Myeloid differentiation factor 88-deficient bone marrow cells improve Alzheimer's disease-related symptoms and pathology

Modulation of Alzheimer's disease brain pathology in mice by gut bacterial depletion: the role of IL-17a

Gut bacteria regulate brain pathology of Alzheimer's disease (AD) patients and animal models; however, the underlying mechanism remains unclear. In this study, 3-month-old APP-transgenic female mice with and without knock-out of Il-17a gene were treated with antibiotics-supplemented or normal drinking water for 2 months. The antibiotic treatment eradicated almost all intestinal bacteria, which led to a reduction in Il-17a-expressing CD4-positive T lymphocytes in the spleen and gut, and to a decrease in bacterial DNA in brain tissue. Depletion of gut bacteria inhibited inflammatory activation in both brain tissue and microglia, lowered cerebral Aβ levels, and promoted transcription of Arc gene in the brain of APP-transgenic mice, all of which effects were abolished by deficiency of Il-17a. As possible mechanisms regulating Aβ pathology, depletion of gut bacteria inhibited β-secretase activity and increased the expression of Abcb1 and Lrp1 in the brain or at the blood-brain barrier, which were also reversed by the absence of Il-17a. Interestingly, a crossbreeding experiment between APP-transgenic mice and Il-17a knockout mice further showed that deficiency of Il-17a had already increased Abcb1 and Lrp1 expression at the blood-brain barrier. Thus, depletion of gut bacteria attenuates inflammatory activation and amyloid pathology in APP-transgenic mice via Il-17a-involved signaling pathways. Our study contributes to a better understanding of the gut-brain axis in AD pathophysiology and highlights the therapeutic potential of Il-17a inhibition or specific depletion of gut bacteria that stimulate the development of Il-17a-expressing T cells.

A novel strategy for bioactive natural products targeting NLRP3 inflammasome in Alzheimer's disease

Alzheimer's disease (AD), the most common type of dementia, is an ageing-related progressive neurodegenerative brain disorder. Extracellular neuritic plaques composed of misfolded amyloid β (Aβ) proteins and intracellular neurofibrillary tangles formed by hyperphosphorylated tau protein are the two classical characteristics of AD. Aβ and tau pathologies induce neurite atrophy and neuronal apoptosis, leading to cognitive, language, and behavioral deficits. For decades, researchers have made great efforts to explore the pathogens and therapeutics of AD; however, its intrinsic mechanism remains unclear and there are still no well-established strategies to restore or even prevent this disease. Therefore, it would be beneficial for the establishment of novel therapeutic strategy to determine the intrinsic molecular mechanism that is interrelated with the initiation and progression of AD. A variety of evidence indicates that neuroinflammation plays a crucial role in the pathogenesis of AD. Nucleotide-binding oligomerization domain (NOD)-like receptor pyrin domain-containing protein 3 (NLRP3) is a key inflammasome sensor of cellular stress and infection that is involved in the innate immune system. In response to a wide range of stimuli like Aβ, NLRP3 assembles apoptosis-associated speck-like protein (ASC) and procaspase-1 into an inflammasome complex to induce the caspase-1 mediated secretion of interleukin (IL)-1β/IL-18 in M1 polarized microglia, triggering the pathophysiological changes and cognitive decline of AD. Therefore, targeting NLRP3 inflammasome seems an efficient path for AD treatment via regulating brain immune microenvironment. Furthermore, accumulating evidence indicates that traditional Chinese medicine (TCM) exerts beneficial effects on AD via NLRP3 inflammasome inactivation. In this review, we summarize current reports on the role and activated mechanisms of the NLRP3 inflammasome in the pathogenesis of AD. We also review the natural products for attenuating neuroinflammation by targeting NLRP3 inflammasome activation, which provides useful clues for developing novel AD treatments.

p38α‐MAPK‐deficient myeloid cells ameliorate symptoms and pathology of APP ‐transgenic Alzheimer's disease mice

Alzheimer's disease (AD), the most common cause of dementia in the elderly, is pathologically characterized by extracellular deposition of amyloid‐β peptides (Aβ) and microglia‐dominated inflammatory activation in the brain. p38α‐MAPK is activated in both neurons and microglia. How p38α‐MAPK in microglia contributes to AD pathogenesis remains unclear. In this study, we conditionally knocked out p38α‐MAPK in all myeloid cells or specifically in microglia of APP‐transgenic mice, and examined animals for AD‐associated pathologies (i.e., cognitive deficits, Aβ pathology, and neuroinflammation) and individual microglia for their inflammatory activation and Aβ internalization at different disease stages (e.g., at 4 and 9 months of age). Our experiments showed that p38α‐MAPK‐deficient myeloid cells were more effective than p38α‐MAPK‐deficient microglia in reducing cerebral Aβ and neuronal impairment in APP‐transgenic mice. Deficiency of p38α‐MAPK in myeloid cells inhibited inflammatory activation of individual microglia at 4 months but enhanced it at 9 months. Inflammatory activation promoted microglial internalization of Aβ. Interestingly, p38α‐MAPK‐deficient myeloid cells reduced IL‐17a‐expressing CD4‐positive lymphocytes in 9 but not 4‐month‐old APP‐transgenic mice. By cross‐breeding APP‐transgenic mice with Il‐17a‐knockout mice, we observed that IL‐17a deficiency potentially activated microglia and reduced Aβ deposition in the brain as shown in 9‐month‐old myeloid p38α‐MAPK‐deficient AD mice. Thus, p38α‐MAPK deficiency in all myeloid cells, but not only in microglia, prevents AD progression. IL‐17a‐expressing lymphocytes may partially mediate the pathogenic role of p38α‐MAPK in peripheral myeloid cells. Our study supports p38α‐MAPK as a therapeutic target for AD patients.

Treatment With CD52 Antibody Protects Neurons in Experimental Autoimmune Encephalomyelitis Mice During the Recovering Phase

Multiple sclerosis (MS) is a chronic autoimmune disease driven by T and B lymphocytes. The remyelination failure and neurodegeneration results in permanent clinical disability in MS patients. A desirable therapy should not only modulate the immune system, but also promote neuroprotection and remyelination. To investigate the neuroprotective effect of CD52 antibody in MS, both C57BL/6J and SJL mice with experimental autoimmune encephalomyelitis (EAE) were treated with CD52 antibody at the peak of disease. Treatment with CD52 antibody depleted T but not B lymphocytes in the blood, reduced the infiltration of T lymphocytes and microglia/macrophages in the spinal cord. Anti-CD52 therapy attenuated EAE scores during the recovery phase. It protected neurons immediately after treatment (within 4 days) as shown by reducing the accumulation of amyloid precursor proteins. It potentially promoted remyelination as it increased the number of olig2/CC-1-positive mature oligodendrocytes and prevented myelin loss in the following days (e.g., 14 days post treatment). In further experiments, EAE mice with a conditional knockout of BDNF in neurons were administered with CD52 antibodies. Neuronal deficiency of BDNF attenuated the effect of anti-CD52 treatment on reducing EAE scores and inflammatory infiltration but did not affect anti-CD52 treatment-induced improvement of myelin coverage in the spinal cord. In summary, anti-CD52 therapy depletes CD4-positive T lymphocytes, prevents myelin loss and protects neurons in EAE mice. Neuronal BDNF regulates neuroprotective and anti-inflammatory effect of CD52 antibody in EAE mice.

Haploinsufficiency of microglial MyD88 ameliorates Alzheimer's pathology and vascular disorders in APP/PS1-transgenic mice

Growing evidence indicates that innate immune molecules regulate microglial activation in Alzheimer's disease (AD); however, their effects on amyloid pathology and neurodegeneration remain inconclusive. Here, we conditionally deleted one allele of myd88 gene specifically in microglia in APP/PS1-transgenic mice by 6 months and analyzed AD-associated pathologies by 9 months. We observed that heterozygous deletion of myd88 gene in microglia decreased cerebral amyloid β (Aβ) load and improved cognitive function of AD mice, which was correlated with reduced number of microglia in the brain and inhibited transcription of inflammatory genes, for example, tnf-α and il-1β, in both brain tissues and individual microglia. To investigate mechanisms underlying the pathological improvement, we observed that haploinsufficiency of MyD88 increased microglial recruitment toward Aβ deposits, which might facilitate Aβ clearance. Microglia with haploinsufficient expression of MyD88 also increased vasculature in the brain of APP/PS1-transgenic mice, which was associated with up-regulated transcription of osteopontin and insulin-like growth factor genes in microglia. Moreover, MyD88-haploinsufficient microglia elevated protein levels of LRP1 in cerebral capillaries of APP/PS1-transgenic mice. Cell culture experiments further showed that treatments with interleukin-1β decreased LRP1 expression in pericytes. In summary, haploinsufficiency of MyD88 in microglia at a late disease stage attenuates pro-inflammatory activation and amyloid pathology, prevents the impairment of microvasculature and perhaps also protects LRP1-mediated Aβ clearance in the brain of APP/PS1-transgenic mice, all of which improves neuronal function of AD mice.

Decreased pH in the aging brain and Alzheimer's disease

Using publicly available data sets, we compared pH in the human brain and the cerebrospinal fluid (CSF) of postmortem control and Alzheimer's disease cases. We further investigated the effects of long-term acidosis in vivo in the APP-PS1 mouse model of Alzheimer's disease. We finally examined in vitro whether low pH exposure could modulate the release of proinflammatory cytokines and the uptake of amyloid beta by microglia. In the human brain, pH decreased with aging. Similarly, we observed a reduction of pH in the brain of C57BL/6 mice with age. In addition, independent database analyses revealed that postmortem brain and CSF pH is further reduced in Alzheimer's disease cases compared with controls. Moreover, in vivo experiments showed that low pH CSF infusion increased amyloid beta plaque load in APP-PS1 mice. We further observed that mild acidosis reduced the amyloid beta 42-induced release of tumor necrosis factor-alpha by microglia and their capacity to uptake this peptide. Brain acidosis is associated with aging and might affect pathophysiological processes such as amyloid beta aggregation or inflammation in Alzheimer's disease.

Tau-Mediated Dysregulation of Neuroplasticity and Glial Plasticity

The inability of individual neurons to compensate for aging-related damage leads to a gradual loss of functional plasticity in the brain accompanied by progressive impairment in learning and memory. Whereas this loss in neuroplasticity is gradual during normal aging, in neurodegenerative diseases such as Alzheimer’s disease (AD), this loss is accelerated dramatically, leading to the incapacitation of patients within a decade of onset of cognitive symptoms. The mechanisms that underlie this accelerated loss of neuroplasticity in AD are still not completely understood. While the progressively increasing proteinopathy burden, such as amyloid β (Aβ) plaques and tau tangles, definitely contribute directly to a neuron’s functional demise, the role of non-neuronal cells in controlling neuroplasticity is slowly being recognized as another major factor. These non-neuronal cells include astrocytes, microglia, and oligodendrocytes, which through regulating brain homeostasis, structural stability, and trophic support, play a key role in maintaining normal functioning and resilience of the neuronal network. It is believed that chronic signaling from these cells affects the homeostatic network of neuronal and non-neuronal cells to an extent to destabilize this harmonious milieu in neurodegenerative diseases like AD. Here, we will examine the experimental evidence regarding the direct and indirect pathways through which astrocytes and microglia can alter brain plasticity in AD, specifically as they relate to the development and progression of tauopathy. In this review article, we describe the concepts of neuroplasticity and glial plasticity in healthy aging, delineate possible mechanisms underlying tau-induced plasticity dysfunction, and discuss current clinical trials as well as future disease-modifying approaches.

NLRP3 Is Involved in the Maintenance of Cerebral Pericytes

Pericytes play a central role in regulating the structure and function of capillaries in the brain. However, molecular mechanisms that drive pericyte proliferation and differentiation are unclear. In our study, we immunostained NACHT, LRR and PYD domains-containing protein 3 (NLRP3)-deficient and wild-type littermate mice and observed that NLRP3 deficiency reduced platelet-derived growth factor receptor β (PDGFRβ)-positive pericytes and collagen type IV immunoreactive vasculature in the brain. In Western blot analysis, PDGFRβ and CD13 proteins in isolated cerebral microvessels from the NLRP3-deficient mouse brain were decreased. We further treated cultured pericytes with NLRP3 inhibitor, MCC950, and demonstrated that NLRP3 inhibition attenuated cell proliferation but did not induce apoptosis. NLRP3 inhibition also decreased protein levels of PDGFRβ and CD13 in cultured pericytes. On the contrary, treatments with IL-1β, the major product of NLRP3-contained inflammasome, increased protein levels of PDGFRβ, and CD13 in cultured cells. The alteration of PDGFRβ and CD13 protein levels were correlated with the phosphorylation of AKT. Inhibition of AKT reduced both protein markers and abolished the effect of IL-1β activation in cultured pericytes. Thus, NLRP3 activation might be essential to maintain pericytes in the healthy brain through phosphorylating AKT. The potential adverse effects on the cerebral vascular pericytes should be considered in clinical therapies with NLRP3 inhibitors.

TLR4 Cross-Talk With NLRP3 Inflammasome and Complement Signaling Pathways in Alzheimer's Disease

Amyloid plaques, mainly composed of abnormally aggregated amyloid β-protein (Aβ) in the brain parenchyma, and neurofibrillary tangles (NFTs), consisting of hyperphosphorylated tau protein aggregates in neurons, are two pathological hallmarks of Alzheimer's disease (AD). Aβ fibrils and tau aggregates in the brain are closely associated with neuroinflammation and synapse loss, characterized by activated microglia and dystrophic neurites. Genome-wide genetic association studies revealed important roles of innate immune cells in the pathogenesis of late-onset AD by recognizing a dozen genetic risk loci that modulate innate immune activities. Furthermore, microglia, brain resident innate immune cells, have been increasingly recognized to play key, opposing roles in AD pathogenesis by either eliminating toxic Aβ aggregates and enhancing neuronal plasticity or producing proinflammatory cytokines, reactive oxygen species, and synaptotoxicity. Aggregated Aβ binds to toll-like receptor 4 (TLR4) and activates microglia, resulting in increased phagocytosis and cytokine production. Complement components are associated with amyloid plaques and NFTs. Aggregated Aβ can activate complement, leading to synapse pruning and loss by microglial phagocytosis. Systemic inflammation can activate microglial TLR4, NLRP3 inflammasome, and complement in the brain, leading to neuroinflammation, Aβ accumulation, synapse loss and neurodegeneration. The host immune response has been shown to function through complex crosstalk between the TLR, complement and inflammasome signaling pathways. Accordingly, targeting the molecular mechanisms underlying the TLR-complement-NLRP3 inflammasome signaling pathways can be a preventive and therapeutic approach for AD.

Neuroprotective Properties and Therapeutic Potential of Bone Marrow-Derived Microglia in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, which is characterized by a progressive cognitive decline and senile plaques formed by amyloid β (Aβ). Microglia are the immune cells of the central nervous system (CNS). Studies have proposed 2 types of microglia, namely, the resident microglia and bone marrow-derived microglia (BMDM). Recent studies suggested that BMDM, not the resident microglia, can phagocytose Aβ, which has a great therapeutic potential in AD. Bone marrow-derived microglia can populate the CNS in an efficient manner and their functions can be regulated by some genes. Thus, methods that increase their recruitment and phagocytosis could be used as a new tool that clears Aβ and ameliorates cognitive impairment. Herein, we review the neuroprotective functions of BMDM and their therapeutic potential in AD.

Deficiency of TLR4 ameliorates hypoperfusion-induced brain pathology

Microglial inflammatory activation contributes to chronic cerebral hypoperfusion-induced brain pathology. This editorial highlights a study by Qin et al. (Theranostics 2018; 8(19):5434-5451. doi:10.7150/thno.27882) that deficiency of TLR4 attenuates cognitive dysfunction and white matter injury by reducing autophagy and pro-inflammatory activation in microglia.

Whole-brain low-intensity pulsed ultrasound therapy markedly improves cognitive dysfunctions in mouse models of dementia - Crucial roles of endothelial nitric oxide synthase

BACKGROUND

Therapeutic focused-ultrasound to the hippocampus has been reported to exert neuroprotective effects on dementia. In the present study, we examined whether the whole-brain LIPUS (low-intensity pulsed ultrasound) therapy is effective and safe in 2 mouse models of dementia (vascular dementia, VaD and Alzheimer's disease, AD), and if so, to elucidate the common underlying mechanism(s) involved.

METHODS

We used bilateral carotid artery stenosis (BCAS) model with micro-coils in male C57BL/6 mice as a VaD model and 5XFAD transgenic mice as an AD model. We applied the LIPUS therapy (1.875 MHz, 6.0 kHz, 32cycles) to the whole brain.

RESULTS

In both models, the LIPUS therapy markedly ameliorated cognitive impairments (Y-maze test and/or passive avoidance test) associated with improved cerebral blood flow (CBF). Mechanistically, the LIPUS therapy significantly increased CD31-positive endothelial cells and Olig2-positive oligodendrocyte precursor cells (OPCs) in the VaD model, while it reduced Iba-1-positive microglias and amyloid-β (Aβ) plaque in the AD model. In both models, endothelium-related genes were significantly upregulated in RNA-sequencing, and expressions of endothelial nitric oxide synthase (eNOS) and neurotrophins were upregulated in Western blotting. Interestingly, the increases in glia cells and neurotrophin expressions showed significant correlations with eNOS expression. Importantly, these beneficial effects of LIPUS were absent in eNOS-knockout mice.

CONCLUSIONS

These results indicate that the whole-brain LIPUS is an effective and non-invasive therapy for dementia by activating specific cells corresponding to each pathology, for which eNOS activation plays an important role as a common mechanism.

Thymoquinone alleviates the experimentally induced Alzheimer’s disease inflammation by modulation of TLRs signaling

Alzheimer’s disease (AD) is characterized by a robust inflammatory response elicited by the accumulation and deposition of amyloid-β (Aβ) within the brain. Aβ induces detrimental inflammatory responses through toll-like receptors (TLRs) signaling pathway. Thymoquinone (TQ), the main active constituent of Nigella sativa oil, has been reported by several previous studies for its potent anti-inflammatory effect. The aim of this study is to elucidate the effect of TQ in improving learning and memory, using a rat model of AD induced by a combination of aluminum chloride (AlCl3) and d-galactose (d-Gal). TQ was administered orally at doses of 10, 20, and 40 mg/kg/day for 14 days after AD induction. Memory functions were assessed using the step through passive avoidance test. Amyloid plaques were shown to be present using hematoxylin and eosin staining. Tumor necrosis factor-alpha (TNF-α) and Interleukin-1beta (IL-1β) levels in brain were assessed via ELISA and profiling TLR-2, TLR-4, myeloid differential factor 88, toll–interleukin-1 receptor domain-containing adapter-inducing interferon-β, interferon regulatory factor 3 (IRF-3), and nuclear factor-κB (NF-κB) expressions via real-time polymerase chain reaction. TQ improved AD rat cognitive decline, decreased Aβ formation and accumulation, significantly decreased TNF-α and IL-1β at all levels of doses and significantly downregulated the expression of TLRs pathway components as well as their downstream effectors NF-κB and IRF-3 mRNAs at all levels of doses ( p < 0.05). We concluded that TQ reduced the inflammation induced by d-Gal/AlCl3 combination. It is therefore reasonable to assign the anti-inflammatory responses to the modulation of TLRs pathway.

Aβ and Inflammatory Stimulus Activate Diverse Signaling Pathways in Monocytic Cells: Implications in Retaining Phagocytosis in Aβ-Laden Environment

Background: Accumulation of amyloid β (Aβ) is one of the main hallmarks of Alzheimer’s disease (AD). The enhancement of Aβ clearance may provide therapeutic means to restrict AD pathology. The cellular responses to different forms of Aβ in monocytic cells are poorly known. We aimed to study whether different forms of Aβ induce inflammatory responses in monocytic phagocytes and how Aβ may affect monocytic cell survival and function to retain phagocytosis in Aβ-laden environment.

Methods: Monocytic cells were differentiated from bone marrow hematopoietic stem cells (HSC) in the presence of macrophage-colony stimulating factor. Monocytic cells were stimulated with synthetic Aβ42 and intracellular calcium responses were recorded with calcium imaging. The formation of reactive oxygen species (ROS), secretion of cytokines and cell viability were also assessed. Finally, monocytic cells were introduced to native Aβ deposits ex vivo and the cellular responses in terms of cell viability, pro-inflammatory activation and phagocytosis were determined. The ability of monocytic cells to phagocytose Aβ plaques was determined after intrahippocampal transplantation in vivo.

Results: Freshly solubilized Aβ induced calcium oscillations, which persisted after removal of the stimulus. After few hours of aggregation, Aβ was not able to induce oscillations in monocytic cells. Instead, lipopolysaccharide (LPS) induced calcium responses divergent from Aβ-induced response. Furthermore, while LPS induced massive production of pro-inflammatory cytokines, neither synthetic Aβ species nor native Aβ deposits were able to induce pro-inflammatory activation of monocytic cells, contrary to primary microglia. Finally, monocytic cells retained their viability in the presence of Aβ and exhibited phagocytic activity towards native fibrillar Aβ deposits and congophilic Aβ plaques.

Conclusion: Monocytic cells carry diverse cellular responses to Aβ and inflammatory stimulus LPS. Even though Aβ species cause specific responses in calcium signaling, they completely lack the ability to induce pro-inflammatory phenotype of monocytic cells. Monocytes retain their viability and function in Aβ-laden brain.

Brain-borne IL-1 adjusts glucoregulation and provides fuel support to astrocytes and neurons in an autocrine/paracrine manner

It is still controversial which mediators regulate energy provision to activated neural cells, as insulin does in peripheral tissues. Interleukin-1β (IL-1β) may mediate this effect as it can affect glucoregulation, it is overexpressed in the 'healthy' brain during increased neuronal activity, and it supports high-energy demanding processes such as long-term potentiation, memory and learning. Furthermore, the absence of sustained neuroendocrine and behavioral counterregulation suggests that brain glucose-sensing neurons do not perceive IL-1β-induced hypoglycemia. Here, we show that IL-1β adjusts glucoregulation by inducing its own production in the brain, and that IL-1β-induced hypoglycemia is myeloid differentiation primary response 88 protein (MyD88)-dependent and only partially counteracted by Kir6.2-mediated sensing signaling. Furthermore, we found that, opposite to insulin, IL-1β stimulates brain metabolism. This effect is absent in MyD88-deficient mice, which have neurobehavioral alterations associated to disorders in glucose homeostasis, as during several psychiatric diseases. IL-1β effects on brain metabolism are most likely maintained by IL-1β auto-induction and may reflect a compensatory increase in fuel supply to neural cells. We explore this possibility by directly blocking IL-1 receptors in neural cells. The results showed that, in an activity-dependent and paracrine/autocrine manner, endogenous IL-1 produced by neurons and astrocytes facilitates glucose uptake by these cells. This effect is exacerbated following glutamatergic stimulation and can be passively transferred between cell types. We conclude that the capacity of IL-1β to provide fuel to neural cells underlies its physiological effects on glucoregulation, synaptic plasticity, learning and memory. However, deregulation of IL-1β production could contribute to the alterations in brain glucose metabolism that are detected in several neurologic and psychiatric diseases.

The contribution of neuroinflammation to amyloid toxicity in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease and the most common cause of dementia. Deposition of amyloid-β (Aβ) remains a hallmark feature of the disease, yet the precise mechanism(s) by which this peptide induces neurotoxicity remain unknown. Neuroinflammation has long been implicated in AD pathology, yet its contribution to disease progression is still not understood. Recent evidence suggests that various Aβ complexes interact with microglial and astrocytic expressed pattern recognition receptors that initiate innate immunity. This process involves secretion of pro-inflammatory cytokines, chemokines and generation of reactive oxygen species that, in excess, drive a dysregulated immune response that contributes to neurodegeneration. The mechanisms by which a neuroinflammatory response can influence Aβ production, aggregation and eventual clearance are now becoming key areas where future therapeutic intervention may slow progression of AD. This review will focus on evidence supporting the combined neuroinflammatory-amyloid hypothesis for pathogenesis of AD, describing the key cell types, pathways and mediators involved. Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the leading cause of dementia worldwide. Deposition of intracellular plaques containing amyloid-beta (Aβ) is a hallmark proteinopathy of the disease yet the precise mechanisms by which this peptide induces neurotoxicity remains unknown. A neuroinflammatory response involving polarized microglial activity, enhanced astrocyte reactivity and elevated pro-inflammatory cytokine and chemokine load has long been implicated in AD and proposed to facilitate neurodegeneration. In this issue we discuss key receptor systems of innate immunity that detect Aβ, drive pro-inflammatory cytokine and chemokine production and influence Aβ aggregation and clearance. Evidence summarized in this review supports the combined neuroinflammatory-amyloid hypothesis for pathogenesis of AD and highlights the potential of immunomodulatory agents as potential future therapies for AD patients.

Long-term treatment with Ginkgo biloba extract EGb 761 improves symptoms and pathology in a transgenic mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by extracellular deposits of amyloid β peptide (Aβ) and microglia-dominated neuroinflammation. The therapeutic options for AD are currently limited. In this study, we investigated the antiinflammatory effects and the underlying molecular mechanisms of Ginkgo biloba extract EGb 761 when administered to TgCRND8 AD mice, which overexpress human Alzheimer's amyloid precursor protein (APP) specifically in neurons. We gave APP-transgenic mice EGb 761 as a dietary supplement for 2 or 5months. Plasma concentrations of EGb 761 components in mice were in the same range as such concentrations in humans taking EGb 761 at the recommended dose (240mg daily). Treatment with EGb 761 for 5months significantly improved the cognitive function of the mice as measured by the Barnes Maze test. It also attenuated the loss of synaptic structure proteins, such as PSD-95, Munc18-1, and SNAP25. Treatment with EGb 761 for 5months inhibited microglial inflammatory activation in the brain. The effects of treatment with EGb 761 for 2months were weak and not statistically significant. Moreover, EGb 761 activated autophagy in microglia. Treatment with EGb 761 decreased Aβ-induced microglial secretion of TNF-α and IL-1β and activation of caspase-1, both of which were abolished by the inhibition of autophagy. Treatment with EGb 761 also reduced the concentrations of NLRP3 protein that colocalized with LC3-positive autophagosomes or autolysosomes in microglia. Additionally, long-term treatment with EGb 761 may reduce cerebral Aβ pathology by inhibiting β-secretase activity and Aβ aggregation. Therefore, long-term treatment with G. biloba extract EGb 761, a clinically available and well-tolerated herbal medication, ameliorates AD pathology by antiinflammatory and Aβ-directed mechanisms.

IKKβ deficiency in myeloid cells ameliorates Alzheimer's disease-related symptoms and pathology

Alzheimer's disease (AD) is characterized by extracellular amyloid-β (Aβ) deposits and microglia-dominated inflammatory activation. Innate immune signaling controls microglial inflammatory activities and Aβ clearance. However, studies examining innate immunity in Aβ pathology and neuronal degeneration have produced conflicting results. In this study, we investigated the pathogenic role of innate immunity in AD by ablating a key signaling molecule, IKKβ, specifically in the myeloid cells of TgCRND8 APP-transgenic mice. Deficiency of IKKβ in myeloid cells, especially microglia, simultaneously reduced inflammatory activation and Aβ load in the brain and these effects were associated with reduction of cognitive deficits and preservation of synaptic structure proteins. IKKβ deficiency enhanced microglial recruitment to Aβ deposits and facilitated Aβ internalization, perhaps by inhibiting TGF-β-SMAD2/3 signaling, but did not affect Aβ production and efflux. Therefore, inhibition of IKKβ signaling in myeloid cells improves cognitive functions in AD mice by reducing inflammatory activation and enhancing Aβ clearance. These results contribute to a better understanding of AD pathogenesis and could offer a new therapeutic option for delaying AD progression.

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta-amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein-coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.

Atherosclerosis and Alzheimer--diseases with a common cause? Inflammation, oxysterols, vasculature

BACKGROUND

Aging is accompanied by increasing vulnerability to pathologies such as atherosclerosis (ATH) and Alzheimer disease (AD). Are these different pathologies, or different presentations with a similar underlying pathoetiology?

DISCUSSION

Both ATH and AD involve inflammation, macrophage infiltration, and occlusion of the vasculature. Allelic variants in common genes including APOE predispose to both diseases. In both there is strong evidence of disease association with viral and bacterial pathogens including herpes simplex and Chlamydophila. Furthermore, ablation of components of the immune system (or of bone marrow-derived macrophages alone) in animal models restricts disease development in both cases, arguing that both are accentuated by inflammatory/immune pathways. We discuss that amyloid β, a distinguishing feature of AD, also plays a key role in ATH. Several drugs, at least in mouse models, are effective in preventing the development of both ATH and AD. Given similar age-dependence, genetic underpinnings, involvement of the vasculature, association with infection, Aβ involvement, the central role of macrophages, and drug overlap, we conclude that the two conditions reflect different manifestations of a common pathoetiology.

MECHANISM

Infection and inflammation selectively induce the expression of cholesterol 25-hydroxylase (CH25H). Acutely, the production of 'immunosterol' 25-hydroxycholesterol (25OHC) defends against enveloped viruses. We present evidence that chronic macrophage CH25H upregulation leads to catalyzed esterification of sterols via 25OHC-driven allosteric activation of ACAT (acyl-CoA cholesterol acyltransferase/SOAT), intracellular accumulation of cholesteryl esters and lipid droplets, vascular occlusion, and overt disease.

SUMMARY

We postulate that AD and ATH are both caused by chronic immunologic challenge that induces CH25H expression and protection against particular infectious agents, but at the expense of longer-term pathology.

Rutin improves spatial memory in Alzheimer's disease transgenic mice by reducing Aβ oligomer level and attenuating oxidative stress and neuroinflammation

Alzheimer's disease (AD) is a progressive, neurodegenerative disease characterized by extracellular β-amyloid (Aβ) plaques and intracellular neurofibrillary tangles in the brain. Aβ aggregation is closely associated with neurotoxicity, oxidative stress, and neuronal inflammation. The soluble Aβ oligomers are believed to be the most neurotoxic form among all forms of Aβ aggregates. We have previously reported a polyphenol compound rutin that could inhibit Aβ aggregation and cytotoxicity, attenuate oxidative stress, and decrease the production of nitric oxide and proinflammatory cytokines in vitro. In the current study, we investigated the effect of rutin on APPswe/PS1dE9 transgenic mice. Results demonstrated that orally administered rutin significantly attenuated memory deficits in AD transgenic mice, decreased oligomeric Aβ level, increased super oxide dismutase (SOD) activity and glutathione (GSH)/glutathione disulfide (GSSG) ratio, reduced GSSG and malondialdehyde (MDA) levels, downregulated microgliosis and astrocytosis, and decreased interleukin (IL)-1β and IL-6 levels in the brain. These results indicated that rutin is a promising agent for AD treatment because of its antioxidant, anti-inflammatory, and reducing Aβ oligomer activities.

Manual versus Automated Rodent Behavioral Assessment: Comparing Efficacy and Ease of Bederson and Garcia Neurological Deficit Scores to an Open Field Video-Tracking System

Animal models of stroke have been crucial in advancing our understanding of the pathophysiology of cerebral ischemia. Currently, the standards for determining neurological deficit in rodents are the Bederson and Garcia scales, manual assessments scoring animals based on parameters ranked on a narrow scale of severity. Automated open field analysis of a live-video tracking system that analyzes animal behavior may provide a more sensitive test. Results obtained from the manual Bederson and Garcia scales did not show significant differences between pre- and post-stroke animals in a small cohort. When using the same cohort, however, post-stroke data obtained from automated open field analysis showed significant differences in several parameters. Furthermore, large cohort analysis also demonstrated increased sensitivity with automated open field analysis versus the Bederson and Garcia scales. These early data indicate use of automated open field analysis software may provide a more sensitive assessment when compared to traditional Bederson and Garcia scales.

Loss of P2Y₂ nucleotide receptors enhances early pathology in the TgCRND8 mouse model of Alzheimer's disease

Neuroinflammation is a prominent feature in Alzheimer's disease (AD) and activation of the brain's innate immune system, particularly microglia, has been postulated to both retard and accelerate AD progression. Recent studies indicate that the G protein-coupled P2Y2 nucleotide receptor (P2Y2R) is an important regulator of innate immunity by assisting in the recruitment of monocytes to injured tissue, neutrophils to bacterial infections and eosinophils to allergen-infected lungs. In this study, we investigated the role of the P2Y2R in progression of an AD-like phenotype in the TgCRND8 mouse model that expresses Swedish and Indiana mutations in amyloid precursor protein (APP). Our results indicate that P2Y 2 R expression is upregulated in TgCRND8 mouse brain within 10 weeks of age and then decreases after 25 weeks of age, as compared to littermate controls expressing low levels of the P2Y 2 R. TgCRND8 mice with homozygous P2Y 2 R deletion survive less than 5 weeks, whereas mice with heterozygous P2Y 2 R deletion survive for 12 weeks, a time point when TgCRND8 mice are fully viable. Heterozygous P2Y 2 R deletion in TgCRND8 mice increased β-amyloid (Aβ) plaque load and soluble Aβ1-42 levels in the cerebral cortex and hippocampus, decreased the expression of the microglial marker CD11b in these brain regions and caused neurological deficits within 10 weeks of age, as compared to age-matched TgCRND8 mice. These findings suggest that the P2Y2R is important for the recruitment and activation of microglial cells in the TgCRND8 mouse brain and that the P2Y2R may regulate neuroprotective mechanisms through microglia-mediated clearance of Aβ that when lost can accelerate the onset of an AD-like phenotype in the TgCRND8 mouse.

CD4+ T cell expression of MyD88 is essential for normal resolution of Chlamydia muridarum genital tract infection

Resolution of Chlamydia genital tract infection is delayed in the absence of MyD88. In these studies, we first used bone marrow chimeras to demonstrate a requirement for MyD88 expression by hematopoietic cells in the presence of a wild-type epithelium. Using mixed bone marrow chimeras we then determined that MyD88 expression was specifically required in the adaptive immune compartment. Furthermore, adoptive transfer experiments revealed that CD4(+) T cell expression of MyD88 was necessary for normal resolution of genital tract infection. This requirement was associated with a reduced ability of MyD88(-/-)CD4(+) T cells to accumulate in the draining lymph nodes and genital tract when exposed to the same inflammatory milieu as wild-type CD4(+) T cells. We also demonstrated that the impaired infection control we observed in the absence of MyD88 could not be recapitulated by deficiencies in TLR or IL-1R signaling. In vitro, we detected an increased frequency of apoptotic MyD88(-/-)CD4(+) T cells upon activation in the absence of exogenous ligands for receptors upstream of MyD88. These data reveal an intrinsic requirement for MyD88 in CD4(+) T cells during Chlamydia infection and indicate that the importance of MyD88 extends beyond innate immune responses by directly influencing adaptive immunity.

Neuron-released oligomeric α-synuclein is an endogenous agonist of TLR2 for paracrine activation of microglia

Abnormal aggregation of α-synuclein and sustained microglial activation are important contributors to the pathogenic processes of Parkinson's disease. However, the relationship between disease-associated protein aggregation and microglia-mediated neuroinflammation remains unknown. Here, using a combination of in silico, in vitro and in vivo approaches, we show that extracellular α-synuclein released from neuronal cells is an endogenous agonist for Toll-like receptor 2 (TLR2), which activates inflammatory responses in microglia. The TLR2 ligand activity of α-synuclein is conformation-sensitive; only specific types of oligomer can interact with and activate TLR2. This paracrine interaction between neuron-released oligomeric α-synuclein and TLR2 in microglia suggests that both of these proteins are novel therapeutic targets for modification of neuroinflammation in Parkinson's disease and related neurological diseases.

Innate immunity and neuroinflammation

Inflammation of central nervous system (CNS) is usually associated with trauma and infection. Neuroinflammation occurs in close relation to trauma, infection, and neurodegenerative diseases. Low-level neuroinflammation is considered to have beneficial effects whereas chronic neuroinflammation can be harmful. Innate immune system consisting of pattern-recognition receptors, macrophages, and complement system plays a key role in CNS homeostasis following injury and infection. Here, we discuss how innate immune components can also contribute to neuroinflammation and neurodegeneration.

Tenascin-C deficiency ameliorates Alzheimer's disease-related pathology in mice

Alzheimer's disease (AD) is a neurodegenerative disease characterized by deposits of amyloid β peptide (Aβ) and microglia-driven inflammatory activation. Tenascin-C (tnc) is an extracellular matrix protein that is upregulated in inflammation and induces further inflammatory responses. We hypothesized that tnc contributes to the inflammatory pathology in AD. Using real-time polymerase chain reaction, we observed that tnc gene transcription was upregulated in cultured microglia after Aβ challenge and in the brain of an AD mouse model that overexpresses mutated amyloid precursor protein (APP) in neural cells. By cross-breeding APP-transgenic mice and tenascin-C-deficient mice, we demonstrated using real-time polymerase chain reaction, Western blot analysis, enzyme-linked immunosorbent assay, and immunohistochemistry that tnc deficiency reduces pro- but enhances anti-inflammatory activation in the mutated APP-transgenic mouse brain, associated with a reduced cerebral Aβ load and higher levels of the postsynaptic density protein 95. Thus, our study indicates that functional inhibition of tnc exerts beneficial effects on AD pathogenesis, suggesting a potential for tnc as a new therapeutic target in AD.

Review: experimental manipulations of microglia in mouse models of Alzheimer's pathology: activation reduces amyloid but hastens tau pathology

The inflammation hypothesis of Alzheimer's pathogenesis has directed much scientific effort towards ameliorating this disease. The development of mouse models of amyloid deposition permitted direct tests of the proposal that amyloid-activated microglia could cause neurodegeneration in vivo. Many approaches to manipulating microglial activation have been applied to these mouse models, and are the subject of this review. In general, these results do not support a direct neuricidal action of microglia in mouse amyloid models under any activation state. Some of the manipulations cause both a reduction in pathology and a reduction in microglial activation. However, at least for agents like ibuprofen, this outcome may result from a direct action on amyloid production, and a reduction in the microglial-provoking amyloid deposits, rather than from reduced microglial activation leading to a decline in amyloid deposition. Instead, a surprising number of the experimental manipulations which increase microglial activation lead to enhanced clearance of the amyloid deposits. Both the literature and new data presented here suggest that either classical or alternative activation of microglia can lead to enhanced amyloid clearance. However, a limited number of studies comparing the same treatments in amyloid-depositing vs. tau-depositing mice find the opposite effects. Treatments that benefit amyloid pathology accelerate tau pathology. This observation argues strongly that potential treatments be tested for impact on both amyloid and tau pathology before consideration of testing in humans.

Loss of interleukin receptor-associated kinase 4 signaling suppresses amyloid pathology and alters microglial phenotype in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is typified by the deposition of amyloid in the brain, which elicits a robust microglial-mediated inflammatory response that is associated with disease exacerbation and accelerated progression. Microglia are the principal immune effector cells in the brain and interact with fibrillar forms of Aβ (fAβ) through a receptor complex that includes Toll-like receptors (TLR) 2/4/6 and their coreceptors. Interleukin receptor-associated kinases (IRAKs) are essential intracellular signaling molecules for transduction of TLR signals. Studies of mouse models of AD in which the individual TLRs are knocked out have produced conflicting results on roles of TLR signaling in amyloid homeostasis. Therefore, we disrupted a common downstream TLR signaling element, IRAK4. We report that microglial IRAK4 is necessary in vitro for fAβ to activate the canonical pro-inflammatory signaling pathways leading to activation of p38, JNK, and ERK MAP kinases and to generate reactive oxygen species. In vivo the loss of IRAK4 function results in decreased Aβ levels in a murine model of AD. This was associated with diminished microgliosis and astrogliosis in aged mice. Analysis of microglia isolated from the adult mouse brain revealed an altered pattern of gene expression associated with changes in microglial phenotype that were associated with expression of IRF transcription factors that govern microglial phenotype. Further, loss of IRAK4 function also promoted amyloid clearance mechanisms, including elevated expression of insulin-degrading enzyme. Finally, blocking IRAK function restored olfactory behavior. These data demonstrate that IRAK4 activation acts normally to regulate microglial activation status and influence amyloid homeostasis in the brain.

The neuroendocrine control of the innate immune system in health and brain diseases

The innate immune reaction takes place in the brain during immunogenic challenges, injury, and disease. Such a response is highly regulated by numerous anti-inflammatory mechanisms that may directly affect the ultimate consequences of such a reaction within the cerebral environment. The neuroendocrine control of this innate immune system by glucocorticoids is critical for the delicate balance between cell survival and damage in the presence of inflammatory mediators. Glucocorticoids play key roles in regulating the expression of inflammatory genes, and they also have the ability to modulate numerous functions that may ultimately lead to brain damage or repair after injury. Here we review these mechanisms and discuss data supporting both neuroprotective and detrimental roles of the neuroendocrine control of innate immunity.

Inflammation and neurodegeneration: the story 'retolled'

Toll-like receptors (TLRs) play a crucial role in innate immunity by recognizing conserved motifs predominantly found in microorganisms. Increasing evidence supports a role for TLRs in sterile inflammation as observed in neurodegenerative disorders. This includes work suggesting a contribution for these receptors to the pathophysiology of Alzheimer's disease (AD), Parkinson's disease (PD), and related disorders. In this review, the potential role of TLRs in the context of protein aggregation, neuronal degeneration, and genetic risk factors is addressed. In particular, we discuss the evidence derived from experimental models of both AD and PD which suggests that activation of TLRs can have beneficial and detrimental effects on pathological features such as protein aggregation and neuronal death. A deeper understanding of these dichotomous observations could be used for therapeutic benefit.

MyD88 deficiency results in both cognitive and motor impairments in mice

The myeloid differentiation primary response gene 88 (MyD88) product is the most common adaptor protein implicated in Toll-like and interleukin receptor (TIR) domain signaling and thus plays an important role in the innate immune system. Despite the fact that the MyD88-dependent pathway has emerged as an important player in cell death processes described in several animal models of neurodegenerative disorders, the contribution of this pathway to specific behavioral phenotypes has been largely ignored. To understand the full implication of this pathway, we tested MyD88(-/-) mice for both motor and cognitive functions in normal conditions. MyD88(-/-) mice displayed impaired spatial and working memory as detected by the Barnes maze, the water T-maze and the passive avoidance tests. Furthermore, MyD88(-/-) mice demonstrated hypolocomotion in the open-field and wheel activity systems, as well as impairments in motor coordination and balance using the pole test and the rotarod. Our findings shed light on behavioral alterations that are associated with the deletion of the MyD88 protein in physiological conditions. These behavioral effects should be taken into consideration when assessing the role of the MyD88-dependent pathway in various infectious and non-infectious conditions.

Toll-like receptor (TLR) and inflammasome actions in the central nervous system

During the past 10 years, much attention has been focused towards elucidating the impact of Toll-like receptors (TLRs) in central nervous system (CNS) innate immunity. TLR signaling triggers the transcriptional activation of pro-interleukin-1β (pro-IL-1β) and pro-IL-18 that are processed into their active forms by the inflammasome. Recent studies have demonstrated inflammasome involvement during CNS infection, autoimmune disease, and injury. This review will address inflammasome actions within the CNS and how cooperation between TLR and inflammasome signaling may influence disease outcome. In addition, the concept of alternative inflammasome functions independent of IL-1 and IL-18 processing are considered in the context of CNS disease.

Bone marrow transplantation confers modest benefits in mouse models of Huntington's disease

Huntington's disease (HD) is caused by an expanded polyglutamine tract in the protein huntingtin (htt). Although HD has historically been viewed as a brain-specific disease, htt is expressed ubiquitously, and recent studies indicate that mutant htt might cause changes to the immune system that could contribute to pathogenesis. Monocytes from HD patients and mouse models are hyperactive in response to stimulation, and increased levels of inflammatory cytokines and chemokines are found in pre-manifest patients that correlate with pathogenesis. In this study, wild-type (WT) bone marrow cells were transplanted into two lethally irradiated transgenic mouse models of HD that ubiquitously express full-length htt (YAC128 and BACHD mice). Bone marrow transplantation partially attenuated hypokinetic and motor deficits in HD mice. Increased levels of synapses in the cortex were found in HD mice that received bone marrow transplants. Importantly, serum levels of interleukin-6, interleukin-10, CXC chemokine ligand 1, and interferon-γ were significantly higher in HD than WT mice but were normalized in mice that received a bone marrow transplant. These results suggest that immune cell dysfunction might be an important modifier of pathogenesis in HD.

Animal Models of Alzheimer's Disease: Utilization of Transgenic Alzheimer's Disease Models in Studies of Amyloid Beta Clearance

Glial cells in Alzheimer’s disease (AD) have been shown to be capable of clearing or at least restricting the accumulation of toxic amyloid beta (Aβ) deposits. Recently, bone marrow (BM)–derived monocytic cells have been recognized in experimental studies to be superior in their phagocytic properties when compared to their brain endogenous counterparts. In human AD, BM-derived monocytic cells may have deficiencies in their capacity to restrict plaque growth. Therefore, enhancement of phagocytic properties of cells of monocyte origin, both brain endogenous microglia and BM-derived monocytic cells, offers an attractive therapeutic approach to fight off AD. Transgenic mouse models with aberrant Aβ deposition offer a valuable tool for discovery of novel pathways to facilitate cell-mediated Aβ uptake. This article reviews the most recent findings on the phagocytic capacity of cells with monocytic origin in various transgenic AD models and describes the methods to study phagocytic activity of these cells.

TLR2 is a primary receptor for Alzheimer's amyloid β peptide to trigger neuroinflammatory activation

Microglia activated by extracellularly deposited amyloid β peptide (Aβ) act as a two-edged sword in Alzheimer's disease pathogenesis: on the one hand, they damage neurons by releasing neurotoxic proinflammatory mediators (M1 activation); on the other hand, they protect neurons by triggering anti-inflammatory/neurotrophic M2 activation and by clearing Aβ via phagocytosis. TLRs are associated with Aβ-induced microglial inflammatory activation and Aβ internalization, but the mechanisms remain unclear. In this study, we used real-time surface plasmon resonance spectroscopy and conventional biochemical pull-down assays to demonstrate a direct interaction between TLR2 and the aggregated 42-aa form of human Aβ (Aβ42). TLR2 deficiency reduced Aβ42-triggered inflammatory activation but enhanced Aβ phagocytosis in cultured microglia and macrophages. By expressing TLR2 in HEK293 cells that do not endogenously express TLR2, we observed that TLR2 expression enabled HEK293 cells to respond to Aβ42. Through site-directed mutagenesis of tlr2 gene, we identified the amino acids EKKA (741-744) as a critical cytoplasmic domain for transduction of inflammatory signals. By coexpressing TLR1 or TLR6 in TLR2-transgenic HEK293 cells or silencing tlrs genes in RAW264.7 macrophages, we observed that TLR2-mediated Aβ42-triggered inflammatory activation was enhanced by TLR1 and suppressed by TLR6. Using bone marrow chimeric Alzheimer's amyloid precursor transgenic mice, we observed that TLR2 deficiency in microglia shifts M1- to M2-inflammatory activation in vivo, which was associated with improved neuronal function. Our study demonstrated that TLR2 is a primary receptor for Aβ to trigger neuroinflammatory activation and suggested that inhibition of TLR2 in microglia could be beneficial in Alzheimer's disease pathogenesis.

Utilization of APPswe/PS1dE9 Transgenic Mice in Research of Alzheimer's Disease: Focus on Gene Therapy and Cell-Based Therapy Applications

One of the most extensively used transgenic mouse model of Alzheimer's disease (AD) is APPswe/PS1dE9 mice, which over express the Swedish mutation of APP together with PS1 deleted in exon 9. These mice show increase in parenchymal Aβ load with Aβ plaques starting from the age of four months, glial activation, and deficits in cognitive functions at the age of 6 months demonstrated by radial arm water maze and 12-13 months seen with Morris Water Maze test. As gene transfer technology allows the delivery of DNA into target cells to achieve the expression of a protective or therapeutic protein, and stem cell transplantation may create an environment supporting neuronal functions and clearing Aβ plaques, these therapeutic approaches alone or in combination represent potential therapeutic strategies that need to be tested in relevant animal models before testing in clinics. Here we review the current utilization of APPswe/PS1dE9 mice in testing gene transfer and cell transplantation aimed at improving the protection of the neurons against Aβ toxicity and also reducing the brain levels of Aβ. Both gene therapy and cell based therapy may be feasible therapeutic approaches for human AD.

The effects of MyD88 deficiency on exploratory activity, anxiety, motor coordination, and spatial learning in C57BL/6 and APPswe/PS1dE9 mice

Toll-like receptors (TLRs) are a family of pattern-recognition receptors in innate immunity and provide a first line defense against pathogens and tissue injuries. In addition to important roles in infection, inflammation, and immune diseases, recent studies show that TLR signaling is involved in modulation of learning, memory, mood, and neurogenesis. Because MyD88 is essential for the downstream signaling of all TLRs, except TLR3, we investigated the effects of MyD88 deficiency (MyD88-/-) on behavioral functions in mice. Additionally, we recently demonstrated that a mouse model of Alzheimer's disease (AD) deficient for MyD88 had decreases in Aβ deposits and soluble Aβ in the brain as compared with MyD88 sufficient AD mouse models. Because accumulation of Aβ in the brain is postulated to be a causal event leading to cognitive deficits in AD, we investigated the effects of MyD88 deficiency on behavioral functions in the AD mouse model at 10 months of age. MyD88 deficient mice showed more anxiety in the elevated plus-maze. In the motor coordination tests, MyD88 deficient mice remained on a beam and a bar for a longer time, but with slower initial movement on the bar. In the Morris water maze test, MyD88 deficiency appeared to improve spatial learning irrespective of the transgene. Our findings suggest that the MyD88-dependent pathway contributes to behavioral functions in an AD mouse model and its control group.

The role of the MYD88-dependent pathway in MPTP-induced brain dopaminergic degeneration

Background

Mounting evidence supports a significant role of inflammation in Parkinson's disease (PD) pathophysiology, with several inflammatory pathways being suggested as playing a role in the dopaminergic degeneration seen in humans and animal models of the disease. These include tumor necrosis factor, prostaglandins and oxidative-related stress components. However, the role of innate immunity has not been established in PD.

Methods

Based on the fact that the myeloid differentiation primary response gene (88) (MyD88) is the most common adaptor protein implicated in toll-like receptor (TLR) signaling, critical in the innate immune response, we undertook a study to investigate the potential contribution of this specific pathway to MPTP-induced brain dopaminergic degeneration using MyD88 knock out mice (MyD88-/-), following our observations that the MyD88-dependent pathway was critical for MPTP dopaminergic toxicity in the enteric nervous system. Post-mortem analyses assessing nigrostriatal dopaminergic degeneration and inflammation were performed using HPLC, western blots, autoradiography and immunofluorescence.

Results

Our results demonstrate that MyD88-/- mice are as vulnerable to MPTP-induced dopamine and DOPAC striatal depletion as wild type mice. Furthermore, MyD88-/- mice show similar striatal dopamine transporter and tyrosine hydroxylase loss, as well as dopaminergic cell loss in the substantia nigra pars compacta in response to MPTP. To evaluate the extent of the inflammatory response created by the MPTP regimen utilized, we further performed bioluminescence imaging using TLR2-luc/gfp transgenic mice and microglial density analysis, which revealed a modest brain microglial response following MPTP. This was accompanied by a significant astrocytic reaction in the striatum, which was of similar magnitude both in wild type and MyD88-/- mice.

Conclusions

Our results suggest that subacute MPTP-induced dopaminergic degeneration observed in the central nervous system is MyD88-independent, in contrast to our recent observations that this pathway, in the same cohort of animals, is critical in the loss of dopaminergic neurons in the enteric nervous system.

MyD88 deficiency ameliorates β-amyloidosis in an animal model of Alzheimer's disease

The accumulation of β-amyloid protein (Aβ) in the brain is thought to be a primary etiologic event in Alzheimer's disease (AD). Fibrillar Aβ plaques, a hallmark of AD abnormality, are closely associated with activated microglia. Activated microglia have contradictory roles in the pathogenesis of AD, being either neuroprotective (by clearing harmful Aβ and repairing damaged tissues) or neurotoxic (by producing proinflammatory cytokines and reactive oxygen species). Aβ aggregates can activate microglia by interacting with multiple toll-like receptors (TLRs), the pattern-recognition receptors of the innate immune system. Because the adapter protein MyD88 is essential for the downstream signaling of all TLRs, except TLR3, we investigated the effects of MyD88 deficiency (MyD88(-/-)) on Aβ accumulation and microglial activation in an AD mouse model. MyD88 deficiency decreased Aβ load and microglial activation in the brain. The decrease in Aβ load in an MyD88(-/-) AD mouse model was associated with increased and decreased protein expression of apolipoprotein E (apoE) and CX3CR1, respectively, compared with that in an MyD88 wild-type AD mouse model. These results suggest that MyD88 deficiency may reduce Aβ load by enhancing the phagocytic capability of microglia through fractalkine (the ligand of CX3CR1) signaling and by promoting apoE-mediated clearance of Aβ from the brain. These findings also suggest that chronic inflammatory responses induced by Aβ accumulation via the MyD88-dependent signaling pathway exacerbate β-amyloidosis in AD.