A convenient method for the relative and absolute quantification of lipid components in liposomes by 1H- and 31P NMR-spectroscopy.

OBJECTIVE: Liposomes are promising delivery systems for pharmaceutical applications and have been used in medicine in the recent past. Preparation of liposomes requires reliable characterization and quantification of the phospholipid components for which the traditional cumbersome molybdate method is used frequently. The objective was to improve relative and absolute quantification of lipid components from liposomes. METHODS: A reliable method for quantification of lipid composition in liposome formulations in the 1-10 µmol range with 1H- and 31P NMR spectroscopy at 600 MHz has been developed. The method is based on three crystalline small-molecule standards (Ph3PO4, (Tol)3PO4, and Ph3PO) in CDCl3. RESULTS: Excellent calibration linearity and chemical stability of the standards was observed. The method was tested in blind fashion on liposomes containing POPC, PEG-ceramide and a pH-sensitive trans-aminocyclohexanol-based amphiphile (TACH).1 Relative quantification (percentage of components) as well as determination of absolute lipid amount was possible with excellent reproducibility with an average error of 5%. Quantification (triplicate) was accomplished in 15 min based on 1H NMR and in 1 h based on 31P NMR. Very little change in mixture composition was observed over multiple preparative steps. CONCLUSION: Liposome preparations containing POPC, POPE, DOPC, DPPC, TACH, and PEG-ceramide can be reliably characterized and quantified by 1H NMR and 31P NMR spectroscopy at 600 MHz in the µmol range.

A pipeline for making 31P NMR accessible for small- and large-scale lipidomics studies.

Detailed molecular analysis is of increasing importance in research into the regulation of biochemical pathways, organismal growth and disease. Lipidomics in particular is increasingly sought after as it provides insight into molecular species involved in energy storage, signalling and fundamental cellular structures. This has led to the use of a range of tools and techniques to acquire lipidomics data. 31P NMR for lipidomics offers well-resolved head group/lipid class analysis, structural data that can be used to inform and strengthen interpretation of mass spectrometry data and part of a priori structural determination. In the present study, we codify the use of 31P NMR for lipidomics studies to make the technique more accessible to new users and more useful for a wider range of questions. The technique can be used in isolation (phospholipidomics) or as a part of determining lipid composition (lipidomics). We describe the process from sample extraction to data processing and analysis. This pipeline is important because it allows greater thoroughness in lipidomics studies and increases scope for answering scientific questions about lipid-containing systems.

Novel Mitochondria-Targeted Triphenylphosphonium Conjugates of Linear ß-Phosphorylated Nitrones : Preparation, 31P NMR Mitochondrial Distribution, EPR Spin Trapping Reporting, and Site-Directed Antiapoptotic Properties.

The mitochondrion can be considered as the metabolic powerhouse of the cell, having a key impact on energy production, cell respiration, and intrinsic cell death. Mitochondria are also the main source of endogenous reactive oxygen species , including free radicals (FR), which are physiologically involved in signaling pathways but may promote cell damage when unregulated or excessively formed in inappropriate locations. A variety of chronic pathologies have been associated with FR-induced mitochondrial dysfunctions , such as cancer, age-related neurodegenerative diseases, and metabolic syndrome.In recent years drug design based on specific mitochondria-targeted antioxidants has become a very attractive therapeutic strategy and, among target compounds, nitrones have received growing attention because of their specific affinity toward FR. Here, we describe protocols dealing with the preparation, mitochondria permeation assessment, electron paramagnetic resonance (EPR) spin trapping setting, and antiapoptotic properties evaluation of a series of new linear nitrones vectorized by a triphenylphosphonium cation and labeled with a diethoxyphosphoryl moiety as 31P nuclear magnetic resonance (NMR) reporter with antioxidant property.

Sensitive Morphological Characterization of Oriented High-Density Lipoprotein Nanoparticles Using 31 P†NMR Spectroscopy.

The biological function of high-density lipoprotein (HDL) nanoparticles, the so-called good cholesterol that is associated with a low risk of heart disease, depends on their composition, morphology, and size. The morphology of HDL particles composed of apolipoproteins, lipids and cholesterol is routinely visualised by transmission electron microscopy (TEM), but higher-resolution tools are needed to observe more subtle structural differences between particles of different composition. Here, reconstituted HDL formulations are oriented on glass substrates and solid-state 31 P NMR spectroscopy is shown to be highly sensitive to the surface curvature of the lipid headgroups. The spectra report potentially functionally important differences in the morphology of different HDL preparations that are not detected by TEM. This method provides new morphological insights into HDL comprising a naturally occurring apolipoprotein A-I mutant, which may be linked to its atheroprotective properties, and holds promise as a future research tool in the clinical analysis of plasma HDL.

Creatine kinase rate constant in the human heart at 7T with 1D-ISIS/2D CSI localization.

PURPOSE: Creatine Kinase (CK) reaction plays an important role in energy metabolism and estimate of its reaction rate constant in heart provides important insight into cardiac energetics. Fast saturation transfer method ([Formula: see text] nominal) to measure CK reaction rate constant (kf) was previously demonstrated in open chest swine hearts. The goal of this work is to further develop this method for measuring the kf in human myocardium at 7T. [Formula: see text] approach is combined with 1D-ISIS/2D-CSI for in vivo spatial localization and myocardial CK forward rate constant was then measured in 7 volunteers at 7T. METHODS: [Formula: see text] method uses two partially relaxed saturation transfer (ST) spectra and correction factor to determine CK rate constant. Correction factor is determined by numerical simulation of Bloch McConnell equations using known spin and experimental parameters. Optimal parameters and error estimate in calculation of CK reaction rate constant were determined by simulations. The technique was validated in calf muscles by direct comparison with saturation transfer measurements. [Formula: see text] pulse sequence was incorporated with 1D-image selected in vivo spectroscopy, combined with 2D-chemical shift spectroscopic imaging (1D-ISIS/2D-CSI) for studies in heart. The myocardial CK reaction rate constant was then measured in 7 volunteers. RESULTS: Skeletal muscle kf determined by conventional approach and [Formula: see text] approach were the same 0.31 ± 0.02 s-1 and 0.30 ± 0.04 s-1 demonstrating the validity of the technique. Results are reported as mean ± SD. Myocardial CK reaction rate constant was 0.29 ± 0.05 s-1, consistent with previously reported studies. CONCLUSION: [Formula: see text] method enables acquisition of 31P saturation transfer MRS under partially relaxed conditions and enables 2D-CSI of kf in myocardium. This work enables applications for in vivo CSI imaging of energetics in heart and other organs in clinically relevant acquisition time.

Dynamic 31 P spectroscopic imaging of skeletal muscles combining flyback echo-planar spectroscopic imaging and compressed sensing.

PURPOSE: Dynamic phosphorus MR spectroscopic imaging (31 P-MRSI) experiments require temporal resolution on the order of seconds to concurrently assess different muscle groups. A highly accelerated pulse sequence combining flyback echo-planar spectroscopic imaging (EPSI) and compressed sensing was developed and tested in a phantom and healthy humans during an exercise-recovery challenge of the lower leg muscles, using a clinical 3T MRI. METHODS: A flyback EPSI readout designed to achieve 2.25 × 2.25 cm2 resolution over a 18 × 18 cm2 field of view (i.e., 8 × 8 matrix) was combined with compressed sensing through the inclusion of pseudorandom gradient blips to sub-sample the ky-kt dimensions by a factor of 2.7×, achieving a temporal resolution of 9 s. The sequence was first tested in a phantom to assess performance compared to fully sampled EPSI (fidEPSI) and phase encoded chemical shift imaging (fidCSI). Then, tests were performed in 11 healthy volunteers during an exercise-recovery challenge of the lower leg muscles. Voxels containing tissue from different muscle groups were evaluated measuring percentage phosphocreatine (%PCr) depletion, time constant of PCr recovery (τPCr) and intracellular pH at rest and following exercise. RESULTS: The sequence was capable to track the dynamic PCr response of multiple muscles simultaneously. No statistical differences were found in the metabolite ratio, pH or linewidth when compared with fidEPSI and fidCSI in the phantom study. Dynamic experiments showed differences in PCr depletion when comparing soleus with gastrocnemius muscles. Intracellular pH, τPCr and %PCr decrease were consistent with reported values. CONCLUSION: Highly accelerated 31 P-MRSI combining flyback EPSI and compressed sensing is capable of assessing concurrent energy metabolism in multiple muscle groups using a clinical 3T MR system.

In vivo bone 31 P relaxation times and their implications on mineral quantification.

PURPOSE: The intersubject variations in bone phosphorus-31 (31 P) T1 and T 2 * , as well as the implications on in vivo 31 P MRI-based bone mineral quantification, were investigated at 3T field strength. METHODS: A technique that isolates the bone signal from the composite in vivo 31 P spectrum was first evaluated via simulation and experiments ex vivo and subsequently applied to measure the T1 of bone 31 P collectively with a spectroscopic saturation recovery sequence in a group of healthy subjects aged 26 to 76 years. T 2 * was derived from the bone signal linewidth. The density of bone 31 P was derived for all subjects from 31 P zero TE images acquired in the same scan session using the measured relaxation times. Test-retest experiments were also performed to evaluate repeatability of this in vivo MRI-based bone mineral quantification protocol. RESULTS: The T1 obtained in vivo using the proposed spectral separation method combined with saturation recovery sequence is 38.4 ± 1.5 s for the subjects studied. Average 31 P density found was 6.40 ± 0.58 mol/L (corresponding to 1072 ± 98 mg/cm3 mineral density), in good agreement with an earlier study in specimens from donors of similar age range. Neither the relaxation times (P = 0.18 for T1 , P = 0.99 for T 2 * ) nor 31 P density (P = 0.55) were found to correlate with subject age. Average coefficients of variation for the repeat study were 1.5%, 2.6%, and 4.4% for bone 31 P T1 , T 2 * , and density, respectively. CONCLUSION: Neither 31 P T1 nor T 2 * varies significantly in healthy adults across a 50-year age range, therefore obviating the need for subject-specific measurements.

Unveiling a hidden 31 P signal coresonating with extracellular inorganic phosphate by outer-volume-suppression and localized 31 P MRS in the human brain at 7T.

PURPOSE: The study was undertaken to demonstrate that there is more than 1 component in the extracellular Pi31 P signal ( Piex) acquired from human head using nonlocalized 31 P MRS. METHODS: Outer-volume-suppression (OVS) saturation and 1D/2D 31 P CSI were utilized to reveal the presence of an additional component in the Piex signal. RESULTS: 67% of the head extracellular Pi signal was attenuated upon OVS saturation of the peripheral meningeal tissues, likely reflecting elimination of the Pi signal in the meningeal fluids (the blood and CSF). Localized 1D/2D CSI data provided further support for this assignment. Upon correction for the meningeal contribution, the extracellular Pi concentration was 0.51 ± 0.07 mM, whereas the intracellular Pi was 0.85 ± 0.10 mM. The extracellular pH was measured as 7.32 ± 0.04 when using OVS, as compared to 7.39 ± 0.03 when measured without OVS (N = 7 subjects). CONCLUSION: The extracellular Pi signal acquired from the human head using nonlocalized 31 P MRS contains a significant component likely contributed by peripheral blood and CSF in meninges that must be removed in order to use this signal as an endogenous probe for measuring extracellular pH and other properties in the brain.

31 P magnetization transfer magnetic resonance spectroscopy: Assessing the activation induced change in cerebral ATP metabolic rates at 3 T.

PURPOSE: In vivo 31 P magnetic resonance spectroscopy (MRS) magnetization transfer (MT) provides a direct measure of neuronal activity at the metabolic level. This work aims to use functional 31 P MRS-MT to investigate the change in cerebral adenosine triphosphate (ATP) metabolic rates in healthy adults upon repeated visual stimuli. METHODS: A magnetization saturation transfer sequence with narrowband selective saturation of γ-ATP was developed for 31 P MT experiments at 3 T. RESULTS: Using progressive saturation of γ-ATP, the intrinsic T1 relaxation times of phosphocreatine (PCr) and inorganic phosphate (Pi) at 3 T were measured to be 5.1 ± 0.8 s and 3.0 ± 1.4 s, respectively. Using steady-state saturation of γ-ATP, a significant 24% ± 14% and 11% ± 7% increase in the forward creatine kinase (CK) pseudo-first-order reaction rate constant, k1 , was observed upon visual stimulation in the first and second cycles, respectively, of a paradigm consisting of 10-minute rest followed by 10-minute stimulation, with the measured baseline k1 being 0.35 ± 0.04 s-1 . No significant changes in forward ATP synthase reaction rate, PCr/γ-ATP, Pi/γ-ATP, and nicotinamide adenine dinucleotide/γ-ATP ratios, or intracellular pH were detected upon stimulation. CONCLUSION: This work demonstrates the potential of studying cerebral bioenergetics using functional 31 P MRS-MT to determine the change in the forward CK reaction rate at 3 T. Magn Reson Med 79:22-30, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

High-resolution 31 P echo-planar spectroscopic imaging in vivo at 7T.

PURPOSE: Conventional 31 P chemical shift imaging is time-consuming and yields only limited spatial resolution. The purpose of this study was to demonstrate feasibility of 31 P echo-planar spectroscopic imaging (EPSI) in vivo at 7T. METHODS: A 3D 31 P EPSI sequence with trapezoidal-shaped gradient pulses was implemented on a 7T MR scanner. To increase spectral width with reduced demand on gradient performance, a multishot approach was chosen. Acquisition weighting and 31 P-{1 H} double resonance for nuclear Overhauser signal enhancement were applied to increase sensitivity. RESULTS: 3D 31 P-{1 H} EPSI data from model solution and from human calf muscle and brain were obtained from voxels with effective sizes of 4.1 to 16.2 cm3 in measurement times of approximately 10 min. Individual spectra showed well-resolved resonances of endogenous 31 P-metabolites without artifacts. Volumetric high-resolution 31 P-metabolite maps in vivo showed metabolic heterogeneity of different tissues. CONCLUSION: In vivo 31 P EPSI at 7T yields high-quality metabolic images. The proposed multishot EPSI technique reduces the measurement times for acquisition of volumetric high-resolution maps of 31 P-metabolites or intracellular pH in human studies. Magn Reson Med 79:1251-1259, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Spatially localized phosphorous metabolism of skeletal muscle in Duchenne muscular dystrophy patients: 24-month follow-up.

OBJECTIVES: To assess the changes in phosphodiester (PDE)-levels, detected by 31P magnetic resonance spectroscopy (MRS), over 24-months to determine the potential of PDE as marker for muscle tissue changes in Duchenne Muscular Dystrophy (DMD) patients. METHODS: Spatially resolved phosphorous datasets were acquired in the right lower leg of 18 DMD patients (range: 5-15.4 years) and 12 age-matched healthy controls (range: 5-14 years) at three time-points (baseline, 12-months, and 24-months) using a 7T MR-System (Philips Achieva). 3-point Dixon images were acquired at 3T (Philips Ingenia) to determine muscle fat fraction. Analyses were done for six muscles that represent different stages of muscle wasting. Differences between groups and time-points were assessed with non-parametric tests with correction for multiple comparisons. Coefficient of variance (CV) were determined for PDE in four healthy adult volunteers in high and low signal-to-noise ratio (SNR) datasets. RESULTS: PDE-levels were significantly higher (two-fold) in DMD patients compared to controls in all analyzed muscles at almost every time point and did not change over the study period. Fat fraction was significantly elevated in all muscles at all time points compared to healthy controls, and increased significantly over time, except in the tibialis posterior muscle. The mean within subject CV for PDE-levels was 4.3% in datasets with high SNR (>10:1) and 5.7% in datasets with low SNR. DISCUSSION AND CONCLUSION: The stable two-fold increase in PDE-levels found in DMD patients in muscles with different levels of muscle wasting over 2-year time, including DMD patients as young as 5.5 years-old, suggests that PDE-levels may increase very rapidly early in the disease process and remain elevated thereafter. The low CV values in high and low SNR datasets show that PDE-levels can be accurately and reproducibly quantified in all conditions. Our data confirms the great potential of PDE as a marker for muscle tissue changes in DMD patients.

Pre-transplantation 31P-magnetic resonance spectroscopy for quality assessment of human pancreatic grafts - A feasibility study.

OBJECTIVE: To investigate the feasibility of using 31P-MRS for objective non-invasive quality assessment of human pancreas grafts prior to transplantation or islet isolation. MATERIALS AND METHODS: Pancreata from 5 human donors, 3 males and 2 females, aged 49-78years, with body mass index (BMI) 22-31kg/m2, were included. Pancreata were perfused with histidine-tryptophan-ketoglutarate solution during procurement and stored in hypothermic condition (4°C) for 21-44h. During the period of hypothermic storage repeated spectra were obtained for each graft by 31P-MRS (1.5Tesla) to measure the cold ischemia time (CIT) dependent changes of the phosphorous metabolites adenosine triphosphate (ATP), phosphomonoesters (PME), phosphodiesters (PDE) and inorganic phosphate (Pi), in the grafts. Graft temperature was measured immediately before and after MR-examination. Reference spectrum for non-viable tissue was obtained after graft exposure to room temperature. RESULTS: PME/Pi, PDE/Pi and ATP/Pi spectral intensities ratios decreased with increasing CIT, reflecting the decreased viability of the grafts. PME/Pi ratio was the most discriminatory variable at prolonged CIT. 31P-MRS could be performed without significantly increasing graft temperature. CONCLUSIONS: 31P-MRS may provide quantitative parameters for evaluating graft viability ex vivo, and is a promising tool for objective non-invasive assessment of the quality of human pancreas grafts prior to transplantation or islet isolation.

Adiabatic excitation for 31 P MR spectroscopy in the human heart at 7 T: A feasibility study.

PURPOSE: Phosphorus magnetic resonance spectroscopy (31 P-MRS) provides a unique tool for assessing cardiac energy metabolism, often quantified using the phosphocreatine (PCr)/adenosine triphosphate (ATP) ratio. Surface coils are typically used for excitation for 31 P-MRS, but they create an inhomogeneous excitation field across the myocardium, producing undesirable, spatially varying partial saturation. Therefore, we implemented adiabatic excitation in a 3D chemical shift imaging (CSI) sequence for cardiac 31 P-MRS at 7 Tesla (T). METHODS: We optimized an adiabatic half passage pulse with bandwidth sufficient to excite PCr and γ-ATP together. In addition, the CSI sequence was modified to allow interleaved excitation of PCr and γ-ATP, then 2,3-DPG, to enable PCr/ATP determination with blood correction. Nine volunteers were scanned at 2 transmit voltages to confirm that measured PCr/ATP was independent of B1+ (i.e. over the adiabatic threshold). Six septal voxels were evaluated for each volunteer. RESULTS: Phantom experiments showed that adiabatic excitation can be reached at the depth of the heart using our pulse. The mean evaluated cardiac PCr/ATP ratio from all 9 volunteers corrected for blood signal was 2.14 ± 0.16. Comparing the two acquisitions with different voltages resulted in a minimal mean difference of -0.005. CONCLUSION: Adiabatic excitation is possible in the human heart at 7 T, and gives consistent PCr/ATP ratios. Magn Reson Med 78:1667-1673, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The pH heterogeneity in human calf muscle during neuromuscular electrical stimulation.

PURPOSE: The aim of the study was to examine pH heterogeneity during fatigue induced by neuromuscular electrical stimulation (NMES) using phosphorus magnetic resonance spectroscopy (31 P-MRS). It is hypothesized that three pH components would occur in the 31 P-MRS during fatigue, representing three fiber types. METHODS: The medial gastrocnemius of eight subjects was stimulated within a 3-Tesla whole body MRI scanner. The maximal force during stimulation (Fstim ) was examined by a pressure sensor. Phosphocreatine (PCr), adenosintriphosphate, inorganic phosphate (Pi), and the corresponding pH were estimated by a nonvolume-selective 31 P-MRS using a small loop coil at rest and during fatigue. RESULTS: During fatigue, Fstim and PCr decreased to 27% and 33% of their initial levels, respectively. In all cases, the Pi peak increased when NMES was started and split into three different peaks. Based on the single Pi peaks during fatigue, an alkaline (6.76 ± 0.08), a medium (6.40 ± 0.06), and an acidic (6.09 ± 0.05) pH component were observed compared to the pH (7.02 ± 0.02) at rest. CONCLUSION: It is suggested that NMES is able to induce pH heterogeneity in the medial gastrocnemius, and that the single Pi peaks represent the different muscle fiber types of the skeletal muscle. Magn Reson Med 77:2097-2106, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Phase transformations during processing and in vitro degradation of porous calcium polyphosphates.

A 2-Step sinter/anneal treatment has been reported previously for forming porous CPP as biodegradable bone substitutes [9]. During the 2-Step annealing treatment, the heat treatment used strongly affected the rate of CPP degradation in vitro. In the present study, x-ray diffraction and (31)P solid state nuclear magnetic resonance were used to determine the phases that formed using different heat treating processes. The effect of in vitro degradation (in PBS at 37 °C, pH 7.1 or 4.5) was also studied. During CPP preparation, ß-CPP and γ-CPP were identified in powders formed from a calcium monobasic monohydrate precursor after an initial calcining treatment (10 h at 500 °C). Melting of this CPP powder (at 1100 °C), quenching and grinding formed amorphous CPP powders. Annealing powders at 585 °C (Step-1) resulted in rapid sintering to form amorphous porous CPP. Continued annealing to 650 °C resulted in crystallization to form a multi-phase structure of ß-CPP primarily plus lesser amounts of α-CPP, calcium ultra-phosphates and retained amorphous CPP. Annealing above 720 °C and up to 950 °C transformed this to ß-CPP phase. In vitro degradation of the 585 °C (Step-1 only) and 650 °C Step-2 annealed multi-phase samples occurred significantly faster than the ß-CPP samples formed by Step-2 annealing at or above 720 °C. This faster degradation was attributable to preferential degradation of thermodynamically less stable phases that formed in samples annealed at 650 °C (i.e. α-phase, ultra-phosphate and amorphous CPP). Degradation in lower pH solutions significantly increased degradation rates of the 585 and 650 °C annealed samples but had no significant effect on the ß-CPP samples.

Design and characterization of chitosan-based composite particles with tunable interfacial properties.

Composite nano-microparticles were prepared in aqueous solution that contain chitosan or modified (carboxymethyl) chitosan with polyanion species such as alginate or tripolyphosphate, respectively. Several types of particles were prepared and characterized by (1)H/(31)P NMR spectroscopy, IR spectroscopy, and DLS. According to DLS, the particle size was observed to increase as the concentration of the aqueous urea solution increased. The average size and polydispersity index (in parentheses) vary and are reported for the chitosan-based particles from 243.0 ± 1 nm (0.28) to 424 ± 14 nm (0.33), according to DLS measurements at ambient conditions. Thus, the particles are herein referred to as nano-microparticles (NMPs) due to the relative size range. The stability of the NMPs is related to the particle composition and the aqueous solution conditions, as evidenced by variable NMP stability on the order of two weeks or more at different ionic strength.

Solid-state 31P NMR investigation on the status of guanine nucleotides in paclitaxel-stabilized microtubules.

Microtubule dynamics is a target for many chemotherapeutic drugs. In order to understand the biochemical effects of paclitaxel on the GTPase activity of tubulin, the status of guanine nucleotides in microtubules was investigated by (31)P cross-polarization magic angle spinning (CPMAS) NMR. Microtubules were freshly prepared in vitro in the presence of paclitaxel and then lyophilized in sucrose buffer for solid-state NMR experiments. A (31)P CPMAS NMR spectrum with the SNR of 25 was successfully acquired from the lyophilized microtubule sample. The broadness of the (31)P spectral lines in the spectrum indicates that the molecular environments around the guanine nucleotides inside tubulin may not be as crystalline as reported by many diffraction studies. Deconvolution of the spectrum into four spectral components was carried out in comparison with the (31)P NMR spectra obtained from five control samples. The spectral analysis suggested that about 13% of the nucleotides were present as GTP and 37% as GDP in the ß-tubulin (E-site) of the microtubules. It was found that most of the GDPs were present as GDP-Pi complex in the microtubules, which seems to be one of the effects of paclitaxel binding.

Investigation of the curvature induction and membrane localization of the influenza virus M2 protein using static and off-magic-angle spinning solid-state nuclear magnetic resonance of oriented bicelles.

A wide variety of membrane proteins induce membrane curvature for function; thus, it is important to develop new methods to simultaneously determine membrane curvature and protein binding sites in membranes with multiple curvatures. We introduce solid-state nuclear magnetic resonance (NMR) methods based on magnetically oriented bicelles and off-magic-angle spinning (OMAS) to measure membrane curvature and the binding site of proteins in mixed-curvature membranes. We demonstrate these methods on the influenza virus M2 protein, which not only acts as a proton channel but also mediates virus assembly and membrane scission. An M2 peptide encompassing the transmembrane (TM) domain and an amphipathic helix, M2(21-61), was studied and compared with the TM peptide (M2TM). Static (31)P NMR spectra of magnetically oriented 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) bicelles exhibit a temperature-independent isotropic chemical shift in the presence of M2(21-61) but not M2TM, indicating that the amphipathic helix confers the ability to generate a high-curvature phase. Two-dimensional (2D) (31)P spectra indicate that this high-curvature phase is associated with the DHPC bicelle edges, suggestive of the structure of budding viruses from the host cell. (31)P- and (13)C-detected (1)H relaxation times of the lipids indicate that the majority of M2(21-61) is bound to the high-curvature phase. Using OMAS experiments, we resolved the (31)P signals of lipids with identical headgroups based on their distinct chemical shift anisotropies. On the basis of this resolution, 2D (1)H-(31)P correlation spectra show that the amide protons in M2(21-61) correlate with the DMPC but not DHPC (31)P signal of the bicelle, indicating that a small percentage of M2(21-61) partitions into the planar region of the bicelles. These results show that the amphipathic helix induces high membrane curvature and localizes the protein to this phase, in good agreement with the membrane scission function of the protein. These bicelle-based relaxation and OMAS solid-state NMR techniques are generally applicable to curvature-inducing membrane proteins such as those involved in membrane trafficking, membrane fusion, and cell division.

Comparison of three reference methods for the measurement of intracellular pH using 31P MRS in healthy volunteers and patients with lymphoma.

31P magnetic resonance spectroscopy (31P MRS) can measure intracellular pH (pHi) using the chemical shift difference between pH-dependent inorganic phosphate (Pi) and a pH-independent reference peak. This study compared three different frequency reference peaks [phosphocreatine (PCr), α resonance of adenosine triphosphate (αATP) and water (using 1H MRS)] in a cohort of 10 volunteers and eight patients with non-Hodgkin's lymphoma (NHL). Well-resolved chemical shift imaging (CSI) spectra were acquired on a 1.5T scanner for muscle, liver and tumour. The pH was calculated for all volunteers and patients using the available methods. The consistency of the resulting pH was evaluated. The direct Pi­PCr method was best for those spectra with a very well-defined PCr, such as muscle (pH=7.05 ± 0.02). In liver, the Pi­αATP method gave more consistent results (pH=7.30 ± 0.06) than the calibrated water-based method (pH=7.27 ± 0.11). In NHL nodes, the measured pH using the Pi­αATP method was 7.25 ± 0.12. Given that the measured range includes some biological variation in individual patients, treatment-related changes of the order of 0.1 pH units should be detectable.

Integrating muscle cell biochemistry and whole-body physiology in humans:(31)P-MRS data from the InSight trial.

We acquired (31)P-MRS data from skeletal muscle of subjects of mixed gender and ethnicity, combined with a panel of physiological characteristics, and tested several long-standing hypotheses regarding relationships between muscle cell biochemistry and whole-body physiology with unusually high statistical power. We hypothesized that i) whole-body VO(2)max would correlate with muscle respiratory capacity, ii) resting muscle phosphocreatine concentration ([PCr]) would negatively correlate with delta efficiency and iii) muscle mitochondrial function would positively correlate with both resting VO(2) and total daily energy expenditure (TDEE). Muscle respiratory capacity explained a quarter of the variation in VO(2)max (r(2) = 26, p < .001, n = 87). There was an inverse correlation between muscle [PCr] and delta efficiency (r = -23, p = 046, n = 87). There was also a correlation between [PCr] recovery halftime and TDEE (r = -23, p = 035, n = 87). Our data not only provide insights into muscle cell chemistry and whole-body physiology but our mixed cohort means that our findings are broadly generalizable.