Calcium occlusion in plasma membrane Ca2+-ATPase

Magnesium enhances aurintricarboxylic acid's inhibitory action on the plasma membrane Ca2+-ATPase

Our research aimed to elucidate the mechanism by which aurintricarboxylic acid (ATA) inhibits plasma membrane Ca2+-ATPase (PMCA), a crucial enzyme responsible for calcium transport. Given the pivotal role of PMCA in cellular calcium homeostasis, understanding how it is inhibited by ATA holds significant implications for potentially regulating physiopathological cellular processes in which this pump is involved. Our experimental findings revealed that ATA employs multiple modes of action to inhibit PMCA activity, which are influenced by ATP but also by the presence of calcium and magnesium ions. Specifically, magnesium appears to enhance this inhibitory effect. Our experimental and in-silico results suggest that, unlike those reported in other proteins, ATA complexed with magnesium (ATA·Mg) is the molecule that inhibits PMCA. In summary, our study presents a novel perspective and establishes a solid foundation for future research efforts aimed at the development of new pharmacological molecules both for PMCA and other proteins.

Epigallocatechin 3-gallate inhibits the plasma membrane Ca2+-ATPase: effects on calcium homeostasis

Flavonoids are natural compounds responsible for the health benefits of green tea. Some of the flavonoids present in green tea are catechins, among which are: epigallocatechin, epicatechin-3-gallate, epicatechin, catechin and epigallocatechin-3-gallate (EGCG). The latter was found to induce apoptosis, reduce reactive oxygen species, in some conditions though in others it acts as an oxidizing agent, induce cell cycle arrest, and inhibit carcinogenesis. EGCG also was found to be involved in calcium (Ca2+) homeostasis in excitable and in non-excitable cells. In this study, we investigate the effect of catechins on plasma membrane Ca2+-ATPase (PMCA), which is one of the main mechanisms that extrude Ca2+ out of the cell. Our studies comprised experiments on the isolated PMCA and on cells overexpressing the pump. Among catechins that inhibited PMCA activity, the most potent inhibitor was EGCG. EGCG inhibited PMCA activity in a reversible way favoring E1P conformation. EGCG inhibition also occurred in the presence of calmodulin, the main pump activator. Finally, the effect of EGCG on PMCA activity was studied in human embryonic kidney cells (HEK293T) that transiently overexpress hPMCA4. Results show that EGCG inhibited PMCA activity in HEK293T cells, suggesting that the effects observed on isolated PMCA occur in living cells.

Natural flavonoids inhibit the plasma membrane Ca2+-ATPase

Research on flavonoids from plant sources has recently sparked increasing interest because of their beneficial health properties. Different studies have shown that flavonoids change the intracellular Ca²⁺ homeostasis linked to alterations in the function of mitochondria, Ca²⁺ channels and Ca²⁺ pumps. These findings hint at plasma membrane Ca²⁺-ATPase (PMCA) involvement, as it transports Ca²⁺ actively to the extracellular medium coupled to ATP hydrolysis, thus maintaining ion cellular homeostasis. The present study aims to investigate the effect of several natural flavonoids on PMCA both in isolated protein systems and in living cells, and to establish the relationship between flavonoid structure and inhibitory activity on PMCA. Our results show that natural flavonoids inhibited purified and membranous PMCA with different effectiveness: quercetin and gossypin were the most potent and their inhibition mechanisms seem to be different, as quercetin does not prevent ATP binding whereas gossypin does. Moreover, PMCA activity was inhibited in human embryonic kidney cells which transiently overexpress PMCA, suggesting that the effects observed on isolated systems could occur in a complex structure like a living cell. In conclusion, this work reveals a novel molecular mechanism through which flavonoids inhibit PMCA, which leads to Ca²⁺ homeostasis and signaling alterations in the cell.

E2P-like states of plasma membrane Ca2+‑ATPase characterization of vanadate and fluoride-stabilized phosphoenzyme analogues

The plasma membrane Ca²⁺‑ATPase (PMCA) belongs to the family of P-type ATPases, which share the formation of an acid-stable phosphorylated intermediate as part of their reaction cycle. The crystal structure of PMCA is currently lacking. Its abundance is approximately 0.1% of the total protein in the membrane, hampering efforts to produce suitable crystals for X-ray structure analysis. In this work we characterized the effect of beryllium fluoride (BeF_x), aluminium fluoride (AlF_x) and magnesium fluoride (MgF_x) on PMCA. These compounds are known inhibitors of P-type ATPases that stabilize E2P ground, E2·P phosphoryl transition and E2·P_i product states. Our results show that the phosphate analogues BeF_x, AlF_x and MgF_x inhibit PMCA Ca²⁺‑ATPase activity, phosphatase activity and phosphorylation with high apparent affinity. Ca²⁺‑ATPase inhibition by AlF_x and BeF_x depended on Mg²⁺ concentration indicating that this ion stabilizes the complex between these inhibitors and the enzyme. Low pH increases AlF_x and BeF_x but not MgF_x apparent affinity. Eosin fluorescent probe binds with high affinity to the nucleotide binding site of PMCA. The fluorescence of eosin decreases when fluoride complexes bind to PMCA indicating that the environment of the nucleotide binding site is less hydrophobic in E2P-like states. Finally, measuring the time course of E → E2P-like conformational change, we proposed a kinetic model for the binding of fluoride complexes and vanadate to PMCA. In summary, our results show that these fluoride complexes reveal different states of phosphorylated intermediates belonging to the mechanism of hydrolysis of ATP by the PMCA.

Aluminum inhibits the plasma membrane and sarcoplasmic reticulum Ca2+-ATPases by different mechanisms

Aluminum (Al³⁺) is involved in the pathophysiology of neurodegenerative disorders. The mechanisms that have been proposed to explain the action of Al³⁺ toxicity are linked to changes in the cellular calcium homeostasis, placing the transporting calcium pumps as potential targets. The aim of this work was to study the molecular inhibitory mechanism of Al³⁺ on Ca²⁺-ATPases such as the plasma membrane and the sarcoplasmic reticulum calcium pumps (PMCA and SERCA, respectively). These P-ATPases transport Ca²⁺ actively from the cytoplasm towards the extracellular medium and to the sarcoplasmic reticulum, respectively. For this purpose, we performed enzymatic measurements of the effect of Al³⁺ on purified preparations of PMCA and SERCA. Our results show that Al³⁺ is an irreversible inhibitor of PMCA and a slowly-reversible inhibitor of SERCA. The binding of Al³⁺ is affected by Ca²⁺ in SERCA, though not in PMCA. Al³⁺ prevents the phosphorylation of SERCA and, conversely, the dephosphorylation of PMCA. The dephosphorylation time courses of the complex formed by PMCA and Al³⁺ (EPAl) in the presence of ADP or ATP show that EPAl is composed mainly by the conformer E₂P. This work shows for the first time a distinct mechanism of Al³⁺ inhibition that involves different intermediates of the reaction cycle of these two Ca²⁺-ATPases.

Regulation of the Plasma Membrane Calcium ATPases by the actin cytoskeleton

Associations between the cortical cytoskeleton and the components of the plasma membrane are no longer considered to be merely of structural and mechanical nature but are nowadays recognized as dynamic interactions that modulate a plethora of cellular responses. Reorganization of actin filaments upon diverse stimuli - among which is the rise in cytosolic Ca²⁺ - is involved in cell motility and adhesion, phagocytosis, cytokinesis, and secretion. Actin dynamics also participates in the regulation of ion transport across the membranes where it not only plays a key role in the delivery and stabilization of channels and transporters in the plasma membrane but also in the regulation of their activity. The recently described functional interaction between actin and the Plasma Membrane Ca²⁺-ATPase (PMCA) represents a novel regulatory mechanism of the pump at the time that unveils a new pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca²⁺ homeostasis. In this review, we summarize the current knowledge on the interaction between the cortical actin cytoskeleton and the PMCA and discuss the possible mechanisms that may explain the pump's modulation.

[SEM visualization of corneal epithelium through lanthanoid staining based on Ca/Nd isomorphous substitution in Ca-dependent molecular systems]

Cumulative biomicroscopic evidence is usually sufficient for the diagnosis of recurrent corneal erosion or bullous keratopathy, however, exploration of the disease pathogenesis requires subcellular-level visualization of corneal structure. In the current study, lanthanoid staining and scanning electron microscopy were employed to visualize quite a number of structures responsible for epithelium organization. In particular, the study proves possible the use of Ca/Nd isomorphous substitution at Ca2+ sites of cytoadherence proteins for visualization of corresponding cellular structures.

AIM

To assess the value of information provided by scanning electron microscopy of corneal epithelium that involves lanthanoid staining based on the Ca/Nd isomorphous substitution in Ca-dependent molecular systems.

MATERIAL AND METHODS

Anterior corneal epithelial scrapes were obtained from patients with recurrent corneal erosion or bullous keratopathy and cadaver eyes with no signs of any ophthalmic disease. Samples were then studied under a scanning electron microscope (Zeiss EVO LS10, BSE, EP - 79 Pa, 20-28 kV, Ln-staining with the BioREE assay kit).

RESULTS

In all cases, lanthanoid staining of biopsy material provided high-contrast SEM images with well-recognizable structural and ultrastructural elements associated with Ca2+ sites of cytoadherence proteins.

CONCLUSION

Lanthanoid staining of biopsy material and subsequent SEM enabled detailed visualization of structural features of the corneal epithelium in various pathologies. Due to the Ca/Nd isomorphism we were able to evaluate structural position of the majority of protein molecules engaged in Ca-dependant processes and, consequently, in cytoadherence. Basing on the neodymium distribution within the basal membrane, we have described local effects of different substances on the lamina densa in the projection of basal layer cell borders that occur after unidirectional ultrafiltration. The results confirm the failure of the junctional adhesion complex in recurrent corneal erosion.

Calcium signaling of pancreatic acinar cells in the pathogenesis of pancreatitis

Pancreatitis is an increasingly common and sometimes severe disease that lacks a specific therapy. The pathogenesis of pancreatitis is still not well understood. Calcium (Ca²⁺) is a versatile carrier of signals regulating many aspects of cellular activity and plays a central role in controlling digestive enzyme secretion in pancreatic acinar cells. Ca²⁺ overload is a key early event and is crucial in the pathogenesis of many diseases. In pancreatic acinar cells, pathological Ca²⁺ signaling (stimulated by bile, alcohol metabolites and other causes) is a key contributor to the initiation of cell injury due to prolonged and global Ca²⁺ elevation that results in trypsin activation, vacuolization and necrosis, all of which are crucial in the development of pancreatitis. Increased release of Ca²⁺ from stores in the intracellular endoplasmic reticulum and/or increased Ca²⁺ entry through the plasma membrane are causes of such cell damage. Failed mitochondrial adenosine triphosphate (ATP) production reduces re-uptake and extrusion of Ca²⁺ by the sarco/endoplasmic reticulum Ca²⁺-activated ATPase and plasma membrane Ca²⁺-ATPase pumps, which contribute to Ca²⁺ overload. Current findings have provided further insight into the roles and mechanisms of abnormal pancreatic acinar Ca²⁺ signals in pancreatitis. The lack of available specific treatments is therefore an objective of ongoing research. Research is currently underway to establish the mechanisms and interactions of Ca²⁺ signals in the pathogenesis of pancreatitis.

Conformational changes produced by ATP binding to the plasma membrane calcium pump

The aim of this work was to study the plasma membrane calcium pump (PMCA) reaction cycle by characterizing conformational changes associated with calcium, ATP, and vanadate binding to purified PMCA. This was accomplished by studying the exposure of PMCA to surrounding phospholipids by measuring the incorporation of the photoactivatable phosphatidylcholine analog 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine to the protein. ATP could bind to the different vanadate-bound states of the enzyme either in the presence or in the absence of Ca(2+) with high apparent affinity. Conformational movements of the ATP binding domain were determined using the fluorescent analog 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate. To assess the conformational behavior of the Ca(2+) binding domain, we also studied the occlusion of Ca(2+), both in the presence and in the absence of ATP and with or without vanadate. Results show the existence of occluded species in the presence of vanadate and/or ATP. This allowed the development of a model that describes the transport of Ca(2+) and its relation with ATP hydrolysis. This is the first approach that uses a conformational study to describe the PMCA P-type ATPase reaction cycle, adding important features to the classical E1-E2 model devised using kinetics methodology only.

Cannabinoid receptor 1 in the vagus nerve is dispensable for body weight homeostasis but required for normal gastrointestinal motility

The cannabinoid receptor 1 (CB(1)R) is required for body weight homeostasis and normal gastrointestinal motility. However, the specific cell types expressing CB(1)R that regulate these physiological functions are unknown. CB(1)R is widely expressed, including in neurons of the parasympathetic branches of the autonomic nervous system. The vagus nerve has been implicated in the regulation of several aspects of metabolism and energy balance (e.g., food intake and glucose balance), and gastrointestinal functions including motility. To directly test the relevance of CB(1)R in neurons of the vagus nerve on metabolic homeostasis and gastrointestinal motility, we generated and characterized mice lacking CB(1)R in afferent and efferent branches of the vagus nerve (Cnr1(flox/flox); Phox2b-Cre mice). On a chow or on a high-fat diet, Cnr1(flox/flox); Phox2b-Cre mice have similar body weight, food intake, energy expenditure, and glycemia compared with Cnr1(flox/flox) control mice. Also, fasting-induced hyperphagia and after acute or chronic pharmacological treatment with SR141716 [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole carboxamide] (CB(1)R inverse agonist) paradigms, mutants display normal body weight and food intake. Interestingly, Cnr1(flox/flox); Phox2b-Cre mice have increased gastrointestinal motility compared with controls. These results unveil CB(1)R in the vagus nerve as a key component underlying normal gastrointestinal motility.