Histone Methyltransferase SETDB1 Regulates the Development of Cortical Htr3a-Positive Interneurons and Mood Behaviors

GABAergic (gamma-aminobutyric acidergic) interneurons (INs) are highly heterogeneous, and Htr3a labels a subpopulation of cortical INs originating from the embryonic caudal ganglionic eminence. SETDB1 is one of the histone H3K9 methyltransferases and plays an essential role in the excitatory neurons, but its role in regulating cortical inhibitory INs remains largely unknown. In this study, we generated transgenic mice with conditional knockout of Setdb1 in neural progenitor cells (Setdb1-NS-cKO) and GABAergic neurons (Setdb1-Gad2-cKO). In addition, we performed RNA sequencing, ATAC-seq (assay for transposase-accessible chromatin with sequencing), chromatin immunoprecipitation sequencing, luciferase assay, chromatin conformation capture, and CRISPR (clustered regularly interspaced short palindromic repeats)/dCas9 to study the epigenetic mechanism underlying SETDB1-mediated transcriptional regulation of Htr3a. We also performed in situ hybridization and whole-cell recording to evaluate the functional properties of cortical Htr3a+ INs and behavioral tests for mood. We detected significant upregulation of Htr3a expression in the embryonic ganglionic eminence of Setdb1-NS-cKO and identified the endogenous retroviral sequence RMER21B as a new target of SETDB1. RMER21B showed enhancer activity and formed distal chromatin interaction with the promoter of Htr3a. In addition, we observed an increased number and enhanced excitability of Htr3a+ INs in the knockout cortex. Moreover, Setdb1-Gad2-cKO mice exhibited anxiety- and depressive-like behaviors, which were partially reversed by a 5-HT3 receptor antagonist. These findings suggest that SETDB1 represses Htr3a transcription via RMER21B-mediated distal chromatin interaction in the embryonic ganglionic eminence and regulates the development of cortical Htr3a+ INs and mood behaviors.