An overview on microglial origin, distribution, and phenotype in Alzheimer's disease

Macrophage Lyn Kinase Is a Sex-Specific Regulator of Post-Subarachnoid Hemorrhage Neuroinflammation

BACKGROUND

Lyn kinase is a member of the Src family of tyrosine kinases, primarily known for its role in regulating immune cell signaling. It can phosphorylate and modulate the activity of various proteins involved in immune responses, including Toll-like receptor 4 (TLR4). TLR4-mediated inflammatory pathways have been extensively studied; however, the sex-specific interaction of TLR4 and Lyn in neuroinflammation after aneurysmal subarachnoid hemorrhage (SAH) has yet to be investigated. SAH occurs due to a ruptured aneurysm, and the consequences often lead to neuroinflammation and cognitive impairments. In our study, we investigated the sex-specific involvement of Lyn kinase in regulating TLR4 signaling to understand the TLR4-mediated inflammatory response after SAH.

METHODS

Cell-specific Lyn knockout mice of both sexes were used for this study. Wild-type and conditional knockout mouse brains were analyzed by multicolor flow cytometry, immunohistochemistry, and western blotting at postoperative day 7 following SAH surgery. An unbiased spatial transcriptomic analysis was performed with the frozen mouse brain tissues. A 3-dimensional brain stroke model and cerebrospinal fluid samples of patients with SAH were also used for this study.

RESULTS

Our overall animal and patient data from flow cytometry, immunohistochemistry, western blot, cognitive function tests, and spatial transcriptomic data revealed that Lyn kinase is a sex-specific regulator in inflammatory cytokine production, red blood cell phagocytosis, neuronal apoptosis, and cognitive function, as well as a negative regulator of TLR4 signaling pathways.

CONCLUSIONS

Our results highlight sex-specific modulation of Lyn kinase activity in TLR4 signaling after hemorrhagic stroke and indicate that successful treatment of neuroinflammation may require sex-specific treatments.

Microglial TLR4-Lyn kinase is a critical regulator of neuroinflammation, Aβ phagocytosis, neuronal damage, and cell survival in Alzheimer's disease

Disease-Associated Microglia (DAM) are a focus in Alzheimer's disease (AD) research due to their central involvement in the response to amyloid-beta plaques. Microglial Toll-like receptor 4 (TLR4) is instrumental in the binding of fibrillary amyloid proteins, while Lyn kinase (Lyn) is a member of the Src family of non-receptor tyrosine kinases involved in immune signaling. Lyn is a novel, non-canonical, intracellular adaptor with diverse roles in cell-specific signaling which directly binds to TLR4 to modify its function. Lyn can be activated in response to TLR4 stimulation, leading to phosphorylation of various substrates and modulation of inflammatory and phagocytosis signaling pathways. Here, we investigated the TLR4-Lyn interaction in neuroinflammation using WT, 5XFAD, and 5XFAD x Lyn-/- mouse models by western blotting (WB), co-immunoprecipitation (co-IP), immunohistochemistry (IHC) and flow cytometric (FC) analysis. A spatial transcriptomic analysis of microglia in WT, 5XFAD, and 5XFAD x Lyn-/- mice revealed essential genes involved in neuroinflammation, Aβ phagocytosis, and neuronal damage. Finally, we explored the effects of a synthetic, TLR4-Lyn modulator protein (TLIM) through an in vitro AD model using primary murine microglia. Our WB, co-IP, IHC, and FC data show an increased, novel, direct protein-protein interaction between TLR4 and Lyn kinase in the brains of 5XFAD mice compared to WT. Furthermore, in the absence of Lyn (5XFAD x Lyn-/- mice); increased expression of protective Syk kinase was observed, enhanced microglial Aβ phagocytosis, increased astrocyte activity, decreased neuronal dystrophy, and a further increase in the cell survival signaling and protective DAM population was noted. The DAM population in 5XFAD mice which produce more inflammatory cytokines and phagocytose more Aβ were observed to express greater levels of TLR4 and Lyn. Pathway analysis comparison between WT, 5XFAD, and 5XFAD x Lyn-/- mice supported these findings via our microglial spatial transcriptomic analysis. Finally, we created an in vitro co-culture system with primary murine microglial and primary murine hippocampal cells exposed to Aβ as a model of AD. When these co-cultures were treated with our TLR4-Lyn Interaction Modulators (TLIMs), an increase in Aβ phagocytosis and a decrease in neuronal dystrophy was seen. Lyn kinase has a central role in modulating TLR4-induced inflammation and Syk-induced protection in a 5XFAD mouse model. Our TLIMs ameliorate AD sequalae in an in vitro model of AD and could be a promising therapeutic strategy to treat AD.

Gut Microbiota Mediates Neuroinflammation in Alzheimer's Disease: Unraveling Key Factors and Mechanistic Insights

The gut microbiota, the complex community of microorganisms that inhabit the gastrointestinal tract, has emerged as a key player in the pathogenesis of neurodegenerative disorders, including Alzheimer's disease (AD). AD is characterized by progressive cognitive decline and neuronal loss, associated with the accumulation of amyloid-β plaques, neurofibrillary tangles, and neuroinflammation in the brain. Increasing evidence suggests that alterations in the composition and function of the gut microbiota, known as dysbiosis, may contribute to the development and progression of AD by modulating neuroinflammation, a chronic and maladaptive immune response in the central nervous system. This review aims to comprehensively analyze the current role of the gut microbiota in regulating neuroinflammation and glial cell function in AD. Its objective is to deepen our understanding of the pathogenesis of AD and to discuss the potential advantages and challenges of using gut microbiota modulation as a novel approach for the diagnosis, treatment, and prevention of AD.

Gender differences in Alzheimer's may be associated with TLR4-LYN expression in damage associated microglia and neuronal phagocytosis

The role of Aβ plaques and neurofibrillary tangles in Alzheimer's disease (AD) pathogenesis have recently come into question due to failure of many pharmaceutical agents targeting these deposits and detection of these misfolded proteins in normal human brains. Therefore, we investigated correlations between microglial activation and toll like receptor 4 (TLR4) and Lck/Yes novel tyrosine (LYN) kinase signaling in an AD mouse model. In this study, we used 5-6-month-old 5XFAD and wild type (WT) male and female mice. Immunohistochemistry (IHC) and flow cytometry (FC) were performed on their brains. Cognitive performance was assessed with the Barnes-Maze. IHC showed more Ab aggregation in microglia of female 5XFAD mice compared to their male counterparts. Increased co-localization of microglial TLR4 and LYN was also observed in AD more than WT and females more than males. IHC also suggests microglial phagocytosis of neurons in AD mice, which is supported by FC data. Our FC data also support the involvement of disease associated microglia (DAMs) in this process based on cytokine secretion. Cognitive assessment by the Barnes maze showed 5XFAD females performed worse than males. In this study, we investigated the relationship between microglial TLR4 and LYN kinase in 5XFAD male and females. Our data reveals a correlation between microglial TLR4 and LYN co-localization and AD pathogenesis, more in females than males. Targeting microglial TLR4 and Lyn in DAMs may offer new therapeutic opportunities in the treatment of AD.

Modified TPP-MoS2 QD Blend as a Bio-Functional Model for Normalizing Microglial Dysfunction in Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease of old age. Accumulation of β-amyloid peptide (Aβ) and mitochondrial dysfunction results in chronic microglial activation, which enhances neuroinflammation and promotes neurodegeneration. Microglia are resident macrophages of the brain and spinal cord which play an important role in maintaining brain homeostasis through a variety of phenotypes, including the pro-inflammatory phenotype and anti-inflammatory phenotypes. However, persistently activated microglial cells generate reactive species and neurotoxic mediators. Therefore, inhibitors of microglial activation are seen to have promise in AD control. The modified TPP/MoS2 QD blend is a mitochondrion-targeted nanomaterial that exhibits cytoprotective activities and antioxidant properties through scavenging free radicals. In the present study, the cell viability and cytotoxicity of the DSPE-PEG-TPP/MoS2 QD blend on microglial cells stimulated by Aβ were investigated. The levels of reactive oxygen species (ROS) and mitochondrial membrane potential (MMP) were also assessed. In addition, pro-inflammatory and anti-inflammatory cytokines, such as tumor necrosis factor α (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β), transforming growth factor beta (TGF-β), inducible nitric oxide synthase (iNOS) and arginase-1 (Arg-I) were measured in the presence or absence of the DSPE-PEG-TPP/MoS2 QD blend on an immortalized microglia cells activated by accumulation of Aβ. We found that the DSPE-PEG-TPP/MoS2 QD blend was biocompatible and nontoxic at specific concentrations. Furthermore, the modified TPP/MoS2 QD blend significantly reduced the release of free radicals and improved the mitochondrial function through the upregulation of MMP in a dose-dependent manner on microglial cells treated with Aβ. In addition, pre-treatment of microglia with the DSPE-PEG-TPP/MoS2 QD blend at concentrations of 25 and 50 μg/mL prior to Aβ stimulation significantly inhibited the release and expression of pro-inflammatory cytokines, such as IL-1β, IL-6, TNF-α, and iNOS. Nevertheless, the anti-inflammatory cytokines TGF-β and Arg-I were activated. These findings suggest that the modified TPP/MoS2 QD blend reduced oxidative stress, inflammation and improved the mitochondrial function in the immortalized microglial cells (IMG) activated by Aβ. Overall, our research shows that the DSPE-PEG-TPP/MoS2 QD blend has therapeutic promise for managing AD and can impact microglia polarization.

The effects of microglia-associated neuroinflammation on Alzheimer's disease

Alzheimer's disease (AD) is defined as a severe chronic degenerative neurological disease in human. The pathogenic mechanism of AD has been convincingly elucidated by the "amyloid cascade hypothesis" with the main focus of the pathological accretion of β-amyloid (Aβ) peptides outside the cell. However, increasing evidence suggests that this hypothesis is weak in explaining the pathogenesis of AD. Neuroinflammation is crucial in the development of AD, which is proven by the elevated levels of inflammatory markers and the identification of AD risk genes relevant to the innate immune function. Here, we summarize the effects of microglia-mediated neuroinflammation on AD, focusing on the temporal and spatial changes in microglial phenotype, the interactions among microglia, Aβ, tau, and neurons, and the prospects and recent advances in neuroinflammation as a diagnostic and therapeutic target of AD.