A splice variant of human Bmal1 acts as a negative regulator of the molecular circadian clock

Jiwon Lee,Eonyoung Park,Kyungjin Kim,Ilmin Kwon,Ga Hye Kim

doi:10.1038/s12276-018-0187-x

Abstract

Bmal1 is one of the key molecules that controls the mammalian molecular clock. In humans, two isoforms of Bmal1 are generated by alternative RNA splicing. Unlike the extensively studied hBmal1b, the canonical form of Bmal1 in most species, the expression and/or function of another human-specific isoform, hBmal1a, are poorly understood. Due to the lack of the N-terminal nuclear localization signal (NLS), hBMAL1a does not enter the nucleus as hBMAL1b does. However, despite the lack of the NLS, hBMAL1a still dimerizes with either hCLOCK or hBMAL1b and thereby promotes cytoplasmic retention or protein degradation, respectively. Consequently, hBMAL1a interferes with hCLOCK:hBMAL1b-induced transcriptional activation and the circadian oscillation of Period2. Moreover, when the expression of endogenous hBmal1a is aborted by CRISPR/Cas9-mediated knockout, the rhythmic expression of hPer2 and hBmal1b is restored in cultured HeLa cells. Together, these results suggest a role for hBMAL1a as a negative regulator of the mammalian molecular clock.

Highlights

The regulation of temporal and spatial coordination of gene expression is crucial for living systems
Here, we report evidence that the human-specific isoform of BMAL1 may function as a negative regulator of the molecular circadian clock
BMAL1, along with its binding partner CLOCK, is an indispensable core clock component that is required for the transcriptional activation of downstream clock genes

Summary

Introduction

The regulation of temporal and spatial coordination of gene expression is crucial for living systems. Heterodimers of CLOCK and BMAL1 (CLOCK:BMAL1) function as transcriptional activators and induce the rhythmic expression of downstream circadian clock genes such as Periods (Pers) and Cryptochromes (Crys)[4,5,6]. Other downstream products of the CLOCK:BMAL1 complex, Rev-erbα and Rorα, constitute additional feedback loops by repressing or activating the transcription of Bmal[1], respectively[3,7]. These autoregulatory feedback mechanisms of core clock components serve as a fundamental driving force that generates and maintains the circadian rhythm. In addition to the core molecular feedback loops, increasing evidence suggests that post-translational modifications such as phosphorylation, sumoylation, and ubiquitination are closely involved in the fine-tuning of circadian clock control[3,8,9,10,11,12]

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