Theme 6 Tissue biomarkers

Abstract

Background: Glutamate excitotoxicity is a longstanding hypothesis in the pathophysiology of ALS. Prior studies have demonstrated increased plasma glutamate levels in ALS patients, which suggest a systemic defect in glutamate metabolism (1). The most abundant amino acid consumed in our diet is glutamate. Studies in healthy human subjects have demonstrated efficient metabolism of dietary glutamate via metabolism by enzymes present in the liver, gut lumen and residential gut bacteria. There is increasing evidence that the gut microbiota has significant CNS effects and intestinal dysbiosis has been found in the ALS transgenic SOD1G93A mouse model (2). If intestinal dysbiosis altered the prevalence of glutamine synthetase (GS) producing bacteria, dietary glutamate homeostasis could be impaired, leading to elevated plasma glutamate levels.Objectives: This study examined the degree of fluctuation in plasma glutamate levels seen with consumption of a protein shake in a cohort of ALS patients and family members from a single ALS center.Methods: Twenty six patients (87% of total cohort followed in the ALS center) underwent measurement of plasma amino acid analysis prior to and 1 hour following consumption of a 75 gm protein shake. A subset of 16 patients went on to receive a probiotic with high GS activity and completed serial protein challenges and amino acid analysis during the study. Ten unaffected family members of ALS patients underwent a similar protein challenge. Glutamate Metabolism Dysfunction (GMD) was defined as >30 umol difference post-prandially compared to fasting, and graded as mild (>30-60), moderate (>60-90) or severe (>90).Results: At baseline, 65.4% (17/26) ALS patients screened were GMD positive, compared to 30% (3/10) of tested family members. The severity of GMD in ALS patients was 41% mild, 29% moderate, 29% severe with only mild severity identified in family members. In the six month treatment phase, 75% (6/8) of patients with stable or improving GMD status saw significant improvements in their ALSFRS-R rate of decline, while 71.4% (5/7) with worsening or remaining severe GMD status experienced worsening of their rate of progression.Discussion and conclusions: Although limited by small sample size, this study does represent an excellent sampling within a single ALS center and is the first of its kind to investigate whether impairment in dietary glutamate metabolism exists in ALS patients. If validated in a larger ALS population, detection of glutamate metabolism dysfunction (GMD) could represent a novel biomarker linked to a potential therapeutic target in ALS patients. Planned microbiome analysis will also help in validating this hypothesis.