SRP182030 PRJNA517055 CDK5-mediated phosphorylation of TET3 ensures robust activation of BRN2 during neuronal differentiation Tet3 converts 5-methylcyotsine to 5-hydroxymethylcytosine (5hmC), although it remains unclear how its functions can be regulated. We showed that Tet3 is phosphorylated by cyclin-dependent kinase 5 at a highly conserved serine residue within its catalytic domain, which leads to an increase in its dioxygenase activity in vitro. Interestingly, when stably expressed in Tet triple-knockout mouse embryonic stem cells (mESCs), wild-type Tet3 elicited higher 5hmC enrichment and expression of genes involved in neurogenesis whereas phosphor-mutant Tet3 caused elevated 5hmC and expression of metabolic pathways genes. Expression of wild-type, but not phosphor-mutant Tet3 in Tet3-knockout mESCs, causes optimal expression of BRN2, Hes1 and Hey2 transcription factors which lead to robust terminal differentiation measured by MAP2 expression. Taken together, our results suggest that cdk5-mediated phosphorylation of Tet3 ensures robust activation of neuronal transcriptional programs during differentiation.