Chapter 3 Ca2+ Channels of Paramecium: A Multidisciplinary Study

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.

Active oscillations in microscale navigation

Living organisms routinely navigate their surroundings in search of better conditions, more food, or to avoid predators. Typically, animals do so by integrating sensory cues from the environment with their locomotor apparatuses. For single cells or small organisms that possess motility, fundamental physical constraints imposed by their small size have led to alternative navigation strategies that are specific to the microscopic world. Intriguingly, underlying these myriad exploratory behaviours or sensory functions is the onset of periodic activity at multiple scales, such as the undulations of cilia and flagella, the vibrations of hair cells, or the oscillatory shape modes of migrating neutrophils. Here, I explore oscillatory dynamics in basal microeukaryotes and hypothesize that these active oscillations play a critical role in enhancing the fidelity of adaptive sensorimotor integration.

Ion Channels of Cilia: Paramecium as a Model

Holotrichous ciliates, like Paramecium, swim through their aqueous environment by beating their many cilia. They can alter swimming speed and direction, which seems to have mesmerized early microscopists of the 1600's. We know from extensive and elegant physiological studies and generation of mutants that these cells can be considered little swimming neurons because their ciliary beating is under bioelectric control of ion channels in the cilia. This chapter will focus on the ionic control of swimming behavior by ciliary ion channels, primarily in the holotrichous ciliate Paramecium. Voltage gated and calcium activated channels for calcium, magnesium, sodium, and potassium are regulated in a closely orchestrated manner that allows cilia to bend and propel the cell forward or backward. Sensory input that generates receptor potentials feeds into the control of this channel activity and allows the cell to turn or speed up. This in turn helps the cell to avoid predators or toxic conditions. While the focus is on P. tetraurelia and P. caudatum, the principles of ciliary ion channel activity and control are easily extendable to other ciliates and protists. The high conservation of channel and ion pump structures also extends the lessons from Paramecium to higher organisms.

A simple device to immobilize protists for electrophysiology and microinjection

We present a simple device to mechanically immobilize motile cells such as ciliates. It can be used in particular for intracellular electrophysiology and microinjection. A transparent filter with holes smaller than the specimen is stretched over an outlet. A flow is induced by either a peristaltic pump or a depressurized tank, mechanically entraining cells to the bottom, where they are immobilized against the filter. The cells start swimming again as soon as the flow is stopped. We demonstrate the device by recording action potentials in Paramecium and injecting a fluorescent dye into the cytosol.

Behavioral Effects of a Chemorepellent Receptor Knockout Mutation in Tetrahymena thermophila

Although many single-cell eukaryotes have served as classical model systems for chemosensory studies for decades, the major emphasis has been on chemoattraction and no chemorepellent receptor gene has been identified in any unicellular eukaryote. This is the first description of a gene that codes for a chemorepellent receptor in any protozoan. Integration of both depolarizing chemorepellent pathways and hyperpolarizing chemoattractant pathways is as important to chemoresponses of motile unicells as excitatory and inhibitory neurotransmitter pathways are to neurons. Therefore, both chemoattractant and chemorepellent pathways should be represented in a useful unicellular model system. Tetrahymena cells provide such a model system because simple behavioral bioassays, gene knockouts, biochemical analysis, and other approaches can be used with these eukaryotic model cells. This work can contribute to the basic understanding of unicellular sensory responses and provide insights into the evolution of chemoreceptors and possible chemorepellent approaches for preventing infections by some pathogenic protozoa.

A conditioned supernatant from Tetrahymena thermophila contains a powerful chemorepellent for wild-type cells, and a gene called G37 is required for this response. This is the first genomic identification of a chemorepellent receptor in any eukaryotic unicellular organism. This conditioned supernatant factor (CSF) is small (<1 kDa), and its repellent effect is resistant to boiling, protease treatment, and nuclease digestion. External BAPTA eliminated the CSF response, suggesting that Ca²⁺ entry is required for the classical avoiding reactions (AR) used for chemorepulsion. A macronuclear G37 gene knockout (G37-KO) mutant is both nonresponsive to the CSF and overresponsive to other repellents such as quinine, lysozyme, GTP, and high potassium concentrations. All of these mutant phenotypes were reversed by overexpression of the wild-type G37 gene in a G37 overexpression mutant. Overexpression of G37 in the wild type caused increased responsiveness to the CSF and underresponsiveness to high K⁺ concentrations. Behavioral adaptation (by prolonged exposure to the CSF) caused decreases in responsiveness to all of the stimuli used in the wild type and the overexpression mutant but not in the G37-KO mutant. We propose that the constant presence of the CSF causes a decreased basal excitability of the wild type due to chemosensory adaptation through G37 and that all of the G37-KO phenotypes are due to an inability to detect the CSF. Therefore, the G37 protein may be the CSF receptor. The physiological role of these G37-mediated responses may be to both moderate basal excitability and detect the CSF as an indicator of high cell density growth.

IMPORTANCE Although many single-cell eukaryotes have served as classical model systems for chemosensory studies for decades, the major emphasis has been on chemoattraction and no chemorepellent receptor gene has been identified in any unicellular eukaryote. This is the first description of a gene that codes for a chemorepellent receptor in any protozoan. Integration of both depolarizing chemorepellent pathways and hyperpolarizing chemoattractant pathways is as important to chemoresponses of motile unicells as excitatory and inhibitory neurotransmitter pathways are to neurons. Therefore, both chemoattractant and chemorepellent pathways should be represented in a useful unicellular model system. Tetrahymena cells provide such a model system because simple behavioral bioassays, gene knockouts, biochemical analysis, and other approaches can be used with these eukaryotic model cells. This work can contribute to the basic understanding of unicellular sensory responses and provide insights into the evolution of chemoreceptors and possible chemorepellent approaches for preventing infections by some pathogenic protozoa.

Novel Insights into the Development and Function of Cilia Using the Advantages of the Paramecium Cell and Its Many Cilia

Paramecium species, especially P. tetraurelia and caudatum, are model organisms for modern research into the form and function of cilia. In this review, we focus on the ciliary ion channels and other transmembrane proteins that control the beat frequency and wave form of the cilium by controlling the signaling within the cilium. We put these discussions in the context of the advantages that Paramecium brings to the understanding of ciliary motility: mutants for genetic dissections of swimming behavior, electrophysiology, structural analysis, abundant cilia for biochemistry and modern proteomics, genomics and molecular biology. We review the connection between behavior and physiology, which allows the cells to broadcast the function of their ciliary channels in real time. We build a case for the important insights and advantages that this model organism continues to bring to the study of cilia.

Novel types of Ca2+ release channels participate in the secretory cycle of Paramecium cells

A database search of the Paramecium genome reveals 34 genes related to Ca(2+)-release channels of the inositol-1,4,5-trisphosphate (IP(3)) or ryanodine receptor type (IP(3)R, RyR). Phylogenetic analyses show that these Ca(2+) release channels (CRCs) can be subdivided into six groups (Paramecium tetraurelia CRC-I to CRC-VI), each one with features in part reminiscent of IP(3)Rs and RyRs. We characterize here the P. tetraurelia CRC-IV-1 gene family, whose relationship to IP(3)Rs and RyRs is restricted to their C-terminal channel domain. CRC-IV-1 channels localize to cortical Ca(2+) stores (alveolar sacs) and also to the endoplasmic reticulum. This is in contrast to a recently described true IP(3) channel, a group II member (P. tetraurelia IP(3)R(N)-1), found associated with the contractile vacuole system. Silencing of either one of these CRCs results in reduced exocytosis of dense core vesicles (trichocysts), although for different reasons. Knockdown of P. tetraurelia IP(3)R(N) affects trichocyst biogenesis, while CRC-IV-1 channels are involved in signal transduction since silenced cells show an impaired release of Ca(2+) from cortical stores in response to exocytotic stimuli. Our discovery of a range of CRCs in Paramecium indicates that protozoans already have evolved multiple ways for the use of Ca(2+) as signaling molecule.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

L-glutamate-induced membrane hyperpolarization and behavioural responses in Paramecium tetraurelia

Paramecium tetraurelia is attracted to L-glutamic acid concentrations of 10(-9) M to 10(-4) M in a behavioural assay. Electrophysiological studies show that P. tetaurelia responds to L-glutamate application with hyperpolarization. This response is transient, even in the continued presence of the stimulus. The concentration dependence of the membrane potential response is similar to that of the behavioural responses, although the threshold concentration of L-glutamate required for hyperpolarization is three orders of magnitude lower than for attraction. The membrane potential response to L-glutamate persists following artificial deciliation of P. tetraurelia. While application of L-glutamate to P. tetraurelia invariably elicits a hyperpolarization, withdrawal of the stimulus frequently results in a second transient membrane response, in the form of either a hyperpolarization or a depolarization. It is suggested that these 'off-responses' may have a significant role in maintaining a behavioural response to L-glutamate.

Calcium activation of macrocilia in the ctenophore Beroë

1. Macrocilia on the lips of the ctenophore Beroë are usually quiescent, but can be activated to beat rapidly and continuously by various stimuli. 2. During feeding, macrocilia beat actively and serve to spread the lips of Beroë over its prey. 3. Vigorous, repetitive mechanical stimulation of the lips evokes widespread activation of macrocilia via a pathway that is probably neural. 4. Extracellular electrical stimulation (DC or bipolar pulse-trains) elicits immediate activation of macrocilia on lip pieces, but not on dissociated cells. 5. Macrocilia on lip pieces are activated to beat by high KCl artificial sea water (ASW), but not by high KCl Ca-free ASW. Continuous beating for long periods is also elicited by high Ca ASW or Mg-free ASW, but not by Ca-Mg-free ASW. Addition of La, Cd, Co or Mn (10 mM) to high KCl ASW reversibly blocks activation. Verapamil, D-600, nifedipine, or BAY K 8644 (10 microM) has no effect on KC1-induced activation, but the anticalmodulin drug W-7 (10 microM) reversibly inhibits beating. 6. Mild heat treatment dissociates macrociliary cells from lip tissue. Such isolated macrociliary cells usually beat continuously in normal sea water, and swim in circular paths. Ca-free ASW, or addition of Co or Mn to ASW, inhibits beating of dissociated cells. High KCl ASW activates beating of quiescent, isolated macrociliary cells. 7. Ca-Mg-free ASW inhibits beating of dissociated macrociliary cells, and return to Mg-free ASW activates motility, allowing one to activate macrocilia on isolated cells simply by addition of Ca.(ABSTRACT TRUNCATED AT 250 WORDS)

Iontophoretic localization of Ca-sensitive sites controlling activation of ciliary beating in macrocilia of Beroë: the ciliary rete

Macrocilia are thick compound ciliary organelles found on the lips of the ctenophore Beroë. Each macrocilium contains several hundred axonemes enclosed by a single common membrane around the shaft of the organelle. Macrocilia are activated to beat rapidly and continuously in the normal direction by stimulus-triggered Ca influx through voltage-dependent Ca channels (Tamm, 1988). Heat-dissociated macrociliary cells are spontaneously active without depolarizing stimuli, providing Ca is present (Tamm, 1988). Here we investigate the spatial distribution of macrociliary Ca channels by iontophoretic application of extracellular Ca to different sites along quiescent, "potentially activated" macrocilia of dissociated cells in Ca-free medium. We find that Ca sensitivity for eliciting motility is highest or resides exclusively on the basal portion of the macrociliary surface. This is the first demonstration of local differences in Ca sensitivity along living cilia or flagella. The Ca-sensitive region coincides morphologically with a reticulum of unfused ciliary membranes at the base of the macrocilium. This ciliary rete is in direct communication with the surrounding sea water. It is likely that the ciliary rete provides the necessary Ca influx to trigger beating by virtue of its greater Ca conductance (i.e., density of Ca channels) and/or greater total membrane area.

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.

A calcium-dependent potassium current is increased by a single-gene mutation in Paramecium

The membrane currents of wild type Paramecium tetraurelia and the behavioral mutant teaA were analyzed under voltage clamp. The teaA mutant was shown to have a greatly increased outward current which was blocked completely by the combined use of internally delivered Cs+ and external TEA+. This, along with previous work (Satow, Y., Kung, C., 1976, J. Exp. Biol. 65:51-63) identified this as a K+ current. It was further found to be a calcium-activated K+ current since this increased outward K+ current cannot be elicited when the internal calcium is buffered with injected EGTA. The mutation pwB, which blocks the inward calcium current, also blocks this increased outward K+ current in teaA. This shows that this mutant current is activated by calcium through the normal depolarization-sensitive calcium channel. While tail current decay kinetic analysis showed that the apparent inactivation rates for this calcium-dependent K+ current are the same for mutant and wild type, the teaA current activates extremely rapidly. It is fully activated within 2 msec. This early activation of such a large outward current causes a characteristic reduction in the amplitude of the action potential of the teaA mutant. The teaA mutation had no effect on any of the other electrophysiological parameters examined. The phenotype of the teaA mutant is therefore a general decrease in responsiveness to depolarizing stimuli because of a rapidly activating calcium-dependent K+ current which prematurely repolarizes the action potential.

Ion channels in yeast

Voltage-dependent ion channels have been found in the plasma membrane of the yeast Saccharomyces cerevisiae. Ion channel activities were recorded from spheroplasts or patches of plasma membrane with the patch-clamp technique. The most prominent activities came from a set of potassium channels with the properties of activation by positive but not negative voltages, high selectivity for potassium over sodium ion, unit conductance of 20 picosiemens, inhibition by tetraethylammonium or barium ions, and bursting kinetics.

A mutation that increases a novel calcium-activated potassium conductance of Paramecium tetraurelia

Under two-electrode voltage clamp, a mutant of P. tetraurelia, restless (rst/rst), showed a large increase in induced current and an outward tail current when compared to the wild-type cell for hyperpolarizing voltage steps. An increase in the induced and tail currents is also observed for depolarizing voltage steps. The larger current during voltage steps and tail in the mutant were eliminated by the use of CsCl-filled electrodes and tetraethylammonium ion (TEA+) in the bath solution, characterizing the lesion as affecting a K+ conductance. Ionophoretic injection of ethylene glycol bis-(beta-aminoethyl ether) n,n,n',n-tetraacetic acid (EGTA) to buffer internal Ca2+ concentration reduced the increased K+ current and tail of the restless cell, indicating Ca2+ activation of the K+ current. Time course and amplitude of remaining currents after blockage of K+ conductances with Cs+ and TEA+ were similar in wild-type and restless cells suggesting no restless defect in entry of calcium. The Ca2+-activated sodium current was similar in the mutant to that in wild type arguing against a defect in calcium regulation activating the K+ channel in the restless cell. We conclude that the restless mutation alters a Ca2+-activated potassium conductance other than the one previously described. The multiplicity of Ca2+-activated potassium conductances in Paramecium is discussed.