A Zebrafish Model for a Rare Genetic Disease Reveals a Conserved Role for FBXL3 in the Circadian Clock System.

Shir Confino,Lior Appelbaum,Zohar Ben-Moshe Livne,Michaela Kolker,Nathalie Geyer,Nicholas S Foulkes,Sohyun Kathy Park,Yoav Gothilf,Yair Wexler,Daniela Vallone,Shimon Edvardson,Odelia Pisanty,Talya Dor,Orly Elpeleg,Joel Reiter,Hagar Mor-Shaked,Adi Tovin

doi:10.3390/ijms23042373

Abstract

The circadian clock, which drives a wide range of bodily rhythms in synchrony with the day–night cycle, is based on a molecular oscillator that ticks with a period of approximately 24 h. Timed proteasomal degradation of clock components is central to the fine-tuning of the oscillator’s period. FBXL3 is a protein that functions as a substrate-recognition factor in the E3 ubiquitin ligase complex, and was originally shown in mice to mediate degradation of CRY proteins and thus contribute to the mammalian circadian clock mechanism. By exome sequencing, we have identified a FBXL3 mutation in patients with syndromic developmental delay accompanied by morphological abnormalities and intellectual disability, albeit with a normal sleep pattern. We have investigated the function of FBXL3 in the zebrafish, an excellent model to study both vertebrate development and circadian clock function and, like humans, a diurnal species. Loss of fbxl3a function in zebrafish led to disruption of circadian rhythms of promoter activity and mRNA expression as well as locomotor activity and sleep–wake cycles. However, unlike humans, no morphological effects were evident. These findings point to an evolutionary conserved role for FBXL3 in the circadian clock system across vertebrates and to the acquisition of developmental roles in humans.

Highlights

An intrinsic timing mechanism in animals, the circadian clock, drives a variety of physiological and behavioral daily rhythms in synchrony with the environmental daynight cycle
We have identified a F-box and leucine rich repeat protein 3 (FBXL3) LOF mutation in a consanguineous family in which homozygous patients were affected with developmental delay, morphological abnormalities and moderate intellectual disability
FBXL3 was shown to serve as a key circadian clock component in mice, but no evidence for morphological defects was reported in that model

Summary

Introduction

An intrinsic timing mechanism in animals, the circadian clock, drives a variety of physiological and behavioral daily rhythms in synchrony with the environmental daynight cycle. The positive elements, the brain and muscle ARNT (aryl hydrocarbon receptor nuclear translocator)- like (BMAL) protein and the circadian locomotor output cycles kaput (CLOCK) protein, which belong to the basic helix–loop–helix transcription factor family drive the expression of clock-controlled genes by binding to E-box enhancer regulatory regions (50 -CACGTG-30 ) within their promoters [3,4]. They drive the expression of the negative clock factors, the period (PER) and cryptochrome (CRY) proteins, which translocate into the nucleus as heterodimers and directly interact with the BMAL-CLOCK heterodimer and inhibit its function, thereby down-regulating their own expression [4,5]. The Fbxl mutant mice exhibit a behavioral phase-delay phenotype and a phase-delay of membrane excitability in neurons of the ventral suprachiasmatic nucleus (SCN) [9], the site of the master clock in mammals

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Conclusion