Physiological mouse brain Aβ levels are not related to the phosphorylation state of threonine-668 of Alzheimer's APP

Background. Amyloid-β peptide species ending at positions 40 and 42 (Aβ40, Aβ42) are generated by the proteolytic processing of the Alzheimer's amyloid precursor protein (APP). Aβ peptides accumulate in the brain early in the course of Alzheimer's disease (AD), especially Aβ42. The cytoplasmic domain of APP regulates intracellular trafficking and metabolism of APP and its carboxyl-terminal fragments (CTFα, CTFβ). The role of protein phosphorylation in general, and that of the phosphorylation state of APP at threonine-668 (Thr⁶⁶⁸) in particular, has been investigated in detail by several laboratories (including our own). Some investigators have recently proposed that the phosphorylation state of Thr⁶⁶⁸ plays a pivotal role in governing brain Aβ levels, prompting the current study. Methodology. In order to evaluate whether the phosphorylation state of Thr⁶⁶⁸ controlled brain Aβ levels, we studied the levels and subcellular distributions of holoAPP, sAPPα, sAPPβ, CTFα, CTFβ, Aβ40 and Aβ42 in brains from "knock-in" mice in which a non-phosphorylatable alanyl residue had been substituted at position 668, replacing the threonyl residue present in the wild-type protein. Conclusions. The levels and subcellular distributions of holoAPP, sAPPα, sAPPβ, CTFα, CTFβ, Aβ40 and Aβ42 in the brains of Thr⁶⁶⁸Ala mutant mice were identical to those observed in wild-type mice. These results indicate that, despite speculation to the contrary, the phosphorylation state of APP at Thr⁶⁶⁸ does not play an obvious role in governing the physiological levels of brain Ab40 or Aβ42 in vivo.