Highlights

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In the cell, the mitotic spindle is a structure that forms during cell division and segregates the chromosomes into the two future daughter cells. Spindles are made of dynamic filaments called microtubules that are continuously transported towards the two opposite poles of the spindle by molecular motors. However, scientists still do not understand how these poleward flows are generated and how they lead to spindle self-organization. Researchers in the group led by Jan Brugués at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG), the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) and the Center for Systems Biology Dresden (CSBD), worked together with Frank Jülicher’s research group at the MPI-PKS to understand how such poleward flows are generated in spindles. In 2018, the Brugués lab was able to show that the size of spindles is controlled by microtubule branching: In the vicinity of DNA, new microtubules branch off from mother microtubules like branches in a tree. However, such a branching process naturally leads to microtubules branching outwards from chromosomes, whereas in real spindles they branch inwards to interact with chromosomes during segregation. In the current study, published in the journal Nature Physics, the scientists combined in vitro experiments with physical models to show that the poleward flows together with a gelation process driven by motor crosslinking, allows for the correct microtubule inward branching observed in spindles. Benjamin Dalton an author in the study, explains, “The spindle is a highly dynamic structure where its building blocks are constantly created, transported and destroyed within seconds. Still, the spindle can survive for as long as an hour and the microtubule flows are remarkably constant throughout the structure. It’s difficult to reconcile these things.” David Oriola, another author in the study explains, “By tracking the movement of single microtubules using fluorescent microscopy, we found that the spindle did not behave as a simple fluid, but rather as a gel.” Combining large-scale simulations with experimental data, the researchers found that gelation is necessary for the generation of the poleward flows, and in turn, such flows are in charge of organizing the microtubule network such that microtubules point inwards rather than outwards.

Dalton et al.: A gelation transition enables the self-organization of bipolar metaphase spindles. Nature Physics, 10 February 2022, doi: 10.1038/s41567-021-01467-x

Steffen Rulands receives funding within the REDESIGN consortium to bring personalized cancer therapy to patients using organoids and predictive models. The Saxon Ministry for Science, Arts and Tourism is funding four research projects for individualized cancer therapy with around 2.3 million euros. The projects will start this year and run for three years. One of the projects is the REDESIGN consortium with the goal to develope personalized medical therapy of Gastric Cancer. This type of cancer is difficult to treat once resistance to standard chemotherapy develops. Therefore, there is a need to tailor the therapy to each patient. The REDESIGN consortium is led by Daniel E. Stange (University Hospital Carl Gustav Carus Dresden), in collaboration with Steffen Rulands, Bon-Kyoung Koo (Institute of Molecular Biotechnology of the Austrian Academy of Sciences), and Mette N. Svendsen (University of Copenhagen). Steffen Rulands will use functional drug response data generated from patient-derived organoids to predict the efficacy of different regimines of chemotherapy. These organoids are initially grown by clinical research labs from patient tumor tissue samples’and then exposed to varying cocktails of chemotherapy. Organoids are sequenced before and after treatment to detect mutations that may affect the outcome of therapy. Moreover, organoid growth speed, among other parameters, are measured during the treatment. These datasets will be used by Steffen's group to develop quantitative predictions of treatment efficiency using two complementary approaches, stochastic models and deep neural networks. The combination of the two methods will allow for high predictive and explanatory power while tackling the complexity of the data. This complexity arises from the presence of mixed populations of cells in the tumor and different constellations of mutations between tumors from different patients. Eventually, the developed model will help understand how resistance to therapy develops. More importantly, it will predict how any given patient would respond to chemotherapy based on the constellation of mutations found in the tumor. It will also assess the likelihood of tumor relapse in patients for each possible therapeutic approach and recommend the therapy that targets the tumor cells carrying mutations that make the tumor more aggressive and relapsing.

Life starts with one cell. When an organism develops, dividing cells specialize to form the variety of tissues and organs that build up the adult body, while keeping the same genetic material – contained in our DNA. In a process known as transcription, parts of the DNA – the genes ¬– are copied into a messenger molecule -the ribonucleic acid (RNA) – that carries the information needed to produce proteins, the building blocks of life. The parts of our DNA that are read and transcribed determine the fate of our cells. The readers of the DNA are proteins called transcription factors: they bind to specific sites on the DNA and activate the transcription process. How they recognize which location on the DNA they need to bind to and how these are distinguished from other random binding sites in the genome remains an open question. Scientists at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG) and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS), both located in Dresden, show that thousands of individual transcription factors team up and interact with each other. They collectively wet the DNA surface by forming liquid droplets that can identify clusters of binding sites on the DNA surface.

Jose A. Morin et al: Sequence dependent surface condensation of a pioneer transcription factor on DNA. Nature Physics. 03. February 2022, doi: 10.1038/s41567-021-01462-2

Application deadline: 13 March 2022. The ELBE postdoctoral fellows program addresses independent researchers on the postdoctoral level, who come with their own research proposal and freely choose which groups to affiliate with. The program provides an ideal springboard to an independent research career in systems biology, theoretical biophysics, computational biology, and related areas. Please click on the link- button to see the full advertisement!

The smallest fish in the world, the Paedocypris, measures only 7 millimeters. This is nothing compared to the 9 meters of the whale shark. The small fish shares many of the same genes and the same anatomy with the shark, but the dorsal and caudal fins, gills, stomach and heart, are thousands of times smaller! How do organs and tissues of this miniature fish stop growing very quickly, unlike those of their giant cousin? A multidisciplinary team led by scientists from the University of Geneva (UNIGE), Switzerland, and the Max Planck Institute for the Physics of Complex Systems (MPIPKS) was able to answer this fundamental question by studying its physics and using mathematical equations, as revealed by their work published in the journal Nature.

M. R. Michaelidi et al., Nature (2021)

Recently, the possibility of inducing superconductivity for electrons in two-dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via nonadiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifetime. For realistic parameters, also including cavity loss, this results in a critical temperature which can be more than 1 order of magnitude larger than the BCS prediction. Moreover, a finite average number of photons (as can be achieved by incoherently pumping the cavity) adds to the fluctuations and leads to a further enhancement of the critical temperature.

A. Chakraborty and F. Piazza, Phys. Rev. Lett. 127, 177002 (2021)

We review the recent developments and the current status in the field of quantum-gas cavity QED. Since the first experimental demonstration of atomic self-ordering in a system composed of a Bose–Einstein condensate coupled to a quantized electromagnetic mode of a high-Q optical cavity, the field has rapidly evolved over the past decade. The composite quantum-gas-cavity systems offer the opportunity to implement, simulate, and experimentally test fundamental solid-state Hamiltonians, as well as to realize non-equilibrium many-body phenomena beyond conventional condensed-matter scenarios. This hinges on the unique possibility to design and control in open quantum environments photon-induced tunable-range interaction potentials for the atoms using tailored pump lasers and dynamic cavity fields. Notable examples range from Hubbard-like models with long-range interactions exhibiting a lattice-supersolid phase, over emergent magnetic orderings and quasicrystalline symmetries, to the appearance of dynamic gauge potentials and non-equilibrium topological phases. Experiments have managed to load spin-polarized as well as spinful quantum gases into various cavity geometries and engineer versatile tunable-range atomic interactions. This led to the experimental observation of spontaneous discrete and continuous symmetry breaking with the appearance of soft-modes as well as supersolidity, density and spin self-ordering, dynamic spin-orbit coupling, and non-equilibrium dynamical self-ordered phases among others. In addition, quantum-gas-cavity setups offer new platforms for quantum-enhanced measurements. In this review, starting from an introduction to basic models, we pedagogically summarize a broad range of theoretical developments and put them in perspective with the current and near future state-of-art experiments.

F. Mivehvar et al. Adv. Phys. 70, 1 (2021)

Quantum phase transitions are central to our understanding of why matter at very low temperatures can exhibit starkly different properties upon small changes of microscopic parameters. Accurately locating those transitions is challenging experimentally and theoretically. Here, we show that the antithetic strategy of forcing systems out of equilibrium via sudden quenches provides a route to locate quantum phase transitions. Specifically, we show that such transitions imprint distinctive features in the intermediate-time dynamics, and results after equilibration, of local observables in quantum chaotic spin chains. Furthermore, we show that the effective temperature in the expected thermal-like states after equilibration can exhibit minima in the vicinity of the quantum critical points. We discuss how to test our results in experiments with Rydberg atoms and explore nonequilibrium signatures of quantum critical points in models with topological transitions.

A. Haldar et al. Phys. Rev. X 11, 031062 (2021)

Quantum spin liquids provide paradigmatic examples of highly entangled quantum states of matter. Frustration is the key mechanism to favor spin liquids over more conventional magnetically ordered states. Here we propose to engineer frustration by exploiting the coupling of quantum magnets to the quantized light of an optical cavity. The interplay between the quantum fluctuations of the electro-magnetic field and the strongly correlated electrons results in a tunable long-range interaction between localized spins. This cavity-induced frustration robustly stabilizes spin liquid states, which occupy an extensive region in the phase diagram spanned by the range and strength of the tailored interaction. This occurs even in originally unfrustrated systems, as we showcase for the Heisenberg model on the square lattice.

A. Chiocchetta et al. Nat. Commun. 12, 5901 (2021)