A Goldilocks approach to leaky blood vessels: Not too stiff, but not too loose

Just like Goldilocks wouldn’t eat porridge that was too hot or too cold, blood vessels won't grow properly in tissues that are too stiff or too loose. (Project Gutenberg/Wikimedia Commons)
In the tale Goldilocks and the Three Bears, Goldilocks tries all of the bears’ porridge, chairs and beds, finding that only the little bear’s things were just right. Everything else was a little off for her…too hot or too cold, too hard or too soft and so on.

Similarly, for everything to work as it should in the body, things need to be just right. Blood pressure shouldn’t be too high or too low; organs can’t be too big or too small, etc.

Donald Ingber, MD, PhD, and his lab in Boston Children’s Vascular Biology Program take this “just right” approach when thinking about how organs and tissues are structured. Recently, he and a member of his research staff, Akiko Mammoto, MD, PhD, discovered that by changing the stiffness of the surrounding tissues—not too loose and not too tight— they could keep blood vessels from leaking. Their finding could have real consequences for people with sepsis or other diseases featuring leaky vessels.

Mechanics of health, disease and blood vessels

Mammoto and Ingber’s road to this finding started with studies of angiogenesis (the process of blood vessel growth). In 2009, they discovered a gene pathway in endothelial cells lining the walls of capillaries that links vessel growth, vessel structure and the mechanical properties of the tissues in which a vessel grows.

Tissue stiffness is one of those mechanical properties. As Mammoto notes, tissues are more rigid in many disease states, like cancer, fibrosis and arteriosclerosis. This, she thinks, creates a therapeutic opportunity.

“People usually think about soluble factors or drugs that directly affect cells when talking about treatments. But it may be possible to change the physical structure of a diseased tissue’s microenvironment, which could lead to therapeutic changes in the tissue itself.”

Just right for vessels

Somewhere between Jello-O and erasers is the right stiffness for blood vessels to grow properly. (stevendepolo/Flickr, Alex Morfin/Wikimedia Commons)
With those ideas in mind, Mammoto and Ingber, who also directs the Wyss Institute for Biologically Inspired Engineering at Harvard, examined the relationship between vascular permeability—how “leaky” blood vessels are—and tissue stiffness, initially by growing endothelial cells on polyacrylamide gels of varying stiffness. The gels mimicked the extracellular matrix (ECM), the structural mesh of proteins on which cells build tissues and organs.

As the pair reported in Nature Communications, the gel experiments produced some intriguing results. When the gels were soft like Jell-O or rigid like a hard rubber eraser, the endothelial cells wouldn’t form tight cell-cell junctions. (These junctions’ integrity is crucial to blood vessels. When they’re too loose, they leak, causing swelling or edema.) But cells grown on gels about as stiff as muscle flattened out and joined together securely.

Their next challenge was to find the same effects in vivo. By turning up expression of lysyl oxidase (LOX)—an enzyme that crosslinks collagen and elastin in the ECM—Mammoto was able to make the lungs more rigid in a mouse model. To do the opposite, she used a LOX inhibitor called BAPN.

When she looked at the blood vessels in the animals’ lungs, she saw the same pattern as in the gel experiments. The vessels in both the over-LOXed and the BAPN-treated animals were leaky, and their endothelial cells did not form tight junctions.

Getting the lungs to loosen up

To their results to a disease state, the pair turned to a model of acute respiratory distress syndrome (ARDS), a common and fatal outcome of sepsis. “The vessels in the lungs of patients with sepsis become porous, letting fluid leak into the lungs and leading to pulmonary edema,” Ingber explains. “It’s why many sepsis patients have to go on ventilators.”

Mammoto found that the lung tissues of mice exposed to endotoxin, a powerful, sepsis-fueling bacterial toxin, grew 35 percent stiffer than normal and exhibited high levels of LOX expression. “No one knew that endotoxin damages the lungs by affecting the LOX pathway,” says Ingber. “This was a whole new finding in and of itself.”

Look at these lung blood vessels. The ones on the left are normal, the ones in the middle have been damaged by endotoxin. BAPN protected the ones on the right from endotoxin's effects. (Akiko Mammoto)

In addition, Mammoto found she could neutralize endotoxin’s stiffening effects in the mice with BAPN and other LOX inhibitors, which in turn prevented toxin-induced pulmonary edema.

It’s a discovery that, if it can be translated to humans, could be a boon for patients with sepsis. Mammoto and Ingber are now on the hunt for treatments based on what they’ve learned.

“Currently, doctors can give antibiotics to treat sepsis causing ARDS, but have no way to treat ARDS itself,” Mammoto says. “But if we can come up with a way to change the lung microenvironment and counteract the effects of endotoxin, it might give other treatments a better chance of success.”

Ingber concurs. “This could open up a new universe of agents for therapeutically targeting edema in any tissue caused by any number of conditions, such as sepsis, smoke inhalation, heart failure and more.”