Regulatory Mechanisms Controlling Maturation of Serotonin Neuron Identity and Function

The brain serotonin (5-hydroxytryptamine; 5-HT) system has been extensively studied for its role in normal physiology and behavior, as well as, neuropsychiatric disorders. The broad influence of 5-HT on brain function, is in part due to the vast connectivity pattern of 5-HT-producing neurons throughout the CNS. 5-HT neurons are born and terminally specified midway through embryogenesis, then enter a protracted period of maturation, where they functionally integrate into CNS circuitry and then are maintained throughout life. The transcriptional regulatory networks controlling progenitor cell generation and terminal specification of 5-HT neurons are relatively well-understood, yet the factors controlling 5-HT neuron maturation are only recently coming to light. In this review, we first provide an update on the regulatory network controlling 5-HT neuron development, then delve deeper into the properties and regulatory strategies governing 5-HT neuron maturation. In particular, we discuss the role of the 5-HT neuron terminal selector transcription factor (TF) Pet-1 as a key regulator of 5-HT neuron maturation. Pet-1 was originally shown to positively regulate genes needed for 5-HT synthesis, reuptake and vesicular transport, hence 5-HT neuron-type transmitter identity. It has now been shown to regulate, both positively and negatively, many other categories of genes in 5-HT neurons including ion channels, GPCRs, transporters, neuropeptides, and other transcription factors. Its function as a terminal selector results in the maturation of 5-HT neuron excitability, firing characteristics, and synaptic modulation by several neurotransmitters. Furthermore, there is a temporal requirement for Pet-1 in the control of postmitotic gene expression trajectories thus indicating a direct role in 5-HT neuron maturation. Proper regulation of the maturation of cellular identity is critical for normal neuronal functioning and perturbations in the gene regulatory networks controlling these processes may result in long-lasting changes in brain function in adulthood. Further study of 5-HT neuron gene regulatory networks is likely to provide additional insight into how neurons acquire their mature identities and how terminal selector-type TFs function in postmitotic vertebrate neurons

Shared Long-Range Regulatory Elements Coordinate Expression of a Gene Cluster Encoding Nicotinic Receptor Heteromeric Subtypes

The nicotinic acetylcholine receptor (nAChR) β4/α3/α5 gene cluster encodes several heteromeric transmitter receptor subtypes that are essential for cholinergic synaptic transmission in adrenal gland, autonomic ganglia, pineal gland, and several nuclei in the central nervous system. However, the transcriptional mechanisms coordinating expression of these subunit genes in different cell populations are unknown. Here, we used transgenic methods to investigate long-range transcriptional control of the cluster. A 132-kb P1-derived artificial chromosome (PAC) encoding the rat cluster recapitulated the neurally- and endocrine-restricted expression patterns of the endogenous β4/α3/α5 genes. Mutation of ETS factor binding sites in an enhancer, β43′, embedded in the β4 3′-untranslated exon resulted in greatly diminished β4, α3, and α5 expression in adrenal gland and to a lesser extent in the superior cervical ganglion (SCG) but not in other tissues. Phylogenetic sequence analyses revealed several conserved noncoding regions (CNRs) upstream of β4 and α5. Deletion of one of them (CNR4) located 20 kb upstream of β4 resulted in a dramatic decrease in β4 and α3 expression in the pineal gland and SCG. CNR4 was sufficient to direct LacZ transgene expression to SCG neurons, which express the endogenous β4α3α5 subunits, and pineal cells, which express the endogenous β4α3 combination. Finally, CNR4 was able to direct transgene expression to major sites of expression of the endogenous cluster in the brain. Together, our findings support a model in which cell type-specific shared long-range regulatory elements are required for coordinate expression of clustered nAChR genes

Pet-1 Deficiency Alters The Circadian Clock And Its Temporal Organization Of Behavior

The serotonin and circadian systems are two important interactive regulatory networks in the mammalian brain that regulate behavior and physiology in ways that are known to impact human mental health. Previous work on the interaction between these two systems suggests that serotonin modulates photic input to the central circadian clock (the suprachiasmatic nuclei; SCN) from the retina and serves as a signal for locomotor activity, novelty, and arousal to shift the SCN clock, but effects of disruption of serotonergic signaling from the raphe nuclei on circadian behavior and on SCN function are not fully characterized. In this study, we examined the effects on diurnal and circadian behavior, and on ex vivo molecular rhythms of the SCN, of genetic deficiency in Pet-1, an ETS transcription factor that is necessary to establish and maintain the serotonergic phenotype of raphe neurons. Pet-1-/- mice exhibit loss of rhythmic behavioral coherence and an extended daily activity duration, as well as changes in the molecular rhythms expressed by the clock, such that ex vivo SCN from Pet-1-/- mice exhibit period lengthening and sex-dependent changes in rhythmic amplitude. Together, our results indicate that Pet-1 regulation of raphe neuron serotonin phenotype contributes to the period, precision and light/dark partitioning of locomotor behavioral rhythms by the circadian clock through direct actions on the SCN clock itself, as well as through non-clock effects. © 2014 Ciarleglio et al

A brain-wide functional map of the serotonergic responses to acute stress and fluoxetine

Central serotonin (5-HT) orchestrates myriad cognitive processes and lies at the core of many stress-related psychiatric illnesses. However, the basic relationship between its brain-wide axonal projections and functional dynamics is not known. Here we combine optogenetics and fMRI to produce a brain-wide 5-HT evoked functional map. We find that DRN photostimulation leads to an increase in the hemodynamic response in the DRN itself, while projection areas predominately exhibit a reduction of cerebral blood volume mirrored by suppression of cortical delta oscillations. We find that the regional distribution of post-synaptically expressed 5-HT receptors better correlates with DRN 5-HT functional connectivity than anatomical projections. Our work suggests that neuroarchitecture is not the primary determinant of function for the DRN 5-HT. With respect to two 5-HT elevating stimuli, we find that acute stress leads to circuit-wide blunting of the DRN output, while the SSRI fluoxetine noticeably enhances DRN functional connectivity. These data provide fundamental insight into the brain-wide functional dynamics of the 5-HT projection system

Main effects of genotype on circadian behavioral characteristics in 12L:12D (a–c) and constant darkness (d–f).

<div><p>| </p><p> </p><p></p><p></p><p></p><p></p><p> </p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p> </p><p>| | </p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p>| | </p><p><a> </a></p><p></p><p><a>| </a></p><p><a> </a></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p><a> </a></p><p></p><p></p><p></p><p><a> </a></p><p></p><p></p><p></p><p></p><p></p><p><a> </a></p><p><a>| | </a></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p><a>| | </a></p><p></p><p></p><p><a> </a></p><p></p><p><a>| <b> </b></a></p><p></p><p></p><p><a><b>| </b></a><b><a> | <b> </b></a></b></p><p></p><p></p><p><b><a><b>| </b></a></b></p><p></p><p></p><p></p><p><b><a><b> </b></a></b></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p><b><a><b><u> </u></b></a></b></p><p></p><p></p><p><b><a><b><u> </u></b></a></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a>| </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p></p><p><b><b><u><a>| </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a> </a></u></b></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p><b><b><u><a>| </a></u></b></b></p><p><b><b><u><a> <u> </u></a></u></b></b></p><p><b><b><u><a> | | | </a></u></b></b></p><p><b><b><u><a> <u> </u></a></u></b></b></p><p></p><p><b><b><u><a> | </a></u></b></b></p><p></p><p><b><b><u><a> </a></u></b></b></p><p></p><p></p><p><b><b><u><a><b> </b></a></u></b></b></p><p></p><p></p><p><b><b><u><a><b>| <b> | <b> </b></b></b></a></u></b></b></p><p></p><p></p><p><u><a><b><b><b>| </b></b></b></a></u></p><p><u><a><b><b><b> </b></b></b></a></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a>| </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p><u><b><b><b><a>| </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a>| </a></b></b></b></u></p><p><u><b><b><b><a> <u> </u></a></b></b></b></u></p><p><u><b><b><b><a> | | | </a></b></b></b></u></p><p><u><b><b><b><a> <u> </u></a></b></b></b></u></p><p></p><p><u><b><b><b><a> | </a></b></b></b></u></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p></p><p></p><p></p><p></p><p><u><b><b><b><a> <b> </b></a></b></b></b></u></p><p><u><b><b><a><b> </b></a></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p><u><b><b><b><a> </a></b></b></b></u></p><p><u><b><b><b><a> </a></b></b></b></u></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p><u><b><b><b><a><u> </u></a></b></b></b></u></p><p></p><p></p><p><u><b><b><b><a><u> </u></a></b></b></b></u></p><p></p><p></p><p><u><b><b><b><a><u><u> </u></u></a></b></b></b></u></p><p><u><b><b><b><u><u><a> </a></u></u></b></b></b></u></p><p><u><b><b><b><u><u><a><i></i></a></u></u></b></b></b></u></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p></div

Main effects of genotype (and photoperiod) on circadian behavioral phenotypes.

<p>Activity monitored by infrared motion detector and wheel in LD and DD, as denoted. <i>Ex vivo Per1::Luc</i> expression is below. Significance is ascribed at <b><i>p</i></b><b>≤0.05</b>.</p