The role of microRNA-132/212 in molecular circadian clock regulation

The mammalian central circadian clock is localized in the suprachiasmatic nucleus (SCN), and is responsible for orchestrating ~24-hour rhythms in behavior and physiology. An additional property of timekeeping in organisms is seasonal adaptation, or adaptation to seasonal variations in environmental conditions. Melatonin signaling has been studied for its role in adapting physiology to seasonal light/dark variations, yet the SCN is responsible for encoding and relaying daylength information by unknown mechanisms. MicroRNAs are small non-coding RNAs that regulate gene expression post-transcriptionally. MicroRNA-132 is part of a microRNA cluster, miR-132/212, that is often expressed in tandem, with well-characterized roles in neuronal plasticity. MiR-132 has been shown to modulate photic entrainment in acute paradigms; however, the role of miR-132/212 in acute and long-term circadian plasticity has not been explored. I aimed to do so in this study using a loss-of-function approach. I found that miR-132/212 ablation alters entrainment to non-24 hour cycles and seasonal photoperiods, and is accompanied by heightened clock protein rhythm amplitude and altered photoperiod-dependent clock protein expression patterns in the SCN. MiR-132/212-deficient SCN neurons show decreased dendritic spine density, which could potentially underlie the perturbed rhythmicity in clock gene expression and behavioral phenotypes. In the seasonal mammal Mesocricetus auratus, adaptation to winter-like photoperiods correlates with decreased dendritic spine density in SCN neurons, and is independent from melatonin signaling. MeCP2 (methyl-CpG binding protein 2) is a miR-132/212 target with roles in brain development and plasticity. MeCP2 expression is altered in miR-132/212-deficient SCN, as is its downstream gene BDNF (brain derived neurotrophic factor) and mTOR (mammalian target of rapamycin) signaling. I used in vitro and in vivo approaches to explore how MeCP2 modulates dendritic morphology phenotypes, and propose that the interaction between miR-132/212-regulated MeCP2, its downstream target BDNF, and the mTOR signaling pathway, is responsible for modulating SCN neuronal plasticity, which could be the basis for encoding seasonal time.Ph.D

GRK2: putting the brakes on the circadian clock

G protein-coupled receptor kinases (GRKs) are a family of serine/threonine protein kinases that terminate G protein-coupled receptor (GPCR) signaling by phosphorylating the receptor and inducing its internalization. In addition to their canonical function, some GRKs can phosphorylate non-GPCR substrates and regulate GPCR signaling in a kinase-independent manner. GPCRs are abundantly expressed in the suprachiasmatic nucleus (SCN), a structure in the mammalian brain that serves as the central circadian pacemaker. Various facets of circadian timekeeping are under the influence of GPCR signaling, and thus are potential targets for GRK regulation. Despite this, little attention has been given to the role of GRKs in circadian rhythms. In this research highlight, we discuss our latest findings on the functional involvement of GRK2 in mammalian circadian timekeeping in the SCN. Using grk2 knockout mice, we demonstrate that GRK2 is critical for maintaining proper clock speed and ensuring that the clock is appropriately synchronized to environmental light cycles. Although grk2 deficiency expectedly alters the expression of a key GPCR in the SCN, our study also reveals that GRK2 has a more direct function that touches the heart of the circadian clock

The Circadian Molecular Clock Regulates Adult Hippocampal Neurogenesis by Controlling the Timing of Cell-Cycle Entry and Exit

The subgranular zone (SGZ) of the adult hippocampus contains a pool of quiescent neural progenitor cells (QNPs) that are capable of entering the cell cycle and producing newborn neurons. The mechanisms that control the timing and extent of adult neurogenesis are not well understood. Here, we show that QNPs of the adult SGZ express molecular-clock components and proliferate in a rhythmic fashion. The clock proteins PERIOD2 and BMAL1 are critical for proper control of neurogenesis. The absence of PERIOD2 abolishes the gating of cell-cycle entrance of QNPs, whereas genetic ablation of bmal1 results in constitutively high levels of proliferation and delayed cell-cycle exit. We use mathematical model simulations to show that these observations may arise from clock-driven expression of a cell-cycle inhibitor that targets the cyclin D/Cdk4-6 complex. Our findings may have broad implications for the circadian clock in timing cell-cycle events of other stem cell populations throughout the body

Rapid-acting antidepressants and the circadian clock.

A growing number of epidemiological and experimental studies has established that circadian disruption is strongly associated with psychiatric disorders, including major depressive disorder (MDD). This association is becoming increasingly relevant considering that modern lifestyles, social zeitgebers (time cues) and genetic variants contribute to disrupting circadian rhythms that may lead to psychiatric disorders. Circadian abnormalities associated with MDD include dysregulated rhythms of sleep, temperature, hormonal secretions, and mood which are modulated by the molecular clock. Rapid-acting antidepressants such as subanesthetic ketamine and sleep deprivation therapy can improve symptoms within 24 h in a subset of depressed patients, in striking contrast to conventional treatments, which generally require weeks for a full clinical response. Importantly, animal data show that sleep deprivation and ketamine have overlapping effects on clock gene expression. Furthermore, emerging data implicate the circadian system as a critical component involved in rapid antidepressant responses via several intracellular signaling pathways such as GSK3β, mTOR, MAPK, and NOTCH to initiate synaptic plasticity. Future research on the relationship between depression and the circadian clock may contribute to the development of novel therapeutic strategies for depression-like symptoms. In this review we summarize recent evidence describing: (1) how the circadian clock is implicated in depression, (2) how clock genes may contribute to fast-acting antidepressants, and (3) the mechanistic links between the clock genes driving circadian rhythms and neuroplasticity

Hypothalamic circuits and aging: keeping the circadian clock updated

Over the past century, age-related diseases, such as cancer, type-2 diabetes, obesity, and mental illness, have shown a significant increase, negatively impacting overall quality of life. Studies on aged animal models have unveiled a progressive discoordination at multiple regulatory levels, including transcriptional, translational, and post-translational processes, resulting from cellular stress and circadian derangements. The circadian clock emerges as a key regulator, sustaining physiological homeostasis and promoting healthy aging through timely molecular coordination of pivotal cellular processes, such as stem-cell function, cellular stress responses, and inter-tissue communication, which become disrupted during aging. Given the crucial role of hypothalamic circuits in regulating organismal physiology, metabolic control, sleep homeostasis, and circadian rhythms, and their dependence on these processes, strategies aimed at enhancing hypothalamic and circadian function, including pharmacological and non-pharmacological approaches, offer systemic benefits for healthy aging. Intranasal brain-directed drug administration represents a promising avenue for effectively targeting specific brain regions, like the hypothalamus, while reducing side effects associated with systemic drug delivery, thereby presenting new therapeutic possibilities for diverse age-related conditions

RFamide-related peptide-3 (RFRP-3) suppresses sexual maturation in a eusocial mammal

Neuroendocrine mechanisms underlying social inhibition of puberty are not well understood. Here, we use a model exhibiting the most profound case of pubertal suppression among mammals to explore a role for RFamide-related peptide-3 [RFRP-3; mammalian ortholog to gonadotropin-inhibitory hormone (GnIH)] in neuroendocrine control of reproductive development. Naked mole rats (NMRs) live in sizable colonies where breeding is monopolized by two to four dominant animals, and no other members exhibit signs of puberty throughout their lives unless they are removed from the colony. Because of its inhibitory action on the reproductive axis in other vertebrates, we investigated the role of RFRP-3 in social reproductive suppression in NMRs. We report that RFRP-3 immunofluorescence expression patterns and RFRP-3/GnRH cross-talk are largely conserved in the NMR brain, with the exception of the unique presence of RFRP-3 cell bodies in the arcuate nucleus (Arc). Immunofluorescence comparisons revealed that central expression of RFRP-3 is altered by reproductive status, with RFRP-3 immunoreactivity enhanced in the paraventricular nucleus, dorsomedial nucleus, and Arc of reproductively quiescent NMRs. We further observed that exogenous RFRP-3 suppresses gonadal steroidogenesis and mating behavior in NMRs given the opportunity to undergo puberty. Together, our findings establish a role for RFRP-3 in preserving reproductive immaturity, and challenge the view that stimulatory peptides are the ultimate gatekeepers of puberty

GRK2 Fine-Tunes Circadian Clock Speed and Entrainment via Transcriptional and Post-translational Control of PERIOD Proteins

The pacemaker properties of the suprachiasmatic nucleus (SCN) circadian clock are shaped by mechanisms that influence the expression and behavior of clock proteins. Here, we reveal that G-protein-coupled receptor kinase 2 (GRK2) modulates the period, amplitude, and entrainment characteristics of the SCN. Grk2-deficient mice show phase-dependent alterations in light-induced entrainment, slower recovery from jetlag, and longer behavioral rhythms. Grk2 ablation perturbs intrinsic rhythmic properties of the SCN, increasing amplitude and decreasing period. At the cellular level, GRK2 suppresses the transcription of the mPeriod1 gene and the trafficking of PERIOD1 and PERIOD2 proteins to the nucleus. Moreover, GRK2 can physically interact with PERIOD1/2 and promote PERIOD2 phosphorylation at Ser545, effects that may underlie its ability to regulate PERIOD1/2 trafficking. Together, our findings identify GRK2 as an important modulator of circadian clock speed, amplitude, and entrainment by controlling PERIOD at the transcriptional and post-translational levels

miR-132/212 Modulates Seasonal Adaptation and Dendritic Morphology of the Central Circadian Clock

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RFamide-related peptide-3 (RFRP-3) suppresses sexual maturation in a eusocial mammal

Neuroendocrine mechanisms underlying social inhibition of puberty are not well understood. Here, we use a model exhibiting the most profound case of pubertal suppression among mammals to explore a role for RFamide-related peptide-3 [RFRP-3; mammalian ortholog to gonadotropin-inhibitory hormone (GnIH)] in neuroendocrine control of reproductive development. Naked mole rats (NMRs) live in sizable colonies where breeding is monopolized by two to four dominant animals, and no other members exhibit signs of puberty throughout their lives unless they are removed from the colony. Because of its inhibitory action on the reproductive axis in other vertebrates, we investigated the role of RFRP-3 in social reproductive suppression in NMRs. We report that RFRP-3 immunofluorescence expression patterns and RFRP-3/GnRH cross-talk are largely conserved in the NMR brain, with the exception of the unique presence of RFRP-3 cell bodies in the arcuate nucleus (Arc). Immunofluorescence comparisons revealed that central expression of RFRP-3 is altered by reproductive status, with RFRP-3 immunoreactivity enhanced in the paraventricular nucleus, dorsomedial nucleus, and Arc of reproductively quiescent NMRs. We further observed that exogenous RFRP-3 suppresses gonadal steroidogenesis and mating behavior in NMRs given the opportunity to undergo puberty. Together, our findings establish a role for RFRP-3 in preserving reproductive immaturity, and challenge the view that stimulatory peptides are the ultimate gatekeepers of puberty