Coupled Response of Membrane Hydration with Oscillating Metabolism in Live Cells: An Alternative Way to Modulate Structural Aspects of Biological Membranes?

Abstract

We propose that active metabolic processes may regulate structural changes in biological membranes via the physical state of cell water. This proposition is based on recent results obtained from our group in yeast cells displaying glycolytic oscillations, where we demonstrated that there is a tight coupling between the oscillatory behavior of glycolytic metabolites (ATP, NADH) and the extent of the dipolar relaxation of intracellular water, which oscillates synchronously. The mechanism we suggest involves the active participation of a polarized intracellular water network whose degree of polarization is dynamically modulated by temporal ATP fluctuations caused by metabolism with intervention of a functional cytoskeleton, as conceived in the long overlooked association-induction hypothesis (AIH) of Gilbert Ling. Our results show that the polarized state of intracellular water can be propagated from the cytosol to regions containing membranes. Since changes in the extent of the polarization of water impinge on its chemical activity, we hypothesize that metabolism dynamically controls the local structure of cellular membranes via lyotropic effects. This hypothesis offers an alternative way to interpret membrane related phenomena (e.g., changes in local curvature pertinent to endo/exocytosis or dynamical changes in membranous organelle structure, among others) by integrating relevant but mostly overlooked physicochemical characteristics of the cellular milieu.

Keywords: 6-acyl-2-(dimethylamino)naphtalenes fluorescence probes; ATP; association-induction hypothesis (AIH); biological membranes; crowding; cytoskeletal proteins; lyotropic mesomorphism; mesophases; water activity.

Figure 1

Oscillations in NADH, intracellular ATP, and the fluorescence of ACDAN, LAURDAN, and Nile red were synchronized. (A) Oscillations in NADH fluorescence, intracellular ATP concentration, ACDAN fluorescence, LAURDAN fluorescence, and Nile red fluorescence in a cell suspension with oscillating glycolysis. The cell suspension was 10% (w/v) S. cerevisiae BY4743 in 100 mM potassium phosphate buffer, pH 6.8, and temperature of 25 °C. Oscillations in glycolysis were induced at approximately t = 100 s by adding 30 mM glucose and 5 mM KCN to the cell suspension. (B) Power spectra of the oscillations in (A). Experimental details can be found in the Materials and Methods section.

Figure 2

Oscillations in the polarization of water in aqueous environments with glycolytic oscillations may induce oscillations in (lipid-rich membranous) regions where no biochemical oscillations are expected to appear. (A) Theoretical phase plot of oscillations of polarization of water in a region with no glycolytic oscillations (p₁) against the polarization of water in a region with glycolytic oscillations (p) generated by a Yang-Ling isotherm-based model of glycolytic oscillations. (B) Phase plot of Nile red fluorescence against the fluorescence of the three DAN probes: ACDAN (blue), PRODAN (red), and LAURDAN (grey). Reproduced from [13] and [15].

Figure 3

Membrane probes such as Mitotracker red may respond to changes other than membrane potential. (A) Fluorescence image of S. cerevisiae BY4743 cells stained with 1 μM Mitotracker red. (B) Oscillations in the fluorescence of NADH and Mitotracker red in a suspension of yeast cells where oscillations in glycolysis were induced by the addition of glucose and KCN as seen in Figure 1. (C) Power spectra of the oscillations in (B). Experimental details can be found in the Materials and Methods section.

Figure 4

Sketch of the coupling between cytosolic and membranous regions. Changes in the activity of water caused by the polarization of intracellular water—via metabolic effects—can induce lyotropic regulation of the membrane structure with, for example, important curvature effects. For more details, see Section 4. Part of the figure is adapted from [19].