Neutral gravitaxis of gliding Loxodes exposed to normal and raised gravity

Swimming Paramecium in magnetically simulated enhanced, reduced, and inverted gravity environments

Earth's gravity exerts relatively weak forces in the range of 10-100 pN directly on cells in biological systems. Nevertheless, it biases the orientation of swimming unicellular organisms, alters bone cell differentiation, and modifies gene expression in renal cells. A number of methods of simulating different strength gravity environments, such as centrifugation, have been applied for researching the underlying mechanisms. Here, we demonstrate a magnetic force-based technique that is unique in its capability to enhance, reduce, and even invert the effective buoyancy of cells and thus simulate hypergravity, hypogravity, and inverted gravity environments. We apply it to Paramecium caudatum, a single-cell protozoan that varies its swimming propulsion depending on its orientation with respect to gravity, g. In these simulated gravities, denoted by f(gm), Paramecium exhibits a linear response up to f(gm) = 5 g, modifying its swimming as it would in the hypergravity of a centrifuge. Moreover, experiments from f(gm) = 0 to -5 g show that the response is symmetric, implying that the regulation of the swimming speed is primarily related to the buoyancy of the cell. The response becomes nonlinear for f(gm) >5 g. At f(gm) = 10 g, many paramecia "stall" (i.e., swim in place against the force), exerting a maximum propulsion force estimated to be 0.7 nN. These findings establish a general technique for applying continuously variable forces to cells or cell populations suitable for exploring their force transduction mechanisms.

Graviperception and gravitaxis in algae

Photosynthetic flagellates are among the most intensely studied unicellular organisms in the field of graviperception and gravitaxis. While the phenomenon of graviorientation has been known for many decades, only recently was the molecular mechanism unveiled. Earlier hypotheses tried to explain the precise orientation by a passive buoy mechanism assuming the tail end to be heavier than the front. In the photosynthetic flagellate Euglena gracilis, the whole cell body is denser than the surrounding medium, pressing onto the lower cell membrane where it seems to activate mechanosensitive ion channels specific for calcium. The calcium entering the cells during reorientation can be visualized by the fluorescence probe, Calcium Crimson. Cyclic AMP is likewise involved in the molecular pathway. Inhibitors of calcium channels and ionophores impair gravitaxis while caffeine, a blocker of the phosphodiesterase, enhances the precision of orientation.

Graviresponses of certain ciliates and flagellates

Protozoa are eukaryotic cells and represent suitable model systems to study the mechanisms of gravity perception and signal transduction due to their clear gravity-induced responses (gravitaxis and gravikinesis). Among protists, parallel evolution for graviperception mechanisms have been identified: either sensing by distinct stato-organelles (e.g., the Müller vesicles of the ciliate Loxodes) or by sensing the density difference between the whole cytoplasm and the extracellular medium (as proposed for Paramecium and Euglena). These two models are supported by experiments in density-adjusted media, as the gravitaxis of Loxodes was not affected, whereas the orientation of Paramecium and Euglena was completely disturbed. Both models include the involvement of ion channels in the cell membrane. Diverse experiments gave new information on the mechanism of graviperception in unicellular systems, such as threshold values in the range of 10% of gravity, relaxation of the responses after removal of the stimulus, and no visible adaptation phenomena during exposure to hypergravity or microgravity conditions for up to 12 days.

Graviorientation in protists and plants

Gravitaxis, gravikinesis, and gravitropism are different graviresponses found in protists and plants. The phenomena have been intensively studied under variable stimulations ranging from microgravity to hypergravity. A huge amount of information is now available, e.g. about the time course of these events, their adaptation capacity, thresholds, and interaction between gravity and other environmental stimuli. There is growing evidence that a pure physical mechanism can be excluded for orientation of protists in the gravity field. Similarly, a physiological signal transduction chain has been postulated in plants. Current investigations focus on the question whether gravity is perceived by intracellular gravireceptors (e.g. the Muller organelle of the ciliate Loxodes, barium sulfate vacuoles in Chara rhizoids or starch statoliths in higher plants) or whether the whole cell acts as a sedimenting body exerting pressure on the lower membrane. Behavioral studies in density adjusted media, effects of inhibitors of mechano-sensitive ion channels or manipulations of the proposed gravireceptor structures revealed that both mechanisms have been developed in protists and plants. The threshold values for graviresponses indicate that even 10% of the normal gravitational field can be detected, which demands a focusing and amplifying system such as the cytoskeleton and second messengers.

Gravitaxis in unicellular microorganisms

Orientation of organisms with respect to the gravitational field of the Earth has been studied for more than 100 years in a number of unicellular microorganisms including flagellates and ciliates. Several hypotheses have been developed how the weak stimulus is perceived. Intracellular statoliths have been found to be involved in gravitaxis of Loxodes, while no specialized organelles have been detected in other ciliates, e.g. Paramecium. Also in the slime mold Physarum no specialized gravireceptors have been identified yet. In the flagellate Euglena gracilis the whole cell body, which is denser than the surrounding medium, seems to act as a statolith pressing onto the lower membrane where it activates mechanosensitive ion channels. Similar results were obtained for the ciliate Paramecium. In contrast to the flagellate Euglena, several ciliates have been found to show gravikinesis, which is defined as a dependence of the swimming velocity on the direction of movement in the gravity field.

Graviresponses of gliding and swimming Loxodes using step transition to weightlessness

Cells of Loxodes striatus were adjusted to defined culturing, experimental solution O2-supply, temperature, and state of equilibration to be subjected to step type transition of acceleration from normal gravity, (1 g) to the weightless condition (microgravity) during free fall in a 500 m drop shaft. Cellular locomotion inside a vertical experimental chamber was recorded preceding transition and during 10 s of microgravity. Cell tracks from video records were used to separate cells gliding along a solid surface from free swimmers, and to determine gravitaxis and gravikenesis of gliding and swimming cells. With O2 concentrations > or = 40% air saturation gliders and swimmers showed a positive gravitaxis. In microgravity gravitaxis of gliders relaxed within 5 s whereas gravitaxis relaxation of swimmers was not completed even after 10 s. Rates of horizontal gliders (319 micrometers/s) exceeded those, of horizontal swimmers (275 micrometers/s). Relaxation of gravikinesis was incomplete after 10 s of microgravity. Analysis of the locomotion rates during the g-step transition revealed that gliders sediment more slowly, than swimmers (14 versus 45 micrometers/s). The gravikinesis of gliders cancelled sedimentation effects during upward and downward locomotion tending to maintain cells at a predetermined level inside sediments of a freshwater habitat. At > or = 40% air saturation, gravikinesis of swimmers augmented the speed of the majority of cells during gravitaxis, which favours fast vertical migrations of Loxodes.

Electric potentiation of gravikinesis in Paramecium is possibly mediated by filaments

Sensitivity of Paramecium to mechanical stress including gravitational force is organized along two opposing gradients of membrane channel distribution: depolarizing Ca channels and hyperpolarizing K channels. Mechanoreceptor channels reside in the membrane of the cell soma and are activated, when the weight of the cytoplasm deforms the "lower" plasma membrane. Channel distribution is such as to generate ciliary activation which can counteract sedimentation of the cells: a reduction in downward swimming rate and an augmentation in upward swimming rate. Application of weak DC fields does not only induce the well-known cathodal orientation and swimming of Paramecium toward the cathode (galvano-taxis). We document that swimming velocity is augmented up to 175% as a function of the voltage gradient between 0.3 V/cm and 0.8 V/cm (galvanokinesis). A gradient of 0.3 V/cm was highly effective in raising the common negative gravikinesis of downward swimmers threefold. The gravikinesis of upward swimmers reversed polarity under field stimulation inducing cells to augment sedimentation effects (positive gravikinesis). Both effects of electric-field stimulation on ciliary activation are of the depolarizing type: reduction in the frequency of normally beating cilia. Analysis of the data shows that a voltage-sensitivity of gravireceptor channels would not account for the observed potentiation of negative gravikinesis. It is suggested that a previously described voltage-dependent Ca channel of the soma membrane interferes with a Ca(2+)-sensitive, peripheral filament system, which directly connects to gravireceptor channels.

Mechanisms of graviperception and response in unicellular systems

This introduction to a symposium considers established principles of mechanoreception and the physiology of excitable cells as a background of gravireception. From the perspective of experimental work in protists, current developments in the treatment and interpretation of graviresponses are addressed.

Influence of accelerations on the spatial orientation of Loxodes and Paramecium

The gravitactic ciliates Paramecium and Loxodes were cultivated for 15 days in space during the IML-2 spacelab mission. At dedicated times their behavioral responses to different accelerations between 10(-3) x g and 1.5 x g were investigated by using a slow rotating centrifuge microscope (NIZEMI). The threshold for gravitaxis of Paramecium was found to be at > 0.16 x g and < or = 0.3 x g. No adaptation of Paramecium to the conditions of weightlessness was observed over the duration of 15 days. Loxodes showed no graviresponses to increasing accelerations, though it demonstrated gravitaxis after return to earth.

A theory of gravikinesis in Paramecium

The archaic eukaryote unicellular microorganism, Paramecium, is propelled by thousands of cilia, which are regulated by modulation of the membrane potential. Ciliates can successfully cope with gravity, which is the phylogenetically oldest stimulus for living things. One mechanism for overcoming sedimentation is negative gravitaxis, an orientational response antiparallel to the gravity vector. We have postulated the existence of a negative gravikinesis in Paramecium, i.e. a modulation of swimming speed as a function of cellular orientation in space. With negative gravikinesis, an upward oriented cell actively augments the rate of forward swimming and depresses active locomotion during downward orientation. A brief outline of the gravikinesis hypothesis is given on a quantitative basis and experimental data are presented which have confirmed the major assumptions.