The plasma membrane calcium ATPase modulates calcium homeostasis, intracellular signaling events and function in platelets

BACKGROUND

The plasma membrane calcium ATPase (PMCA) regulates localized signaling events in a variety of cell types, although its functional role in platelets remains undefined.

OBJECTIVES

To investigate the role of PMCA in determining platelet intracellular calcium concentration ([Ca²(+) ](i) ) at rest and following agonist stimulation, and to define the corresponding effects upon different stages of platelet activation.

METHODS

[Ca²(+) ](i) was continuously measured in Fura-2-loaded platelets and in vitro and in vivo functional analyses performed in the presence of the PMCA inhibitor carboxyeosin (CE).

RESULTS

Concentrations of CE that selectively inhibited Ca²(+) extrusion through PMCA were established in human platelets. [Ca²(+) ](i) was elevated by CE in resting platelets, although collagen-stimulated Ca²(+) release was reduced. Impaired Ca²(+) mobilization upon agonist stimulation was accompanied by reduced dense granule secretion and impaired platelet aggregation. Platelet aggregation responses were also reduced in PMCA4(-/-) mice and in an in vivo mouse model of platelet thromboembolism. Conversely, inhibition of PMCA promoted the early and later stages of platelet activation, observed as enhanced adhesion to fibrinogen, and accelerated clot retraction. Investigations into the signaling mechanisms underlying CE-mediated inhibition of platelet aggregation implicated cGMP-independent vasodilator-stimulated phosphoprotein phosphorylation.

CONCLUSIONS

Disruption of PMCA activity perturbs platelet Ca²(+) homeostasis and function in a time-dependent manner, demonstrating that PMCA differentially regulates Ca²(+) -dependent signaling events, and hence function, throughout the platelet activation process.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

Fluoridation--what the public know and what they want

OBJECTIVE

To describe the knowledge and beliefs on fluoridation of a community served by an unfluoridated water supply prior to consultation on implementing fluoridation.

METHODS

Telephone survey of a sample of residents and businesses utilising semi-structured questionnaires. Comparisons were made among responses of ethnic and age groups.

RESULTS

Residents were more conversant concerning fluoridation than business users. Knowledge and beliefs among ethnic groups differed significantly. Younger persons and those who had never been married were less knowledgeable. Pamphlets supplied by water-provider organisations or health professionals were the preferred methods of acquiring fluoridation information. The majority of respondents wished the opportunity to express their views via a referendum. Health authorities were the favoured decision-makers.

CONCLUSIONS AND IMPLICATIONS

Gaps were identified in this community's knowledge on fluoridation with important differences based on ethnicity and age. Strategies to inform the public on water fluoridation need to address these subgroup differences.

Transient expression and oil body targeting of an Arabidopsis oleosin-GUS reporter fusion protein in a range of oilseed embryos

An Arabidopsis oleosin gene including 860 bp of promoter as well as coding sequence was fused to the reporter, β-glucuronidase (GUS). In transient expression assays following particle bombardment, the Arabidopsis gene in both transcriptional and translational fusions with GUS was functional in a range of oilseed embryos as determined histochemically and fluorimetrically. Significantly, upon homogenization and separation into cytoplasmic and oil body fractions, the complete oleosin-GUS fusion was found to be targeted to the oil bodies in all of the heterologous species tested including canola, flax, soybean, sunflower, safflower, cotton, castor bean, coriander as well as the monocot, corn. The utilization of oleosin protein fusions for recombinant protein expression should be applicable to this wide range of oilseed crops. Additionally, the method of transient expression subcellular targeting should be useful in determining structural motifs controlling intracellular trafficking of these proteins.

Importance of the Chiral Centers of Jasmonic Acid in the Responses of Plants (Activities and Antagonism between Natural and Synthetic Analogs)

The importance of the two chiral centers at C-3 and C-7 in the molecular structure of jasmonic acid in plant responses was investigated. We separated methyl jasmonate (MeJA) into (3R)- and (3S)-isomers with a fixed stereochemistry at C-3, but epimerization at C-7 is possible. The four isomers of the nonepimerizable analog 7-methyl MeJA were synthesized. These six esters and their corresponding acids were tested in three bioassays: (a) senescence in sunflower (Helianthus annuus) cotyledons; (b) proteinase inhibitor II gene expression in transgenic tobacco (Nicotiana tabacum) with [beta]-glucuronidase as a biochemical reporter; and (c) seed germination in Brassica napus and wheat (Triticum aestivum). The esters and acids had similar activities in the three assays, with the ester being more effective than its acid. The (3R)-stereochemistry was critical for jasmonate activity. Although activity was reduced after substituting the C-7 proton with a methyl group, the analogs with (3R,7R)- or (3R,7S)-stereochemistry were active in some of the assays. Although the four isomers of 7-methyl MeJA were inactive or only weakly active in the senescence assay, they could overcome the senescence-promoting effect of (3R)-MeJA. The strongest antagonistic effect was observed with the (3R,7S)-isomer.