Neuroendocrine control of synaptic transmission by PHAC-1 in C. elegans

A dynamic interplay between synaptic and neuromodulatory signalling guarantees flexible but robust neuronal circuits. Presynaptic modulation plays a crucial role in controlling the excitatory-inhibitory balance within networks. Here, we designed a genetic screen to identify genes involved in the neuromodulation of the C. elegans neuromuscular junctions (NMJ) and identified the orthologs of the Protein Phosphatase 1 regulatory subunit PHACTR1 (phac-1) and the presynaptic phosphoproteins Synapsin (snn-1). Five de novo variants of human PHACTR1 are associated with severe early-onset epilepsies (DEE70). To understand the impact of these variants, we introduced the DEE70 mutations into phac-1. These mutations resulted in the formation of a constitutively active PP1-PHAC-1 holoenzyme that disrupts cholinergic signalling at the NMJ. By using quantitative fluorescence imaging, electron microscopy and electrophysiology, we found that the constitutive holoenzyme alters the synaptic vesicle cycle, reduces the synaptic vesicle reserve pool, and increases neuropeptide release by dense-core vesicles. Notably, while SNN-1 phosphoregulation contributes to NMJ signalling, genetic interactions suggest that SNN-1 is not the main effector of PP1-PHAC-1 holoenzyme signalling. Collectively, our results confirm the pathogenicity of DEE70 variants, clarify their dominant-positive effects, and provide evidence of a presynaptic mode of action for DEE70.