Effects of Cryptochrome-modulating compounds on circadian behavioral rhythms in zebrafish

Interplay of Light, Melatonin, and Circadian Genes in Skin Pigmentation Regulation

Circadian regulation of skin pigmentation is essential for thermoregulation, ultraviolet (UV) protection, and synchronization of skin cell renewal. This regulation involves both cell-autonomous photic responses and non-cell-autonomous hormonal control, particularly through melatonin produced in a light-sensitive manner. Photosensitive opsins, cryptochromes, and melatonin regulate circadian rhythms in skin pigment cells. We studied light/dark cycles and melatonin coordination in melanin synthesis and cell proliferation of Xenopus laevis melanophores. In vivo, tadpole pigmentation shows robust circadian regulation mainly hormone-driven, in that isolated melanophores respond strongly to melatonin but only slightly to light. Melanophore proliferation is faster in the dark and slower with melatonin as compared to a 12/12 light/dark cycle. Expression of circadian core genes (clock, bmal1, per1, per2, per3, cry1, cry2, and cry4) in melatonin-treated cells during the light phase mimics dark phase expression. Overexpression of individual Crys did not affect melanization or cell proliferation, likely due to their cooperative actions. Melanin synthesis was inhibited by circadian cycle deregulation through (a) pharmacological inhibition of Cry1 and Cry2 degradation with KL001, (b) continuous light or dark conditions, and (c) melatonin treatment. Our findings suggest that circadian cycle regulation, rather than proliferative capacity, alters melanization of melanophores.

Light-Dependent Circadian Rhythm Governs O-GlcNAc Cycling to Influence Cognitive Function in Adult Zebrafish

This study explores the 24-h rhythmic cycle of protein O-GlcNAcylation within the brain and highlights its crucial role in regulating the circadian cycle and neuronal function based on zebrafish as an animal model. In our experiments, disruption of the circadian rhythm, achieved through inversion of the light-dark cycle or daytime melatonin treatment, not only impaired the rhythmic changes of O-GlcNAcylation along with altering expression patterns of O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) in zebrafish brain but also significantly impeded learning and memory function. In particular, circadian disruption affected rhythmic expression of protein O-GlcNAcylation and OGT in the nuclear fraction. Notably, the circadian cycle induces rhythmic alterations in O-GlcNAcylation of H2B histone protein that correspond to changes in H3 trimethylation. Disruption of the cycle interfered with these periodic histone code alterations. Pharmacological inhibition of OGT with OSMI-1 disrupted the wake-sleep patterns of zebrafish without affecting expression of circadian rhythm-regulating genes. OSMI-1 inhibited the expression of c-fos, bdnf, and calm1, key genes associated with brain function and synaptic plasticity, and decreased the binding of O-GlcNAcylated H2B and OGT to promoter regions of these genes. The collective findings support the potential involvement of circadian cycling of the O-GlcNAc histone code in regulating synaptic plasticity and brain function. Overall, data from this study provide evidence that protein O-GlcNAcylation serves as a pivotal posttranslational mechanism integrating circadian signals and neuronal function to regulate rhythmic physiology.

A methylbenzimidazole derivative regulates mammalian circadian rhythms by targeting Cryptochrome proteins

Background: Impairment of the circadian clock has been associated with numerous diseases, including sleep disorders and metabolic disease. Although small molecules that modulate clock function may form the basis of drug discovery of clock-related diseases, only a few compounds that selectively target core clock proteins have been identified. Three scaffolds were previously discovered as small-molecule activators of the clock protein Cryptochrome (CRY), and they have been providing powerful tools to understand and control the circadian clock system. Identifying new scaffolds will expand the possibilities of drug discovery. Methods: A methylbenzimidazole derivative TH401 identified from cell-based circadian screens was characterized. Effects of TH401 on circadian rhythms were evaluated in cellular assays. Functional assays and X-ray crystallography were used to elucidate the effects of the compound on CRY1 and CRY2 isoforms. Results: TH401 lengthened the period of circadian rhythms and stabilized both CRY1 and CRY2. The compound repressed Per2 reporter activity, which was reduced by Cry1 or Cry2 knockout and abolished by Cry1/Cry2 double knockout, indicating the dependence on CRY isoforms. Thermal shift assays showed slightly higher interaction of TH401 with CRY2 over CRY1. The crystal structure of CRY1 in complex with TH401 revealed a conformational change of the gatekeeper W399, which is involved in isoform-selectivity determination. Conclusions: The present study identified a new small molecule TH401 that targets both CRY isoforms. This compound has expanded the chemical diversity of CRY activators, and will ultimately aid in the development of therapeutics against circadian clock-related disorders.

Molecular connections between circadian clock and health/aging

For decades, considerable efforts have been expended for solving the molecular mechanisms of disease progression. An important clue to tackle this question is the circadian clock. Recent findings have uncovered previously unknown molecular connections between circadian clock and disease incidence, consequently causing the aging process. Furthermore, "chronotherapy" is emerging as a new concept of optimizing the time of the day for drug administration according to target gene expressions in order to maximize therapeutic efficacy and minimize the side effects. This concept will help cure patients and prevent them from suffering evitable pain and side effects. This JB special issue "Molecular connections between circadian clock and health/aging" discusses how the circadian clocks link to health and aging from molecular to organismal levels.

Structural and Chemical Biology Approaches Reveal Isoform-Selective Mechanisms of Ligand Interactions in Mammalian Cryptochromes

Cryptochromes (CRYs) are core components of the circadian feedback loop in mammals, which regulates circadian rhythmicity in a variety of physiological processes including sleep–wake cycles and metabolism. Dysfunction of CRY1 and CRY2 isoforms has been associated with a host of diseases, such as sleep phase disorder and metabolic diseases. Accumulating evidence for distinct roles of CRY1 and CRY2 has highlighted the need for CRY isoform-selective regulation; however, highly conserved sequences in CRY ligand-binding sites have hindered the design of isoform-selective compounds. Chemical biology approaches have been identifying small-molecule modulators of CRY proteins, which act in isoform-non-selective and also isoform-selective manners. In this review, we describe advances in our understanding of CRY isoform selectivity by comparing X-ray crystal structures of mammalian CRY isoforms in apo form and in complexes with compounds. We discuss how intrinsic conformational differences in identical residues of CRY1 and CRY2 contribute to unique interactions with different compound moieties for isoform selectivity.

Modulation of circadian clock by crude drug extracts used in Japanese Kampo medicine

Circadian rhythm is an approximately 24 h endogenous biological rhythm. Chronic disruption of the circadian clock leads to an increased risk of diabetes, cardiovascular disease, and cancer. Hence, it is important to develop circadian clock modulators. Natural organisms are a good source of several medicines currently in use. Crude drugs used in Japanese traditional Kampo medicine or folk medicines are an excellent source for drug discovery. Furthermore, identifying new functions for existing drugs, known as the drug repositioning approach, is a popular and powerful tool. In this study, we screened 137 crude drug extracts to act as circadian clock modulators in human U2OS cells stably expressing the clock reporter Bmal1-dLuc, and approximately 12% of these modulated the circadian rhythm. We further examined the effects of several crude drugs in Rat-1 fibroblasts stably expressing Per2-luc, explant culture of lung from Per2::Luciferase knockin mice, and zebrafish larvae in vivo. Notably, more than half of the major ingredients of these crude drugs were reported to target AKT and its relevant signaling pathways. As expected, analysis of the major ingredients targeting AKT signaling confirmed the circadian clock-modulating effects. Furthermore, activator and inhibitor of AKT, and triple knockdown of AKT isoforms by siRNA also modulated the circadian rhythm. This study, by employing the drug repositioning approach, shows that Kampo medicines are a useful source for the identification of underlying mechanisms of circadian clock modulators and could potentially be used in the treatment of circadian clock disruption.