From radioimmunoassay to mass spectrometry: a new method to quantify orexin-A (hypocretin-1) in cerebrospinal fluid

Detection of reduced orexin-A/hypocretin-1 and its fragments by orexin-A "gold-standard" radioimmunoassay

BACKGROUND

Orexin-A/hypocretin-1 level is determined in the cerebrospinal fluid samples as a part of clinical narcolepsy diagnostics utilizing a specific commercial radioimmunoassay (RIA); this assay is also widely used in research of many other conditions. The specificity of RIAs is in general variable, and little has been firmly disclosed about the specificity of this RIA assay. Thus, the validity of many research results obtained using the kit is unclear. At least metabolites of orexin-A have been proposed as potential interfering substances.

METHODS

Since this issue has not been systematically assessed, we decided to investigate it using synthetic variants of orexin-A and -B (intact peptides and peptide fragments as well as reduced orexin-A).

RESULTS

Our synthetic orexin-A bound correspondingly to the orexin-A standard included in the kit while orexin-B did not bind even at 10000-fold higher concentrations. Reduction of the disulfide bridges in orexin-A (giving orexin-A-SS) decreased its binding 25-fold. C-terminal truncation of orexin-A-SS was well tolerated - some of the fragments actually bound better than orexin-A-SS - while N-terminal truncation was not allowed.

CONCLUSIONS

The results demonstrate that the RIA kit is fairly selective for intact orexin-A among the peptides tested. However, this does not as such prove that it measures intact orexin-A in the physiological samples, and further studies including identification of physiological orexin-A metabolites are thus required. We also suggest that the redox milieu of cerebrospinal fluid - that has been suggested to vary in different diseases - may have an impact on what is measured with the kit.

Enhancing the Management of Hypersomnia: Examining the Role of the Orexin System

Excessive daytime sleepiness (EDS) is common. However, clinical features of excessive sleepiness can have broad and variable presentations. In addition, there can be an increased likelihood of medical or psychiatric comorbidity. Examination of the networks that regulate sleep-wake and circadian control reveals a complex and intricately designed integration system. Dysregulation in the coordination, effectiveness, or efficiency of these systems can contribute to developing EDS, and inform on the endotypes observed and pharmacologic considerations for treatment. The discovery and characterization of the diurnal expression and function of orexin (hypocretin) have led to a transformed understanding of sleep-wake control and EDS, as well as its role beyond sleep. As a result, a novel drug class, orexin agonists, is anticipated to emerge for clinical use in the near future. An understanding of orexin physiology and its transdisciplinary impact is necessary to best prepare for patient selection, use, and anticipated benefit and monitoring of both expected benefits and any other health change. This study provides a review of the range of clinical features and impact of EDS, the relationship between sleep-wake, circadian and other health networks, and an examination of orexin physiology with anticipatory guidance on the potential transdisciplinary role and impact of orexin agonists.

Obstructive sleep apnea syndrome, orexin, and sleep-wake cycle: The link with the neurodegeneration

Obstructive sleep apnea syndrome (OSAS) significantly affects the sleep-wake circadian rhythm through intermittent hypoxia and chronic sleep fragmentation. OSAS patients often experience excessive daytime sleepiness, frequent awakenings, and sleep fragmentation, leading to a disrupted circadian rhythm and altered sleep-wake cycle. These disruptions may exacerbate OSAS symptoms and contribute to neurodegenerative processes, particularly through the modulation of clock gene expression such as CLOCK, BMAL1, and PER. Emerging evidence connects OSAS to cognitive impairment and suggests that these changes may contribute to the development of neurodegenerative disorders such as Alzheimer disease, suggesting that OSAS could be a reversible risk factor for these conditions. Biomarkers, including melatonin and orexin, play crucial roles in understanding these mechanisms. In OSAS patients, melatonin, a marker of circadian rhythmicity, often shows altered secretion patterns that are not fully corrected by continuous positive airway pressure therapy. Orexin, which regulates the sleep-wake cycle, exhibits increased cerebrospinal fluid levels in OSAS patients, possibly due to compensatory mechanisms against sleep impairment and daytime sleepiness. These biomarkers highlight the intricate relationship between circadian rhythm disruptions and neurodegenerative risks in OSAS, emphasizing the need for further research and potential therapeutic strategies to mitigate these effects and improve patient outcomes.

International Union of Basic and Clinical Pharmacology CXIV: Orexin Receptor Function, Nomenclature and Pharmacology

The orexin system consists of the peptide transmitters orexin-A and -B and the G protein-coupled orexin receptors OX1 and OX2 Orexin receptors are capable of coupling to all four families of heterotrimeric G proteins, and there are also other complex features of the orexin receptor signaling. The system was discovered 25 years ago and was immediately identified as a central regulator of sleep and wakefulness; this is exemplified by the symptomatology of the disorder narcolepsy with cataplexy, in which orexinergic neurons degenerate. Subsequent translation of these findings into drug discovery and development has resulted to date in three clinically used orexin receptor antagonists to treat insomnia. In addition to sleep and wakefulness, the orexin system appears to be a central player at least in addiction and reward, and has a role in depression, anxiety and pain gating. Additional antagonists and agonists are in development to treat, for instance, insomnia, narcolepsy with or without cataplexy and other disorders with excessive daytime sleepiness, depression with insomnia, anxiety, schizophrenia, as well as eating and substance use disorders. The orexin system has thus proved an important regulator of numerous neural functions and a valuable drug target. Orexin prepro-peptide and orexin receptors are also expressed outside the central nervous system, but their potential physiological roles there remain unknown. SIGNIFICANCE STATEMENT: The orexin system was discovered 25 years ago and immediately emerged as an essential sleep-wakefulness regulator. This discovery has tremendously increased the understanding of these processes and has thus far resulted in the market approval of three orexin receptor antagonists, which promote more physiological aspects of sleep than previous hypnotics. Further, orexin receptor agonists and antagonists with different pharmacodynamic properties are in development since research has revealed additional potential therapeutic indications. Orexin receptor signaling is complex and may represent novel features.

Exploring the role of Orexin-A neuropeptide in Parkinson's disease: A systematic review and meta-analysis

BACKGROUND

Parkinson's disease (PD) is a progressive neurological condition that affects movement and coordination. Orexin-A (OXA) is an excitatory neuropeptide that is found throughout the central nervous system. There is growing interest in investigating the potential diagnostic and therapeutic utility of OXA in PD. To date, studies have reported a wide range of OXA concentrations in patients with PD. In this review, we discuss the current understanding of the dysregulation of OXA in PD and analyze its levels in the CSF.

METHODS

We searched six databases (PubMed, Scopus, Web of Science, EMBASE, ProQuest, and EBSCOHost) and preprint servers using a predetermined search strategy through 4th March 4, 2023. The search keywords included "Parkinson's disease", "Orexin-A", "Hypocretin-1", "cerebrospinal fluid", and "CSF". Studies that reported OXA/Hypocretin-1 levels in the CSF of patients with PD were included. Two researchers independently reviewed the records and extracted data.

FINDINGS

Eighteen studies involving 244 patients were analyzed. CSF Orexin-A concentrations were lower in patients with Parkinson's disease than in controls, with a mean difference of -59.21 (95 % CI: -89.10 to -29.32). The mean OXA levels were 281.52 (95 % CI: 226.65-336.40).

CONCLUSION

Our analysis reveals lower concentrations of orexin-A in the cerebrospinal fluid of Parkinson's disease patients compared to controls, but within the normal range. These findings suggest a potential, but not significant, disruption in the orexinergic system associated with the disease.

Hypocretin-1/orexin-A, sleep and excessive daytime sleepiness in patients with nonconvulsive status epilepticus: A cross-sectional cohort study

BACKGROUND AND OBJECTIVES

Nonconvulsive status epilepticus (NCSE) manifests as a change in mental status without a coma (NCSE proper) or comatose NCSE. Hypocretin-1/orexin-A (H/O) is involved in alertness and sleep maintenance. Sleep impairment and excessive daytime sleepiness (EDS) have a negative impact on cognitive functions and activities of daily living (ADL).

METHODS

Patients meeting the NCSE criteria underwent cerebrospinal fluid and brain magnetic resonance imaging examinations, polysomnographies (PSG), multiple latency sleep tests (MSLT), and completed Epworth Sleepiness Scale (ESS). Montreal Cognitive Assessment was used to evaluate cognitive functions, and the Barthel Index was used to assess ADL in the acute phase (V1) and three months follow-up (V2).

RESULTS

From May 2020 to May 2023, we enrolled 15 patients, eight (53.3 %) women, with a median age of 69 (14) years. The median H/O CSF concentration was 250 (63.6) pg/ml; however, only three CSF samples (20 %) decreased below the borderline concentration of 200 pg/ml. Fourteen out of 15 patients (93.3 %) completed the PSG study. The median of wakefulness after sleep onset was 167 (173.5) min, sleep efficiency (SE) was 62.9 (63) %, sleep latency (SL) was 6 (32) min, REM sleep was 2.85 (7.2) %, and REM first episode latency was 210.5 (196.5) minutes. The medians of the stages N1 NREM were 4.65 (15) %, N2 NREM 68.4 (29.9) %, and N3 NREM 21.8 (35.5) %. MSLT mean latency was 7.7 (12.6) minutes. A significant negative correlation exists between H/O CSF concentrations and the stage N1 NREM (rs = -0.612, p = 0.02), and the proportion of cumulative sleep time with oxygen saturation below 90 % in total sleep time (TST) t90 (rs = -0.57, p = 0.03). MSLT had significant negative correlation with TST (rs = -0.5369, p = 0.0478), with SE (rs = -0.5897, p = 0.0265), with apnea-hypopnea index (rs = -0.7631, p = 0.0002) and with deoxygenation index (rs = -0.8009, p = 0.0006). A positive correlation exists between MSLT and SL (rs = 0.6284, p = 0.0161) and between ESS and t90 (rs = 0.9014, p = 0.0004). The correlation between H/O CSF concentrations and EDS, cognitive performance, and ADL was not proved.

CONCLUSIONS

Patients after NCSE exhibited sleep impairment and excessive daytime sleepiness. Hypocretin-1/orexin-A concentrations decreased only in 20 % of these cases.

Repeated polysomnography and multiple sleep latency test in narcolepsy type 1 and other hypersomnolence disorders

BACKGROUND

The diagnosis of narcolepsy is based on clinical information, combined with polysomnography (PSG) and the Multiple Sleep Latency Test (MSLT). PSG and the MSLT are moderately reliable at diagnosing narcolepsy type 1 (NT1) but unreliable for diagnosing narcolepsy type 2 (NT2). This is a problem, especially given the increased risk of a false-positive MSLT in the context of circadian misalignment or sleep deprivation, both of which commonly occur in the general population.

AIM

We aimed to clarify the accuracy of PSG/MSLT testing in diagnosing NT1 versus controls without sleep disorders. Repeatability and reliability of PSG/MSLT testing and temporal changes in clinical findings of patients with NT1 versus patients with hypersomnolence with normal hypocretin-1 were compared.

METHOD

84 patients with NT1 and 100 patients with non-NT1-hypersomnolence disorders, all with congruent cerebrospinal fluid hypocretin-1 (CSF-hcrt-1) levels, were included. Twenty-five of the 84 NT1 patients and all the hypersomnolence disorder patients underwent a follow-up evaluation consisting of clinical assessment, PSG, and a modified MSLT. An additional 68 controls with no sleep disorders were assessed at baseline.

CONCLUSION

Confirming results from previous studies, we found that PSG and our modified MSLT accurately and reliably diagnosed hypocretin-deficient NT1 (accuracy = 0.88, reliability = 0.80). Patients with NT1 had stable clinical and electrophysiological presentations over time that suggested a stable phenotype. In contrast, the PSG/MSLT results of patients with hypersomnolence, and normal CSF-hcrt-1 had poor reliability (0.32) and low repeatability.

Diurnal Fluctuations of Orexin-A and -B in Cynomolgus Monkey Cerebrospinal Fluid Determined by a Novel Analytical Method Using Antiadsorptive Additive Treatment Followed by Nanoflow Liquid Chromatography-High-Resolution Mass Spectrometry

Orexin-A (OXA) and -B (OXB) are involved in the regulation of multiple physiological functions including the sleep-wake states; therefore, it is critical to monitor their levels under various conditions. Unfortunately, the widely used radioimmunoassay has insufficient specificity for OXA. Although liquid chromatography-tandem mass spectrometry (LC-MS/MS) has higher specificity for OXA, previously reported OXA levels in human cerebrospinal fluid (CSF) measured using this technique are still inconsistent. Moreover, to the best of our knowledge, OXB has not been detected in the CSF. In this study, we established a novel method for OXA and OXB measurement. We noticed that OXA and OXB in the CSF was sticky; thus, citric acid and Tween 80 were used to prevent their nonspecific binding. Then, highly specific and sensitive nanoflow liquid chromatography-high-resolution mass spectrometry (nanoLC-HRMS) was used to measure OXA and OXB levels. Evaluation of the diurnal fluctuations of OXA and OXB in cisternal and lumbar CSF samples from cynomolgus monkeys revealed a sharp increase in the early light period, followed by a gradual increase to the maximum levels at the end of the light period, and then a sharp drop to the minimum levels during the early dark period. OXB levels were lower than OXA levels in cisternal CSF. Although basal OXA levels in individual monkeys showed substantial variations, the ratios between the maximum and minimum OXA levels of each monkey were similar. Our method for accurate OXA and OXB measurement should help improve our knowledge of orexin biology.

Repeated measures of hypocretin-1 in Danish and Italian patients with narcolepsy and in controls

STUDY OBJECTIVES

The assay currently used worldwide to measure cerebrospinal fluid hypocretin-1 (CSF-hcrt-1) for diagnosing narcolepsy uses a competitive radioimmunoassay with polyclonal anti-hcrt-1 antibodies. This assay detects multiple hypocretin-1 immunoreactive species in the CSF that are all derived from full-length hcrt-1. We aimed to revalidate CSF-hcrt-1 cut-offs for narcolepsy type 1 (NT1) diagnosis and to evaluate temporal changes in CSF-hcrt-1 levels in patients suspected of having central hypersomnia.

METHOD

We carried out a repeat lumbar puncture with a mean follow-up of 4.0 years, to measure CSF-hcrt-1 in patients suspected of having central hypersomnia in a follow-up study. Data from CSF samples of patients with NT1 and of controls without known hypersomnia, from the Italian-Stanford and Danish populations, were examined using a receiver-operating characteristic analysis.

RESULTS

The optimal CSF-hcrt-1 cut-offs for identifying NT1 were 129 pg/ml and 179 pg/ml for the Italian-Stanford and Danish populations, respectively. The sensitivity was 0.93-0.99 and the specificity was 1. Follow-up lumbar puncture measurements of CSF-hcrt-1 were obtained from 73 patients. 30 of 32 patients with low CSF-hcrt-1 levels continued to be categorized as low, with an unaltered diagnosis; two patients showed a marked increase in CSF-hcrt-1, attaining normal values at follow-up. One of these patients relapsed to low CSF-hcrt-1 after follow-up. All 41 patients with normal CSF-hcrt-1 at baseline had normal CSF-hcrt-1 at follow-up.

CONCLUSION

CSF-hcrt-1 measurement can provide an accurate test for diagnosing NT1, although it is important to validate the CSF-hcrt-1 cut-off for specific testing locations. Stable CSF-hcrt-1 levels support the already established prognosis of narcolepsy as permanent once the disorder has fully developed.

Hypocretin-1 measurements in cerebrospinal fluid using radioimmunoassay: within and between assay reliability and limit of quantification

Study Objectives

The most sensitive and specific investigative method for the diagnosis of narcolepsy type 1 (NT1) is the determination of hypocretin-1 (orexin-A) deficiency (≤110 pg/mL) in cerebrospinal fluid using a radioimmunoassay (RIA). We aimed to assess the reliability of the Phoenix Pharmaceuticals hypocretin-1 RIA, by determining the lower limit of quantification (LLOQ), the variability around the cutoff of 110 pg/mL, and the inter- and intra-assay variability.

Methods

Raw data of 80 consecutive hypocretin-1 RIAs were used to estimate the intra- and inter-assay coefficient of variation (CV). The LLOQ was established and defined as the lowest converted concentration with a CV <25%; the conversion is performed using a harmonization sample which is internationally used to minimize variation between RIAs.

Results

The mean intra-assay CV was 4.7%, while the unconverted inter-assay CV was 28.3% (18.5% excluding 2 outliers) and 7.5% when converted to international values. The LLOQ was determined as 27.9 pg/mL. The intra-assay CV of RIAs with lower specific radioactive activity showed a median of 5.6% (n = 41, range 1.6%–17.0%), which was significantly higher than in RIAs with higher specific activity (n = 36; median 3.2%, range 0.4%–11.6%, p = .013). The CV around the 110 pg/mL cutoff was <7%.

Conclusions

Hypocretin-1 RIAs should always be harmonized using standard reference material. The specific activity of an RIA has a significant impact on its reliability, because of the decay of ¹²⁵I radioactivity. Values around the hypocretin-1 cut-off can reliably be measured. Hypocretin-1 concentrations below 28 pg/mL should be reported as “undetectable” when measured with the Phoenix Pharmaceuticals RIA.

Clinical Trial Information

This study is not registered in a clinical trial register, as it has a retrospective database design

50th anniversary of the ESRS in 2022-JSR special issue

This article addresses the clinical presentation, diagnosis, pathophysiology and management of narcolepsy type 1 and 2, with a focus on recent findings. A low level of hypocretin-1/orexin-A in the cerebrospinal fluid is sufficient to diagnose narcolepsy type 1, being a highly specific and sensitive biomarker, and the irreversible loss of hypocretin neurons is responsible for the main symptoms of the disease: sleepiness, cataplexy, sleep-related hallucinations and paralysis, and disrupted nocturnal sleep. The process responsible for the destruction of hypocretin neurons is highly suspected to be autoimmune, or dysimmune. Over the last two decades, remarkable progress has been made for the understanding of these mechanisms that were made possible with the development of new techniques. Conversely, narcolepsy type 2 is a less well-defined disorder, with a variable phenotype and evolution, and few reliable biomarkers discovered so far. There is a dearth of knowledge about this disorder, and its aetiology remains unclear and needs to be further explored. Treatment of narcolepsy is still nowadays only symptomatic, targeting sleepiness, cataplexy and disrupted nocturnal sleep. However, new psychostimulants have been recently developed, and the upcoming arrival of non-peptide hypocretin receptor-2 agonists should be a revolution in the management of this rare sleep disease, and maybe also for disorders beyond narcolepsy.

Orexin 2 receptor (OX2R) protein distribution measured by autoradiography using radiolabeled OX2R-selective antagonist EMPA in rodent brain and peripheral tissues

Orexin, a neuropeptide, performs various physiological functions, including the regulation of emotion, feeding, metabolism, respiration, and sleep/wakefulness, by activating the orexin 1 receptor and orexin 2 receptor (OX2R). Owing to the pivotal role of OX2R in wakefulness and other biological functions, OX2R agonists are being developed. A detailed understanding of OX2R protein distribution is essential for determining the mechanisms of action of OX2R agonists; however, this has been hindered by the lack of selective antibodies. In this study, we first confirmed the OX2R-selective binding of [³H]-EMPA in in vitro autoradiography studies, using brain slices from OX2R knockout mice and their wild-type littermates. Subsequently, OX2R protein distribution in rats was comprehensively assessed in 51 brain regions and 10 peripheral tissues using in vitro autoradiography with [³H]-EMPA. The widespread distribution of OX2R protein, including that in previously unrecognized regions of the retrosplenial cortex, was identified. In contrast, OX2R protein expression was negligible/very low in peripheral tissues, suggesting that orexin exerts OX2R-dependent physiological functions primarily through activation of the central nervous system. These findings will be useful for understanding the wide range of biological functions of OX2R and the application of OX2R agonists in various disorders.

Misdiagnosis of narcolepsy caused by a false positive orexin-A/hypocretin-1 enzyme immune assay

The diagnosis of narcolepsy is based on clinical history, sleep studies, and, in some cases, cerebrospinal fluid orexin-A/hypocretin-1 measurement. The gold standard for orexin measurement is the radioimmune assay (RIA), but other commercial kits are also available, such as the enzyme immune assay (EIA). The specificity of orexin EIA in humans is unknown. We report four cases where orexin levels were measured by EIA and resulted in false positives and the misdiagnosis of narcolepsy. Therefore, orexin EIA measurement should be strongly discouraged in a clinical setting.

Narcolepsy with intermediate cerebrospinal level of hypocretin-1

STUDY OBJECTIVES

To describe the phenotype of narcolepsy with intermediate cerebrospinal hypocretin-1 levels (CSF hcrt-1).

METHODS

From 1600 consecutive patients with narcolepsy from Bologna and Montpellier sleep centers we selected patients with intermediate CSF hcrt-1 levels (110-200 pg/ml). Clinical, neurophysiological and biological data were contrasted for the presence of cataplexy, HLA-DQB1*06:02, and median CSF hcrt-1 levels (149.34 pg/mL).

RESULTS

Forty-five (55% males, aged 35 ± 17 years) patients (2.8% of all cases) were included. Thirty-three (73%) were HLA-DQB1*06:02, 29 (64%) reported cataplexy (21, 72.4% with typical features), and 5 (11%) had presumed secondary etiology. Cataplexy was associated with other core narcolepsy symptoms, increased sleep onset REM periods, and nocturnal sleep disruption. Cataplexy and irrepressible daytime sleep were more frequent in HLA DQB1*06:02 positive patients. Lower CSF hcrt-1 levels were associated with hallucinations.

CONCLUSION

Narcolepsy with intermediate CSF hcrt-1 level is a rare condition with heterogeneous phenotype. HLA DQB1*06:02 and lower CSF hcrt-1 were associated with typical narcolepsy features, calling for future research to distinguish incomplete from secondary narcolepsy forms.

LC-MS/MS Analysis Elucidates a Daily Rhythm in Orexin A Concentration in the Rat Vitreous Body

Orexins are two neuropeptides synthesised mainly in the brain lateral hypothalamic area. The orexinergic system provides arousal-dependent cues for a plethora of brain centres, playing a vital role in feeding behaviour, regulation of the sleep-wake cycle and circadian rhythms. Recently, orexins were found to be produced in the retina of an eye; however, their content in the vitreous body and possible daily pattern of expression have not yet been explored. In this manuscript, we describe the development and validation of a liquid chromatography with tandem mass spectrometry (LC-MS/MS) method designed for quantitative bioanalysis of orexin in the rat vitreous body. Orexin was extracted from vitreous body samples with a water:acetonitrile:formic acid (80:20:0.1; v/v/v) mixture followed by vortexing and centrifuging. Separation was performed on a reverse-phase HPLC column under gradient conditions. Orexin was analysed via multiple-reaction monitoring (MRM) in the positive electrospray mode. The total analysis time for each sample was less than 5.0 min. Once the method was fully optimised, it was then validated, following the 2018 FDA guidance on bioanalytical method validations. The calibration curves for orexin (1-500 ng/mL) were constructed using a linear regression with a 1/x2 weighting. The lower limit of quantitation for orexin was 1.0 pg/mL for the vitreous body. Intra-day and inter-day estimates of accuracy and precision were within 10% of their nominal values, indicating that the method is reliable for quantitation of orexin in the rat vitreous body. From the physiological perspective, our results are the first to show daily rhythm of orexin synthesis by the retina with possible implications on the circadian regulation of vision.

Cerebrospinal fluid orexin in Alzheimer's disease: a systematic review and meta-analysis

OBJECTIVE/BACKGROUND

A growing body of evidence suggests that sleep and Alzheimer's disease (AD) have a bi-directional relationship. Emerging research also suggests that orexin, a key neurotransmitter involved in sleep-wake regulation, may be altered in persons with AD, however results have not been consistent across prior studies. This investigation was conducted to both evaluate the aggregate literature to minimize the risk of bias and identify potential factors associated with heterogeneity across studies.

METHODS

Systematic review identified relevant investigations that compared cerebrospinal fluid orexin in persons with AD and controls. Meta-analysis (random effects model) compared effect size (Hedge's g) for orexin between AD and controls. Meta-regression was additionally performed for key variables of interest to evaluate potential causes of heterogeneity among studies.

RESULTS

17 studies were identified that met inclusion/exclusion criteria. Evidence of publication bias was not identified. Non-significant increases in orexin were observed in AD relative to controls, with moderate to large heterogeneity among studies (Hedge's g = 0.20, p = 0.136, I² = 72.6%). Meta-regression demonstrated both year of publication (β = 0.055, p = 0.020) and effect size for phosphorylated tau in AD versus controls (β = 0.417, p = 0.031) were associated with differences in orexin.

CONCLUSIONS

Results do not support broad differences in orexin in AD compared to controls, however, evolving diagnostic criteria may have affected findings across studies. Future research that examines orexin in AD over the longitudinal course of the disorder and explores potential links between phosphorylated tau and orexin are indicated.

Orexin-A measurement in narcolepsy: A stability study and a comparison of LC-MS/MS and immunoassays

BACKGROUND

Orexin-A and -B are neuropeptides involved in sleep-wake regulation. In human narcolepsy type 1, this cycle is disrupted due to loss of orexin-producing neurons in the hypothalamus. Cerebrospinal fluid (CSF) orexin-A measurement is used in the diagnosis of narcolepsy type 1. Currently available immunoassays may lack specificity for accurate orexin quantification. We developed and validated a liquid chromatography mass spectrometry assay (LC-MS/MS) for CSF orexin-A and B.

METHODS

We used CSF samples from narcolepsy type 1 (n = 22) and type 2 (n = 6) and non-narcoleptic controls (n = 44). Stable isotope-labeled orexin-A and -B internal standards were added to samples before solid-phase extraction and quantification by LC-MS/MS. The samples were also assayed by commercial radioimmunoassay (RIA, n = 42) and enzymatic immunoassay (EIA, n = 72) kits. Stability of orexins in CSF was studied for 12 months.

RESULTS

Our assay has a good sensitivity (10 pmol/L = 35 pg/mL) and a wide linear range (35-3500 pg/mL). Added orexin-A and -B were stable in CSF for 12 and 3 months, respectively, when frozen. The median orexin-A concentration in CSF from narcolepsy type 1 patients was <35 pg/mL (range < 35-131 pg/mL), which was lower than that in CSF from control individuals (98 pg/mL, range < 35-424 pg/mL). Orexin-A concentrations determined using our LC-MS/MS assay were five times lower than those measured with a commercial RIA. Orexin-B concentrations were undetectable.

CONCLUSIONS

Orexin-A concentrations measured by our LC-MS/MS assay were lower in narcolepsy type 1 patients as compared to controls. RIA yielded on average higher concentrations than LC-MS/MS.

Association of CSF orexin-A levels and nocturnal sleep stability in patients with hypersomnolence

Objective

To evaluate the associations between CSF orexin-A (ORX) levels and markers of nocturnal sleep stability, assessed by polysomnography.

Methods

Nocturnal polysomnography data and ORX levels of 300 drug-free participants (55% men, 29.9±15.5 years, ORX level 155.1±153.7 pg/mL) with hypersomnolence were collected. Several markers of nocturnal sleep stability were analyzed: sleep and wake bouts and sleep/wake transitions. Groups were categorized according to ORX levels, in 2 categories (deficient ≤110; >110), in tertiles (≤26, 26–254, >254), and compared using logistic regression models. Results were adjusted for age, sex, and body mass index.

Results

We found higher number of wake bouts (43 vs 25, p < 0.0001), sleep bouts (43 vs 25.5, p < 0.0001), and index of sleep bouts/hour of sleep time, but lower index of wake bouts/hour of wake time (41.4 vs 50.6, p < 0.0001), in patients with ORX deficiency. The percentage of wake bouts <30 seconds was lower (51.3% vs 60.8%, p < 0.001) and of wake bouts ≥1 minutes 30 seconds higher (7.7% vs 6.7%, p = 0.02) when ORX deficient. The percentage of sleep bouts ≤14 minutes was higher (2–5 minutes: 23.7% vs 16.1%, p < 0.0001), and of long sleep bouts lower (>32 minutes 30 seconds: 7.3% vs 18.3%, p < 0.0001), when ORX deficient. These findings were confirmed when groups were categorized according to ORX tertiles, with a dose–response effect of ORX levels in post hoc comparisons, and in adjusted models.

Interpretation

This study shows an association between ORX levels and nocturnal sleep stabilization in patients with hypersomnolence. Sleep and wake bouts are reliable markers of nighttime sleep stability that correlate with CSF ORX levels in a dose-dependent manner.

HPLC analysis of CSF hypocretin-1 in type 1 and 2 narcolepsy

Narcolepsy is a chronic sleep disorder caused by a loss of hypocretin (hcrt) neurons in the hypothalamus. Cerebrospinal fluid (CSF) hcrt-1 measurement has been well established as a gold standard of narcolepsy diagnosis, although some portions of narcoleptic patients show normal hcrt-1 levels. We aimed to examine peptide degradation of hcrt-1 and its abnormality in the CSF of patients by using high performance liquid chromatography (HPLC) followed by radioimmunoassay (RIA). CSF was collected from healthy controls, narcoleptic patients of type 1 with hcrt-1 deficiency, type 1 with normal hcrt-1 level, and type 2 with normal hcrt-1 level. We found that the majority of hcrt-1 immunoreactivity in extracted CSF was derived from unauthentic hcrt-1 peaks, which are predicted to be inactive metabolites, and the intact hcrt-1 peptide was less than 10% of the gross amount, suggesting that the regular RIA for CSF hcrt-1 measures largely reflect the unauthentic hcrt-1-related metabolites rather than the intact one. As expected, all hcrt-1-related peaks were abolished in type 1 with hcrt-1 deficiency. Importantly, we also found that the sum of the authentic hcrt-1 peptide (peaks 3 and 4) significantly decreased in non-deficient type 1 and tended to decrease in type 2 narcoleptic patients although the levels with the regular RIA in non-extracted CSF was equivalent to healthy controls. Immunoreactivity with unauthentic hcrt-1 metabolites may masks the possible decline in authentic hcrt-1 level caused by the partial loss of hcrt neurons. Our findings may provide new insights into the degradation of the hcrt-1 peptide and the pathophysiology of narcolepsy.

Management Of Excessive Sleepiness In Patients With Narcolepsy And OSA: Current Challenges And Future Prospects

Excessive daytime sleepiness (EDS) can be caused by insufficient sleep but is also a manifestation of medical or sleep disorders and a side effect of medications. It impacts quality of life and creates safety concerns in the home, at work, and on the roads. Screening questionnaires can be used to estimate EDS, but further evaluation is necessary. EDS is a common symptom of both narcolepsy and obstructive sleep apnea (OSA). Polysomnography and multiple sleep latency testing are used to diagnose these disorders. However, isolating the primary etiology of EDS can be challenging and may be multifactorial. Untreated OSA can show polysomnographic findings that are similar to narcolepsy. The effects of sleep deprivation and certain medications can also affect the polysomnographic results. These challenges can lead to misdiagnosis. In addition, narcolepsy and OSA can occur as comorbid disorders. If EDS persists despite adequate treatment for either disorder, a comorbid diagnosis should be sought. Thus, despite advances in clinical practice, appropriate management of these patients can be challenging. This review is focused on EDS due to OSA and narcolepsy and addresses some of the challenges with managing this patient population.

What sample preparation should be chosen for targeted MS monoclonal antibody quantification in human serum?

Aim: Monoclonal antibody-based treatment of cancer has been established as one of the most successful therapeutic strategies. Materials & methods: In this work, we developed a workflow based on an automated protein-A capture and LC–MS/MS analysis to quantify bevacizumab on patient serum during treatment. This analytical approach was fully validated and compared with a commercially available Monoclonal antibody-based treatment preparation (nanosurface and molecular-orientation limited kit). Results: The analytical comparison of the two preparative workflows based on protein-A capture gave similar results with a better lower limit of quantification for the nanosurface and molecular-orientation limited kit (0.26986 vs 1.9565 μg/ml). Conclusion: LC–MS/MS has clear advantages compared with ELISA when considering method development time, multiplexing capacities and absolute quantification with internal standardization.

Temporal Changes in the Cerebrospinal Fluid Level of Hypocretin-1 and Histamine in Narcolepsy

Study Objectives:

To follow the temporal changes of cerebrospinal fluid (CSF) biomarker levels in narcoleptic patients with unexpected hypocretin level at referral.

Methods:

From 2007 to 2015, 170 human leukocyte antigen (HLA) DQB1*06:02-positive patients with primary narcolepsy and definite (n = 155, 95 males, 60 females, 36 children) or atypical cataplexy (n = 15, 4 males, 3 children) were referred to our center. Cerebrospinal hypocretin deficiency was found in 95.5% and 20% of patients with definitive and atypical cataplexy, respectively. CSF hypocretin-1 (n = 6) and histamine/tele-methylhistamine (n = 5) levels were assessed twice (median interval: 14.4 months) in four patients with definite and in two with atypical cataplexy and hypocretin level greater than 100 pg/mL at baseline.

Results:

CSF hypocretin levels decreased from normal/intermediate to undetectable levels in three of the four patients with definite cataplexy and remained stable in the other (>250 pg/mL). Hypocretin level decreased from 106 to 27 pg/mL in one patient with atypical cataplexy, and remained stable in the other (101 and 106 pg/mL). CSF histamine and tele-methylhistamine levels remained stable, but for one patient showing increased frequency of cataplexy and a strong decrease (−72.5%) of tele-methylhistamine levels several years after disease onset. No significant association was found between relative or absolute change in hypocretin level and demographic/clinical features.

Conclusions:

These findings show that in few patients with narcolepsy with cataplexy, symptoms and CSF marker levels can change over time. In these rare patients with cataplexy without baseline hypocretin deficiency, CSF markers should be monitored over time with potential for immune therapies in early stages to try limiting hypocretin neuron loss.

Orexin and Alzheimer's Disease

Alzheimer's disease (AD) is the most frequent age-related dementia. It prevalently causes cognitive decline, although it is frequently associated with secondary behavioral disturbances. AD neurodegeneration characteristically produces a remarkable destruction of the sleep-wake cycle, with diurnal napping, nighttime arousals, sleep fragmentation, and REM sleep impairment. It was recently hypothesized that the orexinergic system was involved in AD pathology. Accordingly, recent papers showed the association between orexinergic neurotransmission dysfunction, sleep impairment, and cognitive decline in AD. Orexin is a hypothalamic neurotransmitter which physiologically produces wakefulness and reduces REM sleep and may alter the sleep-wake cycle in AD patients. Furthermore, the orexinergic system seems to interact with CSF AD biomarkers, such as beta-amyloid and tau proteins. Beta-amyloid accumulation is the main hallmark of AD pathology, while tau proteins mark brain neuronal injury due to AD pathology. Investigations so far suggest that orexinergic signaling overexpression alters the sleep-wake cycle and secondarily induces beta-amyloid accumulation and tau-mediated neurodegeneration. Therefore, considering that orexinergic system dysregulation impairs sleep-wake rhythms and may influence AD pathology, it is hypothesized that orexin receptor antagonists are likely potential preventive/therapeutic options in AD patients.

The "Known Unknowns" of Kleine-Levin Syndrome: A Review and Future Prospects

Kleine-Levin syndrome (KLS) is a rare, homogeneous, debilitating sleep disorder characterized by episodic hypersomnia, cognitive impairment, and behavioral changes. The etiology, pathophysiology, and optimal management of KLS remain uncertain. We identify the 5 key areas requiring urgent attention: KLS immunopathogenesis studies, next-generation genetics, multimodal functional imaging, biomarker discovery, and clinical drug trials. A centralized registry of afflicted individuals must be established. Disease uniformity should make the identification of associated genetic or imaging biomarkers easier, but clinical efforts require laboratory-based research to model the disease and generate preclinical data for clinical translation.

Hypocretin measurement: shelf age of radioimmunoassay kit, but not freezer time, influences assay variability

The hypothalamic peptide hypocretin 1 (orexin A) may be assayed in cerebrospinal fluid to diagnose narcolepsy type 1. This testing is not commercially available, and factors contributing to assay variability have not previously been comprehensively explored. In the present study, cerebrospinal fluid hypocretin concentrations were determined in duplicate in 155 patient samples, across a range of sleep disorders. Intra-assay variability of these measures was analyzed. Inter-assay correlation between samples tested at Emory and at Stanford was high (r = 0.79, p < 0.0001). Intra-assay correlation between samples tested in duplicate in our center was also high (r = 0.88, p < 0.0001); intra-assay variability, expressed as the difference between values as a percentage of the higher value, was low at 9.4% (SD = 7.9%). Although both time the sample spent in the freezer (r = 0.16, p = 0.04) and age of the kit used for assay (t = 3.64, p = 0.0004) were significant predictors of intra-kit variability in univariate analyses, only age of kit was significant in multivariate linear regression (F = 4.93, p = 0.03). Age of radioimmunoassay kit affects intra-kit variability of measured hypocretin values, such that kits closer to expiration exhibit significantly more variability.

Narcolepsy

Narcolepsy is a chronic sleep disorder that has a typical onset in adolescence and is characterized by excessive daytime sleepiness, which can have severe consequences for the patient. Problems faced by patients with narcolepsy include social stigma associated with this disease, difficulties in obtaining an education and keeping a job, a reduced quality of life and socioeconomic consequences. Two subtypes of narcolepsy have been described (narcolepsy type 1 and narcolepsy type 2), both of which have similar clinical profiles, except for the presence of cataplexy, which occurs only in patients with narcolepsy type 1. The pathogenesis of narcolepsy type 1 is hypothesized to be the autoimmune destruction of the hypocretin-producing neurons in the hypothalamus; this hypothesis is supported by immune-related genetic and environmental factors associated with the disease. However, direct evidence in support of the autoimmune hypothesis is currently unavailable. Diagnosis of narcolepsy encompasses clinical, electrophysiological and biological evaluations, but simpler and faster procedures are needed. Several medications are available for the symptomatic treatment of narcolepsy, all of which have quite good efficacy and safety profiles. However, to date, no treatment hinders or slows disease development. Improved diagnostic tools and increased understanding of the pathogenesis of narcolepsy type 1 are needed and might lead to therapeutic or even preventative interventions.

Cerebrospinal fluid levels of orexin-A and histamine, and sleep profile within the Alzheimer process

To better understand how sleep/wake dysregulation affects Alzheimer's disease (AD), we compared the cerebrospinal fluid (CSF) orexin and histamine/tele-methylhistamine (HA/t-MHA) levels of 82 patients (41 probable-AD-high level of evidence, 41 mild cognitive impairment MCI-due-to-AD), 24 other neurologic disorders (OND) and 24 controls. We determined the relationships between these biomarkers, the CSF AD biomarkers concentrations, and the clinical sleep profile. CSF orexin-A but not HA/t-MHA levels were higher in MCI and AD than OND and controls. CSF orexin-A is correlated to CSF amyloid-β₄₂in MCI and AD, independently of age, gender, MMSE, total-tau/phosphorylated-tau, HA or sleep parameters. Nighttime sleep duration was longer in MCI and AD patients than controls. In MCI, nighttime sleep duration negatively correlated with CSF amyloid-β₄₂ and MMSE. To conclude, CSF orexin-A but not HA/t-HMA was upregulated in AD and correlated with amyloid-β₄₂ level. Our data suggested a change in the sleep-wake pattern at an early stage of the disease that needs further investigation to deeply explain the mechanistic interplay between sleep and Alzheimer.