Synapses are intricately organized subcellular compartments in which molecular machines cooperate to ensure spatiotemporally precise transmission of chemical signals. Key components of this machinery are P/Q-type voltage-gated Ca2+-channels (VGCCs), specifically UNC-2, that translate electrical signals into a trigger for fusion of synaptic vesicles (SVs) with the plasma membrane. UNC-2 and the Ca2+ microdomains it generates must be located in the right distance to the primed SV, to elicit transmitter release without delay. Rab3 interacting molecule (RIM/UNC-10) and RIM-binding protein (RIM-BP/RIMB-1) were shown in different model systems to contribute to the spatial organization of the active zone protein scaffold, and to localize VGCCs next to docked SVs by binding to each other and to the C-terminal region of the Cav2 VGCC alpha-subunit (UNC-2). We asked how this machinery is organized at the C. elegans neuromuscular junction (NMJ), and whether it can differentially regulate transmission in circuits composed of different neuron types, specifically cholinergic vs. GABAergic motor neurons. Evidence for such differential functionality of the presynaptic release machinery was provided by earlier work using optogenetic stimulation (1,2) and recordings in distinct mutants or pharmacologically affected synapses (3).
rimb-1 mutants had mild synaptic defects, through loosening the anchoring of the UNC-2 VGCC and delaying the onset of SV fusion, while RIM (
unc-10) deletion had much more severe defects.
rimb-1 mutants caused increased cholinergic but reduced GABAergic transmission, while overall transmission at the NMJ was reduced, as shown by voltage imaging. The UNC-2 channel could further be untethered by removing its C-terminal PDZ binding motif, and this untethering could be exacerbated by combining the deltaPDZ mutant with the
rimb-1 mutation. Similar phenotypes resulted from acute degradation of the Cav2 beta-subunit CCB-1, indicating that destabilization of the VGCC complex causes the same phenotypes as its untethering. 1 Liewald et al. (2008) Nat Methods 5(10): 895-902. 2 Liu et al. (2009) PNAS 106(26): 10823-10828. 3 Liu et al. (2018) Cell Rep 22(9): 2334-2345.