Metformin rescues muscle function in BAG3 myofibrillar myopathy models.

ABSTRACT

Dominant de novo mutations in the co-chaperone BAG3 cause a severe form of myofibrillar myopathy, exhibiting progressive muscle weakness, muscle structural failure, and protein aggregation. To elucidate the mechanism of disease in, and identify therapies for, BAG3 myofibrillar myopathy, we generated two zebrafish models, one conditionally expressing BAG3P209L and one with a nonsense mutation in bag3. While transgenic BAG3P209L-expressing fish display protein aggregation, modeling the early phase of the disease, bag3-/- fish exhibit exercise dependent fiber disintegration, and reduced swimming activity, consistent with later stages of the disease. Detailed characterization of the bag3-/- fish, revealed an impairment in macroautophagic/autophagic activity, a defect we confirmed in BAG3 patient samples. Taken together, our data highlights that while BAG3P209L expression is sufficient to promote protein aggregation, it is the loss of BAG3 due to its sequestration within aggregates, which results in impaired autophagic activity, and subsequent muscle weakness. We therefore screened autophagy-promoting compounds for their effectiveness at removing protein aggregates, identifying nine including metformin. Further evaluation demonstrated metformin is not only able to bring about the removal of protein aggregates in zebrafish and human myoblasts but is also able to rescue the fiber disintegration and swimming deficit observed in the bag3-/- fish. Therefore, repurposing metformin provides a promising therapy for BAG3 myopathy.AbbreviationsACTN actinin, alpha; BAG3 BAG cochaperone 3; CRYAB crystallin alpha B; DES desmin; DMSO dimethyl sulfoxide; DNAJB6 DnaJ heat shock protein family (Hsp40) member B6; dpf days post fertilization; eGFP enhanced green fluorescent protein; FDA Food and Drug Administration; FHL1 four and a half LIM domains 1; FLNC filamin C; hpf hours post-fertilization; HSPB8 heat shock protein family B [small] member 8; LDB3/ZASP LIM domain binding 3; MYOT myotilin; TTN titin; WT wild-type.