Abstract P2073: Rbx2-CRL5 Mediates Mitochondrial Ubiquitination And Mitophagy To Maintain Myocardial Homeostasis Independent Of Parkin

Abstract

The ubiquitin (Ub) ligase Parkin has been recognized as a central player in mitophagy. In contrast to its presumed importance in the heart, ablation of Parkin fails to impact homeostatic mitophagy and cardiac function in both young and aging mice, suggesting compensation from other mitochondrial Ub ligases. However, the identities of such additional mitochondrial Ub ligases, as well as their physiological significance in the heart, remain unclear. Here, we demonstrate that Cullin-RING Ub ligase 5 (CRL5) plays a key role in the control of mitochondrial homeostasis in cardiomyocytes. CRL5 belongs to the family of Cullin-RING Ub ligases (CRLs) and consists of the RING box protein RBX2, scaffold protein Cullin 5 (Cul5), and one of 38 substrate receptor proteins. Analysis of the mitochondrial outer membrane-interacting proteome revealed a robust association of CRLs with mitochondria. Pharmacological and genetic perturbations of CRLs inhibited mitochondrial ubiquitination and mitophagy. Subcellular fractionation, live-cell imaging and immunogold electron microscopy further revealed that RBX2 and Cul5, two core components of CRL5, are quickly recruited to mitochondria upon mitochondrial depolarization. Depletion of RBX2 or Cul5 inhibited mitochondrial ubiquitination and turnover, impaired mitochondrial function, and increased cell death in cardiomyocytes. Importantly, RBX2 deficiency did not alter Parkin expression, had a more robust inhibitory effect on mitochondrial ubiquitination compared to Parkin deficiency, and remained effective in the absence of Parkin. Moreover, loss of RBX2 attenuated Parkin-mediated ubiquitination, suggesting its dominant, Parkin-independent actions in mitochondria. In vivo , deletion of RBX2 in adult mouse hearts suppressed mitophagic activity and provoked accumulation of damaged mitochondria in myocardium, leading to rapid development of dilated cardiomyopathy and heart failure. Finally, RBX2 was upregulated in pressure overloaded mouse hearts but downregulated in human failing hearts, further supporting its protective role in the heart. Taken together, our data allow us to propose RBX2-CRL5 as a mitochondrial Ub ligase and a physiological regulator of mitochondrial and cardiac integrity.