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Microglia are crucial for maintaining brain health and neuron function. Here, we report that microglia establish connections with neurons using tunneling nanotubes (TNTs) in both physiological and pathological conditions. These TNTs facilitate the rapid exchange of organelles, vesicles, and proteins. In neurodegenerative diseases like Parkinson's and Alzheimer's disease, toxic aggregates of alpha-synuclein (α-syn) and tau accumulate within neurons. Our research demonstrates that microglia use TNTs to extract neurons from these aggregates, restoring neuronal health. Additionally, microglia share their healthy mitochondria with burdened neurons, reducing oxidative stress and normalizing gene expression. Disrupting mitochondrial function with antimycin A before TNT formation eliminates this neuroprotection. Moreover, co-culturing neurons with microglia and promoting TNT formation rescues suppressed neuronal activity caused by α-syn or tau aggregates. Notably, TNT-mediated aggregate transfer is compromised in microglia carrying Lrrk22(Gly2019Ser) or Trem2(T66M) and (R47H) mutations, suggesting a role in the pathology of these gene variants in neurodegenerative diseases., Competing Interests: Declaration of interests E.L. is a co-founder and advisor at IFM Therapeutics, and M.T.H. serves as an advisory board member at IFM Therapeutics, T3D, and Alector., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Activation of the innate immune system following pattern recognition receptor binding has emerged as one of the major pathogenic mechanisms in neurodegenerative disease. Experimental, epidemiological, pathological, and genetic evidence underscores the meaning of innate immune activation during the prodromal as well as clinical phases of several neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and frontotemporal dementia. Importantly, innate immune activation and the subsequent release of inflammatory mediators contribute mechanistically to other hallmarks of neurodegenerative diseases such as aberrant proteostatis, pathological protein aggregation, cytoskeleton abnormalities, altered energy homeostasis, RNA and DNA defects, and synaptic and network disbalance and ultimately to the induction of neuronal cell death. In this review, we discuss common mechanisms of innate immune activation in neurodegeneration, with particular emphasis on the pattern recognition receptors (PRRs) and other receptors involved in the detection of damage-associated molecular patterns (DAMPs)., Competing Interests: Declaration of interests For M.T.H.: scientific advisory boards: Alector Inc, IFM Therapeutics, Muna Therapeutics, Tiaki Inc. Paris Brain Institute, ICVS Braga University, and UK-DRI. Editorial boards: Alzheimer Research & Therapy, ASN Neuro, and Molecular Neurobiology. Other: Lundbeck Foundation Grants and prize panel., (Copyright © 2024 Elsevier Inc. All rights reserved.)

The ApoE-Trem2 pathway links two of the most important genetic risk variants for sporadic Alzheimer's disease. In this issue of Neuron, Gratuze and colleagues 1 report that Trem2 deficiency further aggravates neurodegeneration in tau mutant mice expressing human ApoE4. Together with previous work, this study points to a complex interaction and highlights the need for studying molecular interactions on all human ApoE variants., Competing Interests: Declaration of interests M.T.H. is a science advisory board member for IFM Therapeutics, Alector Inc., Cyclerion Inc., and Muna Therapeutics. He has received honoraria for oral presentations from Biogen, Takeda, NovoNordisk, and AD Immune., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Microglial research has advanced considerably in recent decades yet has been constrained by a rolling series of dichotomies such as "resting versus activated" and "M1 versus M2." This dualistic classification of good or bad microglia is inconsistent with the wide repertoire of microglial states and functions in development, plasticity, aging, and diseases that were elucidated in recent years. New designations continuously arising in an attempt to describe the different microglial states, notably defined using transcriptomics and proteomics, may easily lead to a misleading, although unintentional, coupling of categories and functions. To address these issues, we assembled a group of multidisciplinary experts to discuss our current understanding of microglial states as a dynamic concept and the importance of addressing microglial function. Here, we provide a conceptual framework and recommendations on the use of microglial nomenclature for researchers, reviewers, and editors, which will serve as the foundations for a future white paper., Competing Interests: Declaration of interests B.A. is the shareholder and member of scientific advisory board of Tranquis Therapeutics. K.B. is an employee and shareholder of AbbVie. M.C. receives research support from Vigil, is a member of the scientific advisory board of Vigil, and has a patent on TREM2. S.C. is a recipient of research funding from Eli Lilly and Company. C.C. is a member of the advisory board of Exalys Therapeutics and is the recipient of a research grant from IONIS therapeutics. B.D.S. is occasionally consulting for different companies. He is founding scientist of Augustin TX and of Muna TX. He is also shareholder of Muna TX. C.H. collaborates with Denali Therapeutics. C.H. is chief advisor of ISAR Bioscience and a member of the advisory board of AviadoBio. J.K. is a scientific advisor and collaborator with PureTech. T.M. is a cofounder of REGAIN Therapeutics, owner of a provisional patent on compositions and methods for treatment and/or prophylaxis of proteinopathies, and owner of a provisional patent on preventing or reverting abnormal amyloid deposition. R.M. has scientific collaborations with Alector, Nodthera, and Alchemab and is a consultant for Sanofi. B.M. has received consultancy fees from AstraZeneca. A. Sierra is a recipient of a research grant from Hoffmann La Roche., (Copyright © 2022 Elsevier Inc. All rights reserved.)

There is an urgent need to improve the understanding of neuroinflammation in Alzheimer's disease (AD). We analyzed cerebrospinal fluid inflammatory biomarker correlations to brain structural volume and longitudinal cognitive outcomes in the DELCODE study and in a validation cohort of the F.ACE Alzheimer Center Barcelona. We investigated whether respective biomarker changes are evident before onset of cognitive impairment. YKL-40; sTREM2; sAXL; sTyro3; MIF; complement factors C1q, C4, and H; ferritin; and ApoE protein were elevated in pre-dementia subjects with pathological levels of tau or other neurodegeneration markers, demonstrating tight interactions between inflammation and accumulating neurodegeneration even before onset of symptoms. Intriguingly, higher levels of ApoE and soluble TAM receptors sAXL and sTyro3 were related to larger brain structure and stable cognitive outcome at follow-up. Our findings indicate a protective mechanism relevant for intervention strategies aiming to regulate neuroinflammation in subjects with no or subjective symptoms but underlying AD pathology profile., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

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., Competing Interests: Declaration of interests Michael T. Heneka serves as an advisory board member at IFM Therapeutics, Alector and Tiaki. He received honoraria for oral presentations from Novartis, Roche, and Biogen. The other authors declare that there is no conflict of interest with regard to the experimental part of this study., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Pathological hyperphosphorylation and aggregation of tau (pTau) and neuroinflammation, driven by interleukin-1β (IL-1β), are the major hallmarks of tauopathies. Here, we show that pTau primes and activates IL-1β. First, RNA-sequence analysis suggests paired-helical filaments (PHFs) from human tauopathy brain primes nuclear factor κB (NF-κB), chemokine, and IL-1β signaling clusters in human primary microglia. Treating microglia with pTau-containing neuronal media, exosomes, or PHFs causes IL-1β activation, which is NLRP3, ASC, and caspase-1 dependent. Suppression of pTau or ASC reduces tau pathology and inflammasome activation in rTg4510 and hTau mice, respectively. Although the deletion of MyD88 prevents both IL-1β expression and activation in the hTau mouse model of tauopathy, ASC deficiency in myeloid cells reduces pTau-induced IL-1β activation and improves cognitive function in hTau mice. Finally, pTau burden co-exists with elevated IL-1β and ASC in autopsy brains of human tauopathies. Together, our results suggest pTau activates IL-1β via MyD88- and NLRP3-ASC-dependent pathways in myeloid cells, including microglia., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

Alzheimer's disease is the world's most common neurodegenerative disorder. It is associated with neuroinflammation involving activation of microglia by β-amyloid (Aβ) deposits. Based on previous studies showing apoptosis-associated speck-like protein containing a CARD (ASC) binding and cross-seeding extracellular Aβ, we investigate the propagation of ASC between primary microglia and the effects of ASC-Aβ composites on microglial inflammasomes and function. Indeed, ASC released by a pyroptotic cell can be functionally built into the neighboring microglia NOD-like receptor protein (NLRP3) inflammasome. Compared with protein-only application, exposure to ASC-Aβ composites amplifies the proinflammatory response, resulting in pyroptotic cell death, setting free functional ASC and inducing a feedforward stimulating vicious cycle. Clustering around ASC fibrils also compromises clearance of Aβ by microglia. Together, these data enable a closer look at the turning point from acute to chronic Aβ-related neuroinflammation through formation of ASC-Aβ composites., Competing Interests: Declaration of Interests Michael T. Heneka serves as an advisory board member at IFM Therapeutics, Alector, and Tiaki. He received honoraria for oral presentations from Novartis, Roche, and Biogen. The authors declare that there is no conflict of interest with regard to the experimental part of this study., (Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.)

In this issue of Immunity, Mrdjen et al. (2018) use high-dimensional single-cell proteomics and high parametric mass cytometry to provide insight into the long-lasting issue of how to identify and characterize both resident and recruited leukocyte populations in healthy, aged, and diseased CNS., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Neuroinflammation is a common feature of aging and neurodegeneration. In this issue of Neuron, Mathur et al. (2017) report that the antiviral drug Ganciclovir induces an interferon type I response in microglia through activation of the STING pathway, inhibiting inflammation and leading to protection in a model of multiple sclerosis., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Homeostatic control of brain metabolism is essential for neuronal activity. Jais et al., (2016) report that reduced brain glucose uptake elicited by a high-fat diet self-corrects by the recruitment of peripheral, VEGF-producing macrophages to the blood-brain barrier. Their findings further suggest that restoring brain glucose availability might help protect from cognitive impairment in Alzheimer's disease., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Part of the inflammatory response in Alzheimer's disease (AD) is the upregulation of the inducible nitric oxide synthase (NOS2) resulting in increased NO production. NO contributes to cell signaling by inducing posttranslational protein modifications. Under pathological conditions there is a shift from the signal transducing actions to the formation of protein tyrosine nitration by secondary products like peroxynitrite and nitrogen dioxide. We identified amyloid β (Aβ) as an NO target, which is nitrated at tyrosine 10 (3NTyr(10)-Aβ). Nitration of Aβ accelerated its aggregation and was detected in the core of Aβ plaques of APP/PS1 mice and AD brains. NOS2 deficiency or oral treatment with the NOS2 inhibitor L-NIL strongly decreased 3NTyr(10)-Aβ, overall Aβ deposition and cognitive dysfunction in APP/PS1 mice. Further, injection of 3NTyr(10)-Aβ into the brain of young APP/PS1 mice induced β-amyloidosis. This suggests a disease modifying role for NOS2 in AD and therefore represents a potential therapeutic target., (Copyright © 2011 Elsevier Inc. All rights reserved.)