Metabolic diseases are a pressing public health issue in the United States. With the meteoric rise of insulin resistance, obesity, and atherosclerosis, mechanistic understanding of lipid metabolism could not be more imperative. Many of these metabolic disorders involve improper storage of lipids into cellular lipid storage organelles known as lipid droplets (LDs). These unique organelles, which contain a core of triacylglycerols and cholesterol esters, accumulate when dietary free fatty acids are abundant, and conversely get consumed when there is a high demand for energy. In this way, LDs are essential for both cellular and whole-body homeostasis. Proteins that reside on the surface of LDs play critical roles in the dynamic turnover of LDs by controlling the intake and release of the lipids stored within. Although the metabolic roles of LDs are well known, the mechanisms employed to regulate the growth and catabolism of individual LDs are poorly understood. We have identified two proteins, DFCP1 and WHAMM, which are poised to function as crucial metabolic switches for LDs. In particular, we found that DFCP1 localizes to compartments that are involved in both LD biogenesis and catabolism. Moreover, we discovered that DFCP1 has unique biochemical properties that allow it to form an oligomeric coat on LDs, which suggests that it could play a role in the tethering of LDs to the ER and other LDs. Forcing DFCP1 to disassemble from LDs leads to misregulated LD catabolism. Thus, we postulate that DFCP1 functions as a molecular switch that releases growing LDs from the ER for catabolism. Interestingly, LD release is coupled to the accumulation of the activator of the Arp2/3 complex, WHAMM, and actin. We found that accumulation of WHAMM and actin to LDs occurs specifically in response to starvation, where it drives actin-mediated mobilization and constriction of LDs. Consequently, the goals of this proposal are to define the molecular mechanisms employed by DFCP1, WHAMM, and the actin cytoskeleton to drive LD catabolism. To fully address these questions, we will use a bottom-up approach to determine the mechanism and molecular properties that allow DFCP1 to switch LDs from growth to catabolism (Aim 1). In parallel, we will examine how WHAMM and the Arp2/3 complex coordinate with DFCP1 to drive LD dynamics and catabolism (Aim 2). The mechanistic insight obtained by these studies will provide new insights into how LD metabolism helps to maintain cellular homeostasis, but also how misregulation of this process contributes to the pathogenesis of metabolic diseases.
Lipid Droplets are specialized transient organelles that serve as reservoirs of neutral lipids that can be tapped for energy or to resupply membranes with lipids. Failure to store or use lipids efficiently is the hallmark of metabolic diseases. The proposed research will elucidate the molecular mechanisms that lipid droplet proteins use to regulate lipid metabolism, as well as how alterations in these processes impact cellular energy homeostasis. Ultimately, understanding the molecular control of lipid storage and catabolism will be paramount to gain new insights into lipid storage diseases that will form a considerable healthcare burden in the near future.
