Lipid sorting by ceramide structure from plasma membrane to ER for the cholera toxin receptor ganglioside GM1.

The glycosphingolipid GM1 binds cholera toxin (CT) on host cells and carries it retrograde from the plasma membrane (PM) through endosomes, the trans-Golgi (TGN), and the endoplasmic reticulum (ER) to induce toxicity. To elucidate how a membrane lipid can specify trafficking in these pathways, we synthesized GM1 isoforms with alternate ceramide domains and imaged their trafficking in live cells. Only GM1 with unsaturated acyl chains sorted efficiently from PM to TGN and ER. Toxin binding, which effectively crosslinks GM1 lipids, was dispensable, but membrane cholesterol and the lipid raft-associated proteins actin and flotillin were required. The results implicate a protein-dependent mechanism of lipid sorting by ceramide structure and provide a molecular explanation for the diversity and specificity of retrograde trafficking by CT in host cells.

Insulin degrading enzyme is localized predominantly at the cell surface of polarized and unpolarized human cerebrovascular endothelial cell cultures.

Insulin degrading enzyme (IDE) is expressed in the brain and may play an important role there in the degradation of the amyloid beta peptide (Abeta). Our results show that cultured human cerebrovascular endothelial cells (HCECs), a primary component of the blood-brain barrier, express IDE and may respond to exposure to low levels of Abeta by upregulating its expression. When radiolabeled Abeta is introduced to the medium of cultured HCECs, it is rapidly degraded to smaller fragments. We believe that this degradation is largely the result of the action of IDE, as it can be substantially blocked by the presence of insulin in the medium, a competitive substrate of IDE. No inhibition is seen when an inhibitor of neprilysin, another protease that may degrade Abeta, is present in the medium. Our evidence suggests that the action of IDE occurs outside the cell, as inhibitors of internalization fail to affect the rate of the observed degradation. Further, our evidence suggests that degradation by IDE occurs on the plasma membrane, as much of the IDE present in HCECs was biotin-labeled by a plasma membrane impermeable reagent. This activity seems to be polarity dependent, as measurement of Abeta degradation by each surface of differentiated HCECs shows greater degradation on the basolateral (brain-facing) surface. Thus, IDE could be an important therapeutic target to decrease the amount of Abeta in the cerebrovasculature.

Upregulation of clusterin/apolipoprotein J in lactacystin-treated SH-SY5Y cells.

Clusterin (apolipoprotein J) is a highly conserved, multifunctional, vertebrate glycoprotein. Several isoforms of clusterin have been described including the predominant secreted isoform (sCLU) and several nuclear isoforms (nCLU) associated with cell death. sCLU has been shown to bind a variety of partly unfolded, stressed proteins including those associated with Lewy bodies (LBs) in patients with Parkinson's disease (PD). The development of familial and sporadic PD has been associated with the ubiquitin-proteasome system (UPS) dysfunction and aberrant protein degradation. This suggests that failure of the UPS to degrade abnormal proteins may underlie nigral degeneration and LB formation in PD. The effects of toxin-mediated proteasomal impairment on changes in gene expression and cell viability were studied in differentiated SH-SY5Y cells. Clusterin expression was increased in cells exposed for 24 hr to the proteasomal inhibitor lactacystin (10 microM) as determined by gene microarray analysis. RT-PCR showed that sCLU, not nCLU, was the major clusterin isoform expressed in both control and lactacystin-treated cells. Western blot analysis identified statistically significant increases in sCLU in total cell lysates after 24 hr of lactacystin exposure and showed that sCLU fractionates with the endoplasmic reticulum. Time-course studies demonstrated that maximal decreases in proteasome activity (4 hr) preceded maximal increases in clusterin expression (24 hr). Together these data suggest that proteasome impairment results in the upregulation of sCLU in SH-SY5Y cells, supporting the hypothesis that the association of clusterin with LBs in PD may be related to UPS failure.

Ethanol increases retinoic acid production in cerebellar astrocytes and in cerebellum.

Several characteristics of fetal alcohol syndrome (FAS) are similar to the teratogenic effects of retinoic acid (RA) exposure. It has been suggested that FAS may result from ethanol-induced alteration in endogenous RA synthesis, leading to abnormal embryonic concentrations of this morphogen. We examined whether ethanol may interfere with RA synthesis in the postnatal cerebellum, as a region of the developing CNS particularly vulnerable to both ethanol and RA teratogenesis. It was found that astrocytes are the predominant source of postnatal RA synthesis in the cerebellum. They express both retinaldehyde dehydrogenase 1 and 2. In vitro cytosolic preparations of astrocytes, as well as live cell preparations, have an increased capacity to synthesize RA in the presence of ethanol. A mechanism by which ethanol could stimulate RA synthesis is via the ethanol-activated short-chain retinol dehydrogenases, which we show to be present in the postnatal cerebellum. To determine whether ethanol stimulated RA synthesis in vivo, a sensitive and highly specific HPLC/MSn technique was used to measure cerebellar RA after administration of ethanol to postnatal day 4 rat pups. Cerebellar RA levels climbed significantly after such treatment. These results suggest that the cerebellar pathology exerted by ethanol may occur, at least in part, through increased production of RA.

Insulin degrading enzyme is expressed in the human cerebrovascular endothelium and in cultured human cerebrovascular endothelial cells.

Insulin degrading enzyme (IDE) is found in the cytosol, peroxisomes and plasma membrane of many cells. Although it preferentially cleaves insulin it can also cleave many other small proteins with diverse sequences including the monomeric form of the amyloid beta peptide (A beta). In the brain, IDE has been reported to be expressed predominantly in neurons. In this study, IDE expression was detected in cultured human cerebrovascular endothelial cells. Using laser capture microdissection followed by PCR analysis, it was found that IDE mRNA is expressed in human brain blood vessels. Using immunofluorescence and multiphoton microscopy IDE was localized to the endothelium of the cerebrovascular blood vessels in human.

Identification of the protein disulfide isomerase family member PDIp in experimental Parkinson's disease and Lewy body pathology.

Parkinson's disease (PD) is a slowly progressing neurodegenerative disorder with no clear etiology. Pathological hallmarks of the disease include the loss of dopaminergic neurons from the substantia nigra (SN) and the presence of Lewy bodies (LBs) (alpha-synuclein and ubiquitin-positive, eosinophilic, cytoplasmic inclusions) in many of the surviving neurons. Experimental modeling of PD neurodegeneration using the neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenyl-pyridinium (MPP(+)) has identified changes in gene expression of different endoplasmic reticulum (ER) stress proteins associated with MPTP- and PD-related neurodegeneration. We show that the protein disulfide isomerase (PDI) family member pancreatic protein disulfide isomerase (PDIp), previously considered exclusively expressed in pancreatic tissue, is uniquely upregulated among PDI family members within 24 h following exposure of retinoic acid (RA)-differentiated SH-SY5Y human neuroblastoma cells to either 1 mM MPP(+) or 10 microM of the highly specific proteasome inhibitor lactacystin. RT-PCR confirms PDIp expression in brain of post-mortem human PD subjects and immunohistochemical studies demonstrate PDIp immunoreactivity in LBs. Collectively, these findings suggest that increased PDIp expression in dopaminergic (DA) neurons might contribute to LB formation and neurodegeneration, and that this increased PDIp expression may be the result of proteasome impairment.

cDNA microarray analysis of changes in gene expression associated with MPP+ toxicity in SH-SY5Y cells.

cDNA microarray analysis of 1-methyl-4-phenyl-pyridinium (MPP+) toxicity (1 mM, 72 h) in undifferentiated SH-SY5Y cells identified 48 genes that displayed a signal intensity greater than the mean of all differentially expressed genes and a two-fold or greater difference in normalized expression. RT-PCR analysis of a subset of genes showed that c-Myc and RNA-binding protein 3 (RMB3) expression decreased by approximately 50% after 72 h of exposure to MPP+ (1 mM) but did not change after 72 h of exposure to 6-hydroxydopamine (25 microM), rotenone (50 nM), and hydrogen peroxide (600 microM). Exposure of retinoic acid (RA)-differentiated SH-SY5Y cells to MPP+ (1 mM, 72 h) also resulted in a decrease in RMB3 expression and an increase in GADD153 expression. In contrast, c-Myc expression was slightly increased in RA-differentiated cells. Collectively, these data provide new insights into the molecular mechanisms of MPP+ toxicity and show that MPP+ can elicit distinct patterns of gene expression in undifferentiated and RA-differentiated SH-SY5Y cells.

Retinoid quantification by HPLC/MS(n).

UNLABELLED: Retinoic acid (RA) mediates most of the biological effects of vitamin A that are essential for vertebrate survival. It acts through binding to receptors that belong to the nuclear receptor transcription factor superfamily (Mangelsdorf et al. 1994). It is also a highly potent vertebrate teratogen. To determine the function and effects of endogenous and exogenous RA, it is important to have a highly specific, sensitive, accurate, and precise analytical procedure. Current analyses of RA and other retinoids are labor intensive, of poor sensitivity, have limited specificity, or require compatibility with RA reporter cell lines (Chen et al. 1995. BIOCHEM: Pharmacol. 50: 1257-1264; Creech Kraft et al. 1994. BIOCHEM: J. 301: 111-119; Lanvers et al. 1996. J. Chromatogr. B Biomed. Appl. 685: 233-240; Maden et al. 1998. DEVELOPMENT: 125: 4133-4144; Wagner et al. 1992. DEVELOPMENT: 116: 55-66). This paper describes an HPLC/mass spectrometry/mass spectrometry product ion scan (HPLC/MS(n)) procedure for the analysis of retinoids that employs atmospheric pressure chemical ionization MS. The retinoids are separated by normal-phase column chromatography with a linear hexane-isopropanol-dioxane gradient. Each retinoid is detected by a unique series of MS(n) functions set at optimal collision-induced dissociation energy (30% to 32%) for all MS(n) steps. The scan events are divided into three segments, based on HPLC elution order, to maximize the mass spectrometer duty cycle. The all-trans, 9-cis, and 13-cis RA isomers are separated, if desired, by an isocratic hexane-dioxane-isopropanol mobile phase. This paper describes an HPLC/MS(n) procedure possessing high sensitivity and specificity for retinoids.

Specific up-regulation of GADD153/CHOP in 1-methyl-4-phenyl-pyridinium-treated SH-SY5Y cells.

Growth arrest DNA damage-inducible 153 (GADD153) expression was increased in 1-methyl-4-phenyl-pyridinium (MPP(+))-treated human SH-SY5Y neuroblastoma cells as determined by gene microarray analysis. GADD153 expression increased after 24 hr of MPP(+) (1 mM) exposure and preceded activation of caspase 3. Comparison of GADD153 expression among cultures treated with other toxins whose primary mode of action is either via mitochondrial impairment (rotenone) or via oxidative stress (6-hydroxydopamine or hydrogen peroxide) showed that GADD153 was uniquely up-regulated by MPP(+). Together these data suggest that a cellular mechanism distinct from mitochondrial impairment or oxidative stress contributes significantly to the up-regulation of GADD153 by MPP(+) and that GADD153 may function as an inducer of apoptosis following MPP(+) exposure. Published 2002 Wiley-Liss, Inc.