Rhythmic spontaneous depolarizations determine a slow-and-fast rhythm in walking of the marine hypotrich Euplotes vannus

Cell motility: Bioelectrical control of behavior without neurons

Single cells are capable of remarkably sophisticated, sometimes animal-like, behaviors. New work demonstrates bioelectric control of motility through the differential regulation of appendage movements in a unicellular organism that walks across surfaces using leg-like bundles of cilia.

Bioelectric control of locomotor gaits in the walking ciliate Euplotes

Diverse animal species exhibit highly stereotyped behavioral actions and locomotor sequences as they explore their natural environments. In many such cases, the neural basis of behavior is well established, where dedicated neural circuitry contributes to the initiation and regulation of certain response sequences. At the microscopic scale, single-celled eukaryotes (protists) also exhibit remarkably complex behaviors and yet are completely devoid of nervous systems. Here, to address the question of how single cells control behavior, we study locomotor patterning in the exemplary hypotrich ciliate Euplotes, a highly polarized cell, which actuates a large number of leg-like appendages called cirri (each a bundle of ∼25-50 cilia) to swim in fluids or walk on surfaces. As it navigates its surroundings, a walking Euplotes cell is routinely observed to perform side-stepping reactions, one of the most sophisticated maneuvers ever observed in a single-celled organism. These are spontaneous and stereotyped reorientation events involving a transient and fast backward motion followed by a turn. Combining high-speed imaging with simultaneous time-resolved electrophysiological recordings, we show that this complex coordinated motion sequence is tightly regulated by rapid membrane depolarization events, which orchestrate the activity of different cirri on the cell. Using machine learning and computer vision methods, we map detailed measurements of cirri dynamics to the cell's membrane bioelectrical activity, revealing a differential response in the front and back cirri. We integrate these measurements with a minimal model to understand how Euplotes-a unicellular organism-manipulates its membrane potential to achieve real-time control over its motor apparatus.

Methods and Measures for Investigating Microscale Motility

Motility is an essential factor for an organism's survival and diversification. With the advent of novel single-cell technologies, analytical frameworks, and theoretical methods, we can begin to probe the complex lives of microscopic motile organisms and answer the intertwining biological and physical questions of how these diverse lifeforms navigate their surroundings. Herein, we summarize the main mechanisms of microscale motility and give an overview of different experimental, analytical, and mathematical methods used to study them across different scales encompassing the molecular-, individual-, to population-level. We identify transferable techniques, pressing challenges, and future directions in the field. This review can serve as a starting point for researchers who are interested in exploring and quantifying the movements of organisms in the microscale world.

The evolution and diversity of actin-dependent cell migration

Many eukaryotic cells, including animal cells and unicellular amoebae, use dynamic-actin networks to crawl across solid surfaces. Recent discoveries of actin-dependent crawling in additional lineages have sparked interest in understanding how and when this type of motility evolved. Tracing the evolution of cell crawling requires understanding the molecular mechanisms underlying motility. Here we outline what is known about the diversity and evolution of the molecular mechanisms that drive cell motility, with a focus on actin-dependent crawling. Classic studies and recent work have revealed a surprising number of distinct mechanical modes of actin-dependent crawling used by different cell types and species to navigate different environments. The overlap in actin network regulators driving multiple types of actin-dependent crawling, along with cortical-actin networks that support the plasma membrane in these cells, suggest that actin motility and cortical actin networks might have a common evolutionary origin. The rapid development of additional evolutionarily diverse model systems, advanced imaging technologies, and CRISPR-based genetic tools, is opening the door to testing these and other new ideas about the evolution of actin-dependent cell crawling.

A unicellular walker controlled by a microtubule-based finite-state machine

Cells are complex biochemical systems whose behaviors emerge from interactions among myriad molecular components. Computation is often invoked as a general framework for navigating this cellular complexity. However, it is unclear how cells might embody computational processes such that the theories of computation, including finite-state machine models, could be productively applied. Here, we demonstrate finite-state-machine-like processing embodied in cells using the walking behavior of Euplotes eurystomus, a ciliate that walks across surfaces using fourteen motile appendages (cirri). We found that cellular walking entails regulated transitions among a discrete set of gait states. The set of observed transitions decomposes into a small group of high-probability, temporally irreversible transitions and a large group of low-probability, time-symmetric transitions, thus revealing stereotypy in the sequential patterns of state transitions. Simulations and experiments suggest that the sequential logic of the gait is functionally important. Taken together, these findings implicate a finite-state-machine-like process. Cirri are connected by microtubule bundles (fibers), and we found that the dynamics of cirri involved in different state transitions are associated with the structure of the fiber system. Perturbative experiments revealed that the fibers mediate gait coordination, suggesting a mechanical basis of gait control.

Origins of eukaryotic excitability

All living cells interact dynamically with a constantly changing world. Eukaryotes, in particular, evolved radically new ways to sense and react to their environment. These advances enabled new and more complex forms of cellular behaviour in eukaryotes, including directional movement, active feeding, mating, and responses to predation. But what are the key events and innovations during eukaryogenesis that made all of this possible? Here we describe the ancestral repertoire of eukaryotic excitability and discuss five major cellular innovations that enabled its evolutionary origin. The innovations include a vastly expanded repertoire of ion channels, the emergence of cilia and pseudopodia, endomembranes as intracellular capacitors, a flexible plasma membrane and the relocation of chemiosmotic ATP synthesis to mitochondria, which liberated the plasma membrane for more complex electrical signalling involved in sensing and reacting. We conjecture that together with an increase in cell size, these new forms of excitability greatly amplified the degrees of freedom associated with cellular responses, allowing eukaryotes to vastly outperform prokaryotes in terms of both speed and accuracy. This comprehensive new perspective on the evolution of excitability enriches our view of eukaryogenesis and emphasizes behaviour and sensing as major contributors to the success of eukaryotes. This article is part of the theme issue 'Basal cognition: conceptual tools and the view from the single cell'.

Feast or flee: bioelectrical regulation of feeding and predator evasion behaviors in the planktonic alveolate Favella sp. (Spirotrichia)

Alveolate (ciliates and dinoflagellates) grazers are integral components of the marine food web and must therefore be able to sense a range of mechanical and chemical signals produced by prey and predators, integrating them via signal transduction mechanisms to respond with effective prey capture and predator evasion behaviors. However, the sensory biology of alveolate grazers is poorly understood. Using novel techniques that combine electrophysiological measurements and high-speed videomicroscopy we investigated the sensory biology of Favella sp., a model alveolate grazer, in the context of its trophic ecology. Favella sp. produced frequent rhythmic depolarizations (∼500 ms long) that caused backward swimming and are responsible for endogenous swimming patterns relevant to foraging. Contact of both prey cells and non-prey polystyrene microspheres at the cilia produced immediate mechano-stimulated depolarizations (∼500 ms long) that caused backward swimming, and likely underlie aggregative swimming patterns of Favella sp. in response to patches of prey. Contact of particles at the peristomal cavity that were not suitable for ingestion resulted in MSDs after a lag of ∼600 ms, allowing time for particles to be processed before rejection. Ingestion of preferred prey particles was accompanied by transient hyperpolarizations (∼1 s) that likely regulate this step of the feeding process. Predation attempts by the copepod Acartia tonsa elicited fast (∼20 ms) animal-like action potentials accompanied by rapid contraction of the cell to avoid predation. We have shown that the sensory mechanisms of Favella sp. are finely tuned to the type, location, and intensity of stimuli from prey and predators.

Ethogram of Aspidisca sedigita

The analysis of behaviour of Aspidisca sedigita has been undertaken to describe the main features of its biology. In drawing the standard ethogram of A. sedigita, several peculiarities have been discovered: (i) the cirri of Aspidisca are thicker and tufted versus the slim and pointed cirri of other hypotrichs; (ii) the side-stepping reaction is performed without its typical backward motion; (iii) a typical clockwise rotation of 90°, followed by a similar but anticlockwise one, is performed frequently and results in a shift of the creeping Aspidisca into a new trajectory, close and parallel to the previous one; (iv) the very rare swimming motion of the species occurs along a regular helicoid, with the ciliary organelles facing in the opposite direction of the centre of the helicoid; (v) the creeping and swimming of conjugating pairs are similar to those of single organisms. The analysis of behaviour of A. sedigita is suggested to contribute to our knowledge of the adaptive strategies of this species.

REGULAR ARTICLES / ARTICLES RÉGULIERSMovement of the cirri during the creeping of Euplotes crassus (Ciliata, Hypotrichida)

The beating cycle of several cirri (frontal cirri 1/0, 3/I, 3/II, 3/III; transverse cirri; one caudal cirrus) of the ciliate Euplotes crassus was studied and described thoroughly in specimens that were actually creeping along the substrate. The beating cycle of the frontal cirri was measured both spatially and temporally, and it was found that (i) the single beating cycle was formed by an active propulsion phase (about 70% of the single step), followed by a recovery phase that so far has never been described and is where the cirri are transferred forwards passively (about 30% of the step); (ii) whenever the euplotes stops, it assumes its "zero position," repositioning all of its frontal cirri to their respective "standard positions"; and (iii) at the beginning of a new creeping phase the frontal cirri were reactivated in a well-defined order. The transverse cirri were kept still during forward creeping, while their angular position was changed with respect to the substrate during the stops and backward movements of the ciliate. The first left caudal cirrus beats constantly and its operating cycle appeared to be independent of the creeping or immobile state of the organism. The findings are discussed from the functional point of view and in the context of available literature on the internal beating potentialities of the different cirri.

Characterisation of the voltage-activated calcium current in the marine ciliate Euplotes vannus

We have isolated the early Ca current (ICa) from the whole cell current that activates upon depolarisations in the marine ciliate Euplotes vannus. The peak of ICa activated within 4.2 ms at depolarisations to 5 mV with an amplitude of 2.5 +/- 0.35 nA and was reduced to 1.0 +/- 0.14 nA (n = 5) when the extracellular Ca concentration was changed from 10 to 1 mM. The voltage-dependent activation curve was steeper and shifted to more negative values when external Ca2+ was replaced by Ba2+. The early inward current inactivated with a double-exponential time course including a fast and a slow component, and no inactivation was recorded with Ba2+. The time constants for the recovery from inactivation varied between 44 and 153 ms according to the depolarisation-dependent Ca influx. At the common resting potential of -25 mV, ICa was not steady-state inactivated; ICa half-inactivated at -14.5 mV, and totally inactivated at -5 mV. ICa was inhibited by 10 mM extracellular Cd2+. The peptides omega-conotoxin-GVIA (20 microM), omega-conotoxin-MVIIC (600 nM), omega-agatoxin-IVA (60 nM) and calciseptine (900 nM) did not block ICa. The benzothiazepine-derivative diltiazem (100 microM) and the dihydropiridine nifedipine (100 microM) inhibited 51% and 33% of ICa, respectively. The naphthalene sulfonamide W7 reduced ICa with an inhibition coefficient of 33 microM.