Cocaine‐Induced Changes in the <scp>BRD</scp> 4 Interactome Identifies Casein Kinase 1 Epsilon as a Therapeutic Target

Recent research has revealed that BRD4, a bromodomain and extra terminal domain (BET) epigenetic "reader" protein, plays an essential role in regulating behavioral and molecular responses to cocaine. To date, the roles of BRD4 and other BET proteins in substance use disorder (SUD) models have been mainly studied using small molecule inhibitors that block the bromodomain interactions with acetylated histones. In other disease models, non-bromodomain BRD4-protein interactions have also been shown to play an important role in BRD4's molecular functions, but these interactions have yet to be studied in SUD models. Here, using BRD4 co-immunoprecipitation coupled to tandem mass spectrometry, we identified several putative BRD4-interacting proteins in the nucleus accumbens (NAc) that are altered by acute or repeated cocaine exposure in male and female Sprague Dawley rats. In BRD4-immunoprecipitated samples, gene ontology molecular function analysis revealed an enrichment of pathways associated with polymerase activity, SH3 domain binding, and chromatin-protein-adaptor activity in the cocaine treated groups. Notably, casein kinase 1 epsilon (CK1ε), a protein previously implicated in SUDs, was identified as a putative BRD4-interacting protein, and this association was increased following repeated cocaine injections. However, no significant change in total CK1ε protein expression was observed in the NAc via western blot following acute or repeated cocaine treatment. In additional experiments, we sought to determine the effects of CK1ε inhibition on the expression of a cocaine-related memory. Pharmacological inhibition of CK1ε with PF-4800567 diminished the expression of cocaine conditioned place preference. In a separate cohort of rats, an acute injection of PF-4800567 did not alter locomotor activity and anxiety-like behavior in the open field test. Collectively, these findings suggest that several novel proteins, including CK1ε, may play a role in BRD4-mediated molecular adaptations to cocaine exposure, and that CK1ε represents a potential target for future investigation in animal models of cocaine-seeking behavior.