Tether-supported biomembranes with α-helical peptide-based anchoring constructs

Supported Biomembrane Systems Incorporating Multiarm Polymers and Bioorthogonal Tethering

To functionalize interfaces with supported biomembranes and membrane proteins, the challenge is to build stabilized and supported systems that mimic the native lipid microenvironment. Our objective is to control substrate-to-biomembrane spacing and the tethering chemistry so proteoliposomes can be fused and conjugated without perturbation of membrane protein function. Furthermore, the substrates need to exhibit low protein and antibody nonspecific binding to use these systems in assays. We have employed protein orthogonal coupling schemes in concert with multiarm poly(ethylene glycol) (PEG) technology to build supported biomembranes on microspheres. The lipid bilayer structures and tailored substrates of the microsphere-supported biomembranes were analyzed via flow cytometry, confocal fluorescence, and super-resolution imaging microscopy, and the lateral fluidity was quantified using fluorescence recovery after photobleaching (FRAP) techniques. Under these conditions, the 4-arm-PEG20,000-NH2 based configuration gave the most desirable tethering system based on lateral diffusivity and coverage.

Nanomechanical Properties of Artificial Lipid Bilayers Composed of Fluid and Polymerizable Lipids

Polymerization enhances the stability of a planar supported lipid bilayer (PSLB) but it also changes its chemical and mechanical properties, attenuates lipid diffusion, and may affect the activity of integral membrane proteins. Mixed bilayers composed of fluid lipids and poly(lipids) may provide an appropriate combination of polymeric stability coupled with the fluidity and elasticity needed to maintain the bioactivity of reconstituted receptors. Previously (Langmuir, 2019, 35, 12483-12491) we showed that binary mixtures of the polymerizable lipid bis-SorbPC and the fluid lipid DPhPC form phase-segregated PSLBs composed of nanoscale fluid and poly(lipid) domains. Here we used atomic force microscopy (AFM) to compare the nanoscale mechanical properties of these binary PSLBs with single-component PSLBs. The elastic (Young's) modulus, area compressibility modulus, and bending modulus of bis-SorbPC PSLBs increased upon polymerization. Before polymerization, breakthrough events at forces below 5 nN were observed, but after polymerization, the AFM tip could not penetrate the PSLB up to an applied force of 20 nN. These results are attributed to the polymeric network in poly(bis-SorbPC), which increases the bilayer stiffness and resists compression and bending. In binary DPhPC/poly(bis-SorbPC) PSLBs, the DPhPC domains are less stiff, more compressible, and are less resistant to rupture and bending compared to pure DPhPC bilayers. These differences are attributed to bis-SorbPC monomers and oligomers present in DPhPC domains that disrupt the packing of DPhPC molecules. In contrast, the poly(bis-SorbPC) domains are stiffer and less compressible relative to pure PSLBs; this difference is attributed to DPhPC filling the nm-scale pores in the polymerized domains that are created during bis-SorbPC polymerization. Thus, incomplete phase segregation increases the stability of poly(bis-SorbPC) but has the opposite, detrimental effect for DPhPC. Overall, these results provide guidance for the design of partially polymerized bilayers for technological uses.

Phase Composition Control in Microsphere-Supported Biomembrane Systems

The popularization of studies in membrane protein lipid phase coexistence has prompted the development of new techniques to construct and study biomimetic systems with cholesterol-rich lipid microdomains. Here, microsphere-supported biomembranes with integrated α-helical peptides, referred to as proteolipobeads (PLBs), were used to model peptide/protein partitioning within DOPC/DPPC/cholesterol phase-separated membranes. Due to the appearance of compositional heterogeneity and impurities in the formation of model PLB assemblies, fluorescence-activated cell sorting (FACS) was used to characterize and sort PLB populations on the basis of disordered phase (Ld) content. In addition, spectral imaging was used to assess the partitioning of FITC-labeled α-helical peptide between fluorescently labeled Ld phase and unlabeled ordered phase (Lo) phase lipid microdomains. The apparent peptide partition coefficient, Kp,app, was measured to be 0.89 ± 0.06, indicating a slight preference of the peptide for the Lo phase. A biomimetic motif of the Lo phase concentration enhancement of the biotinyl-peptide ligand display in proteolipobeads was also observed. Finally, peptide mobility was measured by FRAP separately in each lipid phase, yielding diffusivities of 0.036 ± 0.005 and 0.014 ± 0.003 μm2/s in the Ld and Lo phases, respectively.

Biodegradable, Tethered Lipid Bilayer-Microsphere Systems with Membrane-Integrated α-Helical Peptide Anchors

Supported lipid bilayers (SLBs) are ideally suited for the study of biomembrane-biomembrane interactions and for the biomimicry of cell-to-cell communication, allowing for surface ligand displays that contain laterally mobile elements. However, the SLB paradigm does not include three-dimensionality and biocompatibility. As a way to bypass these limitations, we have developed a biodegradable form of microsphere SLBs, also known as proteolipobeads (PLBs), using PLGA microspheres. Microspheres were synthesized using solvent evaporation and size selected with fluorescence activated cell sorting (FACS). Biomembranes were covalently tethered upon fusion to microsphere supports via short-chain PEG spacers connecting membrane-integrated α-helical peptides and the microsphere surface, affecting membrane diffusivity and mobility as indicated by confocal FRAP analysis. Membrane heterogeneities, which are attributed to PLGA hydrophobicity and rough surface topography, are curtailed by the addition of PEG tethers. This method allows for the presentation of tethered, laterally mobile biomembranes in three dimensions with functionally embedded attachment peptides for mobile ligand displays.