Protein Plays Unexpected Role in Embryonic Stem Cells What if you found out that pieces of your front door were occasionally flying off the door frame to carry out chores around the house? That the kind of surprise scientists at the Salk Institute experienced with their recent discovery that nucleoporinsproteins that act as cellular doorways to help manage what goes in and out of a cell nucleusare actually much bigger players in expressing genes than previously thought. The finding, published June 16, 2015 in the journal Genes & Development, shows that nucleoporins play an important role in maintaining embryonic stem cells before they begin to develop into specific tissues. This discovery gives a new understanding to genetic diseases that are caused by mutations in these proteins. One nucleoporin protein in particular has a dramaticand unanticipatedfunction in the formation of neurons from stem cells. Weve shone a new light on this class of proteins, said Martin Hetzer, a professor in Salk Molecular and Cell Biology Laboratory and senior author of the new paper. I hope people start to accept and realize that nucleoporins are more than just transport proteins. Nucleoporinsof which there are about 30 versionsare typically part of nuclear pore complexes, giant structures that connect the inside of a cell nucleus to the outer cytoplasm. In 2010, Hetzer team first uncovered hints that nucleoporins may also have a role in regulating the timing of when genes inside the nucleus are transcribed into proteins during a cell development. But exactly what that role might be was unclear. In the new work, Hetzer and his colleagues focused on one particular nucleoporin called Nup153, which is known to rapidly move on and off of the nuclear pore complex, suggesting it might be doing something other than providing structural support to the pore. The researchers turned to mouse embryonic stem cellscells that have the potential to differentiate into any cell type in the bodyand deleted Nup153. They expected that if Nup153 played a key role in cell differentiation, then removing it from stem cells would stop them from differentiating. Instead, the opposite happened. The big surprise was that when we took out this gene, the stem cells started to differentiate, said Hetzer. And not only did they start to differentiate, but they started to differentiate into neurons. Nup153, researchers discovered, put the brakes on certain genes that need to be turned on for stem cells to turn into brain cells. When the brakes are lifted, the stem cells start differentiating. This study not only revealed a critical function for nucleoporins in mediating the undifferentiated state of embryonic stem cells by silencing neural genes, but also introduced new mechanistic directions for elucidating the role of these proteins during mammalian development, said Filipe Jacinto, a postdoctoral researcher in Hetzer lab and first author of the paper. Hetzer suspects that other nucleoporins also have roles in gene expression control, but cautions that the roles could be very differenteach nucleoporin, he said, likely targets a different set of genes, and some might activate the genes rather than repress them. Mutations in many nucleoporin genes has been linked to human diseases and developmental disorders, including some forms of leukemia and inherited heart problems. Until now, Hetzer said, researchers have assumed the mutations led to disease by altering the transport of proteins in and out of a cell nucleus. Now, were realizing this is probably not the only explanation, he said. Many of those diseases and developmental disorders might actually be caused by the ability of these genes to regulate gene expression programs. His lab is planning to follow up with studies on Nup153, and exactly how it recruited to genes, as well as investigating the developmental roles of other nucleoporins. The work and the researchers involved were supported by grants from the National Institutes of Health and the National Cancer Institute.