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.