Abstract 909: Determining the Role of the Co-chaperone BAG3 at the Cardiac Myofilament

Bcl-2-associated athanogene 3 (BAG3) is an adaptor protein often associated with protein quality control. In the heart, decreased BAG3 expression is linked to dilated cardiomyopathy. Mutations to BAG3 can result in myofibrillar disarray, indicating a possible role at the sarcomere. One previous study found BAG3 has a role in maintaining sarcomere structure, however this was in neonatal myocytes. Whether BAG3 has any role in the myofilament of the adult myocyte is unknown. To determine if BAG3 is present in the cardiac sarcomere and identify the myofilament-BAG3 interactome, we used co-immunoprecipitation of the myofilament fraction from mouse LV and mass spectrometry. This revealed a robust interaction of BAG3 with the myofilament. Furthermore, BAG3 interacted with a heat shock protein (HSPB8) known to assist in the removal of protein aggregates, which are abundant after MI. To determine the mechanistic role of BAG3, we overexpressed the protein in mice hearts using adeno-associated virus 9. At baseline, overexpression caused a slight decrease in skinned myocyte calcium sensitivity compared to WT. This effect may be due to BAG3’s interaction with β-adrenergic receptors, which can cause PKA phosphorylation of thin filament proteins known to reduce calcium sensitivity. Preliminary data support this. We then exposed these mice to left anterior descending coronary artery ligation to induce MI. Compared to shams, MI decreased maximum calcium activated force (F max ) in the WT mice, but BAG3 overexpression restored F max to near sham levels. We also studied the functional impact of a known disease-causing mutation discovered in humans, P209L. Transgenic overexpression of P209L BAG3 had no effect on F max , calcium sensitivity, or cooperativity, suggesting the mutation may not interfere with domains in BAG3 required for its myofilament role. This work is the first to show a functional role for BAG3 in the adult myofilament. However, our findings reveal a complex interaction in which the effect of altered BAG3 expression varies with context. The P209L mutation findings suggest that not all BAG3 mutations have shared phenotypes, a conclusion supported by recent clinical work. Further research is necessary to understand BAG3’s myofilament role in health and disease.