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Microgravity Research Opportunities for the 1990s: Chapter 5
Microgravity Research Opportunities
for the 1990s
PART II—SCIENTIFIC ISSUES
5
Biological Sciences and Biotechnology
INTRODUCTION
Progress in biology research is often a consequence of the investigator's skill in
maintaining experimental control through the use of diverse chemical, physical, genetic,
and immunological techniques. Major advances in molecular biology and biotechnology are
critically dependent on refinements of existing techniques and the development of new and
better ones.
Microgravity research in the biological sciences can help to further these advances.
It can also provide fundamental knowledge important to NASA's overall goals. To do so,
REPORT MENU
however, a systematic program is needed to identify and explore those cellular and
NOTICE
biomolecular processes, mechanisms, structures, and assemblies that are affected by
MEMBERSHIP
transfer to the microgravity environment. For the several areas of biology and
PREFACE
EXECUTIVE SUMMARY biotechnology, such as cell culture, cell fusion, electrophoretic separation, and protein
PART I crystal growth, in which the effects of microgravity have been demonstrated, further
CHAPTER 1 research is needed to identify and explore mechanisms by which gravitational effects are
CHAPTER 2 realized. New methods and techniques to take advantage of the potential of microgravity
PART II
applications need to be developed.
CHAPTER 3
CHAPTER 4
CHAPTER 5
CHAPTER 6
CHAPTER 7 JUSTIFICATION FOR MICROGRAVITY RESEARCH
PART III
CHAPTER 8
Gravity affects biological systems through its influence on the transfer of mass and
APPENDIX A
heat, particularly in the area of fluid dynamics and transport, as well as its less well
APPENDIX B
understood impact on cell structure and function. The basis of modern research in the
biological sciences is the study of molecules, molecular assemblies, organelles, cells, and
cell assemblies in controlled fluid and chemical environments. The impetus for microgravity
research is that it may lead to new knowledge about biological systems, to improvements in
current experimental techniques, and to the development of new experimental approaches
to biological problems.
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The NASA research program in the biological sciences and biotechnology has
focused on three primary areas of biological interest: (1) separation physics aimed at
providing improved resolution and sensitivity in preparative and bioanalytical techniques; (2)
cell biology, cell function, and cell-cell interactions; and (3) physical chemistry of biological
macromolecules and their interactions, including studies of protein crystal growth directed
at supporting crystallographic structure determinations. Brief discussions of microgravity
research in each of these areas provide an introduction to the work that has been done and
to some of the realized or anticipated scientific and technological benefits.
Separation physics.1 Progress in the biological sciences and biotechnology has
been limited to some extent by the failure of fluid-based techniques of current separation
and analytical methods to achieve the necessary resolution, a failure exacerbated by the
effects of gravity. Two such effects are density-dependent thermal convection and
sedimentation, both of which perturb processes used to separate and purify proteins,
fractionate cell components and organelles, or separate mixtures of naturally or genetically
engineered cells.
Cell biology.2 Modern research in the biological sciences and biotechnology
relies on the manipulation of cells of living organisms. In the case of biotechnology, the
purpose of these manipulations may be to produce useful molecules-both naturally
occurring and of artificial origin-in useful quantities; to develop new organisms or new
biological molecules for specific uses; or to improve yields of plant and animal products
through genetic alteration. Recombinant techniques, for example, make it possible to
produce natural or artificially mutated versions of proteins exhibiting a wide range of
activities and uses, scientific and medical, in heretofore extraordinary quantities. The
techniques essential to these manipulations are applied in aqueous environments and are
subject to fluid dynamics and transport processes. Gravitational effects may have important
consequences-experimental in the case of scientific research and financial in the case of
biotechnological production. Examples include fermentation processes; compartmental
targeting of expressed products within the cell; and the ultimate purity, structural integrity,
and activity of a protein product. Particle sedimentation under the influence of gravity, for
example, can interfere with aggregation processes such as those mediating cell-cell
interactions, cell fusion, cell agglutination, and cellular interactions with substrates.
Molecular structure.3 A full understanding of the functions of biological
macromolecules, and of the chemical and physical effects that they organize and manage
to achieve these functions, is not possible without detailed knowledge of their three-
dimensional architectures. Nor is it possible to engineer new proteins, whether for medical
uses or as complex biomaterials, without an ability to relate molecular structure and
function. Protein crystallography, currently the principal method for determining the
structure of complex biological molecules, requires relatively large, well-ordered single
crystals of useful morphology. Crystals with these qualities may be difficult to produce for a
variety of reasons, some of which may be influenced by gravity, through density-driven
convection and sedimentation. Protein crystal growth experiments conducted aboard the
shuttle have provided persuasive evidence that improvements can, in fact, be realized for a
variety of protein samples.
Another justification for microgravity research concerns the importance of biological
research to technological developments supporting crewed space missions. A number of
ambitious goals for human exploration of the solar system have been advocated. These
include a permanently inhabited base on the Moon and a program of crewed missions to
Mars.
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Experience from a variety of extended missions in Earth orbit has shown that
prolonged exposure to microgravity can have profound effects on human physiology and
that return to terrestrial gravity may require long periods of readjustment and reconditioning.
Research in the life sciences has begun to pinpoint some of the physiological systems most
affected by habituation to reduced-gravity conditions, but little is known about the effects of
microgravity on the underlying cellular processes. Additional biological science research is
required in support of life sciences and human physiology goals if we are to meet the full
range of life-support challenges presented by human space exploration. These challenges
range from the development of methods for ameliorating the physiological effects of
microgravity and the return to terrestrial gravity to the development of technologies for
meeting nutritional, respiratory, and waste disposal needs. This research should explore
microgravity effects on cells and cell processes, particularly those involving human
physiological systems at risk, as well as survey microgravity effects across a wide range of
model systems and organisms.
A final justification for microgravity research in the biological sciences is the
anticipation that new knowledge will be accumulated by the optimized production of small
amounts of microgravity-derived biomaterials such as purified proteins or protein crystals.
Such precious specimens may induce the research community to conduct experiments in
novel and previously inaccessible areas of inquiry. Microgravity research may also facilitate
otherwise labor-intensive, and therefore seldom-attempted, efforts on Earth and make
precious reagents and biomaterials accessible to a wider community. It is possible that
knowledge gained in microgravity experiments will permit improvements in terrestrial
strategies in such areas as bioseparations and materials processing. It is also possible that
methods and processes may be discovered that are possible only in space.
FUNDAMENTAL RESEARCH AREAS
NASA has for some time nurtured a program of flight experiments to delineate
specific advantages and limitations of microgravity research with regard to biological
sciences. Experiments were designed to produce results that were anticipated from model
experiments or analyses but could not be tested in terrestrial laboratories. Because of
broad interest in these studies, scientists in a wide range of disciplines, in both academia
and industry, were encouraged to participate and judge the potential benefits of biology and
biotechnology research in microgravity. Typical of innovative science, preliminary
experiments in electrophoresis, phase partitioning, protein crystal growth, and cell culture
yielded unexpected results. In some cases, additional or unanticipated advantages of a
microgravity environment appeared; in other cases, secondary effects masked in terrestrial
experiments became dominant. Research areas in the biological sciences that have
demonstrated potential benefit from microgravity experimentation are presented below.
Investigations of Mechanisms of Macromolecular
Interactions at the Subcellular Level
Biotechnology is for the most part applied molecular biology in the sense that
molecular events occurring within the cell, or among groups of cells, are altered or
manipulated in some way to achieve a desired end. The methods involved may be based
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on recombinant DNA technology, cell fusion, or a number of other cellular or subcellular
approaches. The objectives may be to produce altered cell types in culture, specific
immunoglobulins, drugs, commercial enzymes, proteins, other pharmacological agents, and
even whole organisms having improved properties. In order to apply molecular biology at
the cellular and subcellular level, where microgravity is intended as a probe or a tool, one
must know with some assurance exactly which processes and mechanisms are, in fact,
affected by gravity. At present, no adequate knowledge base exists.
The underlying premise for performing biology and biotechnology experiments in
space, and for the development of molecular biology tools based on the absence of gravity,
is that the microgravity environment must have direct effects on cells and subcellular
events. A similar premise underlies research to understand the short- and long-term
physiological effects of microgravity on humans and to develop methods and procedures to
minimize or eliminate any negative impacts of the effect of microgravity. The following
questions must be answered: What are the effects of microgravity on cells and subcellular
events, what are their magnitudes, and what are the consequences? Even if the
consequences are insignificant, the answers are needed in order to understand these
systems fully.
A concerted effort should be made to determine convincingly whether microgravity
has an observable effect on the growth, development, and structure of living cells.
Observations and biochemical assays should be conducted on organisms exposed for
various periods of time to a microgravity environment to determine if there are any gross
changes in character and, if so, to what general categories of events these effects should
be ascribed.
Following a careful cataloging of microgravity effects on cells, studies should be
extended to more complex cell-cell systems and the study of longer-term trends. These
might include, depending on the earlier results, investigations of cell-cell and cell-matrix
interactions and how they are affected by microgravity, as well as the transport of materials
between cells. Mixed cell systems should be examined to determine if intercellular
communication is affected. Other areas for investigation include the secretory and transport
properties of cultured endocrine cells, the development of nerve cells, plant cell-wall
structural changes, chemotaxis, virus propagation, adhesion processes, and the assembly
of organized macromolecular aggregations such as viruses, membranes, and cell
organelles.
In the long term, processes such as multigenerational changes, embryogenesis,
evolution, and complex cell interactions approaching the organism level must be
addressed. This level of sophistication would also include viral, fungal, and bacterial
infection of cells and the role that microgravity plays in alterations of these interactions.
Growth of Biological Macromolecular Crystals
Macromolecular crystallography has proved to be a powerful tool for basic research
in biochemistry and molecular biology. As a result, it has attracted the interest and support
of the pharmaceutical, chemical, and biotechnology industries, particularly for use in
structure-based drug design and protein engineering. Crystals of an appropriate size and
quality are a prerequisite for determination of the three-dimensional structures of proteins
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and nucleic acids. The major obstacle in the application of crystallographic methods to
proteins and in the application of protein structure in structure-based drug design is growing
suitable crystals.
Initial research in crystal growth under microgravity conditions concentrated on the
preparation of metals, alloys, and electronic materials and the identification of model
systems. Results of these experiments suggested that growth in microgravity might also
have beneficial effects on the preparation of protein crystals. Rationales included the
elimination of sedimentation, which might create more nearly isotropic or uniform growth
conditions and thereby produce improvements in size and shape, and the elimination of
density-driven convection, which might create conditions closer to diffusion-limited growth
and thereby improve crystalline order and extend x-ray scattering limits to higher resolution.
An extensive program of experiments in protein crystal growth in microgravity has been
conducted in recent years to test these possibilities. This program has been international in
scope and has employed a variety of orbital vehicles, but the majority of these experiments
have been sponsored by NASA and conducted aboard the space shuttle.
In recent years, successful macromolecular crystallization experiments have been
performed by U.S. investigators using a number of instruments of U.S. and European
design. In early Vapor Diffusion Apparatus (VDA) experiments, crystals of g-interferon,
porcine elastase, and isocitrate lyase grew larger, displayed more uniform morphologies,
and yielded diffraction data of higher resolution than equivalent crystals grown on Earth.4
Similar results were obtained for canavalin5 and positive results continue to accumulate
from this apparatus.6 On USML-1, experiments in the glovebox, using a modified vapor
diffusion technique, yielded crystals of malic enzyme of substantially enhanced
properties.7,8 In the Cryostat device provided by the German Space Agency DARA, crystals
were obtained by liquid-liquid diffusion of satellite tobacco mosaic virus (STMV) that were
larger and diffracted to a higher resolution than the best obtained in the laboratory. Data
from these crystals allowed structure determination of STMV at 1.8-Å resolution, the highest
ever achieved for any virus crystal.9-11 On IML-1 in the Cryostat12 and on IML-2 in the
European Space Agency (ESA) Advanced Protein Crystallization Facility (APCF), which
also supports liquid-liquid diffusion experiments, a number of morphological alterations to
crystals of canavalin and another larger virus, turnip yellow mosaic virus (TYMV), were
clearly demonstrated. A third technique, temperature-induced batch crystallization, has
recently produced larger, higher-resolution crystals of insulin.13 A group of U.S.
investigators also carried out protein crystal growth experiments on the Russian Mir, using
a number of different crystallization devices. In agreement with the results cited above, they
reported that experiments on 5 of 21 proteins produced results superior to those obtained
on Earth.14,15 An interesting aspect of the results seen so far is that when the same protein
is crystallized by a variety of different techniques in microgravity, a range of crystalline
samples may be expected,16 pointing up the need for multiple flight experiments and
optimization. These experiments have provided persuasive evidence that growth in
microgravity can produce protein crystals of larger size, better shape, and higher quality
than have been obtained on Earth. They also show that benefits from microgravity crystal
growth can be crucial to success in protein structure determination.17,18 (See Figure 5.1
and Plates 5.1 and 5.2.)
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FIGURE 5.1 Crystal of malic enzyme form parasitic nematode Ascaris suum grown in
interactive glovebox experiments aboard the United States Microgravity Laboratory-1 flight
in 1992. While this crystal (0.20 to 0.25 mm on an edge) is about 20 to 25% of the volume
of good crystals grown on Earth, resolution limits from this crystal exceed by about 0.5 Å
the best data ever collected from crystals grown on Earth.
On the other hand, although protein crystal growth experiments in microgravity have
yielded a variety of encouraging and successful results, they have not shown that protein
crystals uniformly display improved properties when grown in microgravity. A reason for this
result might be the mismatch between protein and technique. Experiments testing a variety
of proteins in a particular apparatus have been illuminating. These experiments could be
expanded to include a broader range of proteins, such as membrane proteins, receptor-
ligand complexes, glycoproteins, and other problematic macromolecules, to understand
better the limits of a given technique and what might be done to overcome them. It is also
important, however, that a broader range of techniques be explored in repetitive
microgravity experiments. More experiments that use liquid-liquid diffusion or batch
methods, for example, will provide a better understanding of the differences among
techniques and the results expected to be produced in microgravity. Another reason that
not all protein crystals grown in microgravity have shown improved properties might be that
crystallization conditions have not been optimized for growth in a microgravity environment.
Past results have shown that, more often than not, optimum conditions for growth on Earth
are not optimum for growth in microgravity and that the periods of time required to allow
crystal growth to go to completion are too short for optimal growth in microgravity. The
glovebox experiments in protein crystal growth on USML-1 were the first to offer
opportunities for iterative experiments in microgravity with an experienced specialist to
interpret results and design subsequent experiments. The results of these experiments
indicated that iteration can increase the production of crystals of high quality in
microgravity.19,20
A goal of future experiments should be to provide a better understanding of the
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fundamental forces and mechanisms governing protein crystal growth on Earth and in
microgravity. While it may continue to be important to survey the crystal growth
performance of a wide variety of proteins and techniques in space, it is becoming
increasingly necessary as well to observe, measure, and manipulate crystal growth
processes so that we understand what they entail. A systematic evaluation of which
macromolecules derive the greatest benefits, and from which techniques, requires repetitive
experiments with a single method to optimize growth parameters for specific proteins, as
well as an expansion of experiments to a wider variety of proteins and methods. Analyses
of chemical and physical characteristics of macromolecules that impact their organization
into crystals merit support. Although a diverse mix of macromolecules might be desirable,
even essential, in future crystal growth experiments, a systematic approach to these
experiments is the only means of delineating the dominant factors involved in protein crystal
growth in microgravity.
Automation and remote control of protein crystal growth will likely be needed for
future microgravity experiments. Automated systems that permit dynamic monitoring and
control of key variables such as temperature, protein concentration, ionic strength, pH, and
precipitant concentration will be useful for both ground-based and space experiments that
are directed at better understanding of protein crystal growth processes. Nevertheless, at
least for the near term, NASA may want to consider support for additional missions such as
USML-1 in which an experienced observer is in control of iterative protein crystal growth
experiments.
In summary, the success of initial experiments suggests that further research is
needed and an expanded program of protein crystal growth experiments deserves support.
It would be helpful if this research were to focus somewhat more sharply on gaining a
clearer understanding of the physical and chemical phenomena involved in the nucleation
and growth of protein crystals, in addition to developing an expanded range of techniques
and methods to secure the benefits of microgravity growth for a broader range of
macromolecular crystals. This implies increased basic research in protein crystal growth on
Earth not only to expand understanding of the fundamental scientific principles, but also to
support and develop the cadre of scientists needed to design and conduct the fundamental
crystal growth experiments in microgravity.
Separation and Purification of
Biological Macromolecules and Assemblies
Biological systems typically contain a large variety of proteins and nucleic acid
molecules that have widely differing properties and that are often combined in
macromolecular complexes, oligomers, and unique molecular assemblies such as viruses
and ribosomes. Purification of macromolecules, complexes, or even cells is often a
necessary first step to more detailed biological characterization and biotechnology
applications. This is true whether isolation is directly from natural sources, such as blood
sera, or from manufactured mixtures, such as culture media, fermentation products,
hybridoma cultures, or synthetic peptide solutions. Furthermore, to exploit the potential of
genetic engineering and engineered cells, it is necessary to purify products from the
heterogeneous mixtures in which they are synthesized. Associated separation and
purification challenges are often major obstacles to advanced biochemical studies and
biotechnological applications.
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Analytical separations commonly use electrophoresis in water-based gels to good
advantage; gels of this type are generally recognized as providing the highest resolution for
analytical separations of proteins and analyses of nucleic acid sequences. Density-driven
thermal convection and sedimentation are well-known phenomena limiting the resolution of
electrokinetic separations on Earth, and suppression of these phenomena in microgravity
environments might be expected to eliminate the need for gels and make it possible to
achieve the ultimate resolving power of electrophoretic experiments in free fluids. The
results of electrophoresis experiments carried out in microgravity with well-characterized
molecules and particles have shown that buoyancy-driven phenomena are diminished, but
have revealed new phenomena at work that may affect purification processes.21 One of the
significant findings has been the role of electrohydrodynamic effects in electrophoresis,
where in certain experiments in microgravity they appear to significantly disrupt
resolution.22 More fundamental research, much of it in the fields of fluid dynamics and
transport phenomena, is needed if investigators are to learn how to moderate or eliminate
disadvantages due to electrohydrodynamic effects in microgravity. Further, while it is
important to use microgravity to minimize negative effects caused by gravitational
acceleration, it would also be useful to harness the unique features of microgravity fluid
processes, such as virtually unlimited capillary rise or interfacial tension effects, for the
enhancement of separation technology.
Separation processes also depend, in some cases, on subtle differences in the
interactions between a molecule of interest and different solvents. Investigations of
partitioning in aqueous polymer two-phase systems on recent shuttle missions have
demonstrated the dependence of separation efficiency on several variables, including
volume fractions of the component phases; container geometry; physical properties of the
component phases, such as interfacial tension and relative phase viscosities; and surface
tension interactions between aqueous phases and container walls.23 Some of these
variables can be minimized; interactions with container walls, for example, may be reduced
by special coatings.
The importance of improvements in resolution in chromatographic and
electrophoretic separations to progress in the biochemical and biological sciences should
not be underestimated. For example, biological sciences and biotechnology will be
influenced significantly over the next decade by the mapping of the human genome-an
effort that will require the isolation and purification of DNA fragments on a scale not
encountered previously. For other applications, resolution requirements for protein
purification have become increasingly stringent as the role of impurities in recombinant
products has become more evident. A continued program of microgravity research in the
separation and purification of biological macromolecules should be supported where it can
contribute to improved techniques and improved results in terrestrial and microgravity
environments.
Cell Culture, Growth, and Differentiation
The culture and manipulation of living cells, particularly the more fragile mammalian
and plant cells, can be scientifically and technologically rewarding, but it can also be
technically challenging. Some of these cell types, which naturally occur as part of
specialized tissues and organs, are dependent for their survival and proper function on the
environment provided by other cells and tissues. While the unique functions of differentiated
cells make them especially valuable for research or for the generation of specific products,
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such cells, as a direct consequence of their interdependence, frequently exhibit sensitivity
to shear, have complex metabolic requirements, and lose cell functions when cultured in
artificial environments. Such sensitivities can complicate the utilization of these cells to
generate desired products or responses. The culture of cells in microgravity offers a
number of potential benefits: sedimentation and density-driven convection are virtually
absent, and changes in physical and chemical properties of the fluids constituting and
nourishing the culture media can be exploited to benefit the culture system. The resulting
environment may, therefore, be relatively quiescent and free of the steep velocity gradients
in fluids and the sedimentation effects characteristic of ground-based cell culture systems.
Improvement of the culture of shear-sensitive mammalian and plant cells would permit the
manipulation of cells in ways that are difficult or impossible to accomplish on Earth. Such
effects could be particularly important in processes that are dependent on cellular
interactions, such as cell fusion or the assembly of three-dimensional multicellular systems.
Prototype culture systems have been developed that are compatible with
microgravity operations and that simulate some of the fluid effects found in microgravity.
Mammalian cell cultures have been used to test the feasibility and potential for extended
microgravity research. Preliminary results indicate that culture devices developed with
NASA support provide a low fluid shear environment with minimal sedimentation.24 Delicate
mammalian cells have been cultured to very high densities in these instruments, and
unique associations of cells into tissue-like aggregates have been observed. Limitations on
the development of multicellular structures due to gravitationally induced disturbances have
also been observed. Microgravity will enable maintenance of a culture environment that
supplies minimum shear and maximum freedom for three-dimensional cellular association.
Cell Fusion and Membrane Assembly
The fusion of cells to produce viable hybrids-a low-frequency event with current
terrestrial technologies-is another area that could benefit from the microgravity
environment. Fused cells are used for genetic studies where natural crosses are impossible
or impractical. Fusion is the key step to monoclonal antibody production. Cell fusion
requires the interaction of two different cell types and the annealing of the cell membranes
to yield one hybrid cell. The fusion of cell membranes is usually accomplished by treatment
with electric current or chemicals, either or both of which may be harmful to the cell. The
cells to be fused are usually of different densities, so sedimentation tends to separate the
cells during the fusion process and leads to aborted fusion or cell death. Microgravity could
reduce the tendency of the cells to separate due to sedimentation and density differences.
This might permit a reduction in the intensity and duration of the high-frequency aligning
field in the case of electrofusion and, possibly, in the concentrations of fusing agents used
for chemically induced cell fusion. Similar considerations may be operative in the synthesis
of liposomes and the deposition of biomembranes.
Further studies concerning the influence of a microgravity environment on cell
fusion are needed. Results from sounding rocket and spacelab experiments25,26 suggested
that studies should be extended to typical mammalian fusion partners. The effects of such
factors as cell viability, growth phase, composition of growth medium, and shear
environment on the fusion process should be evaluated. Other technologies should be
investigated in both ground-based studies and microgravity to determine whether
improvements are observed in microgravity similar to those observed with electrofusion
techniques. Basic research utilizing model artificial membranes and related biopolymer
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analogues may provide a better understanding of the processes and mechanisms at work
in the fusion of natural cell membranes and could possibly lead to the development of more
durable artificial membranes. A thorough characterization of the factors influencing fusion
efficiency will improve Earth-based fusion technologies and will benefit the development of
microgravity-based cell fusion activities.
EXPERIMENTAL REQUIREMENTS
Research areas in the biological sciences with expected sensitivity to the
gravitational environment involve processes that typically take hours, days, or even weeks.
The minimum duration of microgravity required for these experiments is, thus, greater than
that available from drop towers, parabolic airplane flights, or suborbital rockets. If
experiments in the biological sciences are to be conducted in microgravity, they must be
carried out aboard the shuttle, the space station, or other orbiters such as free-flyers.
Little is known about the limits below which accelerations, or frequencies and
durations of accelerations, must be maintained so that the biological effects of microgravity
are not obscured. It must be true, of course, that if the overall acceleration environment of
the experimental system is kept to a fraction of the stochastic acceleration environment of
its molecular components, then interference with studies of biological effects of microgravity
is avoided. In the case of the shuttle, it is known that the background acceleration
environment in orbit, the so-called g-jitter, can reach levels that interfere with biological
experiments. Acceleration environments will vary from vehicle to vehicle and acceleration
limits will vary from experiment to experiment, so that the best match of experiment to
vehicle will have to be judged on a case-by-case basis. It may also be that acceleration
environments that are a constant fraction of unit gravity may be helpful to studies in some
areas of the biological sciences. It is important, however, that acceleration environments be
monitored, not only so that experiments can be interpreted properly but also so that realistic
acceleration scenarios can be included in the experimental design.
RECOMMENDATIONS AND CONCLUSIONS
In support of the biological and biotechnical research areas described above,
priority should be given in the order listed to the following research activities:
1. More research should be devoted to quantifying the role of gravity in protein
crystal growth processes and to identifying those mechanisms affected by gravity.
2. Further experimentation is needed in both terrestrial and microgravity
environments to develop new methods and materials that take advantage of microgravity
for biochemical separations.
3. The potential advantages of the microgravity environment for the study of cellular
interactions, cell fusion, and multicellular assembly processes should be explored to identify
candidate cell systems that show maximum benefit from culture in a microgravity
environment.
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4. Cellular and biomolecular processes, structures, assemblies, and mechanisms
that might be affected by gravity should be systematically assayed to explore the effects of
microgravity on these systems. The objective should be a comprehensive database of cell
types and their experiences in microgravity environments.
The first two topics have some demonstrated successes and the greater likelihood
of future successes. The other two topics are exploratory.
REFERENCES
1. Vanderhoff, J.W., and C.J. van Oss. 1979. Electrophoretic separation of
biological cells in microgravity. Pp. 257-273 in Electrokinetic Separation Methods, P.G.
Righetti, C.J. van Oss, and J.W. Vanderhoff, eds. Elsevier/North-Holland Biomedical Press,
Amsterdam.
2. Cogoli, A. 1987. Cell cultures in space: From basic research to biotechnology II.
Pp. 285-290 in Proceedings of the Third European Symposium on Life Sciences Research
in Space, Graz, Austria, September 14-18. ESA SP-271.
3. Bugg, C.E. 1986. The future of protein crystal growth. J. Cryst. Growth, 76:535-
544.
4. DeLucas, L.J., et al. 1989. Protein crystal growth in microgravity. Science,
246:651-654.
5. McPherson, A., A. Greenwood, and J. Day. 1991. The effect of microgravity on
protein crystal growth. Adv. Space Res., 11(7):343-356.
6. DeLucas, L.J., et al. 1994. Recent results and new hardware developments for
protein crystal growth in microgravity. J. Cryst. Growth, 135:183-195.
7. DeLucas, L.J., et al. 1993. Protein crystal growth results from the United States
Microgravity Laboratory-1 mission. J. Phys. D: Appl. Phys., 26:B100-B103.
8. DeLucas, L.J., et al. 1993. Protein crystal growth results from the United States
Micro-gravity Laboratory-1 Mission. NASA Reference Publication (conference proceedings)
from the USML-1 and USMP-1 Joint L+1 Science Review. Huntsville, Ala., September 22-
24.
9. Day, J., and A. McPherson. 1992. Macromolecular crystal growth experiments on
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on International Microgravity Laboratory-1. Protein Science, 11:1254-1268.
13. Long, M.M., et al. 1994. Protein crystal growth in microgravity-temperature-
induced large-scale crystallization of insulin. Proceedings of the International Symposium of
Microgravity Sciences and Applications, Beijing, China, May 10-13, 1993. Microgravity Sci.
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14. Stoddard, B.L., R.K. Strong, G.K. Farber, A. Arrott, and G. Petsko. 1991. Design
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of biological macromolecules using the Mir space station. J. Cryst. Growth, 110:312-316.
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growth of protein crystal in microgravity aboard the Mir space station. J. Cryst. Growth,
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25. Baumann, T., W. Kreis, W. Mehrle, R. Hampp, and E. Reinhard. 1990.
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PLATE 5.1 Crystals of satellite tobacco mosaic virus (STMV) grown in microgravity during
the flight of International Microgravity Laboratory-1 in January 1992. X-ray diffraction
measurements demonstrated the diffraction of the crystals to a resolution greater than 1.8-
Å Bragg spacings, significantly improved over those grown in Earth laboratories.
PLATE 5.2 Droplets of protein solution in the Vapor Diffusion Apparatus (VDA) on shuttle
mission STS-26. Droplets, about 40 l in size, are poised on double-barreled syringes and
are formed by the mixing of protein and precipitant solutions isolated in separate barrels
prior to launch. Droplets equilibrate by vapor diffusion against a reservoir of precipitant
stabilized in microgravity in a porous high-molecular-weight polyethylene plug (not shown).
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