Terminal differentiation of intercalated cells: the role of hensin

Time course and magnitude of ventilatory and renal acid-base acclimatization following rapid ascent to and residence at 3,800 m over nine days

Rapid ascent to high altitude imposes an acute hypoxic and acid-base challenge, with ventilatory and renal acclimatization countering these perturbations. Specifically, ventilatory acclimatization improves oxygenation, but with concomitant hypocapnia and respiratory alkalosis. A compensatory, renally mediated relative metabolic acidosis follows via bicarbonate elimination, normalizing arterial pH(a). The time course and magnitude of these integrated acclimatization processes are highly variable between individuals. Using a previously developed metric of renal reactivity (RR), indexing the change in arterial bicarbonate concentration (Δ[HCO3-]a; renal response) over the change in arterial pressure of CO2 (Δ[Formula: see text]; renal stimulus), we aimed to characterize changes in RR magnitude following rapid ascent and residence at altitude. Resident lowlanders (n = 16) were tested at 1,045 m (day [D]0) prior to ascent, on D2 within 24 h of arrival, and D9 during residence at 3,800 m. Radial artery blood draws were obtained to measure acid-base variables: [Formula: see text], [HCO3-]a, and pHa. Compared with D0, [Formula: see text] and [HCO3-]a were lower on D2 (P < 0.01) and D9 (P < 0.01), whereas significant changes in pHa (P = 0.072) and RR (P = 0.056) were not detected. As pHa appeared fully compensated on D2 and RR did not increase significantly from D2 to D9, these data demonstrate renal acid-base compensation within 24 h at moderate steady-state altitude. Moreover, RR was strongly and inversely correlated with ΔpHa on D2 and D9 (r≤ -0.95; P < 0.0001), suggesting that a high-gain renal response better protects pHa. Our study highlights the differential time course, magnitude, and variability of integrated ventilatory and renal acid-base acclimatization following rapid ascent and residence at high altitude.NEW & NOTEWORTHY We assessed the time course, magnitude, and variability of integrated ventilatory and renal acid-base acclimatization with rapid ascent and residence at 3,800 m. Despite reductions in [Formula: see text] upon ascent, pHa was normalized within 24 h of arrival at 3,800 m through renal compensation (i.e., bicarbonate elimination). Renal reactivity (RR) was unchanged between days 2 and 9, suggesting a lack of plasticity at moderate steady-state altitude. RR was strongly correlated with ΔpHa, suggesting that a high-gain renal response better protects pHa.

Renal reactivity: acid-base compensation during incremental ascent to high altitude

KEY POINTS

Ascent to high altitude imposes an acid-base challenge in which renal compensation is integral for maintaining pH homeostasis, facilitating acclimatization and helping prevent mountain sicknesses. The time-course and extent of plasticity of this important renal response during incremental ascent to altitude is unclear. We created a novel index that accurately quantifies renal acid-base compensation, which may have laboratory, fieldwork and clinical applications. Using this index, we found that renal compensation increased and plateaued after 5 days of incremental altitude exposure, suggesting plasticity in renal acid-base compensation mechanisms. The time-course and extent of plasticity in renal responsiveness may predict severity of altitude illness or acclimatization at higher or more prolonged stays at altitude.

ABSTRACT

Ascent to high altitude, and the associated hypoxic ventilatory response, imposes an acid-base challenge, namely chronic hypocapnia and respiratory alkalosis. The kidneys impart a relative compensatory metabolic acidosis through the elimination of bicarbonate (HCO₃ ^- ) in urine. The time-course and extent of plasticity of the renal response during incremental ascent is unclear. We developed an index of renal reactivity (RR), indexing the relative change in arterial bicarbonate concentration ([HCO₃ ^- ]_a ) (i.e. renal response) against the relative change in arterial pressure of CO₂ ( P aC O 2 ) (i.e. renal stimulus) during incremental ascent to altitude ( Δ [ HC O 3 - ] a / Δ P aC O 2 ). We aimed to assess whether: (i) RR magnitude was inversely correlated with relative changes in arterial pH (ΔpH_a ) with ascent and (ii) RR increased over time and altitude exposure (i.e. plasticity). During ascent to 5160 m over 10 days in the Nepal Himalaya, arterial blood was drawn from the radial artery for measurement of blood gas/acid-base variables in lowlanders at 1045/1400 m and after 1 night of sleep at 3440 m (day 3), 3820 m (day 5), 4240 m (day 7) and 5160 m (day 10) during ascent. At 3820 m and higher, RR significantly increased and plateaued compared to 3440 m (P < 0.04), suggesting plasticity in renal acid-base compensations. At all altitudes, we observed a strong negative correlation (r ≤ -0.71; P < 0.001) between RR and ΔpH_a from baseline. Renal compensation plateaued after 5 days of altitude exposure, despite subsequent exposure to higher altitudes. The time-course, extent of plasticity and plateau in renal responsiveness may predict severity of altitude illness or acclimatization at higher or more prolonged stays at altitude.

Chronic lithium treatment induces novel patterns of pendrin localization and expression

Prolonged lithium treatment is associated with various renal side effects and is known to induce inner medullary collecting duct (IMCD) remodeling. In animals treated with lithium, the fraction of intercalated cells (ICs), which are responsible for acid-base homeostasis, increases compared with renal principal cells (PCs). To investigate the intricacies of lithium-induced IMCD remodeling, male Sprague-Dawley rats were fed a lithium-enriched diet for 0,1, 2, 3, 6, 9, or 12 wk. Urine osmolality was decreased at 1 wk, and from 2 to 12 wk, animals were severely polyuric. After 6 wk of lithium treatment, approximately one-quarter of the cells in the initial IMCD expressed vacuolar H⁺-ATPase, an IC marker. These cells were localized in portions of the inner medulla, where ICs are not normally found. Pendrin, a Cl^-/[Formula: see text] exchanger, is normally expressed only in two IC subtypes found in the convoluted tubule, the cortical collecting duct, and the connecting tubule. At 6 wk of lithium treatment, we observed various patterns of pendrin localization and expression in the rat IMCD, including a novel phenotype wherein pendrin was coexpressed with aquaporin-4. These observations collectively suggest that renal IMCD cell plasticity may play an important role in lithium-induced IMCD remodeling.

Hypokalemic Distal Renal Tubular Acidosis

Distal renal tubular acidosis (DRTA) is defined as hyperchloremic, non-anion gap metabolic acidosis with impaired urinary acid excretion in the presence of a normal or moderately reduced glomerular filtration rate. Failure in urinary acid excretion results from reduced H⁺ secretion by intercalated cells in the distal nephron. This results in decreased excretion of NH₄⁺ and other acids collectively referred as titratable acids while urine pH is typically above 5.5 in the face of systemic acidosis. The clinical phenotype in patients with DRTA is characterized by stunted growth with bone abnormalities in children as well as nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria, hypocitraturia, and relatively alkaline urine. Hypokalemia is a striking finding that accounts for muscle weakness and requires continued treatment together with alkali-based therapies. This review will focus on the mechanisms responsible for impaired acid excretion and urinary potassium wastage, the clinical features, and diagnostic approaches of hypokalemic DRTA, both inherited and acquired.

Acid-Base Homeostasis

Acid-base homeostasis and pH regulation are critical for both normal physiology and cell metabolism and function. The importance of this regulation is evidenced by a variety of physiologic derangements that occur when plasma pH is either high or low. The kidneys have the predominant role in regulating the systemic bicarbonate concentration and hence, the metabolic component of acid-base balance. This function of the kidneys has two components: reabsorption of virtually all of the filtered HCO3(-) and production of new bicarbonate to replace that consumed by normal or pathologic acids. This production or generation of new HCO3(-) is done by net acid excretion. Under normal conditions, approximately one-third to one-half of net acid excretion by the kidneys is in the form of titratable acid. The other one-half to two-thirds is the excretion of ammonium. The capacity to excrete ammonium under conditions of acid loads is quantitatively much greater than the capacity to increase titratable acid. Multiple, often redundant pathways and processes exist to regulate these renal functions. Derangements in acid-base homeostasis, however, are common in clinical medicine and can often be related to the systems involved in acid-base transport in the kidneys.

Evaluation of cellular plasticity in the collecting duct during recovery from lithium-induced nephrogenic diabetes insipidus

The cellular morphology of the collecting duct is altered by chronic lithium treatment. We have previously shown that lithium increases the fraction of type-A intercalated cells and lowers the fraction of principal cells along the collecting duct. Moreover, type-A intercalated cells acquire a long-row distribution pattern along the tubules. In the present study, we show that these morphological changes reverse progressively after discontinuation of lithium and finally disappear after 19 days from lithium suspension. In this time frame we have identified for the first time, in vivo, a novel cellular type positive for both intercalated and principal cells functional markers, as recognized by colabeling with H+-ATPase/aquaporin-4 (AQP4) and anion exchanger-1 (AE-1)/AQP2 and Foxi1/AQP4. This cell type is mainly present after 6 days of lithium washout, and it disappears in parallel with the long-row pattern of the type-A intercalated cells. It usually localizes either in the middle or at the edge of the long-row pattern. Its ultrastructure resembles the intercalated cells as shown both by differential interference contrast and by electron microscopy. The time course of appearance, the localization along the collecting duct, and the ultrastructure suggest that the cells double labeled for principal and intercalated cells markers could represent a transition element driving the conversion of intercalated cells into principal cells.

Proliferation of acid-secretory cells in the kidney during adaptive remodelling of the collecting duct

The renal collecting duct adapts to changes in acid-base metabolism by remodelling and altering the relative number of acid or alkali secreting cells, a phenomenon termed plasticity. Acid secretory A intercalated cells (A-IC) express apical H(+)-ATPases and basolateral bicarbonate exchanger AE1 whereas bicarbonate secretory B intercalated cells (B-IC) express basolateral (and apical) H(+)-ATPases and the apical bicarbonate exchanger pendrin. Intercalated cells were thought to be terminally differentiated and unable to proliferate. However, a recent report in mouse kidney suggested that intercalated cells may proliferate and that this process is in part dependent on GDF-15. Here we extend these observations to rat kidney and provide a detailed analysis of regional differences and demonstrate that differentiated A-IC proliferate massively during adaptation to systemic acidosis. We used markers of proliferation (PCNA, Ki67, BrdU incorporation) and cell-specific markers for A-IC (AE1) and B-IC (pendrin). Induction of remodelling in rats with metabolic acidosis (with NH(4)Cl for 12 hrs, 4 and 7 days) or treatment with acetazolamide for 10 days resulted in a larger fraction of AE1 positive cells in the cortical collecting duct. A large number of AE1 expressing A-IC was labelled with proliferative markers in the cortical and outer medullary collecting duct whereas no labeling was found in B-IC. In addition, chronic acidosis also increased the rate of proliferation of principal collecting duct cells. The fact that both NH(4)Cl as well as acetazolamide stimulated proliferation suggests that systemic but not urinary pH triggers this response. Thus, during chronic acidosis proliferation of AE1 containing acid-secretory cells occurs and may contribute to the remodelling of the collecting duct or replace A-IC due to a shortened life span under these conditions.

cAMP stimulates apical V-ATPase accumulation, microvillar elongation, and proton extrusion in kidney collecting duct A-intercalated cells

Kidney proton-secreting A-intercalated cells (A-IC) respond to systemic acidosis by accumulating the vacuolar ATPase (V-ATPase) in their apical membrane and by increasing the length and number of apical microvilli. We show here that the cell-permeant cAMP analog CPT-cAMP, infused in vivo, results in an almost twofold increase in apical V-ATPase accumulation in AE1-positive A-IC within 15 min and that these cells develop an extensive array of apical microvilli compared with controls. In contrast, no significant change in V-ATPase distribution could be detected by immunocytochemistry in B-intercalated cells at the acute time point examined. To show a direct effect of cAMP on A-IC, we prepared cell suspensions from the medulla of transgenic mice expressing EGFP in IC (driven by the B1-subunit promoter of the V-ATPase) and exposed them to cAMP analogs in vitro. Three-dimensional reconstructions of confocal images revealed that cAMP induced a time-dependent growth of apical microvilli, starting within minutes after addition. This effect was blocked by the PKA inhibitor myristoylated PKI. These morphological changes were paralleled by increased cAMP-mediated proton extrusion (pHi recovery) by A-IC in outer medullary collecting ducts measured using the ratiometric probe BCECF. These results, and our prior data showing that the bicarbonate-stimulated soluble adenylyl cyclase (sAC) is highly expressed in kidney intercalated cells, support the idea that cAMP generated either by sAC, or by activation of other signaling pathways, is part of the signal transduction mechanism involved in acid-base sensing and V-ATPase membrane trafficking in kidney intercalated cells.

Regulation of the V-ATPase in kidney epithelial cells: dual role in acid-base homeostasis and vesicle trafficking

The proton-pumping V-ATPase is a complex, multi-subunit enzyme that is highly expressed in the plasma membranes of some epithelial cells in the kidney, including collecting duct intercalated cells. It is also located on the limiting membranes of intracellular organelles in the degradative and secretory pathways of all cells. Different isoforms of some V-ATPase subunits are involved in the targeting of the proton pump to its various intracellular locations, where it functions in transporting protons out of the cell across the plasma membrane or acidifying intracellular compartments. The former process plays a critical role in proton secretion by the kidney and regulates systemic acid-base status whereas the latter process is central to intracellular vesicle trafficking, membrane recycling and the degradative pathway in cells. We will focus our discussion on two cell types in the kidney: (1) intercalated cells, in which proton secretion is controlled by shuttling V-ATPase complexes back and forth between the plasma membrane and highly-specialized intracellular vesicles, and (2) proximal tubule cells, in which the endocytotic pathway that retrieves proteins from the glomerular ultrafiltrate requires V-ATPase-dependent acidification of post-endocytotic vesicles. The regulation of both of these activities depends upon the ability of cells to monitor the pH and/or bicarbonate content of their extracellular environment and intracellular compartments. Recent information about these pH-sensing mechanisms, which include the role of the V-ATPase itself as a pH sensor and the soluble adenylyl cyclase as a bicarbonate sensor, will be addressed in this review.

The calcineurin inhibitor FK506 (tacrolimus) is associated with transient metabolic acidosis and altered expression of renal acid-base transport proteins

Calcineurin inhibitors like FK506 (tacrolimus) are routinely used for immunosuppression following transplantation. Its use is limited by many side effects, including renal tubular acidosis (RTA), mainly of the distal type. In this study, rats were treated with FK506 and at baseline (after 9 days) systemic acid-base status was similar to that in control animals. However, FK506-treated rats given NH4Cl in the drinking water for 2 days developed a more severe metabolic acidosis than control animals. Urine pH was more alkaline, but net acid excretion was normal. After 7 days of acid load, all differences related to acid-base homeostasis were equalized in both groups. Protein abundance of type IIa Na-Pi cotransporter, type 3 Na+/H+ exchanger, and electrogenic Na+-bicarbonate cotransporter, and both a4 and B2 subunits of the vacuolar H+-ATPase were reduced under baseline conditions, while induction of metabolic acidosis enhanced protein abundance of these transporters in FK506-treated animals. In parallel, protein expression of AE1 was reduced at baseline and increased together with pendrin during NH4Cl loading in FK506 rats. Protein abundance of the Na+-bicarbonate cotransporter NBCn1 was reduced under baseline conditions but remained downregulated during metabolic acidosis. Morphological analysis revealed an increase in the relative number of non-type A intercalated cells in the connecting tubule and cortical collecting duct at the expense of principal cells. Additionally, subcellular distribution of the a4 subunit of the vacuolar H+-ATPase was affected by FK506 with less luminal localization in the connecting tubule and outer medullary collecting duct. These data suggest that FK506 impacts on several major acid-base transport proteins in the kidney, and its use is associated with transient metabolic acidosis and altered expression of key renal acid-base transport proteins.

Functional regulation of H+-ATPase-rich cells in zebrafish embryos acclimated to an acidic environment

It is important to maintain internal pH homeostasis in biological systems. In our previous studies, H(+)-ATPase-rich (HR) cells were found to be responsible for proton secretion in the skin of zebrafish embryos during development. In this study, zebrafish embryos were exposed to acidic and basic waters to investigate the regulation of HR cell acid secretion during pH disturbances. Our results showed that the function of HR cells on the skin of zebrafish embryos can be upregulated in pH 4 water not only by increasing the cell number but also by enlarging the acid-secreting function of single cells. We also identified an "alveolar-type" apical opening under scanning electron microscopy observations of the apical membrane of HR cells, and the density and size of the alveolar type of apical openings were also increased in pH 4 water. p63 and PCNA immunostaining results also showed that additional HR cells in pH 4 water may be differentiated not only from ionocyte precursor cells but also newly proliferating epithelial stem cells.

Salivary agglutinin/glycoprotein-340/DMBT1: a single molecule with variable composition and with different functions in infection, inflammation and cancer

Salivary agglutinin (SAG), lung glycoprotein-340 (gp-340) and Deleted in Malignant Brain Tumours 1 (DMBT1) are three names for identical proteins encoded by the dmbt1 gene. DMBT1/SAG/gp-340 belongs to the scavenger receptor cysteine-rich (SRCR) superfamily of proteins, a superfamily of secreted or membrane-bound proteins with SRCR domains that are highly conserved down to sponges, the most ancient metazoa. On the one hand, DMBT1 may represent an innate defence factor acting as a pattern recognition molecule. It interacts with a broad range of pathogens, including cariogenic streptococci and Helicobacter pylori, influenza viruses and HIV, but also with mucosal defence proteins, such as IgA, surfactant proteins and MUC5B. Stimulation of alveolar macrophage migration, suppression of neutrophil oxidative burst and activation of the complement cascade point further to an important role in the regulation of inflammatory responses. On the other hand, DMBT1 has been demonstrated to play a role in epithelial and stem cell differentiation. Inactivation of the gene coding for this protein may lead to disturbed differentiation, possibly resulting in tumour formation. These data strongly point to a role for DMBT1 as a molecule linking innate immune processes with regenerative processes.

Genome-wide mapping of Polycomb target genes unravels their roles in cell fate transitions

The Polycomb group (PcG) proteins form chromatin-modifying complexes that are essential for embryonic development and stem cell renewal and are commonly deregulated in cancer. Here, we identify their target genes using genome-wide location analysis in human embryonic fibroblasts. We find that Polycomb-Repressive Complex 1 (PRC1), PRC2, and tri-methylated histone H3K27 co-occupy >1000 silenced genes with a strong functional bias for embryonic development and cell fate decisions. We functionally identify 40 genes derepressed in human embryonic fibroblasts depleted of the PRC2 components (EZH2, EED, SUZ12) and the PRC1 component, BMI-1. Interestingly, several markers of osteogenesis, adipogenesis, and chrondrogenesis are among these genes, consistent with the mesenchymal origin of fibroblasts. Using a neuronal model of differentiation, we delineate two different mechanisms for regulating PcG target genes. For genes activated during differentiation, PcGs are displaced. However, for genes repressed during differentiation, we paradoxically find that they are already bound by the PcGs in nondifferentiated cells despite being actively transcribed. Our results are consistent with the hypothesis that PcGs are part of a preprogrammed memory system established during embryogenesis marking certain key genes for repressive signals during subsequent developmental and differentiation processes.

Zona pellucida domain proteins

Many eukaryotic proteins share a sequence designated as the zona pellucida (ZP) domain. This structural element, present in extracellular proteins from a wide variety of organisms, from nematodes to mammals, consists of approximately 260 amino acids with eight conserved cysteine (Cys) residues and is located close to the C terminus of the polypeptide. ZP domain proteins are often glycosylated, modular structures consisting of multiple types of domains. Predictions can be made about some of the structural features of the ZP domain and ZP domain proteins. The functions of ZP domain proteins vary tremendously, from serving as structural components of egg coats, appendicularian mucous houses, and nematode dauer larvae, to serving as mechanotransducers in flies and receptors in mammals and nonmammals. Generally, ZP domain proteins are present in filaments and/or matrices, which is consistent with the role of the domain in protein polymerization. A general mechanism for assembly of ZP domain proteins has been presented. It is likely that the ZP domain plays a common role despite its presence in proteins of widely diverse functions.

Exploring the role of galectin 3 in kidney function: a genetic approach

Galectin 3 belongs to a family of glycoconjugate-binding proteins that participate in cellular homeostasis by modulating cell growth, adhesion, and signaling. We studied adult galectin 3 null mutant (Gal 3-/-) and wild-type (WT) mice to gain insights into the role of galectin 3 in the kidney. By immunofluorescence, galectin 3 was found in collecting duct (CD) principal and intercalated cells in some regions of the kidney, as well as in the thick ascending limbs at lower levels. Compared to WT mice, Gal 3-/- mice had approximately 11% fewer glomeruli (p < 0.04), associated with kidney hypertrophy (p < 0.006). In clearance experiments, urinary chloride excretion was found to be higher in Gal 3-/- than in WT mice (p < 0.04), but there was no difference in urinary bicarbonate excretion, in glomerular filtration, or urinary flow rates. Under chronic low sodium diet, Gal 3-/- mice had lower extracellular fluid (ECF) volume than WT mice (p < 0.05). Plasma aldosterone concentration was higher in Gal 3-/- than in WT mice (p < 0.04), which probably caused the observed increase in alpha-epithelial sodium channel (alpha-ENaC) protein abundance in the mutant mice (p < 0.001). Chronic high sodium diet resulted paradoxically in lower blood pressure (p < 0.01) in Gal 3-/- than in WT. We conclude that Gal 3-/- mice have mild renal chloride loss, which causes chronic ECF volume contraction and reduced blood pressure levels.

Stable expression of the extracellular domains of rabbit recombinant CD5: development and characterization of polyclonal and monoclonal antibodies

Previous studies in our laboratory suggested that there was positive selection of B cells during early development in the appendix of normal and V(H) mutant (ali/ali) rabbits. Preferential expansion and survival of B lymphocytes was affected by the Ig V(H) frameworks 1 and 3 sequences expressed on the cell surface. We demonstrated a specific interaction between rabbit CD5 and the V region of rabbit heavy chains and suggested that CD5 is a potential selecting ligand for B-cell surface immunoglobulin framework region sequences. To further investigate the role of CD5 in rabbit B-cell selection and survival we prepared recombinant constructs and obtained stable expression of the three scavenger receptor cysteine-rich (SRCR) extracellular domains of rabbit CD5. Here we describe the production and purification of this expressed recombinant CD5 protein, polyclonal antibody obtained by immunization of a goat and initial production and characterization of specific mAbs against peptides selected from each sequenced SRCR domain.

Conversion of ES cells to columnar epithelia by hensin and to squamous epithelia by laminin

Single-layered epithelia are the first differentiated cell types to develop in the embryo, with columnar and squamous types appearing immediately after blastocyst implantation. Here, we show that mouse embryonic stem cells seeded on hensin or laminin, but not fibronectin or collagen type IV, formed hemispheric epithelial structures whose outermost layer terminally differentiated to an epithelium that resembled the visceral endoderm. Hensin induced columnar epithelia, whereas laminin formed squamous epithelia. At the egg cylinder stage, the distal visceral endoderm is columnar, and these cells begin to migrate anteriorly to create the anterior visceral endoderm, which assumes a squamous shape. Hensin expression coincided with the dynamic appearance and disappearance of columnar cells at the egg cylinder stage of the embryo. These expression patterns, and the fact that hensin null embryos (and those already reported for laminin) die at the onset of egg cylinder formation, support the view that hensin and laminin are required for terminal differentiation of columnar and squamous epithelial phenotypes during early embryogenesis.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Cyclosporin A produces distal renal tubular acidosis by blocking peptidyl prolyl cis-trans isomerase activity of cyclophilin

Cyclosporin A (CsA), a widely used immunosuppressant, causes distal renal tubular acidosis (dRTA). It exerts its immunosuppressive effect by a calcineurin-inhibitory complex with its cytosolic receptor, cyclophilin A. However, CsA also inhibits the peptidyl prolyl cis-trans isomerase (PPIase) activity of cyclophilin A. We studied HCO(3)(-) transport and changes in beta-intercalated cell pH on luminal Cl(-) removal in isolated, perfused rabbit cortical collecting tubules (CCDs) before and after exposure to media pH 6.8 for 3 h. Acid incubation causes adaptive changes in beta-intercalated cells by extracellular deposition of hensin (J Clin Invest 109: 89, 2002). Here, CsA prevented this adaptation. The unidirectional HCO(3)(-) secretory flux, estimated as the difference between net flux and that after Cl(-) removal from the lumen, was -6.7 +/- 0.2 pmol.min(-1).mm(-1) and decreased to -1.3 +/- 0.2 after acid incubation. CsA in the bath prevented the adaptive decreases in HCO(3)(-) secretion and apical Cl(-):HCO(3)(-) exchange. To determine the mechanism, we incubated CCDs with FK-506, which inhibits calcineurin activity independently of the host cell cyclophilin. FK-506 did not prevent the acid-induced adaptive decrease in unidirectional HCO(3)(-) secretion. However, [AD-Ser](8) CsA, a CsA derivative, which does not inhibit calcineurin but inhibits PPIase activity of cyclophilin A, completely blocked the effect of acid incubation on apical Cl(-):HCO(3)(-) exchange. Acid incubation resulted in prominent "clumpy" staining of extracellular hensin and diminished apical surface of beta-intercalated cells [smaller peanut agglutinin (PNA) caps]. CsA and [AD-Ser](8) CsA prevented most hensin staining and the reduction of apical surface; PNA caps were more prominent. We suggest that hensin polymerization around adapting beta-intercalated cells requires the PPIase activity of cyclophilins. Thus CsA is able to prevent this adaptation by inhibition of a peptidyl prolyl cis-trans isomerase activity. Such inhibition may cause dRTA during acid loading.

Membrane Trafficking of Angiotensin Receptor Type-1 and Mechanochemical Signal Transduction in Proximal Tubule Cells

Cellular localization and trafficking of the major angiotensin receptor, AT 1 , was studied in mouse proximal tubule cell lines because angiotensin II concentrations in the luminal fluid of proximal tubules are greater than the K d of the receptor and would predict high turnover rates of the receptor. Mouse proximal tubule cells can exist in 2 polarized, differentiated states after confluence: a protoepithelium and a highly differentiated epithelium. The latter is distinguished by greater polarization of the microtubule cytoskeleton and collection of apical microtubule-dependent membrane proteins in condensed apical recycling endosomes (CARE) in proximity to the primary cilium. AT 1 , AT 2 , and the sodium hydrogen exchanger NHE3 are localized to CARE. With fluid movement, AT 1 receptors externalize from CARE to the apical plasma membrane and allow luminal angiotensin II to initiate cell signaling. These data suggest that fluid movement controls receptor externalization and, hence, a model in which ciliary deflection results in transduction of a mechanical stimulus into the chemical signaling of the AT 1 receptor.

Distal renal tubular acidosis in mice that lack the forkhead transcription factor Foxi1

While macro- and microscopic kidney development appear to proceed normally in mice that lack Foxi1, electron microscopy reveals an altered ultrastructure of cells lining the distal nephron. Northern blot analyses, cRNA in situ hybridizations, and immunohistochemistry demonstrate a complete loss of expression of several anion transporters, proton pumps, and anion exchange proteins expressed by intercalated cells of the collecting ducts, many of which have been implicated in hereditary forms of distal renal tubular acidosis (dRTA). In Foxi1-null mutants the normal epithelium with its two major cell types - principal and intercalated cells - has been replaced by a single cell type positive for both principal and intercalated cell markers. To test the functional consequences of these alterations, Foxi1(-/-) mice were compared with WT littermates in their response to an acidic load. This revealed an inability to acidify the urine as well as a lowered systemic buffer capacity and overt acidosis in null mutants. Thus, Foxi1(-/-) mice seem to develop dRTA due to altered cellular composition of the distal nephron epithelium, thereby denying this epithelium the proper gene expression pattern needed for maintaining adequate acid-base homeostasis.

Spatiotemporal expression patterns of sialoglycoconjugates during nephron morphogenesis and their regional and cell type-specific distribution in adult rat kidney

The expression of alpha2,6- and alpha2,3-linked sialic acids on N-glycans was studied in embryonic, postnatal, and adult rat kidney. Histochemistry and blotting using Polyporus squamosus and Sambucus nigra lectins for alpha2,6-linked sialic acids and the Maackia amurensis lectin for alpha2,3-linked sialic acids were performed and sialyltransferase activity was assayed. N-glycans with alpha2,6- and alpha2,3-linked sialic acid were differently expressed in the two embryonic anlagen and early stages of nephron. Metanephrogenic mesenchyme was positive for alpha2,3-linked sialic acid but not for the alpha2,6-linked one, which became detectable initially in the proximal part of S-shaped bodies. Collecting ducts were positive for alpha2,6-linked sialic acid, whereas alpha2,3-linked sialic acid was restricted to their ampullae. Although positive in embryonic kidney, S1 and S2 of proximal tubules became unreactive for alpha2,3-linked sialic acid in postnatal and adult kidneys. In adult kidney, intercalated but not principal cells of collecting ducts were reactive for alpha2,3-linked sialic acid. In contrast, alpha2,6-linked sialic acids were detected in all cells of adult kidney nephron. Blot analysis revealed a different but steady pattern of bands reactive for alpha2,6- and alpha2,3-linked sialic acid in embryonic, postnatal, and adult kidney. Activity of alpha2,6 and alpha2,3 sialyltransferases was highest in embryonic kidney and decreased over postnatal to adult kidney with the activity of alpha2,6 sialyltransferase always being three to fourfold that of alpha2,3 sialyltransferase. Thus, alpha2,6- and alpha2,3-linked sialic acids are differently expressed in embryonic anlagen and mesenchyme-derived early stages of nephron and show regional and cell type-specific differences in adult kidney.