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Paraspeckles are nuclear ribonucleic complex formed of a long non-coding RNA, nuclear-enriched abundant transcript one (Neat1) and associated RNA-binding proteins (RBP) whose cellular known functions are to sequester in the nucleus both proteins and RNAs. However, how RNAs are bound to paraspeckles is largely unknown. It is highly likely that binding of RNAs may occur via interactions with RBPs and accordingly, two structures present in the 3ʹUTR of some RNAs have been shown to allow their association to paraspeckles via protein binding. However, Neat1 could also be involved in the targeting of RNAs through direct RNA–RNA interactions. Using an RNA pull-down procedure adapted to select only RNAs engaged in direct RNA–RNA interactions and followed by RNA-seq we showed that in a rat pituitary cell line, GH4C1 cells, 1791 RNAs were associated with paraspeckles by direct interaction with Neat1. Neat1 was actually found able to bind more than 30% of the total transcripts targeted by the paraspeckles, we have identified in this cell line in a previous study. Furthermore, given the biological processes in which direct RNAs targets of Neat1 were involved as determined by gene ontology analysis, it was proposed that Neat1 played a major role in paraspeckle functions such as circadian rhythms, mRNA processing, RNA splicing and regulation of cell cycle. Finally, we provided evidence that direct RNA targets of Neat1 were preferentially bound to the 5ʹ end of Neat1 demonstrating that they are located in the shell region of paraspeckles. [ABSTRACT FROM AUTHOR]

Circadian clocks regulate rhythmic gene expression levels by means of mRNA oscillations that are mainly driven by post-transcriptional regulation. We identified a new post-transcriptional mechanism, which involves nuclear bodies called paraspeckles. Major components of paraspeckles including the long noncoding RNA Neat1, which is the structural component, and its major protein partners, as well as the number of paraspeckles, follow a circadian pattern in pituitary cells. Paraspeckles are known to retain within the nucleus RNAs containing inverted repeats of Alu sequences. We showed that a reporter gene in which these RNA duplex elements were inserted in the 3’-UTR region displayed a circadian expression. Moreover, circadian endogenous mRNA associated with paraspeckles lost their circadian pattern when paraspeckles were disrupted. This work not only highlights a new paraspeckle-based post-transcriptional mechanism involved in circadian gene expression but also provides the list of all mRNA associated with paraspeckles in the nucleus of pituitary cells. [ABSTRACT FROM AUTHOR]

Most clock-controlled genes (CCGs) lack the specific E-box response element necessary for direct circadian regulation. This is the case for the prolactin (Prl) gene, the expression of which oscillates in individual lactotrope pituitary cells. To characterize the processes underlying this oscillation, we used a lactotrope cell line (GH4C1 cells). In these cells, Prl gene expression fluctuated significantly during 24 h (P=0.0418). Circadian Prl transcription depended on an interaction between the pituitary-specific transcription factor, PIT-1, and the helicase-like transcription factor (HLTF), a SWI/SNF chromatin remodeler, shown here to bind the Prl promoter on an E-box that differs from the specific E-box preferentially bound by clock proteins. Circadian Prl transcription was further accompanied by marked daily chromatin transitions. While neither HLTF nor PIT-1 was rhythmically expressed, NONO and SFPQ, identified as HLTF-associated proteins by mass spectrometry, displayed a circadian pattern and bound rhythmically to the Prl promoter. Furthermore, NONO and SFPQ were functionally involved in circadian Prl transcription since overexpression of both proteins greatly reduced Prl promoter activity (P<0.001) and disrupted its circadian pattern. A mechanism involving a rhythm in paraspeckle protein recruitment is proposed to explain how the core oscillator can generate a circadian pattern of CCGs lacking the specific E-box response element. [ABSTRACT FROM AUTHOR]

Rhythmic biological functions in mammals are orchestrated by a circadian timekeeper in the suprachiasmatic nucleus of the hypothalamus (SCN) which precisely adjusts clock outputs to solar time through the process of photic synchronization. Entrainment to the 24-h light-dark cycle is known to act on the molecular loops which trigger circadian oscillations but is also thought to involve day-night adjustments in the intercellular phasing of the multiple component SCN oscillators. This view is supported by data showing that the SCN undergoes important rearrangements of its neuroglial architecture throughout the 24-h cycle. The present paper highlights our data showing in rat that the two main sources of SCN efferents, composed of neurons synthesizing either vasopressin (AVP) or vasoactive intestinal peptide (VIP), are diffentially involved in day-night SCN neuroglial plasticity. We found that the synaptic inputs received by the VIP neurons, which are major integrators of photic signals in the retinorecipient SCN subregion, increased during the day while those received by the AVP neurons remained unchanged at day and night. Glutamatergic axons, known to convey photic information from the retina, together with nonglutamatergic axons, contribute to the synaptic remodellings on VIP neurons. Experimental data providing strong indication that these plastic events may subserve synchronization of the clock to the light-dark cycle and that the daily fluctuations of plasma glucocorticoid hormones may act as temporal endocrine signals that may modulate SCN neuroglial plasticity through the rhythmic release of serotonin are also reviewed. [ABSTRACT FROM AUTHOR]

The daily temporal organization of rhythmic functions in mammals, which requires synchronization of the circadian clock to the 24-h light–dark cycle, is believed to involve adjustments of the mutual phasing of the cellular oscillators that comprise the time-keeper within the suprachiasmatic nucleus of the hypothalamus (SCN). Following from a previous study showing that the SCN undergoes day/night rearrangements of its neuronal–glial network that may be crucial for intercellular phasing, we investigated the contribution of glutamatergic synapses, known to play major roles in SCN functioning, to such rhythmic plastic events. Neither expression levels of the vesicular glutamate transporters nor numbers of glutamatergic terminals showed nycthemeral variations in the SCN. However, using quantitative imaging after combined immunolabelling, the density of synapses on neurons expressing vasoactive intestinal peptide, known as targets of the retinal input, increased during the day and both glutamatergic and non-glutamatergic synapses contributed to the increase (+36%). This was not the case for synapses made on vasopressin-containing neurons, the other major source of SCN efferents in the non-retinorecipient region. Together with electron microscope observations showing no differences in the morphometric features of glutamatergic terminals during the day and night, these data show that the light synchronization process in the SCN involves a selective remodelling of synapses at sites of photic integration. They provide a further illustration of how the adult brain may rapidly and reversibly adapt its synaptic architecture to functional needs. [ABSTRACT FROM AUTHOR]

Extracellular regulated kinase (ERK) signalling is believed to play roles in various aspects of circadian clock mechanisms. In this study, we show in rat that the nuclear versus cytoplasmic intracellular distribution of the phosphorylated forms of ERK1/2 (P-ERK1/2) in the central clock, namely the suprachiasmatic nucleus (SCN), is proportionally constant across the light/dark cycle while the spatial distribution and neurochemical phenotype of cells expressing these activated forms are time-regulated according to a daily rhythm and light-regulated. P-ERK1/2 was exclusively found in neuronal elements. At daytime, it was detected throughout the dorsoventral extent of the SCN, partly within neurons synthesizing either arginine-vasopressin or vasoactive intestinal peptide (VIP). At night time, it was segregated in the ventrolateral aspect of the nucleus, within a cluster of cells 45% of which were gastrin-releasing peptide (GRP) neurons with or without co-localization with VIP. After a light pulse at night, expression of P-ERK1/2 increased in GRP neurons but also appeared in a population of neurons that stained for VIP only. These data show that the GRP neurons are closely associated with ERK1/2 activation at night and point to the importance of ERK1/2 signalling not only in intra-SCN transmission of photic information but also in maintenance of neuronal rhythms in the SCN. [ABSTRACT FROM AUTHOR]

The question of involvement of glucocorticoid hormones as temporal signals for the synchronization of the timekeeping system was addressed in rats with different corticosterone status. The authors showed that adrenalectomy had no effects on the synchronization of wheel-running activity rhythms to a steadystate LD 12:12 cycle, regardless of whether it was compensated for by a corticosterone replacement therapy that either reinstated constant plasma concentrations of the hormone or mimicked its natural rhythm. However, after a 12-h phase shift (daylight reversal), the lack of circulating corticosterone induced a significant shortening of the resynchronization rate (less than 3 days vs. 7 days). Normalization required restoration of a rhythmic corticosterone secretion that was synchronized to the new photoperiod. Under constant darkness, the corticosterone rhythm did not show any synchronizing effect, providing evidence that it participates in entrainment of the locomotor activity rhythm through modulation of light effects. It is proposed that, under stable lighting conditions, circulating glucocorticoids contribute to stabilizing activity rhythms by reinforcing resistance of the circadian timing system to variations of the photoperiod. Experimental evidence that serotonergic neurons are involved in relaying their modulatory effects to the clock is also presented. [ABSTRACT FROM AUTHOR]

Luteinizing hormone-releasing hormone (LHRH release, which serves as the primary drive to the hypothalamic-pituitary gonaldal axis, is controlled by many neuro-mediators. Serotonin has been implicated in this regulation. However, it is unclear whether the central effect of serotonin on LHRH secretion is exerted directly on LHRH neurosecretory neurons or indirectly via multisynaptic pathways. The present studies were undertaken in order to examine whether LHRH secretion from immortalized LHRH cell lines is directly regulated by serotonin and, if so, to identify the receptor subtype involved. 8-hydroxy-2-(di- n-propylamino)tetralin (8-OH-DPAT), a 5-HT receptor agonist, stimulated LHRH release from GT1-1 cells. This effect was blocked by ritanserin, a 5-HT receptor antagonist, but not by SDZ-216-525, a 5-HT antagonist. Basal LHRH release was not affected by the 5-HT agonist DOI. Reverse transcription and polymerase chain reaction technique (RT-PCR) was used in order to identify 5-HT and 5-HT receptor mRNA in immortalized LHRH cell lines. GT1-1 cells express mRNA for the 5-HT, but not the 5-HT receptor subtypes. These results demonstrate a direct stimulatory effect of serotonin on LHRH release via 5-HT receptor. [ABSTRACT FROM AUTHOR]