Baylor Team Characterizes Key Protein’s Role in Pluripotency

Research led by Baylor College of Medicine describes how a key protein for resetting specialized cells back into pluripotent stem cells accomplishes this task, and the findings that could have significant implications in such fields as regenerative and transplant medicine.  

Pluripotent stem cells have yet to specialize in a particular biological function and maintain the potential to become any of the possible cell types in an organism. The protein NANOG is the telltale marker of pluripotent stem cells and a necessary ingredient to reset specialized cells back into naïve, untrained stem cells. But how human NANOG accomplishes this has largely remained a mystery.

The study, led by Baylor College of Medicine and reported recently in Nature Cell Biology, found that human NANOG activates pluripotency because its “super stickiness” enables it to form large aggregates at very low concentrations. These aggregates interact with chromatin—string of DNA and proteins that coil to form chromosomes carrying the cells’ genetic information—to reshape the genomic landscape in a way that activates a pluripotent state.

“Resetting specialized cells to a pluripotent state requires massive reorganization of the chromatin and changes in gene expression—turning on genes involved in pluripotency and turning off genes that specify specialized cells,” says coauthor Dr. Josephine Ferreon, assistant professor of pharmacology and chemical biology and member of the Dan L Duncan Comprehensive Cancer Center at Baylor. “Furthermore, coordinated gene activation often requires bringing DNA elements that are far apart closer to enable gene expression. We found that NANOG’s properties—its naturally floppy, flexible 3D shape and a C-terminal tail that is structurally akin to prion-like proteins—enable it to achieve this.”

NANOG’s high tendency to self-adhere and aggregate, however, posed a problem for traditional ensemble techniques that require high protein concentrations. To study this very challenging protein, the team resorted to highly sensitive fluorescence approaches.

“In this study, we applied single molecule and fluorescence fluctuation microscopy techniques with which we can visualize whether two molecules interact with each other. The experiments were performed at very small concentrations, picomolar to nanomolar, where we can usually avoid aggregation and investigate highly aggregation-prone proteins,” says co-corresponding author Dr. Allan Chris Ferreon, assistant professor of pharmacology and chemical biology at Baylor. “However, with NANOG, even at extremely low concentrations, we still detected aggregation. Nonetheless, we were able to show that NANOG aggregation is actually essential to its function as a master transcription factor and a mediator of the bridging of DNAs. This phenomenon may be unique to NANOG.”

“We think that this phenomenon is the reason why NANOG expression is key to the establishment of pluripotency. When NANOG’s level is low, cells are prone to differentiate, and when its level is high, the ground pluripotent state or ‘full reset’ is achieved and maintained,” Dr. Josephine Ferreon adds.

The researchers think that NANOG acts like a molecular glue that can initiate and stabilize key chromatin interactions important for the pluripotent state. NANOG’s aggregation behavior also explains its role as a molecular ‘hub’ protein and its interactions with many important chromatin regulators that are involved in opening chromatin and recognizing and modifying specific chromatin regions.

“In the future, we hope to understand more about the role of NANOG and its prion-like region in recruiting or cooperating with important transcription factors, coactivators and epigenetic modulators to reshape the genomic landscape,” Dr. Josephine Ferreon says.