Title

Neuropathogenic role of adenylate kinase-1 in Aβ-mediated tau phosphorylation via AMPK and GSK3β.

Journal

Human molecular genetics

Volume

21

Issue

None

Pages

2725-37

Date

2012-06-15

Authors

Jung YK | Park H | Koh JY | Gwon Y | Kim C | Kim Y | Kam TI | Choi H

Neuronal expression of AK1 is upregulated in AD patients and is induced by Ab42

Therefore, we examined whether oligomeric forms of Ab42 could regulate AK1 expression in neuronal cells.

From western blot analysis, we found that AK1 expression increased 􏰍2-fold in the cortical neurons after exposure to Ab42

As shown in Figure 5A and B, treatment of control cells with Ab42 reduced the levels of AMPK phosphorylation at Thr172 and the inhibitory phosphorylation of GSK3b at Ser9. On the contrary, these alterations triggered by Ab42 were not observed in SH-SY5Y/AK1 knockdown cells, showing no changes in the levels of the phosphorylated AMPK and GSK3b. These results suggest that AK1 plays a crucial role in the regulation of AMPK and GSK3b in the neuronal cells exposed to Ab42.

A number of studies have found that Ab species, including oligomeric Ab42, are important factors responsible for tau pathogenesis

Interestingly, tau phosphor- ylation, which was detected by PHF-1 (Ser 396/404), CP13 (Ser 202) and 12E8 (Ser 262) antibodies, was also increased by Ab42

Among AK isoforms, AK2 was not detected in the primary neurons and HT22 cells, whereas AK3 expression was observed in the neurons but not regulated by Ab42

When analyzed by reverse transcription-polymerase chain reaction, the level of AK1 mRNA, not AK1b mRNA, increased by Ab42 in primary neurons

Compared with control neurons, the effects of Ab42 on tau phosphorylation at CP13, PHF-1 and AT180 epitopes were significantly ameliorated in AK1 knockdown cortical neurons

Interestingly, treatment with Ab42 decreased the phosphorylation of AMPK at Thr172, whereas the total amount of AMPK was not altered in primary cortical neurons

We found, in addition, that enzyme activity of AMPK was sup- pressed in the cortical neurons after exposure to Ab42

These results indicate that AMPK activity is impaired by Ab42 in the neuronal cells in which tau phosphor- ylation increased.

Consistently, the phosphorylation of acetyl Co-A carboxylase, a well-defined downstream substrate of AMPK, was also reduced by Ab42

Soluble oligomers of Ab42 serve as the prominent synapto- toxic form and induce tau hyperphosphorylation

Conversely, treatment with AICAR (5-aminoimidazole-4- carboxamide ribonucleoside), an AMPK activator (34), increased the inhibitory phosphorylation of GSK3b and reduced tau phosphorylation at Ser396/404 in SH-SY5Y cells and cortical neurons

AK1 expression also increased in neuronal cells which were exposed to oxidative stress but not to other toxic insults, such as proteostasis stressors

AK1 overexpression markedly decreased AMPK phosphorylation at Thr172 in the primary neurons, whereas AK1 R132A mutant failed to do so

The treatment of cortical neurons with compound C, a selective AMPK inhibitor (33), reduced the inhibitory phosphorylation of GSK3b at Ser9 and markedly increased tau phosphorylation at Ser396/404

These observations indicate that AMPK negatively regulates GSK3b activity and tau phosphorylation

the AK1 level was higher in the hippocampus of 9-, 12- and 15-month-old Tg2576 mice expressing Swedish mutant of amyloid precursor protein (hAPP)

AK1 also increased in the hippocampus of APP-J20, an AD model mouse expressing familial AD-mutant APP (Supple- mentary Material, Fig. S1A and B).

Ectopic expression of AK1 in mouse cortical neurons increased immunoreactivity against phosphorylated tau at Ser396/404 (PHF-1), Ser202 (CP13) and Ser231/Thr235 (AT180)

The data suggest that the increased expres- sion of AK1 can enhance tau aggregation as well as tau phos- phorylation.

In addition, we found that transient expression of AK1 (but not AK1 R132A mutant) markedly reduced the inhibitory phosphorylation of GSK3b at Ser9 (Fig. 4D and Supplemen- tary Material, S6A), indicating that AK1 may regulate GSK3b activity.

Co-expression of an AK1 transgene with tau in flies (gl-tau2.1/UAS-AK1) markedly enhanced tau-induced retinal degeneration,indicating that AK1 enhances tau toxicity in the fly retina.

These results indicate that AK1 exacerbates rough eye phenotype and tau hyperpho- sphorylation in a tauopathy model organism and that there is a close correlation between the exacerbated rough eye pheno- type and tau phosphorylation.

With the immunohisto- chemical analysis, we found that the expression levels of AK1 markedly elevated in the NeuN-positive hippocampal neurons of AD patients

Interestingly, the phosphorylated form of tau at Ser 396/404 (PHF-1) was detected exclusively in AD patients, although total amounts of tau protein (TG5) were not changed

Moreover, the phosphorylation of AMPK at Thr172 tended to be reduced in AD patients, although statis- tical significance was marginal

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