Dynamic complexity of the microglial activation response in transgenic models of amyloid deposition: implications for Alzheimer therapeutics

The relationship between inflammation, impaired glymphatic system, and neurodegenerative disorders: A vicious cycle

The term "glymphatic" emerged roughly a decade ago, marking a pivotal point in neuroscience research. The glymphatic system, a glial-dependent perivascular network distributed throughout the brain, has since become a focal point of investigation. There is increasing evidence suggesting that impairment of the glymphatic system appears to be a common feature of neurodegenerative disorders, and this impairment exacerbates as disease progression. Nevertheless, the common factors contributing to glymphatic system dysfunction across most neurodegenerative disorders remain unclear. Inflammation, however, is suspected to play a pivotal role. Dysfunction of the glymphatic system can lead to a significant accumulation of protein and waste products, which can trigger inflammation. The interaction between the glymphatic system and inflammation appears to be cyclical and potentially synergistic. Yet, current research is limited, and there is a lack of comprehensive models explaining this association. In this perspective review, we propose a novel model suggesting that inflammation, impaired glymphatic function, and neurodegenerative disorders interconnected in a vicious cycle. By presenting experimental evidence from the existing literature, we aim to demonstrate that: (1) inflammation aggravates glymphatic system dysfunction, (2) the impaired glymphatic system exacerbated neurodegenerative disorders progression, (3) neurodegenerative disorders progression promotes inflammation. Finally, the implication of proposed model is discussed.

Genetic model of selective COX2 inhibition improve learning and memory ability and brain pathological changes in 5xFAD mouse

Alzheimer's disease (AD) is the most common neurodegenerative disease that leads to dementia. Its pathogenesis is very complex, and inflammation is one of the main pathophysiological mechanisms of AD. Non-steroidal anti-inflammatory drugs (NSAIDs), which mainly target cyclooxygenase (COX) activity, are used to reduce the risk of AD, but several side effects limit their application. Here we assess the effect of Cyclooxygenase-2 (COX2) catalytic activity on learning ability and AD pathology using 5x Familial Alzheimer's Disease (FAD) mice with COX2 inhibition (5xFAD/COX2 KO), 5xFAD mice with cyclooxygenase inactivation of COX2 (5xFAD/COX2 Y385F), and 5xFAD mice with peroxidase (POX) inactivation of COX2 (5xFAD/COX2) H374Y), respectively. Our results indicate that learning ability of COX2 KO and mutants is improved compared to 5xFAD mice, further investigations show that Aβ depositions are reduced, microglia and astrocytes homeostasis are changed in COX2 KO and mutants. Especially, there is more responsive microglia in the brain of 5xFAD/COX2 Y385F mice, and Aβ depositions are more effectively cleaned at old age. Taken together, these results identify a role of COX2 Y385F in regulating microglia function and may have important implications for future treatment of AD.

An insight into the neuroprotective effects and molecular targets of pomegranate (Punica granatum) against Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease that still has no permanent cure. The drugs prescribed in the present days are only for symptomatic relief for the patients. Many studies correlating the reduction in the incidence of AD with the diet consumed have been published. These studies showed that a diet rich in polyphenols is associated with a decrease in the incidence of AD. The present review is focused on the ability of pomegranate and its bioactive components to ameliorate the progression of AD and their ability to exert a neuroprotective effect. Various studies showing the ability of pomegranate in inhibiting enzymes, reducing reactive oxygen species, inhibition of microglial activation, inhibition of tau protein hyperphosphorylation, maintenance of synaptic plasticity, anti-inflammatory activity and its ability to inhibit Beta secretase-1 (BACE-1) has been reviewed in this article. In spite of the lack of studies on humans, there are compelling evidence indicating that pomegranate can reduce various risk factors involved in the causation of AD and thus can be used as a persistent nutraceutical to slow ageing and for providing neuroprotection for the treatment of AD.Highlights An overview of traditional and pharmacological uses of pomegranate (POM).Potential of POM in the treatment of neurodegenerative diseases especially in AD.Insight into the molecular mechanisms of neuroprotective effects of POM in AD.Clinical evaluation studies involving POM and its bioactive components.

Loss of Serpin E2 alters antimicrobial gene expression by microglia but not astrocytes

Microglia are the brain-resident immune cells responsible for surveilling and protecting the central nervous system. These cells can express a wide array of immune genes, and that expression can become highly dynamic in response to changes in the environment, such as traumatic injury or neurological disease. Though microglial immune responses are well studied, we still do not know many mechanisms and regulators underlying all the varied microglial responses. Serpin E2 is a serine protease inhibitor that acts on a wide variety of serine proteases, with particularly potent affinity for the blood clotting enzyme thrombin. In the brain, Serpin E2 is highly expressed by many cell types, especially glia, and loss of Serpin E2 leads to behavioral changes as well as deficits in synaptic plasticity. To determine whether Serpin E2 is important for maintaining homeostasis in glia, we performed RNA sequencing of microglia and astrocytes from Serpin E2-deficient mice in a healthy state or under immune activation due to lipopolysaccharide (LPS) injection. We found that microglia in Serpin E2-deficient mice had higher expression of antimicrobial genes, while astrocytes did not display any robust changes in transcription. Furthermore, the lack of Serpin E2 did not affect transcriptional responses to LPS in either microglia or astrocytes. Overall, we find that Serpin E2 is a regulator of antimicrobial genes in microglia.

The effects of enhancing angiotensin converting enzyme in myelomonocytes on ameliorating Alzheimer's-related disease and preserving cognition

This review examines the role of angiotensin-converting enzyme (ACE) in the context of Alzheimer's disease (AD) and its potential therapeutic value. ACE is known to degrade the neurotoxic 42-residue long alloform of amyloid β-protein (Aβ42), a peptide strongly associated with AD. Previous studies in mice, demonstrated that targeted overexpression of ACE in CD115+ myelomonocytic cells (ACE10 models) improved their immune responses to effectively reduce viral and bacterial infection, tumor growth, and atherosclerotic plaque. We further demonstrated that introducing ACE10 myelomonocytes (microglia and peripheral monocytes) into the double transgenic APPSWE/PS1ΔE9 murine model of AD (AD+ mice), diminished neuropathology and enhanced the cognitive functions. These beneficial effects were dependent on ACE catalytic activity and vanished when ACE was pharmacologically blocked. Moreover, we revealed that the therapeutic effects in AD+ mice can be achieved by enhancing ACE expression in bone marrow (BM)-derived CD115+ monocytes alone, without targeting central nervous system (CNS) resident microglia. Following blood enrichment with CD115+ ACE10-monocytes versus wild-type (WT) monocytes, AD+ mice had reduced cerebral vascular and parenchymal Aβ burden, limited microgliosis and astrogliosis, as well as improved synaptic and cognitive preservation. CD115+ ACE10-versus WT-monocyte-derived macrophages (Mo/MΦ) were recruited in higher numbers to the brains of AD+ mice, homing to Aβ plaque lesions and exhibiting a highly Aβ-phagocytic and anti-inflammatory phenotype (reduced TNFα/iNOS and increased MMP-9/IGF-1). Moreover, BM-derived ACE10-Mo/MΦ cultures had enhanced capability to phagocytose Aβ42 fibrils, prion-rod-like, and soluble oligomeric forms that was associated with elongated cell morphology and expression of surface scavenger receptors (i.e., CD36, Scara-1). This review explores the emerging evidence behind the role of ACE in AD, the neuroprotective properties of monocytes overexpressing ACE and the therapeutic potential for exploiting this natural mechanism for ameliorating AD pathogenesis.

Structural analysis of the microglia-interneuron interactions in the CA1 hippocampal area of the APP/PS1 mouse model of Alzheimer's disease

Microglia can interact with glutamatergic neurons and, through control of synaptic elements, regulate their physiological function. Much less is known about the partnership between microglia and GABAergic inhibitory interneurons. Here, we compared the interactions between microglia and parvalbumin (PV+) and somatostatin (SOM+) expressing interneurons in the CA1 hippocampal area of APP/PS1 transgenic mice that mimic certain aspects of the Alzheimer's disease (AD). We first uncovered a high level of interactions between microglia and two types of interneurons, with 98% of SOM+ and 90% of PV+ cells receiving different types of putative microglial contacts. The latter included the microglia soma to the interneuron soma (Soma_MG -to-Soma_IN ), the microglia process to the interneuron soma (Process_MG -to-Soma_IN ) and the microglia process to the interneuron dendrite (Process_MG -to-Dend_IN ) interactions. Moreover, we found significantly larger areas of interaction for the Soma_MG -to-Soma_IN and the Process_MG -to-Dend_IN type of contacts between microglia and SOM+ cells. In contrast, PV+ cells exhibited larger areas for the Process_MG -to-Soma_IN interactions. Second, in APP/PS1 mice, although the overall microglia interactions with interneurons remained preserved, the fraction of interneurons receiving putative microglia contacts on their dendrites was reduced, and larger areas of interactions were observed for somatic contacts, suggesting a stronger modulation of the interneuron output by microglia in AD. In summary, these results reveal microglia as important partners of hippocampal PV+ and SOM+ GABAergic cells, with interneuron type-specific pattern of interactions. Thus, microglia may play an essential role in the operation of interneurons under normal conditions and their dysfunction in disease.

Pathophysiological Ionotropic Glutamate Signalling in Neuroinflammatory Disease as a Therapeutic Target

Glutamate signalling is an essential aspect of neuronal communication involving many different glutamate receptors, and underlies the processes of memory, learning and synaptic plasticity. Despite neuroinflammatory diseases covering a range of maladies with very different biological causes and pathophysiologies, a central role for dysfunctional glutamate signalling is becoming apparent. This is not just restricted to the well-described role of glutamate in mediating neurodegeneration, but also includes a myriad of other influences that glutamate can exert on the vasculature, as well as immune cell and glial regulation, reflecting the ability of neurons to communicate with these compartments in order to couple their activity with neuronal requirements. Here, we discuss the role of pathophysiological glutamate signalling in neuroinflammatory disease, using both multiple sclerosis and Alzheimer's disease as examples, and how current steps are being made to harness our growing understanding of these processes in the development of neuroprotective strategies. This review focuses in particular on N-methyl-D-aspartate (NMDA) and 2-amino-3-(3-hydroxy-5-methylisooxazol-4-yl) propionate (AMPA) type ionotropic glutamate receptors, although metabotropic, G-protein-coupled glutamate receptors may also contribute to neuroinflammatory processes. Given the indispensable roles of glutamate-gated ion channels in synaptic communication, means of pharmacologically distinguishing between physiological and pathophysiological actions of glutamate will be discussed that allow deleterious signalling to be inhibited whilst minimising the disturbance of essential neuronal function.

Dissecting the Crosstalk between Endothelial Mitochondrial Damage, Vascular Inflammation, and Neurodegeneration in Cerebral Amyloid Angiopathy and Alzheimer's Disease

Alzheimer’s disease (AD) is the most prevalent cause of dementia and is pathologically characterized by the presence of parenchymal senile plaques composed of amyloid β (Aβ) and intraneuronal neurofibrillary tangles of hyperphosphorylated tau protein. The accumulation of Aβ also occurs within the cerebral vasculature in over 80% of AD patients and in non-demented individuals, a condition called cerebral amyloid angiopathy (CAA). The development of CAA is associated with neurovascular dysfunction, blood–brain barrier (BBB) leakage, and persistent vascular- and neuro-inflammation, eventually leading to neurodegeneration. Although pathologically AD and CAA are well characterized diseases, the chronology of molecular changes that lead to their development is still unclear. Substantial evidence demonstrates defects in mitochondrial function in various cells of the neurovascular unit as well as in the brain parenchyma during the early stages of AD and CAA. Dysfunctional mitochondria release danger-associated molecular patterns (DAMPs) that activate a wide range of inflammatory pathways. In this review, we gather evidence to postulate a crucial role of the mitochondria, specifically of cerebral endothelial cells, as sensors and initiators of Aβ-induced vascular inflammation. The activated vasculature recruits circulating immune cells into the brain parenchyma, leading to the development of neuroinflammation and neurodegeneration in AD and CAA.

Neuroinflammation in Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disease associated with human aging. Ten percent of individuals over 65 years have AD and its prevalence continues to rise with increasing age. There are currently no effective disease modifying treatments for AD, resulting in increasingly large socioeconomic and personal costs. Increasing age is associated with an increase in low-grade chronic inflammation (inflammaging) that may contribute to the neurodegenerative process in AD. Although the exact mechanisms remain unclear, aberrant elevation of reactive oxygen and nitrogen species (RONS) levels from several endogenous and exogenous processes in the brain may not only affect cell signaling, but also trigger cellular senescence, inflammation, and pyroptosis. Moreover, a compromised immune privilege of the brain that allows the infiltration of peripheral immune cells and infectious agents may play a role. Additionally, meta-inflammation as well as gut microbiota dysbiosis may drive the neuroinflammatory process. Considering that inflammatory/immune pathways are dysregulated in parallel with cognitive dysfunction in AD, elucidating the relationship between the central nervous system and the immune system may facilitate the development of a safe and effective therapy for AD. We discuss some current ideas on processes in inflammaging that appear to drive the neurodegenerative process in AD and summarize details on a few immunomodulatory strategies being developed to selectively target the detrimental aspects of neuroinflammation without affecting defense mechanisms against pathogens and tissue damage.

Effects of concentrated ambient ultrafine particulate matter on hallmarks of Alzheimer's disease in the 3xTgAD mouse model

BACKGROUND

Exposure to air pollution has been identified as a possible environmental contributor to Alzheimer's Disease (AD) risk. As the number of people with AD worldwide continues to rise, it becomes vital to understand the nature of this potential gene-environment interaction. This study assessed the effects of short-term exposures to concentrated ambient ultrafine particulates (UFP, <100 nm) on measurements of amyloid-β, tau, and microglial morphology.

METHODS

Two cohorts of aged (12.5-14 months) 3xTgAD and NTg mice were exposed to concentrated ambient UFP or filtered air for 2 weeks (4-h/day, 4 days/week). Bronchoalveolar lavage fluid and brain tissue were collected twenty-four hours following the last exposure to evaluate lung inflammation, tau pathology, amyloid-β pathology, and glial cell morphology.

RESULTS

No exposure- or genotype-related changes were found with any of the measures of lung inflammation or in the hippocampal staining density of astrocyte marker glial fibrillary acidic protein. The microglia marker, ionized calcium binding adaptor molecule 1, and amyloid-β marker, 6E10, exhibited significant genotype by exposure interactions such that levels were lower in the UFP-exposed as compared to filtered air-exposed 3xTgAD mice. When microglia morphology was assessed by Sholl analysis, microglia from both NTg mouse groups were ramified. The 3xTgAD air-exposed mice had the most ameboid microglia, while the 3xTgAD UFP-exposed mice had microglia that were comparatively more ramified. The 3xTgAD air-exposed mice had more plaques per region of interest as measured by Congo red staining as well as more plaque-associated microglia than the 3xTgAD UFP-exposed mice. The number of non-plaque-associated microglia was not affected by genotype or exposure. Levels of soluble and insoluble human amyloid-β₄₂ protein were measured in both 3xTgAD groups and no exposure effect was found. In contrast, UFP-exposure led to significant elevations in phosphorylated tau in 3xTgAD mice as compared to those that were exposed to air, as measured by pT205 staining.

CONCLUSIONS

Exposure to environmentally relevant levels of ultrafine particulates led to changes in tau phosphorylation and microglial morphology in the absence of overt lung inflammation. Such changes highlight the need to develop greater mechanistic understanding of the link between air pollution exposure and Alzheimer's disease.

Immune Modulation by Inhibitors of the HO System

The heme oxygenase (HO) system involves three isoforms of this enzyme, HO-1, HO-2, and HO-3. The three of them display the same catalytic activity, oxidating the heme group to produce biliverdin, ferrous iron, and carbon monoxide (CO). HO-1 is the isoform most widely studied in proinflammatory diseases because treatments that overexpress this enzyme promote the generation of anti-inflammatory products. However, neonatal jaundice (hyperbilirubinemia) derived from HO overexpression led to the development of inhibitors, such as those based on metaloproto- and meso-porphyrins inhibitors with competitive activity. Further, non-competitive inhibitors have also been identified, such as synthetic and natural imidazole-dioxolane-based, small synthetic molecules, inhibitors of the enzyme regulation pathway, and genetic engineering using iRNA or CRISPR cas9. Despite most of the applications of the HO inhibitors being related to metabolic diseases, the beneficial effects of these molecules in immune-mediated diseases have also emerged. Different medical implications, including cancer, Alzheimer´s disease, and infections, are discussed in this article and as to how the selective inhibition of HO isoforms may contribute to the treatment of these ailments.

The role of the M1/M2 microglia in the process from cancer pain to morphine tolerance

Cancer pain, especially bone cancer pain, is a pain state often caused by inflammation or dysfunctional nerves. Moreover, in the management of cancer pain, opioid especially morphine is widely used, however, it also brings severe side effects such as morphine tolerance to the patient (Deandrea et al., 2008). A growing body of literatures demonstrated that neuroinflammation is mediated by microglia. As the macrophages like immune cells, microglia play an important role in the pathogenesis of cancer pain and morphine tolerance. Microglia acquire different activation states to regulate the function of these cells. As to M1 phenotype, microglia release pro-inflammatory cytokines and neurotoxic molecules that promote inflammation and cytotoxic reactions. Conversely, when microglia represent M2 phenotypes secreting anti-inflammatory cytokines and nutrient factors that promote the function of repair, regeneration and restore homeostasis. A better understanding of microglia activation in cancer pain and morphine tolerance is crucial for the development of hypothesized neuroprotective drugs. Targeting microglia different polarization states by the inhibition of their deleterious pro-inflammatory neurotoxicity and/or enhancing their beneficial anti-inflammatory protective function seems to be an effective treatment for cancer pain and morphine tolerance.

Peripherally derived angiotensin converting enzyme-enhanced macrophages alleviate Alzheimer-related disease

Targeted overexpression of angiotensin-converting enzyme (ACE), an amyloid-β protein degrading enzyme, to brain resident microglia and peripheral myelomonocytes (ACE10 model) substantially diminished Alzheimer's-like disease in double-transgenic APPSWE/PS1ΔE9 (AD+) mice. In this study, we explored the impact of selective and transient angiotensin-converting enzyme overexpression on macrophage behaviour and the relative contribution of bone marrow-derived ACE10 macrophages, but not microglia, in attenuating disease progression. To this end, two in vivo approaches were applied in AD+ mice: (i) ACE10/GFP+ bone marrow transplantation with head shielding; and (ii) adoptive transfer of CD115+-ACE10/GFP+ monocytes to the peripheral blood. Extensive in vitro studies were further undertaken to establish the unique ACE10-macrophage phenotype(s) in response to amyloid-β1-42 fibrils and oligomers. The combined in vivo approaches showed that increased cerebral infiltration of ACE10 as compared to wild-type monocytes (∼3-fold increase; P < 0.05) led to reductions in cerebral soluble amyloid-β1-42, vascular and parenchymal amyloid-β deposits, and astrocytosis (31%, 47-80%, and 33%, respectively; P < 0.05-0.0001). ACE10 macrophages surrounded brain and retinal amyloid-β plaques and expressed 3.2-fold higher insulin-like growth factor-1 (P < 0.01) and ∼60% lower tumour necrosis factor-α (P < 0.05). Importantly, blood enrichment with CD115+-ACE10 monocytes in symptomatic AD+ mice resulted in pronounced synaptic and cognitive preservation (P < 0.05-0.001). In vitro analysis of macrophage response to well-defined amyloid-β1-42 conformers (fibrils, prion rod-like structures, and stabilized soluble oligomers) revealed extensive resistance to amyloid-β1-42 species by ACE10 macrophages. They exhibited 2-5-fold increased surface binding to amyloid-β conformers as well as substantially more effective amyloid-β1-42 uptake, at least 8-fold higher than those of wild-type macrophages (P < 0.0001), which were associated with enhanced expression of surface scavenger receptors (i.e. CD36, scavenger receptor class A member 1, triggering receptor expressed on myeloid cells 2, CD163; P < 0.05-0.0001), endosomal processing (P < 0.05-0.0001), and ∼80% increased extracellular degradation of amyloid-β1-42 (P < 0.001). Beneficial ACE10 phenotype was reversed by the angiotensin-converting enzyme inhibitor (lisinopril) and thus was dependent on angiotensin-converting enzyme catalytic activity. Further, ACE10 macrophages presented distinct anti-inflammatory (low inducible nitric oxide synthase and lower tumour necrosis factor-α), pro-healing immune profiles (high insulin-like growth factor-1, elongated cell morphology), even following exposure to Alzheimer's-related amyloid-β1-42 oligomers. Overall, we provide the first evidence for therapeutic roles of angiotensin-converting enzyme-overexpressing macrophages in preserving synapses and cognition, attenuating neuropathology and neuroinflammation, and enhancing resistance to defined pathognomonic amyloid-β forms.

Therapeutic Trem2 activation ameliorates amyloid-beta deposition and improves cognition in the 5XFAD model of amyloid deposition

BACKGROUND

Triggering receptor expressed on myeloid cell-2 (TREM2) is a lipid and lipoprotein binding receptor expressed by cells of myeloid origin. Homozygous TREM2 mutations cause early onset progressive presenile dementia while heterozygous, point mutations triple the risk of Alzheimer's disease (AD). Although human genetic findings support the notion that loss of TREM2 function exacerbates neurodegeneration, it is not clear whether activation of TREM2 in a disease state would result in therapeutic benefits. To determine the viability of TREM2 activation as a therapeutic strategy, we sought to characterize an agonistic Trem2 antibody (AL002a) and test its efficacy and mechanism of action in an aggressive mouse model of amyloid deposition.

METHODS

To determine whether agonism of Trem2 results in therapeutic benefits, we designed both intracranial and systemic administration studies. 5XFAD mice in the intracranial administration study were assigned to one of two injection groups: AL002a, a Trem2-agonizing antibody, or MOPC, an isotype-matched control antibody. Mice were then subject to a single bilateral intracranial injection into the frontal cortex and hippocampus and euthanized 72 h later. The tissue from the left hemisphere was histologically examined for amyloid-beta and microglia activation, whereas the tissue from the right hemisphere was used for biochemical analyses. Similarly, mice in the systemic administration study were randomized to one of the aforementioned injection groups and the assigned antibody was administered intraperitoneally once a week for 14 weeks. Mice underwent behavioral assessment between the 12- and 14-week timepoints and were euthanized 24 h after their final injection. The tissue from the left hemisphere was used for histological analyses whereas the tissue from the right hemisphere was used for biochemical analyses.

RESULTS

Here, we show that chronic activation of Trem2, in the 5XFAD mouse model of amyloid deposition, leads to reversal of the amyloid-associated gene expression signature, recruitment of microglia to plaques, decreased amyloid deposition, and improvement in spatial learning and novel object recognition memory.

CONCLUSIONS

These findings indicate that Trem2 activators may be effective for the treatment of AD and possibly other neurodegenerative disorders.

Oxiracetam Offers Neuroprotection by Reducing Amyloid β-Induced Microglial Activation and Inflammation in Alzheimer's Disease

Background: Alzheimer's disease (AD) is characterized by amyloid beta (Aβ) accumulation in the brain, which triggers the activation of microglia; in turn, microglia release neuroinflammatory factors capable of damaging neurons. Thus, a therapeutic approach targeting this sustained microglia-induced inflammatory response deserves investigation. Here, we examined whether oxiracetam (ORC), a nootropic of the racetam family, can indirectly prevent Aβ-induced neurotoxicity by attenuating microglial activation.

Methods: Aβ42 oligomers were used to stimulate BV2 microglial cells, and the morphological changes and phagocytic capacity of BV2 cells were evaluated using fluorescence microscopy. We used quantitative reverse transcription polymerase chain reaction to assess the inhibitory effects of ORC on Aβ-induced mRNA levels of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α); enzyme-linked immunosorbent assay was used to examine the productions of these cytokines. We also assessed the mRNA level of inducible nitric oxide synthase and the production of nitric oxide (NO). The conditioned medium from BV2 cells was used to culture hippocampal HT22 cells to assess indirect toxicity using the MTT assay.

Results: ORC prevented the Aβ-induced activation of BV2 cells, as reflected by reduced morphological changes and phagocytic ability. In addition, ORC downregulated the expression of Aβ-induced cytokines (IL-1β, IL-6, and TNF-α) and the production of NO in BV2 cells. Furthermore, ORC protected HT22 cells from indirect damage evoked by Aβ-treated BV2 cell-conditioned medium, but not from direct Aβ-induced toxicity.

Conclusions: ORC suppressed the activation of BV2 cells, decreased the production of Aβ-induced inflammatory molecules and NO in BV2 cells, and protected HT22 cells against indirect toxicity mediated by Aβ-treated BV2 cell-conditioned medium. Thus, ORC may exert a protective role in AD through attenuating the damage caused by inflammation and oxidative stress.

"Olfactory Three-Needle" Enhances Spatial Learning and Memory Ability in SAMP8 Mice

As one of the most important therapies in complementary and alternative medicine, acupuncture has been used in the treatment of Alzheimer's disease (AD). Acupuncture of “olfactory three-needle” manipulation can improve the cognitive ability of AD patients. However, the mechanism of “olfactory three-needle” in AD remains largely unknown. Here, we identified that the “olfactory three-needle” therapy and eugenol olfactory stimulation both reduced the deposition of β-amyloid (Aβ) protein and increased the expression of synaptophysin (SYP), but only the “olfactory three-needle” enhanced the spatial learning and memory ability of SAMP8. Remarkably, the “olfactory three-needle” inhibited the phosphorylation of p38MAPK and the excessive activation of microglia (MG) in the hippocampus. Our study demonstrates that the “olfactory three-needle” enhances spatial learning and memory ability by inhibiting the phosphorylation of p38MAPK and the excessive activation of MG to reduce the neuroinflammatory response and neurotoxicity of Aβ and promote synaptic regeneration, but it was not completely consistent with the stimulation of the olfactory system.

Electroacupuncture attenuates cognition impairment via anti-neuroinflammation in an Alzheimer's disease animal model

Background

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by progressive loss of cognitive abilities and memory leading to dementia. Electroacupuncture (EA) is a complementary alternative medicine approach, applying an electrical current to acupuncture points. In clinical and animal studies, EA causes cognitive improvements in AD and vascular dementia. However, EA-induced changes in cognition and microglia-mediated amyloid β (Aβ) degradation have not been determined yet in AD animals. Therefore, this study investigated the EA-induced molecular mechanisms causing cognitive improvement and anti-inflammatory activity in five familial mutation (5XFAD) mice, an animal model of AD.

Methods

5XFAD mice were bilaterally treated with EA at the Taegye (KI3) acupoints three times per week for 2 weeks. To evaluate the effects of EA treatment on cognitive functions, novel object recognition and Y-maze tests were performed with non-Tg, 5XFAD (Tg), and EA-treated 5XFAD (Tg + KI3) mice. To examine the molecular mechanisms underlying EA effects, western blots, immunohistochemistry, and micro-positron emission tomography scans were performed. Furthermore, we studied synapse ultrastructures with transmission electron microscopy and used electrophysiology to investigate EA effects on synaptic plasticity in 5XFAD mice.

Results

EA treatment significantly improved working memory and synaptic plasticity, alleviated neuroinflammation, and reduced ultrastructural degradation of synapses via upregulation of synaptophysin and postsynaptic density-95 protein in 5XFAD mice. Furthermore, microglia-mediated Aβ deposition was reduced after EA treatment and coincided with a reduction in amyloid precursor protein.

Conclusions

Our findings demonstrate that EA treatment ameliorates cognitive impairment via inhibition of synaptic degeneration and neuroinflammation in a mouse model of AD.

Active immunization with tau epitope in a mouse model of tauopathy induced strong antibody response together with improvement in short memory and pSer396-tau pathology

Abnormal tau hyperphosphorylation and its aggregation into neurofibrillary tangles are a hallmark of tauopathies, neurodegenerative disorders that include Alzheimer's disease (AD). Active and passive Tau-immunotherapy has been proposed as a therapeutic approach to AD with mixed results. One of the limitations of active immunotherapy may be associated with the mediocre immunogenicity of vaccines that are not inducing therapeutically potent titers of antibodies. The aim of this study was to test the efficacy of an anti-tau vaccine, AV-1980R/A composed of N terminal peptide of this molecule fused with an immunogenic MultiTEP platform and formulated in a strong adjuvant, Advax^CpG in a Tg4510 mouse model of tauopathy. Experimental mice were immunized with AV-1980R/A and a control group of mice were injected with adjuvant only. Nontransgenic and tetracycline transactivator (tTA) transgenic littermates were included as baseline controls to contrast with the tau phenotype. Active immunization with AV-1980R/A induced very strong anti-tau humoral immune responses in both nontransgenic and transgenic mice with evidence of IgG in brains of AV-1980R/A vaccinated mice. These experimental animals displayed an improvement in short-term memory during a novel object recognition test. However, impairments in other behavioral tasks were not prevented by AV-1980R/A vaccinations. At the same time, high titers of anti-tau antibodies reduced hyperphosphorylated pSer396 tau but did not lower the level of other phosphorylated tau species in the brains of AV-1980R/A vaccinated mice. These data indicate that active immunotherapy with an N-terminal Tau epitope was only partially effective in improving cognition and reducing pathology in the stringent Tg4510 mouse model of tauopathy.

Periodic dietary restriction ameliorates amyloid pathology and cognitive impairment in PDAPP-J20 mice: Potential implication of glial autophagy

Dietary restriction promotes cell regeneration and stress resistance in multiple models of human diseases. One of the conditions that could potentially benefit from this strategy is Alzheimer's disease, a chronic, progressive and prevalent neurodegenerative disease. Although there are no effective pharmacological treatments for this pathology, lifestyle interventions could play therapeutic roles. Our objectives were 1) to evaluate the effects of dietary restriction on cognition, hippocampal amyloid deposition, adult neurogenesis and glial reactivity and autophagy in a mouse model of familial Alzheimer's disease, and 2) to analyze the role of glial cells mediating the effects of nutrient restriction in an in vitro model. Therefore, we established a periodic dietary restriction protocol in adult female PDAPP-J20 transgenic mice for 6 weeks. We found that dietary restriction, not involving overall caloric restriction, attenuated cognitive deficits, amyloid pathology and microglial reactivity in transgenic mice when compared with ad libitum-fed transgenic animals. Also, transgenic mice showed an increase in the astroglial positive signal for LC3, an autophagy-associated protein. In parallel, hippocampal adult neurogenesis was decreased in transgenic mice whereas dietary-restricted transgenic mice showed a neurogenic status similar to controls. In vitro experiments showed that nutrient restriction decreased astroglial and, indirectly, microglial NFκB activation in response to amyloid β peptides. Furthermore, nutrient restriction was able to preserve astroglial autophagic flux and to decrease intracellular amyloid after exposure to amyloid β peptides. Our results suggest neuroprotective effects of nutrient restriction in Alzheimer's disease, with modulation of glial activation and autophagy being potentially involved pathways.

The small molecular CCR3 antagonist YM344031 attenuates neurodegenerative pathologies and improves learning and memory performance in a mouse model of Alzheimer's disease

The chemokine C-C receptor 3 (CCR3) plays a role in the pathogenesis of Alzheimer's disease (AD). Based on our previous observations that deletion of CCR3 prevented neurodegenerative pathologies in amyloid precursor protein/presenilin 1 (APP/PS1) double-transgenic mice, we hypothesize that CCR3 antagonists may provide therapeutic benefits to AD. To this end, we examined the effect of the brain-penetrable CCR3 antagonist, YM344031, on AD-related pathologies in APP/PS1 double transgenic mice. Treatment of 10-month-old APP/PS1 double-transgenic mice with YM344031 (50 mg/kg, b.i.d.) for two months resulted in dramatic decreases in β-amyloid deposition, tau hyperphosphorylation and synaptic loss in the forebrain, significant attenuation of microgliosis and astrogliosis, and marked improvement of spatial learning and memory performance compared with the vehicle-treated mice. These results support CCR3 antagonism as a potential therapeutic strategy for AD.

Cysteine-rich granulin-3 rapidly promotes amyloid-β fibrils in both redox states

Granulins (GRNs 1-7) are cysteine-rich proteolytic products of progranulin (PGRN) that have recently been implicated in neurodegenerative diseases including frontotemporal dementia (FTD) and Alzheimer's disease (AD). Their precise mechanism in these pathologies remains uncertain, but both inflammatory and lysosomal roles have been observed for GRNs. Among the seven GRNs, GRN-3 is well characterized and is implicated within the context of FTD. However, the relationship between GRN-3 and amyloid-β (Aβ), a protein relevant in AD pathology, has not yet been explored. To gain insight into this mechanism, we investigated the effect of both oxidized and reduced GRN-3 on Aβ aggregation and found that both GRN-3 (oxidized) and rGRN-3 (reduced) bind to monomeric and oligomeric Aβ42 to promote rapid fibril formation with subtle rate differences. As low molecular weight oligomers of Aβ are well-established neurotoxins, rapid promotion of fibrils by GRN-3 mitigates Aβ42-induced cellular apoptosis. These data provide valuable insights in understanding GRN-3's ability to modulate Aβ-induced toxicity under redox control and presents a new perspective toward AD pathology. These results also prompt further investigation into the role(s) of other GRNs in AD pathogenesis.

Neuroinflammation and fractalkine signaling in Alzheimer's disease

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder, and the most common form of dementia. As the understanding of AD has progressed, it is now believed that AD is an amyloid-initiated tauopathy with neuroinflammation serving as the link between amyloid deposition, tau pathology, and neurodegeneration. As microglia are the main immune effectors in the central nervous system, they have been the focus of attention in studies investigating the neuroinflammatory component of AD. Therefore, recent work has focused on immunomodulators, which can alter microglial activation without suppressing activity, as potential therapeutics for AD. Fractalkine (CX3CL1; FKN), a unique chemokine with a one-to-one relationship with its receptor, signals through its cognate receptor (CX3CR1) to reduce expression of pro-inflammatory genes in activated microglia. Disrupting FKN signaling has opposing effects on the two hallmark pathologies of AD, but over-expressing a soluble FKN has been shown to reduce tau pathology while not altering amyloid pathology. Recently, differential signaling has been reported when comparing two cleavage variants of soluble FKN. These differential effects may explain recent studies reporting seemingly conflicting results regarding the effect of FKN over expression on AD pathologies.

How stress mediators can cumulatively contribute to Alzheimer's disease An allostatic load approach

Allostatic load is defined as the frequent activation of the neuroendocrine, immunological, metabolic and cardiovascular systems, which makes individuals more susceptible to stress-related health problems. According to this model, physiological dysregulations start to emerge decades before diseases manifest. Consequently, stress research has shifted its attention to anticipating the degree of this dysregulation to better understand the impact of stress hormones and other biomarkers on disease progression. In view of the growing number of studies that demonstrate the influence of modifiable risk factors on cognitive decline, in addition to the effects of chronic stress mediators, the objective of the present review was to present an overview of the development of cognitive changes based on studies on stress and its mediators.

Pathophysiological Mechanisms Linking Type 2 Diabetes and Dementia: Review of Evidence from Clinical, Translational and Epidemiological Research

BACKGROUND

Type 2 diabetes represents an increasing health burden world-wide and its prevalence in particularly higher in elderly population. Consistent epidemiological evidence suggests an increased risk of dementia associated to type 2 diabetes; the mechanisms underlying these associations, however, remain unclear.

OBJECTIVE

The study aims to review epidemiological, clinical and pre-clinical data that weigh on pathophysiological links, mechanisms of disease and associations between type 2 diabetes and dementia to identify areas of opportunity for future research.

METHODS

We searched the following electronic bibliographic databases: PUBMED, EMBASE, SCIELO, MEDLINE and OVID for clinical, translational and epidemiological research literature that summarize diabetes-related risk factors for dementia, metabolic and neurological changes associated to T2D, evidence of therapeutic approaches in type 2 diabetes and its pathophysiological implications for dementia.

RESULTS

Type 2 diabetes mellitus increases risk for all-cause dementia, vascular dementia and Alzheimer's disease. The most evaluated mechanisms linking both disorders in pre-clinical studies include an increase in neuronal insulin resistance, impaired insulin signaling, pro-inflammatory state, mitochondrial dysfunction and vascular damage which increase deposition of β-amyloid, tau proteins and GSK3β, leading to an earlier onset of dementia in individuals with impairment in the glucose metabolism. Neuroimaging and neuropathology evidence linking cerebrovascular lesions, neurodegeneration and particularly small-vessel disease in the onset of dementia is consistent with the increased risk of incident dementia in type 2 diabetes, but consistent evidence of AD-related pathology is scarce. Epidemiological data shows increased risk of dementia related to hypoglycemic episodes, glycemic control, metabolic syndrome, insulin resistance and genetic predisposition, but the evidence is not consistent and statistical analysis might be affected by inconsistent covariate controlling. Therapeutic approaches for T2D have shown inconsistent result in relation to dementia prevention and delay of cognitive decline; lifestyle intervention, particularly physical activity, is a promising alternative to ameliorate the impact of disability and frailty on T2D-related dementia.

CONCLUSION

Vascular disease, inflammation and impaired brain insulin signaling might occur in T2D and contribute to dementia risk. Evidence from epidemiological studies has not consistently reported associations that could integrate a unified mechanism of disease in humans. Evaluation of the effect of antidiabetic medications and non-pharmacological interventions in dementia prevention in type 2 diabetes is promising but has thus far offered inconsistent results.

Early minor stimulation of microglial TLR2 and TLR4 receptors attenuates Alzheimer's disease-related cognitive deficit in rats: behavioral, molecular, and electrophysiological evidence

At early stages of Alzheimer's disease (AD), soluble amyloid beta (Aβ) accumulates in brain while microglia are in resting state. Microglia can recognize Aβ long after formation of plaques and release neurotoxic mediators. We examined impact of early minor activation of microglia by Toll-like receptors (TLRs) 2 and 4 agonists on Alzheimer's disease-related disturbed synaptic function and spatial memory in rats. Microglial BV-2 cells were treated by 0.1, 1, and 10 μg/mL of the TLRs ligands lipopolysaccharide, monophosphoryl lipid A (MPL), and Pam3Cys for 24 hours. Culture medium was then changed with media containing 1-μM Aβ. Tumour necrosis factor (TNF)-α and CCL3 levels were measured in the supernatant, 24 hours thereafter. One μg of TLRs ligands which was able to release low level of TNF-α and CCL3, was administered intracerebroventricularly (i.c.v) to adult male rats every 3 days for 24 days. At the half of the treatment period, Aβ1-42 was infused i.c.v (0.075 μg/hour) for 2 weeks. Finally, the following factors were measured: memory performance by Morris water maze, postsynaptic potentials of dentate gyrus following perforant pathway stimulation, hippocampal inflammatory cytokines interleukin 1 (IL-1)β and TNF-α, anti-inflammatory cytokines IL-10 and TGF-1β, microglia marker arginase 1, Aβ deposits, and the receptor involved in Aβ clearance, formyl peptide receptor 2 (FPR2). TLRs ligands caused dose-dependent release of TNF-α and CCL3 by BV-2 cells. Aβ-treated cells did not release TNF-α and CCL3, whereas those pretreated with MPL and Pam3Cys significantly released these cytokines in response to Aβ. Low-dose TLRs ligands improved the disturbance in spatial and working memory; restored the impaired long-term potentiation induced by Aβ; decreased TNF-α, and Aβ deposits; enhanced TGF-1β, IL-10, and arginase 1 in the hippocampus of Aβ-treated rats; and increased polarization of hippocampal microglia to the anti-inflammatory phenotype. The ligands increased formyl peptide receptor 2 in both BV-2 cells and hippocampus/cortex of Aβ-treated rats. Microglia can sense/clear soluble Aβ by early low-dose MPL and Pam3Cys and safeguard synaptic function and memory in rats.

Neuronal SphK1 acetylates COX2 and contributes to pathogenesis in a model of Alzheimer's Disease

Although many reports have revealed the importance of defective microglia-mediated amyloid β phagocytosis in Alzheimer’s disease (AD), the underlying mechanism remains to be explored. Here we demonstrate that neurons in the brains of patients with AD and AD mice show reduction of sphingosine kinase1 (SphK1), leading to defective microglial phagocytosis and dysfunction of inflammation resolution due to decreased secretion of specialized proresolving mediators (SPMs). Elevation of SphK1 increased SPMs secretion, especially 15-R-Lipoxin A4, by promoting acetylation of serine residue 565 (S565) of cyclooxygenase2 (COX2) using acetyl-CoA, resulting in improvement of AD-like pathology in APP/PS1 mice. In contrast, conditional SphK1 deficiency in neurons reduced SPMs secretion and abnormal phagocytosis similar to AD. Together, these results uncover a novel mechanism of SphK1 pathogenesis in AD, in which impaired SPMs secretion leads to defective microglial phagocytosis, and suggests that SphK1 in neurons has acetyl-CoA-dependent cytoplasmic acetyltransferase activity towards COX2.

Sphingosine kinase (SphK) converts sphingosine into lipids, and is implicated in inflammation. Here the authors show that SphK1 functions as an acetyltransferase, regulates microglial phagocytosis and is reduced in a model of Alzheimer’s Disease, such that its restoration ameliorates pathology

Amyloid-Beta and Phosphorylated Tau Accumulations Cause Abnormalities at Synapses of Alzheimer's disease Neurons

Amyloid-beta (Aβ) and hyperphosphorylated tau are hallmark lesions of Alzheimer's disease (AD). However, the loss of synapses and dysfunctions of neurotransmission are more directly tied to disease severity. The role of these lesions in the pathoetiological progression of the disease remains contested. Biochemical, cellular, molecular, and pathological studies provided several lines of evidence and improved our understanding of how Aβ and hyperphosphorylated tau accumulation may directly harm synapses and alter neurotransmission. In vitro evidence suggests that Aβ and hyperphosphorylated tau have both direct and indirect cytotoxic effects that affect neurotransmission, axonal transport, signaling cascades, organelle function, and immune response in ways that lead to synaptic loss and dysfunctions in neurotransmitter release. Observations in preclinical models and autopsy studies support these findings, suggesting that while the pathoetiology of positive lesions remains elusive, their removal may reduce disease severity and progression. The purpose of this article is to highlight the need for further investigation of the role of tau in disease progression and its interactions with Aβ and neurotransmitters alike.

Peptides as Potential Therapeutics for Alzheimer's Disease

Intracellular synthesis, folding, trafficking and degradation of proteins are controlled and integrated by proteostasis. The frequency of protein misfolding disorders in the human population, e.g., in Alzheimer's disease (AD), is increasing due to the aging population. AD treatment options are limited to symptomatic interventions that at best slow-down disease progression. The key biochemical change in AD is the excessive accumulation of per-se non-toxic and soluble amyloid peptides (Aβ(1-37/44), in the intracellular and extracellular space, that alters proteostasis and triggers Aβ modification (e.g., by reactive oxygen species (ROS)) into toxic intermediate, misfolded soluble Aβ peptides, Aβ dimers and Aβ oligomers. The toxic intermediate Aβ products aggregate into progressively less toxic and less soluble protofibrils, fibrils and senile plaques. This review focuses on peptides that inhibit toxic Aβ oligomerization, Aβ aggregation into fibrils, or stabilize Aβ peptides in non-toxic oligomers, and discusses their potential for AD treatment.

Targeting Apolipoprotein E/Amyloid β Binding by Peptoid CPO_Aβ17-21 P Ameliorates Alzheimer's Disease Related Pathology and Cognitive Decline

Inheritance of the apolipoprotein E4 (apoE4) genotype has been identified as the major genetic risk factor for late onset Alzheimer's disease (AD). Studies have shown that apoE, apoE4 in particular, binds to amyloid-β (Aβ) peptides at residues 12-28 of Aβ and this binding modulates Aβ accumulation and disease progression. We have previously shown in several AD transgenic mice lines that blocking the apoE/Aβ interaction with Aβ12-28 P reduced Aβ and tau-related pathology, leading to cognitive improvements in treated AD mice. Recently, we have designed a small peptoid library derived from the Aβ12-28 P sequence to screen for new apoE/Aβ binding inhibitors with higher efficacy and safety. Peptoids are better drug candidates than peptides due to their inherently more favorable pharmacokinetic properties. One of the lead peptoid compounds, CPO_Aβ17-21 P, diminished the apoE/Aβ interaction and attenuated the apoE4 pro-fibrillogenic effects on Aβ aggregation in vitro as well as apoE4 potentiation of Aβ cytotoxicity. CPO_Aβ17-21 P reduced Aβ-related pathology coupled with cognitive improvements in an AD APP/PS1 transgenic mouse model. Our study suggests the non-toxic, non-fibrillogenic peptoid CPO_Aβ17-21 P has significant promise as a new AD therapeutic agent which targets the Aβ related apoE pathway, with improved efficacy and pharmacokinetic properties.

Innate Immunity Stimulation via Toll-Like Receptor 9 Ameliorates Vascular Amyloid Pathology in Tg-SwDI Mice with Associated Cognitive Benefits

Alzheimer's disease (AD) is characterized by the presence of parenchymal amyloid-β (Aβ) plaques, cerebral amyloid angiopathy (CAA) and neurofibrillary tangles. Currently there are no effective treatments for AD. Immunotherapeutic approaches under development are hampered by complications related to ineffectual clearance of CAA. Genome-wide association studies have demonstrated the importance of microglia in AD pathogenesis. Microglia are the primary innate immune cells of the brain. Depending on their activation state and environment, microglia can be beneficial or detrimental. In our prior work, we showed that stimulation of innate immunity with Toll-like receptor 9 agonist, class B CpG (cytosine-phosphate-guanine) oligodeoxynucleotides (ODNs), can reduce amyloid and tau pathologies without causing toxicity in Tg2576 and 3xTg-AD mouse models. However, these transgenic mice have relatively little CAA. In the current study, we evaluated the therapeutic profile of CpG ODN in a triple transgenic mouse model, Tg-SwDI, with abundant vascular amyloid, in association with low levels of parenchymal amyloid deposits. Peripheral administration of CpG ODN, both before and after the development of CAA, negated short-term memory deficits, as assessed by object-recognition tests, and was effective at improving spatial and working memory evaluated using a radial arm maze. These findings were associated with significant reductions of CAA pathology lacking adverse effects. Together, our extensive evidence suggests that this innovative immunomodulation may be a safe approach to ameliorate all hallmarks of AD pathology, supporting the potential clinical applicability of CpG ODN.

SIGNIFICANCE STATEMENT

Recent genetic studies have underscored the emerging role of microglia in Alzheimer's disease (AD) pathogenesis. Microglia lose their amyloid-β-clearing capabilities with age and as AD progresses. Therefore, the ability to modulate microglia profiles offers a promising therapeutic avenue for reducing AD pathology. Current immunotherapeutic approaches have been limited by poor clearance of a core AD lesion, cerebral amyloid angiopathy (CAA). The present study used Tg-SwDI mice, which have extensive CAA. We found that stimulation of the innate immune system and microglia/macrophage activation via Toll-like receptor 9 using CpG (cytosine-phosphate-guanine) oligodeoxynucleotides (ODNs) leads to cognitive improvements and CAA reduction, without associated toxicity. Our data indicate that this novel concept of immunomodulation represents a safer method to reduce all aspects of AD pathology and provide essential information for potential clinical use of CpG ODN.

Microglia Responses in Acute and Chronic Neurological Diseases: What Microglia-Specific Transcriptomic Studies Taught (and did Not Teach) Us

Over the last decade, microglia have been acknowledged to be key players in central nervous system (CNS) under both physiological and pathological conditions. They constantly survey the CNS environment and as immune cells, in pathological contexts, they provide the first host defense and orchestrate the immune response. It is well recognized that under pathological conditions microglia have both sequential and simultaneous, beneficial and detrimental effects. Cell-specific transcriptomics recently became popular in Neuroscience field allowing concurrent monitoring of the expression of numerous genes in a given cell population. Moreover, by comparing two or more conditions, these approaches permit to unbiasedly identify deregulated genes and pathways. A growing number of studies have thus investigated microglial transcriptome remodeling over the course of neuropathological conditions and highlighted the molecular diversity of microglial response to different diseases. In the present work, we restrict our review to microglia obtained directly from in vivo samples and not cell culture, and to studies using whole-genome strategies. We first critically review the different methods developed to decipher microglia transcriptome. In particular, we compare advantages and drawbacks of flow cytometry and laser microdissection to isolate pure microglia population as well as identification of deregulated microglial genes obtained via RNA sequencing (RNA-Seq) vs. microarrays approaches. Second, we summarize insights obtained from microglia transcriptomes in traumatic brain and spinal cord injuries, pain and more chronic neurological conditions including Amyotrophic lateral sclerosis (ALS), Alzheimer disease (AD) and Multiple sclerosis (MS). Transcriptomic responses of microglia in other non-neurodegenerative CNS disorders such as gliomas and sepsis are also addressed. Third, we present a comparison of the most activated pathways in each neuropathological condition using Gene ontology (GO) classification and highlight the diversity of microglia response to insults focusing on their pro- and anti-inflammatory signatures. Finally, we discuss the potential of the latest technological advances, in particular, single cell RNA-Seq to unravel the individual microglial response diversity in neuropathological contexts.

Oligomeric tau-targeted immunotherapy in Tg4510 mice

BACKGROUND

Finding ways to reverse or prevent the consequences of pathogenic tau in the brain is of considerable importance for treatment of Alzheimer's disease and other tauopathies. Immunotherapy against tau has shown promise in several mouse models. In particular, an antibody with selectivity for oligomeric forms of tau, tau oligomer monoclonal antibody (TOMA), has shown rescue of the behavioral phenotype in several murine models of tau deposition.

METHODS

In this study, we examined the capacity of TOMA to rescue the behavioral, histological, and neurochemical consequences of tau deposition in the aggressive Tg4510 model. We treated mice biweekly with 60 μg TOMA i.p. from 3.5 to 8 months of age.

RESULTS

Near the end of the treatment, we found that oligomeric tau was elevated in both the CSF and in plasma. Further, we could detect mouse IgG in Tg4510 mouse brain after TOMA treatment, but not after injection with mouse IgG1 as control. However, we did not find significant reductions in behavioral deficits or tau deposits by either histological or biochemical measurements.

CONCLUSIONS

These data suggest that there is some exposure of the Tg4510 mouse brain to TOMA, but it was inadequate to affect the phenotype in these mice at the doses used. These data are consistent with other observations that the rapidly depositing Tg4510 mouse is a challenging model in which to demonstrate efficacy of tau-lowering treatments compared to some other preclinical models of tau deposition/overexpression.

Targeting Microglial Activation States as a Therapeutic Avenue in Parkinson's Disease

Parkinson’s disease (PD) is a chronic and progressive disorder characterized neuropathologically by loss of dopamine neurons in the substantia nigra, intracellular proteinaceous inclusions, reduction of dopaminergic terminals in the striatum, and increased neuroinflammatory cells. The consequent reduction of dopamine in the basal ganglia results in the classical parkinsonian motor phenotype. A growing body of evidence suggest that neuroinflammation mediated by microglia, the resident macrophage-like immune cells in the brain, play a contributory role in PD pathogenesis. Microglia participate in both physiological and pathological conditions. In the former, microglia restore the integrity of the central nervous system and, in the latter, they promote disease progression. Microglia acquire different activation states to modulate these cellular functions. Upon activation to the M1 phenotype, microglia elaborate pro-inflammatory cytokines and neurotoxic molecules promoting inflammation and cytotoxic responses. In contrast, when adopting the M2 phenotype microglia secrete anti-inflammatory gene products and trophic factors that promote repair, regeneration, and restore homeostasis. Relatively little is known about the different microglial activation states in PD and a better understanding is essential for developing putative neuroprotective agents. Targeting microglial activation states by suppressing their deleterious pro-inflammatory neurotoxicity and/or simultaneously enhancing their beneficial anti-inflammatory protective functions appear as a valid therapeutic approach for PD treatment. In this review, we summarize microglial functions and, their dual neurotoxic and neuroprotective role in PD. We also review molecules that modulate microglial activation states as a therapeutic option for PD treatment.

Innate immunity in Alzheimer's disease: the relevance of animal models?

The mouse is one of the organisms most widely used as an animal model in biomedical research, due to the particular ease with which it can be handled and reproduced in laboratory. As a member of the mammalian class, mice share with humans many features regarding metabolic pathways, cell morphology and anatomy. However, important biological differences between mice and humans exist and must be taken into consideration when interpreting research results, to properly translate evidence from experimental studies into information that can be useful for human disease prevention and/or treatment. With respect to Alzheimer’s disease (AD), much of the experimental information currently known about this disease has been gathered from studies using mainly mice as models. Therefore, it is notably important to fully characterise the differences between mice and humans regarding important aspects of the disease. It is now widely known that inflammation plays an important role in the development of AD, a role that is not only a response to the surrounding pathological environment, but rather seems to be strongly implicated in the aetiology of the disease as indicated by the genetic studies. This review highlights relevant differences in inflammation and in microglia, the innate immune cell of the brain, between mice and humans regarding genetics and morphology in normal ageing, and the relationship of microglia with AD-like pathology, the inflammatory profile, and cognition. We conclude that some noteworthy differences exist between mice and humans regarding microglial characteristics, in distribution, gene expression, and states of activation. This may have repercussions in the way that transgenic mice respond to, and influence, the AD-like pathology. However, despite these differences, human and mouse microglia also show similarities in morphology and behaviour, such that the mouse is a suitable model for studying the role of microglia, as long as these differences are taken into consideration when delineating new strategies to approach the study of neurodegenerative diseases.

Islet amyloid polypeptide: Another key molecule in Alzheimer's pathogenesis?

Recent epidemiological evidence reveals that patients suffering from type 2 diabetes mellitus (T2DM) often experience a significant decline in cognitive function, and approximately 70% of those cases eventually develop Alzheimer's disease (AD). Although several pathological processes are shared by AD and T2DM, the exact molecular mechanisms connecting these two diseases are poorly understood. Aggregation of human islet amyloid polypeptide (hIAPP), the pathological hallmark of T2DM, has also been detected in brain tissue and is associated with cognitive decline and AD development. In addition, hIAPP and amyloid β protein (Aβ) share many biophysical and physiological properties as well as exert similar cytotoxic mechanisms. Therefore, it is important to examine the possible role of hIAPP in the pathogenesis of AD. In this article, we introduce the basics on this amyloidogenic protein. More importantly, we discuss the potential mechanisms of hIAPP-induced AD development, which will be beneficial for proposing novel and feasible strategies to optimize AD prevention and/or treatment in diabetics.

High bioavailability curcumin: an anti-inflammatory and neurosupportive bioactive nutrient for neurodegenerative diseases characterized by chronic neuroinflammation

Neuroinflammation is a pathophysiological process present in a number of neurodegenerative disorders, such as Alzheimer's disease, Huntington's disease, Parkinson's disease, stroke, traumatic brain injury including chronic traumatic encephalopathy and other age-related CNS disorders. Although there is still much debate about the initial trigger for some of these neurodegenerative disorders, during the progression of disease, broad range anti-inflammatory drugs including cytokine suppressive anti-inflammatory drugs (CSAIDs) might be promising therapeutic options to limit neuroinflammation and improve the clinical outcome. One of the most promising CSAIDs is curcumin, which modulates the activity of several transcription factors (e.g., STAT, NF-κB, AP-1) and their pro-inflammatory molecular signaling pathways. However, normal curcumin preparations demonstrate low bioavailability in vivo. To increase bioavailability, preparations of high bioavailability curcumin have been introduced to achieve therapeutically relevant concentrations in target tissues. This literature review aims to summarize the pharmacokinetic and toxicity profile of different curcumin formulations.

Young microglia restore amyloid plaque clearance of aged microglia

Alzheimer's disease (AD) is characterized by deposition of amyloid plaques, neurofibrillary tangles, and neuroinflammation. In order to study microglial contribution to amyloid plaque phagocytosis, we developed a novel ex vivo model by co-culturing organotypic brain slices from up to 20-month-old, amyloid-bearing AD mouse model (APPPS1) and young, neonatal wild-type (WT) mice. Surprisingly, co-culturing resulted in proliferation, recruitment, and clustering of old microglial cells around amyloid plaques and clearance of the plaque halo. Depletion of either old or young microglial cells prevented amyloid plaque clearance, indicating a synergistic effect of both populations. Exposing old microglial cells to conditioned media of young microglia or addition of granulocyte-macrophage colony-stimulating factor (GM-CSF) was sufficient to induce microglial proliferation and reduce amyloid plaque size. Our data suggest that microglial dysfunction in AD may be reversible and their phagocytic ability can be modulated to limit amyloid accumulation. This novel ex vivo model provides a valuable system for identification, screening, and testing of compounds aimed to therapeutically reinforce microglial phagocytosis.

Interactions between inflammation, sex steroids, and Alzheimer's disease risk factors

Alzheimer's disease (AD) is an age-related neurodegenerative disorder for which there are no effective strategies to prevent or slow its progression. Because AD is multifactorial, recent research has focused on understanding interactions among the numerous risk factors and mechanisms underlying the disease. One mechanism through which several risk factors may be acting is inflammation. AD is characterized by chronic inflammation that is observed before clinical onset of dementia. Several genetic and environmental risk factors for AD increase inflammation, including apolipoprotein E4, obesity, and air pollution. Additionally, sex steroid hormones appear to contribute to AD risk, with age-related losses of estrogens in women and androgens in men associated with increased risk. Importantly, sex steroid hormones have anti-inflammatory actions and can interact with several other AD risk factors. This review examines the individual and interactive roles of inflammation and sex steroid hormones in AD, as well as their relationships with the AD risk factors apolipoprotein E4, obesity, and air pollution.

Microglial response to LPS increases in wild-type mice during aging but diminishes in an Alzheimer's mouse model: Implication of TLR4 signaling in disease progression

Microglia-mediated clearance of amyloid beta-protein (Aβ) via Toll-like receptor 4 (TLR4) signaling may play an important role in the pathogenesis of Alzheimer's disease (AD). However, as the disease progresses, activated microglia appear to become incapable of clearing Aβ deposits. Because repeated exposure to a TLR4 ligand leads to a diminished response of monocytes/macrophages to lipopolysaccharide (LPS) and because aggregated Aβ is a TLR4 ligand, we hypothesize that chronic exposure of microglia to Aβ deposits may induce a state of Toll-like receptor (TLR) signaling dysfunction, leading to decreased Aβ clearance and accelerated disease progression. LPS or phosphate-buffered saline (PBS) was injected into the hippocampus of AD-model (TgAPP/PS1) and wild-type (non-Tg) mice before and after the onset of Aβ deposition, at age 2 and 12 months, respectively. Brain specimens were collected 7 days post-injection and analyzed for microglial activation and Aβ load. While LPS-injected 2-month-old non-Tg mice showed 48-fold and 11-fold greater Iba1 immunoreactivity in the neocortex and hippocampus, respectively, compared with PBS-injected mice, LPS-injected 2-month-old TgAPP/PS1 mice had 61-fold and 13-fold increases in the neocortex and hippocampus, respectively. LPS injection activated microglia more strongly in TgAPP/PS1 mice than in non-Tg mice at 2 months of age. In contrast, at 12 months of age, Iba1 immunoreactivity of microglia was increased 541-fold and 38-fold in the neocortex and hippocampus, respectively, in LPS-injected non-Tg mice and 2.7-fold and 3.3-fold in the neocortex and hippocampus, respectively, in LPS-injected TgAPP/PS1 mice. Surprisingly, LPS injection decreased CD45 immunoreactivity in TgAPP/PS1 mice but increased it in non-Tg mice at 12 months. Although microglia in 12-month-old non-Tg mice showed stronger response to LPS than 2-month-old non-Tg mice, microglia in TgAPP/PS1 mice exhibited diminished immune response to LPS during aging. Our data indicate that microglial TLR4 signaling is altered in an AD mouse model and suggest that altered TLR4 signaling may contribute to Aβ accumulation in the brain.

Microglial immunophenotype in dementia with Alzheimer's pathology

Background

Genetic risk factors for Alzheimer’s disease imply that inflammation plays a causal role in development of the disease. Experimental studies suggest that microglia, as the brain macrophages, have diverse functions, with their main role in health being to survey the brain parenchyma through highly motile processes.

Methods

Using the Medical Research Council Cognitive Function and Ageing Studies resources, we have immunophenotyped microglia to investigate their role in dementia with Alzheimer’s pathology. Cerebral cortex obtained at post-mortem from 299 participants was analysed by immunohistochemistry for cluster of differentiation (CD)68 (phagocytosis), human leukocyte antigen (HLA)-DR (antigen-presenting function), ionized calcium-binding adaptor molecule (Iba1) (microglial motility), macrophage scavenger receptor (MSR)-A (plaque-related phagocytosis) and CD64 (immunoglobulin Fcγ receptor I).

Results

The presence of dementia was associated positively with CD68 (P < 0.001), MSR-A (P = 0.010) and CD64 (P = 0.007) and negatively with Iba1 (P < 0.001). Among participants without dementia, the cognitive function according to the Mini-Mental State Examination was associated positively with Iba1 (P < 0.001) and negatively with CD68 (P = 0.033), and in participants with dementia and Alzheimer’s pathology, positively with all microglial markers except Iba1. Overall, in participants without dementia, the relationship with Alzheimer’s pathology was negative or not significant, and positive in participants with dementia and Alzheimer’s pathology. Apolipoprotein E (APOE) ε2 allele was associated with expression of Iba1 (P = 0.001) and MSR-A (P < 0.001) and APOE ε4 with CD68, HLA-DR and CD64 (P < 0.001).

Conclusions

Our findings raise the possibility that in dementia with Alzheimer’s pathology, microglia lose motility (Iba-1) necessary to support neurons. Conversely, other microglial proteins (CD68, MSR-A), the role of which is clearance of damaged cellular material, are positively associated with Alzheimer’s pathology and impaired cognitive function. In addition, our data imply that microglia may respond differently to Aβ and tau in participants with and without dementia so that the microglial activity could potentially influence the likelihood of developing dementia, as supported by genetic studies, highlighting the complexity and diversity of microglial responses.

The Rationale for Insulin Therapy in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, with a prevalence that increases with age. By 2050, the worldwide number of patients with AD is projected to reach more than 140 million. The prominent signs of AD are progressive memory loss, accompanied by a gradual decline in cognitive function and premature death. AD is the clinical manifestation of altered proteostasis. The initiating step of altered proteostasis in most AD patients is not known. The progression of AD is accelerated by several chronic disorders, among which the contribution of diabetes to AD is well understood at the cell biology level. The pathological mechanisms of AD and diabetes interact and tend to reinforce each other, thus accelerating cognitive impairment. At present, only symptomatic interventions are available for treating AD. To optimise symptomatic treatment, a personalised therapy approach has been suggested. Intranasal insulin administration seems to open the possibility for a safe, and at least in the short term, effective symptomatic intervention that delays loss of cognition in AD patients. This review summarizes the interactions of AD and diabetes from the cell biology to the patient level and the clinical results of intranasal insulin treatment of cognitive decline in AD.

Drug Development in Alzheimer's Disease: The Contribution of PET and SPECT

Clinical trials aiming to develop disease-altering drugs for Alzheimer’s disease (AD), a neurodegenerative disorder with devastating consequences, are failing at an alarming rate. Poorly defined inclusion-and outcome criteria, due to a limited amount of objective biomarkers, is one of the major concerns. Non-invasive molecular imaging techniques, positron emission tomography and single photon emission (computed) tomography (PET and SPE(C)T), allow visualization and quantification of a wide variety of (patho)physiological processes and allow early (differential) diagnosis in many disorders. PET and SPECT have the ability to provide biomarkers that permit spatial assessment of pathophysiological molecular changes and therefore objectively evaluate and follow up therapeutic response, especially in the brain. A number of specific PET/SPECT biomarkers used in support of emerging clinical therapies in AD are discussed in this review.

In Vivo Detection of Age- and Disease-Related Increases in Neuroinflammation by 18F-GE180 TSPO MicroPET Imaging in Wild-Type and Alzheimer's Transgenic Mice

Alzheimer's disease (AD) is the most common cause of dementia. Neuroinflammation appears to play an important role in AD pathogenesis. Ligands of the 18 kDa translocator protein (TSPO), a marker for activated microglia, have been used as positron emission tomography (PET) tracers to reflect neuroinflammation in humans and mouse models. Here, we used the novel TSPO-targeted PET tracer (18)F-GE180 (flutriciclamide) to investigate differences in neuroinflammation between young and old WT and APP/PS1dE9 transgenic (Tg) mice. In vivo PET scans revealed an overt age-dependent elevation in whole-brain uptake of (18)F-GE180 in both WT and Tg mice, and a significant increase in whole-brain uptake of (18)F-GE180 (peak-uptake and retention) in old Tg mice compared with young Tg mice and all WT mice. Similarly, the (18)F-GE180 binding potential in hippocampus was highest to lowest in old Tg > old WT > young Tg > young WT mice using MRI coregistration. Ex vivo PET and autoradiography analysis further confirmed our in vivo PET results: enhanced uptake and specific binding (SUV75%) of (18)F-GE180 in hippocampus and cortex was highest in old Tg mice followed by old WT, young Tg, and finally young WT mice. (18)F-GE180 specificity was confirmed by an in vivo cold tracer competition study. We also examined (18)F-GE180 metabolites in 4-month-old WT mice and found that, although total radioactivity declined over 2 h, of the remaining radioactivity, ∼90% was due to parent (18)F-GE180. In conclusion, (18)F-GE180 PET scans may be useful for longitudinal monitoring of neuroinflammation during AD progression and treatment.

SIGNIFICANCE STATEMENT

Microglial activation, a player in Alzheimer's disease (AD) pathogenesis, is thought to reflect neuroinflammation. Using in vivo microPET imaging with a novel TSPO radioligand, (18)F-GE180, we detected significantly enhanced neuroinflammation during normal aging in WT mice and in response to AD-associated pathology in APP/PS1dE9 Tg mice, an AD mouse model. Increased uptake and specific binding of (18)F-GE180 in whole brain and hippocampus were confirmed by ex vivo PET and autoradiography. The binding specificity and stability of (18)F-GE180 was further confirmed by a cold tracer competition study and a metabolite study, respectively. Therefore, (18)F-GE180 PET imaging may be useful for longitudinal monitoring of neuroinflammation during AD progression and treatment and may also be useful for other neurodegenerative diseases.

β-Asarone Mitigates Amyloidosis and Downregulates RAGE in a Transgenic Mouse Model of Alzheimer's Disease

Elevated β-amyloid (Aβ) is a hallmark of Alzheimer's disease (AD). Recent evidence has suggested that the receptor of advanced glycation end products (RAGE) is a key target for Aβ-induced perturbation in AD, and blockade of RAGE significantly alleviates synaptic injury. Our previous study has suggested that β-asarone could reduce neuronal apoptosis and improve memory deficits in β-amyloid precursor protein and presenilin-1 (APP/PS1) double transgenic AD-model mice. In the present study, we evaluated the effects of β-asarone on amyloidosis in APP/PS1 mice. We found that the survival of neurons of APP/PS1 mice was improved by β-asarone, meanwhile, β-asarone decreased Aβ deposition and down-regulated Aβ1-42 levels in cortex and hippocampus of APP/PS1 mice brain. Interestingly, the level of RAGE was also significantly down-regulated by β-asarone. Our findings suggest that β-asarone might be effective for the treatment of AD, and the decreasing effects of β-asarone on Aβ might associate with its down-regulation of RAGE.

Cannabinoid CB2 Receptors in a Mouse Model of Aβ Amyloidosis: Immunohistochemical Analysis and Suitability as a PET Biomarker of Neuroinflammation

In Alzheimer's disease (AD), one of the early responses to Aβ amyloidosis is recruitment of microglia to areas of new plaque. Microglial receptors such as cannabinoid receptor 2 (CB2) might be a suitable target for development of PET radiotracers that could serve as imaging biomarkers of Aβ-induced neuroinflammation. Mouse models of amyloidosis (J20APPswe/ind and APPswe/PS1ΔE9) were used to investigate the cellular distribution of CB2 receptors. Specificity of CB2 antibody (H60) was confirmed using J20APPswe/ind mice lacking CB2 receptors. APPswe/PS1ΔE9 mice were used in small animal PET with a CB2-targeting radiotracer, [11C]A836339. These studies revealed increased binding of [11C]A836339 in amyloid-bearing mice. Specificity of the PET signal was confirmed in a blockade study with a specific CB2 antagonist, AM630. Confocal microscopy revealed that CB2-receptor immunoreactivity was associated with astroglial (GFAP) and, predominantly, microglial (CD68) markers. CB2 receptors were observed, in particular, in microglial processes forming engulfment synapses with Aβ plaques. In contrast to glial cells, neuron (NeuN)-derived CB2 signal was equal between amyloid-bearing and control mice. The pattern of neuronal CB2 staining in amyloid-bearing mice was similar to that in human cases of AD. The data collected in this study indicate that Aβ amyloidosis without concomitant tau pathology is sufficient to activate CB2 receptors that are suitable as an imaging biomarker of neuroinflammation. The main source of enhanced CB2 PET binding in amyloid-bearing mice is increased CB2 immunoreactivity in activated microglia. The presence of CB2 immunoreactivity in neurons does not likely contribute to the enhanced CB2 PET signal in amyloid-bearing mice due to a lack of significant neuronal loss in this model. However, significant loss of neurons as seen at late stages of AD might decrease the CB2 PET signal due to loss of neuronally-derived CB2. Thus this study in mouse models of AD indicates that a CB2-specific radiotracer can be used as a biomarker of neuroinflammation in the early preclinical stages of AD, when no significant neuronal loss has yet developed.

Neuroinflammation in Alzheimer's disease

Increasing evidence suggests that Alzheimer's disease pathogenesis is not restricted to the neuronal compartment, but includes strong interactions with immunological mechanisms in the brain. Misfolded and aggregated proteins bind to pattern recognition receptors on microglia and astroglia, and trigger an innate immune response characterised by release of inflammatory mediators, which contribute to disease progression and severity. Genome-wide analysis suggests that several genes that increase the risk for sporadic Alzheimer's disease encode factors that regulate glial clearance of misfolded proteins and the inflammatory reaction. External factors, including systemic inflammation and obesity, are likely to interfere with immunological processes of the brain and further promote disease progression. Modulation of risk factors and targeting of these immune mechanisms could lead to future therapeutic or preventive strategies for Alzheimer's disease.

Potential role of antimicrobial peptides in the early onset of Alzheimer's disease

Cerebral aggregation of amyloid-β (Aβ) is thought to play a major role in the etiology of Alzheimer's disease. Environmental influences, including chronic bacterial or viral infections, are thought to alter the permeability of the blood-brain barrier (BBB) and thereby facilitate cerebral colonization by opportunistic pathogens. This may eventually trigger Aβ overproduction and aggregation. Host biomolecules that target and combat these pathogens, for instance, antimicrobial peptides (AMPs) such as Aβ itself, are an interesting option for the detection and diagnostic follow-up of such cerebral infections. As part of the innate immune system, AMPs are defensive peptides that efficiently penetrate infected cells and tissues beyond many endothelial barriers, most linings, including the BBB, and overall specifically target pathogens. Based on existing literature, we postulate a role for labeled AMPs as a marker to target pathogens that play a role in the aggregation of amyloid in the brain.