Sphingolipid trafficking — sorted out?

The chlamydial inclusion preferentially intercepts basolaterally directed sphingomyelin-containing exocytic vacuoles

Chlamydiae replicate intracellularly within a unique vacuole termed the inclusion. The inclusion circumvents classical endosomal/lysosomal pathways but actively intercepts a subset of Golgi-derived exocytic vesicles containing sphingomyelin (SM) and cholesterol. To further examine this interaction, we developed a polarized epithelial cell model to study vectoral trafficking of lipids and proteins to the inclusion. We examined seven epithelial cell lines for their ability to form single monolayers of polarized cells and support chlamydial development. Of these cell lines, polarized colonic mucosal C2BBe1 cells were readily infected with Chlamydia trachomatis and remained polarized throughout infection. Trafficking of (6-((N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl) amino)hexanoyl)sphingosine) (NBD-C(6)-ceramide) and its metabolic derivatives, NBD-glucosylceramide (GlcCer) and NBD-SM, was analyzed. SM was retained within L2-infected cells relative to mock-infected cells, correlating with a disruption of basolateral SM trafficking. There was no net retention of GlcCer within L2-infected cells and purification of C. trachomatis elementary bodies from polarized C2BBe1 cells confirmed that bacteria retained only SM. The chlamydial inclusion thus appears to preferentially intercept basolaterally-directed SM-containing exocytic vesicles, suggesting a divergence in SM and GlcCer trafficking. The observed changes in lipid trafficking were a chlamydia-specific effect because Coxiella burnetii-infected cells revealed no changes in GlcCer or SM polarized trafficking.

Sorting behavior of a transgenic erythropoietin-growth hormone fusion protein in murine salivary glands

Salivary glands are useful gene transfer target sites for the production of therapeutic proteins, and can secrete proteins into both saliva and the bloodstream. The mechanisms involved in this differential protein sorting are not well understood, although it is believed, at least in part, to be based on the amino acid sequence of the encoded protein. We hypothesized that a transgenic protein, human erythropoietin (hEpo), normally sorted from murine salivary glands into the bloodstream, could be redirected into saliva by fusing it with human growth hormone (hGH). After transfection, the hEpo-hGH fusion protein was expressed and glycosylated in both HEK 293 and A5 cells. When packaged in an adenovirus serotype 5 vector and delivered to murine submandibular cells in vivo via retroductal cannulation, the hEpo-hGH fusion protein was also expressed, albeit at approximately 26% of the levels of hEpo expression. Importantly, in multiple experiments with different cohorts of mice, the hEpo-hGH fusion protein was sorted more frequently into saliva, versus the bloodstream, than was the hEpo protein (p < 0.001). These studies show it is possible to redirect the secretion of a transgenic constitutive pathway protein from salivary gland cells after gene transfer in vivo, a finding that may facilitate developing novel treatments for certain upper gastrointestinal tract disorders.

The significance of caveolin-1 expression in parietal epithelial cells of Bowman's capsule

AIMS

To analyse the expression of caveolin-1 in normal human kidney and during diseases leading to nephrotic syndrome in children and to compare its pattern with those observed in control samples, both human and animal.

METHODS AND RESULTS

The study group was composed of 104 children diagnosed with minimal change disease (MCD), focal segmental glomerulosclerosis (FSGS), lupus glomerulonephritis (LGN) and Schönlein-Henoch glomerulopathy (SH). The research protocol employed direct immunohistochemical assay with the use of mono- and polyclonal antibodies against caveolins. Kidney samples of Wistar rats, wild-type mice and caveolin-1-deficient mice were also analysed. In the control human samples, caveolin-1 was most abundant in the muscle layer of blood vessels and parietal epithelial cells (PECs). Its expression in PECs was significantly lower in children diagnosed with FSGS and LGN than in those with MCD, SH or in controls. In the control animal tissues, except for knock-out mice, caveolin-1 was present in distal convoluted tubules, PECs, endothelial cells and muscle.

CONCLUSIONS

Caveolae are extremely stable elements of PECs and can be excluded from their cell membrane only in response to the dramatic cell reconstruction observed in FSGS and LGN.

An Efficient Synthesis of d-erythro- and d-threo-Sphingosine from d-Glucose: Olefin Cross-Metathesis Approach

[reaction: see text] The D-erythro- and D-threo-sphingosine were synthesized via E-selective olefin cross-metathesis using a D-glucose-derived building block and long-chain terminal alkene.

Role of Caveolae and Caveolins in Health and Disease

Although they were discovered more than 50 years ago, caveolae have remained enigmatic plasmalemmal organelles. With their characteristic “flasklike” shape and virtually ubiquitous tissue distribution, these interesting structures have been implicated in a wide range of cellular functions. Similar to clathrin-coated pits, caveolae function as macromolecular vesicular transporters, while their unique lipid composition classifies them as plasma membrane lipid rafts, structures enriched in a variety of signaling molecules. The caveolin proteins (caveolin-1, -2, and -3) serve as the structural components of caveolae, while also functioning as scaffolding proteins, capable of recruiting numerous signaling molecules to caveolae, as well as regulating their activity. That so many signaling molecules and signaling cascades are regulated by an interaction with the caveolins provides a paradigm by which numerous disease processes may be affected by ablation or mutation of these proteins. Indeed, studies in caveolin-deficient mice have implicated these structures in a host of human diseases, including diabetes, cancer, cardiovascular disease, atherosclerosis, pulmonary fibrosis, and a variety of degenerative muscular dystrophies. In this review, we provide an in depth summary regarding the mechanisms by which caveolae and caveolins participate in human disease processes.

Inhibition of sphingolipid synthesis: effects on glycosphingolipid-GPI-anchored protein microdomains

The idea that the transport and sorting of glycosylphosphatidylinositol (GPI)-anchored proteins depends on their interaction with glycosphingolipids was first proposed five or six years ago. Until recently, only circumstantial evidence was available to support this suggestion. During the past year, compelling support for this hypothesis has been provided by observations that inhibition of sphingolipid synthesis reduces the rate of transport of GPI-anchored proteins in yeast, and abolishes the polarized sorting of a GPI-anchored protein in epithelia.

HIV-1 gp41 envelope residues 650-685 exposed on native virus act as a lectin to bind epithelial cell galactosyl ceramide

The initial step in the interaction between human immunodeficiency virus (HIV-1) and epithelial cells is the binding of HIV-1 envelope glycoproteins to the epithelial cell galactosyl ceramide (GalCer). Here we show that HIV-1 envelope gp41 residues 650-685 bind GalCer in a galactose-specific manner. The gp41 residues that display this lectin activity are highly conserved among HIV-1 isolates and constitute three regions: residues 650-661, which encompass a charged helix; residues 662-667, referred to as the conserved epitope ELDKWA, the epitope recognized by antibodies that neutralize HIV-1 entry in epithelial and CD4(+)-mononucleated cells; and residues 668-685, a hydrophobic Trp-rich sequence that stabilizes the structure of the galactose binding site. Similar to other galactose-specific lectins, the gp41 lectin site is active only as an oligomer. Finally the orientation of the galactose toward the gp41 lectin site appears to be controlled by the lipid microenvironment of the epithelial membrane. From the experimental data we construct a theoretical model of the interaction between gp41 and GalCer based on thermodynamic considerations. This model integrates the dynamics and the spatial organization of the viral envelope glycoproteins, GalCer organized in raft microdomains in the apical region of the epithelial cell membrane and the interfacial water. Characterization of the minimal sequence and structure of gp41 in direct interaction with GalCer may help unravel the still unknown immunogenic determinant able to elicit antibodies against ELDKWA and target of one of the rare neutralizing antibodies against gp41.

Cholesterol depletion of enterocytes. Effect on the Golgi complex and apical membrane trafficking

Intestinal brush border enzymes, including aminopeptidase N and sucrase-isomaltase, are associated with "rafts" (membrane microdomains rich in cholesterol and sphingoglycolipids). To assess the functional role of rafts in the present work, we studied the effect of cholesterol depletion on apical membrane trafficking in enterocytes. Cultured mucosal explants of pig small intestine were treated for 2 h with the cholesterol sequestering agent methyl-beta-cyclodextrin and lovastatin, an inhibitor of hydroxymethylglutaryl-coenzyme A reductase. The treatment reduced the cholesterol content >50%. Morphologically, the Golgi complex/trans-Golgi network was partially transformed into numerous 100-200 nm vesicles. By immunogold electron microscopy, aminopeptidase N was localized in these Golgi-derived vesicles as well as at the basolateral cell surface, indicating a partial missorting. Biochemically, the rates of the Golgi-associated complex glycosylation and association with rafts of newly synthesized aminopeptidase N were reduced, and less of the enzyme had reached the brush border membrane after 2 h of labeling. In contrast, the basolateral Na(+)/K(+)-ATPase was neither missorted nor raft-associated. Our results implicate the Golgi complex/trans-Golgi network in raft formation and suggest a close relationship between this event and apical membrane trafficking.

Polarized secretion of transgene products from salivary glands in vivo

Previously (Kagami et al. Hum. Gene Ther. 1996;7:2177-2184) we have shown that salivary glands are able to secrete a transgene-encoded protein into serum as well as saliva. This result and other published data suggest that salivary glands may be a useful target site for vectors encoding therapeutic proteins for systemic delivery. The aim of the present study was to assess in vivo if transgene-encoded secretory proteins follow distinct, polarized sorting pathways as has been shown to occur "classically" in cell biological studies in vitro. Four first-generation, E1-, type 5 recombinant adenoviruses were used to deliver different transgenes to a rat submandibular cell line in vitro or to rat submandibular glands in vivo. Subsequently, the secretory distribution of the encoded proteins was determined. Luciferase, which has no signal peptide, served as a cell-associated, negative control and was used to correct for any nonspecific secretory protein release from cells. The three remaining transgene products tested, human tissue kallikrein (hK1), human growth hormone (hGH), and human alpha1-antitrypsin (halpha1AT), were predominantly secreted (>96%) in vitro. Most importantly, in vivo, after a parasympathomimetic secretory stimulus, both hK1 and hGH were secreted primarily in an exocrine manner into saliva. Conversely, halpha1AT was predominantly secreted into the bloodstream, i.e., in an endocrine manner. The aggregate results are consistent with the recognition of signals encoded within the transgenes that result in specific patterns of polarized protein secretion from rat submandibular gland cells in vivo.

Sphingolipids and cell signalling

The sphingolipid storage disorders constitute a group of inherited metabolic disorders in which the structure of the stored sphingolipid and the corresponding genetic defect have been established. However, the pathological mechanism(s) behind the disorders has not been fully elucidated. Sphingolipids are known to be recognition molecules involved in intercell communication and altered expression might lead to dyscommunication. The impaired degradation and lysosomal accumulation of specific sphingolipids might influence the metabolism of other molecules and/or intracellular transport, which in turn might alter the distribution of these molecules. However, the progress of these diseases indicates that additional factors, besides the stored sphingolipid itself, might be involved. During the last decade, several sphingolipids have emerged as active participants in intracellular signalling processes such as growth control and apoptosis. Particular interest focused on the sphingolipid metabolites, ceramide and sphingosine, as potential mediators in intracellular events and an altered presence of these metabolites in sphingolipidoses cannot be ruled out. Some sphingolipids have been found to influence cytokine release and thereby might induce immunological processes, which are known to exist in at least one of the sphingolipidoses--Gaucher disease. These processes might already have a pathogenic effect during early development, before significant storage has occurred.

Protein secretion by rat parotid acinar cells. Pathways and regulation

Protein secretion from rat parotid acinar cells occurs in both the absence and presence of secretory agonists. Release takes place by four pathways that are distinguished by combined examination of their timing following biosynthetic labeling, their relative composition of salivary proteins, and their sensitivity to secretagogue stimulation. Following pulse-labeling with a radioactive amino acid, two unstimulated export pathways are detected--a constitutive-like pathway that is coupled to maturation of secretory granules and the later unstimulated exocytosis of secretory granules. In both cases, protein release is insensitive to secretory antagonists. Two regulated secretory pathways are also detected. The major regulated pathway comprises stimulated exocytosis of secretory granules and requires application of beta-adrenergic agonists (> or = 1 microM). A newly discovered minor regulated pathway resembles the constitutive-like pathway in secretory composition but requires low-dose stimulation by either beta-adrenergic or cholinergic agonists. The latter pathway may provide a significant component of basal secretion by the parotid gland during periods between meals.

Functional involvement of proteins, interacting with sphingolipids, in sphingolipid transport to the canalicular membrane in the human hepatocytic cell line, HepG2?

A photoreactive sphingolipid precursor was used to investigate the potential involvement of protein-lipid interactions that may convey specificity to sphingolipid transport in the human hepatoma cell line, HepG2. A 125I-labeled, photoreactive ceramide, 125I-N3-Cer, was incubated with the cells and became incorporated into two sphingolipid products. The major product was photoreactive sphingomyelin (125I-N3-SM) (25% of total radioactivity), while only minor amounts of photoreactive glucosylceramide (125I-N3-GlcCer) were formed (< 2%). After photoactivation, a restricted number of proteins was labeled. Given the absolute amounts of the newly synthesized, photoreactive lipids and their precursor present in the cells, labeling of the proteins can be assumed to be derived from interaction with either ceramide (Cer) or sphingomyelin (SM), or both. To discriminate between these possibilities, photoactivation and protein analysis was performed in cells treated with D-threo-1-phenyl-2-decanoyl amino-3-morpholino-1-propanol (PDMP), an inhibitor of sphingolipid biosynthesis. In treated cells, the radioactive SM pool was reduced by approximately 80%. Concomitantly, labeling of a 60-kd protein, seen in control cells, decreased. Furthermore, the 60-kd protein is membrane-associated and insoluble in detergent at low temperature. Moreover, when cells containing photoreactive sphingolipids after a preincubation with the photoreactive Cer were photoactivated and subsequently incubated with fluorescent sphingolipid analogs, transport of the latter to the bile canalicular membrane, as observed in control cells, was inhibited. Taken together, the data suggest that distinct proteins, among them a 60-kd protein, may play a specific and functional role in sphingolipid transport to the bile canalicular membrane.

Inhibition of cellular uptake of folate by blocking synthesis of the membrane folate receptor

The folate receptor, bound to the plasma membrane through a glycosylphosphatidylinositol anchor, requires both sphingolipids and cholesterol in the membrane for full activity. In recent studies, treatment of cells in culture with the mycotoxin fumonisin B1, which inhibits sphingolipid synthesis, virtually abolished uptake of 5-methyltetrahydrofolate.

Cellular pathology of lysosomal storage disorders

Lysosomal storage disorders are rare, inborn errors of metabolism characterized by intralysosomal accumulation of unmetabolized compounds. The brain is commonly a central focus of the disease process and children and animals affected by these disorders often exhibit progressively severe neurological abnormalities. Although most storage diseases result from loss of activity of a single enzyme responsible for a single catabolic step in a single organelle, the lysosome, the overall features of the resulting disease belies this simple beginning. These are enormously complex disorders with metabolic and functional consequences that go far beyond the lysosome and impact both soma-dendritic and axonal domains of neurons in highly neuron type-specific ways. Cellular pathological changes include growth of ectopic dendrites and new synaptic connections and formation of enlargements in axons far distant from the lysosomal defect. Other storage diseases exhibit neuron death, also occurring in a cell-selective manner. The functional links between known molecular genetic and enzyme defects and changes in neuronal integrity remain largely unknown. Future studies on the biology of lysosomal storage diseases affecting the brain can be anticipated to provide insights not only into these pathogenic mechanisms, but also into the role of lysosomes and related organelles in normal neuron function.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Surface membrane polarity of proximal tubular cells: alterations as a basis for malfunction

The surface membrane of proximal tubular cells is organized into distinct apical and basolateral membrane domains. The establishment and maintenance of these biochemically, structurally and physiologically distinct domains involves a multi-stage process involving cell-cell, cell-ECM interactions, and polarized targeting mechanisms. Ischemia, via cellular ATP depletion, results in a series of structural, biochemical and functional alterations that lead to loss of proximal tubular cell surface membrane polarity. Of central importance is the rapidly-occurring, duration-dependent disruption and dissociation of the actin cytoskeleton and associated surface membrane structures. This results in numerous cellular alterations including loss of cell-cell contact, cell-extracellular matrix adhesion and surface membrane polarity. Redistribution of surface membrane proteins and lipids into the alternate domain results in the cells inability to function properly. Repair of these disorders involves re-establishment of the actin cytoskeleton and apical and basolateral surface membrane domains. Recent information indicates growth factors may play a role in hastening this repair process.

A novel class of cell surface glycolipids of mammalian cells. Free glycosyl phosphatidylinositols

Glycosyl phosphatidylinositol (GPI) lipids function as anchors of membrane proteins, and free GPI units serve as intermediates along the path of GPI-anchor biosynthesis. By using in vivo cell surface biotinylation, we show that free GPIs: 1) can exit the rough endoplasmic reticulum and are present on the surface of a murine EL-4 T-lymphoma and a human carcinoma cell (HeLa), 2) arrive at the cell surface in a time and temperature-dependent fashion, and 3) are built on a base-labile glycerol backbone, unlike GPI anchors of surface proteins of the same cells. The free GPIs described in this study may serve as a source of hormone-sensitive phosphoinositol glycans. The absence of free GPIs from the cell surface may also account for the growth advantage of blood cells in paroxysmal nocturnal hemoglobinuria.

Conjugation rescue of exocytosis mutants in Tetrahymena thermophila indicates the presence of functional intermediates in the regulated secretory pathway

Tetrahymena thermophila possesses a regulated secretory pathway in which mucin proteins are stored in dense-core granules, called mucocysts. Exocytosis-defective mutants exist that fail to secrete mucin in response to secretagogues. Four of the mutants (SB281, SB283, SB285 and SB715) appear to be blocked at different steps of the regulated secretory pathway. SB281 and SB285 accumulate mucin proteins in heterogeneous cytoplasmic organelles which have not yet been identified; SB283 makes mucocyst-like structures but they contain no immunologically identifiable 80-kDa or 50-kDa mucin proteins; and SB715 has more than normal amounts of immature and undocked mucocysts. The organelles that accumulate in exocytosis-defective mutants could be either normal intermediates in the biosynthetic pathway or aberrant structures that form as a result of the mutations. We have used conjugation rescue to analyze steps in the biogenesis of exocytosis-competent mucocysts and to identify functional intermediates. The cytoplasmic organelles that accumulate in SB281 appear to be unidentified biosynthetic intermediates, and the defect is in a cytosolic protein essential for mucocyst maturation. The organelles which accumulate in the other mutants are likely biosynthetic, but their mutations are in proteins which are labile or not free to diffuse into the mutant conjugant.

Ethanol attenuation of ganglioside sialylation and neuritogenesis

Neurite-producing cultured embryonic chick brain neurons in 96-h culture were exposed chronically to 25 mM ethanol for a 72-h period. Neurital plasma membrane extension was markedly attenuated in the ethanol-exposed neurons as compared with ethanol-free control cultures. The rate of biosynthetic sialylation of gangliosides, which are major structural sialoglycosphingolipid components in the exofacial lipid bilayer of the neurital plasma membrane, decreased to about half that in the alcohol-free neurons. The findings show that ganglioside sialylation in the morphodifferentiating central nervous system neuron is significantly attenuated by chronic exposure to a moderate dose of ethanol. Depression of ganglioside-dependent neuritogenesis may represent a mechanism for development of the central nervous system component of the fetal alcohol syndrome.

Is plasma membrane lipid composition defined in the exocytic or the endocytic pathway?

Compared with intracellular membranes, the plasma membrane is rich in cholesterol and sphingomyelin. How does this distinct composition arise? Here David Allan and Karl-Josef Kallen take a critical view of the belief that these lipids arrive at the plasma membrane via vesicular traffic from the Golgi complex and propose instead that they may be accreted in the endocytic recycling pathway.

Transport and sorting of membrane lipids

The lipid composition of cellular membranes may seem unnecessarily complex. However, the lipid composition of each membrane is carefully regulated by local metabolism and specificity in transport, marking the functional significance for the cell. Recent research has revealed unexpected discoveries concerning the topology of lipid synthesis, specificity in lipid transport, and the function of lipid and protein microdomains in sorting.

Glycosphingolipid-enriched, detergent-insoluble complexes in protein sorting in epithelial cells

In simple epithelial cells, the delivery of apical and basolateral proteins to the cell surface is mediated by sorting in the trans-Golgi network and transport via separate vesicular carriers. In order to identify the molecular machinery involved in protein sorting, we have recently studied a detergent-insoluble complex in Madin-Darby canine kidney (MDCK) cells, following CHAPS extraction of exocytic carrier vesicles, specifically including the apical marker protein influenza hemagglutinin (HA). Previously, a Triton X-100 insoluble membrane residue that was enriched in glycosylphosphatidylinositol-anchored (GPI) proteins and glycolipids was characterized and implicated in transport to the apical cell surface [Brown, D., & Rose, J. (1991) Cell 68, 533-544]. In this report, the protein compositions of the CHAPS and Triton complexes have been compared by two-dimensional gel analysis. Only a few major membrane proteins are found in the complexes. The protein compositions are qualitatively similar, but differ quantitatively in the individual components. The CHAPS complex is depleted of GPI-linked proteins and retains a minor fraction of lipids similar in composition to that of the Triton X-100 insoluble complex. We propose that in vivo the complexes form part of a sorting platform that mediates protein segregation and delivery to the apical cell surface.

Monensin inhibits synthesis of plasma membrane sphingomyelin by blocking transport of ceramide through the Golgi: evidence for two sites of sphingomyelin synthesis in BHK cells

The monovalent cationophore monensin, which is known to interfere with vesicular transport through the Golgi apparatus, inhibits synthesis of sphingomyelin in BHK cells by up to 40%. The monensin-sensitive component of sphingomyelin synthesis appears to be the pool which normally reaches the cell surface since treatment of cells with exogenous sphingomyelinase causes an almost identical loss of sphingomyelin. Monensin causes increases in ceramide and glucosylceramide labelling which together are equivalent to the decrease in sphingomyelin labelling. Monensin also increases synthesis of cholesterol ester, probably due to the decreased delivery of sphingomyelin to the plasma membrane. However, monensin has no effect on resynthesis of plasma membrane sphingomyelin which has been degraded by extracellular sphingomyelinase. The results support the idea that synthesis of sphingomyelin destined for the plasma membrane does not occur in the cis- or medial-Golgi but depends on vesicular transport of ceramide to a second synthesis site which is distal to the medial-Golgi.