Omega-3 Fatty Acids Augment the Actions of Nuclear Receptor Agonists in a Mouse Model of Alzheimer's Disease.

Alzheimer's disease (AD) is a highly prevalent disorder for which there are no effective therapies. Accumulation of amyloid ß (Aß) peptides in the brain is associated with impaired cognition and memory, pronounced inflammatory dysregulation, and subsequent amyloid plaque deposition. Thus, drugs that promote the clearance of Aß peptides and resolution of inflammation may represent viable therapeutic approaches. Agonists of nuclear receptors LXR:RXR and PPAR:RXR act to ameliorate AD-related cognitive impairment and amyloid accumulation in murine models of AD. The use of an agonist to the nuclear receptor RXR, bexarotene, as monotherapy against AD, presents potential challenges due to the metabolic perturbations it induces in the periphery, most prominently hypertriglyceridemia. We report that the ω-3 fatty acid docosahexaenoic acid (DHA), in combination with bexarotene, enhances LXR:RXR target gene expression of Abca1 and ApoE, reduces soluble forms of Aß, and abrogates release of pro-inflammatory cytokines and mediators both in vitro and in a mouse model of AD. Moreover, DHA abrogates bexarotene-induced hypertriglyceridemia in vivo. Importantly, dual therapy promotes reductions in AD pathology and resultant amelioration of cognitive deficits. While monotherapy with either bexarotene or DHA resulted in modest effects in vitro and in vivo, combined treatment with both agents produced a significant additive benefit on associated AD-related phenotypes, suggesting that targeted combinatorial agents may be beneficial over single agents alone in treating AD.

Indoleamine 2,3-dioxygenase inhibition attenuates lipopolysaccharide induced persistent microglial activation and depressive-like complications in fractalkine receptor (CX(3)CR1)-deficient mice.

An impaired ability to regulate the activation of microglia by fractalkine (CX3CL1) leads to persistent neuroinflammation and behavioral alterations following lipopolysaccharide (LPS) challenge. While these responses are usually transient, LPS injection caused prolonged depressive-like behavior in fractalkine receptor deficient mice (CX3CR1(-/-)) that was associated with exaggerated microglial activation and induction of the tryptophan (TRP) degrading enzyme indoleamine 2,3-dioxygenase (IDO). IDO activation and subsequent generation of neuroactive kynurenine metabolites may have a pivotal role in the development of depression. Therefore, the purpose of this study was to determine the extent to which LPS-induced depressive-like behavior in CX3CR1(-/-) mice was dependent on IDO activation. CX3CR1(-/-) mice were implanted prior to LPS challenge with a slow release pellet of 1-methyl-tryptophan (1-MT), a competitive inhibitor of IDO. Here we show that the depressive-like behavior evident in CX3CR1(-/-) mice 72 h after LPS injection was abrogated by inhibition of IDO. LPS also decreased body weight and locomotor activity in CX3CR1(-/-) mice, but these effects were independent of 1-MT. Consistent with the increased metabolism of TRP by IDO, the ratio of 3-hydroxykynurenine (3-HK) to TRP was increased in the brain 72 h after LPS. Increased serotonin (5-HT) turnover was also evident in the brain. The LPS-associated increases in both 3-HK:TRP and 5-HIAA:5-HT ratios were prevented by the inhibition of IDO. Last, IDO blockade attenuated microglial activation in the prefrontal cortex and hippocampus 72 h after LPS. Collectively these data indicate that LPS-induced IDO activation contributes to persistent microglial activation and depressive-like behavior in CX3CR1(-/-) mice.

ß-Adrenergic receptor antagonism prevents anxiety-like behavior and microglial reactivity induced by repeated social defeat.

Psychosocial stress is associated with altered immune function and development of psychological disorders including anxiety and depression. Here we show that repeated social defeat in mice increased c-Fos staining in brain regions associated with fear and threat appraisal and promoted anxiety-like behavior in a ß-adrenergic receptor-dependent manner. Repeated social defeat also significantly increased the number of CD11b(+)/CD45(high)/Ly6C(high) macrophages that trafficked to the brain. In addition, several inflammatory markers were increased on the surface of microglia (CD14, CD86, and TLR4) and macrophages (CD14 and CD86) after social defeat. Repeated social defeat also increased the presence of deramified microglia in the medial amygdala, prefrontal cortex, and hippocampus. Moreover, mRNA analysis of microglia indicated that repeated social defeat increased levels of interleukin (IL)-1ß and reduced levels of glucocorticoid responsive genes [glucocorticoid-induced leucine zipper (GILZ) and FK506 binding protein-51 (FKBP51)]. The stress-dependent changes in microglia and macrophages were prevented by propranolol, a ß-adrenergic receptor antagonist. Microglia isolated from socially defeated mice and cultured ex vivo produced markedly higher levels of IL-6, tumor necrosis factor-α, and monocyte chemoattractant protein-1 after stimulation with lipopolysaccharide compared with microglia from control mice. Last, repeated social defeat increased c-Fos activation in IL-1 receptor type-1-deficient mice, but did not promote anxiety-like behavior or microglia activation in the absence of functional IL-1 receptor type-1. These findings indicate that repeated social defeat-induced anxiety-like behavior and enhanced reactivity of microglia was dependent on activation of ß-adrenergic and IL-1 receptors.

Fractalkine receptor (CX3CR1) deficiency sensitizes mice to the behavioral changes induced by lipopolysaccharide.

BACKGROUND: Interactions between fractalkine (CX3CL1) and fractalkine receptor (CX3CR1) regulate microglial activation in the CNS. Recent findings indicate that age-associated impairments in CX3CL1 and CX3CR1 are directly associated with exaggerated microglial activation and an impaired recovery from sickness behavior after peripheral injection of lipopolysaccharide (LPS). Therefore, the purpose of this study was to determine the extent to which an acute LPS injection causes amplified and prolonged microglial activation and behavioral deficits in CX3CR1-deficient mice (CX3CR1-/-). METHODS: CX3CR1-/- mice or control heterozygote mice (CX3CR1+/-) were injected with LPS (0.5 mg/kg i.p.) or saline and behavior (i.e., sickness and depression-like behavior), microglial activation, and markers of tryptophan metabolism were determined. All data were analyzed using Statistical Analysis Systems General Linear Model procedures and were subjected to one-, two-, or three-way ANOVA to determine significant main effects and interactions. RESULTS: LPS injection caused a prolonged duration of social withdrawal in CX3CR1-/- mice compared to control mice. This extended social withdrawal was associated with enhanced mRNA expression of IL-1ß, indolamine 2,3-dioxygenase (IDO) and kynurenine monooxygenase (KMO) in microglia 4 h after LPS. Moreover, elevated expression of IL-1ß and CD14 was still detected in microglia of CX3CR1-/- mice 24 h after LPS. There was also increased turnover of tryptophan, serotonin, and dopamine in the brain 24 h after LPS, but these increases were independent of CX3CR1 expression. When submitted to the tail suspension test 48 and 72 h after LPS, an increased duration of immobility was evident only in CX3CR1-/- mice. This depression-like behavior in CX3CR1-/- mice was associated with a persistent activated microglial phenotype in the hippocampus and prefrontal cortex. CONCLUSIONS: Taken together, these data indicate that a deficiency of CX3CR1 is permissive to protracted microglial activation and prolonged behavioral alterations in response to transient activation of the innate immune system.

ABCA1 is Necessary for Bexarotene-Mediated Clearance of Soluble Amyloid Beta from the Hippocampus of APP/PS1 Mice.

Alzheimer's disease (AD) is characterized by impaired clearance of amyloid beta (Aß) peptides, leading to the accumulation of Aß in the brain and subsequent neurodegeneration and cognitive impairment. ApoE plays a critical role in the proteolytic degradation of soluble forms of Aß. This effect is dependent upon lipidation of ApoE by ABCA1-mediated transfer of phospholipids and cholesterol. ApoE and ABCA1 are induced by the action of the RXR agonist, bexarotene. We have previously shown that bexarotene reduces Aß levels in AD mouse models and we have hypothesized that this effect requires ABCA1-mediated lipidation of ApoE. To test this hypothesis, we crossed ABCA1-deficient (ABCA1 KO) mice with the APP/PS1 model of AD. Aged ABCA1 WT and ABCA1 KO APP/PS1 mice were treated for 7 days with vehicle or bexarotene (100 mg/kg/day). Bexarotene reduced levels of soluble Aß 1-40 and 1-42 in the hippocampus of ABCA1 WT but not ABCA1 KO APP/PS1 mice. In contrast, insoluble levels of Aß, and plaque loads were unaffected by bexarotene in this study. ABCA1 KO mice had increased levels of inflammation compared with ABCA1 WT mice. Bexarotene also increased most inflammatory gene markers evaluated. The effect of bexarotene on microglial inflammatory profiles, however, was independent of ABCA1 genotype. Importantly, bexarotene ameliorated deficits in novel object recognition in ABCA1 WT but not ABCA1 KO APP/PS1 mice. These data indicate that ABCA1-induced lipidation of ApoE is necessary for the ability of bexarotene to clear hippocampal soluble Aß and ameliorate cognitive deficits.

Riluzole partially rescues age-associated, but not LPS-induced, loss of glutamate transporters and spatial memory.

Impaired memory may result from synaptic glutamatergic dysregulation related to chronic neuroinflammation. GLT1 is the primary excitatory amino acid transporter responsible for regulating extracellular glutamate levels in the hippocampus. We tested the hypothesis that if impaired spatial memory results from increased extracellular glutamate due to age or experimentally induced chronic neuroinflammation in the hippocampus, then pharmacological augmentation of the glutamate transporter GLT1 will attenuate deficits in a hippocampal-dependent spatial memory task. The profile of inflammation-related genes and proteins associated with normal aging, or chronic neuroinflammation experimentally-induced via a four-week LPS infusion into the IV(th) ventricle, were correlated with performance in the Morris water maze following treatment with Riluzole, a drug that can enhance glutamate clearance by increasing GLT1 expression. Age-associated inflammation was qualitatively different from LPS-induced neuro-inflammation in young rats. LPS produced a pro-inflammatory phenotype characterized by increased IL-1ß expression in the hippocampus, whereas aging was not associated with a strong central pro-inflammatory response but with a mixed peripheral immune phenotype. Riluzole attenuated the spatial memory impairment, the elevation of serum cytokines and the decrease in GLT1 gene expression in Aged rats, but had no effect on young rats infused with LPS. Our findings highlight the therapeutic potential of reducing glutamatergic function upon memory impairment in neurodegenerative diseases associated with aging.

Cognitive and behavioral consequences of impaired immunoregulation in aging.

A hallmark of the aged immune system is impaired immunoregulation of the innate and adaptive immune system in the periphery and also in the central nervous system (CNS). Impaired immunoregulation may predispose older individuals to an increased frequency of peripheral infections with concomitant cognitive and behavioral complications. Thus, normal aging is hypothesized to alter the highly coordinated interactions between the immune system and the brain. In support of this notion, mounting evidence in rodent models indicate that the increased inflammatory status of the brain is associated with increased reactivity of microglia, the innate immune cells of the CNS. Understanding how immunity is affected with age is important because CNS immune cells play an integral role in propagating inflammatory signals that are initiated in the periphery. Increased reactivity of microglia sets the stage for an exaggerated inflammatory cytokine response following activation of the peripheral innate immune system that is paralleled by prolonged sickness, depressive-like complications and cognitive impairment. Moreover, amplified neuroinflammation negatively affects several aspects of neural plasticity (e.g., neurogenesis, long-term potentiation, and dendritic morphology) that can contribute to the severity of neurological complications. The purpose of this review is to discuss several key peripheral and central immune changes that impair the coordinated response between the immune system and the brain and result in behavioral and cognitive deficits.