Active Trans-Plasma Membrane Water Cycling in Yeast Is Revealed by NMR

Abstract

Plasma membrane water transport is a crucial cellular phenomenon. Net water movement in response to an osmotic gradient changes cell volume. Steady-state exchange of water molecules, with no net flux or volume change, occurs by passive diffusion through the phospholipid bilayer and passage through membrane proteins. The hypothesis is tested that plasma membrane water exchange also correlates with ATP-driven membrane transport activity in yeast (Saccharomyces cerevisiae). Longitudinal (1)H(2)O NMR relaxation time constant (T(1)) values were measured in yeast suspensions containing extracellular relaxation reagent. Two-site-exchange analysis quantified the reversible exchange kinetics as the mean intracellular water lifetime (τ(i)), where τ(i)(-1) is the pseudo-first-order rate constant for water efflux. To modulate cellular ATP, yeast suspensions were bubbled with 95%O(2)/5%CO(2) (O(2)) or 95%N(2)/5%CO(2) (N(2)). ATP was high during O(2), and τ(i)(-1) was 3.1 s(-1) at 25°C. After changing to N(2), ATP decreased and τ(i)(-1) was 1.8 s(-1). The principal active yeast ion transport protein is the plasma membrane H(+)-ATPase. Studies using the H(+)-ATPase inhibitor ebselen or a yeast genetic strain with reduced H(+)-ATPase found reduced τ(i)(-1), notwithstanding high ATP. Steady-state water exchange correlates with H(+)-ATPase activity. At volume steady state, water is cycling across the plasma membrane in response to metabolic transport activity.

DCE-MRI of hepatocellular carcinoma: perfusion quantification with Tofts model versus shutter-speed model—initial experience

“…For example, the FXR-allowed SSM version used in this study allows for a non-infinitely fast transcytolemmal exchange, modeled by the mean intracellular water molecule lifetime, τ i . This parameter has been suggested to reflect cellular metabolic activity, manifest in ATP-driven membrane transport activity in a yeast suspension study [50]. This observation was further validated in a breast DCE-MRI study [51], demonstrating that changes in τ i are not due to changes in cell sizes but rather changes in energetic metabolism-driven cell membrane water permeability.…”

“…This version of the SSM has been used to quantify perfusion of breast [48] and prostate cancer [49], but not in the liver. It has been suggested that τ i may reflect cellular metabolic activity [50, 51]. …”

Dynamic Contrast-Enhanced MRI–Derived Intracellular Water Lifetime (τi): A Prognostic Marker for Patients with Head and Neck Squamous Cell Carcinomas

“…Earlier studies 12, 25, 26 have reported that τ i is inversely correlated with cell membrane ion-pump activity, a measure of mitochondrial metabolism suggesting that τ i may be a sensitive indicator of cellular energy turnover. Similarly, some studies have reported that increased metabolic activity is associated with lower τ i (median=0.16–1.03s) in regions of prostrate, 15, 16 and esophageal cancer 17 compared to normal tissues.…”

Magnetic resonance microdynamic imaging reveals distinct tissue microenvironments

“…Although our findings suggest that, indeed, the cell membrane is sufficient to induce a relatively low diffusivity and, in addition, the intraneuronal and intraglial peaks were well-resolved, we cannot exclude the effects of fast exchange in these two cases. The impact of exchange is expected to increase with in vivo applications because of the elevated temperature, the presence of active water and ion channels (Zhang et al, 2011), and the longer diffusion times necessary when using clinical systems. This issue may be addressed by direct measurement of the exchange rate spectrum, either via relaxation exchange spectroscopy (REXSY) (Lee et al, 1993) or via diffusion exchange spectroscopy (DEXSY) (Callaghan and Fur’o, 2004).…”