Fluorescence spectroscopic studies of pressure effects on Na+,K(+)-ATPase reconstituted into phospholipid bilayers and model raft mixtures

Inactivation of Bacillus subtilis spores by a combination of high-pressure thermal treatment and potassium sorbate

Combining High-pressure Thermal Treatment (HPTT) and Potassium Sorbate (PS) may have a stronger spore inactivation effect. Spores of Bacillus subtilis were subjected to HPTT at 600 MPa-65 °C/75 °C and a combination of HPTT and PS of 0.1% and 0.2% concentrations. After these treatments, different procedures and techniques were employed to investigate the spore's inactivation. The results revealed that 4.92 ± 0.05 log spores were inactivated after treatment at 600 MPa-75 °C, while 5.97 ± 0.09 log spores were inactivated when the HPTT treatment was combined with 0.2% PS. Changes in permeability of the spore's inner membrane were characterized by OD600 value and release rates of nucleic acids, protein, and dipicolinic acid (DPA). Compared with HPTT treatment at 600 MPa-75 °C, the OD600 value of spores decreased further by about 50% after treatment with a combination of HPTT and 0.2% PS. Additionally, the combined treatments resulted in a significant increase in the OD260 and OD280 values, as well as the DPA release. The spore size analysis indicated a significant decrease in the size of spores treated with a combination of HPTT at 600 MPa-75 °C and PS of 0.2% concentration. Furthermore, the flow cytometry analysis and confocal laser scanning microscopy (CLSM) analysis indicated that the inner membrane damage of spores was higher after combined treatments than that after HPTT treatment alone. A significant reduction was also found in the Na+/K+-ATPase activity after the combined treatments. Also, the FTIR analysis revealed that the combined treatments resulted in significant adverse changes in the spores' inner membrane, cell wall, cortex, and nucleic acid. Therefore, the combination of HPTT and PS has a stronger inactivation effect and can be suggested as a promising strategy for the inactivation of Bacillus subtilis spores.

Autophagy-dependent Na+-K+-ATPase signalling and abnormal urate reabsorption in hyperuricaemia-induced renal tubular injury

Increasing evidence indicates that hyperuricaemia (HUA) is not only a result of decreased renal urate excretion but also a contributor to kidney disease. Na⁺-K⁺-ATPase (NKA), which establishes the sodium gradient for urate transport in proximal tubular epithelial cells (PTECs), its impairment leads to HUA-induced nephropathy. However, the specific mechanism underlying NKA impairment-mediated renal tubular injury and increased urate reabsorption in HUA is not well understood. In this study, we investigated whether autophagy plays a key role in the NKA impairment signalling and increased urate reabsorption in HUA-induced renal tubular injury. Protein spectrum analysis of exosomes from the urine of HUA patients revealed the activation of lysosomal processes, and exosomal expression of lysosome membrane protein 2 was associated with increased serum levels and decreased renal urate excretion in patients. We demonstrated that high uric acid (UA) induced lysosome dysfunction, autophagy and inflammation in a time- and dose-dependent manner and that high UA and/or NKA α1 siRNA significantly increased mitochondrial abnormalities, such as reductions in mitochondrial respiratory complexes and cellular ATP levels, accompanied by increased apoptosis in cultured PTECs. The autophagy inhibitor hydroxychloroquine (HCQ) ameliorated NKA impairment-mediated mitochondrial dysfunction, Nod-like receptor pyrin domain-containing protein 3 (NLRP3)-interleukin-1β (IL-1β) production, and abnormal urate reabsorption in PTECs stimulated with high UA and in rats with oxonic acid (OA)-induced HUA. Our findings suggest that autophagy plays a pivotal role in NKA impairment-mediated signalling and abnormal urate reabsorption in HUA-induced renal tubular injury and that inhibition of autophagy by HCQ could be a promising treatment for HUA.

AMPK alleviates high uric acid-induced Na+-K+-ATPase signaling impairment and cell injury in renal tubules

One of the mechanisms in hyperuricemia (HUA)-induced renal tubular injury is the impairment of Na⁺-K⁺-ATPase (NKA) signaling, which further triggers inflammation, autophagy, and mitochondrial dysfunction and leads to cell injury. Here, we used RNA sequencing to screen the most likely regulators of NKA signaling and found that the liver kinase B1(LKB1)/adenosine monophosphate (AMP)-activated protein kinase (AMPK)/ mammalian target of rapamycin (mTOR) pathway was the most abundantly enriched pathway in HUA. AMPK is a key regulator of cell energy metabolism; hence, we examined the effect of AMPK on HUA-induced dysregulation of NKA signaling and cell injury. We first detected AMPK activation in high uric acid (UA)-stimulated proximal tubular epithelial cells (PTECs). We further found that sustained treatment with the AMPK activator 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR), but not the AMPK inhibitor Compound C, significantly alleviated UA-induced reductions in NKA activity and NKA α1 subunit expression on the cell membrane by reducing NKA degradation in lysosomes; sustained AICAR treatment also significantly alleviated activation of the NKA downstream molecules Src and interleukin-1β (IL-1β) in PTECs. AICAR further alleviated high UA-induced apoptosis, autophagy, and mitochondrial dysfunction. Although AMPK activation by metformin did not reduce serum UA levels in hyperuricemic rats, it significantly alleviated HUA-induced renal tubular injury and NKA signaling impairment in vivo with effects similar to those of febuxostat. Our study suggests that AMPK activation may temporarily compensate for HUA-induced renal injury. Sustained AMPK activation could reduce lysosomal NKA degradation and maintain NKA function, thus alleviating NKA downstream inflammation and protecting tubular cells from high UA-induced renal tubular injury.

High serum levels of uric acid cause kidney tissue damage through cellular processes that have now been identified by researchers in China. Uric acid is a common component of urine, but causes damage if it is present in high levels in the blood (hyperuricemia). While investigating the mechanisms behind hyperuricemia, Zhibin Ye and co-workers at Fudan University in Shanghai recently showed that impairment of the Na⁺-K⁺-ATPase (NKA) signaling pathway, which regulates uric acid transportation through the kidneys, is a crucial feature of renal damage progression. The team have now shown that NKA is regulated by the AMP-activated protein kinase (AMPK) pathway, and that AMPK is enriched during the initial phases of hyperuricemia. Studies on rat models indicated that sustained AMPK activation restored NKA signaling, limiting damage from hyperuricemia.

Interrogating the Structural Dynamics and Energetics of Biomolecular Systems with Pressure Modulation

High hydrostatic pressure affects the structure, dynamics, and stability of biomolecular systems and is a key parameter in the context of the exploration of the origin and the physical limits of life. This review lays out the conceptual framework for exploring the conformational fluctuations, dynamical properties, and activity of biomolecular systems using pressure perturbation. Complementary pressure-jump relaxation studies are useful tools to study the kinetics and mechanisms of biomolecular phase transitions and structural transformations, such as membrane fusion or protein and nucleic acid folding. Finally, the advantages of using pressure to explore biomolecular assemblies and modulate enzymatic reactions are discussed.

Impaired Na+-K+-ATPase signaling in renal proximal tubule contributes to hyperuricemia-induced renal tubular injury

Hyperuricemia contributes to renal inflammation. We aimed to investigate the role of Na⁺–K⁺–ATPase (NKA) in hyperuricemia-induced renal tubular injury. Human primary proximal tubular epithelial cells (PTECs) were incubated with uric acid (UA) at increasing doses or for increasing lengths of time. PTECs were then stimulated by pre-incubation with an NKA α1 expression vector or small interfering RNA before UA (100 μg ml⁻¹, 48 h) stimulation. Hyperuricemic rats were induced by gastric oxonic acid and treated with febuxostat (Feb). ATP levels, the activity of NKA and expression of its α1 subunit, Src, NOD-like receptor pyrin domain-containing protein 3 (NLRP3) and interleukin 1β (IL-1β) were measured both in vitro and in vivo. Beginning at concentrations of 100 μg ml⁻¹, UA started to dose-dependently reduce NKA activity. UA at a concentration of 100 μg ml⁻¹ time-dependently affected the NKA activity, with the maximal increased NKA activity at 24 h, but the activity started to decrease after 48 h. This inhibitory effect of UA on NKA activity at 48 h was in addition to a decrease in NKA α1 expression in the cell membrane, but an increase in lysosomes. This process also involved the subsequent activation of Src kinase and NLRP3, promoting IL-1β processing. In hyperuricemic rats, renal cortex NKA activity and its α1 expression were upregulated at the 7th week and both decreased at the 10th week, accompanied with increased renal cortex expression of Src, NLRP3 and IL-1β. The UA levels were reduced and renal tubular injuries in hyperuricemic rats were alleviated in the Feb group. Our data suggested that the impairment of NKA and its consequent regulation of Src, NLRP3 and IL-1β in the renal proximal tubule contributed to hyperuricemia-induced renal tubular injury.

Pressure Effects on Artificial and Cellular Membranes

We review the combined effect of temperature and pressure on the structure, dynamics and phase behavior of lipid bilayers, differing in chain length, headgroup structure and composition as revealed by thermodynamic, spectroscopic and scattering experiments. The effect of additives, such as ions, cholesterol, and anaesthetics is discussed as well. Our data include also reports on the effect of pressure on the lateral organization of heterogeneous lipid membranes and lipid extracts from cellular membranes, as well as the influence of peptide and protein incorporation on the pressure-dependent structure and phase behavior of lipid membranes. Moreover, the effects of pressure on membrane protein function are summarized. Finally, we introduce pressure as a kinetic variable for studying the kinetics of various lipid phase transformations.

Exploring the structure and phase behavior of plasma membrane vesicles under extreme environmental conditions

A protocol was developed to generate GPMVs showing phase separation under ambient conditions and theirp,T-dependent phase behavior was studied.

In situ determination of Clostridium endospore membrane fluidity during pressure-assisted thermal processing in combination with nisin or reutericyclin

This study determined the membrane fluidity of clostridial endospores during treatment with heat and pressure with nisin or reutericyclin. Heating (90°C) reduced laurdan (6-dodecanoyl-2-dimethylaminonaphthalene) general polarization, corresponding to membrane fluidization. Pressure (200 MPa) stabilized membrane order. Reutericyclin and nisin exhibit divergent effects on heat- and pressure-induced spore inactivation and membrane fluidity.

Introduction to high-pressure bioscience and biotechnology

The manipulation of biological materials using elevated pressure is providing an ever-growing number of opportunities in both the applied and basic sciences. Manipulation of pressure is a useful parameter for enhancing food quality and shelf life; inactivating microbes, viruses, prions, and deleterious enzymes; affecting recombinant protein production; controlling DNA hybridization; and improving vaccine preparation. In biophysics and biochemistry, pressure is used as a tool to study intermediates in protein folding, enzyme kinetics, macromolecular interactions, amyloid fibrous protein aggregation, lipid structural changes, and to discern the role of solvation and void volumes in these processes. Biologists, including many microbiologists, examine the utility and basis of pressure inactivation of cells and cellular processes, and conversely seek to discover how deep-sea life has evolved a preference for high-pressure environments. This introduction and the papers that follow provide information on the nature and promise of the highly interdisciplinary field of high-pressure bioscience and biotechnology (HPBB).

The regulation of proximal tubular salt transport in hypertension: an update

PURPOSE OF REVIEW

Renal proximal tubular sodium reabsorption is regulated by sodium transporters, including the sodium glucose transporter, sodium amino acid transporter, sodium hydrogen exchanger isoform 3 and sodium phosphate cotransporter type 2 located at the luminal/apical membrane, and sodium bicarbonate cotransporter and Na+/K+ATPase located at the basolateral membrane. This review summarizes recent studies on sodium transporters that play a major role in the increase in blood pressure in essential/polygenic hypertension.

RECENT FINDINGS

Sodium transporters and Na+/K+ATPase are segregated in membrane lipid and nonlipid raft microdomains that regulate their activities and trafficking via cytoskeletal proteins. The increase in renal proximal tubule ion transport in polygenic hypertension is primarily due to increased activity of NHE3 and Cl/HCO3 exchanger at the luminal/apical membrane and a primary or secondary increase in Na+/K+ATPase activity.

SUMMARY

The increase in renal proximal tubule ion transport in hypertension is due to increased actions by prohypertensive factors that are unopposed by antihypertensive factors.

A quantum dot-labeled ligand-receptor binding assay for G protein-coupled receptors contained in minimally purified membrane nanopatches

A robust method to directly measure ligand-receptor binding interactions using fluorescence cross-correlation spectroscopy (FCCS) is described. The example receptor systems demonstrated here are the human micro-opioid receptor, a representative G protein-coupled receptor (GPCR), and Streptavidin, but these general protocols can be extended for the analysis of many membrane receptors. We present methods for the preparation of GPCR-containing membrane nanopatches that appear to have the shapes of nanovesicles, labeling of proteins in membrane vesicles, in addition to the coupling of quantum dots (QDs) to peptide ligands. Further, we demonstrate that reliable binding information can be obtained from these partially purified receptors.

Effects of temperature and pressure on the lateral organization of model membranes with functionally reconstituted multidrug transporter LmrA

To contribute to the understanding of membrane protein function upon application of pressure, we investigated the influence of hydrostatic pressure on the conformational order and phase behavior of the multidrug transporter LmrA in biomembrane systems. To this end, the membrane protein was reconstituted into various lipid bilayer systems of different chain length, conformation, phase state and heterogeneity, including raft model mixtures as well as some natural lipid extracts. In the first step, we determined the temperature stability of the protein itself and verified its reconstitution into the lipid bilayer systems using CD spectroscopic and AFM measurements, respectively. Then, to yield information on the temperature and pressure dependent conformation and phase state of the lipid bilayer systems, generalized polarization values by the Laurdan fluorescence technique were determined, which report on the conformation and phase state of the lipid bilayer system. The temperature-dependent measurements were carried out in the temperature range 5-70 degrees C, and the pressure dependent measurements were performed in the range 1-200 MPa. The data show that the effect of the LmrA reconstitution on the conformation and phase state of the lipid matrix depends on the fluidity and hydrophobic matching conditions of the lipid system. The effect is most pronounced for fluid DMPC and DMPC with low cholesterol levels, but minor for longer-chain fluid phospholipids such as DOPC and model raft mixtures such as DOPC/DPPC/cholesterol. The latter have the additional advantage of using lipid sorting to avoid substantial hydrophobic mismatch. Notably, the most drastic effect was observed for the neutral/glycolipid natural lipid mixture. In this case, the impact of LmrA incorporation on the increase of the conformational order of the lipid membrane was most pronounced. As a consequence, the membrane reaches a mechanical stability which makes it very insensitive to application of pressures as high as 200 MPa. The results are correlated with the functional properties of LmrA in these various lipid environments and upon application of high hydrostatic pressure and are discussed in the context of other work on pressure effects on membrane protein systems.

Resting membrane potentials recorded on-site in intact skeletal muscles from deep sea fish (Sigmops gracile) salvaged from depths up to 1.000 m

The effect of elevated ambient pressures in deep sea fish residing at certain bottom depths or even covering different depth levels during migration is poorly understood. Elevated pressures are known to influence membrane properties of various excitable tissues in many species. Reliable results on membrane properties require freshly isolated living cells and short decompression times. During a scientific cruise south of Japan, deep sea fish were sampled from depths up to 1.000 m by using the intelligent operative net sampling system IONESS. On-site electrophysiological recordings of resting membrane potentials were performed in freshly isolated skeletal muscles from Sigmops gracile. Experiments were conducted at various extracellular K+ concentrations to derive relative membrane ion permeabilities and estimate intracellular K+ concentrations [K+]i in the muscles studied. With increasing sampling depth, a tendency for depolarized resting membrane potentials was observed. This could be explained by an increase in relative Na+ over K+ resting membrane permeabilities. Fish samples from deeper sites also had larger [K+]i values compared with shallower sites. This study represents a first approach to perform sophisticated physiological live-cell experiments on board a fully operating ship. These data are expected to more realistically reflect the physiological state of biological preparations residing in the deep sea.