Effect of Hspb8 deletion in the skeletal muscle of a mouse model for Amyotrophic Lateral Sclerosis

Introduction Amyotrophic lateral sclerosis (ALS) is a adult-onset progressive motoneuron disease, that mostly appears as a sporadic form; only 10% of ALS are inherited cases (fALS) due to specific gene mutations. The most studied fALSs are those linked to mutations of the SOD1 gene. The mutant SOD1 encodes a protein that misfolds forming protein aggregates toxic to motoneurons and other affected cells. We previously found that the small heat shock protein (HSP) B8 removes toxic protein species of the mutant SOD1 via a degradative process called chaperone assisted selective autophagy (CASA). HSPB8 clears also other ALS-associated misfolded proteins as TDP-43 and the dipeptides derived from C9orf72 mutated gene. Furthermore, HSPB8 is highly expressed in spinal cord motor neurons and in skeletal muscle fibers of ALS mice the end stage of the disease. Results Here, we evaluated the potential protective effect of HSPB8 in a mouse model of ALS. To this aim we generated a new transgenic line of ALS mice lacking HSPB8 expression (Hspb8-/- / SOD1G93A-/+) and evaluated the survival, the onset of the pathology, the expression of different genes involved in the protein quality control and tissue morphology. In general, no worsening of different parameters tested were seen in Hspb8 KO mice, while some behavioral parameters were improved. Survival did not change in Hspb8-/- / SOD1G93A-/+ mice (vs. SOD1G93A-/+), motor performances were slightly improved and disease onset markedly delayed in male mice. Collected tissues at the onset and at the end stage of disease were used to evaluate SOD1 aggregates, the levels of different mRNAs/proteins and their morphology. Our data indicate that the absence of HSPB8 has a dual effect in gastrocnemius muscle and spinal cord, since the lack of HSPB8 worsened SOD1 aggregation in the spinal cord, but decreased SOD1 accumulation in ALS muscle. Furthermore, the absence of HSPB8 modulated the levels of other HSPB proteins, that may compensate the absence of HSPB8 in ALS muscle. No morphological differences were found in the muscles of the two types of mice at the end stage of the disease. Conclusion The tissue specific effects we observed could be due to compensatory mechanisms associated to proteins belonging to the HSPB family and known to be involved in ALS. Our results indicate that HSPB8 and CASA are involved in ALS, but further studies are needed to disclose the effects of HSPB8 in ALS. (Grant: AFM Telethon N° 23236)