The sarcolemmal calcium pump, alpha-1 syntrophin, and neuronal nitric-oxide synthase are parts of a macromolecular protein complex

Platelet PMCA- and SERCA-type Ca2+ -ATPase expression in diabetes: a novel signature of abnormal megakaryocytopoiesis

BACKGROUND

Previous studies have shown platelet Ca(2+) abnormalities in diabetes mellitus and some reports suggest abnormal platelet production. Platelet Ca(2+) homeostasis is controlled by a multi-Ca(2+)-ATPase system that includes two plasma membrane Ca(2+)-ATPase (PMCA) and seven sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) isoforms. In addition, we recently found that the expression of PMCA4b and SERCA3 isoforms may serve as new markers of abnormal megakaryocytopoiesis [Nurden P et al. Impaired megakaryocytopoiesis in type 2B von Willebrand disease with severe thrombocytopenia. Blood 2006; 108: 2587-95].

AIM

To analyze the expression of major platelet Ca(2+)-ATPases in 27 patients with type 1 or type 2 diabetes (T1D or T2D) compared with normal donors.

METHODS

Investigation of protein and mRNA expressions of PMCA1b and PMCA4b, and SERCA2b, SERCA3a and SERCA3b, using specific Western blotting and reverse transcriptase-polymerase chain reaction, respectively.

RESULTS

Remarkably, all patients with T1D were found to present a higher expression of PMCA4b protein (212% +/- 28%; n = 10) and PMCA4b mRNA (155% +/- 16%; n = 17), coupled with a higher expression of SERCA3b mRNA (165% +/- 9%) in some cases. Patients with T2D (n = 10) were also studied for protein expression and were found to present similar major upregulation of the expression of PMCA4b protein (180% +/- 28%; n = 10). Lastly, five of 10 patients with T1D were studied for PMCA4b expression after insulin treatment, with four of five recovering normal expression (96% +/- 15%; n = 5).

CONCLUSIONS

Compared with the expression of PMCA4b upon platelet maturation, platelets from diabetic patients exhibit similarities with immature megakaryocytes. Thus, this study reinforces the idea that abnormal megakaryocytopoiesis can provide additional insights into diabetes and could represent a novel therapeutic target for antithrombotic drugs.

Distinct phenotypes among plasma membrane Ca2+-ATPase knockout mice

Ca2+ gradients across the plasma membrane, required for Ca2+ homeostasis and signaling, are maintained in part by plasma membrane Ca2+-ATPase (PMCA) isoforms 1-4. Gene targeting has been used to analyze the functions of PMCA1, PMCA2, and PMCA4 in mice. PMCA1 null mutant embryos die during the preimplantation stage, and loss of a single copy of the PMCA1 gene contributes to apoptosis in vascular smooth muscle. PMCA2 deficiency in sensory hair cells of the inner ear causes deafness and balance defects, most likely by affecting both intracellular Ca2+ and extracellular Ca2+ in the endolymph. PMCA2 is required for viability of certain neurons, consistent with a major role in maintenance of intracellular Ca2+. Surprisingly, loss of PMCA2 in lactating mammary glands causes a sharp reduction in milk Ca2+, consistent with a macrocalcium secretory function. Although PMCA4 is widely expressed and is the most abundant isoform in some tissues, null mutants appear healthy. However, male PMCA4 null mutants are infertile due to a failure of hyperactivated sperm motility resulting from the absence of PMCA4 in the sperm tail, and Ca2+ signaling in B lymphocytes, involving interactions between PMCA4, CD22, and the tyrosine phosphatase SHP-1, is defective. Studies of bladder smooth muscle from PMCA4 null mutants and PMCA1 heterozygous mice suggest that PMCA1 and PMCA4 play different roles in smooth muscle contractility, with PMCA1 contributing to overall Ca2+ clearance and PMCA4 being required for carbachol-stimulated contraction. These phenotypes indicate that PMCA1 serves essential housekeeping functions, whereas PMCA4 and particularly PMCA2 serve more specialized physiological functions.

The regulation of the Na/Ca exchanger and plasmalemmal Ca2+ ATPase by other proteins

Na/Ca exchanger (NCX) and plasma membrane Ca2+ ATPase are the Ca2+ efflux mechanisms known in mammalian cells. NCX is the main transporter to efflux intracellular Ca2+ in the heart. NCX protein contains nine putative transmembrane domains and a large intracellular loop joining two sets of the transmembrane domains. The intracellular loop regulates the activity of the NCX by interacting with other proteins and nonprotein factors, such as ions, PIP2. Several proteins that are associated with NCX have been identified recently. Similarly, plasmalemmal Ca2+ ATPase (PMCA) has 10 putative transmembrane domains, and the C-terminal intracellular region inhibits transporter activity. There are several proteins associated with PMCA, and the roles of the associated proteins of PMCA vary from specific localization to involving PMCA in signal transduction. Elucidation of structural and functional roles played by these associated proteins of NCX and PMCA will provide opportunities to develop drugs of potential therapeutic value.

Plasma membrane Ca2+ ATPases as dynamic regulators of cellular calcium handling

Plasma membrane Ca2+ ATPases (PMCAs) are essential components of the cellular toolkit to regulate and fine-tune cytosolic Ca2+ concentrations. Historically, the PMCAs have been assigned a housekeeping role in the maintenance of intracellular Ca2+ homeostasis. More recent work has revealed a perplexing multitude of PMCA isoforms and alternative splice variants, raising questions about their specific role in Ca2+ handling under conditions of varying Ca2+ loads. Studies on the kinetics of individual isoforms, combined with expression and localization studies suggest that PMCAs are optimized to function in Ca2+ regulation according to tissue- and cell-specific demands. Different PMCA isoforms help control slow, tonic Ca2+ signals in some cells and rapid, efficient Ca2+ extrusion in others. Localized Ca2+ handling requires targeting of the pumps to specialized cellular locales, such as the apical membrane of cochlear hair cells or the basolateral membrane of kidney epithelial cells. Recent studies suggest that alternatively spliced regions in the PMCAs are responsible for their unique targeting, membrane localization, and signaling cross-talk. The regulated deployment and retrieval of PMCAs from specific membranes provide a dynamic system for a cell to respond to changing needs of Ca2+ regulation.

Isoform-specific up-regulation of plasma membrane Ca2+ATPase expression during colon and gastric cancer cell differentiation

In this work we demonstrate a differentiation-induced up-regulation of the expression of plasma membrane Ca2+ATPase (PMCA) isoforms being present in various gastric/colon cancer cell types. We found PMCA1b as the major isoform in non-differentiated cancer cell lines, whereas the expression level of PMCA4b was significantly lower. Cell differentiation initiated with short chain fatty acids (SCFAs) and trichostatin A, or spontaneous differentiation of post-confluent cell cultures resulted in a marked induction of PMCA4b expression, while only moderately increased PMCA1b levels. Up-regulation of PMCA4b expression was demonstrated both at the protein and mRNA levels, and closely correlated with the induction of established differentiation markers. In contrast, the expression level of the Na+/K+-ATPase or that of the sarco/endoplasmic reticulum Ca2+ATPase 2 protein did not change significantly under these conditions. In membrane vesicles obtained from SCFA-treated gastric/colon cancer cells a marked increase in the PMCA-dependent Ca2+ transport activity was observed, indicating a general increase of PMCA function during the differentiation of these cancer cells. Because various PMCA isoforms display distinct functional characteristics, we suggest that up-regulated PMCA expression, together with a major switch in PMCA isoform pattern may significantly contribute to the differentiation of gastric/colon cancer cells. The analysis of PMCA expression may provide a new diagnostic tool for monitoring the tumor phenotype.

Plasma membrane calcium pump activity is affected by the membrane protein concentration: evidence for the involvement of the actin cytoskeleton

Plasma membrane calcium pumps (PMCAs) are integral membrane proteins that actively expel Ca(2+) from the cell. Specific Ca(2+)-ATPase activity of erythrocyte membranes increased steeply up to 1.5-5 times when the membrane protein concentration decreased from 50 microg/ml to 1 microg/ml. The activation by dilution was also observed for ATP-dependent Ca(2+) uptake into vesicles from Sf9 cells over-expressing the PMCA 4b isoform, confirming that it is a property of the PMCA. Dilution of the protein did not modify the activation by ATP, Ca(2+) or Ca(2+)-calmodulin. Treatment with non-ionic detergents did not abolish the dilution effect, suggesting that it was not due to resealing of the membrane vesicles. Pre-incubation of erythrocyte membranes with Cytochalasin D under conditions that promote actin polymerization abolished the dilution effect. Highly-purified, micellar PMCA showed no dilution effect and was not affected by Cytochalasin D. Taken together, these results suggest that the concentration-dependent behavior of the PMCA activity was due to interactions with cytoskeletal proteins. The dilution effect was also observed with different PMCA isoforms, indicating that this is a general phenomenon for all PMCAs.

Neuronal Nitric Oxide Synthase Signaling in the Heart Is Regulated by the Sarcolemmal Calcium Pump 4b

Background— Neuronal nitric oxide synthase (nNOS) has recently been shown to be a major regulator of cardiac contractility. In a cellular system, we have previously shown that nNOS is regulated by the isoform 4b of plasma membrane calcium/calmodulin-dependent ATPase (PMCA4b) through direct interaction mediated by a PDZ domain (PSD 95, Drosophilia Discs large protein and Zona occludens-1) on nNOS and a cognate ligand on PMCA4b. It remains unknown, however, whether this interaction has physiological relevance in the heart in vivo.

Methods and Results— We generated 2 strains of transgenic mice overexpressing either human PMCA4b or PMCA ct120 in the heart. PMCA ct120 is a highly active mutant form of the pump that does not interact with or modulate nNOS function. Calcium was extruded normally from PMCA4b-overexpressing cardiomyocytes, but in vivo, overexpression of PMCA4b reduced the β-adrenergic contractile response. This attenuated response was not observed in ct120 transgenic mice. Treatment with a specific nNOS inhibitor ( N ω-propyl- l -arginine) reduced the β-adrenergic response in wild-type and ct120 transgenic mice to levels comparable to those of PMCA4b transgenic animals. No differences in lusitropic response were observed in either transgenic strain compared with wild-type littermates.

Conclusions— These data demonstrate the physiological relevance of the interaction between PMCA4b and nNOS and suggests its signaling role in the heart.

The plasma membrane calcium ATPase and disease

The plasma membrane calcium ATPase (PMCA) uses energy to pump calcium (Ca2+) ions out of the cytosol into the extracellular milieu, usually against a strong chemical gradient. This energy expenditure is necessary to maintain a relatively low intracellular net Ca2+ load. Mammals have four genes (ATP2B1-ATP2B4), encoding the proteins PMCA1 through PMCA4. Transcripts from each of these genes are alternatively spliced to generate several variant proteins that are in turn post-translationally modified in a variety of ways. Expressed ubiquitously and with some level of functional redundancy in most vital tissues, only one of the four genes--Atp2b2--has been causally linked through naturally occuring mutations to disease in mammals: specifically to deafness and ataxia in spontaneous mouse mutants. In humans, a missense amino acid substitution in PMCA2 modifies the severity of hearing loss. Targeted null mutations of the Atp2b1 and Atp2b4 genes in mouse are embryonic lethal and cause a sperm motility defect, respectively. These phenotypes point to complex human diseases like hearing loss, cardiac function and infertility. Changes in PMCA expression are associated with other diseases including cataract formation, carciniogenesis, diabetes, and cardiac hypertension and hypertrophy. Severity of these diseases may be affected by subtle changes in expression of the PMCA isoforms expressed in those tissues.