Molecular Mechanisms Of Synapse Development And Plastici

Child Health And Human Development

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Abstract

This laboratory studies the molecules that regulate synapse development and plasticity, with particular emphasis on the role of neurotrophic factors. Traditionally, neurotrophic factors are defined as secretory proteins that regulate neuronal survival and differentiation. Recent studies have established a new concept that neurotrophic factors also play important roles in synapse transmission and plasticity in both developing and adult nervous system. Two types of regulation have been discovered: acute modulation of synaptic transmission and plasticity, and long-term alteration of the structure and function of synapses. 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. This laboratory was among the first to study the synaptic functions of neurotrophic factors. We are continuing the studies on neurotrophic regulation of synapses. In the period covered by this report, we have focused on a number of neurotrophic factors other than neurotrophins: 1) Pre- and post-synaptic effect of GDNF and neurturin on the development of the neuromuscular synapses. Glial cell line-derived neurotrophic factor (GDNF) is known for its potent effect on neuronal survival, but its role in the development and function of synapses is not well studied. Using Xenopus nerve-muscle co-cultures, we show that GDNF and its family member neurturin (NRTN) facilitate the development of the neuromuscular junction (NMJ). Long-term application of GDNF significantly increased the total length of neurites in the motoneurons. GDNF also caused an increase in the number and the size of synaptic vesicle clustering, as demonstrated by synaptobrevin-GFP fluorescent imaging, and FM dye staining. Electrophysiological experiments revealed two effects of GDNF on synaptic transmission at NMJ. First, GDNF markedly increased the frequency of spontaneous transmission and decreased the variability of evoked transmission, suggesting an enhancement of transmitter secretion. Second, GDNF elicited a small increase in the quantal size, without affecting the average rise and decay times of synaptic currents. Imaging analysis showed that the size of acetylcholine receptor (AChR) clusters at synapses increased in muscle cells over-expressing GDNF. Neurturin had very similar effects as GDNF. These results suggest that GDNF and NRTN are new neuromodulators that regulate the development of the neuromuscular synapse through both pre- and postsynaptic mechanisms. (JBC). 2) Regulation of AChR clustering by Dishevelled interacting with MuSK and PAK1. An important aspect of synapse development is the clustering of neurotransmitter receptors in the postsynaptic membrane. The best-known molecule that controls AChR clustering at the NMJ is Agrin. Although MuSK is required for Agrin-induced AChR clustering, the underlying molecular mechanisms remain unclear. We show that in muscle cells, MuSK interacts with Dishevelled (Dvl), a signaling molecule important for planar cell polarity. Disruption of the MuSK-Dvl interaction inhibits Agrin-induced and neuron-induced AChR clustering. Expression of dominant negative Dvl1 in the postsynaptic muscle cells reduces the amplitude of spontaneous synaptic currents at the NMJ. Moreover, Dvl1 interacts with a downstream kinase PAK1. Agrin activates PAK, and this activation requires Dvl. Inhibition of PAK1 activity attenuates AChR clustering. These results demonstrate important roles of Dvl and PAK in Agrin/MuSK-induced AChR clustering, and reveal a novel function of Dvl in synapse development. (Neuron). 3) Synaptic vesicle depletion in mice lacking alpha-synuclein. While mutation of a-synuclein, a protein associated with pre-synaptic vesicles, is implicated in the etiology and pathogenesis of Parkinson's disease (PD), the biological function of the normal protein is unknown. Mice lacking a-synuclein have been generated. Electron microscopic examination of hippocampal synapses revealed a selective deficiency of undocked vesicles without affecting docked vesicles. Field recording of CA1 synapses in hippocampal slices from the mutant mice demonstrated normal basal synaptic transmission, paired pulse facilitation, and response to a brief train of high frequency stimulation (100 Hertz, 40 pulses) that exhausts only docked vesicles. In contrast, the a-synuclein knockout mice exhibited significant impairments in synaptic response to a prolonged train of repetitive stimulation (12.5 Hertz, 300 pulses) capable of depleting docked as well as reserve pool vesicles. Moreover, the replenishment of the docked vesicles by reserve pool vesicles after depletion was slower in the mutant synapses. Thus, a-synuclein may be required for the genesis and/or maintenance of a subset of presynaptic vesicles, those in the "reserve" or "resting" pools. These results reveal, for the first time, the normal function of endogenous a-synuclein in regulating synaptic vesicle mobilization at nerve terminals. (J. Neurosci.)