title: Protein phosphatase 2A methyltransferase links homocysteine metabolism with tau and amyloid precursor protein regulation.
abstract: Alzheimer's disease (AD) neuropathology is characterized by the accumulation of phosphorylated tau and amyloid-beta peptides derived from the amyloid precursor protein (APP). Elevated blood levels of homocysteine are a significant risk factor for many age-related diseases, including AD. Impaired homocysteine metabolism favors the formation of S-adenosylhomocysteine, leading to inhibition of methyltransferase-dependent reactions. Here, we show that incubation of neuroblastoma cells with S-adenosylhomocysteine results in reduced methylation of protein phosphatase 2A (PP2A), a major brain Ser/Thr phosphatase, most likely by inhibiting PP2A methyltransferase (PPMT). PP2A methylation levels are also decreased after ectopic expression of PP2A methylesterase in Neuro-2a (N2a) cells. Reduced PP2A methylation promotes the downregulation of B alpha-containing holoenzymes, thereby affecting PP2A substrate specificity. It is associated with the accumulation of both phosphorylated tau and APP isoforms and increased secretion of beta-secretase-cleaved APP fragments and amyloid-beta peptides. Conversely, incubation of N2a cells with S-adenosylmethionine and expression of PPMT enhance PP2A methylation. This leads to the accumulation of dephosphorylated tau and APP species and increased secretion of neuroprotective alpha-secretase-cleaved APP fragments. Remarkably, hyperhomocysteinemia induced in wild-type and cystathionine-beta-synthase +/- mice by feeding a high-methionine, low-folate diet is associated with increased brain S-adenosylhomocysteine levels, PPMT downregulation, reduced PP2A methylation levels, and tau and APP phosphorylation. We reported previously that downregulation of neuronal PPMT and PP2A methylation occur in affected brain regions from AD patients. The link between homocysteine, PPMT, PP2A methylation, and key CNS proteins involved in AD pathogenesis provides new mechanistic insights into this disorder.
authors.affiliation: Department of Pathology, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA. estelle.sontag@utsouthwestern.edu
authors.forename: Estelle
authors.initials: E
authors.lastname: Sontag
authors.forename: Viyada
authors.initials: V
authors.lastname: Nunbhakdi-Craig
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Sontag
authors.forename: Ramon
authors.initials: R
authors.lastname: Diaz-Arrastia
authors.forename: Egon
authors.initials: E
authors.lastname: Ogris
authors.forename: Sanjana
authors.initials: S
authors.lastname: Dayal
authors.forename: Steven R
authors.initials: SR
authors.lastname: Lentz
authors.forename: Erland
authors.initials: E
authors.lastname: Arning
authors.forename: Teodoro
authors.initials: T
authors.lastname: Bottiglieri
mesh_terms: D016564:Amyloid beta-Protein Precursor / Q000502:physiology*; D000818:Animals; D045744:Cell Line, Tumor; D006710:Homocysteine / Q000235:genetics / Q000378:metabolism*; D008745:Methylation; D008780:Methyltransferases / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D041681:NIH 3T3 Cells; D010749:Phosphoprotein Phosphatases / Q000235:genetics / Q000378:metabolism*; D054648:Protein Phosphatase 2; D016875:tau Proteins / Q000502:physiology*
chemical_list: D016564:Amyloid beta-Protein Precursor; D016875:tau Proteins; D006710:Homocysteine; D008780:Methyltransferases; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2