Upregulation of HSPB8 as potential therapeutic approach in familial and sporadic ALS

Several data suggest that accumulation of aggregated proteins (mutated SOD1, TDP-43 and FUS/TLS) plays an important role in motor neuronal cell death occurring in Amyotrophic Lateral Sclerosis (ALS). Protein aggregation results from the formation of aberrant conformations (misfolding) of aggregate-prone proteins, some of which have been found mutated in the familial forms of ALS. The removal of misfolded (aggregated) proteins is operated by the cells via two major degradative systems the ubiquitin-proteasome pathway (UPP) and the autophagy. Both systems require the assistance of intracellular chaperons. The molecular chaperones recognize and bind misfolded proteins, preventing their aggregation and facilitating their degradation, thus exerting neuroprotective functions. In this project, we will focus on the chaperone HSPB8, which forms a stable complex with the co-chaperone Bag3. Overexpression of HSPB8 (and Bag3) prevents aggregation of mutated SOD1 and TDP-43, associated with familial and sporadic ALS, by increasing their degradation via autophagy, an essential process for aggregate-prone protein clearance and neuronal survival. Thus, HSPB8 (and Bag3) may help motor neurons to cope with misfolded TDP-43 and SOD1 by either directly targeting them to the autophagic vacuoles for degradation and/or restoring/boosting the autophagy flux. Interestingly, deregulated autophagy is amongst the causes for motor neuron diseases (MNDs), further pointing to a link between protein aggregation, protein quality control, HSPB8-Bag3 and autophagy. Besides, mislocalization/aggregation of TDP-43 and FUS/TLS to the mRNA containing cytoplasmic stress granules (SGs) alters RNA metabolism and has been suggested as pathomechanism contributing to ALS. Our preliminary data indicate that HSPB8 is recruited to SG, where it colocalizes with TDP-43. Therefore, HSPB8, by either preventing the mislocalization/aggregation of mutated TDP-43 and FUS to SG and/or by targeting them for degradation may also contribute to disease amelioration by avoiding impairment of specific RNA translation/processing. The hypothesis that HSPB8 may exert essential functions for motor neuron viability is further supported by the observation that HSPB8 is upregulated in surviving motor neurons in both an ALS mouse model and in human ALS tissues, as well as by the finding that mutated forms of HSPB8 (which are found in aggregates) cause dominant hereditary motor neuropathy. In this project, we will investigate the hypothesis that upregulation of HSPB8 (and Bag3) may protect against ALS, using both motor neuronal cells and the mutated SOD1, TDP-43 and FUS/TLS based Drosophila models of ALS. We will also perform a drug screening to find compounds able to induce HSPB8 expression specifically in motor neurons. This work will provide insights in the role of HSPB8 as modulator of ALS and will identify specific sites of action of HSPB8 whose modulation may also represent new therapeutic targets for ALS.