P1-391: Intracellular dynamics of Alcadein family molecules and their metabolites in neuronal cells and tissues

Abstract

Alcadein family proteins (Alcs) are evolutionarily conserved type I membrane proteins mainly expressed in neuronal tissues. Alcs associate with amyloid β-precursor protein (APP) through their cytoplasmic domains via an adaptor scaffolding protein X11-like (J. Biol. Chem. [2003] 278, 49448). Interestingly, despite their difference in the primary structures, Alcs and APP share common functional and biochemical features; Alc functions as a cargo receptor for kinesin-1 (EMBO J. [2007] 26, 1475), and Alcs undergo highly similar metabolism to those observed in APP. Alcs are subjected to be cleaved at primarily jaxtamembrane region by certain metalloproteinases and secondarily intramembrane region by γ-secretase to produce their ectodomain fragments (sAlc), small peptide fragments (β-Alc) and their intracellular domain fragments (AlcICD). This regulated intramembrane proteolysis (RIP) is very similar to APP-RIP, in which APP secretes sAPP, Aβ (or P3) and AICD. These functional and biochemical similarities suggest the possible availability of Alcs as an alternative biochemical marker for AD diagnosis. To analyze how Alcs behave in neuronal cells, we investigated the intracellular dynamics of Alcs and their metabolites. We expressed series of Alcadein family proteins and their deletion mutants mimicking the AlcICDs in neuroblastoma cells, and observed their intracellular dynamics utilizing several lines of biochemical and cell biological methodologies. Their expressions in mouse brain tissues were also examined. Several lines of evidence indicate that Alcs are transported to cell surface as effectively as APP, and Alcs are efficiently processed to generate AlcICDs that have potential to be localized in the nucleus. AlcICD is relatively stable in nucleus, and cytoplasmic AlcICD is susceptible to degradation by proteasome. In mouse brain, each Alc family members are differently expressed. The amount of AlcICDs in mouse brain increases as with its age, suggesting that Alc-RIP signal might function in maturating and/or aging neural cells. Alcs are efficiently processed as APP, providing additional evidence supporting the idea that Alcs would be an alternative biochemical marker reflecting alterations of APP metabolism in AD.