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A
Return on Investment
Understanding
responses to environmental changes and improving the health and lifespan
of ecosystems and people are among the potential benefits of systems
biology. |
As
ORNL researchers seek answers to life sciences' persistent questions,
some are struck by how systems biology applies to life on a variety
of scales. Rich Norby, a physiological ecologist in ORNL's Environmental
Sciences Division (ESD), is one of those scientists. At the annual
American Association for the Advancement of Science meeting in 2004
in Seattle, Norby heard LeRoy Hood, pioneer of the DNA sequencer, "talk
about systems biology in a way that made me think, 'That's what I do.'"
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The effects of elevated levels of carbon dioxide on a forest are being determined at this ORNL sweetgum stand.
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Hood
studies living cells and immunology while Norby focuses on forests
and global change.
"Dr. Hood
was talking about DNA, proteins, and underlying networks in a live
cell and I talked as a panelist at the AAAS conference about trees,
roots, microbes, air temperature, and carbon dioxide in a small forest
ecosystem. There's a huge gulf in scale there, but I think the basic
principles are still the same. In both cases the objective is to identify
components of the system and analyze their interactions to reveal emergent
properties of the system as a whole."
Systems
biology research at ORNL has applications at different scales, as shown
in a few examples in this article. The research described relates to
ecosystems and global change, energy production, bioremediation, and
human health.
Ecosystems
and Global Change
Norby is
one of many ESD researchers who studied individual seedlings in chambers
to determine how their physiology was affected by exposure to one pollutant
such as ozone. The scientists studied a simple, controlled system that
was a long way from today's endpoint of a global forest.
FACE experiment. After
a number of years experimenting with small tree seedlings in growth
chambers and saplings in field chambers, Norby became involved in the
much larger, more complex but controlled Free-Air Carbon Dioxide (CO2)
Enrichment (FACE) experiment, a Department of Energy user facility
at ORNL. The purpose of this experiment, now in its seventh year, has
been to help DOE understand more completely the consequences of elevated
atmospheric CO2. Whereas in the past, the focus of experiments
was on individual components of the system, the current focus is on
integration of component organisms and processes into a system-level
response.
In the
FACE experiment, tons of CO2 are pumped into plots of sweetgum
trees, so that the concentration of CO2 in the tree stand
is almost 50% higher than the ambient level. Norby and his colleagues
then compare the responses of the forest ecosystems exposed to elevated
levels of CO2 with the responses of the forest in ambient
air.
Each year,
ORNL staff measure net primary productivity (NPP), or the total amount
of carbon fixed into organic matter in the ecosystem, above and below
ground. They have found that the NPP of the plots exposed to elevated
CO2 was enhanced by about 23% annually over six years. But
how that carbon was allocated changed over time.
"In the
first year," Norby says, "the trees exposed to elevated CO2 had
a 35% increase in wood production—above ground trunks and branches.
In the second year it was 15% and then in the past four years we measured
only a 5 to 7% annual increase in wood production.
A new experiment at ORNL is measuring the growth of roots of seven species of grasses, herbs, and weeds.
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Instead, the NPP
shows up in fine root production. Our technician Joanne Ledford has
measured and documented increases in production of fine roots over
six years, which is a significant and unprecedented response."
Fine roots
are an important component of a forest system because they regulate
the cycling of carbon, water, and essential nutrients. Unfortunately,
for DOE's interest in removing CO2 from the air, fine roots
do not store carbon for much more than a year, unlike the wood in tree
trunks, which can store carbon for decades.
"Fine roots
have a short life, and when they die, microbes digest them to get energy" Norby
says. "Much of the carbon in fine roots is returned to the atmosphere
as CO2. That's not a good story if your interest is net
removal of CO2 from the atmosphere."
However,
because considerable carbon is moving through the soil system, an opportunity
exists for some of it to be trapped in longer-lived soil organic matter
pools. One important research challenge at the FACE facility is to
quantify the amount of carbon that remains in or gains access to these
pools. Researchers are seeing indications that, compared with the ambient
plots, the FACE plots show an increase in 'protected' carbon—soil
carbon that will not decompose right away.
"Through
understanding the interrelationships between components within the
system and how they work together to get an integrated response, we
will have a stronger basis from which to project the responses of forests
to global change," Norby says. "Systems biology applied at a large
scale can help DOE better understand biological impacts of atmospheric
and climatic change."
Underground
activities. The
FACE experiment demonstrated the importance of whole-system analysis,
including responses below the ground. The experimental system, however,
is very simple—one dominant species (sweetgum) and one environmental
change (CO2).
A new study
at ORNL is investigating the responses of a more complex community
to multiple environmental change factors. Near the FACE facility is
the Old-Field Community, Climate, and Atmospheric Manipulation (OCCAM)
experiment, a new joint project between ORNL and the University of
Tennessee. ESD's Steve DiFazio is the principal investigator of an
internally funded Laboratory Directed Research and Development Program
project on ecosystem genomics that makes use of the OCCAM experiment.
DiFazio
is testing genomics in a systems-level approach on abandoned land to
determine the amount of growth of roots of seven different species
of grasses, herbs, and weeds subjected to three different treatments— ambient
and elevated atmospheric CO2, ambient and increased temperature
(higher by 3°C), and ambient and decreased soil moisture. The ORNL
researchers are interested in how the different combinations of treatments
change the composition of the plant community, and how this transformation
alters ecosystem responses.
The different
species are not difficult to distinguish above ground, but observing
community composition changes below ground presents a challenge. "In
the OCCAM plots, you cannot tell which species the intertwined roots
belong to because there is no easy way to distinguish, based on a root's
appearance, which of the aboveground species it came from," DiFazio
says.
To solve
the problem, he and his colleagues are taking a novel approach based
on the new field of ecosystem genomics. The approach views ecosystems
not as a web of habitats for a variety of species but rather as a stage
on which genes, proteins, and living cells interact.
"We are
investigating a method in which characteristic DNA from the roots of
the individual plants is used to identify the species," DiFazio says. "We
will take a plug of soil, grind it up and, using our DNA-based technique,
determine the relative abundance of each species present. By comparing
the different treatments, we will know more about how different species
respond to changes in carbon dioxide levels, temperature, and soil
moisture."
Another
aspect of the project is an assessment of the indirect effects of the
plant responses to ecosystem perturbations. For example, researchers
are uncertain how microbial populations will respond to increased productivity
and competitiveness of individual plant species. Also, if the plants
have a higher rate of photosynthesis under changing conditions, will
microbial populations that fix nitrogen for plants adapt fast enough
to meet plants' nutritional needs?
To address
these questions, ESD's Jizhong Zhou is using microarrays to assess
the responses of microbial populations in this experiment. His team
will determine how the microbial populations change in response to
the treatments, and whether changes in plant populations are reflected
in the composition and functioning of the microbial communities. Detailed,
integrated studies such as these are required to achieve a systems
level understanding of the effects of climatic change.
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Improving the efficiency of biological hydrogen production is an ORNL research goal.
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Energy Production
A systems
biology approach to understanding a protein complex could unlock a
source of energy, according to Brian Davison, director of ORNL's
Life Sciences Division (LSD). One complex is hydrogenase, an enzyme
that can take electrons and protons from other enzymes and compounds
and use them to release hydrogen for use in power-producing fuel cells. "This
is a way to produce energy in the form of hydrogen using a biological
system," Davison adds.
Systems
biology may enable researchers to find smarter ways to harness the
ability of microbes to produce hydrogen under certain limited conditions. "Everything
in the life of some microorganisms has tended to limit their ability
to produce hydrogen," Davison says. "Some microorganisms,
we think, use hydrogenases to deal with excess energy and prevent the
buildup of protons and other free radicals floating around inside their
cells. We want the process to produce
hydrogen all the time, so we are studying the enzyme's active site
and its partners to see how hydrogen is generated."
Scientists
have found evidence that when hydrogenase is used to produce hydrogen
from water, the oxygen that is formed can hurt the hydrogenase. Research
at ORNL and elsewhere seeks to solve this problem to improve the efficiency
of biological hydrogen production.
"When we
fully understand natural systems," Davison says, "I believe we can
coax them to do our bidding in a smarter, better way than in past biological
approaches where success came from doing experiments and getting desired
results by accident."
Bioremediation
Bioremediation
is the use of microorganisms to eliminate, contain, or reduce the concentration
of contaminants in soil and water. One of DOE's missions is to use
bioremediation to clean up waste sites or immobilize wastes so they
do not migrate.
Computer visualization of a Shewanella bacterial protein.
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Using a systems biology approach, ORNL researchers
have been helping DOE identify bacteria that can immobilize and make
less bioavailable any compounds containing radionuclides, such as uranium
and strontium, and toxic metals, such as chromium, technetium, and
mercury.
ORNL researchers
are helping DOE search for bacteria that show great promise for changing
uranium compounds from a soluble to an insoluble chemical state. Such
transformed uranium compounds are more likely to stay put in soil or
sediments rather than dissolve in groundwater and flow off-site. If
scientists can find a bacterium that is especially effective at reducing
uranium compounds, the discovery could well meet DOE's environmental
goals and save millions of dollars in potential cleanup costs.
Researchers
could characterize the capabilities of this new bacterium and try to
identify the genes that enable the reduction of each uranium atom by
donating two electrons. Such an interaction with metal enables the
bacterium to extract energy from carbon.
Jizhong
Zhou, Dorothea Thompson, and others on Zhou's team have been studying
the bacterium Shewanella oneidensis strain MR-1, whose genome
was completely sequenced by The Institute for Genomic Research (TIGR)
and annotated by TIGR as well as LSD's Frank Larimer and others. Shewanella is
able to make uranium less soluble in the laboratory, but further research
is needed to determine how well the bacterium responds at a toxic waste
site.
"Because
of DOE's mission, we are trying to understand how Shewanella responds
to environmental stresses such as high and low pH, high temperature,
high salt, and metal toxicity," Thompson says "DOE wishes to know how Shewanella transforms,
detoxifies, and reduces metals in the environment. DOE also seeks to
understand the relationship to environmental stresses and which stresses
make the process less effective in bioremediation."
DNA microarray
technology allows Zhou's team to place an array of at least 20,000
DNA probes, each corresponding to a single microbial gene, on a glass
microscope slide. They can look at the global expression—all
the responses of genes at the messenger RNA level—in a bacterial
cell exposed to a toxic metal such as strontium or chromium. Then,
by interacting with the mass spectrometry group at ORNL, they can determine
if the switched-on, or up-regulated, genes produce corresponding increases
in the encoded protein products.
Working
with Steve Brown, an ESD postdoctoral researcher, Thompson found in
some cases that a limited set of genes in Shewanella revealed
greater than a hundredfold increases in expression in response to exposure
to strontium. Some of these differentially expressed genes encode enzymes
that synthesize siderophores, low-molecular-weight compounds that show
a high affinity for binding iron.
"By disrupting
a gene in Shewanella, we have produced a mutant that is unable
to produce the siderophore," Thompson says. "We found this mutant displays
an increased sensitivity or lower tolerance to strontium than the normal
bacterium, suggesting that siderophores may be involved in the resistance
mechanism."
In Zhou's
laboratory, researchers have built microarrays for a mixture of microbes
in contaminated samples to determine which genes in bacteria have been
turned up or down by exposure to the contaminated site. They have found
that many of the different bacteria have adapted to the contaminated
site by changing the amounts of specific proteins produced.
The researchers
have determined how the microbe population changes in soil when nitrogen
is added or when a contaminated site is remediated. No other group
has been able to characterize in detail how a community of different
species of microbes changes in a contaminated soil or groundwater sample
and how that community differs from a community of the same species
in a clean reference sample.
Zhou and
his colleagues have been analyzing microbes present in groundwater
at an Oak Ridge site that has legacy wastes with a high concentration
of toxic metals and radionuclides. The site is a Field Research Center
(FRC) of DOE's Natural and Accelerated Bioremediation Research (NABIR)
Program, located at the Y-12 National Security Complex on DOE's Oak
Ridge Reservation. Zhou believes that microarray analysis of many samples
taken from the NABIR site will produce the microbe that is the most
effective at reducing uranium.
"It is
very expensive to pump and treat groundwater, so DOE would like to
speed the growth of bacteria that can convert uranium and technetium
compounds to materials that are less mobile and less toxic," says ESD's
David Watson, manager of the FRC in Oak Ridge. "We hope that this bioremediation
strategy for wastes containing metals and radionuclides will economically
reduce risks to human health and the environment."
Human Health
Benefits
Live-cell nanobiosensor. The
first observation of programmed cell death in a single live cell, or
apoptosis, was made recently at ORNL by Corporate Fellow Tuan Vo-Dinh
and two colleagues.
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ORNL's nanobiosensor penetrates a cell without destroying it and targets a specific protein.
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Vo-Dinh has led the development of "nanobiosensor" technology
for investigating vital biomolecular processes, including interactions
between proteins in living cells.
Vo-Dinh, leader of LSD's Advanced Biomedical Science and Technology Group; Paul
Kasili, a Ph.D. degree candidate at the UT-ORNL Graduate School of
Genome Science and Technology, and postdoctoral researcher Joon Myong Song recently published papers
on the optical nanobiosensor for measuring apoptosis in a single living
cell in the Journal of the American Chemical Society and in Nature.
"This minimally
invasive nanotechnology allows scientists to go inside a live cell
and follow its molecular processes in real time," Vo-Dinh says.
The nanobiosensor
is a tiny fiber-optic probe drawn to a tip of only 40 nanometers (nm)
across—a thousand times smaller than a human hair. Experiments
have demonstrated that such a probe is small enough to be inserted
into a cell and withdrawn without destroying it. Light from a laser
can be directed through the fiber-optic probe.
Because
the 40-nm width of the probe tip is much more narrow than the 400-nm
wavelength of the light, only molecules near the tip are excited by
the laser signal. In this way, scientists can target specific molecules
inside the cell, such as proteins, enzymes, or DNA strands.
Vo-Dinh
and his colleagues have demonstrated that a fiber-optic probe with
a bioreceptor molecule at its tip can be manipulated inside a cell
to find a target protein. When the protein binds to the bioreceptor,
a laser signal excites the target molecule, causing it to fluoresce.
The resulting glow is detected.
The team
recently detected the signaling process involved in apoptosis—a
key process in an organism's ability to prevent disease. The programmed
cell-death mechanism causes the cell to self-destruct before it can
introduce disease to the organism. "When a cell in our body receives
insults such as toxins or inflammation and is damaged, it kills itself
so that it does not propagate," says Vo-Dinh. "The loss of cells' ability
to undergo apoptosis is one cause of uncontrollable cell growth leading
to cancer. For the first time we have seen apoptosis occur within a
single live cell."
Environmental
and engineered nanoparticles. Some
particles in the air we breathe are smaller than one 100 billionths
of a meter in diameter. To improve our understanding of these "nanoparticles" and
their impacts on human health and the environment, ORNL researchers
led by ESD's Mengdawn Cheng have developed special technologies.
Nanoparticles generated by ultrasonic atomization are studied at ORNL to determine their potential health effects.
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Their inventions produce well-defined nanoparticles of a known size and composition
and measure the responses of lung cells to particles of different sizes. The
research is of particular importance to DOE because emissions from
internal combustion engines in automobiles, trucks, off-road vehicles,
and aircraft are known to contain nanoparticles.
"Our research
suggests that the environmental and health effects of nanoparticles
are different from the impacts of particles in the micrometer size
range, even when particles of different sizes have an identical chemical
composition," Cheng says.
"Using
a direct air-cell exposure approach, we found that human lung cells
exposed to 10-micron titanium dioxide particles showed little damage.
But other lung cells died when exposed briefly to 20-nanometer titanium
dioxide particles. It appears that the size of the particles, their
surface properties, and area of exposure affect cellular response and
increase the nanoparticles' toxicological potency toward biological
tissues."
Cheng believes
that nanoparticles emitted from engines as by-products are potentially
more dangerous than titanium particles. The reason: engine nanoparticles
are complex mixtures of organic chemicals and toxic metals.
The work
of Cheng's group has a systems biology flavor when the group applies
precision aerosol science and technology to detect and characterize
the biomarkers generated by human lung cells exposed to nanoparticles.
The group collaborates with university researchers as well as scientists
at DOE and Department of Defense laboratories. Cheng
expects results from the research will have significant impacts on
future emissions controls, environmental and occupational health regulation,
and defense work.
Combating
blindness. Plant
proteins might someday provide higher-resolution vision for the legally
blind than current and near-term artificial retina implants such as
electrode arrays. Recent research at ORNL showing that plant molecules
can be fused with mammalian cells suggests this exciting possibility
could be realized soon.
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DOE's Artificial Retina Program, previously managed by ORNL, focuses on construction of
microelectrode arrays that would directly stimulate surviving retinal tissue in people who
become blind as a result of retinal degenerative diseases.
Courtesy of Lawrence Livermore National Laboratory.
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The research was led by Eli Greenbaum, a corporate fellow in ORNL's Chemical Sciences
Division (CSD) and former leader of DOE's Artificial Retina Program, which involves research by several
national laboratories.
"Mammalian
and plant systems have been separated by two billion years on the evolutionary
time scale, but we showed it is possible to combine them," Greenbaum
says. "This work fits well into systems biology because large multicomponent
systems are involved."
Greenbaum
and CSD's Tanya Kuritz showed that a spinach protein—a light-absorbing
pigment, or "photosynthetic reaction center," called Photosystem I
(PSI)—could be incorporated in a liposome, an artificial membrane
made of lipids. In collaboration with Professor Ida Lee of the University
of Tennessee, the team demonstrated that a voltage high enough to make
a nerve cell fire is generated by PSI inside a liposome when exposed
to light. They then inserted the PSI-containing liposomes into membranes
of retinablastoma cells, which are cancerous versions of cells in the
eye's retina. The process demonstrated that the presence of PSI molecules
is essential to making eye cancer cells respond to light.
"What we
do not know is whether these spinach proteins are stable enough to
last a long time and whether they would undergo immune rejection by
the eye," Greenbaum says.
Longevity
and genes. "Aging
is a perfect example of systems biology," says Dabney Johnson, leader
of LSD's Mammalian Genetics Group. "Like mice, people are predetermined
to live a long life or a short life, depending on whether they have
a network of longevity or 'shortevity' genes."
Two years
ago, members of the Tennessee Mouse Genome Consortium (TMGC) were surprised
to learn that the National Institute on Aging was less interested in
knowing which diseases shorten a lifespan or impair health and wellness
and more interested in finding out which genes increase a healthy lifespan.
Dabney Johnson and her colleagues are studying old mice to determine which genes increase a healthy lifespan.
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The NIA was responding to findings that 200 genes in a recently sequenced
worm are related to lifespan and that making a mutation in any one
of these genes will alter the worm's longevity. The NIA funded a TMGC
study to identify mouse genes that affect lifespan. TMGC researchers
targeted different longevity genes for mutation in individual mice
by using ENU, the chemical mutagen discovered 25 years ago at ORNL.
The offspring
affected by the ENU treatment are being aged to their full lifespan
at a UT mouse facility. Researchers compared the blood chemistry and
other characteristics of the mice to try to predict which mice might
live longer. Johnson says that two factors related to a network of
genes are known to affect the lifespans of mice, as well as worms,
fruit flies, and—probably— people.
"Individuals
who are smaller and thinner live longer," she notes. "Individuals that
are big, heavy, bulky, and tall for their species tend to die young."
"The other
factor associated with longevity is resistance to stress. Individuals
who are usually on an even keel live longer. The theory is that individuals
subjected to prolonged stress make oxygen radicals inside their cells.
The radicals damage proteins, DNA, and lipids inside cells. Damage
accumulated over a lifetime diminishes the functioning of cells."
In about
two years, the researchers will know which of the mice with "mutated
longevity genes" lived a significantly long life. They then can determine
which gene or genes are involved in longevity.
Cell
Communication. Understanding
information processing within living cells and communication between
them is a goal of researchers in the Molecular-Scale Engineering and
Nanoscale Technologies Research Group of ORNL's Condensed Matter Sciences
and Engineering Science and Technology divisions. Mike
Simpson, who leads this group and holds a joint faculty appointment
at the University of Tennessee and ORNL, is spearheading an effort
to use computational, analytical, and experimental tools to simulate
genetic circuits and genetic networks in cells and predict how they
will respond to signals generated by the environment or other cells.
"Computation
and simulation will help us select the most important experiments to
perform and decide the most intelligent ways to do them to learn more
quickly about information processing within cells," Simpson says.
Instead
of wires, components within a genetic circuit are interconnected by
molecular interactions such as regulatory protein-DNA interactions
to control gene expression; RNA polymerase-DNA interactions that produce
messenger RNA (mRNA) during gene expression; and ribosome-mRNA interactions
that produce proteins that carry out cellular functions. A small subset
of interconnected reactions that carry out a single function is considered
a genetic circuit; a genetic network, which hooks together multiple
genetic circuits, is responsible for complex cellular functions.
Simpson's
group has written mathematical expressions to represent the components
of genetic circuits and genetic networks to understand better the biological
functions of bacterial cells. The approach is one of many used in systems
biology, an emerging discipline that endeavors to apply analytical
tools and approaches more familiar to the physical sciences to biological
problems.
For a U.S.
Defense Advanced Research Projects Agency project funded jointly with
the National Science Foundation, UT graduate students in Simpson's
group have developed software tools that simulate and analyze "stochastic
fluctuations"— random noise in cellular biomolecular populations
that may be important to genetic circuit fluctuations. For example,
stochastic fluctuations are vital to the decision-making process in
the phage virus's infection of E. coli bacteria. To examine
such processes and gain new insight into biological function, the UT-ORNL
collaboration developed and published papers on the Exact Stochastic
Simulator that simulates noise and its effect on biological systems.
The group
is refining an experiment in which genetic components of a cell-cell
communication system found in the marine bacterium Vibrio fischeri
are inserted into E. coli. V. fischeri cells do not give off
light unless a large enough population of cells is present, such as
a squid's "light" organ that offers them a nutrient-rich environment.
When populated with V. fischeri, this otherwise dark organ
suddenly becomes luminescent. Thus, the squid's predators lurking below
cannot distinguish the "camouflaged" squid from starlight above.
Simpson's
group is interested in the genetic circuits that process cell-cell
communication like that found in V. fischeri. "Cell-cell communication
is the mechanism that allows groups of cells to coordinate their activities
and produce the complex group behaviors that lead to infection, biofilm
formation, and functioning tissues, organs, and organisms," Simpson
says. "By looking at the more primitive communication systems in bacteria,
we hope to develop an understanding of information processing in cellular
communication systems of more complex organisms, especially those that
impact human health."
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