Convergent, RIC-8-Dependent Gα Signaling Pathways in the Caenorhabditis elegans Synaptic Signaling Network

Abstract

We used gain-of-function and null synaptic signaling network mutants to investigate the relationship of the G␣ q and G␣ s pathways to synaptic vesicle priming and to each other. Genetic epistasis studies using G␣ q gain-of-function and null mutations, along with a mutation that blocks synaptic vesicle priming and the synaptic vesicle priming stimulator phorbol ester, suggest that the G␣ q pathway generates the core, obligatory signals for synaptic vesicle priming. In contrast, the G␣ s pathway is not required for the core priming function, because steady-state levels of neurotransmitter release are not significantly altered in animals lacking a neuronal G␣ s pathway, even though these animals are strongly paralyzed as a result of functional (nondevelopmental) defects. However, our genetic analysis indicates that these two functionally distinct pathways converge and that they do so downstream of DAG production. Further linking the two pathways, our epistasis analysis of a ric-8 null mutant suggests that RIC-8 (a receptor-independent G␣ guanine nucleotide exchange factor) is required to maintain both the G␣ q vesicle priming pathway and the neuronal G␣ s pathway in a functional state. We propose that the neuronal G␣ s pathway transduces critical positional information onto the core G␣ q pathway to stabilize the priming of selected synapses that are optimal for locomotion. O NE proposed mechanism for establishing and omi and Kidokoro 2000; Waters and Smith 2000). These electrophysiological studies thus show that both modifying behaviors or memories involves presynaptic changes in synaptic strength, and regulating the pathways can positively affect priming, but they do not reveal how the two pathways interact and why they both amount of synaptic-vesicle-mediated neurotransmitter release at synapses is thought to be critical for changing exist. Highlighting this point, two recent studies showed that the G␣ s pathway is not even required for synaptic presynaptic strength (Kandel and Pittenger 1999; Lin vesicle priming, because evoked release remains norand Scheller 2000; Sudhof 2000). Electrophysiologimal, or even increases, when G␣ s null synapses are subcal studies have defined a "readily releasable," or "primed," jected to low frequency stimulation (Hou et al. 2003; pool of synaptic vesicles, the size of which determines Renden and Broadie 2003). release probability and, therefore, presynaptic strength Providing important insights and yet further adding (Sudhof 2000). The term "primed" can be applied eito the puzzlement about the relative roles of the G␣ q ther to an individual synaptic vesicle or to a synapse and G␣ s pathways, recent research suggests that both (i.e., a primed synapse is one containing a relatively large pathways, directly or indirectly, affect the synaptic abunpool of primed synaptic vesicles). A major challenge is dance of UNC-13, a large, conserved protein that binds to understand the layout and logic of the network of diacylglycerol (DAG) (Maruyama and Brenner 1991), signaling pathway...

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