Background

Cells use modular active systems to create their structure, control their mechanics, and respond to their environment. These systems work together to produce sophisticated behaviours such as motility and search. In fact, some single-celled ciliated organisms can perform extraordinary movements, including jumping, avoidance responses and hunting. However, it is often unclear how systems coupling specifies the complex dynamics that define such behaviours.

Lacrymaria olor is unicellular predatory ciliate found in freshwater ponds. Its name derives from its general shape, which is that of a teardrop, with a small head at the end of a long neck. Lacrymaria olor is known for its extreme morphological changes to extend, contract, and whip its ‘cell neck’ over many body lengths to capture prey.

A better understanding of systems coupling resulting in specific motility and search behaviour of ciliates, flagellates, and amoeboid organisms is relevant not only from a cell biology standpoint, but also from a human and veterinary health point of view, as multiple protozoan parasites use complex motility behaviours for intra- or extracellular invasion of different host structures (1-6). In their work, Coyle et al used Lacrymaria olor as a model system, and analysed its hunting strategy (7), discovering previously unknown changes to unique subcellular structures during fast dynamics.

 

Key findings and developments

In their work, Coyle et al established Lacrymaria microcultures for long-term imaging; visualized and digitalized multiple dynamic events, and explored the sub-cellular structures responsible for Lacrymaria hunting behaviour. Imaging showed that Lacrymaria alternates between dormant states, in which the neck of the organism is fully retracted, and hunting states in which the neck undergoes rapid changes in length and shape. During the latter, the cell undergoes extreme morphological changes at the sub-second temporal resolution, to repeatedly extend and whip a slender neck-like proboscis over many body-lengths, which serves to strike and engulf prey (Figure 1).

 

Long-term visualization and Lacrymaria hunting behaviour

Long-term imaging of Lacrymaria’s hunting behaviour showed that during hunts, Lacrymaria attaches its tail-end loosely to the plate surface while the head, neck, and body cilia are actively beating. The authors suggest that this anchorage allows Lacrymaria to use hydrodynamic forces, and allows cortex contractility to deform its cell morphology. Altogether, tail anchorage seems to allow the head and neck to sample multiple locations, arguably for a comprehensive search for food, while the body remains stationary. The authors analysed the tracking data of each subcellular anatomical structure – head, neck and body- to determine how each contribute to the complex hunting behaviour described above.

The head

High-speed imaging showed that the head cilia are highly active during extension, suggesting a strong thrust to pull the neck forward, generating strong flows in the surroundings. Conversely, head cilia are inactive during neck retraction.

The neck

Analysis of the neck showed very dramatic and versatile changes during hunting. Each hunt was initiated by an extension period in which the neck stretches to a mean neck length that was largely consistent across hunts. This was followed by a highly dynamic searching period, lasting up to 15 minutes, in which the neck length rapidly fluctuated about the mean length. Finally, full neck retraction followed, and the organism entered the dormant state. Interestingly, the overall motility during hunting events terminated immediately upon prey finding and engulfment.

The authors explored the sub-cellular structures possibly responsible for the neck’s dramatic behaviour shown during hunting periods. Tubulin and centrin cytoskeletal proteins were analysed in extended and retracted cells, and for the first time, a network of undulating centrin-containing fibres juxtaposed with the microtubules was observed in this organism. Altogether, the analysis suggested that the neck is a dynamic tether containing two active systems – ciliary and centrin elements- supported by a microtubule geometry that facilitates and constrains dynamic neck length changes during hunts.

The body

The Lacrymaria body centroid was found to move very little within the boundary of the search area defined by the head. Despite its relatively limited movement, during hunts, the body’s orientation could change through rotation. Sub-cellular analysis showed that cilia in the helical region of the body apply a torque to the cell, while cilia in the lowest region of the body, interact with the surface for anchoring. Altogether, the geometry of ciliary, microtubular, and centrin elements possibly contribute to the body’s function as an anchor during hunting events.

Characterization of neck dynamics

Based on the observations described above, the authors suggest that the search behaviour during hunting events emerges as a tug-of-war between active ciliated and cytoskeletal structures that extend, retract, and deform the neck. The authors went on to characterize the flexibility and shape dynamics, and identified the natural eigenshape modes of the neck. Main conclusions from this analysis were that a) only a small number of shape modes was necessary to describe neck shape; b) that the head can access most nearby points without moving the body; c) that many locations within the organism’s reach are strongly correlated to contributions from specific shape modes; and d) that neck shape dynamics are not periodic.

Different possible movements were described from this analysis, including low mode steering, for lateral reach; low mode buckling that helps extended necks reorient; and high mode whipping events in which brief oscillations in shape modes cause the neck to wave back and forth during rapid length reduction.

Lacrymaria and shape space

Altogether, the neck is described as a complex dynamic object in which length, shape, and material properties such as stiffness, actively change due to changes in sub-cellular structures. The authors therefore went on to explore how disruption of these different sub-cellular structures would impact Lacrymaria shape and hunting behaviour.

Studies in other organisms have shown that calcium ions regulate ciliary activity and contractility in ciliates, providing a link between the cell’s surrounding and the cell’s interior. Consistent with such findings, exposure of Lacrymaria to increased calcium concentrations led to increased ciliary activity and hyperextended supercoiled neck geometries. Meanwhile, decreased calcium led to an immediate loss of cell tension and abolished ciliary reversals. Further analysis of the effects of calcium manipulation across structures led the authors to conclude that coupling between ciliary and contractile programs is needed to maintain the length/shape relationship; and that neither system alone provides the dynamic repertoire of shapes necessary for comprehensive search during hunting.

Overall

Two modular systems controlling Lacrymaria motility are motile cilia on the surface and contractile protein networks on the cell’s interior cortex. These systems are organized in space through the geometry of the cell’s microtubule cytoskeletal scaffolding anchored to an incompressible membrane; and in time through calcium-dependent signalling controllers that rapidly regulate these activities. Coupled ciliary and contractile active systems act antagonistically and result in dramatic neck dynamics during Lacrymaria hunting.

 

What I like about this paper

I like the questions that it addresses, and the questions that it raises. I particularly like that although interrogating specific biological events from the point of view of various disciplines is slowly happening across research fields, full integration of scientific disciplines is still not common. This work successfully addressed important and exciting biological and biophysical questions. As stated in their preprint, Lacrymaria is a model organism, however the findings that the authors discuss here can be applied to other systems, particularly those relevant to human health – such as parasite and bacteria motility within humans or animal models.

 

Open questions

1. Why did you choose Lacrymaria olor as a model to study the biophysical characteristics, dynamics and coupled mechanisms allowing a complex behaviour such as hunting?

 

2. How do you think the dynamics of this ciliate organism would be affected by physical characteristics of its natural environment (i.e. in freshwater ponds)? Equally, do you observe different behaviours affected by the prey’s dynamics too?

 

3. You did a detailed analysis of the 3 key anatomical parts composing While the neck dynamics are indeed the most dramatic, what would be the effect of loss of anchorage of the tail, loss of the capacity of the body to rotate, or alteration of head motility (without alteration of the neck)?

 

4. Could you explain further your findings from the principle component analysis on coordinate-free length-free parametrizations of the shape data, to identify the natural eigenshape modes of the neck?

 

5. How conserved do you think the organization and control of active systems are in nature? Under this same question, how would you expect the dynamics you observed, to be in parasitic forms (ciliates and flagellates) capable of invading bodies with different microenvironments (eg. Leishmania spp. or Trypanosoma spp.)?

 

6. In your conclusion, you mention the potential of understanding the dynamics you studied, for programming biological systems and engineering molecular machines at the microscale. Can you expand further on this idea?

 

References

1. Hochstetter A, Pfohl T, Motility, force generation, and energy consumption of unicellular parasites, Trends in Parasitology, 32(7), 2016.

2. Mital, J, Ward GE, Current and emerging approaches to studying invasion in apicomplexan parasites, Subcell Biochem, 47:1-32, 2008

3. Soldati D, Meissner M, Toxoplasma as a novel system for motility, Curr Opin Cell Biol, 16(1), 2004

4. Krüger T, Engstler M, Flagellar motility in eukaryotic human parasites, Semin Cell Dev Biol, 46:113-127, 2015

5. Bertiaux E, Bastin P, Dealing with several flagella in the same cell, Cell Microbiol, 2020

6. De Niz M, Burda PC, Kaiser G, del Portillo HA, Spielmann T, Frischknecht F, Heussler VT, Progress in imaging methods: insights gained into Plasmodium biology, Nature Rev Microbiol, 15 (1), 2017

7. Coyle SM, Flaum EM, Li H, Krishnamurthy D, Prakash M, Coupled active systems encode emergent behavioral dynamics of the unicellular predator Lacrymaria olor, bioRxiv, 2018

Tags: biophysics, dynamics, imaging, lacrymaria olor

doi: https://doi.org/10.1242/prelights.16446

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