Inhibition of Protein Phosphatase 2A Overrides Tau Protein Kinase I/Glycogen Synthase Kinase 3β and Cyclin-dependent Kinase 5 Inhibition and Results in Tau Hyperphosphorylation in the Hippocampus of Starved Mouse

Abstract

Hyperphosphorylated tau is the major component of paired helical filaments in neurofibrillary tangles found in Alzheimer's disease (AD) brain. Starvation of adult mice induces tau hyperphosphorylation at many paired helical filaments sites and with a similar regional selectivity as those in AD, suggesting that a common mechanism may be mobilized. Here we investigated the mechanism of starvation-induced tau hyperphosphorylation in terms of tau kinases and Ser/Thr protein phosphatases (PP), and the results were compared with those reported in AD brain. During starvation, tau hyperphosphorylation at specific epitopes was accompanied by decreases in tau protein kinase I/glycogen synthase kinase 3␤ (TPKI/ GSK3␤), cyclin-dependent kinase 5 (cdk5), and PP2A activities toward tau. These results demonstrate that the activation of TPKI/GSK3␤ and cdk5 is not necessary to obtain hyperphosphorylated tau in vivo, and indicate that inhibition of PP2A is likely the dominant factor in inducing tau hyperphosphorylation in the starved mouse, overriding the inhibition of key tau kinases such as TPKI/GSK3␤ and cdk5. Furthermore, these data give strong support to the hypothesis that PP2A is important for the regulation of tau phosphorylation in the adult brain, and provide in vivo evidence in support of a central role of PP2A in tau hyperphosphorylation in AD. Alzheimer's disease (AD)1 is a neurodegenerative disorder characterized by the presence of two histopathological hallmarks called senile plaques and neurofibrillary tangles. The former are deposits of the ␤-amyloid peptide (A␤) (1), whereas neurofibrillary tangles consist of hyperphosphorylated tau protein assembled in paired helical filaments (PHF) (2). Hyperphosphorylation refers to the state that tau is phosphorylated at more sites than tau from adult brain and that, for a given site, a higher than normal percentage of tau molecules is phosphorylated (3). Tau is a microtubule-associated protein, and its normal physiological function is to bind and stabilize microtubules. In vitro studies have shown that PHF-tau fails to promote microtubule assembly (4 -9), and thus it has been proposed to lead to microtubule destabilization, appearance of neurofibrillary tangles, and neurodegeneration in AD brain (10). Phosphorylation of tau can be regulated by many protein kinases and phosphatases in vitro (11-13). These findings, and the changes in kinases and phosphatases observed in AD, suggest that tau hyperphosphorylation in AD brain is likely to be due to an imbalance of the protein phosphorylation and dephosphorylation systems. But to date the mechanism of conversion from normal adult tau to hyperphosphorylated tau, the significance of tau hyperphosphorylation in PHF formation, and its relationship to A␤ deposition remain largely elusive.Tau hyperphosphorylation is a physiological reversible response of the brain to stressful conditions like cold water stress 2 (14), heat-shock (15), or starvation (16). Starvation induces decreases in circulating glucose, insulin, and leptin, and inc...

Alterations in Glucose Metabolism Induce Hypothermia Leading to Tau Hyperphosphorylation through Differential Inhibition of Kinase and Phosphatase Activities: Implications for Alzheimer's Disease

“…Previous reports demonstrating that modulations of glucose or insulin levels alter tau antigenicity in neuronal cultures support this hypothesis (Cheng and Mattson, 1992;Hong and Lee, 1997;Lesort et al, 1999). We also reported that starvation, which decreases the amount of circulating glucose and insulin and lowers glucose utilization (Newsholme, 1976;Cherel et al, 1988), induces AD-like tau hyperphosphorylation in mice (Yanagisawa et al, 1999;Planel et al, 2001).…”

“…Starvation was performed as reported previously (Planel et al, 2001). To investigate the effect of glucose, food was similarly withheld from four mice, but they had free access to a 30% glucose solution.…”

“…Samples were then centrifuged for 20 min at 100,000 ϫ g at 4°C, and protein content of the supernatants was determined. Depending on the antibody used, 7-14 g of protein were analyzed as described previously (Planel et al, 2001).…”

Inhibition of glycogen synthase kinase-3 by lithium correlates with reduced tauopathy and degenerationin vivo

“…1E) in JNPL3 mice (n ϭ 6 for each treatment). PBS was used for control rather than NaCl because our published data have shown that NaCl has no significant effect on tau phosphorylation (29). Furthermore, the use of a second GSK-3 inhibitor strengthened our confidence that the effects seen with LiCl were not due to ionic imbalance.…”

Glucose deprivation increases tau phosphorylation via P38 mitogen‐activated protein kinase

“…We interpret these changes in the activation state of this kinase isoforms as not relevant to the increase in tau phosphorylation we observed under our experimental conditions as it is known that an increase in pGSK3 is an index of a reduced activity of this kinase (Fang et al ., 2000). For this reason, we believe that, as previously reported, the change in pGSK3 in our cells is a generic response to an energy‐deprived condition (Planel et al ., 2001). …”