Conditional immortalization of bicarbonate-secreting intercalated cells from rabbit

The study of intercalated cells using ex vivo techniques: primary cell culture, cell lines, kidney slices, and organoids

This review summarizes methods to study kidney intercalated cell (IC) function ex vivo. While important for acid-base homeostasis, IC dysfunction is often not recognized clinically until it becomes severe. The advantage of using ex vivo techniques is that they allow for the differential evaluation of IC function in controlled environments. Although in vitro kidney tubular perfusion is a classical ex vivo technique to study IC, here we concentrate on primary cell cultures, immortalized cell lines, and ex vivo kidney slices. Ex vivo techniques are useful in evaluating IC signaling pathways that allow rapid responses to extracellular changes in pH, CO2, and bicarbonate (HCO3-). However, these methods for IC work can also be challenging, as cell lines that recapitulate IC do not proliferate easily in culture. Moreover, a "pure" IC population in culture does not necessarily replicate its collecting duct (CD) environment, where ICs are surrounded by the more abundant principal cells (PCs). It is reassuring that many findings obtained in ex vivo IC systems signaling have been largely confirmed in vivo. Some of these newly identified signaling pathways reveal that ICs are important for regulating NaCl reabsorption, thus suggesting new frontiers to target antihypertensive treatments. Moreover, recent single-cell characterization studies of kidney epithelial cells revealed a dual developmental origin of IC, as well as the presence of novel CD cell types with certain IC characteristics. These exciting findings present new opportunities for the study of IC ex vivo and will likely rediscover the importance of available tools in this field.NEW & NOTEWORTHY The study of kidney intercalated cells has been limited by current cell culture and kidney tissue isolation techniques. This review is to be used as a reference to select ex vivo techniques to study intercalated cells. We focused on the use of cell lines and kidney slices as potential useful models to study membrane transport proteins. We also review how novel collecting duct organoids may help better elucidate the role of these intriguing cells.

Intercalated Cells of the Kidney Collecting Duct in Kidney Physiology

The epithelium of the kidney collecting duct (CD) is composed mainly of two different types of cells with distinct and complementary functions. CD principal cells traditionally have been considered to have a major role in Na+ and water regulation, while intercalated cells (ICs) were thought to largely modulate acid-base homeostasis. In recent years, our understanding of IC function has improved significantly owing to new research findings. Thus, we now have a new model for CD transport that integrates mechanisms of salt and water reabsorption, K+ homeostasis, and acid-base status between principal cells and ICs. There are three main types of ICs (type A, type B, and non-A, non-B), which first appear in the late distal convoluted tubule or in the connecting segment in a species-dependent manner. ICs can be detected in CD from cortex to the initial part of the inner medulla, although some transport proteins that are key components of ICs also are present in medullary CD, cells considered inner medullary. Of the three types of ICs, each has a distinct morphology and expresses different complements of membrane transport proteins that translate into very different functions in homeostasis and contributions to CD luminal pro-urine composition. This review includes recent discoveries in IC intracellular and paracrine signaling that contributes to acid-base regulation as well as Na+, Cl-, K+, and Ca2+ homeostasis. Thus, these new findings highlight the potential role of ICs as targets for potential hypertension treatments.

Using Ex Vivo Kidney Slices to Study AMPK Effects on Kidney Proteins

The ex vivo kidney slice technique has been used extensively in the fields of kidney physiology and cell biology. Our group and others have used this method to study epithelial traffic of transport proteins in situ in kidney tissue. In this methodology chapter, we summarize our adaptation of this classic protocol for the study of the effect of AMPK in the modulation of transport protein regulation, especially in kidney epithelial cells. Briefly, slices were obtained by sectioning freshly harvested rodent (rat or mouse) kidneys using a Stadie-Riggs tissue slicer. The harvested kidney and the kidney slices are kept in a physiological buffer equilibrated with 5% CO₂ at body temperature (37 °C) in the presence of different AMPK activating agents vs. vehicle control followed by rapid freezing or fixation of the slices to prevent non-specific AMPK activation. Thus, homogenates of these frozen slices can be used to study AMPK activation status in the tissue as well as the downstream effects of AMPK on kidney proteins via biochemical techniques, such as immunoblotting and immunoprecipitation. Alternatively, the fixed slices can be used to evaluate AMPK-mediated subcellular traffic changes of epithelial transport proteins via immunolabeling followed by confocal microscopy. The resulting micrographs can then be used for systematic quantification of AMPK-induced changes in subcellular localization of transport proteins.

Collecting duct intercalated cell function and regulation

Intercalated cells are kidney tubule epithelial cells with important roles in the regulation of acid-base homeostasis. However, in recent years the understanding of the function of the intercalated cell has become greatly enhanced and has shaped a new model for how the distal segments of the kidney tubule integrate salt and water reabsorption, potassium homeostasis, and acid-base status. These cells appear in the late distal convoluted tubule or in the connecting segment, depending on the species. They are most abundant in the collecting duct, where they can be detected all the way from the cortex to the initial part of the inner medulla. Intercalated cells are interspersed among the more numerous segment-specific principal cells. There are three types of intercalated cells, each having distinct structures and expressing different ensembles of transport proteins that translate into very different functions in the processing of the urine. This review includes recent findings on how intercalated cells regulate their intracellular milieu and contribute to acid-base regulation and sodium, chloride, and potassium homeostasis, thus highlighting their potential role as targets for the treatment of hypertension. Their novel regulation by paracrine signals in the collecting duct is also discussed. Finally, this article addresses their role as part of the innate immune system of the kidney tubule.

Renal acid-base regulation: new insights from animal models

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

Regulation of large-conductance Ca2+-activated K+ channels by WNK4 kinase

Large-conductance, Ca(2+)-activated K(+) channels, commonly referred to as BK channels, have a major role in flow-induced K(+) secretion in the distal nephron. With-no-lysine kinase 4 (WNK4) is a serine-threonine kinase expressed in the distal nephron that inhibits ROMK activity and renal K(+) secretion. WNK4 mutations have been described in individuals with familial hyperkalemic hypertension (FHHt), a Mendelian disorder characterized by low-renin hypertension and hyperkalemia. As BK channels also have an important role in renal K(+) secretion, we examined whether they are regulated by WNK4 in a manner similar to ROMK. BK channel activity was inhibited in a rabbit intercalated cell line transfected with WNK4 or a WNK4 mutant found in individuals with FHHt. Coexpression of an epitope-tagged BK α-subunit with WNK4 or the WNK4 mutant in HEK293 cells reduced BK α-subunit plasma membrane and whole cell expression. A region within WNK4 encompassing the autoinhibitory domain and a coiled coil domain was required for WNK4 to inhibit BK α-subunit expression. The relative fraction of BK α-subunit that was ubiquitinated was significantly increased in cells expressing WNK4, compared with controls. Our results suggest that WNK4 inhibits BK channel activity, in part, by increasing channel degradation through an ubiquitin-dependent pathway. Based on these results, we propose that WNK4 provides a cellular mechanism for the coordinated regulation of two key secretory K(+) channels in the distal nephron, ROMK and BK.

AMP-activated protein kinase regulates the vacuolar H+-ATPase via direct phosphorylation of the A subunit (ATP6V1A) in the kidney

The vacuolar H(+)-ATPase (V-ATPase) in intercalated cells contributes to luminal acidification in the kidney collecting duct and nonvolatile acid excretion. We previously showed that the A subunit in the cytoplasmic V1 sector of the V-ATPase (ATP6V1A) is phosphorylated by the metabolic sensor AMP-activated protein kinase (AMPK) in vitro and in kidney cells. Here, we demonstrate that treatment of rabbit isolated, perfused collecting ducts with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) inhibited V-ATPase-dependent H(+) secretion from intercalated cells after an acid load. We have identified by mass spectrometry that Ser-384 is a major AMPK phosphorylation site in the V-ATPase A subunit, a result confirmed by comparing AMPK-dependent phosphate labeling of wild-type A-subunit (WT-A) with that of a Ser-384-to-Ala A subunit mutant (S384A-A) in vitro and in intact HEK-293 cells. Compared with WT-A-expressing HEK-293 cells, S384A-A-expressing cells exhibited greater steady-state acidification of HCO3(-)-containing media. Moreover, AICAR treatment of clone C rabbit intercalated cells expressing the WT-A subunit reduced V-ATPase-dependent extracellular acidification, an effect that was blocked in cells expressing the phosphorylation-deficient S384A-A mutant. Finally, expression of the S384A-A mutant prevented cytoplasmic redistribution of the V-ATPase by AICAR in clone C cells. In summary, direct phosphorylation of the A subunit at Ser-384 by AMPK represents a novel regulatory mechanism of the V-ATPase in kidney intercalated cells. Regulation of the V-ATPase by AMPK may couple V-ATPase activity to cellular metabolic status with potential relevance to ischemic injury in the kidney and other tissues.

Cell biology of the intercalated cell in the kidney

The intercalated cell of the collecting tubule of the mammalian kidney is specialized for the transport of H(+) and HCO3. They exist in two forms; one specialized for acid secretion and the other secretes HCO3 into the urine. We discovered many years ago that feeding animals an acid diet converts the HCO3 secreting form to an acid secreting type. Here I discuss the molecular basis of this transformation. The conversion of the cell types is mediated by an extracellular matrix protein hensin (also known as DMBT1). However much remains to be identified in the differentiation of these cells.

Differentiation of intercalated cells in the kidney

The intercalated cell of collecting ducts of the kidney is of two forms, the α form secretes acid, whereas the β form secretes HCO(3). Here, we review recent work that shows that the α form is derived from the β form and that the pathway is mediated by an extracellular matrix protein called hensin/DMBT1.

Terminal differentiation in epithelia: the role of integrins in hensin polymerization

Epithelia, the most abundant cell type, differentiate to protoepithelia from stem cells by developing apical and basolateral membrane domains and form sheets of cells connected by junctions. Following this differentiation step, the cells undergo a second step (terminal differentiation), during which they acquire a mature phenotype, which unlike the protoepithelial one is tissue and organ specific. An extracellular matrix (ECM) protein termed hensin (DMBT1) mediates this differentiation step in the kidney intercalated cells. Although hensin is secreted as a soluble monomer, it requires polymerization and deposition in the ECM to become active. The polymerization step is mediated by the activation of inside-out signaling by integrins and by the secretion of two proteins: cypA (a cis-trans prolyl isomerase) and galectin 3.

PKA regulates vacuolar H+-ATPase localization and activity via direct phosphorylation of the a subunit in kidney cells

The vacuolar H(+)-ATPase (V-ATPase) is a major contributor to luminal acidification in epithelia of Wolffian duct origin. In both kidney-intercalated cells and epididymal clear cells, cAMP induces V-ATPase apical membrane accumulation, which is linked to proton secretion. We have shown previously that the A subunit in the cytoplasmic V(1) sector of the V-ATPase is phosphorylated by protein kinase A (PKA). Here we have identified by mass spectrometry and mutagenesis that Ser-175 is the major PKA phosphorylation site in the A subunit. Overexpression in HEK-293T cells of either a wild-type (WT) or phosphomimic Ser-175 to Asp (S175D) A subunit mutant caused increased acidification of HCO(3)(-)-containing culture medium compared with cells expressing vector alone or a PKA phosphorylation-deficient Ser-175 to Ala (S175A) mutant. Moreover, localization of the S175A A subunit mutant expressed in HEK-293T cells was more diffusely cytosolic than that of WT or S175D A subunit. Acute V-ATPase-mediated, bafilomycin-sensitive H(+) secretion was up-regulated by a specific PKA activator in HEK-293T cells expressing WT A subunit in HCO(3)(-)-free buffer. In cells expressing the S175D mutant, V-ATPase activity at the membrane was constitutively up-regulated and unresponsive to PKA activators, whereas cells expressing the S175A mutant had decreased V-ATPase activity that was unresponsive to PKA activation. Finally, Ser-175 was necessary for PKA-stimulated apical accumulation of the V-ATPase in a polarized rabbit cell line of collecting duct A-type intercalated cell characteristics (Clone C). In summary, these results indicate a novel mechanism for the regulation of V-ATPase localization and activity in kidney cells via direct PKA-dependent phosphorylation of the A subunit at Ser-175.

Role of integrins in the assembly and function of hensin in intercalated cells

Epithelial differentiation proceeds in at least two steps: Conversion of a nonepithelial cell into an epithelial sheet followed by terminal differentiation into the mature epithelial phenotype. It was recently discovered that the extracellular matrix (ECM) protein hensin is able to convert a renal intercalated cell line from a flat, squamous shape into a cuboidal or columnar epithelium. Global knockout of hensin in mice results in embryonic lethality at the time that the first columnar cells appear. Here, antibodies that either activate or block integrin beta1 were used to demonstrate that activation of integrin alpha v beta 1 causes deposition of hensin in the ECM. Once hensin polymerizes and deposits into the ECM, it binds to integrin alpha 6 and mediates the conversion of epithelial cells to a cuboidal phenotype capable of apical endocytosis; therefore, multiple integrins play a role in the terminal differentiation of the intercalated cell: alpha v beta 1 generates polymerized hensin, and another set of integrins (containing alpha 6) mediates signals between hensin and the interior of the cells.

Endothelin signaling pathways in rat adrenal medulla

1. We further characterized the effect of endothelins (ETs) on receptor-mediated phosphoinositide (PI) turnover, nitric oxide synthase (NOS) activation, and cGMP formation in whole rat adrenal medulla. 2. The PI hydrolysis was assessed as accumulation of inositol monophosphates (InsP(1)) in the presence of 10 mM LiCl in whole tissue and the analysis of inositol-1-phosphate by Dowex anion exchange chromatography. NOS activity was assayed by monitoring the conversion of radiolabeled L-arginine to L-citrulline. Cyclic GMP formation was assessed as accumulation of cGMP in whole tissue in the presence of phosphodiesterase inhibition, and the amount of cGMP formed was determined by radioimmuno-antibody procedure. 3. ET-1 and ET-3 increased PI turnover by 30% in whole adrenal medulla prelabeled with [(3)H] myoinositol. Both ETs isoforms, at equimolar doses, increased NOS activity and cGMP levels in similar degree. The selective ET(B) receptor agonist, IRL-1620, also increased cGMP formation, mimicking the effects of ETs, while IRL-1620 did not alter the PI metabolism. ETs-induced InsP(1) accumulation and cGMP was dependent on extracellular calcium. The effect of ETs on PI turnover was inhibited by neomycin. The L-arginine analogue, N-nitro-L-arginine (L-NAME), and two inhibitors of soluble guanylyl cyclase, methylene blue and ODQ, significantly inhibited the increase in cGMP production induced by ETs or IRL-1620. The selective ET(A) receptor antagonist, BQ 123, inhibited the ETs-induced increase in PI turnover, while the selective ET(B) receptor antagonist, BQ 788, was ineffective. Likewise, BQ 788, significantly inhibited ET-1- or ET-3-induced NOS activation and cGMP generation but not ETs-induced InsP(1) accumulation. 4. Our data indicate that stimulation of PI turnover and NO-induced cGMP generation constitutes ETs signaling pathways in rat adrenal medulla. The former action is mediated through activation of ET(A) receptor, while the latter through the activation of ET(B) receptor. These results support the role of endothelins in the regulation of adrenal medulla function.

Galectin-3 expression is induced in renal β-intercalated cells during metabolic acidosis

The adaptation of the cortical collecting duct (CCD) to metabolic acidosis requires the polymerization and deposition in the extracellular matrix of the novel protein hensin. HCO3−-secreting β-intercalated cells remove apical Cl−:HCO3−exchangers and may reverse functional polarity to secrete protons. Using intercalated cells in culture, we found that galectin-3 facilitated hensin polymerization, thereby causing their differentiation into the H+-secreting cell phenotype. We examined the expression of galectin-3 in the rabbit kidney and its relationship to hensin during metabolic acidosis. In control kidneys, galectin-3 was expressed in the cortical and medullary collecting ducts. In the outer cortex 26 ± 3% of CCD cells expressed galectin-3 compared with 64 ± 3% of the cells of the inner cortex. In the CCD, galectin-3 was rarely expressed in β-intercalated cells, being primarily present in α-intercalated and principal cells. During metabolic acidosis, the intensity of cellular staining for galectin-3 increased and more cells began to express it; the percentage of CCD cells expressing galectin-3 increased from 26 ± 3 to 66 ± 3% in the outer cortex and from 64 ± 3 to 78 ± 4% in the inner cortex. This was particularly evident in β-intercalated cells where expression was found in only 8 ± 2% in control animals but in 75 ± 2% during metabolic acidosis in the outer cortex and similarly for the inner cortex (26 ± 6 to 90 ± 7%). Importantly, both galectin-3 and hensin were found in the extracellular matrix of microdissected CCDs; and during metabolic acidosis, many more cells exhibited this extracellular colocalization. Thus galectin-3 may play several important roles in the CCD, including mediating the adaptation of β-intercalated cells during metabolic acidosis.

Role of hensin in mediating the adaptation of the cortical collecting duct to metabolic acidosis

PURPOSE OF REVIEW

The cortical collecting duct is able to secrete HCO3-, a state that can be converted to acid secretion during metabolic acidosis. Bicarbonate secretion in this segment is mediated by beta-intercalated cells whereas alpha-intercalated cells perform acid secretion. During metabolic acidosis, the number of beta-intercalated cells is reduced while that of alpha-intercalated cells increases without a change in the total number of intercalated cells, suggesting conversion of one cell type to another. Using an immortalized intercalated cell line we found that this adaptation is mediated by an extracellular protein named hensin. Hensin is secreted as a monomer which is then polymerized in the extracellular environment by a complex process requiring at least three other proteins.

RECENT FINDINGS

We describe that a cyclophilin, via its cis/trans prolyl isomerase activity, is required for this polymerization. This may explain the distal renal tubular acidosis observed with cyclosporin A therapy. In addition, galectin-3 is needed to aggregate the protein. Finally, we recently found that activation of integrins is also necessary for the development of the hensin fiber. Hensin is expressed in all epithelia and deletion of its gene is embryonic lethal at an early stage when the first columnar epithelia develop.

SUMMARY

These studies suggest that the response of intercalated cells to metabolic acidosis uses a pathway that is involved in terminal differentiation of columnar epithelia.

Terminal differentiation of intercalated cells: the role of hensin

During the response to metabolic acidosis, the intercalated cell of the collecting tubule converts from one that secretes HCO3(-) to one that absorbs HCO3(-) by H(+) secretion. The molecular basis of this complex change in phenotype was studied in an immortalized intercalated cell line. We found that it was induced by secretion, polymerization, and deposition of a protein, which we termed hensin, into the extracellular matrix. Surprisingly, this change in phenotype is identical to terminal differentiation of epithelial cells in that it recapitulated all the characteristics of terminal differentiation, including a change in cell shape, acquisition of specialized apical structures (microvilli and ruffles), and the ability to secrete and endocytose materials in a regulated manner from the apical membrane. Hensin is expressed in most epithelia, and others have discovered that it is deleted in a large number of epithelial tumors. These results suggest that conversion of polarity of the intercalated cells represents a process of terminal differentiation.

Terminal differentiation of epithelia from trophectoderm to the intercalated cell: the role of hensin

The intercalated cells of the collecting tubules of mammalian kidneys were discovered by Haggege and Richet to change their morphology in response to a variety of physiologic stimuli related to changes in acid base status. Recent studies showed that the conversion of beta to alpha intercalated cell under the influence of acidification of the medium is due to the deposition of hensin in the extracellular matrix of these cells and activation of a novel inductive signal transduction pathway. The conversion of beta to alpha cells is shown to be a process of terminal differentiation. Hensin is secreted as a monomer, and activation of the cell induces two activities that convert it to a dimer by folding and into a fiber by bundling of the folded dimers by galectin 3. Only the fiber is functional. Hensin is expressed in most epithelial cells, and its staining pattern suggests that it might be involved in the terminal differentiation of most epithelia. There is loss of heterozygosity of hensin in a large number of epithelial and neural tumors, making it likely that it is a tumor suppressor gene.

Endothelin-1 in chronic renal failure and hypertension

Investigation into the role of endothelin-1 (ET-1) in renal function has revealed two major direct actions leading to the control of extracellular volume and blood pressure. These are the regulation of renal hemodynamics and glomerular filtration rate and the modulation of sodium and water excretion. In the rat remnant kidney model of chronic renal failure, ET-1 production is increased in blood vessels and renal tissues. These changes are related to an increase in preproET-1 expression and correlate with the rise in blood pressure, the development of cardiovascular hypertrophy, and the degree of renal insufficiency and injury. Selective ETA receptor blockade prevents the progression of hypertension and the vascular and renal damage, supporting a role for ET-1 in chronic renal failure progression. The increase in ET-1 production can be associated with other local mediators, including angiotensin II, transforming growth factor-beta1 and nitric oxide, the local production of which is also altered in chronic renal failure. In human patients with essential hypertension, atherosclerosis, and nephrosclerosis, plasma ET-1 levels are increased compared with patients with uncomplicated essential hypertension. Similarly, plasma ET-1 concentrations are markedly increased in patients with end-stage renal disease undergoing dialysis, and this correlates with blood pressure, suggesting that ET-1 may contribute to hypertension in these patients. The treatment of anemia in patients with renal failure with human recombinant erythropoietin increases blood pressure by accentuating the underlying endothelial dysfunction and the elevated vascular ET-1 production. Overall, these results support a role for ET-1 in hypertension and the end-organ damage associated with chronic renal failure. ETA receptor blockade may then represent a potential target for the management of hypertension and cardiovascular and renal protection.

Terminal Differentiation of Epithelia

All epithelia form sheets of cells connected by tight and adherent junctions and exhibit polarized distribution of membrane proteins and lipids. During their development, epithelia progress from this 'generic' phenotype to terminally differentiated states characterized by the development of apical structures such as microvilli, apical endocytosis and regulated exocytosis as well as characteristic cell shapes. We have identified an extracellular matrix protein, hensin, which when polymerized into a fiber induces the terminal differentiation of renal cells. Hensin is expressed in most epithelia where it exists in tissue-specific alternately spliced forms. Many epithelial tumors have deletions in the human ortholog of hensin. We propose that hensin mediates terminal differentiation of these epithelia.

Metanephric mesenchyme contains embryonic renal stem cells

Renal epithelial cells derive from either cells of the metanephric mesenchyme or ureteric bud cells, but the origin of other renal cells is unclear. To test whether metanephric mesenchymal cells generate cells other than epithelial, we examined the developmental potential of a metanephric mesenchymal cell line (7.1.1 cells) and of primary cultures of metanephric mesenchymal cells. 7.1.1 Cells express both mesenchymal and epithelial markers and, on confluence, form well-defined monolayers expressing epithelial junctional proteins. However, 7.1.1 cells as well as primary cultures of metanephric mesenchymal cells also generate spindle-shaped cells that are positive for alpha-smooth muscle actin, indicating that they are myofibroblasts and/or smooth muscle; this differentiation pathway is inhibited by collagen IV and enhanced by fetal calf serum or transforming growth factor-beta(1). Transforming growth factor-beta(1) also induces expression of smooth muscle proteins, indicating that the cells differentiate into smooth muscle. 7.1.1 Cells as well as primary cultures of metanephric mesenchymal cells also express vascular endothelial growth factor receptor 2 and Tie-2, suggesting that the metanephric mesenchymal cells that generate epithelia may also differentiate into endothelial cells. The pluripotency of the 7.1.1 cells is self-renewing. The data suggest that the metanephric mesenchyme contains embryonic renal stem cells.

Acid incubation reverses the polarity of intercalated cell transporters, an effect mediated by hensin

Metabolic acidosis causes a reversal of polarity of HCO(3)(-) flux in the cortical collecting duct (CCD). In CCDs incubated in vitro in acid media, beta-intercalated (HCO(3)(-)-secreting) cells are remodeled to functionally resemble alpha-intercalated (H(+)-secreting) cells. A similar remodeling of beta-intercalated cells, in which the polarity of H(+) pumps and Cl(-)/HCO(3)(-) exchangers is reversed, occurs in cell culture and requires the deposition of polymerized hensin in the ECM. CCDs maintained 3 h at low pH ex vivo display a reversal of HCO(3)(-) flux that is quantitatively similar to an effect previously observed in acid-treated rabbits in vivo. We followed intracellular pH in the same beta-intercalated cells before and after acid incubation and found that apical Cl/HCO(3) exchange was abolished following acid incubation. Some cells also developed basolateral Cl(-)/HCO(3)(-) exchange, indicating a reversal of intercalated cell polarity. This adaptation required intact microtubules and microfilaments, as well as new protein synthesis, and was associated with decreased size of the apical surface of beta-intercalated cells. Addition of anti-hensin antibodies prevented the acid-induced changes in apical and basolateral Cl(-)/HCO(3)(-) exchange observed in the same cells and the corresponding suppression of HCO(3)(-) secretion. Acid loading also promoted hensin deposition in the ECM underneath adapting beta-intercalated cells. Hence, the adaptive conversion of beta-intercalated cells to alpha-intercalated cells during acid incubation depends upon ECM-associated hensin.

Endothelin: what does the radiologist need to know?

Endothelin (ET) is a potent endogenous vasoconstrictor peptide. It has been implicated in various pathological states since its discovery in 1988. The cardiovascular system and the kidneys are important sites for the action of this peptide. Two types of ET receptor, ETA and ETB, govern the biological effects of ET. Drugs that can prevent the endogenous synthesis of ET or block its binding to receptors may offer important therapeutic impact to patients with congestive heart failure, pulmonary hypertension and acute renal failure. Areas of particular interest to the radiologist include the role of ET in mediating some of the side effects of contrast media, particularly contrast medium nephropathy, and the involvement of ET in the pathogenesis of restenosis following angioplasty. This review outlines the basic biology of this important mediator and its role in health and disease.

Renal endothelin in hypertension

Due to the potent vasoconstrictor action of endothelin-1 and its synthesis throughout the vasculature and other tissues, most investigators believe that it is an active participant in the pathogenesis of hypertension. However, the autocrine and paracrine nature of the endothelin system has made its role difficult to define. In recent years, it has become apparent that endothelin-1 contributes to the regulation of renal salt and water excretion and that it is a major contributor to the hypertension associated with salt-dependency. Evidence suggests that endothelin-1 within the renal medulla is activated in conditions of salt loading and inhibits reabsorption of sodium in a nitric oxide-dependent manner. Blockade of endothelin A receptors lowers arterial pressure in animal models of salt-dependent hypertension. Furthermore, circulating levels of endothelin-1 are generally higher in African-Americans compared to white Americans as is the prevalence of salt-dependent hypertension. Therefore, it would appear that use of endothelin A-selective receptor antagonists should be targeted to those individuals at risk for salt-dependent hypertension. Blockade of endothelin B receptors would not be desirable because of their important role in eliminating a salt load.

Hensin, the polarity reversal protein, is encoded by DMBT1, a gene frequently deleted in malignant gliomas

The band 3 anion exchanger is located in the apical membrane of a beta-intercalated clonal cell line, whereas the vacuolar H(+)-ATPase is present in the basolateral membrane. When these cells were seeded at confluent density, they converted to an alpha-phenotype, localizing each of these proteins to the opposite cell membrane domain. The reversal of polarity is induced by hensin, a 230-kDa extracellular matrix protein. Rabbit kidney hensin is a multidomain protein composed of eight SRCR ("scavenger receptor, cysteine rich"), two CUB ("C1r/C1s Uegf Bmp1"), and one ZP ("zona pellucida") domain. Other proteins known to have these domains include CRP-ductin, a cDNA expressed at high levels in mouse intestine (8 SRCR, 5 CUB, 1 ZP), ebnerin, a protein cloned from a rat taste bud library (4 SRCR, 3 CUB, 1 ZP), and DMBT1, a sequence in human chromosome 10q25-26 frequently deleted in malignant gliomas (9 SRCR, 2 CUB, 1 ZP). Rabbit and mouse hensin genomic clones contained a new SRCR that was not found in hensin cDNA but was homologous to the first SRCR domain in DMBT1. Furthermore, the 3'-untranslated regions and the signal peptide of hensin were homologous to those of DMBT1. Mouse genomic hensin was localized to chromosome 7 band F4, which is syntenic to human 10q25-26. These data suggest that hensin and DMBT1 are alternatively spliced forms of the same gene. The analysis of mouse hensin bacterial artificial chromosome (BAC) genomic clone by sequencing and Southern hybridization revealed that the gene also likely encodes CRP-ductin. A new antibody against the mouse SRCR1 domain recognized a protein in the mouse and rabbit brain but not in the immortalized cell line or kidney, whereas an antibody to SRCR6 and SRCR7 domains which are present in all the transcripts, recognized proteins in intestine, kidney, and brain from several species. The most likely interpretation of these data is that one gene produces at least three transcripts, namely, hensin, DMBT1, and CRP-ductin. Hensin may participate in determining the polarized phenotype of other epithelia and brain cells.

Only multimeric hensin located in the extracellular matrix can induce apical endocytosis and reverse the polarity of intercalated cells

When an intercalated epithelial cell line was seeded at low density and allowed to reach confluence, it located the anion exchanger band 3 in the apical membrane and an H+-ATPase in the basolateral membrane. The same clonal cells seeded at high density targeted these proteins to the reverse location. Furthermore, high density cells had vigorous apical endocytosis, and low density cells had none. The extracellular matrix of high density cells was capable of inducing apical endocytosis and relocation of band 3 to the basolateral membrane in low density cells. A 230-kDa extracellular matrix (ECM) protein termed hensin, when purified to near-homogeneity, was able to reverse the phenotype of the low density cells. Antibodies to hensin prevented this effect, indicating that hensin is necessary for conversion of polarity. We show here that hensin was synthesized by both low density and high density cells. Whereas both phenotypes secreted soluble hensin into their media, only high density cells localized it in their ECM. Analysis of soluble hensin by sucrose density gradients showed that low density cells secreted monomeric hensin, and high density cells secreted higher order multimers. When 35S-labeled monomeric hensin was added to high density cells, they induced its aggregation suggesting that the multimerization was catalyzed by surface events in the high density cells. Soluble monomeric or multimeric hensin did not induce apical endocytosis in low density cells, whereas the more polymerized hensin isolated from insoluble ECM readily induced it. These multimers could be disaggregated by sulfhydryl reagents and by dimethylmaleic anhydride, and treatment of high density ECM by these reagents prevented the induction of endocytosis. These results demonstrate that hensin, like several ECM proteins, needs to be precipitated in the ECM to be functional.

Actions of endothelin-1 on cultured rat renomedullary interstitial cells are modulated by nitric oxide

1. Cultured renomedullary interstitial cells (RMIC) isolated from 4-week-old Sprague-Dawley rat kidneys possess ETA receptors, as identified by reverse transcription-polymerase chain reaction (RT-PCR). 2. Treatment with endothelin (ET)-1 (10(-6) mol/L) increases the intracellular inositol 1,4,5-trisphosphate concentrations within 10 s and intracellular calcium concentrations after 7 s. 3. Endothelin-1 (10(-7) and 10(-10) mol/L) induced increases in intracellular cAMP concentrations, but only in the presence of N omega-nitro-L-arginine, a nitric oxide synthase (NOS) inhibitor. Addition of ET-1 (10(-10) mol/L) to the RMIC culture led to increases in intracellular cGMP concentrations through activation of NOS. 4. In the presence of ET-1 (10(-7) and 10(-10) mol/L) and during NOS inhibition, RMIC responded with increased cell proliferation and extracellular matrix (ECM) synthesis. These responses were abolished by BQ-123 (10(-6) mol/L), suggesting mediation via the ETA receptor subtype. The proliferative effect of ET-1 was also abolished by atrial natriuretic peptide (10(-6) mol/L). 5. The present study provides evidence that binding of ET-1 to ETA receptors on RMIC activates several intracellular second messenger systems that mediate cell proliferation and ECM synthesis. 6. These results also highlight an important interaction between ET-1 and nitric oxide in the control of RMIC function.

Hensin remodels the apical cytoskeleton and induces columnarization of intercalated epithelial cells: processes that resemble terminal differentiation

Intercalated epithelial cells exist in a spectrum of phenotypes; at one extreme, beta cells secrete HCO3 by an apical Cl/HCO3 exchanger and a basolateral H+ ATPase. When an immortalized beta cell line is seeded at high density it deposits in its extracellular matrix (ECM) a new protein, hensin, which can reverse the polarity of several proteins including the Cl/HCO3 exchanger (an alternately spliced form of band 3) and the proton translocating ATPase. When seeded at low density and allowed to form monolayers these polarized epithelial cells maintain the original distribution of these two proteins. Although these cells synthesize and secrete hensin, it is not retained in the ECM, but rather, hensin is present in a large number of intracellular vesicles. The apical cytoplasm of low density cells is devoid of actin, villin, and cytokeratin19. Scanning electron microscopy shows that these cells have sparse microvilli, whereas high density cells have exuberant apical surface infolding and microvilli. The apical cytoplasm of high density cells contains high levels of actin, cytokeratin19, and villin. The cell shape of these two phenotypes is different with high density cells being tall with a small cross-sectional area, whereas low density cells are low and flat. This columnarization and the remodeling of the apical cytoplasm is hensin-dependent; it can be induced by seeding low density cells on filters conditioned by high density cells and prevented by an antibody to hensin. The changes in cell shape and apical cytoskeleton are reminiscent of the processes that occur in terminal differentiation of the intestine and other epithelia. Hensin is highly expressed in the intestine and prostate (two organs where there is a continuous process of differentiation). The expression of hensin in the less differentiated crypt cells of the intestine and the basal cells of the prostate is similar to that of low density cells; i.e., abundant intracellular vesicles but no localization in the ECM. On the other hand, as in high density cells hensin is located exclusively in the ECM of the terminally differentiated absorptive villus cells and the prostatic luminal cell. These studies suggest that hensin is a critical new molecule in the terminal differentiation of intercalated cell and perhaps other epithelial cells.

Activation of particulate guanylyl cyclase by endothelins in cultured SV-40 transformed cat iris sphincter smooth muscle cells

We investigated the effects of endothelins (ETs) on cGMP production in cultured SV-40 transformed cat iris sphincter smooth muscle (SV-CISM-2) cells. ET-3 increased cGMP formation in a concentration-dependent manner (EC50 = 98nM), which was 2.5 times higher than that of ET-1. The ET(B)receptor agonists sarafotoxin-S6c and IRL 1620 also increased cGMP production, mimicking the effects of the ETs. The ET(B) receptor antagonist BQ 788, but not the ET(A) receptor antagonist BQ610, dose-dependently blocked ET-3-stimulated cGMP formation (IC50=10nM). The phorbol ester, Phorbol 12, 13-dibutyrate (PDBu), which inhibits particulate guanylyl cyclase in smooth muscle, dose-dependently inhibited ET-3-stimulated cGMP accumulation (IC50=66nM). LY83583 and ODQ, inhibitors of soluble guanylyl cyclases, as well as inhibitors of the nitric oxide cascade and of intracellular Ca2+ elevation had no appreciable effect on ET-3-induced cGMP production. ET-3 markedly inhibited carbachol-induced intracellular Ca2+ mobilization. We conclude that ET-3 increases intracellular cGMP levels in SV-CISM-2 cells through activation of the ET(B) receptor subtype and subsequent stimulation of the membrane-bound guanylyl cyclase. Elevation of cGMP by ET and the subsequent inhibition of muscarinic stimulation of intracellular Ca2+ mobilization by the cyclic nucleotide could serve to modulate the contractile effects of Ca2+-mobilizing agonists in the iris sphincter smooth muscle.

Phenotypic plasticity in the intercalated cell: the hensin pathway

The collecting duct of the renal tubule contains two cell types, one of which, the intercalated cell, is responsible for acidification and alkalinization of urine. These cells exist in a multiplicity of morphological forms, with two extreme types, alpha and beta. The former acidifies the urine by an apical proton-translocating ATPase and a basolateral Cl/HCO3 exchanger, which is an alternately spliced form of band 3. This kidney form of band 3, kAE1, is present in the apical membrane of the beta-cell, which has the H+-ATPase on the basolateral membrane. We had suggested previously that metabolic acidosis leads to conversion of beta-types to alpha-types. To study the biochemical basis of this plasticity, we used an immortalized cell line of the beta-cell and showed that these cells convert to the alpha-phenotype when plated at superconfluent density. At high density these cells localize a new protein, which we term "hensin," to the extracellular matrix, and hensin acts as a molecular switch capable of changing the phenotype of these cells in vitro. Hensin induces new cytoskeletal proteins, makes the cells assume a more columnar shape and retargets kAE1 and the H+-ATPase. These recent studies suggest that the conversion of beta- to alpha-cells, at least in vitro, bears many of the hallmarks of terminal differentiation.

The effect of apical and basolateral lipids on the function of the band 3 anion exchange protein

Although many polarized proteins are sorted to the same membrane domain in all epithelial tissues, there are some that exhibit a cell type-specific polarity. We recently found that band 3 (the anion exchanger AE1) was present in the apical membrane of a renal intercalated cell line when these cells were seeded at low density, but its targeting was reversed to the basolateral membrane under the influence of an extracellular matrix protein secreted when the cells were seeded at high density. Because apical and basolateral lipids differ in epithelia, we asked what effect might these lipids have on band 3 function. This question is especially interesting since apical anion exchange in these cells is resistant to disulfonic stilbene inhibitors while basolateral anion exchange is quite sensitive. Furthermore, the apical anion exchanger cannot be stained by antibodies that readily identify the basolateral protein. We used short chain sphingolipid analogues and found that sphingomyelin was preferentially targeted to the basolateral domain in the intercalated cell line. The ganglioside GM1 (Gal 1beta1, 3GalNAcbeta1, 4Gal-NeuAcalpha2, 3Galbeta1, 4Glc ceramide) was confined to the apical membrane as visualized by confocal microscopy after addition of fluorescent cholera toxin to filter grown cells. We reconstituted erythrocyte band 3 into liposomes using apical and basolateral types of lipids and examined the inhibitory potency of 4, 4'-dinitorsostilbene-2,2'-disulfonic acid (DNDS; a reversible stilbene) on 35SO4/SO4 exchange. Although anion exchange in sphingomyelin liposomes was sensitive to inhibition, the addition of increasing amounts of the ganglioside GM1 reduced the potency of the inhibitor drastically. Because these polarized lipids are present in the exofacial surface of the bilayer, we propose that the lipid structure might influence the packing of the transmembrane domains of band 3 in that region, altering the binding of the stilbenes to these chains. These results highlight the role of polarized lipids in changing the function of unpolarized proteins or of proteins whose locations differ in different epithelia.

Hensin, a new collecting duct protein involved in the in vitro plasticity of intercalated cell polarity

Two forms of intercalated cells are present in kidney collecting tubules, the alpha cell has apical endocytosis, apical H+-ATPase and basolateral band 3, while beta cells have reversed polarity of these proteins and no apical endocytosis. When a beta cell line was seeded at high density, it changed into the alpha form. We previously showed that a partially purified 230 kD extracellular matrix protein of high density cells was able to retarget band 3 from apical to basolateral domains and stimulated apical endocytosis in vitro (Van Adelsberg, J., J.C. Edwards, J. Takito, B. Kiss, and Q. Al-Awqati. 1994. Cell. 76:1053-1061). We now purify this protein, which was named hensin, to near homogeneity and find that it belongs to the macrophage scavenger receptor cysteine rich (SRCR) family. An antibody, generated against a fusion protein made from a partial cDNA recognized a 230-kD protein in rabbit kidney and in the intercalated cell line. In vitro, the hensin antibody inhibited expression of apical endocytosis. Hensin was secreted in a polarized manner and bound to the basolateral membrane and extracellular matrix. Immunohistochemistry of the kidney showed that it was expressed only in collecting tubules. Double immunofluorescence with hensin and peanut lectin, H+-ATPase, or band 3 showed many patterns; most alpha-cells had hensin staining while 50% of beta-cells did not. These results suggest that hensin may also be involved in the polarity reversal of intercalated cells in vivo.

Endothelin receptors in normal and diseased kidneys

1. Endothelin-1 (ET-1), the most potent vasoconstrictor yet identified, mediates a multitude of responses in various tissues including the kidney. The biological responses of ET-1 are mediated by specific cell surface receptors classified as ET(A) and ETB. Species differences are observed in the distribution as well as function of these ET receptors. 2. Involvement of ET has been demonstrated in a number of renal diseases, including ischaemia-induced acute renal failure, chronic renal failure, radiocontrast and cyclosporin-induced nephrotoxicity. ET antibodies as well as ET receptor antagonists have been shown to be beneficial in these disease models.

Role of endothelin in renal function and dysfunction

1. Endothelin has significant effects on renovascular, glomerular and tubular function. 2. Endothelin causes severe renal vasoconstriction, resulting in a decrease in renal blood flow and glomerular filtration rate. 3. Endothelin can inhibit sodium reabsorption and, in the rat, vasopressin-induced water transport. 4. The endothelin receptor subtypes mediating renovascular and tubular effects of endothelin may differ between species. 5. Renal endothelin production, metabolism and receptor binding is altered in a number of renal diseases, including acute and chronic renal failure and cyclosporine and radiocontrast nephrotoxicity. 6. Endothelin receptor antagonists or antibodies can attenuate the severity or progression of a number of renal diseases.

Endothelin receptor heterogeneity, G-proteins, and signaling via cAMP and cGMP cascades

1. The endothelins (ETs) are potent vasoactive peptides which are involved in diverse biological processes, such as contraction, neuromodulation, and neurotransmission, as well as in certain pathophysiological conditions including cardiac and renal failure. 2. The diversity of action of ETs may be attributed to (i) the existence of a number of receptor subtypes, and (ii) the G-protein-mediated activation of different signal transduction pathways. 3. The combined action of these two variables modulates the response, since different receptor subtypes can stimulate and/or inhibit the cAMP and cGMP cascades.

Stimulation of Na+/H+ exchange activity by endothelin in opossum kidney cells

Endothelin-1 (ET-1) controls multiple aspects of kidney function. In this study we have analysed the effects of ET-1 on apical Na+/H+ exchange activity in opossum kidney (OK) cells. ET-1 (at 10(-10) M and 10(-8) M) activated Na+/H+ exchange activity within 5 min of exposure. ET-1 (10(-8) M) prevented PTH-induced (parathyroid hormone; 10(-8) M) inhibition of Na+/H+ exchange activity; it also abolished transport inhibition in response to 10(-3) M IBMX (isobutyl-methylxanthine) and 3 x 10(-7) M TPA (phorbol 12-myristate 13-acetate), but had no effect on the 8-bromo-cAMP-induced (10(-4) M) decrease of transport rate. Basal cAMP content, IBMX- and PTH-stimulated cAMP production were unaffected by ET-1 (10(-8) M). The stimulatory action of ET-1 (10(-8) M) on Na+/H+ exchange activity was prevented by calphostin C (10(-8) M). These data document that OK cells might serve as a useful in vitro model for analysis of cellular mechanisms involved in endothelin action; proteine kinase C activation seems to participate in the observed endothelin effects.

An induced extracellular matrix protein reverses the polarity of band 3 in intercalated epithelial cells

The intercalated epithelial cell exists in two interconvertible forms in vivo, one where band 3 protein is apical and the other where it is basolateral. We seeded an immortalized clone of these cells at low density and found that band 3 was apical at confluence. There was little or no apical endocytosis. But when the cells were plated at high density, band 3 was basolateral, and there was vigorous apical endocytosis. Extracellular matrix produced by high density cells was able to retarget band 3 in low density cells and to induce apical endocytosis, as did a 230 kd protein partially purified from this matrix. Therefore, polarized targeting of some proteins is determined by external cues that might determine their polarity by reorganizing the cytoplasm.