Mechanisms of synaptic competition at developing neuromuscular synapses
Synapses made by a neuron with its synaptic partners are malleable during development, and as a consequence of experience, with respect to number, strength, and functional properties such as short and long term plasticity. A well studied model system for developmental, activity-dependent plasticity is mouse neuromuscular synapses, which undergo activity-dependent plasticity in development that is a hallmark of their smaller, less accessible counterparts in the CNS.
During late embryonic and early postnatal life, neuromuscular synapses undergo synapse elimination, in which the synapses of one axon are pitted in competition against the synapses of other axons innervating the same target cell. Based on their activity patterns relative to their competitors, one or a small number of axons will emerge as winners, maintaining their synapses into adult life, while other axons lose the competition and are permanently deleted from neural circuitry.
Despite many structural and a few functional studies of neuromuscular synapse elimination, little is known about the underlying mechanisms and many important questions remain to be addressed. One important set of questions includes how the structural changes in competing inputs are related to progressive changes in input strength, to the outcome of competition, and how activity mediates this process.
To understand the dynamics of the poorly understood presynaptic aspects of competition, including synaptic vesicle release, recycling and trafficking at developing mammalian neuromuscular synapses, we have developed transgenic lines of mice in which the Thy1 promoter drives expression of synaptopHluorin (Thy1-spH). SpH is a pH-sensitive variant of GFP tethered to the luminal domain of the vesicular protein VAMP2 that allows synaptic vesicle recycling to be monitored optically. Our recently published work (Wyatt and Balice-Gordon, 2008) suggests that spH+ synaptic vesicle clusters can be readily visualized within motor axon terminals and vesicle release, recycling and trafficking assessed using optical measurements of activity-induced fluorescence changes in isolated sternomastoid nerve-muscle preparations as well as in vivo.
There are two sub-projects that utilize these mice:
- determine the relationship between vesicle recycling, synaptic strength and synaptic size of competing inputs to developing neuromuscular junctions undergoing synapse elimination; and
- determine how postsynaptic activity blockade retrogradely affects synaptic vesicle recycling at developing and adult neuromuscular junctions, testing several proteins as candidate retrograde factors that may modulate presynaptic neurotransmitter release.
Understanding how activity modulates presynaptic vesicle recycling and trafficking, affecting synaptic structure, strength and survival would provide a mechanism by which plastic changes in synaptic function could permanently alter neural circuitry.