Single-Janus Rod Tracking Reveals the Rock-and-Roll of Endosomes in Living Cells

Endosomes in cells are known to move directionally along microtubules, but their rotational dynamics have rarely been investigated. Even less is known, specifically, about the rotation of nonspherical endosomes. Here we report a single Janus rod rotational tracking study to reveal, the rich rotational dynamics. of rod-shaped endosomes in living cells. The rotational reporters were Janus rods that display patches of different fluorescent colors on opposite sides along their long axes. When the Janus rods are wrapped tightly inside endosomes, their shape and optical anisotropy allow the simultaneous measurements of all three rotational angles (in-plane, out-of-plane, and longitadinal) and the translational motion of single endosomes with high spatiotemporal resolutions. We demonstrate that endosomes undergo, in-plane, rotation and rolling during intracellular transport and that such rotational dynamics are driven by rapid micrcittibule fluctuations. We reveal for the first time the rock-and-roll of endosomes in living cells and how the intracellular environment Modifies such rotational dynamics. This study demonstrates a unique application of Janus particles as imaging probes in the elucidation of fundamental biological questions: