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

Abstract

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 non-spherical 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 the opposite sides along their long axes. When wrapped tightly inside endosomes, the shape and optical anisotropy of these Janus rods enable the simultaneous measurements of all three rotational angles (in-plane, out-of-plane, and longitudinal) 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 microtubule 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.