Ever since his high school days in Philadelphia, Richard Huganir has been enthralled by the idea that the workings of the brain could be explained in molecular terms. So in his 30 years of research, he has focused on nerve cell proteins called postsynaptic receptors. These key components of the brain’s electrical circuits accept chemical messages from neighboring neurons. The messages are transmitted through connections called synapses, which are found by the thousands on each of the brain’s hundred billion neurons. Huganir is determining how modifying the receptors or changing their abundance alters synapses during learning, memory, and other dynamic brain functions. As well as shedding light on the normal activities of the brain, these studies should provide insights into abnormal states, such as schizophrenia, chronic pain, and drug addiction.
Huganir made his first major contribution as a graduate student at Cornell, when he helped isolate and characterize the first postsynaptic receptor from the brain. During the following years, he made the important discovery that adding or removing phosphate groups (phosphorylation and dephosphorylation) can alter the ability of several receptor types to pick up messages from other neurons. Huganir focuses mainly on the receptor that responds to glutamate, one of the chemicals that transmit information across synapses. Several enzymes can add phosphate groups to each of this receptor’s four subunits, and several others can remove them, Huganir discovered. Therefore, many permutations of phosphate groups can appear on a single receptor over time.
Huganir’s group has uncovered some of the physiological consequences of phosphorylation and dephosphorylation. For example, the addition of a phosphate group to the GluR1 subunit of one type of glutamate receptor makes that receptor more responsive to glutamate. So in this instance, phosphorylation strengthens synapses, improving communication between neurons.
Long-lasting changes at synapses are said to involve long-term potentiation (LTP) if synapses become stronger and long-term depression (LTD) if they become weaker. Both LTP and LTD are thought to be essential for learning and memory because they remodel circuits in the brain. To test this hypothesis, Huganir’s group created mice in which the GluR1 subunit could not be phosphorylated.
By directly recording synaptic transmission in the mutant mice, Huganir showed that LTD could not occur in certain parts of the brain. “So we found that phosphorylation of these receptors regulates their properties and functions and is also important for the well-characterized form of plasticity called LTD,” Huganir says.
The mutant mice could learn fairly well, but they couldn’t remember what they had learned as well as normal mice. For example, they could learn to find an underwater platform. But within a couple of days, they forgot where it was. Thus LTD, promoted by receptor phosphorylation, was needed for memory retention. The group has also shown that receptor phosphorylation is important for memory and learning involving LTP.
In collaboration with Robert Malinow at Cold Spring Harbor, Huganir has investigated emotional learning, which enables us to remember what we were doing when, say, 9/11 occurred. This type of learning follows events that provoke a strong emotional response, because trauma boosts the release of stress hormones, such as norepinephrine (noradrenaline). “We found that norepinephrine increases phosphorylation at the same [receptor] sites we had been studying for years,” Huganir says.
To see how the absence of those sites affects emotional learning, Huganir and Malinow examined how the mutant mice learned in stressful situations. Normally, mice freeze when they sense danger, and they learn this fear conditioning response much faster if injected with norepinephrine. But the hormone injections had no effect on the mutant animals’ behavior. “So norepinephrine won’t enhance the learning of the fear conditioning response [if subunit 1 of the glutamate receptor can’t be phosphorylated],” Huganir says.
Huganir’s group is also determining how receptors get to and from synapses, because receptor abundance affects synaptic strength. By stimulating brain slices with electrodes and varying the frequency, the researchers found that receptors can be added to or removed from synapses within minutes. Therefore, they looked for proteins that bind to receptors, escort them through neurons, take them to the correct type of synapse, and remove redundant ones. They have now identified and cloned many of these receptor-interacting proteins. “We try to figure out what they do, where they are interacting with the receptor, what regulates their interaction, and what the functional consequences are,” Huganir says. For example, he discovered that a protein called PICK1 mediates LTD, and helps remove glutamate receptors from certain types of synapses in the cerebellum (which controls complex movements). Recently, the group showed that synapses in mutant mice lacking PICK1 do not exhibit LTD.
“It is clear now that trafficking receptors is a major method in the brain for regulating synaptic transmission and plasticity and that both forms of receptor modulation—phosphorylation and trafficking—are critical not only in learning and memory but also in things like drug addiction,” Huganir says, “because these same mechanisms are involved in strengthening certain synapses in areas of the brain involved in pleasure and reward as well as areas involved in emotion.”
