Isolated perfused tubule. Introduction: background and development of microperfusion technique

Landscape of GPCR expression along the mouse nephron

Kidney transport and other renal functions are regulated by multiple G protein-coupled receptors (GPCRs) expressed along the renal tubule. The rapid, recent appearance of comprehensive unbiased gene expression data in the various renal tubule segments, chiefly RNA sequencing and protein mass spectrometry data, has provided a means of identifying patterns of GPCR expression along the renal tubule. To allow for comprehensive mapping, we first curated a comprehensive list of GPCRs in the genomes of mice, rats, and humans (https://hpcwebapps.cit.nih.gov/ESBL/Database/GPCRs/) using multiple online data sources. We used this list to mine segment-specific and cell type-specific expression data from RNA-sequencing studies in microdissected mouse tubule segments to identify GPCRs that are selectively expressed in discrete tubule segments. Comparisons of these mapped mouse GPCRs with other omics datasets as well as functional data from isolated perfused tubule and micropuncture studies confirmed patterns of expression for well-known receptors and identified poorly studied GPCRs that are likely to play roles in the regulation of renal tubule function. Thus, we provide data resources for GPCR expression across the renal tubule, highlighting both well-known GPCRs and understudied receptors to provide guidance for future studies.

Targeted Single-Cell RNA-seq Identifies Minority Cell Types of Kidney Distal Nephron

BACKGROUND

Proximal tubule cells dominate the kidney parenchyma numerically, although less abundant cell types of the distal nephron have disproportionate roles in water and electrolyte balance.

METHODS

Coupling of a FACS-based enrichment protocol with single-cell RNA-seq profiled the transcriptomes of 9099 cells from the thick ascending limb (CTAL)/distal convoluted tubule (DCT) region of the mouse nephron.

RESULTS

Unsupervised clustering revealed Slc12a3 ⁺/Pvalb ⁺ and Slc12a3 ⁺/Pvalb ^- cells, identified as DCT1 and DCT2 cells, respectively. DCT1 cells appear to be heterogeneous, with orthogonally variable expression of Slc8a1, Calb1, and Ckb. An additional DCT1 subcluster showed marked enrichment of cell cycle-/cell proliferation-associated mRNAs (e.g., Mki67, Stmn1, and Top2a), which fit with the known plasticity of DCT cells. No DCT2-specific transcripts were found. DCT2 cells contrast with DCT1 cells by expression of epithelial sodium channel β- and γ-subunits and much stronger expression of transcripts associated with calcium transport (Trpv5, Calb1, S100g, and Slc8a1). Additionally, scRNA-seq identified three distinct CTAL (Slc12a1 ⁺) cell subtypes. One of these expressed Nos1 and Avpr1a, consistent with macula densa cells. The other two CTAL clusters were distinguished by Cldn10 and Ptger3 in one and Cldn16 and Foxq1 in the other. These two CTAL cell types were also distinguished by expression of alternative Iroquois homeobox transcription factors, with Irx1 and Irx2 in the Cldn10 ⁺ CTAL cells and Irx3 in the Cldn16 ⁺ CTAL cells.

CONCLUSIONS

Single-cell transcriptomics revealed unexpected diversity among the cells of the distal nephron in mouse. Web-based data resources are provided for the single-cell data.

Microfluidic platform for rapid measurement of transepithelial water transport

Water transport across epithelial monolayers is of central importance in mammalian fluid homeostasis, and epithelial aquaporin (AQP) water channels are potential drug targets. Current methods to measure transepithelial water permeability based on indicator dilution have limited accuracy and can require hours for a single measurement. We report here a microfluidics platform for rapid and accurate measurement of water transport across a conventionally cultured epithelial monolayer on a porous filter requiring only a single image obtained using a standard laboratory fluorescence microscope. The undersurface of a porous polyester filter containing cultured epithelial cells on top is contacted with a perfused microfluidic channel of 100 μm width, 20 μm height and 10 cm length with folded geometry, with in-plane size of 3.2 × 3.2 mm² for visualization with a 2× objective lens. Osmotic water permeability is measured from the steady-state concentration profile along the length of the channel of a membrane-impermeant fluorescent dye in the perfusate, in which an osmotic gradient is imposed by an anisosmolar solution overlying the epithelial monolayer; diffusional water permeability is measured using a D₂O/H₂O-sensing fluorescent dye in the perfusate with a D₂O-containing isosmolar solution overlying the cell layer. Permeability values are deduced from single fluorescence images. The method, named fluid transport on a chip (FT-on-Chip), was applied to measure transepithelial osmotic and diffusional water permeability in control and AQP4-expressing epithelial cell monolayers. FT-on-Chip allows for rapid, accurate and repeated measurements of transepithelial water permeability, and is generalizable to transport measurements of ions and solutes using suitable indicator dyes.

50 Years of renal physiology from one man and the perfused tubule: Maurice B. Burg

Technical advancements in research techniques in science are made in slow increments. Even so, large advances from insight and hard work of an individual with a single technique can have astonishing ramifications. Here, we examine the impact of Dr. Maurice B. Burg and the isolated perfused renal tubule technique and celebrate the 50th anniversary of the publication by Dr. Burg and his colleagues of their landmark paper in the American Journal of Physiology in 1966. In this study, we have taken a scientific visualization approach to study the scientific contributions of Dr. Burg and the isolated perfused tubule preparation as determining research impact by the number of research students, postdoctoral fellows, visiting scientists, and national and international collaborators. Additionally, we have examined the research collaborations (first and second generation scientists), established the migrational visualization of the first generation scientists who worked directly with Dr. Burg, quantified the metrics indices, identified and quantified the network of coauthorship of the first generation scientists with their second generation links, and determined the citations analyses of outputs of Dr. Burg and/or his first generation collaborators as coauthors. We also review the major advances in kidney physiology that have been made with the isolated perfused tubule technique. Finally, we are all waiting for the discoveries that the isolated perfused preparation technique will bring during the next 50 years.

Adaptation by the collecting duct to an exogenous acid load is blunted by deletion of the proton-sensing receptor GPR4

We previously reported that the deletion of the pH sensor GPR4 causes a non-gap metabolic acidosis and defective net acid excretion (NAE) in the GPR4 knockout mouse (GPR4−/−) (Sun X, Yang LV, Tiegs BC, Arend LJ, McGraw DW, Penn RB, and Petrovic S. J Am Soc Nephrol 21: 1745–1755, 2010). Since the major regulatory site of NAE in the kidney is the collecting duct (CD), we examined acid-base transport proteins in intercalated cells (ICs) of the CD and found comparable mRNA expression of kidney anion exchanger 1 (kAE1), pendrin, and the a4 subunit of H+-ATPase in GPR4−/− vs. +/+. However, NH4Cl loading elicited adaptive doubling of AE1 mRNA in GPR4+/+, but a 50% less pronounced response in GPR4−/−. In GPR4+/+, NH4Cl loading evoked a cellular response characterized by an increase in AE1-labeled and a decrease in pendrin-labeled ICs similar to what was reported in rabbits and rats. This response did not occur in GPR4−/−. Microperfusion experiments demonstrated that the activity of the basolateral Cl−/HCO3− exchanger, kAE1, in CDs isolated from GPR4−/− failed to increase with NH4Cl loading, in contrast to the increase observed in GPR4+/+. Therefore, the deficiency of GPR4 blunted, but did not eliminate the adaptive response to an acid load, suggesting a compensatory response from other pH/CO2/bicarbonate sensors. Indeed, the expression of the calcium-sensing receptor (CaSR) was nearly doubled in GPR4−/− kidneys, in the absence of apparent disturbances of Ca2+ homeostasis. In summary, the expression and activity of the key transport proteins in GPR4−/− mice are consistent with spontaneous metabolic acidosis, but the adaptive response to a superimposed exogenous acid load is blunted and might be partially compensated for by CaSR.

Renal potassium transport: mechanisms and regulation

The regulation of potassium metabolism involves mechanisms for the appropriate distribution between the intra- and extracellular fluid compartments and for the excretion by the kidney. Clearance and single nephron studies show that renal excretion is determined by regulated potassium secretion and potassium reabsorption, respectively, in principal and intercalated cells of the distal nephron. Measurement of the electrochemical driving forces acting on potassium transport across individual cell membranes and characterization of several ATPases and potassium channels provide insights into the transport and regulation of renal potassium excretion.

NH4+ as a substrate for apical and basolateral Na(+)-H+ exchangers of thick ascending limbs of rat kidney: evidence from isolated membranes

1. We have used highly purified right-side-out luminal and basolateral membrane vesicles (LMVs and BLMVs) isolated from rat medullary thick ascending limb (MTAL) to study directly the possible roles of the LMV and BLMV Na(+)-H+ exchangers in the transport of NH4+. 2. Extravesicular NH4+ ((NH4+)o) inhibited outward H+ gradient-stimulated 22Na+ uptake in both types of vesicles. This inhibition could not be accounted for by alteration of intravesicular pH (pHi). 3. Conversely, in both plasma membrane preparations, the imposition of outward NH4+ gradients stimulated 22Na+ uptake at the acidic pHi (6.60) of MTAL cells, under conditions in which possible alterations in pHi were prevented. All NH4+ gradient-stimulated Na+ uptake was sensitive to 0.5 mM 5-(N,N-dimethyl)-amiloride. 4. The BLMV and LMV Na(+)-H+ exchangers had a similar apparent affinity for internal H+ (Hi+), with pK (-log of dissociation constant) values of 6.58 and 6.52, respectively. 5. These findings indicate that NH4+ interacts with the external and internal transport sites of the LMV and BLMV Na(+)-H+ antiporters, and that both of these exchangers can mediate the exchange of internal NH4+ ((NH4+)i) for external Na+ (Na+o) at the prevailing pHi of MTAL cells. 6. We conclude that operation of the BLMV Na(+)-H+ exchanger on the NH4(+)-Na+ mode may represent an important pathway for mediating the final step of NH4+ absorption, whereas transport of NH4+ on the apical antiporter may provide negative feedback regulation of NH4+ absorption.

Unusual permeability properties of gastric gland cells

Physiologists have long pondered the riddle of why the stomach is itself not digested by the very juice it secretes. One explanation is that a mucus-bicarbonate barrier, coating the stomach lumen as well as superficial portions of gastric glands, prevents autodigestion. However, this leaves unanswered the question of what protects cells deeper in the glands, which seem to lack a mucus barrier. These are the parietal and chief cells, which secrete acid and pepsin. Using perfused single gastric glands from rabbit, we recently found that intracellular pH is uniquely resistant to extreme degrees of luminal acidification, suggesting that the apical (luminal) barrier might also exclude ammonia and carbon dioxide, to which cell membranes are generally highly permeable. We now show that this is indeed the case. There are three reports of membranes with very low permeabilities to NH3 (refs 5-7), and none of membranes impermeable to CO2.

Differentiation in vitro of primary cultures and transfected cell lines of epithelial cells derived from the thick ascending limb of Henle's loop

The use of primary cell cultures derived from defined locations of the kidney has enabled the study of certain kidney cell type-specific characteristics under defined environmental conditions. The use of primary cell cultures, however, has a number of inherent disadvantages, many of which may be overcome by the use of differentiated cell lines of defined origin. In this paper I describe in detail an approach to: (a) the isolation and culture of primary cultures derived from the thick ascending limb of Henle's loop (TALH), and (b) the production of differentiated cell lines by the transfection of these primary cell cultures with early region SV40 virus genes. The characteristics of these cultures and other TALH-derived cell lines are described.

Oxygen deprivation-induced injury to isolated rabbit kidney tubules

The utility of freshly isolated suspensions of rabbit tubules enriched in proximal segments for studying the pathogenesis of oxygen deprivation-induced renal tubular cell injury was evaluated. Oxygenated control preparations exhibited very good stability of critical cell injury-related metabolic parameters including oxygen consumption, cell cation homeostasis, and adenine nucleotide metabolism for periods in excess of 2 h. Highly reproducible models of oxygen deprivation-induced injury and recovery were developed and alterations of injury-related metabolic parameters in these models were characterized in detail. When oxygen deprivation was produced under hypoxic conditions, tubules sustained widespread lethal cell injury and associated metabolic alterations within 15-30 min. However, when oxygen deprivation was produced under simulated ischemic conditions, tubules tolerated 30-60 min with only moderate amounts of lethal cell injury occurring, a situation similar to that seen with ischemia in vivo. Like ischemia in vivo, simulated ischemia in vitro was characterized by a fall in pH during oxygen deprivation. No such fall in pH occurred in the hypoxic model. To test whether this fall in pH could contribute to the protection seen during simulated ischemia in vitro, tubules were subjected to hypoxia at medium pHs ranging from 7.45 to 6.41. Striking protection from hypoxic injury was seen as pH was reduced with maximal protection occurring in tubules made hypoxic at pHs below 7.0. Measurements of injury-associated metabolic parameters suggested that the protective effect of reduced pH may be mediated by pH-induced alterations of tubule cell Ca++ metabolism. This study has, thus, defined and characterized in detail a new and extremely versatile model system for the study of oxygen deprivation-induced cell injury in the kidney and has established that pH alterations play a major role in modulating such injury.

Characteristics of ionic binding by rat renal tissue in vitro

A study has been made of Na+ and Cl- binding in metabolically inhibited slices of rat renal cortex and outer medulla incubated in modified Krebs phosphate-bicarbonate Ringer solution. At pH 7.35 in control media (cortex, 147 mmol Na+/l, 105 mmol Cl-/l; outer medulla, 187 mmol Na+/l, 145 mmol Cl-/l) cortical slices bound (mean) 171 nmol Na+ and 56.7 nmol Cl-/mg solute-free dry weight; outer medullary slices bound 188 nmol Na+/mg and negligible amounts of Cl-. In both regions, Na+ was exchangeable on a 1:1 basis for K+ or Li+ in media containing equal concentrations of each cation: Na+ was completely displaced by La3+. In cortical slices in media containing equimolar Cl- and other monovalent anions, binding occurred according to the sequence acetate less than or equal to salicylate less than or equal to Cl- less than SCN-; Cl- was completely displaced by PO4(3-). When medium pH was lowered, Na+ binding was markedly reduced in both regions, whereas Cl- binding increased (and became significant in outer medulla). In NaCl solutions, Na+ binding capacity was saturated at control Na+ concentrations. When [Na+] was progressively reduced (iso-osmolality being maintained by addition of urea), bound Na+ in both regions was nearly linearly related to log medium [Na+]. Raising medium osmolality with urea caused decreased Na+ binding and increased Cl- binding in both regions. Na+ binding in both regions was significantly reduced by pre-treatment with chondroitinase ABC. Binding of both ions was independent of temperature within the range 2-37 degrees C. The possibility is raised that renal ionic binding might influence vectorial ion transport by affecting free ion activity in the region of the transporting cells.