Immunocytochemical localization of apolipoprotein D in oligodendrocyte precursor-like cells, perivascular cells, and pericytes in the human cerebral cortex

Glycolytically impaired Drosophila glial cells fuel neural metabolism via β-oxidation

Neuronal function is highly energy demanding and thus requires efficient and constant metabolite delivery by glia. Drosophila glia are highly glycolytic and provide lactate to fuel neuronal metabolism. Flies are able to survive for several weeks in the absence of glial glycolysis. Here, we study how Drosophila glial cells maintain sufficient nutrient supply to neurons under conditions of impaired glycolysis. We show that glycolytically impaired glia rely on mitochondrial fatty acid breakdown and ketone body production to nourish neurons, suggesting that ketone bodies serve as an alternate neuronal fuel to prevent neurodegeneration. We show that in times of long-term starvation, glial degradation of absorbed fatty acids is essential to ensure survival of the fly. Further, we show that Drosophila glial cells act as a metabolic sensor and can induce mobilization of peripheral lipid stores to preserve brain metabolic homeostasis. Our study gives evidence of the importance of glial fatty acid degradation for brain function, and survival, under adverse conditions in Drosophila.

The Lipocalin Apolipoprotein D Functional Portrait: A Systematic Review

Apolipoprotein D is a chordate gene early originated in the Lipocalin protein family. Among other features, regulation of its expression in a wide variety of disease conditions in humans, as apparently unrelated as neurodegeneration or breast cancer, have called for attention on this gene. Also, its presence in different tissues, from blood to brain, and different subcellular locations, from HDL lipoparticles to the interior of lysosomes or the surface of extracellular vesicles, poses an interesting challenge in deciphering its physiological function: Is ApoD a moonlighting protein, serving different roles in different cellular compartments, tissues, or organisms? Or does it have a unique biochemical mechanism of action that accounts for such apparently diverse roles in different physiological situations? To answer these questions, we have performed a systematic review of all primary publications where ApoD properties have been investigated in chordates. We conclude that ApoD ligand binding in the Lipocalin pocket, combined with an antioxidant activity performed at the rim of the pocket are properties sufficient to explain ApoD association with different lipid-based structures, where its physiological function is better described as lipid-management than by long-range lipid-transport. Controlling the redox state of these lipid structures in particular subcellular locations or extracellular structures, ApoD is able to modulate an enormous array of apparently diverse processes in the organism, both in health and disease. The new picture emerging from these data should help to put the physiological role of ApoD in new contexts and to inspire well-focused future research.

Neuroprotective Effect of Apolipoprotein D in Cuprizone-Induced Cell Line Models: A Potential Therapeutic Approach for Multiple Sclerosis and Demyelinating Diseases

Apolipoprotein D (Apo D) overexpression is a general finding across neurodegenerative conditions so the role of this apolipoprotein in various neuropathologies such as multiple sclerosis (MS) has aroused a great interest in last years. However, its mode of action, as a promising compound for the development of neuroprotective drugs, is unknown. The aim of this work was to address the potential of Apo D to prevent the action of cuprizone (CPZ), a toxin widely used for developing MS models, in oligodendroglial and neuroblastoma cell lines. On one hand, immunocytochemical quantifications and gene expression measures showed that CPZ compromised neural mitochondrial metabolism but did not induce the expression of Apo D, except in extremely high doses in neurons. On the other hand, assays of neuroprotection demonstrated that antipsychotic drug, clozapine, induced an increase in Apo D synthesis only in the presence of CPZ, at the same time that prevented the loss of viability caused by the toxin. The effect of the exogenous addition of human Apo D, once internalized, was also able to directly revert the loss of cell viability caused by treatment with CPZ by a reactive oxygen species (ROS)-independent mechanism of action. Taken together, our results suggest that increasing Apo D levels, in an endo- or exogenous way, moderately prevents the neurotoxic effect of CPZ in a cell model that seems to replicate some features of MS which would open new avenues in the development of interventions to afford MS-related neuroprotection.

Apolipoprotein D

ApoD is a 25 to 30 kDa glycosylated protein, member of the lipocalin superfamily. As a transporter of several small hydrophobic molecules, its known biological functions are mostly associated to lipid metabolism and neuroprotection. ApoD is a multi-ligand, multi-function protein that is involved lipid trafficking, food intake, inflammation, antioxidative response and development and in different types of cancers. An important aspect of ApoD's role in lipid metabolism appears to involve the transport of arachidonic acid, and the modulation of eicosanoid production and delivery in metabolic tissues. ApoD expression in metabolic tissues has been associated positively and negatively with insulin sensitivity and glucose homeostasis in a tissue dependent manner. ApoD levels rise considerably in association with aging and neuropathologies such as Alzheimer's disease, stroke, meningoencephalitis, moto-neuron disease, multiple sclerosis, schizophrenia and Parkinson's disease. ApoD is also modulated in several animal models of nervous system injury/pathology.

Expression Pattern of Myelin-Related Apolipoprotein D in Human Multiple Sclerosis Lesions

Apolipoprotein D (Apo D) is a key molecule in the lipid transport during homeostasis and repair processes in normal and pathological conditions of the nervous system with a putative neuroprotective effect. In the last decades, huge experimental efforts have been made to know the exact mechanism of action of Apo D, even though, it remains an open question. In this regard, studies in mammals and flies have suggested that Apo D seems to act through a variety of cellular mechanisms related with its ability to selectively bind different lipid ligands. For instance, this apolipoprotein is required to myelin compaction, it participates in axon regeneration/remyelination, and it can control the magnitude and timing of the inflammatory response after injury, promoting myelin clearance, and regulating the number of immune cells recruited to the damaged area. These, among others, are some of the reasons to study Apo D in multiple sclerosis (MS) pathology, where it could be particularly important since the autoimmune reaction against oligodendrocytes (OLGs) and myelin is generally assumed as the most plausible cause of this pathology. The aim of this work was to investigate the Apo D expression pattern in MS lesions, including active and inactive demyelinating plaques, and also remyelinating ones. Human brain tissues with inflammatory demyelination consistent with MS were used to quantify Apo D immunosignal in different lesions. Our results show a clear decrease of Apo D expression in all sclerosis plaques, being lower in the inactive than in active areas but recovers in the remyelination ones. Apo D is mainly produced by the matured OLGs of white matter and is located in cell processes surrounding the myelin sheath. All these data seem to indicate an important role of Apo D in myelination/remyelination processes as a molecule with a neuroprotective potential, and may serve as a good starting point for its study in MS.

Apolipoprotein D Overexpression Protects Against Kainate-Induced Neurotoxicity in Mice

Excitotoxicity due to the excessive activation of glutamatergic receptors leads to neuronal dysfunction and death. Excitotoxicity has been implicated in the pathogenesis of a myriad of neurodegenerative diseases with distinct etiologies such as Alzheimer's and Parkinson's. Numerous studies link apolipoprotein D (apoD), a secreted glycoprotein highly expressed in the central nervous system (CNS), to maintain and protect neurons in various mouse models of acute stress and neurodegeneration. Here, we used a mouse model overexpressing human apoD in neurons (H-apoD Tg) to test the neuroprotective effects of apoD in the kainic acid (KA)-lesioned hippocampus. Our results show that apoD overexpression in H-apoD Tg mice induces an increased resistance to KA-induced seizures, significantly attenuates inflammatory responses and confers protection against KA-induced cell apoptosis in the hippocampus. The apoD-mediated protection against KA-induced toxicity is imputable in part to increased plasma membrane Ca2+ ATPase type 2 expression (1.7-fold), decreased N-methyl-D-aspartate receptor (NMDAR) subunit NR2B levels (30 %) and lipid metabolism alterations. Indeed, we demonstrate that apoD can attenuate intracellular cholesterol content in primary hippocampal neurons and in brain of H-apoD Tg mice. In addition, apoD can be internalised by neurons and this internalisation is accentuated in ageing and injury conditions. Our results provide additional mechanistic information on the apoD-mediated neuroprotection in neurodegenerative conditions.

Apo J and Apo D: Complementary or Antagonistic Roles in Alzheimer's Disease?

Apolipoprotein D (Apo D) and Apolipoprotein J (Apo J) are among the only nine apolipoproteins synthesized in the nervous system. Apart from development, these apolipoproteins are implicated in the normal aging process as well as in different neuropathologies as Alzheimer's disease (AD), where a neuroprotective role has been postulated. Different authors have proposed that Apo D and Apo J could be biomarkers for AD but as far as we know, there are no studies about the relationship between them as well as their expression pattern along the progression of the disease. In this paper, using double immunohistochemistry techniques, we have demonstrated that Apo D is mainly located in glial cells while Apo J expression preferentially occurs in neurons; both proteins are also present in AD diffuse and mature senile plaques but without signal overlap. In addition, we have observed that Apo J and Apo D immunostaining shows a positive correlation with the progression of the disease and the Braak's stages. These results suggest complementary and cell-dependent neuroprotective roles for each apolipoprotein during AD progress.

Apolipoprotein D subcellular distribution pattern in neuronal cells during oxidative stress

Apolipoprotein D (Apo D) is a secreted glycoprotein, member of the lipocalin superfamily, with a related beneficial role in metabolism and lipid transport due to the presence of a binding pocket that allows its interaction with several lipids. Nowadays, it has been clearly demonstrated that Apo D expression is induced and its subcellular location undergoes modifications in stressful and pathological conditions that characterize aging processes and neurodegenerative diseases. The aim of the present work was to study in detail the effect of H2O2 on the subcellular location of Apo D, in the hippocampal cell line HT22, by structural, ultrastructural, immunocytochemical, and molecular techniques in order to characterize the Apo D distribution pattern in neurons during oxidative stress. Our results indicate that Apo D is located in the cytoplasm under physiological conditions but treatment with H2O2 induces apoptosis and causes a displacement of Apo D location to the nucleus, coinciding with DNA fragmentation. In addition, we demonstrated that Apo D tends to accumulate around the nuclear envelope in neurons and glial cells of different brain areas in some neurodegenerative diseases and during human aging, but never inside the nucleus. These data suggest that the presence of Apo D in the nucleus, which some authors related with a specific transport, is a consequence of structural and functional alterations during oxidative stress and not the result of a specific role in the regulation of nuclear processes.

Cerebrospinal fluid proteomics in multiple sclerosis

Multiple sclerosis (MS) is an immune mediated chronic inflammatory disease of the central nervous system usually initiated during young adulthood, affecting approximately 2.5 million people worldwide. There is currently no cure for MS, but disease modifying treatment has become increasingly more effective, especially when started in the first phase of the disease. The disease course and prognosis are often unpredictable and it can be challenging to determine an early diagnosis. The detection of novel biomarkers to understand more of the disease mechanism, facilitate early diagnosis, predict disease progression, and find treatment targets would be very attractive. Over the last decade there has been an increasing effort toward finding such biomarker candidates. One promising strategy has been to use state-of-the-art quantitative proteomics approaches to compare the cerebrospinal fluid (CSF) proteome between MS and control patients or between different subgroups of MS. In this review we summarize and discuss the status of CSF proteomics in MS, including the latest findings with a focus on the last five years. This article is part of a Special Issue entitled: Neuroproteomics: Applications in Neuroscience and Neurology.

Pathways of cholesterol homeostasis in mouse retina responsive to dietary and pharmacologic treatments

Effects of serum cholesterol on cholesterol content in the retina are currently unknown. It is also unclear how cholesterol levels are controlled in the retina. High-cholesterol diet and oral administrations of simvastatin were used to modulate serum cholesterol in mice. These treatments only modestly affected cholesterol content in the retina and had no significant effect on retinal expression of the major cholesterol- and vision-related genes; the sterol-regulatory element binding protein pathway of transcriptional regulation does not seem to be operative in the retina under the experimental conditions used. Evidence is obtained that posttranslational mechanisms play a role in the control of retinal cholesterol. Retinal genes were only upregulated by oral administrations of TO901317 activating liver X receptors. Three of the upregulated genes could be of particular importance (apoD, Idol, and Rpe65) and have not yet been considered in the context of cholesterol homeostasis in the retina. Collectively, the data obtained identify specific features of retinal cholesterol maintenance and suggest additional therapies for age-related macular degeneration, a blinding disease characterized by cholesterol and lipid accumulations in chorioretinal tissues.

Discovery and initial verification of differentially abundant proteins between multiple sclerosis patients and controls using iTRAQ and SID-SRM

In the present study, we aimed to discover cerebrospinal fluid (CSF) proteins with significant abundance difference between early multiple sclerosis patients and controls, and do an initial verification of these proteins using selected reaction monitoring (SRM). iTRAQ and Orbitrap MS were used to compare the CSF proteome of patients with clinically isolated syndrome (CIS) (n=5), patients with relapsing-remitting multiple sclerosis that had CIS at the time of lumbar puncture (n=5), and controls with other inflammatory neurological disease (n=5). Of more than 1200 identified proteins, five proteins were identified with significant abundance difference between the patients and controls. In the initial verification using SRM we analyzed a larger patient and control cohort (n=132) and also included proteins reported as differentially abundant in multiple sclerosis in the literature. We found significant abundance difference for 11 proteins after verification, of which the five proteins alpha-1-antichymotrypsin, contactin-1, apolipoprotein D, clusterin, and kallikrein-6 were significantly differentially abundant in several of the group comparisons. This initial study form the basis for further biomarker verification studies in even larger sample cohorts, to determine if these proteins have relevance as diagnostic or prognostic biomarkers for multiple sclerosis.

Age-related changes of apolipoprotein D expression in female rat central nervous system with chronic estradiol treatment

Aging is associated with a reduction in metabolic functions, increased incidence of neurodegenerative diseases, and memory or cognitive dysfunction. With aging, a decrease in plasma estrogen levels, related to loss of gonadal function, occurs in females. Estrogens have neuroprotective effects and estradiol treatment improves some aspects of neuronal homeostasis affected by aging. In other way, recent studies show that apo D can play a neuroprotective role in some neuropathologies and during aging. The possible relation between estradiol treatment and the expression of apo D, during aging in the CNS, was investigated in female rats. Our results confirm an expression of apo D zone-dependent, in relation with aging, and an overexpression of apo D related to ovariectomy and estradiol treatment. This overexpression strengthens the idea that apo D plays a neuroprotective role in the CNS.

Neuroprotective effect of apolipoprotein D against human coronavirus OC43-induced encephalitis in mice

Apolipoprotein D (apoD) is a lipocalin upregulated in the nervous system after injury or pathologies such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. We previously demonstrated that apoD protects against neuropathology by controlling the level of peroxidated lipids. Here, we further investigated the biological function of apoD in a mouse model of acute encephalitis. Our results show that apoD transcript and protein are upregulated during acute encephalitis induced by the human coronavirus OC43 (HCoV-OC43) infection. The apoD upregulation coincides with glial activation, and its expression returns to normal levels when the virus is cleared, concomitantly to a resolved glial reactivity. In addition, the overexpression of human apoD in the neurons of Thy-1/ApoD transgenic mice results in a threefold increase of the number of mice surviving to HCoV-OC43 infection. This increased survival rate is correlated with an upregulated glial activation associated with a limited innate immune response (cytokines, chemokines) and T-cell infiltration into infected brains. Moreover, the protection seems to be associated with a restricted phospholipase A2 activity. These data reveal a role for apoD in the regulation of inflammation and suggest that it protects from HCoV-OC43-induced encephalitis, most likely through the phospholipase A2 signaling pathways.

Deficit of pericapillary oligodendrocytes in the prefrontal cortex in schizophrenia

OBJECTIVE

Our previous studies have shown a significant decrease of numerical density of oligodendrocytes in the prefrontal cortex in postmortem schizophrenic brains. Deficit of oligodendrocytes was associated with loss of oligodendroglial satellites of pyramidal neurons. In this study we tested the hypothesis that there might be a deficit and loss of pericapillary oligodendrocytes in the prefrontal cortex in schizophrenia.

METHOD

Autopsy samples from the prefrontal cortex (BA 10) were obtained from 12 normal controls and 12 chronic schizophrenic cases. Capillaries and oligodendrocytes were viewed in paraffin sections stained with a Luxol-fast blue and cresyl violet. Electron microscopy was applied to study the ultrastructure of oligodendrocytes. For morphometric analysis, an average of 100 rectilinear capillary segments from layer V was sampled for each individual. The number of oligodendrocytes visible along rectilinear segments was expressed as the number of oligodendrocytes per 0.01 mm of capillary length.

RESULTS

Subjects with schizophrenia had a significantly lower (-23%; P < 0.005) number of pericapillary oligodendrocytes compared to controls. Prominent ultrastructural dystrophic and degenerative alterations of pericapillary oligodendrocytes have been revealed in schizophrenic brains.

CONCLUSION

The present study provides evidence that there is a prominent reduction, damage and loss of pericapillary oligodendrocytes in the prefrontal cortex in schizophrenia. These changes may contribute to the pathophysiological basis for altered blood-brain barrier and lowered metabolic rates in subjects with schizophrenia.

Liver X receptor activation enhances cholesterol loss from the brain, decreases neuroinflammation, and increases survival of the NPC1 mouse

Although cholesterol is a major component of the CNS, there is little information on how or whether a change in sterol flux across the blood-brain barrier might alter neurodegeneration. In Niemann-Pick type C (NPC) disease, a mutation in NPC1 protein causes unesterified cholesterol to accumulate in the lysosomal compartment of every cell, including neurons and glia. Using the murine model of this disease, we used genetic and pharmacologic approaches in an attempt to alter cholesterol homeostasis across the CNS. Genetic deletion of the sterol transporters ATP-binding cassette transporter A1 (ABCA1) and low-density lipoprotein receptor in the NPC1 mouse did not affect sterol balance or longevity. However, deletion of the nuclear receptor, liver X receptor beta (LXRbeta), had an adverse effect on progression of the disease. We therefore tested the effects of increasing LXR activity by oral administration of a synthetic ligand for this transcription factor. Treatment with this LXR agonist increased cholesterol excretion out of brain from 17 to 49 microg per day, slowed neurodegeneration, and prolonged life. This agonist did not alter synthesis of cholesterol or expression of genes associated with the formation of 24(S)-hydroxycholesterol or neurosteroids such as CYP46A1, 3alphaHSD, and CYP11A1. However, levels of the sterol transporters ABCA1 and ATP-binding cassette transporter G1 were increased. Concomitantly, markers of neuroinflammation, CD14, MAC1, CD11c, and inducible nitric oxide synthase, were reduced, and microglia reverted from their amoeboid, active form to a ramified, resting configuration. Thus, LXR activation resulted in increased cholesterol excretion from the brain, decreased neuroinflammation, and deactivation of microglia to slow neurodegeneration and extend the lifespan of the NPC1 mouse.

Apolipoprotein D is a component of compact but not diffuse amyloid-beta plaques in Alzheimer's disease temporal cortex

Apolipoprotein D (apoD) is elevated in Alzheimer's disease (AD) cortex, localizing to cells, blood vessels, and neuropil deposits (plaques). The role of apoD in AD pathology and the extent of its co-distribution with diffuse (amorphous) and compact (dense fibrillar) amyloid-beta (Abeta) plaques are currently unclear. To address this issue, we combined apoD and Abeta immunohistochemistry with ThioS/X-34 staining of the beta-pleated sheet protein conformation in temporal cortex from 36 AD patients and 12 non-demented controls. ApoD-immunoreactive, Abeta-immunoreactive, and ThioS/X-34-stained plaques were detected exclusively in AD tissue. Dual-immunolabeling showed that 63% of Abeta plaques co-localized apoD. All apoD plaques contained Abeta protein and ThioS/X-34 fluorescence. Compared to controls, AD cases showed elevated vascular and intracellular apoD immunostaining which localized primarily to cells clustered within plaques and around large blood vessels. ApoD-immunoreactive cells within plaques morphologically matched MHC-II- and CD-68-immunoreactive microglia, and did not contain the astrocytic marker GFAP, which labeled a subset of apoD-immunoreactive cells surrounding plaques. These data suggest that neuropil deposits of apoD localize only to a subset of Abeta plaques, which contain compact aggregates of fibrillar Abeta. Elevated apoD in AD brain may influence Abeta aggregation, or facilitate phagocytosis and transport of Abeta fibrils from plaques to cerebral vasculature.

Immunohistochemical study of distribution of apolipoproteins E and D in human cerebral beta amyloid deposits

Several molecules are known to be closely associated with amyloid deposits in human brain. Among these, apolipoproteins such as apolipoproteins E (apo E) and J (apo J) have been found in two neuropathological hallmarks of Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA): senile plaques (SPs) and cerebrovascular amyloid. These apolipoproteins may be implicated in amyloid fibrillogenesis. Apo D is a multiligand-multifunctional glycoprotein present in SPs, as we previously reported. The aim of this work is to study the link between immunolocalization of apo E and apo D in AD and CAA brains. Both apolipoproteins were found in all types of SPs, but apo E was observed more often than apo D in mature plaques. Whereas apo E is always located overlapping the amyloid core, apo D seems to situate preferably around and near the amyloid. Immunohistochemistry revealed that these apolipoproteins behave differently in cerebral vessels. Apo E labeling in vessels appears mainly linked to amyloid deposits, whereas apo D shows a distribution almost opposite to that of apo E. This could be an indication of the different roles that each apolipoprotein plays in the pathogenesis of amyloid deposition.

Apolipoprotein D inhibits platelet-derived growth factor-BB-induced vascular smooth muscle cell proliferated by preventing translocation of phosphorylated extracellular signal regulated kinase 1/2 to the nucleus

OBJECTIVE

Elevated apolipoprotein D (apoD) levels are associated with reduced proliferation of cancer cells. We therefore investigated whether apoD, which occurs free or associated with HDL, suppresses vascular smooth muscle cell (VSMC) proliferation, which is related to the pathobiology of disease.

METHODS AND RESULTS

Intense immunoreactivity for apoD was observed in human atherosclerotic plaque but not in normal coronary artery. However, an increase in apoD mRNA was seen in quiescent relative to proliferating fetal lamb aortic VSMCs, and in the rat aortic VSMC line (A10), we demonstrated uptake of apoD from serum. Stable transfection of apoD in A10 cells in the absence of serum did not influence VSMC proliferation assessed by [3H]-thymidine incorporation. ApoD, administered at a dose of 100 ng/mL, completely inhibited basal as well as platelet-derived growth factor (PDGF)-BB-induced VSMC proliferation (P<0.01) but had no effect on fibroblast growth factor-induced VSMC proliferation. ApoD did not suppress PDGF-BB or fibroblast growth factor-2-induced phosphorylation of extracellular signal regulated kinase (ERK) 1/2 but selectively inhibited PDGF-BB-mediated ERK1/2 nuclear translocation.

CONCLUSIONS

Our data suggest that apoD selectively modulates the proliferative response of VSMC to growth factors by a mechanism related to nuclear translocation of ERK1/2.