Deletion of the microglial transmembrane immune signaling adaptor TYROBP ameliorates Huntington’s disease mouse phenotype

BACKGROUND Huntington’s disease (HD) is a fatal neurodegenerative disorder caused by an expansion of the CAG trinucleotide repeat in the huntingtin gene. Immune activation is abundant in the striatum of HD patients. Detection of active microglia at presymptomatic stages suggests that microgliosis is a key early driver of neuronal dysfunction and degeneration. Recent studies showed that deletion of Tyrobp , a microglial-enriched protein, ameliorates neuronal function in Alzheimer’s disease amyloid and tauopathy mouse models while decreasing components of the complement subnetwork, thus raising the possibility that Tyrobp deletion can be beneficial for HD. METHODS To test the hypothesis that Tyrobp deficiency would be beneficial in a HD model, we placed the Q175 HD mouse model on a Tyrobp -null background. We characterized these mice with a combination of behavioral testing, immunohistochemistry, transcriptomic and proteomic profiling. Further, we evaluated the Q175 microglia-specific gene signature, with and without Tyrobp , by purifying microglia from these mice for transcriptomics. RESULTS Comprehensive analysis of publicly available transcriptomic HD human data revealed that TYROBP network is overactivated in HD putamen. The Q175 mice showed morphologic microglial activation, reduced levels of post-synaptic density-95 protein and motor deficits at 6 and 9 months of age, all of which were ameliorated on the Tyrobp -null background. Gene expression analysis revealed that lack of Tyrobp in the Q175 model does not prevent the decrease in the expression of striatal neuronal genes but reduces pro-inflammatory pathways that are specifically active in HD human brain. Integration of transcriptomic and proteomic data identified that astrogliosis and complement system pathway were reduced after Tyrobp deletion. Results were further validated by immunofluorescence analysis. Microglia-specific HD gene dysregulation, characterized by overexpression of neuronal genes, was also not corrected by Tyrobp deletion. CONCLUSIONS Our data provide molecular and functional support demonstrating that Tyrobp deletion prevents many of the abnormalities in the Q175 HD mouse model, in the absence of correction of striatal neuronal gene expression, suggesting that the Tyrobp pathway is a potential therapeutic candidate for Huntington’s disease. ### Competing Interest Statement The authors have declared no competing interest. * HD : Huntington’s disease HTT : Huntingtin human gene MSNs : Medium spiny neurons Htt : Huntingtin mouse protein mHtt : mutant huntingtin Tyrobp : TYRO protein tyrosine kinase-binding protein AD : Alzheimer’s disease LOAD : Late onset Alzheimer’s disease DEGs : Differentially expressed genes PSD-95 : Post-synaptic density 95 DAM : Disease-associated microglia GSEA : Gene set enrichment analysis DIA : Data-independent acquisition FDR : False discovery rate DEPs : Differentially expressed proteins PRC2 : Polycomb repressive complex 2 H3K27me2 : Histone 3 Lysine 27 dimethylation H3K27me3 : Histone 3 Lysine 27 trimethylation Erk : Extracellular signal-regulated kinase Iba1 : Ionized calcium binding adaptor molecule 1 RNAseq : Ribonucleic acid sequencing qPCR : Quantitative polymerase chain reaction SEM : Standard error of the mean TBS : Tris-buffered saline PBS : Phosphate-buffered saline WT : wild-type Gfap : Glial fibrillary acidic protein IL- : Interleukin- PPI : Protein-Protein interaction MCODE : Molecular Complex Detection