Septins regulate heart contractility through modulation of cardiomyocyte store-operated Ca2+ entry
Authors: Benjamin Tripoli; Courtney Petersen; Jeremy Smyth
Affiliation: USUHS - Bethesda, MD
Keywords: j. cardiovascular disease; j. other signaling pathways
Proper calcium handling in cardiomyocytes is paramount to maintaining cardiomyocyte contractility and heart function. Recent data from our lab and others in the field have established that store operated calcium entry (SOCE) is an essential component of cardiomyocyte calcium regulation. SOCE refers to the pathway in which endoplasmic/sarcoplasmic reticulum (ER/SR) calcium store depletion causes influx of calcium to replenish the stores. The pathway is mediated by STIM proteins, which act as calcium sensors in the SR/ER, and ORAI calcium influx channels in the plasma membrane. Both upregulation and suppression of SOCE in cardiomyocytes causes disruption of normal cardiac function, demonstrating that proper regulation of SOCE in cardiomyocytes is essential. However, the mechanisms responsible for the regulation of SOCE in cardiomyocytes is not yet fully understood. To this end, septin GTPases have emerged as regulators of the SOCE pathway in non-cardiac tissues, with septin 1, 2, or 4 suppression resulting in SOCE suppression and septin 7 (PNUT) suppression resulting in SOCE upregulation. Importantly, the role of septins in cardiomyocytes is nearly completely unknown. Through intravital imaging analysis of Drosophila heart contractility, we now show that cardiomyocyte specific RNAi-based depletion of septins 1, 2, or 4 results in dilated cardiomyopathy nearly identical to that caused by SOCE suppression. We further show that co-expression of septin 2 RNAi with a constitutively active Orai channel suppresses the septin 2 phenotype, supporting the hypothesis that septin 2 associated dilated cardiomyopathy is due to SOCE suppression. Interestingly, septin 7 suppression resulted in hypertrophic cardiomyopathy similar to that caused by SOCE upregulation, and micro-computerized tomography (microCT) analysis of heart size further confirmed the hypertrophic phenotype of septin 7 depleted hearts. This septin 7 phenotype was suppressed by Orai depletion, again supporting a role for SOCE dysregulation in the septin depletion-mediated cardiac phenotypes. We are currently developing tools to analyze Stim and Orai localization, sarcomere organization, and t-tubule architecture to further investigate septins and SOCE regulation in cardiomyocytes.