During that visit, the three sat down to see if they could figure out the discrepancy in the data. The “problem,” Silva felt, might in fact be an opportunity: a hint of how they could use CREB as a tool not merely to enhance or suppress memories but to explore each new memory’s precise location—to locate the engram. Maybe after all these years, it would be possible to find true tracks of memory in the brain. Perhaps it was actually necessary for only a small percentage of neurons to be involved in forming a memory. Maybe memory formation is a kind of competitive sport. CREB could play an essential role in recruiting the neurons lucky enough to underlie the memories we form.
By the time Josselyn’s study was published, in 2001, she had already accepted an invitation extended to her and her husband, Paul Frankland, also a postdoc in neuroscience, to join Silva at UCLA.
Once there, Josselyn dreamed up an ingeniously elaborate study with Silva to test their “neuronal competition” theory of memory formation. First they decided to use a mutant mouse engineered to have very low levels of CREB, so that any behavioral effects of injecting extra CREB into the amygdala would jump out. Then Josselyn and Silva devised a twist on the herpesvirus technique Josselyn had used at Yale. Rather than just adding the gene for CREB to the virus, this time they also inserted the gene for green fluorescent protein. That way, any neuron that had extra CREB by virtue of getting infected with the virus would also conveniently light up under the microscope.
But to know for certain if the neurons with the extra CREB—and only the neurons with the extra CREB—were the ones that stored the memory engram, the researchers had another trick up their sleeve: a means to show which neurons had been active in recollecting a memory.
That trick was a tough one to pull off. First Josselyn and Silva placed the mice inside a cage where they were shocked each time they heard a tone. Demonstrating the expected conditioned response, the mice would then freeze in fear whenever they heard the tone—even though shocks were now withheld. Next the researchers sacrificed the animals within five minutes of the time they heard the tone and froze in fear. Why five minutes? Because as soon as a memory forms, bits of RNA that produce a protein called Arc (activity-regulated cytoskeleton-associated protein) are activated inside neurons for precisely five minutes. Detecting Arc inside a neuron, in other words, shows that a memory had just been activated inside it less than five minutes before the animal’s death. To detect the presence of Arc, the scientists used a fluorescent tagging technique, as they had with CREB.
Ok, so they’ve injected their mice’s amygdalas with extra CREB, taught them to recognize a tone that they heard when they got shocked, played the tone again to see that they froze, and then immediately sedated and sacrificed them. Next the scientists removed the mice’s brains, separated out the amygdalas, sliced them thinly, and placed them on glass slides.
Under the microscope, the resulting slices of amygdala lit up like a Christmas tree: green if they had absorbed the extra CREB, red if positive for Arc. While the red and green did not overlap precisely, Josselyn and Silva did find that the neurons fluorescing red (active with Arc) were 3 to 10 times as likely as their neighbors to glow green (positive for CREB) as well. Makes sense, right? When baseball players take steroids, they hit more home runs. CREB, essentially a memory stimulant, is like steroids for neurons.
But this is where things got seriously strange. Imagine if, when home-run kings Barry Bonds, Alex Rodriguez, and Sammy Sosa allegedly used steroids and began hitting better, their teammates who did not take steroids actually started playing worse. Incredibly, that is exactly what happened on the neuronal playing field: Neurons that did not glow green with CREB were about one-twelfth as likely as their CREB-rich counterparts to shine red with Arc, meaning they had not been involved in the memory engram at all.
Silva and Josselyn finally concluded that memory storage among neurons is a zero-sum game. CREB helps form memories not by making all neurons stronger but by turning up the contrast between the haves and the have-nots.
“It is not enough to succeed; others must fail,” says Josselyn, quoting Gore Vidal.
Josselyn happens to like Tom Cruise. “His movie Risky Business changed my life,” she says. Wearing sneakers and a faded Ohio State University T-shirt, Josselyn sits in her small office at the University of Toronto’s Hospital for Sick Children, located kitty-corner to her husband’s office. (Eat your heart out, Mr. Cruise.) In July 2003, they left UCLA together for Toronto, where they were each given their own laboratories and named assistant professors of neuroscience. Next to her office door now stands a “Festivus pole,” which she used in December for an office party to celebrate the fictional secular holiday depicted on Seinfeld.
When they arrived in Toronto, Josselyn says, she and Frankland collaborated on a study aimed at taking the next step in manipulating the CREB-enriched memory engram: killing it. If the neurons with extra CREB were truly essential to maintaining a memory, they reasoned, eliminating those neurons should eliminate the memory as well. But unlike the neurons in the interpositus nucleus that Thompson had surgically removed back in the 1980s, the 15 percent of CREB-enriched neurons didn’t mass together in a huddle that could be targeted with a scalpel. Instead, they were evenly dispersed around the amygdala. The challenge: how to kill them without destroying surrounding neurons.
To do so, Josselyn and Frankland looked for something they could add to the herpesvirus, in addition to CREB, that would eventually cause the infected neurons to die. Finally a collaborator of theirs, the neurobiologist Steven Kushner, learned about a new technique for selectively assassinating neurons using diphtheria toxin. Mice do not naturally carry the receptor that permits the toxin to enter neurons, but monkeys do. By adding the simian gene for the diphtheria receptor to the herpesvirus that would also carry the CREB gene, the scientists now had the means to kill only those neurons with extra CREB.
With diphtheria as their time bomb, the team once again trained mice in auditory fear conditioning. As before, animals that had received injections of the virus carrying the gene for CREB (as well as the toxin receptor) learned better, demonstrating far more fear than those without it. And as predicted, when the husband-wife team infused the mice with diphtheria, selectively killing only those neurons with both extra CREB and the toxin receptor, the mice no longer froze when they heard the tone. Josselyn and Frankland had caused selective amnesia—a historic finding that brought them one step closer to finding a treatment for PTSD.
A year and a half later, Josselyn described these and other findings at the enormous annual meeting of the Society for Neuroscience during a symposium she and Silva cochaired on “mechanisms of memory.” The standing-room-only crowd, comprising hundreds of neuroscientists, buzzed with excitement at the notion that they might finally have a better tool than talk therapy to treat the millions of veterans and others disabled by traumatic memories. A door had been opened: The fear engram had been found. It had been strengthened with CREB and obliterated with diphtheria—and a path now beckoned toward a practical treatment for people.
Not even Silva or Josselyn believe that human subjects will readily accept infusions of CREB loaded with diphtheria anytime soon. “It’s very interesting work but not practical for clinical treatment of patients,” says Denis Paré, a neuroscientist at Rutgers University in New Jersey who is also studying pharmacological approaches to manipulating fearful memories. “These are experimental manipulations that you couldn’t do in a human.”
Silva agrees but remains cautiously optimistic about the prospects for translating his and Josselyn’s research into useful treatments. “We can’t fix something that we don’t understand,” he says. “For the longest time we had no understanding of the physical representation of memory in the brain, of engrams. We now know that CREB has a role in determining where memories go. We can manipulate CREB to funnel memories into specific cells.”
A mouse’s memory of a single fearful event is one thing; the complex associations of human memory, powered by a dense network of neuronal connections, is quite another. “We’re studying a really simple, basic kind of memory,” Josselyn concedes. “With mice, you can’t ask them, ‘Do you remember that cage we put you in yesterday?’ All we can do is observe their behavior to see if they’ve learned something or not. More complex memories, like the recollection of an event that happened to you, are stored in many different areas of the brain. But even for treating PTSD, we wouldn’t want to take away the entire memory, just the part that leaves you disabled with fear. And that comes from the amygdala.”
So it turns out that Karl Lashley’s belief in memory as existing in a distributed network is still alive and well; Silva and Josselyn have not overturned it, only supplemented it, showing that some parts of some memories do exist in a discrete number of neurons.
As for the practical application of using CREB as a memory enhancer, Josselyn is exploring it—but warily. Back in the 1990s, when CREB research was first getting under way, a company called Memory Pharmaceuticals invested millions in an effort to develop a pill that would extend the action of CREB. But the company, along with others that had bet on so-called smart pills, hardly fared well.
“People thought we’d see a pill on the market by now,” Josselyn says. “Companies were set up and folded, trying to find this pill. The field was giddy in the initial stages. It turns out that memory is really tough to strengthen, especially in people.”
Yet cautious progress is being made. Perhaps Josselyn’s most thrilling finding so far, in research published this past July in Neuropsychopharmacology, involves mice with the equivalent of Alzheimer’s disease. When her group injected CREB into the animals’ hippocampus using engineered herpesvirus, the mice regained their ability to learn.
Could a similar process be used to deliver CREB into human brains? “It’s possible,” Josselyn says. “The principle is there, but we need much finer tools. The herpesvirus is probably not a good way to go in people. But we’re really not doing all this just to improve learning in mice. We want to figure out how people learn and remember and how we can help them when they don’t.”
For my middle-aged brain, a breakthrough cannot come too soon. At the conclusion of my meeting with Silva in Los Angeles, he offered to walk me to my car. But in an absurdly ironic twist, I couldn’t remember where I had parked it, on some back street in an underground lot. Determined to help, Silva accompanied me as we searched along Westwood Plaza, past the Semel Institute for Neuroscience and Human Behavior, past the Reed Neurological Research Center, past the Ahmanson Laboratory of Neurobiology—none of them doing me a damn speck of good. Any farther and we’d have ended up in front of Grauman’s, where the trace of Marilyn’s and Jane’s handprints were holding up a lot better than my memory.
And so, like two mice lost in a maze, we walked the streets, the journalist and one of the world’s leading memory researchers, as he tried to help me remember.
Dan Hurley is the author of Diabetes Rising: How a Rare Disease Became a Modern Pandemic and What to do About It. He lives in New Jersey.