Super Resolution Imaging of Highly Curved Membrane Structures in Giant Unilamellar Vesicles Encapsulating Polymer Solutions

Abstract

Giant vesicles encapsulating polymer solutions can undergo morphological transformations upon deflation (Adv. Mater. Interfaces 4:1600451, 2017). The vesicle membrane can initiate budding and nanotube formation. The nanotube diameter is related to the membrane spontaneous curvature. Based on our previous findings (ACS Nano 10:463, 2016), nanotubes with membrane composition of the liquid-disordered phase have a diameter below optical resolution. Direct imaging of the intrinsic contact angle formed by the membrane and the two phases in budded vesicles (PRL, 103:238103, 2009) is also hindered by the poor axial resolution in confocal microscopy. Here, we use a super resolution technique, stimulated emission depletion (STED) microscopy in both 2D and 3D mode combined with microfluidics to study these remarkable membrane morphologies. We first designed a microfluidic device which can dramatically increase the trapping efficiency of giant unilamellar vesicles (GUVs) and improve the solution exchange rate. Then, with a resolution less than 35nm from STED microscopy, we visualize the membrane nanotube structures with unprecedented detail, and compare the directly measured nanotube diameters with previously reported theoretical and experimental ones. Additionally, by manipulating the height of the microfluidic channels, we pinch and orient the budded vesicles to image the intrinsic contact angle with lateral STED resolution. These highly curved membrane structures imaged with super resolution microscopy will serve to deepen and expand our understanding of biomembranes. This work is part of the MaxSynBio consortium, jointly funded by the Federal Ministry of Education and Research of Germany and the Max Planck Society.