Study: How to Produce Mouse-Human Chimeric Embryos

The ability to produce millions of mature human cells in a living organism — called a chimera, which contains the cells of two species — is critical if the ultimate promise of stem cells to treat or cure human disease is to be realized.

But to produce those mature cells, human primed stem cells must be converted back into an earlier, less developed naive state so that the human stem cells can co-develop with the inner cell mass in a mouse blastocyst.

The protocol outlining how to do that was published July 2 in Nature Protocols by the UB scientists. They were invited to publish it because of the significant interest generated by the team’s initial publication describing its breakthrough in May 2020.  

“This paper will enable many scientists to use this new platform to study the human disease of their interest,” says Jian Feng, PhD, professor of physiology and biophysics in the Jacobs School of Medicine and Biomedical Sciences and senior author on the paper.

“Over time, it will transform biomedical research toward a more effective use of the human model system to directly study virtually any inborn condition of an individual,” he says. “It will stimulate unforeseen discoveries and applications that may fundamentally change our understanding of human biology and medicine.”

The protocol will allow scientists to create animal models that Feng says provide a much more realistic picture of embryonic development than has ever been possible.

These more realistic animal models also will have the potential to reveal the mechanisms behind numerous diseases, especially those that afflict individuals from birth.

“This step-by-step protocol will benefit the entire field by enabling other scientists to use our methods to generate chimeras to study human diseases that they are experts in,” Feng says. “It will lead to the generation of better mouse models for various human diseases, such as sickle cell anemia, COVID-19 and many others, or various human developmental disorders.”

The paper demonstrates how to generate naive human pluripotent stem cells from existing induced pluripotent stem cells that may be derived from patients with various diseases, how to generate mouse-human chimeras using these cells and how to quantify the amount of human cells in the chimeras. 

“Using our method, one can now track the development of naive human pluripotent stem cells in mouse-human chimeric embryos in real time,” Feng says.

These stem cells can then be manipulated either genetically or pharmacologically, providing valuable information about human development and disease.

“For example, one can label naive human pluripotent stem cells by inserting green fluorescent protein in a hemoglobin gene to study the development of human red blood cells in mouse-human chimeras,” Feng notes.

Another application is to generate humanized mouse models to study many human diseases.

“These mice contain critical human cells, tissues or even organs so that they more accurately reflect the human condition,” Feng says. “With our method, the human cells are made along with the mouse during the development of the mouse embryo. There would be better matching and no rejections, because there are ways for the human cells to be made where there is no competition from their mouse counterparts.”

By allowing others to improve and adapt the method to eventually generate chimeras in larger animals, this protocol may also lead to the generation of human organs to address the dire shortage of organs available for transplant, according to Feng.

“If naive human pluripotent stem cells are able to generate significant amounts of mature human cells in other larger species, it could be possible to make human tissues or even human organs in chimeric animals,” he notes.

This would be possible using blastocyst complementation where, Feng explains, normal pluripotent stem cells from one species can reconstitute an organ for that species in a blastocyst of another species that have been genetically modified not to grow that particular organ.

“Ultimately, a better understanding of how human cells develop and grow in chimeras may enable the generation of human cells, tissues and organs in a completely artificial system and fundamentally change how we treat many human diseases,” he says. “Research using chimeras is a bridge that must be crossed to reach that possibility.” 

Co-authors from the Department of Physiology and Biophysics are:

• Zhixing Hu, PhD, postdoctoral researcher
• Houbo Jiang, PhD, research scientist
• Boyang Zhang, PhD, postdoctoral associate

Other co-authors are:

• Hanqin Li, PhD, who earned his doctoral degree in the neuroscience program and is now a postdoctoral associate at the Innovative Genomics Institute at the University of California, Berkeley
• Brandon Joseph Marzullo, lead programmer and bioinformatics analyst in the Genomics and Bioinformatics Core of UB’s New York State Center of Excellence in Bioinformatics and Life Sciences
• Aimee B. Stablewski, co-director of the Gene Targeting and Transgenic Shared Resource of the Roswell Park Comprehensive Cancer Center
• Donald A. Yergeau, PhD, associate director of genomic technologies in the Genomics and Bioinformatics Core of UB’s New York State Center of Excellence in Bioinformatics and Life Sciences

The work was supported by NYSTEM and the Buffalo Blue Sky Initiative.