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Gene Therapy: Putting Muscle Into the Research
Bio-artificial muscle implants could attenuate or perhaps prevent muscle loss
among astronauts in orbit and patients with muscle-wasting diseases on Earth.
When astronauts spend time in space, they also spend some of the hard-earned
muscle mass that they've built up on Earth prior to their flights. In the microgravity
environment of a spacecraft in orbit, muscles atrophy quickly. The body perceives
that it does not need muscles to be as strong, resulting in a decrease of muscle
mass by as much as 5 percent a week - up to about 20 percent over time in the
muscles that are used more to fight gravity on Earth, such as those in the calves
and along the spine.
To help attenuate this muscle loss, and perhaps even prevent it, Principal
Investigator Herman Vandenburgh, of Brown University, is studying the effects
of a protein that could be manufactured in small bioartificial muscle implants.
The results of his work, supported by NASA's Fundamental Space Biology Division,
could mean better muscle condition in astronauts on long-term spaceflights. On
Earth, results also could help the elderly suffering from frailty and patients
battling muscle-wasting diseases such as AIDS.
Testing the Strength of an Idea
How well muscles are conditioned depends on how much the muscles are required
to work. When the workload of muscles is reduced, such as when they are not bearing
much weight in space or during long-term bed rest on Earth, the rate at which
the muscles make new proteins decreases, resulting in muscle atrophy. Vandenburgh
learned this cause-and-effect from engineered avian (bird) tissue experiments
he conducted on STS-66 in 1994 and STS-72 in 1995. Based on this knowledge, he
formed a hypothesis for attenuating muscle atrophy. "Having that piece of
information," Vandenburgh remembers, "was really what keyed us into
the possibility that [the protein] insulin-like growth factor 1 (IGF-1) might
be effective, because the major effect of IGF-1 on muscle cells is to increase
synthesis of [other] proteins [needed by the muscles]."
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Left: Bioartificial muscles grown in Vandenburgh's laboratory
are a crucial element in his research. He's testing to find the dosage of IGF-1
that will stimulate new protein production in muscles, thereby attenuating atrophy
in astronauts and in patients with muscle-wasting diseases on Earth.
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To find out just how effective IGF-1 is, Vandenburgh and colleagues Paul Kosnik,
Courtney Powell, and Peter Lee have been developing a model for atrophy using
bioartificial muscles in his laboratory. Vandenburgh explains, "It's an Earth-based
model for what we think happens in space, so that we can grow the bioartificial
muscles in our lab and then we can induce atrophy by reducing the tension on the
muscle, which is what we think happens when [astronauts] go into space. The muscles
generate less force, so they don't make as much protein and they waste away."
The bioartificial muscles Vandenburgh uses on the ground start as avian, mouse,
rat, or human cells. He engineers the cells in a culture chamber, where they multiply
and grow into small muscle-like samples that are large enough to manipulate with
the hardware he has developed. "The hardware is a self-contained, computerized
system that allows us to very precisely control the length of the bioartificial
muscle down to the micron range," he describes. Controlling the length of
the muscle controls the tension on it. Greater tension simulates a muscle that
is in use; less tension, one that is at rest.
"We have a force transducer connected to one end of the bioartificial
muscle," he continues. "This allows us to measure the actual force,
or contraction, that the muscle can generate when we electrically stimulate it,
so that we can actually measure the work performed by the muscle. If the muscle
is atrophying, that would be reflected in a decreased ability to generate force
and do work." With this hardware, Vandenburgh will be able to test IGF-1
and get a better idea of how much of the protein is needed to stimulate new protein
production and attenuate atrophy in tissue grown from avian cells (a good model
system for what happens in human and other mammalian cells) for the next flight
of his experiment in 2004. He explains, "We're pretty confident that something
like IGF-1 will work, but we don't know what dose will be most effective."
Developing a New Delivery System
To administer the doses he is testing, Vandenburgh uses gene therapy, which
should be more effective than injections in delivering the protein. He explains
the problem with injections, a common way of delivering already available growth-factor
treatments for various conditions, such as dwarfism: "These growth factors
turn over very quickly in the body. For instance, IGF-1 probably has a half-life
of a couple hours in the body, so in order to get effective doses when you inject
it, you have to inject high concentrations, and you have to inject it every day.
The problem with injecting high concentrations is that it can lead to adverse
side effects."
So instead, he puts the deoxyribonucleic acid (DNA) sequence for the IGF-1
into a replication-deficient retrovirus. Retroviruses carry ribonucleic acid (RNA)
rather than DNA, but this type of virus cannot trigger replication of its own
RNA in a host cell. Instead, it carries the genetic material of a foreign DNA
for replication. Then Vandenburgh puts the engineered retrovirus into the cells
of engineered muscle tissue to allow the foreign DNA to stably incorporate into
those cells.
"Viruses are very efficient at getting into cells the material you want,"
he explains. The virus attaches to a host muscle cell and then recruits the host
cell's enzymes to copy the genetic material of the IGF-1. As the IGF-1 is manufactured,
it builds up in the cell and then is released to surrounding cells. Secretion
of the factor is at a physiological level, similar to the level at which the body
would supply the factor to muscle cells were they supporting a normal workload.
In the future, the bioartificial muscles could be used in a device implanted
subcutaneously (under the skin) as a living "little protein factory,"
as Vandenburgh calls it, delivering proper dosages of the protein to the whole
body for 6 months or more. This nature-simulating delivery system for IGF-1 could
mean a drastic reduction in how many needles and side effects that patients with
muscle atrophy would have to deal with.
Supporting Basic Research
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When that day comes, huge numbers of people may benefit. Not only might astronauts
keep their muscles in better condition, but elderly people, AIDS patients, and
people with congestive heart failure may also redevelop stronger, healthier muscles.
In addition, the technology could be used with other proteins to treat other protein-deficiency
diseases (see sidebar), such as diabetes.
Right: The hardware Vandenburgh's research group uses
in his ground research controls the length of a bioartifical muscle (made from
engineered tissue), and thereby the tension on it. The strain gauge measures how
much "work" the muscle responds with when it is stimulated by the stepper
motor. Graphic credit - Herman Vandenburgh
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The irony of all the potential this technology has on Earth is that originally,
the IGF-1 research was geared for astronauts only. "We started out with our
primary interest in space biology without any real concept that it might have
direct Earth-based applications. So it was a very basic research project initially
that was funded by NASA to understand what happens in space, and how we might
be able to prevent the wasting that occurs [in astronauts]. Then it really evolved
into a very important potential Earth-based therapy."
Vandenburgh notes the significance of conducting basic research such as his:
"One of the points we try to make is that fundamental biology research is
very important because you never know where it's going to lead. It's like the
molecular biologists back in the '70s who were studying E. coli - really, all
of that evolved into doing the human genome project, sequencing DNA in humans.
No one would have envisioned back then that the research would have that kind
of application. So even when you don't envision how [a microgravity experiment]
could help people on Earth, these basic research problems can go in different
directions and have an impact on treating diseases on Earth."
Implanting for the Future
In conjunction with preparations for the next flight of Vandenburgh's experiment,
he is developing a commercial implant device that could be used on Earth to manufacture
and release a variety of different proteins, such as insulin for diabetics, human
growth hormone for children with dwarfism, or factor VIII for hemophiliacs. The
device, which would be inserted under the skin, is similar in concept to a commercially-available
device used for delivering contraceptives for periods of up to five years. It
could deliver small physiological doses of the needed protein for six months or
more, eliminating the need for daily injections of large pharmaceutical doses.
"The difference," Vandenburgh explains, "is that the new implant
is a living device, not just a repository forwarding manufactured compounds."
His company, Cell-Based Delivery Inc., conducted early tests on rats with their
hind legs suspended to induce atrophy. (The test is a standard model that was
developed by Principal Investigator Emily Morey-Holton, of Ames Research Center.)
Seventy percent of normal muscle wasting was prevented in rats that received growth
factor through implants. Atrophy could not be prevented in rats that received
daily injections of the factor instead of the implant.
Cell-Based Delivery also has already shown feasibility for the device in tests
of over a dozen sheep. In those tests, Vandenburgh and his team were able to deliver
IGF-1 at a constant level for up to 30 days, the length of time they kept the
experiments going. The company has just completed a series of large-animal studies
and is ready to conduct a 6- to 12-month safety (phase I) study for the U.S. Food
and Drug Administration. "Once that's been completed," Vandenburgh projects,
"we hope to be able to start treating patients in the next 12 to 8 months
in a small phase II clinical trial. If it's successful, it will be a revolutionary
way of delivering proteins."
Related Web Sites
Fundamental
Space Biology Program
Fundamental
Biology Research on the ISS
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