Developing neural circuits are actively remodeled as synapses are created in new locations and dismantled in others. These dynamic events are regulated by neuronal activity to produce mature circuits with specific physiological functions. This phenomenon has been observed throughout animal phylogeny which suggests that the underlying pathways are conserved. However, the molecular mechanisms that drive synaptic remodeling are largely unknown. Here we propose a strategy that exploits the simple model organism, C. elegans, to define a development program that remodels the synaptic architecture of a GABAergic circuit. Ventral synapses for DD class GABA neurons are relocated to new sites on the dorsal side during larval development. This synaptic remodeling program is blocked by the UNC-55/COUP-TF transcription factor in VD motor neurons which normally synapse with ventral muscles. We exploited this UNC- 55 function in a powerful cell-specific profiling strategy that identified 19 conserved genes with roles in synaptic remodeling. We have now shown that one of these UNC-55 targets, the DEG/ENaC cation channel, UNC-8, promotes synaptic remodeling in a mechanism that is activated by GABAergic signaling. This finding is important because DEG/ENaC proteins have been implicated in learning and memory but the mechanism that links DEG/ENaC function to synaptic plasticity is poorly understood.
Specific Aim 1 tests the key prediction that UNC-8 is closely associated with GABAergic synapses that are remodeled by UNC-8 activity.
Specific Aim 2 is designed to test the novel hypothesis that a Ca2+-dependent mechanism links neural activity to UNC-8 cation transport in a feedback loop that dismantles the presynaptic machinery.
Specific Aim 3 defines the cellular origin and molecular components of the proposed activity-dependent pathway that regulates UNC-8 and promotes GABAergic synaptic remodeling. Together, these approaches offer a powerful opportunity to delineate an intricate molecular pathway that controls synaptic plasticity. Moreover, the conservation of these remodeling components in mammals argues that the results of this work are likely to reveal fundamental mechanisms that regulate synaptic plasticity in the human brain.
Normal brain development is defined by the active reorganization of connections or synapses between neurons. To identify genes that regulate these complex events, we are studying a synaptic remodeling program in the nematode, C elegans, an organism with a simple, well-defined nervous system and powerful tools for genetic analysis. The results of this work should reveal genes with similar roles in the vertebrate nervous system and thereby provide important clues to the biological basis of human diseases that arise from dysregulated mechanisms of synaptic remodeling.