Silke Kiessling

Adrenal glucocorticoids have a key role in circadian resynchronization in a mouse model of jet lag.

Jet lag encompasses a range of psycho- and physiopathological symptoms that arise from temporal misalignment of the endogenous circadian clock with external time. Repeated jet lag exposure, encountered by business travelers and airline personnel as well as shift workers, has been correlated with immune deficiency, mood disorders, elevated cancer risk, and anatomical anomalies of the forebrain. Here, we have characterized the molecular response of the mouse circadian system in an established experimental paradigm for jet lag whereby mice entrained to a 12-hour light/12-hour dark cycle undergo light phase advancement by 6 hours. Unexpectedly, strong heterogeneity of entrainment kinetics was found not only between different organs, but also within the molecular clockwork of each tissue. Manipulation of the adrenal circadian clock, in particular phase-shifting of adrenal glucocorticoid rhythms, regulated the speed of behavioral reentrainment. Blocking adrenal corticosterone either prolonged or shortened jet lag, depending on the time of administration. This key role of adrenal glucocorticoid phasing for resetting of the circadian system provides what we believe to be a novel mechanism-based approach for possible therapies for jet lag and jet lag-associated diseases.

Light Stimulates the Mouse Adrenal through a Retinohypothalamic Pathway Independent of an Effect on the Clock in the Suprachiasmatic Nucleus

The brains master circadian pacemaker resides within the hypothalamic suprachiasmatic nucleus (SCN). SCN clock neurons are entrained to the day/night cycle via the retinohypothalamic tract and the SCN provides temporal information to the central nervous system and to peripheral organs that function as secondary oscillators. The SCN clock-cell network is thought to be the hypothalamic link between the retina and descending autonomic circuits to peripheral organs such as the adrenal gland, thereby entraining those organs to the day/night cycle. However, there are at least three different routes or mechanisms by which retinal signals transmitted to the hypothalamus may be conveyed to peripheral organs: 1) via retinal input to SCN clock neurons; 2) via retinal input to non-clock neurons in the SCN; or 3) via retinal input to hypothalamic regions neighboring the SCN. It is very well documented that light-induced responses of the SCN clock (i.e., clock gene expression, neural activity, and behavioral phase shifts) occur primarily during the subjective night. Thus to determine the role of the SCN clock in transmitting photic signals to descending autonomic circuits, we compared the phase dependency of light-evoked responses in the SCN and a peripheral oscillator, the adrenal gland. We observed light-evoked clock gene expression in the mouse adrenal throughout the subjective day and subjective night. Light also induced adrenal corticosterone secretion during both the subjective day and subjective night. The irradiance threshold for light-evoked adrenal responses was greater during the subjective day compared to the subjective night. These results suggest that retinohypothalamic signals may be relayed to the adrenal clock during the subjective day by a retinal pathway or cellular mechanism that is independent of an effect of light on the SCN neural clock network and thus may be important for the temporal integration of physiology and metabolism.

Amphetamine in Adolescence Disrupts the Development of Medial Prefrontal Cortex Dopamine Connectivity in a dcc -Dependent Manner

Initiation of drug use during adolescence is a strong predictor of both the incidence and severity of addiction throughout the lifetime. Intriguingly, adolescence is a period of dynamic refinement in the organization of neuronal connectivity, in particular medial prefrontal cortex (mPFC) dopamine circuitry. The guidance cue receptor, DCC (deleted in colorectal cancer), is highly expressed by dopamine neurons and orchestrates their innervation to the mPFC during adolescence. Furthermore, we have shown that amphetamine in adolescence regulates DCC expression in dopamine neurons. Drugs in adolescence may therefore induce their enduring behavioral effects via DCC-mediated disruption in mPFC dopamine development. In this study, we investigated the impact of repeated exposure to amphetamine during adolescence on both the development of mPFC dopamine connectivity and on salience attribution to drug context in adulthood. We compare these effects to those induced by adult exposure to an identical amphetamine regimen. Finally, we determine whether DCC signaling within dopamine neurons is necessary for these events. Exposure to amphetamine in adolescence, but not in adulthood, leads to an increase in the span of dopamine innervation to the mPFC, but a reduction of presynaptic sites present on these axons. Amphetamine treatment in adolescence, but not in adulthood, also produces an increase in salience attribution to a previously drug-paired context in adulthood. Remarkably, DCC signaling within dopamine neurons is required for both of these effects. Drugs of abuse in adolescence may therefore induce their detrimental behavioral consequences by disrupting mesocortical dopamine development through alterations in the DCC signaling cascade.

Enhancing circadian clock function in cancer cells inhibits tumor growth

BackgroundCircadian clocks control cell cycle factors, and circadian disruption promotes cancer. To address whether enhancing circadian rhythmicity in tumor cells affects cell cycle progression and reduces proliferation, we compared growth and cell cycle events of B16 melanoma cells and tumors with either a functional or dysfunctional clock.ResultsWe found that clock genes were suppressed in B16 cells and tumors, but treatments inducing circadian rhythmicity, such as dexamethasone, forskolin and heat shock, triggered rhythmic clock and cell cycle gene expression, which resulted in fewer cells in S phase and more in G1 phase. Accordingly, B16 proliferation in vitro and tumor growth in vivo was slowed down. Similar effects were observed in human colon carcinoma HCT-116 cells. Notably, the effects of dexamethasone were not due to an increase in apoptosis nor to an enhancement of immune cell recruitment to the tumor. Knocking down the essential clock gene Bmal1 in B16 tumors prevented the effects of dexamethasone on tumor growth and cell cycle events.ConclusionsHere we demonstrated that the effects of dexamethasone on cell cycle and tumor growth are mediated by the tumor-intrinsic circadian clock. Thus, our work reveals that enhancing circadian clock function might represent a novel strategy to control cancer progression.

Circadian Clocks and Inflammation: Reciprocal Regulation and Shared Mediators

The immune system is deeply interconnected with the endogenous 24-h oscillators of the circadian system. Indeed, the connection between these two physiological systems occurs at multiple levels and in both directions. On one hand, various aspects of the immune system show daily rhythms, which appear to be essential for healthy immune maintenance and proper immune response. On the other hand, immune responses cause changes in circadian rhythms, disrupting their delicate balance and manifesting in disease. Indeed, immune challenges cause various time-, gene-, and tissue-specific effects on circadian-regulated factors. This article reviews the possible mediators of the cross talk between the circadian clock and the immune system, in particular the inflammatory pathways. The rhythmic expression of cytokines and their receptors, as well as other rhythmically regulated humoral factors such as glucocorticoids, melatonin, leptin, or prostaglandins, could gate the effects of the immune response on the circadian system. In addition, systemic cues such as body temperature and neuronal connections between the brain and peripheral tissues may underlie the immune–circadian communication.

PRL2 links magnesium flux and sex-dependent circadian metabolic rhythms

Magnesium (Mg2+) plays pleiotropic roles in cellular biology, and it is essentially required for all living organisms. Although previous studies demonstrated intracellular Mg2+ levels were regulated by the complex of phosphatase of regenerating liver 2 (PRL2) and Mg2+ transporter of cyclin M (CNNMs), physiological functions of PRL2 in whole animals remain unclear. Interestingly, Mg2+ was recently identified as a regulator of circadian rhythm-dependent metabolism; however, no mechanism was found to explain the clock-dependent Mg2+ oscillation. Herein, we report PRL2 as a missing link between sex and metabolism, as well as clock genes and daily cycles of Mg2+ fluxes. Our results unveil that PRL2-null animals displayed sex-dependent alterations in body composition, and expression of PRLs and CNNMs were sex- and circadian time-dependently regulated in brown adipose tissues. Consistently, PRL2-KO mice showed sex-dependent alterations in thermogenesis and in circadian energy metabolism. These physiological changes were associated with an increased rate of uncoupled respiration with lower intracellular Mg2+ in PRL2-KO cells. Moreover, PRL2 deficiency causes inhibition of the ATP citrate lyase axis, which is involved in fatty acid synthesis. Overall, our findings support that sex- and circadian-dependent PRL2 expression alter intracellular Mg2+ levels, which accordingly controls energy metabolism status.

Clock Genes and Cancer

Mismatch between the external time and the internal circadian time causes loss of circadian organization and is frequently linked to cancer. This chapter describes the role of the molecular circadian clock in the incidence and progression of cancer. The first section will present the strong association between disrupted clock gene expression in either the host or the tumor tissue with cancer progression. Furthermore, it will be evaluated whether timed clock gene expression is a relevant factor for tumor development. Possible processes that are regulated by the circadian clock and may trigger tumor growth during circadian disruption will be summarized in the second section. The last section will highlight the importance of circadian timing for the development of effective cancer therapies.

Circadian Clocks and Immune Functions

In mammals, circadian rhythms modulate many physiological processes, including the immune system. This system is divided into two interconnected arms, the innate and the adaptive immune systems. Immunocompetent cells, such as macrophages, natural killer cells, and lymphocytes, have a functional circadian clock. Indeed, daily variations are observed in numbers of circulating cells, as well as in their capacity to secrete cytolytic factors and cytokines. The daily variation is also observed, for example, in lymphocyte expansion after immunization with an antigen across the day and in effector functions against the antigen. In this chapter, we review the current knowledge of circadian rhythms in the immune system, from the first line of immune defense (the innate immune response) to the pathogen-specific control of infections (the adaptive immune response). We end with some examples of immune pathologies influenced by the circadian system.

Genetic background-dependent effects of murine micro RNAs on circadian clock function.

MicroRNAs (miRs) are important regulators of a wide range of biological processes. Antagomir studies suggest an implication of miR-132 in the functionality of the mammalian circadian clock. miR-212 and miR-132 are tandemly processed from the same transcript and share the same seed region. We found the clock modulator miR-132 and miR-212 to be expressed rhythmically in the central circadian clock. Consequently, mRNAs implicated in circadian functions may likely be targeted by both miRs. To further characterize the circadian role we generated mice with stable deletion of the miR-132/212 locus and compared the circadian behavior of mutant and wild-type control animals on two genetic backgrounds frequently used in chronobiological research, C57BL/6N and 129/Sv. Surprisingly, the wheel-running activity phenotype of miR mutant mice was highly background specific. A prolonged circadian free-running period in constant darkness was found in 129/Sv, but not in C57BL/6N miR-132/212 knockout mice. In contrast, in C57BL/6N, but not in 129/Sv miRNA-132/212 knockout mice a lengthened free-running period was observed in constant light conditions. Furthermore, miR-132/212 knockout mice on 129/Sv background exhibited enhanced photic phase shifts of locomotor activity accompanied by reduced light induction of Period gene transcription in the SCN. This phenotype was absent in miRNA-132/212 knockout mice on a C57BL/6N background. Together, our results reveal a strain and light regimen-specific function of miR-132/212 in the circadian clock machinery suggesting that miR-132 and miR-212 act as background-dependent circadian rhythm modulators.