PROJECT SUMMARY/ABSTRACT
Shortage of organs for transplantation is one of the largest unmet medical needs. Researchers are currently
working on a variety of ways to increase the number of organs available. Recently, the prospect of producing
human organs in animals via interspecies blastocyst complementation has raised important attention and may
constitute an innovative approach to overcome the worldwide shortage of donor organs. Interspecies blastocyst
complementation works by injecting donor pluripotent stem cells (PSCs) from one species into the
organogenesis-disabled blastocyst of a different species. The growing host embryo can provide an emptied
developmental organ niche for donor cells to occupy. Despite the potential, however, recent work revealed that
although donor PSCs can initially engraft to inner cell mass (ICM) of blastocysts from distantly related host
species, their chimeric contribution to early post-implantation stages is low. These results suggest the existence
of embryonic xenogeneic barriers between evolutionarily distant species. Unlike chimera formation within the
same species, a multitude of factors can differ significantly between species, which preclude robust chimerism.
Cell competition, a conserved fitness-sensing process during which fitter cells eliminate neighboring less-fit but
viable cells, has been proposed as a surveillance mechanism to ensure normal development and tissue
homeostasis, and has also been suggested to act as a barrier to interspecies chimerism. During chimera
formation, xenogeneic donor PSCs may be treated as unfit or aberrant cells and targeted for elimination. Most
recently, we developed an interspecies PSC co-culture strategy and discovered a previously unknown mode of
cell competition between species. Interspecies competition between PSCs occurred in primed but not naive
pluripotent cells, and between evolutionarily distant species. By comparative transcriptome analysis, we found
that genes related to the NF-¿B and p53 signaling pathways, among others, were upregulated in less-fit ‘loser’
human cells. Genetic inactivation of TP53, a core component (P65, also known as RELA) and an upstream
regulator (MYD88) of the NF-¿B complex in human PSCs could overcome their competitive elimination by co-
cultured mouse PSCs, thereby improving the survival and chimerism of human cells in early mouse embryos.
Based on extensive preliminary results, in this proposal, we will further dissect the mechanisms underlying
interspecies primed PSC competition in both loser and winner cells. And, by suppressing interspecies PSC
competition, we aim to improve chimerism and provide the proof-of-concept of interspecies blastocyst
complementation from donor PSCs of rhesus monkey, our close kin, in distantly related mouse host.