(H-281) Isoproterenol Induces Sarcomere Damage and Chaperone-Assisted Selective Autophagy Complex Recruitment in a Stress Cardiomyopathy Mouse Model

Heart failure affects over 64 million people worldwide and is commonly caused by injury to the heart, leading to the weakening of contractile function. Sarcomeres are the basic unit of contraction in cardiac muscle, and as such, understanding how its structure changes in injured hearts is essential towards developing therapies for individuals with heart failure. Despite well-characterized research establishing the importance of sarcomeres in normal cardiac function, the events surrounding the damage and repair of sarcomeres following cardiac stress or injury remain understudied. To gain further insight into this process within the context of cardiac pathology, we use a model of isoproterenol (ISO)-induced cardiac injury in mice. Our previous work showed that ISO injections provoke an acute cardiac inflammatory response associated with reversable changes in heart structure and function that mimic stress cardiomyopathy in humans. Using this model, we hypothesized that cardiomyocyte sarcomeres are damaged by ISO and that sarcomere repair is essential for cardiac function recovery. To investigate potential mechanisms behind this repair, we focused on chaperone-assisted selective autophagy (CASA), a pathway traditionally involved in removing damaged and misfolded proteins within muscle cells. Previous research prompted analysis of the CASA complex by indicating that CASA proteins can localize to sarcomeres to perform a crucial role in maintaining sarcomere integrity, as mutations affecting CASA proteins, namely BAG3 and HSPB8, lead to cardiac pathology. This study investigated whether cardiac sarcomeres are damaged in the context of stress cardiomyopathy and the role of CASA in response to sarcomere damage. 

Female, 10-week-old, wild-type mice (C57BL/6J) were injected intraperitoneally with ISO (300 mg/kg in saline), with hearts collected at baseline and at multiple time points after injection: 1-hour, 4-hour, day 1, and day 3. The left ventricles were isolated and snap-frozen for extraction of myofilament-enriched proteins or preparation of skinned myocytes for immunofluorescence staining. Immunoblotting was performed on myofilament-enriched samples to detect the abundance of CASA proteins – BAG3, HSPB8, HSP70, CHIP – as well as actinin and GAPDH as controls to verify myofilament enrichment. For immunofluorescence sarcomere staining, skinned cardiomyocytes were prepared and stained for the same CASA proteins as above, as well as desmin (sarcomere marker), and then visualized using confocal fluorescence microscopy. 

Results/Discussion: Visualization of cardiomyocyte sarcomeres using immunofluorescence staining revealed that they were damaged as early as 1-hour post-ISO, as indicated by increased desmin disorganization relative to baseline. Indeed, our preliminary data (n=2-4 samples/group) with immunoblotting of myofilament-enriched extracts from hearts at 1-hour post-ISO suggest a decrease in CASA protein levels when compared to baseline. This correlates to our previous work where we showed that at 1-hour post-ISO, we observed a peak of serum cardiac troponin I levels and reduced left ventricular ejection fraction. However, at 4 hours post-ISO, there was recovery of desmin organization in the immunofluorescence images, with the return of the structured striation pattern typical of normal cardiac muscle. At this time point, CASA protein abundance in myofilament-enriched extracts increased back to baseline levels and was maintained at least 1 day after ISO injection, corresponding with the timing of the recovery of cardiac function observed in this model.

Conclusions: This preliminary study showed that ISO causes acute sarcomere damage that is accompanied by a decrease in the myofilament-associated abundance of CASA complex proteins. Moreover, increased levels of CASA complex proteins in the myofilament were observed shortly after, corresponding with the recovery of cardiac function in this model. Further work will involve assessment of sarcomeric protein ubiquitination to confirm ISO-induced myofilament damage and loss of function studies targeting critical CASA proteins to investigate the role of this complex in sarcomere repair and cardiac functional recovery post-ISO. Findings from these studies will likely contain implications beyond understanding stress cardiomyopathy, as a catecholamine surge is observed with neurohormonal upregulation in heart failure and is implicated in provoking cellular injury to worsen this condition.