Synaptic plasticity is a hall mark of the nervous system and the foundation for learning, memory formation and the response to injury. Despite many studies in this field, the detailed mechanisms that regulate synaptic plasticity are not fully understood.
In this dissertation, I use the C. elegans motor circuit as a model system to study the molecular mechanisms that regulate synaptic plasticity. In newly born larval animals, Dorsal D (DD) motor neurons receive cholinergic inputs in the dorsal nerve cord. These inputs are switched to the ventral side by the end of the first larval (L1) stage. By exploiting the postsynaptic GABA neuron marker, the acetylcholine receptor (AChR) subunit ACR-12, I discovered that an immunoglobulin super family (IgSF) protein, OIG-1, antagonizes the relocation of postsynaptic AChRs in early DD neurons. This effect is relieved with down-regulation of OIG-1 by the transcription factor, IRX-1/Iroquois, thus allowing the repositioning of synaptic inputs to the ventral side during L1/L2 transition. In the later-born Ventral D (VD) neurons, the transcription factor UNC-55/COUP-TF turns off IRX-1, thereby maintaining high levels of OIG-1 to block the removal of dorsally-located ACR-12 receptors. In contrast, the CUB-domain protein LEV-10 is spatially regulated in developing GABA neurons. LEV-10 is strictly localized on the dorsal side of DD neurons to cluster ACR-12-containing receptors and to inhibit remodeling during the L1 stage. Later, LEV-10 is relocated to the ventral side to maintain AChRs after remodeling. I also describe two muscle-secreted proteins LEV-9 and OIG-4, that function non-cell autonomously to maintain AChRs on GABA neurons during development. Together, these intracellular (OIG-1), transmembrane (LEV-10) and extracellular (OIG-4, LEV-9) proteins act collectively to stabilize postsynaptic clusters of AChRs and thereby modulate synaptic plasticity of GABAergic motor neurons in C. elegans.
In addition, I describe a novel method, NATF (Native And Tissue-specific Fluorescence) that combines CRISPR/Cas-9 genome editing and split-GFP to label proteins at their native expression level. By inserting a GFP11X7 tag into the protein of interest and crossing the resultant knock-in line with tissue specific GFP1-10 reporter lines, I determined that OIG-1 is expressed intracellularly at its native level and resolved LEV-10 at discrete postsynaptic locations in muscle cells, GABA neurons and cholinergic neurons.
Both IgSF and CUB-domain proteins have been implicated in multiple aspects of neural plasticity, such as axon guidance, receptor clustering, etc. Our study provides new insight on how these proteins can be temporally and spatially regulated to modulate receptor clustering and synaptic plasticity during neural development.