Performing sensitive biological experiments is
always a delicate affair. Few researchers, however, contend with the
challenges faced by Cheryl Nickerson, whose working laboratory aboard
the International Space Station (ISS) is located hundreds of miles above
the Earth, traveling at some 17,000 miles per hour.
Nickerson,
a microbiologist at Arizona State University’s Biodesign Institute, is
using the ISS platform to pursue new research into the effects of
microgravity on disease-causing organisms.
Nickerson presented her research findings and charted the course for future investigations aboard the ISS on February 18 at the 2013 annual meeting for the American Association for the Advancement of Science, held in Boston, Mass. Her talk, entitled “Microgravity: A Novel Tool for Advances in Biomedical Research,” is part of a special session devoted to ISS science.
“One important focus of my research
is to use the microgravity environment of spaceflight as an innovative
biomedical research platform. We seek to unveil novel cellular and
molecular mechanisms related to infectious disease progression that
cannot be observed here on Earth, and to translate our findings to novel
strategies for treatment and prevention.”
During an earlier series of NASA
space shuttle and ground-based experiments, Nickerson and her team made a
startling discovery. Spaceflight culture increased the disease-causing
potential (virulence) of the foodborne pathogen Salmonella, yet many of
the genes known to be important for its virulence were not turned on and
off as expected when this organism is grown on Earth. Understanding how
this switching is regulated may be useful for designing targeted
strategies to prevent infection.
For NASA, Nickerson’s findings were
revelatory, given their implications for the health of astronauts on
extended spaceflight missions. Already faced with the potential for
compromised immunity induced by the rigors of space travel, astronauts
may have to further contend with the threat of disease-causing microbes
with amped-up infectious abilities. A more thorough understanding of
infectious processes and host responses under these conditions is
therefore vital for the design of therapeutics and other methods of
limiting vulnerability for those on space missions.
The story however, doesn’t end
there. Further research by Nickerson’s team pointed to important
implications for the understanding of health and disease on Earth. Her
team, including NASA scientists, showed that one of the central factors
affecting the behavior of pathogenic cells is the physical force
produced by the movement of fluid over a bacterial cell’s sensitive
surface. This property, known as fluid shear, helps modulate a broad
range of cell behaviors, provoking changes in cell morphology,
virulence, and global alterations in gene expression, in pathogens like
Salmonella.
“There
are conditions that are encountered by pathogens during the infection
process in the human body that are relevant to conditions that these
same organisms experience when cultured in spaceflight. By studying the
effect of spaceflight on the disease-causing potential of major
pathogens like Salmonella, we may be able to provide insight into
infectious disease mechanisms that cannot be attained using traditional
experimental approaches on Earth, where gravity can mask key cellular
responses,” says Nickerson
Nickerson’s spaceflight studies
also pinpointed an evolutionarily conserved protein—called Hfq—which
appears to act as a global regulator of gene responses to spaceflight
conditions. Further research by her team established that Hfq is a
central mediator in the spaceflight-induced responses of other bacterial
pathogens, including Pseudomonas aeruginosa, thus representing the
first spaceflight-induced regulator acting across bacterial species.
Nickerson’s examination of the
post-spaceflight alterations in bacterial behavior made use of
microarray technology, which allows analysis of gene expression for the
entire 4.8 million base pairs found in Salmonella’s circular chromosome.
Data revealed that 167 distinct genes and 73 proteins had been altered
during growth under microgravity conditions, including (but not limited
to) virulence-associated genes. Of the 167 genes undergoing up- or
down-regulation in response to spaceflight, one third were under the
control of the Hfq master regulator protein.
These microgravity studies open a
new window into the infectious disease mechanisms of Salmonella, an
aggressive pathogen responsible for infecting an estimated 94 million people globally and causing 155,000 deaths annually. In
the U.S. alone, more than 40,000 cases of Salmonellosis are reported
annually, resulting in at least 500 deaths, and health care costs in
excess of $50 million. However, only a small percentage of infections with Salmonellaare
reported, and the estimated two to four million cases of
Salmonella-induced gastroenteritis which occur in the United States each
year constitute a significant economic loss of productive work time,
reported to exceed $2 billion annually.
While
Salmonella has been a pathogen of choice for a broad range of
spaceflight investigations, Nickerson stresses that her findings have
spaceflight and Earth-based implications. Her
confidence is based on her team’s work showing that microgravity
culture also uniquely alters gene expression and pathogenesis-related
responses in other microorganisms.
Nickerson emphasizes that the ISS
provides an unprecedented opportunity to study the infection process
under microgravity conditions, enabling advances in our understanding of
microbial gene expression and accompanying host responses during
infection in fine-grained detail. This novel approach holds the
potential to identify new classes of genes and proteins associated with
infection and disease not possible using traditional experimental
conditions on Earth, where the force of gravity can mask certain
cellular responses. Further, experiments aboard the ISS will permit the
study of microbial transitions and cellular responses to infection over a
prolonged time frame - an important advance not available during
shuttle-based experiments.
Microgravity research may provide
an opportunity to identify novel targets for vaccine development and the
Nickerson team, in collaboration with Roy Curtiss, director of the
Biodesign Institute’s Center for Infectious Diseases and Vaccinology has
been working toward this goal. Based on previous findings, the
scientists hypothesized that results from microgravity experiments might
be used to facilitate vaccine development on Earth.
In a recent spaceflight experiment
aboard space shuttle mission STS-135, the team flew a genetically
modified Salmonella-based anti-pneumoccal vaccine that was developed in
the Curtiss lab. By understanding the effect of microgravity culture on
the gene expression and immunogenicity of the vaccine strain, their goal
is to genetically modify the strain back on Earth to enhance its
ability to confer a protective immune response against pneumococcal
pneumonia.
“Recognizing that the spaceflight
environment imparts a unique signal capable of modifying Salmonella
virulence, we will use this same principle in an effort to enhance the
protective immune response of the recombinant attenuated Salmonella
vaccine strain,” Nickerson says.
Nickerson’s space-based
microgravity experiments are carried out in conjunction with
simultaneous Earth-based controls housed in the same hardware as those
in orbit, to compare the behavior of bacterial cells under normal Earth
gravity. Additional information is also provided using Earth-based cell
cultures which are subjected to a kind of simulated microgravity,
produced by culturing cells in a rotating wall vessel bioreactor (RWV), a
device designed by NASA engineers to replicate aspects of cell culture
in the spaceflight environment.
Back at ASU, RWV reactor
experiments were conducted by Nickerson and her team to help confirm
that Hfq plays a central regulatory role in the Salmonella response to
spaceflight conditions. Nickerson has also used this RWV technology to
grow three dimensional (3-D) cell culture models that mimic key aspects
of the structure and function of tissues in the body. These 3-D models
are being used in the Nickerson lab as human surrogates to provide novel
insight into the infectious disease process not obtainable by
conventional approaches and for drug/therapeutic testing and development
for treatment and prevention.
Nickerson also focuses research
efforts on determining the entire repertoire of environmental factors
that may influence bacterial response to spaceflight culture. For
example, she found that the ion concentration in the cell culture media
played a key role in the resulting effect of spaceflight on Salmonella
virulence. Using the RWV, she was able to identify specific salts that
may be responsible for this effect.
Nickerson’s long list of firsts
(first study to examine the effect of spaceflight on the virulence of a
pathogen, first to obtain the entire gene expression response of a
bacterium to spaceflight, first to profile the infection process in
human cells in spaceflight, first identification of a
spaceflight-responsive global gene regulator acting across bacterial
species), will soon be augmented with a new experiment, that will be
flown on SpaceX Dragon slated for the ISS later this year. Nicknamed
PHOENIX, the project will mark the first time a whole, living
organism—in this case a nematode—will be infected with a pathogen and
simultaneously monitored in real time during the infection process under
microgravity conditions.
This and future studies aboard ISS
will almost certainly deepen science’s understanding of the molecular
and cellular cues underlying pathogenic virulence and open a new chapter
in the understanding of health and disease to benefit the general
public.
“It is exciting to me that our work to discover how to keep astronauts healthy during spaceflight may translate into novel ways to prevent infectious diseases here on Earth,” Nickerson says.