Metabolic Reprogramming during Human Neuron Differentiation Identifies Glutaminase as a Key Determinant in Fragile X Syndrome

Metabolic homeostasis gone awry is a contributor to, if not an underlying cause of, several neurologic disorders. Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by a trinucleotide repeat expansion in FMR1 and consequent loss of the encoded protein FMRP, which results in downstream molecular, neurologic, and mitochondrial deficits that are linked to cognitive impairment. In human postmortem brain, many metabolites and solute carrier proteins are coordinately dysregulated, which also occurs during differentiation of human iPSCs into excitatory neurons. Metabolic tracing in FXS neurons demonstrates a dearth of glutamine deamidation to glutamate, which reduces anaplerosis into the TCA cycle, consequently hindering bioenergetic and biosynthetic functions of mitochondria. Mechanistically, aberrant expression of glutaminase isoforms in FXS is responsible for reduced glutaminolysis, thereby altering glutamate levels which may contribute to FXS.