Role of ceramide in neuronal cell death and differentiation

Acid sphingomyelinase as a pathological and therapeutic target in neurological disorders: focus on Alzheimer's disease

Over the past decade, numerous studies have highlighted the importance of acid sphingomyelinase (ASM) in disease treatment in humans. This enzyme functions primarily to generate ceramide, maintain the cellular membrane, and regulate cellular function. However, in the blood and brain of patients with neurological disorders, including major depression, ischemic stroke, amyotrophic lateral sclerosis, multiple sclerosis, and Alzheimer's disease (AD), elevated ASM levels significantly suggest disease onset or progression. In these diseases, increased ASM is profoundly involved in neuronal death, abnormal autophagy, neuroinflammation, blood-brain barrier disruption, hippocampal neurogenesis loss, and immune cell dysfunction. Moreover, genetic and pharmacological inhibition of ASM can prevent or ameliorate various diseases. The therapeutic effects of ASM inhibition have prompted the urgent need to develop ASM inhibitors, and several ASM inhibitors have been identified. In this review, we summarize the current knowledge on the critical roles and mechanisms of ASM in brain cells and blood that are associated with different neuropathological features, especially those observed in AD. Furthermore, we elucidate the potential possibility and limitations of existing ASM-targeting drugs according to experimental studies in neurological disorder mouse models.

System level modeling and analysis of TNF-α mediated sphingolipid signaling pathway in neurological disorders for the prediction of therapeutic targets

Sphingomyelin (SM) belongs to a class of lipids termed sphingolipids. The disruption in the sphingomyelin signaling pathway is associated with various neurodegenerative disorders. TNF-α, a potent pro-inflammatory cytokine generated in response to various neurological disorders like Alzheimer's disease (AD), Parkinson's disease (PD), and Multiple Sclerosis (MS), is an eminent regulator of the sphingomyelin metabolic pathway. The immune-triggered regulation of the sphingomyelin metabolic pathway via TNF-α constitutes the sphingomyelin signaling pathway. In this pathway, sphingomyelin and its downstream sphingolipids activate various signaling cascades like PI3K/AKT and MAPK/ERK pathways, thus, controlling diverse processes coupled with neuronal viability, survival, and death. The holistic analysis of the immune-triggered sphingomyelin signaling pathway is imperative to make necessary predictions about its pivotal components and for the formulation of disease-related therapeutics. The current work offers a comprehensive in silico systems analysis of TNF-α mediated sphingomyelin and downstream signaling cascades via a model-based quantitative approach. We incorporated the intensity values of genes from the microarray data of control individuals from the AD study in the input entities of the pathway model. Computational modeling and simulation of the inflammatory pathway enabled the comprehensive study of the system dynamics. Network and sensitivity analysis of the model unveiled essential interaction parameters and entities during neuroinflammation. Scanning of the key entities and parameters allowed us to determine their ultimate impact on neuronal apoptosis and survival. Moreover, the efficacy and potency of the FDA-approved drugs, namely Etanercept, Nivocasan, and Scyphostatin allowed us to study the model's response towards inhibition of the respective proteins/enzymes. The network analysis revealed the pivotal model entities with high betweenness and closeness centrality values including recruit FADD, TNFR_TRADD, act CASP2, actCASP8, actCASP3 and 9, cytochrome C, and RIP_RAIDD which profoundly impacted the neuronal apoptosis. Whereas some of the entities with high betweenness and closeness centrality values like Gi-coupled receptor, actS1PR, Sphingosine, S1P, actAKT, and actERK produced a high influence on neuronal survival. However, the current study inferred the dual role of ceramide, both on neuronal survival and apoptosis. Moreover, the drug Nivocasan effectively reduces neuronal apoptosis via its inhibitory mechanism on the caspases.

Vitamin D3 Enriches Ceramide Content in Exosomes Released by Embryonic Hippocampal Cells

The release of exosomes can lead to cell–cell communication. Nutrients such as vitamin D3 and sphingolipids have important roles in many cellular functions, including proliferation, differentiation, senescence, and cancer. However, the specific composition of sphingolipids in exosomes and their changes induced by vitamin D3 treatment have not been elucidated. Here, we initially observed neutral sphingomyelinase and vitamin D receptors in exosomes released from HN9.10 embryonic hippocampal cells. Using ultrafast liquid chromatography tandem mass spectrometry, we showed that exosomes are rich in sphingomyelin species compared to whole cells. To interrogate the possible functions of vitamin D3, we established the optimal conditions of cell treatment and we analyzed exosome composition. Vitamin D3 was identified as responsible for the vitamin D receptor loss, for the increase in neutral sphingomyelinase content and sphingomyelin changes. As a consequence, the generation of ceramide upon vitamin D3 treatment was evident. Incubation of the cells with neutral sphingomyelinase, or the same concentration of ceramide produced in exosomes was necessary and sufficient to stimulate embryonic hippocampal cell differentiation, as vitamin D3. This is the first time that exosome ceramide is interrogated for mediate the effect of vitamin D3 in inducing cell differentiation.

Neutral Sphingomyelinase-2 Deficiency Ameliorates Alzheimer's Disease Pathology and Improves Cognition in the 5XFAD Mouse

Recent evidence implicates exosomes in the aggregation of Aβ and spreading of tau in Alzheimer's disease. In neural cells, exosome formation can be blocked by inhibition or silencing of neutral sphingomyelinase-2 (nSMase2). We generated genetically nSMase2-deficient 5XFAD mice (fro;5XFAD) to assess AD-related pathology in a mouse model with consistently reduced ceramide generation. We conducted in vitro assays to assess Aβ42 aggregation and glial clearance with and without exosomes isolated by ultracentrifugation and determined exosome-induced amyloid aggregation by particle counting. We analyzed brain exosome content, amyloid plaque formation, neuronal degeneration, sphingolipid, Aβ42 and phospho-tau levels, and memory-related behaviors in 5XFAD versus fro;5XFAD mice using contextual and cued fear conditioning. Astrocyte-derived exosomes accelerated aggregation of Aβ42 and blocked glial clearance of Aβ42 in vitro Aβ42 aggregates were colocalized with extracellular ceramide in vitro using a bifunctional ceramide analog preloaded into exosomes and in vivo using anticeramide IgG, implicating ceramide-enriched exosomes in plaque formation. Compared with 5XFAD mice, the fro;5XFAD mice had reduced brain exosomes, ceramide levels, serum anticeramide IgG, glial activation, total Aβ42 and plaque burden, tau phosphorylation, and improved cognition in a fear-conditioned learning task. Ceramide-enriched exosomes appear to exacerbate AD-related brain pathology by promoting the aggregation of Aβ. Reduction of exosome secretion by nSMase2 loss of function improves pathology and cognition in the 5XFAD mouse model.

SIGNIFICANCE STATEMENT

We present for the first time evidence, using Alzheimer's disease (AD) model mice deficient in neural exosome secretion due to lack of neutral sphingomyelinase-2 function, that ceramide-enriched exosomes exacerbate AD-related pathologies and cognitive deficits. Our results provide rationale to pursue a means of inhibiting exosome secretion as a potential therapy for individuals at risk for developing AD.

Repetitive transcranial magnetic stimulation applications normalized prefrontal dysfunctions and cognitive-related metabolic profiling in aged mice

Chronic high-frequency repetitive transcranial magnetic stimulation (rTMS) is a noninvasive brain stimulation technique that has recently received increasing interests as a therapeutic procedure for neurodegenerative diseases. To identify the metabolism mechanism underlying the improving effects of rTMS, we observed that high frequency (25Hz) rTMS for 14 days could reverse the decline of the performance of the passive avoidance task in aged mice. We further investigated the metabolite profiles in the prefrontal cortex (PFC) in those mice and found that rTMS could also reverse the metabolic abnormalities of gamma-aminobutyric acid, N-acetyl aspartic, and cholesterol levels to the degree similar to the young mice. These data suggested that the rTMS could ameliorate the age-related cognitive impairment and improving the metabolic profiles in PFC, and potentially can be used to improve cognitive decline in the elderly.

Involvement of sphingolipids in ethanol neurotoxicity in the developing brain

Ethanol-induced neuronal death during a sensitive period of brain development is considered one of the significant causes of fetal alcohol spectrum disorders (FASD). In rodent models, ethanol triggers robust apoptotic neurodegeneration during a period of active synaptogenesis that occurs around the first two postnatal weeks, equivalent to the third trimester in human fetuses. The ethanol-induced apoptosis is mitochondria-dependent, involving Bax and caspase-3 activation. Such apoptotic pathways are often mediated by sphingolipids, a class of bioactive lipids ubiquitously present in eukaryotic cellular membranes. While the central role of lipids in ethanol liver toxicity is well recognized, the involvement of sphingolipids in ethanol neurotoxicity is less explored despite mounting evidence of their importance in neuronal apoptosis. Nevertheless, recent studies indicate that ethanol-induced neuronal apoptosis in animal models of FASD is mediated or regulated by cellular sphingolipids, including via the pro-apoptotic action of ceramide and through the neuroprotective action of GM1 ganglioside. Such sphingolipid involvement in ethanol neurotoxicity in the developing brain may provide unique targets for therapeutic applications against FASD. Here we summarize findings describing the involvement of sphingolipids in ethanol-induced apoptosis and discuss the possibility that the combined action of various sphingolipids in mitochondria may control neuronal cell fate.

Endoplasmic reticulum (ER) stress inhibitor salubrinal protects against ceramide-induced SH-SY5Y cell death

In the present study, we examined the mechanisms of ceramide-induced cell death in SH-SY5Y human neuroblastoma cells. Our results demonstrate a significant endoplasmic reticulum (ER) stress response in SH-SY5Y cells after short-chain ceramide (C6) treatment. Administration of ceramide (C6) to SH-SY5Y human neuroblastoma cells caused apoptotic cell death, which was inhibited by ER stress inhibitor salubrinal. Further, ceramide-induced cell death reduced significantly in stable SH-SY5Y cells expressing C/EBP homologous protein (CHOP) shRNA. Salubrinal inhibited ceramide-induced inositol-requiring enzyme 1α (IRE1α)/apoptosis signal regulating kinase 1 (ASK1)/c-Jun N-terminal kinase (JNK) phosphorylation. Taken together, these data suggest that ceramide-induced SH-SY5Y cell death may be linked to the ER stress-regulated intrinsic pathway, and proposed the potential protective effects of salubrinal.

Inhibition of ceramide biosynthesis preserves photoreceptor structure and function in a mouse model of retinitis pigmentosa

Retinitis pigmentosa (RP) is a genetic disease causing progressive apoptotic death of photoreceptors and, ultimately, incurable blindness. Using the retinal degeneration 10 (rd10) mouse model of RP, we investigated the role of ceramide, a proapoptotic sphingolipid, in retinal degeneration. We also tested the possibility that photoreceptor loss can be slowed or blocked by interfering with the ceramide signaling pathway of apoptosis in vivo. Retinal ceramide levels increased in rd10 mice during the period of maximum photoreceptor death. Single intraocular injections of myriocin, a powerful inhibitor of serine palmitoyl-CoA transferase, the rate-limiting enzyme of ceramide biosynthesis, lowered retinal ceramide levels to normal values and rescued photoreceptors from apoptotic death. Noninvasive treatment was achieved using eye drops consisting of a suspension of solid lipid nanoparticles loaded with myriocin. Short-term noninvasive treatment lowered retinal ceramide in a manner similar to intraocular injections, indicating that nanoparticles functioned as a vector permitting transcorneal drug administration. Prolonged treatment (10-20 d) with solid lipid nanoparticles increased photoreceptor survival, preserved photoreceptor morphology, and extended the ability of the retina to respond to light as assessed by electroretinography. In conclusion, pharmacological targeting of ceramide biosynthesis slowed the progression of RP in a mouse model, and therefore may represent a therapeutic approach to treating this disease in humans. Transcorneal administration of drugs carried in solid lipid nanoparticles, as experimented in this study, may facilitate continuous, noninvasive treatment of patients with RP and other retinal pathologies.

Synthetic, non-natural analogs of ceramide elevate cellular ceramide, inducing apoptotic death to prostate cancer cells and eradicating tumors in mice

The anticancer effects of synthetic, non-natural analogs of ceramide were tested using human TSU-Pr1 prostate cancer cells in-vitro as well as in-vivo, following their effects on tumors development in mice. When incubated with the cultured cancer cells, the analogs elevated cellular ceramide and induced a cytotoxicity and death by apoptosis. When a ceramide analog was injected intradermally or intraperitoneally into BALB/c-Nude or NOD-SCID mice bearing a human prostate tumor, a considerable regression of the tumor was observed. The synthetic ceramide analogs should thus be further investigated as potential anticancer drugs.

The ceramide transporter and the Goodpasture antigen binding protein: one protein--one function?

The Goodpasture antigen-binding protein (GPBP) and its splice variant the ceramide transporter (CERT) are multifunctional proteins that have been found to play important roles in brain development and biology. However, the function of GPBP and CERT is controversial because of their involvement in two apparently unrelated research fields: GPBP was initially isolated as a protein associated with collagen IV in patients with the autoimmune disease Goodpasture syndrome. Subsequently, a splice variant lacking a serine-rich domain of 26 amino acids (GPBPDelta26) was found to mediate the cytosolic transport of ceramide and was therefore (re)named CERT. The two splice forms likely carry out different functions in specific sub-cellular localizations. Selective GPBP knockdown induces extensive apoptosis and tissue loss in the brain of zebrafish. GPBP/GPBPDelta26 knock-out mice die as a result of structural and functional defects in endoplasmic reticulum and mitochondria. Because both mitochondria and ceramide play an important role in many biological events that regulate neuronal differentiation, cellular senescence, proliferation and cell death, we propose that GPBP and CERT are pivotal in neurodegenerative processes. In this review, we discuss the current state of knowledge on GPBP and CERT, including the molecular and biochemical characterization of GPBP in the field of autoimmunity as well as the fundamental research on CERT in ceramide transport, biosynthesis, localization, metabolism and cell homeostasis.

Evaluation of sphinganine and sphingosine as human breast cancer chemotherapeutic and chemopreventive agents

No comparative study of the effects of sphingolipid metabolites on proliferation and differentiation in normal human breast epithelial cells versus stem cells and tumorigenic cells has been reported. The purpose of this study was to evaluate the chemotherapeutic and chemopreventive potential of sphingoid bases (sphingosine and sphinganine) using a novel cell culture system of normal human breast epithelial cells (HBEC) developed from breast tissues of healthy women obtained during reduction mammoplasty (Type I HBEC with stem cell characteristics and Type II HBEC with basal epithelial cell phenotypes) and transformed tumorigenic Type I HBEC. The results show that sphinganine inhibited the growth and induced apoptosis of transformed tumorigenic Type I HBEC more potently than sphingosine (IC(50) for sphinganine 4 microM; sphingosine 6.4 microM). Both sphinganine and sphingosine at high concentrations (8-10 lM) arrested the cell cycle at G(2)/M. Sphinganine inhibited the growth and caused death of Type I HBEC more strongly than sphingosine. In comparison, Type II HBEC (normal differentiated cells) were less sensitive to the growth-inhibitory effects of sphingoid bases than Type I HBEC (stem cells) or transformed tumorigenic Type I HBEC, suggesting that sphingoid bases may serve as chemotherapeutic agents. At concentrations (0.05, 0.1, and 0.5 microM) that are below the growth-inhibitory range, sphingoid bases induced differentiation of Type I HBEC to Type II HBEC, as detected morphologically and via expression of a tumor suppressor protein, maspin, which is a marker of Type II HBEC. Thus, sphingoid bases may function as chemotherapeutic as well as chemopreventive agents by preferentially inhibiting cancer cells and eliminating stem cells from which most breast cancer cells arise.

N-tosyl-l-phenylalanyl-chloromethyl ketone reduces ceramide during hypoxic–ischemic brain injury in newborn rat

N-tosyl-L-phenylalanyl-chloromethyl ketone (TPCK) suppresses apoptosis and protects neurons from damage in animal models. TPCK is thought to act by inhibiting ceramide production by sphingomyelinase. Ceramide is a proapoptotic intracellular signal that is involved in the cerebral ischemia. We wished to see whether ceramide contributes to TPCK's neuroprotective effects in vivo. Seven-day-old rat pups had the right carotid arteries permanently ligated followed by 2.5 h of hypoxia (8% oxygen). TPCK (10 mg/kg, n=62) or vehicle (n=63) was administered by i.p. 5 min prior to hypoxia. The level of ceramide in brain cortex both in lesioned and unlesioned hemispheres was measured at 8 h, 18 h, 24 h, 2 and 5 days after hypoxia-ischemia using reversed phase high performance liquid chromatography. The level of ceramide significantly increased due to hypoxic-ischemia at 18, 24 h and 2 days after hypoxia (P<0.05 or P<0.01) but not at 8 h or 5 days after hypoxia as compared to the contralateral hemisphere or a sham group. Pretreatment with TPCK reduced this increase. We also examined the level of sphingomyelin and the activities of the ceramide synthesizing sphingomyelinase enzymes by thin layer chromatography. The activities of acidic and neutral sphingomyelinase significantly increased due to hypoxic ischemia at 24 h after hypoxia. TPCK significantly reduced this increase (P<0.05 vs. vehicle) but did not affect the level of sphingomyelin. The results are consistent with the hypothesis that ceramide is involved in TPCK's neuroprotective effects in hypoxic-ischemic brain injury in the newborn rat.

Ceramide induces neuronal apoptosis through mitogen-activated protein kinases and causes release of multiple mitochondrial proteins

Ceramide accumulates in neurons during various disorders associated with acute or chronic neurodegeneration. In these studies, we investigated the mechanisms of ceramide-induced apoptosis in primary cortical neurons using exogenous C(2) ceramide as well as inducing endogenous ceramide accumulation using inhibitors of glucosylceramide synthetase. Ceramide induced the translocation of certain, but not all, pro-apoptotic mitochondrial proteins: cytochrome c, Omi, SMAC, and AIF were released from the mitochondria, whereas Endonuclease G was not. Ceramide also selectively altered the phosphorylation state of members of the MAPK superfamily, causing dephosphorylation of ERK1/2 and hyperphosphorylation of p38 MAP kinases, but not affecting the phosphorylation of JNK or ERK5. Inhibitors of the p38 MAP kinase pathway (SB-202190 or SB-203580) and an inhibitor of the ERK1/2 pathway (U0126) reduced ceramide-induced neuronal death. These p38 and ERK1/2 inhibitors appear to block ceramide-activated apoptotic signaling upstream of the mitochondria, as they attenuated mitochondrial release of cytochrome c, Omi, AIF, and SMAC, as well as reducing ceramide-induced caspase-3 activation.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.

NGF acts via p75 low-affinity neurotrophin receptor and calpain inhibition to reduce UV neurotoxicity

The relative roles of the high-affinity nerve growth factor (NGF) receptor, TrkA, and low-affinity p75 neurotrophin receptor (p75NTR) in neuronal survival are an active research area. We reported previously that UV treatment induces a calpain-dependent, delayed neuronal death. We show here that NGF inhibits this UV-induced cortical neuron death. Interestingly, NGF neuroprotection requires p75NTR. Because it has been reported that NGF binding to p75NTR leads to ceramide generation, we evaluated whether ceramide was also neuroprotective. We found that ceramide also inhibits UV toxicity, and that the actions of ceramide and NGF were not additive. Moreover, cycloheximide inhibited ceramide and NGF neuroprotection, suggesting that their actions require new protein synthesis. Consistent with this possibility, we found that NGF activates the expression of genes such as calbindin. Lastly, we explored the role of calpain in NGF actions. NGF and ceramide both reduced the level of calpain activation after UV treatment. This NGF effect was p75NTR dependent. Overall, we interpret these results as consistent with an NGF neuroprotective pathway wherein p75NTR activation leads sequentially to ceramide generation, new protein synthesis, and inhibition of calpain activation. Overall, these results provide insight into a p75NTR dependent pathway of NGF neuroprotection.

Tissue-specific ceramide response in the chronically hypoxic rat model mimicking cyanotic heart disease

BACKGROUND AND OBJECTIVE

Acute hypoxia is associated with apoptosis and increase in ceramide levels in various organs. To assess the effect of chronic hypoxia on ceramide accumulation in the lungs and kidneys, we utilized an animal model mimicking cyanotic heart disease.

METHODS

Rats were placed in a hypoxic environment at birth and oxygen levels were maintained at 10% in an air-tight Plexiglas chamber. Controls remained in room air. Animals were sacrificed and the lung and kidneys were harvested and weighed at 1 and 4 weeks, respectively. Ceramide levels were measured using a modified diacylglycerol kinase assay.

RESULTS

Significant polycythemia developed in the hypoxic rats at 1 and 4 weeks. Indexed lung and kidney masses were significantly increased in the hypoxic animals as compared to controls at 1 and 4 weeks, respectively. The ceramide levels in the hypoxic lungs and kidneys were not significantly different from control groups at 1 and 4 weeks. [Ceramide/phosphate ratio in the kidneys was 1.28 +/- 0.17 (C) versus 1.18 +/- 0.12 (H) at 1 week; P = 0.39, and 1.46 +/- 0.08 (C) versus 1.33 +/- 0.15 (H) at 4 weeks (P = 0.44)] and [ceramide/phosphate ratio (pmol/nmol) in the lungs was 2.29 +/- 0.14 (C) versus 1.98 +/- 0.12 (H) at 1 week (P = 0.17), and 2.42 +/- 0.16 (C) versus 2.30 +/- 0.05 (H) at 4 weeks, P = 0.34].

CONCLUSION

The response of lungs and kidneys to chronic hypoxia includes increase in indexed mass and lack of ceramide accumulation. This is similar to the response previously reported in the chronically hypoxic brain and heart. Thus, various organs appear to have similar ceramide response pattern to chronic hypoxia.

Involvement of oxidative stress-induced abnormalities in ceramide and cholesterol metabolism in brain aging and Alzheimer's disease

Alzheimer's disease (AD) is an age-related disorder characterized by deposition of amyloid beta-peptide (Abeta) and degeneration of neurons in brain regions such as the hippocampus, resulting in progressive cognitive dysfunction. The pathogenesis of AD is tightly linked to Abeta deposition and oxidative stress, but it remains unclear as to how these factors result in neuronal dysfunction and death. We report alterations in sphingolipid and cholesterol metabolism during normal brain aging and in the brains of AD patients that result in accumulation of long-chain ceramides and cholesterol. Membrane-associated oxidative stress occurs in association with the lipid alterations, and exposure of hippocampal neurons to Abeta induces membrane oxidative stress and the accumulation of ceramide species and cholesterol. Treatment of neurons with alpha-tocopherol or an inhibitor of sphingomyelin synthesis prevents accumulation of ceramides and cholesterol and protects them against death induced by Abeta. Our findings suggest a sequence of events in the pathogenesis of AD in which Abeta induces membrane-associated oxidative stress, resulting in perturbed ceramide and cholesterol metabolism which, in turn, triggers a neurodegenerative cascade that leads to clinical disease.

Synthetic, non-natural sphingolipid analogs inhibit the biosynthesis of cellular sphingolipids, elevate ceramide and induce apoptotic cell death

Numerous studies have demonstrated the participation of sphingolipids in signal transduction and regulation of cell growth. Several cellular stress agents have been shown to elevate ceramide, the basic precursor of all sphingolipids, initiating a cascade of events leading to arrest of the cell cycle, apoptosis and cell death. Aiming at inhibiting metabolic pathways of sphingolipid metabolism that might lead to an increase of cellular ceramide, we have synthesized non-natural analogs of ceramide, sphingosine and trimethylsphingosine. When the respective analogs were applied to HL60 human myeloid leukemic cells they inhibited the biosynthesis of sphingomyelin (SPM) and glycosphingolipids and induced apoptosis that led to cell death. A fluorescent procedure which has been developed for quantifying the biosynthesis of cellular ceramide indicated an increase in the ceramide content following an incubation with the synthetic analogs. These results suggest that the newly synthesized sphingolipid analogs might be valuable for potential application as a therapeutic modality in leukemia and other malignancies.

Hippocampal programmed cell death after status epilepticus: evidence for NMDA-receptor and ceramide-mediated mechanisms

PURPOSE

Status epilepticus (SE) can result in acute neuronal injury with subsequent long-term age-dependent behavioral and histologic sequelae. To investigate potential mechanisms that may underlie SE-related neuronal injury, we studied the occurrence of programmed cell death (PCD) in the hippocampus in the kainic acid (KA) model.

METHODS

In adult rats, KA-induced SE resulted in DNA fragmentation documented at 30 h after KA injection. Ceramide, a known mediator of PCD in multiple neural and nonneural tissues, increased at 2-3 h after KA intraperitoneal injection, and then decreased to control levels before increasing again from 12 to 30 h after injection. MK801 pretreatment prevented KA-induced increases in ceramide levels and DNA fragmentation, whether there was reduction in seizure severity or not (achieved with 5 mg/kg and 1 mg/kg of MK801, respectively).

RESULTS

Both ceramide increases and DNA fragmentation were observed after KA-induced SE in adult and in P35 rats. Ceramide did not increase after KA-induced SE in P7 pups, which also did not manifest any DNA fragmentation. Intrahippocampal injection of the active ceramide analogue C2-ceramide produced widespread DNA fragmentation, whereas the inactive ceramide analogue C2-dihydroceramide did not.

CONCLUSIONS

Our data support the hypotheses that (a) N-methyl-d-aspartate-receptor activation results in ceramide increases and in DNA fragmentation; (b) ceramide is a mediator of PCD after SE; and (c) there are age-related differences in PCD and in the ceramide response after SE. Differences in the ceramide response could, potentially, be responsible for observed age-related differences in the response to SE.

Inverse Genomics as a powerful tool to identify novel targets for the treatment of neurodegenerative diseases

Toward the prevention of neurodegeneration we have used Immusol's Inverse Genomics platform to identify gene targets involved in neuronal cell death. Inverse genomics uses a combinatorial library of unique hairpin ribozymes with randomized substrate binding sequences to cleave unique RNA transcripts, thereby decreasing translation of the encoded proteins. Using the SK-N-MC neuroblastoma cell line a cell based survival selection assay was designed with C2-ceramide or TNFalpha as an induction signal for apoptosis. SK-N-MC cells were stably transduced with a ribozyme vector library, and then exposed to 20 microM C2-ceramide or 50 ng/ml TNFalpha to induce cell death. Surviving cells were harvested, their DNA isolated, and the ribozymes rescued by PCR for re-introduction into fresh cells. After several rounds of selection and ribozyme rescue we have identified individual ribozymes that protect neuronal cells from C2-ceramide induced apoptosis. Three of the cellular targets of these ribozyme sequence tags have been validated. Microarray analysis and yeast two-hybrid screens have also been used to gain insight into the pathways involved by identifying additional players involved in these pathways. These target genes may also serve as therapeutic targets for development of drugs for Alzheimer and Parkinson's diseases.

Ceramide induces neuronal apoptosis through the caspase-9/caspase-3 pathway

C(2)-ceramide, a cell-permeable analog of ceramide, caused cell death in cultured rat cortical neuronal cells. C(2)-ceramide-induced neuronal loss was accompanied by upregulation of caspase-3 activity, measured by cleavage of its fluorogenic substrate Ac-DEVD-AMC. Similar results were obtained when cortical neuronal cultures were treated with sphingomyelinase, an enzyme responsible for ceramide formation in the cell. Morphological evaluation of C(2)-ceramide-treated cortical neurons showed nuclear condensation and fragmentation as visualized by Hoechst 33258 staining. Co-administration of the selective caspase-3 inhibitor z-DEVD-fmk or caspase-9 inhibitor z-LEHD-fmk significantly reduced C(2)-ceramide-induced cell death, while co-application of the caspase-8, inhibitor z-IETD-fmk, was without effect. Immunoblot analysis of protein extracts from C(2)-ceramide-treated cortical neuronal cultures revealed upregulation of active caspase-9 and caspase-3 protein levels, whereas presence of active caspase-8 immunoreactivity was undetectable in this system. Administration of C(2)-ceramide to SH-SY5Y human neuroblastoma cells also caused apoptotic cell death. Moreover, ceramide-induced cell death was significantly decreased in caspase-9 dominant-negative SH-SY5Y cells, while both caspase-8 dominant-negative cultures and mock-transfected cells showed equally high levels of cell death following C(2)-ceramide treatment. Taken together, these data suggest that neuronal death induced by ceramide may be linked to the caspase-9/caspase-3 regulated intrinsic pathway of cellular apoptosis.

Ceramide regulation of apoptosis versus differentiation: a walk on a fine line. Lessons from neurobiology

One of the characteristics of ceramide-mediated biology is the variety of biological outcomes observed in response to its intracellular accumulation. The molecular mechanisms that govern the cell "decision-making" in response to ceramide remain largely unclear. In this perspective, the study of neural models has begun to provide important insight into the understanding of these mechanisms that regulate differentiation and cell death. Indeed, differentiation and cell death are among the most common effects elicited by ceramide in most cell types and in neural cells, too. Therefore, the lessons we may learn from the study of ceramide regulation of neurobiology would also shed light on the regulation of ceramide-mediated biology in other cellular models. Since increasing evidence links aberrant metabolism of ceramide to different pathologies, the understanding of the mechanisms underlying these events may represent the key to the design of novel therapeutic approaches.

Regulation of apoptosis during neuronal differentiation by ceramide and b-series complex gangliosides

Lipid analysis of gestational day E14.5 mouse brain revealed elevation of ceramide to a tissue concentration that induced apoptosis when added to the medium of neuroprogenitor cells grown in cell culture. Elevation of ceramide was coincident with the first appearance of b-series complex gangliosides (BCGs). Expression of BCGs by stable transfection of murine neuroblastoma (F-11) cells with sialyltransferase-II (ST2) resulted in a 70% reduction of ceramide-induced apoptosis. This was most likely due to an 80% reduced expression of prostate apoptosis response-4 (PAR-4). PAR-4 expression and apoptosis were restored by preincubation of ST2-transfected cells with N-butyl deoxinojirimycin (NB-DNJ) or PD98059, two inhibitors of ganglioside biosynthesis or p42/44 mitogen-activated protein (MAPK) kinase, respectively. In sections of day E14.5 mouse brain, the intermediate zone showed intensive staining for complex gangliosides, but only low staining for apoptosis (TUNEL) and PAR-4. Apoptosis and PAR-4 expression, however, were elevated in the ventricular zone which only weakly stained for complex gangliosides. Whole cell patch clamping revealed a 2-fold increased calcium influx in ST2-transfected cells, the blocking of which with nifedipine restored apoptosis to the level of untransfected cells. In serum-free culture, supplementation of the medium with IGF-1 was required to maintain MAPK phosphorylation and the anti-apoptotic effect of BCG expression. BCG-enhanced calcium influx and the presence of insulin-like growth factor-1 may thus activate a cell survival mechanism that selectively protects developing neurons against ceramide-induced apoptosis by up-regulation of MAPK and reduction of PAR-4 expression.

Nuclear localization of neutral sphingomyelinase 1: biochemical and immunocytochemical analyses

To examine the intracellular localization of neutral sphingomyelinase 1 (nSMase 1), a rabbit polyclonal antibody was raised against a recombinant form of the enzyme expressed in E. coli. It has been reported that, in rat liver or in ascites hepatoma AH7974, high activity of neutral sphingomyelinase (SMase) is found at the plasma membrane, with a lesser but significant amount in nucleus and cytoplasm. The biochemical properties, dithiothreitol requirement and high salt concentration dependency, of cloned and expressed nSMase 1 resemble those of previously described nuclear neutral SMase of AH7974. The present study was therefore focused on the nuclear localization of this enzyme. Western blotting of subcellular fractions using anti-rat nSMase 1 antibody revealed most nSMase 1 to be associated with the nuclei and some with microsomes, but not with plasma membranes. Consistently, neutral SMase activity in nuclear extract was immunoprecipitated by the antibody, while that of plasma membranes was not. The results indicate that nSMase 1 mainly resides in the nucleus and may thus differ from neutral SMase in plasma membrane. On gel-filtration column chromatography of nuclear extract, the profile of neutral SMase activity corresponded well with immunoreactive protein bands on western blotting, suggesting that a large part of nuclear neutral SMase may be nSMase 1. Removal of the nuclear envelope by treatment with Triton X-100 did not significantly decrease the amount of nuclear nSMase 1, and western blotting of subnuclear fractions (i.e. nuclear envelope, chromatin, and nuclear matrix) revealed nSMase 1 signal exclusively in the nuclear matrix. Immunocytochemistry with AH7974, as well as rat fibroblast cell line 3Y1, demonstrated nSMase 1 to be localized mainly in the nucleus, with some in the cytoplasm. Moreover, immuno-electron microscopy clearly showed the signal of nSMase 1 to be more dense in the nucleus than in the cytoplasm of AH7974.