Deleting the Arntl clock gene in the granular layer of the mouse cerebellum: impact on the molecular circadian clockwork

Prefrontal cortex molecular clock modulates development of depression-like phenotype and rapid antidepressant response in mice

Depression is associated with dysregulated circadian rhythms, but the role of intrinsic clocks in mood-controlling brain regions remains poorly understood. We found increased circadian negative loop and decreased positive clock regulators expression in the medial prefrontal cortex (mPFC) of a mouse model of depression, and a subsequent clock countermodulation by the rapid antidepressant ketamine. Selective Bmal1KO in CaMK2a excitatory neurons revealed that the functional mPFC clock is an essential factor for the development of a depression-like phenotype and ketamine effects. Per2 silencing in mPFC produced antidepressant-like effects, while REV-ERB agonism enhanced the depression-like phenotype and suppressed ketamine action. Pharmacological potentiation of clock positive modulator ROR elicited antidepressant-like effects, upregulating plasticity protein Homer1a, synaptic AMPA receptors expression and plasticity-related slow wave activity specifically in the mPFC. Our data demonstrate a critical role for mPFC molecular clock in regulating depression-like behavior and the therapeutic potential of clock pharmacological manipulations influencing glutamatergic-dependent plasticity.

Diurnal proteome profile of the mouse cerebral cortex: Conditional deletion of the Bmal1 circadian clock gene elevates astrocyte protein levels and cell abundance in the neocortex and hippocampus

Circadian oscillators, defined by cellular 24 h clock gene rhythms, are found throughout the brain. Cerebral cortex-specific conditional knockout of the clock gene Bmal1 (Bmal1 CKO) leads to depressive-like behavior, but the molecular link from clock gene to altered behavior is unknown. Further, diurnal proteomic data on the cerebral cortex are currently unavailable. With the aim of determining the diurnal proteome profile and downstream targets of the cortical circadian clock, we here performed a proteomic analysis of the mouse cerebral cortex. Proteomics identified approximately 2700 proteins in both the neocortex and the hippocampus. In the neocortex, 15 proteins were differentially expressed (>2-fold) between day and night, mainly mitochondrial and neuronal plasticity proteins. Only three hippocampal proteins were differentially expressed, suggesting that daily protein oscillations are more prominent in the neocortex. The number of differentially expressed proteins was reduced in the Bmal1 CKO, suggesting that daily rhythms in the cerebral cortex are primarily driven by local clocks. The proteome of the Bmal1 CKO cerebral cortex was dominated by upregulated proteins expressed in astrocytes, including GFAP (4-fold) and FABP7 (>20-fold), in both the neocortex and hippocampus. These findings were confirmed at the transcript level. Cellular analyses of astrocyte components revealed an increased number of GFAP-positive cells in the Bmal1 CKO cerebral cortex. Further, BMAL1 was found to be expressed in both GFAP- and FABP7-positive astrocytes of control animals. Our data show that Bmal1 is required for proper cellular composition of the cerebral cortex, suggesting that increased cortical astrocyte activity may induce behavioral changes.

Aberrant single-subject morphological cerebellar connectome in chronic insomnia

BACKGROUND

To systematically investigate the topological organisation of morphological networks of the cerebellum using structural MRI and examine their clinical relevance in chronic insomnia (CI).

METHODS

One hundred and one patients with CI and 102 healthy controls (HCs) were recruited in this study. Individual morphological networks of the cerebellum were constructed based on regional grey matter volume, and topologically characterised using weighted graph theory-based network approaches. Between-group comparisons were performed using permutation tests, and Spearman's correlation was used to examine the relationships between topological alterations and clinical variables.

RESULTS

Compared with HCs, patients with CI exhibited a lower normalised clustering coefficient. Locally, CI patients exhibited lower nodal efficiency in the cerebellar lobule VIIb and vermis regions, but higher nodal efficiency in the right cerebellar lobule VIIIa regions. No correlations were observed between network alterations and clinical variables.

CONCLUSIONS

Individual morphological network analysis provides a new strategy for investigating cerebellar morphometric changes in CI, and our findings may have important implications in establishing diagnostic and categorical biomarkers.

The Circadian Oscillator of the Cerebellum: Triiodothyronine Regulates Clock Gene Expression in Granule Cells in vitro and in the Cerebellum of Neonatal Rats in vivo

The central circadian clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus, but an SCN-dependent molecular circadian oscillator is present in the cerebellar cortex. Recent findings suggest that circadian release of corticosterone is capable of driving the circadian oscillator of the rat cerebellum. To determine if additional neuroendocrine signals act to shape cerebellar clock gene expression, we here tested the role of the thyroid hormone triiodothyronine (T3) in regulation of the cerebellar circadian oscillator. In cultured cerebellar granule cells from mixed-gender neonatal rats, T3 treatment affected transcript levels of the clock genes Per2, Arntl, Nr1d1, and Dbp, suggesting that T3 acts directly on granule cells to control the circadian oscillator. We then used two different in vivo protocols to test the role of T3 in adult female rats: Firstly, a single injection of T3 did not influence clock gene expression in the cerebellum. Secondly, we established a surgical rat model combining SCN lesion with a programmable micropump infusing circadian physiological levels of T3; however, rhythmic infusion of T3 did not reestablish differential clock gene expression between day and night in SCN lesioned rats. To test if the effects of T3 observed in vitro were related to the developmental stage, acute injections of T3 were performed in mixed-gender neonatal rats in vivo; this procedure significantly affected cerebellar expression of the clock genes Per1, Per2, Nr1d1, and Dbp. Developmental comparisons showed rhythmic expression of all clock genes analyzed in the cerebellum of adult rats only, whereas T3 responsiveness was limited to neonatal animals. Thus, T3 shapes cerebellar clock gene profiles in early postnatal stages, but it does not represent a systemic circadian regulatory mechanism linking the SCN to the cerebellum throughout life.

Daytime Restricted Feeding Affects Day-Night Variations in Mouse Cerebellar Proteome

The cerebellum harbors a circadian clock that can be shifted by scheduled mealtime and participates in behavioral anticipation of food access. Large-scale two-dimensional difference gel electrophoresis (2D-DIGE) combined with mass spectrometry was used to identify day–night variations in the cerebellar proteome of mice fed either during daytime or nighttime. Experimental conditions led to modified expression of 89 cerebellar proteins contained in 63 protein spots. Five and 33 spots were changed respectively by time-of-day or feeding conditions. Strikingly, several proteins of the heat-shock protein family (i.e., Hsp90aa1, 90ab1, 90b1, and Hspa2, 4, 5, 8, 9) were down-regulated in the cerebellum of daytime food-restricted mice. This was also the case for brain fatty acid protein (Fabp7) and enzymes involved in oxidative phosphorylation (Ndufs1) or folate metabolism (Aldh1l1). In contrast, aldolase C (Aldoc or zebrin II) and pyruvate carboxylase (Pc), two enzymes involved in carbohydrate metabolism, and vesicle-fusing ATPase (Nsf) were up-regulated during daytime restricted feeding, possibly reflecting increased neuronal activity. Significant feeding × time-of-day interactions were found for changes in the intensity of 20 spots. Guanine nucleotide-binding protein G(o) subunit alpha (Gnao1) was more expressed in the cerebellum before food access. Neuronal calcium-sensor proteins [i.e., parvalbumin (Pvalb) and visinin-like protein 1 (Vsnl1)] were inversely regulated in daytime food-restricted mice, compared to control mice fed at night. Furthermore, expression of three enzymes modulating the circadian clockwork, namely heterogeneous nuclear ribonucleoprotein K (Hnrnpk), serine/threonine-protein phosphatases 1 (Ppp1cc and Ppp1cb subunits) and 5 (Ppp5), was differentially altered by daytime restricted feeding. Besides cerebellar proteins affected only by feeding conditions or daily cues, specific changes in in protein abundance before food access may be related to behavioral anticipation of food access and/or feeding-induced shift of the cerebellar clockwork.

Regulation and function of extra-SCN circadian oscillators in the brain

Most organisms evolved endogenous, so called circadian clocks as internal timekeeping mechanisms allowing them to adapt to recurring changes in environmental demands brought about by 24-hour rhythms such as the light-dark cycle, temperature variations or changes in humidity. The mammalian circadian clock system is based on cellular oscillators found in all tissues of the body that are organized in a hierarchical fashion. A master pacemaker located in the suprachiasmatic nucleus (SCN) synchronizes peripheral tissue clocks and extra-SCN oscillators in the brain with each other and with external time. Different time cues (so called Zeitgebers) such as light, food intake, activity and hormonal signals reset the clock system through the SCN or by direct action at the tissue clock level. While most studies on non-SCN clocks so far have focused on peripheral tissues, several extra-SCN central oscillators were characterized in terms of circadian rhythm regulation and output. Some of them are directly innervated by the SCN pacemaker, while others receive indirect input from the SCN via other neural circuits or extra-brain structures. The specific physiological function of these non-SCN brain oscillators as well as their role in the regulation of the circadian clock network remains understudied. In this review we summarize our current knowledge about the regulation and function of extra-SCN circadian oscillators in different brain regions and devise experimental approaches enabling us to unravel the organization of the circadian clock network in the central nervous system.

Rhythmic Release of Corticosterone Induces Circadian Clock Gene Expression in the Cerebellum

Neurons of the cerebellar cortex contain a circadian oscillator, with circadian expression of clock genes being controlled by the master clock of the suprachiasmatic nucleus (SCN). However, the signaling pathway connecting the SCN to the cerebellum is unknown. Glucocorticoids exhibit a prominent SCN-dependent circadian rhythm, and high levels of the glucocorticoid receptor have been reported in the cerebellar cortex; we therefore hypothesized that glucocorticoids may control the rhythmic expression of clock genes in the cerebellar cortex. We here applied a novel methodology by combining the electrolytic lesion of the SCN with implantation of a micropump programmed to release corticosterone in a circadian manner mimicking the endogenous hormone profile. By use of this approach, we were able to restore the corticosterone rhythm in SCN-lesioned male rats. Clock gene expression in the cerebellum was abolished in rats with a lesioned SCN, but exogenous corticosterone restored the daily rhythm in clock gene expression in the cerebellar cortex, as revealed by quantitative real-time PCR and radiochemical in situ hybridization for the detection of the core clock genes Per1, Per2, and Arntl. On the contrary, exogenous hormone did not restore circadian rhythms in body temperature and running activity. RNAscope in situ hybridization further revealed that the glucocorticoid receptor colocalizes with clock gene products in cells of the cerebellar cortex, suggesting that corticosterone exerts its actions by binding directly to receptors in neurons of the cerebellum. However, rhythmic clock gene expression in the cerebellum was also detectable in adrenalectomized rats, indicating that additional control mechanisms exist. These data show that the cerebellar circadian oscillator is influenced by SCN-dependent rhythmic release of corticosterone.

Circadian regulation of membrane physiology in neural oscillators throughout the brain

Twenty-four-hour rhythmicity in physiology and behavior are driven by changes in neurophysiological activity that vary across the light-dark and rest-activity cycle. Although this neural code is most prominent in neurons of the primary circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus, there are many other regions in the brain where region-specific function and behavioral rhythmicity may be encoded by changes in electrical properties of those neurons. In this review, we explore the existing evidence for molecular clocks and/or neurophysiological rhythms (i.e., 24 hr) in brain regions outside the SCN. In addition, we highlight the brain regions that are ripe for future investigation into the critical role of circadian rhythmicity for local oscillators. For example, the cerebellum expresses rhythmicity in over 2,000 gene transcripts, and yet we know very little about how circadian regulation drives 24-hr changes in the neural coding responsible for motor coordination. Finally, we conclude with a discussion of how our understanding of circadian regulation of electrical properties may yield insight into disease mechanisms which may lead to novel chronotherapeutic strategies in the future.

The Mysterious Food-Entrainable Oscillator: Insights from Mutant and Engineered Mouse Models

The food-entrainable oscillator (FEO) is a mysterious circadian clock because its anatomical location(s) and molecular timekeeping mechanism are unknown. Food anticipatory activity (FAA), which is defined as the output of the FEO, emerges during temporally restricted feeding. FAA disappears immediately during ad libitum feeding and reappears during subsequent fasting. A free-running FAA rhythm has been observed only in rare circumstances when food was provided with a period outside the range of entrainment. Therefore, it is difficult to study the circadian properties of the FEO. Numerous studies have attempted to identify the critical molecular components of the FEO using mutant and genetically engineered mouse models. Herein we critically review the experimental protocols and findings of these studies in mouse models. Several themes emerge from these studies. First, there is little consistency in restricted feeding protocols between studies. Moreover, the protocols were sometimes not optimal, resulting in erroneous conclusions that FAA was absent in some mouse models. Second, circadian genes are not necessary for FEO timekeeping. Thus, another noncanonical timekeeping mechanism must exist in the FEO. Third, studies of mouse models have shown that signaling pathways involved in circadian timekeeping, reward (dopaminergic), and feeding and energy homeostasis can modulate, but are not necessary for, the expression of FAA. In sum, the approaches to date have been largely unsuccessful in discovering the timekeeping mechanism of the FEO. Moving forward, we propose the use of standardized and optimized experimental protocols that focus on identifying genes that alter the period of FAA in mutant and engineered mouse models. This approach is likely to permit discovery of molecular components of the FEO timekeeping mechanism.