Emerging role of lipid droplets in Aedes aegypti immune response against bacteria and Dengue virus.

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.

Crystallization and preliminary X-ray diffraction analysis of HeLp, a heme lipoprotein from the hemolymph of the cattle tick Boophilus microplus.

The main protein present in the hemelymph of the cattle tick Boophilus microplus is a lipoprotein able to bind heme (HeLp). It has an apparent molecular weight of 354 000 Da and is composed of two polypeptide chains found in stoichiometric amounts. It contains 33% lipids. The protein was crystallized using the hanging-drop vapour-diffusion method in the presence of 1,6-hexanediol as a precipitant. X-ray diffraction data were collected to 2.1 A resolution using a synchrotron-radiation source. The crystal belongs to the triclinic space group P1, with unit-cell parameters a = 90.58, b = 105.50, c = 116.14 A, alpha = 112.40, beta = 111.64, gamma = 91.35 degrees. Owing to the lack of information about the amino-acid sequence, the structure of HeLp will be solved by the use of heavy atoms. Several possible derivatives have been collected and analysis is under way.