Researchers have discovered a surprising new function of netrin1, a key protein involved in neural development, acting as a regulator that restricts bone morphogenetic protein (BMP) signaling within the developing spinal cord.
Scientists from the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA have found an unexpected role for the molecule netrin1 in structuring the developing spinal cord.
The researchers found out that netrin1, which is mainly recognized as a guidance cue for growing nerve fibers, also restricts BMP signaling to particular areas of the spinal cord. This role is crucial because this signaling needs to be accurately contained in the dorsal region for sensory neurons to develop effectively.
The findings, published in Cell Reports, change our understanding of how intricate spinal circuits are formed during embryonic development and may influence future approaches for spinal cord repair.
The research was led by senior author Samantha Butler, a neurobiology professor at the David Geffen School of Medicine.
“This is a tale of scientific curiosity—discovering something unusual and trying to make sense of it,” said Butler, who is also part of the UCLA Broad Stem Cell Research Center. “We found that netrin1, known for its strong role in building neural circuits, has an entirely unexpected function in organizing the spinal cord in early development.”
The formation of the dorsal spinal cord, which processes sensory inputs like touch and pain, involves detailed compartmentalization and organization. Sensory functions require specific neurons to develop in precisely defined areas, governed by BMP signaling that occurs strictly within the dorsal spinal cord’s limits.
It is vital that BMP signals are contained to prevent them from spreading to other spine regions, which could disturb the development of different neuron types. The primary boundary regulator discovered by Butler and her team was netrin1.
“The regional specificity of signaling molecules like BMP and netrin1 is crucial for the accurate formation and function of neural networks,” said Sandy Alvarez, a graduate student in Butler’s lab and first author of the study. “Without the regulation provided by netrin1, we could see a disorganized neural network, which might affect whether and how axons reach their targets.”
Control spinal cord (left) where a fluorescent tracer (green) has been introduced into dI1 (red cells) axons (arrows) only on the right side of the spinal cord. Experimental spinal cord (right) where netrin1 (blue) and the tracer (green) have also been introduced on the right side—resulting in a significant reduction in dI1 neurons (red) and no axons from those neurons. | Credit: Samantha Butler Lab/UCLA
By regulating BMP signaling boundaries, netrin1 ensures that sensory neurons develop in the dorsal region, away from motor and interneurons in the ventral region—a division essential for relaying sensory input and motor output in the body.
In 2017, Butler and her team challenged a long-held belief regarding axon growth during embryonic development. Scientists had thought axons—thin fibers connecting cells in the nervous system—were drawn towards or repelled by cues like netrin1 across long distances. However, Butler’s research demonstrated that netrin1 functions more like a sticky adhesive, guiding axon growth directly along pathways rather than acting as a distant signal.
This unexpected revelation led Butler’s team to investigate further. They conducted gain-of-function experiments using chicken and mouse embryos, as well as mouse embryonic stem cells, introducing a traceable version of netrin1 to observe the effects.
Interestingly, they noted that axons had vanished.
Initially, Alvarez feared something had gone wrong—believing her experiments were failing. But when the results consistently repeated, she made a surprising connection.
“We knew BMPs were essential for the dorsal spinal cord’s patterning during embryonic development, but there was hardly any scientific literature on the interaction between netrin1 and BMP signaling,” Alvarez said. “I realized I was observing netrin1’s repression of BMP activity in our animal models.”
Utilizing various genetic strategies in animal models, the team demonstrated that altering netrin1 levels specifically changed the patterning of certain nerve cells in the dorsal spinal cord. When netrin1 levels rose, certain populations of dorsal nerve cells disappeared; without netrin1, these populations flourished.
Further bioinformatics analysis revealed the underlying cause: the researchers found that netrin1 indirectly inhibited BMP activity by regulating RNA translation.
“Netrin1 is the most effective architect of neuronal circuits I’ve ever encountered,” Butler noted. “Our next goal will be to understand how we can use netrin1 to reconstruct circuitry in patients with nerve damage or spinal cord injuries.”
While the team will continue to investigate how these findings can lead to possible clinical applications, including therapies based on netrin1 for neural repair, their results might also extend beyond spinal cord development. Netrin1 and BMP are also present in other organs throughout the body where precise cellular patterning is vital.
“Our results highlight a need to reassess how netrin1 and BMP interact in other systems,” Alvarez said. “This could enhance our understanding of certain types of cancer affecting cells or developmental abnormalities involving BMP and netrin1.”
Other UCLA contributors included Sandeep Gupta, Yesica Mercado-Ayon, Kaitlyn Honeychurch, Cristian Rodriguez, and Riki Kawaguchi.
The research was supported 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 several UCLA graduate fellowships, including support from Eugene V. Cota-Robles, Whitcome & Hilliard Neurobiology awards; the UCLA Broad Stem Cell Research Center (BSCRC) postdoctoral training grant; as well as grants from the National Institutes of Health and innovation awards from the BSCRC.
