Bile acids (BA) are important physiological molecules not only mediating nutrients absorption and metabolism in peripheral tissues, but exerting neuromodulation effect in the central nerve system (CNS). The catabolism of cholesterol to BA occurs predominantly in the liver by the classical and alternative pathways, or in the brain initiated by the neuronal-specific enzyme CYP46A1 mediated pathway. Circulating BA could cross the blood brain barrier (BBB) and reach the CNS through passive diffusion or BA transporters. Brain BA might trigger direct signal through activating membrane and nucleus receptors or affecting activation of neurotransmitter receptors. Peripheral BA may also provide the indirect signal to the CNS via farnesoid X receptor (FXR) dependent fibroblast growth factor 15/19 (FGF15/19) pathway or takeda G protein coupled receptor 5 (TGR5) dependent glucagon-like peptide-1 (GLP-1) pathway. Under pathological conditions, alterations in BA metabolites have been discovered as potential pathogenic contributors in multiple neurological disorders. Attractively, hydrophilic ursodeoxycholic acid (UDCA), especially tauroursodeoxycholic acid (TUDCA) can exert neuroprotective roles by attenuating neuroinflammation, apoptosis, oxidative or endoplasmic reticulum stress, which provides promising therapeutic effects for treatment of neurological diseases. This review summarizes recent findings highlighting the metabolism, crosstalk between brain and periphery, and neurological functions of BA to elucidate the important role of BA signaling in the brain under both physiological and pathological conditions.

Disturbed sleep is closely associated with an increased risk of metabolic diseases. However, the underlying mechanisms of circadian clock genes linking sleep and lipid profile abnormalities have not been fully elucidated. This study aimed to explore the important role of the circadian clock in regulating impaired cholesterol metabolism at an early stage of sleep deprivation (SD). Sleep disturbance was conducted using an SD instrument. Our results showed that SD increased the serum cholesterol levels. Concentrations of serum leptin and resistin were much lower after SD, but other metabolic hormone concentrations (adiponectin, glucagon, insulin, thyroxine, norepinephrine, and epinephrine) were unchanged before and after SD. Warning signs of cardiovascular diseases [decreased high density lipoprotein (HDL)-cholesterol and increased corticosterone and 8-hydroxyguanosine levels] and hepatic cholestasis (elevated total bile acids and bilirubin levels) were observed after SD. Cholesterol accumulation was also observed in the liver after SD. The expression levels of HMGCR, the critical enzyme for cholesterol synthesis, remained unchanged in the liver. However, the expression levels of liver CYP7A1, the enzyme responsible for the conversion of cholesterol into bile acids, significantly reduced after SD. Furthermore, expression of NR1D1, a circadian oscillator and transcriptional regulator of CYP7A1, strikingly decreased after SD. Moreover, NR1D1 deficiency decreased liver CYP7A1 levels, and SD could exacerbate the reduction of CYP7A1 expression in NR1D1^−/− mouse livers. Additionally, NR1D1 deficiency could further increase serum cholesterol levels under SD. These results suggest that sleep disturbance can induce increased serum cholesterol levels and liver cholesterol accumulation by NR1D1 mediated CYP7A1 inhibition.

Dysregulated IL‐10 producing regulatory B cells (Bregs) are associated with the progression of systemic lupus erythematosus. An immunomodulatory role of heat shock proteins (HSPs) is implicated in autoimmune diseases. However, the molecular basis underlying the role of Hspa13 in regulating Bregs function and lupus pathogenesis remains unclear. In this study, Bregs display higher Hspa13 expression than IL‐10⁻ B cells. Induction of IL‐10 production is weakened in B cells with Hspa13 knockdown or knockout. Hspa13 binds to the IL‐10 promoter via the TATA or CAAT box and activates IL‐10 transcription in the nucleus. Furthermore, Hspa13 positive cells are enriched in marginal zone (MZ) B cells to regulate IL‐10 production. Stimulated B220⁺ B or MZ B cells from CD19^creHspa13^fl/fl mice for Breg induction show an impaired capacity to promote CD4⁺Foxp3⁺ regulatory T cells (Treg) differentiation. In lupus MRL/lpr mice, a decline in Treg differentiation is accompanied by decreased Hspa13 expression in both Bregs and MZ B cells. Moreover, adoptive transfusion of Bregs and MZ B cells from CD19^creHspa13^fl/fl mice fails to increase the frequency of Tregs, attenuate renal pathology, or decrease anti‐dsDNA antibody levels. These results explain the unique role of Hspa13 in determining MZ regulatory function and affecting lupus pathogenesis.