Silk Proteins Strengthen Tendon Repair and Tissue Regeneration

Researchers at the Terasaki Institute for Biomedical Innovation (TIBI) have turned to a silk protein, silk fibroin (SF), produced by the Bombyx mori silkworm as a biomaterial for tendon repair. The researchers report their method has yielded significant improvements in mesenchymal stem cells (MSCs) tendon regeneration.

Their findings are published in the journal Small in a paper titled, “Co-Electrospun Silk Fibroin and Gelatin Methacryloyl Sheet Seeded with Mesenchymal Stem Cells for Tendon Regeneration.”

SF is a promising biomaterial for tendon repair, but its relatively rigid mechanical properties and low cell affinity have limited its application in regenerative medicine,” wrote the researchers. “Meanwhile, gelatin-based polymers have advantages in cell attachment and tissue remodeling but have insufficient mechanical strength to regenerate tough tissue such as tendons. Taking these aspects into account, in this study, gelatin methacryloyl (GelMA) is combined with SF to create a mechanically strong and bioactive nanofibrous scaffold (SG).”

Possible treatments for tendon injuries include tendon tissue grafts from patients or donors, but these pose risks such as infections, transplant rejection, or necrosis. Synthetic transplants have been attempted, but mechanical, biocompatibility, and biodegradation issues have hampered these efforts.

Another approach is to use MSCs. However, treatment methods using systemic infusion, direct injection, or genetic modification of MSCs may result in not targeting specificity to the injury site, prohibitively high cell numbers, or cells that are unable to isolate.

The team first turned to silk fibroin because of its strength, durability, biocompatibility, and bio-degradative qualities.

In order to improve the scaffold’s ability for tissue regeneration, the team next paired silk fibroin with GelMA, a gelatin-based, water-retaining gel, due to GelMA’s biocompatibility, controllable degradation, stiffness, and ability to promote cell attachment and growth.

“The synergistic effects of GelMA’s capacity for supporting regenerative tissue formation and the structural advantages of silk fibroin make our composite material well suited for tendon repair,” said HanJun Kim, PhD, DVM, TIBI’s team leader on the project.

They prepared mixtures with varying ratios of silk fibroin and GelMA (SG) and fabricated them into thin nanofiber sheets. They then tested the sheets for fiber structure and stretchiness and chose an optimum formulation with the best mechanical properties. They also observed that the silk fibroin imparted an increased porosity to the material; this enhances tendon repair.

The MSCs on the SG sheets showed an increase in cell viability and proliferation over those on silk fibroin sheets without GelMA (SF). Genetic analysis showed that relevant gene activity in SG MSCs was significantly increased, in contrast to those on SF sheets, which was decreased.

Further tests on a growth factor secreted by MSCs seeded onto nanofiber sheets showed that the growth factors produced by the MSCs on the SG sheets were best able to repair injured tendon tissue cultivated in a culture dish.

Experiments were also conducted on live rats with injured Achilles tendons.

“Tissue remodeling for tendon repair is especially difficult to achieve,” said Ali Khademhosseini, PhD, TIBI’s director and CEO. “The work done here significantly advances that achievement.”

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