graviperception between 0.16 and 0.3 g (Hemmersbach et al. 1996). Using a
centrifuge with an attached microscope on TEXUS and MAXUS sounding rockets,
providing accelerations during 6 min and 13 min of microgravity the threshold could
be measured even more precisely to be 0.12 g for Euglena (Häder et al. 1997). The
direct comparison between Paramecium and Loxodes confirmed the threshold for
Paramecium in the range of 0.16 and 0.3 g and revealed higher sensitivity of 0.15 g
for Loxodes. The swimming velocity of Euglena followed precisely Stokes’ law for
sedimentation (Häder et al. 1996) in contrast to the ciliates Paramecium and Loxodes
which show gravikinesis (Machemer-Röhnisch et al. 1998). Also no adaptation to
the microgravity conditions was found.
3.6 Sensory Transduction Chain for Gravitaxis
If it is correct that the heavier cellular cytoplasm presses onto the lower membrane,
the next question is what is the nature of the gravireceptor which perceives this force
(Häder et al. 2005b). One option could be mechanosensitive ion channels which can
be found in many organisms from bacteria to vertebrates. These membrane-bound
proteins are operated by small mechanical forces and gate the influx of cations
(Jarman and Groves 2013). In order to be effective first an ion gradient has to be
established across the outer membrane. In fact, in Euglena an ATPase was found
which pumps Ca
2+ into the outer medium and generates a gradient of six orders of
magnitude. This pump can be blocked by vanadate and as a result gravitactic orientation is impaired (Lebert et al. 1997). Likewise, the synthetic ionophore
calcimycin (A23187) can be integrated into the cell membrane, which breaks down
the Ca
2+ gradient and reduces gravitactic orientation (Lebert et al. 1996). The presumed mechanosensitive ion channels can be blocked by gadolinium, which hinders
the Ca
2+ influx into the cell during gravitactic stimulation (Häder and Hemmersbach
1997). Likewise, application of potassium or cadmium strongly reduce the precision
of gravitaxis in Euglena (Lebert et al. 1996). Also in Chlamydomonas
mechanosensitive ion channels are thought to be involved in graviperception
(Yoshimura 2011) since mutants which lack these proteins did not show gravitaxis
(Häder et al. 2006).
After stimulation the Ca
2+ gated by the channel protein can be visualized using a
fluorescent chromophore such as Calcium Crimson previously loaded into the cells
by electroporation; a strongly fluorescent signal could be detected near the front end
of the cell. This was confirmed in the colorless E. longa and the likewise colorless,
but gravitactic competent mutants 1F, 9F and FB (Häder and Lebert 2001). The
fluorescent signal could also be detected during hypergravity phases of parabolic
airplane maneuvers (Richter et al. 2002c) as well as on a centrifuge during a
parabolic rocket flight when the threshold was exceeded (Häder and Lebert 1998).
Gadolinium, which blocks stretch-sensitive ion channels (Sachs and Morris 1998)
impaired the signal. The mechanosensitive ion channels are supposed to be located
at the front end of the cell adjacent to the trailing flagellum. In that position they are
36
3 Gravitaxis in Flagellates and Ciliates
centrifuge with an attached microscope on TEXUS and MAXUS sounding rockets,
providing accelerations during 6 min and 13 min of microgravity the threshold could
be measured even more precisely to be 0.12 g for Euglena (Häder et al. 1997). The
direct comparison between Paramecium and Loxodes confirmed the threshold for
Paramecium in the range of 0.16 and 0.3 g and revealed higher sensitivity of 0.15 g
for Loxodes. The swimming velocity of Euglena followed precisely Stokes’ law for
sedimentation (Häder et al. 1996) in contrast to the ciliates Paramecium and Loxodes
which show gravikinesis (Machemer-Röhnisch et al. 1998). Also no adaptation to
the microgravity conditions was found.
3.6 Sensory Transduction Chain for Gravitaxis
If it is correct that the heavier cellular cytoplasm presses onto the lower membrane,
the next question is what is the nature of the gravireceptor which perceives this force
(Häder et al. 2005b). One option could be mechanosensitive ion channels which can
be found in many organisms from bacteria to vertebrates. These membrane-bound
proteins are operated by small mechanical forces and gate the influx of cations
(Jarman and Groves 2013). In order to be effective first an ion gradient has to be
established across the outer membrane. In fact, in Euglena an ATPase was found
which pumps Ca
2+ into the outer medium and generates a gradient of six orders of
magnitude. This pump can be blocked by vanadate and as a result gravitactic orientation is impaired (Lebert et al. 1997). Likewise, the synthetic ionophore
calcimycin (A23187) can be integrated into the cell membrane, which breaks down
the Ca
2+ gradient and reduces gravitactic orientation (Lebert et al. 1996). The presumed mechanosensitive ion channels can be blocked by gadolinium, which hinders
the Ca
2+ influx into the cell during gravitactic stimulation (Häder and Hemmersbach
1997). Likewise, application of potassium or cadmium strongly reduce the precision
of gravitaxis in Euglena (Lebert et al. 1996). Also in Chlamydomonas
mechanosensitive ion channels are thought to be involved in graviperception
(Yoshimura 2011) since mutants which lack these proteins did not show gravitaxis
(Häder et al. 2006).
After stimulation the Ca
2+ gated by the channel protein can be visualized using a
fluorescent chromophore such as Calcium Crimson previously loaded into the cells
by electroporation; a strongly fluorescent signal could be detected near the front end
of the cell. This was confirmed in the colorless E. longa and the likewise colorless,
but gravitactic competent mutants 1F, 9F and FB (Häder and Lebert 2001). The
fluorescent signal could also be detected during hypergravity phases of parabolic
airplane maneuvers (Richter et al. 2002c) as well as on a centrifuge during a
parabolic rocket flight when the threshold was exceeded (Häder and Lebert 1998).
Gadolinium, which blocks stretch-sensitive ion channels (Sachs and Morris 1998)
impaired the signal. The mechanosensitive ion channels are supposed to be located
at the front end of the cell adjacent to the trailing flagellum. In that position they are
36
3 Gravitaxis in Flagellates and Ciliates