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Research:
(Background Information)
Neurotrophic
factors are traditionally viewed as secretory proteins that regulate
neuronal survival and differentiation. However, a series of recent studies
have revealed an unexpected role for these factors in synaptic development
and plasticity. The neurotrophins BDNF and NT-3 have been shown to acutely
potentiate synaptic transmission at the neuromuscular junction and in
the brain. These factors also promote long-term maturation of the neuromuscular
synapses. In the visual system, neurotrophins are involved in the formation
of eye-specific synaptic connections and activity-dependent synaptic competition.
Gene knockout and physiological experiments demonstrated that the neurotrophin
BDNF plays an important role in hippocampal long-term potentiation (LTP),
a long-lasting enhancement in synaptic efficacy often used as a cellular
model for learning and memory. These findings have brought together two
hotly pursued areas of neuroscience, namely, the function of neurotrophic
factors and the mechanisms for synaptic plasticity. Continuous studies
in this emerging field will help understand how synapses develop and function
in the brain, and may have general implications in treating neurological
disorders in both children and adults. Our laboratory was among the first
to study synaptic function of neurotrophic factors. Currently, we are
focusing on the mechanisms by which neurotrophic factors regulate synapses,
using the neuromuscular junction and hippocampus as model systems.
Synapse
formation is a highly organized, multi-stage process. After initial
contact is made between pre- and postsynaptic elements, these components
undergo a series of activity-dependent events, leading a mature synapse.
Retrograde messengers have long been thought to mediate synapse development.
However, so far there is no definitive proof of any molecules as the retrograde
messengers. The NT-3 receptor TrkC has been found in the presynaptic motoneurons.
We have shown that NT-3 gene is expressed in the postsynaptic muscle cells,
and its expression is activity-dependent. Innervation and consequent membrane
depolarization leads to a rapid but specific increase in NT-3 mRNA in
developing muscle cells. Application of NT-3 to nerve-muscle co-culture
induces a series of presynaptic changes indicative of synaptic maturation.
These include an increase in the frequency and amplitude of spontaneous
synaptic currents (SSCs); an increase in the amplitude and a decrease
in the variability of evoked synaptic currents, and characteristic change
in the amplitude distribution of SSCs. NT-3 treatment also lead to a significant
increase in synaptic varicosities as well as an enhancement of the expression
of various synaptic vesicle proteins in the motoneurons. These results
provide direct evidence that muscle-derived NT-3 may serve as a retrograde
messenger for activity-dependent synaptic strengthening at the developing
neuromuscular junction. We are currently investigating the molecular mechanisms
underlying the NT-3 effect, and its physiological relevance in synaptic
competition and elimination. We will also identify other factors that
regulate different stages of synaptogenesis.
Hippocampus
is an area in the brain important for learning and memory. Tetanic stimulation
induces LTP, which is an electrophysiological manifestation of learning
and memory. The expression of BDNF gene is enhanced by tetanic stimulation.
We have discovered that exogenous application of BDNF promotes LTP in
neonatal hippocampus where endogenous BDNF level is low, while application
of the BDNF antagonist TrkB-IgG inhibits LTP in adult hippocampus where
endogenous BDNF level is high. The BDNF effect on LTP is restricted to
tetanized synapse (input specific). This effect is achieved by an attenuation
of the synaptic fatigue induced by high frequency, tetanic stimulation.
We have also shown that BDNF preferentially enhances synaptic transmission
at high frequency, through a presynaptic mechanism. These results provide
the basis for a role of BDNF as a retrograde messenger in the Hebbian
model, which predicts that more active synapses are favorable during synaptic
competition. Using BDNF knockout mice, we have investigated the mechanisms
by which BDNF regulates high frequency synaptic transmission. We found
a severe impairment in hippocampal LTP in the heterozygous mice, primarily
due to deficits in presynaptic properties. These mice show a pronounced
synaptic fatigue induced by tetanus. Synaptic fatigue is known to be due
to a depletion of synaptic vesicles during high frequency stimulation.
Electron microscopic study showed that there are less synaptic vesicles
docked at presynaptic active zone in the mutant mice. Biochemical experiments
indicated that proteins involved in synaptic vesicle docking are markedly
reduced in these mice. Treatment of the mutant slices with BDNF reversed
the electrophysiological and biochemical deficits in the hippocampal synapses.
Using a conditional knockout mouse with specific deletion of the BDNF
receptor TrkB in the CA1 region, we showed that BDNF modulates LTP and
HFS response in the CA1 synapses through mechanisms independent of postsynaptic
CA1 pyramidal neurons. Taken together, these results suggest a novel role
for BDNF in the mobilization and/or docking of synaptic vesicles to presynaptic
active zones. Our studies may have general implications in understanding
the mechanisms of learning and memory, and in treatment of learning disorders
in both children and adults. We are currently using biochemical physiological
approaches to determine the molecular targets as well as signal transduction
mechanism of the BDNF regulation.
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