Human Derived Dermal Fibroblasts as in Vitro Research Tool to Study Circadian Rhythmicity in Psychiatric Disorders

A number of psychiatric disorders are defined by persistent or recurrent sleep-wake disturbances alongside disruptions in circadian rhythm and altered clock gene expression. Circadian rhythms are present not only in the hypothalamic suprachiasmatic nucleus but also in peripheral tissues. In this respect, cultures of human derived dermal fibroblasts may serve as a promising new tool to investigate cellular and molecular mechanisms underlying the pathophysiology of mental illness. In this article, we discuss the advantages of fibroblast cultures to study psychiatric disease. More specifically, we provide an update on recent advances in modeling circadian rhythm disorders using human fibroblasts.

Synergies of Multiple Zeitgebers Tune Entrainment

Circadian rhythms are biological rhythms with a period close to 24 h. They become entrained to the Earth's solar day via different periodic cues, so-called zeitgebers. The entrainment of circadian rhythms to a single zeitgeber was investigated in many mathematical clock models of different levels of complexity, ranging from the Poincaré oscillator and the Goodwin model to biologically more detailed models of multiple transcriptional translational feedback loops. However, circadian rhythms are exposed to multiple coexisting zeitgebers in nature. Therefore, we study synergistic effects of two coexisting zeitgebers on different components of the circadian clock. We investigate the induction of period genes by light together with modulations of nuclear receptor activities by drugs and metabolism. Our results show that the entrainment of a circadian rhythm to two coexisting zeitgebers depends strongly on the phase difference between the two zeitgebers. Synergistic interactions of zeitgebers can strengthen diurnal rhythms to reduce detrimental effects of shift-work and jet lag. Medical treatment strategies which aim for stable circadian rhythms should consider interactions of multiple zeitgebers.

Responses of activity rhythms to temperature cues evolve in Drosophila populations selected for divergent timing of eclosion

Even though the rhythms in adult emergence and locomotor activity are two different phenomena that occur at distinct life stages of the fly life cycle, previous studies have hinted at similarities in certain aspects of the organisation of the circadian clock driving these two rhythms. For instance, the period gene plays an important regulatory role in both rhythms. In an earlier study, we have shown that selection on timing of adult emergence behaviour in populations of Drosophila melanogaster leads to the co-evolution of temperature sensitivity of circadian clocks driving eclosion. In this study, we investigated whether temperature sensitivity of the locomotor activity rhythm evolved in our populations separately from the adult emergence rhythm, with the goal of understanding the extent of similarity (or lack thereof) in circadian organisation underlying the two rhythms. We found that in response to simulated jetlag with temperature cycles, late chronotypes (populations selected for predominant emergence during dusk) indeed re-entrained faster than early chronotypes (populations selected for predominant emergence during dawn) to 6 h phase delays, thereby indicating enhanced sensitivity of the activity/rest clock to temperature cues in these stocks (entrainment is the synchronisation of internal rhythms to cyclic environmental time cues). Additionally, we found that late chronotypes show higher plasticity of phases across regimes, day-to-day stability in phases and amplitude of entrainment, all indicative of enhanced temperature-sensitive activity/rest rhythms. Our results highlight remarkably similar organisation principles between circadian clocks regulating emergence and activity/rest rhythms.

Selection for Timing of Eclosion Results in Co-evolution of Temperature Responsiveness in Drosophila melanogaster

Circadian rhythms in adult eclosion of Drosophila are postulated to be regulated by a pair of coupled oscillators: one is the master clock that is light sensitive and temperature compensated and the other that is a slave oscillator whose period is temperature sensitive and whose phase is reflected in the overt behavior. Within this framework, we reasoned that in populations of Drosophila melanogaster that have been artificially selected for highly divergent phases of eclosion rhythm, there may be changes in this network of the master-slave oscillator system, via changes in the temperature-sensitive oscillator and/or the coupling of the light- and temperature-sensitive oscillators. We used light/dark cycles in conjunction with different constant ambient temperatures and 2 different amplitudes of temperature cycles in an overall cool or warm temperature and analyzed phases, gate width, and normalized amplitude of the rhythms in each of these conditions. We found that the populations selected for eclosion in the morning ( early flies) do not vary their phases with change in temperature regimes, whereas the populations selected for eclosion in the evening ( late flies) show phase lability of up to ~5 h. Our results imply a genetic correlation between timing of behavior and temperature sensitivity of the circadian clock.

Life-history traits of Drosophila melanogaster populations exhibiting early and late eclosion chronotypes

BACKGROUND

The hypothesis that circadian clocks confer adaptive advantage to organisms has been proposed based on its ubiquity across almost all levels of complexity and organization of life-forms. This thought has received considerable attention, and studies employing diverse strategies have attempted to investigate it. However, only a handful of them have examined how selection for circadian clock controlled rhythmic behaviors influences life-history traits which are known to influence Darwinian fitness. The 'early' and 'late' chronotypes are amongst the most widely studied circadian phenotypes; however, life-history traits associated with these chronotypes, and their consequences on Darwinian fitness remain largely unexplored, primarily due to the lack of a suitable model system. Here we studied several life-history traits of Drosophila melanogaster populations that were subjected to laboratory selection for morning (early) and evening (late) emergence.

RESULTS

We report that the late eclosion chronotypes evolved longer pre-adult duration as compared to the early eclosion chronotypes both under light/dark (LD) and constant dark (DD) conditions, and these differences appear to be mediated by both clock dependent and independent mechanisms. Furthermore, longer pre-adult duration in the late chronotypes does not lead to higher body-mass at pupariation or eclosion, but the late females were significantly more fecund and lived significantly shorter as compared to the early females.

CONCLUSIONS

Coevolution of multiple life-history traits in response to selection on timing of eclosion highlights correlations of the genetic architecture governing timing of eclosion with that of fitness components which suggests that timing ecologically relevant behaviors at specific time of the day might confer adaptive advantage.

Molecular Correlates of Circadian Clocks in Fruit Fly Drosophila melanogaster Populations Exhibiting early and late Emergence Chronotypes

Although association of circadian clock properties with the timing of rhythmic behaviors (chronotype) has been extensively documented over several decades, recent studies on mice and Drosophila have failed to observe such associations. In addition, studies on human populations that examined effects of clock gene mutations/polymorphisms on chronotypes have revealed disparate and often contradictory results, thereby highlighting the need for a suitable model organism to study circadian clocks' role in chronotype regulation, the lack of which has hindered exploration of the underlying molecular-genetic bases. We used a laboratory selection approach to raise populations of Drosophila melanogaster that emerge in the morning (early) or in the evening (late), and over 14 years of continued selection, we report clear divergence of their circadian phenotypes. We also assessed the molecular correlates of early and late emergence chronotypes and report significant divergence in transcriptional regulation, including the mean phase, amplitude and levels of period (per), timeless (tim), clock (clk) and vrille (vri) messenger RNA (mRNA) expression. Corroborating some of the previously reported light-sensitivity and oscillator network coupling differences between the early and the late populations, we also report differences in mRNA expression of the circadian photoreceptor cryptochrome (cry) and in the mean phase, amplitude and levels of the neuropeptide pigment-dispersing factor (PDF). These results provide the first-ever direct evidence for divergent evolution of molecular circadian clocks in response to selection imposed on an overt rhythmic behavior and highlight early and late populations as potential models for chronotype studies by providing a preliminary groundwork for further exploration of molecular-genetic correlates underlying circadian clock-chronotype association.

NMR-based investigation of the Drosophila melanogaster metabolome under the influence of daily cycles of light and temperature

This work presents an NMR-based metabolomic approach to study metabolic processes inD. melanogasterthat exhibit a diurnal rhythm.