The missing dimension of cell membrane organization. Study on cubic membrane structure and function

Abstract

Membranes are of fundamental importance for biological systems. They provide for cellular compartmentalization and control of the internal cell environment. The biophysical properties of the membrane lipids and proteins play a key role in determining the membrane morphology and geometry. Far from being a simple flat sheet, cell membrane can fold itself into 3D nano-periodic cubic structures. The same cubic geometry is well studied in other disciplines such as mathematics, physics and polymer chemistry. Although cubic membranes have been observed in numerous cell types and under various stress conditions, knowledge about the mechanism of cubic membrane formation and potential function in biological systems is scarce. Possibly the best-characterized cubic membrane transition was observed in the mitochondrial inner membranes of the free-living giant amoeba (Chaos carolinensis). In this organism, mitochondrial inner membranes undergo dramatic changes in 3D organization upon food depletion, providing a valuable model. As first step toward understanding the factors controlling cubic membrane formation, we developed a method to isolate the mitochondria from amoeba Chaos with integrated cubic membrane organization. Our data shows that it is essential to include high concentrations of EDTA in the isolation medium to enhance the yield of isolated mitochondria with intact cubic membrane organization from amoeba Chaos. Furthermore, our detailed study on lipid profile of cubic membranes uncovered a novel link between cubic membrane formation under starvation conditions in amoeba IX Chaos cultures and the accumulation of long chain polyunsaturated fatty acid (specifically, docosapentaenoic acid) in cellular membrane phospholipids. In attempt to investigate the potential role of cubic membranes in biological systems, our results demonstrate that mitochondria containing ordered cubic membranes readily adsorb short segments of oligonucleotides, in vivo and in vitro with significant molecular uptake suggesting that cubic membranes may play a role in the intracellular macromolecules transportation. Moreover, the adsorbed oligonucleotide molecules within the cubic membranes are protected from the oxidative damage. Further studies on antioxidants activity and oxidative damage biomarkers in both starved amoeba (with cubic membrane organization) and fed amoeba (without cubic membrane organization) shows that total antioxidant system and the amounts of catalase and glutathione peroxidase were quantified in higher levels in fed as compared to starved Amoeba. Furthermore, although the antioxidants activity is lower and ROS levels are higher in starved amoeba as compared to fed amoeba, the levels of RNA oxidation and damage were significantly low in starved amoeba. This surprising finding implies that alternative protective mechanisms might take place to control the oxidative damage within the starved Amoeba Chaos cells. As the appearance of cubic membranes coincide with the cellular oxidative stress, it is probable that the structural transition of the cellular membrane into cubic organization may play an important part of the cell¿s protective response to oxidative stress.