Human iPSC-derived liver bud. [Takanori Takebe]
In the pursuit of regenerative medicine, scientists have long touted the therapeutic potential of induced pluripotent stem cells (iPSCs). Today, researchers from Japan bring the notion of lab-grown organs to bear, reporting their creation of a rudimentary but functional human liver made from these reprogrammed progenitor cells plus a mix of others, which they successfully transplanted into a mouse.
In a proof-of-concept study appearing online today in Nature, the Yokohama City University Graduate School of Medicine’s Takanori Takebe, Ph.D., and his colleagues demonstrate their in vitro generation of vascularized “liver buds,” around 4 mm to 5 mm in size, which recapitulated some human liver functions in vivo.
Creating a cocktail of hepatic endoderm cells derived from iPSCs mixed with HUVECs and human mesenchymal stem cells, the researchers were surprised to find the concoction gave rise to self-organizing 3D structures.
“After plating in a petri dish in vitro, the cells organized, formed a three-dimensional tissue,” Dr. Takebe told reporters during a press briefing. When transplanted into a mouse, these buds quickly hooked up to host vasculature, he noted—within 48 hours after transplantation.
These rudimentary organs also produced human albumin and showed significant hepatic maturation as compared with conventional human iPSCs directed to differentiate into hepatic endoderm cells.
“These results highlight the enormous therapeutic potential using in vitro-grown organ-bud transplantation for treating organ failure,” Dr. Takebe et al. report.
While several groups have successfully generated organ-like tissue in vitro using stem cells, a slew of challenges have impeded efforts aimed at translating these results for potential clinical use.
The University of Pittsburgh’s Stephen Badylak, M.D., Ph.D., who was not involved in the work, says the results reported by Dr. Takebe and his colleagues represent a significant advance for the field.
“This is the first time to be able to take iPS cells, mix them with some other cells, and in combination form something that is a viable, three-dimensional construct in vivo,” Dr. Badylak tells GEN. “Most people who are working in tissue engineering and regenerative medicine, organ engineering have always felt this could happen, but no one has really shown it. …A lot of people will be citing this paper.”
Vascularization has been one major sticking point, notes Dr. Badylak, who is deputy director of Pittsburgh’s McGowan Institute for Regenerative Medicine.
“[For] all of the people working in organ engineering—whether it’s heart, lung, liver, or kidney—the rate-limiting step right now is the ability to vascularize the construct. Everybody can grow liver cells, heart cells, and kidney cells in culture, but when you take them into the body and try to say ‘OK, now go over here and do your job,’ they all die because they don’t have a vascular network,” he says. “People have been trying different approaches to engineer vascular networks into populations of parenchymal cells.”
To date, Dr. Badylak adds, “Our attempts at recreating tissue structure are really very naïve compared to what Mother Nature can do.”
Getting lab-grown tissue to recapitulate the functions of vascularized organs has also stumped many teams.
In their paper, Dr. Takebe and his colleagues present ELISA- and gene expression-based data among other functional analyses to show that these transplanted liver buds performed some human liver functions. “They [Takebe et al.] did a good job of showing that these cells did what liver cells are supposed to do,” Dr. Badylak says.
New Metabolic Model
The researchers also showed that the human iPSC liver bud mouse model showed similar drug metabolism activities as human liver. Administering ketoprofen or debrisoquine, two compounds known to be metabolized between mice and humans, Dr. Takebe et al. found that the human iPSC liver bud mouse model produced a profile similar to that of human liver.
They suggested that this result underscores the potential of predicting human drug metabolite profiles using mice transplanted with human iPSC liver buds, as they closely mimicked human physiology.
“Our approach will also contribute to establish a novel drug screening platform for new drug development,” Dr. Takebe elaborated in an email to GEN. “Mainly, we are seeking two approaches: One is the use of humanized chimeric liver mice, and the other is [an] in vitro drug screening platform by making use of the directly differentiated hepatocyte from human iPSC-liver bud in vitro.”
Leiden University Medical Center’s Christine Mummery, Ph.D., tells GEN that use of these human iPSC liver buds “in detecting toxic effects of drugs and compounds that are metabolized to the toxic state by the liver,” is a particularly promising implication of this study.
Dr. Mummery, a professor of developmental biology who was not involved in this work, adds that efforts aimed at using human iPSCs to model disease states and drug metabolism activities to date have fallen short when it comes to behaving like human organs. Such has been a problem, she says, “when attempting to use these cells … in the case of liver, for example, to metabolize drugs in the way the normal human liver does it.”
Not a Small Step
Commenting on the research, Leiden’s Dr. Mummery says that though it is still early-stage, the work of Dr. Takanebe et al. overcomes key challenges. “Even though this has to be done by growing in a mouse, it demonstrates that the liver-like cells derived from human iPSCs do have the capacity to mature and there is not just some kind of magic block,” she says. “A genuine advance, I would say.”
Adds Pittsburgh’s Dr. Badylak: “Now the challenge becomes how do we take this and go to the next step, which is to create the whole, or something that could legitimately be hooked up to the circulation in a way that replaces the damaged or missing tissue.”
“That’s the next step,” he says. “It’s not a small step.”
Indeed, Dr. Takebe told reporters his team is now working to generate tens of thousands of liver buds in vitro, noting it would likely be around a decade before this human iPSC liver bud approach is primed for clinical use. He also said initial evidence has suggested that this human iPSC-based method might also be used to generate other organs, such as pancreas, kidneys, and lungs.
“Although efforts must ensue to translate these techniques to treatments for patients, this proof-of-concept demonstration of organ-bud transplantation provides a promising new approach to study regenerative medicine,” the researchers concluded.