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.

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.

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.

Ataxin-3 represses transcription via chromatin binding, interaction with histone deacetylase 3, and histone deacetylation.

Ataxin-3 (AT3), the disease protein in spinocerebellar ataxia type 3 (SCA3), has been associated with the ubiquitin-proteasome system and transcriptional regulation. Here we report that normal AT3 binds to target DNA sequences in specific chromatin regions of the matrix metalloproteinase-2 (MMP-2) gene promoter and represses transcription by recruitment of the histone deacetylase 3 (HDAC3), the nuclear receptor corepressor (NCoR), and deacetylation of histones bound to the promoter. Both normal and expanded AT3 physiologically interacted with HDAC3 and NCoR in a SCA3 cell model and human pons tissue; however, normal AT3-containing protein complexes showed increased histone deacetylase activity, whereas expanded AT3-containing complexes had reduced deacetylase activity. Consistently, histone analyses revealed an increased acetylation of total histone H3 in expanded AT3-expressing cells and human SCA3 pons. Expanded AT3 lost the repressor function and displayed altered DNA/chromatin binding that was not associated with recruitment of HDAC3, NCoR, and deacetylation of the promoter, allowing aberrant MMP-2 transcription via the transcription factor GATA-2. For transcriptional repression normal AT3 cooperates with HDAC3 and requires its intact ubiquitin-interacting motifs (UIMs), whereas aberrant transcriptional activation by expanded AT3 is independent of the UIMs but requires the catalytic cysteine of the ubiquitin protease domain. These findings demonstrate that normal AT3 binds target promoter regions and represses transcription of a GATA-2-dependent target gene via formation of histone-deacetylating repressor complexes requiring its UIM-associated function. Expanded AT3 aberrantly activates transcription via its catalytic site and loses the ability to form deacetylating repressor complexes on target chromatin regions.

Gene expression profiling in ataxin-3 expressing cell lines reveals distinct effects of normal and mutant ataxin-3.

Spinocerebellar ataxia type 3 (SCA3) is a late-onset neurodegenerative disorder caused by the expansion of a polyglutamine tract within the gene product, ataxin-3. We have previously shown that mutant ataxin-3 causes upregulation of inflammatory genes in transgenic SCA3 cell lines and human SCA3 pontine neurons. We report here a complex pattern of transcriptional changes by microarray gene expression profiling and Northern blot analysis in a SCA3 cell model. Twenty-three differentially expressed genes involved in inflammatory reactions, nuclear transcription, and cell surface-associated processes were identified. The identified corresponding proteins were analyzed by immunohistochemistry in human disease and control brain tissue to evaluate their implication in SCA3 pathogenesis. In addition to several inflammatory mediators upregulated in mutant ataxin-3 expressing cell lines and pontine neurons of SCA3 patients, we identified a profound repression of genes encoding cell surface-associated proteins in cells overexpressing normal ataxin-3. Correspondingly, these genes were upregulated in mutant ataxin-3 expressing cell lines and in pontine neurons of SCA3 patients. These findings identify for the first time target genes transcriptionally regulated by normal ataxin-3 and support the hypothesis that both loss of normal ataxin-3 and gain of function through protein-protein interacting properties of mutant ataxin-3 contribute to SCA3 pathogenesis.

Transgenic rat model of Huntington's disease.

Huntington's disease (HD) is a late manifesting neurodegenerative disorder in humans caused by an expansion of a CAG trinucleotide repeat of more than 39 units in a gene of unknown function. Several mouse models have been reported which show rapid progression of a phenotype leading to death within 3-5 months (transgenic models) resembling the rare juvenile course of HD (Westphal variant) or which do not present with any symptoms (knock-in mice). Owing to the small size of the brain, mice are not suitable for repetitive in vivo imaging studies. Also, rapid progression of the disease in the transgenic models limits their usefulness for neurotransplantation. We therefore generated a rat model transgenic of HD, which carries a truncated huntingtin cDNA fragment with 51 CAG repeats under control of the native rat huntingtin promoter. This is the first transgenic rat model of a neurodegenerative disorder of the brain. These rats exhibit adult-onset neurological phenotypes with reduced anxiety, cognitive impairments, and slowly progressive motor dysfunction as well as typical histopathological alterations in the form of neuronal nuclear inclusions in the brain. As in HD patients, in vivo imaging demonstrates striatal shrinkage in magnetic resonance images and a reduced brain glucose metabolism in high-resolution fluor-deoxy-glucose positron emission tomography studies. This model allows longitudinal in vivo imaging studies and is therefore ideally suited for the evaluation of novel therapeutic approaches such as neurotransplantation.