Nonspecific effects of the pharmacological probes commonly used to analyze signal transduction in rabbit parietal cells

A SLC4 family bicarbonate transporter is critical for intracellular pH regulation and biomineralization in sea urchin embryos

Efficient pH regulation is a fundamental requisite of all calcifying systems in animals and plants but with the underlying pH regulatory mechanisms remaining largely unknown. Using the sea urchin larva, this work identified the SLC4 HCO₃^- transporter family member SpSlc4a10 to be critically involved in the formation of an elaborate calcitic endoskeleton. SpSlc4a10 is specifically expressed by calcifying primary mesenchyme cells with peak expression during de novo formation of the skeleton. Knock-down of SpSlc4a10 led to pH regulatory defects accompanied by decreased calcification rates and skeleton deformations. Reductions in seawater pH, resembling ocean acidification scenarios, led to an increase in SpSlc4a10 expression suggesting a compensatory mechanism in place to maintain calcification rates. We propose a first pH regulatory and HCO₃^- concentrating mechanism that is fundamentally linked to the biological precipitation of CaCO₃. This knowledge will help understanding biomineralization strategies in animals and their interaction with a changing environment.

Many marine organisms such as mussels, sea urchins or corals, have skeletons and shells, which – due to their beautiful colors and shapes – are often desirable collector pieces. These structures are made from calcium and carbonate ions that react to form calcium carbonate crystals in a process known as biomineralization.

In sea urchin larvae, for example, the skeleton is built by so-called primary mesenchyme cells, which work similar to the bone forming cells in mammals. These mesenchyme cells use calcium from the sea water, which travels to the site where the shell starts to form. About half of the carbonate comes from carbon dioxide that the animals make as they breathe, but it is not known how the other half gets to the site of biomineralization.

Producing a skeleton generates acid, and marine animals need to be able to regulate their pH levels, as too acidic environments can dissolve the calcium carbonate and threatening to destroy the developing shell. How cells accumulate enough carbonate to make their shells, and how they cope with acidity is still poorly understood.

Here, Hu et al. address this problem by studying purple sea urchin larvae, revealing that they use ion transporters to gather bicarbonate from seawater. These structures are part of a group of bicarbonate transporters known as the ‘SLC4 transporter family’, which sit across the membrane of the mesenchyme cells and move the bicarbonate ions along. As the sea urchin larvae develop, the levels of the transporter protein start to rise in mesenchyme cells, peaking around the time they are producing the skeleton.

Stopping the production of the transporter hindered the larvae from building normal skeletons and also made their cells more acidic. It turns out that bicarbonate is more than a skeleton ingredient – it also helps to buffer the acid made in the process. Bicarbonate ions can combine with acidic molecules to form water and carbon dioxide. Bicarbonate pumped in from the sea neutralises the acidic molecules made during calcium carbonate formation, which helps to stabilize pH levels.

When the acidity of the water was experimentally increased, it prompted the sea urchins to produce more of the SLC4 transporters, revealing that they may have another role to play. Their acid-neutralizing capability helped the animals to cope with changes in their environment. Taking on more bicarbonate could therefore help to compensate for rising acidity, allowing skeleton production to carry on as normal.

This last finding is important in the context of ocean acidification. As the amount of carbon dioxide in the atmosphere increases, more of the gas dissolves in the sea. The chemical reactions that follow make the water more acidic and decreases the pH levels of the sea. Understanding how animals make their skeletons and shells, and manage acid, could reveal how they will cope as the environment changes in the future.

Myosin IIB and F-actin control apical vacuolar morphology and histamine-induced trafficking of H-K-ATPase-containing tubulovesicles in gastric parietal cells

Selective inhibitors of myosin or actin function and confocal microscopy were used to test the role of an actomyosin complex in controlling morphology, trafficking, and fusion of tubulovesicles (TV) containing H-K-ATPase with the apical secretory canaliculus (ASC) of primary-cultured rabbit gastric parietal cells. In resting cells, myosin IIB and IIC, ezrin, and F-actin were associated with ASC, whereas H-K-ATPase localized to intracellular TV. Histamine caused fusion of TV with ASC and subsequent expansion resulting from HCl and water secretion; F-actin and ezrin remained associated with ASC whereas myosin IIB and IIC appeared to dissociate from ASC and relocalize to the cytoplasm. ML-7 (inhibits myosin light chain kinase) caused ASC of resting cells to collapse and most myosin IIB, F-actin, and ezrin to dissociate from ASC. TV were unaffected by ML-7. Jasplakinolide (stabilizes F-actin) caused ASC to develop large blebs to which actin, myosin II, and ezrin, as well as tubulin, were prominently localized. When added prior to stimulation, ML-7 and jasplakinolide prevented normal histamine-stimulated transformations of ASC/TV and the cytoskeleton, but they did not affect cells that had been previously stimulated with histamine. These results indicate that dynamic pools of actomyosin are required for maintenance of ASC structure in resting cells and for trafficking of TV to ASC during histamine stimulation. However, the dynamic pools of actomyosin are not required once the histamine-stimulated transformation of TV/ASC and cytoskeleton has occurred. These results also show that vesicle trafficking in parietal cells shares mechanisms with similar processes in renal collecting duct cells, neuronal synapses, and skeletal muscle.

Inhibition of the Na+/dicarboxylate cotransporter by anthranilic acid derivatives

The Na(+)/dicarboxylate cotransporter NaDC1 absorbs citric acid cycle intermediates from the lumen of the small intestine and kidney proximal tubule. No effective inhibitor has been identified yet, although previous studies showed that the nonsteroidal anti-inflammatory drug, flufenamate, inhibits the human (h) NaDC1 with an IC(50) value of 2 mM. In the present study, we have tested compounds related in structure to flufenamate, all anthranilic acid derivatives, as potential inhibitors of hNaDC1. We found that N-(p-amylcinnamoyl) anthranilic acid (ACA) and 2-(p-amylcinnamoyl) amino-4-chloro benzoic acid (ONO-RS-082) are the most potent inhibitors with IC(50) values lower than 15 microM, followed by N-(9-fluorenylmethoxycarbonyl)-anthranilic acid (Fmoc-anthranilic acid) with an IC(50) value of approximately 80 microM. The effects of ACA on NaDC1 are not mediated through a change in transporter protein abundance on the plasma membrane and seem to be independent of its effect on phospholipase A(2) activity. ACA acts as a slow inhibitor of NaDC1, with slow onset and slow reversibility. Both uptake activity and efflux are inhibited by ACA. Other Na(+)/dicarboxylate transporters from the SLC13 family, including hNaDC3 and rbNaDC1, were also inhibited by ACA, ONO-RS-082, and Fmoc-anthranilic acid, whereas the Na(+)/citrate transporter (hNaCT) is much less sensitive to these compounds. The endogenous sodium-dependent succinate transport in Caco-2 cells is also inhibited by ACA. In conclusion, ACA and ONO-RS-082 represent promising lead compounds for the development of specific inhibitors of the Na(+)/dicarboxylate cotransporters.

Nervous control of ciliary beating by Cl-, Ca2+ and calmodulin inTritonia diomedea

In vertebrates, motile cilia line airways, oviducts and ventricles. Invertebrate cilia often control feeding, swimming and crawling, or gliding. Yet control and coordination of ciliary beating remains poorly understood. Evidence from the nudibranch mollusc, Tritonia diomedea, suggests that locomotory ciliated epithelial cells may be under direct electrical control. Here we report that depolarization of ciliated pedal epithelial (CPE)cells increases ciliary beating frequency (CBF), and elicits CBF increases similar to those caused by dopamine and the neuropeptide, TPep-NLS. Further,four CBF stimulants (zero external Cl-, depolarization, dopamine and TPep-NLS) depend on a common mode of action, viz. Ca2+influx, possibly through voltage-gated Ca2+ channels, and can be blocked by nifedipine. Ca2+ influx alone, however, does not provide all the internal Ca2+ necessary for CBF change. Ryanodine receptor(RyR) channel-gated internal stores are also necessary for CBF excitation. Caffeine can stimulate CBF and is sensitive to the presence of the RyR blocker dantrolene. Dantrolene also reduces CBF excitation induced by dopamine and TPep-NLS. Finally, W-7 and calmidazolium both block CBF excitation by caffeine and dopamine, and W-7 is effective at blocking TPep-NLS excitation. The effects of calmidazolium and W-7 suggest a role for Ca2+-calmodulin in regulating CBF, either directly or via Ca2+-calmodulin dependent kinases or phosphodiesterases. From these results we hypothesize dopamine and TPep-NLS induce depolarization-driven Ca2+ influx and Ca2+ release from internal stores that activates Ca2+-calmodulin, thereby increasing CBF.

Interference of calmidazolium with measurement of mitochondrial membrane potential using the tetraphenylphosphonium electrode or the fluorescent probe rhodamine 123

Calmidazolium (CMZ) is a positively charged, hydrophobic compound used as a calmodulin antagonist. It may cause unspecific effects in mitochondria, e.g., a decrease in membrane potential (deltapsi), swelling, and uncoupling. Several groups have advised against use of CMZ in studying signal transduction in mitochondria. We report here that it interferes with measurement of deltapsi in rat liver mitochondria (RLM) when using the tetraphenyl phosphonium (TPP+) electrode. We also found that CMZ reduces the signal, indicating an apparent drop in deltapsi. CMZ itself gave a signal with the TPP+ electrode in the absence of RLM. At high concentrations, > 10 microM, it also reduced the fluorescence quenching of the probe rhodamine 123. This may be due to an interference with mitochondrial uptake and binding of this positively charged probe or to an uncoupling effect. It is concluded that CMZ and similar positively charged calmodulin antagonists such as trifluoperazine may be used in mitochondria if these interferences are controlled and calibration is carried out under the experimental conditions used.

Role of Rho in rabbit parietal cell

Rho is known as an important regulator of actin microfilament formation. We were led to study it because a dynamic rearrangement of actin filaments occurs during activation of gastric acid secretion. In order to use specific probes, the rabbit gastric gland culture system was employed and the various genes were expressed using adenovirus vector. When the constitutive active mutant of Rho (RhoAV14) was expressed, histamine- or carbachol-stimulated acid secretion monitored by (14)C-aminopyrine accumulation was inhibited. Conversely, expression of C3 toxin, the specific inhibitor of Rho, and expression of G(12/13)-specific regulator of G-protein signaling domain, the specific inhibitor of G(12/13) which is considered to be an upstream mediator of Rho, both potentiated acid secretion stimulated by the agonists. F-actin staining of parietal cell expressing RhoAV14 revealed that the microfilament supporting the intracellular canaliculi (not on the basolateral membrane) almost disappeared. No clear changes in the intracellular localization of Rho were observed during stimulation of parietal cell. In resting glands, the endogenous active form of Rho was relatively high, and it decreased during histamine stimulation. The finding that any treatment which inhibit Rho augment acid secretion whereas those that activate Rho inhibit secretion strongly suggests that the Rho-pathway conducts a negatively regulating signal in parietal cell activation, possibly via site-specific regulation of actin microfilaments.

Phosphatidylinositol is essential determinant for K+ permeability involved in gastric proton pumping

Gastric vesicles purified from acid-secreting rabbit stomach display K(+) permeability manifested by the valinomycin-independent proton pumping of H(+)-K(+)-ATPase as monitored by acridine orange quenching. This apparent K(+) permeability is attenuated by the treatment of the membrane with 5 mM Mg(2+), and this phenomenon has been attributed to membrane-bound phosphoprotein phosphatase. However, with the exception of the nonspecific inhibitor pyrophosphate, protein phosphatase inhibitors failed to inhibit the loss of K(+) permeability. Preincubation of the membrane with neomycin, a phospholipase C inhibitor, surrogated the effect of Mg(2+), whereas another inhibitor, U-73122, did not. Phosphatidylinositol 4,5-bisphosphate (PIP(2)) restored the attenuated K(+) permeability by treatment with either Mg(2+) or neomycin. Furthermore, either phosphatidylinositol bound to phosphatidylinositol transfer protein or phosphatidylinositol 4,5,6-trisphosphate (PIP(3)) surrogated the effect of PIP(2). Mg(2+) and neomycin reduced K(+) permeability in the membrane as determined by Rb(+) influx and K(+)-dependent H(+) diffusion. Treatment with Mg(2+) reduced the contents of PIP(2) and PIP(3) in the membrane. These results suggest that PIP(2) and/or PIP(3) maintain K(+) permeability, which is essential for proton pumping in the apical membrane of the secreting parietal cell.

Inhibition of TASK-1 potassium channel by phospholipase C

The two-pore-domain K(+) channel, TASK-1, was recently shown to be a target of receptor-mediated regulation in neurons and in adrenal glomerulosa cells. Here, we demonstrate that TASK-1 expressed in Xenopus laevis oocytes is inhibited by different Ca(2+)-mobilizing agonists. Lysophosphatidic acid, via its endogenous receptor, and ANG II and carbachol, via their heterologously expressed ANG II type 1a and M(1) muscarinic receptors, respectively, inhibit TASK-1. This effect can be mimicked by guanosine 5'-O-(3-thiotriphosphate), indicating the involvement of GTP-binding protein(s). The phospholipase C inhibitor U-73122 reduced the receptor-mediated inhibition of TASK-1. Downstream signals of phospholipase C action (inositol 1,4,5-trisphosphate, cytoplasmic Ca(2+) concentration, and diacylglycerol) do not mediate the inhibition. Unlike the G(q)-coupled receptors, stimulation of the G(i)-activating M(2) muscarinic receptor coexpressed with TASK-1 results in an only minimal decrease of the TASK-1 current. However, additional coexpression of phospholipase C-beta(2) (which is responsive also to G(i) beta gamma-subunits) renders M(2) receptor activation effective. This indicates the significance of phospholipase C activity in the receptor-mediated inhibition of TASK-1.

Inhibition of carbachol stimulated acid secretion by interleukin 1beta in rabbit parietal cells requires protein kinase C

BACKGROUND

Interleukin 1beta (IL-1beta) is a potent inhibitor of gastric acid secretion. Regulatory actions at several levels have previously been demonstrated, including direct inhibition of parietal cell acid secretion. Although IL-1beta may activate several intracellular signalling pathways, the mechanisms responsible for inhibition of carbachol stimulated acid secretion have not been determined.

AIMS

To investigate the roles of protein kinase C (PKC) and the sphingomyelinase signalling pathways in the regulation of acid secretion by IL-1beta.

METHODS

Rabbit parietal cells were obtained by collagenase-EDTA digestion and centrifugal elutriation. Acid secretion stimulated by carbachol and A23187 (to mimic elevations in intracellular calcium) was assessed by 14C aminopyrine uptake in response to IL-1beta, PKC, and sphingomyelinase manipulation.

RESULTS

IL-1beta inhibited carbachol and A23187 stimulated acid secretion in a dose dependent manner. The inhibitory actions were completely reversed by each of three different PKC inhibitors, staurosporine, H-7, and chelerythrine, as well as by PKC depletion with high dose phorbol ester pretreatment. IL-1beta did not downregulate parietal cell muscarinic receptor. IL-1beta significantly increased membrane PKC activity. Activation of the sphingomyelinase/ceramide pathway had no effect on basal or stimulated acid secretion. The inhibitory action of IL-1beta was independent of protein kinase A and protein kinase G activity.

CONCLUSIONS

IL-1beta directly inhibits parietal cell carbachol stimulated acid secretion. This action occurs distal to muscarinic receptor activation and elevations in intracellular calcium and requires PKC.

A proposed mechanism for the potentiation of cAMP-mediated acid secretion by carbachol

Acid secretion in isolated rabbit gastric glands was monitored by the accumulation of [(14)C]aminopyrine. Stimulation of the glands with carbachol synergistically augmented the response to dibutyryl cAMP. The augmentation persisted even after carbachol was washed out and was resistant to chelated extracellular Ca(2+) and to inhibitors of either protein kinase C or calmodulin kinase II. Cytochalasin D at 10 microM preferentially blocked the secretory effect of carbachol and its synergism with cAMP, whereas it had no effect on histamine- or cAMP-stimulated acid secretion within 15 min. Cytochalasin D inhibited the carbachol-stimulated intracellular Ca(2+) concentration ([Ca(2+)](i)) increase due to release from the Ca(2+) store. Treatment of the glands with cytochalasin D redistributed type 3 inositol 1,4,5-trisphosphate receptor (the major subtype in the parietal cell) from the fraction containing membranes of large size to the microsomal fraction, suggesting a dissociation of the store from the plasma membrane. These findings suggest that intracellular Ca(2+) release by cholinergic stimulation is critical for determining synergism with cAMP in parietal cell activation and that functional coupling between the Ca(2+) store and the receptor is maintained by actin microfilaments.

Responsiveness of beta-escin-permeabilized rabbit gastric gland model: effects of functional peptide fragments

We established a beta-escin-permeabilized gland model with the use of rabbit isolated gastric glands. The glands retained an ability to secrete acid, monitored by [14C]aminopyrine accumulation, in response to cAMP, forskolin, and histamine. These responses were all inhibited by cAMP-dependent protein kinase inhibitory peptide. Myosin light-chain kinase inhibitory peptide also suppressed aminopyrine accumulation, whereas the inhibitory peptide of protein kinase C or that of calmodulin kinase II was without effect. Guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS) abolished cAMP-stimulated acid secretion concomitantly, interfering with the redistribution of H+-K+-ATPase from tubulovesicles to the apical membrane. To identify the targets of GTPgammaS, effects of peptide fragments of certain GTP-binding proteins were examined. Although none of the peptides related to Rab proteins showed any effect, the inhibitory peptide of Arf protein inhibited cAMP-stimulated secretion. These results demonstrate that our new model, the beta-escin-permeabilized gland, allows the introduction of relatively large molecules, e.g., peptides, into the cell, and will be quite useful for analyzing signal transduction of parietal cell function.