Amphisome biogenesis couples synaptic autophagy to local protein synthesis

Summary Amphisomes are hybrid organelles that result from fusion of late endosomes with autophagosomes. Here, we report that amphisomes containing BDNF/TrkB and capable of signaling are formed at presynaptic boutons following high-frequency stimulation. Activity dependent bulk-endocytosis serves as a membrane source and autophagy initiation is induced by the energy-sensing enzyme AMPK. Intense neuronal transmission not only induces high membrane exchange and metabolic demands, but it also causes the remodeling of presynaptic molecular composition. We show that formation of BDNF/TrkB signaling amphisomes contribute to the fast turnover of key presynaptic proteins that constitute the synaptic vesicle cluster. Following sustained synaptic activity newly-formed amphisomes enable their degradation as cargo on the one hand and their replenishment by translation of mRNA localized at synaptic boutons on the other. We propose that activity-induced synaptic autophagy to a large degree represents amphisome formation that in turn is required to replace cytomatrix proteins at presynaptic sites.