Traumatic brain injuries, including mild concussions, may set off a chain reaction in the brain that disrupts neuronal communication and impairs memory and cognition, according to University of California, Riverside research published in the Journal of Neuroinflammation on June 12. The study identifies a new interaction between an innate immune receptor called toll-like receptor 4 (TLR4) and an enzyme known as MMP-9 following brain injury.
Deepak Subramanian, assistant professional researcher in the Department of Molecular, Cell and Systems Biology and corresponding author of the study, said TLR4 activation after a concussive injury increases MMP-9 activity. "Brain injury activates TLR4 in neurons," he said. "TLR4 signaling causes MMP-9 levels to increase. Increased MMP-9 alters how neurons talk to each other, resulting in heightened network excitability associated with seizures and impaired cognition. This direct connection between neuronal TLR4 and MMP-9 in the injured brain is the crucial link."
The researchers used rat and mouse models of mild-to-moderate concussive brain injury for their experiments. They observed rapid increases in both TLR4 and MMP-9 levels after injury but found that blocking TLR4 signaling—either pharmacologically or genetically—prevented changes in MMP-9 levels. "That told us TLR4 is upstream of MMP-9," Subramanian said.
Blocking either TLR4 or MMP-9 limited changes to disrupted brain circuits after injury. According to Subramanian, "When inhibition drops or excitation becomes excessive, the network activity patterns lose precision... Instead of meaningful communication, you get excessive noise across the network, which interferes with learning, memory formation, and recall." Injured animals showed reduced synaptic plasticity—a key factor for learning—and deficits in spatial memory one month later; however, those treated early with inhibitors performed significantly better.
The findings suggest that early intervention targeting this pathway could affect long-term neurological outcomes if administered within 48 hours post-injury. Co-corresponding author Viji Santhakumar noted, "By identifying that the TLR4-MMP-9 pathway is activated exclusively after injury, we hope to move closer to pathway-specific preventive treatments without impacting normal brain function." The team also cautioned about complexities because both molecules are necessary for healthy brains.
Future research will investigate downstream molecular targets of MMP-9 as well as mechanisms by which TLR4 shifts from stabilizing neural networks under normal conditions to contributing to disruption following trauma.
