Differential effects of ceramide species on exocytosis in rat PC12 cells

Lipid remodeling in acrosome exocytosis: unraveling key players in the human sperm

It has long been thought that exocytosis was driven exclusively by well-studied fusion proteins. Some decades ago, the role of lipids became evident and escalated interest in the field. Our laboratory chose a particular cell to face this issue: the human sperm. What makes this cell special? Sperm, as terminal cells, are characterized by their scarcity of organelles and the complete absence of transcriptional and translational activities. They are specialized for a singular membrane fusion occurrence: the exocytosis of the acrosome. This unique trait makes them invaluable for the study of exocytosis in isolation. We will discuss the lipids' role in human sperm acrosome exocytosis from various perspectives, with a primary emphasis on our contributions to the field. Sperm cells have a unique lipid composition, very rare and not observed in many cell types, comprising a high content of plasmalogens, long-chain, and very-long-chain polyunsaturated fatty acids that are particular constituents of some sphingolipids. This review endeavors to unravel the impact of membrane lipid composition on the proper functioning of the exocytic pathway in human sperm and how this lipid dynamic influences its fertilizing capability. Evidence from our and other laboratories allowed unveiling the role and importance of multiple lipids that drive exocytosis. This review highlights the role of cholesterol, diacylglycerol, and particular phospholipids like phosphatidic acid, phosphatidylinositol 4,5-bisphosphate, and sphingolipids in driving sperm acrosome exocytosis. Furthermore, we provide a comprehensive overview of the factors and enzymes that regulate lipid turnover during the exocytic course. A more thorough grasp of the role played by lipids transferred from sperm can provide insights into certain causes of male infertility. It may lead to enhancements in diagnosing infertility and techniques like assisted reproductive technology (ART).

Elongation of Very Long-Chain Fatty Acids (ELOVL) in Atopic Dermatitis and the Cutaneous Adverse Effect AGEP of Drugs

Atopic dermatitis (AD) is a common inflammatory skin disease, in particular among infants, and is characterized, among other things, by a modification in fatty acid and ceramide composition of the skin's stratum corneum. Palmitic acid and stearic acid, along with C16-ceramide and 2-hydroxy C16-ceramide, occur strikingly in AD. They coincide with a simultaneous decrease in very long-chain ceramides and ultra-long-chain ceramides, which form the outermost lipid barrier. Ceramides originate from cellular sphingolipid/ceramide metabolism, comprising a well-orchestrated network of enzymes involving various ELOVLs and CerSs in the de novo ceramide synthesis and neutral and acid CERase in degradation. Contrasting changes in long-chain ceramides and very long-chain ceramides in AD can be more clearly explained by the compartmentalization of ceramide synthesis. According to our hypothesis, the origin of increased C16-ceramide and 2-hydroxy C16-ceramide is located in the lysosome. Conversely, the decreased ultra-long-chain and very long-chain ceramides are the result of impaired ELOVL fatty acid elongation. The suggested model's key elements include the lysosomal aCERase, which has pH-dependent long-chain C16-ceramide synthase activity (revaCERase); the NADPH-activated step-in enzyme ELOVL6 for fatty acid elongation; and the coincidence of impaired ELOVL fatty acid elongation and an elevated lysosomal pH, which is considered to be the trigger for the altered ceramide biosynthesis in the lysosome. To maintain the ELOVL6 fatty acid elongation and the supply of NADPH and ATP to the cell, the polyunsaturated PPARG activator linoleic acid is considered to be one of the most suitable compounds. In the event that the increase in lysosomal pH is triggered by lysosomotropic compounds, compounds that disrupt the transmembrane proton gradient or force the breakdown of lysosomal proton pumps, non-HLA-classified AGEP may result.

The pair ceramide 1-phosphate/ceramide kinase regulates intracellular calcium and progesterone-induced human sperm acrosomal exocytosis

Before fertilization, spermatozoa must undergo calcium-regulated acrosome exocytosis in response to physiological stimuli such as progesterone and zona pellucida. Our laboratory has elucidated the signaling cascades accomplished by different sphingolipids during human sperm acrosomal exocytosis. Recently, we established that ceramide increases intracellular calcium by activating various channels and stimulating the acrosome reaction. However, whether ceramide induces exocytosis on its own, activation of the ceramide kinase/ceramide 1-phosphate (CERK/C1P) pathway or both is still an unsolved issue. Here, we demonstrate that C1P addition induces exocytosis in intact, capacitated human sperm. Real-time imaging in single-cell and calcium measurements in sperm population showed that C1P needs extracellular calcium to induce [Ca2+]i increase. The sphingolipid triggered the cation influx through voltage-operated calcium (VOC) and store-operated calcium (SOC) channels. However, it requires calcium efflux from internal stores through inositol 3-phosphate receptors (IP3R) and ryanodine receptors (RyR) to achieve calcium rise and the acrosome reaction. We report the presence of the CERK in human spermatozoa, the enzyme that catalyzes C1P synthesis. Furthermore, CERK exhibited calcium-stimulated enzymatic activity during the acrosome reaction. Exocytosis assays using a CERK inhibitor demonstrated that ceramide induces acrosomal exocytosis, mainly due to C1P synthesis. Strikingly, progesterone required CERK activity to induce intracellular calcium increase and acrosome exocytosis. This is the first report, implicating the bioactive sphingolipid C1P in the physiological progesterone pathway leading to the sperm acrosome reaction.

Drug triggered pruritus, rash, papules, and blisters - is AGEP a clash of an altered sphingolipid-metabolism and lysosomotropism of drugs accumulating in the skin?

Rash, photosensitivity, erythema multiforme, and the acute generalized exanthematous pustulosis (AGEP) are relatively uncommon adverse reactions of drugs. To date, the etiology is not well understood and individual susceptibility still remains unknown. Amiodarone, chlorpromazine, amitriptyline, and trimipramine are classified lysosomotropic as well as photosensitizing, however, they fail to trigger rash and pruritic papules in all individuals. Lysosomotropism is a common charcteristic of various drugs, but independent of individuals. There is evidence that the individual ability to respond to external oxidative stress is crosslinked with the elongation of long-chain fatty acids to very long-chain fatty acids by ELOVLs. ELOVL6 and ELOVL7 are sensitive to ROS induced depletion of cellular NADPH and insufficient regeneration via the pentose phosphate pathway and mitochondrial fatty acid oxidation. Deficiency of NADPH in presence of lysosomotropic drugs promotes the synthesis of C16-ceramide in lysosomes and may contribute to emerging pruritic papules of AGEP. However, independently from a lysosomomotropic drug, severe depletion of ATP and NAD(P)H, e.g., by UV radiation or a potent photosensitizer can trigger likewise the collapse of the lysosomal transmembrane proton gradient resulting in lysosomal C16-ceramide synthesis and pruritic papules. This kind of papules are equally present in polymorphous light eruption (PMLE/PLE) and acne aestivalis (Mallorca acne). The suggested model of a compartmentalized ceramide metabolism provides a more sophisticated explanation of cutaneous drug adverse effects and the individual sensitivity to UV radiation. Parameters such as pKa and ClogP of the triggering drug, cutaneous fatty acid profile, and ceramide profile enables new concepts in risk assessment and scoring of AGEP as well as prophylaxis outcome.

Drugs, Metabolites, and Lung Accumulating Small Lysosomotropic Molecules: Multiple Targeting Impedes SARS-CoV-2 Infection and Progress to COVID-19

Lysosomotropism is a biological characteristic of small molecules, independently present of their intrinsic pharmacological effects. Lysosomotropic compounds, in general, affect various targets, such as lipid second messengers originating from lysosomal enzymes promoting endothelial stress response in systemic inflammation; inflammatory messengers, such as IL-6; and cathepsin L-dependent viral entry into host cells. This heterogeneous group of drugs and active metabolites comprise various promising candidates with more favorable drug profiles than initially considered (hydroxy) chloroquine in prophylaxis and treatment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections/Coronavirus disease 2019 (COVID-19) and cytokine release syndrome (CRS) triggered by bacterial or viral infections. In this hypothesis, we discuss the possible relationships among lysosomotropism, enrichment in lysosomes of pulmonary tissue, SARS-CoV-2 infection, and transition to COVID-19. Moreover, we deduce further suitable approved drugs and active metabolites based with a more favorable drug profile on rational eligibility criteria, including readily available over-the-counter (OTC) drugs. Benefits to patients already receiving lysosomotropic drugs for other pre-existing conditions underline their vital clinical relevance in the current SARS-CoV2/COVID-19 pandemic.

COVID-19/SARS-CoV-2 Infection: Lysosomes and Lysosomotropism Implicate New Treatment Strategies and Personal Risks

In line with SARS and MERS, the SARS-CoV-2/COVID-19 pandemic is one of the largest challenges in medicine and health care worldwide. SARS-CoV-2 infection/COVID-19 provides numerous therapeutic targets, each of them promising, but not leading to the success of therapy to date. Neither an antiviral nor an immunomodulatory therapy in patients with SARS-CoV-2 infection/COVID-19 or pre-exposure prophylaxis against SARS-CoV-2 has proved to be effective. In this review, we try to close the gap and point out the likely relationships among lysosomotropism, increasing lysosomal pH, SARS-CoV-2 infection, and disease process, and we deduce an approach for the treatment and prophylaxis of COVID-19, and cytokine release syndrome (CRS)/cytokine storm triggered by bacteria or viruses. Lysosomotropic compounds affect prominent inflammatory messengers (e.g., IL-1B, CCL4, CCL20, and IL-6), cathepsin-L-dependent viral entry of host cells, and products of lysosomal enzymes that promote endothelial stress response in systemic inflammation. As supported by recent clinical data, patients who have already taken lysosomotropic drugs for other pre-existing conditions likely benefit from this treatment in the COVID-19 pandemic. The early administration of a combination of antivirals such as remdesivir and lysosomotropic drugs, such as the antibiotics teicoplanin or dalbavancin, seems to be able to prevent SARS-CoV-2 infection and transition to COVID-19.

Down-regulation of ceramide kinase via proteasome and lysosome pathways in PC12 cells by serum withdrawal: Its protection by nerve growth factor and role in exocytosis

Ceramide kinase (CerK) phosphorylates ceramide to ceramide-1-phosphate (C1P). CerK is highly expressed in the brain, and its association with the neuronal function has been reported. Previous reports showed that the activity of CerK is regulated by post-translational modifications including phosphorylation, whereas the cellular fate of CerK protein and its role in neuronal functions have not been clearly elucidated. Therefore, we investigated these issues in PC12 cells. Treatment with nerve growth factor (NGF) for 6 h increased the formation of C1P but not CerK mRNA. Knockdown of CerK and overexpression of HA-tagged CerK down- and up-regulated the formation of C1P, respectively. In PC12-CerK-HA cells, serum withdrawal caused ubiquitination of CerK-HA protein and down-regulated both CerK-HA protein and C1P formation within 6 h, and these down-regulations were abolished by co-treatments with NGF or proteasome inhibitors such as MG132 and clasto-lactacystin. Microscopic analysis showed that treatment with the proteasome inhibitors increased CerK-HA in puncture structures, possibly endosomes and/or vesicles, in cells. Treatment with the lysosome inhibitors reduced serum withdrawal-induced down-regulation of CerK-HA protein but not C1P formation. When knockdown or overexpression of CerK was performed, Ca²⁺-induced release of [³H] noradrenaline was reduced or enhanced, respectively, but neurite extension was not modified. There was a positive correlation between noradrenaline release and formation of C1P and/or CerK-HA levels in NGF- and clasto-lactacystin-treated cells. These results suggest that levels of CerK were down-regulated by the ubiquitin/proteasome and lysosome pathways and the former pathway-sensitive pool of CerK was suggested to be linked with exocytosis in PC12 cells.

Ceramide induces a multicomponent intracellular calcium increase triggering the acrosome secretion in human sperm

Exocytosis of spermatozoon's secretory vesicle, named acrosome reaction (AR), is a regulated event that plays a central role in fertilization. It is coupled to a complex calcium signaling. Ceramide is a multitasking lipid involved in exocytosis. Nevertheless, its effect on secretion is controversial and the underlying cellular and molecular mechanisms remain unknown. Human spermatozoa are useful to dissect the role of ceramide in secretion given that the gamete is not capable to undergo any trafficking mechanisms other than exocytosis. We report for the first time, the presence of sphingolipid metabolism enzymes such as neutral-sphingomyelinase and ceramide synthase in sperm. Ceramidases are also present and active. Both the addition of cell-permeable ceramide and the rise of the endogenous one, increase intracellular calcium acting as potent inducers of exocytosis. Ceramide triggers AR in capacitated spermatozoa and enhances the gamete response to progesterone. The lipid induces physiological ultrastructural changes in the acrosome and triggers an exocytosis-signaling cascade involving protein tyrosine phosphatase 1B and VAMP2. Real-time imaging showed an increment of calcium in the cytosol upon ceramide treatment either in the absence or in the presence of extracellular calcium. Pharmacological experiments demonstrate that at early stages the process involves ryanodine receptors, CatSper (calcium channel of sperm), and store-operated calcium channels. We set out the signaling sequence of events that connect ceramide to internal calcium mobilization and external calcium signals during secretion. These results allow the coordination of lipids and proteins in a pathway that accomplishes secretion. Our findings contribute to the understanding of ceramide's role in regulated exocytosis and fertilization.

Interaction of Synaptosomal-Associated Protein 25 with Neutral Sphingomyelinase 2: Functional Impact on the Sphingomyelin Pathway

Neurotransmitter release is mediated by ceramide, which is generated by sphingomyelin hydrolysis. In the present study, we examined whether synaptosomal-associated protein 25 (SNAP-25) is involved in ceramide production and exocytosis. Neutral sphingomyelinase 2 (nSMase2) was partially purified from bovine brain and we found that SNAP-25 was enriched in the nSMase2-containing fractions. In rat synaptosomes and PC12 cells, the immunoprecipitation pellet of anti-SNAP-25 antibody showed higher nSMase activity than the immunoprecipitation pellet of anti-nSMase2 antibody. In PC12 cells, SNAP-25 was colocalized with nSMase2. Transfection of SNAP-25 small interfering RNA (siRNA) significantly inhibited nSMase2 translocation to the plasma membrane. A23187-induced ceramide production was concomitantly reduced in SNAP-25 siRNA-transfected PC12 cells compared with that in scrambled siRNA-transfected cells. Moreover, transfection of SNAP-25 siRNA inhibited dopamine release, whereas addition of C₆-ceramide to the siRNA-treated cells moderately reversed this inhibition. Additionally, nSMase2 inhibition reduced dopamine release. Collectively, our results indicate that SNAP-25 interacts with nSMase2 during ceramide production, which mediates exocytosis and neurotransmitter release.

The 16p11.2 homologs fam57ba and doc2a generate certain brain and body phenotypes

Deletion of the 16p11.2 CNV affects 25 core genes and is associated with multiple symptoms affecting brain and body, including seizures, hyperactivity, macrocephaly, and obesity. Available data suggest that most symptoms are controlled by haploinsufficiency of two or more 16p11.2 genes. To identify interacting 16p11.2 genes, we used a pairwise partial loss of function antisense screen for embryonic brain morphology, using the accessible zebrafish model. fam57ba, encoding a ceramide synthase, was identified as interacting with the doc2a gene, encoding a calcium-sensitive exocytosis regulator, a genetic interaction not previously described. Using genetic mutants, we demonstrated that doc2a^+/− fam57ba^+/− double heterozygotes show hyperactivity and increased seizure susceptibility relative to wild-type or single doc2a^−/− or fam57ba^−/− mutants. Additionally, doc2a^+/− fam57ba^+/− double heterozygotes demonstrate the increased body length and head size. Single doc2a^+/− and fam57ba^+/− heterozygotes do not show a body size increase; however, fam57ba^−/− homozygous mutants show a strongly increased head size and body length, suggesting a greater contribution from fam57ba to the haploinsufficient interaction between doc2a and fam57ba. The doc2a^+/− fam57ba^+/− interaction has not been reported before, nor has any 16p11.2 gene previously been linked to increased body size. These findings demonstrate that one pair of 16p11.2 homologs can regulate both brain and body phenotypes that are reflective of those in people with 16p11.2 deletion. Together, these findings suggest that dysregulation of ceramide pathways and calcium sensitive exocytosis underlies seizures and large body size associated with 16p11.2 homologs in zebrafish. The data inform consideration of mechanisms underlying human 16p11.2 deletion symptoms.

Administration of glucosylceramide ameliorated the memory impairment in aged mice

The function and the role of glucosylceramide have not been well studied in the central nervous system. This study was aimed to investigate the possible roles of glucosylceramide in memory function in aged mice. Glucosylceramide (50 mg/kg, p.o.) showed memory enhancing activity after 3-month treatment in the aged mice (C56BL/6, 18–20 months old) through Y-maze, novel objective test, and Morris water maze test. Long-term treatment of glucosylceramide decreased the expression of iNOS and COX-2 in the brain of aged mice. The LPS-induced mRNA level of iNOS, COX-2, IL-1β, and TNF-α was reduced by the acute treatment with glucosylceramide in adult mice. These results suggest that glucosylceramide plays an important role in anti-inflammatory and memory enhancement, and it could be a potential new therapeutic agent for the treatment of neurodegenerative diseases such as Alzheimer's disease.

Ceramide in plasma membrane repair

The perforation of the plasmalemma by pore-forming toxins causes an influx of Ca(2+) and an efflux of cytoplasmic proteins. In order to ensure cellular survival, lesions have to be identified, plugged and removed from the membrane. The Ca(2+)-driven fusion of lysosomes with the plasma membrane leads to hydrolysis of sphingomyelin by acid sphingomyelinase and a formation of ceramide platforms in the outer leaflet of the lipid bilayer. We propose that the negative curvature, promoted by tighter packing of lipids in the outer layer, leads to an inward vesiculation of the damaged area for its endocytotic uptake and internal degradation. In contrast, the activation of neutral sphingomyelinase triggers the production of ceramide within the inner leaflet of the lipid bilayer, thereby promoting an outward curvature, which enables the cell to shed the membrane-containing toxin pore into the extracellular space. In this process, ceramide is supported by members of the annexin protein family which act as Ca(2+) sensors and as membrane fusion agents.

Ceramide profiles in the brain of rats with diabetes induced by streptozotocin

Diabetes is associated with disturbances of brain activity and cognitive impairment. We hypothesize that ceramides may constitute an important contribution to diabetes-linked neuro-dysfunction. In our study we used rats injected with streptozotocin (STZ) as a model of severe hyperglycemia. Using the gas-liquid chromatography technique we found a significant increase of ceramide content in brains and a decrease in plasma of diabetic rats. The inhibitor of serine palmitoyltransferase, myriocin, reduced ceramide generation in hyperglycemic brains, although injected alone it exerted a paradoxical effect of ceramide upregulation. Myriocin had no impact on ceramide concentration in the plasma of either control or diabetic rats. The level of ceramide saturated fatty acids was elevated whereas the level of ceramide poly-unsaturated fatty acids was downregulated in brains of all experimental groups. The concentration of ceramide mono-unsaturated fatty acids remained unchanged. The pattern of individual ceramide species was altered depending on treatment. We noted an STZ-evoked increase of brain ceramide C16:0, C18:0 and C20:0 and a strong decline in ceramide C18:2 fatty acid levels. Some changes of brain ceramide pattern were modified by myriocin. We found a decreased amount of total ceramide-ω-6 fatty acids in STZ-treated rat brains and no changes in ceramide-ω-3 concentration. We conclude that ceramides may be important mediators of diabetes-accompanied brain dysfunction.

Phytoceramide shows neuroprotection and ameliorates scopolamine-induced memory impairment

The function and the role phytoceramide (PCER) and phytosphingosine (PSO) in the central nervous system has not been well studied. This study was aimed at investigating the possible roles of PCER and PSO in glutamate-induced neurotoxicity in cultured neuronal cells and memory function in mice. Phytoceramide showed neuro-protective activity in the glutamate-induced toxicity in cultured cortical neuronal cells. Neither phytosphingosine nor tetraacetylphytosphingosine (TAPS) showed neuroproective effects in neuronal cells. PCER (50 mg/kg, p.o.) recovered the scopolamine-induced reduction in step-through latency in the passive avoidance test; however, PSO did not modulate memory function on this task. The ameliorating effects of PCER on spatial memory were confirmed by the Morris water maze test. In conclusion, through behavioral and neurochemical experimental results, it was demonstrated that central administration of PCER produces amelioration of memory impairment. These results suggest that PCER plays an important role in neuroprotection and memory enhancement and PCER could be a potential new therapeutic agent for the treatment of neurodegenerative diseases such as Alzheimer's disease.

Important roles of brain-specific carnitine palmitoyltransferase and ceramide metabolism in leptin hypothalamic control of feeding

Brain-specific carnitine palmitoyltransferase-1 (CPT-1c) is implicated in CNS control of food intake. In this article, we explore the role of hypothalamic CPT-1c in leptin's anorexigenic actions. We first show that adenoviral overexpression of CPT-1c in hypothalamic arcuate nucleus of rats increases food intake and concomitantly up-regulates orexigenic neuropeptide Y (NPY) and Bsx (a transcription factor of NPY). Then, we demonstrate that this overexpression antagonizes the anorectic actions induced by central leptin or compound cerulenin (an inhibitor of fatty acid synthase). The overexpression of CPT-1c also blocks leptin-induced down-regulations of NPY and Bsx. Furthermore, the anorectic actions of central leptin or cerulenin are impaired in mice with brain CPT-1c deleted. Both anorectic effects require elevated levels of hypothalamic arcuate nucleus (Arc) malonyl-CoA, a fatty acid-metabolism intermediate that has emerged as a mediator in hypothalamic control of food intake. Thus, these data suggest that CPT-1c is implicated in malonyl-CoA action in leptin's hypothalamic anorectic signaling pathways. Moreover, ceramide metabolism appears to play a role in leptin's central control of feeding. Leptin treatment decreases Arc ceramide levels, with the decrease being important in leptin-induced anorectic actions and down-regulations of NPY and Bsx. Of interest, our data indicate that leptin impacts ceramide metabolism through malonyl-CoA and CPT-1c, and ceramide de novo biosynthesis acts downstream of both malonyl-CoA and CPT-1c in mediating their effects on feeding and expressions of NPY and Bsx. In summary, we provide insights into the important roles of malonyl-CoA, CPT-1c, and ceramide metabolism in leptin's hypothalamic signaling pathways.

Differential effects of lysophospholipids on exocytosis in rat PC12 cells

Secretory phospholipase A2 (sPLA2) activity is present in the CNS and the sPLA2-IIA isoform has been shown to induce exocytosis in cultured hippocampal neurons. However, little is known about possible contributions of various lysophospholipid species to exocytosis in neuroendocrine cells. This study was therefore carried out to examine the effects of several lysophospholipid species on exocytosis on rat pheochromocytoma-12 (PC12) cells. An increase in vesicle fusion, indicating exocytosis, was observed in PC12 cells after external infusion of lysophosphatidylinositol (LPI), but not lysophosphatidylcholine or lysophosphatidylserine by total internal reflection microscopy. Similarly, external infusion of LPI induced significant increases in capacitance, or number of spikes detected at amperometry, indicating exocytosis. Depletion of cholesterol by pre-incubation of cells with methyl beta cyclodextrin and depletion of Ca2+ by thapsigargin and incubation in zero external Ca2+ resulted in attenuation of LPI induced exocytosis, indicating that exocytosis was dependent on the integrity of lipid rafts and intracellular Ca2+. Moreover, LPI induced a rise in intracellular Ca2+ suggesting that this could be the trigger for exocytosis. It is postulated that LPI may be an active participant in sPLA2-mediated exocytosis in the CNS.

Roles of cholesterol in vesicle fusion and motion

Although it is well established that exocytosis of neurotransmitters and hormones is highly regulated by numerous secretory proteins, such as SNARE proteins, there is an increasing appreciation of the importance of the chemophysical properties and organization of membrane lipids to various aspects of the exocytotic program. Based on amperometric recordings by carbon fiber microelectrodes, we show that deprivation of membrane cholesterol by methyl-beta-cyclodextrin not only inhibited the extent of membrane depolarization-induced exocytosis, it also adversely affected the kinetics and quantal size of vesicle fusion in neuroendocrine PC12 cells. In addition, total internal fluorescence microscopy studies revealed that cholesterol depletion impaired vesicle docking and trafficking, which are believed to correlate with the dynamics of exocytosis. Furthermore, we found that free cholesterol is able to directly trigger vesicle fusion, albeit with less potency and slower kinetics as compared to membrane depolarization stimulation. These results underscore the versatile roles of cholesterol in facilitating exocytosis.

Mu-opioid receptor activation in live cells

Interaction of the mu-opioid receptor (MOP) with selected ligands was investigated in live cells using advanced imaging by confocal laser scanning microscopy integrated with fluorescence correlation spectroscopy and fluorescence cross-correlation spectroscopy. In PC12 cells stably transformed to express the fluorescently labeled MOP-enhanced green fluorescent protein construct, two pools of MOP were identified that could be discriminated by differences in their lateral mobility in the cell membrane. The majority of MOP receptors (80+/-10%) were characterized by a diffusion coefficient D(MOP,1) = (4+/-2) x 10(-11) m(2) s(-1), compared with the slowly moving fraction, D(MOP,2) = (4+/-2) x 10(-12) m(2) s(-1). On stimulation with selected agonists ([D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin, enkephalin-heptapeptide Tyr-Gly-Gly-Phe-Met-Arg-Phe, morphine, and methadone), surface density of the MOP decreased, whereas the lateral mobility increased. In contrast, antagonists (naloxone and naltrexone) "froze" the receptor in the membrane, i.e., increased MOP surface density and decreased lateral mobility. Agonist activation was also accompanied by pronounced changes in the dynamics of plasma membrane lipids, as revealed by the general lipid marker 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate dye. The results provide new information about MOP activation in live cells at the molecular level, with a special focus on the dynamics of the intricate interplay between this receptor and the surrounding lipids.

Long-chain ceramide is elevated in presenilin 1 (PS1M146V) mouse brain and induces apoptosis in PS1 astrocytes

The pro-apoptotic sphingolipid ceramide plays an emergent role in the etiology of Alzheimer's disease (AD), although its function for neurodegeneration is not known. We determined the concentration and composition of ceramide in hippocampal tissue from newborn presenilin 1 (PS1) knock-in (PS1M146V) mice, a mouse model for early-onset familial AD. We found that PS1 tissue contains 3.1 (+/-0.5)-fold more total ceramide than wild-type tissue. In particular, the proportion of C20 and C24 ceramide is increased by 4.0- or 8.5-fold, respectively. The ceramide elevation in PS1 brain is consistent with a 3.7 (+/-0.5)-fold increase of the protein level of the neurotrophin receptor p75NTR, which has been suggested to stimulate the hydrolysis of sphingomyelin to generate ceramide. The predominance of C20 and C24 ceramide is concurrent with the elevated gene expression of lass 2 and lass 4, two isoforms of ceramide synthase that generate dihydroceramide with long-chain fatty acid. Our study indicates that primary cultured astrocytes but not neurons from PS1 mice undergo apoptosis when incubated with C20 ceramide. In contrast, wild-type astrocytes remain unaffected. The sensitivity of PS1 astrocytes is most likely due to the 9.5 (+/-0.4)-fold elevated expression of PAR-4 (prostate apoptosis response-4), a protein that inhibits atypical PKC zeta/lambda in the presence of ceramide. Our results suggest that astroglial death due to ceramide/PAR-4-induced apoptosis may critically contribute to the etiology of AD.