APOE Christchurch enhances a disease-associated microglial response to plaque but suppresses response to tau pathology.

Background: Apolipoprotein E ε4 (APOE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (LOAD). A recent case report identified a rare variant in APOE, APOE3-R136S (Christchurch), proposed to confer resistance to autosomal dominant Alzheimer's Disease (AD). However, it remains unclear whether and how this variant exerts its protective effects. Methods: We introduced the R136S variant into mouse Apoe ( ApoeCh ) and investigated its effect on the development of AD-related pathology using the 5xFAD model of amyloidosis and the PS19 model of tauopathy. We used immunohistochemical and biochemical analysis along with single-cell spatial transcriptomics and proteomics to explore the impact of the ApoeCh variant on AD pathological development and the brain's response to plaques and tau. Results: In 5xFAD mice, ApoeCh enhances a Disease-Associated Microglia (DAM) phenotype in microglia surrounding plaques, and reduces plaque load, dystrophic neurites, and plasma neurofilament light chain. By contrast, in PS19 mice, ApoeCh suppresses the microglial and astrocytic responses to tau-laden neurons and does not reduce tau accumulation or phosphorylation, but partially rescues tau-induced synaptic and myelin loss. We compared how microglia responses differ between the two mouse models to elucidate the distinct DAM signatures induced by ApoeCh . We identified upregulation of antigen presentation-related genes in the DAM response in a PS19 compared to a 5xFAD background, suggesting a differential response to amyloid versus tau pathology that is modulated by the presence of ApoeCh . Conclusions: These findings highlight the ability of the ApoeCh variant to modulate microglial responses based on the type of pathology, enhancing DAM reactivity in amyloid models and dampening neuroinflammation to promote protection in tau models. This suggests that the Christchurch variant's protective effects likely involve multiple mechanisms, including changes in receptor binding and microglial programming.

The Abca7V1613M variant reduces Aß generation, plaque load, and neuronal damage.

BACKGROUND: Variants in ABCA7, a member of the ABC transporter superfamily, have been associated with increased risk for developing late onset Alzheimer's disease (LOAD). METHODS: CRISPR-Cas9 was used to generate an Abca7V1613M variant in mice, modeling the homologous human ABCA7V1599M variant, and extensive characterization was performed. RESULTS: Abca7V1613M microglia show differential gene expression profiles upon lipopolysaccharide challenge and increased phagocytic capacity. Homozygous Abca7V1613M mice display elevated circulating cholesterol and altered brain lipid composition. When crossed with 5xFAD mice, homozygous Abca7V1613M mice display fewer Thioflavin S-positive plaques, decreased amyloid beta (Aß) peptides, and altered amyloid precursor protein processing and trafficking. They also exhibit reduced Aß-associated inflammation, gliosis, and neuronal damage. DISCUSSION: Overall, homozygosity for the Abca7V1613M variant influences phagocytosis, response to inflammation, lipid metabolism, Aß pathology, and neuronal damage in mice. This variant may confer a gain of function and offer a protective effect against Alzheimer's disease-related pathology. HIGHLIGHTS: ABCA7 recognized as a top 10 risk gene for developing Alzheimer's disease. Loss of function mutations result in increased risk for LOAD. V1613M variant reduces amyloid beta plaque burden in 5xFAD mice. V1613M variant modulates APP processing and trafficking in 5xFAD mice. V1613M variant reduces amyloid beta-associated damage in 5xFAD mice.

Genetic diversity promotes resilience in a mouse model of Alzheimer's disease.

INTRODUCTION: Alzheimer's disease (AD) is a neurodegenerative disorder with multifactorial etiology, including genetic factors that play a significant role in disease risk and resilience. However, the role of genetic diversity in preclinical AD studies has received limited attention. METHODS: We crossed five Collaborative Cross strains with 5xFAD C57BL/6J female mice to generate F1 mice with and without the 5xFAD transgene. Amyloid plaque pathology, microglial and astrocytic responses, neurofilament light chain levels, and gene expression were assessed at various ages. RESULTS: Genetic diversity significantly impacts AD-related pathology. Hybrid strains showed resistance to amyloid plaque formation and neuronal damage. Transcriptome diversity was maintained across ages and sexes, with observable strain-specific variations in AD-related phenotypes. Comparative gene expression analysis indicated correlations between mouse strains and human AD. DISCUSSION: Increasing genetic diversity promotes resilience to AD-related pathogenesis, relative to an inbred C57BL/6J background, reinforcing the importance of genetic diversity in uncovering resilience in the development of AD. HIGHLIGHTS: Genetic diversity's impact on AD in mice was explored. Diverse F1 mouse strains were used for AD study, via the Collaborative Cross. Strain-specific variations in AD pathology, glia, and transcription were found. Strains resilient to plaque formation and plasma neurofilament light chain (NfL) increases were identified. Correlations with human AD transcriptomics were observed.

MAC2 is a long-lasting marker of peripheral cell infiltrates into the mouse CNS after bone marrow transplantation and coronavirus infection.

Microglia are the primary resident myeloid cells of the brain responsible for maintaining homeostasis and protecting the central nervous system (CNS) from damage and infection. Monocytes and monocyte-derived macrophages arising from the periphery have also been implicated in CNS pathologies, however, distinguishing between different myeloid cell populations in the CNS has been difficult. Here, we set out to develop a reliable histological marker that can assess distinct myeloid cell heterogeneity and functional contributions, particularly in the context of disease and/or neuroinflammation. scRNAseq from brains of mice infected with the neurotropic JHM strain of the mouse hepatitis virus (JHMV), a mouse coronavirus, revealed that Lgals3 is highly upregulated in monocyte and macrophage populations, but not in microglia. Subsequent immunostaining for galectin-3 (encoded by Lgals3), also referred to as MAC2, highlighted the high expression levels of MAC2 protein in infiltrating myeloid cells in JHMV-infected and bone marrow (BM) chimeric mice, in stark contrast to microglia, which expressed little to no staining in these models. Expression of MAC2 was found even 6-10 months following BM-derived cell infiltration into the CNS. We also demonstrate that MAC2 is not a specific label for plaque-associated microglia in the 5xFAD mouse model, but only appears in a distinct subset of these cells in the presence of JHMV infection or during aging. Our data suggest that MAC2 can serve as a reliable and long-lasting histological marker for monocyte/macrophages in the brain, identifying an accessible approach to distinguishing resident microglia from infiltrating cells in the CNS under certain conditions.

Microglia Do Not Restrict SARS-CoV-2 Replication following Infection of the Central Nervous System of K18-Human ACE2 Transgenic Mice.

Unlike SARS-CoV-1 and MERS-CoV, infection with SARS-CoV-2, the viral pathogen responsible for COVID-19, is often associated with neurologic symptoms that range from mild to severe, yet increasing evidence argues the virus does not exhibit extensive neuroinvasive properties. We demonstrate SARS-CoV-2 can infect and replicate in human iPSC-derived neurons and that infection shows limited antiviral and inflammatory responses but increased activation of EIF2 signaling following infection as determined by RNA sequencing. Intranasal infection of K18 human ACE2 transgenic mice (K18-hACE2) with SARS-CoV-2 resulted in lung pathology associated with viral replication and immune cell infiltration. In addition, ∼50% of infected mice exhibited CNS infection characterized by wide-spread viral replication in neurons accompanied by increased expression of chemokine (Cxcl9, Cxcl10, Ccl2, Ccl5 and Ccl19) and cytokine (Ifn-λ and Tnf-α) transcripts associated with microgliosis and a neuroinflammatory response consisting primarily of monocytes/macrophages. Microglia depletion via administration of colony-stimulating factor 1 receptor inhibitor, PLX5622, in SARS-CoV-2 infected mice did not affect survival or viral replication but did result in dampened expression of proinflammatory cytokine/chemokine transcripts and a reduction in monocyte/macrophage infiltration. These results argue that microglia are dispensable in terms of controlling SARS-CoV-2 replication in in the K18-hACE2 model but do contribute to an inflammatory response through expression of pro-inflammatory genes. Collectively, these findings contribute to previous work demonstrating the ability of SARS-CoV-2 to infect neurons as well as emphasizing the potential use of the K18-hACE2 model to study immunological and neuropathological aspects related to SARS-CoV-2-induced neurologic disease. IMPORTANCE Understanding the immunological mechanisms contributing to both host defense and disease following viral infection of the CNS is of critical importance given the increasing number of viruses that are capable of infecting and replicating within the nervous system. With this in mind, the present study was undertaken to evaluate the role of microglia in aiding in host defense following experimental infection of the central nervous system (CNS) of K18-hACE2 with SARS-CoV-2, the causative agent of COVID-19. Neurologic symptoms that range in severity are common in COVID-19 patients and understanding immune responses that contribute to restricting neurologic disease can provide important insight into better understanding consequences associated with SARS-CoV-2 infection of the CNS.

Microglia do not restrict SARS-CoV-2 replication following infection of the central nervous system of K18-hACE2 transgenic mice.

Unlike SARS-CoV-1 and MERS-CoV, infection with SARS-CoV-2, the viral pathogen responsible for COVID-19, is often associated with neurologic symptoms that range from mild to severe, yet increasing evidence argues the virus does not exhibit extensive neuroinvasive properties. We demonstrate SARS-CoV-2 can infect and replicate in human iPSC-derived neurons and that infection shows limited anti-viral and inflammatory responses but increased activation of EIF2 signaling following infection as determined by RNA sequencing. Intranasal infection of K18 human ACE2 transgenic mice (K18-hACE2) with SARS-CoV-2 resulted in lung pathology associated with viral replication and immune cell infiltration. In addition, ∼50% of infected mice exhibited CNS infection characterized by wide-spread viral replication in neurons accompanied by increased expression of chemokine ( Cxcl9, Cxcl10, Ccl2, Ccl5 and Ccl19 ) and cytokine ( Ifn-λ and Tnf-α ) transcripts associated with microgliosis and a neuroinflammatory response consisting primarily of monocytes/macrophages. Microglia depletion via administration of colony-stimulating factor 1 receptor inhibitor, PLX5622, in SARS-CoV-2 infected mice did not affect survival or viral replication but did result in dampened expression of proinflammatory cytokine/chemokine transcripts and a reduction in monocyte/macrophage infiltration. These results argue that microglia are dispensable in terms of controlling SARS-CoV-2 replication in in the K18-hACE2 model but do contribute to an inflammatory response through expression of pro-inflammatory genes. Collectively, these findings contribute to previous work demonstrating the ability of SARS-CoV-2 to infect neurons as well as emphasizing the potential use of the K18-hACE2 model to study immunological and neuropathological aspects related to SARS-CoV-2-induced neurologic disease. IMPORTANCE: Understanding the immunological mechanisms contributing to both host defense and disease following viral infection of the CNS is of critical importance given the increasing number of viruses that are capable of infecting and replicating within the nervous system. With this in mind, the present study was undertaken to evaluate the role of microglia in aiding in host defense following experimental infection of the central nervous system (CNS) of K18-hACE2 with SARS-CoV-2, the causative agent of COVID-19. Neurologic symptoms that range in severity are common in COVID-19 patients and understanding immune responses that contribute to restricting neurologic disease can provide important insight into better understanding consequences associated with SARS-CoV-2 infection of the CNS.

Subventricular zone/white matter microglia reconstitute the empty adult microglial niche in a dynamic wave.

Microglia, the brain's resident myeloid cells, play central roles in brain defense, homeostasis, and disease. Using a prolonged colony-stimulating factor 1 receptor inhibitor (CSF1Ri) approach, we report an unprecedented level of microglial depletion and establish a model system that achieves an empty microglial niche in the adult brain. We identify a myeloid cell that migrates from the subventricular zone and associated white matter areas. Following CSF1Ri, these amoeboid cells migrate radially and tangentially in a dynamic wave filling the brain in a distinct pattern, to replace the microglial-depleted brain. These repopulating cells are enriched in disease-associated microglia genes and exhibit similar phenotypic and transcriptional profiles to white-matter-associated microglia. Our findings shed light on the overlapping and distinct functional complexity and diversity of myeloid cells of the CNS and provide new insight into repopulating microglia function and dynamics in the mouse brain.

Microglial dyshomeostasis drives perineuronal net and synaptic loss in a CSF1R+/- mouse model of ALSP, which can be rescued via CSF1R inhibitors.

Adult-onset leukoencephalopathy with axonal spheroids and pigmented glia is an autosomal dominant neurodegenerative disease caused by mutations in colony-stimulating factor 1 receptor (CSF1R). We sought to identify the role of microglial CSF1R haploinsufficiency in mediating pathogenesis. Using an inducible Cx3cr1 CreERT2/+-Csf1r +/fl system, we found that postdevelopmental, microglia-specific Csf1r haploinsufficiency resulted in reduced expression of homeostatic microglial markers. This was associated with loss of presynaptic surrogates and the extracellular matrix (ECM) structure perineuronal nets. Similar phenotypes were observed in constitutive global Csf1r haploinsufficient mice and could be reversed/prevented by microglia elimination in adulthood. As microglial elimination is unlikely to be clinically feasible for extended durations, we treated adult CSF1R+/- mice at different disease stages with a microglia-modulating dose of the CSF1R inhibitor PLX5622, which prevented microglial dyshomeostasis along with synaptic- and ECM-related deficits. These data highlight microglial dyshomeostasis as a driver of pathogenesis and show that CSF1R inhibition can mitigate these phenotypes.

Childbearing vs. clinical trial participation: is it one or the other?

Recent advances have shed light on the importance of early therapeutic intervention for neurodegenerative diseases. Primary prevention trials present a potential disease-modifying strategy for pre-symptomatic patients of autosomal dominant neurodegenerative diseases (ADND), such as early onset familial Alzheimer's disease (AD) and Huntington's disease (HD). As trials target earlier disease stages, however, prospective participants face new ethical and logistical challenges, namely childbearing and reproductive health decisions. Since pregnancy is an exclusion criteria for such trials, participants of reproductive age must choose between participating in research and having a family. Such decisions carry significant burdens for ADND patients that if left unaddressed could impact patient well-being and the field as whole. We use our perspective as scientists, advocates, and ADND family members to highlight current shortcomings in the field regarding trial participation and family planning issues for ADND patients and call for the establishment of a normative standard to address these concerns.

Effects of long-term and brain-wide colonization of peripheral bone marrow-derived myeloid cells in the CNS.

BACKGROUND: Microglia, the primary resident myeloid cells of the brain, play critical roles in immune defense by maintaining tissue homeostasis and responding to injury or disease. However, microglial activation and dysfunction has been implicated in a number of central nervous system (CNS) disorders, thus developing tools to manipulate and replace these myeloid cells in the CNS is of therapeutic interest. METHODS: Using whole body irradiation, bone marrow transplant, and colony-stimulating factor 1 receptor inhibition, we achieve long-term and brain-wide (~ 80%) engraftment and colonization of peripheral bone marrow-derived myeloid cells (i.e., monocytes) in the brain parenchyma and evaluated the long-term effects of their colonization in the CNS. RESULTS: Here, we identify a monocyte signature that includes an upregulation in Ccr1, Ms4a6b, Ms4a6c, Ms4a7, Apobec1, Lyz2, Mrc1, Tmem221, Tlr8, Lilrb4a, Msr1, Nnt, and Wdfy1 and a downregulation of Siglech, Slc2a5, and Ccl21a/b. We demonstrate that irradiation and long-term (~ 6 months) engraftment of the CNS by monocytes induces brain region-dependent alterations in transcription profiles, astrocytes, neuronal structures, including synaptic components, and cognition. Although our results show that microglial replacement with peripherally derived myeloid cells is feasible and that irradiation-induced changes can be reversed by the replacement of microglia with monocytes in the hippocampus, we also observe that brain-wide engraftment of peripheral myeloid cells (relying on irradiation) can result in cognitive and synaptic deficits. CONCLUSIONS: These findings provide insight into better understanding the role and complexity of myeloid cells in the brain, including their regulation of other CNS cells and functional outcomes.

To Kill a Microglia: A Case for CSF1R Inhibitors.

Microglia, the brain's immune sentinels, have garnered much attention in recent years. Researchers have begun to identify the manifold roles that these cells play in the central nervous system (CNS), and this work has been greatly facilitated by microglial depletion paradigms. The varying degrees of spatiotemporal manipulation afforded by such techniques allow microglial ablation before, during, and/or following insult, injury, or disease. We review the major methods of microglial depletion, including toxin-based, genetic, and pharmacological approaches, which differ in key factors including depletion onset, duration, and off-target effects. We conclude that pharmacological CSF1R inhibitors afford the most extensive versatility in manipulating microglia, making them ideal candidates for future studies investigating microglial function in health and disease.

Microglia facilitate loss of perineuronal nets in the Alzheimer's disease brain.

BACKGROUND: Microglia, the brain's principal immune cell, are increasingly implicated in Alzheimer's disease (AD), but the molecular interfaces through which these cells contribute to amyloid beta (Aß)-related neurodegeneration are unclear. We recently identified microglial contributions to the homeostatic and disease-associated modulation of perineuronal nets (PNNs), extracellular matrix structures that enwrap and stabilize neuronal synapses, but whether PNNs are altered in AD remains controversial. METHODS: Extensive histological analysis was performed on male and female 5xFAD mice at 4, 8, 12, and 18 months of age to assess plaque burden, microgliosis, and PNNs. Findings were validated in postmortem AD tissue. The role of neuroinflammation in PNN loss was investigated via LPS treatment, and the ability to prevent or rescue disease-related reductions in PNNs was assessed by treating 5xFAD and 3xTg-AD model mice with colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 to deplete microglia. FINDINGS: Utilizing the 5xFAD mouse model and human cortical tissue, we report that PNNs are extensively lost in AD in proportion to plaque burden. Activated microglia closely associate with and engulf damaged nets in the 5xFAD brain, and inclusions of PNN material are evident in mouse and human microglia, while aggrecan, a critical PNN component, deposits within human dense-core plaques. Disease-associated reductions in parvalbumin (PV)+ interneurons, frequently coated by PNNs, are preceded by PNN coverage and integrity impairments, and similar phenotypes are elicited in wild-type mice following microglial activation with LPS. Chronic pharmacological depletion of microglia prevents 5xFAD PNN loss, with similar results observed following depletion in aged 3xTg-AD mice, and this occurs despite plaque persistence. INTERPRETATION: We conclude that phenotypically altered microglia facilitate plaque-dependent PNN loss in the AD brain. FUNDING: The NIH (NIA, NINDS) and the Alzheimer's Association.

Aging and Progression of Beta-Amyloid Pathology in Alzheimer's Disease Correlates with Microglial Heme-Oxygenase-1 Overexpression.

Neuroinflammation and oxidative stress are being recognized as characteristic hallmarks in many neurodegenerative diseases, especially those that portray proteinopathy, such as Alzheimer's disease (AD). Heme-oxygenase 1 (HO-1) is an inducible enzyme with antioxidant and anti-inflammatory properties, while microglia are the immune cells in the central nervous system. To elucidate the brain expression profile of microglial HO-1 in aging and AD-progression, we have used the 5xFAD (five familial AD mutations) mouse model of AD and their littermates at different ages (four, eight, 12, and 18 months). Total brain expression of HO-1 was increased with aging and such increase was even higher in 5xFAD animals. In co-localization studies, HO-1 expression was mainly found in microglia vs. other brain cells. The percentage of microglial cells expressing HO-1 and the amount of HO-1 expressed within microglia increased progressively with aging. Furthermore, this upregulation was increased by 2-3-fold in the elder 5xFAD mice. In addition, microglia overexpressing HO-1 was predominately found surrounding beta-amyloid plaques. These results were corroborated using postmortem brain samples from AD patients, where microglial HO-1 was found up-regulated in comparison to brain samples from aged matched non-demented patients. This study demonstrates that microglial HO-1 expression increases with aging and especially with AD progression, highlighting HO-1 as a potential biomarker or therapeutic target for AD.

Sustained microglial depletion with CSF1R inhibitor impairs parenchymal plaque development in an Alzheimer's disease model.

Many risk genes for the development of Alzheimer's disease (AD) are exclusively or highly expressed in myeloid cells. Microglia are dependent on colony-stimulating factor 1 receptor (CSF1R) signaling for their survival. We designed and synthesized a highly selective brain-penetrant CSF1R inhibitor (PLX5622) allowing for extended and specific microglial elimination, preceding and during pathology development. We find that in the 5xFAD mouse model of AD, plaques fail to form in the parenchymal space following microglial depletion, except in areas containing surviving microglia. Instead, Aß deposits in cortical blood vessels reminiscent of cerebral amyloid angiopathy. Altered gene expression in the 5xFAD hippocampus is also reversed by the absence of microglia. Transcriptional analyses of the residual plaque-forming microglia show they exhibit a disease-associated microglia profile. Collectively, we describe the structure, formulation, and efficacy of PLX5622, which allows for sustained microglial depletion and identify roles of microglia in initiating plaque pathogenesis.

Replacement of microglia in the aged brain reverses cognitive, synaptic, and neuronal deficits in mice.

Microglia, the resident immune cell of the brain, can be eliminated via pharmacological inhibition of the colony-stimulating factor 1 receptor (CSF1R). Withdrawal of CSF1R inhibition then stimulates microglial repopulation, effectively replacing the microglial compartment. In the aged brain, microglia take on a "primed" phenotype and studies indicate that this coincides with age-related cognitive decline. Here, we investigated the effects of replacing the aged microglial compartment with new microglia using CSF1R inhibitor-induced microglial repopulation. With 28 days of repopulation, replacement of resident microglia in aged mice (24 months) improved spatial memory and restored physical microglial tissue characteristics (cell densities and morphologies) to those found in young adult animals (4 months). However, inflammation-related gene expression was not broadly altered with repopulation nor the response to immune challenges. Instead, microglial repopulation resulted in a reversal of age-related changes in neuronal gene expression, including expression of genes associated with actin cytoskeleton remodeling and synaptogenesis. Age-related changes in hippocampal neuronal complexity were reversed with both microglial elimination and repopulation, while microglial elimination increased both neurogenesis and dendritic spine densities. These changes were accompanied by a full rescue of age-induced deficits in long-term potentiation with microglial repopulation. Thus, several key aspects of the aged brain can be reversed by acute noninvasive replacement of microglia.

Prevention of C5aR1 signaling delays microglial inflammatory polarization, favors clearance pathways and suppresses cognitive loss.

BACKGROUND: Pharmacologic inhibition of C5aR1, a receptor for the complement activation proinflammatory fragment, C5a, suppressed pathology and cognitive deficits in Alzheimer's disease (AD) mouse models. To validate that the effect of the antagonist was specifically via C5aR1 inhibition, mice lacking C5aR1 were generated and compared in behavior and pathology. In addition, since C5aR1 is primarily expressed on cells of the myeloid lineage, and only to a lesser extent on endothelial cells and neurons in brain, gene expression in microglia isolated from adult brain at multiple ages was compared across all genotypes. METHODS: C5aR1 knock out mice were crossed to the Arctic AD mouse model, and characterized for pathology and for behavior performance in a hippocampal dependent memory task. CX3CR1GFP and CCR2RFP reporter mice were bred to C5aR1 sufficient and knockout wild type and Arctic mice to enable sorting of microglia (GFP-positive, RFP-negative) isolated from adult brain at 2, 5, 7 and 10 months of age followed by RNA-seq analysis. RESULTS: A lack of C5aR1 prevented behavior deficits at 10 months, although amyloid plaque load was not altered. Immunohistochemical analysis showed no CCR2+ monocytes/macrophages near the plaques in the Arctic brain with or without C5aR1. Microglia were sorted from infiltrating monocytes (GFP and RFP-positive) for transcriptome analysis. RNA-seq analysis identified inflammation related genes as differentially expressed, with increased expression in the Arctic mice relative to wild type and decreased expression in the Arctic/C5aR1KO relative to Arctic. In addition, phagosomal-lysosomal gene expression was increased in the Arctic mice relative to wild type but further increased in the Arctic/C5aR1KO mice. A decrease in neuronal complexity was seen in hippocampus of 10 month old Arctic mice at the time that correlates with the behavior deficit, both of which were rescued in the Arctic/C5aR1KO. CONCLUSIONS: These data are consistent with microglial polarization in the absence of C5aR1 signaling reflecting decreased induction of inflammatory genes and enhancement of degradation/clearance pathways, which is accompanied by preservation of CA1 neuronal complexity and hippocampal dependent cognitive function. These results provide links between microglial responses and loss of cognitive performance and, combined with the previous pharmacological approach to inhibit C5aR1 signaling, support the potential of this receptor as a novel therapeutic target for AD in humans.

Intravenous infusion of monocytes isolated from 2-week-old mice enhances clearance of Beta-amyloid plaques in an Alzheimer mouse model.

Alzheimer's disease (AD) is characterized by the deposition of ß-amyloid (Aß) senile plaques and tau-associated neurofibrillary tangles. Other disease features include neuroinflammation and cholinergic neurodegeneration, indicating their possible importance in disease propagation. Recent studies have shown that monocytic cells can migrate into the AD brain toward Aß plaques and reduce plaque burden. The purpose of this study was to evaluate whether the administration of intravenous infusions of 'young' CD11b-positive (+) monocytes into an AD mouse model can enhance Aß plaque clearance and attenuate cognitive deficits. Peripheral monocytes were isolated from two-week-old wildtype mice using the Pluriselect CD11b+ isolation method and characterized by FACS analysis for surface marker expression and effective phagocytosis of 1 µm fluorescent microspheres, FITC-Dextran or FITC-Aß1-42. The isolated monocytes were infused via the tail vein into a transgenic AD mouse model, which expresses the Swedish, Dutch/Iowa APP mutations (APPSwDI). The infusions began when animals reached 5 months of age, when little plaque deposition is apparent and were repeated again at 6 and 7 months of age. At 8 months of age, brains were analyzed for Aß+ plaques, inflammatory processes and microglial (Iba1) activation. Our data show that infusions of two-week-old CD11b+ monocytes into adult APPSwDI mice results in a transient improvement of memory function, a reduction (30%) in Aß plaque load and significantly in small (<20 µm) and large (>40 µm) plaques. In addition, we observe a reduction in Iba1+ cells, as well as no marked elevations in cytokine levels or other indicators of inflammation. Taken together, our findings indicate that young CD11b+ monocytes may serve as therapeutic candidates for improved Aß clearance in AD.

Migration of blood cells to ß-amyloid plaques in Alzheimer's disease.

Alzheimer's disease (AD) is a neurodegenerative disease that leads to the progressive deterioration of cognitive and memory functions. The deposition of extracellular beta-amyloid (Aß) senile plaques and intracellular tau neurofibrillary tangles are considered the cardinal pathological hallmarks of AD, however, accumulating evidence indicates that immune cells may also play an important role in disease pathogenesis. Among these immune cells, blood-derived cells and their infiltration into the CNS towards Aß plaques have been implicated in therapeutic strategies against AD. Here, we review the current literature on blood cell migration into the AD brain and the important players involved in this selective migration towards Aß plaques.

Vascular pathology of 20-month-old hypercholesterolemia mice in comparison to triple-transgenic and APPSwDI Alzheimer's disease mouse models.

Several studies have shown that elevated plasma cholesterol levels (i.e. hypercholesterolemia) serve as a risk factor for late-onset Alzheimer's disease (AD). However, it remains unclear how hypercholesterolemia may contribute to the onset and progression of AD pathology. In order to determine the role of hypercholesterolemia at various stages of AD, we evaluated the effects of high cholesterol diet (5% cholesterol) in wild-type (WT; C57BL6) and triple-transgenic AD (3xTg-AD; Psen1, APPSwe, tauB301L) mice at 7, 14, and 20 months. The transgenic APP-Swedish/Dutch/Iowa AD mouse model (APPSwDI) was used as a control since these animals are more pathologically-accelerated and are known to exhibit extensive plaque deposition and cerebral amyloid angiopathy. Here, we describe the effects of high cholesterol diet on: (1) cognitive function and stress, (2) AD-associated pathologies, (3) neuroinflammation, (4) blood­brain barrier disruption and ventricle size, and (5) vascular dysfunction. Our data show that high dietary cholesterol increases weight, slightly impairs cognitive function, promotes glial cell activation and complement-related pathways, enhances the infiltration of blood-derived proteins and alters vascular integrity, however, it does not induce AD-related pathologies. While normal-fed 3xTg-AD mice display a typical AD-like pathology in addition to severe cognitive impairment and neuroinflammation at 20 months of age, vascular alterations are less pronounced. No microbleedings were seen by MRI, however, the ventricle size was enlarged. Triple-transgenic AD mice, on the other hand, fed a high cholesterol diet do not survive past 14 months of age. Our data indicates that cholesterol does not markedly potentiate AD-related pathology, nor does it cause significant impairments in cognition. However, it appears that high cholesterol diet markedly increases stress-related plasma corticosterone levels as well as some vessel pathologies. Together, our findings represent the first demonstration of prolonged high cholesterol diet and the examination of its effects at various stages of cerebrovascular- and AD-related disease.

Intravenous infusion of nerve growth factor-secreting monocytes supports the survival of cholinergic neurons in the nucleus basalis of Meynert in hypercholesterolemia Brown-Norway rats.

The recruitment of monocytes into the brain has been implicated in Alzheimer's disease and recent studies have indicated that monocytes can reduce amyloid plaque burden. Our previous investigations have shown that hypercholesterolemic rats develop cognitive, cholinergic, and blood-brain barrier dysfunction, but do not develop amyloid plaques. This study was designed to evaluate the effects of repeated intravenous (i.v.) infusion (via the dorsal penile vein) of primary monocytes on cognition, the cholinergic system, and cortical cytokine levels in hypercholesterolemia Brown-Norway rats. In addition, we also transduced the monocytes with nerve growth factor (NGF) to evaluate whether these cells could be used to deliver a neuroprotective agent to the brain. Our results indicate that repeated i.v. infused monocytes migrate into the brains of hypercholesterolemic rats; however, this migration does not translate into marked effects on learning. Animals receiving NGF-loaded monocytes demonstrate slightly improved learning and significantly elevated cholinergic neuron staining compared to treatment with monocytes alone. Furthermore, our data indicate that repeated infusion of monocytes does not lead to elevated cytokine secretion, indicating that no inflammatory response is induced. This study provides an experimental attempt to evaluate the effects of blood-derived primary monocytes in hypercholesterolemia rats.