Investigation of Na(+),K(+)-ATPase on a solid supported membrane: the role of acylphosphatase on the ion transport mechanism

Reaffirmation of Mechanistic Proteomic Signatures Accompanying SGLT2 Inhibition in Patients With Heart Failure: A Validation Cohort of the EMPEROR Program

BACKGROUND

Sodium-glucose cotransporter 2 (SGLT2) inhibitors exert a distinctive pattern of direct biological effects on the heart and kidney under experimental conditions, but the meaningfulness of these signatures for patients with heart failure has not been fully defined.

OBJECTIVES

We performed the first mechanistic validation study of large-scale proteomics in a double-blind randomized trial of any treatment in patients with heart failure.

METHODS

In a discovery cohort from the EMPEROR (Empagliflozin Outcome Trial in Patients With Chronic Heart Failure and Reduced Ejection Fraction) program, we studied the effect of randomized treatment with placebo or empagliflozin on 1,283 circulating proteins in 1,134 patients with heart failure with a reduced or preserved ejection fraction. In a validation cohort, we expanded the number to 2,155 assessed proteins, which were measured in 1,120 EMPEROR participants who had not been studied previously.

RESULTS

In the validation cohort, 25 proteins were the most differentially enriched by empagliflozin (ie, ≥15% between-group difference and false discovery rate <1% at 12 weeks with known effects on the heart or kidney): 1) 13 proteins promote autophagy and other cellular quality-control functions (IGFBP1, OTUB1, DNAJB1, DNAJC9, RBP2, IST1, HSPA8, H-FABP, FABP6, ATPIFI, TfR1, EPO, IGBP1); 2) 12 proteins enhance mitochondrial health and ATP production (UMtCK, TBCA, L-FABP, H-FABP, FABP5, FABP6, RBP2, IST1, HSPA8, ATPIFI, TfR1, EPO); 3) 7 proteins augment cellular iron mobilization or erythropoiesis (TfR1, EPO, IGBP1, ERMAP, UROD, ATPIF1, SNCA); 4) 3 proteins influence renal tubular sodium handling; and 5) 9 proteins have restorative effects in the heart or kidneys, with many proteins exerting effects in >1 domain. These biological signatures replicated those observed in our discovery cohort. When the threshold for a meaningful between-group difference was lowered to ≥10%, there were 58 additional differentially enriched proteins with actions on the heart and kidney, but the biological signatures remained the same.

CONCLUSIONS

The replication of mechanistic signatures across discovery and validation cohorts closely aligns with the experimental effects of SGLT2 inhibitors. Thus, the actions of SGLT2 inhibitors-to promote autophagy, restore mitochondrial health and production of ATP, promote iron mobilization and erythropoiesis, influence renal tubular ion reabsorption, and normalize cardiac and renal structure and function-are likely to be relevant to patients with heart failure. (EMPagliflozin outcomE tRial in Patients With chrOnic heaRt Failure With Preserved Ejection Fraction [EMPEROR-Preserved], NCT03057951; EMPagliflozin outcomE tRial in Patients With chrOnic heaRt Failure With Reduced Ejection Fraction [EMPEROR-Reduced], NCT03057977).

High-throughput metabolomics method based on liquid chromatography-mass spectrometry: Insights into the underlying mechanisms of salinity-alkalinity exposure-induced metabolites changes in Barbus capito

It is critical to investigate the adaptive development and the physiological mechanism of fish in external stimulation. In this study, the response of Barbus capito to salinity-alkalinity exposure was explored by high-throughput nontargeted and liquid chromatography-mass spectrometry-based metabolomics to investigate metabolic biomarker and pathway changes. Meanwhile, the biochemical indexes of Barbus capito were measured to discover the chronic impairment response to salinity-alkalinity exposures. A total of 29 tissue metabolites were determined to deciphering the endogenous metabolic changes of fishes during the different concentration salinity-alkalinity exposures environment, which were mainly involved in the key metabolism including the phenylalanine, tyrosine, and tryptophan biosynthesis, arachidonic acid metabolism, pyruvate metabolism, citrate cycle, and glycerophospholipid metabolism. Finally, we found the amino acid metabolism as key target was associated with the endogenous metabolites and metabolic pathways of Barbus capito to salinity-alkalinity exposures. In conclusion, metabolomics is a potentially powerful tool to reveal the mechanism information of fish in various exposure environments.

Phosphatidylserine flipping by the P4-ATPase ATP8A2 is electrogenic

Phospholipid flippases (P4-ATPases) utilize ATP to translocate specific phospholipids from the exoplasmic leaflet to the cytoplasmic leaflet of biological membranes, thus generating and maintaining transmembrane lipid asymmetry essential for a variety of cellular processes. P4-ATPases belong to the P-type ATPase protein family, which also encompasses the ion transporting P2-ATPases: Ca²⁺-ATPase, Na⁺,K⁺-ATPase, and H⁺,K⁺-ATPase. In comparison with the P2-ATPases, understanding of P4-ATPases is still very limited. The electrogenicity of P4-ATPases has not been explored, and it is not known whether lipid transfer between membrane bilayer leaflets can lead to displacement of charge across the membrane. A related question is whether P4-ATPases countertransport ions or other substrates in the opposite direction, similar to the P2-ATPases. Using an electrophysiological method based on solid supported membranes, we observed the generation of a transient electrical current by the mammalian P4-ATPase ATP8A2 in the presence of ATP and the negatively charged lipid substrate phosphatidylserine, whereas only a diminutive current was generated with the lipid substrate phosphatidylethanolamine, which carries no or little charge under the conditions of the measurement. The current transient seen with phosphatidylserine was abolished by the mutation E198Q, which blocks dephosphorylation. Likewise, mutation I364M, which causes the neurological disorder cerebellar ataxia, mental retardation, and disequilibrium (CAMRQ) syndrome, strongly interfered with the electrogenic lipid translocation. It is concluded that the electrogenicity is associated with a step in the ATPase reaction cycle directly involved in translocation of the lipid. These measurements also showed that no charged substrate is being countertransported, thereby distinguishing the P4-ATPase from P2-ATPases.

Oxadiazon affects the expression and activity of aldehyde dehydrogenase and acylphosphatase in human striatal precursor cells: A possible role in neurotoxicity

Exposure to herbicides can induce long-term chronic adverse effects such as respiratory diseases, malignancies and neurodegenerative diseases. Oxadiazon, a pre-emergence or early post-emergence herbicide, despite its low acute toxicity, may induce liver cancer and may exert adverse effects on reproductive and on endocrine functions. Unlike other herbicides, there are no indications on neurotoxicity associated with long-term exposure to oxadiazon. Therefore, we have analyzed in primary neuronal precursor cells isolated from human striatal primordium the effects of non-cytotoxic doses of oxadiazon on neuronal cell differentiation and migration, and on the expression and activity of the mitochondrial aldehyde dehydrogenase 2 (ALDH2) and of the acylphosphatase (ACYP). ALDH2 activity protects neurons against neurotoxicity induced by toxic aldehydes during oxidative stress and plays a role in neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease. ACYP is involved in ion transport, cell differentiation, programmed cell death and cancer, and increased levels of ACYP have been revealed in fibroblasts from patients affected by Alzheimer's disease. In this study we demonstrated that non-cytotoxic doses of oxadiazon were able to inhibit neuronal striatal cell migration and FGF2- and BDNF-dependent differentiation towards neuronal phenotype, and to inhibit the expression and activity of ALDH2 and to increase the expression and activity of ACYP2. In addition, we have provided evidence that in human primary neuronal precursor striatal cells the inhibitory effects of oxadiazon on cell migration and differentiation towards neuronal phenotype were achieved through modulation of ACYP2. Taken together, our findings reveal for the first time that oxadiazon could exert neurotoxic effects by impairing differentiative capabilities of primary neuronal cells and indicate that ALDH2 and ACYP2 are relevant molecular targets for the neurotoxic effects of oxadiazon, suggesting a potential role of this herbicide in the onset of neurodegenerative diseases.

Inhibitory effect of Pb2+ on the transport cycle of the Na+,K+-ATPase

The effect of Pb(2+) on the transport cycle of the Na(+),K(+)-ATPase was characterized in detail at a molecular level by combining electrical and biochemical measurements. Electrical measurements were performed by adsorbing purified membrane fragments containing Na(+),K(+)-ATPase on a solid-supported membrane. Upon adsorption, the Na(+),K(+)-ATPase was activated by carrying out concentration jumps of different activating substrates, for example, Na(+) and ATP. Charge movements following Na(+),K(+)-ATPase activation were measured in the presence of various Pb(2+) concentrations to investigate the effect of Pb(2+) on different ion translocating steps of the pump cycle. These charge measurements were then compared to biochemical measurements of ATPase activity in the presence of increasing Pb(2+) concentration. Our results indicate that Pb(2+) inhibits cycling of the enzyme, but it does not affect cytoplasmic Na(+) binding and release of Na(+) ions at the extracellular side at concentrations below 10 muM. To explain the inhibitory effect of Pb(2+) on the Na(+),K(+)-ATPase, we propose that Pb(2+) may interfere with the hydrolytic cleavage of the phosphorylated intermediate E(2)P, which occurs in the K(+)-related branch of the pump cycle.

Rapid activation of the melibiose permease MelB immobilized on a solid-supported membrane

Rapid solution exchange on a solid-supported membrane (SSM) is investigated using fluidic structures and a solid-supported membrane of 1 mm diameter in wall jet geometry. The flow is analyzed with a new technique based on specific ion interactions with the surface combined with an electrical measurement. The critical parameters affecting the time course of the solution exchange and the transfer function describing the time resolution of the SSM system are determined. The experimental data indicate that solution transport represents an intermediate situation between the plug flow and the Hagen-Poiseuille laminar flow regime. However, to a good approximation the rise of the surface concentration can be described by Hagen-Poiseuille flow with ideal mixing at the surface of the SSM. Using an improved cuvette design, solution exchange as fast as 2 ms was achieved at the surface of a solid-supported membrane. As an application of the technique, the rate constant of a fast electrogenic reaction in the melibiose permease MelB, a bacterial ( Escherichia coli) sugar transporter, is determined. For comparison, the kinetics of a conformational transition of the same transporter was measured using stopped-flow tryptophan fluorescence spectroscopy. The relaxation time constant obtained for the charge displacement agrees with that determined in the stopped-flow experiments. This demonstrates that upon sugar binding MelB undergoes an electrogenic conformational transition with a rate constant of k approximately 250 s (-1).

Charge transfer in P-type ATPases investigated on planar membranes

Planar lipid bilayers, e.g., black lipid membranes (BLM) and solid supported membranes (SSM), have been employed to investigate charge movements during the reaction cycle of P-type ATPases. The BLM/SSM method allows a direct measurement of the electrical currents generated by the cation transporter following chemical activation by a substrate concentration jump. The electrical current transients provides information about the reaction mechanism of the enzyme. In particular, the BLM/SSM technique allows identification of electrogenic steps which in turn may be used to localize ion translocation during the reaction cycle of the pump. In addition, using the high time resolution of the technique, especially when rapid activation via caged ATP is employed, rate constants of electrogenic and electroneutral steps can be determined. In the present review, we will discuss the main results obtained by the BLM and SSM methods and how they have contributed to unravel the transport mechanism of P-type ATPases.

Electrogenic steps of the SR Ca-ATPase enzymatic cycle and the effect of curcumin

Sarcoplasmic reticulum (SR) vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps in the presence of calcium ions. The resulting capacitive current transients are compared with those calculated on the basis of the enzymatic cycle of the calcium pump. This comparison provides information on the kinetics of the E(2)-E(1) conformational change and on its pH dependence. The alteration in the current transients following ATP concentration jumps in the presence of curcumin is examined. In particular, curcumin decreases the rate of slippage of the Ca-ATPase, and at concentrations above 10 microM reduces calcium transport by this pump.

Pre-steady state electrogenic events of Ca2+/H+ exchange and transport by the Ca2+-ATPase

Native or recombinant SERCA (sarco(endo)plasmic reticulum Ca(2+) ATPase) was adsorbed on a solid supported membrane and then activated with Ca(2+) and ATP concentration jumps through rapid solution exchange. The resulting electrogenic events were recorded as electrical currents flowing along the external circuit. Current transients were observed following Ca(2+) jumps in the absence of ATP and following ATP jumps in the presence of Ca(2+). The related charge movements are attributed to Ca(2+) reaching its binding sites in the ground state of the enzyme (E(1)) and to its vectorial release from the enzyme phosphorylated by ATP (E(2)P). The Ca(2+) concentration and pH dependence as well as the time frames of the observed current transients are consistent with equilibrium and pre-steady state biochemical measurements of sequential steps within a single enzymatic cycle. Numerical integration of the current transients recorded at various pH values reveal partial charge compensation by H(+) in exchange for Ca(2+) at acidic (but not at alkaline) pH. Most interestingly, charge movements induced by Ca(2+) and ATP vary over different pH ranges, as the protonation probability of residues involved in Ca(2+)/H(+) exchange is lower in the E(1) than in the E(2)P state. Our single cycle measurements demonstrate that this difference contributes directly to the reduction of Ca(2+) affinity produced by ATP utilization and results in the countertransport of two Ca(2+) and two H(+) within each ATPase cycle at pH 7.0. The effects of site-directed mutations indicate that Glu-771 and Asp-800, within the Ca(2+) binding domain, are involved in the observed Ca(2+)/H(+) exchange.

Clotrimazole Inhibits the Ca2+-ATPase (SERCA) by Interfering with Ca2+ Binding and Favoring the E2 Conformation

Clotrimazole (CLT) is an antimycotic imidazole derivative that is known to inhibit cytochrome P-450, ergosterol biosynthesis and proliferation of cells in culture, and to interfere with cellular Ca(2+) homeostasis. We found that CLT inhibits the Ca(2+)-ATPase of rabbit fast-twitch skeletal muscle (SERCA1), and we characterized in detail the effect of CLT on this calcium transport ATPase. We used biochemical methods for characterization of the ATPase and its partial reactions, and we also performed measurements of charge movements following adsorption of sarcoplasmic reticulum vesicles containing the ATPase onto a gold-supported biomimetic membrane. CLT inhibits Ca(2+)-ATPase and Ca(2+) transport with a K(I) of 35 mum. Ca(2+) binding in the absence of ATP and phosphoenzyme formation by the utilization of ATP in the presence of Ca(2+) are also inhibited within the same CLT concentration range. On the other hand, phosphoenzyme formation by utilization of P(i) in the absence of Ca(2+) is only minimally inhibited. It is concluded that CLT inhibits primarily Ca(2+) binding and, consequently, the Ca(2+)-dependent reactions of the SERCA cycle. It is suggested that CLT resides within the membrane-bound region of the transport ATPase, thereby interfering with binding and the conformational effects of the activating cation.

Porous membranes for reconstitution of ion channels

Functional biological synthetic composite (BSC) membranes were made using phospholipids, biological membrane proteins and permeable synthetic supports or membranes. Lipid bilayers were formed on porous polycarbonate (PC), polyethylene terephthalate (PETE) and poly (l-lactic acid) (PLLA) membranes and in 10-100 microm laser-drilled pores in a 96-well plastic plate as measured by increased resistance or decreased currents. Bilayers in 50 microm and smaller pores were stable for up to 4 h as measured by resistance changes or a current after gramicidin D reconstitution. Biological membrane transport reconstitution was then carried out. Using vesicles containing Kv1.5 K(+) channels, K(+) currents and decreased resistance were measured across bilayers in 50 microm pores in the plastic plate and PLLA membranes, respectively, which were inhibited by compound B, a Kv1.5 K(+) channel inhibitor. Functional reconstitution of Kv1.5 K(+) channels was successful. Incorporation of membrane proteins in functional form in stable permeable membrane-supported lipid bilayers is a simple technology to create BSC membranes that mimic biological function which is readily adaptable for high throughput screening.

Crystal structure and structural stability of acylphosphatase from hyperthermophilic archaeon Pyrococcus horikoshii OT3

To elucidate the structural basis for the high stability of acylphosphatase (AcP) from Pyrococcus horikoshii OT3, we determined its crystal structure at 1.72 A resolution. P. horikoshii AcP possesses high stability despite its approximately 30% sequence identity with eukaryotic enzymes that have moderate thermostability. The overall fold of P. horikoshii AcP was very similar to the structures of eukaryotic counterparts. The crystal structure of P. horikoshii AcP shows the same fold betaalphabetabetaalphabeta topology and the conserved putative catalytic residues as observed in eukaryotic enzymes. Comparison with the crystal structure of bovine common-type AcP and that of D. melanogaster AcP (AcPDro2) as representative of eukaryotic AcP revealed some significant characteristics in P. horikoshii AcP that likely play important roles in structural stability: (1) shortening of the flexible N-terminal region and long loop; (2) an increased number of ion pairs on the protein surface; (3) stabilization of the loop structure by hydrogen bonds. In P. horikoshii AcP, two ion pair networks were observed one located in the loop structure positioned near the C-terminus, and other on the beta-sheet. The importance of ion pairs for structural stability was confirmed by site-directed mutation and denaturation induced by guanidium chloride.

Calcium transport by sarcoplasmic reticulum Ca-ATPase can be investigated on a solid-supported membrane

Sarcoplasmic reticulum (SR) native vesicles incorporating Ca-ATPase are adsorbed on a solid-supported lipid membrane (SSM). Upon adsorption, the ion pumps are chemically activated by concentration jumps of ATP and the capacitive current transients generated by SR Ca-ATPase are measured under potentiostatic conditions. The Michaelis-Menten constant, K(M), for ATP is evaluated by varying the concentration of ATP in the activating solution. This preliminary result shows that ion transport by SR Ca-ATPase can be suitably investigated by a technique based on concentration jumps on an SSM.

A molecular dynamics study of acylphosphatase in aggregation-promoting conditions: the influence of trifluoroethanol/water solvent

The 98-residue protein acylphosphatase exhibits a high propensity for aggregation under certain conditions. Aggregates formed from wild-type acylphosphatase in the presence of 2,2,2-trifluoroethanol and from highly destabilized mutants are essentially identical in structure. Furthermore, it has been shown by mutational studies that different regions of the protein are important for aggregation and folding. In the present molecular dynamics study, we compare the behavior of the protein in aqueous solution and in a 25% (v/v) 2,2,2-trifluoroethanol/water environment mimicking the experimental conditions. The 2,2,2-trifluoroethanol surrounding affects the structure of the protein mostly in the regions important for aggregation, in good agreement with experimental data. This suggests that the early step of (partly) unfolding, which precedes the aggregation process, has been observed.

Time-resolved charge translocation by sarcoplasmic reticulum Ca-ATPase measured on a solid supported membrane

Sarcoplasmic reticulum vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps both in the absence and in the presence of K(+) ions and at different pH values. Ca(2+) concentration jumps in the absence of ATP were also carried out. The resulting capacitive current transients were analyzed together with the charge under the transients. The relaxation time constants of the current transients were interpreted on the basis of an equivalent circuit. The current transient after ATP concentration jumps and the charge after Ca(2+) concentration jumps in the absence of ATP exhibit almost the same dependence upon the Ca(2+) concentration, with a half-saturating value of approximately 1.5 microM. The pH dependence of the charge after Ca(2+) translocation demonstrates the occurrence of one H(+) per one Ca(2+) countertransport at pH 7 by direct charge-transfer measurements. The presence of K(+) decreases the magnitude of the current transients without altering their shape; this decrease is explained by K(+) binding to the cytoplasmic side of the pump in the E(1) conformation and being released to the same side during the E(1)-E(2) transition.