March 21, 2013
For organisms to develop and grow, asymmetry is essential. New research from Howard Hughes Medical Institute scientists reveals how a localized source of a signaling molecule directs a dividing stem cell to produce two different cells—one identical to its parent, the other a more specialized cell type—and aligns those cells.
In a developing tissue, such oriented divisions will position cells to migrate to the right place to ensure the right architecture.
“This kind of asymmetry is a universal aspect of how organisms grow,” says Roel Nusse, an HHMI investigator at Stanford University, explaining that dividing cells must orient themselves appropriately to create the asymmetrical bodies of complex organisms. In a paper published March 21, 2013, in the journal Science, Nusse and his collaborators show that a protein called Wnt3a coordinates the orientation of the two different cell types that are generated when a dividing stem cell undergoes an asymmetrical division.
Wnt3a is one of a large family of Wnt proteins that play important roles in controlling how organisms develop and grow. Nusse and Harold Varmus discovered the first Wnt gene in mice in Varmus’s lab at the University of California, San Francisco in 1982. Since then, Nusse and others have shown that Wnt proteins play key roles in embryonic development, tissue regeneration, bone growth, stem cell differentiation, as well as many human cancers.
To study how Wnt proteins affect cells, researchers typically add the molecule to the nutrient-rich solution in which laboratory-cultured cells are grown. Nusse and his team recently showed that when Wnt3a is given to embryonic stem cells in this way, it helps the cells maintain their identity as stem cells, rather than differentiating into more specialized cells. But experiments like these don’t really reflect the ways cells in a living organism receive signals, Nusse says.
“Most of the signals that cells in tissues make for each other are received by neighboring cells,” he says. “So there’s an orientation effect: the signal comes from one end of the cell and it only activates the target cell at one side. There’s all kinds of geometry going on, regulated by the signals between cells. But when you add growth factors to a tissue culture medium, there’s no orientation effect.”
Shukry Habib, a postdoctoral researcher in Nusse’s lab, came up with a way to recreate that orientation effect with cells grown in a dish. Rather than adding Wnt3a to the tissue culture medium, he attached it to tiny beads. When he added the Wnt-coated beads to dishes in which embryonic stem cells were growing, the scientists could then watch individual cells that were close enough to a bead to receive a Wnt3a signal, and track the fate of the new cells as they divided.
Nusse says the first experiments with the Wnt3a beads were underway when he attended a meeting of HHMI scientists and met with Eric Betzig, a lab head at the Janelia Farm Research Campus. In 2011, Betzig’s team developed a high-speed, high-resolution, three-dimensional imaging technology that they call the Bessel beam plane illumination microscope. The microscope gives extraordinarily detailed views of cellular processes in action, and as Betzig and Nusse talked, they realized it could be a powerful tool in tracking the stem cells’ response to the Wnt-coated beads.
“The advantage that we have is that we can now look at the behavior of individual cells in real time,” Nusse says. Habib arranged to spend time in Betzig’s lab through Janelia’s visiting scientist program, and the teams set up experiments that combined the power of the microscope with their new Wnt-coated beads.
Under the microscope, they soon observed that each cell in contact with a Wnt-coated bead divided the same way—orienting itself so that when division was complete, one daughter cell was close to the Wnt3a signal and the other cell was further away. Each time, the cell closest to the bead (the proximal cell) maintained the characteristics of an embryonic stem cell, while the distant (or distal) cell turned on genes indicating it had begun to differentiate into the more specialized cell known as an epiblast stem cell. “That makes sense,” Nusse says, “because the Wnt signal is important for stem cell fate, and the lack of a Wnt signal would allow the distal cell to differentiate.”
Further, the scientists could see that Wnt3a influenced the orientation of the structural components that established the axis along which the cell would divide. “So Wnt makes the cells divide so there will be a proximal and distal cell with respect to the origin of the Wnt signal,” Nusse explains. “Because of this orientation, there will always be a cell that is distal from the Wnt signal, and that’s the cell that is going to differentiate, while the cell that’s close maintains the stem cell fate.”
“People have been wondering about asymmetrical cell division for a long time,” Nusse says. “We’ve been able to address these old questions with new tools.” Now, his team will begin to investigate the molecules that link the Wnt signal to the behavior of the cells. With new technology, he says, “we have immense power—so much is possible.”