Gravisensitivity of the acellular slime mold Physarum polycephalum demonstrated on the fast-rotating clinostat

The acellular slime mold Physarum polycephalum was used to investigate a postulated general gravisensitivity of cells. Physarum was subjected i) to a rotation on the fast-rotating clinostat, which enables the simulation of weightlessness (0 g), and ii) to single horizontal turns of 180 degrees. On the fast-rotating clinostat the response consists of a frequency increase in radial contractile activity, an oscillation of the mean values (frequency regulation phenomena) and an increase in standard deviation. A combination of 0 g and respiration impediment inhibits the response to 0 g, i. e., the frequency increase in radial contractile activity during weightlessness simulation. Turning the specimens horizontally about 180 degrees in a normally positioned light microscope also leads to a frequency increase in radial contractile activity, but the temporal pattern of the frequency increase differs in comparison to the experiments performed on the clinostat during weightlessness simulation. These results demonstrate the occurrence of gravisensitivity in Physarum polvcephalum. Regulation phenomena and the possible role of mitochondria in graviperception are discussed.

Paramecium--a model system for studying cellular graviperception

Experiments under varied gravitational accelerations as well as in density-adjusted media showed that sensation of gravity in protists may be linked to the known principles of mechanosensation. Paramecium, a ciliate with clear graviresponses (gravitaxis and gravikinesis) is an ideal model system to prove this hypothesis since the ciliary activity and thus the swimming behaviour is controlled by the membrane potential. It has also been assumed that the cytoplasmic mass causes a distinct stimulation of the bipolarly distributed mechano-sensitive K+ and Ca2+ ion channels in the plasma membrane in dependence of the spatial orientation of the cell. In order to prove this hypothesis, different channel blockers are currently under investigation. Gadolinium did not inhibit gravitaxis in Paramecium, showing that it does not specifically block gravireceptors. Further studies concentrated on the question of whether second messengers are involved in the gravity signal transduction chain. Exposure to 5 g for up to 10 min led to a significant increase in cAMP.

Putative graviperception mechanisms of protists

Many (if not all) free-living cells use the gravity vector for their spatial orientation (gravitaxis). Additional responses may include gravikinesis as well as changes in morphological and physiological parameters. Though using essentially different modes of locomotion, ameboid and ciliated cells seem to rely on common fundamental graviperception mechanisms. Uniquely in the ciliate family Loxodidae a specialized intracellular gravireceptor organelle has been developed, whereas in all other cells common cell structures seem to be responsible for gravisensing. Changes in direction or magnitude of acceleration (from 0 to 5 g) as well as experiments in density-adjusted media strongly indicate that either the whole cytoplasm or dense organelles like nuclei act as statoliths and open directly or via cytoskeletal elements mechano-sensitive ion channels in the cell membrane. A recent spaceflight experiment (S/MM-06) demonstrated that prolonged (9 d) actual weightlessness did not affect the ability of Loxodes to respond to acceleration stimuli. However, prolonged cooling (> or = l4 d, 4-10 degrees C) destroyed the ability for gravitactic orientation of Paramecium. This may reflect a profound effect either on the gravireceptor itself or on the gravity-signal processing. In gravity signalling the ubiquitous second messenger cAMP may be involved in acceleration-stimulus transduction.

Involvement of the second messenger cAMP in gravity-signal transduction in Physarum

The aim of the investigation was to clarify, whether cellular signal processing following graviperception involves second messenger pathways. The test object was a most gravisensitive free-living ameboid cell, the myxomycete (acellular slime mold) Physarum polycephalum. It was demonstrated that the motor response is related to acceleration-dependent changes in the levels of the cellular second messenger cyclic adenosine monophosphate (cAMP). Rotating Physarum plasmodia in the gravity field of the Earth about a horizontal axis increased their cAMP concentration. Depriving the cells for a few days of the acceleration stimulus (near weightlessness in a space experiment on STS-69) slightly lowered plasmodial cAMP levels. Thus, the results provide first indications that the acceleration-stimulus signal transduction chain of Physarum uses an ubiquitous second messenger pathway.

Acceleration-sensitivity threshold of Physarum

Free-living cells show distinct gravisensitivities and often use the gravity ('g') vector for their spatial orientation. The rhythmic contractions of the ameboid Myxomycete (acellular slime mold) Physarum polycephalum are a sensitive parameter which can be modified by external stimuli. Space experiments and ground-based 0 x g simulation studies established that the contraction period transiently decreases after a transition from 1 x g to 0 x g with a back-regulating process starting after 30 min. For determination of the threshold of acceleration sensitivity, a slow-rotating centrifuge microscope (NIZEMI--Niedergeschwindigkeits-Zentrifugenmikroskop) was used, providing in space accelerations from 0 x g to 1.5 x g. A stepwise acceleration increase revealed that the lowest acceleration level capable of inducing a response was 0.1 x g. The response to the acceleration increase was an increase in contraction period, in contrast to a stimulus deprivation, which led to a period decrease. The time schedule of the acceleration responses and back-regulating process seems to be fixed, suggesting that every acceleration being above the threshold can induce a complete response-regulation process. The low acceleration-sensitivity threshold favors rather large and dense cell organelles as candidates for the gravity receptor in Physarum.

Gravity perception and signal transduction in single cells

Cellular signal processing in multi-, as well as in unicellular organisms, has to rely on fundamentally similar mechanisms. Free-living single cells often use the gravity vector for their spatial orientation (gravitaxis) and show distinct gravisensitivities. In this investigation the gravisensitive giant ameboid cell Physarum polycephalum (Myxomycetes, acellular slime molds) is used. Its gravitaxis and the modulation of its intrinsic rhythmic contraction activity by gravity was demonstrated in 180 degrees-turn experiments and in simulated, as well as in actual, near-weightlessness studies (fast-rotating clinostat; Spacelab D1, IML-1). The stimulus perception was addressed in an IML-2 experiment, which provided information on the gravireceptor itself by the determination of the cell's acceleration-sensitivity threshold. Ground-based experiments designed to elucidate the subsequent steps in signal transduction leading to a motor response, suggest that an acceleration stimulus induces changes in the level of second messenger, adenosine 3',5'-cyclic monophosphate (cAMP), indicating also that the acceleration-stimulus signal transduction chain of Physarum uses an ubiquitous second messenger pathway.

Gravitational response of the slime mold Physarum

The acellular slime mold Physarum polycephalum is used as a model system to investigate the graviresponse of single cells which possess no receptors specialized for the perception of gravity. To obtain insights into the gravity-signal transduction mechanism the light response of the cell is used: Macroplasmodia of the slime mold show clear geo- and phototaxes. Gravity increases and white light decreases transiently the contraction frequency of plasmodial strands whereby both responses follow the same time pattern. Since mitochondria play a major role in changing the contraction rhythm in response to light and gravity stimuli, the simultaneous and subsequent inductions of the opposing light and gravity responses and their mutual influences on one another were investigated. The experiments were performed in weightlessness (0 g)--simulated on the fast-rotating clinostat as well as in actual weightlessness during the IML-1 Space Shuttle mission. The results indicate that mitochondria (chondriome) are part of the acceleration-stimulus reaction chain in Physarum. Two models for a direct gravireceptor mechanism are discussed.

Potential sites for the perception of gravity in the acellular slime mold Physarum polycephalum

Recently a gravisensitivity of the acellular slime mold Physarum polycephalum, which possesses no specialized gravireceptor, could be established by conducting experiments under simulated and under real near weightlessness. In these experiments macroplasmodia showed a modulation of their contraction rhythm followed by regulation phenomena. Until now the perception mechanism for the gravistimulus is unknown, but several findings indicate the involvement of mitochondria: A) During the impediment of respiration the 0g-reaction is inhibited and the regulation is reduced. B) The response to a light stimulus and the following regulation phenomena strongly resemble the behavior during exposure to 0g, the only difference is that the two reactions are directed into opposite directions. In the blue-light reaction a flavin of the mitochondrial matrix seems to be involved in the light perception. C) The contraction rhythm as well as its modulations are coupled to rhythmic changes in the levels of ATP and calcium ions, involving the mitochondria as sites of energy production and of Ca(++)-storage. So the mitochondria could be the site of the regulation and they possibly are the receptor sites for the light and gravity stimuli. Also the observation of a morphologic polarity of the slime mold's plasmodial strands has to be considered: Cross-sections reveal that the ectoplasmic wall surrounding the streaming endoplasm is much thinner on the physically lower side than on the upper side of the strand--this applies to strands lying on or hanging on a horizontal surface. So, in addition to the mitochondria, also the morphologic polarity may be involved in the perception mechanism of the observed gravisensitivity and of the recently established geotaxis. The potential role of the nuclei and of the contractile elements in the perception of gravity is also discussed.

Classification of gravity effects on "free" cells

When cell physiologists detect gravity related reactions of their objects it is often difficult to decide where the receptors for the observed effects are located. Answering this question is necessary for any further analysis of a detected gravity effect on cells. In previous papers we have discussed direct and indirect gravity effects in relation to the smallest functional units where the primary receptor, which interacts with gravity, is positioned inside and outside of such a unit, respectively. So, in a first approximation we can conclude that in a multicellular aquatic organism, which changes its metabolism in weightlessness, the primary receptors of gravity are located inside the cells of that organism. A special approach is necessary when free living cells, the density of which may be higher than the one of the (liquid) medium, or even cells living on a free surface are observed. In these two cases also indirect effects have to be taken into account, which will be demonstrated with the aid of the slime mold Physarum polycephalum. Additionally the environment of the organisms can be changed directly and indirectly by gravity.

Confirmation of gravisensitivity in the slime mold Physarum polycephalum under near weightlessness

We have investigated Physarum polycephalum, a unicellular organism with no special gravity receptors, on its ability to react to gravity. The first experiments were 0 g-simulation experiments on the fast-rotating clinostat conducted with plasmodial strands of this acellular slime mold. In these earth-bound experiments the observed parameters were periodicity of the contractions and dilatations of the strand's ectoplasm as well as the periodicity and velocity of the striking cytoplasmic (endoplasmic) shuttle streaming. During 0 g-simulation these parameters showed significant changes indicating the existence of a gravisensitivity of the slime mold. The Space-Shuttle experiment (ESA-Biorack in D 1-Mission) should demonstrate the validity of the 0 g-simulation on the fast-rotating clinostat. The experiment was designed in a way enabling the registration of the same parameters as on the clinostat (using the light microscope in combination with a photo diode and a cinecamera). Only one of the two planned measurement sessions was fully successful and provided us with data confirming the results gained on the fast-rotating clinostat: The slime mold showed under real near weightlessness in the D 1-Space Shuttle Mission a transient frequency increase in tis contraction rhythmicity and a (steady) increase in the streaming velocity of its endoplasm.

[Polar neutral organic compounds in urban aerosols. 1. Chemical characterization and mutagenic effect in relation to various sources]

Airborne particulate matter was collected by high volume samplers at two sampling stations in Berlin (West) between March 1983 and February 1984 (immissions). The stations were choosen so that two main-sources of urban air pollution could be considered (automobile) traffic and private fuel combustion (domesticfuel). Also in February and March 1983 particles were collected from private coal firing (emissions). The collected particles were fractionated into the following particle sizes (immissions, during October 1983-February 1984): greater than 7.2 microns, 7.2-1.5 microns and less than 1.5 micron (aerodynamic diameter). The etherextractable organic matter of the particles (= EEOM) was determined and the organic matter was then separated into acidic, basic, and neutral fractions. The neutral fraction was further separated into aliphatic compounds (= AlP), polycyclic aromatic hydrocarbons (= PAH), and polar neutral compounds (= POCN) by thin layer chromatography. The mutagenic activity of all organic fractions was determined by using the mammalian microsome bioassay by Ames and was compared with the activity of a whole polar organic extract (including POCN, acidic and basic fraction), a whole neutral extract, and a whole ethersoluble organic extract. Also in February 1984 the nitroreductase specific mutagenicity on immission-stations was determined using nitroreductase deficient strains. Some compounds of the POCN-fraction, and of PAH (in comparison) were identified by gas chromatography. The following results were obtained: The neutral fraction of the organic matter amounted to 70-90% of the EEOM (not depending from particle size, immissions and emissions). The separation of the neutral compounds into AlP, PAH and POCN showed a higher amount of AlP at the collecting station which includes mainly automobile traffic. Some cancer and/or mutagenic suspect compounds were identified by the chromatographic analysis of POCN and PAH. A higher amount of several nitrogroups-containing POCN was found at the collecting station which includes mainly private combustion whereas at the station including mainly automobile traffic 1-nitropyrene was the main compound. The POCN-fraction showed the mostly highest mutagenic activity in the Ames bioassay, mainly for lung penetrating particles (less than 1.5 micron diameter). The whole organic extract (EEOM) showed a lower mutagenic activity than the POCN-fraction. Using nitroreductase deficient strains, at the collecting station which includes mainly private fuel combustion a higher reduction of mutagenicity was shown than at the automobile-traffic including measurement station.

The pathway of photosensory transduction in Physarum polycephalum

Irradiation of the plasmodia of Physarum with blue and white light results in a transient change of theie oscillatory contraction frequency. This reaction to light decreases with increasing distance from the illuminated area (Block and Wohlfarth-Bottermann, 1981). The first local appearance of light response in non-illuminated parts of the plasmodia was used to analyse the sensory pathway of the light stimulus modulating the contractile apparatus. Different experimental assays revealed that the direction of photosensory transduction is determined by the momentary direction of protoplasmic shuttle streaming. The endoplasmic flow carriers the signal responsible for photosensory transduction and light reaction to the force generating ectoplasmic tube.

Blue light as a medium to influence oscillatory contraction frequency in Physarum

Blue light (496 nm; threshold intensity approximately 1500 Lux) induces a transient frequency decrease of oscillatory contraction automaticity in Physarum. Many, but not all specimens react by an increase in the force amplitude of longitudinal contraction. The spectral region of 496 nm provokes a photophobic response of the plasmodia. The blue light reaction of radial and longitudinal contraction activities decreases with increasing distance from the irradiated area. The light-induced decrease in frequency can be used for experimental phase shifting, e.g., when studying the nature and the pathway of signal transmission for the spatial phase synchronization of contractile activities.