Analysis of the QTL for sleep homeostasis in mice:Homer1ais a likely candidate

Remembering and forgetting in sleep: Selective synaptic plasticity during sleep driven by scaling factors Homer1a and Arc

Sleep is a conserved and essential process that supports learning and memory. Synapses are a major target of sleep function and a locus of sleep need. Evidence in the literature suggests that the need for sleep has a cellular or microcircuit level basis, and that sleep need can accumulate within localized brain regions as a function of waking activity. Activation of sleep promoting kinases and accumulation of synaptic phosphorylation was recently shown to be part of the molecular basis for the localized sleep need. A prominent hypothesis in the field suggests that some benefits of sleep are mediated by a broad but selective weakening, or scaling-down, of synaptic strength during sleep in order to offset increased excitability from synaptic potentiation during wake. The literature also shows that synapses can be strengthened during sleep, raising the question of what molecular mechanisms may allow for selection of synaptic plasticity types during sleep. Here I describe mechanisms of action of the scaling factors Arc and Homer1a in selective plasticity and links with sleep need. Arc and Homer1a are induced in neurons in response to waking neuronal activity and accumulate with time spent awake. I suggest that during sleep, Arc and Homer1a drive broad weakening of synapses through homeostatic scaling-down, but in a manner that is sensitive to the plasticity history of individual synapses, based on patterned phosphorylation of synaptic proteins. Therefore, Arc and Homer1a may offer insights into the intricate links between a cellular basis of sleep need and memory consolidation during sleep.

Arc and Homer1 are involved in comorbid epilepsy and depression: A microarray data analysis

BACKGROUND

Depression is one of the most common comorbid psychiatric condition associated with epilepsy. It has a negative impact on the patient's quality of life. However, the underlying molecular mechanisms leading to depression are currently unclear. The aim of this study was to determine the hub genes associated with epilepsy and depression.

METHODS

Gene expression profiles (GSE47752 and GSE20388) were downloaded from the gene expression omnibus (GEO) database. Differentially expressed genes (DEGs) for epilepsy and depression groups were separately searched. Subsequently, network analyses methods were employed to establish protein-protein interaction (PPI) networks, and to perform Gene Ontology (GO) terms and pathway enrichment analyses for co-expressed DEGs.

RESULTS

A total of 772 genes were upregulated in patients with epilepsy whereas 91 genes were up-regulated in patients with depression. In addition, 1304 genes were down-regulated in epilepsy whereas 141 genes were down-regulated in patients with depression. Among co-expressed DEGs, 5 DEGs were up-regulated and 19 were down-regulated. Further analysis revealed that the co-expressed DEGs were involved in regulation of vasculature development, regulation of angiogenesis, glutamate receptor signaling pathway, cellular response to interleukin-1 and positive regulation of protein kinase B signaling. The Arc and Homer1 genes were identified as the common candidate genes involved in the pathogenesis of epilepsy and depression.

CONCLUSIONS

Arc and Homer1 may contribute to the comorbidity of epilepsy and depression.

Depriving Mice of Sleep also Deprives of Food

Both sleep-wake behavior and circadian rhythms are tightly coupled to energy metabolism and food intake. Altered feeding times in mice are known to entrain clock gene rhythms in the brain and liver, and sleep-deprived humans tend to eat more and gain weight. Previous observations in mice showing that sleep deprivation (SD) changes clock gene expression might thus relate to altered food intake, and not to the loss of sleep per se. Whether SD affects food intake in the mouse and how this might affect clock gene expression is, however, unknown. We therefore quantified (i) the cortical expression of the clock genes Per1, Per2, Dbp, and Cry1 in mice that had access to food or not during a 6 h SD, and (ii) food intake during baseline, SD, and recovery sleep. We found that food deprivation did not modify the SD-incurred clock gene changes in the cortex. Moreover, we discovered that although food intake during SD did not differ from the baseline, mice lost weight and increased food intake during subsequent recovery. We conclude that SD is associated with food deprivation and that the resulting energy deficit might contribute to the effects of SD that are commonly interpreted as a response to sleep loss.

Hippocampal neurons' cytosolic and membrane-bound ribosomal transcript profiles are differentially regulated by learning and subsequent sleep

Sleep loss disrupts consolidation of hippocampus-dependent memory. To understand the cellular basis for this effect, we quantified RNAs associated with translating ribosomes in cytosol and on cellular membranes of different hippocampal neuron populations. Our analysis suggests that while sleep loss (but not learning) alters numerous ribosomal transcripts in cytosol, learning has dramatic effects on transcript profiles for less–well-characterized membrane-bound ribosomes. We demonstrate that postlearning sleep deprivation occludes already minimal learning-driven changes on cytosolic ribosomes. It simultaneously alters transcripts associated with metabolic and biosynthetic processes in membrane-bound ribosomes in excitatory hippocampal neurons and highly active, putative “engram” neurons, respectively. Together, these findings provide insights into the cellular mechanisms altered by learning and their disruption by subsequent sleep loss.

The hippocampus is essential for consolidating transient experiences into long-lasting memories. Memory consolidation is facilitated by postlearning sleep, although the underlying cellular mechanisms are largely unknown. We took an unbiased approach to this question by using a mouse model of hippocampally mediated, sleep-dependent memory consolidation (contextual fear memory). Because synaptic plasticity is associated with changes to both neuronal cell membranes (e.g., receptors) and cytosol (e.g., cytoskeletal elements), we characterized how these cell compartments are affected by learning and subsequent sleep or sleep deprivation (SD). Translating ribosome affinity purification was used to profile ribosome-associated RNAs in different subcellular compartments (cytosol and membrane) and in different cell populations (whole hippocampus, Camk2a+ neurons, or highly active neurons with phosphorylated ribosomal subunit S6 [pS6+]). We examined how transcript profiles change as a function of sleep versus SD and prior learning (contextual fear conditioning; CFC). While sleep loss altered many cytosolic ribosomal transcripts, CFC altered almost none, and CFC-driven changes were occluded by subsequent SD. In striking contrast, SD altered few transcripts on membrane-bound (MB) ribosomes, while learning altered many more (including long non-coding RNAs [lncRNAs]). The cellular pathways most affected by CFC were involved in structural remodeling. Comparisons of post-CFC MB transcript profiles between sleeping and SD mice implicated changes in cellular metabolism in Camk2a+ neurons and protein synthesis in highly active pS6+ (putative “engram”) neurons as biological processes disrupted by SD. These findings provide insights into how learning affects hippocampal neurons and suggest that the effects of SD on memory consolidation are cell type and subcellular compartment specific.

The Complex Formed by Group I Metabotropic Glutamate Receptor (mGluR) and Homer1a Plays a Central Role in Metaplasticity and Homeostatic Synaptic Scaling

G-protein-coupled receptors can be constitutively activated following physical interaction with intracellular proteins. The first example described was the constitutive activation of Group I metabotropic glutamate receptors (mGluR: mGluR1,5) following their interaction with Homer1a, an activity-inducible early-termination variant of the scaffolding protein Homer that lacks dimerization capacity (Ango et al., 2001). Homer1a disrupts the links, maintained by the long form of Homer (cross-linking Homers), between mGluR1,5 and the Shank-GKAP-PSD-95-ionotropic glutamate receptor network. Two characteristics of the constitutive activation of the Group I mGluR-Homer1a complex are particularly interesting: (1) it affects a large number of synapses in which Homer1a is upregulated following enhanced, long-lasting neuronal activity; and (2) it mainly depends on Homer1a protein turnover. The constitutively active Group I mGluR-Homer1a complex is involved in the two main forms of non-Hebbian neuronal plasticity: "metaplasticity" and "homeostatic synaptic scaling," which are implicated in a large series of physiological and pathologic processes. Those include non-Hebbian plasticity observed in visual system, synapses modulated by addictive drugs (rewarded synapses), chronically overactivated synaptic networks, normal sleep, and sleep deprivation.

High-throughput sleep phenotyping produces robust and heritable traits in Diversity Outbred mice and their founder strains

STUDY OBJECTIVES

This study describes high-throughput phenotyping strategies for sleep and circadian behavior in mice, including examinations of robustness, reliability, and heritability among Diversity Outbred (DO) mice and their eight founder strains.

METHODS

We performed high-throughput sleep and circadian phenotyping in male mice from the DO population (n = 338) and their eight founder strains: A/J (n = 6), C57BL/6J (n = 14), 129S1/SvlmJ (n = 6), NOD/LtJ (n = 6), NZO/H1LtJ (n = 6), CAST/EiJ (n = 8), PWK/PhJ (n = 8), and WSB/EiJ (n = 6). Using infrared beam break systems, we defined sleep as at least 40 s of continuous inactivity and quantified sleep-wake amounts and bout characteristics. We developed assays to measure sleep latency in a new environment and during a modified Murine Multiple Sleep Latency Test, and estimated circadian period from wheel-running experiments. For each trait, broad-sense heritability (proportion of variability explained by all genetic factors) was derived in founder strains, while narrow-sense heritability (proportion of variability explained by additive genetic effects) was calculated in DO mice.

RESULTS

Phenotypes were robust to different inactivity durations to define sleep. Differences across founder strains and moderate/high broad-sense heritability were observed for most traits. There was large phenotypic variability among DO mice, and phenotypes were reliable, although estimates of heritability were lower than in founder mice. This likely reflects important nonadditive genetic effects.

CONCLUSIONS

A high-throughput phenotyping strategy in mice, based primarily on monitoring of activity patterns, provides reliable and heritable estimates of sleep and circadian traits. This approach is suitable for discovery analyses in DO mice, where genetic factors explain some proportion of phenotypic variation.

Mutations in Metabotropic Glutamate Receptor 1 Contribute to Natural Short Sleep Trait

Sufficient and efficient sleep is crucial for our health. Natural short sleepers can sleep significantly shorter than the average population without a desire for more sleep and without any obvious negative health consequences. In searching for genetic variants underlying the short sleep trait, we found two different mutations in the same gene (metabotropic glutamate receptor 1) from two independent natural short sleep families. In vitro, both of the mutations exhibited loss of function in receptor-mediated signaling. In vivo, the mice carrying the individual mutations both demonstrated short sleep behavior. In brain slices, both of the mutations changed the electrical properties and increased excitatory synaptic transmission. These results highlight the important role of metabotropic glutamate receptor 1 in modulating sleep duration.

Dynamic changes in cerebral and peripheral markers of glutamatergic signaling across the human sleep-wake cycle

Sleep and brain glutamatergic signaling are homeostatically regulated. Recovery sleep following prolonged wakefulness restores efficient functioning of the brain, possibly by keeping glutamatergic signaling in a homeostatic range. Evidence in humans and mice suggested that metabotropic glutamate receptors of subtype-5 (mGluR5) contribute to the brain's coping mechanisms with sleep deprivation. Here, proton magnetic resonance spectroscopy in 31 healthy men was used to quantify the levels of glutamate (Glu), glutamate-to-glutamine ratio (GLX), and γ-amino-butyric-acid (GABA) in basal ganglia (BG) and dorsolateral prefrontal cortex on 3 consecutive days, after ~8 (baseline), ~32 (sleep deprivation), and ~8 hours (recovery sleep) of wakefulness. Simultaneously, mGluR5 availability was quantified with the novel radioligand for positron emission tomography, [18F]PSS232, and the blood levels of the mGluR5-regulated proteins, fragile X mental retardation protein (FMRP) and brain-derived neurotrophic factor (BDNF) were determined. The data revealed that GLX (p = 0.03) in BG (for Glu: p < 0.06) and the serum concentration of FMRP (p < 0.04) were increased after sleep loss. Other brain metabolites (GABA, N-acetyl-aspartate, choline, glutathione) and serum BDNF levels were not altered by sleep deprivation (pall > 0.6). By contrast, the night without sleep enhanced whole-brain, BG, and parietal cortex mGluR5 availability, which was normalized by recovery sleep (pall < 0.05). The findings provide convergent multimodal evidence that glutamatergic signaling is affected by sleep deprivation and recovery sleep. They support a role for mGluR5 and FMRP in sleep-wake regulation and warrant further studies to investigate their causality and relevance for regulating human sleep in health and disease. Clinical Trial Registration: www.clinicaltrials.gov (study identifier: NCT03813082).

Recent advances in understanding the genetics of sleep

Sleep is a ubiquitous and complex behavior in both its manifestation and regulation. Despite its essential role in maintaining optimal performance, health, and well-being, the genetic mechanisms underlying sleep remain poorly understood. Here, we review the forward genetic approaches undertaken in the last four years to elucidate the genes and gene pathways affecting sleep and its regulation. Despite an increasing number of studies and mining large databases, a coherent picture on “sleep” genes has yet to emerge. We highlight the results achieved by using unbiased genetic screens mainly in humans, mice, and fruit flies with an emphasis on normal sleep and make reference to lessons learned from the circadian field.

Acute Sleep Loss Upregulates the Synaptic Scaffolding Protein, Homer1a, in Non-canonical Sleep/Wake Brain Regions, Claustrum, Piriform and Cingulate Cortices

Homer proteins are a component of the post-synaptic density of neurons that are necessary for the maintenance and consolidation of behavioral state. The dominant negative protein homer1a is rapidly increased by neuronal activity and sleep loss. Homer1a knockout mice with globally absent homer1a have reduced ability to sustain wakefulness during the active period. It is not known whether homer1a is required globally or in very specific brain regions or neurons for its role in maintaining wake. In this study, we examined the expression of homer1a, an immediate early gene involved in intracellular signaling cascades, in mice subjected to extended wakefulness. We found that mice displayed increased expression of homer1a in the claustrum, a brain region thought to be involved in consciousness, as well as the cingulate and piriform cortices compared to non-sleep deprived mice. In situ hybridization (ISH) studies also indicate that homer1a is not induced in the known wake promoting regions with sleep deprivation, but is instead upregulated primarily in the claustrum and piriform cortex. Examination of homer1a expression levels with recovery sleep after sleep deprivation indicate that baseline homer1a expression levels were restored. Further, we have identified that homer1a is upregulated in excitatory neurons of the claustrum suggesting that homer1a promotes wakefulness through activating excitatory neurons. This work identifies regions previously unknown to be involved in sleep regulation that respond to acute sleep deprivation or enhanced waking.

Sleep and synaptic down-selection

The synaptic homeostasis hypothesis (SHY) proposes that sleep is an essential process needed by the brain to maintain the total amount of synaptic strength under control. SHY predicts that by the end of a waking day the synaptic connections of many neural circuits undergo a net increase in synaptic strength due to ongoing learning, which is mainly mediated by synaptic potentiation. Stronger synapses require more energy and supplies and are prone to saturation, creating the need for synaptic renormalization. Such renormalization should mainly occur during sleep, when the brain is disconnected from the environment and neural circuits can be broadly reactivated off-line to undergo a systematic but specific synaptic down-selection. In short, according to SHY sleep is the price to pay for waking plasticity, to avoid runaway potentiation, decreased signal-to-noise ratio, and impaired learning due to saturation. In this review, we briefly discuss the rationale of the hypothesis and recent supportive ultrastructural evidence obtained in our laboratory. We then examine recent studies by other groups showing the causal role of cortical slow waves and hippocampal sharp waves/ripples in sleep-dependent down-selection of neural activity and synaptic strength. Finally, we discuss some of the molecular mechanisms that could mediate synaptic weakening during sleep.

Cerebral mGluR5 availability contributes to elevated sleep need and behavioral adjustment after sleep deprivation

Increased sleep time and intensity quantified as low-frequency brain electrical activity after sleep loss demonstrate that sleep need is homeostatically regulated, yet the underlying molecular mechanisms remain elusive. We here demonstrate that metabotropic glutamate receptors of subtype 5 (mGluR5) contribute to the molecular machinery governing sleep-wake homeostasis. Using positron emission tomography, magnetic resonance spectroscopy, and electroencephalography in humans, we find that increased mGluR5 availability after sleep loss tightly correlates with behavioral and electroencephalographic biomarkers of elevated sleep need. These changes are associated with altered cortical myo-inositol and glycine levels, suggesting sleep loss-induced modifications downstream of mGluR5 signaling. Knock-out mice without functional mGluR5 exhibit severe dysregulation of sleep-wake homeostasis, including lack of recovery sleep and impaired behavioral adjustment to a novel task after sleep deprivation. The data suggest that mGluR5 contribute to the brain's coping mechanisms with sleep deprivation and point to a novel target to improve disturbed wakefulness and sleep.

Linking Network Activity to Synaptic Plasticity during Sleep: Hypotheses and Recent Data

Research findings over the past two decades have supported a link between sleep states and synaptic plasticity. Numerous mechanistic hypotheses have been put forth to explain this relationship. For example, multiple studies have shown structural alterations to synapses (including changes in synaptic volume, spine density, and receptor composition) indicative of synaptic weakening after a period of sleep. Direct measures of neuronal activity and synaptic strength support the idea that a period of sleep can reduce synaptic strength. This has led to the synaptic homeostasis hypothesis (SHY), which asserts that during slow wave sleep, synapses are downscaled throughout the brain to counteract net strengthening of network synapses during waking experience (e.g., during learning). However, neither the cellular mechanisms mediating these synaptic changes, nor the sleep-dependent activity changes driving those cellular events are well-defined. Here we discuss potential cellular and network dynamic mechanisms which could underlie reductions in synaptic strength during sleep. We also discuss recent findings demonstrating circuit-specific synaptic strengthening (rather than weakening) during sleep. Based on these data, we explore the hypothetical role of sleep-associated network activity patterns in driving synaptic strengthening. We propose an alternative to SHY—namely that depending on experience during prior wake, a variety of plasticity mechanisms may operate in the brain during sleep. We conclude that either synaptic strengthening or synaptic weakening can occur across sleep, depending on changes to specific neural circuits (such as gene expression and protein translation) induced by experiences in wake. Clarifying the mechanisms underlying these different forms of sleep-dependent plasticity will significantly advance our understanding of how sleep benefits various cognitive functions.

Molecular Mechanisms of Sleep Homeostasis in Flies and Mammals

Sleep is homeostatically regulated with sleep pressure accumulating with the increasing duration of prior wakefulness. Yet, a clear understanding of the molecular components of the homeostat, as well as the molecular and cellular processes they sense and control to regulate sleep intensity and duration, remain a mystery. Here, we will discuss the cellular and molecular basis of sleep homeostasis, first focusing on the best homeostatic sleep marker in vertebrates, slow wave activity; second, moving to the molecular genetic analysis of sleep homeostasis in the fruit fly Drosophila; and, finally, discussing more systemic aspects of sleep homeostasis.

Pleiotropic genetic effects influencing sleep and neurological disorders

Research evidence increasingly points to the large impact of sleep disturbances on public health. Many aspects of sleep are heritable and genes influencing traits such as timing, EEG characteristics, sleep duration, and response to sleep loss have been identified. Notably, large-scale genome-wide analyses have implicated numerous genes with small effects on sleep timing. Additionally, there has been considerable progress in the identification of genes influencing risk for some neurological sleep disorders. For restless legs syndrome, implicated variants are typically in genes associated with neuronal development. By contrast, genes conferring risk for narcolepsy function in the immune system. Many genetic variants associated with sleep disorders are also implicated in neurological disorders in which sleep abnormalities are common; for example, variation in genes involved in synaptic homoeostasis are implicated in autism spectrum disorder and sleep-wake control. Further investigation into pleiotropic roles of genes influencing both sleep and neurological disorders could lead to new treatment strategies for a variety of sleep disturbances.

Homer1a drives homeostatic scaling-down of excitatory synapses during sleep

Sleep is an essential process that supports learning and memory by acting on synapses through poorly understood molecular mechanisms. Using biochemistry, proteomics, and imaging in mice, we find that during sleep, synapses undergo widespread alterations in composition and signaling, including weakening of synapses through removal and dephosphorylation of synaptic AMPA-type glutamate receptors. These changes are driven by the immediate early gene Homer1a and signaling from group I metabotropic glutamate receptors mGluR1/5. Homer1a serves as a molecular integrator of arousal and sleep need via the wake- and sleep-promoting neuromodulators, noradrenaline and adenosine, respectively. Our data suggest that homeostatic scaling-down, a global form of synaptic plasticity, is active during sleep to remodel synapses and participates in the consolidation of contextual memory.

Genetic dissection of sleep homeostasis

Sleep is a complex behavior both in its manifestation and regulation, that is common to almost all animal species studied thus far. Sleep is not a unitary behavior and has many different aspects, each of which is tightly regulated and influenced by both genetic and environmental factors. Despite its essential role for performance, health, and well-being, genetic mechanisms underlying this complex behavior remain poorly understood. One important aspect of sleep concerns its homeostatic regulation, which ensures that levels of sleep need are kept within a range still allowing optimal functioning during wakefulness. Uncovering the genetic pathways underlying the homeostatic aspect of sleep is of particular importance because it could lead to insights concerning sleep's still elusive function and is therefore a main focus of current sleep research. In this chapter, we first give a definition of sleep homeostasis and describe the molecular genetics techniques that are used to examine it. We then provide a conceptual discussion on the problem of assessing a sleep homeostatic phenotype in various animal models. We finally highlight some of the studies with a focus on clock genes and adenosine signaling molecules.

The circadian clock gene Csnk1e regulates rapid eye movement sleep amount, and nonrapid eye movement sleep architecture in mice

STUDY OBJECTIVES

Efforts to identify the genetic basis of mammalian sleep have included quantitative trait locus (QTL) mapping and gene targeting of known core circadian clock genes. We combined three different genetic approaches to identify and test a positional candidate sleep gene - the circadian gene casein kinase 1 epsilon (Csnk1e), which is located in a QTL we identified for rapid eye movement (REM) sleep on chromosome 15.

MEASUREMENTS AND RESULTS

Using electroencephalographic (EEG) and electromyographic (EMG) recordings, baseline sleep was examined in a 12-h light:12-h dark (LD 12:12) cycle in mice of seven genotypes, including Csnk1e(tau/tau) and Csnk1e(-/-) mutant mice, Csnk1e (B6.D2) and Csnk1e (D2.B6) congenic mice, and their respective wild-type littermate control mice. Additionally, Csnk1e(tau/tau) and wild-type mice were examined in constant darkness (DD). Csnk1e(tau/tau) mutant mice and both Csnk1e (B6.D2) and Csnk1e (D2.B6) congenic mice showed significantly higher proportion of sleep time spent in REM sleep during the dark period than wild-type controls - the original phenotype for which the QTL on chromosome 15 was identified. This phenotype persisted in Csnk1e(tau/tau) mice while under free-running DD conditions. Other sleep phenotypes observed in Csnk1e(tau/tau) mice and congenics included a decreased number of bouts of nonrapid eye movement (NREM) sleep and an increased average NREM sleep bout duration.

CONCLUSIONS

These results demonstrate a role for Csnk1e in regulating not only the timing of sleep, but also the REM sleep amount and NREM sleep architecture, and support Csnk1e as a causal gene in the sleep QTL on chromosome 15.

Contribution of transcriptional and translational mechanisms to the recovery aspect of sleep regulation

Sleep parallels brain functioning and mental health. Neuronal activity during wakefulness leads to a subsequent increase in sleep intensity as measured using electroencephalographic slow-wave activity (SWA; index of neuronal synchrony in the low-frequency range). Wakefulness, and particularly prolonged wakefulness, also drives important changes in brain gene expression and changes in protein regulation. The role of these two cellular mechanisms in sleep-wake regulation has typically been studied independently, and their exact contribution to SWA remains poorly defined. In this review, we highlight that many transcriptional pathways driven by sleep deprivation are associated to protein regulation. We first describe the relationship between cytokines, clock genes, and markers of sleep need with an emphasis on transcriptional processes. Observations regarding the role of protein metabolism in sleep-wake regulation are then depicted while presenting interconnections between transcriptional and translational responses driven by sleep loss. Lastly, a manner by which this integrated response can feed back on neuronal network activity to determine sleep intensity is proposed. Overall, the literature supports that a complex cross-talk between transcriptional and translational regulation during prolonged wakefulness drives the changes in sleep intensity as a function of the sleep/wake history.

Differential up-regulation of Vesl-1/Homer 1 protein isoforms associated with decline in visual performance in a preclinical glaucoma model

Glaucoma is a multifactorial progressive ocular pathology, clinically presenting with damage to the retina and optic nerve, ultimately leading to blindness. Retinal ganglion cell loss in glaucoma ultimately results in vision loss. Vesl/Homer proteins are scaffolding proteins that are critical for maintaining synaptic integrity by clustering, organizing and functionally regulating synaptic proteins. Current anti-glaucoma therapies target IOP as the sole modifiable clinical parameters. Long-term pharmacotherapy and surgical treatment do not prevent gradual visual field loss as the disease progresses, highlighting the need for new complementary, alternative and comprehensive treatment approaches. Vesl/Homer expression was measured in the retinae of DBA/2J mice, a preclinical genetic glaucoma model with spontaneous mutations resulting in a phenotype reminiscent of chronic human pigmentary glaucoma. Vesl/Homer proteins were differentially expressed in the aged, glaucomatous DBA/2J retina, both at the transcriptional and translational level. Immunoreactivity for the long Vesl-1L/Homer 1c isoform, but not of the immediate early gene product Vesl-1S/Homer 1a was increased in the synaptic layers of the retina. This increased protein level of Vesl-1L/Homer 1c was correlated with phenotypes of increased disease severity and a decrease in visual performance. The increased expression of Vesl-1L/Homer 1c in the glaucomatous retina likely results in increased intracellular Ca(2+) release through enhancement of synaptic coupling. The ensuing Ca(2+) toxicity may thus activate neurodegenerative pathways and lead to the progressive loss of synaptic function in glaucoma. Our data suggest that higher levels of Vesl-1L/Homer 1c generate a more severe disease phenotype and may represent a viable target for therapy development.

Homeostatic and circadian contribution to EEG and molecular state variables of sleep regulation

STUDY OBJECTIVES

Besides their well-established role in circadian rhythms, our findings that the forebrain expression of the clock-genes Per2 and Dbp increases and decreases, respectively, in relation to time spent awake suggest they also play a role in the homeostatic aspect of sleep regulation. Here, we determined whether time of day modulates the effects of elevated sleep pressure on clock-gene expression. Time of day effects were assessed also for recognized electrophysiological (EEG delta power) and molecular (Homer1a) markers of sleep homeostasis.

DESIGN

EEG and qPCR data were obtained for baseline and recovery from 6-h sleep deprivation starting at ZT0, -6, -12, or -18.

SETTING

Mouse sleep laboratory.

PARTICIPANTS

Male mice.

INTERVENTIONS

Sleep deprivation.

RESULTS

The sleep-deprivation induced changes in Per2 and Dbp expression importantly varied with time of day, such that Per2 could even decrease during sleep deprivations occurring at the decreasing phase in baseline. Dbp showed similar, albeit opposite dynamics. These unexpected results could be reliably predicted assuming that these transcripts behave according to a driven damped harmonic oscillator. As expected, the sleep-wake distribution accounted for a large degree of the changes in EEG delta power and Homer1a. Nevertheless, the sleep deprivation-induced increase in delta power varied also with time of day with higher than expected levels when recovery sleep started at dark onset.

CONCLUSIONS

Per2 and delta power are widely used as exclusive state variables of the circadian and homeostatic process, respectively. Our findings demonstrate a considerable cross-talk between these two processes. As Per2 in the brain responds to both sleep loss and time of day, this molecule is well positioned to keep track of and to anticipate homeostatic sleep need.

CITATION

Curie T; Mongrain V; Dorsaz S; Mang GM; Emmenegger Y; Franken P. Homeostatic and circadian contribution to EEG and molecular state variables of sleep regulation. SLEEP 2013;36(3):311-323.

Effects of insufficient sleep on circadian rhythmicity and expression amplitude of the human blood transcriptome

Insufficient sleep and circadian rhythm disruption are associated with negative health outcomes, including obesity, cardiovascular disease, and cognitive impairment, but the mechanisms involved remain largely unexplored. Twenty-six participants were exposed to 1 wk of insufficient sleep (sleep-restriction condition 5.70 h, SEM = 0.03 sleep per 24 h) and 1 wk of sufficient sleep (control condition 8.50 h sleep, SEM = 0.11). Immediately following each condition, 10 whole-blood RNA samples were collected from each participant, while controlling for the effects of light, activity, and food, during a period of total sleep deprivation. Transcriptome analysis revealed that 711 genes were up- or down-regulated by insufficient sleep. Insufficient sleep also reduced the number of genes with a circadian expression profile from 1,855 to 1,481, reduced the circadian amplitude of these genes, and led to an increase in the number of genes that responded to subsequent total sleep deprivation from 122 to 856. Genes affected by insufficient sleep were associated with circadian rhythms (PER1, PER2, PER3, CRY2, CLOCK, NR1D1, NR1D2, RORA, DEC1, CSNK1E), sleep homeostasis (IL6, STAT3, KCNV2, CAMK2D), oxidative stress (PRDX2, PRDX5), and metabolism (SLC2A3, SLC2A5, GHRL, ABCA1). Biological processes affected included chromatin modification, gene-expression regulation, macromolecular metabolism, and inflammatory, immune and stress responses. Thus, insufficient sleep affects the human blood transcriptome, disrupts its circadian regulation, and intensifies the effects of acute total sleep deprivation. The identified biological processes may be involved with the negative effects of sleep loss on health, and highlight the interrelatedness of sleep homeostasis, circadian rhythmicity, and metabolism.

CD40 activation induces NREM sleep and modulates genes associated with sleep homeostasis

The T-cell derived cytokine CD40 ligand is overexpressed in patients with autoimmune diseases. Through activation of its receptor, CD40 ligand leads to a tumor necrosis factor (TNF) receptor 1 (TNFR1) dependent impairment of locomotor activity in mice. Here we report that this effect is explained through a promotion of sleep, which was specific to non-rapid eye movement (NREM) sleep while REM sleep was suppressed. The increase in NREM sleep was accompanied by a decrease in EEG delta power during NREM sleep and by a decrease in the expression of transcripts in the cerebral cortex known to be associated with homeostatic sleep drive, such as Homer1a, Early growth response 2, Neuronal pentraxin 2, and Fos-like antigen 2. The effect of CD40 activation was mimicked by peripheral TNF injection and prevented by the TNF blocker etanercept. Our study indicates that sleep-wake dysregulation in autoimmune diseases may result from CD40 induced TNF:TNFR1 mediated alterations of molecular pathways, which regulate sleep-wake behavior.

Heritability of performance deficit accumulation during acute sleep deprivation in twins

STUDY OBJECTIVES

To determine if the large and highly reproducible interindividual differences in rates of performance deficit accumulation during sleep deprivation, as determined by the number of lapses on a sustained reaction time test, the Psychomotor Vigilance Task (PVT), arise from a heritable trait.

DESIGN

Prospective, observational cohort study.

SETTING

Academic medical center.

PARTICIPANTS

There were 59 monozygotic (mean age 29.2 ± 6.8 [SD] yr; 15 male and 44 female pairs) and 41 dizygotic (mean age 26.6 ± 7.6 yr; 15 male and 26 female pairs) same-sex twin pairs with a normal polysomnogram.

INTERVENTIONS

Thirty-eight hr of monitored, continuous sleep deprivation.

MEASUREMENTS AND RESULTS

Patients performed the 10-min PVT every 2 hr during the sleep deprivation protocol. The primary outcome was change from baseline in square root transformed total lapses (response time ≥ 500 ms) per trial. Patient-specific linear rates of performance deficit accumulation were separated from circadian effects using multiple linear regression. Using the classic approach to assess heritability, the intraclass correlation coefficients for accumulating deficits resulted in a broad sense heritability (h(2)) estimate of 0.834. The mean within-pair and among-pair heritability estimates determined by analysis of variance-based methods was 0.715. When variance components of mixed-effect multilevel models were estimated by maximum likelihood estimation and used to determine the proportions of phenotypic variance explained by genetic and nongenetic factors, 51.1% (standard error = 8.4%, P < 0.0001) of twin variance was attributed to combined additive and dominance genetic effects.

CONCLUSION

Genetic factors explain a large fraction of interindividual variance among rates of performance deficit accumulations on PVT during sleep deprivation.

Hippocampal CA1 transcriptional profile of sleep deprivation: relation to aging and stress

Background

Many aging changes seem similar to those elicited by sleep-deprivation and psychosocial stress. Further, sleep architecture changes with age suggest an age-related loss of sleep. Here, we hypothesized that sleep deprivation in young subjects would elicit both stress and aging-like transcriptional responses.

Methodology/Principal Findings

F344 rats were divided into control and sleep deprivation groups. Body weight, adrenal weight, corticosterone level and hippocampal CA1 transcriptional profiles were measured. A second group of animals was exposed to novel environment stress (NES), and their hippocampal transcriptional profiles measured. A third cohort exposed to control or SD was used to validate transcriptional results with Western blots. Microarray results were statistically contrasted with prior transcriptional studies. Microarray results pointed to sleep pressure signaling and macromolecular synthesis disruptions in the hippocampal CA1 region. Animals exposed to NES recapitulated nearly one third of the SD transcriptional profile. However, the SD -aging relationship was more complex. Compared to aging, SD profiles influenced a significant subset of genes. mRNA associated with neurogenesis and energy pathways showed agreement between aging and SD, while immune, glial, and macromolecular synthesis pathways showed SD profiles that opposed those seen in aging.

Conclusions/Significance

We conclude that although NES and SD exert similar transcriptional changes, selective presynaptic release machinery and Homer1 expression changes are seen in SD. Among other changes, the marked decrease in Homer1 expression with age may represent an important divergence between young and aged brain response to SD. Based on this, it seems reasonable to conclude that therapeutic strategies designed to promote sleep in young subjects may have off-target effects in the aged. Finally, this work identifies presynaptic vesicular release and intercellular adhesion molecular signatures as novel therapeutic targets to counter effects of SD in young subjects.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Role of Homer proteins in the maintenance of sleep-wake states

Sleep is an evolutionarily conserved process that is linked to diurnal cycles and normal daytime wakefulness. Healthy sleep and wakefulness are integral to a healthy lifestyle; this occurs when an organism is able to maintain long bouts of both sleep and wake. Homer proteins, which function as adaptors for group 1 metabotropic glutamate receptors, have been implicated in genetic studies of sleep in both Drosophila and mouse. Drosophila express a single Homer gene product that is upregulated during sleep. By contrast, vertebrates express Homer as both constitutive and immediate early gene (H1a) forms, and H1a is up-regulated during wakefulness. Genetic deletion of Homer in Drosophila results in fragmented sleep and in failure to sustain long bouts of sleep, even under increased sleep drive. However, deletion of Homer1a in mouse results in failure to sustain long bouts of wakefulness. Further evidence for the role of Homer1a in the maintenance of wake comes from the CREB alpha delta mutant mouse, which displays a reduced wake phenotype similar to the Homer1a knockout and fails to up-regulate Homer1a upon sleep loss. Homer1a is a gene whose expression is induced by CREB. Sustained behaviors of the sleep/wake cycle are created by molecular pathways that are distinct from those for arousal or short bouts, and implicate an evolutionarily-conserved role for Homer in sustaining these behaviors.

Disrupted Homer scaffolds mediate abnormal mGluR5 function in a mouse model of fragile X syndrome

Enhanced mGluR5 function is causally associated with the pathophysiology of Fragile X Syndrome (FXS), a leading inherited cause of intellectual disability and autism. Here we provide evidence that altered mGluR5-Homer scaffolds contribute to mGluR5 dysfunction and phenotypes in the FXS mouse model, Fmr1 KO. In Fmr1 KO mice mGluR5 is less associated with long Homer isoforms, but more associated with the short Homer1a. Genetic deletion of Homer1a restores mGluR5- long Homer scaffolds and corrects multiple phenotypes in Fmr1 KO mice including altered mGluR5 signaling, neocortical circuit dysfunction, and behavior. Acute, peptide-mediated disruption of mGluR5-Homer scaffolds in wildtype mice mimics many Fmr1 KO phenotypes. In contrast, Homer1a deletion does not rescue altered mGluR-dependent long-term synaptic depression or translational control of FMRP target mRNAs. Our findings reveal novel functions for mGluR5-Homer interactions in the brain and delineate distinct mechanisms of mGluR5 dysfunction in a mouse model of cognitive dysfunction and autism.

Genome-wide association-, replication-, and neuroimaging study implicates HOMER1 in the etiology of major depression

BACKGROUND

Genome-wide association studies are a powerful tool for unravelling the genetic background of complex disorders such as major depression.

METHODS

We conducted a genome-wide association study of 604 patients with major depression and 1364 population based control subjects. The top hundred findings were followed up in a replication sample of 409 patients and 541 control subjects.

RESULTS

Two SNPs showed nominally significant association in both the genome-wide association study and the replication samples: 1) rs9943849 (p(combined) = 3.24E-6) located upstream of the carboxypeptidase M (CPM) gene and 2) rs7713917 (p(combined) = 1.48E-6), located in a putative regulatory region of HOMER1. Further evidence for HOMER1 was obtained through gene-wide analysis while conditioning on the genotypes of rs7713917 (p(combined) = 4.12E-3). Homer1 knockout mice display behavioral traits that are paradigmatic of depression, and transcriptional variants of Homer1 result in the dysregulation of cortical-limbic circuitry. This is consistent with the findings of our subsequent human imaging genetics study, which revealed that variation in single nucleotide polymorphism rs7713917 had a significant influence on prefrontal activity during executive cognition and anticipation of reward.

CONCLUSION

Our findings, combined with evidence from preclinical and animal studies, suggest that HOMER1 plays a role in the etiology of major depression and that the genetic variation affects depression via the dysregulation of cognitive and motivational processes.

Genetic analysis of sleep

Almost 20 years ago, the gene underlying fatal familial insomnia was discovered, and first suggested the concept that a single gene can regulate sleep. In the two decades since, there have been many advances in the field of behavioral genetics, but it is only in the past 10 years that the genetic analysis of sleep has emerged as an important discipline. Major findings include the discovery of a single gene underlying the sleep disorder narcolepsy, and identification of loci that make quantitative contributions to sleep characteristics. The sleep field has also expanded its focus from mammalian model organisms to Drosophila, zebrafish, and worms, which is allowing the application of novel genetic approaches. Researchers have undertaken large-scale screens to identify new genes that regulate sleep, and are also probing questions of sleep circuitry and sleep function on a molecular level. As genetic tools continue to be refined in each model organism, the genes that support a specific function in sleep will become more apparent. Thus, while our understanding of sleep still remains rudimentary, rapid progress is expected from these recently initiated studies.

A verification of previously identified QTLs for cocaine-induced activation using a panel of B6.A chromosome substitution strains (CSS) and A/J x C57Bl/6J F2 mice

BACKGROUND

The objective of this study was to confirm provisional quantitative trait loci (QTL) for cocaine-induced locomotor activation, on chromosomes 1, 5, 6, 9, 12, 15, 16, 17, and 18, previously identified in the AXB/BXA recombinant inbred (RI) and AcB/BcA recombinant congenic (RC) strains of mice derived from A/J (A) and C57BL/6J (B6) progenitors. This was accomplished through a genetic analysis of cocaine-induced activity in an AxB6 F2 cross and a phenotypic survey across a panel of B6.A chromosome substitution strains (CSS) mice. Mice were tested for cocaine-induced activity, following administration of saline and cocaine (20 mg/kg), utilizing an open-field procedure.

RESULTS

Among AxB6 F2 mice, differences in cocaine-induced activity were associated with loci on chromosome 1 (D1Mit305), 5 (D5Mit409), 16 (D16Mit131), and 18 (D18Mit189). A survey of the CSS panel confirmed cocaine QTLs on chromosomes 5 and 15, previously identified in RI or RC strains. Overall, the regions on chromosomes 5 and 18 represent verification of QTL previously identified in both the RC and RI strains. Additionally, the B6 allele for these QTL was consistently associated with greater relative cocaine activation.

CONCLUSIONS

Collectively, chromosome 5 and 18 QTL have now been replicated in multiple independent crosses derived from the A/J and C57BL/6J progenitors. The use of an in silico analysis highlighted potential candidate genes on chromosomes 5 and 18. The present results complement the targeted gene approach currently prevalent in the study of cocaine and provide a broader empirically based focus for subsequent candidate gene studies.

The genetic and molecular regulation of sleep: from fruit flies to humans

It has been known for a long time that genetic factors affect sleep quantity and quality. Genetic screens have identified several mutations that affect sleep across species, pointing to an evolutionary conserved regulation of sleep. Moreover, it has also been recognized that sleep affects gene expression. These findings have given valuable insights into the molecular underpinnings of sleep regulation and function that might lead the way to more efficient treatments for sleep disorders.

What are microarrays teaching us about sleep?

Many fundamental questions about sleep remain unanswered. The presence of sleep across phyla suggests that it must serve a basic cellular and/or molecular function. Microarray studies, performed in several model systems, have identified classes of genes that are sleep-state regulated. This has led to the following concepts: first, a function of sleep is to maintain synaptic homeostasis; second, sleep is a stage of macromolecule biosynthesis; third, extending wakefulness leads to downregulation of several important metabolic pathways; and, fourth, extending wakefulness leads to endoplasmic reticulum stress. In human studies, microarrays are being applied to the identification of biomarkers for sleepiness and for the common debilitating condition of obstructive sleep apnea.

Genetic analysis of the psychostimulant effects of nicotine in chromosome substitution strains and F2 crosses derived from A/J and C57BL/6J progenitors

Previous research utilizing the AcB/BcA recombinant congenic strains (RCS) of mice mapped provisional quantitative trait loci (QTLs) for the psychostimulant effects of nicotine to multiple regions on chromosomes 7, 11, 12, 14, 16, and 17. The current study was designed to confirm these QTLs in an A/J (A) x C57Bl/6J (B6) F2 cross and a panel of B6.A chromosome substitution strains (CSS). The panel of B6.A CSS consists of 21 strains, each carrying a different A/J chromosome on a B6 background. The A x B6 F2, CSS, A, and B6 mice were tested for sensitivity to the effects of nicotine on locomotor activity using a computerized open-field apparatus. In A x B6 F2 mice two QTLs were identified which confirm those previously observed in the AcB/BcA RCS. Significant differences in the expression of nicotine-induced activity were associated with loci on chromosome 11 (D11Mit62) and chromosome 16 (D16Mit131) in the A x B6 F2. At the chromosome 11 QTL, an A allele was associated with lower nicotine-induced activity scores relative to the B6. In contrast, the A allele was associated with greater relative nicotine activity values for the chromosome 16 QTL. A survey of the CSS panel confirmed the presence of QTLs for nicotine activation on chromosomes 2, 14, 16, and 17 previously identified in the AcB/BcA RCS. In the informative CSS strains, A alleles were consistently associated with greater nicotine-induced activity scores compared to the B6. The results of the present study are the first to validate QTLs for sensitivity to the effects of nicotine across multiple strains of mice. QTLs on chromosomes 2, 11, 14, 16, and 17 were confirmed in CSS and/or F2 mice. Significantly, the identification of a QTL on chromosome 16 has now been replicated in three crosses derived from the A and B6 progenitors.