New Discovery Shows How Synapses Strengthen Through an Unexpected Extracellular Mechanism

Neuroscientists have uncovered a surprising and important mechanism that helps explain how synapses gain strength—a process central to learning, memory, and pain. This new research, led by teams at The University of Texas at Dallas and Tulane University, shines light on a biochemical pathway that has been largely overlooked: extracellular phosphorylation happening right in the synaptic cleft.

The study reveals that a secreted enzyme called vertebrate lonesome kinase (VLK) plays a direct role in reorganizing receptors on the surface of nerve cells. This discovery challenges long-standing assumptions in neuroscience about where and how phosphorylation occurs and opens up new possibilities for understanding—and potentially treating—pain.

A Clear Look at What the Researchers Found

Scientists have long known that phosphorylation, the process of adding a phosphate group to a protein, is essential to how cells regulate function. But until now, phosphorylation was primarily considered an intracellular process—something that happens inside cells.

This research shows that phosphorylation also happens outside cells, particularly at the synapse, through enzymes known as ectokinases.

The focus of the study is VLK, a kinase that neurons can release into the synaptic cleft. Researchers found that when presynaptic neurons release VLK, the enzyme phosphorylates the extracellular side of a receptor on the postsynaptic neuron called EphB2.

This specific phosphorylation event occurs at a particular spot—tyrosine 504, or Y504—on the outer domain of the EphB2 receptor. When EphB2 is phosphorylated at this location, it becomes capable of interacting with the GluN1 subunit of the NMDA receptor (NMDAR).

The interaction between EphB2 and NMDAR leads to NMDAR clustering at the postsynaptic membrane. NMDA receptors are essential for controlling electrical signaling in the brain and are deeply involved in synaptic plasticity. When they cluster at the synapse, they enhance the postsynaptic response, effectively strengthening the synaptic connection.

This process helps explain how synapses adapt, grow stronger, and influence long-term neuronal behavior.

How the Team Proved This Mechanism

To confirm the role of VLK, the researchers used several complementary approaches:

• Genetically engineered mice were created in which VLK was specifically removed from sensory neurons. These mice did not develop normal pain hypersensitivity after surgical injury, implying that VLK is required for injury-induced pain signaling.
• Recombinant VLK was injected into the spinal cord of normal mice, and this alone produced strong pain responses—but only when NMDA receptors were active.
• Human sensory neuron tissue was tested, showing that human neurons also produce and secrete VLK, and that VLK triggers the same EphB2–NMDAR interaction seen in mice.
• The team demonstrated that VLK is released through synaptic vesicles using a typical SNARE-dependent release pathway, meaning neuron activity can directly regulate how much VLK enters the synapse.

Throughout all the experiments, the researchers found that removing VLK did not impair other sensory or motor functions. The affected mice walked normally, reacted normally to heat and chemicals, and showed no issues with coordination or balance. This suggests VLK’s influence is pain-specific, rather than broadly disruptive.

Why This Discovery Is a Big Deal

It challenges textbook neuroscience.

Until now, synaptic plasticity has largely been explained through intracellular mechanisms—how neurons change internally to affect communication. This study shows that neurons also rely on extracellular biochemical modifications to regulate receptor behavior.

Phosphorylation outside the cell isn’t new in biology, but it was poorly understood in the nervous system. This research shows it is far more important than scientists realized.

It identifies a new pain mechanism.

Chronic pain and injury-induced hypersensitivity often involve exaggerated synaptic signaling. Because NMDA receptors are deeply involved in this, they have long been drug targets—but blocking them directly leads to major side effects, since NMDARs influence almost every aspect of brain function.

VLK offers a new target upstream of the NMDAR itself.

Instead of shutting down NMDA receptors everywhere, future pain treatments could potentially block VLK locally, such as in the spinal cord, targeting pain pathways without altering essential brain activity.

It has strong translational potential.

Because the mechanism appears in human tissue, not just mice, it could inform real clinical approaches. A therapy that inhibits VLK in injury-activated circuits might reduce hypersensitivity after surgery or nerve damage, while preserving normal sensation.

Synaptic Plasticity: A Quick Background for Readers

Synaptic plasticity refers to how synapses—the points where neurons connect—change their strength over time. It’s the basis of:

• learning new information
• forming memories
• refining neural circuits
• pain signaling and chronic pain development

Two receptors often appear in discussions of plasticity:

• Eph receptors, which help neurons form and maintain proper connections
• NMDA receptors, which detect patterns of neural activity and trigger internal signaling cascades

The EphB2–NMDAR connection highlighted in this research underscores how complex synaptic signaling is. It also shows that tiny molecular interactions can have huge behavioral consequences—from how we learn to why we feel pain.

More About VLK and Ectokinases

VLK (Vertebrate Lonesome Kinase) was not originally discovered in the nervous system. Earlier studies linked it to:

• platelet function
• bone development
• other extracellular signaling processes

Ectokinases like VLK are secreted enzymes that modify proteins outside cells. Although ectokinase activity was identified nearly 150 years ago, researchers had very limited understanding of what these enzymes did in the brain.

This study provides the most direct evidence yet that ectokinases:

• operate at synapses,
• regulate receptor organization,
• and significantly influence synaptic strength.

That means the field may have been missing a major piece of the synaptic plasticity puzzle all this time.

What This Could Mean for Future Research

This discovery opens several intriguing directions:

• Identifying other ectokinases that may regulate synaptic function
• Investigating whether disorders of learning and memory involve disrupted extracellular phosphorylation
• Exploring new non-opioid pain treatments that target VLK
• Studying how activity-dependent VLK release influences the formation of new neural circuits
• Understanding whether long-term memory relies in part on extracellular phosphorylation events

Because VLK release depends on neural activity, it may also play a role in synaptic strengthening during development or during the acquisition of complex skills.

The authors themselves believe this is only the beginning. The presence of kinase activity in the synaptic cleft could reshape how neuroscience understands receptor regulation and could lead to a rethinking of how synapses function at a biochemical level.

Final Thoughts

This research offers a rare combination of fundamental neuroscientific insight and real clinical potential. It updates a foundational concept—synaptic plasticity—by introducing a new mechanism operating outside the cell. It also points to a promising new target for treating pain without the widespread side effects associated with NMDA receptor inhibition.

As scientists continue exploring extracellular phosphorylation, we may soon find that synapses behave in ways far more dynamic and chemically complex than previously understood.

Research Reference:
The synaptic ectokinase VLK triggers the EphB2–NMDAR interaction to drive injury-induced pain
https://www.science.org/doi/10.1126/science.adp1007