Amphisomes are organelles of the autophagy pathway that result from the fusion of autophagosomes with late endosomes. While biogenesis of autophagosomes and late endosomes occurs continuously at axon terminals, non-degradative roles of autophagy at boutons are barely described. Here, we show that in neurons BDNF/TrkB traffick in amphisomes that signal locally at presynaptic boutons during retrograde transport to the soma. This is orchestrated by the Rap GTPase-activating (RapGAP) protein SIPA1L2, which connects TrkB amphisomes to a dynein motor. The autophagosomal protein LC3 regulates RapGAP activity of SIPA1L2 and controls retrograde trafficking and local signaling of TrkB. Following induction of presynaptic plasticity, amphisomes dissociate from dynein at boutons enabling local signaling and promoting transmitter release. Accordingly, sipa1l2 knockout mice show impaired BDNF-dependent presynaptic plasticity. Taken together, the data suggest that in hippocampal neurons, TrkB-signaling endosomes are in fact amphisomes that during retrograde transport have local signaling capacity in the context of presynaptic plasticity.

Jacob, the protein encoded by the Nsmf gene, is involved in synapto-nuclear signaling and docks an N-Methyl-D-Aspartate receptor (NMDAR)-derived signalosome to nuclear target sites like the transcription factor cAMP-response-element-binding protein (CREB). Several reports indicate that mutations in NSMF are related to Kallmann syndrome (KS), a neurodevelopmental disorder characterized by idiopathic hypogonadotropic hypogonadism (IHH) associated with anosmia or hyposmia. It has also been reported that a protein knockdown results in migration deficits of Gonadotropin-releasing hormone (GnRH) positive neurons from the olfactory bulb to the hypothalamus during early neuronal development. Here we show that mice that are constitutively deficient for the Nsmf gene do not present phenotypic characteristics related to KS. Instead, these mice exhibit hippocampal dysplasia with a reduced number of synapses and simplification of dendrites, reduced hippocampal long-term potentiation (LTP) at CA1 synapses and deficits in hippocampus-dependent learning. Brain-derived neurotrophic factor (BDNF) activation of CREB-activated gene expression plays a documented role in hippocampal CA1 synapse and dendrite formation. We found that BDNF induces the nuclear translocation of Jacob in an NMDAR-dependent manner in early development, which results in increased phosphorylation of CREB and enhanced CREB-dependent Bdnf gene transcription. Nsmf knockout (ko) mice show reduced hippocampal Bdnf mRNA and protein levels as well as reduced pCREB levels during dendritogenesis. Moreover, BDNF application can rescue the morphological deficits in hippocampal pyramidal neurons devoid of Jacob. Taken together, the data suggest that the absence of Jacob in early development interrupts a positive feedback loop between BDNF signaling, subsequent nuclear import of Jacob, activation of CREB and enhanced Bdnf gene transcription, ultimately leading to hippocampal dysplasia.

Since aggregates of the microtubule‐binding protein tau were found to be the main component of neurofibrillary tangles more than 30 years ago, their contribution to neurodegeneration in Alzheimer's disease (AD) and tauopathies has become well established. Recent work shows that both tau load and its distribution in the brain of AD patients correlate with cognitive decline more closely compared to amyloid plaque deposition. In addition, the amyloid cascade hypothesis has been recently challenged because of disappointing results of clinical trials designed to treat AD by reducing beta‐amyloid levels, thus fuelling a renewed interest in tau. There is now robust evidence to indicate that tau pathology can spread within the central nervous system via a prion‐like mechanism following a stereotypical pattern, which can be explained by the trans‐synaptic inter‐neuronal transfer of pathological tau. In the receiving neuron, tau has been shown to take multiple routes of internalisation, which are partially dependent on its conformation and aggregation status. Here, we review the emerging mechanisms proposed for the uptake of extracellular tau in neurons and the requirements for the propagation of its pathological conformers, addressing how they gain access to physiological tau monomers in the cytosol. Furthermore, we highlight some of the key mechanistic gaps of the field, which urgently need to be addressed to expand our understanding of tau propagation and lead to the identification of new therapeutic strategies for tauopathies.

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Using synaptosomes purified from the brains of two transgenic mouse models overexpressing mutated human tau (TgP301S and Tg4510) and brains of patients with sporadic Alzheimer's disease, we showed that aggregated and hyperphosphorylated tau was both present in purified synaptosomes and released in a calcium-and SNAP25-dependent manner. In all mouse and human synaptosomal preparations, tau release was inhibited by the selective mGlu2/3 receptor agonist LY379268, an effect prevented by the selective mGlu2/3 antagonist LY341495. LY379268 was also able to block pathological tau propagation between primary neurons in an in vitro microfluidic cellular model. These novel results are transformational for our understanding of the molecular mechanisms mediating tau release and propagation at synaptic terminals in Alzheimer's disease and suggest these processes could be inhibited therapeutically by the selective activation of presynaptic G-protein-coupled receptors.

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...