Regulation of apolipoprotein E secretion in rat primary hippocampal astrocyte cultures

Impaired astrocytic synaptic function by peripheral cholesterol metabolite 27-hydroxycholesterol

Astrocytes represent the most abundant cell type in the brain, where they play critical roles in synaptic transmission, cognition, and behavior. Recent discoveries show astrocytes are involved in synaptic dysfunction during Alzheimer's disease (AD). AD patients have imbalanced cholesterol metabolism, demonstrated by high levels of side-chain oxidized cholesterol known as 27-hydroxycholesterol (27-OH). Evidence from our laboratory has shown that elevated 27-OH can abolish synaptic connectivity during neuromaturation, but its effect on astrocyte function is currently unclear. Our results suggest that elevated 27-OH decreases the astrocyte function in vivo in Cyp27Tg, a mouse model of brain oxysterol imbalance. Here, we report a downregulation of glutamate transporters in the hippocampus of CYP27Tg mice together with increased GFAP. GLT-1 downregulation was also observed when WT mice were fed with high-cholesterol diets. To study the relationship between astrocytes and neurons, we have developed a 3D co-culture system that allows all the cell types from mice embryos to differentiate in vitro. We report that our 3D co-cultures reproduce the effects of 27-OH observed in 2D neurons and in vivo. Moreover, we found novel degenerative effects in astrocytes that do not appear in 2D cultures, together with the downregulation of glutamate transporters GLT-1 and GLAST. We propose that this transporter dysregulation leads to neuronal hyperexcitability and synaptic dysfunction based on the effects of 27-OH on astrocytes. Taken together, these results report a new mechanism linking oxysterol imbalance in the brain and synaptic dysfunction through effects on astrocyte function.

Hypercholesterolemia and 27-Hydroxycholesterol Increase S100A8 and RAGE Expression in the Brain: a Link Between Cholesterol, Alarmins, and Neurodegeneration

Alterations in cholesterol metabolism in the brain have a major role in the physiology of Alzheimer's disease (AD). Oxysterols are cholesterol metabolites with multiple implications in memory functions and in neurodegeneration. Previous studies have shown detrimental effects of cholesterol metabolites in neurons, but its effect in glial cells is unknown. We used a high-fat/high-cholesterol diet in mice to study the effects of hypercholesterolemia over the alarmin S100A8 cascade in the hippocampus. Using CYP27Tg, a transgenic mouse model, we show that the hypercholesterolemia influence on the brain is mediated by the excess of 27-hydroxycholesterol (27-OH), a cholesterol metabolite. We also employed an acute model of 27-OH intraventricular injection in the brain to study RAGE and S100A8 response. We used primary cultures of neurons and astrocytes to study the effect of high levels of 27-OH over the S100A8 alarmin cascade. We report that a high-fat/high-cholesterol diet leads to an increase in S100A8 production in the brain. In CYP27Tg, we report an increase of S100A8 and its receptor RAGE in the hippocampus under elevated 27-OH in the brain. Using siRNA, we found that 27-OH upregulation of RAGE in astrocytes and neurons is mediated by the nuclear receptor RXRγ. Silencing RXRγ in neurons prevented 27-OH-mediated upregulation of RAGE. These results show that S100A8 alarmin and RAGE respond to high levels of 27-OH in the brain in both neurons and astrocytes through RXRγ. Our study supports the notion that 27-OH mediates detrimental effects of hypercholesterolemia to the brain via alarmin signaling.

Alterations in cholesterol metabolism as a risk factor for developing Alzheimer's disease: Potential novel targets for treatment

Alzheimer's disease (AD) is the most common form of dementia and it is characterized by the deposition of amyloid-β (Aβ) plaques and neurofibrillary tangles in the brain. However, the complete pathogenesis of the disease is still unknown. High level of serum cholesterol has been found to positively correlate with an increased risk of dementia and some studies have reported a decreased prevalence of AD in patients taking cholesterol-lowering drugs. Years of research have shown a strong correlation between blood hypercholesterolemia and AD, however cholesterol is not able to cross the Blood Brain Barrier (BBB) into the brain. Cholesterol lowering therapies have shown mixed results in cognitive performance in AD patients, raising questions of whether brain cholesterol metabolism in the brain should be studied separately from peripheral cholesterol metabolism and what their relationship is. Unlike cholesterol, oxidized cholesterol metabolites known as oxysterols are able to cross the BBB from the circulation into the brain and vice-versa. The main oxysterols present in the circulation are 24S-hydroxycholesterol and 27-hydroxycholesterol. These oxysterols and their catalysing enzymes have been found to be altered in AD brains and there is evidence indicating their influence in the progression of the disease. This review gives a broad perspective on the relationship between hypercholesterolemia and AD, cholesterol lowering therapies for AD patients and the role of oxysterols in pathological and non-pathological conditions. Also, we propose cholesterol metabolites as valuable targets for prevention and alternative AD treatments.

Aggregation of the Inflammatory S100A8 Precedes Aβ Plaque Formation in Transgenic APP Mice: Positive Feedback for S100A8 and Aβ Productions

Inflammation plays an important role in Alzheimer's disease (AD) and other neurodegenerative disorders. Although chronic inflammation in later stages of AD is well described, little is known about the inflammatory processes in preclinical or early stages of the disease prior to plaque deposition. In this study, we report that the inflammatory mediator S100A8 is increased with aging in the mouse brain. It is observed as extracellular aggregates, which do not correspond to corpora amylacea. S100A8 aggregation is enhanced in the hippocampi of two different mouse models for amyloid-β (Aβ) overproduction (Tg2576 and TgAPParctic mice). S100A8 aggregates are seen prior the formation of Aβ plaques and do not colocalize. In vitro treatment of glial cells from primary cultures with Aβ42 resulted in an increased production of S100A8. In parallel, treatment of a neuronal cell line with recombinant S100A8 protein resulted in enhanced Aβ42 and decreased Aβ40 production. Our results suggest that important inflammatory processes are occurring prior to Aβ deposition and the existence of a positive feedback between S100A8 and Aβ productions. The possible relevance of aging- or AD-dependent formation of S100A8 aggregates in the hippocampus thus affecting learning and memory processes is discussed.

Potential neurochemical links between cholesterol and suicidal behavior

The role of cholesterol in psychiatric diseases has aroused the interest of the medical community, particularly in association with violent and suicidal behavior. Herein, we discuss some aspects of brain cholesterol metabolism, exploring possible mechanisms underlying the findings and reviewing the available literature on the possible neurochemical link between suicide and low or reduced levels of serum cholesterol. Most of the current hypotheses suggest a decreased serotonergic activity due to a decrease in cholesterol in the lipid rafts of synaptic membranes. Some aspects and limitations of this assumption are emphasized. In addition to serotonin hypofunction, other mechanisms have been proposed to explain increased impulsivity in suicidal individuals, including steroid modulation and brain-derived neurotrophic factor decrease, which could also be related to changes in lipid rafts. Other putative markers of suicidal behavior (e.g. protein S100B) are discussed in connection with cholesterol metabolism in the brain tissue.

Cholesterol as a causative factor in Alzheimer's disease: a debatable hypothesis

High serum/plasma cholesterol levels have been suggested as a risk factor for Alzheimer's disease (AD). Some reports, mostly retrospective epidemiological studies, have observed a decreased prevalence of AD in patients taking the cholesterol lowering drugs, statins. The strongest evidence causally linking cholesterol to AD is provided by experimental studies showing that adding/reducing cholesterol alters amyloid precursor protein (APP) and amyloid beta-protein (Ab) levels. However, there are problems with the cholesterol-AD hypothesis. Cholesterol levels in serum/plasma and brain of AD patients do not support cholesterol as a causative factor in AD.Prospective studies on statins and AD have largely failed to show efficacy. Even the experimental data are open to interpretation given that it is well-established that modification of cholesterol levels has effects on multiple proteins, not only amyloid precursor protein and Ab. The purpose of this review, therefore, was to examine the above-mentioned issues, discuss the pros and cons of the cholesterol-AD hypothesis, involvement of other lipids in the mevalonate pathway, and consider that AD may impact cholesterol homeostasis.

AP-2β regulates amyloid beta-protein stimulation of apolipoprotein E transcription in astrocytes

Two key players involved in Alzheimer's disease (AD) are amyloid beta protein (Aβ) and apolipoprotein E (apoE). Aβ increases apoE protein levels in astrocytes which is associated with cholesterol trafficking, neuroinflammatory responses and Aβ clearance. The mechanism for the increase in apoE protein abundance is not understood. Based on different lines of evidence, we propose that the beta-adrenergic receptor (βAR), cAMP and the transcription factor activator protein-2 (AP-2) are contributors to the Aβ-induced increase in apoE abundance. This hypothesis was tested in mouse primary astrocytes and in cells transfected with an apoE promoter fragment with binding sites for AP-2. Aβ(42) induced a time-dependent increase in apoE mRNA and protein levels which were significantly inhibited by βAR antagonists. A novel finding was that Aβ incubation significantly reduced AP-2α levels and significantly increased AP-2β levels in the nuclear fraction. The impact of Aβ-induced translocation of AP-2 into the nucleus was demonstrated in cells expressing AP-2 and incubated with Aβ(42). AP-2 expressing cells had enhanced activation of the apoE promoter region containing AP-2 binding sites in contrast to AP-2 deficient cells. The transcriptional upregulation of apoE expression by Aβ(42) may be a neuroprotective response to Aβ-induced cytotoxicity, consistent with apoE's role in cytoprotection.

All-trans retinoic acid can regulate the expressions of gelatinases and apolipoprotein E in glomerulosclerosis rats

Apolipoprotein E (apoE) is an important plasma protein in cholesterol homeostasis and plays a key role in the pathogenesis of glomerulosclerosis (GS). Gelatinases include matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9). The abnormal expressions of gelatinases are implicated in the pathogenesis of extracellular matrix accumulation. All-trans retinoic acid (ATRA) is an import biological agent which can play a protective role against GS. We performed this investigation to explore whether ATRA could regulate the expressions of gelatinases and apoE in the glomerulus of GS rats. 120 Wistar rats were randomly divided into three groups: sham operation group (SHO), glomerulosclerosis model group without treatment (GS) and GS model group treated with ATRA (GA). The GS disease was established by uninephrectomy and adriamycin injection. At the end of 9 and 13 weeks, the relevant samples were collected and determined. Compared with GS group at 9/13 weeks, values of 24-hour urine total protein, 24-hour urine excretion for albumin, blood urea nitrogen, serum creatinine and glomerulosclerosis index, and protein expressions of apoE, transforming growth factor-βl (TGF-β1), α-smooth muscle actin, collagen-IV and fibronectin in glomerulus and mRNA expressions of apoE and TGF-β1 in renal tissue were significantly down-regulated by ATRA (each P<0.01). However, the expressions of MMP-2 and MMP-9 (mRNA, protein and activity) were enhanced in GA group than those in GS group. In conclusion, gelatinases are associated with apoE expression, and ATRA can increase the gelatinases expressions and reduce the accumulation of apoE in glomerulus of GS rats, but the detailed mechanism needs to be elucidated in the future.

Side chain-oxidized oxysterols regulate the brain renin-angiotensin system through a liver X receptor-dependent mechanism

Disturbances in cholesterol metabolism have been associated with hypertension and neurodegenerative disorders. Because cholesterol metabolism in the brain is efficiently separated from plasma cholesterol by the blood-brain barrier (BBB), it is an unsolved paradox how high blood cholesterol can cause an effect in the brain. Here, we discuss the possibility that cholesterol metabolites permeable to the BBB might account for these effects. We show that 27-hydroxycholesterol (27-OH) and 24S-hydroxycholesterol (24S-OH) up-regulate the renin-angiotensin system (RAS) in the brain. Brains of mice on a cholesterol-enriched diet showed up-regulated angiotensin converting enzyme (ACE), angiotensinogen (AGT), and increased JAK/STAT activity. These effects were confirmed in in vitro studies with primary neurons and astrocytes exposed to 27-OH or 24S-OH, and were partially mediated by liver X receptors. In contrast, brain RAS activity was decreased in Cyp27a1-deficient mice, a model exhibiting reduced 27-OH production from cholesterol. Moreover, in humans, normocholesterolemic patients with elevated 27-OH levels, due to a CYP7B1 mutation, had markers of activated RAS in their cerebrospinal fluid. Our results demonstrate that side chain-oxidized oxysterols are modulators of brain RAS. Considering that levels of cholesterol and 27-OH correlate in the circulation and 27-OH can pass the BBB into the brain, we suggest that this cholesterol metabolite could be a link between high plasma cholesterol levels, hypertension, and neurodegeneration.

Critical role of ABCA1 transporter in sphingosine 1-phosphate release from astrocytes

Sphingosine 1-phosphate (S1P) is accumulated in lipoproteins, especially high-density lipoprotein (HDL), in plasma. However, it remains uncharacterized how extracellular S1P is produced in the CNS. The treatment of rat astrocytes with retinoic acid and dibutyryl cAMP, which induce apolipoprotein E (apoE) synthesis and HDL-like lipoprotein formation, stimulated extracellular S1P accumulation in the presence of its precursor sphingosine. The released S1P was present together with apoE particles in the HDL fraction. S1P release from astrocytes was inhibited by the treatment of the cells with glybenclamide or small interfering RNAs specific to ATP-binding cassette transporter A1 (ABCA1). Astrocytes from Abca1-/- mice also showed impairment of retinoic acid/dibutyryl cAMP-induced S1P release in association with the blockage of HDL-like lipoprotein formation. However, the formation of either apoE or lipoprotein itself was not sufficient, and additional up-regulation of ABCA1 was requisite to stimulate S1P release. We conclude that the S1P release from astrocytes is coupled with lipoprotein formation through ABCA1.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Carotenoids and Alzheimer's disease: an insight into therapeutic role of retinoids in animal models

Carotenoids play a pivotal role in prevention of many degenerative diseases mediated by oxidative stress including neurodegenerative diseases like Alzheimer's Disease (AD). The involvement of retinoids in physiology, AD pathology and their therapeutic role in vitro and in vivo has been extensively studied. This review focuses on the role of carotenoids like retinoic acid (RA), all trans retinoic acid (ATRA), lycopene and β-carotene in prevention of AD symptoms primarily through inhibition of amyloid beta (Aβ) formation, deposition and fibril formation either by reducing the levels of p35 or inhibiting corresponding enzymes. The role of antioxidant micronutrients in prevention or delaying of AD symptoms has been included. This study emphasizes the dietary supplementation of carotenoids to combat AD and warrants further studies on animal models to unravel their mechanism of neuroprotection.

All-trans retinoic acid regulates the expression of apolipoprotein E in rats with glomerulosclerosis induced by Adriamycin

Apolipoprotein E (apoE) is an important plasma protein in cholesterol homeostasis and plays a key role in the progression of glomerulosclerosis (GS). We conducted this investigation to explore whether all-trans retinoic acid (ATRA) could regulate the apoE expression in the pathological process of GS. 120 Wistar rats were divided into three groups at random: sham operation group (SHO), glomerulosclerosis model group without treatment (GS), GS model group treated with ATRA (GA); n=40, respectively. The disease of GS in rat was established by uninephrectomy and adriamycin (5mg/kg) injection. At the end of 9 and 13 weeks, 20 rats in each group were killed and the relevant samples were collected. 24-hour urine total protein (24UTP), 24-hour urine excretion for albumin (24Ualb), serum total protein (TP) and serum albumin (Alb), blood urea nitrogen (BUN), serum creatinine (Scr), total cholesterol (TC), triglyceride (TG), high-density lipoprotein (HDL), low-density lipoprotein (LDL), serum and urine apoE and glomerulosclerosis index (GSI) were measured. The protein expressions of collagen IV (Col-IV), fibronectin (FN) and apoE in glomeruli were determined by immunohistochemistry. Real-time reverse transcription polymerase chain reaction (real-time RT-PCR) was used to detect the expression of apoE mRNA in kidney. TP and Alb in GA group in 9/13-week were increased than those of GS group, however, the differences were not statistically significant. Compared with group GS at 9/13 weeks, values of 24UTP, 24Ualb, BUN, Scr, TC, TG, HDL, LDL, serum and urine apoE, and GSI in GA group that were significantly reduced, and protein expressions of Col-IV, FN and apoE in glomeruli and expression of apoE mRNA in renal tissue were significantly down-regulated by ATRA (P<0.01). In conclusion, ATRA can regulate the expression of apoE, reduce the accumulation of extracellular matrix (ECM) and step down the progression of GS.

Apolipoprotein E and Alzheimer's disease: molecular mechanisms and therapeutic opportunities

Multiple genetic and environmental factors are likely to contribute to the development of Alzheimer's disease (AD). The most important known risk factor for AD is presence of the E4 isoform of apolipoprotein E (apoE). Epidemiological studies demonstrated that apoE4 carriers have a higher risk and develop the disease and an early onset. Moreover, apoE4 is the only molecule that has been associated with all the biochemical disturbances characteristic of the disease: amyloid-beta (Abeta) deposition, tangle formation, oxidative stress, lipid homeostasis deregulation, synaptic plasticity loss and cholinergic dysfunction. This large body of evidence suggest that apoE is a key player in the pathogenesis of AD. This short review examines the current facts and hypotheses of the association between apoE4 and AD, as well as the therapeutic possibilities that apoE might offer for the treatment of this disease.

Beta/A4-Amyloid increases nerve growth factor production in rat primary hippocampal astrocyte cultures

Nerve growth factor (NGF), a member of the neurotrophin family, is an essential mediator of neuronal activity and synaptic plasticity of basal forebrain cholinergic neurons (BFCN). In processes of chronic degeneration of BFCN like in Alzheimer's disease (AD), characterized among others by amyloid containing plaques, NGF has been shown to improve cognitive decline and rescue BFCN but also to reduce survival of hippocampal neurons via p75 neurotrophin receptor (p75). Little is known about the mechanisms of NGF regulation in glial cells under pathological conditions in AD. This study investigates the influence of amyloid administration on the NGF protein secretion in rat primary hippocampal astrocytes. Astrocytes were stimulated with "aged" beta/A4-Amyloid (1-40), and NGF was measured in different fractions, such as supernatant, vesicles, and cytosol fraction. Treatment with amyloid at a final concentration of 10 microM for 72 h led to increased NGF protein levels up to 30-fold increase compared to unstimulated controls. This observation may be an endogenous neuroprotective mechanism possibly contributing to a delay of amyloid-dependent loss of cholinergic neurons or contribute to accelerated neuronal death by activation of p75 within Alzheimer pathology.

HDL-like lipoproteins in cerebrospinal fluid affect neural cell activity through lipoprotein-associated sphingosine 1-phosphate

High-density lipoprotein (HDL)-associated sphingosine 1-phosphate mediates a variety of lipoprotein-induced actions in vascular cell systems. However, it remains unknown whether extracellular S1P is associated with lipoproteins to exert biological actions in central nervous system. Human cerebrospinal fluid (CSF) induced rat astrocyte migration in a manner sensitive to S1P receptor antagonist VPC23019 and the migration activity was recovered in S1P fraction by thin-layer chromatography. Density-gradient separation of CSF revealed that the major S1P activity was detected in the HDL fraction. In conditioned medium of rat astrocytes cultured with sphingosine, the S1P activity was recovered again in the HDL fraction. The HDL fraction also induced migration of astrocytes and process retraction of oligodendrocytes in a manner similar to S1P. We concluded that S1P is accumulated in HDL-like lipoproteins in CSF and mediates some of lipoprotein-induced neural cell functions in central nervous system.

Astroglia‐derived retinoic acid is a key factor in glia‐induced neurogenesis

Astroglial cells are essential components of the neurogenic niches within the central nervous system. Emerging evidence suggests that they are among the key regulators of postnatal neurogenesis. Although astrocytes have been demonstrated to possess the potential to instruct stem cells to adopt a neuronal fate, little is known about the nature of the glia-derived instructive signals. Here we propose that all-trans retinoic acid, one of the most powerful morphogenic molecules regulating neuronal cell fate commitment, may be one of the glia-derived factors directing astroglia-induced neurogenesis. According to data obtained from several complementary approaches, we show that cultured astrocytes express the key enzyme mRNAs of retinoic acid biosynthesis and actively produce all-trans retinoic acid. We show that blockage of retinoic acid signaling by the pan-RAR antagonist AGN193109 prevents glia-induced neuron formation by noncommitted stem cells. Therefore, we provide strong in vitro evidence for retinoic acid action in astroglia-induced neuronal differentiation.

Amyloid beta-protein1-42 increases cAMP and apolipoprotein E levels which are inhibited by beta1 and beta2-adrenergic receptor antagonists in mouse primary astrocytes

Amyloid beta-protein (Abeta) increases apolipoprotein E (apoE) levels in astrocytes which could alter lipid trafficking. The mechanism for the Abeta-induced increase in apoE levels is not well understood. It is well established that stimulation of beta-adrenergic receptors (betaARs) increases cAMP levels. Elevation of cAMP levels increases apoE abundance. The current study determined if Abeta(1-42) stimulation of cAMP and apoE levels could be inhibited by betaAR antagonists in astrocytes. We demonstrate that Abeta(1-42) but not the reverse protein Abeta(42-1) or Abeta(1-40) stimulated cAMP formation and this stimulation was inhibited by selective betaAR antagonists in mouse primary cortical astrocytes. Abeta(1-42) significantly increased apoE levels which were significantly inhibited by the betaAR selective antagonists with the greatest inhibition observed with the beta(2) antagonist. Separate lines of evidence have suggested that agonist-induced stimulation of betaARs and increases in apoE abundance may serve a neuroprotective role in astrocytes. Our results indicate a potential interaction between betaARs and apoE which may contribute to reducing Abeta(1-42) neurotoxicity.

Time course of response to estradiol replacement in ovariectomized mice: brain apolipoprotein E and synaptophysin transiently increase and glial fibrillary acidic protein is suppressed

The current study examined the effect of long-term estradiol replacement in ovariectomized mice. Estradiol-17beta (E2) pellets or vehicle pellets were implanted at the time of ovariectomy (OVX) in young adult female mice. Five mice from each group were sacrificed at 5, 14, 28 and 49 days after OVX and pellet replacement. Western blotting of homogenates from somatosensory cortex, hippocampus, olfactory bulb and cerebellum was performed to obtain concentrations of glial fibrillary acidic protein (GFAP), apolipoprotein E (apoE) and synaptophysin (SYN). At 5 days after OVX, GFAP levels were not affected by E2 replacement. In contrast to GFAP, synaptophysin and apoE concentrations were significantly elevated by 15% and 25%, respectively, in the E2-replaced group compared to the vehicle-replaced group at 5 days but by 14 days concentrations were equivalent. Late in the time course of this study, at 49 days, GFAP concentrations were higher in the E2-deprived mice but did not increase in the E2-replaced group. Immunocytochemistry for GFAP confirmed this observation. Of note was that these effects occurred in all four brain regions measured. These observations suggest that estradiol is able to suppress reactive gliosis. In addition, E2 replacement in OVX mice is associated with transiently higher levels of apoE and synaptophysin.

Astrocytes produce and secrete FGF-1, which promotes the production of apoE-HDL in a manner of autocrine action

The astrocytes prepared by 1 week secondary culture after 1 month primary culture of rat brain cells (M/W cells) synthesized and secreted apolipoprotein E (apoE) and cholesterol more than the astrocytes prepared by conventional 1 week primary and 1 week secondary culture (W/W cells) (Ueno, S., J. Ito, Y. Nagayasu, T. Furukawa, and S. Yokoyama. 2002. An acidic fibroblast growth factor-like factor secreted into the brain cell culture medium upregulates apoE synthesis, HDL secretion and cholesterol metabolism in rat astrocytes. Biochim. Biophys. Acta. 1589: 261-272). M/W cells also highly expressed fibroblast growth factor-1 (FGF-1) mRNA. FGF-1 was identified in the cell lysate of both cell types, but M/W cells released more of it into the medium. Immunostaining of FGF-1 and apoE revealed that both localized in the cells that produce glial fibrillary acidic protein. The conditioned media of M/W cells and FGF-1 stimulated W/W cells to release apoE and cholesterol to generate more HDL. Pretreatment with a goat anti-FGF-1 antibody or heparin depleted the stimulatory activity of M/W cell-conditioned medium. The presence of the anti-FGF-1 antibody in the medium suppressed apoE secretion by M/W cells. Differential inhibition of signaling pathways suggested that FGF-1 stimulates apoE synthesis via the phosphoinositide 3-OH kinase for PI3K/Akt pathway. Thus, astrocytes release FGF-1, which promotes apoE-HDL production by an autocrine mechanism. These results are consistent with our in vivo observation that astrocytes produce FGF-1 before the increase of apoE in the postinjury lesion of the mouse brain (Tada, T., J. Ito, M. Asai, and S. Yokoyama. 2004. Fibroblast growth factor 1 is produced prior to apolipoprotein E in the astrocytes after cryo-injury of mouse brain. Neurochem. Int. 45: 23-30).

Role of retinoid signalling in the adult brain

Vitamin A (all-trans-retinol) is the parent compound of a family of natural and synthetic compounds, the retinoids. Retinoids regulate gene transcription in numerous cells and tissues by binding to nuclear retinoid receptor proteins, which act as transcription factors. Much of the research conducted on retinoid signalling in the nervous system has focussed on developmental effects in the embryonic or early postnatal brain. Here, we review the increasing body of evidence indicating that retinoid signalling plays an important role in the function of the mature brain. Components of the metabolic pathway for retinoids have been identified in adult brain tissues, suggesting that all-trans-retinoic acid (ATRA) can be synthesized in discrete regions of the brain. The distribution of retinoid receptor proteins in the adult nervous system is different from that seen during development; and suggests that retinoid signalling is likely to have a physiological role in adult cortex, amygdala, hypothalamus, hippocampus, striatum and associated brain regions. A number of neuronal specific genes contain recognition sequences for the retinoid receptor proteins and can be directly regulated by retinoids. Disruption of retinoid signalling pathways in rodent models indicates their involvement in regulating synaptic plasticity and associated learning and memory behaviours. Retinoid signalling pathways have also been implicated in the pathophysiology of Alzheimer's disease, schizophrenia and depression. Overall, the data underscore the likely importance of adequate nutritional Vitamin A status for adult brain function and highlight retinoid signalling pathways as potential novel therapeutic targets for neurological diseases.

Cholinergic modulation of baker's yeast cell phagocytosis by rat astrocytes

Cholinergic regulation of baker's yeast cell phagocytosis in rat cultured astrocytes was studied. Phagocytic activity was reduced by 1 x 10(-5) M of atropine or pirenzepine, but not by AF-DX116 or 4-DAMP. In addition, carbachol stimulated phagocytosis in a dose-dependent manner. Furthermore, only 1 x 10(-5)M of atropine, pirenzepine and 4-DAMP significantly reduced enhanced activity induced by 1 x 10(-7)M carbachol. It was also observed that L-NMMA, staurosporine, or U-73122, reduced phagocytosis activity while TFP failed to do so. Nitrite levels in astrocyte supernatants increased after baker's yeast cells were incorporated to astrocyte cultures, correlating with enhanced phagocytosis induced by carbachol stimulation, and were reduced by 1 x 10(-5) M of atropine, pirenzepine or aminopiridine, but not by AF-DX116 or 4-DAMP. Enhanced NO production triggered by astrocyte phagocytosis may have pathological consequences.

Apolipoprotein E: diversity of cellular origins, structural and biophysical properties, and effects in Alzheimer's disease

Apolipoprotein E4 (apoE4) is a major risk factor for Alzheimer's disease (AD). Several hypotheses have been proposed to explain the association of the APOE epsilon4 allele with AD; however, the mechanisms underlying this association are largely unknown. Initially, apoE was thought to be synthesized primarily by astrocytes but not by neurons in the brain. However, subsequent studies have demonstrated that central nervous system neurons also express apoE under diverse physiological and pathological conditions. Detailed studies of the structure and biophysical properties of apoE isoforms have demonstrated unique properties distinguishing apoE4 from apoE3. Because the structural and biophysical properties of a protein determine how it functions under normal and abnormal conditions, apoE4, with its multiple cellular origins and multiple structural and biophysical properties, might contribute to the pathology of AD through several different mechanisms. Some of these mechanisms might be suitable targets for the development of new treatments for AD.

Plasticity and the spread of Alzheimer's disease-like changes

Tangles are a major histopathological feature of Alzheimer's disease and their regional location and number correlate significantly with the individual's cognitive decline. Intriguingly, these tangles are formed only in a small subset of nerve cell types and are practically absent in most animal species examined so far. In humans, tangle formation seemingly starts decades before clinical signs of dementia are seen and spread over cortical areas in a regular manner described by the Braak classification. In the present article the role of plasticity-related molecules and mechanisms are discussed considering their putative role in neuronal vulnerability and spread of tangles. Special emphasis is given to some aspects of lipid metabolism, that is, apolipoprotein E polymorphism, statin effects, and lysosomal dysfunction in Alzheimer's and Niemann-Pick C's diseases.

Apolipoprotein E and β-amyloid (1-42) regulation of glycogen synthase kinase-3β

Glycogen synthase kinase-3beta (GSK-3beta) is implicated in regulating apoptosis and tau protein hyperphosphorylation in Alzheimer's disease (AD). We investigated the effects of two key AD molecules, namely apoE (E3 and E4 isoforms) and beta-amyloid (Abeta) 1-42 on GSK-3beta and its major upstream regulators, intracellular calcium and protein kinases C and B (PKC and PKB) in human SH-SY5Y neuroblastoma cells. ApoE3 induced a mild, transient, Ca2+-independent and early activation of GSK-3beta. ApoE4 effects were biphasic, with an early strong GSK-3beta activation that was partially dependent on extracellular Ca2+, followed by a GSK-3beta inactivation. ApoE4 also activated PKC-alpha and PKB possibly giving the subsequent GSK-3beta inhibition. Abeta(1-42) effects were also biphasic with a strong activation dependent partially on extracellular Ca2+ followed by an inactivation. Abeta(1-42) induced an early and potent activation of PKC-alpha and a late decrease of PKB activity. ApoE4 and Abeta(1-42) were more toxic than apoE3 as shown by MTT reduction assays and generation of activated caspase-3. ApoE4 and Abeta(1-42)-induced early activation of GSK-3beta could lead to apoptosis and tau hyperphosphorylation. A late inhibition of GSK-3beta through activation of upstream kinases likely compensates the effects of apoE4 and Abeta(1-42) on GSK-3beta, the unbalanced regulation of which may contribute to AD pathology.

Proteomic analysis of astrocytic secretion in the mouse. Comparison with the cerebrospinal fluid proteome

Astrocytes, the most abundant cell type in the central nervous system, are intimately associated with synapses. They play a pivotal role in neuronal survival and the brain inflammatory response. Some astrocytic functions are mediated by the secretion of polypeptides. Using a proteomic approach, we have identified more than 30 proteins released by cultured astrocytes. These include proteases and protease inhibitors, carrier proteins, and antioxidant proteins. Exposing astrocytes to brefeldin A, which selectively blocks secretory vesicle assembly, suppressed the release of some of these proteins. This indicates that astrocytes secrete these proteins by a classic vesicular mechanism and others by an alternative pathway. Astrocytes isolated from different brain regions secreted a similar pattern of proteins. However, the secretion of some of them, including metalloproteinase inhibitors and apolipoprotein E, was region-specific. In addition, pro-inflammatory treatments modified the profile of astrocytic protein secretion. Finally, more than two thirds of the proteins identified in the astrocyte-conditioned medium were detectable in the mouse cerebrospinal fluid, suggesting that astrocytes contribute to the cerebrospinal fluid protein content. In conclusion, this study provides the first unbiased characterization of the major proteins released by astrocytes, which may play a crucial role in the modulation of neuronal survival and function.

Evidence for defective retinoid transport and function in late onset Alzheimer's disease

The hypothesis of this article is that late onset Alzheimer's disease (AD) is influenced by the availability in brain of retinoic acid (RA), the final product of the vitamin A (retinoid) metabolic cascade. Genetic, metabolic, and environmental/dietary evidence is cited supporting this hypothesis. Significant genetic linkages to AD are demonstrated for markers close to four of the six RA receptors, RA receptor G at 12q13, retinoid X receptor B at 6p21.3, retinoid X receptor G at 1q21, and RA receptor A at 17q21. Three of the four retinol-binding proteins at 3q23 and 10q23 and the RA-degrading cytochrome P450 enzymes at 10q23 and 2p13 map to AD linkages. Synthesis of the evidence supports retinoid hypofunction and impaired transport as contributing factors. These findings suggest testable experiments to determine whether increasing the availability of retinoid in brain, possibly through pharmacologic targeting of the RA receptors and the cytochrome P450 RA-inactivating enzymes, can prevent or decrease amyloid plaque formation.

Genotype-related differences of hippocampal apolipoprotein E levels only in early stages of neuropathological changes in Alzheimer's disease

Inheritance of the epsilon4 allele of apolipoprotein E (APOE, gene; apoE, protein) represents the most common genetic risk factor for developing Alzheimer's disease (AD), but the role of apoE in AD pathogenesis is yet to be clarified. A number of studies investigating apoE expression and protein levels in AD brain in correlation to its genetic polymorphism has yielded controversial results. We designed our approach based on neuropathological characteristics of AD to investigate apoE levels in relation to the APOE genotype and AD-related neurofibrillary changes, and amyloid deposits. We determined hippocampal apoE levels by reducing sodium dodecylsulfate-polyacrylamide gel electrophoresis and immunoblotting in 70 Braak-staged and APOE-genotyped autopsy brains. In our stage-, age- and gender-matched case sample, we found a significant increase of hippocampal apoE in the APOE epsilon3 homozygotes with beginning AD-related pathology (Braak stages I and II) compared with brain samples free of neurofibrillary changes and amyloid deposits. In the APOE epsilon4 allele carriers no such increase was found. In both genotype groups, severely affected brain samples with widespread neurofibrillary changes (Braak stages V and VI) and amyloid deposits (Braak stage C) showed low apoE levels comparable to those found in unaffected brain samples (Braak stage 0). Our data suggests that the isoform-specific impact of apoE on the development of AD may be of crucial importance only in the early stages of the disease. These stages are believed to represent phases of the disease in which the beginning neurodegeneration can be compensated by plastic reorganization.

Apolipoprotein E: a major piece in the Alzheimer's disease puzzle

Alzheimer's disease (AD) is a complex neurodegenerative disorder with multiple etiologies. The presence of the E4 isoform of apolipoprotein E (apoE) has been shown to increase the risk and to decrease the age of onset for AD and is the major susceptibility factor known for the disease. ApoE4 has been shown to intensify all the biochemical disturbances characteristic of AD, including beta amyloid (Abeta) deposition, tangle formation, neuronal cell death, oxidative stress, synaptic plasticity and dysfunctions of lipid homeostasis and cholinergic signalling. In contrast, other apoE isoforms are protective. Here we review and discuss these major hypotheses of the apoE4-AD association.