It’s all about having the right inhibitor

When researchers use an inhibitor to block a kinase’s activity, they generally expect to see less of the downstream effects of that kinase. So when neuroscientists observed the opposite using a new inhibitor for one of the most-studied proteins in learning and memory, they were very surprised. Their results, which suggest that the kinase’s structural activity is more important than the reactions it catalyzes, are leading scientists to rethink its role in learning (Nature 2023, DOI: 10.1038/s41586-023-06465-y).

There have been “dozens and dozens of papers” on the enzymatic activity and substrates of the calcium/calmodulin–dependent protein kinase (CaMKII) in the brain, says Roger Nicoll, a neuroscientist at the University of California, San Francisco. “And the fact is, all of that can be swept away.”

To form and maintain new memories, neurons conduct a synapse- strengthening process called long-term potentiation (LTP). While many biochemical changes happen in a synapse during LTP, decades of neuroscience research have established that CaMKII plays a critical role.

That has been established both by removing CaMKII completely and using a stable of inhibitors that block its activation or its interaction with substrates, says Ulli Bayer, a pharmacologist at the University of Colorado Anschutz Medical Campus. But Bayer, who studies CaMKII and its binding with a neurotransmitter receptor called the NMDA receptor, noticed that for a long time, every available inhibitor also blocked that interaction.

When the now-defunct biotechnology company Allosteros Therapeutics developed a CaMKII-specific inhibitor that could substitute for adenosine triphosphate (ATP), blocking kinase activity without disrupting the NMDA receptor binding site, Bayer saw an opportunity to test how each function contributes to LTP.

“Our expected results were just to validate the last, you know, 30 years of CaMKII research,” says Jonathan Tullis, the paper’s first author. But the data he collected soon told a different story.

Although the inhibitor blocks enzymatic activity by outcompeting ATP, it also increases interaction with the receptor. That, Tullis found, was enough to enable LTP. Because the result was so unexpected, the researchers validated it with two other approaches that do not depend on the new inhibitor. Each suggested that binding the receptor, rather than phosphorylating substrates, is the most important thing CaMKII does.

“It’s one of those studies that does sort of destroy dogma in the field,” says Jason Shepherd, a molecular neuroscientist at the University of Utah who was not involved in the study.

Nicoll, the UCSF neuroscientist, recently announced similar findings in a preprint (bioRxiv 2023, DOI: 10.1101/2023.08.25.554912). His team’s study of several new CaMKII mutations shows that in the absence of receptor binding, even a CaMKII enzyme that is always on cannot instigate LTP. While he’s not pleased about having been scooped, Nicoll says, the two studies are complementary in establishing that “it really is the CaMKII-NMDA receptor complex that is the memory.”

The two studies do not explain precisely how that complex strengthens synapses—a question that Shepherd, Nicoll, and Bayer all agree must now be investigated. It also remains to be determined whether CaMKII activity is as dispensable in behavioral tests of memory as it is in isolated tissue. If it holds up, Bayer says, the finding suggests new opportunities to inhibit CaMKII without disrupting learning and memory in conditions like stroke, Alzheimer’s disease, and heart disease.