Optical flow analysis reveals that Kinesin-mediated advection impacts the orientation of microtubules in the Drosophila oocyte

The orientation of microtubule (MT) networks is exploited by motors to deliver cargoes to specific intracellular destinations and is thus essential for cell polarity and function. Reconstituted in vitro systems have largely contributed to understanding the molecular framework regulating the behavior of MT filaments. In cells, however, MTs are exposed to various biomechanical forces that might impact on their orientation, but little is known about it. Oocytes, which display forceful cytoplasmic streaming, are excellent model systems to study the impact of motion forces on cytoskeletons in vivo. Here we implement variational optical flow analysis as a new approach to analyze the polarity of MTs in the Drosophila oocyte, a cell that displays distinct Kinesin-dependent streaming. After validating the method as robust for describing MT orientation from confocal movies, we find that increasing the speed of flows results in aberrant plus end growth direction. Furthermore, we find that in oocytes where Kinesin is unable to induce cytoplasmic streaming, the growth direction of MT plus ends is also altered. These findings lead us to propose that cytoplasmic streaming - and thus motion by advection - contributes to the correct orientation of MTs in vivo. Finally, we propose a possible mechanism for a specialized cytoplasmic actin network (the actin mesh) to act as a regulator of flow speeds to counteract the recruitment of Kinesin to MTs.