Our research is focused on the cellular mechanisms of neurodegenerative diseases, most notably FTD (frontotemporal dementia) and ALS (amyotrophic lateral sclerosis). FTD and ALS are currently incurable, and affected patients usually die within a few years of disease onset. Post-mortem brains of FTD and ALS patients show characteristic protein inclusions in the cytosol of neurons and glia cells (Fig. 1). The main components of these pathological inclusions are two RNA-binding proteins, TDP-43 (TAR DNA binding protein of 43 kDa) and FUS (Fused in sarcoma), which usually reside in the cell nucleus and regulate transcription and splicing of numerous target genes.
Fig. 1: Neuronal cytoplasmic TDP- 43 and FUS inclusions in post-mortem brains of FTD patients. Reproduced from Dormann & Haass, Trends Neurosci 2011
We have previously shown that proper nuclear import of FUS is crucial for neuronal health (Dormann et al. 2010; Dormann & Haass 2011). This is best exemplified by our finding that ALS-causing point mutations in the FUS gene disrupt the nuclear localization signal of FUS and thus impair nuclear import of FUS mediated by the nuclear import factor Transportin/Karyopherin β2 (Fig. 2). More recently, we found that nuclear import of FUS is regulated by a post-translational modification, arginine methylation of FUS, and that this modification is defective in FTD patients with FUS inclusions (Dormann et al. 2012; Suarez-Calvet et al., 2016).
Fig. 2: ALS-causing FUS mutations impair nuclear import of FUS. Compared to FUS wild-type (WT), which is mostly nuclear, ALS-associated FUS mutations show a nuclear import defect and accumulate in the cytosol. The degree of cytosolic mislocalization (% cyt.) is correlated with disease severity (age of onset/disease duration). Reproduced from Dormann et al., EMBO J 2010.
Furthermore, our previous work has shown that cellular stress is necessary to cause clustering of cytosolic FUS or TDP-43 in so-called stress granules (SGs) (Dormann et al. 2010; Bentmann et al. 2012). As SG marker proteins are frequently found in insoluble FUS or TDP-43 inclusions in post-mortem brains of ALS and FTD patients, we have proposed that stress granules or other RNP granules may act as condensation points and promote TDP-43 or FUS aggregation due to high local protein concentrations (Fig. 3).
Fig. 3: Cellular stress as a “second hit” in ALS/FTD pathogenesis. Two “hits” are required for the deposition of FUS or TDP-43 in cytosolic protein inclusions. Reproduced from Dormann & Haass, Trends Neurosci 2011.
To gain insight into the molecular mechanisms that underlie TDP-43 and FUS mislocalization / inclusion formation and subsequent neurodegeneration, we study the intracellular transport pathways and aggregation behavior of TDP-43 and FUS as well as their interactions with RNAs and other RNP granule components using in vitro and cell culture models (cell lines, primary neurons) and a combination of cell biological methods, molecular biology and protein biochemistry.
Currently, we are focusing on the following research areas:
(1) Nuclear transport defects in ALS/FTD: By which mechanisms are TDP-43 and FUS exported from the nucleus to the cytoplasm? Which additional factors regulate nuclear import of TDP-43 and FUS? Are nuclear transport components or regulatory factors defective in ALS/FTD patients? How does the most frequent genetic cause of ALS and FTD, a repeat expansion in the C9orf72 gene, affect the nucleocytoplasmic transport machinery?
(2) Arginine methylation of FUS and other RGG/RG-domain containing RNA-binding proteins: How does arginine methylation regulate nuclear transport of these proteins? How does it affect protein-protein interactions, RNA binding and protein function? Does loss of FUS arginine methylation, as seen in FTD-FUS patients, promote pathological aggregation of FUS?
(3) Pathological roles of RNP granules: How do FUS mutations alter the composition of cytosolic FUS RNP granules and cytosolic mRNA processing functions of FUS? Which factors suppress or enhance pathological phase transitions of FUS and TDP-43 in RNP granules?
We are supported by an Emmy Noether grant of the Deutsche Forschungsgemeinschaft (DFG) (DO-1804/1-1) and by the Munich Cluster for Systems Neurology (SyNergy, EXC 1010). We also received Seed Funding by the LMU Junior Researcher Fund within the framework of LMUexcellent.