Lipid Metabolic Pathways in the Midgut of Manduca sexta

Abstract

Insects such as Manduca sexta must be efficient in obtaining energy stores in order to survive. The main goal of the M. sexta larva is to eat and store enough energy to reach the adult stage, produce eggs (in the case of females), and reproduce. Triacylglycerol (TAG), the most important energy storage molecule, is stored mainly in adipose tissues in lipid droplets, although other tissues are also able to store TAG in similar organelle structures but to a lesser degree. The phosphatidic acid (PA) pathway and the 2-monoacylglycerol (MAG) pathway are both energy-dependant acyl-CoA processes and are the main synthetic pathways by which TAG is synthesized in adipose and other tissues. My research led to the discovery of an energy-independent pathway for the synthesis of TAG that was present in the M. sexta midgut. Based on partial purification, a transacylase/lipase enzyme is present and responsible for DAG and TAG synthesis in the M.sexta midgut. Lipogenesis and lipolysis in adipose tissue involves a series of enzymes. Adiponutrin and desnutrin, two proteins involved in fat homeostasis in humans and mice, have received a lot of attention since their activities are dependent on the fed or unfed state of the animal. In this study, bioinformatics analyses were performed, which allowed the identification of an insect gene that has an ortholog in human and mice that plays an important role in adipose tissue TAG hydrolysis and synthesis. Only one insect gene ortholog was found to be present in the Aedes aegypti (mosquito), Drosophila melanogaster (fruit fly), Anopheles gambiae (mosquito), Bombyx mori (silk worm), and Tribolium castaneum (red flour beetle) genomes corresponding to genes involved in the regulation of TAG metabolism in mice (adiponutrin, desnutrin) and humans (iPLA-epsilon, iPLA-zeta, and iPLA-eta). Expression of the M. sexta calcium-independent phospholipase A2 (iPLA) ortholog has demonstrated that the protein is able to transfer acyl groups between MAGs in an energy-independent manner, similar to that in human iPLAs. This is the first example of a transacylase identified in insects.