The role of microglia/ macrophages during neuroinflammation and neurodegenerative diseases remains controversial. To date, at least two activations states have been suggested consisting of a classical response (M1) and the alternative response (M2). Identifying selective biomarkers of microglia that representative their functional activation states may help elucidate disease course and understand repair mechanisms. Two cocktails containing either TNF-α, IL-12, and IL-1β (referred to as CKT-1) or IL-13 and IL-4 (referred to CKT-2) were injections into the hippocampus of mice aged 6, 12, or 24 months. Microarray analysis was performed on hippocampal tissue 3 days post injection. Gene transcripts were compared between CKT-1 versus CKT-2 stimulator cocktails. Several selective transcripts expressed for the CKT-1 included CXCL13, haptoglobin, MARCO, and calgranulin B, while a smaller subset of genes was selectively induced by the CKT-2 and consisted of FIZZ1, IGF-1, and EAR 11. Importantly, selective transcripts were induced at all ages by CKT-1, whereas selective gene transcripts induced by CKT-2 decreased with age suggesting an age-related reduction in the IL-4/ IL-13 signaling pathway.

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Tau accumulation remains one of the closest correlates of neuronal loss in Alzheimer's disease. In addition, tau associates with several other neurodegenerative diseases, collectively known as tauopathies, in which clinical phenotypes manifest as cognitive impairment, behavioral disturbances, and motor impairment. Polyamines act as bivalent regulators of cellular function and are involved in numerous biological processes. The regulation of the polyamines system can become dysfunctional during disease states. Arginase 1 (Arg1) and nitric oxide synthases compete for L-arginine to produce either polyamines or nitric oxide, respectively. Herein, we show that overexpression of Arg1 using adeno-associated virus (AAV) in the CNS of rTg4510 tau transgenic mice significantly reduced phospho-tau species and tangle pathology. Sustained Arg1 overexpression decreased several kinases capable of phosphorylating tau, decreased inflammation, and modulated changes in the mammalian target of rapamycin and related proteins, suggesting activation of autophagy. Arg1 overexpression also mitigated hippocampal atrophy in tau transgenic mice. Conversely, conditional deletion of Arg1 in myeloid cells resulted in increased tau accumulation relative to Arg1-sufficient mice after transduction with a recombinant AAV-tau construct. These data suggest that Arg1 and the polyamine pathway may offer novel therapeutic targets for tauopathies.

Background Tau stabilizes microtubules; however, in Alzheimer’s disease (AD) and tauopathies, tau becomes hyperphosphorylated, aggregates, and results in neuronal death. Our group recently uncovered a unique interaction between polyamine metabolism and tau fate. Polyamines exert an array of physiological effects that support neuronal function and cognitive processing. Specific stimuli can elicit a polyamine stress response (PSR), resulting in altered central polyamine homeostasis. Evidence suggests that elevations in polyamines following a short-term stressor are beneficial; however, persistent stress and subsequent PSR activation may lead to maladaptive polyamine dysregulation, which is observed in AD, and may contribute to neuropathology and disease progression. Methods Male and female mice harboring tau P301L mutation (rTg4510) were examined for a tau-induced central polyamine stress response (tau-PSR). The direct effect of tau-PSR byproducts on tau fibrillization and oligomerization were measured using a thioflavin T assay and a N2a split superfolder GFP-Tau (N2a-ssGT) cell line, respectively. To therapeutically target the tau-PSR, we bilaterally injected caspase 3-cleaved tau truncated at aspartate 421 (AAV9 Tau ΔD421) into the hippocampus and cortex of spermidine/spermine- N 1 -acetyltransferase (SSAT), a key regulator of the tau-PSR, knock out (SSAT-/-), and wild type littermates, and the effects on tau neuropathology, polyamine dysregulation, and behavior were measured. Lastly, cellular models were employed to further examine how SSAT repression impacted tau biology. Results Tau induced a unique tau-PSR signature in rTg4510 mice, notably in the accumulation of acetylated spermidine. In vitro, higher-order polyamines prevented tau fibrillization but acetylated spermidine failed to mimic this effect and even promoted fibrillization and oligomerization. AAV9 Tau ΔD421 also elicited a unique tau-PSR in vivo, and targeted disruption of SSAT prevented the accumulation of acetylated polyamines and impacted several tau phospho-epitopes. Interestingly, SSAT knockout mice presented with altered behavior in the rotarod task, the elevated plus maze, and marble burying task, thus highlighting the impact of polyamine homeostasis within the brain. Conclusion These data represent a novel paradigm linking tau pathology and polyamine dysfunction and that targeting specific arms within the polyamine pathway may serve as new targets to mitigate certain components of the tau phenotype. Electronic supplementary material The online version of this article (10.1186/s13195-019-0507-y) contains supplementary material, which is available to authorized users.

The inflammation hypothesis of Alzheimer’s pathogenesis has directed much scientific effort towards ameliorating this disease. The development of mouse models of amyloid deposition permitted direct tests of the proposal that amyloid-activated microglia could cause neurodegeneration in vivo. Many approaches to manipulating microglial activation have been applied to these mouse models, and are the subject of this review. In general, these results do not support a direct neuricidal action of microglia in mouse amyloid models under any activation state. Some of the manipulations cause both a reduction in pathology, and a reduction in microglial activation. However, at least for agents like ibuprofen, this outcome may result from a direct action on amyloid production, and a reduction in the microglial provoking amyloid deposits, rather than from reduced microglial activation leading to a decline in amyloid deposition. Instead, a surprising number of the experimental manipulations which increase microglial activation lead to enhanced clearance of the amyloid deposits. Both the literature and new data presented here suggest that either classical or alternative activation of microglia can lead to enhanced amyloid clearance. However, a limited number of studies comparing the same treatments in amyloid-depositing vs tau-depositing mice find the opposite effects. Treatments that benefit amyloid pathology accelerate tau pathology. This observation argues strongly that potential treatments be tested for impact on both amyloid and tau pathology before consideration of testing in humans.

Table 1). Transcripts reflecting altered arginine metabolism included increased ASS1; ASL1, which promotes arginine synthesis and NOS2; and ARG1, which depletes arginine to produce nitric oxide and ornithine. Transcripts associated with polyamine signaling included increased prosynthetic enzymes SMS and ODC1 and catabolic SAT1, SMOX, and PAOX enzymes, signifying dysregulation along the entire cycle of the polyamine axis. PMF1 also increased, which associates with increase SAT1 transcription together with NRF2 (16). Positive indirect regulators of polyamine production consisted of increased AZIN1, AZIN2, and AGMAT, whereas negative regulators consisted of OAZ1, OAZ3, OAT1, and OTC. Interestingly, AD brains showed elevated vesicular polyamine transporter SLC18B1 (17). Amongst the NMDA receptors, only the glutamate ionotropic receptor NMDA type subunit 2B (GRIN2B) increased. Most notably, of all the gene transcripts, the greatest change was AZIN2 (fold change = 2.4341, P = 0.000048).AZIN2 protein is increased in AD brains. We measured protein in the human postmortem AD brain for tau and AZIN2 levels to extend our transcriptome findings. Using a cluster analysis based on paired helical filament (PHF) phospho-tau (AT8) in AD, we observed significant increases in high molecular weight (HMW) tau, phospho-tau (AT8, PHF1), and AZIN2 in AD brains that were positive for AT8 staining (AT8 + ; Figure 2, A-F). These data further support the correlation between tau neuropathology and dysregulation of polyamine metabolism regarding elevated AZIN2.AZIN2 overexpression precipitates tau-dependent alterations in anxiety and cognition. To determine how chronic polyamine activation (sustained PSR) impacted affect and cognition during tau neuropathology, we overexpressed AZIN2 via adeno-associated viral constructs (AAV9-AZIN2) or an empty capsid plasmid in the cortex and hippocampus of nTg and tau PS19 mice (Figure 3A). Immunohistochemistry confirmed equal exogenous AZIN2 HA-tagged viral expression and AZIN2 in the cortex and hippocampus of nTg and PS19 mice, ensuring similar expression across groups (Figure 3, B and C). Although tau (HT7) remained unchanged after increased AZIN2 (Figure 3D), phospho-tau (AT8) increased in the cortex, hippocampus (CA3), and dentate gyrus (DG) in response to AZIN2 overexpression in PS19 mice (Figure 3E). PS19 mice showed an increase in microglia activation in the hippocampus's DG relative to nTg controls (Figure 3F) and independent of AZIN2. These data suggest that chronic polyamine activation may facilitate tau neuropathology.AZIN2 overexpression induced tau-dependent alterations in affective and cognitive processing (Figure 4, A-F). Overall, locomotor activity was comparable across all groups in terms of the total distance traveled (Figure 4A); however, AZIN2 overexpres-

Alzheimer’s disease (AD) includes several hallmarks comprised of amyloid-β (Aβ) deposition, tau neuropathology, inflammation, and memory impairment. Brain metabolism becomes uncoupled due to aging and other AD risk factors, which ultimately lead to impaired protein clearance and aggregation. Increasing evidence indicates a role of arginine metabolism in AD, where arginases are key enzymes in neurons and glia capable of depleting arginine and producing ornithine and polyamines. However, currently, it remains unknown if the reduction of arginase 1 (Arg1) in myeloid cell impacts amyloidosis. Herein, we produced haploinsufficiency of Arg1 by the hemizygous deletion in myeloid cells using Arg1fl/fl and LysMcreTg/+ mice crossed with APP Tg2576 mice. Our data indicated that Arg1 haploinsufficiency promoted Aβ deposition, exacerbated some behavioral impairment, and decreased components of Ragulator-Rag complex involved in mechanistic target of rapamycin complex 1 (mTORC1) signaling and autophagy. Additionally, Arg1 repression and arginine supplementation both impaired microglial phagocytosis in vitro. These data suggest that proper function of Arg1 and arginine metabolism in myeloid cells remains essential to restrict amyloidosis.

Tauopathies consist of intracellular accumulation of hyperphosphorylated and aggregated microtubule protein tau, which remains a histopathological feature of Alzheimer’s disease (AD) and frontotemporal dementia. l-Arginine is a semi-essential amino acid with a number of bioactive molecules. Its downstream metabolites putrescine, spermidine, and spermine (polyamines) are critically involved in microtubule assembly and stabilization. Recent evidence implicates altered arginine metabolism in the pathogenesis of AD. Using high-performance liquid chromatographic and mass spectrometric assays, the present study systematically determined the tissue concentrations of l-arginine and its nine downstream metabolites in the frontal cortex, hippocampus, parahippocampal region, striatum, thalamus, and cerebellum in male PS19 mice-bearing human tau P301S mutation at 4, 8, and 12–14 months of age. As compared to their wild-type littermates, PS19 mice displayed early and/or prolonged increases in L-ornithine and altered polyamine levels with age. There were also genotype- and age-related changes in L-arginine, L-citrulline, glutamine, glutamate, and γ-aminobutyric acid in a region-and/or chemical-specific manner. The results demonstrate altered brain arginine metabolism in PS19 mice with the most striking changes in L-ornithine, polyamines, and glutamate, indicating a shift of L-arginine metabolism to favor the arginase–polyamine pathway. Given the role of polyamines in maintaining microtubule stability, the functional significance of these changes remains to be explored in future research.