Molecular Mechanisms Of Synapse Development &Plasticity

Child Health And Human Development

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Abstract

Neurotrophic factors are defined as secretory proteins that regulate neuronal survival and differentiation. Recent studies indicate that a major function of neurotrophic factors in the brain is to regulate synaptic transmission and plasticity. This laboratory was among the first to reveal this novel function. We demonstrated, using rodent models, that brain derived neurotrophic factor (BDNF) plays a key role in hippocampal long-term potentiation (LTP), a cellular model for learning and memory. Using the Xenopus neuromuscular synapse as a model, we identified two modes of regulation by neurotrophins: acute modulation of synaptic transmission and plasticity, and long-term alteration of the structure and function of synapses. In the period covered by this report, we have focused our attention on a major cell biological issue: how do diffusible molecules such as neurotrophins elicit local and synapse-specific modulation? We have made two significant discoveries: 1) The BDNF val66met polymorphism affects activity-dependent secretion of BDNF and human memory and hippocampal function. Previous studies by this as well as other laboratories indicate that BDNF is important for hippocampal synaptic plasticity and hippocampus-dependent learning and memory in animals. In collaboration with Daniel Weinberger?s lab at NIMH, we examined the relevance of these studies in human by studying a human SNP (single nucleotide polymorphism) that converts a valine (val) to methionine (met) in the 5? pro-region of the BDNF protein. Subjects with the met/met genotype exhibited poorer performance in tests of episodic memory. Functional magnetic resonance imaging (fMRI) of two separate cohorts both revealed abnormal hippocampal activation during a cognitive task in subjects with a met allele. Furthermore, levels of n-acetyl aspartate (NAA), a putative in vivo measure of neuronal integrity and synaptic abundance, were lower in the hippocampal formation of subjects with met alleles. Rodent hippocampal neurons transfected with val-BDNF-GFP exhibited a punctate distribution pattern throughout the soma and dendrites, while those with met-BDNF-GFP showed large clusters in the peri-nucleus region. Double staining experiments indicated that met-BDNF-GFP failed to be localized in secretory granules or transported to the synapses. Depolarization-induced secretion of met-BDNF was significantly reduced while its constitutive secretion was unchanged. These results represent the first demonstration of a role for BDNF in human memory and hippocampal function, and suggest val/met exerts these effects by impacting synaptic targeting and activity-dependent BDNF secretion. Thus, it is the local and activity-dependent secretion of BDNF at synapses, rather than the level or secretion of BDNF per se, that is critical for hippocampal plasticity and memory function. (Cell) 2) Activity- and tyrosine kinase-dependent facilitation of TrkB receptor internalization in hippocampal neurons. Another mechanism to achieve synapse-specific modulation is to have activity-dependent control of neurotrophin signaling through membrane receptors. We have previously shown that high-frequency neuronal activity facilitates the insertion of the BDNF receptor TrkB into the cell surface, leading to an enhanced response to BDNF in active neurons/synapses. We have now examined the role of neuronal activity on TrkB internalization, a process critical for many aspects of BDNF functions. Using three independent approaches, we show that electric stimulation of hippocampal neurons markedly enhances TrkB internalization. Electric stimulation also potentiates TrkB tyrosine kinase activity. The activity-dependent enhancement of TrkB internalization and its tyrosine kinase requires Ca2+ influx through NMDA receptors and Ca2+ channels. Inhibition of internalization had no effect on TrkB kinase, but inhibition of TrkB kinase prevents the modulation of TrkB internalization, suggesting a critical role of the tyrosine kinase in the activity-dependent receptor endocytosis. Assuming this mechanism is operative at synapses, BDNF may preferentially modulate active synapses by activity- and Ca2+- dependent potentiation of TrkB tyrosine kinase and its internalization.