Localized Na+/H+ exchanger 1 expression protects Ca2+-regulated adenylyl cyclases from changes in intracellular pH

Antidepressants Produce Persistent Gα s-Associated Signaling Changes in Lipid Rafts after Drug Withdrawal

Termination of antidepressant therapy often has negative consequences. Although symptoms of antidepressant withdrawal are widely recognized, the molecular processes that underlie them are not well characterized. We show that certain aspects of Gα _s signaling remain suppressed after antidepressant withdrawal, even after others have reverted to baseline. Antidepressant treatment causes translocation of Gα _s protein from lipid rafts to nonraft membrane regions. This results in augmented Gα _s signaling, including facilitated activation of adenylyl cyclase and increased cAMP accumulation. Using CC6 or SK-N-SH cells and a lipid raft-localized cAMP sensor, we show that Gα _s signaling is reduced in lipid rafts, even while signaling is enhanced elsewhere in the cell. These signaling changes mirror the changes in Gα _s localization observed after antidepressant treatment. Furthermore, we show that suppression of Gα _s signaling in lipid rafts persists at least 24 hours after cessation of antidepressant treatment. Gα _s localization was quantified after membrane isolation and sequential detergent extraction. We show that suppression of lipid raft Gα _s signaling persists for an extended time period after antidepressant withdrawal, whereas increased nonraft membrane Gα _s signaling reverts partially or fully upon cessation of antidepressant treatment. Translocation of Gα _s out of lipid rafts is also persistent. These events may reflect cellular adaptations to antidepressant treatment that contribute to antidepressant discontinuation syndromes and may aid in the discovery of new treatments and strategies to mitigate the symptoms of depression and antidepressant withdrawal. SIGNIFICANCE STATEMENT: This work explores, for the first time, the effects of antidepressants on Gα _s signaling after drug withdrawal. This provides novel insight into the cellular and molecular processes affected by antidepressant drugs and their persistence after discontinuation of treatment.

Rapid Regulation of Human Multidrug and Extrusion Transporters hMATE1 and hMATE2K

Vectorial transport of organic cations (OCs) in renal proximal tubules is mediated by sequential action of human OC transporter 2 (hOCT2) and human multidrug and toxic extrusion protein 1 and 2K (hMATE1 and hMATE2K), expressed in the basolateral (hOCT2) and luminal (hMATE1 and hMATE2K) plasma membranes, respectively. It is well known that hOCT2 activity is subjected to rapid regulation by several signaling pathways, suggesting that renal OC secretion may be acutely adapted to physiological requirements. Therefore, in this work, the acute regulation of hMATEs stably expressed in human embryonic kidney cells was characterized using the fluorescent substrate 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP⁺) as a marker. A specific regulation of ASP⁺ transport by hMATE1 and hMATE2K measured in uptake and efflux configurations was observed. In the example of hMATE1 efflux reduction by inhibition of casein kinase II, it was also shown that this regulation is able to modify transcellular transport of ASP⁺ in Madin–Darby canine kidney II cells expressing hOCT2 and hMATE1 on the basolateral and apical membrane domains, respectively. The activity of hMATEs can be rapidly regulated by some intracellular pathways, which sometimes are common to those found for hOCTs. Interference with these pathways may be important to regulate renal secretion of OCs.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Luminescence-activated nucleotide cyclase regulates spatial and temporal cAMP synthesis

cAMP is a ubiquitous second messenger that regulates cellular proliferation, differentiation, attachment, migration, and several other processes. It has become increasingly evident that tight regulation of cAMP accumulation and localization confers divergent yet specific signaling to downstream pathways. Currently, few tools are available that have sufficient spatial and temporal resolution to study location-biased cAMP signaling. Here, we introduce a new fusion protein consisting of a light-activated adenylyl cyclase (bPAC) and luciferase (nLuc). This construct allows dual activation of cAMP production through temporally precise photostimulation or chronic chemical stimulation that can be fine-tuned to mimic physiological levels and duration of cAMP synthesis to trigger downstream events. By targeting this construct to different compartments, we show that cAMP produced in the cytosol and nucleus stimulates proliferation in thyroid cells. The bPAC-nLuc fusion construct adds a new reagent to the available toolkit to study cAMP-regulated processes in living cells.

Electron Pathways through Erythrocyte Plasma Membrane in Human Physiology and Pathology: Potential Redox Biomarker?

Erythrocytes are involved in the transport of oxygen and carbon dioxide in the body. Since pH is the influential factor in the Bohr-Haldane effect, pHi is actively maintained via secondary active transports Na+/H+ exchange and HC3–/Cl– anion exchanger. Because of the redox properties of the iron, hemoglobin generates reactive oxygen species and thus, the human erythrocyte is constantly exposed to oxidative damage. Although the adult erythrocyte lacks protein synthesis and cannot restore damaged proteins, it is equipped with high activity of protective enzymes. Redox changes in the cell initiate various signalling pathways. Plasma membrane oxido-reductases (PMORs) are trans-membrane electron transport systems that have been found in the membranes of all cells and have been extensively characterized in the human erythrocyte. Erythrocyte PMORs transfer reducing equivalents from intracellular reductants to extracellular oxidants, thus their most important role seems to be to enable the cell respond to changes in intra- and extra-cellular redox environments.

So far the activity of erythrocyte PMORs in disease states has not been systematically investigated. This review summarizes present knowledge on erythrocyte electron transfer activity in humans (health, type 1 diabetes, diabetic nephropathy, and chronic uremia) and hypothesizes an integrated model of the functional organization of erythrocyte plasma membrane where electron pathways work in parallel with transport metabolons to maintain redox homeostasis.

cAMP-dependent secretagogues stimulate the NaHCO3 cotransporter in the villous epithelium of the brushtail possum, Trichosurus vulpecula

In the ileum of the brushtail possum, Trichosurus vulpecula, fluid secretion appears to be driven by electrogenic HCO₃^- secretion. Consistent with this, the cystic fibrosis transmembrane conductance regulator is expressed in the apical membrane of the ileal epithelial cells and the pancreatic or secretory variant of the NaHCO₃ cotransporter in the basolateral membrane. This suggests that in the possum ileum, electrogenic HCO₃^- secretion is driven by basolateral NaHCO₃ cotransporter (NBC) activity. To determine if the NBC contributes to HCO₃^- secretion in the possum ileum, intracellular pH (pHi) measurements in isolated villi were used to demonstrate NBC activity in the ileal epithelial cells and investigate the effect of cAMP-dependent secretagogues. In CO₂/HCO₃^--free solutions, recovery of the epithelial cells from an acid load was Na⁺-dependent and ≈80% inhibited by ethyl-isopropyl-amiloride (EIPA, 10 µmol L^-1), indicative of the presence of an Na⁺/H⁺ exchanger, most likely NHE1. However, in the presence of CO₂/HCO₃^-, EIPA only inhibited ≈ 50% of the recovery, the remainder was inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS, 500 µmol L^-1), indicative of NBC activity. Under steady-state conditions, NHE1 inhibition by EIPA had little effect on pHi in the presence or absence of secretagogues, but NBC inhibition with DIDS resulted in a rapid acidification of the cells, which was increased fivefold by secretagogues. These data demonstrate the functional activity of an NaHCO₃ cotransporter in the ileal epithelial cells. Furthermore, the stimulation of NBC activity by secretagogues is consistent with the involvement of an NaHCO₃ cotransporter in electrogenic HCO₃^- secretion.

The STIM1-ORAI1 microdomain

The regulatory protein STIM1 controls gating of the Ca(2+) channel ORAI1 by a direct protein-protein interaction. Because STIM1 is anchored in the ER membrane and ORAI1 is in the plasma membrane, the STIM-ORAI pathway can support Ca(2+) influx only where the two membranes come into close apposition, effectively demarcating a microdomain for Ca(2+) signalling. This review begins with a brief summary of the STIM-ORAI pathway of store-operated Ca(2+) influx, then turns to the special geometry of the STIM-ORAI microdomain and the expected characteristics of the microdomain Ca(2+) signal. A final section of the review seeks to place the STIM-ORAI microdomain into a broader context of cellular Ca(2+) signalling.

Involvement of tumor acidification in brain cancer pathophysiology

Gliomas, primary brain cancers, are characterized by remarkable invasiveness and fast growth. While they share many qualities with other solid tumors, gliomas have developed special mechanisms to convert the cramped brain space and other limitations afforded by the privileged central nervous system into pathophysiological advantages. In this review we discuss gliomas and other primary brain cancers in the context of acid-base regulation and interstitial acidification; namely, how the altered proton (H⁺) content surrounding these brain tumors influences tumor development in both autocrine and paracrine manners. As proton movement is directly coupled to movement of other ions, pH serves as both a regulator of cell activity as well as an indirect readout of other cellular functions. In the case of brain tumors, these processes result in pathophysiology unique to the central nervous system. We will highlight what is known about pH-sensitive processes in brain tumors in addition to gleaning insight from other solid tumors.

Improved genetically-encoded, FlincG-type fluorescent biosensors for neural cGMP imaging

Genetically-encoded biosensors are powerful tools for understanding cellular signal transduction mechanisms. In aiming to investigate cGMP signaling in neurones using the EGFP-based fluorescent biosensor, FlincG (fluorescent indicator for cGMP), we encountered weak or non-existent fluorescence after attempted transfection with plasmid DNA, even in HEK293T cells. Adenoviral infection of HEK293T cells with FlincG, however, had previously proved successful. Both constructs were found to harbor a mutation in the EGFP domain and had a tail of 17 amino acids at the C-terminus that differed from the published sequence. These discrepancies were systematically examined, together with mutations found beneficial for the related GCaMP family of Ca²⁺ biosensors, in a HEK293T cell line stably expressing both nitric oxide (NO)-activated guanylyl cyclase and phosphodiesterase-5. Restoring the mutated amino acid improved basal fluorescence whereas additional restoration of the correct C-terminal tail resulted in poor cGMP sensing as assessed by superfusion of either 8-bromo-cGMP or NO. Ultimately, two improved FlincGs were identified: one (FlincG2) had the divergent tail and gave moderate basal fluorescence and cGMP response amplitude and the other (FlincG3) had the correct tail, a GCaMP-like mutation in the EGFP region and an N-terminal tag, and was superior in both respects. All variants tested were strongly influenced by pH over the physiological range, in common with other EGFP-based biosensors. Purified FlincG3 protein exhibited a lower cGMP affinity (0.89 μM) than reported for the original FlincG (0.17 μM) but retained rapid kinetics and a 230-fold selectivity over cAMP. Successful expression of FlincG2 or FlincG3 in differentiated N1E-115 neuroblastoma cells and in primary cultures of hippocampal and dorsal root ganglion cells commends them for real-time imaging of cGMP dynamics in neural (and other) cells, and in their subcellular specializations.

An improved targeted cAMP sensor to study the regulation of adenylyl cyclase 8 by Ca2+ entry through voltage-gated channels

Here we describe an improved sensor with reduced pH sensitivity tethered to adenylyl cyclase (AC) 8. The sensor was used to study cAMP dynamics in the AC8 microdomain of MIN6 cells, a pancreatic β-cell line. In these cells, AC8 was activated by Ca(2+) entry through L-type voltage-gated channels following depolarisation. This activation could be reconstituted in HEK293 cells co-expressing AC8 and either the α1C or α1D subunit of L-type voltage-gated Ca(2+) channels. The development of this improved sensor opens the door to the study of cAMP microdomains in excitable cells that have previously been challenging due to the sensitivity of fluorescent proteins to pH changes.

Acidosis-mediated regulation of the NHE1 isoform of the Na⁺/H⁺ exchanger in renal cells

The mammalian Na⁺/H⁺ exchanger isoform 1 (NHE1) is a ubiquitous plasma membrane protein that regulates intracellular pH by removing a proton in exchange for extracellular sodium. Renal tissues are subject to metabolic and respiratory acidosis, and acidosis has been shown to acutely activate NHE1 activity in other cell types. We examined if NHE1 is activated by acute acidosis in HEK293 and Madin-Darby canine kidney (MDCK) cells. Acute sustained intracellular acidosis (SIA) activated NHE1 in both cell types. We expressed wild-type and mutant NHE1 cDNAs in MDCK cells. All the cDNAs had a L163F/G174S mutation, which conferred a 100-fold resistance to EMD87580, an NHE1-specific inhibitor. We assayed exogenous NHE1 activity while inhibiting endogenous activity with EMD87580 and while inhibiting the NHE3 isoform of the Na⁺/H⁺ exchanger using the isoform-specific inhibitor S3226. We examined the activation and phosphorylation of the wild-type and mutant NHE1 proteins in response to SIA. In MDCK cells we demonstrated that the amino acids Ser⁷⁷¹, Ser⁷⁷⁶, Thr⁷⁷⁹, and Ser⁷⁸⁵ are important for NHE1 phosphorylation and activation after acute SIA. SIA activated ERK-dependent pathways in MDCK cells, and this was blocked by treatment with the MEK inhibitor U0126. Treatment with U0126 also blocked activation of NHE1 by SIA. These results suggest that acute acidosis activates NHE1 in mammalian kidney cells and that in MDCK cells this activation occurs through an ERK-dependent pathway affecting phosphorylation of a distinct set of amino acids in the cytosolic regulatory tail of NHE1.

Regulation of the cardiac Na⁺/H⁺ exchanger in health and disease

The Na(+) gradient produced across the cardiac sarcolemma by the ATP-dependent Na(+)-pump is a constant source of energy for Na(+)-dependent transporters. The plasma membrane Na(+)/H(+) exchanger (NHE) is one such secondary active transporter, regulating intracellular pH, Na(+) concentration, and cell volume. NHE1, the major isoform found in the heart, is activated in response to a variety of stimuli such as hormones and mechanical stress. This important characteristic of NHE1 is intimately linked to heart diseases, including maladaptive cardiac hypertrophy and subsequent heart failure, as well as acute ischemic-reperfusion injury. NHE1 activation results in elevation of pH and intracellular Na(+) concentration, which potentially enhance downstream signaling cascades in the myocardium. Therefore, in addition to determining the mechanism underlying regulation of NHE1 activity, it is important to understand how the ionic signal produced by NHE1 is transmitted to the downstream targets. Extensive studies have identified many accessory factors that interact with NHE1. Here, we have summarized the recent progress on understanding the molecular mechanism underlying NHE1 regulation and have shown a possible signaling pathway leading to cardiac remodeling, which is initiated from NHE1. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

A kinase-anchoring proteins and adenylyl cyclase in cardiovascular physiology and pathology

3'-5'-Cyclic adenosine monophosphate (cAMP), generated by adenylyl cyclase (AC), serves as a second messenger in signaling pathways regulating many aspects of cardiac physiology, including contraction rate and action potential duration, and in the pathophysiology of hypertrophy and heart failure. A kinase-anchoring proteins localize the effect of cAMP in space and time by organizing receptors, AC, protein kinase A, and other components of the cAMP cascade into multiprotein complexes. In this review, we discuss how the interaction of A kinase-anchoring proteins with distinct AC isoforms affects cardiovascular physiology.

Na(+)/H(+) exchanger 1 directly binds to calcineurin A and activates downstream NFAT signaling, leading to cardiomyocyte hypertrophy

The calcineurin A (CaNA) subunit was identified as a novel binding partner of plasma membrane Na(+)/H(+) exchanger 1 (NHE1). CaN is a Ca(2+)-dependent phosphatase involved in many cellular functions, including cardiac hypertrophy. Direct binding of CaN to the (715)PVITID(720) sequence of NHE1, which resembles the consensus CaN-binding motif (PXIXIT), was observed. Overexpression of NHE1 promoted serum-induced CaN/nuclear factor of activated T cells (NFAT) signaling in fibroblasts, as indicated by enhancement of NFAT promoter activity and nuclear translocation, which was attenuated by NHE1 inhibitor. In neonatal rat cardiomyocytes, NHE1 stimulated hypertrophic gene expression and the NFAT pathway, which were inhibited by a CaN inhibitor, FK506. Importantly, CaN activity was strongly enhanced with increasing pH, so NHE1 may promote CaN/NFAT signaling via increased intracellular pH. Indeed, Na(+)/H(+) exchange activity was required for NHE1-dependent NFAT signaling. Moreover, interaction of CaN with NHE1 and clustering of NHE1 to lipid rafts were also required for this response. Based on these results, we propose that NHE1 activity may generate a localized membrane microdomain with higher pH, thereby sensitizing CaN to activation and promoting NFAT signaling. In cardiomyocytes, such signaling can be a pathway of NHE1-dependent hypertrophy.

Fibronectin stimulates migration through lipid raft dependent NHE-1 activation in mouse embryonic stem cells: involvement of RhoA, Ca(2+)/CaM, and ERK

BACKGROUND

Extracellular matrix (ECM) components and intracellular pH (pH(i)) may serve as regulators of cell migration in various cell types.

METHODS

The Oris migration assay was used to assess the effect of fibronectin (FN) on cell motility. The Na(+)/H(+) exchanger (NHE)-1 activity was evaluated by measuring pH(i) and [(22)Na(+)] uptake. To examine activated signaling molecules, western blot analysis and immunoprecipitation was performed.

RESULTS

ECM components (FN, laminin, fibrinogen, and collagen type I) increased [(22)Na(+)] uptake, pH(i), and cell migration. In addition, FN-induced increase of cell migration was inhibited by NHE-1 inhibitor amiloride or NHE-1-specific siRNA. FN selectively increased the mRNA and protein expression of NHE-1, but not that of NHE-2 or NHE-3. FN binds integrin β1 and subsequently stimulates caveolin-1 phosphorylation and Ca(2+) influx. Then, NHE-1 is phosphorylated by RhoA and Rho kinases, and Ca(2+)/calmodulin (CaM) signaling elicits complex formation with NHE-1, which is enriched in lipid raft/caveolae microdomains of the plasma membrane. Activation of NHE-1 continuously induces an increase of [(22)Na(+)] uptake and pH(i). Finally, NHE-1-dependent extracellular signal-regulated kinase (ERK) 1/2 phosphorylation enhanced matrix metalloproteinase-2 (MMP-2) and filamentous-actin (F-actin) expression, partially contributing to the regulation of embryonic stem cells (ESCs) migration.

CONCLUSIONS

FN stimulated mESCs migration and proliferation through NHE-1 activation, which were mediated by lipid raft-associated caveolin-1, RhoA/ROCK, and Ca(2+)/CaM signaling pathways.

GENERAL SIGNIFICANCE

The precise role of NHE in the modulation of ECM-related physiological functions such as proliferation and migration remains poorly understood. Thus, this study analyzed the relationship between FN and NHE in regulating the migration of mouse ESCs and their related signaling pathways.

cAMP measurements with FRET-based sensors in excitable cells

The development of FRET (fluorescence resonance energy transfer)-based sensors for measuring cAMP has opened the door to sophisticated insights into single-cell cAMP dynamics. cAMP can be measured in distinct cell populations and even in distinct microdomains within cells. However, there is still only limited information on cAMP dynamics in excitable cells, particularly as a function of the activity of voltage-gated Ca2+ channels. A major reason for this is the pH shifts that can occur in excitable cells and their effects on fluorescent proteins.

Adenylyl cyclase AC8 directly controls its micro-environment by recruiting the actin cytoskeleton in a cholesterol-rich milieu

The central and pervasive influence of cAMP on cellular functions underscores the value of stringent control of the organization of adenylyl cyclases (ACs) in the plasma membrane. Biochemical data suggest that ACs reside in membrane rafts and could compartmentalize intermediary scaffolding proteins and associated regulatory elements. However, little is known about the organization or regulation of the dynamic behaviour of ACs in a cellular context. The present study examines these issues, using confocal image analysis of various AC8 constructs, combined with fluorescence recovery after photobleaching and fluorescence correlation spectroscopy. These studies reveal that AC8, through its N-terminus, enhances the cortical actin signal at the plasma membrane; an interaction that was confirmed by GST pull-down and immunoprecipitation experiments. AC8 also associates dynamically with lipid rafts; the direct association of AC8 with sterols was confirmed in Förster resonance energy transfer experiments. Disruption of the actin cytoskeleton and lipid rafts indicates that AC8 tracks along the cytoskeleton in a cholesterol-enriched domain, and the cAMP that it produces contributes to sculpting the actin cytoskeleton. Thus, an adenylyl cyclase is shown not just to act as a scaffold, but also to actively orchestrate its own micro-environment, by associating with the cytoskeleton and controlling the association by producing cAMP, to yield a highly organized signalling hub.

Cell volume regulation and signaling in 3T3-L1 pre-adipocytes and adipocytes: on the possible roles of caveolae, insulin receptors, FAK and ERK1/2

Caveolae have been implicated in sensing of cell volume perturbations, yet evidence is still limited and findings contradictory. Here, we investigated the possible role of caveolae in cell volume regulation and volume sensitive signaling in an adipocyte system with high (3T3-L1 adipocytes); intermediate (3T3-L1 pre-adipocytes); and low (cholesterol-depleted 3T3-L1 pre-adipocytes) caveolae levels. Using large-angle light scattering, we show that compared to pre-adipocytes, differentiated adipocytes exhibit several-fold increased rates of volume restoration following osmotic cell swelling (RVD) and osmotic cell shrinkage (RVI), accompanied by increased swelling-activated taurine efflux. However, caveolin-1 distribution was not detectably altered after osmotic swelling or shrinkage, and caveolae integrity, as studied by cholesterol depletion or expression of dominant negative Cav-1, was not required for either RVD or RVI in pre-adipocytes. The insulin receptor (InsR) localizes to caveolae and its expression dramatically increases upon adipocyte differentiation. In pre-adipocytes, InsR and its effectors focal adhesion kinase (FAK) and extracellular signal regulated kinase (ERK1/2) localized to focal adhesions and were activated by a 5 min exposure to insulin (100 nM). Osmotic shrinkage transiently inhibited InsR Y(146)-phosphorylation, followed by an increase at t=15 min; a similar pattern was seen for ERK1/2 and FAK, in a manner unaffected by cholesterol depletion. In contrast, cell swelling had no detectable effect on InsR, yet increased ERK1/2 phosphorylation. In conclusion, differentiated 3T3-L1 adipocytes exhibit greatly accelerated RVD and RVI responses and increased swelling-activated taurine efflux compared to pre-adipocytes. Furthermore, in pre-adipocytes, Cav-1/caveolae integrity is not required for volume regulation. Given the relationship between hyperosmotic stress and insulin signaling, the finding that cell volume regulation is dramatically altered upon adipocyte differentiation may be relevant for the understanding of insulin resistance and metabolic syndrome.

Cytosolic H+ microdomain developed around AE1 during AE1-mediated Cl-/HCO3- exchange

Microdomains, regions of discontinuous cytosolic solute concentration enhanced by rapid solute transport and slow diffusion rates, have many cellular roles. pH-regulatory membrane transporters, like the Cl−/HCO3− exchanger AE1, could develop H+ microdomains since AE1 has a rapid transport rate and cytosolic H+ diffusion is slow. We examined whether the pH environment surrounding AE1 differs from other cellular locations. As AE1 drives Cl−/HCO3− exchange, differences in pH, near and remote from AE1, were monitored by confocal microscopy using two pH-sensitive fluorescent proteins: deGFP4 (GFP) and mNectarine (mNect). Plasma membrane (PM) pH (defined as ∼1 μm region around the cell periphery) was monitored by GFP fused to AE1 (GFP.AE1), and mNect fused to an inactive mutant of the Na+-coupled nucleoside co-transporter, hCNT3 (mNect.hCNT3). GFP.AE1 to mNect.hCNT3 distance was varied by co-expression of different amounts of the two proteins in HEK293 cells. As the GFP.AE1–mNect.hCNT3 distance increased, mNect.hCNT3 detected the Cl−/HCO3− exchange-associated cytosolic pH change with a time delay and reduced rate of pH change compared to GFP.AE1. We found that a H+ microdomain 0.3 μm in diameter forms around GFP.AE1 during physiological HCO3− transport. Carbonic anhydrase isoform II inhibition prevented H+ microdomain formation. We also measured the rate of H+ movement from PM GFP.AE1 to endoplasmic reticulum (ER), using mNect fused to the cytosolic face of ER-resident calnexin (CNX.mNect). The rate of H+ diffusion through cytosol was 60-fold faster than along the cytosolic surface of the plasma membrane. The pH environment surrounding pH regulatory transport proteins may differ as a result of H+ microdomain formation, which will affect nearby pH-sensitive processes.

Regulation of early neurite morphogenesis by the Na+/H+ exchanger NHE1

The ubiquitously expressed Na(+)/H(+) exchanger NHE1 plays an important role in regulating polarized membrane protrusion and directional motility in non-neuronal cells. Using NGF-differentiated PC12 cells and murine neocortical neurons in vitro, we now show that NHE1 plays a role in regulating early neurite morphogenesis. NHE1 was expressed in growth cones in which it gave rise to an elevated intracellular pH in actively extending neurites. The NHE1 inhibitor cariporide reversibly reduced growth cone filopodia number and the formation and elongation of neurites, especially branches, whereas the transient overexpression of full-length NHE1, but not NHE1 mutants deficient in either ion translocation activity or actin cytoskeletal anchoring, elicited opposite effects. In addition, compared with neocortical neurons obtained from wild-type littermates, neurons isolated from NHE1-null mice exhibited reductions in early neurite outgrowth, an effect that was rescued by overexpression of full-length NHE1 but not NHE1 mutants. Finally, the growth-promoting effects of netrin-1, but not BDNF or IGF-1, were markedly reduced by cariporide in wild-type neocortical neurons and were not observed in NHE1-null neurons. Although netrin-1 failed to increase growth cone intracellular pH or Na(+)/H(+) exchange activity, netrin-1-induced increases in early neurite outgrowth were restored in NHE1-null neurons transfected with full-length NHE1 but not an ion translocation-deficient mutant. Collectively, the results indicate that NHE1 participates in the regulation of early neurite morphogenesis and identify a novel role for NHE1 in the promotion of early neurite outgrowth by netrin-1.

cAMP signal transduction in the heart: understanding spatial control for the development of novel therapeutic strategies

3'-5'-Cyclic adenosine monophosphate (cAMP) is a pleiotropic intracellular second messenger generated in response to activation of G(s) protein-coupled receptors. In the heart, cAMP mediates the catecholaminergic control on heart rate and contractility but, at the same time, it is responsible for the functional response to a wide variety of other hormones and neurotransmitters, raising the question of how the myocyte can decode the cAMP signal and generate the appropriate functional output to each individual extracellular stimulus. A growing body of evidence points to the spatial organization of the components of the cAMP signalling pathway in distinct, spatially segregated signalling domains as the key feature underpinning specificity of response and data is emerging, indicating that alteration of spatial control of the cAMP signal cascade associates with heart pathology. Most of the details of the molecular organization and regulation of individual cAMP signalling compartments are still to be elucidated but future research should provide the knowledge necessary to develop and test new therapeutic strategies that, by acting on a limited subset of downstream targets, would improve efficacy and minimize off-target effects.

cAMP-dependent and cholinergic regulation of the electrogenic intestinal/pancreatic Na+/HCO3- cotransporter pNBC1 in human embryonic kidney (HEK293) cells

Background

The renal (kNBC1) and intestinal (pNBC1) electrogenic Na⁺/HCO₃^- cotransporter variants differ in their primary structure, transport direction, and response to secretagogues. Previous studies have suggested that regulatory differences between the two subtypes can be partially explained by unique consensus phosphorylation sites included in the pNBC1, but not the kNBC1 sequence. After having shown activation of NBC by carbachol and forskolin in murine colon, we now investigated these pathways in HEK293 cells transiently expressing a GFP-tagged pNBC1 construct.

Results

Na⁺- and HCO₃^--dependent pH_i recovery from an acid load (measured with BCECF) was enhanced by 5-fold in GFP-positive cells compared to the control cells in the presence of CO₂/HCO₃^-. Forskolin (10^-5 M) had no effect in untransfected cells, but inhibited the pH_i recovery in cells expressing pNBC1 by 62%. After preincubation with carbachol (10^-4 M), the pH_i recovery was enhanced to the same degree both in transfected and untransfected cells, indicating activation of endogenous alkalizing ion transporters. Acid-activated Na⁺/HCO₃^- cotransport via pNBC1 expressed in renal cells is thus inhibited by cAMP and not affected by cholinergic stimulation, as opposed to the findings in native intestinal tissue.

Conclusion

Regulation of pNBC1 by secretagogues appears to be not solely dependent on its primary structure, but also on properties of the cell type in which it is expressed.

Stimulation of mammalian G-protein-responsive adenylyl cyclases by carbon dioxide

Carbon dioxide is fundamental to the physiology of all organisms. There is considerable interest in the precise molecular mechanisms that organisms use to directly sense CO₂. Here we demonstrate that a mammalian recombinant G-protein-activated adenylyl cyclase and the related Rv1625c adenylyl cyclase of Mycobacterium tuberculosis are specifically stimulated by CO₂. Stimulation occurred at physiological concentrations of CO₂ through increased k_cat. CO₂ increased the affinity of enzyme for metal co-factor, but contact with metal was not necessary as CO₂ interacted directly with apoenzyme. CO₂ stimulated the activity of both G-protein-regulated adenylyl cyclases and Rv1625c in vivo. Activation of G-protein regulated adenylyl cyclases by CO₂ gave a corresponding increase in cAMP-response element-binding protein (CREB) phosphorylation. Comparison of the responses of the G-protein regulated adenylyl cyclases and the molecularly, and biochemically distinct mammalian soluble adenylyl cyclase revealed that whereas G-protein-regulated enzymes are responsive to CO₂, the soluble adenylyl cyclase is responsive to both CO₂ and bicarbonate ion. We have, thus, identified a signaling enzyme by which eukaryotes can directly detect and respond to fluctuating CO₂.

Regulation of Na+/H+ exchanger 1 allosteric balance by its localization in cholesterol- and caveolin-rich membrane microdomains

The Na+/H+ exchanger 1, which plays an essential role in intracellular pH regulation in most tissues, is also known to be a key actor in both proliferative and apoptotic processes. Its activation by H+ is best described by the Monod-Wyman-Changeux model: the dimeric NHE-1 oscillates between a low and a high affinity conformation, the balance between the two forms being defined by the allosteric constant L(0). In this study, influence of cholesterol- and caveolin-rich microdomains on NHE-1 activity was examined by using cholesterol depleting agents, including methyl-beta-cyclodextrin (MBCD). These agents activated NHE-1 by modulating its L(0) parameter, which was reverted by cholesterol repletion. This activation was associated with NHE-1 relocation outside microdomains, and was distinct from NHE-1 mitogenic and hormonal stimulation; indeed MBCD and serum treatments were additive, and serum alone did not change NHE-1 localization. Besides, MBCD activated a serum-insensitive, constitutively active mutated NHE-1 ((625)KDKEEEIRK(635) into KNKQQQIRK). Finally, the membrane-dependent NHE-1 regulation occurred independently of Mitogen Activated Protein Kinases, especially Extracellular Regulated Kinase activation, although this kinase was activated by MBCD. In conclusion, localization of NHE-1 in membrane cholesterol- and caveolin-rich microdomains constitutes a novel physiological negative regulator of NHE-1 activity.

Caveolins in glial cell model systems: from detection to significance

Glial cells prevail in number and in diversity of cellular phenotypes in the nervous system. They have also gained prominence due to their multiple physiological and pathophysiological roles. Our current knowledge of the asymmetry and heterogeneity of the plasma membrane demands an in depth analysis of the diverse array of membrane microdomains postulated to exist in the context of glial cells. This review focuses and analyzes the studies reported to date on the detection of caveolae membrane rafts and the caveolin family members in glial cell model systems, the conditions leading to changes in their level of expression, and their functional and clinical significance. Outstanding in this work emerge the ubiquitous expression of caveolins, including the typically regarded 'muscle-specific' cav3, in diverse glial cell model systems, their participation in reactive astrogliosis, cancer, and their key relevance to calcium signaling. The knowledge obtained to date demands incorporation of the caveolins and caveolae membrane rafts in our current models on the role of glial cells in heath and neurological disease.

Heterodimers of adenylyl cyclases 2 and 5 show enhanced functional responses in the presence of Galpha s

Recent studies have demonstrated that adenylyl cyclase isoforms can form both homo- and heterodimers and that this may be the basic functional unit of these enzymes (see Cooper, D.M.F. and Crossthwaite, A.J. (2006) Trends. Pharmacol. Sci. 8:426-431). Here, we show that adenylyl cyclases 2 and 5 can form a functional heterodimeric complex in HEK293 cells using a combination of BRET, confocal imaging, co-immunoprecipitation and assays of adenylyl cyclase activity. The AC2/5 complex is formed constitutively and is stable in the presence of receptor or forskolin stimulation. The complex formed by AC2/5 is also much more sensitive to the presence of Galpha(s) and forskolin than either of the parent AC isoforms themselves. Finally, we also show that this complex can be detected in native tissues as AC2 and AC5 were localized to the same structures in adult mouse ventricular myocytes and neonatal mouse cardiac fibroblasts and could be co-immunoprecipitated from lysates of mouse heart.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

Regulation and Role of the Presynaptic and Myocardial Na+/H+Exchanger NHE1: Effects on the Sympathetic Nervous System in Heart Failure

In acute myocardial ischemia and in chronic heart failure, sympathetic activation with excessive norepinephrine (NE) release from and reduced NE reuptake into sympathetic nerve endings is a prominent cause of arrhythmias and cardiac dysfunction. The Na(+)/H(+) exchanger NHE1 is the predominant isoform in the heart. It contributes to cellular acid-base balance, and electrolyte, and volume homeostasis, and is activated in response to intracellular acidosis and/or activation of guanine nucleotide binding (G) protein-coupled receptors. NHE1 mediates its signaling via protein kinases A (PKA) or C (PKC). In cardiomyocytes, NHE1 is restricted to specialized membrane domains, where it regulates the activity of pH-sensitive proteins and modulates the driving force of the Na(+)/Ca(2+) exchanger. During acute ischemia/reperfusion and in heart failure the activity/amount of NHE1 is increased, leading to intracellular Ca(2+) overload and promoting structural (apoptosis, hypertrophy) and functional (arrhythmias, hypercontraction) myocardial damage. In sympathetic nerve endings, increased NHE1 activity results in the accumulation of axoplasmic Na(+) that diminishes the inward and/or favors the outward transport of NE via the neuronal norepinephrine transporter (NET). The increased NE levels within the nerve-muscle junction facilitate the sustained stimulation of myocardial alpha- and beta-adrenoceptors (ARs), which in turn aggravate the increases in myocardial NHE1 activity and the associated deleterious effects. Furthermore, the responsiveness of the beta-AR declines overtime, which results in further release of NE, initiating a vicious cycle. Accordingly, NHE1 is a potential candidate for targeted intervention to suppress this feedback loop.

Regulation of mitogen-activated protein kinase pathways by the plasma membrane Na+/H+ exchanger, NHE1

The mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, play a major role in the regulation of pivotal cellular processes such as cell death/survival balance, cell cycle progression, and cell migration. MAPK activity is regulated by a three-tiered phosphorelay system, which is in turn regulated by a complex network of signaling events and scaffolding proteins. The ubiquitous plasma membrane Na(+)/H(+) exchanger NHE1 is activated by, and implicated in, the physiological/pathophysiological responses to many of the same stimuli that modulate MAPK activity. While under some conditions, NHE1 is regulated by MAPKs, a number of studies have, conversely, implicated NHE1 in the regulation of MAPK activity. Here, we discuss the current evidence indicating the involvement of NHE1 in MAPK regulation, the mechanisms by which this may occur, and the possible physiological and pathophysiological relevance of this phenomenon.

Relationships Between Calcium and pH in the Regulation of the Slow Afterhyperpolarization in Cultured Rat Hippocampal Neurons

The Ca2+-dependent slow afterhyperpolarization (AHP) is an important determinant of neuronal excitability. Although it is established that modest changes in extracellular pH (pHo) modulate the slow AHP, the relative contributions of changes in the priming Ca2+ signal and intracellular pH (pHi) to this effect remain poorly defined. To gain a better understanding of the modulation of the slow AHP by changes in pHo, we performed simultaneous recordings of intracellular free calcium concentration ([Ca2+]i), pHi, and the slow AHP in cultured rat hippocampal neurons coloaded with the Ca2+- and pH-sensitive fluorophores fura-2 and SNARF-5F, respectively, and whole cell patch-clamped using the perforated patch technique. Decreasing pHo from 7.2 to 6.5 lowered pHi, reduced the magnitude of depolarization-evoked [Ca2+]i transients, and inhibited the subsequent slow AHP; opposite effects were observed when pHo was increased from 7.2 to 7.5. Although decreases and increases in pHi (at a constant pHo) reduced and augmented, respectively, the slow AHP in the absence of marked changes in preceding [Ca2+]i transients, the inhibition of the slow AHP by decreases in pHo was correlated with low pHo-dependent reductions in [Ca2+]i transients rather than the decreases in pHi that accompanied the decreases in pHo. In contrast, high pHo-induced increases in the slow AHP were correlated with the accompanying increases in pHi rather than high pHo-dependent increases in [Ca2+]i transients. The results indicate that changes in pHo modulate the slow AHP in a manner that depends on the direction of the pHo change and substantiate a role for changes in pHi in modulating the slow AHP during changes in pHo.

Layers of organization of cAMP microdomains in a simple cell

Based on a variety of single-cell measurements, the notion that cAMP microdomains exist in cells is being increasingly embraced. The cellular and molecular underpinnings of this organization are also steadily being revealed. A dependence of Ca(2+)-sensitive ACs (adenylate cyclases) in HEK-293 cells (human embryonic kidney cells) on capacitative Ca(2+) entry is enforced by their presence in lipid rafts and protein-protein interactions. In these cells, many of the participants in the cAMP cascade, including AC, phosphodiesterase 4, cAMP-dependent protein kinase [PKA (protein kinase A)] and protein phosphatase 2A, are now seen to be involved in higher order assemblies. Moreover, the presence of Na(+)/H(+) exchanger 1 in these domains creates a microclimate, protected against global swings in cellular pH. The Ca(2+)-stimulatable AC8, which is targeted to these regions, can sequester calmodulin for its own regulatory purposes. These devices are a sampling of the multiple layers of organization that are in place -- even in a simple cell -- to ensure faithful and economical communication of the cAMP message.

Cell surface F1Fo ATP synthase: a new paradigm?

The mitochondrial F1Fo adenosine triphosphate (ATP) synthase is one of the most thoroughly studied enzyme complexes known. Yet, a number of new observations suggesting that the enzyme is also located on the cell surface necessitate further investigation. While the mitochondrial synthase utilizes the proton gradient generated by oxidative phosphorylation to power ATP synthesis, the cell surface synthase has instead been implicated in numerous activities, including the mediation of intracellular pH, cellular response to antiangiogenic agents, and cholesterol homeostasis. Intriguingly, a common thread uniting these various models of cell surface ATP synthase functions is the apparently caveolar distribution of the enzyme. Recent studies concerning the cell surface ATP synthase manifest applications in the regulation of serum cholesterol levels, cellular proliferation and antitumor strategies. This review addresses the expression, interactions, functions, and consequences of inhibition of cell surface ATP synthase, an enzyme now displaying a shift in paradigm, as well as of location.

Compartmentalization of adenylate cyclase and cAMP signalling

Concepts of cAMP signalling have changed dramatically from the linear cascades of just a few years ago, with the realization that numerous cellular processes affect this motif. These influences include other signalling pathways--most significantly Ca2+, scaffolding proteins (which are themselves variously regulated) to organize the elements of the pathway, and subcellular targeting of components. An obvious implication of this organization is that global measurements of cAMP may trivialize the complexity of the cAMP signals and obscure the regulation of targets. In this presentation, current developments on the targeting and assembly of ACs (adenylate cyclases) and their delivery to selected raft or non-raft domains of the plasma membrane will be discussed, along with the susceptibility of raft-targeted ACs to very discrete modes of increases in the intracellular Ca2+ concentration. Single-cell explorations of cAMP dynamics, as measured with cyclic nucleotide-gated channels, are also described in this paper, particularly as applied to cells in which the composition of AKAP (A-kinase anchoring protein)-PKA (protein kinase A)-PDE (phosphodiesterase) assemblies is probed by RNA interference ablation of defined AKAPs.