The human liver possesses remarkable regenerative capabilities, yet in cases of end-stage liver disease, this natural healing process can be overwhelmed. Once the liver reaches a critical point of damage, the only viable solution often becomes organ transplantation. However, the scarcity of donor livers poses a significant challenge, prompting researchers to explore alternative strategies for liver restoration.
Innovative approaches are emerging that aim to engineer entire, implantable liver organs. Nevertheless, current laboratory-created liver constructs are still limited in size and therapeutic potential. A collaborative research team from the Wyss Institute at Harvard University, Boston University, and MIT has taken a unique approach to address this issue.
The Concept of Satellite Livers
Christopher Chen, MD, PhD, a core faculty member at the Wyss Institute and a professor at Boston University, proposed an intriguing idea: what if a small liver construct could be implanted and then encouraged to grow within the body? “A sufficiently grown, functional ‘satellite liver’ could alleviate the metabolic burden of a damaged liver and act as a bridge until a transplant is available,” Chen explained.
This initiative is led by Amy Stoddard, PhD, a doctoral student who developed this concept during her research and postdoctoral work. The project integrates tissue engineering with synthetic biology, employing a genetic strategy termed “bioengineered on-demand outgrowth via synthetic biology triggering” (BOOST). By modifying the gene expression of liver cells, the team has successfully activated a tissue growth program in small liver constructs after implantation in mice.
Breakthroughs in Liver Tissue Engineering
In their published research, the team outlined their findings, emphasizing the potential of their technique as a novel approach to cell therapy scale-up. By initially implanting a small liver construct and subsequently inducing it to grow in situ, they aim to bypass the challenges associated with producing large quantities of cellular materials and creating constructs capable of sustaining themselves post-engraftment.
The study highlights the pressing need for alternatives to organ transplants, especially given the limited availability of donor organs. While promising advancements have been made in tissue engineering for cell therapies, scaling these constructs to therapeutic sizes remains a significant barrier.
Induction of Growth in Engineered Constructs
To facilitate the growth of small liver constructs once implanted, the researchers focused on identifying specific growth cues. Recognizing that liver growth is influenced by soluble growth factors, Stoddard explored various candidates for their ability to stimulate cell proliferation in cultured hepatocytes.
The BOOST strategy combines synthetic biology and tissue engineering, allowing for controlled liver growth inside the body. By rewiring the gene expression of hepatocytes and supporting fibroblast cells, the research team developed a method to activate tissue growth in engineered liver constructs post-implantation.
Overcoming Density-Induced Proliferation Challenges
Through diligent experimentation, the team identified a set of four growth factors that effectively promoted hepatocyte growth in culture. However, they soon discovered that these factors were less effective in densely packed three-dimensional liver tissues. This observation led them to investigate the role of the protein YAP, which responds to mechanical signals and influences cell proliferation.
It became evident that YAP’s degradation in high-density conditions was inhibiting cell growth. By introducing a non-degradable version of YAP into hepatocytes, the researchers successfully overcame this barrier, paving the way for effective cell proliferation in dense tissue environments.
Engineering Control Mechanisms for Growth
To achieve controlled proliferation of hepatocytes in living organisms, the team developed a synthetic biology toolkit capable of managing growth factor and YAP signaling within the engineered liver tissues. They engineered fibroblast cell lines to secrete specific growth factors and created hepatocytes that expressed the non-degradable YAP protein, allowing for localized control of cell growth.
Time course experiments indicated that a continuous treatment with doxycycline (DOX) led to significant expansion of the engineered liver tissue. Upon removal of DOX, the hepatocytes reverted to a non-proliferating state, demonstrating the potential for controlled growth.
Testing BOOST in Living Models
The true test of the BOOST-engineered liver constructs came when the team implanted them into living mice treated with DOX. The results were impressive, showing a 500% increase in tissue proliferation and a doubling of engineered hepatocytes. Additionally, the newly formed tissues were well-vascularized, meeting the metabolic demands of the growing construct and exhibiting no adverse effects.
These findings mark a significant milestone, as they demonstrate the ability to induce growth in liver tissue without relying on host liver injury, which has traditionally been a prerequisite for hepatocyte engraftment.
Future Implications of BOOST Technology
Looking ahead, the researchers plan to investigate the potential of BOOST technology in addressing liver injuries. “Our BOOST strategy sets the stage for future solid organ cell therapies that can be non-surgically controlled to meet patient needs,” Bhatia noted. The applications of this approach could extend beyond liver disease, potentially transforming the treatment landscape for other engineered tissues such as heart or pancreatic tissues.
The research team concluded that their work represents an exciting proof of concept, suggesting that tissue scale-up through growth could indeed be achievable. This research lays the groundwork for developing “smart” tissue therapeutics, which could be tailored to individual patient requirements, ultimately providing solutions for previously untreatable diseases.
Key Takeaways
- The BOOST strategy integrates synthetic biology and tissue engineering to induce growth in small liver constructs post-implantation.
- Researchers identified YAP protein and specific growth factors as crucial components in promoting hepatocyte proliferation in dense tissue environments.
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The successful trials in mice demonstrate the potential for engineered liver tissues to expand without necessitating injury to the host liver.
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The implications of BOOST technology could extend to other organ systems, potentially revolutionizing treatments for various diseases.
In summary, the innovative work on engineering liver tissue through the BOOST strategy represents a groundbreaking advancement in the field of cell therapy. This research not only addresses the limitations of current organ transplantation practices but also opens up new avenues for the treatment of liver disease and beyond.
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