Stoichiometry and voltage dependence of the sodium pump in voltage-clamped, internally dialyzed squid giant axon

Individual genomic loci, transcript level, and biochemical profile of immune and antioxidant markers associated with genetically identified bacterial mastitis in Shami goats in Egypt

Background

Mastitis in goats is unquestionably a grave concern, with far-reaching implications for both animal well-being and productivity, while also presenting a potential threat to public health.

Aim

The study aimed to compare culture methods and multiplex PCR (m-PCR) in the detection of the most three common mastitis-causing pathogens (Staphylococcus aureus, Escherichia coli, and Streptococcus spp.) and investigate the gene expression, single nucleotide polymorphisms (SNPs), serum concentrations of immunological and antioxidant indicators linked to mastitis in Shami goats.

Methods

One hundred Shami do (50 Shami goats with clinical mastitis and 50 normal goats taken as control group). The culture methods and m-PCR analysis were used to find the bacteria in the milk samples. Blood samples were obtained to assess some hemato-biochemical parameters, detect SNPs, and determine the expression of certain immunological and antioxidant indicators in the genes.

Results

The culture method detected the pathogens causing mastitis in 90% of the milk samples, but m-PCR detected them in 100% of the milk samples. SNPs linked to mastitis resistance/susceptibility in examined does were detected through DNA sequencing of immunological and antioxidant indicators. The magnitude of gene expression varied significantly between the resistant and mastitis-affected groups. Significant (P ˂ 0.05) elevations were noticed in WBCs count, mainly neutsrophils count, serum levels of BHB, NEFA, triglycerides, LDL-C, AST, ALT, ALP, creatinine, total protein, globulin, Ca, K, GPx, MDA, acute phase proteins, and cytokines in mastitis affected does as compared to control. While RBCs count, PCV%, lymphocytes count, serum concentration of glucose, cholesterol, HDL-C, albumin, Na, Cl, P, GSH, SOD, and catalase significantly (P ˂ 0.05) diminished in mastitis affected does compared to healthy ones. APPs and pro-inflammatory cytokines scored high sensitivities and NPVs but TNF-α and serum amyloid A (SAA) had the highest percentages of increase.

Conclusion

The study confirmed that m-PCR is the most sensitive method for bacteria identification (S. aureus, E. coli, and Strept. spp.) while SNPs in antioxidant and immunological genes may be important genetic indicators for mastitis risk or resistance in Shami does. To establish an effective management plan and forecast the most sensitive risk time for illness onset, gene expression profiles of the tested genes may also be employed as proxy biomarkers. TNF-α and SAA may be precious indicators for the detection of caprine mastitis.

The Na+,K+-ATPase and its stoichiometric ratio: some thermodynamic speculations

 Almost seventy years after its discovery, the sodium-potassium adenosine triphosphatase (the sodium pump) located in the cell plasma membrane remains a source of novel mechanistic and physiologic findings. A noteworthy feature of this enzyme/transporter is its robust stoichiometric ratio under physiological conditions: it sequentially counter-transports three sodium ions and two potassium ions against their electrochemical potential gradients per each hydrolyzed ATP molecule. Here we summarize some present knowledge about the sodium pump and its physiological roles, and speculate whether energetic constraints may have played a role in the evolutionary selection of its characteristic stoichiometric ratio.

Beyond Memristors: Neuromorphic Computing Using Meminductors

Resistors with memory (memristors), inductors with memory (meminductors) and capacitors with memory (memcapacitors) play different roles in novel computing architectures. We found that a coil with a magnetic core is an inductor with memory (meminductor) in terms of its inductance L(q) being a function of charge q. The history of the current passing through the coil is remembered by the magnetization inside the magnetic core. Such a meminductor can play a unique role (that cannot be played by a memristor) in neuromorphic computing, deep learning and brain-inspired computers since the time constant (t0=LC) of a neuromorphic RLC circuit is jointly determined by the inductance L and capacitance C, rather than the resistance R. As an experimental verification, this newly invented meminductor was used to reproduce the observed biological behavior of amoebae (the memorizing, timing and anticipating mechanisms). In conclusion, a beyond-memristor computing paradigm is theoretically sensible and experimentally practical.

An axon-specific expression of HCN channels catalyzes fast action potential signaling in GABAergic interneurons

During high-frequency network activities, fast-spiking, parvalbumin-expressing basket cells (PV⁺-BCs) generate barrages of fast synaptic inhibition to control the probability and precise timing of action potential (AP) initiation in principal neurons. Here we describe a subcellular specialization that contributes to the high speed of synaptic inhibition mediated by PV⁺-BCs. Mapping of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel distribution in rat hippocampal PV⁺-BCs with subcellular patch-clamp methods revealed that functional HCN channels are exclusively expressed in axons and completely absent from somata and dendrites. HCN channels not only enhance AP initiation during sustained high-frequency firing but also speed up the propagation of AP trains in PV⁺-BC axons by dynamically opposing the hyperpolarization produced by Na⁺-K⁺ ATPases. Since axonal AP signaling determines the timing of synaptic communication, the axon-specific expression of HCN channels represents a specialization for PV⁺-BCs to operate at high speed.

The precise subcellular location of ion channels is a key determinant of their functions. Here, subcellular patch-clamp recordings demonstrate that an axon-specific expression of HCN channels facilitates the initiation and propagation of action potentials in parvalbumin-expressing basket cells.

Mitochondrial respiratory chain inhibition and Na+K+ATPase dysfunction are determinant factors modulating the toxicity of nickel in the brain of indian catfish Clarias batrachus L

Nickel is a potential neurotoxic pollutant inflicting damage in living organisms, including fish, mainly through oxidative stress. Previous studies have demonstrated the impact of nickel toxicity on mitochondrial function, but there remain lacunae on the damage inflicted at mitochondrial respiratory level. Deficient mitochondrial function usually affects the activities of important adenosinetriphosphatases responsible for the maintenance of normal neuronal function, namely Na⁺K⁺ATPase, as explored in our study. Previous reports demonstrated the dysfunction of this enzyme upon nickel exposure but the contributing factors for the inhibition of this enzyme remained unexplored. The main purpose of this study was to elucidate the impact of nickel neurotoxicity on mitochondrial respiratory complexes and Na⁺K⁺ATPase in the piscine brain and to determine the contributing factors that had an impact on the same. Adult Clarias batrachus were exposed to nickel treated water at 10% and 20% of the 96 h LC50 value (41 mg.l^–1) respectively and sampled on 20, 40 and 60 days. Exposure of fish brain to nickel led to partial inhibition of complex IV of mitochondrial respiratory chain, however, the activities of complex I, II and III remained unaltered. This partial inhibition of mitochondrial respiratory chain might have been sufficient to lower mitochondrial energy production in mitochondria that contributed to the partial dysfunction of Na⁺K⁺ATPase. Besides energy depletion other contributing factors were involved in the dysfunction of this enzyme, like loss of thiol groups for enzyme activity and lipid peroxidation-derived end products that might have induced conformational and functional changes. However, providing direct evidence for such conformational and functional changes of Na⁺K⁺ATPase was beyond the scope of the present study. In addition, immunoblotting results also showed a decrease in Na⁺K⁺ATPase protein expression highlighting the impact of nickel neurotoxicity on the expression of the enzyme itself. The implication of the inhibition of mitochondrial respiration and Na⁺K⁺ATPase dysfunction was the neuronal death as evidenced by enhanced caspase-3 and caspase-9 activities. Thus, this study established the deleterious impact of nickel neurotoxicity on mitochondrial functions in the piscine brain and identified probable contributing factors that can act concurrently in the inhibition of Na⁺K⁺ATPase. This study also provided a vital clue about the specific areas that the therapeutic agents should target to counter nickel neurotoxicity.

External Ion Access in the Na/K Pump: Kinetics of Na+, K+, and Quaternary Amine Interaction

Na/K pumps build essential ion gradients across the plasmalemma of animal cells by coupling the extrusion of three Na⁺, with the import of two K⁺ and the hydrolysis of one ATP molecule. The mechanisms of selectivity and competition between Na⁺, K⁺, and inhibitory amines remain unclear. We measured the effects of external tetrapropylammonium (TPA⁺) and ethylenediamine (EDA²⁺) on three different Na/K pump transport modes in voltage-clamped Xenopus oocytes: 1) outward pump current (I_P), 2) passive inward H⁺ current at negative voltages without Na⁺ or K⁺ (I_H), and 3) transient charge movement reporting the voltage-dependent extracellular binding/release of Na⁺ (Q_Na). Both amines competed with K⁺ to inhibit I_P. TPA⁺ inhibited I_H without competing with H⁺, whereas EDA²⁺ did not alter I_H at pH 7.6. TPA⁺ competed with Na⁺ in Q_Na measurements, reducing Na⁺-apparent affinity, evidenced by a ∼-75 mV shift in the charge-voltage curve (at 20 mM TPA⁺) without reduction of the total charge moved (Q_tot). In contrast, EDA²⁺ and K⁺ did not compete with Na⁺ to inhibit Q_Na; both reduced Q_tot without decreasing Na⁺-apparent affinity. EDA²⁺ (15 mM) right-shifted the charge-voltage curve by ∼+50 mV. Simultaneous occlusion of EDA²⁺ and Na⁺ by an E2P conformation unable to reach E1P was demonstrated by voltage-clamp fluorometry. Trypsinolysis experiments showed that EDA²⁺-bound pumps are much more proteolysis-resistant than Na⁺-, K⁺-, or TPA⁺-bound pumps, therefore uncovering unique EDA²⁺-bound conformations. K⁺ effects on Q_Na and I_H were also evaluated in pumps inhibited with beryllium fluoride, a phosphate mimic. K⁺ reduced Q_tot without shifting the charge-voltage curve, indicating noncompetitive effects, and partially inhibited I_H to the same extent as TPA⁺ in non-beryllium-fluorinated pumps. These results demonstrate that K⁺ interacts with beryllium-fluorinated pumps inducing conformational changes that alter Q_Na and I_H, suggesting that there are two external access pathways for proton transport by I_H.

Complementary Tuning of Na+ and K+ Channel Gating Underlies Fast and Energy-Efficient Action Potentials in GABAergic Interneuron Axons

Fast-spiking, parvalbumin-expressing GABAergic interneurons (PV⁺-BCs) express a complex machinery of rapid signaling mechanisms, including specialized voltage-gated ion channels to generate brief action potentials (APs). However, short APs are associated with overlapping Na⁺ and K⁺ fluxes and are therefore energetically expensive. How the potentially vicious combination of high AP frequency and inefficient spike generation can be reconciled with limited energy supply is presently unclear. To address this question, we performed direct recordings from the PV⁺-BC axon, the subcellular structure where active conductances for AP initiation and propagation are located. Surprisingly, the energy required for the AP was, on average, only ∼1.6 times the theoretical minimum. High energy efficiency emerged from the combination of fast inactivation of Na⁺ channels and delayed activation of Kv3-type K⁺ channels, which minimized ion flux overlap during APs. Thus, the complementary tuning of axonal Na⁺ and K⁺ channel gating optimizes both fast signaling properties and metabolic efficiency.

Thermal block of action potentials is primarily due to voltage-dependent potassium currents: a modeling study

OBJECTIVE

Thermal block of action potential conduction using infrared lasers is a new modality for manipulating neural activity. It could be used for analysis of the nervous system and for therapeutic applications. We sought to understand the mechanisms of thermal block.

APPROACH

To analyze the mechanisms of thermal block, we studied both the original Hodgkin/Huxley model, and a version modified to more accurately match experimental data on thermal responses in the squid giant axon.

MAIN RESULTS

Both the original and modified models suggested that thermal block, especially at higher temperatures, is primarily due to a depolarization-activated hyperpolarization as increased temperature leads to faster activation of voltage-gated potassium ion channels. The minimum length needed to block an axon scaled with the square root of the axon's diameter.

SIGNIFICANCE

The results suggest that voltage-dependent potassium ion channels play a major role in thermal block, and that relatively short lengths of axon could be thermally manipulated to selectively block fine, unmyelinated axons, such as C fibers, that carry pain and other sensory information.

Intracellular Requirements for Passive Proton Transport through the Na+,K+-ATPase

The Na⁺,K⁺-ATPase (NKA or Na/K pump) hydrolyzes one ATP to exchange three intracellular Na+ (Na⁺_i) for two extracellular K+ (K⁺_o) across the plasma membrane by cycling through a set of reversible transitions between phosphorylated and dephosphorylated conformations, alternately opening ion-binding sites externally (E2) or internally (E1). With subsaturating [Na⁺]_o and [K⁺]_o, the phosphorylated E2P conformation passively imports protons generating an inward current (I_H), which may be exacerbated in NKA-subunit mutations associated with human disease. To elucidate the mechanisms of I_H, we studied the effects of intracellular ligands (transported ions, nucleotides, and beryllium fluoride) on I_H and, for comparison, on transient currents measured at normal Na⁺_o (Q_Na). Utilizing inside-out patches from Xenopus oocytes heterologously expressing NKA, we observed that 1) in the presence of Na⁺_i, I_H and Q_Na were both activated by ATP, but not ADP; 2) the [Na⁺]_i dependence of I_H in saturating ATP showed K_0.5,Na = 1.8 ± 0.2 mM and the [ATP] dependence at saturating [Na⁺]_i yielded K_0.5,ATP = 48 ± 11 μM (in comparison, Na⁺_i-dependent Q_Na yields K_0.5,Na = 0.8 ± 0.2 mM and K_0.5,ATP = 0.43 ± 0.03 μM; 3) ATP activated I_H in the presence of K⁺_i (∼15% of the I_H observed in Na⁺_i) only when Mg²⁺_i was also present; and 4) beryllium fluoride induced maximal I_H even in the absence of nucleotide. These data indicate that I_H occurs when NKA is in an externally open E2P state with nucleotide bound, a conformation that can be reached through forward Na/K pump phosphorylation of E1, with Na⁺_i and ATP, or by backward binding of K⁺_i to E1, which drives the pump to the occluded E2(2K), where free P_i (at the micromolar levels found in millimolar ATP solutions) promotes external release of occluded K⁺ by backdoor NKA phosphorylation. Maximal I_H through beryllium-fluorinated NKA indicates that this complex mimics ATP-bound E2P states.

The vicious circle of hypometabolism in neurodegenerative diseases: Ways and mechanisms of metabolic correction

Hypometabolism, characterized by decreased brain glucose consumption, is a common feature of many neurodegenerative diseases. Initial hypometabolic brain state, created by characteristic risk factors, may predispose the brain to acquired epilepsy and sporadic Alzheimer's and Parkinson's diseases, which are the focus of this review. Analysis of available data suggests that deficient glucose metabolism is likely a primary initiating factor for these diseases, and that resulting neuronal dysfunction further promotes the metabolic imbalance, establishing an effective positive feedback loop and a downward spiral of disease progression. Therefore, metabolic correction leading to the normalization of abnormalities in glucose metabolism may be an efficient tool to treat the neurological disorders by counteracting their primary pathological mechanisms. Published and preliminary experimental results on this approach for treating Alzheimer's disease and epilepsy models support the efficacy of metabolic correction, confirming the highly promising nature of the strategy. © 2017 Wiley Periodicals, Inc.

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.

The α4 isoform of the Na⁺,K⁺-ATPase is tuned for changing extracellular environments

In their journey from the male to the female reproductive tract, spermatozoa are confronted with a constantly changing environment. To cope with the associated challenges, spermatozoa express a distinct set of transporters, channels and pumps. One of the membrane proteins unique to spermatozoa is the α4 isoform of the Na(+) ,K(+) -ATPase. In addition to α4, spermatozoa express the ubiquous α1 variant. To get a detailed understanding of how α1 and α4 differ, and why spermatozoa need an additional Na(+) ,K(+) -ATPase, we have conducted an electrophysiological comparison of the rodent isoforms (rat α4 versus mouse α1-3) using the Xenopus oocyte expression system. We demonstrate isoform-specific differences in the voltage sensitivity of steady-state turnover, with α2 being the more sensitive, and α1 and α2 having faster Na(+) release kinetics than α3 and α4. Our data further show that the α1 and α2 turnover rates are fast compared with those of α3 and α4. Finally, α4 is less influenced by changes in extracellular Na(+) and temperature than α1. Based on these findings, we discuss the possibility that evolution has selected robust activity rather than rapid turnover for α4.

Mussel adhesion is dictated by time-regulated secretion and molecular conformation of mussel adhesive proteins

Interfacial water constitutes a formidable barrier to strong surface bonding, hampering the development of water-resistant synthetic adhesives. Notwithstanding this obstacle, the Asian green mussel Perna viridis attaches firmly to underwater surfaces via a proteinaceous secretion (byssus). Extending beyond the currently known design principles of mussel adhesion, here we elucidate the precise time-regulated secretion of P. viridis mussel adhesive proteins. The vanguard 3,4-dihydroxy-L-phenylalanine (Dopa)-rich protein Pvfp-5 acts as an adhesive primer, overcoming repulsive hydration forces by displacing surface-bound water and generating strong surface adhesion. Using homology modelling and molecular dynamics simulations, we find that all mussel adhesive proteins are largely unordered, with Pvfp-5 adopting a disordered structure and elongated conformation whereby all Dopa residues reside on the protein surface. Time-regulated secretion and structural disorder of mussel adhesive proteins appear essential for optimizing extended nonspecific surface interactions and byssus' assembly. Our findings reveal molecular-scale principles to help the development of wet-resistant adhesives.

Mechanism of potassium ion uptake by the Na(+)/K(+)-ATPase

The Na⁺/K⁺-ATPase restores sodium (Na⁺) and potassium (K⁺) electrochemical gradients dissipated by action potentials and ion-coupled transport processes. As ions are transported, they become transiently trapped between intracellular and extracellular gates. Once the external gate opens, three Na⁺ ions are released, followed by the binding and occlusion of two K⁺ ions. While the mechanisms of Na⁺ release have been well characterized by the study of transient Na⁺ currents, smaller and faster transient currents mediated by external K⁺ have been more difficult to study. Here we show that external K⁺ ions travelling to their binding sites sense only a small fraction of the electric field as they rapidly and simultaneously become occluded. Consistent with these results, molecular dynamics simulations of a pump model show a wide water-filled access channel connecting the binding site to the external solution. These results suggest a mechanism of K⁺ gating different from that of Na⁺ occlusion.

During transport by the Na⁺/K⁺-ATPase, Na⁺ and K⁺ ions become occluded between intra- and extracellular gates. Here Castillo et al. measure transient electrical signals arising from K⁺ occlusion and use molecular simulations to describe a K⁺ gating mechanism fundamentally different to that of Na⁺.

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.

Route, mechanism, and implications of proton import during Na+/K+ exchange by native Na+/K+-ATPase pumps

The Na⁺/K⁺ pump is a hybrid transporter that can also import protons at physiological K⁺ and Na⁺ concentrations.

A single Na⁺/K⁺-ATPase pumps three Na⁺ outwards and two K⁺ inwards by alternately exposing ion-binding sites to opposite sides of the membrane in a conformational sequence coupled to pump autophosphorylation from ATP and auto-dephosphorylation. The larger flow of Na⁺ than K⁺ generates outward current across the cell membrane. Less well understood is the ability of Na⁺/K⁺ pumps to generate an inward current of protons. Originally noted in pumps deprived of external K⁺ and Na⁺ ions, as inward current at negative membrane potentials that becomes amplified when external pH is lowered, this proton current is generally viewed as an artifact of those unnatural conditions. We demonstrate here that this inward current also flows at physiological K⁺ and Na⁺ concentrations. We show that protons exploit ready reversibility of conformational changes associated with extracellular Na⁺ release from phosphorylated Na⁺/K⁺ pumps. Reversal of a subset of these transitions allows an extracellular proton to bind an acidic side chain and to be subsequently released to the cytoplasm. This back-step of phosphorylated Na⁺/K⁺ pumps that enables proton import is not required for completion of the 3 Na⁺/2 K⁺ transport cycle. However, the back-step occurs readily during Na⁺/K⁺ transport when external K⁺ ion binding and occlusion are delayed, and it occurs more frequently when lowered extracellular pH raises the probability of protonation of the externally accessible carboxylate side chain. The proton route passes through the Na⁺-selective binding site III and is distinct from the principal pathway traversed by the majority of transported Na⁺ and K⁺ ions that passes through binding site II. The inferred occurrence of Na⁺/K⁺ exchange and H⁺ import during the same conformational cycle of a single molecule identifies the Na⁺/K⁺ pump as a hybrid transporter. Whether Na⁺/K⁺ pump–mediated proton inflow may have any physiological or pathophysiological significance remains to be clarified.

Transepithelial glucose transport and Na+/K+ homeostasis in enterocytes: an integrative model

The uptake of glucose and the nutrient coupled transcellular sodium traffic across epithelial cells in the small intestine has been an ongoing topic in physiological research for over half a century. Driving the uptake of nutrients like glucose, enterocytes must have regulatory mechanisms that respond to the considerable changes in the inflow of sodium during absorption. The Na-K-ATPase membrane protein plays a major role in this regulation. We propose the hypothesis that the amount of active Na-K-ATPase in enterocytes is directly regulated by the concentration of intracellular Na(+) and that this regulation together with a regulation of basolateral K permeability by intracellular ATP gives the enterocyte the ability to maintain ionic Na(+)/K(+) homeostasis. To explore these regulatory mechanisms, we present a mathematical model of the sodium coupled uptake of glucose in epithelial enterocytes. Our model integrates knowledge about individual transporter proteins including apical SGLT1, basolateral Na-K-ATPase, and GLUT2, together with diffusion and membrane potentials. The intracellular concentrations of glucose, sodium, potassium, and chloride are modeled by nonlinear differential equations, and molecular flows are calculated based on experimental kinetic data from the literature, including substrate saturation, product inhibition, and modulation by membrane potential. Simulation results of the model without the addition of regulatory mechanisms fit well with published short-term observations, including cell depolarization and increased concentration of intracellular glucose and sodium during increased concentration of luminal glucose/sodium. Adding regulatory mechanisms for regulation of Na-K-ATPase and K permeability to the model show that our hypothesis predicts observed long-term ionic homeostasis.

In vivo study of transepithelial potential difference (TEPD) in proximal convoluted tubules of rat kidney by synchronization modulation electric field

Synchronization modulation (SM) electric field has been shown to effectively activate function of Na(+)/K(+) pumps in various cells and tissues, including skeletal muscle cells, cardiomyocyte, monolayer of cultured cell line, and peripheral blood vessels. We are now reporting the in vivo studies in application of the SM electric field to kidney of living rats. The field-induced changes in the transepithelial potential difference (TEPD) or the lumen potential from the proximal convoluted tubules were monitored. The results showed that a short time (20 s) application of the SM electric field can significantly increase the magnitude of TEPD from 1-2 mV to about 20 mV. The TEPD is an active potential representing the transport current of the Na/K pumps in epithelial wall of renal tubules. This study showed that SM electric field can increase TEPD by activation of the pump molecules. Considering renal tubules, many active transporters are driven by the Na(+) concentration gradient built by the Na(+)/K(+) pumps, activation of the pump functions and increase in the magnitude of TEPD imply that the SM electric field may improve reabsorption functions of the renal tubules.

Functional analysis of human Na+/K+-ATPase familial or sporadic hemiplegic migraine mutations expressed in Xenopus oocytes

AIM: Functional characterization of ATP1A2 mutations that are related to familial or sporadic hemiplegic migraine (FHM2, SHM).

METHODS: cRNA of human Na⁺/K⁺-ATPase α₂- and β₁-subunits were injected in Xenopus laevis oocytes. FHM2 or SHM mutations of residues located in putative α/β interaction sites or in the α₂-subunit’s C-terminal region were investigated. Mutants were analyzed by the two-electrode voltage-clamp (TEVC) technique on Xenopus oocytes. Stationary K⁺-induced Na⁺/K⁺ pump currents were measured, and the voltage dependence of apparent K⁺ affinity was investigated. Transient currents were recorded as ouabain-sensitive currents in Na⁺ buffers to analyze kinetics and voltage-dependent pre-steady state charge translocations. The expression of constructs was verified by preparation of plasma membrane and total membrane fractions of cRNA-injected oocytes.

RESULTS: Compared to the wild-type enzyme, the mutants G900R and E902K showed no significant differences in the voltage dependence of K⁺-induced currents, and analysis of the transient currents indicated that the extracellular Na⁺ affinity was not affected. Mutant G855R showed no pump activity detectable by TEVC. Also for L994del and Y1009X, pump currents could not be recorded. Analysis of the plasma and total membrane fractions showed that the expressed proteins were not or only minimally targeted to the plasma membrane. Whereas the mutation K1003E had no impact on K⁺ interaction, D999H affected the voltage dependence of K⁺-induced currents. Furthermore, kinetics of the transient currents was altered compared to the wild-type enzyme, and the apparent affinity for extracellular Na⁺ was reduced.

CONCLUSION: The investigated FHM2/SHM mutations influence protein function differently depending on the structural impact of the mutated residue.

Changes in the Distribution of the α 3 Na(+)/K(+) ATPase Subunit in Heterozygous Lurcher Purkinje Cells as a Genetic Model of Chronic Depolarization during Development

A common assumption of excitotoxic mechanisms in the nervous system is that the ionic imbalance resulting from overstimulation of glutamate receptors and increased Na⁺ and Ca⁺⁺ influx overwhelms cellular energy metabolic systems leading to cell death. The goal of this study was to examine how a chronic Na⁺ channel leak current in developing Purkinje cells in the heterozygous Lurcher mutant (+/Lc) affects the expression and distribution of the α3 subunit of the Na⁺/K⁺ ATPase pump, a key component of the homeostasis system that maintains ionic equilibrium in neurons. The expression pattern of the catalytic α3 Na⁺/K⁺ ATPase subunit was analyzed by immunohistochemistry, histochemistry, and Western Blots in wild type (WT) and +/Lc cerebella at postnatal days P10, P15, and P25 to determine if there are changes in the distribution of active Na⁺/K⁺ ATPase subunits in degenerating Purkinje cells. The results suggest that the expression of the catalytic α3 subunit is altered in chronically depolarized +/Lc Purkinje cells, although the density of active Na⁺/K⁺ ATPase pumps is not significantly altered compared with WT in the cerebellar cortex at P15, and then declines from P15 to P25 in the +/Lc cerebellum as the +/Lc Purkinje cells degenerate.

Sustained maximal voluntary contraction produces independent changes in human motor axons and the muscle they innervate

The repetitive discharges required to produce a sustained muscle contraction results in activity-dependent hyperpolarization of the motor axons and a reduction in the force-generating capacity of the muscle. We investigated the relationship between these changes in the adductor pollicis muscle and the motor axons of its ulnar nerve supply, and the reproducibility of these changes. Ten subjects performed a 1-min maximal voluntary contraction. Activity-dependent changes in axonal excitability were measured using threshold tracking with electrical stimulation at the wrist; changes in the muscle were assessed as evoked and voluntary electromyography (EMG) and isometric force. Separate components of axonal excitability and muscle properties were tested at 5 min intervals after the sustained contraction in 5 separate sessions. The current threshold required to produce the target muscle action potential increased immediately after the contraction by 14.8% (p<0.05), reflecting decreased axonal excitability secondary to hyperpolarization. This was not correlated with the decline in amplitude of muscle force or evoked EMG. A late reversal in threshold current after the initial recovery from hyperpolarization peaked at −5.9% at ∼35 min (p<0.05). This pattern was mirrored by other indices of axonal excitability revealing a previously unreported depolarization of motor axons in the late recovery period. Measures of axonal excitability were relatively stable at rest but less so after sustained activity. The coefficient of variation (CoV) for threshold current increase was higher after activity (CoV 0.54, p<0.05) whereas changes in voluntary (CoV 0.12) and evoked twitch (CoV 0.15) force were relatively stable. These results demonstrate that activity-dependent changes in motor axon excitability are unlikely to contribute to concomitant changes in the muscle after sustained activity in healthy people. The variability in axonal excitability after sustained activity suggests that care is needed when using these measures if the integrity of either the muscle or nerve may be compromised.

Role of AE2 for pHi regulation in biliary epithelial cells

The Cl⁻/HCO⁻₃anion exchanger 2 (AE2) is known to be involved in intracellular pH (pH_i) regulation and transepithelial acid-base transport. Early studies showed that AE2 gene expression is reduced in liver biopsies and blood mononuclear cells from patients with primary biliary cirrhosis (PBC), a disease characterized by chronic non-suppurative cholangitis associated with antimitochondrial antibodies (AMA) and other autoimmune phenomena. Microfluorimetric analysis of the Cl⁻/HCO⁻₃ anion exchange (AE) in isolated cholangiocytes showed that the cAMP-stimulated AE activity is diminished in PBC compared to both healthy and diseased controls. More recently, it was found that miR-506 is upregulated in cholangiocytes of PBC patients and that AE2 may be a target of miR-506. Additional evidence for a pathogenic role of AE2 dysregulation in PBC was obtained with Ae2^−/−_a,b mice, which develop biochemical, histological, and immunologic alterations that resemble PBC (including development of serum AMA). Analysis of HCO⁻₃ transport systems and pH_i regulation in cholangiocytes from normal and Ae2^−/−_a,b mice confirmed that AE2 is the transporter responsible for the Cl⁻/HCO⁻₃exchange in these cells. On the other hand, both Ae2^+/+_a,b and Ae2^−/−_a,b mouse cholangiocytes exhibited a Cl⁻-independent bicarbonate transport system, essentially a Na⁺-bicarbonate cotransport (NBC) system, which could contribute to pH_i regulation in the absence of AE2.

Evaluation of spectrum of peripheral neuropathy in predialysis patients with chronic kidney disease

INTRODUCTION

Neurological complications secondary to the uremic state, contribute largely to the morbidity and mortality in patients with renal failure. The prevalence of peripheral neuropathy remains high in advanced renal dysfunction.

MATERIALS AND METHODS

The present cross-sectional study was conducted on 100 adult patients of chronic kidney disease between 18 and 75 years of age with serum creatinine greater than 2 mg/dL. Apart from routine examination and baseline investigations, detailed history was elicited pertaining to patients' neurological symptoms, and scored according to the Neurological Symptom Score. Motor nerve conduction velocity was measured from right median, ulnar, peroneal, and tibial nerves.

RESULTS

It was observed that neurological symptoms increased steadily with raise in serum creatinine. The mean nerve conduction velocities (NCVs) of right median nerve, ulnar nerve, peroneal nerve, and tibial nerve were 51.34 ± 6.07, 53.04 ± 5.91, 44.72 ± 6.14, and 44.20 ± 5.17, respectively. The NCVs of all the tested nerves decreased significantly with increase in serum creatinine levels (p < 0.01): 70% of the patients had uremic polyneuropathy; 6% had asymptomatic neuropathy, 51% had symptomatic non-disabling neuropathy, while disabling neuropathy was seen in 13% of the patients.

CONCLUSION

Our data suggests that NCV testing when complimented with meticulous neurological assessment can provide invaluable input. These tests apart from helping us detect neuropathy in advanced renal dysfunction; can also detect the disease in largely asymptomatic patients which avoids the necessity to order for detailed neurophysiological investigation.

Attenuation of oxidative hemolysis of human red blood cells by the natural phenolic compound, allylpyrocatechol

The protecting ability of the Piper betle leaves-derived phenol, allylpyrocatechol (APC) against AAPH-induced membrane damage of human red blood cells (RBCs) was investigated. Compared to control, AAPH (50 mM) treatment resulted in significant hemolysis (55%, p < 0.01), associated with increased malondialdehyde (MDA) (2.9-fold, p < 0.001) and methemoglobin (6.1-fold, p < 0.001) levels. The structural deformation due to membrane damage was confirmed from scanning electron microscopy (SEM) images and Heinz bodies formation, while the cell permeability was evident from the K(+) efflux (28.7%, p < 0.05) and increased intracellular Na(+) concentration (8%, p < 0.05). The membrane damage, due to the reduction of the cholesterol/phospholipids ratio and depletion (p < 0.001) of ATP, 2,3-DPG by ˜44-54% and Na(+)-K(+) ATPase activity (43.7%), indicated loss of RBC functionality. The adverse effects of AAPH on all these biochemical parameters and the resultant oxidative hemolysis of RBCs were significantly reduced by pretreating the cells with APC (7 μM) or α-tocopherol (50 μM) for 1 h, prior to incubation with AAPH.

Synchronization modulation increases transepithelial potentials in MDCK monolayers through Na/K pumps

Transepithelial potential (TEP) is the voltage across a polarized epithelium. In epithelia that have active transport functions, the force for transmembrane flux of an ion is dictated by the electrochemical gradient in which TEP plays an essential role. In epithelial injury, disruption of the epithelial barrier collapses the TEP at the wound edge, resulting in the establishment of an endogenous wound electric field (∼100 mV/mm) that is directed towards the center of the wound. This endogenous electric field is implicated to enhance wound healing by guiding cell migration. We thus seek techniques to enhance the TEP, which may increase the wound electric fields and enhance wound healing. We report a novel technique, termed synchronization modulation (SM) using a train of electric pulses to synchronize the Na/K pump activity, and then modulating the pumping cycles to increase the efficiency of the Na/K pumps. Kidney epithelial monolayers (MDCK cells) maintain a stable TEP and transepithelial resistance (TER). SM significantly increased TEP over four fold. Either ouabain or digoxin, which block Na/K pump, abolished SM-induced TEP increases. In addition to the pump activity, basolateral distribution of Na/K pumps is essential for an increase in TEP. Our study for the first time developed an electrical approach to significantly increase the TEP. This technique targeting the Na/K pump may be used to modulate TEP, and may have implication in wound healing and in diseases where TEP needs to be modulated.

Apoptotic microtubules delimit an active caspase free area in the cellular cortex during the execution phase of apoptosis

Apoptotic microtubule network (AMN) is organized during apoptosis, forming a cortical structure beneath plasma membrane, which has an important role in preserving cell morphology and plasma membrane permeability. The aim of this study was to examine the role of AMN in maintaining plasma membrane integrity during the execution phase of apoptosis. We demonstrated in camptothecin-induced apoptosis in H460 cells that AMN delimits an active caspase free area beneath plasma membrane that permits the preservation of cellular cortex and transmembrane proteins. AMN depolymerization in apoptotic cells by a short exposure to colchicine allowed active caspases to reach the cellular cortex and cleave many key proteins involved in plasma membrane structural support, cell adhesion and ionic homeostasis. Cleavage of cellular cortex and plasma membrane proteins, such as α-spectrin, paxilin, focal adhesion kinase (FAK), E-cadherin and integrin subunit β4 was associated with cell collapse and cell detachment. Otherwise, cleavage-mediated inactivation of calcium ATPase pump (PMCA-4) and Na⁺/Ca²⁺ exchanger (NCX) involved in cell calcium extrusion resulted in calcium overload. Furthermore, cleavage of Na⁺/K⁺ pump subunit β was associated with altered sodium homeostasis. Cleavage of cell cortex and plasma membrane proteins in apoptotic cells after AMN depolymerization increased plasma permeability, ionic imbalance and bioenergetic collapse, leading apoptotic cells to secondary necrosis. The essential role of caspase-mediated cleavage in this process was demonstrated because the concomitant addition of colchicine that induces AMN depolymerization and the pan-caspase inhibitor z-VAD avoided the cleavage of cortical and plasma membrane proteins and prevented apoptotic cells to undergo secondary necrosis. Furthermore, the presence of AMN was also critical for proper phosphatidylserine externalization and apoptotic cell clearance by macrophages. These results indicate that AMN is essential to preserve an active caspase free area in the cellular cortex of apoptotic cells that allows plasma membrane integrity during the execution phase of apoptosis.

Physiology and pathophysiology of myelinated nerve fibers

The chief role of the axon is that of impulse conduction, which depends on the electrical cable structure and voltage-dependent ion channels of the axonal membrane. Over recent decades, the development of specialized techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel function to the processes of impulse conduction in myelinated nerves. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps, and ion exchange processes activated during impulse conduction in peripheral axons. These noninvasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and neurological disease, particularly in relation to the pathophysiological mechanisms that underlie neuropathy.

Spontaneous excitation patterns computed for axons with injury-like impairments of sodium channels and Na/K pumps

In injured neurons, “leaky” voltage-gated sodium channels (Nav) underlie dysfunctional excitability that ranges from spontaneous subthreshold oscillations (STO), to ectopic (sometimes paroxysmal) excitation, to depolarizing block. In recombinant systems, mechanical injury to Nav1.6-rich membranes causes cytoplasmic Na⁺-loading and “Nav-CLS”, i.e., coupled left-(hyperpolarizing)-shift of Nav activation and availability. Metabolic injury of hippocampal neurons (epileptic discharge) results in comparable impairment: left-shifted activation and availability and hence left-shifted I_Na-window. A recent computation study revealed that CLS-based I_Na-window left-shift dissipates ion gradients and impairs excitability. Here, via dynamical analyses, we focus on sustained excitability patterns in mildly damaged nodes, in particular with more realistic Gaussian-distributed Nav-CLS to mimic “smeared” injury intensity. Since our interest is axons that might survive injury, pumps (sine qua non for live axons) are included. In some simulations, pump efficacy and system volumes are varied. Impacts of current noise inputs are also characterized. The diverse modes of spontaneous rhythmic activity evident in these scenarios are studied using bifurcation analysis. For “mild CLS injury”, a prominent feature is slow pump/leak-mediated E_Ion oscillations. These slow oscillations yield dynamic firing thresholds that underlie complex voltage STO and bursting behaviors. Thus, Nav-CLS, a biophysically justified mode of injury, in parallel with functioning pumps, robustly engenders an emergent slow process that triggers a plethora of pathological excitability patterns. This minimalist “device” could have physiological analogs. At first nodes of Ranvier and at nociceptors, e.g., localized lipid-tuning that modulated Nav midpoints could produce Nav-CLS, as could co-expression of appropriately differing Nav isoforms.

Nerve cells damaged by trauma, stroke, epilepsy, inflammatory conditions etc, have chronically leaky sodium channels that eventually kill. The usual job of sodium channels is to make brief voltage signals –action potentials– for long distance propagation. After sodium channels open to generate action potentials, sodium pumps work harder to re-establish the intracellular/extracellular sodium imbalance that is, literally, the neuron's battery for firing action potentials. Wherever tissue damage renders membranes overly fluid, we hypothesize, sodium channels become chronically leaky. Our experimental findings justify this. In fluidized membranes, sodium channel voltage sensors respond too easily, letting channels spend too much time open. Channels leak, pumps respond. By mathematical modeling, we show that in damaged channel-rich membranes the continual pump/leak counterplay would trigger the kinds of bizarre intermittent action potential bursts typical of injured neurons. Arising ectopically from injury regions, such neuropathic firing is unrelated to events in the external world. Drugs that can silence these deleterious electrical barrages without blocking healthy action potentials are needed. If fluidized membranes house the problematic leaky sodium channels, then drug side effects could be diminished by using drugs that accumulate most avidly into fluidized membranes, and that bind their targets with highest affinity there.

Quick and effective hyperpolarization of the membrane potential in intact smooth muscle cells of blood vessels by synchronization modulation electric field

Blood vessel dilation starts from activation of the Na/K pumps and inward rectifier K channels in the vessel smooth muscle cells, which hyperpolarizes the cell membrane potential and closes the Ca channels. As a result, the intracellular Ca concentration reduces, and the smooth muscle cells relax and the blood vessel dilates. Activation of the Na/K pumps and the membrane potential hyperpolarization plays a critical role in blood vessel functions. Previously, we developed a new technique, synchronization modulation, to control the pump functions by electrically entraining the pump molecules. We have applied the synchronization modulation electric field noninvasively to various intact cells and demonstrated the field-induced membrane potential hyperpolarization. We further applied the electric field to blood vessels and investigated the field induced functional changes of the vessels. In this paper, we report the results in a study of the membrane potential change in the smooth muscle cells of mesenteric blood vessels in response to the oscillating electric field. We found that the synchronization modulation electric field can effectively hyperpolarize the muscle membrane potential quickly in seconds under physiological conditions.

Polymorphisms of the ATP1A1 gene associated with mastitis in dairy cattle

Mastitis affects the concentrations of potassium and sodium in milk. Since sodium-potassium adenosine triphosphatase (Na(+), K(+)-ATPase) is critical for maintaining the homeostasis of these two ions, and is involved in cell apoptosis and pathogenesis, we presumed that polymorphism of the ATP1A1 gene, which encodes the bovine Na(+), K(+)-ATPase α1 subunit could be associated with mastitis. The ATP1A1 gene was analyzed in 320 Holstein cows using PCR low ionic strength single-strand conformation polymorphism (PCR-LIS-SSCP) and DNA sequencing methods. A C/A SNP was identified at nucleotide position -15,739 in exon 17 of the ATP1A1 gene, but it did not induce any change in amino acids. We examined a possible association of polymorphism of the ATP1A1 gene with somatic cell score and 305-day milk yields. Individuals with genotype CC in ATP1A1 had significantly lower somatic cell scores and 305-day milk yields than those with genotype CA. We also examined changes in Na(+), K(+)-ATPase activity of red cell membranes. The Na(+), K(+)-ATPase activity was significantly higher in dairy cows with genotype CC compared to the other two genotypes, and the Na(+), K(+)-ATPase activity of the resistant group was significantly higher than that of the susceptible group in dairy cows. We conclude that this polymorphism has potential as a marker for mastitis resistance in dairy cattle.

Activity-dependent conduction failure: molecular insights

Weakness and fatigue are commonly encountered symptoms in neurological disorders and significantly impair quality of life. In the case of motor axons, conduction block contributes to weakness and fatigue and may be associated with aberrant nerve activity including fasciculations and cramp. These symptoms result from dysfunction of the constituent channels and pumps of the axonal membrane. In critically conducting axons, impulse conduction can be impaired by the effects of activity or by other mechanisms that produce a significant shift in membrane potential. Conduction failure may be accentuated or relieved by maneuvers that manipulate the time course of the driving current, including the administration of agents that interfere with Na(+) channel function. In patients with inflammatory neuropathies, normal activity may be sufficient to precipitate conduction failure at sites of impaired function in multifocal motor neuropathy (MMN) and chronic inflammatory demyelinating polyneuropathy (CIDP). From a clinical perspective, these features are not assessed adequately by conventional neurophysiological techniques. As weakness and fatigue may only develop following activity or exertion, it is useful to assess the effects of impulse trains to determine the extent of conduction failure and the resulting symptoms in neurological patients. These techniques and the physiological mechanisms underlying the development of activity-dependent hyperpolarization will be critically appraised in this review, with a focus on demyelinating neuropathies, MMN and the neurodegenerative disease, and amyotrophic lateral sclerosis (ALS).

Hyperpolarization of the membrane potential in cardiomyocyte tissue slices by the synchronization modulation electric field

Our previous studies have shown that a specially designed, so-called synchronization modulation electric field can entrain active transporter Na/K pumps in the cell membrane. This approach was previously developed in a study of single cells using a voltage clamp to monitor the pump currents. We are now expanding our study from isolated single cells to aggregated cells in a 3-dimensional cell matrix, through the use of a tissue slice from the rat heart. The slice is about 150 μm in thickness, meaning the slices contain many cell layers, resulting in a simplified 3-dimensional system. A fluorescent probe was used to identify the membrane potential and the ionic concentration gradients across the cell membrane. In spite of intrinsic cell-to-cell interactions and the difficulty in stimulating cell aggregation in the tissue slice, the oscillating electric field increased the intracellular fluorescent intensity, indicating elevation of the cell ionic concentration and hyperpolarization of the cell membrane. Blockage of these changes by ouabain confirmed that the results are directly related to Na/K pumps. These results along with the backward modulation indicate that the synchronization modulation electric field can influence the Na/K pumps in tissue cells of a 3-dimensional matrix and therefore hyperpolarize the cell membrane.

The dynamic relationships between the three events that release individual Na⁺ ions from the Na⁺/K⁺-ATPase

Na(+)/K(+) pumps move net charge through the cell membrane by mediating unequal exchange of intracellular Na(+) and extracellular K(+). Most charge moves during transitions that release Na(+) to the cell exterior. When pumps are constrained to bind and release only Na(+), a membrane voltage-step redistributes pumps among conformations with zero, one, two or three bound Na(+), thereby transiently generating current. By applying rapid voltage steps to squid giant axons, we previously identified three components in such transient currents, with distinct relaxation speeds: fast (which nearly parallels the voltage-jump time course), medium speed (τ(m)=0.2-0.5 ms) and slow (τ(s)=1-10 ms). Here we show that these three components are tightly correlated, both in their magnitudes and in the time courses of their changes. The correlations reveal the dynamics of the conformational rearrangements that release three Na(+) to the exterior (or sequester them into their binding sites) one at a time, in an obligatorily sequential manner.

Energy landscape of the reactions governing the Na+ deeply occluded state of the Na+/K+-ATPase in the giant axon of the Humboldt squid

The Na(+)/K(+) pump is a nearly ubiquitous membrane protein in animal cells that uses the free energy of ATP hydrolysis to alternatively export 3Na(+) from the cell and import 2K(+) per cycle. This exchange of ions produces a steady-state outwardly directed current, which is proportional in magnitude to the turnover rate. Under certain ionic conditions, a sudden voltage jump generates temporally distinct transient currents mediated by the Na(+)/K(+) pump that represent the kinetics of extracellular Na(+) binding/release and Na(+) occlusion/deocclusion transitions. For many years, these events have escaped a proper thermodynamic treatment due to the relatively small electrical signal. Here, taking the advantages offered by the large diameter of the axons from the squid Dosidicus gigas, we have been able to separate the kinetic components of the transient currents in an extended temperature range and thus characterize the energetic landscape of the pump cycle and those transitions associated with the extracellular release of the first Na(+) from the deeply occluded state. Occlusion/deocclusion transition involves large changes in enthalpy and entropy as the ion is exposed to the external milieu for release. Binding/unbinding is substantially less costly, yet larger than predicted for the energetic cost of an ion diffusing through a permeation pathway, which suggests that ion binding/unbinding must involve amino acid side-chain rearrangements at the site.

Effects of oligomycin on transient currents carried by Na+ translocation of Bufo Na+/K(+)-ATPase expressed in Xenopus oocytes

Na(+)/K(+)-ATPase (NKA) exports 3Na(+) and imports 2K(+) at the expense of the hydrolysis of 1ATP under physiological conditions. In the absence of K(+), it can mediate electroneutral Na(+)/Na(+) exchange. In the electroneutral Na(+)/Na(+) exchange mode, NKA produces a transient current containing fast, medium and slow components in response to a sudden voltage step. These three components of the transient current demonstrate the sequential release of Na(+) ions from three binding sites. Our data from oocytes provide further experimental support for the existence of these components. Oligomycin is an NKA inhibitor that favors the 2Na(+)-occluded state without affecting the conformational state of the NKA. We studied the effects of oligomycin on both K(+)-activated currents and transient currents in wild-type Bufo NKA and a mutant form of Bufo NKA, NKA: G813A. Oligomycin blocked almost all of the K(+)-activated current, although the three components of the transient current showed different sensitivities to oligomycin. The oligomycin-inhibited charge movement measured using a P/4 protocol had a rate coefficient similar to the medium transient component. The fast component of the transient current elicited by a short voltage step also showed sensitivity to oligomycin. However, the slow component was not totally inhibited by oligomycin. Our results indicate that the second and third sodium ions might be released to the extracellular medium by a mechanism that is not shared by the first sodium ion.

Tubacin suppresses proliferation and induces apoptosis of acute lymphoblastic leukemia cells

Over the past decade, histone deacetylase inhibitors have increasingly been used to treat various malignancies. Tubacin (tubulin acetylation inducer) is a small molecule that inhibits histone deacetylase 6 (HDAC6) and induces acetylation of α-tubulin. We observed a higher antiproliferative effect of tubacin in acute lymphoblastic leukemia (ALL) cells than in normal hematopoietic cells. Treatment with tubacin led to the induction of apoptotic pathways in both pre-B and T cell ALL cells at a 50% inhibitory concentration (IC(50)) of low micromolar concentrations. Acetylation of α-tubulin increases within the first 30 min following treatment of ALL cells with tubacin. We also observed an accumulation of polyubiquitinated proteins and poly(ADP-ribose) polymerase (PARP) cleavage. Furthermore, the signaling pathways activated by tubacin appear to be distinct from those observed in multiple myeloma. In this article, we demonstrate that tubacin enhances the effects of chemotherapy to treat primary ALL cells in vitro and in vivo. These results suggest that targeting HDAC6 alone or in combination with chemotherapy could provide a novel approach to treat ALL.

NKCC1 controls GABAergic signaling and neuroblast migration in the postnatal forebrain

From an early postnatal period and throughout life there is a continuous production of olfactory bulb (OB) interneurons originating from neuronal precursors in the subventricular zone. To reach the OB circuits, immature neuroblasts migrate along the rostral migratory stream (RMS). In the present study, we employed cultured postnatal mouse forebrain slices and used lentiviral vectors to label neuronal precursors with GFP and to manipulate the expression levels of the Na-K-2Cl cotransporter NKCC1. We investigated the role of this Cl- transporter in different stages of postnatal neurogenesis, including neuroblast migration and integration in the OB networks once they have reached the granule cell layer (GCL). We report that NKCC1 activity is necessary for maintaining normal migratory speed. Both pharmacological and genetic manipulations revealed that NKCC1 maintains high [Cl-]i and regulates the resting membrane potential of migratory neuroblasts whilst its functional expression is strongly reduced at the time cells reach the GCL. As in other developing systems, NKCC1 shapes GABAA-dependent signaling in the RMS neuroblasts. Also, we show that NKCC1 controls the migration of neuroblasts in the RMS. The present study indeed indicates that the latter effect results from a novel action of NKCC1 on the resting membrane potential, which is independent of GABAA-dependent signaling. All in all, our findings show that early stages of the postnatal recruitment of OB interneurons rely on precise, orchestrated mechanisms that depend on multiple actions of NKCC1.

Neurotoxicological effects of low-dose methylmercury and mercuric chloride in developing offspring mice

Mercury is a well-known toxic metal and potently induces severe neurotoxicological effects, especially in infants and children. The purpose of this study was to explore the underlying mechanisms of neurotoxic effects of mercurial compounds on the different stages of developing mice. Low-doses (the probability of human exposure in mercury-contaminated areas) of methylmercury (MeHg) (M, 0.02mg/kg/day) and mercury chloride (HgCl(2)) (H, 0.5mg/kg/day) were administered to mice of the following groups: (1) treatment with distilled water for 7 consecutive weeks after weaning (control-vehicle (CV)); exposure to mercurial compounds at different stages; (2) for 7 consecutive weeks after weaning (control-MeHg (CM) and control-HgCl(2) (CH)); (3) only during perinatal and weaning stages (MeHg-vehicle (MV) and HgCl-vehicle (HV)); and (4) in all experimental stages (MeHg-MeHg (MM) and HgCl(2)-HgCl(2) (HH)). Results revealed the neurobehavioral defects (increased locomotor activities, motor equilibrium impairment, and auditory dysfunction) that correlated with increasing Hg accumulation in CM and CH groups. However, it revealed a decrease and an increase in locomotor activities in MV and HV groups, respectively; these became more severe in MM and HH groups than in MV and HV groups. Motor equilibrium performance in MV and HV groups remained normal, while that in MM and HH groups was decreased. The most severe auditory defects (altered auditory brainstem response, ABR test) found in MM and HH groups than those in the respective CM and CH, MV and HV, including absolute wave III delays and interwave I-III latencies, which suggested that the irreversible auditory dysfunction caused by mercurial compounds. Furthermore, the alteration of lipid peroxidation (LPO), Na(+)/K(+)-ATPase activities, and nitric oxide (NO(x)) in the brain tissues contributed to the observed neurobehavioral dysfunction and hearing impairment. These findings provide evidence that fetuses were much more susceptible to the effects of mercurial compounds with regard to inducing severely neurotoxicological injuries as that found in human beings. The signaling of ROS/Na(+)-K(+)-ATPase/NO(x) plays a crucial role in the underlying mechanism for mercurial compound-induced toxic effects in offspring.

Regulation of Na+/K+ ATPase transport velocity by RNA editing

Editing of Na⁺/K⁺ ATPase mRNAs modulates the Na⁺/K⁺ pump's turnover rate by selectively targeting the release of the final sodium to the outside.

Because firing properties and metabolic rates vary widely, neurons require different transport rates from their Na⁺/K⁺ pumps in order to maintain ion homeostasis. In this study we show that Na⁺/K⁺ pump activity is tightly regulated by a novel process, RNA editing. Three codons within the squid Na⁺/K⁺ ATPase gene can be recoded at the RNA level, and the efficiency of conversion for each varies dramatically, and independently, between tissues. At one site, a highly conserved isoleucine in the seventh transmembrane span can be converted to a valine, a change that shifts the pump's intrinsic voltage dependence. Mechanistically, the removal of a single methyl group specifically targets the process of Na⁺ release to the extracellular solution, causing a higher turnover rate at the resting membrane potential.

In order for excitable cells like neurons and muscles to generate electrical signals, they require ion gradients across their plasma membranes. For example, sodium concentrations are much lower inside a cell than outside, and for potassium it is the opposite case. The job of maintaining these ion gradients falls squarely on a single protein: the Na⁺/K⁺ pump. During each transport cycle, this enzyme moves three sodium ions out of the cell and imports two of potassium. Because this process is the foundation for so many physiological processes, the Na⁺/K⁺ pump is costly to operate, using ∼30% of the ATP generated by an organism. Proper regulation of its turnover rate is vital. In this work, we use the giant nerve cell of squid as a model to show that the Na⁺/K⁺ pump can be regulated by an unsuspected mechanism. Although the gene that codes for this enzyme can make a perfectly functional pump, sometimes its information changes as it passes through the messenger RNA. This is achieved by editing RNA and as a result subtly different versions of the pump can be made, differing at only three amino acids out of more than a thousand. We demonstrate that RNA editing modulates the Na⁺/K⁺ pump's turnover rate and sodium release.

Utilizing natural activity to dissect the pathophysiology of acute oxaliplatin-induced neuropathy

Oxaliplatin is first-line chemotherapy for colorectal cancer, but produces dose-limiting neurotoxicity. Acute neurotoxicity following infusion produces symptoms including cold-triggered fasciculations and cramps, with subsequent chronic neuropathy developing at higher cumulative doses. Axonal excitability studies were undertaken in 15 oxaliplatin-treated patients before and immediately after oxaliplatin infusion to determine whether the mechanisms underlying acute neurotoxicity altered resting membrane potential or Na(+)/K(+) pump function. Excitability properties were assessed before and after maximal voluntary contraction (MVC) of the abductor pollicis brevis. Following oxaliplatin infusion, abnormalities developed in the recovery cycle with refractoriness markedly increased. Following activity, changes developed consistent with axonal hyperpolarization, with proportional changes pre- and post-oxaliplatin in normalized threshold. However, recovery cycle parameters following activity were significantly and disproportionally enhanced post-oxaliplatin, with partial normalization of the recovery cycle curve post-activity. Patients with the most abnormal change in the recovery cycle after infusion demonstrated the greatest changes post-contraction. Prominent abnormalities developed in Na(+) channel-associated parameters in response to natural activity, without significant alteration in axonal membrane potential or Na(+)/K(+) pump function. Findings from the present series suggest that oxaliplatin affects nerve excitability through voltage-dependent mechanisms, with specific effects mediated through axonal Na(+) channel inactivation.

The effect of holding potential on charge translocation by the Na+/K +-ATPase in the absence of potassium

The Na(+)/K(+)-ATPase exports 3Na(+) and imports 2K(+) at the expense of the hydrolysis of 1 ATP. In the absence of K(+), it carries on electroneutral, Na(+)-dependent transient charge movement (also known as "electroneutral Na(+)/Na(+) exchange mode") and produces a transient current containing faster and slower components in response to a sudden voltage step. Components with different speeds represent sequential release of Na(+) ions from three binding sites. The effect of holding potential on slow charge movement was studied in the presence of different concentrations of ADP(i), Na (i) (+) and Na (o) (+) with the intention of improving our understanding of Na (i) (+) binding. However, the manipulation of [ADP](i) and [Na(+)](i) did not cause as pronounced changes as predicted in the magnitude of charge movement (Q (tot)), which indicated that our experimental conditions were not able to backwardly drive reaction across the energy barrier to Na (i) (+) release/rebinding steps. On the contrary, lowering [Na(+)](o) caused evident dependence of Q (tot) on holding potential, with characteristics suggesting that pumps were escaping from E2P through the uncoupled Na(+) efflux activity.

The two C-terminal tyrosines stabilize occluded Na/K pump conformations containing Na or K ions

Interactions of the three transported Na ions with the Na/K pump remain incompletely understood. Na/K pump crystal structures show that the extended C terminus of the Na,K-adenosine triphosphatase (ATPase) alpha subunit directly contacts transmembrane helices. Deletion of the last five residues (KETYY in almost all Na/K pumps) markedly lowered the apparent affinity for Na activation of pump phosphorylation from ATP, a reflection of cytoplasmic Na affinity for forming the occluded E1P(Na3) conformation. ATPase assays further suggested that C-terminal truncations also interfere with low affinity Na interactions, which are attributable to extracellular effects. Because extracellular Na ions traverse part of the membrane's electric field to reach their binding sites in the Na/K pump, their movements generate currents that can be monitored with high resolution. We report here electrical measurements to examine how Na/K pump interactions with extracellular Na ions are influenced by C-terminal truncations. We deleted the last two (YY) or five (KESYY) residues in Xenopus laevis alpha1 Na/K pumps made ouabain resistant by either of two kinds of point mutations and measured their currents as 10-mM ouabain-sensitive currents in Xenopus oocytes after silencing endogenous Xenopus Na/K pumps with 1 microM ouabain. We found the low affinity inhibitory influence of extracellular Na on outward Na/K pump current at negative voltages to be impaired in all of the C-terminally truncated pumps. Correspondingly, voltage jump-induced transient charge movements that reflect pump interactions with extracellular Na ions were strongly shifted to more negative potentials; this signals a several-fold reduction of the apparent affinity for extracellular Na in the truncated pumps. Parallel lowering of Na affinity on both sides of the membrane argues that the C-terminal contacts provide important stabilization of the occluded E1P(Na3) conformation, regardless of the route of Na ion entry into the binding pocket. Gating measurements of palytoxin-opened Na/K pump channels additionally imply that the C-terminal contacts also help stabilize pump conformations with occluded K ions.

Fatigue in multiple sclerosis: mechanisms and management

Multiple sclerosis [MS] is a chronic immune-mediated disorder of the central nervous system [CNS]. Fatigue may be a debilitating symptom in MS patients, adversely impacting on their quality of life. Clinically, fatigue may manifest as exhaustion, lack of energy, increased somnolence, or worsening of MS symptoms. Activity and heat typically serve to exacerbate symptoms of fatigue. There is now strong evidence to suggest that fatigue results from reduced voluntary activation of muscles by means of central mechanisms. Given that axonal demyelination is a pathological hallmark of MS, activity-dependent conduction block [ADCB] has been proposed as a mechanism underlying fatigue in MS. This ADCB results from axonal membrane hyperpolarization, mediated by the Na(+)/K(+) electrogenic pump, with conduction failure precipitated in demyelinated axons with a reduced safety factor of impulse transmission. In addition, Na(+)/K(+) pump dysfunction, as reported in MS, may induce a depolarizing conduction block associated with inactivation of Na(+) channels. These processes may induce secondary effects including axonal degeneration triggered by raised levels of intracellular Ca(2+) through reverse operation of the Na(+)-Ca(2+) exchanger. Restoration of normal conduction in demyelinated axons with selective channel blockers improves fatigue and may yet prove useful as a neuroprotective strategy, in preventing secondary axonal degeneration and consequent functional impairment.

Chromium III exposure inhibits brain Na+K+ATPase activity of Clarias batrachus L. involving lipid peroxidation and deficient mitochondrial electron transport chain activity

The present study elucidated the role of lipid peroxidation and diminished mitochondrial electron transport chain activity in partial dysfunction of brain Na+K+ATPase of Clarias batrachus exposed to chromium III ions. The fish were exposed to 10% and 20% of the derived 96 h LC50 value, 5.69 mg/L and 11.38 mg/L, respectively, and sampled on 20, 40 and 60 days. Exposure to chromium III on fish brain demonstrated an increased lipid peroxidation, production of protein carbonyl and reactive oxygen species and loss of protein thiol groups in synaptosomal fraction with decreased activity of Na+K+ATPase, partial inactivation of mitochondrial electron transport chain activity and energy depletion.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Distribution of the NA/K pumps' turnover rates as a function of membrane potential, temperature, and ion concentration gradients and effect of fluctuations

Because of structural independence of the Na/K pump molecules, the pumping rates of individual pumps may not be the same, instead showing some sort of distribution. Detailed information about the distribution has not previously been reported. The pumping rate of Na/K pumps depends on many parameters, such as membrane potential, temperature, and ion concentration gradients across the cell membrane. Fluctuation of any of the variables will change the pumping rate, resulting in a distribution. On the basis of a simplified six-state model, a steady-state pumping flux and therefore the pumping rate were obtained. Parameters were determined based on previous experimental results on amphibian skeletal muscle and theoretical work. Gaussian fluctuations of all the variables were considered to determine the changes in the pumping rate. These variable fluctuations may be totally independent or correlated to each other. The results showed that the pumping rates of the Na/K pumps are distributed in an asymmetric profile, which has a higher probability at the lower pumping rate. We present a model distribution of pumping rates as a function of temperature, membrane potential, and ion concentration.