Anisotropic Subdiffusion in the Cytoplasm of Living Cells

The cytoplasm of eukaryotic cells is a complex fluid that harbors a variety of membrane-enveloped organelles with distinct spatial distributions and duties. Peroxisomes, for example, are vesicular entities with radii in the sub-micron range, dispersed all over the cytoplasm to provide oxidative reaction vessels across the cell. Here, we have tracked peroxisomes as biologically relevant tracer particles to explore the local structure of the cytoplasm. Focusing on the vast majority of (sub)diffusively moving peroxisomes, we show that these display a significant motion anisotropy even in the absence of intact actin filaments and microtubules or when the endoplasmic reticulum (ER) is disrupted. In untreated cells, the local preference direction aligns with a global axis that correlates with the cell's long axis; disrupting the cytoskeleton or the ER progressively randomizes this alignment. Altogether, our data indicate that the cytoplasm features a weak nematic substructure rather than being an isotropic fluid.