Unexpected regulation of transcription factors critical to development
A team of developmental biologists at the University of Massachusetts Amherst led by Dominique Alfandari, with others at MIT, report in a new paper that they have for the first time described how two transcription factors that are "absolutely essential for human development" are regulated by a cell surface metalloprotease known as ADAM13. The discovery adds to knowledge of how cells migrate in vertebrate embryos, how stem cells differentiate and how cancer cells metastasize.
ADAM13 belongs to a group of proteins called proteases that cut other proteins to change their function. Alfandari says, "Five years ago we discovered that ADAM metalloproteases control gene expression in a cell; no one had done that before. In this new paper, we describe the details of the mechanism by which the ADAM13 protein on the cell surface can affect gene expression in the nucleus, which is remarkable."
"We found that ADAM13 has to work with a series of other proteins and two transcription factors, tfap2- and arid3a. We've described every step they take and how they are changed to get the work done. Arid3a shuttles between ADAM13 on the surface and genes in the nucleus. We've seen that some ADAMs can do this but, for example, ADAM9 can't do it at all," the molecular biologist adds. Details appear now in the online, open-access journal eLIFE.
In studies supported by the NIH's National Institute of Dental and Craniofacial Research, Alfandari and colleagues including co-investigator Helene Cousin and first author and postdoctoral researcher Vikram Khedgikar, are well known for tracking individual cells in frog embryos to learn how the ADAM13 protease controls proteins in the cranial neural crest to form the jaw and face. Cranial neural crest cell migration is common to all vertebrate embryos including humans, and defects in their production or migration lead to severe facial malformations.
After many years of this work, Alfandari and colleagues are now getting more into the detail of what the ADAM family of proteins can do, he notes. "What is exciting about that is we've shown that ADAM13, which is one of more than 30 members of this family, controls two essential transcription factors that bind to DNA to turn on genes."
He explains that arid3a in humans controls cell-fate decisions, that is, it directs uncommitted stem cells to differentiate from a naïve state to a cell that knows its job. In this role, arid3a can be thought of as an "anti-stem-cell factor," because it shuts down the stem cell's ability to remain naïve. Alfandari quips, "It kicks the kids out of home and forces them to go to school and learn a trade."
A special role of arid3a in forcing the cell to specialize has implications for cancer, he adds. When cells differentiate they often stop dividing; they rarely do both. By forcing differentiation, arid3a suppresses cell division. He explains, "In a tumor, this means arid3a forces tumor cells to stay in place and perform work rather than dividing. They are more benign, tending to stay in place and not metastasize." Thus, the prognosis for a patient with a tumor that has a lot of arid3a in it is better than one that has little arid3a, because with less, they proliferate more.
Alfandari adds, "Perhaps it may give us a new tool to control tumors, if you can control ADAM activity from outside of the cell, you could control arid3a as well. This is long-term, I'm dreaming of what could happen. We don't know yet if ADAM can play a role in this way to control cancer, but it is a new possibility."
As for tfap2-, in all species studied so far it controls the cells at the border of the neural plate in the embryo, which go on to form all of the sensory organs for hearing, vision and smell, plus all the cranio-facial structures, so "tfap2- pretty much defines this group of cells during vertebrate evolution," the lab director points out.
"For our work this is very important because it helps us understand how neural crest cells acquire their specialization and learn how and where to move. It also suggests that ADAM proteins in other kinds of cells may regulate similar processes."
The molecular biologist says that ADAM13 has been thought of as "a low-key player, it was not thought to be critical." Indeed, he thinks of ADAM proteins as a fine-tuning mechanism, like the coach on the sidelines of a football game. "If you lose the coach, the team can continue to function even if less efficient. If an embryo is missing ADAM13, it can survive, but with birth defects."
However, the transcription factors arid3a and tfap2- are far more important, more akin to a quarterback. "If you lose the quarterback, the team can't go on," he says. "If you lost arid3a and tfap2-alpha, these are absolutely critical for the embryo and without them it cannot survive."
In the future, Alfandari says, his lab will continue to define the parts of ADAM13 and arid3a that communicate with each other and how arid3a shuttles between ADAM on the cell surface and genes in the nucleus. "Our goal is to figure out the specific protein sequences that control arid3a," he notes. "That will take another five years."