Accumulation of apolipoproteins in the regenerating and remyelinating mammalian peripheral nerve. Identification of apolipoprotein D, apolipoprotein A-IV, apolipoprotein E, and apolipoprotein A-I

Axon-Schwann cell interactions regulate the expression of fibroblast growth factor-5 (FGF-5)

We screened for genes whose expression is significantly up- or downregulated during Wallerian degeneration in adult rat sciatic nerve with cDNA arrays. Fibroblast growth factor-5 (FGF-5) mRNA seemed to be induced. This was confirmed by northern blotting and in situ hybridization, as well as Western blotting for FGF-5 in axotomized nerve. Axon-Schwann cell interactions decreased the steady-state level of FGF-5 mRNA in regenerating sciatic nerves, and forskolin diminished its expression in cultured Schwann cells. We conclude that denervated Schwann cells synthesize FGF-5, which is a secreted, neuronotrophic member of the FGF family.

Altered apolipoprotein D expression in the brain of patients with Alzheimer disease

The etiology of late-onset Alzheimer disease is poorly understood. Predisposing factors such as the apolipoprotein E4 allele, as well as protective factors (e.g., antioxidants) have been proposed to play a role in the disease's process. A search for predisposing factors contributing to sporadic late-onset Alzheimer disease was initiated using the differential display technique. RNA expression profiles of the entorhinal cortex and the cerebellum of Alzheimer-diseased and normal patients were compared. The entorhinal cortex is the first brain region to accumulate neurofibrillary tangles during disease progression, whereas the cerebellum is spared. In the Alzheimer cases of this study, one signal showing preferential expression in the entorhinal cortex corresponded to the apolipoprotein D gene. This preferential expression might be genuine at the RNA level as suggested by the in situ hybridization method used. In addition, immunohistochemical experiments showed higher percentages of Apolipoprotein D reactive pyramidal neurons in the entorhinal cortex and region 1 of Ammon's horn in diseased patients. This increase correlated with the number of neurofibrillary tangles in Alzheimer as well as in normal patients. Colocalization of Apolipoprotein D proteins and neurofibrillary tangles in the same neuron was rare. Thus, these results suggest that in Alzheimer disease and aging, apolipoprotein D gene expression is increased in stressed cortical neurons before they possibly accumulate neurofibrillary tangles.

The human neutrophil lipocalin supports the allosteric activation of matrix metalloproteinases

The human neutrophil lipocalin (HNL), a member of the large family of lipocalins that exhibit various physiological functions, is coexpressed in granulocytes with progelatinase B (MMP-9). Part of it is covalently bound to the proenzyme and therefore may play a possible role in the activation process of promatrix metalloproteinases. We now report that HNL is able to accelerate the direct activation of promatrix metalloproteinases slightly. A significant enhancement of the activity could be demonstrated for the HgCl2- and the plasma kallikrein-induced activation of all three secretory forms of proMMP-9 and of proMMP-8. The same activating effects were exerted by HNL isolated from granulocytes as well as by the recombinant forms expressed by the yeast Pichia pastoris or by Escherichia coli. This demonstrates that the carbohydrate moiety is not essential for the biological activity of HNL. Activation and activity enhancement are obviously mediated by entrapping the remaining N-terminal sequence residues of the partially truncated proenzyme into the hydrophobic binding pocket of the HNL. In conclusion these results document that HNL can exert an enzyme-activating effect in the regulation of inflammatory and pathophysiological responses of granulocytes in the physiological activation of MMPs that have been subject to limited proteolytic processing.

Increased CNS levels of apolipoprotein D in schizophrenic and bipolar subjects: implications for the pathophysiology of psychiatric disorders

Chronic administration of the atypical antipsychotic drug, clozapine, to rodents has been shown to increase the concentration of apolipoprotein D (apoD) in several area of the brain, suggesting that apoD could be involved in the therapeutic effects of antipsychotic drugs and/or the pathology of psychotic illnesses. Here, we measured a significant decrease in the concentration of apoD in serum samples from schizophrenic patients. In contrast, apoD levels were significantly increased (92--287%) in dorsolateral prefrontal cortex (Brodmann's area 9) of schizophrenic and bipolar subjects. Elevated levels of apoD expression were also observed in the caudate of schizophrenic and bipolar subjects (68--89%). No differences in apoD immunoreactivity were detected in occipital cortex (Brodmann's area 18) in either group, or in the hippocampus, substantia nigra, or cerebellum of the schizophrenic group. The low serum concentrations of apoD observed in these patients supports recent hypotheses involving systemic insufficiencies in lipid metabolism/signaling in schizophrenia. Elevation of apoD expression selectively within central nervous system regions implicated in the pathology of these neuropsychiatric disorders suggests a focal compensatory response that neuroleptic drug regimens may augment.

Characterization of transgenic mice expressing apolipoprotein E4(C112R) and apolipoprotein E4(L28P; C112R)

Apolipoprotein E (ApoE), which is genetically polymorphic, is a constituent of different lipoproteins. Two variants, ApoE4(C112R) and ApoE4(L28P; C112R) have been linked to the risk of developing Alzheimer's disease. Transgenic mice carrying ApoE4(C112R) (AD71) and ApoE4(L28P; C112R) (AD61) were generated and compared to wild-type mice. The use of glial fibrillary acidic protein as promoter led to transgene expression mainly in glial cells but also in neurons. Transgene protein levels were approximately three-and-a-half-fold that of endogenous ApoE in the glial fibrillary acidic protein-ApoE4(C112R) (AD71) and nearly twofold in the glial fibrillary acidic protein-ApoE4(L28P; C112R) (AD61) mouse lines. Neither transgenic mouse differed from wild-type in cognitive tests at the age of approximately one-and-a-half years. The locomotor activity of AD61 mice was similar to controls, whereas AD71 mice exhibited a clearly reduced level of motor activity. Immunohistological and biochemical brain protein analyses revealed no difference between strains.Thus, in the absence of morphological changes over-expression of ApoE4(C112R) on a background of endogenous mouse ApoE, may result in behavioral deficits while for the ApoE4(L28P; C112R) transgene higher expression might be required or some compensatory mechanisms might protect these animals from the behavioral abnormalities.

Expression of the apolipoprotein E gene in the skin is controlled by a unique downstream enhancer

A distal enhancer that specifies apolipoprotein E gene expression in the skin was identified and characterized by in situ hybridization in transgenic mice generated with constructs of the human apolipoprotein E/C-I/C-IV/C-II gene cluster. Transgene constructs containing the enhancer expressed high levels of apolipoprotein E mRNA in the germinative cell layer of the sebaceous gland and in epithelial cells of the hair follicle root sheath. Apolipoprotein E mRNA was also detected in basal epithelial cells of the epidermis. Expression of the human apolipoprotein E transgene at these sites was specified by a unique 1.0 kb enhancer domain located 1.7 kb downstream of the apolipoprotein E gene. No transgene expression was detected in skin epithelial cells in transgenic mice when this enhancer was deleted from the apolipoprotein E gene cluster. The enhancer was used to construct a transgene expression vector that faithfully directed a heterologous cDNA to the normal sites of apolipoprotein E gene expression in epithelial cells of the skin.

Bacterially produced apolipoprotein D binds progesterone and arachidonic acid, but not bilirubin or E-3M2H

Apolipoprotein D (ApoD) constitutes an atypical lipoprotein in so far as it is predominantly found associated with HDL particles but belongs to the lipocalin structural family. Apart from its involvement in serum lipid transport it is abundant in various tissues, and differing physiological functions have been ascribed to it. We have now developed an E. coli expression system that permits the efficient production of biochemically homogeneous ApoD via secretion into the bacterial periplasm. Detailed ligand binding studies by fluorescence titration revealed that progesterone and arachidonic acid are complexed with dissociation constants both in the 1 microM range, whereas the presumed ligands pregnenolone, bilirubin and E-3M2H are not recognized by the recombinant protein. In contrast with previous reports it thus appears that ApoD discriminates well in its binding function between closely related compounds.

Astrocyte phenotype and prevention against oxidative damage in neurotoxicity

Astrocytes possess a potent array of protective systems. These are chiefly targeted against oxidised products and radicals, which are frequently present in increased amounts following exposure of nervous tissue to a range of toxic insults. Following exposure to the toxic chemicals astrocytes commonly respond by alteration in phenotype with upregulation of a large number of molecules, including those controlling the protective systems. This article summarizes evidence, largely obtained from in vitro studies, which supports the concept that some of the changes in astrocyte phenotype are associated with increased protection against the cytotoxicity caused by the oxidative damage that results from exposure to range of neurotoxicants.

The bacterial lipocalins

The lipocalins were once regarded as a eukaryotic protein family, but new members have been recently discovered in bacteria. The first bacterial lipocalin (Blc) was identified in Escherichia coli as an outer membrane lipoprotein expressed under conditions of environmental stress. Blc is distinguished from most lipocalins by the absence of intramolecular disulfide bonds, but the presence of a membrane anchor is shared with two of its closest homologues, apolipoprotein D and lazarillo. Several common features of the membrane-anchored lipocalins suggest that each may play an important role in membrane biogenesis and repair. Additionally, Blc proteins are implicated in the dissemination of antibiotic resistance genes and in the activation of immunity. Recent genome sequencing efforts reveal the existence of at least 20 bacterial lipocalins. The lipocalins appear to have originated in Gram-negative bacteria and were probably transferred horizontally to eukaryotes from the endosymbiotic alpha-proteobacterial ancestor of the mitochondrion. The genome sequences also reveal that some bacterial lipocalins exhibit disulfide bonds and alternative modes of subcellular localization, which include targeting to the periplasmic space, the cytoplasmic membrane, and the cytosol. The relationships between bacterial lipocalin structure and function further illuminate the common biochemistry of bacterial and eukaryotic cells.

Apolipoprotein D

Apolipoprotein D (apoD) is a 29-kDa glycoprotein that is primarily associated with high density lipoproteins in human plasma. It is an atypical apolipoprotein and, based on its primary structure, apoD is predicted to be a member of the lipocalin family. Lipocalins adopt a beta-barrel tertiary structure and transport small hydrophobic ligands. Although apoD can bind cholesterol, progesterone, pregnenolone, bilirubin and arachidonic acid, it is unclear if any, or all of these, represent its physiological ligands. The apoD gene is expressed in many tissues, with high levels of expression in spleen, testes and brain. ApoD is present at high concentrations in the cyst fluid of women with gross cystic disease of the breast, a condition associated with increased risk of breast cancer. It also accumulates at sites of regenerating peripheral nerves and in the cerebrospinal fluid of patients with neurodegenerative conditions, such as Alzheimer's disease. ApoD may, therefore, participate in maintenance and repair within the central and peripheral nervous systems. While its role in metabolism has yet to be defined, apoD is likely to be a multi-ligand, multi-functional transporter. It could transport a ligand from one cell to another within an organ, scavenge a ligand within an organ for transport to the blood or could transport a ligand from the circulation to specific cells within a tissue.

Uptake of lipoproteins for axonal growth of sympathetic neurons

Lipoproteins originating from axon and myelin breakdown in injured peripheral nerves are believed to supply cholesterol to regenerating axons. We have used compartmented cultures of rat sympathetic neurons to investigate the utilization of lipids from lipoproteins for axon elongation. Lipids and proteins from human low density lipoproteins (LDL) and high density lipoproteins (HDL) were taken up by distal axons and transported to cell bodies, whereas cell bodies/proximal axons internalized these components from only LDL, not HDL. Consistent with these observations, the impairment of axonal growth, induced by inhibition of cholesterol synthesis, was reversed when LDL or HDL were added to distal axons or when LDL, but not HDL, were added to cell bodies. LDL receptors (LDLRs) and LR7/8B (apoER2) were present in cell bodies/proximal axons and distal axons, with LDLRs being more abundant in the former. Inhibition of cholesterol biosynthesis increased LDLR expression in cell bodies/proximal axons but not distal axons. LR11 (SorLA) was restricted to cell bodies/proximal axons and was undetectable in distal axons. Neither the LDL receptor-related protein nor the HDL receptor, SR-B1, was detected in sympathetic neurons. These studies demonstrate for the first time that lipids are taken up from lipoproteins by sympathetic neurons for use in axonal regeneration.

The synthesis and transport of lipids for axonal growth and nerve regeneration

Neurons are unique polarized cells in which the growing axon is often located up to a meter or more from the cell body. Consequently, the intracellular movement of membrane lipids and proteins between cell bodies and axons poses a special challenge. The mechanisms of lipid transport within neurons are, for the most part, unknown although lipid transport via vesicles and via cholesterol- and sphingolipid-rich 'rafts' are considered likely mechanisms. Very active anterograde and retrograde transport of lipid-containing vesicles occurs between the cell body and distal axons. However, it is becoming clear that the axon need not obtain all of its membrane constituents from the cell body. For example, the synthesis of phosphatidylcholine, the major membrane phospholipid, occurs in axons, and its synthesis at this location is required for axonal elongation. In contrast, cholesterol synthesis appears to occur only in cell bodies, and cholesterol is efficiently delivered from cell bodies to axons by anterograde transport. Cholesterol that is required for axonal growth can also be exogenously supplied from lipoproteins to axons of cultured neurons. Several studies have suggested a role for apolipoprotein E in lipid delivery for growth and regeneration of axons after a nerve injury. Alternatively, or in addition, apolipoprotein E has been proposed to be a ligand for receptors that mediate signal transduction cascades. Lipids are also transported from axons to myelin, although the importance of this process for myelination is not clear.

Presence of apolipoprotein E immunoreactivity in degenerating neurones of mice is dependent on the severity of kainic acid-induced lesion

Apolipoprotein E (apoE) is a major apolipoprotein in the central nervous system (CNS) that may play a role in various CNS disorders. ApoE is primarily localised in astrocytes, but neuronal apoE mRNA expression has been demonstrated in normal and diseased human brain, as well as in ischaemic rat brain. To obtain further insight into the role of apoE in neuronal degeneration in the CNS and conditions of neuronal apoE localisation, we have investigated in mice the distribution of apoE following neuronal injury induced by kainic acid (n=35, 25 or 35 mg kainic acid/kg BW). Consecutive series of brain sections were immunostained for apoE and markers for astroglia (GFAP) and microglia/macrophage cells (CR3). Degenerating neurones were identified with a silver-degeneration staining technique. The intensity and cellular distribution of apoE-immunoreactivity (apoE-ir) was dependent on the severity of neuronal injury. Mice that developed mild neuronal degeneration, restricted to a subset of neurones in the hippocampus, showed increased apoE-ir in astrocytes concomitant with increased GFAP-ir and mild microgliosis. In these mice, no neuronal apoE-ir was detected. In contrast, mice developing severe neuronal injury in the hippocampus - frequently also showing degeneration in other brain regions including cortex, thalamus, striatum and amygdala - showed intense apoE-ir in degenerating neurones. Surrounding the lesion, apoE-ir was increased in neuropil recurrently whereas GFAP-ir astrocytes disappeared. Thus, in mice apoE accumulates in degenerating neurones in conditions of severe neuronal injury putatively in association with disruption of the glial network.

Apolipoprotein E and apolipoprotein D expression in a murine model of singlet oxygen-induced cerebral stroke

Apolipoprotein E (apoE)-deficient mice exhibit neuronal abnormalities similar to those in Alzheimer's disease and enhanced sensitivity to stroke-associated injuries. Here, we show that apoE deficiency results in impaired microglia/macrophage recruitment and accumulation after cerebral infarct. Astrogliosis and apolipoprotein D (apoD) expression are unaffected, suggesting that the neurological abnormalities of apoE-deficient mice could be due to impaired microglia/macrophage recruitment/accumulation, which is important for the clearance of neurodegenerative products via reverse cholesterol transport. To our knowledge, the results presented herein provide the first experimental evidence that brain microglia/macrophage recruitment/accumulation is affected by apoE deficiency. The insights gained from this study should facilitate the elucidation of the role of apoE in neurological disorders such as dementia with stroke and Alzheimer's disease.

Improved post-embedding immunocytochemistry of myelinated nervous tissue for electron microscopy

The particularly high lipid content of normal mature adult myelin sheaths, together with the light fixation protocols usually necessary to retain antigenicity, combine to make white matter nervous tissue an especially problematical subject for post-embedding immuno-electron microscopy using modern acrylic resins. Fixation and infiltration modifications to standard processing schedules for Lowicryl were found to greatly improve the embedding and therefore the resulting morphology. This in turn improved the signal to noise ratio by reducing the high non-specific backgrounds usually found in poorly infiltrated areas. Using Lowicryl HM20, we have been able to obtain satisfactory immunostaining for myelin basic protein with good retention of structural integrity in the myelin of both normal and lesioned adult cortico spinal tract.

Increased expression of apolipoprotein D following experimental traumatic brain injury

Increasing evidence suggests that apolipoprotein D (apoD) could play a major role in mediating neuronal degeneration and regeneration in the CNS and the PNS. To investigate further the temporal pattern of apoD expression after experimental traumatic brain injury in the rat, male Sprague-Dawley rats were subjected to unilateral cortical impact injury. The animals were killed and examined for apoD mRNA and protein expression and for immunohistological analysis at intervals from 15 min to 14 days after injury. Increased apoD mRNA and protein levels were seen in the cortex and hippocampus ipsilateral to the injury site from 48 h to 14 days after the trauma. Immunohistological investigation demonstrated a differential pattern of apoD expression in the cortex and hippocampus, respectively: Increased apoD immunoreactivity in glial cells was detected from 2 to 3 days after the injury in cortex and hippocampus. In contrast, increased expression of apoD was seen in cortical and hippocampal neurons at later time points following impact injury. Concurrent histopathological examination using hematoxylin and eosin demonstrated dark, shrunken neurons in the cortex ipsilateral to the injury site. In contrast, no evidence of cell death was observed in the hippocampus ipsilateral to the injury site up to 14 days after the trauma. No evidence of increased apoD mRNA or protein expression or neuronal pathology by hematoxylin and eosin staining was detected in the contralateral cortex and hippocampus. Our results reveal induction of apoD expression in the cortex and hippocampus following traumatic brain injury in the rat. Our data also suggest that increased apoD expression may play an important role in cortical neuronal degeneration after brain injury in vivo. However, increased expression of apoD in the hippocampus may not necessarily be indicative of neuronal death.

Regional difference of lipid distribution in brain of apolipoprotein E deficient mice

According to recent knowledge, apolipoprotein E (apo E) plays a significant role in the homeostasis of intracellular cholesterol level in various tissues. Apo E deficient mice develop hyperlipidemia, and suffer from atherosclerosis in extracerebral blood vessels and neurodegeneration in the central nervous system. Furthermore, Walker et al. (Am. J. Path., 1997;151:1371-1377) demonstrated cerebral xanthomas of various sizes in the brain of apo E deficient mice. In the present study, it is illustrated that in the homozygous apo E deficient mice of advancing age, a great number of foamy macrophages extravasate from microvessels in thalamus and fimbria hippocampi, and scatter in the perivascular regions and migrate toward the ependyma, fimbria hippocampi, hippocampus, and thalamus. Here, it must be pointed out that under hyperlipidemia, although foamy macrophages made clusters in the perivascular region, the cerebral microvessels did not develop atherosclerosis. On the other hand, in the other cerebral regions such as cerebral cortex, caudoputamen, globus pallidus, and substantia nigra, macrophages did not appear and microvessels retained normal shapes, but the fluorescent granular perithelial (in short, FGP) cells accompanied by these vessels contained a certain amount of lipids. That is, in the cerebral cortex and caudoputamen, lipid components are detected in FGP cells and microglia, while in the globus pallidus and substantia nigra, they are mainly localized in astrocytes. The reason why the astrocytes in such defined regions contain, specifically, a high quantity of lipid components remains unsettled. Axonal degenerations are often represented in thalamus, globus pallidus, and substantia nigra. On the other hand, in the specimens of Wild-type mice, lipid components were observed only in FGP cells, and the vascular architecture took a normal profile. Any lipid laden macrophages and the axonal degenerations could not be detected through the cerebral parenchyma. Furthermore, it is also a noticeable finding that immunohistochemically, the FGP cells express a positive reaction against the antibody of apo E in the Wild-type mice, but those of homozygous apo E deficient mice are immunonegative. FGP cells are not only provided with the scavenger receptor, but also contribute to the lipid metabolism in the brain.

Modulation of apolipoprotein D and apolipoprotein E expression in rat hippocampus after entorhinal cortex lesion

Apolipoprotein (apo) D is a member of the lipocalin family of proteins. Although its physiological function is unknown, apoD is thought to transport one or more small hydrophobic ligands. A second apolipoprotein, apoE is known to play an important role in lipid transport, and apoE genetic polymorphism has been shown to be associated with susceptibility to Alzheimer's disease. Both apoD and apoE are expressed in the central nervous system (CNS) and both proteins accumulate at sites of peripheral nerve injury due to increased local synthesis. The two proteins may have overlapping or complementary functions within nervous tissue. In order to define the role of apoD within the CNS, we have studied the regional distribution of apoD and apoE mRNA and protein within the normal rat brain and the changes in apoD and apoE expression in the hippocampus of rats after entorhinal cortex lesion (EC lesion). Within the brains of normal rats, apoD expression in the hippocampus was as high as 180-fold that of the liver. ApoD mRNA levels in other areas of the rat brain ranged from 40 to 120 times the hepatic levels. The distribution of apoE gene expression within the brain was similar to that of apoD, but was much lower than hepatic apoE expression. When rats were subjected to EC lesion, the apoD message increased by 54% at 4 days post lesion (DPL) in the ipsilateral region of hippocampus while apoE mRNA levels (ipsilateral and contralateral) decreased by 43%. At 6 to 8 DPL apoD mRNA in the ipsilateral hippocampus remained elevated (42% above controls) whereas the apoE mRNA levels increased to about 15% above those of controls. At 14 and 31 DPL, both apoD and apoE expression was similar to controls. The increase in immunoreactive apoD in hippocampal extracts was more dramatic. At 1 DPL, immunoreactive apoD levels were already 16-fold higher than those in extracts of non-lesioned animals and, at 31 DPL, levels were still 8-fold higher than those of control animals. Finally, we have demonstrated that the levels of apoD in the brains of apoE-deficient mice are 50-fold those of wildtype control mice. ApoD clearly has an important function within the CNS in both normal and pathological situations.

Hippocampal alterations of apolipoprotein E and D mRNA levels in vivo and in vitro following kainate excitotoxicity

Alteration in the expression of apolipoprotein E (ApoE) and apolipoprotein D (ApoD) genes was evaluated in rat, 7 days following status epilepticus (SE) induced by intra-amygdala injection of kainate (KA), and in organotypic hippocampal cultures, 2 days after a single 1 h exposure to KA. Global polyadenylated RNA (poly A+) steady state, assessing global regulation of mRNA transcription was first measured in cortices and hippocampi from each animal and in the organotypic cultures. No alteration due to KA treatment was observed and individual concentrations of ApoE and ApoD mRNA species were therefore measured and comparative analysis performed. In the cortices of KA-treated animals, ApoE and ApoD mRNA levels did not show statistically significant changes. In contrast, in hippocampi, 7 days after SE, ApoE and ApoD mRNA levels were significantly increased, respectively, by 123 and 138%. This in vivo effect was confirmed in vitro on organotypic cultures, where KA treatment increased ApoE and ApoD mRNA expressions, respectively, by 72 and 61%. These observations indicate that lipidic metabolism is modified in the lesioned structure and suggest an increased traffic of lipids and a need for more ApoE and D in the hippocampus during the period of recovery and restructuration that follows severe seizures.

Apolipoprotein D gene expression in the rat brain and light and electron microscopic immunocytochemistry of apolipoprotein D expression in the cerebellum of neonatal, immature and adult rats

Apolipoprotein D gene and protein expression were investigated in the rat brain and cerebellum, respectively, during development. Apolipoprotein D gene expression was first observed in embryonic day 12 rat brain, with a moderate increase in apolipoprotein D messenger RNA levels towards the later part (embryonic days 15-17) of gestation. In the postnatal rat brain, a marked induction of apolipoprotein D messenger RNA occurred at postnatal day 10, with progressively higher levels of apolipoprotein D messenger RNA observed up to postnatal day 20. Somewhat lower, but none the less high, levels of apolipoprotein D messenger RNA continued to be present in brains of adult animals. In the immature cerebellum (day 3 up to one- to two-week-old rats), there were many densely labeled apolipoprotein D-immunoreactive cells that had features of oligodendrocyte precursors. Purkinje neurons showed apolipoprotein D immunoreactivity in one- to two-week-old animals, after which there appeared to be some decrease in staining. Oligodendrocytes in the cerebella of two-week-old animals were strongly apolipoprotein D positive, with immunoreactivity declining in older animals. These results reveal a maturation-associated induction of apolipoprotein D gene expression in the rat brain, and expression of apolipoprotein D in glial (immature oligodendrocyte) cells in the immature cerebellum, followed by specific expression of apolipoprotein D in Purkinje neurons.

Localization of Niemann-Pick C1 protein in astrocytes: implications for neuronal degeneration in Niemann- Pick type C disease

Niemann-Pick type C disease (NP-C) is an inherited neurovisceral lipid storage disorder characterized by progressive neurodegeneration. Most cases of NP-C result from inactivating mutations of NPC1, a recently identified member of a family of genes encoding membrane-bound proteins containing putative sterol sensing domains. By using a specific antipeptide antibody to human NPC1, we have here investigated the cellular and subcellular localization and regulation of NPC1. By light and electron microscopic immunocytochemistry of monkey brain, NPC1 was expressed predominantly in perisynaptic astrocytic glial processes. At a subcellular level, NPC1 localized to vesicles with the morphological characteristics of lysosomes and to sites near the plasma membrane. Analysis of the temporal and spatial pattern of neurodegeneration in the NP-C mouse, a spontaneous mutant model of human NP-C, by amino-cupric-silver staining, showed that the terminal fields of axons and dendrites are the earliest sites of degeneration that occur well before the appearance of a neurological phenotype. Western blots of cultured human fibroblasts and monkey brain homogenates revealed NPC1 as a 165-kDa protein. NPC1 levels in cultured fibroblasts were unchanged by incubation with low density lipoproteins or oxysterols but were increased 2- to 3-fold by the drugs progesterone and U-18666A, which block cholesterol transport out of lysosomes, and by the lysosomotropic agent NH4Cl. These studies show that NPC1 in brain is predominantly a glial protein present in astrocytic processes closely associated with nerve terminals, the earliest site of degeneration in NP-C. Given the vesicular localization of NPC1 and its proposed role in mediating retroendocytic trafficking of cholesterol and other lysosomal cargo, these results suggest that disruption of NPC1-mediated vesicular trafficking in astrocytes may be linked to neuronal degeneration in NP-C.

The golden hamster aphrodisin gene. Structure, expression in parotid glands of female animals, and comparison with a similar murine gene

The so-called lipocalins are a family of extracellular proteins that are known to typically fulfill tasks as transport proteins for small hydrophobic molecules. However, in the last decade, a large diversity has been described concerning their functions, for example as enzymes, immunomodulators, or proteins involved in coloration and pheromone action. Aphrodisin belongs to those lipocalins, which are of significant importance for the pheromonal stimulation of copulatory behavior in male hamsters. We recently succeeded in characterizing the corresponding cDNA and demonstrated the expression of the aphrodisin gene in the vagina, uterus, and Bartholin's glands of female hamsters. Here we report the structure of the aphrodisin gene and the functionality of its promoter region. We further compare the aphrodisin gene to the related gene for mouse odorant-binding protein 1a, indicating similar functions of their products. As a novelty, we show that the aphrodisin gene, in addition to the above-mentioned tissues, is also expressed in female hamster parotid glands. In contradiction to the results expected, we finally demonstrate that aphrodisin already occurs in vaginal discharge before the female animals reach fertility. These findings may lead to the identification of as yet unknown aphrodisin functions.

Apolipoprotein E4 induces neuronal cell death under conditions of suppressed de novo cholesterol synthesis

The presence of the apolipoprotein E (apoE) allele epsilon4 is a major risk factor for the development of Alzheimer's disease (AD); however, the molecular mechanism underlying the acceleration of AD development in individuals with epsilon4 remains to be determined. To investigate the isoform-specific effects of apoE on neurons, primary neuron cultures were prepared from fetal rat cerebral cortices. Inhibition by compactin, a 3-hydroxyl-3-methylglutaryl coenzyme A reductase inhibitor of de novo cholesterol synthesis, induced premature neuronal cell death in a dose-dependent manner. In the presence of compactin at a sublethal dose to the cells, rabbit beta-migrating very low density lipoprotein (beta-VLDL) with human apoE4 (the product of epsilon4) induced premature neuronal cell death, while that with apoE3 (the product of epsilon3) did not. Neurons cultured in the presence of apoE4, beta-VLDL, and compactin were shrunken and spherical, containing condensed chromatin and fragmented DNA, features characteristic of apoptosis. The addition of intermediate metabolites of the cholesterol biosynthetic pathway, including mevalonate and squalene, rescued neuronal cells incubated with apoE4 and beta-VLDL, in the presence of compactin. These results strongly suggest that a reduction in the level of endogenously synthesized cholesterol is a prerequisite for apoE4-induced neuronal cell death.

Pattern of apolipoprotein D immunoreactivity in human brain

Presence of intracytoplasmatic apolipoprotein D (apo D), a lipophilic ligand transporter, was investigated in normal human brains between 20 and 55 years, using an anti-human apolipoprotein D antibody and extravidin-biotin-enhanced immunohistochemistry. Apo D immunoreactivity was found in neuroglial cells of white matter in all sampled brain regions studied but also in pial cells and perivascular cells. Immunoreactive neurons do not present a uniform pattern throughout the gray matter. The pons and the brainstem show a high immunoreactivity for apo D in several nuclei (olivary, arciforme, cuneado, raphe). In the cerebellum the immunoreactivity appears in some neurons of the Purkinje layer. Finally in the cerebral cortex apo D positive neurons were not observed. These results suggest that apo D role may vary depending of cellular synthesis or location.

Peripheral sensory nerve defects in apolipoprotein E knockout mice

Apolipoprotein E (apoE), a plasma lipoprotein involved in lipid metabolism, is also proposed to have important functions within the central and peripheral nervous systems. To investigate the function of apoE in the peripheral nervous system, we examined the structure of sciatic nerves in apoE-deficient (apoE KO) mice. In the normal peripheral nervous system, apoE is produced by nonmyelinating Schwann cells, suggesting a role for apoE in the support of unmyelinated thermal and nociceptive sensory afferents. Using electron microscopy, we have found that apoE KO mice have abnormal and reduced numbers of unmyelinated axons within the sciatic nerve. ApoE KO unmyelinated axons are irregularly shaped and separated by very little Schwann cell cytoplasm. ApoE KO myelinated fibers and myelin are ultrastructurally normal. Consistent with these morphological findings, apoE KO mice display reduced sensitivity to noxious thermal stimuli. These data provide in vivo support for the hypothesis that apoE promotes neuronal health and survival.

Apolipoprotein E and multiple sclerosis: a biochemical and genetic investigation

Apolipoprotein E (apo E) is postulated to be a major lipid carrier protein in the brain involved in brain development and repair. Multiple sclerosis (MS) is a major demyelinating disease characterized by destruction of myelin and marked alteration of myelin cholesterol and lipid metabolism. We have determined serum and cerebrospinal fluid (CSF) apo E concentrations using an original time-resolved immunofluorometric assay and calculated intrathecal apo E concentration. Apo E concentrations were determined in 13 control subjects and 129 neurological patients: 34 definite MS patients, 25 with Guillain-Barré syndrome (GBS), 32 with amyotrophic lateral sclerosis (ALS) and 38 with other neurological diseases. Seven clinical parameters (sex, age, age at MS onset, duration of the disease, course, clinical status and disability score) were considered in MS patients. Significant (P < 0.01) decrease in CSF apo E was observed in MS, linked to a decrease in intrathecal apo E. The decreased CSF apo E concentration in MS patients occur independent of the apo E genotype. Apo E is considered as a neurotrophic factor in the brain. Any decrease in intrathecal apo E synthesis would thus contribute to progression of neurological diseases, such as MS.

Apolipoprotein AIV codon 360 mutation increases with human aging and is not associated with Alzheimer's disease

In order to evaluate genetic apolipoprotein polymorphisms as risk factors for Alzheimer's disease (AD), we studied apolipoprotein (apo) AIV after apoE, an apolipoprotein also present in the brain. The allelic distribution of apoAIV codon 360 polymorphism was no different in AD group (n = 120) compared with elderly healthy individuals (n = 119). Surprisingly, this polymorphism was over-represented (11.40%, vs. 5.7% P < 0.005) in all these aged subjects (74.29 +/- 8.46 years) and independently of the clinical and mental status compared with the younger population (39.00 +/- 9.69 years) of the same regional recruitment. These results suggest that the apoAIV (360:His) allele could be a marker of aging and longevity.

A minimally lipidated form of cell-derived apolipoprotein E exhibits isoform-specific stimulation of neurite outgrowth in the absence of exogenous lipids or lipoproteins

Within the central nervous system, apolipoprotein E (apoE) synthesis is increased in response to nerve injury, a finding that may reflect a role for apoE in neuronal remodeling. Recent studies show that apoE3 promotes and apoE4 inhibits neurite outgrowth in cultured neuronal cells. Interestingly, these isoform-specific effects are observed only when apoE is presented to cells in the presence of an exogenous lipid source such as rabbit beta-very low density lipoprotein (beta-VLDL), making it difficult to discern the biologically active form of apoE or to understand the role of the lipid source. In the present study we tested whether a cell-derived lipidated form of apoE can alter neurite outgrowth in the absence of beta-VLDL by constructing Neuro-2a cell lines expressing high levels of apoE. Our results showed that endogenous apoE3 stimulated neurite outgrowth, whereas the endogenous apoE4 isoform was neutral. Furthermore, beta-VLDL antagonized the stimulatory effects of the endogenous apoE3. Characterization of the secreted apoE3 indicated that the neurite outgrowth-stimulating activity could be recovered from culture medium with an anti-apoE immunoaffinity column and was present in a poorly lipidated particle with a density between 1.19 and 1.26 g/ml. These results indicated that the biological activity of apoE3 in stimulating neurite outgrowth was inherent in the cell-derived apoE particle and was not dependent on either (a) an interaction of apoE3 with an artificial lipid source or (b) independent actions of apoE3 and beta-VLDL.

Immunolocalization of apolipoprotein D, androgen receptor and prostate specific antigen in early stage prostate cancers

PURPOSE

To determine the cellular distribution and levels of immunohistochemical staining for apolipoprotein D (Apo-D), prostate specific antigen (PSA) and androgen receptor (AR) in early stage prostate cancers.

MATERIALS AND METHODS

Cellular distribution of Apo-D, PSA and AR in 30 stage A/B prostate cancers and in non-malignant glandular tissue contained in the same sections was detected immunohistochemically, and staining was evaluated by computerized video image analysis.

RESULTS

Staining for Apo-D (percentage positive cellular area) was significantly increased in tumor cells of early stage prostate cancers compared with non-malignant glandular tissue. PSA and AR were present at high levels in both early stage prostate tumors and non-malignant prostate.

CONCLUSIONS

Malignant transformation in the prostate is associated with increased cellular levels of Apo-D.

Increased apolipoprotein E mRNA in the hippocampus in Alzheimer disease and in rats after entorhinal cortex lesioning

The distribution of apolipoprotein E (ApoE) mRNA was characterized in the hippocampus of humans with Alzheimer disease (AD) and in rats with experimental lesions (unilateral ablation of the entorhinal cortex) that model selected features of AD. In both AD and the lesion model, we observed a shift in the location of astrocytes containing prevalent ApoE mRNA from the neuropil to regions with densely packed neurons. The increased abundance of ApoE mRNA in astrocytes close to neuron cell bodies could be indicative of lipid uptake in regions where neurons are degenerating or where synaptic remodeling is taking place.

Elevation of apolipoprotein E in the CSF of cattle affected by BSE

The cerebrospinal fluid (CSF) of patients suffering from Creutzfeldt-Jakob disease (CJD) display two unique polypeptide chains by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). In the absence of a well-defined ante-mortem diagnostic test for bovine spongiform encephalopathy (BSE), spinal fluid samples of eight normal cows and eight cows known to carry BSE by post-mortem histological analysis were investigated to verify if equivalent polypeptides were present. Proteins with similar migration to human CJD polypeptides were not detected. But surprisingly, a cluster of polypeptide spots that was faint or not detected in normal bovine CSF samples was found to be elevated or massively increased in BSE CSF samples (more than 10-fold increase). These elevated polypeptide chains were identified as apolipoprotein E.

Differential expression of apolipoprotein D and apolipoprotein E in the kainic acid-lesioned rat hippocampus

Expression of apolipoprotein D, a member of the lipocalin superfamily of transporter proteins, was investigated in the kainic acid-lesioned rat hippocampus. Using an anti-rat apolipoprotein D antibody and biotin avidin-enhanced immunocytochemistry, in the normal rat hippocampus there was little apolipoprotein D expression, that was restricted mainly to scattered astrocytes. By contrast, kainic acid-injected rats showed apolipoprotein D immunoreactivity in the pyramidal neurons of the affected CA fields 24-48 h after injection of the excitotoxin, at a time when there was no histological evidence of cell death. Apolipoprotein D immunoreactivity peaked by day 3, coincident with neuronal cell death, and declined thereafter, reaching very low levels by day 7. Besides pyramidal neurons, apolipoprotein D immunoreactivity was also observed in a small number of reactive glial cells in the affected CA fields, but not in the vascular compartments at any time-point. In contrast to the neuronal expression of apolipoprotein D, apolipoprotein E immunoreactivity was observed predominantly in degenerating astrocytes. In conclusion, following excitotoxic injury with kainic acid, apolipoprotein D is expressed in hippocampal pyramidal neurons destined for subsequent cell death.

Apolipoprotein E uptake and low-density lipoprotein receptor-related protein expression by the NTera2/D1 cell line: a cell culture model of relevance for late-onset Alzheimer's disease

Apolipoprotein E has been shown to be a risk factor for late-onset Alzheimer's disease, with the apolipoprotein epsilon 4 allele conferring the risk. Apolipoprotein E is found in neurofibrillary tangles and senile plaques, the pathological characteristics of Alzheimer's disease. To date there is no direct evidence that human neurons can take up exogenous apolipoprotein E, which is necessary if apolipoprotein E is involved in the formation of neurofibrillary tangles. To examine apolipoprotein E uptake we employed the human NTera2/D1 cell line, which can be induced by retinoic acid to differentiate into postmitotic NTera2-N neurons, which have the characteristics and morphology of human central nervous system neurons. We defined the cell line as genotype apolipoprotein epsilon 3/3 and demonstrated that the cells do not synthesize apolipoprotein E but can take up and internalize exogenous recombinant apolipoprotein E3. We also confirmed the expression of the low-density lipoprotein receptor-related protein, a known receptor for apolipoprotein E. The NTera2/D1 cell line therefore provides a useful human cell model for examining the effects of other apolipoprotein E isoforms with a view to defining intraneuronal interactions of apolipoprotein E.

Retinoic acid-induced expression of apolipoprotein D and concomitant growth arrest in human breast cancer cells are mediated through a retinoic acid receptor RARalpha-dependent signaling pathway

Apolipoprotein D (apoD) is a human plasma protein, belonging to the lipocalin superfamily, that is produced by a specific subtype of highly differentiated breast carcinomas and that is strongly up-regulated by retinoic acid (RA) in breast cancer cells. In this work, we have examined the molecular mechanisms mediating the induction of apoD gene expression by retinoids in T-47D human breast cancer cells. Northern blot analysis revealed that Ro40-6055, a synthetic retinoid that selectively binds and activates the retinoic acid receptor RARalpha, induced the accumulation of apoD mRNA in breast cancer cells in a time- and dose-dependent manner. The time course analysis demonstrated that apoD mRNA was induced 14-fold over control cells after 48 h of incubation with 10(-8) M Ro40-6055. As little as 10(-11) M of this retinoid induced apoD mRNA 5-fold over the control, whereas incubation with 10(-7) M Ro40-6055 induced maximally 15-fold over control cells. RARalpha-selective antagonists counteracted the inductive effects of all-trans-RA, 9-cis-RA, and Ro40-6055 on the expression of apoD, when present at the same concentration as the retinoid agonists. By contrast, RARbeta-, RARgamma-, and RXR-selective retinoids did not affect apoD gene expression. The retinoid agonist Ro40-6055 had an antiproliferative effect on T-47D cells, with maximal growth inhibition of approximately 60% obtained after 7 days of incubation with 10(-7) M. This antiproliferative effect could be counteracted by a 100-fold excess of the antagonist Ro41-5253. Treatment of the cells with retinoids that do not bind the nuclear retinoic acid receptors did not affect apoD expression, despite the fact that they did have a strong antiproliferative effect on T-47D cells. On the basis of these results, a role for RARalpha on apoD gene expression induction by retinoids in breast cancer cells is proposed.

Alterations in apolipoprotein E expression during aging and neurodegeneration

Apolipoprotein E (apoE) is a 34 kDa protein that plays an important role in cholesterol transport, uptake and redistribution. Within the nervous system, apoE might be involved in maintaining synaptic integrity after injury and during aging. ApoE might help maintain the integrity of the synaptodendritic complex by several different mechanisms. Among them, recent studies have suggested that apoE: (1) stabilizes the neuronal cytoskeleton; (2) plays an important role in transporting esterified cholesterol to neurons undergoing reinnervation where it is taken up by the low density lipoprotein receptor-related protein pathway and used as a precursor for the synthesis of new synaptic terminals; (3) regulates interactions between neurons and the extracellular matrix (e.g. laminin); and (4) regulates levels of intracellular calcium. The main objective of the manuscript is to review the current progress in understanding the functions of apoE in the nervous system and how malfunctioning of this molecule might result in neurodegenerative disorders such as Alzheimer's disease.

A zone immunoelectrophoresis assay method for quantification of apolipoprotein D in human cerebrospinal fluid

A zone immunoelectrophoresis assay (ZIA) has been developed for the quantification of apolipoprotein D (apo D) in human unconcentrated cerebrospinal fluid (CSF). The apo D concentrations of samples of the serum, plasma and CSF were directly proportional to the migration distances of the corresponding zones of immunoprecipitates developed during electrophoresis in glass capillaries filled with antibody-containing agarose gel. A linear standard curve, between about 1 and 12 mg of apo D/1 was obtained using a commercial serum preparation. Seronorm, as apo D standard. The coefficients of variation of the ZIA were below 8% (n = 5 x 6) and 10% (n = 8) for within-run and between-run reproducibility, respectively. Quantification experiments with disulfide-reducing agent, mixtures of CSF and urine as well as frozen and stored CSF samples indicated parallelism between the precipitate-forming immunologic reactions of apo D in different sample matrices when performed with ZIA. Application of this method to quantify apo D of CSF and plasma samples from 51 normal healthy men aged 16-72 years yielded means +/- SD of 5.3 +/- 1.5 mg/l and 128.4 +/- 22.7 mg/l, respectively. No correlation was found between the CSF and plasma apo D concentrations.

Molecular cloning of the mouse apolipoprotein D gene and its upregulated expression in Niemann-Pick disease type C mouse model

Apolipoprotein D (ApoD) is a member of the lipocalin superfamily. The primary structure and diverse expression of ApoD suggest that this protein is a multiligand, multifunctional glycoprotein. Here we report the structure of the mouse ApoD gene, which is composed of six exons spanning approximately 20 kb in length. All the exon-intron splice junctions follow the consensus GT/AG sequence. The 5'-flanking region of the mouse ApoD gene contains several putative regulatory elements, including FSE-2, GRE, SDR, MRE, IL-6RE, and TATA box. Northern blot analysis revealed that ApoD was highly expressed in the brain and adipose tissue in mouse. Lower levels of expression were observed in the heart, lung, thymus, testis, and salivary glands. In situ hybridization for the brain showed that ApoD mRNA was mainly localized in the subarachnoid space including the pia. In the Niemann-Pick disease type C mouse model, ApoD expression was upregulated in many organs such as brain, adipose tissue, heart, and thymus, presumably due to enhanced ApoD synthesis in perivascular fibroblasts.

Secretion and processing of apolipoprotein A-I in the avian sciatic nerve during development

Apolipoprotein A-I (apo A-I), a major apolipoprotein synthesized by liver and intestine to facilitate transport of plasma lipids as lipoproteins, has been detected also in the avian sciatic nerve. The mRNA and protein levels of apo A-I have been shown to increase during the period of rapid myelination (LeBlanc et al.: J Cell Biol 109:1245-1256, 1989). In order to assess the synthesis of apo A-I protein and the processing of apo A-I isoforms during development, endoneurial slices of avian sciatic nerves from chicks during active myelination at 15 and 17 days embryonic and 1 day posthatch age were incubated with [35]S-methionine. The incubations were fractionated into secreted and intracellular fractions, and incorporation of the label was assessed for apo A-I protein. The pattern of labeling of Po protein, as a marker of myelination, was also determined in the intracellular and compact myelin fractions. Methionine incorporation into Po protein was highest in the intracellular compartment at the 15-day embryonic stage and decreased thereafter, with a corresponding increase in the myelin fraction. During these developmental periods, the levels of nascent apo A-I increased in both the secreted and intracellular fractions. The synthesis of apo A-I specifically increases in the secreted fraction compared with total protein synthesis. The processing of the pro-apo A-I is also developmentally regulated. In the intracellular compartment, there are approximately equal proportions of the acidic and basic isoforms. However, with increasing age, a higher proportion of the apo A-I is secreted as acidic isoforms. It is concluded that the secretion and processing of apo A-I is developmentally regulated in the chick sciatic nerve, in parallel with the process of active myelination.

Identification of a circulatory and oocytic avian apolipoprotein D

Apolipoprotein D (apo D) is an unusual apolipoprotein with respect to structure and sites of synthesis. It has been identified in the circulatory system of certain mammals, but its physiological role remains poorly understood. In this report, it is shown that apo D is not exclusively a mammalian apolipoprotein, and evidence is presented which suggests a novel function for this protein during oogenesis in the chicken. The avian apo D which we identify has the same molecular mass (29 kDa) as the human protein and also associates preferentially with the plasma lipoprotein fraction. In addition to the 29 kDa avian apo D species, an immunoreactive 24 kDa protein is observed in chicken serum. The chicken apo D (along with the 24 kDa species) is also demonstrated to be present in the yolk of the rapidly growing chicken oocyte, a cell with high endocytic activity. Clathrin-coated vesicles from chicken oocytes, which we have previously shown to contain specific lipoproteins along with their oocytic receptors (Bujo et al., 1994: EMBO J 13:5165-5175), also contain chicken apo D. Thus, apo D represents a novel candidate for plasma-to-oocyte transport of lipids and/or their mobilization during embryogenesis in oviparous species.

Stationary phase expression of a novel Escherichia coli outer membrane lipoprotein and its relationship with mammalian apolipoprotein D. Implications for the origin of lipocalins

We report a novel outer membrane lipoprotein of Escherichia coli. DNA sequencing between ampC and sugE at the 94.5 min region of the E. coli chromosome revealed an open reading frame specifying 177 amino acid residues. Primer extension analysis demonstrated that the promoter is activated at the transition between exponential and stationary growth phases under control of the rpoS sigma factor gene, and this was confirmed in vivo by monitoring expression of beta-galactosidase activity from a lacZ translational fusion. The amino acid sequence exhibited 31% identity with human apolipoprotein D (apoD), which is a component of plasma high density lipoprotein and belongs to the eukaryotic family of lipocalins. The bacterial lipocalin (Blc) contained a short deletion of 7 amino acid residues corresponding to a hydrophobic surface loop that is thought to facilitate the physical interaction between apoD and high density lipoprotein. However, Blc exhibited a typical prokaryotic lipoprotein signal peptide at its amino terminus. Overexpression, membrane fractionation, and metabolic labeling with [3H]palmitate demonstrated that Blc is indeed a globomycin-sensitive outer membrane lipoprotein. Blc represents the first bacterial member of the family of lipocalins and may serve a starvation response function in E. coli.

Arachidonic acid binds to apolipoprotein D: implications for the protein's function

The lipocalin apolipoprotein D (ApoD) is associated in human plasma with lecithin-cholesterol acyl transferase. It has also been found in high concentration in the fluid of gross cystic disease of the mammary gland. Using protein fluorescence quenching, it is shown that ApoD binds arachidonic acid (Ka of 1.6 x 10(8) M-1) and as previously known progesterone (Ka of 2.5 x 10(6) M-1), but neither cholesterol nor any of the other prostanoid molecules examined had measurable affinity. This specific binding of arachidonate, also observable directly, suggests a role for ApoD in the mobilisation of arachidonic acid, and hence prostaglandin synthesis.