Targeting microglia for the treatment of Alzheimer's Disease.

While histological changes in microglia have long been recognized as a pathological feature of Alzheimer's disease (AD), recent genetic association studies have also strongly implicated microglia in the etiology of the disease. Coding and noncoding polymorphisms in several genes expressed in microglia-including APOE, TREM2, CD33, GRN, and IL1RAP-alter AD risk, and therefore could be considered as entry points for therapeutic intervention. Furthermore, microglia may have a substantial effect on current amyloid ß (Aß) and tau immunotherapy approaches, since they are the primary cell type in the brain to mediate Fc receptor-facilitated antibody effector function. In this review, we discuss the considerations in selecting microglial therapeutic targets from the perspective of drug discovery feasibility, and consider the role of microglia in ongoing immunotherapy clinical strategies. GLIA 2016;64:1710-1732.

Next generation transcriptomics and genomics elucidate biological complexity of microglia in health and disease.

Genome-wide expression profiling technology has resulted in detailed transcriptome data for a wide range of tissues, conditions and diseases. In neuroscience, expression datasets were mostly generated using whole brain tissue samples, resulting in data from a mixture of cell types, including glial cells and neurons. Over the past few years, a rapidly increasing number of expression profiling studies using isolated microglial cell populations have been reported. In these studies, the microglia transcriptome was compared to other cell types, such as other brain cells and peripheral tissue macrophages, and related to aging and neurodegenerative conditions. A commonality found in many of these studies was that microglia possess distinct gene expression signatures. This repertoire of selectively-expressed microglial genes highlight functions beyond immune responses, such as synaptic modulation and neurotrophic support, and open up avenues to explore as-yet-unexpected roles. These data provide improved understanding of disease pathology, and complement not only the aforementioned whole brain tissue transcriptome studies, but also genome- and epigenome-wide association studies. In this review, insights obtained from isolated microglia transcriptome studies are presented, and compared to studies using other genome-wide approaches. The relation of microglia to other tissue macrophages and glial cell populations, as well as the role of microglia in the aging brain and in neurodegenerative conditions, will be discussed. Many more of these types of studies are expected in the near future, hopefully leading to the identification of novel genes and targets for neurodegenerative conditions.

Critical data-based re-evaluation of minocycline as a putative specific microglia inhibitor.

Minocycline, a second generation broad-spectrum antibiotic, has been frequently postulated to be a "microglia inhibitor." A considerable number of publications have used minocycline as a tool and concluded, after achieving a pharmacological effect, that the effect must be due to "inhibition" of microglia. It is, however, unclear how this "inhibition" is achieved at the molecular and cellular levels. Here, we weigh the evidence whether minocycline is indeed a bona fide microglia inhibitor and discuss how data generated with minocycline should be interpreted. GLIA 2016;64:1788-1794.

Type I interferon regulation by USP18 is a key vulnerability in cancer.

Precise regulation of Type I interferon signaling is crucial for combating infection and cancer while avoiding autoimmunity. Type I interferon signaling is negatively regulated by USP18. USP18 cleaves ISG15, an interferon-induced ubiquitin-like modification, via its canonical catalytic function, and inhibits Type I interferon receptor activity through its scaffold role. USP18 loss-of-function dramatically impacts immune regulation, pathogen susceptibility, and tumor growth. However, prior studies have reached conflicting conclusions regarding the relative importance of catalytic versus scaffold function. Here, we develop biochemical and cellular methods to systematically define the physiological role of USP18. By comparing a patient-derived mutation impairing scaffold function (I60N) to a mutation disrupting catalytic activity (C64S), we demonstrate that scaffold function is critical for cancer cell vulnerability to Type I interferon. Surprisingly, we discovered that human USP18 exhibits minimal catalytic activity, in stark contrast to mouse USP18. These findings resolve human USP18's mechanism-of-action and enable USP18-targeted therapeutics.

Blocking IL-1 signaling rescues cognition, attenuates tau pathology, and restores neuronal ß-catenin pathway function in an Alzheimer's disease model.

Inflammation is a key pathological hallmark of Alzheimer's disease (AD), although its impact on disease progression and neurodegeneration remains an area of active investigation. Among numerous inflammatory cytokines associated with AD, IL-1ß in particular has been implicated in playing a pathogenic role. In this study, we sought to investigate whether inhibition of IL-1ß signaling provides disease-modifying benefits in an AD mouse model and, if so, by what molecular mechanisms. We report that chronic dosing of 3xTg-AD mice with an IL-1R blocking Ab significantly alters brain inflammatory responses, alleviates cognitive deficits, markedly attenuates tau pathology, and partly reduces certain fibrillar and oligomeric forms of amyloid-ß. Alterations in inflammatory responses correspond to reduced NF-κB activity. Furthermore, inhibition of IL-1 signaling reduces the activity of several tau kinases in the brain, including cdk5/p25, GSK-3ß, and p38-MAPK, and also reduces phosphorylated tau levels. We also detected a reduction in the astrocyte-derived cytokine, S100B, and in the extent of neuronal Wnt/ß-catenin signaling in 3xTg-AD brains, and provided in vitro evidence that these changes may, in part, provide a mechanistic link between IL-1 signaling and GSK-3ß activation. Taken together, our results suggest that the IL-1 signaling cascade may be involved in one of the key disease mechanisms for AD.

Subchronic treatment with aldosterone induces depression-like behaviours and gene expression changes relevant to major depressive disorder.

The potential role of aldosterone in the pathophysiology of depression is unclear. The aim of this study was to test the hypothesis that prolonged elevation of circulating aldosterone induces depression-like behaviour accompanied by disease-relevant changes in gene expression in the hippocampus. Subchronic (2-wk) treatment with aldosterone (2 µg/100 g body weight per day) or vehicle via subcutaneous osmotic minipumps was used to induce hyperaldosteronism in male rats. All rats (n = 20/treatment group) underwent a modified sucrose preference test. Half of the animals from each treatment group were exposed to the forced swim test (FST), which served both as a tool to assess depression-like behaviour and as a stress stimulus. Affymetrix microarray analysis was used to screen the entire rat genome for gene expression changes in the hippocampus. Aldosterone treatment induced an anhedonic state manifested by decreased sucrose preference. In the FST, depressogenic action of aldosterone was manifested by decreased latency to immobility and increased time spent immobile. Aldosterone treatment resulted in transcriptional changes of genes in the hippocampus involved in inflammation, glutamatergic activity, and synaptic and neuritic remodelling. Furthermore, aldosterone-regulated genes substantially overlapped with genes affected by stress in the FST. This study demonstrates the existence of a causal relationship between the hyperaldosteronism and depressive behaviour. In addition, aldosterone treatment induced changes in gene expression that may be relevant to the aetiology of major depressive disorder. Subchronic treatment with aldosterone represents a new animal model of depression, which may contribute to the development of novel targets for the treatment of depression.

Induction of the phase II detoxification pathway suppresses neuron loss in Drosophila models of Parkinson's disease.

Alpha-synuclein aggregates are a common feature of sporadic Parkinson's disease (PD), and mutations that increase alpha-synuclein abundance confer rare heritable forms of PD. Although these findings suggest that alpha-synuclein plays a central role in the pathogenesis of this disorder, little is known of the mechanism by which alpha-synuclein promotes neuron loss or the factors that regulate alpha-synuclein toxicity. To address these matters, we tested candidate modifiers of alpha-synuclein toxicity using a Drosophila model of PD. In the current work, we focused on phase II detoxification enzymes involved in glutathione metabolism. We find that the neuronal death accompanying alpha-synuclein expression in Drosophila is enhanced by loss-of-function mutations in genes that promote glutathione synthesis and glutathione conjugation. This neuronal loss can be overcome by genetic or pharmacological interventions that increase glutathione synthesis or glutathione conjugation activity. Moreover, these same pharmacological agents suppress neuron loss in Drosophila parkin mutants, a loss-of-function model of PD. Our results suggest that oxidative stress is a feature of alpha-synuclein toxicity and that induction of the phase II detoxification pathway represents a potential preventative therapy for PD.

Drosophila DJ-1 mutants are selectively sensitive to environmental toxins associated with Parkinson's disease.

Parkinson's disease (PD) is a common neurodegenerative disorder that displays both sporadic and inherited forms. Exposure to several common environmental toxins acting through oxidative stress has been shown to be associated with PD. One recently identified inherited PD gene, DJ-1, may have a role in protection from oxidative stress, thus potentially linking a genetic cause with critical environmental risk factors. To develop an animal model that would allow integrative study of genetic and environmental influences, we have generated Drosophila lacking DJ-1 function. Fly DJ-1 homologs exhibit differential expression: DJ-1beta is ubiquitous, while DJ-1alpha is predominantly expressed in the male germline. DJ-1alpha and DJ-1beta double knockout flies are viable, fertile, and have a normal lifespan; however, they display a striking selective sensitivity to those environmental agents, including paraquat and rotenone, linked to PD in humans. This sensitivity results primarily from loss of DJ-1beta protein, which also becomes modified upon oxidative stress. These studies demonstrate that fly DJ-1 activity is selectively involved in protection from environmental oxidative insult in vivo and that the DJ-1beta protein is biochemically responsive to oxidative stress. Study of these flies will provide insight into the critical interplay of genetics and environment in PD.

Transcriptomic analysis of purified human cortical microglia reveals age-associated changes.

Microglia are essential for CNS homeostasis and innate neuroimmune function, and play important roles in neurodegeneration and brain aging. Here we present gene expression profiles of purified microglia isolated at autopsy from the parietal cortex of 39 human subjects with intact cognition. Overall, genes expressed by human microglia were similar to those in mouse, including established microglial genes CX3CR1, P2RY12 and ITGAM (CD11B). However, a number of immune genes, not identified as part of the mouse microglial signature, were abundantly expressed in human microglia, including TLR, Fcγ and SIGLEC receptors, as well as TAL1 and IFI16, regulators of proliferation and cell cycle. Age-associated changes in human microglia were enriched for genes involved in cell adhesion, axonal guidance, cell surface receptor expression and actin (dis)assembly. Limited overlap was observed in microglial genes regulated during aging between mice and humans, indicating that human and mouse microglia age differently.

Drosophila models pioneer a new approach to drug discovery for Parkinson's disease.

Despite the prevalence and severity of Parkinson's disease (PD), little is known about the molecular etiology of this disease, and preventative and disease-modifying therapies remain elusive. Recently, linkage studies have begun to identify single-gene mutations that are responsible for rare, heritable forms of PD, which offer an opportunity to gain insight into the molecular mechanisms of this disorder through the creation and analysis of appropriate animal models. One model system that is tractable for these studies is the fruit fly, Drosophila melanogaster. Analysis of several Drosophila models of PD has revealed some surprising insights into the pathogenesis of PD and begun to highlight potential treatment strategies.

Induction of a common microglia gene expression signature by aging and neurodegenerative conditions: a co-expression meta-analysis.

INTRODUCTION: Microglia are tissue macrophages of the central nervous system that monitor brain homeostasis and react upon neuronal damage and stress. Aging and neurodegeneration induce a hypersensitive, pro-inflammatory phenotype, referred to as primed microglia. To determine the gene expression signature of priming, the transcriptomes of microglia in aging, Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS) mouse models were compared using Weighted Gene Co-expression Network Analysis (WGCNA). RESULTS: A highly consistent consensus transcriptional profile of up-regulated genes was identified, which prominently differed from the acute inflammatory gene network induced by lipopolysaccharide (LPS). Where the acute inflammatory network was significantly enriched for NF-κB signaling, the primed microglia profile contained key features related to phagosome, lysosome, antigen presentation, and AD signaling. In addition, specific signatures for aging, AD, and ALS were identified. CONCLUSION: Microglia priming induces a highly conserved transcriptional signature with aging- and disease-specific aspects.

Tau overexpression impacts a neuroinflammation gene expression network perturbed in Alzheimer's disease.

Filamentous inclusions of the microtubule-associated protein, tau, define a variety of neurodegenerative diseases known as tauopathies, including Alzheimer's disease (AD). To better understand the role of tau-mediated effects on pathophysiology and global central nervous system function, we extensively characterized gene expression, pathology and behavior of the rTg4510 mouse model, which overexpresses a mutant form of human tau that causes Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We found that the most predominantly altered gene expression pathways in rTg4510 mice were in inflammatory processes. These results closely matched the causal immune function and microglial gene-regulatory network recently identified in AD. We identified additional gene expression changes by laser microdissecting specific regions of the hippocampus, which highlighted alterations in neuronal network activity. Expression of inflammatory genes and markers of neuronal activity changed as a function of age in rTg4510 mice and coincided with behavioral deficits. Inflammatory changes were tau-dependent, as they were reversed by suppression of the tau transgene. Our results suggest that the alterations in microglial phenotypes that appear to contribute to the pathogenesis of Alzheimer's disease may be driven by tau dysfunction, in addition to the direct effects of beta-amyloid.

Tau transgenic mice as models for cerebrospinal fluid tau biomarkers.

Levels of tau in cerebrospinal fluid (CSF) are elevated in Alzheimer's disease (AD) patients. It is believed this elevation is related to the tau pathology and neurodegeneration observed in AD, but not all tauopathies have increased CSF tau. There has been little pre-clinical work to investigate mechanisms of increased CSF tau due to the difficulty in collecting CSF samples from mice, the most commonly used pre-clinical models. We developed methods to collect CSF from mice without contamination from tau in brain tissue, which is approximately 50,000 fold more abundant in brain than CSF. Using these methods, we measured CSF tau from 3xTg, Tg4510, and Tau Alone transgenic mice. All three lines of mice showed age-dependent increases in CSF tau. They varied in phenotype from undetectable to severe tau pathology and neurodegeneration, suggesting that degenerating neurons are unlikely to be the only source of pathologic CSF tau. Overall, CSF tau levels mirrored expression levels and changes of tau in the brain, but they did not always correlate exactly. CSF tau was often more sensitive to changes in brain transgene expression and pathology. In addition, we also developed ELISA assays specific to different regions of the tau protein. We used these assays to provide evidence that CSF tau exists as fragments, with little intact C-terminus and partial loss of the N-terminus. Taken together, these assays and mouse models may be used to facilitate a deeper understanding of CSF tau in neurodegenerative disease.

Increased glutathione S-transferase activity rescues dopaminergic neuron loss in a Drosophila model of Parkinson's disease.

Loss-of-function mutations of the parkin gene are a major cause of early-onset parkinsonism. To explore the mechanism by which loss of parkin function results in neurodegeneration, we are using a genetic approach in Drosophila. Here, we show that Drosophila parkin mutants display degeneration of a subset of dopaminergic (DA) neurons in the brain. The neurodegenerative phenotype of parkin mutants is enhanced by loss-of-function mutations of the glutathione S-transferase S1 (GstS1) gene, which were identified in an unbiased genetic screen for genes that modify parkin phenotypes. Furthermore, overexpression of GstS1 in DA neurons suppresses neurodegeneration in parkin mutants. Given the previous evidence for altered glutathione metabolism and oxidative stress in sporadic Parkinson's disease (PD), these data suggest that the mechanism of DA neuron loss in Drosophila parkin mutants is similar to the mechanisms underlying sporadic PD. Moreover, these findings identify a potential therapeutic approach in treating PD.

Activated RIC, a small GTPase, genetically interacts with the Ras pathway and calmodulin during Drosophila development.

The mammalian Rit and Rin proteins, along with the Drosophila homologue RIC, comprise a distinct and evolutionarily conserved subfamily of Ras-related small GTP-binding proteins. Unlike other Ras superfamily members, these proteins lack a signal for prenylation, contain a conserved but distinct effector domain, and, in the case of Rin and RIC, contain calmodulin-binding domains. To address the physiological role of this Ras subfamily in vivo, activated forms of the Drosophila Ric gene were introduced into flies. Expression of activated RIC proteins altered the development of well-characterized adult structures, including wing veins and photoreceptors of the compound eye. The effects of activated RIC could be mitigated by a reduction in dosage of several genes in the Drosophila Ras cascade, including Son of sevenless (Sos), Dsor (MEK), rolled (MAPK), and Ras itself. On the other hand, reduction of calmodulin exacerbated the defects caused by activated RIC, thus providing the first functional evidence for interaction of these molecules. We conclude that the activation of the Ras cascade may be an important in vivo requisite to the transduction of signals through RIC and that the binding of calmodulin to RIC may negatively regulate this small GTPase.