Whether it’s a high school biology class or a college anatomy book, they’ve always offered the same basic description. Arteries are comprised of three basic layers, they say: There’s connective tissue on the outside, endothelial cells lining the interior and smooth muscle cells sandwiched in between.
True, as far as it goes.
But two University of Missouri researchers have discovered there’s a whole added dimension to blood vessel anatomy that goes a long way toward explaining how they work and offering the possibility of novel new treatments for high blood pressure and other diseases.
Gerald Meininger and Michael Hill of MU’s Dalton Cardiovascular Research Center, along with an international group of collaborators, have found an elaborate scaffolding of proteins running through arteries that nobody had fully accounted for.
Meininger and Hill believe the protein, called “elastin,” helps hold artery wall cells together and plays an important role in the cellular signaling that regulates blood flow through the arteries.
Their work has attracted the attention of the online edition of Scientific American and has even appeared on YouTube. And no wonder. The beautiful 3-D images of tiny blood vessels glimpsed through their microscopes reveal an anatomical architecture never seen before.
“Past studies have performed biochemical and electron microscopic analyses of these elements, but we have never had the opportunity to visualize their distribution within the (vessel) wall in such detail,” said William Cole, a cardiovascular researcher at the University of Calgary.
Dyed to fluoresce bright red under laser light, the elastin can be seen as long fibers, like filaments running along the exterior of arteries and laid down as sheets inside the vessels. The arteries are microscopically photographed in thin anatomical slices the way a CT scanner sections a patient’s heart or brain — you can see the ways proteins are arranged in every layer of the vessel wall.
Hill describes it this way: “It’s almost as if you were looking at a city building; the cells are windows, but there is still a structure around them.”
The role elastin plays also depends on where the arteries are located, the researchers found. The sturdy scaffolding is more important to arteries in muscles that are constantly flexing than to brain arteries that are protected from stretching and twisting.
Meininger refers to these structures of cells and proteins as “the wisdom of the architecture of the blood vessel wall. These are not randomly arrayed proteins. There’s a functional logic to this design.”
The Dalton group at MU “is internationally known for such contributions, and this has been true for decades,” said Stephanie Watts, a professor of pharmacology and toxicology and assistant dean of the graduate school at Michigan State University.
“They are looked to as experts, and this study — one which makes you think hard about what you really know about a blood vessel — solidifies their position as such.”
Hill and Meininger have been collaborating on blood vessel research for nearly 30 years, ever since Hill, as a young graduate student from Australia, came to Texas A&M University Health Science Center. There he met Meininger, a newly minted PhD doing postdoctoral studies.
They tend to complete each other’s thoughts, as they speak together about their work. “We’re a couple of science nerds,” Hill jokes.
“I learn from him. He learns from me, and it comes together in new ways,” Meininger says.
They are focusing their attention on microcirculation, the small blood vessels that distribute blood within the body’s tissues and regulate blood flow and pressure.
Scientists have known for some time that elastin is a component of blood vessel walls. But previous research has looked more at how much of the protein is present in vessels, not where it is in the vessels. The technology just wasn’t available to see it.
What researchers had been able to determine is that the amount of elastin in blood vessels declines with age and is replaced by another protein, collagen, which is stiffer. The stiffening of arteries is one of the contributing factors to the rising blood pressure that comes with aging.
The way vessels regulate the amount of blood reaching tissues also is well-established. This is how it works: Let’s say you are working out at the gym, repeatedly lifting a weight to exercise your biceps. Your heart could do all the work of supplying more blood to the muscles by beating faster. But that would be inefficient. So the blood vessels to your biceps also lend a hand. The muscle cells in the arteries relax, widening the diameter of the vessels to let more blood through. Later, when you’re rested, the muscle cells will constrict to reduce blood flow.
Meininger and Hill didn’t feel that explanation told the whole story. How could it be that muscle cells up and down an artery knew when to relax or constrict?
“We’ve had a very simplistic way of looking at these vessels,” Hill says.
“We started to realize the blood vessel walls were much more complicated,” Meininger adds.
What came next involved a bit of serendipity.
In the course of their research, they used an enzyme to break down the elastin on the outer covering of arteries in order to have better access to the muscle cells underneath. That caused the vessels to lose their shape: They stretched out and dilated.
Meininger and Hill wanted to find out if this shape change was happening because the vessel cells had lost their connection to the elastin. “We didn’t know how they were coupled together, and that made us very curious about what was going on,” Meininger says.
But they ran into roadblocks. “No matter w”hat experiments we were trying to design we couldn’t come up with a definitive answer,” Hill says.
Where in the vessel wall were these proteins? How were they attached to the cells? The researchers needed to do microscopic imaging for that.
Meininger and Hill recognized that certain fluorescent dyes that scientists used to stain blood vessel walls actually were binding only to elastin. That discovery opened the way to producing images of intricate detail that reveal how integral elastin is to the structure of vessels.
“With every technology you reach a barrier where you can’t take it any further,” Meininger says. “You get a telescope for the first time and you realize that there are thousands of stars out there. With this microscope, hey, we’re seeing thousands of stars.”
Each of the four microscopy laboratories at the Dalton Center holds a half-million dollars or more of advanced technology. The microscopes are configured to do what the researchers have in mind.
The live-cell imaging laboratory is a hole of a room, painted flat black to absorb any stray light from the lasers that cause the dyes to fluoresce (too much laser light can damage the tissues). The microscope sensors are so sensitive they can capture individual photons.
In another laboratory, a scanning probe microscope can physically “touch” single vessel cells and sense individual molecules to determine their mechanical properties, such as how soft or stiff a vessel may be.
Meininger and Hill use operating microscopes to dissect live vessels a couple millimeters long for their imaging studies. The arteries they work with are very small, less than one hundredth of an inch in diameter; these are the microcirculation “resistance vessels” that regulate blood pressure throughout the body.
The two researchers are developing collaborations with engineers and mathematicians on campus to help them model the vessels’ protein structures and the way they behave mechanically. And they are working with MU architects to make 3-D renderings of vessels in virtual reality, just as architects do for the buildings they design. “We want to be able to walk through the vessels,” Hill says.
Meininger and Hill believe that elastin provides structural support to vessels, maintaining their physical integrity. Vessel cells latch onto to the protein matrix — muscle cells that have been contracting for long periods may even disengage and reorient themselves so they can keep the vessel’s diameter small without continuing to contract.
“Once you have a mature blood vessel, it’s not fixed. It’s continuously adapting,” says Hill.
Elastin also may provide cells with a way to sense the pressure of the blood surging through a vessel. The muscle cells may be able to detect how far the vessel is being stretched by the amount of tension the blood is applying to the elastin structure.
“We think cells are continuously sampling their environment,” Hill says.
“That’s where our research is now,” Meininger added. “That’s a fundamental question, and there are more laboratories interested in it than just us.”
The most important aspect of Hill and Meininger’s work may be the stimulus it provides “for considering how vessel wall structure changes in disease states,” says Cole of the University of Calgary. “We know that the biochemical composition and physical properties of small arteries, such as stiffness, change in diseases including hypertension and diabetes, but how these alterations relate to and impact vessel wall structure is not known.”
Their new insights into elastin have “paved the way,” Cole adds, for research with medical applications.
Elastin is an unexplored frontier for medical research, Meininger and Hill say, and ought to be investigated as a target for new medications. Drugs for high blood pressure, for example, treat the vessel cells to regulate the diameter of vessels. “We may be missing the big picture,” Meininger says. “There could be a whole class of compounds directed at the proteins to improve, increase or protect elastin.”
Hill is studying how the high blood sugar levels found in diabetes may cause elastin in vessels to deteriorate.“We don’t know why there’s less elastin with aging,” Meininger says, “but by looking at enzymes that attack these proteins we may be able to forestall hardening of the arteries.”
Meininger and Hill’s shared fascination with the physiology of the circulatory system dates back to the beginnings of their careers, even earlier.
Meininger, a Michigan native, started out as an engineering student at Central Michigan University but found biology more intellectually stimulating. A basic physiology course “got me really excited about the discipline — it combined physics, biology and chemistry.”
For graduate study, Meininger came to MU, where his advisor was a microcirculation expert. Meininger’s dissertation was on how microcirculation is affected by high blood pressure and how vascular resistance is distributed through a network of blood vessels.
“I could describe the changes, but I couldn’t describe the mechanism,” Meininger says. Already, he was encountering the laboratory challenges that he and Hill would later find ways to overcome.
Meanwhile, back in Australia, Hill was beginning a parallel career. He had been interested in science and the circulatory system ever since high school. Like Meininger, he started out in college as an engineering major but switched to organic and biochemistry.
In 1983, Meininger was a postdoctoral fellowship at Texas A&M. His department head asked if he would be interested in working with a doctoral student from Australia for a couple of months.
They wound up working 18-hour days and six-day weeks for two months. They got a paper out of it. “The studies we were doing all related to how disease affects the ability of vessels to respond to changes in blood pressure,” Meininger says.
Hill returned to Texas A&M in 1985, with his family this time, for a fellowship. Faculty positions at universities in Virginia and Australia followed.
Meininger stayed on at Texas A&M for 25 years, becoming a Regents professor and director of its health science center’s division of vascular biology. In 2005, he came to MU as director of the Dalton Cardiovascular Research Center. “That I did my graduate work here — and now I’m running the building — is ironic to me,” he says.
Meininger soon suggested to Hill that he join him at MU. So Hill packed up and moved from Australia one more time. “It’s a brilliant place to study the physiology and biology of blood vessels,” Hill says. MU offers the opportunity to collaborate with biomedical engineers and researchers in the medical and veterinary schools. “We’ve really captured the field.”
How is it that they’ve been able to collaborate so long and productively? “I think because we complement each other,” Hill says. “He’s organized and I’m not.”
“And I think because we can enjoy a glass of beer together and let work go,” Meininger says.
When Hill isn’t in the laboratory, he’s out golfing or traveling. He has two adult children back in Australia whom he visits regularly. Meininger is an avid bird watcher — he pointed out a bluebird and a pair of Canada geese as he spoke. “I’ve had a lot of meetings where a bird perches on a branch outside the window and I interrupted the meeting.”
Meininger also gardens and is learning dry stone walling. “I want to build one.”
“The antithesis of a blood vessel,” Hill quips. “Something that doesn’t move.”