By Nana O 
Scientists at the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA have uncovered an unexpected role for the molecule netrin1 in organizing the developing spinal cord. This groundbreaking discovery reveals that netrin1, long recognized for its crucial function in guiding the growth of nerve fibers, also acts as a critical boundary regulator for another vital signaling pathway, bone morphogenetic protein (BMP) signaling. This dual functionality is essential for the precise compartmentalization required for the proper development of sensory neurons, a finding that significantly advances our understanding of neurodevelopment and holds promise for future spinal cord repair strategies.
A Paradigm Shift in Understanding Spinal Cord Organization
The intricate architecture of the spinal cord, particularly the dorsal region responsible for processing sensory information such as touch and pain, is established through a highly orchestrated series of developmental events. Central to this organization is the precise confinement of BMP signaling to this dorsal area. This signaling pathway is indispensable for the differentiation and development of specific sensory neurons. However, its uncontrolled spread to other regions of the developing spinal cord could lead to severe disruptions, impairing the formation of other crucial neuron types, such as motor neurons and interneurons, which reside in the ventral regions.
The UCLA research team, led by Professor Samantha Butler of neurobiology at the David Geffen School of Medicine and a distinguished member of the UCLA Broad Stem Cell Research Center, has identified netrin1 as the key molecule responsible for enforcing these critical boundaries. Their findings, recently published in the prestigious journal Cell Reports, demonstrate that netrin1 not only directs the path of growing axons but also actively suppresses BMP signaling in adjacent areas, thereby ensuring that BMP activity remains localized to the dorsal spinal cord.
"This is a story of scientific curiosity — of discovering something odd and trying to understand why it happened," stated Professor Butler. "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."
Unraveling the Mechanism: Netrin1’s Regulatory Power
The research began with an observation that deviated from established understanding. Previous work by Butler’s lab had already challenged long-held beliefs about axon guidance. In 2017, her team demonstrated that netrin1 acts not as a distant attractant or repellent for axons, but rather as a localized adhesive cue, guiding growth directly along its molecular track. This discovery prompted further investigation into netrin1’s multifaceted role.
In subsequent experiments utilizing gain-of-function models in chicken and mouse embryos, alongside studies with mouse embryonic stem cells, the researchers introduced a traceable version of netrin1 into the developing spinal cord. The initial results were perplexing: instead of observing enhanced axon growth, they observed a dramatic disappearance of axons.
Sandy Alvarez, a graduate student in Butler’s lab and the first author of the study, initially interpreted these unexpected results as experimental failure. However, upon repeated replication of the phenomenon, she began to formulate a new hypothesis. "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 explained. "I realized what I was observing was the repression of BMP activity by netrin1 in our animal models."
This realization led the team to explore the interaction between netrin1 and BMP signaling more deeply. Through sophisticated genetic manipulations in animal models, they were able to directly demonstrate that altering netrin1 levels had a profound impact on the patterning of specific nerve cells in the dorsal spinal cord. When netrin1 levels were elevated, certain dorsal nerve cell populations diminished, while conversely, when netrin1 was removed, these same populations expanded. This inverse relationship strongly suggested a regulatory role for netrin1 over BMP-dependent cell development.
Further in-depth bioinformatics analysis provided the mechanistic explanation. The researchers discovered that netrin1 indirectly inhibits BMP activity by modulating RNA translation. This intricate molecular crosstalk ensures that the BMP signal, crucial for dorsal neuronal development, does not spill over into ventral regions, where it could interfere with the development of motor neurons and interneurons. This precise spatial control is fundamental for establishing the distinct functional domains within the spinal cord.
The Importance of Boundaries for Neural Network Formation
The implications of netrin1’s boundary-setting function are far-reaching for the establishment of functional neural circuits. "The regional specificity of signaling molecules like BMP and netrin1 is extremely important for proper neural network formation and function," emphasized Alvarez. "Without netrin1’s regulation, we would likely see a disorganized neural network, potentially affecting how, and even if, axons reach their targets." The visual evidence from their study, depicting a control spinal cord with organized axonal projections versus an experimental one where netrin1 introduction leads to reduced neuron numbers and absent axons, starkly illustrates this point. This image, showing a fluorescent tracer highlighting dI1 axons in the control spinal cord versus a significant reduction in dI1 neurons and their axons in the experimental spinal cord where netrin1 was introduced, serves as a powerful visual testament to the molecule’s critical role.
This discovery highlights the exquisite precision required during embryonic development. The separation of dorsal sensory pathways from ventral motor pathways is not merely a matter of location but is underpinned by complex molecular signaling events that must be meticulously controlled in space and time. Netrin1’s dual role as an axon guidance cue and a BMP signaling suppressor underscores the elegance and efficiency of developmental processes.
A Timeline of Discovery and Future Directions
The journey leading to this discovery can be traced back to foundational research on axon guidance. For decades, the prevailing scientific consensus held that molecules like netrin1 exerted their influence over long distances, either attracting or repelling growing axons. Professor Butler’s 2017 work significantly revised this understanding, proposing a more direct, adhesive interaction. This shift in perspective was the fertile ground upon which the current discovery about netrin1’s role in BMP signaling was cultivated.
The current research, building upon this revised understanding, involved a multi-year investigation:
- 2017: Professor Butler’s lab publishes findings challenging the long-distance guidance paradigm for netrin1, proposing a more direct adhesive mechanism.
- Post-2017: The team, intrigued by netrin1’s localized influence, begins investigating its potential role in developmental patterning beyond axon guidance.
- Experimental Phase: Gain-of-function experiments are initiated using chicken and mouse embryos, along with mouse embryonic stem cells, to observe the effects of introducing traceable netrin1.
- Unexpected Observation: Researchers observe the unexpected disappearance of axons upon netrin1 introduction, leading to a re-evaluation of the experiments.
- Hypothesis Formulation: Sandy Alvarez proposes the hypothesis that netrin1 might be involved in repressing BMP activity.
- Mechanistic Investigation: Genetic approaches and bioinformatics analysis are employed to confirm the interaction between netrin1 and BMP signaling, revealing the RNA translation modulation mechanism.
- Publication: The findings are published in Cell Reports, marking a significant advancement in the field of neurodevelopmental biology.
Looking ahead, Professor Butler expressed a strong commitment to translating these fundamental discoveries into tangible therapeutic applications. "Netrin1 is the most powerful architect of neuronal circuits that I have ever worked with," she stated. "Our next endeavor will be to understand how we can deploy netrin1 to rebuild circuitry in patients with nerve damage or injured spinal cords."
Broader Implications and Potential Therapeutic Avenues
The implications of this research extend beyond the development of the spinal cord. Netrin1 and BMP signaling pathways are conserved across various biological systems and play crucial roles in the development and patterning of numerous organs. Therefore, the newly identified interaction between netrin1 and BMP signaling could have far-reaching consequences for understanding a range of biological processes and diseases.
"Our results suggest a need to re-evaluate how netrin1 and BMP interact in other systems," noted Alvarez. "This could inform our understanding of certain cell type cancers or developmental disruptions where BMP and netrin1 are involved." For instance, dysregulation of BMP signaling is implicated in various cancers, and understanding how netrin1 modulates this pathway could open new avenues for therapeutic intervention. Similarly, developmental disorders arising from mispatterning could be better understood and potentially treated by targeting this newly elucidated molecular interplay.
The potential for netrin1-based therapies for spinal cord repair is particularly exciting. Spinal cord injuries often result in devastating functional deficits due to the loss of neurons and the failure of damaged neural circuits to regenerate. If netrin1 can be effectively harnessed, it could potentially guide the regrowth of axons and promote the formation of organized neural networks in injured areas, offering hope for restoring lost function.
Collaborative Effort and Funding
This pivotal research was a collaborative effort involving a dedicated team of UCLA scientists. Key contributors included Sandeep Gupta, Yesica Mercado-Ayon, Kaitlyn Honeychurch, Cristian Rodriguez, and Riki Kawaguchi, all affiliated with UCLA.
The research received substantial support from various funding sources, underscoring its scientific merit and potential impact:
- UCLA Senior Undergraduate Research Scholarship
- CSUN CIRM Bridges 3.0 Stem Cell Research & Therapy training program
- National Institutes of Health (NIH)
- National Science Foundation (NSF)
- UCLA graduate fellowships, including support from the Eugene V. Cota-Robles, Whitcome, and Hilliard Neurobiology awards
- UCLA Broad Stem Cell Research Center (BSCRC) postdoctoral training grant
- Innovation awards from the BSCRC
This multifaceted support network highlights the collaborative and interdisciplinary nature of modern scientific research, enabling breakthroughs that can fundamentally alter our understanding of biology and pave the way for transformative medical advancements. The discovery of netrin1’s dual role marks a significant step forward, promising to reshape both developmental biology and the future of regenerative medicine.