Homologous cardiac calcium pump regulators phospholamban and sarcolipin adopt distinct oligomeric states in the membrane

Phospholamban (PLN) and Sarcolipin (SLN) are homologous membrane proteins that belong to the family of proteins that regulate the activity of the cardiac calcium pump (sarcoplasmic reticulum Ca2+-ATPase, SERCA). PLN and SLN share highly conserved leucine zipper motifs that control self-association; consequently, it has been proposed that both PLN and SLN assemble into stable pentamers in the membrane. In this study, we used molecular dynamics (MD) simulations and Western blot analysis to investigate the precise molecular architecture of the PLN and SLN oligomers. Analysis showed that the PLN pentamer is the predominant oligomer present in mouse ventricles and ventricle-like human iPSC-derived cardiomyocytes, in agreement with the MD simulations showing stable leucine zipper interactions across all protomer-protomer interfaces and MD replicates. Interestingly, we found that the PLN pentamer populates an asymmetric structure of the transmembrane region, which is likely an intrinsic feature of the oligomer in a lipid bilayer. The SLN pentamer is not favorably formed across MD replicates and species of origin; instead, SLN from human and mouse atria primarily populate coexisting dimeric and trimeric states. In contrast to previous studies, our findings indicate that the SLN pentamer is not the predominant oligomeric state populated in the membrane. We conclude that despite their structural homology, PLN and SLN adopt distinct oligomeric states in the membrane. We propose that the distinct oligomeric states populated by PLN and SLN may contribute to tissue-specific SERCA regulation via differences in protomer-oligomer exchange, oligomer-SERCA dynamics, and noise filtering during β-adrenergic stimulation in the heart.

A hallmark of phospholamban functional divergence is located in the N-terminal phosphorylation domain

Sarcoplasmic reticulum Ca²⁺ pump (SERCA) is a critical component of the Ca²⁺ transport machinery in myocytes. There is clear evidence for regulation of SERCA activity by PLB, whose activity is modulated by phosphorylation of its N-terminal domain (residues 1–25), but there is less clear evidence for the role of this domain in PLB’s functional divergence. It is widely accepted that only sarcolipin (SLN), a protein that shares substantial homology with PLB, uncouples SERCA Ca²⁺ transport from ATP hydrolysis by inducing a structural change of its energy-transduction domain; yet, experimental evidence shows that the transmembrane domain of PLB (residues 26–52, PLB_26–52) partially uncouples SERCA in vitro. These apparently conflicting mechanisms suggest that PLB’s uncoupling activity is encoded in its transmembrane domain, and that it is controlled by the N-terminal phosphorylation domain. To test this hypothesis, we performed molecular dynamics simulations (MDS) of the binary complex between PLB_26–52 and SERCA. Comparison between PLB_26–52 and wild-type PLB (PLB_WT) showed no significant changes in the stability and orientation of the transmembrane helix, indicating that PLB_26–52 forms a native-like complex with SERCA. MDS showed that PLB_26–52 produces key intermolecular contacts and structural changes required for inhibition, in agreement with studies showing that PLB_26–52 inhibits SERCA. However, deletion of the N-terminal phosphorylation domain facilitates an order-to-disorder shift in the energy-transduction domain associated with uncoupling of SERCA, albeit weaker than that induced by SLN. This mechanistic evidence reveals that the N-terminal phosphorylation domain of PLB is a primary contributor to the functional divergence among homologous SERCA regulators.

Metabolic regulation of the PMCA: Role in cell death and survival

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The PMCA is an ATP-driven Ca²⁺ pump critical for the maintenance of low cytosolic calcium.

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The PMCA has an important but paradoxical role in cell death and survival.

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The PMCA can be differentially regulated by caspase/calpain cleavage.

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Glycolytic ATP supply may be sufficient to fuel the PMCA during metabolic stress.

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The ATP sensitivity of the PMCA can be regulated by acidic phospholipids.

The plasma membrane Ca²⁺-ATPase (PMCA) is a ubiquitously expressed, ATP-driven Ca²⁺ pump that is critical for maintaining low resting cytosolic Ca²⁺ ([Ca²⁺]_i) in all eukaryotic cells. Since cytotoxic Ca²⁺ overload has such a central role in cell death, the PMCA represents an essential “linchpin” for the delicate balance between cell survival and cell death. In general, impaired PMCA activity and reduced PMCA expression leads to cytotoxic Ca²⁺ overload and Ca²⁺ dependent cell death, both apoptosis and necrosis, whereas maintenance of PMCA activity or PMCA overexpression is generally accepted as being cytoprotective. However, the PMCA has a paradoxical role in cell death depending on the cell type and cellular context. The PMCA can be differentially regulated by Ca²⁺-dependent proteolysis, can be maintained by a localised glycolytic ATP supply, even in the face of global ATP depletion, and can be profoundly affected by the specific phospholipid environment that it sits within the membrane. The major focus of this review is to highlight some of the controversies surrounding the paradoxical role of the PMCA in cell death and survival, challenging the conventional view of ATP-dependent regulation of the PMCA and how this might influence cell fate.

Wilhelm Hasselbach: a personal tribute

Wilhelm Hasselbach (1921-2015) is best known for his major contribution to the discovery and molecular elucidation of an ATP-driven active ion transport across a biological membrane. He had discovered SERCA, the calcium pump of the sarcoplasmic reticulum (Hasselbach and Makinose in Biochem Z 333:518-528, 1961; Biochem Z 339:94-111, 1963).

Mechanisms of Intracellular Calcium Homeostasis in MC3T3-E1 Cells and Bone Tissues of Sprague-Dawley Rats Exposed to Fluoride

Calcium homeostasis of osteoblasts (OBs) has an important role in the physiology and pathology of bone tissue. In order to study the mechanisms of intracellular calcium homeostasis, MC3T3-E1 cells and Sprague-Dawley rats were treated with different concentrations of fluoride. Then, we examined intracellular-free calcium ion ([Ca(2+)]i) in MC3T3-E1 cells as well as mRNA and protein levels of Cav1.2, the main subunit of L-type voltage-dependent calcium channels (VDCCs), Na(+)/Ca(2+) exchange carriers (NCS), and plasma membrane Ca(2+)-ATPase (PMCA), inositol 1,4,5-trisphosphate receptor (IP3R) channels, sarco/endoplasmic reticulum calcium ATPase 2b (SERCA2b)/ATP2A2 in vitro, and rat bone tissues in vivo. Our results showed that [Ca(2+)]i of fluoride-treated OBs increased in a concentration-dependent manner with an increase in the concentration of fluoride. We also found that the low dose of fluoride led to high expression levels of Cav1.2, NCS-1, and PMCA and low expression levels of IP3R and SERCA2b/ATP2A2, while the high dose of fluoride induced an increase in SERCA2b/ATP2A2 levels and decrease in Cav1.2, PMCA, NCS-1, and IP3R levels. These results demonstrate that calcium channels and calcium pumps of plasma and endoplasmic reticulum (ER) membranes keep intracellular calcium homeostasis by regulating Cav1.2, NCS-1, PMCA, IP3R, and SERCA2b/ATP2A2 expression.

The CRaZy Calcium Cycle

Calcium is an essential cation for a cell. This cation participates in the regulation of numerous processes in either prokaryotes or eukaryotes, from bacteria to humans. Saccharomyces cerevisiae has served as a model organism to understand calcium homeostasis and calcium-dependent signaling in fungi. In this chapter it will be reviewed known and predicted transport mechanisms that mediate calcium homeostasis in the yeast. How and when calcium enters the cell, how and where it is stored, when is reutilized, and finally secreted to the environment to close the cycle. As a second messenger, maintenance of a controlled free intracellular calcium concentration is important for mediating transcriptional regulation. Many environmental stimuli modify the concentration of cytoplasmic free calcium generating the "calcium signal". This is sensed and transduced through the calmodulin/calcineurin pathway to a transcription factor, named calcineurin-responsive zinc finger, CRZ, also known as "crazy", to mediate transcriptional regulation of a large number of genes of diverse pathways including a negative feedback regulation of the calcium homeostasis system.

The effect of TGF-beta-induced epithelial-mesenchymal transition on the expression of intracellular calcium-handling proteins in T47D and MCF-7 human breast cancer cells

The contribution of Ca(2+) in TGF-β-induced EMT is poorly understood. We aimed to confirm the effect of TGF-β on the gene expression of intracellular calcium-handling proteins and to investigate the potential underlying mechanisms in TGF-β-induced EMT. T47D and MCF-7 cells were cultured in vitro and treated with TGF-β. The mRNA expression of EMT marker genes and intracellular calcium-handling proteins were quantified by qRT-PCR. qRT-PCR and Western blot analysis results verified the changes of EMT marker gene expression. Furthermore, we found that TGF-β induced cell morphological changes significantly with an increase of cell surface area and cell length. These results indicated that TGF-β induced EMT. The mRNA expression levels of SPCA1, SPCA2 and MCU were not influenced by TGF-β treatment, while NCX1 expression was decreased in T47D cells. In addition, the mRNA levels of SERCAs and IP3Rs were significantly changed due to TGF-β-induced EMT. The TGF-β-treated T47D cells exhibited markedly greater response to ATP than the control cells, and the descent velocity of cytosolic calcium concentration was faster in TGF-β-treated cells than in control cells. This is the first report to demonstrate that TGF-β-induced EMT in human breast cancer cells is associated with alterations in endoplasmic reticulum calcium homeostasis.

Recent perspectives into biochemistry of decavanadate

The number of papers about decavanadate has doubled in the past decade. In the present review, new insights into decavanadate biochemistry, cell biology, and antidiabetic and antitumor activities are described. Decameric vanadate species (V₁₀) clearly differs from monomeric vanadate (V₁), and affects differently calcium pumps, and structure and function of myosin and actin. Only decavanadate inhibits calcium accumulation by calcium pump ATPase, and strongly inhibits actomyosin ATPase activity (IC₅₀ = 1.4 μmol/L, V₁₀), whereas no such effects are detected with V₁ up to 150 μmol/L; prevents actin polymerization (IC₅₀ of 68 μmol/L, whereas no effects detected with up to 2 mmol/L V₁); and interacts with actin in a way that induces cysteine oxidation and vanadate reduction to vanadyl. Moreover, in vivo decavanadate toxicity studies have revealed that acute exposure to polyoxovanadate induces different changes in antioxidant enzymes and oxidative stress parameters, in comparison with vanadate. In vitro studies have clearly demonstrated that mitochondrial oxygen consumption is strongly affected by decavanadate (IC₅₀, 0.1 μmol/L); perhaps the most relevant biological effect. Finally, decavanadate (100 μmol/L) increases rat adipocyte glucose accumulation more potently than several vanadium complexes. Preliminary studies suggest that decavanadate does not have similar effects in human adipocytes. Although decavanadate can be a useful biochemical tool, further studies must be carried out before it can be confirmed that decavanadate and its complexes can be used as anticancer or antidiabetic agents.

Plasma membrane calcium pumps and their emerging roles in cancer

Alterations in calcium signaling and/or the expression of calcium pumps and channels are an increasingly recognized property of some cancer cells. Alterations in the expression of plasma membrane calcium ATPase (PMCA) isoforms have been reported in a variety of cancer types, including those of breast and colon, with some studies of cancer cell line differentiation identifying specific PMCA isoforms, which may be altered in some cancers. Some studies have also begun to assess levels of PMCA isoforms in clinical tumor samples and to address mechanisms of altered PMCA expression in cancers. Both increases and decreases in PMCA expression have been reported in different cancer types and in many cases these alterations are isoform specific. In this review, we provide an overview of studies investigating the expression of PMCA in cancer and discuss how both the overexpression and reduced expression of a PMCA isoform in a cancer cell could bestow a growth advantage, through augmenting responses to proliferative stimuli or reducing sensitivity to apoptosis.

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.