Throughout the last decades, genetic perturbations massively advanced our molecular understanding of cell biological processes. At the same time however, the spatio-temporal organization of cells and developing embryos is widely believed to also depend on physical transport processes such as diffusion and directed intracellular flows. Problematically, physical models to unite reaction and transport as drivers of morphogenesis remain difficult to test experimentally. As an example: how would one change direction or temporal persistence of cytoplasmic flows to test their role during embryogenesis?
Here we demonstrate focused-light induced cytoplasmic-streaming (FLUCS). FLUCS uses light controlled thermoviscous expansion phenomena to induce well-defined flows in single cells and developing embryos. These non-invasive flows are localized, directed, highly dynamical, probe-free, and non-invasive. By controlling flows inside the cytoplasm of one-cell C. elegans embryos, we directly demonstrate the causal role of flows for the establishment of the head-to-tail body axis (aka PAR-polarization). Specifically, we find that i) cytoplasmic flows transport PAR-2 proteins towards the membrane, thus helping to define the posterior pole. ii) we show that induced flows of the actomyosin cell cortex transport and even invert PAR polarization. This light-controlled re-localization of native proteins reveals iii) the down-stream phenotype of embryos with an inverted body plan, which demonstrates that body axis formation is a bi-stable process.
Moreover, we utilize FLUCS for active and probe-free micro-rheology, revealing a fluid-to-solid transition of the cytoplasm in energy-depleted yeast cells. On the subcellular scale, we show how hydrodynamic flows can be reversibly induced within the cell nucleus.
We conclude by emphasizing the enormous biomedical potential of FLUCS to study and guide the spatio-temporal organization of cells and embryos by light.
i) Mittasch et al, Nature Cell Biology 20 (2018)
News and Views: Kruse, Chiaruttini, Roux, Nature Cell Biol 20 (2018)