Chemosensory signal transduction in paramecium

Ciliate Research. From Myth to Trendsetting Science

This special issue of the Journal of Eukaryotic Microbiology (JEM) summarizes achievements obtained by generations of researchers with ciliates in widely different disciplines. In fact, ciliates range among the first cells seen under the microscope centuries ago. Their beauty made them an object of scientia amabilis and their manifold reactions made them attractive for college experiments and finally challenged causal analyses at the cellular level. Some of this work was honored by a Nobel Prize. Some observations yielded a baseline for additional novel discoveries, occasionally facilitated by specific properties of some ciliates. This also offers some advantage in the exploration of closely related parasites (malaria). Articles contributed here by colleagues from all over the world encompass a broad spectrum of ciliate life, from genetics to evolution, from molecular cell biology to ecology, from intercellular signaling to epigenetics etc. This introductory chapter, largely based on my personal perception, aims at integrating work presented in this special issue of JEM into a broader historical context up to current research.

Paramecium, a Model to Study Ciliary Beating and Ciliogenesis: Insights From Cutting-Edge Approaches

Cilia are ubiquitous and highly conserved extensions that endow the cell with motility and sensory functions. They were present in the first eukaryotes and conserved throughout evolution (Carvalho-Santos et al., 2011). Paramecium has around 4,000 motile cilia on its surface arranged in longitudinal rows, beating in waves to ensure movement and feeding. As with cilia in other model organisms, direction and speed of Paramecium ciliary beating is under bioelectric control of ciliary ion channels. In multiciliated cells of metazoans as well as paramecia, the cilia become physically entrained to beat in metachronal waves. This ciliated organism, Paramecium, is an attractive model for multidisciplinary approaches to dissect the location, structure and function of ciliary ion channels and other proteins involved in ciliary beating. Swimming behavior also can be a read-out of the role of cilia in sensory signal transduction. A cilium emanates from a BB, structurally equivalent to the centriole anchored at the cell surface, and elongates an axoneme composed of microtubule doublets enclosed in a ciliary membrane contiguous with the plasma membrane. The connection between the BB and the axoneme constitutes the transition zone, which serves as a diffusion barrier between the intracellular space and the cilium, defining the ciliary compartment. Human pathologies affecting cilia structure or function, are called ciliopathies, which are caused by gene mutations. For that reason, the molecular mechanisms and structural aspects of cilia assembly and function are actively studied using a variety of model systems, ranging from unicellular organisms to metazoa. In this review, we will highlight the use of Paramecium as a model to decipher ciliary beating mechanisms as well as high resolution insights into BB structure and anchoring. We will show that study of cilia in Paramecium promotes our understanding of cilia formation and function. In addition, we demonstrate that Paramecium could be a useful tool to validate candidate genes for ciliopathies.

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.

Reconsidering the evidence for learning in single cells

The question of whether single cells can learn led to much debate in the early 20th century. The view prevailed that they were capable of non-associative learning but not of associative learning, such as Pavlovian conditioning. Experiments indicating the contrary were considered either non-reproducible or subject to more acceptable interpretations. Recent developments suggest that the time is right to reconsider this consensus. We exhume the experiments of Beatrice Gelber on Pavlovian conditioning in the ciliate Paramecium aurelia, and suggest that criticisms of her findings can now be reinterpreted. Gelber was a remarkable scientist whose absence from the historical record testifies to the prevailing orthodoxy that single cells cannot learn. Her work, and more recent studies, suggest that such learning may be evolutionarily more widespread and fundamental to life than previously thought and we discuss the implications for different aspects of biology.

A Novel Role for Polycystin-2 (Pkd2) in P. tetraurelia as a Probable Mg2+ Channel Necessary for Mg2+-Induced Behavior

A human ciliopathy gene codes for Polycystin-2 (Pkd2), a non-selective cation channel. Here, the Pkd2 channel was explored in the ciliate Paramecium tetraurelia using combinations of RNA interference, over-expression, and epitope-tagging, in a search for function and novel interacting partners. Upon depletion of Pkd2, cells exhibited a phenotype similar to eccentric (XntA1), a Paramecium mutant lacking the inward Ca²⁺-dependent Mg²⁺ conductance. Further investigation showed both Pkd2 and XntA localize to the cilia and cell membrane, but do not require one another for trafficking. The XntA-myc protein co-immunoprecipitates Pkd2-FLAG, but not vice versa, suggesting two populations of Pkd2-FLAG, one of which interacts with XntA. Electrophysiology data showed that depletion and over-expression of Pkd2 led to smaller and larger depolarizations in Mg²⁺ solutions, respectively. Over-expression of Pkd2-FLAG in the XntA1 mutant caused slower swimming, supporting an increase in Mg²⁺ permeability, in agreement with the electrophysiology data. We propose that Pkd2 in P. tetraurelia collaborates with XntA for Mg²⁺-induced behavior. Our data suggest Pkd2 is sufficient and necessary for Mg²⁺ conductance and membrane permeability to Mg²⁺, and that Pkd2 is potentially a Mg²⁺-permeable channel.

Methods for Studying Ciliary-Mediated Chemoresponse in Paramecium

Paramecium is a useful model organism for the study of ciliary-mediated chemical sensing and response. Here we describe ways to take advantage of Paramecium to study chemoresponse.Unicellular organisms like the ciliated protozoan Paramecium sense and respond to chemicals in their environment (Van Houten, Ann Rev Physiol 54:639-663, 1992; Van Houten, Trends Neurosci 17:62-71, 1994). A thousand or more cilia that cover Paramecium cells serve as antennae for chemical signals, similar to ciliary function in a large variety of metazoan cell types that have primary or motile cilia (Berbari et al., Curr Biol 19(13):R526-R535, 2009; Singla V, Reiter J, Science 313:629-633, 2006). The Paramecium cilia also produce the motor output of the detection of chemical cues by controlling swimming behavior. Therefore, in Paramecium the cilia serve multiple roles of detection and response.We present this chapter in three sections to describe the methods for (1) assaying populations of cells for their behavioral responses to chemicals (attraction and repulsion), (2) characterization of the chemoreceptors and associated channels of the cilia using proteomics and binding assays, and (3) electrophysiological analysis of individual cells' responses to chemicals. These methods are applied to wild type cells, mutants, transformed cells that express tagged proteins, and cells depleted of gene products by RNA Interference (RNAi).

New Insights Into the Roles of Retinoic Acid Signaling in Nervous System Development and the Establishment of Neurotransmitter Systems

Secreted chiefly from the underlying mesoderm, the morphogen retinoic acid (RA) is well known to contribute to the specification, patterning, and differentiation of neural progenitors in the developing vertebrate nervous system. Furthermore, RA influences the subtype identity and neurotransmitter phenotype of subsets of maturing neurons, although relatively little is known about how these functions are mediated. This review provides a comprehensive overview of the roles played by RA signaling during the formation of the central and peripheral nervous systems of vertebrates and highlights its effects on the differentiation of several neurotransmitter systems. In addition, the evolutionary history of the RA signaling system is discussed, revealing both conserved properties and alternate modes of RA action. It is proposed that comparative approaches should be employed systematically to expand our knowledge of the context-dependent cellular mechanisms controlled by the multifunctional signaling molecule RA.

Pharmacological characterization of N-methyl-d-aspartic acid (NMDA)-like receptors in the single-celled organism Paramecium primaurelia

Paramecium primaurelia is a unicellular eukaryote that moves in freshwater by ciliary beating and responds to environmental stimuli by altering motile behaviour. The movements of the cilia are controlled by the electrical changes of the cell membrane: when the intraciliary Ca2+ concentration associated with plasma membrane depolarization increases, the ciliary beating reverses its direction, and consequently the swimming direction changes. The ciliary reversal duration is correlated with the amount of Ca2+ influx. Here we evaluated the effects due to the activation or blockade of NMDA receptors on swimming behaviour in Paramecium. Paramecia normally swim forward drawing almost linear tracks. We observed that the simultaneous administration of NMDA and glycine induced a partial ciliary reversal (PaCR) leading to a continuous spiral-like swim. Furthermore, the duration of continuous ciliary reversal (CCR), triggered by high external KCl concentrations, was longer in NMDA/glycine treated cells. NMDA action required the presence of Ca2+, as the normal forward swimming was restored when the ion was omitted from the extracellular milieu. The PaCR and the enhancement of CCR duration significantly decreased when the antagonists of the glutamate site D-AP5 or CGS19755, the NMDA channel blocker MK-801, or the glycine site antagonist DCKA were added. The action of NMDA/glycine was also abolished by Zn2+ or ifenprodil, the GluN2A and the GluN2B NMDA-containing subunit blockers, respectively. Searches of the Paramecium genome database currently available indicate that the NMDA-like receptor with ligand binding characteristics of an NMDA receptor-like complex, purified from rat brain synaptic membranes and found in some metazoan genome, is also present in Paramecium. These results provide evidence that functional NMDA receptors similar to those typical of mammalian neuronal cells are present in the single-celled organism Paramecium and thus suggest that the glutamatergic NMDA system is a phylogenetically old behaviour-controlling mechanism.

Chemo-accumulation without changes in membrane potential in the microstome form of the ciliate Tetrahymena vorax

The swimming behaviour of ciliates is mainly determined by membrane potential and transmembrane fluxes. In a chemical gradient, swimming ciliates may approach or move away from the source. Based on experiments on Paramecium, it is generally assumed that chemical attractants and repellents affect the swimming behaviour of ciliates by specific changes in the membrane potential. We have examined whether there is a causal relationship between membrane potential and chemo-accumulation in the microstome form of the polymorphic ciliate Tetrahymena vorax. Effects of chemo-attractants on the membrane potential of Tetrahymena have not been previously reported. Microstome T. vorax cells aggregated close to a point source of l-cysteine and the complex meat hydrolysate proteose peptone. Chemo-accumulated cells displayed a significantly higher turning frequency than control cells at a similar cell density. A concentration of 20 mmol l(-1) l-cysteine did not evoke any detectable change in the membrane potential whereas 1% proteose peptone depolarised the cells by ∼12 mV. This is contrary to the current model, which predicts agents that induce a moderate depolarisation to be repellents. A solution of 1% proteose peptone contains 21 mmol(-1) Na(+). A solution of 21 mmol(-1) NaCl without organic compounds also caused ∼12 mV depolarisation but had no aggregating effect on the cells. Collectively, the electrophysiological and behavioural data indicate that chemo-accumulation in the microstome form of T. vorax is not governed obligatorily by the membrane potential. We thus suggest that the simple membrane potential model for chemokinesis in Paramecium may not be valid for T. vorax.

cAMP-dependent signal pathways in unicellular eukaryotes

The review summarizes current data about mechanisms of signal transduction with participation of cAMP (cyclic adenosine monophosphate) and elements of the complex cAMP-protein kinase A (PKA) signal pathway in unicellular eukaryotes. Conceptions of evolutionary origin of eukaryotic signal transduction systems are developed.

ParameciumDB: a community resource that integrates the Paramecium tetraurelia genome sequence with genetic data

ParameciumDB (http://paramecium.cgm.cnrs-gif.fr) is a new model organism database associated with the genome sequencing project of the unicellular eukaryote Paramecium tetraurelia. Built with the core components of the Generic Model Organism Database (GMOD) project, ParameciumDB currently contains the genome sequence and annotations, linked to available genetic data including the Gif Paramecium stock collection. It is thus possible to navigate between sequences and stocks via the genes and alleles. Phenotypes, of mutant strains and of knockdowns obtained by RNA interference, are captured using controlled vocabularies according to the Entity-Attribute-Value model. ParameciumDB currently supports browsing of phenotypes, alleles and stocks as well as querying of sequence features (genes, UniProt matches, InterPro domains, Gene Ontology terms) and of genetic data (phenotypes, stocks, RNA interference experiments). Forms allow submission of RNA interference data and some bioinformatics services are available. Future ParameciumDB development plans include coordination of human curation of the near 40 000 gene models by members of the research community.

Nitrogen regulation of root branching

BACKGROUND

Many plant species can modify their root architecture to enable them to forage for heterogeneously distributed nutrients in the soil. The foraging response normally involves increased proliferation of lateral roots within nutrient-rich soil patches, but much remains to be understood about the signalling mechanisms that enable roots to sense variations in the external concentrations of different mineral nutrients and to modify their patterns of growth and development accordingly.

SCOPE

In this review we consider different aspects of the way in which the nitrogen supply can modify root branching, focusing on Arabidopsis thaliana. Our current understanding of the mechanism of nitrate stimulation of lateral root growth and the role of the ANR1 gene are summarized. In addition, evidence supporting the possible role of auxin in regulating the systemic inhibition of early lateral root development by high rates of nitrate supply is presented. Finally, we examine recent evidence that an amino acid, L-glutamate, can act as an external signal to elicit complex changes in root growth and development.

CONCLUSIONS

It is clear that plants have evolved sophisticated pathways for sensing and responding to changes in different components of the external nitrogen supply as well as their own internal nitrogen status. We speculate on the possibility that the effects elicited by external L-glutamate represent a novel form of foraging response that could potentially enhance a plant's ability to compete with its neighbours and micro-organisms for localized sources of organic nitrogen.

Chemotaxis in the green flagellate alga Chlamydomonas

Behavior of the green flagellate alga Chlamydomonas changes in response to a number of chemical stimuli. Specific sensitivity of the cells to different substances might appear only at certain stages of the life cycle. The heterogamous species C. allensworthii demonstrates chemotaxis of male gametes towards pheromones excreted by female gametes. In C. reinhardtii chemotaxis towards tryptone occurs only in gametes, whereas chemotaxis towards ammonium, on the contrary, only in vegetative cells. Chemotaxis to different chemical stimuli might involve different mechanisms of reception and signal transduction, elucidation of which has only recently begun. Indirect evidences show that the cells likely respond to tryptone with changes in the membrane electrical conductance. The recently completed project of sequencing the whole nuclear genome of C. reinhardtii provides the basis for future identification of molecular elements of the chemosensory cascade in this alga.

Second messenger cascade of glial responses evoked by interneuron activity and by a myomodulin peptide in the leech central nervous system

The giant glial cell in the neuropil of segmental ganglia of the leech Hirudo medicinalis responds to the activity of the Leydig interneuron and to a peptide of the myomodulin family, the presumed transmitter mediating the Leydig neuron-to-giant glial cell transmission, with a membrane hyperpolarization due to an increased membrane K+ conductance [Britz et al. (2002) Glia, 38, 215-227]. We have now studied the second messenger cascade initiated by Leydig neuron stimulation and by the endogenous myomodulin (MMHir) in the voltage-clamped giant glial cell. Glial responses to both stimuli are mediated by a G-protein-coupled receptor linked to adenylyl cyclase by the following criteria: (i) injection of GDP-beta-S, but not GDP, resulted in an irreversible decrease of the glial responses to both stimuli; (ii) the responses to both stimuli were reversibly inhibited by the adenylyl cyclase inhibitor SQ22,536; and (3) bath-applied di-butyryl-cyclic AMP, but not di-butyryl-cyclic GMP, elicited an outward current, which reduced the responses elicited by neuronal stimulation or myomodulin. A cocktail of protein kinase (PK) inhibitors (H-8, KT5720), the PKA antagonist Rp-cAMPS, or presumed inhibitors of cyclic nucleotide channels, LY83583 and l-cis-diltiazem, had no effect on the glial responses. Our results suggest that Leydig neuron stimulation and MMHir activate a cAMP-mediated K+ conductance in the glial cell, which appeared neither to be due to the activation of PKA nor of known cyclic nucleotide-gated channels directly.

On the scents of smell in the salamander

Our sense of smell is based on a remarkable chemical-detection system that possesses high sensitivity, broad discriminability and plastic, yet stable, function. Understanding how olfactory stimuli translate into perception is a problem of daunting complexity. How do odour-coding events in single cells correlate with emergent properties from the ensemble, and with behaviour? For comprehensive descriptions of neural function, analysis must extend from examination of how elemental principles relate to the function of the whole. The tiger salamander has long been used as an experimental model in studies of olfaction, enabling general questions about olfactory function to be approached.