Regenerating the nuclear envelope during exit from mitosis
Dr Jeremy Carlton, KCL and Francis Crick Institute, UK
During cell division, as well as separating their duplicated genomes, cells must also deconstruct, separate and then reconstruct many of their cytoplasmic organelles. Mammalian nuclei are surrounded by a double-membraned organelle called the nuclear envelope that is continuous with the endoplasmic reticulum. During mitotic exit, sheets of ER envelope the forming daughter nuclei and become fused together to generate a sealed nuclear envelope. In addition to its role in membrane abscission during cytokinesis, viral budding, endosomal sorting and plasma membrane repair, the endosomal sorting complex required for transport-III (ESCRT-III) machinery has recently been shown to participate in nuclear envelope sealing during mitotic exit. Nuclear envelope localisation of ESCRT-III is dependent upon the ESCRT-III component CHMP7 and the inner nuclear membrane protein LEM2. However, it is unclear how ESCRT-III actually engages nuclear membranes. Here, Dr Carlton shows that the N-terminus of CHMP7 acts as a novel membrane-binding module. This membrane-binding ability allows CHMP7 to bind to the endoplasmic reticulum (ER), an organelle continuous with the NE, and provides a platform to direct NE-recruitment of ESCRT-III during mitotic exit. Dr Carlton also identifies novel activities in the C-terminus of CHMP7 that restrict its activity to the inner-nuclear membrane and help us understand how this complex can help regenerate the nucleus. Dr Carlton finds that mutations that compromise CHMP7 function also prevent assembly of downstream ESCRT-III components at the reforming NE and proper establishment of post-mitotic nucleo-cytoplasmic compartmentalisation. These data identify a novel membrane-binding activity within an ESCRT-III subunit that is essential for post-mitotic nuclear regeneration.
Synthetic biology of minimal cellular systems
Professor Petra Schwille, Max Planck Institute of Biochemistry, Germany
In recent years, biophysics has accumulated an impressive selection of cutting-edge techniques to analyse biological systems with ultimate sensitivity and precision, down to the single molecule level. However, a strictly quantitative application of most of these techniques in living cells or organisms has been extremely challenging, because of the enormous complexity and redundancy of cellular modules and elements. The more physiological a system under study, the harder it is to define a manageable number of relevant control parameters. This renders it necessary to accumulate ever more sophisticated techniques and assays in order to master a single biological problem, and thus, often extends experiments and publications in the life sciences to hardly manageable sizes. An alternative approach is to limit the methodological toolbox in a biological study without sacrificing the biophysical standards of quantitation. Instead, the biological phenomenon will have to be reduced to its fundamental features by reconstituting it in a bottom-up approach. The strive for identifying such minimal biological systems, particularly of subcellular structures or modules, has in the past years been very successful, and crucial in vitro experiments with reduced complexity can nowadays be performed, e.g., on reconstituted cytoskeleton and membrane systems. As a particularly exciting example for the power of minimal systems, we recently demonstrated the self-organization of MinCDE, essential proteins of the bacterial cell division machinery, leading to a protein-based pacemaker and spatiotemporal cue for downstream events, such as the positioning of divisome proteins. In her talk, Petra Schwille will discuss some recent results of her group’s work on membrane-based systems, using single molecule optics and biological reconstitution assays. Petra will further discuss the perspective of assembling a minimal system to reconstitute cell division.
Buckling of an epithelium growing under spherical confinement
Dr Aurelien Roux, University of Geneva, Switzerland
Many organs, such as the gut or the spine are formed through folding of an epithelium. While genetic regulations of cell fates leading to epithelium folding have been investigated, mechanisms by which forces sufficient to deform the epithelium are generated are less studied. Here, Aurelien Roux shows that cells forming an epithelium on to the inner surface of spherical elastic shells protrude inward while growing. By measuring the pressure and local forces applied onto the elastic shell, Aurelien shows that this folding is induced by compressive stresses arising within the epithelial layer: while growing under spherical confinement, epithelial cells are subjected to lateral compression, which induces epithelium buckling. While several fold initiations can be observed within one capsule, final shapes often show a single fold. These findings are recapitulated by an analytical model of the epithelium buckling from which the Roux group can estimate local compressive forces and rigidity. As proposed for gastrulation or neurulation, this study shows that forces arising from epithelium proliferation are sufficient to drive epithelium folding.
Signalling reactions on membrane surfaces: the roles of space, force, and time
Professor Jay Groves, University of California, Berkeley, USA
Most intracellular signal transduction reactions take place on the membrane. The membrane provides much more than just a surface environment on which signalling molecules are concentrated. There is a growing realization that multiple physical and chemical mechanisms allow the membrane to actively participate in the signalling reactions. Using a combination of single molecule imaging and spectroscopic techniques, Professor Jay Groves’ research seeks to directly resolve the actual mechanics of signalling reactions on membrane surfaces both in reconstituted systems and in living cells. These observations are revealing new insights into cellular signalling processes as well as some unexpected functional behaviours of proteins on the membrane surface. The Groves’ lab has recently discovered a type of signalling reaction phenomenon that enables geometrical features of the membrane surface to couple directly to the outcome of a signalling process.
Harnessing Nature's ability to create membrane compartmentalisation
Dr Paul Beales, University of Leeds, UK
Dr Barbara Ciani, University of Sheffield, UK
A biological cell can be thought of as a complex chemical reactor where vast numbers of interactions are simultaneously taking place. To prevent unwanted cross-talk and interference within the ‘noise’ of all these concurrent chemical pathways, a cell compartmentalises these processes localizing different functions within individual membrane-bound structures (organelles). Confinement of chemical processes also allows a cell to maintain incompatible environments that are optimal for each organelle's function, which would not be possible within a single ‘pot’. If we are to mimic this complexity within synthetic ‘nanoreactors’, we need to develop ways of mimicking cellular compartmentalisation within synthetic structures. Here, Dr Barbara Ciani and Dr Paul Beales will show that it is possible to create multi-compartment architectures, in vitro, using a purified membrane remodelling protein complex. Barbara and Paul will also show how this in vitro system allows us to learn what controls the membrane shaping action of these proteins and therefore regulate the encapsulation of cargo.