ISG-like membrane thermodynamics: a stepwise calorimetric investigation

Cells and vesicles functionalities, as well as membrane proteins’ one, are strictly correlated to membrane lipid composition because of its influence on membrane thermodynamic stability. Despite the numerous studies on model membranes carried out through calorimetric, spectroscopic and imaging techniques, model systems were considered with simple composition which does not keep into consideration the complexity and the asymmetry of the real bilayers and the lipid thermodynamic reorganization. In this frame, simulating the composition of real cell membranes might be a key point for the correct interpretation of membrane-based mechanisms in cells since each lipid modification might produce modifications of the molecular structure of such systems, which in turn might lead to variations of their physicochemical properties [6]. In this work we selected Insulin Secretory Granules (ISGs), vesicles located in the pancreatic Langerhans β-cells and which are responsible for insulin and amylin storage and secretion in response to nutrients intake, as reference system and we dissected the influence of several factors that may influence the physicochemical behaviour of real cell membranes, as membrane curvature, phospholipid tails’ length and unsaturation, phospholipid headgroup, etc. We performed a thermodynamic study of different vesicles at physiological pH by using the micro-DSC technique. An ISG-like membrane was therefore obtained by mixing fourteen different lipids, achieving a composition which represents the 80% of the headgroups and an average of 80% of the tails in real ISGs [7]. The influence of cholesterol and different FFAs (stearic acid, oleic acid and elaidic acid) was investigated highlighting the magnitude of the effects on such a detailed membrane.