Failure of the glutamate uptake-ascorbic acid exchange drives seizure susceptibility and severity

Veterans Health Administration, Decatur PA

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Abstract

Co-occurrence of seizures with Alzheimer's disease dramatically alters prognosis, and speeds cognitive de- cline. Even mild seizures that may go undetected by caregivers can increase Alzheimer's disease pathogene- sis and also independently lead to neurodegeneration and cognitive dysfunction. Subclinical (non-scorbutic) vitamin C (ascorbate) deficiency is widespread in the USA, particularly among the elderly, and veteran popula- tions where smoking and oxidative stress-related diseases such as atherosclerosis and diabetes are common. A critical, but understudied, mechanism of neuronal protection during excitatory signaling is the glutamate up- take-ascorbate release exchange in astrocytes. As glutamate is taken up into perisynaptic astrocytes via GLT- 1, ascorbate is released into the extracellular fluid providing acute protection against oxidative damage. If GLT- 1 transporter function is compromised by oxidative damage, or if insufficient ascorbate is available for release, then this protection is diminished and the neurotoxic effects of prolonged glutamate exposure are magnified. Recent data support the role of ascorbate treatments in mitigating the effects of seizures, but previous re- search has been critically limited by the use of rodent models that synthesize ascorbate and thus cannot be- come deficient. Decreased brain ascorbate, at levels relevant to human deficiency, is achieved in mice by ge- netic modification of the ability to synthesize ascorbate in liver (Gulo-/-) combined with differential ascorbate supplementation. These mice can also be crossed with the APP/PSEN1 mouse model of Alzheimer's disease to model dietary insufficiency in combination with a disease in which co-occurrence of seizures is common. Specific Aim 1) Demonstrate the role of ascorbate in supporting glutamate clearance by cultured astrocytes. Using cultured astrocytes from both mice and humans we will investigate how conditions that alter the oxida- tive balance of the culture media (e.g. ascorbate and ?-amyloid) determine the properties of GLT-1 in gluta- mate uptake. We will also test how altered glutamate clearance under these situations impacts neuronal sur- vival using co-culture techniques and measuring survival and dendritic branching of primary cultured neurons following exposure to glutamate. These experiments will provide direct support for the importance of the gluta- mate-ascorbate exchange mechanism in supporting neuronal health and protecting against glutamate toxicity. Specific Aim 2) Determine relationship between low brain ascorbate conditions and susceptibility to spontane- ous and pharmacologically-induced seizures. We will record neuronal hyper excitability and measure seizure events through electroencephalography (EEG) under baseline conditions and following seizure induction using kainic acid. Such studies have not previously been conducted in low ascorbate conditions owing to lack of widespread access to appropriate models. We will use an implantable telemetry (wireless) system that allows long-term measuring of EEG in animals within their home cages. This is a significant advance on more tradi- tional `tethered' EEG systems. We will also determine the extent to which repeated exposures to glutamatergic agonists, even without a clear behavioral (seizure) correlate, can negatively impact both cognitive ability, and ?-amyloid production in the APP/PSEN1 mice to explore the functional outcome of seizures in an Alzheimer's disease model in addition to wild-type and Gulo-/- mice. Specific Aim 3) Establish the extent to which upregulation of GLT-1 improves glutamate clearance and de- creases detrimental effects of kainic acid treatments. GLT-1, and therefore glutamate clearance, represents a targetable strategy to reduce the extent of neural damage following hyperexcitability, or seizure events. We will investigate a compound, ceftriaxone, known to up-regulate GLT-1 expression, and assess its ability to de- crease seizure susceptibility and severity. Our long-term goal is to show that protecting glutamate uptake and avoiding ascorbate deficiency can minimize effects of seizure in specific populations, and provide a cheap pre- ventative strategy against cognitive decline.