Dynamic Force Adaptation of Lipid Droplets during Intracellular Transport in Cos1 Cells

Most bidirectional motion of organelle towards and away from the nucleus (Minus and Plus end of microtubules) inside cell is carried out by the combination cytoplasmic dynein and a Kinesin family of motors on the microtubule tracks. It is generally assumed that transport is static, and does not respond to local opposition to motion. However we observe that this is not the case: lipid droplets stalled by the application of external load adapt, and dynamically increase their force production. The average time for the adaptation is under ten seconds and it improved the ability of LDs to overcome the opposition to motion between three and four-fold. Although there is no observation of such a phenomena at the individual cargo level there is a well known adaptive behavior of the heart muscle explained by the Starling's law of heart1 wherein the more the heart is stretched, the stronger the cardiac muscle contracts. We hypothesize the load dependent adaptation in our case could involve reactivation process by a kinase that controls the activation and inactivation of motors through the switching complex. Adaptation occurs only in the minus-end direction, and requires the presence of NudE/L and Lis1.1. Solaro, R. J. Mechanisms of the Frank-Starling law of the heart: the beat goes on. Biophysical journal 93, 4095- 4096, doi:10.1529/biophysj.107.117200 (2007).