Abstract
This study investigated the effects of elevated fatty acid (FA) supply from adipose tissue on the ultrastructure of cardiac lipid droplets (LDs) and the expression and organization of LD scaffold proteins perilipin-2 (PLIN2) and perilipin-5 (PLIN5). Stimulation of adipocyte lipolysis by fasting (24 h) or β3-adrenergic receptor activation by CL316, 243 (CL) increased cardiac triacylglycerol (TAG) levels and LD size, whereas CL treatment also increased LD number. LDs were tightly associated with mitochondria, which was maintained during LD expansion. Electron tomography (ET) studies revealed continuity of LD and smooth endoplasmic reticulum (SER), suggesting interconnections among LDs. Under fed ad libitum conditions, the cristae of mitochondria that apposed LD were mostly organized perpendicularly to the tangent of the LD surface. Fasting significantly reduced, whereas CL treatment greatly increased, the perpendicular alignment of mitochondrial cristae. Fasting and CL treatment strongly upregulated PLIN5 protein and PLIN2 to a lesser extent. Immunofluorescence and immuno-electron microscopy demonstrated strong targeting of PLIN5 to the cardiac LD-mitochondrial interface, but not to the mitochondrial matrix. CL treatment augmented PLIN5 targeting to the LD-mitochondrial interface, whereas PLIN2 was not significantly affected. Together, our results support the concept that the interface between LD and cardiac mitochondria represents an organized and dynamic "metabolic synapse" that is highly responsive to FA trafficking.