Swimming Euglena respond to confinement with a behavioural change enabling effective crawling

Abstract

Some euglenids, a family of aquatic unicellular organisms, can develop highly concerted, large-amplitude peristaltic body deformations. This remarkable behaviour has been known for centuries. Yet, its function remains controversial, and is even viewed as a functionless ancestral vestige. Here, by examining swimming Euglenagracilis in environments of controlled crowding and geometry, we show that this behaviour is triggered by confinement. Under these conditions, it allows cells to switch from unviable flagellar swimming to a new and highly robust mode of fast crawling, which can deal with extreme geometric confinement and turn both frictional and hydraulic resistance into propulsive forces. To understand how a single cell can control such an adaptable and robust mode of locomotion, we developed a computational model of the motile apparatus of Euglena cells consisting of an active striated cell envelope. Our modelling shows that gait adaptability does not require specific mechanosensitive feedback but instead can be explained by the mechanical self-regulation of an elastic and extended motor system. Our study thus identifies a locomotory function and the operating principles of the adaptable peristaltic body deformation of Euglena cells.

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Fig. 1: Confinement triggers changes in cell behaviour and metaboly.
Fig. 2: Metaboly is an effective crawling mode of locomotion under confinement.
Fig. 3: Mechanism of locomotion during metaboly.
Fig. 4: Computational modelling of crawling in confinement by metaboly.

Code availability

Mathematica (version 11.3.0.0) custom algorithms were developed and used to analyse the theoretical model in Fig. 3 and Supplementary Note 4. Matlab (R2017b) custom algorithms were developed and used to compute sliding displacements from strip curvature (Fig. 1e and Supplementary Note 1) and to implement the computational model in Fig. 4 and Supplementary Note 5. These computer codes are available from the corresponding authors upon reasonable request.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon request.

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Acknowledgements

G.N. and A.D.S. acknowledge the support of the European Research Council (AdG-340685-MicroMotility). M.A. acknowledges the support of the European Research Council (CoG-681434), the Generalitat de Catalunya (2017-SGR-1278 and ICREA Academia prize for excellence in research). We thank S. Guido for helpful discussions in the early stages of this study.

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Authors

Contributions

G.N., M.A. and A.D.S. conceived the study. A.B. provided cells and culture expertise. G.N. performed the experiments. G.N., M.A. and A.D.S. analysed the data, performed theoretical analysis and wrote the paper.

Corresponding authors

Correspondence to Marino Arroyo or Antonio DeSimone.

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The authors declare no competing interests.

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Journal peer review information: Nature Physics thanks Andrew Callan-Jones and other anonymous reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Notes 1–6, Supplementary Figures 1–5 and Supplementary References 36–42.

Reporting Summary

Supplementary Video 1

Video recordings of Euglena gracilis between glass slides separated by a spacer of thickness 80 µm. In dilute cultures (left), cells exhibit flagellar swimming without cell shape changes. In crowded cultures (right), cells display a variety of behaviours, including flagellar swimming, cell rounding and spinning, and large-amplitude periodic cell body deformations typical of metaboly.

Supplementary Video 2

Euglena gracilis cells exhibiting metaboly and directed motion in the anterior-to-posterior direction while confined between glass slides. Observation using bright-field reflected light microscopy reveals the reconfigurations of the striated cell envelope concomitant with cell body deformations in the plane of the glass slide. The separation between the slides is 6 µm.

Supplementary Video 3

Euglena gracilis cells exhibiting metaboly and directed motion in the anterior-to-posterior direction while confined between glass slides. Observation using bright-field reflected light microscopy reveals the reconfigurations of the striated cell envelope concomitant with cell body deformations in the plane of the glass slide. The separation between the slides is 4 µm.

Supplementary Video 4

Video recordings of Euglena gracilis in tapered capillaries. Cells swimming into tapered capillaries transition from flagellar swimming (top left) to developing large-amplitude shape excursions (top right), including rounding (bottom left). When confined in capillary diameters smaller than about twice the free-swimming cell diameter, most cells develop the prototypical peristaltic cell body deformations of metaboly (bottom right).

Supplementary Video 5

Euglena gracilis cell confined in a glass capillary and imaged using bright-field reflected light microscopy. Rounding of the cell body, as determined by the reconfigurations of the pellicle strips, allows the cell to switch its orientation. The microscope was intermittently focused at the pellicle/capillary interface to visualize the pellicle and at the capillary axis to visualize cell shape.

Supplementary Video 6

Video recordings of Euglena gracilis exhibiting metaboly and directed motion in tapered capillaries under increasing confinement, as quantified by the ratio of dcap/dcell, along with kymographs relative to the capillary axis. The movie shows that crawling by metaboly is effective up to very large degree of confinement.

Supplementary Video 7

Video recordings of Euglena gracilis not exhibiting body deformations and acting as hydraulic plugs driven by a known pressure difference, pin, of increasing magnitude between the capillary extremities. Data from these experiments allowed us to quantify a viscous and confinement-dependent friction between cells and the capillary walls.

Supplementary Video 8

Video recordings of Euglena cells stuck in a glass capillary and beating their anterior flagellum.

Supplementary Video 9

Results from the idealized model for the power phase of metaboly in the limit of infinite wall friction relative to hydraulic resistance (top), in the limit of zero wall friction relative to hydraulic resistance (bottom), and for an intermediate case where hydraulic propulsive forces and frictional resistive forces compete (middle). The blue arrows report the average fluid velocity induced by the cell, defined as the flow rate divided by the cross-sectional area of the capillary. The surface of the idealized model is decorated along slip lines by material particles to highlight their motion relative to the capillary walls in the contact region.

Supplementary Video 10

Video recordings of an Euglena cell effectively crawling by metaboly (right) in the presence of an immobile cell, stuck in the capillary and acting as a hydraulic plug (left).

Supplementary Video 11

Video recordings of Euglena gracilis performing metaboly in a capillary and of suspended polystyrene beads by combining bright-field and fluorescence microscopy. Data from these experiments allowed us to quantify the fluid flow around crawling cells by tracking the fluorescent beads. Only beads in the vicinity of the cell undergo rapid motions due to local flows induced by shape changes and flagellar beating.

Supplementary Video 12

Euglena gracilis crawling by metaboly while confined into a glass capillary. Observation using bright-field reflected light microscopy allows for the visualization of the pellicle strips in contact with the capillary wall. The microscope was intermittently focused at the pellicle/capillary interface to visualize the pellicle and at the capillary axis to visualize cell shape. The movie also reports the kymograph relative to the capillary axis. The trajectories of pellicle features reveal sliding between the pellicle and the capillary wall in the contact region.

Supplementary Video 13

Computational results from the theoretical model of crawling by metaboly under confinement. Results are shown for increasing confinement, as quantified by the ratio of dcap/dcell = {0.875, 1.0, 1.375, 1.8}, in the limit of high hydraulic resistance, and during three cycles. The cell motion is reported by black and white features fixed in the frame of the capillary. Notice that the model self-adapts to imposed confinement by developing a limit cycle (gait), which is consistent with the experimental observations on Euglena cells. The four gaits at different degrees of confinement are the result on the same activation pattern, represented as a space–time colour map (left).

Supplementary Video 14

Video recordings of Distigma proteus between glass slides separated on one side by a spacer of thickness 80 µm in order to realize a wedge-shaped fluid chamber. In the absence of confinement (gap between plates 36 µm), cells exhibit flagellar swimming. Significant confinement between the two plates (gap 5 µm) triggers non-reciprocal peristaltic cell deformations, which allow Distigma cells to crawl.

Supplementary Video 15

Video recordings of Peranema trichophorum between glass slides separated on one side by a spacer of thickness 80 µm in order to realize a wedge-shaped fluid chamber. In the absence of confinement (gap between plates 52–43 µm), cells glide on the substrate thanks to the movement of their flagellum. During gliding, cells occasionally bend their body, and this shape change is associated with sharp turns of the cell trajectory. Under high confinement between the glass plates (gap 7 µm), cells are not able to glide and develop periodic, largely reciprocal shape changes.

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Noselli, G., Beran, A., Arroyo, M. et al. Swimming Euglena respond to confinement with a behavioural change enabling effective crawling. Nat. Phys. 15, 496–502 (2019). https://doi.org/10.1038/s41567-019-0425-8

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