Circadian clocks enable organisms to predict and align their behaviors and physiologies to constant daily day-night environmental cycle. Because the ubiquitin ligase Siah2 has been identified as a potential regulator of circadian clock function in cultured cells, we have used SIAH2-deficient mice to examine its function in vivo. Our experiments demonstrate a striking and unexpected sexually dimorphic effect of SIAH2-deficiency on the regulation of rhythmically expressed genes in the liver. The absence of SIAH2 in females, but not in males, altered the expression of core circadian clock genes and drastically remodeled the rhythmic transcriptome in the liver by increasing the number of day-time expressed genes, and flipping the rhythmic expression from nighttime expressed genes to the daytime. These effects are not readily explained by effects on known sexually dimorphic pathways in females. Moreover, loss of SIAH2 in females, not males, preferentially altered the expression of transcription factors and genes involved in regulating lipid and lipoprotein metabolism. Consequently, SIAH2-deficient females, but not males, displayed disrupted daily lipid and lipoprotein patterns, increased adiposity and impaired metabolic homeostasis. Overall, these data suggest that SIAH2 may be a key component of a female-specific circadian transcriptional output circuit that directs the circadian timing of gene expression to regulate physiological rhythms, at least in the liver. In turn, our findings imply that sex-specific transcriptional mechanisms may closely interact with the circadian clock to tailor overt rhythms for sex-specific needs.

The time-dependent degradation of core circadian clock proteins is essential for the proper functioning of circadian timekeeping mechanisms that drive daily rhythms in gene expression and, ultimately, an organism’s physiology. The ubiquitin proteasome system plays a critical role in regulating the stability of most proteins, including the core clock components. Our laboratory developed a cell-based functional screen to identify ubiquitin ligases that degrade any protein of interest and have started screening for those ligases that degrade circadian clock proteins. This screen identified Spsb4 as a putative novel E3 ligase for RevErbα. In this article, we further investigate the role of Spsb4 and its paralogs in RevErbα stability and circadian rhythmicity. Our results indicate that the paralogs Spsb1 and Spsb4, but not Spsb2 and Spsb3, can interact with and facilitate RevErbα ubiquitination and degradation and regulate circadian clock periodicity.

Circadian clocks enable organisms to predict and align their behaviors and physiologies to constant daily day-night environmental cycle. Because the ubiquitin ligase Siah2 has been identified as a potential regulator of circadian clock function in cultured cells, we have used Siah2-deficient mice to examine its function in vivo. Our experiments demonstrate a striking and unexpected sexually dimorphic effect of Siah2 deficiency on the regulation of rhythmically expressed genes. The absence of Siah2 in females, but not in males, altered the expression of core circadian clock genes and drastically remodeled the rhythmic hepatic transcriptome. Siah2 loss, only in females, increased the expression of 100’s of genes selectively at mid-day, resulting in a ∼70% increase in the number of rhythmically expressed genes, and shifted the expression of 100’s of other genes from a mid-night peak, to a mid-day peak. The combined result is a near inversion of overall rhythmicity in gene expression selectively in Siah2-deficient females. This dramatic reorganization created a substantial misalignment between rhythmic liver functions and feeding/behavioral rhythms, and consequently impaired daily patterns of lipid/lipoprotein metabolism and metabolic responses to high-fat diet. Collectively, our results suggest that Siah2 is part of a female-specific circadian mechanism important for maintaining metabolic homeostasis and may play a key role in the establishing sexual dimorphisms in metabolism.Signficance statementCircadian clocks drive daily rhythms in many aspects of our physiology, optimally aligning functions across the day-night cycle. How circadian clocks drives these rhythms is thought to be due to largely similar transcriptional pathways and mechanisms in males and females, although some rhythms are modulated by sex and growth hormones. In this study, we present data that uncover the surprising existence of a female-specific transcriptional mechanism that is essential for the proper rhythmic control of gene expression in the liver. Disrupting this mechanism substantially impairs the circadian regulation of lipid and cholesterol metabolism selectively in females, impairing their resistance to diet-induced obesity. These results reveal that circadian clocks may be broadly coupled to physiological rhythms using unexpected sex-specific mechanisms.