Gravity - a linear acceleration stimulus - is an appropriate reference value for spatial orientation. It thus promoted already in unicellular organisms the development of gravity perceiving structures and of gravisensing organs in higher organisms. Free-living single cells performing clear gravity responses to counteract their sedimentation (passive settling) are perfect model systems to investigate gravitaxis (= spatial orientation) and gravikinesis (= g-dependent regulation of the locomotion velocity). They have the experimental advantage that perception, transduction, and stimulus response are performed in one cell. Ciliates regulate their swimming behavior via the coordinated beat of several thousand cilia. This beat is controlled by the membrane potential and the activation of ion channels, i.e. events also occurring in nerve cells. The amoeboid giant cell Physarum polycephalum (acellular slime mold, plasmodium) regulates its directed movement via the rhythmic contractions of its protoplasmic veins. The contractions are driven by actomyosin, which also drives human muscle contractions.
|Perception via specialized gravisensing organelles (Loxodes)|
|Perception via dense cell organelles (Physarum)|
|Perception of the cell mass via mechanosensitive ion channels (Paramecium)|
To elucidate the mechanism of graviperception and signal transduAction, both cell types were investigated under varying accelerations and their behavior analyzed using computer-controlled image analysis. Methods used to apply varying accelerations were hypergravity, simulated weightlessness achieved using fast-rotating clinostats, and microgravity in sounding rockets (TEXUS 27, 28, 39 und MAXUS 2), the Space Shuttle (D1, IML-1, IML 2, STS-69, S/MM-06) and parabolic flights (www.novespace.fr). In addition, cells were examined in density-modified media and after destruction of receptor candidates.
Experiments concerning graviresponses of ciliates are also perfomed in the DLR_School_Lab
Graviresponses are enhanced in hypergravity, whereas they disappear in weightlessness - the time course of the relaxation indicates to the involvement of elastic cytoskeletal elements.
The threshold for graviresponses is in the range of 0.1 – 0.3xg.
Ciliates do not adapt to weightlessness (at least not within 14 days), whereas the slime mold adapts within a few hours to the new stimulus quality.
The widely distributed second messenger cAMP is somehow involved in the gravistimulus signal-transduction chain.
Present Models of Gravity Perception
Perception via specialized gravisensing organelles. The ciliate Loxodes possesses vacuoles enclosing a heavy mineral complex, attached to a microtubular stick. Upon destruction of this structure by a laser beam the cells swim randomly like in weightlessness.
Perception via dense cell organelles. In the case of Physarum it is postulated that the gravistimulus is perceived via cell organelles which exceed the density of the cytoplasm by at about 0.35 g/cm³. Here primarily the plasmodial multitude of nuclei is proposed as a receptor candidate.
Perception of the cell mass via mechanosensitive ion channels in the cell membrane (e.g. in case of Paramecium). The precondition is a polar channel distribution in the cell membrane: hyperpolarizing mechanosensitive potassium channels at the rear and depolarizing mechanosensitive calcium channels A at the front end (see scheme right). The pressure of the cytoplasm onto the lower membrane induces a specific stimulation, modifying the ciliary beat and thus changing the behavior accordingly.
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