Human embryonic stem cells (hESCs) appear to have inbuilt preferences for the cell lineages they will differentiate into, and these preferences can be ‘read’ in the presence of cell surface markers, scientists claim. Researchers at McMaster University have found that subsets of stem cells within a population of hESCs that carry characteristic markers of pluripotency actually display differences in histone modification that have effectively preprogrammed them to differentiate into either hematopoietic or neural cell lineages.
Moreover, the team claims, which lineage an hESC is most amenable to differentiating into can be identified by the cell surface markers stem cell factor receptor (c-KIT) and A2B5. Reporting in Cell Stem Cell, Mick Bhatia, Ph.D., and colleagues claim their findings could help improve the efficiency of lineage-specific stem cell differentiation for both research and clinical applications.
Initial tests showed that in comparison with the differentiation efficiencies of unfractionated hESCs, stem cells separated according to the identified protein markers generated nearly five times as many blood cells and 12 times as many neural cells. Their paper is titled “Cell Fate Potential of Human Pluripotent Stem Cells Is Encoded by Histone Modifications.”
Tests by Dr. Bhatia’s team on fractionated c-Kit+, c-Kit-, and A2B5+ and A2B5- human ESCs showed that c-KIT+ and A2B5+ hESCs are lineage-biased toward hematopoietic and neural cell fates, respectively. At the epigenetic level, this lineage differentiation bias was found to be encoded by histone modifications of H3K27me3 and H3K4me3 methylation at lineage-associated and pluripotency genes. Activating (H3K4me3) and repressive (H3K27me3) histone methylation marks are associated with the transcription and repression of gene expression, respectively.
Combining c-KIT+ and A2B5+ subfraction analyses with expression profiling and lineage potential assays, the researchers showed that hESC cultures are actually a complex mosaic of cell types that cover the spectrum from self-renewing undifferentiated stem cell to incipient lineage-biased cells.
Moreover, the McMasters team notes, although previous work has found that the acquisition of lineage markers is typically associated with the loss of pluripotency markers, the new work indicates that the relationship between pluripotency and fate markers is not necessarily mutually exclusive. Although A2B5+ hESCs showed marked downregulation of pluripotency markers at the transcript and protein levels, the cells did still express these proteins and demonstrated clonogenic self-renewal capacity. In addition, c-Kit+ cells demonstrated little change in levels of the stem cell markers Oct4 and Nanog, while the plurpiotency marker SSEA3 was equally distributed in c-Kit+ and c-Kit- populations of hESCs.
Instead, the overall findings suggest that while these distinct subsets of stem cells have leanings toward differentiating into one cell lineage or another and therefore are not all equal, they do retain pluripotency, Dr. Bhatia comments. “They are all pluripotent. You can force a cell that normally would love to become a neural cell to turn into blood; they’ll do it, but not very well and not happily.”
The authors conclude, “Our results indicate that cell fate potential is encoded within functionally heterogeneous hESCs, thereby providing a means of better understanding the fundamental processes that underlie cell fate initiation and lineage-priming toward enhancing lineage-specific differentiation from hPSCs.” One of the next steps will be to see whether the same type of preprograming is inherent in induced pluripotent stem cells.