FBXL21 Regulates Oscillation of the Circadian Clock through Ubiquitination and Stabilization of Cryptochromes

Abstract

In the mammalian circadian clockwork, CRY1 and CRY2 repressor proteins are regulated by posttranslational modifications for temporally coordinated transcription of clock genes. Previous studies revealed that FBXL3, an F-box-type E3 ligase, ubiquitinates CRYs and mediates their degradation. Here, we found that FBXL21 also ubiquitinates CRYs but counteracts FBXL3. Fbxl21(-/-) mice exhibited normal periodicity of wheel-running rhythms with compromised organization of daily activities, while an extremely long-period phenotype of Fbxl3(-/-) mice was attenuated in Fbxl3/Fbxl21 double-knockout mice. The double knockout destabilized the behavioral rhythms progressively and sometimes elicited arrhythmicity. Surprisingly, FBXL21 stabilized CRYs and antagonized the destabilizing action by FBXL3. Predominantly cytosolic distribution of FBXL21 contrasts with nuclear localization of FBXL3. These results emphasize the physiological importance of antagonizing actions between FBXL21 and FBXL3 on CRYs, and their combined actions at different subcellular locations stabilize oscillation of the circadian clock.

Investigation of genetic variants in ubiquitin enzyme genes involved in the modulation of neurodevelopmental processes: a role in schizophrenia susceptibility?

“…Furthermore, the circadian pattern of FBXL21 expression in the mouse suprachiasmatic nucleus (region responsible for controlling circadian rhythm) is reminiscent of the expression pattern seen for other circadian pacemaker genes such as Period 1 (PER1) (Dardente et al, 2008). Mutations in either the FBXL21 or FBXL3 genes can lead to a dysfunction of circadian rhythm oscillations and lead to significant behavioral disturbances in individuals and alterations in their sleeping patterns; moreover the absence of FBXL21 causes a short-period phenotype in both mice and cells (Hirano et al, 2013;Yoo et al, 2013 (Bromundt et al, 2011); which is further supported by a microarray study which found a significant downregulation of the circadian pacemaker gene PER1 in the postmortem temporal cortex of schizophrenia patients compared to healthy controls (Aston et al, 2004). Furthermore, an animal model study has shown dysfunction in the synchronization of circadian rhythms between brain cell networks involved in sleep-wake regulation and cognition (Dudley et al, 2003).…”

“…The FBXL21 protein is known to be able to stabilize cryptochrome (CRY) proteins CRY1 and CRY2, which are implicated in regulating mammalian circadian rhythms (Hirano et al, 2013). In fact, the FBXL21 protein was reported to antagonize the FBXL3 protein, another F-box-type E3 ligase, which ubiquitinates CRY proteins and mediates their degradation (Hirano et al, 2013).…”

Spatiotemporal separation of PER and CRY posttranslational regulation in the mammalian circadian clock

“…As a result, we clarified a process by which CKI exerts control over the circadian period, demonstrated through both the hyperphosphorylating CKIe tau mutant and small molecule CKI inhibitors, such as longdaysin. Future work will further explore the posttranslational landscape surrounding PER and CRY, including the recently discovered FBXL21, the paralog of FBXL3 that is responsible for regulating CRY stability in the cytoplasm (30,31). In developing our predictions, we used multiple models and parameterizations to ensure our mechanisms are consistent across many in silico realizations.…”

Phosphorylation of the Cryptochrome 1 C-terminal Tail Regulates Circadian Period Length

“…Phosphorylation of CRY1 by AMP-activated protein kinase (AMPK) promotes ubiquitination by SCF FBXL3 (22). Loss of function mutations in the Fbxl3 gene stabilize the CRY proteins and lengthen the circadian period (17,19,20), whereas mutation in Fbxl21 shortens circadian periods (18,21). In addition, a mechanism that is specific for CRY2 includes the phosphorylation of CRY2 C-terminal tail, first at Ser-557 by the priming kinase DYRK1A and then at Ser-553 by GSK-3, which generates a degradation signal, resulting in proteasomal degradation of CRY2 through an undiscovered mechanism (9, 10).…”