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The intracellular trafficking and proteolytic processing of the membrane-bound amyloid precursor protein (APP) are coordinated events leading to the generation of pathogenic amyloid-beta (Aβ) peptides. The membrane transport of newly synthesized APP from the Golgi to the endolysosomal system is not well defined, yet it is likely to be critical for regulating its processing by β-secretase (BACE1) and γ-secretase. Here, we show that the majority of newly synthesized APP is transported from the trans-Golgi network (TGN) directly to early endosomes and then subsequently to the late endosomes/lysosomes with very little transported to the cell surface. We show that Arl5b, a small G protein localized to the TGN, and AP4 are essential for the post-Golgi transport of APP to early endosomes. Arl5b is physically associated with AP4 and is required for the recruitment of AP4, but not AP1, to the TGN. Depletion of either Arl5b or AP4 results in the accumulation of APP, but not BACE1, in the Golgi, and an increase in APP processing and Aβ secretion. These findings demonstrate that APP is diverted from BACE1 at the TGN for direct transport to early endosomes and that the TGN represents a site for APP processing with the subsequent secretion of Aβ., (© 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

The recruitment of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors underlies the strengthening of neuronal connectivity during learning and memory. This process is triggered by N-methyl-D-aspartate (NMDA) receptor-dependent postsynaptic Ca2+ influx. Synaptotagmin (Syt)-1 and -7 have been proposed as Ca2+ sensors for AMPA receptor exocytosis but are functionally redundant. Here, we identify a cytosolic C2 domain-containing Ca2+-binding protein, Copine-6, that forms a complex with AMPA receptors. Loss of Copine-6 expression impairs activity-induced exocytosis of AMPA receptors in primary neurons, which is rescued by wild-type Copine-6 but not Ca2+-binding mutants. In contrast, Copine-6 loss of function does not affect steady-state expression or tetrodotoxin-induced synaptic upscaling of surface AMPA receptors. Loss of Syt-1/Syt-7 significantly reduces Copine-6 protein expression. Interestingly, overexpression of wild-type Copine-6, but not the Ca2+-binding mutants, restores activity-dependent exocytosis of AMPA receptors in Syt-1/Syt-7 double-knockdown neurons. We conclude that Copine-6 is a postsynaptic Ca2+ sensor that mediates AMPA receptor exocytosis during synaptic potentiation. [Display omitted] • Copine-6 binding to Ca2+ augments its interaction with GluA1 during synaptic potentiation • Copine-6 translocation to dendritic spines and recycling endosomes requires Ca2+ binding • Copine-6 binding to Ca2+ is required for glycine-induced GluA1 exocytosis • Copine-6 overexpression rescues GluA1 exocytosis in Syt-1/Syt-7 knockdown neurons Tan and Jang et al. identify an interaction between the GluA1 subunit of AMPA receptors with the neuron-specific C2 domain-containing protein Copine-6 that is enriched in the somatodendritic compartment. The binding of Copine-6 to calcium mediates activity-dependent insertion of GluA1-containing AMPA receptors to the plasma membrane during synaptic potentiation. [ABSTRACT FROM AUTHOR]