A Mechanism for Cytoplasmic Streaming: Kinesin-Driven Alignment of Microtubules and Fast Fluid Flows

Abstract

The transport of cytoplasmic components can be profoundly affected by hydrodynamics. Cytoplasmic streaming in Drosophila oocytes offers a striking example. Forces on fluid from kinesin-1 are initially directed by a disordered meshwork of microtubules, generating minor slow cytoplasmic flows. Subsequently, to mix incoming nurse cell cytoplasm with ooplasm, a subcortical layer of microtubules forms parallel arrays that support long-range, fast flows. To analyze the streaming mechanism, we combined observations of microtubule and organelle motions with detailed mathematical modeling. In the fast state, microtubules tethered to the cortex form a thin subcortical layer and undergo correlated sinusoidal bending. Organelles moving in flows along the arrays show velocities that are slow near the cortex and fast on the inward side of the subcortical microtubule layer. Starting with fundamental physical principles suggested by qualitative hypotheses, and with published values for microtubule stiffness, kinesin velocity, and cytoplasmic viscosity, we developed a quantitative coupled hydrodynamic model for streaming. The fully detailed mathematical model and its simulations identify key variables that can shift the system between disordered (slow) and ordered (fast) states. Measurements of array curvature, wave period, and the effects of diminished kinesin velocity on flow rates, as well as prior observations on f-actin perturbation, support the model. This establishes a concrete mechanistic framework for the ooplasmic streaming process. The self-organizing fast phase is a result of viscous drag on kinesin-driven cargoes that mediates equal and opposite forces on cytoplasmic fluid and on microtubules whose minus ends are tethered to the cortex. Fluid moves toward plus ends and microtubules are forced backward toward their minus ends, resulting in buckling. Under certain conditions, the buckling microtubules self-organize into parallel bending arrays, guiding varying directions for fast plus-end directed fluid flows that facilitate mixing in a low Reynolds number regime.

Oscillatory fluid flow drives scaling of contraction wave with system size

“…In general, our model broadens the budding understanding of the fundamental role of cytoplasmic flows in a large class of biophysical systems [38], [1], [39], [19]. In very diverse systems, flows appear to be a fundamental part of a self-organized machinery.…”

Microtubule–microtubule sliding by kinesin-1 is essential for normal cytoplasmic streaming in Drosophila oocytes

“…However, the complex relationship between free cytoplasmic microtubules, subcortical dynamic microtubules, and stable cortically anchored microtubules is unclear. We speculate that microtubule sliding by kinesin-1 may contribute to parallel alignment of subcortical microtubules, which is essential for efficient generation of cytoplasmic streams by moving organelles (52). However, microtubule sliding still no doubt contributes directly to ooplasmic streaming, because any moving cargoes including microtubules will exert hydrodynamic forces on the cytoplasm.…”

“…This streaming activity is preceded by a global redistribution of the microtubule network, from a strong concentration at the posterior pole during stage 7 to a gradient of microtubules starting at the anterior pole by stage 10A (50). At the onset of fast ooplasmic streaming in stage 10B, subcortical microtubule arrays form, which undulate with the cytoplasmic flows and eventually disappear as streaming ceases (50)(51)(52). Organelle transport by kinesin-1 along these subcortical microtubule arrays has been proposed to drive ooplasmic streaming, by exerting hydrodynamic forces on the surrounding cytoplasm (8,52).…”

“…At the onset of fast ooplasmic streaming in stage 10B, subcortical microtubule arrays form, which undulate with the cytoplasmic flows and eventually disappear as streaming ceases (50)(51)(52). Organelle transport by kinesin-1 along these subcortical microtubule arrays has been proposed to drive ooplasmic streaming, by exerting hydrodynamic forces on the surrounding cytoplasm (8,52). However, surprisingly, the general kinesin-1 cargo adapter KLC and the microtubule minus-end motor dynein are dispensable for fast ooplasmic streaming (7)(8)(9).…”

“…Next, a population of subcortical microtubules emerges in stage 10B oocytes (50,51). These subcortical microtubules are highly dynamic and undulate and buckle with the flowing cytoplasm (8,52). Ooplasmic streaming is thought to be driven by these subcortical microtubules, because they support long organelle transport events, which cause cytoplasmic movement, and because they appear precisely at the onset of fast streaming.…”