Compensating lipid fluxes generated by the aminophospholipid translocase

Flunitrazepam induces geometrical changes at the lipid-water interface

Flunitrazepam (FNTZ) effects on molecular packing and surface curvature in artificial model membranes were investigated. FNTZ, from the subphase under dipalmitoylphosphatidylcholine (dpPC) monolayers at the air-water interface, expanded the surface pressure-area isotherm and induced an increment in the limiting area; in this conditions, the collapse pressure of dpPC decreased, indicating a lowering in the stability of the monolayer. Thermodynamic-geometric correlations based on molecular parameters predicted a decrement in the aggregation number and stability, and an increase in the curvature of the self-aggregated structure of dpPC in aqueous medium in the presence of FNTZ. Accordingly, negative-staining electron microscopy of dpPC aqueous dispersions showed that the mean diameter of dpPC vesicles decreased 2 and 2.87 times in the presence of 10 nM and 50 µM FNTZ, respectively, compared with control samples. The release of a soluble marker entrapped in dpPC liposomes increased slightly respect to the control in the presence of FNTZ. In dpPC-dpPE mixed liposomes 50 µM FNTZ induced a decrement in the amount of the aminophospholipid exposed to the outer monolayer. Concluding, an FNTZ-induced expansion of dpPC-water interface region affected the constraints imposed on the lipid-water system by the molecular geometry, interacting free energies and entropy that determine the shape of a multimolecular structure. In liposomes composed of a pure phospholipid, the bilayer expansion leaded, through a structure instability, to reduce the liposome size; in mixed liposomes, phospholipid molecules translocation could be observed as another compensating mechanism of the initial perturbation. These results may be relevant for understanding benzodiazepines' effects non-mediated by membrane receptors.

Flunitrazepam partitioning into natural membranes increases surface curvature and alters cellular morphology

In recent studies, we showed that flunitrazepam (FNTZ) and other benzodiazepines interact with artificial phospholipid membranes locating at the polar head group region, inducing a membrane expansion, reducing the molecular packing and reorganising molecular dipoles. In the present paper we investigated the possibility that those phenomena could be transduced into changes in the curvature of membranes from natural origin. Hence we studied the effect of FNTZ on cellular morphology using human erythrocyte as a natural assay system. Shape changes of erythrocytes were evaluated by light microscopy and expressed as a morphological index (MI). FNTZ induced echinocytosis in a time-dependent manner with MI values significantly higher than those of control (without drug) or DMSO (vehicle) samples. Lidocaine, a local anesthetic known to induce stomatocytosis by incorporating in the inner monolayer, counterbalanced the concentration-dependent FNTZ crenating effects. FNTZ induced protective effects, compared with control and DMSO, against time-dependent hemolysis. Hypotonic-induced hemolysis, was also lowered by FNTZ in a concentration-dependent manner. Both antihemolytic effects suggested a drug-induced membrane expansion allowing a greater increase in cell volume before lysis. In such a complex system like a cell, curvature changes triggered by drug partitioning towards the plasma membrane, might be an indirect effect exerted through modifications of ionic-gradients or by affecting cytoskeleton-membrane linkage. In spite of that, the curvature changes can be interpreted as a mechanism suitable to relieve the tension generated initially by drug incorporation into the bilayer and may be the resultant of the dynamic interactions of many molecular fluxes leading to satisfy the spontaneous membrane curvature.

Lipid translocation across the plasma membrane of mammalian cells

The plasma membrane, which forms the physical barrier between the intra- and extracellular milieu, plays a pivotal role in the communication of cells with their environment. Exchanging metabolites, transferring signals and providing a platform for the assembly of multi-protein complexes are a few of the major functions of the plasma membrane, each of which requires participation of specific membrane proteins and/or lipids. It is therefore not surprising that the two leaflets of the membrane bilayer each have their specific lipid composition. Although membrane lipid asymmetry has been known for many years, the mechanisms for maintaining or regulating the transbilayer lipid distribution are still not completely understood. Three major players have been presented over the past years: (1) an inward-directed pump specific for phosphatidylserine and phosphatidylethanolamine, known as aminophospholipid translocase; (2) an outward-directed pump referred to as 'floppase' with little selectivity for the polar headgroup of the phospholipid, but whose actual participation in transport of endogenous lipids has not been well established; and (3) a lipid scramblase, which facilitates bi-directional migration across the bilayer of all phospholipid classes, independent of the polar headgroup. Whereas a concerted action of aminophospholipid translocase and floppase could, in principle, account for the maintenance of lipid asymmetry in quiescent cells, activation of the scramblase and concomitant inhibition of the aminophospholipid translocase causes a collapse of lipid asymmetry, manifested by exposure of phosphatidylserine on the cell surface. In this article, each of these transporters will be discussed, and their physiological importance will be illustrated by the Scott syndrome, a bleeding disorder caused by impaired lipid scrambling. Finally, phosphatidylserine exposure during apoptosis will be briefly discussed in relation to inhibition of translocase and simultaneous activation of scramblase.