Foreword

Biological membranes are highly heterogeneous and differ from each other in terms of lipid and protein compositions. The structure and function of cell membranes do not depend exclusively on the particular proteins, but rather depend on lipid composition and lipid–protein interactions. In particular, the variety of lipid composition in membranes is crucial in determining the biophysical properties of the membranes such as charge, curvature, fluidity, and packing of the hydrocarbon chains.

This Current Topics of Chem. Pharm. Bull. aims to reflect some recent progress in biophysical aspects and techniques to research living cell membranes and non-living model biomembrane systems. This issue contains a number of reviews from experts in physicochemical and biophysical fields on different types of biological membrane systems including phospholipid vesicles, nanodiscs, and cell membranes.

To begin with, two reviews have addressed recent aspects of thermodynamic and molecular dynamic studies on model lipid membrane systems. Phospholipid bilayer membranes vary their membrane states by undergoing phase transitions in response to various external environmental factors. In “Membrane States of Saturated Glycerophospholipids: A Thermodynamic Study of Bilayer Phase Transitions,” Matsuki et al. at Tokushima University have thermodynamically characterized the bilayer phase transitions of three kinds of saturated glycerophospholipids with a different polar head group (phosphatidyl-ethanolamine, -choline, or -glycerol), and have explained the various membrane states depending on temperature and pressure. In “Solution NMR to Quantify Mobility in Membranes: Diffusion, Protrusion, and Drug Transport Processes,” Emiko Okamura of Himeji Dokkyo University has introduced a strategy to quantify molecular dynamics in membranes using solution NMR spectroscopy. In addition to membrane lipids, the mobility of small-sized drugs, chemicals, and peptides inside and outside of membranes has been reviewed. In-cell NMR spectroscopy has also been introduced as a promising technique to monitor drug transport in real time without perturbing the living system.

Since most phospholipids are synthesized in the endoplasmic reticulum and subsequently distributed to each organelle, interbilayer transfer and transbilayer movement (flip-flop) of phospholipids play important roles in determining the composition and function of biomembranes. In “Evaluation of Interbilayer and Transbilayer Transfer Dynamics of Phospholipids Using Time-Resolved Small-Angle Neutron Scattering,” Minoru Nakano of University of Toyama explains how time-resolved small-angle neutron scattering detects the interbilayer and transbilayer transfer of phospholipids in situ and real-time, and introduces recent progress on the evaluation of spontaneous and protein- (or peptide-) mediated lipid transfer in several phospholipid dispersion systems.

Nanodiscs consist of discoidal phospholipid bilayer surrounded by a scaffold protein, and are used as membrane platforms to solubilize a variety of membrane proteins into the phospholipid bilayer environments. In “Nanodisc for Structural Biology in a Membranous Environment,” Masanori Osawa and his colleagues at Keio University have reviewed how nanodiscs or protein-reconstituted nanodiscs are prepared, and how they have been used in the structural biology field to analyze protein structure, dynamics, and interactions with lipid molecules using solution NMR and cryo-electron microscopy.

In cell membranes, protein–lipid interactions have crucial effects on the physiological function of membrane lipids as well as proteins. Fatty acid desaturase introduces cis-double bond into acyl chain of acyl-CoA and regulates the physicochemical property and function of cellular membrane. In “Structure and Function of Δ9-Fatty Acid Desaturase,” Nagao et al. at Kyoto University introduce some recent advances in understanding of the function and regulation of fatty acid desaturase in terms of protein structure. In “Molecular Mechanisms for Protection of Hepatocytes against Bile Salt Cytotoxicity,” Morita et al. at Shiga University of Medical Science Hospital have focused on molecular mechanisms for the protection of hepatocytes against bile salt cytotoxicity. Bile salts play multiple physiological roles but have damaging effects on cell membranes due to their detergent properties. To attenuate the cytotoxicity of bile salts, ABCB4, a member of the ATP-binding cassette (ABC) transporter family, mediates secretion of phospholipids into bile. Finally, it is known that pathogenic bacteria deliver effector proteins for infection across membrane barriers using type III secretion system (T3SS). In “Biophysical Mechanism of Protein Export by Bacterial Type III Secretion System,” Takashi Ohgita and Hiroyuki Saito of Kyoto Pharmaceutical University summarize the current understanding of T3SS, and explaine a novel model of the protein export mechanism of T3SS by which T3SS needle rotates with proton-motive force.

I take this opportunity to thank all the authors and contributors for their efforts in making this Current Topics on the biophysical research of biological membrane systems. I believe that the topics covered in this Current Topics will interest many readers of Chem. Pharm. Bull.