Wavelength-specific artificial light disrupts molecular clock in avian species: A power-calibrated statistical approach

https://doi.org/10.1016/j.envpol.2020.114206Get rights and content

Highlights

  • We developed a power-calibrated meta-analysis approach.

  • Effect size of 2.48 promises the reproducibility of the results as high as 80%.

  • Clock genes exhibited robust circadian rhythmic expression in long wavelength.

  • cClock, cCry1, cCry2, cPer2, and cPer3, were arrhythmic in short wavelength.

  • The expression level of molecular genes were wavelength-dependent.

Abstract

Nighttime lighting is an increasingly important anthropogenic environmental stress on plants and animals. Exposure to unnatural lighting environments may disrupt the circadian rhythm of organisms. However, the sample size of relevant studies, e.g. disruption of the molecular circadian clock by light pollution, was small (<10), which led to low statistical power and difficulties in replicating prior results. Here, we developed a power-calibrated statistical approach to overcome these weaknesses. The results showed that the effect size of 2.48 in clock genes expression induced by artificial light would ensure the reproducibility of the results as high as 80%. Long-wavelength light (560–660 nm) entrained expressions of the positive core clock genes (e.g. cClock) and negative core clock genes (e.g. cCry1, cPer2) in robust circadian rhythmicity, whereas those clock genes were arrhythmic in short-wavelength light (380–480 nm). Further, we found artificial light could entrain the transcriptional-translational feedback loop of the molecular clock in a wavelength-dependent manner. The expression of the positive core clock genes (cBmal1, cBmal2 and cClock), cAanat gene and melatonin were the highest in short-wavelength light and lowest in long-wavelength light. For the negative regulators of the molecular clock (cCry1, cCry2, cPer2 and cPer3), the expression of which was the highest in long-wavelength light and lowest in short-wavelength light. Our statistical approach opens new opportunities to understand and strengthen conclusions, comparing with the studies with small sample sizes. We also provide comprehensive insight into the effect of wavelength-specific artificial light on the circadian rhythm of the molecular clock in avian species. Especially, the global lighting is shifting from “yellow” sodium lamps, which is more like the long-wavelength light, toward short-wavelength light (blue light)-enriched “white” light-emitting diodes (LEDs).

Introduction

The nighttime environment in the majority of the world has greatly been disrupted by nighttime lighting, termed ‘light pollution’. Nighttime lighting shows a variety of impacts on the biological processes of plants and animals (Davies et al., 2017; Knop et al., 2017; Macgregor et al., 2017). Especially for animals, nighttime lighting affects behavioral traits, including reproduction, foraging, sleeping and migration (Swaddle et al., 2015). Nighttime lighting also has been reported to influence physiological function, such as intervention in immune response, cortisol levels, testosterone levels and glucose metabolism (Bedrosian et al., 2016). Moreover, nighttime lighting is more likely to be problematic for animals. Almost all species evolve a circadian system to adapt to cycling changes caused by the Earth rotation (Crane and Young, 2014). Nighttime lighting may interfere with circadian rhythms (de Jong et al., 2016; Raap et al., 2016a; Raap et al., 2016b), and disrupt physiological homeostasis (Dominoni et al., 2013; Jones et al., 2015). As a result, exposure to abnormal light cycles may cause deleterious effects on daily and annual rhythms of organisms' life (Bradshaw and Holzapfel, 2007). However, the consequences of nighttime lighting on the circadian rhythm of organisms remain poorly understood. Until recently, the studies on this subject have accelerated.

Avian species, as a kind of ideal model animal, have achieved the status of a reliable vertebrate model for circadian researches. Avian species have a far more complex visual system than mammals because of their multiple light receptors (Borges et al., 2012). Furthermore, avian species obtains a central circadian system with three independent endogenous circadian oscillators. Comparing with mammals, whose central oscillator is only the suprachiasmatic nucleus (SCN) of the hypothalamus, pineal, and retina of avian species are another two independent central oscillators (Cassone, 2014; Kumar et al., 2004). Circadian oscillation in avian species is generated by a molecular feedback loop with positive and negative limbs formed by a set of circadian clock genes (Jiang et al., 2017). Briefly, three positive clock genes include cClock (circadian locomotor output cycles kaput), cBmal1, and cBmal2 (brain and muscle arnt-like 1/2), as well as four negative clock genes, include cCry1, cCry2 (cryptochrome 1/2), cPer2 and cPer3 (period 2/3) consist of the molecular basis of oscillators in avian. The abundance of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) mRNA in the chicken pineal gland exhibits a circadian rhythm which could be regulated by cBmal1, and cBmal2 gene (Chong et al., 2000). However, messenger RNA levels of both cCry1 and cCry2 have been proved to display circadian oscillation in cultured pineal cells under light/dark and constant darkness conditions. Noticeably, their mRNA levels during the light period were significantly higher than those in the dark period, indicating light-dependent up-regulation of the two Cry genes mediated by photoreceptor(s) intrinsic to the chick pineal cells (Yamamoto et al., 2001). Melatonin serves an important role in the regulation of circadian and seasonal rhythms, immunomodulation and neurotransmission. Circadian oscillation of birds is generated by a transcriptional-translational feedback loop with positive and negative limbs formed by Clock-Bmal1/2 and Per2/3-Cry1/2 genes in Japanese Quail (Yoshimura et al., 2000), House Sparrow (Doi et al., 2001), chicken (Bailey et al., 2002) and redheaded bunting (Singh et al., 2013). Among all the vertebrates, avian species are the most sensitive to light. Their visual system can detect the light information and convert them into neural electrical signals which can be transmitted to the brain through the optic nerve (Barinaga, 2002). The physiology and behavior of avian species can be modulated by the perception of variations in the light cycle (Brandstätter et al., 2001; Singh et al., 2012; Yadav et al., 2015). Although photoperiod is considered as the dominant feature of light information, the spectral composition of the subjective nighttime lighting can also influence avian circadian and photoperiodic responses (Zawilska et al., 1995). Especially, the current trend in global lighting is shifting from “yellow” sodium lamps toward a new generation of energy-efficient “white” light-emitting diodes (LEDs) (Schubert and Kim, 2005). The white LED lighting is rich in blue wavelength, which exerts the most efficient in disrupting the circadian rhythm of organisms (Stevens et al., 2014). However, the entrainment of wavelength-specific artificial light on circadian rhythm is still inconclusive.

In this study, we focused on the influence of wavelength-specific artificial light (blue, green, red) on the molecular biology clock of domesticated birds. Firstly, we developed a power-calibrated statistical approach, termed “micro meta-analysis approach”, which opened new opportunities to understand and strengthen conclusions based on the studies with small sample sizes. The sample size of studies involved in molecular biology, e.g. disruption of the molecular circadian clock by light pollution, was small, which led to low statistical power and difficulties in replicating prior results. Meta-analysis is a well-established statistical technique that synthesizes two or more researches of a similar topic, which is relatively new to the field of molecular biology. By synthesizing those studies with small sample sizes. We expected to generate greater statistical power conclusions. The second aim of this study was to implement the power-calibrated micro meta-analysis approach to elucidate the effects of different light spectra on the melatonin rhythm and circadian rhythmic expression of clock genes of domesticated birds. We expect that lights with short-wavelength (380–480 nm) will be more efficient in disrupting the molecular biology clock involved in the transcriptional-translational feedback loop.

Section snippets

Data collection

In this study, we conducted a standardized literature search (PRISMA) in ISI Web of Knowledge in August 2019 using combinations of the following terms: “light pollution”, “artificial light”, “LED”, “circadian rhythm”, “biological clock”, “clock gene”, “melatonin”, “bird”, and “chick” in abstract and title. No limits regarding study type or date were set. We additionally conducted an exploratory literature search by applying those string in Google Scholar. Overall, seven studies (describing 1155

Power-calibrated sample size for meta-analysis

For high-level effect (ES d = 1–2), there was a logarithmic function relationship between the sample size and the statistical power. With the increase of the sample size, statistical power improved sharply (Fig. 1A). For the medium level of effect size (ES d = 0.3–0.7), there was a linear positive correlation between the sample size and statistical power. With the increase of the sample size, the statistical power increased gradually (Fig. 1B). Fig. 1C summarized the relationship between ES and

Discussion

The sample sizes of the studies involved in molecular biology, e.g. disruption of the molecular circadian clock by light pollution, were small. Small sample sizes led to low statistical power and difficulties in replicating prior results. In order to overcome these weaknesses, we developed a power-calibrated micro meta-analysis approach in this study. With the increase of the sample size in the meta-analysis, statistical power increased sharply for a high level of effect size (ES d = 1–2) but

Author contributions

Y. Y., C. Z., Q. L. and C. P. conceived of the study. Y. Y., and J. P. designed most experiments. C. Z., Q. L. Y. Y., C. P. and T. W. collected. Y. Y., Q. L. and J. P. analyzed the data. Y. Y. wrote the manuscript. J. P. supervised all aspects of the work.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Acknowledgements

The study was funded by the National Natural Science Foundation of China (no. 31772644) and China Agriculture Research System (CARS-40).

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  • Cited by (0)

    This paper has been recommended for acceptance by Wen Chen.

    1

    These authors contributed equally to this work.

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