Abstract
In mammals, lipid droplets (LDs) are ubiquitous organelles that modulate immune and inflammatory responses through the production of lipid mediators. In insects, it is unknown whether LDs play any role during the development of immune responses. We show that Aedes aegypti Aag2 cells – an immune responsive cell lineage – accumulates LDs when challenged with Enterobacter cloacae, Sindbis, and Dengue viruses. Microarray analysis of Aag2 challenged with E.cloacae or infected with Dengue virus revealed high transcripts levels of genes associated with lipid storage and LDs biogenesis, correlating with the increased LDs numbers in those conditions. Similarly, in mosquitoes, LDs accumulate in midgut cells in response to Serratia marcescens and Sindbis virus or when the native microbiota proliferates, following a blood meal. Also, constitutive activation of Toll and IMD pathways by knocking-down their respective negative modulators (Cactus and Caspar) increases LDs numbers in the midgut. Our results show for the first time an infection-induced LDs accumulation in response to both bacterial and viral infections in Ae. Aegypti, and we propose a role for LDs in mosquito immunity. These findings open new venues for further studies in insect immune responses associated with lipid metabolism.
Highlights
Lipid droplets (LDs) are multifunctional structures present in most organisms, from bacteria to eukaryotes
Using Ae. aegypti as a model, we show for the first time that lipid droplets (LDs) accumulate in response to immune challenges with bacteria and viruses, such as Dengue virus
We found that heat-killed Gram-positive bacteria did not alter the number of LDs, but the Gram-negative bacteria Enterobacter cloacae promoted a significant increase in the number of LDs in the Aag[2] cell cytoplasm (Fig. 1D–F and S1 and S2)
Summary
Lipid droplets (LDs) are multifunctional structures present in most organisms, from bacteria to eukaryotes. These structures are abundant in lipid storing cells, such as mammalian adipocytes and insect fat body cells, in which they were first identified. LDs are mainly composed of a fatty acid monolayer and structural proteins, such as Perilipin 1, Perilipin 2 and 31,2 These proteins are involved in LDs biogenesis and degradation, and are exclusively located in LDs, regardless the cell type[3,4,5,6,7]. We show that the constitutive activation of Toll and IMD pathways increase LD content in the mosquito midguts, suggesting that in insects, as in mammals, LDs are directly linked to the activation of immune signaling pathways
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