Metformin rescues muscle function in BAG3 myofibrillar myopathy models

Dominant <i>de novo</i> 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 BAG3^P209L and one with a nonsense mutation in <i>bag3</i>. While transgenic BAG3^P209L-expressing fish display protein aggregation, modeling the early phase of the disease, <i>bag3</i>^-/- fish exhibit exercise dependent fiber disintegration, and reduced swimming activity, consistent with later stages of the disease. Detailed characterization of the <i>bag3</i>^-/- fish, revealed an impairment in macroautophagic/autophagic activity, a defect we confirmed in <i>BAG3</i> patient samples. Taken together, our data highlights that while BAG3^P209L 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 <i>bag3</i>^-/- fish. Therefore, repurposing metformin provides a promising therapy for BAG3 myopathy.<b>Abbreviations:</b>ACTN: 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.