Rab-mediated secretion of lipoproteins in Drosophila melanogaster. Sebastian Dunst, Marko Brankatschk, Anja Zeigerer, Marino Zerial, Suzanne Eaton. MPI-CBG, Dresden, Germany.

   The mammalian liver secretes very low density lipoproteins (VLDL) to export synthesized lipids to extrahepatic tissues. ApolipoproteinB-100 (apoB-100) is a principal scaffold protein that forms VLDL particles. It has an important role for the assembly of VLDL particles and their secretion through the secretory pathway that is essentially coordinated by Rab GTPases and their effectors. Although the disruption of Rab-mediated transport has been implicated in several inherited human disorders, an in vivo model system for studying vesicular transport in tissue development and function is still lacking.
   We used homologous recombination to generate a comprehensive Rab library in Drosophila melanogaster that includes rab genes fused to a fluorophore, modified by a proteolytic cleavage site, as well as precise loss-of-function alleles. We utilized this novel resource to screen for Rabs that are involved in the secretion of lipoproteins from the larval fat body, an organ analogous to the mammalian liver and white adipose tissue. The fat body produces and secretes two apoB-100 functional homologues of systemic lipid carriers, Lipid transfer particle (LTP) and Lipophorin (LPP), which are required to shuttle dietary lipids from the gut to peripheral organs (Palm et al., 2012).
   Our Rab screen identified two Rabs, whose localization suggested a role in trafficking of LPP and LTP. The fat body-specific RNAi-mediated knock down of these Rabs causes late larval lethality due to a specific depletion of LTP. The resulting mis-lipidation of LPP and lipid uptake defect from the midgut resembles the ltp mutant phenotype. Our data further suggests that the two Rabs cause LTP loss-of-function through different mechanisms as indicated by co-localization studies, qPCR and lipoprotein density gradient fractionation. Since Drosophila Rab proteins are more than 80 percent similar to their mammalian homologues, we further assess the role of Rab-mediated apoB-100 VLDL secretion and lipidation in primary mouse hepatocytes.