A central role for the acid sphingomyelinase/ceramide system in neurogenesis and major depression

Understanding the pathophysiology of depression: From monoamines to the neurogenesis hypothesis model - are we there yet?

A number of factors (biogenic amine deficiency, genetic, environmental, immunologic, endocrine factors and neurogenesis) have been identified as mechanisms which provide unitary explanations for the pathophysiology of depression. Rather than a unitary construct, the combination and linkage of these factors have been implicated in the pathogenesis of depression. That is, environmental stressors and heritable genetic factors acting through immunologic and endocrine responses initiate structural and functional changes in many brain regions, resulting in dysfunctional neurogenesis and neurotransmission which then manifest as a constellation of symptoms which present as depression.

Clinical and metabolic consequences of L-serine supplementation in hereditary sensory and autonomic neuropathy type 1C

Hereditary sensory neuropathy type 1 (HSAN1) may be the first genetic neuropathy amenable to a specific mechanism-based treatment, as L-serine supplementation can be used to lower the neurotoxic levels of 1-deoxysphingolipids (1-deoxySL) that cause the neurodegeneration. The treatment is so far untested in HSAN1C caused by variants in the serine palmitoyl transferase subunit 2 (SPTLC2) gene. The aim of this study was to establish whether oral L-serine lowers 1-deoxySL in a patient with HSAN1C, to perform a dose escalation to find the minimal effective dose, and to assess the safety profile and global metabolic effects of the treatment. Our patient underwent a 52-wk treatment in which the L-serine dose was titrated up to 400 mg/kg/day. She was followed up by repeated clinical examination, nerve conduction testing, and skin biopsies to document effects on small nerve fibers. Serum was assayed for 1-deoxySL and metabolomics analysis of 111 metabolites. We found a robust lowering of 1-deoxySL, which correlated in a near-linear fashion with increased serum L-serine levels. Metabolomics analysis showed a modest elevation in glycine and a marked reduction in the level of cytosine, whereas most of the other assayed metabolites did not change. There were no direct side effects from the treatment, but the patient developed a transitory toe ulceration during the course of the study. The Charcot–Marie–Tooth neuropathy score increased by 1 point. We conclude that oral supplementation of L-serine decreases 1-deoxySL in HSAN1C without major global effects on metabolism. L-serine is therefore a potential treatment for HSAN1C.

Plasma sphingolipids and depressive symptoms in coronary artery disease

BACKGROUND

Depression is highly prevalent in individuals with coronary artery disease (CAD) and increases the risk of future cardiac events and mortality. Sphingolipids have been implicated in the pathophysiology of both CAD and depression. This study assessed the association between plasma sphingolipid concentrations and depressive symptoms in CAD subjects.

METHODS

Depressive symptoms were measured using the depression subscale of the self-reported Hospital Anxiety and Depression Scale (HADS). Sphingolipid concentrations were measured from fasting plasma samples using high-performance liquid chromatography-coupled electrospray ionization tandem mass spectrometry (LC/MS/MS). Linear regression models were used to assess associations between log-transformed concentrations of plasma sphingolipids and depressive symptoms.

RESULTS

A total of 111 CAD patients (mean (SD) age = 63.6 ± 6.4, 84.7% male) were included. In linear regression analyses, higher plasma concentrations of ceramides C16:0 (β = 0.204, p = .026) and C18:0 (β = 0.209, p = .023) and sphingomyelin SM18:1 (β = 0.210, p = .024) were significantly associated with higher HADS depression subscale score after adjusting for covariates.

CONCLUSION

Sphingolipids, in particular the ceramide species C16:0 and C18:0 and the sphingomyelin species SM18:1, may be implicated in the pathophysiology of depression in CAD. The association between plasma sphingolipid concentrations and depression should be further examined in CAD patients and in other populations.

Plasma Ceramides and Neuropsychiatric Symptoms of Alzheimer's Disease

BACKGROUND

Various evidence demonstrates the influences of ceramides on Alzheimer's disease (AD) pathogenesis. Furthermore, increased ceramides were also suggested to be related to cognitive decline. However, the association between ceramides and neuropsychiatric symptoms of AD remains unclear.

OBJECTIVE

This study sought to investigate the association between plasma ceramide levels and multiple neuropsychiatric symptoms in AD.

METHODS

A total of 98 patients and 92 cognitively normal controls participated in this study, including 56 with mild AD and 42 with moderate to severe AD. The Neuropsychiatric Inventory (NPI) was used to assess neuropsychiatric symptoms. Considering the influences of dementia severity on ceramide levels and neuropsychiatric symptoms, a subgroup analysis was conducted by dementia severity.

RESULTS

Except for C24 : 0, all ceramide species were significantly higher in AD patients than in controls. After controlling for confounding factors, the C16 : 0 and C20 : 0 levels were positively associated with delusions, and the quartiles of C22 : 0 and C24 : 0 were positively associated with depression. In the subgroup analysis, association between ceramide species and delusions were only observed in mild AD, and the association between ceramides and depression were prominent in moderate to severe AD. In mild AD, after controlling for age, gender, anti-dementia medications, diabetes status, and ApoE ɛ4 status, the C16 : 0, C20 : 0, and quartiles of C24 : 1 were associated with delusions. In moderate to severe AD, depression was associated with C22 : 0 and C24 : 0.

CONCLUSION

There were stage-specific associations between ceramides and neuropsychiatric symptoms of AD. The potential mechanisms deserve further investigation.

Plasma Nervonic Acid Is a Potential Biomarker for Major Depressive Disorder: A Pilot Study

BACKGROUND

Diagnostic biomarkers of major depressive disorder, bipolar disorder, and schizophrenia are urgently needed, because none are currently available.

METHODS

We performed a comprehensive metabolome analysis of plasma samples from drug-free patients with major depressive disorder (n=9), bipolar disorder (n=6), schizophrenia (n=17), and matched healthy controls (n=19) (cohort 1) using liquid chromatography time-of-flight mass spectrometry. A significant effect of diagnosis was found for 2 metabolites: nervonic acid and cortisone, with nervonic acid being the most significantly altered. The reproducibility of the results and effects of psychotropic medication on nervonic acid were verified in cohort 2, an independent sample set of medicated patients [major depressive disorder (n=45), bipolar disorder (n=71), schizophrenia (n=115)], and controls (n=90) using gas chromatography time-of-flight mass spectrometry.

RESULTS

The increased levels of nervonic acid in patients with major depressive disorder compared with controls and patients with bipolar disorder in cohort 1 were replicated in the independent sample set (cohort 2). In cohort 2, plasma nervonic acid levels were also increased in the patients with major depressive disorder compared with the patients with schizophrenia. In cohort 2, nervonic acid levels were increased in the depressive state in patients with major depressive disorder compared with the levels in the remission state in patients with major depressive disorder and the depressive state in patients with bipolar disorder.

CONCLUSION

These results suggested that plasma nervonic acid is a good candidate biomarker for the depressive state of major depressive disorder.

Doxepin Mitigates Noise-induced Neuronal Damage in Primary Auditory Cortex of Mice via Suppression of Acid Sphingomyelinase/Ceramide Pathway

Neuronal damage in primary auditory cortex (A1) underlies complex manifestations of noise exposure, prevention of which is critical for health maintenance. Acid sphingomyelinase (ASM) catalyzes generation of ceramide (Cer) which if over-activated mediates neuronal disorders in various diseases. Tricyclic antidepressants (TCAs), by restraining ASM/Cer, benefits multiple neuronal anomalies, so we aimed to elucidate the effect of TCA on noise induced hearing loss and auditory cortex derangement, unraveling mechanism involved. The mice were exposed to noise with frequencies of 20-20 KHz and intensity of 95 dB. Doxepin hydrochloride (DOX), a kind of TCAs, was given intragastrically by 5 mg kg^-1  days^-1 . Morphology of neurons was examined using hematoxylin-eosin (HE) and Nissl staining. Apoptosis was assayed through transferase-mediated dUTP nick end labeling (TUNEL). The content of ASM, Cer or acid ceramidase (AC) was detected by western blot and immunohistochemistry analysis. We demonstrated intense, broad band noise caused upward shift of auditory brainstem response (ABR) threshold to sound over frequencies 4-32 KHz, with prominent morphologic changes and enhanced apoptosis in neurons of primary auditory cortex (A1) (P < 0.05). DOX partly restored noise-caused hearing loss alleviating morphologic changes or apoptosis remarkably (P < 0.05). Both ASM and Cer abundance were elevated significantly by noise which was reversed upon DOX treatment (P < 0.05), but neither noise nor DOX altered AC content. DOX had no influence on hearing, neuronal morphology or ASM/Cer in control mice. Our result suggests DOX palliates noise induced hearing loss and neuronal damage in auditory cortex by correcting over-activation of ASM/Cer without hampering intrinsic behavior of it. Anat Rec, 300:2220-2232, 2017. © 2017 Wiley Periodicals, Inc.

Crosstalk between sphingolipids and vitamin D3: potential role in the nervous system

Sphingolipids are both structural and bioactive compounds. In particular, ceramide and sphingosine 1-phosphate regulate cell fate, inflammation and excitability. 1-α,25-dihydroxyvitamin D3 (1,25(OH)2 D3 ) is known to play an important physiological role in growth and differentiation in a variety of cell types, including neural cells, through genomic actions mediated by its specific receptor, and non-genomic effects that result in the activation of specific signalling pathways. 1,25(OH)2 D3 and sphingolipids, in particular sphingosine 1-phosphate, share many common effectors, including calcium regulation, growth factors and inflammatory cytokines, but it is still not known whether they can act synergistically. Alterations in the signalling and concentrations of sphingolipids and 1,25(OH)2 D3 have been found in neurodegenerative diseases and fingolimod, a structural analogue of sphingosine, has been approved for the treatment of multiple sclerosis. This review, after a brief description of the role of sphingolipids and 1,25(OH)2 D3 , will focus on the potential crosstalk between sphingolipids and 1,25(OH)2 D3 in neural cells.

Stressful learning paradigm precludes manifestation of cognitive ability in sphingomyelin synthase-2 knockout mice

Sphingomyelin synthases (SMSs) are enzymes converting ceramide to sphingomyelin. The behavioral phenotype attributed to their disruption has not been well described. We examined learning ability and hippocampal synaptic plasticity in mice deficient in SMS2 (SMS2 KO). In context-dependent fear learning and novel object recognition test, no difference in learning ability was detected in SMS2 KO and wild-type (WT) mice. By contrast, achievement of the Morris water maze (MWM) test was deteriorated in SMS2 KO mice. In the hippocampal CA1, while the basic synaptic transmission was normal, both short- and long-term synaptic plasticity was moderately suppressed. We interpret that the MWM test taking place in wet environment may represent learning paradigm under more stressful condition than those performed in dry conditions, and that the learning ability of SMS2 KO mice failed to manifest itself fully in stressful situations. In agreement, forced swimming induced depression-like behavior more easily in SMS2 KO mice. Mass spectrometry suggested a slightly altered species distribution of ceramide in the hippocampus of SMS2 KO mice. These findings support the proposal that altered synthesis of ceramide, which is the substrate of SMS2 and therefore expected to be modified in SMS2 KO mice, is associated with depression-like tendency in animal models and depressive disorder in humans.

Crystal structure of mammalian acid sphingomyelinase

Acid sphingomyelinase (ASMase, ASM, SMPD1) converts sphingomyelin into ceramide, modulating membrane properties and signal transduction. Inactivating mutations in ASMase cause Niemann-Pick disease, and its inhibition is also beneficial in models of depression and cancer. To gain a better understanding of this critical therapeutic target, we determined crystal structures of mammalian ASMase in various conformations. The catalytic domain adopts a calcineurin-like fold with two zinc ions and a hydrophobic track leading to the active site. Strikingly, the membrane interacting saposin domain assumes either a closed globular conformation independent from the catalytic domain, or an open conformation, which establishes an interface with the catalytic domain essential for activity. Structural mapping of Niemann-Pick mutations reveals that most of them likely destabilize the protein's fold. This study sheds light on the molecular mechanism of ASMase function, and provides a platform for the rational development of ASMase inhibitors and therapeutic use of recombinant ASMase.

Protective effect of liquiritigenin on depressive-like behavior in mice after lipopolysaccharide administration

Liquiritigenin (Liq), the main active ingredient of traditional Chinese medicine licorice, possesses anti-inflammatory and neuroprotective properties. The current investigation was designed to explore whether liquiritigenin could relieve lipopolysaccharide (LPS)-induced depression-like behavior in mice and the underlying mechanism. Liquiritigenin (7.5mg/kg, 15mg/kg) and fluoxetine (20mg/kg) were pretreated intragastrically once daily for 7 consecutive days. LPS (0.5mg/kg) was injected subcutaneously to establish the depression model 30min after pretreatment on day 7. Interleukin (IL)-6 and tumor necrosis factor (TNF)-α levels in serum and hippocampus were detected by enzyme-linked immunosorbent assay (ELISA). Behavioral assessment was conduct 24h post LPS injection. The expressions of p65NF-κB, IκBα, brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase B (TrkB) in hippocampus were determined by western blot. The obtained results showed that liquiritigenin effectively reduced the levels of pro-inflammatory cytokines and the expressions of p-p65NF-κB and p-IκBα. Furthermore, liquiritigenin preconditioning could down-regulate the immobility time in tail suspension test (TST), forced swimming test (FST) and up-regulate BDNF and TrkB contents in hippocampus. Thus, it is assumed that the antidepressant activity of liquiritigenin might be attributed to its anti-inflammatory property and BDNF/TrkB signaling pathway.

The role of sphingolipids in neuronal plasticity of the brain

This Editorial highlights a study by Müller et al. in which the authors suggest a new sphingolipid-dependent mechanism for behavioral extinction. Their study should be considered in the broad perspective of sphingolipid metabolic pathways and traffic (depicted in the graphic). Read the highlighted article 'A sphingolipid mechanism for behavioral extinction' on page 589.

Omega-3/omega-6 fatty acid ratios in different phospholipid classes and depressive symptoms in coronary artery disease patients

Depressive symptoms are highly incident among coronary artery disease (CAD) patients and increase mortality. Reduced ratios of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (omega-3 fatty acids) to arachidonic acid (AA, omega-6 fatty acid) concentrations have been linked with depressive symptoms in CAD. It remains unclear whether depressive symptoms are differentially associated with that ratio in different phospholipid classes, and this may have mechanistic implications. This study investigated associations between depressive symptoms in CAD patients and the EPA+DHA to AA ratio in the major phospholipid classes. This was a cross-sectional study of stable CAD patients. Sociodemographic, medical, medication, and cardiopulmonary fitness data were collected from each patient. Each patient was assessed for depressive symptoms using the 17-item Hamilton Depression Rating Scale (HAM-D). The percentage of EPA, DHA, and AA in each erythrocyte phospholipid class was determined using gas chromatography from fasting blood. Relationships between EPA+DHA to AA ratios and depressive symptoms were assessed using linear regression and were corrected for multiple comparisons. Seventy-six CAD patients were included (age=61.9 ± 8.5, 74% male, HAM-D=7.2 ± 5.9). In a backward elimination linear regression model, lower EPA+DHA to AA in erythrocyte phosphatidylinositol (B=-12.71, β=-0.33, p<.01) and sphingomyelin (B=-2.52, β=-0.37, p<.01) was associated with greater depressive symptom severity, independently of other known predictors. Other phospholipid classes were not associated with depressive symptoms. In conclusion, the relationship between EPA+DHA to AA ratios and depressive symptoms in CAD may not be consistent across phospholipid classes. Continued investigation of these potentially differential relationships may clarify underlying disease mechanisms.

P2X7 as a new target for chrysophanol to treat lipopolysaccharide-induced depression in mice

P2X7 receptor is a ligand gated ion channel found peripheral macrophages and microglia in the nervous system. The current study investigated the relationship between the activated P2X7 and depression for the first time. Chrysophanol (Chr) was examined for its protective effects against depression targeting P2X7. Chr (20mg/kg, 40mg/kg) and fluoxetine (20mg/kg) were intragastrically treated once daily for 7 consecutive days. Lipopolysaccharide (LPS, 0.5mg/kg) was intraperitoneally injected to develop depression model 30min after drug administration on day 7. Behavioral tests were measured 24h after LPS injection. Interleukin (IL)-6, IL-1β and tumor necrosis factor (TNF)-α levels in serum and hippocampus were measured by enzyme-linked immunosorbent assay (ELISA). The expressions of P2X7/NF-κB pathway-related proteins were assessed by western blot. The findings showed that Chr remarkably reduced the elevations of IL-6, IL-1β and TNF-α caused by LPS stimulation. The expressions of P2X7, p-IKKα, p-IKKβ, p-IκBα and p-NF-κBp65 were significantly decreased by Chr pretreatment. In addition, immobility time in tail suspension test (TST) and forced swimming test (FST) were reduced by Chr without affecting spontaneous locomotor activity in open filed test (OFT) and the preference for sucrose was also recovered in sucrose preference test (SPT) with Chr preconditioning. Thus, it is reasonable to speculate that Chr might exert antidepressant effect through inhibiting P2X7/NF-κB signaling pathway.