RESEARCH HIGHLIGHTS

CELL BIOLOGY Live-streaming the

A new approach uses beams of light to cell. Perturbations using this focused-light- membrane location, and demonstrated that direct cytoplasmic flows. induced cytoplasmic streaming (FLUCS) altered flow is sufficient to repolarize PAR-2 To get around the busy cell, molecules system matched the theoretical modeling in the worm zygote. Labeling of and don’t just rely on . Traffic can get of thermoviscous flows. Their optical setup revealed that induced flows move swept up in cytosolic movements or take a allows high-resolution fluorescence imaging the entire actomyosin cortex. cytoskeletal path. The study of this bustling in parallel, and to avoid heat stress, they use The researchers also used FLUCS in com- transport has been slowed by a lack of good small local temperature fluctuations while bination with genetically encoded fluores- tools. Because genetics cannot be used to stabilizing global temperatures with a tem- cent reporters in the cytoplasm to explore introduce rapid and precise local changes, perature-controlled stage. in brewer’s yeast. They found Moritz Kreysing and colleagues from the After a proof-of-principle demonstration, that both diffusion-based and flow-induced Max Planck Institute of Molecular Cell the researchers focused on embryogenesis dynamics of the reporters drop when yeast Biology and Genetics and the Technische in the roundworm Caenorhabditis elegans. cells are energy depleted, showing that in Universität Dresden in Germany have devel- Asymmetric cell fate decisions often rely on this low metabolic state, the yeast cytoplasm oped a different approach to investigate cyto- the differential partitioning of cytoplasmic arrests by entering a gel-like state. plasmic flow. factors before cell division; each daughter cell By enabling precise local perturbations, The researchers based their work on pre- receives a unique cytoplasmic inheritance the FLUCS approach expands the opportu- vious observations that laser light can gen- that specifies a different fate. The distribu- nities to study transport mechanisms in cells. erate moving temperature fields in viscous tion of partitioning-defective (PAR) proteins, Tal Nawy media. Using an infrared laser to locally heat for example, is hypothesized to polarize in RESEARCH PAPERS cytoplasm along a scanning axis, they found the zygote because of cortical cytoplasmic Mittasch, M. et al. Non-invasive perturbations of that they could precisely control the timing, flows. Using FLUCS, the research team was intracellular flow reveal physical principles of cell direction and localization of flow within a able to direct the PAR-2 protein to a specific organization. Nat. Cell Biol. 20, 344–351 (2018).

244 | VOL.15 NO.4 | APRIL 2018 | NATURE METHODS