Scientists discover 'entirely unanticipated' role of protein netrin1 in spinal cord development

The researchers discovered that netrin1, which is known primarily as a guidance cue that directs growing nerve fibers, also limits bone morphogenetic protein, or BMP, signaling to specific regions of the spinal cord. This boundary-setting function is critical because this signaling activity must be precisely confined to the dorsal region for sensory neurons to develop properly.

Their findings, published in Cell Reports, reshape our understanding of how complex spinal circuits are established during embryonic development and could inform future therapeutic strategies for spinal cord repair.

The research was led by senior author Samantha Butler, a professor of neurobiology at the David Geffen School of Medicine.

"This is a story of scientific curiosity -- of discovering something odd and trying to understand why it happened," said Butler, who is also a 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, disrupting the formation of other neuron types. 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." note

Control spinal cord (left) where a fluorescent tracer (green) has been introduced into dI1 (red cells) axons (arrows) on the right side of the spinal cord only (i.e. on one side of the spinal cord). Experimental spinal cord (right) where netrin1 (blue) and the tracer (green) has been introduced again on the right side of the spinal cord -- the number of dI1 neurons (red) is much reduced, and there are no axons coming from those neurons. | Credit: Samantha Butler Lab/UCLA

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, a division essential for the proper relay of sensory input and motor output throughout the body.

In 2017, Butler and her colleagues overturned a long-standing paradigm about axon growth during embryonic development. For decades, scientists had believed that axons -- thin fibers that connect cells in the nervous system -- were attracted or repelled by guidance cues like netrin1 over long distances. 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 unexpected 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.

Curiously, they found that axons had disappeared.

Alvarez initially thought something had gone wrong -- that her experiments had failed. But when the results repeated several times over, she made the surprising connection.

"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."

While the team will continue to explore how these findings could inform potential clinical applications, including netrin1-based therapies for neural repair, their 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."

Other UCLA authors include Sandeep Gupta, Yesica Mercado-Ayon, Kaitlyn Honeychurch, Cristian Rodriguez and Riki Kawaguchi.

The research was funded by the UCLA Senior Undergraduate Research Scholarship; the CSUN CIRM Bridges 3.0 Stem Cell Research & Therapy training program; National Institutes of Health, National Science Foundation and UCLA graduate fellowships, including support from the Eugene V. Cota-Robles, Whitcome and Hilliard Neurobiology awards; the UCLA Broad Stem Cell Research Center (BSCRC) postdoctoral training grant; and grants from the National Institutes of Health and innovation awards from the BSCRC.