UC Berkeley

Lipid droplets (LDs) are endoplasmic reticulum-derived neutral lipid storage organelles that consist of a core of neutral lipids bounded by a phospholipid monolayer decorated with associated proteins. The canonical role of LDs is to function as a lipid storage depot, and the accumulation of enlarged LDs is the pathological hallmark of numerous prevalent metabolic diseases, such as obesity and metabolic dysfunction-associated steatotic liver disease (MASLD). Recent findings also implicate LDs in a growing number of surprising cellular roles such as preventing lipotoxicity in neurons, increasing the bioactivation of hydrophobic drugs, and suppressing bacterial invasion as part of a protective innate immune response. Despite these important cellular functions, many details surrounding LD biogenesis and breakdown are incompletely understood. Furthermore, while it is appreciated that the regulation and function of LDs must drastically change under starvation, overnutrition, inflammation, and other altered environments, the pathways that govern these responses remain mostly unexplored.

To address this gap in knowledge, we performed a series of CRISPR-Cas9 screens under different metabolic states to uncover mechanisms of hepatic neutral lipid flux. Clustering of chemical-genetic interactions identified CLIC-like chloride channel 1 (CLCC1) as a critical regulator of neutral lipid storage and secretion. Loss of CLCC1 resulted in the buildup of large LDs in hepatoma cells and knockout in mice caused liver steatosis. Remarkably, the LDs are in the lumen of the ER and exhibit properties of lipoproteins, indicating a profound shift in neutral lipid flux. Finally, remote homology searches identified a domain in CLCC1 that is homologous to yeast Brl1p and Brr6p, factors that promote the fusion of the inner and outer nuclear envelopes during nuclear pore complex assembly. Loss of CLCC1 lead to extensive nuclear membrane herniations, consistent with impaired nuclear pore complex assembly. Thus, we identify CLCC1 as the human Brl1p/Brr6p homolog and propose that CLCC1-mediated membrane remodeling promotes hepatic neutral lipid flux and nuclear pore complex assembly.

One of the other novel genes from these screens is the interleukin IL32, a secreted pro-inflammatory cytokine that the Olzmann Lab unexpectedly identified as an LD protein using proximity labeling proteomics. Our data demonstrate that endogenous IL32 localizes to a subset of LDs following the induction of host defense signaling pathways, suggesting that IL32 may be a key host factor that acts to regulate LDs during an immune response. These findings not only lay a groundwork for elucidating the IL32-mediated mechanisms of LD-bacteria interactions, creating a foundation for an understanding of how targeting lipid storage can be exploited to halt infectious disease pathogenesis, but also highlight the importance of LD heterogeneity in a variety of cellular functions. Together, these studies provide an extensive compendium of lipid storage modulators and provide rich mechanistic insights into lipid droplet flux and LD heterogeneity in the liver.