Extracellular allosteric Na(+) binding to the Na(+),K(+)-ATPase in cardiac myocytes

Predicting Absorption-Distribution Properties of Neuroprotective Phosphine-Borane Compounds Using In Silico Modeling and Machine Learning

Phosphine-borane complexes are novel chemical entities with preclinical efficacy in neuronal and ophthalmic disease models. In vitro and in vivo studies showed that the metabolites of these compounds are capable of cleaving disulfide bonds implicated in the downstream effects of axonal injury. A difficulty in using standard in silico methods for studying these drugs is that most computational tools are not designed for borane-containing compounds. Using in silico and machine learning methodologies, the absorption-distribution properties of these unique compounds were assessed. Features examined with in silico methods included cellular permeability, octanol-water partition coefficient, blood-brain barrier permeability, oral absorption and serum protein binding. The resultant neural networks demonstrated an appropriate level of accuracy and were comparable to existing in silico methodologies. Specifically, they were able to reliably predict pharmacokinetic features of known boron-containing compounds. These methods predicted that phosphine-borane compounds and their metabolites meet the necessary pharmacokinetic features for orally active drug candidates. This study showed that the combination of standard in silico predictive and machine learning models with neural networks is effective in predicting pharmacokinetic features of novel boron-containing compounds as neuroprotective drugs.

Sodium homeostasis in the tumour microenvironment

The concentration of sodium ions (Na+) is raised in solid tumours and can be measured at the cellular, tissue and patient levels. At the cellular level, the Na+ gradient across the membrane powers the transport of H+ ions and essential nutrients for normal activity. The maintenance of the Na+ gradient requires a large proportion of the cell's ATP. Na+ is a major contributor to the osmolarity of the tumour microenvironment, which affects cell volume and metabolism as well as immune function. Here, we review evidence indicating that Na+ handling is altered in tumours, explore our current understanding of the mechanisms that may underlie these alterations and consider the potential consequences for cancer progression. Dysregulated Na+ balance in tumours may open opportunities for new imaging biomarkers and re-purposing of drugs for treatment.

Evidence for heterogeneous subsarcolemmal Na+ levels in rat ventricular myocytes

The intracellular Na⁺ concentration ([Na⁺]) regulates cardiac contractility. Previous studies have suggested that subsarcolemmal [Na⁺] is higher than cytosolic [Na⁺] in cardiac myocytes, but this concept remains controversial. Here, we used electrophysiological experiments and mathematical modeling to test whether there are subsarcolemmal pools with different [Na⁺] and dynamics compared with the bulk cytosol in rat ventricular myocytes. A Na⁺ dependency curve for Na⁺-K⁺-ATPase (NKA) current was recorded with symmetrical Na⁺ solutions, i.e., the same [Na⁺] in the superfusate and internal solution. This curve was used to estimate [Na⁺] sensed by NKA in other experiments. Three experimental observations suggested that [Na⁺] is higher near NKA than in the bulk cytosol: 1) when extracellular [Na⁺] was high, [Na⁺] sensed by NKA was ~6 mM higher than the internal solution in quiescent cells; 2) long trains of Na⁺ channel activation almost doubled this gradient; compared with an even intracellular distribution of Na⁺, the increase of [Na⁺] sensed by NKA was 10 times higher than expected, suggesting a local Na⁺ domain; and 3) accumulation of Na⁺ near NKA after trains of Na⁺ channel activation dissipated very slowly. Finally, mathematical models assuming heterogeneity of [Na⁺] between NKA and the Na⁺ channel better reproduced experimental data than the homogeneous model. In conclusion, our data suggest that NKA-sensed [Na⁺] is higher than [Na⁺] in the bulk cytosol and that there are differential Na⁺ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. NEW & NOTEWORTHY Our data suggest that the Na⁺-K⁺-ATPase-sensed Na⁺ concentration is higher than the Na⁺ concentration in the bulk cytosol and that there are differential Na⁺ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/heterogeneous-sodium-in-ventricular-myocytes/ .

Steady-state analysis of enzymes with non-Michaelis-Menten kinetics: The transport mechanism of Na+/K+-ATPase

Procedures to define kinetic mechanisms from catalytic activity measurements that obey the Michaelis-Menten equation are well established. In contrast, analytical tools for enzymes displaying non-Michaelis-Menten kinetics are underdeveloped, and transient-state measurements, when feasible, are therefore preferred in kinetic studies. Of note, transient-state determinations evaluate only partial reactions, and these might not participate in the reaction cycle. Here, we provide a general procedure to characterize kinetic mechanisms from steady-state determinations. We described non-Michaelis-Menten kinetics with equations containing parameters equivalent to k_cat and K_m and modeled the underlying mechanism by an approach similar to that used under Michaelis-Menten kinetics. The procedure enabled us to evaluate whether Na⁺/K⁺-ATPase uses the same sites to alternatively transport Na⁺ and K⁺ This ping-pong mechanism is supported by transient-state studies but contradicted to date by steady-state analyses claiming that the release of one cationic species as product requires the binding of the other (ternary-complex mechanism). To derive robust conclusions about the Na⁺/K⁺-ATPase transport mechanism, we did not rely on ATPase activity measurements alone. During the catalytic cycle, the transported cations become transitorily occluded (i.e. trapped within the enzyme). We employed radioactive isotopes to quantify occluded cations under steady-state conditions. We replaced K⁺ with Rb⁺ because ⁴²K⁺ has a short half-life, and previous studies showed that K⁺- and Rb⁺-occluded reaction intermediates are similar. We derived conclusions regarding the rate of Rb⁺ deocclusion that were verified by direct measurements. Our results validated the ping-pong mechanism and proved that Rb⁺ deocclusion is accelerated when Na⁺ binds to an allosteric, nonspecific site, leading to a 2-fold increase in ATPase activity.

Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes

Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.

Na+-K+-ATPase, a new class of plasma membrane receptors

The Na(+)-K(+)-ATPase (NKA) differs from most other ion transporters, not only in its capacity to maintain a steep electrochemical gradient across the plasma membrane, but also as a receptor for a family of cardiotonic steroids, to which ouabain belongs. Studies from many groups, performed during the last 15 years, have demonstrated that ouabain, a member of the cardiotonic steroid family, can activate a network of signaling molecules, and that NKA will also serve as a signal transducer that can provide a feedback loop between NKA and the mitochondria. This brief review summarizes the current knowledge and controversies with regard to the understanding of NKA signaling.

Stimulation of the cardiac myocyte Na+-K+ pump due to reversal of its constitutive oxidative inhibition

Protein kinase C can activate NADPH oxidase and induce glutathionylation of the β1-Na(+)-K(+) pump subunit, inhibiting activity of the catalytic α-subunit. To examine if signaling of nitric oxide-induced soluble guanylyl cyclase (sGC)/cGMP/protein kinase G can cause Na(+)-K(+) pump stimulation by counteracting PKC/NADPH oxidase-dependent inhibition, cardiac myocytes were exposed to ANG II to activate NADPH oxidase and inhibit Na(+)-K(+) pump current (Ip). Coexposure to 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) to stimulate sGC prevented the decrease of Ip. Prevention of the decrease was abolished by inhibition of protein phosphatases (PP) 2A but not by inhibition of PP1, and it was reproduced by an activator of PP2A. Consistent with a reciprocal relationship between β1-Na(+)-K(+) pump subunit glutathionylation and pump activity, YC-1 decreased ANG II-induced β1-subunit glutathionylation. The decrease induced by YC-1 was abolished by a PP2A inhibitor. YC-1 decreased phosphorylation of the cytosolic p47(phox) NADPH oxidase subunit and its coimmunoprecipitation with the membranous p22(phox) subunit, and it decreased O2 (·-)-sensitive dihydroethidium fluorescence of myocytes. Addition of recombinant PP2A to myocyte lysate decreased phosphorylation of p47(phox) indicating the subunit could be a substrate for PP2A. The effects of YC-1 to decrease coimmunoprecipitation of p22(phox) and p47(phox) NADPH oxidase subunits and decrease β1-Na(+)-K(+) pump subunit glutathionylation were reproduced by activation of nitric oxide-dependent receptor signaling. We conclude that sGC activation in cardiac myocytes causes a PP2A-dependent decrease in NADPH oxidase activity and a decrease in β1 pump subunit glutathionylation. This could account for pump stimulation with neurohormonal oxidative stress expected in vivo.

Mapping human brain capillary water lifetime: high-resolution metabolic neuroimaging

Shutter-speed analysis of dynamic-contrast-agent (CA)-enhanced normal, multiple sclerosis (MS), and glioblastoma (GBM) human brain data gives the mean capillary water molecule lifetime (τ(b)) and blood volume fraction (v(b); capillary density-volume product (ρ(†)V)) in a high-resolution (1)H2O MRI voxel (40 μL) or ROI. The equilibrium water extravasation rate constant, k(po) (τ(b)(-1)), averages 3.2 and 2.9 s(-1) in resting-state normal white matter (NWM) and gray matter (NGM), respectively (n = 6). The results (italicized) lead to three major conclusions. (A) k(po) differences are dominated by capillary water permeability (P(W)(†)), not size, differences. NWM and NGM voxel k(po) and v(b) values are independent. Quantitative analyses of concomitant population-averaged k(po), v(b) variations in normal and normal-appearing MS brain ROIs confirm P(W)(†) dominance. (B) P(W)(†) is dominated (>95%) by a trans(endothelial)cellular pathway, not the P(CA)(†) paracellular route. In MS lesions and GBM tumors, P(CA)(†) increases but P(W)(†) decreases. (C) k(po) tracks steady-state ATP production/consumption flux per capillary. In normal, MS, and GBM brain, regional k(po) correlates with literature MRSI ATP (positively) and Na(+) (negatively) tissue concentrations. This suggests that the P(W)(†) pathway is metabolically active. Excellent agreement of the relative NGM/NWM k(po)v(b) product ratio with the literature (31)PMRSI-MT CMR(oxphos) ratio confirms the flux property. We have previously shown that the cellular water molecule efflux rate constant (k(io)) is proportional to plasma membrane P-type ATPase turnover, likely due to active trans-membrane water cycling. With synaptic proximities and synergistic metabolic cooperativities, polar brain endothelial, neuroglial, and neuronal cells form "gliovascular units." We hypothesize that a chain of water cycling processes transmits brain metabolic activity to k(po), letting it report neurogliovascular unit Na(+),K(+)-ATPase activity. Cerebral k(po) maps represent metabolic (functional) neuroimages. The NGM 2.9 s(-1) k(po) means an equilibrium unidirectional water efflux of ~10(15) H2O molecules s(-1) per capillary (in 1 μL tissue): consistent with the known ATP consumption rate and water co-transporting membrane symporter stoichiometries.

Sodium and proton effects on inward proton transport through Na/K pumps

The Na/K pump hydrolyzes ATP to export three intracellular Na (Nai) as it imports two extracellular K (Ko) across animal plasma membranes. Within the protein, two ion-binding sites (sites I and II) can reciprocally bind Na or K, but a third site (site III) exclusively binds Na in a voltage-dependent fashion. In the absence of Nao and Ko, the pump passively imports protons, generating an inward current (IH). To elucidate the mechanisms of IH, we used voltage-clamp techniques to investigate the [H]o, [Na]o, and voltage dependence of IH in Na/K pumps from ventricular myocytes and in ouabain-resistant pumps expressed in Xenopus oocytes. Lowering pHo revealed that Ho both activates IH (in a voltage-dependent manner) and inhibits it (in a voltage-independent manner) by binding to different sites. Nao effects depend on pHo; at pHo where no Ho inhibition is observed, Nao inhibits IH at all concentrations, but when applied at pHo that inhibits pump-mediated current, low [Na]o activates IH and high [Na]o inhibits it. Our results demonstrate that IH is a property inherent to Na/K pumps, not linked to the oocyte expression environment, explains differences in the characteristics of IH previously reported in the literature, and supports a model in which 1), protons leak through site III; 2), binding of two Na or two protons to sites I and II inhibits proton transport; and 3), pumps with mixed Na/proton occupancy of sites I and II remain permeable to protons.

Intratumor mapping of intracellular water lifetime: metabolic images of breast cancer?

Shutter-speed pharmacokinetic analysis of dynamic-contrast-enhanced (DCE)-MRI data allows evaluation of equilibrium inter-compartmental water interchange kinetics. The process measured here - transcytolemmal water exchange - is characterized by the mean intracellular water molecule lifetime (τi). The τi biomarker is a true intensive property not accessible by any formulation of the tracer pharmacokinetic paradigm, which inherently assumes it is effectively zero when applied to DCE-MRI. We present population-averaged in vivo human breast whole tumor τi changes induced by therapy, along with those of other pharmacokinetic parameters. In responding patients, the DCE parameters change significantly after only one neoadjuvant chemotherapy cycle: while K(trans) (measuring mostly contrast agent (CA) extravasation) and kep (CA intravasation rate constant) decrease, τi increases. However, high-resolution, (1 mm)(2), parametric maps exhibit significant intratumor heterogeneity, which is lost by averaging. A typical 400 ms τi value means a trans-membrane water cycling flux of 10(13) H2O molecules s(-1)/cell for a 12 µm diameter cell. Analyses of intratumor variations (and therapy-induced changes) of τi in combination with concomitant changes of ve (extracellular volume fraction) indicate that the former are dominated by alterations of the equilibrium cell membrane water permeability coefficient, PW, not of cell size. These can be interpreted in light of literature results showing that τi changes are dominated by a PW (active) component that reciprocally reflects the membrane driving P-type ATPase ion pump turnover. For mammalian cells, this is the Na(+), K(+)-ATPase pump. These results promise the potential to discriminate metabolic and microenvironmental states of regions within tumors in vivo, and their changes with therapy.