J Neurochem. 2015 Oct;135(2):381-94.

Riluzole rescues glutamate alterations, cognitive deficits, and tau pathology associated with P301L tau expression.

Hunsberger HC1, Weitzner, DS1, Rudy, CC1, Hickman, JE1, Libell, EM2, Speer, RR2, Setti, SE3, Gerhardt, GA4, Reed, MN1,3.

 1Behavioral Neuroscience, Department of Psychology, West Virginia University, Morgantown, West Virginia, USA

2Department of Biology, West Virginia University, Morgantown, West Virginia, USA

3 Drug Discovery and Development, School of Pharmacy, Auburn University, Auburn, AL, USA

4Center for Microelectrode Technology (CenMeT), Department of Anatomy and Neurobiology, University of Kentucky Health Sciences Center, Lexington, Kentucky, USA

 

Abstract

Hyperexcitability of the hippocampus is a commonly observed phenomenon in the years preceding a diagnosis of Alzheimer’s disease (AD). Our previous work suggests a dysregulation in glutamate neurotransmission may mediate this hyperexcitability, and glutamate dysregulation correlates with cognitive deficits in the rTg(TauP301L)4510 tau mouse model of AD. To determine whether improving glutamate regulation would attenuate cognitive deficits and AD-related pathology, TauP301L mice were treated with riluzole (~ 12.5 mg/kg/day p.o.), an FDA-approved drug for amyotrophic lateral sclerosis that lowers extracellular glutamate levels. Riluzole-treated TauP301L mice exhibited improved performance in the memory tasks (water radial arm maze and the Morris water maze), associated with a decrease in glutamate release and an increase in glutamate uptake in the dentate gyrus, cornu ammonis 3 (CA3), and cornu ammonis 1 (CA1) regions of the hippocampus. Riluzole also attenuated the TauP301L-mediated increase in hippocampal vesicular glutamate transporter 1, which packages glutamate into vesicles and influences glutamate release, and the TauP301L-mediated decrease in hippocampal glutamate transporter 1, the major transporter responsible for removing glutamate from the extracellular space. The TauP301L-mediated reduction in PSD-95 expression, a marker of excitatory synapses in the hippocampus, was also rescued by riluzole. Riluzole treatment reduced total levels of tau, as well as the pathological phosphorylation and conformational changes in tau associated with the P301L mutation. These findings open new opportunities for the development of clinically applicable therapeutic approaches to regulate glutamate in vulnerable circuits for those at risk for the development of AD.

PMID: 26146790

 

Supplement:

Alzheimer’s disease (AD), the most common form of dementia, progressively impairs memory, cognitive performance, and the ability to function independently. Biologically, AD is characterized by three features upon autopsy. These characteristics include plaques consisting of beta-amyloid protein, tangles consisting of tau protein, and loss of the neurons or brain cells (Figure 1). Many clinical trials targeting these proteins during late stages of the disease have been unsuccessful, presumably because of the excessive neuronal loss associated with later stages of the disease. Using a tau mouse model of AD that develops tau pathology and memory deficits, we sought to therapeutically target an earlier time point in the disease process, when memory deficits and tau pathology are subtle and neuronal loss is not readily detectable. This earlier time point more closely models the intermediate stage between memory declines associated with aging and the severe, more global cognitive deficits associated with AD.

 

 

Figure1Figure 1. Characteristics present in the brains of Alzheimer’s patients vs. healthy patients. The neurons or brain cells in AD are surrounded by amyloid plaques and infected with neurofibrillary tangles.

 

During this intermediate stage of memory loss, humans often exhibit a hyperexcitability or excess firing in the hippocampus, a primary memory region in the brain. This hyperexcitability phenotype correlates with the severity of memory deficits and later the conversion to AD. However, the underlying cause of hyperexcitability is not known. Our laboratory hypothesized that tau pathology may result in hyperexcitability by increasing glutamate levels in the extracellular space. Glutamate is the primary excitatory neurotransmitter in the brain and plays a key role in learning and memory at low extracellular concentrations. However, at high extracellular concentrations, glutamate can cause excessive stimulation of neurons, leading to an increase in neuronal activity (i.e., hyperexcitability) and eventually neuronal death as observed in AD. To test the hypothesis that excessive levels of glutamate may underlie the early memory deficits observed in our tau mouse model of AD, we used a novel procedure, termed microelectrode arrays, to measure glutamate levels in the hippocampus. We observed a robust increase in extracellular glutamate levels in our tau model of AD (Figure 2), and this increase correlated with memory deficits and tau pathology (1), suggesting increases in extracellular glutamate levels may be a causative factor in the development of AD.

The goal of the current study was to determine whether decreasing extracellular glutamate levels would rescue memory deficits and AD-related pathology associated with our AD tau mouse model (2).  We used the drug riluzole, currently approved for treatment of amyotrophic lateral sclerosis (ALS), to decrease extracellular glutamate levels. Riluzole works by blocking glutamate release into the extracellular space and increasing the removal of glutamate from the extracellular space, resulting in overall decreases in extracellular glutamate levels. Whereas untreated tau mice exhibited increased extracellular glutamate levels as previously shown (1), riluzole treatment decreased extracellular glutamate levels in riluzole-treated tau mice. This decrease in extracellular glutamate levels was associated with improvements in memory performance in riluzole-treated tau mice when examined using a hippocampal-dependent memory task designed for use in mice. Moreover, riluzole treatment reduced the pathological phosphorylation and conformational changes in tau associated with AD (2).

Significance: These findings further elucidate the changes in glutamate regulation associated with tau pathology, open new opportunities for the development of clinically applicable therapeutic approaches to regulate glutamate in vulnerable circuits for those at risk for the development of AD, and are an important first step toward the development of an off-label, proof-of-concept investigation of riluzole in AD patients.

 

 

Figure2Figure 2. A representation of glutamate regulation in a healthy neurons vs. a diseased neuron. Glutamate is released from the pre synapse and travels to the post synapse, which promotes learning and memory. Glutamate is then cleared by the astrocyte. Using a microelectrode we found that in the disease process there is an increase in glutamate release and a decrease in glutamate clearance leading to an increase in glutamate in the extracellular space.  These high levels of glutamate promote tau phosphorylation, inflammation, and cell death.

 

Acknowledgements: This work was supported by the National Institute of General Medical Sciences (Reed-U54GM104942), NIA (Reed-R15AG045812), the Alzheimer’s Association (Reed-NIRG-12-242187), a WVU Faculty Research Senate Grant, and a WVU PSCOR grant.

 

References:

  1.         Hunsberger, HC, Rudy, CC, Batten, SR, Gerhardt, GA, Reed, MN 2015 P301L tau expression affects glutamate release and clearance in the hippocampal trisynaptic pathway. Journal of Neurochemistry, 132:169-182.
  2.         Hunsberger, HC, Weitzner, DS, Rudy, CC, Hickman, JE, Libell, EM, Speer, RR, Gerhardt, GA, Reed, MN 2015 Riluzole rescues glutamate alterations, cognitive deficits, and tau pathology associated with P301L tau expression. Journal of Neurochemistry, 135:381-394.

 

 

Multiselect Ultimate Query Plugin by InoPlugs Web Design Vienna | Webdesign Wien and Juwelier SchönmannMultiselect Ultimate Query Plugin by InoPlugs Web Design Vienna | Webdesign Wien and Juwelier Schönmann