Modulating immune system activation in neurodegenerative disease
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Abstract
In Alzheimer’s disease and related dementias (ADRD), chronic neuroinflammation contributes to cellular dysfunction and neurodegeneration. During cellular stress, RNA-binding proteins (RBPs) interact with tau and other disease-related proteins to promote toxic protein aggregation. We hypothesize that modulating these stress pathways can alleviate the contribution of RBPs to disease pathology in models of ADRD. In this dissertation, two different approaches to modulating the immune system were explored. In the first study, the RBP T-cell intracellular 1 (TIA1) was selectively knocked out in microglia of P301S tauopathy mice. TIA1 decreases inflammation in the peripheral immune system; therefore, we hypothesized that TIA1 could regulate inflammation in the central immune system. To explore this, P301S tauopathy mice overexpressing human microtubule associated protein tau (MAPT) with P301S mutation were bred with TIA1 conditional knockout mice in microglia using a CX3CR1 cre recombinase (P301S/TIA1cKO). We assessed the effect of conditionally mutating TIA1 in microglia on immune system activation and tau pathology using immunoblot, transcriptional assays, and immunohistochemistry. TIA1 is required in microglia for activation of the microglial sensome and transcription of immune receptors. Receptors that respond to tau pathology (TREM2, CLEC7A, TYROBP) and inflammatory cytokines (TNFα, IL-1β) were significantly downregulated in P301S/TIA1cKO mice compared to P301S mice. This was corroborated by a distinct lack of microglial morphological changes and CD68+ microgliosis. P301S/TIA1cKO mice also had significantly less tau pathology than P301S mice. In response to lipopolysaccharide stimulation, TIA1 null microglia exhibited decreased microglial activation, in contrast with peripheral TIA1 null macrophages which have exacerbated immune activation. In conclusion, microglial TIA1 is critical for transcription of the microglial sensome and mutating microglial TIA1 reduces tau aggregation.
In the second project, the integrated stress response (ISR) was inhibited using the viral protein open reading frame 57 (ORF57) from Human herpesvirus-8. Activation of the ISR is one of the precipitating events for protein aggregation in ADRDs. The ISR is activated by many stressors, including protein aggregation, dsRNA, viruses, and oxidative stress. In the ISR, protein kinase R (PKR) phosphorylates eIF2α which halts global translation, facilitates stress granule (SG) assembly, and induces apoptosis. Chronic activation of the ISR leads to protein aggregation of TAR DNA-binding protein 43kDa (TDP-43) through this SG pathway. ORF57 binds to and inhibits protein kinase R (PKR), preventing PKR from phosphorylating eIF2α and affecting translation. We hypothesized that ORF57 would inhibit TDP-43 aggregation through this blockade of PKR activation. To study this, the viral protein, ORF57, was optimized for expression in human neuroblastoma cells overexpressing TDP-43. These cells were treated with sodium arsenite oxidative stress causing TDP-43 to form insoluble aggregates. The impact of ORF57 on immune activation and TDP-43 aggregation was explored using immunofluorescence, immunoblot, and proteomics. ORF57 reduced phosphorylation of eIF2α, improved protein synthesis rates, and decreased SG assembly. ORF57 also affected TDP-43 pathology by altering TDP-43 aggregate morphology and increasing solubility of those aggregates. TDP-43 aggregates in cells with ORF57 disassembled at a significantly higher rate and had a faster recovery after photobleaching than those without ORF57. Proteomics revealed that ORF57 interacts with TDP-43, and other RBPs, in the presence of stress. In conclusion, my work demonstrates how the anti-stress functions of viral proteins can be re-engineered to inhibit disease processes. ORF57 blocks the ISR and reduces insoluble TDP-43 aggregation, demonstrating a novel approach to decreasing immune activation in disease. Together these two projects support a benefit of modulating immune activation in ADRDs.
Description
2024