Ionophore-catalyzed cation transport between phospholipid inverted micelles manifest in DNMR

A Futile Cycle?: Tissue Homeostatic Trans-Membrane Water Co-Transport: Kinetics, Thermodynamics, Metabolic Consequences

The phenomenon of active trans-membrane water cycling (AWC) has emerged in little over a decade. Here, we consider H2O transport across cell membranes from the origins of its study. Historically, trans-membrane water transport processes were classified into: A) compensating bidirectional fluxes ("exchange"), and B) unidirectional flux ("net flow") categories. Recent literature molecular structure determinations and molecular dynamic (MD) simulations indicate probably all the many different hydrophilic substrate membrane co-transporters have membrane-spanning hydrophilic pathways and co-transport water along with their substrates, and that they individually catalyze category A and/or B water flux processes, although usually not simultaneously. The AWC name signifies that, integrated over the all the cell's co-transporters, the rate of homeostatic, bidirectional trans-cytolemmal water exchange (category A) is synchronized with the metabolic rate of the crucial Na+,K+-ATPase (NKA) enzyme. A literature survey indicates the stoichiometric (category B) water/substrate ratios of individual co-transporters are often very large. The MD simulations also suggest how different co-transporter reactions can be kinetically coupled molecularly. Is this (Na+,K+-ATPase rate-synchronized) cycling futile, or is it consequential? Conservatively representative literature metabolomic and proteinomic results enable comprehensive free energy analyses of the many transport reactions with known water stoichiometries. Free energy calculations, using literature intracellular pressure (Pi) values reveals there is an outward trans-membrane H2O barochemical gradient of magnitude comparable to that of the well-known inward Na+ electrochemical gradient. For most co-influxers, these gradients are finely balanced to maintain intracellular metabolite concentration values near their consuming enzyme Michaelis constants. The thermodynamic analyses include glucose, glutamate-, gamma-aminobutyric acid (GABA), and lactate- transporters. 2%-4% Pi alterations can lead to disastrous concentration levels. For the neurotransmitters glutamate- and GABA, very small astrocytic Pi changes can allow/disallow synaptic transmission. Unlike the Na+ and K+ electrochemical steady-states, the H2O barochemical steady-state is in (or near) chemical equilibrium. The analyses show why the presence of aquaporins (AQPs) does not dissipate the trans-membrane pressure gradient. A feedback loop inherent in the opposing Na+ electrochemical and H2O barochemical gradients regulates AQP-catalyzed water flux as an integral AWC aspect. These results also require a re-consideration of the underlying nature of Pi. Active trans-membrane water cycling is not futile, but is inherent to the cell's "NKA system" - a new, fundamental aspect of biology.

Metabolic activity diffusion imaging (MADI): I. Metabolic, cytometric modeling and simulations

Evidence mounts that the steady-state cellular water efflux (unidirectional) first-order rate constant (k_io [s^-1 ]) magnitude reflects the ongoing, cellular metabolic rate of the cytolemmal Na⁺ , K⁺ -ATPase (NKA), ^c MR_NKA (pmol [ATP consumed by NKA]/s/cell), perhaps biology's most vital enzyme. Optimal ¹ H₂ O MR k_io determinations require paramagnetic contrast agents (CAs) in model systems. However, results suggest that the homeostatic metabolic k_io biomarker magnitude in vivo is often too large to be reached with allowable or possible CA living tissue distributions. Thus, we seek a noninvasive (CA-free) method to determine k_io in vivo. Because membrane water permeability has long been considered important in tissue water diffusion, we turn to the well-known diffusion-weighted MRI (DWI) modality. To analyze the diffusion tensor magnitude, we use a parsimoniously primitive model featuring Monte Carlo simulations of water diffusion in virtual ensembles comprising water-filled and -immersed randomly sized/shaped contracted Voronoi cells. We find this requires two additional, cytometric properties: the mean cell volume (V [pL]) and the cell number density (ρ [cells/μL]), important biomarkers in their own right. We call this approach metabolic activity diffusion imaging (MADI). We simulate water molecule displacements and transverse MR signal decays covering the entirety of b-space from pure water (ρ = V = 0; k_io undefined; diffusion coefficient, D₀ ) to zero diffusion. The MADI model confirms that, in compartmented spaces with semipermeable boundaries, diffusion cannot be described as Gaussian: the nanoscopic D (D_n ) is diffusion time-dependent, a manifestation of the "diffusion dispersion". When the "well-mixed" (steady-state) condition is reached, diffusion becomes limited, mainly by the probabilities of (1) encountering (ρ, V), and (2) permeating (k_io ) cytoplasmic membranes, and less so by D_n magnitudes. Importantly, for spaces with large area/volume (A/V; claustrophobia) ratios, this can happen in less than a millisecond. The model matches literature experimental data well, with implications for DWI interpretations.

Intracellular water preexchange lifetime in neurons and astrocytes

PURPOSE

To determine the intracellular water preexchange lifetime, τ_i , the "average residence time" of water, in the intracellular milieu of neurons and astrocytes. The preexchange lifetime is important for modeling a variety of MR data sets, including relaxation, diffusion-sensitive, and dynamic contrast-enhanced data sets.

METHODS

Herein, τ_i in neurons and astrocytes is determined in a microbead-adherent, cultured cell system. In concert with thin-slice selection, rapid flow of extracellular media suppresses extracellular signal, allowing determination of the transcytolemmal-exchange-dominated, intracellular T₁ . With this knowledge, and that of the intracellular T₁ in the absence of exchange, τ_i can be derived.

RESULTS

Under normal culture conditions, τ_i for neurons is 0.75 ± 0.05 s versus 0.57 ± 0.03 s for astrocytes. Both neuronal and astrocytic τ_i s decrease within 30 min after the onset of oxygen-glucose deprivation, with the astrocytic τ_i showing a substantially greater decrease than the neuronal τ_i .

CONCLUSIONS

Given an approximate intra- to extracellular volume ratio of 4:1 in the brain, these data imply that, under normal physiological conditions, an MR experimental characteristic time of less than 0.012 s is required for a nonexchanging, two-compartment (intra- and extracellular) model to be valid for MR studies. This characteristic time shortens significantly (i.e., 0.004 s) under injury conditions. Magn Reson Med 79:1616-1627, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Compartmentalization of Gd liposomes: the quenching effect explained

Cationic liposomes carrying high [Gd] can be used as efficient cell-labeling agents. In a compartmentalized state, Gd can cause signal loss (relaxivity quenching). The contributions of liposomal [Gd], size and compartmentalization state to relaxivity quenching were assessed. The dependency of signal intensity (SI) on intraliposomal [Gd] was assessed comparing three different [Gd] (0.3, 0.6 and 1.0 M Gd) in both small (80 nm) and large (120 nm) cationic liposomes. In addition, five compartmentalization states were compared: free Gd, intact Gd liposomes, ruptured Gd liposomes, Gd liposomes in intact cells and Gd liposomes in ruptured cells (simulating cell death). Gd also causes R2 effects, which is often overlooked. Therefore, both R1 and R2 relaxation rates of a dilution range were measured by T1 and T2 mapping on a 7 T clinical scanner. Less is more. As the unidirectional water efflux rate (outbound across the liposome membrane, κle) is proportional to the surface:volume ratio, smaller liposomes yielded a consistently higher R1 than larger liposomes. For equal voxel [Gd] less concentrated liposomes (0.3 M Gd) yielded higher R1/R2 ratio because of the higher extraliposomal water fraction (vl ). Gd exhibits a dualistic behavior: from hypointensity to hyperintensity to hypointensity, with decreasing [Gd]. Regarding compartmentalization, fewer membrane barriers means a higher R1 /R2 ratio. Gd liposomes exhibit a versatile contrast behavior, dependent on the compartmentalization state, liposomal size, intraliposomal [Gd] and liposome number. Both R1 and R2 effects contribute to this. The versatility allows one to tailor the optimal liposomal formulation to desired goals in cell labeling and tracking.

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

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

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

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

Intracellular water-specific MR of microbead-adherent cells: the HeLa cell intracellular water exchange lifetime

Quantitative characterization of the intracellular water (1)H MR signal from cultured cells will provide critical biophysical insight into the MR signal from tissues in vivo. Microbeads provide a robust immobilization substrate for the many mammalian cell lines that adhere to surfaces and also provide sufficient cell density for observation of the intracellular water MR signal. However, selective observation of the intracellular water MR signal from perfused, microbead-adherent mammalian cells requires highly effective suppression of the extracellular water MR signal. We describe how high-velocity perfusion of microbead-adherent cells results in short apparent (1)H MR longitudinal and transverse relaxation times for the extracellular water in a thin slice selected orthogonal to the direction of flow. When combined with a spin-echo pulse sequence, this phenomenon provides highly effective suppression of the extracellular water MR signal. This new method is exploited here to quantify the kinetics of water exchange from the intracellular to extracellular spaces of HeLa cells. The time constant describing water exchange from intracellular to extracellular spaces, also known as the exchange lifetime for intracellular water, is 119 +/- 14 ms.

Equilibrium water exchange between the intra- and extracellular spaces of mammalian brain

This report describes the measurement of water preexchange lifetimes and intra/extracellular content in intact, functioning mammalian brain. Intra- and extracellular water magnetic resonance (MR) signals from rat brain in vivo were quantitatively resolved in the longitudinal relaxation domain following administration of an MR relaxation agent into the extracellular space. The estimated intracellular water content fraction was 81% +/- 8%, and the intra- to extracellular exchange rate constant was 1.81 +/- 0.89 s(-1) (mean +/- SD, N = 9), corresponding to an intracellular water preexchange lifetime of approximately 550 ms. These results provide a temporal framework for anticipating the water exchange regime (fast, intermediate, or slow) underlying a variety of compartment-sensitive measurements. The method also supplies a means by which to evaluate membrane water permeability and intra/extracellular water content serially in intact tissue. The data are obtained in an imaging mode that permits detection of regional variations in these parameters.

Measuring the change in CBV upon cortical activation with high temporal resolution using look-locker EPI and Gd-DTPA

A method of simultaneously measuring the changes in cerebral blood volume (CBV) and T(*) (2) that occur on brain activation with high temporal resolution was developed. The method involves measuring the change in the longitudinal relaxation time (T(1)) that occurs following a bolus injection of Gd-DTPA and converting this measurement to a change in blood volume assuming fast exchange. The sequence was optimized for the measurement of changes in CBV with high temporal resolution. A change in CBV of 27 +/- 4% on activation of the primary visual cortex (V1) was measured across four subjects. The time course of changes in T(*) (2) showed a poststimulus undershoot (P = 0.008) corresponding approximately to a period over which CBV was still elevated above baseline, but falling (P = 0.01). The effects of perfusion, nonfulfillment of the assumption of fast exchange and of intrinsic T(1) changes on activation on the model used to calculate the change in CBV are discussed.

Osmotic effects on the T2 relaxation decay of in vivo muscle

Saline solutions are commonly employed as a vehicle for drugs administered intramuscularly. In this study, in vivo measurements of spin-spin relaxation (T2) processes by magnetic resonance imaging (MRI) were performed to investigate the distribution of water in rat masseter muscle tissue after intramuscular injection of saline solutions of varying tonicity. Prior to saline injection, image-based T2 relaxation decay of muscle was monoexponential. After injection of saline, the T2 relaxation decay became multiexponential. Non-negative least squares (NNLS) analysis of the decay curves revealed two relaxation components: a fast component (T2 = 20-40 ms) and a slow component (T2 = 150-400 ms), which are assigned to intra- and extracellular water protons, respectively. Injection of hypertonic saline solutions significantly increased the extracellular water component in muscle tissue compared to isotonic saline solutions, an effect which lasted for more than 60 min. These findings suggest that MRI techniques may be useful to investigate the effect of hyper- or hypotonic solutions on muscle tissue in vivo.

Equilibrium transcytolemmal water-exchange kinetics in skeletal muscle in vivo

It is commonly assumed that equilibrium transcytolemmal water exchange in tissue is sufficiently frequent as to be fast on any NMR time scale achievable with an extracellular contrast agent (CR) in vivo. A survey of literature values for cell membrane diffusional permeability coefficients (P) and cell sizes suggests that this should not really be so. To evaluate this issue experimentally, we used a programmed intravenous CR infusion protocol for the rat with several rate plateaus, each of which achieved an increased steady-state concentration of GdDTPA(2-) in the blood plasma. Interleaved rigorous measurements of (1)H(2)O inversion recoveries were made from arterial blood and from a region of homogeneous thigh muscle tissue throughout the CR infusion. We made careful relaxographic analyses for the blood and muscle (1)H(2)O longitudinal relaxation times. The combined data from several animals were evaluated with a two-site model for equilibrium transcytolemmal water exchange. An excellent fitting was achieved, with parameters that agreed very well with the relevant physiological properties available in the literature. The fraction of water in the extracellular space, 0.11, is quite consistent with published values, as well as with reported tissue CR concentrations when one accounts for the restriction of CR to this space. The derived average lifetime for a water molecule in the thigh muscle sarcoplasm, 1.1 +/- 0.4 sec, implies a sarcolemmal P of 13 x 10(-4) cm/sec, which is well within the range of literature values determined in vitro. Moreover, we find that because of the exchange, the (1)H(2)O longitudinal relaxation rate constant exhibits a decided nonlinear dependence on the tissue or thermodynamic (extracellular) concentration of GdDTPA(2-). The muscle system departs the fast-exchange limit at a [CR] value of <100 micromol/L. This has significant implications for the quantitative use of CRs as MRI tracers. Magn Reson Med 42:467-478, 1999. Published 1999 Wiley-Liss, Inc.

Aggregation of lasalocid A in membranes: a fluorescence study

The aggregation behavior of the carboxylic ionophore, lasalocid A, has been studied in egg phosphatidylcholine vesicles by monitoring the intrinsic fluorescence of lasalocid A. Self quenching of lasalocid A fluorescence in vesicles of egg phosphatidylcholine suggests aggregation of lasalocid A. When aggregated lasalocid A is treated with increasing concentrations of lipid, there is an increase in fluorescence due to gradual reduction of self quenching on lateral dilution. This confirms the presence of loosely held non-covalent aggregates of lasalocid A in the membrane. This result is relevant in elucidating the molecular mechanism of cation transport by lasalocid A across membranes.

Bulk magnetic susceptibility shifts in NMR studies of compartmentalized samples: use of paramagnetic reagents

The bulk magnetic susceptibility (BMS) shift of a nuclear resonance frequency caused by a paramagnetic compound is of importance in vivo NMR, both magnetic resonance spectroscopy and magnetic resonance imaging. However, since it is a rather complicated phenomenon, it has been the source of many misinterpretations in the literature. We have reworked and organized the theory of the BMS shift. This includes accounting for the important effects of local susceptibility. We have conducted experiments on phantom samples in order to illustrate the principles involved. Our phantoms consist of capillaries and coaxial cylinders. They simulate the situations of blood vessels oriented parallel and perpendicular to the magnetic field and the interstitial spaces surrounding them. In most of our experiments, the paramagnetic compound was one of several different hyperfine shift reagents for cation resonances. These were chosen to cover a range of potencies, in both magnitude and sign, of the shifts they produce. However, we also used a reagent which was incapable of inducing a hyperfine shift and thus could cause only a BMS shift. Although we report only 23Na spectra in this paper, the latter samples simulate the cases where one observes the water 1H resonance in experiments employing hyperfine shift reagents for cations. There have been a number of such investigations recently reported in the literature. The principles considered in this paper allow us to offer new interpretations for the results of several experiments published in the last few years.

Micellar enzymology

Experimental approaches to modelling the enzymatic function of biological membranes are discussed. Emphasis is given to pseudohomogeneous systems such as proteolipid complexes and enzymes in organic solvents; the latter are solubilized with phospholipids or synthetic surfactants. Methods for producing and studying such micellar systems are considered. The key research problems of micellar enzymology are formulated and its relation to enzyme membranology is discussed. Finally, the new potentialities are noted of applied enzymology (biotechnology) offered by application of a colloidal solution of water in organic solvents as a microheterogeneous medium for enzymatic reactions.