Downregulation of microglial activation by apolipoprotein E and apoE-mimetic peptides

Sex steroids, APOE genotype and the innate immune system

Microglia are a primary cellular component of the CNS innate immune system. Their response to conserved pathogen motifs is inherent and leads to the release of cytoactive factors that impact surrounding neurons and glia. The microglial response is modified by the local tissue environment and by "global" factors such as gender. Exposure to estrogen and testosterone, in general, down regulate microglia and peripheral macrophage function, promoting an anti-inflammatory phenotype. Other global factors, however, can "override" the gender-based effects demonstrated by estrogen or testosterone. Apolipoprotein E (APOE) genotype and the expression of specific isoforms of apolipoprotein E differentially regulate microglial and peripheral macrophage function. Our studies have shown that the presence of the APOE4 gene, a known risk factor for AD and other neurodegenerative diseases, promotes a pro-inflammatory macrophage phenotype in neonatal microglia. However, in adult mice, the APOE genotype-specific effect depends on gender. Peritoneal macrophages from female adult APOE3 and APOE4 targeted replacement mice do not demonstrate an APOE genotype-specific response, whereas adult male APOE4 targeted replacement mice show enhanced macrophage responsiveness compared to adult male APOE3 mice. At least part of the altered macrophage response in APOE4 male mice may be due to differences in androgen receptor sensitivity to testosterone. These data re-enforce the concept that classical activation in macrophages has multiple levels of regulation, dictated by competing or synergistic factors and genotype.

A novel therapeutic derived from apolipoprotein E reduces brain inflammation and improves outcome after closed head injury

Although apolipoprotein E4 (APOE4) was initially identified as a susceptibility gene for the development of Alzheimer's disease, the presence of the APOE4 allele is also associated with poor outcome after acute brain injury. One mechanism by which apoE may influence neurological outcome is by downregulating the neuroinflammatory response. Because it does not readily cross the blood-brain barrier, the apoE holoprotein has limited therapeutic potential. We demonstrate that a single intravenous injection of a small peptide derived from the apoE receptor binding region crosses the blood-brain barrier and significantly improves histological and functional outcomes after traumatic brain injury (TBI). The development of an apoE-based intervention represents a novel therapeutic strategy in the management of acute brain injury.

Interactions between Alzheimer's disease and cerebral ischemia--focus on inflammation

Progressive memory impairment, beta-amyloid (Abeta) plaques associated with local inflammation, neurofibrillary tangles, and loss of neurons in selective brain areas are hallmarks of Alzheimer's disease (AD). Although beta-amyloid precursor protein (APP) and Abeta have a central role in the etiology of AD, it is not clear which forms of APP or Abeta are responsible for the neuronal vulnerability in AD brain. Brain ischemia, another cause of dementia in the elderly, has recently been recognized to contribute to the pathogenesis of AD and individuals with severe cognitive decline and possibly underlying AD are at increased risk for ischemic events in the brain. Moreover, the epsilon4 allele of apolipoprotein E (ApoE) is a risk factor for both AD and poor outcome following brain ischemia and hemorrhage. Several factors and molecular mechanisms that lower the threshold of neuronal death in models of AD have recently been described. Among these neuroinflammation seems to play an important role. The development and maturation of both AD neuropathology and ischemic lesions in the central nervous system are characterized by activation of glial cells and upregulation of inflammatory mediators. Indeed, anti-inflammatory approaches have proven to be beneficial in the prevention and treatment of AD-like neuropathology and ischemic injuries in vivo. This review summarizes some of the findings suggesting that neuronal overexpression of human APP renders the brain more vulnerable to ischemic injury and describes the factors that are involved in increased neuronal susceptibility to ischemic stroke.

Apolipoprotein E Modulates Clearance of Apoptotic Bodies In Vitro and In Vivo, Resulting in a Systemic Proinflammatory State in Apolipoprotein E-Deficient Mice

Apolipoprotein E (apoE) is a 34-kDa glycoprotein involved in lipoprotein transport through interaction with the low-density lipoprotein receptor and related receptors. Recently, it has become clear that apoE binding to its receptors plays a role both in development and in control of the immune system. In this study, we show that apoE modulates the rate of uptake of apoptotic cells by macrophages. In vitro, apoE-deficient macrophages ingest less apoptotic thymocytes (but not latex beads) than wild-type macrophages, and this defect can be corrected by addition of exogenous apoE protein. In vivo, the number of dying macrophages is increased in a range of tissues, including lung and brain. Possibly in response to the larger numbers of persistent apoptotic bodies, the number of live macrophages in these tissues are also increased compared with those of wild-type control mice. In addition to the significant changes in macrophage population dynamics we observed, levels of the proinflammatory cytokine TNF-alpha and the positive acute phase reactant fibrinogen are also elevated in the livers from apoE-deficient mice. In contrast, neither deletion of the gene encoding the LDL receptor nor cholesterol feeding of wild-type mice affected either the number of apoptotic bodies or the number of live macrophages. We conclude that apoE deficiency results in impaired clearance of apoptotic cell remnants and a functionally relevant systemic proinflammatory condition in mice, independent of its role in lipoprotein metabolism. Any similar reduction of apoE activity in humans may contribute to the pathogenesis of a wide range of chronic diseases including atherosclerosis, dementia, and osteoporosis.

A dual role for apolipoprotein e in neuroinflammation: anti- and pro-inflammatory activity

Chronically activated glia associated with amyloid plaques might contribute to neuronal dysfunction in Alzheimer's disease (AD) through generation of neuroinflammatory molecules. Apolipoprotein E (apoE), also found associated with amyloid plaques, has been hypothesized to serve an anti-inflammatory role in the CNS through its ability to modulate beta-amyloid (Abeta)-induced glial activation. To further characterize the effect of apoE on inflammation, we examined the ability of exogenously added human apoE3 and apoE4 to modulate neuro inflammatory responses of cultured rat glia. Apolipoprotein E3 (apoE3) and apoE4 suppressed oligomeric Abeta-induced production of inducible nitric oxide synthase and cyclo-oxygenase-2, supporting an anti- inflammatory role for apoE. Exogenous apoE also inhibited Abeta-induced production of endogenous apoE. However, exogenous apoE in the absence of Abeta stimulated production of the pro-inflammatory cytokine interleukin-1beta in an isoform-dependent manner, with apoE4 inducing a significantly greater response than apoE3. These data support the idea that Abeta stimulation of glial apoE limits neuroinflammation but that overproduction of apoE by activated glia might exacerbate inflammation. In addition, the observation that apoE4 has more robust pro-inflammatory activity than apoE3 provides a mechanistic link between the APOE4 allele and AD, and suggests potential apoE-based therapeutic strategies.

Augmented delayed infarct expansion and reactive astrocytosis after permanent focal ischemia in apolipoprotein E4 knock-in mice

Using homozygous human apolipoprotein E2 (apoE2) (2/2)-, apoE3 (3/3)-, or apoE4 (4/4)-knock-in (KI) mice, we aimed to examine whether an apoE isoform-specific exacerbation of delayed infarct expansion occurs after permanent middle cerebral artery occlusion (pMCAO). Compared with 2/2- or 3/3-KI mice, 4/4-KI mice exhibited significantly larger infarct volumes and worse neurologic deficits after pMCAO, with no significant differences between the latter two groups. Infarct volume in 4/4-KI mice was significantly increased from 1 to 5 days after pMCAO, whereas that in 2/2- or 3/3-KI mice was not significantly altered. DNA fragmentation in the peri-infarct area as detected by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphatenick end-labeling was increased to a similar degree in all of the KI mice by 5 days after pMCAO, with no significant differences among the mouse groups. At every time-point examined, human apoE was most markedly expressed in the peri-infarct area, with similar immunoreactivity among the three lines of KI mice. The glial fibrillary acidic protein immunoreactive burden in the peri-infarct area was progressively increased through 7 days in 4/4-KI mice, but not in 2/2- or 3/3-KI mice. Taken together, these data show that the apoE4 isoform acts to aggravate delayed infarct expansion and peri-infarct reactive astrocytosis during the subacute phase of pMCAO in genetically engineered apoE-KI mice.

Non-steroidal anti-inflammatory drugs mediate increased in vitro glial expression of apolipoprotein E protein

Epidemiological studies have shown that use of non-steroidal anti-inflammatory drugs (NSAIDs) by the elderly is associated with a decreased relative risk and a delayed onset of Alzheimer's disease (AD). In contrast, the apolipoprotein E (apoE) gene has proven to be a risk factor for AD with apoE epsilon 4 AD patients having been found to show lower levels of brain apoE. In the present study, treatment of primary rat mixed glial cell cultures with the common NSAIDs, indomethacin and aspirin, induced significant increases in extracellular apoE protein levels. Similarly, treatment of primary rat astrocyte cell cultures with aspirin and a cyclooxygenase (COX)-2-selective aspirin derivative also stimulated significant increases in apoE protein. However, astrocyte and mixed glial apoE protein levels were significantly reduced following exposure to COX-2-specific indomethacin amides and an inactive indomethacin derivative. ApoE protein modulation was observed at physiological and subphysiological concentrations well below the COX inhibition IC50 values of the NSAIDs used, suggestive of a COX-independent mechanism. In contrast to these results, indomethacin and aspirin treatment failed to induce any significant changes in apoE mRNA levels. The failure of NSAIDs to significantly alter apoE expression may have been indicative of a nontranscriptional mechanism of apoE protein induction. Consequently, NSAID-induced increases in apoE protein may enhance apoE-mediated immunosuppression and compensatory synaptic plasticity, potentially resulting in decreased AD risk and delay of disease onset.

APOE genotype and an ApoE-mimetic peptide modify the systemic and central nervous system inflammatory response

Human apolipoprotein E is the major apolipoprotein expressed in the brain and exists as three isoforms, designated E2, E3, and E4. Although evidence suggests that apolipoprotein E plays an important role in modifying systemic and brain inflammatory responses, there is little data investigating apoE isoform-specific effects in vivo. In this study, we compared the inflammatory responses of targeted-replacement mice expressing the human APOE3 and APOE4 genes after intravenous administration of lipopolysaccharide. Animals expressing the E4 allele had significantly greater systemic and brain elevations of the pro-inflammatory cytokines TNFalpha and IL-6 as compared with their APOE3 counterparts, suggesting an isoform-specific effect of the immunomodulatory properties of apoE. Furthermore, intravenous administration of a small apoE-mimetic peptide derived from the receptor-binding region of the apoE holoprotein (apoE-(133-149)) similarly suppressed both systemic and brain inflammatory responses in mice after lipopolysaccharide administration. These results suggest that apoE plays an isoform-specific role in mediating the systemic and brain inflammatory responses. Moreover, because exogenous administration of this apoE mimetic peptide is effective at suppressing both systemic and brain inflammation, it may represent a novel therapeutic strategy for diseases characterized by systemic or central nervous system inflammation, such as septic shock, multiple sclerosis, and traumatic brain injury.

Protective effect of apolipoprotein E-mimetic peptides on N-methyl-D-aspartate excitotoxicity in primary rat neuronal-glial cell cultures

Apolipoprotein E (apoE) is a 34-kD protein with multiple biological properties. Recent clinical and preclinical observations implicate a role for apoE in modifying the response of the brain to focal and global ischemia. One mechanism by which apoE might exert these effects is by reducing glutamate-induced excitotoxic neuronal injury associated with ischemic insults. We demonstrate that human recombinant apoE confers a mild neuroprotective effect in primary neuronal-glial cultures exposed to 100 microM N-methyl-D-aspartate. Furthermore, a peptide derived from the receptor-binding region of apoE (residues 133-149) maintained a significant helical population as assessed by circular dichroism, and completely suppressed the neuronal cell death and calcium influx associated with N-methyl-D-aspartate exposure. Neuroprotection was greatest when the peptide was added concurrently with N-methyl-D-aspartate; however, a significant protection was observed when peptide was preincubated and washed off prior to N-methyl-D-aspartate exposure. These results suggest that one mechanism by which apoE may modify the CNS response to ischemia is by partially blocking glutamate excitotoxicity. Moreover, small peptide fragments derived from the receptor-binding region of apoE have enhanced bioactivity compared with the intact holoprotein, and may represent a novel therapeutic strategy for the treatment of brain ischemia.

Apolipoprotein E expression after spinal cord injury in the mouse

Apolipoprotein E (apo-E), a protein involved in lipid metabolism and cholesterol transport, has been found to be up-regulated in CNS injury and is associated with Alzheimer's disease in humans. In this study, we show that apo-E is also up-regulated after complete spinal cord transection in the C57BL/6 mouse. In the uninjured cord, the cellular localization of apo-E protein is in astrocytes, in individual neurons throughout the laminae except for the dorsal horn, and in endothelial cells of capillaries in the immediate vicinity of those neurons. After injury, RNA levels are elevated as early as 4 days and reach a maximal level between 1 and 2 weeks. Protein levels follow closely but remain up-regulated beyond 3 weeks. Early on, the protein can be found in neutrophils and macrophages at the injury site and only at later times in astrocytes during the remodeling of white matter tracts, most prominently in degenerating parts of the fasciculus gracilis.

Microglial function in human APOE3 and APOE4 transgenic mice: altered arginine transport

The APOE4 genotype is a known risk factor for Alzheimer's disease (AD) and is associated with poorer outcomes after neuropathological insults. To understand APOE's function, we have examined microglia, the CNS specific macrophage, in transgenic mice expressing the human APOE3 and APOE4 gene allele. Our data demonstrate that arginine uptake is enhanced in APOE4 microglia compared to APOE3 microglia. The increased arginine uptake in APOE4 Tg microglia is associated with an increased expression of mRNA for cationic amino acid transporter-1 (Cat1), a constuitively expressed member of the arginine selective transport system (the y+ transport system) found in most cells. The macrophage-associated transporter, cationic amino acid transporter 2B (Cat2B) did not demonstrate a change in mRNA expression. This change in microglial arginine transport suggests a potential impact of the APOE4 gene allele on those biochemical pathways such as NO production or cell proliferation to which arginine contributes.

Estrogen is neuroprotective via an apolipoprotein E-dependent mechanism in a mouse model of global ischemia

Estrogen can ameliorate brain damage in experimental models of focal cerebral ischemia., estrogen increases levels of apolipoprotein E (apoE), which also has neuroprotective effects in brain injury. The authors tested the hypotheses that physiologically relevant levels of 17beta-estradiol are neuroprotective in global cerebral ischemia and that neuroprotection is mediated via apoE. In the first study, subcutaneous implants of 17beta-estradiol were tested in female C57Bl/6J mice (ovariectomized and nonovariectomized) and plasma levels measured by radioimmunoassay to validate that physiologically relevant levels could be achieved. In the second study, female C57Bl/6J and apoE-deficient mice were ovariectomized and implanted with 17beta-estradiol or placebo pellet. Two weeks later, transient global ischemia was induced by bilateral carotid artery occlusion and the mice killed after 72 hours. Ischemic and normal neurons were counted in the caudate nucleus and CA1 pyramidal cell layer and the percentage of neuronal damage was compared between the treated groups. In C57Bl/6J mice, there was less neuronal damage in the 17beta-estradiol-treated group compared with placebo group in the caudate nucleus (15 +/- 20% versus 39 +/- 27%, = 0.02) and in the CA1 pyramidal cell layer (1.8 +/- 2% versus 10 +/- 14%, = 0.08). In contrast, neuronal damage was not significantly different between the 17beta-estradiol and placebo groups in apoE-deficient mice in the caudate nucleus (47 +/- 35% versus 53 +/- 29%, = 0.7) or in the CA1 pyramidal cell layer (24 +/- 19% versus 24 +/- 19%, = 1.0). The data indicate a neuroprotective role for estrogen in global ischemia, the mechanism of which is apoE-dependent.

Physiological expression of macrophage apoE in the artery wall reduces atherosclerosis in severely hyperlipidemic mice

We have previously reported that the introduction of macrophage apoE into mice lacking both apoE and the LDL receptor (apoE(-)(/-)/LDLR(-)(/-)) through bone marrow transplantation (apoE(+)(/+)/LDLR(-)(/-)-->apoE(-)(/-)/LDLR(-)(/-)) produces progressive accumulation of apoE in plasma without affecting lipid levels. This model provides a tool to study the effects of physiologically regulated amounts of macrophage apoE on atherogenesis in hyperlipidemic animals. Ten-week-old male apoE(-)(/-)/LDLR(-)(/-) mice were transplanted with either apoE(+)(/+)/LDLR(-)(/-) (n = 11) or apoE(-)(/-)/LDLR(-)(/-) (n = 14) marrow. Although there were no differences between the two groups in lipid levels at baseline or at 5 and 9 weeks after transplantation, apoE levels in the apoE(+)(/+)LDLR(-)(/-)-->apoE(-)(/-)/LDLR(-)(/-) mice increased to 4 times the apoE levels of normal mice. This resulted in a 60% decrease in aortic atherosclerosis in the apoE(+)(/+)/LDLR(-)(/-)-->apoE(-)(/-)/LDLR(-)(/-) compared with the apoE(-)(/-)/LDLR(-)(/-)-->apoE(-)(/-)/LDLR(-)(/-) controls, (15957 +/- 1907 vs. 40115 +/- 8302 micro m(2) +/- SEM, respectively). In a separate experiment, apoE(+)(/+)/LDLR(-)(/-) mice were transplanted with either apoE(+)(/+)/LDLR(-)(/-) or apoE(-)(/-)/LDLR(-)(/-) marrow and placed on a high-fat diet for 8 weeks. In the absence of macrophage apoE, lesion area was increased by 75% in the aortic sinus and by 56% in the distal aorta. These data show that physiologic levels of macrophage apoE in the vessel wall are anti-atherogenic in conditions of severe hyperlipidemia and can affect later stages of plaque development.

Apolipoprotein E protects against NMDA excitotoxicity

Preclinical and clinical evidence implicates a role for endogenous apolipoprotein E in modifying the response of the brain to focal and global ischemia. To investigate whether apoE modulates the neuronal response to glutamate excitotoxicity, we exposed primary neuronal glial cultures and a neuronal cell line to biologically relevant concentrations of apolipoprotein E prior to NMDA exposure. In both of these paradigms, apolipoprotein E exerted partial protective effects. At neuroprotective concentrations, however, apolipoprotein E failed to block NMDA-induced calcium influx to the same magnitude as the NMDA receptor antagonist MK-801. These results suggest that one mechanism by which apolipoprotein E modifies the central nervous system response to ischemia may be by reducing glutamate-induced excitotoxicity.

Regulating factors for microglial activation

Microglia, residential macrophages in the central nervous system, can release a variety of factors including cytokines, chemokines, etc. to regulate the communication among neuronal and other types of glial cells. Microglia play immunological roles in mechanisms underlying the phagocytosis of invading microorganisms and removal of dead or damaged cells. When microglia are hyperactivated due to a certain pathological imbalance, they may cause neuronal degeneration. Pathological activation of microglia has been reported in a wide range of conditions such as cerebral ischemia, Alzheimer's disease, prion diseases, multiple sclerosis, AIDS dementia, and others. Nearly 5000 papers on microglia can be retrieved on the Web site PubMed at present (November 2001) and half of them were published within the past 5 years. Although it is not possible to read each paper in detail, as many factors as possible affecting microglial functions in in vitro culture systems are presented in this review. The factors are separated into "activators" and "inhibitors," although it is difficult to classify many of them. An overview on these factors may help in the development of a new strategy for the treatment of various neurodegenerative diseases.

Human Apolipoprotein E concentration in response to diseases and therapeutic treatments

Apolipoprotein (Apo) E is an important circulating and tissular protein involved in cholesterol homeostasis and many other functions. The common polymorphism in the coding region of the gene, four polymorphisms in the promoter region, other additional single nucleotide polymorphisms, as well as several ApoE variants have been identified. Outside genetic polymorphism effects, ApoE concentration is modulated in human plasma and tissue through many processes: 1) transcription regulation through hormone responsive elements; 2) cytokines; 3) compartmentalization in particles or linkage to HSPG; 4) degradation after oxidation, glycation, and proteolysis; and 5) through many specific and nonspecific receptor interactions. Is the level of ApoE in tissue or plasma critical in different pathologies such as cardiovascular diseases (CVD) or Alzheimer's disease (AD)? ApoE is able to bind to Aβ, tau, to be an antioxidant, to respond to inflammation, and is involved in cholesterol delivery, uptake, and accumulation. In experimental situations ApoE injection or positive modulation decreases cholesterol and triglycerides and improves cognitive impairment. ApoE peptides are involved in immune response. It seems more and more clear that low vs. high plasma concentration should be evaluated in large epidemiological studies. Only after such studies can the question be answered: Is a low or high concentration of ApoE beneficial or dangerous? This fascinating apolipoprotein will then be an interesting marker and/or drug target. Drug Dev. Res. 56:95–110, 2002. © 2002 Wiley‐Liss, Inc.

Association of apolipoprotein E polymorphism with myocardial infarction in Greek patients with coronary artery disease

Studies in several populations have indicated that genetic variation at the apolipoprotein E (apoE) structural locus influences the risk of coronary artery disease (CAD) and myocardial infarction (MI). This study aimed at investigating whether apoE polymorphism has an allelic and/or genotypic impact on the risk of MI in Greek patients with CAD. We compared apoE gene polymorphism in a group of patients with angiographically confirmed CAD but not MI [CAD/MI (-)-group, n = 143] and a group of age and sex-matched CAD patients who had experienced a non-fatal Ml [CAD/MI (+)-group, n = 124]. The patients were also compared with a group of healthy younger individuals (n = 240) with no family history of CAD. The apoE genotype distribution differed significantly between the two groups of CAD patients (p = 0.02). The epsilon2 allele was 5.3-fold less frequent in the CAD/ MI (+)-group compared with the CAD/MI (-)-group (1.2% vs. 6.3%, p = 0.01). The frequency of the epsilon2 allele in healthy subjects was 8.1%, which is 6.8-fold higher than in CAD/MI (+)-patients (p = 0.001) and twice as high compared with all CAD patients (p = 0.02). No differences in epsilon4 allele frequencies were observed between CAD/MI (+)- and CAD/MI (-)-patients (10.9% vs. 9.8%), or between patients with CAD and healthy subjects (10.3% vs. 10.2%). In summary, the epsilon4 allele was not found to be associated with an increased risk for CAD or MI. In contrast, a negative association of the epsilon2 allele with Ml was observed among Greek patients with CAD.

Apolipoprotein E affects the central nervous system response to injury and the development of cerebral edema

Apolipoprotein E has been implicated in modifying neurological outcome after traumatic brain injury, although the mechanisms by which this occurs remain poorly defined. To investigate the role of endogenous apolipoprotein E following acute brain injury, noninvasive magnetic resonance imaging was performed on anesthetized mice following closed head injury. Effacement of the lateral ventricle was used as a radiographic surrogate for cerebral edema. At 24 hours following injury, apolipoprotein E-deficient animals had a greater degree of cerebral edema as compared to matched controls. In addition, the brains of apolipoprotein E-deficient animals had a significantly greater upregulation of tissue necrosis factor alpha messenger ribonucleic acid as compared to controls as early as 1-hr post injury. Thus, modulation of the endogenous central nervous system inflammatory response may be one mechanism by which apolipoprotein E affects outcome following acute brain injury.

Apolipoprotein E modulates glial activation and the endogenous central nervous system inflammatory response

Apolipoprotein E (apoE) is a 299 amino acid protein that is associated with risk of developing Alzheimer's Disease (AD) and outcome after acute brain injury. To investigate the possibility that apoE modulates glial activation we studied the effect of endogenous apoE on inflammatory gene regulation in vitro and in vivo. Our results indicate that apoE downregulates CNS production of TNFalpha, Il-1beta, and Il-6 mRNA following stimulation with lipopolysaccharide (LPS). This effect of endogenous apoE on inflammatory gene regulation appears to be specific, and may account for the biological role that apoE plays in acute and chronic human neurological disease.