P4-215: Mitochondrial transmembrane protein modulates APP processing

Abstract

Amyloid beta peptides (Abeta) are thought to be generated in mainly in cell surface, endosome and Trans-Golgi network by presenilin complexes with gamma-secretase activities. However, significant amount of Abeta is produced in other subcellular compartments, like early secretary pathway or mitochondria. Gamma-cleavage of amyloid precursor protein in early secretary pathway more preferentially occurs at the position 42 to generate Abeta42 that is toxic to neurons. Thus, in addition to the secreted Abeta, intracellular Abeta may play an important role in the neuronal cell death in Alzheimer's disease. Mitochondria is reported to have essential components of gamma-secretase complexes, presenilin, nicastrin, APH-1 and PEN-2 and work as a mature enzymatic complex in vitro. However, the detail remains unknown.Mitochondria are one of the vulnerable organelles to aging. A lot of proteins in mitochondria are modified and differently translated by the long-lasting stresses, such as an oxidative stress. In addition, recent paper showed the importance of the modulator protein, TMP21, in gamma-secretase complexes. Thus, we tried to find a mitochondrial protein that modulates gamma-secretase activities. Modulator activities of mitochondrial proteins in generation were screened by the knock-down of specific proteins using a siRNA method and by over-expression. To clarify the mechanism of the modulation, the interaction with the components of presenilin complexes, gamma- and epsilon-secretase activities were assessed. One mitochondrial protein was identified to positively modulate secretase activities. Over-expression of the protein showed more than fourfold increase in both Abeta40 and Abeta42 generated from the whole cells. AICD generation was also increased using a reporter assay. The artificial tagging at the C-terminus of the protein loses the modulating functions, suggesting that C-terminus is likely to be essential to the modulating function. Direct interaction with the components of presenilin complexes was not detected. Moreover, in vitro gamma-secretase assay showed no significant modulation. This mitochondrial protein indirectly modulates APP processing through the C-terminus.