Phase Behavior of Synaptosomal Membranes: The Effect of Lipid Composition and Temperature

The phase behavior of synaptosomal membranes isolated from mammalian rat and marine invertebrate squid intact nerve endings was compared. Homogeneous liquid disordered (Lα) phase was observed at their body temperatures of 37 and ∼20°C for the rat and squid, respectively, using fluorescence microscopy and 1H-MAS NMR. Temperature decrease resulted in co-existence of an ordered phase with onset temperatures of 24°C for rat and 8°C for squid. Comprehensive analyses of the lipid composition by LC/MS explained the significant difference in the onset temperature: >2x higher content of 18:0 and 18:1 lipid chains in rat, which contributes to a higher transition temperature. The amount of ω-3 chains was ∼3x higher in squid than in rat due to a difference in the amounts of 22:6 and 20:5: squid membranes contained 2x more total 22:6 in major phospholipids (PC, PE, and PS) and cardiolipin (17:0,22:6,22:6,22:6-species). While prominent in rat, 20:4 was not found in squid. It is known for Lα phase PC that 16:0,22:6 is more ordered and thicker with a smaller area/lipid than 16:0,20:4. The high content of 22:6 in squid may contribute to order in the Lα phase at the body temperature (∼20°C) in contrast to the high content of 20:4 in rat at a higher body temperature (37°C). Fluorescence microscopy data showing the temperature-dependent phase behavior using giant unilamellar vesicles prepared with lipid compositions based on the synaptosomal measurements above will be discussed. Synaptosomal preparations of squid with body temperatures differing by ∼9°C will be compared.