Neuroscience Letters 561 (2014) 41–45

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Hypocretin in cerebrospinal fluid is positively correlated with Tau and pTau Michael Deuschle a,∗ , Claudia Schilling a , F. Markus Leweke a , Frank Enning a , Thomas Pollmächer b,c , Hermann Esselmann d , Jens Wiltfang d,f , Lutz Frölich a , Isabella Heuser a,e a

Central Institute of Mental Health, Medical Faculty Mannheim, Heidelberg University, J5, 68159 Mannheim, Germany Max Planck Institute of Psychiatry, Kraepelinstrasse 10, 80804 München, Germany c Center of Mental Health Klinikum Ingolstadt, Krumenauerstraße 25, 85049 Ingolstadt, Germany d LVR-Hospital Essen, Deparment of Psychiatry and Psychotherapy, Medical Faculty, University of Duisburg-Essen, Virchowstrasse 174, 45147 Essen, Germany e Department of Psychiatry, Charite, Eschenalle 3, 14050 Berlin, Germany f Dept. of Psychiatry and Psychotherapy, University of Göttingen, Von-Siebold-Str. 5, 37075 Göttingen, Germany b

h i g h l i g h t s • Hypocretin concentrations in CSF do not differ between patients with Alzheimer’s disease and major depression. • Hypocretin concentrations in CSF are related to Tau and phosphorylated Tau (pTau) in CSF. • Independent from diagnoses, hypocretin might be related to the regulation of Tau protein.

a r t i c l e

i n f o

Article history: Received 22 August 2013 Received in revised form 9 November 2013 Accepted 16 December 2013 Keywords: Depression Alzheimer’s disease Orexin Hypocretin A␤ Tau protein

a b s t r a c t It has been suggested that sleep–wake regulation as well as hypocretins play a role in the pathophysiology of Alzheimer’s disease. We analyzed A␤40 , A␤42 , Tau protein, phosphorylated Tau (pTau) protein as well as hypocretin-1 concentrations in the CSF of a detection sample of 10 patients with Alzheimer’s disease (AD) as well as 10 age- and gender-matched patients with major depression as a comparison group of different pathology. In order to replicate the findings, we used a confirmation sample of 17 AD patients and 8 patients with major depression. We found hypocretin-1 concentrations in CSF not to differ between patients with depression and AD. However, hypocretin-1 was significantly related to Tau (r = 0.463, p < 0.001) and pTau (r = 0.630, p < 0.0001). These effects were more pronounced in depressed patients when compared to AD patients. We conclude that hypocretin-1 may play a role in the metabolism of Tau proteins across different diagnostic entities including AD. It has to be determined whether there is a causal relationship between hypocretin-1 and Tau as well as pTau. © 2013 Elsevier Ireland Ltd. All rights reserved.

1. Introduction Numerous previous studies have shown altered circadian rest–activity as well as sleep–wake disturbances in patients with Alzheimer’s disease (AD) [26,27]. Sleep disturbances of patients with AD are multifaceted with hypersomnia as well as early and nighttime awakenings as main disturbances [15]. Phase delay and increased nocturnal activity seem to differentiate patients with AD from other types of dementia [10] and it has been shown that sleep–wake regulation is associated with impairment in cognitive and functional measures in AD patients. Especially, wakefulness

∗ Corresponding author. Tel.: +49 621 1703 2331; fax: +49 621 1703 2325. E-mail address: [email protected] (M. Deuschle). 0304-3940/$ – see front matter © 2013 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neulet.2013.12.036

during the night and prolonged REM latencies have been associated with impaired cognition and function in patients with AD [17]. Thus, sleep regulation might be intimately related to the neurobiology of AD rather than to dementia per se. Similarily, disturbed sleep is a core feature of major depression and may play a major role in the pathophysiology of the disorder [24]. The neuropeptides hypocretin-1 and -2 play a major role in the neurobiology of sleep and circadian regulation [18]. The hypocretin (orexin) system of the lateral hypothalamus projects into the locus coeruleus [6] and is involved in the regulation of wakefulness [20]. It is assumed that hypocretins play a particular role in regulating long wake bouts and consolidation of wakefulness [2]. Although not unequivocally [1], both the number of hypothalamic hypocretin-immunoreactive neurons and cerebrospinal fluid hypocretin concentrations have been found to be reduced

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in postmortem analysis of AD patients compared to healthy controls [8]. Moreover, low hypocretin concentrations in cerebrospinal fluid (CSF) have been shown to be associated with daytime napping and, thus, wake fragmentation [7]. Also, in a rodent AD model, it has recently been demonstrated that the amount of tissue amyloid-␤ (A␤) correlated with wakefulness and increased during sleep deprivation as well as after hypocretin-1 infusion. Chronic sleep restriction was associated with increased A␤ plaque formation in amyloid precursor protein transgenic mice suggesting that both the sleep–wake cycle and hypocretin-1 play a role in the pathogenesis of AD [14]. As abnormalities in activity and sleep architecture precede amyloid plaque deposition in transgenic amyloid precursor protein mouse models, they have been considered as endophenotypes of AD [13]. Considering these findings, we wanted to test whether hypocretin-1 in CSF is related to common biomarkers of AD, e.g. amyloid-␤ 42 (A␤42 ), A␤40 , Tau and phosphorylated Tau (pTau). Therefore, we measured hypocretin-1 and the above-mentioned AD markers in the CSF of 10 patients with AD and 10 patients with major depression (MD) serving as a comparison group, matched for age, gender and CSF storing. We used a replication sample to validate our initial findings.

2. Methods 2.1. Subjects The study was carried out in accordance to the Declaration of Helsinki. All subjects provided written informed consent and the study was approved by the local Ethics Committees of the Heidelberg University (detection sample) and the University of Cologne (replication sample). The detection sample included 10 patients with dementia of the Alzheimer’s type (AD) (age: 66.4 ± 7.2 [range: 56–79 yrs]; 7 female/3 male; Mini Mental State Examination [MMSE] 19.8 ± 7.9 [range: 5–27]) and 10 patients with a major depressive episode according to DSM-IV (age: 66.3 ± 9.3 [range: 53–79 yrs]; 6 female/4 male; Hamilton Depression Rating Scale, 21-item [HRDS] 23.6 ± 6.7 [range: 14–36]). Depressed patients had been drug-free for at least 5 days except for lorazepam in case of agitation. Eight AD patients were free of psychopharmacological medication, one patient was treated with alprazolam, one patient with piracetam. All subjects were assessed at the multi-professional memory clinic of the Central Institute of Mental Health with complete medical history including relatives’ information, neuropsychiatric and physical examination, psychometric testing, ECG, standard laboratory evaluations and cerebral imaging with CT or MRI. Inclusion criteria were diagnosis of dementia of probable Alzheimer type according to NINCDS/ADRDA criteria [16] or major depressive episode according to DSM-IV, respectively. Subjects with unstable medical conditions were excluded. All depressed patients were in full remission and without any relevant cognitive impairment after treatment. For validation purposes, the same parameters were studied in a group of 17 patients with AD (age: 78.7 ± 7.7 [range: 62–88 yrs]; 14 female/3 male; MMSE 20.8 ± 6.4) and 8 patients with major depression without evidence for dementia (age: 70.5 ± 6.5 [range: 65–83 yrs]; 6 female/2 male; HDRS [available in 5/8] 19.6 ± 10.4 [range: 8–31]) was characterized in a similar procedure at the University of Cologne, Dept. of Psychiatry and Psychotherapy. AD patients were being treated with rivastigmin (n = 2), rivastigmin and SSRI (n = 1), haloperidol (n = 1), risperidone (n = 5), SSRI and mirtazapine (n = 1), TCA and lithium (n = 1), olanzapine and mirtazapine (n = 1), venlafaxine (n = 1), SSRI and reboxetine (n = 1) or had no psychopharmacological treatment (n = 3). Depressed patients were receiving reboxetine and haloperidol (n = 1), SSRI

(n = 1), venlafaxine (n = 1), mirtazapine (n = 2), venlafaxine and mirtazapine (n = 1), fluoxetine, melperone and pregabalin (n = 1) or citalopram (n = 1). 2.2. Procedures Controlling for circadian variation of CSF A␤ and hypocretin-1 levels, CSF was obtained at 9.00 a.m. after overnight fasting in all subjects. CSF was immediately cooled in dry ice and stored at −80 ◦ C until analysis. No evaluation of sleep was done. 2.3. Lab methods Hypocretin-1 levels were determined by a radioimmunoassay as described earlier ([4]; RIA, Phoenix Pharmaceuticals, Mountain View, CA, USA). PTau, total Tau protein, A␤42 , and A␤40 levels were determined by ELISA. The ELISA for phospho-tau at Thr181 was conducted as previously described [25]. Briefly, the HT7 monoclonal antibody (MAb) directed against both normal Tau and pTau was used for capturing, and biotinylated MAb AT270 for detection. The ELISAs Innotest hTAU Antigen ELISA and Innotest ␤-Amyloid(1–42), ELISA Innogenetics (Ghent, Belgium) served for quantification of CSF Tau and A␤42 , respectively. Both ELISAs were conducted according to published standard methods [12]. For the quantification of A␤40 , the amyloid␤ (1–40) (N) ELISA from IBL (Hamburg, Germany) was used. This ELISA uses Anti-Human Abeta (35–40) (1A10) antibody as capturing antibody and a HRP conjugated Anti-Human Abeta(N) Rabbit IgG polyclonal antibody as detection antibody. 2.4. Statistics We used MANOVA with diagnosis (AD vs. MD) and CSF concentrations of hypocretin-1 as independent and A␤42 , A␤40 , Tau and pTau as dependent variables first for the detection sample and, in a second step, for all patients. In this combined analysis of detection and replication samples, we used sample (detection vs. replication) as an additional independent variable to control for interassay variations. Local effects were analyzed using Student’s ttest and Pearson’s correlation in order to test whether associations are diagnosis-specific. Due to interassay variation, we used normalized hypocretin-1 data of both, detection and replication sample for correlative analysis with ˛ ≤ 0.05 accepted as level of significance. All results are reported as mean ± SD. 3. Results 3.1. Detection sample Age, gender distribution and CSF hypocretin-1 did not differ between diagnostic groups of our detection sample. In contrast, A␤42 , Tau and, by trend, A␤40 and pTau differed between AD and depressed patients in the expected directions: CSF concentrations of A␤42 were lower in patients with AD when compared to depressed patients. In contrast, CSF concentrations of pTau were higher in patients with AD compared to MD (see Table 1). As expected, CSF concentrations of A␤42 were negatively correlated with Tau (r = −0.67, p < 0.001) and pTau (r = −0.53, p < 0.02). In our detection sample, MANOVA revealed significant main effects of CSF hypocretin concentration (Wilks-Lambda 0.486, p = 0.030) and diagnosis (Wilks-Lambda 0.370, p = 0.005) on the dependent variables A␤42 , A␤40 , Tau and pTau. Hypocretin1 was significantly and positively related to A␤40 (F1,19 = 5.46, p = 0.032), Tau (F1,19 = 7.1, p = 0.016) and pTau (F1,19 = 13.3, p = 0.002) but not A␤42 (F1,19 = 2.15, n.s.). Also, diagnosis was related to

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Table 1 Concentrations of hypocretin-1, A␤ and Tau proteins in the CSF of the detection sample (above) and the replication sample (below) with patients with depression and Alzheimer’s disease. Depression

Alzheimer’s disease

Mean ± SD

Correlation with hypocretin-1

Mean ± SD

Correlation with hypocretin-1

Detection sample Age (yrs)

66.3 ± 9.3

66.4 ± 7.2

Gender (f/m) A␤42 [pg/ml]

6/4 687 ± 231

A␤40 [pg/ml]

105.1 ± 37.9

Tau [pg/ml]

237 ± 90

pTau [pg/ml]

53.9 ± 22.9

Hypocretin-1 [pg/ml]

408 ± 33

r = 0.17 n.s. n.s. r = 0.52 n.s. r = 0.44 n.s. r = 0.48 n.s. r = 0.55 p = 0.10 n.a.

r = −0.03 n.s. n.s. r = 0.21 n.s. r = 0.64 p = 0.048 r = 0.59 p = 0.073 r = 0.72 p = 0.019 n.a.

Replication sample Age (yrs)

70.5 ± 6.5

Gender (f/m) A␤42 [pg/ml]

6/2 995 ± 501

A␤40 [pg/ml]

70.5 ± 6.5

Tau [pg/ml]

252 ± 149

pTau [pg/ml]

55.0 ± 23.5

Hypocretin-1 [pg/ml]

238 ± 32

* **

r = 0.17 n.s. n.s. r = 0.27 n.s. r = 0.76 p = 0.030 r = 0.90 p = 0.003 r = 0.88 p = 0.004 n.a.

7/3 459 ± 170** 77.7 ± 24.7* 369 ± 111** 76.0 ± 32.0* 417 ± 49 78.8 ± 7.9** 14/3 533 ± 103 78.8 ± 7.9 487 ± 288** 76.5 ± 34.6 248 ± 29

r = −0.04 n.s. n.s. r = 0.31 n.s. r = 0.45 0.077 r = 0.53 p = 0.035 r = 0.57 p = 0.022 n.a.

p < 0.1. p < 0.05.

A␤40 (F1,19 = 5.58, p = 0.030), A␤42 (F1,19 = 7.44, p = 0.014), Tau (F1,19 = 9.70, p = 0.006), but not pTau (F1,19 = 3.83, p = 0.067). Combining MD and AD patients of the detection sample, CSF hypocretin-1 was related to pTau (r = 0.65, p = 0.002), Tau (r = 0.50, p = 0.024), but not A␤40 (r = 0.41, p = 0.076) and A␤42 (r = 0.24, n.s.). Associations of CSF hypocretin-1 concentration with A␤40 , A␤42 , Tau and pTau in diagnostic subgroups are presented in Table 1. In the group of AD patients, MMSE was not associated with A␤42 , A␤40 , Tau, pTau or hypocretin (all r < 0.63). 3.2. Replication sample Including all patients of detection and replication samples in a MANOVA model, we found strong and significant main effects of both CSF hypocretin-1 concentration (WilksLambda 0.531, p = 0.0001) and diagnosis (Wilks-Lambda 0.581, p = 0.0001) on the dependent variables A␤42 , A␤40 , Tau and pTau. Hypocretin-1 was significantly related to Tau (F1,40 = 11.50, p = 0.002), pTau (F1,40 = 26.54, p = 0.0001) and A␤40 (F1,40 = 11.96, p = 0.001), but not A␤42 (F1,40 = 2.49, n.s.). Additionally, diagnosis was related to A␤42 (F1,40 = 21.82, 0.0001), Tau (F1,40 = 8.1, p = 0.007) and pTau (F1,40 = 5.61, p = 0.023), but not to A␤40 (F1,40 = 3.68, p = 0.062). Using normalized hypocretin values to control for interassay variability, we found hypocretin-1 to be strongly related with pTau (r = 0.630, p = 0.0001) and Tau (r = 0.463, p = 0.001), but only to a lesser extent to A␤40 (r = 0.328, p = 0.028) and not to A␤42 (r = 0.11, n.s.). 4. Discussion While we found CSF hypocretin-1 to be positively correlated to CSF Tau and pTau in our combined analysis of MD and AD patients

and in the MD subgroup, these associations failed to reach significance in the AD group of the detection sample alone. However, the association between hypocretin-1 and Tau as well as pTau was significant in the replication sample in both diagnostic subgroups. AD-related biomarkers differed between depressed and AD patients, indicating the eligibility of the studied subjects. CSF hypocretin-1 did not differ between diagnostic groups, confirming that CSF hypocretin-1 is not a specific marker of AD [1]. A␤40 and A␤42 were not unequivocally correlated to CSF hypocretin-1. In contrast to a recent study in wild-type and APP transgenic mice [14], we cannot confirm an immediate A␤-related pathology in association with CSF hypocretin in human subjects with AD. This might be due to the tissue being investigated: while Kang et al. studied interstitial fluid, we investigated CSF. However, we found a strong association of Tau and pTau with CSF hypocretin1, which, of course, was not determined in the above-mentioned animal study. Interestingly, the correlations of CSF hypocretin-1 with Tau and pTau were seen in depressed as well as AD patients. Thus, the correlations of Tau and pTau were not specific for AD, which might indicate an association, that is independent from diagnosis. Similarly, in APP transgenic as well as wild-type mice A␤ in brain interstitial fluid was found to be associated with hypocretin indicating that the association with AD-related molecular changes is independent from current disease [14]. Interestingly, earlier studies have reported an association of CSF total Tau in healthy controls, but not in patients with AD or dementia with Lewy bodies [28]. In contrast, other authors failed to show an association between hypocretin-1 and Tau, pTau or A␤42 in AD patients and controls although sample size and severity of AD were comparable [21]. We can only speculate that differences in the timing of lumbar punctures and in fasting conditions may explain these different findings (fasting at 9.00 am in our study; 0.30–1.30 p.m. in [21]).

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Regrettably, we had no access to CSF of healthy controls. Also, due to disturbed sleep, patients with major depression are not wellsuited to serve as a control group to AD regarding sleep-related peptides. However, in a careful study comparing depressed patients and healthy controls, it has been shown that CSF hypocretin-1 levels did not differ between depressed patients and healthy controls [22]. Additionally this study showed CSF hypocretin-1 not to depend on clinical features of the affective disorder [22]. Although our study lacks a healthy control group, it clearly shows that the associations between AD-related peptides and hypocretin-1 are not specific to AD. Regarding medication, we found similar associations between hypocretin-1 and Tau/p-Tau in both samples. Since most subjects of the detection sample did not receive psychopharmacological treatment, while most subjects of the replication sample were under treatment, it may be assumed that these associations are independent from treatment. To date, we can only speculate about potential mechanisms mediating the link between hypocretin-1 and Tau/pTau. Hypocretin has been reported to be related to apoptosis [19], but may prevent ischemic neuronal damage [9]. In addition to appetite regulation, hypocretin is closely involved in the regulation of glucose homeostasis by increasing endogenous glucose production and lowering hepatic insulin sensitivity [29]. Glycogen synthase kinase-3␤ (GSK-3␤) is an insulin-regulated kinase [3] that is not only involved in glycogen synthesis, but also in transcription factor regulation and, potentially, Tau phosphorylation [11]. Hypocretin increases neuronal insulin signaling and may, thus, be involved in the regulation of GSK-3␤ and other kinases [23]. Thus, hypocretin could play a role in neuroendangerment, Tau phosphorylation as well as disturbed glucose regulation of AD patients. It is unclear, whether there is a causal relationship between hypocretin-1 and Tau/pTau. Hypocretin antagonists, like suvorexant, will allow testing this hypothesis. Potentially, hypocretin antagonists may lower Tau and pTau pathology, which would imply hypocretin receptors as a future drug target for the treatment or prevention of Alzheimer’s disease. Aside from the small sample size, a major limitation of our study is that no sleep data were recorded. The nature of our data does not allow relating sleep and sleep–wake features to CSF hypocretin-1. However, hypocretin-1 may be directly associated with the physiological function of AD-related biomarkers while sleep variables are a proxy of hypocretin regulation. As another drawback we only performed basic psychometric testing, omitting the possibility to relate CSF hypocretin-1 concentrations to severity and specific domains of impaired cognitive function. Since CSF from healthy controls was not available, we used CSF of depressed patients. As depressed patients frequently suffer from disturbed sleep, the variation of hypocretin in both diagnostic groups may be due to variations in sleep quality. Replication of our findings in healthy controls is thus necessary in order to confirm our assumption of a physiological association between AD related peptides and hypocretin. Taken together, our data provide evidence that CSF hypocretin1 may be related to CSF Tau and pTau. These associations might be independent of AD, but it can be speculated that high hypocretin1 concentrations contribute to symptoms and pathophysiology of AD. Clearly, our data need replication in larger patient samples characterized with regard to sleep and in-depth neuropsychological assessment, in healthy control subjects and in prodromal AD [5]. In addition, further research regarding signaling pathways induced by hypocretin-1 may elucidate the molecular mechanisms mediating the relationship between hypocretin-1 and Tau/pTau. Considering preclinical data [14], hypocretin receptors could be potential drug targets for the prevention or treatment of Tau pathology.

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