In one-thousandth of a second, a neuron dumps up to 5,000 molecules of a chemical messenger, called a neurotransmitter, into the synapse, the junction that transmits signals from one neuron to the next.
A single neuron may receive input at hundreds of synapses, each firing up to hundreds of times per second. To make matters more complex for scientists, the neuron processes these incoming signals differently, depending on when and where they arrive. Bernardo Sabatini's goal is to understand the machinery that makes synapses work efficiently and effectively.
"I think the brain as a whole is way too complicated to understand any time soon," says Sabatini, a neurobiologist at Harvard Medical School. "But we can learn the basic rules for how the brain wires itself." His research group is harnessing powerful optical technologies to peer directly into synapses so that they can observe the changing conditions as information is processed on a millisecond timescale.
Although these explorations demand extreme precision and patience, Sabatini is energized by the challenge of studying synapses. "These are the true basic units, or machines, of the brain that allow us to learn and remember things," he says.
Sabatini's interest in neuroscience blossomed while he was a graduate student in the joint M.D./Ph.D. program offered by Harvard and the Massachusetts Institute of Technology. After completing that program, he went on to become a postdoctoral fellow in Karel Svoboda's HHMI research group at Cold Spring Harbor Laboratory (CSHL). (Svoboda has since left CSHL to become a group leader at HHMI's Janelia Farm Research Campus.)
While in Svoboda's lab at CSHL, Sabatini developed imaging techniques that help visualize neuronal calcium channels in their native environment. Calcium channels are voltage-sensitive molecules that permit calcium to flow into neurons.
But Sabatini was confronted with a major technical obstacle: single-channel patch clamping—a technique widely used to measure electrical current in neurons—could not be applied to study the calcium channels in intact synapses. To overcome this hurdle, Sabatini and his colleagues invented something called the optical fluctuation analysis technique, which uses a laser scanning microscope to visualize calcium channels by measuring the light emitted from a calcium-sensitive fluorescent dye injected into target neurons in slices of rat brain. With this new technique, the researchers deduced the number and properties of the calcium channels.
Shortly after finishing that project, Sabatini was recruited to Harvard Medical School, where he established a research program to study dendritic spines—tiny lollipop-shaped structures that stud dendrites, the tree-like branches of a nerve cell that receive incoming electrical signals. Hundreds or thousands of these spines appear, change, and disappear as synapses are formed.
At Harvard, Sabatini pioneered the development of optical techniques for observing the interactions of dendritic spines during synaptic events. Using these techniques, his research group has uncovered many of the mechanisms that enable individual synapses to control the consequences of their stimulation. Sabatini's overall goal is to understand synaptic plasticity—the changes in the strength of nerve signal transmission across synapses that are crucial to learning and memory.
Using a custom-designed two-photon laser scanning microscope, Sabatini's group can visualize individual spines within synapses in slices of brain tissue and study the role of spines in nerve signaling. Sabatini and his coworkers don't use off-the-shelf microscopes—they build their own, and program them for their day-to-day requirements.
"Part of the reason that we're on the cutting edge is that we're in control of our technology," says Sabatini. "If we have an idea and need to have the microscope perform a new function, we can write the software and make it happen."
His lab is geared toward basic research, but he has recently been collaborating with disease-oriented researchers to look for links between changes in fine-grained synaptic structure and disorders such as Alzheimer's disease and tuberous sclerosis, an autism-spectrum disorder. "I want someday to understand and help cure disease if I can," Sabatini declares.
He is lending his technological expertise to study how amyloid-beta peptide alters the function of synapses. The peptide is found in abundance in the sticky plaques that dot the brains of people who have Alzheimer's.
The researchers have observed that when amyloid-beta is placed on cultured brain cells in the laboratory, "it causes the cell to retract a large proportion of its synapses, and many of the dendritic spines go away." A paper describing the research findings that have grown out of this collaboration has begun to shed light on the cellular pathway involved in this damage. Sabatini intends to pursue these findings further as an HHMI investigator.
Sabatini's group has developed automated image-analysis techniques to measure the density and size of dendritic spines in cultured mammalian neurons. They plan to use image analysis to measure the loss of dendritic spines as a marker of neuronal damage by amyloid-beta. Their goal is to develop a high-throughput screen to identify proteins that can counteract the toxic affects of amyloid-beta peptide. One of Sabatini's long-range plans is to use cultured mammalian neurons to do genome-scale screens for proteins involved in synapse formation.