A functional autoantibody in narcolepsy

Connecting the dots: An updated review of the role of autoimmunity in narcolepsy and emerging immunotherapeutic approaches

BACKGROUND

Narcolepsy type 1 (NT1) is a chronic disorder characterized by pathological daytime sleepiness and cataplexy due to the disappearance of orexin immunoreactive neurons in the hypothalamus. Genetic and environmental factors point towards a potential role for inflammation and autoimmunity in the pathogenesis of the disease. This study aims to comprehensively review the latest evidence on the autoinflammatory mechanisms and immunomodulatory treatments aimed at suspected autoimmune pathways in NT1.

METHODS

Recent relevant literature in the field of narcolepsy, its autoimmune hypothesis, and purposed immunomodulatory treatments were reviewed.

RESULTS

Narcolepsy is strongly linked to specific HLA alleles and T-cell receptor polymorphisms. Furthermore, animal studies and autopsies have found infiltration of T cells in the hypothalamus, supporting T cell-mediated immunity. However, the role of autoantibodies has yet to be definitively established. Increased risk of NT1 after H1N1 infection and vaccination supports the autoimmune hypothesis, and the potential role of coronavirus disease 2019 and vaccination in triggering autoimmune neurodegeneration is a recent finding. Alterations in cytokine levels, gut microbiota, and microglial activation indicate a potential role for inflammation in the disease's development. Reports of using immunotherapies in NT1 patients are limited and inconsistent. Early treatment with IVIg, corticosteroids, plasmapheresis, and monoclonal antibodies has seldomly shown some potential benefits in some studies.

CONCLUSION

The current body of literature supports that narcolepsy is an autoimmune disorder most likely caused by T-cell involvement. However, the potential for immunomodulatory treatments to reverse the autoinflammatory process remains understudied. Further clinical controlled trials may provide valuable insights into this area.

The immunopathogenesis of narcolepsy type 1

Narcolepsy type 1 (NT1) is a chronic sleep disorder resulting from the loss of a small population of hypothalamic neurons that produce wake-promoting hypocretin (HCRT; also known as orexin) peptides. An immune-mediated pathology for NT1 has long been suspected given its exceptionally tight association with the MHC class II allele HLA-DQB1*06:02, as well as recent genetic evidence showing associations with polymorphisms of T cell receptor genes and other immune-relevant loci and the increased incidence of NT1 that has been observed after vaccination with the influenza vaccine Pandemrix. The search for both self-antigens and foreign antigens recognized by the pathogenic T cell response in NT1 is ongoing. Increased T cell reactivity against HCRT has been consistently reported in patients with NT1, but data demonstrating a primary role for T cells in neuronal destruction are currently lacking. Animal models are providing clues regarding the roles of autoreactive CD4+ and CD8+ T cells in the disease. Elucidation of the pathogenesis of NT1 will allow for the development of targeted immunotherapies at disease onset and could serve as a model for other immune-mediated neurological diseases.

Narcolepsy following COVID-19: A case report and review of potential mechanisms

Key Clinical Message

The immune activation in COVID-19 may trigger narcolepsy in vulnerable patients. We suggest clinicians carefully evaluate patients with post-COVID fatigue and hypersomnia for primary sleep disorders, specifically narcolepsy.

Abstract

The patient is a 33-year-old Iranian woman without a significant past medical history with the full range of narcolepsy symptoms that started within 2 weeks after her recovery from COVID-19. Sleep studies revealed increased sleep latency and three sleep-onset rapid eye movement events, compatible with a narcolepsy-cataplexy diagnosis.

Activating autoantibodies against G protein-coupled receptors in narcolepsy type 1

STUDY OBJECTIVES

Narcolepsy type 1 is a rare hypersomnia of central origin, which is caused by loss of hypothalamic neurons that produce the neuropeptides hypocretin-1 and -2. Hypocretin-containing nerve terminals are found in areas known to play a central role in autonomic control and in pain signaling. Cholinergic M2 receptors are found in brain areas involved with the occurrence of hallucinations and cataplexy. In addition to classical symptoms of narcolepsy, the patients suffer frequently from autonomic dysfunction, chronic pain, and hypnagogic/hypnopompic hallucinations. We aimed to test whether narcolepsy type 1 patients have autoantibodies against autonomic β2 adrenergic receptor, M2 muscarinic receptors, or nociception receptors.

METHODS

We tested the serum of ten narcolepsy type 1 patients (five female) for activating β2 adrenergic receptor autoantibodies, M2 muscarinic receptor autoantibodies, and nociception receptor autoantibodies.

RESULTS

Ten of ten patients were positive for muscarinic M2 receptor autoantibodies (P < 0.001), 9/10 were positive for autoantibodies against nociception receptors (P < 0.001), and 5/10 were positive for β2 adrenergic receptor autoantibodies (P < 0.001).

CONCLUSIONS

Narcolepsy type 1 patients harbored activating autoantibodies against M2 muscarinic receptors, nociception receptors, and β2 adrenergic receptors. M2 receptor autoantibodies may be related to the occurrence of cataplexy and, moreover, hallucinations in narcolepsy since they are found in the same brain areas that are involved with these symptoms. The occurrence of nociception receptor autoantibodies strengthens the association between narcolepsy type 1 and pain. The connection between narcolepsy type 1, autonomic complaints, and the presumed cardiovascular morbidity might be associated with the occurrence of β2 adrenergic receptor autoantibodies. On the other hand, the presence of the autoantibodies may be secondary to the destruction of the hypocretin pathways.

[Narcolepsy: From the discovery of a wake promoting peptide to autoimmune T cell biology and molecular mimicry with flu epitopes]

Narcolepsy-cataplexy was first described in the late 19th century in Germany and France. Prevalence was established to be 0.05 % and a canine model was discovered in the 1970s. In 1983, a Japanese study found that all patients carried HLA-DR2, suggesting autoimmunity as the cause of the disease. Studies in the canine model established that dopaminergic stimulation underlies anti-narcoleptic action of psychostimulants, while antidepressants were found to suppress cataplexy through adrenergic reuptake inhibition. No HLA association was found in canines. A linkage study initiated in 1988 revealed in hypocretin (orexin) receptor two mutations as the cause of canine narcolepsy in 1999. In 1992, studies on African Americans showed that DQ0602 was a better marker than DR2 across all ethnic groups. In 2000, hypocretin-1/orexin A levels were measured in the cerebrospinal fluid (CSF) and found to be undetectable in most patients, establishing hypocretin deficiency as the cause of narcolepsy. Decreased CSF hypocretin-1 was then found to be secondary to the loss of the 70,000 neurons producing hypocretin in the hypothalamus, suggesting immune destruction of these cells as the cause of the disease. Additional genetic studies, notably genome wide associations (GWAS), found multiple genetic predisposing factors for narcolepsy. These were almost all involved in other autoimmune diseases, although a strong and unique association with T cell receptor (TCR) alpha and beta loci were observed. Nonetheless, all attempts to demonstrate presence of autoantibodies against hypocretin cells in narcolepsy failed, and the presumed autoimmune cause remained unproven. In 2009, association with strep throat infections were found, and narcolepsy onsets were found to occur more frequently in spring and summer, suggesting upper away infections as triggers. Following reports that narcolepsy cases were triggered by vaccinations and infections against influenza A 2009 pH1N1, a new pandemic strain that erupted in 2009, molecular mimicry with influenza A virus was suggested in 2010. This hypothesis was later confirmed by peptide screening showing higher activity of CD4⁺ T cell reactivity to a specific post-translationally amidated segment of hypocretin (HCRT-_NH2) and cross-reactivity of specific TCRs with a pH1N1-specific segment of hemagglutinin that shares homology with HCRT-_NH2. Strikingly, the most frequent TCR recognizing these antigens was found to carry sequences containing TRAJ24 or TRVB4-2, segments modulated by narcolepsy-associated genetic polymorphisms. Cross-reactive CD4⁺ T cells with these cross-reactive TCRs likely subsequently recruit CD8⁺ T cells that are then involved in hypocretin cell destruction. Additional flu mimics are also likely to be discovered since narcolepsy existed prior to 2009. The work that has been conducted over the years on narcolepsy offers a unique perspective on the conduct of research on the etiopathogeny of a specific disease.

Enteric neuroplasticity and dysmotility in inflammatory disease: key players and possible therapeutic targets

Intestinal functions, including motility and secretion, are locally controlled by enteric neural networks housed within the wall of the gut. The fidelity of these functions depends on the precision of intercellular signaling among cellular elements, including enteric neurons, epithelial cells, immune cells, and glia, all of which are vulnerable to disruptive influences during inflammatory events. This review article describes current knowledge regarding inflammation-induced neuroplasticity along key elements of enteric neural circuits, what is known about the causes of these changes, and possible therapeutic targets for protecting and/or repairing the integrity of intrinsic enteric neurotransmission. Changes that have been detected in response to inflammation include increased epithelial serotonin availability, hyperexcitability of intrinsic primary afferent neurons, facilitation of synaptic activity among enteric neurons, and attenuated purinergic neuromuscular transmission. Dysfunctional propulsive motility has been detected in models of colitis, where causes include the changes described above, and in models of multiple sclerosis and other autoimmune conditions, where autoantibodies are thought to mediate dysmotility. Other cells implicated in inflammation-induced neuroplasticity include muscularis macrophages and enteric glia. Targeted treatments that are discussed include 5-hydroxytryptamine receptor 4 agonists, cyclooxygenase inhibitors, antioxidants, B cell depletion therapy, and activation of anti-inflammatory pathways.

Prostaglandin D2 Receptor DP1 Antibodies Predict Vaccine-induced and Spontaneous Narcolepsy Type 1: Large-scale Study of Antibody Profiling

BACKGROUND

Neuropathological findings support an autoimmune etiology as an underlying factor for loss of orexin-producing neurons in spontaneous narcolepsy type 1 (narcolepsy with cataplexy; sNT1) as well as in Pandemrix influenza vaccine-induced narcolepsy type 1 (Pdmx-NT1). The precise molecular target or antigens for the immune response have, however, remained elusive.

METHODS

Here we have performed a comprehensive antigenic repertoire analysis of sera using the next-generation phage display method - mimotope variation analysis (MVA). Samples from 64 children and adolescents were analyzed: 10 with Pdmx-NT1, 6 with sNT1, 16 Pandemrix-vaccinated, 16 H1N1 infected, and 16 unvaccinated healthy individuals. The diagnosis of NT1 was defined by the American Academy of Sleep Medicine international criteria of sleep disorders v3.

FINDINGS

Our data showed that although the immunoprofiles toward vaccination were generally similar in study groups, there were also striking differences in immunoprofiles between sNT1 and Pdmx-NT1 groups as compared with controls. Prominent immune response was observed to a peptide epitope derived from prostaglandin D2 receptor (DP1), as well as peptides homologous to B cell lymphoma 6 protein. Further validation confirmed that these can act as true antigenic targets in discriminating NT1 diseased along with a novel epitope of hemagglutinin of H1N1 to delineate exposure to H1N1.

INTERPRETATION

We propose that DP1 is a novel molecular target of autoimmune response and presents a potential diagnostic biomarker for NT1. DP1 is involved in the regulation of non-rapid eye movement (NREM) sleep and thus alterations in its functions could contribute to the disturbed sleep regulation in NT1 that warrants further studies. Together our results also show that MVA is a helpful method for finding novel peptide antigens to classify human autoimmune diseases, possibly facilitating the design of better therapies.

Effective treatment of narcolepsy-like symptoms with high-frequency repetitive transcranial magnetic stimulation: A case report

RATIONALE

Narcolepsy is a rare sleep disorder with disrupted sleep-architecture. Clinical management of narcolepsy lies dominantly on symptom-driven pharmacotherapy. The treatment role of repetitive transcranial magnetic stimulation (rTMS) for narcolepsy remains unexplored.

PATIENT CONCERNS

In this paper, we present a case of a 14-year-old young girl with excessive daytime sleepiness (EDS), cataplexy and hypnagogic hallucinations.

DIAGNOSES

After excluding other possible medical conditions, this patient was primarily diagnosed with narcolepsy.

INTERVENTIONS

The patient received 25 sessions of high-frequency rTMS over the left dorsolateral prefrontal cortex (DLPFC).

OUTCOMES

The symptoms of EDS and cataplexy significantly improved after rTMS treatment. Meanwhile, her score in the Epworth sleep scale (ESS) also remarkably decreased.

LESSONS

This case indicates that rTMS may be selected as a safe and effective alternative strategy for treating narcolepsy-like symptoms. Well-designed researches are warranted in future investigations on this topic.

CD4+ T-Cell Reactivity to Orexin/Hypocretin in Patients With Narcolepsy Type 1

Introduction

Narcolepsy type 1 is accompanied by a selective loss of orexin/hypocretin (hcrt) neurons in the lateral hypothalamus caused by yet unknown mechanisms. Epidemiologic and genetic associations strongly suggest an immune-mediated pathogenesis of the disease.

Methods

We compared specific T-cell reactivity to orexin/hcrt peptides in peripheral blood mononuclear cells of narcolepsy type 1 patients to healthy controls by a carboxyfluorescein succinimidyl ester proliferation assay. Orexin/hcrt-specific T-cell reactivity was also determined by cytokine (interferon gamma and granulocyte-macrophage colony-stimulating factor) analysis. Individuals were considered as responders if the cell division index of CD3+CD4+ T cells and both stimulation indices of cytokine secretion exceeded the cutoff 3. Additionally, T-cell reactivity to orexin/hcrt had to be confirmed by showing reactivity to single peptides present in different peptide pools.

Results

Using these criteria, 3/15 patients (20%) and 0/13 controls (0%) showed orexin/hcrt-specific CD4+ T-cell proliferation (p = .2262). The heterogeneous reactivity pattern did not allow the identification of a preferential target epitope.

Conclusions

A significant role of orexin/hcrt-specific T cells in narcolepsy type 1 patients could not be confirmed in this study. Further studies are needed to assess the exact role of CD4+ T cells and possible target antigens in narcolepsy type 1 patients.

Narcolepsy Type 1 as an Autoimmune Disorder: Evidence, and Implications for Pharmacological Treatment

Narcolepsy type 1 (NT1) is a rare sleep disorder caused by the very specific loss of hypothalamic hypocretin (Hcrt)/orexin neurons. The exact underlying process leading to this destruction is yet unknown, but indirect evidence strongly supports an autoimmune origin. The association with immune-related genetic factors, in particular the strongest association ever reported in a disease with an allele of a human leukocyte antigen (HLA) gene, and with environmental factors (i.e., the H1N1 influenza infection and vaccination during the pandemic in 2009) are in favor of such a hypothesis. The loss of Hcrt neurons is irreversible, and NT1 is currently an incurable and disabling condition. Patients are managed with symptomatic medication, targeting the main symptoms (excessive daytime sleepiness, cataplexy, disturbed nocturnal sleep), and they require a lifelong treatment. Improved diagnostic tools, together with an increased understanding of the pathogenesis of NT1, may lead to new therapeutic and even preventive interventions. One future treatment could include Hcrt replacement, but this neuropeptide does not cross the blood-brain barrier. However, Hcrt receptor agonists may be promising candidates to treat NT1. Another option is immune-based therapies, administered at disease onset, with already some initiatives to slow down or stop the dysimmune process. Whether immune-based therapy could be beneficial in NT1 remains, however, to be proven.

Comorbidity of Narcolepsy Type 1 With Autoimmune Diseases and Other Immunopathological Disorders: A Case-Control Study

Background

Several evidences suggest that autoimmune diseases (ADs) tend to co-occur in an individual and within the same family. Narcolepsy type 1 (NT1) is a chronic sleep disorder caused by a selective loss of hypocretin-producing neurons due to a mechanism of neural destruction that indicates an autoimmune pathogenesis, although no evidence is available. We report on the comorbidity of ADs and other immunopathological diseases (including allergy diseases) in narcolepsy.

Methods

We studied 158 Caucasian NT1 patients (60.7% male; mean age 49.4 ± 19.7 years), in whom the diagnosis was confirmed by polysomnography followed by a multiple sleep latency test, or by hypocretin-1 levels measurements.

Results

Thirty out of 158 patients (18.99%; 53.3% female; 29 sporadic and one familial cases) had one or more immunopathological diseases associated. A control group of 151 subjects were matched by gender and age with the narcolepsy patients. Results demonstrated that there was a higher frequency of ADs in our series of narcolepsy patients compared to the sample of general population (odds ratio: 3.17; 95% confidence interval: 1.01 - 10.07; P = 0.040). A temporal relationship with the age at onset of the diseases was found.

Conclusions

Cataplexy was significantly more severe in NT1 patients with immunopathological diseases, and immunopathological diseases are a risk factor for severe forms of cataplexy in our series (odds ratio: 23.6; 95% confidence interval: 5.5 - 100.1).

Autoantibody targets in vaccine-associated narcolepsy

Narcolepsy is a chronic sleep disorder with a yet unknown cause, but the specific loss of hypocretin-producing neurons together with a strong human leukocyte antigen (HLA) association has led to the hypothesis that autoimmune mechanisms might be involved. Here, we describe an extensive effort to profile autoimmunity repertoires in serum with the aim to find disease-related autoantigens. Initially, 57 serum samples from vaccine-associated and sporadic narcolepsy patients and controls were screened for IgG reactivity towards 10 846 fragments of human proteins using planar microarrays. The discovered differential reactivities were verified on suspension bead arrays in the same sample collection followed by further investigation of 14 antigens in 176 independent samples, including 57 narcolepsy patients. Among these 14 antigens, methyltransferase-like 22 (METTL22) and 5'-nucleotidase cytosolic IA (NT5C1A) were recognized at a higher frequency in narcolepsy patients of both sample sets. Upon sequence analysis of the 14 proteins, polymerase family, member 3 (PARP3), acyl-CoA-binding domain containing 7 (ARID4B), glutaminase 2 (GLS2) and cyclin-dependent kinase-like 1 (CDKL1) were found to contain amino acid sequences with homology to proteins found in the H1N1 vaccine. These findings could become useful elements of further clinical assays that aim towards a better phenotypic understanding of narcolepsy and its triggers.

Hypocretin (orexin) biology and the pathophysiology of narcolepsy with cataplexy

The discovery of hypocretins (orexins) and their causal implication in narcolepsy is the most important advance in sleep research and sleep medicine since the discovery of rapid eye movement sleep. Narcolepsy with cataplexy is caused by hypocretin deficiency owing to destruction of most of the hypocretin-producing neurons in the hypothalamus. Ablation of hypocretin or hypocretin receptors also leads to narcolepsy phenotypes in animal models. Although the exact mechanism of hypocretin deficiency is unknown, evidence from the past 20 years strongly favours an immune-mediated or autoimmune attack, targeting specifically hypocretin neurons in genetically predisposed individuals. These neurons form an extensive network of projections throughout the brain and show activity linked to motivational behaviours. The hypothesis that a targeted immune-mediated or autoimmune attack causes the specific degeneration of hypocretin neurons arose mainly through the discovery of genetic associations, first with the HLA-DQB1*06:02 allele and then with the T-cell receptor α locus. Guided by these genetic findings and now awaiting experimental testing are models of the possible immune mechanisms by which a specific and localised brain cell population could become targeted by T-cell subsets. Great hopes for the identification of new targets for therapeutic intervention in narcolepsy also reside in the development of patient-derived induced pluripotent stem cell systems.

History of narcolepsy at Stanford University

Although narcolepsy was first described in the late nineteenth century in Germany and France, much of the research on this disorder has been conducted at Stanford University, starting with Drs. William C. Dement and Christian Guilleminault in the 1970s. The prevalence of narcolepsy was established, and a canine model discovered. Following the finding in Japan that almost all patients with narcolepsy carry a specific HLA subtype, HLA-DR2, Hugh Mac Devitt, F. Carl Grumet, and Larry Steinman initiated immunological studies, but results were generally negative. Using the narcoleptic canines, Dr. Nishino and I established that stimulants increased wakefulness by stimulating dopaminergic transmission while antidepressants suppress cataplexy via adrenergic reuptake inhibition. A linkage study was initiated with Dr. Grumet in 1988, and after 10 years of work, the canine narcolepsy gene was cloned by in 1999 and identified as the hypocretin (orexin) receptor 2. In 1992, studying African Americans, we also found that DQ0602 rather than DR2 was a better marker for narcolepsy across all ethnic groups. In 2000, Dr. Nishino and I, in collaboration with Dr. Lammers in the Netherlands, found that hypocretin 1 levels in the cerebrospinal fluid (CSF) were undetectable in most cases, establishing hypocretin deficiency as the cause of narcolepsy. Pursuing this research, our and Dr. Siegel's group, examining postmortem brains, found that the decreased CSF hypocretin 1 was secondary to the loss the 70,000 neurons producing hypocretin in the hypothalamus. This finding revived the autoimmune hypothesis but attempts at demonstrating immune targeting of hypocretin cells failed until 2013. At this date, Dr. Elisabeth Mellins and I discovered that narcolepsy is characterized by the presence of autoreactive CD4(+) T cells to hypocretin fragments when presented by DQ0602. Following reports that narcolepsy cases were triggered by vaccinations and infections against influenza A 2009 pH1N1, a new pandemic strain that erupted in 2009, our groups also established that a small epitope of pH1N1 resembles hypocretin and is likely involved in molecular mimicry. Although much remains to be done, these achievements, establishing hypocretin deficiency as the cause of narcolepsy, demonstrating its autoimmune basis, and showing molecular mimicry between hypocretin and sequences derived from a pandemic strain of influenza, are likely to remain classics in human immunology.

Etiopathogenesis and neurobiology of narcolepsy: a review

Narcolepsy is a chronic lifelong sleep disorder and it often leaves a debilitating effect on the quality of life of the sufferer. This disorder is characterized by a tetrad of excessive daytime sleepiness, cataplexy (brief loss of muscle tone following strong emotion), hypnogogic hallucinations and sleep paralysis. There are two distinct subgroups of Narcolepsy: Narcolepsy with cataplexy and Narcolepsy without cataplexy. For over 100 years, clinicians have recognised narcolepsy, but only in the last few decades have scientists been able to shed light on the true cause and pathogenesis of narcolepsy. Recent studies have shown that a loss of the hypothalamic neuropeptide Hypocretin/Orexincauses Narcolepsy with cataplexy and that an autoimmune mechanism may be responsible for this loss. Our understanding of the neurophysiologic aspect of narcolepsy has also significantly improved. The basic neural mechanisms behind sleepiness and cataplexy, the two defining symptoms of narcolepsy have started to become clearer. In this review, we have provided a detailed account of the key aspects of etiopathogenesis and neurobiology of narcolepsy, along with a critical appraisal of the more recent and interesting causal associations.We have also looked at the contributions of neuroimaging to the etiopathogenesis of Narcolepsy.

Genetic association, seasonal infections and autoimmune basis of narcolepsy

In recent years, a growing number of potential autoimmune disorders affecting neurons in the central nervous system have been identified, including narcolepsy. Narcolepsy is a lifelong sleep disorder characterized by excessive daytime sleepiness with irresistible sleep attacks, cataplexy (sudden bilateral loss of muscle tone), hypnagogic hallucinations, and abnormalities of Rapid Eye Movement sleep. Narcolepsy is generally a sporadic disorder and is caused by the loss of hypocretin (orexin)-producing neurons in the hypothalamus region of the brain. Studies have established that more than 90% of patients have a genetic association with HLA DQB1*06:02. Genome-wide association analysis shows a strong association between narcolepsy and polymorphisms in the TCRα locus and weaker associations within TNFSF4 (also called OX40L), Cathepsin H and the P2RY11-DNMT1 (purinergic receptor subtype P2Y11 to DNMT1, a DNA methytransferase) loci, suggesting an autoimmune basis. Mutations in DNMT1 have also been reported to cause narcolepsy in association with a complex neurological syndrome, suggesting the importance of DNA methylation in the pathology. More recently, narcolepsy was identified in association with seasonal streptococcus, H1N1 infections and following AS03-adjuvanted pH1N1 influenza vaccination in Northern Europe. Potential immunological pathways responsible for the loss of hypocretin producing neurons in these cases may be molecular mimicry or bystander activation. Specific autoantibodies or T cells cross-reactive with hypocretin neurons have not yet been identified, however, thus narcolepsy does not meet Witebsky's criteria for an autoimmune disease. As the brain is not an easily accessible organ, mechanisms of disease initiation and progression remain a challenge to researchers.

Increased immune complexes of hypocretin autoantibodies in narcolepsy

Background

Hypocretin peptides participate in the regulation of sleep-wake cycle while deficiency in hypocretin signaling and loss of hypocretin neurons are causative for narcolepsy-cataplexy. However, the mechanism responsible for alteration of the hypocretin system in narcolepsy-cataplexy and its relevance to other central hypersomnias remain unknown. Here we studied whether central hypersomnias can be associated with autoantibodies reacting with hypocretin-1 peptide present as immune complexes.

Methodology

Serum levels of free and dissociated (total) autoantibodies reacting with hypocretin-1 peptide were measured by enzyme-linked immunosorbent assay and analyzed with regard to clinical parameters in 82 subjects with narcolepsy-cataplexy, narcolepsy without cataplexy or idiopathic hypersomnia and were compared to 25 healthy controls.

Principal Findings

Serum levels of total but not free IgG autoantibodies against hypocretin-1 were increased in narcolepsy-cataplexy. Increased levels of complexed IgG autoantibodies against hypocretin-1 were found in all patients groups with a further increase in narcolepsy-cataplexy. Levels of total IgM hypocretin-1 autoantibodies were also elevated in all groups of patients. Increased levels of anti-idiotypic IgM autoantibodies reacting with hypocretin-1 IgG autoantibodies affinity purified from sera of subjects with narcolepsy-cataplexy were found in all three groups of patients. Disease duration correlated negatively with serum levels of hypocretin-1 IgG and IgM autoantibodies and with anti-idiotypic IgM autoantibodies.

Conclusion

Central hypersomnias and particularly narcolepsy-cataplexy are characterized by higher serum levels of autoantibodies directed against hypocretin-1 which are present as immune complexes most likely with anti-idiotypic autoantibodies suggesting their relevance to the mechanism of sleep-wake cycle regulation.

Elevated Tribbles homolog 2-specific antibody levels in narcolepsy patients

Narcolepsy is a sleep disorder characterized by excessive daytime sleepiness and attacks of muscle atonia triggered by strong emotions (cataplexy). Narcolepsy is caused by hypocretin (orexin) deficiency, paralleled by a dramatic loss in hypothalamic hypocretin-producing neurons. It is believed that narcolepsy is an autoimmune disorder, although definitive proof of this, such as the presence of autoantibodies, is still lacking. We engineered a transgenic mouse model to identify peptides enriched within hypocretin-producing neurons that could serve as potential autoimmune targets. Initial analysis indicated that the transcript encoding Tribbles homolog 2 (Trib2), previously identified as an autoantigen in autoimmune uveitis, was enriched in hypocretin neurons in these mice. ELISA analysis showed that sera from narcolepsy patients with cataplexy had higher Trib2-specific antibody titers compared with either normal controls or patients with idiopathic hypersomnia, multiple sclerosis, or other inflammatory neurological disorders. Trib2-specific antibody titers were highest early after narcolepsy onset, sharply decreased within 2-3 years, and then stabilized at levels substantially higher than that of controls for up to 30 years. High Trib2-specific antibody titers correlated with the severity of cataplexy. Serum of a patient showed specific immunoreactivity with over 86% of hypocretin neurons in the mouse hypothalamus. Thus, we have identified reactive autoantibodies in human narcolepsy, providing evidence that narcolepsy is an autoimmune disorder.

Potentiation of a functional autoantibody in narcolepsy by a cholinesterase inhibitor

We have recently reported the presence of an immunoglobulin G (IgG) autoantibody (Ab) in patients with narcolepsy with cataplexy that abolishes spontaneous colonic migrating motor complexes (CMMCs) and increases smooth muscle tension and atropine-sensitive phasic contractions in a physiological assay of an isolated colon. In this study, we used the cholinesterase inhibitor, neostigmine, to explore the mechanism of the narcoleptic IgG-mediated disruption of enteric motor function in four patients with narcolepsy with cataplexy and to identify a pharmacological mimic of the Ab. Neostigmine potentiated the narcoleptic IgG-mediated increase in smooth muscle resting tension and phasic smooth muscle contractions by an atropine-sensitive mechanism but exerted no effect on resting tension in the presence of control IgG. Decreased frequency of CMMCs mediated by IgG with anti-M3R activity was reversed by neostigmine. Therefore, a challenge with a cholinesterase inhibitor improves the specificity of the CMMC assay for narcoleptic IgG. Tetrodotoxin (TTX), a neuronal sodium channel blocker, also abolished CMMCs and increased resting tone, and a similar potentiation was observed with neostigmine; thus, TTX is a mimic of the functional effects of the narcoleptic IgG in this bioassay. These findings provide a link to pharmacological studies of canine narcolepsy and are consistent with a functional blockade of both excitatory and inhibitory motor neurons by the narcoleptic Ab, similar to the TTX mimic, presumably by binding to an autoantigenic target expressed in both populations of neurons.

An autoantibody in narcolepsy disrupts colonic migrating motor complexes

Despite strong circumstantial evidence for the autoimmune hypothesis of narcolepsy, conventional immunological methods have failed to detect an autoantibody. This study investigated the real-time effects of narcoleptic immunoglobulins on a spontaneous colonic migrating motor complex (CMMC) preparation. IgG from patients with narcolepsy with cataplexy or healthy controls was added directly to isolated mouse colons undergoing CMMC activity to test for autoantibodies that disrupt colonic motility. The effect of immunoglobulins prepared for clinical intravenous treatment (IVIg) on autoantibody-mediated colonic disruption was also assessed. Narcoleptic IgGs markedly reduced the frequency of CMMCs or irreversibly abolished them. Abrogation of CMMCs was followed by an increase in the resting tension of the colon preparation and appearance of atropine-sensitive phasic smooth muscle contractions. IVIg partially neutralized the inhibitory effect of narcoleptic IgG on the CMMCs. The dramatic effect of narcoleptic IgG on CMMC generation is consistent with an autoantibody-mediated disruption of enteric neural pathways. The ex vivo whole-organ approach allows real-time examination of the physiological effects of the narcoleptic autoantibody and offers a new avenue for exploring the autoimmune basis of narcolepsy. The neutralizing effect of IVIg on the autoantibody provides a rationale for the reported clinical improvement in cataplexy when IVIg are given at disease onset.

Pediatric narcolepsy

Narcolepsy is a disabling disease with a prevalence of 0.05%. It is characterized by excessive daytime sleepiness, cataplexy, sleep paralysis, hypnogogic hallucinations, automatic behavior, and disrupted nocturnal sleep. The presentation can be very variable, making diagnosis difficult. Loss of hypocretin containing neurons in the lateral hypothalamus has been noted in autopsy studies, and the cerebrospinal fluid level of hypocretin is reduced in patients with narcolepsy with cataplexy. New treatment options are available for the many symptoms of this disease. Early recognition and treatment can greatly improve the quality of life of patients with narcolepsy. A detail review of the epidemiology, pathophysiology, and management of narcolepsy in children is presented.

Narcolepsy: immunological aspects

Narcolepsy with cataplexy is a debilitating sleep disorder with an estimated prevalence of about 0.05%. Narcolepsy is caused by a selective loss of hypocretin (orexin) producing neurons in the perifornical hypothalamus. Based on the very strong association with the HLA subtype DQB1*0602, it is currently hypothesized narcolepsy is caused by an autoimmune-mediated process directed at the hypocretin neurons. So far however, studies focusing on general markers of (auto)immune activation, as well as humoral immunity against the hypocretin system have not yielded consistent results supporting this hypothesis.

Screening the human prepro-orexin gene in a single-centre narcolepsy cohort

BACKGROUND AND PURPOSE

Although the orexin system has an established role in narcolepsy, the mechanism of orexin deficiency in human cases is unknown. The strong association with human leukocyte antigen (HLA) DQB1*0602 suggests an autoimmune basis, but supporting evidence is lacking. Although data indicate that HLA status is not the sole genetic factor, only a single case of a functional orexin system mutation has been discovered, in a study with a selection bias designed to increase yield. In this study, we examined the prepro-orexin gene for mutations in a cohort of unrelated patients with narcolepsy from a national UK referral centre.

PATIENTS AND METHODS

Subjects with a diagnosis of narcolepsy were recruited from a patient database. DNA samples were obtained using buccal smear kits. The prepro-orexin gene was amplified using polymerase chain reactions and screened for polymorphisms and mutations.

RESULTS

Eighty-one patients were recruited, of whom 69 provided DNA samples. A previously described intronic single nucleotide polymorphism, of unlikely significance, was identified in one subject who had typical clinical and electrophysiological features of narcolepsy. It was located 16 base pairs downstream from exon 1. No other mutations were found.

CONCLUSION

This result supports existing evidence which indicates that mutations of the prepro-orexin gene are rare and that the genetic contribution to the aetiology of human narcolepsy is likely to be complex.

Muscarine activates the sodium-calcium exchanger via M3receptors in basal forebrain neurons

Neurons of the medial septum/diagonal band of Broca (MSDB) project to the hippocampus. Muscarinic cholinergic mechanisms within the MSDB are potent modulators of hippocampal functions; intraseptal scopolamine disrupts and intraseptal carbachol facilitates hippocampus-dependent learning and memory tasks, and the associated hippocampal theta rhythm. In earlier work, we demonstrated that, within the MSDB, the septohippocampal GABAergic but not cholinergic neurons are the primary target of muscarinic manipulations and that muscarinic activation of septohippocampal GABAergic neurons is mediated directly via M(3) receptors. In the present study, we examined the ionic mechanism(s) underlying the excitatory actions of muscarine in these neurons. Using whole-cell patch-clamp recording techniques in rat brain slices, we demonstrated that M(3) receptor-mediated muscarinic activation of MSDB neurons is dependent on external Na(+) and is also reduced by bath-applied Ni(2+) and KB-R7943 as well as by replacing external Na(+) with Li(+), suggesting a primary involvement of the Na(+)-Ca(2+) exchanger. We conclude that the M(3) receptor-mediated muscarinic activation of MSDB septohippocampal GABA-type neurons, that is important for cognitive functioning, is mediated via activation of the Na(+)-Ca(2+) exchanger.

Hypersomnias of Central Origin

Hypersomnia related to CNS disorders can be due to a variety of conditions. In this review, we discuss the diagnosis and treatment of narcolepsy with and without cataplexy, idiopathic hypersomnia, recurrent hypersomnia, and related illnesses. Research has provided insight into the underlying etiologies of these disorders, such as the genetic influences on disease development and the fundamental role of hypocretins in narcolepsy. We define the current utility of diagnostic testing, including sleep studies, neuroimaging techniques, and laboratory investigations. New treatment options for hypersomnia are discussed.

Genomewide association analysis of human narcolepsy and a new resistance gene

Human narcolepsy is a hypersomnia that is affected by multiple genetic and environmental factors. One genetic factor strongly associated with narcolepsy is the HLA-DRB1*1501-DQB1*0602 haplotype in the human leukocyte antigen region on chromosome 6, whereas the other genetic factors are not clear. To discover additional candidate regions for susceptibility or resistance to human narcolepsy, we performed a genomewide association study, using 23,244 microsatellite markers. Two rounds of screening with the use of pooled DNAs yielded 96 microsatellite markers (including 16 markers on chromosome 6) with significantly different estimated frequencies in case and control pools. Markers not located on chromosome 6 were evaluated by the individual typing of 95 cases and 95 controls; 30 markers still showed significant associations. A strong association was displayed by a marker on chromosome 21 (21q22.3). The surrounding region was subjected to high-density association mapping with 14 additional microsatellite markers and 74 SNPs. One microsatellite marker (D21S0012m) and two SNPs (rs13048981 and rs13046884) showed strong associations (P < .0005; odds ratios 0.19-0.33). These polymorphisms were in a strong linkage disequilibrium, and no other polymorphism in the region showed a stronger association with narcolepsy. The region contains three predicted genes--NLC1-A, NLC1-B, and NLC1-C--tentatively named "narcolepsy candidate-region 1 genes," and NLC1-A and NLC1-C were expressed in human hypothalamus. Reporter-gene assays showed that the marker D21S0012m in the promoter region and the SNP rs13046884 in the intron of NLC1-A significantly affected expression levels. Therefore, NLC1-A is considered to be a new resistance gene for human narcolepsy.

Immunohistochemical screening for autoantibodies against lateral hypothalamic neurons in human narcolepsy

Most human patients with narcolepsy have no detectable hypocretin-1 in their cerebrospinal fluid. The cause of this hypocretin deficiency is unknown, but the prevailing hypothesis states that an autoimmune-mediated mechanism is responsible. We screened for the presence of autoantibodies against neurons in the lateral hypothalamus in 76 patients and 63 controls, using immunohistochemistry. Autoantibodies were present in two patients, but also in two controls. However, one of the patients had a clearly different staining pattern and nerve endings of immunolabeled cells were found to project onto hypocretin-producing neurons, suggesting a possible pathophysiological role. Humoral immune mechanisms appear not to play a role in the pathogenesis of narcolepsy, at least not in the clinically overt stage of the disease.

Narcolepsy with cataplexy in early childhood

Narcolepsy is a rare neurologic sleep disorder with morbidity associated with functional impairment and frequent delay in diagnosis. Symptoms typically manifest in adolescence or early adulthood, but diagnosis of narcolepsy has been reported in early childhood. Diagnosis rates are as low as 50% of the total population of patients with narcolepsy and are delayed as much as 10 years after disease onset due to inadequate patient-physician communication and/or misdiagnosis. I present the complexity of diagnosing narcolepsy in early childhood in a patient with cataplexy that started soon after independent ambulation at age 10 months.

Late-onset leanness in mice with targeted ablation of melanin concentrating hormone neurons

The observation that loss of orexin (hypocretin) neurons causes human narcolepsy raises the possibility that other acquired disorders might also result from loss of hypothalamic neurons. To test this possibility for body weight, mice with selective loss of melanin concentrating hormone (MCH) neurons were generated. MCH was chosen to test because induced mutations of the MCH gene in mice cause hypophagia and leanness. Mice with ablation of MCH neurons were generated using toxin (ataxin-3)-mediated ablation strategy. The mice appeared normal but, after 7 weeks, developed reduced body weight, body length, fat mass, lean mass, and leptin levels. Leanness was characterized by hypophagia and increased energy expenditure. To study the role of MCH neurons on obesity secondary to leptin deficiency, we generated mice deficient in both ob gene product (leptin) and MCH neurons. Absence of MCH neurons in ob/ob mice improved obesity, diabetes, and hepatic steatosis, suggesting that MCH neurons are important mediators of the response to leptin deficiency. These data show that loss of MCH neurons can lead to an acquired leanness. This has implications for the pathogenesis of acquired changes of body weight and might be considered in clinical settings characterized by substantial weight changes later in life.

Narcolepsy: a review of evidence for autoimmune diathesis

Sporadic narcolepsy with cataplexy is a disabling disease that is strongly associated with the major histocompatibility class II allele HLA DQB1*0602 and is characterized by profound reduction in the cerebrospinal fluid (CSF) concentration of hypocretin 1 levels. This article provides a comprehensive review of the evidence that neurologic autoimmunity is the pathogenic basis of narcolepsy with cataplexy. Despite this evidence, specific antibody markers for narcolepsy have been elusive. Clinical trials using intravenous immunoglobulin infusions in recent onset narcolepsy with cataplexy have led to improvement in cataplexy in some patients. Future research must focus on elucidation of immune markers and early ameliorative treatments for narcolepsy.

The hypocretins and sleep

The hypocretins (also called the orexins) are two neuropeptides derived from the same precursor whose expression is restricted to a few thousand neurons of the lateral hypothalamus. Two G-protein coupled receptors for the hypocretins have been identified, and these show different distributions within the central nervous system and differential affinities for the two hypocretins. Hypocretin fibers project throughout the brain, including several areas implicated in regulation of the sleep/wakefulness cycle. Central administration of synthetic hypocretin-1 affects blood pressure, hormone secretion and locomotor activity, and increases wakefulness while suppressing rapid eye movement sleep. Most human patients with narcolepsy have greatly reduced levels of hypocretin peptides in their cerebral spinal fluid and no or barely detectable hypocretin-containing neurons in their hypothalamus. Multiple lines of evidence suggest that the hypocretinergic system integrates homeostatic, metabolic and limbic information and provides a coherent output that results in stability of the states of vigilance.

Hypocretins (orexins) and sleep–wake disorders

Since their discovery in 1998, the hypocretins (orexins)-peptides that are produced by a group of neurons situated in the posterolateral hypothalamus--have been shown to excite many CNS areas including many neuronal systems that regulate sleep and wakefulness. Animal studies indicate that hypocretins play a part in the regulation of various functions including arousal, muscle tone, locomotion, regulation of feeding behaviour, and neuroendocrine and autonomic functions. A link between hypocretin deficiency and narcoleptic symptoms was first shown in canine and rodent models of narcolepsy. Hypocretin deficiency, as shown by low or absent concentrations in CSF, was subsequently found in 90% of patients with sporadic narcolepsy-cataplexy, and less commonly in familial narcolepsy. In most other sleep-wake and neurological disorders, hypocretin concentrations are normal. Low concentrations were also found in hypothalamic disorders, acute traumatic brain injury, and a few other disorders. The exact function of the hypocretin system in sleep-wake regulation and its pathophysiological role in hypocretin-deficient and non-deficient narcolepsy as well as in non-narcoleptic, hypocretin-deficiency syndromes remain unclear.

Studies of humoral immunity to preprohypocretin in human leukocyte antigen DQB1*0602-positive narcoleptic subjects with cataplexy

BACKGROUND

Canine models for narcolepsy have mutations of the hypocretin receptor 2 gene, and preprohypocretin knockout murine lines exhibit narcoleptic-like behaviors. Human narcolepsy with cataplexy is associated with human leukocyte antigen DQB1*0602 and reduced hypocretin levels in cerebrospinal fluid, suggesting an autoimmune diathesis. We tested the hypothesis that DQB1*0602-positive narcoleptic subjects with cataplexy have immunoglobulin (Ig)G reactive to human preprohypocretin and its cleavage products.

METHODS

Serum samples of 41 DQB1*0602-positive narcoleptic subjects with cataplexy and 55 control subjects were studied, as were 19 narcoleptic and 13 control samples of cerebrospinal fluid. We tested for IgG reactive to preprohypocretin and its major cleavage products (including hypocretin 1 and 2), using immunoprecipitation assays (IP), immunofluorescence microscopy (IF) of Chinese hamster ovarian cells expressing preprohypocretin, and Western blots.

RESULTS

There was no evidence for IgG reactive to preprohypocretin or its cleavage products in CSF of subjects with narcolepsy as measured by IPs, Western blots, and IF. Although the IP with CSF and the C-terminal peptide showed significant differences by two methods of comparison, the control subjects had higher counts per minute than narcoleptic subjects, which was opposite to our hypothesis.

CONCLUSIONS

The hypothesis that DQB1*0602-positive narcoleptic subjects with cataplexy have IgG reactive to preprohypocretin or its cleavage products was not supported.