Eliza Morris

Motion in Cytoskeletal Networks

Inside our cells molecular motors transport materials, enabling growth and other cellular functions. While transporting cargo these motors walk along filamentous tracks that act as roadways throughout the cell. The environment in which these motors walk is a robust mechanical system with stiffness similar to that of Jell-O. This research seeks to help elucidate the effects of this complex cellular environment on the cargo transport process. The molecular motors inside our cells are moving along a tangled mess of filamentous tracks and through an enormously large number of other filaments. Despite being comprised of roughly 70% water the cell is actually a robust mechanical system with stiffness similar to that of Jell-O. Its mechanical properties are primarily governed by the cytoskeleton a complex networks of filaments of varying size and stiffness. Extending throughout the cytoplasm, the cytoskeleton provides structural integrity and stability (or flexibility, depending on the circumstances) to the cell. As the cell is more like Jell-O than water, and the cytoskeleton is largely responsible for this alteration, we attempt to understand cargo transport inside the cell through observing directed motion inside a network of filaments. In this research we aim to observe, measure, interpret, and ultimately understand the effects of cargo transport on the surrounding environment, as well as the impact of that environment on the transport process. To do this we use both in vivo and in vitro experiments to characterize drag and flow induced by moving objects in the cytoskeleton, or, in the in vitro case, in extracted filament networks. Using magnetic tweezers we study the relationship between bead motion and filament response in various biopolymer networks (shown in Fig. 1). We observe actual cargo transport in the presence of surrounding networks (shown in Fig. 2).

Movie 1: Confocal images of bead displacements and recovery in response to applied forces in 12 uM partially labeled entangled F-actin (1 labeled filament : 300 unlabeled filaments). Red indicates initial position at zero force. Green indicates displacement at a given force.

Figure 2: Time series of fluorescence images of a bead attached to kinesin traveling along a surface attached microtubule. There is no surrounding networking. The velocity is roughly 1 um/s. Using optical trapping we confine beads being carried as cargo by motors inside cells enabling the determination of the forces required to halt cargo transport in vivo (Shown in Fig. 3).

Figure 3: Bright field image of vesicles containing magnetic beads being transported by molecular motors along microtubules within a cell.