Access Type

Open Access Dissertation

Date of Award

January 2012

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Biochemistry and Molecular Biology

First Advisor

William Brusilow

Abstract

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease characterized by motor neuron death and a corresponding loss of neuromuscular connections resulting in muscle atrophy. Patients become paralyzed shortly after symptom onset and typically die within one to five years of pulmonary complications. ALS is a relatively rare disease, with an overall incidence of approximately 2 in 100,000 people per year and a prevalence of about 5 in 100,000 people. It is typically associated with increasing age and has a slight male prevalence, with a male to female ratio of approximately 3:2. ALS is classified as either familial (the less common form of the disease that has a hereditary component) or sporadic (the more common form that has no identifiable genetic component), with both forms of the disease exhibiting a great deal of heterogeneity with respect to causes and underlying molecular mechanisms. A wide variety of gene mutations have been associated with both forms of ALS, including mutations in superoxide dismutase 1 (SOD1), TDP-43, and FUS, as well as several other genes that have yet to be identified. These genes encode a variety of proteins that encompass several processes within the cell, leading to an array of molecular mechanisms that can become dysfunctional during pathogenesis. These mechanisms include, glutamate excitotoxicity, mitochondrial dysfunction and oxidative stress, dysfunctional protein quality control, aberrant mRNA processing, neurotrophic factor dysregulation, and neuroinflammation. ALS is difficult to diagnose due to the heterogeneity of symptoms exhibited by patients and the lack of reliable biomarkers of the disease. As a result, a positive diagnosis typically takes up to a year from symptom onset, at which time a significant amount of irreversible damage has occurred.

Since ALS is a neurodegenerative disease, most research focuses on the central nervous system, however data obtained from ALS patients and animal models suggests that there are metabolic abnormalities associated with the disease. The experiments conducted in this study first examined gender differences in the survival response to an irreversible inhibitor of glutamine synthetase, L-methionine sulfoximine (MSO), which targets excitotoxicity using the SOD1-G93A transgenic mouse model of ALS. Human studies have shown that there are gender differences in basic metabolism, and studies of drug metabolism in rats and humans suggest that males and females may respond differently to treatment. The results of these experiments show that female SOD1 mice respond better to MSO treatment, showing both an extension in the average lifespan as well as improved neuromuscular function with MSO treatment. In addition, ovariectomy removes the beneficial effects of MSO treatment on both survival and neuromuscular function. The second portion of this study was designed to test the hypothesis that the SOD1 mutation disrupts nitrogen metabolism, specifically the urea cycle. The results of these studies show that untreated presymptomatic SOD1 mice have altered nitrogen metabolism and show decreased levels of several urea cycle intermediates in the plasma compared to their wild-type counterparts. These plasma metabolites also show gender-specific differences in wild-type and SOD1 mice, supporting the idea that male and female mice have differences in metabolism. Plasma metabolites change with age and suggest that there are compensatory mechanisms that aid in normalizing plasma metabolites in SOD1 mice. Again, these metabolites show gender-specificity and MSO treatment normalizes the levels of these metabolites in a gender-specific fashion in SOD1 mice. Studies of nitrogen handling enzymes also show that there are gender-specific differences in CPSI activity levels that are not associated with expression of the transgene. Male SOD1 mice begin to show changes in nitrogen handling enzymes in the liver, specifically glutamine synthetase and CPSI, suggesting that males may begin to experience alterations in nitrogen metabolism prior to symptom onset that may help to explain the gender-specific effects of MSO treatment in this mouse model.

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