One of the secondary objectives was to fly the National Institutes of Health Cells 4 payload (NIH.C4), sponsored by Ames Research Center. Kennedy Space Center sponsored an additional secondary payload called Biological Research in Canisters 6 (BRIC-06).
Cellular signal processing in all organisms is probably based on fundamentally similar mechanisms. The stimulus interacts with a primary receptor in order to initiate a response, mediated by signal transduction pathways. In the case of gravity as a stimulus, it has been shown that free-living single eukaryotic cells, such as slime molds, often use this vector for their spatial orientation (gravitaxis) and, in addition, show distinct sensitivities to gravity. For this experiment, slime mold cells were used to locate the gravireceptor and determine the interaction between signal perception and the cell's response (signal transduction and processing).
Life Sciences Payload
Organisms
To investigate the acceleration-stimulus signal transduction chain, a gravisensitive Myxomycete, acellular slime mold (Physarum polycephalum), was used. Its ameboid locomotion represents one of the two major types of cellular motility (the other being microtubule-based). The plasmodia, giant cells, display a distinct gravitaxis and their intrinsic rhythmic contractions and cytoplasmic streaming are modulated by gravity.
Petri dishes were flown in BRIC-60 canisters. For a general description of the BRIC-60, see BRIC-01. A Gaseous Nitrogen (GN2) Freezer was flown to allow freezing of canister contents inflight. For a general description of the GN2 Freezer, see BRIC-03.
Operations
Preflight
Several endoplasmic drops of slime mold were added to a nutrient agar in petri dishes and allowed to grow for two to three hours. Thirty-six such petri dishes were then wrapped in parafilm and loaded into two BRIC-60 canisters. Lythium hydroxide pouches were included in each canister to scrub carbon dioxide.
Inflight
A 24-hour asynchronous ground control was conducted in the Orbiter Environmental Simulator (OES). For a general description, see BRIC-01.
BRIC canisters were frozen in the GN2 Freezer at two different points in the flight: nine petri dishes after two days in space and an additional nine after three days in space. The remaining 18 petri dishes were returned to Earth and frozen after recovery.
Postflight
The flight samples were removed from the canister upon recovery and then analyzed and compared with the ground controls. Cellular secondary messenger levels (the cyclic nucleotides cAMP and cGMP) were analyzed.
Results
BRIC-06 demonstrated that only the level of the secondary messenger cAMP drops significantly with time spent in microgravity in the force-generating part of the cell. Therefore, contrary to short-term gravi-stimulations, adaptation to microgravity leads to permanently reduced cAMP levels. The results indicated that in Physarum polycephalum, the secondary messenger cAMP is involved in gravity signaling.
Additional Reading
NASA. STS-69 Press Kit, August 1995. Contained in NASA Space Shuttle Launches Web site:
http://www.ksc.nasa.gov/shuttle/missions/missions.html.