Decision at the crossroads
The study, headed by scientist Hannes Schmidt, focused on an area of the spinal cord of mice called the dorsal root entry zone (DREZ). This region along the back of the spine is an important relay station for sensory information that starts all over the body and is destined for the brain. For example, heat-detecting neurons under the skin stretch long axons all the way to the spine, where they enter the DREZ. Upon arrival the axons fork into two branches, one heading toward the brain, and the other down. Without this split, sense information is not properly transmitted through the body.
Blue staining reveals that the CNP signal is expressed in the spine of mouse embryos during the phase at which sensory axons enter the DREZ. Encountering the signal causes them to branch.
Rathjen says the DREZ is ideal for studying branching behavior. "It's an either-or situation; forking happens or it doesn't. You can easily observe it under the microscope. This gives us a clear test to find genes that are crucial for the process."
The researchers thought that axons entering the DREZ might encounter a molecular signal that changes their behavior. This would activate a signaling pathway in the cell that changes its chemistry and structure. Over the past few years, Schmidt and his colleagues have found some of the molecules in this pathway by developing strains of mice without particular genes. First they discovered how the signal is passed along inside a neuron once it arrives, via messenger molecules called cGMPs. In 2007 they found the molecule on the surface of axons that receives the message: a protein called Npr2. With the current study, they have finally identified the signal itself: a protein called CNP, which is produced in the dorsal spinal cord. When axons enter the zone, the Npr2 proteins on their surface recognize CNP and launch a program that starts the branching process.
A close look at normal mice showed that CNP proteins are produced in the dorsal spinal cord early in animal development, as most sensory axons prepare to enter. This pattern had been seen before, but CNP's functions had not been explained.
Mice without CNP experienced the same problems as earlier strains, investigated by Schmidt and his colleagues, which lacked the Npr2 receptor and cGMP signaling proteins. In all three cases, axons entering the DREZ had failed to fork. This confirmed that CNP activates Npr2 and triggers the signaling program in the cell.
Rathjen says that team is not yet sure whether CNP is the only signal, or whether it works in concert with other protein cues. One thing that remains to be explained is why the axon branches only once when it has entered the DREZ. The signal might stop, or forking might cause changes in the axon which make it unable to respond to CNP. Further experiments are underway to provide an answer.
Removing CNP from the mice did not disturb other modes of axon branching. This likely means that there is no universal branching signal; the process can probably be triggered in several ways, depending on the context. Rathjen says that unraveling these information pathways is important.
"After a spinal cord injury, the connections between axons and their target cells are often broken," he says. "Understanding the signals that control axon forking and growth might lead to a way to regenerate some of those connections."
- Russ Hodge
Highlight Reference:
C-type natriuretic peptide (CNP) is a bifurcation factor for sensory neurons. Schmidt H, Stonkute A, Jüttner R, Koesling D, Friebe A, Rathjen FG. Proc Natl Acad Sci U S A. 2009 Sep 29;106(39):16847-52. Epub 2009 Sept. 17
Link to the original article
2002 article on cGMP's role in axonal branching
2007 article on Npr2's role in branching
Wikipedia article on axons
