Publications in Acta Neuropathologica Communications by NOMIS researchers

Frontotemporal dementia is characterized by progressive atrophy of frontal and/or temporal cortices at an early age of onset. The disorder shows considerable clinical, pathological, and genetic heterogeneity. Here we investigated the proteomic signatures of frontal and temporal cortex from brains with frontotemporal dementia due to GRN and MAPT mutations to identify the key cell types and molecular pathways in their pathophysiology. We compared patients with mutations in the GRN gene (n = 9) or with mutations in the MAPT gene (n = 13) with non-demented controls (n = 11). Using quantitative proteomic analysis on laser-dissected tissues we identified brain region-specific protein signatures for both genetic subtypes. Using published single cell RNA expression data resources we deduced the involvement of major brain cell types in driving these different protein signatures. Subsequent gene ontology analysis identified distinct genetic subtype- and cell type-specific biological processes. For the GRN subtype, we observed a distinct role for immune processes related to endothelial cells and for mitochondrial dysregulation in neurons. For the MAPT subtype, we observed distinct involvement of dysregulated RNA processing, oligodendrocyte dysfunction, and axonal impairments. Comparison with an in-house protein signature of Alzheimer’s disease brains indicated that the observed alterations in RNA processing and oligodendrocyte function are distinct for the frontotemporal dementia MAPT subtype. Taken together, our results indicate the involvement of different brain cell types and biological mechanisms in genetic subtypes of frontotemporal dementia. Furthermore, we demonstrate that comparison of proteomic profiles of different disease entities can separate general neurodegenerative processes from disease-specific pathways, which may aid the development of disease subtype-specific treatment strategies.

Hexanucleotide repeat expansion in C9orf72 is the most common genetic cause of frontotemporal dementia and amyotrophic lateral sclerosis, but the pathogenic mechanism of this mutation remains unresolved. Haploinsufficiency has been proposed as one potential mechanism. However, insights if and how reduced C9orf72 proteins levels might contribute to disease pathogenesis are still limited because C9orf72 expression, localization and functions in the central nervous system (CNS) are uncertain, in part due to the poor specificity of currently available C9orf72 antibodies.Here, we generated and characterized novel knock-out validated monoclonal rat and mouse antibodies against C9orf72. We found that C9orf72 is a low abundant, cytoplasmic, highly soluble protein with the long 481 amino acid isoform being the predominant, if not exclusively, expressed protein isoform in mouse tissues and human brain. As consequence of the C9orf72 repeat expansion, C9orf72 protein levels in the cerebellum were reduced to 80% in our series of C9orf72 mutation carriers (n = 17) compared to controls (n = 26). However, no associations between cerebellar protein levels and clinical phenotypes were seen. Finally, by utilizing complementary immunohistochemical and biochemical approaches including analysis of human iPSC derived motor neurons, we identified C9orf72, in addition to its association to lysosomes, to be localized to the presynapses and able to interact with all members of the RAB3 protein family, suggestive of a role for C9orf72 in regulating synaptic vesicle functions by potentially acting as guanine nucleotide exchange factor for RAB3 proteins.In conclusion, our findings provide further evidence for haploinsufficiency as potential mechanism in C9orf72 pathogenesis by demonstrating reduced protein levels in C9orf72 mutation carriers and important novel insights into the physiological role of C9orf72 in the CNS. Moreover, the described novel monoclonal C9orf72 antibodies will be useful tools to further dissect the cellular and molecular functions of C9orf72.

Frontotemporal Lobar Degeneration (FTLD) encompasses certain related neurodegenerative disorders which alter behaviour, personality and language. Heterogeneous ribonuclear proteins (hnRNPs) maintain RNA metabolism and changes in their function may underpin the pathogenesis of FTLD. Immunostaining for hnRNP A1, A2/B1 and A3 was performed on sections of temporal cortex with hippocampus from 61 patients with FTLD, stratified by pathological hallmarks into FTLD-tau and FTLD-TDP type A, B and C subtypes, and by genetics into patients with C9orf72 expansions, MAPT or GRN mutations, or those without known mutation. Four patients with Motor Neurone Disease (MND) with C9orf72 expansions and 10 healthy controls were also studied. Semi-quantitative analysis assessed hnRNP staining intensity in dentate gyrus (DG) and CA4 region of hippocampus, and temporal cortex (Tcx) in the different pathological and genetic groups.Immunostaining for hnRNP A1, A2/B1 and A3 revealed no consistent changes in pattern or amount of physiological staining across any of the pathological or genetic groups. No immunostaining of any inclusions resembling TDP-43 immunoreactive neuronal cytoplasmic inclusions or dystrophic neurites, was seen in either Tcx or DG of the hippocampus in any of the FTLD cases investigated for hnRNP A1, A2/B1 and A3. However, immunostaining for hnRNP A3 showed that inclusion bodies, resembling those TDP-43 negative, p62-immunopositive structures containing dipeptide repeat proteins (DPR) were variably observed in hippocampus and cerebellum. The proportion of cases showing hnRNP A3-immunoreactive DPR, and the number of hnRNP A3-positive inclusions within cases, was significantly greater in DG than in cells of CA4 region and cerebellum, but the latter was significantly less in all three regions compared to that detected by p62 immunostaining.