Alterations of the Sphingolipid Pathway in Alzheimer’s Disease: New Biomarkers and Treatment Targets?

The public health burden of Alzheimer disease (AD), the most common neurodegenerative disease, threatens to explode in the middle of this century. Current FDA-approved AD treatments (e.g. cholinesterase inhibitors, NMDA-receptor agonists) do not provide a “cure”, but rather a transient alleviation of symptoms for some individuals. Other available therapies are few and of limited effectiveness so additional avenues are needed. Sphingolipid metabolism is a dynamic process that modulates the formation of a number of bioactive metabolites, or second messengers critical in cellular signaling and apoptosis. In brain, the proper balance of sphingolipids is essential for normal neuronal function, as evidenced by a number of severe brain disorders that are the result of deficiencies in enzymes that control sphingolipid metabolism. Laboratory and animals studies suggest both direct and indirect mechanisms by which sphingolipids contribute to amyloid-beta production and Alzheimer pathogenesis but few studies have translated these findings to humans. Building on the laboratory and animal evidence demonstrating the importance of sphingolipid metabolism in AD, this review highlights relevant translational research incorporating and expanding basic findings to humans. A brief biological overview of sphingolipids (sphingomyelins, ceramides, and sulfatides) in AD is first described, followed by a review of human studies including post-mortem studies, clinical and epidemiological studies. Lastly, the potential role of peripheral ceramides in AD pathogenesis is discussed, as well as the possible use of sphingolipids as biomarkers for AD.     

Interactions between neural membrane glycerophospholipid and sphingolipid mediators: a recipe for neural cell survival or suicide

The neural membranes contain phospholipids, sphingolipids, cholesterol, and proteins. Glycerophospholipids and sphingolipids are precursors for lipid mediators involved in signal transduction processes. Degradation of glycerophospholipids by phospholipase A(2) (PLA(2)) generates arachidonic acid (AA) and docosahexaenoic acids (DHA). Arachidonic acid is metabolized to eicosanoids and DHA is metabolized to docosanoids. The catabolism of glycosphingolipids generates ceramide, ceramide 1-phosphate, sphingosine, and sphingosine 1-phosphate. These metabolites modulate PLA(2) activity. Arachidonic acid, a product derived from glycerophospholipid catabolism by PLA(2), modulates sphingomyelinase (SMase), the enzyme that generates ceramide and phosphocholine. Furthermore, sphingosine 1-phosphate modulates cyclooxygenase, an enzyme responsible for eicosanoid production in brain. This suggests that an interplay and cross talk occurs between lipid mediators of glycerophospholipid and glycosphingolipid metabolism in brain tissue. This interplay between metabolites of glycerophospholipid and sphingolipid metabolism may play an important role in initiation and maintenance of oxidative stress associated with neurologic disorders as well as in neural cell proliferation, differentiation, and apoptosis. Recent studies indicate that PLA(2) and SMase inhibitors can be used as neuroprotective and anti-apoptotic agents. Development of novel inhibitors of PLA(2) and SMase may be useful for the treatment of oxidative stress, and apoptosis associated with neurologic disorders such as stroke, Alzheimer disease, Parkinson disease, and head and spinal cord injuries.     

Spinal muscular atrophy associated with progressive myoclonus epilepsy

A rare syndrome characterized by lower motor neuron disease associated with progressive myoclonic epilepsy, referred to as “spinal muscular atrophy associated with progressive myoclonic epilepsy” (SMA‐PME), has been described in childhood and is inherited as an autosomal recessive trait. SMA‐PME is caused by mutation in the ASAH1 gene encoding acid ceramidase. Ceramide and the metabolites participate in various cellular events as lipid mediators. The catabolism of ceramide in mammals occurs in lysosomes through the activity of ceramidase. Three different ceramidases (acid, neutral and alkaline) have been identified and appear to play distinct roles in sphingolipid metabolism. The enzymatic activity of acid ceramidase is deficient in two rare inherited disorders; Farber disease and SMA‐PME. Farber disease is a very rare and severe autosomal recessive condition with a distinct clinical phenotype. The marked difference in disease manifestations may explain why Farber and SMA‐PME diseases were not previously suspected to be allelic conditions. The precise molecular mechanism underlying the phenotypic differences remains to be clarified. Recently, a condition with mutation in CERS1, the gene encoding ceramide synthase 1, has been identified as a novel form of PME. This finding underlies the essential role of enzymes regulating either the synthesis (CERS1) or degradation (ASAH1) of ceramide, and the link between defects in ceramide metabolism and PME.     

Plasma phosphatidylcholine and sphingomyelin concentrations are associated with depression and anxiety symptoms in a Dutch family-based lipidomics study

The central nervous system has the second highest concentration of lipids after adipose tissue. Alterations in neural membrane phospho- and sphingolipid composition can influence crucial intra- and intercellular signalling and alter the membrane's properties. Recently, the polyunsaturated fatty acids (PUFA) hypothesis for depression suggests that phospho- and sphingolipid metabolism includes potential pathways for the disease. In 742 people from a Dutch family-based study, we assessed the relationships between 148 different plasma phospho- and sphingolipid species and depression/anxiety symptoms as measured by the Hospital Anxiety and Depression Scales (HADS-A and HADS-D) and the Centre for Epidemiological Studies Depression Scale (CES-D). We observed significant differences in plasma sphingomyelins (SPM), particularly the SPM 23:1/SPM 16:0 ratio, which was inversely correlated with depressive symptom scores. We observed a similar trend for plasma phosphatidylcholines (PC), particularly the molar proportion of PC O 36:4 and its ratio to ceramide CER 20:0. Absolute levels of PC O 36:4 were also associated with depression symptoms in an independent replication. To our knowledge this is the first study on depressive symptoms that focuses on specific phospho- and sphingolipid molecules in plasma rather than total PUFA concentrations. The findings of this lipidomic study suggests that plasma sphingomyelins and ether phospholipids should be further studied for their potential as biomarkers and for a better understanding of the underlying mechanisms of this systemic disease.     

Increased expression of serine palmitoyltransferase (SPT) in balloon-injured rat carotid artery

The response to vascular injury is a complex wound healing response involving cell proliferation, migration, remodeling and inflammation. In the present studies we employed a rat balloon angioplasty model of vascular injury to investigate the potential role of sphingolipid signaling in the response to vascular injury. The enzyme serine palmitoyltransferase (SPT) catalyzes the first committed step in de novo sphingolipid biosynthesis. We observed marked upregulation of expression of both SPT subunits in actively proliferating cells in injured vessels. This enhanced SPT expression occurs in de-differentiated fibroblasts and proliferating vascular smooth muscle cells. The upregulation is particularly apparent in the proliferating luminal edge of the neointima and the adventitial de-differentiated fibroblasts and may serve as a hallmark of this process. The possible functional consequences of this enzyme upregulation and its role in the response to vascular injury are suggested but remain to be determined.     

Neuronopathic juvenile glucosylceramidosis due to sap-C deficiency: clinical course, neuropathology and brain lipid composition in this Gaucher disease variant

Glucosylceramide lipidosis results from a defective lysosomal degradation of this glycolipid. Lipid degradation is controlled by two components, the enzyme β-glucocerebrosidase and a sphingolipid activator protein. While most Gaucher cases are due to mutations within the gene that codes for the lysosomal enzyme, only two patients have been described with normal enzyme levels and mutations in the gene for the sphingolipid activator protein C (sap-C). Here we present the detailed neurological manifestations, neuropathological findings and brain lipid composition in one sap-C-deficient patient. The patient was an 8-year-old boy who presented with transient losses of consciousness, myoclonic jerks and generalized seizures resistant to all antiepileptic drugs. He developed progressive horizontal ophthalmoplegia, pyramidal and cerebellar signs, and died at the age of 15.5 years. Neuropathological studies demonstrated neuronal cell loss and neuronophagia, massive intraneuronal lipid storage and lack of perivascular Gaucher cells. Electron microscopy examination showed different types of storage including lipofuscin granules as well as the cytosomes with parallel arrays of bilayers that are assumed to be formed by stored lipids. General brain lipid composition did not show a remarkable increase or loss of any of the major lipid fractions but the glucosylceramide concentration in the cortex of several anatomical regions showed a striking increase. Fatty acid composition of the ceramide moiety clearly suggests that gangliosides are the main precursors in the cerebral cortex, while it implies an additional and distinct source in the cerebellum. Studying the phenotypic consequences of mutant sphingolipid activator proteins is critical to a better understanding of the physiological significance of these proteins. Received: 28 July 1997 / Revised: 22 June 1998 / Revised, accepted: 10 July 1998     

Fabry's disease: new therapeutic options for this lysosomal storage disorder

Fabry's disease is an x-linked, recessive, lysosomal storage disorder that results from deficient alpha-galactosidase A activity with pathological sphingolipid deposition mainly in endothelium, smooth muscle cells, kidneys, central and peripheral nervous system, and myocardium. Clinical manifestation mostly occurs during childhood and adolescence with severe pain attacks or chronic pain mainly in hands and feet, hypohydrosis, and skin lesions (angiokeratoma). In more advanced disease stages, renal and cerebrovascular complications develop with proteinuria and later renal failure and cerebral ischemia caused by cerebral microangiopathy, dilatative arteriopathy, or cardiac embolism. Heterozygote female carriers are severely affected more often than was previously considered. The diagnosis is based on the detection of deficient alpha-galactosidase A activity in leukocytes, fibroblasts, or tissue biopsies. Two randomised placebo-controlled studies showed that enzyme replacement is effective by demonstrating either reduced pain or reduced tissue sphingolipid deposition. Early diagnosis of Fabry's disease is important in view of these new causal therapeutic options.     

Evidence for disruption of sphingolipid metabolism in schizophrenia

As the field of glycobiology grows, important roles for glycolipids and glycoproteins in neurological disorders are being increasingly appreciated. However, few studies have explored the involvement of these molecules in the pathology of psychiatric illnesses. We investigated molecular differences related to glycobiology in subjects with schizophrenia by analyzing gene expression profiles using a focused glycogene chip, a custom-designed oligonucleotide array containing genes encoding proteins related to glycobiology, including glycosyltransferases, carbohydrate-binding proteins, proteoglycans, and adhesion molecules. We measured expression profiles in prefrontal cortical (BA46) samples from schizophrenic subjects and matched controls. We find differential expression of genes particularly related to glycosphingolipid/sphingolipid metabolism and N- and O-linked glycan biosynthesis in subjects with schizophrenia. Expression decreases of seven genes associated with these pathways, UGT8, SGPP1, GALC, B4GALT6, SPTLC2, ASAH1, and GAL3ST1, were validated by quantitative PCR in schizophrenic subjects with short-term illness. Only one of these genes, SPTLC2, showed differential expression in chronic schizophrenic subjects, although an increase in expression was observed. Covariate analysis showed that the expression of five of these genes was significantly positively correlated with age in schizophrenic, but not control, subjects. These changing patterns of expression could represent an adaptive response to pathology with disease progression or a compensatory effect of antipsychotic medication, although no significant correlations between gene expression levels and drug doses were observed. Disruption of sphingolipid metabolism early in illness could result in widespread downstream effects encompassing diverse pathological deficits already described in schizophrenia, especially those involving myelination and oligodendrocyte function; hence, this system may represent an important link in schizophrenia pathology.     

A 1H magnetic resonance spectroscopy study in adults with obsessive compulsive disorder: relationship between metabolite concentrations and symptom severity

1H magnetic resonance spectroscopy (1H MRS) studies exploring brain metabolites, especially glutamine + glutamate (Glx), in obsessive compulsive disorder (OCD) are of vital interest for trying to understand more about the pathophysiology of OCD. Therefore, we conducted the present 1H MRS study with the aims of (1) comparing MRS metabolites in a group of adult patients with OCD and a group of healthy controls, and (2) examining the relationship between MRS metabolite concentrations and symptom severity in the patient group. Three brain regions were studied, the right caudate nucleus, the anterior gyrus cinguli and the occipital cortex bilaterally. Since multivariate analysis is a highly useful tool for extraction of 1H MRS data, we applied principal component analysis (PCA) and partial least square projection to latent structures (PLS) to the MRS data. PLS disclosed a strong relationship between several of the metabolites and OCD symptom severity, as measured with Yale-Brown obsessive-compulsive scale (YBOCS): the YBOCS score was found to be positively correlated to caudate creatine, Glx, glutamate, and choline compounds as well as occipital cortex myoinositol, and negatively correlated to occipital cortex Glx. The negative correlation between occipital cortex Glx and YBOCS was the most impressive. PCA did not reveal any tendency for a separation between the patients with OCD and controls with respect to MRS metabolites. The results are discussed in relation to corticostriatothalamocortical feedback and previous observations of poor visuospatial ability in OCD.     

Sphingolipids in Neurodegeneration

Although the brain contains a high content of sphingolipids, we know relatively little about the roles that sphingolipids play in regulating neural functions. Once regarded only for their structural roles in maintaining the integrity of cellular and sub-cellular compartments, it is now apparent that many sphingolipid species are biologically active and play important roles in regulating signaling events. Recent technological and scientific advances are rapidly increasing our knowledge of the roles that sphingolipids play in regulating normal neural activity. Likewise, we are beginning to understand how perturbations in sphingolipid metabolism contribute to the pathogenesis of a variety of neurodegenerative conditions. In this special issue of NeuroMolecular Medicine, we present a series of review articles that summarize new and emerging technologies for the analysis of sphingolipids, sphingolipid metabolic pathways, and how dysfunctions in sphingolipid metabolism contribute to neurodegeneration in lysosomal storage disorders, Alzheimer’s disease and Multiple Sclerosis.     

Proton magnetic resonance spectroscopy in diagnostics of tumorous and nontumorous lesions in brain

Whether proton magnetic resonance spectroscopy (1H-MRS) can be used in the diagnosis of tumorous and nontumorous lesions of the brain was studied. The results of studies were analyzed in 80 patients, including 54 patients with brain tumors (astrocytomas and meningiomas), 20 with nontumorous lesions of the brain, and 6 apparently healthy individuals (controls). The nontumorous lesions of the brain involved cerebral circulatory disorders as ischemia (n = 14) and postradiation changes (n = 6). The studies were performed on a 1.5-T Signa-Horizont magnetic resonance tomograph by using the PROBE/SV programme package. The peak ratios of the following metabolites: N-acetylaspartate (NAA), choline (Cho), creatine, lactate (Lac), lipids (Lip) were examined in the spectra obtained. The spectra of astrocytomas showed a reduced peak of NAA, an elevated peak Cho; there was an increase in the peak of Lac along with higher tumor malignancy stage. The spectra of meningiomas presented a high peak of Cho along with a noticeable reduction in the peaks of other metabolites, the peak of NAA was not visualized. In ischemic lesion of the brain, the peak of Lac appeared within the first hours when the peaks of other metabolites were unchanged. A rise in the peak of Lac and a reduction in that of other metabolites were detectable with time. In radiation-induced lesions, the spectrum displayed a high peak of Lac and the peak of Lip appeared with virtually complete reduction in the peaks of other metabolites. 1H-MRS should be considered as an axillary technique used in combination with routine MRI in the diagnosis of various lesions of the brain, which can estimate chemical compositions, time-course of metabolic changes in the study tissue.     

Kinetic and equilibrium analyses of [(123)I]epidepride binding to striatal and extrastriatal dopamine D(2) receptors.

Quantitative SPECT measures of dopamine D(2) like receptors with [(123)I]epidepride is complicated by its high affinity and lipophilic metabolites. The purpose of this study was to use both parent (P) and lipophilic metabolites (M) as input functions in a kinetic paradigm and in comparison to the results of equilibrium studies. Kinetic studies on eleven healthy human subjects, ages 32+/- 10 were performed following i.v. injection of approximately 370 MBq of [(123)I]epidepride. Images were acquired for 13.5+/-1.0 hours. Equilibrium studies were done on seven of eleven subjects with a bolus injection of approximately 140 MBq, bolus/infusion ratio of 10 hours, and infusion for 30-32 hours. High (striatum) and low (temporal cortex) density regions were studied. Two (P and M) and one (P) input function models were applied in the kinetic studies. In receptor-rich regions, the distribution volumes in nondisplaceable compartments were fixed to those in cerebellum. In addition, in the two input function model, K(1)(P)/K(1)(M) was fixed to the values in the cerebellum. The one input function model provided V'(3) values (=f(1)*B'(max)/K(D)) which were consistent with those obtained in equilibrium studies in both receptor-rich regions, while the two input function model provided consistent values only in striatum. Poor identifiability of the rate constants of metabolites seemed to be the source of errors in the two input function model. These results suggest that correct V'(3) values can be obtained with the one input function model both in high- and low-density regions.     

Sphingolipids in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic autoimmune demyelinating disease of the CNS. Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), are target cells in MS. Although the etiology of MS is poorly known, new insights suggest oligodendrocyte apoptosis as one of the critical events followed by glial activation and infiltration of lymphocytes and macrophages. A major breakthrough in delineation of the mechanism of cell death, perivascular cuffing, and glial activation came from elucidation of the sphingolipid signal transduction pathway. The sphingolipid signal transduction pathway induces apoptosis, differentiation, proliferation, and growth arrest depending upon cell and receptor types, and downstream targets. Sphingomyelin, a major component of myelin membrane formed by mature oligodendrocytes, is abundant in the CNS and ceramide, its primary catabolic product released by activation of either neutral or acidic sphingomyelinase, serves as a potential lipid second messenger or mediator molecule modulating diverse cellular signaling pathways. Similarly, under certain conditions, sphingosine produced from ceramide by ceramidase is phosphorylated by sphingosine kinases to sphingosine-1 phosphate, another potent second messenger molecule. Both ceramide and sphingosine-1 phosphate regulate life and death of many cell types including brain cells and participate in pathogenic processes of MS. In this review, we have made an honest attempt to compile recent findings made by others and us relating to the role of sphingolipids in the disease process of MS.     

Chemokines in cerebrospinal fluid correlate with cerebral metabolite patterns in HIV-infected individuals

Chemokines influence HIV neuropathogenesis by affecting the HIV life cycle, trafficking of macrophages into the nervous system, glial activation, and neuronal signaling and repair processes; however, knowledge of their relationship to in vivo measures of cerebral injury is limited. The primary objective of this study was to determine the relationship between a panel of chemokines in cerebrospinal fluid (CSF) and cerebral metabolites measured by proton magnetic resonance spectroscopy (MRS) in a cohort of HIV-infected individuals. One hundred seventy-one stored CSF specimens were assayed from HIV-infected individuals who were enrolled in two ACTG studies that evaluated the relationship between neuropsychological performance and cerebral metabolites. Concentrations of six chemokines (fractalkine, IL-8, IP-10, MCP-1, MIP-1β, and SDF-1) were measured and compared with cerebral metabolites individually and as composite neuronal, basal ganglia, and inflammatory patterns. IP-10 and MCP-1 were the chemokines most strongly associated with individual cerebral metabolites. Specifically, (1) higher IP-10 levels correlated with lower N-acetyl aspartate (NAA)/creatine (Cr) ratios in the frontal white matter and higher MI/Cr ratios in all three brain regions considered and (2) higher MCP-1 levels correlated with lower NAA/Cr ratios in frontal white matter and the parietal cortex. IP-10, MCP-1, and IL-8 had the strongest associations with patterns of cerebral metabolites. In particular, higher levels of IP-10 correlated with lower neuronal pattern scores and higher basal ganglia and inflammatory pattern scores, the same pattern which has been associated with HIV-associated neurocognitive disorders (HAND). Subgroup analysis indicated that the effects of IP-10 and IL-8 were influenced by effective antiretroviral therapy and that memantine treatment may mitigate the neuronal effects of IP-10. This study supports the role of chemokines in HAND and the validity of MRS as an assessment tool. In particular, the findings identify relationships between the immune response—particularly an interferon-inducible chemokine, IP-10—and cerebral metabolites and suggest that antiretroviral therapy and memantine modify the impact of the immune response on neurons.     

Human sphingolipid activator protein-1 and sphingolipid activator protein-2 are encoded by the same gene

Mixed oligonucleotide primers complementary to the translation product of the sphingolipid activator protein (SAP)-2 were used to generate a 144-base pair (bp) complementary DNA (cDNA). This cDNA probe was used to isolate a 2,649-nucleotide-long cDNA that was sequenced and found to contain coding sequences for two known activators of lysosomal enzymes, namely, the sphingolipid activator protein (SAP)-1 and SAP-2. The cDNA contains an open reading frame of 1,482 nucleotides and 1,167 nucleotides of 3'-nontranslated region, followed by a stretch of 24 residues of adenylic acid. At 20 nucleotides upstream from the poly(A) tail there is a consensus AATAAA polyadenylation signal that is preceded by another potential polyadenylation signal. The cDNA, designed SAP-1/SAP-2 cDNA, hybridizes with two human mRNA species of approximately 3 kb in length, which most probably arise from polyadenylation at different sites. There are higher amounts of steady-state RNA levels of the SAP-1/SAP-2 mRNA in skin fibroblasts in comparison to B cells. The steady-state SAP-1/SAP-2 mRNA levels in Gaucher B cells are higher than in their normal counterparts. There is one human SAP-1/SAP-2 gene that has been cloned and is localized on two approximately 5 kb BamHI fragments.     

Blood-brain barrier penetration and pharmacokinetics of amitriptyline and its metabolites in p-glycoprotein (abcb1ab) knock-out mice and controls

In earlier studies with P-gp (abcb1) knock-out mice, we showed that P-gp exports the antidepressants citalopram, paroxetine, venlafaxine and amitriptyline and its metabolites across the blood-brain barrier, thereby reducing cerebral bioavailability of some substances up to 9 times. The present study investigated the pharmacokinetics of amitriptyline and whether abcb1ab double knock-out mice metabolize amitriptyline and its metabolites differently. P-gp knock-out mice and controls received a s.c. injection of 10mug amitriptyline/g of body weight. The animals were sacrificed after 30, 60, 120 and 240min and concentrations of amitriptyline and its metabolites were measured with HPLC in brain, plasma, liver, kidney, spleen, lung, muscle, fat and ovaries. Cerebral concentrations of amitriptyline and its metabolites were higher in P-gp-deficient mice compared to controls. No significant group effect was found for spleen, liver, lung, kidney and fat tissue. The results of our study indicate that amitriptyline and its metabolites are substrates of P-gp. Overall pharmacokinetics between knock-outs and controls were very similar. This confirms the validity of the P-gp knock-out model and allows for a continued research of the interactions between P-gp, the blood-brain barrier and CNS substances such as antidepressants, neuroleptics and others.     

Evidence that accumulation of ceramides and cholesterol esters mediates oxidative stress-induced death of motor neurons in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of motor neurons in the spinal cord resulting in progressive paralysis and death. The pathogenic mechanism of ALS is unknown but may involve increased oxidative stress, overactivation of glutamate receptors, and apoptosis. We report abnormalities in sphingolipid and cholesterol metabolism in the spinal cords of ALS patients and in a transgenic mouse model (Cu/ZnSOD mutant mice), which manifest increased levels of sphingomyelin, ceramides, and cholesterol esters; in the Cu/ZnSOD mutant mice, these abnormalities precede the clinical phenotype. In ALS patients and Cu/Zn-SOD mutant mice, increased oxidative stress occurs in association with the lipid alterations, and exposure of cultured motor neurons to oxidative stress increases the accumulation of sphingomyelin, ceramides, and cholesterol esters. Pharmacological inhibition of sphingolipid synthesis prevents accumulation of ceramides, sphingomyelin, and cholesterol esters and protects motor neurons against death induced by oxidative and excitotoxic insults. These findings suggest a pivotal role for altered sphingolipid metabolism in the pathogenesis of ALS.     

Inhibition of sphingolipid synthesis affects kinetics but not fidelity of L1/NgCAM transport along direct but not transcytotic axonal pathways

Glycosphingolipids are constituents of lipid rafts which might function in sorting apical and axonal cargoes in the trans-Golgi network. In fact, two GPI-linked proteins, Thy1 and PrPC, require lipid raft lipids for sorting to the axon. It was previously shown that inhibition of glycosphingolipid synthesis by FumonisinB1 (FB1) impairs axon outgrowth but not axon specification, leading to the hypothesis that formation of axonally-targeted vesicles is coupled to sphingolipid synthesis. Since the axonal cell adhesion molecule L1/NgCAM can partition into membrane rafts biochemically, we asked whether correct targeting to the axon is FB1-sensitive, similarly to GPI-linked proteins. We previously showed that cultured hippocampal neurons use more than one trafficking pathway to the axon: a transcytotic pathway and a direct pathway. We show here that reducing raft lipid levels does not disrupt axonal targeting of L1/NgCAM along either pathway. Unexpectedly, FB1 selectively slowed the kinetics of surface expression of a truncated NgCAM using the direct pathway, but not of NgCAM using the transcytotic pathway. Therefore, the formation and/or transport of a subset of axonally-targeted vesicles are coupled to sphingolipid synthesis. Our results yield a mechanism for the axon outgrowth defect observed in FB1.     

Identification of Metabolites of Valproic Acid in Serum of Humans, Dog, Rat, and Mouse

In kinetic studies of VPA in humans, dogs, rats, and mice, as well as in clinical routine analysis of serum concentrations of VPA in epileptic patients, 2--4 peaks (depending on the species examined) were regularly found in the gas chromatograms in addition to VPA. Comparison with control serum indicated that these peaks resulted from metabolism of VPA. By GC--MS, two of these metabolites could be identified as 5-hydroxy-2-propyl-pentanoic acid (5-OH-VPA) and 4-hydroxy-2-propyl-pentanoic acid (4-OH-VPA), using synthesized reference substances. Both metabolites results from omega (omega1, omega2) oxidation of VPA, 5-OH-VPA only occurring in serum of mice, and 4-OH-VPA in serum of mice and humans. With the aid of low- and high-resolution mass spectra, likely structures of the two remaining metabolites, both of which were found in serum of all the species examined, were proposed. One of these, 3-hydroxy-2-propyl-pentanoic acid (3-OH-VPA) confirms the involvement of beta-oxidation in the metabolism of VPA. The fourth metabolite, whose identity is uncertain, indicates a substance not described previously as a metabolite of VPA.     

The role of sphingolipids in the control of skeletal muscle function: a review

In this review, potential roles for the endogenous sphingolipid, sphingosine, and its derivatives are described for muscle cells. Sphingosine modulates the function of important calcium channels in muscle, including the ryanodine receptor (RyR) calcium release channel of the sarcoplasmic reticulum (SR). Sphingosine blocks calcium release through the SR ryanodine receptor and reduces the activity of single skeletal muscle RyR channels reconstituted into planar lipid bilayers. Sphingosine-blocked calcium release is coincident with the inhibitory effects of sphingosine on [3H]ryanodine binding to the RyR. The sphingomyelin signal transduction pathway has also been identified in both skeletal and cardiac muscle. A neutral form of sphingomyelinase (nSMase) enzyme has been localized to the junctional transverse tubule membrane. The high turnover of the SMase is responsible for the production of ceramide and sphingosine. HPLC analyses indicate that significant resting levels of sphingosine are present in muscle tissue. A model of excitation-contraction coupling is presented suggesting a potential role for this endogenous sphingolipid in normal muscle function. Putative roles for sphingolipid mediators in skeletal muscle dysfunction are also discussed. We hypothesize that sphingosine plays important roles in malignant hyperthermia and during the development of muscle fatigue.