26 THE SCIENTIST | the-scientist.com
past experiences. Scientists are also examining how memory forma-
tion and retrieval change with age, how those processes are altered
in animal models of Alzheimer’s disease, and how accessing memo-
ries can influence an animal’s emotional state.
“Almost every neurological disease or psychiatric disease—every-
thing from autism to stress to PTSD to Alzheimer’s to epilepsy—they
all affect the memory system,” says Denise Cai, a neuroscientist at the
Icahn School of Medicine at Mount Sinai.
A little more than a decade ago, such memory manipulations
might have seemed like science fiction—and in terms of applying
them to people, they mostly still are. But in two watershed papers,
published in 2009^2 and 2012,^3 researchers blew open the doors to
memory control in lab animals. In addition to optogenetic control
over neuronal firing, transgenic techniques allowed scientists to
prime or modify the specific cells that were activated when the
animals first stored a new memory. That collection of neurons,
called a memory trace, will fire again during recollection. In the
first of these two seminal papers, Josselyn’s team showed they
could control, ahead of time, which neurons would join a trace,
and then kill those cells to eliminate the memory. Shortly there-
after, Susumu Tonegawa’s group at MIT presented techniques to
identify and reactivate memory traces.
Since then, “it’s reached a fever pitch,” says Boston University
neuroscientist Steve Ramirez, a Tonegawa lab alum and coauthor
on the 2012 paper.Forgetting and remembering
In Toronto, Josselyn is one-half of a memory-manipulating duo
with her husband, Paul Frankland. In their 2009 Science study,^2
the pair aimed to make mice forget a specific memory: that in a
particular chamber, the sound of a tone preceded a foot shock. Fear
conditioning is a common technique in the field because the mem-
ory forms quickly, within a few trials. Rodents that recall the expe-
rience freeze in fear upon hearing the tone again, while those that
forget are more likely to carry on exploring their cage as normal.
Memory traces incorporate neurons throughout the brain,
touching parts that process sights, sounds, smells, and emo-
tions. For simplicity, researchers usually focus their studies
on one brain area of interest. In this case, the Toronto team
zeroed in on the amygdala, which processes emotions such
as fear. The amygdala also takes part in storing memories of
events and emotions.
Rather than wait to see which neurons would join the mem-
ory trace and identify those, the researchers primed certain
neurons to join that trace. To do so, they took advantage of
the fact that the production of the transcription factor CREB
makes neurons excitable, and that excitability makes them
more likely to take part in memory storage. Using a virus-
delivered genetic construct, the researchers randomly overex-
pressed CREB in a subset of neurons in the amygdala. When
the team trained the mice to link the tone with a shock, those
high-CREB neurons were three times more likely to join the
trace than unaltered cells.The genetic construct Josselyn, Frankland, and their col-
leagues used also made the engineered neurons—many of which
joined the memory trace—vulnerable to a toxin produced by
the bacterium that causes diphtheria. Once the team injected
the toxin to kill those trace neurons, the mice didn’t freeze in
response to the tone. “The memory was gone,” says Josselyn.
That study showed that memory scientists could sit in the
driver’s seat, bending traces and their associated memories to
their will. Meanwhile, the MIT team set out to force mice to
recollect a fearful experience whenever the scientists wanted
them to. Doing so would prove that the cells in the trace they
manipulated were indeed behind the memory.Their work depended on the optogenetic tool channel-
rhodopsin, a protein that spans cell membranes and responds
to light. Blue light, delivered via an implanted optic fiber, causes
the channel to open, letting in positive ions and making neu-
rons fire. The researchers developed a system to express chan-
nelrhodopsin in mouse neurons involved in a particular memory
trace in the dentate gyrus, a brain structure closely associated
with the hippocampus. The hippocampus is involved in learn-
ing and memory as well as emotion and motivation; the dentate
gyrus integrates sensory inputs during memory formation. Like
Josselyn and Frankland, the MIT group used fear conditioning,
training mice to associate a specific place and tone with a shock.
The team designed their transgenic mice so that memory
traces would not be labeled if the animals’ diets included the anti-
biotic doxycycline. The researchers removed doxycycline from the
mice’s meals for a short time to create a memory with channel-
rhodopsin expressed in the relevant trace cells, then reinstated
the doxycycline treatment to avoid labeling any other memory
traces. When they later shone blue light into the brain to activate
the target trace while the mice were in a different place, the ani-
mals would recall the foot-shock situation and freeze. The team
concluded in their 2012 Nature study^3 that those cells truly rep-
resented the memory in the brain. (See illustration on page 28.)
Those early days of memory manipulation were “incredibly
exciting,” recalls neuroscientist Tomás Ryan, who trained at MIT
with Tonegawa during that time before starting his own lab at Trin-
ity College Dublin. The new techniques “entirely changed how we
can do things.”
Since then, many researchers have adopted Tonegawa’s sys-
tem, or created variations, to ask their own questions about
memory. For example, rather than force a recollection, a par-Life, in the real world, is an accumulation
of an almost infinite number of memories
across a lifetime.
—Denise Cai, Icahn School of Medicine at Mount Sinai