How does the brain record a memory? Somehow our experiences and interactions can be imprinted in the mind, but exactly how neurons alter their connections to enable memory has been murky. Now scientists say they have identified the molecular machinery that links experience with learning--and it all comes down to one microscopic motor.
Scientists believe that recording memories involves a process, called long-term potentiation (LTP), that enhances communication between pairs of neurons. Neurons communicate by releasing neurotransmitters that stimulate receptors on their neighbors, and LTP triggers more receptors to accumulate at the receiving cell's membrane--making it more sensitive to incoming messages.
Previous research had suggested that actin and myosin, two proteins with key roles in muscle contraction, might play a role in clustering receptors in neurons. To investigate this possibility, Michael Ehlers, a neurobiologist at Duke University Medical Center in Durham, North Carolina, and his colleagues used time-lapse imaging and biochemical methods to examine brain slices from rats. These experiments revealed what happens after an incoming signal triggers a rush of calcium into a neuron. The calcium activates a myosin protein called myosin Vb, prompting it to latch onto packets of receptors stored deeper in cells and drag them out to the neuron's signaling site, where the receptors can receive neurotransmitters and contribute to LTP.
To make sure that myosin Vb was indeed the motor that made the learning process possible, Ehlers and his colleagues chemically inhibited myosin Vb in neurons. The cells were incapable of generating LTP, the researchers report today in Cell. "We were surprised that one motor molecule could account for the bulk of membrane trafficking events," says Ehlers. "It may in fact be a motor that makes memories."
"What they've done is connect a lot of dots," says Marie Wooten, a neurobiologist who studies cell signaling at Auburn University in Alabama. The paper shows step by step how neurons move receptors to their outer membrane during LTP, Wooten says. Bettina Winckler, a neuroscientist at the University of Virginia, Charlottesville, agrees. "This paper is like a jewel," says Winckler--or, perhaps, a well-engineered motor. "It all fits everything together so beautifully," she says.