The chemical reprogramming of unipotent adult germ cells towards authentic pluripotency and de novo establishment of imprinting

Although somatic cells can be reprogrammed to pluripotent stem cells (PSCs) with pure chemicals, authentic pluripotency of chemically induced pluripotent stem cells (CiPSCs) has never been achieved through tetraploid complementation assay. Spontaneous reprogramming of spermatogonial stem cells (SSCs) was another non-transgenic way to obtain PSCs, but this process lacks mechanistic explanation. Here, we reconstructed the trajectory of mouse SSC reprogramming and developed a five-chemical combination, boosting the reprogramming efficiency by nearly 80- to 100-folds. More importantly, chemical induced germline-derived PSCs (5C-gPSCs), but not gPSCs and chemical induced pluripotent stem cells, had authentic pluripotency, as determined by tetraploid complementation. Mechanistically, SSCs traversed through an inverted pathway of in vivo germ cell development, exhibiting the expression signatures and DNA methylation dynamics from spermatogonia to primordial germ cells and further to epiblasts. Besides, SSC-specific imprinting control regions switched from biallelic methylated states to monoallelic methylated states by imprinting demethylation and then re-methylation on one of the two alleles in 5C-gPSCs, which was apparently distinct with the imprinting reprogramming in vivo as DNA methylation simultaneously occurred on both alleles. Our work sheds light on the unique regulatory network underpinning SSC reprogramming, providing insights to understand generic mechanisms for cell-fate decision and epigenetic-related disorders in regenerative medicine.

Rapid conversion of human ESCs into mouse ESC-like pluripotent state by optimizing culture conditions

The pluripotent state between human and mouse embryonic stem cells is different. Pluripotent state of human embryonic stem cells (ESCs) is believed to be primed and is similar with that of mouse epiblast stem cells (EpiSCs), which is different from the naïve state of mouse ESCs. Human ESCs could be converted into a naïve state through exogenous expression of defined transcription factors (Hanna et al., 2010). Here we report a rapid conversion of human ESCs to mouse ESC-like naïve states only by modifying the culture conditions. These converted human ESCs, which we called mhESCs (mouse ESC-like human ESCs), have normal karyotype, allow single cell passage, exhibit domed morphology like mouse ESCs and express some pluripotent markers similar with mouse ESCs. Thus the rapid conversion established a naïve pluripotency in human ESCs like mouse ESCs, and provided a new model to study the regulation of pluripotency.