Reduced early hypoxic/ischemic brain damage is associated with increased GLT-1 levels in mice expressing mutant (P301L) human tau

Abstract

Mutations in tau proteins are associated with a group of neurodegenerative diseases, termed tauopathies. To investigate whether over-expressing human tau with P301L mutation also affects stroke-induced brain damage, we performed hypoxia/ischemia (H/I) in young adult P301L tau transgenic mice. Surprisingly, brain infarct volume was significantly smaller in transgenic mice compared to wild-type mice 24 h after H/I induction. TUNEL staining also revealed less brain apoptosis in transgenic mice following H/I. H/I resulted in a significant increase in tau fragments generated by caspase activation and a marked decrease in tau phosphorylation at residue T231 in cortex of wild-type but not transgenic mice. Activation of calpain and caspase-3 following H/I was also reduced in transgenic compared to wild-type mice, as reflected by lower levels of the specific spectrin breakdown products generated by calpain or caspase-3. Finally, basal levels of the glial glutamate transporter, GLT-1, were higher in brains of transgenic as compared to wild-type mice. These results support the idea that enhanced levels of GLT-1 in transgenic mice are responsible for reducing H/I-induced brain damage by decreasing extracellular glutamate accumulation and subsequent calpain and caspase activation.

Introduction

Stroke and dementia are leading neurological disorders in the elderly population. Epidemiological studies have revealed a close correlation between ischemia and Alzheimer's disease-(AD) type dementia, as more than half of AD patients exhibit cerebrovascular pathology at autopsy, a coincidence that is more than by chance alone (Kalaria, 2000). Animal studies indicate that cerebral ischemia upregulates the expression of the amyloid precursor protein and enhances the production and accumulation of the amyloid β-peptide in adult rats (Nihashi et al., 2001, Shi et al., 2000). Ischemic insults also affect tau proteins, which are the main components of neurofibrillary tangles, as indicated by a recent study showing that transient cerebral ischemia induces tau hyperphosphorylation in wildtype mice (Wen et al., 2004). However, whether predisposition conditions for AD or for dementia due to tauopathies affect stroke-induced brain damage has not been thouroughly investigated. Recently, a trangenic mouse model (hereafter referred to as P301L tau trangenic mice) overexpressing human tau with a mutation (P301L) found in frontotemporal dementia patients has been developed (Lewis et al., 2000). These mice exhibit neurofibrillary tangles in neurons of amygdala, hypothalamus, brainstem and spinal cord, and pre-tangles in hippocampus and cortex. This trangenic model thus provides a unique opportunity to investigate whether ischemia insults exacerbate tauopathies and whether overexpression of mutant tau affects ischemia-induced brain damage. This study represents the first attempt to answer these questions.

Numerous reports have indicated that spreading of neurological damage following ischemia may result from a process known as excitotoxicity, in which neurons die as a consequence of overstimulation of NMDA receptors due to excessive glutamate release (Choi, 1988, Nakanishi, 1992, Olney et al., 1986). Stimulation of NMDA receptors results in widespread increases in cytosolic calcium, which in turn activates cytotoxic cascades (Arundine and Tymianski, 2003, Choi, 1995), including the activation of calcium-dependent proteases, calpains (Czogalla and Sikorski, 2005), and apoptotic protease caspases (Love, 2003, Mattson, 2000). In addition to neuronal glutamate transporters, glial specific glutamate transporters, glutamate transporter 1 [GLT-1, also known as excitatory amino acid transporter 2 (EAAT2)] and glutamate–aspartate transporter (GLAST, also known as EAAT1) regulate extracellular glutamate levels under normal and pathological conditions (Rossi et al., 2007, Schousboe and Waagepetersen, 2006, Sheldon and Robinson, 2007, Yi and Hazell, 2006). Because excitotoxicity has been proposed to be a major contributor to hypoxia/ischemia-induced neuronal death, the present study also investigated the expression of GLT-1 and its potential relationship to hypoxia/ischemia-induced brain damage in tau transgenic mice.