Amazing stuff!
"In a major brain science breakthrough, New Orleans-based researchers and collaborators have discovered a nerve signaling mechanism that takes place outside the cell, flipping on a 'pain switch.' This significantly updates the way we understand how pain receptors work in the brain, and it could shed light on a path to safer pain medication that can effectively provide relief without the usual side effects. ...
They found that nerve cells communicate outside the cell with an enzyme called vertebrate lonesome kinase (VLK); this enzyme alters proteins in the space between neurons. When active neurons release VLK, it boosts the function of a pain receptor. ..."
"... The researchers found that mice genetically engineered to lack VLK in sensory neurons involved in pain did not develop acute hypersensitivity to pain after surgery. Conversely, administering VLK to normal mice induced robust pain hypersensitivity that was mediated by NMDA receptor activation. ..."
From the editor's summary and abstract:
"Editor’s summary
In the brain, extracellular kinases are responsible for phosphorylating the extracellular domain of proteins involved in synaptic transmission. Srikanth et al. focused on the mechanism of action of the synaptic kinase VLK (vertebrate lonesome linase) in sensory neurons. VLK is released from presynaptic sites and phosphorylates the extracellular domain of ephrin-B receptor kinases, thereby promoting the clustering of N-methyl-D-aspartic acid receptors (NMDARs) at synapses. Furthermore, VLK appears indispensable for neuronal viability and plays a crucial role in NMDAR-dependent injury-induced pain. These results suggest that VLK-mediated signaling might be targeted for managing NMDAR-dependent pain. ...
Structured Abstract
INTRODUCTION
Protein phosphorylation is a central mechanism of intracellular regulation, but its function outside the cell remains poorly understood. Although the secreted milk protein casein was shown to be phosphorylated more than a century ago in the 1880s, the biological relevance of extracellular phosphorylation has remained largely unexplored.
Recent studies have identified two families of secreted kinases and revealed that many synaptic proteins are phosphorylated in their extracellular domains. However, whether such phosphorylation events modulate synaptic signaling or behavior in vivo has remained an open question.
RATIONALE
We sought to define a role for extracellular phosphorylation in regulating receptor interactions at synapses. Specifically, we tested whether a secreted kinase could control the interaction between the receptor tyrosine kinase EphB2 and the N-methyl-d-aspartate receptor (NMDAR), a key regulator of glutamate signaling, and pain. Because EphB2 extracellular phosphorylation at the evolutionarily conserved tyrosine Y504 is required for binding to the NMDAR, we asked whether this modification might be controlled by secreted tyrosine kinases, and whether this signaling could modulate pain behaviors in vivo. Our goal was to define a synaptic signaling pathway linking presynaptic activity to extracellular receptor phosphorylation and downstream NMDAR function.
RESULTS
We identified vertebrate lonesome kinase (VLK/Pkdcc) as the only member of a six-gene ectokinase family that is both secreted and sufficient to induce EphB2–NMDAR interaction.
VLK phosphorylated EphB2 at Y504 in an ATP-dependent manner, and this effect was blocked by extracellular phosphatase and absent with VLK-kinase–dead mutants. Recombinant VLK (rVLK) induced EphB2–NMDAR complex formation in cultured neurons and spinal tissue, whereas Pkdcc knockout abolished the interaction. VLK localized to synaptic vesicles and was released in a SNARE-dependent manner following ephrin-B stimulation or elevated neuronal activity, consistent with regulated synaptic release.
In vivo, conditional deletion of Pkdcc from presynaptic sensory neurons disrupted EphB2 phosphorylation and prevented EphB2–NMDAR interaction in dorsal horn projection neurons after injury, without altering baseline expression of either protein.
Mice lacking VLK in sensory neurons failed to develop mechanical hypersensitivity following surgical injury but retained normal motor coordination and responses to heat and chemical stimuli.
Intrathecal injection of rVLK induced EphB2–NMDAR interaction and pain-like behaviors, and this effect required NMDAR activity.
Postsynaptic deletion of EphB2 in spinothalamic neurons blocked injury-induced pain behaviors, confirming the necessity of this signaling pathway for pain sensitization.
CONCLUSION
Our findings reveal an extracellular phosphorylation pathway that links presynaptic activity to postsynaptic receptor organization and behavior.
By releasing a kinase that acts on the outside of the postsynaptic membrane, neurons can modulate NMDAR function in a spatially and behaviorally specific manner—without invoking canonical intracellular pathways—highlighting an underappreciated dimension of synaptic signaling that occurs in the extracellular space. Given the essential role of NMDARs in synaptic plasticity and the embryonic lethality of neuronal VLK knockout, this pathway likely extends beyond pain and may offer new therapeutic opportunities for modulating receptor function."
Pain Research Reveals New Detail of How Synapses Strengthen (original news release 1)
Tulane scientists uncover new pain-signaling switch (original news release 2)
The synaptic ectokinase VLK triggers the EphB2–NMDAR interaction to drive injury-induced pain (no public access)
VLK drives extracellular phosphorylation of EphB2 to govern the EphB2-NMDAR interaction and injury-induced pain (preprint, open access)
A synaptic ectokinase induces the EphB2–NMDAR interaction and mechanical hypersensitivity.
Fig. 1 VLK is sufficient to induce the EphB2-NMDAR interaction.
Two of the study's authors: Hajira Elahi & Ted Price
