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Current
Projects:
Mechanisms
by which BDNF regulates long-term potentiation (LTP)
We have previously demonstrated that BDNF facilitates LTP by enhancing
ability of hippocampal synapses to follow high frequency, LTP-inducing
tetanic stimulation (Nature, '96; J. Neurosci., '98). This is achieved,
at least in part, by promoting synaptic vesicle docking, possibly through
regulation of the levels of the synaptic proteins synaptophysin and synaptobrevin
in the presynaptic terminals (J. Neurosci., '99). Using a conditional
knockout mouse line in which the BDNF receptor TrkB has been selectively
deleted in postsynaptic neurons, we demonstrated this year that at CA1
hippocampal synapses, BDNF acts exclusively on presynaptic neurons but
not on postsynaptic neurons, an issue under considerable debate (J. Neurosci.
'00). We have also extended our work to long-term, cAMP and protein synthesis
dependent regulation of hippocampal synapses by BDNF (J. Biological Chemistry,
'01). We are currently investigating the role of BDNF in the long-lasting
LTP, using transgenic and knockout approaches.
Signal
transduction mechanisms for acute effects of neurotrophins
In the hippocampus, BDNF modulation of synaptic plasticity is mediated
by signaling pathways involving MAP kinase and Phosphoinositide-3 kinase
(PI3K), but not Phospholipase C-gamma (Learning and Memory, 1999). We
demonstrated that acute modulation of NT3 on transmitter release at the
NMJ uses an unusual mechanism which involves calcium release from intracellular
stores through inositol 1, 4, 5-trisphosphate (IP3) and/or ryanodine receptors,
leading to an activation of calcium/calmodulin kinase II (CaMKII) (J.
Cell Biol., '00). Further, we demonstrated using photo-uncaging that simultaneous
activation of PI3K and IP3 receptors is not only necessary but also sufficient
to mediate the effect of NT3 (Nature Neurosci., '01). We are continuing
to investigate the key signaling mechanisms underlying acute and long-term
neurotrophic regulation, as well as mechanisms by which acute effects
can be converted into long-term modulation.
Activity-dependent
modulation of neurotrophin receptors
To understand how synapse specific neurotrophic regulation is achieved,
we have examined the role of neuronal/synaptic activity in the trafficking
of the neurotrophin receptors TrkB. We revealed an activity-dependent
modulation of the insertion of TrkB into the plasma membrane of cultured
hippocampal neurons. This effect requires Ca2+ influx through NMDA-type
glutamate receptors and Ca2+ channels, and involves CaMKII (J. Cell Biol.,
'00). We are actively pursuing this line of research, with particular
emphasis on activity-dependent regulation of TrkB internalization, and
intracellular signaling. These studies may not only provide insights into
the mechanistic link between activity-dependent and neurotrophic modulation
of synaptic efficacy, but also have general implications in the cell biology
of growth factor signaling.
Role
of GDNF family proteins on neuronal and synaptic functions
We have preciously cloned a new GDNF receptor (Neuroscience, '98) and
demonstrated that differential signaling of GDNF and BDNF may mediate
differential functions by these two factors in the midbrain dopaminergic
neurons (Neuroscience, '99; Mol. Brain Res., '99). Recently, we discovered
an unexpected, acute effect of GDNF on A-type potassium channels, leading
to a potentiation of neuronal excitability, in the dopaminergic neurons
in culture as well as in adult brain slices (Nature Neurosci., '01). Further,
we show that GDNF regulates the K+ channels through a mechanism that involves
activation of MAP kinase. We have also used the Xenopus NMJ as a model
system to study the synaptic effects of GDNF. We found that long-term
application of GDNF to the NMJ facilitates synaptic transmission by enhancing
release probability. This effect is mediated by up-regulation of the calcium
binding protein frequenin, which interacts with the N-type calcium channels
at the nerve terminals (Neuron, '01). We are further characterizing the
pre- and postsynaptic effects of the GDNF family of proteins on neuromuscular
development.
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