Heat shock protein family B member 8 (HSPB8) is a chaperone involved in the Chaperone Assisted Selective Autophagy (CASA) complex. HSPB8 in conjunction with BAG3 recognizes and promotes the autophagy-mediated removal of misfolded proteins associated with motor neuron disease including amyotrophic lateral sclerosis (ALS) and spinal and bulbar muscular atrophy (SBMA). Mutations in HSPB8 gene, which has previously been associated with the axonal type of Charcot Marie Tooth, have recently been associated with autosomal dominant rimmed vacuolar myopathy. Affected patients have adult-onset limb girdle myopathy with muscle biopsy showing fatty replacement, endomysial fibrosis, and rimmed vacuoles leading to progressive muscle atrophy and early demise.
We have previously demonstrated reduced expression of HSPB8, disrupted autophagy and TDP-43 in patient fibroblasts and iPSC-derived skeletal muscle progenitor cells (SMPC). There is no treatment available for this debilitating disease, an unmet need which we have tried to address by upregulation of HSPB8 and enhancement of autophagy. High throughput screening identified trehalose, a naturally occurring disaccharide, as a potent HSPB8 inducer and autophagy facilitator.
In this study, we assessed the effect of trehalose in the HSPB8 patient iPSC-derived skeletal muscle progenitor cells (SMPC) Trehalose was well tolerated up to 100 mM concentration. The qPCR and western blot data showed that trehalose induced gradual increase over 48h of the expression of HSPB8, autophagic proteins LC3B and p62, and decrease of TDP-43 expression.
The knock-in Hspb8 mouse model with the c.515dupC fs variant shows significant muscle weakness at 15 months of age. Muscle pathology reveals increased TDP-43, and autophagy pathology recapitulating disease phenotype. We treated 12 to 15-month-old Hspb8 mutant mice in addition to WT littermates with 2% trehalose in drinking water for 4 months and compared motor performance to untreated Hspb8 mutant and WT mice. Preliminary results from behavioral motor testing using rotarod, inverted screen test, and grip strength showed improvement in motor function starting after one month of treatment. Biochemical and histological studies are pending for the effects on typical muscle pathology
Our preliminary results show promising results from trehalose treatment for HSPB8-associated rimmed vacuolar myopathy and may have potential in related neuromuscular disorders.