Effect of high hydrostatic pressure on the bacterial mechanosensitive channel MscS

Control of pathogenic and spoilage bacteria in meat and meat products by high pressure: Challenges and future perspectives

High-pressure processing is among the most widely used nonthermal intervention to reduce pathogenic and spoilage bacteria in meat and meat products. However, resistance of pathogenic bacteria strains in meats at the current maximum commercial equipment of 600 MPa questions the ability of inactivation by its application in meats. Pathogens including Escherichia coli, Listeria, and Salmonelle, and spoilage microbiota including lactic acid bacteria dominate in raw meat, ready-to-eat, and packaged meat products. Improved understanding on the mechanisms of the pressure resistance is needed for optimizing the conditions of pressure treatment to effectively decontaminate harmful bacteria. Effective control of the pressure-resistant pathogens and spoilage organisms in meats can be realized by the combination of high pressure with application of mild temperature and/or other hurdles including antimicrobial agents and/or competitive microbiota. This review summarized applications, mechanisms, and challenges of high pressure on meats from the perspective of microbiology, which are important for improving the understanding and optimizing the conditions of pressure treatment in the future.

Genomic Characteristics and Potential Metabolic Adaptations of Hadal Trench Roseobacter and Alteromonas Bacteria Based on Single-Cell Genomics Analyses

Heterotrophic bacteria such as those from the Roseobacter group and genus Alteromonas dominate the hadal zones of oceans; however, we know little about the genomic characteristics and potential metabolic adaptations of hadal trench-dwelling bacteria. Here, we report multiple single amplified genomes (SAGs) belonging to Roseobacter and Alteromonas, recovered from the hadal zone of the Mariana Trench. While phylogenetic analyses show that these hadal SAGs cluster with their surface relatives, an analysis of genomic recruitment indicates that they have higher relative abundances in the hadal zone of the Mariana Trench. Comparative genomic analyses between the hadal SAGs and reference genomes of closely related shallow-water relatives indicate that genes involved in the mobilome (prophages and transposons) are overrepresented among the unique genes of the hadal Roseobacter and Alteromonas SAGs; the functional proteins encoded by this category of genes also shows higher amino acid sequence variation than those encoded by other gene sets within the Roseobacter SAGs. We also found that genes involved in cell wall/membrane/envelope biogenesis, transcriptional regulation, and metal transport may be important for the adaptation of hadal Roseobacter and Alteromonas lineages. These results imply that the modification of cell surface-related proteins and transporters is the major direction of genomic evolution in Roseobacter and Alteromonas bacteria adapting to the hadal environment, and that prophages and transposons may be the key factors driving this process.

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.

Hydrostatic pressure enhances mitomycin C induced apoptosis in urothelial carcinoma cells

OBJECTIVES

Urothelial carcinoma (UC) of the bladder is the second most common cancer of the genitourinary system. Clinical UC treatment usually involves transurethral resection of the bladder tumor followed by adjuvant intravesical immunotherapy or chemotherapy to prevent recurrence. Intravesical chemotherapy induces fewer side effects than immunotherapy but is less effective at preventing tumor recurrence. Improvement to intravesical chemotherapy is, therefore, needed.

METHODS AND MATERIALS

Cellular effects of mitomycin C (MMC) and hydrostatic pressure on UC BFTC905 cells were assessed. The viability of the UC cells was determined using cellular proliferation assay. Changes in apoptotic function were evaluated by caspase 3/7 activities, expression of FasL, and loss of mitochondrial membrane potential.

RESULTS

Reduced cell viability was associated with increasing hydrostatic pressure. Caspase 3/7 activities were increased following treatment of the UC cells with MMC or hydrostatic pressure. In combination with 10 kPa hydrostatic pressure, MMC treatment induced increasing FasL expression. The mitochondria of UC cells displayed increasingly impaired membrane potentials following a combined treatment with 10 μg/ml MMC and 10 kPa hydrostatic pressure.

CONCLUSIONS

Both MMC and hydrostatic pressure can induce apoptosis in UC cells through an extrinsic pathway. Hydrostatic pressure specifically increases MMC-induced apoptosis and might minimize the side effects of the chemotherapy by reducing the concentration of the chemical agent. This study provides a new and alternative approach for treatment of patients with UC following transurethral resection of the bladder tumor.

The protective effect of osmoprotectant TMAO on bacterial mechanosensitive channels of small conductance MscS/MscK under high hydrostatic pressure

Activity of the bacterial mechanosensitive channels of small conductance MscS/MscK of E. coli was investigated under high hydrostatic pressure (HHP) using the "flying-patch" patch-clamp technique. The channels were gated by negative pipette voltage and their open probability was measured at HHP of 0.1 to 80 MPa. The channel open probability decreased with increasing HHP. When the osmolyte methylamine N-oxide (TMAO) was applied to the cytoplasmic side of the inside-out excised membrane patches of E. coli giant spheroplasts the inhibitory effect of HHP on the channel activity was suppressed at pressures of up to 40 MPa. At 40 MPa and above the channel open probability decreased in a similar fashion with or without TMAO. Our study suggests that TMAO helps to counteract the effect of HHP up to 40 MPa on the MscS/MscK open state by "shielding" the cytoplasmic domain of the channels.

Differential effects of lipids and lyso-lipids on the mechanosensitivity of the mechanosensitive channels MscL and MscS

Mechanosensitive (MS) channels of small (MscS) and large (MscL) conductance are the major players in the protection of bacterial cells against hypoosmotic shock. Although a great deal is known about structure and function of these channels, much less is known about how membrane lipids may influence their mechanosensitivity and function. In this study, we use liposome coreconstitution to examine the effects of different types of lipids on MscS and MscL mechanosensitivity simultaneously using the patch-clamp technique and confocal microscopy. Fluorescence lifetime imaging (FLIM)-FRET microscopy demonstrated that coreconstitution of MscS and MscL led to clustering of these channels causing a significant increase in the MscS activation threshold. Furthermore, the MscL/MscS threshold ratio dramatically decreased in thinner compared with thicker bilayers and upon addition of cholesterol, known to affect the bilayer thickness, stiffness and pressure profile. In contrast, application of micromolar concentrations of lysophosphatidylcholine (LPC) led to an increase of the MscL/MscS threshold ratio. These data suggest that differences in hydrophobic mismatch and bilayer stiffness, change in transbilayer pressure profile, and close proximity of MscL and MscS affect the structural dynamics of both channels to a different extent. Our findings may have far-reaching implications for other types of ion channels and membrane proteins that, like MscL and MscS, may coexist in multiple molecular complexes and, consequently, have their activation characteristics significantly affected by changes in the lipid environment and their proximity to each other.

Flying-patch patch-clamp study of G22E-MscL mutant under high hydrostatic pressure

High hydrostatic pressure (HHP) present in natural environments impacts on cell membrane biophysical properties and protein quaternary structure. We have investigated the effect of high hydrostatic pressure on G22E-MscL, a spontaneously opening mutant of Escherichia coli MscL, the bacterial mechanosensitive channel of large conductance. Patch-clamp technique combined with a flying-patch device and hydraulic setup allowed the study of the effects of HHP up to 90 MPa (as near the bottom of the Marianas Trench) on the MscL mutant channel reconstituted into liposome membranes, in addition to recording in situ from the mutant channels expressed in E. coli giant spheroplasts. In general, against thermodynamic predictions, hydrostatic pressure in the range of 0.1-90 MPa increased channel open probability by favoring the open state of the channel. Furthermore, hydrostatic pressure affected the channel kinetics, as manifested by the propensity of the channel to gate at subconducting levels with an increase in pressure. We propose that the presence of water molecules around the hydrophobic gate of the G22E MscL channel induce hydration of the hydrophobic lock under HHP causing frequent channel openings and preventing the channel closure in the absence of membrane tension. Furthermore, our study indicates that HHP can be used as a valuable experimental approach toward better understanding of the gating mechanism in complex channels such as MscL.

Bubbles, gating, and anesthetics in ion channels

We suggest that bubbles are the bistable hydrophobic gates responsible for the on-off transitions of single channel currents. In this view, many types of channels gate by the same physical mechanism-dewetting by capillary evaporation-but different types of channels use different sensors to modulate hydrophobic properties of the channel wall and thereby trigger and control bubbles and gating. Spontaneous emptying of channels has been seen in many simulations. Because of the physics involved, such phase transitions are inherently sensitive, unstable threshold phenomena that are difficult to simulate reproducibly and thus convincingly. We present a thermodynamic analysis of a bubble gate using morphometric density functional theory of classical (not quantum) mechanics. Thermodynamic analysis of phase transitions is generally more reproducible and less sensitive to details than simulations. Anesthetic actions of inert gases-and their interactions with hydrostatic pressure (e.g., nitrogen narcosis)-can be easily understood by actions on bubbles. A general theory of gas anesthesia may involve bubbles in channels. Only experiments can show whether, or when, or which channels actually use bubbles as hydrophobic gates: direct observation of bubbles in channels is needed. Existing experiments show thin gas layers on hydrophobic surfaces in water and suggest that bubbles nearly exist in bulk water.

Nav channel mechanosensitivity: activation and inactivation accelerate reversibly with stretch

Voltage-gated sodium channels (Nav) are modulated by many bilayer mechanical amphiphiles, but whether, like other voltage-gated channels (Kv, HCN, Cav), they respond to physical bilayer deformations is unknown. We expressed human heart Nav1.5 pore alpha-subunit in oocytes (where, unlike alphaNav1.4, alphaNav1.5 exhibits normal kinetics) and measured small macroscopic currents in cell-attached patches. Pipette pressure was used to reversibly stretch the membrane for comparison of I(Na)(t) before, during, and after stretch. At all voltages, and in a dose-dependent fashion, stretch accelerated the I(Na)(t) time course. The sign of membrane curvature was not relevant. Typical stretch stimuli reversibly accelerated both activation and inactivation by approximately 1.4-fold; normalization of peak I(Na)(t) followed by temporal scaling ( approximately 1.30- to 1.85-fold) resulted in full overlap of the stretch/no-stretch traces. Evidently the rate-limiting outward voltage sensor motion in the Nav1.5 activation path (as in Kv1) accelerated with stretch. Stretch-accelerated inactivation occurred even with activation saturated, so an independently stretch-modulated inactivation transition is also a possibility. Since Nav1.5 channel-stretch modulation was both reliable and reversible, and required stretch stimuli no more intense than what typically activates putative mechanotransducer channels (e.g., stretch-activated TRPC1-based currents), Nav channels join the ranks of putative mechanotransducers. It is noteworthy that at voltages near the activation threshold, moderate stretch increased the peak I(Na) amplitude approximately 1.5-fold. It will be important to determine whether stretch-modulated Nav current contributes to cardiac arrhythmias, to mechanosensory responses in interstitial cells of Cajal, to touch receptor responses, and to neuropathic (i.e., hypermechanosensitive) and/or normal pain reception.

Modulation of channel activity and gadolinium block of MscL by static magnetic fields

The magnetic field of the Earth has for long been known to influence the behaviour and orientation of a variety of living organisms. Experimental studies of the magnetic sense have, however, been impaired by the lack of a plausible cellular and/or molecular mechanism providing meaningful explanation for detection of magnetic fields by these organisms. Recently, mechanosensitive (MS) ion channels have been implied to play a role in magnetoreception. In this study we have investigated the effect of static magnetic fields (SMFs) of moderate intensity on the activity and gadolinium block of MscL, the bacterial MS channel of large conductance, which has served as a model channel to study the basic physical principles of mechanosensory transduction in living cells. In addition to showing that direct application of the magnetic field decreased the activity of the MscL channel, our study demonstrates for the first time that SMFs can reverse the effect of gadolinium, a well-known blocker of MS channels. The results of our study are consistent with a notion that (1) the effects of SMFs on the MscL channels may result from changes in physical properties of the lipid bilayer due to diamagnetic anisotropy of phospholipid molecules and consequently (2) cooperative superdiamagnetism of phospholipid molecules under influence of SMFs could cause displacement of Gd(3+) ions from the membrane bilayer and thus remove the MscL channel block.