Chlamydia Subversion of Host Lipid Transport: Interactions with Cytoplasmic Lipid Droplets

Abstract

The Chlamydiaceae are Gram-negative, obligate intracellular bacteria that are significant pathogens of humans and animals. Intracellularly, the bacteria reside in a membrane-bound vacuole, called the inclusion, from which they manipulate host processes to create a niche optimal for survival and propagation. Acquisition of host-derived lipids is essential for chlamydial growth, yet the source of lipids and mechanisms of trafficking to the inclusion are not well-established. The inclusion avoids interaction with several classical membrane and lipid transport pathways. In a functional genomic screen to identify host modulating chlamydial proteins, our lab identified cytosolic lipid droplets (LDs) as potential target organelles of Chlamydia. LDs are postulated to function in many cellular processes, such as lipid metabolism and transport, membrane trafficking, and cell signaling; therefore, we hypothesized that LDs may be important for Chlamydia pathogenesis as a source of lipids or as a platform for regulating other cellular functions. Here, we characterize the interaction between eukaryotic LDs and the chlamydial inclusion. We find that LDs are recruited to the Chlamydia inclusion, chlamydial infection disrupts neutral lipid homeostasis, and pharmacological prevention of LD formation inhibits chlamydial replication. Chlamydia produces proteins (Ldas) that localize with LDs in yeast and mammalian cells when transiently expressed and are exported out of the inclusion to peripheral lipid-rich structures during infection. By electron microscopy and live cell imaging, we observe the translocation of intact LDs into the Chlamydia inclusion lumen. Biochemical and microscopic analysis of LDs from infected cells reveals that LD translocation may occur at specialized subregions of the inclusion membrane. The Chlamydia Lda3 protein is implicated in LD tethering to the inclusion membrane, and displacement of the protective coat protein, ADRP, from LD surfaces. This phenomenon could provide access for lipases to the LD core for utilization by the replicating bacteria. Additionally, the functional domains of Lda3 involved in binding to LD and inclusion membranes are identified. In these studies, we identify eukaryotic lipid droplets (LDs) as a novel target organelle important for Chlamydia pathogenesis and describe a unique mechanism of whole organelle sequestration not previously observed for bacterial pathogens. These results represent a fundamental shift in our understanding of host interactions with the chlamydial inclusion, and may represent a new area for research in the field of cellular microbiology.