Characterizing microglial senescence: Tau as a key player.

The highest risk factor for the development of neurodegenerative diseases like tauopathies is aging. Many physiological decrements underlying aging are linked to cellular senescence. Senescent cells are characterized by an irreversible growth arrest and formation of a senescence-associated secretory phenotype (SASP), a proinflammatory secretome that modifies the cellular microenvironment and contributes to tissue deterioration. Microglia, the innate immune cells in the brain, can enter a senescent state during aging. In addition, senescent microglia have been identified in the brains of tau-transgenic mice and patients suffering from tauopathies. While the contribution of senescent microglia to the development of tauopathies and other neurodegenerative diseases is a growing area of research, the effect of tau on microglial senescence remains elusive. Here, we exposed primary microglia to 5 and 15 nanomolar (nM) of monomeric tau for 18 h, followed by a recovery period of 48 h. Using multiple senescence markers, we found that exposure to 15 nM, but not 5 nM of tau increased levels of cell cycle arrest and a DNA damage marker, induced loss of the nuclear envelope protein lamin B1 and the histone marker H3K9me3, impaired tau clearance and migration, altered the cell morphology and resulted in formation of a SASP. Taken together, we show that exposure to tau can lead to microglial senescence. As senescent cells were shown to negatively impact tau pathologies, this suggests the presence of a vicious circle, which should be further investigated in the future.

NLRP3-directed antisense oligonucleotides reduce microglial immunoactivities in vitro.

Alzheimer's disease (AD) is associated with the cerebral deposition of Amyloid-ß (Aß) peptide, which leads to NLRP3 inflammasome activation and subsequent release of interleukin-1ß (IL-1ß) and interleukin-18 (IL-18). NLRP3 reduction has been found to increase microglial clearance, protect from synapse loss, and suppress both the changes to synaptic plasticity and spatial memory dysfunction observed in murine AD models. Here, we test whether NLRP3-directed antisense oligonucleotides (ASOs) can be harnessed as immune modulators in primary murine microglia and human THP-1 cells. NLRP3 mRNA degradation was achieved at 72 h of ASO treatment in primary murine microglia. Consequently, NLRP3-directed ASOs significantly reduced the levels of cleaved caspase-1 and mature IL-1ß when microglia were either activated by LPS and nigericin or LPS and Aß. In human THP-1 cells NLRP3-targeted ASOs also significantly reduced the LPS plus nigericin- or LPS plus Aß-induced release of mature IL-1ß. Together, NLRP3-directed ASOs can suppress NLRP3 inflammasome activity and subsequent release of IL-1ß in primary murine microglia and THP-1 cells. ASOs may represent a new and alternative approach to modulate NLRP3 inflammasome activation in neurodegenerative diseases, in addition to attempts to inhibit the complex pharmacologically.

Microglial PD-1 stimulation by astrocytic PD-L1 suppresses neuroinflammation and Alzheimer's disease pathology.

Chronic neuroinflammation is a pathogenic component of Alzheimer's disease (AD) that may limit the ability of the brain to clear amyloid deposits and cellular debris. Tight control of the immune system is therefore key to sustain the ability of the brain to repair itself during homeostasis and disease. The immune-cell checkpoint receptor/ligand pair PD-1/PD-L1, known for their inhibitory immune function, is expressed also in the brain. Here, we report upregulated expression of PD-L1 and PD-1 in astrocytes and microglia, respectively, surrounding amyloid plaques in AD patients and in the APP/PS1 AD mouse model. We observed juxtamembrane shedding of PD-L1 from astrocytes, which may mediate ectodomain signaling to PD-1-expressing microglia. Deletion of microglial PD-1 evoked an inflammatory response and compromised amyloid-ß peptide (Aß) uptake. APP/PS1 mice deficient for PD-1 exhibited increased deposition of Aß, reduced microglial Aß uptake, and decreased expression of the Aß receptor CD36 on microglia. Therefore, ineffective immune regulation by the PD-1/PD-L1 axis contributes to Aß plaque deposition during chronic neuroinflammation in AD.

Microglia jointly degrade fibrillar alpha-synuclein cargo by distribution through tunneling nanotubes.

Microglia are the CNS resident immune cells that react to misfolded proteins through pattern recognition receptor ligation and activation of inflammatory pathways. Here, we studied how microglia handle and cope with α-synuclein (α-syn) fibrils and their clearance. We found that microglia exposed to α-syn establish a cellular network through the formation of F-actin-dependent intercellular connections, which transfer α-syn from overloaded microglia to neighboring naive microglia where the α-syn cargo got rapidly and effectively degraded. Lowering the α-syn burden attenuated the inflammatory profile of microglia and improved their survival. This degradation strategy was compromised in cells carrying the LRRK2 G2019S mutation. We confirmed the intercellular transfer of α-syn assemblies in microglia using organotypic slice cultures, 2-photon microscopy, and neuropathology of patients. Together, these data identify a mechanism by which microglia create an "on-demand" functional network in order to improve pathogenic α-syn clearance.

Microglial NLRP3 Inflammasome Activation upon TLR2 and TLR5 Ligation by Distinct α-Synuclein Assemblies.

Parkinson's disease (PD) is the second most common age-related neurodegenerative disorder and is characterized by the formation of cellular inclusions inside neurons that are rich in an abnormal form of the protein α-synuclein (α-syn). Microglia are the CNS resident immune cells that react to misfolded proteins through pattern recognition receptor ligation and activation of signaling transduction pathways. Here, we studied activation of primary microglia isolated from wild-type mouse by distinct α-syn forms and their clearance. Internalization of α-syn monomers and oligomers efficiently activated the NOD-like receptor pyrin domain containing 3 (NLRP3) inflammasome via TLR2 and TLR5 ligation, thereby acting on different signaling checkpoints. We found that primary microglia effectively engulf α-syn but hesitate in its degradation. NLRP3 inhibition by the selective inhibitor CRID3 sodium salt and NLRP3 deficiency improved the overall clearance of α-syn oligomers. Together, these data show that distinct α-syn forms exert different microglial NLRP3 inflammasome activation properties, thereby compromising its degradation, which can be prevented by NLRP3 inhibition.

Inhibition of formyl peptide receptors improves the outcome in a mouse model of Alzheimer disease.

BACKGROUND: An important hallmark of Alzheimer's disease (AD) is the increase of Aß1-42 burden and its accumulation to senile plaques, leading the reactive gliosis and neurodegeneration. The modulation of glia cell function represents an attractive therapeutic strategy, but is currently limited by an incomplete understanding of its relevance for AD. The chemotactic G-protein coupled formyl peptide receptor (FPR), which is known to modulate Aß1-42 uptake and signal transduction, might be one candidate molecule regulating glia function in AD. Here, we investigate whether the modulation of FPR exerts beneficial effects in an AD preclinical model. METHODS: To address this question, APP/PS1 double-transgenic AD mice were treated for 20 weeks with either the pro-inflammatory FPR agonist fMLF, the FPR1/2 antagonist Boc2 or the anti-inflammatory FPR2 agonist Ac2-26. Spatial learning and memory were evaluated using a Morris water maze test. Immunohistological staining, gene expression studies, and flow cytometry analyses were performed to study neuronal loss, gliosis, and Aß-load in the hippocampus and cortex, respectively. RESULTS: FPR antagonism by Boc2-treatment significantly improved spatial memory performance, reduced neuronal pathology, induced the expression of homeostatic growth factors, and ameliorated microglia, but not astrocyte, reactivity. Furthermore, the elevated levels of amyloid plaques in the hippocampus were reduced by Boc2-treatment, presumably by an induction of amyloid degradation. CONCLUSIONS: We suggest that the modulation of FPR signaling cascades might be considered as a promising therapeutic approach for alleviating the cognitive deficits associated with early AD. Additional studies are now needed to address the downstream effectors as well as the safety profile of Boc2.

NLRP3 inflammasome activation drives tau pathology.

Alzheimer's disease is characterized by the accumulation of amyloid-beta in plaques, aggregation of hyperphosphorylated tau in neurofibrillary tangles and neuroinflammation, together resulting in neurodegeneration and cognitive decline1. The NLRP3 inflammasome assembles inside of microglia on activation, leading to increased cleavage and activity of caspase-1 and downstream interleukin-1ß release2. Although the NLRP3 inflammasome has been shown to be essential for the development and progression of amyloid-beta pathology in mice3, the precise effect on tau pathology remains unknown. Here we show that loss of NLRP3 inflammasome function reduced tau hyperphosphorylation and aggregation by regulating tau kinases and phosphatases. Tau activated the NLRP3 inflammasome and intracerebral injection of fibrillar amyloid-beta-containing brain homogenates induced tau pathology in an NLRP3-dependent manner. These data identify an important role of microglia and NLRP3 inflammasome activation in the pathogenesis of tauopathies and support the amyloid-cascade hypothesis in Alzheimer's disease, demonstrating that neurofibrillary tangles develop downstream of amyloid-beta-induced microglial activation.

Microglia-derived ASC specks cross-seed amyloid-ß in Alzheimer's disease.

The spreading of pathology within and between brain areas is a hallmark of neurodegenerative disorders. In patients with Alzheimer's disease, deposition of amyloid-ß is accompanied by activation of the innate immune system and involves inflammasome-dependent formation of ASC specks in microglia. ASC specks released by microglia bind rapidly to amyloid-ß and increase the formation of amyloid-ß oligomers and aggregates, acting as an inflammation-driven cross-seed for amyloid-ß pathology. Here we show that intrahippocampal injection of ASC specks resulted in spreading of amyloid-ß pathology in transgenic double-mutant APPSwePSEN1dE9 mice. By contrast, homogenates from brains of APPSwePSEN1dE9 mice failed to induce seeding and spreading of amyloid-ß pathology in ASC-deficient APPSwePSEN1dE9 mice. Moreover, co-application of an anti-ASC antibody blocked the increase in amyloid-ß pathology in APPSwePSEN1dE9 mice. These findings support the concept that inflammasome activation is connected to seeding and spreading of amyloid-ß pathology in patients with Alzheimer's disease.

NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice.

Alzheimer's disease is the world's most common dementing illness. Deposition of amyloid-ß peptide drives cerebral neuroinflammation by activating microglia. Indeed, amyloid-ß activation of the NLRP3 inflammasome in microglia is fundamental for interleukin-1ß maturation and subsequent inflammatory events. However, it remains unknown whether NLRP3 activation contributes to Alzheimer's disease in vivo. Here we demonstrate strongly enhanced active caspase-1 expression in human mild cognitive impairment and brains with Alzheimer's disease, suggesting a role for the inflammasome in this neurodegenerative disease. Nlrp3(-/-) or Casp1(-/-) mice carrying mutations associated with familial Alzheimer's disease were largely protected from loss of spatial memory and other sequelae associated with Alzheimer's disease, and demonstrated reduced brain caspase-1 and interleukin-1ß activation as well as enhanced amyloid-ß clearance. Furthermore, NLRP3 inflammasome deficiency skewed microglial cells to an M2 phenotype and resulted in the decreased deposition of amyloid-ß in the APP/PS1 model of Alzheimer's disease. These results show an important role for the NLRP3/caspase-1 axis in the pathogenesis of Alzheimer's disease, and suggest that NLRP3 inflammasome inhibition represents a new therapeutic intervention for the disease.

PPARγ/RXRα-induced and CD36-mediated microglial amyloid-ß phagocytosis results in cognitive improvement in amyloid precursor protein/presenilin 1 mice.

Alzheimer's disease (AD) is characterized by the extracellular deposition of amyloid-ß (Aß), neurofibrillary tangle formation, and a microglial-driven inflammatory response. Chronic inflammatory activation compromises microglial clearance functions. Because peroxisome proliferator-activated receptor γ (PPARγ) agonists suppress inflammatory gene expression, we tested whether activation of PPARγ would also result in improved microglial Aß phagocytosis. The PPARγ agonist pioglitazone and a novel selective PPARα/γ modulator, DSP-8658, currently in clinical development for the treatment of type 2 diabetes, enhanced the microglial uptake of Aß in a PPARγ-dependent manner. This PPARγ-stimulated increase of Aß phagocytosis was mediated by the upregulation of scavenger receptor CD36 expression. In addition, combined treatment with agonists for the heterodimeric binding partners of PPARγ, the retinoid X receptors (RXRs), showed additive enhancement of the Aß uptake that was mediated by RXRα activation. Evaluation of DSP-8658 in the amyloid precursor protein/presenilin 1 mouse model confirmed an increased microglial Aß phagocytosis in vivo, which subsequently resulted in a reduction of cortical and hippocampal Aß levels. Furthermore, DSP-8658-treated mice showed improved spatial memory performance. Therefore, stimulation of microglial clearance by simultaneous activation of the PPARγ/RXRα heterodimer may prove beneficial in prevention of AD.

Mrp14 deficiency ameliorates amyloid ß burden by increasing microglial phagocytosis and modulation of amyloid precursor protein processing.

Neuroinflammation plays a fundamental role in the pathogenesis of Alzheimer's disease (AD), resulting in the extensive activation of microglial and astroglial cells. Here we describe the role of myeloid-related protein Mrp14, a recently described amplifier of inflammation, in Alzheimer's disease and in the related amyloid precursor protein/presenilin1 (APP/PS1) mouse model. Detection of Mrp14 in control, mildly cognitive impaired, and AD patients revealed a strong induction of Mrp14 in protein extracts as well as in the cerebrospinal fluid, but not in blood plasma. In APP/PS1 mice, Mrp14 and its heterodimeric partner Mrp8 was found to be upregulated in microglial cells surrounding amyloid plaques. Functionally, loss of Mrp14 led to increased phagocytosis of fibrillar amyloid ß (Aß) in microglia cells in vitro and in vivo. Generating APP/PS1-transgenic mice deficient for Mrp14, we observed a decrease of key cytokines involved in APP processing, a reduction of BACE1 expression and activity, and consequently overall Aß deposition. We therefore conclude that Mrp14 promotes APP processing and Aß accumulation under neuroinflammatory conditions.