Studies of the Interaction of Cardiac Lipid Droplets with Mitochondria Using Electron Microscopy

Abstract

In the United States and many developed countries, obesity is a rising health problem. Obesity is the result of an imbalance in lipid homeostasis and is evidenced by the excessive accumulation of cytoplasmic lipid droplets (CLDs) in cardiac muscles. The CLD is a dynamic organelle that functions in maintaining cellular lipid storage and metabolism [1]. It interacts with various cell organelles such as the endoplasmic reticulum, endosomes and mitochondria for lipid transport, storage and beta oxidation to generate energy. To date, only a few lipid droplet-associated proteins have been identified; among these, perilipin proteins are the most abundant. In cardiac myocytes, perilipin 5 has been shown to play a role in the recruitment of mitochondria to CLDs and is a negative regulator of lipolysis [2]. Our ultimate goal is to understand how CLDs in cardiac muscle cooperate, spatially and metabolically with mitochondria and the role of perilipin 5 in the dynamic interactions between these two organelles. Due to the small size of CLDs, electron microscopy (EM) techniques are required to characterize their physical and molecular interactions with mitochondria. However, lipids are prone to be extracted during traditional EM and light microscopy sample processing steps. Using fluorescence microscopy, it has been shown previously that methanol and acetone fixation can induce lipid droplets fusion and alter the size and morphology of CLDs [3]. In the electron microscope, lipid droplets can be seen both as electron dense or electron opaque vacuoles in cells or tissues prepared by different fixation and staining methods. Here we summarize our findings on the effect of various fixation and staining methods on the morphology of CLDs in cardiac muscles.