Alkaline pH- and cAMP-induced V-ATPase membrane accumulation is mediated by protein kinase A in epididymal clear cells

Soluble adenylyl cyclase coordinates intracellular pH homeostasis and biomineralization in calcifying cells of a marine animal

Biomineralizing cells concentrate dissolved inorganic carbon (DIC) and remove protons from the site of mineral precipitation. However, the molecular regulatory mechanisms that orchestrate pH homeostasis and biomineralization of calcifying cells are poorly understood. Here, we report that the acid-base sensing enzyme soluble adenylyl cyclase (sAC) coordinates intracellular pH (pH_i) regulation in the calcifying primary mesenchyme cells (PMCs) of sea urchin larvae. Single-cell transcriptomics, in situ hybridization, and immunocytochemistry elucidated the spatiotemporal expression of sAC during skeletogenesis. Live pH_i imaging of PMCs revealed that the downregulation of sAC activity with two structurally unrelated small molecules inhibited pH_i regulation of PMCs, an effect that was rescued by the addition of cell-permeable cAMP. Pharmacological sAC inhibition also significantly reduced normal spicule growth and spicule regeneration, establishing a link between PMC pH_i regulation and biomineralization. Finally, increased expression of sAC mRNA was detected during skeleton remineralization and exposure to CO₂-induced acidification. These findings suggest that transcriptional regulation of sAC is required to promote remineralization and to compensate for acidic stress. This work highlights the central role of sAC in coordinating acid-base regulation and biomineralization in calcifying cells of a marine animal.

Purinergic signaling in the male reproductive tract

Purinergic receptors are ubiquitously expressed throughout the body and they participate in the autocrine and paracrine regulation of cell function during normal physiological and pathophysiological conditions. Extracellular nucleotides activate several types of plasma membrane purinergic receptors that form three distinct families: P1 receptors are activated by adenosine, P2X receptors are activated by ATP, and P2Y receptors are activated by nucleotides including ATP, ADP, UTP, UDP, and UDP-glucose. These specific pharmacological fingerprints and the distinct intracellular signaling pathways they trigger govern a large variety of cellular responses in an organ-specific manner. As such, purinergic signaling regulates several physiological cell functions, including cell proliferation, differentiation and death, smooth muscle contraction, vasodilatation, and transepithelial transport of water, solute, and protons, as well as pathological pathways such as inflammation. While purinergic signaling was first discovered more than 90 years ago, we are just starting to understand how deleterious signals mediated through purinergic receptors may be involved in male infertility. A large fraction of male infertility remains unexplained illustrating our poor understanding of male reproductive health. Purinergic signaling plays a variety of physiological and pathophysiological roles in the male reproductive system, but our knowledge in this context remains limited. This review focuses on the distribution of purinergic receptors in the testis, epididymis, and vas deferens, and their role in the establishment and maintenance of male fertility.

Cimetidine-induced androgenic failure causes cell death and changes in actin, EGF and V-ATPase immunoexpression in rat submandibular glands

Submandibular gland (SMG) is responsive to androgens via androgen receptor (AR). We verified whether cimetidine induces androgenic dysfunction in SMG, and evaluated the structural integrity, cell death and immunoexpression of actin, EGF and V-ATPase in androgen-deficient SMG. Male rats received cimetidine (CMTG) and control animals (CG) received saline. Granular convoluted tubules (GCTs) diameter and number of acinar cell nuclei were evaluated. TUNEL and immunofluorescence reactions for detection of AR, testosterone, actin, EGF and V-ATPase were quantitatively analysed. In CG, testosterone immunolabelling was detected in acinar and ductal cells cytoplasm. AR-immunolabelled nuclei were observed in acinar cells whereas ductal cells showed AR-immunostained cytoplasm, indicating a non-genomic AR action. In CMTG, the weak testosterone and AR immunoexpression confirmed cimetidine-induced androgenic failure. A high cell death index was correlated with decreased number of acinar cells, GCTs diameter and EGF immunoexpression under androgenic dysfunction. Actin immunofluorescence decreased in the SMG cells, but an increased and diffuse cytoplasmic V-ATPase immunolabelling was observed in striated ducts, suggesting a disruption in the actin-dependent V-ATPase recycling due to androgenic failure. Our findings reinforce the androgenic role in the maintenance of SMG histophysiology, and point to a potential clinical use of cimetidine against androgen-dependent glandular tumour cells.

Regulation and function of V-ATPases in physiology and disease

The vacuolar H⁺-ATPases (V-ATPases) are essential, ATP-dependent proton pumps present in a variety of eukaryotic cellular membranes. Intracellularly, V-ATPase-dependent acidification functions in such processes as membrane traffic, protein degradation, autophagy and the coupled transport of small molecules. V-ATPases at the plasma membrane of certain specialized cells function in such processes as bone resorption, sperm maturation and urinary acidification. V-ATPases also function in disease processes such as pathogen entry and cancer cell invasiveness, while defects in V-ATPase genes are associated with disorders such as osteopetrosis, renal tubular acidosis and neurodegenerative diseases. This review highlights recent advances in our understanding of V-ATPase structure, mechanism, function and regulation, with an emphasis on the signaling pathways controlling V-ATPase assembly in mammalian cells. The role of V-ATPases in cancer and other human pathologies, and the prospects for therapeutic intervention, are also discussed.

The Vacuolar H+ ATPase α3 Subunit Negatively Regulates Migration and Invasion of Human Pancreatic Ductal Adenocarcinoma Cells

Increased metabolic acid production and upregulation of net acid extrusion render pH homeostasis profoundly dysregulated in many cancers. Plasma membrane activity of vacuolar H+ ATPases (V-ATPases) has been implicated in acid extrusion and invasiveness of some cancers, yet often on the basis of unspecific inhibitors. Serving as a membrane anchor directing V-ATPase localization, the a subunit of the V0 domain of the V-ATPase (ATP6V0a1-4) is particularly interesting in this regard. Here, we map the regulation and roles of ATP6V0a3 in migration, invasion, and growth in pancreatic ductal adenocarcinoma (PDAC) cells. a3 mRNA and protein levels were upregulated in PDAC cell lines compared to non-cancer pancreatic epithelial cells. Under control conditions, a3 localization was mainly endo-/lysosomal, and its knockdown had no detectable effect on pHi regulation after acid loading. V-ATPase inhibition, but not a3 knockdown, increased HIF-1 expression and decreased proliferation and autophagic flux under both starved and non-starved conditions, and spheroid growth of PDAC cells was also unaffected by a3 knockdown. Strikingly, a3 knockdown increased migration and transwell invasion of Panc-1 and BxPC-3 PDAC cells, and increased gelatin degradation in BxPC-3 cells yet decreased it in Panc-1 cells. We conclude that in these PDAC cells, a3 is upregulated and negatively regulates migration and invasion, likely in part via effects on extracellular matrix degradation.

How Reciprocal Interactions Between the Tumor Microenvironment and Ion Transport Proteins Drive Cancer Progression

Solid tumors comprise two major components: the cancer cells and the tumor stroma. The stroma is a mixture of cellular and acellular components including fibroblasts, mesenchymal and cancer stem cells, endothelial cells, immune cells, extracellular matrix, and tumor interstitial fluid. The insufficient tumor perfusion and the highly proliferative state and dysregulated metabolism of the cancer cells collectively create a physicochemical microenvironment characterized by altered nutrient concentrations and varying degrees of hypoxia and acidosis. Furthermore, both cancer and stromal cells secrete numerous growth factors, cytokines, and extracellular matrix proteins which further shape the tumor microenvironment (TME), favoring cancer progression.Transport proteins expressed by cancer and stromal cells localize at the interface between the cells and the TME and are in a reciprocal relationship with it, as both sensors and modulators of TME properties. It has been amply demonstrated how acid-base and nutrient transporters of cancer cells enable their growth, presumably by contributing both to the extracellular acidosis and the exchange of metabolic substrates and waste products between cells and TME. However, the TME also impacts other transport proteins important for cancer progression, such as multidrug resistance proteins. In this review, we summarize current knowledge of the cellular and acellular components of solid tumors and their interrelationship with key ion transport proteins. We focus in particular on acid-base transport proteins with known or proposed roles in cancer development, and we discuss their relevance for novel therapeutic strategies.

Novel role of proton-secreting epithelial cells in sperm maturation and mucosal immunity

Epithelial cells are immune sensors and mediators that constitute the first line of defense against infections. Using the epididymis, a model for studying tubular organs, we uncovered a novel and unexpected role for professional proton-secreting 'clear cells' in sperm maturation and immune defense. The epididymal epithelium participates in the maturation of spermatozoa via the establishment of an acidic milieu and transfer of proteins to sperm cells, a poorly characterized process. We show that proton-secreting clear cells express mRNA transcripts and proteins that are acquired by maturing sperm, and that they establish close interactions with luminal spermatozoa via newly described 'nanotubes'. Mechanistic studies show that injection of bacterial antigens in vivo induces chemokine expression in clear cells, followed by macrophage recruitment into the organ. Injection of an inflammatory intermediate mediator (IFN-γ) increased Cxcl10 expression in clear cells, revealing their participation as sensors and mediators of inflammation. The functional diversity adopted by clear cells might represent a generalized phenomenon by which similar epithelial cells decode signals, communicate with neighbors and mediate mucosal immunity, depending on their precise location within an organ.

Reciprocal Regulation of V-ATPase and Glycolytic Pathway Elements in Health and Disease

The ability of cells to adapt to fluctuations in glucose availability is crucial for their survival and involves the vacuolar proton-translocating ATPase (V-ATPase), a proton pump found in all eukaryotes. V-ATPase hydrolyzes ATP via its V₁ domain and uses the energy released to transport protons across membranes via its V_o domain. This activity is critical for pH homeostasis and generation of a membrane potential that drives cellular metabolism. A number of stimuli have been reported to alter V-ATPase assembly in yeast and higher eukaryotes. Glucose flux is one of the strongest and best-characterized regulators of V-ATPase; this review highlights current models explaining how glycolysis and V-ATPase are coordinated in both the Saccharomyces cerevisiae model fungus and in mammalian systems. Glucose-dependent assembly and trafficking of V-ATPase, V-ATPase-dependent modulations in glycolysis, and the recent discovery that glucose signaling through V-ATPase acts as a molecular switch to dictate anabolic versus catabolic metabolism are discussed. Notably, metabolic plasticity and altered glycolytic flux are critical drivers of numerous human pathologies, and the expression and activity of V-ATPase is often altered in disease states or can be pharmacologically manipulated as treatment. This overview will specifically discuss connections between V-ATPase and glycolysis in cancer.

Relative contribution of clear cells and principal cells to luminal pH in the mouse epididymis

While spermatozoa undergo epididymal maturation, they remain quiescent thanks to the establishment of a low luminal pH. This study is aimed at determining how epithelial cells lining the epididymal lumen work together to maintain and regulate this acidic milieu. In particular, we examined the relative contribution of clear cells (CCs) and principal cells (PCs) to this process. Functional analysis in the mouse cauda epididymidis (Cd) perfused in vivo showed that the pH of a control solution remained constant at pH 6.6 after perfusion through the Cd lumen. In contrast, the pH of both an acidic (pH 5.8) and alkaline (pH 7.8) perfusate was progressively restored toward the control acidic pH. Pharmacological studies indicated the contribution of cystic fibrosis transmembrane regulator, previously shown to be present in the apical membrane of PCs, to the recovery from an acidic pH of 5.8. In addition, we found that CCs and PCs equally contribute to the recovery from an alkaline of 7.8, via the H+ pumping vacuolar ATPase (V-ATPase) located in CCs, and the Na+/H+ exchanger type 3 (NHE3) located in PCs. Immunofluorescence labeling showed apical membrane accumulation of the V-ATPase in CCs at pH 7.8, and its internalization at pH 5.8 compared to pH 6.6. Immunofluorescence showed expression of NHE3, but absence of NHE2, in PCs located in the Cd. RT-PCR and western blotting showed expression of NHE3 in all epididymal regions. Luminal 8-(4-chlorophenylthio)adenosine 3΄,5΄-cyclic monophosphate (cpt-cAMP) partially inhibited luminal pH recovery from pH 7.8. However, cpt-cAMP induced an increase in V-ATPase apical membrane accumulation at this pH. Cell fractionation studies showed the apical accumulation of NHE3 from intracellular vesicles at pH 7.8 versus 6.6, and prevention of this effect by cpt-cAMP. These results indicate the participation of both CCs and PCs in the regulation of luminal pH in the epididymis. Our study also shows the dual role of PCs in HCO3− and H+ secretion, and that this switch from base to acid secretion depends on the luminal environment. Characterization of the respective roles of CCs and PCs in the regulation of the optimal luminal condition for epididymal sperm maturation should provide new frameworks for the evaluation and treatment of male infertility.

The inhibitors of soluble adenylate cyclase 2-OHE, KH7, and bithionol compromise mitochondrial ATP production by distinct mechanisms

Soluble adenylate cyclase (sAC) is a non-plasma membrane-bound isoform of the adenylate cyclases signaling via the canonical second messenger, 3',5'-cyclic AMP (cAMP). sAC is involved in key physiological processes such as insulin release, sperm motility, and energy metabolism. Thus, sAC has attracted interest as a putative drug target and attempts have been made to develop selective inhibitors. Since sAC has a binding constant for its substrate, ATP, in the millimolar range, reductions in mitochondrial ATP production may be part of the mechanism-of-action of sAC inhibitors and the potential of these compounds to study the physiological outcomes of inhibition of sAC might be severely hampered by this. Here, we evaluate the effects of two commonly employed inhibitors, 2-OHE and KH7, on mitochondrial ATP production and energy metabolism. For comparison, we included a recently identified inhibitor of sAC, bithionol. Employing mitochondria isolated from mouse brain, we show that all three compounds are able to curb ATP production albeit via distinct mechanisms. Bithionol and KH7 mainly inhibit ATP production by working as a classical uncoupler whereas 2-OHE mainly works by decreasing mitochondrial respiration. These findings were corroborated by investigating energy metabolism in acute brain slices from mice. Since all three sAC inhibitors are shown to curb mitochondrial ATP production and affect energy metabolism, caution should be exercised when employed to study the physiological roles of sAC or for validating sAC as a drug target.

Functional Significance of the Adcy10-Dependent Intracellular cAMP Compartments

Mounting evidence confirms the compartmentalized structure of evolutionarily conserved 3'⁻5'-cyclic adenosine monophosphate (cAMP) signaling, which allows for simultaneous participation in a wide variety of physiological functions and ensures specificity, selectivity and signal strength. One important player in cAMP signaling is soluble adenylyl cyclase (sAC). The intracellular localization of sAC allows for the formation of unique intracellular cAMP microdomains that control various physiological and pathological processes. This review is focused on the functional role of sAC-produced cAMP. In particular, we examine the role of sAC-cAMP in different cellular compartments, such as cytosol, nucleus and mitochondria.

Extracellular Adenosine Stimulates Vacuolar ATPase-Dependent Proton Secretion in Medullary Intercalated Cells

Acidosis is an important complication of AKI and CKD. Renal intercalated cells (ICs) express the proton pumping vacuolar H⁺-ATPase (V-ATPase) and are extensively involved in acid-base homeostasis. H⁺ secretion in type A intercalated cells (A-ICs) is regulated by apical vesicle recycling and stimulated by cAMP. In other cell types, cAMP is increased by extracellular agonists, including adenosine, through purinergic receptors. Adenosine is a Food and Drug Administration-approved drug, but very little is known about the effect of adenosine on IC function. Therefore, we investigated the role of adenosine in the regulation of V-ATPase in ICs. Intravenous treatment of mice with adenosine or agonists of ADORA2A and ADORA2B purinergic P1 receptors induced V-ATPase apical membrane accumulation in medullary A-ICs but not in cortical A-ICs or other IC subtypes. Both receptors are located in A-IC apical membranes, and adenosine injection increased urine adenosine concentration and decreased urine pH. Cell fractionation showed that adenosine or an ADORA2A or ADORA2B agonist induced V-ATPase translocation from vesicles to the plasma membrane and increased protein kinase A (PKA)-dependent protein phosphorylation in purified medullary ICs that were isolated from mice. Either ADORA2A or ADORA2B antagonists or the PKA inhibitor mPKI blocked these effects. Finally, a fluorescence pH assay showed that adenosine activates V-ATPase in isolated medullary ICs. Our study shows that medullary A-ICs respond to luminal adenosine through ADORA2A and ADORA2B receptors in a cAMP/PKA pathway-dependent mechanism to induce V-ATPase-dependent H⁺ secretion.

Soluble adenylyl cyclase is essential for proper lysosomal acidification

Lysosomes, the degradative organelles of the endocytic and autophagic pathways, function at an acidic pH. Lysosomes are acidified by the proton-pumping vacuolar ATPase (V-ATPase), but the molecular processes that set the organelle's pH are not completely understood. In particular, pH-sensitive signaling enzymes that can regulate lysosomal acidification in steady-state physiological conditions have yet to be identified. Soluble adenylyl cyclase (sAC) is a widely expressed source of cAMP that serves as a physiological pH sensor in cells. For example, in proton-secreting epithelial cells, sAC is responsible for pH-dependent translocation of V-ATPase to the luminal surface. Here we show genetically and pharmacologically that sAC is also essential for lysosomal acidification. In the absence of sAC, V-ATPase does not properly localize to lysosomes, lysosomes fail to fully acidify, lysosomal degradative capacity is diminished, and autophagolysosomes accumulate.

Regulation of epithelial function, differentiation, and remodeling in the epididymis

The epididymis is a single convoluted tubule lined by a pseudostratified epithelium. Specialized epididymal epithelial cells, the so-called principal, basal, narrow, and clear cells, establish a unique luminal environment for the maturation and storage of spermatozoa. The epididymis is functionally and structurally divided into several segments and sub-segments that create regionally distinct luminal environments. This organ is immature at birth, and epithelial cells acquire their fully differentiated phenotype during an extended postnatal period, but the factors involved in this complex process remain incompletely characterized. In the adult epididymis, the establishment of an acidic luminal pH and low bicarbonate concentration in the epididymis contributes to preventing premature activation of spermatozoa during their maturation and storage. Clear cells are proton-secreting cells throughout the epididymis, but principal cells have distinct acid/base transport properties, depending on their localization within the epididymis. Basal cells are located in all epididymal segments, but they have a distinct morphology depending on the segment and species examined. How this structural plasticity of basal cells is regulated is discussed here. Also, the role of luminal factors and androgens in the regulation of epithelial cells is reviewed in relation to their respective localization in the proximal versus distal regions of the epididymis. Finally, we describe a novel role for CFTR in tubulogenesis and epithelial cell differentiation.

Activation of the metabolic sensor AMP-activated protein kinase inhibits aquaporin-2 function in kidney principal cells

Aquaporin-2 (AQP2) is essential to maintain body water homeostasis. AQP2 traffics from intracellular vesicles to the apical membrane of kidney collecting duct principal cells in response to vasopressin [arginine vasopressin (AVP)], a hormone released with low intravascular volume, which causes decreased kidney perfusion. Decreased kidney perfusion activates AMP-activated kinase (AMPK), a metabolic sensor that inhibits the activity of several transport proteins. We hypothesized that AMPK activation also inhibits AQP2 function. These putative AMPK effects could protect interstitial ionic gradients required for urinary concentration during metabolic stress when low intravascular volume induces AVP release. Here we found that short-term AMPK activation by treatment with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR; 75 min) in kidney tissue prevented baseline AQP2 apical accumulation in principal cells, but did not prevent AQP2 apical accumulation in response to the AVP analog desmopressin (dDAVP). Prolonged AMPK activation prevented AQP2 cell membrane accumulation in response to forskolin in mouse collecting duct mpkCCD_c14 cells. Moreover, AMPK inhibition accelerated hypotonic lysis of Xenopus oocytes expressing AQP2. We performed phosphorylation assays to elucidate the mechanism by which AMPK regulates AQP2. Although AMPK weakly phosphorylated immunoprecipitated AQP2 in vitro, no direct AMPK phosphorylation of the AQP2 COOH-terminus was detected by mass spectrometry. AMPK promoted Ser-261 phosphorylation and antagonized dDAVP-dependent phosphorylation of other AQP2 COOH-terminal sites in cells. Our findings suggest an increasing, time-dependent antagonism of AMPK on AQP2 regulation with AICAR-dependent inhibition of cAMP-dependent apical accumulation and AVP-dependent phosphorylation of AQP2. This inhibition likely occurs via a mechanism that does not involve direct AQP2 phosphorylation by AMPK.

Temperature Modulates the Effects of Ocean Acidification on Intestinal Ion Transport in Atlantic Cod, Gadus morhua

CO₂-driven seawater acidification has been demonstrated to enhance intestinal bicarbonate secretion rates in teleosts, leading to an increased release of CaCO₃ under simulated ocean acidification scenarios. In this study, we investigated if increasing CO₂ levels stimulate the intestinal acid–base regulatory machinery of Atlantic cod (Gadus morhua) and whether temperatures at the upper limit of thermal tolerance stimulate or counteract ion regulatory capacities. Juvenile G. morhua were acclimated for 4 weeks to three CO₂ levels (550, 1200, and 2200 μatm) covering present and near-future natural variability, at optimum (10°C) and summer maximum temperature (18°C), respectively. Immunohistochemical analyses revealed the subcellular localization of ion transporters, including Na⁺/K⁺-ATPase (NKA), Na⁺/H⁺-exchanger 3 (NHE3), Na⁺/HCO3− cotransporter (NBC1), pendrin-like Cl⁻/HCO3− exchanger (SLC26a6), V-type H⁺-ATPase subunit a (VHA), and Cl⁻ channel 3 (CLC3) in epithelial cells of the anterior intestine. At 10°C, proteins and mRNA were generally up-regulated for most transporters in the intestinal epithelium after acclimation to higher CO₂ levels. This supports recent findings demonstrating increased intestinal HCO3− secretion rates in response to CO₂ induced seawater acidification. At 18°C, mRNA expression and protein concentrations of most ion transporters remained unchanged or were even decreased, suggesting thermal compensation. This response may be energetically favorable to retain blood HCO3− levels to stabilize pH_e, but may negatively affect intestinal salt and water resorption of marine teleosts in future oceans.

Vacuolar-ATPase (V-ATPase) Mediates Progesterone-Induced Uterine Fluid Acidification in Rats

We hypothesized that progesterone-induced decrease in uterine fluid pH involves V-ATPase. In this study, expression and functional activity of V-ATPase in uterus were investigated under progesterone influence. Ovariectomized adult female rats received subcutaneous injection of estradiol-17β (1 µg/kg/day) or progesterone (20 mg/kg/day) for 3 days or 3 days estradiol-17β followed by 3 days vehicle, progesterone, or estradiol-17β plus progesterone. Mifepristone, a progesterone receptor blocker, was concomitantly given to the rats which received progesterone. A day after last injection, rate of uterine fluid secretion, its HCO3 (-) concentration, and pH were determined via in vivo uterine perfusion in rats under anesthesia. V-ATPase inhibitor, bafilomycin, was introduced into the perfusion buffer, and changes in these parameters were observed. Expression of V-ATPase A1 and B1/2 proteins and mRNAs in uterus were quantified by Western blotting and real-time PCR, respectively. Distribution of these proteins was observed by immunohistochemistry. Our findings showed that under progesterone influence, uterine fluid secretion rate, HCO3 (-) concentration, and pH were significantly reduced. Administration of bafilomycin did not cause significant changes in fluid secretion rate; however, HCO3 (-) concentration and pH were significantly elevated. In parallel with these changes, expression of V-ATPase A1 and B1/2 proteins and mRNAs were significantly increased with these proteins highly distributed in uterine luminal and glandular epithelia. In conclusion, increased expression and functional activity of V-ATPase were most likely responsible for the decreased in uterine fluid pH observed under progesterone influence.

Protein kinase A activity is necessary for fission and fusion of Golgi to endoplasmic reticulum retrograde tubules

It is becoming increasingly accepted that together with vesicles, tubules play a major role in the transfer of cargo between different cellular compartments. In contrast to our understanding of the molecular mechanisms of vesicular transport, little is known about tubular transport. How signal transduction molecules regulate these two modes of membrane transport processes is also poorly understood. In this study we investigated whether protein kinase A (PKA) activity regulates the retrograde, tubular transport of Golgi matrix proteins from the Golgi to the endoplasmic reticulum (ER). We found that Golgi-to-ER retrograde transport of the Golgi matrix proteins giantin, GM130, GRASP55, GRASP65, and p115 was impaired in the presence of PKA inhibitors. In addition, we unexpectedly found accumulation of tubules containing both Golgi matrix proteins and resident Golgi transmembrane proteins. These tubules were still attached to the Golgi and were highly dynamic. Our data suggest that both fission and fusion of retrograde tubules are mechanisms regulated by PKA activity.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Histological and morphometric study of the epididymus of Chinchilla lanigera Grey under controlled conditions in captivity

Chinchilla lanigera, native to the Andean Mountains of Perú, Chile, Bolivia and Argentina, is a specimen of great economic importance because of its fur. In mammals, spermatozoa originate in testes and are transported to the epididymis, where they undergo morphological and biochemical modifications known as sperm maturation, a basic step in the acquisition of their fertilizing ability. The aim of this work is the macroscopic and microscopic analysis of the epididymis of Chinchilla lanigera Grey and its sectorization based on a histomorphological study. The epididymis presents a clear segmentation into four regions: initial segment, caput, corpus and cauda. The epithelium lining the seminiferous tubules is pseudostratified, with principal cells with stereocilia and basal, clear, apical, narrow and halo cells. The histological analysis showed that principal and basal cells are the prevailing populations in all regions, also revealing narrow cells and the absence of clear cells in the initial segment. Each segment presents its different histological and morphometric characteristics, which supports the idea of the specific behaviour of each region, giving a segment-specific character to the process of sperm maturation in this species. No significant differences were found in the morphometric measurements or in the histological evaluation of the epididymis of samples collected in April and October. The fact that no differences were found between the samples collected during the two periods when the reproductive ability in nature is different suggests the importance of external factors in the control of the reproductive cycle of Chinchilla lanigera.

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.

Established and potential physiological roles of bicarbonate-sensing soluble adenylyl cyclase (sAC) in aquatic animals

Soluble adenylyl cyclase (sAC) is a recently recognized source of the signaling molecule cyclic AMP (cAMP) that is genetically and biochemically distinct from the classic G-protein-regulated transmembrane adenylyl cyclases (tmACs). Mammalian sAC is distributed throughout the cytoplasm and it may be present in the nucleus and inside mitochondria. sAC activity is directly stimulated by HCO3(-), and sAC has been confirmed to be a HCO3(-) sensor in a variety of mammalian cell types. In addition, sAC can functionally associate with carbonic anhydrases to act as a de facto sensor of pH and CO2. The two catalytic domains of sAC are related to HCO3(-)-regulated adenylyl cyclases from cyanobacteria, suggesting the cAMP pathway is an evolutionarily conserved mechanism for sensing CO2 levels and/or acid/base conditions. Reports of sAC in aquatic animals are still limited but are rapidly accumulating. In shark gills, sAC senses blood alkalosis and triggers compensatory H(+) absorption. In the intestine of bony fishes, sAC modulates NaCl and water absorption. And in sea urchin sperm, sAC may participate in the initiation of flagellar movement and in the acrosome reaction. Bioinformatics and RT-PCR results reveal that sAC orthologs are present in most animal phyla. This review summarizes the current knowledge on the physiological roles of sAC in aquatic animals and suggests additional functions in which sAC may be involved.

sAC from aquatic organisms as a model to study the evolution of acid/base sensing

Soluble adenylyl cyclase (sAC) is poised to play multiple physiological roles as an acid/base (A/B) sensor in aquatic organisms. Many of these roles are probably similar to those in mammals; a striking example is the evolutionary conservation of a mechanism involving sAC, carbonic anhydrase and vacuolar H⁺-ATPase that acts as a sensor system and regulator of extracellular A/B in shark gills and mammalian epididymis and kidney. Additionally, the aquatic environment presents unique A/B and physiological challenges; therefore, sACs from aquatic organisms have likely evolved distinct kinetic properties as well as distinct physiological roles. sACs from aquatic organisms offer an excellent opportunity for studying the evolution of A/B sensing at both the molecular and whole organism levels. Moreover, this information could help understand and predict organismal responses to environmental stress based on mechanistic models.This article is part of a Special Issue entitled "The Role of Soluble Adenylyl Cyclase in Health and Disease," guest edited by J. Buck and L. R. Levin.

Role of the bicarbonate-responsive soluble adenylyl cyclase in pH sensing and metabolic regulation

The evolutionarily conserved soluble adenylyl cyclase (sAC, adcy10) was recently identified as a unique source of cAMP in the cytoplasm and the nucleus. Its activity is regulated by bicarbonate and fine-tuned by calcium. As such, and in conjunction with carbonic anhydrase (CA), sAC constitutes an HCO(-) 3/CO(-) 2/pH sensor. In both alpha-intercalated cells of the collecting duct and the clear cells of the epididymis, sAC is expressed at significant level and involved in pH homeostasis via apical recruitment of vacuolar H(+)-ATPase (VHA) in a PKA-dependent manner. In addition to maintenance of pH homeostasis, sAC is also involved in metabolic regulation such as coupling of Krebs cycle to oxidative phosphorylation via bicarbonate/CO2 sensing. Additionally, sAC also regulates CFTR channel and plays an important role in regulation of barrier function and apoptosis. These observations suggest that sAC, via bicarbonate-sensing, plays an important role in maintaining homeostatic status of cells against fluctuations in their microenvironment.

Regulation of luminal acidification by the V-ATPase

Specialized cells in the body express high levels of V-ATPase in their plasma membrane and respond to hormonal and nonhormonal cues to regulate extracellular acidification. Mutations in or loss of some V-ATPase subunits cause several disorders, including renal distal tubular acidosis and male infertility. This review focuses on the regulation of V-ATPase-dependent luminal acidification in renal intercalated cells and epididymal clear cells, which are key players in these physiological processes.

pH sensing via bicarbonate-regulated "soluble" adenylyl cyclase (sAC)

Soluble adenylyl cyclase (sAC) is a source of the second messenger cyclic adenosine 3′, 5′ monophosphate (cAMP). sAC is directly regulated by bicarbonate (HCO⁻₃) ions. In living cells, HCO⁻₃ ions are in nearly instantaneous equilibrium with carbon dioxide (CO₂) and pH due to the ubiquitous presence of carbonic anhydrases. Numerous biological processes are regulated by CO₂, HCO⁻₃, and/or pH, and in a number of these, sAC has been shown to function as a physiological CO₂/HCO₃/pH sensor. In this review, we detail the known pH sensing functions of sAC, and we discuss two highly-studied, pH-dependent pathways in which sAC might play a role.

The role of cAMP-mediated intracellular signaling in regulating Na+ uptake in zebrafish larvae

In the current study, the role of cAMP in stimulating Na(+) uptake in larval zebrafish was investigated. Treating larvae at 4 days postfertilization (dpf) with 10 μM forskolin or 1 μM 8-bromo cAMP significantly increased Na(+) uptake by three-fold and twofold, respectively. The cAMP-dependent stimulation of Na(+) uptake was probably unrelated to protein trafficking via microtubules because pretreatment with 200 μM colchicine or 30 μM nocodazole did not attenuate the magnitude of the response. Na(+) uptake was stimulated markedly following acute (2 h) exposure to acidic water. The acid-induced increase in Na(+) uptake was accompanied by a twofold elevation in whole body cAMP levels and attenuated by inhibiting PKA with 10 μM H-89. Knockdown of Na(+)-H(+) exchanger 3b (NHE3b) attenuated, but did not abolish, the stimulation of Na(+) uptake during forskolin treatment. In glial cell missing 2 morphants, in which the role of NHE3b in Na(+) uptake is diminished and the Na(+)-Cl(-) cotransporter (NCC) becomes the predominant route of Na(+) entry, forskolin treatment continued to increase Na(+) uptake. These data suggest that at least NHE3b and NCC are targeted by cAMP in zebrafish larvae. Staining of larvae with fluorescent forskolin and propranolol revealed the presence of transmembrane adenylyl cyclase within multiple subtypes of ionocytes expressing β-adrenergic receptors. Taken together, results of the present study demonstrate that cAMP-mediated intracellular signaling may regulate multiple Na(+) transporters and plays an important role in regulating Na(+) uptake in zebrafish larvae during acute exposure to an acidic environment.

Characterization of CFTR High Expresser cells in the intestine

The CFTR High Expresser (CHE) cells express eightfold higher levels of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel compared with neighboring enterocytes and were first identified by our laboratory (Ameen et al., Gastroenterology 108: 1016, 1995). We used double-label immunofluorescence microscopy to further study these enigmatic epithelial cells in rat intestine in vivo or ex vivo. CHE cells were found in duodenum, most frequent in proximal jejunum, and absent in ileum and colon. CFTR abundance increased in CHE cells along the crypt-villus axis. The basolateral Na(+)K(+)Cl(-) cotransporter NKCC1, a key transporter involved in Cl(-) secretion, was detected at similar levels in CHE cells and neighboring enterocytes at steady state. Microvilli appeared shorter in CHE cells, with low levels of Myosin 1a, a villus enterocyte-specific motor that retains sucrase/isomaltase in the brush-border membrane (BBM). CHE cells lacked alkaline phosphatase and absorptive villus enterocyte BBM proteins, including Na(+)H(+) exchanger NHE3, Cl(-)/HCO3(-) exchanger SLC26A6 (putative anion exchanger 1), and sucrase/isomaltase. High levels of the vacuolar-ATPase proton pump were observed in the apical domain of CHE cells. Levels of the NHE regulatory factor NHERF1, Na-K-ATPase, and Syntaxin 3 were similar to that of neighboring enterocytes. cAMP or acetylcholine stimulation robustly increased apical CFTR and basolateral NKCC1 disproportionately in CHE cells relative to neighboring enterocytes. These data strongly argue for a specialized role of CHE cells in Cl(-)-mediated "high-volume" fluid secretion on the villi of the proximal small intestine.

Functional vacuolar ATPase (V-ATPase) proton pumps traffic to the enterocyte brush border membrane and require CFTR

Vacuolar ATPases (V-ATPases) are highly conserved proton pumps that regulate organelle pH. Epithelial luminal pH is also regulated by cAMP-dependent traffic of specific subunits of the V-ATPase complex from endosomes into the apical membrane. In the intestine, cAMP-dependent traffic of cystic fibrosis transmembrane conductance regulator (CFTR) channels and the sodium hydrogen exchanger (NHE3) in the brush border regulate luminal pH. V-ATPase was found to colocalize with CFTR in intestinal CFTR high expresser (CHE) cells recently. Moreover, apical traffic of V-ATPase and CFTR in rat Brunner's glands was shown to be dependent on cAMP/PKA. These observations support a functional relationship between V-ATPase and CFTR in the intestine. The current study examined V-ATPase and CFTR distribution in intestines from wild-type, CFTR(-/-) mice and polarized intestinal CaCo-2BBe cells following cAMP stimulation and inhibition of CFTR/V-ATPase function. Coimmunoprecipitation studies examined V-ATPase interaction with CFTR. The pH-sensitive dye BCECF determined proton efflux and its dependence on V-ATPase/CFTR in intestinal cells. cAMP increased V-ATPase/CFTR colocalization in the apical domain of intestinal cells and redistributed the V-ATPase Voa1 and Voa2 trafficking subunits from the basolateral membrane to the brush border membrane. Voa1 and Voa2 subunits were localized to endosomes beneath the terminal web in untreated CFTR(-/-) intestine but redistributed to the subapical cytoplasm following cAMP treatment. Inhibition of CFTR or V-ATPase significantly decreased pHi in cells, confirming their functional interdependence. These data establish that V-ATPase traffics into the brush border membrane to regulate proton efflux and this activity is dependent on CFTR in the intestine.

Regulated traffic of anion transporters in mammalian Brunner's glands: a role for water and fluid transport

The Brunner's glands of the proximal duodenum exert barrier functions through secretion of glycoproteins and antimicrobial peptides. However, ion transporter localization, function, and regulation in the glands are less clear. Mapping the subcellular distribution of transporters is an important step toward elucidating trafficking mechanisms of fluid transport in the gland. The present study examined 1) changes in the distribution of intestinal anion transporters and the aquaporin 5 (AQP5) water channel in rat Brunner's glands following second messenger activation and 2) anion transporter distribution in Brunner's glands from healthy and disease-affected human tissues. Cystic fibrosis transmembrane conductance regulator (CFTR), AQP5, sodium-potassium-coupled chloride cotransporter 1 (NKCC1), sodium-bicarbonate cotransporter (NBCe1), and the proton pump vacuolar ATPase (V-ATPase) were localized to distinct membrane domains and in endosomes at steady state. Carbachol and cAMP redistributed CFTR to the apical membrane. cAMP-dependent recruitment of CFTR to the apical membrane was accompanied by recruitment of AQP5 that was reversed by a PKA inhibitor. cAMP also induced apical trafficking of V-ATPase and redistribution of NKCC1 and NBCe1 to the basolateral membranes. The steady-state distribution of AQP5, CFTR, NBCe1, NKCC1, and V-ATPase in human Brunner's glands from healthy controls, cystic fibrosis, and celiac disease resembled that of rat; however, the distribution profiles were markedly attenuated in the disease-affected duodenum. These data support functional transport of chloride, bicarbonate, water, and protons by second messenger-regulated traffic in mammalian Brunner's glands under physiological and pathophysiological conditions.

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.

Bicarbonate disruption of the pulmonary endothelial barrier via activation of endogenous soluble adenylyl cyclase, isoform 10

It is becoming increasingly apparent that cAMP signals within the pulmonary endothelium are highly compartmentalized, and this compartmentalization is critical to maintaining endothelial barrier integrity. Studies demonstrate that the exogenous soluble bacterial toxin, ExoY, and heterologous expression of the forskolin-stimulated soluble mammalian adenylyl cyclase (AC) chimera, sACI/II, elevate cytosolic cAMP and disrupt the pulmonary microvascular endothelial barrier. The barrier-disruptive effects of cytosolic cAMP generated by exogenous soluble ACs are in contrast to the barrier-protective effects of subplasma membrane cAMP generated by transmembrane AC, which strengthens endothelial barrier integrity. Endogenous soluble AC isoform 10 (AC10 or commonly known as sAC) lacks transmembrane domains and localizes within the cytosolic compartment. AC10 is uniquely activated by bicarbonate to generate cytosolic cAMP, yet its role in regulation of endothelial barrier integrity has not been addressed. Here we demonstrate that, within the pulmonary circulation, AC10 is expressed in pulmonary microvascular endothelial cells (PMVECs) and pulmonary artery endothelial cells (PAECs), yet expression in PAECs is lower. Furthermore, pulmonary endothelial cells selectively express bicarbonate cotransporters. While extracellular bicarbonate generates a phosphodiesterase 4-sensitive cAMP pool in PMVECs, no such cAMP response is detected in PAECs. Finally, addition of extracellular bicarbonate decreases resistance across the PMVEC monolayer and increases the filtration coefficient in the isolated perfused lung above osmolality controls. Collectively, these findings suggest that PMVECs have a bicarbonate-sensitive cytosolic cAMP pool that disrupts endothelial barrier integrity. These studies could provide an alternative mechanism for the controversial effects of bicarbonate correction of acidosis of acute respiratory distress syndrome patients.

Circulating aldosterone induces the apical accumulation of the proton pumping V-ATPase and increases proton secretion in clear cells in the caput epididymis

Clear cells express the vacuolar proton-pumping H(+)-ATPase (V-ATPase) and acidify the lumen of the epididymis, a process that is essential for male fertility. The renin-angiotensin-aldosterone system (RAAS) regulates fluid and electrolyte balance in the epididymis, and a previous study showed binding of aldosterone exclusively to epididymal clear cells (Hinton BT, Keefer DA. Steroid Biochem 23: 231-233, 1985). We examined here the role of aldosterone in the regulation of V-ATPase in the epididymis. RT-PCR showed expression of the mineralocorticoid receptor [MR; nuclear receptor subfamily 3, group C member 2 (NR3C2)] and 11-β-dehydrogenase isozyme 2 (HSD11β2) mRNAs specifically in clear cells, isolated by fluorescence-activated cell sorting from B1-enhanced green fluorescent protein (EGFP) mice. Tail vein injection of adult rats with aldosterone, 1,2-dioctanoyl-sn-glycerol (DOG), or 8-(4-chlorophenylthio)-cAMP (cpt-cAMP) induced V-ATPase apical membrane accumulation and extension of V-ATPase-labeled microvilli in clear cells in the caput epididymis but not in the cauda. V-ATPase activity was measured in EGFP-expressing clear cells using the intracellular pH (pHi)-sensing dye seminaphthorhodafluor-5F-5-(and 6)-carboxylic acid, acetoxymethyl ester acetate (SNARF-5F). Aldosterone induced a rapid increase in the rate of Na(+)- and bicarbonate-independent pHi recovery following an NH4Cl-induced acid load in clear cells isolated from the caput but not the cauda. This effect was abolished by concanamycin A, spironolactone, and chelerythrine but not myristoylated-protein kinase inhibitor (mPKI) or mifepristone. Thus aldosterone increases V-ATPase-dependent proton secretion in clear cells in the caput epididymis via MR/NR3C2 and PKC activation. This study, therefore, identifies aldosterone as an active member of the RAAS for the regulation of luminal acidification in the proximal epididymis.

Regulation of BK-α expression in the distal nephron by aldosterone and urine pH

In the distal nephron, the large-conductance Ca-activated K (BK) channel, comprised of a pore-forming-α (BK-α) and the BK-β4 subunit, promotes K excretion when mice are maintained on a high-K alkaline diet (HK-alk). We examined whether BK-β4 and the acid-base status regulate apical membrane expression of BK-α in the cortical (CCD) and medullary collecting ducts (MCD) using immunohistochemical analysis (IHC) and Western blot. With the use of IHC, BK-α of mice on acontrol diet localized mostly cytoplasmically in intercalated cells (IC) of the CCD and in the perinuclear region of both principle cells (PC) and IC of the MCD. HK-alk wild-type mice (WT), but not BK-β4 knockout mice (β4KO), exhibited increased apical BK-α in both the CCD and MCD. When given a high-K acidic diet (HK-Cl), BK-α expression increased but remained cytoplasmic in the CCD and perinuclear in the MCD of both WT and β4KO. Western blot confirmed that total BK-α expression was enhanced by either HK-alk or HK-Cl but only increased in the plasma membrane with HK-alk. Compared with controls, mice drinking NaHCO3 water exhibited more apical BK-α and total cellular BK-β4. Spironolactone given to mice on HK-alk significantly reduced K secretion and decreased total cellular BK-α but did not affect cellular BK-β4 and apical BK-α. Experiments with MDCK-C11 cells indicated that BK-β4 stabilizes surface BK-α by inhibiting degradation through a lysosomal pathway. These data suggest that aldosterone mediates a high-K-induced increase in BK-α and urinary alkalinization increases BK-β4 expression, which promotes the apical localization of BK-α.

Altered V-ATPase expression in renal intercalated cells isolated from B1 subunit-deficient mice by fluorescence-activated cell sorting

Unlike human patients with mutations in the 56-kDa B1 subunit isoform of the vacuolar proton-pumping ATPase (V-ATPase), B1-deficient mice (Atp6v1b1(-/-)) do not develop metabolic acidosis under baseline conditions. This is due to the insertion of V-ATPases containing the alternative B2 subunit isoform into the apical membrane of renal medullary collecting duct intercalated cells (ICs). We previously reported that quantitative Western blots (WBs) from whole kidneys showed similar B2 protein levels in Atp6v1b1(-/-) and wild-type mice (Păunescu TG, Russo LM, Da Silva N, Kovacikova J, Mohebbi N, Van Hoek AN, McKee M, Wagner CA, Breton S, Brown D. Am J Physiol Renal Physiol 293: F1915-F1926, 2007). However, WBs from renal medulla (including outer and inner medulla) membrane and cytosol fractions reveal a decrease in the levels of the ubiquitous V-ATPase E1 subunit. To compare V-ATPase expression specifically in ICs from wild-type and Atp6v1b1(-/-) mice, we crossed mice in which EGFP expression is driven by the B1 subunit promoter (EGFP-B1(+/+) mice) with Atp6v1b1(-/-) mice to generate novel EGFP-B1(-/-) mice. We isolated pure IC populations by fluorescence-assisted cell sorting from EGFP-B1(+/+) and EGFP-B1(-/-) mice to compare their V-ATPase subunit protein levels. We report that V-ATPase A, E1, and H subunits are all significantly downregulated in EGFP-B1(-/-) mice, while the B2 protein level is considerably increased in these animals. We conclude that under baseline conditions B2 upregulation compensates for the lack of B1 and is sufficient to maintain basal acid-base homeostasis, even when other V-ATPase subunits are downregulated.

Separate roles of PKA and EPAC in renal function unraveled by the optogenetic control of cAMP levels in vivo

Cyclic AMP (cAMP) is a ubiquitous second messenger that regulates a variety of essential processes in diverse cell types, functioning via cAMP-dependent effectors such as protein kinase A (PKA) and/or exchange proteins directly activated by cAMP (EPAC). In an intact tissue it is difficult to separate the contribution of each cAMP effector in a particular cell type using genetic or pharmacological approaches alone. We, therefore, utilized optogenetics to overcome the difficulties associated with examining a multicellular tissue. The transgenic photoactive adenylyl cyclase bPAC can be activated to rapidly and reversibly generate cAMP pulses in a cell-type-specific manner. This optogenetic approach to cAMP manipulation was validated in vivo using GAL4-driven UAS-bPAC in a simple epithelium, the Drosophila renal (Malpighian) tubules. As bPAC was expressed under the control of cell-type-specific promoters, each cAMP signal could be directed to either the stellate or principal cells, the two major cell types of the Drosophila renal tubule. By combining the bPAC transgene with genetic and pharmacological manipulation of either PKA or EPAC it was possible to investigate the functional impact of PKA and EPAC independently of each other. The results of this investigation suggest that both PKA and EPAC are involved in cAMP sensing, but are engaged in very different downstream physiological functions in each cell type: PKA is necessary for basal secretion in principal cells only, and for stimulated fluid secretion in stellate cells only. By contrast, EPAC is important in stimulated fluid secretion in both cell types. We propose that such optogenetic control of cellular cAMP levels can be applied to other systems, for example the heart or the central nervous system, to investigate the physiological impact of cAMP-dependent signaling pathways with unprecedented precision.

β-Adrenergic regulation of Na+ uptake by larval zebrafish Danio rerio in acidic and ion-poor environments

The potential role of adrenergic systems in regulating Na(+) uptake in zebrafish (Danio rerio) larvae was investigated. Treatment with isoproterenol (a generic β-adrenergic receptor agonist) stimulated Na(+) uptake, whereas treatment with phenylephrine (an α(1)-adrenergic receptor agonist) as well as clonidine (an α(2)-adrenergic receptor agonist) significantly reduced Na(+) uptake, suggesting opposing roles of α- and β-adrenergic receptors in Na(+) uptake regulation. The increase in Na(+) uptake associated with exposure to acidic water (pH = 4.0) was attenuated in the presence of the nonselective β-receptor antagonist propranolol or the β(1)-receptor blocker atenolol; the β(2)-receptor antagonist ICI-118551 was without effect. The stimulation of Na(+) uptake associated with ion-poor water (32-fold dilution of Ottawa tapwater) was unaffected by β-receptor blockade. Translational gene knockdown of β-receptors using antisense oligonucleotide morpholinos was used as a second method to assess the role of adrenergic systems in the regulation of Na(+) uptake. Whereas β(1)- or β(2B)-receptor knockdown led to significant decreases in Na(+) uptake during exposure to acidic water, only β(2A)-receptor morphants failed to increase Na(+) uptake in response to ion-poor water. In support of the pharmacology and knockdown experiments that demonstrated an involvement of β-adrenergic systems in the control of Na(+) uptake, we showed that the H(+)-ATPase-rich (HR) cell, a subtype of ionocyte known to be a site of Na(+) uptake, is innervated and appears to express β-adrenergic receptors (propranolol binding sites) at 4 days postfertilization. These data indicate an important role of adrenergic systems in regulating Na(+) uptake in developing zebrafish.

Phosphodiesterase inhibitor modulation of brain microvascular endothelial cell barrier properties

BACKGROUND AND PURPOSE

Brain microvascular disorders, including cerebral microscopic hemorrhage, have high prevalence but few treatment options. To develop new strategies for these disorders, we analyzed the effects of several phosphodiesterase (PDE) inhibitors on human brain microvascular endothelial cells (HBECs).

METHODS

We modified barrier properties and response to histamine of HBECs using cilostazol (PDE3 inhibitor), rolipram (PDE4 inhibitor), and dipyridamole (non-specific PDE inhibitor).

RESULTS

Cilostazol and dipyridamole altered the distribution of endothelial F-actin. Cilostazol increased expression of tight junction protein claudin-5 by 118% compared to control (p<.001). Permeability to albumin was decreased by cilostazol (21% vs control, p<.05), and permeability to dextran (70Kd) was decreased by both cilostazol (37% vs control, p<.001) and dipyridamole (44% vs control, p<.0001). Cilostazol increased trans-endothelial electrical resistance (TEER) after 12h by 111% compared to control (p<.0001). Protein kinase A (PKA) inhibitors H89 and KT5720 attenuated the TEER increase by cilostazol. Transient increased permeability in response to histamine was significantly mitigated by cilostazol, but not by other PDE inhibitors.

CONCLUSIONS

These findings demonstrate distinctive effects of cilostazol and other PDE inhibitors on HBECs, including enhanced barrier characteristics and mitigation of response to histamine. PKA-mediated effects of cilostazol were prominent in this model. These in vitro findings are consistent with therapeutic potential of PDE inhibitors in human brain microvascular disorders.

ATP secretion in the male reproductive tract: essential role of CFTR

Extracellular ATP is essential for the function of the epididymis and spermatozoa, but ATP release in the epididymis remains uncharacterized. We investigated here whether epithelial cells release ATP into the lumen of the epididymis, and we examined the role of the cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) and HCO(3)(-) conducting ion channel known to be associated with male fertility, in this process. Immunofluorescence labelling of mouse cauda epididymidis showed expression of CFTR in principal cells but not in other epithelial cells. CFTR mRNA was not detectable in clear cells isolated by fluorescence-activated cell sorting (FACS) from B1-EGFP mice, which express enhanced green fluorescent protein (EGFP) exclusively in these cells in the epididymis. ATP release was detected from the mouse epididymal principal cell line (DC2) and increased by adrenaline and forskolin. Inhibition of CFTR with CFTR(inh172) and transfection with CFTR-specific siRNAs in DC2 cells reduced basal and forskolin-activated ATP release. CFTR-dependent ATP release was also observed in primary cultures of mouse epididymal epithelial cells. In addition, steady-state ATP release was detected in vivo in mice, by measuring ATP concentration in a solution perfused through the lumen of the cauda epididymidis tubule and collected by cannulation of the vas deferens. Luminal CFTR(inh172) reduced the ATP concentration detected in the perfusate. This study shows that CFTR is involved in the regulation of ATP release from principal cells in the cauda epididymidis. Given that mutations in CFTR are a leading cause of male infertility, we propose that defective ATP signalling in the epididymis might contribute to dysfunction of the male reproductive tract associated with these mutations.

Cell- and region-specific expression of sugar chains in the mouse epididymal epithelium using lectin histochemistry combined with immunohistochemistry

To understand the cytochemical properties of epididymal epithelial cells, the characteristics of glycoconjugates in the mouse epididymis were examined using the technique of lectin histochemistry combined with immunohistochemistry. Characteristic staining patterns depending on the type of lectins were observed in the epididymal epithelium. Principal cells expressed N-acetyl-D-galactosamine (GalNAc), N-acetyl-D-glucosamine (GlcNAc), and Fucose in the proximal region of the epididymis and Mannose, Glucose, and Galactose in the distal region of the epididymis. Basal cells expressed Mannose, Glucose, Galactose, and GlcNAc in the proximal region and Galactose in the distal region. On the other hand, clear cells expressed various sugar residues and differences among regions were not observed. Interestingly, principal cells, clear cells, and basal cells specifically reacted with Ulex Europaeus-Agglutinin I (UEA-I lectin), Maackia Amurensis-Lectin I (MAL-I lectin), and Griffonia simplicifolia Lectin I-B4 (GS-I B lectin), respectively. These findings indicate that the selectivity in lectin reactivity for distinct cell types and segment-dependent staining in the epididymis may be related to cellular and regional differences in function. Furthermore, because some lectins stain particular cells or cellular compartments selectively, these lectins could be useful markers for histopathological evaluation of diseases or diagnosis of male infertility.

Acidosis-induced V-ATPase trafficking in salivary ducts is initiated by cAMP/PKA/CREB pathway via regulation of Rab11b expression

Changes in systemic acid-base homeostasis cause a series of organ-specific cellular responses, among them changes of acid-base transporter activities, and recruitment or retrieval of these transporters from intracellular pools to the plasma membrane and vice versa. The purpose of this study was to investigate the impact of protein phosphorylation in the acidosis-induced translocation of vacuolar-type H(+)-ATPase (V-ATPase) in salivary ducts and to identify molecular targets. Therefore, the human submandibular gland cell line HSG was exposed to acidosis and V-ATPase trafficking was investigated in the presence or absence of inhibitors and activators of sAC/PKA and Src/ERK signaling pathways. Putative target genes have been identified by RT-PCR and immunoblotting, and validated by loss-of-function experiments. Acidosis caused activation of cAMP/PKA and Src signaling and inhibition of either pathway significantly impaired acidosis-induced V-ATPase redistribution and incorporation into the plasma membrane. Activation of ERK1/2 was Src-independent, whereas activation of PKA caused phosphorylation of cAMP response element-binding (CREB) and activation to regulate Rab11b transcription. Loss-of-function of CREB down-regulated Rab11b transcript and protein and significantly impaired acidosis-induced V-ATPase translocation in HSG cells. These data demonstrate that the cAMP/PKA/CREB signaling pathway initiates acidosis-induced V-ATPase trafficking in salivary ducts via regulation of Rab11b expression and provide first evidence for a molecular mechanism underlying cAMP/PKA-dependent transporter trafficking that could account for accumulation and activity of transporters in other cellular systems as well.

New insights into the dynamic regulation of water and acid-base balance by renal epithelial cells

Maintaining tight control over body fluid and acid-base homeostasis is essential for human health and is a major function of the kidney. The collecting duct is a mosaic of two cell populations that are highly specialized to perform these two distinct processes. The antidiuretic hormone vasopressin (VP) and its receptor, the V2R, play a central role in regulating the urinary concentrating mechanism by stimulating accumulation of the aquaporin 2 (AQP2) water channel in the apical membrane of collecting duct principal cells. This increases epithelial water permeability and allows osmotic water reabsorption to occur. An understanding of the basic cell biology/physiology of AQP2 regulation and trafficking has informed the development of new potential treatments for diseases such as nephrogenic diabetes insipidus, in which the VP/V2R/AQP2 signaling axis is defective. Tubule acidification due to the activation of intercalated cells is also critical to organ function, and defects lead to several pathological conditions in humans. Therefore, it is important to understand how these "professional" proton-secreting cells respond to environmental and cellular cues. Using epididymal proton-secreting cells as a model system, we identified the soluble adenylate cyclase (sAC) as a sensor that detects luminal bicarbonate and activates the vacuolar proton-pumping ATPase (V-ATPase) via cAMP to regulate tubular pH. Renal intercalated cells also express sAC and respond to cAMP by increasing proton secretion, supporting the hypothesis that sAC could function as a luminal sensor in renal tubules to regulate acid-base balance. This review summarizes recent advances in our understanding of these fundamental processes.

Targeting reversible disassembly as a mechanism of controlling V-ATPase activity

Vacuolar proton-translocating ATPases (V-ATPases) are highly conserved proton pumps consisting of a peripheral membrane subcomplex called V1, which contains the sites of ATP hydrolysis, attached to an integral membrane subcomplex called Vo, which encompasses the proton pore. V-ATPase regulation by reversible dissociation, characterized by release of assembled V1 sectors into the cytosol and inhibition of both ATPase and proton transport activities, was first identified in tobacco hornworm and yeast. It has since become clear that modulation of V-ATPase assembly level is also a regulatory mechanism in mammalian cells. In this review, the implications of reversible disassembly for V-ATPase structure are discussed, along with insights into underlying subunit-subunit interactions provided by recent structural work. Although initial experiments focused on glucose deprivation as a trigger for disassembly, it is now clear that V-ATPase assembly can be regulated by other extracellular conditions. Consistent with a complex, integrated response to extracellular signals, a number of different regulatory proteins, including RAVE/rabconnectin, aldolase and other glycolytic enzymes, and protein kinase A have been suggested to control V-ATPase assembly and disassembly. It is likely that multiple signaling pathways dictate the ultimate level of assembly and activity. Tissue-specific V-ATPase inhibition is a potential therapy for osteoporosis and cancer; the possibility of exploiting reversible disassembly in design of novel V-ATPase inhibitors is discussed.

Intracellular cAMP signaling by soluble adenylyl cyclase

Soluble adenylyl cyclase (sAC) is a recently identified source of the ubiquitous second messenger cyclic adenosine 3',5' monophosphate (cAMP). sAC is distinct from the more widely studied source of cAMP, the transmembrane adenylyl cyclases (tmACs); its activity is uniquely regulated by bicarbonate anions, and it is distributed throughout the cytoplasm and in cellular organelles. Due to its unique localization and regulation, sAC has various functions in a variety of physiological systems that are distinct from tmACs. In this review, we detail the known functions of sAC, and we reassess commonly held views of cAMP signaling inside cells.

Establishment of cell-cell cross talk in the epididymis: control of luminal acidification

Male infertility is often caused by sperm that have low motility and interact poorly with the oocyte. Spermatozoa acquire these crucial functions in the epididymis. A low luminal bicarbonate (HCO(3)(-)) concentration and low pH keep sperm quiescent during their maturation and storage in this organ. This review describes how epididymal epithelial cells work in a concerted manner, together with spermatozoa, to establish and maintain this acidic luminal environment. Clear cells express the proton-pumping ATPase (V-ATPase) in their apical membrane and actively secrete protons. HCO(3)(-) induces V-ATPase accumulation in apical microvilli in clear cells via HCO(3)(-)-sensitive adenylyl cyclase-dependent cAMP production. HCO(3)(-) is secreted from principal cells following basolateral stimulation, to transiently "prime" spermatozoa before ejaculation. Luminal ATP and adenosine also induce V-ATPase apical accumulation in clear cells via activation of P2 and P1 receptors, respectively. ATP is released into the lumen from sperm and principal cells and is then metabolized into adenosine by local nucleotidases. In addition, the V-ATPase is regulated by luminal angiotensin II via activation of basal cells, which can extend narrow body projections that cross the tight junction barrier. Basal cells then secrete nitric oxide, which diffuses out to stimulate proton secretion in clear cells via activation of the cGMP pathway. Thus, an elaborate communication network is present between principal cells and clear cells, and between basal cells and clear cells, to control luminal acidification. Monitoring and decoding these "intercellular conversations" will help define pathophysiologic mechanisms underlying male infertility.

Cyclic nucleotide phosphodiesterase 1 regulates lysosome-dependent type I collagen protein degradation in vascular smooth muscle cells

OBJECTIVE

The phenotypic modulation of vascular smooth muscle cells (VSMCs) to a synthetic phenotype is vital during pathological vascular remodeling and the development of various vascular diseases. An increase in type I collagen (collagen I) has been implicated in synthetic VSMCs, and cyclic nucleotide signaling is critical in collagen I regulation. Herein, we investigate the role and underlying mechanism of cyclic nucleotide phosphodiesterase 1 (PDE1) in regulating collagen I in synthetic VSMCs.

METHODS AND RESULTS

The PDE1 inhibitor IC86340 significantly reduced collagen I in human saphenous vein explants undergoing spontaneous remodeling via ex vivo culture. In synthetic VSMCs, high basal levels of intracellular and extracellular collagen I protein were markedly decreased by IC86340. This attenuation was due to diminished protein but not mRNA. Inhibition of lysosome function abolished the effect of IC86340 on collagen I protein expression. PDE1C but not PDE1A is the major isoform responsible for mediating the effects of IC86340. Bicarbonate-sensitive soluble adenylyl cyclase/cAMP signaling was modulated by PDE1C, which is critical in collagen I degradation in VSMCs.

CONCLUSIONS

These data demonstrate that PDE1C regulates soluble adenylyl cyclase/cAMP signaling and lysosome-mediated collagen I protein degradation, and they suggest that PDE1C plays a critical role in regulating collagen homeostasis during pathological vascular remodeling.

Channels and transporters in salivary glands

According to the two-stage hypothesis, primary saliva, a NaCl-rich plasma-like isotonic fluid is secreted by salivary acinar cells and its ionic composition becomes modified in the duct system. The ducts secrete K(+) and HCO (3) (-) and reabsorb Na(+) and Cl(-) without any water movement, thus establishing a hypotonic final saliva. Salivary secretion depends on the coordinated action of several channels and transporters localized in the apical and basolateral membrane of acinar and duct cells. Early functional studies in perfused glands, followed by the molecular cloning of several transport proteins and the subsequent analysis of mutant mice, have greatly contributed to our understanding of salivary fluid and the electrolyte secretion process. With a few exceptions, most of the key channels and transporters involved in salivary secretion have now been identified and characterized. However, the picture that has emerged from all these studies is one of a complex molecular network characterized by redundancy for several transport proteins, compensatory mechanisms, and adaptive changes in health and disease. Current research is directed to the molecular interactions between the determinants and the ways in which they are regulated by extracellular signals and intracellular mediators. This review focuses on the functionally and molecularly best-characterized channels and transporters that are considered to be involved in transepithelial fluid and electrolyte transport in salivary glands.