|
 |
|
 |
|
 |
|
 |
| |
UC Berkeley | |
UC Berkeley MCB Department | |
Helen Wills Neuroscience Institute | |
Biophysics |
Robert S. Zucker
Professor of Neurobiology
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
Lab Phone: (510) 642-5297
Fax Line: (510) 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.
