Ronin takes a place beside, but not with, Oct4, Nanog, and Sox2
(This is out in Cell today. The research highlight will appear on our site on July 10)
Embryonic stem cells exist in a state of suspended differentiation; they are able to become any cell type but resist taking on any particular fate. A trio of transcription factors is widely recognized as essential to this pluripotent state: Oct4, Nanog and Sox2. These proteins interact closely with each other and often bind the same or nearby sites on DNA. Now, work by Thomas Zwaka of the Baylor College of Medicine in Houston, Texas, and colleagues reveals another protein that works outside this pluripotency network1. He dubbed it Ronin, the word for a samurai warrior without a master.
Zwaka came upon Ronin serendipitously. His previous work had shown that caspase-3, an enzyme that regulates cell death, also affects pluripotency by cleaving Nanog. Zwaka suspected that this enzyme might have additional roles to play in pluripotency, so his lab carried out a laborious large-scale screen called a yeast two-hybrid assay to figure out what other proteins the caspase interacted with. This screen pulled out the protein that is now called Ronin, and Zwaka’s team set about conducting a series of experiments to find out what it does.
Ronin is expressed only in oocytes, early embryos and some regions of the brain where cells proliferate. Neither embryos nor embryonic stem (ES) cells lacking Ronin can survive. If Ronin was overexpressed, mouse stem cells failed to differentiate even under conditions (the removal of a key growth factor) that caused all stem cells with normal Ronin expression to do so.
To really know that overexpression of Ronin can maintain ES cells without growth factors, the cells should be grown over several passages, says Qi-Long Ying, a stem cell scientist at the University of Southern California, in Los Angeles, who recently reported a cocktail of small molecules that allow mouse ES cells to proliferate without the growth factors typically required2. “I don’t know that they tested for prolonged culture.” Nonetheless, he says, Ronin seems to be a new essential factor for maintaining both pluripotency and early embryonic development. “It’s very important and also very interesting.”
Ronin seems to act through chromatin regulation. It has a zinc-finger DNA-binding motif called THAP that is often found in proteins that modify chromatin; Ronin also binds another protein called host cell factor-1 (HCF-1) that, along with other proteins, helps to modify histones, the structures within cells that package DNA into chromatin and help regulate gene expression. Ronin also seems highly conserved across species: the gene that encodes it is quite similar in humans and zebrafish, and there is even an apparent homologue in sea urchins.
It makes sense that important functions like self-renewal are regulated by a network and not a handful of factors, says Sadhan Majumder at the M.D. Anderson Cancer Center, in Houston, Texas, who has identified components of pluripotency networks3. “I think scientists are focusing on a few molecules that have so far been discovered, but I am sure that the network is controlled by many,” he says. Any of these network components could shift a cell’s equilibrium between self-renewal and differentiation.
Zwaka says that the next steps are to search for a Ronin regulatory network and other Ronin-related proteins. He’s also testing to see if Ronin can reprogram cells to pluripotency in the absence of other known reprogramming factors.
Repressing microRNAs for pluripotency
1. Dejosez, M. et al. Ronin is essential for embryogenesis and the pluripotency of mouse embryonic stem cells. Cell 133, 1162–1174 (2008). | Article |
2. Ying, Q.-L. et al. The ground state of embryonic stem cell self-renewal. Nature 453, 519–523 (2008). | Article |
3. Singh, S. et al. REST/NRSF maintains self-renewal and pluripotency of embryonic stem cells. Nature 453, 223–227 (2008). | Article |