Hypergravity signal transduction in HeLa cells with concomitant phosphorylation of proteins immunoprecipitated with anti-microtubule-associated protein antibodies

Microtubule associated protein (MAP1A) mRNA was up-regulated by hypergravity in the rat inner ear

Differential display analysis of differential mRNA expression in the rat inner ear under hypergravity identified two down- and four up-regulated genes. The up-regulation of microtubule associated protein 1A (MAP1A) in one of these was confirmed by real-time polymerase chain reaction. Since MAP1A is believed to work as a cell stabilizer connecting the actin with microtubule, this is possibly a response to strengthen this stabilizer under hypergravity. The MAP1A gene is the first found to be affected by gravity change in the inner ear.

Stimulation of DNA polymerase alpha by hypergravity generated by centrifugal acceleration

Gravity alteration is known to influence cell proliferation. Here we tested the effects of hypergravity on the action of DNA polymerase alpha, one of the DNA replication enzymes in eukaryotes. Hypergravity was produced by horizontal centrifugal acceleration with a hand-made rotator. The reaction rate of DNA polymerase alpha in centrifuge tubes increased along with the acceleration up to 4g, when a plateau was reached. In contrast, no stimulation was observed with primase, DNA polymerase epsilon, and the E. coli DNA polymerase I Klenow fragment. Kinetic analysis of DNA polymerase alpha reactions revealed that, under high gravity conditions, the K(m) value for template DNA decreased while the V(max) stayed constant. In contrast, the centrifugal acceleration did not affect the K(m) values for deoxyribonucleoside triphosphates. These results suggest that the hypergravity enhances the activity of DNA polymerase alpha by increasing the affinity of the enzyme for template DNA. Such enhancement was more prominent with a low concentration of DNA polymerase alpha under low ionic conditions.

Fertilization of sea urchin eggs and sperm motility are negatively impacted under low hypergravitational forces significant to space flight

Sperm and other flagellates swim faster in microgravity (microG) than in 1 G, raising the question of whether fertilization is altered under conditions of space travel. Such alterations have implications for reproduction of plant and animal food and for long-term space habitation by man. We previously demonstrated that microG accelerates protein phosphorylation during initiation of sperm motility but delays the sperm response to the egg chemotactic factor, speract. Thus sperm are sensitive to changes in gravitational force. New experiments using the NiZeMi centrifugal microscope examined whether low hypergravity (hyperG) causes effects opposite to microG on sperm motility, signal transduction, and fertilization. Sperm % motility and straight-line velocity were significantly inhibited by as little as 1.3 G. The phosphorylation states of FP130, an axonemal phosphoprotein, and FP160, a cAMP-dependent salt-extractable flagellar protein, both coupled to motility activation, showed a more rapid decline in hyperG. Most critically, hyperG caused an approximately 50% reduction in both the rate of sperm-egg binding and fertilization. The similar extent of inhibition of both fertilization parameters in hyperG suggests that the primary effect is on sperm rather than eggs. These results not only support our earlier microG data demonstrating that sperm are sensitive to small changes in gravitational forces but more importantly now show that this sensitivity affects the ability of sperm to fertilize eggs. Thus, more detailed studies on the impact of space flight on development should include studies of sperm function and fertilization.

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.

Comparative studies of the graviresponses of Paramecium and Loxodes

Gravitactic protozoa offer advantages in studying how the gravity stimulus is perceived on the cellular level. By means of a slow rotating centrifuge microscope in space the acceleration thresholds for gravitaxis of Loxodes striatus and Paramecium biaurelia were determined: < or = 0.15 x g for Loxodes and 0.3 x g for Paramecium, indicating different sensitivities of these species. Neutral-buoyant densities of immobilized cells were determined using media of different densities, revealing densities of 1.03 to 1.035 g/cm3 for Loxodes and 1.04 g/cm3 to 1.045 g/cm3 for Paramecium. Behavioral studies revealed that gravitaxis of Loxodes persisted independent of the density of the medium. In contrast, negative gravitaxis of Paramecium was no longer measurable if the density of the medium approached the density of the cell. The results suggest that in the case of Loxodes gravity is perceived by an intracellular receptor and, in the case of Paramecium by its own mass via the pressure on the lower cell membrane.

Activation and proliferation of lymphocytes and other mammalian cells in microgravity

The experimental findings reviewed in this chapter support the following conclusions: Proliferation. Human T-lymphocytes, associated with monocytes as accessory cells, show dramatic changes in the centrifuge, in the clinostat and in space. In free-floating cells the mitogenic response is depressed by 90% in microgravity, whereas in cells attached to a substratum activation is enhanced by 100% compared to 1-G ground and inflight controls. The duration of phase G1 of the mitotic cycle of HeLa cells is reduced in hypergravity, resulting in an increased proliferation rate. Other systems like Friend cells and WI38 human embryonic lung cells do not show significant changes. Genetic expression and signal transduction. T-lymphocytes and monocytes show important changes in the expression of cytokines like interleukin-1, interleukin-2, interferon-gamma and tumor necrosis factor. The data from space experiments in Spacelab, Space Shuttle mid-deck, and Biokosmos have helped to clarify certain aspects of the mechanism of T-cell activation. Epidermoid A431 cells show changes in the genetic expression of the proto-oncogenes c-fos and c-jun in the clinostat and in sounding rockets. Membrane function, in particular the binding of ligates as first messengers of a signal, is not changed in most of the cell systems in microgravity. Morphology and Mortility. Free cells, lymphocytes in particular, are able to move and form aggregates in microgravity, indicating that cell-cell contacts and cell communications do take place in microgravity. Dramatic morphological and ultrastructural changes are not detected in cells cultured in microgravity. Important experiments with single mammalian cells, including immune cells, were carried out recently in three Spacelab flights, (SL-J, D-2, and IML-2 in 1992, 1993, and 1994, respectively). The results of the D-2 mission have been published in ref. 75; those of the IML-2 mission in ref. 76. Finally, many cell biology experiments in space have suffered in the past from a lack of adequate controls (like 1-G centrifuges) and of proper experimental conditions (like well-controlled temperature). In this respect the availability of Biorack, outfitted with proper incubators with 1-G control centrifuge as well as a glovebox with a microscope, is a great advantage. It is also desirable that cell biology experiments in space are accompanied or even preceded by a program of ground-based investigations in the fast rotating clinostat and in the centrifuge, and that preparatory experiments be done in parabolic flights and sounding rockets, whenever possible. Proper publication of the results of space experiments is another important need. A great number of data have been published in proceedings and reports that are not available to the broad scientific community. To guarantee the credibility and the international recognition of space biology it is important that the results be published in international, peer reviewed journals.

The C5 Unit: a semi-automatic cell culture device suitable for experiments under microgravity

This paper presents data on a novel, semi-automatic cell culturing device called 'C5 Unit' (connectable circuit cell culture chamber) which was developed and adapted to the quality and size criteria set by the characteristics of the ESA Biorack. The suitability of the hardware for culturing cells under microgravity conditions was demonstrated by successful culture of primary mouse cells from neonatal cerebellum and testis aboard the Space Shuttle during the IML-2 mission.

Microgravity induces prostaglandin E2 and interleukin-6 production in normal rat osteoblasts: role in bone demineralization

It has been suggested that microgravity alters bone metabolism. Evidence for this phenomenon includes the negative calcium balance and decreased bone density in astronauts, as well as, inhibition of bone formation in rats flown for 2 to 3 weeks. However, the specific mechanisms that modulate these changes in microgravity are unknown. The purpose of this study was to clarify the mechanism of microgravity-induced bone demineralization using normal rat osteoblasts obtained from femur marrow cultures. The osteoblasts were cultured for 5 days during a Shuttle-Spacelab flight (STS-65). After collection of the culture medium, the cellular DNA and RNA were fixed on board. Enzyme-immunoassay of the culture medium for prostaglandin E2 (PGE2) indicated that microgravity induced a 4.5- to 136-fold increase in flight samples as compared to the ground control cultures. This increase of PGE2 production was consistent with a 3.3- to 9.5-fold elevation of inducible prostaglandin G/H synthase-2 (PGHS-2) mRNA, quantitated by reverse transcription-polymerase chain reaction (RT-PCR). The mRNA induction for the constitutive isozyme PGHS-1 was less than that for PGHS-2. The interleukin-6 (IL-6) mRNA was also increased (6.4- to 9.3-fold) in microgravity as compared to the ground controls. Since PGE2 and IL-6 are both known to play a role in osteoclast formation and bone resorption, these data provide molecular mechanisms that contribute to our understanding of microgravity-induced alterations in the bone resorption process.

Risk characterization and the extended spaceflight environment

Recent trends toward prolonged space flights have highlighted concern over potential health hazards for crews occupying a confined habitat for an extended duration mission. Previous spaceflight and remote habitat experiences have identified a large number of compounds that pose potential hazards as space habitat contaminants. A project is underway to prioritize these compounds according to: 1) the degree of potential exposure, 2) known or potential health risks, 3) process- and mission-specific determination of sources of contamination, and 4) preliminary evaluation of mission performance degradations associated with compound contaminant or substitution. An effort is underway to establish a comprehensive database making use of all available national and international data in order to provide a basis from which to evaluate both the impact and the likelihood of long term or transient exposure to these agents. The database will define projected capabilities of Space Station Freedom from early assembly phase through man-tended and permanently-manned capability, previous knowledge of the lunar base environment, and the limited data available from the Viking mission for the Mars environment. This information will then form the basis for mitigation measures including: 1) future process modification or substitution, 2) special provisions for compound containment or use restrictions, 3) re-evaluation of current exposure standards, and 4) reprioritization of information needs (e.g., toxicology, effects on human performance, or re-evaluation of alternative-process mission requirements).