Netrin1 is a protein that was first characterized for its axon guidance activities during embryonic development. The netrin family has been shown to play many critical roles in developmental and physiological processes beyond axon guidance. Netrin1 is involved in the progression of cancers, diabetes, and inflammatory bowel diseases. It also directs cellular differentiation across organ systems. However, no role for netrin1 directing cell fate in the developing nervous system in vivo has been described. Now, scientists at the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at the University of California, Los Angeles (UCLA), have uncovered an unexpected role for netrin1 in organizing the developing spinal cord.
Their findings are published in Cell Reports in an article titled, “Netrin1 patterns the dorsal spinal cord through modulation of Bmp signaling,” and may reshape our understanding of how complex spinal circuits are established during embryonic development.
“This is a story of scientific curiosity—of discovering something odd and trying to understand why it happened,” said senior author Samantha Butler, PhD, a professor of neurobiology at the David Geffen School of Medicine and member of the UCLA Broad Stem Cell Research Center. “We found that netrin1, which we’ve long known as a powerful architect of neural circuits, has an entirely unanticipated role in organizing the spinal cord during early development.”
The development of the dorsal spinal cord, where sensory inputs like touch and pain are processed, is characterized by precise compartmentalization and organization. For these sensory processes to function, specific neurons must form in carefully defined regions. This activity is orchestrated by BMP signaling, which occurs only within the boundaries of the dorsal spinal cord.
BMP signals must be carefully contained to ensure they don’t spread to other regions of the spine. The critical boundary keeper, Butler and her team discovered, was netrin1.
“The regional specificity of signaling molecules like BMP and netrin1 is extremely important for proper neural network formation and function,” said Sandy Alvarez, a graduate student in Butler’s lab and first author of the study. “Without netrin1’s regulation, we would likely see a disorganized neural network, potentially affecting how, and even if, axons reach their targets.”
By setting boundaries on BMP signaling, netrin1 plays a pivotal role in making sure that sensory neurons develop in the dorsal region away from motor and interneurons in the ventral region.
Butler’s research revealed, however, that netrin1 acts more like a sticky adhesive surface, guiding axon growth directly along pathways rather than acting as a distant cue. This discovery prompted Butler’s team to explore further. In gain-of-function experiments with chicken and mouse embryos, along with mouse embryonic stem cells, they introduced a traceable version of netrin1 to the developing spinal cord to observe the resulting changes.
The researchers found that axons had disappeared. “We knew that BMPs play a key role in patterning the dorsal spinal cord during embryonic development, but there was virtually no scientific literature about the interaction between netrin1 and BMP signaling,” Alvarez said. “I realized what I was observing was the repression of BMP activity by netrin1 in our animal models.”
Using a combination of genetic approaches in animal models, the team demonstrated that manipulating netrin1 levels specifically altered the patterning of certain nerve cells in the dorsal spinal cord. When netrin1 levels increased, certain dorsal nerve cell populations disappeared; when netrin1 was removed, these populations expanded.
Further bioinformatics analysis helped establish why this was occurring: The researchers found that netrin1 was indirectly inhibiting BMP activity by controlling RNA translation.
“Netrin1 is the most powerful architect of neuronal circuits that I have ever worked with,” Butler said. “Our next endeavor will be to understand how we can deploy netrin1 to rebuild circuitry in patients with nerve damage or injured spinal cords.”
The findings could have implications beyond spinal cord development. Netrin1 and BMP are also expressed in other organs throughout the body where precise cell patterning is crucial.
“Our results suggest a need to re-evaluate how netrin1 and BMP interact in other systems,” Alvarez said. “This could inform our understanding of certain cell type cancers or developmental disruptions where BMP and netrin1 are involved.”