the Na-dependent Cl/HCO3 exchanger contributes 48.7 +/- 7.3% (n = 13, P < 0.05 vs. adult), to the rate of recovery. tMCs (days 6-12) yielded transporter activity that was not statistically different than adult MCs (37.8 +/- 4.9 and 54.3 +/- 10.2% for Na/H and Na-dependent Cl/HCO3, respectively). The magnitude of the stimulated response to angiotensin II by Na/H and Na-dependent Cl/HCO3 exchanger in adult and tMCs is unchanged throughout development. The Na/H exchanger appears to play a greater role in pHi homeostasis earlier on in development, and this may reflect developmental needs of the maturing cell.