Alternative titles; symbolsOREXIN; OXPREPROOREXINOther entities represented in this entry:OREXIN A, INCLUDEDOREXIN B, INCLUDEDHYPOCRETIN 1, INCLUDED; HCRT1, INCL...
Alternative titles; symbols
Other entities represented in this entry:
HGNC Approved Gene Symbol: HCRT
Cytogenetic location: 17q21.2 Genomic coordinates (GRCh38): 17:42,184,059-42,185,451 (from NCBI)
The HCRT gene encodes hypocretin, also known as orexin (OX), a neuropeptide expressed in various brain regions, first identified in the hypothalamus. The hypothalamus acts as a major regulatory center for autonomic and endocrine homeostasis. Structurally, it is a confederation of nuclei that regulate a broad array of physiologic and behavioral activities. For some of these activities, particular peptides are major products of individual nuclei. These peptides exert their actions by transport to the pituitary, by entering the general circulation, or by secretion within the central nervous system (CNS) (summary by Gautvik et al., 1996).
▼ Cloning and Expression
Gautvik et al. (1996) used directional tag PCR subtraction to identify 38 rat mRNAs selectively expressed within the hypothalamus. Preliminary in situ hybridization studies revealed that one of these, called clone 35 by them, was expressed exclusively by a bilaterally symmetric structure within the posterior hypothalamus. De Lecea et al. (1998) showed that rat clone 35 mRNA encodes the precursor of 2 putative peptides, the hypocretins, that share substantial amino acid identities with each other and with the gut hormone secretin (182099). The designation hypocretin (HCRT) was adopted to indicate that it is a hypothalamic member of the incretin family. HCRT mRNA, which accumulates primarily after postnatal week 3, is restricted to neuronal cell bodies of the dorsal and lateral hypothalamus. At least 1 of the peptides had neuroexcitatory activity. The observations suggested to de Lecea et al. (1998) that HCRT mRNA encodes mouse peptides that act endogenously within the CNS as homeostatic regulators, with a possible role in nutritional homeostasis.
Using high-resolution HPLC fractions from various tissue extracts for agonists of 7-transmembrane G protein-coupled cell surface receptors, Sakurai et al. (1998) identified 2 neuropeptides that bind and activate 2 closely related G protein-coupled receptors. The polypeptides, named orexin A and orexin B, are derived from the same precursor (orexin) by proteolytic processing. The orexin-A peptide comprises gln33 to gly66, and the orexin-B peptide arg69 to met96, of the precursor protein. The predicted peptide for orexin A is identical in human, rat, mouse, and bovine. The predicted peptide for human orexin B shows only 2 amino acid differences when compared with the rodent sequence. Preproorexin mRNA and immunoreactive orexin A are localized in neurons within and around the lateral and posterior hypothalamus in the adult rat brain. Northern blot analysis revealed that in the rat a 0.7-kb preproorexin mRNA species is expressed predominantly in the brain and in a small amount in testis. The orexin-1 receptor (HCRTR1; 602392) binds orexin A only, whereas the orexin-2 receptor (HCRTR2; 602393) binds both orexin A and orexin B. When administered intracerebroventricularly to rats, orexin A and orexin B stimulated food consumption. In addition, preproorexin mRNA levels were upregulated upon fasting. Sakurai et al. (1998) concluded that these peptides are mediators in the central feedback mechanism that regulates feeding behavior.
▼ Gene Structure
Miyoshi et al. (2001) determined that the mouse Hcrt gene contains 2 exons and spans 31.2 kb. The human HCRT gene also contains 2 exons.
By radiation hybrid mapping, Sakurai et al. (1998) localized the human preproorexin gene between markers WI-6595 and UTR9641 on chromosome 17q21.
De Lecea et al. (1998) stated that the mouse Hcrt gene maps to chromosome 11. Miyoshi et al. (2001) determined that the order and orientation of genes at mouse chromosome 11, Ptrf (603198)--Stat3 (102582)--Stat5a (601511)--Stat5b (604260)--Lgp1 (608587)--Hcrt, are identical in the syntenic region of human chromosome 17q21.
▼ Gene Function
The localization of orexin neuropeptides in the lateral hypothalamus focused attention on their role in ingestion. The orexigenic neurons in the lateral hypothalamus, however, project widely in the brain, and thus the physiologic role of orexins is likely to be complex. Hagan et al. (1999) investigated the action of orexin A in modulating the arousal state of rats by using a combination of tissue localization and electrophysiologic and behavioral techniques. They showed that the brain region receiving the densest innervation from orexinergic nerves is the locus ceruleus, a key modulator of attentional state, where application of orexin A increased cell firing of intrinsic noradrenergic neurons. Orexin A increased arousal and locomotor activity and modulated neuroendocrine function. The data suggested that orexin A plays an important role in orchestrating the sleep-wake cycle.
Kirchgessner and Liu (1999) demonstrated the presence of orexin-immunoreactive neurons in rat, mouse, guinea pig, and human intestinal mucosa, observing that about 25% of neurons in both the myenteric and submucosal plexi, which also expressed leptin receptors, displayed orexin-like immunoreactivity. Orexin excited secretomotor neurons in the guinea pig submucosal plexus and increased motility. Fasting upregulated the phosphorylated form of cAMP response element-binding protein (see 123810) in orexin-immunoreactive neurons, indicating a functional response to food status in these cells. Kirchgessner and Liu (1999) concluded that orexin in the gut may play an even more intimate role in regulating energy homeostasis than it does in the central nervous system.
In rat brain slices and neurons, Lambe and Aghajanian (2003) showed that hypocretin directly excited prefrontal thalamocortical synapses. This activation was demonstrated electrophysiologically and by detection of induced calcium transients selectively in single spines that were postsynaptic to thalamocortical boutons. The findings suggested that hypocretin may have effects on the thalamocortical interactions that underlie arousal and attention in the prefrontal cortex.
Sikder and Kodadek (2007) found that orexin-1 stimulation of human embryonic kidney cells expressing OX1R resulted in significant upregulation of a host of genes, including HIF1A (603348), a transcription factor normally activated under hypoxic conditions. Orexin-1 stimulation also caused a concomitant downregulation of VHL (608537), an E3 ubiquitin ligase that mediates HIF1A turnover via the ubiquitin-proteasome pathway. Chromatin immunoprecipitation assays revealed increased HIF1A occupancy on promoters of HIF1A target genes following orexin stimulation. The spectrum of HIF1A-induced genes differed in normoxic cells stimulated with orexin from those induced by hypoxia. Orexin-mediated activation of HIF1A resulted in increased glucose uptake and higher glycolytic activity, similar to what was observed in hypoxic cells. However, OX1R-expressing cells favored ATP production through the tricarboxylic acid cycle and oxidative phosphorylation rather than through anaerobic glycolysis. Sikder and Kodadek (2007) concluded that HIF1A, in addition to responding to hypoxia, has a role in hormone-mediated regulation of hunger and wakefulness.
Adamantidis et al. (2007) directly probed the impact of Hcrt neuron activity on sleep state transitions with in vivo neural photostimulation, genetically targeting channelrhodopsin-2 to Hcrt cells and using an optical fiber to deliver light deep in the brain, directly into the lateral hypothalamus, of freely moving mice. Adamantidis et al. (2007) found that direct, selective, optogenetic photostimulation of Hcrt neurons increased the probability of transition to wakefulness from either slow wave sleep or rapid eye movement (REM) sleep. Notably, photostimulation using 5 to 30 Hz light pulse trains reduced latency to wakefulness, whereas 1 Hz trains did not. Adamantidis et al. (2007) concluded that their study established a causal relationship between frequency-dependent activity of a genetically defined neural cell type and a specific mammalian behavior central to clinical conditions and neurobehavioral physiology.
Silva et al. (2009) showed that Foxa2 (600288), a downstream target of insulin signaling, regulates the expression of orexin and melanin-concentrating hormone (MCH; 176795). During fasting, Foxa2 binds to MCH and orexin promoters and stimulates their expression. In fed and in hyperinsulinemic obese mice, insulin signaling led to nuclear exclusion of Foxa2 and reduced expression of MCH and orexin. Constitutive activation of Foxa2 in the brain resulted in increased neuronal MCH and orexin expression and increased food consumption, metabolism, and insulin sensitivity. Spontaneous physical activity of these animals in the fed state was significantly increased and was similar to that in fasted mice. Conditional activation of Foxa2 through the T156A mutation expression in the brain of obese mice also resulted in improved glucose homeostasis, decreased fat, and increased lean body mass. Silva et al. (2009) concluded that Foxa2 can act as a metabolic sensor in neurons of the lateral hypothalamic area to integrate metabolic signals, adaptive behavior, and physiologic responses.
Role in Neurologic Disorders
Nishino et al. (2000) measured immunoreactive hypocretin in the cerebrospinal fluid of 9 patients with narcolepsy (161400) and 8 age-matched controls. All patients were positive for HLA-DR2/DQB1*0602. HCRT1 was detectable in all controls; in 7 of 9 patients, HCRT concentrations were below the detection limit of the assay. The authors proposed that an HLA-associated autoimmune-mediated destruction of HCRT-containing neurons in the lateral hypothalamus might produce narcolepsy in these patients.
Thannickal et al. (2000) studied the hypothalamus of 16 human brains, including those of 4 narcoleptics. The human narcoleptics had an 85 to 95% reduction in the number of HCRT neurons. Melanin-concentrating hormone (176795) neurons, which are intermixed with HCRT cells in the normal brain, were not reduced in number, indicating that cell loss was relatively specific for HCRT neurons. The presence of gliosis in the hypocretin cell region is consistent with a degenerative process being the cause of the HCRT loss in narcolepsy.
In 31 patients with narcolepsy, Dalal et al. (2001) found reduced or undetectable levels of CSF hypocretin compared to controls. Plasma levels of hypocretin, however, were at normal levels, similar to controls, suggesting that systemic hypocretin derived from CNS-independent sources is preserved in narcolepsy. The authors noted that a potential autoimmune mechanism for the disorder is unlikely to be directed against the hypocretin molecule.
Ripley et al. (2001) reported undetectably low levels of CSF hypocretin-1 in 37 of 42 adults with narcolepsy and 3 patients with Guillain-Barre syndrome (139393). Low but detectable CSF hypocretin-1 levels were found in patients with acute lymphocytic leukemia, intracranial tumors, head trauma, and CNS infections.
Arii et al. (2004) found very low CSF hypocretin-1 levels in 6 of 6 children with narcolepsy ranging in age from 6 to 16 years. All were HLA-DR2-positive. Decreased levels of CSF hypocretin-1 were also found in children with Guillain-Barre syndrome, head trauma, brain tumor, and CNS infection. The authors concluded that measurement of CSF hypocretin-1 is diagnostically useful in children.
Orexin-producing neurons also produce other signaling molecules, including the endogenous opiate dynorphin (see PDYN; 131340) and NARP (NPTX2; 600750), a protein that regulates AMPA glutamate receptor clustering (see, e.g., GRIA1, 138248) (Crocker et al., 2005). Independently, Crocker et al. (2005) and Blouin et al. (2005) used immunohistochemistry to demonstrate colocalization of orexin and NARP protein within neurons in the posterior, lateral, dorsal, and dorsomedial hypothalamic nuclei of normal control brains. In addition, Crocker et al. (2005) showed colocalization of prodynorphin mRNA, NARP, and orexin within neurons in the same hypothalamic regions. In contrast, combined results from both studies showed that most neurons in the paraventricular and supraoptic nuclei expressed only prodynorphin mRNA or only NARP protein with no orexin, suggesting a different cell type. Similar studies of a total of 6 patients with documented narcolepsy with cataplexy showed a marked reduction (5 to 11% of normal) of hypothalamic orexin-, prodynorphin-, and NARP-positive neurons, whereas paraventricular neurons containing only prodynorphin or NARP alone were similar to controls. The findings of Crocker et al. (2005) and Blouin et al. (2005) indicated that narcolepsy is associated with a loss of the orexin-producing neurons themselves, rather than a failure to produce the orexin protein. The results were consistent with selective neurodegeneration of these cells or an autoimmune process.
▼ Molecular Genetics
Peyron et al. (2000) explored the role of hypocretins in human narcolepsy through histopathology of 6 narcolepsy brains and mutation screening of HCRT, HCRTR1, and HCRTR2 in 74 patients of various HLA and family history status. One HCRT mutation (602358.0001), impairing peptide trafficking and processing, was found in a single case with early-onset narcolepsy. In situ hybridization of the perifornical area and peptide radioimmunoassays indicated global loss of hypocretins, without gliosis or signs of inflammation in all human cases examined. Peyron et al. (2000) concluded that although hypocretin loci do not contribute significantly to genetic predisposition, most cases of human narcolepsy are associated with a deficient hypocretin system.
Gencik et al. (2001) screened the entire preproorexin gene for mutations or polymorphisms in 133 patients with narcolepsy. They identified a 3250C-T sequence variation in the 5-prime untranslated region, 22 bp 5-prime from the start codon. The 3250T allele was present in 6 of 178 narcoleptic patients, as well as in 1 of 189 healthy control subjects. All patients were heterozygous for 3250T.
Hungs et al. (2001) identified a common HCRT polymorphism (-909C-T) and tested it in 502 subjects, including 105 trio families (both parents and 1 affected child), 80 Caucasian narcolepsy patients, and 107 Caucasian control subjects. This polymorphism was not associated with the disease. Furthermore, no association was found with -22T, a rare 5-prime untranslated region polymorphism previously thought to be associated with narcolepsy. Hungs et al. (2001) concluded that the HCRT locus is not a major narcolepsy susceptibility locus.
▼ Animal Model
Neurons containing hypocretin are located exclusively in the lateral hypothalamus and send axons to numerous regions throughout the central nervous system, including the major nuclei implicated in sleep regulation. Chemelli et al. (1999) reported that, by behavioral and electroencephalographic criteria, homozygous Hcrt knockout mice exhibited a phenotype strikingly similar to that in human narcolepsy patients as well as in canarc-1 mutant dogs, the only known monogenic model of narcolepsy. Moreover, modafinil, an antinarcoleptic drug with ill-defined mechanisms of action, activates hypocretin-containing neurons. They proposed that hypocretin regulates sleep/wakefulness states, and that Hcrt knockout mice are a model of human narcolepsy, a disorder characterized primarily by rapid eye movement (REM) sleep dysregulation.
Ostrander and Giniger (1999) reviewed the molecular biology of inherited narcolepsy in the Doberman pinscher dog. They cited the work of Lin et al. (1999), who mapped and cloned the gene responsible by breeding a pedigree of Doberman pinschers with autosomal recessive narcolepsy; Ostrander and Giniger (1999) reproduced the pedigree. Lin et al. (1999) identified the HCRTR2 gene as a possible candidate for narcolepsy. Siegel (1999) reviewed the nature of narcolepsy and the HCRT system.
To examine the phenotype caused by postnatal loss of orexin-containing neurons, Hara et al. (2001) expressed a toxic transgene specifically in orexin-containing neurons of transgenic mice. Using antiorexin immunostaining, Hara et al. (2001) showed that hypothalamic orexin-containing neurons gradually decreased in number, and were almost completely absent in the brains of the transgenic mice by 15 weeks of age. Infrared video photography and electroencephalographic/electromyographic (EEG/EMG) recording demonstrated cataplexy-like behavioral arrests, premature entry into rapid eye movement (REM) sleep, and poorly consolidated sleep patterns in the transgenic mice, a phenotype similar to human narcolepsy. Additionally, Hara et al. (2001) observed a late-onset obesity in the transgenic mice despite reduced food intake. Hara et al. (2001) concluded that orexinergic neurons are important in the regulation of sleep/wake states and play a role in the regulation of feeding behavior and energy homeostasis. Hara et al. (2001) suggested that the orexin/ataxin-3 transgenic mice provide a model for studying the pathophysiology and treatment of narcolepsy.
Boutrel et al. (2005) found that intracerebroventricular infusions of orexin A, which they called Hcrt1, led to a dose-related reinstatement of cocaine-seeking behavior in rats. Hcrt1 also elevated intracranial self-stimulation thresholds, suggesting that it negatively regulates the activity of brain reward circuitry. Hcrt1-induced reinstatement of cocaine seeking was prevented by blockade of noradrenergic and corticotropin-releasing hormone (CRH; 122560) systems, suggesting that Hcrt1 reinstated drug seeking through induction of a stress-like state. The findings suggested a role for hypocretins in driving drug-seeking behavior.
Huntington disease (HD; 143100) is a neurodegenerative disorder caused by an expanded CAG repeat in the gene encoding huntingtin (613004). Mutant huntingtin forms intracellular aggregates and is associated with neuronal death in select brain regions. The R6/2 mouse model of HD is well studied and replicates many features of the disease. Petersen et al. (2005) described a dramatic atrophy and loss of orexin-producing neurons in the lateral hypothalamus of R6/2 Huntington mice and in Huntington patients. Similar to animal models and patients with impaired orexin function, the R6/2 mice were narcoleptic. Both the number of orexin neurons in the lateral hypothalamus and the levels of orexin in the cerebrospinal fluid were reduced by 72% in end-stage R6/2 mice compared with wildtype littermates, suggesting that orexin could be used as a biomarker reflecting neurodegeneration.
Brisbare-Roch et al. (2007) observed dose-dependent decreased alertness and increased NREM and REM sleep, but no signs of cataplexy, in rats, dogs, and humans during transient pharmacologic reduction in orexin function using an orally administered HCRTR1/HCRTR2 receptor antagonist that penetrates into the brain.
Using in vivo microdialysis in mice, Kang et al. (2009) found that during orexin infusion the amount of brain interstitial fluid (ISF) amyloid-beta (see APP, 104760) was significantly increased. The amount of ISF amyloid-beta also significantly increased during acute sleep deprivation, but decreased with infusion of a dual orexin receptor antagonist. Chronic sleep restriction significantly increased, and a dual orexin receptor antagonist decreased, amyloid-beta plaque formation in APP transgenic mice. Kang et al. (2009) concluded that the sleep-wake cycle and orexin play a role in the pathogenesis of Alzheimer disease (104300).
Johnson et al. (2010) showed that orexin may be involved in panic disorder (167870) and anxiety (see 607834), both of which are associated with increased arousal, hypervigilance, and stimulation of the autonomic nervous system. A rat model of panic disorder showed increased activation of Hcrt-positive cells in the dorsomedial-perifornical hypothalamus after sodium lactate administration that correlated with anxious behavior compared to nonpanic rats. This response was attenuated with siRNA against the Hcrt gene, as well as by antagonists to the Hcrt receptor injected directly into the stria terminalis. These attenuating effects mimicked treatment with benzodiazepines, which result in increased GABAergic activity. Finally, cerebrospinal fluid levels of Hcrt were increased among 53 individuals with panic anxiety compared to controls, suggesting that the hypocretin system may be involved in the pathophysiology of panic anxiety.
▼ ALLELIC VARIANTS ( 1 Selected Example):
.0001 NARCOLEPSY 1 (1 patient)
In a patient with early-onset narcolepsy (161400), Peyron et al. (2000) found a G-to-T transversion changing leucine-16 to a highly charged arginine in the poly-leucine hydrophobic core of the HCRT signal peptide. This allele was not present in 212 control chromosomes, including the patient's unaffected mother. DNA from the unaffected father was unavailable to establish this allele as a de novo mutation. The patient had severe cataplexy (5 to 20 attacks per day when untreated), daytime sleepiness, sleep paralysis, and hypnagogic hallucinations. He first demonstrated cataplexy (expressed as head dropping when laughing) at age 6 months. Sudden episodes of imperative sleep (a few minutes to 1 hour) were also noted at this early age. Sleep-onset rapid eye movement (REM) periods were first documented at 2.5 years of age. Over a 16-year follow-up period, testing consistently showed extremely short sleep latencies and multiple sleep-onset REMs. His symptoms were partially controlled with methylphenidate and imipramine, chlomipramine, or fluoxetine. Additional clinical features included severe periodic leg movements and episodic nocturnal bulimia focused on sweets from the age of 5 years. HCRT concentrations were undetectable in lumbar cerebrospinal fluid. Magnetic resonance imaging focused on the hypothalamic region was unremarkable.