Immune response to pathogens is energetically expensive to the host; however, the cellular source of energy to fuel immune response remains unknown. In this study, we show that Caenorhabditis elegans exposed to pathogenic Gram-positive and Gram-negative bacteria or yeast rapidly utilizes lipid droplets, the major energy reserve. The nematode’s response to the pathogenic bacterium Enterococcus faecalis entails metabolic rewiring for the upregulation of several genes involved in lipid utilization and downregulation of lipid synthesis genes. Genes encoding acyl-CoA synthetase ACS-2, involved in lipid metabolism, and flavin monooxygenase FMO-2, involved in detoxification, are two highly upregulated genes during E. faecalis infection. We find that both ACS-2 and FMO-2 are necessary for survival and rely on NHR-49, a peroxisome proliferator-activated receptor alpha (PPARα) ortholog, for upregulation during E. faecalis infection. Thus, NHR-49 regulates an immunometabolic axis of survival in C. elegans by modulating breakdown of lipids as well as immune effector production upon E. faecalis exposure.

Compartmentalization is crucial for the functioning of cells. Membranes enclose and protect the cell, regulate the transport of molecules entering and exiting the cell, and organize cellular machinery in subcompartments. In addition, membraneless condensates, or coacervates, offer dynamic compartments that act as biomolecular storage centers, organizational hubs, or reaction crucibles. Emerging evidence shows that phase‐separated membraneless bodies in the cell are involved in a wide range of functional interactions with cellular membranes, leading to transmembrane signaling, membrane remodeling, intracellular transport, and vesicle formation. Such functional and dynamic interplay between phase‐separated droplets and membranes also offers many potential benefits to artificial cells, as shown by recent studies involving coacervates and liposomes. Depending on the relative sizes and interaction strength between coacervates and membranes, coacervates can serve as artificial membraneless organelles inside liposomes, as templates for membrane assembly and hybrid artificial cell formation, as membrane remodelers for tubulation and possibly division, and finally, as cargo containers for transport and delivery of biomolecules across membranes by endocytosis or direct membrane crossing. Here, recent experimental examples of each of these functions are reviewed and the underlying physicochemical principles and possible future applications are discussed.

Immune responses to pathogenic microbes include activation of resistance and tolerance mechanisms in the host both of which are energetically expensive. In this study, we show that C. elegans exposed to Gram positive bacteria Enterococcus faecalis and Staphylococcus aureus, rapidly utilizes lipid droplets, the major energy reserve in the nematode. Feeding on E. faecalis causes developmental arrest in C. elegans larvae and growth arrest in adults, pointing to starvation response. We find that nematode's early response to infection entails upregulation of 25 genes involved in lipid hydrolysis and downregulation of 13 lipid synthesis genes as early as 8 hours following exposure. We also show that lipid droplets play a protective role in C. elegans during infection. NHR-49, a PPARα ortholog, is required for E. faecalis induced betaoxidation of fatty acids and immune effector production. It regulates an immunometabolic axis required for survival of the nematode on E. faecalis. Our findings reveal a facet of nutritional immunity wherein lipid droplet homeostasis plays a central role in nematode microbe interactions.