Metabolic pathways modulate the neuronal toxicity associated with fragile X-associated tremor/ataxia syndrome

Abstract

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder that affects premutation carriers (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. Much remains unknown regarding the metabolic alterations associated with FXTAS, especially in the brain, and the most affected region, the cerebellum. Investigating the metabolic changes in FXTAS will aid in the identification of biomarkers as well as in understanding the pathogenesis of disease. To identify the metabolic alterations associated with FXTAS, we took advantage of our FXTAS mouse model that expresses 90 CGG repeats in cerebellar Purkinje neurons and exhibits the key phenotypic features of FXTAS. We performed untargeted global metabolic profiling of age-matched control and FXTAS mice cerebella at 16-20 weeks and 55 weeks. Out of 506 metabolites measured in cerebellum, we identified 186 metabolites that demonstrate significant perturbations due to the (CGG) 90 repeat (P<0.05) and found that these differences increase dramatically with age. To identify key metabolic changes in FXTAS pathogenesis, we performed a genetic screen using a Drosophila model of FXTAS. Out of 28 genes that we tested in the f ly, 8 genes showed significant enhanced neuronal toxicity associated with CGG repeats, such as Schlank (ceramide synthase), Sk2 (sphingosine kinase) and Ras (IMP dehydrogenase). By combining metabolic profiling with a Drosophila genetic screen to identify genetic modifiers of FXTAS, we demonstrate an effective method for functional validation of high-throughput metabolic data and show that sphingolipid and purine metabolism are significantly perturbed in FXTAS pathogenesis.

Metabolic Alterations in FMR1 Premutation Carriers

“…In the cerebellum of FXTAS mice expressing r(CGG) 90 in Purkinje cells, our group also observed alterations of amino acids (41 out of 115) (Kong et al, 2019). For example, 5oxoproline was decreased in aged FXTAS compared to aged WT mice, suggesting that this metabolite is altered due to CGG-associated toxicity.…”

“…Recently, sphingolipid metabolism was found to be altered in the cerebellum of FXTAS mice (Figure 1, orange box) (Kong et al, 2019). Specifically, levels of sphingosine, sphingosine 1-phosphate, and sphingomyelin were increased, while levels of ceramide were decreased in FXTAS mice compared to wildtype (Kong et al, 2019). Further pathway analysis and subsequent validation in a FXTAS Drosophila model confirmed two genes related to sphingosine metabolism, Schlank and Sk2 (Kong et al, 2019).…”

“…Specifically, levels of sphingosine, sphingosine 1-phosphate, and sphingomyelin were increased, while levels of ceramide were decreased in FXTAS mice compared to wildtype (Kong et al, 2019). Further pathway analysis and subsequent validation in a FXTAS Drosophila model confirmed two genes related to sphingosine metabolism, Schlank and Sk2 (Kong et al, 2019). Schlank is the Drosophila ortholog of ceramide synthase, the enzyme synthesizing ceramide from sphingosine.…”

“…Interestingly, subsequent genetic screening using a Drosophila model of FXTAS revealed that knockdown of CG4306, the fly ortholog of Gamma-glutamylcyclotransferase (Ggct), which encodes the enzyme responsible for catalyzing the formation of 5-oxoproline (Oakley et al, 2008), resulted in suppression of (CGG) 90 toxicity in Drosophila. This finding suggests Ggct as a genetic modifier of CGG-associated neurotoxicity (Kong et al, 2019), providing a novel therapeutic target for FXTAS. Giulivi et al (2016b) found overall fatty acids levels decreased in the plasma of premutation carriers.…”

“…Sphingolipids are structural lipids for eukaryotic cell membranes, consisting of the sphingoid backbone, which is N-acylated with various fatty acids to form ceramide species (Maceyka and Spiegel, 2014). Recently, sphingolipid metabolism was found to be altered in the cerebellum of FXTAS mice (Figure 1, orange box) (Kong et al, 2019). Specifically, levels of sphingosine, sphingosine 1-phosphate, and sphingomyelin were increased, while levels of ceramide were decreased in FXTAS mice compared to wildtype (Kong et al, 2019).…”