Endogenous substrates of sphingosine-dependent kinases (SDKs) are chaperone proteins: heat shock proteins, glucose-regulated proteins, protein disulfide isomerase, and calreticulin

Trifunctional Sphinganine: A New Tool to Dissect Sphingolipid Function

Functions and cell biology of the sphingolipids sphingosine and sphinganine in cells are not well understood. While some signaling roles for sphingosine have been elucidated, the closely related sphinganine has been described only insofar as it does not elicit many of the same signaling responses. Here, we prepared multifunctionalized derivatives of the two lipid species that differ only in a single double bond of the carbon backbone. Using these novel probes, we were able to define their spatiotemporal distributions within cells. Furthermore, we used these tools to systematically map the protein interactomes of both lipids. The lipid-protein conjugates, prepared through photo-crosslinking in live cells and extraction via click chemistry to azide beads, revealed significant differences in the captured proteins, highlighting their distinct roles in various cellular processes. This work elucidates mechanistic differences between these critical lipids and sets the foundation for further studies of the cellular functions of sphingosine and sphinganine.

The dynamics and role of sphingolipids in eukaryotic organisms upon thermal adaptation

All living beings have an optimal temperature for growth and survival. With the advancement of global warming, the search for understanding adaptive processes to climate changes has gained prominence. In this context, all living beings monitor the external temperature and develop adaptive responses to thermal variations. These responses ultimately change the functioning of the cell and affect the most diverse structures and processes. One of the first structures to detect thermal variations is the plasma membrane, whose constitution allows triggering of intracellular signals that assist in the response to temperature stress. Although studies on this topic have been conducted, the underlying mechanisms of recognizing thermal changes and modifying cellular functioning to adapt to this condition are not fully understood. Recently, many reports have indicated the participation of sphingolipids (SLs), major components of the plasma membrane, in the regulation of the thermal stress response. SLs can structurally reinforce the membrane or/and send signals intracellularly to control numerous cellular processes, such as apoptosis, cytoskeleton polarization, cell cycle arresting and fungal virulence. In this review, we discuss how SLs synthesis changes during both heat and cold stresses, focusing on fungi, plants, animals and human cells. The role of lysophospholipids is also discussed.

Mammalian sphingoid bases: Biophysical, physiological and pathological properties

Sphingoid bases encompass a group of long chain amino alcohols which form the essential structure of sphingolipids. Over the last years, these amphiphilic molecules were moving more and more into the focus of biomedical research due to their role as bioactive molecules. In fact, free sphingoid bases interact with specific receptors and target molecules and have been associated with numerous biological and physiological processes. In addition, they can modulate the biophysical properties of biological membranes. Several human diseases are related to pathological changes in the structure and metabolism of sphingoid bases. Yet, the mechanisms underlying their biological and pathophysiological actions remain elusive. Within this review, we aimed to summarize the current knowledge on the biochemical and biophysical properties of the most common sphingoid bases and to discuss their importance in health and disease.

Mammalian sphingoid bases: Biophysical, physiological and pathological properties

Sphingoid bases encompass a group of long chain amino alcohols which form the essential structure of sphingolipids. Over the last years, these amphiphilic molecules were moving more and more into the focus of biomedical research due to their role as bioactive molecules. In fact, free sphingoid bases interact with specific receptors and target molecules, and have been associated with numerous biological and physiological processes. In addition, they can modulate the biophysical properties of biological membranes. Several human diseases are related to pathological changes in the structure and metabolism of sphingoid bases. Yet, the mechanisms underlying their biological and pathophysiological actions remain elusive. Within this review, we aimed to summarize the current knowledge on the biochemical and biophysical properties of the most common sphingoid bases and to discuss their importance in health and disease.

Chaperone proteins identified from synthetic proteasome inhibitor-induced inclusions in PC12 cells by proteomic analysis

Chaperone proteins are significant in Lewy bodies, but the profile of chaperone proteins is incompletely unraveled. Proteomic analysis is used to determine protein candidates for further study. Here, to identify potential chaperone proteins from agent-induced inclusions, we carried out proteomic analysis of artificially synthetic proteasome inhibitor (PSI)-induced inclusions formed in PC12 cells exposed to 10 microM PSI for 48 h. Using biochemical fractionation, 2-D electrophoresis, and identification through peptide mass fingerprints searched against multiple protein databases, we repeatedly identified eight reproducible chaperone proteins from the PSI-induced inclusions. Of these, 58 kDa glucose regulated protein, 75 kDa glucose regulated protein, and calcium-binding protein 1 were newly identified. The other five had been reported to be consistent components of Lewy bodies. These findings suggested that the three potential chaperone proteins might be recruited to PSI-induced inclusions in PC12 cells under proteasome inhibition.

Differential signaling of sphingosine derivatives in U937 human monocytes depends on the degree of N-methylation

Previously, we studied N,N-dimethyl-D-erythro-sphingosine (DMS)-induced cell death and signaling in U937 human monocytes; we found that DMS-induced sphingosine kinase- and PKC-independent apoptosis. In the present study, we studied apoptotic responses by three N-methyl derivatives of sphingosine: N-monomethyl-D-erythro-sphingosine (MMS), N,N,N-trimethyl-D-erythro-sphingosine (TMS), and D-erythro-sphingosine (SPH). The potency order in the apoptotic response was DMS>or=MMS>TMS>SPH. We compared cellular responses to the derivatives in terms of activities of MAPK signaling molecules, mitochondrial membrane potential (DeltaPsi(m)), and reactive oxygen species (ROS) generation. Our results suggest that the degree of N-methylation affects the apoptosis-inducing capacity and other related responses including MAPK modulation, DeltaPsi(m), and ROS generation. Dimethylation and monomethylation on the C2 amine of sphingosine enhance the apoptotic response; however, trimethylation induces differential modulation of signaling molecules and less cytotoxicity. Our investigation will be useful for understanding the actions of sphingolipids in apoptosis and for developing chemotherapeutics based on DMS structure.

Structure and function of glycosphingolipids and sphingolipids: recollections and future trends

Based on development of various methodologies for isolation and characterization of glycosphingolipids (GSLs), we have identified a number of GSLs with globo-series or lacto-series structure. Many of them are tumor-associated or developmentally regulated antigens. The major question arose, what are their functions in cells and tissues? Various approaches to answer this question were undertaken. While the method is different for each approach, we have continuously studied GSL or glycosyl epitope interaction with functional membrane components, which include tetraspanins, growth factor receptors, integrins, and signal transducer molecules. Often, GSLs were found to interact with other carbohydrates within a specific membrane microdomain termed "glycosynapse", which mediates cell adhesion with concurrent signal transduction. Future trends in GSL and glycosyl epitope research are considered, including stem cell biology and epithelial-mesenchymal transition (EMT) process.

Calreticulin in rainbow trout: a limited response to endoplasmic reticulum (ER) stress

Calreticulin (CRT) is a resident protein of the endoplasmic reticulum where it serves as a calcium modulator and chaperone to newly synthesized glycoproteins. In mammals, CRT is a structurally conserved 46 kDa protein that demonstrates anomalous migration at 60 kDa on SDS polyacrylamide gels and can be up-regulated by A23187 and thapsigargin due to the endoplasmic reticulum stress elements (ERSE) in the promoter region of its gene. CRT has numerous proposed functions and has been localized to the surface of PHA-stimulated T lymphocytes. CRT has been identified in mammals, plants and more recently from rainbow trout. Here, we report the cloning of the CRT proximal promoter from rainbow trout which includes elements typical of genes transcribed by RNA polymerase II including a TATA box, an Sp1 binding site, CCAAT boxes and the conservation of promoter stress elements (ERSE) demonstrated to be responsible for calcium modulation in mammals. This report demonstrates that the anomalous 60 kDa gel migration of mammalian CRT is conserved in rainbow trout and that CRT exists primarily as a dimer or oligomer in all tissues tested, excluding muscle and sperm in which it exists as a single polypeptide. Although it contains a potential N-glycosylation site, rainbow trout CRT is not subject to N-type glycosylation. Through the use of reverse transcriptase (RT) PCR along with western blotting, in both primary cultured leukocytes and the macrophage cell line RTS11, this report demonstrates that, unlike mammals, rainbow trout CRT is not strongly up-regulated by the calcium homeostasis antagonists, A23187 and thapsigargin, but is present on the cell surface of PHA-stimulated leukocytes. Taken together, this data suggests that CRT may have an alternative mode of regulation or function in fish.

Essential requirement for sphingosine kinase 2 in a sphingolipid apoptosis pathway activated by FTY720 analogues

The clinical immunosuppressant FTY720 is a sphingosine analogue that, once phosphorylated by sphingosine kinase 2 (Sphk2), is an agonist of multiple receptor subtypes for sphingosine 1-phosphate. Short exposures to FTY720 afford long term protection in lymphoproliferative and autoimmune disease models, presumably by inducing apoptosis in subsets of cells essential for pathogenesis. Sphingosine itself is pro-apoptotic, and apoptosis induced with FTY720 or sphingosine is thought to proceed independently of their phosphorylation. Following chemical mutagenesis of Jurkat cells we isolated mutants that are selectively resistant to FTY720 analogue AAL(R), as well as natural sphingolipid bases, including sphingosine. Cells lacking functional Sphk2 were resistant to apoptosis induced with AAL(R), indicating that apoptosis proceeds through AAL(R) phosphorylation. Phosphorylation of AAL(R) was also required for induction of lymphocyte apoptosis in mice, as apoptosis was not induced with the non-phosphorylatable chiral analogue, AAL(S). Apoptosis was induced in the spleen but not the thymus of mice administered 1 mg/kg AAL(R), correlating with levels of AAL(R)-phosphate (AFD(R)) in organ extracts. AFD(R) did not induce apoptosis when added to the cell culture medium, indicating that it induces apoptosis through an intracellular target. NBD-labeled AAL(R) localized to the endoplasmic reticulum, and AAL(R) treatment resulted in elevated cytosolic calcium, Bax redistribution from cytosol to mitochondrial and endoplasmic reticulum membranes, and caspase-independent mitochondrial permeabilization in Jurkat cells. We therefore describe an apoptotic pathway triggered by intracellular accumulation of sphingolipid base phosphates and suggest that sphingoid base substrates for Sphk2 acting intracellularly could be useful in the treatment of lymphoproliferative diseases.

A proteomic screen identified stress-induced chaperone proteins as targets of Akt phosphorylation in mesangial cells

The serine-threonine kinase Akt regulates mesangial cell apoptosis, proliferation, and hypertrophy. To define Akt signaling pathways in mesangial cells, we performed a functional proteomic screen for rat mesangial cell proteins phosphorylated by Akt. A group of chaperone proteins, heat shock protein (Hsp) 70, Hsp90alpha, Hsp90beta, Glucose-regulated protein (Grp) Grp78, Grp94, and protein disulfide isomerase (PDI) were identified as potential Akt substrates by two techniques: (a) in vitro phosphorylation of mesangial cell lysate by recombinant active Akt followed by protein separation by SDS-PAGE or 2-DE and phosphoprotein identification by peptide mass fingerprinting using MALDI-MS, or (b) immunoblot analysis of proteins from PDGF-stimulated mesangial cells using an anti-Akt phospho-motif antibody. In vitro kinase reactions using recombinant proteins confirmed that Akt phosphorylates Hsp70, Hsp90alpha and beta, Grp94, and PDI. Immunoprecipitation of Akt from mesangial cell lysate coprecipitated Grp78 and Hsp70. PDGF stimulation of mesangial cells caused an acidic shift in the isoelectric point of Hsp70, Hsp90, and PDI that was dependent on PI-3K activity for Hsp70 and Hsp90. The data suggest that Akt-mediated phosphorylation of stress-induced chaperones represents a mechanism for regulation of chaperone function during mesangial cell responses to physiologic and pathologic stimuli.

Extracellular disulfide exchange and the regulation of cellular function

An emerging concept is that disulfide bonds can act as a dynamic scaffold to present mature proteins in different conformational and functional states on the cell surface. Two examples are the conversion of the receptor, integrin alphaIIbbeta3, from a low affinity to a high affinity state, and the interaction of CD4 receptor with the HIV-1 envelope glycoprotein gp120 to promote virus-cell fusion. In both of these cases there is a remodeling of the protein disulfide bonding pattern. The formation and rearrangement of disulfide bonds is modulated by a family of enzymes known as the thiol isomerases, which include protein disulfide isomerase (PDI), ERp5, ERp57, and ERp72. While these enzymes were reported originally to be restricted in location to the endoplasmic reticulum, in some cells thiol isomerases are found on the cell surface. This may indicate a wider role for these enzymes in cell function. In platelets it has been shown that reagents that react with cell surface sulfhydryl groups are capable of blocking a number of functional responses, including integrin-mediated aggregation, adhesion, and granule secretion. Furthermore, the use of function blocking antibodies to either PDI or ERp5 causes inhibition of these functional responses. This review summarizes current knowledge of the extracellular regulation of disulfide exchange and the implications of this in the regulation of cell function.

Dimethylsphingosine Regulates Intracellular pH and Ca2+ in Human Monocytes

Dimethylsphingosine (DMS) was first reported as an inhibitor of protein kinase C and later has been used as a specific inhibitor of sphingosine kinase. Furthermore, its anti-cancer effect has become a basis for development of chemotherapy. Nevertheless, its anti-neoplastic mechanism has poorly been understood. In the present study, we observed that DMS increased intracellular pH and Ca(2+) concentration in U937 human monocytes. To further characterize these DMS-induced actions, we employed structurally-related sphingolipids and specific pharmacological tools such as inhibitors of protein kinase C and Na(+)/H(+) exchanger and found that the two responses of DMS were mimicked by four stereoisomers of sphingosine and two isomers to dihydrosphingosine, but not with sphingosine 1-phosphate, sphingosyl-phosphorylcholine, and C2-ceramide. Furthermore, DMS-induced pH increase was independent of Na(+)/H(+) exchanger activity. We also characterized the interrelationship between DMS-induced pH increase and DMS-induced Ca(2+) increase. Since DMS is considered to be a good anti-cancer candidate, our characterization of DMS actions provides useful information for development of DMS chemotherapy.

Sphingosine activates protein kinase A type II by a novel cAMP-independent mechanism

Protein kinase A (PKA) has long been recognized as playing a major role in many regulatory processes in cells through its activation by the ubiquitous second messenger cAMP. We show here a novel mode of activation of PKA type II that is independent of cAMP and is, instead, dependent on sphingosine. PKA type II is specifically activated by sphingosine and its analog, dimethylsphingosine, but not by sphingosine-1-phosphate or other lipids. Like cAMP, sphingosine activates PKA holoenzyme but not the catalytic subunit alone, suggesting that the activation is mediated by the regulatory subunits. However, sphingosine-activated PKA, but not cAMP-activated PKA, is inhibited by phosphatidylserine, suggesting a distinct mechanism of activation. Furthermore, unlike cAMP, sphingosine does not induce the dissociation of PKA holoenzyme into catalytic and regulatory subunits. Modulation of sphingosine levels in vivo results in alteration in basal membrane-associated PKA activity consistent with a direct effect of membrane sphingosine on PKA type II. Importantly, sphingosine-dependent but not cAMP-dependent activation of PKA specifically phosphorylates Ser58 of the multifunctional adapter protein 14-3-3zeta, promoting the conversion of dimeric 14-3-3 to a monomeric state, thus potentially modulating several biological functions. These results define a new mode of PKA activation that is sphingosine-dependent and mechanistically different from the classical cAMP-dependent activation of PKA. Furthermore, they suggest that stimuli that induce sphingosine accumulation and modulate phospholipid content at the cell membrane have the potential to activate PKA, thereby inducing the phosphorylation of distinct substrates and biological activities.

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.

Characterization of a novel type of endogenous activator of soluble guanylyl cyclase

Nitric oxide (NO) remains the only firmly established endogenous modulator of soluble guanylyl cyclase (sGC) activity, but physiological, structural, and biochemical evidence now suggests that in vivo regulation of sGC involves direct interaction with other factors. We searched for such endogenous modulators in human umbilical vein endothelial cells and COS-7 cells. The cytosolic fraction of both cell types stimulated the activity of semipurified sGC severalfold in the absence or presence of a saturating concentration of NO. The cytosolic factor was sensitive to proteinase K and destroyed by boiling, suggesting that it contains a protein component. Size exclusion chromatography revealed peaks of activity between 40 and 70 kDa. The sGC-activating effect was further purified by ion exchange chromatography. In the presence of the benzylindazole YC-1 or NO, the partially purified factor synergistically activated sGC, suggesting that this factor had a mode of activation different from that of YC-1 or NO. Four candidate activators were identified from the final purification step by matrix-assisted laser desorption ionization mass spectrometry analysis. Using an sGC affinity matrix, one of them, the molecular chaperone Hsp70, was shown to directly interact with sGC. This interaction was further confirmed by co-immunoprecipitation in lung tissues and by co-localization in smooth muscle cells. sGC and Hsp70 co-localized at the plasma membrane, supporting the idea that sGC can be translocated to the membrane. Hsp70 co-purifies with the sGC-activating effect, and immunodepletion of Hsp70 from COS-7 cytosol coincided with a marked attenuation of the sGC-activating effect, yet the effect was not rescued by the addition of pure Hsp70. Thus, Hsp70 is a novel sGC-interacting protein that is responsible for the sGC-activating effect, probably in association with other factors or after covalent modification.

Sphingosine-dependent apoptosis: a unified concept based on multiple mechanisms operating in concert

Exposure of 3T3/A31 cells to serum-free medium, one type of apoptotic stimulus, causes a rapid increase in the sphingosine (Sph) level, which initiates a series of processes: (i) activation of caspase 3 through an enhanced "cascade" of caspases, (ii) release of the C-terminal-half kinase domain of PKCdelta (PKCdelta KD) by caspase 3, and (iii) activation of Sph-dependent kinase 1 (SDK1), which was previously identified as PKCdelta KD. The activation of caspase 3 and release of PKCdelta KD are inhibited strongly by the incubation of cells with the ceramidase inhibitor D-erythro-2-tetradecanoylamino-1-phenyl-1-propanol and, to a much lesser extent, by L-cycloserine, an inhibitor of de novo ceramide synthesis. Exogenous addition of Sph or N,N-dimethyl-Sph to U937 cells causes caspase 3 activation and release of PKCdelta KD (SDK1), leading to apoptosis. The Sph-induced apoptotic process associated with activation of caspase 3 and release of PKCdelta KD (SDK1) may promote the proapoptotic effect of BAD or BAX through an increase of phosphorylated 14-3-3. In addition, Sph induces apoptosis through a separate process: the blocking of "survival signal" through the Akt kinase pathway induced by alpha3beta1-mediated cell adhesion to laminin 10/11 in extracellular matrix. We hereby propose a unified concept of Sph-dependent apoptosis based on these multiple mechanisms operating in concert.

Sphingosine-dependent protein kinase-1, directed to 14-3-3, is identified as the kinase domain of protein kinase C delta

Some protein kinases are known to be activated by d-erythro-sphingosine (Sph) or N,N-dimethyl-d-erythro-sphingosine (DMS), but not by ceramide, Sph-1-P, other sphingolipids, or phospholipids. Among these, a specific protein kinase that phosphorylates Ser60, Ser59, or Ser58 of 14-3-3beta, 14-3-3eta, or 14-3-3zeta, respectively, was termed "sphingosine-dependent protein kinase-1" (SDK1) (Megidish, T., Cooper, J., Zhang, L., Fu, H., and Hakomori, S. (1998) J. Biol. Chem. 273, 21834-21845). We have now identified SDK1 as a protein having the C-terminal half kinase domain of protein kinase Cdelta (PKCdelta) based on the following observations. (i). Large-scale preparation and purification of proteins showing SDK1 activity from rat liver (by six steps of chromatography) gave a final fraction with an enhanced level of an approximately 40-kDa protein band. This fraction had SDK1 activity approximately 50000-fold higher than that in the initial extract. (ii). This protein had approximately 53% sequence identity to the Ser/Thr kinase domain of PKCdelta based on peptide mapping using liquid chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry data. (iii). A search for amino acid homology based on the BLAST algorithm indicated that the only protein with high homology to the approximately 40-kDa band is the kinase domain of PKCdelta. The kinase activity of PKCdelta did not depend on Sph or DMS; rather, it was inhibited by these sphingoid bases, i.e. PKCdelta did not display any SDK1 activity. However, strong SDK1 activity became detectable when PKCdelta was incubated with caspase-3, which releases the approximately 40-kDa kinase domain. PKCdelta and SDK1 showed different lipid requirements and substrate specificity, although both kinase activities were inhibited by common PKC inhibitors. The high susceptibility of SDK1 to Sph and DMS accounts for their important modulatory role in signal transduction.

A sphingosine-dependent protein kinase that specifically phosphorylates 14-3-3 (SDK1) is identified as the kinase domain of PKCdelta: a preliminary note

A specific protein kinase that phosphorylates Ser60, Ser59, or Ser58 of 14-3-3beta, eta, or zeta, respectively, only in the presence of sphingosine (Sph) or N,N-dimethyl-Sph (DMS), was termed "sphingosine-dependent protein kinase-1" (SDK1) [J. Biol. Chem. 273(34) (1998) 21834]. We have now identified SDK1 as a protein having the same amino acid sequence as in the C-terminal-half kinase domain of PKCdelta, with approximately 40 kDa molecular mass, based on large-scale purification of a protein from rat liver, and partial sequence using three different combinations of LC-MS or LC-MS/MS with respective search engine. PKCdelta did not display any SDK1 activity and PKCdelta activity was inhibited by Sph and DMS. However, strong SDK1 activity, only in the presence of Sph or DMS, became detectable when PKCdelta was incubated with caspase-3, which releases the approximately 40 kDa kinase domain.

Sphingosine in apoptosis signaling

The sphingolipid metabolites ceramide, sphingosine, and sphingosine 1-phosphate contribute to controlling cell proliferation and apoptosis. Ceramide and its catabolite sphingosine act as negative regulators of cell proliferation and promote apoptosis. Conversely, sphingosine 1-phosphate, formed by phosphorylation of sphingosine by a sphingosine kinase, has been involved in stimulating cell growth and inhibiting apoptosis. As the phosphorylation of sphingosine diminishes apoptosis, while dephosphorylation of sphingosine 1-phosphate potentiates it, the role of sphingosine as a messenger of apoptosis is of importance. Herein, the effects of sphingosine on diverse signaling pathways implicated in the apoptotic process are reviewed.

Proteins of the PDI family: unpredicted non-ER locations and functions

Protein disulfide isomerases (PDIs) constitute a family of structurally related enzymes which catalyze disulfide bonds formation, reduction, or isomerization of newly synthesized proteins in the lumen of the endoplasmic reticulum (ER). They act also as chaperones, and are, therefore, part of a quality-control system for the correct folding of the proteins in the same subcellular compartment. While their functions in the ER have been thoroughly studied, much less is known about their roles in non-ER locations, where, however, they have been shown to be involved in important biological processes. At least three proteins of this family from higher vertebrates have been found in unusual locations (i.e., the cell surface, the extracellular space, the cytosol, and the nucleus), reached through an export mechanism which has not yet been understood. In some cases their function in the non-ER location is clearly related to their redox properties, but in most cases their mechanism of action has still to be disclosed, although their propensity to associate with other proteins or even with DNA might be the main factor responsible for their activities.

Conformational studies of a novel cationic glycolipid, glyceroplasmalopsychosine, from bovine brain by NMR spectroscopy

A novel glycosphingolipid containing a long chain aldehyde conjugated to galactose and glycerol, Gro1(3)-O-CH((CH(2))(n)CH(3))-O-6Galbeta-sphingosine (glyceroplasmalopsychosine) has been studied by NMR spectroscopy (Hikita et al. J. Biol. Chem. 2001, 276, 23084-23091). We further report here on the conformation showing the galactose and the glycerol at the end of two parallel hydrophobic chains, i.e. the sphingosine and the fatty aldehyde. This is proposed based on the interproton distances derived from ROESY experiments and 3 J (H,H) coupling constants. The absence of any intraresidual NOEs between protons in the glycerol residue suggested that the C-C-2 and C-C-3 bonds in the glycerol may be rotating freely, supporting the proposed conformation in which the unique terminal glycerol is in an environment with a minimal steric hindrance. The present study proposes a conformation of glyceroplasmalopsychosine greatly different from the two conventional plasmalopsychosines possessing a fatty aldehyde chain oriented in an opposite direction to the sphingosine.

Cationic glycosphingolipids in neuronal tissues and their possible biological significance

During the course of studies on natural occurrence of sphingosine base in brain, cationic glycosphingolipids bound to carboxymethyl-Sephadex and eluted with triethylamine in organic solvents were isolated and characterized. Four classes of compounds were identified: (i) plasmalopsychosine-A and -B; (ii) glyceroplasmalopsychosine; (iii) glycosphingolipids having de-N-acetyl-hexosamine, e.g., de-N-acetyl-Lc3Cer; (iv) glycosylsphingosine, i.e., lysoglycosphingolipid. Only two kinds, galactosylsphingosine (psychosine) and lactosylsphingosine, were found to occur naturally in brain. All these compounds were isolated from extract of brain white matter. Their occurrence, quantity, and distribution pattern differ from one species to another. Their quantity is much lower than that of regular acidic and neutral glycosphingolipids. They may interact with regular glycosphingolipids in glycosphingolipid-enriched microdomains to elicit signal transduction, to modify cellular phenotype, although studies along this line are highly limited at this time.

GRP94-associated enzymatic activities. Resolution by chromatographic fractionation

GRP94 (gp96), which performs established functions as a molecular chaperone and immune system modulator, has been reported to display a number of intrinsic enzymatic activities, including ATP hydrolysis, protein phosphorylation, and aminopeptidase. In observing that GRP94 co-purified with bacterial beta-galactosidase through multiple chromatographic steps, we have examined the hypothesis that the reported enzymatic activities of GRP94 may reflect co-purification of contaminant enzymes, rather than intrinsic catalytic functions. In subjecting GRP94 to increasingly stringent chromatographic purification, we report that a GRP94 carboxyl-terminal directed protein kinase activity could be separated from GRP94 by heparin affinity chromatography. Analysis of the kinase substrate specificity indicates that this kinase is distinct from casein kinase II, which is known to co-purify with GRP94. Electrophoretically pure GRP94 displayed low, but significant levels of aminopeptidase activity. Further purification of GRP94 by anion exchange and heparin affinity chromatography yielded resolution of GRP94 from the aminopeptidase activity. Furthermore, exhaustive trypsinolysis of GRP94 preparations displaying aminopeptidase activity yielded complete proteolysis of GRP94 but did not affect aminopeptidase activity. These results are discussed with respect to current models for GRP94 function and the role of such co-purifying (poly)peptides in the generation of GRP94-dependent cellular immune responses.

A novel plasmal conjugate to glycerol and psychosine ("glyceroplasmalopsychosine"): isolation and characterization from bovine brain white matter

A novel plasmal conjugate of glycosphingolipid having cationic lipid properties was isolated from the white matter of bovine brain. Linkage analysis of galactosyl residue by methylation, liquid secondary ion, and electrospray ionization mass spectrometry of intact and methylated derivatives, and by (1)H- and (13)C-NMR spectroscopy, identified the structure unambiguously as an O-acetal conjugate of plasmal to the primary hydroxyl group of glycerol and to the 6-hydroxyl group of galactosyl residue of beta-galactosyl 1-->1 sphingosine (psychosine). This novel compound is hereby termed "glyceroplasmalopsychosine"; its structure is shown below (see text).

Increase in vulnerability of middle-aged rat brain to lead by cerebral energy depletion

The neurotoxic effects of low-level lead (Pb) during senescence are increasing interests of importance. We investigated the effects of low-level Pb on the brain in a normal condition and a pathophysiological condition of energy shortage that is commonly found in age-related neurological diseases. Middle-aged rats (15 months old) were exposed to 200 mg/l Pb acetate in drinking water for 2 months and thereafter received bilateral intracerebroventricular injections of streptozotocin (STZ). After 1 month's additional exposure to the same level of Pb solution as before the rats were sacrificed. Blood and brain Pb levels were measured by graphite furnace atomic absorption spectrophotometry. Energy-rich phosphate levels in the brain were determined by high-performance liquid chromatography equipped with a UV detector. Astroglial activation and glucose-regulated protein (GRP)94 expression were examined immunohistochemically. Exposure to Pb increased the blood Pb level to 10.8 microg/dl and the brain Pb level to 0.052 microg/g. But a significant additional increase in the brain Pb level, to 0.101 microg/g, became obvious in rats treated with Pb + STZ. Both Pb and STZ induced perturbation in brain energy metabolism, but no further alteration in energy metabolite levels was found in rats treated with Pb + STZ. Astroglial activation and GRP94-positive astrocytes and neurons were found only in the brains of Pb + STZ-treated rats. These results suggest that exposure to low-level Pb can perturb brain energy metabolism and the brain becomes more vulnerable to Pb when it is under energy stress.