Upregulation of Store Operated Ca2+ Entry pathologically impairs Drosophila cardiac function

More than 300,000 people in the United States die of heart failure each year. Heart failure occurs when heart muscle cannot fully contract, and invariably involves dysregulation of the Ca²⁺ transport mechanisms that drive cardiomyocyte contractility. Thorough understanding of cardiomyocyte Ca²⁺ transport mechanisms is therefore essential for the development of novel heart failure therapeutics. To this end, our lab recently demonstrated that the store operated Ca²⁺ entry (SOCE) mechanism of Ca²⁺ transport is essential for proper Drosophila heart contractility. SOCE is mediated by STIM proteins, which function as Ca²⁺ sensors in the ER/SR, and Orai Ca²⁺ influx channels in the plasma membrane. Numerous animal studies have demonstrated that upregulation of SOCE in cardiomyocytes induces pathological cardiac hypertrophy and heart failure. Interestingly however, humans with gain-of-function mutations in STIM1 or Orai1 exhibit varying degrees of skeletal muscle myopathy and platelet dysfunction, but not cardiac hypertrophy or other cardiac complications. To better understand how upregulated SOCE alters cardiomyocyte physiology, we generated a transgenic Drosophila model that expresses a Stim mutant which confers constitutive Ca²⁺ influx activity (Stim^CA) due to two aspartate to alanine changes in the Ca²⁺ binding EF-hand domain.Heart-specific expression of Stim^CA also significantly impaired animal development, as 80% of these animals died as larvae and pupariation of surviving larvae was delayed by approximately two days compared to W¹¹¹⁸ driven Stim^CA controls. Intravital imaging of adult heart contractility revealed that heart specific expression of Stim^CA reduced end-diastolic and end-systolic dimensions and decreased heart rate, consistent with significantly impaired heart function. Furthermore, microCT analysis showed heart specific expression of Stim^CA increased heart wall thickness, suggesting that decreased heart diameters observed in intravital imaging may result from increases in heart wall thickness. Collectively our results demonstrate that heart specific upregulation of the SOCE pathway pathologically impairs Drosophila cardiac function resulting in cardiac hypertrophy and hypertrophic cardiomyopathy. Moving forward, we plan to directly measure cardiomyocyte Ca²⁺ in vivo to determine how Stim^CA expression affects Ca²⁺ physiology and dynamics.