 |
 |
|
 |
|
 |
|
 |
| |
UC Berkeley | |
UC Berkeley MCB Department | |
Helen Wills Neuroscience Institute | |
Biophysics |
Robert S. Zucker
Professor (Emeritus) of Neurobiology and Professor of the
Graduate School
University of California, Berkeley
Department of Molecular and Cell Biology, Division of
Neurobiology
Neuroscience Institute
Biophysics Graduate Program
Contact Information:
Professor Robert Zucker
University of California
Department of Molecular and Cell Biology
111 Life Sciences Addition
Berkeley, CA 94720-3200 USA
Office Phone: (510) 642-3407
Mobile: (510) 599-4809
Fax Line: (510) 642-3407, or 643-6791
E-Mail: zucker@berkeley.edu
Synapses are not static; they are highly regulated. The strength
of synaptic transmission can wax and wane after a bout of
activity, and this shapes the information processing properties
of neural circuits. Our lab concentrates on studying the
mechanisms regulating synaptic transmission.
We use biophysical methods such as control of second messengers
by photolysis of light sensitive caged compounds, measurement of
second messengers with fluorescent dyes, detection of molecular
rearrangements using fluorescence resonance energy transfer
(FRET), tracking of vesicle movements with vital vesicle
membrane labeling dyes, and mathematical modeling and
computational simulations, as well as electrophysiological
techniques, to study the mechanisms underlying regulation of
synaptic transmission.
We study the roles of calcium in triggering transmitter
release at fast synapses, and in modulating release following
prior activity. Presynaptic calcium not only triggers
exocytosis, but also facilitates release after a single action
potential, and enhances release after a train of action
potentials (post-tetanic potentiation, PTP), by binding to
different target molecules in presynaptic terminals. Presynaptic
mitochondrial loading and Na+ accumulation also play roles in
PTP: the post-tetanic extrusion of Ca2+ by Na+/Ca2+
exchange is reversed, prolonging the enhancement of transmitter
release by the lingering residual Ca2+ leaking out of
mitochondria and in from the outside. Computer simulations of Ca2+
diffusion away from channel mouths to docked vesicles and
facilitatory effectors can account for the time course of
transmitter release, magnitude of facilitation, and measurements
of presynaptic [Ca2+]. We are also exploring the
rearrangements of presynaptic proteins triggered by Ca2+
using FRET. And we are exploring the Ca2+-dependence
of transmitter release and short-term synaptic plasticity at
neuromuscular junctions and cultured hippocampal synapses by
photolysis of photosensitive Ca2+ chelators (caged Ca2+).
We have also studied forms of synaptic plasticity called long-term
potentiation
(LTP) and long-term depression (LTD), which are involved
in memory consolidation in the mammalian brain. We have found
that LTP is selectively triggered by short-lasting high [Ca2+]i
elevations in hippocampal CA1 pyramidal neurons, while prolonged
modest [Ca2+]i elevations selectively
activate LTD. We have also explored a possible role for
postsynaptic Ca2+ in CA3 pyramidal cells.
Depolarization-induced suppression of inhibition (DSI) is
another form of cortical synaptic plasticity, in which
postsynaptic activity suppresses the release of transmitter from
presynaptic inhibitory interneurons. We have shown that cortical
DSI is caused by micromolar levels of [Ca2+]
operating at a target distant from Ca2+ channel
mouths.
At cholinergic synapses made by a multi-transmitter
Aplysia neuron, Ca2+ acts quite locally, nonlinearly,
and at high levels to secrete transmitter. In contrast, at peptidergic
synapses from the same neuron, we have found that Ca2+
acts more linearly to release pre-docked vesicles located
somewhat farther from Ca2+ channel mouths.
Serotonin is a neuromodulator of fast
glutamatergic synapses, both in the brain and at crayfish
neuromuscular junctions. Using membrane-labeling fluorescent
dyes, we found that serotonin enhances transmission by
increasing the pool of presynaptic vesicles available for
release. We recently discovered that serotonin activates the
production of cyclic AMP to activate a pathway involving
exchange protein activated by cAMP (Epac) and to open
presynaptic hyperpolarization and cyclic nucleotide-activated
(HCN) channels to enhance secretion evoked by action potentials
by an actin-dependent process.
Repetitive activity at crayfish neuromuscular junctions
activates a long-term facilitation (LTF) of
transmission. This process also depends on HCN channel
activation. Na+ accumulates in nerve terminals, is
extruded by an electrogenic Na+ pump whose
hyperpolarization activates HCN channels to cause LTF. LTF also
requires Ca2+ influx during stimulation, integrity of
actin filaments, local presynaptic protein synthesis, the
calcium-dependent dephosphorylation of a target protein, and the
activity of numerous protein kinases that influence protein
synthesis.
The docking and fusion of synaptic vesicles are functions of a
complex of presynaptic proteins called SNAREs. By labeling
the SNARE proteins VAMP-2 and SNAP-25B with GFPs, we have been
able to detect changes in fluorescence resonance energy transfer
(FRET) that may report a tightening or reorientation of the
SNARE complex on vesicle fusion as cis-SNAREs are formed, followed by disassembly
of SNAREs after SNAREs have dispersed from the active zone but
prior to endocytosis, and finally reassembly of SNAREs as new
vesicles are docked and primed.
For more information on some of these projects, click here, or on the Projects button from any page.
