Tet Controls Axon Guidance in Early Brain Development through Glutamatergic Signaling

Mutations in human TET proteins have been found in individuals with neurodevelopmental disorders. Here we report a new function of Tet in regulating Drosophila early brain development. We found that mutation in the Tet DNA-binding domain (TetAXXC) resulted in axon guidance defects in the mushroom body (MB). Tet is required in early brain development during the outgrowth of MB {beta} axons. Transcriptomic study shows that glutamine synthetase 2 (Gs2), a key enzyme in glutamatergic signaling, is significantly downregulated in the TetAXXC mutant brains. CRISPR/Cas9 mutagenesis or RNAi knockdown of Gs2 recapitulates the TetAXXC mutant phenotype. Surprisingly, Tet and Gs2 act in the insulin-producing cells (IPCs) to control MB axon guidance, and overexpression of Gs2 in these cells rescues the axon guidance defects of TetAXXC. Treating TetAXXC with the metabotropic glutamate receptor antagonist MPEP can rescue while treating with glutamate enhances the phenotype confirming Tet function in regulating glutamatergic signaling. TetAXXC and the Drosophila homolog of Fragile X Messenger Ribonucleoprotein protein mutant (Fmr13) have similar axon guidance defects and reduction in Gs2 mRNA levels. Interestingly, overexpression of Gs2 in the IPCs also rescues the Fmr13 phenotype, suggesting functional overlapping of the two genes. Our studies provide the first evidence that Tet can control the guidance of axons in the developing brain by modulating glutamatergic signaling and the function is mediated by its DNA-binding domain.