Aqueous-filled polymer microcavity arrays: versatile & stable lipid bilayer platforms offering high lateral mobility to incorporated membrane proteins.

A key prerequisite in an ideal supported lipid bilayer based cell membrane model is that the mobility of both the lipid matrix and its components are unhindered by the underlying support. This is not trivial and with the exception of liposomes, many of even the most advanced approaches, although accomplishing lipid mobility, fail to achieve complete mobility of incorporated membrane proteins. This is addressed in a novel platform comprising lipid bilayers assembled over buffer-filled, arrays of spherical cap microcavities formed from microsphere template polydimethoxysilane. Prior to bilayer assembly the PDMS is rendered hydrophilic by plasma treatment and the lipid bilayer prepared using Langmuir Blodgett assembly followed by liposome/proteoliposome fusion. Fluorescence Lifetime Correlation Spectroscopy confirmed the pore suspended lipid bilayer exhibits diffusion coefficients comparable to free-standing vesicles in solution. The bilayer modified arrays are highly reproducible and stable over days. As the bilayers are suspended over deep aqueous reservoirs, reconstituted membrane proteins experience an aqueous interface at both membrane interfaces and attain full lateral mobility. Their utility as membrane protein platforms was exemplified in two case studies with proteins of different dimensions in their extracellular and cytoplasmic domains reconstituted into DOPC lipid bilayers; Glycophorin A, and Integrin αIIbß3. In both cases, the proteins exhibited 100% mobility with high lateral diffusion coefficients.

The lateral diffusion and fibrinogen induced clustering of platelet integrin αIIbß3 reconstituted into physiologically mimetic GUVs.

Platelet integrin αIIbß3 is a key mediator of platelet activation and thrombosis. Upon activation αIIbß3 undergoes significant conformational rearrangement, inducing complex bidirectional signalling and protein recruitment leading to platelet activation. Reconstituted lipid models of the integrin can enhance our understanding of the structural and mechanistic details of αIIbß3 behaviour away from the complexity of the platelet machinery. Here, a novel method of αIIbß3 insertion into Giant Unilamellar Vesicles (GUVs) is described that allows for effective integrin reconstitution unrestricted by lipid composition. αIIbß3 was inserted into two GUV lipid compositions that seek to better mimic the platelet membrane. First, "nature's own", comprising 32% DOPC, 25% DOPE, 20% CH, 15% SM and 8% DOPS, intended to mimic the platelet cell membrane. Fluorescence Lifetime Correlation Spectroscopy (FLCS) reveals that exposure of the integrin to the activators Mn(2+) or DTT does not influence the diffusion coefficient of αIIbß3. Similarly, exposure to αIIbß3's primary ligand fibrinogen (Fg) alone does not affect αIIbß3's diffusion coefficient. However, addition of Fg with either activator reduces the integrin diffusion coefficient from 2.52 ± 0.29 to µm(2) s(-1) to 1.56 ± 0.26 (Mn(2+)) or 1.49 ± 0.41 µm(2) s(-1) (DTT) which is consistent with aggregation of activated αIIbß3 induced by fibrinogen binding. The Multichannel Scaler (MCS) trace shows that the integrin-Fg complex diffuses through the confocal volume in clusters. Using the Saffman-Delbrück model as a first approximation, the diffusion coefficient of the complex suggests at least a 20-fold increase in the radius of membrane bound protein, consistent with integrin clustering. Second, αIIbß3 was also reconstituted into a "raft forming" GUV with well defined liquid disordered (Ld) and liquid ordered (Lo) phases. Using confocal microscopy and lipid partitioning dyes, αIIbß3 showed an affinity for the DOPC rich Ld phase of the raft forming GUVs, and was effectively excluded from the cholesterol and sphingomyelin rich Lo phase. Activation and Fg binding of the integrin did not alter the distribution of αIIbß3 between the lipid phases. This observation suggests partitioning of the activated fibrinogen bound αIIbß3 into cholesterol rich domains is not responsible for the integrin clustering observed.