The generation of amyloid-β (Aβ) peptides through proteolytic processing of the amyloid precursor protein (APP) is a key pathogenic event in Alzheimer’s disease (AD). Aβ generation begins with APP endocytosis, which is mediated by the endocytic YENPTY sequence located in the cytoplasmic tail of APP. Mints, a family of cytosolic adaptor proteins, directly bind to the YENPTY motif of APP and facilitate APP endocytosis and amyloidogenic processing. In addition, loss of any one of the three Mint proteins decreases Aβ production in aging mouse models of AD, supporting the hypothesis that the APP-Mint interaction may provide a novel therapeutic target to selectively reduce Aβ production in AD. Characterizing the biochemical and cellular dynamics of the APP-Mint interaction is critical for understanding Aβ generation. Thus, we generated Mint1 mutants that bind with high affinity (Mint1Y633A) or low affinity (Mint1Y549A/F610A) to APP. These Mint1 mutants exhibited profound alterations in cellular localization, APP endocytosis, and Aβ production. Therapeutically, we generated a novel cell-permeable APP mimetic peptide (APPMP) that interferes with the APP-Mint interaction. This APPMP was designed to outcompete endogenous APP binding, with a 46-fold improved affinity to Mint. Treatment of primary neurons from an AD mouse model with several cell permeable APPMP variants reduced Aβ production with minimal cellular toxicity, supporting Mints as a promising novel therapeutic target for AD. The PTB domain of Mint1 that mediates APP binding is autoinhibited by an adjacent C-terminal α-helix. However, the molecular mechanisms underlying the relief of Mint1 autoinhibition are unclear. Since post-translational modification is one mechanism for alleviating protein autoinhibition, and Mint1 is highly regulated by phosphorylation, we performed mass spectrometry and identified several Mint1 phosphosites. In addition, we found constitutively-active Src kinase, a kinase implicated in Mint phosphorylation, enhanced APP-Mint1 binding. These results suggest that Src kinase-mediated phosphorylation of Mint1 may relieve Mint1 autoinhibition and promote APP-Mint1 interaction. Overall, this work biochemically characterized the Mint-APP interaction and how it affects amyloidogenic processing, provided a proof of concept for targeting the APP-Mint1 interaction as an AD therapeutic target, and suggested a novel mechanism for the relief of Mint1 autoinhibition.

Th The kinetics of exocytotic secretion depend not only on the spatial relationship between calcium channels and the exocytotic apparatus, but also on the total amount of Ca2+ influx through Ca2+ channels, the free Ca2+ around the release site and the filling state of the release-ready vesicles. These factors may differ between neurons and endocrine cells. Bovine chromaffin cells (BCCs) are neuroendocrine cells responsible for catecholamine release from the adrenal glands. Ca2+ imaging experiments have shown that localized zones of Ca2+ influx exist on BCC membranes, but how different Ca2+ channel subtypes are distributed, and the mechanisms by which they are targeted, remain to be elucidated. CASK (calcium, calmodulin associated serine kinase) and Mint1 (Munc-18-interacting protein 1), which are modular adaptor proteins involved in synaptic targeting, have recently been found to function in targeting of á1B Ca2+ channels in hippocampal neurons. These data led to the proposal that Ca2+ channels are clustered in BCCs and that CASK and Mint1 play important roles in targeting and/or anchoring channels to their proper location. p*Using RT-PCR and Western blotting, CASK is demonstrated present in isolated BCCs. Mint1 is shown to be present by Western blotting as well. Immunocytochemical experiments and experiments in which BCCs were transfected with plasmids expressing á1A, á1B, and á1C subunits labeled with green fluorescent protein, have shown that á1A and á1B subunits are clustered on the plasma membranes of BCCs, while the á1C subunit is distributed in diffuse patches. With immunoprecipitation, it was determined that CASK interacts biochemically with á1A and á1B Ca2+ channels. Transfection of BCCs with NC3-GFP, which codes for the sequence of the á1B Ca2+ channel that interacts with CASK and Mint1, results in a punctate pattern of fluorescence, which is consistent with the binding of GFP labeled peptide to complexes of CASK and Mint1 at sites of release. Furthermore, immunocytochemical analysis of cells transfected with NC3-GFP showed that á1B Ca2+ channels have a dispersed distribution suggesting that they have been displaced from the binding sites. These data suggest that CASK and Mint1 are important in clustering and targeting Ca2+ channels in the BCC plasma membrane. This study is the first to show the existence and function of CASK and Mint1 in BCCs, and may contribute to our understanding of the exocytotic process in neuroendocrine cells

Th The kinetics of exocytotic secretion depend not only on the spatial relationship between calcium channels and the exocytotic apparatus, but also on the total amount of Ca2+ influx through Ca2+ channels, the free Ca2+ around the release site and the filling state of the release-ready vesicles. These factors may differ between neurons and endocrine cells. Bovine chromaffin cells (BCCs) are neuroendocrine cells responsible for catecholamine release from the adrenal glands. Ca2+ imaging experiments have shown that localized zones of Ca2+ influx exist on BCC membranes, but how different Ca2+ channel subtypes are distributed, and the mechanisms by which they are targeted, remain to be elucidated. CASK (calcium, calmodulin associated serine kinase) and Mint1 (Munc-18-interacting protein 1), which are modular adaptor proteins involved in synaptic targeting, have recently been found to function in targeting of á1B Ca2+ channels in hippocampal neurons. These data led to the proposal that Ca2+ channels are clustered in BCCs and that CASK and Mint1 play important roles in targeting and/or anchoring channels to their proper location. p*Using RT-PCR and Western blotting, CASK is demonstrated present in isolated BCCs. Mint1 is shown to be present by Western blotting as well. Immunocytochemical experiments and experiments in which BCCs were transfected with plasmids expressing á1A, á1B, and á1C subunits labeled with green fluorescent protein, have shown that á1A and á1B subunits are clustered on the plasma membranes of BCCs, while the á1C subunit is distributed in diffuse patches. With immunoprecipitation, it was determined that CASK interacts biochemically with á1A and á1B Ca2+ channels. Transfection of BCCs with NC3-GFP, which codes for the sequence of the á1B Ca2+ channel that interacts with CASK and Mint1, results in a punctate pattern of fluorescence, which is consistent with the binding of GFP labeled peptide to complexes of CASK and Mint1 at sites of release. Furthermore, immunocytochemical analysis of cells transfected with NC3-GFP showed that á1B Ca2+ channels have a dispersed distribution suggesting that they have been displaced from the binding sites. These data suggest that CASK and Mint1 are important in clustering and targeting Ca2+ channels in the BCC plasma membrane. This study is the first to show the existence and function of CASK and Mint1 in BCCs, and may contribute to our understanding of the exocytotic process in neuroendocrine cells