Richard Tsien, PhD

We are studying how the location and identity of presynaptic calcium channels is regulated. Voltage-gated Ca2+ channels provide the critical link between the firing of a presynaptic nerve terminal and its release of neurotransmitter. The Ca2+ channels must be positioned very close to sites of vesicle fusion, and come in diverse forms with distinct activity-dependence, responsiveness to GABA, dopamine, acetylcholine and other neuromodulators, and susceptibility to neurological disorders such as migraine, ataxia or dystonia. Our working hypothesis involves molecular "slots" for particular types of channels. Slots regulate the mix of channel types and also help explain how defective channels might displace normal ones in genetically dominant disorders.

Our lab is particularly interested in studying multiple modes of synaptic vesicle fusion. The opening of Ca2+ channels drives at least two distinct forms of fusion. In the classical mode, the vesicle membrane fully merges with and flattens into the presynaptic membrane (full collapse fusion). In a newly characterized mode, termed "kiss-and-run," the connection between the vesicle interior and the external medium lasts long enough to allow passage of neurotransmitter, but the connection is severed before the identity of the vesicle is lost. We study the dynamic properties and functional implications of both fusion modes by loading single synaptic vesicles with single photoluminescent reporter particles?quantum dots. Sharp distinctions between full collapse fusion and kiss-and-run are now in hand. Experiments are underway to monitor the same fusion event optically and electrophysiologically.

One area of intense attention in our lab is the fundamental unit of cell-cell communication between brain neurons: quantal synaptic transmission. Presynaptic release of a packet of neurotransmitter, for example, glutamate, causes activation of postsynaptic receptors and a brief flow of current that promotes firing of the postsynaptic cell. We work on neuronal mechanisms that allow synapses to adapt to a sudden or long-lasting change in their level of activity. For example, blockade of impulses or of postsynaptic glutamate receptors causes a cascade of biochemical events that eventually leads to readjustment of critical molecular players on both sides of the synapse. We use state-of-the art methods to pin down the cell biology of changes in synaptic strength, of importance for adaptation of brain networks in learning and memory. Ongoing work in cultures of isolated neurons and brain slices.

We study how synaptic transmission and depolarization cause changes in neuronal gene expression. Despite its importance, signaling from synapse or surface membrane to nucleus is only partly understood. One example of such signaling involves a local increase in Ca2+ concentration near a class of Ca2+ channels (L-type) different from those that trigger presynaptic transmitter release, subsequently leading to activation of an exemplar transcription factor, CREB, a regulator of transcription of many important neuronal genes. Our approach is to combine physiological approaches (how fast, how steeply voltage-dependent, how is signal transduced) and biochemical experiments using cDNA microarrays (which genes, in what context, what relationship to learning and memory).