Two Pathways for the Activity-Dependent Growth and Differentiation of Synaptic Boutons in Drosophila

• Published in: eNeuro 2019-07, Vol.6 (4), p.ENEURO.0060-19.2019
• Main Authors: Vasin, Alexander, Sabeva, Nadezhda, Torres, Carol, Phan, Sébastien, Bushong, Eric A, Ellisman, Mark H, Bykhovskaia, Maria
• Format: Article
• Language: English
• Subjects: Animals; Animals, Genetically Modified; Cell Differentiation; Cyclic AMP-Dependent Protein Kinases - physiology; Drosophila; Drosophila Proteins - physiology; Female; Locomotion; Male; Neuromuscular Junction - cytology; Neuromuscular Junction - growth & development; Neuromuscular Junction - physiology; Neuromuscular Junction - ultrastructure; New Research; Presynaptic Terminals - physiology; Presynaptic Terminals - ultrastructure; Synapsins - physiology; Synaptic Vesicles - physiology
• ISSN: 2373-2822; 2373-2822
• DOI: 10.1523/ENEURO.0060-19.2019

Synapse formation can be promoted by intense activity. At the larval neuromuscular junction (NMJ), new synaptic boutons can grow acutely in response to patterned stimulation. We combined confocal imaging with electron microscopy and tomography to investigate the initial stages of growth and differentiation of new presynaptic boutons at the NMJ. We found that the new boutons can form rapidly in intact larva in response to intense crawling activity, and we observed two different patterns of bouton formation and maturation. The first pathway involves the growth of filopodia followed by a formation of boutons that are initially devoid of synaptic vesicles (SVs) but filled with filamentous matrix. The second pathway involves rapid budding of synaptic boutons packed with SVs, and these more mature boutons are sometimes capable of exocytosis/endocytosis. We demonstrated that intense activity predominantly promotes the second pathway, i.e., budding of more mature boutons filled with SVs. We also showed that this pathway depends on synapsin (Syn), a neuronal protein which reversibly associates with SVs and mediates their clustering via a protein kinase A (PKA)-dependent mechanism. Finally, we took advantage of the temperature-sensitive mutant to demonstrate that seizure activity can promote very rapid budding of new boutons filled with SVs, and this process occurs at scale of minutes. Altogether, these results demonstrate that intense activity acutely and selectively promotes rapid budding of new relatively mature presynaptic boutons filled with SVs, and that this process is regulated via a PKA/Syn-dependent pathway.