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The Journal of Neuroscience, March 14, 2007, 27(11):2751-2759; doi:10.1523/JNEUROSCI.3316-06.2007

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Neurobiology of Disease
Protein Phosphatase 2A Methyltransferase Links Homocysteine Metabolism with Tau and Amyloid Precursor Protein Regulation

Estelle Sontag,¹ Viyada Nunbhakdi-Craig,¹ Jean-Marie Sontag,¹ Ramon Diaz-Arrastia,² Egon Ogris,³ Sanjana Dayal,⁴ Steven R. Lentz,⁴ Erland Arning,⁵ and Teodoro Bottiglieri⁵

¹Departments of Pathology and ²Neurology, University of Texas Southwestern Medical Center, Dallas, Texas 75390, ³Department of Medical Biochemistry, Max F. Perutz Laboratories, Medical University of Vienna, A-1030 Vienna, Austria, ⁴Department of Internal Medicine, University of Iowa and Veterans Affairs Medical Center, Iowa City, Iowa 52242, and ⁵Institute of Metabolic Disease, Baylor University Medical Center, Dallas, Texas 75226

Correspondence should be addressed to Dr. Estelle Sontag, Department of Pathology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-9073. Email: estelle.sontag{at}utsouthwestern.edu

Alzheimer's disease (AD) neuropathology is characterized by the accumulation of phosphorylated tau and amyloid-ß 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-containing holoenzymes, thereby affecting PP2A substrate specificity. It is associated with the accumulation of both phosphorylated tau and APP isoforms and increased secretion of ß-secretase-cleaved APP fragments and amyloid-ß 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 -secretase-cleaved APP fragments. Remarkably, hyperhomocysteinemia induced in wild-type and cystathionine-ß-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.

Key words: Alzheimer's disease; cystathionine-ß-synthase; folate; homocysteine; methylation; PP2A

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Received Aug. 1, 2006; revised Dec. 29, 2006; accepted Jan. 30, 2007.

Correspondence should be addressed to Dr. Estelle Sontag, Department of Pathology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-9073. Email: estelle.sontag{at}utsouthwestern.edu

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