Tau phosphorylation in the mouse brain during aversive conditioning

Abstract

Stress response is intimately involved in memory formation. Stress has been shown to cause reversible Alzheimer-like tau phosphorylation in the brain of experimental animals, but it is not known whether tau phoshorylation takes place during memory acquisition. As an initial investigation we chose contextual fear conditioning paradigm involving electric shocks, and studied tau phosphorylation in the hippocampus and a neighboring limbic region of the mouse brain. Quantitative immunoblot analyses of tissue extracts rapidly prepared from animals undergoing the conditioning showed statistically significant increases in the phosphorylation level at Thr231/Ser235 of tau in both tissues. The reaction reached statistical significance after 10 but not 3 shocks of 0.8 mA. Ten shocks of 0.2 mA were ineffective. Concurrent increases in phosphorylation of protein kinase TPKI/GSK3β at Ser9 and of CaMKIIα at Thr286 were observed. These results suggest involvement of tau and TPKI/GSK3β phosphorylation in an early phase of memory formation in the hippocampus and amygdala, raising a possibility that a dysregulation of tau phosphorylation may underlie memory impairment in incipient Alzheimer's disease.

Introduction

A large effort by many laboratories has been made studying multiple phosphorylation of tau protein in the hope of understanding the etiology of Alzheimer's disease (AD) (Grundke-Iqbal et al., 1986, Goedert, 1996). Tau phosphorylation has turned out to be a highly complex phenomenon involving more than 20 sites on tau, a number of protein kinases and protein phosphatases (Imahori and Uchida, 1997, Gong et al., 2000). However, its relationship to the pathogenesis and memory impairment of AD still remains unclear.

Recent demonstration of reversible phosphorylation of tau in the hippocampus and other brain regions of wild-type mice during starvation (Yanagisawa et al., 1999), as well as after forced cold water swimming or ether exposure (Okawa et al., 2003, Ikeda et al., 2007) indicated that tau phosphorylation is a normal process associated with the stress reaction (Ikeda et al., 2007). Stress-induced tau phosphorylation was observed at several serine and threonine sites on tau that are among those phosphorylated in AD brains. The reaction was robust in the hippocampus and weak in the cerebellum in correspondence with the known regional difference in susceptibility to tangle pathology in AD brains (Braak and Braak, 1991), prompting us to suspect a possible relationship between this physiological phenomenon and the etiology of AD.

Incipient AD manifests as deficits in recent episodic memory (Sclan, 1995), a function particularly associated with the hippocampal formation. It has been known that stress hormones play critical roles in establishing memory (McEwen, 1999, McGaugh, 2004), and certain severe and chronic forms of stress can damage hippocampal neurons (Sapolsky, 1994). We were thus led to studying tau's possible role in memory formation, by first determining whether tau phosphorylation takes place in the hippocampus and related brain regions during an experimental paradigm of learning. Footshock fear conditioning was chosen, because it is a widely used and established experimental paradigm for studying contextual memory in rodents, in which robust memory can be established within a short defined time.