RESEARCH ARTICLE Effects of FABP knockdown on flight performance of the desert locust, Schistocerca gregaria Sanjeeva Rajapakse, David Qu, Ahmed Sayed Ahmed, Jutta Rickers-Haunerland and Norbert H. Haunerland* ABSTRACT During migratory flight, desert locusts rely on fatty acids as their predominant source of energy. Lipids mobilized in the fat body are transported to the flight muscles and enter the muscle cells as free fatty acids. It has been postulated that muscle fatty acid binding protein (FABP) is needed for the efficient translocation of fatty acids through the aqueous cytosol towards mitochondrial β-oxidation. To assess whether FABP is required for this process, dsRNA was injected into freshly emerged adult males to knock down the expression of FABP. Three weeks after injection, FABP and its mRNA were undetectable in flight muscle, indicating efficient silencing of FABP expression. At rest, control and treated animals exhibited no morphological or behavioral differences. In tethered flight experiments, both control and treated insects were able to fly continually in the initial, carbohydrate-fueled phase of flight, and in both groups, lipids were mobilized and released into the hemolymph. Flight periods exceeding 30 min, however, when fatty acids become the main energy source, were rarely possible for FABP-depleted animals, while control insects continued to fly for more than 2 h. These results demonstrate that FABP is an essential element of skeletal muscle energy metabolism in vivo. KEY WORDS: RNAi, Fatty acid binding protein, Insect flight, Lipid transport INTRODUCTION For many centuries, locusts have inflicted severe damage to human populations in African and Asian countries. Every few years, when weather conditions are favorable, locusts that normally develop dispersed in their solitary stage accumulate in large numbers and undergo a phase transformation to their gregarious form (Pener and Simpson, 2009). As adults, gregarious locusts form gigantic swarms that can migrate in a coordinated manner for several hundred kilometers, touching down for feeding and eradicating much of the vegetation along their path. Migratory flight of locusts is among the most energy demanding of activities, and insects have developed an efficient mechanism to fuel this metabolic activity (Wegener, 1996). In the initial phase of flight, the readily available disaccharide trehalose serves as the main energy source for muscle contraction, but within 30–60 min, lipids become the predominant metabolic fuel (Mayer and Candy, 1969). Lipids are stored as triglycerides in the fat body. Their mobilization is initiated by the release of adipokinetic hormone (AKH), which activates a signal transduction pathway that triggers the action of a lipase in the fat body. One fatty acid chain is cleaved from the triacylglycerol molecule, and the resulting diacylglycerol (DAG), which is the major transport form of lipids in insects, is released into the hemolymph (Van der Horst and Rodenburg, 2010). Locusts use an effective transport system, often referred to as the ‘lipophorin shuttle’, to assure sustained delivery of DAG to the flight muscle (Van der Horst and Rodenburg, 2010). In resting insects, the predominant hemolymph lipoprotein is the high-density form of lipophorin (HDLp), a protein composed of the two apoproteins apoLp-I (∼250 kDa) and apoLp-II (∼80 kDa), as well as phospholipids, DAG and smaller amounts of other lipids, which together amount to around 20% of the mass of the lipophorin particle. Upon their release from the fat body, numerous DAG molecules associate with HDLp, and the lipid-enriched particle is stabilized by the binding of several molecules of a third apoprotein, apoLp-III (∼18 kDa). The resulting low-density lipophorin (LDLp) has a density of ∼1.02 g ml -1 and contains more than 40% lipid, mostly in the form of DAG. A lipoprotein lipase located at the flight muscle membrane hydrolyzes DAG; free fatty acids enter the flight muscle, while glycerol and apoLp-III are released into the hemolymph. Lipophorin returns to the high-density form HDLp, which remains in the hemolymph and can resume transporting DAG from the fat body to the flight muscle (Van der Horst and Rodenburg, 2010). Although the transport of lipids through the hemolymph has been studied extensively, less is known about how fatty acids enter the flight muscle cells and translocate through the aqueous cytosol to the mitochondria, where beta-oxidation takes place. It is widely believed that fatty acid binding proteins (FABPs) play a role in intracellular transport of fatty acids, especially in muscle cells (Haunerland and Spener, 2004). FABPs belong to an ancient family of genes now called the intracellular lipid binding protein (iLBP) family, which originated more than a billion years ago (Schaap et al., 2002). The first gene duplication appears to have occurred approximately 900 mya, long before the vertebrate–invertebrate divergence, and hence all animals seem to have at least two distinct FABPs, reflecting the two major branches of the phylogenetic tree. Subsequent gene and genome duplications gave rise to the variety of FABP found today (Schaap et al., 2002). In mammals, more than 14 different members of the gene family have been identified, with distinct differences in tissue-specific expression patterns. In contrast, fewer paralogs have been characterized in insects, which appear to express only one or two isoforms on each of the two branches (Haunerland and Thakrar, 2009). In locusts, only one FABP has been characterized to date, but recent expressed sequence tag or genome sequencing projects suggest a potentially larger number of paralogs. FABP was first discovered in the flight muscle of adults of the desert locust, Schistocerca gregaria (Haunerland and Chisholm, 1990), and later in Locusta migratoria (Van der Horst, 1990; Maatman et al., 1994). FABP is the most abundant Received 15 March 2019; Accepted 1 October 2019 Department of Biological Sciences, Simon Fraser University, Burnaby, BC V5A1S6, Canada. *Author for correspondence (haunerla@sfu.ca) N.H.H., 0000-0002-0499-9400 1 © 2019. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2019) 222, jeb203455. doi:10.1242/jeb.203455 Journal of Experimental Biology