Calmodulin effect on purified rat cortical plasma membrane Ca(2+)-ATPase in different phosphorylation states

Calcineurin is part of a negative feedback loop in the InsP3/Ca²⁺ signalling pathway in blowfly salivary glands

The ubiquitous InsP3/Ca(2+) signalling pathway is modulated by diverse mechanisms, i.e. feedback of Ca(2+) and interactions with other signalling pathways. In the salivary glands of the blowfly Calliphora vicina, the hormone serotonin (5-HT) causes a parallel rise in intracellular [Ca(2+)] and [cAMP] via two types of 5-HT receptors. We have shown recently that cAMP/protein kinase A (PKA) sensitizes InsP3-induced Ca(2+) release. We have now identified the protein phosphatase that counteracts the effect of PKA on 5-HT-induced InsP3/Ca(2+) signalling. We demonstrate that (1) tautomycin and okadaic acid, inhibitors of protein phosphatases PP1 and PP2A, have no effect on 5-HT-induced Ca(2+) signals; (2) cyclosporin A and FK506, inhibitors of Ca(2+)/calmodulin-activated protein phosphatase calcineurin, cause an increase in the frequency of 5-HT-induced Ca(2+) oscillations; (3) the sensitizing effect of cyclosporin A on 5-HT-induced Ca(2+) responses does not involve Ca(2+) entry into the cells; (4) cyclosporin A increases InsP3-dependent Ca(2+) release; (5) inhibition of PKA abolishes the effect of cyclosporin A on the 5-HT-induced Ca(2+) responses, indicating that PKA and calcineurin act antagonistically on the InsP3/Ca(2+) signalling pathway. These findings suggest that calcineurin provides a negative feedback on InsP3/Ca(2+) signalling in blowfly salivary glands, counteracting the effect of PKA and desensitizing the signalling cascade at higher 5-HT concentrations.

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

Cross-talk between intracellular calcium ([Ca(2+)](i)) signaling and cAMP defines the specificity of stimulus-response coupling in a variety of cells. Previous studies showed that protein kinase A (PKA) potentiates and phosphorylates the plasma membrane Ca(2+)-ATPase (PMCA) in a Ca(2+)-dependent manner in parotid acinar cells (Bruce, J. I. E., Yule, D. I., and Shuttleworth, T. J. (2002) J. Biol. Chem. 277, 48172-48181). The aim of this study was to further investigate the spatial regulation of [Ca(2+)](i) clearance in parotid acinar cells. Par-C10 cells were used to functionally isolate the apical and basolateral PMCA activity by applying La(3+) to the opposite side to inhibit the PMCA. Activation of PKA (using forskolin) differentially potentiated apical [Ca(2+)](i) clearance in mouse parotid acinar cells and apical PMCA activity in Par-C10 cells. Immunofluorescence of parotid tissue slices revealed that PMCA1 was distributed throughout the plasma membrane, PMCA2 was localized to the basolateral membrane, and PMCA4 was localized to the apical membrane of parotid acinar cells. However, in situ phosphorylation assays demonstrated that PMCA1 was the only isoform phosphorylated by PKA following stimulation. Similarly, immunofluorescence of acutely isolated parotid acinar cells showed that the regulatory subunit of PKA (RIIbeta) translocated to the apical region following stimulation. These data suggest that PKA-mediated phosphorylation of PMCA1 differentially regulates [Ca(2+)](i) clearance in the apical region of parotid acinar cells because of a dynamic translocation of PKA. Such tight spatial regulation of Ca(2+) efflux is likely important for the fine-tuning of Ca(2+)-dependent effectors close to the apical membrane important for the regulation of fluid secretion and exocytosis.

Effects of Phenylalanine and its Metabolites on Cytoplasmic Free Calcium in Cortical Neurons

Classic phenylketonuria (PKU) is characterized by brain lesions. However, its underlying neurotoxic mechanisms remain unknown. Based on our previous studies, we hypothesized that calcium might participate in PKU-associated neuropathy. In cultured cortical neurons, cytoplasmic free calcium concentration ([Ca(2+)](i)) decreased dramatically when treatment with phenylalanine (Phe) and phenyllactic acid, while phenylacetic acid treatment immediately increased [Ca(2+)](i), which began to decrease after 3 min. Moreover, [Ca(2+)](i) decreased dramatically after Phe treatment in the presence of EGTA suggesting that Phe might increase [Ca(2+)](i) efflux. Phe-induced [Ca(2+)](i) decrease was strongly inhibited by vanadate, a non-specific plasma membrane Ca(2+)-ATPase (PMCA) antagonist, suggesting that Phe might increase [Ca(2+)](i) efflux throught modulating PMCA. These findings were further supported by the facts that Phe could increase membrance (45)Ca-uptake capability and PMCA activity. In contrast, treatment of KBR7943 or thapsigargin, antagonists to Na/Ca Exchanger (NCX) and Sarco/Endoplasmic reticulum Ca(2+)-ATPase (SERCA), respectively, did not elicit any changes in [Ca(2+)](i). Specific siRNA against PMCA had an effect similar to vanadate. Since the brain injury induced by phenylalaninemia was thought to be a chronic process, we cultured cortical neurons in the presence of Phe for 2 weeks and measured [Ca(2+)](i), PMCA activity and (45)Ca-uptake capability at days 3, 7, 9 and 14, respectively. PMCA activity and (45)Ca-uptake capability decreased from day 9, at the same time [Ca(2+)](i) increase was observed. In conclusion, PMCA participate in regulating Phe-induced initial rapid decrease in [Ca(2+)](i) and subsequent long-term increase in [Ca(2+)](i).

Ca2+/calmodulin-dependent protein kinase II is an essential mediator in the coordinated regulation of electrocyte Ca2+-ATPase by calmodulin and protein kinase A

The aim of this study was to investigate (a) whether Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) participates in the regulation of plasma membrane Ca2+-ATPase and (b) its possible cross-talk with other kinase-mediated modulatory pathways of the pump. Using isolated innervated membranes of the electrocytes from Electrophorus electricus L., we found that stimulation of endogenous protein kinase A (PKA) strongly phosphorylated membrane-bound CaM kinase II with simultaneous substantial activation of the Ca2+ pump (approximately 2-fold). The addition of cAMP (5-50 pM), forskolin (10 nM), or cholera toxin (10 or 100 nM) stimulated both CaM kinase II phosphorylation and Ca2+-ATPase activity, whereas these activation processes were cancelled by an inhibitor of the PKA alpha-catalytic subunit. When CaM kinase II was blocked by its specific inhibitor KN-93, the Ca2+-ATPase activity decreased to the levels measured in the absence of calmodulin; the unusually high Ca2+ affinity dropped 2-fold; and the PKA-mediated stimulation of Ca2+-ATPase was no longer seen. Hydroxylamine-resistant phosphorylation of the Ca2+-ATPase strongly increased when the PKA pathway was activated, and this phosphorylation was suppressed by inhibition of CaM kinase II. We conclude that CaM kinase II is an intermediate in a complex regulatory network of the electrocyte Ca2+ pump, which also involves calmodulin and PKA.

Crosstalk between cAMP and Ca2+ signaling in non-excitable cells

An impressive array of cytosolic calcium ([Ca2+](i)) signals exert control over a broad range of physiological processes. The specificity and fidelity of these [Ca2+](i) signals is encoded by the frequency, amplitude, and sub-cellular localization of the response. It is believed that the distinct characteristics of [Ca2+](i) signals underlies the differential activation of effectors and ultimately cellular events. This "shaping" of [Ca2+](i) signals can be achieved by the influence of additional signaling pathways modulating the molecular machinery responsible for generating [Ca2+](i) signals. There is a particularly rich source of potential sites of crosstalk between the cAMP and the [Ca2+](i) signaling pathways. This review will focus on the predominant molecular loci at which these classical signaling systems interact to impact the spatio-temporal pattern of [Ca2+](i) signaling in non-excitable cells.

Ca2+-dependent protein kinase--a modulation of the plasma membrane Ca2+-ATPase in parotid acinar cells

Cross-talk between cAMP and [Ca(2+)](i) signaling pathways represents a general feature that defines the specificity of stimulus-response coupling in a variety of cell types including parotid acinar cells. We have reported recently that cAMP potentiates Ca(2+) release from intracellular stores, primarily because of a protein kinase A-mediated phosphorylation of type II inositol 1,4,5-trisphosphate receptors (Bruce, J. I. E., Shuttleworth, T. J. S., Giovannucci, D. R., and Yule, D. I. (2002) J. Biol. Chem. 277, 1340-1348). The aim of the present study was to evaluate the functional and molecular mechanism whereby cAMP regulates Ca(2+) clearance pathways in parotid acinar cells. Following an agonist-induced increase in [Ca(2+)](i) the rate of Ca(2+) clearance, after the removal of the stimulus, was potentiated substantially ( approximately 2-fold) by treatment with forskolin. This effect was prevented completely by inhibition of the plasma membrane Ca(2+)-ATPase (PMCA) with La(3+). PMCA activity, when isolated pharmacologically, was also potentiated ( approximately 2-fold) by forskolin. Ca(2+) uptake into the endoplasmic reticulum of streptolysin-O-permeabilized cells by sarco/endoplasmic reticulum Ca(2+)-ATPase was largely unaffected by treatment with dibutyryl cAMP. Finally, in situ phosphorylation assays demonstrated that PMCA was phosphorylated by treatment with forskolin but only in the presence of carbamylcholine (carbachol). This effect of forskolin was Ca(2+)-dependent, and protein kinase C-independent, as potentiation of PMCA activity and phosphorylation of PMCA by forskolin also occurred when [Ca(2+)](i) was elevated by the sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid and was attenuated by pre-incubation with the Ca(2+) chelator, 1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA). The present study demonstrates that elevated cAMP enhances the rate of Ca(2+) clearance because of a complex modulation of PMCA activity that involves a Ca(2+)-dependent step. Tight regulation of both Ca(2+) release and Ca(2+) efflux may represent a general feature of the mechanism whereby cAMP improves the fidelity and specificity of Ca(2+) signaling.