The realization of human embryonic stem cells (hESC) as a model for human developmental hematopoiesis and in potential cell replacement strategies relies on an improved understanding of the extrinsic and intrinsic factors regulating hematopoietic-specific hESC differentiation. Human mesenchymal stem cells (hMSCs) are multipotent cells of mesodermal origin that form part of hematopoietic stem cell niches and have an important role in the regulation of hematopoiesis through production of secreted factors and/or cell-to-cell interactions.

We have previously shown that hESCs may be successfully maintained feeder-free using hMSC-conditioned media (MSC-CM). Scientits hypothesized that hESCs maintained in MSC-CM may be more prone to differentiation towards hematopoietic lineage than hESCs grown in standard human foreskin fibroblast (HFF)-conditioned media (HFF-CM). They report that specification into hemogenic progenitors and subsequent hematopoietic differentiation and clonogenic progenitor capacity is robustly enhanced in hESC lines maintained in MSC-CM. Interestingly, co-culture of hESCs on hMSCs fully abrogates hematopoietic specification of hESCs suggesting that the improved hematopoietic differentiation is mediated by MSC-secreted factors rather than by MSC-hESC physical interactions.

To investigate the molecular mechanism involved in this process, they analyzed global (LINE-1) methylation and genome-wide promoter DNA methylation. Human ESCs grown in MSC-CM showed a decrease of 17% in global DNA methylation and a promoter DNA methylation signature consisting in 45 genes commonly hypomethylated and 102 genes frequently hypermethylated.

This data indicates that maintenance of hESCs in MSC-CM robustly augments hematopoietic specification and that the process seems mediated by MSC-secreted factors conferring a DNA methylation signature to undifferentiated hESCs which may influence further predisposition towards hematopoietic specification.