By boosting levels of a single protein normally found in mice, scientists have created a fruit fly with an exceptionally long memory. The feat sheds light on how memories are maintained and suggests that the cellular machinery for maintaining memories may be remarkably similar across a wide range of animals.
The recollection of your first kiss and all other memories are encoded in the connections, or synapses, between your brain cells. When a memory is reinforced, scientists believe, the synapses that encode the memory are strengthened. But exactly how neurons strengthen synapses--or choose which ones to strengthen--isn't known.
Previous studies suggested that a certain family of enzymes might be important for memory maintenance. To investigate the role of one of these enzymes, called PKM, neuroscientist Jerry Yin and colleagues at Cold Spring Harbor Laboratory in New York state engineered fruit flies to produce extra PKM when exposed to heat. They then trained the flies to avoid an odor at one end of a T-shaped maze by pairing the smell with an electric shock.
Normal flies learn to associate the odor with the shock and avoid the arm of the maze that has the smell--but they forget the association after 24 hours. When the PKM gene was activated in the engineered flies 30 minutes after training, the flies had no trouble remembering the association the next day, the team reports in the April issue of Nature Neuroscience. However, switching on PKM before training had no effect. That suggests to the team that the molecule is more important for maintaining existing memories than for making new ones.
In the same issue, another Cold Spring Harbor Laboratory team, led by Todd Sacktor, reports that PKM can help maintain strong synaptic connections between recently active neurons in the brains of rats. Together, the two studies hint at a cellular mechanism for maintaining memories, says Yin: "We believe that PKM may be involved in a process that tags synapses during memory formation. This way, ... only those synapses corresponding to a particular memory are strengthened in response to experience." According to neuroscientist Ole Paulsen of Oxford University, U.K., that model is "a significant step forward in understanding the molecular mechanisms involved in maintaining memory." Moreover, Paulsen says, the findings suggest the same mechanism may be at work in creatures as different as flies and mammals.