Loss of Flower/FLWR-1 induces an increase in neuronal excitability and causes defective recycling of synaptic vesicles

The Flower protein is proposed to couple synaptic vesicle fusion to recycling in different model organisms. It is supposed to trigger activity-dependent bulk endocytosis by conducting Ca ²⁺ at endocytic sites. However, this mode of action is debated. Here, we investigate the role of the nematode homolog (FLWR-1) in neurotransmission in Caenorhabditis elegans . Our results confirm that FLWR-1 facilitates the recycling of synaptic vesicles at the neuromuscular junction. Ultrastructural analysis of synaptic boutons after hyperstimulation surprisingly reveals an accumulation of endosomal structures in flwr-1 mutants. These findings do not support a role of FLWR-1 in the formation of bulk endosomes but rather a function in their breakdown following cleavage from the plasma membrane. Unexpectedly, loss of FLWR-1 conveys increased neuronal excitability which causes an excitation-inhibition imbalance. Finally, we obtained evidence that this increased transmission at the neuromuscular junction might be caused by deregulation of MCA-3, the nematode homolog of the plasma membrane Ca ²⁺ ATPase (PMCA).