The circadian system controls a large array of physiological and metabolic functions. The molecular organization of the circadian clock is complex, involving various elements organized in feedback regulatory loops. Here we demonstrate that the RelB subunit of NFκB acts as a repressor of circadian transcription. RelB physically interacts with the circadian activator BMAL1 in the presence of CLOCK to repress circadian gene expression at the promoter of the clock-controlled gene Dbp. The repression is independent of the circadian negative regulator CRY. Notably, RelB -/- fibroblasts have profound alterations of circadian genes expression. These findings reveal a previously unforeseen function for RelB as an important regulator of the mammalian circadian system in fibroblasts.

Liver regeneration offers a distinctive opportunity to study cell proliferation in vivo Mammalian silent information regulator 1 (SIRT1), a NAD⁺-dependent histone deacetylase, is an important regulator of various cellular processes, including proliferation, metabolism, and circadian rhythms. In the liver, SIRT1 coordinates the circadian oscillation of clock-controlled genes, including genes that encode enzymes involved in metabolic pathways. We performed partial hepatectomy in WT and liver-specific Sirt1-deficient mice and analyzed the expression of cell cycle regulators in liver samples taken at different times during the regenerative process, by real time PCR, Western blotting analysis, and immunohistochemistry. Lipidomic analysis was performed in the same samples by MS/HPLC. We showed that G₁/S progression was significantly affected by absence of SIRT1 in the liver, as well as circadian gene expression. This was associated to lipid accumulation due to defective fatty acid beta-oxidation. Our study revealed for the first time the importance of SIRT1 in the regulation of hepatocellular proliferation, circadian rhythms, and lipid metabolism during liver regeneration in mice. These results represent an additional step toward the characterization of SIRT1 function in the liver.

Major mood disorders have been linked to abnormalities in circadian rhythms, leading to disturbances in sleep, mood, temperature, and hormonal levels. We provide evidence that ketamine, a drug with rapid antidepressant effects, influences the function of the circadian molecular machinery. Ketamine modulates CLOCK:BMAL1-mediated transcriptional activation when these regulators are ectopically expressed in NG108-15 neuronal cells. Inhibition occurs in a dose-dependent manner and is attenuated after treatment with the GSK3β antagonist SB21673. We analyzed the effect of ketamine on circadian gene expression and observed a dose-dependent reduction in the amplitude of circadian transcription of the Bmal1, Per2, and Cry1 genes. Finally, chromatin-immunoprecipitation analyses revealed that ketamine altered the recruitment of the CLOCK:BMAL1 complex on circadian promoters in a time-dependent manner. Our results reveal a yet unsuspected molecular mode of action of ketamine and thereby may suggest possible pharmacological antidepressant strategies.

Accumulating evidence highlights intriguing interplays between circadian and metabolic pathways. We show that PER2 directly and specifically represses PPARγ, a nuclear receptor critical in adipogenesis, insulin sensitivity, and inflammatory response. PER2-deficient mice display altered lipid metabolism with drastic reduction of total triacylglycerol and nonesterified fatty acids. PER2 exerts its inhibitory function by blocking PPARγ recruitment to target promoters and thereby transcriptional activation. Whole-genome microarray profiling demonstrates that PER2 dictates the specificity of PPARγ transcriptional activity. Indeed, lack of PER2 results in enhanced adipocyte differentiation of cultured fibroblasts. PER2 targets S112 in PPARγ, a residue whose mutation has been associated with altered lipid metabolism. Lipidomic profiling demonstrates that PER2 is necessary for normal lipid metabolism in white adipocyte tissue. Our findings support a scenario in which PER2 controls the proadipogenic activity of PPARγ by operating as its natural modulator, thereby revealing potential avenues of pharmacological and therapeutic intervention.

The environmental light/dark cycle has left its mark on the body's physiological functions to condition not only our inner biology, but also the interaction with external cues. In this scenario, the circadian regulation of the immune response has emerged as a critical factor in defining the host-pathogen interaction and the identification of the underlying circuitry represents a prerequisite for the development of circadian-based therapeutic strategies. The possibility to track down the circadian regulation of the immune response to a metabolic pathway would represent a unique opportunity in this direction. Herein, we show that the metabolism of the essential amino acid tryptophan, involved in the regulation of fundamental processes in mammals, is regulated in a circadian manner in both murine and human cells and in mouse tissues. By resorting to a murine model of pulmonary infection with the opportunistic fungus Aspergillus fumigatus, we showed that the circadian oscillation in the lung of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO)1, generating the immunoregulatory kynurenine, resulted in diurnal changes in the immune response and the outcome of fungal infection. In addition, the circadian regulation of IDO1 drives such diurnal changes in a pre-clinical model of cystic fibrosis (CF), an autosomal recessive disease characterized by progressive lung function decline and recurrent infections, thus acquiring considerable clinical relevance. Our results demonstrate that the circadian rhythm at the intersection between metabolism and immune response underlies the diurnal changes in host-fungal interaction, thus paving the way for a circadian-based antimicrobial therapy.

The aromatic amino acid L-tryptophan (Trp) is essentially metabolized along the host and microbial pathways. While much is known about the role played by downstream metabolites of each pathways in intestinal homeostasis, their role in lung immune homeostasis is underappreciated. Here we have examined the role played by the Trp hydroxylase/5-hydroxytryptamine (5-HT) pathway in calibrating host and microbial Trp metabolism during Aspergillus fumigatus pneumonia. We found that 5-HT produced by mast cells essentially contributed to pathogen clearance and immune homeostasis in infection by promoting the host protective indoleamine-2,3-dioxygenase 1/kynurenine pathway and limiting the microbial activation of the indole/aryl hydrocarbon receptor pathway. This occurred via regulation of lung and intestinal microbiota and signaling pathways. 5-HT was deficient in the sputa of patients with Cystic fibrosis, while 5-HT supplementation restored the dysregulated Trp partitioning in murine disease. These findings suggest that 5-HT, by bridging host-microbiota Trp partitioning, may have clinical effects beyond its mood regulatory function in respiratory pathologies with an inflammatory component.

Cystic fibrosis (CF) is an autosomal recessive disorder characterized by respiratory failure due to a vicious cycle of defective Cystic Fibrosis Transmembrane conductance Regulator (CFTR) function, chronic inflammation and recurrent bacterial and fungal infections. Although the recent introduction of CFTR correctors/potentiators has revolutionized the clinical management of CF patients, resurgence of inflammation and persistence of pathogens still posit a major concern and should be targeted contextually. On the background of a network-based selectivity that allows to target the same enzyme in the host and microbes with different outcomes, we focused on sphingosine-1-phosphate (S1P) lyase (SPL) of the sphingolipid metabolism as a potential candidate to uniquely induce anti-inflammatory and antifungal activities in CF. As a feasibility study, herein we show that interfering with S1P metabolism improved the immune response in a murine model of CF with aspergillosis while preventing germination of Aspergillus fumigatus conidia. In addition, in an early drug discovery process, we purified human and A. fumigatus SPL, characterized their biochemical and structural properties, and performed an in silico screening to identify potential dual species SPL inhibitors. We identified two hits behaving as competitive inhibitors of pathogen and host SPL, thus paving the way for hit-to-lead and translational studies for the development of drug candidates capable of restraining fungal growth and increasing antifungal resistance.