Cell Biology, Neuroscience
University of Wisconsin
Dr. Chapman is also a professor of neuroscience at the University of Wisconsin–Madison.
Membrane Trafficking in Neurons
Neurons communicate with each other via chemical signals called neurotransmitters. When a neuron fires, synaptic vesicles filled with neurotransmitters fuse with a nerve cell’s plasma membrane to release their contents into the synaptic cleft. The neurotransmitters then bind to receptors on postsynaptic cells. Edwin Chapman and his team study proteins and molecular triggers that mediate this chemical release. In parallel, his team also investigates the mechanisms that mediate the release of hormones from neurons and neuroendocrine cells.
Chapman’s lab was the first to reconstitute calcium-triggered membrane fusion in vitro, and his team has pioneered studies to look at how the calcium-binding protein synaptotagmin 1 (syt1) triggers membrane fusion. They discovered that syt1 and a set of proteins that mediate vesicle-membrane fusion, called SNAREs, are the minimum complement of proteins essential for calcium triggered fusion. More recently, his laboratory showed that the exocytotic fusion pore is a hybrid structure, composed of both lipids and the transmembrane domains of SNARE proteins. The team is now focused on discerning the functions of a number of additional synaptic and membrane trafficking proteins, which have already been cloned. For example, they recently discovered that a protein named Doc2 drives the slow, asynchronous phase of synaptic transmission.
Chapman is also interested in the botulinum neurotoxins (BoNTs), which block exocytosis. BoNTs inhibit vesicle-membrane fusion by cleaving SNARE proteins, which halts fusion and neurotransmitter release. Chapman’s team discovered the receptors for most of the BoNTs, and showed that they sneak into an open vesicle when it’s exposed during exocytosis. Now, they are looking at what happens once the toxin has infiltrated the neuron, investigating the possibility of “distal effects,” or the toxin’s ability to jump from neuron to neuron in a circuit, doing damage along the way.
In recent years, Chapman has also started to explore organelle trafficking in neurons, looking at how mitochondria and lysosomes move through cells to regulate synaptic function, and to understand how neurons become so highly polarized.
Ed Chapman likes to work with his hands. He’s been a mechanic, carpenter, rope maker, and a combine operator, harvesting peas. He brings this same mechanical penchant to his laboratory’s studies of how neuronal cells traffic molecules, release their chemical messengers, and connect with other cells to, ultimately, determine how we think, feel, and move.
As a postdoctoral fellow with Reinhard Jahn at Yale University, Chapman studied the presynaptic nerve terminal, where neurotransmitters are released. All the various protein players had been identified, but no one had figured out how they worked together. “It was like a puzzle with all the pieces laid out before me,” says Chapman. “Figuring out how they fit together has been the rate-limiting step.”
For almost two decades, Chapman’s own lab has been chipping away at the puzzle, using the latest optical reporters and molecular techniques to reconstitute membrane fusion in a test tube and determine the minimum components needed. But fitting together the nuts and bolts of the membrane fusion machinery is just a starting point for his lab team. They take those molecular findings and translate them into genetic and electrophysiology experiments using mouse models.
A key aspect of research in the Chapman laboratory is moving "back and forth" between fusion experiments in a test tube and in cultured mouse neurons. For example, his group discovered that in response to binding Ca2+, the synaptic vesicle protein synaptotagmin 1 penetrates the target membrane and triggers rapid exocytosis in neurons. There are 17 isoforms of synaptotagmin, and optical experiments from the Chapman group indicate that most of these isoforms mark specific subsets of large dense-core vesicles that release hormones to modulate synaptic transmission and neuronal function. Using in vitro and cell-based approaches, Chapman’s lab established a key role for another Ca2+ sensor, Doc2, during the slow phase of synaptic transmission. They then went on to create hybrid sensors that selectively tune the kinetics of transmission. Another major focus in Chapman’s lab is understanding the structure and dynamics of fusion pores, which form the first aqueous connection between the vesicle lumen and the extracellular space.
Chapman relies on his strong background in chemistry and physics to help his team straddle the intersection of biophysics, electrophysiology, and genetics. He says working in such diverse fields is a bit like mixing performance art and sculpture. But it allows his group to do things like use “old-fashioned chemistry techniques” to measure the kinetics of physical interactions between molecules that trigger membrane fusion.
Chapman is now turning his attention toward higher-level cellular mechanics – the connections between neurons that build circuits. His group grows neurons on diversely patterned surfaces to see how their connectivity influences their properties. For example, he is studying a process called reverberation, which results when a multiconnected circuit of neurons persistently fires nerve transmissions. The process is thought to play a role in memory.
There are plenty of questions Chapman still wants to ask. Do organelles – such as mitochondria, lysosomes, or lipid droplets – become specialized to serve distinct functions in different parts of the neuron? How are large dense-core granules created, and what underlies the observed variation in this class of organelles? “There’s all this crazy stuff you can do,” he says. More chances to get his hands dirty again.