Cilia-like beating of active microtubule bundles - PubMed (original) (raw)

Cilia-like beating of active microtubule bundles

Timothy Sanchez et al. Science. 2011.

Abstract

The mechanism that drives the regular beating of individual cilia and flagella, as well as dense ciliary fields, remains unclear. We describe a minimal model system, composed of microtubules and molecular motors, which self-assemble into active bundles exhibiting beating patterns reminiscent of those found in eukaryotic cilia and flagella. These observations suggest that hundreds of molecular motors, acting within an elastic microtubule bundle, spontaneously synchronize their activity to generate large-scale oscillations. Furthermore, we also demonstrate that densely packed, actively bending bundles spontaneously synchronize their beating patterns to produce collective behavior similar to metachronal waves observed in ciliary fields. The simple in vitro system described here could provide insights into beating of isolated eukaryotic cilia and flagella, as well as their synchronization in dense ciliary fields.

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Figures

Fig. 1

A minimal system of MTs, molecular motors, and depleting polymer assembles into actively beating MT bundles. (A) Schematic illustration of all components required for assembly of active bundles. (B) A sequence of images illustrating the beating pattern of an active bundle over one beat cycle. Scale bar, 30 μm. (C) The conformation of the bending MT bundle indicates a fairly symmetric beating pattern that is reminiscent of those found in cilia and flagella. (D) Sequence of images showing one beating cycle of a flagellum from Chlamydomonas reinhardtii. Scale bar, 5 μm.

Fig. 2

A short MT bound to a longer filament by the depletion force exhibits thermally driven sliding motion. (A) A time-lapse observation of two sliding MTs over a period of 15 min. Scale bar, 3 μm. At this depletant concentration, unbinding events are very rare. (B) A trajectory of a shorter MT diffusing while bound to the longer MT. The position is measured relative to the end point of a longer MT. (C) MSD of the relative diffusion between the two MTs. The subdiffusive behavior indicates viscoelastic coupling between the two filaments. The MSD data are averaged for six filament pairs in which shorter filaments were between 3 and 4 μm long. In each case, the dynamics were observed for anywhere between 10 and 30 min.

Fig. 3

Propagation of metachronal waves in a densely packed array of active MT bundles. (A) Interface of an air bubble covered with a dense monolayer of beating bundles (see also movie S7). The bundles are trapped at their bases between the glass and an air bubble. Regions of the edge that are covered with a sufficiently high density of active MT bundles exhibit methachronal traveling waves. The length of bundles is ~10 μm. Scale bar, 40 μm. (B) Time sequence (25-s time lapse) of one section from (A) shows local propagation of metachronal waves. Scale bar, 20 μm. (C) Image of a ciliary field on the cell surface of the ciliate Opalina ranarum, which generates traveling metachronal waves. Scale bar, 20 μm. (D) A kymograph of the fluorescence intensity represents the local MT density of the bundle field, with higher densities displayed in red and lower densities in blue. Easily distinguishable metachronal waves travel in either a clockwise or counterclockwise direction at different regions of interface. Low surface coverage at the top of the interface results in asynchronous beating of active bundles. Dashed red line in (A) indicates the position at which the kymograph was taken. (E) A kymograph of the ciliate field shown in (C) is presented for comparison. Dashed red line in (C) indicates the position at which the kymograph was taken. [With permission from IWF gGMBH, images in (C) and (E) are extracted from (33).]

Fig. 4

Quantitative analysis of the spatial and temporal properties of a travelling metachronal wave. (A) A 3D plot of the correlation function of the density, ρ, is plotted as a function of spatial separation (Δ_x_) and temporal time lag (Δ_t_). (B) The spatial correlation function at zero time lag (Δ_t_ = 0) corresponds to the red slice of the 3D surface plot in (A). The functional form indicates a periodic structure with a wavelength of 54 μm. (C) The temporal autocorrelation plot corresponds to the blue slice of the surface plot in (A) at zero spatial separation (Δ_x_ = 0). The periodicity of this correlation highlights the time propagation of the density wave, as each point becomes anticorrelated with itself when 1/2 of a wavelength has passed and correlated once again when a full wavelength has passed.

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Cilia-like beating of active microtubule bundles - PubMed (original) (raw)