NASA - National Aeronautics and Space Administration

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]."

Image to right: 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.

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

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.

Image to 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

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."

Resources:

Space Biology Program
Fundamental Biology Research on the ISS
NASA's Space Biology Outreach Program - Web of Life