Differential regulation of the apical plasma membrane Ca(2+) -ATPase by protein kinase A in parotid acinar cells

Postsynaptic Calcium Extrusion at the Mouse Neuromuscular Junction Alkalinizes the Synaptic Cleft

Neurotransmission is shaped by extracellular pH. Alkalization enhances pH-sensitive transmitter release and receptor activation, whereas acidification inhibits these processes and can activate acid-sensitive conductances in the synaptic cleft. Previous work has shown that the synaptic cleft can either acidify because of synaptic vesicular release and/or alkalize because of Ca2+ extrusion by the plasma membrane ATPase (PMCA). The direction of change differs across synapse types. At the mammalian neuromuscular junction (NMJ), the direction and magnitude of pH transients in the synaptic cleft during transmission remain ambiguous. We set out to elucidate the extracellular pH transients that occur at this cholinergic synapse under near-physiological conditions and identify their sources. We monitored pH-dependent changes in the synaptic cleft of the mouse levator auris longus using viral expression of the pseudoratiometric probe pHusion-Ex in the muscle. Using mice from both sexes, a significant and prolonged alkalization occurred when stimulating the connected nerve for 5 s at 50 Hz, which was dependent on postsynaptic intracellular Ca2+ release. Sustained stimulation for a longer duration (20 s at 50 Hz) caused additional prolonged net acidification at the cleft. To investigate the mechanism underlying cleft alkalization, we used muscle-expressed GCaMP3 to monitor the contribution of postsynaptic Ca2+ Activity-induced liberation of intracellular Ca2+ in muscle positively correlated with alkalization of the synaptic cleft, whereas inhibiting PMCA significantly decreased the extent of cleft alkalization. Thus, cholinergic synapses of the mouse NMJ typically alkalize because of cytosolic Ca2+ liberated in muscle during activity, unless under highly strenuous conditions where acidification predominates.SIGNIFICANCE STATEMENT Changes in synaptic cleft pH alter neurotransmission, acting on receptors and channels on both sides of the synapse. Synaptic acidification has been associated with a myriad of diseases in the central and peripheral nervous system. Here, we report that in near-physiological recording conditions the cholinergic neuromuscular junction shows use-dependent bidirectional changes in synaptic cleft pH-immediate alkalinization and a long-lasting acidification under prolonged stimulation. These results provide further insight into physiologically relevant changes at cholinergic synapses that have not been defined previously. Understanding and identifying synaptic pH transients during and after neuronal activity provides insight into short-term synaptic plasticity synapses and may identify therapeutic targets for diseases.

Dysregulation of the microbiota-brain axis during long-term exposure to polystyrene nanoplastics in rats and the protective role of dihydrocaffeic acid

Polystyrene nano-plastics (PS-NPs) can be accumulated in the food chain and can penetrate biological barriers to affect multiple physiological functions. However, the adverse effects of nano-plastics on mammals and the underlying mechanism still remain unknown. To fill the gaps, our study administrated low-dose PS-NPs (50 and 100 μg/L) for 24 consecutive weeks in rats. Behavioral and morphological evaluations were performed to assess the neurobehavoirs. A combined analysis of multiple omics was used to evaluate the dysfunctions of the gut-microbe-brain axis. After dihydrochalcone(NHDC) treatment in the PS-NPs rat model, the inflammation response and apoptosis process were assessed and proteomics was used to explore the underlying mechanism. Our results indicated that long-term exposure to low-dose PS-NPs could induce abnormal neurobehaviors and amygdaloid nucleus impairment, and stimulate inflammatory responses and apoptosis. Metagenomics results revealed that four microbial phyla including Proteobacteria, Firmicutes, Defferibacteres, and Bacteroidetes changed significantly compared to the control. Targeted metabolomics analysis in the feces showed alteration of 122 metabolites induced by the PS-NPs exposure, among which the content of dihydrocaffeic acid was significantly associated with the different microbial genera and pivotal differential metabolites in the amygdaloid nucleus. And NHDC treatment significantly alleviated PS-NP-induced neuroinflammation and apoptosis and the cyclic adenosine monophosphate(cAMP)/protein kinase A(PKA)/phosphorylated cAMP-response element binding protein(p-CREB)/plasma membrane calcium-transporting ATPase 2(Atp2b2) signaling pathway was identified in the proteomics. In conclusion, long-term exposure to low-dose PS-NPs has adverse effects on emotion through the dysregulation of the gut-brain axis, and dihydrocaffeic acid can alleviate these effects via the cAMP/PKA/p-CREB/Atp2b2 signaling pathway.

PMCA Ca2+ clearance in dental enamel cells depends on the magnitude of cytosolic Ca2

Enamel formation (amelogenesis) is a two-step process whereby crystals partially grow during the secretory stage followed by a significant growth expansion during the maturation stage concurrent with an increase in vectorial Ca2+ transport. This requires tight regulation of cytosolic Ca2+ (c Ca2+ ) concentration in the enamel forming ameloblasts by controlling Ca2+ influx (entry) and Ca2+ extrusion (clearance). Gene and protein expression studies suggest that the plasma membrane Ca2+ -ATPases (PMCA1-4) are likely involved in c Ca2+ extrusion in ameloblasts, yet no functional analysis of these pumps has been reported nor whether their activity changes across amelogenesis. PMCAs have high Ca2+ affinity and low Ca2+ clearance which may be a limiting factor in their contribution to enamel formation as maturation stage ameloblasts handle high Ca2+ loads. We analyzed PMCA function in rat secretory and maturation ameloblasts by blocking or potentiating these pumps. Low/moderate elevations in c Ca2+ measured using the Ca2+ probe Fura-2-AM show that secretory ameloblasts clear Ca2+ faster than maturation stage cells through PMCAs. This process was completely inhibited by an external alkaline (pH 9.0) solution or was significantly delayed by the PMCA blockers vanadate and caloxin 1b1. Eliciting higher c Ca2+ transients via the activation of the ORAI1 Ca2+ channel showed that the PMCAs of maturation ameloblasts were more efficient. Inhibiting PMCAs decreased the rate of Ca2+ influx via ORAI1 but potentiation with forskolin had no effect. Our findings suggest that PMCAs are functional Ca2+ pumps during amelogenesis regulating c Ca2+ upon low and/or moderate Ca2+ stimulus in secretory stage, thus participating in amelogenesis.

Plasma Membrane Ca2+ Pump PMCA4z Is More Active Than Splicing Variant PMCA4x

The plasma membrane Ca²⁺ pumps (PMCA) are P-ATPases that control Ca²⁺ signaling and homeostasis by transporting Ca²⁺ out of the eukaryotic cell. Humans have four genes that code for PMCA isoforms (PMCA1-4). A large diversity of PMCA isoforms is generated by alternative mRNA splicing at sites A and C. The different PMCA isoforms are expressed in a cell-type and developmental-specific manner and exhibit differential sensitivity to a great number of regulatory mechanisms. PMCA4 has two A splice variants, the forms "x" and "z". While PMCA4x is ubiquitously expressed and relatively well-studied, PMCA4z is less characterized and its expression is restricted to some tissues such as the brain and heart muscle. PMCA4z lacks a stretch of 12 amino acids in the so-called A-M3 linker, a conformation-sensitive region of the molecule connecting the actuator domain (A) with the third transmembrane segment (M3). We expressed in yeast PMCA4 variants "x" and "z", maintaining constant the most frequent splice variant "b" at the C-terminal end, and obtained purified preparations of both proteins. In the basal autoinhibited state, PMCA4zb showed a higher ATPase activity and a higher apparent Ca²⁺ affinity than PMCA4xb. Both isoforms were stimulated by calmodulin but PMCA4zb was more strongly activated by acidic lipids than PMCA4xb. The results indicate that a PMCA4 intrinsically more active and more responsive to acidic lipids is produced by the variant "z" of the splicing site A.

Cross-Talk Between the Adenylyl Cyclase/cAMP Pathway and Ca2+ Homeostasis

Cyclic AMP and Ca²⁺ are the first second or intracellular messengers identified, unveiling the cellular mechanisms activated by a plethora of extracellular signals, including hormones. Cyclic AMP generation is catalyzed by adenylyl cyclases (ACs), which convert ATP into cAMP and pyrophosphate. By the way, Ca²⁺, as energy, can neither be created nor be destroyed; Ca²⁺ can only be transported, from one compartment to another, or chelated by a variety of Ca²⁺-binding molecules. The fine regulation of cytosolic concentrations of cAMP and free Ca²⁺ is crucial in cell function and there is an intimate cross-talk between both messengers to fine-tune the cellular responses. Cancer is a multifactorial disease resulting from a combination of genetic and environmental factors. Frequent cases of cAMP and/or Ca²⁺ homeostasis remodeling have been described in cancer cells. In those tumoral cells, cAMP and Ca²⁺ signaling plays a crucial role in the development of hallmarks of cancer, including enhanced proliferation and migration, invasion, apoptosis resistance, or angiogenesis. This review summarizes the cross-talk between the ACs/cAMP and Ca²⁺ intracellular pathways with special attention to the functional and reciprocal regulation between Orai1 and AC8 in normal and cancer cells.

Ca2+ Signaling in Exocrine Cells

Calcium (Ca²⁺) and cyclic AMP (cAMP) signaling cross talk and synergize to stimulate the cardinal functions of exocrine cells, regulated exocytosis, and fluid and electrolyte secretion. This physiological process requires the organization of the two signaling pathways into complexes at defined cellular domains and close placement. Such domains are formed by membrane contact sites (MCS). This review discusses the basic properties of Ca²⁺ signaling in exocrine cells, the role of MCS in the organization of cell signaling and in cross talk and synergism between the Ca²⁺ and cAMP signaling pathways and, finally, the mechanism by which the Ca²⁺ and cAMP pathways synergize to stimulate epithelial fluid and electrolyte secretion.

Hydrogen peroxide inhibits Ca2+ efflux through plasma membrane Ca2+-ATPase in mouse parotid acinar cells

Intracellular Ca²⁺ mobilization is closely linked with the initiation of salivary secretion in parotid acinar cells. Reactive oxygen species (ROS) are known to be related to a variety of oxidative stress-induced cellular disorders and believed to be involved in salivary impairments. In this study, we investigated the underlying mechanism of hydrogen peroxide (H₂O₂) on cytosolic Ca²⁺ accumulation in mouse parotid acinar cells. Intracellular Ca²⁺ levels were slowly elevated when 1 mM H₂O₂ was perfused in the presence of normal extracellular Ca²⁺. In a Ca²⁺-free medium, 1 mM H₂O₂ still enhanced the intracellular Ca²⁺ level. Ca²⁺ entry tested using manganese quenching technique was not affected by perfusion of 1 mM H₂O₂. On the other hand, 10 mM H₂O₂ induced more rapid Ca²⁺ accumulation and facilitated Ca²⁺ entry from extracellular fluid. Ca²⁺ refill into intracellular Ca²⁺ store and inositol 1,4,5-trisphosphate (1 µM)-induced Ca²⁺ release from Ca²⁺ store was not affected by 1 mM H₂O₂ in permeabilized cells. Ca²⁺ efflux through plasma membrane Ca²⁺-ATPase (PMCA) was markedly blocked by 1 mM H₂O₂ in thapsigargin-treated intact acinar cells. Antioxidants, either catalase or dithiothreitol, completely protected H₂O₂-induced Ca²⁺ accumulation through PMCA inactivation. From the above results, we suggest that excessive production of H₂O₂ under pathological conditions may lead to cytosolic Ca²⁺ accumulation and that the primary mechanism of H₂O₂-induced Ca²⁺ accumulation is likely to inhibit Ca²⁺ efflux through PMCA rather than mobilize Ca²⁺ ions from extracellular medium or intracellular stores in mouse parotid acinar cells.

The serum protein renalase reduces injury in experimental pancreatitis

Acute pancreatitis is a disease associated with inflammation and tissue damage. One protein that protects against acute injury, including ischemic injury to both the kidney and heart, is renalase, which is secreted into the blood by the kidney and other tissues. However, whether renalase reduces acute injury associated with pancreatitis is unknown. Here, we used both in vitro and in vivo murine models of acute pancreatitis to study renalase's effects on this condition. In isolated pancreatic lobules, pretreatment with recombinant human renalase (rRNLS) blocked zymogen activation caused by cerulein, carbachol, and a bile acid. Renalase also blocked cerulein-induced cell injury and histological changes. In the in vivo cerulein model of pancreatitis, genetic deletion of renalase resulted in more severe disease, and administering rRNLS to cerulein-exposed WT mice after pancreatitis onset was protective. Because pathological increases in acinar cell cytosolic calcium levels are central to the initiation of acute pancreatitis, we also investigated whether rRNLS could function through its binding protein, plasma membrane calcium ATPase 4b (PMCA4b), which excretes calcium from cells. We found that PMCA4b is expressed in both murine and human acinar cells and that a PMCA4b-selective inhibitor worsens pancreatitis-induced injury and blocks the protective effects of rRNLS. These findings suggest that renalase is a protective plasma protein that reduces acinar cell injury through a plasma membrane calcium ATPase. Because exogenous rRNLS reduces the severity of acute pancreatitis, it has potential as a therapeutic agent.

Importance of bicarbonate transport in pH control during amelogenesis - need for functional studies

Dental enamel, the hardest mammalian tissue, is produced by ameloblasts. Ameloblasts show many similarities to other transporting epithelia although their secretory product, the enamel matrix, is quite different. Ameloblasts direct the formation of hydroxyapatite crystals, which liberate large quantities of protons that then need to be buffered to allow mineralization to proceed. Buffering requires a tight pH regulation and secretion of bicarbonate by ameloblasts. Many investigations have used immunohistochemical and knockout studies to determine the effects of these genes on enamel formation, but up till recently very little functional data were available for mineral ion transport. To address this, we developed a novel 2D in vitro model using HAT-7 ameloblast cells. HAT-7 cells can be polarized and develop functional tight junctions. Furthermore, they are able to accumulate bicarbonate ions from the basolateral to the apical fluid spaces. We propose that in the future, the HAT-7 2D system along with similar cellular models will be useful to functionally model ion transport processes during amelogenesis. Additionally, we also suggest that similar approaches will allow a better understanding of the regulation of the cycling process in maturation-stage ameloblasts, and the pH sensory mechanisms, which are required to develop sound, healthy enamel.

Evidence for Bicarbonate Secretion by Ameloblasts in a Novel Cellular Model

Formation and growth of hydroxyapatite crystals during amelogenesis generate a large number of protons that must be neutralized, presumably by HCO3 (-)ions transported from ameloblasts into the developing enamel matrix. Ameloblasts express a number of transporters and channels known to be involved in HCO3 (-)transport in other epithelia. However, to date, there is no functional evidence for HCO3 (-)transport in these cells. To address questions related to HCO3 (-)export from ameloblasts, we have developed a polarized 2-dimensional culture system for HAT-7 cells, a rat cell line of ameloblast origin. HAT-7 cells were seeded onto Transwell permeable filters. Transepithelial resistance was measured as a function of time, and the expression of transporters and tight junction proteins was investigated by conventional and quantitative reverse transcription polymerase chain reaction. Intracellular pH regulation and HCO3 (-)transport were assessed by microfluorometry. HAT-7 cells formed epithelial layers with measureable transepithelial resistance on Transwell permeable supports and expressed claudin-1, claudin-4, and claudin-8-key proteins for tight junction formation. Transport proteins previously described in maturation ameloblasts were also present in HAT-7 cells. Microfluorometry showed that the HAT-7 cells were polarized with a high apical membrane CO2 permeability and vigorous basolateral HCO3 (-)uptake, which was sensitive to Na(+)withdrawal, to the carbonic anhydrase inhibitor acetazolamide and to H2DIDS inhibition. Measurements of transepithelial HCO3 (-)transport showed a marked increase in response to Ca(2+)- and cAMP-mobilizing stimuli. Collectively, 2-dimensional HAT-7 cell cultures on permeable supports 1) form tight junctions, 2) express typical tight junction proteins and electrolyte transporters, 3) are functionally polarized, and 4) can accumulate HCO3 (-)ions from the basolateral side and secrete them at the apical membrane. These studies provide evidence for a regulated, vectorial, basolateral-to-apical bicarbonate transport in polarized HAT-7 cells. We therefore propose that the HAT-7 cell line is a useful functional model for studying electrolyte transport by ameloblasts.

Rats undernourished in utero have altered Ca2+ signaling and reduced fertility in adulthood

Epidemiological and animal studies have shown that placental undernutrition impairs reproduction in adult offspring, but the underlying molecular mechanisms within the male genital tract remain unknown. Due to its special physiological characteristics in transport and the modulation of the environment to which its luminal content is exposed, we hypothesized that the vas deferens would be a highly sensitive target. The goals were to investigate whether intrauterine malnutrition affects molecular mechanisms related to Ca(2+)- and oxidative stress-modulated processes and causes structural alterations in the adult rat vas deferens that could attenuate fecundity and fertility. Male adult rats malnourished in utero had increased vas deferens weight associated with thickening of the muscular coat, a decrease in the total and haploid germ cells, a marked increase in the immature cells, and a decline in the numbers of pregnant females and total offspring per male rat. The ex vivo response of vas deferens from malnourished rats demonstrated an accentuated decrease in the contractile response to phenylephrine. The vas deferens had a marked decrease in Ca(2+) transport due to the uncoupling of Ca(2+)-stimulated ATP hydrolysis and ATP-driven Ca(2+) flux, and the downregulation of both sarco-endoplasmic reticulum Ca(2+)-ATPase 2 and the coupling factor 12-kDa FK506-binding protein. An increase in protein carbonylation (a marker of oxidative damage) and an imbalance between protein kinases C and A were observed as a legacy of undernutrition in early life. These results provide the structural and molecular basis to explain at least in part how maternal undernutrition affects fecundity and fertility in adult male rats.

Homer2 protein regulates plasma membrane Ca²⁺-ATPase-mediated Ca²⁺ signaling in mouse parotid gland acinar cells

Homer proteins are scaffold molecules with a domain structure consisting of an N-terminal Ena/VASP homology 1 protein-binding domain and a C-terminal leucine zipper/coiled-coil domain. The Ena/VASP homology 1 domain recognizes proline-rich motifs and binds multiple Ca(2+)-signaling proteins, including G protein-coupled receptors, inositol 1,4,5-triphosphate receptors, ryanodine receptors, and transient receptor potential channels. However, their role in Ca(2+) signaling in nonexcitable cells is not well understood. In this study, we investigated the role of Homer2 on Ca(2+) signaling in parotid gland acinar cells using Homer2-deficient (Homer2(-/-)) mice. Homer2 is localized at the apical pole in acinar cells. Deletion of Homer2 did not affect inositol 1,4,5-triphosphate receptor localization or channel activity and did not affect the expression and activity of sarco/endoplasmic reticulum Ca(2+)-ATPase pumps. In contrast, Homer2 deletion markedly increased expression of plasma membrane Ca(2+)-ATPase (PMCA) pumps, in particular PMCA4, at the apical pole. Accordingly, Homer2 deficiency increased Ca(2+) extrusion by acinar cells. These findings were supported by co-immunoprecipitation of Homer2 and PMCA in wild-type parotid cells and transfected human embryonic kidney 293 (HEK293) cells. We identified a Homer-binding PPXXF-like motif in the N terminus of PMCA that is required for interaction with Homer2. Mutation of the PPXXF-like motif did not affect the interaction of PMCA with Homer1 but inhibited its interaction with Homer2 and increased Ca(2+) clearance by PMCA. These findings reveal an important regulation of PMCA by Homer2 that has a central role on PMCA-mediated Ca(2+) signaling in parotid acinar cells.

cAMP and Ca²⁺ signaling in secretory epithelia: crosstalk and synergism

The Ca(2+) and cAMP/PKA pathways are the primary signaling systems in secretory epithelia that control virtually all secretory gland functions. Interaction and crosstalk in Ca(2+) and cAMP signaling occur at multiple levels to control and tune the activity of each other. Physiologically, Ca(2+) and cAMP signaling operate at 5-10% of maximal strength, but synergize to generate the maximal response. Although synergistic action of the Ca(2+) and cAMP signaling is the common mode of signaling and has been known for many years, we know very little of the molecular mechanism and mediators of the synergism. In this review, we discuss crosstalk between the Ca(2+) and cAMP signaling and the function of IRBIT (IP3 receptors binding protein release with IP3) as a third messenger that mediates the synergistic action of the Ca(2+) and cAMP signaling.

Glycolytic ATP fuels the plasma membrane calcium pump critical for pancreatic cancer cell survival

Pancreatic cancer is an aggressive cancer with poor prognosis and limited treatment options. Cancer cells rapidly proliferate and are resistant to cell death due, in part, to a shift from mitochondrial metabolism to glycolysis. We hypothesized that this shift is important in regulating cytosolic Ca²⁺ ([Ca²⁺]_i), as the ATP-dependent plasma membrane Ca²⁺ ATPase (PMCA) is critical for maintaining low [Ca²⁺]_i and thus cell survival. The present study aimed to determine the relative contribution of mitochondrial versus glycolytic ATP in fuelling the PMCA in human pancreatic cancer cells. We report that glycolytic inhibition induced profound ATP depletion, PMCA inhibition, [Ca²⁺]_i overload, and cell death in PANC1 and MIA PaCa-2 cells. Conversely, inhibition of mitochondrial metabolism had no effect, suggesting that glycolytic ATP is critical for [Ca²⁺]_i homeostasis and thus survival. Targeting the glycolytic regulation of the PMCA may, therefore, be an effective strategy for selectively killing pancreatic cancer while sparing healthy cells.

Growth factors polymerized within fibrin hydrogel promote amylase production in parotid cells

Salivary gland cell differentiation has been a recurring challenge for researchers as primary salivary cells show a loss of phenotype in culture. Particularly, parotid cells show a marked decrease in amylase expression, the loss of tight junction organization and proper cell function. Previously, Matrigel has been used successfully as an extracellular matrix; however, it is not practical for in vivo applications as it is tumorigenic. An alternative method could rely on the use of fibrin hydrogel (FH), which has been used extensively in biomedical engineering applications ranging from cardiovascular tissue engineering to wound-healing experiments. Although several groups have examined the effects of a three-dimensional (3D) environment on salivary cell cultures, little is known about the effects of FH on salivary cell cultures. The current study developed a 3D cell culture model to support parotid gland cell differentiation using a combination of FH and growth factor-reduced Matrigel (GFR-MG). Furthermore, FH polymerized with a combination of EGF and IGF-1 induced formation of 3D spheroids capable of amylase expression and an agonist-induced increase in the intracellular Ca(2+) concentration ([Ca(2+)]i) in salivary cells. These studies represent an initial step toward the construction of an artificial salivary gland to restore salivary gland dysfunction. This is necessary to reduce xerostomia in patients with compromised salivary function.

Painful nerve injury increases plasma membrane Ca2+-ATPase activity in axotomized sensory neurons

BACKGROUND

The plasma membrane Ca2+-ATPase (PMCA) is the principal means by which sensory neurons expel Ca2+ and thereby regulate the concentration of cytoplasmic Ca2+ and the processes controlled by this critical second messenger. We have previously found that painful nerve injury decreases resting cytoplasmic Ca2+ levels and activity-induced cytoplasmic Ca2+ accumulation in axotomized sensory neurons. Here we examine the contribution of PMCA after nerve injury in a rat model of neuropathic pain.

RESULTS

PMCA function was isolated in dissociated sensory neurons by blocking intracellular Ca2+ sequestration with thapsigargin, and cytoplasmic Ca2+ concentration was recorded with Fura-2 fluorometry. Compared to control neurons, the rate at which depolarization-induced Ca2+ transients resolved was increased in axotomized neurons after spinal nerve ligation, indicating accelerated PMCA function. Electrophysiological recordings showed that blockade of PMCA by vanadate prolonged the action potential afterhyperpolarization, and also decreased the rate at which neurons could fire repetitively.

CONCLUSION

We found that PMCA function is elevated in axotomized sensory neurons, which contributes to neuronal hyperexcitability. Accelerated PMCA function in the primary sensory neuron may contribute to the generation of neuropathic pain, and thus its modulation could provide a new pathway for peripheral treatment of post-traumatic neuropathic pain.

Plasma membrane calcium pump (PMCA) isoform 4 is targeted to the apical membrane by the w-splice insert from PMCA2

Local Ca(2+) signaling requires proper targeting of the Ca(2+) signaling toolkit to specific cellular locales. Different isoforms of the plasma membrane Ca(2+) pump (PMCA) are responsible for Ca(2+) extrusion at the apical and basolateral membrane of polarized epithelial cells, but the mechanisms and signals for differential targeting of the PMCAs are not well understood. Recent work demonstrated that the alternatively spliced w-insert in PMCA2 directs this pump to the apical membrane. We now show that inserting the w-insert into the corresponding location of the PMCA4 isoform confers apical targeting to this normally basolateral pump. Mutation of a di-leucine motif in the C-tail thought to be important for basolateral targeting did not enhance apical localization of the chimeric PMCA4(2w)/b. In contrast, replacing the C-terminal Val residue by Leu to optimize the PDZ ligand site for interaction with the scaffolding protein NHERF2 enhanced the apical localization of PMCA4(2w)/b, but not of PMCA4x/b. Functional studies showed that both apical PMCA4(2w)/b and basolateral PMCA4x/b handled ATP-induced Ca(2+) signals with similar kinetics, suggesting that isoform-specific functional characteristics are retained irrespective of membrane targeting. Our results demonstrate that the alternatively spliced w-insert provides autonomous apical targeting information in the PMCA without altering its functional characteristics.

Apical scaffolding protein NHERF2 modulates the localization of alternatively spliced plasma membrane Ca2+ pump 2B variants in polarized epithelial cells

The membrane localization of the plasma membrane Ca(2+)-ATPase isoform 2 (PMCA2) in polarized cells is determined by alternative splicing; the PMCA2w/b splice variant shows apical localization, whereas the PMCA2z/b and PMCA2x/b variants are mostly basolateral. We previously reported that PMCA2b interacts with the PDZ protein Na(+)/H(+) exchanger regulatory factor 2 (NHERF2), but the role of this interaction for the specific membrane localization of PMCA2 is not known. Here we show that co-expression of NHERF2 greatly enhanced the apical localization of GFP-tagged PMCA2w/b in polarized Madin-Darby canine kidney cells. GFP-PMCA2z/b was also redirected to the apical membrane by NHERF2, whereas GFP-PMCA2x/b remained exclusively basolateral. In the presence of NHERF2, GFP-PMCA2w/b co-localized with the actin-binding protein ezrin even after disruption of the actin cytoskeleton by cytochalasin D or latrunculin B. Surface biotinylation and fluorescence recovery after photobleaching experiments demonstrated that NHERF2-mediated anchorage to the actin cytoskeleton reduced internalization and lateral mobility of the pump. Our results show that the specific interaction with NHERF2 enhances the apical concentration of PMCA2w/b by anchoring the pump to the apical membrane cytoskeleton. The data also suggest that the x/b splice form of PMCA2 contains a dominant lateral targeting signal, whereas the targeting and localization of the z/b form are more flexible and not fully determined by intrinsic sequence features.

Plasma membrane calcium pump regulation by metabolic stress

The plasma membrane Ca²⁺-ATPase (PMCA) is an ATP-driven pump that is critical for the maintenance of low resting [Ca²⁺]_i in all eukaryotic cells. Metabolic stress, either due to inhibition of mitochondrial or glycolytic metabolism, has the capacity to cause ATP depletion and thus inhibit PMCA activity. This has potentially fatal consequences, particularly for non-excitable cells in which the PMCA is the major Ca²⁺ efflux pathway. This is because inhibition of the PMCA inevitably leads to cytosolic Ca²⁺ overload and the consequent cell death. However, the relationship between metabolic stress, ATP depletion and inhibition of the PMCA is not as simple as one would have originally predicted. There is increasing evidence that metabolic stress can lead to the inhibition of PMCA activity independent of ATP or prior to substantial ATP depletion. In particular, there is evidence that the PMCA has its own glycolytic ATP supply that can fuel the PMCA in the face of impaired mitochondrial function. Moreover, membrane phospholipids, mitochondrial membrane potential, caspase/calpain cleavage and oxidative stress have all been implicated in metabolic stress-induced inhibition of the PMCA. The major focus of this review is to challenge the conventional view of ATP-dependent regulation of the PMCA and bring together some of the alternative or additional mechanisms by which metabolic stress impairs PMCA activity resulting in cytosolic Ca²⁺ overload and cytotoxicity.

The contribution of AKAP5 in amylase secretion from mouse parotid acini

A-kinase (PKA) anchoring proteins (AKAPs) are essential for targeting type II PKA to specific locales in the cell to control function. In the present study, AKAP5 (formerly AKAP150) and AKAP6 were identified in mouse parotid acini by type II PKA regulatory subunit (RII) overlay assay and Western blot analysis of mouse parotid cellular fractions, and the role of AKAP5 in mouse parotid acinar cell secretion was determined. Mice were euthanized with CO(2). Immunofluorescence staining of acinar cells localized AKAP5 to the basolateral membrane, whereas AKAP6 was associated with the perinuclear region. In functional studies, amylase secretion from acinar cells of AKAP5 mutant [knockout (KO)] mice treated with the beta-adrenergic agonist, isoproterenol, was reduced overall by 30-40% compared with wild-type (WT) mice. In contrast, amylase secretion in response to the adenylyl cyclase (AC) activator, forskolin, and the cAMP-dependent protein kinase (PKA) activator, N(6)-phenyl-cAMP, was not statistically different in acini from WT and AKAP5 KO mice. Treatment of acini with isoproterenol mimicked the effect of the Epac activator, 8-(4-methoxyphenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8-pMeOPT-2'-O-Me-cAMP), in stimulating Rap1. However, in contrast to isoproterenol, treatment of acini with 8-pMeOPT-2'-O-Me-cAMP resulted in stimulation of amylase secretion from both AKAP5 KO and WT acinar cells. As a scaffolding protein, AKAP5 was found to coimmunoprecipitate with AC6, but not AC8. Data suggest that isoproterenol-stimulated amylase secretion occurs via both an AKAP5/AC6/PKA complex and a PKA-independent, Epac pathway in mouse parotid acini.

Oxidant-induced inhibition of the plasma membrane Ca2+-ATPase in pancreatic acinar cells: role of the mitochondria

Impairment of the normal spatiotemporal pattern of intracellular Ca(2+) ([Ca(2+)](i)) signaling, and in particular, the transition to an irreversible "Ca(2+) overload" response, has been implicated in various pathophysiological states. In some diseases, including pancreatitis, oxidative stress has been suggested to mediate this Ca(2+) overload and the associated cell injury. We have previously demonstrated that oxidative stress with hydrogen peroxide (H(2)O(2)) evokes a Ca(2+) overload response and inhibition of plasma membrane Ca(2+)-ATPase (PMCA) in rat pancreatic acinar cells (Bruce JI and Elliott AC. Am J Physiol Cell Physiol 293: C938-C950, 2007). The aim of the present study was to further examine this oxidant-impaired inhibition of the PMCA, focusing on the role of the mitochondria. Using a [Ca(2+)](i) clearance assay in which mitochondrial Ca(2+) uptake was blocked with Ru-360, H(2)O(2) (50 microM-1 mM) markedly inhibited the PMCA activity. This H(2)O(2)-induced inhibition of the PMCA correlated with mitochondrial depolarization (assessed using tetramethylrhodamine methylester fluorescence) but could occur without significant ATP depletion (assessed using Magnesium Green fluorescence). The H(2)O(2)-induced PMCA inhibition was sensitive to the mitochondrial permeability transition pore (mPTP) inhibitors, cyclosporin-A and bongkrekic acid. These data suggest that oxidant-induced opening of the mPTP and mitochondrial depolarization may lead to an inhibition of the PMCA that is independent of mitochondrial Ca(2+) handling and ATP depletion, and we speculate that this may involve the release of a mitochondrial factor. Such a phenomenon may be responsible for the Ca(2+) overload response, and for the transition between apoptotic and necrotic cell death thought to be important in many disease states.

Proinflammatory cytokines tumor necrosis factor-alpha and interferon-gamma alter tight junction structure and function in the rat parotid gland Par-C10 cell line

Sjögren's syndrome (SS) is an autoimmune disorder characterized by inflammation and dysfunction of salivary glands, resulting in impaired secretory function. The production of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) is elevated in exocrine glands of patients with SS, although little is known about the effects of these cytokines on salivary epithelial cell functions necessary for saliva secretion, including tight junction (TJ) integrity and the establishment of transepithelial ion gradients. The present study demonstrates that chronic exposure of polarized rat parotid gland (Par-C10) epithelial cell monolayers to TNF-alpha and IFN-gamma decreases transepithelial resistance (TER) and anion secretion, as measured by changes in short-circuit current (I(sc)) induced by carbachol, a muscarinic cholinergic receptor agonist, or UTP, a P2Y(2) nucleotide receptor agonist. In contrast, TNF-alpha and IFN-gamma had no effect on agonist-induced increases in the intracellular calcium concentration [Ca(2+)](i) in Par-C10 cells. Furthermore, treatment of Par-C10 cell monolayers with TNF-alpha and IFN-gamma increased paracellular permeability to normally impermeant proteins, altered cell and TJ morphology, and downregulated the expression of the TJ protein, claudin-1, but not other TJ proteins expressed in Par-C10 cells. The decreases in TER, agonist-induced transepithelial anion secretion, and claudin-1 expression caused by TNF-alpha, but not IFN-gamma, were reversible by incubation of Par-C10 cell monolayers with cytokine-free medium for 24 h, indicating that IFN-gamma causes irreversible inhibition of cellular activities associated with fluid secretion in salivary glands. Our results suggest that cytokine production is an important contributor to secretory dysfunction in SS by disrupting TJ integrity of salivary epithelium.