Intracellular cholesterol and phospholipid trafficking: comparable mechanisms in macrophages and neuronal cells

Astaxanthin enhances autophagy, amyloid beta clearance and exerts anti-inflammatory effects in in vitro models of Alzheimer's disease-related blood brain barrier dysfunction and inflammation

Defective degradation and clearance of amyloid-β as well as inflammation per se are crucial players in the pathology of Alzheimer's disease (AD). A defective transport across the blood-brain barrier is causative for amyloid-β (Aβ) accumulation in the brain, provoking amyloid plaque formation. Using primary porcine brain capillary endothelial cells and murine organotypic hippocampal slice cultures as in vitro models of AD, we investigated the effects of the antioxidant astaxanthin (ASX) on Aβ clearance and neuroinflammation. We report that ASX enhanced the clearance of misfolded proteins in primary porcine brain capillary endothelial cells by inducing autophagy and altered the Aβ processing pathway. We observed a reduction in the expression levels of intracellular and secreted amyloid precursor protein/Aβ accompanied by an increase in ABC transporters ABCA1, ABCG1 as well as low density lipoprotein receptor-related protein 1 mRNA levels. Furthermore, ASX treatment increased autophagic flux as evidenced by increased lipidation of LC3B-II as well as reduced protein expression of phosphorylated S6 ribosomal protein and mTOR. In LPS-stimulated brain slices, ASX exerted anti-inflammatory effects by reducing the secretion of inflammatory cytokines while shifting microglia polarization from M1 to M2 phenotype. Our data suggest ASX as potential therapeutic compound ameliorating AD-related blood brain barrier impairment and inflammation.

Transcriptome analysis of human induced excitatory neurons supports a strong effect of clozapine on cholesterol biosynthesis

Antipsychotics are known to modulate dopamine and other neurotransmitters which is often thought to be the mechanism underlying their therapeutic effects. Nevertheless, other less studied consequences of antipsychotics on neuronal function may contribute to their efficacy. Revealing the complete picture behind their action is of paramount importance for precision medicine and accurate drug selection. Progress in cell engineering allows the generation of induced pluripotent stem cells (iPSCs) and their differentiation to a variety of neuronal types, providing new tools to study antipsychotics. Here we use excitatory cortical neurons derived from iPSCs to explore their response to therapeutic levels of Clozapine as measured by their transcriptomic output, a proxy for neuronal homeostasis. To our surprise, but in agreement with the results of many investigators studying glial-like cells, Clozapine had a very strong effect on cholesterol metabolism. More than a quarter (12) of all annotated cholesterol genes (46) in the genome were significantly changed at FDR < 0.1, all upregulated. This is a 35-fold enrichment with an adjusted p = 8 × 10^-11. Notably no other functional category showed evidence of enrichment. Cholesterol is a major component of the neuronal membrane and myelin but it does not cross the blood brain barrier, it is produced locally mostly by glia but also by neurons. By singling out increased expression of cholesterol metabolism genes as the main response of cortical excitatory neurons to antipsychotics, our work supports the hypothesis that cholesterol metabolism may be a contributing mechanism to the beneficial effects of Clozapine and possibly other antipsychotics.

A high through-put reverse genetic screen identifies two genes involved in remote memory in mice

Previous studies have revealed that the initial stages of memory formation require several genes involved in synaptic, transcriptional and translational mechanisms. In contrast, very little is known about the molecular and cellular mechanisms underlying later stages of memory, including remote memory (i.e. 7-day memory). To identify genes required for remote memory, we screened randomly selected mouse strains harboring known mutations. In our primary reverse genetic screen, we identified 4 putative remote memory mutant strains out of a total of 54 lines analyzed. Additionally, we found 11 other mutant strains with other abnormal profiles. Secondary screens confirmed that mutations of integrin β2 (Itgβ2) and steryl-O-acyl transferase 1 (Soat1) specifically disrupted remote memory. This study identifies some of the first genes required for remote memory, and suggests that screens of targeted mutants may be an efficient strategy to identify molecular requirements for this process.

cAMP-induced expression of ABCA1 is associated with MAP-kinase-pathway activation

Several lines of evidence suggest that the ATP binding cassette A1 (ABCA1) is also involved in other degenerative processes such as brain neurodegeneration. Cholesterol and cAMP activate ABCA1 in a cell-specific manner. We employed a cell culture model of murine monocytes (P388) and neuroblastoma cells (N2A) and studied the differential induction of the ABCA1-gene product by modifying the cholesterol acceptor and by inhibition of the MAP-kinase pathway. Our study reveals a rise of ABCA1-expression in both N2A and P388 by cAMP. This increase is accompanied by a higher activation of the MAP-kinase-pathway. The inhibition of the MAP-kinase activation disrupts the stimulating effect of cAMP but increases the base line expression of ABCA1. Our data suggest a negative feedback between the MAP-kinase-system and ABCA1. We conclude that the interaction of the MAP-kinase pathway and the ABCA1 system might affect the function of neuronal and microglial cells in the brain.

Monocyte cholesterol homeostasis correlates with the presence of detergent resistant membrane microdomains

BACKGROUND

Lipid membrane microdomains are involved in the regulation of biological functions of monocyte membrane proteins. These microdomains show a relative resistance to non-ionic detergents providing an easy analytical tool to study them.

METHODS

Here, we applied a rapid detergent-based flow cytometric assay to investigate microdomain association of proteins on monocytes from whole blood samples. The association of known surface antigens with detergent resistant fraction of membranes (DRMs) was compared using monocytes from healthy blood donors, patients with genetic disorders affecting cellular cholesterol traffic and patients with systemic inflammatory response.

RESULTS

All investigated surface antigens of Niemann-Pick type C (NPC)-mutant monocytes with impaired cholesterol influx and defective late endosome cholesterol trafficking, presented a strongly increased DRM-association. Though, membrane antigens of ATP binding cassette transporter A1 (ABCA1)-mutant monocytes with impaired cholesterol efflux did not show alterations in DRM-association. Differential CD14-dependent receptor clustering within microdomains was also investigated in response to in vivo lipopolysaccharide (LPS) and/or atherogenic lipoprotein activation. Increased DRM-association of the GPI-anchored proteins CD14, CD55, the Fcgamma receptor CD64, the scavenger receptors CD36, CD91 and CD163, the integrin CD11a, and complement receptor 3 complex CD11b/CD18 were observed from patients with systemic inflammatory response syndrome (SIRS)/sepsis or coronary artery disease (CAD)/myocardial infarction. Interestingly, the tetraspanin CD81 presented increased DRM-association in SIRS/sepsis patients, but not in CAD patients. Moreover, the pentaspanin CD47 and the Fcgamma RIII CD16 showed an increased DRM partition in CAD patients but disassembled from DRMs in SIRS/sepsis patients.

CONCLUSIONS

Our results demonstrate that flow cytometric analysis of short time in situ detergent extraction provides a powerful tool for rapid screening of blood monocyte DRMs to preselect patients with potential raft/microdomain abnormalities for more detailed analysis.

Cholesterol dysfunction in neurodegenerative diseases: Is Huntington's disease in the list?

Brain cholesterol is an essential component of cell membranes, and involved in a number of biological functions such as membrane trafficking, signal transduction, myelin formation and synaptogenesis. Given these widespread activities and the knowledge that all brain cholesterol derives from local synthesis, it is not surprising that dysfunctions in cholesterol synthesis, storage, transport and removal may lead to human brain diseases. Some of these diseases emerge as a consequence of genetic defects in the enzymes involved in cholesterol biosynthesis; in other cases, such as Alzheimer's disease, there is a link between cholesterol metabolism and the formation and deposition of amyloid-beta peptide. Emerging evidence indicates that changes in cholesterol synthesis may also occur in Huntington's disease, an inherited, autosomal dominant neurodegenerative disorder that primarily affects striatal neurons of the brain. We here provide an overview of the involvement of cholesterol in normal brain function and its impact on neurodegenerative diseases. In particular, we consider the available clinical, biological and molecular evidence indicating a potential dysregulation of cholesterol homeostasis in Huntington's disease.

Cholesterol-enriched diet affects spatial learning and synaptic function in hippocampal synapses

The aim of the present study was to determine the effect of a cholesterol-rich diet on learning performance and monitor possible related changes in synaptic function. To this purpose, we compared controls with rats fed with a cholesterol-enriched diet (CD). By using a Morris water-maze paradigm, we found that CD rats learned a water-maze task more quickly than rats fed with a regular diet (RD). A longer period of this diet tended to alter the retention of memory without affecting the improvement in the acquisition of the task. Because of the importance of the hippocampus in spatial learning, we hypothesized that these behavioral effects of cholesterol would involve synaptic changes at the hippocampal level. We used whole-cell patch-clamp recording in the CA1 area of a hippocampal rat slice preparation to test the influence of the CD on pre- and postsynaptic function. CD rats displayed an increase in paired-pulse ratio in both glutamatergic synapses (+48 +/- 9%) and GABAergic synapses (+41 +/- 8%), suggesting that the CD induces long-lasting changes in presynaptic function. Furthermore, by recording NMDA-receptor-mediated currents (I(NMDA)) and AMPA-receptor-mediated currents (I(AMPA)) in the same set of cells we found that CD rats display a lower I(NMDA)/I(AMPA) ratio (I(NMDA)/I(AMPA) = 0.75 +/- 0.32 in RD versus 0.10 +/- 0.03 in CD), demonstrating that cholesterol regulates also postsynaptic function. We conclude that a cholesterol-rich diet affects learning speed and performance, and that these behavioral changes occur together with robust, long-lasting, synaptic changes at both the pre- and postsynaptic level.

Why is 11beta-hydroxysteroid dehydrogenase type 1 facing the endoplasmic reticulum lumen? Physiological relevance of the membrane topology of 11beta-HSD1

11Beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) is essential for the local activation of glucocorticoid receptors (GR). Unlike unliganded cytoplasmic GR, 11beta-HSD1 is an endoplasmic reticulum (ER)-membrane protein with lumenal orientation. Cortisone might gain direct access to 11beta-HSD1 by free diffusion across membranes, indirectly via intracellular binding proteins or, alternatively, by insertion into membranes. Membranous cortisol, formed by 11beta-HSD1 at the ER-lumenal side, might then activate cytoplasmic GR or bind to ER-lumenal secretory proteins. Compartmentalization of 11beta-HSD1 is important for its regulation by hexose-6-phosphate dehydrogenase (H6PDH), which regenerates cofactor NADPH in the ER lumen and stimulates oxoreductase activity. ER-lumenal orientation of 11beta-HSD1 is also essential for the metabolism of the alternative substrate 7-ketocholesterol (7KC), a major cholesterol oxidation product found in atherosclerotic plaques and taken up from processed cholesterol-rich food. An 11beta-HSD1 mutant adopting cytoplasmic orientation efficiently catalyzed the oxoreduction of cortisone but not 7KC, indicating access to cortisone from both sides of the ER-membrane but to 7KC only from the lumenal side. These aspects may be relevant for understanding the physiological role of 11beta-HSD1 and for developing therapeutic interventions to control glucocorticoid reactivation.

Regulatory effects of synthetic liver X receptor- and peroxisome-proliferator activated receptor agonists on sterol transport pathways in polarized cerebrovascular endothelial cells

The blood-brain barrier contributes to maintain brain cholesterol metabolism and protects this uniquely balanced system from exchange with plasma lipoprotein cholesterol. Brain capillary endothelial cells, representing a physiological barrier to the central nervous system, express apolipoprotein A-I (apoA-I, the major high-density lipoprotein (HDL)-associated apolipoprotein), ATP-binding cassette transporter A1 (ABCA1), and scavenger receptor, class B, type I (SR-BI), proteins that promote cellular cholesterol mobilization. Liver X receptors (LXRs) and peroxisome-proliferator activated receptors (PPARs) are regulators of cholesterol transport, and activation of LXRs and PPARs has potential therapeutic implications for lipid-related neurodegenerative diseases. To clarify the functional impact of LXR/PPAR activation, sterol transport along the: (i) ABCA1/apoA-I and (ii) SR-BI/HDL pathway was investigated in primary, polarized brain capillary endothelial cells, an in vitro model of the blood-brain barrier. Activation of LXR (24(S)OH-cholesterol, TO901317), PPARalpha (bezafibrate, fenofibrate), and PPARgamma (troglitazone, pioglitazone) modulated expression of apoA-I, ABCA1, and SR-BI on mRNA and/or protein levels without compromising transendothelial electrical resistance or tight junction protein expression. LXR-agonists and troglitazone enhanced basolateral-to-apical cholesterol mobilization in the absence of exogenous sterol acceptors. Along with the induction of cell surface-located ABCA1, several agonists enhanced cholesterol mobilization in the presence of exogenous apoA-I, while efflux of 24(S)OH-cholesterol (the major brain cholesterol metabolite) in the presence of exogenous HDL remained unaffected. Summarizing, in cerebrovascular endothelial cells apoA-I, ABCA1, and SR-BI represent drug targets for LXR and PPAR-agonists to interfere with cholesterol homeostasis at the periphery of the central nervous system.

Appropriate Function of 11β-Hydroxysteroid Dehydrogenase Type 1 in the Endoplasmic Reticulum Lumen Is Dependent on Its N-terminal Region Sharing Similar Topological Determinants with 50-kDa Esterase

By interconverting glucocorticoids, 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) exerts an important pre-receptor function and is currently considered a promising therapeutic target. In addition, 11beta-HSD1 plays a potential role in 7-ketocholesterol metabolism. Here we investigated the role of the N-terminal region on enzymatic activity and addressed the relevance of 11beta-HSD1 orientation into the endoplasmic reticulum (ER) lumen. Previous studies revealed that the luminal orientation of 11beta-HSD1 and 50-kDa esterase/arylacetamide deacetylase (E3) is determined by their highly similar N-terminal transmembrane domains. Substitution of Lys(5) by Ser in 11beta-HSD1, but not of the analogous Lys(4) by Ile in E3, led to an inverted topology in the ER membrane, indicating the existence of a second topological determinant. Here we identified Glu(25)/Glu(26) in 11beta-HSD1 and Asp(25) in E3 as the second determinant for luminal orientation. Our results suggest that the exact location of specific residues rather than net charge distribution on either side of the helix is critical for membrane topology. Analysis of charged residues in the N-terminal domain revealed an essential role of Lys(35)/Lys(36) and Glu(25)/Glu(26) on enzymatic activity, suggesting that these residues are responsible for the observed stabilizing effect of the N-terminal membrane anchor on the catalytic domain of 11beta-HSD1. Moreover, activity measurements in intact cells expressing wild-type 11beta-HSD1, facing the ER lumen, or mutant K5S/K6S, facing the cytoplasm, revealed that the luminal orientation is essential for efficient oxidation of cortisol. Furthermore, we demonstrate that 11beta-HSD1, but not mutant K5S/K6S with cytoplasmic orientation, catalyzes the oxoreduction of 7-ketocholesterol. 11beta-HSD1 and E3 constructs with cytosolic orientation of their catalytic moiety should prove useful in future studies addressing the physiological function of these proteins.

Postnatal development of inflammation in a murine model of Niemann–Pick type C disease: immunohistochemical observations of microglia and astroglia

Niemann-Pick type C (NPC) is a rare and fatal neurovisceral storage disorder that is currently untreatable. In most cases, NPC is caused by mutations of the NPC1 gene, which encodes a glycoprotein playing an important role in cholesterol transport. Mice lacking the NPC1 gene exhibit several pathological features of NPC patients and have been widely used to provide insights into the mechanisms of the disease. In the present study, we analyzed the postnatal development of pathological manifestations of inflammation in several brain regions of NPC1-/- mice. Brain sections from NPC1-/- and wild-type (NPC1+/+) mice were immunostained with the MAC1 antibody, which recognizes microglia, with antibodies against glial fibrillary acidic protein (GFAP), which recognize astrocytes, and with antibodies against the cytokine interleukin-1beta (IL-1beta). Numbers of MAC1 immunopositive cells were markedly increased in several brain regions of NPC1-/- mice as early as 2 weeks of age. This effect was particularly evident in globus pallidus, ventral lateral thalamus, medial geniculate nucleus, and cerebellum. MAC1-immunopositive cells had enlarged cell bodies and shorter processes, suggesting they were in an active state. By 4 weeks, most brain structures exhibited enhanced microglial activation in NPC1-/- mice, and this was maintained at 12 weeks. At 2 weeks, reactive astrocytes were only observed in the ventral lateral thalamus while they were present throughout the brain of NPC1-/- mice at 4 weeks of age. Moreover, the astroglial reaction coincided with up-regulation of the cytokine, interleukin-1beta, in most, but not all brain regions. In particular, no interleukin-1beta up-regulation was observed in regions devoid of neuronal degeneration. These results suggest that microglial activation precedes and might be causally related to neuronal degeneration, while astrocyte activation might be a consequence of neuronal degeneration.

Widespread distribution of PLTP in human CNS: evidence for PLTP synthesis by glia and neurons, and increased levels in Alzheimer's disease

Plasma phospholipid transfer protein (PLTP) is one of the key proteins in lipid and lipoprotein metabolism. We examined PLTP distribution in human brain using PLTP mRNA dot-blot, Northern blot, immunohistochemistry (IHC), Western blot, and phospholipid transfer activity assay analyses. PLTP mRNA of 1.8 kb was widely distributed in all the examined regions of the central nervous system at either comparable or slightly lower levels than in the other major organs, depending on the region. Cerebrospinal fluid phospholipid transfer activity represented 15% of the plasma activity, indicating active PLTP synthesis in the brain. Western blot and phosholipid transfer activity assay demonstrated secretion of active PLTP by neurons, microglia, and astrocytes in culture. IHC demonstrated PLTP presence in neurons, astrocytes, microglia, and oligodendroglia. Some neuronal groups, such as nucleus hypoglossus and CA2 neurons in hippocampus, ependymal layer, and choroid plexus were particularly strongly stained, with substantial glial and neuropil immunostaining throughout the brain. Comparison between brain tissues from patients with Alzheimer's disease (AD) and nonAD subjects revealed a significant increase (P = 0.02) in PLTP levels in brain tissue homogenates and increased PLTP immunostaining in AD.

Role of cholesterol in synapse formation and function

Cholesterol is a multifaceted molecule, which serves as essential membrane component, as cofactor for signaling molecules and as precursor for steroid hormones. Consequently, defects in cholesterol metabolism cause devastating diseases. So far, the role of cholesterol in the nervous system is less well understood. Recent studies showed that cultured neurons from the mammalian central nervous system (CNS) require glia-derived cholesterol to form numerous and efficient synapses. This suggests that the availability of cholesterol in neurons limits the extent of synaptogenesis. Here, I will summarize the experimental evidence for this hypothesis, describe what is known about the structural and functional role of cholesterol at synapses, and discuss how cholesterol may influence synapse development and stability.

Outsourcing in the brain: do neurons depend on cholesterol delivery by astrocytes?

Brain function depends on the cooperation between highly specialized cells. Neurons generate electrical signals and glial cells provide structural and metabolic support. Here, I propose a new kind of job-sharing between neurons and astrocytes. Recent studies on primary cultures of highly purified neurons from the rodent central nervous system (CNS) suggest that, during development, neurons reduce or even abandon cholesterol synthesis to save energy and import cholesterol from astrocytes via lipoproteins. The cholesterol shuttle may be restricted to compartments distant from the soma including synapses and may be regulated by electrical activity. Testing these hypotheses will help to improve our still insufficient understanding of brain cholesterol metabolism and its role in neurodegeneration.

Cholesterol attenuates the membrane perturbing properties of beta-amyloid peptides

Growing evidence indicates a significant linkage between Abeta and cholesterol metabolism, although the exact role of cholesterol in brain aging and in the pathogenesis of AD is still unknown. Recently, in vitro and in vivo modification of cell cholesterol and its effect on Abeta-generation became a straight focus in the research of AD. In the present study, we discretely modulated the cholesterol contents of neuronal membranes from mice of different ages in vivo and in vitro using lovastatin and methyl-beta-cyclodextrin, respectively. The aim of the study was to investigate whether this modulation results in altered physico-chemical membrane properties. Therefore, we performed membrane fluidity measurements using three fluorescent dyes labeling different membrane regions. Furthermore, we evaluated the effects of cholesterol modulation on the membrane disturbing properties of Abeta. Modulation of membrane cholesterol content in vivo and in vitro was linked to changes in membrane properties. Very interestingly, cholesterol content of in vitro modulated neuronal membranes was negatively correlated with the membrane perturbing effects of Abeta.