Role of Microglia TLRs in Neurodegeneration

Toll-like receptors (TLRs) are a group of receptors widely distributed in the organism. In the central nervous system, they are expressed in neurons, astrocytes and microglia. Although their involvement in immunity is notorious, different articles have demonstrated their roles in physiological and pathological conditions, including neurodegeneration. There is increasing evidence of an involvement of TLRs, especially TLR2, 4 and 9 in neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS). In this sense, their expression in microglia might modulate the activity of these cells, which in turn, lead to protective or deleterious effects over neurons and other cells. Therefore, TLRs might mediate the link between inflammation and neurodegenerative diseases. However, further studies have to be performed to elucidate the role of the other TLRs in these diseases and to further prove and confirm the pathophysiological role of all TLRs in neurodegeneration. In this article, we revise and summarize the current knowledge regarding the role of TLRs in neurodegeneration with the focus on the possible functions of these receptors in microglia.

Regulation of prostaglandin E2 synthase expression in activated primary rat microglia: evidence for uncoupled regulation of mPGES-1 and COX-2

Prostaglandin E2 (PGE2) is among the most important mediators involved in neuroinflammatory processes. The final step of its synthesis is regulated by enzymes termed prostaglandin E2 synthases (PGES). Three PGES are known, cytosolic (c)PGES, membrane-associated (m)PGES-1 and mPGES-2. The expression of mPGES-1 is induced by inflammatory stimuli such as lipopolysaccharide (LPS), interleukin (IL)-1beta, and tumor necrosis factor (TNF)-alpha. Although some roles of mPGES-1 have already been suggested, its function in the CNS and the signaling pathways involved in its upregulation are poorly understood. In this study, we examined the regulation of mPGES-1 in primary rat microglia and the signaling pathways involved in its expression. Whereas the expression of cPGES and mPGES-2 was not stimulated by LPS, low doses of LPS (0.1-1 ng/mL) sufficiently stimulated mPGES-1 mRNA expression. A corresponding protein synthesis, however, was obtained only with higher doses (10-100 ng/mL). The LPS-induced increase of mPGES-1 was inhibited by different signaling pathway inhibitors, such as SP600125, LY294002, GF109203X, and SC-514, suggesting the involvement of c-Jun N-terminal kinase (JNK), phosphatidylinositol 3-kinase (PI-3K)/Akt, protein kinase C (PKC) pathways, and the nuclear factor (NF)-kappaB, respectively. In contrast to other reports, LPS-induced mPGES-1 synthesis was not invariably coupled to the synthesis of COX-2, since inhibition of PI-3K with LY294002 decreased mPGES-1 but increased COX-2 levels. This detailed view of the intracellular signaling pathways involved in mPGES-1 expression in activated microglia opens a new avenue in the search for novel potential therapeutic targets to reduce neuroinflammation, and demonstrates that mPGES-1 expression is not strictly coupled to the expression of COX-2.

Role of prostaglandins in neuroinflammatory and neurodegenerative diseases

Increasing data demonstrates that inflammation participates in the pathophysiology of neurodegenerative diseases. Among the different inflammatory mediators involved, prostaglandins play an important role. The effects induced by prostaglandins might be mediated by activation of their known receptors or by nonclassical mechanisms. In the present paper, we discuss the evidences that link prostaglandins, as well as the enzymes that produce them, to some neurological diseases.

The 5-lipoxygenase (5-LOX) Inhibitor Zileuton Reduces Inflammation and Infarct Size with Improvement in Neurological Outcome Following Cerebral Ischemia

Stroke is one of the most frequent causes of death and disability worldwide causing a major clinical and socioeconomic impact. Although the pathophysiology of brain ischemia and reperfusion is complex, the inflammatory process plays an important role in pathogenesis, contributing to the expansion of brain injury. The 5-lipoxygenase (5-LOX) is a key enzyme in the biosynthesis of the leukotrienes and has been implicated and in the central nervous system (CNS) disorders such as Alzheimer's disease and acute ischemic stroke. Zileuton, a selective 5-LOX inhibitor, has antiinflammatory properties and exerts an inhibitory effect on inflammatory diseases. The objective of this study was to evaluate the effects of blocking 5-LOX activity in a murine model of transient and global brain ischemia. Zileuton improved neurological deficits and significantly decrease volume and density of lesion, compared to vehicle-ischemic animals measured by magnetic resonance imaging (MRI). In addition, the blockage of 5-LOX reduced infarct area and histopathological changes. Furthermore, by enzyme immunoassay (ELISA) increased brain levels of tumor necrosis factor-alpha (TNFalpha), interferon-gamma (IFN-gamma), interleukin-1beta (IL-1beta), interleukin-6 (IL-6), chemokine (C-X-C motif) ligand 1 (CXCL1), chemokine (C-C motif) ligand 3 (CCL3) and chemokine (C-C motif) ligand 5 (CCL5) were detected in the vehicle-ischemic group, whereas in Zileuton-ischemic group presented reduction of these mediators. The concentration of the antiinflammatory cytokine interleukin-10 (IL-10) was increased after 5-LOX inhibition. Our results suggest that Zileuton decreases brain damage and reduces inflammatory cytokines expression in the CNS which contributes, at least in part, to improve the neurological outcome of brain ischemia.

Activation of EP2 receptor suppresses poly(I: C) and LPS-mediated inflammation in primary microglia and organotypic hippocampal slice cultures: Contributing role for MAPKs

Brain inflammation is a critical factor involved in neurodegeneration. Recently, the prostaglandin E₂ (PGE₂ ) downstream members were suggested to modulate neuroinflammatory responses accompanying neurodegenerative diseases. In this study, we investigated the protective effects of prostaglandin E₂ receptor 2 (EP₂ ) during TLR3 and TLR4-driven inflammatory response using in vitro primary microglia and ex vivo organotypic hippocampal slice cultures (OHSCs). Depletion of microglia from OHSCs differentially affected TLR3 and TLR4 receptor expression. Poly(I:C) induced the production of prostaglandin E₂ in OHSCs by increasing cyclooxygenase (COX-2) and microsomal prostaglandin E synthase (mPGES)-1. Besides, stimulation of OHSCs and microglia with Poly(I:C) upregulated EP₂ receptor expression. Co-stimulation of OHSCs and microglia with the EP₂ agonist butaprost reduced inflammatory mediators induced by LPS and Poly(I:C). In Poly(I:C) challenged OHSCs, butaprost almost restored microglia ramified morphology and reduced Iba1 immunoreactivity. Importantly, microglia depletion prevented the induction of inflammatory mediators following Poly(I:C) or LPS challenge in OHSCs. Activation of EP₂ receptor reversed the Poly(I:C)/LPS-induced phosphorylation of the mitogen activated protein kinases (MAPKs) ERK, p38 MAPK and c-Jun N-terminal kinase (JNK) in microglia. Collectively, these data identify an anti-inflammatory function for EP₂ signaling in diverse innate immune responses, through a mechanism that involves the mitogen-activated protein kinases pathway.

Resveratrol inhibits prostaglandin formation in IL-1beta-stimulated SK-N-SH neuronal cells

Resveratrol, a polyphenol present in grapes and red wine, has been studied due to its vast pharmacological activity. It has been demonstrated that resveratrol inhibits production of inflammatory mediators in different in vitro and in vivo models. Our group recently demonstrated that resveratrol reduced the production of prostaglandin (PG) E₂ and 8-isoprostane in rat activated microglia. In a microglial-neuronal coculture, resveratrol reduced neuronal death induced by activated microglia. However, less is known about its direct roles in neurons. In the present study, we investigated the effects of resveratrol on interleukin (IL)-1β stimulated SK-N-SH cells. Resveratrol (0.1-5 μM) did not reduce the expression of cyclooxygenase (COX)-2 and microsomal PGE₂ synthase-1 (mPGES-1), although it drastically reduced PGE₂ and PGD₂ content in IL-1β-stimulated SK-N-SH cells. This effect was due, in part, to a reduction in COX enzymatic activity, mainly COX-2, at lower doses of resveratrol. The production of 8-iso-PGF_2α, a marker of cellular free radical generation, was significantly reduced by resveratrol. The present work provides evidence that resveratrol reduces the formation of prostaglandins in neuroblastoma cells by reducing the enzymatic activity of inducible enzymes, such as COX-2, and not the transcription of the PG synthases, as demonstrated elsewhere.

Cannabinoid receptor 1/2 double-knockout mice develop epilepsy

The endocannabinoid system has gained attention as an important modulator of activity in the central nervous system. Initial studies focused on cannabinoid receptor 1 (CB1), which is widely expressed in the brain, but recent work also implicates cannabinoid receptor 2 (CB2) in modulating neuronal activity. Both receptors are capable of reducing neuronal activity, generating interest in cannabinoid receptor agonists as potential anticonvulsants. CB1 (Cnr1) and CB2 (Cnr2) single-knockout mice have been generated, with the former showing heightened seizure sensitivity, but not overt seizures. Given overlapping and complementary functions of CB1 and CB2 receptors, we queried whether double-knockout mice would show an exacerbated neurological phenotype. Strikingly, 30% of double-knockout mice exhibited provoked behavioral seizures, and 80% were found to be epileptic following 24/7 video-electroencephalographic monitoring. Single-knockout animals did not exhibit seizures. These findings highlight the importance of the endocannabinoid system for maintaining network stability.

Neuroprotective effects of intrastriatal injection of rapamycin in a mouse model of excitotoxicity induced by quinolinic acid

Background

The mammalian target of rapamycin (mTOR) is a kinase involved in a variety of physiological and pathological functions. However, the exact role of mTOR in excitotoxicity is poorly understood. Here, we investigated the effects of mTOR inhibition with rapamycin against neurodegeneration, and motor impairment, as well as inflammatory profile caused by an excitotoxic stimulus.

Methods

A single and unilateral striatal injection of quinolinic acid (QA) was used to induce excitotoxicity in mice. Rapamycin (250 nL of 0.2, 2, or 20 μM; intrastriatal route) was administered 15 min before QA injection. Forty-eight hours after QA administration, rotarod test was performed to evaluate motor coordination and balance. Fluoro-Jade C, Iba-1, and GFAP staining were used to evaluate neuronal cell death, microglia morphology, and astrocytes density, respectively, at this time point. Levels of cytokines and neurotrophic factors were measured by ELISA and Cytometric Bead Array 8 h after QA injection. Striatal synaptosomes were used to evaluate the release of glutamate.

Results

We first demonstrated that rapamycin prevented the motor impairment induced by QA. Moreover, mTOR inhibition also reduced the neurodegeneration and the production of interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α induced by excitotoxic stimulus. The lowest dose of rapamycin also increased the production of IL-10 and prevented the reduction of astrocyte density induced by QA. By using an in vitro approach, we demonstrated that rapamycin differently alters the release of glutamate from striatal synaptosomes induced by QA, reducing or enhancing the release of this neurotransmitter at low or high concentrations, respectively.

Conclusion

Taken together, these data demonstrated a protective effect of rapamycin against an excitotoxic stimulus. Therefore, this study provides new evidence of the detrimental role of mTOR in neurodegeneration, which might represent an important target for the treatment of neurodegenerative diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0793-x) contains supplementary material, which is available to authorized users.

Postictal alterations induced by intrahippocampal injection of pilocarpine in C57BL/6 mice

Temporal lobe epilepsy (TLE) is the most common form of epilepsy in adults. The pilocarpine (PILO) experimental model of TLE portrays behavioral and pathophysiological changes in rodents that are very similar to those found in humans with TLE. However, this model is associated with an unfortunate high mortality rate. Studies have shown that intrahippocampal injection of PILO, while having a much smaller mortality rate, induces status epilepticus (SE) that secondarily leads to TLE. To the best of our knowledge, the present study was the first to evaluate the cognitive and histological alterations 72h after intrahippocampal microinjection of PILO in C57BL/6 mice. Seventy percent of mice developed status epilepticus (SE) after PILO administration, and all animals survived after SE. Seventy-two hours after SE, mice presented memory impairment in both Novel Object Recognition (recognition index - vehicle: 67.57±4.46% vs PILO: 52.33±3.29%) and Contextual Fear Conditioning (freezing time - vehicle: 203±20.43 vs PILO: 107.80±25.17s) tasks. Moreover, using Nissl and NeuN staining, we observed in PILO-treated mice a significant decrease in cell viability and an increase in neuronal loss in all three hippocampal regions analyzed, cornus ammonis (CA) 1, CA3, and dentate gyrus (DG), in comparison with the control group. Additionally, using Iba-1 staining, we observed in PILO-treated mice a significant increase in microglial proliferation in CA1, CA3, and DG of the hippocampus. Therefore, intrahippocampal PILO microinjection is an efficient route to induce SE and acute postictal epileptogenic-like alterations in C57BL/6 mice.

PI3Kγ deficiency enhances seizures severity and associated outcomes in a mouse model of convulsions induced by intrahippocampal injection of pilocarpine

Phosphatidylinositol 3-kinase (PI3K) is an enzyme involved in different pathophysiological processes, including neurological disorders. However, its role in seizures and postictal outcomes is still not fully understood. We investigated the role of PI3Kγ on seizures, production of neurotrophic and inflammatory mediators, expression of a marker for microglia, neuronal death and hippocampal neurogenesis in mice (WT and PI3Kγ(-/-)) subjected to intrahippocampal microinjection of pilocarpine. PI3Kγ(-/-) mice presented a more severe status epilepticus (SE) than WT mice. In hippocampal synaptosomes, genetic or pharmacological blockade of PI3Kγ enhanced the release of glutamate and the cytosolic calcium concentration induced by KCl. There was an enhanced neuronal death and a decrease in the doublecortin positive cells in the dentate gyrus of PI3Kγ(-/-) animals after the induction of SE. Levels of BDNF were significantly increased in the hippocampus of WT and PI3Kγ(-/-) mice, although in the prefrontal cortex, only PI3Kγ(-/-) animals showed significant increase in the levels of this neurotrophic factor. Pilocarpine increased hippocampal microglial immunolabeling in both groups, albeit in the prelimbic, medial and motor regions of the prefrontal cortex this increase was observed only in PI3Kγ(-/-) mice. Regarding the levels of inflammatory mediators, pilocarpine injection increased interleukin (IL) 6 in the hippocampus of WT and PI3Kγ(-/-) animals and in the prefrontal cortex of PI3Kγ(-/-) animals 24h after the stimulus. Levels of TNFα were enhanced in the hippocampus and prefrontal cortex of only PI3Kγ(-/-) mice at this time point. On the other hand, PI3Kγ deletion impaired the increase in IL-10 in the hippocampus induced by pilocarpine. In conclusion, the lack of PI3Kγ revealed a deleterious effect in an animal model of convulsions induced by pilocarpine, suggesting that this enzyme may play a protective role in seizures and pathological outcomes associated with this condition.

A high-refined carbohydrate diet facilitates compulsive-like behavior in mice through the nitric oxide pathway

Obesity is characterized by abnormal adipose tissue expansion and is associated with chronic inflammation. Obesity itself may induce several comorbidities, including psychiatric disorders. It has been previously demonstrated that proinflammatory cytokines are able to up-regulate inducible nitric oxide synthase (iNOS) and nitric oxide (NO) release, which both have a role in compulsive related behaviors.

OBJECTIVE

To evaluate whether acute or chronic consumption of a high-refined carbohydrate-containing (HC) diet will modify burying-behavior in the Marble Burying Test (MBT) through augmentation of NO signaling in the striatum, a brain region related to the reward system. Further, we also verified the effects of chronic consumption of a HC diet on the reinforcing effects induced by cocaine in the Conditioned Place Preference (CPP) test.

METHODS

Male BALB/c mice received a standard diet (control diet) or a HC diet for 3 days or 12 weeks.

RESULTS

An increase in burying behavior occurred in the MBT after chronic consumption of a HC diet that was associated with an increase of nitrite levels in the striatum. The pre-treatment with Aminoguanidine (50 mg/kg), a preferential inhibitor of iNOS, prevented such alterations. Additionally, a chronic HC diet also induced a higher expression of iNOS in this region and higher glutamate release from striatal synaptosomes. Neither statistical differences were observed in the expression levels of the neuronal isoform of NOS nor in microglia number and activation. Finally, the reinforcing effects induced by cocaine (15 mg/kg, i.p.) during the expression of the conditioned response in the CPP test were not different between the chronically HC diet fed mice and the control group. However, HC diet-feeding mice presented impairment of cocaine-preference extinction.

CONCLUSION

Altogether, our results suggest that the chronic consumption of a HC diet induces compulsive-like behavior through a mechanism possibly associated with NO activation in the striatum.

Neuroprotection by the cannabidiol aminoquinone VCE-004.8 in experimental ischemic stroke in mice

Synthetic cannabidiol (CBD) derivative VCE-004.8 is a peroxisome proliferator-activated receptor gamma (PPARγ) and cannabinoid receptor type 2 (CB₂) dual agonist with hypoxia mimetic activity. The oral formulation of VCE-004.8, termed EHP-101, possesses anti-inflammatory properties and is currently in phase 2 clinical trials for relapsing forms of multiple sclerosis. The activation of PPARγ or CB₂ receptors exerts neuroprotective effects by dampening neuroinflammation in ischemic stroke models. However, the effect of a dual PPARγ/CB₂ agonist in ischemic stroke models is not known. Here, we demonstrate that treatment with VCE-004.8 confers neuroprotection in young mice subjected to cerebral ischemia. Male C57BL/6J mice, aged 3-4 months, were subjected to 30-min transient middle cerebral artery occlusion (MCAO). We evaluated the effect of intraperitoneal VCE-004.8 treatment (10 or 20 mg/kg) either at the onset of reperfusion or 4h or 6h after the reperfusion. Seventy-two hours after ischemia, animals were subjected to behavioral tests. Immediately after the tests, animals were perfused, and brains were collected for histology and PCR analysis. Treatment with VCE-004.8 either at the onset or 4h after reperfusion significantly reduced infarct volume and improved behavioral outcomes. A trend toward reduction in stroke injury was observed in animals receiving the drug starting 6h after recirculation. VCE-004.8 significantly reduced the expression of pro-inflammatory cytokines and chemokines involved in BBB breakdown. Mice receiving VCE-004.8 had significantly lower levels of extravasated IgG in the brain parenchyma, indicating protection against stroke-induced BBB disruption. Lower levels of active matrix metalloproteinase-9 were found in the brain of drug-treated animals. Our data show that VCE-004.8 is a promising drug candidate for treating ischemic brain injury. Since VCE-004.8 has been shown to be safe in the clinical setting, the possibility of repurposing its use as a delayed treatment option for ischemic stroke adds substantial translational value to our findings.

CB2 and toll-like receptors crosstalk in microglia

Microglia play a major role in certain neuropathological conditions. In a recent paper, Reusch et al. demonstrated how signaling pathways downstream of cannabinoid type 2 (CB₂) and toll-like receptors (TLRs) converge in these cells. The findings suggest that CB₂ receptors play a permissive role in microglia activation mediated by TLRs.

Pharmacological inhibition of receptor-interacting protein kinase 2 (RIPK2) elicits neuroprotective effects following experimental ischemic stroke

Ischemic stroke induces a debilitating neurological insult, where inflammatory processes contribute greatly to the expansion and growth of the injury. Receptor-interacting protein kinase 2 (RIPK2) is most well-known for its role as the obligate kinase for NOD1/2 pattern recognition receptor signaling and is implicated in the pathology of various inflammatory conditions. Compared to a sham-operated control, ischemic stroke resulted in a dramatic increase in the active, phosphorylated form of RIPK2, indicating that RIPK2 may be implicated in the response to stroke injury. Here, we assessed the effects of pharmacological inhibition of RIPK2 to improve post-stroke outcomes in mice subjected to experimental ischemic stroke. We found that treatment at the onset of reperfusion with a RIPK2 inhibitor, which inhibits the phosphorylation and activation of RIPK2, resulted in marked improvements in post-stroke behavioral outcomes compared to the vehicle-administered group assessed 24 h after stroke. RIPK2 inhibitor-treated mice exhibited dramatic reductions in infarct volume, concurrent with reduced damage to the blood-brain barrier, as evidenced by reduced levels of active matrix metalloproteinase-9 (MMP-9) and leakage of blood-borne albumin in the ipsilateral cortex. To explore the protective mechanism of RIPK2 inhibition, we next pretreated mice with RIPK2 inhibitor or vehicle and examined transcriptomic alterations occurring in the ischemic brain 6 h after stroke. We observed a dramatic reduction in neuroinflammatory markers in the ipsilateral cortex of the inhibitor-treated group while also attaining a comprehensive view of the vast transcriptomic alterations occurring in the brain with inhibitor treatment through bulk RNA-sequencing of the injured cortex. Overall, we provide significant novel evidence that RIPK2 may represent a viable target for post-stroke pharmacotherapy and potentially other neuroinflammatory conditions.

Molecular Mechanisms Linking Osteoarthritis and Alzheimer's Disease: Shared Pathways, Mechanisms and Breakthrough Prospects

Alzheimer's disease (AD) is a progressive neurodegenerative disease mostly affecting the elderly population. It is characterized by cognitive decline that occurs due to impaired neurotransmission and neuronal death. Even though deposition of amyloid beta (Aβ) peptides and aggregation of hyperphosphorylated TAU have been established as major pathological hallmarks of the disease, other factors such as the interaction of genetic and environmental factors are believed to contribute to the development and progression of AD. In general, patients initially present mild forgetfulness and difficulty in forming new memories. As it progresses, there are significant impairments in problem solving, social interaction, speech and overall cognitive function of the affected individual. Osteoarthritis (OA) is the most recurrent form of arthritis and widely acknowledged as a whole-joint disease, distinguished by progressive degeneration and erosion of joint cartilage accompanying synovitis and subchondral bone changes that can prompt peripheral inflammatory responses. Also predominantly affecting the elderly, OA frequently embroils weight-bearing joints such as the knees, spine and hips leading to pains, stiffness and diminished joint mobility, which in turn significantly impacts the patient's standard of life. Both infirmities can co-occur in older adults as a result of independent factors, as multiple health conditions are common in old age. Additionally, risk factors such as genetics, lifestyle changes, age and chronic inflammation may contribute to both conditions in some individuals. Besides localized peripheral low-grade inflammation, it is notable that low-grade systemic inflammation prompted by OA can play a role in AD pathogenesis. Studies have explored relationships between systemic inflammatory-associated diseases like obesity, hypertension, dyslipidemia, diabetes mellitus and AD. Given that AD is the most common form of dementia and shares similar risk factors with OA-both being age-related and low-grade inflammatory-associated diseases, OA may indeed serve as a risk factor for AD. This work aims to review literature on molecular mechanisms linking OA and AD pathologies, and explore potential connections between these conditions alongside future prospects and innovative treatments.