 |
Narcolepsy, Sleep, and Hypocretin (Orexin)
Summary: Emmanuel Mignot studies sleep and sleep disorders, most notably narcolepsy and the neuromodulator hypocretin (orexin). His approaches involve animal model, human population, and human clinical studies.
Sleep and Sleep Disorders
Sleep, a fundamental need, consumes more than half of our first year of life and roughly a third of our adult life. A role for sleep in decreasing energy metabolism, improving brain maturation, and consolidating memory has been suggested. In rats, a complete lack of sleep increases appetite, body temperature, and metabolic expenditure. In a matter of weeks, sleep deprivation leads to death.
A heterogeneous state, sleep is classically separated into rapid eye movement (REM) and non-REM sleep. Non-REM sleep can be subdivided into light non-REM sleep (stage I and II) and slow-wave sleep (stage III and IV). Independent of this organization, the propensity to sleep is regulated by homeostatic (sleep-debt-dependent) and circadian (clock-dependent) processes. The importance of circadian factors is best illustrated in the absence of time cues. In these conditions, sleep, wakefulness, core temperature, and various other behaviors still fluctuate with a periodicity close to 24 hours, called the free-running circadian period. Circadian factors regulating sleep are mostly localized in the hypothalamic suprachiasmatic nuclei, while the neural basis of homeostatic sleep regulation is unknown. Finally, sleep is associated with a host of physiological changes, including sleep-state-specific or circadian-controlled changes in endocrine release, convulsive thresholds, regulation of breathing, cardiovascular control, gastrointestinal physiology, and muscle tone.
The complexity of sleep is matched by an increasing number of pathologies (more than 50 now cataloged). The evolution of sleep disorders medicine into a distinct specialty is proceeding rapidly. In 1970, only a few specialists studied sleep disorders. Now, more than 600 sleep centers and 2,000 physicians are accredited by the American Board of Sleep Medicine. Sleep disorders are prevalent and associated with significant morbidity. Sleep-disordered breathing (snoring and gasping during sleep), which produces sleepiness and is now recognized as a major risk factor for cardiovascular morbidity, affects 5–20 percent of the population at various ages. More than 10 percent of the population consistently complain of severe chronic insomnia. Severe restless legs syndrome (RLS)—associated with difficulty in falling asleep because of discomfort in the legs and sleep that is fragmented because of periodic limb movement—affects 5 percent of the population.
In a 24-hour society that encourages sleep deprivation, accidents due to sleepiness are a major public hazard. Evidence also shows that sleep deprivation has adverse consequences on hormonal regulation. Daytime sleepiness is often secondary to obstructive sleep apnea, a condition that affects more than 10 percent of the population, with adverse cardiovascular and metabolic consequences. Narcolepsy, a neurological disorder, affecting ~0.05 percent of the population, also causes severe sleepiness.
Current treatments for sleep disorders are all symptomatically based. Our goal is to discover the true cause of sleep disorders and to use this knowledge to design new treatments.
Narcolepsy
Narcolepsy offers sleep researchers a unique opportunity to gather information on the mechanisms regulating REM sleep and alertness. The main symptoms of narcolepsy are sleepiness and abnormal REM sleep. REM sleep, which normally occurs in the middle of the night, is associated with dreaming and muscle paralysis. Since the 1960s it has been known that most narcolepsy symptoms, such as sleep paralysis, cataplexy, and hypnagogic hallucinations, are pathological equivalents of REM sleep. This has led to the development of the multiple sleep latency test (MSLT), a test where the patient is asked to nap 4 or 5 times during the day and REM sleep is recorded. In narcolepsy, most patients have brief naps with REM sleep; more than two naps containing REM sleep are diagnostic for narcolepsy.
In sleep paralysis, the patient is suddenly unable to move for a few minutes, most often just before falling asleep or when waking up. During hypnagogic hallucinations, patients experience dream-like hallucinations while dozing or falling asleep. Cataplexy, a pathological equivalent of REM sleep paralysis unique to narcolepsy, is a striking sudden episode of muscle weakness triggered by emotions. Typically, the patient's knees buckle and may give way upon laughing, elation, surprise, or anger. In other attacks, the head may drop or the jaw may become slack. In severe cases, the patient might fall down and become completely paralyzed for a few seconds to several minutes.
The study of narcolepsy has been facilitated by the existence of a canine model of the disorder with autosomal-recessive transmission. In 1999, we discovered the disorder is caused by a mutation in the hypocretin (also known as orexin) receptor 2 gene. Masashi Yanagisawa (HHMI, University of Texas Southwestern Medical Center at Dallas) and his colleagues also discovered that mice genetically manipulated to lack the ligand of this receptor (the hypocretin peptide) had symptoms reminiscent of narcolepsy. In 2000, we discovered that human narcolepsy (when with cataplexy) is associated with very low to undetectable levels of hypocretin in the cerebrospinal fluid (CSF) and brain tissue. These findings indicate that most human cases of narcolepsy-cataplexy are caused by hypocretin deficiency. The current treatments for narcolepsy are purely symptomatic, mostly using stimulants and antidepressants that stimulate monoamine (dopamine, norepinephrine, serotonin) neurotransmission. Hypocretin replacement therapies may provide novel therapeutic avenues.
Hypocretin and Sleep
Hypocretin is a neuropeptide encoded by a single gene. Two receptors (hypocretin receptor-1 and -2) are known. Surprisingly, almost all hypocretin cells are located in the hypothalamus, yet they project widely in the brain and spinal cord. Hypocretin's direct effect on target cells is also almost always excitatory. Dense projections on monoaminergic cells—especially histaminergic neurons, a neuronal group rich in hypocretin receptor-2—provide a possible anatomical framework for the effects of hypocretins on sleep regulation.
The finding that hypocretin cells are located in the hypothalamus initially suggested a role for these peptides in the regulation of food intake. More recently, the role of hypocretins in regulating sleep in concert with other physiological functions has been emphasized. For example, hypocretin cells have leptin receptors and are sensitive to ghrelin and glucose. It has been suggested that hypocretin neurons integrate metabolic signals to regulate alertness. This could explain sleepiness after meals and fasting-induced alertness. It is also known that the central administration of hypocretin stimulates sympathetic outflow and increases metabolic expenditure. Hypocretin may participate in the regulation of overall energy metabolism, with primary effects on sleep.
We are particularly interested in delineating the role of hypocretin in homeostatic, sleep-stage, and circadian sleep regulation. To do so, we believe it is essential to study not only rodents but also multiple other species. Rodents are nocturnal, have polyphasic sleep, and are likely to have a fairly specific ecological reaction to short-term food deprivation or forced exercise. A model of particular interest to us is the squirrel monkey, a New World monkey with daytime consolidation of wakefulness similar to humans. Using multiple animal models, we found that hypocretin release is controlled by both circadian and homeostatic influences in various species. We also found that sleep debt is a major regulator of hypocretin release, suggesting that hypocretin is essential to maintain alertness in the face of small amounts of sleep deprivation. This important property may explain why narcoleptic patients who do not have hypocretin are typically unable to stay awake for more than 2 hours.
Clinical Applications in Narcolepsy
Hypocretin cell loss causes narcolepsy-cataplexy. In humans, narcolepsy is a genetically complex disorder with environmental influences. Our studies have shown a tight genetic association with HLA-DQB1*0602 in cases with cataplexy and hypocretin deficiency, suggesting the involvement of the immune system. More recently, using a genome-wide association design, we found that specific polymorphisms within the TCR@ locus (J segment) are also associated with narcolepsy. Because the TCR is only expressed in immune T cells, the finding is the strongest evidence to date that the disease is due to an autoimmune attack against hypocretin cells. We are now searching for the particular TCR idiotype that may be involved, together with HLA-peptide presentation, in triggering autoimmunity. As more than 100 autoimmune diseases are associated with HLA, and yet how HLA predisposes to autoimmunity is unclear, we believe this work will shed light not only on narcolepsy but on how HLA predisposes to organ-specific autoimmune attacks.
As a result of our research, narcolepsy is now occasionally diagnosed by measuring CSF hypocretin-1. More than 95 percent of patients with cataplexy and HLA-DQB1*0602 have low hypocretin levels, delineating a biologically homogenous entity. We are also finding rare cases atypical either clinically (e.g., no cataplexy or normal sleep tests) or biologically (normal hypocretin or DQB1*0602 negative). Low CSF hypocretin is also found when narcolepsy has a secondary origin, in the context of specific disorders with daytime sleepiness (Prader-Willi syndrome, Niemann-Pick type C disease), head trauma, or hypothalamic tumors. This research is slowly redefining the clinical landscape of hypersomnia.
We are administrating hypocretin peripherally and centrally in animal models to test whether hypocretin replacement might provide therapeutic relief in humans. An alternative treatment could involve replacing hypocretin-producing cells in the central nervous system. If successful, these approaches will be rapidly tried in human narcolepsy.
Fishing for Novel Hypocretin Pathway Genes
A recent area of interest is the use of animal models such as the zebrafish to isolate novel genes regulating the hypocretin and histamine systems. Zebrafish are transparent early in life, facilitating whole-mount in situ hybridization, and uniquely suited to large-scale mutagenesis. We have found that zebrafish contain hypocretin and histamine-producing cells, and we have used automated videorecording to characterize sleep-like behavior in this animal model. Furthermore, we recently identified a hypocretin receptor fish and found its sleep to be fragmented, suggesting involvement of the hypocretin neuropeptide in fish sleep regulation. Future studies are aimed at mapping the neuronal circuitry mediating sleep regulation in zebrafish. We are also making use of the transparency of this model to study synaptic plasticity and sleep in vivo.
Genetic Evaluation of Sleep and Metabolic Studies in a Population-Based Sample
One of our major interests is the study of normal sleep variation and sleep disorders in the general population. To achieve this aim, we collaborate with Terry Young (University of Madison–Wisconsin). A major interest in this collaboration is the study of milder narcolepsy and sleep phenotypes. We are systematically conducting MSLT sleep testing in a general population sample and correlating the results with known narcolepsy genetic factors such as DQB1*0602.
We are also interested in genetic factors that influence sleep-disordered breathing (SDB) and the development of high blood pressure in association with this disorder. We found and replicated the observation that APOE ε4, a genetic factor known to increase risk for Alzheimer's disease, also increases the risk of SDB around middle age, suggesting brain influences on the regulation of breathing during sleep.
Our lab is also evaluating the study of hormonal changes with sleep restriction, and its consequences for general health and obesity. An association between sleep restriction and obesity is increasingly recognized. We found clear association between habitual sleep amounts, polysomnographically defined sleep, and appetite regulatory hormone levels such as leptin and ghrelin. Finally, we are also attempting to determine whether genetic variation in known circadian genes such as clock influences morningness-eveningness tendencies in this population.
This work is also funded by grants from the National Institutes of Health.
Last updated May 28, 2009
|
|
|
|
