DNMT3L affects synaptic function and induces neurobehavioral alterations in Drosophila via Sqd

Autism spectrum disorder (ASD) is highly comorbid with Down syndrome (DS), yet the shared molecular mechanisms remain unclear. DNA Methyltransferase 3-like (DNMT3L) is overexpressed in DS brain tissue and plays a role in neural development. However, the neurobiological functions of the heterogeneous nuclear ribonucleoprotein D (HNRNPD) family are still largely unknown. In this study, we generated Drosophila lines that overexpress DNMT3L. Using mass spectrometry, we identified Sqd (Squid), the Drosophila homolog of human HNRNPD, as a binding partner of DNMT3L, and we confirmed their physical interaction. We found that DNMT3L overexpression suppresses Sqd function, increases the mRNA stability of factor of interpulse interval (fipi), and raises its expression level. These changes led to neurobehavioral alterations and altered synaptic plasticity in flies. We further tested this in mammalian cells and confirmed the interaction between DNMT3L and HNRNPD, as well as their regulatory effects on neural cell adhesion molecule 1/2 (NCAM1/2), the mammalian homologs of fipi. Together, our findings reveal a DNMT3L-Sqd/HNRNPD-fipi/NCAM regulatory axis. This work provides a basis for identifying therapeutic targets to treat ASD that occurs alongside DS.