Transbilayer coupling of lipid dynamics.

Lipids are organized in plasma membranes in a bilayer with distinctive outer exoplasmic and inner cytoplasmic leaflets. These leaflets differ greatly in terms of their lipid and attached protein compositions. In addition to this asymmetry, membranes are also heterogeneous with respect to their lateral organization (1,2). Recently, model membranes have been developed that are able to mimic lateral heterogeneity as well as transbilayer asymmetry of their constituent lipids (3,4). In these systems, the exoplasmic mimicking leaflets contain high-chain-melting temperature lipids, low-chain-melting temperature lipids, and cholesterol. These ternary mixtures, by themselves, form phase-separated membranes with microscopic lipid-raft-like liquid-ordered and liquid-disordered domains. It turns out that in asymmetric model membranes, these domains can induce the formation of domains in the opposing leaflet even when they are composed of lipids that, by themselves, wouldn’t separate into different phases. This is important, because it might be one mechanism by which signaling (raft-) components are brought together at the two sides of the plasma membrane. Recently, it has been shown that this process modulates the spatial distribution of the anionic lipids phosphatidylserine and phosphatidylinositol-4,5-bisphosphate, which then modulates the binding of C2 domains of the synaptic vesicle-membrane protein synaptotagmin1 (5). Which interactions are responsible for this lipid-mediated transbilayer coupling? A few theoretical models have been published in which acyl-chain interdigitation, midplane surface tension, transbilayer cholesterol dynamics, and electrostatic interactions are discussed as possible mechanisms (6,7). However, these interactions had not been systematically explored previously. In this volume of the Biophysical Journal, Chiantia and London (8) are taking first steps toward a deeper understanding of this phenomenon by reporting results about the influence of acyl-chain length and saturation on the interleaflet coupling in asymmetric bilayers. The authors make use of their recently developed liposome-based asymmetric membrane system (9,10), with which they can determine diffusion coefficients (by fluorescence correlation spectroscopy) and order parameters (from fluorescence lifetime) in the two leaflets of the lipid bilayer independently. Simple lipid compositions were used in both leaflets: the inner leaflet contained only a single species of phosphatidylcholine (PC) and the outer leaflet contained a mixture of sphingomyelin and PC. Stronger coupling, measured by a reduced diffusion coefficient in the inner PC leaflet, was achieved by sphingomyelin lipids with longer acyl chains in the outer leaflet and/or by PC lipids with one saturated and one unsaturated acyl chain in the inner leaflet. Surprisingly, the packing order of the PC-leaflet was not influenced by the more ordered outer leaflet. Even though these results cannot explain the coupling between coexisting lipid phases in asymmetric lipid raft membranes, they are exciting because they emphasize the role of lipid clusters or domains that go beyond those of lipid rafts. Under the described conditions, clusters of slowly diffusing lipids can induce transient domains that are defined by their diffusion dynamics rather than their acyl-chain order. This type of coupling should happen independently of how the inducing lipid cluster was formed in the first place. Possible mechanisms include interactions with integral or peripheral membrane proteins, or the already mentioned phase separation in raftlike lipid mixtures. Chiantia and London present a methodology that, in the future, perhaps in combination with recent developments in super-resolution microscopy (11), will give us a better understanding of the molecular underpinning of lipid-mediated interleaflet coupling. In particular, it will be interesting to see how the inclusion of cholesterol or proteins will affect lipid dynamics and membrane order in model and cell membranes.