Plum is a novel regulator of synaptic function and muscle size in D. melanogaster

Mammalian Myostatin (MST) and GDF11, members of the TGF-β superfamily of growth factors, are powerful regulators of muscle size in both model organisms and humans. Myoglianin (MYO), the Drosophila homolog of MST and GDF11, is a strong inhibitor of synaptic function and structure at the neuromuscular junction (NMJ), and a negative regulator of body weight and muscle size and function in flies. Here, we identified Plum, a cell surface immunoglobulin homologous to mammalian developmental regulators Protogenin and Nope, as a modulator of MYO function in the larval neuromuscular system. Reduction of Plum specifically in the larval body-wall muscles abolishes the previously demonstrated positive effect of attenuated MYO signalling on both muscle size and neuromuscular junction structure and function, likely by de-sequestrating the remaining MYO. In addition, downregulation of Plum on its own results in decreased synaptic strength and body weight, classifying Plum as a (novel) regulator of neuromuscular function and body (muscle) size. These findings offer new insights into possible regulatory mechanisms behind ageing- and disease-related neuromuscular dysfunctions in humans and identify potential targets for therapeutic interventions.