4C (chromosome conformation capture on chip) analyses using the clock-controlled Dbp gene as bait reveal the existence of circadian long-range interactions in mouse embryonic fibroblasts. The Dbp circadian interactome is dependent on a functional circadian clock and contains a number of clock-related DNA elements. Dynamic transitions in the epigenome have been associated with regulated patterns of nuclear organization. The accumulating evidence that chromatin remodeling is implicated in circadian function prompted us to explore whether the clock may control nuclear architecture. We applied the chromosome conformation capture on chip technology in mouse embryonic fibroblasts (MEFs) to demonstrate the presence of circadian long-range interactions using the clock-controlled Dbp gene as bait. The circadian genomic interactions with Dbp were highly specific and were absent in MEFs whose clock was disrupted by ablation of the Bmal1 gene (also called Arntl). We establish that the Dbp circadian interactome contains a wide variety of genes and clock-related DNA elements. These findings reveal a previously unappreciated circadian and clock-dependent shaping of the nuclear landscape.

MLL1 regulates circadian promoters by depositing H3K4 trimethyl marks, whose levels are also modulated by the NAD+-dependent deacetylase SIRT1. SIRT1 is now shown to promote circadian deacetylation of MLL1, thus affecting MLL1's methyltransferase activity. The circadian clock controls the transcription of hundreds of genes through specific chromatin-remodeling events. The histone methyltransferase mixed-lineage leukemia 1 (MLL1) coordinates recruitment of CLOCK–BMAL1 activator complexes to chromatin, an event associated with cyclic trimethylation of histone H3 Lys4 (H3K4) at circadian promoters. Remarkably, in mouse liver circadian H3K4 trimethylation is modulated by SIRT1, an NAD+-dependent deacetylase involved in clock control. We show that mammalian MLL1 is acetylated at two conserved residues, K1130 and K1133. Notably, MLL1 acetylation is cyclic, controlled by the clock and by SIRT1, and it affects the methyltransferase activity of MLL1. Moreover, H3K4 methylation at clock-controlled-gene promoters is influenced by pharmacological or genetic inactivation of SIRT1. Finally, levels of MLL1 acetylation and H3K4 trimethylation at circadian gene promoters depend on NAD+ circadian levels. These findings reveal a previously unappreciated regulatory pathway between energy metabolism and histone methylation.

Conventional antidepressants usually require several weeks to achieve a full clinical response in patients with major depressive disorder, an illness associated with dysregulated circadian rhythms and a high incidence of suicidality. Two rapid-acting antidepressant strategies, low-dose ketamine (KT) and sleep deprivation (SD) therapies, dramatically reduce depressive symptoms within 24 hours in a subset of major depressive disorder patients. However, it is unknown whether they exert their actions through shared regulatory mechanisms. To address this question, we performed comparative transcriptomics analyses to identify candidate genes and relevant pathways common to KT and SD. We used the forced swim test, a standardized behavioral approach to measure antidepressant-like activity of KT and SD. We investigated gene expression changes using high-density microarrays and pathway analyses (Gene Ontology, Kyoto Encyclopedia of Genes and Genomes, Gene Set Enrichment Analysis) in KT- and SD-treated mice compared with saline-treated control male mice. We show that KT and SD elicit common transcriptional responses implicating distinct elements of the circadian clock and processes involved in neuronal plasticity. There is an overlap of 64 genes whose expression is common in KT and SD. Specifically, there is downregulation of clock genes including Ciart, Per2, Npas4, Dbp, and Rorb in both KT- and SD-treated mice. We demonstrate a potential involvement of the circadian clock in rapid antidepressant responses. These findings could open new research avenues to help design chronopharmacological strategies to treat major depressive disorder.

The circadian clock orchestrates daily rhythms in many physiological, behavioral and molecular processes, providing means to anticipate, and adapt to environmental changes. A specific role of the circadian clock is to coordinate functions of the immune system both at steady-state and in response to infectious threats. Hence, time-of-day dependent variables are found in the physiology of immune cells, host-parasite interactions, inflammatory processes, or adaptive immune responses. Interestingly, the molecular clock coordinates transcriptional-translational feedback loops which orchestrate daily oscillations in expression of many genes involved in cellular functions. This clock function is assisted by tightly controlled transitions in the chromatin fiber involving epigenetic mechanisms which determine how a when transcriptional oscillations occur. Immune cells are no exception, as they also present a functional clock dictating transcriptional rhythms. Hereby, the molecular clock and the chromatin regulators controlling rhythmicity represent a unique scaffold mediating the crosstalk between the circadian and the immune systems. Certain epigenetic regulators are shared between both systems and uncovering them and characterizing their dynamics can provide clues to design effective chronotherapeutic strategies for modulation of the immune system.

Circadian rhythms drive the temporal organization of a wide variety of physiological and behavioral functions in ∼24-h cycles. This control is achieved through a complex program of gene expression. In mammals, the molecular clock machinery consists of interconnected transcriptional–translational feedback loops that ultimately ensure the proper oscillation of thousands of genes in a tissue-specific manner. To achieve circadian transcriptional control, chromatin remodelers serve the clock machinery by providing appropriate oscillations to the epigenome. Recent findings have revealed the presence of circadian interactomes, nuclear “hubs” of genome topology where coordinately expressed circadian genes physically interact in a spatial and temporal-specific manner. Thus, a circadian nuclear landscape seems to exist, whose interplay with metabolic pathways and clock regulators translates into specific transcriptional programs. Deciphering the molecular mechanisms that connect the circadian clock machinery with the nuclear landscape will reveal yet unexplored pathways that link cellular metabolism to epigenetic control.