Memory is critical to how we learn, form identities, and interact with others and the world around us. It is essential to the functions we perform in our everyday lives. Yet, despite well over a century of investigations in various scientific inquiry fields, we still have much to learn about what memory is and how it works.
Over the past decades, innovations in science and technology have allowed investigators to explore the physiological processes involved in memory formation, learning, and cognition at both the cellular and intracellular levels. Neuroscience has shown how synapses connecting neurons in the brain can change, becoming stronger or weaker in response to chemical signals released during activities that require higher or lower levels of cognitive function and thus increasing or decreasing activity between neurotransmitters and receivers. This "synaptic plasticity" comes in multiple forms, including long-term potentiation (LTP)—the increase in synaptic strength that makes it easier for neurons to communicate with one another-and long-term depression (LTD)—the weakening of synaptic strength, which has the opposite effect. These changes occur due to exposure to high- and low-frequency bursts of calcium ions, respectively, and are widely thought to be involved in neurochemical processes that occur during learning and memory formation.
Portland State University assistant professor of chemistry, Steve Reichow, was recently awarded a three-year, $225,000 grant from the Whitehall Foundation to study the molecular mechanisms of the Ca2+/Calmodulin-dependent protein kinase II (CaMKII) an isoform, a cell-signaling complex that plays a vital role in regulating synaptic plasticity. Reichow and his research team specialize in using the tools of electron cryomicroscopy (Cryo-EM). Cryo-EM requires cooling biological samples with liquid nitrogen. At such temperatures, scientists can study individual proteins' structure in their native environments under the microscope. The images of near-atomic scale molecules acquired using these methods and tools allow Reichow to study proteins such as CaMKII, characterize their compositions, determine their molecular architectures, and examine the relationships between the complex structures give them their distinct shapes and enable them to perform tasks essential to life.
As Reichow explained, the CaMKII protein is a complex molecule with a distinctive architecture that operates within neurons and is responsible for transmitting the calcium ion signals that trigger LTP and LTD (synaptic plasticity), making connections between synapses either stronger or weaker. CaMKII can decode various conditions of high-frequency calcium signals and then deliver these messages to initiate LTP. Alternatively, when CaMKII detects low calcium input levels from the synapse, it sends the required signals to activate LTD. This particular protein also has a kind of molecular memory in that, after it has become activated with high or low levels of calcium, it remembers this message so that it can complete the task of letting neurons know whether they should be transmitting stronger or weaker signals between synapses without further input from neurotransmitters. It wasn't too long ago that the structure of this protein wasn't understood. However, Reichow's recent work gave shape to the structure of this critical protein, which opened the door for investigators to ask: How is the architecture of the CaMKII protein related to its ability to complete these complicated tasks?
"X-ray crystallography is the typical way of studying protein structures like this," Reichow said, pulling up a three-dimensional model of the CaMKII protein on his computer screen. "But, for that to work, you have to have a relatively stable molecule. Others have tried using that technique and found that this particular protein wasn't amenable to those methods, presumably because it wasn't stable enough, which hinted at its complex behaviors."
The CaMKII molecule, Reichow noted, has to move around within a cell. It also has to chemically-modify itself to get its job done. "Its complex structure makes it difficult to crystallize," he said, "which is why electron cryomicroscopy seemed ideally suited for studying it."
A paper recently published in Nature Communications details how Reichow and his team could use the tools of electron cryomicroscopy to take pictures of the CaMKII protein in its natural environment and use computational image rendering to elucidate the protein's structure in an inactive (basal) state. As is his current work, that study was in collaboration with Ulli Bayer, a pharmacology professor at the University of Colorado's School of Medicine.
"By taking a closer look at the structure of CaMKII, we can start asking how it is that it does all of the work it needs to do," Reichow said. "So, how does it know the difference between high-frequency and low-frequency calcium streams? How does this molecular memory work? That's the beauty of the techniques we use in the lab. We're able to characterize proteins like this at the atomic-level. We're able to show that there's a lot of flexibility between its central hub and the star-like halo of twelve outer domains connected to it. And we believe that this molecular flexibility is why it can respond in different ways to different types of cell stimulus."
With funding from the Whitehall Foundation, Reichow and his team are going back to the microscope to get images of the CaMKII protein in its active states under high- and low-frequency impulses of calcium ions. By stimulating their sample cells with calcium, they hope to capture the protein activation process and gain new insights into how this particular molecule's plasticity enables synaptic plasticity.
The structural and mechanical characterization of the CaMKII protein could contribute significantly to our understanding of the neurochemical processes underlying learning, memory, cognition, and even neurological diseases such as Alzheimer's, depression, and schizophrenia. That knowledge, in turn, could spur further discoveries in labs like Bayer's where researchers studying higher brain functions hope to one day unlock the secrets of neurological disorders associated with abnormal synaptic plasticity.