Sci. Aging Knowl. Environ., 9 November 2005
Engineers and biologists are making progress toward creating custom-made artificial tissues to rejuvenate aging bodies
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/2005/45/ns3
Abstract: Aging takes a toll on organs and tissues. Skin rumples, hearts fail, and muscles weaken. Drugs and other interventions can sometimes slow the devastation but rarely repair existing damage. Furthermore, although transplants can replace ruined organs, demand for donors outstrips supply, and immune rejection is a risk. To circumvent these problems, researchers are working to create tissues in the lab that can substitute for defective ones in the body. Building on previous success in designing simple tissues such as skin and bone, they are seeking new biomaterials and methods to craft more complicated ones.
Deteriorating cartilage can keep a 40-something runner out of weekend 10K races, and any aging athlete would appreciate spruced-up tissues to stay competitive. Furthermore, similar technology could help many elderly people continue climbing stairs or throwing balls for their dogs--and younger folks who suffer from disease might get some relief. Body-part replacements might conjure up thoughts of the Six Million Dollar Man, but real-life biologists and engineers are devising ways to create engineered tissues that bring flagging ones back to life.
Their success could revolutionize medicine, potentially reversing illnesses such as diabetes, heart disease, and liver failure. Researchers have grappled with the problem for several decades without fulfilling the field's exaggerated early expectations. But recent breakthroughs are bringing them closer. They're developing new scaffolds on which to build tissues, figuring out how to grow cells on those frames, and deciphering the cues that cells need to form complex structures.
Filling a Void
Drugs, surgery, and other treatments can slow some of aging's decline, but they can't stop it. And in some diseases, organs fail outright, so people of any age can suffer from degenerating tissues. Transplants meet only a fraction of the need for replacement organs and tissues. Even if donated material is available, a recipient's immune system can reject it, and antirejection drugs render patients susceptible to infections.
In some cases, artificial parts can substitute for those that have worn out. Plastic heart valves can replace leaky ones, and titanium joints can step in for crumbling knees. But the body often treats such implants as invaders and attacks them. In tissue engineering, researchers strive to create the best substitute for the original components: living flesh, which avoids the problem of persuading human tissue to accept metal or plastic. A flurry of research activity and business ventures in the early 1990s prompted speculation about a host of engineered tissues that soon would be available.
Researchers did deliver on a few fronts, developing techniques to replace cartilage and skin. However, technicians need to grow a patient's own cartilage or skin cells on a meshlike backbone before doctors can apply it. Although this strategy eliminates rejection, it also means that patients must wait days or weeks before the new tissue is ready. For some conditions, that delay might be acceptable, but for others it could mean death. And the scheme doesn't allow mass-production of a part that's ready to use and sitting on hospital shelves. As a result, few companies have achieved financial success making engineered tissues, says engineer Robert Nerem of the Georgia Institute of Technology in Atlanta. The products are effective, says surgeon and tissue engineer Joseph Vacanti of Harvard University. But they tend to be relatively simple tissues such as skin or bone, and researchers are aiming to design more complicated replacements for kidneys and blood vessels.
Nearly all tissue engineering relies on growing cells--in a lab dish or in the body--on an artificial framework, and researchers are concocting ways to improve these supports. Simply injecting cells into a patient would work only for illnesses in which a small number of cells malfunction or a single type of cell disappears, says Anthony Atala, a reconstructive urologist at Wake Forest University in Winston-Salem, North Carolina. For instance, providing diabetics with insulin-producing cells might reinvigorate manufacture of the sugar-controlling hormone, he says, but "if you had a patient with end-stage heart failure, cell therapy is not going to do the job."
To repair extensive damage, engineered tissues require a system that can guide regrowth. Materials provide structure, but they also transmit chemical messages that influence how cells grow, says biomedical engineer Gordana Vunjak-Novakovic of Columbia University. And these structural and informational functions intersect: Cells grow and move in response to signals conveyed by the surface on which they lie, she says, and they are finicky, demanding a material similar to what the body normally weaves. Cardiac cells need a soft, tough, and elastic surface, Vunjak-Novakovic says, and bone needs a material that's brittle and porous.
Sending the Right Signals
Studies this year have provided new insights into how physical properties influence a cell's behavior. For instance, cells pull harder on stiffer substrates, and that pulling drives them to divide, chemical engineer David Mooney of Harvard University and colleagues reported online in the 14 March Proceedings of the National Academy of Sciences (PNAS). This finding hints that researchers must tailor scaffold flexibility for each cell type. And cells more readily take up DNA when they cling to stiffer materials, the same group reported in the June issue of Nature Materials. To direct tissue renewal, researchers often coat materials with genes for proteins that control cell behavior, and Mooney's findings suggest that more rigid meshes might entice cells to absorb those genes.
Scientists must also recreate how the body molds tissues as an organism develops, which requires uncovering which signals to transmit to cells. For instance, various proteins in an embryo nudge nascent cells to specialize into heart cells, prod them to multiply and organize into a tube, and finally push them to fold into the chambers and arteries of a complete heart. "We're trying to mimic that process," says Mooney. But scientists might not need to replay the entire program, he adds. They need to understand "What is the minimum that we need to mimic? And how much can we let [an organism] sort out?" In some cases, one cue might be enough to prompt cells on a scaffold to form a tissue. Vunjak-Novakovic says, "If you pick one important factor and provide it, that's a clue to the cells, and they do the rest." But researchers will also likely devise ways to provide more lifelike--and potentially more complicated--patterns of growth-spurring molecules, she says. New materials will provide "multiple signals to cells, release growth factors in gradients, or release factors at different times." Scientists will "really make materials tailored to the custom needs of cells."
Recent studies have shed light on another key issue: how engineered tissues coexist with normal ones in the body. Molecules known as matricellular proteins interpose between cells and the extracellular matrix, the fibrous web that holds cells together in a tissue and controls inflammation and other processes. These proteins also dictate how an organism responds to implanted material, cell biologist E. Helene Sage of the Benaroya Research Institute in Seattle, Washington, and colleagues have found. Devices such as insulin pumps, which dispense the hormone into the bloodstream, work until the body revolts against the foreign body and walls it off. Removing either of two matricellular proteins disrupts this reaction, Sage and colleagues reported in the June issue of The American Journal of Physiology. And eliminating both proteins boosts the number of blood vessels surrounding a foreign body. Manipulating these proteins could improve how well implants work, says Sage, by increasing their access to the bloodstream instead of isolating them. "We struggle with what kind of materials to use, but we're not paying attention to the body's reactions," she says. "We have to engineer against some of these as well."
Putting Tissues Into Circulation
Creating the right materials and identifying the right signals might help scientists surmount other obstacles to constructing replacement parts. The biggest challenge in making complete organs is size, says Vacanti. Blood vessels aren't necessary to grow thin tissues, but they are required to build thicker tissues or organs. In an effort to solve the problem, his team exploits microchip-manufacturing technology. In the January issue of Tissue Engineering, the researchers described etching a circuit of blood vessels into a silicon wafer and then transferring that pattern to a polymer film, creating a web of tubes. Coating those tubes with blood vessel cells resulted in a blood-delivery network. Now they're working on growing liver cells on top of this network, with the eventual goal of manufacturing full-sized organs.
Other teams are confronting another major challenge: coaxing different types of cells to integrate with each other. For instance, in June, chemical engineer Robert Langer of the Massachusetts Institute of Technology (MIT) in Cambridge and colleagues reported the results of combining cells from muscles, blood vessels, and connective tissue on a three-dimensional scaffold. Cultures containing connective tissue cells harbored increased quantities of a protein that encourages blood vessel growth and contained more blood vessel-like structures than did cultures without them, the team reported in Nature Biotechnology. The researchers also implanted tissues grown either with or without connective tissue cells into rodents. More blood vessels hooked into the animals' circulatory system when the implants contained connective tissue cells, and muscle cells grew more readily.
Other approaches might also spur blood-vessel growth. In one engineering strategy, researchers inject a scaffold material coated with cells into failing tissue rather than growing a complete organ in the lab. Ideally, the cells will reproduce and rejuvenate the tissue. But the cells often die, in part because they need blood vessels to deliver nutrients. Researchers often start with artificial scaffolds built from chemicals used to make plastics, but "the human body never makes those kind of polymers," says MIT biomedical engineer Shuguang Zhang.
Instead, Zhang and colleagues use a segment of zuotin, a DNA-grabbing protein found in yeast that congeals into a gel. Although people don't manufacture this substance, at least its amino acid building blocks are familiar to humans. Cells stick to the gel and grow. The team is now determining whether the material can kindle regeneration of different tissues. In work published in February in Circulation, the researchers injected the gel into rodent hearts and found that it attracted more of the animals' own heart muscle cells than did a commonly used substance. In addition, blood vessel cells flocked to it. These findings suggest that the gel might stimulate blood-vessel growth as well as regenerate heart muscle. One key advantage to the zuotin-based scaffold, says Zhang, is that the spaces between the molecules are smaller than in other materials. That arrangement means that cells lie among many strands of the gel rather than crawling along a single one, which mimics how cells interact with the body's natural protein matrix. "To foster tissue engineering, you must understand how a cell interacts with its own environment," says Zhang. That knowledge should permit scientists to recreate the environment best suited for growing a particular type of cell.
The Right Stuff
To fabricate any kind of tissue, researchers must first isolate the kind of cell that can generate it. That cell might come from the person who will receive the engineered tissue, from another person, or even from another species. Although the patient's cells are the first choice, they might not be healthy enough, a setback that could be especially serious in the elderly, says Nerem. "Are cells from a patient who has developed disease the right cells to use? We don't know enough to answer that question," he says. As people age, he points out, their blood vessels become more likely to soak up cholesterol and develop atherosclerosis, behavior that might compromise the utility of a material constructed from those cells.
Adult or embryonic stem cells might provide an alternative. They can mature into many kinds of cells, allowing scientists to cultivate multiple tissues. Using stem cells from another person would circumvent concerns about the health of a patient's own cells, and researchers are optimistic that stem cell-based tissues might slip past the immune system. However, researchers are far from knowing how to direct stem cells to form the tissues they want. "There's so much focus on stem cells, but we don't really understand how stem cells work," says Zhang. They will "need a lot of instruction," says Mooney, "and we'll need to prevent them from doing things that are undesirable," such as forming tumors. Researchers are beginning to make progress, however. Two years ago, Langer and colleagues enticed embryonic stem cells--the most pliable type--to grow and specialize on a scaffold. The cells activated genes crucial for the function of different tissues, depending on which signals the researchers provided, suggesting that stem cells might prove useful in engineering replacement parts.
Regardless of where cells originate, researchers need to produce large quantities of them. That challenge requires that scientists alter their thinking, says Zhang. Typically, cultured cells grow on a flat plate. But "2D isn't representing what is going on in the body," he says. Cells misbehave when moved from a three-dimensional body tissue to a culture plate. For instance, cartilage cells stop producing collagen, a primary component of connective tissue, he says.
But growing cells on three-dimensional scaffolds of interwoven fibers containing nooks and crannies poses challenges. Scientists need innovative ways to feed them, for instance. One method is a bioreactor, a device that continually adds nutrients to a chamber containing cells and a scaffold, and researchers are putting new twists on this approach. Cato Laurencin, an engineer at the University of Virginia, Charlottesville, and his team use a spinning bioreactor to keep nutrients moving past bone cells rather than just pumping in food. The technique allows cells to coat the inner reaches of the scaffold, not just its exterior, and the cells crank up bone genes to a greater extent, the researchers reported last year in PNAS.
Other scientists are exploiting bioreactors to create more complicated tissues. For instance, Peter McFetridge, an engineer at the University of Oklahoma in Norman, and colleagues use them to craft blood vessels. They start with umbilical veins, which they shave down with a device resembling a lathe, leaving a thin, tubular protein scaffold. Then they pass blood vessel cells through the middle of the tube and circulate smooth muscle cells, which coat the vessel, around the outside of the tube. This system recapitulates how vessels form in the body, says McFetridge. Oxygen concentrations differ for cells on the inside and outside of a blood vessel, and his bioreactor permits him to control the oxygen content on both surfaces. In addition, subjecting cells to the flow rates they would encounter in a working vessel causes them to stick tighter to the protein scaffold, potentially creating a sturdier vessel. That advance could improve engineered vessels. Researchers have achieved some success replacing veins with engineered vessels, but those devices need to withstand only low pressure in the body, says Nerem. Now, scientists must develop replacements that can endure the higher pressure in arteries, he says.
Other teams are building miniature bioreactors inside animals instead of on the lab bench. To spur bone growth in rabbits, Langer and his team created a space between the shinbone and the surrounding tissue, which contains bone cell progenitors. They injected a synthetic matrix material into the chamber. After 12 weeks, normal-looking bone filled the area, they reported online 29 July in PNAS. The team harvested the tissue and used it to repair bone damage in other parts of the animal. Langer says the approach could improve existing methods for bone grafts. They typically come from the hip bone, and removing the material can leave patients with chronic pain; they often experience more discomfort there than where the tissue is implanted. The new strategy could allow doctors to grow bone for grafting in a more accessible location, says Langer.
Research in the coming years should merge improved materials with a better understanding of how to nurture cells into functioning tissues. Those efforts could generate the kinds of successful engineered tissues that scientists, doctors, and patients have been waiting for, helping aging athletes--and other folks--keep up the pace.
November 9, 2005
R. John Davenport is a science writer and former associate editor of SAGE KE based in Rhode Island. He's in the market for knee-replacement technology.
Science of Aging Knowledge Environment. ISSN 1539-6150