Abstract
Most eukaryotic cells contain varying amounts of cytosolic lipidic inclusions termed lipid bodies (LBs) or lipid droplets (LDs). In mammalian cells, such as macrophages, these lipid-rich organelles are formed in response to host-pathogen interaction during infectious diseases and are sites for biosynthesis of arachidonic acid (AA)-derived inflammatory mediators (eicosanoids). Less clear are the functions of LBs in pathogenic lower eukaryotes. In this study, we demonstrated that LBs, visualized by light microscopy with different probes and transmission electron microscopy (TEM), are produced in trypomastigote forms of the parasite Trypanosoma cruzi, the causal agent of Chagas’ disease, after both host interaction and exogenous AA stimulation. Quantitative TEM revealed that LBs from amastigotes, the intracellular forms of the parasite, growing in vivo have increased size and electron-density compared to LBs from amastigotes living in vitro. AA-stimulated trypomastigotes released high amounts of prostaglandin E2 (PGE2) and showed PGE2 synthase expression. Raman spectroscopy demonstrated increased unsaturated lipid content and AA incorporation in stimulated parasites. Moreover, both Raman and MALDI mass spectroscopy revealed increased AA content in LBs purified from AA-stimulated parasites compared to LBs from unstimulated group. By using a specific technique for eicosanoid detection, we immunolocalized PGE2 within LBs from AA-stimulated trypomastigotes. Altogether, our findings demonstrate that LBs from the parasite Trypanosoma cruzi are not just lipid storage inclusions but dynamic organelles, able to respond to host interaction and inflammatory events and involved in the AA metabolism. Acting as sources of PGE2, a potent immunomodulatory lipid mediator that inhibits many aspects of innate and adaptive immunity, newly-formed parasite LBs may be implicated with the pathogen survival in its host.
Highlights
The dynamic nature of lipid bodies (LBs), known as lipid droplets has lead to their recognition as highly active organelles within most cell types involved in different biological functions and containing lipids and many proteins
Metacyclic trypomastigote forms cultured in the presence of peritoneal macrophages for 1 h showed a significant increase of LB numbers compared to trypomastigotes alone (Fig 1A and 1B)
We investigated the ultrastructure of LBs in these intracellular forms of the parasite by transmission electron microscopy (TEM) in both peritoneal and heart inflammatory macrophages
Summary
The dynamic nature of lipid bodies (LBs), known as lipid droplets has lead to their recognition as highly active organelles within most cell types involved in different biological functions and containing lipids and many proteins (reviewed in [1,2,3]). Host-pathogen interaction leads to increased formation of LBs within cells from the immune system mainly macrophages. In these cells, LBs serve as intracellular sites for metabolic transformation of arachidonic acid (AA) into biologically active inflammatory mediators (eicosanoid derivatives) (reviewed in [5,6,7]). LBs in mammalian cells are remarkably linked to inflammatory responses and are considered structural markers of inflammation [5, 6, 8]
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