HSPB8 is recruited into stress granules upon stress and protects against TDP-43 mediated toxicity in vivo in drosophila melanogaster

Background: Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease comprising clinically indistinguishable sporadic (s) and familial (f) forms, associated with a number of genes (SOD1, TDP-43, FUS, UBQLN2, VCP, FIG4, CHMP2B, SQSTM1, C9ORF72). Most ALS proteins are involved in protein degradation, ubiquitin proteasome pathway (UPP) and autophagolysosomal pathway (APLP), RNA processing or stress granule (SG) response. In ALS, motoneurons accumulate protein aggregates that contain RNAbinding proteins markers of SGs. Both aggregated proteins and persistent SGs are cleared by autophagy (1). Thus, proteostasis (mediated by the protein quality control, PQC) and ribostasis (involving SGs) may be interconnected and their imbalance may participate in ALS. Thus, upregulation of key players of the PQC, including molecular chaperones (heat shock proteins), which can assist the autophagy-mediated degradation of specifi c clients, thereby restoring proteostasis (and indirectly, ribostasis), may be benefi cial in ALS. Objectives: We previously published that overexpression of the small heat shock protein HSPB8 protects against TPD-43 mediated toxicity in immortalized motoneurons. In fact, HSPB8 facilitates the autophagy-mediated degradation of truncated TDP-43, thereby decreasing its cytosolic aggregation (2). Here we further investigated HSPB8 mode of action, focusing on its potential role both at SGs and protein aggregates. We also investigated the protective role of HSPB8 against TDP-43 mediated toxicity in vivo in Drosophila melanogaster . Methods: For the in vitro work, HeLa cells were treated with SG inducing agents, including arsenite, MG132, heat shock. Cells were fi xed and HSPB8 subcellular localization, as well as SG formation was investigated by immunofl uorescence using specifi c antibodies. HSPB8 colocalization with mutated FUS containing SGs was studied in HeLa cells transfected with HA-tagged R518K FUS. For the in vivo work, the following transgenic fl ies were used: GMR-mut-NLS-TDP-43/ CyO; UAS-HSP67Bc/TM3; UAS-HSPB8/CyO and UASHSPB8/TM3. Flies were grown and crossed at 25 ° C. Results: We found that HSPB8 is recruited into SGs upon various stress. HSPB8 also colocalizes with mutated FUS at SGs, suggesting that it may act both at the level of protein aggregates (previous results) and SGs. Next, we tested HSPB8 protective role in vivo , using a Drosophila model expressing aNLS-TDP-43 mutant that accumulates in the cytosol, thereby causing eye degeneration (kindly provided by Prof. JP Taylor). We found that overexpression in vivo of both human HSPB8 and its Drosophila functional ortholog HSP67Bc protects against TDP-43 mediated toxicity. This is associated with a decrease in NLS-TDP-43 levels, supporting HSP67Bc/HSPB8 role in facilitating client degradation. Discussion and conclusion: Our results support that HSPB8 may act both at the level of aggregate-prone proteins as well as SGs, facilitating the degradation of mutant ALS proteins (eg, TDP-43) and decreasing their cytosolic accumulation, thereby exerting protective functions. Acknowledgements: AriSLA,