Aluminium in Alzheimer’s disease: are we still at a crossroad?

Aluminium, an environmentally abundant non-redox trivalent cation has long been implicated in the pathogenesis of Alzheimers disease (AD). However, the definite mechanism of aluminium toxicity in AD is not known. Evidence suggests that trace metal homeostasis plays a crucial role in the normal functioning of the brain, and any disturbance in it can exacerbate events associated with AD. The present paper reviews the scientific literature linking aluminium with AD. The focus is on aluminium levels in brain, region-specific and subcellular distribution, its relation to neurofibrillary tangles, amyloid beta, and other metals. A detailed mechanism of the role of aluminium in oxidative stress and cell death is highlighted. The importance of complex speciation chemistry of aluminium in relation to biology has been emphasized. The debatable role of aluminium in AD and the cross-talk between aluminium and genetic susceptibility are also discussed. Finally, it is concluded based on extensive literature that the neurotoxic effects of aluminium are beyond any doubt, and aluminium as a factor in AD cannot be discarded. However, whether aluminium is a sole factor in AD and whether it is a factor in all AD cases still needs to be understood.Received 22 July 2004; received after revision 3 September 2004; accepted 16 September 2004     

Ceramide function in the brain: when a slight tilt is enough

Ceramide, the precursor of all complex sphingolipids, is a potent signaling molecule that mediates key events of cellular pathophysiology. In the nervous system, the sphingolipid metabolism has an important impact. Neurons are polarized cells and their normal functions, such as neuronal connectivity and synaptic transmission, rely on selective trafficking of molecules across plasma membrane. Sphingolipids are abundant on neural cellular membranes and represent potent regulators of brain homeostasis. Ceramide intracellular levels are fine-tuned and alteration of the sphingolipid–ceramide profile contributes to the development of age-related, neurological and neuroinflammatory diseases. The purpose of this review is to guide the reader towards a better understanding of the sphingolipid–ceramide pathway system. First, ceramide biology is presented including structure, physical properties and metabolism. Second, we describe the function of ceramide as a lipid second messenger in cell physiology. Finally, we highlight the relevance of sphingolipids and ceramide in the progression of different neurodegenerative diseases.     

Cardiolipin, the heart of mitochondrial metabolism

Cardiolipin is a unique phospholipid, which is almost exclusively localized in the mitochondrial inner membrane where it is synthesized from phosphatidylglycerol and cytidinediphosphate-diacylglycerol. After primary synthesis, the mature acyl chain composition of cardiolipin is achieved by at least two remodeling mechanisms. In the mitochondrial membrane cardiolipin plays an important role in energy metabolism, mainly by providing stability for the individual enzymes and enzyme complexes involved in energy production. Moreover, cardiolipin is involved in different stages of the mitochondrial apoptotic process and in mitochondrial membrane dynamics. Cardiolipin alterations have been described in various pathological conditions. Patients suffering from Barth syndrome have an altered cardiolipin homeostasis caused by a primary deficiency in cardiolipin remodeling. Alterations in cardiolipin content or composition have also been reported in more frequent diseases such as diabetes and heart failure. In this review we provide an overview of cardiolipin metabolism, function and its role in different pathological states. Received 16 January 2008; received after revision 26 February 2008; accepted 26 March 2008     

Inhibition of cholesterol recycling impairs cellular PrPSc propagation

The infectious agent in prion diseases consists of an aberrantly folded isoform of the cellular prion protein (PrP^c), termed PrP^Sc, which accumulates in brains of affected individuals. Studies on prion-infected cultured cells indicate that cellular cholesterol homeostasis influences PrP^Sc propagation. Here, we demonstrate that the cellular PrP^Sc content decreases upon accumulation of cholesterol in late endosomes, as induced by NPC-1 knock-down or treatment with U18666A. PrP^c trafficking, lipid raft association, and membrane turnover are not significantly altered by such treatments. Cellular PrP^Sc formation is not impaired, suggesting that PrP^Sc degradation is increased by intracellular cholesterol accumulation. Interestingly, PrP^Sc propagation in U18666A-treated cells was partially restored by overexpression of rab 9, which causes redistribution of cholesterol and possibly of PrP^Sc to the trans-Golgi network. Surprisingly, rab 9 overexpression itself reduced cellular PrP^Sc content, indicating that PrP^Sc production is highly sensitive to alterations in dynamics of vesicle trafficking.     

Protein profiling of 3T3-L1 adipocyte differentiation and (tumor necrosis factor α-mediated) starvation

The increased incidence of obesity and related disorders in Western societies requires a thorough understanding of the adipogenic process. Data at the protein level of this process are scarce. Therefore we performed a proteome analysis of differentiating and starving 3T3-L1 cells using two-dimensional gel electrophoresis combined with mass spectrometry. Effects of different starvation conditions were examined by subjecting 3T3-L1 adipocytes to caloric restriction, either in the absence or the presence of the lipolysis inducer tumor necrosis factor-. Ninety-three differentially expressed proteins were found during differentiation and starvation of 3T3-L1 cells, 50 of which were identified. GenMAPP/MAPP-finder software revealed a non-reciprocal regulation of the glycolytic pathway during 3T3-L1 differentiation followed by starvation. Furthermore, proteins involved in growth regulation, cytoskeletal rearrangements and protein modification, 16 of which have not been described before in 3T3-L1 cells, were identified. In conclusion, our data provide valuable information for further understanding of the adipogenic process.Received 9 November 2004; received after revision 21 December 2004; accepted 28 December 2004     

Pharmacodynamic profile of CQP 201-403, a novel 8α-amino-ergoline

Summary The profile of action in animals of CQP 201-403, a novel 8-amino-ergoline, is in most aspects that of a very potent dopaminomimetic, both as a prolactin secretion inhibitor, and at the levels of the CNS and the cardiovascular system. Qualitatively CQP 201-403 differs slightly from bromocriptine and apomorphine in its effects on the CNS (no influence on serotonin metabolism in the rat cortex; induction of masculine mounting behavior in rats) and the cardiovascular system of the dog (reflex tachycardia in response to a blood-pressure fall). In man the new compound proved to be highly active in lowering prolactin serum levels and to be more potent than bromocriptine (Parlodel^®).In memory of Dr Annemarie Closse, who died 14 June 1987.     

The expression levels of three raft-associated molecules in cultivated vascular cells are dependent on culture conditions

Relaying a signal across the plasma membrane requires functional connections between the partner molecules. Membrane microdomains or lipid rafts provide an environment in which such specific interactions can take place. The integrity of these sites is often taken for granted when signalling pathways are investigated in cell culture. However, it is well known that smooth muscle and endothelial cells undergo cytoskeletal rearrangements during monolayer culturing. Likewise affected – and with potentially important consequences for signalling events – is the organization of the plasma membrane. The expression levels of three raft markers were massively upregulated, and raft-associated 5-nucleotidase activity increased in conventional monolayer cultures as compared with a spheroidal coculture model, shown to promote the differentiation of endothelial cells. Our data point to a shift of raft components in monolayer cultures and demonstrate potential advantages of the spheroid coculture system for investigation of raft-mediated signalling events in endothelial cells.Received 4 August 2003; received after revision 18 September 2003; accepted 25 September 2003     

Changes in erythrocyte membrane lipid composition affect the transient decrease in membrane order which accompanies insulin receptor down-regulation.

We have recently demonstrated, using electron paramagnetic resonance (EPR) spectroscopy, that insulin receptor internalization in response to insulin incubation (down-regulation) in human erythrocytes is accompanied by a transient decrease in membrane order, as measured by the 2T' parallel order parameter. Since membrane lipids play such an important role in receptor internalization, we investigated the possible effects that an alteration of the normally-occurring lipid profile might have on down-regulation and the concomitant transient decrease in membrane order. Consequently, human erythrocytes enriched with cholesterol and erythrocytes from cirrhotic patients were examined, because both of these groups of cells have a higher cholesterol/phospholipid molar ratio (CH/PL) than controls. The 5-nitroxystearate spin label, which inserts into the lipid bilayer of cell membranes, was used to monitor changes in 2T' parallel for a 3-h period at 37 degrees C. We report here that both cholesterol-enriched and cirrhotic erythrocytes do not down-regulate, as demonstrated by binding assays, and that they do not show the typical transient decrease in membrane order observed in controls. The results seem to indicate that a more ordered membrane inhibits internalization of the insulin receptor in erythrocytes, and that an increase in membrane disorder is necessary for insulin receptor down-regulation.     

P-glycoprotein and ‘lipid rafts’: some ambiguous mutual relationships (floating on them, building them or meeting them by chance?)

P-glycoprotein (P-gp) is an active membrane transporter responsible for cell detoxification against numerous amphiphilic compounds, leading to multidrug resistance in tumor cells. It displays entangled connections with its membrane environment since it recognizes its substrates within the cytosolic leaflet and it also translocates some endogenous lipids to the exoplasmic leaflet. Regarding its relationships with membrane microdomains, ‘lipid rafts’, a literature analysis concludes that (i) P-gp also exists in rafts and non-raft membrane domains, depending on the cell considered, the experimental conditions and the method used to test it; (ii) cholesterol has a positive influence on P-gp function, and this may be a direct effect of the free cholesterol present in membrane or an indirect effect mediated by the cholesterol-enriched microdomains; (iii) when present in rafts, P-gp interacts with protein partners regulating its activity; (iv) P-gp is a lipid translocase that handles the raft-constituting lipids with particular efficiency, and it also influences membrane trafficking in the cell. Received 18 November 2005; received after revision 23 December 2005; accepted 12 January 2006     

Bridging the molecular and biological functions of the oxysterol-binding protein family

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large eukaryotic gene family that transports and regulates the metabolism of sterols and phospholipids. The original classification of the family based on oxysterol-binding activity belies the complex dual lipid-binding specificity of the conserved OSBP homology domain (OHD). Additional protein- and membrane-interacting modules mediate the targeting of select OSBP/ORPs to membrane contact sites between organelles, thus positioning the OHD between opposing membranes for lipid transfer and metabolic regulation. This unique subcellular location, coupled with diverse ligand preferences and tissue distribution, has identified OSBP/ORPs as key arbiters of membrane composition and function. Here, we will review how molecular models of OSBP/ORP-mediated intracellular lipid transport and regulation at membrane contact sites relate to their emerging roles in cellular and organismal functions.     

Abnormal lipid composition of microsomes from cirrhotic rat liver—does it contribute to decreased microsomal function?

We determined to what extent a change in the lipid composition of the smooth endoplasmic reticulum contributes to altered microsomal function in cirrhosis. Rats were rendered cirrhotic either by chronic exposure to phenobarbital/CCl₄ (MCIR) or by bile duct ligation (BCIR). Microsomal function was tested in vivo by the aminopyrine breath test (ABT), then microsomes were prepared and their phospholipid and cholesterol composition analysed. ABT was reduced by 35 and 41% in BCIR and MCIR, respectively. Cholesterol in microsomes was increased in both cirrhotic groups. (BCIR +154%, MCIR +75%) while total phospholipid content was not affected. As shown in other membrane systems, the phospholipid/cholesterol (PL/XOL) ratio showed an excellent inverse correlation with fluorescence anisotropy determined by diphenylhexatriene fluorescence polarization (r=–0.896). The PL/XOL ratio was significantly correlated with aminopyrine N-demethylation in vivo (r=0.649). Alterations in the composition of phospholipid groups (an increase in sphingomyelin in both cirrhotic groups, and a decrease in phosphatidylcholine and an increase in phosphatidylethanolamine in BCIR) also contributed to increased membrane rigidity. We conclude that altered membrane fluidity contributes to diminished microsomal function but that other factors must also be involved since the PL/XOL ratio explained only 42% of the variance in aminopyrine N-demethylation.     

Unraveling the pathogenesis of Parkinson’s disease – the contribution of monogenic forms

The field of Parkinsons disease pathogenesis is rapidly evolving from the one of a monolithic and obscure entity into the one of a complex scenario with several known molecular players. The ongoing systematic exploration of the genome holds great promise for the identification of the genetic factors conferring susceptibility to the common non-Mendelian forms of this disease. However, most of the progress of the last 5 years has come from the successful mapping and cloning of genes responsible for rare Mendelian variants of Parkinsons disease. These discoveries are providing tremendous help in understanding the molecular mechanisms of this devastating disease. Here we review the genetics of the monogenic forms of Parkinsons disease. Moreover, we focus on the mechanisms of disease caused by -synuclein and parkin mutations, and the implications of this growing body of knowledge for understanding the pathogenesis of the common forms of the disease. Received 10 March 2004; received after revision 26 April 2004; accepted 29 April 2004     

Molecular basis of Alzheimer's disease

Despite an exponential production of data, Alzheimer's disease (AD) remains an enigma. Unresolved questions persist in the face of the heterogeneity of this neuropathology. Recent progress in understanding mechanisms for AD results from the study of amyloid precursor protein (APP) metabolism and the involvement of senile plaque-associated proteins. In addition to the amyloid cascade hypothesis, alternative schemes emerge, in which the amyloid peptide is not the primary effector of the disease. Perturbations of vesicular trafficking, the cytoskeletal network, and membrane cholesterol distribution could be central events. Furthermore, since the physiological role of APP, presenilins, and apolipoprotein E in the central nervous system are not completely understood, their involvement in AD etiology remains speculative. New actors have to be found to try to explain sporadic cases and non-elucidated familial cases.     

Bile Acids: Chemistry, Pathochemistry, Biology, Pathobiology, and Therapeutics

Bile acids and bile alcohols in the form of their conjugates are amphipathic end products of cholesterol metabolism with multiple physiological functions. The great variety of bile acids and bile alcohols that are present in vertebrates are tabulated. Bile salts have an enterohepatic circulation resulting from efficient vectorial transport of bile salts through the hepatocyte and the ileal enterocyte; such transport leads to the accumulation of a pool of bile salts that cycles between the liver and intestine. Bile salt anions promote lipid absorption, enhance tryptic cleavage of dietary proteins, and have antimicrobial effects. Bile salts are signaling molecules, activating nuclear receptors in the hepatocyte and ileal enterocyte, as well as an increasing number of G-protein coupled receptors. Bile acids are used therapeutically to correct deficiency states, to decrease the cholesterol saturation of bile, or to decrease the cytotoxicity of retained bile acids in cholestatic liver disease.     

Pancreastatin,a chromogranin A-derived peptide,inhibits leptin and enhances UCP-2 expression in isolated rat adipocytes

Leptin, the ob gene product, is an adipocyte-secreted hormone that centrally regulates weight by decreasing caloric intake and increasing energy expenditure. Expression of leptin is regulated by dietary status, insulin, glucocorticoids and catecholamines. Pancreastatin (PST), a chromogranin A-derived peptide, correlates with catecholamine levels, and may play a role in the physiology of stress, modulating endocrine secretion and metabolism. Thus, PST has been found to exert a lipolytic and anti-insulin effect in white adipocytes. The aim of the present work was to investigate a possible role of PST modulating the expression of key genes involved in lipid storage and metabolism: leptin, PPAR-2, UCP-1 and UCP-2. We incubated isolated rat epididymal adipocytes with 100 nM PST for 16 and 24 h. Leptin, UCP-2 and UCP-1 mRNA levels were assessed by RT-PCR, followed by Southern blot. Leptin secretion was also measured by ELISA. PST inhibited leptin expression and secretion at 16-h incubation, but this effect was no longer observed after 24 h. On the other hand, PST stimulated the expression of UCP-2 after 16 h. However, the effect was still significant after 24 h. The inhibitory effect of PST on leptin expression and secretion and the stimulation of UCP-2 expression were prevented by blocking PKC. UCP-1 and PPR-2 expression did not change after PST stimulation. Leptin differentially regulates the expression of key genes in the rat adipocyte, upregulating the expression of UCP-2 and inhibiting the expression and secretion of leptin by a mechanism that involves PKC activity. These effects may contribute to the metabolic action of catecholamines in physiological and pathophysiological conditions with increased sympathetic activity.Received 5 September 2003; received after revision 6 October 2003; accepted 14 October 2003     

Human and rodent type 1 11β-hydroxysteroid dehydrogenases are 7β-hydroxycholesterol dehydrogenases involved in oxysterol metabolism

Interconversion between cortisone and the glucocorticoid receptor ligand cortisol is carried out by 11-hydroxysteroid dehydrogenase (11-HSD)isozymes and constitutes a medically important example of pre-receptor control of steroid hormones. The enzyme 11-HSD type 1 (11-HSD1) catalyzes the conversion of cortisone to its active receptor-binding derivative cortisol, whereas 11-HSD type 2 performs the reverse reaction. Specific inhibitors against the type 1 enzyme lower intracellular levels of glucocorticoid hormone, with an important clinical application in insulin resistance and other metabolic disorders. We report here on the in vitro oxysterol-metabolizing properties of human and rodent 11-HSD1. The enzyme, either as full-length, membrane-attached, or as a transmembrane domain-deleted, soluble form, mediates exclusively conversion between 7-ketocholesterol and 7-hydroxycholesterol with similar k_cat values as observed with glucocorticoid hormones. Thus, human, rat, and mouse 11-HSD1 have dual enzyme activities like the recently described 7-hydroxysteroid dehydrogenase/11-hydroxysteroid dehydrogenase from hamster liver, but differ fundamentally from the latter in that 7-OH rather than 7-OH dehydrogenase constitutes the second activity. These results demonstrate an enzymatic origin of species differences in 7-oxysterol metabolism, establish the origin of endogenous 7-OH cholesterol in humans, and point to a possible involvement of 11-HSD1 in atherosclerosis.Received 30 December 2003; received after revision 16 February 2004; accepted 16 February 2004     

Abnormal lipid composition of microsomes from cirrhotic rat liver--does it contribute to decreased microsomal function?

We determined to what extent a change in the lipid composition of the smooth endoplasmic reticulum contributes to altered microsomal function in cirrhosis. Rats were rendered cirrhotic either by chronic exposure to phenobarbital/CCl4 (MCIR) or by bile duct ligation (BCIR). Microsomal function was tested in vivo by the aminopyrine breath test (ABT), then microsomes were prepared and their phospholipid and cholesterol composition analysed. ABT was reduced by 35 and 41% in BCIR and MCIR, respectively. Cholesterol in microsomes was increased in both cirrhotic groups. (BCIR + 154%, MCIR + 75%) while total phospholipid content was not affected. As shown in other membrane systems, the phospholipid/cholesterol (PL/XOL) ratio showed an excellent inverse correlation with fluorescence anisotropy determined by diphenylhexatriene fluorescence polarization (r = -0.896). The PL/XOL ratio was significantly correlated with aminopyrine N-demethylation in vivo (r = 0.649). Alterations in the composition of phospholipid groups (an increase in sphingomyelin in both cirrhotic groups, and a decrease in phosphatidylcholine and an increase in phosphatidylethanolamine in BCIR) also contributed to increased membrane rigidity. We conclude that altered membrane fluidity contributes to diminished microsomal function but that other factors must also be involved since the PL/XOL ratio explained only 42% of the variance in aminopyrine N-demethylation.