Smart Scaffolds

A leader in the field of “bioinspired” tissue engineering, Kristi Anseth creates customized three-dimensional (3-D) cell implants designed to repair or replace damaged body parts, such as bone and cartilage and even heart valves. Her 3-D cell cultures consistently show important differences from two-dimensional (2-D) plates. With Natalie Ahn, she’s exploring metastasis as well.

A major source of inspiration for Anseth, an HHMI investigator at the University of Colorado at Boulder, is the extracellular matrix (ECM), the biologically active scaffolding of connective tissue that surrounds cells. Along with supporting and protecting cells, the ECM stores molecular signals and growth factors that regulate cells’ development, shape, and function. Anseth uses the makeup of the ECM as a jumping-off point in fabricating customizable “intelligent scaffolds” containing living cells designed to repair or replace damaged tissues.

“What I’m excited about is, how do we make synthetic mimics of the ECM that recapitulate aspects of wound healing,” Anseth says. “How can we introduce the right cues at the right time and place? This is really biology in the fourth dimension.”

Her basic repair kit is a synthetic material called a hydrogel that has properties of the ECM and serves as a scaffold for living cells. For example, a piece of hydrogel seeded with cartilage cells taken from an athlete with a damaged knee would be implanted into the joint. As the cells grow, migrate, and multiply, the scaffolding degrades to make way for the new cartilage to form over the bone and restore the joint’s function. In 2003, the Anseth group published on the first such intelligent, degradable scaffold that could be injected into joints; after several years of animal tests, it is now in clinical trials.

Anseth and her colleagues are now applying the method to make bionic heart valve leaflets (the flaps that open and close the valve). They reported in the journal Biomaterials in late 2009 on experiments in which valvular interstitial cells, a component of heart valve leaflets, were inserted into the 3-D hydrogel culture system. In contrast to how they behave in a 2-D culture, the cells differentiated, proliferated, and migrated through the gel and responded to growth factors added to the culture. The researchers concluded that the culture system is a good model for developing tissue engineered heart valves.

In their current phase of work, the researchers are designing hydrogels with metabolic activities, such as regulating insulin production. When they cultured pancreatic insulin-producing islet cells in gels containing the basement membrane proteins collagen and laminin, the islets secreted six times more insulin than those cultured without the proteins.

Anseth is also developing cell-containing liquid precursors that can be injected into the body and then hardened by means of a light beam through a process called photopolymerization, providing the strength, stability, and flexibility to grow new cells in a three-dimensional framework. These polygels slowly dissolve, like surgical stitches, allowing the implanted cells to form new tissue in place of the gel matrix.

Watching Metastasis

With HHMI Investigator Natalie Ahn, a University of Colorado colleague, Anseth is investigating an aspect of cancer cell invasion. “Natalie had a paper about melanoma cell polarity—they redistribute molecules to one side of the cell,” Anseth explains. “This polarity might be a factor in how cancer cells move in a certain direction in metastasis.”

Ahn explains: “In response to a polarization cue, the ligand Wnt5a, proteins in the cells undergo a rapid, asymmetric reorganization into a novel intracellular structure that controls membrane retraction at the tail end of cells, allowing them to move directionally. Because Wnt5a strongly promotes invasion in melanoma cells, this regulation of cell polarity could influence directional movement of cancer cells in metastasis. Two postdoctoral fellows in our labs got together to examine how these cells would behave in a 3-D environment.”

When melanoma cells are suspended in a 3-D matrix, Ahn adds, “The cells show a remarkable degree of directional movement and stable localization of this novel polarity structure at the cell posterior, in a manner that is far more pronounced than in 2D culture.”

In a paper published in January 2010 in Integrative Biology Anseth described modifying a synthetic matrix so that the pores through which objects moved were smaller than the diameter of a cancer cell—in this case, fibrosarcoma cells. If the cell was going to become invasive, the scientists assumed, it would have to use protein-digesting enzymes to chew through the matrix fibers.

That’s not what the researchers saw, however. Instead, the rounded cells traveling in the direction of their “leading edge” morphed into an amoeba-like form, squeezing and flowing through the pores with ease. Because this phenomenon could be seen only in the ECM-mimicking 3-D culture, wrote the researchers, the finding “is exciting and demonstrates that an engineering strategy combined with current biological methods could broaden our understanding of cancer progression and metastasis.”

-- Richard Saltus
HHMI Bulletin, August 2010

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