Dissecting the effects of free fatty acids on the thermodynamic stability of complex model membranes mimicking insulin secretory granules

Abstract

A stepwise micro-DSC study of Small, Large and Giant Unilamellar Vesicles prepared as pure and mixed systems of DMPC, DPPC, DSPC and DOPC was performed, achieving the preparation of final model membranes whose phospholipid compositions represent the 75% in terms of the phospholipids tails and the 50% headgroups of the Insulin Secretory Granules (vesicles located in the pancreatic Langerhans β-cells and which are responsible for insulin and amylin storage and secretion in response to nutrient intake). Moreover, the effect of Free Fatty Acids, whose levels are recurrently altered in diabetic and/or obese subjects, on the thermodynamic stability of the final membranes was eventually investigated. The results allowed to discriminate each single thermodynamic contribution among the main factors that dictate the overall thermodynamic stability of these complex unilamellar systems evidencing mainly entropic effects hierarchically summarized as phospholipid unsaturations > phospholipid tail length > membrane curvature. The effect of the Free Fatty Acids highlighted a strong stabilizing effect on the membranes as well as more pronounced phase segregations in the case of saturated acids (palmitic and stearic), whereas the opposite effect was observed in the case of an unsaturated one (oleic).

Lipid Modulation in the Formation of β-Sheet Structures. Implications for De Novo Design of Human Islet Amyloid Polypeptide and the Impact on β-Cell Homeostasis

“…The content of anionic phospholipids in β-cell membranes is reported to range from 2.5% to 13.2%, a proportion mainly situated in the inner leaflet of the membrane [38], proportion much similar as found in insulin secretory granules where a higher content of PS and phosphatidylinositol has been described [17,42], which are derived from ER membranes during insulin maturation. This contributes to the possibility that hIAPP aggregation could be increased at the intracellular space, especially during the process of maturation of insulin secretory granules.…”

Influence of Free Fatty Acids on Lipid Membrane–Nisin Interaction

“…Hence, the absence of relevant extra enthalpic contribution once again confirms that the thermotropic behavior of these vesicles is mainly entropically driven and follows a composition-dependent proportionality. 25 , 26 …”

“…SUVs samples were diluted up to 3.2 mM phospholipid concentration, also for vesicles which included FFAs. 25 , 26 The phospholipid concentration was derived by accurately considering the lipid weight and the dilution volumes at each step of the liposome preparation protocol (the validity of such an approach was assessed in previous works 25 , 26 ).…”

“…In order to quantitatively compare and discuss the transition cooperativity between different systems, we adopted the transition average temperature, T̅ , and the average cooperativity index, ACI, defined elsewhere. 25 Briefly, the transition average temperature, T̅ , is defined as where T 0 and T f are the initial and final limits of the observable peak, respectively, and the frequency function f ( T ) is the normalized calorimetric peak distribution whereas the average cooperativity index, ACI, is defined as The higher the ACI value, the lower the transition cooperativity.…”

“… 7 , 8 , 16 − 19 However, though low percentages of FFAs are naturally present in plasma and in cell membranes (around 0.3–10% of total lipids), 20 altered levels of FFAs are recurrent in pathological cases (e.g., diabetic and/or obese subjects) and may cause strong alterations in cell metabolism 21 − 23 as well as in cell phospholipid bilayers’ physicochemical properties and thermodynamics. 24 , 25 …”