Understanding the mechanisms of supported lipid membrane reshaping into tubular networks using quantitative DIC microscopy

TL;DR

This study demonstrates that spin-coated, glass-supported lipid bilayers can spontaneously form tubular networks, which are quantitatively analyzed by qDIC to measure tube radii and infer 3D geometry. By comparing DOPC (Ld) and dcpc (So) lipids, the work reveals how surface tension and phase transitions regulate tube formation and radius, including a bilamellar-to-unilamellar lipid transfer at triple junctions that reshapes the membrane network. The authors derive a radius-tension relation $r=\sqrt{\eta/(2\sigma)}$ to explain observed changes during the dcpc Ld-So transition, and they demonstrate a versatile model system for dissecting lipid-lipid interactions governing membrane reshaping. Overall, the approach provides a powerful platform to study the biophysics of lipid tubes and their relevance to cellular membrane filaments, with potential to inform understanding of organelle and cytoskeletal membranes.

Abstract

Biological membranes are known to form various structural motifs, from lipid bilayers to tubular filaments and networks facilitating e.g. adhesion and cell-cell communication. To understand the biophysical processes underpinning lipid-lipid interactions in these systems, synthetic membrane models are crucial. Here, we demonstrate the formation of tubular networks from supported lipid membranes of controlled lipid composition on glass. We quantify tube radii using quantitative differential interference contrast (qDIC) and propose a biophysical mechanism for the formation of these structures, regulated by surface tension and lipid exchange with connected supported membranes. Two lipid types are investigated, namely DOPC and DC15PC, exhibiting a liquid disordered and a solid ordered phase at room temperature, respectively. Tube formation is studied versus temperature, revealing bilamellar layers retracting and folding into tubes upon DC15PC lipids transitioning from liquid to solid phase, which is explained by lipid transfer from bilamellar to unilamellar layers. This study introduces a novel model system for bilayer tubes, allowing to elucidate the biophysics of lipid-lipid interactions governing lipid membrane reshaping into tubular structures, important for our understanding of biological membrane filaments.

Figures (8)

• Figure 1: a) A qDIC $\delta(\mathbf{r})$ image showing a region of a spin-coated DOPC slb stack in which tubular networks have formed, scaled from $m$ = $-6$ mrad to $M$ = $+6$ mrad. b) The corresponding fluorescence image, scaled from $m$ = 80 to $M$ = 260 counts. c) The same region shown as a minimised qDIC phase $\phi(\mathbf{r})$ image, scaled from $m$ = -12 mrad to $M$ = 8 mrad. d) The phase profile across a tube; the line cut is shown in the inset. The yellow line is a fit using Eq.(\ref{['BLTubes:eq:birefringentfit']}) with the parameters shown, and the grey dashed line shows the fit function using $f=0$, i.e. without birefringence.
• Figure 2: Minimised qDIC phase image containing the region shown in Fig. \ref{['BLTubes:fig:DOPCBilayerTubes']}, scaled from -18 mrad to 6 mrad. Lines show measured profiles across tubes, with a colour encoding the resulting tube circumference on a scale as indicated.
• Figure 3: a) Minimised qDIC phase image showing a region with tubes formed on top of other bilayers, as well as tubes formed on glass, scaled from $m$ = 0 mrad to $M$ = 32 mrad. b,c) Line cuts through two different tubes, with phase profiles and fits shown in d). e) Phase step $2fa$ due to bilayer birefringence versus the length of the bilayer cross section for tubes formed on glass (blue) and on other bilayers (magenta). Coloured lines show the expected phase step for different cross-sectional shapes (see upper row of g) as a function of circumference. f) Fitted peak amplitude $a$ against width $c$, from the fitting to the DOPC tube phase profiles, with simulated curves for different cross-section shapes shown as coloured lines. The positions of the points corresponding to the phase profiles shown in d) are denoted by crosses. g) Illustration of the different cross-section shapes simulated in e) and f), with corresponding frame colour.
• Figure 4: a) Phase image of a unilamellar region with a border of higher optical thickness. b) Illustration of the proposed cross-section topology at the border, with a bilayer triple junction structure. c) Phase profile along the region indicated in a), and fit.
• Figure 5: a–f) qDIC phase images showing shape changes in a dcpc bilayer during cooling ($m = -33$ mrad, $M = 23$ mrad). False colour indicates lamellarity: unilamellar (red), bilamellar (green), and multilamellar (blue). Red circles mark selected membrane–surface pinning points. g,h) Illustrations of bilayer arrangements at locations marked by blue and green dashed lines in (a). i) Phase profile across a bilamellar edge with structural illustration. j) Illustration of the mechanism of bilamellar-to-tubular rearrangement, shown as plan-view and cross-section.