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.