Network Analysis Of The PGRN-deficient Mouse Brain Proteome Reveals Pathogenic Mechanisms Shared In Human FTD Caused By GRN Mutations

Objective:Heterozygous,loss-of-function mutations in the granulin gene(GRN)encoding progranulin(PGRN)are a common cause of Frontotemporal dementia(FTD).However,it is unclear how deficiency of PGRN and granulins causes neurodegeneration.In this study,we explored the main content of protein modification in FTD-GRN based on animal model,to find potential pathogenesis mechanisms of FTD-GRN.Methods:We used Grn knockout mice as an animal model of FTD,utilized Tandem Mass Tag(TMT)isobaric labeling mass spectrometry to perform an unbiased quantitative proteomic analysis of whole-brain tissue from wild-type(Grn+/+)and Grn knockout(Grn-/-)mice at 3 and 19months of age.We found highly co-expressed protein modules through Weighted correlation network analysis(WGCNA)and identified characters of modules through Gene Ontology(GO)and cell type enrichment.Then we used western blotting,ELISA,immunohistochemistry and immunofluorescence staining to identify specific proteins,further,we verified pathological changes in human brain samples and bio-fluid.Results:1.In mouse brain proteomic analysis:1)Lysosomal proteins(i.e.Gns,Scarb2,Hexb)were selectively upregulated and proteins involved in lipid metabolism(Acly,Apoc3,Asahl,Gpld1,Ppt1,Naaa)were decreased in Grn-/-mouse.2)PGRN deficiency was correlated with lysosomal dysfunction and neuroinflammation.3)Occurrence of disease was related to microglia.4)Synaptic loss and demyelination were late onset of Grn deficiency.2.Through biological process:1)CatD and CatZ aggregation happened in old Grn-/-mice brain.2)Up-regulated expression levels of GPNMB and galectin-3 in microglia were found in Grn-/-mice and both proteins were co-localized with microglia.3)We found similar up-regulation levels of GPNMB and galectin-3 in FTD-GRN patients’frontal tissue to mice brain.4)GPNMB levels were significantly increased in the cerebrospinal fluid of FTD-GRN patients,but not in MAPT or C9orf72 carriers.Conclusion:Our findings support the idea that insufficiency of PGRN and granulins in humans causes neurodegeneration through lysosomal dysfunction and neuroinflammation,which could be targeted to develop effective therapies.