Abstract 297: Caveolae and Mitochondria: a Subcellular Symbiotic Relationship that Facilitates Cardiac Myocyte Function
Caveolae, membrane microdomains composed of lipids and various proteins, including the protein caveolin (Cav), are involved in trafficking and compartmentation of signaling molecules. The possible relationship between caveolae and intracellular organelles is poorly defined. Here, we used whole hearts and cardiac myocytes (CM) and sought to determine whether caveolae and mitochondria associate and if this association has a functional consequence. Electron microscopy of CM revealed close apposition of subsarcolemmal mitochondria and caveolae. Subsarcolemmal mitochondria have greater expression of Cavs-1 and -3 relative to interfibrillar mitochondria and expression of Cav-3 in mitochondria was confirmed by immunoelectron microscopy. Immunoblotting of sucrose density gradient fractions prepared from CM, which facilitates the isolation of caveolae from other cellular membranes, detected enrichment of multiple enzymes that can serve as antioxidants (heme oxygenase-1, eNOS, cytochrome P450 reductase, and Cu-Zn and Mn superoxide dismutase) in caveolin-enriched buoyant fractions. Hearts of aged animals (24+ months), which have mitochondrial dysfunction, showed a 48% reduction in Cav-3 expression (p<0.01) relative to young animals (6 months). Adenoviral overexpression of Cav-3 in CM resulted in increased formation of caveolae and reduced generation of superoxide (as assessed by dihydroethidium staining) in response to hypoxia/reperfusion stress compared to control and LacZ- treated CM. Cav-3 overexpressing myocytes had endogenously elevated levels of phosphorylated Akt and glycogen synthase kinaseβ, two downstream mediators of cardiac protection. The close apposition of caveolae and mitochondria, the enrichment of redox-enzymes in caveolae, the ability of increased Cav expression to form caveolae and attenuate free radical generation and loss of Cav expression with age suggest that caveolae are microdomains that may serve as “antioxidant sinks” for free radicals produced by adjacent mitochondria in CM. The close association of caveolae and mitochondria may thus represent an important symbiotic subcellular organization that helps promote function and limit oxidative damage in the heart.