Early and rapid de novo synthesis of Alzheimer beta A4-amyloid precursor protein (APP) in activated microglia

Autosomal Dominant Alzheimer's Disease Mutations in Human Microglia Are Not Sufficient to Trigger Amyloid Pathology in WT Mice but Might Affect Pathology in 5XFAD Mice

Several genetic variants that affect microglia function have been identified as risk factors for Alzheimer's Disease (AD), supporting the importance of this cell type in disease progression. However, the effect of autosomal dominant mutations in the amyloid precursor protein (APP) or the presenilin (PSEN1/2) genes has not been addressed in microglia in vivo. We xenotransplanted human microglia derived from non-carriers and carriers of autosomal dominant AD (ADAD)-causing mutations in the brain of hCSF1 WT or 5XFAD mice. We observed that ADAD mutations in microglia are not sufficient to trigger amyloid pathology in WT mice. In 5XFAD mice, we observed a non-statistically significant increase in amyloid plaque volume and number of dystrophic neurites, coupled with a reduction in plaque-associated microglia in the brain of mice xenotransplanted with ADAD human microglia compared to mice xenotransplanted with non-ADAD microglia. In addition, we observed a non-statistically significant impairment in working and contextual memory in 5XFAD mice xenotransplanted with ADAD microglia compared to those xenotransplanted with non-ADAD-carrier microglia. We conclude that, although not sufficient to initiate amyloid pathology in the healthy brain, mutations in APP and PSEN1 in human microglia might cause mild changes in pathological and cognitive outcomes in 5XFAD mice in a manner consistent with increased AD risk.

Neuroinflammation in Alzheimer's Disease: A Potential Role of Nose-Picking in Pathogen Entry via the Olfactory System?

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by progressive cognitive decline and memory impairment. Many possible factors might contribute to the development of AD, including amyloid peptide and tau deposition, but more recent evidence suggests that neuroinflammation may also play an-at least partial-role in its pathogenesis. In recent years, emerging research has explored the possible involvement of external, invading pathogens in starting or accelerating the neuroinflammatory processes in AD. In this narrative review, we advance the hypothesis that neuroinflammation in AD might be partially caused by viral, bacterial, and fungal pathogens entering the brain through the nose and the olfactory system. The olfactory system represents a plausible route for pathogen entry, given its direct anatomical connection to the brain and its involvement in the early stages of AD. We discuss the potential mechanisms through which pathogens may exploit the olfactory pathway to initiate neuroinflammation, one of them being accidental exposure of the olfactory mucosa to hands contaminated with soil and feces when picking one's nose.

Knockdown of Amyloid Precursor Protein: Biological Consequences and Clinical Opportunities

Amyloid precursor protein (APP) and its cleavage fragment Amyloid-β (Aβ) have fundamental roles in Alzheimer's disease (AD). Genetic alterations that either increase the overall dosage of APP or alter its processing to favour the generation of longer, more aggregation prone Aβ species, are directly causative of the disease. People living with one copy of APP are asymptomatic and reducing APP has been shown to lower the relative production of aggregation-prone Aβ species in vitro. For these reasons, reducing APP expression is an attractive approach for AD treatment and prevention. In this review, we will describe the structure and the known functions of APP and go on to discuss the biological consequences of APP knockdown and knockout in model systems. We highlight progress in therapeutic strategies to reverse AD pathology via reducing APP expression. We conclude that new technologies that reduce the dosage of APP expression may allow disease modification and slow clinical progression, delaying or even preventing onset.

Synapses, Microglia, and Lipids in Alzheimer's Disease

Alzheimer's disease (AD) is characterised by synaptic dysfunction accompanied by the microscopically visible accumulation of pathological protein deposits and cellular dystrophy involving both neurons and glia. Late-stage AD shows pronounced loss of synapses and neurons across several differentially affected brain regions. Recent studies of advanced AD using post-mortem brain samples have demonstrated the direct involvement of microglia in synaptic changes. Variants of the Apolipoprotein E and Triggering Receptors Expressed on Myeloid Cells gene represent important determinants of microglial activity but also of lipid metabolism in cells of the central nervous system. Here we review evidence that may help to explain how abnormal lipid metabolism, microglial activation, and synaptic pathophysiology are inter-related in AD.

Quercetin-biapigenin nanoparticles are effective to penetrate the blood-brain barrier

Search for efficient therapeutic agents for central nervous system (CNS) disorders has been extensive. Nevertheless, blood-brain barrier (BBB) is an obstacle that prevents the majority of compounds to act in these diseases. It is, thus, of extreme relevance the BBB overcome, in order to deliver a drugs therapeutically active concentration to the action site, with the least losses and interaction with other organs, tissues, or cells. The present study aimed to investigate the potential protective effect of quercetin-biapigenin encapsulated into poly(Ɛ-polycaprolactone) (PCL) nanoparticles against t-BOOH-induced oxidative stress in several brain cell lines, as well as evaluate the permeability of those active molecules through an in vitro BBB model. The three cell lines under study (BV-2, hcmec/D3, and U87) presented different reactions to t-BOOH. In general, quercetin-biapigenin PCL-loaded nanoparticles were able to minimize compound toxicity they convey, regardless the cell line. Quercetin-biapigenin PCL-loaded nanoparticles (Papp of approximately 80 × 10-6 cm/s) revealed to be more permeable than free compounds (Papp of approximately 50 × 10-6 cm/s). As of our knowledge, this is the first report of quercetin-biapigenin PCL-loaded nanoparticle activity in brain cells. It is also the first determining its permeability through BBB, as an effective nanocarrier for brain delivery.

Microglial autophagy defect causes parkinson disease-like symptoms by accelerating inflammasome activation in mice

Microglial activation-induced neuroinflammation is closely associated with the development of Parkinson disease (PD). Macroautophagy/autophagy regulates many biological processes, but the role of autophagy in microglial activation during PD development remains largely unclear. In this study, we showed that deletion of microglial Atg5 caused PD-like symptoms in mice, characterized by impairment in motor coordination and cognitive learning, loss of tyrosine hydroxylase (TH) neurons, enhancement of neuroinflammation and reduction in dopamine levels in the striatum. Mechanistically, we found that inhibition of autophagy led to NLRP3 (NLR family pyrin domain containing 3) inflammasome activation via PDE10A (phosphodiesterase 10A)-cyclic adenosine monophosphate (cAMP) signaling in microglia, and the sequential upregulation of downstream IL1B/IL-1β in turn increased the expression of MIF (macrophage migration inhibitory factor [glycosylation-inhibiting factor]), a pro-inflammatory cytokine. Inhibition of NLRP3 inflammasome activation by administration of MCC950, a specific inhibitor for NLRP3, decreased MIF expression and neuroinflammatory levels, and rescued the loss of TH neurons in the substantial nigra (SN). Interestingly, we found that serum MIF levels in PD patients were significantly elevated. Taken together, our results reveal an important role of autophagy in microglial activation-driven PD-like symptoms, thus providing potential targets for the clinical treatment of PD. Abbreviations: ATG: autophagy related; cAMP: cyclic adenosine monophosphate; cKO: conditional knockout; NOS2/INOS: nitric oxide synthase 2, inducible; IL1B: interleukin 1 beta; ITGAM/CD-11b: integrin alpha M/cluster of differentiation molecule 11B; MAP1LC3: microtubule-associated protein 1 light chain 3; MIF: macrophage migration inhibitory factor (glycosylation-inhibiting factor); NLRP3: NLR family pyrin domain containing 3; PBS: phosphate-buffered saline; PD: parkinson disease; PDE10A: phosphodiesterase 10A; SN: substantial nigra; TH: tyrosine hydroxylase; TNF: tumor necrosis factor; WT: wild type.

Neuroimmune interactions in Alzheimer's disease-New frontier with old challenges?

The perceived role of the immune system in neurodegenerative diseases has undergone drastic changes over time. Initially considered as a passive bystander, then condemned as a mediator of neurodegeneration and now established as an important player in the pathogenetic cascade, neuroimmune interactions have come a long way to arrive center stage in Alzheimer's disease research. Despite major breakthroughs in recent years, basic questions remain unanswered as conflicting data describe immune overactivation, inadequate response or exhaustion of the immune system in neurodegenerative diseases. Furthermore, difficulties in translating in vitro and in vivo studies in model systems to the complex human disease condition with multiple overlapping pathologies and the long disease duration in patients suffering from neurodegenerative diseases have hampered progress. Development of novel, advanced model systems, as well as new technologies to interrogate existing disease models and valuable collections of human tissue samples, including brain tissue in parallel with improved imaging and biomarker technologies are guiding the way to better understand the role of the immune system in Alzheimer's disease with hopes for more effective interventions in the future.

PSEN1ΔE9, APPswe, and APOE4 Confer Disparate Phenotypes in Human iPSC-Derived Microglia

Here we elucidate the effect of Alzheimer disease (AD)-predisposing genetic backgrounds, APOE4, PSEN1ΔE9, and APPswe, on functionality of human microglia-like cells (iMGLs). We present a physiologically relevant high-yield protocol for producing iMGLs from induced pluripotent stem cells. Differentiation is directed with small molecules through primitive erythromyeloid progenitors to re-create microglial ontogeny from yolk sac. The iMGLs express microglial signature genes and respond to ADP with intracellular Ca²⁺ release distinguishing them from macrophages. Using 16 iPSC lines from healthy donors, AD patients and isogenic controls, we reveal that the APOE4 genotype has a profound impact on several aspects of microglial functionality, whereas PSEN1ΔE9 and APPswe mutations trigger minor alterations. The APOE4 genotype impairs phagocytosis, migration, and metabolic activity of iMGLs but exacerbates their cytokine secretion. This indicates that APOE4 iMGLs are fundamentally unable to mount normal microglial functionality in AD.

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APOE4 genotype has a profound impact on several functions of microglia-like cells

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Inflammatory responses are aggravated in cells with APOE4 genotype

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Metabolism, phagocytosis, and migration are decreased in APOE4 microglia-like cells

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Familial mutations APPswe and PSEN1ΔE9 have only minor effects on functionality

Iron Dyshomeostasis Induces Binding of APP to BACE1 for Amyloid Pathology, and Impairs APP/Fpn1 Complex in Microglia: Implication in Pathogenesis of Cerebral Microbleeds

As a putative marker of cerebral small vessel disease, cerebral microbleeds (CMBs) have been associated with vascular cognitive impairment. Both iron accumulation and amyloid protein precursor (APP) dysregulation are recognized as pathological hallmarks underlying the progression of CMBs, but their cross-talk is not yet understood. In this study, we found a profound increase of amyloid formation with increasing FeCl₃ treatment, and a distinct change in APP metabolism and expression of iron homeostasis proteins (ferritin, Fpn1, iron regulatory protein) was observed at the 300 uM concentration of FeCl₃. Further results revealed that extracellular iron accumulation might potentially induce binding of APP to BACE1 for amyloid formation and decrease the capability of APP/Fpn1 in mediating iron export. Our findings in this study, reflecting a probable relationship between iron dyshomeostasis and amyloid pathology, may help shed light on the underlying pathogenesis of CMBs in vascular cognitive impairment.

Amyloid precursor protein modulates macrophage phenotype and diet-dependent weight gain

It is well known that mutations in the gene coding for amyloid precursor protein are responsible for autosomal dominant forms of Alzheimer’s disease. Proteolytic processing of the protein leads to a number of metabolites including the amyloid beta peptide. Although brain amyloid precursor protein expression and amyloid beta production are associated with the pathophysiology of Alzheimer’s disease, it is clear that amyloid precursor protein is expressed in numerous cell types and tissues. Here we demonstrate that amyloid precursor protein is involved in regulating the phenotype of both adipocytes and peripheral macrophages and is required for high fat diet-dependent weight gain in mice. These data suggest that functions of this protein include modulation of the peripheral immune system and lipid metabolism. This biology may have relevance not only to the pathophysiology of Alzheimer’s disease but also diet-associated obesity.

Autophagy regulates MAVS signaling activation in a phosphorylation-dependent manner in microglia

Mitochondrial antiviral signaling (MAVS) protein has an important role in antiviral immunity and autoimmunity. However, the pathophysiological role of this signaling pathway, especially in the brain, remains elusive. Here we demonstrated that MAVS signaling existed and mediated poly(I:C)-induced inflammation in the brain. Along with the MAVS signaling activation, there was an induction of autophagic activation. Autophagy negatively regulated the activity of MAVS through direct binding of LC3 to the LIR motif Y(9)xxI(12) of MAVS. We also found that c-Abl kinase phosphorylated MAVS and regulated its interaction with LC3. Interestingly, tyrosine phosphorylation of MAVS was required for downstream signaling activation. Importantly, in vivo data showed that the deficiency of MAVS or c-Abl prevented MPTP-induced microglial activation and dopaminergic neuron loss. Together, our findings reveal the molecular mechanisms underlying the regulation of MAVS-dependent microglial activation in the nervous system, thus providing a potential target for the treatment of microglia-driven inflammatory brain diseases.

APP Regulates Microglial Phenotype in a Mouse Model of Alzheimer's Disease

Prior work suggests that amyloid precursor protein (APP) can function as a proinflammatory receptor on immune cells, such as monocytes and microglia. Therefore, we hypothesized that APP serves this function in microglia during Alzheimer's disease. Although fibrillar amyloid β (Aβ)-stimulated cytokine secretion from both wild-type and APP knock-out (mAPP(-/-)) microglial cultures, oligomeric Aβ was unable to stimulate increased secretion from mAPP(-/-) cells. This was consistent with an ability of oligomeric Aβ to bind APP. Similarly, intracerebroventricular infusions of oligomeric Aβ produced less microgliosis in mAPP(-/-) mice compared with wild-type mice. The mAPP(-/-) mice crossed to an APP/PS1 transgenic mouse line demonstrated reduced microgliosis and cytokine levels and improved memory compared with wild-type mice despite robust fibrillar Aβ plaque deposition. These data define a novel function for microglial APP in regulating their ability to acquire a proinflammatory phenotype during disease.

SIGNIFICANCE STATEMENT

A hallmark of Alzheimer's disease (AD) brains is the accumulation of amyloid β (Aβ) peptide within plaques robustly invested with reactive microglia. This supports the notion that Aβ stimulation of microglial activation is one source of brain inflammatory changes during disease. Aβ is a cleavage product of the ubiquitously expressed amyloid precursor protein (APP) and is able to self-associate into a wide variety of differently sized and structurally distinct multimers. In this study, we demonstrate both in vitro and in vivo that nonfibrillar, oligomeric forms of Aβ are able to interact with the parent APP protein to stimulate microglial activation. This provides a mechanism by which metabolism of APP results in possible autocrine or paracrine Aβ production to drive the microgliosis associated with AD brains.

REVIEW ■ : Activation of Microglia: A Dangerous Interlude in Immune Function in the Brain

Microglial cells are representatives of the immune system in the CNS parenchyma. Their most characteristic property is their ability to modify their behavior in response to diverse signals from other cells in a variety of experimental conditions and human diseases, both acute and chronic. The transformation from a quiescent state into phagocytic brain macrophages is under strict control and accompanied by the production of several secretory products. These include cytokines, excitatory amino acids, and reactive oxygen metabolites by which the activated microglial cells correspond with other cells of the brain and immune system. Thus, they represent an essential host defense and repair system, and may be responsible for tissue destruction and neuronal death, depending on the balance of activating and inhibitory signals. NEUROSCIENTIST 2:293-299, 1996

Neurosteroidogenesis Today: Novel Targets for Neuroactive Steroid Synthesis and Action and Their Relevance for Translational Research

Neuroactive steroids are endogenous neuromodulators synthesised in the brain that rapidly alter neuronal excitability by binding to membrane receptors, in addition to the regulation of gene expression via intracellular steroid receptors. Neuroactive steroids induce potent anxiolytic, antidepressant, anticonvulsant, sedative, analgesic and amnesic effects, mainly through interaction with the GABAA receptor. They also exert neuroprotective, neurotrophic and antiapoptotic effects in several animal models of neurodegenerative diseases. Neuroactive steroids regulate many physiological functions, such as the stress response, puberty, the ovarian cycle, pregnancy and reward. Their levels are altered in several neuropsychiatric and neurological diseases and both preclinical and clinical studies emphasise a therapeutic potential of neuroactive steroids for these diseases, whereby symptomatology ameliorates upon restoration of neuroactive steroid concentrations. However, direct administration of neuroactive steroids has several challenges, including pharmacokinetics, low bioavailability, addiction potential, safety and tolerability, which limit its therapeutic use. Therefore, modulation of neurosteroidogenesis to restore the altered endogenous neuroactive steroid tone may represent a better therapeutic approach. This review summarises recent approaches that target the neuroactive steroid biosynthetic pathway at different levels aiming to promote neurosteroidogenesis. These include modulation of neurosteroidogenesis through ligands of the translocator protein 18 kDa and the pregnane xenobiotic receptor, as well as targeting of specific neurosteroidogenic enzymes such as 17β-hydroxysteroid dehydrogenase type 10 or P450 side chain cleavage. Enhanced neurosteroidogenesis through these targets may be beneficial not only for neurodegenerative diseases, such as Alzheimer's disease and age-related dementia, but also for neuropsychiatric diseases, including alcohol use disorders.

Brain tissue responses to neural implants impact signal sensitivity and intervention strategies

Implantable biosensors are valuable scientific tools for basic neuroscience research and clinical applications. Neurotechnologies provide direct readouts of neurological signal and neurochemical processes. These tools are generally most valuable when performance capacities extend over months and years to facilitate the study of memory, plasticity, and behavior or to monitor patients’ conditions. These needs have generated a variety of device designs from microelectrodes for fast scan cyclic voltammetry (FSCV) and electrophysiology to microdialysis probes for sampling and detecting various neurochemicals. Regardless of the technology used, the breaching of the blood–brain barrier (BBB) to insert devices triggers a cascade of biochemical pathways resulting in complex molecular and cellular responses to implanted devices. Molecular and cellular changes in the microenvironment surrounding an implant include the introduction of mechanical strain, activation of glial cells, loss of perfusion, secondary metabolic injury, and neuronal degeneration. Changes to the tissue microenvironment surrounding the device can dramatically impact electrochemical and electrophysiological signal sensitivity and stability over time. This review summarizes the magnitude, variability, and time course of the dynamic molecular and cellular level neural tissue responses induced by state-of-the-art implantable devices. Studies show that insertion injuries and foreign body response can impact signal quality across all implanted central nervous system (CNS) sensors to varying degrees over both acute (seconds to minutes) and chronic periods (weeks to months). Understanding the underlying biological processes behind the brain tissue response to the devices at the cellular and molecular level leads to a variety of intervention strategies for improving signal sensitivity and longevity.

Effects of caspase-1 knockout on chronic neural recording quality and longevity: insight into cellular and molecular mechanisms of the reactive tissue response

Chronic implantation of microelectrodes into the cortex has been shown to lead to inflammatory gliosis and neuronal loss in the microenvironment immediately surrounding the probe, a hypothesized cause of neural recording failure. Caspase-1 (aka Interleukin 1β converting enzyme) is known to play a key role in both inflammation and programmed cell death, particularly in stroke and neurodegenerative diseases. Caspase-1 knockout (KO) mice are resistant to apoptosis and these mice have preserved neurologic function by reducing ischemia-induced brain injury in stroke models. Local ischemic injury can occur following neural probe insertion and thus in this study we investigated the hypothesis that caspase-1 KO mice would have less ischemic injury surrounding the neural probe. In this study, caspase-1 KO mice were implanted with chronic single shank 3 mm Michigan probes into V1m cortex. Electrophysiology recording showed significantly improved single-unit recording performance (yield and signal to noise ratio) of caspase-1 KO mice compared to wild type C57B6 (WT) mice over the course of up to 6 months for the majority of the depth. The higher yield is supported by the improved neuronal survival in the caspase-1 KO mice. Impedance fluctuates over time but appears to be steadier in the caspase-1 KO especially at longer time points, suggesting milder glia scarring. These findings show that caspase-1 is a promising target for pharmacologic interventions.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Hippocampal plasticity after a vagus nerve injury in the rat

Stimulation of the vagus nerve has been previously reported to promote neural plasticity and neurogenesis in the brain. Several studies also revealed plastic changes in the spinal cord after injuries to somatosensory nerves originating from both the brachial and lumbo-sacral plexuses. However, the neurogenic responses of the brain to the injury of the viscerosensory innervation are not as yet well understood. In the present study, we investigated whether cells in the dentate gyrus of the hippocampus respond to a chemical and physical damage to the vagus nerve in the adult rat. Intraperitoneal capsaicin administration was used to damage non-myelinated vagal afferents while subdiaphragmatic vagotomy was used to damage both the myelinated and non-myelinated vagal afferents. The 5-bromo-2-deoxyuridine (BrdU) incorporation together with cell-specific markers was used to study neural proliferation in subgranular zone, granule cell layer, molecular layer and hilus of the dentate gyrus. Microglia activation was determined by quantifying changes in the intensity of fluorescent staining with a primary antibody against ionizing calcium adapter-binding molecule 1. Results revealed that vagotomy decreased BrdU incorporation in the hilus 15 days after injury compared to the capsaicin group. Capsaicin administration decreased BrdU incorporation in the granular cell layer 60 days after the treatment. Capsaicin decreased the number of doublecortin-expressing cells in the dentate gyrus, whereas vagotomy did not alter the expression of doublecortin in the hippocampus. Both the capsaicin- and the vagotomy-induced damage to the vagus nerve decreased microglia activation in the hippocampus at 15 days after the injury. At 30 days post injury, capsaicin-treated and vagotomized rats revealed significantly more activated microglia. Our findings show that damage to the subdiaphragmatic vagus in adult rats is followed by microglia activation and long-lasting changes in the dentate gyrus, leading to alteration of neurogenesis.

Amyloid precursor protein and proinflammatory changes are regulated in brain and adipose tissue in a murine model of high fat diet-induced obesity

Background

Middle age obesity is recognized as a risk factor for Alzheimer's disease (AD) although a mechanistic linkage remains unclear. Based upon the fact that obese adipose tissue and AD brains are both areas of proinflammatory change, a possible common event is chronic inflammation. Since an autosomal dominant form of AD is associated with mutations in the gene coding for the ubiquitously expressed transmembrane protein, amyloid precursor protein (APP) and recent evidence demonstrates increased APP levels in adipose tissue during obesity it is feasible that APP serves some function in both disease conditions.

Methodology/Principal Findings

To determine whether diet-induced obesity produced proinflammatory changes and altered APP expression in brain versus adipose tissue, 6 week old C57BL6/J mice were maintained on a control or high fat diet for 22 weeks. Protein levels and cell-specific APP expression along with markers of inflammation and immune cell activation were compared between hippocampus, abdominal subcutaneous fat and visceral pericardial fat. APP stimulation-dependent changes in macrophage and adipocyte culture phenotype were examined for comparison to the in vivo changes.

Conclusions/Significance

Adipose tissue and brain from high fat diet fed animals demonstrated increased TNF-α and microglial and macrophage activation. Both brains and adipose tissue also had elevated APP levels localizing to neurons and macrophage/adipocytes, respectively. APP agonist antibody stimulation of macrophage cultures increased specific cytokine secretion with no obvious effects on adipocyte culture phenotype. These data support the hypothesis that high fat diet-dependent obesity results in concomitant pro-inflammatory changes in brain and adipose tissue that is characterized, in part, by increased levels of APP that may be contributing specifically to inflammatory changes that occur.

Microglial alterations in human Alzheimer's disease following Aβ42 immunization

AIMS

In Alzheimer's disease (AD), microglial activation prompted by the presence of amyloid has been proposed as an important contributor to the neurodegenerative process. Conversely following Aβ immunization, phagocytic microglia have been implicated in plaque removal, potentially a beneficial effect. We have investigated the effects of Aβ42 immunization on microglial activation and the relationship with Aβ42 load in human AD.

METHODS

Immunostaining against Aβ42 and microglia (CD68 and HLA-DR) was performed in nine immunized AD cases (iAD - AN1792, Elan Pharmaceuticals) and eight unimmunized AD (cAD) cases.

RESULTS

Although the Aβ42 load (% area stained of total area examined) was lower in the iAD than the cAD cases (P=0.036), the CD68 load was higher (P=0.046). In addition, in the iAD group, the CD68 level correlated with the Aβ42 load, consistent with the immunization upregulating microglial phagocytosis when plaques are present. However, in two long-surviving iAD patients in whom plaques had been extensively cleared, the CD68 load was less than in controls. HLA-DR quantification did not show significant difference implying that the microglial activation may have related specifically to their phagocytic function. CD68 and HLA-DR loads in the pons were similar in both groups, suggesting that the differences in microglial activation in the cortex were due to the presence of AD pathology.

CONCLUSION

Our findings suggest that Aβ42 immunization modifies the function of microglia by increasing their phagocytic activity and when plaques have been cleared, the level of phagocytosis is decreased below that seen in unimmunized AD.

Monocytes and Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by extracellular amyloid beta (Aβ) deposition and intracellular neurofibrillary tangle formation. Monocyte is part of the innate immune system and can effectively remove dead cells and debris. It has been suggested that Aβ can recruit monocytes into brain in AD mice, resulting in restriction of cerebral amyloidosis. However, monocyte may act as a double-edged sword, either beneficial (e.g., clearance of Aβ) or detrimental (e.g., secretion of neurotoxic factors). In addition, recent studies indicate that in AD patients, Aβ phagocytosis by monocytes is ineffective. The present review mainly summarized the current knowledge on monocytes and their potential roles in AD.

Glial amyloid precursor protein expression is restricted to astrocytes in an experimental toxic model of multiple sclerosis

The amyloid precursor protein is rapidly induced in reactive glia in response to pathological stimuli and inflammation. In this study, we investigated its expression in an experimental multiple sclerosis animal model, the cuprizone mouse model which reveals massive myelin loss. Cuprizone intoxication for 5 weeks induced immense demyelination of the corpus callosum and resulted in hypertrophic and hyperplastic astrocytosis accompanied by microglia/macrophage invasion. Using double-immunofluorescence, real-time quantitative PCR and Western Blot, we observed that activated astrocytes are the main source of amyloid precursor protein during demyelination. In order to rule out astrocytes, in general, responding to inflammatory and toxic compounds by amyloid precursor protein expression, neonatal astroglia cultures were exposed to various stimuli. Under control conditions, astroglial amyloid precursor protein was only moderately expressed. None of the treatments had a significant effect on its expression in vitro. Our results suggest that amyloid precursor protein is specifically up-regulated under cuprizone-induced demyelination. It remains to be further elucidated whether amyloid precursor protein-positive astrocytes are directly implicated in the pathological mechanism of demyelination.

Neuronal damage is much delayed and microgliosis is more severe in the aged hippocampus induced by transient cerebral ischemia compared to the adult hippocampus

Activation of astrocytes and microglia in the post-ischemic hippocampus has been investigated using ischemia models. The aim of this study was to investigate differences of delayed neuronal death and gliosis in the hippocampal CA1 region (CA1) between adult and aged gerbils. Delayed neuronal death in the CA1 was later in the aged gerbil than in the adult gerbil after ischemia/reperfusion (I/R). GFAP-immunoreactive ((+)) astrocytes and Iba-1(+) microglia were activated following neuronal damage in both adult and aged gerbils after I/R. Changes in GFAP immunoreactivity and protein levels were similar in both groups: they were distinctly increased from 3 days after I/R. Iba-1 immunoreactivity and protein levels in the aged sham gerbil were much higher than those in the adult sham gerbil. Activation of microglia in the CA1 of the aged group was slower, lower 4 days and much higher 7 days than that in the adult gerbil after I/R. These observations indicate that delayed neuronal death in the CA1 of the aged group is slower than that in the adult group after I/R. In addition, microglial activation, not astrocytes, in the aged ischemia group is slower and more intense than that in the adult ischemia group.

Amyloid precursor protein mediates a tyrosine kinase-dependent activation response in endothelial cells

Amyloid precursor protein (APP) is a ubiquitously expressed type 1 integral membrane protein. It has the ability to bind numerous extracellular matrix components and propagate signaling responses via its cytoplasmic phospho-tyrosine, (682)YENPTY(687), binding motif. We recently demonstrated increased protein levels of APP, phosphorylated APP (Tyr682), and beta-amyloid (Abeta) in brain vasculature of atherosclerotic and Alzheimer's disease (AD) tissue colocalizing primarily within the endothelial layer. This study demonstrates similar APP changes in peripheral vasculature from human and mouse apoE(-/-) aorta, suggesting that APP-related changes are not restricted to brain vasculature. Therefore, primary mouse aortic endothelial cells and human umbilical vein endothelial cells were used as a model system to examine the function of APP in endothelial cells. APP multimerization with an anti-N-terminal APP antibody, 22C11, to simulate ligand binding stimulated an Src kinase family-dependent increase in protein phospho-tyrosine levels, APP phosphorylation, and Abeta secretion. Furthermore, APP multimerization stimulated increased protein levels of the proinflammatory proteins, cyclooxygenase-2 and vascular cell adhesion molecule-1 also in an Src kinase family-dependent manner. Endothelial APP was also involved in mediating monocytic cell adhesion. Collectively, these data demonstrate that endothelial APP regulates immune cell adhesion and stimulates a tyrosine kinase-dependent response driving acquisition of a reactive endothelial phenotype. These APP-mediated events may serve as therapeutic targets for intervention in progressive vascular changes common to cerebrovascular disease and AD.

Dissecting molecular mechanisms in the living brain of dementia patients

Understanding the molecular mechanisms associated with the development of dementia is essential for designing successful interventions. Dementia, like cancer and cardiovascular disease, requires early detection to potentially arrest or prevent further disease progression. By the time a neurologist begins to manage clinical symptoms, the disease has often damaged the brain significantly. Because successful treatment is the logical goal, detecting the disease when brain damage is still limited is of the essence. The role of chemistry in this discovery process is critical. With the advent of molecular imaging, the understanding of molecular mechanisms in human neurodegenerative diseases has exploded. Traditionally, knowledge of enzyme and neurotransmitter function in humans has been extrapolated from animal studies, but now we can acquire data directly from both healthy and diseased human subjects. In this Account, we describe the use of molecular imaging probes to elucidate the biochemical and cellular bases of dementia (e.g., Alzheimer's disease) and the application of these discoveries to the design of successful therapeutic interventions. Molecular imaging permits observation and evaluation of the basic molecular mechanisms of disease progression in the living brains of patients. 2-Deoxy-2-[(18)F]fluoro-d-glucose is used to assess the effect of Alzheimer's disease progression on neuronal circuits projecting from and to the temporal lobe (one of the earliest metabolic signs of the disease). Recently, we have developed imaging probes for detection of amyloid neuropathology (both tau and beta-amyloid peptide deposits) and neuronal losses. These probes allow us to visualize the development of pathology in the living brain of dementia patients and its consequences, such as losses of critical neurons associated with memory deficits and other neuropsychiatric impairments. Because inflammatory processes are tightly connected to the brain degenerative processes, inflammation is now emerging as an important target for new molecular imaging probes. The combination of molecular probes targeting various processes of dementia is a useful tool for detailed monitoring of disease mechanism, progression, and diagnosis, as well as for the development of rational strategies for promising therapeutic interventions.

Differential immunoreactivity of microglial and astrocytic marker protein in the hippocampus of the seizure resistant and sensitive gerbils

In the present study, we compared differences in ionized calcium-binding adapter molecule 1 (Iba-1) and glial fibrillary acidic protein (GFAP) immunoreactivities for microglia and astrocytes, respectively, in the hippocampus of the seizure-resistant (SR) and seizure-sensitive (SS) gerbils. The density of Iba-1 immunoreactive microglia in the hippocampal CA1 region (CA1) and dentate gyrus (DG) of the SS gerbil was higher than that in the SR gerbil, and many Iba-1 immunoreactive microglia in the SS gerbil were hypertrophied in morphology. In contrast, we could not find significant difference in the density of GFAP immunoreactive astrocytes between the SR and SS gerbils. This result indicates that Iba-1 immunoreactive microglia in CA1 and DG of the SS gerbil are activated compared to those in the SR gerbil.

Amyloid precursor protein mediates monocyte adhesion in AD tissue and apoE(-)/(-) mice

Amyloid precursor protein (APP) is a type 1 integral membrane protein, which is highly conserved and ubiquitously expressed. Numerous data suggest it functions in cellular adhesion. For example, APP binds components of the extracellular matrix to propagate intracellular signaling responses. In order to investigate adhesion-related changes in inflamed vasculature, brains from apolipoprotein E(-/-) (apoE(-/-)) mice were examined for changes related to APP then compared to human Alzheimer's disease (AD) brains. Cerebrovasculature from mouse apoE(-)/(-) and human AD brains revealed strong immunoreactivity for APP, APP phosphorylated at tyrosine residue 682 (pAPP) and Aβ. Further, Western blot analyses from mouse apoE(-/-) and AD brains showed statistically higher protein levels of APP, pAPP and increased APP association with the tyrosine kinase, Src. Lastly, utilizing a modified Stamper-Woodruff adhesion assay, we demonstrated that adhesion of monocytic cells to apoE(-/-) and AD brain endothelium is partially APP dependent. These data suggest that endothelial APP function coupled with increased Aβ production are involved in the vascular dysfunction associated with atherosclerosis and AD.

What determines the molecular composition of abnormal protein aggregates in neurodegenerative disease?

Abnormal protein aggregates, in the form of either extracellular plaques or intracellular inclusions, are an important pathological feature of the majority of neurodegenerative disorders. The major molecular constituents of these lesions, viz., beta-amyloid (Abeta), tau, and alpha-synuclein, have played a defining role in the diagnosis and classification of disease and in studies of pathogenesis. The molecular composition of a protein aggregate, however, is often complex and could be the direct or indirect consequence of a pathogenic gene mutation, be the result of cell degeneration, or reflect the acquisition of new substances by diffusion and molecular binding to existing proteins. This review examines the molecular composition of the major protein aggregates found in the neurodegenerative diseases including the Abeta and prion protein (PrP) plaques found in Alzheimer's disease (AD) and prion disease, respectively, and the cellular inclusions found in the tauopathies and synucleinopathies. The data suggest that the molecular constituents of a protein aggregate do not directly cause cell death but are largely the consequence of cell degeneration or are acquired during the disease process. These findings are discussed in relation to diagnosis and to studies of to disease pathogenesis.

Modulation of gene expression and cytoskeletal dynamics by the amyloid precursor protein intracellular domain (AICD)

Amyloidogenic processing of the amyloid precursor protein (APP) results in the generation of beta-amyloid, the main constituent of Alzheimer plaques, and the APP intracellular domain (AICD). Recently, it has been demonstrated that AICD has transactivation potential; however, the targets of AICD-dependent gene regulation and hence the physiological role of AICD remain largely unknown. We analyzed transcriptome changes during AICD-dependent gene regulation by using a human neural cell culture system inducible for expression of AICD, its coactivator FE65, or the combination of both. Induction of AICD was associated with increased expression of genes with known function in the organization and dynamics of the actin cytoskeleton, including alpha2-Actin and Transgelin (SM22). AICD target genes were also found to be differentially regulated in the frontal cortex of Alzheimer's disease patients compared with controls as well as in AICD/FE65 transiently transfected murine cortical neurons. Confocal image analysis of neural cells and cortical neurons expressing both AICD and FE65 confirmed pronounced changes in the organization of the actin cytoskeleton, including the destabilization of actin fibers and clumping of actin at the sites of cellular outgrowth. Our data point to a role of AICD in developmental and injury-related cytoskeletal dynamics in the nervous system.

Reproductive hormones modulate oxidative stress in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by gradual cognitive decline, impairments in speech and language, and dysfunction in the sensorimotor systems, culminating in complete reliance on nursing care. Oxidative stress, caused by an imbalance in the pro-oxidant/antioxidant mechanisms in the body, has been implicated in AD pathogenesis, as in many other age-associated diseases such as atherosclerosis, Parkinson disease, and amyotrophic lateral sclerosis. Although the hormones estrogen, progesterone, testosterone, and luteinizing hormone are best known for their roles in reproduction, many studies show these hormones have other roles, including neuroprotection. Changes in the levels of these hormones that occur in reproductive senescence are hypothesized to increase risk of AD, as a result of reduced protection against oxidative insults. The Abeta peptide, overproduction of which is thought to be a key pathogenic event in the development of AD, is neurotoxic, most likely due to its ability to promote oxidative stress. The reproductive hormones are known to influence Abeta metabolism, and this review discusses the beneficial and detrimental effects these hormones have on Abeta production and oxidative stress, and their relevance in potential AD therapies.

Microglial activation in Alzheimer disease: Association with APOE genotype

Microglial cells are considered to play an important role in the pathogenesis of Alzheimer disease. Apart from producing the Alzheimer amyloid precursor (APP) as an acute phase protein, microglial cells seem to be involved in the deposition of its amyloidogenic cleavage product, the amyloid-beta peptide (Abeta). Abeta is bound by apolipoprotein E (APOE) in an isoform-specific manner, and it has been demonstrated that inheritance of the AD susceptibility allele, APOE epsilon4, is associated with increased deposition of Abeta in the cerebral cortex. However, the relationship between APOE epsilon4 gene dose and microglial activation is unknown. Using microglial expression of major histocompatibility complex class II molecules as a marker, we have performed a quantitative genotype-phenotype analysis on microglial activation in frontal and temporal cortices of 20 APOE genotyped AD brains. The number of activated microglia and the tissue area occupied by these cells increased significantly with APOE epsilon4 gene dose. When a model of multiple linear regression was used to compare the relative influence of APOE genotype, sex, disease duration, age at death, diffuse and neuritic plaques as well as neurofibrillary tangles on microglial activation, only APOE genotype was found to have a significant effect. Thus, the APOE gene product represents an important determinant of microglial activity in AD. Since microglial activation by APP has been shown to be modulated by apoE in vitro, a direct role of microglia in AD pathogenesis is conceivable.

Amyloid precursor protein cross-linking stimulates beta amyloid production and pro-inflammatory cytokine release in monocytic lineage cells

Beta amyloid peptide-containing neuritic plaques are a defining feature of Alzheimer's disease pathology. Beta amyloid are 38-43 residue peptides derived by proteolytic cleavage of amyloid precursor protein. Although much attention has focused on the proteolytic events leading to beta amyloid generation, the function of amyloid precursor protein remains poorly described. Previously, we reported that amyloid precursor protein functions as a pro-inflammatory receptor on monocytic lineage cells and defined a role for amyloid precursor protein in adhesion by demonstrating that beta(1) integrin-mediated pro-inflammatory activation of monocytes is amyloid precursor protein dependent. We demonstrated that antibody-induced cross-linking of amyloid precursor protein in human THP-1 monocytes and primary mouse microglia stimulates a tyrosine kinase-based pro-inflammatory signaling response leading to acquisition of a reactive phenotype. Here, we have identified pro-inflammatory mediators released upon amyloid precursor protein-dependent activation of monocytes and microglia. We show that amyloid precursor protein cross-linking stimulated tyrosine kinase-dependent increases in pro-inflammatory cytokine release and a tyrosine kinase-independent increase in beta amyloid 1-42 generation. These data provide much needed insight into the function of amyloid precursor protein and provide potential therapeutic targets to limit inflammatory changes associated with the progression of Alzheimer's disease.

Decreased expression of inflammation-related genes following inhalation exposure to manganese

Excessive exposure to manganese (Mn) by inhalation can induce psychosis and Parkinsonism. The clinical manifestations of Mn neurotoxicity have been related to numerous physiological and cellular processes, most notably dopamine depletion. However, few studies have explored the molecular events that are triggered in response to exposure to Mn by inhalation. In this current study, the transcriptional patterns of genes related to oxidative stress or inflammation were examined in the brain rats of exposed to inhaled Mn during either gestation or early adulthood. The expression of genes encoding for proteins critical to an inflammatory response and/or possessing pro-oxidant properties, including TGFbeta and nNOS, were slightly depressed by prenatal exposure, whereas inhalation exposure to Mn during adulthood markedly down-regulated their transcription. However, when exposures to manganese occurred during gestation, the extent of altered gene expression induced by subsequent exposure to Mn in adulthood was reduced. This suggests that prior exposure to Mn may have attenuated the effects of inhalation exposure to Mn in adulthood, in which the expression of inflammation-related genes were suppressed.

Positron emission tomography and single-photon emission computed tomography in central nervous system drug development

In this review, the value of functional imaging [positron emission tomography (PET)/single-photon emission computed tomography (SPECT)] in drug development is considered. Radionuclide imaging can help establish the diagnosis of neurodegenerative disorders where this is in doubt and provides a potential biomarker for following drug effects on disease progression. PET and SPECT can help understand mechanisms of disease and determine the functional effects of therapeutic approaches on neurotransmission and metabolism. Synthesizing radiotracer analogs of novel drugs can provide proof of principle that these agents reach their enzyme or receptor targets and delineate their regional brain distribution. If such radiotracers do not prove to have ideal properties for imaging, the concept of microdosing potentially allows multiple other drug analogs to be tested with less stringent regulatory requirements than for novel medicinals. Finally, PET tracers can provide receptor and enzyme active site dose occupancy profiles, thereby guiding dosage selection for phase 1 and phase 2 trials. The eventual hope is that radiotracer imaging will provide a surrogate marker for drug efficacy, although this has yet to be realized, and progress the concept of personalized medicine where receptor/enzyme binding profiles help predict therapeutic outcome.

Microglial responses to amyloid beta peptide opsonization and indomethacin treatment

Background

Recent studies have suggested that passive or active immunization with anti-amyloid β peptide (Aβ) antibodies may enhance microglial clearance of Aβ deposits from the brain. However, in a human clinical trial, several patients developed secondary inflammatory responses in brain that were sufficient to halt the study.

Methods

We have used an in vitro culture system to model the responses of microglia, derived from rapid autopsies of Alzheimer's disease patients, to Aβ deposits.

Results

Opsonization of the deposits with anti-Aβ IgG 6E10 enhanced microglial chemotaxis to and phagocytosis of Aβ, as well as exacerbated microglial secretion of the pro-inflammatory cytokines TNF-α and IL-6. Indomethacin, a common nonsteroidal anti-inflammatory drug (NSAID), had no effect on microglial chemotaxis or phagocytosis, but did significantly inhibit the enhanced production of IL-6 after Aβ opsonization.

Conclusion

These results are consistent with well known, differential NSAID actions on immune cell functions, and suggest that concurrent NSAID administration might serve as a useful adjunct to Aβ immunization, permitting unfettered clearance of Aβ while dampening secondary, inflammation-related adverse events.

Quantitative morphological study of microglial cells in the ischemic rat brain using principal component analysis

Pathogenic stimuli induce alterations in the morphology of microglial cells. We analysed changes in lectin-stained cells on the 1st, 3rd, 7th or 14th day after transient global ischemia. Three areas differing in the degree of microglial reaction were selected for analysis: the upper cerebral cortex, the hippocampal CA1 area, and the hilus of the dentate gyrus. Nine morphological parameters, including fractal dimension, lacunarity, self-similarity range, solidity, convexity and form factor were determined. Then the resultant data were processed using principal component analysis (PCA). We found that the two first principal components together explained more than 73% of the observed variability, and may be sufficient both to describe the morphological diversity of the cells, and to determine the dynamics and direction of the changes. In both hippocampal areas, the transformation to hypertrophied and phagocytic cells was observed, but changes in the hilus were faster than in the CA1. In contrast, in the cortex, a microglial reaction was characterised by an increase in the complexity of processes. The results presented show that the quantitative morphological analysis can be an effective tool in research on the reactive behaviour of microglia and, particularly, in the detection of small and early changes in the cells.

Amyloid precursor protein expression in circulating monocytes and brain macrophages from patients with HIV-associated cognitive impairment

We examined amyloid precursor protein (APP) surface expression on circulating leukocytes and in brain tissues from normal individuals and HIV+ subjects with cognitive impairment. Most monocytes, and a subset of B-lymphocytes, expressed APP, while T-lymphocytes, granulocytes, and natural killer (NK) cells did not. CD14bright/CD16+ monocytes expressed the highest levels, and CD14dim/CD16+ cells were negative, suggesting a relationship with activation. Higher APP+ monocyte levels correlated with increased numbers of CD16+ monocytes, but not with the degree of cognitive impairment. Treatment of monocytes with M-CSF, but not LPS, upregulated APP expression. In the brain, APP appeared as axonal immunoreactivity and diffuse plaques, and APP+ perivascular macrophages were seen in cases with severe dementia. APP may facilitate monocyte entry into the brain.

The facial nerve axotomy model

Experimental models such as the facial nerve axotomy paradigm in rodents allow the systematic and detailed study of the response of neurones and their microenvironment to various types of challenges. Well-studied experimental examples include peripheral nerve trauma, the retrograde axonal transport of neurotoxins and locally enhanced inflammation following the induction of experimental autoimmune encephalomyelitis in combination with axotomy. These studies have led to novel insights into the regeneration programme of the motoneurone, the role of microglia and astrocytes in synaptic plasticity and the biology of glial cells. Importantly, many of the findings obtained have proven to be valid in other functional systems and even across species barriers. In particular, microglial expression of major histocompatibility complex molecules has been found to occur in response to various types of neuronal damage and is now regarded as a characteristic component of "glial inflammation". It is found in the context of numerous neurodegenerative disorders including Parkinson's and Alzheimer's disease. The detachment of afferent axonal endings from the surface membrane of regenerating motoneurones and their subsequent displacement by microglia ("synaptic stripping") and long-lasting insulation by astrocytes have also been confirmed in humans. The medical implications of these findings are significant. Also, the facial nerve system of rats and mice has become the best studied and most widely used test system for the evaluation of neurotrophic factors.

Acetylcholinesterase induces the expression of the β-amyloid precursor protein in glia and activates glial cells in culture

Acetylcholinesterase (AChE) activities in CNS physiopathology are increasingly diverse and range from neuritogenesis, through synaptogenesis, to enhancement of amyloid fiber assembly. In Alzheimer's disease, senile plaques and neurodegeneration specially affect regions enriched for cholinergic synapses. In this study we show an effect of AChE that could contribute to the increased deposition of Abeta in certain regions. Affinity-purified AChE induced the expression of amyloid-beta-precursor protein (beta-APP) in glial cells in a concentration-dependent manner up to 5 nM. In glia, AChE also increased inducible nitric oxide synthase (iNOS) assessed by immunocytochemistry and decreased reductive metabolism as evidence of cell activation. AChE could increase the expression of beta-APP in astrocytes and microglia as result of the activation of glial cells. As a whole, we found that AChE has additional effects that could result in an increased synthesis of Abeta, both by increasing beta-APP expression of astrocytes and by further activating glial cells.

Critical role of microglial NADPH oxidase-derived free radicals in the in vitro MPTP model of Parkinson's disease

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) damages dopaminergic neurons as seen in Parkinson's disease. Although increasing evidence suggests an involvement of glia in MPTP neurotoxicity, the nature of this involvement remains unclear. Exploiting the advantage of cell culture systems, we demonstrated that microglia, but not astroglia, significantly enhanced the progression of MPTP-induced dopaminergic neurodegeneration. Characterization of the temporal relationship between neurodegeneration and microglial activation demonstrates that reactive microgliosis resulting from MPTP-initiated neuronal injury, but not direct activation, underlies the microglia-enhanced MPTP neurotoxicity. Mechanistically, through the release of NADPH oxidase-derived reactive oxygen species, microglia contribute to the progressive neuronal damage. Among the factors measured, the production of extracellular superoxide was the most prominent. NADPH oxidase inhibitor, apocynin, attenuated MPTP-induced dopaminergic neurodegeneration only in the presence of glia. More importantly, dopaminergic neurons from mice lacking NADPH oxidase, a key enzyme for superoxide production in immune cells, are significantly more resistant to MPTP neurotoxicity than those from wild-type controls, and microglia dictate the resistance. This study demonstrates that reactive microgliosis triggered by MPTP-induced neuronal injury and NADPH oxidase-mediated superoxide production in microglia constitute an integral component of MPTP neurotoxicity. This study also suggests that NADPH oxidase may be a promising target for therapeutic interventions in Parkinson's disease.

Chronic glial activation, neurodegeneration, and APP immunoreactive deposits following acute administration of double-stranded RNA

Several neurodegenerative disorders, including Alzheimer's and Parkinson's diseases, are associated with immunocompetent microglia, leading to the suggestion that chronic glial-mediated inflammation contributes to the neurodegeneration seen in these diseases. Little direct evidence supports this hypothesis, and no suitable rodent models exist that do not involve the use of blunt trauma or ischaemia, events that are infrequently encountered in the human disease state. In the present study, we report that administration of double-stranded RNA, a classical inducer of interferon-gamma (IFN-gamma), causes rapid and persistent activation of microglia and astrocytes, as well as induction of interleukin-1beta (IL-beta) and nitric oxide synthase. In close temporal succession to glial activation, there is neurodegeneration, with neuron loss involving apoptosis in selected brain regions including the septal nucleus, hippocampus, cortex and thalamus, along with hippocampal atrophy. This neuronal loss is accompanied by punctate deposits of material that are immunoreactive for amyloid precursor protein, beta-amyloid peptide (Abeta), and apolipoprotein E. The findings may have clinical relevance, since the administration of the nonsteroidal antiinflammatory agent (NSAID) ibuprofen markedly reduces the neurodegeneration observed in the absence of significant glial inhibition. These findings may be relevant to the pathogenesis of Alzheimer's disease in particular, and to other neurodegenerative diseases involving inflammation.

Glial neuronal signaling in the central nervous system

Glial cells are known to interact extensively with neuronal elements in the brain, influencing their activity. Astrocytes associated with synapses integrate neuronal inputs and release transmitters that modulate synaptic sensitivity. Glial cells participate in formation and rebuilding of synapses and play a prominent role in protection and repair of nervous tissue after damage. For glial cells to take an active part in plastic alterations under physiological conditions and pathological disturbances, extensive specific signaling, both within single cells and between cells, is required. In recent years, intensive research has led to our first insight into this signaling. We know there are active connections between astrocytes in the form of networks promoting Ca2+ and ATP signaling; we also know there is intense signaling between astrocytes, microglia, oligodendrocytes, and neurons, with an array of molecules acting as signaling substances. The cells must be functionally integrated to facilitate the enormous dynamics of and capacity for reconstruction within the nervous system. In this paper, we summarize some basic data on glial neuronal signaling to provide insight into synaptic modulation and reconstruction in physiology and protection and repair after damage.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Evidence for binding of the ectodomain of amyloid precursor protein 695 and activated high molecular weight kininogen

To identify ligands that bind to the N-terminal portion of human amyloid precursor protein (APP), we sought binding partners for a fragment of the ectodomain of human APP695 (sAPP(695)T). The probe bound to fragments of high molecular weight kininogen (HK) in rat cortical membrane preparations in vitro. Laser confocal microscopy indicated that APP and HK colocalize near cerebral blood vessels, in the neuropil, and in many neurons of rat brain. sAPP(695)T bound to human activated kininogen (HKa) (K(d)=0.3+/-0.1 nM), but not to inactivated or low molecular weight kininogen. Binding was specific for the light chain sequence of HKa. Biotinylated human HKa also bound to sAPP(695) (K(d)=0.3+/-0.5 nM). sAPP(695) and HKa form tight complexes in solution that can be coimmunoprecipitated. These results support the hypothesis that forms of APP and kininogen can interact in brain tissue. Considering the implications of APP in neurite outgrowth, the APP-HKa interaction could modulate neurogenesis.

Air pollution and brain damage

Exposure to complex mixtures of air pollutants produces inflammation in the upper and lower respiratory tract. Because the nasal cavity is a common portal of entry, respiratory and olfactory epithelia are vulnerable targets for toxicological damage. This study has evaluated, by light and electron microscopy and immunohistochemical expression of nuclear factor-kappa beta (NF-kappaB) and inducible nitric oxide synthase (iNOS), the olfactory and respiratory nasal mucosae, olfactory bulb, and cortical and subcortical structures from 32 healthy mongrel canine residents in Southwest Metropolitan Mexico City (SWMMC), a highly polluted urban region. Findings were compared to those in 8 dogs from Tlaxcala, a less polluted, control city. In SWMMC dogs, expression of nuclear neuronal NF-kappaB and iNOS in cortical endothelial cells occurred at ages 2 and 4 weeks; subsequent damage included alterations of the blood-brain barrier (BBB), degenerating cortical neurons, apoptotic glial white matter cells, deposition of apolipoprotein E (apoE)-positive lipid droplets in smooth muscle cells and pericytes, nonneuritic plaques, and neurofibrillary tangles. Persistent pulmonary inflammation and deteriorating olfactory and respiratory barriers may play a role in the neuropathology observed in the brains of these highly exposed canines. Neurodegenerative disorders such as Alzheimer's may begin early in life with air pollutants playing a crucial role.

Brain plasticity and microglia: is transsynaptic glial activation in the thalamus after limb denervation linked to cortical plasticity and central sensitisation?

Microglia are a subset of tissue-macrophages that are ubiquitously distributed throughout the entire CNS. In health, they remain largely dormant until activated by a pathological stimulus. The availability of more sensitive detection techniques has allowed the early measurement of the cell responses of microglia in areas with few signs of active pathology. Subtle neuronal injury can induce microglial activation in retrograde and anterograde projection areas remote from the primary lesion focus. There is also evidence that in cases of long-standing abnormal neuronal activity, such as in patients after limb amputation with chronic pain and phantom sensations, glial activation may occur transsynaptically in the thalamus. Such neuronally driven glial responses may be related to the emergence central sensitisation in chronic pain states or plasticity phenomena in the cerebral cortex. It is suggested, that such persistent low-level microglial activation is not adequately described by the traditional concept of phagocyte-mediated tissue damage that largely evolved from studies of acute brain lesion models or acute human brain pathology. Due to the presence of signal molecules that can act on neurons and microglia alike, the communication between neurons and microglia is likely to be bi-directional. Persistent subtle microglial activity may modulate basal synaptic transmission and thus neuronal functioning either directly or through the interaction with astrocytes. The activation of microglia leads to the emergence of microstructural as well as functional compartments in which neurokines, interleukins and other signalling molecules introduce a qualitatively different, more open mode of cell-cell communication that is normally absent from the healthy adult brain. This 'neo-compartmentalisation', however, occurs along predictable neuronal pathways within which these glial changes are themselves under the modulatory influence of neurons or other glial cells and are subject to the evolving state of the pathology. Depending on the disease state, yet relatively independent of the specific disease cause, fluctuations in the modulatory influence by non-neuronal cells may form the cellular basis for the variability of brain plasticity phenomena, i.e. the plasticity of plasticity.

Differential expression of Bcl-2 and APP immunoreactivity after intrastriatal injection of MPP+ in the rat

While there is growing evidence that Bcl-2 proto-oncogene and beta-amyloid precursor proteins (APP) are neuroprotective in function, our recent studies have demonstrated that Bcl-2 and APP may be co-expressed and co-regulated in retinal neurons or glia under normal or experimental conditions. Whether Bcl-2 and APP are functionally coupled in other neuronal systems is not clear. This issue was investigated further in the present experiments by examining the expression pattern of two molecules after unilateral intrastriatal injection of 1-methyl-4-phenyl-pyridinium (MPP(+)), a neurotoxic metabolite that selectively damages dopaminergic neurons. One hour to 2 months after MPP(+) injection into rat striatum, a significant increase in Bcl-2 expression was observed in distinct populations of neurons, astrocyte-like and OX-42-positive cells not only in traumatic regions but also in remote areas including the ipsilateral cortex and substantia nigra (SN). No detectable change was observed in the striatum, cortex or SN on the contralateral side of the brain. The immunoreactive pattern and time-dependent APP increase was similar to that of Bcl-2 in the severely injured striatum and cortex. However, an up-regulation of Bcl-2 expression, but not APP, appears in dopaminergic neurons in the ipsilateral SN pars compacta where there was retrograde degeneration. In contrast, APP immunoreactivity was decreased in the hippocampus following intrastriatal injury, whereas, no alteration in Bcl-2 expression was detected. The differential changes in Bcl-2 and APP expression in nigral neurons and some other brain tissues suggest that these proteins may not be co-regulated by a common mechanism, at least in certain neuronal pathways.