Folate Deficiency InducesIn Vitroand Mouse Brain Region-Specific Downregulation of Leucine Carboxyl Methyltransferase-1 and Protein Phosphatase 2A Bα Subunit Expression That Correlate with Enhanced Tau Phosphorylation

Abstract

Altered folate homeostasis is associated with many clinical and pathological manifestations in the CNS. Notably, folate-mediated onecarbon metabolism is essential for methyltransferase-dependent cellular methylation reactions. Biogenesis of protein phosphatase 2A (PP2A) holoenzyme containing the regulatory B␣ subunit, a major brain tau phosphatase, is controlled by methylation. Here, we show that folate deprivation in neuroblastoma cells induces downregulation of PP2A leucine carboxyl methyltransferase-1 (LCMT-1) expression, resulting in progressive accumulation of newly synthesized demethylated PP2A pools, concomitant loss of B␣, and ultimately cell death. These effects are further accentuated by overexpression of PP2A methylesterase (PME-1) but cannot be rescued by PME-1 knockdown. Overexpression of either LCMT-1 or B␣ is sufficient to protect cells against the accumulation of demethylated PP2A, increased tau phosphorylation, and cell death induced by folate starvation. Conversely, knockdown of either protein accelerates folate deficiencyevoked cell toxicity. Significantly, mice maintained for 2 months on low-folate or folate-deficient diets have brain-region-specific alterations in metabolites of the methylation pathway. Those are associated with downregulation of LCMT-1, methylated PP2A, and B␣ expression and enhanced tau phosphorylation in susceptible brain regions. Our studies provide novel mechanistic insights into the regulation of PP2A methylation and tau. They establish LCMT-1-and B␣-containing PP2A holoenzymes as key mediators of the role of folate in the brain. Our results suggest that counteracting the neuronal loss of LCMT-1 and B␣ could be beneficial for all tauopathies and folate-dependent disorders of the CNS.

Does Inorganic Mercury Play a Role in Alzheimer's Disease? A Systematic Review and an Integrated Molecular Mechanism

“…PP2A is responsible for de-phosphorylation of tau and a decrease in its activity leads to tau hyperphosphorylation and formation of NFTs. Methylation of the catalytic subunit of PP2A, increases its activity and decreases tau phosphorylation, while folate-deficiency, which lowers methionine synthase activity, has the opposite effect [184]. Reduced PP2A activity also increases Aβ production, so impaired methylation can contribute to both NFTs and amyloid plaque formation [182].…”

PP2A methylation controls sensitivity and resistance to β-amyloid–induced cognitive and electrophysiological impairments

“…8 and 9). Impaired methyl-donor metabolism is a risk factor for AD (10, 11), and PP2A dysregulation caused by impaired methylation is thought to be one of the molecular mechanisms contributing to this increased risk (12)(13)(14). Methylation promotes the formation of PP2A holoenzymes that contain Bα regulatory subunits (7,13,(15)(16)(17)(18)(19), and these forms of PP2A exhibit the greatest tau phosphatase activity (6, 7).…”

The Role of PP2A Methylation in Susceptibility and Resistance to TBI and AD-Induced Neurodegeneration

“…The data we obtained from our analysis are consistent with impaired PP2A methylation contributing to the increased AD risk observed in hyperhomocysteinemic individuals, and suggest that reduced PP2A methylation may do so by altering the susceptibility of these individuals to the pathological effects of elevated Aβ concentrations. These results, however, do not exclude the possible contribution of additional downstream effects of elevated homocysteine in increasing AD risk (Fleming et al, 2012;Fuso et al, 2012a;Marlatt et al, 2008;Sontag et al, 2007;Sontag et al, 2008;Troen et al, 2008;Wei et al, 2011;Zhang et al, 2009;Zhuo et al, 2010). They also do not exclude the possibility that PME-1 over expression may affect PP2A activity and Aβ sensitivity via a mechanism that is independent of PME-1's methylesterase activity .…”

“…Elevated homocysteine levels in the blood or CSF of human subjects has been linked to an increased risk for AD (reviewed in : Zhuo et al, 2011(Zhuo et al, 2011, and this link between elevated levels of homocysteine and AD-like pathology or cognitive impairment has been reproduced in cell and animal models (Bernardo et al, 2007;Fuso et al, 2012b;Hasegawa et al, 2005;Kruman et al, 2002;Pacheco-Quinto et al, 2006;Rhodehouse et al, 2013;Sontag et al, 2007;Wei et al, 2011;Zhang et al, 2009;Zhuo et al, 2010;Zhuo and Pratico, 2010a, b). Several potential mechanisms have been proposed to explain the link between hyperhomocysteinemia and AD including oxidative stress, cerebrovascular damage, altered DNA methylation, Aβ elevation and tau protein phosphorylation (Fleming et al, 2012;Fuso et al, 2012a;Marlatt et al, 2008;Sontag et al, 2007;Sontag et al, 2008;Troen et al, 2008;Wei et al, 2011;Zhang et al, 2009;Zhuo et al, 2010). Homocysteine is a metabolite in the biochemical pathway that produces S-adenosly methionine, which acts as the donor for methylation reactions in the cell, so one of the consequences of hyperhomocysteinemia is impaired methylation (Schalinske and Smazal, 2012).…”

“…PP2A is also the principal tau phosphatase, and tau hyperphosphorylation is also thought to play a critical role in AD (Martin et al, 2013). Manipulations that impair homocysteine metabolism both reduce PP2A methylation and increase tau phosphorylation suggesting that one of the ways that PP2A might link homocysteine metabolism to AD is via its action as a tau phosphatase (Nicolia et al, 2010;Sontag et al, 2007;Sontag et al, 2008;Yoon et al, 2007;Zhang et al, 2008).…”