SIPA1L2 controls trafficking and signaling of TrkB-containing amphisomes at presynaptic terminals

Abstract

2 SummaryAmphisomes are transient organelles that derive from fusion of autophagosomes with late endosomes. They rapidly transform into degradative autolysosomes, whereas non-degradative roles of the autophagic pathway have been barely described. Here we show that in neurons BDNF/TrkB receptor bearing Rab7 / Light chain 3 (LC3) -positive amphisomes signal at presynaptic boutons during retrograde trafficking to the soma. Local signaling and inward transport essentially require the Rap GTPase-activating (RapGAP) protein SIPA1L2, which directly binds to TrkB and Snapin to connect TrkB-containing amphisomes to dynein.Association with LC3 regulates the RapGAP activity of SIPA1L2 and thereby retrograde trafficking. Following induction of presynaptic plasticity amphisomes dissociate from dynein at boutons, and this enables local signaling and promotes transmitter release. Accordingly, sipa1l2 knockout mice show impaired BDNF-dependent presynaptic plasticity. Collectively, the data suggest that TrkB-signaling endosomes are in fact amphisomes that during retrograde transport have local signaling capacity in the context of presynaptic plasticity.3 TrkB signaling endosomes that carry axonal neurotrophic signals are generated at axon terminals and constitute an important long-range retrograde signaling mechanism conveying information of synaptic activity (1). Upon binding to BDNF, BDNF/TrkB have been shown to be internalized either via pinocytosis (2,3) or in a clathrin-dependent manner (4) to specialized vesicular compartments and sorted to diverse pathways by yet unknown mechanisms. TrkB signaling is required for different aspects of neuronal function including the expression of presynaptic LTP at mossy fiber (MF) synapses (5). By binding to different adaptor proteins, TrkB activates three major molecular cascades: the Ras-MAPK pathway, the phosphatidyl inositol-3 (PI3)-kinase cascade and the phospholipase C-γ1 pathway (6).Rap-1 based signaling ensures sustained ERK activation since prenylated Rap1 is associated to TrkB containing endosomes. These are long-lived and persist during transport from nerve terminals to the cell soma (7, 8 9,10). SIPA1L2 (also known as SPAR2) is a member of the SIPA1L family of neuronal RapGAPs ( Fig. S1) (11). The protein is most abundant in granule cells of the dentate gyrus (DG) and cerebellum and shows RapGAP activity for the small GTPases Rap1 and 2 (12). Here, we report that SIPA1L2 binds directly to TrkB, and links the receptor tyrosine kinase to a Dynein motor for retrograde trafficking via a direct interaction with the adaptor protein Snapin.Interestingly, SIPA1L2 concurrently associates via LC3b to Rab7-positive amphisomes and binding of LC3b promotes RapGAP activity. The amphisome trafficks retrogradely along axons, it stops at presynaptic boutons and both motility and signaling are controlled by SIPA1L2 whose RapGAP activity reduces the velocity of amphisome transport. Presynaptic long-term potentiation (LTP) induces a Protein kinase A (PKA)-dependent dissociation of the SIPA1L2/Snap...

Impairment of autophagy-lysosomal activity near the chronically implanted microelectrodes

“…Similar to lysosomes, autophagosomes can fuse with late endosomes at axonal terminals, forming an intermediate organelle called amphisome [127]. Amphisomes are involved in TrkB signaling and local activation of ERK1/2 signaling at boutons for neurotransmitter release [128]. The autophagy marker LC3B can be recruited to low pH amphisome, which may express RFP-only signals [66].…”

Brain‐specific functions of the endocytic machinery

“…Following the endocytosis at distal axons, activated TRK complexes are internalized into a so-called "signaling endosome", an organelle of endosome/amphisome-like identity, which continues to signal moving in a microtubule-mediated manner to the cell body [288,[291][292][293][294][295]. Via its non-canonical function, endocytosis contributes not only to the internalization of activated TRK complexes into signaling endosomes but can also regulate their intracellular trafficking (for more details please see [237]).…”