Shape fluctuations in biomembranes: the physics and the biology

By Dean Williams on July 6, 2016

Shape fluctuations of lipid membranes have intrigued cell biologists and physicists alike. Many biological membranes including the cell plasma membrane are known to ‘jiggle’ and ‘twitch’, that is to say fluctuate, with at least some energy consumption. But the purpose of these fluctuations has only been speculated upon, partly because they are very hard to measure. Authors Cornelia Monzel and Kheya Sengupta introduce their latest research, published in Journal of Physics D: Applied Physics:

The physics of a free, soft, thermally fluctuating membrane is well known but measurements on single membranes are highly challenging. In long standing collaborations with the groups of Rudolf Merkel (Research Centre Jülich), Ana Smith (University of Erlangen) and Udo Seifert (University of Stuttgart), we have been developing experimental and theoretical means to quantify and analyze fluctuations, either thermal or active, of simple membranes under controlled conditions.

Reflection Interference Contrast Microscopy (RICM) has traditionally been a powerful tool to quantify membrane adhesion. In early work in the lab of Erich Sackmann in the 70s and 80s, RICM was already used to quantify fluctuations. With the later introduction of Dual-Wavelength RICM (DW-RICM), the accuracy and robustness of RICM was significantly improved, especially in the context of fluctuations. The power of RICM lies in the capacity to measure the distance between a membrane and a wall quasi-instantaneously and with nanometer resolution (for standard camera and light source equipment the spatio-temporal resolution is 5 nm and 50 ms).

Working in the laboratory of Rudolf Merkel, and in collaboration with him, we recently developed a complementary technique called dynamic optical displacement spectroscopy (DODS) which improves on the temporal resolution (20 µs) and the spatial accessibility of RICM, but has a lower spatial resolution at 20 nm. DODS enables time-correlation measurements at almost every point on a biomembrane. We used it to measure membrane fluctuations in giant unilamellar vesicles (GUVs) and living cells.

Figure 1: GUV Structured Adhesion on Micropatterned Substrates. (a) Sketch of a GUV adhered to a protein micro-pattern. The membrane is free to fluctuate within the square region inside the protein grid, but on the grid, fluctuations are suppressed. Membrane fluctuations are measured at the distal and proximal surface (coloured boxes). (b) RICM micrograph of the GUV adhesion zone. The zoom depicts the reconstructed topography of the membrane above the substrate, rising to 75 nm at the centre, and the corresponding fluctuation map [nm²]. Taken from C Monzel and K Sengupta 2016 J. Phys. D: Appl. Phys. 49 243002 (c) DODS measurement of membrane fluctuations at positions indicated in (a). Analysis of the displacement autocorrelation function (dACF) yields fluctuation amplitude, membrane tension σ, interaction potential strength γ, and membrane-substrate distance h0. Ⓒ IOP Publishing, All Rights Reserved.

In a series of work focusing on adhesion to homogeneously distributed ligands we exploited the advantages of RICM to explore both adhesion and fluctuations. In collaboration with Laurent Limozin, we set in place a protocol to follow the membrane fluctuations as a GUV adhered. Two distinct regimes could be identified – adhesion first takes place at constant tension and then later enters a regime where the tension increases and finally saturates. In a separate work, in collaboration with Susanne Fenz and Ana Smith, we used suppression of fluctuations as a signature of the on-set of vesicle adhesion. New kinds of bond assemblies became visible and led us to predict the existence of adhesion patches with a non-compact distribution of bonds. In another series of experiments in collaboration with Rudolf Merkel, we controlled the geometry of the adhered membrane using micro-patterns, enabling systematic quantification of fluctuations for different tension and adhesive conditions. We found that fluctuation modes depend on the underlying adhesion pattern and that amplitudes and the membrane substrate distance can be systematically tuned by osmotic deflation. We combined DODS and RICM measurements on adhered patterned membranes to shed light on ultra-weak non-specific interactions.

In the case of cells, DODS enabled quantification of membrane undulations in red blood cells (RBCs) and human macrophages. We showed that membrane fluctuations carry signatures of cellular activity: stimulation by ATP (RBCs) and the cytokine interferon gamma (macrophages) demonstrated significant effects on fluctuation amplitude and relaxation time.

About the authors

Cornelia Monzel has been inspired by the physics of biological systems since her Master studies at Cambridge University (UK). During her PhD in the labs of Prof. R. Merkel (Research Centre Jülich (D)) and Dr. K. Sengupta (University Aix-Marseille II (F)) she specialized on model membrane fluctuations and their adhesion to micropatterned substrates. In 2012 she moved to the lab of Prof. M. Tanaka (Heidelberg University (D)) with a postdoc fellowship by the excellence cluster CellNetworks. She investigated hematopoietic stem cell adhesion and migration as well as apoptosis signalling of cancer cells on well-defined model membranes. In 2015 she moved to the lab of Prof. M. Dahan (Institut Curie (F)) with an IPGG and DAAD fellowship to develop a novel approach termed Magnetogenetics to control intracellular signalling with tailored magnetic nanoparticles applying magnetic fields. Her current interest comprises the (active) magnetogenetic modulation of cell signalling and the modelling of cellular processes by the design of (passive) cell mimetic model systems.

Kheya Sengupta did her PhD in Bangalore with V. A, Raghunathan on liquid-crystalline phases of phospholipids. In 2001 she moved to the laboratory of Erich Sackmann in Munich with an Alexander von Humboldt fellowship. She studied bio-mimetic systems and contributed to the development of multi-wavelength RIC microscopy. In 2004 she moved to University of Pennsylvania, where she worked with Paul Janmey on cell mechanics and cell adhesion. In 2005 she came back to Germany as a research associate at the Forschungszentrum Julich in the laboratory of Rudolf Merkel, where she continued her work on adhesion and continued to improve RIC microscopy. Since 2007 she is a researcher at the National Scientific Research Council of France (CNRS) and works at CINaM, Marseille, France. Her current interests include adhesion and mechanical properties of cells and cell mimetic model systems, and the interaction of such soft and living matter with nanoscale objects.